| Abstract
| - The two-photon fluorescence excitation spectrum of 1,6-methano-[10]annulene has been measured in fluidsolution at room temperature between 25 000 and 40 000 cm-1 and in rigid solution at 77 and 15 K between25 000 and 28 000 cm-1. The two-photon polarization ratios of both spectra have been determined. Spectralassignments have been made on the basis of two-photon polarization data, ab initio calculations, and comparisonwith 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 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 ≈1540cm-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.
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