Anal. Chem. 1907, 59, 1636-1638
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(23) Dovichi, N. J.; Nolan, T. G.; Weimer, W. A. Anal. Chern. 1084, 56. 1700- 1704. (24) Nolen, T. G.; Weimer. W. A.: Dovichi, N. J. Anal. Chem. 1084, 56. 1704-1707. (25) Weimer, W. A.; Dovichi, N. J. J . Appl. phvs. 1086, 59, 225-230. (26) Weimer, W. A.; Dovichi, N. J. Appl. Opt. 1985, 24, 2981-2986. (27) Weimer, W. A.; Dovichi. N. J. Appl. Spectrosc. 1085, 39, 1009- 10 13. (28) Burgi, D. S.;Nolan, T. G.; Risefelt, J. A,; Dovichi, N. J. Opt. f n g . 1084, 23, 756-758. (29) Jackson, W. B.; Amer, N. M.; Boccara, A. C.; Fournier, D. Appl. Opt. 1081, 20, 1333-1344.
(30) Wetsel, G. C.; Stotts, S. A. Appl. Phys. Lett. 1083, 42, 931-933. (31) Dovichi, N. J.; Harris, J. M. Proc. SPIE-Int. SOC. Opt. fng. 1981, 288, 372-375. (32) Woodruff, S. D.; Yeung. E. S. Anal. Chem. 1082, 54, 1174-1178. (33) Wilson, S. A.; Yeung, E. S.Anal. Chem. 1085, 5 7 , 2611-2614.
RECEIVED for review December 23, 1986. Accepted March 2, 1987. This work was funded by the Natural Sciences and Engineering Research Council of Canada.
Fluorescence Emission as a Probe To Investigate Electrochemical Polymerization of 9-Vinylanthracene Prashant V. Kamat
Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556
The cationic polymerization of 9-vinylanthracene can be InC tiated at a transparent Sno, electrode with the application of anodic potentials ( E > 1.1 V vs. saturated sodium chloride calomel electrode) In acetonitrile solutions. The exclmer emlsslon (emission maxlmum 500 nm) of poiy(9-vinylanthracene) whlch is dlstingulshabie from the monomer fluorescence emlsslon (emlsslon maxima 410,430 nm) has been wed to probe the eiectrochemkai polymerization process directly. The in situ spectroelectrochemkai technique, which wouM be useful in obtalning kinetic and mechanistic Information of the electropdymerizatlon process, Is described.
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In recent years, much attention has been drawn to the development and characterization of conducting polymers (1-4). Many aromatic heterocyclic molecules, upon electrochemical oxidation, lead to the formation of an electrically conducting organic polymer film at the electrode surface (1). Recently we reported ( 5 ) electrochemical polymerization of 1-vinylpyrene a t a conducting SnOpelectrode achieved with the application of anodic potentials. Such polymers with aryl molecules as pendent groups often exhibit excimer emission which is distinctively different from the monomer fluorescence emission. It is of interest to see whether one could utilize this photophysical property to probe the polymerization process directly. The purpose of this study is to demonstrate the feasibility of an in situ spectroelectrochemical technique in monitoring the electropolymerization process by using such fluorescence emission as a probe. As shown earlier (610), a technique could be useful for electrochemists to study the electrode surface as well as the transients and products generated in an electrochemical reaction.
EXPERIMENTAL SECTION Materials. 9-Vinylanthracene (Aldrich),acetonitrile (Aldrich, gold label), and tetrabutylammonium perchlorate, TBAP (Alfa), were used as supplied. SnOz electrodes were cut from an antimony-doped NESA glass obtained from PPG industries and were cleaned as described earlier (11). Instrumentation. Electrochemical and spectroelectrochemicd measurements were done with a Princeton Applied Research (PAR) Model 173 potentiostat/galvanostat, a PAR Model 175 universal programmer, and a Kipp and Zonnen X-Y recorder. Experiments were performed in a standard three-compartment
cell with a Pt wire as a counter electrode (CE) and a saturated sodium chloride calomel electrode (SSCE) as a reference electrode (RE). The potentiostat had a provision to compensate for the iR drop and this was employed during the electrochemical measurements. For spectroelectrochemical measurements a poly(tetrafluoroethy1ene) block was machined to accommodate a 0.5 cm X 4 cm SnOz plate (WE), a Pt wire (CE), and an Ag wire (RE) and was inserted into the 1 x 1 x 4.5 cm cuvette (Figure 1). The distance between the SnOP and the front wall of the cuvette, which was about 0.5 mm, allowed a thin layer of the sample. The whole assembly could then be inserted into the sample chamber of the fluorometer such that the excitation beam was at 45' to the SnOz electrode. In principle this spectroelectrochemical cell was similar to a previously described OTTLE-optically transparent thin layer electrochemical-cell (9, IO). Fluorescence measurements were carried out in the front face geometry. A SLM single photon counting fluorescence spectrometer was used t o monitor the fluorescence emission.
RESULTS AND DISCUSSION Electropolymerization of 9-Vinylanthracene. Cyclic voltammograms observed under anodic scans are shown in Figure 2. At potentials around 1.1 V (vs. SSCE) 9-vinylanthracene exhibited an irreversible oxidation and led to a curve crossing in the reverse scan. Successive anodic scans led to the decrease in the peak current and simultaneous growth of the polymer film a t the electrode surface. This behavior was similar to the electrochemical polymerization of 1-vinylpyrene and is induced by the radical cation of 9vinylanthracene formed during electrochemical oxidation The
polymer is insoluble in acetonitrile but soluble in solvents such as T H F or CH2C12. Absorption and Emission Characteristics. The excitation and emission spectra of 9-vinylanthracene in CH&N and poly(9-vinylanthracene) film coated on the SnO, electrode are shown in Figure 3. (The polymer film coated S n 0 2plate was carefully washed with CH&N to remove any traces of the monomer.) The monomer exhibits absorption maxima at 350,368, and 388 nm and emission maxima at 420 and 428 nm. The polymer film has absorption maxima a t 358, 376, and 396 nm which is red-shifted compared to its monomer
0003-2700/87/0359-1636$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
p- 9.9mm--/
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(a)
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WAVELENGTH ( n m l
I
Figure 1. Design of the cell used in spectroelectrochemical measurements.
W
t -I W
300
500
400
600
I
WAVELENGTH ( n m )
Figure 3. Excitation and emission spectra of (a) 9-vinylanthracene in acetonitrile and (b) poly(9-vinylanthracene) film formed at'the SnO, electrode.
?
I
Emirrion at SPOnm
Figure 2. Cyclic voltammograms of 0.1 M 9-vinylanthracene in acetonitrile which exhibit electropdymerizatknat the SnO, electrode during repetitive scans (scan rate, 50 mV/s; electrolyte, 0.1 M TBAP).
absorption maxima. The emission of poly(9-vinylanthracene) is broad with a maximum around 500 nm and is distinctively different from the monomer emission. Such an emission which is commonly observed with many anthracene polymers is attributed to the formation of the excimers (14, 15). Two anthracene units attached to the C-C chain can partially overlap to induce an excimer emission in such polymers. Since the emission bands of the monomer and the polymer are well separated from each other (e.g., a t 520 nm the fluorescence emission is dominated only by the polymer), it should be possible to probe the polymerization process directly by monitoring the fluorescence emission. Spectroelectrochemistry T o Probe the Electropolymerization Process. It has been demonstrated earlier (8-13) that spectroelectrochemistry can be useful in investigating the
0.5
I
I
I
I
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0.9
1.1
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Figure 4. Fluorescence emission monitored at 520 nm during anodic cycling (WE, SnO,; electrolyte, 0.2 M 9-vinylanthracene, 0.1 M TBAP; scan rate, 10 mV/s).
events that follow the electrochemical oxidation or reduction step. We have employed a similar approach to monitor the electrochemical polymerization of 9-vinylanthracene. The effect of anodic cycling on the fluorescence emission a t 520 nm is shown in Figure 4. During the anodic scan, a sudden increase in the emission at 520 nm was observed as the potential was increased beyond 1.0 V. Such an increase in the
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 13, JULY 1, 1987
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,
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Flgure 5. (a) Increase in the relative emission intensity at 520 nm after the application of an anodic potential of 1.1 V and passage of 1.2 X lo-* C through the circuit. The growth in the emission was monitored after disconnecting the applied potential at t = 0 s (WE, SnO,; electrolyte, 0.2 M 9-vinylanthracene, 0.1 M TBAP in acetonitrile at 295 K). (b) Plot of log (I-' - 1') vs. time representlng the pseudo-first-order fit of the fluorescence emission growth.
fluorescence emission at 520 nm represents the formation of poly(9-vinylanthracene) at the SnOpelectrode. When observed separately, the emission a t 420 nm was found to decrease during the anodic scan in a similar way which indicated the depletion of the monomer during the electropolymerization process. The potential at which the sudden increase in the excimer emission at 520 nm (or decrease in the monomer fluorescence emission at 420 nm) could be seen matched the oxidation of 9-vinylanthracene observed in the cyclic voltammogram. Kinetics of Polymerization. Electrochemical oxidation of the vinylaryl compound leads to the formation of its radical cation which is capable of initiating the polymerization process. The mechanism of the polymerization process can be described by the steps initiation
M L P , propagation
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n = 1, 2, ... (2) P,+l where M is a monomer and P, and P,+l are growing radical-cations having n and ( n + 1) monomeric units linked P,
+M
together covalently. The application of anodic potential results in the generation of P1 at the electrode (reaction 1). Subsequent chemical reaction of electrogenerated species (reaction 2) can be monitored optically to investigate the kinetics of polymerization. If the mechanism described by reactions 1 and 2 is true, one would expect these radical cations to continue to propagate and add on monomeric molecules until all the monomer is depleted. Indeed, when the applied potential (- 1.1V) at the SnO, electrode was disconnected, the fluorescence emission a t 520 nm continued to increase until the monomer is exhausted. A typical growth in the excimer emission observed with the electrochemical initiation is shown in Figure 5a. ( t = 0 represents the time a t which the applied potential was disconnected.) The observed exponential growth in the excimer emission represented the propagation of the polymer chain to add on monomeric units. This experimental trace can be treated with pseudo-first-order kinetics as represented - If, vs. t , where Imf, the in Figure 5b. The plot of log (Imf fluorescence intensity at infinite time, and If, the fluorescence intensity at any given time t , was a straight line and gave a
value of 0.004 s-l as the apparent pseudo-first-order rate constant for the chain propagation reaction. The concentration of the monomer was 0.2 M (30 @molin 0.15 mL) and the concentration of the initiation, which was controlled by monitoring the charge passed through the circuit, was 1.2 X mol. The feasibility of an in situ spectroelectrochemical technique to investigate the electropolymerization process has been demonstrated in this paper. Further details regarding the kinetics and the mechanistic features of the electrochemical polymerization of 9-vinylanthracene will be published elsewhere (16).
ACKNOWLEDGMENT Helpful discussions with Santosh K. Gupta, Department of Chemical Engineering, are greatly appreciated. Registry No. SnOz,18282-10-5;9-vinylanthracene,2444-68-0; poly(9-vinylanthracene), 29659-51-6. LITERATURE CITED Waltman, R. J.; Bargon, J. Can. J . Chem. 1986, 6 4 , 76. Frommer, J. E. Acc. Chem. Res. 1986, 19, 2, and references cited therein. Waltman, R. J.; Diaz, A. F.; Eargon, J. J . Electrochem. SOC. W64, 131, 740. Kamat, P. V.: Easheer, R. A. Chem. Phys. Lett. 1984, 103, 503. Kamat, P. V.; Easheer, R. A,; Fox, M. A. Macromolecules lg65, 18, 1386. Murray, R. W.; Heinemann, W. R.: O'Dorn, G. W. Anal. Chem. 1967, 39, 1666. Kuwana. T.;Winograd, N . In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1974; Vol. 7, p 1. Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1976, 9 , 241, and references clted therein. Heineman, W. R . Anal Chem. 1978, 50, 390A. and references cited therein. De Angelis, J. P.; Heineman, W. R. J . Chem. Educ. 1976, 53, 594. Kamat, P. V.; Fox, M. A.; Fatiadi, A. J . J . A m . Chem. Soc, 1984, 106, 1191. Albertson, D. E.; Elount. H. N.; Hawkrldge, R. M. Anal. Chem. 1979, 57, 556. Elubaugh, E. A.; Yacynych, A. M.; Heineman. W. R. Anal. Chem. 1979, 57, 561. Hargreaves, J. S.; Webber S. E. Macromokcules 1984, 17, 235. Semerak, S. N.; Frank, W . C. A&. Po/ym. Scl. 1983, 5 4 , 31. Kamat. P. V.; Gupta, S. K., submitted for publication in J . Electroanal. Chem .
RECEIVED for review October 16, 1986. Accepted March 19, 1987. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Document No. NDRL-2914 from the Notre Dame Radiation Laboratory.
