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J. Phys. Chem. C 2009, 113, 5358–5367
Experimental and Theoretical Studies of the Photoreduction of Metal Ion-Dendrimer Complexes: Observation of a Delocalized Organic Radical Haiying Wan, Shenggang Li, Tatyana A. Konovalova, Yangliu Zhou, Joseph S. Thrasher, David A. Dixon,* and Shane C. Street* Chemistry Department, The UniVersity of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama 35487-0336 ReceiVed: May 28, 2008; ReVised Manuscript ReceiVed: December 4, 2008
Following complexation with certain dendrimer functionalities, metal ions can be reduced to zerovalent metal nanoparticles via UV irradiation and with the dendrimer oxidized to a radical cation. EPR-silent Zn(II) ions can serve as the oxidizing agent, enabling the nature of the dendrimer radical cation to be examined. Spectral simulations and quantum chemical calculations were carried out to elucidate the nature of the free radicals. Spectral simulations in conjunction with electronic structure calculations suggest that the electron spin density is localized on the central N-C-C-N core structure and delocalized over the N and C atoms in the core. The radical cations of model structures with the ethylenediamine (EDA) core and that of the G0-NH2 polyamidoamine (PAMAM) were found to have a weak central one-electron C-C bond. The description of the molecular structure of the cation falls between the limit of two iminium-type ions with a charge of +0.5 e on each (1/2+R2N ) CH21/2•) interacting by a one-electron C-C bond and the other limit of a 1/2+1/2• NR2-CH2-CH2-NR21/2+1/2• structure with a spin of 1/2 and a charge of 1/2 on each N. For EDA, our calculated ionization energies and heats of formation at the coupled cluster (CCSD(T)) level are in good agreement with available experimental data. The ionization energy of the G0-NH2 PAMAM was found to be substantially lower than that of EDA. The reduction in the ionization energies for the dendrimers and other effects such as metal-ligand interaction and solvation contribute to the reduction of metal cations by dendrimers with UV irradiation. Similar experiments with the G0-NH2 poly(propylene imine) (PPI) did not produce metal nanoparticles, indicating these effects are not as favorable as those for G0-NH2 PAMAM. Introduction At the nanoscale, the properties of matter can differ significantly from those of individual atoms or molecules and bulk materials. Metal nanoparticles have a broad range of applications in electronic and magnetic materials, catalysis, pharmaceuticals, and powder metallurgy.1-5 There are two broad synthetic approaches for nanoscale materials: physical methods6 and chemical methods.7 Photochemical methods have not been widely used8 until recently for the production of transition metal nanoparticles; examples include Pd,9 Mo,10 Cd,11 Pt,12 and Fe13 nanoparticles. Most of these synthetic reactions, however, require either high energy light source ( 200 nm) following complexation of precursor metal ions with the PAMAM dendrimer. On the PAMAM dendrimer, only the amide and amine groups in PAMAM can absorb photons at wavelengths longer than 200 nm.25 Absorption can also occur by metal d-d or charge-transfer transitions. However, even though the synthesis works, the detailed mechanism and kinetics by which metal ions are converted to metal clusters via photoreduction
10.1021/jp804717u CCC: $40.75 2009 American Chemical Society Published on Web 03/18/2009
Photoreduction of Metal Ion-Dendrimer Complexes in dendrimers remain largely unclear. For example, what species are formed during the photoreduction? We have used electron paramagnetic resonance (EPR) spectroscopy26,27 to study free radicals in Cu(II)-dendrimer complexes.28,29 We used photochemical reactions to synthesize copper nanoparticles by UV irradiation following complexation of the precursor Cu(II) ion to amine-terminated G0 or G4 PAMAM dendrimers in aqueous solution.24 We studied copper(II) complexation to the G0 PAMAM dendrimer as a function of pH by using EPR spectroscopy, and the interpretation of the results was aided by density functional theory30 (DFT) calculations. However, the presence of the EPR-active Cu(II) makes it more difficult to know if organic-based radicals are also formed. Zn offers a benefit to the analysis of the mechanism, as the Zn(II) ion is EPR silent, which lets us probe for the presence of organic radicals without the complicating paramagnetic Cu(II) signal. We now report the synthesis of Zn nanoparticles by the same procedure.24 The resulting Zn nanoparticles were studied with high resolution transmission electron microscopy (HRTEM). EPR was used to identify potential intermediates of the photochemical reaction of the Zn PAMAM dendrimer system as well as those for the Cu dendrimer system. To better understand the role of the PAMAM dendrimer, we also investigated the Cu(II)-PPI and Zn(II)-PPI dendrimer systems. Unlike the case of the PAMAM system, nanoparticles are not formed following complexation of PPI with the precursor metal ions Cu(II) and Zn(II) and subsequent UV irradiation. DFT and coupled cluster with single and double excitations and a perturbative triples correction (CCSD(T)) calculations31-34 were used to predict the structures of the free radicals observed in the EPR experiments. Experimental and Computational Methods In a typical experiment, 10 mM aqueous (Milli-Q, Millipore) solution of ZnCl2 (Aldrich) was mixed with 0.1 mM G0 NH2terminated PAMAM dendrimer (Aldrich). The molar ratio of Zn to G0 dendrimer was kept 1:1. UV irradiation was provided by eight lamps (8 W per lamp at 253.7 nm) in a Rayonet photochemical reactor (model RPR-600, Southern New England Ultraviolet Inc.) at ambient temperature for 12 h. The solution was purged with nitrogen gas for ∼30 min prior to the irradiation. G4 PPI (Aldrich) was used in the Cu and Zn PPI dendrimer systems with the same metal to dendrimer molar ratio as described above. Images of the Zn-dendrimer nanocomposites were obtained with a FEI Technai Supertwin transmission electron microscope. EPR measurements were performed with a Varian E-12 spectrometer equipped with a rectangular cavity. The magnetic field was measured with a Bruker ESR 035 M gaussmeter, and the microwave frequency was measured with a HP 5245 L frequency counter. EPR spectra were recorded by using the Scientific Software Services’ setup consisting of a PIDM, a data acquisition computer connected to a Varian E-12 spectrometer, and the EWWIN data acquisition software. Diphenylpicrylhydrazyl (DPPH) (g ) 2.00351) was used to calibrate the magnetic field. Dendrimer samples were placed in quartz EPR tubes (o.d. 4 mm, WILMAD) and measured at 77 K. The experimental spectra were simulated by using the XSophe computer simulation software with a powder-matrix diagonalization approach and the Gaussian line shape.35 DFT calculations were performed to predict the molecular structures, energetics, and spin properties of the free radicals. We used the B3LYP hybrid exchange-correlation functional36,37 and the DFT-optimized polarized double-ζ DZVP2 basis set.38
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5359 Due to the potential presence of multiple local minima for some of the model complexes, we optimized the structures starting from different initial geometries. Both steric effects and hydrogen bonding interactions were considered in choosing the starting geometries as well as our previous work on model Cu-dendrimer complexes.24 In general, the structures do not have symmetry, so there is substantial flexibility in the optimization procedure for finding low energy conformers. For the largest model complexes, we used the SAM1/d method as implemented in the AMPAC 8 program to perform the initial geometry optimization.39 Geometries were optimized and harmonic vibrational frequencies were calculated using analytic second derivative techniques to ensure that the optimized geometries are minima on the potential energy hypersurface and to obtain the zero-point vibrational energies and other thermodynamic properties. The g-tensors and hyperfine coupling constants A were calculated at the same level. In addition, single point energies were calculated at the CCSD(T) level with the aug-cc-pVDZ (aVDZ) basis set.40 For open-shell systems, the R/UCCSD(T) formalism is employed, where a restricted open-shell Hartree-Fock calculation is initially performed, and spin restriction is relaxed when calculating the correlation energy.41-43 For ethylenediamine (EDA), a model for the core of the PAMAM dendrimer, and its cation, the geometries were also calculated at the MP244 level with the aug-cc-pVTZ (aVTZ) basis set, and the energies were calculated at the CCSD(T) level with the aVXZ (X ) D, T, Q) basis sets and extrapolated to the complete basis set (CBS) limit using a mixed Gaussian/exponential formula,45 following previous work on the accurate prediction of thermodynamic properties.46 Corevalence corrections were calculated at the CCSD(T)/aug-ccpwCVTZ level,47,48 and scalar relativistic corrections were calculated at the CISD/aVTZ level. ∆ESR is taken as the sum of the mass-velocity and one-electron Darwin (MVD) terms in the Breit-Pauli Hamiltonian.49 For the calculations of the heats of formation,50 the zero-point energies for EDA calculated at the MP2/aVTZ level were corrected by scaling the C-H and N-H stretching frequencies by the ratios CH4(expt)/CH4(MP2/ aVTZ) and NH3(expt)/NH3(MP2/aVTZ), respectively.51 The MP2 and CCSD(T) calculations were done to benchmark the DFT results and provide scaling factors for the larger molecules. Ionization energies were calculated at the various computational levels and compared with available experimental results. All DFT and MP2 calculations were carried out with the Gaussian 03 program package,52 and the CCSD(T) calculations were performed with the MOLPRO 2006.1 program package.53 The calculations were carried out on the Opteron-based Cray XD1 and Itanium 2-based SGI Altix supercomputers at the Alabama Supercomputer Center, the Xeon-based Dell Linux cluster at the University of Alabama, the local Opteron-based Parallel Quantum Solutions Linux cluster, and the Itanium 2-based Linux cluster at the Molecular Science Computing Facility from the Pacific Northwest National Laboratory. Results and Discussion Nanoparticle Characterization. Figure 2 (see also Supporting Information Figure SM-3) shows an electron micrograph demonstrating the formation of Zn nanoparticles generated in the presence of G0-NH2 PAMAM after irradiation. The value of the spatial frequency (Supporting Information) is ∼0.25 nm, consistent with the Zn hexagonal (002) plane.54,55 The diffraction patterns in the selected area were analyzed, and the particles were confirmed as metallic Zn nanoparticles. The cell parameters derived from the diffraction pattern are a ) 0.2738 nm and c
5360 J. Phys. Chem. C, Vol. 113, No. 14, 2009
Figure 2. TEM image of Zn/G0-NH2 nanoparticles at low amplification with diffraction pattern.
) 0.5017 nm, consistent with the reported values of the Zn hexagonal phase.54,55 Further evidence for the production of Zn nanoparticles (UV-vis and X-ray photoelectron (XPS) spectra) is provided in the Supporting Information. There was no evidence for the formation of Zn nanoparticles in the G4-NH2 PPI dendrimer system after UV irradiation and, consistent with this observation, there was no EPR signal. In similar experiments with Cu,24 no evidence for Cu nanoparticles was found after irradiation of the G4-NH2 PPI dendrimer system. EPR Spectral Analysis. The EPR spectra of the Cu(II)/G0NH2 PAMAM system before and after irradiation are shown in Figure 3 for comparison purposes. The initial spectrum before irradiation exhibits a characteristic signal of the Cu(II) ion (ICu ) 3/2)56,57 with an axial g-tensor (g| ) 2.25 and g⊥ ) 2.08) (Figure 3a). Irradiation of the sample resulted in a decrease in the intensity of the Cu(II) EPR signal with a concomitant increase in the intensity of a new signal at higher magnetic fields (Figure 3b). The line shape and the parameters of this signal (giso ) 2.0035) are similar to those previously reported for organic nitrogen-centered radicals.58-61 A similar observation was made by Ottaviani et al. for Cu(II) solutions with starburst dendrimers.28 They detected a multiline EPR signal with g ≈ 2 in dendrimer solutions, which increased in intensity in the presence of the Cu(II) ions,28 consistent with our observations. In order to improve the signal-to-noise ratio for the EPR spectrum of this radical and to minimize the interference from an EPR-active metal, we replaced the paramagnetic Cu(II) ion complexing to the dendrimer with the EPR silent Zn(II) ion. A Zn(II)/G0-NH2 PAMAM solution was prepared at neutral pH (∼7). No EPR signal was detected before irradiation. The sample was irradiated, and EPR spectra were measured at 77 K. After irradiation, the sample exhibited an EPR signal (Figure 4a) similar to that observed in the high field region for the Cu(II) system. The EPR signal for this radical obtained in the Zn(II)/ dendrimer system is much better resolved (Figure 4b) than that in the Cu(II)/dendrimer system, allowing for better characterization of this radical. The XSophe program35 uses a perturbative approach to simulate EPR spectra. The Simplex optimization method was
Wan et al.
Figure 3. EPR spectra of the Cu(II)/G0-NH2 PAMAM solutions in toluene at 77 K: (a) before irradiation and (b) after irradiation for 45 min. Parameters: microwave power, 5 mW; modulation frequency, 100 kHz; microwave frequency, 9.102 GHz; modulation amplitude, 10 G; gain, 2 × 104.
used with a raw data error function and the peak extrema normalization. Spectral optimization was carried out with 1000 iterations. Four chemical models (Table 1) were considered in the spectral simulations based on radicals localized in the EDA core (Figure 5). The adjustable parameters in the simulations are the g-tensor, the nitrogen and hydrogen hyperfine tensors, and the Gaussian line width. The parameters used in the simulations for these models are listed in Table 1. Model (A) consists of a [(RCH2)2NCH2R′]•+ radical cation with electron spin density localized on half of the core with the same hyperfine coupling constants on all six neighboring hydrogens. Model (A) is a nitrogen-centered radical cation with three equivalent -CH2R groups attached. Both the nitrogen and the hydrogen hyperfine tensors are slightly anisotropic. It provides a poor fit to the experimental spectrum. Model (B) consists of a similar [(RCH2)2NCH2R′]•+ radical cation as in model (A) but with dominant hyperfine coupling constants on two of the neighboring hydrogens on the same CH2 group. Model (B) is similar to model (A) except that the two hydrogens in one of the -CH2R groups have slightly different isotropic hyperfine tensors. Furthermore, the contribution from the other two -CH2R groups is much smaller. This simulation gives the best fit for the outer lines but not the central lines. Model (C) consists of a [(RCH2)2NCH2CH2N(CH2R)2]•+ radical cation with spin density delocalized over the EDA core with equivalent hyperfine coupling constants on the two nitrogens and dominant but inequivalent hyperfine coupling constants on the four hydrogens between the nitrogens. Model (C), unlike models (A) and (B), has two nitrogen centers with the same isotropic hyperfine tensors and the two -CH2R groups between them which are almost equivalent and isotropic. This simulation gives the best fit for the central lines but cannot account for the outer lines. Model (D) consists of a similar [(RCH2)2NCH2CH2N(CH2R)2]•+ radical cation as in model (C) but with different hyperfine coupling constants on the two nitrogens. Model (D) is similar to model (C) except that the two nitrogen centers are inequivalent and have highly anisotropic hyperfine couplings and the g-tensor also shows an increase in anisotropy. The model
Photoreduction of Metal Ion-Dendrimer Complexes
J. Phys. Chem. C, Vol. 113, No. 14, 2009 5361 TABLE 1: EPR Tensor Element (XX, YY, ZZ) Parameters for Photogenerated G0-NH2 PAMAM Radical Cation Used in the Spectral Simulation: g-Tensor, Hyperfine Coupling Constant (hfc) Tensors, and Gaussian Line Widths property
XX
YY
ZZ
•+
Figure 4. EPR spectra of Zu(II)-containing dendrimer after 45 min irradiation measured at 77 K: (a) broad field sweep and (b) narrow field sweep.
(D) simulation does not fit the central portion of the experimental spectrum as well as that for model (C) but does show some intensity in the outer part of the spectrum. On the basis of the simulations, the photogenerated paramagnetic species is best assigned as a combination of models (B) and (C). Model (B) is a limit of model (C) where all of the spin density is localized on a single nitrogen. The need for some contribution from both models to account for the experimental spectrum suggests that there are a range of nitrogen radical species with spin delocalized on two centers to those with a localized spin on a single nitrogen. In both models, only a single CH2 group (model (B)) or two CH2 groups connecting the two equivalent N centers (model (C)) need to be present to reproduce the spectra. Because model (D) accounts for some of the outer part of the spectrum, the contributions from two inequivalent nitrogens connected by two methylene groups cannot be completely eliminated. On the basis of their spectral simulations, Ottaviani et al. assigned the organic radical they observed for the Cu(II) starburst dendrimer system to a nitrogen-based radical with two nonequivalent CH2 groups from the amidoamine units.28 The assignment to a nitrogen radical center with inequivalent CH2 groups is reasonably consistent with our assignments. The detection of such nitrogen-based radicals at 77 K in the Cu(II)
(A) [(RCH2)2NCH2R′] g-tensor 2.00228 2.00327 hfc (N)/G 21.3241 14.2724 hfc (H)/G 11.6714 11.5419 Gaussian line width/G 3.7781 3.7781
2.00413 21.8172 14.3474 3.7781
(B) [(RCH2)2NCH′H′′R′]•+ g-tensor 2.00248 2.00301 hfc (N)/G 22.6052 26.2319 hfc (H′)/G 32.6134 38.7497 hfc (H′′)/G 32.9502 34.6930 hfc (H)/G -0.3588 3.05454 Gaussian line width/G 3.43977 3.43977
2.00396 27.9530 38.9271 37.8468 4.71777 3.43977
(C) [(RCH2)2NCH′2CH′′2N(CH2R)2]•+ g-tensor 2.00238 2.00311 hfc (N)/G 12.6180 10.4113 hfc (H′)/G 12.1647 13.0538 hfc (H′′)/G 8.28605 7.42857 hfc (H)/G -0.3729 3.06955 Gaussian line width/G 3.46948 3.46948
2.00426 10.7386 10.7386 5.56146 5.01648 3.46948
(D) [(RCH2)2N′CH′2CH′′2N′′(CH2R)2]•+ g-tensor 2.00256 2.00587 hfc (N′)/G 12.4693 11.1191 hfc (N′′)/G 9.28793 4.47295 hfc (H′)/G 9.23464 9.26353 hfc (H′′)/G 11.6558 13.5189 Gaussian line width/G 3.64056 3.64056
2.00662 35.0150 30.5965 11.5721 5.50160 3.64056
and Zn(II) G0-NH2 PAMAM systems after UV irradiation and at room temperature by Ottaviani et al. for higher generation starburst dendrimers28 is a clear indication of its relative stability. Electronic Structure Calculations: Geometries and Orbitals. In order to gain a better understanding of the structures of these nitrogen radicals, we performed DFT and CCSD(T) calculations. Figure 6 shows the optimized structures for several molecules and their cations with the EDA core. For neutral EDA, it has previously been predicted at the B3LYP/6-311+G(d,p) level that the most stable form is the gauche conformer with one hydrogen from one of the amine groups forming a weak hydrogen bond to the nitrogen on the other amine group, and the trans conformer with C2h symmetry shown in Figure 6a lies ∼1.0 kcal/mol higher in energy.62 As the MP2 method is superior to the B3LYP method in describing hydrogen bonding and other weak nonbonding interactions, we reoptimized the geometries of the gauche and trans conformers for both the neutral and cation of EDA (Supporting Information Figure SM-4) at the MP2/aVTZ level and calculated their energies at the CCSD(T) level. The structures of the trans conformers of the neutral and cation optimized at the MP2/aVTZ level are very similar to those calculated at the B3LYP/DZVP2 level, with the bond lengths at the MP2 level being slightly shorter (
