2) above, wherein R1 is an alkyl group having 3-20 carbon atoms; M is Zn ion; 2 R2 is an acetyl group; and Im is Im1 (wherein R3 is a methyl group). [0018] (4) A mercapto-substituted imidazolylporphyrin metal complex dimer represented by a general formula (2): [0019] wherein each of R1, M, R2, X and Im including R3 has the same meaning as defined for the general formula (1) mentioned in item (1) a