A naphthalocyanine dye of formula (I) is provided:whereinM is a metal group or is absent;R1 and R2 are independently selected from hydrogen or C1-12 alkoxy;X is selected from O, S or —NH—;Ar1, Ar2, Ar3, Ar4, Ar5, Ar6, Ar7 and Ar8 are selected from phenyl, naphthyl, pyridyl, furanyl, pyrollyl, thiophenyl, each of Ar1, Ar2, Ar3, Ar4, Ar5, Ar6, Ar7 and Ar8 being optionally substituted with 1, 2, 3, 4 or 5 groups, the or each group being independently selected from C1-12 alkyl, C1-12 alkoxy, C1-12 arylalkyl, C1-12 arylalkoxy, —(OCH2CH2)dORd, cyano, halogen, amino, hydroxyl, thiol, —SRv, —NRuRv, nitro, phenyl, phenoxy, —CO2Rv, —C(O)Rv, —OCORv, —SO2Rv, —OSO2Rv, —NHC(O)Rv, —CONRuRv, —CONRuRv, sulfonic acid, sulfonic acid salt and sulfonamide;d is an integer from 2 to 5000;Rd is H, C1-8 alkyl or C(O)C1-8 alkyl; andRu and Rv are independently selected from hydrogen, C1-12 alkyl, phenyl or phenyl-C1-8 alkyl.