Photochromic Compounds

A photochromic compound has the formula (I) wherein R?1¿ is selected from the group including substituted amino aryl moieties, substituted cyclic amino aryl moieties and substituted $i(N)-indolinoaryl or $i(N)-tetrahydroquinolino aryl groups, R?3¿ is selected from the group including linear or branched C¿1?-C¿20? alkyl, linear or branched C¿2?-C¿20? alkenyl or polyalkenyl, C¿2?-C¿20? alkynyl or polyalkynyl, aryl, heteroaryl, linear or branched C¿1?-C¿20? alkyl aryl, linear or branched C¿1?-C¿20? alkyl-cycloalkyl or -bicycloalkyl or -tricycloalkyl or substituted -cycloalkyl or -bicycloalkyl or -tricycloalkyl, linear or branched C¿1?-C¿20? haloalkyl or perhaloalkyl, linear or branched C¿1?-C¿20? alkoxyalkyl or peralkoxyalkyl, linear or branched C¿1?-C¿20? alkylthioalkyl or peralkylthioalkyl, linear or branched C¿1?-C¿20? amino alkyl or peraminoalkyl, C¿3?-C¿20? cycloalkyl or substituted cycloalkyl, C¿4?-C¿20? bicycloalkyl or substituted bicycloalkyl, C¿6?-C¿20? tricycloalkyl or substituted tricycloalkyl or linear or branched C¿1?-C¿20? hydroxyalkyl or perhydroxyalkyl; R?4¿ and R?5¿ which may be the same or different are R?3¿ or R?4¿ and R?5¿ are conjoined to form a spiro linked C¿3?-C¿20? cyclic system or substituted cyclic system; and wherein R?2¿ and R?6¿ are each selected from R?3¿ or hydrogen, halogen, nitro, nitroso, amino, acetamido, C¿2?-C¿10? $i(N)-alkylamido, alkoxycarbonyl, nitrile, carboxy, hydroxy, aryloxy, C¿1?-C¿10? alkoxy, a heteroatom which is O or N or S or P which in turn can be bonded either singly or multiply to carbon, oxygen, sulphur or nitrogen, C¿1?-C¿5? alkoxy or alkylthio or aryl or aryloxy or multiples thereof.