Quinoxaline-Based Semiconducting Polymer Dots for in Vivo NIR-II

Jul 22, 2019 - Here, we have synthesized three NIR-II fluorescent polymer dots ..... (b) Ex vivo biodistribution of P1-Pdots in tumor and organs after...
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Quinoxaline-Based Semiconducting Polymer Dots for in Vivo NIR-II Fluorescence Imaging Ye Liu,† Jinfeng Liu,† Dandan Chen,‡ Xiaosha Wang,† Zhenbao Liu,§ Hui Liu,† Lihui Jiang,† Changfeng Wu,‡ and Yingping Zou*,†,∥ †

College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China Department of Biomedical Engineering, Southern University of Science and Technology, Shenzhen, Guangdong 518055, China § Department of Pharmaceutics, Xiangya School of Pharmaceutical Sciences, Central South University, Changsha 410013, China ∥ Molecular Imaging Research Center of Central South University, Changsha, Hunan 410008, China

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‡

S Supporting Information *

ABSTRACT: In vivo fluorescence imaging within the second near-infrared region (NIR-II, 1000−1700 nm) has advantages of a higher signal-to-background ratio (SBR), spatial resolution, and deeper tissue penetration depth than that in the visible (400−650 nm) and the first near-infrared window (NIR-I, 650−1000 nm). Here, we have synthesized three NIR-II fluorescent polymer dots (P1-Pdots, P2-Pdots, and P3-Pdots) for the NIR-II imaging. These Pdots were designed and optimized by using benzodithiophene as a donor unit and quinoxaline derivatives as acceptor units. The backbone and side chains of the quinoxaline acceptor units were varied to optimize the fluorescence performance. We found that the substituted position of alkoxy groups in the side chains plays an important role in enhancing the NIR-II window quantum yield (QY). In one case, the resulting nanoparticles (P1-Pdots) exhibited an emission peak at ∼1100 nm and a high QY of ∼1%. P1Pdots possesses additional advantages for bioimaging, including deep tissue penetration depth, good stability, and biocompatibility. The blood vessel imaging of the mouse by P1-Pdots could be clearly observed with high spatial resolution and displayed an SBR of ∼2.1. Besides, P1-Pdots has further demonstrated its applications for tumor imaging of tumor-bearing nude mice, such as assessing the in vivo angiography and monitoring tumor vasculatures. Our results indicate the Pdots afford high fluorescence signals and spatial resolution for imaging deep tissues.

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probes in visible and NIR-I regions.21−23 Conjugated polymers are playing an increasingly important role in many fields, for instance, polymer solar cells24−26 and biomedicine. In the past decade, conjugated polymers have been extensively used for in vivo imaging and nanomedicine because of their tunable optical properties by modification of their backbone structures.27,28 However, conjugated polymers are largely unexplored for NIRII fluorescence imaging as QYs of polymers are quite low. In this study, three conjugated polymers were designed and synthesized with benzodithiophene (BDT) as donor and triazole[4,5-g]-quinoxaline (TQ) derivatives as acceptors (named P1, P2, and P3, respectively). We varied structures of the acceptors to optimize the properties of conjugate polymers. Small fluorescent nanoparticles prepared from these polymers exhibit fluorescence emission longer than 1000 nm, among which P1-Pdots shows an emission peak at ∼1100 nm and a QY with ∼1%. P1-Pdots not only is easy to be synthesized (P1 only take three steps) but also owns additional advantages for

INTRODUCTION In the past decade, fluorescence bioimaging within the second near-infrared (NIR-II, 1000−1700 nm) region has attracted considerable interest, for the deep tissue penetration depth, high resolution, and signal-to-background ratio (SBR).1−4 Although imaging quality for the first near-infrared (NIR-I, 650−900 nm) range is much more superior than that in the visible range (400− 650 nm) in vivo biological imaging, the penetration depth of light in vivo tissues is quite low. Therefore, imaging in NIR-II is a highly promising in vivo imaging modality that can provide high resolution, SBR, and deep tissue penetration depth.5−7 To date, various NIR-II fluorophores have been developed, including single-walled carbon nanotubes, 8,9 quantum dots,10−12 rare-earth doped nanoparticles,13,14 and organic materials.15−18 Because of the nanomaterials are excreted slowly, resulting in anxious about long-term toxicity for heavy metals in inorganic materials may be retained in the body.19,20 Alternatively, organic compounds, including small molecules and polymers, have also been investigated for NIR-II fluorescence imaging. Nowadays, mostly near-infrared organic fluorophores are small organic molecule. However, their quantum yields (QYs) are low as compared with fluorescent © XXXX American Chemical Society

Received: June 4, 2019 Revised: July 10, 2019

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DOI: 10.1021/acs.macromol.9b01142 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthetic Route of Polymers

fluorescence bioimaging in the NIR-II region. The blood vessel imaging of mouse by P1-Pdots could be clearly observed with high spatial resolution and displayed an SBR of ∼2.1. Besides, P1-Pdots has been further applied for tumor imaging with a subcutaneous tumor model in NIR-II, indicating its potential for the in vivo angiography and monitoring tumor vasculatures with high resolution.

onto TQ to enhance close packing of the polymer backbone and improve the optical properties. Therefore, the conjugated polymers have been designed and synthesized with BDT derivatives as donor and TQ derivatives as acceptor. To investigate the effect of π-bridge with Pdots, we introduction the thiophene moiety into P3. Besides, it has been concluded that the lower QYs of fluorophores are mainly limited by the interactions with H2O in water solution. Also, the alkoxyl chains incorporated onto the polymer backbone possibly decrease the chance of H2O interactions because of the hydrophobicity and steric hindrance.17 Therefore, the conjugated alkoxyphenyl groups were introduced onto the TQ unit to modify the energy gap and improve the QY. Also, the alkoxyl chains anchored on different positions were designed and synthesized. Three conjugated polymers were successfully designed and synthesized through the Stille coupling polymerization reaction (Schemes 1 and S1). These compounds and polymers were characterized by NMR spectroscopy (see Figure S2−S13) and exhibited good solubility. Furthermore, the number average molecular weights (Mn) of P1, P2, and P3 were 25.8, 24.2, and 21.9 kDa, respectively. Also, polydispersity index of P1, P2 and P3 were 1.58, 1.65 and 2.33, respectively. Preparation and Characterizations of Pdots. To prepare Pdots (Figure 1a), conjugated polymer P1, or P2, or P3 was dissolved in anhydrous tetrahydrofuran (THF). Then, the THF solution with polymers and PS−PEG−COOH was rapidly injected in ultrapure aqueous solution with vigorous sonication and THF was then evaporated to obtain Pdots (named as P1Pdots, P2-Pdots, and P3-Pdots)29,30. The color of these Pdots solutions are green (Figure 1b), and the color of P1-Pdots solution is the lightest in these solutions. Dynamic light scattering (DLS) showed that P1-Pdots, P2-Pdots, and P3Pdots possessed an average size of 28, 37, and 24 nm, respectively (Figures 1c and S1a). Impressively, the particle sizes of these Pdots were almost unchanged for 1 month,

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RESULTS AND DISCUSSION Synthesis and Characterizations of Conjugated Polymers. As illustrated in Scheme 1, conjugated polymers employ a D−A structure with electron-donating monomer (4,4,9,9tetrakis(4-hexylphenyl)-4,9-dihydro-sindaceno[1,2-b:5,6-b′]dithiophene-2,7-diyl)bis(trimethylstannane)) (BDT) and electron-withdrawing monomer, 4,9-dibromo-2-(2-ethylhexyl)-6,7bis(3-((2-ethylhexyl)oxy)phenyl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline) (M1), or 4,9-dibromo-2-(2-ethylhexyl)-6,7-bis(4((2-ethylhexyl)oxy)phenyl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline (M2), or 4,9-bis(5-bromothiophen-2-yl)-2-(2ethylhexyl)-6,7-bis(4-((2-ethylhexyl)oxy)phenyl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline (M3), for in vivo NIR-II window imaging. The key to obtain NIR-II Pdots is mainly determined by the selection of donor and acceptor in polymers. As a donor unit, BDT is widely used to fabricate copolymer with a benzyl core in the center and two flanking thiophene rings which could achieve much less steric hindrance with neighboring acceptor units, resulting in more planar polymer structures.33 Quinoxaline (Qx) is a typical electron-withdrawing unit for optoelectronic applications as it can be easily modified through flexible side chains and conjugated backbones.34 As one of the Qx derivative, TQ is a strong electron-poor unit with four imine groups (CN), which can make the structure more planar by strong intermolecular π−π interaction and red shift of spectra.35 Impressively, TQ derivatives conjugated with BDT can exhibit bathochromic shifts to the NIR region absorption and fluorescence. Solubilizing alkyl chains have been incorporated B

DOI: 10.1021/acs.macromol.9b01142 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

The absorption spectrum of P2-Pdots was red-shifted almost ∼200 nm as compared with that of P3-Pdots. Removal of thiophene is beneficial for polymer to improve the red shift of spectra because thiophene may destroy the planarity of the polymer backbone. Compared with P2-Pdots, the absorption spectrum of P1-Pdots was red-shifted about 50 nm and the fluorescence signals of P1-Pdots were markedly improved, indicating that the alkoxy chain anchored on the 4-position was better than that on the 3-position for NIR-II region bioimaging, which could be attributed to the fact that the alkoxy chain anchored on the 4-position has more steric hindrance and thus increase the dihedral angle between D and A and, meanwhile, may reduce the possibility of water interactions. As a result, the absorption peak (923 nm) and emission peak (1095 nm) of P1Pdots are in the NIR-II region through optimization of acceptor units, which indicated that the optimization of acceptor units is important to realize enhanced absorption and fluorescence signals in the NIR-II region. In the result, P1-Pdots not only is easy to synthesize (P1 only take 3 steps) but also owns great potential applied to NIR-II window bioimaging. The fluorescence signals of P1-Pdots in water and IR-26 in 1,2-dichloroethane with the same absorbance value at 808 nm under 1000 nm LP filter showed that the fluorescence signal of P1-dots was much stronger than that of IR-26 (Figure 1e). The fluorescence QY of P1-Pdots with an excitation at 808 nm is 0.92%, which was measured by standard IR-26 (QY = 0.5%) as reference (Figure S1d,e).31,32 Furthermore, the cytotoxicity of P1-Pdots was evaluated by measuring the MCF-7 cell viability with various P1-Pdots concentrations (0−100 μg mL−1) using MTT analysis. The MCF-7 cell viability was more than 90% even in the high concentrations of 100 μg mL−1 after 24 h incubation, demonstrating its low cytotoxicity (Figure S1c). All these results indicated that P1-Pdots is a promising NIR-II fluorescence probe, which is stable and bright for biological imaging.

Figure 1. (a) Schematic of preparation for Pdots. (b) Photograph for Pdots solutions with the same concentration. From left to right: P1Pdots, P2-Pdots, and P3-Pdots. (c) Size distribution of Pdots measured by DLS and transmission electron microscopy. (d) NIR-II signals of Pdots with the same concentration at 1000 nm long-pass (LP) filters. (e) NIR-II signals of IR-26 and P1-Pdots (with the same absorbance value at 808 nm) in 1000 LP filters.

indicating that they own good stability in water. The zeta potential of the P1-Pdots was −35 mV (Figure S1b). UV−vis−NIR absorption peaks of P1-Pdots, P2-Pdots, and P3-Pdots are at 923, 870, and 703 nm, respectively (Figure 2a). Meanwhile, the emission spectra of these Pdots are in the NIR-II window (Figure 2b), and emission peaks of P1-Pdots, P2Pdots, and P3-Pdots are at 1095, 1063, and 1058 nm, respectively. All these Pdots exhibit large Stokes shift of more than 170 nm due to the strong charge interaction between the donor and acceptor units. Besides, we compared the fluorescence signals of these Pdots (50 μg mL−1) under the 1000 nm LP filter, showing that the fluorescence signal of P1Pdots is the brightest (Figure 1d).

Figure 2. Optical properties of Pdots. (a) UV−vis−NIR absorption spectra of Pdots in water. (b) Fluorescence spectra of the Pdots solution. (c) Emission spectra of P1-Pdots solutions under an excitation of 808 nm. (d) Integrated fluorescence intensity plotted as a function of absorbance at 808 nm for P1-Pdots solutions. C

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Figure 3. Blood vessel NIR-II fluorescence imaging of BALB/c nude mice injected with P1-Pdots (100 μL, 200 μg mL−1) through the tail vein under excitation wavelength of 808 nm (1000 nm LP and 50 ms) at 5 min: (a) images of the vasculature for tumor. (b) Images of blood vessel for intestines. (c) NIR-II images of nude mice. (d) Cross-sectional intensity profile measured along the red-dashed line and vasculature images of hindlimb.

Figure 4. (a) Real-time NIR-II fluorescence imaging MCF-7 tumors in living BALB/c nude mice injected with P1-Pdots (100 μL, 200 μg mL−1) through the tail vein under excitation wavelength of 808 nm (1000 nm LP and 50 ms). (b) Ex vivo biodistribution of P1-Pdots in tumor and organs after 24 h under excitation wavelength of 808 nm (1000 nm LP and 50 ms). (c) Average NIR-II fluorescence of P1-Pdots in the tumor and organs.

Fluorescence Bioimaging with P1-Pdots in the NIR-II Window. To study in vivo vascular, tumor imaging, and biodistribution with P1-Pdots, three MCF-7 tumor-bearing BALB/c nude mice were injected with 100 μL P1-Pdots (200 μg mL−1) through the tail vein. Immediately, a video imaging of the nude mouse with supine position was performed in the NIR-II region under very short exposure time of 50 ms, 1000 nm LP filter, and a fast frame rate of 20 fps. From this video, the vasculature of the nude mice was clearly observed (Supporting Information Movie I and Figure 3). Impressively, the blood vessels of intestines and bowel moving in the nude mice could be clearly visualized in the video. This phenomenon was first time observed through NIR-II fluorescence bioimaging by Pdots. Furthermore, the vasculature for tumor and hindlimb could be clearly observed with high spatial resolution and fluorescence signal of P1-Pdots kept for a few hours, indicating that P1-Pdots is suitable for vascular imaging during surgery. The small hindlimb blood vessel with a diameter of ∼0.42 mm (extract the

width based on the FWHM of Gaussian peak) displayed an SBR of ∼2.1 (Figure 3d). All these results indicate that the P1-Pdots could afford high SBR and spatial resolution at deeper tissue penetration depths. Tumor NIR-II imaging in living mice by using P1-Pdots was validated (Figure 4a). From the bioimaging with the 1000 nm LP filter and a very short exposure time of 50 ms, we can note that a fluorescence signal was observed inside the tumor after P1-Pdots injected a few minutes, and the intensity of fluorescence signal in the tumor area increased with time (0− 24 h) because of the nonspecific diffusion and accumulation. This indicates the passive tumor targeting mechanism and enhanced permeability and retention effect (EPR) of P1-Pdots in tumor. With great promising NIR-II imaging results of P1Pdots, this Pdots could be applied to observing tumor vessels and EPR effect, and the phenomenon was not obtained with Pdots fluorescence imaging in NIR-II before. D

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Macromolecules To evaluate the distribution of P1-Pdots in vivo fluorescence NIR-II imaging, the ex vivo distribution studies were evaluated after injection of the P1-Pdots for 24 h. Organs (liver, lung, spleen, kidney, heart, and intestines) and tumor for the nude mice injected with P1-dots were dissected for ex vivo NIR-II imaging (Figure 4b). From Figure 4b,c, we can note that P1Pdots mostly existed in the liver and spleen, indicating that the metabolic pathway of P1-Pdots was mainly by the hepatobiliary system. Besides, we found that an obvious accumulation was in tumor, suggesting that P1-Pdots has an EPR effect and could be applied to image-guided tumor surgery. All these results indicated that P1-Pdots is applicable for NIR-II bioimaging. Hence, P1-Pdots could have a lot of potential contrast agents in the NIR-II region.

(JCYJ20170307110157501), Z.Liu acknowledges the Huxiang Young Talent Program of Hunan Province (2018RS3005); Y.L. acknowledges the Fundamental Research Funds of Central South University (2017zzts067).

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CONCLUSIONS In summary, we have successfully developed NIR-II fluorescent polymers with Qx derivatives acceptors for in vivo imaging. In one case, the P1-Pdots exhibited the emission peak at ∼1100 nm and high QY of ∼1%. Additional advantages of the P1-Pdots applied to in vivo NIR-II imaging include good colloidal stability, deep tissue penetration depth, good stability, and biocompatibility. The blood vessels NIR-II fluorescence imaging of mouse by P1-Pdots could be clearly observed with high spatial resolution and displayed an SBR of ∼2.1. Besides, P1-Pdots has been found potential applications for NIR-II tumor imaging of tumor-bearing nude mice, such as assessing the in vivo angiography. The results indicate the great potential of the polymer dots because of the high SBR and spatial resolution for imaging deep tissues.

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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.9b01142. Details of synthesis of monomers, polymers, and Pdots; supporting experimental section; characterization; DLS data; MTT assay tests, and 1H and 13C NMR spectral data(PDF) Observation of the vasculature of a nude mice in the NIRII region under a very short exposure time of 50 ms, a 1000 nm LP filter, and a fast frame rate of 20 fps (AVI)

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REFERENCES

(1) Hong, G.; Antaris, A. L.; Dai, H. Near-infrared fluorophores for biomedical imaging. Nat. Biomed. Eng. 2017, 1, 0010. (2) Wang, S.; Fan, Y.; Li, D.; Sun, C.; Lei, Z.; Lu, L.; Wang, F.; Zhang, F. Anti-quenching NIR-II molecular fluorophores for in vivo highcontrast imaging and pH sensing. Nat. Commun. 2019, 10, 1058. (3) Diao, S.; Blackburn, J. L.; Hong, G.; Antaris, A. L.; Chang, J.; Wu, J. Z.; Bo, Z.; Kuo, C. J.; Dai, H. Fluorescence Imaging In Vivo at Wavelengths beyond 1500 nm. Angew. Chem., Int. Ed. 2015, 54, 14758− 14762. (4) Tang, Y.; Li, Y.; Hu, X.; Zhao, H.; Ji, Y.; Chen, L.; Hu, W.; Zhang, W.; Li, X.; Lu, X. “Lock-and-Key”-Controlled Nanoprobes for Ultrahigh Specific Fluorescence Imaging in the Second Near-Infrared Window. Adv. Mater. 2018, 30, 1801140. (5) Wang, S.; Liu, J.; Feng, G.; Ng, L. G.; Liu, B. NIR-II Excitable Conjugated Polymer Dots with Bright NIR-I Emission for Deep In Vivo Two-Photon Brain Imaging Through Intact Skull. Adv. Funct. Mater. 2019, 29, 1808365. (6) Chen, Y. S.; Zhao, Y.; Yoon, S. J.; Gambhir, S. S.; Emelianov, S. Miniature gold nanorods for photoacoustic molecular imaging in the second near-infrared optical window. Nat. Nanotechnol. 2019, 14, 465− 472. (7) Tuncel, D.; Demir, H. V. Conjugated polymer nanoparticles. Nanoscale 2010, 2, 484. (8) Hong, G.; Wu, J. Z.; Robinson, J. T.; Wang, H.; Zhang, B.; Dai, H. Three-dimensional imaging of single nanotube molecule endocytosis on plasmonic substrates. Nat. Commun. 2012, 3, 700. (9) Welsher, K.; Liu, Z.; Sherlock, S. P.; Robinson, J. T.; Chen, Z.; Daranciang, D.; Dai, H. A route to brightly fluorescent carbon nanotubes for near-infrared imaging in mice. Nat. Nanotechnol. 2009, 4, 773−780. (10) Dong, B.; Li, C.; Chen, G.; Zhang, Y.; Zhang, Y.; Deng, M.; Wang, Q.; Qiangbin. FacileSynthesis of Highly Photoluminescent Ag2Se Quantum Dotsas a New Fluorescent Probe in the Second NearInfraredWindow for in Vivo Imaging. Chem. Mater. 2013, 25, 2503− 2509. (11) Hong, G.; Robinson, J. T.; Zhang, Y.; Diao, S.; Antaris, A. L.; Wang, Q.; Dai, H. In vivo fluorescence imaging with Ag2S quantum dots in the second near-infrared region. Angew. Chem., Int. Ed. 2012, 51, 9818−9821. (12) Zhang, Y.; Zhang, Y.; Hong, G.; He, W.; Zhou, K.; Yang, K.; Li, F.; Chen, G.; Liu, Z.; Dai, H. Biodistribution, pharmacokinetics and toxicology of Ag2S near-infrared quantum dots in mice. Biomaterials 2013, 34, 3639−3646. (13) Naczynski, D. J.; Tan, M. C.; Zevon, M.; Wall, B.; Kohl, J.; Kulesa, A.; Chen, S.; Roth, C. M.; Riman, R. E.; Moghe, P. V. Rare-earth-doped biological composites as in vivo shortwave infrared reporters. Nat. Commun. 2013, 4, 1345−1346. (14) Li, Y.; Zeng, S.; Hao, J. Non-Invasive Optical Guided Tumor Metastasis/Vessel Imaging by Using Lanthanide Nanoprobe with Enhanced Down-Shifting Emission Beyond 1500 nm. ACS Nano 2019, 13, 248−259. (15) Wan, H.; Yue, J.; Zhu, S.; Uno, T.; Zhang, X.; Yang, Q.; Yu, K.; Hong, G.; Wang, J.; Li, L. A bright NIR-II nanofluorophore for threedimensional imaging into biological tissues. Nat. Commun. 2018, 9, 1171. (16) Wang, S.; Fan, Y.; Li, D.; Sun, C.; Lei, Z.; Lu, L.; Wang, T.; Zhang, F. Anti-quenching NIR-II molecular fluorophores for in vivo highcontrast imaging and pH sensing. Nat. Commun. 2019, 10, 1058. (17) Wan, H.; Ma, H.; Zhu, S.; Wang, F.; Tian, Y.; Ma, R.; Yang, Q.; Hu, Z.; Zhu, T.; Wang, W. a Bright NIR-II Fluorophore with Fast Renal Excretion and Its Application in Molecular Imaging of Immune Checkpoint PD-L1. Adv. Funct. Mater. 2018, 28, 1804956.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hui Liu: 0000-0002-4792-9285 Yingping Zou: 0000-0003-1901-7243 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.Z. acknowledges the National Natural Science Foundation of China (51673215) and Science Fund for Distinguished Young Scholars of Hunan Province (2017JJ1029). C.W. acknowledges the National Natural Science Foundation of China (61335001; 81771930), The National Key Research and Development Program of China (2018YFB0407200), and Shenzhen Science and Technology Innovation Commission E

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Macromolecules (18) Miao, Q.; Pu, K. Organic Semiconducting Agents for DeepTissue Molecular Imaging: Second Near-Infrared Fluorescence, SelfLuminescence, and Photoacoustics. Adv. Mater. 2018, 30, 1801778. (19) Zhang, Y.; Hong, G.; Zhang, Y.; Chen, G.; Li, F.; Dai, H.; Wang, Q. Ag2S Quantum Dot: A Bright and Biocompatible Fluorescent Nanoprobe in the Second Near-Infrared Window. ACS Nano 2012, 6, 3695−3702. (20) Zhu, C.-N.; Jiang, P.; Zhang, Z.-L.; Zhu, D.-L.; Tian, Z.-Q.; Pang, D.-W. Ag2Se Quantum Dots with Tunable Emission in the Second Near-Infrared Window. ACS Appl. Mater. Interfaces 2013, 5, 1186− 1189. (21) Tao, Z.; Hong, G.; Shinji, C.; Chen, C.; Diao, S.; Antaris, A. L.; Zhang, B.; Zou, Y.; Dai, H. Biological Imaging Using Nanoparticles of Small Organic Molecules with Fluorescence Emission at Wavelengths Longer than 1000 nm. Angew. Chem., Int. Ed. 2013, 52, 13002−13006. (22) Antaris, A. L.; Chen, H.; Cheng, K.; Sun, Y.; Hong, G.; Qu, C.; Diao, S.; Deng, Z.; Hu, X.; Zhang, B. small-molecule dye for NIR-II imaging. Nat. Mater. 2016, 15, 235−242. (23) Feng, F.; Fang, H.; An, L.; Shu, W.; Li, Y.; Zhu, D. Fluorescent Conjugated Polyelectrolytes for Biomacromolecule Detection. Adv. Mater. 2008, 20, 2959−2964. (24) Yuan, J.; Zhang, Y.; Zhou, L.; Zhang, C.; Lau, T. K.; Zhang, G.; Lu, X.; Yip, H. L.; So, S. K.; Beaupré, S.; Mainville, M.; Johnson, P. A.; Leclerc, M.; Chen, H.; Peng, H.; Li, Y.; Zou, Y. Fused Benzothiadiazole: A Building Block for n-Type Organic Acceptor to Achieve HighPerformance Organic Solar Cells. Adv. Mater. 2019, 31, 1807577. (25) Yuan, J.; Huang, T.; Cheng, P.; Zou, Y.; Zhang, H.; Yang, J. L.; Chang, S.; Zhang, Z.; Huang, W.; Meng, D.; Gao, F.; Yang, Y. Enabling low voltage losses and high photocurrent in fullerene-free organic photovoltaics. Nat. Commun. 2019, 10, 570. (26) Xie, Y.; Gong, Y.; Han, M.; Zhang, F.; Peng, Q.; Xie, G.; Li, Z. Tetraphenylcyclopentadiene-Based Hyperbranched Polymers: Convenient Syntheses from One Pot “A4+ B2” Polymerization and High External Quantum Yields up to 9.74% in OLED Devices. Macromolecules 2019, 52, 896−903. (27) Hong, G.; Zou, Y.; Antaris, A. L.; Diao, S.; Wu, D.; Cheng, K.; Zhang, X.; Chen, C.; Liu, B.; He, Y. Ultrafast fluorescence imaging in vivo with conjugated polymer fluorophores in the second near-infrared window. Nat. Commun. 2014, 5, 4206. (28) Shou, K.; Tang, Y.; Chen, H.; Chen, S.; Zhang, L.; Zhang, A.; Fan, Q.; Yu, A.; Cheng, Z. Diketopyrrolopyrrole-based semiconducting polymer nanoparticles forin vivosecond near-infrared window imaging and image-guided tumor surgery. Chem. Sci. 2018, 9, 3105−3110. (29) Wu, C.; Chiu, D. T. Highly fluorescent semiconducting polymer dots for biology and medicine. Angew. Chem., Int. Ed. 2013, 52, 3086− 3109. (30) Wu, C.; Bull, B.; Szymanski, C.; Christensen, K.; Mcneill, J. Multicolor conjugated polymer dots for biological fluorescence imaging. ACS Nano 2008, 2, 2415−2423. (31) Williams, A. T. R.; Winfield, S. A.; Miller, J. N. Relative fluorescence quantum yields using a computer-controlled luminescence spectrometer. Analyst 1983, 108, 1067−1071. (32) Murphy, J. E.; Beard, M. C.; Norman, A. G.; Ahrenkiel, S. P.; Johnson, J. C.; Yu, P.; Mićić, O. I.; Ellingson, R. J.; Nozik, A. J. PbTe colloidal nanocrystals: synthesis, characterization, and multiple exciton generation. J. Am. Chem. Soc. 2006, 128, 3241−3247. (33) Chen, H.-C.; Chen, Y.-H.; Liu, C.-C.; Chien, Y.-C.; Chou, S.-W.; Chou, P.-T. Prominent Short-Circuit Currents of Fluorinated Quinoxaline-Based Copolymer Solar Cells with a Power Conversion Efficiency of 8.0%. Chem. Mater. 2012, 24, 4766−4772. (34) Lee, S. K.; Lee, W.-H.; Cho, J. M.; Park, S. J.; Park, J.-U.; Shin, W. S.; Lee, J.-C.; Kang, I.-N.; Moon, S.-J. Synthesis and Photovoltaic Properties of Quinoxaline-Based Alternating Copolymers for HighEfficiency Bulk-Heterojunction Polymer Solar Cells. Macromolecules 2011, 44, 5994−6001. (35) Yuan, J.; Ouyang, J.; Cimrová, V.; Leclerc, M.; Najari, A.; Zou, Y. Development of Quinoxaline Based Polymers for Photovoltaic Applications. J. Mater. Chem. C 2017, 5, 1858−1879.

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DOI: 10.1021/acs.macromol.9b01142 Macromolecules XXXX, XXX, XXX−XXX