Electronic absorption, fluorescence and fluorescence excitation spectra, and vibrational Raman spectra of selected homogeneous solution phase, free-base tetraaryl-substituted porphyrinsspecifically, tetraphenylporphine (TPPH2), tetra(p-pyridyl)porphine (TPyPH2), tetra(p-N-methylpyridyl)porphine (TMPyPH2), tetra(p-carboxylphenyl)porphine (TCPPH2), and tetra(p-sulfonatophenyl)porphine (TSPPH2)as well as their protonated diacids are acquired and analyzed. N-protonation induces a structural change in which the free-base porphyrin is converted from a configuration in which the aryl moiety is twisted relative to the macrocycle plane to one in which it is nearly coplanar, thus promoting aggregation. However, spectroscopic investigations of the tetraaryl-substituted porphyrins reveal that coplanarity, although necessary, is insufficient for aggregation to occur. Also required is that the doubly protonated porphyrin have a net zero charge, i.e., be zwitterionic. The structural alignments of monomers in tetraaryl-substituted porphine aggregates are suggested to be similar to those of J-aggregates of cyanine dyes.

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Fluorescence decay dynamics and structure of a meso-tetrakis(p-sulfonatophenyl)porphyrin, referred to herein as either H2TSPP4- or TSPP, which aggregates in highly acidic homogeneous solution, are discussed. The fluorescence lifetime of the aggregate is found to depend on whether electronic excitation is to the B (Soret) state or the Q state. This finding is interpreted as indicating that the aggregate's effective size differs in the two cases. Also, a discussion is provided of possible nonradiative pathways that might affect measured fluorescence lifetimes. Specific structural changes that occur within the porphyrin upon N-protonation and incorporation within the aggregate are also discussed. Additionally, vibrational frequencies are assigned to specific bonds of the porphinato macrocycle, and comparisons are made between bond lengths in the aggregate environment and those of the monomeric dianion.