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

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Abstract
Photoinduced electron transfer to 2,4,6-triphenylpyrylium tetrafluoroborate (P + BF 4 - ) 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)×10 10 dm3 mol -1 s -1 , whereas those for quenching of the triplet state ( 3* P + ) are significantly smaller, (0.5–3)×10 9 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 P 2 are proposed and consequences for preparative work are discussed.

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