Coupling between Harmonic Vibrations Influences Quantum Beating Signatures in Two-Dimensional Electronic Spectra

Abstract

Launching and tracking wavepacket dynamics with two-dimensional electronic spectroscopy (2DES) provides insight into the complex interactions that underlie coherent processes within photoactive materials. With the ever-growing interest in how electronic-vibrational (vibronic) interactions direct ultrafast photophysics, methods for translating 2DES results into meaningful descriptions of the molecular potential energy landscape must evolve correspondingly. The interpretation of quantum beatmaps, which provide direct insight into the intra- and interchromophoric couplings within a chemical system, frequently relies on physical models that account for a single nuclear coordinate. However, several recent works suggest that coupling between wavepackets borne from several different vibrational motions affects 2DES data in meaningful ways. We build upon these insights by directly comparing simulations using single- and multicomponent vibronic Hamiltonians against experimental 2DES results from the organic semiconductors terrylenediimide and ITIC, as well as the biomedical dyes methylene blue and Nile blue A. We show that the experimental beatmaps and Fourier power spectra are well-reproduced when both low- and high-frequency vibrational motions are included in the simulation. Moreover, we demonstrate that the interaction of harmonic wavepackets increases quantum beat amplitudes in the positive-frequency rephasing signals, which significantly complicates standard methods for separating ground- and excited-state vibrational coherence signatures from 2DES data. These findings illustrate that coupling between purely harmonic vibrational wavepackets can have significant and prevalent effects on experimental 2DES results.