Harry D. Gafney Queens College of CUNY Flushing, New York 11367 I Chemiluminescence

and Arthur W. Adamson University of Southern California

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 plants and animals. These known as biolumi- 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 excita- tion 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 chemilumi- nescence and fluorescence spectra indicate the emitting level is the excited singlet state of the dianion of 3-nitro- phthalic acid (3).

The identification of the luminescent species is of prin- cipal interest in a mechanistic study of a chemilumines- cent 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 elec- tronic 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(hi- py)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

riment

Figure 1. A schematic representation of the relative energies of the ground and excited states of Ru(bipy)?+ and R u ( b i p y ) ~ ~ + involved in 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 photolumi- nescence of Ru(bipy).?+. The equivalence of the photolu- minescence 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 experi- mental error, that all e-,,,, initially enter the ligand s * orhital. The intermediate thus formed containing the re- ducing 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 chemi- cal reaction (6).

Although a number of chemiluminescent experiments have been published in this Journal (7), there have been

480 I Journal of Chemical Education

few involving transition metal complexes. This paper de- scribes a very simple, bright chemiluminescent reaction between trisbipyridalmthenium(IIl), Ru(hipy)a3+, and so- dium horohydride, NaBH4. The bright orange phosphores- cence, associated with the transition from the metal to ligand triplet charge-transfer s tate 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 photolumines- cence of Ru(hipy)a2+ have established t ha t the emission process is associated with the transfer of electron density from a ligand centered r* orbital to a metal tz, orbital (5). The equivalence of the emission spectra observed in the photoluminescence and chemiluminescence experi- ments suggests tha t the reducing electron initially p p u - lates a ligand r* orbital rather than direct transfer to a metal tzg orbital. The 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 than the ground state of R ~ ( b i p y ) 3 ~ + . The free energy change asso- ciated with the reversible oxidation of Ru(bipy)az+ to Ru- (hipy)33+, 1.2 eV, is obtained from electrochemical data (8). From Figure 1, i t is apparent tha t an additional 1.0 eV of energy is necessary t o populate the emitting level. Thus the reduction potential of the electron donor must be of sufficient magnitude, 21.0 eV, such tha t sufficient energy is available during electron transfer t o populate the emitting excited state.

In addition to energetic requirements, kinetic require- ments must also be satisfied t o observe chemilumines- cence. The rate of reaction producing the emitting level must be rapid enough and the lifetime of the emitting level long enough t o form a sufficient steady state concen- tration of the excited state species, 3 R ~ ( h i p y ) 3 ~ + , such tha t emission intensity will be visible. Considering the en- ergetic and kinetic requirements tha t must be met, i t is not surprising tha t chemiluminescent reactions are not common.

Experimental

Ru(bipy)~Clz was obtained fmm G . Frederick Smith and used without further purification (91. An alternative supplier of the re- agent, 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 solu- tion is stable and a larger volume can be prepared and stored in- definitely for later use. R~(hipy)~3* is 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 or- ange solution rapidly becomes dark green, characteristic of Ru(hi- p y ) ~ ~ + . Although Ru(bipy)~~+ is indefinitely stable, Ru(bipy),ai is slowly reduced by a reaction with the solvent. The solution half-life of Ru(b i~y )~s+ depends 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 pre- pared hy adding slightly more NaBHd than necessary for a satu- rated 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.

Demonstration

Althnneh direct addition of the Ru(hiovL3+ solution to the ~ - ~ ~ - ~ ~ - n ~ ~ - ~ ~ ~ ~ ~ ~ ~ ~ ~ - ~ ~~ ~~~~ - ~ ~ , .,,- NaBH* solution produces a bright orange flash, considerable fm- thing occurs. Two procedures have been developed to circumvent the problem of frothing. The first procedure makes use of readily

NaBH4 solution is placed in 125-ml Erlenmeyer flask containing a magnetic stirring bar. The solution is stirred at a rate which 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 separa- tory 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 or- ange ribbon of light is observed to traverse the barrel of the appa- ratus. 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 It is not known whether the

reductant producing the ex- cited 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 reac- tion using Figure 1 is valid and illustrates how spectral data can he used in conjunc- tion with other thermody- namic data (electrochemical data) to understand the ener- getics of the reaction. The demonstration is an illustra- tion of a chemiluminescent reaction of a transition metal complex, hut more impor- tantly illustrates how chemi- luminescence can be used to

Figure 2. Apparatus used for the probe the energetics and chemiluminescence demonstra- mechanism of a chemical re- tion. action.

Literature Cited

Ill (a) Arnon. D. I.. Sci. Amsr., 2M. lM (1960): (bl Calvin, M.. and Bassham. J. M.. "The Patch of Carbon in Photo~ynthesis." Pronfice-Hail. Englewood Cliffs, N.J.. L9Si: ic i Parks. R. B.. J. CHEM. EDUC.. 39. 424 119611: (dl While. E. H.. " ~ i i h r a n d L 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); (h l Rsuhut. M . M.. Accts. Cham. Reg., 2, 30 11969): I c ) White, E. H.. and Roswell. D. F.. Acclr. Chem. R M . I.SdiL9701: id1 Hercu1es.D. M . A c r f 8 . Chem. Res . 2. 301 11969). ~~ , ~ . ~ ~ , ~ ,. . .

(3) White. E. H.. and Burrey. M. M.. J . Amen Chsm. S a c . 86. 941 119641, and relel- sneerthelei".

(4) Manin, J. E.. Hart. E. J.. Adamson. A. W.. Gainey. H. D.. and Hnlpern, .I.. J Amrr 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 Chrm Voc. 95. 264 119731. and relerencos

therein. (7) (a) Huntress, E. H.. Stanley. 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 Che- Istes." It'difors: ower. F. P.. and Meliar. 0. P I . Academic Prem. New York.

availahle laboratory glassware. Approximately 50 ml of the

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