In the Laboratory
A Room-Temperature Emission Lifetime Experiment for the Physical Chemistry Laboratory1 Shelly Roalstad, Chad Rue, and Clifford B. LeMaster* Department of Chemistry, Boise State University, Boise, ID 83725 Carol Lasko Department of Chemistry, Humboldt State University, Arcata, CA 95521 Modern physical chemistry courses include an increasingly large spectroscopy component, but complementary laboratory experiments are harder to find. Certain spectroscopic methods, such as infrared and NMR, are used routinely in undergraduate experiments, but these suffer as educational demonstrations because of the complexity of the instrumentation or lack of student access to instrumental inner components. Therefore, they are simply “black box” techniques to many students, and while the students are able to complete the experiment successfully, their understanding of the actual processes involved may be marginal. The most direct spectroscopic experiments with respect to theory and equipment involve absorption and emission of visible light; but while there are many absorption experiments to choose from, there are few experiments involving the emission of light. An experiment that involves luminescence with a relatively long lifetime provides an opportunity to collect spectra, monitor emission intensity after excitation as a function of time, and work up the data to derive kinetic parameters. There are emission experiments that involve fluorescence (1, 2, for example), but fluorescence lifetimes are so short that most undergraduate laboratories lack equipment to do anything more than simply record spectra. Phosphorescence or delayed fluorescence, on the other hand, typically exhibits a longer lifetime that would allow the calculation of kinetic parameters, but liquid nitrogen temperatures are usually required. Some room-temperature phosphorescence experiments have been reported (3, 4), but the experiment described here combines ease of sample preparation, availability of instrumental components, and long emission lifetimes. In this experiment instrumentation is kept to a minimum, requiring only an excitation source (shuttered mercury lamp or nitrogen laser), a sharp cutoff glass filter after the sample to remove any exciting light, a detector, a chart recorder, and an oscilloscope. A monochromator rather than the filter is useful but not necessary. In place of the chart recorder and oscilloscope, computerized data acquisition and data handling techniques can be introduced if desired. For laboratories fortunate enough to have laser capabilities, we describe our extension of the experiment to use a nitrogen-pumped dye laser as the excitation source. The organic molecule fluorescein in boric acid is known to be a strong emitter at room temperature, but its two long-lived emission routes, delayed fluorescence and phosphorescence, are unresolved at room temperature (5). A related fluorescein compound, fluorescein mercuric acetate (FMA), appears more suitable. Resolution of the two emission routes at room temperature is quite
good, allowing examination of each emission process independently. Figure 1 shows the emission spectra of both fluorescein and FMA at room temperature. Using FMA in a boric acid glass as the sample, physical chemistry students collected a room-temperature emission spectrum and identified delayed fluorescence and phosphorescence based on comparison with published spectra (6). Wavelengths at the emission maxima were used to determine energies of the first excited singlet and triplet states in FMA, and a simplified energy level diagram for FMA was constructed. Finally, students measured emission intensity as a function of time, then fit the data to the first order rate equation I = I oe{ kt
(1)
where I is the intensity (voltage) at time t, Io is the intensity at time zero, k is the rate constant to be determined, and t is time. The observed lifetime, τ, is defined as the inverse of the rate constant: τ = 1/ k
(2)
Experimental Procedure
Sample Preparation Fluorescein mercuric acetate was obtained from the Eastman Kodak Company and sodium fluorescein from the Aldrich Chemical Company. Boric acid is available from many sources, including the Aldrich Chemical Company. Since fluorescein mercuric acetate is classified as highly toxic and sodium fluorescein is listed as an irritant, the instructor may wish to personally prepare the sample. Safety precautions and personal protection guidelines are outlined in the material safety data sheets for these compounds and are available from chemical suppliers. A bulk sample of the dye in boric acid was prepared by thoroughly mixing the appropriate quantities of dye and boric acid to give an approximately 10{5 M dye concentration. To accommodate our standard cuvette holder, a glass microscope slide was broken lengthwise. A small amount (approximately 0.5 g) of the dye/boric acid mixture was placed on one half of the slide; the other half was used as a cover. The slides were heated in a muffle furnace at about 250 °C for 3–4 min to melt the mixture. Very thin samples may be obtained by pressing down on the top slide while the boric acid mixture is still hot. The result is a yellow-green transparent glass. The same sample can be used repeatedly with no loss of emission intensity.
*Corresponding author.
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In the Laboratory
Figure 1. Room-temperature emission spectra of fluorescein (sodium salt) and fluorescein mercuric acetate.
Experimental Instrumentation This experiment was originally performed with a broadband UV light source and manual shutter for excitation, and a phototransistor with a 10-V DC power source was used for detection. The phototransistor was connected to the analog inputs of an AcJr data acquisition card (manufactured by Strawberry Tree, Inc.). A strip-chart recorder could be substituted for the data acquisition board. The decay data were saved to an ASCII file, imported to Microsoft Excel, and fit with the built-in function “Growth” (Microsoft’s term for exponential curve fitting) to determine the emission lifetime. The decay data presented here were acquired using a nitrogen-pumped dye laser (Photon Technology International [PTI], Model PL2300, PL201,) and laser dye LD 437 (PTI, PLD 437) operating at 437 nm as the excitation source. The emission was collected at right angles using a scanning monochromator (PTI, bandpass 2 nm), equipped with a mechanically ruled grating of 1200 lines/mm (Edmund Scientific) and a photomultiplier tube (PTI, Model 01614, bandpass 2 nm). The visible spectra were obtained by incrementing a stepper motor (Superior Electric Slo-Syn) on the monochromator by 1 nm after each pulse and subsequent acquisition. In this manner the emission maxima were determined, and the monochromator was set at these maxima to collect the emitted light for decay measurements. All experimental parameters (laser firing, monochromator setting, and signal input) were controlled using the data acquisition board described above and the PC Workbench TM software. The digitization rate for the decay data was 1000 points per second. Between five and ten data sets were collected and averaged to reduce noise.
Figure 2. Delayed fluorescence in FMA at 470 nm. Data from 0.5 seconds after the excitation pulse to five times the expected lifetime are plotted. The lifetime calculated from these data is 0.83 s (dotted line).
shows a student’s experimental emission intensity decay data collected at 470 nm and single exponential fit of the data. Analysis of typical student data using eqs 1 and 2 gives a value of 0.8 (± 0.3) s for the observed lifetime of the delayed fluorescence at 470 nm. The lifetime of the phosphorescence emission at 561 nm is 1.1 (± 0.5) s. The literature value is 1.0 (± 0.5) s (6) for room-temperature delayed fluorescence and ranges from 1.7 to 3.1 s for room-temperature phosphorescence (7). The range of the phosphorescence lifetime in boric acid is attributed to water content, pH, and inhomogeneities in the boric acid glass matrix, and is sample dependent (7, 8). Although the students’ value for the phosphorescence lifetime is somewhat shorter than that reported in the literature, this difference is attributed to sample condition and preparation. Acknowledgment We would like to thank the National Science Foundation for funding under the Instrumentation and Laboratory Improvement Grant Program (Grant #USE9251587). Note 1. A preliminary version of this article was presented at the 207th National Meeting of the American Chemical Society, San Diego, CA, March 14, 1994; paper CHED 48.
Literature Cited
Results At room temperature FMA exhibits delayed fluorescence and phosphorescence at two distinct emission maxima: 470 nm and 561 nm, respectively. The roomtemperature emission spectra of both fluorescein and FMA in the visible region were collected by physical chemistry students and are shown in Figure 1. Figure 2
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