Photoactive Perylenediimide-Bridged Silsesquioxane Functionalized

Mar 10, 2009 - ... synthesized via sol−gel self-assembly of 1,2-bis(triethoxysilyl)ethane and perylene-bridged silsesquioxane, using micelles of plu...
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Photoactive Perylenediimide-Bridged Silsesquioxane Functionalized Periodic Mesoporous Organosilica Thin Films (PMO-SBA15): Synthesis, Self-Assembly, and Photoluminescent and Enhanced Mechanical Properties M. Abdul Wahab,* H. Hussain, and Chaobin He* Department of Materials Synthesis and Integration, Institute of Materials Research and Engineering (IMRE), A*STAR (the Agency for Science, Technology, and Research), 3 Research Link, Singapore 117602, Republic of Singapore Received November 27, 2008. Revised Manuscript Received February 12, 2009 Well-organized periodic mesoporous organosilica thin films (designated as PMO-SBA15), having covalently bonded perylene-bridged silesquioxane (PTCDBS) inside their pore channels, are successfully synthesized via sol-gel self-assembly of 1,2-bis(triethoxysilyl)ethane and perylene-bridged silsesquioxane, using micelles of pluronic surfactant (P123) as a template for the first time. The surfactant is successfully removed from the pore channels of PMO-SBA15 by an acidic solvent extraction procedure. The final PMO-SBA15 thin films are characterized by high resolution X-ray diffraction (HRXRD), transmission electron microcopy (TEM), solidstate 29Si and 13C NMR CP/MAS NMR spectroscopy, nitrogen adsorption-desorption measurements, photoluminescence (PL) spectroscopy, and nanoindentation. HRXRD data reveal the formation of wellorganized hexagonal channels in the pure PMO-SBA15 films. The intensity of the diffracted X-ray, however, systematically attenuates after incorporation of the perylene functionality inside the hexagonal channels. This is attributed to the low X-ray scattering contrast between the mesostructured organosilica walls and organic moieties (perylene) inside the channels, suggesting the successful incorporation of the photoactive perylene molecules inside the nanochannels. This was further confirmed by photoluminescence spectroscopy and nitrogen adsorption-desorption measurements. Additionally, the mechanical hardness of the functionalized PMOSBA15 thin films, measured by nanoindentation, is significantly enhanced as compared with that of the pure PMO film. Thermogravimetric analysis (TGA) and elemental analysis suggested the functionalized PMO-SBA15 materials with PTCDBS.

1. Introduction On the basis of host-guest chemistry, recently the research on self-assembled mesoporous transparent thin films has *Corresponding authors. Fax: 65 6872 7528. E-mail: a-wahab@ imre.a-star.edu.sg (M.A.W.); [email protected] (C.H.). (1) Ogawa, M. J. Am. Chem. Soc. 1994, 116, 7941–7942. (2) Yang, H.; Kupermann, A. K.; Coombs, N.; Afra, S. M.; Ozin, G. A. Nature 1996, 379, 703–705. (3) Sellinger, A.; Weiss, P. M.; Nguyen, N.; Lu, Y.; Assink, R. A.; Gong, W.; Brinker, C. J. Nature 1998, 394, 256–261. (4) (a) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 1999, 121, 9611–9614. (b) Zhang, L.; Hendrikus, C.; Abbenhuis, L.; Gerritsen, G.; Bhriain, N.; Magusin, P. C. M. M.; Brahim, M.; Wei, H.; Rutger, A. S.; Yang, Q.; Li, C. Chem.;Eur. J. 2007, 13, 1210–1221. (5) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Nature 1999, 402, 867–871. (6) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Chem. Mater. 1999, 11, 3302–3308. (7) (a) Minoofar, P. N.; Hernandez, R.; Chia, S.; Dunn, B.; Zink, J. I. J. Am. Chem. Soc. 2001, 123, 1248–1249. (b) Alvaro, M.; Benitez, M.; Das, D.; Ferrer, B.; Garca, H. Chem. Mater. 2004, 16(11), 2222–2228. (8) (a) Carreon, M. A.; Choi, S. Y.; Mamak, M.; Chopra, N.; Ozin, G. A. J. Mater. Chem. 2007, 17, 82–89. (b) Goettman, F.; Sanchez, C. J. Mater. Chem. 2007, 17, 24–30. (9) Lu, Y.; Fan, H.; Doke, N.; Loy, D. A.; Assink, R. A.; LaVan, D. A.; Brinker, C. J. J. Am. Chem. Soc. 2000, 122, 5258–5261. (10) (a) Hatton, B. D.; Landskron, K.; Whitnall, W.; Perovic, D. D.; Ozin, G. A. Adv. Funct. Mater. 2005, 15, 823–829. (b) Dag, O.; Ishii, C. Y.; Asefa, T.; McLachlan, M. J.; Grondey, H.; Coombs, N.; Ozin, G. A. Adv. Funct. Mater. 2001, 11, 213–217.

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expanded to the area of templated mesostructured thin films.1-18 Particularly well-oriented mesochannels in mesostructured transparent thin films on solid substrates are of great interest because the functionalization of nanochannels in mesostructures has allowed one to confine organic functions in nanostructures, which have potential applications for hard coatings,3 energy conversion,7 solar cells,11 molecular separation,12 pH sensors by incorporating pH-sensitive dyes,14 dielectric layers,9,10,18 and sensing of chemical and biological analytes.15,16 The confinement effects of functional groups in the pores of the mesostructured films are crucial (11) Zulkalova, M.; Zukul, A.; Kavan, L.; Nazeeruddin, M. K.; Liska, P.; Gratzel, M. Nano Lett. 2005, 5, 1789–1792. (12) Park, D. H.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Ind. Eng. Chem. Res. 2001, 40, 6105–6110. (13) Yang, C. M.; Cho, A. T.; Pan, F. M.; Tsai, T. G.; Chao, K. Adv. Mater. 2001, 13, 1099–1102. (14) Wirnsberger, G.; Scott, B. J.; Stucky, G. D. Chem. Commun. 2001, 119–120. (15) Angelome, P. A.; Soler-Illia, G. J. D. A. A. Chem. Mater. 2005, 17, 322–331. (16) Li, L. L.; Fang, C. J.; Xu, J.; Yan, C. H. J. Mater. Chem. 2007, 17, 4492–4498. (17) Martini, I. B.; Craig, I. M.; Molenkamp, W. C.; Miyata, H.; Tolbert, S. H.; Schwartz, B. Z. Nat. Nanotechnol. 2007, 2, 647–652. (18) (a) Wong, E. V. A.; Markowitz, M. A.; Qadriq, S. B.; Golledge, S. L.; Castner, D. G.; Gaber, B. P. J. Phys. Chem. B 2002, 106, 6652–6658. (b) Liu, K.; Fu, H.; Shi, K.; Xiao, F.; Jing, L.; Xin, B. J. Phys. Chem. B 2005, 109, 18719–18722.

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which directly affect the final properties such as emission, mechanical, and structural.3-10,19-28 These studies have suggested that simply infiltrated organic functions in nanostructures via infiltration or postgrafting methods have shown to be inefficient for the high control and homogeneous distribution of organic functions within mesochannels with several problems associated with the pore blockage, function leaching, poor mechanical stability and structural properties, and inhomogeneity of organic functions. These problems become very severe when the relatively large guest molecules are accommodated into the narrow channels of hosts. For example, mesotructured silica powders functionalized with polyamidoamine (PAMAM) dendrimers,26a tris(8-hydroxyquinolinate)lanthanide(III) complexes,27c heterocyclic-bridged isocyanurate-containing mesostructured organosilicas,26f,26g bis-silylated azobenzene, binaphthyl and cyclohexadiyl groups, and bis-silylated complexes.7 The confinement effects of organic functions inside mesochannels via a direct co-condensation self-assembly process have showed better control over the functional properties such as structural,1-7,18-26 mechanical performance,3,9,10,19,22 and photoluminescent,7,15,19-21,29,30 but so far only few studies7,9,10,19-21 have been found on the covalently confined mesostructured organosilica thin films with emission and mechanical properties, which are yet to be studied convincingly.

(19) (a) Wahab, M. A.; Sudhakar, S.; Elaine, Y.; Sellinger, A. Chem. Mater. 2008, 20, 1855–1861. (b) Wahab, M. A.; He, C. B. Langmuir 2009, 25, 832–838. (20) Kirmayer, S.; Dovgolevsky, E.; Kalina, M.; Lakin, E.; Cadars, E.; Epping, J. D.; Arteaga, A. F.; Abreu, C. R.; Chmelka, B. F.; Frey, G. L. Chem. Mater. 2008, 20, 3745–3756. (21) Wahab, M. A.; Sellinger, A. Chem. Lett. 2006, 11, 1240–1241. (22) (a) Ogawa, M.; Kurodda, K.; Mori, J. Langmuir 2002, 18, 744–749. (b) Yui, T.; Tsuchino, T.; Itoh, T.; Ogawa, M.; Fukushima, Y.; Takagi, K. Langmuir 2005, 21, 2644–2646. (c) Ogawa, M. Langmuir 1995, 11, 4639–4641. (23) Domenech, A.; Alvaro, M.; Ferrer, B.; Garcia, H. J. Phys. Chem. B 2003, 107, 12781–12788. (24) (a) Wang, Y. Q.; Yang, C. M.; Zibrowins, B.; Spiethoff, B.; Linden, M.; Fchuth, F. Chem. Mater. 2003, 15, 5029–5035. (b) Yang, C. M.; Lin, H. A.; Zibrowins, B.; Spiethoff, B.; Fchuth, F.; Liou, S. C.; Chu, M. W.; Chen, C. H. Chem. Mater. 2007, 19, 3205–3211. (25) (a) Wahab, M. A.; Imai, I.; Kawakami, Y.; Ha, C. S. Chem. Mater. 2005, 17, 2165–2174. (b) Wahab, M. A.; Ha, C. S. J. Mater. Chem. 2005, 15, 508–516. (c) Wahab, M. A.; Kim, I.; Ha, C. S. J. Nanosci. Nanotechnol. 2008, 8, 3529–3539. (d) Wahab, M. A.; Imai, I.; Kawakami, Y.; Ha, C. S. Microporous Mesoporous Mater. 2006, 92, 201–211. (e) Wahab, M. A.; Kim, I.; Ha, C. S. J. Solid State Chem. 2004, 117, 3439–3447. (f) Wahab, M. A.; Kim, I.; Ha, C. S. Microporous Mesoporous Mater. 2004, 69, 19–27. (26) (a) Raynhardt, J. K. P.; Yang, Y.; Sayari, A.; Alper, H. Chem. Mater. 2004, 16, 4095–4102. (b) Mercier, L.; Pinnavaia, T. J. Chem. Mater. 2000, 12, 188–196. (c) Liang, D.; Gao, J.; Yang, Q.; Yang, J.; Li, C. Chem. Mater. 2006, 18, 6012–6018. (e) Blin, J. L.; Gerardin, C.; Rodehuser, L.; Selve, C.; Stebe, M. J. Chem. Mater. 2004, 16, 5071–5080. (f) Oksana, O.; Jaroniec, M. Ind. Eng. Chem. Res. 2007, 46, 1745–1751. (g) Oksana, O.; Jaroniec J. Am. Chem. Soc. 2005, 127(1), 60–61. (d) Richer, R.; Mercier, L. Chem. Commun. 1998, 1775–1776. (27) (a) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024–6036. (b) Kazuki, N.; Yuki, K.; Tomohiko, A.; Kazuyuki, H.; Tetsuya, K. Chem. Mater. 2004, 16, 3652–3658. (c) Sun, L. N.; Zhang, H. J.; Bo, J.; Yu, B.; Yu, S. Y.; Peng, C. Y.; Dang, S.; Guo, X. M.; Feng, J. Langmuir 2008, 24, 5500–5507. (28) (a) Celer, E. B.; Kruk, M.; Zuzek, Y.; Jaroniec, M. J. Mater. Chem. 2006, 16, 2824–2833. (b) Cho, E. B.; Kim, D.; Jaroniec, M. Langmuir 2007, 23, 11844–11849. (c) Giband, A.; Bardeau, J. F.; Colas, M. D.; Bellour, M.; Balasubramanian, V. V.; Robert, A.; Mehdi, A.; Reye, C.; Robert, R. J. J. Mater. Chem. 2004, 14, 1854–1860. (d) Martin, C. F.; Roser, S. J.; Edler, K. J. J. Mater. Chem. 2008, 18, 1222–1231. (29) Suzuki, T.; Miyata, H.; Kuroda, K. J. Mater. Chem. 2008, 18, 1239–1244. (30) Goto, Y.; Mizoshira, N.; Ohtani, O.; Okada, T.; Shimada, T.; Tani, T.; Inagaki, S. Chem. Mater. 2008, 20, 4495–4498.

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Recently, organic functions such as photoluminescent,7,19-21 hole-transporting, and light emitting materials,19 chromophore/dye segments,31,32 and incorporated silica mesostructures are also very imperative and of keen interest for optoelectronics, sensors, and membranes, but only few reports on such functional groups containing periodic mesoporous organosilica films (PMOFs) have been reported9,10,19-22 whereas mechanical9,10,19,21,22 and optical properties19-21,29,30 are hardly reported. These reports have also showed advantages for the design for functional thin films, since it allows the mesopores of a mesostructured framework to be covalently modified with different active functions via an in situ sol-gel self-assembly process, and the organic-inorganic function in the framework of PMOFs could interact with guest species inside the pores, which may lead to a possible appearance of novel properties. Therefore, PMOFs with desired organic functions present many novel functional nanostructured materials based applications such as energy conversion, optical, and sensors, to a name few.7,19,29,30 In this paper, we have successfully prepared well-organized large pores containing periodic mesoporous organosilica silica (PMO-SBA15) films, of which mesochannels are covalently bonded with sol-gel photoactive perylenediimide (perylene-3,4,9,10-tetracarboxylic dianhydride)-bridged silsesquioxane (PTCDBS, Scheme 1) via an in situ sol-gel selfassembly process for the first time as shown in Scheme 2. The main objective here is to in situ organize and chemically attach the PTCDBS function into pore channels of PMO-SBA15 for producing mechanically enhanced functional PMOSBA15 films. This PTCDBS is expected to be able to chemically attach into pore channels via in situ co-condensation selfassembly of 1,2-bis(triethoxysilyl)ethane, perylene diimide bridged silsesquioxane, and nonionic pluronic surfactant P123. The high resolution X-ray diffraction (XRD) data, nitrogen sorption, and transmission electron microscopy (TEM) images show very consistent results for functionalized PMO-SBA15 films. Photoluminescent and nanoindentation studies clearly indicate that the covalently confined PTCDBS function into the PMO-SBA15 pore channels shows blueshifted emission and improved hardness over the pure PMOSBA15 films. Elemental and thermogravimetric analysis (TGA) studies have suggested the incorporation of the PTCDBS function in the mesoporous PMO framework.

2.

Experimental Section

2.1. Materials. 1,2-Bis(triethoxysilyl)ethane (BTSE), perylene-3,4,9,10-tetracarboxylic dianhydride (PTCD), 3-aminopropyltriethoxysilane (APTEOS), and pluronic surfactant (P123) were purchased from Aldrich and used as received. Tetrahydrofuran (THF), ethanol, acetone, and petroleum ether (PTE) were also used in this study. All chemical synthetic reactions were carried out using Schlenk techniques in nitrogen atmosphere with anhydrous solvents for synthesizing perylenebridged silsesquioxane. 2.2. Synthesis of Photoluminescent Perylene-3,4,9,10tetracarboxylic Dianhydride Bridged Silsesquioxane (PTCDBS). Scheme 1 shows the synthetic diagram and chemical structure of photoluminescent perylene-3,4,9,10-tetracarboxylic dianhydride bridged silsesquioxane (PTCDBS). It was carried out in a single step reaction from 1 mL of (31) Wei, M. D.; Wang, K. X.; Yanagida, M.; Sugihara, H.; Morris, M. A.; Holmes, J. D.; Zhou, H. S. J. Mater. Chem. 2007, 17, 3888–3887. (32) Poyraz, A. S.; Albayrak, C.; Dag, O. Microporous Mesoporous Mater. 2008, 115, 548–555.

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3-aminopropyltriethoxysilane (3-APTEOS) and 1 mmol of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCD). The mixture was stirred for several minutes under nitrogen atmosphere and then heated to 130 ( 3 °C in an oil bath. The reaction was held there for few hours. After cooling to room temperature, Scheme 1. Schematic Diagram of the Synthesis of Perylene-3,4,9, 10-Tetracarboxylic Dianhydride Bridged Silsesquioxane (PTCDBS)

Scheme 2. Sketch for the Preparation of Pure and PTCDBS Function Containing PMO-SBA15 Filmsa

a (a) Functional starting sols, (b) spin-coated functional periodic PMO-SBA15 film, (c) hexagonal periodic mesochannels in which the TEM image shows hexagonal ordering for the PMO-SBA15PTCDBS3 sample (scale bar is 10 nm in the inset image), and (d) covalently bonded PTCDBS function inside the pore channels of pure PMO-SBA15 to make a PMO-SBA15 film with the PTCDBS function.

the product was washed with petroleum ether to remove the excess 3-APTEOS. The acetone/petroleum ether mixture (v/v= 1:4) was also used to extract residue. The solvent was then slowly evaporated by using a rotary evaporator to afford the desired product PCTDBS. The final product was characterized by matrix assisted laser desorption ionization time-of-flight (MALDI-TOF), NMR, and Fourier transform infrared (FT-IR) spectroscopy (see the Supporting Information), and we used this characterized final chemical structure (see the Supporting Information Figure S1) as coprecursor for preparing functional thin films. The yield was about 87.4%. 2.3. Preparation of Sols and PMO-SBA15 Thin Films. At first, the stock solution of BTSE, EtOH, water, and HCl (as 0.07 M HCl) was made from the molar ratio 1.00:6.37:2.12:8.63  10-5.19 The mixed components were refluxed at 60 ( 1 °C for 90 min to form the BTSE organosilica precursor containing solution (designated as stock solution A). Stock solution A was then further diluted with THF (20 mL), 0.07 M HCl (1.20 mL), and water (0.2 mL). For homogeneous mixing, this diluted solution was stirred for 10 min. Then 4.16 wt % of surfactant P123 was added to the dilute solutions. Different weight percentages (3, 6, 9, and 13%) of the perylene tetracarboxylicdiimide bridged silsesquioxane (PCTDBS) as shown in Scheme 2 were added for preparing multifunctional organosilica thin films. Before preparing thin films, all surfactant containing solutions were filtered through 0.45 μm PTFE syringe filters and filtrated solutions were deposited on precleaned glass substrates by spin-coating at 1500-1600 rpm according to Scheme 2. The deposited thin films were kept at 80 °C for about 12-14 h, followed by washing with acidic ethanol (H2SO4) for 15-20 min (thin films) and again with a mixture of THF and ethanol to remove any unreacted PTCDBS molecules. Sample code ends with PCTDBS molecule in Table 1 indicate that samples are functionalized with the PTCDBS molecule, whereas the number shows the added amount of PCTDBS molecules (wt %) in the initial mixtures. 2.4. Cleaning of Glass Substrate. Prior to thin film deposition, the glass substrates were cut into 2.5 cm2 pieces, submerged in 1 M H2SO4, and ultrasonicated for 0.5 h. The substrates were next ultrasonicated for 30 min each in deionized water, acetone, and isopropyl alcohol. 2.5. Characterization and Measurements. MALDI-TOF (matrix assisted laser desorption ionization time-of-flight) mass spectra were recorded on a Bruker Autoflex TOF/TOF instrument using dithranol as the matrix for determining the chemical structure weight of the synthesized molecule PTCDBS. The XRD patterns of all thin films were obtained on a PANalytical X’Pert PRO high resolution XRD (HRXRD) instrument with Cu KR radiation (λ = 0.15406 nm). All samples were scanned under same conditions in the range of 2θ = 0-4°. Sample surface areas and pore structures were determined by nitrogen sorption at 77 K using a Quantachrome BET surface area analyzer (model NOVA 1000) and applying the BrunauerEmmett-Teller (BET) and Barret-Joyner-Halenda (BJH) methods. The pore size distributions were evaluated from the desorption branch of the isotherm by using the BJH method. All of the studied samples were dehydrated at 150 °C for 12 h prior to nitrogen adsorption experiments. TEM (Philips CM300 FEG TEM microscope; operated at 300 kV) images were obtained by dispersing sample particles using ultrasonication in methanol

Table 1. Structural and Nitrogen Sorption Properties of Pure and Functional PMO-SBA15 Materials sample code

d100 (nm)

BET surface area (m2/g)

pore volume (cm3/g)

pore size (nm)

hardness (GPa)

contraction of d100 (%)

pure PMO-SBA15 PMO-SBA15-PTCDBS3 PMO-SBA15-PTCDBS6 PMO-SBA15-PTCDBS9 PMO-SBA15-PTCDBS13

9.20 9.11 8.92 7.60 (broad) not detectable

956 878 668 479 301

1.21 0.78 0.59 0.41 0.28

4.98 4.53 3.89 very broad not obtained

0.10 0.16 0.21 0.13 not determined

not applicable 0.97 3.05 17.3 not obtained

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and then pouring the solution onto a holey carbon grid. Luminescent measurements were carried out using a PerkinElmer LS50B luminescence spectrometer. The photoluminescence (PL) excitation wavelength (488 nm) was selected from the UVmax absorption using a UV-vis photospectrometer (UV3101PC). A MTS Nano XP instrument with a Berkovich (threeside pyramid) indenter was used to carry out the nanoindentation tests. A constant strain rate of 5% s-1 was applied during the loading segment. A 60 s holding time was used after the indenter reached maximum depth. The same rate was applied for the unloading segment. The reported hardness values are averaged over a depth range from 20 to 75 nm where the total film thicknesses were about 700-800 nm. Solid-state NMR spectra (Bruker DPX 400 MHz) were recorded on the same NMR spectrometer which is equipped with a solid-state probe using the following experimental conditions: 13C CP/MAS NMR, 6 μs prescan delay time, 2000 μs contact pulse, 5 kHz MAS rate, and 2000 scans; 29Si CP/MAS NMR, 30.0 μs delay time, 1200 μs contact pulse, 5 kHz MAS rate, and 2500 scans. Fourier transforms infrared spectroscopy (FT-IR) was also used for structural characterization. Thermogravimetric analysis was carried out by using a TA Instruments TGA Q500 apparatus in high-resolution mode with a heating rate 10 °C min-1 from 50 to 900 °C. An elemental analyzer, EA (Flash 1112 Series), was used for determining the percentage of C and N in the final products.

3. Results and Discussion The sol-gel photoluminescent precursor, perylenediimidebridged silsesquioxane (PTCDBS) is synthesized according to Scheme 1. The MALDI-TOF spectrum confirms the molecular structure of the used PTCDBS (see the Supporting Information for MALDI-TOF, NMR, and FT-IR data) in this study. The procedure used here to attach hydrolyzable triethoxysilane groups is simple and straightforward, which enables one to introduce various functional aromatic based bridged-organosilane, instead of traditional complex multistep approaches that use organomagnesium, butyllithium, or other chemistries.33,34 PTCDBS is a molecule with a perylene segment as a bridging photoactive functional ligand, and both ends of the perylene segment are extended with alkoxysilanes, which were exploited for covalent attachment to the framework of BTSE based PMO-SBA15 via the sol-gel self-assembly process. Thin films of PMO-SBA15, functionalized with PTCDBS, were deposited on cleaned glass substrates by spin-coating. To remove the template (surfactant molecules) and the residual trapped PTCDBS molecules inside the pore channels of PMO-SBA15, the obtained films were washed with acidic (H2SO4) ethanol and finally a mixture of THF/petroleum ether. The repeated washing procedure ensures that the covalently bonded PTCDBS molecules should be within the pore channels of the PMO-SBA15 as shown in Scheme 2. 3.1. Structural and Surface Properties. Figure 1 shows high-resolution X-ray diffraction patterns of the pure asprepared and surfactant-extracted PMO-SBA15 thin films. Both films (before functionalization with PTCDBS) show two peaks for PMO-SBA15 films within the 2θ range of 0.7-2.5°. Indexing of the HRXRD patterns for the pure PMO-SBA15 samples (d100 = 9.50 nm and d200= 4.74 nm for the as-prepared samples, whereas d100 = 9.20 nm and d200 = 4.60 nm for the acidic-washed samples) shows the (33) Shea, K. J.; Loy, D. A. Chem. Mater. 2001, 13, 3306–3319. (34) Luo, Y.; Duan, H.; Zhang, J.; Lin, C. Chem. Mater. 2005, 17, 2234–2236.

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Figure 1. High resolution XRD patterns of (A) (a) as-prepared and (b) acidic-washed pure PMO-SBA15 films and (B) (a) acidic-washed pure PMO-SBA15, (b) PMO-SBA15-PTCDBS3, (c) PMO-SBA15PTCDBS6, (d) PMO-SBA15-PTCDBS9, and (e) PMO-SBA15PTCDBS13 films. well-oriented periodic mesochannels of PMO-SBA15 materials. The pure samples show the large d-spacing of ∼9.209.50 nm which directly corresponds to the large unit-cell parameter of ∼10.97-10.50 nm for the as-prepared and washed samples, respectively. Previously, Wong et al.18a and Liu et al.18b have reported the similar XRD patterns and mesochannels for the pluronic surfactant (P123) templated 2D hexagonal mesostructured PMO-SBA15 thin films. Washing the pure samples with acidic ethanol increases intensity and decreases d-spacing by 3%. This is most likely due to the removal of surfactant molecules from the pore channels and accompanying co-condensation of silanols groups in PMO-SBA15 films.1-7,18-20 However, the functionalization of pore channels of PMO-SBA15 with PTCDBS attenuates the intensity of the diffractions peaks in Figure 1 for the acidic-washed samples. The intensity shows direct relation with respect to the incorporated amount of PTCDBS in the pore channels. Stein et al.6 have reported reduced XRD intensity when the vinyl organic function is incorporated inside periodic mesostructures via a co-condensation self-assembly method. Similar reduced XRD intensities have also been reported by other authors for other organic functions containing periodic mesostructures via a self-assembly process.5,6,19a,25,26,28a So, based on the previous observations, it could also be concluded that the attenuation of the HRXRD diffraction peak intensities for functionalized PMO-SBA15 films is mostly due to the contrast matching between the silicate framework and organic moieties that are located inside the channels of mesoporous structures.3-6,19a,25,26,28a On the other hand, the disruption of local order may also contribute, in part, to the reduction of the intensity. As the loading of PTCDBS Langmuir 2009, 25(8), 4743–4750

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Figure 2. TEM images of (a) acidic-ethanol washed pure PMO-SBA15, (b) PMO-SBA15PTCDBS3, and (c) PMO-SBA15PTCDBS6. increased from 0 to 13 wt % in the initial mixtures, the position of the main XRD peak (100) shifts to higher angles, corresponding to the gradual decrease in d-spacing from 9.20 to 7.60 nm for the acidic-washed films (see the Supporting Information), indicating that the higher loadings of PTCDBS in the initial mixtures disrupt the ability for the P123 surfactant to form micelles and corresponding mesophases for the formation of the resulting PMO-SBA15 films.3-7,9-11,18-22,25-28 Very recently, Wahab et al.19a reported that the large hydrophobic segment containing organic functional molecules such as (N,N0 -bis(4-tertbutylphenyl)-N,N0 -bis(4-((E)-2-(triethoxysilyl)vinyl)phenyl) biphenyl-4,40 -diamine) (M3) enables an increase of the dspacing of final mesoporous films because of the incorporation of this hydrophobic molecule within the hydrophobic micellar cores of the liquid crystalline mesophases due to a “so called swelling effect”. In the present work, the loading of PTCDBS increased in the initial mixtures leads to the gradual decrease in d-spacing. The discrepancy between the two studies suggests that d-spacing depends on the nature of organic functional groups.24-26 The present molecule PTCDBS is structurally different in terms of its nature and the bridging organic functional groups in the main backbone. For example, due to the presence of the carbonyl and amine functional groups at both sides of the perylene segment (see Scheme 1), this precursor is more hydrophilic in nature than the aforementioned hydrophobic (M3) molecule. So, concerning materials functionalized with the present precursor, the situation is quite different. Therefore, the behavior of the surfactant in aqueous solution is the driving force in the formation of the regular mesophase which would be different in the presence of relatively hydrophilic PTCDBS functions. In this study, it is very plausible that the higher loading of PTCDBS in the mixtures could affect micelle formation for mesophases, since PTCDBS contains hydrophilic and flexible larger groups which could slowly approach themselves at the interface of organosilicate/ surfactant micelles and ultimately disrupt the mesostructuration. This will cause the micelle structure to “open-up” and allow functional silane molecules to migrate deeper within the micelle, where the functional silane molecules subsequently cross-link with the organosilane functionalities. The perturbations thus caused to the micelle organization thus results in the assembly of more disordered materials with shorter d-spacings. Richer and Mercier26d have described that lypophilic interactions between the organosilane molecules and the hydrophobic core of the micelles are likely to result in the deeper penetration of the organosilane molecules within the micelle interface. The final cross-linking of incorporated functional silanes at that assembled micelle leads to the less-organized disordered mesostructures with Langmuir 2009, 25(8), 4743–4750

diminished d-spacings. These findings enable us to explain the observed decreased structural surface properties as listed in Table 1. Importantly, it can be seen in Table 1 (see the Supporting Information Figure S2) the changes in full width at half-maximum (fwhm) and d100-spacing of functionalized films as a function of loading substantiate that the homogeneous distribution of organic function inside the pore channels of PMO-SBA15 could be achieved until a certain loading of PTCDBS.5-7,10,18 The contraction of d100spacings is also listed in Table 1. The obtained results are consistent with the recently organically functionalized mesochannels.3-7,9-11,18-28 The structure of the few acidic ethanol washed samples shown in Figure 2 was examined by TEM. Samples show well-organized mesochannels throughout the samples. The regular pore spacing of the pores in Figure 2a and b is also observed. Such clear and highly oriented mesochannels were previously reported for 2D hexagonal periodic PMO-SBA15 materials,10,18-20,24-28 whereas Figure 2c shows less ordering because of higher loading of organic function in the framework of mesostructured PMO. The calculated pore sizes for the shown images are consistent with pore sizes obtained by the BJH method. Wong et al.18a and Liu et al.18b reported similar TEM images for pluronic surfactant (P123) templated 2D hexagonal mesostructured materials. These results are also well-consistent with the other functionalized mesochannels with various organic functions.3-7,18-20,25-28 The nitrogen adsorption-desorption isotherms were measured to determine the BET surface area, pore volume (PV), and pore size distribution (PSD) of pure and PTCDBS functionalized PMO-SBA15 materials, as outlined in Table 1. Two samples (before and after functionalization) show type-IV isotherms with obvious H1-type hysteresis loops, which are typical for mesostructured SBA-15 silica materials (see the Supporting Information).14,18,24,27,28 These isotherms are very consistent with those of previous reported pluronic surfactant P123 templated mesostructured materials. The isotherms for pure PMO-SBA15 and PMO-SBA15 PTCDBS3 are almost identical, but the shape of isotherms is different while the loading of PTCDBS is g6 wt % (isotherms for higher loadings not shown here). These isotherms are very consistent with those of previous tris(8-hydroxyquinolinate)lanthanide(III) complex functionalized mesoporous SBA-15 materials,27c supported by other functionalized mesochannels with various sizes of organic functions.10-14,24-28,36 As shown in Table 1, when the loading exceeds 6 wt %, it shows a significant impact on surface area, pore volume, and pore size.18-28 This change arises due to the functionalization of PMO-SBA15 with PTCDBS via a coassembly process. Other researchers have DOI: 10.1021/la900042g

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also reported the decreased surface area, pore volume, and pore size for functionalized mesostructures when different sizes of functional silanes were used.26 For example, Sayari et al.26a reported polyamidoamine (PAMAM) dendrimer functionalized mesostructures with reduced surface area, the pore volume, and pore size due to the incorporation of PAMAM inside the pore channels. Pinnavaia and Mercier26b have demonstrated functional mesostructured materials from the co-condensation of tetraethoxysilane (TEOS) and various types of organotrialkoxysilane functions. They have also reported reduced surface area, pore volume, and pore size for all studied mesotructures. It is found that the BET surface properties largely depend on the nature and size of functional trialkoxysilanes. Importantly, the pore sizes for these materials are found to systematically decrease with increasing incorporation of functional silane moiety. This case is more pronounced with large incorporated groups. Based on the observations, they have clearly demonstrated that the direct self-assembly approach has allowed the incorporation of functional groups within the pore channel of mesostructures, resulting in the total surface area, pore volume, and pore size being decreased. These findings could also be supported by the reasons discussed above for the diminishing d-spacings for the functionalized PMO structures. Therefore, we can consider that such a phenomenon can also occur with increasing functional precursor loading in the initial mixtures for this study.19a,24-28 In this study, acidic ethanol was used to wash the final functionalized materials. Again, THF and petroleum ether were also used for final washing of the samples to remove any trapped residual PTCDBS from the pore channels.19,24-28 So, expectedly all nonbonded or simply trapped PTCDBS molecules inside the channels of PMO-SBA15 host should be washed away, since THF/petroleum ether and acidic ethanol was used extensively for washing the final samples. Therefore, the obtained results happen only while the PTCDBS is part of the PMO-SBA15 framework.5a,19a,26,34,35 3.2. Solid-State NMR Characterization for Functional Groups. The solid-state 29Si and 13C CP/MAS NMR analyses for various samples (as-prepared pure PMO-SBA15, acid-washed pure PMO-SBA15, and acid-washed PMOSBA15-PTCDBS6 samples) shown in Figures 3 and 4 were carried out to verify the structure of covalently bonded bridging organic functions in the framework of PMOSBA15, because 29Si NMR spectra have been widely used to obtain a measure of the relative number of different kinds of silicon sites and 13C NMR spectra have also been extensively employed for the determination of functional groups in the PMO samples.3-7,9,10,19,20,25-28,36 The solid-state 29Si NMR spectra in Figure 3 show a small shoulder resonance at -55.8 ppm with a major resonance around -66.4 ppm. These resonances can be attributed to the T2 for (C-Si (OSi)2(OH)) and T3 for (C-Si(OSi)3) framework sites for all studied samples.3-6,9,10,19-22,25,28 However, this study uses two different precursors. The silicon atoms in PTCDBS and BTSE are covalently bonded to similar hybridized (35) (a) Bordoloi, A.; Mathew, N. T.; Lefebvre, F.; Halligudi, S. B. Microporous Mesoporous Mater. 2008, 115, 345–355. (b) Gomez, S.; Giraldo, O.; Garces, L. J.; Villegas, J.; Suib, S. L. Chem. Mater. 2004, 16, 2411–2417. (c) Vinu, A.; Dedecek, J.; Murugesan, V.; Hartmann, M. Chem. Mater. 2002, 14, 2433–2435. (d) Hartmann, M.; Bischof, C.; Luan, Z; Kevan, L. Microporous Mesoporous Mater. 2001, 44-45, 385–394. (36) Cornelius, M.; Haffman, F.; Ufer, B.; Behrans, P.; Froba, M. J. Mater. Chem. 2008, 18, 2587–2592.

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Figure 3. Solid-state 29Si NMR spectra of (a) as-synthesized PMOSBA15, (b) acidic ethanol washed prepared PMO-SBA15, and (c) acidic ethanol washed PMO-SBA15-PTCDBS6.

Figure 4. Solid-state 13C NMR spectra of (a) as-synthesized pure PMO-SBA15, (b) acidic ethanol washed pure PMO-SBA15, and (c) acidic ethanol washed PMO-SBA15-PTCDBS6.

carbon atoms in their own framework. For example, the Si atoms in PTCDBS (O3Si-C-C) and BTSE (O3Si-C-C) are attached to similar sp3 hybridized carbon atoms and the difference is only organic segments. So, the spectra may show little different chemical shifts in terms of peak shifting and broadening as observed in Figure 3. The absence of Qn silicon resonances between -90 and -120 ppm suggests that no cleavage of the carbon-silicon bond (tSi;CH2;) in the framework has occurred during the self-assembly process under the conditions used here.3-6,9,10,24-28 Solid-state 13C CP/MAS NMR spectroscopy was further used in this study for the identification of functional groups in the framework of PMO-SBA15 materials. Figure 4 shows systematic investigations of as-prepared pure PMO-SBA15, acid-washed pure PMO-SBA15, and acid-washed PMO-SBA15PTCDBS6 samples. A prominent resonance at ∼5.3 ppm in Figure 4 is observed for all samples, which is directly attributed to the ethane carbon atoms covalently bonded to silicon atoms in the main framework (tS;CH2;CH2; Sit). Figure 4 also shows the ether cleavage of the surfactant P123 and the effect of the acidic solvent extraction procedure on the samples. In the 13C CP/MAS NMR spectrum of the as-prepared PMO-SBA15 in Figure 4a, the resonance at 17.5 ppm can be assigned to the methyl groups, and the resonances of the main-chain carbons of the propylene oxide (PO, 75.13 ppm) and ethylene oxide (EO, 70.7 ppm) blocks of P123 are also clearly observed. Importantly, peaks responsible for the surfactant P123 are hardly seen (Figure 4b) after extensive washing with acidic ethanol (H2SO4), implying that the surfactant molecules have been decomposed and removed from the sample.20,24 This is in accordance with high surface area and pore volume results in Table 1 for the Langmuir 2009, 25(8), 4743–4750

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acidic-washed samples. On the other hand, the 13C CP/MAS NMR spectrum in Figure 4c for the PTCDBS functionalized PMO-SBA15 sample shows direct evidence for the covalent incorporation of PTCDBS into the pore channels of the PMO-SBA15 framework, since this spectrum provides very significant resonances which could clearly be assigned to the carbons, nitrogens, carbonyl groups, and perylene segment of the PTCDBS structure. For example, the existence of carbonyl groups is confirmed by the resonance at 162.7 ppm, where resonances at 136.2, 131.7, 120-119 (small), and 59.1 ppm could be assigned to OCH2 group.34 The small resonance at 42.3 ppm could be responsible for N-CH2,24 whereas the other two resonances at 18.1 and 49.8 ppm can be assigned to the remnant ethanol during the course of the self-assembly or acid ethanol surfactant extraction procedure. The CH2 group from the employed precursors can produce other small resonances in the range of 13-33 ppm. The NMR spectra in Figures 3 and 4 substantiate that both constituent building units from the used organosilica precursors have taken part in forming the functional mesochannels of the PMO-SBA15 mesostructured framework.3-6,9,10,24-28 3.3. Mechanical and Photoluminescent Properties. Nanoindentation experiments were carried out to find out the effect of the covalently attached PTCDBS inside the pore channels on mechanical properties of the PMO-SBA15 film.9,10,19,22,37 Functionalized (with PTCDBS) and unfunctionalized PMO-SBA15 thin films deposited on glass substrate were studied, and the results are shown in Table 1. To avoid the influence of the substrate on nanoindentation results, measurements were done over depth ranges from 25 to 70 nm on 700-800 nm thick films. The data in Table 1 reveal that the mechanical hardness increases linearly until 6 wt % loading. The hardness value for the 9 wt % PTCDBS incorporated PMO-SBA15 film is relatively lower but still higher than that of the pure PMO-SBA15 film. The data show that PTCDBS enhances the hardness by 1.71 times for the PMO-SBA15PTCDBS3 sample, 2.21 times for the PMO-SBA15PTCDBS6 sample, and 1.3 times for the PMOSBA15PTCDBS9 sample.9,10,19,22 The data thus indicate that the covalently incorporated PTCDBS function significantly improves the hardness of the functionalized thin film. Previously, organic functions containing organized mesostructured films have shown a higher modulus than the less organized disordered mesostructures or amorphous functional silica structures, indicating how the degree of mesoodering in the final films could influence mechanical properties.9,10a,19a These authors have incorporated various organic functions in the organized mesoporous framework via the self-assembly method and found that functionalized mesostructures have shown improved mechanical response over that of the counterpart pure mesoporous matrix. Recently, we have also reported similar results for the functional organized mesostructured materials with improved mechanical response.19a On the basis of the studies described above, at present, we attribute this to its uniform, well-ordered nanostructure, whereas the disordered pore skeletons and relatively soft segment in the final disordered framework due to the presence of a higher amount of functional silanes may contribute to the reduced hardness. (37) Hardness values are obtained from the following equation: H = P/Amax (H = hardness, P = maximum indentation load, Amax = maximum contact area).

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Figure 5. Emission spectra of (a) pure PTCDBS and (b) PMOSBA15PTCDBS6. Photoluminescence (PL) spectroscopy was also employed to investigate the influence of nanoconfinment on the PL functionality inside the pore channels.7,19-21 This study compares three films: first is the PTCDBS functionalized PMO-SBA15PTCDBS6 film, second is the bulk PTCDBS film, and third is the PTCDBS+BTSE film without surfactant (see the Supporting Information). The PL spectra are shown in Figure 5 for comparison. The acidic-washed PTCDBS function containing PMO-SBA15 film shows a blue-shift by about 8-9 nm over that of the bulk PTCDBS film, whereas the PTCDBS+BTSE amorphous film without surfactant shows also a more amorphous structure with a very broad PL spectrum (see the Supporting Information), similar to the bulk PTCDBS film, supported by other similar studied samples.7,20 Importantly, the PTCDBS functionalized PMO film shows a more narrow fwhm of 66 nm, whereas the bulk sample shows a broad fwhm of 72-74 nm, strongly indicating that PTCDBS functions have very different environment after assembling PTCDBS molecules inside the pore channels of PMO-SBA15. Zink et al. and Wahab and co-workers have reported the blueshifted PL spectra while PL functional precursors are confined inside the nanochannels of mesopores via covalent bonding.7a,19-21 These consistent blue-shifted PL results could be used for supporting the present studied mesoporous films. 3.4. Elemental and Thermogravimetry Analysis. The contents of nitrogen and carbon for the PMO-SBA15PTCDBS6 sample were obtained from elemental analysis. From elemental analysis, the percentage values of N and C are 2.98% and 36.94%, respectively, for the final product. The main distinguishable structural component for PCTDBS is N, which is found to be slightly less, 2.98%, than the expected value of 3.7%, substantiating the incorporation of PTCDBS function in the mesoporous framework. The reported value is consistent with those of previously reported nitrogen functionality containing mesostructured materials.25e,26,26f,26g,38 Gaber et al.38 reported the incorporation of N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTS) into the mesoporous organosilica products and found that the nitrogen content is less than the expected value because some AAPTS molecules undergo homocondensation instead of condensing with the main silica matrix during the self-assembly process, and then they are unable to form the siloxane linkages necessary to incorporate them (38) Burleigh, M. C.; Markowitz, M. A.; Spector, M. S.; Gaber, B. P. Chem. Mater. 2001, 13(12), 4760–4766.

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into the matrix. The resulting clusters would be washed away during the acid solvent extraction process. The TG weight change curves of pure and functionalized PMO samples in Figure S5 of the Supporting Information were conducted in the range of 50-900 °C. The first step weight loss (2.3%) below 150 °C is due to the loss of physisorbed water, which often happens in these types of materials, indicating the presence of hydroxyl groups in PMO-SBA15. The observed second weight loss that occurred in the range from 150 to 400 °C can be assigned to the decomposition of organic groups, trapped surfactant molecules, and the loosely bound Si-(OR)1-3 groups. Then the weight loss found in the range of 400-900 °C is due to the decomposition of both the organic matrix, BTSE and PTCDBS. Exceptionally, the PMO-SBA15PTCDBS6 sample showed less weight loss below 150 °C as compared to pure PMO, which could be supported by the incorporation of the hydrophobic perylene segment in the framework of PCTDBS, which is also a part of the functionalized PMOSBA15 framework. This significant difference (less weight loss) for the functionalized PMO-SBA156 could be explained by the fact that the hydroxyl groups of the PMO framework are functionalized with the PTCDBS function, also supported by elemental analysis. In contrast, the weight loss differences between the two samples in the range of 400900 °C could be aroused for different functionalities in the final mesoporous framework. Similar results have been reported previously for tris(3-(trimethoxysilyl)propyl)-isocyanurate and other amine group functionalized mesostructured materials.25,26g,26f,38 The ceramic yields of SiO2 for the pure PMO-SBA15 and PMO-SBA15PTCDBS6 samples are 83% and 73.66%, respectively. The ceramic yield for pure PMO-SBA15 is very close to the expected value, while the value of PMO-SBA15PTCDBS6 is bit higher than the expected value because the self-assembly process likely allows that some guest molecules could be placed somewhere in the interior of the hydrophobic micelle which are unable to reach and react with the silanol interface and are thus washed

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away during the acid ethanol Soxhlet solvent extraction process, and that could impact the final ceramic yield.

4.

Conclusions

In this study, we have demonstrated the successful incorporation of photoluminescent perylene-bridged silsesquioxane into the periodic large pores of hybrid periodic mesoporous organosilica (PMO-SBA15) films via a sol-gel self-assembly process. HRXRD data have showed well-organized hexagonal mesophases which are consistent with TEM images. The 13C and 29Si solid-state NMR spectra and elemental analysis have indicated the incorporation of the PTCDBS function in the framework of PMO-SBA15. The obtained PTCDBS functionalized PMO-SBA15 showed increased hardness by 1.3-2.21 times that of the pure PMOSBA15 film to strongly suggest that the mesopore channels are covalently functionalized with the PCTDBS molecule. The decrease in surface properties (surface area, pore volume, and pore size) and the blue-shift in PL spectra also suggested that the PCTDBS function is already confined inside the mesochannels of PMO-SBA15. The TG weight loss curves also gave insight into the hydrophilic character imparted on the surfaces of the PMO structures by estimating the amount of weight loss (1%) below 150 °C when PMO-SBA15 was functionalized with PTCDBS. Acknowledgment. Financial support was provided by the Institute of Materials Research and Engineering (IMRE) and the Agency for Science, Technology and Research (A*STAR), Republic of Singapore. Supporting Information Available: MALDI-TOF spectrum, 1H NMR and FT-IR data of PTCDBS, d100-spacings, nitrogen adsorption desorption isotherms and pore size distribution curves, PL spectra of BTSE+PTCDBS without surfactant, and TGA curves before and after functionalization. This material is available free of charge via the Internet at http://pubs.acs.org.

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