Rotationally resolved electronic spectra of 9,10-dihydrophenanthrene. A “floppy” molecule in the gas phase Available to Purchase

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 $(φ)$ 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 $φ∼8.5°$⁠. Three prominent Franck-Condon progressions appear in the low resolution spectrum, all with fundamental frequencies lying below $300cm−1$⁠. 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 $∼2650cm−1$ along the bridge deformation mode and is substantially reduced by excitation of the molecule along the (orthogonal) ring twisting coordinate.