Photoinduced electron transfer from isoprenoid polyalkene acetates to dicyanoarenes
Photoinduced electron transfer from electron donors, such as prenyl acetate (D-0), geranyl acetate (D-1), all-trans-farnesyl acetate (D-2) or all-trans-geranylgeranyl acetate (D-3) to 1,4-dicyano-2,3,5,6-tetramethylbenzene (A-1), 1,4-dicyanonaphthalene (A-3) and 9,10-dicyanoanthracene (A-5) in the presence of 1,1'-biphenyl (BP) as co-donor was studied in acetonitrile by time-resolved UV–VIS spectroscopy and conductivity. The main transients observed for A-1, A-3 (λexc = 308 nm) and A-5 (λexc = 353 nm) in deoxygenated acetonitrile are the radical cation of BP (BP˙+) and the radical anion of the respective electron acceptors. Oxidation potentials of 2.1, 1.8 and 1.7 V for D-0, D-1 and D-2, respectively, are consistent with data obtained from steady-state fluorescence quenching of the electron acceptors and decay of BP·+ both occurring via electron transfer. A conductivity component in the millisecond time range upon excitation of A-1 or A-3 in the presence of BP and D-n in aqueous acetonitrile is ascribed to a proton which is released upon nucleophilic addition of water to the radical cation of the donor (D-n˙+) and subsequently reacts with an intermediate in the cyclization pathway. The mechanism of the photocyclization of D-1, involving initially a reaction of D-1˙+ with water, is discussed in detail.