Water soluble sapphyrins: potential fluorescent phosphate anion sensors

Abstract

As part of an ongoing effort to study the anion binding properties of sapphyrins in various media, a number of previously reported water solubilized sapphyrins were studied in methanol and in buffered, neutral aqueous solutions and found to undergo self-aggregation under these solution phase conditions. The nature of the species produced as the result of self-aggregation and the processes leading to their formation were studied via UV-vis absorbance and fluorescence spectroscopy. In previous work (V. Král, H. Furuta, K. Shreder, V. Lynch, and J. L. Sessler, J. Am. Chem. Soc., 1996, 118, 1595–1607.) it was found that the addition of phosphate-type anions to water soluble sapphyrins at pH 6.1 gives rise to visible spectroscopic changes consistent with the binding to the aggregated form and the concurrent formation of an anion-bound dimer with effective equilibrium constants on the order of 100–300 M⁻¹. In this study we show that at high phosphate-to-sapphyrin ratios in neutral, buffered aqueous solutions further deaggregation occurs to produce an anion-bound monomeric form. This highly fluorescent species is formed with effective equilibrium constants on the order of 6–19 M⁻¹ leading to considerations that sapphyrins could function as fluorescent phosphate anion sensors. In an effort to modulate the deaggregation properties, several new sapphyrin derivatives bearing two meso aryl substituents were prepared and studied. The aggregation properties of these latter systems were analyzed in methanol and, in the case of one water solublized system, in neutral aqueous media. In analogy to what was observed for the β-alkyl substituted sapphyrins, H-type aggregates were seen for the water solubilized meso-substituted system in aqueous media. However, in contrast to the H-type dimers seen in the case of the meso-free systems in methanol, J-type dimers were observed in the case of the sapphyrins bearing two meso substituents in this solvent. The effective dimerization constants of several of the meso-diaryl sapphyrins were determined in methanol as were their pK_a values in aqueous media. The solid state structure of the bis HBr salt of one of the meso-diaryl sapphyrins, specifically 10,15-bis(3,5-di-tert-butylphenyl)-3,22-diethyl-2,23-dimethylsapphyrin, was also determined and found to show elements in common with those of previously reported sapphyrin derivatives. In particular, the two counter anions were found to be hydrogen bound above and below the diprotonated sapphyrin plane.