Modeling Nonlocal Electron-Phonon Coupling in Organic Crystals Using Interpolative Maps: The Spectroscopy of Crystalline Pentacene and 7,8,15,16-Tetraazaterrylene

TL;DR

This paper introduces a novel mixed quantum-classical approach to model nonlocal electron-phonon coupling in organic crystals, revealing its significant impact on absorption spectra and providing insights into experimental observations of crystalline pentacene and tetraazaterrylene.

Contribution

The authors develop a new method to include nonlocal electron-phonon coupling in spectroscopic models without assuming linearity, enhancing understanding of organic crystal photophysics.

Findings

Nonlocal coupling broadens absorption spectra non-uniformly.

Lower Davydov component broadens more than the upper in pentacene.

Correlations between charge-transfer fluctuations cause selective broadening.

Abstract

Electron-phonon coupling plays a central role in the transport properties and photophysics of organic crystals. Successful models describing charge- and energy-transport in these systems routinely include these effects. Most models for describing photophysics, on the other hand, only incorporate local electron-phonon coupling to intramolecular vibrational modes, while nonlocal electron-phonon coupling is neglected. One might expect nonlocal coupling to have an important effect on the photophysics of organic crystals, because it gives rise to large fluctuation in the charge-transfer couplings, and charge-transfer couplings play an important role in the spectroscopy of many organic crystals. Here, we study the effects of nonlocal coupling on the absorption spectrum of crystalline pentacene and 7,8,15,16-tetraazaterrylene. To this end, we develop a new mixed quantum-classical approach for including nonlocal coupling into spectroscopic and transport models for organic crystals. Importantly, our approach does not assume that the nonlocal coupling is linear, in contrast to most modern charge-transport models. We find that the nonlocal coupling broadens the absorption spectrum non-uniformly across the absorption line shape. In pentacene, for example, our model predicts that the lower Davydov component broadens considerably more than the upper Davydov component, explaining the origin of this experimental observation for the first time. By studying a simple dimer model, we are able to attribute this selective broadening to correlations between the fluctuations of the charge-transfer couplings. Overall, our method incorporates nonlocal electron-phonon coupling into spectroscopic and transport models with computational efficiency, generalizability to a wide range of organic crystals, and without any assumption of linearity.