15min:

JAIME A. STEARNS, ALOKE DAS, TALITHA M. SELBY AND TIMOTHY S. ZWIER, Department of Chemistry, Purdue University, West Lafayette, IN, 47907; DAVID F. PLUSQUELLIC, National Institute of Standards and Technology, Gaithersburg, MD, 20899.

Diphenylmethane is a simple bichromophore in which two phenyl rings are connected only by a methylene group, giving considerable flexibility in the ring torsion coordinates as well as substantial electronic coupling between the rings. According to DFT calculations, the minimum energy structure has C₂ symmetry with a ring torsion angle near 60^o. There are two such minima connected by a C_2v structure 176 cm^-1 higher in energy. The rotationally-resolved fluorescence excitation spectrum of the S₀-S₁ origin indicated a 70% a-type, 30% c-type transition moment, suggestive of an excitonic state in which the zero-order transition moment is rotated significantly from that of toluene. CIS underestimates the a-type character of the band, while time-dependent DFT overestimates it. The ground state experimental rotational constants are consistent with those predicted by DFT. A Franck-Condon progession in the torsional coordinate was observed in the experimental jet-cooled resonant two-photon ionization and dispersed fluorescence spectra. This progression was well-fit using a harmonic Franck-Condon analysis indicating a 3^o change in the ring torsion angle upon excitation to S₁. The S₀-S₂ origin was assigned to a vibration 123 cm^-1 above the S₀-S₁ origin. The dispersed fluorescence spectrum from this transition showed a great deal of activity in low frequency vibrations which were not present in the excitation spectrum, indicating that a vibronic band of the S₁ state may be resonant with the S₀-S₂ origin. To test the effect of asymmetry on the excitonic coupling, the spectroscopy of 4-methyldiphenylmethane has also been studied. Addition of a methyl group to one chromophore completely localizes the electronic excitation, demonstrated by the fact that the two electronic origins are very near those of toluene and para -xylene. Dispersed fluorescence from the S₀-S₂ origin showed only S₀-S₁ origin-like emission broadened by IVR, indicative of extremely rapid electronic energy transfer.