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J. Phys. Chem. C 2011, 115, 1044–1050
Anchoring of Pt(II) Pyridyl Complex to Mesoporous Silica Materials: Enhanced Photoluminescence Emission at Room Temperature and Photooxidation Activity using Molecular Oxygen† Kohsuke Mori,‡ Kentaro Watanabe,‡ Masayoshi Kawashima,‡ Michel Che,§ and Hiromi Yamashita*,‡ DiVision of Materials and Manufacturing Science, Graduate School of Engineering, Osaka UniVersity, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan, and Institut UniVersitaire de France and Laboratoire de Re´actiVite´ de Surface, UniVersite´ Pierre et Marie Curie-Paris 6, CNRS-UMR 7197, Paris, France ReceiVed: June 17, 2010; ReVised Manuscript ReceiVed: August 6, 2010
Chloro(2,2′:6′,2′′-terpyridine)platinum(II) ([Pt(tpy)Cl]Cl) complex was successfully anchored to a series of (3-aminopropyl)trimethoxysilane-modified mesoporous silica materials (MCM-41, SBA-15, and MCM-48). Pt LIII-edge X-ray absorption fine structure (XAFS) measurements reveal that the Pt complex reacts with amino groups anchored on the mesoporous silica to create a new Pt-N bond. Upon anchoring, the nonemissive Pt(II) complex exhibits strong photoluminescence at room temperature, which is maximized near 530 nm due to ligand-centered (3LC) and/or metal-to-ligand charge transfer (3MLCT) transitions. The intensities of the emission increase in the order of MCM-41 < SBA-15 < MCM-48. In the case of MCM-48, the emission intensity is the highest at 0.42 wt % Pt loading, while concentration quenching is observed accompanied with a new emission due to the metal-metal-to-ligand charge-transfer (3MMLCT) transition at high Pt loading. These results correspond well with the photocatalytic activities in the selective oxidation of styrene derivatives using molecular oxygen (O2). It can be supposed that the enhanced excitation rate and quantum efficiency of the anchored Pt complex, due to the differences in nanoconfinement, increase the energy and/or electron transfer to O2, which ultimately enhances the photooxidation activity. The 3D-connected channel structure of the MCM-48 silica also accounts for the high photocatalytic activity, where the diffusion of O2 toward the anchored Pt complex occurs smoothly compared to the one-dimensional MCM-41 and SBA-15 silicas, as demonstrated by the quenching rate constants obtained from Stern-Volmer plots. 1. Introduction Luminescent polypyridine metal complexes have received much attention due to their unique spectroscopic and photophysical properties.1 Most of these studies have concerned d6 transition metal complexes exemplified by Ru(bpy)32+ with potential capacity for application in chemical sensing, solar energy conversion, and photocatalysis.2 Recently, investigations into emissive square-planar d8 Pt(II) pyridyl complexes have been numerous due to the sensitivity of their spectroscopic properties to the local environment.3 The photoluminescence from these compounds originates from a variety of lowest triplet excited states, including ligand-centered (LC), metal-to-ligand charge-transfer (MLCT), and metal-metal-to-ligand chargetransfer (MMLCT) excited states.4 A low-lying MMLCT excited state occurs in dimers and aggregates with short Pt · · · Pt separation, and the resulting emission is observed at longer wavelengths than those of unimolecular 3LC and 3MLCT emissions. Much work has focused on understanding the nature of these states from spectroscopic examination of the crystals,5 immobilization of solid supports,6 influence of solvents,7 and the choice of ligands and/or substituents.8 The incorporation of guest species into porous inorganic materials, such as clays, zeolites, and mesoporous materials, †
Part of the “Alfons Baiker Festschrift”. * To whom correspondence should be addressed. Fax & Tel: +81-66879-7457. E-mail:
[email protected]. ‡ Osaka University. § Universite´ Pierre et Marie Curie-Paris.
has been extensively studied with respect to synthesizing functional inorganic-organic supramolecular materials.9 The resulting materials exhibit unique physicochemical properties that are controlled by the state of the guest molecules, in addition to the nature of the functional groups on the host surface. In the field of optical materials, there have been numerous reports utilizing zeolites as attractive host materials that can host small molecules and micrometer-scale photonic band gap materials within their restricted supercages (ca. 1.3 nm).10 Both steric and electrostatic constraints can influence the structure and reactivity of the enclosed species, which frequently results in a lifetime enhancement of short-lived reaction intermediates. On the other hand, mesoporous silica materials can be easily tailored to yield desired properties by anchoring functional groups to the surface or doping metal atoms into the framework.11 Moreover, the abundant hydroxyl groups on the pore surface contribute to effective interaction with the guest molecules.12 The mesopore size range (2-50 nm) also presents promising advantages for optical applications; (i) the high surface area creates the potential to dope large materials at higher concentrations without selfinteraction, and (ii) the ordered mesoporous channels can produce size-confined structures such as quantum dot and nanowires. In this study, new luminescent inorganic-organic hybrid materials are developed by anchoring the chloro(2,2′:6′,2′′terpyridine)platinum(II) ([Pt(tpy)Cl]Cl) complex to different mesoporous silica hosts (MCM-41, SBA-15, and MCM-48) modified with (3-aminopropyl)trimethoxysilane. MCM-41 has
10.1021/jp105577f 2011 American Chemical Society Published on Web 08/23/2010
Anchoring of Pt(II) Pyridyl Complex a one-dimensional, hexagonally ordered, unconnected, but regular cylindrical pore structure.13 SBA-15 exhibits a regular one-dimensional array of tubular channels connected through lateral connections.14 Beside its large hexagonal channels ranging from 5-30 nm in diameter, SBA-15 has thicker pore walls (3-6 nm) than MCM-41. Additionally, this molecular sieve often exhibits surface roughness including a certain amount of disordered micropores and small mesopores.15 MCM-48 consists of a uniform array of 3D-connected tubular pores.16 The differences in the pore dimensions and structures of these materials make it possible to investigate the effect of nanoconfinement on anchored guest molecules. In addition, we herein report that the anchoring of this Pt complex enables selective photooxidation of styrene derivatives by molecular oxygen (O2). To the best of our knowledge, this is the first application of this Pt (II) complex to selective photooxidation. The turnover number (TON) based on Pt is found to be dependent on the type of mesoporous silica material. The relationship between the luminescence characteristics and photocatalytic activities is also examined as a function of the amount of Pt complex loading to define the spatial distribution of the Pt complex in the mesoporous channel. The exploitation of host-guest interactions within the restricted pore arrangement is expected to confer new and more effective modes of reactivity on the anchored guest molecules. 2. Experimental Section 2.1. Materials. K2PtCl4 and cetyltrimethylammonium bromide (CTAB) were purchased from Wako Pure Chemical Ind., Ltd. 2,2′:6′,2′′-terpyridine (tpy) was purchased from Tokyo Kasei Kogyo Co., Ltd. Pluronic P123 (EO20PO70EO20) and (3aminopropyl)trimethoxysilane (APTMS) were obtained from Aldrich Chemical Co. Tetraethyl orthosilicate (TEOS) and NaF were obtained from Nakarai tesque. Solvents and all commercially available organic compounds for catalytic reactions were purified using standard procedures. 2.2. Sample Preparation. 2.2.1. Synthesis of MCM-41.17 A typical synthesis involves dissolving 4.4 g of CTAB in 200 g of deionized water containing 1.1 g of NaOH with heating to 303 K. To this solution was added 14.8 mL of TEOS. The mixture was vigorously stirred at room temperature for 2 h and then aged at 373 K for 72 h in an autoclave under static conditions. The product was isolated by vacuum filtration and washed with deionized water and methanol. The resulting white powder was dried at 383 K overnight and calcined at 823 K for 10 h. 2.2.2. Synthesis of SBA-15.18 A total of 4.0 g of Pluronic P123 was dissolved in 30 g of deionized water and 120 mL of 2 M HCl solution with stirring at 313 K. Then 8.5 g of TEOS was added into the above solution with stirring for 24 h. The mixture was aged at 353 K overnight without stirring, and then the product was recovered by vacuum filtration and washed with deionized water. The resulting white powder was dried at 383 K overnight and calcined at 823 K for 10 h. 2.2.3. Synthesis of MCM-48.19 A total of 10.0 g of TEOS was mixed with 50 mL of deionized water and the mixture was vigorously stirred at 308 K for 40 min, then 0.9 g of NaOH and 0.19 g of NaF were added into the mixture. After 60 min of vigorous stirring, 10.6 g of CTAB was added to the mixture and continued stirring for 60 min. Finally, the mixture was aged at 393 K for 24 h under static condition. The resulting white powder was dried at 383 K overnight and calcined at 823 K for 10 h. 2.2.4. Synthesis of [Pt(tpy)Cl]Cl.20 A 15 mL of aqueous solution of 0.5 g of K2PtCl4 and 0.33 g of tpy was stirred under
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1045 reflux condition for 72 h. The red solution was filtered and concentrated to about 3 mL. The red precipitate was collected by vacuum filtration and washed with 0.1 M HCl and acetone, and the resulting solid product was dried under vacuum overnight. 2.2.5. Surface Modification of Silica Supports. To remove physisorbed water before surface modification, the silica support (MCM-41, SBA-15, and MCM-48) was dried at 393 K for 1 h under vacuum. 40 mL of toluene mixture containing 1.0 g of support and 0.27 g of APTMS was stirred at room temperature for 20 h and then refluxed for 4 h. The product was recovered by vacuum filtration, washed with ethanol, and dried under vacuum overnight. 2.2.6. Anchoring of Pt Complex. A total of 200 mL of chloroform solution of 0.012 g of [Pt(tpy)Cl]Cl was stirred with 1.0 g of APTMS-modified MCM-41 sample at room temperature for 24 h. The product was recovered by vacuum filtration, washed with chloroform, and dried under vacuum overnight to give Pt(tpy)/MCM-41 (Pt: 0.43 wt %). Pt(tpy)/SBA-15 (Pt: 0.41 wt %) and Pt(tpy)/MCM-48II (Pt: 0.42 wt %) were prepared by the same method using SBA-15 and MCM-48, respectively. In the case of MCM48, other samples with different Pt loadings were also prepared to give Pt(tpy)/MCM-48I (Pt: 0.20 wt %), Pt(tpy)/MCM-48III (Pt: 0.83 wt %), and Pt(tpy)/MCM-48IV (Pt: 1.24 wt %), respectively. The Pt loadings were determined by inductively coupled plasma (ICP) analysis. 2.3. Characterization. Powder X-ray diffraction patterns were recorded using a Rigaku RINT2500 diffractometer with Cu KR radiation (λ ) 1.5406 Å). BET surface area measurements were performed using a BEL-SORP max (Bel Japan, Inc.) at 77 K. The sample was degassed in vacuum at 373 K for 2 h prior to data collection. UV-vis diffuse reflectance spectra of powdered samples were collected using a Shimadzu UV-2450 spectrophotometer. The reference was BaSO4, and the absorption spectra were obtained by using the Kubelka-Munk function. Infrared spectra were obtained with a JASCO FTIR-6100. Samples were diluted with KBr and compressed into thin diskshaped pellets. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements were performed using a Nippon Jarrell-Ash ICAP-575 Mark II. Photoluminescence measurements were carried out on a fluorolog-3 spectrofluorometer (Horiba) at 293 K. Pt LIII-edge XAFS spectra were recorded at room temperature in fluorescence mode at the BL7C facilities of the Photon Factory at the National Laboratory for High-Energy Physics, Tsukuba (2009G221). A Si(111) double crystal was used to monochromatize the X-rays from the 2.5 GeV electron storage ring. In a typical experiment, the sample was loaded into the in situ cell with plastic windows. EXAFS data were examined using the EXAFS analysis program, Rigaku EXAFS. The pre-edge peaks in the XANES regions were normalized for atomic absorption, based on the average absorption coefficient of the spectral region. Fourier transformation (FT) of k3-weighted normalized EXAFS data was performed over 3.5 Å < k/Å-1 < 12 Å range to obtain the radial structure function. CN (coordination number of scatters), R (distance between the absorbing atom and the scatterer), and Debye-Waller factor were estimated by curve-fitting analysis with the inverse FT in the 0.8 < R/Å < 2.8 range assuming single scattering. 2.4. Photocatalytic Liquid-Phase Oxidation. The powdered Pt catalyst (0.01 g), styrene (10.0 mmol), and acetonitrile (15 mL) were introduce into a quartz reaction vessel (30 mL) which was then sealed with a rubber septum. The resulting mixture was bubbled with oxygen for 30 min in dark conditions. Subsequently the sample was irradiated from a sideways
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Mori et al.
TABLE 1: Textural Properties of Various Silica Supports and Pt Complex-Anchored Samples sample MCM-41 Pt(tpy)/MCM-41 SBA-15 Pt(tpy)/SBA-15 MCM-48 Pt(tpy)/MCM-48II
Pt-loading/wt % SBET/m2 g-1 Vp/cm3 g-1 dave/nm 0.43 0.41 0.42
753 486 703 342 1037 782
0.89 0.62 0.84 0.55 0.78 0.48
2.6 2.0 6.3 6.1 2.5 1.9
SCHEME 1: Anchoring of [Pt(tpy)Cl]+ onto the Modified Silica Surface
Figure 1. (A) Pt LIII-edge XANES spectra and (B) FT-EXAFS spectra of (a) powdered Pt(tpy)Cl, (b) Pt(tpy)/MCM-41, (c) Pt(tpy)/SBA-15, (d) Pt(tpy)/MCM-48I, (e) Pt(tpy)/MCM-48II, (f) Pt(tpy)/MCM-48III, and (g) Pt(tpy)/MCM-48IV.
direction using a Xe lamp (500 W; SAN-EI ERECTRIC CO., Ltd. XEF-501S) for 24 h with magnetic stirring at ambient pressure and temperature. After the reaction, the resulting solution was recovered by filtration and analyzed by an internal standard technique using a Shimadzu GC-14B with a flame ionization detector equipped with TC-1 columns. The turnover number (TON) was defined as follows: TON ) products [mol]/ Pt atoms on catalyst [mol]. 3. Results and Discussion The mesoporous silica materials (MCM-41, SBA-15, and MCM-48) were prepared according to a previously reported procedure via the surfactant self-assembly approach.17-19 The formation of mesoporous structure was confirmed by X-ray diffraction (XRD) patterns and N2 adsorption/desorption isotherms. For example, the low angle XRD pattern of MCM-41 exhibited diffraction peaks assigned to (100), (110), and (200) reflections of the hexagonal mesostructure. N2 adsorption/ desorption isotherm are of type IV without a hysteresis loop. The specific surface area, pore volume, and pore diameter of MCM-41 were determined to be 753 m2 g-1, 0.89 cm3 g-1, and 2.6 nm, respectively. The textural properties of the mesoporous silicas are presented in Table 1. The obtained mesoporous silica materials were dehydrated under dynamic vacuum and the surface OH sites were modified with (3-aminopropyl)trimethoxysilane (APTMS). The attachment of the aminopropyl group to the silica surface was evidenced by elemental analysis and Fourier transform-infrared (FT-IR) spectroscopy. The [Pt(tpy)Cl]Cl complex was synthesized by refluxing 2,2′;6′,2′′-terpyridine (tpy) and K2PtCl4 in aqueous solution.20 The [Pt(tpy)Cl]Cl complex was anchored onto the silica support materials by reacting a chloroform solution of the complex (1.5 × 10-4 M) with APTMS-modified mesoporous silica (1.0 g) at room temperature to generate a new Pt-N bond via the loss of HCl (Scheme 1). In this way, Pt(tpy)/MCM-41 (Pt: 0.43 wt %), Pt(tpy)/SBA-15 (Pt: 0.41 wt %), and Pt(tpy)/ MCM-48II (Pt: 0.42 wt %) were obtained. Table 1 shows that the high BET surface areas and pore volumes were retained even after deposition of the Pt complex. The diffraction peaks due to the hexagonally packed mesoporous structure were almost at the same locations, which indicated that the long-range order was still kept. Other anchoring experiments were performed with different ratios of the [Pt(tpy)Cl]Cl complex to APTMS-
modified MCM-48 to yield Pt(tpy)/MCM-48I (Pt: 0.20 wt %), Pt(tpy)/MCM-48III (Pt: 0.83 wt %), and Pt(tpy)/MCM-48IV (Pt: 1.24 wt %). The Pt content in the resulting materials was in close agreement with the nominal target composition. The estimated diameter of the [Pt(tpy)Cl]Cl complex (ca. 0.9 nm) is less than half that of the MCM-48 channel. Therefore, the Pt precursors should diffuse to a sufficient extent to form surfaceattached complexes that are homogeneously distributed across the channel of mesoporous silica materials under this loading level. X-ray absorption measurements were conducted to elucidate the electronic structure and chemical environment of the deposited Pt. Figure 1A shows normalized X-ray absorption near-edge structure (XANES) spectra at the Pt LIII-edge of the [Pt(tpy)Cl]Cl complex and anchored samples. The white line at around 11565 eV is an absorption threshold resonance attributed to the electronic transitions from 2p3/2 to unoccupied states above the Fermi level and is intensified with an increase in the d-band vacancies as a result of oxidation.21 Therefore, the white line absorption peaks of more oxidized Pt species gave higher intensity than those of reduced species. Figure 1A also shows that all anchored samples afforded higher intensity peaks, which suggests that the surface anchored Pt species appear to be in a slightly electron-deficient state when compared to free [Pt(tpy)Cl]Cl, because of the decrease in the σ-donor electron by replacement of the fourth coordinated ligand from chloride to a nitrogen atom. Fourier transforms (FT) of the Pt LIII-edge X-ray absorption fine structure (EXAFS) data for the [Pt(tpy)Cl]Cl complex and anchored samples are depicted in Figure 1B. All spectra show a strong peak at around 1.5 Å attributable to a Pt-N bond and a small second shell at ca. 2.4 Å, which is assigned to neighboring carbon atoms. This corroborates a tridentate binding structure for Pt(II). In the case of the [Pt(tpy)Cl]Cl complex, an additional peak due to the Pt-Cl bond was observed at around 1.8 Å, which completely disappeared after reaction with APTMS-modified mesoporous silica to generate the new Pt-N bond accompanied by elimination of the Pt-Cl bond. Moreover, the first peaks of Pt(tpy)/MCM-48 were slightly shifted toward shorter interatomic distances with increasing Pt content. To obtain quantitative information regarding the Pt atom coordination, the first peak was fitted using the Pt-N shell parameter in
Anchoring of Pt(II) Pyridyl Complex
J. Phys. Chem. C, Vol. 115, No. 4, 2011 1047
TABLE 2: Results of EXAFS Curve-Fitting Analysis sample
C.N Pt-N
R/Å
∆σ2/Å2
[Pt(tpy)Cl]Cl Pt(tpy)/MCM-41 Pt(tpy)/SBA-15 Pt(tpy)/MCM-48I Pt(tpy)/MCM-48II Pt(tpy)/MCM-48III Pt(tpy)/MCM-48IV
3.0 4.2 4.1 4.2 4.0 4.1 3.9
2.06 2.06 2.05 2.06 2.05 2.03 2.02
0 0.006 0.007 0.003 0.003 0.004 0.005
[Pt(tpy)Cl]Cl as a standard (CN ) 3.0 and R ) 2.05 Å). Table 2 summarizes the curve-fitting analysis, in which the Pt-anchored samples exhibit average distances of 2.02-2.06 Å with coordination numbers of 3.9-4.2. Although it is not possible to rule out the involvement of different types of Pt species in the mesoporous silica channel, the Pt complex mainly exists in a monomeric Pt(II) surrounded by four nitrogen atoms. The average Pt-N distances decreased as the Pt loading amount increased, which suggests that the anchored Pt complexes may undergo slight distortion within the channel, due to the steric constraints at high Pt loading. UV-vis diffuse reflectance spectra of Pt-anchored samples and absorption spectra of [Pt(tpy)Cl]Cl in chloroform solution are shown in Figure 2A. No absorption is observed in the absence of the Pt complex. The absorption spectrum of the [Pt(tpy)Cl]Cl solution exhibits intense absorption bands in the high energy region (λ < 350 nm), which are assigned to LC (π-π*) transition, and moderately intense low-energy bands ranging from 350 to 450 nm, which can be ascribed to spinallowed MLCT transitions involving a 5dπ Pt orbital as the donor orbital and a π* terpyridine orbital as the acceptor orbital.22 All of the absorption bands of the Pt-anchored samples are in good agreement with those of the free complex in chloroform, although a slight shift toward shorter wavelength is observed. In order to gain an insight into the long-wavelength absorption properties of the Pt-anchored samples, room temperature diffuse reflectance UV-vis spectra of the solid [Pt(tpy)Cl]Cl complex and the anchored materials at different Pt loading levels were recorded and the results are depicted in Figure 2B. The spectrum of the solid [Pt(tpy)Cl]Cl complex is characteristically broad with a tailing long-wavelength profile. Nevertheless, there are
Figure 2. UV-vis spectra. (A) Pt(tpy) complex-anchored silica samples: (a) Pt(tpy)/MCM-48II, (b) Pt(tpy)/SBA-15, (c) Pt(tpy)/MCM41, and (d) Pt(tpy)Cl in chloroform. (B) Pt(tpy) complex-anchored on MCM48 at various Pt loadings: (a) Pt(tpy)/MCM-48I, (b) Pt(tpy)/MCM48II, (c) Pt(tpy)/MCM-48III, (d) Pt(tpy)/MCM-48IV, and (e) powdered Pt(tpy)Cl.
Figure 3. Photoluminescence spectra (λex )330 nm). (A) Pt(tpy)Cl complex in acetonitrile at (a) 77 K and (b) at room temperature. The inset shows the spectral dependence on the concentration at 77 K. (B) Pt(tpy)-anchored silica samples: (a) Pt(tpy)/MCM-41, (b) Pt(tpy)/SBA15, (c) Pt(tpy)/MCM-48II, and (d) powdered Pt(tpy)Cl.
additional long-wavelength absorption features at around 510 nm that are not present in the dilute solutions. This band is assigned to a MMLCT [dσ*(Pt)-π*(terpyridine)] excited sate that originates from relatively short Pt · · · Pt separations (
