Synthesis of 2,7,12,17-tetraaryl-3,8,13,18-tetranitroporphyrins; electronic effects on aggregation properties of porphyrins

Abstract

Novel porphyrins substituted with aryl and nitro substituents at the β-position were prepared by the tetramerization of 2-hydroxymethyl-3-aryl-4-nitropyrroles. Aggregation properties of these porphyrins are investigated by means of UV–VIS, ¹H-NMR and EPR spectroscopy. Porphyrin 4a substituted with 4-methoxyphenyl and nitro groups was found to form a strong cofacial aggregate in solution (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 dimer structure of 4a, and the distance between two porphyrins is estimated to be about 4 Å.