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 0 00 cm(-1). The two-photon polarization ratios of both spectra have been det ermined. Spectral assignments have been made on the basis of two-photon pol arization data, ab initio calculations, and comparison with one-photon data . The four excited states below 5 eV have been identified. Calculated one- and two-photon allowed intensities are in good agreement with results deriv ed from the alternant hydrocarbon theory. The S-0 --> S-1 two-photon intens ity is mainly vibronically induced by bl modes. The most active bl vibratio n, the "Kekule"-type mode responsible for double bond localization in 1,6-m ethnno-[10]annulene, shifts to approximate to 1540 cm(-1) in S-1 from the g round state 1355 cm(-1) value. Applying two-photon polarization results to 1,6-methano[10]annulene (C-2v symmetry), the tensor elements of the stronge st totally symmetric S-0 --> S-1 (B-1 x b(1) x a(1)) transition have been e stimated.