Naphthopyran Photochromic Dyes Sensitive to pH

Naphthopyrans of formula (I) or (II) reversibly change their optical properties (colour, induced optical density and/or colourability) with changes of pH. In the formulae, R?1¿ and R?2¿ are hydrogen or certain hydrocarbyl or heterocyclic groups, R?3¿ is an amino functional group or certain oxygen, sulphur or phosphorus groups; and R?4¿ may be certain C¿1?-C¿20? linear or branched alkoxy or alkyl (substituent) groups or is chosen from R?1¿, R?2¿ or R?3¿; and each n is 0 or 1 to 6, the total of all n's being no more than 6.