Theoretical Studyon the Solvent-Dependent OpticalProperties of 2‑Aryl-3H–1,3-benzazaphosphole Oxide

Photoresponsive molecules that respond to the surrounding environment are expected to be utilized as optical functional materials, such as sensors. Recently reported 2-aryl-3H–1,3-benzazaphosphole oxide with a diphenylamino group (ABPO) is one such molecule having interesting solvent-dependent properties. The absorption spectra of ABPO in nonpolar and polar solvents are almost identical in shape and excitation energy, whereas the fluorescence spectrum is red-shifted as the solvent polarity increases. In addition, the fluorescence quantum yield drastically decreases in methanol and acetonitrile solvents. In this study, the solvent-dependent optical properties of ABPO were investigated using quantum chemical calculations with solvent models. To describe the solvation structures explicitly, the QM/MM reweighting free energy self-consistent field method was used. The calculated absorption and fluorescence energies qualitatively reproduced the experimental trends. The <i>S</i>₁ excited state has charge-transfer (CT) character with a large dipole moment, which is responsible for the solvent dependency of fluorescence spectra. Furthermore, in addition to the CT state, the twisted intramolecular charge transfer (TICT) state plays an important role in the fluorescence quenching. It was indicated that the stabilization of the TICT state, which has a larger dipole moment than the CT state, affects the decrease in fluorescence quantum yield in polar solvents.