Pyrene polarity scale

In order to explore the interactions between calixarene and SAIL, we next used fluorescence from pyrene as the indicator. The concentration of the three calixarenes, (1), (2), and (3), respectively, were varied within the pyrene solution in 1M aqueous NaOH in the presence of 50mM (post-cmc) concentration of SAIL [Cj(,mim][Cl]. We extracted information from both pyrene polarity scale and the changes in the fluorescence signal of the pyrene as the concentration of the calixarene is changed. 

Karpovich DS, Blanchard GJ (1995) Relating the polarity dependent fluorescence response of pyrene to vibronic coupling. Achieving a fundamental understanding of the py polarity scale. J Phys Chem 99 3951-3958... 

Karpovich D. S. and Blanchard G. J. (1995) Relating the Polarity-Dependent Fluorescence Response of Pyrene to Vibronic Coupling. Achieving a Fundamental Understanding of the Py Polarity Scale,... 

Pyrene is one of the most widely studied neutral fluorescence probes, and accordingly, this was sometimes used to determine the polarity of some ILs. The polarity scale of the IL analyzed with pyrene is defined as the emission intensity... 

Solvatochromic fluorescent probe molecules have also been used to establish solvent polarity scales. The solvent-dependent fluorescence maximum of 4-amino-V-methylphthalimide was used by Zelinskii et al. to establish a universal scale for the effect of solvents on the electronic spectra of organic compounds [80, 213], More recently, a comprehensive Py scale of solvent polarity including 95 solvents has been proposed by Winnik et al. [222]. This is based on the relative band intensities of the vibronic bands I and III of the % - n emission spectrum of monomeric pyrene cf. Section 6.2.4. A significant enhancement is observed in the 0 0 vibronic band intensity h relative to the 0 2 vibronic band intensity /m with increasing solvent polarity. The ratio of emission intensities for bands I and III serves as an empirical measure of solvent polarity Py = /i/Zm [222]. However, there seems to be some difficulty in determining precise Py values, as shown by the varying Py values from different laboratories the reasons for these deviations have been investigated [223]. 

The molecular probe pyrene is commonly employed to elucidate solute-solvent interactions in normal liquids (18,35). Because of the high molecular symmetry, the transition between the ground and the lowest excited singlet state is only weakly allowed, subject to strong solvation effects (36-39). As a result, in the fluorescence spectrum of pyrene the relative intensities of the first (/i) and third I2) vibronic bands vary with changes in solvent polarity and polarizability. The ratio h/h serves as a convenient solvation scale, often referred to as the Py solvent polarity scale. Py values for an extensive list of common liquid solvents have been tabulated (15,16). 

As expected, pyrene has also been used to characterize supercritical fluid-cosolvent mixtures. For example, Zagrobelny and Bright used the Py polarity scale and pyrene excimer formation to study supercritical C02-methanol and C02-acetonitrile mixtures (160). Their results suggest the clustering of cosolvent molecules around pyrene. Similarly, Brennecke and coworkers measured Py values in CO2, CHF3, and CO2-CHF3 mixtures (43). 

The polarity of some l-alkyl-3-methylimidazolium ionic liquids has been determined using the solvatochromic dyes Nile Red and Reichardt s dye [16, 17], Measurements with Nile Red do not give absolute values of polarity but provide a useful scale to estimate the relative polarity of the ionic liquids. Similar measurements have been made using a number of other solvatochromic dyes (dansylamide, pyrene, pyrenecarboxyaldehyde, and bromonapthalene) for [BMIM][PF6], and gave results consistent with those obtained with Nile Red. Values for Ej obtained for ionic liquids generally fall between the values of 0.6 and 1.0, as shown in Table 4.3. 

Differential solvent interactions with ground- and excited-state molecules not only lead to shifts in the fluorescence maxima but also to perturbation of the relative intensities of the vibrational fine structure of emission bands. For instance, symmetry-forbidden vibronic bands in weak electronic transitions can exhibit marked intensity enhaneements with increasing solute/solvent interaction [320, 359]. A particularly well-studied ease is the solvent-influenced fluorescence spectrum of pyrene, first reported by Nakajima [356] and later used by Winnik et al. [357] for the introduction of an empirical solvent polarity parameter, the so-called Py scale cf. Section 7.4. 

The concept of immobilized ionic liquids entrapped, for instance, on the surface and pores of various porous solid materials (supported ionic liquid phase, SILP) is rapidly become an attractive alternative. In addition, the SILPs can also answer other important issues, such as the difficult procedures for product purification or IL recycling, some toxicity concerns and the problems for application in fixed-bed reactors, which should be addressed for future industrial scale-up. This new class of advanced materials shares the properties of true ILs and the advantages of a solid support, in some cases with an enhanced performance for the solid material. Nevertheless, a central question for the further development of this class of materials is to understand how much the microenvironment provided by the functional surfaces is similar or not to that imparted by ILs. Recent studies carried out using the fluorescence of pyrene to evaluate the polarities of a series of SILPs based on polymeric polystyrene networks reveal an increase in polarity of polymers, whereas the polymer functional surfaces essentially maintain the same polarity as the bulk ILs. However, this is surely not a simple task, in particular if we consider that the basic knowledge of pure ILs is still in its infancy, and we are just starting to understand the fundamentals of pure ILs when used as solvents. 

Pyrene derivatives are the widest used probes for qualitative solubilization [365] by virtue of the solvatochromic shifts of the absorption bands [255], the excimer formation [145,186], the polarity dependent quantum yields [197] and fluorescence life-times [185-187, 196, 197, 202, 215, 292], and the pyrene fluorescence fine structure [65, 74,78,103,112,167, 224, 363, 371] the intensity ratio of the fluorescence bands I at 372 nm and III at 383 nm is a convenient measure for the polarity of the environment of the pyrene label ( py -scale I/III values increase with polarity, cf. Fig. 27). As, however, the fluorescence of pyrene is very sensitive to the experimental set-up [372], absolute I/III values reported by different groups are difficult to compare. 

The Py scale is based on the ratio between the intensity of components (0,0) Ii and (0,2) I3 of the fluorescence of monomeric pyrene (3) in various solvents. It was initially established from 95 solvents and spans values from 0.41 for the gas phase to 1.95 for DMSO. This scale is primarily used in biochemical studies, which usually involve fluorescent probes. However, it poses problems arising largely from the difficulty of obtaining precise values of the above-mentioned intensity ratio this has resulted in divergences among Py values determined by different laboratories. One further hindrance is that the mechanism via which low-polar solvents enhance the intensity of symmetry-forbidden vibronic transitions through a reduction in local symmetry is poorly understood. 

Betaine Dye and Nile Red. As shown in Section Pyrene , the solvent polarity in sol-gel maUices is characterized by the dieleclric constant (e). Actual solute/solvent interaction, however, cannot always be explained quantitatively by the sinple physical parameter. Therefore, many empirical parameters of solvent polarity have been proposed such as Z scale, n scale, and Ey 30) (Reichardt, 1994). t(30) is defined as Eq. (20-1) ... 

Figure 1.16. Polarized absorption spectra of pyrene in stretched polyethylene a) observed absorbances Ez and Ey parallel and perpendicular to the polarization direction of the light, after baseline correction b) reduced spectra A X) = Ez- 1.0 Ey (—) and A/A) = 0.625 (2.8 Ey - Ez) The absorbance scale is in arbitrary units, different for the different sections of the spectrum (by permission from Michl and Thulstrup, 1986).

It was recognized as early as 1981 [54] that in order to achieve the ferroelectric state, a chiral liquid crystal is necessary. Chirality puts two molecules in the ideal unit cell, and this allows new modes of fluctuations and also new forms of organisation which also give possible minima in the free energy [ structure ] function. An electric field can now take the liquid crystal into one of these alternative forms of molecular packing, some of which can have a net ferroelectric moment per unit cell and therefore an overall ferroelectric moment. Columnar pyrenes made recently by Bock and Helfrich [55] have saturation polarization of -llOnCcm" and switch from the anti- to the ferro-electric state on a time scale of = 10 s with switching fields -10 V tm . Similar behavior has been... 

Several research groups have used pyrene as a fluorescent probe in the study of supercritical fluid properties (2,3,40-48). In particular, the density dependence of the Py scale has been examined systematically in a number of supercritical fluids such as CO2 (2,3,40-43,45,46), ethylene (40,41,47), fluoro-form (3,40,41,43,47), and C02-fluoroform mixtures (43). The Py values obtained in various supercritical fluids correlate well with the polarity or polarizability parameters of the fluids (3,40,41,43,47). For example, Brennecke et al. (40) found that the Py values obtained in fluoroform were consistently larger than those obtained in CO2, which were, in turn, consistently larger than those found in ethylene over the entire density region examined. In addition, the Py values obtained in the liquid-like region (reduced density 1.8) indicate that the local polarity of fluoroform is comparable to that of liquid methanol, CO2 with xylenes, and ethane with simple aliphatic hydrocabons (15,16). 

Molecular spectroscopy methods have also been applied to the study of the entrainer effect in supercritical fluid-cosolvent mixtures. Again, the molecular probes employed for absorption and fluorescence measurements include the Kamlet-Taft u polarity/polarizability scale probes (13,14), pyrene (15,16), and TICT molecules (17). 

DC Dong, MA Winnik. The Py scale of solvent polarities—solvent effects on the vibronic fine-structure of pyrene fluorescence and empirical correlations with ET-value and Y-value. Photochem Photobiol 35 17, 1982. 

The shape of the fine structure in the absorption and fluorescence spectra of pyrene is very sensitive to local polarity (15a). The so-called Py--scale" of solvent polarities (15b) is a manifejstation of the Hamm effect (15c), whereby locally anisotropic electric fields relax the forbiddenness of the (0,0) band in Si -> So transitions of symmetrical aromatic chromophores. Thomas has used pyrene fluorescence to probe local polarity in aqucious micelles and in aqueous solutions of polyelec trolytes (15d). 

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