Heterochiral Doped Supramolecular Coordination Networks forHigh-Performance OptoelectronicsXiaobo Shang,†,⊥ Inho Song,†,⊥ Jeong Hyeon Lee,‡ Wanuk Choi,§ Hiroyoshi Ohtsu,#

Gwan Yeong Jung,‡ Jaeyong Ahn,† Myeonggeun Han,∥ Jin Young Koo,Δ 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 andTechnology (UNIST), Ulsan 44919, Republic of Korea§Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, ∥Department ofChemical Engineering, and ΔDepartment of Chemistry, 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

*S Supporting Information

ABSTRACT: Chiral self-sorting has great potential forconstructing new complex structures and determiningchirality-dependent properties in multicomponent mixtures.However, it is still of great challenge to achieve high fidelitychiral self-discrimination. Besides, the researches on thecoordination polymers or metal−organic frameworks formicro/nanooptoelectronics are still rare due to their lowconductivity and difficulty in developing a rapid and simplescale-up synthetic method. Here, heterochiral supramolecularcoordination networks (SCNs) were synthesized by thesolvothermal reaction of naphthalene diimide enantiomersand cadmium iodide, using the chirality as a synthetic tuningparameter to control the morphologies. Intriguingly, hetero-chiral micro/nanocrystals exhibited photochromic and photo-detecting properties. Furthermore, we also developed a simple and efficient doping method to enhance the conductivity andphotoresponsivity of micro/nanocrystals using hydrazine. From experimental and theoretical studies, the mechanism wassuggested as follows: the radicals in the singly occupied molecular orbital level of the ligands provide charge carriers that canundergo “through-space” transport between π−π stacked ligands and the electron transfer from adsorbed hydrazine to the SCNsresults in reduction of energy gap, leading to increased conductivity. Our findings demonstrate a simple and powerful strategyfor implementing coordination networks with redox ligands for micro/nanooptoelectronic applications.

KEYWORDS: chiral self-discrimination, supramolecular coordination network, optoelectronics, doping, micro/nanodevices

1. INTRODUCTIONChirality is of great importance in biochemistry, pharmaceut-ical chemistry, environmental chemistry, agricultural chemistry,and supramolecular chemistry.1 Chiral self-sorting, categorizedas chiral self-recognition or self-discrimination in complexmixtures,2 is a promising tool to construct new complexstructures, as intrinsically different structures can be producedby the interactions between molecules of the same or theopposite chirality with different interaction energies.3 Chiralself-sorting occurs among synthetic molecules and naturalsystems, leading to the amplification of discrete species.4

Achieving high fidelity chiral self-discrimination can bechallenging due to the similarity between the recognition

sites of enantiomers and common conformational lability.4 Inprinciple, two extreme cases are possible for metal-mediatedcoordination polymers (CPs) or metal−organic frameworks(MOFs) when starting from heterochiral ligands. One is ligandself-recognition, and the other is ligand self-discrimination,which produces homochiral and heterochiral complexes,respectively. To date, most studies have explored the syntheticroutes to, and applications of, pure enantiomeric ligands. Incontrast, a more complex and less explored situation arises

Received: March 15, 2019Accepted: May 2, 2019Published: May 2, 2019

Research Article

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© XXXX American Chemical Society A DOI: 10.1021/acsami.9b04653ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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when two enantiomeric ligands are mixed.4 Compared withCPs/MOFs using enantiomeric ligands, the CPs/MOFssynthesized from the racemic mixtures of the ligands do notnecessarily replicate the topology of the frameworks,5 and theoutcomes are more complicated. As noncovalent interactionscan interlink two coordination complexes with ligands of thesame or different chirality, it is of great significance tothoroughly investigate how different factors affect theformation of homo/heterochiral CPs/MOFs by the self-assembly of racemic ligands.6

One of the most important, yet least explored, properties ofCPs/MOFs is their ability to conduct electrical charges for useas active functional materials for next-generation electronic oroptoelectronic devices due to their low conductivity.7 To date,several methods have been reported to improve the electricalconductivity of CPs/MOFs, including the use of conjugatedorganic linkers, different metal cations, molecular dopants, andmixed valence doping.8−13 Besides, ligands that produce stableorganic radicals can be used to obtain conductive CPs/MOFs,as they provide free charge carriers that can improve chargedensity.14 Although an understanding of the mechanisms ofcharge transport in CPs/MOFs is in its infancy,15 three chargetransport mechanisms in CPs/MOFs have been identified:“through-bond” conduction, “through-space” conduction, andconduction via guest molecules.7 Recently, through-spaceintervalence charge transfer (IVCT) has been proposed as amechanism for charge delocalization in MOFs.16,17

As a new type of materials, micro/nanostructured CPs/MOFs have received great attention because they contain bothorganic and inorganic species, which can play parallel roles asboth a precursor and a template under appropriatecondition.18,19 Although the controllability of the size andshape of micro/nanoscaled CPs/MOFs renders them as idealcandidates for a variety of applications, in particular for low-dimensional photonics, electronics, and optoelectronics, theresearches in this field are still rare, and it is of great challengeand high demand to develop a rapid and simple scale-up andenvironmentally friendly synthetic method for micro/nano-scaled CPs/MOFs.20−22 In addition, chirality has rarely beenused as a tuning parameter in controlling the morphology ofmicro/nanoscaled CPs/MOFs.In this study, we synthesized micro/nanoscaled heterochiral

supramolecular coordination networks (SCNs) that are highlysuitable for optoelectronic devices. The AlaNDI-Cd SCNswere synthesized by the solvothermal reaction of naphthalenediimide enantiomers and cadmium iodide, using the chirality asa synthetic tuning parameter to control the morphologies ofSCNs. The heterochiral SCNs exhibited multifunctionalproperties, such as chiral self-discrimination, photochromic,and good photodetecting characteristics, owing to their higherthermodynamic stability over homochiral analogues andphotogenerated radicals. To improve the conductivity andphotoresponsivity of SCNs, we developed an efficient methodwhereby the device with SCNs was exposed to a reducingagent (hydrazine vapor) for 5 min. Notably, the conductivity ofthe n-type doped AlaNDI-Cd micro/nanocrystals was greatlyincreased by a factor of 2500. The photoresponsivity (R) andexternal quantum efficiency (EQE) of the doped SCNs were41.8 A W−1 and 1.4 × 104%, respectively, around 10 timeshigher than those of the undoped system under UV lightexposure. Our experimental and theoretical studies revealedthat 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 transporttakes place in the “through-space” mode. In addition, theadsorption of hydrazine onto the SCN surface reduces theenergy gap, which contributes to the enhanced conductivity.Our pioneering findings demonstrate a highly feasiblemethodology to increase the conductivity of CPs/MOFswith redox ligands by hydrazine doping for a short time andpaves a new way for applications in micro/nanoscaledoptoelectronics.

2. EXPERIMENTAL SECTION2.1. Synthesis of Homochiral AlaNDI-Cd. CdI2 (0.1 mmol) and

enantiopure ligand H2AlaNDI23 (0.1 mmol) in 3 mL of DMF were

stirred for 30 min at room temperature for homogenization and thensealed in a stainless steel tube with a Teflon liner and heated at 120°C for 72 h for the solvothermal reaction. The crude product wasfiltered and washed with DMF to give the final single crystals with theyield of 34% and 38% for (R)- and (S)-AlaNDI-Cd, respectively.

(R)-AlaNDI-Cd calcd 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 calcd 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 3mL of DMF were stirred for 30 min at room temperature forhomogenization and then sealed in a stainless steel tube with a Teflonliner and heated at 120 °C for 72 h for the solvothermal reaction. Thecrude product was filtered and washed with DMF to give the finalsingle crystals with the yield of 24%.

(Rac)-AlaNDI-Cd calcd 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 Ω·cm) with a thermally grown 300 nm thick SiO2 (Ci = 11.5nF cm−2) were used as substrates for electronic devices. The waferswere washed with toluene, acetone, and isopropyl alcohol and driedunder nitrogen gas. The Cr/Au electrodes (4/40 nm) were thermallyevaporated and patterned via conventional photolithography on thesubstrate. The electrodes had a channel length (L) of 10 μm and achannel width (W) of 200 μm (W/L = 20). To fabricate theelectronic device, the synthesized AlaNDI-Cd micro/nanocrystalswere cleaned and collected using vacuum filtration then dispersed inDMF. Sequentially, the dispersed crystals were drop-casted on thesubstrate. The substrates were annealed at 60 °C in a vacuum oven toevaporate the residual DMF.

2.4. Electrical Measurement. The current−voltage character-istics of the SCNs were measured in ambient condition and N2condition using a Keithley 4200-SCS parametric analyzer. Theelectrical conductivity of the SCNs was estimated by the two-probemethod as follows

σ = = ×GlA

IV

lA (1)

where G is the electrical conductance, and l and A are the length andarea of the conduction channel, respectively, which were measuredusing the SEM image of AlaNDI-Cd micro/nanocrystals.

2.5. Rise and Decay Time Estimation. Rise and decay times ofphotodetectors were estimated using following equations

= + +τ τI I Ae Bet tlight dark

/ /r1 r2 (2)

= + +τ τ− −I I Ae Bet tlight dark

/ /d1 d2 (3)

where t is the light switching time, A and B are the scaling constants,and τ1 and τ2 are the time constants for rising and decaying rates,respectively.

2.6. Material Analysis. Elemental analyses were performed on anElementar vario MICRO cube at Technical Support Center in PohangUniversity of Science and Technology. The absorption spectra weremeasured on a Cary 5000 UV−vis−NIR and a Cary 6000i UV−vis−

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NIR spectrophotometer for ligands and SCNs, respectively. PLspectra were recorded on an FP-6500 spectrofluorometer (JASCO).The CD results were obtained using J-815 spectropolarimeter(JASCO). ESR spectra were measured on a Bruker Biospin A200spectrometer. Crystals (3 mg) were put into a thin wall tube for ESRmeasurements.2.7. Single Crystal Analysis. The single-crystal X-ray diffraction

data were collected on a Bruker APEX II QUAZAR instrument. Allthe 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)Å, b = 23.7170(8) Å, c = 14.1718(5) Å, V = 5628.3(3) Å3, Z = 2, ρcalcd= 1.617 g cm−3, μ = 0.842 mm−1, λ = 0.71073 Å (Mo Kα), T =100(2) K, flack parameter: 0.03(4), 14,425 unique reflections out of17,255 with F0 > 4σ(F0), 791 parameters, 56 restraints, 1.437 < θ <30.560°, final R factors R1 = 0.0618 and wR2 = 0.1362, GOF = 1.073.CCDC deposit number: 1531448.2.9. X-ray Crystallographic Data for (Rac)-AlaNDI-Cd Single

Crystal. C104H104N16O40Cd4 Mr = 2740.72, crystal dimensions 0.03 ×0.03 × 0.03 mm3, orthorhombic, space group P22121, a = 14.1619(4)Å, b = 16.7951(5) Å, c = 23.6544(8) Å, V = 5626.2(3) Å3, Z = 2, ρcalcd= 1.618 g cm−3, μ = 0.842 mm−1, λ = 0.71073 Å (Mo Kα), T =100(2) K, flack parameter: 0.39(7), 15,179 unique reflections out of21,529 with F0 > 4σ(F0), 793 parameters, 257 restraints, 1.438 < θ <33.228°, final R factors R1 = 0.1158 and wR2 = 0.2272, GOF = 1.096.CCDC deposit number: 1531449.2.10. Model Systems for Calculation. The unit cell structures

of homochiral and heterochiral SCNs, obtained from ourexperimental XRD patterns and previously reported literature,24

were optimized by density functional theory (DFT) calculation. Toinvestigate the formation energy of SCN and the changes of electronicstructure by hydrazine doping, two different model systems wereconstructed (i.e., cluster and surface systems). To adopt the π−π

interactions between four AlaNDI ligands interconnected by a Cdcenter, catenane motif containing 8 Cd atoms was taken for thecluster model. Note that alanine molecules were substituted forneighboring ligands (i.e., Cd8(DMF)16(AlaNDI)4(alanine)8) toremove the periodicity of SCNs without dangling bonds. The surfacemodel was constructed by introducing the vacuum slab along (100)direction where the AlaNDI ligands are stacked (i.e. ,Cd8(DMF)16(AlaNDI)8).

2.11. DFT Calculations. All DFT calculations were conductedwith DMol3 program.25,26 We used the Perdew−Burke−Ernzerhof(PBE) exchange-correlation functional27 and DNP 4.4 basis set withDFT semicore pseudopotential core treatment. To adopt thedispersion correction for van der Waals interactions, semiempiricalGrimme scheme was employed.28 The convergence criteria forenergy, force, and displacement were set as 1.0 × 10−5 Ha, 0.002 Ha/Å, and 0.005 Å, respectively. The Brillouin zone was sampled by aMonkhorst−Pack29 as Γ-point for the surface system. The atomiccharges were obtained from Mulliken population analysis.30 For thedensity of states (DOS) analysis, the smearing width was set to be0.05 eV. The formation energy (ΔEf) of the (Rac)-AlaNDI-Cd clusterfrom homochiral counterparts was calculated using the followingequation

Δ = − +− − −E E E E( )/2Rac R Sf Cd ( ) Cd ( ) Cd ( ) (4)

where ECd‑(Rac), ECd‑(R), and ECd‑(S) represent the total energy of clustermodeled (Rac)-, (R)-, and (S)-AlaNDI-Cd, respectively. The bindingstrength of adsorbed hydrazine molecules on the (Rac)-AlaNDI-Cdsurface were investigated through binding energy (ΔEbind) anddifferential binding energy (ΔΔEbind) calculations as follows

Δ = − −+ ·E E E nERac n Racbind ( ) Hz ( ) Hz (5)

ΔΔ = − −+ · + − ·E E E ERac n Rac nbind ( ) Hz ( ) ( 1) Hz Hz (6)

where E(Rac)+n·Hz and E(Rac)+(n−1)·Hz represent the total energy of the(Rac)-AlaNDI-Cd surface with “n” and “(n−1)” adsorbed hydrazine

Figure 1. (a, b) Crystal structures of SCNs (a) (Rac)-AlaNDI-Cd; top left shows a-axis projection, top middle shows unit description containingtwo AlaNDI units in 1D chain with description of chirality, top right shows polycatenation of 1D chains, bottom left shows π−π stacking schemewith centroid-centroid distances, and bottom right shows chiral configuration in this SCN and (b) (R)-AlaNDI-Cd; top left shows c-axis projection,top right shows unit description containing two AlaNDI units in 1D chain with description of chirality, bottom left shows π−π stacking schemewith centroid-centroid distances, bottom right shows chiral configuration in this SCN, and bottom right shows polycatenation of 1D chains.Scheme on the right demonstrate the chemical structure of two enantiomeric AlaNDI ligands, which have coordination with cadmium in SCN. (c)Formation energy (ΔEf) calculation results for chiral discrimination of AlaNDI-Cd cluster models. Blue- and red-colored clusters represent eachunit of catenane motif, which is composed of two AlaNDIs connected by a Cd center. For clear view of cluster models, top and front views arepresented, and the arrangements of chiral centers are shown by small letters in the front view.

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molecules, E(Rac) represents the total energy of the bare (Rac)-AlaNDI-Cd surface, and EHz is the total energy of an isolatedhydrazine molecule.2.12. Monte Carlo Simulations. Monte Carlo simulations were

carried out by Sorption program31 to sample the stable adsorptionsites on the (Rac)-AlaNDI-Cd surface for hydrazine molecules. Forthe increase of the surface coverage of hydrazine molecules, werepeated the procedure of sequential addition of one hydrazinemolecule onto the surface of the DFT-optimized structure andreoptimized it. For the potential energy parameters, universalforcefield32 was employed with Mulliken atomic charges obtainedfrom DFT calculations. A total of 1.0 × 105 MC steps for each casewere performed.

3. RESULTS AND DISCUSSION

Homochiral and heterochiral AlaNDI-Cd crystals weresynthesized using the solvothermal reaction, and theirmorphologies were characterized using scanning electronmicroscopy (SEM). The homochiral AlaNDI-Cd crystalsexhibited thick (∼10 μm) block-shaped morphologies (FigureS1a,b), while the heterochiral AlaNDI-Cd crystals formed long,thin (<500 nm) ribbon-shaped structures, which were moresuitable for the fabrication of micro/nanoelectronic devices(Figure S1c,d). Therefore, chirality can be used as a newtuning parameter in controlling the morphologies of CPs/MOFs. Compared with ligands (Figure S2a), the UV−visspectra of the homochiral and heterochiral AlaNDI-Cddisplayed a broad absorption peak below 500 nm, which canbe assigned to n−π* and π−π* transitions of naphthalenediimide (NDI) ligands (Figure S2b). Besides, the circulardichroism (CD) analysis of ligands and homochiral SCNs(Figure S2c,d) indicated that the chirality was transferred fromthe ligand to the homochiral SCNs. From thermogravimetric(TGA) and Brunauer−Emmett−Teller (BET) analyses,

AlaNDI-Cd showed relatively good stability without accessiblepores, despite having internal void (Figures S3 and S4).Single-crystal X-ray diffraction analysis revealed that all the

materials, the (S)-AlaNDI-Cd, (R)-AlaNDI-Cd, and (Rac)-AlaNDI-Cd crystallized as orthorhombic P22121 space groups.This structure comprises 1D polymeric chains of[Cd2(AlaNDI)2] metallomacrocycles connected by a cadmi-um-carboxylate bridge (Figure 1a,b). The structure of thenewly obtained enantiomer, (R)-AlaNDI-Cd, was the mirrorimage of the reported (S)-AlaNDI-Cd structure,24 and (Rac)-AlaNDI-Cd had the same connectivity as (S)-AlaNDI-Cd. Theflack parameter of the newly obtained enantiomer, (R)-AlaNDI-Cd, was 0.03(4), indicating that it had the correctabsolute structure. Within each of these macrocycles, the twoAlaNDI ligands displayed different coordination modes. Oneligand had carboxylate groups coordinating in a chelating κ2-Omode, whereas the other had carboxylate groups in a bridgingμ-O,O′ mode. There were two unique cyclic motifs within thestructure with four unique halves of AlaNDI ligands in theasymmetric unit. The cadmium ions adopted distortedoctahedral coordination geometries, due in part to thepresence of the chelating carboxylate groups, and eachcadmium ion had two ligated DMF molecules to completetheir coordination spheres. The structure had a unique featurein the polycatenation of nonparallel chains by the formation of[2]-catenane motifs between the [Cd2(AlaNDI)2] rings, givingrise to an overall inclined polycatenation from 1D to 2Dstructures. These SCNs had an internal, closed void with a sizeof 594 Å3 (11% of unit cell, without solvent) for the (Rac)structure and 563 Å3 (10% of unit cell, without solvent) for the(R) structure. There were four individual π−π interactions ineach catenane motif with NDI core centroid-to-centroiddistances of 3.39, 3.53, 3.57, and 3.68 Å for (R)-AlaNDI-Cd

Figure 2. (a) UV−vis spectra of heterochiral AlaNDI-Cd micro/nanocrystals before and after UV light exposure (λ = 365 nm, 150 μW cm−2) for 1h. Inset shows the color change of heterochiral AlaNDI-Cd. (b) ESR spectra of heterochiral AlaNDI-Cd micro/nanocrystals before and after UVlight exposure (λ = 365 nm, 150 μW cm−2) for 1 h. (c) I−V characteristics of heterochiral AlaNDI-Cd micro/nanocrystals before and after UV lightexposure (λ = 365 nm, 150 μW cm−2). Inset shows the SEM image of heterochiral AlaNDI-Cd micro/nanocrystals on prepatterned Cr/Auelectrodes. (Length = 10 μm). (d) Time-dependent current change of heterochiral AlaNDI-Cd micro/nanocrystals under UV light exposure (λ =365 nm, 150 μW cm−2).

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and 3.40, 3.54, 3.58, and 3.65 Å for (Rac)-AlaNDI-Cd,corresponding to the same distances of 3.39, 3.53, 3.58, and3.68 Å for (S)-AlaNDI-Cd. In the (Rac)-AlaNDI-Cd, the NDIcore was arranged along a axis in the form ···RRSSRR···, in turncausing the whole crystal to be racemic. To gain a betterunderstanding of the chiral self-discrimination phenomenon ofAlaNDI-Cd with an enantiomeric mixture of ligands, wecalculated the formation energy (ΔEf) via the densityfunctional theory (DFT) method to assess the thermodynamicstabilities of the homochiral and heterochiral AlaNDI-Cdcluster models (Figure 1c). The formation of (Rac)-AlaNDI-Cd, where the constituting chiral centers were arranged in theorder of RSSR repeatedly, was found to be favored by 4.85 kcal

mol−1 over its homochiral counterparts (i.e., RRRR and SSSSarrangements) due to the well-aligned stacking of the AlaNDIligands, which interacted with one another via π−πinteractions, implying that the formation of the heterochiralstructure is thermodynamically preferable.Interestingly, (Rac)-AlaNDI-Cd micro/nanocrystals were

sensitive to UV light and underwent a photochromictransformation from yellowish to black upon 1 h of UVirradiation (λ = 365 nm, 150 μW cm−2) (Figure 2a). TheirUV−vis spectra showed new broad peaks at 500, 615, and 755nm, which may be related to the formation of radical anionsupon UV irradiation.33 However, their FT-IR spectra andpowder X-ray diffraction (PXRD) revealed that the crystal

Figure 3. (a) I−V characteristics of hydrazine-doped heterochiral AlaNDI-Cd micro/nanocrystals in ambient condition. (b) Temperature-dependent conductivity of heterochiral AlaNDI-Cd micro/nanocrystals before and after hydrazine doping. (c) UV−vis spectra of heterochiralAlaNDI-Cd micro/nanocrystals before and after hydrazine doping for 5 min. (d) ESR spectra of quantitative experiments for (Rac)-AlaNDI-Cdmicro−/nanocrystals after UV illumination (λ = 365 nm, 150 μW cm−2) for 1 h and hydrazine doping for 5 min. (e) I−V characteristics ofhydrazine-doped heterochiral AlaNDI-Cd micro/nanocrystals before and after UV light exposure (λ = 365 nm, 150 μW cm−2). (f) Time-dependentcurrent change of hydrazine-doped heterochiral AlaNDI-Cd micro/nanocrystals under UV light exposure (λ = 365 nm, 150 μW cm−2). (g)Photocurrent, photoresponsivity (R), (h) photosensitivity (P), and external quantum efficiency (EQE) of heterochiral AlaNDI-Cd micro/nanocrystals under UV exposure (λ = 365 nm, 150 μW cm−2) before and after hydrazine doping for 5 min.

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structures were identical irrespective of UV irradiation,indicating that their photoresponsive behaviors may resultfrom an electron transfer-related chemical process (FigureS5).34 In addition, the electron spin response (ESR) spectrumshowed a g value of 2.008, supporting that the photochromicprocess may arise from photoinduced radical generation ofredox NDI ligands (Figure 2b).To examine their photoconductivity, (Rac)-AlaNDI-Cd

micro/nanocrystals were placed on a silicon wafer with Cr/Au electrodes prepatterned using photolithography. (Rac)-AlaNDI-Cd micro/nanocrystals showed a sharp increase in thecurrent under UV light illumination (λ = 365 nm, 150 μWcm−2) (Figure 2c), likely due to the formation of radicals at theligands of (Rac)-AlaNDI-Cd. To the best of our knowledge,the research on CPs/MOFs-based photoswitching is still in itsinfancy, and there are rare reports on photoswitchable chiralCPs/MOFs with redox-active ligands instead of ligands withreversible geometric and structural changes.22,35 The photo-switching of electrical conductivity of (Rac)-AlaNDI-Cdmicro/nanocrystals was observed through on/off UV light (λ= 365 nm, 150 μW cm−2) switching test (Figure 2d), and theirrise and decay times were fitted. The conductivity increasedwith a rise time of 76 s upon illumination, whereas thephotocurrent decreased with a decay time of 155 s in the dark,which were remarkably shorter than those of reported AlaNDI-Zn supramolecular biocoordination polymers (SBCPs) (tr =369 s, td = 505 s),22 likely due to their different crystalstructures with different metal sources. The photoswitchedconductivity was reversible upon on/off switching of UV lightillumination. To investigate the reversibility of the UV lightdetection under a longer time light exposure, we obtained I−Vcurves of (Rac)-AlaNDI-Cd micro/nanocrystals under UVlight (λ = 365 nm, 150 μW cm−2) exposure (Figure S6). The(Rac)-AlaNDI-Cd micro/nanocrystals exhibited good photo-stability without breaking down under UV light exposure for 2h and return to the original conductivity in the dark underambient condition. It was reported that the conductance ofNDI-based material can be increased when the potential wasswept to change its neutral species into radicals.36 Therefore,the photoconduction may be related to photoinduced electrontransfer, which generated additional charge carriers with theformation of radicals.As the radical form of an NDI-based material typically leads

to higher conductivity than the neutral species,36 and stableorganic radicals formed by the ligands of MOFs can providefree charge carriers to improve charge density,14 therefore, wehypothesized that the conductivity of insulative NDI-basedAlaNDI-Cd micro/nanocrystals could be enhanced by areducing dopant such as hydrazine. To elucidate the dopingeffect of hydrazine on (Rac)-AlaNDI-Cd, the micro/nano-crystals drop-casted onto prepatterned electrodes wereexposed to saturated hydrazine gas for 5 min. FT-IR spectraand PXRD analysis revealed that the AlaNDI-Cd crystalssurvived the doping process with hydrazine gas (Figure S7).The doped (Rac)-AlaNDI-Cd micro/nanocrystals exhibited anelectrical conductivity of 1.3 × 10−3 S m−1 in the dark, whichsurpassed the value of 5.2 × 10−7 S m−1 before hydrazinedoping by a factor of 2500, indicating the successful transferfrom insulator to semiconductor of SCNs (Figure 3a). Theenhanced conductivity started to decrease in ambientconditions, reaching a value similar to the undopedconductivity after 10 h (Figures S8 and S9a). However, theconductivity changed very slowly in a nitrogen atmosphere,

retaining ∼80% of the enhanced conductivity after 100 h.When (Rac)-AlaNDI-Cd crystal was doped again withhydrazine for 5 min, the conductivity was increased again(Figure S8). This result indicated that the hydrazine dopingprocess was reversible, presumably due to the intact crystalstructure. To investigate the cyclic stability of the hydrazinedoping, we tested the reversibility of the conductivity recoverywith repeated hydrazine doping (5 min) and exposure to highheat in vacuum condition (100 °C, 1 h) (Figure S10). The(Rac)-AlaNDI-Cd micro/nanocrystals exhibited the conduc-tivity recovery for 10 cycles of doping and even for a longertime doping (∼2 h) under ambient condition.Charge hopping between components is generally believed

to be dominant for semiconducting CPs/MOFs,15 in whichconductivity enhances with increasing temperature.7 There-fore, we measured the temperature-dependent electricalconductivity of SCNs. The electrical conductivities wereexponentially enhanced as the temperature increased, demon-strating the semiconducting property of doped SCNs (FigureS11). Activation energies of 578.2 and 80.8 meV for chargehopping were obtained from the Arrhenius fitting of SCNconductivities before and after hydrazine doping, respectively,which indicated the facilitated charge hopping for the dopedSCNs (Figure 3b). As discussed above, the four individual π−πinteractions in heterochiral SCNs with centroid-to-centroidwere around 3.3−3.7 Å. Such close adjacent ligand interactionssuggested that the charge transport of SCNs could dominantlyadopt the “through-space” conduction.We carried out UV−vis spectroscopic analysis of (Rac)-

AlaNDI-Cd crystals before and after hydrazine doping tofurther understand the doping effect (Figure 3c). Comparedwith the original crystals, new absorption peaks were observedat 495, 520, and 745 nm after doping with hydrazine, withsimilar peaks observed in the solid UV−vis spectra afterirradiation with UV light (λ = 365 nm, 150 μW cm−2) for 1 h.Therefore, the increased conductivity after UV light irradiationor hydrazine doping was highly likely due to the formation ofradicals in (Rac)-AlaNDI-Cd. To compare the amount ofradicals formed by these two methods, we carried out ESRexperiments using the same amount of heterochiral SCNsunder UV illumination (λ = 365 nm, 150 μW cm−2) for 1 hand hydrazine doping for 5 min (Figure 3d). We discoveredthat SCNs under hydrazine doping had a higher ESR intensitythan those under UV illumination, demonstrating thathydrazine doping formed a larger amount of radicals. Thesefindings corresponded well to the results from conductivitymeasurements. For both homochiral and heterochiral SCNs, asimilar g factor of 2.008 was observed (Figure 3d and FigureS12) after hydrazine doping. To study the relationshipbetween the formation of radicals and electrical conductivity,the time-dependent ESR spectra were also measured underambient conditions and a nitrogen atmosphere (Figure S9).The ESR intensity decreased sharply under ambient con-ditions, whereas the intensity in a nitrogen atmosphereremained constant, demonstrating the well-matched trendbetween the conductivity of the (Rac)-AlaNDI-Cd micro/nanocrystals and the formation of the radicals in NDI ligandsfrom UV light illumination or hydrazine doping.To quantify the photoresponses of (Rac)-AlaNDI-Cd

micro−/nanocrystals before and after hydrazine doping, wecalculated the photocurrent, photoresponsivity (R), photo-sensitivity (P), and external quantum efficiency (EQE) valuesusing transfer characteristics coupled with UV light irradi-

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ation.22,37 Interestingly, the hydrazine-doped (Rac)-AlaNDI-Cd micro/nanocrystals exhibited much higher photorespon-sivity than undoped ones under UV light illumination for 1 h(Figure 3e−h). After hydrazine doping, micro/nanocrystals of(Rac)-AlaNDI-Cd yielded the maximum R and EQE values of41.8 A W−1 and 1.4 × 104%, respectively, at an applied bias of50 V under UV light illumination (λ = 365 nm, 150 μW cm−2).In contrast, the micro/nanocrystals of (Rac)-AlaNDI-Cdbefore hydrazine doping displayed much poorer photo-responsivity than after doping, with maximum R and EQEvalues of 4.48 A W−1 and 1.4 × 103%, respectively.Furthermore, the maximum R and EQE values were muchhigher than that of previous reported AlaNDI-Zn SBCPs (R =0.92 A W−1, EQE = 312%),22 presumably due to the differentcrystal structures and the resulting device structures. Theirphotoresponsivity was enhanced by about 10 times afterhydrazine doping, indicating that more radicals were formedafter doping under an identical UV light illumination intensity.However, P was relatively reduced from 11.2 to 1.67 because ofthe higher dark current after doping. The current enhancementreached the saturated one within a rise time of 351 s and adecay time of 873 s for original conductivity. The extended

time for saturation of the photocurrent was related to theformation of more radicals from (Rac)-AlaNDI-Cd afterdoping by hydrazine under UV light illumination. Theseresults provide a simple way to increase the conductivity andphotoresponsivity of CPs/MOFs for the application to micro/nanooptoelectronic devices.To understand the mechanism of radical formation by

hydrazine doping, we theoretically investigated the molecularorbitals (MOs) of (Rac)-AlaNDI-Cd according to the chargestate via DFT calculation (Figure 4a). To do this, wecompared the MOs of the (Rac)-AlaNDI-Cd cluster modelin the neutral and electron-accepted (which can be regarded asthe radical anion) states (denoted as Cd-(Rac) and [Cd-(Rac)]−1, respectively). Note that the highest occupied, lowestunoccupied, and singly occupied MO levels were denoted asHOMO, LUMO, and SOMO levels, respectively. As a result,unoccupied MO levels of Cd-(Rac) near the LUMO levelcorresponded to π* orbitals of each AlaNDI ligand, which weremore likely to be reduced due to their electron-deficientcharacteristics. Compared with the orbital distribution at theneutral state, the SOMO level of the [Cd-(Rac)]−1 appeared asa degenerate state composed of two π* orbitals of AlaNDI

Figure 4. (a) DFT calculation of molecular orbitals and corresponding energy levels of (Rac)-AlaNDI-Cd cluster models at neutral (Cd-(Rac)) andelectron-accepted states ([Cd-(Rac)]−1). Blue- and red-colored clusters represent each unit of catenane motif, which composed of two AlaNDIsconnected by a Cd center. The energy states having the similar orbital distribution from each neutral and electron-accepted states were indicated bythe same color (i.e., sky blue, green, and violet). (b−e) DFT calculation results for hydrazine doping on the (Rac)-AlaNDI-Cd surface. (b) Top(left) and front views (right) of the (Rac)-AlaNDI-Cd surface. The carbon, hydrogen, oxygen, nitrogen, and cadmium atoms are colored in gray,white, red, blue, and ivory, respectively. (c) Electronic density of states (DOS) of AlaNDI (black line) and hydrazine (red line) before (dotted line)and after adsorption of hydrazine (solid line) on the (Rac)-AlaNDI-Cd surface. (d) Accumulated negative charge of the (Rac)-AlaNDI-Cd surfacewith increasing number of adsorbed hydrazine molecules. (e) Band gap of the (Rac)-AlaNDI-Cd surface with increasing number of adsorbedhydrazine molecules.

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ligands, each corresponding to the LUMO and LUMO+1states of the Cd-(Rac). Thus, the occurrence of the SOMOlevel by radical formation was predicted to provide theaforementioned “through-space” transport route via π−πinteraction between AlaNDI ligands.We investigated the changes in electronic structure before

and after the adsorption of hydrazine molecules onto the(Rac)-AlaNDI-Cd surface in order to further verify the originof the enhanced conductivity produced by hydrazine doping(Figure 4b,c). When a hydrazine molecule was adsorbed ontothe SCN surface, the electronic density of states (DOS)analysis showed that the peak intensity of the valence bandmaximum (VBM, from hydrazine) was decreased, and theenergy gap was reduced by a downshifting of the conductionband minimum (CBM, from AlaNDI ligands), implying thepossibility of the donor−acceptor electron transfer processfrom hydrazine into the AlaNDI ligands. While the surfacecoverage of hydrazine molecules increased, the electrons wereconsistently transferred from hydrazine as confirmed by thenegatively accumulated charge on the (Rac)-AlaNDI-Cdsurface (Figure 4d). We also noticed that as the adsorbedamount of hydrazine increased, the binding energy (ΔEbind)increased linearly and the differential binding energy(ΔΔEbind) gradually decreased and showed a saturationbehavior (Figures S13 and S14). Continuous electron transferled to a gradual decrease in the energy gap, which eventuallyreached saturation at a level of 0.43 eV (Figure 4e). Theseresults demonstrated that the narrow energy gap induced byhydrazine doping was the origin of increased conductivity.

4. CONCLUSIONSIn conclusion, compared with homochiral analogues, hetero-chiral AlaNDI-Cd micro/nanocrystals were more thermody-namically stable from DFT calculation and exhibited thinnermorphologies, which were more suitable for optoelectronicdevices. Unlike the photoswitching behaviors of conventionalCPs/MOFs, which used ligands with reversible geometric orstructural changes, the synthesized SCNs with redox ligandsexhibited excellent photochromic and photoswitchable proper-ties due to the formation of radicals by the ligands. Afterdoping with hydrazine for 5 min, their electrical conductivity ofAlaNDI-Cd micro/nanocrystals sharply increased by 2500 fold(from 5.2 × 10−7 to 1.3 × 10−3 S m−1), indicating that theelectrical properties of AlaNDI-Cd micro/nanocrystals weresuccessfully changed from insulator to semiconductor levels,due to the reduced energy gap after adsorption of hydrazine onthe SCN surface. Moreover, the R and EQE values of thehydrazine-doped heterochiral AlaNDI-Cd reached 41.8 A W−1

and 1.4 × 104%, respectively, which were approximately 10times higher than those obtained from undoped ones. DFTcalculations indicated that the high conductivity was closelyrelated to “through-space” charge carrier transport betweenπ−π stacked ligands via the formation of radicals, which wereformed by either UV light or hydrazine doping. Our findingsnot only provide a simple way to enhance the conductivity andphotoresponsivity of insulative CPs/MOFs but also demon-strate that the developed micro/nanoscaled CPs/MOFs havehigh potential for use in practical optoelectronic applications.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b04653.

CCDC-1531449 (for (Rac)-AlaNDI-Cd single crystal)(CIF)SEM images; UV−Vis, PL, and CD spectra; TGA andBET results; FT-IR spectra; PXRD results; I−Vcharacteristics; ESR spectra; simulation results(PDF)CCDC-1531448 (for (R)-AlaNDI-Cd single crystal)(CIF)Check CIF for CCDC-1531448 (PDF)Check CIF for CCDC-1531449 (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: skkwak@unist.ac.kr (S.K.K.).*E-mail: joonhoh@snu.ac.kr (J.H.O.).ORCIDHiroyoshi Ohtsu: 0000-0002-2911-3151Sang Kyu Kwak: 0000-0002-0332-1534Joon Hak Oh: 0000-0003-0481-6069Author Contributions⊥X.S. and I.S. contributed equally to this work. The manuscriptwas written through contributions of all authors. All authorshave given approval to the final version of the manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by the Samsung Research FundingCenter of Samsung Electronics under project number SRFC-MA1602-51. We are grateful to the computational resourcesfrom UNIST-HPC. Single-crystal XRD experiments at PLS-IIwere supported in part by MSICT and POSTECH.

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