(K = 900–1300 dm3 mol–1). The extent of aggregation decreases in the following order: porphyrin 4a (nitro and 4-methoxyphenyl substituents) > 4b (nitro and phenyl substituents) > TMTP (methyl and phenyl substituents) > OEP (ethyl substituents). EPR spectra of Cu–4a support the dimerstructure of 4a, and the distance between two porphyrins is estimated to be about 4 Å.