Experiment 3: Radiative Lifetime Determination by Laser-Induced Fluorescence

Transcription

1 Physical Measurements - Core I GENERAL REFERENCES Experiment 3: Radiative Lifetime Determination by Laser-Induced Fluorescence 1. Barrow, G. M. Physical Chemistry McGraw-Hill Book Company: New York, 1988; Demas, J.N. Luminescence Decay Times and Bimolecular Quenching, J. Chem Educ. 1976, 53, PURPOSE To determine the radiative lifetime of a complexed metal ion in aqueous solution and to determine the quenching efficiency of oxygen dissolved in the solution. INTRODUCTION The radiative lifetime of an electronically excited specie can be an extremely valuable probe in determining the local electronic environment surrounding the emitting molecule. For example, fluorescent chromophores are bound to proteins and other large molecules of biochemical interest in order to probe and identify the active binding sites within the molecules. Shifts in the fluorescent wavelengths and changes in the fluorescent lifetimes provide information about the environment of the site. In this experiment we will perform an introductory radiative decay experiment to determine the effect of a perturbing molecule (O 2 ) on the radiative lifetime of a metal complex. THEORY Here, we follow the development of Barrow (1). The sequence of elementary reactions relating to the excitation and decay of a hypothetical molecule D are: D + hν D* Excitation (1) k e D* D + hν Fluorescence (2) The fluorescent lifetime is defined as the time τ observed for the concentration of D* to fall to 1/e of its initial value once the excitation has ceased (e.g., pulsed excitation). The

2 lifetime may be shortened by the presence of a variety of substances. This shortening of the lifetime is referred to as quenching the fluorescence and may occur due to several intermolecular and intramolecular interactions. Quenching may be modeled as a unimolecular or bimolecular process: k q D* D + thermal energy (3) Quenching k 2 D* + Q D + Q + thermal energy (4) The rate constant for the radiative emission (eq. 2) is k e, that for the other first order nonradiative quenching process (eq. 3) is k q, and that for the bimolecular quenching by specie Q (eq. 4) is k 2. The rate equation for the overall process is: - d[d*] dt = (k e + k q + k 2 [Q]) [D*] (5) Solving this first-order differential equation results in a simple expression describing the elapsed time evolution of [D*] following the termination of the excitation. Using τ to symbolize the observed lifetime, where [D*] t = [D*] o e -(t/τ) (6) τ = Taking the natural logarithm of equation 6 results in: 1 k e + k q + k 2 [Q] (7) ln ([D*] t ) = ln [D*] o - (1/τ) t (8) Equation 8 provides a convenient analysis to determine the lifetime τ. Plotting the natural logarithm of the fluorescence amplitude (a measure of D*) versus time t, produces a straight line plot of intercept, ln [D*] o, and slope, -1/τ. In the absence of any quencher, the lifetime, reported as τ ο, is given by τ ο = 1 k e + k q. (9)

3 Combining and simplifying equations 7 and 9 to: 1 " = 1 " o + k 2 [Q], (10) provides a second convenient equation useful in determining the basic parameters of the experiment. Plotting the reciprocal of the observed lifetime 1/τ versus [Q] produces a straight line with intercept 1/τ ο and quenching rate constant k 2. An alternative development can be made for the experiment using continuous excitation by invoking standard steady state assumptions. Here the continuous fluorescence signal φ is proportional to the concentration of D* (φ ο represents the signal in the absence of quencher Q). The resulting equation, the Stern-Volmer equation, is " o " - 1 = k 2 k e + k q [Q] = K SV [Q], (11) where the Stern-Volmer quenching constant, K sv, is given by (via eq. 9, τ ο = 1/[k e + k q ] ): K SV = k 2 τ ο. (12) Examining equation 11 illustrates that the Stern-Volmer quenching constant, K SV, represents the ratio of the second-order deactivation rate constant to the sum of the firstorder rate constants. In our experiment we will directly measure τ for three complexed metal ion solutions (Ru(bipy) 3 2+ ) of different quencher (O 2 ) concentrations to evaluate τ ο, k 2, and K SV. The excitation will be caused by the absorption of nitrogen laser light at nm. This is a pulsed source of short duration (~5-7ns) and corresponding high power. EXPERIMENTAL The basic experimental arrangement is shown in Figure 1 note a monochromator has recently been inserted between the sample and the PMT. Locate all the pertinent parts of the figure on the laboratory bench top. Turn on the nitrogen laser (power only) according to the procedures posted on it and allow it to warm up for at least 10 minutes. Since the water cooling for this laser is part of another apparatus, the instructor will ensure that the cooling water is adequate.

4 Physical Measurements - Core I - Exp. 4 - Figure 1. Laser-Induced Fluorescence Lifetime Apparatus

5 Cuvette Preparation: 1. Obtain the ~1 x 10-4 M stock solution from the instructor as well as the PMMA cuvette. 2. Fill the cuvette 2/3 full of the stock solution using a Pasteur pipette and carefully clamp it into the gas bubbling apparatus using the small 3-prong clamp - do not overtighten! Do not lower the Pasteur pipette into the solution (yet). 3. Connect the open tube at the top of the flask to the nitrogen supply and slowly adjust the flow so that only a few bubbles pass into the water in the flask per second. After this adjustment, slowly lower the upper Pasteur pipette into the cuvette to deoxygenate the solution. Allow nitrogen to flow for at least 5 minutes. 4. Following the gas saturation of step 3, remove the cuvette from the clamp, place it on the lab bench top and using a small square of parafilm, seal the cuvette. Do not allow the parafilm to extend more than 1/8 down its sides. 5. You will repeat steps 3 & 4 (after the recording the fluorescence decay - below) with the compressed air supply (next to the nitrogen supply) and you will carry the bubbler into the Analytical lab to oxygenate the solution (remove the green gas supply line to the torch and connect the latex tube in its place). Complete instructions on the use of compressed gas cylinders and pressure regulators will be given by the instructor. Recording the Fluorescence Decay Spectrum. 1. Turn on the computer system according to the posted instructions as well as the HP digital storage oscilloscope (rocker switch at upper center rear). The scope should be already set at the basic parameters required for the experiment. 2. Open the top screw cap of the black ABS tubing apparatus and carefully place the cuvette into the sample holder inside. Replace the cap and screw it down a few turns - there is no need to make it very tight - please don't. Set the monochromator to 610 nm to pass the fluorescence onto the PMT. 3. Allow nitrogen to flow into the laser (250 Torr on the laser gauge) and switch from Standby/Reset to 5 pulses per second. Open the laser shutter (fluorescent orange) by pulling it toward you until it hits its screw stop. Make sure the scope is triggering (triggering message in the upper left hand corner of the screen). 4. This is important - before applying high voltage to the extremely sensitive photomultiplier tube (PMT) check to make sure all caps are in place on the apparatus. EXPOSING THE PMT TO ROOMLIGHT WITH THE HIGH VOLTAGE ON WILL DESTROY IT AND POSSIBLY OTHER PIECES OF EQUIPMENT CONNECTED TO IT. THIS PARTICULAR PMT IS WORTH ~$1000!

6 5. While watching the scope screen apply ~-1000 V to the PMT. An inverted decay pattern should be visible. The instructor will provide you instructions as to the scope and HV adjustment. DO NOT EXCEED V without instructor authorization. Basically you want to get a decay pattern that fills the screen. 6. When a 'good' signal has been achieved, save the waveform to memory location 1, using the SAVE WAVEFORM button, followed by the store button along the right side of the screen (once this menu 'pops-up'). NOTE - this instruction may change during this semester - ask the instructor! At this point, you will transfer the data to the computer following the directions of the instructor. 7. TURN OFF THE HIGH VOLTAGE TO THE PMT. SHUT THE LASER SHUTTER. PUT THE LASER IN A STANDBY MODE BY SWITCHING TO STANDBY/RESET AND SHUTTING OFF THE NITROGEN VALVE ON THE LASER. 8. Remove the cuvette and proceed to the compressed air saturation of the solution in the cuvette. 9. Repeat these procedures for the air saturated and oxygen saturated runs of your sample. 10. After all three runs have been completed, turn all instrumentation completely off using the posted instructions. 11. Be sure to record the ambient pressure of the laboratory using the barometer on the east wall. You need this for Henry s law. CALCULATIONS: 1. Calculate the concentration of O 2 (M) in the oxygen-saturated and air-saturated samples using Henry's Law, i.e., M oxygen m oxygen = k H P (in mm Hg), where the Henry's law constant, k H, for oxygen is 1.68 x 10-6 molal / mm of Hg at 25 C. The molarity is approximated closely by the molality for these solutions - why? Use your recorded lab atmospheric pressure and % as the oxygen volume content of air for your air sample. What % should you use for your pure oxygen sample? 2. Using your data files and an electronic spreadsheet program (least-squares) - determine the lifetime of the fluorescence for each run using Equation 8.

7 3. From the results of step 2 and using Equation 10, determine τ ο, k 2, and K SV and the associated errors in each of these determinations. Be sure to indicate the correct units for each parameter.