I J. N. Demas University of Virginia CharlottesviHe. Virginia 22901
I I
Luminescence Spectroscopy and Bimolecular Quenching A physical chemistry experiment
For the past two years we have used a multifaceted luminescence experiment in our physical chemistry laboratory. A relativelyhw-cost ~~ectroflunrirneter is used to carry out elementary emission measurements, to study some analytical aspects of fluorimetry, to measure a relative spectral distribution for a light source, and to determine the rate constant for an exceedingly fast bimolecular reaction, the deactivation of an excited state by a metal complex. The experiment, having something for almost everyone, is very popular in our second-semester laboratory. The entire experiment is based around one transition metal complex, tris(2,Z'-bipyridine)ruthenium(II) chloride, [R~(hipy)~]Cln. This luminescent complex is an important new photosensitizer in organic and inorganic photochemistry (1-7) and has been used as the basis of an interesting oscillating reaction with a luminescent indicator (8). Theory
For monochromatic exciting light, the fundamental equation (9,10) of luminescence spectroscopy is (1) O(X) = K'I(h)Q(X)F(X) where O(X) is the observed luminescence intensity when the sample is being excited a t wavelength X with an incident intensity (quantafs) of NX); the luminescence quantum yield, Q(X), is the probability that once a molecule has absorbed a quantum of wavelength X i t will emit (Q 5 1). F(X) is the fraction of the exciting light absorbed by the sample, and K' is a proportionality constant. For most pure substances the emission spectrum is independent of the excitation wavelength, although in a few rare instances it does vary with X (11, 12). Therefore, K' is usually a constant as long as the detector viewing wavelength, the system gain, and the geometry do not change. For right angle viewing in the normal spectrofluorimeter, F(h) is given by Beer's Law (9,10,12) F(X) = - 10-dAICl = - e-2.30L'AC! (2)
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F(X) 2303dX)Cl r(X)Cl 0.5). The highly reactive singlet oxygen formed can be detected hv its reaction with a varietv of eood singlet oxygen scavengers (e.g. tetramethylethylene, trimethvlethvlene. . (4). . . The ~ r o d u c t sare " . 1.2-cvclohexadiene) . all easily analyzed by nmr, ir, or gc. A number of interesting kinetic or mechanistic experiments are possihle. Organics, such as stilhene, can also he used as energy transfer quenchers and, if desired, the sensitized isomerization can also be followed (5). I t should not, however, be necessary to use the elaborate degassing procedure of the original reference; a stream of solvent saturated Nz should be adequate. + also arise by electron Quenching of * R ~ ( b i p y ) 3 ~can transfer to organics (6) or inorganics (Fe3+ (6), T13+ (19), and C O ( C ~ O ~(lb), ) ~ ~the - latter two with efficient destruction of the auencher). A variety of flash photolysis experiments is po&ble; the thermal back reaction to form the original clound state molecules following electron transfer canbe readily followed ( 6 ) . Numerous other metal complexes are also good quenchers: Ni(CNh2-, C O ( C Z O ~ ) ~Cr(C~04)3~-, ~-, Cr(CN)a3-, and in methanol Cu(acac)z (7, 15,20,21). I t is, however, necessary with some complexes absorbing in the excitation-emission region to apply a correction for the trivial absorption of light (Ib). The primary ionic strength effects can also he studied (20, 23). I t is best to use a fixed concentration of quencher and vary the ionic strength with KN03 or KC104. The failure to get really good fits to the theory with anionic quenchers can be traced in part to the presence of quenching both by diffusion and by formation of a non-luminescent donor-quencher ion-pair (22,23). Indeed, i t is possible t o ohtain ion-pairing constants by comhining quenching data ohtained by intensity and decay time measurements (22,24). Indeed, virtually any molecule which has excited states that lie below -50 kcal or is a good electron acceptor ~ + . studies on the quenchshould quench R ~ ( b i ~ y ) 3 Thus, ing and sensitized photochemistry could make an interesting special project or senior thesis. This point is especially cogent since work on quenching of Ru(bipy)z2+ and related complexes is currently one of the forefront areas of solution photochemical research (1-7). Althoueh we have not tried it, a filter fluorimeter could probahlylbe used for both the analytical and luminescence ouenchine studies. The optimum excitation wavelength for t'he analysis would have to be determined euperimekally, but 436 nm should probably he used for the quenching study to minimize trivial absorption of exciting light. ~~~
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Procedure
An Aminco SPF 125 spectrofluorimeter equipped with a xenon lamp and R-106 phototube was used, although any similar spectrofluorimetar would be adequate. An adaptor for cylindrical cuvettes permitted use of very inexpensive Pyrex cells and thus eliminated a major expense due to student breakage of square cuvettes.
A smaller hlank and greater sensitivity would, however, have resulted with the mare expensive cells. M) of Ru(hipy)sChGHzO (G.FrederA stock solution (4 X ick Smith Chemical Co.) was suoolied as well as a 0.500 M .. K4Fe(CN)6solution (wed in our magnetic auseeptibiliry experiment). An absorption spectrum of the stock Rulbipy)?' was SUP. plied since we do nut have a scanning absorption instrument in the laboratory. The Luminescence spectrum was run on the undiluted stock Ru(bipy)32Cwhile the excitation spectrum was m using a g s dilution. For the analytical plot, this diluted stack was sequentially diluted to yield ih, gs, 'hzs, and 'kzs of 1.6 X M; all five solutions were used. For the emission spectrum, h,,= 450 nm with 4-mm slits (largest) and the emission monochromator was kept at 2 mm or less. For the excitation spectrum, 4-mm emission and 1-mm excitation slits were used. Most students recognized that the largest excitation and emission slits yielded the highest sensitization analytical sensitivity. Both emission and excitation spectra were ohtained manually, point-by-point. All data were corrected for the solvent hlank. For the quenching experiments,six 25.1111 volumetric flasks were prepared, each with 5-ml aliquots of the 4 X 10W M Ru(bipy)a2+ stock. A diluted stock KSe(CNh was prepared and 0, 1, 2, 3, 4, and 5 ml of it were added to a volumetric flask and diluted to the mark; the highest concentration of Fe(CN)e4-was limited to -2 X M. The entire experiment including warm up uf the instrument requires ahout 2 hr, even for the less competent. If would therefore be possihle to include additional varintions. Acknowledgment
Acknowledement is made to the Deoartment of ChemisCottrell Research try, the ~ e s e a r c hCorporation for Grant, the donors of the Petroleum Research Fund, administered hy the American Chemical Society, and the National Institutes of Health for a Biomedical Support .. Grant administered by the University of Virginia. I especially thank the students for their patience in the early developmental stages of this experiment.
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Literature Cited (1) ( a ) Dcmas. J. N.. and Adamson, A. W., J Amen Chem Soe.. 5159 (1973).
93, U W (1971): ( b ) 95.
(21 Natarsjan.P.. and Endicott, J. F.. J. Amer Chpm. Soc., 91.3635 (19721. (31 Gafney, H., and Adamaon, A. W., J. Arne Chem. Soe., 94,8238 (1972). (4) D e m . J. N.. Dismente, D., and Harria, E. W.. J. Amer Chem. Soc, 95, 8864
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Wrighton, M., and Markham, J., J Phys. Chsm., 77.3042 (19731. Baek, C. R., Meyer, T. L a n d Whitten, D. G., J Amen Chem Sm., 95,4710 (19741. Balzani. V.. Monei. L... Manfrin. M. F.. Bouctta. F., and Lawenre. G. S., Coord. Chem R."., in p.mr. Dcmas, J. N., and Diemente. D., J. CHEM. EDUC.. 50,357 (1973). Weber, G., and Tealo, F. W. J., Trans. FomdqvSoe., 54, MO (1958). Demas, J, N.,andCmsby,G, A. J Phys Cham., 15,991 (19711. F1etchcr.A. N., J. Phya. Chem.. 72.2742 (1968). Parker, C. A.,"Photolumine.eenaofSol"fion.:Elaevier. N w Y0.k. tM8. Arsauer. R. J.. and White.. C. E... A w l . Chsm.. 36.368 (19MI. (14) Dema5.J. N.,submitted. (15) Bollctta, F.,Maestri, M., and Moggi, F., J. Phya. Chem., 77,861 (19731. (16) Debp. P., Tmw. Elecfmchsm. Soc., 82,265 (1942). (17) Eigen, M., Druse, W.. Maa.8, G.. and Dc Maeyer, L.."Pin Readon Kinet4,s " Vnt 2 (Editor: Porter. G.I. o. 285. .~ ..MaeMillsn Co... 1964.~. (181 oemas. J. N.,*"d Ha"i8, E. W.. work in progress. (19) Lamenee,G. S., and Balzani.V..Inore. Chem., t3.2976(19741. (20) Domas, J. N..and Addington. J. W.. submitted. (21) Fujita, I., and K~baysnhi,H., Ber Bum~nges.Phys. Chem.. 76.115 (19721. (22) Dernas. J. N..and Addington. J. W., J. Amer Chem Soc., 96.3663 (1974. (23) Bolleta. F., Maestri, M., Moggi, L.,and B&.%ni, V., J. A m r . Chem Soe, 95.7866 (19731. (24)BolMta. F..Maestri, M., Mogbi, L.. and Balnani. v., J. Phys Chem.. 78, 1374
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Volume 52, Number 10, October 1975 / 679
