NANO LETTERS
Multielectron Transfer at Heme-Functionalized Nanocrystalline TiO2: Reductive Dechlorination of DDT and CCl4 Forms Stable Carbene Compounds
2006 Vol. 6, No. 6 1284-1286
Jonathan R. Stromberg, Joshua D. Wnuk, Rachelle Ann F. Pinlac, and Gerald J. Meyer* Departments of Chemistry and Materials Science and Engineering, Johns Hopkins UniVersity, 3400 North Charles Street, Baltimore, Maryland 21218 Received March 22, 2006; Revised Manuscript Received April 27, 2006
ABSTRACT Hemin (chloro(protoporhyrinato)iron(III)) was found to bind to mesoporous nanocrystalline (anatase) TiO2 thin films from dimethyl sulfoxide solution, Keq ) 105 M-1 at 298 K. Band gap illumination in methanol reduced hemin to heme and led to the appearance of TiO2 electrons, heme/TiO2(e-). Reactions of heme/TiO2(e-) with CCl4 or 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) led to the formation of stable carbene products in greater than 60% yield. The spectroscopic data are fully consistent with a dissociative two-electron organohalide reduction mechanism of CCl4 and DDT to yield (protoporhyrinato)FeIICCl2 and (protoporhyrinato)FeIICdC(p-Cl-phenyl)2 respectively.
Photon-driven multielectron transfer (MET) reactions represent an important goal in the photochemical sciences.1-5 MET processes avoid high-energy free radical intermediates and can yield desired reaction products under mild, environmentally relevant reaction conditions. Considering their importance, it is surprising that so few clear examples of MET reactivity are documented in the nanoscience literature.1,6 This may be due to the fact that it is often experimentally difficult to establish whether reaction products at illuminated semiconductor interfaces resulted from MET reactions or from secondary photochemical reactions.6 Here we describe direct evidence for MET reduction of organohalide pollutants by heme catalysts anchored to mesoporous nanocrystalline (anatase) TiO2 thin films. More specifically, reactions of heme/TiO2(e-) with DDT (or CCl4) were found to form carbene products through two-electron-transfer processes. Significantly, the carbene product was stable on the anatase nanocrystallites and could be removed from the contaminated solution. Mesoporous nanocrystalline TiO2 films were prepared by the hydrolysis of Ti(i-OPr)4 with a sol-gel method previously described in the literature.7 Anatase TiO2 nanoparticles prepared in this manner were ∼15 nm in diameter, and the film thickness was ∼10 µm. Chloro(protoporhyrinato)iron(III) (hemin, >99.0%, Fluka), 1,1-bis(p-chlorophenyl)-2,2,2* To whom correspondence may be addressed. E-mail:
[email protected]. 10.1021/nl060646a CCC: $33.50 Published on Web 05/19/2006
© 2006 American Chemical Society
trichloroethane (DDT, 98%, Aldrich), and CCl4 (>99.5%, Aldrich) were used as received. Carbenes were synthesized by stoichiometric reactions of hemin with DDT (or CCl4) in the presence of excess reducing agent, as has been previously described.8 Heme, hemin, and the carbenes were independently characterized and anchored to TiO2 thin films. Figure 1 shows the absorption spectra of these porphyrin compounds attached to nanocrystalline TiO2 thin films immersed in methanol.8,9 Significant differences in the Soret bands as well as the R and β bands (Q-bands) were clearly observed, which allowed heme reaction products to be clearly identified. The hemin surface coverage could be independently tuned from zero to a saturation surface coverage of 1 × 10-8 mol/cm2 through equilibrium binding in DMSO solutions, Keq ) 1.0 × 105 M-1 at room temperature. In this study, the surface coverage was maintained at 1 × 10-9 mol/cm2. In typical MET experiments, hemin-functionalized TiO2 films were immersed in N2-saturated methanol and illuminated with ultraviolet light, λ 600 nm) was found to decrease on the same time scale as the heme Soret band (λ ∼418 nm). Significant absorption increases in the Q-band region and at λ 60%) yields that remained on the nanoparticle surface for periods of days. Similar carbenes have previously been synthesized and X-ray crystallographically characterized by organometallic chemists.8 Carbene intermediates have also been invoked in the bioremediation of organohalide pollutants.17,18 In this regard, it is remarkable that the anatase nanocrystallites can provide the multielectron-transfer pathway necessary to produce these same carbenes and at the same time stabilize them such that they can be removed from the contaminated solution. Future studies will focus on elucidating the molecular level mechanistic details of multielectrontransfer reactivity at these nanocrystalline semiconductor interfaces under conditions where the relative concentrations of TiO2(e-) and hemes are systematically varied. Acknowledgment. This work was supported by the National Science Foundation and Collaborative Research Activities for Environmental Molecular Science (CRAEMS) Program. References (1) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (2) Hirakawa, T.; Kamat, P. V. J. Am. Chem. Soc. 2005, 127, 3928. (3) Heyduk, A. F.; Nocera, D. G. Science 2001, 293, 1639. (4) Konduri, R.; de Tacconi, T. R.; Rajeshwar, K.; MacDonnell, F. M. J. Am. Chem. Soc. 2004, 126, 11621. (5) Fukuzumi, S.; Okamoto, K.; Tokuda, Y.; Gros, C. P.; Guilard, R. J. Am. Chem. Soc. 2004, 126, 17059. (6) Choi, W.; Hoffmann, M. R. J. Phys. Chem. 1996, 100 (6), 2161. (7) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Inorg. Chem. 1996, 35, 5319. (8) (a) Lange, M.; Chottard, J. C.; Guerin, P.; Mansuy, D. J. C. S. Chem. Commun. 1977, 648. (b) Mansuy, D.; Lange, M.; Chottard, J. C.; Bartoli, J. F.; Chevrier, B.; Weiss, R. Angew. Chem., Int. Ed. Engl. 1978, 17, 781. (c) Lange, M.; Chottard, J. C.; Mansuy, D. J. Am. Chem. Soc. 1978, 100, 3213. (d) Mansuy, D. Pure Appl. Chem. 1980, 52, 681. (9) Obare, S. O.; Ito, T.; Balfour, M. H.; Meyer, G. J. Nano Lett. 2003, 3 (8), 1151. (10) Rothenberger, G.; Fitzmaurice, D.; Gratzel, M. J. Phys. Chem. 1992, 96, 5983. (11) Tamaki, Y.; Furube. A.; Murai, M.; Hara, K.; Katoh, R.; Tachiya, M. J. Am. Chem. Soc. 2006, 128, 416. (12) Saveant, J.-M. J. Am. Chem. Soc. 1992, 114, 10595. (13) Obare, S. O.; Ito, T.; Meyer, G. J. J. Am. Chem. Soc. 2006, 128, 712. (14) For recent reviews see: (a) Gra¨tzel, M. Nature 2001, 414, 338. (b) Watson, D. F.; Meyer, G. J. Annu. ReV. Phys. Chem. 2005, 56, 119. (15) Cao, F.; Oskam, G.; Searson, P. C.; Stipkala, J.; Farzhad, F.; Heimer, T. A.; Meyer, G. J. J. Phys. Chem. 1995, 99, 11974. (16) Oskam, G.; Bergeron, B. V.; Meyer, G. J.; Searson, P. C. J. Phys. Chem. B 2001, 105, 6867. (17) Wade, R. S.; Castro, C. E. J. Am. Chem. Soc. 1973, 95, 226. (18) Brault, D. EnViron. Health Perspect. 1985, 64, 53.
NL060646A Nano Lett., Vol. 6, No. 6, 2006
