Picosecond Infrared Spectroscopy of Electronically Excited trans-Stilbene in Solution in the Fingerprint Region

The picosecond transient infrared spectrum of the lowest excited singlet (S 1 ) state of trans -stilbene in solution has been recorded in the fingerprint region. Several absorption bands (molar absorption coefficients ε ≲ 100 mol -1 dm 3 cm -1 ) attributable to in-plane vibrational modes of the S 1 species have been observed. The tentative assignments of the vibrational bands are described. Major spectral features are common between the infrared spectrum taken in n -heptane solution and that in acetonitrile solution. The mutual exclusion rule between infrared and Raman spectra of the S 1 species seems to hold in n -heptane solution, which suggests a molecular structure with a center of symmetry. On the other hand, additional weak infrared bands are found in acetonitrile, whose positions coincide with those of the strong Raman bands of the S 1 species. This result suggests that the S 1 species in acetonitrile is distorted slightly from the centrosymmetric structure. Polarized structures of excited species stabilized in polar solvents are discussed in relation to the distorted S 1 trans -stilbene in acetonitrile.