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Using a Push-Pull Azobenzene Haptan to Probe Surface-Core Electronic Communication in Surface-Functionalized CdS Quantum Dots† Abdiaziz A. Farah,‡ Christopher Dares, and William J. Pietro* Department of Chemistry, York UniVersity, 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3 ReceiVed: May 12, 2010; ReVised Manuscript ReceiVed: August 4, 2010
Synthesis and characterization of a novel azobenzene derivative 4-(4-nitrophenylazo) thiophenol is reported. The preliminary investigation of this ligand as a probe for the demonstration of surface-core electronic communication in CdS quantum dot nanoparticles is discussed. Introduction During the past two decades, research on semiconductor clusters of nanometer size dimensionality, frequently referred to as “quantum dots”, has revealed many unique characteristics for these quasi-zero-dimensional systems.1 The physical and chemical properties of semiconductors, including the lowest exciton transition,2,3 emission,4 and redox potential,5,6 undergo drastic changes in this size regime and are significantly modified from those of the corresponding bulk materials, primarily because of quantum confinement effects.7 As the semiconductor particle size approaches that of the Bohr radius of the lowest 1S exciton state,8 the energy bands split into discrete quantized levels, and the band gap Eg widens. The shift of the optical absorption to higher energies provides experimental evidence of such quantum confinement effect. Herron et al.9 have previously demonstrated that for the absorption spectra of several representative CdS nanoclusters, as the particle size decreases, the absorption band edge of the excitonic shoulder blue shifts. Furthermore, the manifestation of the quantum size effect can be easily observed by simply comparing the physical appearance of the deeply orange colored bulk CdS versus the yellow/white color of CdS nanocrystallites reported by us10 and others.11 New materials based on quantum-confined nanoclusters may lead to the development of novel molecular electronics.12 Prior to the realization of unique devices based on quantized semiconducting materials, it is important to functionalize the surface of these quantum dot nanoparticles with chemically active moieties and subsequently derivatize the surface with optical and or electronic molecular reporters. Furthermore, the eventual utilization of these fascinating nanoparticles on a technological scale requires demonstration of electronic communication between the semiconductor core and a remote surface substituent covalently bound to the cluster surface. This crucial criterion was addressed by our group10c following the observation that aniline-functionalized quantum dot decomposed steadily upon exposure to room light, whereas its amidized counterpart did not, even after prolonged exposure to room light. This suggested to us that the rate of decomposition may be controlled by the nature of the para substituent on the capping agent, and we undertook a detailed study of the photodecomposition kinetics of arrange of thiophenolate capped CdS †
Part of the “Mark A. Ratner Festschrift”. * Corresponding author. E-mail:
[email protected]. ‡ Present address: Edward S. Rogers Sr. Department of Electrical and Computer Engineering, University of Toronto, 10 Kings College, Toronto, Ontario, Canada, M5S 3G4.
nanoclusters. In such study, a constant rate of photoinduced decomposition process was observed in all cases, and a single organic product was found and identified the corresponding symmetric disulfide. Strongly electron-donating groups and strongly electron-withdrawing groups both enhanced the photodecomposition rate relative to the unsubstituted thiophenolatecapped nanoclusters. Through our ongoing efforts in fabricating CdS nanoclusters bearing functionalizable sites that can be bound into wide array of substrates, herein, we report the synthesis and characterization of a novel azobenzene derivative, 4-(4-nitrophenylazo) thiophenol (4). The preliminary investigation of this ligand as a probe for the demonstration of surface-core electronic communication in CdS quantum dot nanoparticles will also be described. Experimental Methods Methods and Materials. All chemicals and solvents were purchased from Aldrich chemicals (Canada) or Caledon laboratories (Georgetown, Ontario, Canada) and were purified where necessary according to conventional laboratory techniques.13 All deuterated solvents used for 1H NMR spectroscopy were purchased in sealed ampules from Cambridge Isotope Laboratories (Andover, MA) and used immediately upon opening the ampules. FT-IR-grade potassium bromide was purchased from Aldrich and stored in a glass desiccator over drierite. Infrared spectra were recorded as KBr pellets with a Mattson 3000 Fourier transform infrared spectrometer. 1H NMR spectra were performed on a Bruker ARX 400 MHz nuclear magnetic resonance spectrometer. Electronic spectroscopy of all samples was performed in 1 cm quartz cuvettes using a Hewlett-Packard 84523 diode array spectrophotometer. Mass spectrometry was acquired with Kratos 902 spectrometer using the electron impact ionization method. Computational studies using Gaussian 0914 on 4-(4-nitophenylazo) thiophenol B and the thiophenolate B′ molecules were carried out employing density functional theory (DFT) using the hybrid exchange-correlation functional B3LYP15 and the Los Alamos National Laboratory 2-double-z basis-set LANL2DZ.16 A tight convergence (10-8 au) was used in all calculations. Solvent effects were accounted for using the integral-equation-formalism polarizable continuum model (IEFPCM) using a dielectric constant of 35.69. Optimizations were performed as closed-shell singlets, and the stability of the wave functions was determined using the stable)opt keyword. Vibrational frequency calculations were performed on converged structures to confirm that an energy minimum had been achieved. Time-dependent DFT (TD-DFT)
10.1021/jp104336t 2010 American Chemical Society Published on Web 08/24/2010
Push-Pull Azobenzene Haptan methods were utilized to determine the predicted electronic transitions. Absorption profiles to generate the electronic spectra were calculated using the SWarlock program.17 4-(4-Nitrophenylazo) Phenol (1). We prepared 1 via modification of a method previously described.18 4-Nitroanaline (3.53 g, 25 mmol) was dissolved in 500 mL of 0.5 M hydrochloric acid and cooled to 5 °C. Sodium nitrite (5.82 g, 84 mmol) dissolved in 30 mL of water was added to the solution of 4-nitroaniline hydrochloride, and the mixture was allowed to stand at ice bath temperatures. Phenol (3.58 g, 38 mmol) was dissolved in 500 mL of 2 M sodium hydroxide and cooled to 5 °C. The diazonium salt solution was slowly added with stirring to the phenol solution, resulting in the evolution of a deep red solution. The mixture was acidified to pH 4 (litmus) with 2 M hydrochloric acid, and the resulting bright-orange precipitate was isolated by vacuum filtration. The solid was washed with copious amounts of water and then dried in high vacuum overnight at room temperature. Yield: 5.33 g (86%). 1H NMR (CDCl3, δ): 7.00 (d, 2H, aromatic), 7.98 (m, 4H, aromatic), 8.39 (m, 2H, aromatic), 5.24 (s, 1H, OH). IR (KBr pellet, υ in cm-1): 3409 (s, br, OH), 1512 (vs, asym ArNO2 stretch), 1341 (vs, sym ArNO2 stretch), 852 (vs, C-N ArNO2 stretch), 1934, 1799 (w, aromatic 1,4-disubstitution pattern). UV-vis: (EtOH) λmax (nm) 382, (EtOH/OH-) λmax (nm) 504. EI-MS (m/z): 243, 121, 93. O-4-(4-Nitrophenylazo) N,N-Dimethylthiocarbamate (2). 1,4-Diazabicyclo[2.2.2]octane (DABCO) (6.69 g, 60 mmol) was added to 60 mL of DMF solution of 1 (5.15 g, 21 mmol). To this well-stirred mixture, N,N-dimethylthiocarbamoyl chloride (4.85 g, 39 mmol) was added and subsequently heated and maintained at 60-70 °C under nitrogen for 12 h. The resulting dark solution was poured in crushed ice, giving a red/orange slurry, which was acidified to pH 4 (litmus) with 2 M hydrochloric acid. The yellow/orange precipitate formed was collected by vacuum filtration and dried in high vacuum to yield 5.89 g (81%). 1H NMR (acetonitrile-d3, δ): 3.37 (s, 3H, MeN-Me), 3.44 (s, 3H, Me-N-Me), 7.32 (d, 2H, aromatic), 8.06 (m,4H, aromatic), 8,42 (d, 2H, aromatic). IR (KBr pellet, υ in cm-1): 1512 (vs, asym ArNO2 stretch), 1391 (vs, CdS stretch), 1337 (vs, sym ArNO2 stretch), 854 (vs, C-N ArNO2 stretch). EI-MS (m/z): 330, 88, 72. S-4-(4-Nitrophenylazo) N,N-Dimethylthiocarbamate (3). A well-ground sample of 2 (3.76 g, 11 mmol) was heated at 210 °C under blanket of nitrogen for 4 h. Upon cooling of the reaction mixture, the rearranged product (3) was obtained in near quantitative yield. Recrystallization from 2-propanol afforded a pure orange solid. Yield: 2.74 g (73%). 1H NMR (CDCl3, δ): 3.10 (s, 3H, Me-N-Me), 7.71 (d, 2H, aromatic), 8.06 (m,4H, aromatic), 7.99 (d, 2H, aromatic), 8.06 (d, 2H, aromatic), 8.41 (d, 2H, aromatic). IR (KBr pellet, υ in cm-1): 1668 (vs, CdO stretch), 1519 (vs, asym ArNO2 stretch), 1341 (vs, sym ArNO2 stretch), 856 (vs, C-N ArNO2 stretch). EIMS (m/z): 330, 72. 4-(4-Nitrophenylazo) Thiophenol (4). To a rapidly stirring 30 mL THF solution of 3 (2.11 g, 6.4 mmol) was added 35 mL of 2.4 M methanolic potassium hydroxide solution. The resulting bright purple mixture was stirred at room temperature overnight to completion hydrolysis. The mixture was poured in crushed ice and subsequently acidified to pH 4 (litmus) with 2 M hydrochloric acid. The orange/brown precipitate formed was collected by suction filtration and initially purified by repeated washings with water. Following recrystallization from 2-propanol, the thiophenol derivative was dried in high vacuum at room temperature to yield 1.35 g (65%) of 4 as pale-orange
J. Phys. Chem. C, Vol. 114, No. 48, 2010 20411 solid. 1H NMR (CDCl3, δ): 7.72 (d, 2H, aromatic), 7.96 (m,4H, aromatic), 8.05 (d, 2H, aromatic), 8.40 (d, 2H, aromatic). IR (KBr pellet, υ in cm-1): 1518 (vs, asym ArNO2 stretch), 1340 (vs, sym ArNO2 stretch), 848 (vs, C-N ArNO2 stretch), 1927, 1799 (w, aromatic 1,4-disubstitution pattern). UV-vis: (EtOH) λmax (nm), 380 (EtOH/OH-) λmax (nm) 320, 504. EI-MS (m/z): 259, 137, 109. 4-Nitrothiophenol and 4-(4-Nitrophenylazo) Thiophenol (4) Mixed-Ligand Surface-Functionalized CdS Nanoclusters. A cadmium acetate solution (2.50 g, 11 mmol) was prepared in 75 mL of mixed solvent 1:1:2 MeOH/MeCN/H2O (v/v/v). A second solution containing sodium sulfide (1.25 g, 5.3 mmol), 4-nitrothiophenol (2.48 g, 1.58 mmol), and 4-(4-nitrophenylazo) thiophenol (4) (25 mg, 0.01 mmol) in 75 mL of the solvent mixture and thoroughly degassed with N2 was added, with rapid stirring to the cadmium acetate solution. The reaction mixture was stirred for 16 h under N2 and light-protected environment. The resultant orange precipitate was isolated by centrifugation and repeatedly washed with sonication/centrifugation cycles in water, acetone, and diethyl ether. The solid was subsequently dried at 50 °C in vacuo and subdued light. Yield: 22%: 1H NMR (CdCl3, δ): 7.43-8.49 (m, broad, aromatics). IR (KBr pellet, υ in cm-1): 3094 (w, arom, C-H stretch), 1927, 1799 (w, aromatic 1,4- disubstitution pattern), 1573 (vs, asym ArNO2 stretch), 1474 (s, ring CdC), 1340 (vs, sym ArNO2 stretch), 1107 (m), 1088 (s), 1009 (m, arom-S), 848 (vs, C-N ArNO2 stretch). UV-vis (EtOH) excitonic shoulder at 330 (nm). Results and Discussion Our first attempt to synthesize the desired azo ligand, 4-(4nitrophenylazo) thiophenol (4), involved diazotization of 4-nitroaniline, followed by azo coupling with thiophenol. However, the major bright-yellow product from this reaction was identified by mass spectroscopy as the diphenylsulfide (m/z ) 218). The second route employed toward the synthesis of 4 consisted of the straightforward preparation of 4-(4-nitrophenylazo) phenol (1) (Scheme 1) and the subsequent conversion of the phenol to thiophenol via dialkylthiocarbamates.19 Compounds 1, 2, 3, and 4 were fully characterized by 1H NMR, IR, and EI-MS, and their expected molecular structures were ascertained. Synthesis of a CdS nanocluster using 4-(4-nitrophenylazo) thiophenol (4) ligand as the sole-capping ligand turned out to be quite cumbersome. Differences in the solubility properties of the 4-(4nitrophenylazo) thiophenol (4) ligand in comparison with the other organic caps previously attached to CdS Q-dots may explain the inability to form the 100% azobenzene-functionalized quantum dots.20 Steric effects may also play a role, 4-(4-nitrophenylazo) thiophenol (4) ligand being much larger than the thiophenolate moieties previously studied. Analyses of the sort described above can be realized only upon the preparation of mixed surfacefunctionalized quantum dots, as shown in Scheme 2, containing 1 to 2 mol % of our 4-(4-nitrophenylazo) thiophenol (4) and thiophenolate ligand bearing either electron-donating or electronwithdrawing substituents at the para position. A slight color change was observed upon the addition of cadmium acetate to a solution of sodium sulfide, thiophenol, and 1 to 2% of 4-(4nitrophenylazo) thiophenol (4) ligand from red to orange/yellow, indicating the precipitation of CdS nanoclusters. The 1H NMR spectrum of the collected product exhibited broadened aromatic signals characteristic of CdS nanoclusters as expected. The electronic absorption spectrum of the mixed-ligand CdS nanocluster suspension in DMF is presented in Figure 1A. It reveals results typical of a thiolate-capped CdS quantum confined
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SCHEME 1: Synthesis of 4-(4-Nitrophenylazo) Thiophenol (4) Ligand
SCHEME 2: Synthesis of Mixed-Ligand CdS Nanoclusters Containing 1 mol % of 4-(4-Nitrophenylazo) Thiophenol (4) Ligand
nanocluster with an onset of band-to-band absorption at ca. 330 nm, giving a band gap of 3.75 eV. The difference of 1.22 eV between the bandgap of these mixed-ligand nanoclusters and that of the bulk CdS (Eg ) 2.53 eV) is attributed to quantum confinement effects and can be used to estimate a diameter of 24 Å in accordance with tight-binding analyses.20 TEM images of the mixed-ligand nanoclusters, however, as shown in
Figure 1B, demonstrate that the nanoclusters tend to aggregate into small clusters of various sizes imbedded in fields of larger spheroid clusters that are approximately 10 times larger than the non-mixed-ligand CdS nanocluster counterparts. This observed aggregating behavior for this class of CdS nanoclusters is also a common occurrence in capped metal sulfide nanoclusters.9,10a
Figure 1. (A) Electronic spectrum and (B) TEM of mixed-ligand CdS nanoclusters.
Push-Pull Azobenzene Haptan
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SCHEME 3: Proposed Photodecomposition Mechanism of Surface-Confined 4-(4-Nitrophenylazo) thiophenol (4) Ligand-Containing CdS Nanoclusters
A mechanism for the observed photodecomposition was proposed and is outlined in Scheme 3 for strongly electronwithdrawing substituents; however, an analogous mechanism exists for electron-withdrawing substituents. CdS nanoclusters exhibit a characteristic emission that results from radiative
recombination of a photogenerated exciton.4 According to the mechanism proposed in Scheme 3, nanoclusters functionalized with either electron-donating or electron-withdrawing substituents should suffer fluorescence quenching as a consequence of the competitive surface-induced hole or electron capture
Figure 2. Surface-substituent-induced band-bending in substituted thiolate-capped CdS nanoclusters (adapted from ref 10e).
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Farah et al. TABLE 1: Select Bond Lengths and Orders for Calculated Geometries for B and B′
NdN Nazo-Cs Nazo-Cnitro C-S C-Nnitro
Figure 3. Canonical structures of 4-(4-nitrophenylazo) thiophenol (4) surface-functionalized CdS nanoclusters.
Figure 4. Protonated and deprotonated structures of 4-(4-nitrophenylazo) thiophenol (4) ligand.
illustrated in the photodecomposition mechanism. This surfaceinduced fluorescence quenching can be rationalized using a simple band-bending model depicted in Figure 2. Illustrated in this model are essentially energy level diagrams of the semiconductor core, with the energy of the orbitals along the vertical axis and the radius along the horizontal axis. The valence band edge, analogous to the HOMO, and the conduction band edge, analogous to the LUMO, are highlighted. For a strongly electron-donating surface, the bands bend upward near the surface. (The energy of the orbitals is raised.) A photogenerated electron will sink to the core, and the corresponding hole will float to the surface, thus separating the exciton. The opposite situation occurs with nanoclusters bearing an electron-withdrawing surface. In this case, the bands bend downward near the surface (the energy of the orbitals is lowered), and the exciton will again separate as the hole floats toward the core and the electron sinks to the surface. Once at the surface, either carrier can be captured by a surface group and initiate the decomposition. Exciton separation ultimately results in fluorescence quenching. Not only do these results provide evidence of surface-core electronic communication in these semiconducting nanoclusters but also this electronic communication must be facile on the order of the lifetime of the exciton (∼10-9 s).21 A final experiment that would complement the studies described above is the surface functionalization of CdS Q-Dots with the “push-pull” azobenzene derivative, 4-(4-nitrophenylazo) thiophenol (4). Azobenzenes have long been utilized in the dye industry because of their characteristic strong absorptions in the visible region and in fact comprise the most important class of
bond lengths (Å)
Mayer bond orders
B 1.30 1.42 1.43 1.83 1.46
B 1.89 1.25 1.28 1.23 0.99
B′ 1.32 1.39 1.41 1.78 1.44
B′ 1.70 1.34 1.42 1.56 1.04
dyes today.22 This ligand would function as an ideal probe for the demonstration of surface-core electronic communication in functionalized nanoclusters, as drawn in Figure 3. Delocalization of cadmium-sulfur bonding electron pair of A can be represented by the hyperconjugative state A′. It is proposed that stabilization of this hyperconjugative state (i.e., an electron-donating surface) will lower its energy; therefore, a red shift in the absorption maximum of the azobenzene capping agent is expected. Conversely, destabilization of this hyperconjugative state (i.e., an electron withdrawing surface) will raise its energy, and thus a blue shift in the absorption maximum is anticipated. Ideally, the shift in the absorption maximum of the 4-(4-nitrophenylazo) thiophenol (4) ligand should be a function of the electron demand at the nanocluster surface in the event that there definitely exists surface-core electronic communication. The utilization of 4-(4-nitrophenylazo) thiophenol (4) as a probe for surface-core electronic communication relies on the demonstration of A′ (the hyperconjugative state illustrated in (Figure 3) as an important resonance contributor of A. This has inspired the investigation of 4-(4-nitrophenylazo) thiophenol (4), which can exist in either one of two forms, as shown in Figure 4. The protonated form B or deprotonated form B′, where B′ can be viewed as the extreme case of stabilization of the hyperconjugative state A′ were studied by DFT methods. The calculated geometries of both B and B′ converged with Cs symmetry. Upon deprotonation, the azo NdN bond lengthens by 0.03 Å, whereas the C-N(azo) bonds contract from 0.02 to 0.03 Å, and the C-S bond contracts by 0.06 Å (Table 1). This is in agreement with the Mayer bond orders, which showed a decrease in the azo NdN bond from 1.89 to 1.70 and an increase in the remaining bonds previously mentioned. This implies that B′ and thus A′ may be important resonance of B and A, respectively. B and B′ can be visually distinguished by comparing their colors in solution. An ethanolic solution of 4-(4nitrophenylazo) thiophenol (4) ligand is bright orange in color,
Figure 5. Experimental spectrum (a) of B and (b) after addition of acid to the cuvette in MeOH and (c) spectrum of B′ in MeOH.
Push-Pull Azobenzene Haptan
Figure 6. Predicted electronic transitions and absorption profile for B and B′. Transitions 1 and 1′ are assigned as HOMO f LUMO, whereas 2 and 2′ are assigned as HOMO f LUMO+1.
and upon the addition of base, a drastic color change from orange to violet is observed. This difference in color is manifested in the electronic absorption spectra of B and B′, as shown in Figure 5.
J. Phys. Chem. C, Vol. 114, No. 48, 2010 20415 Immediately apparent is the bathochromic shift of the maximum of the lowest energy charge transfer transition (HOMO to LUMO), which occurs at 372 nm for B and 504 nm for B′. The loss of the shoulder centered at ∼500 nm in the spectrum of B following the addition of acid suggests that there may exist equilibrium between the protonated and deprotonated forms in solution. This hypothesis was further corroborated by pH dependence studies of such equilibrium that exists between the two species. The calculated electronic spectrum of B and B′ (Figure 6) was generated using an average bandwidth of 2600 and 1800 cm-1 for the predicted electronic transitions of B and B′, respectively (Figure 7). It can be seen that the predicted electronic transition energies and relative intensities are similar, with the lower energy transition being more intense than the transition at higher energy. The lower energy transition in each case is assigned as a HOMO f LUMO π f π* transition (thiophenol (77%) f nitrophenyl (76%) for B and thiophenol (72%) f nitrophenyl (74%) for B′). The bathochromic shift in the lowest energy charge-transfer upon deprotonation of B is also observed in the calculated spectra. Figure 6 shows the differences in electron density between the occupied and unoccupied states for the two lowest charge
Figure 7. Predicted electronic transitions (a,c) 1 and (b,d) 2, as shown in Figure 5 for (a,b) B and (c,d) B′. Red indicates a region of excess electron density in the ground state, whereas green indicates regions of excess electron density in the excited state.
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transfer bands for B and B′, which illustrates the push-pull effect between the thiophenol and nitrophenyl groups very clearly in B but less so in B′. The higher energy transition in both cases involves the same orbitals for both B and B′ and is mostly due to a HOMO f LUMO+1 transition, which is also a π f π* transition but does not show a flow of electron density from one end of the molecule to the other for either B or B′. If we return to our initial assumption that B′ represents the extreme condition for stabilization of hyperconjugative state A′, then the experimental data support the premise that 4-(4nitrophenylazo) thiophenol (4) can be used as a probe for demonstration of surface-core electronic communication in functionalized CdS nanoclusters. The absorption maximum of 4-(4-nitrophenylazo) thiophenol (4) when covalently bound to CdS quantum dots can be monitored (i.e., between 372 and 504 nm) as function of the electron demand at the surface. The more strongly electron-donating the surface, the greater the stabilization of the hyperconjugative state A′, and thus a red shift in the absorption maximum is expected. The more strongly electronwithdrawing the surface, the less stabilization of the hyperconjugative state A′, and thus a blue shift in the absorption maximum is expected. In conclusion, we prepared and characterized CdS nanoclusters having remote surface electron-withdrawing groups bearing azobenzene moiety. Characterization based on (UV-vis, DFT, TEM, and emission studies leads us to believe that electronic communication might indeed be mediated by CdS mixed-ligand nanoclusters containing novel 4-(4-nitrophenylazo) thiophenol (4) via a core-mediated inductive mechanism, a vital criterion and a step forward for the realization of nanocluster-based electronic devices. Acknowledgment. We thank the Natural Science and Engineering Research Council of Canada (NSERC) for financial support. References and Notes (1) (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Bawendi, M. C.; Steigerwald, M. L.; Brus, L. E. Annu. ReV. Phys. Chem. 1990, 41, 477. (c) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737. (d) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (e) Somers, R. C.; Bawendi, M. G.; Nocera, D. G. Chem. Soc. ReV. 2007, 36, 579. (2) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (3) Rossetti, R.; Ellison, J. L.; Gibson, J. M.; Brus, L. E. J. Chem. Phys. 1984, 80, 4464. (4) Eychmueller, A.; Haesselbarth, A.; Katsikas, L.; Weller, H. Ber. Bunsen-Ges. 1991, 95, 79.
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