Ionic liquids, solvatochromic probes

Fletcher, K.A., Storey, L, Hendricks, A.E., Pandey, S., Pandey, S., Behavior of the solvatochromic probes Reichardt s dye, pyrene, dansylamide, Nile Red and 1-pyrenecarbaldehyde within the room-temperature ionic liquid bmimPF(6), Green Chem., 3, 210-215, 2001. 

Koel, M., Solvatochromic probes within ionic liquids, Proc. Est. Acad. Sci. Chem., 54, 3-11, 2005. 

Another nonreactive route to the characterization of solvent polarity is the study of the optical absorption and emission spectra of chromophores [188]. These spectra are sensitive to the molecular environment, and because different solvatochromic probes may have different capacities for specific interactions, it is possible to characterize the solvent environment in detail and to construct LFERs analogous to those described above. Studies of the spectra of solvatochromic probes in ionic liquids have in general been consistent with the results of partitioning studies described above [8-10, 69, 70, 198-200], though we will discuss one observed anomaly below [198]. 

Several methods have been applied to determine the solvent power of ionic liquids determined by solvation polarity and nucleophilicity (donor power). A common approach for determining the solvent polarity and setting up an empirical scale is to evaluate the UV-Vis spectra of optical probes such as solvatochromic dyes or transition metal complexes in the solvents under investigation [53], The absorption or emission bands of the probe show a strong shift in their optical spectra according to the polarity of the solvent in which they are dissolved [54-57],... 

Recently, several amino-acid-functionalised solvatochromic probes (Scheme 9) have been suggested and also applied for ionic liquids [121], The probes are characterized by a push-pull aromatic ir-electron system and the amino acid moieties are especially prone to hydrogen bond donor/acceptor interactions. 

Solvatochromic experiments have shown [BMIM]+-based ionic liquids to be good hydrogen-bond donors [95], however, complementary GC-based experiments have shown these same ionic liquids to be poor hydrogen-bond donors [96]. This discrepancy has been resolved, and traced back to the nature of the probe molecules. 

The polarities of different solvents can be compared using fluorescence probe molecules or solvatochromic dyes. When such molecules are dissolved in a solvent, the frequency of the maximum fluorescence or absorption, respectively, changes as a function of the polarity of the medium, and this property can then be compared with the result for standard solvents. The polarities of several ionic liquids were estimated using 5-dimethylamino-isoindole-l,3-dione 34 as a fluorescence probe and Reichardt s dye 35 for visible absorbance measurements, allowing the determination of the normalized parameter ( . (tetramethylsilane) = 0,... 

Fletcher, K. A. and Pandey, S. 2003. Solvatochromic probe behavior within ternary room-temperature ionic liquid l-butyl-3-methylimidazolium hexafluorophosphate + ethanol + water solutions. J. Phys. Chem. B 107, 13532-13539. 

The polarizability of solvents, combined with their polarity, are measured by the Kamlet-Taft n values, obtained most profitably with the solvatochromic probe 4-nitroanisole and shown, as compiled by Jessop et al. [410] and supplemented with values from Kochly et al. [414] and Spange et al. [416] in Table 6.16. The jt values are in a narrow range, 0.95 0.15, i.e., the RTILs are all very polar as expected for ionic liquids. The disadvantage of having both these properties in raie parameter is removed by the separate Catalan parameters SP for polarizability and SdP for dipolarity. These use results from two solvatochromic probes a thiazoline one and 2-(4-(N,N-dimethylamino)benzylidene)malononitrile according to Schade et al. [413], and the values of SP and SdP are also shown in Table 6.16. The polarizability of RTILs is not affected much by the acidity or basicity of its constituent ions, but their polarity does depend on these properties. 

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