Electronic–Vibrational Coupling and Electron Transfer | The Journal of

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Electronic−Vibrational Coupling and Electron Transfer Baxter Abraham,† Luis G. C. Rego,*,‡ and Lars Gundlach*,§,∥ †

SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States Department of Physics, Universidade Federal de Santa Catarina, SC, CEP 88040-900, Brazil § Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States ∥ Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, United States

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‡

ABSTRACT: This Feature Article describes our recent work on implementing pumpdegenerate four-wave mixing for studying electronic vibrational coupling in solid-state systems and in particular in interfacial electron transfer systems. Interfacial electron transfer (ET) reactions constitute key physical phenomena central to a variety of lightinduced energy transport and conversion processes such as vision, photocatalysis, photovoltaics, energy storage, molecular electronics, etc. Heterogeneous material systems like organic/inorganic interfaces are of particular interest due to the variety of possible modifications to their properties. Although ET dynamics has been studied intensively in heterogeneous systems, many questions remain unanswered. The significance of electronic−vibrational coupling and coherence is one aspect that has yet to be fully explored. It is well-known that ET between a molecule and a semiconductor can be accompanied by vibrational excitation due to Franck−Condon overlap of the excited state and cation. The energy necessary for exciting these high-energy vibrational modes is taken from the excess energy of the transferred electron and leads to an ET spectrum. This spectrum carries information that is associated with the vibrational wavepacket that is involved in the ET process, including the coherent beating that results from periodic attempts to overcome the transition state. The dynamics of the wavepacket, and in particular its coherence time, provides insight into the strength of the coupling between electron donor and acceptor states and the adiabaticity of the process. This information is important but not readily available for interfacial ET systems comprising a large number of possible electron acceptor states because neither the rate of the reaction nor the electronic spectrum allows unambiguous assignment. These recent studies give access to vibrational wave packet dynamics triggered by ET reaction and have the potential to allow direct measurement of Huang−Rys parameters and study fundamental properties of interfacial ET like the adiabaticity and the amount of coherenceelectronic and vibrationalin the system.

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INTRODUCTION The study of excitation and coherence of high-energy vibrational modes in molecular ET systems has a long tradition starting with refinements of Marcus’ classical ET theory by including vibrational quantum modes.2,3 Ulstrup and Jortner predicted oscillatory structures in ET spectra comparable to the Franck−Condon progression in radiative transitions. Their predictions for molecular systems were partly confirmed experimentally by Miller et al.4,5 For heterogeneous systems consisting of a molecular donor interfaced with a semiconductor surface, a similar ET spectrum has been predicted by Ramakrishna et al. using a fully quantum mechanical (FQM) model that includes the density of states (DOS) of the semiconductor conduction band.6,7 The overall shape of the ET spectrum could be confirmed experimentally;8,9 however, the coherent beating pattern that has also been predicted was not. Coherence and structural dynamics in molecular homogeneous systems as well as their influence on ET dynamics have been discussed since the advent of ultrafast lasers.10,11 For nonadiabatic ET two cases can be distinguished (Figure 1(a)). An excellent in-depth discussion of coherence and adiabaticity in ultrafast electron transfer can be found in © XXXX American Chemical Society

Figure 1. Graphical representation of coherence in (a) nonadiabatic ET from a single donor state D* to a distribution of acceptor states. Wavepacket in the product (excited) state (red) and in the cation (blue); (b) adiabatic ET. Due to the strong coupling, a single donor state splits into several ET pathways in the molecule−semiconductor complex that can interfere and result in electronic coherence. Based on ref 1.

ref 12. Two general scenarios can be distinguished. First, coherent wavepackets in the reactant state can be generated whenever the energy width of the excitation pulse is sufficiently Received: April 24, 2019 Revised: June 19, 2019 Published: July 8, 2019 A

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broad ground state absorption and complicated intramolecular relaxation pathways. Second, most systems have been measured in a solvent environment. Coupling to solvent modes obstructs the signature of quantum modes, accelerates decoherence, and complicates measurements with sub-20 fs time resolution.2 Third, standard pump−probe techniques cannot distinguish between coherence in the ground and excited state and are not sensitive to noncoherent vibrational excitation in the product state. This Feature Article presents our recent progress in measuring electronic vibrational coupling in excited states of model sensitizer molecules bound to colloidal TiO2 in vacuum using ultrafast techniques. Measurments of Electronic−Vibrational Coupling. Vibrational excitation in ET has been investigated mostly by time-resolved (TR) Raman, CARS, and TR-IR spectroscopy. Bates and Meyer applied TR-resonance Raman to an intramolecular charge transfer complex.38 Mathies et al. employed femtosecond stimulated Raman spectroscopy (FSRS) to measure structural rearrangements in coumarin after HET.39 TR-Raman measurements are challenging due to a weak signal and a complex background that requires careful subtraction.40,41 Spears et al. investigated the influence of vibrational modes on ET in an ion pair employing TR-IR and TR-Vis spectroscopy.42 Lian’s group employed TR-IR for investigating a coumarine−TiO2 system and uncovered the dynamics of electrons injected into TiO2.43 Zanni et al. have assigned, by 2D-IR, vibrational modes to donor molecules inhomogeneously bound to TiO2.44 Pump-2D IR is sensitive to all vibrational modes and in general can not distinguish between those that are coupled to an electronic transition and those that are not involved in an ET reaction. Recently, two-dimensional electronic spectroscopy (2DES) techniques based on different four-wave mixing approaches have been developed. For example, the groups of Fleming and Scholes have shown how two-time anisotropy spectroscopy29 and two-dimensional photon echo experiments45 can be used to study vibrational and electronic coherence during electronic energy transfer in the purple bacteria reaction center and in artificial polymers, respectively. Furthermore, 2DES has been applied to study structural and energy transfer dynamics in photosynthetic systems,46,47 to investigate exciton dynamics in semiconductor nanostructures,48 and to demonstrate the separation of ground and excited state vibrational coherence in a polymer film.49 The aforementioned discovery of coherence in biomolecules triggered an ongoing discussion about identifying and disentangling vibrational and electronic coherence.50 The influence of weakly coupled modes on oscillations in 2D spectra was investigated at room temperature as well as low temperatures.51 For the light-harvesting complex, a 90° phase shift between diagonal and cross peak oscillations was assigned to coupling between population and coherence of the delocalized exciton states.52 However, a similar phase shift has also been observed in a monomeric system, suggesting that such a phase shift is not necessarily an indicator for coherence.53 Fleming’s group combined a visible pump pulse pair from a pulse shaper with an IR probe pulse to study the role of nuclear motions involved in nonradiative relaxation dynamics in molecular systems.54 2DES has also been used to study the porphyrin ground state dynamics.55 Despite the number of aforementioned 2DES studies on molecular systems, only a few reports of HET at molecule−semiconductor interfaces are available. For example, two-dimensional correlation spectroscopy has been applied to study

large. The second case, namely, coherent nuclear motion in the product state, is the more interesting process and has been observed in several isomerization reactions, in photodissociation reactions of small molecules, in some ET systems, and in proton transfer reactions.13−21 However, it is still unclear under which conditions coherence can be observed in heterogeneous electron transfer (HET) systems. The general assumption is that modulations due to vibrational coherence in the product state are small for nonadiabatic ET, i.e., weak electronic coupling between electron donor (D) and acceptor (A), since the nuclei spend little time at the curve-crossing region between A and D potential energy surfaces.10,12,22−24 Accordingly, reports of coherence in the product state in the case of nonadiabatic HET reactions are rare.25 However, the FQM model clearly reproduces wavepacket oscillation along the time axis for a nonadiabatic system. In the case of predominantly adiabatic ET (Figure 1(b)), it is assumed that the population of the product state is strongly modulated by vibrational and electronic coherences.23,26 Electronic coherence has been reported for several adiabatic and nonadiabatic molecular systems, e.g., mixed valence dimers27 and fluorinated benzene,28 in polymers29 and proteins.30,31 Theory has predicted similar behavior for the HET across the Alizarin/ TiO2 interface.32 Moreover, for a similar system, quantum mechanical calculations suggested that ET is partly adiabatic in nature.33 The level of adiabaticity is an open question in HET. The strength of electronic coupling is in general not directly accessible by experiments, and since even moderate electronic coupling can lead to ultrafast subpicosecond ET, the HET rate is not a sufficient measure for the adiabaticity. Adiabatic ET follows a classical Arrhenius rate equation governed by an activation energy barrier kA ∝

−Ea 1 exp τ kBT

(1)

where Ea is the height of the barrier and τ is the donor to acceptor reconfiguration time along the reaction coordinate.34,35 In the nonadiabatic case, the ET rate is described by the Fermi Golden Rule kNA =

2π 2 |V | FCWD ℏ

(2)

where V is the electronic coupling strength between the donor and acceptor potential energy surface (PES), and FCWD is the Franck−Condon weighted density of states which describes the nuclear reconfiguration of the involved vibronic states.2,3 Expanding the FCWD at the high-temperature limit relevant for solvent-free HET leads to a semiclassical Marcus equation equation ÅÄÅ ÑÉ Å −(λ + ΔG° + nℏω)2 ÑÑÑ 2π |V |2 Sne−S ÑÑ kNA = ∑ expÅÅÅÅ ÑÑ ÅÅÇ 4λkBT ÑÖ n ℏn ! 4πλkBT (3)

where the overall rate is summed over each vibrational channel, n, and λ is the reorganization energy.36,37 The Huang−Rhys factor, S, represents the strength of electronic−vibrational coupling and is given by the ratio of the nuclear reorganization energy and the vibrational mode frequency, ℏω. Most of the results discussed above are predictions from theory. Experimental results are rare because, first, many HET systems were designed for solar energy conversion featuring B

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The Journal of Physical Chemistry C vibrational interaction of an organic acid on TiO2;56 HET of a dye to TiO2 has been studied via FSRS;57 and 2D-IR revealed coherent oscillations in a dye/TiO2 system.58 Almost all 2DES investigations of vibrational and electronic coherence employed transitions from the ground state to one or more excited states in a molecular complex or molecule.55 Coherence in HET reactions can be studied by comparing spectra acquired before and after electron transfer. This requires applying an actinic pump pulse before probing the molecule with a resonant probe pulse sequence. The probe sequence will typically be resonant either with the excited state absorption (S1−Sn) or the cation absorption. Zewail and Motzkus suggested that pump-degenerate four-wave mixing (DFWM) can be used to study transition state dynamics,59 and later, Motzkus applied this technique to study internal conversion in β-carotene60 and more recently structural dynamics of retinal.61 Compared to 2DES, DFWM has a faster acquisition time and spectra reconstruction because the 1 excitation frequency (ω1 = τ , Figure 4) is not resolved. Thus,

of vibrational bands during a reaction, directly in the time domain. The remainder of this Feature Article summarizes our effort to employ pump-DFWM for measuring the coupling between electronic and vibrational modes, their coherence times, and their effect on the transfer times. Starting with a brief overview of the method we will continue with a discussion of previously published results and finish with some recent preliminary data followed by an outlook that also includes the application of pump-DFWM to 2D materials where electron−phonon coupling is of major interest.

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RECENT CONTRIBUTIONS The Method. Our method for measuring electronic− vibrational couplings is based on the application of a degenerate four-wave mixing probe pulse sequence to a system that was photoexcited by an initial actinic pump pulse. The initial pump pulse is not phase matched with respect to the FWM pulse sequence, and the possible excitation pathways can be described by the same Feynman diagrams as regular DFWM. The actinic pump pulse allows the measurement of electronic and structural dynamics on an intermediate PES. By tuning the DFWM pulse sequence into resonance with the absorption of a higher excited state, the signal is strongly enhanced with respect to the nonresonant contribution. Individual intermediate state PESs can therefore be separately probed through selective resonance.69 Pump-DFWM is suitable for investigating vibrational excitation during ET reactions because it is possible to probe the dynamics before (excited state) and after (cation) HET. The ability to detect high-frequency vibrational modes beyond the fingerprint region depends on very short femtosecond pulses. The pulse duration must be sufficiently short to capture vibrational oscillations in the time domain. For instance, a 2000 cm−1 mode corresponds to a period of 17 fs. A 10 kHz regenerative Ti:sapphire laser system is used to drive two home-built NOPAs that provide the broadband tunable DFWM pulses (sub-10 fs, 2−7 μJ) and the actinic pump pulse (420−440 nm, 0.8 μJ, sub-20 fs after SHG). The actinic pump pulse is delayed via a stepper motor stage. The output of the first NOPA is split into three parts via beam splitters to achieve a DFWM pulse sequence. The probe pulse is delayed against the Stokes and pump pulse (delay τ2 in Figure 4) via a piezo stage, while τ1 is fixed at zero. The scan range of τ2 defines the achievable resolution for the vibrational part of the signal, which is 5−10 cm−1 in our setup when scanned at 10 Hz. The beams are arranged in the standard folded BOXCARS configuration, focused onto the sample via a concave mirror, and result in a signal beam that is scattered in direction ksignal = kpump − kStokes + kprobe. The DFWM signal is detected with a photomultiplier tube after a monochromator. A chopper wheel blocks every second actinic pump pulse for subtraction of nonresonant signal and scattered light. The high repetition rate allows for incorporation of an additional chopper to measure and subtract scattered light if needed.67 Pump-DFWM pulse intensities are comparable to those employed in pump−probe measurements. For example, white-light pump−probe measurements on PeTiO2 samples are typically performed at 30 nJ pump and 300 pJ probe energy at an OD of around 0.7 at 440 nm, while pump-DFWM measurements on PE are taken with an initial

1

only two delay times have to be scanned for a measurement, i.e., the delay of the actinic pump and the τ2 time (waiting time). However, DFWM still allows for identifying vibrational modes and associated coherence as long as the emission wavelength is resolved.62−64 In an ω3-resolved DFWM measurement, vibrational contributions can be resolved along the τ2 waiting time, whereas in an ω1 and ω3 integrated transient absorption (TA) measurement all vibrational contributions interfere destructively. Electronic contributions, on the other hand, are in phase and survive ω1 as well as ω3 integration. This method has been suggested to distinguish between vibrational and electronic coherences.62,63 The shorter acquisition time of DFWM data compared to 2DES enables the addition of an actinic pump-pulse (pump-DFWM) and allows to study the dynamics on the excited potential energy surfaces where structural dynamics that is triggered by the change of the electronic state, e.g., electronic excitation or electron transfer, can be studied. Figure 2 exemplifies different

Figure 2. (a) Albrecht diagrams for DFWM pulse sequences for excited state resonant measurement before ET (red) and cation resonant measurement of the hot cation shortly after ET (green) and (b) cation resonant measurement of the relaxed cation (cyan).

resonant conditions in a HET system. It should be noted that this method does not rely on impulsively excited vibrational modes, and thus vibrational dynamics that is initiated by ET from an electronic excited but vibrationally cold state can be studied. An extensive comparison between pump-DFWM, pump-impulsive vibrational spectroscopy (IVS), and FSRS has recently been published.65 While all techniques give similar information, pump-DFWM and pump-IVS reveal the evolution C

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Vibrational frequencies are extracted from the oscillatory part of the signal by standard procedures of Fourier transform spectroscopy. It should be noted that the subtraction of pumpon and pump-off spectra has to be performed very carefully. Various processes that are triggered by the actinic pump can change the OD of the sample and result in over- or undersubtraction that manifests as peaks in the subtracted Raman spectrum. Since these peaks show the same time dependence as the underlying linear process, e.g., transient absorption or ground state bleach, these contributions are easily mistaken for real vibrational dynamics. In the case of a strong Raman scatterer and strong resonant enhancement, it is possible to align the setup such that the DFWM signal from the ground state is negligible, and subtraction can be avoided altogether. However, for most systems a subtraction is necessary. In these cases a careful rescaling of the spectra is required before subtraction. A full pump-DFWM measurement results in maps of vibrational spectra vs actinic pump delays at different emission wavelength. The exponential part in pumpDFWM can be analyzed by rate models convoluted with the IRF, which gives results comparable to Bloch’s solution to Liouville’s equation for pulses separated in time and in resonance with a specific transition.72,73 The application of pump-DFWM to solid state samples requires some additional precautions. The colloidal films need to be prepared as scatterfree as possible. Spatial filtering and an additional optical chopper are used to suppress the remaining scattered light. The molecules that are attached to the colloids are prone to photodegradation in the presence of water and oxygen and have to be protected by vacuum. Electronic Vibrational Coupling in the Molecular Excited State. Zinc(II)-tetraphenylporphyrin (Zn-TPP) is a prototypical cyclic tetrapyrrole that has been intensely studied with ultrafast techniques in past decades. Metalloporphyrins have been used as photosensitizers and are known to undergo efficient photoinduced HET into metal oxide semiconductors.74,75 Because of its importance for photochemical processes, the optical properties are of particular interest, and accordingly numerous studies have focused on light absorption and excited state dynamics of Zn-TPP. Relaxation after photoexcitation in the Soret band involves internal conversion that is preceded by an ultrafast process. This relaxation process takes place within the first picosecond before S2 → S1 internal conversion occurs (cf. Figure 5) and has been observed by several groups.76−81 Previously, it has not been determined if it involves a higher lying “dark” state or simply vibrational relaxation in the excited S2 state. We employed pump-DFWM to this model system of a photoexcited molecule in solution to gain insight into this initial excited state relaxation process and settle the dispute about the contending relaxation models. Figure 6(a) displays the presence of two major Raman active modes in the first several hundred femtoseconds after excitation of the S2 state, attributed to stretching motions at the metal center (390 cm−1) and aromatic carbon stretching around the porphyrin perimeter (1350 cm−1). The mode at 900 cm−1 is a THF solvent induced contribution.82,83 Figure 6(b) shows the evolution of the spectrum shown in (a) as a function of actinic pump delay. The Zn-TPP peaks exhibit a rapid rise in amplitude in ∼300 fs. Zooming into the 1352 cm−1 mode shows that this mode undergoes a 12 cm−1 blue shift in about 85 fs (Figure 6(c)). This time scale agrees well with the relaxation process observed previously in other

pump energy of 40 nJ, DFWM-pump and Stokes pulse of 15 nJ each, and a probe energy of 10 nJ at an OD of 1 at 440 nm. Signal Analysis. Resonant contributions in the pumpDFWM signal typically exceed nonresonant signals by orders of magnitude. By adjusting actinic pump and DFWM wavelengths, resonant contributions can be selectively achieved for specific excited or charged states (cf. Figure 2). The remaining nonresonant ground state signal can be subtracted on a shot to shot basis by blocking every second actinic pump pulse. A typical pump-DFWM signal consists of an oscillatory contribution along τ2 that is superimposed on a rising or decaying contribution. The latter constitutes the population dynamics of the electronic intermediate state, while the oscillatory part contains information on molecular structural dynamics and dephasing times of the particular PES. DFWM is mostly employed in homodyne, i.e., self-heterodyne, detection.70 However, the setup in Figure 3 has the option to use

Figure 3. Pump-DFWM instrument utilized the passively phase-stable setup that has been employed for 2DES measurements.66,67 This figure is reprinted with permission from ref 68.

Figure 4. Timing of the pulse sequence (left). For pump-DFWM τ1 = 0, pump-2DES requires scanning of τ1. Wave vectors in folded BOXCARS geometry for the setup (right).

the fourth beam as an external local oscillator. The observed oscillatory part of the DFWM signal beam is linear in the response to the Raman-active nuclear modes.69,70 Due to the large spectral width of the DFWM pulses, it is possible to measure wavepacket dynamics at different energies by detecting the signal at different wavelengths simultaneously. For example, wavepacket dynamics in a hot cation state just after ET can be detected at lower photon energies when compared to wavepacket dynamics in the relaxed cation that can be observed at higher photon energies. DFWM is commonly modeled using Liouville’s equation and solving it perturbatively up to third order.71 However, for resonant DFWM and at delay times larger than the pulse width, the relevant contributions to the signal are largely reduced in number. Thus, signal analysis is possible by subtracting the nonresonant background and partitioning the signal in exponential and oscillatory parts as mentioned above. D

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inner ring stretching mode.85 The rise of the amplitude of the signal can be explained by a shift of the S2 excited state absorption into resonance with the DFWM pulse sequence while the mode relaxes. This is supported by the TA kinetics that show a rise in the excited state absorption on a similar time scale. The interpretation of the frequency shift and of the increase in amplitude are confirmed by pump-DFWM measurements that were performed with a lower actinic pump energy of 430 nm. In these experiments the electronic excited state was prepared in the vibrational ground state (Figure 6(d)). Under these conditions no relaxation dynamics is observed. This pump-DFWM study showed unambiguously that the underlying process constitutes vibrational relaxation in the anharmonic S2 potential.84 Zn-porphyrin Bound to TiO2 Excited State and Cation Vibrational Dynamics. We employed a porphyrin-based dye for the first pump-DFWM studies of an interfacial ET system (Figure 7). The Zn-TPP chromophore, described above, has been extensively investigated with a range of different bridge and anchor groups, mainly with TA, by our group in the past.86 This specific bridge group forms a rigid connection between the chromophore and the isophthalic acid anchor group; however, it allows for rotations of the phenyl groups and the chromophore around the long molecular axis. The isophthalic acid has been shown to bind efficiently to metal oxides through the carboxylate groups.87,88

Figure 5. Jablonski diagram of Zn-TPP. The initial 140 fs process was alternatively ascribed to a dark excited state or to a vibrational relaxation process.

studies. For the measurement shown in Figure 6(c) the excitation wavelength of 420 nm was above the optical gap and resulted in a vibrationally hot excited state. Thus, the continuous shift toward higher wavenumbers can be explained by vibrational relaxation in an anharmonic potential. The magnitude of the shift agrees well with comparable anharmonic corrections. For example, the benzene ring stretching mode at 1494 cm−1 shows a correction of 10 cm−1, similar to the 12 cm−1 correction that was observed here for the 1352 cm−1

Figure 6. (a) Excited state resonant pump-DFWM spectra (T = 100 fs). Dominant features are 385 cm−1 metal−pyrrrole stretch, 1352 cm−1 inner ring stretch, and 910 cm−1 solvent mode. (b), (c), and (d) Actinic pump delay dependent DFWM spectra and magnification of the 1350 cm−1 mode at (c) 420 nm excitation and (d) 430 nm excitation. The black dotted line is a fit with a 12 cm−1/85 fs time constant. This figure is reprinted with permission from ref 84. E

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Figure 8. Pump-DFWM spectrum of Zn-PE-(COOH)2 bound to colloidal TiO2 5 ps after photoexcitation of the S2 band. This figure is reprinted with permission from ref 68.

Figure 7. Structure of Zn-PE-(COOH)2, bond labeling, and labeling of bridge phenyl groups. The molecule is anchored via the two carboxylic acid groups to TiO2. This figure is reprinted with permission from ref 68.

unpumped sample, and the observed vibrational features are those that were triggered by optical excitation. The molecular vibrations present on the cation must either be initiated by or survive the photoinduced HET process. In the event of an adiabatic electron transfer from a vibrationally hot prepared molecular state, it could be expected for excess vibrational energy to remain in the oxidized molecular state after HET coherently.13 Alternatively, nonadiabtic HET can induce vibrations as a result of intramolecular reorganization.10 The fact that Zn-PE-(COOH)2 was excited at its absorption maximum to a Franck−Condon state preceding HET allows for both scenarios in this case. The Zn-TPP chromophore does not show a localized cation absorption in the accessible spectral range; instead, a long-lived component in the TA spectra that is spread over the whole TA spectrum has been attributed to cation absorption. Thus, resonance enhancement of the cation species cannot be achieved here, and an increase in signal intensity by up to a factor of 104 can be expected for suitable model dyes, as will be discussed in the following section.91 Although the weak signal prevented a full scan of the actinic pump delay, a tentative discussion of the pump-DFWM spectrum at 5 ps delay shown in Figure 8 can be attempted. Comparison with DFT calculations of the unbound Zn-PE(COOH) molecule shows that the oxidized Zn-PE-(COOH)+2 should exhibit vibrational modes in the region between 400 and 1200 cm−1. Raman spectra calculated for the ground state and for the cation are displayed in Figure 9. The spectrum that was calculated for Zn-PE-(COOH)+2 is closely related to the pump-DFWM measurement and shows the Raman-active modes of the isolated cation that are produced by HET. The calculated spectrum of the neutral form is clearly dominated by the two modes around 400 and 1300 cm−1, in very good agreement with the measured spectra and with much less intense modes dispersed in between. In contrast, the cation shows higher Raman cross-section for modes with frequencies below 1300 cm−1. These modes are assigned to a C−C stretching mode at 964 cm−1 between the two phenyl groups in the linker furthest away from the chromophore, a C−H wagging mode on the phenyl group at 885 cm−1 within the linker nearest to the chromophore, and a C−C twisting mode

The main difference between measurements on solid-state colloidal films and in the solution phase is replenishment of the photodegraded sample and increased scattering of the colloidal film. Our group has long-standing experience in ultrafast measurements of molecular sensitizers on crystalline and colloidal substrates. Careful sample preparation that minimizes the time between heating of the substrate, sensitization from solution, and transfer into high vacuum protects the bound molecules from photodamage due to the presence of oxygen or water. This technique has been refined over the course of numerous studies89 and has proven reliable. Scattering due to the colloidal nature of the substrate is considerably more challenging for a background-free technique like DFWM when compared to TA. Since pump-DFWM relies on pulse to pulse subtraction of pump-on/pump-off measurements, lock-in techniques are not applicable. Additional background subtraction by blocking pulses from the DFWM pulse sequence using an additional chopper is in principle applicable, but at the cost of increased measurement time. Our approach is based on reducing scattering by preparing the highest quality colloidal films that show no visible scattering and careful spatial filtering of stray light using apertures and lenses. Using this method, we demonstrated the application of pump-DFWM to HET of nanostructured solid-state samples in vacuum.68 Figure 8 shows the pump-DFWM spectrum of the sensitized film. The intense vibrational features known from ground state Raman spectroscopy at 385 and 1352 cm−1 are no longer found after the film has been photoexcited. Instead, peaks at 765, 890, and 935 cm−1 are most prominent. Less intense modes are observable as well, including those found in the film’s ground state, but with a much lower amplitude. The observation of the new vibrational modes following photoexcitation is attributed to the molecular cation, which is the dominant species present after electron transfer occurs. It has been shown previously that the excited state lifetime is reduced to 80 fs by ET when the molecule is adsorbed on TiO2.90 The measured signal was recorded at a delay of 5 ps as a shot-toshot difference between measurements of the pumped and F

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Figure 9. DFT-calculated Raman frequency spectra for Zn-PE(COOH)2 and its oxidized counterpart. The cation is calculated to have more peaks spread throughout the measured region of more equal intensity. This figure is reprinted with permission from ref 68.

at 753 cm−1 in the two phenyl rings of the linker group nearest the chromophore. The calculated modes agree very well with the experimental results overall. It is interesting that except for the mode near 1300 cm−1 on the porphyrin ring the other modes are located on the bridge that provides the pathway for electron transfer, away from the localized initial absorption site. Coupling between vibrational modes on bridge groups in charge and energy transfer has recently regained attention, for example, as a way of controlling the transfer dynamics via external excitation of these modes.92−95 Perylene Chromophore. Perylene represents a good model system because the ground, excited, and cation states each feature strong distinct absorption peaks.96 In contrast with porphyrin-based dyes and many other chromophores, this spectral separation allows for selective resonant enhancement of each state as discussed above. Solution-phase pump-DFWM measurements of the unsubstituted perylene chromophore were performed in THF (100 μM) with a 437 nm actinic pump, centered at the ground state absorption maximum. The DFWM probe sequence was set to overlap with the excited state absorption peak at 640 nm. Without actinic excitation, multiple vibrational modes were recorded in the ground state spectrum, displayed in Figure 10. Good agreement is found with the solvent-subtracted steady-state Raman spectrum at 785 nm. It should be noted that the amplitudes of the peaks in the DFWM and Raman spectra are not directly comparable because of the different probe wavelengths. Additionally the temporal width of the DFWM pulses obscures oscillation in the signal at high wavenumbers more strongly than at lower wavenumbers, which reduces the amplitude in the Fouriertransformed spectrum. DFWM measurements after actinic excitation show three prominent modes at 550, 800, and 1600 cm−1. Vibrational relaxation is complete after 2 ps,97 indicating that these peaks persist from the Franck−Condon state and are representative of the stabilized S1 state. Notably, the 1600 cm−1 peak is blueshifted from its location at 1570 cm−1 in the ground state. This vibrational mode is assigned to scissoring at the ring exterior by DFT calculation (Figure 11) and offers experimental evidence of altered electron density at the perimeter in the π* state. The major vibrational mode at 550 cm−1 is assigned to a high-symmetry overall ring breathing and is not displaced from

Figure 10. From top to bottom: pump-DFWM spectrum of Pe in THF 2 ps after excitation, DFWM spectrum of Pe in THF in the ground state, and conventional Raman spectrum of the ground state. Asterisks denote solvent peaks.

Figure 11. Illustrations of the DFT-calculated 550 (left) and 1570 cm−1 (right) perylene vibrational modes.

the ground state. Additionally, there is an enhancement of the feature at 880 cm−1 that has a small amplitude in the ground state. This mode is therefore strongly coupled to the S0 → S1 transition. Weaker vibrational features may be present in the 1200−1400 cm−1 region of the excited state, but they cannot be confidently resolved with the achieved signal-to-noise level.

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FUTURE OUTLOOK Perylene-benzoic Acid on TiO2. Recently, we measured the perylene-based sensitizer di-tert-butyl-perylene-benzoic acid (DTB-Pe-benzoic acid) in solution and bound to TiO2. As discussed above, Pe derivatives show a well-separated absorption of the ground state, excited state, and cation that is necessary for selective resonant enhancement in pump-DFWM experiments. Functionalization with different bridge and acid anchor groups that are used to chemisorb the sensitizer to a semiconductor surface retains these optical properties of the chromophore.72 Photoexcitation of Pe is almost exclusively a S0−S1 transition without the involvement of triplet states. The excited state lifetime is around 5 ns. These well-defined characteristics make it an ideal candidate for detailed HET investigations. Furthermore, the binding geometry of the chromophore attached with common anchor groups to TiO2 is known from IR and polarization-dependent 2PPE measureG

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The Journal of Physical Chemistry C ments and supported by DFT calculations.98 We have prepared and characterized multiple perylene-benzoic acids with steady state and time-resolved spectroscopy. The transient absorption spectra of the excited perylene dye in solution, following excitation at 450 nm, show only the longlived excited state. The excited state absorption at ∼730 nm is nearly constant, and the region between 520 and 640 nm shows no contributions from the excited state. This range is where the cation absorption is observed after HET. DFWM spectra of DTB-Pe-benzoic acid bound to TiO2 in vacuum are presented in Figure 12. The ground state shows similar

transfer to occur fastertypically