Selective Deposition of Metal Complex Nanocrystals onto the Surfaces of Organic Single Crystals Bearing Pyridine Moieties Yuzo Fujiki, Seiji Shinkai, and Kazuki Sada* Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity 744 Motooka, Nishi, Fukuoka 819-0395, Japan ReceiVed December 19, 2008; ReVised Manuscript ReceiVed February 27, 2009 ABSTRACT: Nanocrystals of a palladium complex 3 were deposited onto the specific surfaces of the single crystals of CT complex (CT1) between pyrene and naphthyldiimide bearing pyridine groups. They were characterized by X-ray powder diffraction (XRD) and scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDX). Nanocrystals of 3 were deposited selectively onto {011 j } faces. Comparison between the crystal structures and deposition preference of the crystal faces revealed that deposition of 3 occurred on the most hydrophobic face of the single crystals of CT1 that exposed no pyridine moieties. This indicated that accessibility or coordination of metal complexes to the pyridine moieties apparently suppressed deposition of the nanocrystals. As a result, the nanocrystals were easily deposited onto the hydrophobic crystal surfaces, and rapid growth provides the thin-film decoration of the selective crystal faces. Introduction Organic single crystals are well-defined supramolecular assemblies made by organic molecules constructed by nonco- valent bonds. Their sizes range from nanometer to millimeter but their shapes are basically identical and independent from their sizes. Moreover, they are mostly anisotropic polyhedrons, and their crystal faces provide various interfaces with different chemical properties because of the low symmetric shape of organic molecules and their anisotropic arrangements in the single crystals. Utility of anisotropic interfaces of organic single crystals as a tool of supramolecular chemistry have attracted growing interest because progress of crystal engineering enables us partially to predict and design crystal structures from molecular structures. 1,2 On the other hand, control of crystal growth of organic and inorganic compounds has been a long-standing problem with respect to biominerallization, and effects of the crystallization by the external additive have been widely investigated. 3-6 However, the role of the functional groups on the interface of organic single crystal for crystallization and crystal growth still remain unclear. In this report, we attempted deposition and crystal growth of nanocrystals of metal complex onto specific crystal faces of organic compounds with pyridine moieties by the metal-coordination equilibrium. Results and Discussion Design of Organic Crystal Substrates. As an organic single- crystal substrate, the charge-transfer (CT) complex of naphtha- lene diimide 1 7 with pyrene 2 was selected. The naphthalene diimide moiety is a strong electron acceptor with the large aromatic plane and capable to form CT complexes with various donors. 8 The pyridine moieties of 1 have the potential to make metal complexes through coordination. The charge-transfer complexes CT1 were prepared by mixing 1 and pyrene 2 at a 1:1 stoichiometry in DMF. As expected, both of the resulting CT complexes were sparingly soluble in water and the common organic solvents such as acetonitrile and chloroform. Single crystals of CT1 were prepared by recrystallization from DMF- methanol mixture to yield red octahedral prisms. They had well- defined crystal faces and their colors originated from the absorption of the CT complex in the solid state. Under the SEM observation, they had flat surfaces in the scale of micrometers. After the crystals were placed in a sample vial and immersed in acetonitrile for an hour at room temperature, SEM observa- tions revealed that they did not change the edges of the single crystals and kept the crystal faces flat. Thus, the single crystals had enough stability against solubilization in such organic solvents. Crystal packing of CT1 was determined by a single-crystal X-ray analysis, as shown in Figure 1. In the crystal structure, they are packed is in a columnar arrangement by the alternating stacking of 1 and 2 by π-π interaction or CT complexation along the crystallographic a axis, but their aromatic planes were slightly tilted from the axis as shown in Figure 1b. The one- dimensional columns were arranged in a two-dimensional fashion by weak C-H ··· O hydrogen bonds (distances; 2.554 * Corresponding author. E-mail: [email protected]. Fax: (81)92-802-2820. Figure 1. Crystal packing diagram of CT complex CT1 viewed from (a) {100} face, (b) {010} face, (c) {001} face, and (d) {011 j } face. CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 6 2751–2755 10.1021/cg801384c CCC: $40.75 2009 American Chemical Society Published on Web 04/14/2009
Transcript
Selective Deposition of Metal Complex Nanocrystals onto theSurfaces of Organic Single Crystals Bearing Pyridine Moieties
Yuzo Fujiki, Seiji Shinkai, and Kazuki Sada*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu UniVersity 744Motooka, Nishi, Fukuoka 819-0395, Japan
ReceiVed December 19, 2008; ReVised Manuscript ReceiVed February 27, 2009
ABSTRACT: Nanocrystals of a palladium complex 3 were deposited onto the specific surfaces of the single crystals of CT complex(CT1) between pyrene and naphthyldiimide bearing pyridine groups. They were characterized by X-ray powder diffraction (XRD)and scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM-EDX). Nanocrystals of 3 were depositedselectively onto {011j} faces. Comparison between the crystal structures and deposition preference of the crystal faces revealed thatdeposition of 3 occurred on the most hydrophobic face of the single crystals of CT1 that exposed no pyridine moieties. This indicatedthat accessibility or coordination of metal complexes to the pyridine moieties apparently suppressed deposition of the nanocrystals.As a result, the nanocrystals were easily deposited onto the hydrophobic crystal surfaces, and rapid growth provides the thin-filmdecoration of the selective crystal faces.
Introduction
Organic single crystals are well-defined supramolecularassemblies made by organic molecules constructed by nonco-valent bonds. Their sizes range from nanometer to millimeterbut their shapes are basically identical and independent fromtheir sizes. Moreover, they are mostly anisotropic polyhedrons,and their crystal faces provide various interfaces with differentchemical properties because of the low symmetric shape oforganic molecules and their anisotropic arrangements in thesingle crystals. Utility of anisotropic interfaces of organic singlecrystals as a tool of supramolecular chemistry have attractedgrowing interest because progress of crystal engineering enablesus partially to predict and design crystal structures frommolecular structures.1,2
On the other hand, control of crystal growth of organic andinorganic compounds has been a long-standing problem withrespect to biominerallization, and effects of the crystallizationby the external additive have been widely investigated.3-6
However, the role of the functional groups on the interface oforganic single crystal for crystallization and crystal growth stillremain unclear. In this report, we attempted deposition andcrystal growth of nanocrystals of metal complex onto specificcrystal faces of organic compounds with pyridine moieties bythe metal-coordination equilibrium.
Results and Discussion
Design of Organic Crystal Substrates. As an organic single-crystal substrate, the charge-transfer (CT) complex of naphtha-lene diimide 17 with pyrene 2 was selected. The naphthalenediimide moiety is a strong electron acceptor with the largearomatic plane and capable to form CT complexes with variousdonors.8 The pyridine moieties of 1 have the potential to makemetal complexes through coordination. The charge-transfercomplexes CT1 were prepared by mixing 1 and pyrene 2 at a1:1 stoichiometry in DMF. As expected, both of the resultingCT complexes were sparingly soluble in water and the commonorganic solvents such as acetonitrile and chloroform. Singlecrystals of CT1 were prepared by recrystallization from DMF-
methanol mixture to yield red octahedral prisms. They had well-defined crystal faces and their colors originated from theabsorption of the CT complex in the solid state. Under the SEMobservation, they had flat surfaces in the scale of micrometers.After the crystals were placed in a sample vial and immersedin acetonitrile for an hour at room temperature, SEM observa-tions revealed that they did not change the edges of the singlecrystals and kept the crystal faces flat. Thus, the single crystalshad enough stability against solubilization in such organicsolvents.
Crystal packing of CT1 was determined by a single-crystalX-ray analysis, as shown in Figure 1. In the crystal structure,they are packed is in a columnar arrangement by the alternatingstacking of 1 and 2 by π-π interaction or CT complexationalong the crystallographic a axis, but their aromatic planes wereslightly tilted from the axis as shown in Figure 1b. The one-dimensional columns were arranged in a two-dimensionalfashion by weak C-H · · ·O hydrogen bonds (distances; 2.554
10.1021/cg801384c CCC: $40.75 2009 American Chemical SocietyPublished on Web 04/14/2009
Å) between phenyl H of 2 and CdO of the imide groups of 1to yield three-dimensional crystal structures.
Furthermore, the X-ray crystallography provided the Millerindices of the crystal faces of the red octahedral prism with sixrectangles as the side faces and two hexagons as the top andbottom faces. The symmetry of P1j space group indicated thatthey are classified four sets of the paired faces. The Millerindices of the crystal faces were shown in Figure 2. The twohexagonal faces were assigned to the crystallographic {100},or {1j00} faces, and the six rectangle faces were to thecrystallographic {011j} or {01j1}, {010} or {01j0}, and {001}or {001j} faces. From the molecular packing diagrams of thecrystal structure, these four sets of the crystal faces should havedifferent chemical properties because of differences in exposureof the pyridine groups, as shown in Figure 3. The angles ofthe long molecular axis of 1 toward the crystal faces definedare summarized in Figure 3. The {001} face had the widestangle and the {011j} had the smallest. In particular, {011j} facewas just parallel to the long molecular axis. On the latter face,the nitrogen atom of the pyridine moiety was not exposed atall, but on the other faces, the pyridine moieties were exposed,and accessibility of the metal complexes should be controlledby the following order: {001}, {001j} > {010}, {01j0} > {100},{1j00} . {011j}, {01j1}.
Adhesion or Decoration of Some Metal Complexes andSalts onto Crystals Surface CT1. Decoration or adhesion ofmetal complexes onto the surfaces of CT1 was carried out bysimple immersion of red octahedral single-crystals (size ca. 0.3mm) in the solutions of various salts and metal complexessummarized in Table 1. They kept in the solution for 1 h atroom temperature by either a batch method or a mounted
method. After enough washing with the solvents, in all cases,they had no apparent changes in color and shape by naked eyes.We then observed the red octahedral crystals by SEM. In thecase of 39 as a metal complex, the crystal surfaces of CT1 weredecorated by the thin film with 200-300 nm thickness as shownin Figure 4, and in the cases of the other metal complexes listedin Table 1, amorphous aggregates that were partially deposited.Interestingly, they were composed of star-shaped nanocrystals.On the surface of the single crystals immersed for 30 min thestar-shaped nanocrystals were scattered all over the surface.Thus, the star-shaped nanocrystals were initially deposited andthen grew to the layers (Figure 3b-d). On the other hand,aqueous solution of the various metal salts under higherconcentration (6 mM) and the other Pd complexes in acetonitrileand chloroform solution did not decorate the crystal faces ofCT1 uniformly. SEM observations illustrated that a smallamount of the large microsized aggregates were partiallydeposited on the crystal surfaces. This result suggests thatcoordination of metal complexes to pyridine moieties did notalways promote decoration of the crystal surfaces.
We then further characterized the star-shaped nanocrystalsby X-ray powder diffraction and EDX in SEM observation. Thedeposited crystals were collected from the decorated CT1crystals by careful scraping. Figure 5 shows the XRPD patternsof the scraped crystals, together with CT1 and 3 as references.The XRPD pattern of the deposited crystals was mostlysummation of those of 3 and CT1. The diffraction peaks ofCT1 originated from contamination by scrapping the depositedcrystals off. This suggested that the deposited crystals were madefrom 3. This observation was confirmed by SEM-EDX andelementary mapping. In SEM-EDX, palladium was detected inboth the star-shaped nanocrystals and the thin film on the crystalsurfaces of CT1 as shown Figure 6. Element mapping shownFigure 7 also indicated that they contained carbon, nitrogen,oxygen, and palladium. Therefore, we believe that depositedcrystals onto the crystal surfaces were composed of 3.
Crystal Growth of 3 onto CT1. Morphology changes fromthe star-shaped nanocrystals to thin film structures of 3 on thecrystal surface of CT1 prompted us to investigate the changesor growth of 3 on the surface of CT1. We investigateddependence of incubate conditions such as temperature andconcentration for deposition and crystal morphology. When theconcentration was reduced from 0.6 to 0.06 mM, no crystal-lization of 3 was observed on the CT1 crystal surfaces eitherat ambient temperature (25 °C) or higher (80 °C). Too lowconcentration induced no deposition or crystallization onto CT1.On the other hand, at the higher concentrations (0.6 and 6 mM),nanocrystals was deposited onto CT1, but their shapes weredifferent. The star-shaped and octahedral nanocrystals werechiefly deposited in a 0.6 and 6.0 mM solution, respectively,as shown Figure 8. In the latter condition, cross-shaped crystals
Figure 2. Single crystal of CT1; (a) SMART face indexing graphic,(b) crystal as mounted for face indexing.
Figure 3. Images of the angle between the long molecular axis of 3and each surface: (a) {100}, (b) {010}, (c) {001}, and (d) {011j}.
Table 1. List of Various Kind of Metal Complexes We Decoratedwith CT1 Surfaces
entry metal complex solvent concentration (mM)
1 Ni(CH3COO)2 water 6.02 Mn(CH3COO)2 water 6.03 Zn(CH3COO)2 water 6.04 KPdCl4 water 6.05 Pybox-Pda acetonitrile 0.66 Terpy-Pdb acetonitrile 0.67 DiphPy-Ptc CHCl3 0.58 Pd(CH3CN)4BF4 acetonitrile 0.6
a Pd(2,6-bis[(S)-4-isopropyloxazolin-2′-yl]pyridine)(CH3CN)(BF4)210).
b Pd(2,2′:6,2′′-Terpyridine)(CH3CN)](BF4)211). c (2,6-Diphenylpyridine)-
were also observed, which indicates that the star-shapednanocrystals grew to octahedral crystals. Higher temperatureand higher concentration promoted growth or crystallization of3 from star-shaped nanocrystals to octahedral ones or thin crystallayers. These results indicated that the star-shaped crystalsshould be intermediate and formed kinetically.
Selective Decoration of Crystal Faces of 3. Nanocrystals 3were deposited selectively on the specific crystal face of CT1.Figure 9 shows SEM images of one single crystal of CT1decorated with 3. The similarity of the crystal shapes betweenin millimeter-sized single crystals and in micrometer-sized
Figure 4. SEM image of the crystal surface of CT1 decorated by 3.
Figure 5. XRPD pattern of (a) powder prepared from CT1 surfacescoated by nanocrystals 3, (b) powder of palladium complex 3, (c)CT1.
Figure 6. EDX spectrum of CT1 coated by nanocrystal 3. Inset: SEMimage (each EDX spectrum was depend on the area encircled by redline). (a, b) Star-shaped assemblies were included with the palladiumelement. The peak of the Pt element comes from the deposition forpreventing electrocharging on SEM observation.
Selective Deposition of Metal Complex Nanocrystals Crystal Growth & Design, Vol. 9, No. 6, 2009 2753
crystals provided the Miller indices of the crystal faces in theCT1. Nanocrystals were deposited densely onto the {100} face,but sparingly onto the {010}. The thin film that was formed byassembly of the nanocrystals was observed only onto {011j}face. The amount of the deposited nanocrystals was obviouslydifferent and dependent on the faces of the hexagonal prism ofCT1 crystal. This result clearly indicated that nanocrystal 3 werecrystallized or deposited onto only {011j} face more easily thanthose of the other faces. These nanocrystals should grow intothe layers of 3 on {011j} faces through the tetrahedral interme-diate.
The selective deposition of the nanocrystals 3 on the {011j}face of CT1 crystal should be originated from differences ofthe chemical properties among the crystal faces. As discussedearlier, the order of the accessibility of the pyridine moietieson the crystal faces from the crystal packing diagrams were{001}, {001j} > {010}, {01j0} > {100}, {1j00} . {011j}, {01j1}.Unexpectedly, this order is different from that of the order ofdeposition of the star-shaped nanocrystals of 3. In particular,crystal packing diagrams illustrated that {01j1} faces have noexposure of the pyridine groups on the surface. This indicatesthat accessibility or coordination of metal complexes to thepyridine moieties apparently suppressed the deposition of thenanocrystals, although the pyridine moieties should be expectedto increase the concentration of the metal complex 3 on theinterface and have more chance of nucleation of the nanocrys-tals. The coordination equilibrium between the ligation sites onthe surface and labile metal complexes should compete withnucleation of nanocrystals and interfere on deposition ofnanocrystals 3. As a result, the star-shaped nanocrystals were
deposited much more quickly onto more hydrophobic surfacesand rapid growth provides the thin-film decoration of the crystalfaces.
Conclusion
We succeeded in preparing charge-transfer crystals CT1 asa substrate that has low solubility to water and organic solvents,and in preparing the decorated crystals with palladium complexnanocrystal 3. Controlled experimental conditions such astemperature and concentration indicated initial nucleation of 3to form star-shaped nanocrystals and then growth of the star-shaped nanocrystals to thin film on the crystal faces. Moreover,we revealed that the nanocrystals deposited selectively on thesurfaces and, under the equilibrium of coordination with pyridinemoieties in acetonitrile solution, suppressed the nucleation of3.
Experimental Section
General. All chemicals and solvents used in this study werecommercially available and used without purification. Powder X-raydiffraction (XRD) data were collected on a Rigaku RINT-1100 usinggraphite-monochromatized Cu KR radiation (λ ) 1.54178 Å) at roomtemperature. Scanning electron micrograph (SEM) observation wasperformed on a Hitachi S-5500. Compounds 1 and 3 were preparedaccording to the reported method.7,9
Figure 7. Element mapping of nanocrytals 3 deposited onto the surfacesof CT1. These images indicated that star-shaped nanocrystals werecomposed of palladium, carbon, oxygen, and nitrogen element.
Figure 8. SEM images of nanocrystals 3. These were dependent onthe conditions of concentration and temperature.
Figure 9. SEM image of (a) CT1 crystal decorated by 3, and (b) thecorner of the decorated crystal, (c) the (100) face, (d) the (001j) face,(e) the (011j) face, (f) the (010) face. This results indicated that thereis a marked tendency that nanocrystal 3 was likely to deposit onto the(011j) surface.
Table 2. Condition of Solution Dissolved by Palladium Complex 3
X-ray Crystallographic Study. X-ray diffraction data were collectedon a Bruker SMART APEX CCD detector with graphite-monochro-matized Mo KR radiation (λ ) 0.71073 Å). The data frames wereintegrated using SAINT and merged to give a unique data set for thestructure determination. Empirical absorption corrections by SADABSwere carried out. Direct methods were used for the structure solutions.The structures were refined by a full matrix least-squares procedureusing all the observed reflections based on F2 using the SHELX suiteof programs. All non-hydrogen atoms were refined with anisotropicdisplacement parameters. The crystal data for the solid solution fromCT1 was as follows; C40H22N4O4, M ) 602.86, triclinic, a ) 7.9341(10)Å, b ) 9.1661(11) Å, c ) 10.3306(13) Å, R ) 89.439(3)°, � )88.443(3)°, γ ) 72.669(3)°, V ) 716.92(15) Å3, temperature 298 K,space group P1j, Z ) 1, µ(Mo KR) ) 0.118 cm-1, Dc ) 1.549 g cm-3.There were 1866 unique reflections, and 1501 observed reflections withFo
2 > 3σFo were used for further calculations after Lorenz andpolarization corrections. The final R1 and Rw values were 0.0410 and0.1304, respectively. The crystal was deposited at the CambridgeCrystallographic Data Center, and the deposition number is CCDC2883836.
Deposition of Metal Complexes onto CT1. Deposition of metalcomplexes was carried out by the following immersion method undervarious conditions; red octahedral crystals (15 mg) of CT1 wereimmersed in an appropriate solution (6.0 mM, ca. 3.0 mL) of metalcomplexes as summarized in Table 2 and the solution was kept for1 h. The crystals were washed well with the solvents. The resultingcrystals were dried on a SEM probe in vacuo and observed by SEM.
Acknowledgment. Financial support for this research wasprovided by the Ministry of Education, Culture, Sports, Science
and Technology of Japan for K.S. and S.S. Y.F. expresses hisspecial thanks for the JSPS Research Fellowships for YoungScientists and the Kyushu University GCOE program of“Science for Future Molecular Systems” for financial support.A part of this work was supported by the “NanotechnologyNetwork Project” of MEXT.
Supporting Information Available: Crystallgraphic information file(CIF). This material is available free of charge via the Internet at http://pubs.acs.org.
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CG801384C
Figure 10. Pyridine moieties exposed on the {010}, {001}, and {100}surfaces can interact with 3 in solution in equilibrium. Deposition speedof nanocrystal 3 onto {010}, {001}, and {100} surfaces was delayedbecause of this equilibrium, whereas the deposition onto the {011j}surface occurred immediately. Anisotropy of the coverage of star-shapednanocrystal 3 onto CT1 surfaces resulted from inhibition of the kineticdeposition of 3 by metal coordination between the pyridine moietieson the CT1 surface and 3, and the nanocrystals deposited easily onthe lipophilic crystal surfaces without pyridine moieties. Red, blue, gray,and white spheres represent oxygen, nitrogen, carbon, and hydrogenatoms, respectively. The pyridine moiety of 1 is shadowed by blueand dark blue, and the green skeletons represent pyrene 2.
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