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Luminescent Hyperbranched Polymers: Combining Thiol-Yne Chemistry with Gold-Mediated C-H Bond Activation Dominik Konkolewicz ,† Sylvain Gaillard,‡ Andrew G. West,†,§ Yuen Yap Cheng ,§ Angus Gray-Weale,|| Timothy W. Schmidt,§ Steven P. Nolan,*,‡ and Sebastien Perrier*,† †
Key Centre for Polymers and Colloids, School of Chemistry, Building F11, University of Sydney, New South Wales 2006, Australia EaStCHEM School of Chemistry, University of St Andrews, North Haugh, St Andrews, KY16 9ST, U.K. § School of Chemistry, Building F11, University of Sydney, New South Wales 2006, Australia School of Chemistry, Monash University, Victoria 3800, Australia
)
‡
bS Supporting Information ABSTRACT: A highly efficient and straightforward synthetic method, leading to the formation of a luminescent hyperbranched polymer, is described. The simple polymer functionalization step takes advantage of the reactivity of relatively acidic alkyne C-H bonds with a basic [Au(NHC)(OH)] (NHC = N-heterocyclic carbene) synthon.
H
ighly branched polymers, such as dendrimers and hyperbranched polymers, exhibit attractive properties, such as low viscosity, high density, and the presence of a high number of terminal end groups.1 These properties render such polymers excellent candidates for applications in targeted delivery, viscosity modification, and possible catalyst supports.2 The presence of numerous end groups is ideal for the generation of high local concentrations of specific functionalities.3 Methods to functionalize such end groups have been explored4 and remain of interest, as atom-economical means of synthesis are paramount in the sustainable development of useful polymeric materials. In this context, our current interest in gold coordination chemistry and catalysis led us to explore the possibility of functionalizing hyperbranched polymers with gold. More precisely, our aim is to provide a controlled functionalization method of acetylenic fragments using simple gold synthons. Such a functionalization should lead to interesting luminescent properties, which could originate from “aurophilic” properties involving weak interactions between gold centers,1a,c,e,5 from a conjugated π-system,6 or from a combination of both.7 The gold(I) acetylide function has been extensively studied and has led to a plethora of novel organo-gold(I) compounds.8 Although gold complexes are now widely used in organic synthesis,9 their use in materials application has been much less studied. Of note in gold-mediated catalysis are complexes bearing N-heterocyclic carbene (NHC) ligands.10 The first synthesis of [Au(NHC)(CtCR)] complexes11 was reported by Singh et al., who reacted ethylmagnesium chloride with [Au(NHC)X] (X = halide).12 In an alternative method, Gray and co-workers r 2011 American Chemical Society
Scheme 1. Synthetic Methods Leading to [Au(NHC)(CtCR)] Complexes
used NaOtBu as a base to form the desired gold acetylide complexes starting from (NHC)gold(I) halide and a terminal alkyne.13 Another more general approach has been to substitute a phosphine ligand by a stronger σ-donating NHC ligand.14 Recently, Nolan et al. have reported the isolation of a versatile gold synthon, [Au(IPr)(OH)]15 (2; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), which proves basic enough to directly deprotonate terminal alkynes to form [Au(IPr)(CtCR)] complexes (and water), providing an alternative, user-friendly synthetic pathway to such complexes (Scheme 1).16 Received: February 3, 2011 Published: February 11, 2011 1315
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Scheme 2. Synthesis of poly(PYMP) (1) and poly(Au-PYMP) (3)
Figure 2. Emission and excitation spectra of poly(Au-PYMP). Inset: photograph of the photoluminescence of 3 upon irradiation at 365 nm. Figure 1. Infrared spectra (KBr pellet) of poly(PYMP) (1; blue) and poly(Au-PYMP) (3; red).
This approach would make the polymer a site of high local concentration of Au-acetylide functions. In this manner, the only side product generated in the alkyne C-H bond functionalization would be water. Recently, Perrier and co-workers have reported the synthesis of an interesting hyper-branched polymer, poly(PYMP) (1), making use of a thiol-yne reaction presenting terminal alkynes at the extremities of the polymer branches.17 The use of the hyperbranched polymer 1 as a σ-bonded ligand of gold(I) centers generating a high local concentration of metal centers appeared to be an interesting approach to increase the “aurophilicity” and therefore the possibility of improved photoluminescent properties of such a metal-functionalized polymer. Combining the methodology employing the [Au(IPr)(OH)] 2 as synthon and this poly(PYMP) 1, we identified an opportunity to develop an interesting approach to luminescent polymeric materials. Here, we report the synthesis and luminescence measurements of the new σ-bonded (NHC)gold poly(PYMP) (3),
a poly(Au-PYMP). The hyperbranched polymer, poly(PYMP), was synthesized as reported.17 As shown earlier, the thiol-yne synthesis of hyperbranched polymers gives very high degrees of branching and essentially no intermediate alkene units. A schematic depiction of the thiol-yne synthesis of poly(PYMP) from the small-molecule PYMP 1 is shown in Scheme 2. To obtain a gold-functionalized polymer, poly(Au-PYMP) (3), poly(PYMP) (1), and [Au(IPr)(OH)] (2) were reacted in THF at 60 °C for 16 h. The disappearance of the IR band at 3282 cm-1, attributed to the C-H bond stretch of terminal alkynes present in the starting material 2, allowed us to monitor the extent of functionalization of terminal alkynes by [Au(IPr)] moieties. Under these conditions and on the basis of IR and NMR data, we estimate the reaction to be complete and complete alkyne C-H bond functionalization is achieved. The synthesis of poly(Au-PYMP) by the activation of the terminal alkyne protons by [Au(IPr)] groups is shown in Scheme 2.18 The infrared spectra of poly(PYMP) (1) and poly(Au-PYMP) (3) are shown in Figure 1. The luminescence properties of the functionalized poly (Au-PYMP) (3) material were studied. 3 was dissolved in chloroform 1316
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Organometallics and placed in a fluorescence cuvette; after thorough deoxygenation, the solution of 3 shows a distinct blue-violet luminescence when placed under ultraviolet radiation (irradiating at 365 nm). This blue-violet luminescence is shown in the inset of Figure 2. Figure 2 shows the emission and excitation spectra of the deoxygenated solution of 3 in chloroform. The emission spectrum shows two distinct bands, one in the UV region at 382 nm and one in the visible at 402 nm. In addition, the emission spectrum shows an addition shoulder in the range 420-430 nm. This emission spectrum is consistent with the blue-violet luminescence observed qualitatively. The excitation spectrum of 3 shows a peak at 347 nm and a second shoulder at approx 335 nm. Such complex spectra have been observed previously and are attributed to vibronic effects.19 The luminescence properties of 3 were further investigated by determining the quantum yield and monitoring the decay in the luminescence intensity. The quantum yield of 3 was measured as 0.25 ( 0.05 in chloroform, and the luminescence lifetime was 6.4 ( 0.7 μs. A similar solution of poly(Au-PYMP) was prepared but was not deoxygenated. This second solution showed no luminescence, nor did it have an excitation or emission spectrum. The relatively long luminescent lifetimes (∼6 μs) and the quenching of luminescence by oxygen, an observation consistent with triplet-triplet quenching, confirm that the luminescence is due to phosphorescence, rather than fluorescence. These emission spectra and lifetimes are also consistent with literature reports of the luminescence properties of gold(I) acetylide complexes.20 In a most relevant recent contribution, Nolan and co-workers reported on gold acetylide complexes centered on a single aromatic ring; the complex obtained with 1,4-diethynylbenzene and 2 displayed an absorbance at 285 nm and an emission spectrum with a peak at 496 nm, close to the blue side of green.16 Other congeners with 1,3-diethynylbenzene and 1,3,5-triethynylbenzene showed similar absorption spectra. Emission spectra were slightly shifted to lower wavelength in the blue region with the most intense peaks at 439 and 448 nm, respectively. The measured quantum yield of 3 (0.25) is much higher than those for typical gold alkynyl complexes (0.01-0.05)20a and is closer to those measured by Lu et al.20c (0.2-0.4) for starburst gold acetylide systems. In addition, the quantum yield for poly(Au-PYMP) is higher than those for the mononuclear [Au(IPr)(CtCR)] complexes.21 These results demonstrate that having a molecule with many gold acetylide groups in close proximity gives rise to a strong luminescence, an observation that is consistent with the concept of aurophilicity. In conclusion, we have shown that the numerous alkyne end groups of poly(PYMP) dendritic polymers are attractive and efficient scaffolds for functionalization, leading to gold alkynyl complexes containing polymeric materials. This feature was illustrated here by the facile functionalization of these end groups with gold via a C-H bond activation reaction, generating materials exhibiting highly luminescent properties. The advantages of the approach outlined in this contribution are that both the synthesis of the hyperbranched material by the thiol-yne reaction and the functionalization of the terminal alkynes with a [Au(IPr)] synthon moiety are very straightforward, require no additional reagent, and generate water as the sole byproduct. The simplicity of an environmentally friendly synthesis and interesting spectroscopic properties of the resulting polymer suggest that such an approach might prove very attractive as a method to synthesize luminescent materials. Ongoing work is aimed at testing the versatility of this gold loading protocol onto polymer
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and surface end groups as well as exploring how the luminescence properties can be further controlled via rational ligand design.
’ ASSOCIATED CONTENT
bS
Supporting Information. Text, figures, and a table giving all experimental data, including synthesis and characterization of all materials. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (S.P.); snolan@ st-andrews.ac.uk (S.N.).
’ ACKNOWLEDGMENT D.K. is grateful to the Australian government for funding. We are grateful to the Polymer Laboratories for SEC equipment. S.P.N. gratefully acknowledges the ERC (FUNCAT-Advanced Investigator Award) and EPSRC for support. S.P.N. is a Royal Society-Wolfson Research Merit Award holder. ’ REFERENCES (1) (a) Aulenta, F.; Hayes, W.; Rannard, S. Eur. Polym. J. 2003, 39, 1741–1771. (b) Frechet, J. M. J.; Hawker, C. J.; Gitsov, I.; Leon, J. W. J. Macromol. Sci., Part A: Pure Appl. Chem. 1996, A33, 1399–1425. (c) Hawker, C. J.; Lee, R.; Frechet, J. M. J. J. Am. Chem. Soc. 1991, 113, 4583–4588. (d) Holter, D.; Burgath, A.; Frey, H. Acta Polym. 1997, 48, 30–35. (e) Voit, B. I.; Lederer, A. Chem. Rev. 2009, 109, 5924–5973. (2) (a) Uhrich, K. Trends Polym. Sci. 1997, 5, 388–393. (b) Jikei, M.; Kakimoto, M.-A. Prog. Polym. Sci. 2001, 26, 1233–1285. (c) Qiu, L. Y.; Bae, Y. H. Pharm. Res. 2006, 23, 1–30. (3) (a) Lyulin, A. V.; Davies, G. R.; Adolf, D. B. Macromolecules 2000, 33, 6899–6900. (b) Konkolewicz, D. Aust. J. Chem. 2009, 62, 823–829. (4) For examples of the use of gold nanoparticles (Au-NPs) as cores for dendrimer construction, see: (a) Astruc, D.; Daniel, M.-C.; Ruiz, J. Chem. Commun. 2004, 2637–2649. For a recent example of dendrimer end-group functionalization, see: (b) Ornelas, C.; Broichhagen, J.; Weck, M. J. Am. Chem. Soc. 2010, 132, 3923–3931. (5) For reviews on aurophilicity, see: (a) Schmidbaur, H. Gold Bull. 1990, 23, 11–21. (b) Schmidbaur, H. Chem. Soc. Rev. 1995, 24, 391–400. (c) Gade, L. Angew. Chem., Int. Ed. 1997, 36, 1171–1173. (d) Pyykk€o, P. Angew. Chem., Int. Ed. 2004, 43, 4412–4456. For selected examples of aurophilicity, see: (e) Hunks, W. J.; MacDonald, M.-A.; Jennings, M. C.; Puddephatt, R. J. Organometallics 2000, 19, 5063–5070. (f) He, X.; Lam, W. H.; Zhu, N.; Yam, V. W.-W. Chem. Eur. J. 2009, 15, 8842–8851. (6) (a) Nguyen, M.-H.; Yip, J. H. K. Organometallics 2010, 29, 2422–2429. (b) Lin, Y.; Yin, J.; Yuan, J.; Hu, M.; Li, Z.; Yu, G.-A.; Liu, S. H. Organometallics 2010, 29, 2808–2814. (7) (a) Ferrer, M.; Gutierrez, A.; Rodriguez, L.; Rossell, O.; Lima, J. C.; Font-Bardia, M.; Solans, X. Eur. J. Inorg. Chem. 2008, 2899–2909. (b) Vicente, J.; Gil-Rubio, J.; Barquero, N.; Jones, P. G.; Bautista, D. Organometallics 2008, 27, 646–659. (c) Constable, E. C.; Housecroft, C. E.; Kocik, M. K.; Neuburger, M.; Schaffner, S.; Zampese, J. A. Eur, J. Inorg. Chem. 2009, 4710–4717. (8) (a) Flugge, S.; Anoop, A.; Goddard, R.; Thiel, W.; Furstner, A. Chem. Eur. J. 2009, 15, 8558–8565. (b) Long, N. J.; Williams, C. K. Angew. Chem., Int. Ed. 2003, 42, 2586–2617. (c) Yam, V. W.-W.; Choi, S. W.-K. J. Chem. Soc., Dalton Trans. 1996, 4227–4232. (d) Payne, N. C.; Ramachandran, R.; Treurnicht, I.; Puddephatt, R. J. Organometallics 1990, 9, 880–882. (e) Feilchenfeld, H.; Weaver, M. J. J. Phys. Chem. 1989, 93, 4276–4282. 1317
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(9) (a) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896–7936. (b) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239–3265. (c) Arcadi, A. Chem. Rev. 2008, 108, 3266–3325. (d) Jimenez-Nu~nez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326–3350. (10) (a) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776–1782. (b) Nolan, S. P. Acc. Chem. Res. 2011, 44, DOI: 10.1021/ar1000764. (11) For the use of NHCs in late-transition-metal catalysis see: Díez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612– 3676. (12) Singh, S.; Kumar, S. S.; Jancik, V.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M. Eur. J. Inorg. Chem. 2005, 3057–3062. (13) Partyka, D. V.; Gao, L.; Teets, T. S.; Updegraff, J. B.; Deligonul, N.; Gray, T. G. Organometallics 2009, 28, 6171–6182. (14) For industrial interest in the area, see: (a) Fujimura, O.; Fukunaga, K.; Honma, T.; Machida, T.; Takahashi, T. (Ube Industries, Ltd., Japan). WO 2006080515, 2006, 102 pp. (b) Fujimura, O.; Fukunaga, K.; Honma, T.; Machida, T. (Ube Industries, Ltd., Japan). WO 2007139001, 2007, 42 pp. (c) Fujimura, O.; Fukunaga, K.; Honma, T.; Machida, T. (Ube Industries, Ltd., Japan). WO 2008050733, 2008, 16 pp. (d) Fujimura, O.; Fukunaga, K.; Honma, T.; Machida, T. (Ube Industries, Ltd., Japan). JP 2008179550, 2008, 41 pp. (15) Gaillard, S.; Slawin, A. M. Z.; Nolan, S. P. Chem. Commun. 2010, 46, 2742–2744. (16) Fortman, G. C.; Poater, A.; Levell, J. W.; Gaillard, S.; Slawin, A. M. Z.; Samuel, I. D. W.; Cavallo, L.; Nolan, S. P. Dalton Trans. 2010, 39, 10382–10390. (17) Konkolewicz, D.; Gray-Weale, A.; Perrier, S. J. Am. Chem. Soc. 2009, 131, 18075–18077. (18) The reaction stoichiometry used leading to the functionalized polymer is one gold unit per alkyne polymer unit. (19) See for example: (a) Li, D.; Hang, X.; Che, C.-M.; Lob, W.-C.; Peng, S.-M. J. Chem. Soc., Dalton Trans. 1993, 2929–2932. (b) Miller, T. E.; Choi, S. W.-K.; Ming, D. M. P.; Murphy, D.; Williams, D. J.; Yam, V. W.-W. J. Organomet. Chem. 1994, 484, 209–224. (20) (a) Li, D.; Hong, X.; Che, C.-M.; Lo, W.-C.; Peng, S.-M. J. Chem. Soc., Dalton Trans. 1993, 2929–2932. (b) Mueller, T. E.; Choi, S. W.-K.; Mingos, D. M. P.; Murphy, D.; Williams, D. J.; Yam, V. W.-W. J. Organomet. Chem. 1994, 484, 209–224. (c) Lu, W.; Zhu, N.; Che, C.-M. J. Organomet. Chem. 2003, 670, 11–16. (21) The quantum yield of systems based on the aryl-acetylide moiety and 2 showed quantum yields between 0.11 and 0.17, depending on the molecular structure.16
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