4-Nitrophenol, solvatochromic probes

Before using a solvatochromic probe in the SCF state, it is important to perform calibrations in well-defined one-phase liquid systems. A variety of solvatochromic indicators have been used previously to probe AOT reverse micelles at atmospheric pressure. For example El Seoud et al. (2 investigated malachite green and thymol blue, which are hydrophobic. They are insoluble in heptane despite their hydrophobicity. Accordingly it was found that these probes are absorbed at the surfactant/oil interface, but do not partition into the water pool. The Xmax and UV absorbance for thymol blue change significantly for Wo below 6, but become constant for Wq from 8.3 to 22.2. UV-VIS and positron annihilation studies showed that even less hydrophobic probes such as nitrophenols are also solubilizied in the surfactant/oil interface. 

Eor another example at the liquid/liquid interface. Steel and Walker used two different solvatochromic probe molecules, para-nitrophenol (PNP) and 2,6-dimethyl-para-nitrophenol (dmPNP), to study the polarity of the water-cyclohexane interface. These probes give spectral shifts as a function of bulk solvent polarity that are very similar because both solutes are mainly sensitive to the nonspecific solvent dipolar interactions. However, when these two dye molecules are adsorbed at the water/cyclohexane interface, they experience quite different polarities. The more polar solute (PNP) has a maximum SHG peak that is close to that of bulk water, and thus it reports a high-polarity environment. In contrast, the less polar solute (dmPNP) reports a much lower interface polarity, having a maximum SHG peak close to that of bulk cyclohexane. Clearly, the more polar solute is adsorbed on the water side of the interface, keeping most of its hydration shell, and thus reports a higher polarity than does the nonpolar solute. Other examples of the surface polarity dependence on probe molecules are discussed in Ref. 363. 

The Lewis basicity or HB A ability of solvents is described by the Kamlet-Taft parameter, measured by means of the couples of solvatochromic probes 4-nitrophenol compared to 4-nitroanisole or 4-nitroaniline compared to 4-nitro-N, N-diethyaniline, the second probe of the couple serving to eliminate the effects of the solvent polarity and polarizability on its first probe. The resulting values of from the compilation of Jessop et al. [410], supplemented with data from Spange etal. [416] are shown inTable 6.16. An alternative measure of this property that has been applied by Schade et al. [413] to RTILs is Catalan s SB parameter, using the solvatochromic probe N.N-dimethyl-4-aminobenzodifuranone, and the resulting values are shown in Table 6.16. The SB values are linear with the values SB = 0.126 +1.056/. The basicity decreases in a series of RTILs with a common anion phosphonium > ammonium > pyrrolidinium > pyridinium > imidazolium, but more moderately than the acidity increases in the opposite direction shown above as shown by Spange et al. [416]. 

Solvatochrotnic shifts of the five probe indicators, 2-nitroaniline, 4-nitroaniline, 4-nitroanisole, 4-nitrophenol, and N,N-dimethyl-4-nitroaniline, have been used to specify the solvatochromic solubility parameters k and beta of the five pure fluids CO2,... 

The solvatochromic effects on UV/visible spectra of certain solutes are so large, that they can conveniently be employed as probes for certain solvating properties of the solvents. Those that have enjoyed widespread application in this capacity are discussed in Chapter 4. They include 2,6-diphenyl -4-(2,4,6-triphenyl-l-pyridino)-phenoxide, 4-methoxynitrobenzene, 4-(dimethylamino)-nitrobenzene, for the estimation of the polarity of solvents, acetylacetonato-N,N,N, N -tetramethylethylenediamino-copper(II) perchlorate, 4-nitrophenol, and 4-nitroaniline, for the estimation of the electron pair donicity of solvents, 4-carboxymethyl-l-ethylpyridinium iodide, 4-cyano-l-ethylpyridinium iodide, and bis-c/.v-1, lO-phenanthrolinodicyano-iron(II) for the estimation of the hydrogen bond donation abilities of solvents (Marcus 1993). 

A solvatochromic scale, based on the ultraviolet-visible, rather than the infrared, spectral band of suitable probes is that based on the Kamlet -Taft P parameter. This is again an averaged quantity, for which the wavenumber shifts of several protic indicators relative to structurally similar but aprotic homomorphs are used (Kamlet et al. 1983 Kamlet and Taft 1976). It is assumed that the nonspecific effect of a solvent on the protic probe is the same as that on the aprotic one, and that it can be expressed in terms of the n parameter for the solvent, so that the donicity of the solvent, if it is a Lewis base, causes the difference between the responses of the two probes towards the solvent. The probes originally employed were 4-nitrophenol (vs 4-nitroanisole) and 4-nitroaniline (vs 4-nitroN,N-diethylaniline), but once a n scale is known, the need for the specific aprotic homomorph values no longer exists, since the general expression ... 

Figure 19. Solvatochromic parameters of five supercritical fluids from measurements of Maiwald compared with literature data for some selected liquid solvents according to Kamlet, Abboud, and Taft. Filled symbols refer to the probe molecules 4-nitroaniline / N,N-dimethyl-4-nitroaniline (NH-hydrogen bonding), open symbols to 4-nitrophenol / 4-nitroanisole probes (OH-hydrogen bonding) adapted from [75].

Similarly, UV spectroscopic scales have been defined from solvatochromic shifts upon hydrogen bonding of the electronic transitions of probes such as 4-nitroaniline and 4-nitrophenol. Among these scales, it is essential to make a distinction between solute and solvent scales. A solute scale is obtained when the HBD probe is smdied in dilute solutions of a series of bases in the same given solvent, whereas a solvent scale comes from measurements on binary systems of the HBD probe dissolved in the pure bases. 

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