Substituent Effect of 1,4-Benzenedicarbonitriles as Sensitizers on the Photoinduced Electron Transfer Reactions in Alcohol

In the photosensitized electron transfer reaction of 6,6-diphenyl-1,4-dioxaspiro[4.5]decane in methanol (MeOH), the quantum yield is increased by the use of 2-methylbenzene-1,4-dicarbonitrile (2-methyl-BDC) or 2,5-dimethylbenzene-1,4-dicarbonitrile (2,5-dimethyl-BDC) as compared to that by the use of 1,4-benzenedicarbonitrile (BDC), though the excited methyl-substituted BDCs have lower reduction potentials than the excited BDC. Detailed study on this substituent effect reveals that the electron transfer between MeOH and the excited BDC occurs to some extent. By the use of methylated BDCs, electron transfer between the excited sensitizers and MeOH is suppressed enough due to its lower reduction potential. Some typical photoinduced electron transfer reactions in alcohol proceed more efficiently by the use of 2-methyl-BDC or 2,5-dimethyl-BDC instead of BDC.