Cage formation subsequent to intramolecular triplet-energy transfer

Abstract

Although most work on intramolecular energy-transfer between nonconjugated chromophores has relied on measurements of donor and/or acceptor emission, it is evident that by proper choice of molecular system, a photochemical reaction of the acceptor could replace its emission. In the Diels–Alder adduct of 1,4-naphthoquinone with cyclopentadiene, the donor tetralin-1,4-dione (D) has been shown to transfer intramolecularly its lowest triplet energy to the isolated norbornylene double bond (N). Following energy migration, the excited olefinic acceptor attacks the π-system of the benzene nucleus in (D) to form a cage product by intramolecular cycloaddition. Sensitization experiments, control reactions, and low-temperature emission spectra verified the path followed by the excitation energy. Apparently the mechanism requires a change in steric conformation prior to cage formation. The intramolecular photochemical cycloaddition in the analogous p-benzoquinone–cyclopentadiene adduct is also discussed.