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>Franckndash;Condon analysis of theS0rarr;T1absorption and phosphorescence spectra of biphenyl and bridged derivatives
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Franckndash;Condon analysis of theS0rarr;T1absorption and phosphorescence spectra of biphenyl and bridged derivatives
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机译：Franckndash;Condon analysis of theS0rarr;T1absorption and phosphorescence spectra of biphenyl and bridged derivatives
The equilibrium geometry and the vibrational force field of the ground and the lowest triplet electronic states of biphenyl and three bridged derivativesminus;biphenylene, fluorene and phenanthreneminus;are computed by using an updated version of the QCFF/PI (Quantum Chemical Force Field/pgr; electron) Hamiltonian. The displacement parameters betweenT1andS0are obtained and used to model theS0rarr;T1absorption and the phosphorescence spectra. The calculated Franckndash;Condon envelopes are found to be in excellent agreement with the vibrational structure of the observed spectra. The common features of the phosphorescence spectra of biphenyl and fluorene are related to the same orbital nature of the lowest triplet state. The observed asymmetry between the phosphorescence and singletndash;triplet absorption spectra of biphenyl is reproduced when the twisted equilibrium geometry ofS0is considered. It is shown that evidence of the nonplanarity of the ground state of biphenyl is manifested by the lower intensity of the band observed in the phosphorescence at 747 cmminus;1with respect to the intensity of the same band in fluorene. The increased vibrational activity calculated in the lower frequency region for biphenylene and phenanthrene agrees with the observed spectra and reflects the different orbital nature of the lowest triplet state of the two strongly perturbed bridged derivatives with respect to biphenyl and fluorene. From the analysis of the computed vibrational frequencies, it is suggested that the false origin of the symmetry forbidden phosphorescence of biphenylene is due to the lowest outhyphen;ofhyphen;plane mode ofausymmetry.
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