15min:

NATHAN R. PILLSBURY, JAIME A. STEARNS, CHRISTIAN W. MÜLLER, ALOKE DAS, TALITHA M. SELBY AND TIMOTHY S. ZWIER, Department of Chemistry, Purdue University, West Lafayette, IN 47907-2084.

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The close proximity of two identical ultraviolet chromophores render diphenylmethane~(DPM) an interesting case for the study of the dependence of excitonic coupling on the distortion along low-frequency large-amplitude vibrational coordinates, in particular the phenyl ring torsional coordinates present in~DPM.

We have studied the fluorescence excitation spectrum and several single vibronic level fluorescence~(SVLF) spectra of the A¹B(\mathrmS₁) X¹A(\mathrmS₀) and the B¹A(\mathrmS₂) X¹A(\mathrmS₀) transition of~DPM cooled in a supersonic jet.

The band in the excitation spectrum blue-shifted by~123 \mathrmcm^-1 from the \mathrmS₁ \mathrmS₀ origin was assigned to the \mathrmS₂ \mathrmS₀ origin. Its SVLF spectrum shows two-region fluorescence reminiscent of that previously observed in the gas-phase and supersonic jet spectra of naphthalene¹ and~ovalene². The high-energy region of this \mathrmS₂~0⁰~SVLF spectrum was tentatively assigned to transitions originating in vibrationally excited levels of the \mathrmS₁~state. This finding indicates the presence of efficient internal mixing of the \mathrmS₂~0⁰ level with the sparse manifold of \mathrmS₁ vibronic background levels. The quantum number changes in the non-totally symmetric torsional mode~(see figure) upon internal mixing suggest that additional to the internal conversion transitions accounted for by the Jortner-Berry v = \pm 1 rule higher order vibronic mechanisms³ have to be considered as well.

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(2) A.~Amirav, U.~Even and J.~Jortner, J.~Chem. Phys. , \textbf1981, 74 , 3745.
(3) B.~Scharf, Chem. Phys. , \textbf1975, 7 , 478.