[2+2] Photodimerization of bispyridylethylenes by a controlled shift of the protonation equilibrium

Irradiation of trans-4,4′-bispyridylethylene in the presence of 1 equiv of concd HCl produced a syn dimer with high selectivity, whereas irradiation in the presence of more than 2 equiv of concd HCl or in the absence of HCl gave a mixture of dimers and by-products with much lower selectivity. This indicated that a suitable amount of acid served as a catalyst for the [2+2] photodimerization of BPEs through cation–π interactions between the pyridinium and pyridine rings.