A Specific Interaction between Ionic Liquids' Cations and Reichardt's Dye

Abstract

Solvatochromic probes are often used to understand solvation environments at the molecular scale. In the case of ionic liquids constituted by an anion and a cation, which are designed and paired in order to obtain a low melting point and other desirable physicochemical properties, these two indivisible components can interact in a very different way with the probe. This is the case with one of the most common probes: Reichardt's Dye. In the cases where the positive charge of the cation is delocalized on an aromatic ring such as imidazolium, the antibonding orbitals of the positively charged aromatic system are very similar in nature and energy to the LUMO of Reichardt's Dye. This leads to an interesting, specific cation-probe interaction that can be used to elucidate the nature of the ionic liquids' cations. Parallel computational and experimental investigations have been conducted to elucidate the nature of this interaction with respect to the molecular structure of the cation.

Keywords: Reichardt’s Dye; charge transfer; ionic liquids; molecular probes; solvatochromism.

Figure 1

Some common IL ions, and some typical structures of IL cations and anions: (a) alkylimidazolium, often R₂ = H, R₃ = CH₃ and R₁ is a longer chain; (b) alkylpyrazolium; (c) alkylpyridinium; (d) tetramethylguanidinium; (e) alkylpyrrolidinium; (f) alkylpiperidinium; (g) alkylmorpholinium; (h) tetraalkylammonium; (i) tetraalkylphosphonium; (j) trialkylsolphonium; (k) an example of imidazolium-based dication; (l) tetrafluoroborate; (m) hexaflurophosphate; (n) glyceroborate, an example of an organic anion; and (o) bis-(trifluoromethane)sulfonimide, which has the short name bistriflimide and acronyms Tf₂N and TFSI (a very common anion). In (b,e–g), R₁ and R₂ are different lengths. In (h–j), three (or two) sidechains are generally the same, and the 4th (or 3rd) is a very different length. Small organic/inorganic ions, such as Cl⁻, Br⁻, RCOO⁻, ⁻OOC(CH₂)_nCOO⁻, RSO₃⁻, etc., are also common.

Figure 2

(a) Reichardt’s dye structure. The betaine core (in blue) presents five phenyl substituents. Steric hindrance does not allow the planarity of the molecule. (b) Isodensity map (threshold vale 0.01 a.u.) is mapped with the molecular electrostatic potential. False color scale goes from blue (electrophilic) to red (nucleophilic) while green (weak electrophilic) and yellow (weak nucleophilic) show regions with intermediate values. On the right is the phenate moiety (indicated by the red arrow). At the center is one of the sides of the pyridinium pocket (indicated by the black arrow). (c) Schematic representation is of the first solvation shell of Reichardt’s dye in ionic liquid.

Figure 3

Cation and anion structures.

Figure 4

Definition of the labels used in Table 1. C1 represents the carbon atom of Reichardt’s Dye that is bonded to the oxygen O; C2 represents the carbon atom in α position with respect to the positive center nearest to the dye; and H represents the hydrogen atom bound to C2 nearest to the dye.

Figure 5

Cation-dye spatial arrangements for some of the cations studied in this paper. (a) NaphC₁Im, (b) TOMP, (c) C₄C₁Im, (d) C₄Im-C₆-C₄Im, (e) o-C₄Pic, (f) m-C₄Pic, (g) p-C₄Pic. In the case of the sterically hindered TOMP cation (b), the size and the absence of orbital of the π kind did not allow the specific interaction described in this paper. Flat imidazolium- or pyridinium-based aromatic compounds (a,c–g) could approach the oxygen moiety at the center of the Reichardt’s Dye pocket more efficiently. In the case of the three picoline isomers (e–g), when the methyl substituent was nearby the positively charged center (ortho and meta), it forced a different interaction, weakening the effect. The sidechains that presented an aromatic π system (a,d) stacked efficiently on one of the two lateral phenylic groups of the dye molecule. In the case of the dicationic system (d), steric hindrance did not allow for the simultaneous interaction of both the positive centers with the phenolic oxygen: one interacted, and the other acted as sidechain. Small sidechains (c,g) on imidazolium did not affect the position and orientation of the cation with respect to the dye molecule.

Figure 5

Cation-dye spatial arrangements for some of the cations studied in this paper. (a) NaphC₁Im, (b) TOMP, (c) C₄C₁Im, (d) C₄Im-C₆-C₄Im, (e) o-C₄Pic, (f) m-C₄Pic, (g) p-C₄Pic. In the case of the sterically hindered TOMP cation (b), the size and the absence of orbital of the π kind did not allow the specific interaction described in this paper. Flat imidazolium- or pyridinium-based aromatic compounds (a,c–g) could approach the oxygen moiety at the center of the Reichardt’s Dye pocket more efficiently. In the case of the three picoline isomers (e–g), when the methyl substituent was nearby the positively charged center (ortho and meta), it forced a different interaction, weakening the effect. The sidechains that presented an aromatic π system (a,d) stacked efficiently on one of the two lateral phenylic groups of the dye molecule. In the case of the dicationic system (d), steric hindrance did not allow for the simultaneous interaction of both the positive centers with the phenolic oxygen: one interacted, and the other acted as sidechain. Small sidechains (c,g) on imidazolium did not affect the position and orientation of the cation with respect to the dye molecule.

Figure 6

Orbitals involved in the transitions. Right: free Reichardt’s dye. The HOMO ➝ LUMO transition (blue arrow) involved an internal charge transfer. In the dye-cation cluster, the LUMO + 2 orbital mixed with the LUMO of the cation, leading to shared orbitals where there was dye-cation charge transfer (purple arrow). The actual transition also involved LUMO + 3 and LUMO + 4 orbitals, similar to LUMO + 2, but they were not reported here.

Figure 7

UV-VIS spectra. All Spectra were renormalized with respect to the area of the peak related to the π ⟶π^* transition (its maximum is at λ₁ wavelength of Table 3). This was for a better comparison of the several systems studied. Thus, the vertical axis corresponds to arbitrary units.