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Measuring Coherently Coupled Intramolecular Vibrational and Charge Transfer Dynamics with Two-Dimensional Vibrational-Electronic Spectroscopy Trevor L. Courtney, Zachary W. Fox, Laura Estergreen, and Munira Khalil J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b00356 • Publication Date (Web): 20 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015

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The Journal of Physical Chemistry Letters

Measuring Coherently Coupled Intramolecular Vibrational and Charge Transfer Dynamics with Two-Dimensional Vibrational-Electronic Spectroscopy Trevor L. Courtney, Zachary W. Fox, Laura Estergreen, and Munira Khalil* Department of Chemistry, University of Washington, Seattle, Washington 98195, United States AUTHOR INFORMATION Corresponding Author *Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT.

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We demonstrate Fourier transform (FT) 2D vibrational-electronic (2D VE)

spectroscopy employing a novel mid-IR and optical pulse sequence. This new femtosecond third-order nonlinear spectroscopy provides the high time and frequency resolutions of existing 2D FT techniques; however, resulting 2D VE spectra contain IR and electronic dipole moment cross terms. We use 2D VE spectroscopy to help understand the vibrational-electronic couplings in the cyanide-bridged transition metal mixed valence complex [(CN)5FeIICNRuIII(NH3)5]dissolved in formamide. The amplitude of the cross-peaks in the 2D VE spectra reveal that three of the intramolecular cyanide stretching vibrations lying along the charge transfer axis are coherently coupled to the metal-to-metal charge transfer electronic transition with differing strengths. Analysis of the 2D VE lineshapes reveals positive and negative correlations of the cyanide stretching modes with the charge transfer transition depending on the physical orientation of the vibration in the molecule and its interaction with the solvent. The insights found thus far into the vibronic couplings in the mixed valence model system indicate that the 2D VE technique will be a valuable addition to the existing multidimensional spectroscopy toolbox. TOC GRAPHIC

KEYWORDS Fourier transform, femtosecond vibronic dynamics, nonlinear multidimensional, metal-to-metal charge transfer, mixed valence


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The direct probing of coupled electronic and vibrational motions in ultrafast photoinduced processes in the condensed phase is key to understanding fundamental energy conversion phenomena in molecules and materials. This is an experimentally challenging task requiring coherent probes with high time and frequency resolution across varying length and time scales.

Over the last two decades, the development and application of coherent 2D

electronic and 2D infrared (IR) spectroscopies have provided researchers with tools to measure the couplings, correlated dynamics, and energy transfer pathways between coupled electronic states and vibrational modes, respectively.1-3 Both of these spectroscopies have enhanced our understanding of complex phenomena such as hydrogen bonding dynamics in liquids and proteins and exciton dynamics in light harvesting and solid-state complexes at the molecular level.4-7 However, third-order multidimensional coherent techniques that combine vibrational (V) and electronic (E) spectroscopies and can directly probe cross-peaks between vibrational and electronic transitions have yet to be fully realized. A notable exception is the recent 2D-EV work of Fleming and coworkers,8 in which evolutions of nuclear modes are probed on electronically excited potential energy surfaces using a sequence of femtosecond optical and IR pulses.

In this paper, we report the development of Fourier transform (FT), coherent 2D

vibrational-electronic (2D VE) spectroscopy and use it to measure cross-peaks between cyanide stretching vibrations (νCN)

and the metal-to-metal charge transfer (MMCT) transition in

[(CN)5FeII–CN–RuIII(NH3)5]− (FeRu) complex.

Determining how the excitation of specific

vibrational modes modulates charge transfer transitions will help researchers better understand correlated vibrational and electronic motions in molecular, biological, and materials systems. While no coherent multidimensional VE techniques have been reported to date, several groups have developed spectroscopies involving IR excitation and optical probe pulses. Park


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and Cho introduced a theoretical description of a coherent 2D IR spectroscopy (COTIRS) experiment, in which two individually tunable, resonant IR pulses are used to generate two consecutive vibrational coherence states.9 A nonresonant optical pulse at a fixed delay probes these coherence states, and the scattered field is measured and plotted against the two IR excitation frequency axes to form a quasi-frequency domain 2D IR spectrum. A similar pulse sequence – two tunable IR excitation pulses and a nonresonant optical probe pulse – is used in the doubly vibrationally enhanced (DOVE) four-wave-mixing techniques developed by Wright and coworker.10 In DOVE, selective enhancement of vibrational cross-peaks for coupled modes helps to measure overtone and combination bands in both IR and Raman techniques. Tahara et al. performed a fully resonant femtosecond 1D VE pump-probe experiment, in which they pumped the OH stretch in the molecule quinizarin and detected the induced low-frequency vibrational coherence via oscillations of the absorbance change of a visible probe.11 Combining both the time and frequency resolutions of previous work, we demonstrate 2D VE spectroscopy, which is a femtosecond FT technique employing a sequence of two mid-IR pump pulses that excite high frequency vibrations and one optical probe field resonant with a charge transfer transition. The resultant cross-peaks measure the dynamics of initially excited vibrations and associated low-frequency vibrational modes with coupled electronic motions in a transition metal mixed valence complex. The 2D VE experiments introduced here use the pump-probe 2D geometry12-15 to simplify the phase-matching of the two-color experiment. Our pulse sequence (Fig. 1a) includes two collinear mid-IR pumps, k1 and k2, from the bright output of a Mach-Zehnder interferometer followed by a noncollinear near-IR continuum probe, kpr. Pulses k1 and k2 are separated by the vibrational coherence time, τ1. During the waiting time, τ2, a population exists in either the first


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Figure 1. 2D FT vibrational-electronic spectroscopy. (a) Collinear IR pumps, k1 and k2, (separated by the vibrational coherence time, τ1) and the noncollinear optical probe, kpr, (after a population time, τ2) impinge on the sample, generating a 3rd-order signal at detection time, τ3. Nonrephasing (NR) and rephasing (R) signals are detected with the local oscillator (LO). (b) NR excited state absorption (ESA) diagram: vibrational excitation with ω1 from ground state, g , 0 , to g ,1 , and electronic excitation to e,1' . (c) NR ground state bleach (GSB) diagram: signal is at frequency equal to the spacing between vibrational ground states g , 0 and e, 0' . (d) Potential energy surface with maximum Franck-Condon overlap between g , 0 and e, 0' . Arrows indicate pump and probe interactions; ∆, is positive (blue-shifted vibrational transitions in the electronic excited state). (e) 2D VE simulation based on (d): coupling of a νCN vibration to the Fe-to-Ru charge transfer in FeRu. ESA and GSB are set to equal strength; the exact value of ∆ is obscured by overlapping ESA and GSB peaks. Simulation parameters are in the Supporting Information. vibrationally excited state, g ,1 , as in the excited state absorption (ESA) example in Fig. 1b, or in the ground state, g , 0 , as in the ground state bleach (GSB) case (Fig. 1c). The optical probe impinges on the sample and generates a third-order signal at detection time, τ3. In this partially collinear geometry, the rephasing and nonrephasing signals copropagate with the probe,


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k sig = k1 ± k 2 + k pr , and we detect the fully absorptive 2D signal.12 The probe pulse acts as the local oscillator (LO); the signal and LO are frequency dispersed in the ω3 dimension by a spectrometer onto a CCD detector. The experimental setup is shown in Fig. S1 of the Supporting Information. A 2D VE spectrum for a fixed τ2 delay results from the FT with respect to τ1; the spectrum is purely real when sampling at and symmetrically about τ1 = 0.12, 16 The third-order signal is located at the ω1 excitation frequency of the IR-active mode responsible for the change in electronic probe transmission. For example, in the ESA scenario of Fig. 1b, a negative signal would be present at (ω1 = ωg,1, ω3 = ωe,1'–g,1). As shown in Fig. 1d-e, this negative (blue) signal would be shifted from the positive (red) GSB signal at (ω1 = ωg,1, ω3 = ωe,0'–g,0). We define the parameter ∆ as the difference in frequency of the IR active vibration in the ground and excited electronic states. Fully resonant 2D VE spectra contain IR (µ10) and electronic (µeg) transition dipole moment cross terms. However, optically nonresonant 2D VE signals, including those from solvents with broad mid-IR absorptions but no corresponding electronic transitions, may give spectral features corresponding to couplings between dipole and polarizability operators. These optically nonresonant 2D VE signals provide similar information to that in the COTIRS experiment.9 To understand coupled electronic and vibrational motions on an ultrafast time scale, we are studying a cyanide-bridged mixed valence charge-transfer complex, FeRu, shown in Fig. 2a along with a simplified representation of its four νCN modes. This is one of several complexes containing two or more metal centers with different oxidation states that have served as model systems in both steady-state and time-resolved spectroscopic experiments to probe the coupling between electronic and vibrational motions during ultrafast electron transfer reactions.17-25 Transient IR, optical, and resonance Raman studies of FeRu in polar solvents have suggested the


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involvement of some of the high frequency νCN vibrations in the charge transfer process19, 23 and significant excitation in the νCN modes upon ultrafast back electron transfer.22 We have extensively studied the vibrational anharmonic couplings among νCN modes and the role of solvent in modulating the molecular structure, anharmonic couplings, and the oxidation states of the metals in the FeRu complex.26-27 This work introduces 2D VE spectroscopy to connect the complementary electronic and vibrational spectroscopic signatures collected previously in FeRu. The goal of this work is to demonstrate the feasibility of the 2D VE technique and its selectivity in measuring the differing coupling strengths of the specific νCN modes to the MMCT transition. We dissolve FeRu in

Figure 2. Mixed-valence complex of [(CN)5FeII–CN–RuIII(NH3)5]− (FeRu). (a) Molecular structure of FeRu with colored arrows corresponding to simplified depictions of four νCN modes. The transition dipole moment of the νradial (red) mode is the only one not in line with the central MMCT axis of FeRu. (b) Linear, solvent-subtracted spectra of FeRu in formamide (black lines, share left vertical axis) with laser spectra (gray lines, right vertical axis). At mid-IR frequencies: FeRu FTIR and pump spectra (νmax mid-IR = 2050 cm-1) with vertical dotted lines of corresponding colors to modes in (a) indicating central frequencies of the four νCN modes: νbridge (purple), νaxial (green), νradial (red), and νtrans (blue). At optical frequencies: near-IR FeRu and probe spectra (νmax near-IR = 11750 cm-1) with vertical dotted line indicating νmax MMCT = 8547 cm-1.


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formamide (FA) and excite four previously assigned νCN vibrations26 – νbridge (2089.4 cm-1), νaxial (2065 cm-1), νradial (2050.6 cm-1), and νtrans (2002.0 cm-1) – with mid-IR pulse pairs centered spectrally at νmax mid-IR = 2050 cm-1 (Fig 2b). The effects of the resulting IR excitations on the MMCT transition centered at ~8547 cm-1 are probed with a near-IR continuum (νmax

near-IR

=

11750 cm-1) to the blue of the broad electronic transition (Fig 2b). Previous work has assigned the electronic transition in FeRu to be a MMCT excitation from the FeII to the RuIII, resulting in FeIII and RuII.22 As shown in Fig. 1d-e, the 2D VE signal is sensitive to the shift, Δ, in the cyanide stretching frequency upon photoexcitation to the transient FeIIIRuII complex. Spectroscopic studies of cyanoferrates in solution have found νCN stretches in FeIII to be blueshifted versus those in FeII compounds.28 This blue shift is 74.3 cm-1 in ferricyanide versus ferrocyanide, both in FA; thus, we approximate the Δ for each νCN in the FeRu complex dissolved in FA as 75 cm-1 for 2D VE simulations (Fig. 1e). The experimental 2D VE spectra are presented in Fig. 3 for FeRu in FA and solvent only samples. Since the third-order signals extend out to the wings of the probe spectrum, division by the intensity or field of the probe (LO) introduces considerable noise. Thus, we present the 2D (3) * data as the cross product Eˆ (ω3 ) Eˆ LO (ω3 ) + c.c. The signal field can then be thought of as

having nearly constant magnitude across, and a line width much larger than the width of the LO spectrum (solid gray lines, Fig. 3c,f). The 2D VE spectra measure the spectrally resolved change in transmission of the probe pulse following IR excitation of all four νCN modes in FeRu. This broad spectral signal across the probe is mostly negative (blue contours) at small τ2 delay, which implies ESA (Fig. 3d,e). A key advantage of the 2D VE spectrum is the resolution of the signal in ω1 into peaks at specific FeRu νCN modes (Fig. 3d,e). At τ2 = 50 fs (Fig. 3c), a positive peak slightly blue-shifted (17 cm-1) from the negative νbridge mode peak exists. This could indicate a


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Figure 3. 2D vibrational-electronic (VE) spectroscopy of [(CN)5FeII–CN–RuIII(NH3)5]− (FeRu) in formamide (FA). (a, b) IR pump pulse spectra (solid gray lines) and FTIR spectra of both FA (dashed black lines) and solvent-subtracted FeRu (solid black lines) plotted with vertical dotted lines denoting four νCN modes in FeRu: νbridge (purple), νaxial (green), νradial (red), and νtrans (blue). (c, f) Near-IR probe pulse spectra (solid gray lines); (c) linear electronic spectrum of FeRu in FA (solid black line). All pulse (linear) spectra are plotted in black (gray) to correspond with absorbance (intensity, Int.) axes. (d, e) Early waiting time 2D VE spectra of FeRu. (g, h) 2D VE spectra of neat FA All four 2D VE spectra are normalized to panel (d). Contour levels are at steps of 0.1 over a range of [-1.0, 1.0]. 2D VE signal from a low-frequency mode coupled to the νbridge mode. Additional spectral features in the 2D VE spectra shown in Figs. 3d-e are attributed to the solvent response, which is detailed at a later point in this discussion and in Fig. S2 of the Supporting Information. The selectivity of the 2D VE technique is evident from the experimental 2D spectra of FeRu in Fig. 3. In both the τ2 = 50 fs (Fig. 3d) and 150 fs (Fig. 3e) spectra, the vibrationally and electronically resonant signals are dominated by amplitude at the νtrans and νbridge modes, which are also the two Raman active νCN modes in the molecule.23 At 150 fs waiting time, a third mode, νaxial, which is not observed in resonance Raman experiments described in Ref. 23, clearly


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exhibits a 2D VE signal. The three modes with 2D VE signal lie along the MMCT axis in FeRu (Fig. 2a). The νradial mode, which is perpendicular to the MMCT axis in FeRu, does not show any signal in the 2D VE spectra. At τ2 = 50 fs, the νtrans mode dominates, but νtrans and νbridge peaks are at the same contour level by τ2 = 150 fs. However, solvent signals, notably at τ2 = 50 fs (Fig. 3g), complicate the identification of weak 2D VE peaks and the relative amplitude assignments of all peaks. Additionally, we expect oscillations of cross-peak amplitude in τ2 based on coupling of high frequency νCN modes to each other or to low-frequency modes. To help us better quantify the recovered signals of the VE cross peaks and find the coupling between IR and optical dipoles, we introduce an integrated projection of the τ2 = 150 fs 2D VE spectrum onto the ω1 excitation axis (Fig. 4a), and compare it with the FTIR spectrum (gray line). For the spectral region plotted in Fig. 3e, the ω1-resolved pixel slices are summed along ω3 to form the 1D projection (open circles, Fig. 4a). For ease of comparison to the FTIR spectrum, the 1D projection has been multiplied by a factor of -1. Also in Fig. 4a is an interpolated 1D projection (black line) obtained by first performing a cubic spline across ω1 at each pixel slice prior to integration along ω3. We have previously reported the relative IR transition dipole strengths of the νCN modes in FeRu in FA and with respect to the νtrans dipole moment, |μradial| = 0.56|μtrans|, |μaxial| = 0.30|μtrans|, and |μbridge| = 0.22|μtrans|.26 For a specific vibrational mode, the integrated area of the IR lineshape in the FTIR spectrum is proportional to |μIR|2 while the area of a peak in the 2D VE integrated projection onto the ω1 axis is proportional to µIR

2

2

eff eff . Here, we define the effective dipole moment, µelec , to include the electronic µelec

transition dipole moment, Franck-Condon factors, non-Condon effects, and the orientational dependence of the angles between the IR and electronic transition dipole moments. Together,


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Figure 4. 1D representations of 2D VE data of [(CN)5FeII–CN–RuIII(NH3)5]− (FeRu) in formamide. (a) The solvent-subtracted FTIR spectrum of FeRu (solid gray line) is plotted with a projection of the τ2 = 150 fs 2D VE FeRu spectrum. Pixel slices in the probe spectral region of either the 2D VE spectrum (open circles) or a cubic spline interpolated (across ω3) 2D VE spectrum (solid black line) are summed, multiplied by -1 for ease of comparison to the FTIR spectrum, and plotted on ω1. The νtrans, νradial, νaxial , and νbridge modes are denoted with blue, red, green, and purple dotted vertical lines, respectively. (b,c) ω1 positions of the maximum magnitudes of the νbridge (b) and νtrans (c) signals in interpolated 2D VE spectra at τ2 = 50 fs and τ2 = 150 fs at each pixel slice are plotted as functions of ω3 (gray traces). The solid black lines in (b,c) are the linear fits with slopes reported in the text.

eff these components of µelec determine the vibronic coupling in the system and therefore the

relative amplitudes of the peaks in the 2D VE spectra and corresponding 1D projections. A eff detailed examination of each of the contributions to µelec and their manifestations in 2D VE

spectra will be explored in future publications. From Fig. 4a we can immediately see that the νCN modes in the projection of the 2D VE spectrum have different relative strengths compared to the


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FTIR spectrum. Fitting the peaks in the 2D VE projection to find their relative areas allows us to eff eff eff eff extract µelec for each mode with respect to νtrans as follows: µradial,elec = 0 µ trans,elec , µaxial,elec =

eff eff eff 0.7 µ trans,elec , and µbridge,elec = 2.2 µ trans,elec . This analysis shows us that the bridge mode is the

most strongly coupled to the MMCT transition among the four νCN modes at τ2 =150 fs and the vibronic couplings between the MMCT transition and the high frequency νCN modes in FeRu are specific to each νCN mode. The evolution of the shape and position of each 2D VE peak provides insight into the coupled vibrational-electronic dynamics of the molecular system. Several techniques have been developed to find the frequency-frequency correlation functions (FFCFs) in 2D spectra, including nodal plane slope, ellipticity, and center line slope (CLS) methods.29-32 In this Letter, we introduce a method of determining the correlation between the IR and optical experimental frequencies for the FeRu νtrans and νbridge mode peaks by performing a procedure that most closely resembles the CLS ωτ method.31-32. The ω1 frequency corresponding to the maximum of a νCN peak is determined for each ω3 slice (parallel to the ω1 axis) of a 2D VE spectrum. In this work, each detection pixel defines a ω3 slice that is cubic spline interpolated in ω1 to refine the location of the maximum. These ω1 maxima for a given peak are plotted against ω3. For the two 2D VE spectra of FeRu (Fig. 3d,e), the peak maxima are plotted for both the νbridge (Fig. 4b) and

νtrans (Fig. 4c) peaks. Neglecting higher-order terms, the peak maxima position data is fit to the form ω1 = ωo + m(ω3 − ωc ) , where ωo is the ω1 frequency at ωc , a central ω3 frequency in the 2D VE peak. The slopes, m, of these frequency correlation plots and how they change as a function of τ2 should report on the coupled vibrational-electronic dynamics of FeRu. At τ2 = 0, the slope is limited by the ratio of the vibrational line width (obtained from FTIR spectra) to the 2D VE


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electronic line width (filtered by the spectral width of the probe spectrum), which gives a maximum magnitude of much less than unity. At long waiting times, the slope for a given νCN peak is expected to reach zero, as the 2D VE lineshape becomes symmetric with respect to ω1. While the slopes of the peaks have clear nonlinear components, we will limit our discussion to the linear fits (black lines, Fig 4b,c). A notable increase in slope in the τ2 = 150 fs trace is evident compared to the τ2 = 50 fs trace, especially for the νbridge mode (0.007 versus 0.002, Fig. 4b between 11100 cm-1 and 11800 cm-1). For the νtrans mode, a negative slope is found (-0.003 for both τ2 delays between 11100 cm-1 and 11700 cm-1). From Fig. 4b,c, it can be seen that at ωc = 11500 cm-1, ωo is much closer to the FTIR peak center for both the νbridge (2089.4 cm-1) and

νtrans (2002 cm-1) modes in the τ2 = 150 fs spectrum. In the τ2 = 50 fs 2D VE spectrum, the magnitude and nonlinearity of each peak tilt and the displacement of each ω1 peak location could be influenced by a combination of the following effects: sampling other nonlinear signals during pulse overlap, contamination by nonresonant solvent signals (e.g., peaks below 2000 cm-1, refer to Fig. S3 in the Supporting Information), or the presence of the positive feature blue-shifted from the νbridge frequency. From the geometry of the molecule, we propose that the positive correlation of the νbridge mode with the MMCT transition reflects how the νCN stretch modulates the distance between the two metal centers and the efficacy of the charge transfer. The negative correlation of the νtrans mode and the MMCT transition could reflect how the solvent modulates the charge transfer between the two metal centers. It has been shown that the νtrans mode interacts most strongly with the solvent because the trans CN ligand is the most basic, i.e., most negatively charged of the CN ligands in FeRu.23, 26 The sharing of the electron density on the trans CN ligand with the solvent affects the metal-ligand backbonding along the Fe-CN-Ru backbone resulting in a negative correlation of the νtrans mode and the MMCT transition. These


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experiments directly measure how inter- and intramolecular vibrational motions modulate the charge density along the MMCT axis. We would also expect that low-frequency modes coupled to the IR and MMCT transitions would affect the tilt of the 2D lineshape. These effects are currently under investigation. As mentioned previously, the IR pump – optical probe pulse ordering includes an inherent sensitivity to nonresonant solvent response.

Although the pump-probe geometry

implies that the copropagating signal and LO are in phase (and τ3 = 0), experiments in this geometry with nonresonant probe interactions have been shown to be sensitive to the strong birefringent portion of the signal in quadrature as described in detail by previous studies.11, 33-34 The lower panels in Fig. 3 present 2D VE spectra of neat FA taken immediately following the FeRu data to explore the effect of the nonresonant solvent response on the measured 2D VE spectra. To provide the most direct comparison between the spectra, the copropagating signal and LO were attenuated by a neutral density filter after the neat solvent sample to match the LO intensity to that after FeRu absorption. Figure S2 of the Supporting Information shows 2D VE spectra of only formamide (3g-h) subtracted from the 2D VE spectra of FeRu in FA (3d-e). The nonresonant 2D VE signals from FA (and possibly CaF2 sample cell windows) are both negative and positive, exist at early waiting time, are smaller in magnitude than the FeRu signals by a factor of 4, and are broad across ω1 (Fig. 3g,h, both normalized to Fig. 3d). Thus, not only does the excitation frequency resolution of 2D VE spectroscopy enable the measurement of couplings between specific vibrational modes and electronic transitions, but it proves advantageous in detecting weak, resonant VE signals above a broad, nonresonant background. Missing from the 2D VE spectra in Fig. 3 are the corresponding positive (GSB) peaks along the probe dimension, with centers vertically displaced from the absorption ω3 centers by Δ


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as shown in the 2D VE simulation in Fig. 1e. Given the estimated ∆ of +75 cm-1, we would still expect to see non-overlapping GSB and ESA contributions with a large ω3 separation in a 2D VE spectrum. The simplified vibronic structure recovered in the 2D VE spectra may also arise from a Franck-Condon overlap that significantly favors an electronic transition between the vibrational states involved in generating the ESA signal as opposed to a transition between states involved in generating the GSB signal.

Probing with a supercontinuum that could access

multiple quanta of νCN or with a tunable source to reach each side of the electronic absorption peak would provide insight into the ability to resolve the underlying vibrational structure in the

ω3 domain. In summary, we have demonstrated a novel 2D vibrational-electronic spectroscopy to measure coupled vibrational and electronic motions in a molecular system.

We have

investigated the couplings of the four νCN stretches to the MMCT transition in a mixed valence compound in solution. Analysis of the 2D VE peaks representing IR and electronic transition dipole moment cross terms reveals that the three modes along the charge transfer axis are each coherently coupled to the MMCT transition while the perpendicular radial mode is not. The four modes in decreasing order of mode-specific electronic coupling strength are as follows:

νbridge>νtrans>νaxial>>νradial.

This 2D VE spectroscopic measurement of strongest coupling

between the νbridge mode and the forward charge transfer transition in FeRu agrees with results from a previous, complementary transient IR experiment of a related cyanide-bridged mixed valence charge-transfer complex, in which the νbridge mode was preferentially excited during the ultrafast back electron transfer.21 The lineshapes of the cross-peaks in the 2D VE measurement are sensitive to the correlation of the vibrational and electronic transitions. We find positive (negative) correlations of the νbridge (νtrans) mode with the MMCT transition at early waiting


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times, which could be attributed to the modulation of the charge transfer by the bridging and trans vibrations along the Fe-Ru backbone and the coupling of the solvent to the νtrans ligand. We expect this technique to find wide applicability in fields of chemistry, biology, and material science, specifically in mode-selective chemistry, coupled vibrational and electron transfer, proton and electron transfer processes, and the study of materials with strong electronic correlations where vibrations have been shown to coherently drive electronic phase transitions. The development of 2D VE spectroscopy reported in this Letter and the recent development of 2D EV spectroscopy extend the reach and applications of the field of coherent femtosecond multidimensional spectroscopy. EXPERIMENTAL SECTION Sample Preparation. Starting materials and solvents were purchased from Sigma Aldrich and Alfa Aesar and were used without further purification. Steady-state infrared spectra were collected with a JASCO FT/IR-4100 with a spectral resolution of 0.5 cm−1; steady-state electronic spectra were collected with a Cary 5000 (Agilent) spectrophotometer with a spectral resolution of 1 nm. The infrared absorption spectra (νCN region) of FA is included in Fig. 3a,b. The mixed valence dimer, FeRu, was synthesized according to literature methods with a modified purification procedure.26, 35 An aqueous solution of ∼0.01 M K4[Fe(CN)6] and ∼0.01 M [RuCl(NH3)5]Cl2 was heated at 60 °C for 2 hours. Following the removal of solid impurities

by vacuum filtration, methanol was added to the aqueous sample in a 2:1 volume ratio, and the solution with precipitate was chilled overnight. The solid, green FeRu was filtered out of solution, run through an ion-exchange column packed with Dowex and charged with 1.5 M NaCl, and condensed. Finally, the sample passed through a size-exclusion column prepared with


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Bio-Gel P2 to remove starting materials. A nearly saturated solution of FeRu was prepared in FA with a maximum optical density (OD) of 0.4 in the νCN region (140 µm path length) after solvent subtraction (Fig. 2). The solvent-subtracted electronic absorption spectrum of FeRu in FA has a peak OD of 0.85 and an OD of ~0.3 at the peak of the probe spectrum (Fig 2). 2D VE Spectroscopy. The output from a 1 kHz Ti:Sapphire regenerative amplifier (800 nm, 35 fs, 2.5 W) is used to generate the pulses in this experiment. Amplified 800 nm light pumps a dual-pass optical parametric amplifier (OPA-800C, Newport) to produce short-wave IR signal and idler pulses; difference frequency mixing in a 0.5-mm thick AgGaS2 crystal yields 70 fs mid-IR pulses tuned to νmax mid-IR ≈ 2050 cm-1 (λmax mid-IR ≈ 4.88 µm) with a bandwidth of 300 cm-1. This mid-IR radiation enters a Mach-Zehnder interferometer with a gold-coated cubic retroreflector mounted to a computer-controlled translation stage in each arm for precise control of the τ1 delay. The vertically polarized IR pump pulse pairs are focused at the sample (0.4 μJ each, 1/e2 spot size of 180 μm) by a 150-mm focal length CaF2 lens. In a separate path, a portion of the amplified Ti:Sapphire output is routed through a computer-controlled delay stage to control the τ2 delay. The near-IR probe is generated by focusing ~500 nJ of 800 nm light in a 3mm thick sapphire window and compressing (fused silica prism pair) and filtering (850-nm longpass) the continuum (νmax near-IR = 11750 cm-1). The resulting vertically polarized, transformlimited, 35 fs near-IR probe pulses (~10 nJ/pulse) are focused to a 100 μm spot size at the sample by a 100-μm focal length BK7 lens. The sample is between two 1-mm thick CaF2 windows that are separated by a 140 µm Teflon spacer, and the temporal response of the experiment (FWHM of IR-optical pulse overlap in sample) is 160 fs. The low fluence of the probe pulse and the translation of the sample perpendicularly to the beam path combine to prevent any damage to the sample.


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The collinear IR pumps are chopped at 500 Hz; alternating laser shots of I LO (ω3 ) and

Eˆ LO (ω3 ) + Eˆ 2(3)D (ω3 ,τ1 ,τ 2 )

2

are fiber coupled (single mode, ThorLabs 780HP) to a 0.303 m

Czerny-Turner spectrometer (Andor Shamrock SR-303i, 600 groove/mm grating, f/4 optics) and a 1600×200 pixel, fan-cooled (-80° C), front-illuminated CCD array (Andor DU970P-UV). The third-order signal was found to have a linear dependence on IR pump power. A symmetric scan of IR pump pulses over τ1 = (-2048 fs, 2048 fs) with 4 fs steps is used to generate a 2D FT spectrum for a fixed τ2 delay. Time steps of 1 fs near τ1 = 0 and an integrated time-domain interference of IR pumps are used to find τ1 = 0 to within ~1/16 of a cycle. After a cubic spline interpolation creates equal τ1 steps, multiple scans are averaged. Fourier transformation with (3) * respect to τ1 isolates the interference term Eˆ 2 D (ω3 , ω1 ,τ 2 ) Eˆ LO (ω3 ) + c.c. ; this term is plotted

against ω1 and ω3 to form an absorptive 2D spectrum for a given τ2.

ASSOCIATED CONTENT Supporting Information. Experimental setup, solvent-subtracted 2D VE spectra and corresponding 1D representations, and simulation parameters for 2D VE spectrum (Fig. 1e). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This work was supported by the Office of Basic Energy Sciences of the U.S. Department of Energy (Grant Nos. DE-SC0002190 and DE-SC0012450), the National Science Foundation (Grant No. CHE 0847790) and the David and Lucile Packard Fellowship for Science and Engineering.


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