Heterocyclic compound or salt thereof, active material, electrolytic solution and redox flow battery

A heterocyclic compound represented by the following formula (1), (2) or (3), or a salt thereof:

• wherein:
• R1 to R8 are each independently a hydrogen atom, an acidic group, an alkoxy group, an alkyl group, an amino group, an amide group or a group represented by formula (a), and at least one of R1 to R8 is a group represented by formula (a);
• X1 represents an oxygen atom, a sulfur atom or NY2;
• Y1 is a group represented by formula (b);
• Y2 is a hydrogen atom, an alkyl group, a carbonyl group, a sulfonyl group or a group represented by formula (b);
• R9 and R10 are each independently a hydrogen atom or a substituent;
• Z1 is an acidic group;
• n is an integer of 1 to 7;
• *a represents a bonding site to formula (1);
• and
• *b represents a bonding site to X1 of formula (a),
• wherein:
• X2 is an oxygen atom, a sulfur atom or NR11;
• Y2 is a linker;
• Z2 is an acidic group;
• n1 is an integer of 1 to 6;
• R11 is a hydrogen atom or an alkyl group;
• each R12 is independently an alkyl group or an acidic group;
• and
• n2 is an integer of 0 to 5, and
• wherein:
• X3 is an oxygen atom or a sulfur atom;
• Y3 is a linker;
• Z3 is an acidic group;
• n3 is an integer of 3 to 8;
• each R13 is independently an alkyl group or an acidic group;
• and
• n4 is an integer of 0 to 5.