4. O.N. Emanuel' and I. P. Skibida, Izv. Akad. Nauk SSSR, Ser. Khim., 61 (1976). 5. O.N. ~manu~l', D. Kh. Kitaeva, and I. P. Skibida, Izv. Akad. Nauk SSSR, Ser. Khim.,

56 (1976).

CHEMILUMINESCENCE IN ALKALINE SOLUTIONS OF HYDROGEN PEROXIDE

AND o-PHENANTHROLINE

O. S. Fedorova and V. M. Berdnikov UDC 541.124:535.379:547.836.3:546.215

We have shown earlier [i] that the oxidation of o-phenanthroline by hydrogen peroxide in neutral and weakly alkaline solutions containing cobalt ions gives rise to chemilumines- cence (CL) in the visible region of the spectrm. A similar effect is observed when the oxi- dation is catalyzed by Cu 2+ ions. Chemical luminescence is also observed, but at a very low intensity, in the uncatalyzed oxidation. We have attempted to develop the mechanism of chem- iluminescence in the o-phenanthroline oxidation through study of the reaction kinetics by the method of [i].

The variation of the chemiluminescence intensity and light sum with the pH of the medium is illustrated in Fig. i. Maximum intensity of 3.5-i0 e quanta/cm3-sec was reached at pH 9.5. The light sum remained constant at 1.8-10 I~ quanta/cm s over the pH interval from 6.7 to 8.5. Under further increase in the pH, the light sum fell, approaching zero at pHi-- 11. The curves showing the time dependence of ICL passed through a maximum, as reported earlier [i], the time required for reaching Ima x diminishing with increasing pH, with instant establishment of the maximum on mixing the reagents at pH~11

Addition of ethanol to the system increased both the CL intensity and the light sum (Fig. 2). Increasing the EtOH concentration from 0 to 40 vol. % increased Ima x by a whole order, its value rising from 2.8.108 to 3.2.109 quanta/cm3-sec. The light sum rose from 1.8.10 I~ to 7.3.10 I~ quanta/cm s when the EtOH concentration was built up to 70 vol. %. The time required for reaching Imax remained practically constant at 0.5• min. The curves showing the variation of Ima x and the light sum with the H202 concentration both passed through maxima (Fig. 3). The total light sum increased linearly with an increase in the o- phenanthroline concentration, while Ima x remained constant.

The fact that the light sum increased linearly with an increase in the o-phenanthroline concentration suggested that the observed chemiluminescencearose as the result of oxida- tional destruction of the o-phenanthroline. This point was checked by measuring the rate of o-phenanthroline consumption in experiments giving rise to chemiluminescence. Here l-ml ali- quots, removed from the reaction system as fixed time intervals, were treated with a phos- phate buffer (0.2 mole/liter, pH 7) and catalase (0.i mg/liter), to decompose the unreacted H202, and the optical densities of the resulting mixtures measured at 265 nm. It was found that the optical density fell off as reaction proceeded, eventually reaching a constant val- ue which was ascribed to weakabsorption in the reaction products. The spectrum showed iso- bestic points at %~235 and ~255 nm. The concentration of the unreacted o-phenanthroline was calculated from the equation

[o-phen] = 3 (D - - D~)/(ephen - - 8~ (1)

h e r e e~ = D=/[o-phen] , and t h e f a c t o r 3 a r i s e s f rom d i l u t i o n o f t h e s o l u t i o n o v e r t h e c o u r s e o f t h e m e a s u r e m e n t s . F i g u r e 4 shows t h e t i m e v a r i a t i o n o f t h e mean r a t e o f 0 - p h e n a n t h r o l i n e

c o n s u m p t i o n , c a l c u l a t e d f rom t h e e x p r e s s i o n wPhen h [ o ' p h e n ] / A t , and t h e change i n t h e r a t e me an o f c h e m i l u m i n e s c e n c e c a l c u l a t e d f rom t h e e q u a t i o n

~L= ~L .i03/~CLNAv (2)

I~ L being the CI intensity at time t, expressed in quanta/cmS.sec, and ~CL being a factor

given by

Institute of Chemical Kinetics and Combustion, Siberian Branch of the Academy of Sci- ences of the USSR, Novosibirsk. Translated from Izvestiya Akademii NaukSSSR, Seriya Khimi- cheskaya, No. 4, pp. 745-749, April, 1979. Original article submitted November 28, 1977.

692 0568-5230/79/2804-0692507.50 �9 1979 Plenum Publishing Corporation

5

% 0

= J

t o r

8 /0 ~fl

r ,

t y ' .

m 1

o 1-q

,~ o

2

I

0 g17 qo 80 80 100 [EtOH], vol. %

Fig. 1 Fig. 2

6~ &

Fig. i. Variation of I (i) and S(2) with the pH, [o-phen] -- 4. 10 -5, [H202] = 2.0 moles/liter.

Fig. 2. Variation of I (I) and S(2) with the ethanol concen- tration. [o-phen] = 4,10 -5 , [Ha02] = 2.0 moles/liter, pH 9.5.

~

f "e ,~

" g

I I

2 J [I-120~ ], mole/liter

F i g . 3. V a r i a t i o n o f I (1) and S (2) w i t h [Ha02] . [ o - p h e n ] = 4~ -5 m o l e / l i t e r ; [Et0H] = 30 v o l . %; pH 1 0 . 6 .

(pCL = S0.tOa/[o_phenl0NAV

where So i s t h e t o t a l CL l i g h t sum, i n q u a n t a / c m 3 ; [ o - p h e n ] i s t h e i n i t i a l c o n c e n t r a t i o n o f t h e o - p h e n a n t h r o l i n e ; and V i s t h e volume of t h e r e a c t i o n m i x t u r e , i n mE

It can be seen from Fig. 4 that the rate of consumption of o-phenanthroline was identi- cal with the rate of chemiluminescence in the reaction, chemiluminescence being obviously associated with the disappearance of the o-phenanthroline. The o-phenanthroline could be destroyed by either direct interaction with the Ha0a, or by interaction with 0H and 02- rad- icals resulting from H202 breakdown. Since the CL intensity increased on adding alcohol and 0H radical acceptor to the solution, it was assumed that OH radical reactions were not re- sponsible for the chemiluminescence.

We suggested earlier [2] that the H02 radical, in its 02- dissociated form (pKHO 2 =

4.75), also fails to participate in the elementary act leading to chemiluminescence. Cer- tain subsequent data (see [3, 4] indicate, however, that extensive oxidational destruction can result, from the reaction of aromatic compounds with 02- ion-radicals. It is also known that the 02- ion-radical is relatively stable in alkaline solutions [2], giving a rather strong ESR signal in frozen alkaline H202 solutions [5].

in order to decide whether or not the 02- radical particates in the elementary act lead- ing to chemiluminescence, experiments ~$ere carried out with solutions containing nitro blue tetrazolium (NBT), a specific 02- acceptor [6]. A clear-cut, 2.3-+0.3 rain induction period appeared on the kinetic CL curve for a system containing NBT at a concentration of 5.4-10 -5 mole/liter, measurements being carried out at [o-phen] = 4.10-~, [H~O~] = 0,1 mole/liter [EtOH] = 30 vol. %, and pH 11.2. The rate of 02- radical formation was calculated from the length of

693

G O

%

e l �9 |

50 100 150 rain

F i g , 4

L

G

~s /

0

f 1 I 1 I 1 1 1

3 6 9 [Cii ~+] "ID 7, mo ~s/Umr

Fig. 5

Fig. 4. Time variation of W CL (full-line curve), and wphen (points). [o-phen] = 4.10 -s mole/liter; [H202] = 0.I mole/liter; [EtOH] = 30%; pH 10.6.

Fig. 5. Variation of the rate of reduction of nitro blue tetrazo- lium with the Cu 2+ ion concentration. [o-phen] = 4,10 -5 , [H202] = 0.i, [NBT] = 5.10 -5 mole/liter, [EtOH] = 30 vol. %, pH 10.8, 22~

the induction period, assuming that each radical formed within this period enters into reac- tion with the NBT. Since four electrons are required for reducing NBT to diformazan, the stable final product, it follows that W i = 4[NBT]o/mind. From this, one finds that Wi = (1.6+0.2).10 -6 mole/liter.sec. The initiation rate was also obtained from the measured in- crease in the optical density of the solution at 560 nm and thel known value of the c at max- imum formazan absorption, 3.10 ~ liters/mole.cm [7]. Although the value obtained here, 9. 10 -7 mole/liter-sec, agreed rather well with the value calculated from the length of the in- duction period, it is only a lower limit, diformazan being only slightly soluble and tending to precipitate out of solution.

It is a well-known fact that the ascorbic acid anion is also an 62- anion-radical ac- ceptor [8]. In fact, the addition of ascorbic acid to the present system led to the appear- ance of an induction period on the kinetic CL curve.

Although it is clear that the 02- ion-radicals play a very important role in the phen- anthroline oxidation, we do not, at this point, have any information concerning the mechan- ism of their formation. One might simply assume that they arise from interaction with trace metal ion impurities. In fact, chemiluminescence is not observed in systems containing "i0 -s mole/liter of ethylenediaminepentaacetic acid (DTPA), a compound which forms a stable complex with metal ions, the situation here being similar to that met in the NBT reduction. Experiments with various metal ion solutions indicated that it is probably the Cu 2+ ion which is responsible for the appearance of the 02- ion-radicals. The concentration of cop- per contaminants in the various solutions was determined from the results of spectrophoto- metric studies of the relation between the rate of NBT reduction and the Cu 2+ ion concentra- tion (Fig. 5). It can be seen from Fig. 5 that the concentration of copper ions in the work- ing solutions was of the order of 2.10 -7 mole/liter.

Let us now consider a possible mechanism for the elementary act leading to chemilumines- Cence. It is simplest, and probably most nearly correct, to assume that the 62- radical at- tacks the o-phenanthroline molecule, adding to the latter's aromatic system to form a perox- ide radical

H HH O--O"

_7i\ ~ O- ~+~//'--~\ t - ' - % ~=_~ ~ _ / T .~ ~ _ , ' - \ _ i "-~---N N'-~-"

After passing through its maximum, the intensity of CL' falls off according to a kinetic law of the form dlldt = --~I s/2. This type of kinetics suggests that CL might arise from the re- combination of intermediate radicals: d [R0~]/dt----- k [R02] 2, f----4k ~ [R02]~/~ 2. If, on the other hand, all of the peroxide radicals recombine, the intensity of CL would be proportional to the rate of initiation, and the decrease in the CL intensity should follow a zeroth-order

694

law. Measured by the inhibitor method, the initiation rate proved to be more than one order higher than the rate of the CL reaction and the o-phenanthroline consumption. One must, ob- viously, assume that the addition of 02- radicals to the o-phenanthroline molecule is a re- versible process, so that only a very small.fraction of the radicals enter into adduct forma- tion with the o-phenanthroline. Since the 02- ion-radical is an active reducing agent (E0 = -0.16 V [9]), it should not be prone to undergo electrophilic addition to the o-phenanthroline molecule, and the assumption of reversibility seems rather plausible. Beyond this point, one could assume that two peroxide radicals would interact through disproportionation, form- ing a relatively unstable dioxetane derivative which could then break down in a nonlimiting step to form an excited dialdehyde

O--O H HH O--O" H HH OOH H H

\ l i..- \ l i / I / " t o \ / \ * 2 /--x, i ~ \ --> .

i~ N X=N/ \'N = / i N N - - ~

In o x i d i z i n g m e d i a , t h e l a t t e r wou ld p r o b a b l y be c o n v e r t e d i n t o 2 , 2 ' - d i p y r i d y l - 3 , 3 ' - e a r b o x y l - i c a c i d .

More d e t a i l e d i n f o r m a t i o n on t h e r e a c t i o n mechan i sm c o u l d be o b t a i n e d t h r o u g h the s t u d y of the excited intermediates and final products resulting from the oxidational destruction of o-phenanthroline.

CONCLUSIONS

i. Chemiluminescence arises from the oxidational destruction of o-phenanthroline.

2. Destruction of o-phenanthroline results from its interaction with O2- ion-radicals.

i, 2. 3. 4. 5. 6. 7. 8. 9.

LITERATURE CITED

O. S. Zhuravleva and V. M. Berdnikov, Izv. Akad. Nauk SSSR, Ser. Khim., 1755 (1977). B. H. J. Bielski and A. O. Allen, J. Phys. Chem., 81, 1048 (1977). J. Moro-oka and C. S. Foote, J. Am. Chem. Sot., 98, 1510 (1976). J. Rosenthal and A. Frimer, Tetrahedron Lett., 2805 (1976). Zh. P. Kachanova, Yu. I. Kozlov, and A. P. Purmal', Zh. Fizo Khim., 43, 2680 (1969). C. Beauchamp and J. Fridovich, Anal. Biochem., 44, 276 (1971). R. W. Miller and C. F. Kerr, J. Biol. Chem., 241, 5597 (1966). B. H. J. Bielski and H. W. Richter, J. Am. Chem. Sot., 99, 3019 (1977). V. M. Berdnikov and O. S. Zhuravleva, Zh. Fiz. Khimo, 46, 2658 (1972).

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