Chirality from the crystal lattice: The achiral diarylethene 1 a crystallizes in a monoclinic chiral space group, that is, two different kinds of crystals, (M)-1 a and (P)-1 a, are obtained. Photoirradiation of (P)-1 a furnished optically pure closed-ring isomer (R,R)-1 b in the single crystal (see scheme). The photochemical cycles can be repeated by alternating UV and visible photoirradiation.