Spin dynamics of organic acids and bases in their nonphosphorescent and short-lived excited triplet states

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1. The effects of protonation on the excited states of trans-3-styrylpyridine (StP) and trans-4,4'-dipyridylethylene (DPE) have been studied through measurements of the time-resolved electron paramagnetic resonance (EPR), UV absorption, and fluorescence spectra in methanol-water mixtures at 77 K. From the analysis of these spectra it is concluded that the single protonation appears to have little effect on the zero-field splitting parameters and the anisotropy in the sublevel populating rates of the lowest excited triplet (T_1) states of StP and DPE. The decay rate constants of the fluorescent states decrease and fluorescence quantum yields increase on single protonation. These experimental results suggest that the single protonation causes a decrease in the intersystem crossing rates for the three T_1 sublevels. These results are explained in terms of the vibronic mixing between the ^1nπ^* and ^1ππ^* states in the lowest excited singlet state.2. The esters of trans-p-methoxycinnamic acid are the most widely used UV absorbers in cosmetic sunscreens. To be effective as a UV protective compound in skin-care sunscreens, the UV absorber must be able to harmlessly transform the absorbed UV energy into vibrational energy. The deactivation mechanism of trans-cinnamic acids (CAs) and their esters have been an object of continuous interest for many years. However, the property of the T_1 states of CAs and their esters is poorly known. This is because of their weakly phosphorescent character and short T_1 lifetimes. In the present research, the effects of deprotonation on the excited states of trans-p-methylcinnamic acid have been studied through measurements of the time-resolved EPR and fluorescence in rigid organic glasses and in stretched poly(vinyl alcohol) films at 77 K. The deprotonation appears to have little effect on the zero-field splitting parameters. However, the triplet sublevel kinetics changes on deprotonation.