4-([2,2′-Bi(1,3-dithiolylidene)]-4-yl)benzonitrile undergoes a [2 + 2] photocycloaddition reaction upon irradiation with polychromatic light, an unusual single-crystal-to-single-crystal transformation for a TTF derivative. In contrast, the closely related pyridylnitrile derivative adopts a different packing motif and is stable to light under the same conditions.