Evaluating the Electronic Structure of Coexisting Excitonic and Multiexcitonic States in Periodic Systems: Significance for Singlet Fission

• Published in: Journal of chemical theory and computation Vol. 18; no. 1; pp. 394 - 405
• Main Authors: Xie, Xiaoyu, Troisi, Alessandro
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 11.01.2022
• Subjects: Charge transfer; Clusters; Couplings; Density functional theory; Electronic structure; Electrons; Fission; Organic chemistry; Spectroscopy and Excited States; Wave functions
• ISSN: 1549-9618; 1549-9626
• DOI: 10.1021/acs.jctc.1c00831

Singlet fission (SF) in organic molecular solids is an example of a process that is challenging to describe with the most common electronic structure approaches. It involves optically bright singlet excited states delocalized over many molecules, which could be efficiently treated by density functional theory, and multiexcitonic localized states that have to be studied with wavefunction methods, usually with small clusters considering their expensive computational costs. In this work, we propose a methodology to combine multiconfigurational wavefunction calculations with reduced Hamiltonian to investigate the electronic structure of large clusters or fully periodic systems. The method is applied to the prototypical SF materials tetracene and pentacene. The results allow one to study how states of different natures (excitonic, charge-transfer, and multiexcitonic) coexist and are contaminated by their couplings in large or periodic systems. Novel insights are therefore possible. For example, because the excitonic bands are relatively broad with respect to the multiexcitonic states, there are limited regions of the crystal momentum space where the transition between the two is more likely.