[4 + 4] photodimerization of azaanthracenes in both solution and solid phase controlled by cation-π interactions.

Regio- and stereoselective [4 + 4] photodimerization reactions of 1- and 2-azaanthracenes were performed in both methanol solution and solid phases to give anti-HT dimers in high yields. In these reactions, intermolecular cation-π interactions between the pyridinium cation and the benzene ring play a key role in preorientation prior to the photodimerization reactions.