Harry D. Gafney Queens College of CUNY Flushing, New York 11367 and Arthur W. Adamson University of Southern California
I
Chemiluminescence riment
The dependence of man's way of life on light energy is well documented (I). This dependence, reflected in man's 'earliest religions and writings, has developed within him a long fascination with lieht. its sources, and its effects on his su&undings. This fas&tion has been stimulated by the natural phenomena of emission of light from various living known as biolumiplants and animals. These nescence, are a specific type of a general categoly of chemical reactions known as chemiluminescent reactions (2).
A chemiluminescent reaction is a chemical reaction in which chemical energy is converted to electronic excitation energy, thus forming an excited state intermediate which deactivates by the emission of a photon of light. Perhaps the best known example of a chemiluminescent reaction is the intense blue light observed upon alkaline oxidation of luminol (3). Comparison of the chemiluminescence and fluorescence spectra indicate the emitting level is the excited singlet state of the dianion of 3-nitrophthalic acid (3). The identification of the luminescent species is of principal interest in a mechanistic study of a chemiluminescent reaction. This is usually accomplished by matching the photoluminescent spectrum of a product or suspected product to the chemiluminescent spectrum. Identification of the excited state intermediate formed in the reaction can then he used as a probe of the energetics and electronic changes occurring during the chemical reaction. For example, consider the chemiluminescent reaction between the hydrated electron, e-(aq), produced by the gamma radiolysis of water, and trishipyridalruthenium(III), Ru(hipyk3+ (4) (eqn. (1)) The spectrum of the light emitted during this reaction, huemitted, is identical to the phosphorescence spectrum of R ~ ( b i p y ) , ~ +Studies . of the photoluminescence of Ru(hip y ) 2 + have led to the assignment of the bright orange phosphorescence, hvem,tt,d, to a transition from the metal to ligand triplet charge transfer state of Ru(bipy)s2+ to the ground state (5). i.e., a r* to tzg transition. The chemiluminescent yield of reaction (11, i.e., the number of
480 I Journal of Chemical Education
Figure 1. A schematic representation of the relative energies of the + in ground and excited states of Ru(bipy)?+ and R u ( b i p y ) ~ ~involved the chemiluminescent reaction.
photons emitted per number of reactions between e-~,,, and Ru(hipy)33+, is within experimental error of the quantum yield of emission obtained from the photoluminescence of Ru(bipy).?+. The equivalence of the photoluminescence and chemiluminescence spectra indicates the reducing electron, e r i a q , , does not directly enter the metal tz, orbitals hut rather enters a ligand n* orbital. The equivalence of the chemiluminescence yield and the photoluminescent quantum yield indicates, within experimental error, that all e-,,,, initially enter the ligand s * orhital. The intermediate thus formed containing the reducing electron in the s * orbital then relaxes to the ground state of Ru(bipy)32+ by an intramolecular ( s * to tz,) electron transfer resulting in the emission of a photon. The chemiluminescence observed during the reaction can he used as a probe of the intimate mechanism of a chemical reaction (6). Although a number of chemiluminescent experiments have been published in this Journal (7), there have been
NaBH4 solution is placed in
few involving transition metal complexes. This paper describes a very simple, bright chemiluminescent reaction between trisbipyridalmthenium(IIl), Ru(hipy)a3+, and sodium horohydride, NaBH4. T h e bright orange phosphorescence, associated with the transition from the metal to ligand triplet charge-transfer state of R u ( b i ' ~ y ) 3 ~to+ the gmund state, is readily observed in a dimly lit room. As previously mentioned, studies of the photoluminescence of Ru(hipy)a2+ have established t h a t the emission process is associated with the transfer of electron density from a ligand centered r* orbital to a metal tz, orbital (5). T h e equivalence of the emission spectra observed in the photoluminescence and chemiluminescence experiments suggests t h a t the reducing electron initially p p u lates a ligand r* orbital rather than direct transfer to a metal tzg orbital. T h e energetics of the reaction can be understood from a consideration of Figure 1. The emitting state, 3Ru(hipy)az+, is 2.2 eV higher in energy t h a n the ground state of R ~ ( b i p y ) 3 ~The + . free energy change associated with the reversible oxidation of Ru(bipy)az+ to Ru(hipy)33+, 1.2 eV, is obtained from electrochemical data (8). From Figure 1, it is apparent t h a t a n additional 1.0 eV of energy is necessary t o populate the emitting level. Thus the reduction potential of the electron donor must b e of sufficient magnitude, 21.0 eV, such t h a t sufficient energy is available during electron transfer t o populate the emitting excited state. In addition to energetic requirements, kinetic requirements must also be satisfied t o observe chemiluminescence. T h e rate of reaction producing t h e emitting level must be rapid enough and the lifetime of the emitting level long enough t o form a sufficient steady state concentration of t h e excited state species, 3 R ~ ( h i p y ) 3 ~ such +, t h a t emission intensity will be visible. Considering the energetic and kinetic requirements t h a t must be met, it is not surprising t h a t chemiluminescent reactions are not common.
produces a vortex in the solution. A funnel with folded filter paper is placed in the neck of the flask. The stem of the funnel should he 1 in. above the NaBHI solution. The freshly prepared Ru(hipy)s3+ solution is poured onto the filter. As the Ru(bipy)2* drips into the NaBHl solution the luminescence is visible as hright orange flashes in the vortex of the NaBHa solution. The second procedure, where a striking hright orange hand of luminescence is observed, makes use of the apparatus shown in Figure 2. The separatory funnel, labelled A in Figure 2, and the body of the apparatus are filled with the NaBH, solution. Freshly prepared Ru(bipy)2+ is carefully dicanted into the other separatory funnel to prevent clogging the valve with PhOz. The valves at the base of the apparatus and at the base af the separator9 funnel containing NaBH* are adjusted to give a slow steady flow of solution through the apparatus. The valve on the separator9 funnel containing the Ru(hipy)2+ is slowly opened until a n orange ribbon of light is observed to traverse the barrel of the apparatus. It may be necessary to partially open the valve at the top of the barrel to allow gases to escape. The reaction mixture can be recovered and a technique for the recovery of the Ru(hipy)?+ could be worked out. Discussion
Experimental
Ru(bipy)~Clzwas obtained fmm G . Frederick Smith and used without further purification (91. An alternative supplier of the reagent, which costs approximately $25/g, is J. T. Baker. A solution of 10-3M in Ru(bipy)aCIa and 1.OM in HzS04 is prepared. At least 10 ml is necessary for each experiment. However this solution is stable and a larger volume can be prepared and stored indefinitely for later use. R ~ ( h i p y ) ~ 3is*prepared in sifu by a lead dioxide, PhOz oxidation. Solid Pb02 is added to the Ru(bipy)s2+ solution and the reaction mixture is shaken. The initial bright orange solution rapidly becomes dark green, characteristic of Ru(hip y ) ~ ~Although +. R u ( b i p y ) ~ ~is+indefinitely stable, Ru(bipy),ai is slowly reduced by a reaction with the solvent. The solution half-life of R u ( b i ~ y ) ~depends s+ on the pH of the solution and in this acidic solution is approximately 2 hr. Thus, the solution should be used immediately after preparation for the maximum brightness of the luminescence. A suspension of NaBH4 in approximately 50 ml of water is prepared hy adding slightly more NaBHd than necessary for a saturated solution. Since NaBH* undergoes hydrolysis, the reagent solution is prepared just prior to use and one or two pellets of NaOH are added to retard hydrolysis.
Figure 2. Apparatus used for the chemiluminescence demonstration.
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It is not known whether the reductant producing the excited state, 3Ru(bipy)a2+, is BH4- or a hydrogen atom, a product of the one electron oxidation of BHI-. In either case explanation of the reaction using Figure 1 is valid and illustrates how spectral data can he used in conjunction with other thermodynamic data (electrochemical data) to understand the energetics of the reaction. T h e demonstration is a n illustration of a chemiluminescent reaction of a transition metal complex, hut more importantly illustrates how chemiluminescence can be used to probe the energetics and mechanism of a chemical reaction.
Literature Cited Ill (a) Arnon. D. I.. Sci. Amsr., 2M. l M (1960): (bl Calvin, M.. and Bassham. J . M.. "The Patch of Carbon in Photo~ynthesis." Pronfice-Hail. Englewood Cliffs, N.J.. L9Si: i c i Parks. R. B.. J . CHEM. EDUC.. 39. 424 119611: (dl While. E. H.. " ~ i i h r a n dL X ~ ; " i~diiars:McEImy. W. D., and Giar3, B.) The .John Hopkin3 PIOII. Baltimore, Md.. 1961. p. 169. (2) (a) H a s . .I. W., J. C H E M EDUC., 11, 396 11967); (hl Rsuhut. M . M.. Accts. Cham. Reg., 2, 30 11969): Ic) White, E. H.. and Roswell. D. F.. Acclr. Chem. R M ., I.SdiL9701: M . A c r f 8 . Chem. R e s . 2. 301 11969). ~ . ,. ~ id1 , ~ . .~Hercu1es.D. (3) White. E. H.. and Burrey. M. M.. J . Amen Chsm. S a c . 86. 941 119641, and relel~~
sneerthelei".
Demonstration
Althnneh direct addition to the ~ of the~ Ru(hiovL3+ ~~ ~ ~ - ~.,,, solution NaBH* solution produces a bright orange flash, considerable fmthing occurs. Two procedures have been developed to circumvent the problem of frothing. The first procedure makes use of readily availahle laboratory glassware. Approximately 50 ml of the
125-ml Erlenmeyer flask containing
a magnetic stirring bar. The solution is stirred at a rate which
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(4) Manin, J. E.. Hart. E. J.. Adamson. A. W.. Gainey. H. D.. and Hnlpern, .I.. J A m r r Cham. Sor.. 91,9238 11972). (5) oernas. J. N..snd Cmsbv. G . A . J M o i Specbore., 26.72(19681. (6) Turro, N. J.. and Lechlken. P.. J. Amer C h r m Voc. 95. 264 119731. and relerencos therein. ~ Huntress, ~ E. H.. Stanley. ~ ~ (7) (a) L. -N.. and Parker. A. S.. J. CHEM. EDUC.. 11. 142 (19341: (b) White. E. H.. J. C H E M EDUC.. 34, M6 (1957): lei Sehneider. J.
CHEM. EDUC.,B.519119701 (8) Buckingharn, D. A,. and Saxenson, A . M.. "Cheiarinp. Aeenfn and Metal CheIstes." It'difors: ower. F. P.. and Meliar. 0. P I . Academic Prem. New York.
Volume 52, Number 7. July 1975 / 481
