Electronic structure of the excited triplet state of nonphosphorescent tropolone studied by time-resolved EPR

Time-resolved EPR measurements and semi-empirical MO calculations have been carried out to clarify the electronic structure of the T₁ state of nonphosphorescent tropolone. The observed zero-field splitting parameters indicate that the T₁ state has a pure ππ* character and the energy order among the spin sublevels was T_γ (in-plane) > T_x (in-plane) > 0 > T_z (out-of-plane), which is the same as tropone for a parent molecule. However, the principal X-axis slightly turns out of the carbonyl bond direction so that the intersystem crossing from the S₁ state to the T_γ sublevel becomes more active. The spin densities obtained from hyperfine splittings of protons and MO calculation of AM1 method show an asymmetric distribution of unpaired π-electrons over two oxygens as well as the seven-membered ring. This unbalanced distribution makes the deviation of the X-axis from the carbonyl bond clear. We found two sites that can exchange each other by intramolecular proton transfer in a durene single crystal.