State-specific studies of internal mixing in a prototypical flexible bichromophore: Diphenylmethane

Laser-induced fluorescence, resonant two-photon ionization, UV-UV hole burning, UV depletion, and single vibronic level fluorescence (SVLF) spectra of jet-cooled diphenylmethane (DPM) have been recorded over the 37 300 - 38 400 cm(-1) region that encompasses the S-1 <- S-0 and S-2 <- S-0 transitions. All transitions in the laser-induced fluorescence excitation spectrum are due to a single conformational isomer of DPM with C-2 symmetry. The S-1 <- S-0 origin transition occurs at 37 322 cm(-1), supporting a short progression in the symmetric torsion T with spacing of 28 cm(-1). The S-2 <- S-0 origin transition occurs 123 cm(-1) above the S-1 origin and possesses very weak torsional structure, observable only under saturating laser power conditions. A combination of SVLF spectroscopy and hot band studies is used to assign the frequencies of the symmetric torsion (T), antisymmetric torsion ((T) over bar), and butterfly (beta) vibrations in the S-0, S-1, and S-2 states. The emission from the S-2 origin is composed of two components, a set of sharp transitions ascribable to the S-2 state and a dense clump of transitions ending in ground-state levels 81, 88, and 93 cm(-1) above the S-0 zero-point level ascribable to S-1(upsilon) emission. Assignment of the transitions in the clump leads to the conclusion that the single vibronic level responsible for the emission has mixed S-2/S-1 character. The mixing involves several torsional vibronic levels in the S-1 manifold close in energy to the S-2 origin, with the correct symmetry to couple the two states. These levels involve significant torsional excitation. The close energetic proximity of these levels leads to a breakdown of typical vibronic coupling selection rules. (c) 2008 American Institute of Physics. [DOI: 10.1063/1.2977730]