| abstract |
Squarylium compounds of the formula: <IMAGE> (in which Q1 and Q2 are each a chromophoric group having an aromatic unsaturated system conjugated with the squarylium ring and such that in the compounds of formulae Q1CH2R1 and Q2CH2R2 the methylene hydrogens are active hydrogens, R1 and R2 are each independently a hydrogen atom or an aliphatic or cycloaliphatic group, and R3 and R4 are each independently a hydrogen atom, or an acyl, aliphatic, cycloaliphatic, aromatic or heterocyclic group, subject to the proviso that one of R3 and R4 may be an amino or substituted amino group, or one of R3 and R4 is a hydrogen atom and the other is an organosulfonyl group, or R3 and R4 together with the intervening nitrogen atom form a nitrogenous heterocyclic ring) are useful as near infra-red absorbers. The presence of the amino group on the squarylium ring enables minor changes in absorption wavelength to be achieved by modifications of this group, and also allows functional groups to be incorporated into the dye without changing the chromophoric groups. |