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Electrogenerated chemiluminescence in mechanistic investigation of electroorganic reactions
%.k~fruad.
Chet=n..133 (1982) 173-182
Eke+
Sequois SA.
FRITZ
PRAGST-am.!
-.
173
Lausaisne-_Printed in The Netherlands
h&-EAEL
VON LbWIS
Sektion Chenrie der HumWdMAuke&tiir.
:
DbR-1040~IBerli,~ Hessische Strusse I-_’ (G.D. R.)
(Received 19th May 1981. in revised form 21s~ September 1981)
ABSTRACT In the intensity-potential curves of the electrogenerated chemihuninescence at the dropping mercury electrode spikes are observed, which are due to the reaction of excessive anion radicals A-’ at the fd!ing mercury drop. The luminescence trace of the drop exhibits a characteristic intensity pattern. These oscillations arise from an interplay of a concentration difference of A-’ between the front and back sides or the falling drop. which is hydrodynamically caused and leads to a corresponding difference in the electrochemical potential and in the-surface tension. and a surface movement of the mercury as the retroactive effect. The influence of the experimental conditions and the structural effect of the investigated cathodic luminescence systems on the oscillating behavior are studied.
INTRODUCTION
_ In the cathodic cleavage of single bonds in the presence of suitable fluorescent compounds, A luminescence can be observed if some energetic and structural conditions are willed [l-4]. This electrogenerated chemiluminescence (ECL) arises from the homcigeneous electron transfer (eqn. 5) between the anion radical of A and the. radical R’ foqned as a cleavage product: R-X+e
ti
R-X-‘
(1)
R-X---
2
R’+x-
(2)
-A-te R-X-:+=
P 4
A-’ _R-X+R7
(3) ‘(44)
A-‘+i
-
‘A+R’
(5)
+
‘A*+&
--a
A+hu,.
2 ‘A. ‘A*
’ -:
0022_072iT/82/0000-hjSO.275
:
(6)
-w . 0 1982 F&evier Sequoia S.A.
174
Instead of reaction (1) the anion radical R-X - _ can also be indirectly formed via A-‘: A-‘+R-X=iA+R.-X--
@I
It was shown in a previous paper [4], that the luminesccnc&potential curves of- this process at the dropping mercury electrode (DME) have a characteristic shape (Fig. 2 in ref. 4). In addition to a luminescence maximum at the beginning of the polaro-graphic wave of A, which was attributed to a change from the direct,(l) to the indirect reduction of R-X (eqn. 8), luminescence spikes at the end of the drop are found. In order to elucidate the reason of these spikes the ECL of some R-X/A systems was investigated at the DME. 9,10-Diphenylanthracene (DPA), 9;10dimethylanthracene (DMA) and rubicene (RC) were used as luminescent compounds A. A suitable cleavage process is the cathodic reduction of some arylsulfonyl compounds Ia-Id
nr-Sq-X
Id
Ib, R=CI
Ia
Ic, R=COOCHg
+
,k--SOz’+X-+A(Ia.
A--.\
Ar-SOi
+X-
+
A
(Lb,
Id) ic )
(9)
EW’ERIMENTAL
The arylsulfonyl compounds Ia-Id were prepared by known methods. Their melting points were in agreement with the literature data. A 0.1 M solution of tetraethylammonium perchlorate (TEAP) in dimethylfoxmamide (DMF) or in a 1: 1 mixture of acetonitrile and toluene (AN/TOL) was used as supporting electrolyte. The experimental arrangement for the luminescence investigations is shown in Fig. 1. The voltammetric cell was placed in a black box. Two slits Sl and S2 were placed between the cell and the photomultiplier, which could, therefore, observe only a small part (1 mm) of the luminescence trace of the falling drop. For measuring the intensity along the trace the capillary could be lifted or lowered by a micrometer screw. For obtaining the intensity-potential curves (see Fig. 2) and the intensity-time curves (see Figs. 4 and 5) the slits were removed and the capillary was adjusted to a position where the whole path of the drop (about 30 mm) could be observed by the photomultiplier. The distances x of the luminescence maxima from the capillary were calculated from the intensity-time curves by way of the relationships (lo)-( 12), v&i& describe the velocity of the drop as a function of the time u(t) with the assumption of the
;
.;
V Fig. I. Experimental arrangement for the.measurement of the luminescence (PM) photomultiplier; (9. S2) slits; (CE) counter electrode; (SCE) saturated (DME) dropping mercmy electrode.
trace of the mercury drop. aqueous calomel electrode:
Stokes limiting case. The best agreement with the experiments was obtained with as found by fitting the calculated x(t) *09=60 cm s-t in 0.1 M TEAP/AN/TOL, curve to the experimental curve, which was measured by use of the arrangement shown in Fig. 1.
x(t) =g/u2(e-ar
+at- 1)
(10) .
v(t) = -g/a(e-uf - 1) a=
act
= g/*o!J 2~*bHg- Psod
(12)
where x is the distance from the capillary tip, g the acceleration due to gravity; q the viscosity of the solution, 0, ihe velocity according to Stokes law, r the radius of the drop and p&, psol;the density of mercury and &the soLution. A GWP 673,(ZWG of’the AdW,- Berlin) electrochen&al system wti used for the eIectrochcmi&l mc.&ureme&. The intensity4me curves were measured with an OG 2-23 (VEB &fe&eIek&ik, Berlin) double-channel oscilloscope, the tune base of .-. which was triggered by- 4 d&zrease in ~&.rrent~ at. th& ,he i&ant as the drop 3s disconnected from, the. cap&&y. ..A.h;@yestigations w&r: $arried out -at the free droPping ek&@ie~in &der to’@+ the @stu&ncc .eaused-by _a drop. -knocker.The pot&&Is given i&&is paper refer to the saturated &iiomel electiode~(SCP). .’ ~.
..
.:
__
.
176 RESULTS
A typical example of an intensity-potential curve at the DME is shown in Fig. 2. The luminescence spikes at the end of the drop life are observed in the potential
region of the first reduction wave of A. It can be seen from a photograph (Fig. 3, exposure I5 min, about 500 drop times) that as well as the luminescence photo of thehanging drop the falling drop also~produces a luminescence trace with a characteristic intensity pattern. After a short dark zone a broad maximum, then another dark zone and a series of smaller and more intense maxima is found. In the first part, a thin luminescence line is seen in the centre of the trace. This behavior is confirmed by measuring the intensity distribution along the drop path with the arrangement shown in Fig. 1. At the beginning the capillary is adjusted in a position in which the drop couid be observed by the photomultiplier, and then the capiliary is lifted by the micrometer screw and the intensity is measured at each 0.25 mm. The huninescence signal at the oscilloscope was limited to the time during which the drop passes the viewing region of the photomultiplier and after that no slow intensity decay was detected for the DPA/Ia ECL system. The intensity-time curves of this ECL system at different electode potentials are
I
4
-1.8
-1.7 E/V
I
-1.9
Fig. 2. Into’Gty-potentialcuw (2) andpolarogram (1) of p-carbmethoxyphenyl-tosyiate and DPA (IO m-3M) in 0.1 M TEAP/DMF.
Ic (4X 10 -’ Mj
5
1IO
Q :
.c
Fig 3. Luminescence trace of ttie hIercury drop in solutions of DPA (10m3 hf) and pchlorobenzenesulfochloride Ia [(i) 2.5 X IO -’ M: (b) 5 X 10 -’ M: (c) Id --3 M ] in 0. I XI TEAP/AN/ToL at -2.05 V. Exposure IS min. drop time I, = 1.77 s. mercury consumption 2.37 mg s-I_
shown in Fig. 4. Within the.region of the DPA wave a negative shift of the potential leads to an increase in intensity and to an extension of the luminescence trace with new maxima. Thei time difference between the maxima is 254ms. It slightly decreases in later parts of the path (increasing velocity of the drop) and slightly increases at more’ negative potentials. The spikes .are obierved only if the concentration of A is at least about 50% of that of R-X. With increasing.c, or .decreasingcn_x the luminescence trace becomes longer and more structured (Fig. 3). An increase of the dropping time at constant potential and concentrations by lowering the level of the mercury vessel leads to a higher spike intensity and to an extension of the luminescence trace. The time difference between the maxima is not significantly changed. A luminescence-time behavior. similar to Fig. 4 was also observed for other ECL systems, if ‘E$(+).,k Er$(R-X) .and if A was in excess (Table 1). This is the c&e with the DPA/6is-(Z2,4,5-triphenylimidazolyl)-1,2’~and DPA/diphenyldisulfide systems; the ECL. of --which was described in previous papers [3,4]. Obviously, the chemical structure,of R-X only has.an effect.on the total intensity of the-spikes, but the intensity pattern of the trace is not characteristically changed. .However, ,,a‘chemical effect .can be found, if,_both A land R-X -have similar cathodic half-wave potentials (Fig. 5). In the series DPA/Ic, DMA/Ib and DPA/Ib
178 x/mm 0
1
20
10
35
60
80
30
100
t fms
Fig. 4. Intensity- time curves at the falling mercury drop of chlorobcnzenrsulfochloride Ia (2 X 10 m-4M) in 0. I M TEAP/AN/TOL
the difference between ,!$$A)
and E,$(R-X)
DPA (2 X 10m3 bf) at different potentials.
md
p-
decreases. In the same senes the
luminescence-time curves become less structured_ In the case of RC/Id (Fig. 5d) the half-wave potential of A is more positive than that of R-X. For this system the emission continues longer than the time when the drop falls. For comparison, 9, IO-dichloro-9, IO-diphenyl-9, IO-dihydroanthracene (DPACI, ) [6] was also investigated. In contrast to the very bright emission at the growing drop no spikes were observed with this compound_ TABLi: 1 Investigated ECL systems. A/R-X
U
Solvent ”
DPA/Ia
DPA/ Ib DMA/Ib DPA,‘Ic RC/M DPA/L= [3] DPA/Ph-S-SPh DPACl 1 [6]
[4)
AN/TOL DMF DMF DMF AN/TOL DMF DMF DMF
Ir.tensity-time curve
E,$ /V(SCE) A
R-X
-LOS - 1.84
-0.17 - 1.80
- I.% - 1.84
- 1.80 - 1.70 - 1.28 -1.11 -0.85 -0.18
-
1.12 1.84 1.84 1.84
Strong oscillations
No oscillations Weak oscillations Medium oscillations No oscillation& slow decay Strong oscillations
Strong oscillations Nospikes
” For abbreviations see experimental section. L z =bis-(2.4,5-triphenylimidarolyl)-1.3’; dichloro-9.1~diphenyl-9.I@dihydroanthracene.
DPAClz =9.10-
:
-. I
=:.
-:
-
*
’
..
..Q -.
..
80
0
179
Lo
120
80
0 t/ms
loo
200
120
-.-
:..-
..->.
:
300
Fig. 5. Intensity-t&e curves at the falling mercury drop of (a) p-carbmethoxyphenyltosylate Ic/DPA at -2.0 V. (b) pchlorophenyltosylate Ib/DMA at -2.1 V. (c) pchlorophenyltos$late Ib/DPA at -LO V and (d) N-p-tosylphthalimide Id/RC at - 1.4 V in 0.1 M TEAP/DMF (curves a-c) and 0.1 M TEAP/AN/TOL (curved). Concentrations of all components 10 -’ hf. tiISCUSSION It is obvious from Fig. 3 that the luminescence spikes of the intensity-potential curves (Fig. 2) are caused by the falling mercury drop. During the lifetime of -the drop, R-X is irreversibly cleaved in a luminescent reaction layer, which moves from
the electrode surface into the solution. Between this reaction zone, and the electrode a layer of ‘A--. is formed, which is the broader the longer is the dropping time and the higher the bulk concentration ratio of A and R-X. Owing to this excess of A’ _ the luminescence continues .via reactions (8) and (5)-(7) after the drop is disconnected from the electrode. The falling mercury drop carries a.solution shell, the thickness of which decreases with increasing velocity. In the first part of .the drop path, the solution shell is much thlcker~than the layer of A-’ and. the intensity. remains at about the .same level’ as during the growth of the drop.. This appears to be a dark zone on the photograph (Fig. 3), since the luminescence time of the growing- drop (e.g;2s) is much longer thanthat of the falling drop ((20 ms mm-t)_ After about 2 mm the solution shell in front of the drop is dliriinislied to about. the same dimension-as the layer-of A-‘Therefore, A- _ reacts more intensively with the surrounding solution of R-X. This gives rise to the_Fist broad intensity maxinmm~of.tbe luminescence trace. ~_The’concentr&ion.;bf,A-“decrea&s more in the front of the’drop than &the b&k side. The c@fferencein &i&ntratiou ctius& a’ difference of the electrochemical potential according to the b&mstian equation with the.more.positive value in front: .AE=&-E,‘& E, 7 E&
(RT/F).ln([Ajr[A-.-lb/[A-l];[Al,). Uf5esb.
~.
~>.
:
‘:
-
1.
<
(13).
180
Since all measurements were made in the far negative part of the electrocapillary curve, the potential difference results in a higher surface tension in the front than on the back side of the drop. From this follows a surface movement of the mercury from the back to the top (Fig. 6), which transports A-- from the reservoir at the back in the same direction. Since the surface movement is opposite to the relative movement of the solution, it involves an increased rate of the reaction of A- _ and R-X, and a new luminescence maximum. As seen from Fig. 3, this emission is particularly intense at the equator of the drop.
II
I Fig. 6. Surface movement front and back sides.
of the falling
mercury
drop
arising
from
a potential
difference
-I E between
Because of the inertia of the mercury and of the limited diffusion rate no steady state is attained between the transport of A-- to the front and its consumption at the front, but this state is exceeded, and the increased transport almost compensates the potential difference. Consequently. the surface movement decelerates and stops. As the drop continues its path, the surface concentration of A-’ in front again rapidly decreases and the process of arising and compensating the potential difference is repeated several times as long as there is any A-’ available on the back side of the drop. These oscillations are the reason for the sharp maxima in the luminescence trace. There are other known cases in which a movement of mercury electrodes is generated by a change or a difference of the electrochemical potential, e.g. the “beating mercury heart” [7] or the first -kind polarographic maxima [S-IO]. In the case of the polarographic maxima a tangential velocity u, of the mercury surface up to 40 cm s-’ was measured. A rough estimation of ut can also be made for these experiments. Between two luminescence maxima (2.5-4 tns) the surrounding~solution should be moved by at least a quarter of the drop periphery (0.67 mm at a
-
,,
:
. . . . :
181
diameter-of.0.85 n&-backward-due to the ‘fall. of the drop. and forward. due to -the surface movement of the &&ury ,From the symmetry of the, maxima. (F&4) it can be. assk+d.‘thai -both $m& take about the same -time (1.3-2 -ms). Therefore, Up should be between about’ 30.‘and: SC! cm s- ’ during the .phase bf the intensity increase. This is about as fast as the drop falls-in this region of-the trace (30-55 cm s-l), as determined from the.intensity-time curves measured tith the arrangement of Fig. 1 and calc$ated from eqns, (IO)-( 12). Such oscillations of the falling mercury drop should occur in each case in which a relatively stable product of a reversible reduction is formed. Chemiluminescence is merely a convenient method -of visual&zing the oscillating behavior,.which is mainly determined by hydrodynamic parameters and by the.diffusion. .A che.mical reaction e.g. the cleavage of R-X, leads to its earlier of the reduction product A-‘, consumption, and therefore shortens the part of the drop path in which the oscillaticn can be observed. On the other hand, the rate of reactions (2)-(S) must be fast enough to follow the frequency of the surface movement of the drop. In some cases (Fig. 4) a tenfold increase or decrease of the intensity occurs within 3 ms. Because of the bimolecular reactions, and with respect to the experimental boundary conditions, a quantitative kinetic treatment of the reaction scheme is rather complicated. Qualitatively, it can be distinguished between two cases according to the difference between the half-wave potentials of A- and R-X. If E,‘;d,(A) K Efs(R-X), the luminescence decay is mainly determined by the diffusion of A-’ and R-X between the layer of A-’ and the bulk solution, since the electron-transfer reactions (S), (4) and (5) are usually very fast. At similar bulk concentrations of A and R-X the luminescence decay is much faster than the oscillation frequency_ It can be shown that under these conditions the slower cleavage reaction (2) has no decisive effect on the decay time. However, the intensity jncreases with increasing k,, since then in the competition of reactions (8) and (5) with respect to A-‘, and of reactions (4) and (5) with respect to R’, the formation of 3A (reaction 5) is more favoured. If E,Tz(A) = E;jd,(R-X) and the cleavage is slow, the concentrations of R-X -. and A-’ are always determined by the equilibrium (a), and the luminescence decay should mainly depend on the rate of the cleavage. However, also in this case a simple first-order decay according to k, cannot be expected from the reaction scheme. In addition, the diffusion .disturbancc of the bimolecular processes (4)-(6) and side reactions of A-‘ with impurities-e.g.. traces of water and oxygen or -quenching of 3A by A-' or other species- render a quantitative interpretation of the decay curve more difficult. The latter difficulties bccome’more serious at the falling drop because of .the considerable ~dilution of A-. and R-X - - on .the trace. From these reasons a weakening of the oscillation structure of the intensity-time curve (Fig. 5) or a slow decay of the. intensity after the drop fall, as in the -case of the rubicene/iV-ptosylphthalimide system; points only qualitatively to a slow cleavage of R-X, but does not .enable the determination of thti exact iate constant k,. In the case of N-p-tosylphthalimide, k, was determined by cyclic voltammetry at 0.28 s_’ in acetonitriie [S]. ~.
-.: : 182
‘.
In sum, data on the mechanism of the cleavage reaction, which ake ktilabkfromthe oscillating behavior of the drop, seem to be rather poor. N&vektheless, it& a simply realized example of an oscillating process, which may be useful.for a.study of relationships between the surface movement and the potential difference in thecontext of the polarographic maxima. REFERENCES I D.T. Santa Crw. D.L. Akins and R.L. Birke, J. Am. Chem. Sot.. 98 (1976) i677. 2 K. Iraya. M. Kawai and S. Toshima, J. Am. Chcm. Sot.. 100 (1978) 59%. 3 F. Pragst and B. Kaltofen. J. Electroanal. Chem.. 112 (1980) 339. 4 F. Pragst. J. Electroanal. Chem., I19 (1981) 315. 5 F. Pragst and B. Kaltoien, Electrochim. Acta. submitted. 6 T.&f. Siegel and H.B. Mark. Jr., J. Am. Chem. Sot.. 94 (1972) 9020; K-G. Boto and AJ. Bard. Electroanal. Chem.. 65 (1975) !US: F. Pngst and 8. Brandt. J. Electroanal. Chem.. 99 (1979) 357. 7 J. Keizer. P.A. Rock and S.-W. Lin. 3. Amer. Chem. See.. 101 (1979) 5637 and refs. therein. 8 J. HeyrovskG and J. Kuta. Grundlagen $er Polarographie. Akademie Verlag. Berlin 1965. p. 401. 9 J. Prow, V. Cielcszky and K. Gyorbiro, Polarographie, Akadcmiai Kiad& Budapest 1967. p. 184. IO hf. v. Stackclberg. Fortschr. Chcm. Forsch.. 2 (1951) 229.
J.
Chet=n..133 (1982) 173-182
Eke+
Sequois SA.
FRITZ
PRAGST-am.!
-.
173
Lausaisne-_Printed in The Netherlands
h&-EAEL
VON LbWIS
Sektion Chenrie der HumWdMAuke&tiir.
:
DbR-1040~IBerli,~ Hessische Strusse I-_’ (G.D. R.)
(Received 19th May 1981. in revised form 21s~ September 1981)
ABSTRACT In the intensity-potential curves of the electrogenerated chemihuninescence at the dropping mercury electrode spikes are observed, which are due to the reaction of excessive anion radicals A-’ at the fd!ing mercury drop. The luminescence trace of the drop exhibits a characteristic intensity pattern. These oscillations arise from an interplay of a concentration difference of A-’ between the front and back sides or the falling drop. which is hydrodynamically caused and leads to a corresponding difference in the electrochemical potential and in the-surface tension. and a surface movement of the mercury as the retroactive effect. The influence of the experimental conditions and the structural effect of the investigated cathodic luminescence systems on the oscillating behavior are studied.
INTRODUCTION
_ In the cathodic cleavage of single bonds in the presence of suitable fluorescent compounds, A luminescence can be observed if some energetic and structural conditions are willed [l-4]. This electrogenerated chemiluminescence (ECL) arises from the homcigeneous electron transfer (eqn. 5) between the anion radical of A and the. radical R’ foqned as a cleavage product: R-X+e
ti
R-X-‘
(1)
R-X---
2
R’+x-
(2)
-A-te R-X-:+=
P 4
A-’ _R-X+R7
(3) ‘(44)
A-‘+i
-
‘A+R’
(5)
+
‘A*+&
--a
A+hu,.
2 ‘A. ‘A*
’ -:
0022_072iT/82/0000-hjSO.275
:
(6)
-w . 0 1982 F&evier Sequoia S.A.
174
Instead of reaction (1) the anion radical R-X - _ can also be indirectly formed via A-‘: A-‘+R-X=iA+R.-X--
@I
It was shown in a previous paper [4], that the luminesccnc&potential curves of- this process at the dropping mercury electrode (DME) have a characteristic shape (Fig. 2 in ref. 4). In addition to a luminescence maximum at the beginning of the polaro-graphic wave of A, which was attributed to a change from the direct,(l) to the indirect reduction of R-X (eqn. 8), luminescence spikes at the end of the drop are found. In order to elucidate the reason of these spikes the ECL of some R-X/A systems was investigated at the DME. 9,10-Diphenylanthracene (DPA), 9;10dimethylanthracene (DMA) and rubicene (RC) were used as luminescent compounds A. A suitable cleavage process is the cathodic reduction of some arylsulfonyl compounds Ia-Id
nr-Sq-X
Id
Ib, R=CI
Ia
Ic, R=COOCHg
+
,k--SOz’+X-+A(Ia.
A--.\
Ar-SOi
+X-
+
A
(Lb,
Id) ic )
(9)
EW’ERIMENTAL
The arylsulfonyl compounds Ia-Id were prepared by known methods. Their melting points were in agreement with the literature data. A 0.1 M solution of tetraethylammonium perchlorate (TEAP) in dimethylfoxmamide (DMF) or in a 1: 1 mixture of acetonitrile and toluene (AN/TOL) was used as supporting electrolyte. The experimental arrangement for the luminescence investigations is shown in Fig. 1. The voltammetric cell was placed in a black box. Two slits Sl and S2 were placed between the cell and the photomultiplier, which could, therefore, observe only a small part (1 mm) of the luminescence trace of the falling drop. For measuring the intensity along the trace the capillary could be lifted or lowered by a micrometer screw. For obtaining the intensity-potential curves (see Fig. 2) and the intensity-time curves (see Figs. 4 and 5) the slits were removed and the capillary was adjusted to a position where the whole path of the drop (about 30 mm) could be observed by the photomultiplier. The distances x of the luminescence maxima from the capillary were calculated from the intensity-time curves by way of the relationships (lo)-( 12), v&i& describe the velocity of the drop as a function of the time u(t) with the assumption of the
;
.;
V Fig. I. Experimental arrangement for the.measurement of the luminescence (PM) photomultiplier; (9. S2) slits; (CE) counter electrode; (SCE) saturated (DME) dropping mercmy electrode.
trace of the mercury drop. aqueous calomel electrode:
Stokes limiting case. The best agreement with the experiments was obtained with as found by fitting the calculated x(t) *09=60 cm s-t in 0.1 M TEAP/AN/TOL, curve to the experimental curve, which was measured by use of the arrangement shown in Fig. 1.
x(t) =g/u2(e-ar
+at- 1)
(10) .
v(t) = -g/a(e-uf - 1) a=
act
= g/*o!J 2~*bHg- Psod
(12)
where x is the distance from the capillary tip, g the acceleration due to gravity; q the viscosity of the solution, 0, ihe velocity according to Stokes law, r the radius of the drop and p&, psol;the density of mercury and &the soLution. A GWP 673,(ZWG of’the AdW,- Berlin) electrochen&al system wti used for the eIectrochcmi&l mc.&ureme&. The intensity4me curves were measured with an OG 2-23 (VEB &fe&eIek&ik, Berlin) double-channel oscilloscope, the tune base of .-. which was triggered by- 4 d&zrease in ~&.rrent~ at. th& ,he i&ant as the drop 3s disconnected from, the. cap&&y. ..A.h;@yestigations w&r: $arried out -at the free droPping ek&@ie~in &der to’@+ the @stu&ncc .eaused-by _a drop. -knocker.The pot&&Is given i&&is paper refer to the saturated &iiomel electiode~(SCP). .’ ~.
..
.:
__
.
176 RESULTS
A typical example of an intensity-potential curve at the DME is shown in Fig. 2. The luminescence spikes at the end of the drop life are observed in the potential
region of the first reduction wave of A. It can be seen from a photograph (Fig. 3, exposure I5 min, about 500 drop times) that as well as the luminescence photo of thehanging drop the falling drop also~produces a luminescence trace with a characteristic intensity pattern. After a short dark zone a broad maximum, then another dark zone and a series of smaller and more intense maxima is found. In the first part, a thin luminescence line is seen in the centre of the trace. This behavior is confirmed by measuring the intensity distribution along the drop path with the arrangement shown in Fig. 1. At the beginning the capillary is adjusted in a position in which the drop couid be observed by the photomultiplier, and then the capiliary is lifted by the micrometer screw and the intensity is measured at each 0.25 mm. The huninescence signal at the oscilloscope was limited to the time during which the drop passes the viewing region of the photomultiplier and after that no slow intensity decay was detected for the DPA/Ia ECL system. The intensity-time curves of this ECL system at different electode potentials are
I
4
-1.8
-1.7 E/V
I
-1.9
Fig. 2. Into’Gty-potentialcuw (2) andpolarogram (1) of p-carbmethoxyphenyl-tosyiate and DPA (IO m-3M) in 0.1 M TEAP/DMF.
Ic (4X 10 -’ Mj
5
1IO
Q :
.c
Fig 3. Luminescence trace of ttie hIercury drop in solutions of DPA (10m3 hf) and pchlorobenzenesulfochloride Ia [(i) 2.5 X IO -’ M: (b) 5 X 10 -’ M: (c) Id --3 M ] in 0. I XI TEAP/AN/ToL at -2.05 V. Exposure IS min. drop time I, = 1.77 s. mercury consumption 2.37 mg s-I_
shown in Fig. 4. Within the.region of the DPA wave a negative shift of the potential leads to an increase in intensity and to an extension of the luminescence trace with new maxima. Thei time difference between the maxima is 254ms. It slightly decreases in later parts of the path (increasing velocity of the drop) and slightly increases at more’ negative potentials. The spikes .are obierved only if the concentration of A is at least about 50% of that of R-X. With increasing.c, or .decreasingcn_x the luminescence trace becomes longer and more structured (Fig. 3). An increase of the dropping time at constant potential and concentrations by lowering the level of the mercury vessel leads to a higher spike intensity and to an extension of the luminescence trace. The time difference between the maxima is not significantly changed. A luminescence-time behavior. similar to Fig. 4 was also observed for other ECL systems, if ‘E$(+).,k Er$(R-X) .and if A was in excess (Table 1). This is the c&e with the DPA/6is-(Z2,4,5-triphenylimidazolyl)-1,2’~and DPA/diphenyldisulfide systems; the ECL. of --which was described in previous papers [3,4]. Obviously, the chemical structure,of R-X only has.an effect.on the total intensity of the-spikes, but the intensity pattern of the trace is not characteristically changed. .However, ,,a‘chemical effect .can be found, if,_both A land R-X -have similar cathodic half-wave potentials (Fig. 5). In the series DPA/Ic, DMA/Ib and DPA/Ib
178 x/mm 0
1
20
10
35
60
80
30
100
t fms
Fig. 4. Intensity- time curves at the falling mercury drop of chlorobcnzenrsulfochloride Ia (2 X 10 m-4M) in 0. I M TEAP/AN/TOL
the difference between ,!$$A)
and E,$(R-X)
DPA (2 X 10m3 bf) at different potentials.
md
p-
decreases. In the same senes the
luminescence-time curves become less structured_ In the case of RC/Id (Fig. 5d) the half-wave potential of A is more positive than that of R-X. For this system the emission continues longer than the time when the drop falls. For comparison, 9, IO-dichloro-9, IO-diphenyl-9, IO-dihydroanthracene (DPACI, ) [6] was also investigated. In contrast to the very bright emission at the growing drop no spikes were observed with this compound_ TABLi: 1 Investigated ECL systems. A/R-X
U
Solvent ”
DPA/Ia
DPA/ Ib DMA/Ib DPA,‘Ic RC/M DPA/L= [3] DPA/Ph-S-SPh DPACl 1 [6]
[4)
AN/TOL DMF DMF DMF AN/TOL DMF DMF DMF
Ir.tensity-time curve
E,$ /V(SCE) A
R-X
-LOS - 1.84
-0.17 - 1.80
- I.% - 1.84
- 1.80 - 1.70 - 1.28 -1.11 -0.85 -0.18
-
1.12 1.84 1.84 1.84
Strong oscillations
No oscillations Weak oscillations Medium oscillations No oscillation& slow decay Strong oscillations
Strong oscillations Nospikes
” For abbreviations see experimental section. L z =bis-(2.4,5-triphenylimidarolyl)-1.3’; dichloro-9.1~diphenyl-9.I@dihydroanthracene.
DPAClz =9.10-
:
-. I
=:.
-:
-
*
’
..
..Q -.
..
80
0
179
Lo
120
80
0 t/ms
loo
200
120
-.-
:..-
..->.
:
300
Fig. 5. Intensity-t&e curves at the falling mercury drop of (a) p-carbmethoxyphenyltosylate Ic/DPA at -2.0 V. (b) pchlorophenyltosylate Ib/DMA at -2.1 V. (c) pchlorophenyltos$late Ib/DPA at -LO V and (d) N-p-tosylphthalimide Id/RC at - 1.4 V in 0.1 M TEAP/DMF (curves a-c) and 0.1 M TEAP/AN/TOL (curved). Concentrations of all components 10 -’ hf. tiISCUSSION It is obvious from Fig. 3 that the luminescence spikes of the intensity-potential curves (Fig. 2) are caused by the falling mercury drop. During the lifetime of -the drop, R-X is irreversibly cleaved in a luminescent reaction layer, which moves from
the electrode surface into the solution. Between this reaction zone, and the electrode a layer of ‘A--. is formed, which is the broader the longer is the dropping time and the higher the bulk concentration ratio of A and R-X. Owing to this excess of A’ _ the luminescence continues .via reactions (8) and (5)-(7) after the drop is disconnected from the electrode. The falling mercury drop carries a.solution shell, the thickness of which decreases with increasing velocity. In the first part of .the drop path, the solution shell is much thlcker~than the layer of A-’ and. the intensity. remains at about the .same level’ as during the growth of the drop.. This appears to be a dark zone on the photograph (Fig. 3), since the luminescence time of the growing- drop (e.g;2s) is much longer thanthat of the falling drop ((20 ms mm-t)_ After about 2 mm the solution shell in front of the drop is dliriinislied to about. the same dimension-as the layer-of A-‘Therefore, A- _ reacts more intensively with the surrounding solution of R-X. This gives rise to the_Fist broad intensity maxinmm~of.tbe luminescence trace. ~_The’concentr&ion.;bf,A-“decrea&s more in the front of the’drop than &the b&k side. The c@fferencein &i&ntratiou ctius& a’ difference of the electrochemical potential according to the b&mstian equation with the.more.positive value in front: .AE=&-E,‘& E, 7 E&
(RT/F).ln([Ajr[A-.-lb/[A-l];[Al,). Uf5esb.
~.
~>.
:
‘:
-
1.
<
(13).
180
Since all measurements were made in the far negative part of the electrocapillary curve, the potential difference results in a higher surface tension in the front than on the back side of the drop. From this follows a surface movement of the mercury from the back to the top (Fig. 6), which transports A-- from the reservoir at the back in the same direction. Since the surface movement is opposite to the relative movement of the solution, it involves an increased rate of the reaction of A- _ and R-X, and a new luminescence maximum. As seen from Fig. 3, this emission is particularly intense at the equator of the drop.
II
I Fig. 6. Surface movement front and back sides.
of the falling
mercury
drop
arising
from
a potential
difference
-I E between
Because of the inertia of the mercury and of the limited diffusion rate no steady state is attained between the transport of A-- to the front and its consumption at the front, but this state is exceeded, and the increased transport almost compensates the potential difference. Consequently. the surface movement decelerates and stops. As the drop continues its path, the surface concentration of A-’ in front again rapidly decreases and the process of arising and compensating the potential difference is repeated several times as long as there is any A-’ available on the back side of the drop. These oscillations are the reason for the sharp maxima in the luminescence trace. There are other known cases in which a movement of mercury electrodes is generated by a change or a difference of the electrochemical potential, e.g. the “beating mercury heart” [7] or the first -kind polarographic maxima [S-IO]. In the case of the polarographic maxima a tangential velocity u, of the mercury surface up to 40 cm s-’ was measured. A rough estimation of ut can also be made for these experiments. Between two luminescence maxima (2.5-4 tns) the surrounding~solution should be moved by at least a quarter of the drop periphery (0.67 mm at a
-
,,
:
. . . . :
181
diameter-of.0.85 n&-backward-due to the ‘fall. of the drop. and forward. due to -the surface movement of the &&ury ,From the symmetry of the, maxima. (F&4) it can be. assk+d.‘thai -both $m& take about the same -time (1.3-2 -ms). Therefore, Up should be between about’ 30.‘and: SC! cm s- ’ during the .phase bf the intensity increase. This is about as fast as the drop falls-in this region of-the trace (30-55 cm s-l), as determined from the.intensity-time curves measured tith the arrangement of Fig. 1 and calc$ated from eqns, (IO)-( 12). Such oscillations of the falling mercury drop should occur in each case in which a relatively stable product of a reversible reduction is formed. Chemiluminescence is merely a convenient method -of visual&zing the oscillating behavior,.which is mainly determined by hydrodynamic parameters and by the.diffusion. .A che.mical reaction e.g. the cleavage of R-X, leads to its earlier of the reduction product A-‘, consumption, and therefore shortens the part of the drop path in which the oscillaticn can be observed. On the other hand, the rate of reactions (2)-(S) must be fast enough to follow the frequency of the surface movement of the drop. In some cases (Fig. 4) a tenfold increase or decrease of the intensity occurs within 3 ms. Because of the bimolecular reactions, and with respect to the experimental boundary conditions, a quantitative kinetic treatment of the reaction scheme is rather complicated. Qualitatively, it can be distinguished between two cases according to the difference between the half-wave potentials of A- and R-X. If E,‘;d,(A) K Efs(R-X), the luminescence decay is mainly determined by the diffusion of A-’ and R-X between the layer of A-’ and the bulk solution, since the electron-transfer reactions (S), (4) and (5) are usually very fast. At similar bulk concentrations of A and R-X the luminescence decay is much faster than the oscillation frequency_ It can be shown that under these conditions the slower cleavage reaction (2) has no decisive effect on the decay time. However, the intensity jncreases with increasing k,, since then in the competition of reactions (8) and (5) with respect to A-‘, and of reactions (4) and (5) with respect to R’, the formation of 3A (reaction 5) is more favoured. If E,Tz(A) = E;jd,(R-X) and the cleavage is slow, the concentrations of R-X -. and A-’ are always determined by the equilibrium (a), and the luminescence decay should mainly depend on the rate of the cleavage. However, also in this case a simple first-order decay according to k, cannot be expected from the reaction scheme. In addition, the diffusion .disturbancc of the bimolecular processes (4)-(6) and side reactions of A-‘ with impurities-e.g.. traces of water and oxygen or -quenching of 3A by A-' or other species- render a quantitative interpretation of the decay curve more difficult. The latter difficulties bccome’more serious at the falling drop because of .the considerable ~dilution of A-. and R-X - - on .the trace. From these reasons a weakening of the oscillation structure of the intensity-time curve (Fig. 5) or a slow decay of the. intensity after the drop fall, as in the -case of the rubicene/iV-ptosylphthalimide system; points only qualitatively to a slow cleavage of R-X, but does not .enable the determination of thti exact iate constant k,. In the case of N-p-tosylphthalimide, k, was determined by cyclic voltammetry at 0.28 s_’ in acetonitriie [S]. ~.
-.: : 182
‘.
In sum, data on the mechanism of the cleavage reaction, which ake ktilabkfromthe oscillating behavior of the drop, seem to be rather poor. N&vektheless, it& a simply realized example of an oscillating process, which may be useful.for a.study of relationships between the surface movement and the potential difference in thecontext of the polarographic maxima. REFERENCES I D.T. Santa Crw. D.L. Akins and R.L. Birke, J. Am. Chem. Sot.. 98 (1976) i677. 2 K. Iraya. M. Kawai and S. Toshima, J. Am. Chcm. Sot.. 100 (1978) 59%. 3 F. Pragst and B. Kaltofen. J. Electroanal. Chem.. 112 (1980) 339. 4 F. Pragst. J. Electroanal. Chem., I19 (1981) 315. 5 F. Pragst and B. Kaltoien, Electrochim. Acta. submitted. 6 T.&f. Siegel and H.B. Mark. Jr., J. Am. Chem. Sot.. 94 (1972) 9020; K-G. Boto and AJ. Bard. Electroanal. Chem.. 65 (1975) !US: F. Pngst and 8. Brandt. J. Electroanal. Chem.. 99 (1979) 357. 7 J. Keizer. P.A. Rock and S.-W. Lin. 3. Amer. Chem. See.. 101 (1979) 5637 and refs. therein. 8 J. HeyrovskG and J. Kuta. Grundlagen $er Polarographie. Akademie Verlag. Berlin 1965. p. 401. 9 J. Prow, V. Cielcszky and K. Gyorbiro, Polarographie, Akadcmiai Kiad& Budapest 1967. p. 184. IO hf. v. Stackclberg. Fortschr. Chcm. Forsch.. 2 (1951) 229.
J.