EXCITONIC COUPLING AND PHENYL RING TORSION IN DIPHENYLMETHANE
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Date
2005
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Publisher
Ohio State University
Abstract
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 C$_{2}$ 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 S$_{0}$-S$_{1}$ 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 S$_{1}$. The S$_{0}$-S$_{2}$ origin was assigned to a vibration 123 cm$^{-1}$ above the S$_{0}$-S$_{1}$ 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 S$_{1}$ state may be resonant with the S$_{0}$-S$_{2}$ 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 {\it para}-xylene. Dispersed fluorescence from the S$_{0}$-S$_{2}$ origin showed only S$_{0}$-S$_{1}$ origin-like emission broadened by IVR, indicative of extremely rapid electronic energy transfer.
Description
Author Institution: Department of Chemistry, Purdue University, West Lafayette, IN, 47907; National Institute of Standards and Technology, Gaithersburg, MD, 20899