J-aggregate formation in bis-(4-carboxyphenyl)porphyrins in water : pH and counterion dependence

Abstract

The self aggregation behaviour of meso-tetraarylporphyrins containing two carboxyphenyl units in adjacent and opposite positions, respectively, 5,10-bis(4-carboxyphenyl)-15,20-diphenylporphyrin (DiCPP-adj) and 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (DiCPP-opp), was investigated at different pH values. Ordered porphyrin architectures are obtained at pH = 0.8 and pH = 12 for both compounds studied, through an easy self assembly approach. The type of architecture and the extent of aggregation are related with the relative positions of the 4-carboxyphenyl groups attached to the porphyrin core and with the counterions present. The aggregates obtained exhibit spontaneous chirality, resonance light scattering, short fluorescence lifetimes and low fluorescence quantum yields. The data gives evidence that at pH = 0.8 with NO₃⁻ the preferred aggregation is more favoured for DiCPP-adj than for DiCPP-opp, whereas at pH = 12 the aggregate of DiCPP-opp is induced by Na⁺ and Li⁺ cations and stabilized by hydrophobic interactions. Deposition of aqueous solutions at key pHs on glass surfaces enables the detection of circular and ring aggregates viewed by fluorescence lifetime imaging microscopy (FLIM) with dimensions of a few micrometres.