Compounds

A compound represented by the general formula (1)

• wherein Y1, Y2, Y3, and Y4 may be the same or different and represent carboxy group, an esterified carboxy group or a group of —(CONH)n—X (wherein X is an optionally branched and optionally substituted hydrocarbon group which may have unsaturated bonds, an optionally substituted aromatic group or a heterocyclic group having conjugated double bonds);
• n is an integer of 1 or 2;
• Q and Q′ each represent an optionally branched alkyl or alkoxy group of 1-6 carbon atoms, a halogen atom, nitro group or nitroso group;
• m and m′ each represent an integer of 0-3;
• and
• A represents an optionally substituted and optionally branched bivalent hydrocarbon group of 1-20 carbon atoms which may have unsaturated bonds, wherein said hydrocarbon group may have a moiety of an optionally substituted aromatic group or a heterocyclic group having conjugated double bonds;
• or a salt thereof.