Electronic Spectra and Photolysis of Bisphenol A in Water

A quantum chemical study of the spectral and luminescent properties of the BPA + 2H₂O complex was carried out. Calculations were performed by the semi-empirical method of intermediate neglect of differential overlap using a program complex and a special parameterization. The spectral behavior of BPA in water was modeled by a complex with water molecules in the ratio 1: 2 forming the hydrogen bond. The calculated data were compared with the results of investigation of the isolated BPA molecule. The nonplanar BPA structure leads to the strong mixing of the π- and σ-type atomic wave functions. The main reason for the low quantum yield of the BPA fluorescence is the efficient process of singlet-triplet conversion in the S₁(ππ*) ≳ T_n(πσ*) channel of the BPA molecule and its complex with water. A study of the photolysis of the isolated BPA molecule upon exposure to solar radiation, the short-wavelength boundary of which on the Earth’s surface is located at ~290 nm (~34480 cm^–1), showed that the energy of the photodissociative state localized on the O–H bond is much higher than this value for BPA. The binding curve is characteristic of the S₁(ππ*) state, while the singlet and triplet states of the πσ* type, localized on the single C–C bonds of the central fragment of the molecule, are repulsion curves with a barrier. From our point of view, the low efficiency of the BPA degradation under the influence of solar radiation is due to the presence of a significant potential barrier to the photolysis in the singlet or triplet state. The mechanisms of bond breaking in the BPA + 2H₂O complex are different for the singlet and triplet states, namely, for the S₃(πσ*) state, the break occurs by the predissociation mechanism, and for the T_n(πσ*) state, due to its population through the singlet-triplet conversion in the S₁(ππ*) → Т_n(πσ*) channel.