Probe solvatochromic

Dimroth et al. introduced 8 as a solvatochromic probe of solvent polarity having absorption in the visible region it shows the largest solvatochromic shift of any substance yet reported. Ey (30) is calculated with Eq. (8-76), like Z. (The peculiar symbolism arose because compound 8 happened to be No. 30 on the list of substances studied by Dimroth et al.) The shift is hypsochromic as solvent polarity is increased. Table 8-16 gives some Ey (30) values. - (30) is linearly... 

Other solvatochromic probes have been proposed. Mukerjee et al. used nitrox-ides for this purpose, finding that their transition energies correlate linearly with Z and t (30). Brooker et al. prepared a polar merocyanine that shows a blue shift... 

The relative importance of the hafide anion - HO - Cell interactions can be inferred from application of the Taft-Kamlet-Abboud equation to the UV-Vis absorbance data of solvatochromic probes, dissolved in cellulose solutions in different solvent systems, including LiCl/DMAc and LiCl/N-methyl-2-pyrrolidinone [96]. According to this equation, the microscopic polarity measured by the indicator, Ej (indicator), in kcalmol is correlated with the properties of the solvents by Eq. 1 ... 

Solvatochromic probes have been used for a variety of applications like the study polarity of pure and mixed solvents [99], and the retention behavior in reverse-phase liquid chromatography [100] among other applications. Frechet et al. used 4-(N-methylamino)-l-nitrobenzene (p-MANB), as the chromophore, to probe the microenvironment of polyaromatic ether based dendrimers [101]. 

Solvation properties, of supercritical solvents, 14 80-81 Solvatochromic materials, 22 708t Solvatochromic probes, 26 853—855 Solvatochromic spectral shifts, 23 96 Solvatochromy, 20 517 Solvay, 7 641 Solvay process, 15 63... 

Frechet et al [26] studied the microenvironment in dendritic molecules by covalently attaching a solvatochromic probe namely iV-methylamino-p-... 

Hydrogen bond donor solvents are simply those containing a hydrogen atom bound to an electronegative atom. These are often referred to as protic solvents, and the class includes water, carboxylic acids, alcohols and amines. For chemical reactions that involve the use of easily hydrolysed or solvolysed compounds, such as AICI3, it is important to avoid protic solvents. Hydrogen bond acceptors are solvents that have a lone pair available for donation, and include acetonitrile, pyridine and acetone. Kamlet-Taft a and ft parameters are solvatochromic measurements of the HBD and HBA properties of solvents, i.e. acidity and basicity, respectively [24], These measurements use the solvatochromic probe molecules V, V-die lliy I -4-n i in tan iline, which acts as a HBA, and 4-nitroaniline, which is a HBA and a HBD (Figure 1.17). 

Solvatochromic probes report on the polarity of the local medium through changes in their UV vis absorption spectra. Perhaps the most famous example of these solvatochromic probes is provided by the Ex30 probe developed by Reichardt (Scheme 8). 

As reporters of local medium properties, polarity in particular, solvatochromic dyes have found obvious use in the study of micellar pseudophasesand are often used in combination with kinetic studies (see, e.g., Rodriguez et al. ). For such combined studies, it should be noted, however, that the binding site of a reactant or reactants may differ significantly from the binding site of the solvatochromic probe. Hence, one has to be careful with direct comparisons. 

As mentioned in Section 4, the analysis of rate data resulting from unimolecular reactions is considerably easier than the analysis of such data for bimolecular reactions, and the same is true for pseudounimolecular reactions. Kinetic probes currently used to study the micellar pseudophase showing first-order reaction kinetics are almost exclusively compounds undergoing hydrolysis reactions showing in fact pseudofirst-order kinetics. In these cases, water is the second reactant and it is therefore anticipated that these kinetic probes report at least the reduced water concentration (or better water activity in the micellar pseudophase. As for solvatochromic probes, the sensitivity to different aspects of the micellar pseudophase can be different for different hydrolytic probes and as a result, different probes may report different characteristics. Hence, as for solvatochromic probes, the use of a series of hydrolytic probes may provide additional insight. 

Solvatochromic probes have found applications in polymer characterisation, where they can be used to look for localised polar features at the molecular level. Two distinct approaches can be adopted ... 

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. 

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

Another well-represented category was that of self-assembled monolayers (SAMS) and other supramolecular systems. The experiments on the SAMS included studies of the surface pKa of one system (110), the kinetics and thermodynamics of the self-assembly process (111), and the characterization of the SAM surface by study of solution contact angles (112). The experiments on supramolecular systems included studies on chemical equilibria in such systems (113, 114, 115), the kinetics of inclusion phenomena (116), and the use of solvatochromic probes in studying inclusion phenomena (117). 

With modified mobile phases, it is somewhat more difficult to estimate their polarity. The polarity can be measured with solvatochromic probes (19), but the results may sometimes be misleading due to specific probe-fluid interactions. 

Application of Solvatochromic Probes to Supercritical and Mixed Fluid Solvents... 

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). 

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]. 

While the study of solvatochromic dyes is well established as a means of probing solvent polarity, these are not the only solutes that can be used in this fashion. A more exotic solvatochromic probe is an excess electron in solution. Optical absorption studies of the thermalized (solvated) electron generated in the pulse radiolysis of a series of ILs show a strong dependence on cation character, with a relatively low frequency for tetraalkylammonium systems and a higher frequency for cyclic (pyrrolidinium-based) cations [48, 207]. The solvated electron spectrum is often interpreted in a particle-in-a-box framework, which would imply that the cyclic cations (which possess smaller ionic volumes) simply coordinate more closely with the electron and so create a smaller domain in which the electron must localize. The breadth of the absorptions and their maximum fall within the range of values expected for moderately polar organic solvents. 

The polarities of binary solvent mixtures with limited mutual miscibility such as zz-BuOH/H20 and c-CeH /DMF [191] as well as of solid polymer mixtures (organic glasses) [195] have also been studied using solvatochromic probes such as the betaine dye (44). 

In 1994, a review on the further development and improvement of the n scale was given by Laurence, Abboud et al. [227], They redetermined n values for a total of 229 solvents, this time using only two (instead of seven) solvatochromic nitroaromatics as indicator compounds, i.e. 4-nitroanisole and A,A-dimethylamino-4-nitroaniline, for good reasons see later and reference [227] for a more detailed discussion. A thermodynamic analysis of the n scale [and the t(30) scale] has been reported by Matyushov et al. [228]. Using six novel diaza merocyanine dyes of the type R-N=N-R (R = N-methylpyridinium-4-yl or A-methylbenzothiazolium-2-yl, and R = 2,6-disubstituted 4-phenolates or 2-naphtholate) instead of nitroaromatics as positively solvatochromic probe compounds, an analogous n azo scale was developed by Buncel et al., which correlates reasonable well with the n scale, but has some advantages for a detailed discussion, see references [333], Another n scale, based solely on naphthalene, anthracene, and y9-carotene, was constructed by Abe [338], n values are mixed solvent parameters, measuring the solvent dipolarity and polarizability. The differences in the various n scales are caused by the different mixture of dipolarity and polarizability measured by the respective indicator. The n scale of Abe is practically independent of the solvent dipolarity, whereas Kamlet-Taft s n and Buncel s n azo reflect different contributions of both solvent dipolarity and polarizability. 

With the use of solvatochromic probes, other non-specific forces (dispersion, dipole-induced dipole, and dipole-dipole) and specific acid-base forces have been explored in SCF solvents. In an effort to compare liquid and supercritical carbon dioxide, Hyatt(ll) measured UV-visible spectra of several solvatochromic probes. There was little difference between the Ex in the liquid and SCF states however, the data can not be interpreted fully since the density and the pressure were not given at the supercritical condition. The results indicated that the... 

Acid-base interactions have been explored as a function of density in SCFs using solvatochromic probes. The basicity of SCF CO2 was found to be relatively constant with respect to density(13L although the polarizability/volume is a linear function of density. The acidity of SCF CHF3 was also observed to be density insensitive, over a range from 4 to 18 mol/L(S). Although it appears that hydrogen-bond interactions become fairly saturated at a relatively low density, further experimental and theoretical work is required to understand this behavior more quantitatively. 

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