Coherent Vibrational Probes of Hydrogen Bond Structure Following

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Coherent Vibrational Probes of Hydrogen Bond Structure Following Ultrafast Electron Transfer Aaron S. Rury, Shayne A Sorenson, and Jahan M Dawlaty J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05239 • Publication Date (Web): 19 Aug 2016 Downloaded from http://pubs.acs.org on August 19, 2016

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The Journal of Physical Chemistry

Coherent Vibrational Probes of Hydrogen Bond Structure Following Ultrafast Electron Transfer Aaron S. Rury∗ , Shayne Sorenson, and Jahan M. Dawlaty∗ Department of Chemistry, University of Southern California E-mail: [email protected]

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To whom correspondence should be addressed

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Abstract We report measurements, ab initio calculations, and analytical models that reveal the coupling of low frequency phonons to localized vibrations of a hydrogen-bonded organic crystal driven by ultrafast electron transfer. Fourier analysis of spectrally resolved oscillations in the transient reflectivity of mid-infrared probe pulses from single crystals of quinhydrone show coupling between Raman and IR-active intermolecular lattice phonons of this material. In addition, phonon coherence spectra of two of these lattice modes show that at least two qualitatively different vibrational transitions near 3000 cm−1 exist in the charge-separated state of quinhydrone. This combined experimental and theoretical approach allows us to estimate the displacement of the O-H stretching vibration of quinhydrone along its different lattice phonon normal coordinates. These results will help better understand and approach a plethora of problems in condensed phases in which electronic and vibrational coupling over vastly different energy scales plays a significant role.

Keywords: hydrogen bonds, coherent spectroscopy, lattice dynamics, electron transfer, vibrational anharmonicity

Introduction Hydrogen bonding is a relatively weak interaction that plays a central role in physical chemical processes ranging from coupled proton-electron transfer reactions, 1–5 solvation processes in condensed phase, 6 structure in biological systems, 7 and formation and function of technologically important materials. 8 Despite the importance of hydrogen bonding in molecules and materials, deterministic exploitation of hydrogen bonding from first principles is still challenging, especially in an excited electronic state and in the presence of charge transfer. Much of the impediment to the detailed understanding of cooperative electronic and hydrogen bond interactions stems from the inhomogeneity of the systems in which these processes have so far been investigated, many coming from biology. Material science has offered a new 2


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class of crystalline materials whose properties make them candidates for next generation electronics and photonics applications that have been purported to take advantage of both electronic and protonic interactions. 9–12 However, in general, the characterization of hydrogen bonding structure in an excited or charge separated electronic state remains under-explored. Such a characterization remains totally unknown in the case of crystalline hydrogen-bonded materials to our knowledge. The direct characterization of hydrogen bonding structures in molecular materials is often hindered by the complex coupling of high frequency vibrations to their low frequency counterparts, thus clouding the interpretation of steady-state spectroscopic data. This physical picture of the complex interaction of high and low frequency vibrations has been established in the excited electronic states of molecules by femtosecond stimulated Raman spectroscopy, 13–15 but has yet to be applied to organic solids. In contrast to the limited experimental investigations of hydrogen bonding dynamics in electronically excited states, several groups have applied advanced ultrafast spectroscopic techniques to study the dynamics and couplings of hydrogen bonding in the ground electronic states of DNA, 16 proteins, 17 and solvated vibrational chromophores 18,19 as well as neat solvents. 20,21 Often used to understand intermolecular hydrogen bonding, molecular homo- and hetero-dimers have been extensively studied using ultrafast pump-probe, photon echo, and multidimensional spectroscopies in the mid-infrared. 22–24 These studies have found that the O-H and N-H stretching vibrations in dimers become displaced along intermolecular coordinates upon vibrational excitation due to anharmonic coupling between these modes. 22 Such a displacement is analogous to the displacement of excited electronic states along totally symmetric nuclear coordinates leading to Franck-Condon progressions in electronic absorption spectroscopy 25 as well as influencing the intensity of these modes in resonance Raman spectroscopy. 26 These experimental and theoretical studies have laid some ground work on how low frequency modes influence high frequency stretching vibrations and hydrogen bonding, which are central to bond breaking and formation in proton transfer reactions.

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Despite the effort to use sophisticated spectroscopic techniques to study the dynamics of hydrogen bonding in electronic ground states, the experimental studies cited above have not been able to extract parameters that characterize the structure and coupling of hydrogenbonding to other vibrations analogous to the electronic case, such as the Huang-Rhys or other electron-nuclear coupling parameters. In addition, to our knowledge no study thus far has directly characterized coupling between delocalized and localized vibrations in an excited electronic state of a hydrogen-bonded material. Understanding the dependence of hydrogen bonding structures on electronic state represents a necessary step toward optimally engineering the coupled proton-electron processes of hydrogen-bonded charge-transfer (HBCT) materials for specific properties and functions. To develop an experimental tool for extracting important information on the structure of hydrogen bonding in the presence of an excited electronic density, we report two-color, ultrafast transient reflectivity measurements that probe the vibrational response of the HBCT material quinhydrone to an optically induced electron transfer process. Quinhydrone is a 1:1 co-crystal of the electron donor hydroquinone (HQ) and the electron acceptor pbenzoquinone (BQ) that forms in a pseudo-1-dimensional mixed stack structure where each monomer alternates along a chain, as seen in several similar organic CT materials. 27 Low dimensional organic hydrogen-bonded materials have recently shown room temperature ferroelectric phases 11 and coupled hydrogen bond-electron transfer phase transitions dictated by isotopic substitution of proton sites. 12 Previous studies have examined the vibrational spectra of quinhydrone for signatures of coupled proton-electron behavior 28 and electronphonon coupling. 29,30 Upon ultrafast optical excitation of electron transfer in quinhydrone, we find that midinfrared probe pulses become modulated at frequencies corresponding to the lattice phonons of this material. Previously, the work of several authors 29,31–37 has shown that broadband detection of these vibrational coherences in the visible region contains rich information on electron-nuclear coupling and can be related to multimode quantum dynamics of coupled

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electronic-vibrational systems. 38 By examining the probe energy dependence of the amplitude and phase of the oscillations in the pump-probe signal, one can assess the coupling between the degree of freedom producing the coherences and the transitions being probed resonantly in the measurement. In contrast to these previous studies, we have applied these physical insights to the case of a mid-infrared probe pulse in resonance with an O-H stretching transitions. This experimental approach is depicted schematically in Figure 1 for a model system. First, a visible pump pulse excites an electronic transition. Second, the electronic excitation impulsively excites Raman-active lattice phonons of the crystal. Last, the lattice phonons modulate the O-H stretching transition energy and report the magnitude of anharmonic coupling. As an example, the bottom panel of Figure 1 shows the anticipated effect of a phonon whose normal coordinate changes the intermolecular separation of the electron donor and acceptor sites of quinhydrone. At one extreme of the oscillation shown as blue points in the wave form of the impulsive excitation of the lattice phonon, the intermolecular distance maximizes leading to less charge transfer between sites and an O-H stretch whose frequency is larger than that of the equilibrium state. In the opposite extreme shown as the red points on the same wave form, the intermolecular distance minimizes causing increased charge transfer from donor to acceptor and an O-H stretch whose frequency is smaller than that of the equilibrium state. By analyzing the probe energy-dependent amplitude and phase of phonon coherences during this non-equilibrium process, we can establish the coupling between their associated phonons and different types of O-H stretching transitions resonant with our probe pulses. This examination finds at least two transitions near 3000 cm−1 directly coupled to the lattice phonons of quinhydrone. Further theoretical analysis of phonon coherence spectra supports the conclusion that a transition coupled to the phonon found at 170 cm−1 qualitatively differs from a transition coupled to at least four other lattice phonons at frequencies between 70 cm−1 and 330 cm−1 . This theoretical analysis allows us to provide the first

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experimentally established estimate of the displacement of the O-H stretching vibration of quinhydrone along different lattice phonon normal coordinates of a hydrogen-bonded molecular crystal. Furthermore, based on our analysis we tentatively assign the transition coupled to the 170 cm−1 phonon as a proton transfer transition, which, based on our previous results, would mean that this phonon modulates both proton and electron transfer in the charge-separated band of quinhydrone. These results establish a new foundation for the spectroscopic investigation and understanding of the interplay between three different types of resonances spanning over 2 orders of magnitude in energy. These methods and the fundamental understanding of the coupling between the different degrees of freedom that they establish in quinhydrone is anticipated to bring hydrogen-bonded materials closer to the next generation applications in photonics, electronics, and catalysis.

Experimental and Computational Design Monoclinic quinhydrone single crystals were formed by two methods for separate spectroscopic measurements. First, for Fourier transform infrared (FTIR) absorption measurements, a saturated solution of commercial polycrystalline quinhydrone powder (Sigma Aldrich 97% pure) in HPLC grade acetonitrile (EDM) was dropcast on a calcium fluoride (CaF2 ) substrate and allowed to dry. This method produced an ensemble of sub-mm sized, randomly oriented single crystals whose vibrational spectrum could be measured in the region of 1500 cm−1 to 4000 cm−1 . Both the polycrystalline quinhydrone and acetonitrile were used without further purification. Second, larger single crystals for resonance Raman and ultrafast spectroscopic measurements were grown by allowing a similar saturated solution of quinhydrone in acetonitrile to slowly evaporate. These crystals possessed typical dimensions of 3x0.3x0.1 mm and were elongated along the a-axis of the crystal, as shown previously. 29,39,40 A schematic of the manner in which the electron donor hydroquinone (HQ) and and electron acceptor p-benzoquinone (BQ) stack is shown in the right panel of Figure 1. 28 Steady-state

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Figure 1: Top: Schematic comparison of the energy scales of model resonances taking part in a two-color pump-probe measurements like that undertaken in this study. Ultrafast excitation of the electron transfer resonance above 10000 cm−1 impulsively excites coherences of the intermolecular lattice phonon vibrations below 500 cm−1 . These coherent superpositions of lattice phonon states in turn modulate the energy of the localized O-H stretching vibration near 3000 cm−1 . Bottom: Schematic of a physical mechanism that can couple an intermolecular phonon to the O-H stretching vibration of quinhydrone. As the phonon modulates the intermolecular distance, the stretching frequency of the O-H vibration is shown to vary.

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Figure 2: Left: Steady-state Fourier transform infrared (FTIR) absorption spectrum of an ensemble of monoclinic quinhydrone single crystals. The central frequency of the three probe pulses are shown as arrows. Right: Schematic of the arrangement of p-benzoquinone and hydroquinone molecules in the (001) plane of monoclinic quinhydrone with interchain hydrogen-bonds shown in red. FTIR measurements were carried out on a Bruker Vertex 80 spectrometer under vacuum conditions. A commercial Raman microscope system (Horiba Xplora One) was used to carry out polarized resonance Raman (rR) spectroscopy measurements excited at 2.33 eV. All measurements were made with ∼1 mW of incident laser power focused to a spot diameter of roughly 10 µm by a 10x microscope objective. We controlled the sample temperature using a stage designed for a backscattering geometry fitted with a silver block that was simultaneously liquid nitrogen cooled and heated using a resistor (Linkam Scientific Instruments). Servo feedback allowed temperature control with 0.1 K precision. Raman spectroscopic measurements were made in nitrogen gas. For determination of rR features due to hydrogen-bonding that might appear in our ultrafast spectroscopic measurements, we focused on rR spectra in the region below 300 cm−1 for different crystal structures and isotopic substitution. To test for the presence of hydrogen bonding in the character of different lattice phonon modes,

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deuterated quinhydrone crystals were formed from slow evaporation of a solution of equal amounts of 99% pure BQ crystals and 97% pure d6 -HQ (Cambridge Isotopes) in acetonitrile. Due to the amount of time necessary to completely evaporate the solvent, some deuteriumhydrogen exchange occurred in the solution, resulting in an average 1:1 ratio of deuteration of HQ hydroxides in these crystals, which we will refer to as d5 -quinhydrone. The structure of these deuterated crystals was determined to be monoclinic from single crystal X-ray diffraction studies. For further comparison, we also grew fully hydrogenated triclinic crystals of quinhydrone following the procedure reported by Sakurai. 39 Ab initio density functional theory electronic structure calculations were carried out using the Becke 1988 exchange functional 41 in combination with the Perdew-Wang generalized gradient approximation correlation functional, 42 denoted B3PW, as implemented in the CRYSTAL14 software package 43 using polarizable electronic basis sets previously reported and a geometry extracted from the room temperature x-ray diffraction pattern of quinhydrone. 40,44,45 The irreducible Brillouin zone of quinhydrone was set on a mesh according to Pack-Monkhorst sampling using a shrinking factor of 8 for all three crystallographic directions. Vibrational frequencies allowing the gross assignment of features in experimental spectra were calculated using a coupled perturbed Kohn-Sham method. 46,47 A schematic showing the general design of the ultrafast spectroscopic measurements highlighted below is shown in Figure S2. Ultrafast pulses from a titanium-doped sapphire oscillator seed a regenerative amplifier system centered at 1.56 eV (795 nm), which is pumped at a repetition rate of 1 kHz. We split the output beam into three arms. Two of the arms each pump two-stage optical parametric amplifiers (OPA). One OPA outputs tunable sub-50 fs pump pulses in the visible region while we use the other OPA to generate our mid-infrared probe pulses. We then steer the pump and probe pulses to a metallic parabolic mirror that focuses both beams onto the sample where they are spatially and temporally overlapped. In order to achieve high time resolution, the pump pulse is delayed relative to the probe pulse via a mechanical stage that adds to the path length of the beam with step precision

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of 100 nm. Inspired by the methods of Kubarych and co-workers, we up-convert the midIR probe pulse to the visible for spectral resolution. 48,49 After reflecting from the sample, the probe beam is re-collimated and steered to a spherically curved mirror that focuses it into a magnesium oxide-doped lithium niobate (MgO:LiNbO3 ) crystal where it overlaps with the third arm of the amplifier output narrowed to a bandwidth of approximately 15 cm−1 . When the 1.56 eV (795 nm) and probe beams spatially and temporally overlap in the MgO:LiNbO3 crystal they generate another beam in a phase-matched direction at the sum of their respective frequencies, in the visible region, whose spectral width is dictated by the bandwidth of the IR probe pulse. The upconversion of the probe pulse to the visible region allows shot-to-shot detection of the pump-probe signal with a high speed silicon CCD camera, as detailed previously. 29

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Raman Shift [cm ] Figure 3: Comparison of the resonance Raman spectra in the low frequency lattice region of monoclinic quinhydrone formed from h6 hydroquinone (blue), triclinic quinhydrone formed from h6 hydroquinone (green) and monoclinic quinhydrone formed from d6 hydroquinone (red) for 2.33 eV excitation. The bottom spectrum shows that the lattice feature near 260 cm−1 for the fully hydrogenated crystal becomes two peaks upon deuteration.

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Results and Discussion Figure 2 shows the steady-state FTIR spectrum of monoclinic quinhydrone crystals dropcast on a CaF2 substrate in the region between 2300 cm−1 and 3650 cm−1 , showing the hydrogen-bonded O-H stretch of HQ. This spectrum qualitatively matches those reported previously. 28,50 The line shape of the peak corresponding to this vibration is highly asymmetric, with a substantial tail extending out from the low frequency side of the peak. This asymmetry can be explained by an electrical anharmonic coupling between the O-H stretch and a low frequency lattice phonon of quinhydrone. 51 On this low frequency tail, sidebands appear to be equally spaced at a frequency close to that of a lattice phonon observed in far-IR absorption measurements and polarized along the b-axis of the crystal. 50 However, closer inspection reveals that these features are more complex than simple sidebands and likely result from a complex interplay of anharmonic interactions between the intramolecular O-H stretch and several lattice phonons. Three arrows denote the central frequency of the pulses we used to probe the response of quinhydrone to ultrafast electron transfer. Figure 3 shows the resonance Raman spectra of monoclinic h6 -quinhydrone, triclinic h6 -quinhydrone, and d5 -quinhydrone excited at 2.33 eV after cooling each crystal to a temperature of 98 K. Our previous results have shown that all the features excited in the region under 300 cm−1 behave as one would expect from crystal lattice phonons. 29,30,52,53 Upon deuteration of HQ, we find that two peaks appear in the rR spectrum at 247 cm−1 and 261 cm−1 whereas only a single peak appears at 259 cm−1 and 265 cm−1 in the rR spectra of monoclinic and triclinic h6 -quinhydrone, respectively. Given that the relative mass difference between h6 -HQ and d5 -HQ is significantly less than the frequency difference between these two features in the bottom spectrum of Figure 3, this isotope effect seems to indicate that this feature possesses substantial character of the hydroxyl hydrogen motion. One would anticipate that such a mode plays a significant role in the hydrogen-bonding network of quinhydrone in its electronic ground state. To understand how electronic excitation and hydrogen bonding couple in the solid-state 11



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on ultrafast time scales, we have excited a room temperature single crystal of monoclinic quinhydrone at 2.64 eV and probed its response with pulses centered at three positions near the frequency of its ground state O-H stretching vibration between 2400 cm−1 and 3600 cm−1 , shown as arrows in Figure 2. The 2.64 eV pump pulse lies at a higher center energy than the peak of the CT transition of quinhydrone, as shown in Figure S1 of the SI. However, our previous work shows that while the pump pulse at this energy may resonantly excite an intramolecular transition of BQ, this state rapidly relaxes into the electron transfer state in less than 100 fs. 30 As in previous studies of the ultrafast dynamics of quinhydrone, 29,30 the pump-probe signal integrated over probe energy shows oscillations in the vibrational transient reflectivity (vTR), shown in the top panel of Figure 4. These oscillations correspond to coherences between lattice phonon vibrational states of the crystal in response to the ultrafast pump pulse. The full vTR spectrum can be found in Figures S3 and S4 of the SI. Previously, the appearance of coherent oscillations in ultrafast transient reflectivity measurements corresponding to lattice phonons was explained by Zeiger et al. in terms of a displaced excited electronic state or band. 54 This result was later confirmed by Merlin and co-workers in the limit of high intensity resonant excitation. 55 However, our mid-IR pulses probe the O-H stretch, significantly lower in energy than any electronic resonance in quinhydrone. Thus, unlike previous studies the observed coherent oscillations inform us about the coupling between the lattice vibrations and hydrogen bonding in the electronically excited state of this material. We expect that anharmonic coupling between the O-H stretch of HQ and lattice phonons would displace the excited vibrational levels of the O-H stretch along the lattice coordinates, similar to the case of displacement of electronic states of small molecules along vibrational coordinates using the Born-Oppenheimer approximation. However, the physical mechanisms leading to displacement in these two situations differ. In the electronic case for a small molecule, the excited state becomes positively displaced along the vibrational coordinate because this electronic state typically carries more weight from the anti-bonding molecular

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%$Figure 4: Top: Ultrafast, frequency-integrated vibrational dynamics of quinhydrone pumped at 2.64 eV and probed between 2400 cm−1 and 3600 cm−1 showing prominent oscillations due to lattice phonon coherences. Bottom: Schematic of the energy levels associated with the electronic, O-H stretching and lattice vibrations and the displacements (d) that account for the appearance of coherent oscillations in ultrafast visible pump, mid-infrared probe measurements.

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orbital and pushing the nuclei apart. In the case of a hydrogen-bonded stretching vibration, as in the current study, the excitation of the O-H stretch increases the amplitude of its motion and its probability of a proton transfer transition. In response, the lattice phonons move the atoms of the proton donor and acceptor site in preparation for this event due to the anharmonic coupling of the nuclear potential energy surface between these eigenmodes. Given our current measurements, we cannot decipher a positive or negative relative displacement of the excited O-H stretching state along the lattice phonon coordinates of interest. However, Fig. 4 shows this relative displacement as negative since we expect that these motions would assist with proton transfer from HQ to BQ in the case of quinhydrone. Champion and co-workers have shown the Born-Oppenheimer approximation holds in the case of proton, hydrogen and hydride transfer for lattice vibrations possessing frequencies below 500 cm−1 . 56 Since the resulting separation of the stretching and lattice phonon vibrations creates effective potential energy surfaces defining quantized states of the low frequency coordinates, the displacement of the excited state of the O-H stretch allows for the excitation of a resonance Raman processes similar to those observed for electronic transitions 22 and analogous to the mechanism proposed by Zeiger et al . A similar mechanism has been used to explain the ability of IR-active vibrations to excite Raman-active lattice phonons in complex oxides through a process termed stimulated ionic Raman scattering. 57–59 Therefore, we argue that the appearance of oscillations in our visible-mid-IR pump-probe measurements stems from the anharmonic coupling of the O-H stretch of HQ in quinhydrone to lattice phonons of the crystal in the charge-separated excited state of the material. The bottom panel of Figure 4 depicts one such anharmonic coupling mechanism for a simple 2-dimensional model of harmonic potential energy surfaces. As shown in the figure, we propose that in direct analogy to the familiar displacement of the ground and excited electronic potential energy surfaces along vibrational coordinates, the O-H stretch vibrational states are also displaced along the low frequency lattice modes with each vibrational potential energy surface possessing quantized lattice phonon states. The concept of having potential

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energy surfaces for each vibrational state of the O-H stretch along the phonon coordinate is justified by the one order of magnitude energy separation between these degrees of freedom in agreement with the standards recently set by Champion and co-workers. 56

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Figure 5: Top: Comparison of the room temperature Fourier spectrum of ultrafast visible pump, mid-IR probe transient reflectivity of monoclinic quinhydrone excited at 2.64 eV (blue) to its spontaneous resonance Raman spectrum excited at 2.33 eV (green) in the spectral region of 45-350 cm−1 . Bottom: Comparison of the atomic motion comprised in the 66 cm −1 Bu and 200 cm−1 Ag vibrational modes found in DFT calculations assigned to the peaks labeled as 1 and 2, respectively, in the top panel. To assess the coupling of the lattice phonons producing the observed oscillations to midinfrared transitions of quinhydrone, we have applied a singular value decomposition (SVD) algorithm in conjunction with Fourier analysis, as used previously. 29 The top panel of Figure 5 compares the Fourier transform from this SVD analysis integrated over all of the probe 15



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energies for the first 4 ps following the arrival of the pump pulse to the resonance Raman (rR) spectrum of monoclinic quinhydrone excited at 2.33 eV at T=273 K. The appearance of many peaks in the Fourier spectrum implies that the O-H stretch of quinhydrone exists on a multidimensional energy landscape strongly dictated by anharmonic coupling to the lattice phonons of this material as well as the displacement of the chargeseparated state along these phonons. 60 We believe the main reason for the appearance of many more features in the Fourier spectrum derived from the ultrafast pump-probe measurements than present in the steady-state resonance Raman spectrum is the nature of each measurement. The frequency domain spontaneous light scattering measurement probes the equilibrium coupling of electronic and vibrational structure present in both the ground and excited states of quinhydrone. This physical picture stems from the fact that the resonantly enhanced scattering process occurs on a time scale proportional to the inverse of the detuning of the incident light field from the resonance condition, which is just a few fs in the case of our 2.33 eV laser. 26 Therefore, effects that necessitate time evolution on the excited state to manifest themselves, such as the anharmonic coupling of low frequency phonons of different irreducible symmetry, will not appear under the experimental conditions of a steady-state resonantly enhanced light scattering measurement. In contrast, the ultrafast transient reflectivity experiment uses a broadband pump pulse to create vibrational coherences in the excited state of quinhydrone. Therefore, a mid-IR probe that interacts with the generated coherence can directly characterize vibrational evolution on the excited state that may result in the appearance oscillatory features due to anharmonic vibrational coupling mechanisms such as nonlinear electron-phonon coupling not present under equilibrium conditions. Upon inspecting the two spectra in the top panel of Figure 5, one notices several similarities and differences between them. First, prominent peaks appear in both spectra near 90 cm−1 that have been observed in previous coherent ultrafast measurements on quinhydrone using a broadband visible probe pulse. 29,30 Second, a peak in the Fourier spectrum appears at 263 cm−1 whose frequency approaches that of the 254 cm−1 of the rR spectrum. Third,

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several peaks appear in the Fourier spectrum that do not appear in the rR spectrum, perhaps most interestingly at 72 cm−1 and 144 cm−1 . Previous vibrational spectroscopic studies of quinhydrone assigned IR-active lattice phonons to peaks near these frequencies. 50,61 Given the mechanism of lattice phonon excitation in ultrafast reflectivity measurements we proposed above, the appearance of these IR-active phonons in our measurement points to a coupling between lattice modes of different symmetry in the excited state of quinhydrone. A similar effect is observed at higher vibrational frequencies in the resonance Raman spectra of quinhydrone recently discovered by Rury. 62 Using a nonlinear Peierls-Hubbard Hamiltonian, this study found that Raman-active and IR-active vibrations of quinhydrone’s lattice couple coherently via nonlinear electron-phonon coupling to produce asymmetric line shapes characteristic of Fano interference, but without the presence of a continuum of states. The ultrafast spectroscopic measurements presented above show that the excited state potential energy surface of quinhydrone may also possess nonlinear electron-phonon coupling to the extent that Raman-active phonons can coherently excite their IR-active counterparts. In addition to the calculation of the integrated Fourier spectrum shown in the top panel of Figure 4, our SVD analysis allows us to produce spectra that show the amplitude and phase of each oscillation in the vTR spectrum of quinhydrone resolved as a function of the probe frequency. We will further refer to these as phonon coherence spectra (PCS). To our knowledge, such spectra from similar ultrafast two-color electronic pump, vibrational probe measurements have not been reported previously. As will be discussed shortly, and in analogy to a previous model for electronic-vibrational coupling developed by Champion and co-workers, 31,32 these PCS contain rich information that is otherwise inaccessible from analysis of the oscillations seen in the top panels of Figures 4 and 5. In particular, for a given phonon mode, a dip in the amplitude with a coincident phase shift at a given probe frequency points to a transition modulated by the nuclear motion comprising that particular phonon vibration. Of the modes whose PCS spectra show attributes consistent with the modulation of the

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energy of a transition, we focus on the 72 cm−1 and 170 cm−1 peaks of Figure 5. This focus is partially justified based on the presence of a 172 cm−1 peak in our previous visible pump, visible probe measurements whose electron-phonon coupling spectrum shows that this mode modulates an electronic transition resonant near ∼2.0 eV (∼16100 cm−1 ). 29 Therefore, any coupling of this lattice phonon to vibrational transitions related to proton dynamics could mean that this mode couples to both electron and proton motion in the same excited electronic state of quinhydrone, as examined in detail below. The atomic motion of two modes found from DFT calculations we believe correspond to the 72 cm−1 and 170 cm−1 modes are shown as blue arrows in sub-panels denoted as (1) and (2), respectively, in the bottom panel of Figure 5. The PCS of 8 different lattice modes present in the Fourier spectrum of Figure 5 can be found in Figure S6 of the supporting information. 3

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Probe Frequency [cm ] Figure 6: The experimental amplitude (solid red) and phase (dashed red) of the phonon coherence spectrum of the 170 cm−1 oscillation derived from a spectrally resolved ultrafast visible pump, mid-IR probe measurement. The experimental data is compared to a predicted spectrum with amplitude (solid blue) and phase (dashed blue) based on a model. The peak in the Fourier spectrum at 170 cm−1 coincides with a totally symmetric Raman peak observed experimentally at 169 cm−1 . As shown in Figure 3, this mode does not shift or split to a detectable amount upon sample preparation from a deuterated form of HQ. 18


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Similarly, we find a peak at 200 cm−1 from DFT calculations which is strongly though not completely localized on the electron acceptor of quinhydrone, p-benzoquinone. Figure 6 shows the PCS of the 170 cm−1 phonon. One sees that a dip in the amplitude and sharp phase shift, demonstrating that this phonon couples to a transition in the mid-IR band near the ground state frequency of the O-H stretch of quinhydrone. The dashed vertical line indicates the spectral position at which probe pulses of two different central frequencies have been overlapped to construct a broadband vTR spectrum, as explained in detail in the supporting information. Figure 6 also shows the behavior of a model derived from the analysis of the spectral properties of wave packets pioneered by Champion and coworkers. 31,32 This model presumes that the 170 cm−1 phonon couples to a transition that is resonant with the probe pulse and has a Gaussian line shape broadened by the displacement of the O-H stretch excited state along this phonon coordinate, as depicted in the bottom panel of Figure 4. The inputs to the model are the central frequency of the transition, the displacement of the O-H excited vibrational state, in ˚ A, and a temperature, T . Full details on the model calculation are found in supporting information. Figure 7 shows the amplitude and phase of the PCS spectrum for the peak we find at a Fourier frequency of 72 cm−1 , as solid and dashed red lines, respectively. Of note in Figure 7 are a dip in the amplitude near 2800 cm−1 in addition to a coincident π phase shift. Additionally, we have plotted the amplitude and phase of a model PCS that allows us to extract the characteristics of the transition coupled to this phonon. Unlike the transition coupled to the 170 cm−1 phonon, in order to qualitatively reproduce the structure of the PCS, we have phenomenologically added an asymmetry to the line shape of the transition, as proposed previously. 63 All the parameters used to calculate model phonon coherence spectra for both the 72 cm−1 and 170 cm−1 peaks of Figure 5 can be found in Table 1. Theoretical modeling of the PCS of the features at 72 cm−1 , 119 cm−1 , 264 cm−1 , and 324 cm−1 seems to support a hypothesis that all of these phonons couple to the same localized vibrational transition of the charge-separated state of quinhydrone. Figure 8 shows

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−1 3100

Probe Frequency [cm ] Figure 7: The experimental amplitude (solid red) and phase (dashed red) of the phonon coherence spectrum of the 72 cm−1 oscillation derived from a spectrally resolved ultrafast visible pump, mid-IR probe measurement. The experimental data is compared to a predicted spectrum with amplitude (solid blue) and phase (dashed blue) based on a model.

Table 1: Tabular comparison of the inputs to the model phonon coherence spectra for the 72 cm−1 peak to those of the 170 cm−1 peak, as described in the SI. νph (cm−1 ) 72 170

Ag 0.07 0.01

Ae 0.93 0.99

d (˚ A) 2.8 1.4

∆ 3.4 2.6

Smv 5.9 3.5

20

σT (cm−1 ) 419 494


a -0.0015 0

ν0 (cm−1 ) 3310 3065

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Figure 8: Left: Experimental phonon coherence spectra (PCS) of the oscillations observed in ultrafast visible pump, mid-IR probe measurements of monoclinic quinhydrone at frequencies of 72 cm−1 (top), 119 cm−1 (2nd from top), 264 cm−1 (2nd from bottom), and 324 cm−1 (bottom) Right: Modeled PCS for the same four frequencies using a single asymmetric transition, as explained in text.

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the experimental and model PCS for each of these phonons to establish this conclusion in the frequency window of 2400-3200 cm−1 . To calculate these model spectra, we have coupled all of the phonons to a single model, asymmetric localized vibrational transition using the formalism detailed in the SI. The only difference between the four model PCS is the displacement of the O-H excited state for each respective phonon. As the phonon energy increases, we have made this displacement smaller. Inspection of Figure 8 shows that by simply varying the absolute displacement of the involved excited vibrational state of the O-H stretch, we can qualitatively reproduce the trend observed in the data. Notably, the position of the dip and phase shift of the PCS shift to lower frequency as the energy of the phonon increases. One should note that the low frequency edge of our probe pulse overlaps with the absorption edge of CO2 , distorting the shape of the PCS of the 264 cm−1 and 324 cm−1 modes and reducing the agreement of the experimental and model results. Even with this technical point, the qualitative agreement between the phonon frequency-dependent trend in experimental and theoretical results provides evidence that these four highlighted phonon modes couple to the same asymmetric transition in the excited state of quinhydrone. Figure 9 compares the transition coupled to the 72 cm−1 phonon to that coupled to the 170 cm−1 phonon. Immediately one notices that both transitions are quite broad. Broad vibrational absorption spectra have been observed in other hydrogen-bonded systems such as water 64,65 and ice. 66,67 In these systems, the O-H stretch can broaden over a wide range of frequencies due to hydrogen bond making and breaking as well as proton transfer. The spectra in the top panel of Figure 9 inferred from analytical models of the PCS may be due to similar effects taking place in the excited state of quinhydrone. The phonons found from DFT calculations and shown in the bottom panel of Figure 5 should sensitively probe the dynamics of hydrogen bond formation and proton transfer given their significant contribution from the motion of oxygen atoms in both HQ and BQ. In addition, supporting evidence for the existence of a broad background of vibrational transitions is also apparent in the steady state FTIR spectrum, as shown in the bottom panel of Figure 9. This spectrum contains several

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Frequency [cm ] Figure 9: Top: Comparison of the transition coupled to the 72 cm−1 phonon (blue) to that coupled to the 170 cm−1 phonon (red). Bottom: FTIR spectrum of quinhydrone crystals in the region between 1000 cm−1 and 2000 cm−1 showing many peaks with Fano line shapes likely due to a broad background possibly arising from delocalized protons.

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peaks at intermediate frequencies between the lattice phonons and O-H stretch that possess Fano line shapes, indicating coupling between these modes and a continuum of vibrational states in quinhydrone whose peak lies at a frequency higher than 2000 cm−1 . Furthermore, Mitani et al. showed that optical absorption above 4000 cm−1 increases substantially upon the application of 38 kbar of pressure along the a-axis of quinhydrone, which they argued was consistent with a phase transition between an ordered and a disordered proton lattice. 28 Based on these considerations, it is plausible to suggest that optical excitation of the crystal results into a similar broadening of the O-H stretching transition to both higher and lower frequencies consistent with our model calculations. Given the asymmetry necessary to reproduce our experimental results, it is reasonable to conclude that the transition modulated by the 72 cm−1 phonon is a hydrogen-bonded O-H stretch in the excited state of quinhydrone. This conclusion is supported by our ab initio calculations of the phonons of quinhydrone from electronic structure, which find an IR-active mode at 66 cm−1 . As shown in the left side of the bottom panel of Figure 5, this mode directly modulates the separation of the electron donor and acceptor quinhydrone. Therefore, it should perturb electron delocalization between molecular sites, which in turn would directly affect the frequency of the O-H stretch, as depicted schematically in the bottom panel of Figure 1. Unlike the transition coupled to the 72 cm−1 phonon, we do not need to introduce any asymmetry into the transition coupled to the 170 cm−1 phonon in our model to describe the experimental PCS. This variation likely points to the difference in the physical origin of the mid-IR transitions that couple to these two highlighted phonons. To better understand the nature of the transition that couples to the 170 cm−1 mode, we consider the electronic excitation initiated by the pump pulse in our experiments. Upon the transfer of an electron from HQ to BQ, the electronic structures of the two molecules become more like each other. Consistent with chemical intuition, the excess electronic charge on BQ renders it more basic while removal of electronic charge from HQ renders it more acidic. In such a case, one would

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expect that the potential energy surfaces dictating the position of the hydroxyl proton at each site become quite similar. Delocalization of protons via a tunneling mechanism was proposed to occur in quinhydrone upon application of static pressure, which Mitani et al. argued was driven by a hydrostatically driven electron transfer from HQ to BQ. 28 Based on this reasoning, we anticipate that the line shape of a direct tunneling transition between sites driven by mid-infrared electromagnetic radiation following optically induced intermolecular electron transfer would reflect this symmetry in the proton potential energy surfaces. In such a case, the line shape of the transition consistent with the PCS of the 170 cm−1 phonon may indicate that this low frequency vibration modulates a tunneling transition by lowering the energy barrier between proton sites. This tentative conclusion is supported by ab initio electronic structure calculations that show that the totally symmetric Raman-active phonon at 200 cm−1 directly modulates the intermolecular O··O distance along the a-axis of the crystal. One would expect such an atomic motion to influence the energy necessary to transfer a proton from one site to another. Further support comes from our previous ultrafast studies using visible probe pulses, in which the 170 cm−1 phonon was found to directly modulate an electronic transition in the charge transfer band. 29 One would anticipate if such a phonon modulates electron transfer between HQ and BQ, its propensity to modulate proton transfer between these molecules would be rather large. If more sophisticated theoretical descriptions of the ultrafast processes taking place in our experiments can provide further insight into the accuracy of this assignment, then the results of our study could lay a foundation for the future capabilities of hydrogenbonded charge transfer materials. Identification of a mode that couples to both proton and electron transfer represents an important step in designing materials that take advantage of these intermolecular processes. In addition to being able to determine a line width, center frequency and phenomenological asymmetry that characterize the transitions coupled to these two lattice modes, the models used to analyze the PCS allow us to delve deeper into the physical source of these

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characteristics. The numerical characteristics of the transitions used to model the PCS qualify an effective Huang-Rhys parameter, Smv , that parametrizes the anharmonic coupling of the O-H stretch and lattice vibrations of quinhydrone. We use the same equation for this parameter as proposed by Huang and Rhys, 68 Smv = ∆2 /2, where ∆ is the dimensionless displacement of the ν = 1 state of the O-H stretch calculated using dHB , as shown schematically in Figure 4, and in conformity with the electronic case defined previously. 69 In the case of the 72 cm−1 phonon, qualitative agreement of experiment and model necessitates a displacement of the excited O-H vibrational state by over 2.0 ˚ A along this coordinate, meaning Smv > 5. For the 170 cm−1 phonon, we need a displacement of at least 1.0 ˚ A to reproduce the features of the experimental coupling spectrum in our model, leading to Smv > 3 for this mode. The quantities for these parameters are found in Table I. We stress that these numerical values represent gross estimations of the coupling between these degrees of freedom of quinhydrone and should be taken as qualitative information. That is, we believe that these numbers indicate that upon excitation of the O-H stretch vibration in the excited electronic state of quinhydrone, the lattice of the crystal responds significantly. Such a response may be a key requisite for proton transfer between HQ and BQ during or immediately following electron transfer.

Conclusions In conclusion, we have undertaken steady-state vibrational and two-color, ultrafast spectroscopic measurements that probe the vibrational response of a hydrogen-bonded material to optically-induced intermolecular electron transfer. Our experiment and models unveil a new perspective in understanding of hydrogen bonding in the presence of electron transfer. In particular, we find that low frequency lattice modes impulsively driven by an ultrafast pump pulse modulate the hydrogen bonded O-H stretching excitations. Thus, in analogy to electronic potential energy surfaces these results highlight the importance of potential

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energy surfaces for vibrations of O-H with respect low frequency phonons. Furthermore, by spectrally resolving our probe pulse and using an extension of a previously derived model, we have uncovered at least two separate transitions resonant in the mid-infrared that become modulated by the lattice phonons of quinhydrone. Using ab initio and analytical model calculations, we show these transitions differ in their characteristics, in particular in the symmetry of their line shapes. Using the characteristics of each transition extracted from analytical models and atomic motion associated with different phonons determined by ab initio electronic structure calculations, we motivate a physical picture in which at least one of these features corresponds to proton tunneling in the charge-separated band of quinhydrone. However, this assignment is tentative at the moment and further theoretical tools are needed to establish its accuracy more thoroughly. Lastly, based on modeling of phonon coherence spectra, we have estimated effective Huang-Rhys parameters that qualify the displacement of the O-H stretching vibrations along low frequency lattice phonons in the presence of intermolecular electronic charge separation. We believe these results will inspire a new perspective on the fundamentals of coupled proton-electron processes in organic solids, provide an avenue to interrogate the coupling of localized and delocalized vibrations in molecules and materials as well as shed new insight into the the design of functional organic materials utilizing a wide array of intermolecular interactions.

Acknowledgement The authors acknowledge support from the University of Southern California start up grant and the AFOSR YIP Award (FA9550-13-1-0128). ASR was partially supported by the Rose Hills Foundation Research Fellowship. JMD and SS were partially supported by the NSF CAREER Award (1454467). The authors thank Dr. Jon Dieringer and Eric Driscoll for contributions to the development of the software used in the reported ultrafast spectroscopic measurements, Dr Ralf Haiges for x-ray structure determination of d5 -quinhydrone crystals, Dr. Frank Devlin for technical assistance with the Raman microscope, and Prof. Sean 27



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Roberts for bringing to our attention the physical importance of the presence of a continuum of vibrational states in materials like quinhydrone.

The authors declare no competing

financial interest.

Supporting Information Available Details on 2D contour vibrational transient reflectivity spectra, the phonon coherence spectra of 8 lattice modes, and the theoretical analysis that leads to model spectra shown in Figures 5,6, and 7 can be found on-line: This material is available free of charge via the Internet at http://pubs.acs.org/.

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