Rotationally resolved electronic spectra of 9,10-dihydrophenanthrene. A 'floppy' molecule in the gas phase

摘要

Rotationally resolved fluorescence excitation spectra of several bands in the S-1 <- S-0 electronic spectrum of 9,10-dihydrophenanthrene (DHPH) have been observed and assigned. Each band was fit using rigid rotor Hamiltonians in both electronic states. Analyses of these data reveal that DHPH has a nonplanar configuration in its S-0 state with a dihedral angle between the aromatic rings (phi) of similar to 21.5 degrees. The data also show that excitation of DHPH with UV light results in a more planar structure of the molecule in the electronically excited state, with phi similar to 8.5 degrees. Three prominent Franck-Condon progressions appear in the low resolution spectrum, all with fundamental frequencies lying below 300 cm(-1). Estimates of the potential energy surfaces along each of these coordinates have been obtained from analyses of the high resolution spectra. The remaining barrier to planarity in the S-1 state is estimated to be similar to 2650 cm(-1) along the bridge deformation mode and is substantially reduced by excitation of the molecule along the (orthogonal) ring twisting coordinate. (c) 2007 American Institute of Physics.