Quantum-Chemical Study of the Nature of Electronically Excited States of Hydroxyaromatic Compounds

Spectroscopy methods make it possible to record the features of specific photo-induced processes occurring inside and between molecules and, moreover, chemical and structural changes of such objects can be monitored and controlled interactively with the help of luminescent molecular probes with known spectral properties. In order to solve the general fundamental problem of establishing the connection of the photophysical and spectral-luminescent properties of organic molecules with their structure, a comparative analysis was made of the nature of electronically excited states and photoprocesses occurring in phenol and its related compounds (p-cresol and bisphenol A). To understand the mechanism of forming the absorption and fluorescence spectra of hydroxyaromatic compounds, the electronically excited states of the molecules under study are presented and analyzed together with the localization of molecular orbitals and the local minima of the electrostatic potential. The results of these calculations have shown that the proton-acceptor ability of isolated molecules is associated with an excess of electron density on oxygen atoms and increases in the series: phenol < n-cresol < bisphenol A. This order is maintained when excited to the S₁ and S₂ states. At the same time, the proton-acceptor ability of phenols decreases in comparison with the ground state. The consequence of the non-planar structure of the bisphenol A (BPA) molecule is the mixed orbital nature of the electronically excited states. The non-planar structure of the BPA leads to an increase in the spin-orbit interaction and in the intercrossing conversion constant between S₁(ππ*) and T₆(πσ*) states compared to phenol.