Pyrene, complexes concentration

Fig. 16. Polarographic data for complex formation as a function of donor concentration (in CHaCls at 25°). Hexamethylbenzene complexes of A, tetracyanoethylene B, tetra-cyanobenzene. Pyrene complexes of C, chloranil D, tetracyanobenzene E, tetracyano-quinondimethide. (Data from ref. 64)...

Elegant evidence that free electrons can be transferred from an organic donor to a diazonium ion was found by Becker et al. (1975, 1977a see also Becker, 1978). These authors observed that diazonium salts quench the fluorescence of pyrene (and other arenes) at a rate k = 2.5 x 1010 m-1 s-1. The pyrene radical cation and the aryldiazenyl radical would appear to be the likely products of electron transfer. However, pyrene is a weak nucleophile the concentration of its covalent product with the diazonium ion is estimated to lie below 0.019o at equilibrium. If electron transfer were to proceed via this proposed intermediate present in such a low concentration, then the measured rate constant could not be so large. Nevertheless, dynamic fluorescence quenching in the excited state of the electron donor-acceptor complex preferred at equilibrium would fit the facts. Evidence supporting a diffusion-controlled electron transfer (k = 1.8 x 1010 to 2.5 X 1010 s-1) was provided by pulse radiolysis. 

Different aromatic hydrocarbons (naphthalene, pyrene and some others) can form excimers, and these reactions are accompanying by an appearance of the second emission band shifted to the red-edge of the spectrum. Pyrene in cyclohexane (CH) at small concentrations 10-5-10-4 M has structured vibronic emission band near 430 nm. With the growth of concentration, the second smooth fluorescence band appears near 480 nm, and the intensity of this band increases with the pyrene concentration. At high pyrene concentration of 10 2 M, this band belonging to excimers dominates in the spectrum. After the act of emission, excimers disintegrate into two molecules as the ground state of such complex is unstable. 

Figure 3. Critical concentration behavior of actin self-assembly. For the top diagram depicting the macroscopic critical concentration curve, one determines the total amount of polymerized actin by methods that measure the sum of addition and release processes occurring at both ends. Examples of such methods are sedimentation, light scattering, fluorescence assays with pyrene-labeled actin, and viscosity measurements. Forthe bottom curves, the polymerization behavior is typically determined by fluorescence assays conducted under conditions where one of the ends is blocked by the presence of molecules such as gelsolin (a barbed-end capping protein) or spectrin-band 4.1 -actin (a complex prepared from erythrocyte membranes, such that only barbed-end growth occurs). Note further that the barbed end (or (+)-end) has a lower critical concentration than the pointed end (or (-)-end). This differential stabilization requires the occurrence of ATP hydrolysis to supply the free energy that drives subunit addition to the (+)-end at the expense of the subunit loss from the (-)-end.

In aqueous acetic acid, the disproportionation of the platinum still occurs quite rapidly, and it can be suppressed further by adding mineral acid. Hydrochloric acid is often used, but this has a disadvantage in that the exchange rate is inversely proportional to the chloride ion concentration. Perchloric acid has been found to be more satisfactory (55). The platinum(II) catalyst most used is sodium or potassium tetrachloropla-tinate(II). An aromatic compound added to the reaction mixture also inhibits disproportionation of the platinum(II) complex—benzene, pyrene, and other aromatics have been used. A comparative study of the effect of various aromatics on the H—D exchange in alkanes has been carried out (55). Even under optimum conditions, the disproportionation [Eq. (4)] still takes place, and the catalytic platinum(II) is slowly removed from the reaction mixture. To get useful rates of exchange in alkanes, temperatures of 100° to 120°C have to be used, and the disproportionation rate increases with temperature. 

In some cases, simultaneously with the quenching of the normal fluorescence a new structureless emission band appeals at about 6000 cm-1 to the red side of the monomer fluorescence spectrum (Figure 6.4). This phenomenon was first observed in pyrene solution by Forster and was explained as due to transitory complex formation between the ground and the excited state molecules since the absorption spectrum was not modified by increase in concentration. Furthermore, cryoscopic experiments gave negative results for the presence of ground state dimers. These shortlived excited state dimers are called pxcimers to differentiate them from... 

Anthracene [19] and pyrene [20] derivatives (AMAC and PBAC, respectively) intercalated into BAZrP galleries show enhanced ground-state dimer and excimer formation. The dimer shows a broad new absorption band, the intensity of which increases quadratically with concentration. In contrast, a 1 1 complex formed... 

The CD architecture is particularly fitting for excimer-based reporter sites. As long as the bucket is sufficiently large to accept two guests, monomers will cofacially organize within the interior of a CD to form excimer. Early spectroscopic studies revealed an enhanced excimer emission upon the addition of y-CD to solutions of naphthalene [361], a result that was subsequently observed for anthracene [362] and pyrene [363,364], Pyrene excimer formation has been examined in particular detail. Proper concentrations of pyrene and y-CD afford 2 1 or 2 2 excimer inclusion complexes. In both cases, pyrenes interact in a cofacial manner and excimer efficiently forms. Alcohol addition can disrupt the excimer by the preference to form a 1 1 1 complex of alcohol, pyrene, and CD. 

Binding of pyrene, fluoranthene, and anthracene to humic substances has been performed by following the decrease in fluorescence intensity of the polluants in the presence of increased humic substance concentrations. Figure 10.12 shows an analysis of the data based on calculations using Equation (10.22). Since the slopes of the plots are equal to the association constants of the complexes, one can say that pyrene has the highest affinity to humic substances, followed by fluoranthene and anthracene. 

Many conventional texts express regression equations in the form of summations rather than matrices, but both approaches are equivalent with modern spreadsheets and matrix oriented programming environments it is easier to build on the matrix based equations and the summations can become rather unwieldy if the problem is more complex. In Figure 5.2, the absorbance of the 25 spectra at 335 nm is plotted against the concentration of pyrene. The graph is approximately linear, and provides a best fit slope calculated by... 

Quantitative approaches to describing reactions in micelles differ markedly from treatments of reactions in homogeneous solution primarily because discrete statistical distributions of reactants among the micelles must be used in place of conventional concentrations [74], Further, the kinetic approach for bimolecular reactions will depend on how the reactants partition between micelles and bulk solution, and where they are located within the microphase region. Distinct microphase environments have been sensed by NMR spectrometry for hydrophobic molecules such as pyrene, cyclohexane and isopropylbenzene, which are thought to lie within a hydrophobic core , and less hydrophobic molecules such as nitrobenzene and N,N-dimethylaniline, which are preferentially located at the micelle-water interface [75]. Despite these complexities, relatively simple kinetic equations for electron-transfer reactions can be derived for cases where both donors and acceptors are uniformly distributed inside the micelle or on its surface. 

Solute/solute Interactions are revealed by the forinatlon of exclmers. Exclmers are excited state dimers that result In a broad structureless band at significantly longer wavelengths than the normal fluorescence. While they are not dimers In the sense of a ground state conqplex, their existence does Indicate that there Is sufficient Interaction In the approximately 10 second lifetime of the excited state (33) to form the excited state complex. We have observed the formation of pyrene exclmers even at extremely low concentrations In supercritical fluids. Figure 7 shows the spectra of pyrene In SCF CO2 at two concentrations. 

An important source of error in the quantification of PAHs is the ubiquitous presence of humic substances (HS) in natural waters (Kile et al., 1994). These substances have been shown to form associations with PAHs and as such to play an important role in PAH transport and fate in the environment (McCarthy and Jimenez, 1985 Kile et al., 1994 Kopinke et al., 2001). These interactions are the basis of competitive complex-ation that reduces the uptake of PAHs by solid phase extraction (Stum et al., 1998 Li and Kee Lee, 2001). This effect is illustrated in Figure 4.3.4 from Algarra et al. (2005), where the impact of humic substances is shown in a Stem-Volmer plot of the pyrene signal obtained by using the present method against the concentration of Suwanne River Humic acid. In addition to the humic substances many other organic substances and particulate matter may interact with PAHs and thus could significantly... 

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