Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Intermolecular Charge Separation in Aggregated Rhodamine Dyes Used in Solar Hydrogen Production Michael F. Mark,† Mark W. Kryman,‡ Michael R. Detty,‡ and David W. McCamant*,† †
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260, United States
‡
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ABSTRACT: Various modern solar light-harvesting systems, including those used in photovoltaics and solar fuel production, depend on efficient electron transfer from a surface-bound molecular dye to nanoscopic semiconductor particles. However, the productive electron transfer competes with a variety of other relaxation pathways for the dye, and the dominant pathway can change dramatically depending on its environment. A new sulfursubstituted thiorhodamine dye was synthesized having exceptional light-harvesting qualities for solar energy applications and for solar hydrogen production in particular. The dye was created with a thiophene spacer bearing a phosphonate-ester (1-Ester) or phosphonic-acid (1-Acid) allowing for excellent solubility in MeCN or the ability to functionalize metal oxide semiconductor nanoparticles such as TiO2. While 1-Ester is found to be fully monomeric in MeCN, 1-Acid readily forms H-aggregated dimers which, upon photoexcitation, undergo charge separation to an ion pair (IP) in 1.5 ps. For 1-Acid dimers, the stabilization of the IP causes an increase in lifetime to 270 ps compared to the 75 ps lifetime of the monomer. When 1-Acid is attached to TiO2, the inhomogeneous surface creates a distribution of chromophore packing structures where a range of transition dipole coupling environments is present such that both excimers and IPs can form. In a variety of solvent environments, ultrafast electron injection was found to occur in 2 ns, persistent formation of the charge-separated state following charge injection to TiO2 only accounts for ∼10% of the photoexcited population, with the dominant relaxation pathways being IP and excimer formation. IP and excimer formation lower the total energy of the aggregate below the conduction band edge of TiO2, deactivating the electron transfer process. The implications of IP and excimer formation in systems for solar light harvesting are discussed.
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dipole moments (TDMs) on neighboring molecules.10−12 Depending on the orientation, the TDM coupling splits the excited state into upper and lower exciton bands where Haggregates form an optically active upper state and a forbidden lower state. Conversely, J-aggregates have allowed lower energy states and forbidden upper states. Experimental results further supported these findings where H-aggregates typically exhibit no fluorescence, whereas J-aggregates are often bright. Recently, Hestand and Spano expanded the treatment of the coupling to show that the spectral shifts were influenced by vibronic coupling between neighboring molecules, creating an enhancement of different transitions within the vibrational progression.13,14 The degree of the coupling can then be assessed via the Huang-Rhys (S) parameter, and a comparison of the intensity of the 0−0/0−1 peaks in the monomer and aggregate absorption spectra.
INTRODUCTION Harnessing and storing sunlight for alternative energy has become interesting in the ongoing search for sustainable and clean energy sources.1 One possible way to achieve this is through the light-driven formation of chemical bonds, specifically, formation of hydrogen−hydrogen bonds in molecular H22 whose combustion byproducts include only water and heat. The first solar hydrogen production systems using a molecular sensitizer date back over 40 years and were composed of a sensitizer, molecular catalyst, sacrificial donor, and redox mediator.3 Since then, light-harvesting systems have evolved including diverse homogeneous molecular systems as well as heterogeneous mixtures of dyes attached to functionalized semiconductors,4−9 most notably TiO2. Upon binding to a semiconductor surface, changes in a dye’s absorption spectrum associated with aggregation are often observed creating either a hypsochromatic (H-aggregate) or a bathochromatic (J-aggregate) shift.10 This splitting was first presented by Kasha who described systems in which close molecular proximity induced coupling between transition © XXXX American Chemical Society
Received: March 30, 2018 Revised: June 26, 2018 Published: June 27, 2018 A
DOI: 10.1021/acs.jpcc.8b03045 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Scheme 1. Synthesis of Thiorhodamines 1-Ester and 1-Acid
The benefit of aggregation in light-harvesting systems has been debated in the recent literature. In some cases, aggregated systems have been shown to work as well as15 or better than monomeric systems16−18 via enhanced light-harvesting capabilities measured as an increase in incident photon to current efficiencies (IPCEs) in dye-sensitized solar cells (DSSCs). Mechanistically, this has been ascribed to the increased mobility of excitons and enhanced charge transfer19,20 or from increased energy transfer rates.20 Recently, we reported a series of chalcogen-substituted rhodamines, which readily form H-aggregates on the surface of TiO2 that were able to produce over 180 turn over numbers of hydrogen under white light irradiation in 24 h.21 This same series of dyes, under DSSC conditions, was also shown to produce increased IPCE values when aggregated on TiO2, possibly from the enhanced coupling to TiO2 due to coplanarity of the donor and bridge of the dye.16,17,22,23 On the contrary, aggregation has also been shown to accelerate access of excited-state decay pathways,24 where the excited state is often quenched,25−27 leading to the need for coadsorbents to achieve high IPCE values.28,29 Recently, excitedstate deactivation was found to be a consequence of TDM coupling between molecules in H-aggregates of methylene blue30,31 where aggregation decreases the singlet lifetime from 370 to ∼10 ps.32 Similarly, rhodamines have been reported to succumb to increased excited-state deactivation from exciton coupling resulting in fast (>50 ps) decay.33 Deactivation of the excited state occurs also by formation of unproductive lower energy states. As an example, several perylene dimers have been synthesized which show complex excited-state dynamics in which reactive pathways facilitate exciton delocalization, forming excimers34−37 and ion pairs (IPs).35,37 In the present study, we present a novel sulfur-substituted rhodamine, 1-X, which readily forms H-aggregates in solution and on the surface of TiO2. Ultrafast transient absorption (TA) spectroscopy with global analysis is employed to compare solutions of monomers and dimers with solid-state samples and to assign dynamics of each species in each sample. The ultrafast time resolution of the experiment (800 nm) was performed with a 780 nm long pass filter to prevent 2nd order diffractions from interfering with the measurement. Cross-correlations were measured via the optical Kerr effect by rotating the pump pulse polarization 45° from that of the probe and measured in a 1 mm glass slide.44 Values were found to be ∼70 fs for both 565 and 620 nm pump pulses. Solution samples were diluted to a maximum absorbance of ∼0.7−0.9 in a 2 mm fused-silica cuvette and were translated vertically at ∼2 mm/s while under irradiation by the laser. Thin films were prepared via submersion of TiO2 slides in a monomeric solution until the bound concentrations reached a maximum absorbance of ∼0.5−0.9, allowing adequate light to transverse the sample. Prior studies have established the strength of the phosphonate to TiO2 linkage.45,46 TA measurements were taken by submerging the dye-coated slides into either neat acetonitrile, acetonitrile with 0.5 M lithium perchlorate (LiClO4) or water (adjusted to pH 3 with HCl) in a 2 mm cuvette. Slide samples were translated vertically and horizontally continuously to minimize irradiating the same spot. When direct comparison was necessary between two samples or excitation wavelengths, the data were scaled to account for absorptance (1 − T) at the pump wavelength. The TA signal was collected using both the parallel and perpendicular pump to probe polarizations, with the isotropic (magic angle) signal calculated via
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RESULTS Absorption and Emission Spectroscopy. The absorption spectra of the dyes in MeCN and on TiO2 are shown in Figure 1. 1-Ester (phosphonate ester, blue shaded) shows an
Figure 1. Absorption spectra of 1-Ester monodispersed in solution (blue), 1-Acid aggregated in MeCN solution (black line), and 1-Acid aggregated on a thin film of TiO2 in MeCN (maroon shaded).
absorption λmax at 605 nm, the S0(v = 0) → S1(v = 0) transition, hereafter “0−0”, and higher energy electronic transitions at 384 nm and in the near-UV. λmax of 1-Acid in MeCN (phosphonic acid, black) appears at 594 nm, the 0−0 transition, while there is an enhancement of the 0−1 absorption appearing at 567 nm. Additional absorption peaks are found including a broad, higher energy absorption band at 368 nm and several more in the near-UV. The enhancement of the higher energy vibronic transition at 567 nm indicates formation of face-to-face stacking or H-type aggregation10,12 which has been previously noted for other rhodamines.17,21,22,48−50 Because higher-ordered species (dimers, trimers, etc.) appear at higher concentrations, the absorption spectrum of 1-Acid will reflect populations of both monomeric and multimeric species. Hence, to distinguish between these species, we will address them as either 1-Acid-Monomer or 1-Acid-Dimer. As seen in the emission and excitation scans in Figure S1, excitation of the aggregated solution sample at 565 nm results in a single emission band centered at 624 nm is indicative of monomer emission, while an excitation scan reveals a monomer-like excitation spectrum with λmax at 596 nm. The observation of only monomeric emission from the sample that contains both monomers and dimers indicates that the dimers are nonemissive, as is expected for H-aggregates.
(ΔA + 2ΔA⊥)
ΔA iso =
3
(1)
Transient anisotropy traces, ρ(t), were calculated via ρ (t ) =
(ΔA − ΔA⊥) (ΔA + 2ΔA⊥)
(2)
Wavelength-specific kinetic data were fit to a series of exponentials with varying amplitudes, convoluted with a Gaussian instrument response function. Global fitting was performed with the Glotaran software package.47 The time and wavelength dependence of the decay associated spectra [DAS, Ai(λ)] was determined by fitting the entire spectrum, I(t,λ), to a set number, n, of exponential functions in which the preexponential amplitudes (Ai) varied for each individual wavelength n
I (t , λ ) =
∑ Ai(λ) exp −(t /τi) i=1
(3)
In this manner, each DAS describes the spectral changes that occur across the entire spectrum with a specific time constant. C
DOI: 10.1021/acs.jpcc.8b03045 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C Our assignment of the aggregates to dimers rather than to other higher-ordered species is addressed in the TA section. Upon binding to a thin film of TiO2 in MeCN, 1-Acid exhibits an absorption spectrum similar to the solution-based aggregate as seen in the maroon-shaded spectrum in Figure 1. Here, λmax of the lower and higher energy absorption bands in the aggregate shifts to 593 and 566 nm, respectively, while the diffuse high-energy band shifts to 418 nm. The emission and excitation spectra of 1-Acid on TiO2 is shown in Figure S1 for various solvents. Excitation at 570 nm shows an emission band with λmax at 636 nm, and a broad tail that trails to the edge of the spectrum is indicative of excimer formation.51 Similarly, the excitation spectrum of 1-Acid on TiO2 presents an absorption spectrum where the 0−1/0−0 ratio of intensities is slightly increased compared to that of a monomer showing TDM coupling between neighboring dyes. Electrochemistry. Figure S2 and Table S1 display the cyclic voltammogram of 1-Ester which has a reversible reduction at −0.678 V and a quasi-reversible oxidation wave at 1.127 V (vs SCE). Spectroelectrochemistry (Figure 2)
Figure 3. (a) TA spectra of 1-Ester excited at 620 nm. (b) Single wavelength kinetic traces in MeCN. Early times are seen on a linear axis from −0.5 to 5 ps and a logarithmic scale from 6 to 1700 ps. (c) DAS spectra of 1-Ester.
feature appears to the blue from an excited-state absorption (ESA) with a maximum at 468 nm. Fitting of the TA data was accomplished via global analysis with the resulting kinetic traces and DAS shown in Figure 3b,c, respectively. The resulting time constants are presented in Table 1. Here, two ultrafast components are found with values of 164 fs (red) and 1.15 ps (blue) that exhibit a loss in the amplitude of the ESA at 468 nm and SE at 623 nm. The two
Figure 2. Spec-echem difference spectrum of the reduction (a) and oxidation (b) of 1-Ester. Oxidation and reduction were carried out at voltages greater than 1.20 V and less than −0.70 V, respectively.
Table 1. TA Time Constants for Dyes in Solution shows the differential absorption spectra of the reduced and the oxidized species of dye 1-Ester. At voltages more negative than the reduction potential, formation of the reduced dye is accompanied by a gradual loss of the signal seen across the visible portion of the spectrum, while new absorption features can be seen only in the near-UV. For the oxidized species, however, a new broad absorption appears spanning from 400 to 700 nm, with the exception of the negative signal representing the bleached chromophore. The signal grows in at the same rate as the bleach, which is consistent with the formation of the cationic species. Solution Dynamics. The TA spectrum of 1-Ester, when excited at 620 nm, is presented in Figure 3a. Here, the bleach maximum at 605 nm mirrors the ground-state absorption spectrum. Negative amplitude signal is seen to the red of the bleach from stimulated emission (SE) and a broad positive
1-Ester excitation wavelength (nm) excited state reorganization (ps) internal reorganization (ps) monomer lifetime (ps) IP formation (ps) IP lifetime (ps) rotational correlation time (ps)
1-AcidMonomera
1-Acid-Dimera
620
620
565
620
565
0.164, 1.15 12.5
0.218
0.295
0.218
0.295
11.8
11.1
76.1
70.6
79.1 1.69 274 134
1.51 276 144
67
a
1-Acid-Monomer and 1-Acid-Dimer data are listed separately but are extracted from the same data set after analysis of single TA experiments of 1-Acid in neat MeCN solution.
D
DOI: 10.1021/acs.jpcc.8b03045 J. Phys. Chem. C XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry C
loss of intensity of the lower energy bleach peak and at SE wavelengths. The spectrum finally evolves to exhibit a bleach spectrum where the peak at 595 nm is 92% of the intensity of the 565 nm peak. The entire spectrum decays to zero in ∼1 ns. TA anisotropy values were obtained from the bleach kinetics presented in Figure S3. For 1-Ester, a rotational correlation time (RCT) of 67 ps was found across the entire spectrum with an initial amplitude of 0.28 at most wavelengths. The RCT values for the aggregated 1-Acid sample were also obtained at 565 and 595 nm when exciting at 620 and 565 nm, respectively, to avoid pump scattering. This yielded an average value of 139 ps with an initial amplitude of just above 0.3. The doubling of the RCT for aggregated 1-Acid compared to monomeric 1-Ester indicates the presence of dimers rather than larger species. The lack of a 67 ps RCT in the 1-Acid samples, however, is likely due to excitation at 565 nm being dominating by the signal from dimers from their large absorption at that wavelength. Conversely, excitation at 620 nm would excite a large portion of the monomer; however, probing at 565 nm would still be sampling mostly dimers because of their large absorbance at that wavelength. Figure 5 shows the DAS from global fitting with corresponding time constants from the data presented in
time constants have different values but exhibit similar spectra, suggesting that there is a range of timescales over which excited-state reorganization takes place. The following 12.5 ps DAS (green) shows an increase in the amplitude over the bleach region and loss in amplitude over redder wavelengths. The decrease from 580 to 620 nm with simultaneous increase from 630 to 700 nm is consistent with the evolution of an ESA band that initially overlaps the GS absorption but then redshifts in 12.5 ps. A similar time constant of 14.7 ps has been previously reported for a series of oxygen substituted rhodamines and has been attributed to the rotation of the thienyl group toward a coplanar geometry with the accompanying formation of an intramolecular-charge-transfer state with electron density shifting to the thienyl group.21,50 However, control experiments performed on the same day as the 1-Ester experiments indicate that the positive feature in the 12.5 ps DAS is unique to the dyes in this study and is not observed in the dimethyl-amine parent compounds of prior studies. The final DAS decays, showing the singlet lifetime of 76.1 ps. In Figure 4a, we present the TA spectra of 1-Acid in MeCN after excitation with 620 nm light. Upon excitation, both the
Figure 4. Transient spectrum of 1-Acid aggregated in MeCN excited at 620 nm (a) and the kinetic traces for important wavelengths (b) fit from −1 to 5 ps linearly and from 6 to 1700 ps logarithmically.
Figure 5. TA DAS of 1-Acid in MeCN when excited at 620 nm. DAS with short time constants are presented in (a) and those associated with time constants >20 ps are in (b) for clarity.
0−0 and the enhanced 0−1 absorption bands are instantly bleached, showing that 620 nm excitation depletes both populations of monomers and dimers despite excitation being resonant only with the lower energy absorption band. The initial TA features appear to be like those of 1-Ester in solution. These features include a bleach mirroring the groundstate absorption, SE at longer wavelengths than the bleach, and a broad ESA from 425 to 540 nm which is centered at 460 nm. As seen in Figure 4b, the kinetic traces show an immediate loss in amplitude following photoexcitation (
