Py scale

Polarity-induced changes in vibronic bands. The Py scale of polarity... 

In contrast, the Py scale, based on the relative intensities of vibronic bands of pyrene, appears to be relatively insensitive to hydrogen bonding ability of solvents. 

Table 10.13 shows that Z and T(30) values are generally in the same order as Y values. Other scales, the tt scale,4"0 the tt scale,4111 and the Py scale,402 are also based on spectral data.403... 

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. 

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]. 

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. 

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). 

Y-P Sun, CE Bunker, NB Hamilton. Py scale in vapor phase and in supercritical carbon dioxide. Evidence in snpport of a three-density-region model for solvation in supercritical fluids. Chem Phys Lett 210 111, 1993. 

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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