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DOI: 10.1039/A607978D (Paper) Reference Section for: J. Chem. Soc., Faraday Trans., 1997, 93, 1523-1530

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

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Klaus-Dieter Warzecha, Martin Demuth and Helmut Görner

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 (λmax=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 (λexc=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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