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 ...