Zwitterionic vs porphyrin free-base structures in 4-phenylsulfonic acid meso-substituted porphyrins

Abstract

The 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin (TPPS4) and 5-phenyl-10,15,20-tris(4-sulfophenyl)porphyrin (TPPS3)] were obtained for the first time as the pure sulfonic acid derivatives and characterized for their constitutional structure in the solid state and in water solution. The zwitterionic species in water solutions, above a critical micellar concentration, are stabilized by side-to-side homoassociation to J-aggregates (B-band: 490 nm), but in solids they only give the J-aggregates when water is present in enough amount. Severe dried solids show a zwitterionic species, stabilized by side-to-side interactions, that probably corresponds to the dimer (B-band: 455 nm). The monomeric zwitterion (B-band: 434 nm) is detected in dilute water solutions and its acid/base properties (concentration pK a (1/2) ≈ 5.0) are not significantly different from those of the acidified solutions of their sodium sulfonate derivatives. However, pure water solutions of TPPS n , in contrast to those of their sodium salts, show important interactions with hydrophilic surfaces: e.g. TPPS4 adsorbs on fused quartz forming a monolayer of the free-base porphyrin together with its counter cations. Dilution of the title porphyrins in a solid matrix (e.g. KBr ) leads to a free-base porphyrin species with a red-shifted B-band (424 nm) that points to a side-to-side homoassociation, which is in contrast with the typical π-stacked aggregates of the corresponding alkaline metal sulfonato salts.