Effect of off-diagonal exciton-phonon coupling on intramolecular singlet fission

TL;DR

This study develops a microscopic model to understand how off-diagonal exciton-phonon coupling influences intramolecular singlet fission, revealing conditions under which it enhances or facilitates the process.

Contribution

It introduces a non-perturbative model incorporating both diagonal and off-diagonal phonon couplings, providing new insights into their roles in singlet fission.

Findings

Off-diagonal coupling can create additional fission channels.

Both couplings can aid fission if excitonic coupling is weak.

Fission is facilitated by diagonal coupling when excitonic coupling is strong.

Abstract

Intramolecular singlet fission (iSF) materials provide remarkable advantages in terms of tunable electronic structures, and quantum chemistry studies have indicated strong electronic coupling modulation by high frequency phonon modes. In this work, we formulate a microscopic model of iSF with simultaneous diagonal and off-diagonal coupling to high-frequency modes. A non-perturbative treatment, the Dirac-Frenkel time-dependent variational approach is adopted using the multiple Davydov trial states. It is shown that both diagonal and off-diagonal coupling can aid efficient singlet fission if excitonic coupling is weak, and fission is only facilitated by diagonal coupling if excitonic coupling is strong. In the presence of off-diagonal coupling, it is found that high frequency modes create additional fission channels for rapid iSF. Results presented here may help provide guiding principles for design of efficient singlet fission materials by directly tuning singlet-triplet interstate coupling.