Time-resolved electron paramagnetic resonance study of the lowest excited triplet state of trans-cinnamic acid

Abstract

Time-resolved and steady-state electron paramagnetic resonance spectra and luminescence spectra have been observed for the lowest excited states of trans-cinnamic acid in rigid solutions at 77 K. The zero-field splitting parameters in the lowest excited triplet (T 1) state are D = 0.1061 cm −1 and E = −0.0497 cm −1. The T 1 state is estimated to be 20 100 cm −1 and the T 1 lifetime is 45 ms in ethanol at 77 K. The T 1 lifetime decreases as the solvent polarity becomes less polar. The sublevel preferentially populated by intersystem crossing is T y ( y close to in-plane short axis and to the CO direction). These results are explained in terms of vibronic and spin-orbit mixings between the 3 nπ ∗ and 3ππ ∗ states in the T 1 state.