Head-to-tail interactions in tyrosine/benzophenone dyads in the ground and the excited state: NMR and laser flash photolysis studies

Abstract The formation of head‐to‐tail contacts in de novo synthesized benzophenone/tyrosine dyads, bp∪Tyr, was probed in the ground and excited triplet state by NMR techniques and laser flash photolysis, respectively. The high affinity of triplet‐excited ketones towards phenols was used to trace the geometric demands for high reactivity in the excited state. A retardation effect on the rates with increasing hydrogen‐bond‐acceptor ability of the solvent is correlated with ground‐state masking of the phenol. In a given solvent the efficiencies of the intramolecular hydrogen‐atom‐transfer reaction depend strongly on the properties of the linker: rate constants for the intramolecular quenching of the triplet state cover the range of 10 5 to 10 8 s −1 . The observed order of reactivity correlates to a) the probability of close contacts (from molecular‐dynamics simulations) and b) the extent of the electronic overlap between the π systems of the donor and acceptor moieties (from NMR). A broad survey of the NMR spectra in nine different solvents showed that head‐to‐tail interactions between the aromatic moieties of the bp∪Tyr dyads already exist in the ground state. Favourable aromatic–aromatic interactions in the ground state appear to correspond to high excited‐state reactivity.