Photosensitive Cyclomer Formation of 1,1'-(1,2-Ethanediyl)bis(pyridinyl) Diradical and Its Derivatives

Article abstract of DOI:10.1021/ja00197a043

Two-electron reduction of 1,1'-(1,1-ethanediyl)bis(pyridinium) dibromide (1a) with sodium amalgam affords the meso and dl cyclomers formed by intramolecular cyclization of the diradical (3a).The meso cyclomer (4a) is thermally converted into the dl cyclomer (5a), while inversely 5a is photolytically converted into 4a.Reduction of the 4,4'-dimethyl and 4,4'-di-tert-butyl derivatives (1b and 1c, respectively) of 1a similarly affords the corresponding meso (4b and 4c) and dl (5b and 5c) cyclomers, which are convertible to each other.One-electron reduction products (2band 2c) of 1b and 1c are detected at room temperature as shown by the ESR spectra in acetonitrile, which exhibited intramolecular rapid spin exchange between the two pyridine rings in the cation radical.The activation energy for the meso to dl conversion was obtained by NMR spectroscopy to be 22.4, 26.9, and 28.8 kcal*mol^-1 for 4a-c, respectively.Photodissociation of the cyclomers gives the diradicals, which were characterized by the ESR triplet spectra at -196 deg C with the zero-field parameters: <*>D<*> = 0.0222 and <*>E<*> = 0.0012 cm^-1 for 3a, <*>D<*> = 0.0210 and <*>E<*> = 0.0008 cm^-1 for 3b, and <*>D<*> = 0.0219 and <*>E<*> = 0.0016 cm^-1 for 3c.Calculations of the D value suggest that the dihedral angle of the NCH2CH2N group of the diradical ina rigid glass is about 95⁰ for these three diradicals.