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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Selective Formation of End-On Orientation between Polythiophene and Fullerene Mediated by Coordination Nanospaces Takashi Kitao, Yotaro Sasaki, Susumu Kitagawa, Yutaka Imamura, Masahiko Tsujimoto, Shu Seki, and Takashi Uemura J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08120 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018
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Selective Formation of End-on Orientation between Polythiophene and Fullerene Mediated by Coordination Nanospaces Takashi Kitao,†,‡ Yotaro Sasaki,§ Susumu Kitagawa, ⊥ Yutaka Imamura, # Masahiko Tsujimoto, ⊥ Shu Seki,¶ and Takashi Uemura*,†,‡,ǁ †
Department of Advanced Materials Science, Graduate School of Frontier Sciences, The
University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan ‡
Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan §
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering,
Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ⊥
Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Yoshida, Sakyo-ku,
Kyoto 606-8501, Japan #
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan
University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan ¶
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University,
Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
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CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-
0012, Japan
ABSTRACT
It is essential for the design and optimization of optoelectrical devices to control heterointerfacial structures of electron donor and acceptor materials at the molecular level, because it is here that critical photophysical processes occur. In the present study, we have demonstrated that the accommodation of polythiophene and fullerene without any peripheries in one-dimensional nanochannels of a porous coordination polymer (PCP) enabled selective formation of end-on orientation relative to polythiophene-fullerene interfaces. We studied the charge carrier dynamics of one-dimensional assemblies of polythiophene and fullerene confined in the PCP. It is noteworthy that a long-lived charge-separated state arises uniquely in the end-on orientation. Theoretical analysis showed a small electron coupling between polythiophene and fullerene in the end-on orientation, suppressing charge recombination and prolonging charge carrier lifetime.
INTRODUCTION
Porous coordination polymers (PCPs) or metal-organic frameworks (MOFs), which are composed of transition metal ions and bridging organic ligands, have been recognized as an excellent platform for host-guest chemistry, including gas storage, separation, catalysis, and sensing.1-8 One of the most striking features of PCPs lies in their tunable nanospace, which can provide an ideal compartment for accommodating a variety of guest species ranging from small gas molecules to macromolecules, and control their assembly structures.9-11 The tailor-made
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nanospaces of PCPs allow us to regulate the relative orientation of neighboring guest species, which should have a profound effect on collective properties that are not present in individual species. Consequently, unprecedented molecular assemblies can be formed to elicit potentially useful, yet unexplored, properties.12-17 Heterojunctions, occurring between electron donor and acceptor materials, are expected to provide fascinating optoelectrical properties that are difficult to produce using homojunctions, making them a key structure in various applications.18-21 The spatial control of donor-acceptor interfaces in the devices is highly requisite because it is here that critical photophysical processes, including energy migration and electron transfer, take place. This is even true for organic photovoltaics (OPVs), which have attracted attention for their promise of low-cost, lightweight, large-area, and flexible solar energy conversion systems.22-26 Primary processes leading to free charge carrier generation upon light excitation have been investigated in several major stages: exciton diffusion into the interfaces, charge separation, and migration of free electrons and holes. In this field of research, most investigated OPVs are composed of polythiophene and fullerene derivatives as donors and acceptors, respectively.27,28 Despite the isotropic nature of the electron diffusion and migration processes on fullerene aggregates, strongly anisotropic hole diffusion on the conjugated backbones of the polythiophenes especially has an impact on the yield of free charge carriers. This suggests a crucial role for the conjugated backbone orientation relative to isotropic fullerene molecules in the primary processes and hence approaching the fundamental landscape of the processes. While simple and potentially inexpensive from a fabrication standpoint, mixing polythiophenes and fullerenes often leads to a highly complex morphology.29 Control of morphological characteristics (domain size, crystallinity, and so on) has been achieved by designing substitutions of component materials, incorporation of additives, and
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optimizing the fabrication condition of cells.30-32 However, precise control of orientation relative to the donor-acceptor heterointerfaces remains a challenging task, which is essential not only for providing the guidelines for achieving high device performance but also for understanding the nature of the excited state and charge dynamics at the interfaces.33,34 In particular, to our knowledge, there has been no report in which end-on orientation between polythiophenes and fullerenes is preferentially formed, because these two molecules tend to stack on one another through π-π interactions.29 In our efforts to control the interfacial structures, we considered that the geometrical constraint of the PCP framework with the one-dimensional channel, whose size is comparable to that of a fullerene molecule, can induce the selective formation of end-on orientation between polythiophenes and fullerenes.35,36 In the present study, unsubstituted polythiophene (PTh) and fullerene (C60) were accommodated in the nanochannels of [La(BTB)]n (1; BTB = 1,3,5benzenetrisbenzoate, channel size = 10.7 × 10.7 Å2), resulting in a one-dimensional donoracceptor assembly (Figure 1).37 PTh is unfusible, nonmelting, and insoluble; hence, the hybridization of PTh with other functional materials has remained challenging at the molecular level. It was noteworthy that the formation of end-on orientation relative to PTh/C60 interfaces in 1 gave rise to a dramatic increase in the lifetime of the charge-separated state, which was 3 orders of magnitude longer than that in a bulk heterojunction film consisting of an archetypal donor-acceptor pair, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Although long-lived charge separated states have been observed in covalently linked donor-acceptor dyads and triads, our use of standard organic photovoltaic materials sets this work apart.38 The results obtained indicate that our methodology is an important approach,
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not only for enhancing the functions of donor and acceptor materials, but also for broadening the understanding of the charge carrier dynamics at heterointerfaces.
Figure 1. Formation of the end-on orientation relative to the PTh/C60 interface in the onedimensional channels of 1.
EXPERIMENTAL SECTION
Materials All the reagents were purchased from commercial sources and used as received without further purification. 1 was prepared according to previously described methods.37 Introduction of C60 into 1 1 was heated at 150 °C under vacuum (
