Two-photon fluorescence excitation spectrum of 1,6-methano-[10]annulene

The two-photon fluorescence excitation spectrum of 1,6-methano-[10]annulene has been measured in fluid solution at room temperature between 25 000 and 40 000 cm-1 and in rigid solution at 77 and 15 K between 25 000 and 28 000 cm-1. The two-photon polarization ratios of both spectra have been determined. Spectral assignments have been made on the basis of two-photon polarization data, ab initio calculations, and comparison with one-photon data. The four excited states below 5 eV have been identified. Calculated one- and twophoton allowed intensities are in good agreement with results derived from the alternant hydrocarbon theory. The S0 - S1 two-photon intensity is mainly vibronically induced by b1 modes. The most active b1 vibration, the “Kekule”-type mode responsible for double bond localization in 1,6-methano-[10]annulene, shifts to 1540 cm-1 in S1 from the ground state 1355 cm-1 value. Applying two-photon polarization results to 1,6-methano- [10]annulene (C2V symmetry), the tensor elements of the strongest totally symmetric S0 - S1 (B1 b1 a1) transition have been estimated.