The absorption spectra of cis- and trans- 1,3,5-hexatriene are reported for the region of the first allowed singlet-singlet electronic transition, 2600 to 2100 A. A least-squares procedure in which a Lorentzian line shape is assumed for each band was used to fit the spectra. Progressions are found of 408, 1262, and 1626 cm−1 for the cis isomer and of 350, 1232, and 1628 cm−1 for the trans isomer. The 0–0 transition for the cis compound, 39668 cm−1, lies about 130 cm−1 lower than that for the trans, 39800 cm−1. The fact that the 0–0 transition energy for cis is less than that for trans is interpreted as evidence that the cis isomer has a non-planar geometry in the electronic ground state. The extinction coefficient for the 0–0 band of the trans isomer is about 1.5 times that for the cis.

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The polarized fluorescence spectrum of triphenylene as a guest in a n-heptane single crystal has been studied at 4 K. The transitions in the fluorescence spectrum are analyzed in terms of known ir and Raman active vibrations in the ground state. A small matrix induced deformation of triphenylene, leading to an in-plane change of geometry is proposed. Polarization data are analyzed in terms of this model.