CUCURBIT[7]URIL HOST-VIOLOGEN GUEST COMPLEXES:

ELECTROCHROMIC AND PHOTOCHEMICAL PROPERTIES

by

MARINA FREITAG

A dissertation submitted to the

Graduate School – Newark

Rutgers, The State University of New Jersey

in partial fulfillment of requirements

for the degree of

Doctor of Philosophy

Graduate Program in Chemistry

Written under the direction of

Professor Elena Galoppini

and approved by

______

______

______

______

Newark, New Jersey

October, 2011

ABSTRACT OF THE DISSERTATION

Abstract

Cucurbituril[7] Host - Viologen Guest Complexes:

Electrochromic and Photochemical Properties

By MARINA FREITAG

Dissertation Director:

Professor Elena Galoppini

In this thesis, we demonstrated that a molecular host, cucurbit[7]uril, provides an alternative method of adsorbing molecules on semiconductors and shields the guest from the hetereogenous interface. These novel hybrid systems exhibited photophysical and electrochemical properties that differ from the properties of layers obtained by directly attaching the chromophore to the semiconductor through binding groups.

This thesis describes the host-guest chemistry between cucurbit[7]uril (CB[7]) and various series of viologen guests. Methylviologen (1,1'-dimethyl-4,4'-bipyridinium dichloride, MV2+), 1-methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+), and 1,1'-di- p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+) were encapsulated in the macrocyclic host cucurbit[7]uril, CB[7].

The complexes MV2+@CB[7] and MTV2+@CB[7] were physisorbed to the surface of

1 TiO2 nanoparticle films. The complexation into CB[7] was monitored by H NMR. TiO2 films functionalized with the complexes were studied by FT-IR-ATR and UV-Vis

ii absorption. The electrochemical and spectroelectrochemical properties of MV2+@CB[7] and MTV2+@CB[7] were studied in solution and in electrochromic windows (ECDs), where the complexes were bound to TiO2 films cast on FTO. The ECDs prepared from

2+ 2+@ MV @CB[7]/TiO2/FTO and MTV CB[7]/TiO2/FTO electrodes showed reversible, sharp and fast color switching upon application of -0.8 V.

Viologen derivative DTV2+ exhibited enhanced fluorescence upon encapsulation.

Aqueous solutions of DTV2+ were weakly fluorescent (Φ = 0.02, τ < 20 ps), whereas the emission of the DTV2+@2CB[7] complex was enhanced by one order of magnitude (Φ =

0.29, τ = 0.7 ns) and was blue-shifted by 35 nm. DTV2+ in polymethylmethacrylate

(PMMA) matrix was fluorescent with a spectrum similar to that observed for the complex in solution. DFT and CIS calculations suggested that the increased planarity of the aromatic rings and a quinonoid structure of the S1 state, induced by encapsulation in the host, can explain the observed emission enhancement. The absorption and emission spectra of DTV2+@2CB[7] in water exhibited a large Stokes shift (ΔSt ~ 10,000 cm-1) and no fine structure. 1H NMR and UV-Vis titration indicated that the DTV2+@2CB[7]

4 complex is formed in aqueous solutions with a complexation constant of K1W = 1.2×10

-1 4 -1 4 -1 4 -1 M , K2W = 1.0×10 M in water, and K1NaCl = 1.1×10 M , K2NaCl = 0.8×10 M in 0.05 M

NaCl aqueous solution.

iii Acknowledgement

I would like to take this opportunity, first and foremost, to thank my supervisor, Professor

Elena Galoppini, for her kindness, help, guidance, and patience during my studies. It has been a pleasure studying in her research group.

I would like to thank my committee members, Prof. Phillip Huskey, Prof. Jenny Lockard of Rutgers University, Newark and Prof. Angel E. Kaifer of University of Miami, Florida for their effort and time in reading and correcting my thesis.

It has been a pleasure working with members of the Galoppini group, both past and present, Dr. Jonathan Rochford, Dr. Olena Taratula, Dr. Sujatha Thyagarajan, Dr. Yongyi

Zhang, Andrew Kopecky, Keyur Chitre, Yan Cao, and Agnieszka Klimczak. I thank them for their constant support and motivation.

I would also like to thank Prof. Piotr Piotrowiak and Prof. Lars Gundlach for their kindness and helpful discussions regarding DFT and CIS calculations.

My sincere thanks also goes to Prof. Carlo A. Bignozzi and Dr. Stefano Caramori from the University of Ferrara, Italy, for offering me the research opportunity in his group and leading me working on diverse exciting projects.

I wish to also thank the faculty members of the Department of Chemistry, Rutgers

University for their guidance, excellent teaching and research advice, and also the staff of both the Department of Chemistry and Rutgers University, especially Judy Slocum and

Monika Dabrowski, for their helpfulness.

In addition, I greatly appreciate the financial assistance that has been provided by the

Donors of the American Chemical Society Petroleum Research Fund for support of this research (ACS PRF #46663-AC10).

iv And a special thanks goes to Richard Freitag for bearing with me during the completion of this manuscript.

v Table of Contents

Abstract ...... ii

Acknowledgement ...... iv

Table of Contents ...... vi

Lists of Figures ...... xi

List of Schemes ...... xxii

Lists of Tables ...... xxiii

Lists of Abbreviations ...... xxiv

Chapter A ...... 1

A.1 Introduction ...... 2

A.1.2 Nanostructured Metal Oxide Interfaces ...... 7

A.2. Supramolecular Hosts on Semiconductor Surfaces ...... 10

A.2.1 Hemicarceplexes ...... 12

A.2.2 Cyclodextrins ...... 14

A.2.3 Calixarenes ...... 21

A.2.4 Zeolites ...... 24

A.2.6 Redox Active Compounds Bound to TiO2 ...... 26

A.3 The Cucurbituril Family...... 28

A.3.1 Cucurbit[7]uril, a Molecular Host ...... 32

vi A.5 References ...... 36

Chapter B ...... 54

Introduction ...... 55

B.2 Synthesis of Viologen Derivatives ...... 59

B.2.1 Synthesis of Alkyl Viologens ...... 59

B.2.2 Synthesis of Aryl Viologens ...... 61

B.3 Host-Guest Complexes of Viologens with Cucurbit[7]uril...... 63

B.3.1 1H NMR study of MV2+@CB[7] and MTV2+@CB[7] ...... 65

B.3.2 Complexation Constant* ...... 68

B.4 Cucurbituril Complexes Bound to TiO2 ...... 69

B.4.1 Preparation of nanostructured TiO2 Films ...... 71

B.4.2 Binding on Nanostructured TiO2 Films ...... 72

B.5 Electrochemistry...... 73

B.5.1 Cyclic Voltammetry Measurements in Solution ...... 73

B.6 UV-Vis Absorption Spectroscopy ...... 77

B.6.1 UV-Vis Absorption Spectra in Solution...... 77

B.6.2 UV-Vis Absorption Measurements of Electrochromic Windows ...... 79

B.7 FT-IR-ATR...... 80

B.7.1 Measurements of Solid Complex Samples...... 80

B.7.2 FT-IR-ATR Measurements of nanostructured TiO2 films ...... 83

vii B.8 Experimental Section ...... 84

B.8.1 General ...... 84

B.8.2 Synthesis and Characterization ...... 85

B.8.2.1 Synthesis of Alkyl Viologen Derivatives ...... 85

B.8.2.2 Synthesis of Aryl Viologen Derivatives...... 86

B.8.3 Inclusion of MV2+ and MTV2+ into CB[7] ...... 88

B.8.4 Titanium Dioxide Synthesis ...... 88

B.8.5 Titanium Dioxide Film Preparation ...... 91

B.8.6 Binding to Titanium Dioxide Films ...... 91

B.8.7 Preparation of Electrochromic Windows ...... 92

B.8.8 Electrochemistry in Solution of MV2+ and MTV2+ and the Corresponding

CB[7] Complexes...... 93

B.8.9 Electrochemistry of Electrochromic Windows ...... 94

B.8.10 Spectroscopic Measurements in Solution of MV2+ and MTV2+, Their

Complexes and the Corresponding Complexation Constant of MTV2+ ...... 94

B.9 Conclusions ...... 95

B.10 References ...... 97

Chapter C ...... 104

C.1 Introduction ...... 105

C.2 Results and Discussion ...... 110

viii C.2.1 Synthesis of Symmetric Viologen DTV2+ ...... 110

C.2.2 Synthesis of Cucurbit[7]uril ...... 111

C.2.3 Host guest complexes of DTV2+ with CB[7] ...... 113

C.2.4 Steady State UV-Vis Absorption and Fluorescence Measurements ...... 118

C.2.4.2 Quantum yields and Lifetime measurements ...... 122

C.2.4.3 Binding Constant and the Influence of NaCl ...... 124

2+ C.2.4.6 Properties of DTV @CB[7] On Nanostructured ZrO2 Films ...... 129

C.2.5 FT-IR-ATR Spectroscopy of Solid Complex Samples and Nanostructured

Films ...... 130

C.2.6 Electrochemistry ...... 132

C.2.6.1 Electrochromic Properties of DTV2+ ...... 134

C.2.7 DFT and CIS Calculations ...... 137

C.3 Experimental Section ...... 140

C.3.1 General ...... 140

C.3.2 Synthesis and Characterization ...... 141

C.3.2.1 Cucurbit[7]uril synthesis ...... 141

C.3.2.2 Synthesis of DTV2+ ...... 143

C.3.2.3 1H NMR Titration-Inclusion of Viologen DTV2+ into CB[7]61-63 ...... 144

C.3.2.3.1 Complexation constants ...... 145

C.3.3 Synthesis of nanostructured ZrO2 ...... 145

ix C.3.4 Preparation of DTV2+ in PMMA Polymer Matrix films ...... 146

C.3.5. Electrochemistry...... 146

C.3.6 Spectroscopic Measurements ...... 147

C.3.7 DFT and CIS calculations ...... 148

C.4 Conclusions ...... 149

C.5 References ...... 151

Appendix ...... 161

Curriculum Vitae ...... 172

x Lists of Figures

Figure A - 1 Differences in the bandgap between metals, semiconductors (metal oxides) and insulators...... 3

Figure A - 2 Principle mechanism of charge separation in TiO2 nanoparticle ...... 4

Figure A - 3 Band positions of several semiconductors in contact with aqueous electrolyte at pH 1, (taken from reference 48)...... 5

Figure A - 4 ATM image of TiO2 thin films (left picture, taken from reference 59) and

SEM image of ZrO2 thin film (right picture, taken from references 42 and 43)...... 6

Figure A - 5 Steps of film preparation of nanocrystalline MOn ...... 7

Figure A - 6 Schematic illustrations of various surface functionalization modes by molecules...... 8

Figure A - 7 Functional anchoring groups for MOn attachment...... 9

Figure A - 8 Illustration of strategies to control the active compound distribution ...... 10

Figure A - 9 Structure of the water soluble hemicarcerand octaacid 1 and hemicarceplex

Az@1. The COOH groups are acting as ancoring groups to TiO2 nanoparticles, (taken from reference 117)...... 12

Figure A - 10 Scheme of charge transfer in an azulene@hemicarcerand bound to TiO2 colloidal solution and comparison with the directly bound dye, (taken from references

115 and 116)...... 13

Figure A - 11 Formation of the hemicarceplex Az@1 monitored by UV-Vis spectroscopy, (taken from reference 115)...... 13

xi Figure A - 12 Chemical structure of the most prominent cyclodextrins: α, β and γ, made of 6, 7, and 8 D(+)-glycopyranose units, respectively, (taken from reference 88)...... 15

Figure A - 13 Structure of the bound azobenzene dye 2, and suggested binding to the nanoparticles of complex 1@α-CD bound to TiO2, (taken from reference 129)...... 16

Figure A - 14 UV-Vis absorption spectra of TiO2 nanoparticle films (curve A, unsensitized film), sensitized with 1 (curve B), reference compound 2 (Curve C) and the complex 1@α-CD (Curve D). The figure shows a picture of the corresponding slides,

(taken from reference 129)...... 17

Figure A - 15 Structure of dye JK-2, encapsulated into β-CD, and proposed binding to

TiO2, (taken from reference 88)...... 18

Figure A - 16 Two different methods of functionalization of TiO2 with CD. The imprinting method for BPA involves physisorption of BPA@β-CD (1:2) and removal of the template, (taken from reference 88)...... 19

Figure A - 17 QCM frequency shifts of alternate adsorption of Ti(OBu)4 and BPA@β-

CD, (taken from reference 131)...... 20

Figure A - 18 Chemical structure of calixarenes and adsorption of a guest- t-Bu- calix[4]arene complex onto silica surface, (taken from reference 134,135)...... 22

Figure A - 19 Binding of calixarenes onto metal oxide (M = Si and Ti) surfaces, (taken from reference 139)...... 23

Figure A - 20 Left: Diffuse reflectance UV-Visabsorption of the calixarene-TiO2 materials. Right: Steady-state PL emission spectra ( ex = 200 nm) of (A) 1a, control materials 5a and 6a and (B) calixarene-TiO2 materials 1a-4a, (taken from reference

136,137) ...... 24

xii Figure A - 21 a) Top: dye-loaded zeolite L antenna; blue-emitting donors inside the zeolite L transfer electronic excitation energy to red-emitting acceptors at the ends.

Middle and bottom: fluorescence microscope images of an approximately 2000-nm-long crystal containing a neutral blue-emitting dye (DMPOPOP) in the middle part and cationic dye Ox+ at both ends (red, polarizer perpendicular) on selective DMPOPOP excitation. b) Top: antenna system with stopcock molecules as external traps and bottom: a schematic representation of a stopcock at the end of a zeolite L channel. The stopcock consists of a head, a spacer, and a label. c) Energy transfer (EnT) from a photonic antenna to a semiconductor, creating an electron–hole pair in the semiconductor

(radiationless near-field process), (taken from reference 91,143)...... 25

Figure A - 22 Switching between colorless and colored form of a viologen derivative with allyl and anchoring group substituents bound to nanocrystalline TiO2 films ...... 27

Figure A - 23 The cucurbit[n]uril family ...... 29

Figure A - 24 Pathways for synthesis of Cucurbit[n]uril...... 30

Figure A - 25 CB[7] (left) binding regions and electrostatic map of CB[7](right), (taken from reference 182)...... 31

Figure A - 26 Coordination of sodium to the portals of CB[7] ...... 32

Figure A - 27 Selected fluorescent dyes for CB[7] guest-host formation, (taken from reference 208, 209) ...... 34

Figure B - 1 Electrochromic Window Designed by Cinnsealach et al. based on Viologen

Modified Nanostructured TiO2 and Conducting Glass Electrodes, (taken from reference

7)...... 57

xiii Figure B - 2 Principle of signal amplification in a nanocrystalline film by high surface area and adsorbed redox chromophores, (taken from reference 29)...... 57

Figure B - 3 Cucurbit[7]uril and viologens studied in this work ...... 59

Figure B - 4 The viologens (MV2+ and MTV2+) and their complexes with CB[7]. Only one of the possible encapsulation modes is shown for MTV2+@CB[7]...... 64

1 2+ Figure B - 5 H NMR (500 MHz) in D2O of 1 mM MV a) in absence of CB[7] and b) in presence of 1 equivalent CB[7]...... 65

Figure B - 6 Chemical shift differences of MV2+ upon complexation...... 66

1 2+ Figure B - 7 H NMR (500 MHz) in D2O of 1 mM MTV a) in absence of CB[7] and b) in presence of 1 equivalent CB[7]...... 67

Figure B - 8 Chemical shift differences of MTV2+ upon complexation ...... 67

Figure B - 9 a) Absorption spectra of 1 μM MTV2+ at increasing concentrations of CB[7] b) Fitted experimental absorbance data at λmax=218 nm, (taken from reference 36)...... 69

Figure B - 10 Illustration of the physisorption for the complexes MV2+@CB[7] and

2+ 1 MTV @CB[7] on TiO2, considering the binding modes found in H NMR ...... 70

Figure B - 11 Schematic Preparation of modified TiO2 electrodes ...... 72

Figure B - 12 Schematic cross section of an electrochromic window ...... 73

Figure B - 13 Cyclic voltammograms in 0.1 M phosphate buffer (pH 7.0) of 0.05 mM

MV2+ a) in absence of CB[7] (black solid line) and b) in presence of 1 equivalent

CB[7](dashed blue line)...... 75

Figure B - 14 Cyclic voltammograms in 0.1 M phosphate buffer (pH 7.0) of 0.05 mM

MTV2+a) in absence of CB[7] (black solid line) and b) in presence of 1 equivalent

CB[7](dashed blue line)...... 76

xiv Figure B - 15 Viologens@CB[7] studied and possible electron transfer between complexes and semiconductor ...... 76

Figure B - 16 Cyclic voltammograms of electrochromic windows prepared from the

CB[7] complexes of 1 (solid line) and 2 (dotted line), (taken from reference 37)...... 77

Figure B - 17 Visible region of the absorption spectra of dication species of (a) MV2+ and

MV2+@CB[7 and (b) MTV2+ and MTV2+@CB[7 in phosphate buffer solution (pH 7.0) in a spectroelectrochemical cell. Solid and dotted lines show spectra in the absence and presence of equimolar amounts of CB[7], respectively...... 78

Figure B - 18 Visible region of the absorption spectra of one-electron reduced species of

(a) MV+ and MV+@CB[7 and (b) MTV+and MTV+@CB[7 in phosphate buffer solution (pH 7.0) in a spectroelectrochemical cell. Solid and dotted lines show spectra in the absence and presence of equimolar amounts of CB[7], respectively...... 79

2+ 2+ Figure B - 19 Absorption spectra of MV @CB[7/TiO2 and MTV @CB[7/TiO2 measured in an electrochromic window after application of -0.8 V...... 80

Figure B - 20 Picture of color changes of an electrochromic window prepared from

2+ MTV @CB[7/TiO2/FTO (a) before and (b) after application of a 800 mV potential. .. 80

Figure B - 21 FT-IR-ATR of MV2+ (blue solid line), MV2+@CB[7 (black solid line) and

CB[7] (red dashed line) ...... 82

Figure B - 22 FT-IR-ATR of MTV2+ (blue solid line), MTV2+@CB[7 (black solid line) and CB[7] (red dashed line) ...... 82

Figure B - 23 FT-IR-ATR of adsorbed complexes MV2+@CB[7 (black solid line),

2+ MTV @CB[7 (red dash line) and CB [7] (blue dotted line) on TiO2/FTO...... 83

Figure B - 24 Experimental set up for the hydrolysis...... 89

xv Figure B - 25 Picture of the custom-made titanium autoclave for the sol gel process of

TiO2 nanoparticles...... 90

Figure B - 26 Construction of electrochromic windows, (taken from reference 37)...... 93

Figure C - 1 Illustration of the formation of a fluorescent guest-host inclusion complex.

...... 105

Figure C - 2 Molecular structure of DTV2+, the viologen derivative studied in this work.

The counter ion is chloride. Molecular structure and dimensions of cucurbit[7]uril and the corresponding inclusion complex...... 106

Figure C - 3 Fluorescent oxo-pyridone compounds ...... 107

Figure C - 4 Proposed Binding Interactions in the ThT, CB7, and Metal Ion System

Leading to the Highly Fluorescent Supramolecular capsule. (Taken from reference 26)

...... 108

1 Figure C - 5 H NMR of Cucurbit[7]uril (CB[7]) in D2O (commercially available,

Sigma-Aldrich, cat.# 545201)...... 113

1 Figure C - 6 H NMR of Cucurbit[7]uril (CB[7]) in D2O...... 113

2+ Figure C - 7 Viologen region of the 1H NMR spectra in D2O of DTV , and after addition of 0.5, 1.0, 2.0, and 3.0 equivalents of CB[7]. The 3-6 ppm region, the CB and solvent signals were omitted for clarity...... 114

Figure C - 8 Chemical shift differences of DTV2+ upon complexation with 2 equivalents of CB[7] in D2O. Similar shifts were observed in the presence of NaCl...... 114

Figure C - 9 Schematic representation of shuttling of a 1:1 complex DTV2+@CB[7] ... 115

2+ Figure C - 10 Viologen region of the 1H NMR spectra of DTV in 0.05 M NaCl in D2O, and after addition of 0.5, 1.0, 2.0, and 3.0 equivalents of CB[7]...... 116

xvi 1 Figure C - 11 H NMR of Cucurbit[7]uril region (6.5 ppm - 3.5 ppm) in D2O, at host- guest ratios (0.5:1, 1:1, 1:2, 1:3) of DTV2+:CB[7] following the viologen encapsulation.

...... 117

Figure C - 12 Absorption spectra of DTV2+ (black solid line) and the corresponding complex DTV2+@CB[7] (red dashed line) ...... 118

Figure C - 13 Emission spectra of DTV2+ (5M) upon addition of CB[7] (concentration range: 1-15 M). λex = 350nm...... 119

Figure C - 14 Job’s plot for complexation of CB[7] and DTV2+. The total concentration of [CB[7]]+[DTV2+] was kept at 0.6 mM...... 120

Figure C - 15 Fluorescence emission titration curve of 5 µM DTV2+ with CB[7]in water

(circles, blue line) and in presence of 0.05 M NaCl (squares, dashed black line). λex = 350 nm ...... 121

Figure C - 16 Normalized absorption and emission spectra of DTV2+@2CB[7] in aqueous solution...... 121

Figure C - 17 Fluorescence lifetime measurements ...... 123

Figure C - 18 Emission spectra of DTV2+ (8.0 wt %) in PMMA polymer matrix ...... 124

Figure C - 19 Changes in the UV-vis spectrum during the complexation of DTV2+ (5 µM) with CB[7] ...... 126

Figure C - 20 Fitted (red solid line, 1:2 model) experimental data (squares) of absorbance

2+ change (Aobs) of DTV at 335 nm against the concentration of CB[7] in water ...... 126

Figure C - 21 Fitted (red solid line, 1:2 model) experimental data (squares) of absorbance

2+ change (Aobs) of DTV at 335 nm against the concentration of CB[7] in 0.05 M NaCl

...... 127

xvii Figure C - 22 Integrated Fluorescence titration of a) DTV2+ and b) DTV2+@CB[7] 1:1

(blue line) and 1:2 (black line) g:h complex with NaCl ...... 127

Figure C - 23 Emission spectra for fluorescence titration of DTV2+@CB[7] complex (3

µM DTV2+ and 1mM CB[7] excess) with NaCl ...... 128

2+ Figure C - 24 Emission spectrum of DTV @2CB[7] on ZrO2 nanoparticle thin film; exc

= 350 nm...... 130

Figure C - 25 FT-IR-ATR of DTV2+ (blue solid), CB[7] (black dashed) and

DTV2+@CB[7] (red solid) ...... 131

Figure C - 26 FT-IR-ATR of adsorbed complex DTV2+@CB[7] (black solid), CB[7]

(blue dashed) on ZrO2 films and ZrO2 blank (red dotted) ...... 132

Figure C - 27 Cyclic voltammograms in 0.1 M phosphate buffer (pH 7.3) of 0.05 mM solutions of DTV2+ in the absence (black solid line) and in the presence (red dashed line) of CB[7] ...... 134

Figure C - 28 Absorption and spectra of DTV2+ in water before (black solid line) and after (red dashed line) one-electron reduction to radical cation DTV●+ by application of –

1.0V...... 135

Figure C - 29 Emission spectra of DTV2+ in water before (black solid line) and after (red dashed line) one-electron reduction to radical cation DTV●+@2CB[7] by application of –

1.0V. exc= 350 nm ...... 135

Figure C - 30 Absorption spectra of DTV2+@2CB[7] in water before (black solid line) and after (red dashed line) one-electron reduction to radical cation DTV●+@2CB[7] by application of –1.0V...... 136

xviii Figure C - 31 Emission spectra of DTV2+@2CB[7] in water before (black solid line) and after (red dashed line) one-electron reduction to radical cation DTV●+@2CB[7] by application of –1.0V. exc = 350 nm ...... 136

2+ Figure C - 32 Optimized structures of the S0, S1 and T1 states of DTV indicating the inter-ring dihedral angles, as well as the (S0-S1) and (S1-T1) intramolecular reorganization energies. The dashed box indicates the almost planar geometry of three consecutive rings in the S1 state and the two bipyridyl central rings in the T1 state...... 138

2+ Figure C - 33 Illustration of bond lengths in the S1 state of DTV . The three semi planar rings exhibit alternating bond lengths. The last p-tolyl ring’s bond lengths are all identical. Bonds with length changing by more than 0.025 Å are shown in color

(contraction in red and lengthening in green), and bonds in black are all identical in length. For bond lengths see Table C - 2...... 139

Figure C - 34 Synthesis and purification overview for CB[7] ...... 143

Figure 1 Structure of 1-butyl-4-(pyridin-4-yl)pyridinium bromide (2) ...... 162

1 Figure 2 H NMR of 1-butyl-4-(pyridin-4-yl)pyridinium bromide (2) in D2O ...... 162

Figure 3 Structure of 1-butyl-1'-(2-(diethoxyphosphoryl)ethyl)-4,4'-bipyridine-1,1'-diium bromide (3)...... 162

Figure 4 1H NMR of 1-butyl-1'-(2-(diethoxyphosphoryl)ethyl)-4,4'-bipyridine-1,1'- diium (3) in D2O ...... 163

Figure 5 Structure of 1-butyl-1'-(2-phosphonoethyl)-4,4'-bipyridine-1,1'-diium (4) ... 163

Figure 6 1H NMR of 1-butyl-1'-(2-phosphonoethyl)-4,4'-bipyridine-1,1'-diium (4) in

D2O ...... 163

xix Figure 7 Structure of 1-butyl-1'-(2-carboxyethyl)-4,4'-bipyridine-1,1'-diium bromide (5)

...... 164

Figure 8 1H NMR of 1-butyl-1'-(2-carboxyethyl)-4,4'-bipyridine-1,1'-diium bromide .. 164

Figure 9 Structure of 1-(2,4-dinitrophenyl)-4-(pyridin-4-yl) pyridinium chloride (7) ... 165

Figure 10 1H NMR of 1-(2,4-dinitrophenyl)-4-(pyridin-4-yl) pyridinium chloride (7) . 165

Figure 11 13C NMR of 1-(2,4-dinitrophenyl)-4-(pyridin-4-yl) pyridinium chloride (7) 165

Figure 12 Structure of 4-(pyridin-4-yl)-1-p-tolylpyridinium chloride (10) ...... 166

Figure 13 1H NMR of 4-(pyridin-4-yl)-1-p-tolylpyridinium chloride (10) ...... 166

Figure 14 13C NMR of 4-(pyridin-4-yl)-1-p-tolylpyridinium chloride (10) ...... 166

Figure 15 Structure of 1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+, 11) . 167

Figure 16 1H NMR of 1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+, 11) .. 167

Figure 17 13C NMR of 1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+, 11) 167

Figure 18 Structure of 1,1'-Bis(2,4-dinitrophenyl)-(4,4'-bipyridine)-1,1'-diium dichloride

(12) ...... 168

Figure 19 1H NMR of 1,1'-Bis(2,4-dinitrophenyl)-(4,4'-bipyridine)-1,1'-diium dichloride

(12) ...... 168

Figure 20 13C NMR of 1,1'-Bis(2,4-dinitrophenyl)-(4,4'-bipyridine)-1,1'-diium dichloride (12) ...... 168

Figure 21 Structure of 1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+, 14)

...... 169

Figure 221H NMR of 1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+, 14)

...... 169

xx Figure 23 13C NMR of 1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+,

14) ...... 169

Figure 24 MALDI-MS of 1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+, 11)

...... 170

Figure 25 MALDI-MS of 1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+,

14) ...... 171

xxi List of Schemes

Scheme B - 1 Electrochemical processes of methylviologen ...... 56

Scheme B - 2 Illustration of the Inclusion of MV2+ into CB[7]...... 58

Scheme B - 3 Formation of an ammonium bromide salt via the Menshutkin Reaction ... 60

Scheme B - 4 Synthesis of phosphonated viologen (4) and carboxylated viologen (5) by

Menshutkin reaction...... 60

Scheme B - 5 Mechanism of synthesis for pyridinium salt MTV2+using Zincke reaction,

(taken from reference 59-61)...... 62

Scheme B - 6 Synthesis of MTV2+ ...... 62

Scheme C - 1 Synthesis of DTV2+ ...... 111

Scheme C - 2 Mechanism of formation for cucurbituril homologues from oligomers .. 112

Scheme C - 3 One-electron redox processes of DTV2+ and associated color changes ... 133

xxii Lists of Tables

Table A - 1 Performance of DSSCs prepared from JK-2 ...... 18

Table B - 1 Voltammetric parameters for free and encapsulated MTV2+and MV2+, (taken from references 36,37) ...... 74

Table C - 1 Selected photophysical properties of 5µM aqueous solutions of DTV2+ and its

CB[7]complexes ...... 120

Table C - 2 Voltammetric parameters for free and encapsulated DTV2+ ...... 130

Table C - 3 Calculated C-C and C-N bond lengths of the conjugated core of DTV2+ in the

S0, S1 and T1 electronic states. Refer Figure C - 32 for bond assignment. All values are in Å...... 134

xxiii Lists of Abbreviations

1:1 a host guest complex composed of one guest and one host

1:2 a host guest complex composed of one guest and two hosts

1:3 a host guest complex composed of one guest and three hosts

Ag/AgCl silver/silver chloride electrode

C.B. conduction band

CB[7] cucurbit[7]uril

CB[n] cucurbit[n]uril

CIS a single excitation configuration interaction calculation

CNTs carbon nanotubes

CV cyclic voltammetry

CV2+ 1-butyl-1'-(2-carboxyethyl)-4,4'-bipyridine-1,1'-diium

D2O deuterium oxide

DBO 2,3-diazabicyclo[2.2.2]oct-2-ene

DFT density functional theory

DMSO dimethyl sulfoxide

DSSC dye-sensitized solar cell

DTV2+ 1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium

Ebg band gap

ECW electrochromic window equiv. equivalents

xxiv ESI-MS electrospray ionization mass spetroscopy

ET electron transfer

Et ethyl eV electron volt

FTO fluorine doped tin oxide g:h guest to host ratio

HOMO Highest Occupied Molecular Orbital

Hz Hertz

IR infrared

J coupling constant in Hz

K complexation constant

LUMO Lowest Occupied Molecular Orbital

M Molar concentration in mol per Litre m/z mass-to-charge ratio

MALDI-TOF Matrix Assisted Laser Desorption Ionization- Time Of Flight

Me methyl

MOn metal oxide nanopacticle

MS mass spectroskopy

MTV2+ 1-Methyl-1'-p-tolyl-4,4'-bipyridinium

MV2+ methylviologen

NMR nuclear magnetic resonance spectroscopy ppm parts per million ps pico second

xxv PV2+ 1-butyl-1'-(2-phosphonoethyl)-4,4'-bipyridine-1,1'-diium

SCE standard calomel electrode

ThT Thioflavin

TLC thin layer chromatography

Trp Tryptophan

UV-Vis Ultraviolet–visible spectroscopy

V.B. valence band wt. % weight percent

δ chemical shift in ppm

Δδ induced shift change in ppm

λmax wavelength of maximum absorbance

λex excitation wavelength

xxvi 1

Chapter A

Introduction

Host - Guest Complexes on

Nanostructured Metal Oxide Interfaces

2

A.1 Introduction

In December 1959 in a meeting of the American Physical Society Richard Feynman gave a classic science lecture with the title ―There is plenty of room at the bottom‖.1,2 Feynman proposed manipulations at a smaller and smaller scale down to the scale of single atoms.3

Nanoscience, studying matter at the 10-9 m scale, provides new opportunities for the development of innovative nanostructured materials, opening new possibilities in science and technology. A partial list includes molecular electronic memories, circuits based on nanowires and carbon nanotubes, novel circuit architectures, new photonic materials and new patterning approaches. Phenomena occurring on this order of magnitude are of interest to physicists, chemists, biologists, electrical and mechanical engineers, and computer scientists, making research in nanotechnology a frontier activity in materials science.4-7 A fast developing part of science is nanochemistry, which involves synthesis, characterization, and modification of nanoparticles and nanostructures. The changes of chemical properties and reactivity of such structures have major impact on their chemical and physical properties.8-12

First of all, literature does not consistently define the term ―nanoparticle‖.13,14 Some descriptions involve the nanometer size (10-9 m) of those particles, number of atoms in a cluster and intermediates between molecules or atoms and the solid phase of a material.6,7,15,16 Studies are being conducted on the potential use of nanomaterials in diverse applications, including hydrogen storage,17 ion-18 and gas-sensing,19,20 surface- modified nanoparticles for enhanced oil recovery,21 adsorption of chemical and biological agents onto nanoparticles,22 active electrode materials for lithium-ion batteries,23 light- emitting16 and electrochromic devices,24-26 and many more.15 3

This thesis will focus on wide band gap metal oxide (particularly TiO2, and ZrO2) nanoparticles.16,27-30 Nanostructured wide band gap metal oxides are important semiconducting materials for renewable energy applications, including artificial

31-34 35-38 39-41 photosynthesis, DSSCs, and photocatalysis. The band gap (Ebg) between the conduction and valence band determines the properties and the difference between a semiconductor and an insulator (Figure A - 1).

Figure A - 1 Differences in the bandgap between metals, semiconductors (metal oxides) and insulators.

The wide band gap metal oxides that are the focus of this thesis are the n-type

42 semiconductors, Titanium dioxide (TiO2). The ability of TiO2 to act as a photocatalyst for water splitting was discovered in 1973.43 When the semiconductor absorbs light of energy larger than the band gap, an electron is excited from the valence to the conduction band, leaving a hole in the valence band. The charge-hole recombination has to be suppressed by trapping the generated electrons or holes using a donor or an acceptor,

(Figure A - 2). 4

Figure A - 2 Principle mechanism of charge separation in TiO2 nanoparticle

0 In TiO2, the titanium is in oxidation state IV (d ). The oxide exists in several polymorphs of which two of them are more relevant: anatase (tetragonal) and rutile (tetragonal). For anatase the band gap is 3.2 eV, slightly narrower is that of the rutile with 3.0 eV. This transition in the UV region results in an absorption at 390 – 400 nm. Rutile is the most

44-46 stable bulk form of TiO2, but anatase is reported to have better photoactivity and is more stable in nanoparticle form. Recent studies have demonstrated that is possible to improve the photocatalytic activity of anatase TiO2 by transition metal doping (with V,

Cr, Fe), nonmetal doping (with N, C, S) and by adsorption of small cations (H+, Li+, Na+,

Mg+).47 5

Figure A - 3 Band positions of several semiconductors in contact with aqueous electrolyte at pH 1, (taken from reference 48).

Another important class of metal oxide nanoparticles is zirconium dioxide (ZrO2).

Zirconium oxide exists in three stable polymorphs: monoclinic is the most stable form up

49-51 to 1100°C, followed by tetragonal (≤ 2370°C) and cubic (≥ 2370°C). ZrO2 is technologically very important for a wide range of applications including laser mirrors,52

53-55 and optical, insulating, and wear resistant coatings. ZrO2 nanoparticles have a similar

56 morphology to TiO2. However, given the wide band gap of 5.0 eV and absorption at

330 nm, ZrO2 it is considered an insulator. These properties make nanocrystalline films of

ZrO2 an excellent substrate for steady state and fluorescence studies of excited state dyes or chromophores on nanoparticle surface, as the fluorescence emission is not quenched.29,38,51,57,58 6

Figure A - 4 ATM image of TiO2 thin films (left picture, taken from reference 59) and

SEM image of ZrO2 thin film (right picture, taken from references 42 and 43).

Metal oxide nanoparticles are often prepared using the sol-gel method, where the reaction is stopped right before gelation occurs, (Figure A - 5).6 Typically it involves hydrolysis and condensation of a metal alkoxide in an alcoholic solution. During the process a gel is formed consisting of a metal oxide network, which results in a nanoporous structure after calcination. Autoclaving of the sol-gels allows controlled growth of crystalline particles.

Pore size, distribution and interconnection are determined by the reaction conditions. To improve the porosity and allow deposition on a substrate, a polymeric binder, polyethylene glycol (PEG), is added to form a homogeneous paste. After the deposition of the MOn paste on a suitable substrate, which can be conducting fluorine SnO2 (FTO) glass substrate, the films are sintered at 450 °C, leading to film coatings with a high surface area and optical transparency. The properties of the nanoparticles are determined by the nucleation, growth and aging. The resulting nanocrystalline films of ZrO2 or TiO2 have a typical thickness of 10 μm and average particle size of 10 to 20 nm.60 7

Figure A - 5 Steps of film preparation of nanocrystalline MOn

A.1.2 Nanostructured Metal Oxide Interfaces

Molecular functionalization (with chromophores, redox compounds, biomolecules etc.)

28,29,61 of the MOn interface enables the use of metal oxide semiconductors in many currently relevant technologies, including DSSCs,35,37,60 photocatalysts,39,62-66 hybrid bioorganic sensors,67-69 optoelectronics70-72 and electrochromic devices.26,73-83 The design of these functional interfaces requires controlled molecular engineering. Most of the interfaces require the following two components: the nanoparticles and a directly 8

connected active compound (chromophore, redox compound, dopant etc.), as illustrated in Figure A - 6.84,85

Figure A - 6 Schematic illustrations of various surface functionalization modes by molecules.

The functionalization of interfaces can be achieved by various methods, most of them requiring structural modifications of the attached molecules. The active compounds can be attached to the surface by covalent binding, ion association, physisorption, trapping in hosts and pores, or by hydrophobic interactions. The most dominant variation for modifying metal oxide nanoparticle is covalent binding. The active compound is attached through several functional groups, silanyl (-O-Si-), sulfide (-SH), amide (NH(C=O)-),

34,48,56,58,60 phosphonate (-P(=O)(OH)2) or the carboxylic group (-COOH), Figure A - 7.

The phosphonate and the carboxylic acid groups are especially efficient and stable attachments as they form covalent bonds with the hydroxyl groups on the MOn surface. 9

Figure A - 7 Functional anchoring groups for MOn attachment.

The binding of dyes/chromophores influences the semiconductor surface properties and can lead to aggregation, dimerization and excimers.35,61,86,87 The ability to control the distribution of such sensitizer on nanocrystalline films is an important challenge in molecular engineering of functionalized nanoparticles. Several strategies can be applied, including the addition of additives or coadsorbents, typically fatty acids,88-90 substitution with bulky groups, trapping in pores91 and spacing trough tripodal linkers.58,60 Each of these strategies can influence the system‘s properties and chromophore‘s performance.

This thesis describes a new strategy by binding the active compound through a molecular cage molecule.

10

Figure A - 8 Illustration of strategies to control the active compound distribution

A.2. Supramolecular Hosts on Semiconductor Surfaces

In order to tune and control the properties of the nanostructured interfaces, redox-active compounds or dyes are used for the surface modifications, which is a key step to developing new functional nanomaterials for display13,58,92-94 and sensor16,92-95 applications. Dye-binding requires the use of functional groups to form covalent bonds and to provide strong electronic coupling with the semiconductor surface. Alternative methods to adsorb molecules on semiconductor surfaces have been developed. They involve the encapsulation of guest molecules in supramolecular organic cages or macrocyclic hosts, including hemicarceplexes,5,96 cyclodextrins,97-100 calixarenes,101,102 and cucurbiturils91,103-105 followed by the binding of the resulting host@guest complexes to the semiconductor surface.91 Additional supramolecular structures are zeolites, a microporous aluminosilicate mineral.

IMPORTANT NOTE: The sentences or paragraphs with asterisks in this thesis were cited verbatim or slightly modified from papers of which I am a co-author. 11

The complexation behavior of many organic macrocycles and supramolecular cages is very well known so that matching hosts can be selected for a variety of guests. These include, but are not limited to, metal ions, redox active compounds, and organic chromophores.91 In many cases the complexation constants are high, leading to stable guest@host complexes. While the approach is limited by the properties of the guest including size and lipophilicity as well as their ability to form stable complexes, a big advantage of this approach is that a structural modification of the guest with anchor groups may not be needed, since the binding occurs through the host. In many cases, the encapsualation leads to significant changes in the molecule‘s chemical, electrochemical, or photophysical properties.4,106,107 In some cases these changes have an advantageous effect on the semiconductor‘s interface properties like increased fluorescence lifetime, the prevention of aggregate formation on the semiconductor surface, an increased dye stability, prevention of excited state quenching, or other processes resulting from close dye-to-dye contact. Finally, encapsulation may shield the guest on the highly heterogeneous semiconductor surface.

In summary, the ability to bind host@guest complexes onto nanostructured semiconductors could lead to a much better control of interfaces that are important for renewable energy, sensor and display technologies and,86,108-113 as shown in some of the results and examples here, offer entirely new applications. 12

A.2.1 Hemicarceplexes

Carcerands, originally developed by Donald J. Cram in 1985, are cage molecules with a hydrophobic interior that can completely entrap their guests so that they do not escape.114

In contrast, hemicarcerands such as octacarboxylic acid 1 in Figure A - 10,115,116 which can be synthetically modified and are more widely studied, allow hydrophobic guests to enter and exit the interior at high temperatures, but form stable complexes, called hemicarceplexes, at room temperature.115,116A series of organic chromophores encapsulated within hemicarcerands were studied by Deshayes,116 Ramamurthy,117 and others118,119to investigate donor-acceptor energy-transfer processes.

Figure A - 9 Structure of the water soluble hemicarcerand octaacid 1 and hemicarceplex

Az@1. The COOH groups are acting as ancoring groups to TiO2 nanoparticles, (taken from reference 117).

Piotrowiak and coworkers, were the first to study interfacial charge transfers in a hybrid assembly composed of a chromophore@hemicarcerand complex covalently bound to nanoparticles of TiO2. The effect of encapsulation on the charge transfer dynamics was probed by comparisons with a directly bound azulene dye, 1-carboxyazulene, Figure A - 13

10.91,115,116 The host was the water soluble hemicarcerand. It has eight COOH anchoring groups substitutents on the exterior, a hydrophobic cavity, wide portals for access of small organic chromophores, and formes a stable complex in aqueous solutions with azulene, which is hydrophobic, Figure A - 9 and Figure A - 10.115,116

Figure A - 10 Scheme of charge transfer in an azulene@hemicarcerand bound to TiO2 colloidal solution and comparison with the directly bound dye, (taken from references

115 and 116).

Figure A - 11 Formation of the hemicarceplex Az@1 monitored by UV-Vis spectroscopy, (taken from reference 115). 14

The complexation was monitored by UV-Vis (Figure A - 11) and 1 H NMR spectroscopy.

For hemicarceplex Az@1, the binding constant was 1  10 8 M 1 . In solution, fluorescence emission of azulene and the reference compound 1-carboxyazulene from the

S1 state had a quantum yield of 0.02 and a lifetime of 1.5 ns. The hemicarceplexes were bound to diluted colloidal aqueous solutions of TiO2 or ZnO nanoparticles. The binding of hemicarceplex Az@1 to colloidal TiO2 nanoparticles was probed by following the spectral changes in the FT-IR spectra in the C=O stretch region.

In case of 1-carboxyazulene directly bound to TiO2, the photoinduced electron transfer was also rapid and efficient. The recombination kinetics of the encapsulated chromophore, however, was three orders of magnitude slower. The exceptional interfacial kinetics of Az@1/TiO2 were ascribed to slow tunneling through the walls of the cage, and consequent averaging out of the normally observed wide distribution of rates. Finally, it was observed that, while Az-COOH/TiO2 diluted colloidal solutions are stable, Az-COOH attached to TiO2 nanoparticle thin films rapidly decomposed.

However, Az@1 was stable on thin films, indicating that encapsulation into the host can indeed lead to improved stability of the dyes on semiconductors.91,115,116

A.2.2 Cyclodextrins

Cyclodextrins (CDs) are colorless water soluble cyclic oligosaccharides, consisting of six to eight D(+)-glycopyranose units connected through  -1,4 glycosidic bonds. They form host-guest complexes with hydrophobic compounds that fit in the hydrophobic cavity.

Depending on the CD‘s and the guest‘s size, 1:1, 1:2, or 2:1 guest@CD complexes are 15

formed. Cyclodextrins are commercially available, and have found a large number of applications in supramolecular chemistry120-124 and nanotechnology.121 Dye@CDs complexes have been used to directly functionalize nanoporous TiO2 nanoparticle films and semiconductor quantum dots for the development of dye-sensitized solar cells125 and sensors.126

Figure A - 12 Chemical structure of the most prominent cyclodextrins: α, β and γ, made of 6, 7, and 8 D(+)-glycopyranose units, respectively, (taken from reference 88).

The anchoring group of the CDs to the surface of metal oxides is the covalent attachment of the OH group on the macrocycle‘s rims.

127 Functionalization of TiO2 by using CDs was studied by Haque and coworkers.

Azobenzene dye 1 was complexed with α-CD, then capped with bulky groups to prevent de-complexation and then bound to TiO2 films. For a comparison, azobenzene dye 2, which has a sulphonate anchor group substituent, was bound directly. 16

The UV-Vis spectra of the sensitized films are shown in Figure A - 14, and indicate clearly that for dye 2, and complex 1@α-CD did bind to TiO2. The charge recombination dynamics observed for 1@α-CD/TiO2 were about two orders of magnitude slower than for the dye 2 direcly bound (τ1/2 ~300 and 4 µs), but the magnitude of the transient absorption signal did not decrease. These studies proved that the molecular encapsulation provides a viable control of interfacial charge transfer.

1@α-CD/TiO2 2/TiO2

Figure A - 13 Structure of the bound azobenzene dye 2, and suggested binding to the nanoparticles of complex 1@α-CD bound to TiO2, (taken from reference 129). 17

Figure A - 14 UV-Vis absorption spectra of TiO2 nanoparticle films (curve A, unsensitized film), sensitized with 1 (curve B), reference compound 2 (Curve C) and the complex 1@α-CD (Curve D). The figure shows a picture of the corresponding slides,

(taken from reference 129).

A second and more recent example was developed by Ko and coworkers.128 They encapsulated a push-pull dye (3-{5‗-[N,N-bis(9,9-dimethylfluorene-2-yl)phenyl]-2,2‗- bisthiophene-5-yl}-2-cyano-acrylic acid (JK-2)) into  -CD and attached this complex onto nanostructured TiO2 films. This TiO2 functionalization had a similar effect as coadsorbents by shielding the chromophoric unit, Figure A - 15. Studies of the TiO2 films and of DSSCs prepared from JK-2@β-CD/TiO2 showed that the encapsulation prevents aggregation of the dye, enhances the adsorbtion of the dye to TiO2, and produces long- lived charge separation.125

18

Figure A - 15 Structure of dye JK-2, encapsulated into β-CD, and proposed binding to

TiO2, (taken from reference 88).

Table A - 1 Performance of DSSCs prepared from JK-2 [a]125

J sc V oc  Dye assembly ff [mAcm 2 ] [V] [ % ] JK-2 14.51 0.70 0.73 7.42 JK2@  -CD 15.34 0.76 0.74 8.65

Table A - 1 summarizes the DSSCs data for JK-2 bound alone, for JK-2 in the presence

125 of a non-complexating co-binder (DCA), and for JK-2@β-CD on TiO2.

In summary, improved efficiencies and stability of JK-2, and slower recombination processes for the of JK-2@β-CD DSSCs, although moderate, were clearly ascribed to the effect of the encapsulation of the dye inside the cyclodextrin, rather than a simple

‗dilution‘ effect as shown by comparisons with DCA as co-adsorber (Figure A - 16). 19

In a third example, sensors based on adsorption of analyte@CD complexes onto TiO2/Pt electrodes were developed to detect organic compounds such as bisphenol A (BPA) or nitroaromatic compounds used in landmines.129,130 The electrodes, incorporated in quartz crystal microbalance (QCM) sensors were able to detect low concentrations of analytes

(detection limit ~1 x 10-8 M). Two approaches were used: either modification of the

TiO2/Pt electrode with CDs or modification with the complex BPA@-CD, and removal of BPA to form a layer that is ‗imprinted‘ for that particular template host, Figure A - 16.

Figure A - 16 Two different methods of functionalization of TiO2 with CD. The imprinting method for BPA involves physisorption ofBPA@β-CD (1:2) and removal of the template, (taken from reference 88).

The TiO2/Pt electrode was functionalized by immersion in 10 mM aqueous solutions of

BPA@β-CD (1:2). The guest BPA was removed by rinsing with organic solvents, to form the β-CD/TiO2/Pt electrode. The whole process was called ‘molecular imprinting‗, since the host-functionalized layer retained a shape memory, and thereby enhanced selectivity for the guest used as the template. The TiO2 layer formation and the template adsorption 20

steps were repeated in cycles on a modified QCM to form layered films of functionalized

TiO2. The layering process was monitored by measuring average frequencies after the adsorptions of Ti(OBu)4 (16  7 ) Hz and binding of the BPA@β-CD complex ( 25  7 )

Hz. Removal of BPA from the template resulted in a 2-3 Hz difference.

Figure A - 17 QCM frequency shifts of alternate adsorption of Ti(OBu) 4 and BPA@β-

CD, (taken from reference 131).

The binding was detected by cyclic surface polarization impedance (cSPI), a method used to monitor adsorption or desorption of chemical substances onto the electrode. Impedance measurements in the presence of BPA indicated that the response of the imprinted sensor was much larger than that obtained for bare Pt or Pt/TiO2 electrodes. Competition experiments with other structurally related guests indicated that the imprinted sensor surface provided a specific binding site for BPA.

In the direct approach, without imprinting, TiO2 layers functionalized with β-CD were commonly employed to detect nitroaromatics used as landmine explosives, including 1,3- 21

dinitrobenzene, 2,4-dinitrotoluene (2,4-DNT), and trinitrotoluene. The modification of the Pt/TiO2 films with β-CD was monitored by QCM, SEM and XPS, observed changes for the measurements were consistent with the binding of β-CD.QCM measurements were recorded for alternating adsorptions of TiO2 layers and β-CD. The largest frequency shifts were observed for 2,4-DNT, with an estimated amount about 3 to 6 times higher compared to other nitroaromatics, indicating some level of selectivity.

A.2.3 Calixarenes

Calixarenes are the recent representatives of molecular hosts, since they were developed later than cyclodextrins.131-133 Macrocycles of calix[n]arenes are constructed by linking a number of phenol residues via methylene bridges, Figure A - 18. Upon rotation around the methylenic units, the aromatic rings can point up or down, leading to different possible conformations of the calixarene ring, from a perfectly conical shape to twisted cones. Calixarenes comprise of two major classes: phenol-derived and resorcinol-derived calixarenes, depending on the position of the OH group. The great freedom to structurally modify calixarenes allows creating various types of host structures, since the phenolic hydroxyl groups and the number of repeating units can be modified in various ways.

Therefore it is possible to design an array of functionalized hosts.134,135 The cavity of calixarenes is hydrophobic, and large enough to form complexes with small organic molecules, such as benzene, or phenol. Calixarenes have found extensive applications from sensors for heavy metal ions to the development of fluorescent probes, to enzyme mimetics.134,135 22

Figure A - 18 Chemical structure of calixarenes and adsorption of a guest- t-Bu- calix[4]arene complex onto silica surface, (taken from reference 134,135).

Modification of metal oxides with calixarenes for sensitization processes was unexplored until recently. Katz and coworkers had first explored binding of t-Bu-calixarenes and encapsulation of small organic guests onto insulating silica surfaces,

136 Figure A - 20. The silica surface was pre-treated with SiCl 4 to activate the anchoring, and t-Bu-calixarenes were bound from a solution. The films were again immersed in a solution of the guest. The π-electron-rich calixarene cavity was used to immobilize on the surface small organic aromatic guests including toluene, benzene, phenol, and nitrobenzene. Solid-state NMR, thermogravimetric analysis (TGA), diffuse-reflectance

UV spectroscopy, and nitrogen physisorption were used to characterize the modified silica surface. Adsorption isotherms, binding constants, and thermal desorption spectroscopy were measured for the encapsulation of various guests. 23

Figure A - 19 Binding of calixarenes onto metal oxide (M = Si and Ti) surfaces, (taken from reference 139).

Calixarene-TiO2 nanocrystals were prepared by refluxing toluene suspensions of TiO2 nanocrystals and calixarenes. Surface coverage studies suggest that the TiO2 surface is about 40 % covered by calixarenes. The binding was strong and calixarene-TiO2 materials did not show significant desorption even in protic solvents. Interestingly, while the individual components did not absorb in the visible region, the calixarene-TiO2 hybrid materials 1a-4a absorbed light in the visible region at ~560 nm by ligand-to-metal charge transfer between the calixarenes and the surface Ti centers, Figure A - 24.

In summary, the work demonstrated a simple route to covalently and electronically coupled calixarene hosts to TiO2 nanoparticles, and tune the interactions of the calixarene with the semiconductor through synthetic modifications of the host. The calixarene-TiO2 hybrids are potentially very interesting materials for semiconductor sensitization.

24

Figure A - 20 Left: Diffuse reflectance UV-Visabsorption of the calixarene-TiO2 materials. Right: Steady-state PL emission spectra ( ex = 200 nm) of (A) 1a, control

136,137 materials 5a and 6a and (B) calixarene-TiO2 materials 1a-4a, (taken from reference )

A.2.4 Zeolites

Zeolites are a large group of minerals consisting of hydrated aluminosilicates of sodium, potassium, calcium, and barium, with periodic, communicating channel systems. These porous minerals are used in a broad range of applications including ion exchange, catalysis, and photochemistry.138-141 They can be readily dehydrated and rehydrated, and are used as cation exchangers and molecular sieves. Calzaferri and co-workers have developed light harvesting host-guest antennas made of organic chromophores entrapped as organized assemblies in the channels of zeolite L. Zeolite type L is made of unidirectional 1.26 nm wide channels.142,143 Size and aspect ratio of the crystallites can be controlled synthetically from 30 to several micrometers, and numerous dyes can be included (anthracene, stilbene, perylenimides, carbazoles etc.). The one-dimensional channels of zeolite-L crystals can be filled with a single dye or, in sequence, with layers 25

of different fluorescent dyes. Bulky molecules (―stopcocks‖) were used to close the channels ends, which are 0.71 nm wide, and the stopcock molecules were synthetically modified to act as acceptors, or to carry anchor groups for surface attachment, (Figure A -

21).

Figure A - 21 a) Top: dye-loaded zeolite L antenna; blue-emitting donors inside the zeolite L transfer electronic excitation energy to red-emitting acceptors at the ends. Middle and bottom: fluorescence microscope images of an approximately 2000-nm-long crystal containing a neutral blue-emitting dye (DMPOPOP) in the middle part and cationic dye Ox+ at both ends (red, polarizer perpendicular) on selective DMPOPOP excitation. b) Top: antenna system with stopcock molecules as external traps and bottom: a schematic representation of a stopcock at the end of a zeolite L channel. The stopcock consists of a head, a spacer, and a label. c) Energy transfer (EnT) from a photonic antenna to a semiconductor, creating an electron–hole pair in the semiconductor (radiationless near-field process), (taken from reference 91,143).

Being able to attach and orient dye-loaded zeolites onto surfaces, including semiconductors, is an important development, since they provide a unidirectional antenna system, and a new design concept for functional interfaces. Nanosized zeolite L crystals 26

were bound as perpendicularly oriented monolayers to ITO etched silicon wafers,143 gold,144,145 and quartz,146 opening the way for optoelectronic applications, including artificial antenna systems, thin layer solar cells and light-emitting electrochemical cells

(LEECs).147 As an example, De Cola and coworkers have coated zeolitic antennas with silica shells to obtain chemically stable and highly versatile multifunctional dye-loaded capsules. They employed microcontact transfer printing methods to form monolayer of such capsules on various surfaces.148-150 This example illustrates the importance of the immobilization and orientation effect, and the exciting possibilities for optoelectronic as well as biological applications148,151 offered by the new interfaces.148-150

A.2.6 Redox Active Compounds Bound to TiO2

Nanocrystalline films of wide bandgap metal oxide are important components in electrochromics and other optical devices and sensors.152 The high surface area of nanoparticles and their film transparency is ideal for high surface coverage with redox active compounds for optical displays. For instance, in electrochromic systems the optical properties between, for example, colorless and colored state can be controlled by an electrochemical potential.26,75 In this case, a redox active molecule is attached to the electronically conducting metal oxide and is switched between two redox states.

Commercial applications require electrochromic materials with high contrast ratio, coloration efficiency and, most importantly, reversibility of the redox process. More recently 4,4‘-bypiridium salts, viologens, appeared as possible candidates with sharp color changes from colorless in oxidized form to a deep blue or violet colored in the 27

reduced form. For electrochromic device application it is useful to bind the viologens to

26,75-77,153-155 27,73,156 26 the surface of nanocrystalline TiO2. Fitzmaurice, Campus,

Grätzel26,75 and coworkers synthetically modified viologens with an anchoring group, either a phosphonate or a carboxylic acid, and attached them to the semiconductor.

Application of negative potentials, between -0.6 and -1.0V, on the modified TiO2 electrodes led to an efficient reversible reduction of the viologen derivatives.

Figure A - 22 Switching between colorless and colored form of a viologen derivative with allyl and anchoring group substituents bound to nanocrystalline TiO2 films

To date, the electrochromic devices based on Viologen-TiO2 nanocrystalline films compositions are constructed without applying any of the strategies to control their distribution on the nanoparticle surface. This approach results in several side effects such as dimerization of viologens or precipitation of the fully reduced species. Since substitution with bulky substituents or coadsorbants of viologens can lead to variation of 28

the electrochromic properties and affect the reversibility of the redox process, other options need to be tested.

A major focus of this thesis is the attachment of viologen derivatives through encapsulation into a supramolecular cucurbituril host. The host can successfully act as coadsorbent and anchoring unit in this system and, at the same time, shield the redox unit from the surface heterogeneity and prevent aggregates. The properties of the complex, the resulting semiconductors interface, and electrochromic device are explored and fully described in the following chapter.

A.3 The Cucurbituril Family

Cucurbit[n]urils (CB[n]) are a family of cyclic oligomers named for their shape, after the latin name for pumpkins(Cucurbitaceae).The macrocycles are formed from an acid- catalyzed condensation reaction of formaldehyde and glycoluril.103,157-159 The first member, CB[6], was synthesized and recrystallized by Behrend in 1905.160 The structure of this substance remained unclear until 1981 when Mock and coworkers revealed that it consisted of six glycoluril units bridged by twelve methylene groups.159,161 Almost twenty years later the cucurbituril family was expanded by the discovery of additional homologues, CB[5], CB[7], CB[8] and CB[10] by Kim, Day and coworkers, (Figure A -

23).157,158,162-164 Research on cucurbiturils has become very popular in the past decade since it overcame most of the early problems such as a lack of synthetic methods, poor solubility, and the lack of homologues. Methods for the synthesis of substituted cucurbiturils have been developed, providing an outstanding platform for fundamental 29

and applied molecular recognition and self-assembly studies.157-159,161-175 CBs are attracting increasing interest as host molecules in a variety of fields (supramolecular chemistry, sensor development, new nanostructures, and biological applications), because of their high affinity for positively charged or nitrogen-containing compounds.171-173,176-

179

Figure A - 23 The cucurbit[n]uril family

A.3.1 Properties and Synthesis of Cucurbituril Homologues

Most recently, efficient, convenient and practical ways for the synthesis of large-scale

CB[n] mixtures have been described by Day,163,164,180,181 Isaacs,103,182 and

Yamaguchi,158,183 ranging from mild experimental conditions to microwave-assisted 30

methods.184 The synthesis of all homologues employs the condensation of glycoluril and formaldehyde in concentrated HCl or 9M H2SO4. The key point is the control of reaction temperature. Temperatures of 110 °C exclusively generate CB[6] as a product, while lowering the temperature to 75-90 °C allows the formation of another homologue combination. The mixture of different homologues can be separated by fractional crystallization and dissolution.

Figure A - 24 Pathways for synthesis of Cucurbit[n]uril.

CB[n] molecules have been fully characterized by various analytical methods including

X-ray crystallography.158,162 The CB[n] homologues are typically characterized by 1H and

13C NMR, as NMR spectroscopy which is also the main characterization tool to investigate their complexation behavior. The resonances of the methylene protons of

CB[n] move downfield with an increasing number of glycoluril units

The cavity of Cucurbituril homologues has the same total depth (9.1 Å), while their widths at various depths and cavity volumes increase with the number of glycoluril subunits, (Figure A - 23). Generally, the diameter of the portals is approximately 2 Å 31

narrower than that of the cavity itself, which provides significant steric barriers to guest association and dissociation.103,158,159,180

The solubility of CB[n] homologues is relatively low in both water and organic solvents, and remains the major limitation for their use. Exceptions are CB[5] and CB[7], which possess moderate solubility in water. However, due to the weak Lewis basicity of the carbonyl portals and their affinity for cations, all CB[n] homologues have greater solubility in acidic solutions or solutions containing alkali metal ions.

The electrostatic properties of CB[n] are crucial for their recognition and complex building behavior. The carbonyl portal and the cavity of a CB[n] have negative electrostatic potential, which consequently leads to a preference of cationic guests.

Figure A - 25 CB[7] (left) binding regions and electrostatic map of CB[7](right), (taken from reference 182).

A.3.1 Cucurbit[7]uril, a Molecular Host

Cucurbit[7]uril has some major advantages over the other homologues belonging to the cucurbituril family. First of all, its size of 7.2 Å is wider than that of CB[5] or CB[6] and allows to encapsulate a large variety of positively charged guests. Among them are 32

adamantanes,185-187 naphthalene,188-190 stilbene,191-193 viologens,105,194-197 ferrocene198-201 nitroxide radicals,202 anticancer drug oxaliplatin203 and corresponding derivatives.103

Kaifer and coworkers executed a number of in-depth studies, by inclusion of neutral and cationic guests and demonstrating that CB[7] complexes possess unusual properties. For example the concentration of cations, Na+ and Ca+, in the solution can have a major influence on the encapsulation properties, in most cases the cations are competing for the carbonyl portal at the CB[7] with the hosts. As a result, the complexation constants are lower.176,194

Figure A - 26 Coordination of sodium to the portals of CB[7]

Furthermore Kaifer and coworkers were able to show that CB[7] can be on different positions over the included molecule. The guests were viologen derivatives with different chain length substituents. The results showed that two modes of binding interaction between viologen guests and the CB[7] host exist, depending on the length of the alkyl chain. For short alkyl chains, a pseudorotaxane inclusion complex was formed, where the

CB[7] is on the viologen nucleus. Longer alkyl chains, on the other hand, result in a complex where CB[7] is docked on the aliphatic substituent.176,204,205 33

Wagner, Nau and coworkers have studied the influence of cucurbit[7]uril of mechanisms in fluorescence quenching and fluorescent enhancement upon binding.206-209 In one of the first studies on fluorophores (2,3-diazabicylo[2.2.2]octa-2-ene, DBO) with CB[7], the encapsulation led to a two-fold increase in emission and life time.190,206,209 The explanation for this phenomenon is that cucurbituril can shield a fluorescent guest from quenchers, including oxygen. In recent studies, more fluorescent dyes were encapsulated into a cavity of CB[7]. The inclusion lead to alternate properties and chemical behavior of the complexes compared to free fluorophores. There are many positive aspects of encapsulating fluorescent dyes into the molecular host CB[7], one of them being the solubilization and preventing aggregation with CB[7] of entirely insoluble dyes. This process has been representatively demonstrated for rhodamine6G, RH6G and coumarin

102. CB[7] was also able to prevent the dyes from adsorption to any surface. Another interesting effect is the fluorescence enhancement upon encapsulation, since it is relevant for sensor applications. This was first observed by Wagner and coworkers for curcurmin,

Figure A - 27. The complex of curcurmin and CB[7] showed a five times higher emission intensity compared to the free dye. This enhancement is likely an effect of the position of the fluorophore in the more hydrophobic cavity, space confinement, restriction of conformation, charge dipole interaction, as well as the polarity effect. They all can lead to a decrease in nonradiative decay. The most characteristic effect of the fluorescence enhancement, consequently, is longer emission lifetimes. The fluorescence enhancement could find applications in fluorescence lifetime imaging spectroscopy (FLIM). Further, as the dye is encapsulated within the cavity of the macromolecular host, it is ―shielded‖ from external fluorescence quenchers, reactants or oxidizers and consequently makes the 34

inserted guest more photostable. All these aspects are related to the low polarity, the spatial confinement of the CB[7] cavity and low chemical as well as photochemical reactivity. Encapsulating fluorescent or redox active dyes within cucurbit[7]uril opens the possibility for a larger number of new applications.

Figure A - 27 Selected fluorescent dyes for CB[7] guest-host formation, (taken from reference 208, 209)

In summary, the modification of nanostructured semiconductor TiO2 with guest-host complexes of photo- and redox-active compounds in molecular hosts results in new, favorable properties. This new concept for the functionalization of TiO2 nanoparticle can be applied to viologen encapsulated into cucurbit[7]uril, where of particular interest is the reversibility and stability that can be gained as side reactions are suppressed through the encapsulation. The following chapter describes and studies the electrochromic properties of CB[7] encapsulated viologen bound to the semiconductor.

35

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54

Chapter B

Electrochromic Properties of Cucurbituril

Complexes with Viologen

55

B.1 Introduction

Electrochromic compounds reversibly change their optical properties upon a redox process.1-3 Typically, an electrochromic compound can be switched between a transparent

(bleached) state, and a colored state where the redox active compound absorbs light in the visible region.4,5 This phenomenon can be used to build flexible electrochromic displays

(ECD).6-10 Commercial and research interests focus on various types of inorganic11,12 and organic13-16 molecules exhibiting electrochromism throughout the visible spectrum. They

17-19 20 are often interfaced with transition-metal oxides, especially nanostructured TiO2. In conjunction with electrochromic molecules, nanostructured TiO2 films are used to develop electrochromic displays21-23 and devices.24,25

It is nearly a century ago that Michaelis first reported on the electrochemical behavior of viologens.26 Viologens are a well-known class of electrochromic compounds and are excellent electron acceptors. The one electron reduction of methylviologen (1,1‟- dimethyl-4,4‟-bipyridinium dichloride) and derivatives leads to a blue to green colored radical cation (MV+). The single reduction process is highly reversible and can be cycled many times without significant side reaction. A second, non-reversible reduction produces a yellow neutral species (MV0) (Scheme B - 1), which is insoluble. Synthetic

2,27,28 modifications of the viologen are necessary to form the chemical bond with TiO2, as described in Chapter A section 2.6, (Figure B - 2).5,7,9,29 Side reactions include the dimerization of the radical cation (MV+), and the aggregation of the neutral species

(MV0).30-34 Methylviologen dications decompose in the presence of base and methanol and as their radical cations can decompose in acetonitrile.27 56

Scheme B - 1 Electrochemical processes of methylviologen

Cinnsealach and coworkers first reported electrochromic windows (ECW) based on

5,9 nanostructured TiO2 films. Their electrochromic windows consisted of a nanostructured

TiO2/ ITO electrode modified with a viologen, sandwiched with a counter electrode, with a liquid electrolyte (LiClO4 and ferrocene as the redox mediator in γ-butyrolactone,

Figure B - 1). The application of a potential lead to the reduction of the viologens, which were chemisorbed to the nanostructured TiO2. ECWs having different colored states (blue to green) were prepared using modified viologens bound at the surface of a

5,9 2 -1 nanostructured TiO2 films. The ECWs had a coloration efficiency of 170 cm C at 608 nm with fast switching time of < 0.5 s.

Campus et al. described ECWs from an electrode consisting of a viologen modified nanostructured TiO2 film with counter electrodes consisting of Prussian Blue either electrodeposited on conducting glass or nanostructured TiO2. The resulting ECW had relatively slow switching times (1-3 s) and also very low transmission in the colored state.24,29 57

Figure B - 1 Electrochromic Window Designed by Cinnsealach et al. based on Viologen Modified Nanostructured TiO2 and Conducting Glass Electrodes, (taken from reference 7).

Figure B - 2 Principle of signal amplification in a nanocrystalline film by high surface area and adsorbed redox chromophores, (taken from reference 29).

Overall, recent developments have been directed towards improving the performance characteristics of viologen/TiO2 ECWs. Using a viologen modified semiconducting nanostructured metal oxide film has proved to be excellent combination to construct ultrafast electrochromic windows.

In this thesis we studied a new method of interfacing methylviologen and derivatives with the nanostructured TiO2 by encapsulation of the redox active compounds into a macrocyclic host, CB[7], and anchoring the complex onto the semiconductor surface through the host.35,36 It is the first successful approach to bind a cucurbituril complex 58

with a redox dye directly to a semiconductor surface. The strategy is based on the excellent electrochromic properties of the viologen/TiO2 systems and the fact that methylviologen forms a very stable complex with cucurbit[7]uril, CB[7], Scheme B -

2.36-48 These complexes are kinetically and thermodynamically very stable, with a typical complexation constant of K ~ 2×105 M-1. The ion-dipole interaction between the dication

(MV2+) and the carbonyl oxygens at the CB[7] portals are responsible for the specific inclusion behavior.41,44,49-52 The electrochemical study demonstrated that the previously described side reactions are suppressed by protecting the viologen within the cavity of the cucurbituril.36,37,43,50,51,53 This, combined with the better control of surface modifications of the nanostructured titanium dioxide through the supramolecular host, gives the viologen@cucurbituril complexes ideal properties as molecular adsorbents.

Scheme B - 2 Illustration of the Inclusion of MV2+ into CB[7].

This chapter is divided in three main parts: the synthesis of methylviologen derivatives,

1-methyl-10-p-tolyl-4,4‟-bipyridinium dichloride, MTV2+, is described in sections B.2.2 and B.8.2.2. Two additional viologen derivatives containing anchoring groups (1-butyl-

1'-(2-phosphonoethyl)-4,4'-bipyridine-1,1'-diium bromide (PV2+) and 1-butyl-1'-(2- carboxyethyl)-4,4'-bipyridine-1,1'-diium bromide (CV2+) were synthesized and studied for comparison. The viologens derivatives were then encapsulated into CB[7] and their 59

electrochemical and optical properties were studied in solution and on TiO2 surface. This is described in Sections B.5 and B.6 of this chapter, where an overview of the properties of MV2+@CB[7] as reported in literature. Finally, We describe the assembly and study of an electrochromic window (ECW) prepared using the CB[7] complexes bound to TiO2

(Sections B.5, B.6.2 and B.8.7 - B.8.9).

B.2 Synthesis of Viologen Derivatives

Three electrochromic compounds were prepared, two consisting of alkyl viologen with

2+ 2+ COOH (CV ) and P(O)(OH)2(PV ) substituents for anchoring to TiO2 surface and one aryl viologen (MTV2+) without any anchoring group. The viologen derivative MTV2+ was the first in a series of aryl viologens tested for electrochromic properties.

Figure B - 3 Cucurbit[7]uril and viologens studied in this work

B.2.1 Synthesis of Alkyl Viologens

The phosphonated viologen (PV2+),291-(2-phosphonoethyl)-1'-propyl-4,4'-bipyridine-1,1'- diium (4), and the carboxylated viologen (CV2+),1-(2-carboxyethyl)-1'-propyl-4,4'- bipyridine-1,1'-diiumdibromide (5) (Scheme B - 3), was prepared with moderate yield but in pure form from 4,4‟-bipyridine (1) with an alkyl bromide in a Menshutkin-type

54,55 reaction. The Menshutkin SN2 reaction allows the alkylation of an amine by an alkyl 60

halide to form an ammonium halide salt (Scheme B - 4). The Menshutkin reaction‟s mechanism is unknown.56,57 The reactants are neutral and soluble in non-polar organic solvents, while the products are salts and are soluble in polar solvents such as ethanol or water. Hence, the isolation and purification of the products from the reaction mixture is easily carried out by precipitation.

Scheme B - 3 Formation of an ammonium bromide salt via the Menshutkin Reaction

Scheme B - 4 Synthesis of phosphonated viologen (4) and carboxylated viologen (5) by Menshutkin reaction 61

B.2.2 Synthesis of Aryl Viologens

Recently Yamaguchi and coworkers adapted the Zincke reaction,27,58 which is used in organic synthesis to transform pyridine into pyridinium salt, to synthesize N-arylated pyridinium salts. We report the synthesis of an asymmetric viologen derivative having a tolyl group. The substitution with the tolyl group follows the mechanism of the Zincke59 reaction, which can be divided into two main steps, (Scheme B - 5).38,60,61 The first step is the formation of the N-2,4-dinitrophenyl-pyridinium salt (7) from a pyridine, 4,4‟- bipyridine (1), and chloro-2,4-dinitrobenzene (6). The second reaction proceeds through opening of the pyridinium ring (8) after the introduction of a primary amine, p-toluidine, and further a displacement (9) of the dinitroaniline group. After further amine elimination, the pyridinium salt (10) is formed. To obtain the desired viologen derivative a second substitution is necessary and is achieved through the reaction with iodomethane, yielding in 1-methyl-10-p-tolyl-4,4‟-bipyridinium dichloride, MTV2+ (11, Scheme B - 6).

Overall the pyridinium salt N-2,4-dinitrophenyl-4-pyridyl-pyridinium chloride (7) was obtained in 45% yield by reaction of 2,4-dinitro-chlorobenzene with 4,4‟-bipyridyl.

Reaction of p-toluidine with the Zincke salt resulted (10) in 70% yield. Reaction of (10) with iodomethane produced (11), an orange solid in quantitative yields, (Scheme B - 6).

The product, MTV2+ (11) was stable on air and soluble in protic solvents, methanol, ethanol and water.

62

Scheme B - 5 Mechanism of synthesis for pyridinium salt MTV2+using Zincke reaction, (taken from reference 59-61).

Scheme B - 6 Synthesis of MTV2+ 63

B.3 Host-Guest Complexes of Viologens with Cucurbit[7]uril

The most common examples of host–guest complexes formed by cucurbit[7]uril involve cationic guests. Their binding affinity is explained by hydrophobic interactions and through cation-dipole interactions between positive charges on the guest and the carbonyl groups on the rims of CB[7].41,62-64 Among the guest molecules that have demonstrated high binding constants with CB[n], are the N-arylated viologen cations. CB[7] shows high affinities for viologens and derivatives. The complexation of methylviologen, MV2+, with CB[7] was first reported by Kim and coworkers.37,43,65 Kaifer and coworkers further investigated the encapsulation of viologen derivatives30,41 and viologen dendrimers.66-68

All viologen derivatives formed very stable inclusion complexes with binding constants for viologen@CB[7] complexes of K = 2×105L/mol.62 More recently, Kaifer and coworkers reported that the binding mode of CB[7] with alkyl viologen derivatives depends on the chain length of the alkyl substituents.41 For viologens with a shorter aliphatic chain, CB[7] resided over the aromatic part of the viologen, and in the case of a longer than three carbons chain, the CB[7] cavity tends to include the aliphatic chain.69

The behavior was explained by favorable hydrophobic interactions between the aliphatic chain and the CB[7]‟s inner cavity. 64

MV2+@CB[7] MTV2+@CB[7]

Figure B - 4 The viologens (MV2+ and MTV2+) and their complexes with CB[7]. Only one of the possible encapsulation modes is shown for MTV2+@CB[7].

This work lead us to assume that many other viologen derivatives, including the synthesized MTV2+ would form a stable inclusion complex with CB[7].

Host-guest complexes of MV2+ and MTV2+ in CB[7] were studied in solution.37,43,70 We observed by 1H NMR and UV-Vis spectroscopy, that MTV2+ as well forms a very stable

5 1:1 complex with CB[7] with KMTV@CB = 1.06 ×10 L/mol. The complexation position of

CB[7] in case of MTV2+ is at the tolyl group and a part of the viologen moiety, as indicated by the chemical shift of the 1H NMR signals. Here we report the inclusion of

MV2+ and MTV2+ and their reduced species behavior upon encapsulation with CB[7], studied by electrochemical and spectrochemical experiments in solution. The electrochemical studies (cyclic voltammetry) of the complexes and free viologens in 0.1

M phosphate buffer indicated that the unsubstituted methylviologen underwent a reversible two electron reduction between ±1.0 V, whereas the behavior of MTV2+was irreversible. 65

B.3.1 1H NMR study of MV2+@CB[7] and MTV2+@CB[7]

The formation of the complexes MV2+@CB[7 and MTV2+@CB[7was monitored by 1H

2+ NMR in D2O. Kim and coworkers were the first to study this encapsulation of MV into

CB[737 and we used the same method to study MTV2+@CB[7. The inclusion of methylviologen into CB[7] leads to a significant shift of the β-proton belonging to the bipyridinium moiety to higher field and the methyl signal to the lower field, (Figure B - 5 and Figure B - 6). At the same time the position of the α-proton undergoes a smaller downfield shift of 0.32 ppm. These results are in agreement with the previously reported

1H NMR spectral changes for this complexation.

1 2+ Figure B - 5 H NMR (500 MHz) in D2O of 1 mM MV a) in absence of CB[7] and b) in presence of 1 equivalent CB[7] 66

Figure B - 6 Chemical shift differences of MV2+ upon complexation

The complexation behavior of the newly synthesized viologen derivative MTV2+ with

CB[7] was investigated by 1H NMR. The 1H NMR spectra of MTV2+ were recorded in absence and in presence of one equivalent of CB[7]. Since MTV2+ is an asymmetric compound, it was of interest to determine its complexation position within the cavity of

CB[7]. Upon complexation of MTV2+ in CB[7], significant upfield shifts of the MTV2+ protons occurred, while the chemical shift of the CB[7] protons remained unchanged

(Figure B - 7). The chemical shift difference (Δδ) of the MTV2+ protons increased in the ring system as shown in Figure B - 8, with the chemical shift of the methyl group almost unchanged, and the largest difference (Δδ = -0.69 and -0.98) was observed for the benzene ring protons of the tolyl unit.

The complexation induces larger upfield chemical shifts and can be associated with the inclusion of the protons into the interior cavity of the cucurbit[7]uril. The smaller chemical shifts and the downfield shifts are related to proton signals, which are located near or outside of the carbonyl portal of CB[7]. This result suggests that in

MTV2+@CB[7] a part of the bipyridinium moiety and the methyl group remains outside of the CB[7] cavity, and that the tolyl group is encapsulated. Considering the negative surface potential of the interior of CB[7], we expected that the bipyridinium moiety would be fully encapsulated in the host, but we cannot exclude the occurrence of other 67

complexation modes, depending on experimental conditions such as the presence of salts, pH and the character of viologen substituents.36,50,65,71-73 Apparently, the hydrophobic interactions between CB[7] cavity and the tolyl substituent are more favorable than the ion-dipole interaction between the carbonyl rims with the positively charged part of

MTV2+.

1 2+ Figure B - 7 H NMR (500 MHz) in D2O of 1 mM MTV a) in absence of CB[7] and b) in presence of 1 equivalent CB[7].

Figure B - 8 Chemical shift differences of MTV2+ upon complexation 68

B.3.2 Complexation Constant*

The binding constants of MV2+and MTV2+ were determined following a procedure described by Kaifer and co-workers.40-42,51,72 The procedures involved the fiiting of the experimental data to a 1:1 complexation model.74,75 This method can be used to quantitatively determine the binding constant and stoichiometry of non-bonding interactions in macromolecular systems and has mostly been used for formations of one- to-one complexes. The extinction coefficient corresponding at the absorption maximum of the guest (viologen) will decrease upon encapsulation into the host (CB[7]) (Figure B -

9). The decrease in the molar absorption is presumably caused by the viologen moiety being complexed in the hydrophobic interior cavity and by ion-dipole interactions at the carbonyl rims of the CB[7].40,42,46-48,72

In the present case, a solution of MV2+ or MTV2+ was titrated with CB[7] until no further change in the absorptions spectra at absorption maximum was observed. The slope of the best fit for a plot of absorbance vs. host concentration data determines the binding constant between viologen and cucurbit[7]uril. The complexation constant of

2+ 5 MTV @CB[7of Kmtv= (1.06 ±0.7 )×10 L/mol was measured. The complexation

5 37 constant of Kmv= 2×10 was previously reported by Kim and coworkers. 69

Figure B - 9 a) Absorption spectra of 1 μM MTV2+ at increasing concentrations of CB[7] b) Fitted experimental absorbance data at λmax=218 nm, (taken from reference 36).

B.4 Cucurbituril Complexes Bound to TiO2

Nanostructured TiO2 films can be easily modified and tuned by absorbing of sensitizers with binding groups or molecular host and electrochromic dyes.76,77 Viologens with anchoring groups, for example phosphonic or carboxylic groups, bind to TiO2 nanoparticle films.5,9,24,27,29,36,78 An anchor group is necessary to form a strong bond with the semiconductor surface, ensuring an effective contact, and to avoid desorption as described in the introductory chapter A. In this work we describe this new approach to anchor an unsubstituted methylviologen and the viologen derivative MTV2+ encapsulated into a macrocyclic host, cucurbit[7]uril, onto a semiconductor surface of nanostructured

TiO2. This alternative approach of adsorbing redox active compounds on semiconductors offers two major benefits: it allows the use of non-functionalized viologens, and it suppresses possible side reactions such as aggregation and dimerization. Only few examples of guest-host complexes bound to semiconductors exist and here we show a proof of concept, probing the strategy outlined in chapter A for viologen@cucurbituril complexes. 70

Figure B - 10 Illustration of the physisorption for the complexes MV2+@CB[7] and 2+ 1 MTV @CB[7] on TiO2, considering the binding modes found in H NMR

Two viologens were studied: methylviologen, and the newly synthesized derivative, 1- methyl-1-p-tolyl-4,4‟-bipyridinium dichloride, MTV2+. In the presented cases, CB[7] serves a dual role. On the one hand it prevents the redox compound from undergoing diverse side reactions, dimerization and aggregation, and on the other hand it serves as a

2+ 2+ binding unit to TiO2. Synthetic adaptations of MV and the derivative MTV are not necessary. The viologens did not bind on TiO2 films unless they were encapsulated into

CB[7]. The surface modification of TiO2 nanostructured films is performed from an aqueous solution and further confirmed by FT-IR-ATR and electrochemical measurements. Electrochromic windows (ECW) were prepared using the modified

2+ 2+ semiconductor films. The ECWs in case of MV @CB[7]/TiO2 and MTV @CB[7]/TiO2 showed reversible color change upon application of a potential and a semireversible two electron reduction process. 71

B.4.1 Preparation of nanostructured TiO2 Films

The sol-gel process produces transparent MOn thin films that can adhere to glass substrate, and are resistant against acidic and alkaline corrosion.20,77 Titanium dioxide, particularly in the anatase form is a non-toxic and inert low-cost material which is prepared through the hydrolysis of titanium(IV) isopropoxide (TiP) in acidic aqueous media (Eq. 1). The main parameters to control the reaction are the ratio between water and titanium (r=[water]/[titanium]), the pH, as well as the calcination temperature. A water to titanium ratio of 10 and higher leads to larger spheres and further to colloids and aggregates. The pH has to be lower than 2, since a higher pH would lead to the formation of large aggregates. Also, lower calcination temperatures lead to smaller sized anatase nanoparticles.79,80

Ti[OCH(CH3)2]4 + 2H2O → TiO2 + (CH3)2CHOH Eq.1

PEG is then added to allow casting on a substrate. The nanocrystalline TiO2 paste was applied as a thin film onto the conductive side of FTO glass using a glass rod, followed by sintering at 450 °C under oxygen flow for 30 min to burn the polymer. A TiO2 film prepared following this procedure are in average 10 µm thick with a particle size of about

20 nm (AFM, light scattering).

B.4.2 Binding on Nanostructured TiO2 Films

The electrodes of sensitized nanostructured TiO2 films were prepared by dipping the

TiO2/FTO films into a 0.5 mM solution of the appropriate viologen@CB[7],

MV2+@CB[7] or MTV2+@CB[7], for a period of 24 h and washed with water and dried. 72

Figure B - 11 Schematic Preparation of modified TiO2 electrodes

B.4.3 Electrochromic Windows

Electrochromic windows were built using the nanostructured TiO2 electrodes modified with viologen@CB[7]. The counter electrode consisted of nanostructured TiO2, modified with ferrocene carboxylic acid Fc-COOH. Windows prepared using the anchored Fc-

COOH exhibited higher stability and reversibility compared to windows prepared using ferrocene in the electrolyte solution.

It is well known that ferrocene and derivatives can be encapsulated into the CB[7] host.81

Ferrocene forms highly stable 1:1 inclusion complexes with CB[7] and the complexation

6 constant was estimated from the UV-Vis data by Kaifer and coworkers as KFC@CB[7]> 10

M-1.82 The reduction of viologen results in changes of the host-guest of the complexation constant.41,42,51 At this point the competitive guest exchange between the ferrocene and the viologen in the physisorbed CB[7] complex cannot be excluded. We observed that the stability and the performance of the assembled ECWs improved after ferrocene was excluded from the electrolyte and Fc-COOH bound to the counter electrode.51,52,72,82

The electrodes were combined in a closed cell with the counter electrode and electrolyte

2,4,7,78,83-85 consisting of LiClO4 in γ-butyrolactone. The proposed assembly was according 73

to the design of ESW prepared by Fitzmaurice and coworkers.5,7,9 γ-Butyrolactone was used, since it has a high boiling point and is chemically inert towards viologens, as opposed to acetonitrile.27

Figure B - 12 Schematic cross section of an electrochromic window

B.5 Electrochemistry

B.5.1 Cyclic Voltammetry Measurements in Solution

Cyclic voltammograms (CVs) of MV2+ and MTV2+ and their respective CB[7] complexes were collected in 0.1 M phosphate buffer at room temperature. Aqueous solutions of free

MV2+ and the corresponding complex (MV2+@CB[7]) exhibited the characteristic two reversible one-electron reduction waves (Figure B - 13), consistent with published data.37,43,65,70 The complex exhibited a shift to more negative potentials for the formation of the radical cation MV•+ and the fully reduced MV. These negative shifts were also observed by Kim and coworkers, and were attributed to the smaller complexation

IMPORTANT NOTE: The sentences or paragraphs within asterisks in this thesis were cited verbatim or slightly modified from papers of which I am a co-author. 74

affinities in CB[7] of MV•+ and MV, compared to MV2+. In the experiment the shift of -

20 mV was comparable and suggested that the complexation affinities of MV•+ and MV are similar (Table B - 1) to the results reported by Kim and approximately one order of magnitude smaller than for dication. In addition, the second reduction process was quasireversible, suggesting precipitation of the reduction product or other processes at the electrode upon reduction. Relatively slow scan rates (0.1 Vs-1) were selected to avoid a competition between complexation equilibria and the time scale of the measurements.

Table B - 1 Voltammetric parameters for free and encapsulated MTV2+and MV2+, (taken from references 36,37)

1 2 E 1/2,V E 1/2, V

Compound (ΔEp, mV (ΔEp, mV

vs Ag/AgCl) vs Ag/AgCl) MV2+ -0.661 -0.975 MV2+@CB[7 -0.681 -0.992 MTV2+ -0.704

MTV2+@CB[7 -0.767

In the CVs of MTV2+ and MTV2+@CB[7], a similar negative shift occurred upon complexation. However, the reduction process for MTV2+ (alone or in the presence of

CB[7]) showed distortion is the voltammogram, as no cathodic wave and a single reduction were observed (Figure B - 14). Redox potentials of bipyridinium dications depend on the anions and substituents on the nitrogen. MTV2+ in solution has mixed anions, I- and Cl-, and an aryl as a substituent, this can widely vary the reduction potentials. Especially the electron withdrawing substituents can lead to a more anodic 75

2 1 second halfwave potential E 1/2 than E 1/2, resulting in the neutral species as the immediate product. One of the possible explanations for the observed behavior is therefore the neutral species MV0. It is an insoluble salt, which can lead deposition in solution or at the electrodes and therefore lead to a chemical irreversibility of the reaction and the distortion in voltammetric measurements. Another kind of irreversibility encountered would be the electrochemical irreversibility, which depends on the rate at which the electron transfer occurs between the working electrode and the solution redox species.

1.5 MV2+@CB[7] (0.05 mM) MV2+ (0.05 mM)

1.0 A]

 0.5 Current[ 0.0

-0.5

-500 -600 -700 -800 -900 -1000 -1100 -1200 Potential [mV], vs Ag/AgCl

Figure B - 13 Cyclic voltammograms in 0.1 M phosphate buffer (pH 7.0) of 0.05 mM MV2+ a) in absence of CB[7] (black solid line) and b) in presence of 1 equivalent CB[7](dashed blue line). 76

1 MTV2+ MTV2+@CB[7] 0

-1

A]  -2

Current [ -3

-4

-5

0 -200 -400 -600 -800 -1000 -1200 Potential [mV], vs Ag/AgCl

Figure B - 14 Cyclic voltammograms in 0.1 M phosphate buffer (pH 7.0) of 0.05 mM MTV2+a) in absence of CB[7] (black solid line) and b) in presence of 1 equivalent CB[7](dashed blue line).

B.5.2 Cyclic Voltammetry of Electrochromic Windows

Figure B - 15 Viologens@CB[7] studied and possible electron transfer between complexes and semiconductor

The CVs of the electrochromic windows prepared from MV2+@CB[7 and

MTV2+@CB[7 differ in position of cathodic and anodic peaks and shapes. Both complexes, displayed a semi-reversible two-electron reduction process, with applied 77

potential from 0 to -1.2V at a scan rate of 0.1V/s (Figure B - 15 and Figure B - 16). The electrochromic characteristics (reversible color change) were observed by application of -

0.8 V corresponding to the first reduction potential. These results are also indicating that the irreversible behavior encountered in solution measurements is most probably the result of a chemical irreversibility, since it is not present in case of ECWs.

3 MV2+@CB[7] (ECW) MTV2+@CB[7] (ECW) 2

1

Current (mA) 0

-1

-2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 Potential (V, vs. SCE)

Figure B - 16 Cyclic voltammograms of electrochromic windows prepared from the CB[7] complexes of 1 (solid line) and 2 (dotted line), (taken from reference 37).

B.6 UV-Vis Absorption Spectroscopy

B.6.1 UV-Vis Absorption Spectra in Solution

The electrochromic properties of viologen solutions were studied by UV-Vis spectroscopy. The viologen dication is colorless, while the viologen radical cations,

+ formed upon electron reduction, are intensely colored (λmax= 622 nm for MV and λmax=

610 nm for MTV+) after a charge transfer between the (formally) +1 and zero valent 78

nitrogens.5,9,27,85-87 The one-electron-reduced species were studied by spectroelectrochemistry. Absorption spectra were measured in solution at 0 V for

MV2+and MTV2+ and at -0.08 V for MV+and MTV+ in the presence and in the absence of one equivalent of CB[7]. The electrochemical reduction immediately resulted in the formation of the radical cation, and was accompanied by rapid color changes from the transparent to deep blue with the absorption band centered at 600 nm.

Figure B - 17 Visible region of the absorption spectra of dication species of (a) MV2+ and MV2+@CB[7 and (b) MTV2+ and MTV2+@CB[7 in phosphate buffer solution (pH 7.0) in a spectroelectrochemical cell. Solid and dotted lines show spectra in the absence and presence of equimolar amounts of CB[7], respectively. 79

1.00 1.00 (b) .+ MV.+ (a) MTV MTV.+@CB[7] MV.+@CB[7] 0.75 0.75

0.50 0.50

Absorbance Absorbance

0.25 0.25

0.00 0.00 500 600 700 800 500 600 700 800 Wavelength, nm Wavelength, nm

Figure B - 18 Visible region of the absorption spectra of one-electron reduced species of (a) MV+ and MV+@CB[7 and (b) MTV+and MTV+@CB[7 in phosphate buffer solution (pH 7.0) in a spectroelectrochemical cell. Solid and dotted lines show spectra in the absence and presence of equimolar amounts of CB[7], respectively.

B.6.2 UV-Vis Absorption Measurements of Electrochromic Windows

The absorption spectra of electrochromic windows prepared from MV2+@CB[7 and

MTV2+@CB[7 were measured at -800mV and are shown in Figure B - 19. In both cases we observed a broad band centered at 600 nm. This is consistent with the absorption spectrum of the corresponding radical cations in solution. In the bleached state the ECWs were slightly yellow and almost colorless, respectively. The sharp color change and the redox process were not observed for windows prepared using solutions of MV2+or

2+ MTV , suggesting that the complex formation and the physisorption to TiO2 are necessary to attach the unsubstituted viologens to the semiconductor surface. The redox process was reversible for several (> 20) switching cycles between bleached and colored state, and no degradation was observed, Figure B - 20. Extensive stability studies were not conducted, since it was not within the scope of this work. 80

2.0

MTV.+@CB[7] MV.+@CB[7] 1.5

1.0 absorbance

0.5

0.0 400 500 600 700 800 wavelength (nm)

2+ 2+ Figure B - 19 Absorption spectra of MV @CB[7/TiO2 and MTV @CB[7/TiO2 measured in an electrochromic window after application of -0.8 V.

(a) (b)

Figure B - 20 Picture of color changes of an electrochromic window prepared from 2+ MTV @CB[7/TiO2/FTO (a) before and (b) after application of a 800 mV potential.

B.7 FT-IR-ATR

B.7.1 Measurements of Solid Complex Samples

FT-IR-ATR spectra of solid samples of MV2+, MTV2+and their respective complexes with CB[7] were recorded. FT-IR-ATR spectra showed aromatic C=C and C=N stretching bands of the pyridine and phenyl rings in the 1600–1430 cm-1range. Upon encapsulation of the guests in CB[7] the FT-IR-ATR spectra of the bound complexes 81

were not additive. Rather, the spectra of the complexes were almost indistinguishable from the FT-IR-ATR spectrum of CB[7], with a strong band assigned to the carbonyl

-1 as(C=O)stretch at 1725 cm (Figure B - 21 and Figure B - 22). Decomplexation can be excluded, because the solid samples of the viologen@CB[7] were prepared by evaporation of the solvents from equimolar solution of guests and host. In any case the

FT-IR-ATR spectrum should show the signals of the present viologens. Borguet and coworkers presented a possible explanation for this result. The research group recently reported a similar case for molecules encapsulated into carbon nanotubes, CNTs.88 The encapsulation lead to a dramatic reduction of the IR intensity (10-fold) for the guest signals. The molecules became practically invisible in the IR spectra within the cavity of

CNTs. A comparison between IR intensity reductions of internally and externally adsorbed guests indicated the occurrence of a screening effect.89,90 This effect develops in solids and semiconductors, consisting in a reduction of the electrostatic field of a guest inside a solid. Carbon nanotube sidewalls are reportedly able to dielectrically screen electric charges in their vicinity, so that the dipole moment of the internal guest is reduced, leading to a reduction of IR intensity.

The formation of supramolecular host–guest inclusion complexes in aqueous solution is highly complex. In case of cucurbituril complexes, hydrogen bonding, ion-dipole and hydrophobic interactions with guests included within the internal cavities of this molecular host can influence the stability and the photophysical behavior of the complex, leading to the screening effect of the host.62,63,91 Therefore we postulate that the disappearance of the viologen‟s IR signal upon encapsulation into CB[7] can be explained by the guest:host interactions and the screening effect of the cucurbituril. 82

Additional FTIR and TPD (Temperature-Programmed Desorption) studies with alternative guests are necessary to fully characterize the formation of cucurbituril complexes.

0.15 MV2+@CB[7] (solid) MV2+ (solid) CB[7] (solid) 0.12

0.09 1462 1720 1657

0.06 1584 Absorbance

0.03

0.00 2000 1800 1600 1400 1200 1000 wavenumbers (cm-1)

Figure B - 21 FT-IR-ATR of MV2+ (blue solid line), MV2+@CB[7 (black solid line) and CB[7] (red dashed line)

0.09 MTV2+@CB[7] (solid) 0.08 MTV2+ (solid) CB[7] (solid) 0.07 1722 1462 0.06

0.05

0.04 1637 Absorbance 0.03

0.02

0.01

0.00 2000 1800 1600 1400 1200 1000 Wavenumbers (cm-1)

Figure B - 22 FT-IR-ATR of MTV2+ (blue solid line), MTV2+@CB[7 (black solid line) and CB[7] (red dashed line) 83

B.7.2 FT-IR-ATR Measurements of nanostructured TiO2 films

FT-IR-ATR was used for the characterization of TiO2 films with attached complexes

2+ 2+ MV @CB[7], MTV @CB[7] or CB[7], Figure B - 23. Upon physisorption to TiO2 the

as(C=O) carbonyl band broadened and shifted to a lower energy by about 10

-1 wavenumbers (1735cm ) compared to that of neat CB[7. While the shift of the as(C=O) carbonyl band to lower energies seems to be consistent with this picture, the quality of IR spectra (broad, low intensity bands) does not allow to draw any conclusion other than the presence of the CB[7] macrocycle on the semiconductor films.88

Figure B - 23 FT-IR-ATR of adsorbed complexes MV2+@CB[7 (black solid line), 2+ MTV @CB[7 (red dash line) and CB [7] (blue dotted line) on TiO2/FTO. 84

B.8 Experimental Section

B.8.1 General

1H NMR (499.90 MHz) and 13C (124.98 MHz) spectra were recorded on a Varian

INOVA 500 spectrometer. The 1H and 13C NMR chemical shifts () are given in ppm and are referenced to the central line of the solvent. NMR spectra were recorded in the indicated solvents at room temperature. Coupling constants (J) are reported in Hz. Mass spectra (ESI) were recorded on the Bruker Daltonics FTMS departmental facility.

Chemicals used in this experiment were all analytical grade and used as received.

Acetone, methylviologen (1), CB[7], 4,4‟-bipyridine, 1-bromobutane, diethyl 2- bromoethylphosphonate, 1-chloro-2,4-dinitrobenzene, ferrocene carboxylic acid, lithium perchlorate, γ-butyrolactone, p-toluidine and iodomethane were purchased from Aldrich or Sigma-Aldrich. 1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+) was used after preparation using the previously described procedure. Nanocrystalline titanium dioxide was prepared via sol–gel hydrolysis and condensation of isopropanol solutions of titanium(IV) isopropoxide and cast on conductive glass FTO, TEC 7 by Pilkington, 2.2 mm thickness, with a sheet resistance of 8-10 Ω/ sq and~80% visible transmittance.

Electrochromic materials, MTV2+ and MV2+ were used as previously prepared and described. All synthetic procedures were carried out under nitrogen atmosphere. 85

B.8.2 Synthesis and Characterization

B.8.2.1 Synthesis of Alkyl Viologen Derivatives

1-butyl-4-(pyridin-4-yl) pyridinium bromide(2): 4,4‟-bipyridine (2.00 g; 26mmol) and

1-bromobutane (2.80 ml; 26mmol) were dissolved in toluene (60 ml). The solution was refluxed for 48 h. The yellow precipitate, which started to appear after a reaction time of

30 min, was collected by filtration and washed twice with hot toluene. The crude product

1 was dried in vacuo. Yield: 3.45 g (50 % yield). H (D2O): 9.03 (d, J= 6.7, 2 H); 8.79 (t,

J= 4.2, 2 H); 8.43 (t, J = 4.2, 2 H); 7.94 (t, J = 4.2, 2 H); 4.70 (t, J= 7.4, 2 H); 2.08 (p, J =

7.5, 2 H); 1.44 (h, J = 7.4, 2 H); 1.00 (t, J = 7.4, 2 H) ppm.

1-butyl-1'-(2-(diethoxyphosphoryl)ethyl)-4,4'-bipyridine-1,1'-diium(3):(2) (0.50 g;

1.7mmol) and diethyl 2-bromoethylphosphonate(0.40 g; 2.0mmol) were dissolved in water (30 ml). The solution was refluxed for 72 h. The solution became yellow and the solvent was slowly evaporated at 100° C.The crude product was dried in vacuo. Yield:

1 0.37 g (40 % yield). H (D2O): 9.06 (d, J = 6.5, 2 H); 8.99 (d, J = 6.4, 2 H); 8.45 (d,

J=6.4, 2 H); 8.40 (d, J= 6.4, 2 H); 4.90 (dt, J1 = 16.4, J2 = 6.6, 2 H); 4.59 (m, 2 H);4.01 (p,

J = 7.5, 3 H); 2.68 (dt, J1 = 18.1, J2 = 7.2, 2 H);1.90 (p, J = 7.5, 2 H); 1.25 (h, J = 7.3, 2

H); 1.10 (t, J = 7.1, 3 H); 0.81 (t, J = 7.4, 2 H) ppm.

1-butyl-1'-(2-phosphonoethyl)-4,4'-bipyridine-1,1'-diium (4, PV2+): (3) (0.90 g; 1.7 mmol) was dissolved in 3M HCl (150 ml). The solution was refluxed for 72 h. The solution became orange and the solvent was slowly evaporated at 100° C. The crude

1 product was dried in vacuo. Yield: 0.81 g (quantitative). H (D2O): 9.20 (d, J = 6.8, 2

H); 9.13 (d, J = 6.7, 2 H); 8.56 (d, J=6.7, 4 H); 4.96 (dt, J1 = 12.6, J2 = 7.5, 2 H); 4.74 (t, J 86

= 7.4, 2 H); 2.51(dt, J1 = 17.3, J2 = 7.6, 2 H); 2.08 (p, J = 7.5, 2 H); 1.42 (h, J = 7.4, 2 H);

0.99 (t, J = 7.4, 3 H) ppm; FT-IR-ATR: = 3463, 3342 (aromatic C-H, N-H), 2987

(aliphatic C-H), 1639 (aromatic C=C), 1231 (P-O), 1024, 972 (P-OH), 832 cm−1;

1-butyl-1'-(2-carboxyethyl)-4,4'-bipyridine-1,1'-diium bromide (5, CV2+): (2) (0.50 g;

1.7 mmol) and diethyl 1-bromopropionic acid (0.26g; 1.7mmol) were dissolved in water

(20 ml). The reaction mixture was refluxed for 72 h, during which time a yellow precipitate formed. The precipitate was collected by filtration and washed with hot

1 ethanol. The crude product was dried in vacuo. Yield: 0.16 g (35 % yield). H (D2O):

9.24 (d, J = 6.2, 2 H); 9.13 (d, J = 6.3, 2 H); 8.55 (d, J=6.4, 4 H); 5.02 (t, J = 6.1, 2 H);

4.74 (t, J = 7.3, 2 H); 3.25 (t, J= 8.4, 2 H); 2.10 (p, J = 7.5, 2 H); 1.42 (q, J = 7.3, 2 H);

1.01 (t, J = 7.3, 3 H) ppm; FT-IR-ATR: = 3003 (aromatic C-H, N-H), 2941 (aliphatic C-

H), 1744 (C=O), 1639 (aromatic C=C), 1177, 819 cm−1.

B.8.2.2 Synthesis of Aryl Viologen Derivatives

1-(2,4-dinitrophenyl)-4-(pyridin-4-yl) pyridinium chloride (7): 4,4‟-bipyridine (1.55 g; 10 mmol) and 2,4-dinitrochlorobenzene (2.03 g; 10 mmol) were dissolved in acetone

(15 ml). The solution was refluxed for 12 h. The brown precipitate was collected by filtration and washed twice with n-pentane. The crude product was dried in vacuo. Yield:

1 1.56 g (45 % yield). H (D2O): 9.47 (d, J = 2.4, 1 H); 9.32 (d, J = 6.7, 2 H); 9.01 (dd, J1

= 2.4, J2 = 8.6, 1 H); 8.92 (d, J = 6.0, 2 H); 8.76 (d, J = 6.7, 2 H); 8.35 (d, J = 8.6, 1 H);

13 8.11 (d, J = 6.0, 2 H) ppm; C NMR (D2O):  159.4, 152.7, 152.2, 148.3, 145.4, 144.5,

140.9, 133.7, 133.2, 128.6, 125.2 ppm. 87

4-(pyridin-4-yl)-1-p-tolylpyridinium chloride (8): p-Toluidine (0.55 g, 4 mmol) was dissolved with 7(0.72 g, 2 mmol) in ethanol (3 ml) under nitrogen atmosphere. The resulting solution was refluxed for 16 h. The precipitate formed was filtered and discarded. The filtrate was dried in vacuo and the yellow crude product was triturated

1 with acetone (250 ml), filtered and dried in vacuo. Yield: 0.38 g (70 %). H (D2O): 9.16

(d, J = 6.7, 2 H); 8.78 (d, J = 6.0, 2 H); 8.53 (d, J = 6.7, 2 H); 7.96 (d, J= 6.0, 2 H); 7.59

13 (dd, J1 = 48.0; J2 = 8.4, 4 H); 2.46 (s, 3 H) ppm; C NMR (500 MHz, D2O): 156.4, 152.5,

146.9, 145.3, 144.5, 142.2, 133.5, 128.4, 126.0, 125.0, 22.8; FT-IR-ATR: = 3005

(aromatic C-H, N-H), 2994 (aliphatic C-H), 1635 (aromatic C=C), 1606, 1535 (C-N),

1342, 819 cm−1.

1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (11, MTV2+): compound 4 (0.15 g,

0.5 mmol) was dissolved in 10 ml ethanol, and iodomethane (0.09 g, 0.6 mmol) was added with a syringe into the solution. The reaction mixture was stirred at 43 C for 24 h, during which time an orange precipitate formed. The precipitate was collected by filtration and washed with acetone. The crude product was dried in vacuo. Yield: 0.20 g

1 2+ (99 %). H (D2O) MTV : 9.37 (d, J = 6.7, 2 H); 9.11 (d, J = 6.7, 2 H); 8.71 (d, J = 6.7,

2 H); 8.62 (d, J = 6.3, 2 H); 7.72 (d, J = 8.4, 2 H); 7.61 (d, J = 8.4, 2 H); 4.54 (s, 3 H);

13 2.52 (s, 3 H) ppm; C NMR (500 MHz, D2O): 152.9, 152.1, 148.9, 147.8, 145.6, 142.6,

142.4, 133.6, 129.6, 129.4, 129.4, 126.4, 51.1, 23.0; FT-IR-ATR: = 3102 (aromatic C-

H, N-H), 2981 (aliphatic C-H), 1635 (aromatic C=C), 1492 (C-N), 819 cm−1; MALDI-

MS: m/z (%): 262.15 (90), 247.13(80). 88

B.8.3 Inclusion of MV2+ and MTV2+ into CB[7]

The inclusion of viologens, MV2+and MTV2+in CB[7] was done following reported procedures.37,92 Briefly, equimolar amounts of methylviologen (or MTV2+) and

CB[7]were dissolved together in distilled water (100 ml) to form a 1 mM solution of the guest@host complex and stirred overnight. This solution was used as the binding solution for the TiO2 films. Evaporation of water led to the solid complex that was used for the

FT-IR-ATR spectra. Formation of the complexes in solution was monitored by 1H NMR in D2O. It was observed that in both cases the complexation is fast (minutes).

2+ 1 2+ MV @CB[7: H NMR (500 MHz, D2O): (a) MV δ 4.54 (s, 3 H);δ 8.55 (d, 2H, J =

15.5), δ 9.04 (d, 2H, J = 15.5); (b) CB [7] 4.25 (d, 14H, J = 15.5), 5.56 (s, 14H), 5.81 (d,

14H, J = 15.5);FT-IR-ATR: = 3000 (broad for N-H, C-H, water), 1722 (C=O), 1638

(C=C), 1462 (C-N), 1375, 1321, 1212 cm−1.

2+ 1 2+ MTV @CB[7: H (500 Hz, D2O): (a) MTV : 9.31 (d, J = 6.0, 2 H); 8.91 (d, J = 5.5, 2

H); 8.63 (d, J = 5.5, 2 H); 8.49 (d, J = 5.5, 2 H); 7.03 (d, J = 7.5, 2 H); 6.63 (d, J = 7.5, 2

H); 3.95 (s, 3 H);1.97 (s, 3 H);(b) CB[7]: 5.80 (d, J = 15.3, 14 H); 5.54 (s, 14 H); 4.24

(d, J = 15.3, 14 H) ppm; FT-IR-ATR: = 3000 (broad for N-H, C-H, water), 1720 (C=O),

1636 (C=C), 1462 (C-N), 1370, 1320, 1214cm−1.

B.8.4 Titanium Dioxide Synthesis

Sol-gel preparation of TiO2 nanoparticles followed the hydrolysis and condensation of titanium(IV) isopropoxide in an aqueous nitric acid solution. A three necked round bottom flask was set up with a thermometer, dropping funnel in the middle neck and

Dean-Stark apparatus, Figure B - 21. The round bottom flask contained a solution of 100 89

ml water containing 0.69 mL of concentrated nitric acid (68 – 70 %). The dropping funnel contained titanium(IV) isopropoxide20 mL with isopropanol 80mL. Both the aqueous and the alkoxide solution were deaerated with nitrogen for at least 10min. The acidic solution was rigorously stirred using a magnetic stirrer and in the meantime the alkoxide solution was added through the dropping funnel at a rate not faster than one drop per second. The additions lead to an immediate formation of a white precipitate in the round bottom flask, indicating the hydrolysis to TiO2.

N2

N2

c d

e b

a

Figure B - 24 Experimental set up for the hydrolysis.

After all the titanium(IV) isopropoxide solution was consumed and the hydrolysis was complete, changes in the set have to be made: the dropping funnel is removed and the

Dean-Stark apparatus as well as the flask should be covered by aluminum foil. The insulation is necessary during the following distillation. The isopropanol needs to be removed from the aqueous solution and can be collected as isopropanol/water mixture of 90

approximately 130 mL at a temperature range of 86-95 °C. The distillation has to be terminated as soon as the distillate reaches 100 °C, indicating that only water remains in the solution. At this point the set-up is reduced to the three neck round bottom flask with a condenser in the middle. The reaction mixture remains overnight to reflux. Following the reflux also the condenser is removed and the reaction volume is reduced to 45 mL by allowing the water to evaporate. The reaction mixture was allowed to cool down and sonicated for 5 min and transferred to the glass beaker with a magnetic stirring rod belonging to the titanium autoclave (Figure B - 22). The titanium autoclave (Model 4760,

Parr) is programmed to heat the sol gel at 200 °C for 12 hours at a pressure reaching 17-

18 bar.

Figure B - 25 Picture of the custom-made titanium autoclave for the sol gel process of TiO2 nanoparticles. 91

The white sol-gel was cooled to room temperature, sonicated (5 min), and transferred from the autoclave beaker to a graduated beaker. At this point the TiO2 concentration in the gel was determined by the „glass dies‟ method. The method involves weighing a small amount of sol-gel on a glass slide before and after drying the gel at 100 °C to remove the containing water. The difference determines the wt. % of TiO2 in the sol-gel.

The concentration should be in a range of 13-17 wt % in order to be processed.

Poly(ethylene glycol) (PEG 2,000; amount: 6 g/L) was added to the colloid to yield a white viscous paste and the mixture was stirred for at least 72 h to reach a good homogeneity for the casting of TiO2 paste on a substrate. The paste was left covered from light exposure for approximately up to one month for thin film preparation. Evidence of a degraded paste is a yellow discoloration. TiO2 pastes older that one month were discarded.

B.8.5 Titanium Dioxide Film Preparation

The TiO2 films were casted on conducting glass (FTO). Briefly, TiO2 paste was spread using a glass test tube on the precut conductive glass, followed by sintering at 450 °C for

30 min under oxygen flow. The films were allowed to cool down before immediate use or were stored in a dark desiccator. No difference was observed when using films stored for weeks as indicated.

B.8.6 Binding to Titanium Dioxide Films

Physisorption of MV2+@CB[7] or MTV2+@CB[7] was done by immersing the films in an aqueous solution with the complex (0.5 mM) for 24 h. Afterwards, the films were 92

rinsed with DIUF water and dried at 105 °C for 20 min. Transparent TiO2 films/FTO modified with ferrocene carboxylic acid (Fc-COOH) were used as counter electrodes for the electrochromic windows. The Fc-COOH-modified electrodes were prepared by immersing the nanostructured TiO2/FTO films into an ethanolic 0.05 mM solution of Fc-

COOH for 1 h, rinsing with ethanol, and drying at 100 °C for 20 min prior use.

B.8.7 Preparation of Electrochromic Windows

The electrochromic windows were assembled following procedures similar to those used by others. The steps, as illustrated in Figure B - 23, are as follows: (1) The FTO glass was cut (3.0 x 2.5 cm) and cleaned with ethanol in a sonicator for several hours. (2) The glass was masked using a self-adhesive vinyl label with a square cutout (1.8 cm side). (3) The

TiO2 paste was cast on the glass using a glass rod and dried in the air for 10 min, and then the vinyl mask was removed. The film was sintered at 450 °C for 30 min under oxygen flow and then cooled slowly to room temperature. (4) The TiO2/FTO films were immersed into the binding solution for 24 h. (5) The counter electrode was prepared by immersing the TiO2/FTO films into a 0.05 mM ethanol solution of Fc-COOH. (6) The electrodes were bound together using a thermoplastic polymer (Surlyn).

93

Figure B - 26 Construction of electrochromic windows, (taken from reference 37).

The thermoplastic was cut to form a frame leaving a small side opening for filling the window with the electrolyte (see below), deposited around the functionalized TiO2 film, and heated to 60 °C. The counter electrode was simultaneously heated, and then the two electrodes were pressed together to form the window. The window was placed in a desiccator, in vacuo, and in the dark for 24 to 48 h. (7) A drop of electrolyte (0.05 M

LiClO4 in anhydrous, freshly distilled γ-butyrolactone) was placed on the side opening.

The window was filled by placing the cell in a desiccator in vacuo for a few seconds and then admitting to air. Finally, the side opening of the window was sealed using epoxy.

B.8.8 Electrochemistry in Solution of MV2+ and MTV2+ and the Corresponding

CB[7] Complexes

The electrochemical properties of MV2+, MTV2+and their CB[7] complexes in solution were studied by cyclic voltammetry on a BAS CV27 potentiostat. The experiments were conducted in aqueous 0.1 M phosphate buffer (pH 7.3) at room temperature. The 94

solutions were deaerated by bubbling nitrogen in a standard three-electrode arrangement with glassy carbon (2 mm diameter), Pt gauze counter electrode, and Ag/AgCl (1.0 M

KCl) as the reference electrode. Each cycle was measured between ± 1.0 V at a scan rate of 50 mV/s with a sensitivity of 10 mA/V. Spectroelectrochemical measurements (Figure

B - 11) were obtained in a quartz cell (ALS Japan, distributed by CH instruments, 011240

SEC-C Thin Layer Quartz Glass Spectroelectrochemical cell Kit, Pt gauze working electrode) and using Ag/AgCl (1.0 M KCl) for aqueous solutions as the reference electrode.

B.8.9 Electrochemistry of Electrochromic Windows

The electrochromic windows were assembled as described. Each CV was recorded between 0 V and -1.0 V with a sensitivity of 1 mA/V at a scan rate of 100 mV/s and a potential of -0.8 V was applied to form the colored state (MV•+or MTV•+) and a potential of + 0.1 V to oxidize the species back to the bleached state (MV2+or MTV2+).

B.8.10 Spectroscopic Measurements in Solution of MV2+ and MTV2+, Their

Complexes and the Corresponding Complexation Constant of MTV2+

FT-IR ATR spectra were acquired on a Thermo Electron. Corp. Nicolet 6700 FT-IR. UV-

VIS absorbance spectra were collected on an Ocean Optics USB4000+ Miniature Fiber

Optic Spectrometer in combination with a PX-2 Pulsed Xenon Light Source (150 ms for integration time and 20 scans to average). To 1000 μL of an aqueous solution 30 μM in

MTV2+were added 50 μL aliquots of a 100 μM aqueous solution of CB[7] using a micropipet. Both solutions were buffered with a 0.1 M phosphate buffer (pH 7.32). The 95

UV-VIS absorption spectra were measured after each addition. The absorbance at

λmax=218 nm was fitted against the concentration of CB[7] (1:1 complexation model) to

5 obtain the equilibrium constant Kmtv= (1.06 ±0.7 )×10 L/mol.

B.8.11 Spectroscopic Measurements of Electrochromic Windows

FT-IR ATR spectra were acquired on a Thermo Electron. Corp. Nicolet 6700 FT-IR. UV-

Vis absorbance spectra were collected on an Ocean Optics USB4000þ miniature fiber- optic spectrometer in combination with a PX-2 pulsed xenon light source (150 ms for integration time and 20 scans to average).

B.9 Conclusions

The organization of photo- and redox-active molecules in semiconductor nanoparticles

(TiO2) hosts produces new properties and collective effects that are useful for the development of functional materials and devices. The proof-of-concept experiments described in this chapter demonstrate the electrochromic properties of two viologen guests encapsulated inside a cucur[7]bituril host where the host was bound to the surface of the semiconductor. Negative effects of the otherwise heterogeneous and complex surfaces can be shielded by the encapsulation of redox-active dyes. The added stability prevents the dimerization as well as aggregation processes and potentially avoids the need for synthetic modifications of the molecules with binding groups. Complexes of methylviologen (MV2+) and 1-methyl-1‟-p-tolyl-4,4‟-bipyridinium dichloride (MTV2+)

were encapsulated in a molecular host, CB[7], and physisorbed onto the surface of TiO2 nanoparticle films. In the absence of CB[7], methylviologen and viologen derivative did 96

not bind to the surface of TiO2 nanoparticle films due to the lack of anchoring groups.

Electrochromic windows were prepared using viologen@CB[7]-modified TiO2 films cast on FTO electrodes. These windows exhibited reversible color switching upon application of -0.8 V, corresponding to the formation of intensely blue radical cations. Further studies of the binding of CB[7] are in progress, to explore more photochemical applications of the described encapsulation, and to further develop this binding approach with other dyes and viologen derivatives.

97

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(45) Kaifer, A. E.; Li, W.; Yi, S.: Cucurbiturils as Versatile Receptors for Redox Active Substrates. Israel Journal of Chemistry 2011, 51, 496-505. (46) Wyman, I. W.; Macartney, D. H.: Host-Guest Complexes and Pseudorotaxanes of Cucurbit[7]uril with Acetylcholinesterase Inhibitors. Journal of Organic Chemistry 2009, 74, 8031-8038. (47) Wyman, I. W.; Macartney, D. H.: Cucurbit[7]uril host-guest and pseudorotaxane complexes with a,x-bis(pyridinium)alkane dications. Organic and Biomolecular Chemistry 2009, 7, 4045-4051. (48) Sindelar, V.; Silvi, S.; Kaifer, A. E.: Switching a molecular shuttle on and off: simple, pH-controlled pseudorotaxanes based on cucurbit[7]uril. Chemical Communications 2006, 2185-2187. (49) Kim, K.; Selvapalam, N.; Ko, Y. H.; Park, K. M.; Kim, D.; Kim, J.: Functionalized cucurbiturils and their applications. Chemical Society Reviews 2007, 36, 267. (50) Lee, J. W.; Samal, S.; Selvapalam, N.; Kim, H.-J.; Kim, K.: Cucurbituril Homologues and Derivatives: New Opportunities in Supramolecular Chemistry. Accounts of Chemical Research 2003, 36, 621-630. (51) Cui, L.; Gadde, S.; Li, W.; Kaifer, A. E.: Electrochemistry of the Inclusion Complexes Formed Between the Cucurbit[7]uril Host and Several Cationic and Neutral Ferrocene Derivatives. Langmuir 2009, 25, 13763-13769. (52) Philip, I. E.; Kaifer, A. E.: Electrochemically Driven Formation of a Molecular Capsule around the Ferrocenium Ion. Journal of American Chemistry Society 2002, 124. (53) Lagona, J.; Mukhopadhyay, P.; Chakrabarti, S.; Isaacs, L.: The Cucurbit[n]uril Family. Angewandte Chemie International Edition 2005, 44, 4844-4870. (54) Smith, M. B.; March, J.: March's Advanced Organic Chemistry; Wiley- Interscience: New York, 2001. (55) Jaeger, W.; Bohrisch, J.; Laschewsky, A.: Synthetic polymers with quaternary nitrogen atoms--Synthesis and structure of the most used type of cationic polyelectrolytes. Progress in Polymer Science 2010, 35, 511-577. (56) Arnett, E. M.; Reich, R.: Electronic effects on the Menshutkin reaction. A complete kinetic and thermodynamic dissection of alkyl transfer to 3- and 4-substituted pyridines. Journal of the American Chemical Society 1980, 102, 5892-5902. (57) Zhu, X.; Zhang, D.; Liu, C.: New insight into the formation mechanism of imidazolium-based halide salts. Journal of Molecular Modeling 2011, 17, 2099-2102. (58) Yamaguchi, I.; Higashi, H.; Shigesue, S.; Shingai, S.; Sato, M.: N-Arylated pyridinium salts having reactive groups. Tetrahedron Letters 2007, 48, 7778-7781. (59) Zincke, T. H.; Weisspfenning, G. J.: Liebigs Ann. 1913, 396, 103. 101

(60) Gnecco, D.; Marazano, C.; EnrIquez, R. l. G.; Ter·n, J. L.; S·nchez S, M. d. R.; Galindo, A.: Oxidation of chiral non-racemic pyridinium salts to enantiopure 2- pyridone and 3-alkyl-2-pyridones. Tetrahedron: Asymmetry 1998, 9, 2027-2029. (61) Becher, J.: Synthesis Synthesis 1980, 589. (62) Marquez, C.; Huang, F.; Nau, W. M.: Cucurbiturils: Molecular Nanocapsules for Time-Resolved Fluorescence-Based Assays. IEEE TRANSACTIONS ON NANOBIOSCIENCE 2004, 3, 39-45. (63) Marquez, C.; Hudgins, R. R.; Nau, W. M.: Mechanism of Host-Guest Complexation by Cucurbituril. Journal of American Chemistry Society 2004, 126, 5806-5816. (64) Mock, W. L.; Shih, N. Y.: Structure and selectivity in host-guest complexes of cucurbituril. The Journal of Organic Chemistry 1986, 51, 4440-4446. (65) Kim, K.: Mechanically interlocked molecules incorporating cucurbituril and their supramolecular assemblies. Chemical Society Reviews 2002, 31, 96-107. (66) Castro, R.; Cuadrado, I.; Alonso, B.; Casado, C. M.; Moràn, M. s.; Kaifer, A. E.: Multisite Inclusion Complexation of Redox Active Dendrimer Guests. Journal of the American Chemical Society 1997, 119, 5760-5761. (67) Wang, W.; Kaifer, A. E.: Electrochemical Switching and Size Selection in Cucurbit[8]uril-Mediated Dendrimer Self-Assembly. Angewandte Chemie International Edition 2006, 45, 7042-7046. (68) Wang, Y.; Cardona, C. M.; Kaifer, A. E.: Molecular Orientation Effects on the Rates of Heterogeneous Electron Transfer of Unsymmetric Dendrimers. Journal of the American Chemical Society 1999, 121, 9756-9757. (69) Yang, H.; Hao, J.; Tan, Y.: Cucurbit[7]uril moving on side chains of polypseudorotaxanes: Synthesis, characterization, and properties. Journal of Polymer Science Part A: Polymer Chemistry 2011, 49, 2138-2146. (70) Kim, H.-Y. H.; Voehler, M.; Harris, T. M.; Stone, M. P.: Detection of an Interchain Carbinolamine Cross-Link Formed in a CpG Sequence by the Acrolein DNA Adduct γ-OH-1,N 2-Propano-2„-deoxyguanosine. Journal of the American Chemical Society 2002, 124, 9324-9325. (71) Koner, A. L.; Nau, W. M.: Cucurbituril Encapsulation of Fluorescent Dyes. Supramolecular Chemistry 2007, 19, 55-66. (72) Woo Sung Jeon, K. M., Sang Hyun Park, Hyungpil Chun, Young Ho Ko, Jin Yong Lee, Eun Sung Lee, S. Samal, N. Selvapalam, Mikhail V. Rekharsky, Vladimir Sindelar, David Sobransingh, Yoshihisa Inoue, Angel E. Kaifer, Kimoon Kim: Complexation of Ferrocene Derivatives by the Cucurbit[7]uril Host: A Comparative Study of the Cucurbituril and Cyclodextrin Host Families. Journal of American Chemistry Society 2005, 127, 12984-12989. 102

(73) Bohne, C.: Supramolecular Dynamics Studied Using Photophysics. Langmuir 2006, 22, 9100-9111. (74) Yang, C.; Liu, L.; Mu, T.-W.; Guo, Q.-X.: The Performance of the Benesi- Hildebrand Method in Measuring the Binding Constants of the Cyclodextrin Complexation. Analytical Sciences 2000, 16, 537-539. (75) Legouin, B.; Gayral, M.; Uriac, P.; Cupif, J.-F.; Levoin, N.; Toupet, L.; van de Weghe, P.: Molecular Tweezers: Synthesis and Formation of Host–Guest Complexes. European Journal of Organic Chemistry 2010, 2010, 5503-5508. (76) Galoppini, E.: Linkers for anchoring sensitizers to semiconductor nanoparticles. Coordination Chemistry Reviews 2004, 248, 1283-1297. (77) Rochford, J.; Chu, D.; Hagfeldt, A.; Galoppini, E.: Tetrachelate Porphyrin Chromophores for Metal Oxide Semiconductor Sensitization: Effect of the Spacer Length and Anchoring Group Position. Journal of the American Chemical Society 2007, 129, 4655-4665. (78) Choi, S. Y.; Mamak, M.; Coombs, N.; Chopra, N.; Ozin, G. A.: Electrochromic Performance of Viologen-Modified Periodic Mesoporous Nanocrystalline Anatase Electrodes. Nano Letters 2004, 4, 1231-1235.

(79) Li, G.; Li, L.; Boerio-Goates, J.; Woodfield, B. F.: High Purity Anatase TiO2 Nanocrystals: Near Room-Temperature Synthesis, Grain Growth Kinetics, and Surface Hydration Chemistry. Journal of the American Chemical Society 2005, 127, 8659- 8666.

(80) Barnard, A. S.; Curtiss, L. A.: Prediction of TiO2 Nanoparticle Phase and Shape Transitions Controlled by Surface Chemistry. Nano Letters 2005, 5, 1261-1266. (81) Ong, W.; Kaifer, A. E.: Unusual Electrochemical Properties of the Inclusion Complexes of Ferrocenium and Cobaltocenium with Cucurbit[7]uril. Organometallics 2003, 22, 4181-4183. (82) Jeon, W. S.; Moon, K.; Park, S. H.; Chun, H.; Ko, Y. H.; Lee, J. Y.; Lee, E. S.; Samal, S.; Selvapalam, N.; Rekharsky, M. V.; Sindelar, V.; Sobransingh, D.; Inoue, Y.; Kaifer, A. E.; Kim, K.: Complexation of Ferrocene Derivatives by the Cucurbit[7]uril Host:‚Äâ A Comparative Study of the Cucurbituril and Cyclodextrin Host Families. Journal of the American Chemical Society 2005, 127, 12984-12989. (83) Santa-Nokki, H.; Kallioinen, J.; Korppi-Tommola, J.: A dye-sensitized solar cell driven electrochromic device. Photochemical & Photobiological Sciences 2007, 6, 63. (84) Rosseinsky, D. R.; Mortimer, R. J.: Electrochromic Systems and the Prospects for Devices. Advanced Materials 2001, 13, 783-793. (85) Jain, V.; Khiterer, M.; Montazami, R.; Yochum, H. M.; Shea, K. J.; Heflin, J. R.: High-Contrast Solid-State Electrochromic Devices of Viologen-Bridged Polysilsesquioxane Nanoparticles Fabricated by Layer-by-Layer Assembly. Applied Materials and Interfaces 2009, 1, 83-89. 103

(86) Braunschweig, A. B.; Ronconi, C. M.; Han, J.-Y.; Aricó, F.; Cantrill, S. J.; Stoddart, J. F.; Khan, S. I.; White, A. J. P.; Williams, D. J.: Pseudorotaxanes and Rotaxanes Formed by Viologen Derivatives. European Journal of Organic Chemistry 2006, 2006, 1857-1866. (87) Lim, J.; Ko, H.; Lee, H.: Single- and dual-type electrochromic devices based on polycarbazole derivative bearing pendent viologen. Synthetic Metals 2006, 156, 695- 698. (88) Kazachkin, D. V.; Nishimura, Y.; Witek, H. A.; Irle, S.; Borguet, E.: Dramatic Reduction of IR Vibrational Cross Sections of Molecules Encapsulated in Carbon Nanotubes. Journal of American Chemistry Society 2011, 133, 8191-8198. (89) Zou, J.; He, H.-Y.; Dong, J.-p.; Long, Y.-c.: A Novel LiCl/H-STI Zeolite Guest/Host Assembly Material with Superior Humidity Sensitivity: Fabrication and Characterization. Chemistry Letters 2001, 30, 810-811. (90) Zou, D.; Andersson, S.; Zhang, R.; Sun, S.; Åkermark, B.; Sun, L.: A Host- Induced Intramolecular Charge-Transfer Complex and Light-Driven Radical Cation Formation of a Molecular Triad with Cucurbit[8]uril. Journal of Organic Chemistry 2008, 73. (91) Pluth, M. D.; Raymond, K. N.: Reversible guest exchange mechanisms in supramolecular host-guest assemblies. Chemical Society Reviews 2007, 36, 161-171. (92) Ko, Y. H.; Kim, E.; Hwang, I.; Kim, K.: Supramolecular assemblies built with host-stabilized charge-transfer interactions. Chemical Communications 2007, 1305- 1315.

104

Chapter C

Fluorescent Properties of di-p-Tolyl-Viologen

by Complexation with Cucurbit[7]uril

105

C.1 Introduction

Fluorescent dyes1-3 have emerged as crucial components in many applications such as secure banknotes,1,4 lasing media,5-7 optical data storage,8,9 biomedical probes,10,11 and environmental sensors.12-15 A critical issue is dye performance, which has to satisfy certain requirements such as chemical and photochemical stability, color intensity, fluorescence quantum yield, solubility and toxicity.16-19 The most obvious way to tune such properties is to synthetically modify the structure.20-24 An alternative approach is the use of host-guest chemistry. It is possible to improve the properties of fluorescent dyes by encapsulating them in hosts, as the supramolecular self-assembly results in changes of the chemical and photochemical properties.25-27 The macrocyclic host, cucurbit[7]uril, is an example, as it has been used as a stabilizing additive and enhancement agent for fluorescent dyes. This property was extensively investigated by Nau and coworkers in the last decade.21,23,28 It was found that CB[7] improved photostability, fluorescence lifetime, quantum yield of the included fluorophores by shielding them from quenchers, Figure C -

1.5,21,23,28-32

Figure C - 1 Illustration of the formation of a fluorescent guest-host inclusion complex.

In the present chapter we have investigated the complexation and fluorescence behavior of a viologen derivative, 1,1'-di-p-tolyl-(4,4'-bipyridine)-1,1'-diium (DTV2+) dichloride, 106 free and encapsulated in cucurbit[7]uril (CB[7]), Viologens (1,1‘-disubstituted-4,4‘- bipyridinium salts) forms a stable and intensely colored radical cation in a fast and reversible redox process.33 They are excellent electron acceptors and because of their properties, viologens are widely used as acceptors in electron transfer studies,34-36, as components of novel electrochromics,37-41 molecular electronics,42,43 redox sensors.44,45

Viologens are also used in the design of cyclophane,46,47 rotaxane48-51 and catenane52,53 molecular hosts.

Figure C - 2 Molecular structure of DTV2+, the viologen derivative studied in this work. The counter ion is chloride. Molecular structure and dimensions of cucurbit[7]uril and the corresponding inclusion complex.

Methylviologen (MV2+, 1,1'-dimethyl-4,4'-bipyridine-1,1'-diium) dichloride and

5 - cucurbit[7]uril (CB[7]) form a stable complex with a complexation constant Kc ~ 10 M 107

1.The redox properties of viologen, viologen@CB[7], and the MV2+@CB[7] complex have been extensively investigated.27,54-59,58-63 More recently, we reported the electrochromic properties of CB[7]complexes of MV2+ and 1-methyl-1'-p-tolyl-4,4'-

2+ bipyridine-1,1'-diium (MTV ), physisorbed on nanoparticle TiO2 thin films (see chapter

B in this thesis).64,65

For many years any account of fluorescent behavior of viologens was attributed to the presence of highly fluorescent pyridone impurities, Figure C - 3.33,66-70 The fluorescence and excited state properties of MV2+ were thoroughly characterized for the first time a decade ago by Kohler and coworkers.71 The solvent strongly influenced the S1 excited state dynamics, with  ~ 0.03 and ~ 1ns decay in acetonitrile, in contrast with several ps decay in water, ascribed to a non-radiative channel, and even faster decay in methanol,

2+ where quenching of the S1 excited state of MV involved electron transfer from the solvent. Fluorescence was also reported for MV2+ embedded in zeolites,72-74 and for 2,7- dimethylthieno(2,3-c:5,4-c’) dipyridinium, a methyl viologen with a thienyl bridge locking the pyridinium rings in a planar and rigid structure.73,75-77

Figure C - 3 Fluorescent oxo-pyridone compounds33,40,66-68

The supramolecular complexes are generally held together by non-covalent interactions, which are relatively weak, and consequently it is important to control the structure and stoichiometry of the complex. This was recently demonstrated in a study of Thioflavin T

(ThT) complexes of CB[7].22,26 Encapsulation of ThT in CB[7] resulted in a large (20 108 fold) fluorescence enhancement which was explained by the restriction of torsional motion, which leads to a reduction of the non-radiative processes of ThT. The largest fluorescence enhancement was observed for complexes consisting of one guest ThT with two hosts CB[7]. Attempts to reach a competitive displacement by further addition of Na+ ions only contributed to fluorescence enhancement, by forming a highly fluorescent supramolecular capsule, (Figure C - 4).

Figure C - 4 Proposed Binding Interactions in the ThT, CB7, and Metal Ion System Leading to the Highly Fluorescent Supramolecular capsule. (Taken from reference 26)

Nau and coworkers reported a two-fold increase in emission lifetime of 2,3- diazabicyclo(2.2.2)oct-2-ene (DBO) encapsulated in CB[7], caused by the guest being shielded by the host from quenchers like oxygen.28,32 In general, emission enhancement of CB[n] complexes with fluorescent dyes has attracted research, potentially leading to applications as fluorescent probes for biological systems,28,32 in imaging microscopy

(FLIM),78 as water soluble sensors,79-81 and in supramolecular photochemistry studies.82,83

This thesis describes an unreported emission enhancement of complexes from CB[7] and

1,1'-di-p-tolyl-4,4'-bipyridine-1,1'-diium (DTV2+) dichloride in aqueous solutions.

Viologen DTV2+ carries two p-tolyl groups on the quaternized nitrogens and the emissive properties of DTV2+@CB[7] are interesting for several reasons. Hybrid organic/inorganic 109 layers of emissive CB[7] complexes could lead to possible applications as light emitting diodes (LED). DTV2+ solutions were weakly fluorescing, and encapsulation in CB[7] produced a strong, blue emission, a similar behavior observed for MTV2+.64,65 No fluorescence emission enhancement was observed for MV2+ and phenyl viologens upon encapsulation in CB[7]. Although minimum traces of impurities cannot be excluded completely, comparisons of different batches of DTV2+ after repeated purification steps, combined with quantum yield and emission lifetime studies, elemental analysis, and titration experiments with CB[7], confirmed that the observed fluorescence enhancement reported here is caused by encapsulation of DTV2+.

Viologen DTV2+ was synthesized as part of the studies of electrochromic properties of viologens@CB[7] complexes bound to TiO2. Using this type of host-guest photochemistry is important, since it has been demonstrated that CB[7] and its viologen complexes bind to nanostructured metal oxide films used for fully reversible electrochromic windows.64,65 Also, the advantages of the host-guest chemistry (inhibited quenching, chemical stability, etc.) are important for fundamental charge transfer studies at semiconductor interfaces.65

It is interesting, that DTV2+ exhibited enhanced fluorescence in CB[7], while methylviologen and the structurally similar phenyl viologen did not. This suggests that the p-tolyl group is important for the observed fluorescence enhancement.

Experimentally the complexation constants, electrochemical properties, the stoichiometry, as well as static and time resolved emission properties of DTV2+ and its complexes with CB[7] were determined. Theoretically the calculattions DFT and CIS of the S1 and T1 excited states of DTV2+ were performed. Since cucurbiturils have multiple 110 carbonyl binding sites for positive ions and tend to complex strongly with salts, especially NaCl,61,84,85 we also investigated the effects of excess NaCl on the complexation of DTV2+ in CB[7]. Salts generally influence the complexation equilibria, while in some cases it even influences the emissive properties of chromophoric guests.85,27,54,55

C.2 Results and Discussion

C.2.1 Synthesis of Symmetric Viologen DTV2+

The symmetric viologen derivative DTV2+ was previously reported, but no characterization was reported.86 The Zincke reaction conditions were used to synthesize

DTV2+according to the previously described mechanism, (chapter B).87,88 The synthesis consists of two steps, first the reaction of 2,4-dinitro-chlorobenzene (6) and 4,4‘- bipyridine (1) yielding the N-4,4-dinitrophenylpyridinium salt (12), followed by reaction with an aniline derivative, p-toluidine (13), yielding the final product the dichloride bipyridinium salt of DTV2+ (14) (36% total yield). The product is a yellow-grey powder, which is soluble in most protic solvents, water, ethanol and methanol. 111

Scheme C - 1 Synthesis of DTV2+

C.2.2 Synthesis of Cucurbit[7]uril

Cucurbiturils were first synthesized in 1905 by Behrend et al.89 and characterized by

Freeman90 and Mock in 1981.90,91 The original reaction conditions were relatively harsh

(110 °C/ 72h) and only resulted in the major product cucurbit[6]uril, CB[6]. In 2001 Kim and coworkers developed a synthetic approach that used milder conditions and allowed the synthesis of new cucurbituril homologues, ranging from CB[5] to CB[8].59,92-94 Kim‘s synthesis involved the condensation reaction between glycoluril and formaldehyde in 9

M sulfuric acid at temperature ranging from 75 °C to 100 °C. The reaction mixture of

CB[n] contained an average of 60% CB[6], 10% CB[5], 20% CB[7], and 10% other higher CB homologues. Day and coworkers further examined the controlling factors for 112 the cucurbituril homologue synthesis, such as acid concentration, acid type, reaction temperature, and reactant concentration. The isolation of these homologues has dramatically increased the applicability of cucurbiturils as hosts, since it is possible to find the appropriate cucurbituril host by cavity size for a given guest. Cucurbiturils CB[5] and CB[7], have better solubility in water compared to their homologues, which have even numbers of glycoluril units. 92,95-9896,97

Scheme C - 2 Mechanism of formation for cucurbituril homologues from oligomers

We observed that in commercial samples of CB[7], salts (MS) and other homologues

(MS, 1H NMR) are often present, (

Figure C - 5). Such impurities influence the complexation behavior of guests with

CB[7],26,61,92,94,96,99-103 and therefore was synthesized and purified through multiple recrystallization steps until the purity was satisfactory (1H NMR), (Figure C - 6). 113

1 Figure C - 5 H NMR of Cucurbit[7]uril (CB[7]) in D2O (commercially available,

Sigma-Aldrich, cat.# 545201).

1 Figure C - 6 H NMR of Cucurbit[7]uril (CB[7]) in D2O.

C.2.3 Host guest complexes of DTV2+ with CB[7]

The formation of host-guest complexes between CB[7] and viologens was monitored by

1 H NMR spectroscopy in D2O. The viologen protons exhibit characteristic shift upon encapsulation within the CB[7] cavity.58 Protons encapsulated within lead to an upfield shift, while protons near the electron rich portals lead to a downfield shift of the proton signals. 1H NMR titration can provide useful information regarding the location of the

IMPORTANT NOTE: The sentences or paragraphs within asterisks in this thesis were cited verbatim or slightly modified from papers of which I am a co-author. 114

CB[7] macrocycles along the viologen thread, based on the complexation-induced shifts.62,63 The 1H NMR titration was repeated in the presence of a large excess of sodium chloride (0.05 M NaCl), (Figure C - 10).

2+ Figure C - 7 Viologen region of the 1H NMR spectra in D2O of DTV , and after addition of 0.5, 1.0, 2.0, and 3.0 equivalents of CB[7]. The 3-6 ppm region, the CB and solvent signals were omitted for clarity.

Figure C - 8 Chemical shift differences of DTV2+ upon complexation with 2 equivalents of CB[7] in D2O. Similar shifts were observed in the presence of NaCl.

115

Upon addition of 0.5 equivalents of CB[7] the tolyl group‘s doublets Hc and Hb broadened considerably and a significant upfield shift of all signals assigned to the tolyl

group (Ha, Hb and Hc) was observed. The pattern pattern

Figure C - 8, indicates that CB[7] preferably encapsulates the p-tolyl moiety. During the initial stages of titration with CB[7], spectral changes were observed, especially a broadening of the 4,4‘-bipyridyl protons signals. This indicates that the CB[7] moves back and forth, shuttling over the viologen derivative on the NMR timescale, (Figure C -

9). A similar shuttling behavior for CB[7] complexes of viologens has been reported by

Kaifer and coworkers.48,62,63,104 It is also possible that the viologen completely exits the cucurbiturils cavity and encapsulates again on the same timescale until a stable 1:2 complex is possible to form.

Figure C - 9 Schematic representation of shuttling of a 1:1 complex DTV2+@CB[7]

However, upon addition of a second CB[7] to the molecule to form the 1:2 g:h complex, the CB[7]s were located over the tolyl group. The electron rich portals of the cucurbiturils can repel each other so that the presence of a second CB[7] will cause the first CB[7]s to shift along the guest.26 After the ratio of 1:2 g:h was reached, the spectrum‘s signals were no longer broaded and the spectrum was sharper. The p-tolyl group protons exhibited the largest shifts (Δδ Hb = -0.97 ppm, Δδ Hc= -0.77 ppm, and Δδ

Ha= -0.50 ppm), and the 4,4‘-bipyridyl protons Hd and He exhibited shifts that are typically observed upon encapsulation of viologens in CB[7], (Figure C - 10) Further 116 addition of CB[7] did not lead to further changes in the spectrum, indicating that shuttling is inhibited upon complete encapsulation of DTV2+, starting at a 1:2 g:h ratio. In the presence of a large excess of NaCl, the spectrum of free DTV2+ was unchanged. Upon addition of CB[7] the proton signals broadened and became sharp at a 1:3 g:h ratio. This suggests that, in the presence of NaCl, a higher host to guest ratio is needed for full complexation, (Figure C - 10).26,49,50,62,105 The two different host molecules on the viologen thread can be distinguished, (Figure C - 11). Another possibility is that a large excess of host led to aggregates, as suggested by the formation of a precipitate, and the shifts and broadening of signals in the CB[7] region o, (Figure C - 11).106-108

2+ Figure C - 10 Viologen region of the 1H NMR spectra of DTV in 0.05 M NaCl in D2O, and after addition of 0.5, 1.0, 2.0, and 3.0 equivalents of CB[7].

117

1 Figure C - 11 H NMR of Cucurbit[7]uril region (6.5 ppm - 3.5 ppm) in D2O, at host- guest ratios (0.5:1, 1:1, 1:2, 1:3) of DTV2+:CB[7] following the viologen encapsulation.

Similar behavior was demonstrated in the case of rotaxanes and pseudorotaxanes using

CB[5], CB[6], CB[7], CB[8] as the cyclic component.51,55,84,109 While some of these rotaxanes and pseudorotaxanes consist of one cucurbituril and one guest as the linear thread, examples have also been prepared with multiple cucurbiturils bound to the thread and even combinations of different cucurbiturils.49-51,110 Additionally, the shuttling of cucurbituril along the molecule thread can be controlled by pH, temperature and solvent effects.54,55,111 118

C.2.4 Steady State UV-Vis Absorption and Fluorescence Measurements

The absorption of DTV2+ and the corresponding complex DTV2+@CB[7] in aqueous solution showed maxima at 250 nm and 335 nm, (Figure C - 12). Aqueous solutions of

2+ DTV display low fluorescence quantum yield (λex = 350 nm,  = 0.02).

0.4 DTV2+ DTV2+@CB[7]

335 250

0.2 Absorbance

0.0 300 400 500 Wavelength [nm]

Figure C - 12 Absorption spectra of DTV2+ (black solid line) and the corresponding complex DTV2+@CB[7] (red dashed line)

The addition of CB[7] leads to extraordinary one-order of magnitude emission enhancement (ex=350 nm) and the maximum increase was reached when two equivalents of CB[7] were present, further suggesting the formation of a DTV2+@2CB[7] complex in water, (Figure C - 13). The titration displayed in Figure C - 15, shows the integrated emission intensity vs. equivalents of CB[7] in presence and absence of 0.05 M

NaCl. In the presence of NaCl (black line, Figure C - 15) maximum emission enhancement was observed at a g:h ratio of 1:3. Overall the presence of NaCl did not, as earlier expected, lead to a dramatic influence of the complexation or the fluorescence 119 properties, but the results still suggest that controlling the content of metal ions is important as implied by the work of Inoue, Kaifer and others61,79,112

450  = 470 nm em 400

350 M 300 CB[7]

250

200 Emissiona.u. 150

100

 = 350 nm ex 50  = 505 nm em 2+ 0 DTV 400 450 500 550 600 650 Wavelength [nm]

Figure C - 13 Emission spectra of DTV2+ (5M) upon addition of CB[7] (concentration range: 1-15 M). λex = 350nm.

The Job‘s plot, based on a continuous variation method, further suggests that the binding stoichiometry is simply 1:2 g:h, Figure C - 14. A Job plot is used to determine the stoichiometry of a binding event. In this method, the total molar concentration of host and guest are held constant, but their mole fractions are varied. A measurable parameter that is proportional to complex formation (such as absorption or emission signal) is plotted against the mole fractions of these two components. The maximum on the plot corresponds to the stoichiometry of the two species. 120

70000 in Water in 0.05 M NaCl

60000

50000

40000

30000

20000

10000

IntegratedFluoresence a.u. Intensity 0.33

0 0.0 0.2 0.4 0.6 0.8 1.0

n 2+/(n 2++n ) DTV DTV CB[7]

Figure C - 14 Job‘s plot for complexation of CB[7] and DTV2+. The total concentration of [CB[7]]+[DTV2+] was kept at 0.6 mM.

2+ The absorption (λmax= 335nm) and emission (λem=505 nm) spectra of DTV @2CB[7] in water exhibited a very large Stokes shift (St ~ 10,000 cm-1) and no fine structure was present in the room temperature fluorescence spectrum. This is an interesting property as large Stokes shift dyes, a large separation between absorption and emission, find applications in multiplexed FRET (Förster resonance energy transfer), and are especially important in biological applications such as fluorescence microscopy and imaging.113-115

121

DTV2+ with CB[7] 45000

40000

35000

30000

25000

20000

15000

10000 1:1 1:2 in 0.05M NaCl 5000

IntegratedFluorescence a.u. Intensity in water 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 CB[7]/[DTV2+]

Figure C - 15 Fluorescence emission titration curve of 5 µM DTV2+ with CB[7]in water (circles, blue line) and in presence of 0.05 M NaCl (squares, dashed black line). λex = 350 nm

DTV2+@CB[7] Absorption DTV2+@CB[7] Emission 1.2

1.0 135 nm

0.8

0.6

0.4 Normalizeda.u. Intensity 0.2  =335 nm  =470 nm max em

0.0 250 300 350 400 450 500 550 600 650 Wavelength [nm]

Figure C - 16 Normalized absorption and emission spectra of DTV2+@2CB[7] in aqueous solution. 122

C.2.4.2 Quantum yields and Lifetime measurements

Quantum yields were measured in presence and absence of NaCl for DTV2+ as well as the complexes DTV2+@CB[7], DTV2+@2CB[7] and DTV2+@3CB[7]. The formation of the

CB[7] complexes resulted in a significant fluorescence quantum yield and lifetime enhancement. The quantum yield of DTV2+ was relatively low at = 0.02 and the corresponding complexes with CB[7] were1:1= 0.12 and 1:2= 0.29. The largest Φ increase was observed for DTV2+@2CB[7] in water.

Table C - 1 Selected photophysical properties of 5µM aqueous solutions of DTV2+ and its CB[7]complexes

(a) (b) em   Compound DTV2+:CB[7] (nm) ± 0.01 (ns) DTV2+ 505 0.02 <0.02 2+

DTV @CB[7] 1:1 482 0.12 0.4 2+

DTV @2CB[7] 1:2 470 0.29 0.7 2+

DTV @3CB[7] 1:3 470 0.28 0.7

(a) ex350 nm (b) Tryptophan was used as the reference with  0.14 in water116,117

The fluorescence lifetime of 2 µM aqueous solutions of DTV2+ was < 20 ps, which is lower than the time resolution of the instrument. The DTV2+ sample also showed bleaching during the measurement, which was ascribed to sample degradation. This result is consistent with the observation that aqueous solutions of samples left in the laboratory environment for prolonged periods of time (weeks) were no longer emissive. It changes dramatically upon addition of CB[7], where the DTV2+ displayed lifetime patterns for complex stoichiometries of 1:1 and 1:2 (guest : host), (Table C - 1). Lifetimes were determined to be 1:1 = 0.4 ns and 1:2 = 0.7 ns respectively. The increase in fluorescence 123 lifetime is quantitatively consistent with the increase in emission intensity. The time resolved experiments were repeated in the presence of a large excess of NaCl (0.05 M) and the results were, within the experimental error, identical to those observed in water.

Fluorescence lifetime measurements in water

DTV2+ 1.0 2+ DTV @CB[7] 1:1 DTV2+@CB[7] 1:2 Al Reference 0.8

0.6

Intesity a.u. Intesity 0.4

0.2

0.0 0 1 2 3 4 5 6 7 8 Time [ns]

Figure C - 17 Fluorescence lifetime measurements

To further explore the possible influence of conformation restrictions of DTV2+ on the emission properties, DTV2+ was combined with poly(methyl methacrylate) (PMMA) polymer, and spin coated into films. The emission spectra of DTV2+ in PMMA matrix showed a blue-shifted, structureless emission band (em = 465 nm) similar to that observed upon encapsulation into CB[7].118 124

140 DTV2+ in PMMA  = 465 nm em 120

100

80

60

Emissiona.u. 40

20  = 350 nm ex 0

400 450 500 550 600 Wavelength [nm]

Figure C - 18 Emission spectra of DTV2+ (8.0 wt %) in PMMA polymer matrix

C.2.4.3 Binding Constant and the Influence of NaCl

Complexation constants were determined by UV-Vis spectroscopic measurements as previously described, (chapter B). The absorption change (Aobs, Eq. 3) at the UV absorption band (max=350 nm) of viologen was fitted against the concentration of CB[7].

CB[7] was added until the stable complex form was reached, (1:2 guest:host model,

4 -1 4 -1 Figure C - 19). The complexation constants were K1W = 1.2×10 M , K2W = 1.0×10 M

4 -1 4 -1 in water, and K1NaCl = 1.1×10 M , K2NaCl = 0.8×10 M in 0.05 M NaCl aqueous solution, (Figure C - 20 and Figure C - 21). The calculation of the binding constant therefore takes into account the concentrations of the free guest and the free host, as well as the concentration of the host-guest complex. For a 1:1 complex the equilibrium Eq.1. is:

125

The binding stoichiometry between the host and guest varies, such as indicated in the

Job‘s plot, a second host is able to bind to the guest giving the equilibrium between the

1:1 complex that was already formed, and the additional host host as in Eq. 2.:

UV-Vis changes upon titration of DTV2+ with CB[7]:

Increased concentrations of metal ions in solutions generally lead to decreased complexation constants without changing the stoichiometry between guest and host.61-63

Results from Inoue and coworkers suggest that coordination of sodium or other alkali cations also depend on the guest dimensions.85 In case of longer guests, the resulting complexes were monocationic [CB[6] ∙Na]+ with increased complexation constants.

Naturally, the highest complexation affinity was reached without salts in pure water.

The influence of NaCl was studied for DTV2+, DTV2+ @CB[7] and DTV2+ @2CB[7]. In each case the fluorescence already dropped with the introduction of NaCl at very small concentrations, interestingly for DTV2+ without the host present. The decrease in fluorescence reached a plateau, where no further changes were observed even by further increasing the NaCl concentration. After this point no difference could be measured for different host stoichiometries. In case of the 1:2 g:h complex, the initial decrease in fluorescence was followed by a minor spike in the fluorescence intensity between 25 and

250 µM of NaCl. 126

1.0 335 nm 0.9 250 nm 0.8

0.7

0.6

0.5

0.4 Absorbancea.u. 0.3

0.2

0.1

0.0 250 300 350 400 450 500 Wavelength [nm]

Figure C - 19 Changes in the UV-vis spectrum during the complexation of DTV2+ (5 µM) with CB[7]

Figure C - 20 Fitted (red solid line, 1:2 model) experimental data (squares) of absorbance 2+ change (Aobs) of DTV at 335 nm against the concentration of CB[7] in water

127

0.060

0.055

0.050 ]

0.045

obs A

 0.040

0.035

0.030

0.025

0.020

0.015 Absorbancechange [ 0.010

0.005

0.000 0 20 40 60 80 100 120 140 160 CB[7] [M]

Figure C - 21 Fitted (red solid line, 1:2 model) experimental data (squares) of absorbance 2+ change (Aobs) of DTV at 335 nm against the concentration of CB[7] in 0.05 M NaCl

DTV2+ : CB[7] 1:1 70000 1300 DTV2+ : CB[7] 1:2

60000 1200

50000 1100 40000

1000 30000

900 20000

800 IntergratedFluorescence Intensity 10000 IntegratedFluorescence Intensity

700 0 0 10 20 30 40 50 60 0 200 400 600 NaCl [M] NaCl [M]

Figure C - 22 Integrated Fluorescence titration of a) DTV2+ and b) DTV2+@CB[7] 1:1 (blue line) and 1:2 (black line) g:h complex with NaCl

In a second set of experiments, a solution containing an excess of CB[7] 1 mM to DTV2+

3 µM content was titrated with NaCl and the corresponding emission spectra were recorded. The results show no influence of NaCl on emission intensity given that CB[7] is present in excess. 128

2+ 1mM CB[7] + 3M DTV 200 No NaCl 0.001 M NaCl 0.004 M NaCl 150 0.02 M NaCl 0.05 M NaCl 0.1 M NaCl

100 Emissiona.u.

50

0 400 450 500 550 600 650 Wavelength [nm]

Figure C - 23 Emission spectra for fluorescence titration of DTV2+@CB[7] complex (3 µM DTV2+ and 1mM CB[7] excess) with NaCl

The experimental results in accordance with previous findings by Kaifer61-63 and

Inoue85,119 suggest that DTV2+ forms a very stable complex with CB[7] at a ratio of 1:2 g:h. Metal cations easily bind in the central region of the complex between the two CB[7] units, a region possessing extended negative charge density. The presence of metalcations will also reduce the repulsion between the CB[7]s because of their positive charge and therefore further stabilize the complex. The terminal CB[7] portals are most probably sealed by Na+ ions, (Error! Reference source not found.). The increased positive charge in the central region also reduces the repulsion between the two carbonyl portals, further stabilizing the complex into a more rigid and planar structure within a metal ion.

Such an arrangement would indeed restrict the non-radiative torsional motion of the pyridine rings. The initial decrease in the fluorescence intensity with salt can be explained due to the dissociation of a 1:1 complex. 129

2+ C.2.4.6 Properties of DTV @CB[7] On Nanostructured ZrO2 Films

The nanoparticle films were prepared from ZrO2. Since ZrO2 has a much wider bandgap

(Eb = 5 eV) and morphology similar to TiO2, it is often used to study excited states on nanostructured films. Binding to metal oxide nanostructured thin films was done by immersing the films cast on glass substrates in 1 mM DTV2+@2CB[7] aqueous solutions.

Control experiments showed that free DTV2+ does not bind (FT-IR). The films were prepared from semiconducting TiO2 (Eb = 3.2 eV) and from ZrO2, an insulator (Eb = 5 eV) with a morphology similar to TiO2. The functionalized films were dried, and absorption and fluorescence spectra were collected. The absorption spectra of bound

DTV2+@2CB[7] (not shown) were obscured by the absorption of the semiconductor, i.e. about < 400 nm for TiO2 (absorption edge), and < 320 nm for ZrO2. A broad, structureless emission centered at about 480 nm was observed on insulating ZrO2, whereas selective excitation of the complex bound on TiO2 was not possible. Although the binding solutions were prepared from the 1:2 complex, it is not possible to tell whether the 1:2 stoichiometry is retained upon binding. However, the blue shift in the emission (480 nm), and control experiments indicating that free DTV2+ does not bind or physisorbed to metal oxide surfaces, suggests that DTV2+ binds as a complex in CB[7].

2+ The binding of DTV @2CB[7] to ZrO2 films were monitored as a function of geometric surface coverage. For the surface coverage measurements ZrO2 films were immersed in the binding solution containing 1mM DTV2+ and 2mM CB[7], 1:2 ratio. The

2+ DTV @2CB[7]/ZrO2 films were immersed in 2 mL solutions of NaOH (pH 11) for2 days. The immersion of synthesized films in basic solution leads to desorption of the chromophore or in this case the complex from the nanostructured surface. The surface 130 coverage of the films was estimated using the extinction coefficient of DTV2+, 5.36 M−1 cm−1. Surface coverage for DTV2+@CB[7] is 0.014mM/cm2 and was determined using

2+ the absorption intensity max = 335 nm of DTV . At this point it is only possible to estimate the surface coverage for the complex DTV2+@2CB[7], but not for the case of physisorbed CB[7] without a guest.

8 DTV2+@CB[7] on ZrO  = 480 nm 2 em 7

6

5

4

3 Emissiona.u. 2

1  = 350 nm ex 0

400 450 500 550 600 Wavelength [nm]

2+ Figure C - 24 Emission spectrum of DTV @2CB[7] on ZrO2 nanoparticle thin film; exc = 350 nm.

C.2.5 FT-IR-ATR Spectroscopy of Solid Complex Samples and Nanostructured

Films

FT-IR-ATR characterizes the solid samples of DTV2+, CB[7], and their complexes. The spectra showed aromatic C=C and C=N stretching bands of the pyridine and phenyl rings in the 1600-1430 cm-1 range. The FT-IR-ATR spectra of encapsulated guests in CB[7] were similar to the FT-IR-ATR spectrum of CB[7], with a strong band assigned to the carbonyl vas(C=O) at 1725cm-1. In Chapter B we have already discussed that the 131 encapsulation leads to a strong reduction of the IR intensity for signals that were previously associated with the guest molecules. The host inclusion disrupts the guest- solvent hydrogen bonding interaction through the screening effect of the host. This disruption leads to the dramatic effects on the stability of the complex and the photophysics and spectroscopy. The screening effect of the cucurbituril interaction can explain the disappearance of the viologen‘s IR signal upon encapsulation into CB[7].

0.6 CB[7] (solid) 808 DTV2+@CB[7] (solid) 2+ 0.5 DTV (solid) 1373

0.4 1717 1477

0.3

0.2 1635 Absorbancea.u. 3387 3005

0.1

0.0 4000 3500 3000 2500 2000 1500 1000 Wavenumbers, cm-1 .

Figure C - 25 FT-IR-ATR of DTV2+ (blue solid), CB[7] (black dashed) and DTV2+@CB[7] (red solid)

132

0.20 DTV2+@CB[7] on Zr0 2 0.18 CB[7] on Zr0 2 Zr0 (blank) 0.16 2

0.14

0.12 1728 0.10

0.08 1465

Absorbancea.u. 1323 0.06 1377 0.04

0.02

0.00 2000 1800 1600 1400 1200 1000 800 Wavenumbers, cm-1

Figure C - 26 FT-IR-ATR of adsorbed complex DTV2+@CB[7] (black solid), CB[7] (blue dashed) on ZrO2 films and ZrO2 blank (red dotted)

C.2.6 Electrochemistry

The redox behavior of DTV2+in presence and absence of CB[7] was studied through cyclic voltammetry (CV).33,120 DTV2+ showed the same reversibility as methyl viologen while having a more positive reduction potential, which indicates that DTV2+ is the better electron acceptor, (Scheme C - 3).121 The one-electron reduction process of viologens is also fast, reversible, and the radical cation intensely colored.58,63,65,67 The effect of CB[7] on the cyclic voltammetric behavior of DTV2+ in Figure C - 27 shows a pronounced anodic shift (10mV) in the position of the corresponding E1/2 value for the reduction of the viologen residue and the expected decrease of current level caused by the guest-host association. The weakened stability of the inclusion complex upon reduction of the viologen derivative was explained by Kim and coworkers as the result of a decreased affinity of the viologen radical cation for CB[7] as compared to the di-cations.58,63,122 133

DTV2+@2CB[7] is a strong photooxidant. The singlet excited state reduction potential

2+ ●+ E(DTV* /DTV ) was estimated to be +2.93 eV vs. NHE, from the singlet energy E00 =

3.09 eV, and the ground state reduction potential E(DTV2+@2CB /DTV●+@2CB) = -

0.156 V vs. NHE. These results are consistent with Kohler‘s estimate of +3.65 eV vs.

NHE for the singlet excited state reduction potential for methyl viologen.71

. d i -c a t io n D TV 2 + ra di c a l - c at io n DT V +

+ e- N N N N - e-

ye ll o w b r ow n / re d i n wa te r m a x= 33 0 nm - 0. 3 V v s Ag / Ag C l in w at e r m a x= 40 0 nm

Scheme C - 3 One-electron redox processes of DTV2+ and associated color changes

Table C - 2 Voltammetric parameters for free and encapsulated DTV2+

0 a Compound E 1/2(V)

DTV2+ -0.35

DTV2+@2CB[7] -0.36

a) Half-wave potentials for the first reduction process vs. saturated Ag/AgCl (3.0 M

NaCl) reference electrode in aqueous 0.1 M phosphate buffer (pH 7.3).

134

0.00012 DTV2+ 0.00010 DTV2+@CB[7] 0.00008

0.00006

0.00004

0.00002

0.00000 E11/2 =-355

Current(A) DTV -0.00002 E11/2 =-365 DTV@CB[7] -0.00004

-0.00006

-0.00008

-100 -200 -300 -400 -500 Potential (mV)

Figure C - 27 Cyclic voltammograms in 0.1 M phosphate buffer (pH 7.3) of 0.05 mM solutions of DTV2+ in the absence (black solid line) and in the presence (red dashed line) of CB[7]

C.2.6.1 Electrochromic Properties of DTV2+

Table C - 2 and Figure C - 30 show the absorption and emission spectra of the electrochemically generated radical cations DTV●+ and of DTV●+@2CB[7]. The complexed radical cation DTV●+@2CB[7] showed an absorption spectrum that was

●+ essentially identical to that of free DTV , and was significantly blue shifted (max = 400 nm) compared to the radical cation of methyl viologen, free and complexed in CB[7]

2+ (max = 600 nm). This property makes DTV a good candidate for electrochromic windows. DTV●+ is not emissive as indicated by the fluorescence spectra of the one- electron reduced complex, DTV●+@CB[7], (Figure C - 31), and the free DTV●+, (Figure

C - 29). The oxidation of DTV●+ and DTV●+@CB[7] leads to a reestablishment of the emission properties. In summary, it is not only possible to switch between two colored states, but also between fluorescent and non-fluorescent state at the same time. This property is potentially attractive for display applications. 135

1.5

1.0

 =335 nm  =400 nm max max

+ DTV2+ DTV.

0.5 Absorbancea.u.

0.0 300 400 500 600 Wavelength [nm]

Figure C - 28 Absorption and spectra of DTV2+ in water before (black solid line) and after (red dashed line) one-electron reduction to radical cation DTV●+ by application of – 1.0V.

25

 =527 nm em 20 2+ DTV 15

10 Intensity a.u. Intensity

5 + DTV.

0 400 450 500 550 600 650 Wavelength [nm]

Figure C - 29 Emission spectra of DTV2+ in water before (black solid line) and after (red dashed line) one-electron reduction to radical cation DTV●+@2CB[7] by application of – 1.0V. exc= 350 nm 136

1.5

1.0  =330 nm  =400 nm max max

2+ 0.5

Absorbancea.u. DTV + DTV

0.0 300 400 500 600 Wavelength [nm]

Figure C - 30 Absorption spectra of DTV2+@2CB[7] in water before (black solid line) and after (red dashed line) one-electron reduction to radical cation DTV●+@2CB[7] by application of –1.0V.

 =475 nm 600 ex

2+ 500 DTV

400

300 Intensity a.u. Intensity 200

100

+ DTV 0 400 450 500 550 600 650 Wavelength [nm]

Figure C - 31 Emission spectra of DTV2+@2CB[7] in water before (black solid line) and after (red dashed line) one-electron reduction to radical cation DTV●+@2CB[7] by application of –1.0V. exc = 350 nm 137

C.2.7 DFT and CIS Calculations

The S1 state and the following intersystem crossing to the T1 state are associated with significant changes of the dihedral angles between the four aromatic rings of the DTV2+, as calculated by DFT and CIS. A PMMA matrix and encapsulation in a CB[7] host definitely hinder and slow down these large amplitude relaxation processes. The 34.4° dihedral angle of the ground state equilibrium conformation of DTV2+ core is close to the

35° dihedral angle of the ground state isoelectronic biphenyl. The twofold symmetry of the system is spontaneously broken in the relaxed state S1, showing the intramolecular

CT nature of the transition. One of the p-tolyl moieties as well as the viologen core are more planar and form a strongly coupled sequence of three rings. The fourth p-tolyl moiety is twisted at an angle of 65.5° with respect to the rest of the molecule. The dihedral angles change from S0 to S1, from 45.4, 34.4 and 45.4 in S0 to 16.1, 19.8 and 67.7 in S1.Figure C - 32illustrates the quinonoid structure of the planarized section of the ring sequence Figure C - 33 lists the complete set of calculated bond length, notable though is the length of the bond ―d‖ which connects the nearly coplanar p-moiety to the viologen. This ―d‖ bond decreases from 1.45 Å to 1.34 Å which is much more double bond like.

Table C - 3 Calculated C-C and C-N bond lengths of the conjugated core of DTV2+ in the S0, S1 and T1 electronic states. Refer Figure C - 33 for bond assignment. All values are in Å.

a b c d e f g h g’ f’ e’ d’ c’ b’ a’

S0 1.41 1.39 1.40 1.45 1.36 1.38 1.41 1.48 1.41 1.38 1.36 1.45 1.40 1.39 1.41

S1 1.42 1.36 1.44 1.34 1.42 1.34 1.43 1.45 1.41 1.37 1.34 1.46 1.39 1.38 1.40

T1 1.42 1.38 1.42 1.42 1.39 1.36 1.44 1.42 1.44 1.36 1.39 1.42 1.41 1.38 1.42 138

The non-emissive T1 state results in another large geometry change. The D2 symmetry of the molecule is restored since both p-tolyl groups rotate out of plane around a completely planar quinonoidal viologen core. These geometry changes also result in large intramolecular reorganization energies (S0-S1) = 0.74 eV and (S1-T1) = 0.40 eV. The large reorganization energy S0-S1 also explains the lack of vibronic structure in the absorption and emission spectra of DTV2+.

S45.4 S45.4 S34.4 SS0

S= 0.74 eV

S16.1 119.8 67.7 SS1

S= 0.40 eV

S34.2 0.7 34.2 ST1

2+ Figure C - 32 Optimized structures of the S0, S1 and T1 states of DTV indicating the inter-ring dihedral angles, as well as the (S0-S1) and (S1-T1) intramolecular reorganization energies. The dashed box indicates the almost planar geometry of three consecutive rings in the S1 state and the two bipyridyl central rings in the T1 state.

139

2+ Figure C - 33 Illustration of bond lengths in the S1 state of DTV . The three semi planar rings exhibit alternating bond lengths. The last p-tolyl ring‘s bond lengths are all identical. Bonds with length changing by more than 0.025 Å are shown in color (contraction in red and lengthening in green), and bonds in black are all identical in length. For bond lengths see Table C - 2.

Calculations predict a 0.74 eV red shift of the S1-S0 transition upon relaxation from the

Franck-Condon state to the S1 geometry, which is consistent with the emission spectra changes of DTV2+ in solution and in a confining media (PMMA or host). The blue shift could be explained by the different polarity in environment and dielectric constant experienced by DTV2+ in CB and in water, but the shift is also predicted by the calculations. This is consistent with the observations for excited states that are not stabilized by reorientation of the solvent molecules when going from a fluid to a confining medium.

A 4-fold reduction of the oscillator strength of the S1-S0 transition from 0.79 in the geometry to 0.20 in the relaxed geometry, as predicted by the calculations, cannot fully account for the essentially complete quenching of the fluorescence of DTV2+ in water and other non-viscous media. Without further non-radiative decay, an oscillator strength of

0.2 in the fully relaxed geometry would lead to a substantial emission. The observed low 140 emission quantum yield and the lifetime of the excited state (<20 ps) can be explained by the rapid intersystem crossing on a timescale similar to the S1 relaxation. The lengthening of the S1 lifetime and increased fluorescence quantum yield as observed in the experiments are results of a slowed down intersystem crossing caused by the hindrance or retardation of the rotation of the four rings of the DTV2+ through confinement in a host or polymer matrix. The observed emission quenching can additionally be explained by the chloride counter-ion which is more tightly associated to DTV2+ in aqueous solution than in the complex. In summary, the excited state rotational dynamics of the DTV2+ seems to play a key role in the enhancement and blue shift of the emission.

C.3 Experimental Section

C.3.1 General

1H and (499.90 MHz) 13C NMR (124.98 MHz) spectra were recorded on a Varian

1 13 INOVA 500 spectrometer in D2O at room temperature. The H and C NMR chemical shifts () are reported in ppm and are referenced to the central line of the solvent (4.82 ppm for D2O). Coupling constants (J) are reported in Hz. High resolution mass spectra

(ESI) were recorded using a Bruker Daltonics FT MS. FT-IR-ATR spectra were acquired on a Thermo Electron Nicolet 6700 FT-IR.UV-VIS absorbance spectra were collected using an Ocean Optics USB4000+ Spectrometer in combination with a PX-2 Pulsed

Xenon Light Source (150 ms for integration time and 20 scans to average). All chemicals used in this experiment were analytical grades and used as received. Nanocrystalline zirconium dioxide was prepared via sol–gel hydrolysis and condensation of isopropanol solutions of zirconium(IV)propoxide and cast on glass substrate, microscopy cover glass 141 slides. The chemicals and solvents, isopropanol, DIUF water, γ-butyrolactone, zirconium(IV)propoxide, acetone, methylviologen, 4,4‘-bipyridine, 1-chloro-2,4- dinitrobenzene, p-toluidine and methyl iodomethane were purchased from Aldrich or

Sigma-Aldrich and used without further purification. The following syntheses were performed under nitrogen atmosphere in anhydrous solvents.

C.3.2 Synthesis and Characterization

C.3.2.1 Cucurbit[7]uril synthesis

CB[7] was prepared and characterized using the method developed by Nau and coworkers.31,32 The separation of CB[n] homologues was adapted from the procedure described by Kim, Day and coworkers.92,96 Glycoluril 15 (10 g, 70 mmol) and formaldehyde (11 mL, excess) were stirred in 15 mL ice cold conc. HCl, (1). This mixture was stirred until it set as a gel, (2), and was then allowed to stand for approximately one hour, then heated to 75 °C for approximately 72 hours. Upon heating the gel quickly melted to give an orange solution, (3). At the end of the reaction, the cloudy solution was allowed to cool to room temperature, and then further cooled in an ice bath. Upon cooling a precipitate formed, (4). This mixture was then filtered, resulting in an orange, viscous filtrate and a pasty white solid. The filtrate was allowed to stand for approximately one hour at room temperature, and a grey-white, semi-crystalline, precipitate was observed, which was again removed by filtration. The filtrate was then placed in a rotary evaporator, and its volume was reduced to between one half and one quarter of its original volume. Then 200 mL of water was added to the solution, more white precipitate formed and it was subsequently filtered. The filtrate was poured into 1 L 142 of acetone, (5). After the formation of a heavy white precipitate the suspension was left to settle for 30 min and then the acetone layer decanted and another 1.5 L mixture of acetone:water 4:1 was added, (6), decanted and finally filtered. The filtered white precipitate was suspended in 200 mL of acetone:water 1:1, stirred for two hours and filtered, (7). The precipitate was washed with 100 mL water and can be discarded as it is was mostly CB[6]. To the filtrate acetone (0.8 L) was added and a cloudy white precipitate formed, (8). The cloudy suspension was decanted and filtered to give a white solid, which was composed mainly of CB[7] and CB[5]. The white solid was dissolved in

100 mL water and poured into 100 mL of methanol, (9 and 10). A white solid formed, which was collected by filtration. This process adding methanol to the filtrate, and subsequent filtration, was normally repeated several times, in order to obtain as many fractions of the product as possible. Further purity of CB[7] was achieved by recrystallization by vapor diffusion in THF/ acetone mixture. In this case the white precipitate was dissolved in a minimum amount of sulfuric acid acidified water and placed into the diffusion container for 5-7 days, (11). The formed white crystals were collected by filtration and subsequently washed with methanol and acetone several times,

(12). The product was dried in vacuo to yield CB[7] as a white powder (2.2 g, 1.75 mmol)

1 H NMR (D2O, 25ºC):  = 4.25 (d, J = 15.5, 14 H, CH2), 5.56 (s, 14 H, CH), 5.81 ppm (d,

J = 15.5, 14 H, CH2), FT-IR-ATR: 3434 (N-H), 2930 (C-H), 1717 (C=O), 1643 (C=C),

1477 (C-N), 1373, 1319, 1214, 1192, 968, 799 cm−1, were consistent with those reported in literature, and were compared with data obtained from a commercial CB[7]sample. 143

Figure C - 34 Synthesis and purification overview for CB[7]

C.3.2.2 Synthesis of DTV2+

1,1'-Bis(2,4-dinitrophenyl)-(4,4'-bipyridine)-1,1'-diium dichloride (12).88,123 4,4‘-

Bipyridine (2, 0.25 g, 1.6 mmol) and 2,4-dinitrochlorobenzene (1.30 g, 6.4 mmol) were dissolved in acetone (20 mL). The solution was refluxed for 24 h, during which time a pale grey precipitate formed. The precipitate was collected by filtration and triturated twice with n-pentane and acetone. The product was dried in vacuo to yield the Zincke salt

1 12as a yellow-grey powder (0.4 g, 44% yield). H NMR (D2O, 25ºC):  = 9.55 (d, J =

6.46, 4H, ArH), 9.49 (d, J = 2.0, 2 H, ArH), 9.03 (d, J = 2.0, 2 H, ArH), 9.00 (d, J = 6.47,

13 4 H, ArH), 8.38 ppm (d, J = 2.0 Hz, 2 H, ArH); C NMR (D2O): δ = 152.60, 149.82,

146.82, 142.77, 138.21, 131.12, 130.72, 127.33, 122.82 ppm.

144

1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+) (14). Compound

12(0.50 g, 9.0 mmol) was dissolved with p-toluidine (5, 0.24 g, 22.5 mmol) in ethanol

(20 mL) under nitrogen atmosphere. The resulting solution was refluxed for 24 h and the formed precipitate was filtered and discarded. The filtrate was evaporated in vacuo and the pale yellow crude product was triturated with acetone (250 mL), filtered, triturated again with acetone (2 x 500 mL), and dried in vacuo to yield 1 as a very pale yellow

1 powder (0.33 g, 90% yield). H NMR (D2O, 25ºC):  = 9.44 (d, J = 6.0, 2 H, ArH), 8.83

(d, J = 6.0, 2 H, ArH), 7.77 (d, J = 8.0, 2 H, ArH), 7.66 (d, J = 8.0, 2 H, ArH), 2.57 (s, 3

13 H, CH3); C NMR (D2O): δ =150.20, 145.25, 143.15, 139.15, 131.01, 126.93, 123.69,

20.31 ppm. FT-IR-ATR: 3387 (N-H), 3005 (C-H), 1635 (aromatic C=C), 1472 (C-N),

−1 + 808 cm ; MALDI-MS: m/z (%): 338.1787 (90) (M ), calcd. for C24H22Cl2N2: 338.1783.

2+ Elem. Anal. Calcd. for DTV (C24H22Cl2N2∙3H2O): C 62.21 %, H 6.08 %, N 6.05 %.

Found: C 62.41 %, H 5.94 %, N 6.09 %.

C.3.2.3 1H NMR Titration-Inclusion of Viologen DTV2+ into CB[7]61-63

2+ Stock solutions of DTV (1 mM) and CB[7] (2 mM) were prepared in D2O. The stock solutions were combined in an NMR tube to obtain the desired guest:host ratio. The same experiment was repeated in the presence of 50 mM NaCl. 145

C.3.2.3.1 Complexation constants

2+ Complexation constants (Kc) of DTV in CB[7] were measured following published methods62,63,124,125 by collecting UV-Vis absorption spectra of DTV2+ (2 ml of 5 µM solution) upon addition of 5 μL aliquots of a 2 mM aqueous solution of CB[7]. The absorbance intensity at λmax = 335 nm was plotted against the concentration of CB[7] and

4 -1 4 fitted to obtain Kcw. The complexation constants were K1W = 1.2×10 M , K2W = 1.0×10

-1 4 -1 4 -1 M in water, and K1NaCl = 1.1×10 M , K2NaCl = 0.8×10 M in 0.05 M NaCl aqueous solution were obtained by fitting the experimental data according to Eq. 3 and a 1:2 g:h complexation model.

C.3.3 Synthesis of nanostructured ZrO2

Sol-gel synthesis of ZrO2 nanoparticles is very similar to the TiO2 nanoparticles synthesis in chapter B.126-131 A three necked round bottom flask was set up with a thermometer, dropping funnel and Dean-Stark apparatus. To theround bottom flask containing 100 mL solution of water containing 0.69 ml conc. HNO3 (68 – 70 %) a 20 mL solution of 70 % zirconium(IV) isopropoxide and 80 mL of isopropanol was added drop wise through a dropping funnel under nitrogen atmosphere and stirred vigorously. The slow combination of the two solutions lead to a fast formation of white precipitate of the zirconia dioxide nanoparticles.After the addition step, the reaction mixture was brought to reflux and ca.

140 ml of isopropanol-water mixture were collected by distillation. Unnecessary equipment, Dean-Stark apparatus and the dropping funnel were removed from the set upand the condenser was moved to the middle neck of the flask. The reaction was allowed to reflux overnight. Following reflux, the water was evaporated from the flask 146 until a final volume of ca. 30 mL was reached and allowed to cool down to RT. The resulting white sol was sonicated for 2 min, transferred to a glass beaker belonging to the autoclave. The sol was allowed to stir in the titanium autoclave with heating at 200 ºC for

12 h, reaching atypical pressure of 17 – 18 bar. After the sintering procedure, the content of ZrO2 was determined by the ‗glass dies‘ method. To the sol gel poly(ethyleneglycol)

(avg. Mol. Wt. 2,000) was added (ca. 1.5 g PEG per 25 cm3 sol-gel) to improve the viscosity for better casting on a substrate. The PEG/ZrO2 mixture was stirred for at least 4 days to ensure homogeneity. The prepared white paste was directly used to be casted on glass substrate before sintering and removing the polymer at 450 °C for 30 min. The prepared ZrO2 films were directly used or stored in desiccator.

C.3.4 Preparation of DTV2+ in PMMA Polymer Matrix films

Polymer coatings were prepared by dissolving polymethylmethacrylate (PMMA) in formic acid (1.0 wt. %) and adding DTV2+ (0.08 wt. %) to the solution. Formic acid was a good spin coating solvent for both PMMA and DTV2+. An SCS|G3P-8 Spin Coat

System from Specialty Coating Systems Inc. was used to prepare thin films on a 1 cm x 1 cm cleaned glass slide at 2000 rpm. The resulting PMMA films contained about 8 wt. % of DTV2+.

C.3.5. Electrochemistry

Cyclic voltammograms were collected on a BAS CV27 potentiostat. The measurements were conducted on 0.5 mM DTV2+ aqueous solutions with 0.1 M phosphate buffer (pH

7.3), following literature methods.58,65 The solutions were deaerated before and during the measurements by bubbling nitrogen in a standard three-electrode arrangement with glassy 147 carbon (2 mm diameter), Pt gauze auxiliary electrode, and Ag/AgCl (3.0 M NaCl) as the reference electrode. The scan rate was 100 mV/sec at a sensitivity of 100 mA/V between

0.00 V and -0.05V. All values are reported vs. Ag/AgCl.

C.3.6 Spectroscopic Measurements

FT-IR-ATR spectra were acquired on a Thermo Electron Nicolet 6700 FT-IR spectrometer equipped with a Smart iTR ATR accessory with ZnSe HATR. UV-VIS absorbance spectra were collected at room temperature on a VARIAN Cary-500 spectrophotometer, or on an Ocean Optics USB4000+ Miniature Fiber Optic

Spectrometer in combination with a PX-2 Pulsed Xenon Light Source (150 ms for integration time and 20 scans to average). Steady-state fluorescence spectra were acquired and recorded at room temperature on a VARIAN Cary-Eclipse fluorescence spectrophotometer calibrated with a standard NIST tungsten-halogen lamp. The samples were excited at 350 nm, close to the absorption maximum. Fluorescence quantum yields

() were calculated using Equation (1),

 A I     ref  sample  sample  A I  ref  sample ref  Eq. (1)

A is the absorbance at ex = 350 nm and I is the integrated emission intensity. Tryptophan in aqueous solution was used as the reference (ref = 0.14 in water at pH 7, λex = 270 nm,

116,117 λem = 355 nm). No significant changes in the emission quantum yield or fluorescence lifetime measurements were observed in deaerated solutions (freeze-pump- thaw) or in the presence of air, hence all spectra reported were collected in the presence of air. The titration experiment was conducted by recording the integrated emission 148 spectra of DTV2+ (2 ml of a 5M aqueous solution) upon addition of 0.002 μmol aliquots of CB[7] (100µM in water).

Time-resolved fluorescence measurements were collected after excitation with 320 nm,

~30 fs pulses (~ 2 nJ) from a Ti:sapphire pumped non-collinear optical parametric amplifier. Picosecond time-resolved measurements were performed in a 10 mm cuvette.

The fluorescence was collected via a lens and focused onto a Hamamatsu H5783-01 photomultiplier. For some of the measurements a 500 nm band pass filter was employed

(FWHM 40 nm). The signal was recorded with a Becker & Hickl PCS 150 sampling card and a PC (50 s sampling time). Femtosecond time-resolved measurements were performed in a 1 mm cuvette. The fluorescence was detected in a Kerr-gated fluorescence spectrometer.132

C.3.7 DFT and CIS calculations

Geometry optimization and spectral calculations were done using the Spartan ‘10 software package (Wavefunction, Inc.). The DFT geometry optimization of the S0 ground state and the T1 triplet state of the dication was performed using the B3LYP pseudopotential and 31-G* basis set. Optimization of the S1 state, as well as the calculation of the S0-S1 transition energies in the equilibrium geometry of the S0 and S1 states were performed at the CIS level and utilized the same 31-G* basis set.

149

C.4 Conclusions

An unprecedented fluorescence enhancement of p-tolyl viologen, DTV2+, upon encapsulation in CB[7] in aqueous solutions or upon casting in PMMA films has been described. 1H NMR spectra revealed that DTV2+ in aqueous solution formed a g:h 1:2 complex with CB[7]. The complexation constant for 1:2 complexation model was K1W =

4 -1 4 -1 4 -1 4 -1 1.2×10 M , K2W = 1.0×10 M in water, and K1NaCl = 1.1×10 M , K2NaCl = 0.8×10 M in 0.05 M NaCl aqueous solution. The different g:h complexation constants show that the presence of NaCl needs to be considered when studying the host-guest chemistry of

CB[n]. Lower ratios of guest:host resulted in shuttling or complexation/decomplexation of the CB[7] unit between the DTV2+ rings as suggested by the results of 1H NMR titration.

Complexation of DTV2+@2CB[7] resulted in an enhanced fluorescence of about one order of magnitude compared to free DTV2+. Emission lifetimes increased from 0.02 to

0.29 and from <20 ps to 0.7 ns. Adding increased amounts of CB[7] to DTV2+, 0 to 2 equivalents, resulted in a blue shift of ~35nm. This blue shift as well as fluorescence emission of DTV2+ were also observed in PMMA films, while the presence of NaCl or oxygen did not significantly change the emissive behavior of the complex.

Since these enhancements have not been observed for alkyl or phenyl viologen complexes of CB[7], we concluded that the structure of the quaternizing moieties, in this case the p-tolyl, is important. Encapsulation of DTV2+ in CB[7] or casting in PMMA films leads to a the semi-planar geometry and a delocalized quinonoid geometry of the emissive S1 state. This has been confirmed by DFT and CIS calculations which predict that the S1 state has a strongly coupled sequence of three nearly coplanar rings (the p- 150 tolyl and the 4,4‘-bipyridium rings). These rings exhibit quinonoid structure with alternating bond lengths. The absorption and emission spectra of the radical cations, which is the electrochemical one-electron reduction of DTV2+ and of DTV2+@2CB[7] to

DTV●+ and DTV●+@2CB[7] respectively, suggests that DTV2+ and DTV2+@2CB[7] could be useful to develop electrochromic materials as well as fluorescence switches.

The results are suggesting that the methyl group on the p-tolyl moiety of a viologen in combination with conformational constraints are shares the reponsibility for the fluorescnt property. In the future, the emissive and electrochemical properties of similar viologen derivatives will be investigated, especially making comparisons between substituted phenyl viologens. Kohler and coworkers found that the choice of counter ions can have a great influence on emission properties of viologens. Chlorides are quenching the emission and hexafluorophosphate leads to improved solubilty. Due to these, and other effects, the influence of different counter ions will be studied.

151

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APPENDIX

162

Figure 1 Structure of 1-butyl-4-(pyridin-4-yl)pyridinium bromide (2)

1 Figure 2 H NMR of 1-butyl-4-(pyridin-4-yl)pyridinium bromide (2) in D2O

Figure 3 Structure of 1-butyl-1'-(2-(diethoxyphosphoryl)ethyl)-4,4'-bipyridine-1,1'-diium bromide (3) 163

Figure 41H NMR of 1-butyl-1'-(2-(diethoxyphosphoryl)ethyl)-4,4'-bipyridine-1,1'-diium (3) in D2O

Figure 5 Structure of 1-butyl-1'-(2-phosphonoethyl)-4,4'-bipyridine-1,1'-diium (4)

Figure 6 1H NMR of 1-butyl-1'-(2-phosphonoethyl)-4,4'-bipyridine-1,1'-diium (4) in D2O 164

Figure 7 Structure of 1-butyl-1'-(2-carboxyethyl)-4,4'-bipyridine-1,1'-diium bromide (5)

Figure 81 H NMR of 1-butyl-1'-(2-carboxyethyl)-4,4'-bipyridine-1,1'-diium bromide (5) in D2O

165

Figure 9 Structure of 1-(2,4-dinitrophenyl)-4-(pyridin-4-yl) pyridinium chloride (7)

Figure 10 1H NMR of 1-(2,4-dinitrophenyl)-4-(pyridin-4-yl) pyridinium chloride (7)

Figure 11 13C NMR of 1-(2,4-dinitrophenyl)-4-(pyridin-4-yl) pyridinium chloride (7)

166

Figure 12 Structure of 4-(pyridin-4-yl)-1-p-tolylpyridinium chloride (10)

Figure 131H NMR of 4-(pyridin-4-yl)-1-p-tolylpyridinium chloride (10)

Figure 1413C NMR of 4-(pyridin-4-yl)-1-p-tolylpyridinium chloride (10)

Figure 15 Structure of 1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+, 11) 167

Figure 161H NMR of 1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+, 11)

Figure 1713C NMR of 1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+, 11)

Figure 18 Structure of 1,1'-Bis(2,4-dinitrophenyl)-(4,4'-bipyridine)-1,1'-diium dichloride (12) 168

Figure 19 1H NMR of 1,1'-Bis(2,4-dinitrophenyl)-(4,4'-bipyridine)-1,1'-diium dichloride (12)

Figure 2013C NMR of 1,1'-Bis(2,4-dinitrophenyl)-(4,4'-bipyridine)-1,1'-diium dichloride (12)

Figure 21 Structure of 1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+, 14)

169

Figure 221H NMR of 1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+, 14)

Figure 2313C NMR of 1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+, 14) 170

Figure 24. MALDI-MS of 1-Methyl-1'-p-tolyl-4,4'-bipyridinium dichloride (MTV2+, 11) 171

Figure 25. MALDI-MS of 1,1'-Di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV2+, 14) 172

Marina Freitag Blücherstr. 7, Phone: (+49) 160 - 94913727 80634 München, Email: [email protected] Germany Website: www.galoppinigroup.com EDUCATION AND PROFESSIONAL EXPERIENCE

01/2007- 10/2011 DEPARTMENT OF CHEMISTRY, RUTGERS UNIVERSITY Newark, NJ, USA Ph.D. Candidate  “Cucurbituril[7] Host-Viologen Guest Complexes: Electrochromic and Photochemical Properties”  Advisor: Prof. Elena Galoppini 05/2009- 06/2009 DEPARTMENT OF CHEMISTRY, UNIVERSITY OF FERRARA Ferrara, Italy Visiting Student  Advisor: Prof. Carlo A. Bignozzi 07/2006-09/2006 FRAUNHOFER INSTITUTE FOR APPLIED POLYMER RESEARCH (IAP) Bachelor’s Thesis  “Photochromism of Selected Fulgimids in Solution and Polymer Blends”  Advisor: Dr. Stumpe 10/2005-06/2006 INSTITUTE OF CRYSTAL GROWTH Berlin, Germany Work Student  Studies on the preparation of nutrient solution for the crystallization of gallium monophosphate.

08/2005-10/2005 INSTITUTE OF CRYSTAL GROWTH Berlin, Germany Professional Practical Training  Preparation of nutrient solution for the crystallization of gallium monophosphate. 10/2003-09/2006 FREE UNIVERSITY OF BERLIN Berlin, Germany Bachelor of Science in Chemistry

CONFERENCES

11/10/2006-13/10/2006 INTERNATIONAL SYMPOSIUM ON PHOTOCHROMISM Vancouver, British Columbia, Canada , Poster session  “Photochromism and Photoinduced Dichroism in a Polymer Blend of a Fulgimide” 12/10/2008-17/10/2008 214TH MEETING OF THE ELECTROCHEMICAL SOCIETY (ECS) Honolulu, Hawaii, USA , Poster session  “Cucurbituril[7] Host - Viologen Guest Complexes: Inclusion and Chemisorption on TiO2” 11/07/2010-16/07/2011 XXIII IUPAC SYMPOSIUM ON PHOTOCHEMISTRY Ferrara, Italy , Organization and Logistics PUBLICATIONS

2010 CUCURBITURIL COMPLEXES OF VIOLOGENS BOUND TO TIO2 FILMS Article M. Freitag, E. Galoppini, Langmuir 2010 26 (11), 8262-8269 2010 MOLECULAR HOST-GUEST COMPLEXES: SHIELDING OF GUESTS ON SEMICONDUCTOR SURFACES Advance Article M. Freitag, E. Galoppini, Energy Environ. Sci., 2010 2011 VIOLOGEN BASED FLUOROPHORES Article Marina Freitag and Elena Galoppini, in preparation 2011 HEMICARSCEPLEX COMPLEXES BOUND TO NANOSTRUCTURED METAL OXIDE FILMS Article M. Porel, M. Freitag, A. Klimzcak, E. Galoppini and V. Rammamurthy in preparation