A Remarkable Isotope Effect on the Photochromism of trans-Biindenylidenedione in the Solid State

A remarkable deuterium isotope effect on the photochromism of 3,3′-diaryl-3H,3′H-[2,2′]biindenylidene-1,1′-dione (2) in the solid state has been found. The yellow crystals of 2 and 2-d2 were immediately changed to reddish purple crystals upon photoirradiation in the solid state. The reddish purple crystals of 2-d2 returned back to the original yellow crystals under dark for 72 h, whereas that of 2 faded within 2 h.