Observing Vibrational Wavepackets during an Ultrafast Electron

Article pubs.acs.org/JPCA

Observing Vibrational Wavepackets during an Ultrafast Electron Transfer Reaction Shahnawaz Rafiq, Jacob C. Dean, and Gregory D. Scholes* Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States S Supporting Information *

ABSTRACT: Recent work has proposed that coherent effects impact ultrafast electron transfer reactions. Here we report studies using broadband pump−probe and two-dimensional electronic spectroscopy of intramolecular nuclear motion on the time scale of the electron transfer between oxazine 1 (Ox1) and dimethylaniline (DMA). We performed time−frequency analysis on the time domain data to assign signal amplitude modulations to ground or excited electronic states in the reactive system (Ox1 in DMA) relative to the control system (Ox1 in chloronaphthalene). It was found that our ability to detect vibrational coherence via the excited electronic state of Ox1 diminishes on the time scale that population is lost by electron transfer. However, the vibrational wavepacket is not damped by the electron transfer process and has been observed previously by detecting the Ox1 radical transient absorption. The analysis presented here indicates that the “addition” of an electron to the photoexcited electron acceptor does not significantly perturb the vibrational coherence, suggesting its presence as a spectator, consistent with the Born−Oppenheimer separation of electronic and nuclear degrees of freedom. reaction center of purple bacteria by Vos et al.37−39 This work triggered intense interest in the nature and possible consequences of coherence in electron transfer.40−45 Recent work by the van Grondelle group has shown that electron transfer in the inverted regime in a photosynthetic reaction center (photosystem II) is expedited when vibrational levels bridge electronic gaps.46 This work collectively highlights the importance of elucidating how molecular vibrations can aid ultrafast electron transfer reactions. Intramolecular vibrations certainly influence electron transfer. Jortner and Bixon,47 for instance, summarize how vibrational levels multiply the number of reaction paths and some of these vibronic paths can have lower activation energies than predicted from purely electronic states. On the contrary, the matrix element for electron transfer is reduced by Franck− Condon factors. Barbara and co-workers48 have shown that it is a combination of classical degrees of freedom (solvent polarization) and quantum effects from the intramolecular modes that speed up electron transfer reactionsby orders of magnitude in the case they studied. Questions remain about how superpositions of vibronic states (coherence) produced by ultrafast broad-band laser pulses provides new opportunities for probing vibrational effects in electron transfer reactions. In this report, we revisited the oxazine 1 (Ox1) and dimethylaniline (DMA) system to understand the time−

1. INTRODUCTION Electron transfer is a simple and ubiquitous reaction.1,2 In the condensed phase, various system (intramolecular) and environmental (intermolecular) degrees of freedom participate in determining the rate and mechanism of the electron transfer process making this a multidimensional problem.3−23 In one extreme is solvent-controlled electron transfer reactions wherein molecular degrees of freedom become unimportant. In the other extreme, however, when electron transfer becomes much faster than the solvent relaxation rate, a situation is found where the solvent degrees of freedom are effectively frozen and much faster motions, such as high frequency intermolecular vibration, drive the electron transfer process.3−23 Recent work has suggested that effects collectively called “coherence” can be harnessed much more widely and prominently than previously imagined, even when electronic coupling is not the dominant energy scale.24−32 For example, Friend and co-workers have reported charge separation in conjugated polymer−fullerene blends that appears to be extraordinarily fast;33 Burghardt and co-workers have used advanced quantum dynamics calculations to study ultrafast charge transfer in conjugated oligomer−fullerene complexes and find that electronic coherence is at play;34 and Lienau and co-workers propose that quantum coherence is behind charge separation in a molecular triad, and that electron transfer appears to be much faster than previously realized.35 This latter group further reports ultrafast coherent charge transfer in an organic photovoltaic blend.36 Vibrational coherences were first observed in an electron transfer system for the photosynthetic © 2015 American Chemical Society

Received: September 25, 2015 Revised: November 17, 2015 Published: November 20, 2015 11837

DOI: 10.1021/acs.jpca.5b09390 J. Phys. Chem. A 2015, 119, 11837−11846

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The Journal of Physical Chemistry A

generate a narrowband pump pulse centered at 660 nm. A portion of the output from OPA and a very small portion of fundamental output from laser amplifier was directed to a commercial pump−probe spectrometer (Ultrafast Systems, Helios). The 800 nm output was focused into the sapphire plate to generate white light continuum from ca. 450 to 750 nm. The time delay between the pump and probe was achieved by means of a translation stage in the probe beam. The pump and the probe beams were focused into the sample cell of 1 mm width and the residual fundamental 800 nm in the white light continuum was removed by using appropriate filters positioned after the sample. For broadband pump−probe and two-dimensional electronic spectroscopy measurements, the experimental setup is explained in detail elsewhere;56−58 however, a brief description is presented here. The 150 fs, 800 nm laser light at a repetition rate of 5 kHz from a Ti:sapphire seeded regenerative amplifier (Spectra-Physics, Spitfire) was used to pump a home-built noncollinear optical parametric amplifier (NOPA). The NOPA output was sent into a folded grating compressor and then into a prism compressor (one prism and a retroreflector) to compress the pulses, which have a bandwidth of 17 fs in broadband pump−probe experiments and 12 fs in 2D electronic measurements as diagnosed with polarization-gated and transient grating frequency resolved optical grating (PG-, TG-FROG).59,60 For the broadband pump−probe experiment, the compressed output was split into three beams by a wedge beam splitter. The two beams reflected from the front and back surfaces of the wedge, having less than 1% total intensity, were used as reference and probe beams. The transmitted beam was used as a pump beam. The time delay between the pump and the probe beams was achieved by means of a translation stage in the pump path. Scatter contributions in the measurements, which appear as fringes on top of the transient absorption spectra and change frequency as the pump is scanned, were filtered by performing an inverse Fourier transform at each delay time. This operation brings the frequency domain data in the time domain wherein a super-Gaussian filter was applied over the probe pulse to remove the scatter from the pump pulse that is moving away from the probe pulse as the time delay between the two increases. A fast Fourier transform was applied to bring back the filtered data into the frequency domain. The step size along the waiting time was kept to 4 fs. 2DES is a third-order nonlinear optical spectroscopy that spreads the information contained in typical pump−probe spectroscopy into two frequency axes and allows elucidation of a frequency−frequency correlation map where each excitation frequency is correlated to the detection frequency for a given waiting time, t2. Three incoming fields (E1, E2, E3) interact with the sample in a particular phase matching condition leading to emission of a third-order signal that is heterodyne-detected by spectral interferometry with a fourth laser field, the local oscillator ELO, so that both amplitude and phase information is obtained. These four fields (E1, E2, E3, and ELO) were generated by focusing the compressed NOPA output on a 2D diffractive optic generating four phase-stable beams in a box geometry. Three beams are passed through pairs of 1° fused silica wedges, each of which incorporates one mounted on a translation stage (Newport VP-25XL) to change the amount of glass a particular beam has to pass through. The third beam is passed through a chopper operating at 50 Hz to remove the scatter. The interaction of the first field with the sample creates a coherence

frequency evolution of a superposition of vibronic states during ultrafast electron transfer process by performing broadband pump−probe and two-dimensional electronic spectroscopy. We find that our ability to detect vibrational coherence via the excited electronic state of Ox1 diminishes on the time scale corresponding to the loss of population by electron transfer. Nevertheless, the wavepacket is not dephased during the electron transfer time scale. We discuss the implications of these observations for electron transfer reactions. This system undergoes forward electron transfer from DMA in its ground state to the excited state of oxazine 1 (Ox1*) on a time scale of ca. 80 fs.49−54 Oxazine forms a weakly bound ground state complex with DMA55 and the optical excitation of oxazine triggers electron transfer from the donor DMA to oxazine, forming a charge separated state. The charge separated state then returns to the ground state through a back electron transfer process in about 3−4 ps.

2. EXPERIMENTAL SECTION Two solvents were chosen for comparison: the electron donating N,N-dimethylaniline (DMA) (Sigma-Aldrich, USA), and an inert control solvent, 1-chloronaphthalene (CN) (TCI, Japan). Oxazine 1 (Lambda Physik, USA) was chosen as the electron acceptor. The absorption spectra of oxazine1 in DMA and CN along with the pulse spectrum is shown in Figure 1.

Figure 1. Normalized absorption spectra of oxazine 1 in chloronaphthalene (black solid line) and dimethylaniline (red solid line). The blue shaded spectrum represents the pulse spectrum used for carrying out broadband pump−probe and 2D measurements. The cyan shaded spectrum is another pulse spectrum that was used for the broadband pump−probe experiment to access a different spectral region.

The absorption maximum of Ox1 is at 15 170 cm−1 (659 nm) in both solvents, whereas the spectrum is broader in DMA than in CN, indicating the formation of a weakly bound ground state complex between Ox1 and DMA.55 The experiments were carried out in a 1 mm cuvette with a maximum optical density of around 0.20. The pulse spectrum has an approximate bandwidth of about 70 nm (full-width at half-maximum, fwhm) and was used for both broadband pump−probe and 2D measurements. In addition, pump−probe measurements were also performed with another pulse spectrum that was shifted toward the high energy spectral region. Narrowband pump−probe measurements were performed on a commercial setup. Briefly, 40 fs, 800 nm laser light at a repetition rate of 1 kHz from a Ti:sapphire regenerative amplifier (Libra, Coherent) was used to pump a commercial optical parametric amplifier (OPerA Solo, Coherent) to 11838


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Figure 2. Broadband pump−probe measurements performed on oxazine 1 in chloronaphthalene (a, b, c) and dimethylaniline (d, e, f). Panels a and d show the residual time domain data after population has been removed by fitting the kinetic traces to multiexponential decay. Panels b and e are the power spectra obtained by Fourier transforming the time domain residual data at each probe frequency. Panels c and f are the power spectra obtained by performing FT of the residual data acquired using the second pulse spectrum. The data have been shown only until 1000 cm−1. For more thorough details regarding other oscillatory features, consult Figures S2 and S6 in the Supporting Information.

23 000 cm−1. These transient signals do not decay completely in the experimental time window of 2 ns. Contrarily, in the Ox1/DMA system the transient signals decay significantly faster due to the electron transfer quenching. The ESA signal, peaked in the range 18 000−19 000 cm−1 decays within the duration of pump pulse (∼100 fs) and another transient signal (peaked at ca. 21 000 cm−1) appears on the high energy side of the ESA on a similar time scale. This time scale corresponds to the forward electron transfer from DMA to Ox1 in the excited state (Ox1*) and leads to the formation of the reaction product (oxazine 1 radical). This photoinduced absorption signal then decays with a time constant of 3−4 ps that is assigned to back electron transfer from Ox1 radical to DMA cation.51,52

between the ground and excited state that is clocked by the time period, t1, with respect to the second field. The second field onsets a population or another coherence and marks the beginning of the waiting time, t2, and allows the system to evolve dynamically. The third pulse effectively ends the waiting time and marks the beginning of another coherence time, t3, and provides information about the instantaneous state of the system prepared by the interaction with the initial two pulses. The fourth beam acts as the local oscillator and is passed through a neutral density filter attenuating it by 4 orders of magnitude and also imparting a delay such that it arrives ∼250 fs before the final laser field. Although the heterodyned detected signal is dispersed directly on the camera giving the detection axis, ν3, the excitation axis, ν1, is retrieved by performing a Fourier transform along the first coherence time (t1) giving a frequency-frequency correlated 2D map at a particular waiting time. The coherence time between the first two fields was scanned from −50 to +50 fs in steps of 0.2 fs and the waiting time was scanned from 0 to 420 fs with 4 fs step size. Depending upon the ordering of the two pump fields, rephasing and nonrephasing signals are obtained.

4. BROADBAND PUMP−PROBE MEASUREMENTS 4.1. Oscillatory Features in Reactive and Nonreactive Systems. Pump−probe spectroscopy with broadband excitation creates a superposition of vibrational levels (coherences) in the excited or ground electronic state.56,59−64 The wavelength dependence of the amplitude of oscillatory features depends mainly on Franck−Condon factors. Excited state coherences are detected by modulations in stimulated emission and excited state absorption signals by wavepacket evolution. For ground state coherences, ground state bleach is modulated. In oxazine, we find that both types of wavepacket motion are prepared. For the nonreactive system Ox1/CN, Fourier transformation (FT) of the residuals obtained after removing the population contribution to the dynamics (via fitting the time domain data with a multiexponential function56) shows oscillations for more than 3 ps, Figure 2a. It is clearly seen that some probe wavelengths expose strong amplitude oscillations compared to other regions and at some spectral regions there is no amplitude at all. The FT (Figure 2b) shows peaks at 560, 567, and 608 cm−1. Figure S2 of the Supporting Information

3. NARROWBAND PUMP−PROBE MEASUREMENTS The electron transfer dynamics of oxazine system was investigated first by carrying out narrowband transient absorption measurements on the nonreactive Ox1/CN and the reactive Ox1/DMA system. The transient spectra of OX1/ CN and OX1/DMA at increasing pump−probe delay times are shown in Figure S1 of Supporting Information. In Ox1/CN, one can clearly see the bleach of the ground state and the stimulated emission from excited state overlap in the spectral region around 15 000 cm−1. In addition, an excited state absorption (ESA) signal appears on the high energy side of the bleach extending over a large spectral range from ca. 17 000− 11839


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Figure 3. Time−frequency analysis performed on pump−probe measurements using continuous wavelet transformation on the time domain residual data of oxazine 1 in chloronaphthalene. The frames are the probe frequency−oscillatory frequency correlation maps at different waiting times. The white dashed line on the top panel of the frames is drawn at the probe frequency corresponding to the excited state minimum, whereas the yellow dashed line on the bottom panel of frames is drawn at the probe frequency corresponding to the ground state minimum. All the frames are scaled to the same value along the z-axis.

the modulations in that spectral region with our experimental setup. In the nonreactive system, the two frequencies 560 and 567 cm−1, have been assigned to the excited and ground state vibrations, respectively, mostly on the basis of their dephasing times.51−54 Further evidence comes from our data by the presence of two nodes in the residual map (Figure 2a), one of which lies at the fluorescence maximum and the other at absorption maximum. The node at the fluorescence maximum corresponds to the wavepacket motion in the excited state with a frequency 560 cm−1. The node at the absorption maximum appears due to the wavepacket motion on the ground state and is associated with 567 cm−1. Though the strong amplitude oscillations in the inert solvent persisted for several picoseconds (Figure 2a), amplitude oscillations are lost concomitant with the excited state population in the electron donating solvent. In Figure 2d, the oscillations are weak, and a distinct nodal region characterized by a phase change and almost zero amplitude, is observed. The Fourier transform of these residual oscillations in Ox1/ DMA system shows multiple frequencies (Figure 2e and Figure S6), including a contribution from solvent in addition to the intramolecular vibrations. Besides the dominant oscillations around 567 and 611 cm−1, our pump−probe measurements also shows other frequencies at 1132, 1430, and 1650 cm−1 (Figure S6 in the Supporting Information) that were earlier observed only in the Raman spectrum. The frequency components at 740 and 1000 cm−1 are due to dimethylaniline. 4.2. Continuous Wavelet Transform (CWT) Analysis. To investigate the time development of these oscillatory features, a wavelet transformation (WT) of the pump−probe data was performed. This analysis allows us to map the temporal evolution of the frequency spectrum within the limits of the frequency−time uncertainty relation.65−67 Briefly, WT uses a zero mean and short-time oscillating function called a

shows the Fourier transform of individual time domain residual traces at different probe frequencies and distinctly reveal two close frequencies 560 and 567 cm−1. Besides those frequencies, other high frequency modes are evident, but they have a very small amplitude (Figure S2). The pulse spectrum (light blue spectrum in Figure 1) used here resolved the ground state bleach and stimulated emission transient signals of Ox1/CN system, as shown in Figure S3 of the Supporting Information. Additional measurements were performed with a different pulse spectrum (cyan spectrum in Figure 1), centered at ca. 18 000 cm−1 to detect the excited state absorption of Ox1. The measurement shows an excited state absorption signal centered around 17 850 cm−1 (560 nm), and the amplitude in this spectral region is also modulated (Figures S3 and S4). The frequency of these amplitude modulations, Figure 2c, was found to be 560 cm−1, the same as one of the frequencies in the bleach/stimulated emission spectral region. In the Ox1/DMA system, the residual pump−probe data (Figure 2d) obtained by removing population dynamics was Fourier transformed to reveal two frequencies at 567 and 611 cm−1 in the bleach/stimulated emission region (Figure 2e). The pump−probe spectrum of Ox1/DMA is shown in Figure S5 of the Supporting Information. In this case, due to fast electron transfer quenching of the initially prepared excited state of Ox1, stimulated emission and excited state absorption signals decay rapidly, in ca. 100 fs. Therefore, amplitude modulations in the ESA region were not observed (Figure 2f). Zinth and co-workers previously52 have observed amplitude modulations by tracking the absorption changes in the 21 000 cm−1 spectral region. These coherences lasted for about 1 ps, much longer than the time scale of the electron transfer from DMA to Ox1*. This spectral region is dominated by the transient signal from photoinduced absorption of the Ox1 radical (electron transfer product). We were not able to study 11840


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The Journal of Physical Chemistry A “mother wavelet”, to decompose a multidimensional (real) signal into different frequency bands known as scales, which are then subsequently converted into oscillatory frequencies. Among the two most common WTs available (discrete and continuous), we used a continuous wavelet transform (CWT) to expand the FT information into the time domain. Broadening in the frequency domain is one of the drawbacks of this technique. However, in the current scenario, we are interested in 560 and 567 cm−1 that are relatively isolated in the frequency domain giving us confidence in the CWT results. CWT was applied to the residual pump−probe data of Ox1/ CN. As shown in different frames of Figure 3, the amplitude of the coherence at 560 cm−1 is distributed in the form of two distinct contours separated by a node at around 14 800 cm−1. This node is approximately at the fluorescence maximum of Ox1 in chloronaphthalene. The node continues to persist at this probe frequency until ∼1 ps, and during this time scale a continuous decrease in amplitude of the contour at the lower frequency region relative to the node is observed. After ca. 1 ps, a distinctly separate node starts appearing on the higher probe frequency around 15 100 cm−1 with contours on either side of it. It persists for several picoseconds (Figure 3). The node observed after 1 ps is located at the frequency corresponding to the absorption maximum of the Ox1/CN system, suggestive of ground state coherence. This clear transformation from the node at 14 800 to the node at 15 100 cm−1 is explained by assuming that immediately after impulsive excitation, a dominant wavepacket oscillates in the excited state with a minor one in the ground state. The phase change at the minimum of the excited state potential (which coincides with the fluorescence maximum) results in the appearance of a node around 14 800 cm−1.37,56 Within 1 ps, however, the wavepacket on the excited state completely dephases, leaving only the wavepacket on the ground state. The disappearance of the node at one position and its reappearance at other position on this time scale corroborates well with the dephasing time of the oscillations in a different spectral region; the excited state absorption region centered at 17 850 cm−1 (Figure S4), which is exclusively due to wavepacket motion in the excited electronic state. In Ox1/DMA the wavelet analysis shows that the 560 cm−1 frequency on the excited state is lost in about 100 fs. In Figure 4, the frames at different waiting times depict that the amplitude of this oscillatory mode is localized toward the stimulated emission at the early time, and immediately afterward the amplitude is shifted toward the absorption region. We cannot see the node distinctly at the frequency corresponding to stimulated emission in this system as we saw in the nonreactive system, primarily due to the rapid decay of excited state owing to the electron transfer quenching. Residual weak amplitude oscillations persist for several picoseconds, but in the spectral region of the absorption maximum, meaning these are ground state oscillations produced by the pump pulse.

Figure 4. Time−frequency analysis performed on pump−probe measurement using CWT on the time domain residual data of oxazine 1 in dimethylaniline. The changes are very rapid, and the signals dephase at a much faster rate. No prominent excited state minimum could be located, but a ground state minimum appears much earlier than in the nonreactive system.

Representative 2D absorptive maps (sum of phased real rephasing and nonrephasing) of Ox1 in CN and DMA at various waiting times are shown in Figure 5. One can observe a significant decrease in the amplitude of third-order signal from 40 to 400 fs in Ox1/DMA relative to the nonreactive Ox1/CN system, suggestive of a very rapid electron transfer process. The 2D maps of Ox1/CN (Figure 5, top panel) show features corresponding to the bleach/stimulated emission of the main band and the vibronic sub-bands. The small differences observed in the 2D map of Ox1 in DMA relative to Ox1 in CN (specifically at 40 fs) are mostly due to the small magnitude of third-order signal in the Ox1/DMA system relative to the nonreactive system. Because of this, the residual solvent response is overexpressed and eventually adds some structure to the 2D map at early time in the dynamics, which have no relevant physical significance. The time scale of the electron transfer process in this system is on the order of ∼100 fs, so with the very high resolution (12 fs from FROG) of our 2D measurements, we can probe the early electron transfer dynamics. An area integrated kinetic trace was extracted at the main bleach band (around ν1 = ν3 = 15 000 cm−1) from the Ox1/DMA data set and also from the neat DMA as a control system. It was found that the decay in Ox1/DMA could be fitted by a biexponential function with two time constants as 5 and 100 fs, whereas the kinetic trace of the neat DMA had a sharp decay with a time constant of 9 fs, as shown in Figure S7 of the Supporting Information. The two time constants, 5 and 100 fs, are due to the coherent artifact around time zero (may also have some small contribution from electron transfer, because it is slight faster than the time constant of 9 fs in pure solvent) and the electron transfer, respectively. We could not reproduce the 30 and 80 fs time constants reported earlier by Zinth and co-workers.52 The amplitude of the third-order signal in the 2D electronic spectroscopy data is strongly modulated along the waiting time.

5. TWO-DIMENSIONAL ELECTRONIC SPECTROSCOPY Two-dimensional electronic spectroscopy (2DES) provides additional insights by spreading the information along the excitation (ν1) and detection axis (ν3), wherein each excitation frequency is correlated to the detection frequency.68−77 Additionally, the nonlinear signal can be separated into its rephasing and nonrephasing components, which eventually helps in assigning observed coherences to the ground and/or excited state. 11841


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Figure 5. Two-dimensional absorptive excitation−detection correlation maps of Ox1 in chloronaphthalene (top panel) and dimethylaniline (bottom panel) at different waiting times. All the 2D maps are scaled to the same limits of z-axis to make an easy comparison.

Figure 6. Absorptive, rephasing, and nonrephasing beating maps corresponding to 580 cm−1 obtained by performing Fourier analysis of the 2D data along the waiting time. The top panels a−c are the excitation−detection 2D correlation maps of oxazine 1 in chloronaphthalene and plots d−f are the maps of oxazine 1 in dimethylaniline.

The population dynamics were removed by fitting the time domain data with a multiexponential function, and the residuals were Fourier transformed with respect to the waiting time, t2. The analysis reveals a strong frequency at 580 cm−1 and a weak oscillation at 1150 and 1420 cm−1, in Figures S8 and S9 of the Supporting Information. The oscillatory frequencies obtained from 2DES measurements are slightly different from those of pump−probe measurements as a result of added ν2 uncertainty

in 2D experiments due to the shorter waiting time window (t2max = 420 fs). Specifically, the mode at ca. 580 cm−1 appears to be composed of two wavepackets 560 and 608 cm−1 (pump−probe measurements above show that both of these wavepackets modulate the signal at the earlier waiting time); however, due to our small t2 time window, the ν2 bandwidths are large, and hence, the two frequencies appear overlapped. 11842


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excited state of Ox1. This addition of electron to the Ox1* strongly quenches the excited state population but has little effect on the nuclear degrees of freedom because the mass of the electron is small compared to that of the molecule. Preparing the excited state of Ox1 (electron acceptor) using a broadband ultrashort pulse produces a coherent superposition of vibrational levels, described as a wavepacket, in the Franck− Condon region of excited state. By performing time−frequency analysis, we have clearly isolated the contribution of wavepackets in the excited and the ground state. In the control system (no electron transfer), the excited state vibrational coherence was detected in the excited state absorption and stimulated emission regions. The presence of a node at the emission maximum until about ∼1 ps revealed by wavelet transformation assigns vibrational coherence to the excited state. The analysis shows repositioning of the node to the absorption maximum after 1 ps, enabling us to assign the coherence to the ground state at later time. However, it does not mean that ground state coherence appears only after excited state vibrational coherence has dephased. Rather, the ground state coherence has always been there from the beginning of the impulsive excitation, but due to having a small amplitude than the excited state coherence, it appears as a minor wavepacket. Contrarily, the excited state population is depleted very rapidly due to electron transfer quenching in the electron transfer system and hence the observation of a node at the excited state minimum was not possible. Instead, WT shows the presence of node at the absorption maximum immediately after a couple of hundred femtoseconds persisting for a much longer time scale. It does not, however, necessarily mean that the excited state wavepacket has completely dephased. It may also imply that we have lost the ability to probe the vibrational coherence in the excited state absorption and stimulated emission region. That is expected because the excited state is completely depopulated on a time scale of a few hundred femtoseconds, so the vibrational wavepacket can no longer modulate signal amplitude from the excited electronic state. There are two possibilities: either the excited state wavepacket is completely dephased during electron transfer or it is still active and instead now modulating the absorption changes of the species formed after the excited state has accepted an electron from DMA. Zinth and co-workers52 observed modulations in the photoinduced absorption of the oxazine radical formed after electron transfer, having the same frequency as the excited state vibrational coherence in the control system. These modulations lasted longer than the time scale of electron transfer and also have the same phase as the oscillations in the excited state absorption change in control system. There have been cases previously where amplitude modulations were observed on the product surface like during isomerization of retinal80,81 and back electron transfer from charge separated state to the ground state in tetracyanoethylene-pyrene complex,82 and the events were ascribed to vibrationally coherent processes. However, in the current scenario, during the electron transfer process, an electron is added to the excited state of Ox1 and this addition will have little effect on the nuclear motion, supported by the previous observation52 that the frequency and the phase of the vibrational coherence are same in the product state of electron transfer system and the excited state of control system.

To rule out any oscillatory contribution from the solvent, a control 2D experiment on chloronaphthalene was performed under similar conditions using the same laser spectrum. The Fourier analysis of the time domain solvent data indicated frequencies at 570, 900, and 1420 cm−1. A 570 cm−1 mode occupies different position in the 2D map than the 580 cm−1 mode in Ox1/CN and its amplitude is very small in comparison, so it does not interfere with the analysis. In the Ox1/DMA system, the frequencies observed in the Fourier analysis are ca. 570, 760, and 1020 cm−1 (Figures S7 and S8). The 760 and 1020 cm−1 modes were assigned to DMA on the basis of Fourier analysis of the 2DES measurements carried out on neat DMA as a control experiment and also on the basis of earlier Raman measurements.52 Beating map construction has recently been shown to greatly aid in the assignment of coherences, particularly by mapping out each set of Feynman pathways giving rise to the oscillatory signal.72−77 Only those Feynman pathways that generate a coherence during the waiting time will predict the oscillatory signal and hence the corresponding beating map of a particular oscillatory mode will have amplitude at distinct ν1, ν3 positions defined by those particular pathways, which will subsequently aid in proper assignment of the coherences. Beating maps, in the form of excitation−detection correlation maps were constructed for the oscillatory mode in the inert and electron donating system, as shown in Figure 6. For the 580 cm−1 coherence in Ox1/CN, the beating map generated from the absorptive 2DES data shows amplitude at the bleach/stimulated emission region of the main absorption band. The beating map distinctly shows one diagonal peak and two cross peaks and it is somewhat ambiguous to assign the coherence to the ground or the excited state (Figure 6a). The details, however, become clearer by constructing the rephasing (Figure 6b) and nonrephasing (Figure 6c) beating maps for this particular mode. In the rephasing map, the amplitude is on the lower and upper cross peaks, whereas in the nonrephasing map, modulations are on the diagonal. On the basis of analysis of the beating maps,78,79 the 580 cm−1 wavepacket oscillates predominantly in the excited state. In contrast, in Ox1/DMA (Figure 6d−f), the amplitude of this oscillatory mode is weak and the spectral positions in the correlation map suggest a significant contribution from the ground state. In the rephasing beating map, the upper cross peak, which appears only if coherence exists on the excited state, is not clearly visible. This increases the possibility that the wavepacket has a significant contribution from the ground state itself. These observations are in accordance with the pump−probe measurements. Temporal information on the amplitude in the beating map of 580 cm−1 was obtained by performing wavelet transform analysis on the 2D data along the waiting time similar to the pump−probe analysis. In Ox1/CN, the amplitude at the upper cross peak in the rephasing map persists over the time scale of the experimental waiting time, which shows that the wavepacket is oscillating on the excited state for >420 fs, whereas in Ox1/DMA, the weak amplitude toward the upper cross peak vanishes within 200 fs and then only the below diagonal peak persists. These results are in accordance with the pump−probe data analysis and indicate clearly that the excited state is lost at a significantly faster rate in the Ox1/DMA system.

6. DISCUSSION Photoexcitation of Ox1 in DMA solvent stimulates the transfer of an electron from DMA in its ground electronic state to the 11843


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The Journal of Physical Chemistry A The high frequency nuclear vibrations may indeed act as promoter modes, as suggested by Bixon and Jorner,44 to enable such rapid transfer of an electron from DMA to oxazine in the adiabatic regime as reported earlier.52 Thus, the vibrational coherence observed in the photoinduced absorption change of the oxazine radical is not an outcome of coherence transfer through reactant and product curve crossing as has been previously proposed for certain system.83,84 Apparently, it is merely a “spectator” that is largely orthogonal to the reaction coordinate and is present in the electron transfer system with the only difference that our capability to observe it in the stimulated emission and excited state absorption region is lost due to rapid loss of the population from the excited state. The same vibrational coherence is now observed in the photoinduced absorption of the species formed after electron transfer. Similar evolution of vibrational coherence was recently reported by Song et al.85 while studying electron transfer in polymer−fullerene blends.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported as part of the Photosynthetic Antenna Research Center (PARC), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0001035. S.R. thanks Marius Koch and Margherita Maiuri for many insightful discussions.

7. CONCLUSIONS The time evolution of vibrational coherence was determined by performing wavelet transform analysis on the time domain transient absorption and two-dimensional electronic spectroscopic data. The utility of this analysis was shown by revealing the evolution of coherence in real time and position, i.e., temporally and spectrally. In the control system, where the electron donating solvent was replaced by an inert solvent (chloronaphthalene), the vibrational coherence in the excited state persisted for ca. 1 ps, strongly modulating the stimulated emission and excited state absorption signals. The presence of a node (where the oscillation amplitude goes to zero) at the fluorescence maximum during this time frame is indicative of excited state wavepacket motion. Afterward, the node is repositioned at the absorption maximum, enunciating the dephasing of the excited state coherence while the wavepacket on the ground state continues to oscillate over a longer time scale (>3 ps). Contrarily, in the electron transfer system where dimethylaniline was used as an electron donating solvent, the node is no longer present at the excited state minimum, while a node at the ground state minimum appears within 200 fs. This result suggests that our capacity to monitor the evolution of wavepacket motion on the reactant state is lost due to the electron transfer process. It does not, however, mean that the wavepacket is dephased during the electron transfer process, rather, it is still active as a spectator and now modulating the photoinduced absorption of the product state. Such an observation is in agreement with the Born−Oppenheimer separation of electronic and nuclear motion, which infers that the “instantaneous” addition of an electron to the photoexcited reactant does not significantly perturb the vibrational coherence.

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obtained with two different laser spectra, Fourier transform at various probe frequency in Ox1/DMA system, fitting of kinetic traces extracted from 2D data of Ox1/DMA and neat DMA, Fourier transform of 2D kinetic traces at various excitation−detection correlations frequencies in Ox1/CN and Ox1/DMA system, Fourier transform plots integrated over the excitation and detection frequencies of absorptive 2D data of Ox1/ CN and Ox1/DMA system. (PDF)

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REFERENCES

(1) Jortner, J.; Bixson, M. Electron Transfer: From Isolated Molecules to Biomolecules. Advances in Chemical Physics; John Willey & Sons, Inc.: New York, 1999; Vol. 106, Part 1. (2) Jortner, J.; Bixson, M. Electron transfer: From Isolated Molecules to Biomolecules. Advances in Chemical Physics; John Willey & Sons, Inc.: New York, 1999; Vol. 107, Part 2. (3) Libby, W. F. Theory of Electron Exchange Reactions in Aqueous Systems. J. Phys. Chem. 1952, 56, 863−868. (4) Marcus, R. A. On the Theory of Oxidation-Reductions Reactions Involving Electron Transfer. I. J. Chem. Phys. 1956, 24, 966−978. (5) Marcus, R. A. On the Theory of Electron Transfer Reactions. VI Unified Treatment for Homogeneous and Electrode Reactions. J. Chem. Phys. 1965, 43, 679−701. (6) Marcus, R. A.; Sutin, N. Electron Transfer in Chemistry and Biology. Biochim. Biophys. Acta, Rev. Bioenerg. 1985, 811, 265−322. (7) Hush, N. S. Adiabatic Rate Processes at Electrodes. I. Energy Charge Relations. J. Chem. Phys. 1958, 28, 962−972. (8) Onuchic, J. N.; Beratan, D. N.; Hopfield, J. J. Some Aspects of Electron Transfer Reaction Dynamics. J. Phys. Chem. 1986, 90, 3707− 3721. (9) Zusman, L. D. The Theory of Transitions between Electronic States, Applications to Radiationless Transitions in Polar Solvents. Chem. Phys. 1983, 80, 29−43. (10) Zusman, L. D. The Theory of Electron Transfer Reactions in Solvents with Two Characteristic Relaxation Times. Chem. Phys. 1988, 119, 51−61. (11) Calef, D. F.; Wolynes, P. G. Classical Solvent Dynamics and Electron Transfer. I. Continuum Theory. J. Phys. Chem. 1983, 87, 3387−3400. (12) Hynes, J. T. Outer Sphere Electron Transfer Reactions and Frequency Dependent Friction. J. Phys. Chem. 1986, 90, 3701−3706. (13) Rips, I.; Jortner, J. Dynamic Solvent Effects on Outer Sphere Electron Transfer. J. Chem. Phys. 1987, 87, 2090−2104. (14) Jortner, J.; Bixson, M. Intramolecular Vibrational Excitations Accompanying Solvent-Controlled Electron Transfer Reactions. J. Chem. Phys. 1988, 88, 167−170. (15) Bixon, M.; Jortner, J. Solvent Relaxation Dynamics and Electron Transfer. Chem. Phys. 1993, 176, 467−481. (16) Yan, Y. J.; Sparpaglione, M.; Mukamel, S. Solvent Dynamics in Electron Transfer, Isomerization, and Nonlinear Optical Processes. A Unified Liouville Space Theory. J. Phys. Chem. 1988, 92, 4842−4853. (17) Kobayashi, T.; Takagi, Y.; Kandori, H.; Kemnitz, K.; Yoshihara, K. Femtosecond Intermolecular Electron Transfer in Diffusionless,

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.5b09390. The narrowband pump−probe measurements of Ox1/ CN and Ox1/DMA, individual Fourier transform at various probe frequencies in the Ox1/CN system, pump−probe data of Ox1/CN obtained with two different laser spectra, FT at the excited state absorption in Ox1/CN system, pump−probe data of Ox1/DMA 11844


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Prototypical Artificial Light-Harvesting System. Nat. Commun. 2013, 4, 1602. (36) Falke, S. M.; Rozzi, C. A.; Brida, D.; Maiuri, M.; Amato, M.; Sommer, E.; De Sio, A.; Rubio, A.; Cerullo, G.; Molinari, E.; Lienau, C. Coherent Ultrafast Charge Transfer in an Organic Photovoltaic Blend. Science 2014, 344, 1001−1005. (37) Vos, M. H.; Rappaport, F.; Lambry, J. C.; Breton, J.; Martin, J. L. Visualization of Coherent Nuclear Motion in a Membrane-Protein by Femtosecond Spectroscopy. Nature 1993, 363, 320−325. (38) Vos, M. H.; Jones, M. R.; Hunter, C. N.; Breton, J.; Martin, J. L. Coherent Nuclear-Dynamics at Room-Temperature in Bacterial Reaction Centers. Proc. Natl. Acad. Sci. U. S. A. 1994, 91, 12701− 12705. (39) Vos, M. H.; Jones, M. R.; Hunter, C. N.; Breton, J.; Lambry, J. C.; Martin, J. L. Coherent Dynamics During the Primary ElectronTransfer Reaction in Membrane-Bound Reaction Centers of Rhodobacter Sphaeroides. Biochemistry 1994, 33, 6750−6757. (40) Walker, G. C.; Åkesson, E.; Johnson, A. E.; Levinger, N. E.; Barbara, P. F. Interplay of Solvent Motion and Vibrational Excitation in Electron-Transfer Kinetics. J. Phys. Chem. 1992, 96, 3728−3736. (41) Barbara, P. F.; Walker, G. C.; Smith, T. P. Vibrational-Modes and the Dynamic Solvent Effect in Electron and Proton-Transfer. Science 1992, 256, 975−981. (42) Wynne, K.; Reid, G. D.; Hochstrasser, R. M. Vibrational Coherence in Electron Transfer: The Tetracyanoethylene-Pyrene Complex. J. Chem. Phys. 1996, 105, 2287−2297. (43) Lucke, A.; Mak, C. H.; Egger, R.; Ankerhold, J.; Stockburger, J.; Grabert, H. Is the Direct Observation of Electronic Coherence in Electron Transfer Reactions Possible? J. Chem. Phys. 1997, 107, 8397− 8408. (44) Bixon, M.; Jortner, J. Vibrational Coherence in Nonadiabatic Dynamics. J. Chem. Phys. 1997, 107, 1470−1482. (45) Ried, P. J.; Silva, C.; Barbara, P. F.; Karki, L.; Hupp, J. T. Electronic Coherence, Vibrational Coherence, and Solvent Degrees of Freedom in the Femtosecond Spectroscopy of Mixed-Valence Metal Dimers in H2O and D2O. J. Phys. Chem. 1995, 99, 2609−2616. (46) Romero, E.; Augulis, R.; Novoderezhkin, V. I.; Ferretti, M.; Thieme, J.; Zigmatus, D.; van Grondelle, R. Quantum Coherence in Photosynthesis for Efficient Solar-Energy Conversion. Nat. Phys. 2014, 10, 676−682. (47) Jortner, J.; Bixon, M. Intramolecular Vibrational Excitations Accompanying Solvent-Controlled Electron Transfer Reactions. J. Chem. Phys. 1988, 88, 167−170. (48) Walker, G. C.; Akesson, E.; Johnson, A. E.; Levinger, N. E.; Barbara, P. F. Interplay of Solvent motion and Vibrational Excitation in Electron-transfer Kinetics: Experiment and Theory. J. Phys. Chem. 1992, 96, 3728−3736. (49) Yoshihara, K.; Nagasawa, Y.; Yartsev, A.; Kumazaki, S.; Kandori, H.; Johnson, A. E.; Tominaga, K. Femtosecond Intermolecular Electron Transfer in Condensed Systems. J. Photochem. Photobiol., A 1994, 80, 169−175. (50) Rubtsov, I. V.; Shirota, H.; Yoshihara, K. Ultrafast Photoinduced Solute-Solvent Electron Transfer: Configuration Dependence. J. Phys. Chem. A 1999, 103, 1801−1808. (51) Seel, M.; Engleitner, S.; Zinth, W. Wavepacket Motion and Ultrafast Electron Transfer in the System Oxazine 1 in N,NDdimethylaniline. Chem. Phys. Lett. 1997, 275, 363−369. (52) Engleitner, S.; Seel, M.; Zinth, W. Nonexponentialities in the Ultrafast Electron Transfer Dynamics in the System Oxazine 1 in N,NDimethylaniline. J. Phys. Chem. A 1999, 103, 3013−3019. (53) Nagasawa, Y.; Yoneda, Y.; Nambu, S.; Muramatsu, M.; Takeuchi, E.; Tsumori, H.; Morikawa, S.; Katayama, T.; Miyasaka, H. Coherent Wavepacket Motion in an Ultrafast Electron Transfer System Monitored by Femtosecond Degenerate Four-Wave Mixing and Pump Probe Spectroscopy. Chem. Phys. 2014, 442, 68−76. (54) Nagasawa, Y.; Yoneda, Y.; Nambu, S.; Muramatsu, M.; Takeuchi, E.; Tsumori, H.; Miyasaka, H. Femtosecond Degenerate Four-Wave Mixing Measurements of Coherent Intermolecular

Weakly Polar Systems: Nile Blue in Aniline and N,N-Dimethylaniline. Chem. Phys. Lett. 1991, 180, 416−422. (18) Kandori, H.; Kemnitz, K.; Yoshihara, K. Subpicosecond Transient Absorption Study of Intermolecular Electron Transfer between Solute and Electron Donating Solvents. J. Phys. Chem. 1992, 96, 8042−8048. (19) Yartsev, A.; Nagasawa, Y.; Douhal, A.; Yoshihara, K. Solvent and Nuclear Dynamics in Ultrafast Intermolecular Electron Transfer in a Diffusionless, Weakly Polar System. Chem. Phys. Lett. 1993, 207, 546− 550. (20) Nagasawa, Y.; Yartsev, A. P.; Tominaga, K.; Johnson, A. E.; Yoshihara, K. Substituent Effects on Intermolecular Electron Transfer: Coumarins in Electron Donating Solvents. J. Am. Chem. Soc. 1993, 115, 7922−7923. (21) Tominaga, K.; Kliner, D. A. V.; Johnson, A. E.; Levinger, N. E.; Barbara, P. F. Femtosecond Experiments and Absolute Rate Calculations on Intervalence Electron Transfer of Mixed Valence Compounds. J. Chem. Phys. 1993, 98, 1228−1243. (22) Nagasawa, Y.; Yartsev, A. P.; Tominaga, K.; Bisht, P. B.; Johnson, A. E.; Yoshihara, K. Dynamical Aspects of Ultrafast Intermolecular Electron Transfer Faster than Solvation Process: Substituent Effects and Energy Gap Dependence. J. Phys. Chem. 1995, 99, 653−662. (23) Yoshihara, K.; Tominaga, K.; Nagasawa, Y. Effects of the Solvent Dynamics and Vibrational Motions in Electron Transfer. Bull. Chem. Soc. Jpn. 1995, 68, 696−712. (24) Engel, G. S.; Calhoun, T. R.; Read, E. L.; Ahn, T.-K.; Mancal, T.; Cheng, Y.-C.; Blankenship, R. E.; Fleming, G. R. Evidence for Wavelike Energy Transfer through Quantum Coherence in Photosynthetic Systems. Nature 2007, 446, 782−786. (25) Cheng, Y. C.; Fleming, G. R. Dynamics of Light Harvesting in Photosynthesis. Annu. Rev. Phys. Chem. 2009, 60, 241−262. (26) Ishizaki, A.; Calhoun, T. R.; Schlau-Cohen, G. S.; Fleming, G. R. Quantum Coherence and Its Interplay with Protein Environments in Photosynthetic Electronic Energy Transfer. Phys. Chem. Chem. Phys. 2010, 12, 7319−7337. (27) Panitchayangkoon, G.; Hayes, D.; Fransted, K. A.; Caram, J. R.; Harel, E.; Wen, J. Z.; Blankenship, R. E.; Engel, G. S. Long-Lived Quantum Coherence in Photosynthetic Complexes at Physiological Temperature. Proc. Natl. Acad. Sci. U. S. A. 2010, 107 (29), 12766− 12770. (28) Collini, E.; Scholes, G. D. Coherent Intrachain Energy Migration in a Conjugated Polymer at Room Temperature. Science 2009, 323, 369−373. (29) Collini, E.; Wong, C. Y.; Wilk, K. E.; Curmi, P. M. G.; Brumer, P.; Scholes, G. D. Coherently Wired Light-Harvesting in Photosynthetic Marine Algae at Ambient Temperature. Nature 2010, 463, 644−648. (30) Harrop, S. J.; Wilk, K. E.; Dinshaw, R.; Collini, E.; Mirkovic, T.; Teng, C.-Y.; Oblinsky, D.; Green, B. R.; Hoef-Emden, K.; Hiller, R. G. Single-Residue Insertion Switches the Quaternary Structure and Exciton States of Cryptophyte Light-Harvesting Proteins. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, E2666. (31) Fassioli, F.; Dinshaw, R.; Arpin, P. C.; Scholes, G. D. Photosynthetic Light Harvesting: Excitons and Coherence. J. R. Soc., Interface 2014, 11, 20130901. (32) Scholes, G. D.; Fleming, G. R.; Olaya-Castro, A.; van Grondelle, R. Lessons from Nature About Solar Light Harvesting. Nat. Chem. 2011, 3, 763−774. (33) Gelinas, S.; Rao, A.; Kumar, A.; Smith, S. L.; Chin, A. W.; Clark, J.; van der Poll, T. S.; Bazan, G. C.; Friend, R. H. Ultrafast Long-Range Charge Separation in Organic Semiconductor Photovoltaic Diodes. Science 2014, 343, 512−516. (34) Tamura, H.; Martinazzo, R.; Ruckenbauer, M.; Burghardt, I. Quantum Dynamics of Ultrafast Charge Transfer at an Oligothiophene-Fullerene Heterojunction. J. Chem. Phys. 2012, 137, 22A540. (35) Rozzi, C. A.; Falke, S. M.; Spallanzani, N.; Rubio, A.; Molinari, E.; Brida, D.; Maiuri, M.; Cerullo, G.; Schramm, H.; Christoffers, J.; Lienau, C. Quantum Coherence Controls the Charge Separation in a 11845


Article

The Journal of Physical Chemistry A Vibrations in an Ultrafast Electron Transfer System. Vib. Spectrosc. 2014, 70, 58−62. (55) Schneider, S.; Stammler, W.; Bierl, R.; Jager, W. Ultrafast Photoinduced Charge Separation and Recombination in Weakly Bound Complexes between Oxazine Dyes and N,N-Dimethylaniline. Chem. Phys. Lett. 1994, 219, 433−439. (56) McClure, S. D.; Turner, D. B.; Arpin, P. C.; Mirkovic, T.; Scholes, D. Coherent Oscillations in the PC577 Cryptophyte Antenna Occur in the Excited State. J. Phys. Chem. B 2014, 118, 1296−1308. (57) Turner, D. B.; Wilk, K. E.; Curmi, P. M. G.; Scholes, G. D. Comparison of Electronic and Vibrational Coherence Measured by Two-Dimensional Electronic Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 1904−1911. (58) Anna, J. M.; Song, Y.; Dinshaw, R.; Scholes, G. D. Two Dimensional Electronic Spectroscopy for Mapping Molecular Photophysics. Pure Appl. Chem. 2013, 85, 1307−1319. (59) Fork, R. L.; Cruz, C. H. B.; Becker, P. C.; Shank, C. V. Compression of Optical Pulses to Femtoseconds by Using Cubic Phase Compensation. Opt. Lett. 1987, 12, 483−485. (60) Trebino, R.; DeLong, K. W.; Fittinghoff, D. N.; Sweetser, J. N.; Krumbugel, M. A.; Richman, B. A.; Kane, D. J. Measuring Ultrashort Laser Pulses in the Time-Frequency Domain Using FrequencyResolved Optical Gating. Rev. Sci. Instrum. 1997, 68, 3277−3295. (61) Jonas, D. M.; Fleming, G. R. Vibrationally Abrupt Pulses in Pump-Probe Spectroscopy. In Ultrafast Processes in Chemistry and Photobiology; El-Sayed, M. A., Tanaka, I., Molin, Y., Eds.; Oxford, Blackwell Scientific Publications: Oxford, U.K., 1995; pp 225−256. (62) Bardeen, C. J.; Wang, Q.; Shank, C. V. Femtosecond Chirped Pulse Excitation of Vibrational Wavepackets in LD690 and Bacteriorhodopsin. J. Phys. Chem. A 1998, 102, 2759−2766. (63) Kraack, J. P.; Buckup, T.; Motzkus, M. Vibrational analysis of excited and ground electronic states of all-trans retinal protonated Schiff-bases. Phys. Chem. Chem. Phys. 2011, 13, 21402−21410. (64) Arpin, P. C.; Turner, D. B.; McClure, S. D.; Jumper, C. J.; Mirkovic, T.; Challa, J. R.; Lee, J.; Teng, C. Y.; Green, B. R.; Wilk, K. E.; Curmi, P. M. G.; Hoef-Emden, K.; McCamant, D. W.; Scholes, G. D. Spectroscopic Studies of Cryptophyte Light Harvesting Proteins: Vibrations and Coherent Oscillations. J. Phys. Chem. B 2015, 119, 10025−10034. (65) Van der Berg, J. Wavelets in Physics; Cambridge University Press: Cambridge, U.K., 2004. (66) Mallat, S. A Wavelet Tour of Signal Processing; Academic Press: New York, 1999. (67) Prior, J.; Castro, E.; Chin, A. W.; Almeida, J.; Huelga, S. F.; Plenio, M. B. Wavelet Analysis of Molecular Dynamics: Efficient Extraction of Time-Frequency Information in Ultrafast Optical Processes. J. Chem. Phys. 2013, 139, 224103−224109. (68) Jonas, D. M. Two Dimensional Femtosecond Spectroscopy. Annu. Rev. Phys. Chem. 2003, 54, 425−463. (69) Brixner, T.; Mancal, T.; Stopkin, I. V.; Fleming, G. R. PhaseStabilized Two-Dimensional Femtosecond Spectroscopy. J. Chem. Phys. 2004, 121, 4221−4236. (70) Brixner, T.; et al. Two-Dimensional Spectroscopy of Electronic Couplings in Photosynthesis. Nature 2005, 434, 625−628. (71) Turner, D. B.; et al. Quantitative Investigation of Quantum Coherence for a Light Harvesting Protein at Conditions Simulating Photosynthesis. Phys. Chem. Chem. Phys. 2012, 14, 4857−4874. (72) Ostroumov, E. E.; Mulvaney, R. M.; Cogdell, R. J.; Scholes, G. D. Broadband 2D Electronic Spectroscopy Reveals a Carotenoid Dark State in Purple Bacteria. Science 2013, 340, 52−56. (73) Seibt, J.; Pullerits, T. Beating Signals in 2D Spectroscopy: Electronic or Nuclear Coherences? Applications to a Quantum Dot Model System. J. Phys. Chem. C 2013, 117, 18728−18737. (74) Fuller, F. D.; Pan, J.; Gelzinis, A.; Butkus, V.; Senlik, S. S.; Wilcox, D. E.; Yocum, C. F.; Valkunas, L.; Abramavicius, D.; Ogilvie, J. P. Vibronic Coherence in Oxygenic Photosynthesis. Nat. Chem. 2014, 6, 706−711. (75) Dean, J. C.; Rafiq, S.; Oblinksy, D. G.; Cassette, E.; Jumper, C. C.; Schole, G. D. Broadband Transient Absorption and Two-

Dimensional Electronic Spectroscopy of Methylene Blue. J. Phys. Chem. A 2015, 119, 9098−9108. (76) Butkus, V.; Genzinis, A.; Augulis, R.; Gall, A.; Buchel, C.; Robert, B.; Zigmantas, D.; Valkunas, L.; Abramavicius, D. Coherence and Population Dynamics of Chlorophyll Excitation in FCP Complex: two dimensional spectroscopic study. J. Chem. Phys. 2015, 142, 212414. (77) Egorova, D. Detection of Dark State in Two-Dimensional Electronic Photon-Echo Signals via Ground-State Coherence. J. Chem. Phys. 2015, 142, 212452. (78) Turner, D. B.; Wilk, K. E.; Curmi, P. M. G.; Scholes, G. D. Comparison of Electronic and Vibrational Coherence Measured by Two-Dimensional Electronic Spectroscopy. J. Phys. Chem. Lett. 2011, 2, 1904−1911. (79) Egorova, D. Self-analysis of Coherent Oscillations in Time Resolved Optical Signals. J. Phys. Chem. A 2014, 118, 10259−10267. (80) Peteanu, L. A.; Schoenlein, R. W.; Wang, Q.; Mathies, R. A.; Shank, C. V. The First Step in Vision Occurs in Femtoseconds: Complete Blue and Red Spectral Studies. Proc. Natl. Acad. Sci. U. S. A. 1993, 90, 11762−11766. (81) Wang, Q.; Schoenlein, W.; Peteanu, L. A.; Mathies, R. A.; Shank, C. V. Vibrationally Coherent Photochemistry in the Femtosecond Primary Event of Vision. Science 1994, 266, 422−424. (82) Wynne, K.; Reid, G. D.; Hochstrasser, R. M. Virbrational Coherence in Electron Transfer: The Tetracyanoethylene-Pyrene Complex. J. Chem. Phys. 1996, 105, 2287−2297. (83) Jean, J. M.; Fleming, G. R. Competition between Energy and Phase Relaxation in Electronic Curve Crossing Processes. J. Chem. Phys. 1995, 103, 2092−2101. (84) Jean, J. M. Vibrational Coherence Effects on Electronic Curve Crossing. J. Chem. Phys. 1996, 104, 5638−5646. (85) Song, Y.; Clafton, S. N.; Pensack, R. D.; Kee, T. W.; Scholes, G. D. Vibrational Coherence Probes the Mechanism of Ultrafast Electron Transfer in Polymer-Fullerene Blends. Nat. Commun. 2014, 5, 4933.

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Observing Vibrational Wavepackets during an Ultrafast Electron

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