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Fluorescence-Tuned Silicone Elastomers for Multicolored UV-LEDs: Realizing the Processibility of Polyhedral Oligomeric Silsesquioxane-Based Hybrid Porous Polymers Ruixue Sun, Shengyu Feng, Dengxu Wang, and Hongzhi Liu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02514 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018
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Chemistry of Materials
Fluorescence-Tuned Silicone Elastomers for Multicolored UV-LEDs: Realizing the Processibility of Polyhedral Oligomeric SilsesquioxaneBased Hybrid Porous Polymers Ruixue Sun,†‡ Shengyu Feng,†‡ Dengxu Wang,*†‡ Hongzhi Liu‡ †
National Engineering Research Center for Colloidal Materials, Shandong University, Jinan 250100, P. R. China
‡
Key Laboratory of Special Functional Aggregated Materials, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China
ABSTRACT: Processibility has been always a challenge for most of insoluble porous materials. Herein, we report a facile approach to realize their processibility by physically blending them into a thiol-containing polysiloxane matrix (PMMS) followed by an efficient thiol-ene crosslinking reaction, resulting in novel silicone elastomers. The applied porous materials are polyhedral oligomeric silsesquioxane-based hybrid porous polymers (HPPs), which were prepared by the Heck reactions of octavinylsilsesquioxane with 4,4’-dibrombiphenyl and/or 1,3,6,8-tetrabromopyrene. They exhibit tunable fluorescence with a continuous color change from blue to red by altering the molar ratio of biphenyl and pyrene units. This remarkable fluorescence modulation endows the elastomers incorporated with HPP materials similar multicolor emissions from blue to red depending on the added HPP. It was observed that HPP materials were well dispersed in the polymeric matrix when the amount of HPPs was below 20 mg per gram of PMMS. Furthermore, multicolored UV-LEDs based on these silicone elastomers were constructed by an in-situ crosslinking method; the devices show color-transformable property by controlling the light switches. These results reveal that blending insoluble porous materials with polymer matrix is an effective strategy to realize their processibility and result in novel functional composites. This simple strategy could be certainly expanded to other insoluble porous materials.
INTRODUCTION Porous organic polymers (POPs) have attracted significant attention in recent years due to their great potential in extensive applications such as gas storage, gas separation, catalysis, sensing, etc.1-4 However, most of these materials, except polymers of intrinsic microporosity, are difficult to process due to their highly crosslinked and stiff networks, which cannot be dissolved or even well dispersed in common solvents.1-4 This severe drawback leads to a large limitation of their assembling into versatile forms such as films, fibers, spheres, or any other desired shapes,5 and thus makes their application extension very difficult. To realize their facile processibility, researchers have attempted four strategies: i) introduction of long alkyl chains into the porous networks, which can impart the networks flexibility and thus solubilize the polymer;6-8 ii) fabrication of soluble porous hyperbranched or dendrimer-like polymers by lowering the crosslinking degree;9, 10 iii) fabrication of solutiondispersible porous polymers by miniemulsion polymerization;11 and iv) direct synthesis of porous thin films/membranes at the liquid-electrode interface.12 However, the first two would sacrifice partial porosity of the resultant materials and the latter two commonly require specific synthetic techniques with sophisticated process. Processibility still remains highly challenged for most of porous materials.
An efficient strategy for manipulating intractable materials (e.g., inorganic nanofillers and metal-organic frameworks (MOFs)) is composition or hybridization with flexible polymers. This strategy can not only overcome the processing difficulty by incorporating them into soft polymer matrix, but also may integrate individual advantages and produce new and versatile materials with peculiar or enhanced properties such as photostability, gas adsorption and moisture stability, which is hard to realize with the individual components.13 However, this approach has been rarely utilized to handle the insoluble POP materials and only few POP materials have been blended with polymers for functional hybrid membranes with enhanced performances for gas separation, proton conductivity, and desalination.14-17 Processing POP materials with polymers for novel composites with intriguing and tunable properties still needs to be explored. In this report, we present a novel approach to deal with the difficult processability of POP materials by physical blending with a thiol-containing polysiloxane and subsequent crosslinking by thiol-ene reaction, resulting in novel organosilicon elastomers. POP materials are chosen from polyhedral oligomeric silsesquioxane (POSS)-based porous polymers due to their high fluorescent performance and thermal stability.18 The selection of polysiloxanes is motivated by their unique advantages including high flexibility of Si-O-Si backbone, low surface energy,
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low toxicity, good biocompatibility, physiological inertness, etc.19, 20 The resultant elastomers exhibit tunable fluorescence colors from blue to red by taking advantage of the facile fluorescence tunability of porous materials. Furthermore, the mixture before crosslinking can be easily coated on various substrates and thus multicolored UVLED lights have been successfully fabricated by an in-situ crosslinking method.
RESULTS AND DISCUSSION As illustrated in Scheme 1, POSS-based hybrid porous polymers were prepared by the Heck reactions of octavinylsilsesquioxane (OVS) with 4,4’-dibrombiphenyl (BP) and/or 1,3,6,8-tetrabromopyrene (Py) using Pd(PPh3)4 as the catalyst, K2CO3 as the base and DMF as the solvent at 120°C for 48 h.21, 22 To tune the properties of porous polymers, pyrene and biphenyl were simultaneously used as comonomers with different molar ratio, resulting in nine products HPP-1 to HPP-9 (Table 1); this comonomer doping strategy has been proved its efficiency in some reports.23, 24 HPP-122 and HPP-921 have been synthesized in our previous reports; all other networks in the series are first reported here. As expected, all the polymers were afforded as solids showing various colors including light yellow (HPP-1), yellow (HPP-2 and HPP-3), light orange (HPP-4~HPP-6), dark orange (HPP-7 and HPP-8), and brick-red (HPP-9) (Fig. S1). It is worthy to note that the colors become darker as the content of pyrene increases in the frameworks, which can be explained by the bathochromic effect of the pyrene chromophores. Scheme 1. Synthesis of fluorescent hybrid porous polymers, HPP-1 to HPP-9
The HPP structures were determined by FT-IR, solidstate 13C and 29Si CP/MAS NMR and elemental analysis (see supporting information). HPP-1, HPP-8 and HPP-9 were selected as examples. In the FT-IR spectra, the characteristic peaks from 1650 to 1400 cm-1 are assigned to C=C stretching vibrations from vinyl and phenyl groups, and the strong peak at around 1110 cm-1 is caused by the Si-O-Si stretching vibration (Fig. S2). In the solid-state 13C NMR spectra, the peaks at around 120 and 150 ppm attributable to ethenylene carbons (SiCH=CH and SiCH=CH) confirm the formation of internal double bonds between vinyl and brominated aromatic groups (Fig. S3a). In the 29Si NMR spectra, the major peaks at around -71 and -80 ppm are ascribed to T2 and T3 units (Tn: CSi(OSi)n(OH)3-n) in the framework (Fig. S3b). The presence of T2 units indicates partial collapse of POSS
cages during the reaction. In addition, the broad peak at around -110 ppm may be attributed to Qn units (Si(OSi)n(OH)4-n), suggesting the partial cleavage of Si-C bonds. These results revealed that target products have been successfully achieved. Thermogravimetric analysis (TGA) reveals the high thermal stability of these polymers with Td (5% mass loss) at ca. 390°C (Fig.S4), comparable to other POSS-based porous polymers.25, 26 As expected, they are amorphous and have no long-range crystallographic order as proved by powder X-ray diffraction (PXRD), which exhibits a broad peak at around 22º 2θ associated with Si-O-Si linkages (Fig.S5), in accordance with other POSS-based nanoporous materials.25, 26 As shown in FE-SEM images, all polymers exhibit similar nanostructured pellets with a relatively uniform diameter of ca. 50 nm (Fig. S6), leading to a good dispersion stability in solvents. For example, the dispersion in THF (0.5 mg mL-1) can keep steady for ca. 8 h, evaluated by natural sedimentation (Fig. S7). The porosity was estimated by N2 adsorptiondesorption isotherms (Fig. 1a, Fig. S8a and Table S1). All the materials exhibit similar sorption tendencies with high uptakes at low relative pressures and a hysteresis loop at high relative pressures, indicating the co-existence of micropores and mesopores. For HPP-9 containing only pyrene, the BET surface area (SBET) and the total pore volume (Vtotal) are 358 m2 g-1 and 0.25 cm3 g-1, while HPP-1 containing only biphenyl shows higher porosity with SBET of 592 m2 g-1 and Vtotal of 0.79 cm3 g-1. HPP-8 containing 11.11 mol% of pyrene exhibits lower porosity than HPP-1, but higher than HPP-9. When the content of pyrene decreases to 2.56 mol% (HPP-8 to HPP-6), the porosity is similar to HPP-1. Interestingly, when the content of pyrene decreases to 0.50 mol% (HPP-5 and HPP-4), the porosity continuously changes with decreased SBET of 379 m2 g-1 but enhanced Vtotal of 0.80 cm3 g-1 (HPP-4). But further decreasing the content to 0.15 mol % (HPP-3 and HPP-2), the porosity increases with SBET of 778 m2 g-1 and Vtotal of 1.24 cm3 g-1 (HPP-2), which is even higher than HPP-1. The data reliability was proved by two measurements for all the samples. This unexpected porosity trend may be explained by the unpredictable effect of pyrene on the formation of the porous framework. Compared with biphenyl units, pyrene units possesses more connectable sites, which result in a higher local crosslinking density and expectedly afford higher porosity.27 However, due to the high rigidity and steric hindrance between pyrene and POSS units, this effect might only work in a certain content of pyrene, suggesting controlling their porosity is still a challenge. All of the polymers exhibit similar pore size distributions (PSDs) with a narrow distribution of micropores centered at ~1.4 and 1.7 nm, and a broad distribution of mesopores centered at ~2.7 nm (Fig. 1b and S8b), evaluated by nonlocal density functional theory (NL-DFT). The PSDs are consistent with the shape of the nitrogen isotherms, suggesting the presence of micro- and mesopores. Moreover, these polymers show low microporosity with Vmicro/Vtotal ratios from 0.001 (HPP-5) to
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Chemistry of Materials 0.40 (HPP-9), indicating the predominance of mesopores in the networks. Table 1. Relative molar ratios of monomers, BET surface area (SBET), optical band gaps, maximum emission wavelength (λmax), quantum yields and average lifetime of HPP-1 to HPP-9. HPPs
Molar ratios of monomers OVS
Molar ratios
SBET
BP
Py
of Py/(BP+Py)
(m g )
Optical band a gap (eV)
2
-1
Quantum
λmax b (nm)
yield (%)
c
Average lifetime (ns)
HPP-1
1
4
0
0%
592
2.46
464
22.42
1.98
HPP-2
1
3.988
0.006
0.15%
778
2.37
485
6.18
2.48
HPP-3
1
3.98
0.01
0.25%
544
2.26
503
17.07
2.70
HPP-4
1
3.96
0.02
0.50%
379
2.24
528
14.59
2.94
HPP-5
1
3.88
0.06
1.52%
388
2.21
545
14.30
2.60
HPP-6
1
3.80
0.1
2.56%
604
2.13
555
5.61
2.91
HPP-7
1
3.60
0.2
5.26%
595
2.04
570
11.96
3.01
HPP-8
1
3.20
0.4
11.11%
576
2.01
592
3.81
4.25
HPP-9
1
0
2
100%
358
1.91
605
2.46
2.66
a
b
Calculated from the onset of the absorption spectra (Fig. S9 and Table S2). Fluorescent emission maximum peak recorded c in the solid state excited at 365 nm. The absolute quantum yields were estimated using Wrighton-Ginley-Morse’s method.
the spectra show a red-shift in the optical absorption onset from 500 to 650 nm with an increase of pyrene content (Table S2). This yields to tunable band gaps from 2.46 eV (HPP-1) to 1.91 eV (HPP-9) (Table 1) when going from HPP-1 to HPP-9. For instance, compared to HPP-1, the absorption maxima of HPP-4 and HPP-9 containing 0.5 and 100 mol% of pyrene were red-shifted to 388 and 410 nm with the absorption edge at 553 and 650 nm, and band gaps of 2.24 and 1.91 eV. This finding is apparently due to the higher π-conjugated system of pyrene than biphenyl units.
Fig. 1. Nitrogen adsorption and desorption isotherms (a) and pore size distribution curves (b) of HPP-1 and HPP-7 to HPP9; (c) Fluorescent emission spectra of HPP-1 to HPP-9 in the solid-state (λex = 365 nm); (d) Photographs of HPP-1~HPP-9 -1 dispersed in ethanol (0.5 mg mL ) under irradiation with UV light at 365 nm; (e) Photographs of HPP-1 to HPP-9 in the solid-state under irradiation with UV light at 365 nm; (f) Photograph of the CIE chromaticity diagram for HPP-1 to HPP-9.
The photophysical properties of HPP-1 to HPP-9 were evaluated by UV/Vis absorption and fluorescent emission spectroscopy. All the polymers exhibit similar absorbance behaviors of strong and broad bands from 200 nm to 700 nm (Fig. S9). HPP-1 exhibits the narrowest absorption with absorption maxima at 329 nm and absorption edge at about 504 nm. The absorption gradually broadens and
The fluorescent emission spectrum in the solid state for HPP-1 containing only biphenyl is blue with the maximum wavelength (λmax) at 464 nm (Fig. 1c). With an increase of pyrene content (HPP-2 to HPP-9), the emission is gradually red-shifted with emission colors from bluishgreen at 485 nm (HPP-2) to green at 528 nm (HPP-4), yellow at 555 nm (HPP-6) and further to red at 605 nm (HPP-9) (Fig. 1c). The bathochromic shift can be visually observed in the HPPs suspensions (0.5 mg mL-1 in ethanol) (Fig. 1d) and in the solid state (Fig. 1e) under UV light upon irradiation at 365 nm. This phenomenon can be explained by the increase of the conjugation length of isolated Py units with adding up to four olefin moieties after the Heck reaction. The emission color change can be easily observed in the CIE-1931 RGB coordinates, which were calculated from the emission spectra. These materials show a continuous color changing from blue (0.15, 0.13), cyan (0.17, 0.29), bright green ((0.36, 0.54), (0.29, 0.50) and (0.22, 0.38)), bright yellow ((0.46, 0.52) and (0.41, 0.53)), orange (0.46, 0.52) to brick red (0.51, 0.47) (Fig. 1f). The absolute fluorescence qantum yield in the solid state were from 2.46% (HPP-9) to 22.42% (HPP-1), estimated using Wrighton-Ginley-Morse’s method.28 The values are comparable or higher than other fluorescence porous polymers such as CP-CMP4c (1.82%)23 and PPCPPyS-PAF-3 (17%).29 In addition, pyrene-containing polymers also exhibit a weak peak centered at 430 nm (Fig.
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S10a), which may be due to few free pyrene at the terminal of the crosslinked network. This speculation can be proved by the emission spectrum of pyrene monomer with λmax at 430 nm (Fig. S10b). Furthermore, to evaluate the stability of the fluorescence properties, their fluorescence lifetimes were measured by time-resolved fluorescence experiments. All the materials follow biexponential fluorescence decay kinetics with average lifetimes (τavg) ranging from 1.98 ns (HPP-1) to 4.25 ns (HPP-8) (Fig. S11, Table 1 and Table S3). The τavg values are comparable to, or higher than those for other fluorescent porous polymers.6, 7, 30 The dual-exponential effect could be explained by re-absorption and re-emission, which is common for fluorescent porous polymers6, 7 because of their broad absorption and their spectral overlaps between absorption and fluorescence bands (Fig. S9 and 1c). To realize the processibility of these polymers, they were blended into a polysiloxane matrix to construct cross-linking fluorescent organosilicon composites. Fortunately, the good dispersibility of these polymers in organic solvents is helpful to the processing and beneficial to their homogeneous distribution in the final composites. A thiol-containing polysiloxane, i.e., poly[(mercaptopropyl)methylsiloxane] (PMMS) was chosen because it can be easily crosslinked through the highly efficient thiol-ene click reaction using tetramethyltetravinylcyclotetrasiloxane (D4Vi) as a crosslinker and 2,2dimethoxy-2-phenylacetophenone (DMPA) as a photoinitiator under UV light. HPP-1 to HPP-9 were first blended with PMMS, D4Vi and DMPA in THF under ultrasonic irradiation for 2 h and then transferred into a Teflon mold. After THF was removed, the mixture was exposed to UV light at 365 nm for 15 min and a series of polysiloxanebased composites (E1 to E9), which are a typical class of silicone elastomers, were obtained (Scheme 2). Crosslinking was evaluated by rinsing the curing films with THF, which is a good solvent for uncured system; the obtained elastomers were insoluble in THF, indicating the successful crosslinking.31
(E5 to E7), orange (E8) and red (E9) (Scheme 2), which meet the requirements of colors (in addition to purple) for modern industry. To further test their fluorescence, E1, E4, E7 and E9 are selected as examples covering blue, green, yellow and red spectral regions (Fig. S12). It is found that high fluorescence of the elastomers can be achieved at a very low HPPs/PMMS concentration of 0.03 mg/g, suggesting that the porous properties of HPPs are maintained after the blending because of the relationship between porosity and fluorescence. It has been proved that the porosity can efficiently enhance the luminescent activity thanks to the interwoven porous network. 21, 22 In other words, if blending can’t maintain the porosity, the elastomers could not exhibit high fluorescence at a low HPPs/PMMS concentration. As expected, with increasing HPPs content from 0.5 to 5 mg/g, the fluorescence intensity was obviously enhanced (Fig. 2a and Fig. S12). For example, E1 with HPP-1 content of 5 mg/g gives the highest emission intensity, being 1.5, 2.8 and 5.8 times higher than that with HPP-1 contents of 2, 1 and 0.5 mg/g (Fig. 2a). This difference can be visually observed under UV light. E1 emits blue fluorescence with brightness enhancement when increasing HPP-1 content from 0.5 to 5 mg/g (Fig. 2a and Fig. S12 insets). Additionally, by lowering the HPP-1 content within the elastomers, especially below 1 mg/g, a slight blue-shift was observed. This finding may be caused by the aggregation effect of HPPs at high concentration while they can be well dispersed in the polysiloxane matrix at low concentration.
Scheme 2. (top) Synthetic routes of polysiloxane-based fluorescent composites; (bottom) Photographs of the composites with a concentration of HPPs/PMMS at 1 mg/g under UV light irradiation at 365 nm. Fig. 2. (a) Fluorescent emission spectra of E1 (λex = 365 nm) with various contents from 0.5 to 5 mg/g of HPP-1. The insets show the photographs of the composite elastomers under UV light at 365 nm; (b) Photographs of the composite elastomers with various contents from 0 to 200 mg/g of HPP-1 under daylight; (c) FE-SEM images of the surface of the composite elastomers with various contents from 0 to 200 mg/g of HPP1.
The color emission of resultant elastomers can be flexibly tuned within a wide range of the visible spectra by altering the composition of the different HPPs. By selecting corresponding HPPs, E1 to E9 display multicolored emission from blue (E1), cyan (E2), green (E3, E4), yellow
The compatibility between polysiloxane and porous materials was further evaluated by blending higher concentration of HPP materials into the composites (HPP-1 as an example). As illustrated in Fig. 2c, when the HPP-1 content is lower than 20 mg/g, a homogenous mixture is obtained and after crosslinking no phase separation was
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Chemistry of Materials observed on the surface of the resulting elastomers after crosslinking, indicating their good compatibility at this concentration. The good compatibility between HPPs and the polysiloxane matrix is apparently mainly due to their similar backbone structures, i.e., Si-O-Si chain segments. Another important feature for these elastomers is their transparency. When the content is higher than 20 mg/g, the elastomers become increasingly opaque (Fig. 2b). This finding can be clearly demonstrated in FE-SEM images for the surface of elastomers (Fig. 2c). At the concentration of
