Temperature, 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. 

Spectroscopic measurements of solvatochromic and fluorescent probe molecules in room temperature ILs provide an insight into solvent inter-molecular interactions, although the interpretation of the different and generally uncorrelated polarity scales is sometimes ambiguous [23]. It appears that the same solvatochromic probes work in ILs as well [24], but up to now only limited data are available on the behavior of electronic absorption and fluorescence solvatochromic probes within ILs and IL-organic solvent mixtures. 

The solvatochromic probe molecule chosen for this work was 2-nitroanisole (Aldrich Chemical Co.). The s value reported in the literature for 2-nitroanisole is -2.428 + 0.195 ( 1 ). A known s value for the solute allows one to calculate the change in the supercritical fluid solvents it value as temperature or pressure changes. The reference absorption maxima for 2-nitroanisole is 32.56 x 10J cm"1 (vQ) in cyclohexane (1). 

Nicolet and Laurence11 studied four phenyl — NALk2 probes not mentioned previously nor included in Table 1 iV,iV-diethyl-3,4-dinitroaniline (38), 4-iV,iV-diethylaminobenzal-dehyde (39), V,lV-dimethylaminobenzonitrile (40) and methyl 4-iV,iV-dimethylamino-benzoate (41), the former two at several temperatures from 0 to 105 °C, the latter two only at 25 °C. The last three probes as well as 6, 42 and 43 mentioned in Table 1 show that the nitro or nitroso chromophore group is not necessary for obtaining useful solvatochromic probes. 

Solvatochromic probes based on aniline derivatives have been used in a host of environments other than liquid solvents and their mixtures, supercritical fluids and room-temperature liquid salts. All these uses cannot be reviewed here and only examples of them are provided. 

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. 

Wetzler and coworkers123 employed 4-aminophthalimide (63) and 4-amino-lV-methyl-phthalimide (64) as solvatochromic (and thermochromic) fluorescent probes in solvent mixtures. A bathochromic shift of the emission spectra was found in mixtures of toluene with ethanol and with acetonitrile123 when the more polar solvent was added to toluene, but raising the temperature causes a relative hypsochromic effect. Mixtures of benzene and acetonitrile were studied by Nevecna and coworkers124 for their polarity by means of the probes 46 and 47 and with respect to the correlation of this with the rate constants of the reaction of triethylamine with ethyl iodide. The fluorescence of the ammonium salt of 4-(l-naphthylsulfonate)aniline (84) in dioxane and water mixtures was studied by Hiittenhain and Balzer125. 

The solute-solvent interactions independent of the stationary phase are obviously of major importance to understanding the retention mechanism in SFC. The solvatochromic behavior of supercritical fluid probes has been used to investigate the specific and non-specific solute-solvent interactions occurring in a fluid as a function of density at different temperatures. Yonker et al. (12.,... 

Ex(30) values show a good, often linear, correlation with a large number of other solvent sensitive processes, such as reaction rates and shifts of chemical equilibria. The betaine dye (Scheme 3) and specially designed derivatives are useful molecular probes in the study of micellar interfaces, microemulsions and phospholipid bUayers, of rigid rod-Uke isocyanide polymers, and the retention behaviour in reversed-phase chromatography. In addition to its solvatochromic behaviour, the dye is sensitive to temperature ( thermosolvatochromism ) and pressure changes ( piezosolvatochromism ) and also to the presence of electrol)d es ( halosolvatochromism ). 

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