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Probing the Aggregation and Photodegradation of

Rhodamine on TiO2

Journal: The Journal of Physical Chemistry

Manuscript ID jp-2017-04604d.R1

Manuscript Type: Article

Date Submitted by the Author: 05-Jul-2017

Complete List of Authors: Cassidy, James; The College of William and Mary, Chemistry Tan, Jenna; The College of William and Mary, Chemistry Wustholz, Kristin; The College of William and Mary, Chemistry

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1 2 3 4 5 6 7 8 9 10 11 12 13 Probing the Aggregation and Photodegradation of 14 15 16 17 Dyes on TiO2 18 19 20 21 James P. Cassidy, Jenna A. Tan, Kristin L. Wustholz* 22 23 The College of William and Mary, Department of Chemistry, 540 Landrum Drive, 24 25 Williamsburg, VA 23185 26 27 28 * Author to whom correspondence should be addressed. Email: [email protected], Phone: 29 30 Phone: (757) 221-2675, Fax: (757) 221-2715 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 1

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1 2 3 ABSTRACT 4 5 6 The formation and photophysical properties of rhodamine derivatives adsorbed to TiO2 7 8 are investigated using diffuse reflectance spectroscopy, steady-state , and time- 9 10 correlated single photon counting (TCSPC) measurements. Rhodamine derivatives containing 11 12 13 substituted amines (i.e., 5-ROX, R101, RB) exhibit an ~50 nm hypsochromic shift in (3 upon 14 15 adsorption to TiO2 relative to solution. By examining a rhodamine derivative with primary 16 17 18 amines (i.e., R560) as well as control experiments on insulating ZrO2 substrates, we demonstrate 19 20 that photocatalyzed N-de-alkylation is largely responsible for the spectral changes observed upon 21 22 surface adsorption to TiO2. For R560, which does not undergo N-de-alkylation, diffuse 23 24 25 reflectance spectra show that mainly monomers and J-aggregates are present on TiO2. 26 27 Comparative lifetime measurements for R560 on TiO2 and ZrO2 show that the injection yield for 28 29 R560/TiO is increased with -loading concentration (i.e., from 0.63 for monomers to ~0.80 30 2 31 32 for heavily-doped films), indicating that the presence of aggregates enhances electron injection. 33 34 The residual fluorescence of R560/TiO2 is attributed to subpopulations of monomers and weakly 35 36 fluorescent J-aggregates of R560 that do not undergo efficient electron injection to TiO The 37 2. 38 39 fluorescence intensity, energy, and lifetime of R560 on TiO2 and insulating ZrO2 films are 40 41 dependent on dye concentration, consistent with a resonance energy transfer quenching process. 42 43 44 This study shows that contributions due to molecular photodegradation and energy transfer 45 46 interactions must be considered when pursuing the development of a controlled aggregation 47 48 strategy for solar energy conversion materials and devices. 49 50 51 52 53 54 55 56 57 58 59 60 2

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1 2 3 INTRODUCTION 4 5 6 By 2050 the world’s population is expected to exceed 9.4 billion with a corresponding 7 1,2 8 global energy demand of 27.6 TW. Coupled with the alarming rise in atmospheric CO2 levels 9 10 over the last half-century due to fossil fuel consumption there is an urgent need for low cost, 11 12 13 sustainable, carbon-neutral energy. Solar energy conversion is a promising strategy for supplying 14 15 the world’s projected energy demand in a sustainable manner. Indeed, the total solar energy 16 17 2,3 18 reaching the earth in one day could power the planet for one year. The conversion of solar 19 20 energy to electricity using dye sensitized solar cells (DSSCs) that employ organic chromophores 21 22 are promising alternatives to silicon-based solar cells.4-6 Yet, the device efficiencies of organic- 23 24 6-8 25 dye-based DSSCs have stalled at ~13%. Controlled surface aggregation of organic dye 26 27 sensitizers offers a promising approach to optimize the efficiency of DSSCs, since the formation 28 29 of molecular aggregates on the semiconductor film is known to impact light harvesting, electron 30 31 9-27 32 transfer (ET) kinetics, and corresponding photocurrents. 33 34 Several organic dyes are known to form molecular aggregates on semiconductor films, 35 36 which can either impede21-24 or enhance9-20 DSSC performance. The formation of H-aggregates 37 38 39 (i.e., from head-to-head dipole interactions) results in the emergence of an absorption peak that is 40 41 hypsochromically shifted relative to that of the monomer.28-30 J-aggregates are characterized by a 42 43 44 head-to-tail dipole interaction and exhibit absorption peaks that are bathochromically shifted 45 46 relative to the monomer. Although the formation of organic dye aggregates on TiO2 is commonly 47 48 reported to lower photocurrents due to self-quenching,31-33 light attenuation,23 decreased excited- 49 50 34 35 51 state lifetime, and weak electronic coupling, aggregation has the potential to enhance light 52 15-19,32,36 32 53 harvesting via spectral broadening For example, J-dimers of RB on SnO2 and 54 55 merocyanine dyes on TiO 36 demonstrate enhanced light harvesting and electron injection as 56 2 57 58 59 60 3

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1 2 3 compared to the corresponding monomer-sensitized films, due to an increased absorption edge of 4 5 6 the aggregates. A series of chalcogenorhodamine dyes on TiO2 demonstrated enhanced electron- 7 8 injection yield and incident photon-to-current efficiency (IPCE) values, which was attributed to 9 10 the formation of surface H-aggregates.16-19 However, rhodamine derivatives that possess 11 12 13 substituted amines are known to undergo a photocatalyzed N-de-alkylation on TiO2 to form dyes 14 15 with absorption spectra that are hypsochromically shifted relative to the original 16 17 37,38 18 chromophore. Better understanding of the connections among dye structure, aggregation, 19 20 photodegradation, and photophysics are crucial to the development of organic-dye-based 21 22 materials for solar energy conversion. 23 24 25 Recently, we examined the photophysical properties of individual rhodamine sensitizers 26 39 27 on TiO2. In the course of these studies, we observed concentration-dependent modifications to 28 29 the diffuse reflectance spectra, consistent with molecular aggregation. Here, a systematic study 30 31 32 of rhodamine aggregates on TiO2 is performed using diffuse reflectance, steady-state 33 34 fluorescence, and time-correlated single-photon counting (TCSPC) measurements. A series of 35 36 rhodamine derivatives (Figure 1) with varying structures, adsorption affinities to TiO , and 37 2 38 39 potential for photoinduced N-de-alkylation were investigated: 5-carboxy-X-rhodmaine (5-ROX), 40 41 rhodamine 101 (R101), (RB), and rhodamine 560 (R560). For the rhodamine 42 43 44 derivatives that contain substituted amines (i.e., 5-ROX, R101, RB), spectral broadening and 45 46 hypsochromic shifting is observed on TiO2, consistent with photodegradation as well as 47 48 molecular aggregation. To circumvent the complication of N-de-alkylated photoproducts, the 49 50 51 impact of dye concentration and substrate (i.e., TiO2 and ZrO2) on molecular photophysics is 52 53 examined using R560, which contains only primary amines. At high dye-loading concentrations, 54 55 monomers and aggregates of R560 are present on TiO , with weakly fluorescent J-aggregates 56 2 57 58 59 60 4

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1 2 3 representing the most abundant population. The fluorescence intensity, energy, and lifetime of 4 5 6 these species are highly dependent on dye concentration, consistent with a self-quenching 7 8 mechanism. 9 10 11 12 13 EXPERIMENTAL 14 15 Materials and Sample Preparation 16 17 18 R560 (99+%, Exciton), R101 (99+%, inner salt, Exciton), RB (99+%, Acros Organics), 19 20 and 5-ROX (≥97%, Thermo Fisher Scientific) were used as obtained from the manufacturer. 21 22 Solutions of R560, R101, and RB were prepared in HPLC grade acetonitrile (99.8%, EMD 23 24 25 Millipore). 5-ROX solutions were prepared in ethyl alcohol (Pharmco-Aaper) due to limited 26 27 in acetonitrile. Thin films of mesoporous nanocrystalline titania (>99.5+%, P25, Acros 28 29 Organics) and zirconia (>99.5+%, 20 nm particle diameter, Sigma Aldrich) were prepared on 30 31 40 32 microscope slides (Fisherbrand) using the doctor blading technique. After doctor blading, thin 33 34 films were placed in a muffle furnace at 300°C for 1.5 to 2 h. Dyes were adsorbed onto TiO2 and 35 36 ZrO films by soaking the coated microscope slides in 15 mL of dye solution in a covered Petri 37 2 38 39 dish. The resulting dyed films were rinsed repeatedly with solvent to ensure removal of unbound 40 41 chromophores. After rinsing and drying, samples were stored in the dark. 42 43 44 45 46 Absorption and Fluorescence Measurements 47 48 Solution-phase UV-Vis and fluorescence measurements were obtained using a 49 50 51 PerkinElmer Lambda 35 and PerkinElmer LS-55 spectrometer, respectively. Diffuse reflectance 52 53 measurements of dyes on TiO2 and ZrO2 films were acquired using a Cary 60 spectrometer with 54 55 a fiber-optic coupler and diffuse reflectance probe. The Kubelka-Munk (K-M) function was 56 57 58 59 60 5

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1 2 3 applied to all diffuse reflectance spectra to correct for light scattering. Diffuse reflectance spectra 4 5 6 were fit to several Gaussian curves corresponding to each species (e.g., monomers, H-, and J- 7 8 aggregates) using OriginPro 9.1 (OriginLab). The full-width-at-half-maximum (FWHM), 9 10 amplitude, and position of the Gaussian functions were determined by using the Levenberg- 11 12 2 13 Marquardt algorithm to minimize the reduced χ statistic. To establish that an appropriate 14 15 number of Gaussian functions were used in fitting, the residuals were plotted in a histogram to 16 17 2 18 confirm that they were randomly distributed around zero with an R value close to unity. Steady- 19 20 state fluorescence measurements of R560/TiO2 and R560/ZrO2 films were acquired using an 21 22 excitation wavelength of 490 nm and 250 nm/minute scan speed. 23 24 25 26 27 Fluorescence Lifetime Studies 28 29 R560/TiO2 and R560/ZrO2 samples were prepared on cover glass (Fisherbrand) by doctor 30 31 41 32 blading and then placed atop an inverted confocal microscope (Nikon, TiU). Excitation was 33 34 provided by a 470-nm pulsed (PicoQuant PDL 800-D LDH) operating at a 10 MHz 35 36 repetition frequency. Laser excitation was sent through a 488-nm dichroic beam splitter 37 38 39 (Semrock, Di02-R488-25x36) and then focused to the sample by a 100x oil-immersed objective 40 41 (Nikon Plan Fluor, NA = 1.3). Excitation powers at the sample were adjusted between ~1 nW 42 43 44 and ~3 µW to prevent the “pile-up” effect, where early photons are over-represented. 45 46 Epifluorescence from the sample was collected through the objective, spectrally filtered 47 48 (Semrock, BLP01-488R-25), and focused onto an avalanche photodiode detector (APD) with a 49 50 51 50-µm aperture (MPD, PDM050CTB). Fluorescence decay curves were collected using a 52 53 TCSPC module (PicoQuant, PicoHarp 300) at ten different locations for each sample to obtain 54 55 average values and associated error. 56 57 58 59 60 6

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1 2 3 Fluorescence dynamics were fit using the nonlinear least-squares reconvolution of the 4 5 6 instrument response function (IRF, full width at half maximum (FWHM) ~130 ps) with multi- 7 8 exponential and stretched exponential functions. The least-squares method was conducted per the 9 10 Marquardt-Levenberg algorithm with the global fit option (PicoQuant, FluoFit V. 4.6.6). Good 11 12 2 13 fit criteria were described by a χ ~ 1, and a random distribution of weighted residuals around 14 15 zero. A sub-IRF lifetime component was present in each of the fluorescence decays, which was 16 17 42 18 attributed to an artifact of the fitting procedure. 19 20 21 22 RESULTS & DISCUSSION 23 24 25 Aggregation and Photodegradation of 5-ROX, R101, and RB on TiO2 26 27 To examine the adsorption of dyes to TiO2, thin films of TiO2-on-glass were immersed in 28 29 dye solutions for 18 hours and then thoroughly rinsed with solvent to remove any unbound 30 31 32 chromophores. Dye solutions were prepared in acetonitrile and at sub-millimolar 33 34 concentrations to avoid the formation of aggregates in solution prior to film sensitization.43,44 35 36 Figure 1A-1C presents the resulting diffuse reflectance spectra of 5-ROX, R101, and RB on TiO 37 2 38 39 along with the corresponding solution-phase spectra of the dyes. The UV-vis spectra of 5-ROX, 40 41 R101, and RB in solution exhibit absorption maxima ((3) at 578, 560, and 555 nm, with 42 43 -1 44 corresponding FWHM values of 1079, 1149, and 1072 cm , respectively. The diffuse reflectance 45 46 spectra of the dye/TiO2 films are markedly different relative to their solution-phase spectra and 47 48 are highly dependent on dye-loading concentration (Table 1). For example, Figure 1A shows that 49 50 -4 51 5-ROX/TiO2 films prepared from 10 M dye loading demonstrate a broad, hypsochromically- 52 -1 53 shifted peak (i.e., (3 = 560 nm, FWHM = 3149 cm on TiO2) relative to solution (i.e., (3 = 54 55 56 57 58 59 60 7

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1 2 3 -1 578 nm, FWHM = 1079 cm in ethanol). The diffuse reflectance spectra of R101/TiO2 and 4 5 6 RB/TiO2 are also broadened and blue shifted as compared to the spectra obtained in solution. 7 8 As dye-loading concentration is increased from 5x10-6 M to 10-4 M, the diffuse 9 10 reflectance spectra exhibit bathochromic shifts and broadening. For example, 5-ROX/TiO films 11 2 12 -6 13 prepared by soaking in a 5x10 M dye solution exhibit an (3 at 528 nm with a corresponding 14 15 -1 -4 FWHM of 2923 cm . As 5-ROX loading concentration increased to 10 M, the (3 is red 16 17 -1 18 shifted to 560 nm and broadened to a FWHM of 3149 cm . Similar results are observed for 19 20 dye/TiO2 films prepared by soaking thin films of TiO2-on-glass in dye solutions for increasing 21 22 periods of time (Figure S1, Supporting Information). To examine the stability of the dye/TiO2 23 24 45 25 films, each film was submerged in water to intentionally desorb the dyes. Only 5-ROX/TiO2 26 27 films demonstrated persistent coloration following water exposure, consistent with the fact that 28 29 5-ROX possesses an additional carboxylate linkage at the para-position on the xanthylium 30 31 39 32 backbone for potential binding to TiO2, as compared to R101 and RB. 33 34 Previous studies of rhodamine dyes on silica46 and laponite clay47 observed 35 36 37 hypsochromic shifts in (3 (i.e., of up to ~25 nm) upon surface adsorption, consistent with the 38 39 formation of H-aggregates as well as modifications to the local dielectric environment. However, 40 41 in the present study, substantial hypsochromic shifts of ~35-50 nm are observed for dye/TiO2 42 43 44 films relative to solution. Detty and Watson have attributed the significant blue-shifting and 45 16-19 46 broadening observed for chalcogenorhodamine dyes on TiO2 to H-aggregation. Another 47 48 possibility is that the photocatalytic degradation of rhodamine dyes on TiO contributes to the 49 2 50 5,6 37 51 observed spectral changes. For example, previous studies have shown that RB and 52 38 53 sulforhodamine-B can undergo photocatalytic N-de-alkylation on TiO2 to form chromophores 54 55 56 with significantly blue-shifted absorption maxima (i.e., by ~50 nm). To explore the possibility 57 58 59 60 8

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1 2 3 that the substantial changes to  upon adsorption to TiO2 films are due to photocatalytic N- 4 (3 5 6 de-alkylation and not H-aggregation, we studied: 1) R560, the structural analog of RB that 7 8 contains only primary amines, and 2) RB on an insulating ZrO2 substrate, where photocatalysis is 9 10 not expected to occur. 11 12 13 Figure 1D shows the diffuse reflectance spectra of R560/TiO2 films relative to solution. 14 15 The absorption spectrum of R560 in acetonitrile exhibits a (3 at 500 nm, with a FWHM of 16 17 -1 -6 18 1201 cm . Corresponding diffuse reflectance spectra of R560/TiO2 films prepared from 5x10 19 20 M, 10-5 M, and 10-4 M dye-loading concentrations exhibit maxima at 503, 504, and 511 nm, with 21 22 FWHM values of 1822, 1934, and 2655 cm-1, respectively (Figure 1D). The reflectance spectra 23 24 25 of R560/TiO2 are red-shifted and broadened relative to solution and the magnitude of these 26 27 changes are modest in comparison to those observed for 5-ROX, R101, and RB (Figure 1A-1C). 28 29 To further investigate the possible photodegradation of 5-ROX, R101, and RB on TiO the 30 2, 31 32 diffuse reflectance spectra of dyed TiO2 films were measured after prolonged exposure to light 33 34 and water. The diffuse reflectance spectra of 5-ROX, R101, and RB on TiO2 exhibited 35 36 37 significant hypsochromic shifts after exposure to room lights for 1 h (Figure S1, Supporting 38 37,38 39 Information), consistent with photocatalyzed N-de-alkylation. In contrast, the diffuse 40 41 reflectance spectrum of R560/TiO2 is relatively unaltered following light exposure. 42 43 44 When RB/TiO2 films are submerged in water to intentionally desorb the dyes from TiO2, 45 46 the extracted solution exhibits absorption and fluorescence maxima at 497 and 520 nm, 47 48 respectively, consistent with the presence of R560 (Figure 2). On the other hand, the aqueous 49 50 51 extracts obtained from RB/ZrO2 films demonstrate absorption and fluorescence spectra that are 52 53 consistent with RB (i.e., with maxima at 554 and 578 nm, respectively) and not its de-alkylated 54 55 photoproduct. Furthermore, RB-sensitized ZrO films exhibit values that are relatively 56 2 (3 57 58 59 60 9

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1 2 3 unchanged relative to solution (Figure S2, Supporting Information). Altogether, these 4 5 6 observations support the interpretation that the significant hypsochromic shifts observed for 5- 7 8 ROX, R101, and RB upon surface adsorption to TiO2 are due to photocatalyzed N-de-alklyation 9 10 and not the formation of H-aggregates. Therefore, to probe rhodamine aggregates on TiO and 11 2 12 13 circumvent the complication of N-de-alkylated photoproducts, we focused on concentration- 14 15 dependent studies of R560/TiO2. 16 17 18 19 20 Diffuse Reflectance Spectroscopy of R560/TiO2 21 22 To study the formation of rhodamine aggregates on TiO2, we measured the 23 24 25 concentration-dependent diffuse reflectance spectra of R560/TiO2. Corresponding control 26 27 experiments are performed on ZrO2 films as a noninjecting substrate. The diffuse reflectance 28 29 spectra of R560/TiO2 films are bathochromically-shifted and broadened as R560 concentration is 30 31 32 increased (Figure 1D and Table 2), consistent with molecular aggregation. To estimate the 33 34 relative population of R560 monomers and aggregates on TiO2 as a function of dye 35 36 concentration, the diffuse reflectance spectra are deconvolved to several Gaussian curves 37 38 39 corresponding to each species (i.e., monomers, H-dimers, J-dimers, and higher-order 40 41 aggregates). Diffuse reflectance signal is observed for R560/TiO2 films prepared from dye- 42 43 -7 44 loading concentrations as low as 10 M. At this concentration, R560/TiO2 films exhibit a 45 46 reflectance spectrum that is only modestly broadened relative to solution (Table 2), consistent 47 48 with the presence of predominately dye monomers. Accordingly, peak-fitting analysis of the 49 50 -7 51 diffuse reflectance spectra of 10 M R560/TiO2 films provides an estimate of the spectral 52 -1 53 contribution due to monomers (i.e., at (3 = 502 nm, FWHM = 1470 cm with vibronic 54 55 shoulder at = 465 nm, FWHM = 1993 cm-1). 56 (3 57 58 59 60 10

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1 2 3 The diffuse reflectance spectra of R560/TiO2 films prepared using dye-loading 4 5 -7 -5 6 concentrations of between 10 M and 5×10 M are deconvolved in several Gaussian 7 8 subpopulations corresponding to the monomer, H-dimers (i.e., (3 = 473 ± 4 nm, FWHM = 9 10 -1 -1 11 1250 ± 20 cm ), and J-dimers (i.e., at (3 = 522 ± 3 nm, FWHM = 1100 ± 10 cm ), consistent 12 13 with previous studies of rhodamine aggregates in water, ethyl glycol, laponite clay, and silica 14 15 gels.25,31,46-49 For films made using the highest dye-loading concentration of 10-4 M R560, the 16 17 18 reflectance spectra are well modeled using two additional Gaussian subpopulations at (3 19 20 values of 446 ± 9 nm (FWHM = 1710 ± 70 cm-1) and 538 ± 1 nm (FWHM = 1281 ± 5 cm-1), 21 22 corresponding to higher-order H- and J-aggregates, respectively.30,47 Figure 3A shows a 23 24 25 representative spectral deconvolution for a R560/TiO2 film. The area under the curves of these 26 27 reflectance bands and the extinction coefficient ( Z ) of rhodamine (i.e., ͩͩͨ = 93,000, 28 ĠĔī 29 -1 -1 30 ͩͦͦ = 39,500, and ͩͪͬ = 9,600 M cm for the monomer, H-dimer, and J-dimer, respectively, 31 50 32 of RB in H2O) were used to approximate the relative population of monomers, H-aggregates, 33 34 and J-aggregates as a function of dye-loading concentration (Figure 3B). The data show that 35 36 37 multiple forms of R560 are present on TiO2, with J-aggregates representing the most abundant 38 39 -6 population for films prepared using >10 M dye. For example, R560/TiO2 films prepared using 40 41 -4 42 10 M dye contain approximately 22% monomers, 9% H-aggregates, and 69% J-aggregates. The 43 44 co-existence of multiple forms of R560 on TiO2 is consistent with the micro- and nano-scale 45 46 heterogeneity of the physiochemical properties of the substrate.43,47,51 The observation that J- 47 48 49 aggregates are preferentially adsorbed onto TiO2 is consistent with previous studies of rhodamine 50 46 36 51 6G on mesoporous silica and merocyanine dyes on TiO2 films. 52 53 54 55 56 57 58 59 60 11

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1 2 3 Fluorescence of R560 on TiO2 and ZrO2 Films 4 5 6 To gain insight to the effects of aggregation on R560 photophysics, we measured the 7 8 steady-state fluorescence spectra and fluorescence lifetimes of R560 as a function of dye-loading 9 10 concentration and substrate. Figure 4A presents the fluorescence spectra of R560/TiO films at 11 2 12 13 various dye-loading concentrations. The fluorescence spectrum of R560 in acetonitrile is 14 15 characterized by an emission maximum ( ) at 519 nm. At the lowest dye-loading 16 17 -7 18 concentration of 10 M, where films of R560/TiO2 contain predominately monomers, a single 19 20 fluorescence peak is observed at 529 nm, close to the  of R560 in solution. Fluorescence 21 22 intensity is increased as dye-loading concentration goes from 10-7 M to 10-6 M, consistent with 23 24 25 the addition of predominately R560 monomers to the film (i.e., based on the relative populations 26 27 shown in Figure 3B). However, further increases in dye content result in bathochromic shifting 28 29 and fluorescence quenching. The observation of weak, bathochromically-shifted emission from 30 31 32 R560/TiO2 films at high dye-loading concentrations is consistent with the formation of J- 33 34 aggregates,30,52 which has been observed for in ethyl glycol,31 laponite clay,47 silica 35 36 46,53 27 37 gels, and TiO2. 38 39 The fluorescence lifetimes of R560 on TiO2 as a function of dye-loading concentration 40 41 were examined using TCSPC measurements. Figure 4B presents the fluorescence decays of 42 43 -7 -6 -5 44 R560/TiO2 films prepared using 10 M, 10 M, and 10 M dye along with the emission decay 45 46 of R560 in solution. Consistent with previous work,54 the fluorescence decay of R560 in 47 48 acetonitrile exhibits an exponential dependence on time with a lifetime () of 3.36 ± 0.04 ns. 49 50 51 Corresponding intensity decays for R560/TiO2 films are best fit to the sum of two exponential 52 53 functions given by: 54 55 / / ͯ Ɵc ͯ Ɵc 56 ̓ʚͨʛ = ̻͙ͥ u + ̻͙ͦ v 57 58 59 60 12

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1 2 3 where ̻ and  are the amplitude and lifetime, respectively, of the ith exponential decay. 4 $ $ 5 6 Attempts to fit the data to stretched exponential functions required a minimum of six free fitting 7 8 parameters and resulted in poor χ2 values (i.e., > 1.2) once the dye-loading concentration 9 10 exceeded 10-7 M. Table S1 in the Supporting Information presents the best-fit parameters for the 11 12 13 lifetime measurements of R560 on TiO2 and ZrO2. The time-averaged lifetime (〈〉) is 14 15 ∑  cv determined from the fit parameters of each decay according to: 〈〉 = Ĝ Ĝ . Table 2 presents the 16 ∑  c 17 Ĝ Ĝ 18 19 〈〉 values for R560/TiO2 as a function of dye-loading concentration. The lifetime of R560 is 20 21 shorter on TiO2 relative to solution and is also dependent on dye concentration. For example, 〈〉 22 23 is decreased from 1.5 ± 0.1 ns to 1.34 ± 0.04 ns when the dye-loading concentration is increased 24 25 -7 -5 -5 26 from 10 M to 10 M, respectively. For dyed films prepared using >10 M R560, the 27 28 fluorescence decays are instrument-response limited (i.e., ≤130 ps). 29 30 31 To examine the extent to which photoinduced electron injection contributes to the 32 33 observed fluorescence quenching of R560/TiO2 films, we examined the concentration-dependent 34 35 fluorescence spectra and lifetimes of R560 on insulating ZrO2 substrates. Figure 5A presents the 36 37 38 fluorescence spectra of R560/ZrO2 films at various dye-loading concentrations. Similar to the 39 40 observations for R560/TiO2, the fluorescence spectra of R560 on ZrO2 demonstrate substantial 41 42 red shifting and quenching as dye concentration is increased. Figure 5B shows that the 43 44 -7 45 fluorescence decay of R560/ZrO2 prepared using 10 M dye-loading concentration is close to 46 47 that observed in solution (i.e., 〈〉 is 3.36 ± 0.04 ns and 3.5 ± 0.1 ns for acetonitrile and ZrO2, 48 49 50 respectively). Values for 〈〉 are decreased from 3.5 ± 0.1 ns to 2.1 ± 0.4 ns as dye-loading 51 -7 -5 52 concentration is increased from 10 M to 5×10 M, respectively (Table S1, Supporting 53 54 Information). As expected, fluorescence is more intense and with longer 〈〉 values for R560 on 55 56 57 58 59 60 13

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1 2 3 ZrO2 relative to TiO2, consistent with a reduction in non-radiative deactivation via electron 4 5 6 transfer on the insulating substrate. 7 8 Previous studies have shown that the injection quantum yield can be quantified by 9 10 integrating the emission decays over time for TiO as compared to ZrO , where the smaller 11 2 2 12 56,57 13 integrated area for TiO2 relative to ZrO2 is attributed to electron injection. Using this 14 15 approach, the injection quantum yield of R560/TiO2 films containing predominately monomers 16 17 -7 18 (i.e., prepared using 10 M dye) was determined to be 0.63. Corresponding analysis of 19 -6 20 R560/TiO2 films prepared using >10 M dye exhibited injection yields as high as 0.80, 21 22 indicating that the formation of aggregates enhances injection to TiO2. However, the 23 24 25 concentration-dependent reflectance spectra of R560/ZrO2 indicate that different quantities of H- 26 27 and J-aggregates are present on ZrO2 relative to TiO2 (Figure S2). Therefore, the calculated 28 29 injection yields for heavily-dyed R560/TiO2 films may be overestimates, since the self- 30 31 32 quenching processes for R560 on TiO2 and ZrO2 are probably not equivalent. 33 34 35 36 Origins of Fluorescence Quenching 37 38 39 Diffuse reflectance measurements of R560/TiO2 revealed the formation of both H-type 40 41 and J-type aggregates on TiO2. However, H-aggregates are nonfluorescent (e.g., the quantum 42 43 -4 31 44 yield of rhodamine 6G H-aggregates in water is ~10 ) and are therefore not expected to 45 29,30,58 46 contribute to the observed emission. Rhodamine J-aggregates are weak emitters that exhibit 47 48 lower quantum yields and bathochromic emission maxima relative to their monomer form.44,47,59 49 50 51 Emissive contributions from a charge-transfer complex on TiO2 are unlikely, since the 52 53 absorption and fluorescence spectra of R560 on TiO2 and ZrO2 are equally red-shifted and 54 55 broadened upon surface adsorption as well as with increased dye loading.60 Therefore, the 56 57 58 59 60 14

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1 2 3 fluorescence of R560/TiO2 is likely to originate from subpopulations of monomers and/or J- 4 5 6 aggregates of R560 that do not undergo efficient electron injection to TiO2. Furthermore, the 7 8 diffuse reflectance and fluorescence data support the interpretation that different species are 9 10 responsible for emission in different R560/TiO samples. At dye concentrations <10-6 M, 11 2 12 13 fluorescence at ~529 nm is mainly due to emissive R560 monomers. Fluorescence from these 14 15 monomers is quenched upon surface adsorption to TiO2, consistent with an increase in non- 16 17 -6 18 radiative decay through photoinduced electron transfer. At dye-loading concentrations ≥10 M, 19 20 emission from monomers at 529 nm is relatively modest and the fluorescence spectra are 21 22 dominated by weak, red-shifted emission from J-aggregates. 23 24 25 The observations that the fluorescence intensity, energy, and lifetime of R560 on TiO2 26 27 are highly dependent on dye concentration is consistent with a self-quenching mechanism, which 28 29 can occur by either electron or energy transfer. However, oxidative and reductive self-quenching 30 31 32 of the R560 singlet excited state are thermodynamically unfavorable (i.e., ∆̿° = −0.6 ͐, 33 34 assuming the redox potentials of R560 are similar to those for ).39,61 Therefore, 35 36 37 energy transfer represents the most plausible explanation for the observed concentration- 38 39 dependent quenching. Energy transfer can occur via an electron exchange or Coulombic (i.e., 40 41 dipole-dipole) interactions, with the latter mechanism favored for systems that possess a large 42 43 44 spectral overlap between donor emission and acceptor absorption. Several examples of 45 46 fluorescence quenching due to dipole-dipole resonance energy transfer (RET) interactions 47 48 between the monomers and aggregates of rhodamine dyes have been reported.31-33 For example, 49 50 31 32 51 previous studies of rhodamines in and on glass reported a Förster-type RET 52 53 mechanism from excited monomers to dimer acceptors. Long-range energy transfer from H- 54 55 56 57 58 59 60 15

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1 2 3 aggregates to monomers for rhodamine 6G in water62 and silicate dispersions63 has also been 4 5 6 described. 7 8 For R560 on TiO2 and ZrO2 films, significant spectral overlap exists between the 9 10 fluorescence of R560 monomers and the absorbance of the J-aggregates (Figure S3, Supporting 11 12 13 Information). Therefore, Förster-type RET from monomers to weakly fluorescent J-aggregates 14 15 represents a plausible explanation for the observations of concentration-dependent fluorescence 16 17 18 quenching as well as the emergence of a red-shifted emission peak at ~587 nm. However, the 19 -7 20 excitation spectra of R560/TiO2 films prepared using >10 M dye indicate that the species 21 22 responsible for the red-shifted emission absorbs strongly at approximately 470 nm, 510 nm, and 23 24 25 530 nm, corresponding to H-aggregates, monomers, and J-aggregates, respectively (Figure S3). 26 27 This observation suggests that energy transfer interactions between H-aggregates and monomers 28 29 as well as monomers and J-aggregates are operative. In addition, the relative decay times of 30 31 32 R560/TiO2 as a function of dye concentration are not well represented by an exponential function 33 34 as predicted by the exchange mechanism for energy transfer (data not shown),64 further 35 36 supporting the interpretation that energy transfer proceeds through a dipole-dipole RET 37 38 39 mechanism. Ultimately, however, due to the heterogeneity of R560/TiO2 films (i.e., with head- 40 41 to-head, head-to-tail, as well as herringbone, brickwork, staircase, ladder geometries 42 43 59,65 44 possible) and the complex contributions of both injection- and RET-induced quenching for 45 46 these species, further studies are necessary to fully understand the underlying quenching 47 48 mechanisms. 49 50 51 52 53 CONCLUSION 54 55 56 57 58 59 60 16

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1 2 3 Diffuse reflectance, steady-state fluorescence, and fluorescence lifetime measurements 4 5 6 were used to investigate the aggregation of a series of rhodamine dyes on TiO2. Rhodamine 7 8 derivatives containing substituted amines (i.e., 5-ROX, R101, and RB) exhibited an ~50 nm 9 10 hypsochromic shift in upon adsorption to TiO due to photoinduced N-de-alkylation. To 11 (3 2 12 13 circumvent this complication, we focused on aggregation studies of R560, a rhodamine 14 15 derivative containing primary amines. Diffuse reflectance measurements of R560/TiO2 16 17 18 demonstrated that multiple forms of the dye are present on TiO2, with J-aggregates representing 19 20 the most abundant population for heavily-dyed films. Fluorescence quenching due to electron 21 22 injection as well as resonance energy transfer interactions are operative on TiO2. The electron 23 24 25 injection yield of R560/TiO2 is increased with dye-loading concentration, indicating that 26 27 aggregate formation enhances electron injection, but further studies are necessary to directly 28 29 probe injection from the R560 aggregates.15,16 Ultimately, the formation of organic dye 30 31 32 aggregates on TiO2 is well known to impact photocurrents, with some studies reporting that 33 34 aggregate formation can increase both light harvesting and injection yields. The present study 35 36 demonstrates that contributions due to molecular photodegradation and energy transfer 37 38 39 interactions must be considered when pursuing the development of a controlled aggregation 40 41 strategy for enhanced light harvesting in organic-dye-based solar cells. 42 43 44 45 SUPPORTING INFORMATION 46 47 Diffuse reflectance spectra of RB, 5-ROX, R101, and R560 on TiO2 as a function of time and 48 49 50 light exposure (Figure S1); Diffuse reflectance spectra of RB and R560 on ZrO2 (Figure S2); 51 52 Fluorescence lifetime data for R560 on TiO2 and ZrO2 (Table S1); Spectral overlap and 53 54 fluorescence excitation spectra of R560/TiO (Figure S3). This information is available free of 55 2 56 57 charge via the Internet at http://pubs.acs.org. 58 59 60 17

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1 2 3 4 5 6 ACKNOWLEDGMENTS 7 8 This work was supported by the National Science Foundation (CHE-1664828) and the Research 9 10 Corporation (MI-CSSA #22491). We thank the NASA Virginia Space Grant Consortium for 11 12 13 support of J.A.T. through a Graduate Research Fellowship. We acknowledge William R. 14 15 McNamara for helpful discussions and access to the diffuse reflectance spectrophotometer. 16 17 18 19 20 All authors have given approval for the final version of the manuscript. 21 22 23 24 25 The authors declare no competing financial interest. 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 18

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 Figure 1. Normalized diffuse reflectance spectra of (A) 5-ROX, (B) R101, (C) RB, and (D) 42 43 -6 -5 44 R560 adsorbed onto TiO2 at varying dye-loading concentrations: (red) 5x10 M, (green) 10 M, 45 -4 46 (blue) 10 M. Reflectance measurements were converted using the Kubelka-Munk function. 47 48 Corresponding absorption spectra of ~10-5 M dye in solution are shown in dashed lines. 49 50 51 52 53 54 55 56 57 58 59 60 19

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1 2 3 4 5 λ FWHM sample max 6 (nm) (cm-1) 7 8 EtOH 578 1079 9 5x10-6 M 528 2923 10 5-ROX 11 10-5 M 541 3236 12 -4 13 10 M 560 3149 14 15 CH3CN 560 1149 16 5x10-6 M 522 3253 17 R101 -5 18 10 M 528 3289 19 10-4 M 555 2974 20 21 CH3CN 555 1072 22 -6 23 5x10 M 505 1926 RB 24 10-5 M 506 2233 25 26 10-4 M 514 3432 27 28 29 30 Table 1. Summary of absorption and diffuse reflectance data for 5-ROX, R101, and RB in 31 32 solution and on TiO2 at varying dye-loading concentrations. 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 20

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 Figure 2. Normalized (solid lines) absorption and (dashed lines) fluorescence spectra of RB and 41 42 R560 in acetonitrile are compared to the spectra of dyes desorbed from RB-sensitized TiO2 and 43 44 45 ZrO2 films using water. (A) Solution-phase spectra of (red) R560 ((3 = 500 ͢͡, !' = 46 47 519 ͢͡) and (blue) RB ((3 = 555 ͢͡, !' = 589 ͢͡). Corresponding absorption and 48 49 -5 50 fluorescence spectra of the aqueous solutions extracted from (B) 10 M RB/TiO2 ((3 = 51 52 -5 498 ͢͡, !' = 520 ͢͡) and (C) 10 M RB/ZrO2 ((3 = 554 ͢͡, !' = 578 ͢͡) 53 54 55 demonstrate N-de-alkylation of RB occurs on TiO2 but not ZrO2. 56 57 58 59 60 21

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 -5 32 Figure 3. (A) Diffuse reflectance spectrum of R560/TiO2 prepared using (green) 5x10 M R560 33 34 is fit to Gaussian curves corresponding to the (black) monomers, (blue) H-aggregates and (red) J- 35 36 37 aggregates. Reflectance measurements were converted using the Kubelka-Munk (K-M) function. 38 39 (B) The area under these curves is used to approximate the percent of the total population due to 40 41 (black) monomers, (blue) H-aggregates, and (red) J-aggregates on R560/TiO films at various 42 2 43 -7 44 dye-loading concentrations. R560/TiO2 films prepared using 10 M dye are approximated as 45 46 containing 100% monomer and 0% aggregates. Error bars correspond to the standard deviation 47 48 from the mean. 49 50 51 52 53 54 55 56 57 58 59 60 22

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1 2 3 4 5 6 R560 λmax FWHM λFL 7 -1 < τ > (ns) 8 Sample (nm) (cm ) (nm) 9 10 CH3CN 500 1201 519 3.36 ± 0.04 11 -7 12 10 M 502 1590 529 1.5 ± 0.1 13 -6 14 10 M 503 1981 529 1.6 ± 0.1 15 -6 16 5x10 M 503 1822 537 1.6 ± 0.1 17 10-5 M 504 1934 547 1.34 ± 0.04 18 19 5x10-5 M 503 2260 550 20 IRF limited 21 10-4 M 511 2654 586 22 23 24 25 Table 2. Summary of absorption, diffuse reflectance, and fluorescence data for R560 in 26 27 acetonitrile and on TiO2 at varying dye-loading concentrations. 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 23

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Figure 4. (A) Fluorescence spectra of R560/TiO2 at varying dye-loading concentrations: (red) 38 39 10-7 M, (orange) 10-6 M, (green) 5x10-6 M, (blue) 10-5 M, (purple) 5x10-5 M, and (cyan) 10-4 M. 40 41 42 (B) Fluorescence decays of R560 (black) in acetonitrile and on TiO2 at dye-loading 43 44 concentrations of (red) 10-7 M, (orange) 10-6 M, and (blue) 10-5 M. (gray) IRF has a FWHM of 45 46 ~130 ps. 47 48 49 50 51 52 53 54 55 56 57 58 59 60 24

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 Figure 5. (A) Normalized fluorescence spectra of R560/ZrO2 at several dye-loading 39 40 concentrations: (red) 10-7 M, (orange) 10-6 M, (green) 5x10-6 M, (blue) 10-5 M, (purple) 5x10-5 41 42 M, and (cyan) 10-4 M. (B) Fluorescence decays of R560 (black) in acetonitrile and on ZrO at 43 2 44 -7 -6 -5 45 dye-loading concentrations of (red) 10 M, (orange) 10 M, and (blue) 10 M. (gray) IRF with a 46 47 FWHM of ~130 ps. 48 49 50 51 52 53 54 55 56 57 58 59 60 25

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1 2 3 25. Vogel, R.; Meredith, P.; Harvey, M.; Rubinsztein-Dunlop, H. Absorption and Fluorescence 4 5 Spectroscopy of Rhodamine 6G in Titanium Dioxide Nanocomposites. Spectrochim. Acta Mol. 6 Biomol. Spectrosc. 2004, 60, 245-249. 7 8 26. Pelet, S.; Grätzel, M.; Moser, J. Femtosecond Dynamics of Interfacial and Intermolecular 9 Electron Transfer at Eosin-Sensitized Metal Oxide Nanoparticles. J. Phys. Chem. B 2003, 107, 10 3215-3224. 11 12 13 27. Lewkowicz, A.; Bojarski, P.; Synak, A.; Grobelna, B.; Akopova, I.; Gryczyński, I.; Kułak, L. 14 Concentration-Dependent Fluorescence Properties of Rhodamine 6G in Titanium Dioxide and 15 Silicon Dioxide Nanolayers. J. Phys. Chem. C 2012, 116, 12304-12311. 16 17 18 28. McRae, E. G.; Kasha, M. The Molecular Exciton Model. In Physical Processes in Radiation 19 Biology: Proceedings of an International Symposium Sponsored by the U.S. Atomic Energy 20 Commission and Held at the Kellogg Center for Continuing Education, Michigan State 21 University, on may 6 - 8, 1963; Augenstein, L., Mason, R., Rosenberg, B., Eds.; Academic Press: 22 New York, 1964, pp 23. 23 24 25 29. McRae, E. G.; Kasha, M. Enhancement of Phosphorescence Ability upon Aggregation of 26 Dye Molecules. J. Chem. Phys. 1958, 28, 721-722. 27 28 30. Kasha, M.; Rawls, H.; Ashraf El-Bayoumi, M. The Exciton Model in Molecular 29 Spectroscopy. Pure Appl. Chem. 1965, 11, 371-392. 30 31 32 31. Bojarski, P.; Matczuk, A.; Bojarski, C.; Kawski, A.; Kukliński, B.; Zurkowska, G.; Diehl, H. 33 Fluorescent Dimers of Rhodamine 6G in Concentrated Ethylene Glycol Solution. Chem. Phys. 34 1996, 210, 485-499. 35 36 37 32. Itoh, K.; Chiyokawa, Y.; Nakao, M.; Honda, K. Fluorescence Quenching Processes of 38 Rhodamine B on Oxide Semiconductors and Light-Harvesting Action of its Dimers. J. Am. 39 Chem. Soc. 1984, 106, 1620-1627. 40 41 33. Van der Auweraer, M.; Verschuere, B.; De Schryver, F. Absorption and Fluorescence 42 Properties of Rhodamine B Derivatives Forming Langmuir-Blodgett Films. Langmuir 1988, 4, 43 44 583-588. 45 46 34. Kay, A.; Graetzel, M. Artificial Photosynthesis. 1. Photosensitization of TiO2 Solar Cells 47 with Chlorophyll Derivatives and Related Natural Porphyrins. J. Phys. Chem. 1993, 97, 6272- 48 6277. 49 50 51 35. Sengupta, S.; Bromley, L.; Velarde, L. Aggregated States of Chalcogenorhodamine Dyes on 52 Nanocrystalline Titania Revealed by Doubly-Resonant Sum Frequency Spectroscopy. J. Phys. 53 Chem. B 2017, 121, 3424-3436. 54 55 56 57 58 59 60 28

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1 2 3 47. Arbeloa, F. L.; Martínez, V. M.; Arbeloa, T.; Arbeloa, I. L. Photoresponse and Anisotropy of 4 5 Rhodamine Dye Intercalated in Ordered Clay Layered Films. J. Photochem. Photobiol. C 2007, 6 8, 85-108. 7 8 48. Martínez Martínez, V.; López Arbeloa, F.; Bañuelos Prieto, J.; Arbeloa López, T.; López 9 Arbeloa, I. Characterization of Rhodamine 6G Aggregates Intercalated in Solid Thin Films of 10 Laponite Clay. 1. Absorption Spectroscopy. J. Phys. Chem. B 2004, 108, 20030-20037. 11 12 13 49. Selwyn, J. E.; Steinfeld, J. I. Aggregation of Equilibriums of Xanthene Dyes. J. Phys. Chem. 14 1972, 76, 762-774. 15 16 50. Rohatgi, K.; Singhal, G. Nature of Bonding in Dye Aggregates. J. Phys. Chem. 1966, 70, 17 18 1695-1701. 19 20 51. Martínez Martínez, V.; López Arbeloa, F.; Bañuelos Prieto, J.; López Arbeloa, I. 21 Characterization of Rhodamine 6G Aggregates Intercalated in Solid Thin Films of Laponite 22 Clay. 2 Fluorescence Spectroscopy. J. Phys. Chem. B 2005, 109, 7443-7450. 23 24 25 52. McRae, E. Molecular Vibrations in the Exciton Theory for Molecular Aggregates. I. General 26 Theory. Aust. J. Chem. 1961, 14, 329-343. 27 28 53. Anedda, A.; Carbonaro, C.; Corpino, R.; Ricci, P.; Grandi, S.; Mustarelli, P. Formation of 29 Fluorescent Aggregates in Rhodamine 6G Doped Silica Glasses. J. Non Cryst. Solids 2007, 353, 30 31 481-485. 32 33 54. Zhang, X.; Zhang, Y.; Liu, L. Fluorescence Lifetimes and Quantum Yields of Ten 34 Rhodamine Derivatives: Structural Effect on Emission Mechanism in Different Solvents. J 35 Lumin 2014, 145, 448-453. 36 37 38 55. Kemnitz, K.; Yoshihara, K. Entropy-Driven Dimerization of Xanthene Dyes in Nonpolar 39 Solution and Temperature-Dependent Fluorescence Decay of Dimers. J. Phys. Chem. 1991, 95, 40 6095-6104. 41 42 43 56. Koops, S. E.; Durrant, J. R. Transient Emission Studies of Electron Injection in Dye 44 Sensitised Solar Cells. Inorg. Chim. Acta 2008, 361, 663-670. 45 46 57. Koops, S. E.; Barnes, P. R.; O’regan, B. C.; Durrant, J. R. Kinetic Competition in a 47 Coumarin Dye-Sensitized Solar Cell: Injection and Recombination Limitations upon Device 48 Performance. J. Phys. Chem. C 2010, 114, 8054-8061. 49 50 51 58. Nasr, C.; Liu, D.; Hotchandani, S.; Kamat, P. V. Dye-Capped Semiconductor Nanoclusters. 52 Excited State and Photosensitization Aspects of Rhodamine 6G H-Aggregates Bound to SiO2 53 and SnO2 Colloids. J. Phys. Chem. 1996, 100, 11054-11061. 54 55 56 57 58 59 60 30

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1 2 3 59. Kaiser, T. E.; Wang, H.; Stepanenko, V.; Würthner, F. Supramolecular Construction of 4 5 Fluorescent J‐Aggregates Based on Hydrogen‐Bonded Perylene Dyes. Angew. Chem. Int. Ed. 6 2007, 46, 5541-5544. 7 8 60. Ramakrishna, G.; Ghosh, H. N. Emission from the Charge Transfer State of Xanthene Dye- 9 Sensitized TiO2 Nanoparticles: A New Approach to Determining Back Electron Transfer Rate 10 and Verifying the Marcus Inverted Regime. J. Phys. Chem. B 2001, 105, 7000-7008. 11 12 13 61. Park, S. M.; Bard, A. J. Electrogenerated Chemiluminescence: Part XXVII. ECL and 14 Electrochemical Studies of Selected Laser Dyes. J. Electroanal. Chem. Interfacial Electrochem. 15 1977, 77, 137-152. 16 17 18 62. Arbeloa, F. L.; Ojeda, P. R.; Arbeloa, I. L. Dimerization and Trimerization of Rhodamine 6G 19 in Aqueous Solution: Effect on the Fluorescence Quantum Yield. J. Chem. Soc. Faraday Trans. 20 1988, 84, 1903-1912. 21 22 63. Bujdák, J.; Iyi, N.; Sasai, R. Spectral Properties, Formation of Dye Molecular Aggregates, 23 24 and Reactions in Rhodamine 6G/Layered Silicate Dispersions. J. Phys. Chem. B 2004, 108, 25 4470-4477. 26 27 64. Maza, W. A.; Morris, A. J. Photophysical Characterization of a Ruthenium (II) Tris (2, 2′- 28 Bipyridine)-Doped Zirconium UiO-67 Metal–Organic Framework. J. Phys. Chem. C 2014, 118, 29 8803-8817. 30 31 32 65. Würthner, F.; Kaiser, T. E.; Saha‐Möller, C. R. J‐Aggregates: From Serendipitous Discovery 33 to Supramolecular Engineering of Functional Dye Materials. Angew. Chem. Int. Ed. 2011, 50, 34 3376-3410. 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 31

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47 48 90x131mm (300 x 300 DPI) 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment The Journal of Physical Chemistry Page 38 of 38

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26 82x43mm (300 x 300 DPI) 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment