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 is an element of less than or similar to 100 mol(-1) dm(3) cm(-1)) attributable to in-plane vibrational modes of the SI 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 SI 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 SI 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.