Quenching of Triplet State Formation by Electron Transfer for

J. Phys. Chem. C 2008, 112, 11973–11977

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Quenching of Triplet State Formation by Electron Transfer for Merocyanine/TiO2 Systems Martin O. Lenz and Josef Wachtveitl* Institute of Physical and Theoretical Chemistry, Institute for Biophysics, Johann Wolfgang Goethe-UniVersity Frankfurt, Max-Von-Laue Str. 7, 60438 Frankfurt/Main, Germany ReceiVed: NoVember 8, 2007; ReVised Manuscript ReceiVed: May 13, 2008

Specific control of photoinduced molecular reactions is desired for various reasons, not least to increase photostability by avoiding harmful, reactive intermediate states. In this paper we will present a way to specifically deactivate the excited-state of merocyanines by selectively addressing additional pathways. The photodynamics for different merocyanines in solution as well as coupled to semiconductor nanoparticles have primarily been investigated by transient absorption spectroscopy. For the free dye, two main fast deactivation pathways exist, one involving an isomerization around the central polymethine chain of the molecule, and the other involving the population of a triplet state. The coupling to a semiconductor nanoparticle provides a way to change photoinitiated dynamics. An ultrafast electron transfer from the dye to the TiO2 nanoparticle takes place, and the isomerization reaction is accelerated by a factor of 20. The population of the triplet state is quenched and does not take place at all. In contrast to the well-known function of TiO2 as photocatalyst, we report here a molecular mechanism that quenches possible degradation pathways and thus might increase the photostability of the adsorbate in a dye/semiconductor system. Introduction Undesired chemical reactions often impose serious problems in reactive molecular systems. This leads not only to a reduced yield of desired reaction products, but might also result in dangerous and destructive side products. Therefore, significant effort has to be applied to minimize side reactions and to maximize the sought final product. This problem arises not only in chemical processing, but also in nature. Therefore, for living systems pathways have to be “directed” to achieve the desired result. This is true not only for the “normal” situation, but especially if unusual events occur. In photosynthesis, for example, the organism has to adapt to varying conditions such as changing light intensity, since excessive amounts of light energy lead to the population of highly reactive triplet states. For the photosystems of various organisms, this protection mechanism is still subject of ongoing research due to its importance and possible applications in artificial light/energy converting systems. Consequently, the study of modulation and control of photoreactions such as triplet state formation is of great interest and might have significant impact on the further development of solar-light-conversion devices. Besides the standard silicon solar cells, a number of different implementations exist to convert light into electrical energy.1 Among these, the Gra¨tzel-cell has been shown to achieve efficiencies close to silicon solar cells2–4 while being significantly less expensive.5 Gra¨tzel cells consist of a dye absorbing light in the visible spectral range, combined with a semiconductor, mostly TiO2, which acts as an electron acceptor of the excited electron and facilitates ultrafast charge separation.5 To increase the efficiency of the dye-sensitized solar cell, a variety of different dyes and different semiconductors with diverse spectral characteristics as well as different chemical stability has been studied.6–17 * Corresponding author phone: +49 69 798 29351; fax +49 69 798 29709; e-mail: [email protected].

SCHEME 1: Chemical structure of the investigated merocyanines

The photophysics of merocyanines and the possible utilization in DSSC has been extensively studied.18–20 Merocyanines containing carboxylic groups are known to readily bind to semiconductor nanoparticles.9 These adsorbates are of considerable physical interest, because the special energetic situation allows for an effcient ultrafast electron injection from discrete energy levels into a continuum of acceptor states. Pure merocyanines are also used as photosensitizers for photodynamic therapy21 or in the in field of detoxification and purification of water.22 The family of merocyanine comprises a large number of different molecules, for some of which the primary photophysics has been investigated and reaction dynamics has been determined.23–25 They are known to undergo efficient photoisomerization upon excitation.26 Due to their central polymethine chain connecting a nitrogen and an oxygen atom (Scheme 1), isomerization can take place once the first excited singlet state is populated. Additionally, an excited triplet state can be populated, leading to more complex deactivation dynamics via distinct reaction pathways.23,24,27,28 So far, no extensive study has been reported that compares the ultrafast dynamics of pure and semiconductor coupled merocyanines. We performed ultrafast transient absorption spectroscopy on various coupled and uncoupled merocyanine dyes. The analysis of the primary dynamics allows us to determine the energetic situation. In particular, we will show that the coupling of the dyes to the semiconductor reduces the population of the energetically low-lying triplet state. Conse-

10.1021/jp7106928 CCC: $40.75  2008 American Chemical Society Published on Web 07/02/2008

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quently, the coupling of the dye to a semiconductor provides an efficient way to selectively direct chemical photoinduced reactions into specific reaction pathways. This enhancement or suppression of specific photoproducts can be considered as an example for “excited state engineering” and is related to “radiative decay engineering”, which is commonly used in biotechnology to tune, for example, fluorescence quantum yields of nanocrystal adsorbates.29 Material and Methods Measurements were performed on samples containing different merocyanine dyes (MC1 and MC2, structures given in Scheme 1) either uncoupled in methanol or coupled to TiO2 colloids in methanol. Typical concentrations used were 200 µM for MC and 10 g/L for TiO2 colloids. The nanoparticles were prepared as described by Moser et al.30 via hydrolysis of TiCl4 and further dialysis. The average particle diameter was about 10 nm as measured by dynamic light scattering. Merocyanines were used as provided. Solvents used for the spectroscopic experiments were spectroscopic grade and were degassed prior to use. Stationary absorption spectra were recorded with an Analytik Jena, Specord S100 spectrometer equipped with a 3 mm fused silica cuvette. Steady-state fluorescence data have been collected in a Varian Cary eclipse spectrophotometer using concentrations of 2 µM MCs. Transient absorption measurements were performed using a femtosecond visible pump white light probe setup as described elsewhere.31 In brief, pulses stemming from a Clark CPA 2001 (repetition rate: 1 kHz, central frequency: 775 nm, pulse width: 150 fs) were converted to 550 or 600 nm for exciting the MC1 and MC2 samples, respectively, employing a NOPA.32 Cross correlation widths have been determined to be below 40 fs. Typical excitation energies were about 240 nJ, and the spot size at the sample position was approximately 100 µM. Part of the amplifier pulses were used to probe the sample with spectrally broad CaF2-generated white light, covering a spectral range from 400 to 700 nm33 and focused down to a diameter of 50 µM in the sample. The white light continuum pulses were dispersed by two vis-spectrometers (signal and reference) and recorded with two 42-segment diode arrays resulting in a spectral resolution of 8 nm. Data acquisition was performed in a single pulse detection mode as balanced and referenced measurement, providing signal-to-noise ratios up to 104. Excitation and probe pulse were polarized parallel with respect to each other. To account for possible sample degradation and long-term drifts, the data were normalized. For this, the ratio between probe and reference signals for the nonexcited sample has been determined every third pulse.34 A continuous exchange of the sample between successive laser pulses was achieved using a combination of a flow cuvette (path length 500 µM) and a syringe pump. This prevents multiple excitation of the same molecules and accumulation of possible photoproducts within the excitation volume. Before further analysis, the data were corrected for group velocity dispersion as well as for coherent signals around delay time zero.35 Data were then analyzed, taking into account the instrument response function and optimizing n exponential decays using a Levenberg-Marquardt algorithm for all wavelengths simultaneously, and the amplitudes for each kinetic component are wavelength-dependent fitting parameters. The obtained fit amplitudes are similar to decay-associated spectra. Results Steady-state Characterization. The absorption spectra for both merocyanine dyes exhibit a similar shape as for standard

Figure 1. Stationary absorption and fluorescence spectra of pure MC2 and coupled to TiO2 in methanol. A minor shift of the main absorption maximum of ∼15 nm and a strongly quenched fluorescence can be seen.

merocyanine 540.36 The absorption for MC1 and MC2 in MeOH peaks at 520 and 560 nm, respectively. A small shoulder on the blue side of the 0-0 transition might be due to higher vibronic excitations. Upon mixing MC and TiO2 nanoparticles, the dye adsorbs onto the surface of the semiconductor via the carboxylic group.37 This chemical reaction can be seen by a red shift of the main absorption band of MC by 15 nm (Figure 1). Because the spectral shift upon adsorption is an indication for the coupling strength, this indicates a weak coupling. For strong coupling dyes such as alizarin, larger shifts of ∼70 nm have been reported.38 In general, the coupling strength for binding via carboxylic groups is significantly smaller than for other types of chemical bonds in dye/semiconductor systems.9,10 Merocyanine dyes are known to easily aggregate in solution due to strong van der Waals attractive forces. Consequently, the concentrations used here were chosen low enough to prevent significant aggregation. No spectral signatures of oligomerization, such as blue-shifted H-aggregates or red-shifted Jaggregates,39 could be detected. Therefore, the assumption of noninteracting dye molecules for the coupled and uncoupled case is justified. In the stationary emission spectra of bound and unbound dye, differences are even more obvious than in the absorption spectra. For the dye/semiconductor system fluorescence is strongly quenched (Figure 1), indicating an efficient coupling and the appearance of additional, nonradiative deactivation channels. Remaining fluorescence might be due to unbound MC, because shape and maximum coincide with features for the unbound dye. As has been shown before,39 the energetic situation for the coupled systems allows for an efficient electron transfer from the excited-state of the dye into the conduction band of the semiconductor, and electron transfer becomes a major deactivation pathway. According to Marcus theory, the electronic coupling strength determines the electron transfer (ET) rate between donor and acceptor. In consequence, the ET time should be significantly increased for molecules attached via a carboxylic group in comparison to stronger-coupled systems such as alizarin. Dynamics of Free MC. In Figure 2, transient spectra after photoexcitation at 600 nm as well as fit amplitudes of MC2 are given. Spectra for all recorded delay times are dominated by the strong ground-state bleach signal around 600 nm, which is partially compensated by the broad absorption of the excited state. A clear signature of the excited-state absorption is only visible as a positive contribution below 500 nm. Within a couple

Quenching of Triplet State Formation

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Figure 3. Transient spectra for free and TiO2-coupled MC1. The signature of the excited triplet state can clearly be seen for the free dye. The lines are drawn to guide the eye.

Figure 2. Transient spectra of MC2 after photoexcitation at 600 nm (top) at different delay times and fit amplitudes of the global fitting procedure (bottom).

of picoseconds, positive absorption develops superimposed on the negative contribution from the depopulated ground state. As has been worked out by Nu¨esch and co-workers,40 this spectral signature is characteristic for a twisted isomer that is most likely rotated along the central polymethine chain of the MC. Out of the three essential time constants, necessary to represent the data appropriately, the fit amplitude for τ2 ) 40 ps shows growing absorption around 575 nm and can therefore be ascribed to the isomerization reaction. The longest time constant of τ3 ) 2.7 ns is mostly necessary to describe a long-lasting photoproduct and can be treated as infinity for the investigated time range. The decay-associated spectra or fit amplitudes for the shortest time constant τ1 ) 2 ps shows spectral features that are characteristic for a population for an triplet state. For different MCs, triplet states energetically located below the first excitedstate have been reported.27 Preliminary time-dependent density functional theory (TDDFT) calculations for MC1 support this assignment, because a manifold of energetically lower-lying triplet states within the vicinity of the first excited-state are predicted. As was shown not only by Nu¨esch and co-workers,25,27,40 for different MC molecules a low-lying triplet state exhibits transient absorption around 700 nm that can be quenched by triplet oxygen. To substantiate our previous assignment, MC1 has been additionally studied by ultrafast-pump/probe experiments in the spectral region between 600 and 800 nm. Transient spectra (Figure 3) clearly show an additional absorption after 200 ps that is only partially decaying within the observation time of 1 ns. This supports the previous finding that the triplet state is populated within a few tens of picoseconds and remains populated until at least 1 ns. The photoinduced dynamics of the pure merocyanine after photoexcitation can be summarized as follows. By the absorption

of the incoming visible photon, the first excited, singlet state is populated. The deactivation can proceed via two distinct pathways, where the splitting either occurs at or shortly out of the Franck-Condon region. With a reaction time of 40 ps an isomerization reaction toward a twisted isomer takes place. In accordance with previous nanosecond-pump-probe measurements,39 these molecules are twisted around the central polymethine chain and reach a fully isomerized state within a few nanoseconds. The second pathway is comprised of no isomerized MC at all, but the fast formation of an excited triplet state. With a time constant of 2 ps, an ultrafast intersystem crossing occurs. As El Sayed showed in 1963,41 an intersystem crossing becomes probable, and therefore fast, if a spin flip is accompanied by a change in the overall molecular configuration. Furthermore, high spin-orbit-coupling, as in the case for sulfur, facilitates fast ISC. Because molecular orbitals partially located close to the sulfur nuclei in the MC are involved in the HOMO-LUMO transition of merocyanines, fast ISC becomes reasonable. Furthermore, the preliminary TDDFT calculations mentioned before showed that the low lying triplet states exhibit nπ* character in contrast to a ππ* character of the LUMO. Taking into account all of the above mentioned arguments, the surprisingly fast ISC on the order of 2 ps can be well-explained and becomes understandable. Dynamics of MC/TiO2. Transient spectra to illustrate the photoinduced dynamics of the merocyanine/TiO2 system are shown in Figure 4. In addition to the ground-state bleach and the excited-state absorption observed as for the free dye, the transient spectra show an evolution of the excited-state absorption (symmetric absorption feature, maximum around 470 nm, clearly visible from 1 ps on) to the absorption of the merocyanine cation (asymmetric, maximum around 525 nm, clearly visible at 20 ps). The asymmetric absorption as observed here is typical for the merocyanine cation and has been reported before.20 The transition from an excited but still neutral dye/TiO2 system toward a charge-separated state occurs on timescales from femtoseconds to picoseconds. As can be seen in the absorbance changes for the single wavelength at 523 nm (Figure 4, bottom), the spectral signature of the cation is not only emerging on the time scale of a few picoseconds, but also on the order of a few tens of femtoseconds. To model the data and describe all essential features, five exponential functions, respectively, and five time constants are necessary. The associated fit amplitudes are given in Figure 5. The two fastest ones (τ1 ) 80 fs, τ2 ) 1.6 ps) describe the onset of the cation absorption and thus describe the electron

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Figure 6. Model for the primary reaction of the coupled merocyanine semiconductor system. After photoexcitation, two distinct reaction pathways lead to charge separated states.

the long-lived photoproduct (τ5 ) 2.7 ns), respectively. Undoubtedly, the spectral signature of the triplet state can not be seen for the coupled system (see also Figure 3). Discussion

Figure 4. Transient absorbance changes for TiO2-coupled MC2 after photoexcitation for different delay times (top) and a single transient at 523 nm (bottom).

Figure 5. Fit amplitudes of the global fitting procedure for the MC2/ TiO2 system.

injection from the merocyanine to the conduction band of the semiconductor. Because of a small extinction coefficient for the electron in the conduction band of the TiO2 nanoparticle in conjunction with spectrally unstructured features, the absorption can not be seen in the long wavelength part of the spectrum. Spectral similarities between the fit amplitude of τ2 of the pure dye and mostly τ2 of the coupled dye imply that the occurring processes are related. Although clearly dominated by ground-state bleach and electron injection, the DAS of τ1 might also contain spectral features that could be attributed to an isomerization. It can therefore be concluded that isomerization takes place mostly within 2 ps for the coupled dye as well. Although the fit amplitude of time constant τ3 ) 11 ps exhibits a mixture of the spectral features of the shorter and longer time constants, the two longest time constants can clearly be ascribed to the recombination of electron and cation (τ4 ) 163 ps) and

Reaction dynamics for the pure dye and the dye/semiconductor system are clearly different. Therefore, a different reaction model has to be developed for the coupled system, and more reaction pathways have to be taken into account to understand the complex deactivation dynamics. As obvious by the spectral signature of the model functions, independent reaction paths exist. A model that is able to explain all experimental observation is given in Figure 6. The sketched potential energy surfaces represent combined surfaces for the dye-semiconductor system as a function of two principal reaction coordinates, indicating the isomerization and the electron transfer pathway of the MC. After photoexcitation into the Franck-Condon region, two deactivation pathways exist, which differ from the pathways of the free dye. The bifurcation into these two pathways appears immediately either in the FC region directly or in close proximity. With a time constant of 80 fs, which is comparable to other dye/semiconductor systems, an electron transfer from the localized excited-state of MC into the conduction band of TiO2 takes place, leading to the charge-separated state that can be identified by the characteristic absorption of the MC cation. The electron back reaction can be seen to occur within the observed time window on the order of 160 ps, indicated by the decay of the spectral features of the cation. However, because the cation has also been observed on longer time scales (>1000 ns), this is not the only deactivation channel of the charge-separated state. A further bifurcation must occur, and other recombination channels exist. The second, independent deactivation channel involves an isomerization of the bound merocyanine on the order of 2 ps. In comparison to the uncoupled dye, this represents an acceleration of the reaction time by a factor of 20. Coupling via the carboxylic group to the semiconductor might change the molecular orbitals in such a way that the rigidity of the central polymethin chain in the excited-state decreases and therefore the isomerization is accelerated. Out of this isomerized state an electron transfer can occur within a few picoseconds, leading to a charge-separated state with an isomerized dye molecule. The decay of this charge-separated state can not be distinguished from the nonisomerized state in the current study; therefore, the decay is assumed to happen for both states with 160 ps.

Quenching of Triplet State Formation No indication for the triplet state can be found for the coupled dye. This deactivation mechanism, which is dominant for the pure dye, is completely quenched in the adsorbate, due to the fact that deactivation after photoexcitation takes place much faster via the two other reaction channels. The electron injection in conjunction with the accelerated isomerization prevents ISC and thus might lead to an increased photostability. Conclusions With UV-vis femtosecond transient absorption spectroscopy the initial dynamics of uncoupled merocyanine in solution and coupled to TiO2-nanoparticles has been investigated. For the free dye, two deactivation channels could be determined, one including an isomerization around the central carbon chain on the order of 40 ps and the other leading to a fast population of an energetically close triplet state on the order of 2 ps. These pathways are independent from each other, and neither triplet state formation out of the isomerized form nor a reaction to the twisted molecule from the triplet state occurs. Upon adsorption of the merocyanine dye to semiconductor nanoparticles via a carboxylic group, the fast triplet formation is quenched, and no spectral signature of the triplet state can be identified. This is due to the fact that two faster reaction pathways exist. The isomerization as observed in the free merocyanine is accelerated by a factor of 20 due to a change in electron configuration upon adsorption and takes place within 2 ps, leading to a twisted isomer. In addition, a new deactivation pathway becomes accessible. The excitation energy can be used for an ultrafast electron injection from the dye molecule into the semiconductor with a time constant of 80 fs. The weak coupling between merocyanine and the semiconductor in comparison to other dye/semiconductor systems results in a lower reaction rate than for strongly coupled systems. These two deactivation channels depopulate the excited-state either fast enough to avoid the ISC or the energetic situation is changed so that the for the pure dye the energetically accessible triplet state is shifted to higher energies and can no longer be populated. Therefore, the coupling of merocyanine to semiconductor nanoparticles represents an efficient way to specifically control ultrafast reaction dynamics on a molecular scale. By suppressing the population of undesired and maybe harmful states, the photostability can be significantly increased. This molecular control of reactions together with the general understanding may prove helpful for the future design of solar cells with longer life spans. Acknowledgment. We like to thank Professor Jaques Moser, Professor Michael Gra¨tzel, and Dr. Alexander Ogrodnik for providing the MC2 and the TiO2-nanoparticles. Helpful discussions and preliminary results provided by Michael Thoss and Ivan Kondov are also acknowledged. This work has been financially supported by the German Science Foundation (DFG, WA 1012/2-2). References and Notes (1) (2) Energy (3)

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