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_l<-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 -21.5°. 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~8.5°. 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 -2650 cm~(-1) along the bridge deformation mode and is substantially reduced by excitation of the molecule along the (orthogonal) ring twisting coordinate.