TIME-RESOLVED ELECTRON-PARAMAGNETIC RESONANCE STUDY OF THE TRIPLET-STATES OF NITRONAPHTHALENES

Time-resolved electron paramagnetic resonance (EPR) and steady state EPR spectra have been observed for the lowest excited triplet (T1) states of nitronapthalenes (NNs) (1- and 2-NN) and dinitronaphthalenes (DNNs) (1,3-, 1,4- 1,5-, 1,8- and 2,7-DNN) in diethylether + isopentane + ethanol at 77 K. With the aid of computer simulations, it is concluded that the sublevel preferentially populated by intersystem crossing is T(y) in 1-NN and 1,3-, 1,4-, 1,5- and 1,8-DNN, while it is mainly T(z) in 2-NN and 2,7-DNN (y and z close to in-plane long and short axes of the napththalene ring respectively). A strong correlation between the zero-field splitting parameters and the T1 lifetimes suggests that there is a strong vibronic coupling between the 3-pi-pi* and the close-lying 3n-pi* states in the T1 state of 1,4-DNN.