Unexpected results of spectral emission and photophysical study of S2-excited 4H-1-benzopyrane-4-thione (BPT) in water solution were explained by intermolecular hydrogen bonding. There are at least two types of BPT–water complexes that participate in the S2-fluorescence similarly as in S0 → S2 absorption. They both deactivate with high efficiency in internal conversion S2 → S1 and in intersystem crossing S1 → T1 processes.
