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

Rotationally resolved fluorescence excitation spectra of several bands in the S1<--S0 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 S0 state with a dihedral angle between the aromatic rings (phi) of approximately 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 approximately 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 S1 state is estimated to be approximately 2650 cm(-1) along the bridge deformation mode and is substantially reduced by excitation of the molecule along the (orthogonal) ring twisting coordinate.