Intensity, microenvironment polarity

In polar solvents, the structure of the acridine 13 involves some zwitterionic character 13 a [Eq. (7)] and the interior of the cleft becomes an intensely polar microenvironment. On the periphery of the molecule a heavy lipophilic coating is provided by the hydrocarbon skeleton and methyl groups. A third domain, the large, flat aromatic surface is exposed by the acridine spacer unit. This unusual combination of ionic, hydrophobic and stacking opportunities endows these molecules with the ability to interact with the zwitterionic forms of amino acids which exist at neutral pH 24). For example, the acridine diacids can extract zwitterionic phenylalanine from water into chloroform, andNMR evidence indicates the formation of 2 1 complexes 39 such as were previously described for other P-phenyl-ethylammonium salts. Similar behavior is seen with tryptophan 40 and tyrosine methyl ether 41. The structures lacking well-placed aromatics such as leucine or methionine are not extracted to measureable degrees under these conditions. 

FIG. 8.4 Determination of the microenvironment of a molecule (a) a portion of the ultraviolet spectrum of benzene in (1) heptane, (2) water, and (3) 0.4 M sodium dodecyl sulfate and (b) ratio of the intensity of the solvent-induced peak to that of the major peak for benzene versus the index of solvent polarity. The relative dielectric constant is also shown versus the index of polarity. (Redrawn, with permission, from P. Mukerjee, J. R. Cardinal, and N. R. Desai, In Micellization, Solubilization and Microemulsions, Vols. 1 and 2 (K. L. Mittal, Ed.), Plenum, New York, 1976.)... 

Pyrene shown a number of photophysical features that made it an attractive fluorophore to probe the microenvironment in micellar aggregates [19]. For the peaks of pyrene PL, two important peaks at about 373 nm and 390 nm among the five dominant peaks of pyrene fluorescence were numbered as 1 and III, respectively [20]. It has been known that intensity ratio of peak 111 to I (III/I) increased as the polarity at the solubilization site of pyrene decreases. Figure 6 shows fluorescence spectra (A.ex = 310 nm) of pyrene in precursor gel containing TPA and I-IV samples denoted as (a), (b), (c), (d) and (e), respectively. The value of 111/1 of pyrene does not change under silicalite-1 gel due to no formation of micelle. However, in the Fig. 6d (sample II), III/I ratio is rapidly increased, while sample III and IV are decreased slightly again. Previously, Park et al. have reported that 111/1 ratio of pyrene for... 

Symmetry-lowering effects of the solvent are referred to as the Ham effect (Ham, 1953 Platt, 1962). Thus, in rigid or fluid solutions, symmetry-forbidden vibrational components of the Bju ( Lb) absorption band of benzene appear with increasing intensity as the polarizability and the polarity of the solvent increase. The fact that for pyrene and 2-methylpyrene the direction of the Lb transition moment is inclined on the average 40° and 20° away from the y axis, respectively, has also been ascribed to a symmetrylowering perturbation by the environment, related to the Ham effect (Lang-kilde et al., 1983). The associated intensification of otherwise weak vibronic peaks in the Lb band can be used for an investigation of microenvironments such as micelles. 

Pyrene is extremely sensitive to the polarity of the microenvironment surrounding it. As the polarity of the microenvironment increases, the emission intensity of the first vibronic band (/,) increases, while the emission intensity of the third vibronic band (I3) decreases. Thus, ///. is related to the dipolarity of the microenvironment surrounding the pyrene molecules for example /,//3 shifts from 0.58 in cyclohexane to 1.87 in water.72 73 PRODAN and DCM are solvatochromic fluorophores whose fluorescence band position is extremely sensitive to the polarity of the surrounding microenvironment. For example, the fluorescence emission maximum of PRODAN shifts from 401 nm in cyclohexane to 531 nm in water.74-75... 

Since the dynamics of the twisted intramolecular charge transfer (TICT) process is very sensitive to the polarity of the medium, the local polarity of an organized medium may also be determined from the rate of the HCT process. For TNS, which is nearly nonfluorescent in water ((j)f = 10 and Xf = 60 ps), the emission quantum yield and lifetime increases nearly 50 times on binding to cyclodextrins and more than 500 times on binding to a neutral micelle, TX -100 [86]. Such a dramatic increase in the emission intensity and lifetime arises because of the marked reduction of the nonradiative HCT process inside the less polar microenvironment of the cyclodextrins and the micelle. Determination of the micropolarities of various organized assemblies using TICT probes has been surveyed quite extensively in several recent reviews [5b-d,f,86]. Therefore, in this chapter we will focus only on some selected works not covered in the earlier reviews. 

Fluorescence is very sensitive to the chemical environment and may be utilized to provide information about the microenvironment surrounding the probe (5, 6). The fluorescence intensity (quantum yield) (1-3), the maximum emission wavelength (1-3), the fluorescence lifetime (6-9), or the polarization (10-12) may all be monitored for specific changes that are induced as a result of changes in polarity, pH, ion concentration, membrane potential, or ligand binding. 

Pyrene is a useful probe for studying the polarity of the medium because its fluorescence vibronic structure is very sensitive to the microenvironment [92,93]. In particular, the ratio of the intensities of the first (373 nm) to the third (384 nm) fluorescence vibronic maxima (R = I/III) strongly decreases with the decreasing polarity of the medium. The interaction of pyrene (13) with the three CDs has been thoroughly investigated by photophysical methods. There are considerable discrepancies among the results of the various reports, which are probably related to the very low solubility of 13 in H2O (<5 X 10 M). The low solubility imposes the need for special precautions when preparing the samples [94]. 

Sequential complexation was confirmed in reference 97, where it was reported that P-CD addition decreases the R value and increases the intensity of the two vibronic bands. Solutions of P-CD containing 13 always exhibited a biexponential decay the shortest, t, = 130 ns, has the same lifetime of 13 in water, the largest, Tj = 300 ns, indicates that 13 experiences a hydrophobic environment. The ratio of the preexponentials /42Mi grew monotonically with [/ -CD]. The data are consistent with a sequential complexation, as in the complexation of 13 with a-CD. In the 1 1 complex, the included pyrene has the same lifetime as 13 in water because a substantial portion of the molecule is still exposed to the solvent when 13 is encapsulated by two cyclodextrins, it experiences a low-polarity microenvironment and its lifetime consequently increases. In the same paper, the complexing ability of a polymer-supported P-CD of the general formula... 

Luminescence of Probe Molecules. These studies permit evaluation of polymer properties. In particular, measurement of the relative Intensities of fluorescence of a probe molecule polarized parallel to and perpendicular to the plane of linearly polarized exciting radiation as a function of orientation of a solid sample yields Information concerning the ordering of polymer chains. In solution, similar polarization studies yield Information on the rotational relaxation of chains and the viscosity of the microenvironment of the probe molecule. More recently, the study of luminescence Intensity of probe molecules as a function of temperature has been used as a method of studying transition temperatures and freeing of subgroup motion in polymers. 

Fluorescence parameters such as the intensity and the position of the emission maximum are sensitive to the modifications occurring in the microenvironment of the fluorophore. In fact, emission energy is important when the environment of the fluorophore shows a low polarity, inducing by that a spectrum with a maximum located at low wavelength. Since the fluorescence intensity is proportional to the number of emitted photons, less the surrounding environment is polar, more the nimiber of the emitted photons is important and much higher will be the intensity. 

The monomer MPI was not found to be suitable for measuring the polarity of the polymer microenvironment. In the region most interesting for measuring either the solvent polarity or the polarity of the polymer microenvironment, the solvatochromic band of compounds MPI and EPI is not separated from a much more intense adjoining band. 

Microenvironment of the emissive species plays an important part in determining the characteristics of the emission. Thus, it is not surprising that supramolecular associations with luminescent molecules often result in altered emission spectra, or lifetime. This is clearly seen in host molecules with relatively hydrophobic cavities that interact with fluorescent guest molecules. Encapsulation, typically, rigidities the guest molecules within the confines of the cavity, which generally results in an increased emission intensity. In addition, hydrophobic pockets of cyclodextrins may also induce shifts in the emission wavelength. For example, ANS (l-anilino-8-naphthalenesnlfonate) and TNS (6-p-toluidinylnaphthalene-2-sulfonate), which are also known as polarity probes, show remarkable complexation-induced fluorescence intensity increases. ... 

The intensity ratio of the third to fin peaks in the vibrational fine structures in pyrene fluorescence spectra (/3//1) is often used as an indicates for the microenvironmental polarity about pyrene. In general, the /3//1 ratio is larger in less polar microenvironments (50). The /3//1 ratio of the pyrene-labeled polymers (Chart 2) with varying /ood increases in the region 10 < fyod < 30 mol %, reaching a... 

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