Solute-solvent complex

T Higuchi, S Dayal, I Pitman. Effects of solute-solvent complexation reactions on dissolution kinetics Testing of a model by using a concentration jump technique. J Pharm Sci 61 695, 1972. 

Figure 10. Schematic diagram of the ESIPT process via solute/solvent complexation.

The time-resolved spectroscopy is a sensitive tool to study the solute-solvent interactions. The technique has been used to characterize the solvating environment in the solvent. By measuring the time-dependent changes of the fluorescence signals in solvents, the solvation, rotation, photoisomerization, or excimer formation processes of a probe molecule can be examined. In conventional molecular solutions, many solute-solvent complexes. 

If 1 1 complexes are formed (1) in the ground state, then two types of photoreactions will occur parallel to each other, that of the bare and that of the complexed solute, each in its own type of cage. Mixed alcohol/ alkane systems, for example, show an indication of preformed solute-solvent complexes as evidenced by the picosecond experiments of Wang and Eisenthal80 81 on DMABN. Planar model systems like the indolines 3 and 5 (Sec. II.A.l) indicate that an additional channel opens for the B state in alcoholic solvents which increases the nonradiative decay path. This can explain the observed reduction of the fluorescence quantum yield in jirotic solvents about 0.1 in the aprotic polar solvent n-butyl chloride, about 0.01 in 1,2-propanediol, and about 0.001 in water.228... 

Other attempts at characterizing the deviation from ideal solubility theory have been made. Anderson et al. [50] showed that solubilities that could not be rationalized from the regular solution theory could be rationalized by assuming the formation of speciLc solute-solvent complexes. Yalkowsky et al. [51,52] showed that the deviation from the ideal solubility equation could be expressed in terms of interfacial tension and surface area. In equation form,... 

The oxidation of meta- and para-substituted anilines with imidazolium fluorochro-mate (IFC)18 and nicotinium dichromate (NDC),19 in several organic solvents, in the presence of p-toluenesulfonic acid (TsOH) is first order in the oxidant and TsOH and is zero order with respect to substrate. A correlation of rate data in different solvents with Kamlet-Taft solvatochromic parameters suggests that the specific solute-solvent interactions play a major role in governing the reactivity, and the observed solvent effects have been explained on the basis of solute-solvent complexation. The oxidation rates with NDC exhibited negative reaction constants, while the oxidation with IFC did not correlate well with any linear free energy relationships. 

Use of diastereotopic probes for determination of absolute (as distinct from relative, e.g. meso vs. dl) stereochemistry is rare an example relating to chiral amine oxides is shown in Fig. 39 26b>. The solute-solvent complex shown, composed of the (S)-amine oxide and (S)-phenyltrifluoromethylcarbinol, has the ethyl group of the... 

In addition to unimolecular reactions, the excited state may participate in several bimolecular processes. At high concentrations, dimer formation, excimer formation, exciplex formation, solute-solvent complexation, energy transfer, and collosional deactivation may occur. The high-concentration conditions are often experienced when the guest molecules are loaded onto the layered materials with high coverages and specific examples will be provided shortly. 

Bayburt, T.M. and Sharp, R.R. (1993) NMR spin relaxation as a probe of the formation of long-lived, geometrically well-defined solute-solvent complexes - solutions of tris(acetylacetonato) manganese(III)./. Phys. Chem., 97, 4558-4566. 

Specific effects on spectroscopy and photophysics induced by complexation of the D-A chromophores with various solvent molecules have been examined for all the compounds under consideration. The idea of the beam work is to generate n solute-solvent complexes and to determine thereby the relation between the solute-solvent interactions and the excited-state CT process. Kajimoto et al. [81a,c, 89], Phillips and co-workers [82], Peng et al. [83], Bernstein and co-workers [84] and others [85, 88, 90-92] have shown that solute-solvent complexes of CDMA were readily produced by varying the partial pressure of the compounds and the stagnation pressure of the carrier gas. Cyclohexane, chloroform, carbon tetrachloride, methyl fluoride, trifluoromethane, dichloromethane, acetone, acetonitrile, metha-... 

Solute-solvent complexes of different stoichiometry have been observed between all the D-A compounds under consideration and various solvent molecules. Some of the clusters show structured excitation spectra and a narrow short-wave emission that has been assigned to the primary excited state of the vdW complex. The broad, red-shifted emission of other clusters can be explained in terms of the transformation of the vdW complexes of stoichiometry l n (n > 0) into excimers or the transition into an intramolecular CT state of the D-A chromophore which is induced by its polar partner(s) (for the complexes of stoichiometry 1 ). The main conclusion from the fluorescence behaviour of the jet-cooled vdW clusters is that dual luminescence is obviously connected with the preference of specific solute-solvent geometries. 

A peculiarity of the cluster beams discussed above which has to be taken into account in the luminescence studies of intramolecular electron transfer processes is that even under beam conditions optimized for the production of small D-A solute-solvent complexes, the simultaneous appearance of homogeneous clusters is nearly inevitable. Hence excimer fluorescence and the fluorescence of the mixed clusters are often superimposed. Therefore, only the careful characterization of the clusters with respect to their constitution, size and structure can allow the unequivocal assignment of the processes observed. 

Available literature solubility data of pure lipids belonging to major (fatty acids, mono-, di- and triacylglycerols, and fatty acid esters) and minor lipid classes (pigments, sterols, vitamins, and hydrocarbons) in SCCO2 were compiled (26, 27). These references (26, 27) contain exhaustive bibliography on lipid + SCCO2 binary systems. Literature data were correlated using Chrastil s equation, which is an empirical model used quite commonly to correlate the solubility of lipid components (30). This model is based on the formation of a solute-solvent complex on association of the solute and solvent molecules and establishes a linear relationship between In(solubility) and In(density) as follows ... 

Obviously the dipole characteristics of the fluorescent probe and the solvent together with the rigidity of the solvent environment will determine the possibility and extent of any reorientation process. The sensitivity of these fluorescent probes to environmental relaxation processes emphasise that care is required in interpreting the ftuo-res ence behaviour in biodiemical sterns purely in terms of the polarity of the bonding site. The possibility that intramolecular diarge-transfer may occur in the excited states of ANS or that specific solute-solvent complexes may form further com-... 

The stability of solute-solvent complexes is governed by the capacity of the solvent to form the specific hydrogen bonds. A comparative study of the chemical shifts of N in adenosine and ATP in dimethylsulfoxide-de and water was performed to gain insight into the HB effects. The N-1 resonance of ATP in water is cu 12 ppm more shielded than the N-1 of adenosine in dimethylsulfoxide-dg. This clearly indicates significant interaction between the N-1 atom and the water molecule. For the protonated molecules, the chemical shift of N-1 for ATP in water is almost equivalent to that for adenosine in DMSO-de-... 

Absorption and emission spectra of radical copolymer of methyl methacrylate wi h N,N-dimethylaminobenzoyloxyethylmethacrylate ( < 0.017=, Mn = 3 X 10 )(l) and ethyl N,N-dlmethylaminobenzoate (ll) are compared in Figure 2. Except for a small red shift ( 3 nm) in I, the absorption spectra are almost identical. Although the presence of a ground state dimer or solute-solvent complex is known for N,N-dimethylaminobenzonitrile, the best studied TICT compound, there seems to be no such complications for the present studies. This is a favorable condition to study the TICT chromophore in polymers since polymers tend to enhance complicated molecular association even in extremely dilute solution [12] which overshadows the aimed photophysical phenomenon. 

Three emission bands (a, p and y in the order of decreasing energy) are observed in CH2CI2 solution and are found to be the emission from the excited state of 1, from the excited state of a solute-solvent complex and from a relaxed twisted excited state of the solute-solvent complex, respectively. Model compound studies show that squaraine forms strong solute-solvent complexes with alcoholic solvent molecules. Analogous complexation process between 1 and the OH groups in PVF is also shown to occur. A model for the stabilization of particles of 1 in polymer solution is put forward where we propose that the stabilization mechanism is a steric effect achieved by adsorption of PVF macromolecules onto particles of 1 via the formation of the PVF 1 complex. 

Here we report preliminary results on the multiple fluorescence emission of 1 and 2. From structure-property relationships, solvent effect and temperature effect studies, we are able to show that the multiple emission is from the emission of free squaraine in solution, the emission of the solute-solvent complex and the emission of a twisted relaxed excited state. Further solvent effect study using 2 as a model shows that squaraine forms strong solute-solvent complexes with alcoholic solvent molecules. Analogous complex-ation process is also detected between 1 and the hydroxy groups on the macromolecular chains of poly(vinyl formal). The Important role of this complexation process on the stabilization mechanism of particles of 1 in polymer solution is discussed. 

As the steric hindrance around the OH group decreases, solute-solvent complexation increases, resulting in the bathochromic shift of Xmax and an increase in p-emission intensity. The general solvent effect on the Xmax and the emission composition of 2 (Figure 2) suggests that the complexation process is very general and complexation becomes very pronounced in solvents of TT > 0.65. Accordingly, a-band is the Franck-Condon emission of the excited state of the solute and P-band is the Franck-Condon emission of the excited state of the solute-solvent complex. 

Further bathochromic shifts on the Xmax and Xp and further increase in Ip are observed at [CHCl3]>.2.24 M. Absorption curves and emission curves no longer pass through their isosbestic or isoemissive points. This observation can either be attributed to the preferred solvation of the solute-solvent complex by chloroform as the concentration of chloroform increases or to the formation of a 1 n solute-solvent complex. The occurrence of these two events is presumably due to the highly localized concentration of chloroform in the solvation shell of squaraine, a consequence of the solute-solvent complexation process. 

For the BOHLM systems (see Chapter 5) with water-immiscible carriers, the concentration gradient-driven solute-solvent complexation/ decomplexation interactions are the dominant driving forces. For the BAHLM systems, Donnan membrane potential [18-26, 32-36], osmotic pressure gradient [27, 37], and possibly pressure gradient [38-40], have to be added as driving forces. Therefore, the theory should take into account both diffusive and convective transport. 

A similar type of solute-solvent complex has been proposed by Benezra and Ourisson " to account for the solvent shifts of many phosphonates. The solvent shifts d( = 5cDci3 C6H6) representa-... 

The solute-solvent complexation of squaraines has also been studied by NMR spectroscopy using 16 as a model compound [59]. Based on the correlation between the differences in the chemical shifts of the C4 (a) and C5 (P) protons (5 -5p) and the quantum yield of fluorescence of 16, it was proposed that increase in the solute-solvent complex leads to increased nonplanarity of the squaraine. From the NMR and fluorescence lifetime smdies it was proposed that the main nonradiative decay process involves rotation of the C-C bond between the C4O2 unit and the phenyl group. 

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