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Solutions of free base meso -tetraphenylporphyrin (H 2 TPP), meso -tetra(4-carboxyphenyl)porphyrin (H 2 TPPC), and meso -tetra(4-pyridyl)porphyrin (H 2 TPyP) under various conditions generate aggregates whose absorption spectra are characterized by invariant Soret bands with bandwidths that are independent of the preparative method. One of the Soret bands is blue-shifted (H-aggregate) relative to the monomeric porphyrin band; other Soret bands are red-shifted (J-aggregates). The aggregates are characterized by different nonradiative rate constants for excited singlet-state decay and by different efficiencies of singlet−singlet annihilation at the high energies of laser excitation. The quantum yields of fluorescence vary between 10 -5 and 10 -2, and the corresponding fluorescence lifetimes vary in the range from 10 -12 to 10 -9 s; they are more than 1 order of magnitude smaller than those of the corresponding monomeric porphyrins. Lifetimes (τ) correlate with the characteristic ground-state absorption recovery times of the aggregates. The sizes of the H-aggregate and aggregates that are characterized by a minimal blue shift of the Soret band range from 15 to 27 Å.
Khairutdinov et al. (Fri,) studied this question.