Microscopic theory of singlet exciton fission. II. Application to pentacene dimers and the role of superexchange

TL;DR

This paper develops a theoretical framework for singlet fission, applies it to pentacene dimers, and finds superexchange as the primary mechanism, predicting rapid fission consistent with experiments and highlighting the role of molecular vibrations.

Contribution

It introduces a superexchange-mediated mechanism for singlet fission in pentacene dimers, advancing understanding of the process and its rapid timescale.

Findings

Superexchange mechanism predicts efficient sub-picosecond singlet fission.

Molecular vibrations have a limited effect on relaxation and decoherence.

Direct coupling yields slower fission rates compared to superexchange.

Abstract

In the preceding paper, we assembled the theoretical components necessary for a unified framework of singlet fission, a type of multiexciton generation producing two triplet excitons from one singlet exciton. In this paper, we apply our methodology to molecular dimers of pentacene, a widely studied material that exhibits singlet fission. We address a longstanding theoretical issue, namely whether singlet fission proceeds via two sequential electron transfer steps mediated by a charge-transfer state or via a direct two-electron transfer process. We find evidence for a superexchange mediated mechanism, whereby the fission process proceeds through virtual charge-transfer states which may be very high in energy. In particular, this mechanism predicts efficient singlet fission on the sub-picosecond timescale, in reasonable agreement with experiment. We investigate the role played by molecular vibrations in mediating relaxation and decoherence, finding that different physically reasonable forms for the bath relaxation function give similar results. We also examine the competing direct coupling mechanism and find it to yield fission rates slower in comparison with the superexchange mechanism for the dimer. We discuss implications for crystalline pentacene, including the limitations of the dimer model.