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Photoluminescent and redox active periodic mesoporous organosilicas based on 2,7-diazapyrene
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Microporous and Mesoporous Materials 112 (2008) 1–13 www.elsevier.com/locate/micromeso
Photoluminescent and redox active periodic mesoporous organosilicas based on 2,7-diazapyrene Kenneth J. Balkus Jr. *, Thomas J. Pisklak, Greg Hundt, John Sibert, Yangfeng Zhang Department of Chemistry and the NanoTech Institute, The University of Texas at Dallas, Richardson, TX 75083-0688, USA Received 25 May 2006; received in revised form 1 May 2007; accepted 2 May 2007 Available online 31 May 2007
Abstract A novel redox-active, luminescent periodic mesoporous organosilica (PMO) based on 2,7-bis(3-trimethoxysilylpropyl)diazapyrinium diiodide (BDAP) was prepared. A 2,7-diazapyrene grafted mesoporous material (DAP-DAM-1) was also synthesized. BDAP loadings as high as 2.3% were achieved in well ordered BDAP-PMO materials. The photoluminescence properties of BDAP-PMO and DAPDAM-1, along with a PMO based on 4,4 0 -bis(triethoxysilyl)-1,1 0 -biphenyl (BTEBp-PMO) were determined. The photoluminescence of BDAP-PMO, DAP-DAM-1, and BTEBp-PMO was quenched by nitrated explosive taggants including o-nitrotoluene, nitrobenzene, 2,3-dimethyl-2,3-dinitrobutane (DMNB), and nitromethane, suggesting the potential use of these PMOs as optical sensors for explosives. In the case of BDAP-PMO and DAP-DAM-1, exposure to nitrobenzene results in 81% quenching of fluorescence. Ó 2008 Published by Elsevier Inc. Keywords: Mesoporous molecular sieves; PMO; DAM-1; Optical sensor
1. Introduction Periodic mesoporous organosilicas (PMO) as well as hybrid mesoporous materials have been widely reported in the literature [1,2]. These materials have either bridging or pendant organic groups in the pore walls. Historically, the mesoporous structure of all silica frameworks, such as MCM-41, SBA-15, and DAM-1 have been functionalized with a diverse range of organic groups [3–35,94].These materials may have the organic functional groups incorporated in the framework during synthesis or by post-synthetic grafting. In 1997 we reported one of the first examples of MCM-41 decorated with organosilanes and metal complexes by post-synthesis modification [36]. This is typically accomplished using organosilanes which are either self-condensed or combined with silica precursors to form covalent linkages between the organics and the inorganic framework [1,2]. Potential problems with this *
Corresponding author. Tel.: +1 972 883 2659; fax: +1 972 883 2925. E-mail address: [email protected] (K.J. Balkus Jr.).
1387-1811/$ - see front matter Ó 2008 Published by Elsevier Inc. doi:10.1016/j.micromeso.2007.05.025
approach are low loadings, heterogeneous distribution and loss of pore volume. In contrast, PMOs are composed of bridging organosilanes which are incorporated into the pore walls and can make up as much as 100% of the framework. The accessible surface areas and density of organic groups in PMOs are much larger than for grafted materials. This provides more free volume in the pores, heterogeneous dispersion, as well as easier access to the organic groups. Theoretically, any organic compound that could be silanated might be incorporated into a mesoporous framework, provided it is stable under synthesis conditions. Most efforts to date have focused on acid/base groups or aromatic groups that impart specific surface properties, such as hydrophilicity/hydrophobicity or act as binding sites for metal ions. Ha and co-workers [19] incorporated a vanadyl Schiff base complex into a mesoporous framework which has the ability to catalyze the cyanosilylation of carbonyl groups, and Reye et al. used a cobalt Schiff base complex to absorb dioxygen [11]. There have also been other metal containing ligands co-condensed into
2
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mesoporous materials such as a carbapalladacycle complex as a catalyst for Suzuki cross-coupling [9]. Also, various alkyl, aromatic, and sulfur containing groups have been disilanated and incorporated into mesoporous materials. Due to the ability to incor porate various organic compounds into the framework of mesoporous materials there is a potential for the application of PMOs as solid catalysts and sensors [1]. Some applications could benefit from a redox active framework; however, there are relatively few examples of such PMO materials. Recently a PMO was synthesized by co-condensing disilylated viologen (N,N 0 bis(triethoxysilylpropyl)-4,4 0 -bipyridinium) with tetraethyl orthosilicate [32,35,37]. Upon irradiation, or mild heating (100 °C) this PMO generates the radical cation. In related work, an electrochromic PMO was prepared using a viologen-modified periodic mesoporous nanocrystalline anatase [38]. Here we present the synthesis of a redox active, fluorescent di-silylated 2,7-diazapyrene mesoporous material (BDAP-PMO) shown in Scheme 1. Ordered BDAP-PMO was prepared with loadings up to 5% with redox activity verified by cyclic voltammetry. For comparison, a tethered 2,7-diazapyrene (DAP-DAM-1) has been prepared from iodopropyl functionalized DAM-1 (Scheme 2). The BDAP-PMO fluorescence can be quenched by certain molecules, which suggests this material might be exploited for optical sensing. A growing concern for homeland security is the detection of explosives. Explosives manufacturers add identification markers, called taggants, to their explosives to aid in detection and tracking. There are two types of taggants: detection taggants and identification taggants. Identification taggants are various types of materials that have been placed in the explosive which allow the explosive to be traced, post-detonation, to its point of origin and possibly determine how the explosive was acquired [40–42]. Because plastic explosives have low vapor pressures and are therefore difficult to detect through chemical means prior to detonation, detection taggants are added. These taggants are high vapor pressure compounds
Scheme 2. Synthesis of DAP-DAM-1.
SiOR 3
I-
+
N
N
~0.9nm
N2
+ 5eq.
I
SiOR3
Δ, 72 hrs +
N
N
SiOR 3
Scheme 1. Synthesis of 2,7-bis(trimethoxysilylpropyl)diazapyrinium diiodide.
I-
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whose volatility enables the explosive to be detected more easily [40–49,41,50–69,52,70–82]. We have chosen to study the sensing ability of our new materials with four common taggants: nitromethane, 2,3-dimethyl-2,3-dinitrobutane (DMNB), nitrobenzene, and ortho-nitrotoluene. Results for fluorescence quenching of BDAP-PMO and DAPDAM-1 powders upon exposure to the taggants is described. BTEBp, first reported by Inagaki et al. [39], is a fluorescent PMO composed of 4,4 0 -bis(triethoxysilyl)1,1 0 -biphenyl. The fluorescensce properties of BTEBp were also investigated and compared with BDAP-PMO and DAPDAM-1. 2. Experimental 2.1. Synthesis of 2,7-diazapyrene The preparation of 2,7-diazapyrene from 1,4,5,8-napthalenetetracarboxylic dianhydride was based on the combination and modification of published procedures [83,84]. 2.1.1. Preparation of N,N 0 -dimethyl-1,4,5,8napthalenetetracarboxylic diimide (1) A 2 L round bottom flask equipped with a reflux condenser was charged with 800 mL of 40% aqueous methylamine. To the solution 25 g (93 mmol) of 1,4,5,8napthalenetetracarboxylic dianhydride (Aldrich, 99%) was added slowly. The suspension was refluxed for 3 h, cooled to 25 °C, and filtered. The pink powder was washed with deionized water and then methanol to yield 26.6 g (90 mmol) of N,N 0 -dimethyl-1,4,5,8-napthalenetetracarboxylic diimide (1) (97% yield). 1H NMR (CF3COOD): d 8.50 (s, 4H, ArH), 3.26 (s, 6H, NCH3) and 13C NMR (CF3COOD): d 166.61, 133.47, 127.98, 127.77, 28.43. 2.1.2. Preparation of 1,6,8-tetrahydro-2,7-dimethyl-2,7diazapyrene To a rigorously dried round bottom flask was added 350 mL of 1 M lithium aluminum hydride (LAH) solution in dry tetrahydrofuran (THF). The reaction vessel was cooled in an ice bath. 18.66 g of AlCl3 were slowly added. The addition of AlCl3 was exothermic and violent. The reaction was removed from the ice bath and compound 1 (17.5 g) was added slowly so that the reaction comes to a weak boil. The reaction was refluxed for 3.5 h under argon. After 30 min of heating the reaction changed color from deep green to gold. After 3 h of heating, the reaction was cooled to 0 °C. To quench any remaining LAH, a suspension of ice (70 g) in 350 mL THF was added to the reaction slowly, then immediately suction filtered. The yellow solid was washed with 70 mL THF and dried under vacuum. Additional product may be recovered from the filtrate (700 mL) by reducing the volume to 15 mL and adding 90 mL chloroform. The solution was then brought to reflux and hot suction filtered. The yellow solids were added to those previously obtained. 1,6,8-Tetrahydro-2,7-dimethyl2,7-diazapyrene (2) was obtained as gold crystals (70%
3
yield). 1H NMR (CDCl3): d 7.01 (s, 4H, ArH), 3.85 (s, 8H, NCH2), 2.55 (s, 6H, NCH3) and 13C NMR (CDCl3): d 131.74, 127.34, 121.56, 58.62, 45.52. 2.1.3. Preparation of 2,7-diazapyrene A series of five traps were constructed to trap gas formed during the reaction. The traps consisted of 500 mL two-neck round bottom flasks with a rubber septum in one port and a greased, ground glass vacuum adapter in the other port. The rubber septa were pierced with a long tip disposable pipette such that the tip of the pipette was at least halfway into the round bottom flask. The glass vacuum adapter of one vessel was connected via TygonÒ tubing to the pipette of the next vessel so that all flasks were connected in series. The first and third vessels were filled with a 30% solution of lead(II) acetate. The second, fourth, and fifth traps were all 10 M NaOH solutions. The reaction vessel was a 250 mL round bottom flask equipped with a reflux condenser. The condenser was topped with a ground glass vacuum adapter connected by TygonÒ tubing to the first trap. The final trap of the series was vented into a fume hood. Compound 2 (5 g, 21 mmol) and 10 g selenium metal (Fluka, 99.9%)(0.13 mol) were mixed and added to the reaction vessel with stirring. The reaction vessel was rapidly heated to 265 °C in a sand bath for 4 h then to 300 °C for 15 min or until bubbling through the trap system ceased. The mixture was then cooled to room temperature. A solution of 1 N HCl (100 mL) was added to the crude reaction and brought to reflux for 10 min. The solution was cooled to room temperature and vacuum filtered. This extraction was repeated until the filtrate no longer fluoresced blue under 365 nm light. The filtrates were combined and made strongly basic with a solution of saturated NaOH. The resulting precipitate was filtered, dried, and ground to a powder. The product (3) purified by column chromatography (80% yield). 1H NMR (CDCl3, 270 MHz) d 9.39 (s, 4H, ArH), 8.06 (s, 4H, ArH). 13C NMR (CDCl3): d 145.18, 126.18, 125.99, 125.60. 2.2. Synthesis of 2,7-bis(3-trimethoxysilylpropyl) diazapyrinium diiodide (BDAP) As shown in Scheme 1, 2,7-diazapyrene (3) (0.20 g, 0.98 mmol) was refluxed in acetonitrile (100 mL) under nitrogen [85] while a fivefold excess of 3-iodopropyltrimethoxysilane (1.42 g, 4.9 mmol) was slowly added to ensure complete conversion to the disilylated product. The reaction was refluxed and stirred for 3 days resulting in an orange solution. The reaction was cooled to 0 °C and an equal volume of petroleum ether was added. An orange precipitate formed and was suction filtered. The orange solid was washed with warm acetonitrile several times and dried under vacuum at 50 °C to yield 0.262 g of 2,7-bis(3-trimethoxysilylpropyl) diazapyrinium diiodide (54% yield). 1H NMR (D2O, 270 MHz), d 10.38 (s, 4H),
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8.91 (s, 4H), 5.10 (t, 4H), 3.56 (s, 18H), 2.49 (br quin, 4H), 0.77 (br t, 4H).
2.5. 4,4 0 -Bis(triethoxysilyl)-1,1 0 -biphenyl periodic mesoporous silica (BTEBp-PMO)
2.3. 2,7-Bis(3-trimethoxysilylpropyl) diazapyrinium diiodide PMO (BDAP-PMO) synthesis
To prepare BTEBp-PMO, 1.65 g of octadecyltrimethylammonium chloride (ODTMAC) was added to 7.5 mL of 6 M sodium hydroxide dissolved in 89.5 mL of deionized water. To this was added 1.8 g of 4,4 0 -bis(triethoxysilyl) biphenylylene, with stirring at RT. This solution was then sonicated and heated to 98 °C for 48 h. The white precipitate was suction filtered, washed with deionized water, and dried at 50 °C to obtain BTEBp-PMO. The removal of the template (ODTAMC) was accomplished by refluxing the as-synthesized BTEBp-PMO in a solution composed of 4 mL of 2 M HCl in 150 mL deionized water for 24 h. The template free BTEBp-PMO was filtered dried at 50 °C for 6 h.
The BDAP-PMO was synthesized following a modified procedure of the previously reported recipe for Dallas Amorphous Material (DAM-1) [86–89]. BDAP and tetramethyl orthosilicate (TMOS) (Aldrich) were used as the silica sources and vitamin E tocopheryl polyethylene glycol 1000 succinate (TPGS) as the template. The molar ratio of BDAP to TMOS was varied from 100% to 1.6%. In a typical synthesis, 0.3 g of vitamin E TPGS (Eastman Chemical) was dissolved in 16.0 mL of deionized H2O. To this mixture, 3.7 mL of 12 M hydrochloric acid was added and the solution was stirred for 30 min. While the solution was stirring, 0.85 g (5.6 mmol) of TMOS along with 0.15 g (0.19 mmol) BDAP was added. The solution (pH 0.2) was aged, with stirring, for 20 min at room temperature and then aged at 40 °C for 24 h followed by heating at 90 °C for 48 h. After cooling to room temperature the product was isolated by filtration and washed with deionized water. To remove the vitamin E TPGS, the as synthesized BDAP-PMO was refluxed in ethanol for 6 h. 2.4. N-(3-Trimethoxysilylpropyl)-2,7-diazapyrene iodide grafted mesoporous material synthesis (DAP-DAM-1) To prepare DAP-DAM-1, TMOS was co-condensed with 3-iodopropyltrimethoxy silane following the DAM-1 synthesis procedure to produce a hybrid mesoporous material with iodopropyl groups present in the pores. 2,7-Diazapyrene was then reacted with the iodopropyl groups to covalently bind it to the framework (Scheme 2). To synthesize the iodopropyl functionalized framework, 0.60 g vitamin E TPGS was dissolved in 32.0 mL of deionized water. To this mixture, 7.4 mL of 12 M hydrochloric acid was added and the solution was stirred for 30 min. While the solution was stirring, 1.70 g (11.2 mmol) of TMOS along with 0.11 g (0.40 mmol) 3-iodopropyltrimethoxy silane (Gelest, 99.9%) was added. The solution was aged, with stirring, for 20 min at room temperature and then aged at 40 °C for 24 h followed by heating at 90 °C for 48 h. After cooling to room temperature the product was isolated by filtration and washed with deionized w ater. To remove the vitamin E TPGS, the as-synthesized iodopropyl DAM-1 was refluxed in ethanol for 6 h (Si/ I = 16.8). Template extracted iodopropyl DAM-1 (0.50 g) was then added to an acetonitrile (200 mL) solution containing 0.04 g (0.18 mmol) 2,7-diazapyrene and refluxed for 72 h. The solution was then suction filtered and washed with warm acetonitrile. To ensure that all unreacted 2,7diazapyrene had been removed from the solids, the material was refluxed in fresh acetonitrile for 6 h and hot gravity filtered.
2.6. Instrumentation EDX analysis and scanning electron microscope images of samples coated with Pd/Au were collected with a Leo 1530VP field-emission scanning electron microscope. Transmission electron microscope images were obtained using a JEM-2100 TEM, operating at 200 kV. X-ray diffractograms were obtained using a Scintag XDS 2000 Xray diffractometer with Cu Ka radiation and a Rigaku Ultima III. Nitrogen adsorption isotherms were measured at 77 K on a Quantachrome Autosorb 1. All samples were outgassed at 100 °C and 106 Torr prior to measurements. A seven-point BET equation was used to calculate surface areas. Pore size distribution was calculated from the desorption data using the BJH method. Luminescence data were collected using a Perkin–Elmer LS 50B luminescence spectrometer. The solid samples were adhered to a coverslip with FluorolubeÒ. They were then treated with 100 lL of various solutions prior to collecting the luminescence data. NMR data were obtained with a JEOL Eclipse 270. Cyclic voltammograms were collected using a Bioanalytical Systems CV-50W. Samples for transmission electron microscopy (TEM) were prepared by suspending the particles in 99% isopropyl alcohol and ultrasonicating for 10 min. The suspensions were allowed to dry on carbon coated Cu grids. Images were collected using a JEM 2100 TEM operating at 200 kV. 3. Results and discussion 3.1. BDAP-PMO The 2,7-diazapyrene, prepared from 1,4,5,8-napthalenetetracarboxylic dianhydride following published procedures [83,84], was then diquarternized with 3-iodopropyltrimethoxysilane by refluxing in acetonitrile (Scheme 1) to form BDAP. The BDAP was not photoluminescent as a solid, but was readily soluble in water and emitted blue light with a kmax at 416 nm.
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To form a redox-active hybrid mesoporous material, BDAP was used as the primary silica source in a modified version of the recipe for DAM-1 [86–89]. The mol% of BDAP present in the synthesis was varied from 100 to 1.6 using TMOS as the secondary silica source. Table 1 summarizes the results of the materials formed. Similar to observations made by Alvaro et al. for the synthesis of viologen PMO [35]; BDAP did not form an ordered mesoporous material at mole percents greater than 5 under the conditions studied. The molar ratios shown in Table 1 are those of the synthesis gel. The percentage of BDAP present in the materials after synthesis was determined using EDAX based on iodine content. Material A had a 2.3 mol% BDAP while B, C, and D were 1.5, 1.0, and 0.7, respectively. The loading was determined by EDAX from Si/I ratios of 21.6, 33.4, 48.7, and 74.2 for materials A, B, C, and D, respectively, based on 2 I per BDAP. The BDAP-PMO powders were characterized by powder X-ray diffraction. Fig. 1A–D shows the X-ray patterns of the luminescent mesoporous materials formed from BDAP (see Table 1). Materials A–D all show similar patterns with one large, broad reflection located at 1.1° 2h. For the hybrid mesoporous materials to be useful as adsorbents or sensors the template must be removed without collapsing the pores or damaging the organic materials present in the mesoporous material. To accomplish this, the vitamin E TPGS was exhaustively extracted by refluxing in ethanol for several hours. Fig. 1A, which corresponds to Table 1 Properties of mesoporous materials synthesized with varying amounts of BDAP BDAP theory (mol%)
BDAP actual (mol%)
Pore size ˚) (A
Surface area (m2/ g)
Fluorescent
A B C D
5.0 3.4 2.2 1.6
2.3 1.5 1.0 0.7
47 48 45 43
501 663 568 577
Yes Yes Yes Yes
Intensity
Sample
(a)
(A)
(b)
(B)
(c)
(d)
0.8
1.8
material A from Table 1, shows that the peak at 1.1° 2h is present after extraction, which indicates that the mesoporosity is retained after template extraction. Also, the transmission electron micrograph of BDAP-PMO after extraction (Fig. 2) clearly shows that the mesoporosity is ˚ pore size retained. The TEM is consistent with the 47 A and well ordered channels running in parallel. This is true for materials B, C, and D (Fig. 1B–D, respectively) as well. Materials B and D also show low intensity reflections (Fig. 1B and D) located 2.2° 2h. The 1.1° (d spac˚ ) and 2.2° (d spacing = 40.1 A ˚ ) 2h peaks index ing = 80.3 A as the (1 0 0) and (2 0 0) peaks associated with 2-D hexagonal type mesoporous materials. The scanning electron micrographs (SEM) of these materials are shown in Fig. 3. Materials A, B, C, and D all show a spherical morphology in the range of 2.0 to 5.5 lm. The pure DAM-1 prepared under these conditions (not shown) also forms in the spherical morphology with a similar size distribution. The pore sizes and surface areas of the BDAP-PMO materials were determined by N2 adsorption. Table 1 shows that in all cases the PMOs containing BDAP have ˚ ) prepared under similar larger pores than DAM-1 (38 A conditions. There may be a slight trend towards larger pores with an increase in BDAP loading. There is also a modest increase in wall thickness in BDAP-PMOs (35, ˚ for A, B, C, and D, respectively) especially 36, 33, and 31 A ˚ for DAM-1. The wall thickness of compared with 26 A PMOs tend to be greater than pure silica materials [95] which is consistent with the trend in wall thickness as the BDAP loading increases. Liang et al. have shown that aromatic compounds such as 1,3,5-triisopropylbenzene can intercalate into the hydrophobic area of the micelle during synthesis and increase the size of the micelle consequently increasing the pore size of the resulting mesoporous
(C)
(D)
2.8
3.8
4.8
5.8
6.8
7.8
Degrees 2
Fig. 1. Powder XRD patterns of BDAP-PMO with (A) 4.4 mol%, (B) 2.9 mol%, (C) 2.0 mol%, and (D) 1.3 mol% BDAP.
5
Fig. 2. Transmission electron micrograph of BDAP-PMO sample A.
6
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Fig. 3. Scanning electron micrographs of BDAP-PMO samples A, B, C and D.
material [96]. The BDAP may also be associated with the micelle, which causes an increase in pore size relative to DAM-1. Table 1 also shows that the surface areas for the BDAP-PMOs are less than for DAM-1 (998 m2/g) but there is no clear trend with loading. The known redox activity of diazapyrinium derivatives allows for an electrochemical means to determine if the BDAP is incorporated into the framework. Fig. 4A shows the cyclic voltammogram of free BDAP dissolved in 0.1 M tetraethylammonium tetrafluoroborate in acetonitrile using a platinum working electrode, silver/silver chloride reference electrode, and a platinum auxiliary electrode. This voltammogram is similar to those reported for N,N 0 dimethyl-2,7-diazapyrinium [90,91] with a reduction peak at (0.50 V) and a small, broad oxidation peak at 0.46 V. Fig. 4B shows the cyclic voltammogram of sample A (2.3 mol% BDAP-PMO) recorded under the same conditions. To obtain the cyclic voltammogram, the BDAP-PMO was adhered to the glassy carbon electrode surface using ParaloidTM B-72 (Aldrich). The cyclic voltammogram of the BDAP-PMO shows a reduction peak at 0.50 V and a smaller, broader oxidation at 0.40 V. These results indicate that the BDAP-PMO is redox active and thereby providing direct evidence for the incorporation of the BDAP into the material framework. 3.2. 2,7-Diazapyrene grafted mesoporous material (DAPDAM-1) In an effort to verfy that the diazapyrene in BDAP-PMO is indeed bridging in the pore wall, we prepared DAM-1
with DAP dangling from the pore wall. This fluorescent material was prepared by reacting 2,7-diazapyrene with an iodopropyl trimethoxysilane functionalized mesoporous DAM-1 material. The functionalized DAM-1 was prepared by co-condensing TMOS with iodopropyl trimethoxysilane, the powder XRD for this material is shown in Fig. 5A. Table 2 (material E) shows the BET surface area and BJH ˚, (desorption branch) pore size was 577 m2/g and 34 A respectively. Organosilanes can interact with the surfactant micelles and reduce their size [97], which could account for the smaller size of the pores as compared to DAM-1. When added to 2,7-diazapyrene in refluxing acetonitrile, the 2,7diazapyrene was mono-quaternized with the dangling iodopropyl functional groups present in the pores. This formed a fluorescent mesoporous material (DAP-DAM-1) whose powder XRD is shown in Fig. 5B. The BET surface area and BJH (desorption branch) pore size for material F ˚ , respectively. The reduc(Table 2) was 545 m2/g and 33 A tion in surface area from 577 to 545 m2/g (materials E and F, respectively) is due to the inclusion of 2,7-diazapyrene in the pores. The loading of mono quaternized 2,7diazapyrene was determined by EDAX. The Si/I ratio for material F was determined to be 16.8, which corresponds to a loading of 3 mol% 2,7-diazapyrene. Fig. 6A and B shows the SEM images of materials E and F, respectively. As can be seen, the spherical morphology of these materials is similar to that of both DAM-1 and BDAP-PMO. There was no change in morphology after the grafting of 2,7-diazapyrene. To ensure that the 2,7-diazapyrene was covalently bound to the mesoporous material and not just adsorbed,
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7
Table 2 Properties of (E) iodopropyl functionalized DAM-1, (F) DAP-DAM-1, (G) DAM-1, and (H) DAM-1 after adsorption of 2,7-diazapyrene Sample
DAP (mol%)a
Pore size ˚) (A
Surface area (m2/ g)
Fluorescent
E F G H
0 3 0 0
34 33 37.8 37.5
577 545 998 850
No Yes No No
a
Fig. 4. Cyclic voltammogram of (A) BDAP in acetonitrile and (B) BDAPPMO.
(A)
Intensity
(B)
functionalized material, except 100% TMOS was used with no iodopropyl trimethoxysilane (material G, Table 2). The powder XRD of material G is shown in Fig. 5C. This sample exhibited a surface area of 998 m2/g and a BJH pore ˚ . Material G was then treated to size distribution of 38 A the same procedure as E by stirring with 2,7-diazapyrene in refluxing acetonitrile (material H). After adsorption of 2,7 diazapyrene there was no change in the XRD pattern (Fig. 5D) while the BET surface area (850 m2/g) decreased ˚ ). Fig. 6C and D slightly with no change in pore size (38 A shows the SEM images of samples G and H, respectively. As can be seen, the morphology of the materials is similar to that of DAM-1, DAP-DAM-1, and the BDAP-PMOs. Samples F and H were treated to similar conditions to determine if the 2,7-diazapyrene was tightly bound or merely adsorbed in the materials. Both materials were, separately stirred in refluxing acetonitrile for 6 h to extract any adsorbed 2,7-diazapyrene. Then they were filtered and placed at 90 °C in a vacuum oven for 16 h to further remove any unbound 2,7-diazapyrene by sublimation. After these treatments, sample F (DAP-DAM-1) was luminescent with a kmax = 424 nm, while sample H showed no luminescence. This confirms the fact that any 2,7-diazapyrene not covalently bound to the pore surface was easily removed. The kmax of DAPDAM-1, which contains mono-cationic 2,7-diazapyrene, was slightly red-shifted relative to BDAP-PMO (kmax = 424 nm versus 420 nm, respectively). This shift in fluorescence due to quaternization is similar to the shift seen in N-methyl-2,7-diazapyrene versus N,N 0 -dimethyl-2,7-diazapyrene [98]. These differences between DAP-DAM-1 and BDAP-PMO seem to support our contention that BDAP is bridging in BDAPPMO.
(C)
3.3. BTEBp periodic mesporous organosilica
(D) 0.5
In the as-synthesized DAM-1.
1
1.5
2.0
2.5
3
3.5
4.0
4.5
5.0
Degrees 2θ
Fig. 5. Powder XRD patterns of (A) iodopropyl functionalized mesoporous material, (B) DAP-DAM-1, (C) DAM-1, and (D) DAM-1 after 2,7diazapyrene adsorption.
a control experiment was performed. A mesoporous material was prepared in the same manner as the iodopropyl
For further comparison with a well-established PMO, we prepared the biphenyl derivative (MTEBp-PMO) [39]. Fig. 7 shows the powder XRD of the extracted BTEBp with the mesopore peak located at 1.9° 2h. The reflections located at 7.4, 14.9, 22.8, 30.8, 37.4° 2h are attributed to long range periodicity due to p stacking. The broad peak at 20.6° 2h is due to Si–O–Si [39]. BTEBp-PMO exhibits a BET surface area of 848 m2/g and a BJH pore size of ˚ . The SEM images (Fig. 8) of this material reveal 24 A
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Fig. 6. Scanning electron micrographs of (A) iodopropyl functionalized mesoporous material, (B) DAP-DAM-1, (C) DAM-1, and (D) DAM-1 after 2,7diazapyrene adsorption.
nanoparticles in the range of 20 to 100 nm. The BTEBp-PMO is also fluorescent (kmax = 375 nm). 3.4. Fluorescence The results discussed above are consistent with the BDAP being incorporated into the mesoporous silica framework. The fact that an ordered phase is not obtained with BDAP loadings greater than 3% is in agreement with incorporation of the bridging silane into the pore walls. To further show that BDAP is not simply self-condensed as an impurity phase but rather incorporated into the pore walls, the as-synthesized BDAP-PMO (pores filled with template)
(
)
Si
Intensity
Si
Fig. 8. Scanning electron micrographs of BTEBp-PMO. 1
6
11
16
21
26
31
36
41
Degrees 2θ
Fig. 7. Powder XRD pattern of BTEBp-PMO.
46
along with the template extracted BDAP-PMO were treated with 120 lM nitrobenzene to quench fluorescence.
K.J. Balkus Jr. et al. / Microporous and Mesoporous Materials 112 (2008) 1–13
The as-synthesized sample showed a marginal reduction in luminescent intensity (<1%) while the sample with the template removed showed a large reduction (81%). With the pores filled with template, nitrobenzene’s access to the BDAP is limited to a few sites at the particle surface. This is consistent with the BDAP being located in the mesopores. Fig. 9A–C shows an optical image of the three materials BDAP-PMO, DAP-DAM-1, BTEBp-PMO excited at 266 nm, respectively. BDAP-PMO is a yellow powder but, as seen in Fig. 9A, when excited at 266 nm, it emits blue light with a kmax = 420 nm. DAP-DAM-1 (Fig. 9B) is a white powder with a slight yellow tint and when excited emits blue fluorescence with a kmax = 424. BTEBp-PMO
9
(Fig. 9C) is also a white powder and when excited also emits blue fluorescence with a kmax = 377 nm. The diazapyrene will interact in a p fashion with aromatic ring compounds. Therefore, fluorescence quenching by nitrobenzene was used a probe of DAP inclusion as described above. This is based on the electron withdrawing properties of nitrated aromatics to form charge transfer complexes with fluorophors, which leads to fluorescence quenching [59,92,93]. Since nitrobenzene is a common taggant for nitrated explosives, the DAP containing PMOs may prove to be effective optical sensors for explosives. The quenching ability of four (Fig. 10) common nitrated explosives taggants was determined for BDAP-PMO, DAP-DAM-1, and BTEBp-PMO. The four taggants used in this study were 2,3-dimethyl-2,3-dinitrobutane [45], nitrobenzene [58], nitromethane [59], and o-nitrotoluene [52] which were diluted with acetonitrile to control concentration. As shown in Fig. 11A, when treated with 30, 60, and 120 lM solutions of o-nitrotoluene, the luminescence of BDAP-PMO decreases. When treated with 30 lM onitrotoluene the luminescent intensity decreased by 10%, with 60 lM it decreased by 22%, and with 120 lM o-nitrotoluene the luminescence was quenched by 34%. Similar results were obtained when nitrobenzene was used as the quencher. For 30, 60, and 120 lM concentrations of nitrobenzene the BDAP-PMO was quenched by 29%, 50%, and 81%, respectively (Fig. 11B). The nitro-alkanes DMNB (Fig. 11C) showed less quenching (12%, 17%, and 25% for 30, 60, and 120 lM, respectively) but were still detectable. However, nitromethane (Fig. 11D) did not show any appreciable change (3%, 5%, and 7%, respectively). Calculated using the lowest concentration of analytes d etected, the detection limits for BDAP-PMO would be 3.7, 4.1, and 5.3 ppm for nitrobenzene, o-nitrotoluene, and DMNB, respectively.
O N
+
O
O N
+
+
O
N O
2,3-Dimethyl-2,3-dinitrobutanee
NO2
Fig. 9. Digital images of (A) BDAP-PMO, (B) DAP-DAM-1, and (C) BTEBp-PMO excited at 266 nm.
Nitrobenzene
O Nitromethane
NO2
O-nitrotoluene
Fig. 10. Molecular structure of nitrated explosives taggants.
10
K.J. Balkus Jr. et al. / Microporous and Mesoporous Materials 112 (2008) 1–13
NO NO 2
NO2
2
BDAP 30 µM
NO2
60 µM 120 µM
NO NO 2 2
+
Intensity
Intensity
O
400
410
420
430 440 450 Wavelength (nm)
460
400
470
N
410
430
420
440
450
O
460
470
Wavelength (nm)
Fig. 11. Quenching of photoluminescence of BDAP-PMO with (A) o-nitrotoluene, (B) nitrobenzene, (C) DMNB, and (D) nitromethane.
Fig. 12A–D shows the emission spectra for DAP-DAM1 when treated with o-nitrotoluene, nitrobenzene, DMNB, and nitromethane, respectively. DAP-DAM-1 showed a decrease in emission intensity (Fig. 12A) of 19%, 36%, and 44% for o-nitrotoluene concentrations of 30, 60, and 120 lM, respectively. This is slightly better than for BDAP-PMO. In the case of nitrobenzene (Fig. 12B), the DAP-DAM-1 0 s luminescence decreased by 29%, 68%, and 81% for 30, 60, and 120 lM concentrations. The nitro-alkanes barely quenched the luminescence of DAP-
DAM-1 and even less than for BDAP-PMO. For the same concentrations, DMNB reduced the intensity by 3%, 8%, and 15% (Fig. 12C) while the nitromethane reduced it by 1%, 3%, and 3% (Fig. 12D), respectively. Fig. 13 shows the emission spectra of BTEBp-PMO which decrease in intensity when treated with 30, 60, and 120 lM solutions of the nitrated taggants. The 30 lM onitrotoluene solution decreased the intensity by 37%, while 60 lM decreased the intensity by 53%, and 120 lM o-nitrotoluene by 69%. Similar results were obtained with nitro-
NO2
NO NO2
2
DAPDAM-1 30 M
NO2
60 M 120 M
NO2 NO2
400
Intensity
Intensity
O
410
420
430
440
450
Wavelength (nm)
460
470
480
400
+
N O
410
420
430
440
450
460
470
480
Wavelength (nm)
Fig. 12. Quenching of photoluminescence of DAP-DAM-1 with (A) o-nitrotoluene, (B) nitrobenzene, (C) DMNB, and (D) nitromethane.
K.J. Balkus Jr. et al. / Microporous and Mesoporous Materials 112 (2008) 1–13
NO NO 2 2
11
NO2
BTEBp 30 M
NO2
60 M 120 M
NO NO 2 2
O Intensity
Intensity 300
+
320
340
360
380
400
420
440
460
480
300
N
320
340
Wavelength (nm)
360
380
400
420
440
O
460
480
Wavelength (nm)
Fig. 13. Quenching of photoluminescence of BTEBp-PMO with (A) o-nitrotoluene, (B) nitrobenzene, (C) DMNB, and (D) nitromethane.
benzene where 30, 60, and 120 lM concentrations quenched the fluorescence by 13%, 25%, and 67%, respectively (Fig. 13B). Interestingly, both DMNB (Fig. 13C) (4%, 17%, and 26% for 30, 60, and 120 lM, respectively) and nitromethane (Fig. 13D) (7%, 15%, and 21%, respectively) partially quenched the emission from BTEBp-PMO. The fluorescence spectra shown in Figs. 11–13 suggest that the aromatic nitrotoluene, and nitrobenzene are easily detected by all three materials. DMNB and nitromethane quench the fluorescence of BTEBp-PMO by a modest amount. There are other explosive sensors based on fluorescence quenching of fluorophors such as pyrene [59]. Although the detection limits for these techniques are much lower than BDAP-PMO (71 ng/mL versus 5300 ng/mL, respectively) these systems require the use of liquid chromatography and laser induced fluorescence which would be difficult to incorporate into a portable device. Conversely, BDAP-PMO, DAP-DAM-1, and BTEBp-PMO are solid based systems which could easily be configured in a solid state device. 4. Conclusions A novel redox active mesoporous material was synthesized by co-condensation of a BDAP based disilane with TMOS at concentrations as high as 2.3 mol%. The characterization of BDAP-PMO is consistent with the pore wall location of the diazapyrene, especially when compared to the DAP-DAM-1 with surface grafted diazapyrene. BDAP-PMO, DAP-DAM-1 and BTEBp-PMO were found to be highly fluorescent. These materials were shown to be effective optical sensors for nitrated explosive taggants based on the quenching of the PMO emission. As high as
81% quenching upon exposure to nitrobenzene was observed for BDAP-PMO and DAP-DAM-1. Further studies of the redox and optical properties are in progress but these results suggest that these PMOs may effectively interact with various guest species in the pores. Acknowledgement We thank the Robert A. Welch Foundation for support of this research. References [1] A. Vinyu, K. Z Hossain, K. Ariga, J. Nanosci. Nanotechnol. 5 (2005) 347–371. [2] B. Hatton, K. Landskron, W. Whitnall, D. Perovic, G.A. Ozin, Acc. Chem. Res. 38 (2005) 305–312. [3] K.Z. Hossain, L. Mersier, Adv. Mater. 14 (2002) 1053. [4] M.P. Kapoor, S. Inagaki, S. Ikeda, K. Kakiuchi, M. Suda, T. Shimada, J. Am. Chem. Soc. 127 (2005) 8174–8178. [5] S. Guan, S. Inagaki, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 5660. [6] M.P. Kapoor, Q. Yang, Y. Goto, S. Inagaki, Chem. Lett. 32 (2003) 914. [7] W.J. Hunks, G.A. Ozin, Adv. Funct. Mater. 15 (2005) 259–266. [8] Q. Yang, M.P. Kapoor, S. Inagaki, J. Am. Chem. Soc. 124 (2002) 9694. [9] A. Corma, D. Das, H. Garcı´a, A. Leyva, J. Catal. 229 (2005) 322–331. [10] M. Kuroki, T. Asefa, W. Whitnal, M. Kruk, C. Yoshina-Ishii, M. Jaroniec, G.A. Ozin, J. Am. Chem. Soc. 124 (2002) 13886. [11] R.J.P. Corriu, E. Lancelle-Beltran, A. Mehdi, C. Reye’, S. Brande‘s, R. Guilard, Chem. Mater. 15 (2003) 3152–3160. [12] Z. Wang, J.M. Heising, A. Clearfield, J. Am. Chem. Soc. 125 (2003) 10375. [13] Q. Yang, M.P. Kapoor, N. Shirokura, M. Ohashi, S. Inagaki, J.N. Kondo, K. Domen, J. Mater. Chem. 15 (2005) 666–673.
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Microporous and Mesoporous Materials 112 (2008) 1–13 www.elsevier.com/locate/micromeso
Photoluminescent and redox active periodic mesoporous organosilicas based on 2,7-diazapyrene Kenneth J. Balkus Jr. *, Thomas J. Pisklak, Greg Hundt, John Sibert, Yangfeng Zhang Department of Chemistry and the NanoTech Institute, The University of Texas at Dallas, Richardson, TX 75083-0688, USA Received 25 May 2006; received in revised form 1 May 2007; accepted 2 May 2007 Available online 31 May 2007
Abstract A novel redox-active, luminescent periodic mesoporous organosilica (PMO) based on 2,7-bis(3-trimethoxysilylpropyl)diazapyrinium diiodide (BDAP) was prepared. A 2,7-diazapyrene grafted mesoporous material (DAP-DAM-1) was also synthesized. BDAP loadings as high as 2.3% were achieved in well ordered BDAP-PMO materials. The photoluminescence properties of BDAP-PMO and DAPDAM-1, along with a PMO based on 4,4 0 -bis(triethoxysilyl)-1,1 0 -biphenyl (BTEBp-PMO) were determined. The photoluminescence of BDAP-PMO, DAP-DAM-1, and BTEBp-PMO was quenched by nitrated explosive taggants including o-nitrotoluene, nitrobenzene, 2,3-dimethyl-2,3-dinitrobutane (DMNB), and nitromethane, suggesting the potential use of these PMOs as optical sensors for explosives. In the case of BDAP-PMO and DAP-DAM-1, exposure to nitrobenzene results in 81% quenching of fluorescence. Ó 2008 Published by Elsevier Inc. Keywords: Mesoporous molecular sieves; PMO; DAM-1; Optical sensor
1. Introduction Periodic mesoporous organosilicas (PMO) as well as hybrid mesoporous materials have been widely reported in the literature [1,2]. These materials have either bridging or pendant organic groups in the pore walls. Historically, the mesoporous structure of all silica frameworks, such as MCM-41, SBA-15, and DAM-1 have been functionalized with a diverse range of organic groups [3–35,94].These materials may have the organic functional groups incorporated in the framework during synthesis or by post-synthetic grafting. In 1997 we reported one of the first examples of MCM-41 decorated with organosilanes and metal complexes by post-synthesis modification [36]. This is typically accomplished using organosilanes which are either self-condensed or combined with silica precursors to form covalent linkages between the organics and the inorganic framework [1,2]. Potential problems with this *
Corresponding author. Tel.: +1 972 883 2659; fax: +1 972 883 2925. E-mail address: [email protected] (K.J. Balkus Jr.).
1387-1811/$ - see front matter Ó 2008 Published by Elsevier Inc. doi:10.1016/j.micromeso.2007.05.025
approach are low loadings, heterogeneous distribution and loss of pore volume. In contrast, PMOs are composed of bridging organosilanes which are incorporated into the pore walls and can make up as much as 100% of the framework. The accessible surface areas and density of organic groups in PMOs are much larger than for grafted materials. This provides more free volume in the pores, heterogeneous dispersion, as well as easier access to the organic groups. Theoretically, any organic compound that could be silanated might be incorporated into a mesoporous framework, provided it is stable under synthesis conditions. Most efforts to date have focused on acid/base groups or aromatic groups that impart specific surface properties, such as hydrophilicity/hydrophobicity or act as binding sites for metal ions. Ha and co-workers [19] incorporated a vanadyl Schiff base complex into a mesoporous framework which has the ability to catalyze the cyanosilylation of carbonyl groups, and Reye et al. used a cobalt Schiff base complex to absorb dioxygen [11]. There have also been other metal containing ligands co-condensed into
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mesoporous materials such as a carbapalladacycle complex as a catalyst for Suzuki cross-coupling [9]. Also, various alkyl, aromatic, and sulfur containing groups have been disilanated and incorporated into mesoporous materials. Due to the ability to incor porate various organic compounds into the framework of mesoporous materials there is a potential for the application of PMOs as solid catalysts and sensors [1]. Some applications could benefit from a redox active framework; however, there are relatively few examples of such PMO materials. Recently a PMO was synthesized by co-condensing disilylated viologen (N,N 0 bis(triethoxysilylpropyl)-4,4 0 -bipyridinium) with tetraethyl orthosilicate [32,35,37]. Upon irradiation, or mild heating (100 °C) this PMO generates the radical cation. In related work, an electrochromic PMO was prepared using a viologen-modified periodic mesoporous nanocrystalline anatase [38]. Here we present the synthesis of a redox active, fluorescent di-silylated 2,7-diazapyrene mesoporous material (BDAP-PMO) shown in Scheme 1. Ordered BDAP-PMO was prepared with loadings up to 5% with redox activity verified by cyclic voltammetry. For comparison, a tethered 2,7-diazapyrene (DAP-DAM-1) has been prepared from iodopropyl functionalized DAM-1 (Scheme 2). The BDAP-PMO fluorescence can be quenched by certain molecules, which suggests this material might be exploited for optical sensing. A growing concern for homeland security is the detection of explosives. Explosives manufacturers add identification markers, called taggants, to their explosives to aid in detection and tracking. There are two types of taggants: detection taggants and identification taggants. Identification taggants are various types of materials that have been placed in the explosive which allow the explosive to be traced, post-detonation, to its point of origin and possibly determine how the explosive was acquired [40–42]. Because plastic explosives have low vapor pressures and are therefore difficult to detect through chemical means prior to detonation, detection taggants are added. These taggants are high vapor pressure compounds
Scheme 2. Synthesis of DAP-DAM-1.
SiOR 3
I-
+
N
N
~0.9nm
N2
+ 5eq.
I
SiOR3
Δ, 72 hrs +
N
N
SiOR 3
Scheme 1. Synthesis of 2,7-bis(trimethoxysilylpropyl)diazapyrinium diiodide.
I-
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whose volatility enables the explosive to be detected more easily [40–49,41,50–69,52,70–82]. We have chosen to study the sensing ability of our new materials with four common taggants: nitromethane, 2,3-dimethyl-2,3-dinitrobutane (DMNB), nitrobenzene, and ortho-nitrotoluene. Results for fluorescence quenching of BDAP-PMO and DAPDAM-1 powders upon exposure to the taggants is described. BTEBp, first reported by Inagaki et al. [39], is a fluorescent PMO composed of 4,4 0 -bis(triethoxysilyl)1,1 0 -biphenyl. The fluorescensce properties of BTEBp were also investigated and compared with BDAP-PMO and DAPDAM-1. 2. Experimental 2.1. Synthesis of 2,7-diazapyrene The preparation of 2,7-diazapyrene from 1,4,5,8-napthalenetetracarboxylic dianhydride was based on the combination and modification of published procedures [83,84]. 2.1.1. Preparation of N,N 0 -dimethyl-1,4,5,8napthalenetetracarboxylic diimide (1) A 2 L round bottom flask equipped with a reflux condenser was charged with 800 mL of 40% aqueous methylamine. To the solution 25 g (93 mmol) of 1,4,5,8napthalenetetracarboxylic dianhydride (Aldrich, 99%) was added slowly. The suspension was refluxed for 3 h, cooled to 25 °C, and filtered. The pink powder was washed with deionized water and then methanol to yield 26.6 g (90 mmol) of N,N 0 -dimethyl-1,4,5,8-napthalenetetracarboxylic diimide (1) (97% yield). 1H NMR (CF3COOD): d 8.50 (s, 4H, ArH), 3.26 (s, 6H, NCH3) and 13C NMR (CF3COOD): d 166.61, 133.47, 127.98, 127.77, 28.43. 2.1.2. Preparation of 1,6,8-tetrahydro-2,7-dimethyl-2,7diazapyrene To a rigorously dried round bottom flask was added 350 mL of 1 M lithium aluminum hydride (LAH) solution in dry tetrahydrofuran (THF). The reaction vessel was cooled in an ice bath. 18.66 g of AlCl3 were slowly added. The addition of AlCl3 was exothermic and violent. The reaction was removed from the ice bath and compound 1 (17.5 g) was added slowly so that the reaction comes to a weak boil. The reaction was refluxed for 3.5 h under argon. After 30 min of heating the reaction changed color from deep green to gold. After 3 h of heating, the reaction was cooled to 0 °C. To quench any remaining LAH, a suspension of ice (70 g) in 350 mL THF was added to the reaction slowly, then immediately suction filtered. The yellow solid was washed with 70 mL THF and dried under vacuum. Additional product may be recovered from the filtrate (700 mL) by reducing the volume to 15 mL and adding 90 mL chloroform. The solution was then brought to reflux and hot suction filtered. The yellow solids were added to those previously obtained. 1,6,8-Tetrahydro-2,7-dimethyl2,7-diazapyrene (2) was obtained as gold crystals (70%
3
yield). 1H NMR (CDCl3): d 7.01 (s, 4H, ArH), 3.85 (s, 8H, NCH2), 2.55 (s, 6H, NCH3) and 13C NMR (CDCl3): d 131.74, 127.34, 121.56, 58.62, 45.52. 2.1.3. Preparation of 2,7-diazapyrene A series of five traps were constructed to trap gas formed during the reaction. The traps consisted of 500 mL two-neck round bottom flasks with a rubber septum in one port and a greased, ground glass vacuum adapter in the other port. The rubber septa were pierced with a long tip disposable pipette such that the tip of the pipette was at least halfway into the round bottom flask. The glass vacuum adapter of one vessel was connected via TygonÒ tubing to the pipette of the next vessel so that all flasks were connected in series. The first and third vessels were filled with a 30% solution of lead(II) acetate. The second, fourth, and fifth traps were all 10 M NaOH solutions. The reaction vessel was a 250 mL round bottom flask equipped with a reflux condenser. The condenser was topped with a ground glass vacuum adapter connected by TygonÒ tubing to the first trap. The final trap of the series was vented into a fume hood. Compound 2 (5 g, 21 mmol) and 10 g selenium metal (Fluka, 99.9%)(0.13 mol) were mixed and added to the reaction vessel with stirring. The reaction vessel was rapidly heated to 265 °C in a sand bath for 4 h then to 300 °C for 15 min or until bubbling through the trap system ceased. The mixture was then cooled to room temperature. A solution of 1 N HCl (100 mL) was added to the crude reaction and brought to reflux for 10 min. The solution was cooled to room temperature and vacuum filtered. This extraction was repeated until the filtrate no longer fluoresced blue under 365 nm light. The filtrates were combined and made strongly basic with a solution of saturated NaOH. The resulting precipitate was filtered, dried, and ground to a powder. The product (3) purified by column chromatography (80% yield). 1H NMR (CDCl3, 270 MHz) d 9.39 (s, 4H, ArH), 8.06 (s, 4H, ArH). 13C NMR (CDCl3): d 145.18, 126.18, 125.99, 125.60. 2.2. Synthesis of 2,7-bis(3-trimethoxysilylpropyl) diazapyrinium diiodide (BDAP) As shown in Scheme 1, 2,7-diazapyrene (3) (0.20 g, 0.98 mmol) was refluxed in acetonitrile (100 mL) under nitrogen [85] while a fivefold excess of 3-iodopropyltrimethoxysilane (1.42 g, 4.9 mmol) was slowly added to ensure complete conversion to the disilylated product. The reaction was refluxed and stirred for 3 days resulting in an orange solution. The reaction was cooled to 0 °C and an equal volume of petroleum ether was added. An orange precipitate formed and was suction filtered. The orange solid was washed with warm acetonitrile several times and dried under vacuum at 50 °C to yield 0.262 g of 2,7-bis(3-trimethoxysilylpropyl) diazapyrinium diiodide (54% yield). 1H NMR (D2O, 270 MHz), d 10.38 (s, 4H),
4
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8.91 (s, 4H), 5.10 (t, 4H), 3.56 (s, 18H), 2.49 (br quin, 4H), 0.77 (br t, 4H).
2.5. 4,4 0 -Bis(triethoxysilyl)-1,1 0 -biphenyl periodic mesoporous silica (BTEBp-PMO)
2.3. 2,7-Bis(3-trimethoxysilylpropyl) diazapyrinium diiodide PMO (BDAP-PMO) synthesis
To prepare BTEBp-PMO, 1.65 g of octadecyltrimethylammonium chloride (ODTMAC) was added to 7.5 mL of 6 M sodium hydroxide dissolved in 89.5 mL of deionized water. To this was added 1.8 g of 4,4 0 -bis(triethoxysilyl) biphenylylene, with stirring at RT. This solution was then sonicated and heated to 98 °C for 48 h. The white precipitate was suction filtered, washed with deionized water, and dried at 50 °C to obtain BTEBp-PMO. The removal of the template (ODTAMC) was accomplished by refluxing the as-synthesized BTEBp-PMO in a solution composed of 4 mL of 2 M HCl in 150 mL deionized water for 24 h. The template free BTEBp-PMO was filtered dried at 50 °C for 6 h.
The BDAP-PMO was synthesized following a modified procedure of the previously reported recipe for Dallas Amorphous Material (DAM-1) [86–89]. BDAP and tetramethyl orthosilicate (TMOS) (Aldrich) were used as the silica sources and vitamin E tocopheryl polyethylene glycol 1000 succinate (TPGS) as the template. The molar ratio of BDAP to TMOS was varied from 100% to 1.6%. In a typical synthesis, 0.3 g of vitamin E TPGS (Eastman Chemical) was dissolved in 16.0 mL of deionized H2O. To this mixture, 3.7 mL of 12 M hydrochloric acid was added and the solution was stirred for 30 min. While the solution was stirring, 0.85 g (5.6 mmol) of TMOS along with 0.15 g (0.19 mmol) BDAP was added. The solution (pH 0.2) was aged, with stirring, for 20 min at room temperature and then aged at 40 °C for 24 h followed by heating at 90 °C for 48 h. After cooling to room temperature the product was isolated by filtration and washed with deionized water. To remove the vitamin E TPGS, the as synthesized BDAP-PMO was refluxed in ethanol for 6 h. 2.4. N-(3-Trimethoxysilylpropyl)-2,7-diazapyrene iodide grafted mesoporous material synthesis (DAP-DAM-1) To prepare DAP-DAM-1, TMOS was co-condensed with 3-iodopropyltrimethoxy silane following the DAM-1 synthesis procedure to produce a hybrid mesoporous material with iodopropyl groups present in the pores. 2,7-Diazapyrene was then reacted with the iodopropyl groups to covalently bind it to the framework (Scheme 2). To synthesize the iodopropyl functionalized framework, 0.60 g vitamin E TPGS was dissolved in 32.0 mL of deionized water. To this mixture, 7.4 mL of 12 M hydrochloric acid was added and the solution was stirred for 30 min. While the solution was stirring, 1.70 g (11.2 mmol) of TMOS along with 0.11 g (0.40 mmol) 3-iodopropyltrimethoxy silane (Gelest, 99.9%) was added. The solution was aged, with stirring, for 20 min at room temperature and then aged at 40 °C for 24 h followed by heating at 90 °C for 48 h. After cooling to room temperature the product was isolated by filtration and washed with deionized w ater. To remove the vitamin E TPGS, the as-synthesized iodopropyl DAM-1 was refluxed in ethanol for 6 h (Si/ I = 16.8). Template extracted iodopropyl DAM-1 (0.50 g) was then added to an acetonitrile (200 mL) solution containing 0.04 g (0.18 mmol) 2,7-diazapyrene and refluxed for 72 h. The solution was then suction filtered and washed with warm acetonitrile. To ensure that all unreacted 2,7diazapyrene had been removed from the solids, the material was refluxed in fresh acetonitrile for 6 h and hot gravity filtered.
2.6. Instrumentation EDX analysis and scanning electron microscope images of samples coated with Pd/Au were collected with a Leo 1530VP field-emission scanning electron microscope. Transmission electron microscope images were obtained using a JEM-2100 TEM, operating at 200 kV. X-ray diffractograms were obtained using a Scintag XDS 2000 Xray diffractometer with Cu Ka radiation and a Rigaku Ultima III. Nitrogen adsorption isotherms were measured at 77 K on a Quantachrome Autosorb 1. All samples were outgassed at 100 °C and 106 Torr prior to measurements. A seven-point BET equation was used to calculate surface areas. Pore size distribution was calculated from the desorption data using the BJH method. Luminescence data were collected using a Perkin–Elmer LS 50B luminescence spectrometer. The solid samples were adhered to a coverslip with FluorolubeÒ. They were then treated with 100 lL of various solutions prior to collecting the luminescence data. NMR data were obtained with a JEOL Eclipse 270. Cyclic voltammograms were collected using a Bioanalytical Systems CV-50W. Samples for transmission electron microscopy (TEM) were prepared by suspending the particles in 99% isopropyl alcohol and ultrasonicating for 10 min. The suspensions were allowed to dry on carbon coated Cu grids. Images were collected using a JEM 2100 TEM operating at 200 kV. 3. Results and discussion 3.1. BDAP-PMO The 2,7-diazapyrene, prepared from 1,4,5,8-napthalenetetracarboxylic dianhydride following published procedures [83,84], was then diquarternized with 3-iodopropyltrimethoxysilane by refluxing in acetonitrile (Scheme 1) to form BDAP. The BDAP was not photoluminescent as a solid, but was readily soluble in water and emitted blue light with a kmax at 416 nm.
K.J. Balkus Jr. et al. / Microporous and Mesoporous Materials 112 (2008) 1–13
To form a redox-active hybrid mesoporous material, BDAP was used as the primary silica source in a modified version of the recipe for DAM-1 [86–89]. The mol% of BDAP present in the synthesis was varied from 100 to 1.6 using TMOS as the secondary silica source. Table 1 summarizes the results of the materials formed. Similar to observations made by Alvaro et al. for the synthesis of viologen PMO [35]; BDAP did not form an ordered mesoporous material at mole percents greater than 5 under the conditions studied. The molar ratios shown in Table 1 are those of the synthesis gel. The percentage of BDAP present in the materials after synthesis was determined using EDAX based on iodine content. Material A had a 2.3 mol% BDAP while B, C, and D were 1.5, 1.0, and 0.7, respectively. The loading was determined by EDAX from Si/I ratios of 21.6, 33.4, 48.7, and 74.2 for materials A, B, C, and D, respectively, based on 2 I per BDAP. The BDAP-PMO powders were characterized by powder X-ray diffraction. Fig. 1A–D shows the X-ray patterns of the luminescent mesoporous materials formed from BDAP (see Table 1). Materials A–D all show similar patterns with one large, broad reflection located at 1.1° 2h. For the hybrid mesoporous materials to be useful as adsorbents or sensors the template must be removed without collapsing the pores or damaging the organic materials present in the mesoporous material. To accomplish this, the vitamin E TPGS was exhaustively extracted by refluxing in ethanol for several hours. Fig. 1A, which corresponds to Table 1 Properties of mesoporous materials synthesized with varying amounts of BDAP BDAP theory (mol%)
BDAP actual (mol%)
Pore size ˚) (A
Surface area (m2/ g)
Fluorescent
A B C D
5.0 3.4 2.2 1.6
2.3 1.5 1.0 0.7
47 48 45 43
501 663 568 577
Yes Yes Yes Yes
Intensity
Sample
(a)
(A)
(b)
(B)
(c)
(d)
0.8
1.8
material A from Table 1, shows that the peak at 1.1° 2h is present after extraction, which indicates that the mesoporosity is retained after template extraction. Also, the transmission electron micrograph of BDAP-PMO after extraction (Fig. 2) clearly shows that the mesoporosity is ˚ pore size retained. The TEM is consistent with the 47 A and well ordered channels running in parallel. This is true for materials B, C, and D (Fig. 1B–D, respectively) as well. Materials B and D also show low intensity reflections (Fig. 1B and D) located 2.2° 2h. The 1.1° (d spac˚ ) and 2.2° (d spacing = 40.1 A ˚ ) 2h peaks index ing = 80.3 A as the (1 0 0) and (2 0 0) peaks associated with 2-D hexagonal type mesoporous materials. The scanning electron micrographs (SEM) of these materials are shown in Fig. 3. Materials A, B, C, and D all show a spherical morphology in the range of 2.0 to 5.5 lm. The pure DAM-1 prepared under these conditions (not shown) also forms in the spherical morphology with a similar size distribution. The pore sizes and surface areas of the BDAP-PMO materials were determined by N2 adsorption. Table 1 shows that in all cases the PMOs containing BDAP have ˚ ) prepared under similar larger pores than DAM-1 (38 A conditions. There may be a slight trend towards larger pores with an increase in BDAP loading. There is also a modest increase in wall thickness in BDAP-PMOs (35, ˚ for A, B, C, and D, respectively) especially 36, 33, and 31 A ˚ for DAM-1. The wall thickness of compared with 26 A PMOs tend to be greater than pure silica materials [95] which is consistent with the trend in wall thickness as the BDAP loading increases. Liang et al. have shown that aromatic compounds such as 1,3,5-triisopropylbenzene can intercalate into the hydrophobic area of the micelle during synthesis and increase the size of the micelle consequently increasing the pore size of the resulting mesoporous
(C)
(D)
2.8
3.8
4.8
5.8
6.8
7.8
Degrees 2
Fig. 1. Powder XRD patterns of BDAP-PMO with (A) 4.4 mol%, (B) 2.9 mol%, (C) 2.0 mol%, and (D) 1.3 mol% BDAP.
5
Fig. 2. Transmission electron micrograph of BDAP-PMO sample A.
6
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Fig. 3. Scanning electron micrographs of BDAP-PMO samples A, B, C and D.
material [96]. The BDAP may also be associated with the micelle, which causes an increase in pore size relative to DAM-1. Table 1 also shows that the surface areas for the BDAP-PMOs are less than for DAM-1 (998 m2/g) but there is no clear trend with loading. The known redox activity of diazapyrinium derivatives allows for an electrochemical means to determine if the BDAP is incorporated into the framework. Fig. 4A shows the cyclic voltammogram of free BDAP dissolved in 0.1 M tetraethylammonium tetrafluoroborate in acetonitrile using a platinum working electrode, silver/silver chloride reference electrode, and a platinum auxiliary electrode. This voltammogram is similar to those reported for N,N 0 dimethyl-2,7-diazapyrinium [90,91] with a reduction peak at (0.50 V) and a small, broad oxidation peak at 0.46 V. Fig. 4B shows the cyclic voltammogram of sample A (2.3 mol% BDAP-PMO) recorded under the same conditions. To obtain the cyclic voltammogram, the BDAP-PMO was adhered to the glassy carbon electrode surface using ParaloidTM B-72 (Aldrich). The cyclic voltammogram of the BDAP-PMO shows a reduction peak at 0.50 V and a smaller, broader oxidation at 0.40 V. These results indicate that the BDAP-PMO is redox active and thereby providing direct evidence for the incorporation of the BDAP into the material framework. 3.2. 2,7-Diazapyrene grafted mesoporous material (DAPDAM-1) In an effort to verfy that the diazapyrene in BDAP-PMO is indeed bridging in the pore wall, we prepared DAM-1
with DAP dangling from the pore wall. This fluorescent material was prepared by reacting 2,7-diazapyrene with an iodopropyl trimethoxysilane functionalized mesoporous DAM-1 material. The functionalized DAM-1 was prepared by co-condensing TMOS with iodopropyl trimethoxysilane, the powder XRD for this material is shown in Fig. 5A. Table 2 (material E) shows the BET surface area and BJH ˚, (desorption branch) pore size was 577 m2/g and 34 A respectively. Organosilanes can interact with the surfactant micelles and reduce their size [97], which could account for the smaller size of the pores as compared to DAM-1. When added to 2,7-diazapyrene in refluxing acetonitrile, the 2,7diazapyrene was mono-quaternized with the dangling iodopropyl functional groups present in the pores. This formed a fluorescent mesoporous material (DAP-DAM-1) whose powder XRD is shown in Fig. 5B. The BET surface area and BJH (desorption branch) pore size for material F ˚ , respectively. The reduc(Table 2) was 545 m2/g and 33 A tion in surface area from 577 to 545 m2/g (materials E and F, respectively) is due to the inclusion of 2,7-diazapyrene in the pores. The loading of mono quaternized 2,7diazapyrene was determined by EDAX. The Si/I ratio for material F was determined to be 16.8, which corresponds to a loading of 3 mol% 2,7-diazapyrene. Fig. 6A and B shows the SEM images of materials E and F, respectively. As can be seen, the spherical morphology of these materials is similar to that of both DAM-1 and BDAP-PMO. There was no change in morphology after the grafting of 2,7-diazapyrene. To ensure that the 2,7-diazapyrene was covalently bound to the mesoporous material and not just adsorbed,
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7
Table 2 Properties of (E) iodopropyl functionalized DAM-1, (F) DAP-DAM-1, (G) DAM-1, and (H) DAM-1 after adsorption of 2,7-diazapyrene Sample
DAP (mol%)a
Pore size ˚) (A
Surface area (m2/ g)
Fluorescent
E F G H
0 3 0 0
34 33 37.8 37.5
577 545 998 850
No Yes No No
a
Fig. 4. Cyclic voltammogram of (A) BDAP in acetonitrile and (B) BDAPPMO.
(A)
Intensity
(B)
functionalized material, except 100% TMOS was used with no iodopropyl trimethoxysilane (material G, Table 2). The powder XRD of material G is shown in Fig. 5C. This sample exhibited a surface area of 998 m2/g and a BJH pore ˚ . Material G was then treated to size distribution of 38 A the same procedure as E by stirring with 2,7-diazapyrene in refluxing acetonitrile (material H). After adsorption of 2,7 diazapyrene there was no change in the XRD pattern (Fig. 5D) while the BET surface area (850 m2/g) decreased ˚ ). Fig. 6C and D slightly with no change in pore size (38 A shows the SEM images of samples G and H, respectively. As can be seen, the morphology of the materials is similar to that of DAM-1, DAP-DAM-1, and the BDAP-PMOs. Samples F and H were treated to similar conditions to determine if the 2,7-diazapyrene was tightly bound or merely adsorbed in the materials. Both materials were, separately stirred in refluxing acetonitrile for 6 h to extract any adsorbed 2,7-diazapyrene. Then they were filtered and placed at 90 °C in a vacuum oven for 16 h to further remove any unbound 2,7-diazapyrene by sublimation. After these treatments, sample F (DAP-DAM-1) was luminescent with a kmax = 424 nm, while sample H showed no luminescence. This confirms the fact that any 2,7-diazapyrene not covalently bound to the pore surface was easily removed. The kmax of DAPDAM-1, which contains mono-cationic 2,7-diazapyrene, was slightly red-shifted relative to BDAP-PMO (kmax = 424 nm versus 420 nm, respectively). This shift in fluorescence due to quaternization is similar to the shift seen in N-methyl-2,7-diazapyrene versus N,N 0 -dimethyl-2,7-diazapyrene [98]. These differences between DAP-DAM-1 and BDAP-PMO seem to support our contention that BDAP is bridging in BDAPPMO.
(C)
3.3. BTEBp periodic mesporous organosilica
(D) 0.5
In the as-synthesized DAM-1.
1
1.5
2.0
2.5
3
3.5
4.0
4.5
5.0
Degrees 2θ
Fig. 5. Powder XRD patterns of (A) iodopropyl functionalized mesoporous material, (B) DAP-DAM-1, (C) DAM-1, and (D) DAM-1 after 2,7diazapyrene adsorption.
a control experiment was performed. A mesoporous material was prepared in the same manner as the iodopropyl
For further comparison with a well-established PMO, we prepared the biphenyl derivative (MTEBp-PMO) [39]. Fig. 7 shows the powder XRD of the extracted BTEBp with the mesopore peak located at 1.9° 2h. The reflections located at 7.4, 14.9, 22.8, 30.8, 37.4° 2h are attributed to long range periodicity due to p stacking. The broad peak at 20.6° 2h is due to Si–O–Si [39]. BTEBp-PMO exhibits a BET surface area of 848 m2/g and a BJH pore size of ˚ . The SEM images (Fig. 8) of this material reveal 24 A
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Fig. 6. Scanning electron micrographs of (A) iodopropyl functionalized mesoporous material, (B) DAP-DAM-1, (C) DAM-1, and (D) DAM-1 after 2,7diazapyrene adsorption.
nanoparticles in the range of 20 to 100 nm. The BTEBp-PMO is also fluorescent (kmax = 375 nm). 3.4. Fluorescence The results discussed above are consistent with the BDAP being incorporated into the mesoporous silica framework. The fact that an ordered phase is not obtained with BDAP loadings greater than 3% is in agreement with incorporation of the bridging silane into the pore walls. To further show that BDAP is not simply self-condensed as an impurity phase but rather incorporated into the pore walls, the as-synthesized BDAP-PMO (pores filled with template)
(
)
Si
Intensity
Si
Fig. 8. Scanning electron micrographs of BTEBp-PMO. 1
6
11
16
21
26
31
36
41
Degrees 2θ
Fig. 7. Powder XRD pattern of BTEBp-PMO.
46
along with the template extracted BDAP-PMO were treated with 120 lM nitrobenzene to quench fluorescence.
K.J. Balkus Jr. et al. / Microporous and Mesoporous Materials 112 (2008) 1–13
The as-synthesized sample showed a marginal reduction in luminescent intensity (<1%) while the sample with the template removed showed a large reduction (81%). With the pores filled with template, nitrobenzene’s access to the BDAP is limited to a few sites at the particle surface. This is consistent with the BDAP being located in the mesopores. Fig. 9A–C shows an optical image of the three materials BDAP-PMO, DAP-DAM-1, BTEBp-PMO excited at 266 nm, respectively. BDAP-PMO is a yellow powder but, as seen in Fig. 9A, when excited at 266 nm, it emits blue light with a kmax = 420 nm. DAP-DAM-1 (Fig. 9B) is a white powder with a slight yellow tint and when excited emits blue fluorescence with a kmax = 424. BTEBp-PMO
9
(Fig. 9C) is also a white powder and when excited also emits blue fluorescence with a kmax = 377 nm. The diazapyrene will interact in a p fashion with aromatic ring compounds. Therefore, fluorescence quenching by nitrobenzene was used a probe of DAP inclusion as described above. This is based on the electron withdrawing properties of nitrated aromatics to form charge transfer complexes with fluorophors, which leads to fluorescence quenching [59,92,93]. Since nitrobenzene is a common taggant for nitrated explosives, the DAP containing PMOs may prove to be effective optical sensors for explosives. The quenching ability of four (Fig. 10) common nitrated explosives taggants was determined for BDAP-PMO, DAP-DAM-1, and BTEBp-PMO. The four taggants used in this study were 2,3-dimethyl-2,3-dinitrobutane [45], nitrobenzene [58], nitromethane [59], and o-nitrotoluene [52] which were diluted with acetonitrile to control concentration. As shown in Fig. 11A, when treated with 30, 60, and 120 lM solutions of o-nitrotoluene, the luminescence of BDAP-PMO decreases. When treated with 30 lM onitrotoluene the luminescent intensity decreased by 10%, with 60 lM it decreased by 22%, and with 120 lM o-nitrotoluene the luminescence was quenched by 34%. Similar results were obtained when nitrobenzene was used as the quencher. For 30, 60, and 120 lM concentrations of nitrobenzene the BDAP-PMO was quenched by 29%, 50%, and 81%, respectively (Fig. 11B). The nitro-alkanes DMNB (Fig. 11C) showed less quenching (12%, 17%, and 25% for 30, 60, and 120 lM, respectively) but were still detectable. However, nitromethane (Fig. 11D) did not show any appreciable change (3%, 5%, and 7%, respectively). Calculated using the lowest concentration of analytes d etected, the detection limits for BDAP-PMO would be 3.7, 4.1, and 5.3 ppm for nitrobenzene, o-nitrotoluene, and DMNB, respectively.
O N
+
O
O N
+
+
O
N O
2,3-Dimethyl-2,3-dinitrobutanee
NO2
Fig. 9. Digital images of (A) BDAP-PMO, (B) DAP-DAM-1, and (C) BTEBp-PMO excited at 266 nm.
Nitrobenzene
O Nitromethane
NO2
O-nitrotoluene
Fig. 10. Molecular structure of nitrated explosives taggants.
10
K.J. Balkus Jr. et al. / Microporous and Mesoporous Materials 112 (2008) 1–13
NO NO 2
NO2
2
BDAP 30 µM
NO2
60 µM 120 µM
NO NO 2 2
+
Intensity
Intensity
O
400
410
420
430 440 450 Wavelength (nm)
460
400
470
N
410
430
420
440
450
O
460
470
Wavelength (nm)
Fig. 11. Quenching of photoluminescence of BDAP-PMO with (A) o-nitrotoluene, (B) nitrobenzene, (C) DMNB, and (D) nitromethane.
Fig. 12A–D shows the emission spectra for DAP-DAM1 when treated with o-nitrotoluene, nitrobenzene, DMNB, and nitromethane, respectively. DAP-DAM-1 showed a decrease in emission intensity (Fig. 12A) of 19%, 36%, and 44% for o-nitrotoluene concentrations of 30, 60, and 120 lM, respectively. This is slightly better than for BDAP-PMO. In the case of nitrobenzene (Fig. 12B), the DAP-DAM-1 0 s luminescence decreased by 29%, 68%, and 81% for 30, 60, and 120 lM concentrations. The nitro-alkanes barely quenched the luminescence of DAP-
DAM-1 and even less than for BDAP-PMO. For the same concentrations, DMNB reduced the intensity by 3%, 8%, and 15% (Fig. 12C) while the nitromethane reduced it by 1%, 3%, and 3% (Fig. 12D), respectively. Fig. 13 shows the emission spectra of BTEBp-PMO which decrease in intensity when treated with 30, 60, and 120 lM solutions of the nitrated taggants. The 30 lM onitrotoluene solution decreased the intensity by 37%, while 60 lM decreased the intensity by 53%, and 120 lM o-nitrotoluene by 69%. Similar results were obtained with nitro-
NO2
NO NO2
2
DAPDAM-1 30 M
NO2
60 M 120 M
NO2 NO2
400
Intensity
Intensity
O
410
420
430
440
450
Wavelength (nm)
460
470
480
400
+
N O
410
420
430
440
450
460
470
480
Wavelength (nm)
Fig. 12. Quenching of photoluminescence of DAP-DAM-1 with (A) o-nitrotoluene, (B) nitrobenzene, (C) DMNB, and (D) nitromethane.
K.J. Balkus Jr. et al. / Microporous and Mesoporous Materials 112 (2008) 1–13
NO NO 2 2
11
NO2
BTEBp 30 M
NO2
60 M 120 M
NO NO 2 2
O Intensity
Intensity 300
+
320
340
360
380
400
420
440
460
480
300
N
320
340
Wavelength (nm)
360
380
400
420
440
O
460
480
Wavelength (nm)
Fig. 13. Quenching of photoluminescence of BTEBp-PMO with (A) o-nitrotoluene, (B) nitrobenzene, (C) DMNB, and (D) nitromethane.
benzene where 30, 60, and 120 lM concentrations quenched the fluorescence by 13%, 25%, and 67%, respectively (Fig. 13B). Interestingly, both DMNB (Fig. 13C) (4%, 17%, and 26% for 30, 60, and 120 lM, respectively) and nitromethane (Fig. 13D) (7%, 15%, and 21%, respectively) partially quenched the emission from BTEBp-PMO. The fluorescence spectra shown in Figs. 11–13 suggest that the aromatic nitrotoluene, and nitrobenzene are easily detected by all three materials. DMNB and nitromethane quench the fluorescence of BTEBp-PMO by a modest amount. There are other explosive sensors based on fluorescence quenching of fluorophors such as pyrene [59]. Although the detection limits for these techniques are much lower than BDAP-PMO (71 ng/mL versus 5300 ng/mL, respectively) these systems require the use of liquid chromatography and laser induced fluorescence which would be difficult to incorporate into a portable device. Conversely, BDAP-PMO, DAP-DAM-1, and BTEBp-PMO are solid based systems which could easily be configured in a solid state device. 4. Conclusions A novel redox active mesoporous material was synthesized by co-condensation of a BDAP based disilane with TMOS at concentrations as high as 2.3 mol%. The characterization of BDAP-PMO is consistent with the pore wall location of the diazapyrene, especially when compared to the DAP-DAM-1 with surface grafted diazapyrene. BDAP-PMO, DAP-DAM-1 and BTEBp-PMO were found to be highly fluorescent. These materials were shown to be effective optical sensors for nitrated explosive taggants based on the quenching of the PMO emission. As high as
81% quenching upon exposure to nitrobenzene was observed for BDAP-PMO and DAP-DAM-1. Further studies of the redox and optical properties are in progress but these results suggest that these PMOs may effectively interact with various guest species in the pores. Acknowledgement We thank the Robert A. Welch Foundation for support of this research. References [1] A. Vinyu, K. Z Hossain, K. Ariga, J. Nanosci. Nanotechnol. 5 (2005) 347–371. [2] B. Hatton, K. Landskron, W. Whitnall, D. Perovic, G.A. Ozin, Acc. Chem. Res. 38 (2005) 305–312. [3] K.Z. Hossain, L. Mersier, Adv. Mater. 14 (2002) 1053. [4] M.P. Kapoor, S. Inagaki, S. Ikeda, K. Kakiuchi, M. Suda, T. Shimada, J. Am. Chem. Soc. 127 (2005) 8174–8178. [5] S. Guan, S. Inagaki, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 122 (2000) 5660. [6] M.P. Kapoor, Q. Yang, Y. Goto, S. Inagaki, Chem. Lett. 32 (2003) 914. [7] W.J. Hunks, G.A. Ozin, Adv. Funct. Mater. 15 (2005) 259–266. [8] Q. Yang, M.P. Kapoor, S. Inagaki, J. Am. Chem. Soc. 124 (2002) 9694. [9] A. Corma, D. Das, H. Garcı´a, A. Leyva, J. Catal. 229 (2005) 322–331. [10] M. Kuroki, T. Asefa, W. Whitnal, M. Kruk, C. Yoshina-Ishii, M. Jaroniec, G.A. Ozin, J. Am. Chem. Soc. 124 (2002) 13886. [11] R.J.P. Corriu, E. Lancelle-Beltran, A. Mehdi, C. Reye’, S. Brande‘s, R. Guilard, Chem. Mater. 15 (2003) 3152–3160. [12] Z. Wang, J.M. Heising, A. Clearfield, J. Am. Chem. Soc. 125 (2003) 10375. [13] Q. Yang, M.P. Kapoor, N. Shirokura, M. Ohashi, S. Inagaki, J.N. Kondo, K. Domen, J. Mater. Chem. 15 (2005) 666–673.
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