Remarkable rate acceleration of [4 + 2] cyclodimerization of an ethynylanthracene derivative on the product crystal surfaces.

Slow evaporation of a chloroform-n-hexane solution of 2-(anthracene-9-ylethynyl)-1-methylbenzimidazole (BzIm-An) yields cyclodimer (BzIm-An)(2) as crystals; the cyclodimerization is then dramatically accelerated on the product crystal faces.