Comparative Studies on Electronic Spectra and Redox Behaviors of Isomeric Benzo[1,2-b:4,5-b′]difurans and Benzo[1,2-b:5,4-b′]difurans

Electronic absorption/emission spectra, absolute fluorescence quantum yields, and oxidation potentials of isomeric benzo[1,2-b:4,5-b′]difurans (1a) and benzo[1,2-b:5,4-b′]difurans (2a) along with their α,α′-di-n-butyl (1b and 2b) and bis(3,5-dihexyloxyphenyl) derivatives (1c and 2c) were studied. The longest wavelength absorption maxima were very close between 1a and 2a and between 1b and 2b; however, the maximum absorption of 1c was significantly red-shifted compared to that of 2c, due to cross-conjugation in the latter. Unlike related compounds, the fluorescence quantum yields of syn (1a−c) and anti (2a−c) isomers were virtually identical. On the other hand, the oxidation potentials of the syn isomers were significantly lower than those of the anti isomers. Molecular orbital calculations revealed that this is likely to be characteristic of benzodifurans, because HOMO energy levels of the [1,2-b:4,5-b′] and [1,2-b:5,4-b′] isomers were estimated to be virtually identical in the other benzodichalcogenophenes.