X-ray diffraction analysis elucidated that a pair of hexaarylbiimidazoles was transformed to a complex composed of two lophyl radicals and a piezodimer on irradiation at low temperatures. The piezodimer, the existence of which has been assumed spectroscopically as an unstable isomer, was produced via molecular swapping of two photo-induced lophyl radicals with drastic conformational change. The black crystal reverted to the original structure on warming to 25 degreesC.