Polarization spectrum pyrene

Figure 14. (a) Absorption polarization spectrum of pyrene-2-carbonic methyl ester (PCME). Solvent 1.85 1 by weight mixture of cholesteryl chloride and cholesteryl laurate 30°C). Concentrations lO M and 3.3. 10 M, respectively. The spectrum PIIE was taken with the linear polarizers P oriented parallel to the electric field E (or parallel to the optix axis L). 

The relative changes in intensity of the vibronic bands in the pyrene fluorescence spectrum has its origin in the extent of vibronic coupling between the weakly allowed first excited state and the strongly allowed second excited state. Dipole-induced dipole interactions between the solvent and pyrene play a major role. The polarity of the solvent determines the extent to which an induced dipole moment is formed by vibrational distortions of the nuclear coordinates of pyrene (Karpovich and Blanchard, 1995). 

Figure 8.2 presents the fluorescence of pyrene on silica gel. The loading is low so that pyrene is predominantly adsorbed as nonaggregated monomers (Mi). The backward fluorescence spectrum Fb of this sample is very comparable to the spectrum in polar solvents and not distorted by reabsorption. However, the forward spectrum Ft is almost completely suppressed in the region of overlap with the o -transition and hot sidebands of the weak first absorption band Si. The absorption coefficients of the sample vary widely from k" = 0.1 cm 1 (Si-band, Aa = 350-370 nm) to k = 25 cm-1 (S2-band, 1 290-340 nm), and in a first approximation the excitation spectrum of Fh reflects this variation correctly (Figure 8.2, left). The Ff-excitation spectrum, however, has only little in common with the real absorption spectrum of the sample. 

Fig. 70a and b. Absorption polarization spectra (a) and the ICD spectrum (b) of pyrene-2-carboxylic acid methyl ester 249)... 

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]. 

The solvent sensitivity of the emission spectrum and fluorescence quantum yield of pyrene and its derivatives have been used to sense the polarity of microphase interiors. By these methods, pyrene in SDS micelles is located within a microenvironment less polar than water, but more polar than typical hydrocarbons [53]. 

Pyrene has been used widely as a photophysical probe because of its long fluorescence lifetime and great tendency for excimer formation. Emission characteristics of pyrene molecules depend on the nature of the solvent. The ratio of relative intensities of the 1st (373 nm) and lllrd (383 nm) peaks, Ijjj/Ij, in a pyrene emission spectrum decreases as the polarity of the solvent increases. This... 

Another extremely useful method for cac determination, especially in the light of high sensitivity, is fluorescence emission spectroscopy [15]. Some aromatic water-insoluble dyes that are present in trace amounts in mixed polyelectrolyte-surfactant solutions have an ability to solubilize within the self-assembled surfactant aggregates and to change their photophysical properties because of the change of environmental polarity. Through this, they offer a very sensitive method for the determination of cac values. A typical and lately frequently used compound is pyrene, which is used as a fluorescence probe to assess various micellar properties. Pyrene exhibits a polarity dependent fluorescence spectrum with the ratio /,//3 (the ratio of the intensity... 

A well-known characteristic of pyrene is that its fluorescence emission spectrum is sensitive to the environment in which it is dispersed [63,64], In particular, the relative fluorescence intensities of two vibronic bands, termed 3 and 1, respectively, change markedly with solvent polarity. Estimation of the ratio of band 3 to 1 can consequently serve as a sensor of hydrophobicity a ratio of 1.7, for example, would be expected when pyrene is dispersed in ra-pentane [65], whereas in aqueous solution a value of 0.55 is observed [6,20]. The sensitivity of the vibrational fine structure of the fluorescence of pyrene to its environment has been exploited by spectroscopists keen to probe the polarity, of a variety of media such as micelles [66], microemulsions [67],... 

For example, McCormick and coworkers [173] have dispersed pyrene in block copolymers of AMPS and 3-(acrylamido)-3-methylbutanoate (AMBA). The pH response of the system was investigated by using the sensitivity of the fluorescence spectrum of the probe to the polarity of the medium. Figure 2.12a shows an example of the spectra at two pH extremes the decrease in the intensity of band 1 from pH 9.0 to 1.0 is indicative of pyrene, which resides in a more hydrophobic environment and is consistent with the formation of micelles under acidic conditions. Figure 2.12b shows a plot of the resultant /1 to /3 ratio across the pH range. A dramatic decrease in the ratio is observed between pH 5.0 and 6.0. This supports NMR data, which indicates that dehydration of the PAMBA block occurs at pH 5.5, which serves to create near monodisperse micelles, which can solubilize low molar mass material. 

A number of workers have attempted to study the polarity of ionic liquids using the fluorescence spectra of polycyclic aromatic hydrocarbons. Of these, the most commonly applied has been that of pyrene [23-26]. The measurements are of the ratio of the intensities of the first and third vibronic bands in the tt-tt emission spectrum of monomer pyrene h/h). The increase in Ii/h values in more polar solvents has... 

Pyrene h/h values measurements have generally placed the ionic liquids in the polarity range of moderately polar solvents. [BMIM][Pp6] has a particularly high value (1.84 [25] and 2.08 [24] have been reported). This can be compared to water (1.96), acetonitrile (1.88) and methanol (1.50) [24]. It should be noted that the spectrum of pyrene would be expected to be sensitive to HF, which could well be present in these [PFs]" ionic liquids, and would lead to artificially high values of h/h- Other fluorescence probes that have been used give broadly similar results [23-25,29]. 

Beside these bipolar, surface active compounds a broad spectrum of polar compounds, sulfonated azo dyes [159, 160], sulfonates [161], additives in commercial dyes [145], benzo[a]pyrene conjugates [162, 163], DNA adducts [164], ozonization products of surfactants [165, 166], explosives [167] and so forth applying FAB or CF-FAB were qualitatively determined or even quantified with success. Grange et al. [159] used accurate mass measurements for identification and confirmation of e.g. sulfonated azo dyes [159]. 

In order to check the drop-weight method, static fluorescence measurements were performed. It is shown [15] that the ratio between the third and the first vibronic peaks (denoted the m/I-ratio) in the pyrene steady-state fluorescence spectrum is dependent on the environmental polarity of the dissolved pyrene nx>lecule. As aggregation starts, pyrene will be dissolved in the less polar aggregate and, thus, the Ilt -ratio will change. 

Aromatic hydrocarbons such as pyrene have also been employed as a luminescence probe of polarity and microviscosity in a variety of organized assemblies (109). Pyrene is a good excimer-forming probe due to the long lifetime of fluorescence and formation of excited-state dimers (excimers) at low concentration. Figure 9 shows an example pyrene luminescence spectrum. The ratio of excimer to monomer fluorescence intensity is often utilized as a measure of pyrene mobility and proximity. The vibronic fine structure of the pyrene monomer is sensitive... 

Without interactions with potential host molecules and in diluted solutions to avoid excimeric formations, pyrene presents in solution an intense and anisotropic fluorescence, as well as a high fluorescence quantum yield [34-37], Direct evidence of ground-state interactions of pyrene with potential host molecules is provided by the emission spectra. The vibrational structure of the emission spectrum of pyrene is constituted by five fine peaks, named I, I2, h, I4, and I5 (Fig. 13.2) [38]. An increase of the intensity of peak Ii is observed in polar solvents, while I, is solvent insensitive. Thus, the evolution of the ratio of intensities /1//3 gives information on the evolution of the polarity of the environment close to molecular pyrene, and consequently on the encapsulation of this guest in a host molecular or supramolecular object [39]. This sensitivity of pyrene, and of peri-fused polycyclic aromatic hydrocarbon molecules in general, to the polarity of the environment is a photophysic property that is extensively used to study host-guest interactions [40]. 

Another remarkable property of the pyrene fluorescence is that a change of polarity in the vicinity of molecular pyrene leads to a dramatic modification of the intensity of the 0-0 vibronic band (/i peak), without affecting the intensity of the /j peak. The /1//3 ratio of the pyrene emission spectrum is therefore an extensively used probe parameter to determine the micropolarity of the environment in which it resides. 

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