COLOUR PHOTOTHERMOGRAPHIC ELEMENTS CONTAINING LEUCO DYES

A photothermographic element comprising a support bearing a photothermographic medium, the medium comprising a light sensitive silver halide in reactive association with a silver salt of an organic acid and a colour generating reducing agent which is a leuco compound which is oxidisable by silver ions into a coloured dye of the general formula: in which: n = 0, 1 or 2, R represents H, CN, lower alkyl of 1 to 5 carbon atoms, aryl or COOR in which R is lower alkyl of 1 to 5 carbon atoms or aryl, R and R independently represent CN, NO2, COOR , SO2R , CO.R in which R6 is as defined above or R and R together represent the necessary atoms to form a 5- or 6-membered carbocyclic or heterocyclic ring having ring atoms selected from C, N, O and S atoms, which carbocyclic or heterocyclic rings possess at least one conjugated electron withdrawing substituent, R and R independently represent H, CN or lower alkyl of 1 to 5 carbon atoms or together represent the necessary atoms to complete a 5 or 6-membered carbocyclic ring, and Ar represents: a) a thienyl group which may be substituted with one or more lower alkyl groups of 1 to 5 carbon atoms, b) a furyl goup which may be substituted with one or more lower alkyl groups of 1 to 5 carbon atoms or c) a phenyl group which may be substituted with one or more groups selected from halogen, hydroxy, lower alkyl of 1 to 5 carbon atoms, lower alkoxy of 1 to 5 carbon atoms, NR R in which R and R are independently selected from H, lower alkyl group of 1 to 5 carbon atoms which may possess substituents selected form CN, OH, halogen and phenyl, and phenyl group optionally substituted with substituents selected from OH, halogen, lower alkyl of 1 to 5 carbon atoms or lower alkoxy of 1 to 5 carbon atoms or R and R together represent the necessary atoms to complete a morpholino group, or said phenyl group may be part of a larger ring structure comprising two or more rings which may be aromatic or heterocyclic containing up . to 20 ring atoms selected from C, N, 0 and S. o