pendently 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 absorber