On the origin of nonvertical triplet excitation transfer:: The relative role of double-bond and phenyl-vinyl torsions in the stilbenes

The results of theoretical calculations at the B3LYP-DFT level using the 6-31+G(d,p) basis set on the ground and triplet states of the stilbenes (c-1, t-1) and of 2,3-diphenylnorbornene (2) are described. They are not consistent with the premise that structural constraints require that the stilbene moiety of 2 assume a planar geometry in its relaxed triplet state. Pronounced pyramidalization at the olefinic carbon atoms gives a PhCCPh dihedral angle of 51.0 degrees in (3)2*. Furthermore, the dihedral angles of (3)2* that involve phenyl-vinyl torsions, 23.3 and 26.5 degrees, do not attain the assumed value of 0 degrees. Thus, it is likely that 2 was improperly used as a rigid model for c-1. The calculations predict that in 2 and in 1 torsional motion about the CC double bond affords the most stabilization in the triplet state. As suggested earlier, it is that motion and not phenyl-vinyl torsion that most facilitates nonvertical energy transfer to the stilbenes. A single energy minimum is found on the stilbene triplet energy surface, close to the postulated geometry of the "phantom" (perpendicular, (3)p-1*) triplet.