Electronic Spectrum of 2,2′-Paracyclophane

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Abstract
The polarized absorption and fluorescence spectra of single crystals of 2,2′‐paracyclophane at 20°K were recorded in the region 3300–3100 Å. The spectra consist of narrow lines of which 87 could be measured in absorption and nine in emission. Single crystals having two different planes developed could be prepared sufficiently thin for measurements. The spectra obtained could be divided into two components, one characteristic of a transition moment parallel and the other perpendicular to the unique axis of the tetragonal crystal. These polarization directions are characteristic of molecular directions since in this case no Davydov splitting is to be expected in view of the high symmetry of the crystal. The absorption system is analyzed satisfactorily as representing the forbidden component of the benzene 1A1g→1B2u transition in the ``dimer'' molecule. A totally symmetric progression‐forming frequency of 240 cm—1 is assigned as the intermoiety vibration. The appearance of a weak 0–0 transition is interpreted as due to a small‐amplitude twist of the moieties relative to one another.