Photocyclization of terpenoid polyalkenes upon electron transfer to a triphenylpyrylium salt A time-resolved study

Abstract

Photoinduced electron transfer to 2,4,6-triphenylpyrylium tetrafluoroborate (P⁺BF₄^-) from terpenoid polyalkenes bearing electron-withdrawing substituents, e.g. 2,6-dimethylhepta-1,5-diene-1,1-dicarbonitrile (D-1) and homologues thereof (D-2, D-3) as well as from other donors, such as 1,1′-biphenyl (BP) and trans-stilbene, has been studied in solution by laser flash photolysis. The main transient in the presence of D-n (n=1, 2, 3) is the triphenylpyranyl radical (P). Fluorescence quenching of ^1*P⁺ by D-n in acetonitrile occurs with rate constants of (0.6–1.7)×10¹⁰ dm3 mol^-1 s^-1, whereas those for quenching of the triplet state (^3*P⁺) are significantly smaller, (0.5–3)×10⁹ dm3 mol^-1 s^-1. The corresponding half-concentrations for fluorescence and triplet quenching are [D]_1/2=20–40 and 0.02–0.2 mmol dm^-3, respectively. BP and trans-stilbene were used as donors since their radical cations can be detected spectroscopically (λ_max=660 and 460 nm, respectively). In addition, the role of BP as a co-donor was examined; the [D]_1/2 values for the quenching of its radical cation are similar to those for the quenching of ^3*P⁺. These findings demonstrate the existence of the polyalkene radical cation (e.g. D-1⁺), although it is not directly detectable, even upon photoionization (λ_exc=248 nm). The quantum yields of photodecomposition of D-1 and formation of photoproducts were measured in acetonitrile in the presence of either water or methanol. P⁺ is not directly restored from P. Two mechanisms for electron back-transfer involving the dimer P₂ are proposed and consequences for preparative work are discussed.

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