Pyrene excimer formation reduced density

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). 

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. 

On the low density side of these traces (below pr = 1), the rise in WIm (as one approaches a reduced density of unity) is a result of pyrene continually being solubilized by the fluid (Figure 5). This observation is consistent with Johnston s report of pyrene solubility in CO2 (17). Again, recall that excimer formation is a generally believed (10,11) to be a bimolecular process, i.e., the rate is concentration dependent. Thus, because the actual analytical concentration of solubilized pyrene in the solution is lower due to solubility, the amount (fraction) of excimer is "artificially" lower. Thus, in the low density region (pr = 0.5 -0.8) the observed trends are simply a result of solubility effects. However, solubility does not help to explain the results seen over the remainder (pr = 0.8 - 1.8) of the density range investigated. 

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. 

---