Singlet fission tinkertoys based on N-heterocyclic carbenes: а study with the spin-flip simplified time-dependent density functional theory method

Singlet fission is a photophysical process in organic matter, in which one absorbed photon is converted in two triplet excitons. Discovered over 50 years ago, today singlet fission attracts scientific interest because it opens new horizons for the development of highly efficient organic-based solar cells beyond the Shockley–Queisser limit. However, a relatively small amount of singlet fission chromophores is available and, with respect to molecular design, little is known about the relationship between compounds’ structure and singlet fission propensity. Here, we report the molecular design of new singlet fission materials based on dimers of N-heterocyclic carbenes (NHC) and their ‘locked’ analogues. The study applies the recently introduced spin-flip simplified time-dependent density functional theory (SF-sTD-DFT) approach, which allows the modelling of sizeable molecules with diradical character like most of the successful singlet fission chromophores. To motivate the choice of method for the study NHC-based chromophores, the SF-sTD-DFT results are benchmarked against high level calculations using the equation-of-motion spin-flip coupled-cluster method with single and double substitutions. Armed with a reliable computational strategy, we have explored in detail the singlet fission propensity variation of the compounds due to the following structural factors: topology, type of substituents, cis-trans conformation, extension of the conjugation and modulation of the heterocyclic aromaticity, as well as the length of the latch in the ‘locked’ analogues. The results reveal new strategies for the design of singlet fission materials and contribute to the deeper understanding of the excited state tunability at the molecular level.