Electronic State-Resolved Electron–Phonon Coupling in an Organic Charge Transfer Material from Broadband Quantum Beat Spectroscopy

Abstract

The coupling of electron and lattice phonon motion plays a fundamental role in the properties of functional organic charge-transfer materials. In this Letter we extend the use of ultrafast vibrational quantum beat spectroscopy to directly elucidate electron-phonon coupling in an organic charge-transfer material. As a case study, we compare the oscillatory components of the transient reflection (TR) of a broadband probe pulse from single crystals of quinhydrone, a 1:1 cocrystal of hydroquinone and p-benzoquinone, after exciting nonresonant impulsive stimulated Raman scattering and resonant electronic transitions using ultrafast pulses. Spontaneous resonance Raman spectra confirm the assignment of these oscillations as coherent lattice phonon excitations. Fourier transforms of the vibrational quantum beats in our broadband TR measurements allow construction of spectra that we show report the ability of these phonons to directly modulate the electronic structure of quinhydrone. These results demonstrate how coherent ultrafast processes can characterize the complex interplay of charge transfer and lattice motion in materials of fundamental relevance to chemistry, materials sciences, and condensed matter physics.

Resonant interactions between discrete phonons in quinhydrone driven by nonlinear electron-phonon coupling

“…Samples of monoclinic quinhydrone single crystals and temperature-dependent, polarized Raman spectra were formed and gathered in the same manner as described previously 17,32 . Figure 2 shows the temperature-dependent Raman scattering from a monoclinic quinhydrone single crystal in the region between 640 cm −1 and 750 cm −1 excited at 2.33 eV for the z(xx)z polarization configuration 33 .…”

“…We assign both features as phonons of quinhydrone. Given the previous notation of the Raman-active lattice phonons of monoclinic quinhydrone 17 , we denote the lower and higher frequency peaks as ν 6 and ν 7 , respectively.…”

“…Upon applying pressure along its a-axis, Mitani et al found that the behavior of O-H stretching vibration of HQ changes dramatically and posited a cooperative proton-electron tunneling phase of quinhydrone that must be mediated by at least one lattice phonon 16 . Rury et al have also shown that unlike other CT crystals, a Raman-active lattice phonon modulates electron transfer in the charge separated state of quinhydrone 17 . However, phonon-phonon interactions in quinhydrone and other HBCT materials remain totally unexplored territory thus far.…”

Exciton-phonon interactions in the Cs3Bi2I9crystal structure revealed by Raman spectroscopic studies

“…In recent years, various theoretical models have been developed to explain the physical properties of semiconductors through exciton–phonon interactions, but some experimental results regarding the dependence on the size of the particles are still incomprehensible . Raman scattering occurring under resonant optical excitation, accompanied by the photoluminescence emission of the material, is an efficient experimental technique to explain the exciton–phonon interaction, and its use has been previously established in various semiconductors , carbon nanotubes , and organic materials . The main observations based on the Raman effect that were not fully understood regarding the Fröhlich exciton–phonon interaction were based on the unexplained different enhancements of the Raman lines in the Stokes and anti‐Stokes branches and also the distinct strength of the exciton–phonon interaction in various morphological forms.…”

Quantum Control of Ultrafast Internal Conversion using Nano-Confined Virtual Photons

“…Given the prominence of non-adiabatic coupling mechanisms in the photophysics of metalloporphyrins, a specialized model appropriately accounting for frequency shifts and non-Condon vibronic coupling effects may be necessary to explain our results. These effects may emerge from more advanced ultrafast spectroscopic experimental in which one coherently modulates the structure of cavity-embedded molecules to directly assess how polaritonic PESs change relative to their free space counterparts [31][32][33][34]. However, such a development is beyond the scope of the current study.…”