Diffusivity, pyrene

Since increasing temperature leads to increased fluidity and thus to a faster probe diffusion, pyrene Hpids have been frequently used to study phase transition in membranes [161,162]. Phospholipid phase separation increases the local concentration of dye labeled Hpids and can, therefore, be investigated via the characterization of exdmer formation. The binding of proteins or ions, however, may induce phase separation as well as decreasing lateral lipid diffusion. Since these two effects are opposing in terms of excimer formation, the binding of such proteins or ions cannot be studied by the (Ex/Mo)-ratio. The time-resolved analysis of the monomer fluorescence of the labeled lipid, however, allows for the separation of... 

The attachment of pyrene or another fluorescent marker to a phospholipid or its addition to an insoluble monolayer facilitates their study via fluorescence spectroscopy [163]. Pyrene is often chosen due to its high quantum yield and spectroscopic sensitivity to the polarity of the local environment. In addition, one of several amphiphilic quenching molecules allows measurement of the pyrene lateral diffusion in the mono-layer via the change in the fluorescence decay due to the bimolecular quenching reaction [164,165]. 

Equations (4-5) and (4-7) are alternative expressions for the estimation of the diffusion-limited rate constant, but these equations are not equivalent, because Eq. (4-7) includes the assumption that the Stokes-Einstein equation is applicable. Olea and Thomas" measured the kinetics of quenching of pyrene fluorescence in several solvents and also measured diffusion coefficients. The diffusion coefficients did not vary as t) [as predicted by Eq. (4-6)], but roughly as Tf. Thus Eq. (4-7) is not valid, in this system, whereas Eq. (4-5), used with the experimentally measured diffusion coefficients, gave reasonable agreement with measured rate constants. 

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. 

The investigation by Becker et al. (1977 b) also included work on the effect of pyrene added as electron donor. Pyrene has an absorption maximum at 335 nm (e = 55000 M-1cm-1, in petroleum). Much more hydro-de-diazoniation takes place in the presence of pyrene with irradiation at 365 nm, and even more on irradiation with light of wavelength <313 nm. Photoexcited pyrene has a half-life of 300 ns and is able to transfer an electron to the diazonium ion. This electron transfer is diffusion-controlled (k= (2-3) X 1010 m 1s 1, Becker et al., 1977a). The radical pairs formed (ArN2 S +) can be detected by 13C- and 15N-CIDNP experiments (Becker et al., 1983, and papers cited there). 

Transport Properties Although the densities of SCFs can approach those of conventional liquids, transport properties are more favorable because viscosities remain lower and diffusion coefficients remain higher. Furthermore, CO2 diffuses through condensed-liquid phases (e.g., adsorbents and polymers) faster than do typical solvents which have larger molecular sizes. For example, at 35°C the estimated pyrene diffusion coefficient in polymethylmethacrylate increases by 4 orders of magnitude when the CO2 content is increased from 8 to 17 wt % with pressure [Cao, Johnston, and Webber, Macromolecules, 38(4), 1335-1340 (2005)]. 

Alvaro, M., Atienzar, P., Bourdelande, J.L., and Garda, H. (2004) An organically modified single wall carbon nanotube containing a pyrene chromophore Fluorescence and diffuse reflectance laser flash photolysis study. Chem. Phys. Lett. 384, 119-123. 

The serious drawback of the methods of evaluation of fluidity based on intermolecular quenching or excimer formation is that the translational diffusion can be perturbed in constrained media. It should be emphasized that, in the case of biological membranes, problems in the estimation of fluidity arise from the presence of proteins and possible additives (e.g. cholesterol). Nevertheless, excimer formation with pyrene or pyrene-labeled phospholipids can provide interesting in-... 

Hresko R. C., Sugar I. P., Barenhoiz Y. and Thompson T. E. (1986) Lateral Diffusion of a Pyrene-Labeled Phosphatidylcholine in Phosphatidylcholine Bilayers/Fluorescence Phase and Modulation Study, Biochemistry 25, 3813-3823. 

Vanderkoi J. M. and Callis J. B. (1974) Pyrene. A Probe of Lateral Diffusion in the Hydrophobic Region of Membrane, Biochemistry 13, 4000—4006. 

J. M. Vanderkooi and J. B. Callis, Pyrene. A probe of lateral diffusion in the hydrophobic region of membranes, Biochemistry 13, 4000-4007 (1974). 

H.-J. Galla and E. Sackmann, Lateral diffusion in the hydrophobic region of membranes Use of pyrene excimers as optical probes, Biochim. Biophys. Acta 339, 103-115 (1974). 

The diffusion-related molecular processes occurring within a Cig stationary-phase have also been investigated using pyrene as a fluorescent probe [169]. Particular spectral bands were attributed to pyrene excimers formed in a diffusion-limited reaction. Rate constants for this formation were then used to estimate the microviscosity of the stationary-phase. A similar application of total internal reflection fluorescence... 

Figure 6 Rate constants for electron attachment to solutes in cyclohexane at 295 K. Solutes are 1—CCI4, 2— -dinitrobenzene, 3—benzoquinone, 4—o-dinitrobenzene, 5— nitrobenzene, 6—O2, 7— perfluoromethylcyclohexane, 8—pyrene, 9—anthracene, 10— biphenyl, 11— naphthalene, 12—CO2, 13— -difluorobenzene, 14—ethylbromide. Dotted line indicates calculated diffusion rate. References for rate data [19,108-111] references for electron affinities [112-115].

Excimer formation has been shown to be a diffusion controlled process 41-43 in which a sandwich or face-on configuration of the two interacting molecules is required.44-48 It has been deduced that intermolecular distance in the excimer state is smaller than for the same configuration with both molecules in their electronic ground states.44-48 Apart from pyrene, excimer-like emission has been observed from a wide range of aromatic compounds including many alkyl derivatives of such hydrocarbons43-47 and vinyl polymers.48-80... 

A fourfold decrease in the IDIIM ratio was observed for the 5.3% peracetylated pyrenylmethyl polyethylenimine derivative in glycerol compared to methanol. The higher viscosity of the glycerol limits the mobility of the attached pyrene group necessary to form excimer, decreases the association rate, and hence lowers ID/IM. These samples at 77°C showed essentially no excimer emission. Clearly, diffusion of the pyrene moieties attached to the polymer side chains is necessary for excimer formation. 

A detailed study of the structure of the aggregates of the ionic surfactants in polyelectrolyte networks was presented in Refs. [66,68]. The dynamics of the changes in the microenvironment of the fluorescent probe, pyrene, in slightly crosslinked networks of poly(diallyldimethylammonium bromide) (PDADMAB) during diffusion of sodium dodecyl sulfate (SDS) in the gel phase has been investigated by means of fluorescence spectroscopy. In Ref. [66], an analogous investigation was reported for complexes formal by the sodium salt of PMAA with cetyltrimethylammonium bromide (CTAB). 

Figure 36. Diffuse reflectance and excitation spectra of pyrene included in Na+X. Note the monomer and excimer emissions possess different excitation spectra. This difference may be the result of nonuniform distribution of pyrene molecules within cages.

Application of MIP chemosensors imprinted with two different templates can also improve detectability. For instance, polycyclic aromatic hydrocarbons (PAHs) have been determined using polymers molecularly imprinted with two different PAH templates [155]. Compared with MIPs of a single-template imprint, the detection signal of the resonant frequency change was enhanced by a factor of five and LOD for pyrene was as low as 60 pg L 1. This two-template largely improved detectability can be attributed to easier accessibility of the recognition sites through different diffusion pathways. 

We report on steady-state and time-resolved fluorescence of pyrene excimer emission in sub- and supercritical C02. Our experimental results show that, above a reduced density of 0.8, there is no evidence for ground-state (solute-solute) interactions. Below a reduced density of 0.8 there are pyrene solubility complications. The excimer formation process, analogous to normal liquids, only occurs for the excited-state pyrene. In addition, the excimer formation process is diffusion controlled. Thus, earlier reports on pyrene excimer emission at rather "dilute pyrene levels in supercritical fluids are simply a result of the increased diffusivity in the supercritical fluid media. There is not any anomalous solute-solute interaction beyond the diffusion-controlled limit in C02. 

In this paper, we present a preliminary analysis of the steady-state and time-resolved fluorescence of pyrene in supercritical C02. In addition, we employ steady-state absorbance spectroscopy to determine pyrene solubility and determine the ground-state interactions. Similarly, the steady-state excitation and emission spectra gives us qualitative insights into the excimer formation process. Finally, time-resolved fluorescence experiments yield the entire ensemble of rate coefficients associated with the observed pyrene emission (Figure 1). From these rates we can then determine if the excimer formation process is diffusion controlled in supercritical C02. 

Figure 9 shows the temperature dependence of the recovered kinetic rate coefficients for the formation (k bimolecular) and dissociation (k unimolecular) of pyrene excimers in supercritical CO2 at a reduced density of 1.17. Also, shown is the bimolecular rate coefficient expected based on a simple diffusion-controlled argument (11). The value for the theoretical rate constant was obtained through use of the Smoluchowski equation (26). As previously mentioned, the viscosities utilized in the equation were calculated using the Lucas and Reichenberg formulations (16). From these experiments we obtain two key results. First, the reverse rate, k, is very temperature sensitive and increases with temperature. Second, the forward rate, kDM, 1S diffusion controlled. Further discussion will be deferred until further experiments are performed nearer the critical point where we will investigate the rate parameters as a function of density. 

Arguments have been advanced elsewhere that the quenching of pyrene on silica gel by 2-halonaphthalenes is diffusion controlled. 1 This Is in contrast to the behavior In solution where the quenching Is Inefficient. It was postulated1 that on the surface the rate of separation of encounter pairs Is slowed to the point where the rate of quenching is determined by the encounter rate. Central to this argument is the... 

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