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), 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)(v) 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.