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Conjugated polymers have been widely used in optical sensors, light-emitting diodes and solar cells, due to their attractive optical and semiconducting properties. It is widely accepted that the optical and electrical properties of conjugated polymer molecules depend on the conjugated segments, i.e., chromophores in conjugated polymer molecule. The study of the evolution of the absorption and emission properties of single conjugated polymer molecules is essential to provide complementary information for the influence of conformation of conjugated polymer on its energy transfer process, as well as on the performance of optoelectronic devices based on conjugated polymers. Although the extensive studies have been reported to elucidate the optical properties of conjugated polymers with single molecule spectroscopy, simultaneous revealing their absorption and emission properties and their real-time evolution are rarely reported. In this paper, we simultaneously measure the absorption and emission properties of chromophores in single Poly[2,7-(9,9-dioctylfluorene)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole](PFO-DBT) conjugated polymer molecules and their real-time evolution by frequency-domain reconstructed defocused wide-field imaging. The emission dipole orientation of chromophore is achieved by applying defocused wide-field fluorescence imaging. The change of defocused patterns of individual polymer chain describes the angular distribution of emitted light and thus the emitting dipole orientation. Meanwhile, the absorption dipole orientation of chromophore in single conjugated PFO-DBT polymer molecule can be clarified in reconstructed frequency-domain imaging by modulating the relative phase of the pulse pairs and performing Fourier transform to the photoluminescence response. The population density of excited state of absorbing chromophore depends both on the relative phase between the ultrashort pulse pairs and on the orientation of absorption transition dipole moment of the chromophore. By extracting the frequency-domain information of fluorescence that is proportional to the population density of excited state, the evolution of absorption dipole orientation of chromophore can be derived. We distinguish three cases for the evolution of chromophores of single PFO-DBT conjugated polymer molecules: the absorption and emission chromophores both keep constant in single PFO-DBT conjugated polymer molecules; one of the dipole orientations of absorption and emission changes, while the other remains unchanged; both of them change simultaneously. The results may pave the way for the further understanding of the role of conformation in the energy transfer pathway in both natural and artificial light harvesting systems at nano- and micro-level.
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Keywords:
- conjugated polymers /
- single molecule spectroscopy /
- defocused imaging /
- transition dipole orientation
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图 1 (a)实验装置示意图; (b)利用相对相位调制的脉冲对激发的共轭聚合物单分子散焦宽场成像原理示意图
Figure 1. (a) Schematic of the experimental setup; (b) schematic diagram of defocused wide-field imaging of single conjugated polymer molecules excited with phase-modulated ultrashort laser pulse pairs.
图 2 共轭聚合物单分子散焦宽场荧光成像时域序列图与利用傅里叶变换频域信息重构的成像序列图 (a)—(c)上半部分为实验测得的散焦宽场荧光成像随时间变化序列, 下半部分为相应的拟合结果; (d)—(f)为与散焦宽场荧光成像同样区域分子的频域信息重构成像图, 不同颜色代表相位的差异, 其中红色代表正相位, 白色代表负相位, 上半部分为直接重构成像结果, 下半部分为拟合结果
Figure 2. Schematic of the time-domain imaging sequence and reconstructed frequency-domain imaging by Fourier transform for single conjugated polymer molecules based on defocused wide-field fluorescence imaging. The upper part of (a), (b) and (c) gives the experimental results of defocused wide-field fluorescence imaging, while the lower part shows the simulation results. (d), (e) and (f) are the reconstructed frequency-domain imaging at the same area, where red color represents positive phase and white represents negative phase, the upper part gives the results of reconstructed imaging, while the lower part shows the simulation results.
图 3 共轭聚合物单分子Ⅰ, Ⅱ和Ⅲ的荧光调制轨迹和相应发射偶极取向发射角的变化 (a)施加在EOM上的锯齿波信号; (b), (c), (d)每个分子的荧光调制轨迹; (e), (f), (g)角度的变化
Figure 3. Modulated fluorescence trajectories and corresponding emission angle of single conjugated polymer molecules Ⅰ, Ⅱ and Ⅲ: (a) The sawtooth wave signal applied on EOM that used for phase modulation of pulse pairs; (b), (c), and (d) the modulated fluorescence trajectories of each molecule; (e), (f), and (g) the change of the angle for molecules Ⅰ, Ⅱ and Ⅲ, respectively.
图 4 (a), (b)共轭聚合物单分子散焦宽场成像时域序列图和相应模式的拟合结果; (c), (d)频域信息重构成像和相应的拟合结果, 其中红色代表正相位, 白色代表负相位; (e)施加在EOM上的锯齿波信号; (f), (g)分别显示分子1和2的荧光调制轨迹; (h), (i)分别显示分子1和2的发射角的变化; 图中展示了共轭聚合物单分子吸收和发射偶极取向均保持恒定(分子1)以及吸收和发射偶极取向同时发生变化(分子2)的情况
Figure 4. (a) and (b) are the snapshots of time-domain imaging based on defocused wide-field fluorescence imaging of single conjugated polymer molecules and corresponding simulation results; (c) and (d) show the reconstructed frequency-domain imaging and corresponding simulation results, where red color represents positive phase and white represents negative phase; (e) the sawtooth wave signal applied on EOM that used for phase modulation of pulse pairs; (f) and (g) show the fluorescence modulation trajectories of molecules 1 and 2, respectively; (h) and (i) show the change of emission angle of molecules 1 and 2, respectively. The absorption and emission dipole orientation of single conjugated polymer molecule 1 keep constant, while that of molecule 2 change simultaneously.
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[1] Hofmann F J, Vogelsang J, Lupton J M 2016 Appl. Phys. Lett. 108 1074
[2] Moliton A, Hiorns R C 2004 Polym. Int. 53 1397
Google Scholar
[3] Pron A, Rannou P 2002 Prog. Polym. Sci. 27 135
Google Scholar
[4] Vohra V, Anzai T 2017 J. Nanomate. 2017 1
[5] Li X, Li Z, Yang Y W 2018 Adv. Mater. 30 7
[6] Rochat S, Swager T M 2013 ACS Appl. Mater. Interfaces 5 4488
Google Scholar
[7] Alsalhi M S, Alam J, Dass L A, Raja M 2011 Int. J. Mol. Sci. 12 2036
Google Scholar
[8] Vohra V, Giovanella U, Tubino R, Murata H, Botta C 2011 ACS Nano 5 5572
Google Scholar
[9] Huang Y, Kramer E J, Heeger A J, Bazan G C 2014 Chem. Rev. 114 7006
Google Scholar
[10] Li G, Chang W H, Yang Y 2017 Nat. Rev. Mater. 2 17043
Google Scholar
[11] Xiao S, Zhang Q, You W 2017 Adv. Mater. 29 1601391
Google Scholar
[12] Vacha M, Habuchi S 2010 NPG Asia Mater. 2 134
Google Scholar
[13] 秦亚强,陈瑞云,石莹,周海涛,张国峰,秦成兵,高岩,肖连团,贾锁堂 2017 66 248201
Google Scholar
Qin Y Q, Chen R Y, Shi Y, Zhou H T, Zhang G F, Qin C B, Gao Y, Xiao L T, Jia S T 2017 Acta Phys. Sin. 66 248201
Google Scholar
[14] Orrit M, Bernard J 1990 Phys. Rev. Lett. 65 2716
Google Scholar
[15] Orrit M, Ha T, Sandoghdar V 2014 Chem. Soc. Rev. 43 973
Google Scholar
[16] Chen R Y, Wu R X, Zhang G F, Gao Y, Xiao L T, Jia S T 2014 Sensors 14 2449
Google Scholar
[17] 李斌, 张国峰, 景明勇, 陈瑞云, 秦成兵, 高岩, 肖连团, 贾锁堂 2016 65 218201
Google Scholar
Li B, Zhang G F, Jing M Y, Chen R Y, Qin C B, Gao Y, Xiao L T, Jia S T 2016 Acta Phys. Sin. 65 218201
Google Scholar
[18] Vanden Bout D A, Yip W T, Hu D H, Fu D K, Swager T M, Barbara P F 1997 Science 277 1074
Google Scholar
[19] Becker K, Lupton J M 2006 J. Am. Chem. Soc. 128 6468
Google Scholar
[20] Huser T, Yan M, Rothberg L J 2000 Proc. Natl. Acad. Sci. USA 97 11187
Google Scholar
[21] Schroeyers W, Vallee R, Patra D, Hofkens J, Habuchi S, Vosch T, Cotlet M, Mullen K, Enderlein J, de Schryver F C 2004 J. Am. Chem. Soc. 126 14310
Google Scholar
[22] Habuchi S, Onda S, Vacha M 2009 Chem. Commun. 334 4868
[23] Müller J G, Lupton J M, Feldmann J, Lemmer U, Scherf U 2004 Appl. Phys. Lett. 84 1183
Google Scholar
[24] Hu D H, Yu J, Wong K, Bagchi B, Rossky P J, Barbara P F 2000 Nature 405 1030
Google Scholar
[25] Ebihara Y, Vacha M 2008 J. Phys. Chem. B 112 12575
Google Scholar
[26] Kobayashi H, Onda S, Furumaki S, Habuchi S, Vacha M 2012 Chem. Phys. Lett. 528 1
Google Scholar
[27] Camacho R, Tubasum S, Southall J, Cogdell R J, Sforazzini G, Anderson H L, Pullerits T, Scheblykin I G 2015 Sci. Rep. 5 11
[28] Lin H, Tabaei S R, Thomsson D, Mirzov O, Larsson P O, Scheblykin I G 2008 J. Am. Chem. Soc. 130 7042
Google Scholar
[29] Hou L, Adhikari S, Tian Y, Scheblykin I G, Orrit M 2017 Nano Lett. 17 1575
Google Scholar
[30] Hou Q, Xu Y S, Yang W, Yuan M, Peng J B, Cao Y 2002 J. Mater. Chem. 12 2887
Google Scholar
[31] Dedecker P, Muls B, Deres A, Ujii H, Hotta J, Sliwa M, Soumillion J P, Müllen K, Enderlein J, Hofkens J 2009 Adv. Mater. 21 1079
Google Scholar
[32] Patra D, Gregor I, Enderlein J 2004 J. Phys. Chem. A 108 6836
Google Scholar
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