Regio- and stereospecific [2+2] photocyclodimerization of a crown-containing butadienyl dye via cation-induced self-assembly in solution.

Self-assembled pseudocyclic structures consisting of two molecules of a crown-containing butadienyl dye and two Mg(2+) ions readily undergo regio- and stereospecific [2+2] photocycloaddition in MeCN to produce a single cyclobutane stereoisomer in almost quantitative yield.