Water-assisted assembly of (E)-arylvinylpyridine hydrochlorides: effective substrates for solid-state [2+2] photodimerization.

Water molecules assist the assembly of (E)-arylvinylpyridine hydrochlorides in a head-to-tail and face-to-face fashion by way of N-HO hydrogen bonds in combination with cation-π interactions between the pyridinium and aromatic rings. Photolysis of the pyridinium salt hydrates provided synHT dimers in high yields.