yl group -CH=CH; the sulfoxide group SO; the azo group N N-; the saturated fluorocarbon groups -CF CF organic phosphine oxides R1:=0 where R is a hydrocarbon group, and the ethylidene group where X can be hydrogen or halogen or divalent groups which can activate halogens on the same ring such as with difiuorobenzoquinone, 1,4- or 1,5- or 1,8-difluoroanthraquinone.