The Conformers of 2,2′-Bipyridine in the Phosphorescent Triplet State as Studied by Electron Spin Resonance and Phosphorescence

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Abstract
Two sets of triplet-state ESR spectra corresponding to the s-cis and s-trans conformers of 2,2′-bipyridine have been observed in mixtures of water with various alcohols at 77K. The intensity ratio of the ESR signal for the s-cis conformer to that for the s-trans conformer depends on the composition of the mixture and on the sample-cooling rate. From the analysis of the phosphorescence spectra, the lowest triplet state of the s-cis conformer is estimated to be close to that of the s-trans conformer. An anomalous ESR signal which cannot be ascribed to either the s-cis conformer or the s-trans conformer has been observed in a 1-propanol-water mixture. The geometries of these conformers are discussed from their observed zero-field splitting parameters, together with their potential energy diagrams, as obtained by an ab initio calculation.