Solvent composition, effect fluorescence

Attention should be paid to the possible existence of several complexes having different stoichiometries. A necessary preliminary experiment consists of recording the fluorescence and/or excitation spectra under experimental conditions (nature of the solvent, composition of the medium, ionic strength, pH (if it has an effect on the stability constant), etc.) as close as possible to the medium in which a cation must be detected. The variations in the fluorescence intensity for an appropriate couple of excitation and emission wavelengths (or for several emission or excitation wavelengths) as a function of cation concentration must be analyzed in order to determine the stoichiometry and the stability constant of the complexes (Appendix B). As in the case of pH determination (see Section 10.2.1), ratiometric measurements are recommended. 

To date, there have been only a handful of time-resolved studies in dense fluid media (33,34,69-72). Of these, the bulk have focused on understanding a particular chemical reaction by adjusting the solvent environment (69-71). Only over the past two years have there been experiments directed toward studying the peculiar effects of supercritical fluids on these solvation processes (33,34,72). The initial work (33,34) showed that 1) time-resolved fluorescence can be used to improve our understanding of solvation in supercritical fluids and 2) the local solvent composition, about a solute molecule, could change significantly on a subnanosecond time scale. 

Weller observed the dual emission in the fluorescence spectrum of SA and MSA in methylcyclohexane (lO" mol/L), characterized by a strong emission with a maximum at 450 nm and a shoulder at 350 to 370 nm. The features of the emission spectrum of salicylic acid and its derivatives in solution are sensitive to the concentration of the solute and the solvent composition. This effect could be due to self-dimerization " or the existence of intermolecular hydrogen bonds formed by the carboxyhc add influencing the intramolecular hydrogen bond, C=0 -H0 in salicylic acid providing ESIPT. 

The description provided in this section should include, as applicable, information on the following (a) organoleptic properties (e.g., appearance, odor, taste) (b) solid-state form (i.e., the preferred crystalline polymorph) (c) solubility profile (limit data to aqueous solubility, pH effect, and at most one or two organic solvents (d) pH, pKa, or pKb (e) melting and boiling range (f) specific gravity or bulk density (g) spectroscopical characteristics such as a specific rotation, refractive index, and fluorescence and (h) isomeric composition. 

Typical spectra obtained for the probed attached at the network junction are shown in Figs. 2 and 3 The probe in THF possesses the highest ratio for CT/LE. An increased intensity for the LE was measured for the swollen sample. This effect can be explained by different polarity /mobility of the probe One can assume that covalent bonded probes possess another probe mobility than free dissolved probe molecules. Furthermore, the covalent bonded probe molecule that shows a higher polarity in comparison to the siloxane chains is located at the network junction. The attached probe molecule is surrounded mainly by siloxane chains of the network. Addition of polar swelling solvents leads to an increase of the CT-emission and the ratio CT/LE is mainly influenced by the composition of polymer and swelling agent (compare spectra for dried and swollen N1 samples in Fig. 2). Therefore, the covalently bonded probe shows another fluorescence behavior in comparison to the free dissolved probes that can be surrounded also by solvent molecules. 

P>MA Particles. PIB-stabilized PbWA particles were prepared containing naphthalene [N] groups covalently attached to the fTWA chains. This was effected quite simply by adding l naphthylmethyl methacrylate to the NWA polymerization step of the particle synthesis. From reactivity ratios, one knows that the N groups are randomly distributed along the P>MA chains. The particles were purified by repeated centrifugation, replacement of the supernatant serum with fresh solvent (isooctane) and redispersion. A fluorescence spectrum of the dispersion was typical of that of a 1-alkyl-naphthalene. Chemical analysis indicated a particle composition IB/NWA/N of 13/100/10. 

Figure 6 shows the fluorescence excitation and emission spectra of Sq2-Sq5 in CH2CI2. In each case, the excitation spectrum was found to be identical to the absorption spectrum and is independent of the monitoring wavelength. The spectral results are summarized in Table 3. Although the effect of chain length on Xp may be small, it has a profound effect on the composition of the emission band. For example, for N = CH3, the intensities of the a- and p-bands are about the same (Fig. 1). As the chain length is increased, the intensity of the a-band decreases whereas the opposite is observed for the p-band (Fig. 6). The gradual dominance of the P-emission indicates that the equilibrium constant for the solute-solvent complex increases as the chain length increases. This is actually consistent with the solute-solvent complex model discussed above. Namely as the CT D-A-D state of squaraine is stabilized (by the electron-releasing N-alkyl group), the tendency for complexation increases [6].

A number of attempts were made to correlate the solvent effects with different solvent parameters, such as the dielectric constant Ej. [46], Z [47], 6 [48], Py [49], n [50], and so forth. The relationships between and these solvent parameters are quite scattered except n. The plot of of Sq4 as a function of solvent parameter n is given in Fig. 10. Along with the red-shift on a systematic and gradual change in the composition of the multiple emission band is observed (see insets in Fig. 10). Sq4 exhibits primarily a-emission in diethyl ether. As the solvent polarity increases, the intensity of the P-emission increases. The p-emission eventually dominates the fluorescence. Because the P-emission is the emission from the solute-solvent complex, the overall spectral results suggest that the solvent effect on may be due to the shift in equilibrium for the complex formation as n increases. For solvents with 7t ranging from 0.273 to 0.567, both a- and P-emission bands are discernible simultaneously. Assuming that the spectral bandwidths of these two bands are similar and that they are not sensitive to solvent. Law [30] has deconvoluted the contribution of the a- and P-bands in the multiple emissions. The relative intensity of these two bands can then be used to estimate the relative concentrations of the free squaraine and the complex. From the ratio of the a- and P-emissions and the molar concentration of the solvent, the equilibrium constants (K in these solvents are calculated. A plot of versus n is depicted in Fig. 11, and a linear plot is obtained. The result simply indicates that the equilibrium constant for solute-solvent complexation increases as n increases. 

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