Spectral-luminescent properties of charged forms of some substituted benzaldehydes

The spectral-luminescent properties of the charged (anionic and cationic) forms of three biologically active substituted benzaldehydes, namely, $o$ -anisaldehyde (2-methoxybenzaldehyde), syringaldehyde (3,5-dimethoxy-4-hydroxybenzaldehyde), and vanillin (3-methoxy-4-hydroxybenzaldehyde), are studied by quantum-chemical methods. Calculations show that the $S_1$ state of the charged forms of the studied molecules, in contrast to the neutral forms, is of the $\pi\pi$* type, and its localization is similar to the localization of the $S_{2}$ $(\pi\pi^{*})$ state of the neutral molecules (anisaldehyde and vanillin) or of the $S_{3}$ $(\pi\pi^{*})$ state of syringaldehyde. According to the calculation results, the spectral range of 240–420 nm contains no new absorption bands corresponding to electronic transitions differing in nature and localization from the electronic transition of the neutral molecules. The calculated fluorescence characteristics of the charged forms of the studied molecules show that the radiative decay rate of the charged forms is considerably higher than that of the neutral forms, which is related to the different orbital nature of the $S_1$ state in the neutral and charged forms. Analysis of the calculated and experimental data on the fluorescence of the studied substituted benzaldehydes in alcohol solutions reveals that the fluorescence at 410 nm belongs to the cationic forms. Vanillin and syringaldehyde may also exhibit weak fluorescence of their anionic forms.