Computer Modeling and Simulations on Flexible Bifunctional Systems:  Intramolecular Energy Transfer Implications^†

A conformational search of the potential energy surface using the single coordinate driving method CICADA, molecular dynamics calculations, and quantum mechanical studies using the 6-31G* basis set were used for a detailed analysis of the conformational behavior of various flexible bichromophoric compounds Ph−CO−(CH₂)_x−O−Ar (x = 3−14; Ar = 2-naphthyl, 2-biphenylyl, 3-biphenylyl, 4-biphenylyl). The results were used for the estimation of end-to-end intramolecular (exchange) energy transfer efficiency and for comparison to the data recently obtained from steady-state quenching and quantum yield measurements (Wagner, P. J.; Klán, P. J. Am. Chem. Soc. 1999, 121, 9626−9635). The conformational search clearly supported the dominance of a through-space interaction in longer molecules (x = 5−14), which was still remarkably high even for x = 14. Comparison of both computational and experimental results suggests that through-bond coupling is responsible for 90% of the energy transfer in the shortest (x = 3) bichromophores. The molecular dynamics calculations seemed to validate the conclusion that only a small fraction of the energy transfer involved ground-state control (static quenching) by ground-state conformations with interchromophore distances within 4 Å. Rate-determining bond rotations to such geometries should be then responsible for the energy transfer within the lifetime of the excited donor. The influence of chromophore orientation was found insignificant for long-tether molecules, but important in short-tether ones due to different “reactive volumes” of different acceptors, such as naphthalene or biphenyl. In addition, a correlation of the calculated average distances between the γ-hydrogen and the carbonyl oxygen with the experimental hydrogen abstraction rate constants in the Norrish type II process strongly supported the right choice of the computational method.