Singlet fission tinkertoys based on N-heterocyclic carbenes: the role of stereoisomerism

Context This theoretical study reports excited states molecular design of potential singlet fission materials based on homo- and heterodimers of N-heterocyclic carbenes (NHCs) and their “locked” analogues. We found that the electronic properties of the investigated compounds can be finely adjusted by an interplay of the following factors: diradical character, topology, type of substituents, extension of the π-conjugation, excited state aromaticity, length of the latch in the “locked” analogues, conformation, and even stereoisomerism. With focus on the stereochemical aspects, this work provides new insights into the molecular design principles in diradicaloid organic materials with attractive electronic and excited state properties. Based on the comprehensive analysis, nine compounds were identified as promising candidates for efficient singlet fission. Methods Due to the large molecular size of the compounds and the importance of both static and dynamic correlation effects, the theoretical study is performed using simplified quantum chemistry (sQC) methods as screening tools, demonstrating their good performance in singlet fission aspect. Excited-state properties and singlet fission propensity of the NHC dimers were evaluated using spin-flip simplified time-dependent density functional theory (SF-sTD-DFT) and unrestricted simplified TDA (UsTDA) calculations. Vertical excitation energies and diradical characters were determined to assess the energetic conditions for singlet fission and to rationalize the effect of topology, substitution, and stereochemistry. The paper is dedicated to the late Prof. Masayoshi Nakano, whose seminal studies on diradicaloids opened a new chapter in the molecular design of modern materials.