A novel method for preparation of 8-hydroxyquinoline functionalized mesoporous silica: Aluminum complexes and photoluminescence studies

Applied Surface Science 257 (2011) 4912–4918

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A novel method for preparation of 8-hydroxyquinoline functionalized mesoporous silica: Aluminum complexes and photoluminescence studies Alireza Badiei a,∗ , Hassan Goldooz b , Ghodsi Mohammadi Ziarani c a b c

School of Chemistry, College of Science, University of Tehran, P.O. Box 14155-6455, Tehran, Iran Department of Chemistry, Tarbiat Modares University, Tehran, Iran Department of Chemistry, Faculty of Science, Alzahra University, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 17 July 2010 Accepted 28 December 2010 Available online 4 January 2011 Keywords: Mesoporous silica SBA-15 Tris(8-hydroxyquinoline)aluminum Sulfonamide Emission spectra

a b s t r a c t 8-Hydroxyquinoline (8-HQ) was attached to mesoporous silica by sulfonamide bond formation between 8-hydroxyquinoline-5-sulfonyl chloride (8-HQ-SO2 Cl) and aminopropyl functionalized SBA-15 (designated as SBA-SPS-Q) and then aluminum complexes of 8-HQ was covalently bonded to SBA-SPS-Q using coordinating ability of grafted 8-HQ.The prepared materials were characterized by powder X-ray diffraction (XRD), nitrogen adsorption–desorption, Fourier transform infrared (FT-IR), thermal analysis (TGA–DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM), elemental analysis and fluorescence spectra. The environmental effects on the emission spectra of grafted 8-HQ and its complexes were studied and discussed in details. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The functionalized mesoporous materials have attracted vast interest due to their potential applications such as catalysts [1,2], adsorption processes [3], micro extraction [4], separation [5] and sensing [6,7]. The incorporation of organic molecules into the channels of mesoporous silicas has attracted much attention in recent years because of their applications to prepare new functional materials [8–10]. One of the interesting molecules for this purpose is 8-hydroxyquinoline (8-HQ) and its derivatives due to their chelating ability towards various metal ions and high luminescence efficiency of resulting metal complexes [11–16]. Among the 8hydroxyquinoline complexes, Tris(8-hydroxyquinoline)aluminum (AlQ3 ) has attracted great interest in the development of luminescence and electroluminescent materials due to its high thermal stability and electroluminescent properties for the construction of reliable organic light-emitting devices (OLED) [17,18]. 8-HQ ligand can be incorporated into mesoporous materials, either by noncovalent (physical interaction) or by covalent (chemical bonding) approaches [11–14]. In covalent approach, the 8-HQ ligand can be firmly bonded to inorganic solids via homogeneous (one step) [11,12] or heterogeneous (two or more steps) procedures [13,14]. A heterogeneous method for binding 8-HQ on mesoporous silica in two steps is a Schiff base reaction between the aldehyde

∗ Corresponding author. Tel.: +98 21 61112614; fax: +98 21 66405141. E-mail address: [email protected] (A. Badiei). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.12.146

derivative of 8-HQ and the amino functionalized mesopore [11]. Regarding the susceptibility of this linkage to hydrolysis in acidic condition, the imine linkage can be reduced to amine by a reducing agent such as NaBH4 to improve the hydrolytic stability [13]. The amine linkage formed in this step should be significantly more stable to acid or base hydrolysis than the imine linkage, and should greatly reduce the cleavage of the grafted ligands from the solid support. In this work, we present a new method for covalently linking 8-HQ in nanochannels of SBA-15 in two steps: attachment of aminopropyl on SBA-15, and then covalent linkage of 8-HQ via stable sulfonamide bond. Furthermore, we investigate the interaction of aluminum ion with this fluorophore material using fluorescence spectroscopy. NMR, XRD, N2 adsorption–desorption, FT-IR, TGA–DTA, elemental analysis, SEM and TEM are employed to characterize the obtained materials.

2. Experimental 2.1. Materials Pluronic P123 with composition EO20 PO70 EO20 and average molecular weight of 5800 was purchased from Aldrich. Tetraethyl orthosilicate (TEOS), the silica source, 3-aminopropyltriethoxysilane (APTES), 8-HQ, 8-hydroxyquinoline-5-sulfonic acid (8-HQS) and chlorosulfonic acid were purchased from Merck. All of the other reagents and solvents were of analytical reagent grade and used as received.

A. Badiei et al. / Applied Surface Science 257 (2011) 4912–4918

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SO2Cl

SO3H chlorosulfonic acid stirring overnight

N

N OH

OH

(8-HQS)

(8-HQ-SO2Cl)

+ OH OH OH

Toluene

SBA-15

+ Et O Et O Si Et O

O O Si O

Reflux

NH2

(SBA-NH2)

NH2 (APTES)

O O Si O

NH Et3N

SO2

+ CH2Cl2

Reflux

(SBA-SPS-Q)

N OH

Scheme 1. Synthesis route of SBA-SPS-Q.

2.2. Synthesis and aminopropyl functionalization of mesoporous silica SBA-15 SBA-15 type mesoporous silica was prepared according to the literature with slight modification [19]. The aminopropyl functionalization of SBA-15 was carried out using APTES as the silylation reagent. In a typical synthesis, about 1.0 g of calcined SBA-15 was added to 100 ml of dry toluene in a 250 ml flask, and the flask was flushed with nitrogen for 20 min. Then 0.54 ml of APTES was added and refluxed. After 4 h, the solid was filtrated, washed with toluene and ethanol, and dried in air. The received solid was denoted as SBA-NH2 .

Fig. 1. XRD patterns for (a) SBA-15, (b) SBA-NH2 and (c) SBA-SPS-Q.

tate was filtered off and dried to obtain desired sulfonyl chloride derivative. 1 H NMR (DMSO, 500 MHz): ı H 7.37 (q, 1 H, arom), 8.1 (q, 1H, arom), 8.14 (m, 1H, arom), 9.09 (t, 1H, arom), 9.8 (q, 1H, arom). IR data: max (KBr pellets)/cm−1 3415 (br, CH, arom), 3085 (br, CH, arom), 1620, 1600, 1588, 1552, 1499 (m, C C and C N), 1367 (s, asym SO2 str.), 1194 (s, sym SO2 str.).

2.3. Synthesis of 8-hydroxyquinoline-5-sulfonylchloride (8-HQ-SO2 Cl)

2.4. Synthesis of 8-HQ functionalized SBA-15

In a 100-ml round-bottom flask fitted with a magnetic stirrer were placed 10 mmol of 8-HQS and 8 ml chlorosulfonic acid and stirred at ambient temperature overnight. Then, the reaction mixture was poured into ice water; the formed yellow precipi-

The synthesis route of SBA-SPS-Q is depicted in Scheme 1. In a typical synthesis, 1 g of SBA-NH2 was dispersed in 60 ml of

O 2S

O2 S

OH AlCl3

NH

NH4OH

O

HN Ethanol

(Q=8-HQ or 8-HQS) NH4OH

Reflux N

Si

Si

Ethanol OO O

AlQ2

N

N

OO O

OH

Scheme 2. Synthesis route of SBA-SPS-AlQ3 and SBA-SPS-Al(QS)3 .

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Fig. 2. SEM (a) and TEM (b) image of compound SBA-SPS-Q.

dichloromethane, and then an excess amount of 8-HQ-SO2 Cl and triethylamine were added to the above solution. The reaction mixture was refluxed for 3 h. Then, it was filtered and washed with dichloromethane and ethanol several times. The final product was obtained as a light yellow powder and designated as SBA-SPS-Q. 2.5. Synthesis of aluminum complex-containing of 8-HQ functionalized SBA-15 An appropriate amount of AlCl3 and NH4 OH was added to an ethanolic suspension of SBA-SPS-Q and mixture was stirred for 24 h at room temperature, then resulting precipitate was filtered, washed carefully with ethanol several times and air-dried (SBASPS-QAl). In the next step the required amount of NH4 OH and 8-HQ or 8-HQS was added to an ethanolic suspension of SBA-SPS-QAl and mixture filtered after two hours stirring in room temperature, then resulting precipitate washed with ethanol several times and air dried. The final yellow powder was designated as SBASPS-AlQ3 (Scheme 2). The other aluminum complex-containing of functionalized mesoporous material denoted as SBA-SPS-Al(QS)3 was prepared in the same manner except that 8-HQS was used instead of 8-HQ (Scheme 2). 3. Instruments and spectroscopic measurements The 1 H and 13 C NMR experiments were operated on a set of Bruker Avance DRS 500 spectrometer at 298 K. Low-angle X-ray diffraction (XRD) patterns were recorded with a Philips X Pert MPD diffractometer using Cu K␣ radiation (40 kV, 40 mA) at a step width of 0.02◦ . N2 adsorption–desorption isotherms were measured using a BELSORP mini-II. FT-IR spectra were recorded within a 4000–400 cm−1 region on a Bruker Vector 22 infrared spectrophotometer. SEM analysis was performed on a Philips XL30 field-emission scanning electron microscope operated at 16 kV

Fig. 3. N2 adsorption–desorption isotherms of (a) SBA-15, (b) SBA-NH2 , (c) SBASPS-Q, and (d) SBA-SPS-AlQ3 (inset: BJH pore size distribution curves of (a) SBA-15, (b) SBA-NH2 , (c) SBA-SPS-Q, and (d) SBA-SPS-AlQ3 ).

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Fig. 4. FT-IR spectra of (a) SBA-NH2 , (b) SBA-SPS-Q, and (c) SBA-SPS-AlQ3 .

while TEM was carried out on a Tecnai G2 F30 at 300 kV. The emission spectra of different samples were recorded by Perkin Elmer LS-50 and a dilute suspension of compounds in ethanol–water solvent (0.2 g/L) was used for fluorescence investigation. Elemental analyses of the silicates containing organic material were carried out in a Rapid (Germany) elemental analyzer. The TGA and DTA thermograms were obtained from a PL-Thermal science PL-STA 1500 instrument. 4. Results and discussion Fig. 1 shows the low angle XRD patterns of SBA-15 and the functionalized SBA-15 with amine and 8-HQ groups. All the samples have a single intensive reflection at 2 angle around 0.85◦ similar to the typical SBA-15 materials, that is generally attributed to the long-range periodic [19]. For the SBA-15 material, two additional peaks corresponding to the higher ordering (1 1 0) and (2 0 0) reflections are also observed, which is associated with a two-dimensional hexagonal (p6mm) structure. However, in the case of functionalized SBA-15 materials the peak (1 0 0) intensity decreases after immobilizations due to the difference in the scattering contrast of the pores and the walls, and to the erratic coating of organic groups on the nanochannels. Fig. 2 illustrates the SEM and TEM images of SBA-SPS-Q. The SEM image (Fig. 2a) shows uniform particles about 1 ␮m and the same morphology was observed for SBA-15, indicating that the morphology of solid was saved without change during the surface modifications. On the other hand, the TEM image (Fig. 2b) reveals the parallel channels, which resemble the pores configuration of SBA-15. This indicates that the pore of SBA-SPS-Q was not collapsed during two steps reactions and this is quite in agreement with the XRD results.

Fig. 5. TGA–DTA analysis of (a) SBA-NH2 , (b) SBA-SPS-Q, and (c) SBA-SPS AlQ3 .

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A. Badiei et al. / Applied Surface Science 257 (2011) 4912–4918 Table 1 Textural parameters of prepared compounds.a Sample

SBET (m2 g−1 )

V (cm3 g−1 )

DBJH (nm)

SBA-15 SBA-NH2 SBA-SPS-Q SBA-SPS-AlQ3

464 129 79 64

0.5578 0.3059 0.1885 0.1432

5.4 5.3 5.2 5.1

a SBET is the BET surface area; V is the total pore volume; DBJH is the average pore diameter calculated using BJH method.

Fig. 6. Emission spectra excited at 377 nm of (a) SBA-SPS-Q, (b) SBA-SPS-QAl, (c) SBA-SPS-AlQ3 , (d) AlQ3 , and (e) SBA-SPS-Al(QS)3 .

The textural properties of the samples were evaluated from the nitrogen adsorption–desorption isotherms (Fig. 3) and the respective specific surface areas (BET method), pore diameters (BJH method) and total pore volumes are given in Table 1. The isotherms show a type-IV isotherm with an obvious H1-type hysteresis loop that is representative for the mesoporous cylindrical or rod-like pores. These results clearly indicate that the mesostructure is preserved during the surface modification. It is noteworthy that the specific surface area, total pore volume and average pore diameter of the samples decrease as the extent of organic lining increases, indicating that the major surface modification takes place on the nanochannels surface of SBA-15. In addition, textural properties of SBA-SPS-AlQ3 demonstrate that the formation of aluminum complex into the nanochannels of SBA-SPS-Q reduces the pore width from 5.4 to 5.1 nm but full accessibility to the modified pores is still retained. The incorporation of organic functional groups in the SBA-15 framework was confirmed by FT-IR spectra (Fig. 4). In the FT-IR spectra of SBA-NH2 , the doublets at 3410 and 3340 cm−1 can be assigned to the asymmetric and symmetric stretching vibrations of NH2 groups, while the doublets at 2945 and 2890 cm−1 are assigned to C–H stretching vibrations of the methylene groups and the strong peak around 1580 and 1650 cm−1 corresponds to N–H bending vibration of –NH2 and –NH3 + groups, which is overlapped by the bending vibration of adsorbed H2 O. In the case of SBA-SPSQ, the peaks at 1610, 1580, 1500, 1471 and 1387 cm−1 are related to the C N and C C ring skeletal vibrations, the band appearing at 1334 cm−1 can be ascribed to asymmetric vibration of O S O group and the other IR bands, appearing at 771 and 796 cm−1 , are

Fig. 7. Proposed interaction of grafted 8-HQ with the surface of functionalized mesoporous silica.

A. Badiei et al. / Applied Surface Science 257 (2011) 4912–4918 Table 2 Elemental analysis data of prepared compounds. Sample

C%

N%

S%

SBA-NH2 SBA-SPS-Q

4.4 6.1

1.7 2.5

– 1.7

assigned to the ring vibrations of HQ and C–O in-plane bending [20,21]. It should be noticed that the –NH2 and –NH3 + vibration bands were still observed after reaction of SBA-NH2 with 8-HQSO2 Cl. Therefore, this indicates that the amine conversion to the sulfonamide was not entirely accomplished, and a part of amine groups was protonated during the reaction. The chemical compositions of the organo-functional groups covalently linked into mesoporous silica materials were determined by elemental analyses (Table 2). After drying of SBA-NH2 under vacuum, the quantity of aminopropyl groups attached to it (L1 = 1.2 mmol/g) was calculated from the nitrogen percentage, as estimated by elemental analysis, using the Eq. (1): L1 =

N% × 10 nitrogen atomic weight

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in longer wavelengths (red shift) can be attributed to the complexation of Al3+ ions and deprotonated form of grafted 8-HQ ligand. There is no obvious difference between PL spectra of SBA-SPS-QAl and SBA-SPS-AlQ3 , except that emission peak of SBA-SPS-AlQ3 is a little broader and has higher intensity. 8-HQ is a fluorogenic ligand and shows a very low fluorescence quantum yield in many solvents in wide ranges of solvent acidity. On the other hand, when 8-HQ reacts with many metal ions, the formed complexes exhibit much higher fluorescence [22]. The poor fluorescence emission of 8-HQ has been assigned to a photoinduced tautomerization reaction followed by deexcitation of the tautomer which occurs mainly via a nonradiative pathway [22]. In aqueous solutions intrinsic intramolecular proton transfer between the two functions and the intermolecular proton transfers between each of the two functions of 8-HQ (–OH and N) and surrounding water molecules can lead to the ketonic tautomeric form of 8-HQ [22,23]. The presence of the intramolecular hydrogen bond in a five-membered ring in the 8-HQ is an important cause of the poor fluorescence of the molecule via intramolecular proton transfer between the two functions [22,24].

(1)

The C/N molar ratio of SBA-NH2 , calculated from the elemental analysis data, is 3 which confirms that aminopropyl groups are grafted into nanochannels. The amount of HQ (L2 = 0.54) in SBASPS-Q was calculated from the sulfur percentage using the Eq. (2): S% × 10 L2 = sulfor atomic weight

(2)

With comparison of L1 and L2 , it can be concluded that the reaction yield of SBA-NH2 with HQ-SO2 Cl is about 45%. The grafted amount of organic species determined by TGA and DTA (Fig. 5) analysis is in a good agreement with the organic content determined by elemental analysis. In the DTA curve of SBA-NH2 (Fig. 5a), the observed peak at 280 ◦ C is correspond to the decomposition of aminopropyl groups. In the case of SBA-SPS-Q and SBA-SPSAlQ3 , The broad peak centered about 500 ◦ C in the DTA curve is probably attributed to the decomposition of the grafted HQ and the two neighbor peaks about 280 and 350 ◦ C can be ascribed to the non-reacted aminopropyl groups on the surface (which is also observed in the SBA-NH2 ) and to the decomposition of the some protonated amine groups (–NH3 + ). In view of these observations, that is quite in agreement with the FT-IR and elemental analysis results, it can be concluded that a half of amine groups was sheltered by the immobilized HQ groups due to the most probable interaction between O2 S–NH– and –NH2 (via acid–base reaction). In order to investigate environmental effects on the fluorescence properties of the grafted molecules, the emission spectra of prepared compounds were studied. Fig. 6 shows the photoluminescence (PL) spectra of all samples which contain HQ, and the maximum emission wavelengths of these materials are summarized in Table 3. In the emission spectra of SBA-SPS-Q (Fig. 6a), intense luminescence was remarkably observed around 477 nm, while SBA-SPS-QAl and SBA-SPS-AlQ3 exhibit a very intense fluorescence around 505 nm in the mixed solution (Fig. 6b and c). The more intense fluorescence of SBA-SPS-QAl and SBA-SPS-AlQ3 Table 3 Emission peak maxima of AlQ3 , SBA-SPS-Q, SBA-SPS-QAl, SBA-SPS-AlQ3 and SBASPS-Al(QS)3 . Compound

Emission (nm)

AlQ3 SBA-SPS-Q SBA-SPS-QAl SBA-SPS-AlQ3 SBA-SPS-Al(QS)3

510 477 505 505 483

N O H In the channels of nanoporous compound, interaction between the free –NH2 or SiOH groups of surface and the phenolic –OH groups (or heterocyclic nitrogen atoms) of 8-HQ can lead to disruption of the intramolecular hydrogen bonding of 8-HQ and cause a large fraction of grafted 8-HQ in the excited state to emit light before the quenching tautomerization process is achieved (Fig. 7). In 8-hydroxyquinoline metal complexes the lowest electronic transitions are ␲–␲* transitions in the quinolate rings, involving partial charge transfer from the phenoxide side to the pyridyl side [25] and photoexcitation of AlQ3 gives rise to fluorescence [26]. The emission of SBA-SPS-AlQ3 is similar to that of AlQ3 in the solution but as could be predicted displays slightly blue shifts. Considering nanometer pore size of SBA-15, it can be plausible that AlQ3 molecules should be presented like monomers in the channels of mesoporous silica; therefore, molecular interactions of AlQ3 on the surface must be improved, resulting in the blue shift in the emission spectra of grafted AlQ3 molecules [13]. In order to study the effect of coordinated ligand on the emission spectra of prepared compound, SBA-SPS-QAl was reacted with 8-HQS to prepare SBA-SPS-Al(QS)3 and as could be expected the emission spectra of this compound (Fig. 6e) exhibits blue shift in comparison with SBA-SPS-AlQ3 . This observed blue shift can be attributed to the presence of electronegative –SO3 group on the 5-position of coordinated 8-HQS ligand [27–29]. 5. Conclusion A new method for grafting of 8-HQ into the nanochannel of mesoporous silica was used and a fluorescent material with highly ordered 2D hexagonal nanostructure was obtained (SBA-SPS-Q). The results demonstrated that during this grafting process only half of the surface amine groups were converted to sulfonamide and grafted 8-HQ ligands in SBA-SPS-Q showed different fluorescence behaviors comparing to 8-HQ molecules in aqueous solution due to the environmental effects of surface in the nanochannels of SBA-15. The observed blue shift in the fluorescence spectra of SBA-SPS-Al(QS)3 indicates that emission spectra of grafted complexes on the SBA-SPS-Q can be easily tuned through the ligand exchange reaction and this is a promising method for the prepa-

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