Anal. Chem. 1998, 70, 5079-5084
Development of a Dual-Electrolysis Stopped-Flow Method for the Observation of Electrogenerated Chemiluminescence in Energy-Sufficient Systems Munetaka Oyama and Satoshi Okazaki*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
New methodology for the measurement of electrogenerated chemiluminescence (ECL) has been developed by using a dual-electrolysis stopped-flow system. Using this method, the ECL from “energy-sufficient” systems composed of different kinds of ion radicals can be easily observed by mixing both the electrolyzed solutions directly. The apparatus and method have been described in detail as well as the ECL observations for various energy-sufficient systems. In particular, in the reaction between the thianthrene (TH) cation radical and the pyrene (PY) anion radical, it was found that the emission spectra changed with the addition of the precursors, reflecting both complex electron- and energy-transfer processes in solution. The present results indicate that the electron- and energy-transfer reactions changed significantly, depending on the type of molecules. The ECL observation with the addition of the third molecule was also informative to compare the ease of the formation of the excited states. It was clarified that the excited states of 9,10-diphenylanthracene and rubrene are easily formed compared with those of TH and PY. Electrogenerated chemiluminescence (ECL) can be observed through the reactions between cation and anion radicals of aromatic substances in aprotic solvents, for example, via the reactions expressed by eqs 1 and 2. The ECL observation is useful
R•+ + R•- f 1R* + R 1
R* f R + hν
(1) (2)
for elucidating electron-transfer and energy-transfer reactions of ion radicals in homogeneous solution, so that numerous studies have been devoted to the ECL reactions so far.1-3 If the ECL is observed in the reactions between the cation and anion radicals of an identical precursor as in eq 1, the reaction * Corresponding author: (Tel) +81-75-753-5882; (fax) +81-75-753-4718; (e-mail)
[email protected]. (1) Bard, A. J.; Faulkner, L.R. Electrochemical Methods, Fundamentals and Applications; Wiley: New York, 1980; pp 621-629. (2) Faulkner, L. R.; Bard, A. J. Electroanalytical Chemistry; Marcel Dekker: New York, 1977; Vol. 10, pp 1-95. (3) Tachikawa, H.; Faukner, L. R. Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T., Heineman, W. R., Eds.; Marcel Dekker: New York, 1984; pp 637-674. 10.1021/ac980692y CCC: $15.00 Published on Web 10/28/1998
© 1998 American Chemical Society
Figure 1. Redox potentials of aromatic compounds used in the ECL reactions: RU ) rubrene, TMPD ) N,N,N′,N′-tetramethyl-p-phenylenediamine, and DPA ) 9,10-diphenylanthracene. Solid gray arrows indicate that the ECL reactions between ion radicals at both ends can be observed by the conventional method. Dotted gray arrows indicate that the ECL reactions have been difficult to observe so far.
scheme is relatively simple as in eqs 1 and 2. In contrast, for the reactions between ion radicals of different molecules, the reaction becomes more complex, for example, via a triplet state as expressed by eqs 3-5.
R1•+ + R2•- f 3R1* + R2 3
R1* + 3R1* f 1R1* + R1 1
R1* f R1 + hν
(3) (4) (5)
For the ECL observation between cation and anion radicals of different precursors, however, there have been a significant restriction that arises from the energy levels of molecules concerned in the conventional electrolysis methods, such as a potential step method with a single electrode and a ring-disk electrode method, which are utilized mostly for the ECL observation. To explain this, the redox potentials of aromatic substances commonly used in the ECL measurements are shown in Figure 1. It is very difficult to observe the reaction between 9,10diphenylanthracene anion radical (DPA•-) and rubrene cation radical (RU•+) by conventional methods. This is because the reduction of RU, which is concurrent with the reduction of DPA, interferes with the observation of the reaction between DPA•- and RU•+. Thus, for the reactions between the different kinds of ion radicals, ECL has only been reported for reaction systems having a smaller redox potential difference (i.e., ∆G) than those of one Analytical Chemistry, Vol. 70, No. 23, December 1, 1998 5079
Figure 2. Structural formulas of molecules used in this work: RU ) rubrene, DPA ) 9,10-diphenylanthracene, PY ) pyrene, TH ) thianthrene, XPA ) 9-halogeno-10-phenylanthracene, and DXA ) 9,10-dihalogenoanthracene.
precursor molecule, e.g., the reaction between N,N,N′,N′-tetramethyl-p-phenylenediamine cation radical (TMPD•+) and DPA•-, which are so-called “energy-deficient” systems.1,2 Thus, the energy-sufficient reactions, e.g., the reaction between DPA•+ and pyrene anion radical (PY•-), have never been observed. However, it was anticipated that the measurement of such a reaction would involve emission directly from the singlet state (S-route), thus providing explicit information on the excited states. To make possible ECL measurements on energy-sufficient systems composed of different kinds of radical ions, in a previous letter,4 we reported a novel approach using a pulse-electrolysis stopped-flow method having dual-electrolysis columns. In this apparatus, after generating cation radicals and anion radicals separately, the ECL can be observed in an optical cell. Therefore, various reactions of the energy-sufficient systems can be targeted including sort-lived species. In the present paper, the details of the apparatus are described together with the results of the ECL measurements for various energy-sufficient systems. In particular, the features of the proposed method are described for the analyses of ECL reactions between thianthrene cation radical (TH•+) and PY•-, and those between halogenoanthracene derivative cation radicals (DXA•+ or XPA•+) and PY•-. The structural formulas of the molecules used in this work are given in Figure 2. EXPERIMENTAL SECTION Apparatus and Method. The details of the pulse-electrolysis stopped-flow method have been described previously.5 The use (4) Oyama, M.; Okazaki, S. J. Electrochem. Soc. 1997, 144, L326-L328. (5) Oyama, M.; Nozaki, K.; Okazaki, S. Anal. Chem. 1991, 63, 1387-1392.
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Figure 3. Schematic diagram of the cells for ECL measurement. (A) Cross-sectional view of dual electrolysis stopped-flow cell: CW, carbon wool working electrode; PG, porous grass tube; WE, Pt lead wire to carbon wool working electrode; CE, Pt wire counter electrode; JM, jet mixer; OC, optical cell; OF, optical fiber; SS, sample solution; CS, counter solution. (B) Cross-sectional view of optical cell for ECL observation: OF, optical fiber; QW, quartz window; JO, jet-mixing optical cell. (C) Top view of the jet-mixing optical cell in (B).
of this apparatus with a single-column electrode has enabled the analysis of the kinetics and mechanisms of the reactions of electrooxidized species with nucleophiles6,7 and those of electoreduced species with metal cations.8,9 In the present work, based on the same principle, we have newly constructed a dualelectrolysis stopped-flow apparatus having two electrolysis lines for the purpose of ECL observation. Figure 3A shows the cross-sectional view of the dualelectrolysis stopped-flow cell. Carbon wool having a feltlike structure (CW, Toho Rayon Co. Ltd.) was used as the working electrode. It was packed tightly in a microporous glass diaphragm tube (PG; 35-mm length × 6-mm i.d.). The carbon wool was in contact electrically with a Pt lead wire (WE, 0.3-mm diameter). Platinum wire (0.3 mm diameter) wound around the PG served as a counter electrode (CE). By using this column electrode, quantitative electrolysis can be achieved within several tens of milliseconds,5 so that in the present apparatus a solution of cation radical can be generated in one column and that of anion radical in the other quantitatively within a very short time. While the constant-current pulse electrolysis was used mainly in this work, (6) Oyama, M.; Nozaki, K.; Okazaki, S. J. Electroanal. Chem. 1991, 304, 6173. (7) Oyama, M.; Sasaki, T.; Okazaki, S. J. Electroanal. Chem. 1997, 420, 1-4. (8) Oyama, M.; Takei, A.; Okazaki, S. J. Chem. Soc., Chem. Commun. 1995, 1909-1910. (9) Oyama, M.; Hoshino, T.; Okazaki, S. J. Electroanal. Chem. 1996, 401, 243246.
controlled-potential pulse electrolysis can be performed with reference electrodes, which can be settled by inserting the backside of the electrolysis columns. The dual electrolysis was performed by using two potentiostats, HECS 311B, with a timer unit (Huso Co. Ltd., Kawasaki, Japan) designed especially for the present purpose. That is, the timer unit made it possible to set the electrolysis time and the electrolysis current (or potential) independently for each electrolysis line, under the conditions that the dual pulses finished at the same time. Hence, the solutions of ion radicals can be mixed effectively just after the pulse electrolysis, which is useful for observation of the ECL reactions of ion radicals with various stabilities. Immediately after imposing the pulses onto the dual-column electrodes, the electrolyzed solutions were mixed rapidly in the double two jet mixer (JM) and delivered into an optical cell (OC). Figure 3A shows the optical arrangement for the absorption measurement with two optical fibers and as an optical cell with a light path of 2.0 mm. UV-visible measurements were utilized for identification of the generation of ion radicals and for optimization of the electrolysis conditions to perform the quantitative electrolysis. For the ECL observation, while the ECL reactions could be observed using the mixer and the optical cell in Figure 3A, we refined the optical cell part to collect the ECL emission as much as possible. As a consequence, we adopted a jet mixing optical cell (JO) as shown in Figure 3B. This cell was designed to collect the emission light directly from the point where the two solutions were mixed, so that high-sensitivity measurement could be performed compared with the measurement using the jet mixer and the optical cell. The spectrophotometer used was a USP-501 (Unisoku Co. Ltd., Hirakata, Japan), which was used for the observation of UVvisible spectra and ECL emission spectra. For observation of ECL emission, we used both a photon-counting system and an imageintensified multichannel photodiode array detector (II + MCPD). The former was used for detecting the time course of the amount of the emitting light without diffracting the light. The latter was used for observing ECL spectra with a spectrophotometer. Both detection systems, the electrolysis cell, and the stoppedflow apparatus were constructed with the help of Unisoku Co. Ltd. The whole system was controlled by a microcomputer (PC9801 FA, NEC). In the ECL measurement, in particular, special attention was paid to degassing the solutions. For each solution, using a flask with two stopcocks, the oxygen was removed and replaced with nitrogen with a vacuum pump. The solutions were directly delivered to the reservoirs of the stopped-flow apparatus without contacting air. Reagents. Rubrene (RU, Nacalai tesque), 9,10-diphenylanthracene (DPA, Nacalai tesque), pyrene (PY, Wako chemicals), and thianthrene (TH, Wako chemicals) were used as received. Synthetic and purification methods for 9-chloro-10-phenylanthracenes (XPA) and 9,10-dihalogenoanthracenes (DXA) were described previously.6,10 The purification methods of acetonitrile (AN) and tetrabutylammonium perchlorate (TBAP) were also described previously.6 (10) Oyama, M.; Okazaki, S. J. Electroanal. Chem. 1991, 297, 557-563.
Figure 4. Absorption spectra of (A) DPA•+ and (B) DPA•- observed at the optimized electrolysis conditions after the electrolysis was performed only for single-electrolysis column. Sample solution: [DPA] 1.0 mM and [TBAP] 0.1 M in AN. Electrolysis time, 1.0 s. Electrolysis current: (A) 5.0 mA; (B) -7.0 mA.
RESULTS AND DISCUSSION Fundamental Evaluation of Dual-Electrolysis StoppedFlow Method for ECL Observation. We first examined how ECL reactions can be observed in the developed dual-electrolysis stopped-flow apparatus by measuring the reactions between DPA•+ and DPA•-. Before ECL measurements, electrolysis conditions were optimized by measuring the UV-visible absorption spectra. Parts A and B of Figure 4 show the absorption spectra observed at optimized electrolysis conditions for the generation of DPA•+ and DPA•-, respectively. These spectra were observed after electrolysis was performed only for a single electrolysis line. In these measurements, the sample solution contained 1.0 mM DPA and 0.1 M TBAP in AN, and the electrolysis time was 1.0 s. For maximum generation, the electrolysis current was 5.0 mA for DPA•+, and -7.0 mA for DPA•-. The difference in the current values presumably reflects the difference in the residual currents, of which the reduction is larger than the oxidation in the electrolysis of DPA. For other ECL reaction systems, the conditions for maximum generation of ion radicals were optimized before the ECL measurements. Figure 5 shows the time course of the absorbance and the ECL observation. Parts A and B of Figure 5 are the results measured after electrolysis with a single column as in the above measurement of absorption spectra. These show that DPA•+ and DPA•- were delivered into the optical cell by a piston drive and remained in the cell without decaying. Figure 5C shows the result of photon-counting measurement after generating both solutions of DPA•+ and DPA•-, and then they were mixed together in the optical cell. The emission was clearly detected on the time domains when the solutions were flowing into the optical cell. Since the ECL reactions are so fast as to be described by the diffusion-controlled reactions, it is inferred that the emission disappeared immediately after the solutions were stopped. However, even for such a fast reaction, owing to the high concentration generation of homogeneous ion radical solutions using the column electrolysis method, the proposed method was found to be utilized for the observation of ECL reactions. By preliminary measurements using photon-counting detection, the optical part was refined to achieve the effective ECL observaAnalytical Chemistry, Vol. 70, No. 23, December 1, 1998
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Figure 7. Redox potentials of the energy-sufficient ECL reaction systems examined in the present work and the results of the emissions. Solid gray arrows indicate that the ECL emission arose from 1RU*. Dotted gray arrows indicate that the ECL emission came from 1DPA*.
Figure 5. Time course of the absorbance (A, B) and the ECL emission (C). (A, B) Absorbance at 650 nm measured before and after the piston drive synchronized with the generation of DPA•+ and DPA•-, respectively. Measured after the electrolysis for only single column. (C) Changes in the emission light with time light detected by a photon-counting method. Measured after generating both the solutions of DPA•+ and DPA•- and mixing them together in the optical cell. The conditions of piston drives are the same in the three measurements.
to proceed via the S-route,1 it is assumed that the ECL was observed directly through 1RU* as expressed by eq 6.
RU•+ + DPA•- f 1RU* + DPA
The ECL reaction between DPA•+ and PY•- is analogous to that of the RU•+-DPA•- system. In the reaction between DPA•+ and PY•-, DPA is easier to reduce than PY (Figure 7). When DPA•+ and PY•- were mixed, the emission source was found to be 1DPA*, as shown in Figure 6A. Also, judging from the ∆G of the DPA•+-DPA•- system that proceeds via the S-route,2,3 the reaction of the DPA•+-PY•- system is expressed by eq 7.
DPA•+ + PY•- f 1DPA* + PY
Figure 6. Emission spectra from (A) 1DPA* and (B) 1RU*. The shapes of the emission from 1DPA* were changed slightly according to the ECL system as described in the text. The shape of the emission from 1RU* was similar for all the systems reported in the present work.
tion as shown in Figure 3B. For the measurements of ECL emission spectra, the signal was accumulated to cover the whole emission sufficiently. The gate time of MCPD was 200 ms. Figure 6A shows the emission spectra of the ECL reaction between DPA•+ and DPA•-. The ECL from 1DPA* was clearly observed. In the following sections, the electron-transfer and the energy-transfer reactions are mainly discussed through the ECL emission spectra. ECL Observation for Energy-Sufficient Systems. Next, we observed the ECL reactions of several energy-sufficient systems, which have not been investigated previously. Figure 7 summarizes the ECL reactions of the targeted systems. For example, the reaction between RU•+ and DPA•- is “energy sufficient” because the ∆G is larger than that of RU•+-RU•- system. This ECL observation has been difficult with conventional methods because RU is more easily reduced than DPA (see Figure 7). However, the ECL could be successfully observed with the proposed method by preparing the solutions of RU•+ and DPA•independently and mixing them directly. For this case, the observed emission spectrum was that from 1RU* as shown in Figure 6B. Because the ∆G in the RU•+-DPA•- system is greater than that in the reaction between RU•+ and RU•-, which is known 5082 Analytical Chemistry, Vol. 70, No. 23, December 1, 1998
(6)
(7)
For the emission spectra observed in this ECL system, it is interesting to point out that a minor but remarkable change was observed in the emission spectra 1DPA*. Namely, the emission intensity of the shoulder peak at the shortest wavelength, i.e., highest energy, around 410 nm was apparently increased compared with the ECL spectra observed in the DPA•+-DPA•- system. This change is inferred to be a reflection that sufficient energy has been supplied in the DPA•+-PY•- reaction system. However, in this case, the emission from 1PY* was not observed even though the ∆G is sufficient or comparable to produce 1PY* as to produce 1DPA*. Six ECL reaction systems having large ∆G values, summarized in Figure 7, were observed by using the proposed method. As a result, from the emission spectra, the reactions were clarified to be expressed by the following equations.
RU•+ + PY•- f 1RU* + PY •+
DPA TH
•+
•+
TH
+ RU
•-
+ RU
•-
+ DPA
•-
(8)
1
(9)
1
(10)
1
(11)
f RU* + DPA f RU* + TH f DPA* + TH
These results indicate that, even in ECL reaction systems having large ∆G, which are studied here for the first time, the ECL was emitted from the lower available singlet state after the direct electron transfer.
wavelength region around 460 nm was observed to increase significantly, as shown in Figure 8C. These results indicate that the emission generated after the reaction between TH•+ and PY•is surely a superposition and is affected by the composition of the mixed solutions. To clarify the sources of the emission spectra, we measured the fluorescent spectra. In the case of PY, the emission from 1PY* was observed at ∼390 nm with three peaks, and a broad fluorescent band was observed around 480 nm for the exicimer (1PY2*). The emission from TH shows the fluorescent band at 440 nm. These results are similar to the reported ones.11,12 By comparing the fluorescent components of the spectra, the following reactions are proposed to explain the observed ECL emission.
TH•+ + PY•- f 1PY* + TH Figure 8. Emission spectra observed in the TH•+-PY•- system: (A) spectrum observed in the reaction between 1.0 mM TH•+ and 1.0 mM PY•-; (B) spectrum observed after the solution containing 1.0 mM TH•+ and 4.0 mM TH was mixed with the solution of 1.0 mM PY•-; (C) spectrum observed after the solution containing 1.0 mM TH•+ was mixed with the solution of 1.0 mM PY•- and 4.0 mM PY.
Because the excitation energies for the singlet states are 2.2, 2.8, 3.0, and 3.4 eV for RU, TH, DPA, and PY, respectively,2,11 energetically favorable emission was found to be observed in many cases. The only exception was the TH•+-DPA•- system (eq 11), in which only the emission from 1DPA*, not from 1TH*, was observed. This result implied that the electron-transfer reaction in ECL is affected not only by the excitation energies of the molecules but by the ease of formation of the excited state, depending on the molecules. ECL Reaction between TH•+ and PY•-. In the reaction between TH•+ and DPA•-, the observed emission was found to be that of 1DPA*. To examine the electron-transfer reactions concerning TH•+, we observed the ECL reactions of TH•+ and PY•-. The ECL reaction concerning TH•+ has received only little attention so far,11 and the ECL reaction between TH•+ and PY•has never been observed previously using the conventional methods. In the present work, the dual-electrolysis stopped-flow method has permitted ECL observation in the reaction between TH•+ and PY•-. Figure 8A shows the emission spectrum observed when 1.0 mM TH•+ was mixed with 1.0 mM PY•-. It was inferred from the complexity of the spectrum that the emission is not from a singlet state of only one component. To study the emission in the reaction between TH•+ and PY•in more detail, the ECL emission was observed in the presence of ion precursors in the solution. The effect of the precursors on the emission could be observed easily by controlling the concentration of generated species, which is one of the great advantages of the proposed method. Figure 8B shows the emission observed after the solution containing 1.0 mM TH•+ and 4.0 mM TH was mixed with the solution of 1.0 mM PY•-. This figure clearly shows the increase of emission around 440 nm. On the other hand, when 4.0 mM neutral PY existed in the solution, emission at the longer (11) Keszthelyi, C. P.; Tachikawa, H.; Bard, A. J. J. Am. Chem. Soc. 1972, 94, 1522-1527.
(12)
followed by 1
PY* + TH f 1TH* + PY 1
PY* + PY f 1PY2*
(13) (14)
Since the energy difference was sufficient or comparable to produce 1PY* in eq 12, the emission from 1PY* with three sharp emission peaks is assumed to be always observed around 390 nm. The changes in the emission spectra by adding the ion precursors demonstrate the existence of the following energy-transfer reactions as in eqs 13 and 14. While other excited states as in eqs 15 and 16 might be generated, the energy transfer from 1PY* would
TH•+ + PY•- f 3PY* + TH TH
•+
+ PY
•-
1
f TH* + PY
(15) (16)
be a major process because the effect of the precursors is significant, as shown in Figure 8. Energy-Sufficient ECL Reactions in the Presence of a Third Component. The result in the TH•+-PY•- reaction system is quite different from that of the DPA•+-PY•- system, whose emission is definitely from 1DPA*, even though the formal oxidation potentials and the emission maximums are similar between DPA and TH. When DPA exists as one component of the emission system, its excited state is assumed to be easily formed so as not to transfer the energy to form 1PY2*. RU also seems to produce a stable excited state because the emitted light in the RU•+-DPA•- system was found to come from 1RU*. In contrast, in the TH•+-PY•- system, it is assumed there are no definite excited states such as 1DPA* and 1RU*. Therefore, the excited state once formed tends to transfer the energy easily, as in eqs 12-14. This indicates that the energy-transfer reactions change significantly, depending on the type of molecules and the ease of formation of the excited state, both of which reflect the appearance of the emission light. To inspect this hypothesis, we observed the changes in the emission spectra of the energy-sufficient ECL systems with the addition of a small amount of RU. Figure 9 shows the emission (12) Maloy, J. T.; Bard, A. J. J. Am. Chem. Soc. 1971, 93, 5968-5981.
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Figure 9. Emission spectrum observed in the energy-sufficient systems with the addition of a small amount of RU for the (A) DPA•+DPA•- and (B) TH•+-PY•- systems. These emissions were observed after mixing a solution of 1.0 mM anion radical with a solution containing both 1.0 mM cation radical and ∼0.05 mM RU•+.
spectra observed for the DPA•+-DPA•- and TH•+-PY•- systems (the concentration of all ion radicals was 1.0 mM) in the presence of ∼0.05 mM RU•+ as the third component. In the dual-electrolysis stopped-flow method, this complicated measurement could be easily performed by adding the third component into one electrolysis line. The emission spectra observed for the TH•+-DPA•- and DPA•+-PY•- systems were almost similar to that of the DPA•+DPA•- system in Figure 9A. Thus, in the cases where a definite excited state of 1DPA* existed in the systems, the emission spectra were proved to mainly come from 1DPA*. On the other hand, a significant change was seen in the TH•+-PY•- system: i.e., the ECL emission spectrum had changed completely from Figure 8A to Figure 9B. The emission observed was from 1RU*, which can easily be recognized by comparing with the spectrum in Figure 6B. Therefore, a very small amount of RU is assumed to scavenge the energies in this system effectively to generate the emission. From these results, it was clarified experimentally that the excited states of 1DPA* and 1RU* are easily formed compared with 1PY* and 1TH*. The scavenge is assumed to be caused by the energy-transfer reaction from the energy supplied as a gap of the redox potentials in the case where there are no definite existing states. It seems that this is also related to the quantum efficiencies, which for DPA and RU are known to be high. ECL Reactions between Halogenoanthracene Derivative Cation Radicals and PY•-. In the dual-electrolysis stopped flow method, it is advantageous that ECL reactions of short-lived ion radicals can be analyzed. Therefore, we observed the ECL reactions concerning halogenoanthracene derivative cation radicals (9-halogeno-10-phenylanthracene cation radical (XPA•+) or 9,10-dihalogenoanthracene cation radical (DXA•+); X ) Cl, Br). These cation radicals are relatively short-lived in AN; e.g., the halflife of DClA•+ was ∼1 s.10 However, quantitative generation could be achieved with the proposed method by the 50-ms electrolysis pulse. Because XPA•- or DXA•- is quite unstable due to the cleavage reactions of X-, there have been no studies on the ECL reactions of these compounds. In the present work, by adopting PY•- as a partner of the ECL reactions, the emission spectra were observed clearly. The emission spectra observed for the ClPA•+-PY•- and DClA•+-PY•- systems are shown in Figure 10. In all cases, the ECL spectra showed that the emissions were from the anthracene derivatives rather than from the excited states of PY, because the 5084 Analytical Chemistry, Vol. 70, No. 23, December 1, 1998
Figure 10. Emission spectrum observed in the (A) ClPA•+-PY•and (B) DClA•+-PY•- systems. These emissions were observed after mixing a solution of 1.0 mM cation radical with a solution containing 1.0 mM PY•-.
typical emission of 1PY* as in Figure 8 was not observed at all. However, while the shape of the emission spectrum of ClPA•+ is similar to that from 1DPA*, the emission spectrum of DClA•+ had a broad emission at the longer wavelength region, which did not coincide with the absorption spectrum.10 For BrPA•+ and DBrA•+, the emission spectra had complex profiles with broad emission bands at the longer wavelength, although their absorption spectra were similar in both substituents (Cl and Br), as we reported previously.10 On the basis of the principle of the proposed method that direct mixing can be carried out in multicomponent systems, the formation of the exciplex is presumed to be a most probable pathway for interpreting these emission spectra. The present results clearly demonstrated that the excited states were affected by the molecular properties, such as the stability of the ion radicals and the type of substituents. CONCLUSIONS The dual-electrolysis stopped-flow method with dual-electrolysis columns has been successfully applied to the analysis of the energy-sufficient ECL systems between the radical ions of different molecules, which has previously been difficult with the conventional electrochemical methods. Since both the electrooxidation and electroreduction states can be formed quantitatively and independently, the electron- and energy-transfer reactions in solution can be observed explicitly with this method. In addition, by selecting reaction species with a sufficient ∆G gap, the details of the ECL reactions through the singlet state can be analyzed. In the present work, it was shown that the emitted light is easily controllable by the composition of the solution and that the characteristics of the emission were clearly dependent on the type of molecule. These observations for controlling the emitting light will be valuable to expand the field and applications of ECL reactions, especially for adopting the multicomponent emission systems. ACKNOWLEDGMENT This work was supported in part by Grants-in Aid Scientific Research, 05554025, 07555567, and 09237235, from the Ministry of Education, Science and Culture, Japan. The authors thank Dr. Toshihiko Nagamura, Unisoku Co. Ltd., for his help in the construction of the apparatus. Received for review June 29, 1998. Accepted September 17, 1998. AC980692Y
