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Spectroscopic studies on conjugated polymers in mesoporous channels: influence of polymer side-chain length
Journal of Physics and Chemistry of Solids 64 (2003) 2451–2455 www.elsevier.com/locate/jpcs
Spectroscopic studies on conjugated polymers in mesoporous channels: influence of polymer side-chain length Hongan Xia, Baohu Wangb, Yanbo Zhangb, Xuefeng Qianb,*, Jie Yinb, Zikang Zhub a
Research and Development Center of Functional Ceramics, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China b School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, China Received 27 August 2002; revised 23 June 2003; accepted 19 August 2003
Abstract The influence of mesoporous environment on the conjugated polymers was studied by UV – Vis absorption and Photoluminescence spectroscopy. The applied polymers were three novel poly( p-phenylenevinylene) derivatives (DDMAPPV). These polymers have dibenzothiophen-5,5-dioxide units in their backbones, but are different from each other in the length of alkoxy side-chains. The polymers were incorporated into the mesoporous channels of SBA-15 by sorption from their dilute solutions. The confined polymers exhibited different trends in the shifts of the absorption onsets and the emission peaks depending on the length of the side-chains. The polymer with shorter side-chain showed red-shifts in both the absorption and emission spectra, whereas the polymer with longer side-chain showed blue-shifts. These phenomena were caused by the combined influences from the electronic confinement and the conformation distortion. Moreover, these trends were enhanced when the polymers were loaded in amine-modified SBA-15. q 2003 Published by Elsevier Ltd. Keywords: D. Optical properties
1. Introduction Host – guest materials show many interesting properties due to their specific intermolecular interactions [1] and the ‘boxing effect’ [2]. The hosts, such as zeolites and mesoporous materials, possess nano-periodic and sizetunable cages or channels which can act as versatile templates for preparing guests with diameter from several angstroms to hundred nanometers. Such nano-sized guests, especially semiconductors, exhibit some special properties due to the quantum-size effects. Moreover, the ordered and oriented guests may show magnetic or charge transport properties different from their bulk phase [3,4]. Besides, it is interesting to study the interactions between hosts and guests, since the large interfacial area and confined * Corresponding author. Tel.: þ86-21-54743268; fax: þ 86-2154741297. E-mail address: [email protected] (X. Qian). 0022-3697/03/$ - see front matter q 2003 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2003.08.002
environment may enhance the intermolecular forces, leading to significant changes in the molecular properties of embedded guests, and these changes may provide valuable information for catalysis [2], optical [5] and electrical applications. So far, extensive studies have been focused on the dyedoped porous silicas [6]. Dyes can be incorporated into silica matrices before or after sol– gel processes. In the former pathway, dyes can be grafted on surfactant molecules to act as structure-directing agents [7], or simply dissolved in the sols without contribution to the formation of ordered pores [5]. Such direct synthetic pathway facilitates the fabrication of devices with special forms, but has possible disadvantages in the remaining solvents [8] and surfactants. These remainders might destroy the properties of the devices. In the latter pathway, porous silicas were previously calcined, and dyes were loaded in the silica pores by physical adsorption from gas or solution, or by chemical reaction to be grafted onto the pore surface. This method has
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N2 atmosphere for 8 h. After the reaction mixture was cooled to room temperature, the solid was recovered by filtration, washed with ethanol, and dried under vacuum at 70 8C. The obtained sample was designated as SBA-15(m). 2.2. Preparation of guest polymers
Fig. 1. Chemical structure of the repeating units in DDMA-PPVs.
several advantages. First, the remaining solvents, surfactants and other organic agents from sol – gel process can completely decompose during calcination, leading to relatively pure hosts. Second, the hydroxyl groups provided reaction sites for grafting dye molecules or for modification of the pore surface to match the polarity of the incorporated guests. But this method has limitation in loading of guests with special molecular shapes. Comparing with the studies on silica-dye systems, fewer papers reported the properties of the conjugated polymers in meso-channels. Several types of conjugated polymers, such as polyaniline [9] and polypyrrole [10], have been synthesized in the channels of host materials, while Poly( p-phenylenevinylene) (PPV) derivatives have been incorporated into oriented mesoporous silicas to study the interchain and intrachain exciton transport [11]. However, no sufficient works were devoted to investigate the properties of the confined conjugated polymers, such as the confinement effect on the spectroscopic properties and the influences of pore surface polarity on the optical properties and the charge transfer. These basic studies would provide valuable information for the designing of the host– guest type devices. In this paper, three soluble poly( p-phenylenevinylene) derivatives (DDMA-PPV) (Fig. 1) were used as guest materials to investigate the confinement effect on their spectroscopic properties. These polymers all contain dibenzothiophene-5,5-dioxide units in the backbones, but are different from each other in the length of alkoxy sidechain.
2. Experimental
In this paper, three employed guest polymers were synthesized for the first time in our laboratory. The detailed description of the synthesis processes will be presented elsewhere [13]. These polymers were the derivatives of poly( p-phenylenevinylene) (DDMA-PPV) (Fig. 1), with the alkoxy side-chains containing four, six and nine carbon atoms, respectively. The molecular weight of each polymer is listed below. P1 : Mn ¼ 2328; Mw ¼ 3166; PDI ¼ 1:36: P2 : Mn ¼ 1406; Mw ¼ 2520; PDI ¼ 1:79: P3 : Mn ¼ 2150; Mw ¼ 3536; PDI ¼ 1:62: 2.3. Incorporation of polymers into mesoporous hosts 0.005 g polymers were dissolved in 20 ml tetrahydrofuran (THF), then 0.2 g SBA-15 or SBA-15(m) was added and stirred at room temperature for 24 h. The solids were recovered by filtration, and dried under vacuum at 70 8C. These solid samples were denoted as P1-S15, P2-S15 and P3-S15 for the SBA-15 incorporated with P1, P2 and P3, as P1-S15(m), P2-S15(m) and P3-S15(m) for the SBA-15(m) incorporated with P1, P2 and P3, respectively. 2.4. Characterization Small-angle X-ray diffraction (SXRD) analysis was conducted on a Rigaku D/max 2550V X-ray diffractometer with Cu Ka irradiation, at 40 kV and 100 mA. Nitrogen adsorption– desorption data were obtained at 77.35 K on a Micromeritics Tristar 3000 analyzer. The samples were dried in flowing nitrogen at 120 8C for at least 24 h before measurement. Diffuse reflectance UV –Vis spectra were measured on a Varian Cary 500 Spectrophotometer. PL spectra were recorded at room temperature on a Hitachi 850 Fluorescence Spectrophotometer. The absorption and emission spectroscopy for all samples were measured at their powder state, except that those of pure polymers were obtained from their thin films coated on silica plates.
2.1. Preparation of host materials The mesoporous silica, SBA-15, was prepared under acidic condition by using an amphiphilic block copolymer as the structure-directing agent according to the literature [12]. The amine modified SBA-15 was prepared by treating SBA-15 with ethylenediaminepropyltriethoxysilane (EDPTES). Typically, 1 g SBA-15 and 5 ml EDPTES were added to 100 ml dried toluene, and refluxed under
3. Results SBA-15 shows three diffraction peaks in its SXRD pattern (Fig. 2), and can be identified as mesoporous silica with highly ordered hexagonal structure. In the SXRD pattern of SBA-15(m), these peaks still existed with a slight decrease in intensity, suggesting the retaining of highly
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Fig. 2. SXRD patterns of SBA-15 and amine modified SBA-15.
ordered mesoporous structure after the modification with EDPTES. The pore parameters of SBA-15 and SBA-15(m), obtained from the nitrogen adsorption – desorption analyses, were listed in Table 1. The diameters of the channels were ˚ for SBA-15 and SBA-15(m), respectively, 70.8 and 65.7 A which were larger than the diameters of the possible DDMA-PPV chains (Fig. 3). Considering the smaller molecular size of the polymers and the flexibility of their side-chains, the polymer molecules can crawl into the channels in the hosts when SBA-15 or SBA-15(m) powder was immerged in the solution of DDMA-PPV. The content of the polymer in each host– guest sample was estimated to be ca. 0.2– 0.4 wt% by the difference between the UV absorbance of the initial polymer solution before mixed with SBA-15 or SBA15(m) and that of the final solution before filtration. 3.1. Absorption and emission spectroscopy of pure DDMA-PPVs
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Fig. 3. Structure diagrams for P3: the repeating units with possible configuration (A and B) and the optimized polymer chain structure (C).
3.2. Absorption and emission spectroscopy of DDMA-PPVs incorporated in SBA-15 As incorporated into SBA-15, DDMA-PPVs showed shifts of their emission peaks. For P1 and P2, the emission peak red-shifted from 527 to 532 nm (Fig. 5, right) and 535 to 537 nm (Fig. 6, right), with a shift value of 5 and 2 nm, respectively. But for P3, as a contrast, the emission peak blue-shifted from 537 to 528 nm, with a shift value of 9 nm (Fig. 7, right). P1-S15 and P2-S15 also showed a red-shift of their absorption onsets (Figs. 5 and 6, left), comparing with the P1 and the P2 film. The shift value for P1-S15 and P2-S15 was 23 and 15 nm, respectively. Correspondingly, P3-S15 showed a blue-shift in its absorption onset comparing with the P3 film, although the shift is relatively small (Fig. 7, left).
The pure DDMA-PPV films showed intensive emission around 530 nm when excited by the light with wavelength of 400 nm (Fig. 4, right). As the length of alkoxy side-chain increased from 4 to 9 carbon atoms, the wavelength of the emission peak increased from 527 to 537 nm. The similar trend was also observed in their UV – Vis absorption spectra. As shown in Fig. 4 (left), the absorption onset of DDMAPPV films moved from 510 to 530 nm as the increase of side-chain length. Table 1 Pore parameters of SBA-15 and amine modified SBA-15 Hosts
Surface area (m2 g21)
Pore volume (cm3 g21)
Pore diameter ˚) (A
SBA-15 SBA-15(m)
603 494
1.053 0.822
70.8 65.7
Fig. 4. Photoluminescence (right) and diffuse reflectance UV –Vis absorption (left) spectra of the DDMA-PPV films.
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Fig. 5. Photoluminescence (right) and diffuse reflectance UV– Vis absorption (left) spectra of P1 and its host– guest composites.
Fig. 7. Photoluminescence (right) and diffuse reflectance UV –Vis absorption (left) spectra of P3 and its host–guest composites.
3.3. Absorption and emission spectroscopy of DDMA-PPVs incorporated in SBA-15(m)
surface with no or a lower degree of aggregation comparing with its pure solid-state. The result is a blue-shift in its absorption and emission spectra. A large amount of literatures have reported that organic dyes in silica matrix showed the absorption and emission spectra similar to those of their dilute solution [8]. But there is another influence, namely, electronic confinement, which would lead to a redshift of 0 – 0 transition of the confined molecules. This effect becomes stronger as the size of the guest molecules approaches the pore dimension [2]. Zeolites are aromatic chemicals were the frequently used host and guest materials to investigate such electronic confinement effect. This effect was confirmed by computer simulation and spectroscopic evidence [2,14]. However, such investigation was limited to the hosts with small pore size and the guests with small molecule size, no articles mentioned about such effect on the conjugated polymers in mesoporous environment. Here, three novel PPV derivatives were selected to investigate the possible host – guest effects. With the difference in side-chain length, P1, P2 and P3 showed different changes in their absorption and emission spectroscopy when they were incorporated in SBA-15. As shown in Fig. 5 – 7 (right), P1-S15 and P2-S15 exhibited a red-shift of emission peak, whereas P3-S15 showed a blue-shift, comparing with the P1, P2 and P3 film, respectively. Moreover, P1-S15 showed a larger shift than P2-S15. The similar trends were also exhibited in the absorption spectra (Fig. 5 – 7, left). The absorption and emission spectra of the polymers in SBA-15 were significantly different from those of the polymers in solutions. In chloroform, the absorption onsets blue-shifted to ca. 482 nm for all polymers, while the emission peaks blue-shifted to 504, 510 and 494 nm for P1, P2 and P3, respectively. The spectroscopic shifts of the polymers confined in SBA-15 are hardly explained by electrostatic effect from such as the pore surface and the intermolecules [2]. First, SBA-15 is a type of all-silica mesoporous material with Si –OH groups on the pore
When DDMA-PPVs were incorporated in amine modified SBA-15, such trends in their absorption and emission spectra were enhanced. As shown in Fig. 5 – 7 (right), the emission peaks of P1-S15(m) and P2-S15(m) had a red-shift of 6 nm and 3 nm comparing with those of P1-S15 and P2S15, respectively, whereas the emission peak of P3-S15(m) had a blue-shift of 2 nm comparing with that of P3-S15. In Fig. 5 – 7 (right), the polymers in SBA-15(m) also showed a red or blue-shift of absorption onset comparing with them in SBA-15, but the shifts were relatively small. Moreover, P3-S15(m) showed a continuous drop in the absorption from 210 to 520 nm.
4. Discussion As organic molecules were incorporated into mesopores at a low content, they would be absorbed on the large pore
Fig. 6. Photoluminescence (right) and diffuse reflectance UV– Vis absorption (left) spectra of P2 and its host– guest composites.
H. Xi et al. / Journal of Physics and Chemistry of Solids 64 (2003) 2451–2455
surface, and its polarity is less than that of ethanol, therefore the electrostatic forces from pore surface could not induce a red-shift in the absorption and emission spectra of the confined polymers. Second, as the polymers incorporated into the mesopores at a low content, the polymer molecules would be absorbed with no or a lower degree of aggregation comparing with their pure solid-state, an the Coulombic interaction energies between molecules would become smaller, leading to a wider band gap [14] and a blue-shift of the absorption edge. From the above analyses, the electronic confinement effect should be used to rationalize the spectroscopic phenomena of these host– guest systems. According to the electronic confinement theory, when P1 molecules were confined in mesoporous channels, their molecular orbitals were deformed due to the confinement, leading to increasing the orbital energies. Since the HOMO (highest occupied molecular orbital) is more sensitive than the LUMO (lowest unoccupied molecular orbitals), the overall effect is a reduction on the band gap of the frontier orbitals [2]. The reduction on the band gap can be proved by the red-shift of the onset in the absorption spectrum of P1-S15 (Fig. 5, left). Similarly, in P2-S15, the molecular orbitals of P2 also suffered from such electronic confinement effect. The molecule size of P2 was enlarged as increasing the sidechain length, and it seems the electronic confinement should be more effective. But in fact, the shift values in the absorption and emission spectra of P2-S15 were smaller than those in the absorption and emission spectra of P1-S15. This irregular result may be attributed to the influence of the steric hindrance from the side-chains. Different from the small aromatic molecules, the DDMA-PPVs possess long molecular chains and nonrigid molecular planes, so the molecular conformation of the polymers could be influenced by the steric hindrance. The distortion of the molecular conformation would decrease the conjugate degree of whole molecules, leading to an increase of the band gap. Since the influence from the distortion of the molecular conformation partly neutralized the influence from electronic confinement, the whole results are smaller red-shifts in the absorption and emission spectra (Fig. 6). By the same token, in P3-S15, the influence from the distortion of the molecular conformation exceeded that from the electronic confinement due to the steric hindrance from the much longer side-chain, and therefore leading to blueshifts in the absorption and emission spectra (Fig. 7). We also investigated the spectroscopy of P1, P2 and P3 in mesopores with strong polarity. When the conjugated polymers were incorporated into amine modified SBA-15, such trends of the red or blue-shifts in their absorption and emission spectra were enhanced. P1-S15(m) and P2-S15(m) showed a more red-shift of 6 and 3 nm in their emission spectra (Figs. 5 and 6, right), respectively, whereas P3S15(m) showed a more blue-shift of 2 nm (Fig. 7, right). These results indicate that the polarity of pore surface
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influenced the energy bands of the confined polymers. In addition, it should be noticed that the absorption pattern of P3-S15(m) was significantly different from that of P3-S15, suggesting a remarkable distortion of the polymer chain occurred due to the repulsive forces between the side-chains of P3 and the amine groups on the pore surface.
5. Conclusions We have provided the convincing spectroscopic evidence for the electronic confinement of conjugated polymers in SBA-15. Also, we have found that the confined polymers suffered from the conformation distortion due to their steric hindrance and their nonrigid conjugate plane. The electronic confinement tended to deform the molecular orbitals of polymers, leading to decreasing of the band gap of the frontier orbitals, whereas the steric hindrance from the side-chains tended to distort the conformation of polymer chains, leading to the increase of the band gap. The different trends of the spectroscopic shifts of the polymers in SBA-15 were the combined results from these opposite influences. Further investigation indicated that increasing the polarity of the pore surface would enhance the trends of the red or blue-shifts in the absorption and emission spectra.
References [1] P.J. Langley, J. Hulliger, Chem. Soc. Rev. 28 (1999) 279. [2] F. Marquez, H. Garcia, E. Palomares, L. Fernandez, A. Corma, J. Am. Chem. Soc. 122 (2000) 6520. [3] T.-Q. Nguyen, J. Wu, S.H. Tolbert, B.J. Schwartz, Adv. Mater. 13 (2001) 609. [4] J. Wu, A.F. Gross, S.H. Tolbert, J. Phys. Chem. B 103 (1999) 2374. [5] B.J. Scott, G. Wirnsberger, M.D. McGehee, B.F. Chmelka, G.D. Stucky, Adv. Mater. 13 (2001) 1231. [6] G. Schulz-Ekloff, D. Wo¨hrle, B. van Duffel, R.A. Schoonheydt, Microporous Mater. 51 (2002) 91. [7] I. Honma, H.S. Zhou, Adv. Mater. 10 (1998) 1532. [8] T.M.H. Costa, V. Stefani, N. Balzaretti, L.T.S.T. Francisco, M.R. Gallas, J.A.H. da Jornada, J. Non-Cryst. Solids 221 (1997) 157. [9] C.-G. Wu, T. Bein, Science 264 (1997) 1757. [10] C. Perruchot, M.M. Chehimi, M. Delamar, P.C. Lacaze, A.J. Eccles, T.A. Steele, C.D. Mair, Synth. Metals 102 (1999) 1194. [11] T.Q. Nguyen, J. Wu, V. Doan, B.J. Schwartz, S.H. Tolbert, Science 288 (2000) 652. [12] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [13] B.H. Wang, H.A. Xi, J. Yin, X.F. Qian, Z.K. Zhu, Synthetic metals, 139 (2003) 187. [14] L.Z. Zhang, Y. Xiong, P. Cheng, G.-Q. Tang, D.-Z. Liao, Chem. Phys. Lett. 358 (2002) 278.
Spectroscopic studies on conjugated polymers in mesoporous channels: influence of polymer side-chain length Hongan Xia, Baohu Wangb, Yanbo Zhangb, Xuefeng Qianb,*, Jie Yinb, Zikang Zhub a
Research and Development Center of Functional Ceramics, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China b School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai 200240, China Received 27 August 2002; revised 23 June 2003; accepted 19 August 2003
Abstract The influence of mesoporous environment on the conjugated polymers was studied by UV – Vis absorption and Photoluminescence spectroscopy. The applied polymers were three novel poly( p-phenylenevinylene) derivatives (DDMAPPV). These polymers have dibenzothiophen-5,5-dioxide units in their backbones, but are different from each other in the length of alkoxy side-chains. The polymers were incorporated into the mesoporous channels of SBA-15 by sorption from their dilute solutions. The confined polymers exhibited different trends in the shifts of the absorption onsets and the emission peaks depending on the length of the side-chains. The polymer with shorter side-chain showed red-shifts in both the absorption and emission spectra, whereas the polymer with longer side-chain showed blue-shifts. These phenomena were caused by the combined influences from the electronic confinement and the conformation distortion. Moreover, these trends were enhanced when the polymers were loaded in amine-modified SBA-15. q 2003 Published by Elsevier Ltd. Keywords: D. Optical properties
1. Introduction Host – guest materials show many interesting properties due to their specific intermolecular interactions [1] and the ‘boxing effect’ [2]. The hosts, such as zeolites and mesoporous materials, possess nano-periodic and sizetunable cages or channels which can act as versatile templates for preparing guests with diameter from several angstroms to hundred nanometers. Such nano-sized guests, especially semiconductors, exhibit some special properties due to the quantum-size effects. Moreover, the ordered and oriented guests may show magnetic or charge transport properties different from their bulk phase [3,4]. Besides, it is interesting to study the interactions between hosts and guests, since the large interfacial area and confined * Corresponding author. Tel.: þ86-21-54743268; fax: þ 86-2154741297. E-mail address: [email protected] (X. Qian). 0022-3697/03/$ - see front matter q 2003 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2003.08.002
environment may enhance the intermolecular forces, leading to significant changes in the molecular properties of embedded guests, and these changes may provide valuable information for catalysis [2], optical [5] and electrical applications. So far, extensive studies have been focused on the dyedoped porous silicas [6]. Dyes can be incorporated into silica matrices before or after sol– gel processes. In the former pathway, dyes can be grafted on surfactant molecules to act as structure-directing agents [7], or simply dissolved in the sols without contribution to the formation of ordered pores [5]. Such direct synthetic pathway facilitates the fabrication of devices with special forms, but has possible disadvantages in the remaining solvents [8] and surfactants. These remainders might destroy the properties of the devices. In the latter pathway, porous silicas were previously calcined, and dyes were loaded in the silica pores by physical adsorption from gas or solution, or by chemical reaction to be grafted onto the pore surface. This method has
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N2 atmosphere for 8 h. After the reaction mixture was cooled to room temperature, the solid was recovered by filtration, washed with ethanol, and dried under vacuum at 70 8C. The obtained sample was designated as SBA-15(m). 2.2. Preparation of guest polymers
Fig. 1. Chemical structure of the repeating units in DDMA-PPVs.
several advantages. First, the remaining solvents, surfactants and other organic agents from sol – gel process can completely decompose during calcination, leading to relatively pure hosts. Second, the hydroxyl groups provided reaction sites for grafting dye molecules or for modification of the pore surface to match the polarity of the incorporated guests. But this method has limitation in loading of guests with special molecular shapes. Comparing with the studies on silica-dye systems, fewer papers reported the properties of the conjugated polymers in meso-channels. Several types of conjugated polymers, such as polyaniline [9] and polypyrrole [10], have been synthesized in the channels of host materials, while Poly( p-phenylenevinylene) (PPV) derivatives have been incorporated into oriented mesoporous silicas to study the interchain and intrachain exciton transport [11]. However, no sufficient works were devoted to investigate the properties of the confined conjugated polymers, such as the confinement effect on the spectroscopic properties and the influences of pore surface polarity on the optical properties and the charge transfer. These basic studies would provide valuable information for the designing of the host– guest type devices. In this paper, three soluble poly( p-phenylenevinylene) derivatives (DDMA-PPV) (Fig. 1) were used as guest materials to investigate the confinement effect on their spectroscopic properties. These polymers all contain dibenzothiophene-5,5-dioxide units in the backbones, but are different from each other in the length of alkoxy sidechain.
2. Experimental
In this paper, three employed guest polymers were synthesized for the first time in our laboratory. The detailed description of the synthesis processes will be presented elsewhere [13]. These polymers were the derivatives of poly( p-phenylenevinylene) (DDMA-PPV) (Fig. 1), with the alkoxy side-chains containing four, six and nine carbon atoms, respectively. The molecular weight of each polymer is listed below. P1 : Mn ¼ 2328; Mw ¼ 3166; PDI ¼ 1:36: P2 : Mn ¼ 1406; Mw ¼ 2520; PDI ¼ 1:79: P3 : Mn ¼ 2150; Mw ¼ 3536; PDI ¼ 1:62: 2.3. Incorporation of polymers into mesoporous hosts 0.005 g polymers were dissolved in 20 ml tetrahydrofuran (THF), then 0.2 g SBA-15 or SBA-15(m) was added and stirred at room temperature for 24 h. The solids were recovered by filtration, and dried under vacuum at 70 8C. These solid samples were denoted as P1-S15, P2-S15 and P3-S15 for the SBA-15 incorporated with P1, P2 and P3, as P1-S15(m), P2-S15(m) and P3-S15(m) for the SBA-15(m) incorporated with P1, P2 and P3, respectively. 2.4. Characterization Small-angle X-ray diffraction (SXRD) analysis was conducted on a Rigaku D/max 2550V X-ray diffractometer with Cu Ka irradiation, at 40 kV and 100 mA. Nitrogen adsorption– desorption data were obtained at 77.35 K on a Micromeritics Tristar 3000 analyzer. The samples were dried in flowing nitrogen at 120 8C for at least 24 h before measurement. Diffuse reflectance UV –Vis spectra were measured on a Varian Cary 500 Spectrophotometer. PL spectra were recorded at room temperature on a Hitachi 850 Fluorescence Spectrophotometer. The absorption and emission spectroscopy for all samples were measured at their powder state, except that those of pure polymers were obtained from their thin films coated on silica plates.
2.1. Preparation of host materials The mesoporous silica, SBA-15, was prepared under acidic condition by using an amphiphilic block copolymer as the structure-directing agent according to the literature [12]. The amine modified SBA-15 was prepared by treating SBA-15 with ethylenediaminepropyltriethoxysilane (EDPTES). Typically, 1 g SBA-15 and 5 ml EDPTES were added to 100 ml dried toluene, and refluxed under
3. Results SBA-15 shows three diffraction peaks in its SXRD pattern (Fig. 2), and can be identified as mesoporous silica with highly ordered hexagonal structure. In the SXRD pattern of SBA-15(m), these peaks still existed with a slight decrease in intensity, suggesting the retaining of highly
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Fig. 2. SXRD patterns of SBA-15 and amine modified SBA-15.
ordered mesoporous structure after the modification with EDPTES. The pore parameters of SBA-15 and SBA-15(m), obtained from the nitrogen adsorption – desorption analyses, were listed in Table 1. The diameters of the channels were ˚ for SBA-15 and SBA-15(m), respectively, 70.8 and 65.7 A which were larger than the diameters of the possible DDMA-PPV chains (Fig. 3). Considering the smaller molecular size of the polymers and the flexibility of their side-chains, the polymer molecules can crawl into the channels in the hosts when SBA-15 or SBA-15(m) powder was immerged in the solution of DDMA-PPV. The content of the polymer in each host– guest sample was estimated to be ca. 0.2– 0.4 wt% by the difference between the UV absorbance of the initial polymer solution before mixed with SBA-15 or SBA15(m) and that of the final solution before filtration. 3.1. Absorption and emission spectroscopy of pure DDMA-PPVs
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Fig. 3. Structure diagrams for P3: the repeating units with possible configuration (A and B) and the optimized polymer chain structure (C).
3.2. Absorption and emission spectroscopy of DDMA-PPVs incorporated in SBA-15 As incorporated into SBA-15, DDMA-PPVs showed shifts of their emission peaks. For P1 and P2, the emission peak red-shifted from 527 to 532 nm (Fig. 5, right) and 535 to 537 nm (Fig. 6, right), with a shift value of 5 and 2 nm, respectively. But for P3, as a contrast, the emission peak blue-shifted from 537 to 528 nm, with a shift value of 9 nm (Fig. 7, right). P1-S15 and P2-S15 also showed a red-shift of their absorption onsets (Figs. 5 and 6, left), comparing with the P1 and the P2 film. The shift value for P1-S15 and P2-S15 was 23 and 15 nm, respectively. Correspondingly, P3-S15 showed a blue-shift in its absorption onset comparing with the P3 film, although the shift is relatively small (Fig. 7, left).
The pure DDMA-PPV films showed intensive emission around 530 nm when excited by the light with wavelength of 400 nm (Fig. 4, right). As the length of alkoxy side-chain increased from 4 to 9 carbon atoms, the wavelength of the emission peak increased from 527 to 537 nm. The similar trend was also observed in their UV – Vis absorption spectra. As shown in Fig. 4 (left), the absorption onset of DDMAPPV films moved from 510 to 530 nm as the increase of side-chain length. Table 1 Pore parameters of SBA-15 and amine modified SBA-15 Hosts
Surface area (m2 g21)
Pore volume (cm3 g21)
Pore diameter ˚) (A
SBA-15 SBA-15(m)
603 494
1.053 0.822
70.8 65.7
Fig. 4. Photoluminescence (right) and diffuse reflectance UV –Vis absorption (left) spectra of the DDMA-PPV films.
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Fig. 5. Photoluminescence (right) and diffuse reflectance UV– Vis absorption (left) spectra of P1 and its host– guest composites.
Fig. 7. Photoluminescence (right) and diffuse reflectance UV –Vis absorption (left) spectra of P3 and its host–guest composites.
3.3. Absorption and emission spectroscopy of DDMA-PPVs incorporated in SBA-15(m)
surface with no or a lower degree of aggregation comparing with its pure solid-state. The result is a blue-shift in its absorption and emission spectra. A large amount of literatures have reported that organic dyes in silica matrix showed the absorption and emission spectra similar to those of their dilute solution [8]. But there is another influence, namely, electronic confinement, which would lead to a redshift of 0 – 0 transition of the confined molecules. This effect becomes stronger as the size of the guest molecules approaches the pore dimension [2]. Zeolites are aromatic chemicals were the frequently used host and guest materials to investigate such electronic confinement effect. This effect was confirmed by computer simulation and spectroscopic evidence [2,14]. However, such investigation was limited to the hosts with small pore size and the guests with small molecule size, no articles mentioned about such effect on the conjugated polymers in mesoporous environment. Here, three novel PPV derivatives were selected to investigate the possible host – guest effects. With the difference in side-chain length, P1, P2 and P3 showed different changes in their absorption and emission spectroscopy when they were incorporated in SBA-15. As shown in Fig. 5 – 7 (right), P1-S15 and P2-S15 exhibited a red-shift of emission peak, whereas P3-S15 showed a blue-shift, comparing with the P1, P2 and P3 film, respectively. Moreover, P1-S15 showed a larger shift than P2-S15. The similar trends were also exhibited in the absorption spectra (Fig. 5 – 7, left). The absorption and emission spectra of the polymers in SBA-15 were significantly different from those of the polymers in solutions. In chloroform, the absorption onsets blue-shifted to ca. 482 nm for all polymers, while the emission peaks blue-shifted to 504, 510 and 494 nm for P1, P2 and P3, respectively. The spectroscopic shifts of the polymers confined in SBA-15 are hardly explained by electrostatic effect from such as the pore surface and the intermolecules [2]. First, SBA-15 is a type of all-silica mesoporous material with Si –OH groups on the pore
When DDMA-PPVs were incorporated in amine modified SBA-15, such trends in their absorption and emission spectra were enhanced. As shown in Fig. 5 – 7 (right), the emission peaks of P1-S15(m) and P2-S15(m) had a red-shift of 6 nm and 3 nm comparing with those of P1-S15 and P2S15, respectively, whereas the emission peak of P3-S15(m) had a blue-shift of 2 nm comparing with that of P3-S15. In Fig. 5 – 7 (right), the polymers in SBA-15(m) also showed a red or blue-shift of absorption onset comparing with them in SBA-15, but the shifts were relatively small. Moreover, P3-S15(m) showed a continuous drop in the absorption from 210 to 520 nm.
4. Discussion As organic molecules were incorporated into mesopores at a low content, they would be absorbed on the large pore
Fig. 6. Photoluminescence (right) and diffuse reflectance UV– Vis absorption (left) spectra of P2 and its host– guest composites.
H. Xi et al. / Journal of Physics and Chemistry of Solids 64 (2003) 2451–2455
surface, and its polarity is less than that of ethanol, therefore the electrostatic forces from pore surface could not induce a red-shift in the absorption and emission spectra of the confined polymers. Second, as the polymers incorporated into the mesopores at a low content, the polymer molecules would be absorbed with no or a lower degree of aggregation comparing with their pure solid-state, an the Coulombic interaction energies between molecules would become smaller, leading to a wider band gap [14] and a blue-shift of the absorption edge. From the above analyses, the electronic confinement effect should be used to rationalize the spectroscopic phenomena of these host– guest systems. According to the electronic confinement theory, when P1 molecules were confined in mesoporous channels, their molecular orbitals were deformed due to the confinement, leading to increasing the orbital energies. Since the HOMO (highest occupied molecular orbital) is more sensitive than the LUMO (lowest unoccupied molecular orbitals), the overall effect is a reduction on the band gap of the frontier orbitals [2]. The reduction on the band gap can be proved by the red-shift of the onset in the absorption spectrum of P1-S15 (Fig. 5, left). Similarly, in P2-S15, the molecular orbitals of P2 also suffered from such electronic confinement effect. The molecule size of P2 was enlarged as increasing the sidechain length, and it seems the electronic confinement should be more effective. But in fact, the shift values in the absorption and emission spectra of P2-S15 were smaller than those in the absorption and emission spectra of P1-S15. This irregular result may be attributed to the influence of the steric hindrance from the side-chains. Different from the small aromatic molecules, the DDMA-PPVs possess long molecular chains and nonrigid molecular planes, so the molecular conformation of the polymers could be influenced by the steric hindrance. The distortion of the molecular conformation would decrease the conjugate degree of whole molecules, leading to an increase of the band gap. Since the influence from the distortion of the molecular conformation partly neutralized the influence from electronic confinement, the whole results are smaller red-shifts in the absorption and emission spectra (Fig. 6). By the same token, in P3-S15, the influence from the distortion of the molecular conformation exceeded that from the electronic confinement due to the steric hindrance from the much longer side-chain, and therefore leading to blueshifts in the absorption and emission spectra (Fig. 7). We also investigated the spectroscopy of P1, P2 and P3 in mesopores with strong polarity. When the conjugated polymers were incorporated into amine modified SBA-15, such trends of the red or blue-shifts in their absorption and emission spectra were enhanced. P1-S15(m) and P2-S15(m) showed a more red-shift of 6 and 3 nm in their emission spectra (Figs. 5 and 6, right), respectively, whereas P3S15(m) showed a more blue-shift of 2 nm (Fig. 7, right). These results indicate that the polarity of pore surface
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influenced the energy bands of the confined polymers. In addition, it should be noticed that the absorption pattern of P3-S15(m) was significantly different from that of P3-S15, suggesting a remarkable distortion of the polymer chain occurred due to the repulsive forces between the side-chains of P3 and the amine groups on the pore surface.
5. Conclusions We have provided the convincing spectroscopic evidence for the electronic confinement of conjugated polymers in SBA-15. Also, we have found that the confined polymers suffered from the conformation distortion due to their steric hindrance and their nonrigid conjugate plane. The electronic confinement tended to deform the molecular orbitals of polymers, leading to decreasing of the band gap of the frontier orbitals, whereas the steric hindrance from the side-chains tended to distort the conformation of polymer chains, leading to the increase of the band gap. The different trends of the spectroscopic shifts of the polymers in SBA-15 were the combined results from these opposite influences. Further investigation indicated that increasing the polarity of the pore surface would enhance the trends of the red or blue-shifts in the absorption and emission spectra.
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