The jet-cooled S-0 -> S-1 excitation spectrum of 1,6-epoxy-[10]annulene

The excitation spectrum of 1,6-epoxy-[10]annulene in the region of the lowest electronic transition (≈417-390 nm) has been measured under supersonic expansion conditions. The observed vibronic structure, with onset at 24008 cm-1, is discussed mostly in terms of allowed transitions. Comparison with MCSCF/CAS calculations of vibrational frequencies in S0 and S1 allows the assignment of several excited state fundamentals and correlation with ground state modes. Both the experimental and theoretical results point to the conclusion that equilibrium geometry changes upon excitation, with the C1 and C6 atoms of the molecular ring, connected by the epoxy bridge, moving slightly apart going from S0 to S1.