Multiple Stimuli-Responsive and White-Light Emission of One-Pot

Jul 24, 2015 - Radhakanta GhoshSujoy DasKalishankar BhattacharyyaDhruba P. ChatterjeeAtosi BiswasArun K. Nandi. Langmuir 2018 34 (41), 12401- ...
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Multiple Stimuli-Responsive and White-Light Emission of One-Pot Synthesized Block Copolymers Containing Poly(3-hexylthiophene) and Poly(triethyl glycol allene) Segments Yan-Yu Hu, Ming Su, Cui-Hong Ma, Zhipeng Yu, Na Liu, Jun Yin, Yunsheng Ding, and Zong-Quan Wu* Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Anhui Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology, Anhui Province, Hefei 230009, China S Supporting Information *

ABSTRACT: Conjugated block copolymers with tunable properties have attract considerable research interests in recent years. Herein, we report a series of novel block copolymers containing conjugated poly(3-hexylthiophene) (P3HT) and poly(triethyl glycol allene) (PTA) segments which were synthesized in one pot using nickel complex as a single catalyst via distinct polymerization mechanisms. Interestingly, the P3HT-b-PTA diblock copolymers exhibit excellent thermoresponsive properties in water, and the lower critical solution temperature (LCST) is dependent on polymer concentration and the block ratio. Moreover, the diblock copolymers showed pH-responsive properties in CHCl3 with the emission color shuttled between orange and deep green upon the alternate additions of trifluoroacetic acid and triethylamine. Both P3HT-b-PTA and P3HT-b-PTA-b-P3HT block copolymers exhibit solvatochromism properties. The emission of the block copolymers can be facilely tuned through variation on solvents with the emission color spanned widely from red to blue. Very interestingly, white-light emission can be readily achieved from the P3HTb-PTA-b-P3HT triblock copolymer in the mixture of THF and methanol with 1/3 volume ratio.

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INTRODUCTION Poly(3-hexylthiophene) (P3HT) has attract considerable research interests in recent years owing to its excellent optical properties, environmental stability, and synthetic accessibility as well as the numerous methods available for its modifications.1 Well-defined regioregular P3HT was usually prepared through a quasi-living Kumada catalyst transfer polymerization (KCTP), independently discovered by Yokozawa et al.2 and McCullough et al.3 The polymerization mechanism was further confirmed by the important milestone works reported by Kiriy,4 McNeil,5 Luscombe,6 and Koeckelberghs.7 Thanks to the living nature of the KCTP, versatile synthesis of chain-end-functionalized P3HTs as well as block copolymers having P3HT segment(s) can be achieved.8 The combination of P3HT with a stimuliresponsive segment would produce novel multifunctional materials with tunable properties.9 For example, Park and collaborators synthesized poly(3-octylthiophene)-b-poly(ethylene oxide) copolymer which changed the emission color varying on the solvent polarity or ionic additives.10 Chen et al. developed P3HT-b-poly(2-(dimethylamino)ethyl methacrylate) rod−coil block copolymers with the distinct variations on the surface structure and their photophysical properties through solvent and temperature stimulus.11 White-light-emitting materials have received considerable research interest in recent decades because of their potential applications in full-color flat panel displays, etc.12 In this © XXXX American Chemical Society

context, organic white-light-emitting materials based on conjugated polymers has been the focus of intense research efforts due to their great potential of flexibility and easy solution processing.13 General strategy for preparing such materials is based on polymers blended with luminescent dyes that simultaneously emit over the whole visible range.14 A convenient alternative is to make copolymers containing moieties emitting at different wavelengths to avoid phase separation of the polymer and the dyes.15 However, to the best of our knowledge, the white light emissions readily achieved from a polymer containing a single polythiophene chromophore with tunable properties are very rare. Nomura and Takamizu developed white lighting materials through the mixed of two oligo-(thiophene)-coated star-shaped polymers with blue and orange-red light emissions.16 We found that the amphiphilic rod−rod poly(3-triethylene glycol thiophene)-bpoly(phenyl isocyanide) copolymer exhibit white-light emission in given solvents.17 However, the synthetic method for facile preparation of P3HT-containing block copolymers that cannot be polymerized with the same mechanism still remains a challenge in the field of polymer synthesis. General strategies involve either Received: May 24, 2015 Revised: July 10, 2015

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Macromolecules Scheme 1. Synthesis of P3HT, P3HT-b-PTA, and P3HT-b-PTA-b-P3HT Block Copolymers and PTA Homopolymer

Table 1. Polymerization Results for P3HT, P3HT-b-PTA, and P3HT-b-PTA-b-P3HT through Sequential Living Block Copolymerization Using Ni(dppp)Cl2 as Catalyst in One-Pota P3HTb (poly-1m) run 1 2 3 4 5 6 7 8

Mnc (Da)

Mw/Mnc

× × × × × × × ×

1.28 1.21 1.18 1.20 1.21 1.18 1.17 1.16

3.0 6.6 5.3 7.0 9.8 5.1 5.4 5.0

103 103 103 103 103 103 103 103

P3HT-b-PTA-b-P3HT (poly(1m-b-2n-b-1o))

P3HT-b-PTAb (poly(1m-b-2n)) Mnc (Da)

Mw/Mnc

× × × × × × × ×

1.35 1.20 1.24 1.21 1.27 1.27 1.29 1.19

12.1 12.0 9.0 11.5 14.0 13.2 11.5 7.1

103 103 103 103 103 103 103 103

Mnc (Da)

18.6 17.0 16.2 8.8

× × × ×

103 103 103 103

Mw/Mnc

yieldd (%)

block ratio (m:n:o)e

1.32 1.29 1.30 1.26

87 85 88 84 81 79 78 77

20:45 40:30 30:20 40:20 60:20:30 30:40:20 30:30:30 30:10:10

a

The polymers were prepared according to Scheme 1. bMn and Mw/Mn were determined by SEC analysis of aliquots removed from the respective reaction mixture prior to the additions of the corresponding new monomers. cMn and Mw/Mn were determined by SEC and reported as their polystyrene equivalents. dIsolated yields over the two or three steps as indicated. eEstimated by integral analysis on 1H NMR spectroscopies.

one-pot synthesis of novel water-soluble, amphiphilic block copolymers containing hydrophobic P3HT and hydrophilic poly(triethylene glycol allene) (PTA) blocks which have never been reported. The P3HT-b-PTA diblock copolymers and P3HT-b-PTA-b-P3HT triblock copolymers with different Mn and tunable compositions were facilely prepared through the tandem polymerizations of 2-bromo-3-hexyl-5-chloromagnesiothiophene (1) and triethylene glycol allene (2) (and 1) with Ni(dppp)Cl2 as a single catalyst under controlled manners (Scheme 1). The afforded block copolymers exhibit multiple stimuli-responsive properties including temperature, pH, and solvents. Moreover, white-light emission with high quantum yield can be readily achieved on the P3HT-b-PTA-b-P3HT triblock copolymer in the mixed solvents of THF and methanol.

elaborating an end-functionalized P3HT into an appropriate macroinitiator for the chain extension of a second block18 or coupling preformed homopolymers with complementary endfunctionalities.19 Such methodologies, however, can be complex and inefficient, frequently resulting materials that contain inseparable homopolymer impurities. Taking advantage of the living nature of the KCTP, we developed a facile synthetic strategy for one-pot synthesis of well-defined P3HT-bpolyisocyanide block copolymers through copolymerization of isocyanide using Ni(II)-terminated P3HT as macroinitiators.20 Recently, we extended this synthetic method for synthesis of P3HT-b-polyallene block copolymers.21 Although the monomers were polymerized via distinct mechanisms, the one-pot block copolymerization was revealed to proceed under living/ controlled manners, affording the well-defined P3HT block copolymers in high yields with controlled molecular weights (Mns), narrow molecular weight distributions (Mw/Mns), and tunable compositions. The development of water-soluble conjugated polymers is of great desirable, particularly in applications that would benefit from environmentally friendly processing steps or those applications that focus on the use of conducting polymers in biological environmental.22 In this contribution, we report on

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RESULTS AND DISCUSSION Synthesis and Characterization. As illuminated in Scheme 1, the P3HT-b-PTA and P3HT-b-PTA-b-P3HT block copolymers were facilely prepared in one-pot using Ni(dppp)Cl2 (dppp = 1,3-bis(diphenylphosphino)propane) as a catalyst.21 First, monomer 1 (generated in situ from 2,5dibromo-3-hexylthiophene and isopropylmagnesium chloride in B

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larger than that of the poly(160-b-220) and poly-160 while the Mw/Mn remains narrow confirms the formation of expected triblock copolymer. Compared to the extensive investigations on P3HT polymerization, kinetic studies on the block copolymerization of allene derivatives with Ni(II)-terminated P3HT have never been reported. Thus, polymerization of monomer 2 with Ni(II)terminated poly-130 (Mn = 5.3 kDa, Mw/Mn = 1.23) was followed by 1H NMR and SEC measurements of the aliquots taken out from the reaction mixture at appropriate time intervals to estimate the monomer conversion as well as the Mn and Mw/Mn of the generated block copolymer (see Supporting Information for details). As shown in Figure 2a, the copolymerization was relatively fast, and >85% of the monomer was consumed within 6 h. Both conversion−M n and conversion−Mw/Mn relationships are plotted in Figure 2b. The Mn values of the P3HT-b-PTA block copolymers isolated at different polymerization stages are linearly correlated with

THF) was polymerized by Ni(dppp)Cl2 in THF at room temperature to afford Ni(II)-terminated P3HT (poly-160) which was then treated with monomer 2 to yield Ni(II)terminated P3HT-b-PTA block copolymer poly(160-b-220) (the footnotes indicate the initial feed ratio of monomer to initiator). The block copolymerization of 2 with Ni(II)terminated poly-160 was proceed in living/controlled chaingrowth manner as the Mn of the block copolymers can be easily controlled through the variation on the initial feed ratio of the monomer to catalyst (Figure S2, Supporting Information). The P3HT-b-PTA-b-P3HT triblock copolymer poly(160-b-220-b-130) was obtained in high yield via the copolymerization of 1 with freshly generated Ni(II)-terminated P3HT-b-PTA diblock copolymer poly(160-b-220) as a macroinitiator in THF at room temperature. Because of living nature of the sequential copolymerization of monomers 1 and 2 with Ni(II)-terminated polymers, a series of P3HT-b-PTA diblock copolymer and P3HT-b-PTA-b-P3HT triblock copolymers were isolated in high yields with controlled Mn and narrow Mw/Mns (Table 1). Because of the amphiphilic character, the synthesized block copolymers showed good solubility in most common organic solvents, such as THF, CHCl3, toluene, and methanol. It should be noted that all the block copolymers have good solubility in water and the concentration can be higher than 4.0 g/L. For control experiments, homopolymer of PTAs (poly-2ms) with different Mn were prepared through the polymerization of monomer 2 in THF at room temperature according to the procedure we reported previously (Scheme 1).21 The one-pot synthetic block copolymers were first examined by size exclusion chromatography (SEC) analyses. The recorded SEC traces of poly(160-b-220-b-130) and poly(160-b220) block copolymers with the poly-160 precursor were shown in Figure 1. SEC trace of poly(160-b-220) appeared at higher

Figure 1. Size exclusion chromatograms of the one-pot synthesized poly-160, poly(160-b-220) diblock copolymer, and poly(160-b-220-b-130) triblock copolymer and the homopolymer of poly-230 (Mn = 5.9 kDa, Mw/Mn = 1.18). SEC conditions: eluent = THF; temperature = 40 °C.

molecular weight region than that of poly-160. The Mn and Mw/ Mn of poly(160-b-220) were estimated to be 14.0 kDa and 1.27, respectively. The Mn is larger than that of the poly-160 precursor (Mn = 9.8 kDa, Mw/Mn = 1.21) supports the formation of the expected diblock copolymer. The SEC trace of the corresponding P3HT-b-PTA-b-P3HT triblock copolymer poly(160-b-220-b-130) was located at the highest-molecularweight region as compared with poly-160 and poly(160-b-220) precursors. The Mn and Mw/Mn of poly(160-b-220-b-130) were estimated to be 18.6 kDa and 1.32 by SEC analysis. The Mn is

Figure 2. (a) Plot of the conversion of monomer 2 with polymerization time initiated by Ni(II)-terminated poly-130 (Mn = 5.3 kDa, Mw/Mn = 1.23) in THF at 25 °C ([2]0 = 0.10 M, [2]0/[Ni]0 = 80). (b) Plot of Mn and Mw/Mn values of the isolated poly(1m-b-2n) as a function of the conversion of 2. (c) First-order kinetic plot for the polymerization of 2 initiated by Ni(II)-terminated poly-130 in THF at 25 °C. C

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initial feed ratio of the monomers used and confirms the formation of expected triblock copolymer. Note that the structures of the synthetic block copolymers are quite stable although they contain olefin moieties. No change could be detected on their 1H NMR spectra in CDCl3 even the samples were deposited for 1 week at room temperature. The UV−vis spectra of the synthesized block copolymers exhibit strong absorptions around 210 and 440 nm come from PTA and P3HT blocks, respectively (Figure S5, Supporting Information). FT-IR spectra of the di- and triblock copolymers showed signals ascribe to both P3HT and PTA segments (Figure S6, Supporting Information). Collectively, these results confirm the formation of desired di- and triblock copolymers containing conjugated rigid-rod P3HT and coil PTA segments. Thermoresponsive Properties of the Synthetic Polymers. The PTA segment contains triethyl glycol chains; thus, it may have lower critical solution temperature (LCST) character and thermoresponsive property due to the polymer−solvent interaction and hydrophobic/hydrophilic balance of the polymer in water. To verify, a clear aqueous solution of poly260 (c = 2.0 g/L) was slowly heated up. Interestingly, the transparent solution turned to opaque upon heating, suggesting the polymer is thermoresponsive (Figure 4a). The transmittance (at 750 nm) versus temperature curves of poly-260 in water indicates the LCST is 50.8 °C. The optical transmittance curve of the polymer upon the heating and cooling cycle is reversible with small hysteresis, suggesting an excellent thermoresponsive behavior of the polymer attribute to the variation on the hydrophilic/hydrophobic balance. At its LCST, PTA undergoes a rapid and reversible conformational change from an extended hydrated coil to a collapsed hydrophobic globule that is insoluble in water. It has been revealed that the LCST value of thermoresponsive polymers could be independent of, directly dependent on, or inversely dependent on Mn.23 Thus, the correlations of LCST with Mn and concentration of poly-2m in water were then investigated. Because of the living nature of the polymerization of 2, poly230, poly-260, and poly-290 with different Mn and narrow Mw/Mn were readily prepared.21 The transmittance versus temperature curves of these three polymers at the same weight concentration (c = 2.0 g/L) are shown in Figure 4b. The LCSTs of poly-230, poly-260, and poly-290 were estimated to be 53.5, 50.8, and 47.4 °C, respectively, which indicate that the LCST of poly-2m is inversely dependent on the Mn. That is, the higher Mn PTA shows a lower LCST in water. In order to test the dilution effect on the LCST of poly-2m, the concentration dependence on LCST of poly-2m was then examined. As shown in Figure 4c, the LCST of poly-290 is 47.4 °C when the concentration is 2.0 g/L. However, it increased to 50.6 and 54.1 °C with the concentration decreased to 1.0 and 0.5 g/L. This result suggests the LCST of poly-290 is dependent on the concentration; a higher concentration will lead to a decreased LCST. Probably, the higher concentration will facilitate polymer collapse and subsequent interchain aggregation, which lead to the decrease of optical transmittance. Since the PTA homopolymer exhibits excellent thermoresponsive property, the thermoresponsive behaviors of the P3HT-b-PTA and P3HT-b-PTA-b-P3HT block copolymers were then investigated. As shown in Figure 4d, a red transparent aqueous solution of P3HT-b-PTA diblock copolymer poly(120-b-245) (c = 1.0 g/L) turns to opaque upon heating. A plot of the transmittance (at 750 nm) versus temperature revealed the LCST is 56.0 °C, which indicates the

the conversion of 2 and kept a narrow distribution, further confirming the living nature of the block copolymerization. Kinetic studies revealed the one-pot block copolymerization of 2 with Ni(II)-terminated poly-130 as macroinitiator obey the first-order rate law. The rate constant was estimated to be ∼1.85 × 10−4 s−1 according to the kinetic plot (Figure 2c). The chemical structures of the one-pot synthesized block copolymers were further characterized by 1H NMR, FT-IR, and UV−vis spectroscopies. The 1H NMR spectra of P3HT homopolymer, P3HT-b-PAT diblock copolymer, and P3HTb-PTA-b-P3HT triblock copolymer measured in CDCl3 at room temperature are shown in Figure 3a−c. For comparison,

Figure 3. 1H NMR (600 MHz) spectra of poly-130 (a), poly(130-b-240) (b), poly(130-b-240-b-120) (c), and poly-230 (d) in CDCl3 at 25 °C.

the 1H NMR spectrum of PTA homopolymer poly-230 (Mn = 5.9 kDa, Mw/Mn = 1.18) is shown in Figure 3d. It was found that both di- and triblock copolymers exhibit characteristic signals attributable to P3HT and PTA segments. For example, the diblock copolymer poly(130-b-240) showed resonances around 6.97 and 2.80 ppm, corresponding to the repeating thiophene units of P3HT block, and the resonances at 6.10− 5.72 and 3.82−3.44 ppm, respectively, corresponding to OCH and −OCH2− protons of PTA segment. The block ratio of P3HT to PTA segment deduced from the integral analysis of signals at 6.97 ppm (aryl proton of P3HT) and 5.82 ppm (OCH of PTA) on the 1H NMR spectrum is about 3:4, which is consistent with the initial feed ratio of the monomers added to the reaction vessel. The 1H NMR profile of the resulting poly(130-b-240-b-120) triblock copolymer is similar to that of the poly(130-b-240) diblock copolymer; however, the block ratio of P3HT to PTA is increased to 5:4, which agrees well with the D

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Figure 4. Transmittance vs temperature curves and photographs of the phase transition for aqueous solution of (a) poly-260 (c = 2.0 g/L) and (d) poly(120-b-245) (c = 1.0 g/L). Transmittance vs temperature curves for aqueous solution of (b) poly-230, poly-260, and poly-290 (c = 2.0 g/L) and (e) poly(135-b-265), poly(135-b-290), and poly(135-b-2115) (c = 2.0 g/L). Transmittance vs temperature curves for aqueous solution of (c) poly-290 and (f) poly(135-b-290) measured at 0.5, 1.0, and 2.0 g/L.

LCST (56.0 °C), while it increased to 461 nm when the temperature exceeds the LCST, further confirming the excellent thermoresponsive property of P3HT-b-PTA diblock copolymer. The heating-induced phase transition of the one-pot synthesized P3HT-b-PTA-b-P3HT triblock copolymer was then investigated in ways similar to that of the PTA homopolymer and the P3HT-b-PTA diblock copolymers. However, no phase transition was observed upon heating on the aqueous solution of triblock copolymers at different concentrations. The clear reddish solution of poly(130-b-230-b130) in water could not be turned to turbid upon heating in the temperature range 25.0−95.0 °C. In addition, UV−vis studies of the poly(130-b-230-b-130) in water (c = 2.0 g/L) revealed that there is almost no change on optical transmittance upon heating. Further attempts to explore the thermoresponsive property of triblock copolymers with different Mn and block ratios all failed even when it contained the same chemical composition as that of the diblock copolymers with excellent thermoresponsive property. These studies suggest that the incorporation two P3HT chains on both chain ends of PTA segment completely destroy the thermoresponsive property. Solvent Effects. The amphiphilic P3HT-b-PTA diblock copolymers compose of a hydrophobic rod P3HT segment and hydrophilic coil PTA segment. Thus, because of the immiscibility of the two segments, the block copolymers may exhibit interesting optical and self-assembly properties in solvents that selective to one of the two segments. Here, block copolymer poly(135-b-265) was initially dissolved in THF, a good solvent for both blocks. As shown in Figure 5a, the block copolymer gives a yellow color, similar to that of the P3HT homopolymer. The UV−vis spectrum measured in THF shows an intense absorption around 440 nm corresponding to the P3HT segment. When the solvent transferred from THF to methanol, the color of the block copolymer was changed to violet. The absorption maximum at 440 nm was shifted to 510 nm, accompanied by new absorptions at 550 and 605 nm,

incorporation of a P3HT segment on PTA did not destroy the thermoresponsive property. Note that the optical absorption edge of the P3HT segment is around 680 nm and thus would not affect the observation of LCST by optical transmission at 750 nm. The optical transmittance curve of the block copolymer upon heating and cooling cycle is reversible. Owing to the well-defined structure and narrow molecular weight distribution of the poly(120-b-245), quite fast phase transitions and very small hysteresis were observed. To get more detail, three P3HT-b-PTA block copolymers of poly(135b-265), poly(135-b-290), and poly(135-b-2115) with the same chain length of P3HT block but differ in chain length of PTA block were prepared. The optical transmittance versus of temperature curves of these polymers were shown in Figure 4e. The LCST of poly(135-b-265), poly(135-b-290), and poly(135-b2115) were estimated to be 50.9, 49.7, and 48.7 °C, respectively. This result indicates that the LCST value of the P3HT-b-PTA block copolymer decrease with the increased chain length of PTA segment. Comparing the LCST values of poly-290 and the block copolymer poly(135-b-290) containing the same chain length of PTA segment, the LCST of poly-290 is 47.4 °C, while the LCST of poly(135-b-290) is 49.7 °C, suggesting the incorporated hydrophobic P3HT enhances the LCST value. Moreover, LCST of the diblock copolymer can be further tuned through variation on the concentration. As illuminated in Figure 4f, diluting the aqueous solution of poly(135-b-290) will elevate the LCST. For example, the LCST of poly(135-b-290) was increased from 49.7 to 52.8 and 55.4 °C when the concentration was diluted from 2.0 to 1.0 and 0.5 g/L. The thermoresponsive behavior of the P3HT-b-PTA block polymer was further investigated by dynamic light scattering (DLS). Figure S7 in the Supporting Information shows the DLS results on aqueous solution of poly(120-b-245) (c = 1.0 g/ L) at temperature range of 50−60 °C. It clearly exhibits that the hydrodynamic diameters of the polymer micelles are enhanced by increasing temperature. The diameter of the block copolymer was ca. 37 nm as the temperature fell below E

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shown in Figure 5d. In THF, the block copolymer showed a maximum emission at 570 nm with 46% quantum yield (QY). In methanol, the emission was shift to 550 nm, and the QY decreased to 8.0%. The emission maximum was shifted to 610 nm in water with 3.7% of QY. The different color and emission properties of the poly(135-b-265) in THF, methanol, and water probably due to the different assembly structures formed in these solvents. DLS studies on poly(135-b-265) in THF, methanol, and water provided further understanding of assembly structures, as analyzed using the CONTIN method. In THF, a narrow size distribution with an average hydrodynamic diameter of 5 nm was observed, consistent with the large majority of chains being well dissolved (Figure 6a). In the selective solvents, the hydrodynamic diameters were estimated to be 122 nm in methanol and 91 nm in water, probably due to the different self-assembly model or compact intensity of the block copolymer in these solvents. The block copolymer poly(135b-265) showed narrow size distributions in these solvents owing to the narrow molecular weight distribution. These results indicate the formation of supramolecular structures through the self-assembly of amphiphilic P3HT-b-PTA block copolymer in solvents selective to the PTA segment. Transition electron microscopy (TEM) images of the self-assembled structure formed from block copolymer poly(135-b-265) in methanol and water are shown in Figures 6b and 6c. It was found that the rod−coil amphiphilic block copolymer self-assembled into spherical nanoparticles in both methanol and water. The dimensions of the assemblies were estimated to be ca. 130 nm in methanol and ca. 90 nm in water which agree well with the DLS analyses. Probably, in these selective polar solvents, poly(135-b-265) self-assembled into core/shell-type nanostructures with P3HT in the core and PTA in the shell, exposing hydrophilic PTA at the exterior. The interesting emission properties of the block copolymer in different solvents may be ascribed to the distinct self-assembled structures which affect not only the stacking of the conjugated P3HT strand but also the effective conjugation length of P3HT due to the changes in the conformation driven by steric interaction as well as changes in the solvophobic environment near the P3HT main chain.25 These studies indicate that the facilely one-pot synthesized, water-soluble amphiphilic P3HT-b-PTA block copolymers are solvent responsive. In addition, they also exhibit excellent temperature- and pH-responsiveness and other interesting properties (see below). pH-Responsive. Interestingly, the synthetic P3HT-b-PTA block copolymer exhibit pH-responsive properties in CHCl3. As shown in Figure 7a, the yellow solution of poly(135-b-265) was

Figure 5. (a) Photographs of P3HT-b-PTA block copolymer poly(135b-265) in THF, methanol, and water (c = 0.5 g/L). (b) Normalized absorption spectra of poly(135-b-265) in THF, methanol, and water (c = 0.15 g/L). (c) Photographs of poly(135-b-265) in THF, methanol, and water under UV light at 365 nm at 25 °C (c = 0.5 g/L). (d) Normalized emission spectra of poly(135-b-265) in THF, methanol, and water at 25 °C with excitation at 380 nm (c = 0.15 g/L).

indicate the formation of π−π interactions associated with semicrystalline aggregate in methanol (Figure 5a,b). In water, the block copolymer poly(135-b-265) showed carnation color, and the absorption maximum appeared at 480 nm with two new absorption peaks at 550 and 605 nm, similar to that in methanol solution. Probably, the amphiphilic rod−coil block copolymer self-assembled into micellar supramolecular structures in the selective solvents. The light-emitting properties of the one-pot synthetic P3HTb-PTA block copolymer were highly dependent on the solvents used. In nonselective THF solution, poly(135-b-265) shows a bright orange emission under UV illumination at 365 nm ascribe to the P3HT chromophore (Figure 5c). However, in the selective methanol solution, in contrast to the blue or weak reddish emissions commonly observed on the reported amphiphilic P3HT block copolymers,10,17,20d,24 the one-pot synthetic P3HT-b-PTA block copolymer exhibits a deep green light emission. This distinct emission was probably ascribe to the change in effective conjugation length as the P3HT chains adopt partially twisted conformation in methanol. Generally, the emissions of amphiphilic P3HT block copolymers were quenched or exhibit very weak red emission in water as described in the literature,10,24 while the one-pot synthetic poly(135-b-265) block copolymer gives bright red emission in water under UV light at 365 nm due to energy transfer to lowenergy sites in the packed P3HT strand.25 The emission spectra of the block copolymer poly(135-b-265) in different solvents are

Figure 6. (a) Hydrodynamic diameter of P3HT-b-PTA diblock copolymer poly(135-b-265) measured in THF, CH3OH, and water at 25 °C (c = 0.1 g/L). TEM images of the self-assembled structure formed from poly(135-b-265) in methanol (b) and in water (c). F

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Figure 7. (a) Photographs of the CHCl3 solution of poly(135-b-265) upon addition of TFA and TEA under room light and UV light at 365 nm (c = 0.15 g/L). (b) Emission spectra of poly(135-b-265) in CHCl3 (c = 0.15 g/L) upon addition of TFA and TEA at 25 °C with excitation at 365 nm. (c) Reversible emission changes at 575 nm of poly(135-b-265) in CHCl3 (c = 0.15 g/L) upon alternate addition of TFA and TEA at 25 °C with excitation at 365 nm.

Figure 8. Photographs of poly(130-b-240-b-120) in different solvents under room light (a) and UV light at 365 nm (b) (c = 0.10 g/L). Absorption (c) and emission (d) spectra of poly(130-b-240-b-120) measured in different solvents at 25 °C (c = 0.10 g/L, excited at 365 nm).

additions of TFA and TEA to the resulting solution of the block copolymer in CHCl3, the emission color was shuttled between orange and deep green (Figure 7a,c). It should be noted that such acid−base induced reversible color and emission changes of the P3TH-b-PTA block copolymer could not be observed in THF solution. To get more details, studies on the pHresponsive behaviors of both P3HT and PAT segments were then performed in CHCl3 under the same experimental conditions as that of the P3HT-b-PTA block copolymer. Upon addition of TFA to the CHCl3 solution of poly-230 (Mn = 5.9 kDa, Mw/Mn = 1.18) at 25 °C, no substantial changes could be observed on the absorption and emission, suggesting the interaction between poly-230 with the added protons is negligible. However, in contrast to poly-230, the intense absorption of poly-130 at 440 nm in CHCl3 was gradually decreased upon the addition of TFA, accompanied by the growth of new peak around 840 nm (Figure S9, Supporting Information). Moreover, the emission of poly-130 at 575 nm was gradually quenched by the addition of TFA. By neutralizing the acidic solution of P3HT by TEA, both the absorption and emission can be recovered (Figure S9, Supporting Information). These studies suggest that the pH-responsive behavior of the P3HT-b-PAT block copolymer was ascribed to the interaction of the P3HT with the added protons.26

turned to colorless upon addition of trifluoroacetic acid (TFA) at room temperature. In addition, the orange-color emission of poly(135-b-265) in CHCl3 under UV illumination at 365 nm turned to deep green (Figure 7a). The emission spectrum of poly(135-b-265) in CHCl3 (c = 0.15 g/L) is shown in Figure 7b, which shows an maximum emission at 575 nm. After addition of TFA to this solution, the maximum emission was quenched stepwise with the increase of TFA concentration and became constant when the TFA concentration reached 0.12 M. The changes of the emission intensity at 575 nm showed a linear correlation to the concentration of the TFA until it was saturated (Figure S8, Supporting Information). To examine the reversibility of the acid-induced color and emission changes, the acidified solution of the poly(135-b-265) in CHCl3 was then neutralized with triethylamine (TEA). The colorless solution was gradually turned back to yellow upon the addition of TEA (Figure 7a). Accordingly, the deep green color emission was turned back to orange under UV light at 365 nm upon the neutralization as confirmed by the photographs and emission spectra shown in Figure 7a,b. However, the emission intensity at 575 nm could not be completely recovered after the neutralization. The slightly decrease of emission may be attributed to the salt effect which was formed in the solution during the acidification and neutralization. Further alternate G

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Macromolecules White-Light Emission of P3HT-b-PTA-b-P3HT Copolymer. The effect of solvents on the optical properties of the one-pot synthesized amphiphilic P3HT-b-PTA-b-P3HT block copolymers was then investigated. In contrast to the P3HT-bPTA diblock copolymer, no thermo- and pH-responsive properties were observed on the corresponding P3HT-bPTA-b-P3HT triblock copolymer under the same experimental conditions even when it contains the same compositions of P3HT and PTA segments as that of the P3HT-b-PTA diblock copolymers. In nonselective THF solution, the triblock copolymer exhibited similar behavior to that of the P3HT-bPTA diblock copolymer (Figure 8a). The absorption spectrum of poly(130-b-240-b-120) in THF is shown in Figure 8c. When the solvent transferred to methanol, a selective solvent for hydrophilic PTA segment, the maximum absorption at 440 nm was red-shifted to 515 nm, accompanied by new absorptions at 555 and 605 nm, indicative of the intermolecular π−π interactions of the P3HT segment. Remarkably, the orangecolor emission of the poly(130-b-240-b-120) under UV light at 365 nm was transfer to blue when the solvent changed from THF to methanol (Figure 8b). Emission spectra of poly(130-b240-b-120) in THF and methanol are shown in Figure 8d. It was found that the triblock copolymer exhibited a strong maximum emission at 570 nm with weak emissions at 410, 430, and 460 nm. When the solvent was changed to methanol, the emission at long wavelength region was substantially quenched, and the emission at short wavelength region was increased, thus giving a blue-light emission. The short wavelength emissions at 410, 430, and 460 nm of the block copolymer were similar to that of the thiophene oligomer, and the intensities were dependent on the solvents used.16 Probably the effective conjugation length of some π-conjugated P3HT strand of the block copolymer was partially reduced caused by rotational defects in the aggregates or the immiscibility of the two segments of P3HT and PTA.10,17 Furthermore, when the methanol solution of the block copolymer was dried and redissolved in THF, the orangelight emission was recovered. This study indicates the solventinduced reversible emission changes were ascribed to the transitions of the conformations of the block copolymer, not the changes on chemical structures. The self-assembled structures of P3HT-b-PTA-b-P3HT in methanol were further investigated by TEM and DLS. The TEM image of the sample casted from methanol solution of poly(130-b-240-b-120) at room temperature is shown in Figure 9a. Some donut-like morphologies with ca. 180 nm in diameters were clearly observed, suggesting the amphiphilic triblock copolymer was probably self-assembled into hollowed

micelles or vesicles in this solvent; however, the exact selfassembly mode is not clear. The thick well of the aggregates may ascribed to the specific bilayer assemblies (Figure S10, Supporting Information).27 DLS analysis of poly(130-b-240-b120) in methanol indicated the hydrodynamic diameter of the assemblies is ca. 190 nm, which is consistent with that obtained from TEM observation (Figure 9b). These results demonstrated that the different emission of the triblock copolymer poly(130-b-240-b-120) in THF and methanol may be ascribed to the distinct self-assembled supramolecular structures. Interestingly, white-light emission was facilely achieved from the solution of poly(130-b-240-b-120) in THF and methanol at 1:3 volume ratios upon the irradiation at 365 nm (Figure 8b). The emission spectra are shown in Figure 8d; it was found that the block copolymer showed whole emission from the 390 to 710 nm region. The QY of the white-light emission was estimated to be 53%. The emission intensities at 570 nm showed a linear correlation to the concentration of the triblock copolymers at the range of 0.10−0.50 g/L, and the ratio of the emission intensities at 570 nm to that at 430 nm was maintained during the variation on the concentrations. Moreover, heating the white-light emission solution from room temperature to 55 °C, no obvious color changes on the emission were observed, suggesting the white-light emission of the P3HT-b-PTA-b-P3HT block copolymer was quite stable and independent of the concentration and temperature at the current experimental conditions. These results demonstrated that the P3HT-b-PTA-b-P3HT block copolymers exhibited tunable light emissions with the emission color spanned widely from orange red to blue, and white light emission can be readily obtained through the regulation on the solvents composition. Compared to the tunable light-emissions of the block copolymers in solution, the emissions of both P3HT-b-PTA diblock copolymer and P3HT-b-PTA-b-P3HT triblock copolymer were quenched in the solid state.

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CONCLUSIONS In summary, amphiphilic P3HT-b-PTA and P3HT-b-PTA-bP3HT block copolymers were facilely prepared in one-pot through sequential polymerization of the two (or three) monomers using Ni(dppp)Cl2 as a single catalyst via distinct polymerization mechanisms. The P3HT-b-PTA diblock copolymer exhibited excellent thermoresponsive property in water, and the LCST can be facilely tuned through the variations on concentration and block ratios. In addition, the diblock copolymer showed highly tunable emission properties depending on the solvents used and the pH value of the solution. In contrast to P3HT-b-PTA diblock copolymer, the P3HT-b-PTA-b-P3HT triblock copolymer shows no thermoand pH-responsive properties. However, the triblock copolymer exhibit tunable light emissions with emission color widely expanded from red to blue due to the distinct self-assembly structures in different solvents. Interestingly, white light emission was readily achieved in the mixture solvent of THF and methanol in 1/3 volume ratios. We believe the present studies will provide not only a facile synthetic method for multiresponsive conjugated block polymers but also a family of novel semiconducting materials for many potential applications like sensor, fluorescence thermometer, optoelectronic, and bioelectronics devices.

Figure 9. (a) TEM images of the self-assembled structure formed from poly(130-b-240-b-120) in methanol (inset is an enlarged structure). (b) Hydrodynamic diameter of P3HT-b-PTA-b-P3HT triblock copolymer poly(130-b-240-b-120) measured in THF and CH3OH at 25 °C (c = 0.10 g/L). H

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental procedures, spectral data, and SEC chromatograms. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.macromol.5b01120.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.-Q.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by National Natural Scientific Foundation of China (21104015, 21172050, 21371043, and 51303044). Z.W. thanks the Thousand Young Talents Program of China for Financial Support.

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DOI: 10.1021/acs.macromol.5b01120 Macromolecules XXXX, XXX, XXX−XXX