Photoexcitation dynamics of azaphosphole compounds: effects of substituents and solvents

Abstract Photoexcitation dynamics of a series of 2-aryl-3H-1,3-benzazaphosphole oxides (ABPO) were studied using ultrafast time-resolved fluorescence spectroscopy and transient absorption spectroscopy. The fluorescence quantum yields and lifetimes strongly depended on the substituents on the phenyl ring at the 2-position, and generally fluorescence lifetimes were very short. Compound (1d) that has a 4-(N,N-diphenylamino)phenyl group at the 2-position showed intense fluorescence in nonpolar solvents, although the fluorescence was quite weak in polar solvents. The time-resolved fluorescence spectrum of 1d showed a large dynamic shift, accompanied by a decrease in fluorescence intensity. The transient absorption spectrum of 1d showed complex spectral dynamics. From the singular value decomposition analysis of the time-resolved spectra, the spectral dynamics were assigned to the charge transfer formation accompanying the rotation of the phenyl ring at 2-position predicted by the theoretical calculations (K. Sugiyama et al., J. Phys. Chem. B, 2025, 129, 2701). A compound with an ortho-hydroxyphenyl at the 2-position (1e-OH) showed intramolecular proton transfer in aprotic solvents, showing dual fluorescence from the normal and tautomer bands. In methanol, a new fluorescence band appeared besides the bands from the normal and tautomer states, which was assigned to the hydrogen-bonding complex with the solvent by the theoretical calculations.