Solvatochromism phenol blue

Solvatochromic shift data have been obtained for phenol blue in supercritical fluid carbon dioxide both with and without a co-solvent over a wide range in temperature and pressure. At 45°C, SF CO2 must be compressed to a pressure of over 2 kbar in order to obtain a transition energy, E, and likewise a polarizability per unit volume which is comparable to that of liquid n-hexane. The E,j, data can be used to predict that the solvent effect on rate constants of certain reactions is extremely pronounced in the near critical region where the magnitude of the activation volume approaches several liters/mole. 

Solvatochromic data, specifically absorption or transition energies (E s), have been obtained for the dye phenol blue in supercritical fluids as a function of both temperature and pressure. These data will be used to compare the "solvent strength" of these fluids with liquid solvents. He will use the terms "solvent strength" and "Et" synonymously in this paper such that they include the magnitude of the polarizability/volume as well as the dipole moment. The "solvent strength" has been characterized by the spectroscopic solvatochromic parameter, E, for numerous liquid solvents (9 JU, J7,JJ3). 

For phenol blue, the E, is zero for ethylene and CF3CI, but nonzero for the Lewis acids CF3H and CO2. One of the attractive features of solvatochromic scales is that the non-specific and specific interactions may be separated since the former can be calculated straightforwardly using Eq. 5. 

The final set of solvatochromic data are shown in Figure 6 for phenol blue in SF C02 doped with various amounts of the co-solvent or entrainer, methanol. Consider a pressure of 100 bar where the Et of phenol blue in C02 is 54 kcal/mol. The red shift is increased more by the addition of 3.5 mole percent methanol at constant pressure than by an increase in the pressure of pure C02 of over 200 bar. The large specific "solvent strength" of methanol causes this behavior. The red shift caused by the co-solvent is in... 

As shown by the NMR chemical shifts of negatively solvatochromic mero-polymethine dyes e.g. phenol blue), the electronic ground-state structure of these dyes changes from a polymethine-like state (b) in non-polar solvents to a polyene-like state (c) in polar solvents [50, 78]. 

Z values have been widely used to correlate other solvent-sensitive processes with solvent polarity, e.g. the a absorption of haloalkanes [61], the n n and n n absorption of 4-methyl-3-penten-2-one [62], the n n absorption of phenol blue [62], the CT absorption of tropylium iodide [63], as well as many kinetic data (Menschutkin reactions, Finkelstein reactions, etc. [62]). Copol5mierized pyridinium iodides, embedded in the polymer chain, have also been used as solvatochromic reporter molecules for the determination of microenvironment polarities in synthetic polymers [173]. No correlation was observed between Z values and the relative permittivity e, or functions thereof [317]. Measurement of solvent polarities using empirical parameters such as Z values has already found favour in textbooks for practical courses in physical organic chemistry [64]. 

For these purposes we have studied the solvatochromic dye of Phenol Blue (PB, Scheme 1) in trifluoromethane (CF3H) near the critical density. Although betaine-30 is more preferable for the comparison with liquid solvent, it was impossible due to the extremely low solubility of betaine-30 to CF3H. PB is more soluble in CF3H, and has absorption around 560 nm which is accessible by our laser system. Unfortunately the photophysics of PB has not been studied until now even in liquid solution. Therefore we have also studied the ground state recovery in several liquid solvents. According to the theoretical calculation, the charge... 

The purpose of these scales is to provide a guide for choosing SCF solvents and co-solvents to achieve a desired solvent strength, for example in a separation or reaction process. An example of a solvatochromic scale is presented in Figure 1 for the UV-vis absorption of phenol blue in ethylene as a function of density at two temperatures (S). The scale is defined as the transition energy, Ej = he/ Xmax where Xmax is the wavelength of maximum absorption. 

The final application of solvatochromic solvent strength scales is the correlation of reaction rate and equilibrium constants in SCF solvents. Solvatochromic scales are often quantitative indicators of the solvent effect on rate constants for a variety of reaction mechanismsU) In a SCF, this solvent effect may be achieved conveniently with a single solvent using pressure. Based on solvatochromic data, it was predicted that an activation volume can reach thousands of mL/mol in a SCF(8). This prediction was confirmed for various types of reactionsClSbZl). For example, the solvatochromic parameter Ex for phenol blue... 

Figure 4. Comparison of local density and bulk density versus pressure based on the solvatochromism of phenol blue in ethylene at 25 C (data from ref. 8.).

Mixed solvents The addition of a small amount of a co-solvent to a supercritical fluid can increase solubilities of certain substances from several percent to several orders of magnitude (32.33.34.35). Spectroscopic data, which have been obtained recently, indicate that preferential solvation by a co-solvent contributes to the large increases. The co-solvents acetone, methanol, ethanol, and n-octane, were investigated by Kim and Johnston(36) using the solute phenol blue as a solvatochromic indicator. In Figure 7, it is apparent that the red shift (solvent strength) exceeds the value which is obtained from linear behavior, i.e. the concavity is positive. This means that the local concentration of co-solvent near the solute... 

Another attempt to relate the results from the use of a solvatochromic probe (Phenol Blue (66)) to the inherent properties of solvent mixtures was made by Phillips and Brennecke122. They obtained the interaction energies (required for the application of the non-random two-liquid (NRTL) approach) of 66 with each of the solvent components from its solubility in the neat solvent. The mixtures studied contained cyclohexane as one component and acetone, triethylamine, ethyl butyrate, cyclohexanone, toluene and acetophenone as the other. Then the local compositions deduced from the solvatochromism of 66 were compared with those calculated by the NRTL equation and reasonable agreement was found. 

One of the most popular and successful scales has been developed by Dimroth and Reichardt. It is based on the pyridlnlum-N-phenoxide betaine [3], which exhibits one of the largest solvatochromic effects ever observed. The solvatochromism of this dye is negative since its ground state is considerably more polar than the excited state and is stabilized by polar solvents. Thus, in diphenylether the dye absorbs at 810 ran and appears blue-green, whereas in water the absorption is centred at 453 nm, giving an orange impression. The transition energy, expressed in kcal mol, is the so-called Ej(30) value of the solvent. Ex(30) values have been determined and tabulated for more than 270 pure solvents and many different solvent mixtures. Protonation converts the dye (Scheme 3) into a phenol as a consequence, Et(30) values cannot be measured for acidic solvents, such as carboxylic acids. 

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