AGGREGATION OF TETRAARYL-SUBSTITUTED PORPHYRINS IN HOMOGENEOUS SOLUTION

Electronic absorption, fluorescence and fluorescence excitation spectr a, and vibrational Raman spectra of selected homogeneous solution phas e, free-base tetraaryl-substituted porphyrins-specifically, tetrapheny lporphine (TPPH2), tetra(p-pyridyl)porphine (TPyPH(2)), tetra(p-N-meth ylpyridyl)porphine (TMPyPH(2)), tetra(p-carboxylphenyl)porphine (TCPPH 2), and tetra(p-sulfonatophenyl)porphine (TSPPH2)-as well as their pro tonated diacids are acquired and analyzed. N-protonation induces a str uctural change in which the free-base porphyrin is converted from a co nfiguration in which the aryl moiety is twisted relative to the macroc ycle plane to one in which it is nearly coplanar, thus promoting aggre gation. However, spectroscopic investigations of the tetraaryl-substit uted porphyrins reveal that coplanarity, although necessary, is insuff icient for aggregation to occur. Also required is that the doubly prot onated porphyrin have a net zero charge, i.e., be zwitterionic. The st ructural alignments of monomers in tetraaryl-substituted porphine aggr egates are suggested to be similar to those of J-aggregates of cyanine dyes.