Aggregation properties of hyperporphyrins with hydroxyphenyl substituents.

Acidification of tetrakis(4-hydroxyphenyl)porphyrin (THPP) and tetrakis(3,5-dihydroxyphenyl)porphyrin (OHPP) in dichloromethane solutions has been investigated as a function of the nature of the counteranion. These porphyrins exhibit different patterns of behavior, and extended aggregates are formed displaying broad extinction features together with intense components due to resonant light scattering. Especially in the case of haloacids, J-aggregated species are obtained exhibiting large bathochromic shifts both for B- and Q-bands, which extend in the far red region. The optical characteristics of the aggregated and monomeric protonated species are strongly influenced by the nature of the counteranions. A comparison with tetrakis(4-methoxyphenyl)porphyrin (TMPP), which remains always in a monomeric form, demonstrates the key role played by the peripheral hydroxyl groups in stabilizing various porphyrin aggregates.