On-the-fly ab initio semiclassical dynamics: Identifying degrees of freedom essential for emission spectra of oligothiophenes

TL;DR

This paper introduces an efficient on-the-fly ab initio semiclassical method combining TGA to accurately simulate vibrationally resolved emission spectra of oligothiophenes, revealing key vibrational modes and electronic character interactions.

Contribution

It develops a novel systematic approach to identify and analyze the importance and coupling of vibrational degrees of freedom in large molecular systems.

Findings

Accurate reproduction of experimental emission spectra for oligothiophenes

Identification of key vibrational modes influencing spectral line shapes

Observation of quinoid and aromatic character interplay during dynamics

Abstract

Vibrationally resolved spectra provide a stringent test of the accuracy of theoretical calculations. We combine the thawed Gaussian approximation (TGA) with an on-the-fly ab initio (OTF-AI) scheme to calculate the vibrationally resolved emission spectra of oligothiophenes with up to five rings. The efficiency of the OTF-AI-TGA permits treating all vibrational degrees of freedom on an equal footing even in pentathiophene with 105 vibrational degrees of freedom, thus obviating the need for the global harmonic approximation, popular for large systems. Besides reproducing almost perfectly the experimental emission spectra, in order to provide a deeper insight into the associated physical and chemical processes, we also develop a novel systematic approach to assess the importance and coupling between individual vibrational degrees of freedom during the dynamics. This allows us to explain how the vibrational line shapes of the oligothiophenes change with increasing number of rings. Furthermore, we observe the dynamical interplay between the quinoid and aromatic characters of individual rings in the oligothiophene chain during the dynamics and confirm that the quinoid character prevails in the center of the chain.