Vibrational analysis of the phosphorescence of a trans-stilbene single crystal. Is the phosphorescent state twisted?

Abstract

The vibrationally well-resolved phosphorescence spectrum of a trans-stilbene single crystal at 4.2 K is observed for the first time. There is no progression of the ethylenic double-bond torsional mode in the spectrum. This observation shows that there is little geometry change in the twist angle around the central double bond in the phosphorescent triplet state relative to the ground state. The possibility of torsion around C(ethylene)-C(phenyl), rather than C(ethylene)=C(ethylene), is indicated. The intensity distribution over the progression of ethylenic C=C stretching differs very much from that in the SI0-TI1 absorption already reported. The anomaly of the absence of mirror image is attributed to the existence of the next triplet state at about a few thousand wavenumbers higher than the T 1 origin