Biindanylidenes: role of central bond torsion in nonvertical triplet excitation transfer to the stilbenes.

The stilbenes were proposed to function as nonvertical triplet excitation (NVET) acceptors for energy-deficient donors because rotation about the central bond diminishes the energy gap between ground and triplet energy surfaces. Recently, the role of central bond torsion in facilitating NVET to cis-stilbene (c-St) was questioned because the behavior of 2,3-diphenylnorbornene as a triplet energy acceptor is similar to that of cis-stilbene. On the basis of the assumption that the rigidity of the norbornene skeleton precludes torsional displacement of the phenyl rings in the triplet state, an alternative mechanism was proposed involving phenyl-vinyl torsion as the key reaction coordinate for NVET to c-St. However, this proposal is inconsistent with theory, which predicts that the triplet state energy minimum corresponds to a geometry with significant displacement of the phenyl rings of 2,3-diphenylnorbornene from a common plane. We now provide experimental evidence demonstrating that central bond torsion is the key coordinate for NVET to stilbenes. Comparison of the activation parameters for the two rigid stilbene analogues, cis- and trans-1,1'-biindanylidene (c-Bi and t-Bi) to those for the stilbenes, shows that the excitation transfer processes remain nonvertical despite the strong structural inhibition of phenyl-vinyl torsion; the relatively small preexponential factors of the respective isomers are almost identical. Their magnitude is a measure of the attenuation introduced by Franck-Condon overlap factors which decrease as the torsional state quantum number corresponding to the transition state increases. These results and results from theoretical calculations are consistent with central bond torsion as the key reaction coordinate in NVET to the biindanylidenes and the stilbenes. The crystal structure of t-Bi shows it to be strictly planar, eliminating phenyl-vinyl torsion toward planarity as a crucial NVET reaction coordinate.