Aggregation of Tetraaryl-Substituted Porphyrins in Homogeneous Solution

Electronic absorption, fluorescence and fluorescence excitation spectra, and vibrational Raman spectra of selected homogeneous solution phase, free-base tetraaryl-substituted porphyrins specifically, tetraphenylporphine (TPPH 2 ), tetra( p -pyridyl)porphine (TPyPH 2 ), tetra( p - N -methylpyridyl)porphine (TMPyPH 2 ), tetra( p -carboxylphenyl)porphine (TCPPH 2 ), and tetra( p -sulfonatophenyl)porphine (TSPPH 2 ) 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.