Photochromic naphtho (2,1-B)pyrans

A naphthopyran compound of general formula (I) (I) wherein R1 represents an alkyl group or an aryl group; each of R4 and R5, which may be the same or different, independently represents an alkyl, alkenyl, carbocyclic or heterocyclic group, or R4 and R5 taken together with the carbon atom to which they are attached form a carbocyclic ring or a heterocyclic ring; and R6 represents a hydrogen atom or a substituent selected from alkyl, alkoxy, aryl, aryloxy, heteroaryl, halogen, amino, substituted amino, azo, imino, amide, carboxylate, ester, cyano, trifluromethyl or nitro, and in addition R6 may represent a carbocyciic or heterocyclic ring fused to ring A. The naphthopyran compounds of the invention are useful as photochromic materials in lenses, e.g. sunglasses, and photochromic transparencies for cars and aircraft.