Multiple effects of aromatic substituents on excited-state properties and singlet fission process in azaquinodimethane systems†

Abstract

Singlet fission (SF) could offset the thermalization loss of high-energy photons via multiexciton generations, thus holding great potential in improving the power conversion efficiency of solar cells. However, the development of SF-based devices has basically remained stagnant so far owing to the limited scope of practical SF materials. Therefore, designing and developing practical SF material systems have been imperative, yet challenging so far. In this work, we comprehensively investigated the effects of aromatic substituents on excited-state properties and SF process of azaquinodimethane systems. Results indicated that the aromatic substituents have a significant influence on molecular diradical characters, thereby determining the excited-state energetics of the SF material system, including optical band gaps and triplet energy. Moreover, the aromatic substituents influenced charge transfer coupling interactions by adjusting molecular packing in the aggregate state to shunt the excited-state population to exert SF process or trap in excimer species. These results not only offer a deep insight into the multiple regulatory effects of the aromatic substituents on excited-state properties and SF process but also provide a practical SF material system, which could lay the foundation for the discovery of new SF-active chromophores and practical applications of new-generation light-harvesting materials.