The singlet states of both trans- and cis-anethole are moderately long-lived (8.5 and 6.1 ns, respectively) and undergo photodimerization as well as cis, trans isomerization. Both the singlet lifetimes and fluorescence rate constants are larger than those for related phenylalkenes. Dimerization of singlet trans-anethole yields exclusively a syn head-to-head dimer with a rate constant of 3.2 X 108M-1 s-1. Reaction of singlet trans-anethole with ground-state cis-anethole also yields a syn head-to-head dimer with a lower rate constant. Dimerization of singlet c/>anethole yields an anti head-to-head dimer with a rate constant of 0.8 X 108M-1 s-1. Whereas the two former reactions occur with retention of anethole stereochemistry, dimerization of cis-anethole occurs with retention at one double bond and inversion at the other. Dimerization of cis-anethole is proposed to occur via sequential formation of a singlet exciplex with optimized π-orbital overlap and a singlet 1,4-biradical intermediate.
ASJC Scopus subject areas
- Colloid and Surface Chemistry