J. Phys. Chem. 1995,99, 7689-7693
7689
Charge-Transfer-to-Solvent Photochemistry of Electrode-Confined Ferrocene- and Cobaltocene-Based Polymers: Photoelectrochemical Reduction of Halocarbons Helen B. Tatistcheff, Lawrence F. Hancock,?and Mark S. Wrighton* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received: August 17, 1994; In Final Form: March 6, 1995@
Like the metallocenes themselves, metallocene-based polymers exhibit near-W (280-370 nm) charge-transferto-solvent (CTTS) absorption in the presence of CC4, CHCl3, CHzC12, CBr4, and CHBr3. The photoelectrochemistry of charge transfer complexes of two ferrocene-containing polymers and one cobaltocene-containing polymer has been studied. The polymers are poly(2-ferrocenylethyl methacrylate), poly(3-(octamethylferroceny1)propyl methacrylate), and poly( 1,l'-bis[( (3-(triethoxysilyl)propyl)amino)carbonyl]cobaltocene). Photoexcitation of metallocenes and metallocene-based polymers in the presence of many halocarbons yields oxidation of the metallocene and reduction of the halocarbon. When the metallocene-based polymer is confined to the surface of an electrode that is held at a potential negative of the formal potential of the metallocene, near-UV excitation results in sustained cathodic current in electrolyte solutions containing halocarbons. The wavelength, acceptor, and potential dependences are in accord with a sustained current that is due to a metallocene-to-halocarbon CTTS absorption where the photoprocess results in the reduction of the halocarbon at an electrode potential significantly positive of where electrochemical reduction occurs in the dark. The octamethyiferrocene-basedsystem shows a more negative potential onset and a longer wavelength offset of photocurrent than the simple ferrocene-based system, consistent with the electron-releasing nature of the methyl substituents. The onset of photocurrent in the cobaltocene-based system occurs at the most negative potential of the three, consistent with the cobaltocene-based system having the most negative formal potential of the metallocenes studied.
Introduction We report sustained photoreduction of halocarbons via charge-transfer-to-solvent (CTTS) excitation of electrodeconfined ferrocene-based polymers made by polymerization of monomers I (FcEMA), I1 (MesFcPMA), and 111 (CcSiO).
I
I1
do
I11
Ferrocene, cobaltocene, and their derivatives are known to form charge transfer complexes with halocarbons, and these have characteristic electronic spectra.'s2 Upon irradiation resulting in excitation, the metallocene-halocarbon charge transfer complexes undergo a photoreaction which begins with charge transfer to yield oxidized metallocene and reduced halocarbon.
* Author to whom correspondence should be addressed.
t Present address: W. R. Grace & Co.-Conn., Lexington, MA 02173. Abstract published in Advunce ACS Absrructs, May 1, 1995.
@
The photoelectrochemistry of charge transfer complexes of FcEMA with CC4, CHC13: CHBr3, &d CBr4 was characterized in this study, as well as MesFcPMA-CC4 and CcSiO-CH2C12 complexes. Our study elaborates earlier findings that irradiation of electrode-confined poly(viny1ferrocene) in the presence of CC4 and CHC13 yields cathodic photocurrent due to the CTTS photochemi~try.~ Electronic spectra of CT complexes of ferrocene with numerous halocarbon acceptors have been reported: as have a few similar complexes of cobaltocene and nickelocene.2 Complexes of ferrocene and its derivatives with CC4 have received the most a t t e n t i ~ n . ~Reactions .~ that occur following charge transfer from ferrocene to CC4 have been studied,' and quantum yields as high as 1.9 for oxidation of ferrocene have been measured. Because CC4 decomposes rapidly upon reduction, there is little chance for back electron trnasfer to occur, so the quantum yield for the initial photoprocess is high. The quantum yield for ferrocene oxidation is further increased by reactions of CCY- decomposition products (Cl- and 'CCl3). The 'CCl3 or some other radical species is probably responsible for oxidation of a second ferrocene. Addition of acrylamide as a radical trap reduces the quantum yield for ferrocene oxidation to -1 and yields polyacrylamide.4" Similarly, polypyrrole can be made by irradiation of ferrocene-CC4 solutions containing pyrrole.8 If radicals generated by the photoprocess are not otherwise trapped, they can attack ferrocenium. Ferrocenium may react with 'CCl3 andor solvent to give cyclopentadienyl ring substitution products or FeC13.' Electrochemical reduction of ferrocenium could reduce the extent of decomposition reactions because ferrocene is more robust than ferrocenium and also makes the ferrocene center available for repeated photooxidation. Thus, if ferrocenium is electrochemically reduced, the process of halocarbon reduction becomes catalytic in f e r r ~ c e n e . ~
0022-365419512099-7689$09.0010 0 1995 American Chemical Society
Tatistcheff et al.
7690 J. Phys. Chem., Vol. 99, No. 19, 1995 Our work was undertaken with the objective of using the CTTS absorption to effect photoreduction of CC4 and other halocarbons in a system that is catalytic in ferrocene, octamethylferrocene, or cobaltocene. Rapid reduction, facilitated by confinement of the metallocene to an electrode surface, should minimize the extent of side reactions which result in irreversible decomposition of the metallocene centers. Regeneration of the reduced metallocene makes it available for subsequent CTTS reactions. A cathodic photocurrent results from irradiation of electrodeconfined metallocenes when the electrode is held at a potential where the metallocene is reduced. Thus, a metallocene with a more negative formal potential will exhibit an onset of photocurrent at more negative potentials. For different derivatives of a given metallocene a shift to more negative reduction potential is accompanied by a shift in CTTS absorbance to longer wavelength. The wavelength of the CTTS absorbance can also be manipulated by selection of the halocarbon acceptor, with more potent acceptors having lower energy CTTS absorbances. The magnitude of the photocurrent depends on excitation wavelength, the identities of the metallocene and the halocarbon in solution, the concentration of halocarbon, and the electrode potential.
Experimental Section 2-Ferrocenylethanol, VII. (Dimethy1amino)methyl)ferrocene (25 mL) was added dropwise to 10 mL of chilled CH3I in methanol. The mixture was refluxed for 1 h. ((Trimethylammonium)methyl)ferrocene iodide, IV, was precipitated by dropwise addition of ethyl ether and collected by filtrati~n.~ IV was then added to a chilled aqueous solution containing 6 equiv of KCN and refluxed for 1.5 h to give ferroceneacetonitrile, V. An adaptation of the method of Gonsalves, Zhan-ru, and Rausch was used to prepare VI1 from V.Io V was extracted into ether, which was extracted with H20 and percolated through Na2S04. A slurry of V in ethanol was added to 10% aqueous KOH and refluxed for 5 h. The solid dissolved slowly followed by evolution of NH3. Most of the ethanol was removed by rotary evaporation, and the mixture was diluted with water and extracted with ether, filtered, cooled in an ice bath, and acidified with 85% H3P04. The product, 2-ferrocenylacetic acid, VI, was collected by filtration and dried under vacuum at 100 "C. Dry ether (500 mL) was placed in a flask with 4.3 g of LiAlH4, and 25 g of VI was placed in a Soxhlet extractor. The ether was refluxed under Ar for 20 h and then extracted with 200 mL of 20% HC1, washed with water, and dried over Na2S04. Solvent was removed, and the product, VII, was recrystallized from petroleum ether: mp 41.0-41.5 "C; 'H NMR 4.09 (m), 3.71 (q), 2.57 (t), 1.6 0). Ferrocenylethyl Methacrylate, I. One equivalent of VI1 was dissolved in 50 mL of CH2C12 with 1 equiv of dimethylphenylamine and 1.5 equiv of methacryloyl chloride and refluxed until TLC indicated consumption of W.The reaction mixture was washed two times each with H20, NazCO3(aq), and H20, dried over MgS04, and filtered. The product was further purified by silica gel chromatography to afford an 83% yield of bright gold crystals. /3-OctamethylferrocenylEthylacrylate,VIII, was prepared according to the method of Zou and Wrighton." 3-(Octamethylferrocenyl)propen-l-ol, IX. In a roundbottomed flask 1.9 g of L i A l h and 150 mL of ether were combined and purged with Ar. A deaerated solution of 9.0 g of VI11 in 100 mL of ether was then added dropwise via an addition funnel. The mixture was stirred for 1 h at room temperature. After quenching unreacted L i A l b with H20, a
solution containing 10 g of N&C1 and 100 mL of H2O was added. The organic layer was extracted with H20 and saturated aqueous NaCl and dried over Na2S04. Rotary evaporation of the solvent resulted in the collection of an orange solid in quantitative yield: mp 59-62 "C. 3-(Octamethylferrocenyl)-l-propanol,X. To a threenecked 500 mL round-bottomed flask was added 8.0 g of M (0.023 mol) and 1.0 g of 10% Pd on activated carbon catalyst (0.94 mmol Pd). After purging the flask with Ar, 250 mL of deaerated ethyl acetate was transferred via cannula. The solution was bubbled with H2 for 1 h, and an HZ atmosphere was maintained for an additional 2 h. The reaction mixture was then filtered through Celite to reveal a yellow-colored solution. Upon rotary evaporation a yellow oil was observed, which solidified upon pumping in vucuo: mp 64-66 "C. (0ctamethylferrocenyl)propylMethacrylate, 11. X was treated directly with distilled, degassed methacryloyl chloride and stirred overnight. Product was isolated in the same manner as I. Polymerization. Polymers based on I and I1 were prepared by addition of monomers to 2,2'-azobis(2-methylpropionitrile) in benzene, which was degassed by freeze/pump/thaw three times and then heated to 80 "C for 12 h. The polymer precipitated upon addition of the benzene solution to CH30H and was collected by filtration. Ferrocene Polymer Deposition. Electrodes were prepared by oxidative precipitation of polymer onto a platinum flag electrode from solutions in CH2C12 (in the case of I) or THF (in the case of 11) containing 0.1 M [n-B~@]C104. Coverages, which were determined by integration of cyclic voltammograms taken in CH~CN/[~-BQN]C~O~ or CH30W[n-Bu4N]C104,were usually iO-9-iO-8 mol/cm2. Cobaltocene Polymer Deposition. Generation of I11 in situ and its electrochemical deposition have been reported.I2 Equipment and Procedures. All electrochemical experiments utilized one channel of a Pine Inst. Co. RDE4-Xl bipotentiostat. Monochromatic illumination for wavelength dependence measurements was provided by placing the quartz electrochemical cell in a Perkin-Elmer MF-44 fluorescence spectrometer equipped with a 150 W Xe lamp while the electrode was held at a potential negative of the formal potential of the metallocene, but positive of the onset of halocarbon reduction in the dark. Extended irradiation and potential dependence experiments utilized a Bausch & Lomb 250 W highpressure Hg lamp equipped with a 325 nm long pass filter. UV-vis spectra were recorded on a Hewlett-Packard Model 8452A spectrophotometer. In order to distinguish CT absorbances, a two-compartment mixing cell was used in which each compartment had a path length of 4 mm (Wilmad Corp.). One compartment contained the metallocene, either in solution or as a film of the metallocene polymer electrodeposited or cast onto an indium-tin oxide/quartz electrode in an electrolyte solution with no halocarbon. Charge transfer absorbance was taken to be the difference between the sum of the absorbances of the unmixed donor and acceptor solutions and the absorbance of the mixed solution. Pt flag electrodes were made from Si wafers coated with 5000 8, Si02 and 1500 8, Si3N4 onto which 50 8, Ti and then 1000 8, Pt were deposited by e- beam evaporation.
Results and Discussion We have characterized the photoresponse of metallocene polymers confined to the surfaces of Pt electrodes in the presence of halocarbon acceptors. When the electrode is held at a potential that is negative of the redox potential of the
Photoelectrochemical Reduction of Halocarbons
J. Phys. Chem., Vol. 99, No. 19, 1995 7691
Wavelength Dependence of Photocurrent for FcEMA I " " I " " I " " I " I Fc E MA (electrode posi ied f i l m ) - in CH 3 C N 10.I M C n Bu4N I C I O4 0.4 -i n C H 3 C N/ 5 0 % CC 14/0.1MCn -Bu4NIC IO4
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Wovelength Dependence of Photocurrent for MeaFc P M A MegFcPMA (cost film) in MeOH/O.l M[n-Bu4N1C104 e, V
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Figure 1. (a) Electronic spectra of a film of FcEMA electrodeposited on an indium-tin oxide/quartz electrode in the presence and absence of CCL. (b) Difference between spectra in part a, showing absorbance due to formation of the CT complex. (c) Wavelength dependence of photocurrent resulting from irradiation of a Pt electrode derivatized with FcEMA in CH3CN/SO% CCuO.1 M [n-BuaN]ClO4.
metallocene, irradiation at the appropriate wavelength causes charge transfer from the metallocene center to the halocarbon in solution. Very small background photocurrents were observed in the absence of either the surface-confined metallocene or the halocarbon acceptor. Figures 1 and 2 show absorption spectra of FcEMA and MeaFcPMA in the presence and absence of CC4 as well as the wavelength dependence of photocurrents (action spectra) resulting from irradiation of electrode-confined polymers in electrolyte solutions containing CC4. The action spectra correspond to the difference spectra, indicating that it is the CTTS absorbances that are responsible for the photocurrents. Similar absorbances and photoresponses were observed for FcEMA in CHCl3, CBr4, and CHBrs and for CcSiO in CH2C12. Maxima of the difference and action spectra are given in Table 1. For the series of FcEMA-solvent systems studied, the CTTS absorption shifts to longer wavelengths with stronger electron acceptor solvents. Also, changing from FcEMA to MegFcPMA shifts the CTTS absorbance with CC4 to longer wavelength and the photocurrent onset is at a more negative potential. Figure 3 shows the dependence of photocurrent from an FcEMA-derivatized elec-
0 300
400 500 Wovelength (nm)
600
Figure 2. (a) Electronic spectra of a film of MeBFcPMA cast onto a quartz plate in the presence and absence of C c 4 . (b) Difference between spectra in part a, showing absorbance due to formation of the CT complex. (c) Wavelength dependence of photocurrent resulting from irradiation of a Pt electrode derivatized with MesFcPMA in CH3OW50% CCuO.1 M [n-Bu,&]C104. trode on CC4 concentration. Photocurrent is linear with CC4 concentration from approximately 0.1 to 3 M. The potential dependence of photocurrents in the FcEMACC4, MegFcPMA-CC4, and CcSiO-CH2C12 systems is shown in Figure 4. Photocurrent (the difference between the light and dark curves) is higher at more negative potentials and drops dramatically as the metallocene is oxidized. This indicates that it is, in fact, the reduced form of the metallocene that is responsible for the photocurrent. The variation in redox potential of the metallocene centers is paralleled by a shift in the potential of the onset of photocurrent. The potential dependence of photocurrents resulting from irradiation of FcEMA in CHC13, CBr4, and CHBr3 is similar to that shown for CC4 in that photocurrent is only observed when the metallocene is present in its reduced state. Data in Table 1 include the formal potential for the metallocenes used; these are the potentials where one can find photocathodic current onsets. Data are also included showing the potential at which one observes the halocarbon reduction in the dark at a Pt electrode. The difference between the potential where one sees
Tatistcheff et al.
7692 J. Phys. Chem., Vol. 99, No. 19,1995 TABLE 4: Metallocene-Halocarbon CTTS
donor-acceptor FcEMA-CHCI~ FcEMA-CC14 FcEMA-CHBr3
FcEMA-CB~~ MegFcPMA-CC14 CcSiO-CH2C12
Potential Dependence of Photocurrent
approx onset metallocene of halocarbon photocurrent absorbance E"2 reduction peak (nm) (V vs SCE) (V vs SCE) peak (nm) +0.35' - 1.5' 215" 284b 335d 340" 352" 365< 330h
+0.35' +0.35' +0.35' +0.059 -0.629
320' 3 166 312b 35v 320'
b FcEMA ( 1 1
6ot
1
'
CH30H/50%CC~/0.1 M [n - BuqN] ClO,
-0.6c -0.9' -0.9' -0.69
