We report on B←N coordination to support a single-crystal-to-single-crystal reaction in the solid state. A [2 + 2] photodimerization is achieved with face-to-face π-stacks of monotopic B←N adducts composed of a phenylboronic acid catechol ester and an alkene with a terminal thiophene group. The photoreaction generates a ditopic B-adduct involving a head-to-tail cyclobutane regio- and stereoselectively. The photodimerization is accompanied by an increase in the tetrahedral character of the B atom. The resulting boron enables channel confinement of chloroform upon recrystallization.