Pyrene excimer

Sahoo D, Narayanaswami V, Kay CM, Ryan RO (2000) Pyrene excimer fluorescence a spatially sensitive probe to monitor lipid-induced helical rearrangement of apolipophorin III. Biochemistry 39 6594-6601... 

Lehrer, S. S. (1997). Intramolecular pyrene excimer fluorescence A probe of proximity and protein conformational change. Fluorescence Spectroscopy 278, 286-295. 

C.H. Lochmiiller, A.S. Colborn, M.L. Hunnicutt and J.M. Harris, Bound pyrene excimer photophysics and the organization and distribution of reaction sites on silica, J. Am. Chem. Soc., 106 (1984) 4077. 

In order to study the molecular dynamics of the outer segments of a dendrimer, one pyrene moiety was selectively and covalently attached to one dendron of poly(aryl ester) dendrimers by Adams (in total three pyrene molecules per dendrimer) [24]. The fluorescence decay of pyrene in the THF solution of the labeled dendrimers provided details of the pyrene excimer formation, such as the excimer formation rate, the excimer decomposition rate constant and the equilibrium constant of the excimer formation. These parameters were utilized to evaluate the diffusional mobility of the dendrimer branches. 

The structure of the pyrene excimer is sandwich-shaped, with the distance between the planes of the two rings being of the order of 0.35nm (Figure 6.3). 

Kim SK, Bok JH, Bartsch RA et al (2005) A fluoride-selective PCT chemosensor based on formation of a static pyrene excimer. Org Lett 7 4839-4842... 

Bodenant B, Fages F, Delville MH (1998) Metal-induced self-assembly of a pyrene-tethered hydroxamate ligand for the generation of multichromophoric supramolecular systems. The pyrene excimer as switch for iron(III)-driven intramolecular fluorescence quenching. J Am Chem Soc 120 7511-7519... 

Yang JS, Lin CS, Hwang CY (2001) Cu2+-induced blue shift of the pyrene excimer emission a new signal transduction mode of pyrene probes. Org Lett 3 889-892... 

Schazmann B, Alhashimy N, Diamond D (2006) Chloride selective calix[4]arene optical sensor combining urea functionality with pyrene excimer transduction. J Am Chem Soc 128 8607-8614... 

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... 

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... 

Fluorescence spectra and quantum yields of pyrene in supercritical CO2 have been determined systematically as functions of temperature, CO2 density, and pyrene concentration. Under near-critical conditions, contributions of the pyrene excimer emission in observed fluorescence spectra are abnormally large. The results cannot be explained in the context of the classical photophysical mechanism well established for pyrene in normal liquid solvents. The photophysical behavior of pyrene in a supercritical fluid is indeed unusual. The experimental results can be rationalized with a proposal that the local concentration of pyrene monomer in the vicinity of an excited pyrene molecule is higher than the bulk in a supercritical solvent environment. It is shown that the calculated ratios between the local and bulk concentrations deviate from unity more significantly under near-critical conditions (Sun and Bunker, 1995). 

Excimer formation can serve as a sensitive probe of group proximities Excimers make evident the interaction of an excited molecule M, (typically an aromatic hydrocarbon), with a molecule in the ground state M producing an excited dimer Mf (or D ). The dimer must be formed within the lifetime of the excited species (e.g., for pyrene derivatives, about 100 nsec). For molecules such as pyrene, excimer formation and fluorescence are contingent on attainment of a well-defined steric arrangement in the dimer.41... 

Nanosecond Time-Resolved Fluorescence Spectroscopy in the Physical Chemistry Laboratory Formation of Pyrene Excimer in Solution 16... 

Birks 68) has proposed that the only change between the unexcited and excited pyrene pair is a reduction in the interplanar distance from 3.53 to 3.37 A, i.e. that the pyrene excimer is not a completely eclipsed sandwich pair either in solution or in the crystal. This proposal is consistent with the observed similarity of the excimer band position for the crystal and solution environment, and with the emission of excimer fluorescence from the crystal even at 4 K. For naphthalene, the greater separation and the nonparallel structure of nearest-neighbor pairs in the crystal apparently prohibits the formation of the sandwich excimer during the naphthalene singlet monomer lifetime. Thus, no excimer fluorescence is observed from defect-free naphthalene crystals. 

As noted earlier, the limiting lifetime of pyrene excimer fluorescence from concentrated solutions in PS and PMMA glasses was found to be the same as that of pyrene in cyclohexane solution. There have been no similar studies of naphthyl compounds in rigid glasses. Values of k and Q for the [2,6]-naphthalenophanes have not yet been determined for any solvent system. The bis(2-naphthyl) compounds have not been quantitatively characterized in rigid matrices, probably because excimer fluorescence is weak and difficult to detect under such conditions. Given such limited data, it can only be assumed that the values of QD and kD of 2-naphthyl excimers remain the same in rigid solution as in fluid solution. 

Laser Flash Photolysis. The instrumental set-up is similar to that used for transient absorption, except that there is no need for a monitoring beam. Figure 7.32 shows the rise and decay of the pyrene excimer in solution. 

A number of photochromic systems have been extensively investigated that undergo cis-trans isomerization (indigos, azo compounds) cleavage (spiropyrans), electrocyclic processes (fulgides, 1,2-diarylethenes) [8.229, 8.244, 8.245], For instance, cis-trans isomerization of a thio-indigo derivative allows the reading of pyrene excimer or monomer fluorescence [8.246]. The 1,2-dithienylethene system presents particularly attractive interconversion properties by photoreversible cyc-litation [8.245],... 

Figure 13.11 Potential energy vs. intermolecular distance for the first excited singlet and the ground state of pyrene excimer. Fronj T. Forster, Angew. Chem. Int. Ed., 8, 333 (1968). Reproduced by permission of Verlag Chemie, GmbH.

Steady-state fluorescence spectroscopy has also been used to study solvation processes in supercritical fluids. For example, Okada et al. (29) and Kajimoto and co-workers (30) studied intramolecular excited-state complexation (exciplex) and charge-transfer formation, respectively, in supercritical CHF3. In the latter studies, the observed spectral shift was more than expected based on the McRae theory (56,57), this was attributed to cluster formation. In other studies, Brennecke and Eckert (5,31,44,45) examined the fluorescence of pyrene in supercritical CO2, C2HSteady-state emission spectra were used to show density augmentation near the critical point. Additional studies investigated the formation of the pyrene excimer (i.e., the reaction of excited- and ground-state pyrene monomers to form the excited-state dimer). These authors concluded that the observance of the pyrene excimer in the supercritical fluid medium was a consequence of increased solute-solute interactions. 

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. 

ZAGROBELNY BRIGHT Investigation of Pyrene Excimer Formation... 

Figure 1. Energy-level diagram for excimer formation. Symbols represent hv, absorbed photon k emissive rate from the monomer species k, bimolecular rate coefficient for formation of the pyrene excimer k, unimolecular rate coefficient for dissociation of the excimer and k, emissive rate from the excimer species. Note no ground-state association is indicated.

Figure 8 shows a pair of typical time-resolved fluorescence decay traces for 100 / M pyrene in supercritical CO2 (Tr = 1.02 pr = 1.17). Note that the ordinate is logarithmic. The upper and lower panels show results for selective observation in the monomer (400 +. 10 nm) and excimer (460 + 10 nm) regions of the pyrene emission spectrum. Several interesting features are apparent from these traces. First, both decay processes are not single exponential. Second, the excimer emission has a significant contribution from a species that "grows in" between 30 - 75 ns this is a result of the excimer taking time to form (i.e., k in Figure 1). Third, the fits between the experimental data and the model shown in Figure 1 are good. Detailed analysis of these decay traces (10,11,21-26) yields the entire ensemble of photophysical kinetic parameters for the pyrene excimer 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. 

The formation of pyrene excimer in normal liquids has been well documented... 

As has been previously reported, when pyrene is adsorbed on silica gel there is evidence for ground state association which is not present In solution or the vapor phase.9-13 but which has been described as being present when pyrene is dissolved in a plastic medium. This is also a manifestation of surface inhomogeneity -some sites enhance the tendency to form a ground state bimolecular complex, whereas other sites contain isolated pyrene molecules. The interaction differences are sufficient to yield significant spectral shifts in absorption and the ground state complex emits with the characteristic pyrene excimer fluorescence. Fig. 5 shows a typical set of spectra Illustrating this association and Fig. 6 presents evidence that this observation Is not due to microcrystal formation. 

Figure 5. Ground state association of pyrene and pyrene "excimer" emission. Excitation and emission opectra of pyrene on silica gel ( excitation (observed at 390 nml...

Table II. Pyrene Excimer-like Emission on Decanol-Covered Si02 as a Function of Temperature...

Figure 10. Pyrene excimer formation on a decanol covered silica...

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