Biphenyl-bridged periodic mesoporous organosilicas: Synthesis and in situ charge-transfer properties

Microporous and Mesoporous Materials 198 (2014) 92–100

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Biphenyl-bridged periodic mesoporous organosilicas: Synthesis and in situ charge-transfer properties Meng Gao, Shuhua Han ⇑, Xiaoyong Qiu, Hui Wang Key Lab of Colloid and Interface Chemistry Ministry of Education, Shandong University, Jinan 250100, PR China

a r t i c l e

i n f o

Article history: Received 23 February 2014 Received in revised form 3 June 2014 Accepted 15 July 2014 Available online 23 July 2014 Keywords: PMOs Cationic oligomer surfactant Biphenyl Charge-transfer complex

a b s t r a c t Using a novel cationic oligomer surfactant as the structure-directing agent, the fluorescent biphenylbridged periodic mesoporous organosilicas (BpUPMOs) were successfully synthesized through the cohydrolysis and cocondensation of 4,40 -bis(3-triethoxysilyl propylureido) biphenyl (BpUSi) and tetraethoxysilane (TEOS). The structure characterization of the samples was obtained by FT-IR spectra, small-angle X-ray scattering (SAXS), high resolution transmission electron microscopy (HRTEM), and nitrogen adsorption–desorption analyses. The results showed that ordered mesoporous structure was obtained below 40 mol% of the content of BpUSi. With the increase of the amount of BpUSi, the ordering degree of mesoporous structure of BpUPMOs decreased. Fluorescence emission spectra of BpUPMOX (X is a mole fraction of BpUSi, 0%, 10%, 20%, 30%, 40%, 50%, respectively) showed gradually enhanced and red-shifted bands from 357 to 372 nm as X was less than 30 mol%. However, with further increase of X value, the bands at 359 nm for BpUPMO40 and at 354 nm for BpUPMO50 exhibited blue-shift compared with that of the sample BpUPMO30 (372 nm). This fluorescence phenomenon induced by the amount of organosilicone in the PMOs has not been reported. Furthermore, Charge-transfer (CT) complex composed of BpUPMO20 and decylviologen was obtained, in which the biphenyl groups were as electron donors in the pore walls and the decylviologen molecules were as electron acceptors in the mesochannels of BpUPMO. UV–vis diffuse reflectance spectra and fluorescence spectra showed the existence of CT complex. Besides, the color of CT complex was over a wide range from the salmon pink to red brown. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Periodic mesoporous organosilicas (PMOs) are a new type of organic–inorganic hybrid materials in which the organic groups are located within the channel walls as bridges between Si centers since the discovery in 1999 [1–3]. PMOs possess unique properties because various functional organic groups are densely and covalently embedded within the silica walls to form the organic–inorganic hybrid materials in the molecular dimension. Therefore, the PMOs hybrid materials are potentially applied in various fields, such as catalysts [4], adsorbents [5,6], drug delivery [7], biochemical reactions [8], energy transfer and luminescent probes [9,10], and optical and fluorescent materials [11–13]. It is well known that biphenyl groups are a system of two interacting benzene rings that are linked together by a CAC bond [11,14]. PMOs can also be made from biphenyl-bridged ⇑ Corresponding author. Tel.: +86 531 88365450; fax: +86 531 88564464. E-mail address: [email protected] (S. Han). http://dx.doi.org/10.1016/j.micromeso.2014.07.021 1387-1811/Ó 2014 Elsevier Inc. All rights reserved.

organosilanes, which have drawn broad interests because the materials possess a controllable wall structure and a high intensity of fluorescence emission. Inagaiki et al. showed the selforganization of biphenylene-silica hybrid solids with crystal-like long-range-ordered structure using 4,40 -bis(triethoxysilyl) biphenyl as organosilica precursor in basic solution [15,16]. Wahab et al. reported the mechanical and optical properties of surfactant-mediated self-assembled biphenyl-bridged PMO films [17]. Recently, the use of energy and electron transfer has broadened potential applications of periodic mesoporous organosilicas, such as photocatalytic biphenyl-bridged PMOs [18], light-harvesting antenna properties [19], and enhanced photocatalytic CO2 reduction [20]. Based on special molecular structures, oligomeric surfactants have some particular characteristics, such as high surface activity, strong self-assembly ability, and special rheologic behavior [21,22]. Thus they can be fittingly used in fields of construction of porous materials [23]. In this work, a novel oligomeric cationic surfactant as the structure-directing agent (see Fig. 1a) was

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a N N

1-Bromotetradecane N

Br N

2,4,6-Tris(bromomethyl)mesitylene 6Br N

N

C14H29

N

N

C14H29

N

N C14H29

b O H2 N

NH2

+

O

C

N

Si O

O

N2 O

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O O Si O

O N H

N H

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N H

Si O

O

Fig. 1. Synthesis of (a) oligomeric cationic surfactant and (b) biphenyl-bridged organosilane precursors (BpUSi).

employed in the preparation of biphenyl-bridged PMO powders in order to enhance the ordering of the hybrid mesoporous structure. 4,40 -Bis(3-triethoxysilyl propylureido) biphenyl (BpUSi) as one of two Si sources was synthesized by the reaction of 4,40 -diaminobiphenyl and 3-(triethoxysilyl) propyl isocyanate, which contained ureido and biphenyl groups to dominate the self-assembly process through the hydrogen bonds and p–p stacking interaction (Fig. 1b). The effect of the molar ratio of BpUSi precursor to BpUPMOs (X value) on the optical properties of BpUPMOs has been discussed in detail. Charge-transfer (CT) donor and acceptor systems are widely used as organic semiconductors in field effect transistors (FETs), charge injection and transport materials in organic light-emitting diodes (OLEDs) and organic photovoltaic (OPV) cells [24–27]. The organization of different groups in PMOs is important for the fabrication of highly functional donor–acceptor structures [18,28,29]. These PMOs could not only provide uniform distribution of organic groups but also serve to protect the organic entities inside and allow their optical properties to be probed because of the chemical inertness and optical transparency of silica. However, the charge-transfer between pore channels and pore walls phenomena are less explored [30]. PMOs allow the acceptor and donor groups to be located in two spatially separated regions: in the pore walls and in the mesochannels, respectively, thus the chargetransfer behavior should be more controllable by the flexible molecular design. Based on the unique properties of the BpUPMOs materials which were successfully synthesized above, the charge-transfer BpUPMOs/decylviologe complex materials with biphenyl groups as the electron donors in the pore walls and decylviologe as the electron acceptors in the pore channels were also prepared.

2. Experimental 2.1. Chemicals and reagents 4,40 -diaminobipheny, 3-(triethoxysilyl) propyl isocyanate and N,N,N0 ,N0 -tetramethylethylenediamine were purchased from Shanghai Jingchun Reagent Company. Acetone and ethanol were obtained from Tianjin Fuyu Reagent Company. Tetraethoxy orthosilane (TEOS) and aqueous ammonia were bought from Tianjin Guangcheng Reagent Company.

2.2. Synthesis 2.2.1. Preparation of oligomeric cationic surfactant 23.24 g (200 mmol) N,N,N0 ,N0 -tetramethylethylenediamine was dissolved in 100 mL ethanol and refluxed for 10 min, and then 27.73 g (100 mmol) 1-bromotetradecane dissolved in 80 mL ethanol was added to the above solution. After refluxing for 2 days, the yellow waxy intermediate was obtained by reduction vaporization. The resulted intermediate was washed with n-hexane three times, followed by drying in vacuum. The yield is 61.60%. 23.61 g (60 mmol) intermediate was dissolved in 100 mL acetone and refluxed for 10 min, and then 6.14 g (15.38 mmol) 2,4,6-tris(bromomethyl)mesitylene acetone solution was added by constant pressure funnel. After 30 min white solid was observed. The mixture was then heated under reflux for 2 days. After cooling to room temperature, the resultant solid was collected and recrystallized for three times to provide oligomeric cationic surfactant (yield 83.02%). Spectral data for 1H NMR (DMSO, d/ppm) were as follows: 5.03 (d, 6H, ArCH2), 4.49

M. Gao et al. / Microporous and Mesoporous Materials 198 (2014) 92–100

(t, 6H, NCH2), 4.19 (t, 6H, CH2N), 3.44–3.10 (m, 36H, NCH3), 2.75 (d, 9H, ArCH3), 2.81–0.84 (C14) (Fig. S1a). 2.2.2. Preparation of 4,40 -bis(3-triethoxysilylpropylureido) biphenyl (BpUSi) 1.84 g (10 mmol) 4,40 -diaminobiphenyl and 5.90 g (24 mmol) 3-(triethoxysilyl) propyl isocyanate were dissolved in anhydrous ethanol (150 mL), and kept refluxing in the N2 atmosphere at 80 °C for 3 days. After cooling to room temperature, a grayish solid was obtained, and then the solid was filtered and washed with hexane for three times, dried in vacuo for 3 days. Spectral data for 1H NMR (DMSO, d/ppm) were as follows: 8.43 (s, 2H, PhNHCO), 7.47 (d, 4H, ArH), 7.42 (d, 4H, ArH), 6.16 (t, 2H, NHCH2), 3.75 (q, 12H, ethoxy CH2), 3.06 (q, 4H, CH2NH), 1.15 (t, 18H, ethoxy CH3), and 0.56 (t, 4H, Si-CH2) (Fig. S1b). 2.2.3. Synthesis of biphenyl-bridged periodic mesoporous organosilicas (BpUPMOs) 0.40 g oligomeric cationic surfactant was dissolved in 50 mL 1-butanol/water solution (4/46 v/v) at 45 °C, and then adjusted pH value to 10 with 2.50 mL ammonia (28% w/w). 2.00 mL DMF mixture solution of TEOS and BpUSi was quickly added under vigorous stirring (the total Si amount is 6 mmol; X is a mole fraction of BpUSi to the total Si, 0%, 10%, 20%, 30%, 40%, 50%, respectively). After stirring for 15 min, the suspension was kept at 50 °C for 5 h, subsequently at 80 °C in oven for another 72 h. The resulting product was recovered by filtration, washed with distilled water and ethanol, and dried in air at ambient temperature (denoted as BpUPMOX). To remove the oligomeric cationic surfactants, the as-synthesized samples were extracted with the methanol solution for 5 days. 2.2.4. Synthesis of charge-transfer (CT) complex composed of BpUPMO20 and decylviologen The preparation and chemical concentrations used were identical to the synthesis of BpUPMOs, except that different amounts of decylviologen (Fig. S7) were dissolved in 1-butanol/water solution together with oligomeric cationic surfactant as the mixed templates which were not extracted after synthesis. The value of Y was used to describe the weight of decylviologen, which was 0.10, 0.20, 0.40, and 0.80 g, respectively. The as-products were denoted as BpUPMO20-Y.

of nitrogen at a heating rate of 10 °C/min in the range of ambient temperature to 600 °C. The solid-state 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded on a Bruker DSX 300 MHz spectrometer (5000 transients, spin speed 6 kHz, acquisition time 0.02 s, pulse delay 3 s). UV–vis absorption spectra were recorded in a quartz cell (light path of 10 mm) on a HP 8453E spectrometer. UV–vis diffuse reflectance spectra were taken on a Shimadzu UV 2550 recording spectrophotometer using BaSO4 as the reference. Fluorescence spectra of the suspension were obtained using a LSS55 spectrometer. The fluorescence microscopy images were performed with an Olympus IX81 + ICCD fluorescence microscope. Time-resolved fluorescence spectra were obtained using an Edinburgh FLS920 fluorescence spectrophotometer. The instrument was operated with a thyratron-gated flash lamp filled with hydrogen at a pressure of 0.4 bar. The lifetimes were estimated from the measured fluorescence decay curves using deconvolution method fitting procedure. 4. Results and discussion 4.1. Structural properties FT-IR spectrum of BpUSi precursor in chloroform (20 mg/mL) clearly exhibits peaks at 3419, 1662 and 1593 cm1 (Fig. 2a),

(a)

BpUSi

1593 1662

3419 BpUPMO0 BpUPMO10 BpUPMO20 BpUPMO30 BpUPMO40 BpUPMO50

Intensity(a.u.)

94

3367 1595 1652

3500

3000

3. Characterization H NMR analyses were carried out on a Bruker DPX-300 NMR spectrometer. FT-IR spectra were recorded on a Nicolet 700 FTIR spectrometer (Thermo-Fisher Scientific, Inc., Waltham, MA). Small-angle X-ray scattering (SAXS) was performed using the SAXSess mc2 by Anton Paar, the sample-detector distance was set to 264.5 mm. High-resolution transmission electron microscopy (HRTEM) images were obtained from a JEM-2100 electron microscopy operating at 200 kV. Prior to the HRTEM measurement, the samples were first dispersed in absolute ethanol by sonication, and then deposited on a copper microgrid. N2 adsorption/desorption measurements were determined at 196 °C on ASAP 2020 Surface Area and Porosimetry Analyzer. The samples were degassed at 150 °C for 8 h under a pressure of about 105 Pa before measured. The surface area was calculated using the Brunauer– Emmett–Teller (SBET) method in a relative pressure (P/P0) range of 0.10–0.30, and the pore size distribution was evaluated using the Barrett–Joyner–Halenda (BJH) method and calculated from the adsorption branch of isotherm. TGA/DSC analyses were carried out using a SDT-Q600 V8.3 Build 101 Simultaneous DSC-TGA Instrument (TA). Studies were conducted under inert atmosphere

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2500

Wavenumber(cm-1 )

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1467

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1400

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Wavenumber(cm-1 ) Fig. 2. FT-IR spectra of (a) BpUSi and BpUPMOs after extraction; (b) oligomeric cationic surfactant (black), BpUPMO20 before (blue) and after extraction (red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Intensity (a.u.)

X10

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2

3

4

5

q (nm-1)

Intensity (a.u.)

(b)

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X10

1

2

3

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5

q (nm-1)

Fig. 3. SAXS patterns of (a) BpUPMOs before extraction (b) BpUPMOs after extraction. From top being BpUPMO0, BpUPMO10, BpUPMO20, BpUPMO30 and BpUPMO40, respectively.

attributing to vibrations of free ureido groups: mNAH, mC@O (amide I) and dNAH (amide II), respectively. Compared with those of BpUSi, the vibration bands of BpUPMOs are shifted towards lower frequencies: 3367 cm1 for mNAH (detailed discussed see below) and 1652 cm1 for mC@O, respectively. At the same time, the dNAH band is shifted towards higher frequencies (1595 cm1) [31–33]. This behaviour is characteristic of strong association of the ureido groups by hydrogen bonds in the BpUPMOs. Besides, the peaks at 1502 and 1544 cm1 correspond to the C@C stretching vibration of the biphenyl rings for both of BpUSi and BpUPMOs. The peaks at 1070 cm1 correspond to the stretching vibrations of SiAOASi frameworks. After extraction, the above peaks still exist and the characteristic biphenyl bands at 1700–1500 cm1 increase with the increasing amount of BpUSi, indicating that the biphenyl units are stably incorporated into the pore walls of the hybrid materials. In addition, for the oligomeric cationic surfactant (Fig. 2b), the observed bending vibrations of the CAH bonds of quaternary ammonium salt are around 1467 cm1. The disappearance of the peak (left inset in Fig. 2b) suggests that oligomeric cationic surfactant has been removed from the sample after extraction. Small-angle X-ray scattering (SAXS) patterns of BpUPMO powders before and after extraction are shown in Fig. 3. For BpUPMO0, two well-resolved peaks at 1.64 and 3.24 nm1 can be indexed to (1 0 0) and (2 0 0) diffractions of an ordered mesophase. With an increase of the amount of BpUSi, the intensities of the two peaks gradually reduce and disappear until X (X is the mole fraction of BpUSi) is 50 mol% (the curve not shown), suggesting a decrease of mesoscale ordering of BpUPMOs. On the other hand, the position of the (1 0 0) scattering peak is shifted to wide-angle, corresponding d-spacing values of 3.83, 3.69, 3.51, 3.41 and 3.34 nm for samples BpUPMO0, BpUPMO10, BpUPMO20, BpUPMO30 and BpUPMO40 decrease also, respectively. The reason is most likely that the self-assembly among BpUSi molecules dominated by both of face-to-face p–p stacking between biphenyl groups and H-bonds between ureido groups disrupts the ability for the surfactant to form micelles and corresponding mesophases [34,35]. After extraction of the template, the periodic mesostructure of BpUPMOs is still retained (Fig. 3b), and the d-spacing values have a slight decrease during the extracting process due to the shrinkage of mesoporous structure. The formation of mesoscale porous structures is further confirmed by HRTEM observation of the extracted BpUPMO10 and BpUPMO20 (Fig. 4). For BpUPMO10, lamellar ordered mesoporous structure and wormhole-like disordered

mesoporous structure are observed simultaneously. For BpUPMO20, the wormhole-like disordered mesophases are observed for majority of the particles. It is very plausible that the higher loading of BpUSi in the mixtures could affect micelle formation for mesophases, resulting in the assembly of more disordered materials [34]. The mesophase of the lamellar system is not stable after surfactant

Fig. 4. HRTEM images of BpUPMO10 (a) and BpUPMO20 (b) after extraction.

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4.2. Optical properties

removal due to the lack of 3-dimensional connection. In this case, the wormhole-like mesophases make the mesoscale porous structures more stable. Although the image is indistinct due to the deterioration of the mesopore framework by irradiation of electron beam. The mesoscale periodicity of BpUPMO observed by TEM is about 3.0 nm. The SEM image of BpUPMO20 is shown in Fig. 5. A representative nanosphere morphology can be seen from the SEM observation. The porosities of the extracted BpUPMOs were examined by nitrogen adsorption/desorption isotherm measurements. As we can see from Fig. S2, type IV nitrogen adsorption/desorption isotherms are exhibited, indicating that the pore sizes of BpUPMOs are in the range of mesoporous scale except curve e (BpUPMO40). The positions of capillary condensation steps move forward to low relative pressure, suggesting that the pore sizes of the obtained materials are gradually decreased from mesopore to micropore scales as the contents of BpUSi increase, especially, the isotherms of BpUPMO40 (curve e) change from type IV to type I. At the same time, surface areas and pore volumes of BpUPMOs also decrease gradually (Table S1). The pore size distribution calculated from the adsorption branches of the isotherms with the BJH method is centered at 2.30 and 2.11 nm for BpUPMO0 and BpUPMO10, respectively. These results are consistent with those of SAXS and HRTEM. The 29Si MAS NMR spectrum of BpUPMO20 (Fig. S3) shows five signals at 56.8, 65.8, 90.5, 100.2 and 109.2 ppm of silicon sites. The signals at 56.8 and 65.8 ppm arise from the Si species of T2 (RSi(OSi)OH2) and T3 (RSi(OSi)3) (R: organic groups), respectively. The signals at 90.5, 100.2 and 109.2 ppm are the Si species of Q2, Q3 and Q4 (Qn: Si(OSi)n(OH)4n). There is no detection of uncondensed silicon species, suggesting that both sides of the silyl groups are attached on the framework. The results of NMR spectroscopy further prove that biphenyl groups are incorporated in the mesoporous network, which are consistent with the results of FTIR spectroscopy. Fig. S4 displays the TGA and DSC curves of BpUPMO20 sample. About 8.37% mass loss attributed to physisorbed water is observed between 20 and 100 °C. The mass decreases by 2.07% from 100 to 250 °C, due to a small loss of water by the condensation of residual silanol and ethoxy groups. The mass continues to decrease gradually to 62.67% at 800 °C, which is ascribed to decomposition of the organic moieties. From the above analysis, the thermal stability of BpUPMOs hybrid materials is maintained at least 250 °C.

291

a

Absorbance/a.u.

Fig. 5. SEM image of BpUPMO20 after extraction.

For the UV–vis spectra (Fig. 6) of BpUSi in ethanol, one peak at 291 nm is attributed to the aromatic p–p⁄ transition of the biphenyls rings. A red-shift of 8 nm from 291 to 299 nm occurs for BpUSi in DMSO compared with BpUSi in ethanol, attributed to the hydrogen bonding between ureido groups in the ethanol solution. While for BpUPMO20, the spectrum shows a broadened and blue-shifted peak compared with that of BpUSi in DMSO. The blue-shifted phenomenon illustrates the existence of interaction between organic groups (the p–p stacking between biphenyl groups and H-bonds between ureido groups) in the frameworks in the ground state. In addition, the broadened peak might be attributed to the lightscattering against the BpUPMO particles. Fluorescence emission spectra of BpUSi and the samples BpUPMOX (X = 10, 20, 30, 40 and 50) are shown in Fig. 7. The emission bands of BpUSi are at 363 nm in DMSO, at 352 nm in ethanol, respectively (Fig. 7a). The blue-shift is ascribed to the hydrogen bonding of ureido units in the ethanol solution. For the BpUPMOs (Fig. 7b), gradual red-shifts from 357 to 372 nm and broadened bands are observed for BpUPMO10, BpUPMO20 and BpUPMO30, because the dense packing of the biphenyl groups permits their neighboring aromatic to achieve intermolecular p–p interactions and thus to form excimer [11]. However, the bands of BpUPMO40 and BpUPMO50 are at 359 and 354 nm, respectively, both of which have a hypsochromic shift compared with that of BpUPMO30 (372 nm). Accordingly the fluorescence intensities of BpUPMO40 and BpUPMO50 also decrease (inset of Fig. 7b). The above phenomenon that the emission bands have a red-shift first and then a hypsochromic shift with fluorescence intensities increasing first and then decreasing with the increase of the amount of organic groups has not been reported in the PMOs materials. Two key roles are played in the special optical properties of BpUPMOs. Firstly, biphenyl, which is twisted for the two benzene rings in the ground state, is known to exhibit geometry change in the singlet excited state toward a planar conformation by internal rotation around the CAC single bond in the excited state [36–38]. As the biphenyl units are incorporated into the silica framework of the mesoporous materials, the internal rotation around the CAC single bond is inhibited by the silica framework walls of the hybrid materials, making fluorescence intensities enhance. The phenomena of red-shift and broadening of emission peaks are attributed to the formation of excimers. Secondly, the self-quenching of BpUPMOs induced by hydrogen bonding is observed. These are different from the previous report [11], in which enhancement of fluorescence

299

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294

c 250

300

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Wavelength(nm) Fig. 6. UV–vis absorption spectra of BpUSi in ethanol (a) and DMSO ((b) 1  105 g/ ml) and BpUPMO20 in ethanol ((c) 1  104 g/ml).

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300

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wavelength(nm)

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fluorescence intensity

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BpUSi content(mol%)

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Wavelength(nm) Fig. 7. Fluorescence Emission spectra (a) BpUSi (4  107 g/mL) in ethanol (black) and DMSO (red) (b) BpUPMO10 (black), BpUPMO20 (red), BpUPMO30 (blue), BpUPMO40 (green) and BpUPMO50 (mulberry) in ethanol solution (4  106 g/mL). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

was exhibited for high-content biphenyl-PMOs due to the aggregation-induced enhanced emission. The fluorescence behavior of biphenyl-based compounds is strongly affected by hydrogen bonding [39–41]. Intermolecular and intramolecular hydrogen bondings are general two types of hydrogen bondings. For the organosilicone precursor BpUSi, intramolecular hydrogen bonding is gradually predominated with an increase of the amount of BpUSi that contains two ureido groups. The molecular chains of BpUSi might fold and twist in the synthesis process of BpUPMOs due to the steric hindrance when the content of BpUSi is high. The hydrogen bonding network systems generated through intramolecular hydrogen bonds may hinder intramolecular rotation of the benzene ring accompanying in the excited state fluorescence quenching. In addition, further evidence is provided by FTIR spectra. Fig. S5 shows that infrared absorption bands of the BpUPMOs are in the range of 3200–3500 cm1, which corresponds to the NH stretching region of ureido groups. This broad band is interpreted as corresponding to two vibrations of hydrogenbonded. The one at 3415 cm1 is attributed to intermolecular hydrogen-bonded of NH stretching region. Another at 3320 cm1 can be assigned to intramolecular interactions of the NH groups [42]. For BpUPMO10, only one band at 3415 cm1 is present,

showing appearance of the intermolecular hydrogen-bonded of NH stretching region. For other BpUPMOX (X = 20, 30, 40 and 50), both of bands at 3320 and 3415 cm1 are simultaneously shown. However, the relative absorption intensities of intramolecular hydrogen bonding at 3320 cm1 increase, and those of band at 3415 cm1 are almost unchanged with the increase of the amount of BpUSi, suggesting that there exist the intra/intermolecular hydrogen bonding in the system, and the amount of intramolecular hydrogen bonding rises, which enhances the interaction of hydrogen bonding. The fluorescence decay curves of the precursor and BpUPMOs are shown in Fig. S6. The fluorescence decay of the precursor is well fitted to a mono-exponential profile with the lifetime of approximately 2.61 ns, which reflects the absence of eximers in the concentration of the precursor BpUSi (1  105g/mL). For the precursor BpUSi, intermolecular interactions do not exist and the two benzene rings with the single carbon–carbon bond linking can rotate freely in the dilute solution. Different from the precursor BpUSi, the decay curves for BpUPMOs are analyzed coincidentally as three-component exponential functions due to the existence of emissive excimer. For the BpUPMOs, the shorter decay parameters (s1) are attributed to the emission of the monomers of biphenyl groups. Besides, longer fluorescence lifetimes (s2 and s3) are ascribed to excimers [30,43]. As listed in Table S2, the s values (s1, s2 and s3) gradually increase in accordance with the sequence of BpUPMO10, BpUPMO20, and BpUPMO30. However, BpUPMO40 and BpUPMO50 exhibit shorter lifetimes by degrees compared with those of BpUPMO30. From BpUPMO10 to BpUPMO30, the rotation of biphenyl groups on the framework is hindered due to the existences of intermolecular interaction between biphenyl groups and steric hindrance of silica framework, and such interaction would induce a slow nonradiation decay process by suppressing molecular motion [29,44]. It is conceivable that the increase of the incorporation of biphenyl groups will enhance the probability of the rotational restriction, thus the s values of fluorescence lifetimes become larger from BpUPMO10 to BpUPMO30. On the other hand, the s values of fluorescence lifetimes of BpUPMO40 and BpUPMO50 are obviously shorter than those of BpUPMO30 (see Table S2). The faster decay process is ascribed to the fluorescence solid-quenching in the tight stacking biphenyl groups due to the intramolecular hydrogen bonds and steric hindrance of silica framework of BpUPMO40 and BpUPMO50 (see Fig. S5). Therefore, the results of the fluorescence quenching are in agreement with those of the steady-state fluorescence spectra (see Fig. 7)

BpUPMO20 BpUPMO20-0.1 BpUPMO20-0.2 BpUPMO20-0.4 BpUPMO20-0.8

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Wavelength(nm) Fig. 8. Diffuse reflectance UV–vis spectra of BpUPMO20 and BpUPMO20-Y.

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Fig. 9. The pictures of BpUPMO20-Y samples.

(a) Fluorescence Intensity

Biphenyl and viologens are typical electrical donor–acceptor pairs and fluorescence of biphenyl derivatives can be quenched by alkyl viologens through charge transfer mechanism. Decylviologen molecules (Fig. S7) are locked into pore channels and 20 mol% BpUSi molecules are doped in the pore walls by a one-step selfassembly process. Fig. 8 is diffuse reflectance UV–vis spectra of BpUPMO20 and BpUPMO20-Y. Only one peak at 300 nm is attributed to the aromatic p–p⁄ transition of the biphenyl rings for BpUPMO20. For all BpUPMO20-Y, the second peak at 420 nm is the characteristic peak of the decylviologen and bromide ion CT complex [29,45]. In addition, the other peak at 515 nm is ascribed to the CT complex of the decylviologen in the pore channels and biphenyl groups within the pore walls [46]. Meanwhile, the absorption intensity of CT complex is enhanced continually as the content of decylviologen increases. Combined with the external appearance of CT complex (Fig. 9), it is obviously observed that the color of appearance transforms from salmon pink to red brown with the increase of the amount of decylviologen group. This external colour change makes a further proof of the formation of CT complex between BpUPMO20 and decylviologen, which can absorb light of the longer wavelength, and make the color of CT complex appearance alter. Fluorescence emission spectra of BpUPMO20 and BpUPMO20-Y are shown in Fig. 10. It is obvious that fluorescence emission of the biphenyl groups can be gradually quenched under 280 nm wavelength excitation as decylviologen is gradually introduced into the BpUPMO20 hybrid materials. Fluorescence quenching is induced by the electron transfer from biphenyl groups to viologens (Fig. 10a). For BpUPMO20-0.8, a weak peak at 580 nm is assigned to the fluorescence emission peak of the CT complex of biphenyl and decylviologen (inset of Fig. 10a), suggesting the existence of the energy transfer between biphenyl and CT complex [47]. However, the energy transfer efficiency is very low. One of the reasons is that the overlap between the acceptor absorption and the donor emission spectra in this system is not efficient enough based on the mechanism of the fluorescence resonance energy transfer (FRET). In previous reports, there was no fluorescence emission for pure viologen aqueous solution [29]. A fast radiationless deactivation pathway of the singlet excited state through rotation around CAC single bond connecting the two pyridinium rings has been invoked as responsible for this nonemissive decay. However, viologen can form CT complexes with electron-rich species and the complexes emit at longer wavelength. Further evidences that the CT complexes could emit the fluorescence at the longer wavelength are shown in Fig. 10b. At the excitation wavelength of 490 nm, the emission peaks of CT complexes at 580 nm are observed clearly, while the intensity is enhanced with the increase of decylviologen amount, indicating that the fluorescence at 580 nm originated from CT complexes in the system [47]. Fig. 11 exhibits the fluorescence microscopy images of the BpUPMO20–0.8 in ethanol

suspensions of 105 g/mL. Every red color particles are observed with mercury lamp excitation (with a 470–490 nm filter) on a micro-scope. The result also demonstrates the existence of the fluorescent CT complexes in which charge transfer is from biphenyl-bridged silica framework to decylviologen in the pore channels. The optical performance of biphenyl-bridged PMOs will make it valuable for the development of optoelectronic materials. In general, surfactant-templated mesosotructured materials contain three spatially separated regions: the silica framework, the hydrophobic micelle core, and the ionic interface between the surfactant and the silica [45]. For BpUSi, it was covalently

BpUPMO20 BpUPMO20-0.1 BpUPMO20-0.2 BpUPMO20-0.4 BpUPMO20-0.8

0

570

300

580

400

500

Wavelength(nm)

(b)

BpUPMO20 BpUPMO20-0.1 BpUPMO20-0.2 BpUPMO20-0.4 BpUPMO20-0.8

Fluorescence Intensity

4.3. In situ charge-transfer (CT) from BpUPMO20 to decylviologen

550

600

650

700

Wavelength(nm) Fig. 10. Fluorescence emission spectra of BpUPMO20 and BpUPMO20-Y in ethanol (2  105g/ml) (a) kex = 280 nm; (b) kex = 490 nm.

M. Gao et al. / Microporous and Mesoporous Materials 198 (2014) 92–100

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Fig. 11. Fluorescence microscopy image of the BpUPMO20–0.8, left in dark field and right in bright field.

NH3·H2O(28%) cocondensation

oligomeric cationic surfactant Decylviologen TEOS

+N

N+

e-

O N H

O N H

BpUSi Fig. 12. Possible binding model of CT complex. Designed donor in the pore wall and acceptor in the pore channel.

incorporated in the pore walls through the co-hydrolysis and co-condensation with TEOS. Besides, BpUSi has ureido and biphenyl groups within the organic bridging units, encouraging build-up of the strong hydrogen-bonded array and p–p stacking, which was proved by FT-IR and fluorescence emission spectra. After extracted, stably mesoporous structure was retained, which depended primarily on the establishment of strong and ordered hydrogen bonding and p–p stacking interactions. On the basis of above PMOs method, we use oligomeric cationic surfactant and decylviologen as structure directors together, building a CT donor–acceptor system (Fig. 12). As explained before, biphenyl molecules were placed in the framework by making them a building block of the framework. The electron acceptor was placed selectively in the ionic interface by utilizing the surfactant properties of decylviologen. Decylviologen is known to form micelles by itself as well as to fully incorporate into oligomeric cationic surfactant micelles while retaining its ability to function as a reversible electron acceptor. The electro-active part of the molecule is the polar headgroup, and it is anchored in place by its association with the micellar core by its decyl tail. Proper distance between donor and acceptor is provided by this configuration which is highly important for the effective charge transfer. Furthermore, the strength of charge-transfer could be modulated by the loading content of decylviologen groups. Obviously, this approach is promising to construct effective electron and energy transfer

systems through simple organic synthesis to substitute the biphenyl and viologen for alternative different donor and acceptor groups to realize wide use in optoelectronic materials.

5. Conclusions In this study, we successfully prepared a series of biphenylbridged PMO materials with a phase mixture of the wormhole-like phase and the lamellar phase. The high loading of BpUSi would accelerate the formation of wormhole-like disordered mesopores. The excimer emission was exhibited when the amount of bridged biphenyl-based precursors was lower than 30% (molar ratio). As the amount of precursors was increased to 40%, the fluorescence quenching caused by hydrogen bond interaction was observed. The existence of excimer was also demonstrated in the fluorescence decay test. The s values of fluorescence lifetimes gradually became larger from BpUPMO10 to BpUPMO30. This result was ascribed to the fact that the rotation of biphenyl rings was restricted by intermolecular interactions between biphenyl groups and steric hindrance of silica framework with the increase of the amount of BpUSi. The faster fluorescence decay caused by intramolecular hydrogen bond was observed for BpUPMO40 and BpUPMO50, which was consistent with the results of the fluorescence spectra. Moreover, a charge transfer donor–acceptor system

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with the biphenyl groups as electron donors in the pore walls and the decylviologen groups as electron acceptors in the mesochannels was built. The color of CT complex could be controlled by the doping amount of decylviologen in the above charge transfer system. Acknowledgements This research is financially supported by the Natural Science Foundation of China (no. 50572057, 21033005) and the Natural Science Foundation of Shandong Province (no. ZR2010BM026). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.micromeso.2014. 07.021. References [1] S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc. 121 (1999) 9611–9614. [2] B.J. Melde, B.T. Holland, C.F. Blanford, A. Stein, Chem. Mater. 11 (1999) 3302– 3308. [3] T. Asefa, M.J. MacLachlan, N. Coombs, G.A. Ozin, Nature 402 (1999) 867–871. [4] Q. Yang, J. Liu, J. Yang, M.P. Kapoor, S. Inagaki, C. Li, J. Catal. 228 (2004) 265– 272. [5] F. Hoffmann, M. Cornelius, J. Morell, M. Fröba, Angew. Chem. Int. Ed. 45 (2006) 3216–3251. [6] V. Rebbin, R. Schmidt, M. Fröba, Angew. Chem. Int. Ed. 45 (2006) 5210–5214. [7] Q. Ji, M. Miyahara, J.P. Hill, S. Acharya, A. Vinu, S.B. Yoon, J.S. Yu, K. Sakamoto, K. Ariga, J. Am. Chem. Soc. 130 (2008) 2376–2377. [8] P.C. Angelomé, G.J.A.A. Soller-Illia, Chem. Mater. 17 (2005) 322–331. [9] R. Hernandez, A.C. Franville, P. Minoofar, B. Dunn, J.I. Zink, J. Am. Chem. Soc. 123 (2001) 1248–1249. [10] P.N. Minoofar, R. Hernandez, S. Chia, B. Dunn, J.I. Zink, A.C. Franville, J. Am. Chem. Soc. 124 (2002) 14388–14396. [11] Y. Goto, N. Mizoshita, O. Ohtani, T. Okada, T. Shimada, T. Tani, S. Inagaki, Chem. Mater. 20 (2008) 4495–4498. [12] N. Tanaka, N. Mizoshita, Y. Maegawa, T. Tani, S. Inagaki, Y.R. Jorapurac, T. Shimada, Chem. Commun. 47 (2011) 5025–5027. [13] X.M. Li, D.K. Shen, J.P. Yang, C. Yao, R.C. Che, F. Zhang, D.Y. Zhao, Chem. Mater. 25 (2013) 106–112. [14] M.P. Kapoor, Q. Yang, S. Inagaki, J. Am. Chem. Soc. 124 (2002) 15176–15177. [15] K. Okamoto, Y. Goto, S. Inagaki, J. Mater. Chem. 15 (2005) 4136–4140. [16] N. Mizoshita, Y. Goto, M.P. Kapoor, T. Shimada, T. Tani, S. Inagaki, Chem. Eur. J. 15 (2009) 219–226.

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