Evaluation of Single-Reference DFT-Based Approaches for the Calculation of Spectroscopic Signatures of Excited States Involved in Singlet Fission

Abstract

Singlet fission (SF) has the potential to dramatically increase solar cell efficiency by converting one singlet exciton to two free triplet excitons via a correlated triplet pair intermediate. Identification and characterization of excited states involved in SF are of great importance for understanding the fundamentals of SF. Despite their importance, it is still non-trivial to distinguish various species in transient absorption spectra due to their spectral overlaps and ultra-short lifetimes. Theoretical modeling of SF and its electronically excited state absorption (ESA) is generally challenging due to the multi-exciton nature of the correlated triplet pair, which usually requires description by expensive high-level ab initio methods. In this work, taking TIPS-pentacene monomer and its covalently linked dimer as representative examples, we demonstrate the use of single-reference DFT-based approaches to simulate the ESA spectra during SF. In particular, the singlet and triplet ESA are evaluated by TDDFT, QR-TDDFT, SLR-TDDFT, SF-TDDFT, and UTDDFT, in combination with ten different exchange-correlation functionals. The correlated triplet pair and its ESA are characterized by broken-symmetry DFT and TDDFT, and the role of orbital relaxation is highlighted. With a rational choice of exchange-correlation functionals, we found the resulting spectra to show good agreement with transient absorption experiments and certain improvements over high-order CI methods.