Porphyrin Assemblies for Use in Photoelectrochemical

DESIGN, SYNTHESIS, AND CHARACTERIZATION OF

PHOSPHORUS(V) PORPHYRIN ASSEMBLIES FOR USE

IN PHOTOELECTROCHEMICAL CELLS

A THESIS

SUBMITTED TO THE FACULTY OF THE

UNIVERSITY OF MINNESOTA

MICHAEL SHEA

IN PARTIAL FLFILLMENT OF THE REQUIREMENTS

FOR THE DEGREE OF

MASTER OF SCIENCE

ADVISER: PRASHANTH K. PODDUTOORI

AUGUST 2020

©Michael Shea 2020 ACKNOWLEDGEMENTS

I would like to thank my advisor, Dr. Poddutoori, for all the help that they have provided me. From him I have learned many things and been shown a great amount of kindness.

I would also like to thank the Board of Examiners for Thesis Defense, Dr. Poddutoori, Dr.

Yoshimura, and Dr. Kiprof, for taking the time and effort to review my thesis.

I would like to thank the other members of the group, Ben Boe, Brandon Bayard, Noah

Holzer, and Niloofar Zarabi, for all their help.

I would also like to thank Dr. Pluimer and Dr. Christiansen for their help, encouragement, and kindness throughout my time in the materials science program.

DEDICATION

I would like to dedicate this thesis to God, my parents (Anita Sandford-Shea and Kevin

Shea), my friends (Jacob Meyer, Cody Haala, Tom Brumbaugh, Adam Wentz, Sam

Betland, Jordan Toole, and Nathan Kovach), and my girlfriend (Jenna Swenson). I could not have done this without their continuous support.

ABSTRACT

A series of axially substituted phosphorus(V) porphyrin esters were synthesized. One of these compounds was converted to a carboxylic acid. This carboxylic acid derivative was evaluated as a photosensitizer. The structures of the synthesized compounds were confirmed by 1H and 31P NMR spectroscopy. The optical properties were evaluated using

UV-VIS absorption and fluorescence spectroscopy, as well as solid state diffuse reflectance spectroscopy. The redox potentials were assessed with cyclic voltammetry

(CV) and differential pulse voltammetry (DPV). The characterization data were used to construct energy level diagrams, laying the groundwork for a theoretical abstraction of how these molecules might function in a prototype photoelectrochemical cell.

iii

TABLE OF CONTENTS

ACKNOWLEDGEMENTS...... i

DEDICATION...... ii

ABSTRACT...... iii

TABLE OF CONTENTS...... iv

LIST OF TABLES...... vii

LIST OF FIGURES...... viii

ABBREVIATIONS...... x

1. INTRODUCTION...... 1

1.1 GREEN ENERGY ALTERNATIVES ARE NEEDED...... 1

1.2 H2 IS A CLEAN BURNING FUEL...... 1

1.3 PV AND PEC...... 4

1.3.1 WATER SPLITTING...... 6

1.3.2 N-TYPE SEMICONDUCTORS...... 7

1.3.3 P-TYPE SEMICONDUCTORS...... 9

1.4 PORPHYRINS...... 11

1.4.1 PORPHYRIN STRUCTURE...... 11

1.4.2 PORPHYRIN SYMMETRY...... 12

1.4.3 SPECTRAL FEATURES OF PORPHYRINS...... 13

1.5 PHOSPHORUS(V) PORPHYRINS...... 15

1.5.1 PHOTOSWITCHES...... 16

1.5.2 PHOTOSENSITIZERS/PHOTODYNAMIC THERAPY...... 18

1.6 ENERGY TRANSFER (DEXTER AND FÖRSTER)...... 18

1.6.1 ENERGY AND ELECTRON TRANSFER...... 18

1.6.2 ELECTRON TRANSFER (OXIDATIVE, REDUCTIVE, AND MARCUS

THEORY)...... 22

1.6.2.1 OXIDATIVE AND REDUCTIVE ELECTRON TRANSFER...... 22

1.6.2.2 MARCUS THEORY...... 24

2 RESULTS AND DISCUSSION...... 26

2.1 UV-VIS SPECTROSCOPY...... 26

2.2 FLUORESCENCE SPECTROSCOPY...... 29

2.3 ELECTROCHEMISTRY...... 31

2.4 SOLID-STATE REFLECTANCE SPECTROSCOPY...... 34

2.5 ENERGY LEVEL DIAGRAMS...... 35

2.6 CONCLUSION...... 41

3. FUTURE WORK...... 41

4. EXPERIMENTAL DETAILS...... 42

4.1 CHEMICALS...... 42

4.2 STRUCTURAL CHARACTERIZATION...... 42

4.3 OPTICAL STUDIES...... 42

4.4 ELECTROCHEMISTRY...... 43

4.5 SOFTWATE...... 44

5. SYNTHESIS...... 44

6. REFERENCES...... 51

7. APPENDICES...... 57

7.1 NMR...... 57

LIST OF TABLES

Table 1: UV-VIS absorption data of the phosphorus(V) porphyrins...... 29

Table 2: Fluorescence data of the phosphorus(V) porphyrins...... 31

Table 3: Summary of the oxidation and reduction DPV data and estimates of the HOMO-

LUMO gap (Energy (eV)) from those values...... 33

Table 4: Wavelength and corresponding energy of the overlapped Q band of the UV-VIS spectra and the fluorescence spectra...... 38

Table 5: Summary of the energy levels and transitions...... 38

vii

LIST OF FIGURES

Figure 1: General overview of the electrolysis of water...... 3

Figure 2: Basic photovoltaic (PV) Cell Design...... 5

Figure 3: Photoelectrochemical (PEC) cell...... 5

Figure 4: Majority and minority carriers in n-type semiconductors...... 8

Figure 5: Band diagram for an n-type semiconductor...... 8

Figure 6: Majority and minority carriers in p-type semiconductors...... 10

Figure 7: Band diagram for a p-type semiconductor...... 10

Figure 8: Variation and similarity of aromaticity in porphyrin and porphyrin-like structures...... 11

Figure 9: Typical axial and peripheral substitutions of phosphorus(V) porphyrins...... 12

Figure 10: Structure of the dianion tetraphenylporphyrin (TPP2-) and related symmetry operations...... 13

Figure 11: Characteristic features in the UV-VIS absorption spectra of porphyrins...... 14

Figure 12: Gouterman’s four orbital model and the relationship of orbital energy levels to spectral transitions of a porphyrin system...... 15

Figure 13: Phosphorus(V) porphyrin-azobenzene photoswitch system...... 17

Figure 14: A general diagram of the Dexter energy transfer mechanism...... 19

Figure 15: A general diagram of the Förster energy transfer mechanism...... 20

Figure 16: Diagram of spectral overlap...... 21

Figure 17: A general diagram of the oxidative electron transfer mechanism...... 23

Figure 18: A general diagram of the reductive electron transfer mechanism...... 23

viii

Figure 19: Marcus theory diagram of a “normal case” region...... 25

Figure 20: The phosphorus(V) porphyrins that were synthesized...... 27

Figure 21: Comparison of the UV-VIS spectra of the prepared samples...... 28

Figure 22: Fluorescence spectra of the phosphorus(V) complexes...... 30

Figure 23: Compilation of the CVs (black) of all of the porphyrins as well as the oxidation

(blue) and reduction (red) DPVs...... 32

+ - Figure 24: Solid-state absorbance spectra of [TPPA-P(OMe)2] Cl ...... 35

Figure 25: The overlapped UV-VIS and fluorescence spectra for the synthesized compounds...... 37

+ - Figure 26: Energy level diagram of [TPPA-P(OMe)2] PF6 on a SnO2 semiconductor.... 39

+ - Figure 27: Energy level diagram of [TPPE-P(OMe)2] PF6 ...... 40

ABBREVIATIONS

UV = Ultraviolet

UV-VIS = Ultraviolet-visible

PV = Photovoltaic

PEC = Photoelectrochemical

TPP = Tetraphenylporphyrin

HOMO = Hight occupied molecular orbital

LUMO = Lowest unoccupied molecular orbital

DNA = Deoxyribonucleic acid

Å = Angstrom (1E-10 meters)

µ = Micro (1E-6) nm = Nanometer (1E-9 meters)

M = Molar mM = Millimolar

CV = Cyclic voltammetry/Cyclic voltammogram

DPV = Differential pulse voltammetry/differential pulse voltammogram eV = Electron volts

V = Volts

SCE = Saturated calomel electrode

NHE = Normal hydrogen electrode

1. INTRODUCTION

1.1 GREEN ENERGY ALTERNATIVES ARE NEEDED

Gases that remain in the atmosphere and trap heat are known as greenhouse gases.1

Such gases do not have a physical or chemical change caused by temperature and are difficult to remove from the atmosphere.2 One of these gases, carbon dioxide, comes from burning fossil fuels. Industrialization has caused atmospheric carbon dioxide levels to increase from 280 ppm to 412 ppm over the last 150 years.2

The earth receives enough solar energy to meet the power consumption needs of the world. There are practical limitations that prevent enough solar energy from being utilized to meet these needs. Some of these limitations include the sun not shining all day, the amount of light that actual reaches the ground, influences of the weather, and variable intensity of light. Due to the variability in sunlight and the limitations on being able to collect it, the need to be able to store it, and develop better methods for storage, becomes apparent.3

1.2 H2 IS A CLEAN BURNING FUEL

All hydrocarbons produce carbon dioxide and water as byproducts of combustion reactions (Equation 1). The combustion of hydrogen produces water as a byproduct

(Equation 2). This makes hydrogen an ideal alternative to fossil fuels.

퐶퐻4 + 2푂2 → 퐶푂2 + 2퐻2푂 (Eq. 1)

2퐻2 + 푂2 → 2퐻2푂 (Eq. 2)

In 1972, Bockris published the first paper regarding a “hydrogen economy”.4-5

NASA started using liquid hydrogen as rocket fuel in the 1950s.6 A number of vehicle manufacturers have started producing hydrogen fuel cell electric vehicles.6-8 As hydrogen can be produced domestically, used for high efficiency, zero-emission fuel cells in electric cars, and a fuel cell is two to three times more efficient than combustion engines, it is a major research interest as an alternative to fossil fuels.6

The majority of hydrogen is produced from the thermal process of reforming natural gas.9 There are two popular processes for doing this. The first is the most popular, steam-methane reforming. In the first step, methane is reacted with steam in the presence of a catalyst to produce carbon monoxide and hydrogen (Equation 3). In the second step, the carbon monoxide is reacted with steam in the presence of a catalyst to produce carbon dioxide and hydrogen, this is known as the “water-gas shift reaction” (Equation 4). In the third step, impurities are removed through what is known as “pressure-swing absorption”.10

This results in carbon dioxide being produced.

Step 1: 퐶퐻4 + 퐻2푂 + 퐻푒푎푡 → 퐶푂 + 3퐻2 (Eq. 3)

Step 2: 퐶푂 + 퐻2푂 → 퐶푂2 + 퐻2 (Eq. 4)

Step 3: Removing impurities

The other commonly used method is partial oxidation. A less than stoichiometric amount of oxygen is used to oxidize methane, or other hydrocarbons, to produce carbon

2 monoxide, hydrogen, and a small amount of carbon dioxide (Equation 5). The next step is the water-gas shift reaction.10 Again, this still results in carbon dioxide being produced.

1 Step 1: 퐶퐻 + 푂 → 퐶푂 + 2퐻 (Eq. 5) 4 2 2 2

Step 2: 퐶푂 + 퐻2푂 → 퐶푂2 + 퐻2 (Eq. 4)

As mentioned previously, hydrogen can also be produced through electrolysis, using water. This method eliminates the production of carbon dioxide. The process of electrolysis of water was developed in the 18th century (Figure 1).11 Electrolysis can only be as clean as the methods that produces the voltage sources that are applied to drive the, electrolysis, process.

Figure 1: General overview of the electrolysis of water.

1.3 PHOTOVOLTAIC (PV) AND PHOTOELECTROCHEMICAL

(PEC) CELLS

Both PV cells and PEC cells convert light into energy. PV cells use solar energy to produce electrical energy which can then be stored in batteries. PEC cells use solar energy to directly do chemical work. PEC cells contain an electrolyte phase where ions carry the moving charge and electrode/electrolyte interfaces where electrochemical reactions transpire. Alternatively, PV cells are solid-state devices where holes or electrons carry the moving charge without a chemical change.12

PEC and PV cells are both designed to utilize the energy of hole-electron pairs created by absorption of light. A junction across which there is an electrochemical potential difference at equilibrium that is critically important for both types of cells. This is because the combination of the electric and chemical potential differences across the junction is what causes holes and electrons to flow in opposite direction. This produces a photocurrent.

The junction in a PV cells is usually a p-i-n junction or a p-n homo- or heterojunction which are formed from doped intrinsic semiconductors that are also the light-absorbing phases

(Figure 2). The junction in a PEC cell can be formed between a semiconductor, that is sometimes dye-sensitized, and an electrolyte solution (Figure 3).12 The type of semiconductors used can either be n-type or p-type.

Figure 2: Basic photovoltaic (PV) cell design. The p-type semiconductor is where the electron holes are. The n-type semiconductor is where the free electrons are.

Figure 3: Photoelectrochemical (PEC) cell.

1.3.1 WATER SPLITTING

Hydrogen is a clean-burning fuel that can be produced using solar energy. Among the ways (such as light13-15, electricity16, and heat17-18) that solar energy may be applied to produce hydrogen, its use as light is the most efficient. This is because the inefficiencies associated with thermal transformation or with conversion of solar energy to electricity prior to electrolysis are not present when light is used. For these reasons, water splitting resulting from the implementation of a photoelectrochemical or photocatalytic system is the most promising means of hydrogen production derived from a renewable energy source.19 Such systems need to be able to efficiently absorb light to allow them to indirectly split water as photons from the sun, by themselves, cannot be used to directly split water molecules due to their inability to electronically excite water molecules.20

One photoelectrochemical system that can be used for splitting water, utilizes two electrodes in an aqueous electrolyte. One electrode, the photoanode, is a photocatalyst that when it absorbs light electron-hole pairs are produced. These electron-hole pairs are separated by an electric field within the semiconductor which allows for the holes to move to the surface of the semiconductor and the electrons to move through an electrical circuit to the counter electrode. At the surface of the semiconductor the holes can oxidize the water molecules to oxygen (Equation 6). The electrons are involved in the reduction of H+ to hydrogen (Equation 7). With this system an additional potential driving-force is needed and is implemented as a bias voltage, either externally or internally. But not all systems require a voltage bias to function. One such photoelectrochemical system is based on the use of semiconducting materials as both photoelectrodes. One photoelectrode, the

6 photoanode, uses n-type semiconducting materials. The other photoelectrode, the photocathode, uses p-type semiconducting materials. This allows for photovoltages to be generated on both electrodes which leads to a significant increase in solar energy conversion. Additionally, the resulting voltage is sufficient for water splitting without the need for a voltage bias.19 In order for the voltage to be deemed sufficient for water splitting the band-gap energy must be at least 1.23V against NHE.

+ 2H2O → 4H + O2 (Eq. 6)

+ - 4H + 4e → 2H2 (Eq. 7)

For this to be the case a couple of requirements must be met. The top of the valence band must be at a more positive potential than the oxidation potential of water (1.23V against NHE at pH = 0). The bottom of the conduction band must be at a more negative potential than the reduction potential of protons (0.0V against NHE at pH = 0).9 21

1.3.2 N-TYPE SEMICONDUCTORS

What are termed as “donor atoms” are the impurities, or dopants, in n-type semiconductors that provide the extra electrons. An example of this is phosphorus.22-23 As electrons are the majority carriers, in n-type semiconductors, their density is much greater than that of the minority carriers, holes (Figure 4). In turn, electrons move from lower to

7 higher potentials. In n-type semiconductors, away from the valence band, the Fermi level is between the conduction band and the donor energy level (Figure 5).22,24

Figure 4: Majority and minority carriers in n-type semiconductors. The green circles represent the majority carriers, the free electrons, while the white circles represent the minority carriers, the holes.

24 Figure 5: Band diagram for an n-type semiconductor. EC is the energy of the conduction band, EF is the Fermi level, ED is the donor level, EV is the valence band energy, EG is the energy level gap, and ΔE is the amount of energy required to inject an electron from the donor to the conduction band.

N-types act as anodes. One metal oxide that can be used for such a system is TiO2.

This metal oxide would then be dyed with a sensitizer which binds to the surface of the

TiO2. When the dye absorbs a photon from light an electron in the HOMO level of the dye is excited to a higher energy LUMO level. This allows them to move from the LUMO of the dye to the lower energy conduction band of the TiO2. The amount of energy that goes to the photocathode is the difference between the HOMO level of the dye and the conduction band of the metal oxide. Upon leaving the counter electrode, the electron enters the electrolyte and converts the triiodide ion into three iodide ions (Equations 7 and 8).

− − − 3퐼 → 퐼3 + 2푒 (Eq. 7)

− − − 퐼3 + 2푒 → 3퐼 (Eq. 8)

The electron is then transferred from the iodide ion into the dye. This reduces the dye back to its original state, thus allowing the process to occur again.

1.3.3 P-TYPE SEMICONDUCTORS

What are termed as “acceptor atoms” are the impurities, or dopants, in p-type semiconductors that provide the extra holes. An example of an acceptor atom is boron.23

As holes are the majority carriers, in p-type semiconductors, their density is much greater than that of the minority carriers, electrons (Figure 6). In turn, holes move from higher to lower potentials. In p-type semiconductors, away from the conduction band, the Fermi level is between the acceptor energy level and the valence band (Figure 7).22,24

Figure 6: Majority and minority carriers in p-type semiconductors. The white circles represent the majority carriers, the holes, while the green circles represent the minority carriers, the electrons.

24 Figure 7: Band diagram for a p-type semiconductor. EC is the energy of the conduction band, EF is the Fermi level, EA is the acceptor level, EV is the valence band energy, EG is the energy level gap, and ΔE is the amount of energy required to inject an electron from the valence band to the acceptor.

1.4 PORPHYRINS

1.4.1 PORPHYRIN STRUCTURE

Porphyrins are substituted porphines. Porphyrins are aromatic macrocycles that have 22 π electrons (Figure 8). The conjugated system present in porphyrins is similar to that of [18]-annulene (Figure 8).25

Figure 8: Variation and similarity of aromaticity in porphyrin and porphyrin-like structures.

Porphyrins can be substituted in a number of locations peripheral to the core aromatic azoannulene moiety previously described and therefore do not interrupt the aromaticity of said moiety. There are 12 peripheral locations at which porphyrins can be

11 substituted, eight β-pyrrolic positions and four meso positions. Porphyrins are also able to host a wide array of metal atoms and main-group elements (which displace the two central hydrogens bonded to two of the four nitrogen atoms) in the central cavity. Further axial substitution can be done on the element of these metallo-, metalloid, or main-group porphyrins. The number and type of axial bonds that can be formed varies depending on the central element. In the case of phosphorus(V) porphyrins, two covalent axial bonds are always formed (Figure 9).

Figure 9: Typical axial and peripheral substitutions of phosphorus(V) porphyrins.

1.4.2 PORPHYRIN SYMMETRY

Deprotonated (dianionic) TPP exhibits D4h symmetry. In neutral TPP, the presence of the central hydrogens removes the two σd planes, which reduces the symmetry to D2h.

When a metal ion is inserted the two inner hydrogens are displaced the D4h symmetry is restored. The D4h symmetry is maintained in porphyrins when the axial substituents are the same, such as dihydroxy-substituted phosphorus porphyrin. The σh plane is removed if there is only one axial substituent, which reduces the symmetry to C4v (Figure 10).

Figure 10: Structure of the dianion tetraphenylporphyrin (TPP2-) and related symmetry operations.

1.4.3 SPECTRAL FEATURES OF PORPHYRINS

Due to π-π* transitions originating in the extended conjugated azoannulene system mentioned previously, porphyrins exhibit strong absorption in the visible-light spectrum.

As shown in Figure 11, two major bands in the visible light absorption spectrum result from these transitions, one around 420 nm and one around 560 nm.

1.0 Soret Band

0.8

0.6 Q Bands

II 0.4

x10

Absorbance Intensity (a.u.) Intensity Absorbance 0.2 I

0.0 400 500 600 Wavelength (nm)

Figure 11: Characteristic features in the UV-VIS absorption spectrum of porphyrins. The

+ - porphyrin shown is [TPPA-P(OMe)2] PF6 ). The region containing the Q-bands is magnified by a factor of 10. (solvent = acetonitrile).

The Gouterman four-orbital model has been used to rationalize the mechanics of these transitions (Figure 12).26-27 The absorption spectra of porphyrins result from the transitions between two HOMO and two LUMO. A higher and lower energy state are produced by the orbital mixing of excited states. As the transition to the S2 state is strongly allowed the S0-S2 transition (Soret band) is more intense than the S0-S1 transition (Q band),

26-27 as the S1 transition is forbidden.

Figure 12: Gouterman’s four orbital model and the relationship of orbital energy levels to spectral transitions of a porphyrin system. Q-band = β(a1u → eg) – α(a2u → eg). Soret band

26-27 = β(a1u → eg) + α(a2u → eg).

The symmetry of metalloporphyrins can be altered by the presence of axial ligands.

The presence of one axial ligand causes the metal-center to be pulled out-of-plane, the resulting symmetry is a C4v point group which influences the energetics of the system.

These changes to the energetics can be seen in the fluorescence spectra and be described by the four-orbital model. The symmetry can be restored to D4h by a second axial ligand pulls the metal-center back in-plane, restoring D4h symmetry.

1.5 PHOSPHORUS(V) PORPHYRINS

Most porphyrins are not strong enough oxidants to be used for water splitting.

Typically, electron withdrawing groups are incorporated in order to increase the redox potentials of the porphyrin. Alternatively, phosphorus can be inserted into the cavity of the porphyrin. Phosphorus(V) porphyrins have a hexavalent phosphorus atom at the center.28

These central phosphorus atoms have a formal oxidation state of +5, and, therefore, are very electron deficient.18 This also makes the phosphorus easy to reduce.19 Having a

15 phosphorus atom at the center provides more than just influence to the redox potential of the porphyrin.

The phosphorus in these porphyrins also provides two axial locations for covalently binding substituents.29 These substituents along with the four pyrrole nitrogen of the porphyrin ring create an octahedral geometry for the phosphorus atom.30-31 Ligand substitutions allow for the porphyrin to be modified as well as significantly reduces π-π stacking of the porphyrin rings, which can alter the photochemistry.28 These axial ligands influence the overall photophysical properties of the porphyrin.31 These ligands also greatly influence singlet oxygen generation and charge transfer and allows switching between the two processes.31 The ability to attach axial ligands allows for a larger range of donors and acceptors to be attached.29 Based on these properties phosphorus porphyrins could be effective dye-sensitized anodes for water oxidation.29

1.5.1 PHOTOSWITCHES

Molecular machines are molecules that can switch between two distinct forms when subject to an external stimulus. Molecular machines are critically important for developing the building block of nanotechnology.32-33 Photoswitches are a category of molecular machines that are sensors that detect light or changes in the intensity of light. Therefore, a photoswitchable compound must be sensitive to light. Some photoswitchable/photoreversible compounds can have their fluorescence and redox properties controlled by photochemically induced reversible cis-trans isomerization.34

Porphyrin-based systems are an example of such compounds. A phosphorus phorphyrin- azobenzene system was designed to exhibit such a process (Figure 13).34

Figure 13: Phosphorus(V) porphyrin-azobenzene photoswitch system.34

Successful synthesis a characterization of such compounds demonstrates the usefulness of the ability of phosphorus porphyrins to form axial bonds as they allow the porphyrin to be used as a photoswitch. Phosphorus(V) porphyrins can function as more than just photoswitches.

1.5.2 PHOTOSENSITIZERS/PHOTODYNAMIC THERAPY

Phosphorus porphyrins are also able to function as photosensitizers. Molecules that produce a physical change in other molecules as a result of a photochemical process are known as photosensitizers.35 Typically, photosensitizers work by absorbing light and transferring it to nearby molecules. They also usually have large, delocalized π-systems.

Porphyrins can be used as photosensitizers.

Charged porphyrin-based dyes can be utilized for a variety of medical applications as a result of their charged nature. Cationic porphyrin derivatives are typically made by creating a positive charge at peripheral locations. Another method is to insert an element, with an oxidation state greater than two, into the porphyrin cavity. Phosphorus is such an element.31 The use of phosphorus porphyrins as photosensitizers in DNA binding and cleaving can be attributed to their strong oxidizing ability of phosphorus porphyrins in the singlet and triplet excited states, their ability to generate singlet oxygen, and their charged nature.31,36 Chemotherapy and antimicrobial photodynamic therapy are other medical applications that take advantage of the ability of phosphorus porphyrins to act as photonucleases.36

1.6 ENERGY AND ELECTRON TRANSFER

1.6.1 ENERGY TRANSFER (DEXTER AND FÖRSTER)

There are two energy transfer mechanisms, the Dexter mechanism and the Förster mechanism. In the Dexter mechanism a double electron transfer occurs, as shown in Figure

14 below. The higher energy singly occupied excited state of the donor transfers its electron 18 to the LUMO of the acceptor. One of the HOMO electrons from the acceptor transfers to the hole of the lower singely occupied MO of the donor. These transfers cause the once excited donor to be in its ground state and the once ground state acceptor to be in its excited state.

Figure 14: A general diagram of the Dexter energy transfer mechanism.

Due to the occurrence of electron transfer, electronic coupling and orbital overlap are important factors that govern the rate of the process. The energy transfer rate for the Dexter mechanism can be calculated using Equation 9. Where HDA is the electronic coupling, and

JD is the normalized absorption and emission integral.

4휋2퐻2 퐾퐷 = 퐷퐴 퐽퐷 (Eq. 9)37 퐸퐸푇 ℎ

The further apart the donor and acceptor are, the slower the transfer. Furthermore, the distance dependence, when it can be accurately measured, is a means of distinguishing between dipole-dipole interactions and electron transfer interactions. This is because

19 electron transfer interactions typically decrease exponentially with increasing distance between the donor and acceptor.

In the Förster mechanism, coulombic interactions where transition dipole moments couple occur. This can occur when the donor and acceptor are 100 Å apart. This is a much larger distance than the Dexter mechanism which operates when the distance between donor and acceptor is ~10 Å apart. As shown in Figure 15 below, as the one excited state goes down, the other excited state goes up.

Figure 15: A general diagram of the Förster energy transfer mechanism.

The efficiency of the Förster energy transfer can be determined using Equation 10.

1 38 퐸 = 6 (Eq. 10) 1+(푟+푅0)

Where R0 is the Förster distance when E = 0.5, and r is the distance between a donor

(fluorescent unit) and an acceptor. The Förster distance can be determined using Equation

11.

2 −4 1/6 39 푅0(Å) = 0.211 ∗ (휅 푛 휙퐷퐽) (Eq. 11) 20

2 Where κ is the orientation factor, n is the refractive index of the medium, ϕD is the fluorescence quantum yield of the donor, and J is the overlap integral. The overlap integral,

J, can be determined using Equation 12.39

퐹 (휆)휀 (휆)휆4푑휆 퐽 = ∫ 퐷 퐴 (Eq. 12)39 ∫ 퐹퐷(휆)푑휆

Where FD(λ) is the fluorescence intensity of the donor as a function of wavelength and

εA(λ) is the molar extinction coefficient of the acceptor at that wavelength, as illustrated in

Figure 16.

1.0 eA

FD 0.8

0.6

0.4 Normalized Intensity Normalized

0.2

0.0 500 550 600 650 700 750 800 Wavelength (nm)

Figure 16: Diagram of spectral overlap. εA is the absorbance of the acceptor and FD is the fluorescence of the donor.

For the Förster mechanism to be possible, spectral overlap is required. The larger the spectral overlap integral is, the better the energy transfer is.

1.6.2 ELECTRON TRANSFER (OXIDATIVE, REDUCTIVE, AND

MARCUS THEORY)

1.6.2.1 OXIDATIVE AND REDUCTIVE ELECTRON TRANSFER

In the case of the electron transfer mechanisms, a single electron is transferred from the donor to the acceptor. The result of this transfer is a pair of charges. The donor becomes cationic and the acceptor becomes anionic. There are two pathways through which this can occur, oxidative and reductive.

In oxidative electron transfer the donor is the photosensitizer, which gets oxidized.

As shown in Figure 17, the donor absorbs a photon which excites one of the HOMO electrons of the ground state donor to its LUMO. The resulting excited state donor then transfers an electron from its LUMO to the LUMO of the ground state acceptor. The result of this, as previously mentioned, is a cationic donor and an anionic acceptor. This is also known as a charge separated state, or radical ion pair. The reductive electron transfer pathway differs slightly from this.

Figure 17: A general diagram of the oxidative electron transfer mechanism. 22

In reductive electron transfer (or hole transfer) the acceptor is the photosensitizer, which gets reduced. As shown in Figure 18, the acceptor absorbs a photon which excites one of the HOMO electrons of the ground state acceptor to its LUMO, thus creating a hole in its HOMO. One of the HOMO electrons from the ground state donor is then transferred to this hole in the HOMO of the excited acceptor, causing the hole to be filled. Again, as previously mentioned, the result of this is a cationic donor and an anionic acceptor.

Figure 18: A general diagram of the reductive electron transfer mechanism.

These oxidative and reductive electron transfer reactions are the basis of the elementary steps of the light reactions of photosynthesis and they can be modeled by the

Rehm-Weller equation (Equation 13)40 where the overall free energy change for the process (ΔGel) depends on the oxidation potential of the donor and the reduction potential of the acceptor. The driving force for the reaction is the absorption of light.

0 + 0 − 40 훥퐺푒푙 = 23.06[퐸 (퐷 /퐷) − 퐸 (퐴/퐴 )] − 푤푝 − 훥퐺00 (Eq. 13)

In the Rehm-Weller equation, E0(D+/D) represents the redox potential for the following half reaction: D+→D; E0(A/A-) represents the redox potential for the following half 23 reaction: A→A-; ΔG is the energy of the first excited singlet state (which represents the

HOMO-LUMO transition) and, wp represents the coulombic energy needed to generate the ion pair.

1.6.2.2 MARCUS THEORY

In 1956, Rudolph A. Marcus developed a theory, now termed Marcus theory, to explain the rate at which an electron is transferred from an electron donor to an electron acceptor. Some assumptions that were made were that the reactants and products are spheres connected by springs. This allows for parabolas to be drawn for the reactants and products which represent their energy as they move around the nuclei of one another

(Figure 19). As such, the energy is dependent on the square of the distance from the equilibrium positions (x-axis), which are different for the reactants and products due to the changes in charge of atoms, or molecules, involved in charge transfer. Changes in charge impact whether the species are being attracted or repelled, and by extension, their location.

Marcus realized that by knowing the thermodynamic parameters of the system and solving for the point where the parabolas cross that he could derive the activation energy and rate constant.41

Figure 19: Marcus theory diagram of a “normal case”.

The activation energy can be calculated using Equation 14.

(∆퐺+ ∆휆)2 ∆퐸 = (Eq. 14) 4휆

The activation energy is represented by ΔE. The Gibbs free energy change between the reactant and products is represented by ΔG. The amount of energy required for the reactants to have the same nuclear configuration as the products without electron transfer, also known as the reorganization energy, is represented by λ.

2. RESULTS AND DISCUSSION

Optical and electrochemical studies were done on the prepared samples. This characterization allowed for energy level diagrams to be made for each compound. These diagrams support the viability of these systems to be used for water splitting by harnessing solar energy. This is important as water splitting allows for the production of H2 gas which is a clean burning fuel.

2.1 UV-VIS SPECTROSCOPY

UV-VIS spectroscopy measures the absorbance of light, in the ultraviolet and visible light regions, by a compound as a function of wavelength. When a compound absorbs a photon, it is excited from its ground state (HOMO) to an electronic excited state

(LUMO). The HOMO is commonly a π orbital of a conjugated functional group and the

LUMO is commonly a π* orbital of the conjugated functional group. The smaller the energy difference between HOMO and LUMO, the less energy is needed and the longer the wavelength that will be absorbed.42 From this, it makes sense that the largest factor in determine the HOMO-LUMO gap is how conjugated the system is.42

UV-VIS spectroscopy was used to investigate each of the four phosphorus(V) porphyrin complexes. The results are shown in Figure 20 and summarized in Table1. The electron-withdrawing nature of the phosphorus(V) results in a red-shift in the UV-VIS spectra, relative to the free-base porphyrin for each of the spectra. This is because the

HOMO-LUMO energy gap is decreased as a result of the better electron delocalization that electron-withdrawing groups provide.43 26

Figure 20: The phosphorus(V) porphyrins that were synthesized.

300000 + - [TPPA-P(OMe)2] PF6 + - [TPPE-P(OMe)2] PF6 250000 + - [TPPE-P(OPh)2] PF6 + - [TPPE-P(OTPy)2] PF6

200000

)

-1 cm

-1 150000

(M e

100000 x10

50000

0 200 300 400 500 600 700 Wavelength (nm)

Figure 21: Comparison of the UV-VIS spectra of the prepared samples.

All of the porphyrins show normal type UV-VIS spectra with two Q bands in the range of 500-630 nm and a Soret band in the range of 415-432 nm, see Figure 21 and Table

28 + - 1. In the case of [TPPE-P(OTPy)2] PF6 , there is an additional signal whose peak absorbance is observed at 285 nm which corresponds to the absorbance of the terpyridine ligand.44-45

Table 1: UV-VIS absorption data of the phosphorus(V) porphyrins.

Q Bands Soret Band Compounds λmax (log10[ε]) λmax (log10[ε]) + - [TPPA-P(OMe)2] PF6 599 (3.54), 559 (4.13) 427 (5.46) + - [TPPE-P(OMe)2] PF6 599 (3.52), 558 (4.07) 427 (5.38) + - [TPPE-P(OPh)2] PF6 603 (3.64), 560 (4.14) 433 (5.26) + - [TPPE-P(OTPy)2] PF6 605 (3.66), 564 (4.17) 433 (5.17)

2.2 FLUORESCENCE SPECTROSCOPY

Fluorescence spectroscopy can be used to complement UV-VIS absorption data.

Fluorescence spectroscopy probes over the same wavelength range as UV-VIS spectroscopy. Unlike in UV-VIS absorption, however, the fluorescence spectroscopy measures the emission of a photon from an excited state to a lower energy than it was absorbed.42

It is important to select a wavelength at which the compounds absorb strongly when performing a fluorescence assay. When assaying porphyrins there are two main characteristic options for such a wavelength, the Soret and Q bands. The Q bands are in the visible light region and the Soret bands are in the near-ultraviolet region. Excitation further into the UV region would degrade the molecule as it would be too energetic. Meanwhile, photons from the infrared region are not energetic enough to drive the process of interest.

Each of the compounds were investigated by fluorescence spectroscopy. This was done by excitation at 565 nm and the emission was measured over 575-800 nm. The results are presented in Figure 22 and are summarized in Table 2.

350000 + - [TPPA-P(OMe)2] PF6 [TPPE-P(OMe) ]+PF - 300000 2 6 + - [TPPE-P(OPh)2] PF6 [TPPE-P(OTPy) ]+PF - 250000 2 6

200000

150000

100000 Fluorescence Intensity (a.u.) Intensity Fluorescence 50000

0 600 650 700 750 800 Wavelength (nm)

Figure 22: Fluorescence spectra of the phosphorus(V) complexes. Concentration = 5E-6

M. Solvent = acetonitrile. Excitation wavelength = 565 nm.

Although it was not quantitatively measured, it can be seen from Figure 22 that

+ - + - [TPPE-P(OPh)2] PF6 and [TPPE-P(OTPy)2] PF6 , which have aromatic ligands, that quenching is occurring. This is because the phenoxy and terpyridoxy ligands are more oxidizable than the methoxy ligand, as they are more conjugated and provide more stabilization. This also indicates that they are stronger electron donors, while phosphorus(V) porphins are good electron acceptors.

Table 2: Fluorescence data of the phosphorus(V) porphyrins.

Compound Wavelength (nm)

+ - [TPPA-P(OMe)2] PF6 615, 669

+ - [TPPE-P(OMe)2] PF6 615, 669

+ - [TPPE-P(OPh)2] PF6 621, 675

+ - [TPPE-P(OTPy)2] PF6 623, 675

2.3 ELECTROCHEMISTRY

The redox potentials of the compounds were investigated using cyclic voltammetry

(CV) and differential pulse voltammetry (DPV). The important trends of the oxidation and reduction DPVs of the compounds are shown in Figure 23. The corresponding oxidation and reduction potentials are presented in Table 3.

Figure 23: Compilation of the CVs (black) of all of the porphyrins as well as the oxidation

+ - (blue) and reduction (red) DPVs. Plot a corresponds to [TPPA-P(OMe)2] PF6 . Plot b

+ - + - corresponds to [TPPE-P(OMe)2] PF6 . Plot c corresponds to [TPPE-P(OPh)2] PF6 . Plot d

+ - corresponds to [TPPE-P(OTPy)2] PF6 . The signal at 0.4V corresponds to the ferrocene internal reference. Solvent = acetonitrile. Electrolyte = 0.1 mM TBAP in acetonitrile.

It can be noticed from the data presented in Figure 23, and in Table 3, is that there is virtually no change to the oxidation potential between an ester and the carboxylic acid moiety. Additionally, the more conjugated the axial ligand, the higher the oxidation potential. The higher the oxidation potential that a compound has, the more likely that compound is to lose electrons and become oxidized.46 All of the porphyrins exhibit one 32 oxidation peak (Figures 23). The other peak present in all of these voltammograms corresponds to the ferrocene internal standard.

From the data in Figure 23 and Table 3, the more conjugated the axial ligand, the higher (less negative) the reduction potential. The higher the reduction potential a compound has, the more likely that compound is to gain electrons and be reduced.47 It can also be seen that all of these porphyrins have two reduction peaks (Figures 23).

Table 3: Summary of the oxidation and reduction DPV data and estimates of the HOMO-

LUMO gap (Energy (eV)) from those values. aThese values are vs SCE. bThese values are vs NHE.

Oxidation Energy Compound Reduction Potential (eV) Potential (eV) (eV)

[TPPA- 1.662a (1.902)b -0.454a (-0.214)b -0.934a (-0.694)b 2.116 + - P(OMe)2] PF6

[TPPE- 1.664a (1.904)b -0.448a (-0.208)b -0.888a (-0.648)b 2.112 + - P(OMe)2] PF6

[TPPE- 1.7a (1.94)b -0.356a (-0.116)b -0.78a (-0.54)b 2.056 + - P(OPh)2] PF6

[TPPE- 1.714a (1.954)b -0.318a (-0.078)b -0.866a (-0.626)b 2.032 + - P(OTPy)2] PF6

2.4 SOLID-STATE REFLECTANCE SPECTROSCOPY

As the final photoanode will be a solid-state component of the PEC, it is important to investigate the solid-state properties of the material. To do this, photosensitized slides had to be made. This was done by taking quartz slides, making a frame of the slide using tape, pipetting a slurry of SnO2 onto the slide, and then a SnO2 layer of uniform thickness was created by spreading the slurry with a razorblade. The tape was then careful removed, and the slides were sintered in an oven at 450oC for 2 h. The slides were then soaked in a

0.1 mM solution of the photosensitizing dye overnight. After drying these slides, they were used for solid-state reflectance spectroscopy (Figure 24). Originally the data was recorded via reflectance but was converted to absorbance data (Figure 24).

0.8 Slide 1 Scan 1 Slide 1 Scan 2 Slide 1 Scan 3 Slide 2 Scan 1 Slide 2 Scan 2 0.6 Slide 2 Scan 3 Slide 3 Scan 1 Slide 3 Scan 2

0.4

0.2 Absorbance Intensity (a.u.) Intensity Absorbance

0.0 200 300 400 500 600 700 Wavelength (nm)

+ - Figure 24: Solid-state absorbance spectra of [TPPA-P(OMe)2] Cl . This data was originally recorded as reflectance data due to it being a solid-state sample. The [TPPA-

+ - P(OMe)2] Cl was bound to SnO2 on a quartz slide.

The Q-bands and Soret band were maintained (Figure 24). However, it can also be seen that they are broader than the liquid-state sample (Figure 21). This is likely due to the energy differences associated with solid- and liquid-state samples, as well as the accompanying differences in molecular movement.

2.5 ENERGY LEVEL DIAGRAMS

+ The S0 /S0 (HOMO) energy was obtained directly from the oxidation potential from

+ the DPV. One method to determine energy level of the lowest excited singlet state, S1 /S1

+ (LUMO) is to use first reduction potential from the DPV. The different between the S0 /S0

+ energy level and the S1 /S1 energy level would represent the S0→S1 transition energy

(HOMO-LUMO gap).48

+ A second method to determine the S0→S1 transition energy and the S1 /S1 energy level is to use the fluorescence spectra. The S0→S1 transition energy can be determined by using Equation 15 to convert the maximum wavelength of the blue edge of the fluorescence

+ spectra from nm to eV. This transition energy is then subtracted from the S0 /S0 to get the

+ 29,49 S1 /S1 energy level. This method is more accurate than the previously mentioned method.

ℎ푐 퐸 = (Eq. 15)50 휆

A third, and more accurate, method to determine the S0→S1 transition energy is to use the Q band region of the UV-VIS spectrum in combination with the fluorescence spectrum. This is done by normalizing the maxima of both spectra to 1. Then the spectra are overlapped (Figure 25) and the wavelength at which they intersect is then converted

+ from nm to eV, using Equation 15 (Table 4). As mentioned previously, the S1 /S1 energy level is then determined from this by subtracting the determined S0→S1 transition energy

+ 51 from the S0 /S0 energy level. This method was used to obtain the energies shown in Table

4, as it is the most accurate. The S0→S2 transition energy can be determined by converting the maximum wavelength of the Soret band of the UV-spectrum from nm to eV, using

+ Equation 15. The second excited state, S2 /S2, energy level can be determined from this by

+ 29 subtracting the S0→S2 transition energy from the S0 /S0 energy level. These values are summarized in Table 5.

Figure 25: The overlapped UV-VIS and fluorescence spectra for the synthesized compounds. The blue spectra are the UV-VIS spectra and the red spectra are the

+ - fluorescence spectra. Plot a corresponds to [TPPA-P(OMe)2] PF6 . Plot b corresponds to

+ - + - [TPPE-P(OMe)2] PF6 . Plot c corresponds to [TPPE-P(OPh)2] PF6 . Plot d corresponds to

+ - [TPPE-P(OTPy)2] PF6 .

Table 4: Wavelength and corresponding energy of the overlapped Q band of the UV-VIS spectra and the fluorescence spectra.

Compounds Wavelength (nm) Energy (eV)

+ - [TPPA-P(OMe)2] PF6 598.43 2.0718

+ - [TPPE-P(OMe)2] PF6 598.43 2.0718

+ - [TPPE-P(OPh)2] PF6 603.63 2.0540

+ - [TPPE-P(OTPy)2] PF6 604.04 2.0526

Table 5: Summary of the energy levels and transitions. + + + S0 /S0 S0→S1 S1 /S1 S0→S2 S2 /S2 Compound (eV) (eV) (eV) (eV) (eV)

[TPPA- 1.902 2.0718 -0.1698 2.9036 -1.0016 + - P(OMe)2] PF6

[TPPE- 1.904 2.0718 -0.1678 2.9036 -0.9996 + - P(OMe)2] PF6

[TPPE- 1.94 2.054 -0.114 2.8634 -0.9234 + - P(OPh)2] PF6

[TPPE- 1.952 2.0526 -0.1006 2.8634 -0.9114 + - P(OTPy)2] PF6

+ - Figure 26: Energy level diagram of [TPPA-P(OMe)2] PF6 on a SnO2 semiconductor.

+ - Figure 27: Energy level diagram of [TPPE-P(OMe)2] PF6 . The energy level diagrams of the other TPPE derivatives look similar to this energy level diagram.

Once again, it can be seen that the conversion of the ester to the carboxylic acid does not have a significant impact on the redox nor the electrical properties of the molecules. Based on these diagrams and other work regarding phosphorus(V) porphyrins as photoanodes, it is believed that these systems will also be able to be used similarly.

2.6 Conclusion

The objective to develop a phosphorus(V) porphyrin system that can be utilized as a photoanode was accomplished. The findings of this work also suggest that once the

+ - terpyridoxy derivative, [TPPE-P(OTPy)2] PF6 , is converted to the carboxylic acid derivative it can be utilized for water splitting. The ground state properties of the systems were investigated using UV-VIS spectroscopy, solid-state reflectance spectroscopy, and electrochemistry. UV-VIS and solid-state reflectance spectroscopy also show that the material absorbs light efficiently in the visible light region and absorbs over a large range of wavelengths in this region as well. The excited state was also investigated using fluorescence spectroscopy. This also showed that the material is photoactive.

Electrochemistry was used to obtain redox values and showed that the material is redox- active with potentials that are suitable for water oxidation as was represented by the energy level diagrams

3. FUTURE WORK

In the future more of the samples will be converted from esters to carboxylic acids.

The terpyridoxy derivative is of particular interest as it can coordinate to metals and form pre/catalysts that are useful for water oxidation. This will also be done. These samples will be bond to SnO2 oxide surfaces on FTO slides. This will allow for them to be utilized as photoanodes. These will then be used for PEC cells. The efficiency of these cells will then be tested and allow for an increased understanding to be gained as to make improvements

41 for the next wave of systems. Additionally, due to the location of the S0, S1, and S2 energy levels, for these compounds, relative to the valence band of NiO it is also possible for them to be used as photocathodes, which will also be investigated in the future.

4. EXPERIMENTAL DETAILS

4.1 CHEMICALS

All chemicals were purchased, and used as received, from Alfa Aesar, Fisher

Chemicals, or MilliporeSigma. Materials used for column chromatography were purchased from either MilliporeSigma or Silicycle Inc. Solvents were not freshly distilled. Solvents described as “dry” had the water removed from them using molecular sieves. In cases where an inert atmosphere is mentioned this was achieved using N2 gas delivered via

Schlenk line. “Dark” conditions were accomplished by wrapping the reaction vessel in aluminum foil.

4.2 STRUCTURAL CHARACTERIZATION

All NMR were recorded using a 400 MHz Bruker Avance NEO. The chemical shifts are reported in ppm (parts per million).

4.3 OPTICAL STUDIES

The UV-Vis spectra were gathered with an Agilent Cary 300 (instrument v12.00) scan software v4.20(470). The fluorescence spectra were recorded using a Photon

Technologies International Quanta Master 8075-11 spectrofluorimeter with a 75 W xenon lamp, running with FelixGX software. The SnO2 paste was made by stirring 1.1g of dry

SnO2 nanoparticles with 2 mL od DI water at room temperature for 30 minutes. The paste was the doctor-bladed onto an FTO and quartz slides. The slides were then sintered at

o + - 450 C for 2 h. The slides were then soaked in 25 mL of 0.1 mM [TPPA-P(OMe)2] Cl in ethanol for 12 h.29 The quartz slides were used to measure reflectance while the FTO slides will function as photoanodes in PEC cells and used to for efficiency testing.

4.4 ELECTROCHEMISTRY

Both cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were performed. Electrochemical data was collected using a BASi potentiostat with a platinum working electrode, platinum wire auxiliary, and silver reference electrode. Ferrocene was used as an internal standard. Tetrabutylammonium hexafluorophosphate was used as a supporting electrolyte. HPLC grade acetonitrile was used as the solvent. The CV setting used were the following: scan rate = 100 mV/s, current full scale = 100 µA, filter = 10 Hz, quiet time = 2 seconds, initial potential = 0 mV, switching potential 1 = -1200 mV, switching potential 2 = 1900 mV, and final potential = 0 mV. The DPV settings were the following: step E = 4 mV, pulse width 50 ms, pulse period = 200 ms, pulse amplitude = 50 mV, scan rate = 20 mV/s, current full scale = 100 µA, and quiet time = 2 seconds.

4.5 SOFTWARE

MesReNova v12.0.0-20080 was used to analyze the NMR spectra. Origin 2018 v95E was used to analyze and plot the optical and electrochemical data. Images of molecules were made using ChemDraw Professional version 16.0.1.4 (77). Microsoft

Word was used in the preparation of this document. Microsoft Excel and PowerPoint were used to make tables and some figures, respectively.

5. SYNTHESIS

Synthesis of Methyl-4-(10,15,20-triphenylporphyrin-5-yl)benzoate (TPPE):

A mixture of 2.195g methyl-4-formylbenzoate (1.83 mL, 13.37 mmol), 4.08 mL benzaldehyde (4.256g, 40.11 mmol), and 3.71 mL pyrrole (3.588g, 53.48 mmol) were refluxed in 200 mL of propanoic acid for 45 minutes while open to air. The reaction mixture was then cooled to room temperature. The excess propanoic acid was then removed by distillation. Initial purification was done on a basic alumina column using 100% dichloromethane as an eluent. Final purification was done on an alumina column using

75% dichloromethane, 25% hexane mixture. The eluent was evaporated under reduced pressure to afford 0.63g (7.0%) of purple solid.

+ - [TPPE-PCl2] Cl :

A mixture of 1.000g TPPE (1.49 mmol) and 2.8 mL POCl3 were refluxed in 20 mL of dry pyridine for 24 h under an inert N2 atmosphere and dark conditions. Then, the reaction mixture was cooled to room temperature. The excess POCl3 and pyridine were then removed under reduced pressure to afford a purple solid. The solid was purified on a silica column using a 95% dichloromethane, 5% methanol mixture to afford the desired product of purple solid.

4-([2,2’:6’,2’’-terpyridin]-4’-yl)phenol (TPyOH):

A mixture of 2.4 mL 4-methoxybenzaldehyde (2.686g, 19.7 mmol), 4.4 mL of 2- acetylpyridine, and 3g of KOH were stirred in 200 mL of ethanol at room temperature until the KOH dissolved. Then 58 mL of NH4OH was added and the mixture was allowed to stir overnight at room temperature. A precipitate formed. The mixture was filtered and the solid was kept and dried under vacuum. To the solid was added 10 mL acetic acid and 15 mL

48% HBr. The mixture was then refluxed for 4 h. After the mixture was cooled to room temperature a precipitate formed. The mixture was then filtered and the solid was kept.

After dissolving the solid in water, triethylamine was added dropwise until the pH was ~7.

A precipitate formed, and the mixture was filtered. The white solid was dried under vacuum to yield 0.6132g (10%).

+ - [TPPE-P(OMe)2] PF6 :

+ - A mixture of 0.100g [TPPE-PCl2] Cl (0.124 mmol) and 5 mL of methanol were refluxed in 10 mL of dry pyridine under an inert N2 atmosphere and dark conditions for 24 h. Then, the reaction mixture was cooled to room temperature and concentrated under reduced pressure. The compound was then purified on a silica column using a 90%

+ - dichloromethane, 10% methanol mixture to afford 0.080g (81%) of [TPPE-P(OMe)2] Cl

+ - as a purple solid. Then, 0.030g of the [TPPE-P(OMe)2] Cl was dissolved in 2 mL of methanol. After 0.500g tetrabutylammonium hexafluorophosphate was added to the solution it was stirred at room temperature for 15 minutes. Then, 20 mL of water was added, and the mixture was stirred at room temperature for 15 minutes. The resulting mixture was purified by a dichloromethane and water extraction. The resulting organic layer was concentrated under reduced pressure to give 0.020g of the product.

+ - [TPPE-P(OPh)2] PF6 :

+ - A mixture of 0.110g [TPPE-PCl2] Cl (0.136 mmol) and 0.200g of phenol (2.125 mmol) were refluxed in 10 mL of dry pyridine under an inert N2 atmosphere and dark conditions for 24 h. Then, the reaction mixture was cooled to room temperature and concentrated under reduced pressure. The compound was then purified on a silica column using a 85% dichloromethane, 15% methanol mixture to afford 0.091g (72%) of [TPPE-

+ - + - P(OPh)2] Cl as a purple solid. Then, 0.045g of the [TPPE-P(OPh)2] Cl was dissolved in

2 mL of methanol. After 0.500g tetrabutylammonium hexafluorophosphate was added to the solution it was stirred at room temperature for 15 minutes. Then, 20 mL of water was added, and the mixture was stirred at room temperature for 15 minutes. The resulting mixture was filtered with water and then dried under vacuum to give 0.030g of the product.

+ - [TPPE-P(OTPy)2] PF6 :

+ - A mixture of 0.150g [TPPE-PCl2] Cl (0.186 mmol) and 0.363g of TPyOH (1.116 mmol) were refluxed in 10 mL of dry pyridine under an inert N2 atmosphere and dark conditions for 24 h. Then, the reaction mixture was cooled to room temperature and concentrated under reduced pressure. The compound was then purified on a silica column using a 85% dichloromethane, 15% methanol mixture to afford 0.100g (39%) of [TPPE-

+ - + - P(OTPy)2] Cl as a purple solid. Then, 0.050g of the [TPPE-P(OTPy)2] Cl was dissolved in 5 mL of methanol. After 0.500g tetrabutylammonium hexafluorophosphate was added to the solution it was stirred at room temperature for 15 minutes. Then, 30 mL of water was added, and the mixture was stirred at room temperature for 15 minutes. The resulting mixture was purified by a dichloromethane and water extraction. The resulting organic layer was concentrated under reduced pressure to give 0.036g of the product.

+ - [TPPA-P(OMe)2] PF6 :

+ - To a solution 0.056g (0.07 mmol) of [TPPE-P(OMe)2] Cl in 5 mL of dry acetonitrile, 1 mL of 1 M LiOH was added. The solution was heated for 6 h at 50oC. Then, the mixture was cooled to room temperature. The mixture was concentrated under reduced pressure. It was then purified on a silica column using a 85% dichloromethane, 15%

+ - methanol mixture. This afforded 0.036g (65%) of [TPPA-P(OMe)2] Cl . Then, 0.036g of

+ - the [TPPA-P(OMe)2] Cl was dissolved in 2 mL of methanol. After 0.500g tetrabutylammonium hexafluorophosphate was added to the solution it was stirred at room temperature for 15 minutes. Then, 20 mL of water was added, and the mixture was stirred at room temperature for 15 minutes. The resulting mixture was purified by a dichloromethane and water extraction. The resulting organic layer was concentrated under reduced pressure to give 0.019g (30%) of the product.

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7. APPENDICES

7.1 NMR

1 TPyOH: H NMR (400 MHz, DMSO-d6): δ 8.76 (d, J = 2.5 Hz, 2H), 8.67 (d, J = 5 Hz,

4H), 8.04 (t, J = 5 Hz, 2H), 7.80 (d, J = 2.5 Hz, 2H), 7.53 (t, J = 2.5 Hz, 2H), 6.98 (d, J =

5 Hz, 2H).

1 TPPE: H NMR (400 MHz, Chloroform-d3): δ 8.86 (m, J = 2.5 Hz, 5H), 8.79 (d, J = 2.5

Hz, 2H), 8.44 (d, J = 5 Hz, 2H), 8.31 (d, J = 2.5 Hz, 2H), 8.22 (d, J = 2.5 Hz, 6H), 7.77 (m,

J = 2.5 Hz, 10H), 4.11 (s, 3H), -2.78 (br. s, 2H).

+ - 1 [TPPE-PCl2] Cl : H NMR (400 MHz, Chloroform-d3): δ 9.17 (m, J = 2.5 Hz, 6H), 9.11

(t, J = 2.5 Hz, 2H), 8.45 (d, J = 5 Hz, 2H), 8.15 (d, J = 5 Hz, 2H), 8.00 (d, J = 2.5 Hz, 6H),

7.80 (m, J = 2.5 Hz, 9H), 4.06 (s, 3H).

+ - 1 [TPPE-P(OMe)2] PF6 : H NMR (400 MHz, Chloroform-d3): δ 9.07 (s, 5H), 9.00 (s,

2H), 8.42 (d, J = 5 Hz, 2H), 7.94 (d, J = 5 Hz, 6H), 7.77 (m, J = 5 Hz, 10H), 4.06 (s, 3H),

-1.85 (d, J = 15 Hz, 6H).

+ - 31 [TPPE-P(OMe)2] PF6 : P NMR (400 MHz, Chloroform-d3): δ -144.75, -177.67.

+ - 1 [TPPE-P(OPh)2] PF6 : H NMR (400 MHz, Chloroform-d3): δ 9.04 (m, J = 2.5 Hz, 6H),

8.98 (m, J = 2.5 Hz, 2H), 8.38 (d, J = 5 Hz, 2H), 7.88 (d, J = 5 Hz, 3H), 7.75 (m, J = 4.5

Hz, 14H), 6.12 (t, J = 5 Hz, 2H), 5.95 (t, J = 5 Hz, 4H), 4.06 (s, 3H), 2.21 (d, J = 5 Hz,

4H).

+ - 31 [TPPE-P(OPh)2] PF6 : P NMR (400 MHz, Chloroform-d3): δ -144.31, -194.60.

+ - 1 [TPPE-P(OTPy)2] PF6 : H NMR (400 MHz, Chloroform-d3): δ 9.11 (m, J = 2.5 Hz,

6H), 9.03 (m, J = 2.5 Hz, 2H), 8.64 (d, J = 2.5 Hz, 4H), 8.55 (d, J = 5 Hz, 4H), 8.42 (d, J

= 5 Hz, 2H), 8.13 (s, 4H), 7.92 (d, J = 5 Hz, 2H), 7.83 (m, J = 3.75 Hz, 10H), 7.75 (m, J =

2.5 Hz, 8H), 7.33 (m, J = 3.75 Hz, 5H), 6.59 (d, J = 5 Hz, 4H), 4.03 (s, 3H), 2.45 (d, J = 5

Hz, 4H).

+ - 31 [TPPE-P(OTPy)2] PF6 : P NMR (400 MHz, Chloroform-d3): δ -144.26, -196.79.

+ - 1 [TPPA-P(OMe)2] PF6 : H NMR (400 MHz, Chloroform-d3): δ 9.05 (m, 8H), 8.50 (d,

J = 5 Hz, 2H), 8.05 (d, J = 2.5 Hz, 2H), 7.94 (d, J = 5 Hz, 6H), 7.76 (m, J = 4.2 Hz, 9H), -

1.85 (d, J = 15 Hz, 6H).

+ - 31 [TPPA-P(OMe)2] PF6 : P NMR (400 MHz, Chloroform-d3): δ -144.66, -177.74.