ARTICLE pubs.acs.org/crystal
Solid State Structural Characterization and Solution Spectroscopy of a Dodecyloxy Copper Nanoball† John J. Perry, IV,‡ Victor Ch. Kravtsov,§ Michael J. Zaworotko,‡ and Randy W. Larsen*,‡ ‡
Department of Chemistry, University of South Florida, 4202 East Fowler Avenue Tampa, Florida 33620, United States Institute of Applied Physics, Academy of Sciences of Moldova. Academiei Street 5 Chisinau, MD-2028 Republic of Moldova
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bS Supporting Information ABSTRACT: The synthesis and properties of discrete nanometer scale materials is of critical importance for the development of advanced nanoscale materials. Here we report the solid structural characterization and solution photophysical properties of self-assembled [Cu2(OC12-bdc)2 L2]12 [(OC12-bdc)2 = 5-(dodecyloxy)-1,3-benzenedicarboxylate; L = solvent] nanoballs (hereafter OC12-nanoball or OC12NB). The crystal packing reveals that selected OC12 moieties thread through the windows of adjacent OC12NB molecules and thereby sustain a 1-fold diamondoid network. Unlike other NBs the OC12NBs are readily soluble in a variety of hydrocarbon solvents including benzene, toluene, and hexane. The solution properties of the OC12NB were examined in toluene using optical absorption, and both steady-state and time-resolved fluorescence methods. These results indicate that the electronic properties of the OC12NB are similar to those of the OH-nanoball previously examined (Larsen et al. et al. Inorg. Chem. 2007, 46(5), 59045910). In addition, fluorescence polarization results further indicate that the OC12NBs remain intact in both toluene and hexane solutions. Overall, these data suggest a general stability for Cu(II) NB’s containing a variety of substituted isophthalic acid ligands in solution.
’ INTRODUCTION Discrete nanoscale assemblies are an important class of molecular structures that may find wide ranging applications including biocompatible drug delivery systems, advanced nanoscale sensor arrays, and even nanomolecular “machines”.15 Of the many types of discrete nanoscale assemblies (e.g., fullerenes, metal shell nanoparticles, etc.) metal organic polyhedra (MOP) are emerging as an important class of discrete nanoscale structures that form via selfassembly under mild solution conditions and can be readily functionalized providing extreme diversity in molecular properties.610 Metal organic polyhedra commonly referred to as nanoballs (NB) are composed of metal or metal cluster building units linked through organic ligands with fixed angle functional groups. NBs represent an attractive target class of structure in that they possess high molecular weight weights (MW 7000 and upward), they can exhibit high solubility, the organic ligands, or axial metal sites can be easily functionalized leading to decoration of the exterior surface, they have external windows and large interior cavities that are hydrophobic in nature, and they can be inherently fluorescent. The prototypal “first generation” NBs are based upon faceted polyhedra, namely the small rhombihexahedron.11 Geometrically the small rhombihexahedron consists of 12 squares linked through each of their four vertices at an angle of 120° in such a manner to enclose space. In first generation NBs the 12 square building units are r 2011 American Chemical Society
M2(RCO2)4 paddlewheel clusters (where M is a metal ion and RCO2 represents an organic carboxylate ligand).12 These 12 molecular squares are subsequently linked through the 24 benzene-1,3-dicarboxylate, bdc, ligands at the 120° angle subtended between the two carboxylate moieties of bdc. Using this design principle a variety of NB’s have been prepared using bdc’s substituted at the 5-position: amino, nitro, hydroxyl,7a dodecyloxy,8c sulphonato,8d methoxy,8d naphthyl (Figure 1) and tert-butyl (M = Mo2þ rather than Cu2þ).13 Decorated NBs have relatively large molecular volumes (9.254.4 nm3 depending upon the type of substitution on the aromatic ring) and internal cavity volumes of ∼0.9 nm3. In addition, several are soluble in a variety of organic solvents including alcohols, DMF and acetonitrile and their exterior decoration can be exploited to prepare “suprasupermolecular” networks in which the NBs are in effect supermolecular building blocks for high symmetry metal organic frameworks.14 The solubility of the NBs in a variety of solvents offers an opportunity to examine the solution properties of these structures. We have previously reported the photophysical properties of a first generation hydroxy NB (OH-NB), [Cu2(5-OH-bdc)2 L2]12 (where (5-OH-bdc)2- = 5-hydroxy benzene-1,3-dicarboxylate Received: April 4, 2011 Published: May 16, 2011 3183
dx.doi.org/10.1021/cg200421y | Cryst. Growth Des. 2011, 11, 3183–3189
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Figure 2. Geometry optimized structure of the OC12BDC ligand.
Figure 1. Schematic of a small Rhombihexahedron depicting the peripheral nature of the functional groups (black balls) when the 1,3-BDC moiety is derivatized in the 5th-position. Atoms are color coded as follows: carbon (gray), oxygen (red), hydrogen (white), nitrogen (blue), sulfur (yellow), transition metal (salmon).
and L is a DMSO or methanol ligand) in methanol.15 The optical spectrum of the OH-NB is dominated by ligand absorbance at 305 nm and a weaker Cu2þ to ligand charge transfer transition at ∼695 nm. The corresponding weak emission spectrum of the OH-NB originates from the emission of the ligand centered at ∼360 nm. Addition of bases such as imidazole results in an increase in emission intensity of the OH-NB solution indicating dissociation of the [Cu2(5-OH-bdc)2L2]12 units. The mechanism of (5-OH-bdc)2 quenching within the OHNB is likely due to excited state charge transfer between the ligand π-system and the Cu d-orbitals. Fluorescence polarization studies further suggest that the OHNB retains a spherical shape in solution. The optical properties of the OHNB also provide a spectroscopic method through which to begin to examine the mechanism through which MOP form in solution. With this in mind we have also utilized rapid mixing techniques together with fast optical spectroscopy to probe the kinetics of [Cu2(5-OH-bdc)2 L2]12 formation in methanol.16 These results reveal at least five distinct kinetic steps associated with the formation of the OHNB in methanol with lifetimes of Kþ > Rbþ > Csþ which suggests that ion association to the OC12NB is the dominate factor in ion transport. In this report, we examine the solution self-assembly, stability and photophysical properties of OC12NB, as well as report a new X-ray crystal structure of OC12NB.
’ MATERIALS AND METHODS All reagents were purchased from Sigma-Aldrich and used without further purification. 5-(dodecyloxy)-1,3-benzenedicarboxylic acid (OC12BDC) was synthesized from commercially available dimethyl 5-hydroxyisophthalate and 1-bromododecane via established procedures from the literature for the alkylation of phenols.18 The spectroscopic data for OC12BDC was consistent with results previously reported for this compound.19 OC12BDC: Yield: 5.015 g (60%); 1H NMR (250 MHz, DMSO-d6, δ): 0.9(t, J = 6.3 Hz, 3H, CH3), 1.2(m, 18 H, CH2), 1.7(m, 2H, CH2), 4.1(t, J = 6.4 Hz, 2H, O CH2), 7.6(s, 2H, ArH), 8.1(s, 1H, ArH), 13.3(br, 2H, COOH); mp 166168 °C (lit. 163166 °C). Two methods were utilized to prepare the OC12NB. Blue, plate-like single crystals of OC12NB suitable for X-ray diffraction were prepared by a two step process. In a typical reaction, 699 mg of Cu(NO3)2 3 2.5 H2O (3.01 mmol) dissolved in 10 mL of methanol was added to a refluxing solution of 1.078 g of OC12BDC (3.076 mmol) in 50 mL of methanol. To this solution 1.08 mL of 2,6-lutidine (9.27 mmol) was added and the mixture was allowed to continue refluxing for one hour. Upon cooling, the solution and precipitate were separated via vacuum filtration and the filtrand was allowed to air-dry overnight resulting in 1.027 g of a turquoise-blue microcrystalline powder, OC12NB, being collected for an overall yield of 82%. This microcrystalline powder was observed to be soluble in several common organic solvents (see Supporting Information). In the second step, single crystals were obtained by the slow-diffusion of acetonitrile into a solution of OC12NB’s dissolved in tetrahydrofuran. OC12NB samples for solution studies were prepared by dissolving equimolar Cu(NO3)2 3 2.5 H2O and OC12BDC (0.2 mmol each) in neat methanol. Two equivalents of aniline were then added resulting in a blue precipitate. The methanol suspension, including the blue precipitate was added to equal volumes (10 mL) of hexane and shaken. This solution was allowed to stand for several hours after which a blue hexane layer formed. This layer was collected and layered over neat dimethyl sulphoxide (DMSO). The solid blue material was collected by centrifugation and washed twice with neat DMSO. The solid material was soluble in both hexane and toluene and had optical properties identical to those obtained by dissolving 3184
dx.doi.org/10.1021/cg200421y |Cryst. Growth Des. 2011, 11, 3183–3189
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Figure 3. Absorption (350 nm traces) of OC12BDC in hexane (10 μM) with 314 nm excitation. Both the absorption and emission traces were normalized to the maximum intensity for illustrative purposes. the crystalline material (from which the X-ray diffraction was obtained) in either hexane or toluene. UV/vis spectra were recorded using a Shimadzu UV2401 spectrometer. Steady-state fluorescence measurements were performed with an ISS PC1 (ISS, Inc., Champaign, IL) single photon counting spectrofluorimeter. Fluorescence quantum yields for both the OC12BDC and OC12NB were obtained using OH-BDC as a reference (Φ = 0.014 with 314 nm excitation)14 and ΦC12 BDC=C12 NB ¼ ΦOHBDC ðI C12 BDC=C12 NB =I OHBDC Þðntoluene 2 =nMethanol 2 Þ ð1Þ where I is the total integrated intensity of the given species at λExcit = 314 nm and n is the refractive index of the solutions (OH-BDC was solubilized in methanol while the C12BDC and C12NB were solubilized in toluene). The integrated intensities (from 320 to 500 nm) were obtained for solutions having matched optical densities at 314 nm (OD of 0.07 for each). Molar extinction coefficients were determined using a calculated molecular weight of 10.27 kDa. Fluorescence lifetimes were obtained by excitation of the sample with a Continuum Leopard I frequency quadroupled Nd:YAG laser (λExc = 266 nm,
