New Organic–Inorganic Perovskite Materials with Different Optical

New Organic–Inorganic Perovskite Materials with Different Optical Properties Modulated by Different Inorganic Sheets Yinyan Li, Guoli Zheng, Cuikun Lin, and Jun Lin* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, China

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 6 1990–1996

ReceiVed January 14, 2008

ABSTRACT: New organic–inorganic perovskites with different PbBr perovskite sheets stabilized by 3- or 4-amidinopyridine were synthesized and structurally characterized. 4-Amidinopyridine constructs -oriented perovskite with inorganic sheets made up of typical corner-sharing octahedra of PbBr2. Analogous chemistry in the presence of 3-amidinopyridine under the same conditions results in an unusual hybrid perovskite with the inorganic sheets showing a novel framework including both corner-sharing and edge-sharing PbBr2, which is different from any previously reported ones. Fine thin films were obtained through a spin-coating process which offers the compounds potential photoelectric application. Optical properties investigation for the two compounds revealed different absorption excitonic peaks of hybrid perovskites at 399 and 411 nm, respectively, which can be attributed to their different inorganic sheets. Band structures of the compounds were calculated by the CASTEP code based on density functional theory.

1. Introduction Metal halide based hybrid perovskite materials have been given much attention due to their diverse structure frameworks, interesting magnetic, optical, and electrical properties, as well as the possibility of processing the materials using lowtemperature techniques.1 Typical hybrid perovskite has a layered structure that consists of an MX6 (M ) bivalent metal, X ) halogen atom) corner-sharing octahedral layer alternating with organic alkylammonium. Metal halides with set up hybrid perovskites include some first row transitional metal, Group (VIA) metal, and even some divalent rare earth metals, etc.2 What we are interested in is the Group (IVA) metals based hybrid perovskite. The key aspect of the perovskite materials containing Group (IVA) metals is the existence of twodimensional (2D) semiconducting sheets.3 Their unusual semiconducting properties have been widely studied and used for photoluminescence materials, thin-film field-effect transistors, electroluminescent devices, etc.4 Absorption spectra of these compounds exhibit excitonic peaks that are attributed to the inorganic perovskite sheets. Characteristic excitonic absorption peaks in this materials can be tuned to virtually any wavelength in the visible spectrum through the appropriate choice of metal atom (Ge, Sn, and Pb), halogen (Cl, Br, and I), or perovskite sheet thickness.1 In addition to tailing the position of the optical features by making substitutions on the metal or halogen sites, control over the specific structure of the metal halide sheets through the appropriate choice of the organic cation should also provide a means of influencing the optical properties.5 First, organic cations with different dielectric constants can influence the optical properties due to the dielectric confinement effect.6 Second, inorganic sheets can be transformed by choosing different organic cations and finally bringing about changes of the optical properties. Some special sheets including a MoO3type tin(II) iodide layer and an organic anionic-cationic cotemplated staircase-like inorganic network, etc.7 have been prepared with special optical properties. In this work we * Corresponding author. E-mail: [email protected].

obtained hybrid perovskite material with new inorganic sheets different from any reported ones, which display different optical properties by selecting special organic cations. Organic cations of hybrid perovskite are generally simply alkylammonium and aromatic ammonium confined by the framework of the inorganic sheets. Some complicated ammoniums including imidazolium,8–10 quaternary ammonium,11 and formamidinium12 have been introduced into hybrid perovskite to template more complicated layered materials with interesting physical properties. Formamidinium itself or with methylammonium has been used to construct novel -oriented perovskites.12 Also, another formamidinium analogue CH3SC()NH2)NH2 has been introduced into PbX2 (X ) Cl, Br) perovskite building up new semiconducting materials.13 Amidinium is actually an ideal organic ligand for building up hybrid perovskite due to its special structure. Derivatives of amidinium may bring about distinct perovskite frameworks, and these structural differences should be reflected in the optical properties of the materials. So in this work we introduce a special amidinium-amidinopyridinium into PbBr2-based hybrid perovskites. New materials with special optical properties may be expected when pyridinium cooperates with amidinium in one ligand of amidinopyridinium to build up hybrid perovskite. The amidinium substituted pyridine has a very special framework in forming hydrogen bonds that are important interactions in hybrid perovskites. So it is vital to introduce the two new organic ligands to hybrid perovskite to construct new structures with special properties. Two amidiniums, 3- or 4-amidinopyridiniums (Scheme 1), were included into PbBr2-based hybrid perovskite and different materials were constructed. Novel layered hybrid perovskite with unprecedented inorganic sheets containing both corner- and edge-shared octahedra (compound 1) was obtained using 3-amidinopyridinium and PbBr2. However, 4-amidinopyridinium constructed common -oriented perovskite (compound 2). Different excitonic absorption peaks were found in the two compounds with different inorganic sheets. Special inorganic sheets in compound 1 make its excitonic absorption (399 nm) show a blue shift with respect to the typical -oriented hybrid perovskite compound 2 (411 nm). Band structures of

10.1021/cg800047p CCC: $40.75  2008 American Chemical Society Published on Web 04/29/2008

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Scheme 1. The Growing and Formation Processes for Compounds 1 and 2

the compounds were studied through calculation by the CASTEP code based on density functional theory (DFT).

2. Experimental Section 2.1. Materials and Synthesis. PbBr2 (99.99%, Aldrich), 3-amidinopyridnium (99%, Alfa Aesar), and 4-amidinopyridnium (97%, Alfa Aesar) were used without further purification. Hydrobromic acid (40%) was used as received from Beijing Chemical Industry Co., Ltd. The growing and formation processes for crystals of compounds 1 and 2 are shown in Scheme 1. Compound 1: Evaporation of the solution of equimolar amounts of PbBr2 (0.367 g, 0.1 mmol) and 3-amidinopyridnium (0.157 g, 0.1 mmol) in 5 mL of hydrobromic acid (40%) at room temperature resulted in the formation of colorless crystals of (C6H9N3)PbBr4 (1). IR (cm-1KBr): ν(NH3+): 3325, 3169, 3109, 2711; ν(pyridinium): 1537, 1497; δ (NH3+): 1687, 1612; δ (Ar-H): 780, 741, 688. Elemental analysis: Calcd for C6H9N3PbBr4: C, 11.09%; H, 1.40; N, 6.46%. Found: C, 11.26%; H, 1.56%; N, 6.27%. Compound 2: Similar method to that for compound 1. Evaporation of the solution of equimolar amounts of PbBr2 (0.367 g, 0.1 mmol) and 2-amidinopyridnium (0.157 g, 0.1 mmol) in 5 mL of hydrobromic acid (40%) at room temperature resulted in the formation of colorless crystals of (C6H9N3)PbBr4 (2). IR (cm-1 KBr): ν(NH3+): 3310, 3204, 3085, 2682; ν(pyridinium): 1558, 1495, 1469; δ (NH3+): 1681, 1646, 1630; δ (Ar-H): 786, 729, 685. Elemental analysis: C6H9N3PbBr4: C, 11.09%; H, 1.40; N, 6.46%. Found: C, 11.30%; H, 1.26; N, 6.61%. 2.2. Thin Film Deposition. Thin films of compounds 1 and 2 were prepared through a spin-coating technique. The coating solutions were prepared by dissolving 10 mg of recrystallized compounds 1 and 2 in 4 and 3 mL of dried DMF, respectively. The films were prepared by flooding the silicon substrate with the solutions and a spinning cycle with 1 s ramping to 1200 rmp and dwelling for 50 s at 1200 rpm. Then the substrate was heated at 100 °C for 20 min to remove residual solvent. 2.3. Characterizations. Diffraction intensities were collected on a Rigaku RAXIS-RAPID image plate diffractometer using the ω-scan technique with Mo KR radiation (λ ) 0.71069 Å). Absorption corrections were applied using the multiscan technique.14a The structures were solved by direct methods using SHELXS-9714b and refined by means of full-matrix least-squares techniques using the SHELXL-97 program14c as implemented in WINGX.14d Non-hydrogen atoms were refined anisotropically. The positions of hydrogen atoms attached to carbon were generated geometrically. Analytical expressions of neutralatom scattering factors were employed, with anomalous dispersion corrections incorporated therein.14e FT-IR spectra were measured with Perking-Elmer 580B infrared spectrophotometer with the KBr pellet technique. Elemental analyses were carried out on Vario EL instrument. The X-ray diffraction (XRD) was examined on a Rigaku-Dmax 2500 diffractometer using Cu KR radiation (λ ) 0.15405 nm). The morphology of the film was inspected using a field emission scanning electron microscope (FESEM, XL30, Philips) equipped with an energy dispersive X-ray spectrometer (EDS, JEOL JXA-840). UV–vis absorption spectra were taken on a Hitachi F-4100 spectrofluorimeter. The band structures of the perovskites were calculated using the CASTEP code (version 3.0, Accelrys) based on DFT. The k-points integrations over the Brillouin zone (BZ) using the Monkhorst and Pack grid are performed. Here, a 1 × 1 × 1 mesh parameters grid is taken.

Figure 1. Crystal packing structure of compound 1 along the a-axis. Table 1. Crystal Data of Compound 1 and 2

empirical formula wavelength/ Å space group crystal system space group a/Å b/Å c/Å R/° β/° γ/° volume/Å3 Z density/Mg/m3 reflections collected final R indices [I > 2σ(I)]

compound 1

compound 2

C6H9N3PbBr4 649.99 0.71073 monoclinic C2/c 16.416(2) 9.0828(11) 18.674(2) 90 101.831(2) 90 2725.1(6) 8 3.169 7262 R1 ) 0.0764, wR2 ) 0.1864

C6H9N3PbBr4 649.99 0.71073 orthorhombic Pbca 16.8125(9) 8.4528(4) 19.4257(10) 90 90 90 2760.6(2) 8 3.128 16045 R1 ) 0.0414 wR2 ) 0.0818

3. Results and Discussion 3.1. Crystal Structure. 3-Amidinopyridinium coordinated with PbBr2 in hydrobromic acid yields a very special hybrid perovskite (compound 1), as shown in Figure 1. Compound 1 belongs to the monoclinic crystal system, crystallizing in space group C2/c (Table 1). This compound consists of a layered structure: amidinopyridinium layers alternated with lead bromide layers. 3-Amidinopyridinium adopts a new method lining up between inorganic sheets, which results in an inorganic layer different from any previously reported hybrid perovskites. Generally in this type of material, MX6 octahedra share opposite corners to form perovskite sheets,2 whereas the inorganic sheet in compound 1 is a combination of corner-shared and edgeshared octahedral layer, as shown in Figure 2. First, the PbBr6 octahedra share neighboring corner through bridging atom Br2 different from the common opposite corner-sharing cotahedra. Second, the PbBr6 octahedra share Br4-Br5 edges at the opposite of the Br2 atoms (Figure 2a). By sharing corners alternating with sharing edges the PbBr6 octahedra connect to form a novel PbBr42- sheet. Within one octahedron, bond lengths of Pb-Br varying from 2.9394(13) to 3.1462(14) Å and bond angles Br-Pb-Br varying from 79.53(4) to 98.696(16)° (Table 2), indicating the stereochemistry of the lone pair of the Pb(II). The corner-shared bridging angle of Pb-Br2-Pb is 171.89(6)° nearly equal to straight angle. One of the edge-shared angle Pb-Br4-Pb [88.46(5)°] is close to the other Pb-Br5-Pb

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Figure 2. (a, b) Inorganic sheets of compound 1. Table 2. Selected Bond Lengths (Å) and Angles (°) for Compounds 1 and 2a Compound 1 Br(1)-Pb Br(3)-Pb Br(4)-Pb Br(5)-Pb#2 Pb-Br(2)-Pb#1 Br(2)-Pb-Br(4) Br(5)-Pb-Br(4) Pb-Br(5)-Pb#2 Br(3)-Pb-Br(2)#3 Br(2)-Pb-Br(2)#3 Br(2)#3-Pb-Br(4) Br(2)#3-Pb-Br(1)

3.1462(14) 2.9394(13) 3.0784(13) 2.9547(12) 171.89(6) 175.59(4) 89.16(4) 93.22(5) 94.53(5) 98.696(16) 85.59(4) 79.53(4) Compound 2

Br(2)-Pb Br(2)-Pb#1 Br(3)-Pb Br(4)-Pb#2 Pb-Br(2)-Pb#1 Pb-Br(4)-Pb#2 Br(3)-Pb-Br(1) Br(4)-Pb-Br(2) Br(3)-Pb-Br(2)#4 Br(1)-Pb-Br(2)#4 Br(4)#3-Pb-Br(2)#4

3.0227(10) 3.1239(10) 2.9616(10) 3.0564(10) 153.04(4) 160.45(4) 178.00(3) 84.16(3) 87.70(3) 91.62(3) 96.15(3)

a Symmetry transformations used to generate equivalent atoms: Compound 1: #1 -x + 5/2, y - 1/2, -z + 1/2; #2 -x + 2, y, -z + 1/ 2; #3 -x + 5/2, y + 1/2, -z + 1/2. Compound 2: #1 -x + 2, y + 1/2, -z + 1/2; #2 -x + 3/2, y + 1/2, z; #3 -x + 3/2, y - 1/2, z; #4 -x + 2, y - 1/2, -z + 1/2.

[93.22(5) °], and the angle of Br4-Pb-Br5 is 89.16(4)°, all of which are close to a right angle. Therefore, through edgesharing, the Pb-Br4-Pb-Br5 builds up a quasi-square. The overall structure of the inorganic sheet displays a very regular and ideal pattern. Such a distinct inorganic sheet comes from the special organic dications.5 In compound 1, 3-amidinopyridinium dications take the configuration that the plane of the pydinium ring is roughly parallel to the approximate plane of the inorganic sheet (Figure 1). Different from others such as substituted phenethylammonium-based perovskites, in which the plane of the benzene ring is almost upright to the plane of the inorganic sheet.15 The distinct arrangement of the organic ligand makes the neighboring inorganic sheets close to each other with the shortest Br-Br distance between two layers of 4.037 Å (distance between Br1-Br1), which is extremely close to the sum of the van der Waals radii.11 This is a very short interlayer distance between

Br-Br in lead(II) bromide layered perovskite, indicative of an enhanced interaction between the layers. The noteworthy aspect of compound 1 is that there are two layers of organic dications between adjacent inorganic sheets to balance the electric charge of the novel inorganic sheets, which is different from typical hybrid perovskite.1 The organic configuration is clearly shown in the packing structure of Figure 3a. Planes of pyridiniums are parallel to each other. Two adjacent organic layers between neighboring inorganic sheets along the c-axis direct different orientations and display approximate antisymmetric formation. Comparison of the organic components of compound 1 with the following compound 2 (which is the 4-amidinopyridinium coordinated one) is shown in Figure 3. Compound 2 is a typical corner-sharing inorganic sheet perovskite, in which there is one layer of organic dications between adjacent inorganic sheets. This is an evident dissimilitude to that of the double layers in compound 1. The diprotonated 3-amidinopydrinium cation bonds with the PbBr42- layer through formation of N-H · · · Br hydrogen bonds (shown in Figure 4). The four hydrogen atoms of the amidinium (H1A, H1B, H2A, H2B) each forms a hydrogen bond that is distinctly different from the typical bridging or terminal halogen configuration hydrogen bonds in hybrid perovskite.1 The H1A atom on N1 forms a hydrogen bond with a terminal halogen Br1, whereas the H1B atom bonds with the other terminal halogen Br3 in the adjacent inorganic layer, which may result in a shorter distance of the adjacent inorganic layers. The H2A, H2B on N2 atoms form hydrogen bonds with a terminal halogen Br1 and a bridging halogen Br4, respectively. The H3A atom on pyridinium forms a hydrogen bond with a terminal halogen Br1. In all these hydrogen bonds, the bond lengths of H-Br vary from 2.481 to 2.982 Å and the bond angles of H-Br-N vary from 144.690 to 170.591°, indicating the strong interactions between the inorganic and the organic components. Compound 2 is a -oriented layered perovskite, belonging to the orthorhombic crystal system and crystallizing in space group Pbca (Table 1). Figure 5 shows the layered crystal structure of compound 2, which displays layers of amidinopyridinium alternating with corner-sharing (PbBr42-) octahedral perovskite sheets. Actually, compared with simple primary ammonium or R,ω-diammonium cations the amidinopyridinium dication has steric hindrance coming from the pyridinium that prevents the inorganic components from regularly building up perovskite sheets.2c However, the amidinium on the opposite position of the pyridinium may play an important role in

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Figure 3. Comparison of the arrangement of organic cations between compound 1 (a) and compound 2 (b).

Figure 4. Structure of compound 1 with hydrogen bonds denoted and atoms numbered.

cooperating with pyridinium that stabilizes the layered provskite sheets to some extent. Different from compound 1, in compound 2 between two adjacent inorganic layers there is one layer of amidinopyridinium dications that bonds them (the inorganic layers) together (Figure 3). In compound 2, 4-amidinopyridinium forms special hydrogen bonds with an adjacent (PbBr42-) layer, which are shown in Figure 6. Both hydrogen atoms on amidinium and pyridium in compound 2 form hydrogen bonds with their vicinal terminal or bridging halogen atoms. Amidinium on the opposite position of the pyridinium makes the organic ligand 4-amidinopyridinium more linear than 3-amidinopyridinium as far as the formation of hydrogen bonds (N-H · · · Br) is concerned. This difference in organic ligand brings about two different perovskite sheets of the compounds to some extent, which further proves the important role of the hydrogen bonds in forming coordinated compounds. In compound 2, the dihedral angle between the plane of the pyridinium in 4-amidinopyridinium and the

Figure 5. Crystal packing structure of compound 2.

approximate plane of the inorganic sheet is about 45°, which is different from the parallel relationship between the two planes in compound 1. 3.2. Film Morphology. While single crystals are often the most useful form of the organic–inorganic perovskite for examining structural and physical properties, many applications require the ability to process materials in the form of thin-films.1 This requirement is certainly true for the optoelectronic devices,4 where miniaturization is the dominant trend and the ability to deposit and pattern thin films on an ever shrinking scale is crucial. Hybrid peroskites can be easily processed into thin film by a simple spin-coating technique that provides their potential application for device fabrication. Figure 7 shows the field emission scanning electron microscopy (FESEM) images of thin films of compounds 1 and 2. The FESEM images indicate that

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Figure 8. X-ray powder diffraction of the thin films for compounds 1 (a) and 2 (b). Figure 6. Crystal structures of compound 2 with hydrogen bonds denoted and atoms numbering.

Figure 9. UV–vis absorption spectra for compound 1 (a) and compound 2 (b).

Figure 7. FESEM images for the thin films of compounds 1 (a) and 2 (b).

the thin films have a smooth surface, consisting of fine and even grains with sizes ranging from 500 nm to 1 µm. The X-ray powder diffraction (XRD) study reveals that the films are wellcrystallized and highly oriented, exhibiting well-defined and

equally spaced (00h) (h ) 2, 4, 6, . . .) diffraction peaks, as shown in Figure 8. 3.3. Optical Properties. Hybrid perovskites exhibit sharp resonances in their room-temperature optical absorption spectra, arising from an exciton state associated with the semiconducting inorganic sheets.3 The relative mobile excitons are sampling in the extended perovskite sheet (the inorganic potential well).16 An optical properties study of the two compounds (1, 2) indicates that both compounds show characteristic excitonic absorption of hybrid perovskite. Figure 9 shows the absorption spectra of the title compounds, in which excitonic absorption at 399 and 411 nm for compound 1 and compound 2 can be observed, respectively. Electronic structure of compounds 1 and 2 were calculated by the CASTEP code based on DFT. Exchange and correlation have been treated by local density approximation (LDA).17 The

New Organic–Inorganic Perovskite Materials

Figure 10. Calculated band structures of compounds 1 (a) and 2 (b).

calculated band structures are shown in Figure 10 with their corresponding calculated density of states (DOS) shown in Figures S1 and S2, Supporting Information. Study of band structures of Group (IVA) metal halide based hybrid perovskite have been reported in detail.18 However, in these two hybrid perovskites we find both exciton transition and π-π* transition of the organic ligand due to the conjugated effect of the aromatic ring. Organic–inorganic perovskite structure has several possible energy-level schemes, as shown in Figure 11, panels a, b, c, respectively.19 The most common arrangement is shown in Figure 11a, in which the semiconducting inorganic sheets alternate with organic layers having much wider band gaps, resulting in a type I quantum well structure. In Figure 11b, wider band gap inorganic layers and organic cations with a smaller HOMO–LUMO gap result in the well/barrier roles of the organic and inorganic layers being switched. In Figure 11c, by shifting the electron affinity of the organic layers relative to the inorganic layers, a staggering of the energy levels leads to a type II quantum well structure. Both of the two compounds belong to the rare type II quantum well structure according to previously reported.2a The band structures can be assigned according to total and partial DOS. The valence bands (VBs) of compound 1 localized at about 0 and –5.0 eV are mainly made up of Pb 6s and Br 4p, mixing with small C2p(π) and N2p(π). VBs of the C2p(π) and N2p(π) are located at about –2.0 and –5.0 eV, while that of the Pb6s and Br4p are located at high energy between about –2.0 and 0 eV. The conduction bands (CBs) of compounds 1 between 0 and 5.0 eV are mostly formed from C2p(π*), N2p(π*), and Pb6p, in which the energy between 1 and 2.5 eV are mainly composed of C2p(π*), N2p(π*), while the Pb6p occupying the higher energy between 2.5 and 5.0 eV. Therefore, the characteristic excitonic absorption peak at 399 nm is mainly ascribed to the charge transition from the Pb6s and Br4p to Pb6p. Note that there is an absorption peak at 354 nm in compound 1 (Figure 9a), which is mainly aroused from

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Figure 11. Schematic organic–inorganic perovskite structure and several possible energy-level schemes that can arise within these structures (reprinted with permission from ref 19. Copyright 2001 International Business Machines).

the charge transition from C2p(π) and N2p(π) to C2p(π*) and N2p(π*) due to the absorption of the organic ligands. Similar to compound 1, the VBs of compound 2 that are localized at about 0.0 and –5.0 eV are mainly made up of Pb6p and Br4p, mixing with small C2p(π) and N2p(π) in lower energy. The higher energy between –2.0 and 0 eV is also formed from Pb6p. The CBs are mainly made up of C2p(π*) and N2p(π*) with a little contributions from Pb6p. The absorption spectrum of compound 2 shows a broadband of organic ligands centered at 390 nm (Figure 9b), which is attributed to the charge transition from C2p(π) and N2p(π) to C2p(π*) and N2p(π*), and a narrow sharp peak of characteristic excitonic absorption at 411 nm due to the charge transition from Pb6s and Br4p to Pb6p. The excitonic absorption of compound 1 exhibits a slightly blue shift compared with typical -oriented PbBr2-based perovskite (compound 2) due to its special inorganic sheets as previously mentioned. The perovskite sheet has been considered as a potential well of the quantum well structures where the excitonic absorption results from.1 So the excitonic absorption is in close relation with the structure of the inorganic sheet. Special inorganic sheets reported previously (including MoO3type tin(II) iodide layer and staircase-like inorganic network)7,18a have also shown a shift of the excitonic absorption compared to common inorganic in absorption spectra. Therefore, it is understandable that special inorganic sheets of compound 1 bring about a blue shift in its excitonic absorption compared to compound 2 with a typical -oriented inorganic sheet. The calculated excitonic band gap of compound 1 is about 2.8 eV and that of compound 2 is about 2.0 eV, which matches with the red shift in the absorption spectra. It should be noted that the calculated band gaps in semiconductors may be underestimated using this first-principles energy band calculation, probably due to many factors such as the action of the crystal field, the structure model, the limits of method itself, etc.20

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4. Conclusions New PbBr2-based hybrid perovskite with special both cornerand edge-sharing octahedral inorganic sheets (compound 1) is built up using 3-amidinopyridinium as the organic dication. Compared with the common corner-sharing -oriented one, which is constructed by 4-aminopyridinium (compound 2), the special inorganic sheets in compound 1 bring about a blue shift of the excitonic absorption. Hybrid perovskite is a structural flexible family. By varying the inorganic components-metal halides and organic components-ammoniums, many kinds of hybrid perovskite can be obtained. These different structures often produce diverse special properties of the materials. Therefore, new materials are still expected in the hybrid perovskite family.

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Acknowledgment. This project is financially supported by the foundation of “Bairen Jihua” of Chinese Academy of Sciences,theMOSTofChina(No.2003CB314707,2007CB935502), and National Natural Science Foundation of China (20431030).

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Supporting Information Available: Crystal data of compounds 1 and 2 in cif format; the calculated density of states (DOS) of compounds 1 (Figure S1) and 2 (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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