Efficient, Scalable, and Solvent-free Mechanochemical Synthesis of

Communication pubs.acs.org/crystal

Efficient, Scalable, and Solvent-free Mechanochemical Synthesis of the OLED Material Alq3 (q = 8‑Hydroxyquinolinate) Xiaohe Ma,† Gin Keat Lim,‡,§ Kenneth D. M. Harris,*,‡ David C. Apperley,∥ Peter N. Horton,⊥ Michael B. Hursthouse,⊥ and Stuart L. James*,† †

Centre for the Theory and Application of Catalysis, School of Chemistry and Chemical Engineering, Queen’s University Belfast, Stranmillis Road, Belfast BT9 5AG, Northern Ireland ‡ School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, Wales § School of Chemical Sciences, Universiti Sains Malaysia, 11800 USM, Pulau Pinang, Malaysia ∥ Department of Chemistry, Durham University, Durham DH1 3LE, England ⊥ Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, England S Supporting Information *

ABSTRACT: The aluminum complex Alq3 (q = 8-hydroxyquinolinate), which has important applications in organic lightemitting diode materials, is shown to be readily synthesized as a pure phase under solvent-free mechanochemical conditions from Al(OAc)2OH and 8-hydroxyquinoline by ball milling. The initial product of the mechanochemical synthesis is a novel acetic acid solvate of Alq3, and the α polymorph of Alq3 is obtained on subsequent heating/desolvation of this phase. The structure of the mechanochemically prepared acetic acid solvate of Alq3 has been determined directly from powder X-ray diffraction data and is shown to be a different polymorph from the corresponding acetic acid solvate prepared by solution-state crystallization of Alq3 from acetic acid. Significantly, the mechanochemical synthesis of Alq3 is shown to be fully scalable across two orders of magnitude from 0.5 to 50 g scale. The Alq3 sample obtained from the solvent-free mechanochemical synthesis is analytically pure and exhibits identical photoluminescence behavior to that of a sample prepared by the conventional synthetic route.

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regard to the processing characteristics of materials for optoelectronic applications. The effectiveness of metal acetates as starting materials for mechanochemical synthesis of metal complexes from moderately acidic ligand precursors has been demonstrated previously, although mainly in the context of divalent metals.4 Such salts are reasonably basic, and the acetic acid released during the reaction could be regarded, in some respects, as playing the role of “internal solvent”. In the present work, solid basic aluminum(III) diacetate [Al(OAc)2(OH)] and solid 8hydroxyquinoline (Hq) were found to react readily at 2 mmol scale without any added solvent by ball milling for 15 min in a small shaker-type mill (Retsch MM400; details in Supporting Information) (Scheme 1). The occurrence of a reaction was apparent from the change of color from the white starting materials to the characteristic bright yellow color of Alq3. Powder XRD data indicate that the reaction occurs with quantitative conversion to a single phase of a crystalline productsee Figure 1 and Figure S1 (figures labeled S are in the Supporting Information). As discussed below, this powder

olvent-free mechanochemical synthesis offers several potential advantages over conventional solvent-based synthetic methods, including the reduction or elimination of solvent use, access to new materials or alternative solid forms, convenience, and shorter reaction times.1,2 However, the technique remains rather poorly understood, and as yet, its scope is barely explored. In addition, reports of mechanochemical synthesis have tended to concentrate on small scales (up to a few grams), and knowledge about scale-up of mechanochemical synthesis is currently very sparse. In this paper, we describe a solvent-free mechanochemical synthesis of the aluminum complex Alq3 (q = 8-hydroxyquinolinate) from solid starting materials by ball milling at scales from 0.5 to 50 g. Alq3 is an important electroluminescent material which is used as the emissive and electron transporting layer in organic lightemitting diodes (OLEDs) and has been central to the development of these devices since the late 1980s.3 In this regard, although not the primary focus of the current paper, we note that tuning the experimental variables involved in mechanochemical synthesis can lead to optimization of particle sizes and morphologies (as well as the possibility of inducing the formation of amorphous phases), and this additional control of the product phase may be of significant interest with © 2012 American Chemical Society

Received: September 5, 2012 Revised: October 22, 2012 Published: October 24, 2012 5869

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amenable to structure determination using single-crystal XRD. Instead, structure determination must be tackled directly from powder XRD data. Although the task of carrying out structure determination from powder XRD data is considerably more challenging than from single-crystal XRD data, recent advances in techniques for analysis of powder XRD data6 are such that the structural properties of materials of moderate complexity can now be determined relatively routinely by this approach. In the case of molecular solids, the development of the directspace strategy for structure solution has created considerable opportunities in this regard. Indeed, such techniques have been used in a number of cases to determine the structural properties of new materials prepared by mechanochemical synthesis.4d,7 In the present work, we have exploited this strategy to determine the crystal structure of Alq3·AcOH-m directly from powder XRD data. Specifically, the powder XRD data8 were indexed using the program DICVOL,9 followed by profile fitting using the Le Bail technique10 and space group assignment. The structure was then solved using the direct-space genetic algorithm program EAGER,11 followed by Rietveld refinement using GSAS.12 Further details of the structure determination procedure are given in the Supporting Information. In the crystal structure13 (Figure 2) of Alq3·AcOH-m, the Alq3 molecules (which exhibit the expected mer geometry) are

Scheme 1

Figure 1. Powder XRD patterns of (a) the as-prepared mechanochemical product Alq3·AcOH-m, (b) the acetic acid solvate Alq3·AcOH-s obtained by crystallization from acetic acid, (c) the desolvated mechanochemical product Alq3-m, and (d) the α polymorph of Alq3 (simulated from the known crystal structure; CCDC code: QATMON01).

Figure 2. Crystal structure of Alq3·AcOH-m determined directly from powder XRD data. Gray spheres and black spheres represent the two independent Al atoms in the asymmetric unit; the two independent acetic acid molecules in the asymmetric unit are shown in green and purple.

product is a previously unknown acetic acid solvate of Alq3, which we denote Alq3·AcOH-m (m represents mechanochemical synthesis). High-resolution solid-state 13C and 27Al NMR results also confirm (from the absence of any signals due to starting materials; Figures S2−S7) that the conversion is quantitative. Furthermore, the powder XRD pattern is distinctly different from the simulated powder XRD patterns for any of the known polymorphs or solvates of Alq3.5 Results from microanalysis, thermogravimetric analysis (TGA), and solution-state 1H NMR (see experimental details and Figure S8) suggest that the product is a cocrystal phase containing Alq3 and the acetic acid byproduct in a 1:1 molar ratio, implying that the other byproducts (a second equivalent of acetic acid and one equivalent of water) are released during the reaction. In the solid-state 13C CPMAS NMR spectrum (Figure S5), two resonances (174.7 and 169.3 ppm) are observed for the COOH group of the acetic acid component, suggesting that this molecule is present in two crystallographically distinct environments. As new materials prepared by solid-state mechanochemical procedures are generally microcrystalline powders, they are not

arranged in layers parallel to the ab-plane. The acetic acid molecules are located in “cavities” within the interlamellar region at the interface between these layers of Alq3 molecules. For one of the two independent acetic acid molecules in the asymmetric unit, the cavity is occupied singly, whereas, for the other acetic acid molecule, the cavity is occupied by a pair of symmetry-equivalent molecules related across an inversion center (this pair of acetic acid molecules are arranged in a hydrogen-bonded dimer, which differs from the planar geometry of the classical double hydrogen-bonded carboxylic acid dimer). However, evidence from several aspects of the Rietveld refinement14 suggests that there may be some degree of disorder of the acetic acid molecules, and thus, we refrain from any attempt to overinterpret the structural details concerning the acetic acid molecules (including their interaction with the Alq3 molecules). In addition to the acetic acid solvate Alq3·AcOH-m resulting from the mechanochemical synthesis, another polymorph of 5870

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Alq3·AcOH (with the same 1:1 molar ratio) was obtained in the present work by vapor diffusion of acetone into a solution of Alq3 in acetic acid and is denoted Alq3·AcOH-s (s represents solvent-based synthesis). Structure determination of Alq3·AcOH-s by single-crystal XRD confirmed the expected mer geometry with the acetic acid molecule engaged in hydrogen bonding to a coordinated quinolic O atom (O···O, 2.66 Å; OH···O, 164.3°; Figure S8). The powder XRD pattern of crushed crystals of Alq3·AcOH-s is in good agreement with the powder XRD pattern simulated from the crystal structure (Figure S10), indicating that the bulk material is monophasic. As expected, the powder XRD patterns of Alq3·AcOH-s (Figure S11) and Alq3·AcOH-m (Figure 1) differ substantially. These observations illustrate the fact that solid-state mechanochemical syntheses often yield different solid forms from those obtained from solution-state preparation procedures.1,2 Alq3·AcOH-m was observed to undergo partial desolvation under ambient conditions over several months. Full desolvation was achieved by maintaining Alq3·AcOH-m at 200 °C for 2 h to give a material denoted Alq3-m, the powder XRD pattern of which is consistent with the α polymorph of Alq3 (Figures 1, S12 and S13). Loss of acetic acid on heating was confirmed by solution-state 1H NMR, solid-state 13C CPMAS NMR, and microanalysis (see Figures S14 and S15 and the experimental details in Supporting Information). Comparison of SEM images taken before and after desolvation reveals a very distinct change in morphology from particles of diameter ca. 100−300 nm to a more contiguous morphology of larger elongated particles up to 2−3 μm in size (Figure S17). The solid-state photoluminescence spectrum of Alq3-m has the characteristic emission at 520 nm, and significantly, the absorption and emission spectra of Alq3-m are indistinguishable from those of Alq3 prepared by the conventional aqueous-based method15 (Figure 3).

powder XRD data recorded at different stages during this process suggest that the large-scale reaction may be complete in as little as 10 min; Figure S18). The Alq3·AcOH-m obtained in this scaled-up procedure was readily converted to the α polymorph of Alq3-m by heating, as for the samples prepared on smaller scale described above. Analytical data (powder XRD, solid-state fluorescence, solid-state 13C CPMAS NMR, microanalysis, TGA) for the samples of Alq3-m prepared on scale up were in agreement with those for the samples prepared on smaller scale (see Figures S19−S22 and the experimental details in Supporting Information). Conventional solvent-based syntheses of Alq3 typically involve Al(NO3)3·9H2O, 8-hydroxyquinoline, acidified aqueous solutions, and ammonium salts as precipitants, generating solvent waste and salt waste.15a,b A solvent-free procedure has also been reported based on heating a solid mixture of Al(OiPr)3 and 8-hydroxyquinoline, but this procedure requires subsequent removal of unreacted Al(OiPr)3 by distillation and washing of the product with water.15c Thus, the mechanochemical synthesis described here is notable in several regards. First, it is very straightforward to perform the synthesis rapidly at scales of ca. 50 g, and there is clearly potential for further scale up. Second, the material Alq3-m obtained after desolvation is analytically pure and exhibits the same photoluminescence characteristics as the product from solvent-based synthesis, suggesting that it has the requisite properties to be used directly in applications. Third, if samples of very high purity are required, Alq3 can be sublimed after synthesis. Thus, adding a sublimation step to the mechanochemical preparation provides an entirely solvent-f ree manufacturing route to very high purity Alq3.16 Finally, based on the efficient solvent-free synthesis of the OLED material Alq3 reported in this paper, we emphasize the following salient points: (i) while mechanochemical synthesis can often be carried out without any solvent for the reaction step, it is important to recall that, in many cases, solvent is still required for purificationhowever, in some cases, such as that reported here, purification may not be required or may be achieved simply by sublimation, and thus the entire preparation process to give the functional material is completely solventfree; (ii) in the present work, it was straightforward to scale up the mechanochemical process to yield 50 g batches of product by using a larger planetary ball mill, and we anticipate that further scale up should be readily achieved; (iii) the structure determination of Alq3·AcOH-m reported here demonstrates the real opportunities that now exist for elucidating the structural properties of microcrystalline mechanochemical products directly from powder XRD data.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 3. Solid-state photoluminescence absorption and emission spectra recorded for samples of Alq3 prepared mechanochemically (Alq3-m) and prepared by conventional solution-based synthesis (Alq3-s).

Crystallographic information (cif files) for the two polymorphs Alq3·AcOH-m and Alq3·AcOH-s, details of experimental procedures, results of analysis of samples, powder XRD data, solid-state and solution-state NMR data, and SEM data. This material is available free of charge via the Internet at http:// pubs.acs.org.

Another significant result from the present work concerns our attempts to scale up the mechanochemical synthesis using a large planetary ball mill (employing a Retsch PM100 mill at 450 rpm with a 250 mL steel jar containing the reactants and 50 steel balls of 10 mm diameter). This experimental setup was found to enable the solvent-free preparation of 50 g batches of Alq3·AcOH-m in quantitative yield within 35 min (in fact,

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AUTHOR INFORMATION

Corresponding Author

*E-mail: K.D.M.H., HarrisKDM@cardiff.ac.uk; S.L.J., s.james@ qub.ac.uk. 5871

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(13) Parameters from the final Rietveld refinement (see also Figure S0) for Alq3·AcOH-m: P21/n; a = 21.9948(16) Å, b = 13.1962(14) Å, c = 21.2276(15) Å, β = 128.0685(33)°; V = 4850.6(8) Å3 (2θ range, 4°−70°; 3867 profile points; 419 refined variables). (14) First, we note that the refined isotropic displacement parameters are somewhat larger than usual for structures determined at ambient temperature, particularly for one of the acetic acid molecules, suggesting that disorder may be a feature of this structure at ambient temperature. Second, during the Rietveld refinement, potential disorder of the acetic acid molecules was assessed by removing these molecules from the structural model followed by inspection of the resultant difference Fourier map. The difference Fourier map shows clearly that there is significant electron density in the regions identified as the locations of the two acetic acid molecules. However, this electron density is considerably smeared out, and well-defined electron density maxima (corresponding to well-defined atomic positions) could not be identified. The combined evidence suggests that the acetic acid molecules probably exhibit some degree of disorder in this structure, and hence the atomic positions of the acetic acid molecules in the final refinement should be interpreted in this context, while also recognizing that the accuracy of structural information derived from powder XRD data is inherently lower that that expected in structure determination from single-crystal XRD data. (15) (a) Mahakhode, J. G.; Bahirwar, B. M.; Dhoble, S. J.; Moharil, S. V. Proc. ASID ’06 (Asian Symposium on Information Display) 2006, 8− 12 Oct, 237. (b) Chirnside, R. C.; Rooksby, H. P.; Pritchard, C. F. Analyst 1941, 66, 399. (c) Saxena, A. K. Synth. React. Inorg., Met.-Org. Chem. 1999, 29, 1747. (16) Huskić, I.; Halasz, I.; Frišcǐ ć, T.; Vančik, H. Green Chem. 2012, 14, 1597.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Government of Malaysia for a studentship (to G.K.L.). REFERENCES

(1) James, S. L.; Adams, C. J.; Bolm, C.; Braga, D.; Collier, P.; Frišcǐ ć, T.; Grepioni, F.; Harris, K. D. M.; Hyett, G.; Jones, W.; Krebs, A.; Mack, J.; Maini, L.; Orpen, A. G.; Parkin, I. P.; Shearouse, W. C.; Steed, J. W.; Waddell, D. C. Chem. Soc. Rev. 2012, 41, 413. (2) (a) Fernandez-Bertran, J. F. Pure Appl. Chem. 1999, 71, 581. (b) Tanaka, K.; Toda, F. Chem. Rev. 2000, 100, 1025. (c) Cave, G. W. V.; Raston, C. L.; Scott, J. L. Chem. Commun. 2001, 2159. (d) Kaupp, G. CrystEngComm 2003, 5, 117. (e) Braga, D.; Giaffreda, S. L.; Grepioni, F.; Pettersen, A.; Maini, L.; Curzi, M.; Polito, M. Dalton Trans. 2006, 1249. (f) Rodriguez, B.; Bruckmann, A.; Rantanen, T.; Bolm, C. Adv. Synth. Catal. 2007, 349, 2213. (g) Lazuen-Garay, A.; Pichon, A.; James, S. L. Chem. Soc. Rev. 2007, 36, 846. (h) Kaupp, G. CrystEngComm 2009, 11, 388. (i) Frišcǐ ć, T.; Jones, W. Cryst. Growth Des. 2009, 9, 1621. (3) Tang, C. W.; van Slyke, A. Appl. Phys. Lett. 1987, 51, 913. (4) (a) Pichon, A.; James, S. L. CrystEngComm 2008, 10, 1839. (b) Pichon, A.; Lazuen-Garay, A.; James, S. L. CrystEngComm 2006, 8, 21. (c) Yuan, W.; Lazuen-Garay, A.; Pichon, A.; Clowes, R.; Wood, C. D.; Cooper, A. I.; James, S. L. CrystEngComm 2010, 12, 4063. (d) Fujii, K.; Lazuen Garay, A.; Hill, J.; Sbircea, E.; Pan, Z.; Xu, M.; Apperley, D. C.; James, S. L.; Harris, K. D. M. Chem. Commun. 2010, 46, 7572. (5) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi, N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147. (6) (a) Harris, K. D. M.; Tremayne, M.; Lightfoot, P.; Bruce, P. G. J. Am. Chem. Soc. 1994, 116, 3543. (b) Kariuki, B. M.; Zin, D. M. S.; Tremayne, M.; Harris, K. D. M. Chem. Mater. 1996, 8, 565. (c) Chernyshev, V. V. Russ. Chem. Bull. 2001, 50, 2273. (d) Harris, K. D. M. Cryst. Growth Des. 2003, 3, 887. (e) Tremayne, M. Philos. Trans. R. Soc. A 2004, 362, 2691. (f) Favre-Nicolin, V.; Č erný, R. Z. Kristallogr. 2004, 219, 847. (g) Tsue, H.; Horiguchi, M.; Tamura, R.; Fujii, K.; Uekusa, H. J. Synth. Org. Chem. Jpn. 2007, 65, 1203. (h) David, W. I. F.; Shankland, K. Acta Crystallogr., Sect. A 2008, 64, 52. (7) (a) Cheung, E. Y.; Kitchin, S. J.; Harris, K. D. M.; Imai, Y.; Tajima, N.; Kuroda, R. J. Am. Chem. Soc. 2003, 125, 14658. (b) Karki, S.; Fábián, L.; Frišcǐ ć, T.; Jones, W. Org. Lett. 2007, 9, 3133. (c) Frišcǐ ć, T.; Meštrović, E.; Šamec, D. Š.; Kaitner, B.; Fábián, L. Chem.Eur. J. 2009, 15, 12644. (d) Fujii, K.; Ashida, Y.; Uekusa, H.; Hirano, S.; Toyota, S.; Toda, F.; Pan, Z.; Harris, K. D. M. Cryst. Growth Des. 2009, 9, 1201. (8) Powder XRD data for Alq3·AcOH-m were recorded at ambient temperature on a Bruker D8 diffractometer (transmission; Gemonochromated Cu Kα1; λ = 1.5406 Å; Vantec detector covering 12° in 2θ; 2θ range, 4°−70°; step size, 0.017°; data collection time, 17 h). (9) Boultif, A.; Louër, D. J. Appl. Crystallogr. 2004, 37, 724. (10) Le Bail, A.; Duroy, H.; Fourquet, J. L. Mater. Res. Bull. 1988, 23, 447. (11) (a) Habershon, S.; Harris, K. D. M.; Johnston, R. L. J. Comput. Chem. 2003, 24, 1766. (b) Harris, K. D. M.; Habershon, S.; Cheung, E. Y.; Johnston, R. L. Z. Kristallogr. 2004, 219, 838. (c) Tedesco, E.; Turner, G. W.; Harris, K. D. M.; Johnston, R. L.; Kariuki, B. M. Angew. Chem., Int. Ed. 2000, 39, 4488. (d) Albesa-Jové, D.; Kariuki, B. M.; Kitchin, S. J.; Grice, L.; Cheung, E. Y.; Harris, K. D. M. ChemPhysChem 2004, 5, 414. (e) Guo, F.; Harris, K. D. M. J. Am. Chem. Soc. 2005, 127, 7314. (f) Fujii, K.; Young, M. T.; Harris, K. D. M. J. Struct. Biol. 2011, 174, 461. (12) Larson, A. C.; Von Dreele, R. B. Los Alamos National Laboratory Report, LAUR 86-748; 2004.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on October 30, 2012, with an error to Scheme 1. The corrected version was reposted on November 1, 2012.

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