Molecular design for efficient singlet fission

Singlet fission is a photophysical process in molecules and molecular aggregates, in which a singlet exciton generated by irradiation splits into two triplet excitons. Recently, singlet fission has attracted a great deal of attention from the viewpoint of applications in organic photovoltaic cells, where singlet fission has a possibility of improving the photoelectric conversion efficiency. Although singlet fission was first observed about 50 years ago in anthracene crystals, and the mechanism has been investigated in detail for a small number of molecular systems such as tetracene and pentacene crystals, the relationships between molecular or crystal structures and singlet fission efficiency are yet to be precisely clarified. Thus, molecular structure - singlet fission relationships and molecular or crystal design guidelines for efficient singlet fission are intensely desired for realizing efficient photovoltaic energy conversion, and experimental and theoretical investigations advance rapidly. We introduce three investigation steps, which are based on bottom-up theoretical modeling from a molecule to a molecular aggregate or crystal. The modelling involves energy level matching at the molecular level, electronic coupling at the aggregate level, and singlet fission dynamics including exciton-phonon (vibronic) coupling, by emphasizing the importance of interplay between each step. From the modelling, we present several design guidelines for efficient singlet fission, which is together with practical molecular structures, chemical modifications and molecular configurations. (C) 2018 Elsevier B.V. All rights reserved.