Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
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Design and Synthesis of Luminescent Liquid Crystalline Polymers with “Jacketing” Effect and Luminescent Patterning Applications Ji-Chun Zhu,†,# Ting Han,‡,§,# Yang Guo,† Ping Wang,† He-Lou Xie,*,† Zhen-Gong Meng,‡ Zhen-Qiang Yu,*,‡ and Ben Zhong Tang*,§
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†
Key Lab of Environment-friendly Chemistry and Application in Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, Hunan Province, China ‡ School of Chemistry and Environmental Engineering, College of Materials Science and Engineering, Center for AIE Research, Shenzhen University, Shenzhen 518060, China § Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China S Supporting Information *
ABSTRACT: To fabricate luminescent liquid crystalline polymers (LLCs), two monomers 2,5-bis[(4′,4″-dibutyloxy)tetraphenylphthalate]styrene (M1) and 2,5-bis[(4′,4″-dibutyloxy)tetraphenylethylene]styrene (M2) have been successfully designed and synthesized. Although M1 and M2 show no LC property and M1 is nonemissive in the solid state, M2 shows very strong solidstate emission with a fluorescence quantum yield (ΦF) of 27.7%. The better solid-state luminescence behavior of M2 than M1 can be attributed to the elimination of the photoinduced electron transfer effect as suggested by the theoretical calculation results. The structural difference between M1 and M2 also results in a dramatical difference of polymerizability. While M1 can be readily homopolymerized using the radical polymerization method, M2 can only be copolymerized under harsh conditions. The resulting homopolymer poly{2,5-bis[2-(4,4′-dibutyloxy)tetraphenylphthalate]styrene} (P0) and copolymers poly{2,5-bis[(4′,4″-dibutyloxy)tetraphenylethylene] styrene}x−{[2,5-di(hexylformate)]styrene}y (Pns, n = 1, 2, 3) all show typical columnar liquid crystal phase (ColH) as demonstrated by the variable-temperature 1D wide-angle X-ray diffraction results. Similar to their corresponding monomers, P0 is weakly emissive with a low ΦF of 2.0% in the solid state whereas Pns exhibit strong solid-state fluorescence with ΦF in the range 18.0−45.1%. The ΦF value of the copolymers increases with the increasing contents of composition M2. The obtained Pns with good solution processability can be used to prepare highly luminescent two-dimensional patterns with high resolution through nanoimprint lithography, which reveals that Pns find potential applications in advanced optoelectronic and biophotonic devices.
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INTRODUCTION
efficiency has become a very interesting and significant research field.14−20 Aggregation-induced emission (AIE) proposed by Tang and co-workers in 2001 is a photophysical phenomenon that is completely opposite to ACQ.21 Generally, luminogens with AIE effect (AIEgens) are weakly or even nonemissive in dilute solutions due to the intramolecular motions which results in the dissipation of excited-state energy via nonradiative relaxation channel approach.22−24 However, the intramolecular motions can be restricted due to physical constraints in the aggregate or solid states, and thus the excited state of the luminogen can decay radiatively to the ground state to induce or enhance the fluorescence of the luminogens.25−30 Thus, it becomes a good strategy to solve the emission quenching issue of conventional LLCs and fabricate highly luminescent LLCs
Luminescent liquid crystals (LLCs) combining luminescent properties and ordered structures of liquid crystals (LC) have attracted great attention due to their significant applications in organic light-emitting diodes, luminescent liquid crystal displays, sensors, and optical storage information.1−7 The formation of ordered LC structure for LLCs generally requires the regular packing in solid state. Unfortunately, conventional luminophores in LLCs with rod- or disc-like shapes tend to form excimeric species due to the presence of strong π−π stacking interactions in solid states. The excited states of the luminophore aggregates usually descent to the ground state through nonradiative relaxation pathways to lead to the emissive quenching of the luminophores, and thus conventional LLCs often show an aggregation-caused quenching (ACQ) phenomenon.8−13 This phenomenon seriously limits the practical applications of LLCs. Therefore, how to reasonably design and fabricate LLCs with high emission © XXXX American Chemical Society
Received: January 31, 2019 Revised: April 8, 2019
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DOI: 10.1021/acs.macromol.9b00221 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules by introducing AIEgen as the substituent groups into the LC mesogens.31−37 As a typical AIEgen, tetraphenylethylene (TPE) shows highly efficient solid-state luminescence because of its twisted “propeller-like” structure.38−42 By modifying the periphery of TPE molecule with suitable substituent groups, such as alkane and LC mesogens, different LLCs have been fabricated.43,44 These LLCs can show high luminescence efficiency and various LC phase structures.45−49 For example, LLCs with TPE as the luminescent core have been reported to show rectangular columnar and hexagonal columnar phase dependent on the tail chains. The emission color can be rapidly and reversibly tuned from sky blue to green.50 The LLC with rodshaped LC moieties as the periphery and TPE as core not only shows high solid-state luminescence efficiency but also exhibits a unique biaxial oriented structure.51 In addition to small molecules, luminescent liquid crystalline polymers (LLCPs) have also been successfully prepared by copolymerizing the mesogens and TPE luminogens.52 On the basis of the AIE and “jacket” effect, we have successfully fabricated a series of TPE-containing LLCPs in our previous work.53 The “jacket” effect comes from the special mesogen-jacketed liquid crystalline polymers (MJLCPs),54 which is a kind of LCPs with bulky side groups attached to the polymer main chain through a very short spacer or without spacer. The bulky side groups of MJLCPs do not have to be mesogenic and can serve as a “jacket” around each chain backbone. Because of the steric requirements of the “jacket”, the main chain of the resultant LCPs can be forced to show an extended conformation.55,56 The obtained LLCPs with different spacer length can show smectic A phase and the columnar phase. Meanwhile, with the decrease of the spacer length, the fluorescence quantum yield (ΦF) of the LLCPs dramatically increases. On the basis of this idea, herein we intend to further design and synthesize new highly luminescent LLCPs without spacers by directly connecting the TPE moiety on the 2- and 5position of a styrene through the ester bonds or carbon− carbon bonds. The resulting monomer without ester groups (M2) shows a much higher solid-state ΦF value of 27.7% than that of the monomer with ester groups (M1). Although both monomers possess vinyl groups, M1 can be easily polymerized by radical homopolymerizations while M2 is hard to be homopolymerized possibly due to the poorer solubility and larger structural steric hindrance of M2. Therefore, further copolymerization was implemented for monomer M2. The chemical structures of the resultant polymers are shown in Scheme 1. All the obtained polymers show typical LC behavior, and all copolymers (Pns) show obvious AIE properties. By utilizing the good solution processability and high solid-state fluorescence of Pns, well-resolved fluorescent two-dimensional patterns were readily fabricated through the nanoimprint lithography.
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Scheme 1. Molecular Structures of P0 and Pns
chloride was carried away with dichloromethane through the rotary evaporator, and this procedure was repeated three times until the oxalyl chloride was completely removed. Then 4,4′-dibutyloxytetrastyrene (4.20 g, 7.58 mmol) was dissolved in 150 mL of distilled THF in a 250 mL magnet-containing round-bottom flask followed by the addition of triethylamine (5.2 mL, 37.90 mmol). Then the prepared vinyl terephthalate chloride was dissolved in THF and added dropwise into the reaction an ice bath. After the dropwise addition, the reaction was performed at room temperature for 12 h. After completion of the reaction, 1 mL of water was added dropwise to quench the reaction, and the resulting white precipitate was removed by suction filtration. Then THF solvent was removed by rotary evaporation. The product was purified by column chromatography on silica gel with dichloromethane:petroleum ether = 2:1 (v/v) as eluent. Yield: 85%. 1H NMR (CDCl3) δ (ppm): 8.42 (d, 1H, Ar−H), 8.13 (d, 2 H, Ar−H), 7.21−6.61 (m,34 H, Ar−H), 7.41 (t, 1 H, −CH CH2−), 5.83 (t, 1 H, −CHCH2−), 5.54 (m, 1 H, −CHCH2−), 3.95 (m, 8 H, −OCH2−), 1.77 (t, 8 H, −CH2−). 1.32−1.51 (t, 8 H, −CH2−), 0.95 (t, 12 H, −CH3). 13C NMR δ (ppm): 164.8 (CO), 164.1 (CO), 157.77−157.85 (phenyl C−O), 148.71−148.81 (phenyl C−O−CO), 142.35−142.39 (CC), 140.60−140.84 (phenyl C−CC), 137.92 (aromatic C−CHCH2), 137.92 (phenyl C−CC), 135.94 (aromatic C−CO), 134 (aromatic −C−CCH−), 133.23 (aromatic meta C−CHCH2), 118.41− 132.61 (phenyl C), 114 (CH2), 67.52−67.54 (−OCH2−), 29.71− 31.39 (−CH2−), 19.27 (−CH2−), 13.89 (−CH3). IR (ATR, cm−1): 3051, 985.57 (HCCH2) 2930.51, 2857.41 (−CH2−), 3051.32, 1603.64, 1491.57, 759.93, 746.77, 696.18 (−Ar), 1716.25 (CO), 1380.06 (−CH3), 1387.71, 842.45 (−CC−). Melting point (mp): 200 °C. Synthesis of Poly(2,5-bis[2-(4,4′-dibutyloxy)tetraphenylphthalate]styrene) (P0). The monomer 2,5-bis[2-(4,4′dibutyloxytetrastyrene)]vinyl terephthalate (1.052 g, 0.92 mmol) and initiator AIBN (1 mg/mL, 0.1 mL) were sequentially added to the polymerization tube, and distilled THF was added as the solvent to make the mass fraction of the monomer be 30%. After three times of freezing and three times of melting, the tube was sealed under vacuum. The polymerization tube was stirred in an oil bath at 60 °C for 12 h. Afterward, the polymerization tube was opened, and the polymerization solution was diluted with three times of THF solvent. The diluted polymerization solution was added dropwise into methanol to precipitate the polymeric products, and the sedimentation process was repeated twice to remove the monomer. The obtained precipitate was collected and dried in a vacuum oven at 30 °C overnight. Yield: 75%. Synthesis of M2 and Pns. The synthetic routes of M2 and Pns are shown in Scheme 3. The detailed synthesis information about the intermediates is shown in the Supporting Information, and the information about monomer and polymer is described as follows.
EXPERIMENTAL SECTION
Synthesis of M1 and P0. The synthetic route of M1 and P0 is shown in Scheme 2. The detailed synthesis information about the intermediates is shown in the Supporting Information, and the information about monomer and polymer is described as follows. Synthesis of 2,5-Bis[2-(4,4′-Dibutyloxytetraphenylphthalate)]styrene (M1). In a 250 mL round-bottom flask equipped with a magnetic stirrer was added vinyl terephthalic acid (0.63 g, 3.30 mmol), 120 mL of dichloromethane, 1.3 mL of oxalyl chloride, and 2 drops of DMF. The acid chloride reaction was complete until the solution becomes clear. The remaining oxalyl B
DOI: 10.1021/acs.macromol.9b00221 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 2. Synthetic Routes of M1 and P0
Scheme 3. Synthetic Routes of M2 and Pns
Synthesis of 2,5-Bis[(4′,4″-dibutyloxy)tetraphenylethylene]styrene (M2). 4′,4″-Dibutyloxytetraphenylethylene-4-borate (1.5 g, 2.50 mmol), potassium carbonate (0.7 g, 5.10 mmol), phase transfer catalyst (methyltrioctylammonium chloride), 2,5-dibromostyrene (0.325 g, 1.25 mmol), and tetrakis(triphenylphosphine)palladium (0.1 g, 0.087 mmol) were added into a 250 mL three-necked flask. The mixture was evacuated first and then bubbled through N2 for three consecutive times. Finally, 90 mL of toluene/H2O (2:1) was injected into the mixture as solvent, and the mixture was stirred in an oil bath at 90 °C for 24 h. The obtained crude product was extracted with H2O/DCM (3:1) and then purified by column chromatography on silica gel (petroleum ether:DCM = 4:1). Yield: 50%. 1H NMR (CDCl3) δ (ppm): 7.79 (s, 1 H, Ar−H), 7.48 (m, 2 H, Ar−H), 7.4 (m, 1 H, Ar−H), 7.32 (m, 1 H, Ar−H), 7.12 (m, 16 H, Ar−H), 6.95 (m, 8 H, Ar−H), 6.64 (m, 8 H, Ar−H), 6.7 (m, 1 H, −CHCH2), 5.69 (d, 1 H, −CHCH2), 5.2 (d, 1 H, −CHCH2), 3.89 (m, 8 H, −OCH2−), 1.73 (s, 8 H, −CH2−), 1.46 (s, 8 H, −CH2−), 0.98 (s, 12 H, −CH3). 13CNMR: 158 (TPE C−O), 148.71−148.81 (TPE CHCH−C), 142.35−142.39 (TPE −C C−), 138.69−139.67 (phenyl C−CC), 138.53 (aromatic C−CH
CH2), 137.95 (phenyl C−CC), 136.20 (aromatic −C−CCH−), 133.95 (aromatic meta C−CHCH2), 113.48−132.68 (phenyl C), 114.68 (CH 2), 67.48 (−OCH 2−), 31.4 (−CH 2−), 19.30 (−CH2−), 13.94 (−CH3). IR (ATR, cm−1): 3023.12, 1287.24 (HCCH2), 2920.16, 2864.14 (−CH2−), 3023.95, 1600.31, 1569.78, 1501.66, 1244.23, 1170.63, 1011.19, 912.32, 814.97 (−Ar), 2957.36, 1385.51 (−CH3), 1385.51, 864.22 (−CC−). MS (MALDI-TOF MS): m/z [M]+ calcd for C76H76O4, 1052.6; found, 1052.6. Melting point (mp): 150 °C. Synthesis of Copolymer Poly(2,5-bis[(4′,4″-dibutyloxy)tetraphenylethylene]styrene) x −(2,5-Di(hexylformate)]styrene)y (Pns). Into a polymerization tube was added 0.5 g (0.475 mmol) of 2,5-bis[(4′,4″-dibutyloxy)tetraphenylethylene]styrene (M2), 5 μL of tert-butyl hydroperoxide, and 0.7 g (1.9 mmol) of [2,5-di(hexylformate)]styrene (HCS). Subsequently, the reaction was purged with argon, and the reaction was polymerized at 170 °C for 10 h. Yield for P1: 60%. The quantities of M2 and HCS used for the synthesis of copolymer P2 are 0.5 g (0.475 mmol) and 0.264 g (0.712 mmol), respectively. Yield for P2: 56%. The quantities of M2 and C
DOI: 10.1021/acs.macromol.9b00221 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules HCS used for the synthesis of copolymer P3 are 0.5 g (0.475 mmol) and 0.117 g (0.317 mmol), respectively. Yield for P3: 54%.
Compared to its analogue M1, M2 showed much poorer solubility in most organic solvents due to its more rigid structure with larger steric hindrance, which could be the crucial reason for the poor homopolymerizability of M2. To solve this problem, a bulk polymerization was employed and the polymerization was conducted at ∼170 °C, which slightly exceeded the melting point of M2 (150 °C). tert-Butyl hydroperoxide with a half-life period of 10 h at 170 °C was used as the initiator. To facilitate the polymerization, comonomers with smaller steric hindrance were used to copolymerize with M2. Because of the low boiling point of the commonly used commercial monomers such as styrene and methyl methacrylate, the lab-made [2,5-di(hexylformate)]styrene (HCS)57 with high boiling point was employed in this system. By controlling the feed ratios, three copolymers (Pns, n = 1, 2, 3) with different contents of M2 and HCS were successfully achieved. The resulting copolymers showed very good solubility in common solvents, such as acetone, THF, toluene, chlorobenzene, and dichloromethane. As shown in Figure S2, the resonance signals of monomer M2 at 7.79 ppm (peak a) and 7.48 ppm (peak b) are assigned as the characteristic peaks of the phenyl protons adjacent to the vinyl group while those at 5.20 ppm (peak i), 5.72 ppm (peak j), and 6.70 ppm (peak k) are characteristic peaks of the vinyl substituent. The resonance peaks in the region of 6.64−7.40 ppm (peaks c, d, e, f, g, and h) were ascribed to the characteristic peaks of the TPE substituents, and the rest of peaks at 3.92 ppm (peak l), 1.76 ppm (peak m), 1.45 ppm (peak n), and 0.98 ppm (peak o) were related to the resonances of alkoxy protons. After the copolymerizations, peaks i, j, and k associated with the vinyl substituent clearly disappeared in the 1H NMR spectra of Pns (Figure 1b). Meanwhile, the peaks of the polymers became broader compared to the peaks of corresponding monomer, which was in good consistence with the formation of expected polymeric structures. The peaks at “s” and “t” positions were assigned to the characteristic resonances of HCS, and the broad peak at 3.75 ppm was related to the characteristic resonance of the “l” and “p” protons in HCS and M2. On the basis of the integrals of the circled area, we further calculated the content of each composition in the copolymers according to the equations
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RESULTS AND DISCUSSION Synthesis and Characterization of Polymers. Monomers M1 and M2 were successfully prepared according to the synthetic route shown in Schemes 2 and 3, whose chemical structures were verified by combined characterization methods. Although both monomers were styrene derivatives, only M1 was able to be readily polymerized using the radical polymerization method. After polymerization, all characteristic peaks associated with the resonances of the vinyl substituent in M1 at 5.50 ppm (peak a), 5.80 ppm (peak b), and 7.52 ppm (peak c) disappeared (see Figure S1 and Figure 1a). In the
8y = 1 8x + 4y = a
where a represents the integral area of the “l” and “p” peaks as labeled in Figure 1. As summarized in Table 1, the final content of M2 in copolymers P1, P2, and P3 is calculated to be 18.0, 32.4, and 45.1%, respectively. It was worth noting that Figure 1. 1H NMR spectra of (a) P0 and (b) Pns (P1, P2, and P3) in CDCl3.
Table 1. Composition and GPC Results of P0, P1, P2, and P3
meantime, the resonance peaks of the polymer P0 were much broader than those of the corresponding monomer (M1), suggesting its polymeric nature. GPC results showed that the number-average molecular weight (Mn) and polydispersity index (PDI) of P0 were 9.10 × 105 and 1.98, respectively. However, the same polymerization process did not work for M2 whose structure was similar to M1 but without ester groups. We also tried to conduct the polymerizations of M2 in different solvents and different polymerization temperatures, but all experiments failed.
P0 P1 P2 P3
monomer feed molar ratio [HCS]:[M2]
the content of HCSa (%)
the content of M2a (%)
Mnb (×104)
4:1 3:2 2:3
82.0 67.6 54.9
18.0 32.4 45.1
91 1.25 1.04 0.67
a
Determined by the calculation results according to the 1H NMR spectra. bDetermined by GPC measurements in THF on the basis of a linear polystyrene calibration. D
DOI: 10.1021/acs.macromol.9b00221 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 2. DSC thermograms of P0, P1, P2, and P3 recorded under nitrogen at a rate 10 °C/min during the first cooling cycle (a) and the second heating cycle (b).
the content of each composition in the polymer structures did not match the corresponding feed ratio because of the different activity of these two monomers. The GPC results of the obtained polymers are also depicted in Table 1. Phase Transitions and LC Property. TGA results suggested that the obtained LLCPs possessed very good thermal stabilities. The decomposition temperature of P0 and Pns at 5% weight loss was higher than 300 °C in an air atmosphere and was ∼350 °C in a nitrogen atmosphere. Subsequently, the DSC experiments were performed to investigate the phase transitions. First heating to 230 °C was performed at a rate of 20 °C/min and then followed by an isothermal annealing process for 5 min to eliminate the thermal history. The DSC trace were further recorded at 10 °C/min. As shown in Figure 2, the DSC thermograms of the polymers only show a glass transition temperature (Tg), which is the typical characteristic of MJLCPs. Moreover, the Tg value increased with the increase of the content of M2, which could be attributed to the larger side group of M2 than that of HCS. PLM results indicated that the obtained two monomers could show typical crystalline birefringence phenomenon below the melting point. The birefringence of the crystal disappears once the temperature exceeded the melting point. However, obvious birefringence phenomenon can be observed for P0 as the temperature was beyond Tg. The clearing point did not appear before the decomposition, and meanwhile the birefringence phenomenon remained in the cooling process. These results indicated that its liquid crystal phase was relatively stable. For P1−P3, all copolymers did not exhibit birefringence during the heating process. However, when shear force was implemented, the birefringence phenomenon quickly appeared (see Figure 3). This observation could be explained by the change of the size of the LC domains. The LC domains were too small to be observed in the initial state. The application of external inducing could prompt the small LC domains to agglomerate into big domains with sizes that are enough to be observed. To gain further insight into phase structures, one-dimensional temperature-variable X-ray diffraction (1D-WAXD) experiments were carried out. The 1D-WAXD results revealed
Figure 3. Representative textures of P0 (a), P1 (b), P2 (c), and P3 (d) at 140 °C (50× magnification).
that M1 and M2 formed a crystalline state (Figure S3). For P0, during the variable temperature process, a strong diffraction peak at around 2θ = 3.00° and a very weak diffraction peak at around 2θ = 5.11° in the low angle region together with a diffuse diffraction halo at 2θ = 19.0° in the high angle region were observed (see Figure 4a,b). Although the two diffraction peaks were slightly shifted due to the cold shrinkage and thermal expansion, the ratio of the two peaks always remained at 1:31/2, indicating that a stable hexagonal columnar phase was formed during the whole variable temperature process. Further calculating results showed the columnar diameter was ∼3.43 nm. As the temperature increased, the intensity of the diffraction peak in the low-angle region became stronger and the half-peak width became narrower, suggesting that the domain region of the supramolecular structure was improved. A similar phenomenon was observed for the copolymers. As shown in Figure 4c,d, two peaks at around 2θ = 4.01° and 2θ = E
DOI: 10.1021/acs.macromol.9b00221 Macromolecules XXXX, XXX, XXX−XXX
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Figure 4. 1D WAXD profiles of P0 (a, b) and P3 (c, d) recorded at various temperatures upon heating (a, c) and cooling (b, d).
M2 in THF and THF/water mixtures with different f w were investigated. As depicted in Figure 5a,b, with the gradual addition of water, a nonsolvent of M1 into the THF solution, the PL spectra first remained almost unchanged when f w was lower than 60%, and then an obvious emission band peaked at ∼520 nm was observed at higher f w. Although the PL intensity of M1 aggregates in THF/water mixture with 90% water fraction at 520 nm was detected to be over 10-fold higher than that in THF solution, no obvious luminescence can be observed by naked eyes (see the inset in Figure 5b). The ΦF value of M1 was measured to be almost zero in both the solution and solid state. Therefore, both the THF solution and the aggregated states of M1 were nonemissive despite the presence of the typical AIE-active moiety in its structure. By contrast, M2 showed an obvious AIE phenomenon. As shown in Figure 5c,d, the THF solution of M2 is almost nonemissive, and the corresponding PL spectrum is basically a flat line parallel to the abscissa. When the f w of the THF/water mixture reached 60%, M2 became emissive. Further increase of the water content greatly enhances the PL intensity. Bright luminescence could be observed at f w of 80% and 90% with
7.11° in low angle region and one diffused halo at approximately 2θ = 20.0° were observed for P3 during the whole variable temperature process, which also proves the formation of typical hexagonal columnar phase with the diameter of 2.55 nm. The 1D-WAXD results of P1 and P2 (see Figure S4) showed peaks at similar positions to those of P3 in the small- and wide-angle regions, indicating that the change in the content of M2 had little effect on the stacked way of the molecules. Photophysical Properties of the Monomers. The photophysical properties of the monomers were investigated in THF solution and in THF/water mixtures with different water fractions (f w). As shown in Figure S5, the absorption spectra of the THF solutions of M1 and M2 are peaked at 316 and 350 nm, respectively. The red-shifted absorption wavelength of M2 in comparison to M1 can be attributed to the larger conjugated structure of M2 arising from the elimination of ester bonds. With the well-known AIEgen TPE in the structures, both monomers were expected to be AIE-active. To verify this conjecture, the photoluminescence (PL) spectra of M1 and F
DOI: 10.1021/acs.macromol.9b00221 Macromolecules XXXX, XXX, XXX−XXX
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Figure 5. (a, c) Emission spectra of M1 (a) and M2 (c) in THF and THF/water mixtures with varied volume fraction of water ( f w). (b, d) Plots of relative PL intensity (I/I0) at 520 nm (for M1) and at 502 nm (for M2) versus the composition of the aqueous mixtures of M1 (b) and M2 (d). Inset: fluorescence photos of M1 and M2 in THF with a f w of 0 and 90% taken under 365 nm UV light irradiation. Solution concentration: 10−5 M; excitation wavelength: 365 nm.
basis set. The calculated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) plots of M1 and M2 are depicted in Figure 6. For M1, the LUMO was localized over the TPE and styrene moieties while the HOMO was mainly concentrated on the TPE moiety. The obvious variation in the electron density distributions of the HOMO and LUMO of M1 indicated that M1 possesses a donor−acceptor structure and thus allows it to undergo a photoinduced electron transfer (PET) process.58−60 Therefore, the weak luminescence of M1 in both solution and aggregated states could be possibly attributed to the fluorescence quenching effect of the PET process. For M2, the removal of ester groups enabled the electron communication between the TPE and styrene moieties and thus resulted in the extended π−π conjugated structure. The DFT result of M2 revealed that the electron cloud distribution of its HOMO and LUMO was similar, suggesting that the PET process was blocked by the small variation of the molecular structure. Therefore, M2 was
the wavelength of the emission maximum (λem) at 502 nm. The maximum PL enhancement of M2 was attained at a f w of 90%, whose PL intensity was almost 130-fold that in the THF solution. The ΦF value of M2 in THF solution and solid state was measured to be 0.1% and 27.7%, respectively. These results clearly indicated that M2 was AIE-active. Compared to our previous work, the luminous efficiency was almost 5.6-fold higher than that of the previously reported best monomers.53 From the structural point of view, the only difference between M1 and M2 was the presence of an ester group in M1. The TPE moiety and the styrene moiety were linked via the ester group in M1 while the TPE moiety was directly connected to the styrene moiety in the structure of M2. However, M1 and M2 showed obviously different luminescent properties especially in the solid state. To investigate the electronic structure effect on the luminescence behavior of M1 and M2, the density functional theory (DFT) calculations were performed using the B3LYP functional with the 6-31G(d,p) G
DOI: 10.1021/acs.macromol.9b00221 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. Molecular orbitals of the ground states of M1 and M2 calculated by B3LYP/6-31G(d,p).
and in THF solution (A90/A0) was calculated to be 4.5, 17.2, 22.0, and 23.0. These results suggested that the luminescence performance of P0 was much poorer than that of other polymers (P1, P2, and P3). The corresponding DFT calculation results for the simplified P0 structure implied that the PET effect was not blocked after the polymerization of M1 (see Figure S8). This explained why the emission of P0 was still weaker than the polymers prepared from M2. An obvious difference between M1 and P0 was that the luminescence becomes slightly stronger after polymerization, and greenish-blue luminescence could be observed in the aggregate or solid states of P0 by the naked eye, albeit in a relatively weak intensity. This observation could be rationalized by the amplification effect of polymers. The larger steric hindrance and more rigid conformation of polymer structures compared to the corresponding monomers could further restrict the molecular motions in the aggregated states and thus resulted in the relatively stronger luminescence. On the other hand, the results in Table 2 also implied that the increase of M2 content is favorable for the solid-state luminescence of the corresponding copolymers. Both the ΦF and the emission enhancement (I90/I0 and A90/A0) of the copolymer gradually increase as the M2 content increases. Highly Luminescent Patterning of the Copolymers. The generation of highly luminescent two-dimensional (2D) patterning is important for the fabrication of optoelectronic and biophotonic devices.61 Because the gel-like saturated solutions of Pns exhibited highly efficient solid-state luminescence and good solution processability, we tried the possibility of patterning Pns to generate highly luminescent 2D patterns with a well-ordered structure. Herein, high throughput nanoimprint lithography was employed to fabricate the luminescent 2D patterns.62 Taking P3 as an example, the saturated solution of P3 in chloroform was first drop-casted onto a clean silicon wafer. Then a poly(dimethylsiloxane) (PDMS) template with the periodicity of 24 μm and feature size of 12 μm was imprinted onto the drop-casting film. Finally, the highly ordered 2D pattern was successfully achieved on the substrate after evaporating the solvent and lifting off the template. As shown in Figure 8a, the SEM image reveals that imprinted pattern presents highly ordered line arrays with the
able to show strong luminescence in the solid state. In addition to the electronic structure, the molecular conformations and motions also have a great influence on the luminescent properties. In the solution state, M1 and M2 underwent free and active molecular motions, which would act as a nonradiative decay pathway for the excitons to result in the weak luminescence of both M1 and M2 in THF solutions and aqueous mixtures with a low f w. Upon aggregation formation or in the solid state, these active motions would be restricted due to the physical constraint and thus blocked the nonradiative relaxation pathway, rendering the monomers to show stronger luminescence. The appearance of the obvious emission band peaked at ∼520 nm in the PL spectra of M1 at higher f w and the strong luminescence of M2 in the aggregated states all supported this mechanism. Photophysical Properties of the Polymers. The absorption spectra of P0 and copolymer Pns were measured and are shown in Figure S5. The absorption maximum of P0 and Pns (P1−P3) was located at 330 and 346 nm, respectively. The PL behaviors of these polymers were then investigated. As depicted in Figure 7 and Figure S7, all polymers showed an aggregation-enhanced emission (AEE) phenomenon, although the emission enhancement was obviously different between P0 and Pns due to their structural difference. In comparison to the monomers, the molecular motions of the luminogens were partially restricted in nature due to the polymeric structures. Therefore, these polymers were emissive in their THF solutions although the intensity was weak. With the addition of water into the THF solution, the PL intensity of these polymers gradually increased and reached the maximum at f w of 90%. Their aggregates showed a blue-green light emission with at a λem at 502 nm. The ΦF value of the solid powders of P0, P1, P2, and P3 was measured to be 2.0, 18.4, 21.0, and 24.3%, respectively, which is much higher than their ΦF in THF solutions (Table 2). These results clearly indicated these polymers are AEE-active. In addition, as summarized in Table 2, the emission enhancement (I90/I0) of P0, P1, P2, and P3 from the THF solution (I0) to the aqueous mixture with f w of 90% (I90) was determined to be about 5.0, 18.6, 23.8, and 25.3, respectively. The integral ratio of the emission area in THF/water mixture with 90% water content H
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Figure 7. (a, c) Emission spectra of P0 (a) and P3 (c) in THF and THF/water mixtures with varied water fractions (f w). (b, d) Plots of relative PL intensity (I/I0) at 502 nm versus the composition of the aqueous mixtures of P0 (b) and Pns (d). Inset: fluorescence photos of P0 and P3 in THF with a f w of 0 and 90% taken under 365 nm UV light irradiation. Solution concentration: 10−5 M; excitation wavelength: 365 nm.
microscope. As depicted in Figure 8b, the fluorescence image shows obvious alternative bright and dark stripes with a high contrast, and the scale of the stripes was in good consistence with that of the pattern as shown in SEM image. The bright and dark stripes in this pattern can be readily assigned to the polymer and the blank space, respectively. This result demonstrated that this kind of LLCPs could be used for the fabrication of highly luminescent 2D patterns.
Table 2. Fluorescent Properties of the Monomers and Polymersa M1 M2 P0 P1 P2 P3
I90/I0
A90/A0
Φsoln (%)
Φsolid (%)
13.6 126.5 5.0 18.6 23.8 25.3
8.7 100.4 4.5 17.2 22.0 23.0
∼0 0.1 ∼0 0.7 0.8 1.1
0 27.7 2.0 18.4 21.0 24.3
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CONCLUSIONS In summary, highly luminescent monomer and LLCPs with the AIE effect were successfully prepared by rational molecular design. The obtained two monomers (M1 and M2) only differed in the presence of ester groups in their chemical structures, but they showed significantly different photophysical behaviors. M1 with ester groups was almost nonemissive due to the PET effect, while M2 without ester groups showed strong luminescence emission in solid state. After polymerization, all the obtained polymers showed typical
a
Abbreviation: I90 and I0 = intensity at f w = 90% and 0%, respectively; A90 and A0 = the integral of the PL spectrum of samples at f w = 90% and 0%, respectively; Φsoln and Φsolid = the fluorescent quantum yield of the THF solution and solid powder of the samples, respectively.
same periodicity and feature size as the template. Meanwhile, the morphology shows a perfect opposite pattern of the corresponding template. Furthermore, the luminescence behavior of the pattern was investigated using a fluorescence I
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Figure 8. SEM image (a) and the fluorescence microscope image (b) of the regular line arrays of polymer P3 patterned on the Si substrate.
and 21674065), Beijing National Laboratory for Molecular Sciences (BNLMS201815), and the Hunan 2011 Collaborative Innovation Center of Chemical Engineering & Technology with Environmental Benignity and Effective Resource Utilization.
columnar liquid crystal phase (ColH) that was independent of the chemical structure and component content. Meanwhile, these polymers showed a typical AEE phenomenon. Although the homopolymer P0 prepared from the radical homopolymerization of M1 still showed weak solid-state emission, the copolymers (P1−P3) prepared from the copolymerizations of M2 and HCS were highly luminescent in the aggregated states. The ΦF of the copolymers gradually increased (18.4, 21.0, 24.3%) with the increasing M2 content (18.0, 32.4, 45.1%) in the copolymer structures. The obtained Pns was able to be used for preparation of highly luminescent 2D patterns through the nanoimprint lithography, indicating that this kind of polymers were promising materials for applications in optoelectronics, nanophotonics, and biological imaging and sensing.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00221. Materials; instruments and measurements; experiment about some intermediate products; experiment results of 1 H NMR, variable-temperature 1D-WAXD, spectra absorption spectra, emission spectra, molecular orbitals of the ground states of the simplified P0 and P(M2) (PDF)
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REFERENCES
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (H.L.X.). *E-mail:
[email protected] (Z.Q.Y.). *E-mail:
[email protected] (B.Z.T.). ORCID
He-Lou Xie: 0000-0003-4103-2634 Zhen-Qiang Yu: 0000-0002-0862-9415 Ben Zhong Tang: 0000-0002-0293-964X Author Contributions #
J.-C.Z. and T.H. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (NNSFC 21674088, 21374092, J
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