Unraveling the variation of photoluminescence by pyridine N-addition in thiazolo[5,4-d]thiazole-based fluorophores

Abstract

We investigate the underlying mechanism of the variation in photoluminescence properties by pyridine N-addition to the parent 2,5-bis(4-pyridinium)thiazolo[5,4-d]thiazole molecule (1). Energetic aspects of singlet and triplet manifolds indicate an efficient triplet formation via the symmetry-allowed S1-T4 intersystem crossing (ISC) in 1. An increased energy gap and symmetry mismatch hinders this ISC pathway in N-protonated (H2-1) and N-methylated (Me2-1) molecules, resulting in the S1 to S0 decay as the dominant process in their photophysics. Consequently, H2-1 and Me2-1 emit fluorescence with a near unity quantum yield. The S3-T5 ISC pathway can induce triplet formation; however, an extremely rapid S3 to S1 internal conversion decay inhibits the triplet formation in the latter molecules.