Structural Effects on Twisted Olefin Triplet Lifetimes. Styrene Derivatives

Article abstract of DOI:10.1021/ja00387a002

The triplet lifetimes in methanol of a series of anisylalkenes and anisylcycloalkenes, including several phenyl analogues, have been determined by nanosecond flash kinetic absorption spectrophotometry with nitrogen laser excitation.Electron-transfer scavenging by paraquat dication, PQ²⁺, (methyl viologen) afforded the readily detectable reduced species PQ^+*.Both triplet lifetimes and rate constants for electron transfer from triplet to PQ²⁺ were determined.The results indicate that acyclic styrene triplets prefer the twisted, "1,2-biradical" structure.Lifetimes are increased by vinyl deuteration, by increasing alkylation, and by constraint within a small ring; in the absence of geometric constraints, the dominant influence on the triplet lifetimes is the extent of substitution at the double bond.Mechanisms for intersystem crossing are discussed, and the tentative conclusion is reached that nuclear motions are important.