J-aggregation of ionic liquid solutions of meso-tetrakis(4-sulfonatophenyl)porphyrin

Abstract

The title porphyrin was dissolved in the hydrophilic ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF(4)], and triggered to assemble into J-aggregates by the addition of incremental volumes of water containing various amounts of acid (0.1, 0.2, or 1.0 M HCl). In contrast to recent studies, the current investigation is unique in that it centers on media that contain a predominant ionic liquid component (2.9-5.4 M [bmim][BF(4)]), as opposed to an aqueous electrolyte containing a small fraction of ionic liquid as dissociated solute. Complex aggregation and underlying photophysical behavior are revealed from absorption spectroscopy, steady-state fluorescence, and resonance light scattering studies. Upon addition of aqueous HCl, the efficient formation of H(4)TPPS(2-) J-aggregates from the diprotonated form of meso-tetrakis(4-sulfonatophenyl)porphyrin (H(2)TPPS(4-)) occurs in [bmim][BF(4)]-rich media in a manner highly dependent upon the acidity, TPPS concentration, and solvent composition. The unique features of TPPS aggregation in this ionic liquid were elucidated, including the surprising disassembly of J-aggregates at higher aqueous contents, and our results are described qualitatively in terms of the molecular exciton theory. Finally, the potential of this system for the optical sensing of water at a sensitivity below 0.5 wt% is demonstrated. Overall, our findings accentuate how little is known about functional self-assembly within ionic liquids and suggest a number of avenues for exploring this completely untouched research landscape.

Binding of the ionic liquid cation 1-alkyl-3-methylimidazolium to p-tetranitrocalix[4]arene probed by fluorescent indicator displacement

“…4). The reduction is due to the [1⋅AO] adduct formation whereby the close proximity of the four adjacent NO 2 groups on the calix [4] arene framework serves to quench the AO fluorescence [20][21][22][23][24]. Notably, AO quenching at low NO 2 group concentrations is usually insignificant, as evidenced by no statistically meaningful quenching in the presence of as high as Fig.…”

“…A representative series of AO emission profiles is shown in Table 1 2. The F/F 0 increase for AO in the presence of 1 was fit to an expression for single exponential growth to a maximum: 4 ] was increased (data not shown). This provides further evidence that AO forms an inclusion complex with the calixerene from which the AO is displaced by the presence of [bmim] + .…”

“…The compounds p-5,11,17,23-tetranitro-25,26,27,28-tetrahydroxy-calix [4]arene (1) and 4,6,10,12,16,18,22,24-octahydroxy-2,8,14,20-tetraethyl-calix [4]resorcinarene (2) were prepared in-house following procedures described elsewhere [17][18][19]. AO (high purity), 1-hexyl-3-methylimidazolium bromide [hmim][Br] (high purity), and potassium tetrafluroborate (KBF 4 ) were purchased from Acros Organics.…”

“…Although sparingly soluble in water, Liu et al [13] showed that CB [6] can be quickly dissolved through the encapsulation of an imidazolium-based IL guest in neutral water and, furthermore, that the alkyl chain length of the IL dictates both the relative binding affinity and the stoichiometry of the hostguest complex. Bates et al [14] showed that N-alkylpyridinium halide ILs form complexes with meso-octamethylcalix [4]pyrrole, with both halide and organic cation binding to the tetrapyrrole macrocycle, the latter within the calix [4]pyrrole cup. Miskolczy and Biczók [15] used the natural isoquinoline alkaloid berberine as a fluorescent probe to study the encapsulation of 1-alkyl-3-methylimidazolium ILs in 4-sulfonatocalix[n]arene (n04, 6) hosts at pH 2.…”

“…In this context, an aqueous IL solution offers a unique milieu for conducting chemical processes, particularly biocatalysis and separations, although interesting recognition, switching, and signaling (sensing) events are possible as well. Over the past couple of years, our groups have highlighted the novel role played by IL species in aqueous solution in relation to their common inorganic electrolyte counterparts in modulating solvation, assembly, and biomolecular behavior [3][4][5][6][7][8][9][10].…”