Pyrene excimer formation supercritical

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. 

A study has been made of both naphthalene/trimethylamine exciplex and pyrene excimer formation in supercritical COj and ethylene fluids. This is a most... 

As expected, pyrene has also been used to characterize supercritical fluid-cosolvent mixtures. For example, Zagrobelny and Bright used the Py polarity scale and pyrene excimer formation to study supercritical C02-methanol and C02-acetonitrile mixtures (160). Their results suggest the clustering of cosolvent molecules around pyrene. Similarly, Brennecke and coworkers measured Py values in CO2, CHF3, and CO2-CHF3 mixtures (43). 

Eckert s group was the first to report pyrene-excimer formation in supercritical fluids at pyrene concentrations significantly below those required in normal liquid solutions (Figure 19) (40,41). Taking into account the difference in viscosity and molecular diffusion in supercritical CO2 (150 bar and 35°C) as opposed to normal liquid cyclohexane, they concluded that the observed yield for excimer formation in CO2 exceeded what might be expected from the higher... 

Bright and coworkers investigated pyrene-excimer formation in supercritical fluids from a somewhat different angle using not only steady-state but also time-resolved fluorescence techniques (47,167). They measured fluorescence lifetimes of the pyrene monomer and excimer at a pyrene concentration of 100 p,M in supercritical ethane, CO2, and fluoroform at reduced densities higher than 0.8. Since the kinetics for pyrene-excimer formation was found to be diffusion controlled in ethane and CO2 and less than diffusion controlled in fluoroform, they concluded that there was no evidence for enhanced pyrene-pyrene interactions in supercritical fluids. The less efficient excimer formation in fluoroform was discussed in terms of the influence of solute-solvent clustering on excimer lifetime and stability. Experimentally, their fluorescence measurements were influenced by extreme inner-filter (self-absorption) effects due to the high pyrene concentration in the supercritical fluid solutions (35). 

Rollins et al. investigated the intramolecular excimer formation of 1,3-di(2-naphthyl)propane in supercritical CO2 (172) and compared the results with intermolecular pyrene-excimer formation recorded under similar conditions (46). Their results show that the ratio of excimer emission to monomer emission decreases gradually with increasing CO2 density (Figure 23), in a pattern that agrees well with that predicted from viscosity changes in terms of the classical photophysical model for excimer formation (35). In a comparison of l,3-di(2-naphthyl)propane and pyrene in the same fluid, the ratio of excimer emission to... 

J Zagrobelny, TA Betts, FV Bright. Steady-state and time-resolved fluorescence investigations of pyrene excimer formation in supercritical CO2. J Am Chem Soc 114 5249, 1992. 

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. 

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. 

Formation of pyrene excimer (a complex between a photoexcited and a ground-state pyrene molecule Scheme 4) is an extensively characterized and well-understood bimolecular process (35). Because the process is known to be diffusion controlled in normal liquid solutions, it serves as a relatively simple model system for studying solvent effects on bimolecular reactions. In fact, it has been widely employed in the probing of the solute-solute clustering in supercritical fluid solutions (40-42,46,47,160,166-168). (See Scheme 4.)... 

An excimer is a special case of exciplex—a complex between an excited-state molecule and a ground-state molecule, where the two molecules have different identities. Exciplex formation has been used as a model bimolecular process in the study of solute-solute clustering in supercritical fluid solutions. Brennecke et al. reported the investigation of naphthalene-triethylamine exciplex formation in supercritical CO2 at 35 C and 50°C (166). Their results show that the exciplex emission can be observed, even at low triethylamine concentrations (5 X 10 -5 X 10 M). Similarly, Inomata et al. investigated the formation of pyrene-dimethylaniline excimer in supercritical CO2 at 45°C (169). They... 

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