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Functional Inorganic Materials and Devices

Heterochiral Doped Supramolecular Coordination Networks for High-Performance Optoelectronics Xiaobo Shang, Inho Song, Jeong Hyeon Lee, Wanuk Choi, Hiroyoshi Ohtsu, Gwan Yeong Jung, Jaeyong Ahn, Myeonggeun Han, Jin Young Koo, Masaki Kawano, Sang Kyu Kwak, and Joon Hak Oh ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04653 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Applied Materials & Interfaces

Heterochiral Doped Supramolecular Coordination Networks for HighPerformance Optoelectronics

Xiaobo Shang,†, Inho Song,†, Jeong Hyeon Lee,‡ Wanuk Choi,§ Hiroyoshi Ohtsu,# Gwan Yeong Jung,‡ Jaeyong Ahn,† Myeonggeun Han,$ Jin Young Koo, & Masaki Kawano,# Sang Kyu Kwak,*,‡ and Joon Hak Oh*,†

†School

of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National

University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea ‡Department

of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National

Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea §Center

for Ordered Nanoporous Materials Synthesis, School of Environmental Science and

Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 37673, Republic of Korea #Department

of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1 Ookayama,

Meguro-ku, Tokyo 152-8550, Japan $Department

of Chemical Engineering, Pohang University of Science and Technology

(POSTECH), Pohang, Gyeongbuk 37673, Republic of Korea &Department

of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang,

Gyeongbuk 37673, Republic of Korea

Keywords: chiral self-discrimination, supramolecular coordination network, optoelectronics, doping, micro-/nano-devices

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Abstract Chiral self-sorting has great potential for constructing new complex structures and determining chirality-dependent properties in multicomponent mixtures. However, it is still of great challenge to achieve high fidelity chiral self-discrimination. Besides, the researches on the coordination polymers or metal-organic frameworks (CPs/MOFs) for micro-/nano-optoelectronics are still rare due to their low conductivity and difficulty in developing a rapid and simple scale-up synthetic method. Here, heterochiral supramolecular coordination networks (SCNs) were synthesized by the solvothermal reaction of naphthalene diimide enantiomers and cadmium iodide, using the chirality as a synthetic tuning parameter to control the morphologies. Intriguingly, heterochiral micro/nanocrystals exhibited photochromic and photodetecting properties. Furthermore, we also developed a simple and efficient doping method to enhance the conductivity and photoresponsivity of micro-/nanocrystals using hydrazine. From experimental and theoretical studies, the mechanism was suggested as follows: the radicals in the singly occupied molecular orbital (SOMO) level of the ligands provide charge carriers that can undergo “through-space” transport between K–K stacked ligands and the electron transfer from adsorbed hydrazine to the SCNs results in reduction of energy gap, leading to increased conductivity. Our findings demonstrate a simple and powerful strategy for implementing coordination networks with redox ligands for micro-/nanooptoelectronic applications.


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1. Introduction Chirality is of great importance in biochemistry, pharmaceutical chemistry, environmental chemistry, agricultural chemistry, and supramolecular chemistry.1 Chiral

self-sorting, categorized as chiral self-recognition or self-discrimination in complex mixtures,2 is a promising tool to construct new complex structures, as intrinsically different

structures can be produced by the interactions between molecules of the same or the opposite chirality with different interaction energies.3 Chiral self-sorting occurs among

synthetic molecules and natural systems, leading to the amplification of discrete species.4 Achieving high fidelity chiral self-discrimination can be challenging, due to the similarity between the recognition sites of enantiomers and common conformational lability.4 In principle, two extreme cases are possible for metal-mediated coordination polymers (CPs) or metal-organic frameworks (MOFs) when starting from heterochiral ligands. One is ligand self-recognition and the other is ligand self-discrimination, which produces homochiral and heterochiral complexes, respectively. To date, most studies have explored the synthetic routes to, and applications of, pure enantiomeric ligands. In contrast, a more complex and less explored situation arises when two enantiomeric ligands are mixed.4 Compared with CPs/MOFs using enantiomeric ligands, the CPs/MOFs synthesized from the racemic mixtures of the ligands do not necessarily replicate the topology of the frameworks5 and the outcomes are more complicated. As non-covalent interactions can interlink two coordination complexes with ligands of the same or different chirality, it is of



great significance to thoroughly investigate how different factors affect the formation of homo/heterochiral CPs/MOFs by the self-assembly of racemic ligands.6 One of the most important, yet least explored properties of CPs/MOFs is their ability to conduct electrical charges for use as active functional materials for next-generation electronic or optoelectronic devices due to their low conductivity.7 To date, several methods have been reported to improve the electrical conductivity of CPs/MOFs, including the use of conjugated organic linkers, different metal cations, molecular dopants and mixed valence doping.8-13 Besides, ligands that produce stable organic radicals can be used to obtain conductive CPs/MOFs, as they provide free charge carriers that can improve charge density.14 Although an understanding of the mechanisms of charge transport in CPs/MOFs is in its infancy,15 three charge transport mechanisms in CPs/MOFs have been identified: “through-bond” conduction, “through-space” conduction, and conduction via guest molecules.7 Recently, “through-space” intervalence charge transfer (IVCT) has been proposed as a mechanism for charge delocalization in MOFs.16-17 As a new type of materials, micro-/nano-structured CPs/MOFs have received great attention because they contain both organic and inorganic species, which can play parallel roles as both a precursor and a template under appropriate condition.18-19 Although the controllability of the size and shape of micro-/nano-scaled CPs/MOFs renders them ideal candidates for a variety of applications, in particular for low-dimensional photonics, electronics, and optoelectronics, the researches in this field are still rare and it is of great challenge and high demand to develop a rapid and simple scale-up and environmentally friendly synthetic method for micro-/nano-scaled CPs/MOFs.20-22 In addition, chirality has rarely been used as a tuning parameter in controlling the morphology of micro-/nano-scaled CPs/MOFs.


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In this study, we synthesized micro-/nano-scaled heterochiral supramolecular coordination networks (SCNs) that are highly suitable for optoelectronic devices. The AlaNDI-Cd SCNs were synthesized by the solvothermal reaction of naphthalene diimide enantiomers and cadmium iodide, using the chirality as a synthetic tuning parameter to control the morphologies of SCNs. The heterochiral SCNs exhibited multifunctional properties, such as chiral self-discrimination, photochromic, and good photodetecting characteristics owing to their higher thermodynamic stability over homochiral analogues and photo-generated radicals, respectively. To improve the conductivity and photoresponsivity of SCNs, we developed an efficient method whereby the device with SCNs was exposed to a reducing agent (hydrazine vapor) for 5 min. Notably, the conductivity of the n-type doped AlaNDI-Cd micro-/nanocrystals was greatly increased by a factor of 2500. The photoresponsivity (R) and external quantum efficiency (EQE) of the doped SCNs were 41.8 A W–1 and 1.4 × 104%, respectively, around ten times higher than those of the undoped system under UV light exposure. Our experimental and theoretical studies revealed that the radicals in the singly occupied molecular orbital (SOMO) level of the ligands generate charge carriers after hydrazine doping or UV-light irradiation and charge transport takes place in the “through-space” mode. In addition, the adsorption of hydrazine onto the SCN surface reduces the energy gap, which contributes to the enhanced conductivity. Our pioneering findings demonstrate a highly feasible methodology to increase the conductivity of CPs/MOFs with redox ligands by hydrazine doping for a short time and paves a new way for applications in micro-/nano-scaled optoelectronics.

2. Experimental Section



2.1 Synthesis of Homochiral AlaNDI-Cd. CdI2 (0.1 mmol) and enantiopure ligand H2AlaNDI23 (0.1 mmol) in 3 mL DMF were stirred for 30 min at room temperature for homogenization, and then sealed in a stainless-steel tube with a Teflon liner and heated at 120 oC for 72 h for the solvothermal reaction. The crude product was filtered and washed with DMF to give the final single crystals with the yield of 34% and 38% for (R)- and (S)-AlaNDI-Cd, respectively. (R)-AlaNDI-Cd Calc. for [Cd(AlaNDI)(DMF)1.9]·1.4H2O : C: 45.07%; H: 4.14%; N:7.98%. Found. C: 45.10%, H: 3.98%, N: 8.32%. (S)-AlaNDI-Cd Calc. for [Cd(AlaNDI)(DMF)1.75]·2H2O : C: 44.29%; H: 4.16%; N:7.67%. Found. C: 44.13%, H: 3.78%, N: 7.78%. 2.2 Synthesis of Heterochiral AlaNDI-Cd. CdI2 (0.1 mmol), (R)-H2AlaNDI (0.05 mmol), and (S)-H2AlaNDI (0.05 mmol) in 3 mL DMF were stirred for 30 min at room temperature for homogenization, and then sealed in a stainless-steel tube with a Teflon liner and heated at 120 oC for 72 h for the solvothermal reaction. The crude product was filtered and washed with DMF to give the final single crystals with the yield of 24%. (Rac)-AlaNDI-Cd Calc. for [Cd(AlaNDI)(DMF)1.9]·1.4H2O: C: 45.07%; H: 4.14%; N:7.98%. Found. C: 45.05%, H: 3.93%, N: 7.95%. 2.3 Device Fabrication. Heavily doped silicon wafers (n-type, < 0.004 XU),= with thermally grown 300 nm thick SiO2 (Ci = 11.5 nF cm–2) were used as substrates for electronic devices. The wafers were washed with toluene, acetone and isopropyl alcohol, and dried under nitrogen gas. The Cr/Au electrodes (4/40 nm) were thermally evaporated and patterned via conventional photolithography on the substrate. The electrodes had a channel length (L) of 10 µm and a channel width (W) of 200 µm (W/L = 20). To fabricate the electronic device, the synthesized AlaNDI-Cd micro-/nanocrystals were cleaned and collected using vacuum filtration, then dispersed in DMF.


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Sequentially, the dispersed crystals were drop-casted on the substrate. The substrates were annealed at 60 °C in a vacuum oven to evaporate the residual DMF. 2.4 Electrical Measurement. The current-voltage characteristics of the SCNs were measured in ambient condition and N2 condition using a Keithley 4200-SCS parametric analyzer. The electrical conductivity of the SCNs was estimated by the two-probe method as follows: =

(1)

= ×

where G is electrical conductance, and l and A are length and area of the conduction channel, which were measured using the SEM image of AlaNDI-Cd micro-/nanocrystals. 2.5 Rise and Decay Time Estimation. Rise and decay times of photodetectors were estimated using following equations: =

+

=

+

(2)

+

(3)

+

where t is the light switching time, A and B are scaling constants, and

1

and

2

are time constants

for rising and decaying rate, respectively. 2.6 Material Analysis. Elemental analyses were performed on an Elementar vario MICRO

cube at Technical SupportCentre in Pohang University of Science and Technology. The

absorption spectra were measured on Cary 5000 UV-Vis-NIR and Cary 6000i UV-Vis-

NIR spectrophotometer for ligands and SCNs, respectively. PL spectra were recorded

on FP-6500 Spectrofluorometer (JASCO). The CD results were obtained using J-815



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Spectropolarimeter (JASCO). ESR spectra were measured on a Bruker Biospin A200

spectrometer. Crystals (3 mg) were put into a thin wall tube for ESR measurements.

2.7 Single Crystal Analysis. The single crystal X-ray diffraction data were collected on a

Bruker APEX II QUAZAR instrument. All the structures were solved by direct methods (SHELXS-97/SHELXS-2014) and refined by full-matrix least squares calculations on F2

(SHELXL-2014) using the SHELX-TL program package. 2.8 X-ray Crystallographic Data for (R)-AlaNDI-Cd Single Crystal. C104H104N16O40Cd4 Mr = 2740.72, crystal dimensions 0.03 × 0.03 × 0.03 mm3, orthorhombic, space group P21212,

a = 16.7454(6), b = 23.7170(8) Å, c = 14.1718(5) Å, V = 5628.3(3) Å3, Z = 2, g cm–3,

= 0.842 mmE ,

calcd

= 1.617

= 0.71073 Å (Mo K ), T = 100(2) K, Flack parameter: 0.03(4),

14425 unique reflections out of 17255 with F0 > 4 (F0), 791 parameters, 56 restraints, 1.437
4 (F0), 793 parameters, 257 restraints, 1.438

Heterochiral Doped Supramolecular Coordination ... - ACS Publications

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