Molecular and electronic structures of the short-lived excited triplet state of tropone

Time-resolved electron paramagnetic resonance experiments on the short-lived T₁ state of nonphosphorescent tropone were carried out using mixed crystals. Well-resolved hyperfine splittings due to allowed and forbidden transitions were observed at principal axes of the zero-field tensor. The spin density distribution, C-H bond angles, and the sign of the D value were deduced from the hyperfine spectra. It has been concluded that the TI state of tropone mainly has a ³ΠΠ* configuration and holds its molecular structure of C_2v symmetry. The experiments and molecular orbital calculations indicate that the large Franck-Condon factor between the T₁ and the So states arises from the change of the potential surfaces due to the antibonding character of the Π* orbital at the C₄-C₅ bond in the T₁ state.