Efficient Ultraviolet Light Detector Based on a Crystalline Viologen

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An efficient ultraviolet light detector based on a crystalline viologen-based metal-organic framework with rapid visible color change under irradiation Shuzhi Hu, Jie Zhang, Shuhuang Chen, Jing-Cao Dai, and Zhiyong Fu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13367 • Publication Date (Web): 31 Oct 2017 Downloaded from http://pubs.acs.org on November 4, 2017

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ACS Applied Materials & Interfaces

An efficient ultraviolet light detector based on a crystalline

viologen-based

metal-organic

framework with rapid visible color change under irradiation Shuzhi Hu,a,b Jie Zhang,a Shuhuang Chen,a Jingcao Daic and Zhiyong Fu*,a,b a

The Key Lab of Fuel Cell Technology of Guangdong Province, Guangdong, School of

Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China. b

State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University,

Fuzhou, 350002, P. R. China. c

Institute of Materials Physical Chemistry, Huaqiao University, Quan-zhou, Fujian 362021,

China.

ABSTRACT: A convenient colorimetric molecular system constructed by the zinc viologencarboxylate framework is developed for naked eye detection of instantaneous UV exposure levels. Only narrow-band absorption in UV regions and unique interpenetrated structure of its colorless crystal enable the system give fast response toward UV irradiance with intensity as low as 0.001 mw/cm2. KEYWORDS: metal organic framework, zinc-viologen framework, crystalline material, UV photodetector, visible-blind, thermal stability.

ACS Paragon Plus Environment

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The importance of UV photodetectors is growing because of the demand for flame detection, chemical sensing, health care, astronomy and astrophysics, environmental and military applications.1 Those visible-blind and solar-blind UV photodetectors are of particular value which can be used in devices requiring full exposure to the visible light.2 Without light-response in visible and infrared (IR) range ensure the device having precise detection capabilities with minimal background in UV region.3 Traditional visible-blind UV photodetectors are conventionally constructed by wide-band gap inorganic semiconductors which are attractive but their complicated and expensive manufacturing restrict them to large-area applications. Therefore, to fulfill the rapidly increasing needs for high-performance UV photodetectors, tremendous efforts have been devoted to achieve convenient devices with visible-blind, high photoresponsiblity, adequate resolution, and fast response time.4,

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Recently, metal organic

frameworks (MOFs) with their advantages in structural control and the combination of different functional units have witnessed the systematic design and construction of

conductivity,

luminescent, temperature and explosive sensors.6-10 These kind of complexes also may readily be used to construct new type of UV photodetectors based on suitable multicomponent metal-ligand assembly. Viologens are a kind of compounds that are used for photochromic systems for their reversible color change ability upon reduction and oxidation.11, 12 The self-assembly of MOFs involving viologen ligands has been used to build novel photonic materials for X-ray radiation protection and response.13, 14 For the attractive photocoloration characters of viologens in the detection of outer stimuli,15, 16 UV photodetectors built based on metal-viologen connections would avoid the requirement of electronic accessories and multi-step analysis to obtain signal information as that in inorganic semiconductors17 and the signal can be directly “visualized” by color change.18-20 However, the challenges confronted in the construction of an efficient


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viologen-based UV light detector are: (a) the designed complex should be colorless to prevent the absorption in visible region for the visible-blind requirement; (b) the molecular system has fast photoresponding rate toward UV light irradiation; (c) the molecular system is stable in the condition of high-temperature and high-humidity. Herein, an interesting colorless MOF-based donor-acceptor complex [Zn(p-CPBPY)(HBTC)]•H2O 1 is prepared with

N-(para-

carboxyphenyl)-4, 4’-bipyridinium (p-CPBPY) and 1, 3, 5-benzenetricarboxylate (BTC) as mixed ligands. The designed structural arrangements make it as a valuable candidate to detect UV ray in a convenient way. Without strong π-π stacking between the aromatic rings in its structure guarantees the complex being colorless. The introduction of para-substituted viologen ligand into the photosensitive complex framework is favorable for building a fast-response photocoloration system with a rate constant of 2.4. Its linear color changing under UV irradiation allows the crystal of 1 to be used as qualified UV index indicator. The robust three-fold interpenetrating structural motif creates a stable photoactive system up to 380 oC confirmed by PXRD，TG and IR data. Colorless crystals of 1 are prepared via the reaction of Zn(NO3)2•6H2O, p-CPBPY, H3BTC, DMF and water. Single crystal data indicates that the photoresponsive material contains a twodimensional structural motif with the zinc ions as connected nodes and the mixed organic ligands as bridges. The Zn2+ center is tetracoordinated with one oxygen atom and one nitrogen atom of two p-CPBPY ligands respectively and two oxygen atoms of two unidentate carboxylate groups of HBTC ligands (Figure 1a). The forming structure presents a three-fold motif with independent equivalent networks (Figure 1b). Thus, the infinite catenane of two dimensional architecture parallel interpenetrated with its neighbors results in a three dimensional array in the packing structure. It is 3.853 Å between the pyridinium ring of guest viologen cation and the benzene


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ring of the host framework (Figure S1). Such packing arrangement suggests the colorless character of 1 due to the weak π-π interactions between the aromatic rings.21 The interspacing distance from one oxygen atom of the carboxyl group to one N atom of viologen ring is 3.749(5) Å (Figure S1), satisfying the requirement for an intermolecular electron transfer from donor to acceptor.22

a

b

Figure 1. The structure of 1: (a) the asymmetric unit; (b) the packing diagram to show the three-fold interpenetrated structure with linear p-CPBPY molecules as bridges and coordinated HBTC ligands as edges and Zn(II) ions as connected nodes at the corners.

Powder X-ray diffraction (PXRD) data approved phase purity of the as-synthesized bulky sample. (Figure S2a). Intense absorption at ultraviolet region is observed in the solid-state diffuse reflectance spectrum of 1. (Figure 2a). Upon ultraviolet light irradiation (5 mw/cm2), a new strong band centered 676 nm gradually emerged with proportional to the irradiation time (Figure S3a), which belongs to the characteristic absorption of viologen radicals generated from electron transfer (ET) reaction.23 Its crystal color turns from colorless to green at the same time. ESR measurement for the colored crystal gives a g value of 2.0085 (Figure S4), which can be


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attributed to organic free radicals generated by electrons transit from electron rich carboxyl groups to electron deficient viologen components.24, 25 The facile ET reaction is anticipated for the use of para-substituted viologen ligand, which is more electron deficient in the pyridinium nitrogen position than that in meta-substituted derivatives.26 These structural characters and weak π-π interactions between the aromatic rings make 1 changing its color quickly upon light irradiation. So far as we know, compound 1 is one of the fastest photo-responding MOF based donor-acceptor type photochromic materials with coloration time less than 0.1s under sunlight (comparison

of

coloration

time,

[Zn3(H2PTIA)(bibp)(H2O)2]·4.5H2O:

2min;14

[Zn2(DPNDI)2(DMA)4F3(H2PW12O40)]n: 5s;27 [PV][Zn3(m-BDC)4]·H2O: 1s28). Its solid state light reversion calculated at 676 nm exhibits first order kinetics with a rate constant of 2.4 for the photocoloration process (Figure S3b). The remarkable thermal stability of the metal-viologen framework is confirmed by thermogravimetric analysis (TGA) and powder X-ray diffraction

a

b

Figure 2. (a) Solid state UV-vis spectral changes of 1 upon UV and visible light illumination (>400 nm); (b) optical band gap energy of compound 1.


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(PXRD) data (Figure S2b and Figure S5). Its structure remains intact until 380 oC by the comparison of PXRD patterns obtained at different temperatures. Although compound 1 is thermochromic, the thermo-coloration temperature is above 100 oC which excludes its influence on the photocoloration (Figure S3c). The photoactive system is also fairly stable in a humidity condition. Its crystal keeps the structural stability and works well even being covered in water (Figure S2c, Figure S8b). Interestingly, the photocoloration reaction is not observed under visible light illumination ( > 400 nm) (Figure 2a), which ensures the system belongs to a visibleblind light switch. A relatively high optical band gap energy (Eg) (3.26 eV) is observed in compound 1 via the calculation based on the diffuse reflectance spectrum (Figure 2b). These

a

b

Figure 3. (a) Plot of absorption change at 676 nm in line with irradiation time at the following UV index (from bottom to top): 2, 4, 6, 8 and 10; (b) photographs indicate different UV exposure levels of 1 via different color change. results support that 1 is an efficient UV photodetector based on the color changes of viologen donor-acceptor molecular system. While the absorbance of 1 at 676 nm as a function of time upon UV irradiation, the measured values are plotted in Figure 3a for the detection of UV radiation. The resulting feature is attractive with respect to personal UV monitoring. Its color


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changing from colorless to green depends on different intensities of UV irradiation, which corresponds to the UV index from 0 to 10 (Figure 3b).29 It is worth noting that the sensitive detecting system appears to work well as a way for measuring UV irradiances with intensity as low as 0.001 mw/cm2, as illustrated by the results in Figure S6. As known, two drawbacks of viologen complexes used as sensors are their recongnised toxicity30 and color instability11 due to the highly oxidation sensitive of reduced radical. In the robust three-fold interpenetrating structure of 1, the viologen components connecting with the metal ions eliminate these two concerns, which enable the stable detecting system operate well even at 100% humidity condition and the color changing remains at least a month (Figure S7). In conclusion, an efficient UV photodetector is constructed based on a metal-viologen stimuli-responsive coloration molecular system. The device changes its color from colorless to green in proportional to outer UV irradiation. Appropriate wide band gap of the material enable it exclude the absorption of visible light. The metal-ligand framework with three-folded interpenetrated structural arrangements provides a stable photosensitive system working well in humidity condition. Integrating para-substituted CPBPY as ligand creates a fast responding system with minimum detection limit as 0.001 mw/cm2 for the UV light intensity. The UV detection based on the color change of metal-viologen framework appears to offer a convenient system to use and a new way to the monitoring of UV radiation. ASSOCIATED CONTENT Supporting Information


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Details about synthesis and general characterizations, PXRD and TG data, additional graphics, crystal data, IR, ESR and UV Spectra are included in Electronic Supplementary Information (ESI). This material is available free of charge via the Internet at http://pubs.acs.org Accession Codes CCDC 1559415 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Fax: +86-20-8711-2965. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors thank the program for the NSFC (21573076), NSF (Guangdong, 2015A030312007), NSF (Fujian, 2015J01202), NCET(130209), CPSF (2016M602456), FRFCU (2017BQ064), and the SRP program (2017s10) for financial support. REFERENCES (1) Gedamu, D.; Paulowicz, I.; Kaps, S.; Lupan, O.; Wille, S.; Haidarschin, G.; Mishra, Y. K.; Adelung, R. Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV Sensors. Adv. Mater. 2014, 26, 1541-1550.


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SYNOPSIS TOC (For Table of Contents Use Only)

An efficient ultraviolet light detector based on a crystalline viologen-based metal-organic framework with rapid visible color change under irradiation Shuzhi Hu, Jie Zhang, Shuhuang Chen, Jingcao Dai and Zhiyong Fu*


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A convenient colorimetric molecular system constructed by the zinc viologencarboxylate framework is developed for naked eye detection of instantaneous UV exposure levels. Only narrow-band absorption in UV regions and unique interpenetrated structure of its colorless crystal enable the system give fast response toward UV irradiance with intensity as low as 0.001 mw/cm2.


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Efficient Ultraviolet Light Detector Based on a Crystalline Viologen

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