論文の公開元へHeterogeneity in Low-dimensional Coordination Nanomaterials
概要
Introduction
Two are better than one. Combination of different constituents provides heterogeneity in nanomaterials. Many scientific achievements have been made with this concept so far. For example, alloy nanoparticles often have higher catalytic efficiencies compared with the monometallic counterparts. Also, organic polymers benefit a lot from the synergistic effects derived from heterostructures or copolymer configurations to boost their functionalities. Random, block, and sequence-controlled copolymers are studied for various purposes such as fabrication of energy-efficient organic solar cells, development of stimuli-responsive materials, and surface functionalization.
Meanwhile, coordination polymers are promising nanomaterials in that desired architectures can be constructed easily just by mixing their building blocks, organic ligands and metal ions. This feature allows researchers to develop many interesting frameworks, e.g., dendrimers, capsules, nanowires, nanosheets, and porous materials. Previously, our group has reported a bis(dipyrrinato)zinc(II) coordination nanowire and a bis(dithiolato)nickel(II) nanosheet which show photo- and electronic-functionalities, respectively. Based on these backgrounds, heterogeneity was introduced into these low-dimensional coordination nanomaterials in my Ph.D. research with the aim of cultivating the research field of coordination nanoscience.
Coordination copolymer chains
While naturally produced DNAs effectively use their single copolymer strands to designate their primary structures of proteins, single strands of one-dimensional artificial polymers have not been developed for useful applications. The difficulty lies in exfoliating them into single strands. Only few examples observed artificial polymers as single strands under ambient conditions, and none has had its heterostructure visualized. In the present research, heterogeneity was imported into the exfoliable dipyrrin coordination nanowires and characterization of the heterostructure in the molecular chain was attempted.
A series of one-dimensional coordination copolymers Co-k was fabricated from two different dipyrrin bridging ligands (BL1 and BL2) and zinc(II) ion (Figure 1). The formation of bis(dipyrrinato)zinc(II) motifs was confirmed by UV/vis absorption spectroscopy, X-ray photoelectron spectroscopy, elemental analysis and inductively coupled plasma atomic emission spectrometry. Also, these analyses showed that the composition of the random copolymers can be tuned by changing the mixing ratio of the two ligands.
The non-planar, rigid and steric frameworks of dipyrrin nanowires enabled us to exfoliate them into single-molecular chains in organic solvents. Thanks to this feature, the thickness of the molecular chains could be investigated by atomic force microscopy (AFM). The observed topographical height of Co-k represented the average thickness of the copolymer strands and the height value showed a strong correlation with the copolymer composition, indicative of the copolymer configuration of the nanowires (Figure 2a). This microscopic investigation became the first attempt to characterize heterostructure in an artificial molecular chain with AFM under ambient conditions.
The coordination copolymers showed notable photoluminescence properties derived from the constitutive bis(dipyrrinato)zinc(II) complexes. Compared with the corresponding homopolymers, the copolymer samples exhibited more efficient luminescence. Here, the heterostructure played a key role to enhance the photoluminescence efficiency (PL) of the dipyrrin nanowires because formation of non-emissive charge-separated state is prohibited owing to the manipulated energy levels of the frontier orbitals at a heteroleptic complex unit.
Assuming that a bis(dipyrrinato)zinc(II) unit is independent of each other in a chain, the PL should be mainly determined by the ratio of the heteroleptic complex units in molecular nanowires. However, the photoluminescence quantum yield plot showed an unexpected bell curve when BL2 was excited (Figure 2b). This experimental result showed that an exciton generated at a dipyrrin unit of BL2 can hop to another unit, verifying the intrachain exciton migration behavior in the dipyrrin nanowires (Figure 2c).
This project not only showed a potential applicability of dipyrrin coordination nanowires as photonic wires but also proposed a new research strategy to fundamental nanoscience.
Multinuclear dipyrrin complexes
For a better understanding of the exciton migration behavior in dipyrrin nanowires, sequence-determined coordination copolymers are desired rather than random ones. Here, preparation of multinuclear dipyrrin-zinc complexes was attempted considering that they can be regarded as the repeating units of periodically arranged dipyrrin copolymers (Figure 3a).
Bridging ligand BL3 and -extended terminal ligand L1 were mixed together with zinc acetate and the resulting mixture was subjected to gel permeation chromatography (GPC) separation (Figure 3b). As a result, 16 complexes CN (from mononuclear C1 to hexadecanuclear C16) were fractioned and characterized by nuclear magnetic resonance spectroscopy and UV/vis absorption spectroscopy. In addition, electron spray ionization mass spectrometry could detect the complexes (up to the decanuclear complex). Please note that the hexadecanuclear complex represents the largest example for a discrete linear coordination chain to the best of our knowledge (Figure 3c). Then their luminescence properties were examined in organic solvents. Thanks to the defined structure of CN, we could carry out a numerical simulation for the photoluminescence efficiency which could reproduce the experimental PL-N plot, verifying our theoretical model of fast intrachain exciton migration.
Heterogeneity in dipyrrin ligand
As the design of the above described coordination nanowires is based on our detailed knowledge on mononuclear bis(dipyrrinato)zinc(II) complexes, study on a novel complex motif leads to development of functional coordination nanomaterials. Here, another heterogeneity was introduced in a dipyrrin ligand to create a new constituent.
Asymmetric dipyrrin ligand L2 was synthesized from two kinds of pyrroles and then the ligand was employed to form homoleptic bis(dipyrrinato)zinc(II) complex M1 (Figure 4). Because of the steric hindrance provided by the methyl groups at the -positions of the dipyrrinato ligand, the complex had two atropisomers. The isomers could be separated by chiral column chromatography and spectroscopic measurements revealed their chiroptical property with intense fluorescence ability (PL = 0.70). These findings may be utilized for construction of new luminescent coordination polymers.
Heterogeneity in coordination nanosheets
Coordination nanosheets with -conjugated planar structures are of great interest. Among them, bis(dithiolato)metal nanosheets (MDTs) are one of the most advantageous materials for device applications due to their remarkably high electrical conductivities.
This work aimed at manipulation of electrical properties of MDT films by incorporating two kinds of metal ions. Mixed-metal dithiolene nanosheet CuxNi1-xDT (x: mixing ratio of copper ion) was prepared at a liquid-liquid interface between a dichloromethane layer of benzenehexathiol (BHT) and an aqueous layer containing both copper and nickel ions (Figure 5a). It was demonstrated that the electrical conductivity of the film could be controlled with the mixing ratio of the metal ions, indicating a possible application of CuxNi1-xDT films as thermoelectric conversion materials. Also, the heterogeneity induced significant improvement in the film crystallinity as observed in GIXD and PXRD patterns (Figure 5b,c).
Conclusion
In the series of research, three types of heterostructure were introduced in low-dimensional coordination systems, producing heteroleptic dipyrrin coordination chains, a complex motif bearing asymmetric dipyrrin ligands, and heterometallic dithiolene nanosheets (Figure 6). Each example proved the significance of heterogeneity in exploring the nanoworld.
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