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Vibronic π-π* transition bands of pyrene, 4,5-dihydropyrene (DHP), and 4,5,9,10-tetrahydropyrene (THP) were assigned by means of dichroism analysis using stretched poly(vinyl alcohol) films. The absorption spectra of the samples in the non-stretched films were divided into two component spectra polarized along the long (x) and short (z) axes of the molecular plane. For pyrene, in addition to the four conventionally-assigned band systems (1Lb, 1La, 1Bb, and 1Ba), some forbidden bands were found in the respective band regions. Most of the vibronic DHP and THP bands were also reasonably well assigned from comparisons with the calculated results for phenanthrene and biphenyl, respectively.
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Yoshinaga et al. (1977) studied this question.
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