Optical and time-resolved electron paramagnetic resonance studies of the excited states of para-methylcinnamic acid and para-methylcinnamate anion

Time-resolved electron paramagnetic resonance (EPR), fluorescence, and phosphorescence spectra have been observed for the excited states oftrans-p-methylcinnamic acid (p-MeCA) andtrans-p-methylcinnamate anion (p-MeCA^?) in rigid organic glasses at 77 K. With a stretched-polymer-film technique, we assigned the resonance fields in the time-resolved EPR spectra of the lowest excited triplet (T₁) states ofp-MeCA andp-MeCA^?. From the analysis of these spectra we concluded that the deviation from planar structure in the T₁ state is small inp-MeCA andp-MeCA^? and the direction of C=O iss-cis to the ethylenic C=C bond inp-MeCA. The deprotonation appears to have little effect on the zero-field splitting (ZFS) parameters. The ZFS parameters were calculated theoretically using semiempirical molecular orbitals. The sublevel preferentially populated by intersystem crossing (ISC) is T _y in bothp-MeCA andp-MeCA^?. However, following the deprotonation ofp-MeCA, (P _y?P _z)/(P _x?P _z) changes from 5.7 inp-MeCA to 2.1 inp-MeCA^? (P _i are relative populating rates; thex andy axes are close to the long and short in-plane molecular axes, respectively, andz-axis is perpendicular to the molecular plane). The fluorescence lifetimes indicate that the deprotonation has little effect on the sum of the three ISC rate constants for the three T₁ sublevels. A decrease in acidity ofp-MeCA upon excitation has been observed.