Recently, supramolecular assemblies of porphyrins have been of growing interests because the aggregate packing structure can be easily tuned through ionic interactions among the cationic core, charged substituents and/or the nature of the inorganic anion. Controlling the packing structure of functional dye molecules by supramolecular methods is a challenging task for tailoring functional materials with desired properties.
The association in aqueous solutions tetrasodium 5,10,15,20-tetrakis(4-sulfonatophenyl) porphyrin (H2TPPS4), which was detected by visible spectra has been not clarified. In acidic media it has been described that new species appear with absorptions at 490 and 706 nm. This behaviour in the acidic region has even been described as very complicated. As a result of interchromophoric interactions, perturbations in the absorption and fluorescence spectra of dyes occur. In terms of excitonic coupling, aggregated dyes with blue- and red-shifted absorption bands are referred to as H- and J-aggregates. Due to the distinct optical properties, control of the formation of H- and J-aggregated states of dyes has attracted much research interest.
H2TPPS4 is a water-soluble tetrapyrrole molecule (with four sulfo group substitutes), interconnected at a atom via methine bridges. H2TPPS4 molecule form changes accordingly to the pH of the solution. In alkaline solutions H2TPPS4 is in tetra-anion form with no additional protons attached to the free nitrogen atoms in the center. However, in acidic solutions it can form J-aggregates as it starts self-assembling from its zwitterionic form. The J-aggregates form when positively charged porphyrin rings and negatively charged sulfo groups begin to interact.
In this study, we investigated how different pH affects the formation of ionic species of H2TPPS4 molecules, their absorption and fluorescence spectra. The solutions were made by diluting H2TPPS4 in deionized water and then changing pH accordingly, with aqueous NaOH (for the pH 7), HCl (for pH 4 to pH 1), or H2SO4 (for pH -1) solutions. At the pH 7 tetra-anion form of H2TPPS4 was observed with Soret band at 414 nm and Q bands at 516, 553, 594 and 634 nm, respectively. Under 414 nm excitation, fluorescence was observed at 644 nm and 702 nm. By lowering to the pH 4, the H2TPPS4 changed to diacid form as two additional protons attached to the free nitrogens at the center of H2TPPS4 molecules. Bathochromic shift of the Soret band (435 nm) was observed in the absorption spectrum and the Q bands were reduced to 645 nm with the shoulders around 550 nm, 594 nm. H2TPPS4 diacid form under 435 nm excitation had a fluorescence peak at 670 nm. By lowering the pH even more at pH 1 the H2TPPS4 formed the zwitterionic form with not only the center nitrogens blocked with protons, but two substitute sulfo groups as well. In this form the H2TPPS4 molecules start to actively self-aggregate forming J-aggregates with a distinguished absorption spectrum and Soret band at 490 nm, and Q band at 708 nm. Under 490 nm excitation, the fluorescence peak of the J-aggregates measured at 714 nm. Then, using H2SO4 the H2TPPS4 solution of pH -1 was reached and at this pH no self-aggregation was observed, as not only all the center nitrogens, but even all the substitute sulfo groups of H2TPPS4 were blocked by protons. This led to an absorption spectrum with the Soret band at 438 nm and Q band at 649 nm (bathochromic shift compared to the diacid form of the H2TPPS4) and fluorescence (under 434 nm excitation) peak at 682 nm.
The results showed that at certain acidic conditions H2TPPS4 can form J-aggregates due to positive electrostatic interactions, however if pH gets too low, all SO3 groups are blocked and the aggregates can no longer form.