Chapter 6
Investigation of Pyrene Excimer Formation in Supercritical C O 2
JoAnn Zagrobelny and Frank V. Bright
Downloaded by UNIV OF ROCHESTER on August 31, 2017 | http://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0488.ch006
Department of Chemistry, Acheson Hall, State University of New York at Buffalo, Buffalo, NY 14214
We report on steady-state and time-resolved fluorescence of pyrene excimer emission in sub- and supercritical CO . 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 CO . 2
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For any pure chemical species, there exists a critical temperature (TJ and pressure (P ) immediately below which an equilibrium exists between the liquid and vapor phases (1). Above these critical points a two-phase system coalesces into a single phase referred to as a supercritical fluid. Supercritical fluids have received a great deal of attention in a number of important scientific fields. Interest is primarily a result of the ease with which the chemical potential of a supercritical fluid can be varied simply by adjustment of the system pressure. That is, one can cover an enormous range of, for example, diffusivities, viscosities, and dielectric constants while maintaining simultaneously the inherent chemical structure of the solvent (1-6). As a consequence of their unique solvating character, supercritical fluids have been used extensively for extractions, chromatographic separations, chemical reaction processes, and enhanced oil recovery (2-6). Many of the present models used to describe fluid-solid phase equilibria require one to assume that the solute is at infinite dilution. That is, researchers have often assumed that solute-solute interactions are nonexistent. Recently, Brennecke et al. used the fluorescent probe pyrene to investigate the possibility of solute-solute interactions in C 0 , C H , and C F H (7-9). Pyrene is an interesting probe because it can form a characteristic excited-state dimer (excimer) during its excited-state c
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0097-6156/92/0488-0073$06.00/0 © 1992 American Chemical Society
Bright and McNally; Supercritical Fluid Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
Downloaded by UNIV OF ROCHESTER on August 31, 2017 | http://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0488.ch006
74
SUPERCRITICAL
FLUID T E C H N O L O G Y
lifetime (10,11). In normal liquids, this bimolecular process is diffusion controlled (10,11) and is often described using the energy-level diagram shown in Figure 1. For all liquid systems studied to date, significant excimer is only observed at pyrene concentrations in the high millimolar range. Brennecke et al. (7-9) observed pyrene excimer in pure supercritical fluids when the pyrene concentrations were in the high micromolar range and concluded that there were solute-solute interactions occurring on the submicrosecond time scale. However, the origin of the observed excimer emission was not completely determined in this earlier work. Specifically, it was not clear if the apparent enhancement in excimer (on a per mole pyrene basis) was a consequence simply of: 1) the increased diffusivity in the supercritical fluid (i.e., the excited- and ground-state pyrene molecules could diffuse faster, thus increasing the probability of interaction during the excited-state lifetime) or 2) actual static (ground-state) solute-solute interactions. To address these issues one must use steady-state and time-resolved fluorescence in concert (10,11). In this paper, we present a preliminary analysis of the steady-state and timeresolved fluorescence of pyrene in supercritical C 0 . In addition, we employ steadystate 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, timeresolved 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 C0 . 2
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Experimental Instrumentation. A schematic of the instrumentation used for the time-resolved fluorescence experiments is shown in Figure 2. A pulsed nitrogen laser (337 nm; Laser Science, model LS 337) is used for excitation, and a 340 nm band-pass filter (BPF) is used to eliminate extraneous plasma discharge. Typically, we operate at a repetition rate of 10-15 Hz. A portion (10%) of the laser is split off using a beam splitter (BS) and directed to a reference photodiode (PD) that serves to trigger the boxcar averager. Prior to entering the sample cell, the laser beam is conditioned with a 150 mm focal length lens. The sample chamber consists of a light-tight aluminum housing and was constructed in house specifically for our supercritical fluid high pressure optical cells (12). These cells are constructed from 303 stainless steel with four fused-silica windows oriented at 90° to one another. The window seals are made using a set of in house designed lead and brass ring seals and the total cell volume is about 5 mL. Initially, viton o-ring seals were used, but a fluorescent impurity was continually extracted into the cell. Attempts to use Teflon seals alleviated the impurity problems; however, pyrene sorbed strongly to or into the Teflon seals. The metal-based seals alleviated all these problems. Following excitation the resulting fluorescence is collected by a lens (50 mm focal length) and collimated, filtered using a bandpass filter (10 nm bandpass), and focussed with a second lens (50 mm focal length) onto the photocathode of a
Bright and McNally; Supercritical Fluid Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
6.
ZAGROBELNY & BRIGHT
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Investigation of Pyrene Excimer Formation
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Downloaded by UNIV OF ROCHESTER on August 31, 2017 | http://pubs.acs.org Publication Date: May 8, 1992 | doi: 10.1021/bk-1992-0488.ch006
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Ground State Figure 1. Energy-level diagram for excimer formation. Symbols represent: h*>, absorbed photon; 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. Temperature
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