Probing the Aggregation and Photodegradation of Rhodamine Dyes

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Probing the Aggregation and Photodegradation of Rhodamine Dyes on TiO James P. Cassidy, Jenna A. Tan, and Kristin L. Wustholz J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04604 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 10, 2017

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The Journal of Physical Chemistry

Probing the Aggregation and Photodegradation of Rhodamine Dyes on TiO2 James P. Cassidy, Jenna A. Tan, Kristin L. Wustholz* The College of William and Mary, Department of Chemistry, 540 Landrum Drive, Williamsburg, VA 23185 * Author to whom correspondence should be addressed. Email: [email protected], Phone: Phone: (757) 221-2675, Fax: (757) 221-2715

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ABSTRACT The formation and photophysical properties of rhodamine derivatives adsorbed to TiO2 are investigated using diffuse reflectance spectroscopy, steady-state fluorescence, and timecorrelated single photon counting (TCSPC) measurements. Rhodamine derivatives containing substituted amines (i.e., 5-ROX, R101, RB) exhibit an ~50 nm hypsochromic shift in upon adsorption to TiO2 relative to solution. By examining a rhodamine derivative with primary amines (i.e., R560) as well as control experiments on insulating ZrO2 substrates, we demonstrate that photocatalyzed N-de-alkylation is largely responsible for the spectral changes observed upon surface adsorption to TiO2. For R560, which does not undergo N-de-alkylation, diffuse reflectance spectra show that mainly monomers and J-aggregates are present on TiO2. Comparative lifetime measurements for R560 on TiO2 and ZrO2 show that the injection yield for R560/TiO2 is increased with dye-loading concentration (i.e., from 0.63 for monomers to ~0.80 for heavily-doped films), indicating that the presence of aggregates enhances electron injection. The residual fluorescence of R560/TiO2 is attributed to subpopulations of monomers and weakly fluorescent J-aggregates of R560 that do not undergo efficient electron injection to TiO2. The fluorescence intensity, energy, and lifetime of R560 on TiO2 and insulating ZrO2 films are dependent on dye concentration, consistent with a resonance energy transfer quenching process. This study shows that contributions due to molecular photodegradation and energy transfer interactions must be considered when pursuing the development of a controlled aggregation strategy for solar energy conversion materials and devices.


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INTRODUCTION By 2050 the world’s population is expected to exceed 9.4 billion with a corresponding global energy demand of 27.6 TW.1,2 Coupled with the alarming rise in atmospheric CO2 levels over the last half-century due to fossil fuel consumption there is an urgent need for low cost, sustainable, carbon-neutral energy. Solar energy conversion is a promising strategy for supplying the world’s projected energy demand in a sustainable manner. Indeed, the total solar energy reaching the earth in one day could power the planet for one year.2,3 The conversion of solar energy to electricity using dye sensitized solar cells (DSSCs) that employ organic chromophores are promising alternatives to silicon-based solar cells.4-6 Yet, the device efficiencies of organicdye-based DSSCs have stalled at ~13%.6-8 Controlled surface aggregation of organic dye sensitizers offers a promising approach to optimize the efficiency of DSSCs, since the formation of molecular aggregates on the semiconductor film is known to impact light harvesting, electron transfer (ET) kinetics, and corresponding photocurrents.9-27 Several organic dyes are known to form molecular aggregates on semiconductor films, which can either impede21-24 or enhance9-20 DSSC performance. The formation of H-aggregates (i.e., from head-to-head dipole interactions) results in the emergence of an absorption peak that is hypsochromically shifted relative to that of the monomer.28-30 J-aggregates are characterized by a head-to-tail dipole interaction and exhibit absorption peaks that are bathochromically shifted relative to the monomer. Although the formation of organic dye aggregates on TiO2 is commonly reported to lower photocurrents due to self-quenching,31-33 light attenuation,23 decreased excitedstate lifetime,34 and weak electronic coupling,35 aggregation has the potential to enhance light harvesting via spectral broadening 15-19,32,36 For example, J-dimers of RB on SnO232 and merocyanine dyes on TiO236 demonstrate enhanced light harvesting and electron injection as



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compared to the corresponding monomer-sensitized films, due to an increased absorption edge of the aggregates. A series of chalcogenorhodamine dyes on TiO2 demonstrated enhanced electroninjection yield and incident photon-to-current efficiency (IPCE) values, which was attributed to the formation of surface H-aggregates.16-19 However, rhodamine derivatives that possess substituted amines are known to undergo a photocatalyzed N-de-alkylation on TiO2 to form dyes with absorption spectra that are hypsochromically shifted relative to the original chromophore.37,38 Better understanding of the connections among dye structure, aggregation, photodegradation, and photophysics are crucial to the development of organic-dye-based materials for solar energy conversion. Recently, we examined the photophysical properties of individual rhodamine sensitizers on TiO2.39 In the course of these studies, we observed concentration-dependent modifications to the diffuse reflectance spectra, consistent with molecular aggregation. Here, a systematic study of rhodamine aggregates on TiO2 is performed using diffuse reflectance, steady-state fluorescence, and time-correlated single-photon counting (TCSPC) measurements. A series of rhodamine derivatives (Figure 1) with varying structures, adsorption affinities to TiO2, and potential for photoinduced N-de-alkylation were investigated: 5-carboxy-X-rhodmaine (5-ROX), rhodamine 101 (R101), rhodamine B (RB), and rhodamine 560 (R560). For the rhodamine derivatives that contain substituted amines (i.e., 5-ROX, R101, RB), spectral broadening and hypsochromic shifting is observed on TiO2, consistent with photodegradation as well as molecular aggregation. To circumvent the complication of N-de-alkylated photoproducts, the impact of dye concentration and substrate (i.e., TiO2 and ZrO2) on molecular photophysics is examined using R560, which contains only primary amines. At high dye-loading concentrations, monomers and aggregates of R560 are present on TiO2, with weakly fluorescent J-aggregates


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representing the most abundant population. The fluorescence intensity, energy, and lifetime of these species are highly dependent on dye concentration, consistent with a self-quenching mechanism.

EXPERIMENTAL Materials and Sample Preparation R560 (99+%, Exciton), R101 (99+%, inner salt, Exciton), RB (99+%, Acros Organics), and 5-ROX (≥97%, Thermo Fisher Scientific) were used as obtained from the manufacturer. Solutions of R560, R101, and RB were prepared in HPLC grade acetonitrile (99.8%, EMD Millipore). 5-ROX solutions were prepared in ethyl alcohol (Pharmco-Aaper) due to limited solubility in acetonitrile. Thin films of mesoporous nanocrystalline titania (>99.5+%, P25, Acros Organics) and zirconia (>99.5+%, 20 nm particle diameter, Sigma Aldrich) were prepared on microscope slides (Fisherbrand) using the doctor blading technique.40 After doctor blading, thin films were placed in a muffle furnace at 300°C for 1.5 to 2 h. Dyes were adsorbed onto TiO2 and ZrO2 films by soaking the coated microscope slides in 15 mL of dye solution in a covered Petri dish. The resulting dyed films were rinsed repeatedly with solvent to ensure removal of unbound chromophores. After rinsing and drying, samples were stored in the dark.

Absorption and Fluorescence Measurements Solution-phase UV-Vis and fluorescence measurements were obtained using a PerkinElmer Lambda 35 and PerkinElmer LS-55 spectrometer, respectively. Diffuse reflectance measurements of dyes on TiO2 and ZrO2 films were acquired using a Cary 60 spectrometer with a fiber-optic coupler and diffuse reflectance probe. The Kubelka-Munk (K-M) function was



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applied to all diffuse reflectance spectra to correct for light scattering. Diffuse reflectance spectra were fit to several Gaussian curves corresponding to each species (e.g., monomers, H-, and Jaggregates) using OriginPro 9.1 (OriginLab). The full-width-at-half-maximum (FWHM), amplitude, and position of the Gaussian functions were determined by using the LevenbergMarquardt algorithm to minimize the reduced χ2 statistic. To establish that an appropriate number of Gaussian functions were used in fitting, the residuals were plotted in a histogram to confirm that they were randomly distributed around zero with an R2 value close to unity. Steadystate fluorescence measurements of R560/TiO2 and R560/ZrO2 films were acquired using an excitation wavelength of 490 nm and 250 nm/minute scan speed.

Fluorescence Lifetime Studies R560/TiO2 and R560/ZrO2 samples were prepared on cover glass (Fisherbrand) by doctor blading41 and then placed atop an inverted confocal microscope (Nikon, TiU). Excitation was provided by a 470-nm pulsed laser (PicoQuant PDL 800-D LDH) operating at a 10 MHz repetition frequency. Laser excitation was sent through a 488-nm dichroic beam splitter (Semrock, Di02-R488-25x36) and then focused to the sample by a 100x oil-immersed objective (Nikon Plan Fluor, NA = 1.3). Excitation powers at the sample were adjusted between ~1 nW and ~3 µW to prevent the “pile-up” effect, where early photons are over-represented. Epifluorescence from the sample was collected through the objective, spectrally filtered (Semrock, BLP01-488R-25), and focused onto an avalanche photodiode detector (APD) with a 50-µm aperture (MPD, PDM050CTB). Fluorescence decay curves were collected using a TCSPC module (PicoQuant, PicoHarp 300) at ten different locations for each sample to obtain average values and associated error.


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Fluorescence dynamics were fit using the nonlinear least-squares reconvolution of the instrument response function (IRF, full width at half maximum (FWHM) ~130 ps) with multiexponential and stretched exponential functions. The least-squares method was conducted per the Marquardt-Levenberg algorithm with the global fit option (PicoQuant, FluoFit V. 4.6.6). Good fit criteria were described by a χ2 ~ 1, and a random distribution of weighted residuals around zero. A sub-IRF lifetime component was present in each of the fluorescence decays, which was attributed to an artifact of the fitting procedure.42

RESULTS & DISCUSSION Aggregation and Photodegradation of 5-ROX, R101, and RB on TiO2 To examine the adsorption of dyes to TiO2, thin films of TiO2-on-glass were immersed in dye solutions for 18 hours and then thoroughly rinsed with solvent to remove any unbound chromophores. Dye solutions were prepared in acetonitrile and ethanol at sub-millimolar concentrations to avoid the formation of aggregates in solution prior to film sensitization.43,44 Figure 1A-1C presents the resulting diffuse reflectance spectra of 5-ROX, R101, and RB on TiO2 along with the corresponding solution-phase spectra of the dyes. The UV-vis spectra of 5-ROX, R101, and RB in solution exhibit absorption maxima ( ) at 578, 560, and 555 nm, with corresponding FWHM values of 1079, 1149, and 1072 cm-1, respectively. The diffuse reflectance spectra of the dye/TiO2 films are markedly different relative to their solution-phase spectra and are highly dependent on dye-loading concentration (Table 1). For example, Figure 1A shows that 5-ROX/TiO2 films prepared from 10-4 M dye loading demonstrate a broad, hypsochromicallyshifted peak (i.e., = 560 nm, FWHM = 3149 cm-1 on TiO2) relative to solution (i.e., =



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578 nm, FWHM = 1079 cm-1 in ethanol). The diffuse reflectance spectra of R101/TiO2 and RB/TiO2 are also broadened and blue shifted as compared to the spectra obtained in solution. As dye-loading concentration is increased from 5x10-6 M to 10-4 M, the diffuse reflectance spectra exhibit bathochromic shifts and broadening. For example, 5-ROX/TiO2 films prepared by soaking in a 5x10-6 M dye solution exhibit an at 528 nm with a corresponding FWHM of 2923 cm-1. As 5-ROX loading concentration increased to 10-4 M, the is red shifted to 560 nm and broadened to a FWHM of 3149 cm-1. Similar results are observed for dye/TiO2 films prepared by soaking thin films of TiO2-on-glass in dye solutions for increasing periods of time (Figure S1, Supporting Information). To examine the stability of the dye/TiO2 films, each film was submerged in water to intentionally desorb the dyes.45 Only 5-ROX/TiO2 films demonstrated persistent coloration following water exposure, consistent with the fact that 5-ROX possesses an additional carboxylate linkage at the para-position on the xanthylium backbone for potential binding to TiO2, as compared to R101 and RB.39 Previous studies of rhodamine dyes on silica46 and laponite clay47 observed hypsochromic shifts in (i.e., of up to ~25 nm) upon surface adsorption, consistent with the formation of H-aggregates as well as modifications to the local dielectric environment. However, in the present study, substantial hypsochromic shifts of ~35-50 nm are observed for dye/TiO2 films relative to solution. Detty and Watson have attributed the significant blue-shifting and broadening observed for chalcogenorhodamine dyes on TiO2 to H-aggregation.16-19 Another possibility is that the photocatalytic degradation of rhodamine dyes on TiO2 contributes to the observed spectral changes.5,6 For example, previous studies have shown that RB37 and sulforhodamine-B38 can undergo photocatalytic N-de-alkylation on TiO2 to form chromophores with significantly blue-shifted absorption maxima (i.e., by ~50 nm). To explore the possibility


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that the substantial changes to upon adsorption to TiO2 films are due to photocatalytic Nde-alkylation and not H-aggregation, we studied: 1) R560, the structural analog of RB that contains only primary amines, and 2) RB on an insulating ZrO2 substrate, where photocatalysis is not expected to occur. Figure 1D shows the diffuse reflectance spectra of R560/TiO2 films relative to solution. The absorption spectrum of R560 in acetonitrile exhibits a at 500 nm, with a FWHM of 1201 cm-1. Corresponding diffuse reflectance spectra of R560/TiO2 films prepared from 5x10-6 M, 10-5 M, and 10-4 M dye-loading concentrations exhibit maxima at 503, 504, and 511 nm, with FWHM values of 1822, 1934, and 2655 cm-1, respectively (Figure 1D). The reflectance spectra of R560/TiO2 are red-shifted and broadened relative to solution and the magnitude of these changes are modest in comparison to those observed for 5-ROX, R101, and RB (Figure 1A-1C). To further investigate the possible photodegradation of 5-ROX, R101, and RB on TiO2, the diffuse reflectance spectra of dyed TiO2 films were measured after prolonged exposure to light and water. The diffuse reflectance spectra of 5-ROX, R101, and RB on TiO2 exhibited significant hypsochromic shifts after exposure to room lights for 1 h (Figure S1, Supporting Information), consistent with photocatalyzed N-de-alkylation.37,38 In contrast, the diffuse reflectance spectrum of R560/TiO2 is relatively unaltered following light exposure. When RB/TiO2 films are submerged in water to intentionally desorb the dyes from TiO2, the extracted solution exhibits absorption and fluorescence maxima at 497 and 520 nm, respectively, consistent with the presence of R560 (Figure 2). On the other hand, the aqueous extracts obtained from RB/ZrO2 films demonstrate absorption and fluorescence spectra that are consistent with RB (i.e., with maxima at 554 and 578 nm, respectively) and not its de-alkylated photoproduct. Furthermore, RB-sensitized ZrO2 films exhibit values that are relatively



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unchanged relative to solution (Figure S2, Supporting Information). Altogether, these observations support the interpretation that the significant hypsochromic shifts observed for 5ROX, R101, and RB upon surface adsorption to TiO2 are due to photocatalyzed N-de-alklyation and not the formation of H-aggregates. Therefore, to probe rhodamine aggregates on TiO2 and circumvent the complication of N-de-alkylated photoproducts, we focused on concentrationdependent studies of R560/TiO2.

Diffuse Reflectance Spectroscopy of R560/TiO2 To study the formation of rhodamine aggregates on TiO2, we measured the concentration-dependent diffuse reflectance spectra of R560/TiO2. Corresponding control experiments are performed on ZrO2 films as a noninjecting substrate. The diffuse reflectance spectra of R560/TiO2 films are bathochromically-shifted and broadened as R560 concentration is increased (Figure 1D and Table 2), consistent with molecular aggregation. To estimate the relative population of R560 monomers and aggregates on TiO2 as a function of dye concentration, the diffuse reflectance spectra are deconvolved to several Gaussian curves corresponding to each species (i.e., monomers, H-dimers, J-dimers, and higher-order aggregates). Diffuse reflectance signal is observed for R560/TiO2 films prepared from dyeloading concentrations as low as 10-7 M. At this concentration, R560/TiO2 films exhibit a reflectance spectrum that is only modestly broadened relative to solution (Table 2), consistent with the presence of predominately dye monomers. Accordingly, peak-fitting analysis of the diffuse reflectance spectra of 10-7 M R560/TiO2 films provides an estimate of the spectral contribution due to monomers (i.e., at = 502 nm, FWHM = 1470 cm-1 with vibronic shoulder at = 465 nm, FWHM = 1993 cm-1).


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The diffuse reflectance spectra of R560/TiO2 films prepared using dye-loading concentrations of between 10-7 M and 5×10-5 M are deconvolved in several Gaussian subpopulations corresponding to the monomer, H-dimers (i.e., = 473 ± 4 nm, FWHM = 1250 ± 20 cm-1), and J-dimers (i.e., at = 522 ± 3 nm, FWHM = 1100 ± 10 cm-1), consistent with previous studies of rhodamine aggregates in water, ethyl glycol, laponite clay, and silica gels.25,31,46-49 For films made using the highest dye-loading concentration of 10-4 M R560, the reflectance spectra are well modeled using two additional Gaussian subpopulations at values of 446 ± 9 nm (FWHM = 1710 ± 70 cm-1) and 538 ± 1 nm (FWHM = 1281 ± 5 cm-1), corresponding to higher-order H- and J-aggregates, respectively.30,47 Figure 3A shows a representative spectral deconvolution for a R560/TiO2 film. The area under the curves of these reflectance bands and the extinction coefficient ( ) of rhodamine (i.e., = 93,000, = 39,500, and = 9,600 M-1 cm-1 for the monomer, H-dimer, and J-dimer, respectively, of RB in H2O)50 were used to approximate the relative population of monomers, H-aggregates, and J-aggregates as a function of dye-loading concentration (Figure 3B). The data show that multiple forms of R560 are present on TiO2, with J-aggregates representing the most abundant population for films prepared using >10-6 M dye. For example, R560/TiO2 films prepared using 10-4 M dye contain approximately 22% monomers, 9% H-aggregates, and 69% J-aggregates. The co-existence of multiple forms of R560 on TiO2 is consistent with the micro- and nano-scale heterogeneity of the physiochemical properties of the substrate.43,47,51 The observation that Jaggregates are preferentially adsorbed onto TiO2 is consistent with previous studies of rhodamine 6G on mesoporous silica46 and merocyanine dyes on TiO2 films.36



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Fluorescence of R560 on TiO2 and ZrO2 Films To gain insight to the effects of aggregation on R560 photophysics, we measured the steady-state fluorescence spectra and fluorescence lifetimes of R560 as a function of dye-loading concentration and substrate. Figure 4A presents the fluorescence spectra of R560/TiO2 films at various dye-loading concentrations. The fluorescence spectrum of R560 in acetonitrile is characterized by an emission maximum ( ) at 519 nm. At the lowest dye-loading concentration of 10-7 M, where films of R560/TiO2 contain predominately monomers, a single fluorescence peak is observed at 529 nm, close to the of R560 in solution. Fluorescence intensity is increased as dye-loading concentration goes from 10-7 M to 10-6 M, consistent with the addition of predominately R560 monomers to the film (i.e., based on the relative populations shown in Figure 3B). However, further increases in dye content result in bathochromic shifting and fluorescence quenching. The observation of weak, bathochromically-shifted emission from R560/TiO2 films at high dye-loading concentrations is consistent with the formation of Jaggregates,30,52 which has been observed for rhodamines in ethyl glycol,31 laponite clay,47 silica gels,46,53 and TiO2.27 The fluorescence lifetimes of R560 on TiO2 as a function of dye-loading concentration were examined using TCSPC measurements. Figure 4B presents the fluorescence decays of R560/TiO2 films prepared using 10-7 M, 10-6 M, and 10-5 M dye along with the emission decay of R560 in solution. Consistent with previous work,54 the fluorescence decay of R560 in acetonitrile exhibits an exponential dependence on time with a lifetime () of 3.36 ± 0.04 ns. Corresponding intensity decays for R560/TiO2 films are best fit to the sum of two exponential functions given by:

= ! + !#


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where $ and $ are the amplitude and lifetime, respectively, of the ith exponential decay. Attempts to fit the data to stretched exponential functions required a minimum of six free fitting parameters and resulted in poor χ2 values (i.e., > 1.2) once the dye-loading concentration exceeded 10-7 M. Table S1 in the Supporting Information presents the best-fit parameters for the lifetime measurements of R560 on TiO2 and ZrO2. The time-averaged lifetime (〈〉) is ∑ ( #

determined from the fit parameters of each decay according to: 〈〉 = ∑ () ) . Table 2 presents the ) )

〈〉 values for R560/TiO2 as a function of dye-loading concentration. The lifetime of R560 is shorter on TiO2 relative to solution and is also dependent on dye concentration. For example, 〈〉 is decreased from 1.5 ± 0.1 ns to 1.34 ± 0.04 ns when the dye-loading concentration is increased from 10-7 M to 10-5 M, respectively. For dyed films prepared using >10-5 M R560, the fluorescence decays are instrument-response limited (i.e., ≤130 ps). To examine the extent to which photoinduced electron injection contributes to the observed fluorescence quenching of R560/TiO2 films, we examined the concentration-dependent fluorescence spectra and lifetimes of R560 on insulating ZrO2 substrates. Figure 5A presents the fluorescence spectra of R560/ZrO2 films at various dye-loading concentrations. Similar to the observations for R560/TiO2, the fluorescence spectra of R560 on ZrO2 demonstrate substantial red shifting and quenching as dye concentration is increased. Figure 5B shows that the fluorescence decay of R560/ZrO2 prepared using 10-7 M dye-loading concentration is close to that observed in solution (i.e., 〈〉 is 3.36 ± 0.04 ns and 3.5 ± 0.1 ns for acetonitrile and ZrO2, respectively). Values for 〈〉 are decreased from 3.5 ± 0.1 ns to 2.1 ± 0.4 ns as dye-loading concentration is increased from 10-7 M to 5×10-5 M, respectively (Table S1, Supporting Information). As expected, fluorescence is more intense and with longer 〈〉 values for R560 on



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ZrO2 relative to TiO2, consistent with a reduction in non-radiative deactivation via electron transfer on the insulating substrate. Previous studies have shown that the injection quantum yield can be quantified by integrating the emission decays over time for TiO2 as compared to ZrO2, where the smaller integrated area for TiO2 relative to ZrO2 is attributed to electron injection.56,57 Using this approach, the injection quantum yield of R560/TiO2 films containing predominately monomers (i.e., prepared using 10-7 M dye) was determined to be 0.63. Corresponding analysis of R560/TiO2 films prepared using >10-6 M dye exhibited injection yields as high as 0.80, indicating that the formation of aggregates enhances injection to TiO2. However, the concentration-dependent reflectance spectra of R560/ZrO2 indicate that different quantities of Hand J-aggregates are present on ZrO2 relative to TiO2 (Figure S2). Therefore, the calculated injection yields for heavily-dyed R560/TiO2 films may be overestimates, since the selfquenching processes for R560 on TiO2 and ZrO2 are probably not equivalent.

Origins of Fluorescence Quenching Diffuse reflectance measurements of R560/TiO2 revealed the formation of both H-type and J-type aggregates on TiO2. However, H-aggregates are nonfluorescent (e.g., the quantum yield of rhodamine 6G H-aggregates in water is ~10-4)31 and are therefore not expected to contribute to the observed emission.29,30,58 Rhodamine J-aggregates are weak emitters that exhibit lower quantum yields and bathochromic emission maxima relative to their monomer form.44,47,59 Emissive contributions from a charge-transfer complex on TiO2 are unlikely, since the absorption and fluorescence spectra of R560 on TiO2 and ZrO2 are equally red-shifted and broadened upon surface adsorption as well as with increased dye loading.60 Therefore, the


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fluorescence of R560/TiO2 is likely to originate from subpopulations of monomers and/or Jaggregates of R560 that do not undergo efficient electron injection to TiO2. Furthermore, the diffuse reflectance and fluorescence data support the interpretation that different species are responsible for emission in different R560/TiO2 samples. At dye concentrations 10-7 M dye indicate that the species responsible for the red-shifted emission absorbs strongly at approximately 470 nm, 510 nm, and 530 nm, corresponding to H-aggregates, monomers, and J-aggregates, respectively (Figure S3). This observation suggests that energy transfer interactions between H-aggregates and monomers as well as monomers and J-aggregates are operative. In addition, the relative decay times of R560/TiO2 as a function of dye concentration are not well represented by an exponential function as predicted by the exchange mechanism for energy transfer (data not shown),64 further supporting the interpretation that energy transfer proceeds through a dipole-dipole RET mechanism. Ultimately, however, due to the heterogeneity of R560/TiO2 films (i.e., with headto-head, head-to-tail, as well as herringbone, brickwork, staircase, ladder geometries possible)59,65 and the complex contributions of both injection- and RET-induced quenching for these species, further studies are necessary to fully understand the underlying quenching mechanisms.

CONCLUSION


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Diffuse reflectance, steady-state fluorescence, and fluorescence lifetime measurements were used to investigate the aggregation of a series of rhodamine dyes on TiO2. Rhodamine derivatives containing substituted amines (i.e., 5-ROX, R101, and RB) exhibited an ~50 nm hypsochromic shift in upon adsorption to TiO2 due to photoinduced N-de-alkylation. To circumvent this complication, we focused on aggregation studies of R560, a rhodamine derivative containing primary amines. Diffuse reflectance measurements of R560/TiO2 demonstrated that multiple forms of the dye are present on TiO2, with J-aggregates representing the most abundant population for heavily-dyed films. Fluorescence quenching due to electron injection as well as resonance energy transfer interactions are operative on TiO2. The electron injection yield of R560/TiO2 is increased with dye-loading concentration, indicating that aggregate formation enhances electron injection, but further studies are necessary to directly probe injection from the R560 aggregates.15,16 Ultimately, the formation of organic dye aggregates on TiO2 is well known to impact photocurrents, with some studies reporting that aggregate formation can increase both light harvesting and injection yields. The present study demonstrates that contributions due to molecular photodegradation and energy transfer interactions must be considered when pursuing the development of a controlled aggregation strategy for enhanced light harvesting in organic-dye-based solar cells.

SUPPORTING INFORMATION Diffuse reflectance spectra of RB, 5-ROX, R101, and R560 on TiO2 as a function of time and light exposure (Figure S1); Diffuse reflectance spectra of RB and R560 on ZrO2 (Figure S2); Fluorescence lifetime data for R560 on TiO2 and ZrO2 (Table S1); Spectral overlap and fluorescence excitation spectra of R560/TiO2 (Figure S3). This information is available free of charge via the Internet at http://pubs.acs.org. 17 ACS Paragon Plus Environment


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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE-1664828) and the Research Corporation (MI-CSSA #22491). We thank the NASA Virginia Space Grant Consortium for support of J.A.T. through a Graduate Research Fellowship. We acknowledge William R. McNamara for helpful discussions and access to the diffuse reflectance spectrophotometer.

All authors have given approval for the final version of the manuscript.

The authors declare no competing financial interest.


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Figure 1. Normalized diffuse reflectance spectra of (A) 5-ROX, (B) R101, (C) RB, and (D) R560 adsorbed onto TiO2 at varying dye-loading concentrations: (red) 5x10-6 M, (green) 10-5 M, (blue) 10-4 M. Reflectance measurements were converted using the Kubelka-Munk function. Corresponding absorption spectra of ~10-5 M dye in solution are shown in dashed lines.



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sample

λmax (nm)

FWHM (cm-1)

EtOH

578

1079

5x10 M

528

2923

10-5 M

541

3236

10-4 M

560

3149

CH3CN

560

1149

5x10 M

522

3253

-5

10 M

528

3289

-4

10 M

555

2974

CH3CN

555

1072

5x10 M

505

1926

-5

10 M

506

2233

-4

514

3432

-6

5-ROX

-6

R101

-6

RB

10 M

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Table 1. Summary of absorption and diffuse reflectance data for 5-ROX, R101, and RB in solution and on TiO2 at varying dye-loading concentrations.


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Figure 2. Normalized (solid lines) absorption and (dashed lines) fluorescence spectra of RB and R560 in acetonitrile are compared to the spectra of dyes desorbed from RB-sensitized TiO2 and ZrO2 films using water. (A) Solution-phase spectra of (red) R560 ( = 500 56, 78 = 519 56) and (blue) RB ( = 555 56, 78 = 589 56). Corresponding absorption and fluorescence spectra of the aqueous solutions extracted from (B) 10-5 M RB/TiO2 ( = 498 56, 78 = 520 56) and (C) 10-5 M RB/ZrO2 ( = 554 56, 78 = 578 56) demonstrate N-de-alkylation of RB occurs on TiO2 but not ZrO2.



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Figure 3. (A) Diffuse reflectance spectrum of R560/TiO2 prepared using (green) 5x10-5 M R560 is fit to Gaussian curves corresponding to the (black) monomers, (blue) H-aggregates and (red) Jaggregates. Reflectance measurements were converted using the Kubelka-Munk (K-M) function. (B) The area under these curves is used to approximate the percent of the total population due to (black) monomers, (blue) H-aggregates, and (red) J-aggregates on R560/TiO2 films at various dye-loading concentrations. R560/TiO2 films prepared using 10-7 M dye are approximated as containing 100% monomer and 0% aggregates. Error bars correspond to the standard deviation from the mean.


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R560 Sample

λmax (nm)

FWHM (cm-1)

λFL (nm)

< τ > (ns)

CH3CN

500

1201

519

3.36 ± 0.04

10-7 M

502

1590

529

1.5 ± 0.1

10-6 M

503

1981

529

1.6 ± 0.1

5x10-6 M

503

1822

537

1.6 ± 0.1

10-5 M

504

1934

547

1.34 ± 0.04

5x10-5 M

503

2260

550

10-4 M

511

2654

586

IRF limited

Table 2. Summary of absorption, diffuse reflectance, and fluorescence data for R560 in acetonitrile and on TiO2 at varying dye-loading concentrations.



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Figure 4. (A) Fluorescence spectra of R560/TiO2 at varying dye-loading concentrations: (red) 10-7 M, (orange) 10-6 M, (green) 5x10-6 M, (blue) 10-5 M, (purple) 5x10-5 M, and (cyan) 10-4 M. (B) Fluorescence decays of R560 (black) in acetonitrile and on TiO2 at dye-loading concentrations of (red) 10-7 M, (orange) 10-6 M, and (blue) 10-5 M. (gray) IRF has a FWHM of ~130 ps.


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Figure 5. (A) Normalized fluorescence spectra of R560/ZrO2 at several dye-loading concentrations: (red) 10-7 M, (orange) 10-6 M, (green) 5x10-6 M, (blue) 10-5 M, (purple) 5x10-5 M, and (cyan) 10-4 M. (B) Fluorescence decays of R560 (black) in acetonitrile and on ZrO2 at dye-loading concentrations of (red) 10-7 M, (orange) 10-6 M, and (blue) 10-5 M. (gray) IRF with a FWHM of ~130 ps.



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TOC graphic.


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Figure 1 79x110mm (300 x 300 DPI)

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Probing the Aggregation and Photodegradation of Rhodamine Dyes

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