A stable luminescent hybrid mesoporous copper complex–silica

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Chemical Communications www.rsc.org/chemcomm

Volume 48 | Number 71 | 14 September 2012 | Pages 8849–8980

ISSN 1359-7345

COMMUNICATION Jesús R. Berenguer, Javier Garcia-Martinez et al. A stable luminescent hybrid mesoporous copper complex–silica

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A stable luminescent hybrid mesoporous copper complex–silicaw Marisa Rico,a Angel E. Sepu´lveda,b Santiago Ruiz,b Elena Serrano,a Jesu´s R. Berenguer,*b Elena Lalindeb and Javier Garcia-Martinez*a Received 25th April 2012, Accepted 14th June 2012 DOI: 10.1039/c2cc32963h Surfactant-assisted co-condensation of an emissive tetranuclear alkynyl-phosphine copper cluster with TEOS affords a hydrothermally stable blue-emitter mesoporous hybrid metal complex– silica material. Mesoporous materials are widely used as catalyst supports because of their large surface area, controllable surface chemistry and porosity, excellent stability (chemical and thermal) and good accessibility.1 However, advantages of these structures are not limited to such applications. Functionalized mesoporous silica materials can display other interesting properties such as recognition and selection, sensing, controlling release and biochemical or photo-electronic functions.2 Among them, functionalized mesoporous luminescent materials are gaining a great deal of attention due to their potential biomedical applications (such as optical imaging or phototherapy), photonic applications (which include tuneable lasers and components of multilayer OLEDs) or their photocatalytic activity.3 Two of the most common approaches to the synthesis of these luminescent materials consist of the use of the mesoporous material as a host, incorporating the organic or inorganic chromophores in solution by impregnation or covalently by post-synthesis grafting.3b,4 Nevertheless, these two methods often present leaching problems, and normally result in a decrease in the surface area and the pore volume. As an alternative, a good number of luminescent periodic mesoporous organosilicas (PMOs) have been prepared by total or partial co-condensation of organic chromophores bearing alkoxysilyl groups.5 This method has several advantages, like high pore volumes or absence of leaching, but the materials lack the tunability that coordination complexes possess. Herein, we extend this approach by using a luminescent organometallic tecton, instead of an organic chromophore, for a

Laboratorio de Nanotecnologı´a Molecular, Departamento de Quı´mica Inorga´nica, Universidad de Alicante, Carretera San Vicente s/n, E-03690, Alicante, Spain. E-mail: [email protected]; Web: www.ua.es/grupo/nanolab; Fax: +34 965903454; Tel: +34 965903400 ext. 2224 b Departamento de Quı´mica-Grupo de Sı´ntesis Quı´mica de La Rioja, UA-CSIC, Universidad de La Rioja, E-26006, Logron˜o, Spain. E-mail: [email protected]; Web: www.unirioja.es/dptos/dq/grupos/materiales; Fax: +34 941299621; Tel: +34 941299646 w Electronic supplementary information (ESI) available: Synthetic procedures, full set of characterization and photophysical data, FTIR, DRUV and emission spectra. See DOI: 10.1039/c2cc32963h

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the preparation of luminescent mesoporous materials. Recently, we have reported the incorporation of various complexes bearing ligands with triethoxysilyl terminal groups into the framework of mesoporous silica and organosilica materials, using a facile solvent-free one-pot method.6 Because of the growing interest that luminescent polynuclear d10 metal complexes are attracting,7 here we report a new tetrametallic alkynyl Cu(I) chromophore, and its co-condensation with tetraethoxysilane (TEOS) to yield emissive MSU-X type mesoporous copper complex–silica, with remarkable hydrothermal stability. Treatment of an anhydrous suspension of the polymeric [Cu(CRCTol)]n with the equimolecular amount of PPETS (PPETS = PPh2(CH2)2Si(OEt)3) affords complex [Cu(CRCTol)(PPETS)]4 1, as a yellow solid in moderate yield (49%), tetrametallic nature of which can be inferred by ESI+ Mass Spectroscopy (m/z 1957 [Cu4(CRCTol)3(PPETS)4 2OEt 2Et]+ 100%; see Fig. S1, ESIw). Although we have not been able to obtain adequate mono-crystals of 1 for an X-ray diffraction study, its formulation as a ‘‘step-like’’ tetracopper compound (Scheme 1), similar to that reported by Che and co-workers for [Cu4(CRCPh)4(P4P)2],8 can be inferred by the presence of two different n(CRC) (2049, 1898 cm 1) and n(Cu–P) (222, 214 cm 1) absorptions in the solid state FT-IR, and two broad singlets at 1.2 and 6.9 ppm in the r.t. solution 31 1 P{ H} spectrum. However, although these two signals are the only ones observed in the whole temperature range recorded (295 to 223 K), the 1H and 13C{1H} NMR spectra suggest the occurrence of some dynamic process in solution, a rather common feature in copper phosphine complexes (see ESIw). Complex 1 is stable in the solid state, but presents a very limited stability in solution or suspension under aerobic conditions. The new mesoporous hybrid luminescent metal complex–silica materials (IS-MSU_1) were synthesized as pale-yellow solids, using an aerobic one-pot procedure, by surfactant-directed co-condensation of a solution of complex 1 in THF with the silica precursor (TEOS) at room temperature, in the presence of Triton X-100, and using NaF as catalyst for the condensation. The surfactant was easily removed by solvent extraction in ethanol, as confirmed by IR spectroscopy. For comparison purposes, complex 1 was grafted on the surface of MSU silica (G-MSU_1, see ESIw).9 The FT-IR spectra of both types of materials present the characteristic bands of the phosphine ligand (see ESIw) and two distinct n(CRC) (2019, 1896 cm 1) and n(Cu–P) (B220, 212 cm 1) absorptions, thus confirming the incorporation of the precursor 1, keeping its structural integrity. Chem. Commun., 2012, 48, 8883–8885

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Scheme 1 Schematic representation of the preparation of luminescent hybrid mesoporous copper–silica materials. Table 1 Metal incorporation and textural parameters of mesoporous metal-complex silica materials Sample

Metala (% mass)

dpb (nm)

ABETc (m2 g 1)

Vpored (cm3 g 1)

MSU-X (blank) IS-MSU_1 G-MSU_1

— 1.4 (1.7) 1.9 (1.7)

3.5 (2–6) 3.8 (3–6) 17 (8–40)

1000 600 120

0.9 0.4 0.4

a

Calculated by ICP-OES analysis (see ESI). Values in brackets represent the theoretical values. b Average pore diameter from the adsorption branch according to the BJH method (values in brackets represent the range of the pore size distribution). c BET surface area estimated by using a multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05–0.30. d Mesopore volume from the isotherms at a relative pressure of 0.95 (see ESI).

The copper incorporation into IS-MSU_1, determined by ICP-OES (Table 1), is ca. 82%, very close to that observed for the grafting materials G-MSU_1 (B100%), thus indicating the effectiveness of the synthetic route herein described. The luminescent hybrid mesoporous materials obtained by the in situ methodology (IS-MSU_1) present homogeneous metal complex incorporation and good textural properties. The incorporation of the metal complex into the MSU-X silica causes a decrease in surface area (Table 1, Fig. 1), most notably in the case of the grafted material due to the partial blocking of the mesopores by the grafted complex. In contrast, both the isotherm and the pore size distribution of the in-situ material (IS-MSU_1) are similar to that corresponding to the complex-free MSU-X,

Fig. 1 Representative nitrogen adsorption isotherms (left) and the corresponding pore size distribution (right) of the materials prepared by the in situ incorporation (circles) and grafting methods (triangles) compared with the complex-free MSU type silica (squares).

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Fig. 2 Representative TEM images of the materials prepared by the in situ incorporation (left, scale bar = 50 nm) and grafting methods (right, scale bar = 20 nm). The inset shows the TEM image for complex-free MSU-X silica (scale bar = 20 nm).

thus confirming the incorporation of the complex into the framework of the silica. It is worth mentioning that IS-MSU_1 presents a surface area of 600 m2 g 1, making this material a good candidate for a wide range of applications. The TEM analysis of these materials confirms their highly mesoporous nature and controlled pore size distribution. Both complex-free MSU-X and IS-MSU_1 show very similar structures (Fig. 2, left), whereas the presence of darker spots on the mesopores of G-MSU_1 (Fig. 2, right) confirms the partial blocking of this material, due to some extent of self-condensation of complex 1, and likely caused by the required presence of sodium fluoride during the synthesis procedure (see ESIw). The photophysical data for complex 1 and the copper–silica materials (IS-MSU_1 and G-MSU_1) are listed in Table S1 (ESIw). The DRUV spectra (Fig. S3, ESIw) present high-energy absorptions (220–300, B325 nm) ascribed, according to previous assignments,8 to intraligand and/or metal-to-ligand charge transfer (MLCT) [3d(Cu) - phosphine] transitions. Complex 1 and G-MSU_1 exhibit an additional low energy feature centered at B406 nm (not clearly observed for IS-MSU_1), which could be tentatively assigned to mixed metal-to-alkynyl charge transfer [3d(Cu) - p*(CRCTol)] and metal-centered (MC) [3d - 4s/p(Cu)] transitions. Excitation of solid 1 in the range of 325–460 nm at 298 K gives rise to a long-lived bright green (f = 0.93) asymmetric structureless emission (lem = 510 nm), which becomes slightly structured upon cooling at 77 K, exhibiting a typical n(CRC) This journal is

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Fig. 3 Emission spectra of complex 1 and the copper–silica materials (IS-MSU_1 and G-MSU_1) in solid state at room temperature (lexc = 365 nm).

vibrational spacing (B1900 cm 1, Fig. S4, ESIw). In line with previous assignments related to alkynyl-phosphine copper systems, the emission likely comes from an emissive state derived from a LMCT [p(CRCTol) - 4s/p(Cu)] origin.8,10 In contrast to the very high quantum yield observed in the solid state (f = 0.93), complex 1 is not emissive in solution; a fact probably favoured by its dynamic behaviour, which could enhance additional non-radiative decay pathways. Both copper–silica materials present lower quantum yields (f B 0.10), which could be related to the presence of ca. ten percent in weight of the tetranuclear copper complex 1. Nevertheless, while the grafting materials G-MSU_1 present a similar emissive behaviour to complex 1 (Fig. 3), the in situ hybrid materials IS-MSU_1 display a dual blue-shifted emission. At r.t., excitation at 365 nm produces a broad structureless emission centered at 490 nm, while excitation at high energy (330 nm) shifts the maximum at 476 nm, giving a more intense emission (Fig. S5a, ESIw). This behaviour becomes more evident at 77 K, excitation at 365 nm gives a similar feature to that at r.t., while excitation at 330 nm produces again a more intense emission, but with two maxima at 454 and 490 nm (Fig. S5b, ESIw). The emission lifetime monitoring of both bands fits to two very different components (B139, B30 ms, Table S1, ESIw). This fact, and the observation of different excitation profiles depending on the monitored wavelength (Fig. S5, ESIw), suggests the presence of two close non-equilibrated emissive states. The peak at 454 nm (139.2 (48%), 36.8 (52%) ms) is tentatively ascribed to a ligand centered [p–p*(CRCTol)] emissive state, while the band at 490 nm is likely due to a mixed LMCT/IL transition [p(CRCTol) - 4s/p(Cu)/ p*(CRCTol)]. The behaviour of IS-MSU_1 materials and the observed blue-shift in relation to 1 may be caused by the occurrence of some degree of distortion on the Cu4(CRCTol)4 core associated with its incorporation into the framework of the silica material. The expected lesser structural distortion in G-MSU_1 will produce no significant change in the emission compared to 1. Interestingly, in contrast to the low stability of 1 in solution, the in situ hybrid materials IS-MSU_1 exhibit remarkably high stability in suspension in the most common organic solvents and even in water, where they present similar emissive profiles to that observed in the solid state (Fig. 4, Table S2, ESIw). In fact, refluxed ethanol suspensions of IS-MSU_1 remain stable for

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Fig. 4 A 5  10 5 M solution of complex 1 in EtOH (A) and a suspension of IS-MSU_1 in EtOH containing the equivalent amount of 1 (B) under visible (left) or UV (right) light.

more than 3 days, without losing their emissive properties. The very high hydrothermal stability of these materials is surely derived from the final protection that the Cu(I) centres acquire upon the incorporation of the complex into the silica framework. In support of this suggestion, the grafted materials G-MSU_1 exhibit lower stability than IS-MSU_1, with evident signals of decomposition after several hours in ethanolic reflux. In summary, the synthesis of a luminescent hydrothermally stable hybrid mesoporous metal complex-silica material has been easily achieved by co-condensation of a tetra-nuclear alkynyl copper cluster (1) with TEOS, thus, opening the possibility of obtaining new materials with great applicability potential. We are currently studying the influence of the chromophore concentration on the emissive properties of the final materials obtained. We thank the Spanish MICINN (Projects CTQ2011-28954C02-01 and CTQ2008-06669-C02-02/BQU) and the CAR (COLABORA project 2009/05) for financial support. E.S. acknowledges financial support from JCI-2008–2165 and BEST2011/223 projects. A. E. S. is grateful for a UR grant.

Notes and references 1 L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133. 2 (a) N. Linares, E. Serrano, M. Rico, A. M. Balu, E. Losada, R. Luque and J. Garcı´ a-Martı´ nez, Chem. Commun., 2011, 47, 9024; (b) K. Ariga, A. Vinu, J. P. Hill and T. Mori, Coord. Chem. Rev., 2007, 251, 2562. 3 (a) A. L. Pe´nard, T. Gacoin and J. P. Boilot, Acc. Chem. Res., 2007, 40, 895; (b) D. Aiello, A. M. Talarico, F. Teocoli, E. I. Szerb, I. Aiello, F. Testa and M. Ghedini, New J. Chem., 2011, 35, 141. 4 (a) K. Binnemans, Chem. Rev., 2009, 109, 4283; (b) L. D. Carlos, R. A. S. Ferreira, V. Zea Bermudez and S. J. L. Ribeiro, Adv. Mater., 2009, 21, 509. 5 N. Mizoshita, T. Tani and S. Inagaki, Chem. Soc. Rev., 2011, 40, 789. 6 (a) N. Linares, A. E. Sepu´lveda, M. C. Pacheco, J. R. Berenguer, E. Lalinde, C. Na´jera and J. Garcı´ a-Martı´ nez, New J. Chem., 2011, 35, 225; (b) A. I. Carrillo, J. Garcı´ a-Martı´ nez, R. Llusar, E. Serrano, I. Sorribes, C. Vicent and J. A. Vidal-Moya, Microporous Mesoporous Mater., 2012, 151, 380; (c) N. Linares, A. E. Sepu´lveda, J. R. Berenguer, E. Lalinde and J. Garcı´ a-Martı´ nez, Microporous Mesoporous Mater., 2012, 158, 300. 7 (a) X. He, N. Zhu and V. W. W. Yam, Dalton Trans., 2011, 9703; (b) S. S. Y. Chui, M. F. Y. Ng and C. M. Che, Chem.–Eur. J., 2005, 11, 1739. 8 (a) W. H. Chan, Z. Z. Zhang, T. C. W. Mak and C. M. Che, J. Organomet. Chem., 1998, 556, 169; (b) Y. G. Ma, W. H. Chan, X. M. Zhou and C. M. Che, New J. Chem., 1999, 23, 263. 9 As suggested by one referee, an in situ hybrid Cu–silica gel has been also prepared without employing surfactant (IS-GEL_1, see ESIw). 10 C. L. Chan, K. L. Cheung, W. H. Lam, E. C. C. Cheng, N. Zhu, S. W. K. Choi and V. W. W. Yam, Chem.–Asian J., 2006, 1, 273.

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