Azastilbenes. 2. Photodimerization

Article abstract of DOI:10.1021/jo01297a003

The photochemical behavior of several azastilbenes has been followed in concentrated solution and in the solid state.In acetonitrile and benzene isomerization and dimerization occur, the reactions being generally faster in acetonitrile.In methanol, however, photoreduction as well as photoaddition of the solvent intervene and are important processes.With irradiation in the solid state, dimerization occurs only for some azastilbenes and their quaternary salts, depending on the orientations of the molecules within the crystal lattice and the distances between adjacent double bonds (3.5-4.2 Angstroem).X-ray analysis has shown that trans-1,2-di(2-pyrazinyl)ethylene crystallizes in two distinct modifications of which only one has a crystal stacking suitable for topochemical dimer formation.The dimers were characterized by 1H and 13C NMR, IR, and mass spectroscopy.The crystalline and molecular structures of five of them were determined by X-ray diffraction, namely cyclobutane dimers of 1,2-di(4-pyridyl)ethylene, 1,2-di(2-pyridyl)ethylene, 1,2-di(2-pyrazinyl)ethylene (all three r-ctt dimers), and 1-(3-pyridyl)-2-(2-pyrazinyl)ethylene (r-ctt head-to-head and head-to-tail dimers).