Spectral-luminescent properties of some benzaldehyde derivatives

Substituted benzaldehydes widely distributed in nature exhibit antibacterial, anti-inflammatory, antiviral, anti-carcinogenic activity with low toxicity, which make their promising in medicine. The results of an experimental and theoretical study of the spectral-luminescent properties of four neutral forms of hydroxy- and methoxy-substituted benzaldehydes are presented. The absorption spectra of the studied compounds were obtained in hexane and ethanol. Measurements of fluorescence spectra and fluorescence excitation of solutions were carried out in ethanol. Based on quantum chemical calculations, the orbital nature of electronic transitions, the features of the electron density distribution upon excitation and changes in the structure of the compound are determined and the rate constants of photophysical processes are estimated. According to calculations, for all compounds studied, the $S_1$ state is formed by the $n\pi$-type transition, and the intensity of the long-wavelength bands in these compounds is formed by one or two $\pi\pi^*$-transitions. The fluorescence quantum yield values are estimated from experimental and theoretical data. As a result of the analysis and comparison of experimental data and the results of quantum chemical calculations it was concluded that the neutral forms of the studied substituted benzaldehydes are not responsible for the observed fluorescence in experiments with ethanol solutions.