Evidence of Ultrafast Charge Transfer Driven by ... - ACS Publications

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Evidence of Ultrafast Charge Transfer Driven by Coherent Lattice Vibrations Aaron S. Rury,* Shayne A. Sorenson, and Jahan M. Dawlaty* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States S Supporting Information *

ABSTRACT: We report evidence that intermolecular vibrations coherently drive charge transfer between the sites of a material on ultrafast time scales. Following a nonresonant stimulated Raman pump pulse that excites the organic material quinhydrone, we observe the initial appearance of oscillations due to intermolecular lattice vibrations and then the delayed appearance of a higher-frequency oscillation that we assign to a totally symmetric intramolecular vibration. We use the coherent dynamics of the transient reflectivity signal to propose that coherence transfer drives excitation of this intramolecular vibration. Furthermore, we conclude that the dynamical frequency shift of the intramolecular vibration reports the formation of a quasi-stable charge-separated state on ultrafast time scales. We calculate model dynamics using the extended Hubbard Hamiltonian to explain coherence transfer due to vibrationally driven charge transfer. These results demonstrate that the coherent excitation of low-frequency vibrations can drive charge transfer in the solid state and control material properties. Following the initial excitation of features below 400 cm−1 induced by a nonresonant ultrafast pump pulse, we observe the delayed appearance of a 450 cm−1 coherence corresponding to a totally symmetric intramolecular vibration of quinhydrone’s electron donor, HQ. Additionally, the frequency of this vibration shifts down by ∼10 cm−1 over the course of 1.5 ps and then shifts back to its initial value before its coherence completely dephases. We use the probe energy dependence of the coherence amplitude of this vibration and analytic modeling of the CT transition probability to motivate the conclusion that these changes are consistent with a time-averaged delocalization of the frontier electron density of the donor site to quinhydrone’s acceptor, that is, coherent vibrationally induced CT. This approach shows that low-frequency modes of molecular and organic materials can act as transducers of CT for a wide variety of physical and chemical processes in crystalline and polycrystalline solids. Experimentally, ∼80 fs pump pulses derived from a widely tunable optical parametric amplifier were centered at 0.95 eV (∼7700 cm−1) to ensure a nonresonant interaction with a monoclinic quinhydrone single crystal, determined from past studies of the dielectric constant of this material.14 The dominant optical process expected is impulsive stimulated Raman scattering that produces coherences of any Ramanactive vibrations whose frequencies lie within the bandwidth of the ultrafast pulse.15,16 Given an interaction off resonance with electronic transitions, we expect these vibrational coherences to appear in the ground electronic state of quinhydrone.

T

he transfer of charge from a donor to an acceptor is central to a vast array of chemical phenomena in both natural1 and artificial molecular machinery.2 Therefore, new methods to control charge transfer (CT) in molecular materials are always needed. This is especially true in the solid state where the efficiency of charge transport often dictates the importance of a material for future applications in electronics, photonics, and biotechnologies. In molecular and organic materials, a variety of chemical and physical stimuli have been used to induce CT. As a simple example, consider the methods used to study the CT-driven neutral-to-ionic phase transition in co-crystals of tetrathiafulvalene and p-tetrachlorobenzoquinone, which include temperature,3,4 hydrostatic pressure,5 and resonant light fields.6−8 Despite this plethora of approaches, the coupling of light directly to vibrations remains an unexplored method to drive CT. With an approach similar to that which Mathies and coworkers used to understand anharmonic vibrational coupling,9,10 we show that nonresonant stimulated Raman pumping of an organic crystal provides a mechanism to drive CT between its donor and acceptor sites. For this demonstration, we investigate the ultrafast vibrational dynamics of a mixed stack co-crystal formed from the ubiquitous reduction−oxidation pair hydroquinone (HQ) and p-benzoquinone (BQ), known as quinhydrone. Our previous ultrafast spectroscopic studies have shown that optical excitation of intermolecular CT leads to oscillatory features in time-domain signals corresponding to the quantum evolution of coherent superpositions of lattice vibrational states, known as vibrational coherences.11−13 The two constituent molecules of quinhydrone are shown in the inset of the top panel of Figure 1. © XXXX American Chemical Society

Received: October 29, 2016 Accepted: December 14, 2016 Published: December 14, 2016 181

DOI: 10.1021/acs.jpclett.6b02523 J. Phys. Chem. Lett. 2017, 8, 181−187

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The Journal of Physical Chemistry Letters

resolved transient reflectivity signal is shown in Figure S1 of the Supporting Information. Immediately following the temporal overlap of the pump and probe pulses, clear oscillations around ΔR/R = 0 appear. As seen in the bottom panel of Figure 1, comparison of the Fourier transform of these oscillations to the z(xx)z-polarized spontaneous Raman spectrum of monoclinic ̅ quinhydrone excited at 1.58 eV (∼12750 cm−1) demonstrates that the time-domain features correspond to coherences of Raman-active vibrations, as anticipated above. The prominent features of the Fourier spectrum appear at frequencies that correspond to wavenumber values of 91, 114, 172, and 450 cm−1. The features found below 200 cm−1 have previously been assigned as the ν2, ν3, and ν4 intermolecular lattice vibrations of monoclinic quinhydrone in order of increasing frequency.11 Given the atomic motion found in density functional theory (DFT) calculations described in the Methods section, close to these experimental peaks, the ν2, ν3, and ν4 modes likely modulate the electron hopping integral, t, of quinhydrone through the electron−phonon (e−ph) interaction. As shown in Figure S2 of the Supporting Information, the spontaneous Raman peak at 450 cm−1 splits in two upon formation of a monoclinic quinhydrone crystal using isotopically substituted HQ. Such a splitting in the peak can only occur if this mode derives its intensity from the electron donor of quinhydrone, HQ. We will refer to this feature at 450 cm−1 as νHQ. The atomic motions comprising a totally symmetric Ag vibration localized on HQ derived from DFT calculations whose frequency lies near νHQ are shown in the inset of the bottom panel of Figure 1. We observe a physically distinct feature in the dynamics of the νHQ coherence. Figure 2 shows a spectrogram of the

Figure 1. (Top) Integrated transient reflectivity of a near-IR region of a white light continuum probe pulse from a single monoclinic quinhydrone crystal following a nonresonant pump pulse centered at 0.95 eV [∼7700 cm−1]. (Inset) Molecular structures of the electron donor HQ and acceptor BQ of quinhydrone. (Bottom) Comparison of the Fourier transform of near-IR integrated transient reflectivity (blue) seen in the top panel to a spontaneous resonance Raman scattering spectrum excited at 1.58 eV [∼12750 cm−1] (red). (Inset) Atomic motions comprising the totally symmetric intramolecular vibration of HQ derived from ab initio electronic structure calculations whose frequency lies close to a value corresponding to 450 cm−1.

Figure 2. Ultrafast evolution of vibrational coherences excited in a single monoclinic crystal of quinhydrone by a 0.95 eV (∼7700 cm−1) pump pulse and probed in the region between 1.58 and 1.71 eV (12750−13800 cm−1). Note the delayed appearance of the coherence that we associated with an Ag intramolecular vibration of HQ near 450 cm−1.

According to theoretical estimates, the charge density of interest in this study largely localizes on the HQ donor site in the ground state of quinhydrone at ambient conditions.17 To determine the effect of the coherences on the electronic structure of quinhydrone, we probe its CT resonance by reflecting a broad-band white light continuum pulse from the pumped area of the crystalline sample. The top panel of Figure 1 shows the transient reflectivity signal from a single monoclinic quinhydrone crystal integrated over probe energies from 1.58 to 1.71 eV (12750 to 13800 cm−1) in the region of the CT resonance. The spectrally

nonresonant transient reflectivity signal derived from a sliding Fourier transform calculation of the waveform of the top panel of Figure 1. We see that while features below 350 cm−1 appear with the arrival of the pump pulse, there is a delay on the order of 250 fs in the appearance of the νHQ coherence. To determine if this is a technical effect due to the characteristics of our pump and probe pulses, we consider Figure S1, which shows that the 182


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The Journal of Physical Chemistry Letters white light continuum pulse possesses minimal spectral phase in the near-IR region of interest. Because this minimal phase cannot account for the delay shown in Figure 2, we believe that the delayed appearance of the νHQ coherence represents a real physical process taking place in quinhydrone during our measurements. Similar physical processes have been termed coherence transfer in the literature of molecular vibrations.18−20 This process physically represents the driving of a coherent superposition of vibrational states along one normal coordinate into a similar superposition along a different coordinate. We further substantiate this assignment by comparing the dynamics of the contributions to the spectrogram of Figure 2 at the positions of the ν2 (91 cm−1) and νHQ (452 cm−1) modes, as shown in Figure 3. One sees that as the ν2 coherence amplitude

Figure 4. Comparison of the ultrafast evolution of vibrational coherences of a single monoclinic crystal of quinhydrone excited by a 0.95 eV (∼7700 cm−1) pump pulse and probed in the region between 1.58 and 1.71 eV (12750−13800 cm−1), zoomed in on the region between 200 and 400 cm−1 to the time-independent integrated Fourier spectrum of Figure 1 shown on a frequency axis relative to ω0/ 2πc = 186 cm−1. The guidelines highlight the alignment of these spectra.

Figure 3. Comparison of ultrafast evolution of vibrational coherence amplitudes that we assign to the ν2 and νHQ modes found at 91 and 450 cm−1, respectively, following excitation of a single monoclinic crystal of quinhydrone by a 0.95 eV (∼7700 cm−1) pump pulse and probed in the region between 1.58 and 1.71 eV (12750−13800 cm−1).

decays, that of the νHQ modes increases, consistent with a coherence transfer mechanism. Our DFT calculations assign a Raman-active mode near 90 cm−1 to librational motion of the donor site, HQ, whose distortion may lead to coherent excitation of νHQ in the presence of an appropriate interaction, as we discuss subsequently. In addition to the appearance of several intermolecular vibrations and νHQ, a broad feature appears in the region between 200 and 350 cm−1 of Figure 2, which does not correspond to any single vibration found from the DFT calculations. Notably, two distinct features appear in the spectrogram at 277 and 358 cm−1 for probe delays larger than 1 ps. As shown in Figure 4, close comparison between the spectrogram in this region to an integrated Fourier spectrum displayed on an offset axis shows that these features almost exactly align with the frequencies that correspond to 186 cm−1 plus ω2/2πc and 186 cm−1 plus ω4/2πc, respectively. Furthermore, 277 and 358 cm−1 also correspond to ω2/2πc and ω4/2πc subtracted from ωHQ/2πc, respectively. From these equalities, we infer that νHQ mixes with both ν2 and ν4 to produce signals at the beat frequencies between these modes. This fact motivates us to propose that νHQ, ν2, and ν4 mix with a fourth excitation near 186 cm−1 to produce the dynamics of Figures 2−4. We explore the assignment of this excitation below. Most surprisingly, the frequency of the νHQ coherence shifts downward following its excitation, as shown in Figure 5. The portion of the spectrogram shown in Figure 5 highlights that while the νHQ coherence initially appears near 462 cm−1

Figure 5. Ultrafast evolution of the νHQ coherence excited in a single monoclinic crystal of quinhydrone excited by a 0.95 eV (∼7700 cm−1) pump pulse and probed in the region between 1.58 and 1.71 eV (12750−13800 cm−1), zoomed in for Fourier frequencies corresponding to values between 420 and 485 cm−1. The arrows highlight the change in the position of this coherence over the probed time window, which may indicate dynamic changes in the localization of electron density on the donor site of quinhydrone, as described in the text.

following coherence transfer, this feature shifts down to 450 cm−1 by a delay of ∼1.5 ps. The νHQ coherence remains near 450 cm−1 for several 100s of fs and then shifts back toward its initial value near 460 cm−1 before it completely dephases. Two experimental considerations led us to conclude that this feature of the data corresponds to a microscopic physical process taking place in quinhydrone during our experiment. First, the ability to recover an oscillatory component of the discrete time waveform of our ultrafast pump−probe data is determined by the size of the step between delay points. The 183


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electron−nuclear coupling mechanism.11,24−27 Pioneering work interpreting the physical meaning of these spectra by Champion and co-workers showed that the width of the amplitude dip and magnitude of the phase shift can be related to the displacement of an excited electronic state along a given Raman-active normal-mode coordinate. Figure 6 shows the VCS for the νHQ mode across the spectral region of quinhydrone’s CT transition. The corresponding

so-called Nyquist frequency sets the upper limit on what components can be resolved. In the case of our measurements, we moved the delay stage to produce 25 fs steps between each data point, corresponding to a Nyquist frequency of 20 THz (ω/2πc ≈ 667 cm−1). Second, the frequency resolution of the spectrogram, Δν, is set by the total time span over which the Fourier transform is calculated, T, through the relation Δν = 1/ T. For the spectrogram analysis, we slide a 10.5 ps window across a 15 ps total span of pump−probe delay times. This provides a frequency resolution that corresponds to 3.2 cm−1. Under these experimental conditions, we sample the waveform of the pump−probe signal fast enough and far enough to resolve changes in the frequency of a vibration on the order of 0.1 THz (ω/2πc ≈ 3 cm−1), clearly allowing us to resolve changes on the order of those shown in Figure 5. The symmetry of νHQ indicates that its ultrafast frequency shift reports a change in the localization of charge on the donor site of quinhydrone. By comparison with Figure S3, one sees that the frequency of νHQ in both the steady-state and ultrafast Raman measurements of Figure 1 is down-shifted by a value corresponding to 15 cm−1 from an Ag vibration found in bulk HQ crystals. Several studies have shown that the frequency of Ag intramolecular vibrations of mixed stacked CT crystals report the amount of electronic population in the frontier molecular orbitals of the donor and acceptor sites.21−23 Predominantly, the frequency of the totally symmetric vibrations of the donor shift downward as more electron density transfers to the acceptor site.22 Explicitly, Painelli and Girlando showed that the amount of frequency shift expected from an Ag mode depends on the strength of the linear electron−molecular vibration (e−mv) coupling constant, g.22 For a nonzero g, the Ag mode of interest must couple to CT via a Herzberg−Teller mechanism. Therefore, to determine if νHQ reports the localization of electron density on the donor site of quinhydrone, we need evidence that it couples to CT via this mechanism and not due to the displacement of the charge-separated state along this coordinate. When we consider the nature of electronic excitations in cocrystals of aromatic molecules like quinhydrone, a small displacement of the charge-separated state along νHQ is physically reasonable. In contrast to intramolecular excitations, intermolecular CT excitations of materials like quinhydrone deplete population in the highest occupied molecular orbital (HOMO) of the donor site without populating its lowest unoccupied molecular orbital (LUMO), that is, ionization. This depletion during the ionization process most strongly couples to the bonds that connect regions of electron density of opposite sign. On the basis of this reasoning, one would anticipate that the displacement of the ionized state of HQ along this vibration is not significant because the νHQ mode largely changes the angles between the carbon bonds of the aromatic ring of HQ without changing bond distances. The spectral content of the ultrafast measurements provides further evidence of this physical picture. By examining the spectral dependence of the amplitude and phase of a given vibrational coherence, one can gain insight into the mechanism that couples this vibration to a transition resonant with the probe pulse. Specifically, the amplitude and phase show a dip and shift, respectively, across the peak of a modulated transition. For the case of a probe pulse resonant with electronic transitions, the spectral characteristics of this vibrational coherence spectrum (VCS) directly report an

Figure 6. Spectra of the amplitude (blue) and phase (red) of the νHQ vibrational coherence of a single monoclinic quinhydrone crystal measured following a nonresonant ultrafast pump pulse centered at 0.95 eV (∼7700 cm−1).

VCSs for the intermolecular vibrations have been presented in our previous work.11 Like those spectra, Figure 6 shows an amplitude dip and phase shift centered between 1.675 and 1.68 eV, which we assign as the peak of the CT transition. While the dips and shifts of the amplitude and phase of the intermolecular vibrational coherences found from our previous study are wide and shallow, the νHQ coherence shows a narrow dip in its amplitude and a large, sharp shift in its phase. On the basis of the previous work highlighted above, these differences indicate that unlike the large displacement of quinhydrone’s chargeseparated state along the intermolecular vibrational coordinates, there is likely minimal displacement of this state along the νHQ coordinate. Despite this, one observes that νHQ is by far the most intense feature in the spontaneous Raman spectrum excited upon resonance with the CT transition of quinhydrone shown in the bottom panel of Figure 1. In combination with the physical picture into the coupling of vibrations and CT explained above, the VCS of νHQ indicates that this mode likely draws its activity in CT from a Herzberg−Teller mechanism. On the basis of this conclusion, we believe that the shift in the frequency of the νHQ coherence is consistent with vibrationally driven CT between the donor and acceptor sites of quinhydrone. Furthermore, the persistence of its frequency near 450 cm−1 for almost 1 ps indicates that the transferred electron density remains on the acceptor site to create a quasistable charge-separated state. Therefore, the measured dynamics provide meaningful evidence that coherent vibrational excitation can lead to significant changes to the electronic structure of CT materials. We use the extended Holstein−Peierls−Hubbard Hamiltonian to understand additional effects of vibrational excitation of charge separation between the donor and acceptor sites of quinhydrone, generally written as 184


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The Journal of Physical Chemistry Letters ĤHPH = Ĥ e + Ĥ ph + Ĥ e−mv + Ĥ e−ph

(1)

m

For a simple two-site model under the conditions of two electrons mostly localized in the HOMO of the donor site, the constituents of eq 1 can be expressed as

∂t

⎜

†

l

Ĥ e−mv =

∑ ∑ ∑ cî†,σcî ,σ i = 1,2

Ĥ e−ph =

1 ⎞⎟ + 2⎠

σ

l

⎛

∂ϵi ∂ql

⎜

m

m

1 ⎟⎞ 2⎠

(2b)

(2c)

0

∂t Qm ∂Q m 0

(2d)

where sites 1 and 2 are taken as donor and acceptor molecules, respectively. In eq 2a, ϵ and U are the energy and on-site Coulomb repulsion on each site of the lattice, respectively, while t is the intersite electron hopping integral. ĉi (ĉ†i ) corresponds to the electron annihilation (creation) operator for the frontier molecular orbital of the ith site such that the operator n̂i = ĉ†i ĉi counts the number of electrons on that site. In eq 2b, the vibrational coordinates are separated into intramolecular and intermolecular contributions with normal coordinates ql and Qm that lead to their respective creation operators, bl̂ and B̂ m. Equations 2c and 2d correspond to the e− mv coupling and e−ph coupling terms, respectively. This model allows us to calculate the probability of making a CT transition in the presence of a time-dependent vibrational interaction between the donor and acceptor sites. Immediately following the interaction with the nonresonant pump pulse, the electronic structure of quinhydrone is perturbed through the e−ph coupling term shown in Equation 2d where now the normal coordinates are time-dependent, Qm(τ) = Q(0) m sin(ωmτ) for an intermolecular vibration with a frequency ωm/2π. Equation 2d shows that this interaction affects the localization of electron density through its coupling to the hopping integral, t. By modulating t, the intermolecular vibrations directly affect a parameter of the extended Hubbard Hamiltonian of eq 2a that determines the amount of electron density delocalized between the donor and acceptor sites of quinhydrone. To calculate the influence of the coherent lattice vibrations, we use a phenomenological approach to the two-level quantum system in the presence of multiple, time-dependent interactions between electronic states. In such a case, we can write the total wave function of the two-site, donor−acceptor system as |Ψ(τ )⟩ = C L(τ )e

− i E Lτ / ℏ

|L⟩ + CCT(τ )e

− i ECTτ / ℏ

|CT⟩

exp( −τ /τm), 0

1

ql

∑ ∑ (c1,̂†σc2,̂ σ + c2,̂ † σc1,̂ σ ) σ

†

∑ ℏωm⎝Bm̂ Bm̂ +

m

Ωm = ℏ 4Vm 2 + Δm 2 , and Δm = ℏωm − Egap, where Egap = ECT − EL. The time τm characterizes the dephasing of the mth vibrational coherence. The probability of finding the system in the charge-separated state |CT⟩ is |CCT(τ)|2, which now results in interference between the contributions due to each independent lattice vibration capable of modulating t that is coherently excited by our stimulated Raman pump pulse. The top panel of Figure 7 shows our calculation of |CCT(τ)|2 for the case that we excite vibrations at the frequencies of the

(2a)

⎛

(4)

couples to t. In eq 4, Vm = Q m(0) ∂Q

σ

∑ ℏωl⎝bl̂ b l̂ +

Vm i Δmτ /2 e sin(Ωmτ /2) ℏΩm

where the subscript m corresponds to each lattice vibration that

Ĥ e = ϵ1n1̂ + ϵ2n2̂ + U1n1,̂ ↑n1,̂ ↓ − t ∑ [c1,̂ †σc 2,̂ σ + c 2,̂ † σc1,̂ σ ]

Ĥ ph =

∑i

CCT(τ ) =

Figure 7. (Top) Time evolution of the probability of finding the model donor−acceptor system in the charge-separated state due to the e−ph interaction as dictated by eq 2d. (Bottom) Fourier transformation of the time evolution of the probability of finding the model system in the charge-separated state showing the dominance of oscillations at a frequency corresponding to 186 cm−1.

ν3,

ν2, ∂t ∂Q 2

= 0

∂t ∂Q 3

ν4 1 ∂t − 2 ∂Q 4

and = 0

modes

of

quinhydrone,

; Egap = 1.6775 eV to match the 0

results of Figure 6, and dephasing times of the three vibrations are 0.6, 1.75, and 1.5 ps in order of increasing frequency to match the dephasing of ν2, ν3, and ν4 found in Figure 2. We have left the transition probability in arbitrary units because we do not have a reasonable way to estimate the derivative of the hopping integrals with respect to the different lattice vibrations, which largely control its value. Under these conditions, we see that the probability of finding the system in |CT⟩ peaks at a time similar to that at which we see the largest deviation in the frequency of the νHQ mode in Figure 5, that is, ∼1.5 ps. We point out that because Egap ≫ ℏωm for all m, the results of

(3)

so that one is left to find the time-dependent coefficients CL(τ) and CCT(τ) to determine the ability of vibrations to induce CT from the charge-localized state |L⟩ to the charge-separated state |CT⟩. Assuming the rotating wave approximation, independent time-varying interactions due to each vibration that induce transitions, and that CL(0) = 1, the coefficient that defines the probability of finding the donor−acceptor system in |CT⟩ becomes 185


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The Journal of Physical Chemistry Letters Figure 7 lie in the large detuning limit, that is, Δ ≫ 4Vm. In this limit, the probability of finding the system in |CT⟩ due to eq 2d is finite but small. Despite this, Figure 5 is clearly consistent with significant population in the charge-separated state of our quinhydrone sample on ultrafast time scales following a nonresonant light−matter interaction, indicating a large enough probability of transiently populating this state due to vibrational perturbations. Furthermore, the top panel of Figure 7 shows oscillatory behavior consistent with an electronic population oscillation often observed in driven two-level quantum systems. The frequency of this oscillation corresponds to an exact match to the value necessary to align the lattice vibrations with the region between 250 and 400 cm−1 shown in Figure 4, that is, 186 cm−1. This equality raises an interesting physical possibility. In the presence of the time-dependent intermolecular vibrational modulation of the electronic structure of quinhydrone, the electronic and vibrational structure mix to coherently excite an intramolecular vibration of the donor site. Given the phenomenological approach used to calculate the dynamics in Figure 7, future work will be necessary to more fundamentally establish the coupling between any electronic population oscillations and vibrations in CT materials and the range of effects that this coupling may induce. Despite this, the results in our study suggest that lattice vibrations can work in concert with intermolecular CT to produce new types of anharmonic couplings to drive coherence transfer. In conclusion, we have shown strong evidence that nonresonant, coherent pumping of the intermolecular lattice vibrations of the organic CT material quinhydrone leads to charge separation between the donor and acceptor sites of this material on ultrafast time scales. This conclusion stems from the dynamics of the frequency of an Ag intramolecular vibration whose excitation is consistent with coherence transfer. We use the extended Holstein−Peierls−Hubbard Hamiltonian to model how low-frequency intermolecular vibrations drive CT through modulation of the electron hopping integral. The results of model calculations find oscillatory behavior of the probability of populating the charge-separated state, indicating that the electronic and vibrational structure mix to drive vibrational coherence transfer in a manner that has not been observed previously. While this approach has been applied to an organic solid in this study, there is reason to believe that this technique can be easily extended to a vast array of materials in which electron transfer plays a vital role in chemical and physical properties to create fundamental knowledge and nextgeneration vibrationally tailored technologies.

implemented in CRYSTAL14.32 The positions of the atoms of monoclinic quinhydrone were set to those found in the room-temperature X-ray diffraction pattern reported previously.33 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. Both IR- and Raman-active vibrational frequencies of quinhydrone were found via the Coupled Perturbed/Kohm− Sham (CHKS) algorithm.34,35

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02523. Measured transient reflectivity signal and Raman spectra of isotopologues of quinhydrone and bulk hydroquinone (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Aaron S. Rury: 0000-0002-1836-1424 Jahan M. Dawlaty: 0000-0001-5218-847X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the University of Southern California start up grant and the AFOSR YIP Award (FA9550-13-1-0128). A.S.R. was partially supported by the Rose Hills Foundation Research Fellowship. J.M.D. and S.A.S. 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.

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REFERENCES

(1) Blankenship, R. Molecular Mechanisms of Photosynthesis; Blackwell Science Inc.; Malden, MA, 2002. (2) Erbas-Cakmak, S.; Leigh, D. A.; McTernan, C. T.; Nussbaumer, A. L. Artificial Molecular Machines. Chem. Rev. 2015, 115, 10081− 10206. (3) Torrance, J. B.; Vazquez, J. E.; Mayerle, J. J.; Lee, V. Y. Discovery of a Neutral-to-Ionic Phase Transition in Organic Materials. Phys. Rev. Lett. 1981, 46, 253−257. (4) Torrance, J. B.; Girlando, A.; Mayerle, J. J.; Crowley, J. I.; Lee, V. Y.; Batail, P.; LaPlaca, S. J. Anomalous Nature of Neutral-to-Ionic Phase Transition in Tetrathiafulvalene-Chloranil. Phys. Rev. Lett. 1981, 47, 1747−1750. (5) Lemée-Cailleau, M. H.; Le Cointe, M.; Cailleau, H.; Luty, T.; Moussa, F.; Roos, J.; Brinkmann, D.; Toudic, B.; Ayache, C.; Karl, N. Thermodynamics of the Neutral-to-Ionic Transition as Condensation and Crystallization of Charge-Transfer Excitations. Phys. Rev. Lett. 1997, 79, 1690−1693. (6) Koshihara, S.; Takahashi, Y.; Sakai, H.; Tokura, Y.; Luty, T. Photoinduced Cooperative Charge Transfer in Low-Dimensional Organic Crystals. J. Phys. Chem. B 1999, 103, 2592−2600. (7) Okamoto, H.; Ishige, Y.; Tanaka, S.; Kishida, H.; Iwai, S.; Tokura, Y. Photoinduced Phase Transition in Tetrathiafulvalene-p-chloranil Observed in Femtosecond Reflection Spectroscopy. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 165202.

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METHODS Single crystals of fully hydrogenated and partially deuterated monoclinic quinhydrone were formed using methods described previously.11,13 Similarly, the ultrafast and steady-state Raman spectroscopic systems have been described elsewhere.11 Both the pump and probe pulses were polarized along the a-axis of the crystal, and all measurements were made at room temperature in atmospheric conditions. To assign the atomic motion corresponding to the experimentally measured coherences, we undertook ab initio electronic structure calculations using DFT. We used the Perdew−Burke− Ernzerhof generalized gradient functional for both exchange and correlation28 as implemented for solids by Adamo and Barone.29 These calculations use polarizable electronic basis sets for hydrogen,30 carbon,30 and oxygen31 atoms as 186


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