Communication pubs.acs.org/IC
Ultrasensitive Photochromic Iodocuprate(I) Hybrid Jun-Ju Shen, Xiang-Xia Li, Tan-Lai Yu, Feng Wang, Peng-Fei Hao, and Yun-Long Fu* School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, China S Supporting Information *
EDAMP2+ as an electron acceptor, a wavelength-dependent photochromic iodoplumbate hybrid (EDAMP)2n(Pb7I18)n· 4nH2O reported by Guo et al.8 indicates the important role of a new acceptor in the assembly of new photochromic systems with novel performance. On the other hand, besides coordination ability as a class of versatile ligands, nicotinohydrazide (NH) derivatives can also exhibit excellent biological activity due to their moderate electron accepting ability and should be able to be used for the construction of photochromic hybrids.9 In this communication, by employing methylnicotinohydrazide dication (MNH2+, Figure 1a) as a new electron acceptor, we obtained a difunctional iodocuprate(I) hybrid {[MNH][Cu2I3]2}n (1) with fast-responsive photochromism, a wide photoresponse
ABSTRACT: By employing in situ methylnicotinohydrazide dication (MNH2+) as an electron acceptor, we have constructed an iodocuprate(I) hybrid {[MNH][Cu2I3]2}n (1), which exhibits charge transfer (CT) thermochromism due to the intense absorption of CT and electron transfer (ET) photochromism with high photocoloration contrast and fast response to UV irradiation due to the synergetic effect of valence change of copper ions.
P
hotochromic materials have manifested promising prospects for their potential applications, such as displays, switches, sensors, data storage, protection, and decoration.1 Except for their diverse photoresponsive mechanisms, the response rate, reversibility, sensitivity, duration, spectral response range, and thermal stability constitute the core issues, and how to optimize these photochromic performances is still a challenge for practical utilization. Recently, organic−inorganic hybrids have been of increasing interest owing to the facile tunability of components, structures, and functionalities, especially for a photochromic system based on the intermolecular electron transfer (ET).2 Featured as typical photoresponsive ET and generation of colored radicals, photochromic hybrid materials have been intensively investigated in the past decade. Generally, these hybrid materials preferentially employ strong electron acceptors, such as viologens and 1,4,5,8-naphthalenediimides (NDIs), and strong Lewis bases as donors, such as halometallates, metallophosphates, metal cyanides, metal−organic complexes, and zeolites.2,3 Meanwhile, their photochromic behavior reveals good modulating ability dependent on the variation of components and structures.2c,3a,4 However, these hybrid materials usually show a slow photoresponse rate and narrow photoresponse range (mainly response to UV light). To date, only several hybrids have been observed with an eye-detectable color change within 10 s.3a,5 For an electron donor−acceptor system, variations of structural features and electron accepting/ donating ability play a key role in the modulation of performance. Therefore, the introduction of new electron acceptors and donors should be an effective strategy to overcome present shortcoming and improve photochromic performance. On one hand, halometallates possess flexible and diverse structures as well as tunable electron donating ability. However, to the best of our knowledge, most of the photochromic hallometallates are mainly viologen chlorometallates and a few viologen bromometllates.2a,c,3a,4b−e,5,6 Rare ET photochromic iodometallate hybrids have been reported so far, which is usually ascribed to the so-called heavy-atom effect.7 By using © 2016 American Chemical Society
Figure 1. (a) The molecular unit of MNH2+ dication. (b) The structures of two different 1D [Cu2I3]nn− chains of 1 (A chain: Cu−I, 2.519(2)−3.144(3) Å; I−Cu−I, 97.15(9)−123.16(12)°; and Cu···Cu, 2.484(3)−2.636(4) Å. B chain: Cu−I, 2.541(3)−2.882(3) Å; I−Cu−I, 97.43(8)−122.42(9)°; and Cu···Cu, 2.492(3)−2.684(5) Å). (c) The unit cell representation of 1. N−H···I hydrogen bonds are displayed by red dotted lines. Only the hydrogen atoms involved in hydrogen bonds are shown for clarity. Received: July 5, 2016 Published: August 19, 2016 8271
DOI: 10.1021/acs.inorgchem.6b01599 Inorg. Chem. 2016, 55, 8271−8273
Communication
Inorganic Chemistry
product displays a new broader absorption band in the range of 500−780 nm in the UV−vis spectrum. Meanwhile, the EPR spectra of 1 and the decolored sample display small signals (Figure 2c), which should be caused by indoor light during sample handling due to the high photosensitivity of 1 as supported by its responses to further light irradiation and heating in vacuo (see photochromic part and Supporting Information). Contrastively, 1P shows a strong and broad EPR signal centered at g = 2.12, which is similar to those of pqCu2Cl4 (g = 2.17) and pqFeCl4 (g = 2.02; pq = N,N′dimethyl-4,4′-bipyridinium) and can be attributed to the combination of signals of Cu2+ ions and organic radicals arising from photoinduced ET.14 To gain some insight into the ET process, XPS experiment was performed. After irradiation, the Cu 2p core-level spectrum exhibited a pair of new peaks at 932.4 and 954.5 eV with the weakening of the peaks at 932.4 and 952.2 eV, which confirms the presence of Cu2+ and suggests that Cu+ loses electrons (Figure 3). The I 3d core-level spectrum shows a new pair of
range from UV to visible light, as well as low temperature thermochromism. The title compound was prepared from an in situ solvothermal reaction of CuI and NH in methanol under acidic conditions (see Supporting Information). Single crystal X-ray analysis reveals that 1 crystallizes in the orthorhombic space group Pbca and its asymmetric unit has four copper(I) atoms, six iodine atoms, and one MNH2+ dication (Figure S1). The structure of 1 contains two similar [Cu2I3]nn− chains composed of edge- and face-shared CuI4 tetrahedra (chains A and B as shown in Figure 1b), analogous to the chains observed in [N(CH3)4][Cu2I3].10 Two chains present slightly different conformations with different Cu−I bond lengths, I−Cu−I bond angles, and Cu···Cu separations due to the distinct environments provided by MNH2+ counterions (Figure 1c). MNH2+ connects with adjacent A chains only by electrostatic interaction, while B chains and MNH2+ are linked by electrostatic interaction and N−H···I hydrogen bonds (N···I, 3.655(14) Å). The phase purity of as-synthesized 1 has been confirmed by the well matched PXRD patterns of experimental and simulated ones (Figure S2). The TGA curve under a N2 atmosphere (Figure S3) shows 1 can be stable up to about 185 °C. The weight loss of 38.3% from 185 to 450 °C corresponds to the loss of [MNH]I2 (calcd 37.1%). The weight loss from 450 to 700 °C is due to the decomposition of CuI.11 Compound 1 displays high photosensitivity under ambient conditions (Figure 2a). Under UV light irradiation, brown-
Figure 3. Cu 2p and I 3d XPS core-level spectra of 1 and 1P.
peaks lying at 620.1 and 631.6 eV, indicating that I− also loses electrons. Moreover, the C 1s and O 1s core-level spectra also show clear changes after irradiation, while the N 1s core-level spectrum is almost unchanged, which may be due to the electron delocalization in the whole organic molecule6b or the obscuration effect of two hydrazine N atoms15 (Figure S6). Therefore, the photochromism of 1 should originate from the ET between [Cu2I3]nn− anion chains and MNH2+ cations. The generation of Cu2+ ions is consistent with the panchromatic absorption of 1P and contributes to the high photocoloration contrast and fast response to UV irradiation. Although the photoinduced ET and generated radicals have been seen in several viologen/NDI iodometallate hybrids, no photochromism is observed due to their original dark color.13b,16 The high photochromic performance of 1 implies that new acceptor MNH2+ can effectively modify the internal electron behavior of hybrid and match well with donor [Cu2I3]nn−. Additionally, 1 also exhibits obvious low temperature reversible thermochromism due to the existence of intense intermolecular CT in 1. Upon being immersed into liquid nitrogen (77 K), crystals of 1 immediately turned yellow (Figure 2a), then gradually reverted to the initial color as the temperature rose to room temperature (298 K). The marked decrease of absorption intensity from 420 to 530 nm at 77 K (Figure 2b) suggests that the thermochromism of 1 results
Figure 2. (a) Photographs showing the thermochromic and photochromic behavior of 1. (b) UV−vis diffuse reflectance spectra of 1, 1P, and the decolored sample at 298 K as well as 1 at 77 K. (c) EPR spectra of 1, 1P, and the decolored sample.
yellow 1 changes to black 1P for 7 s, which tends to be saturated for 50 s (Figure S4). 1P can be completely decolored by keeping it in dark for about 3 days or heating at 110 °C for 8 min in air. This reversible process can be repeated at least 10 times. More interestingly, 1 can also show similar photochromic behavior under visible light illumination (λ > 420 nm, saturated state needs about 40 min) (Figure S5), which is of significance for the utilization of solar energy in practical application.12 The single crystal XRD and PXRD data for 1 and 1P (Tables S1 and S2 and Figure S2) have ruled out the possible structural change after light irradiation. The solid UV−vis diffuse reflectance spectrum of 1 at 298 K shows broad absorption in the range of 420−600 nm, which corresponds to the brownyellow color of 1 (Figure 2b) and implies an intense intermolecular charge transfer (CT) between MNH2+ cations and [Cu2I3]nn− anions with respect to that of colorless [N(CH3)4][Cu2I3].10,13 Comparatively, 1P as an irradiated 8272
DOI: 10.1021/acs.inorgchem.6b01599 Inorg. Chem. 2016, 55, 8271−8273
Communication
Inorganic Chemistry
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from a cold-induced decrease of CT population as reported in our previous work.13a,17 As internal electron behaviors, both of the intermolecular CT and ET originate from the interaction between donors and acceptors, which are of primary importance for new functional materials and have attracted much attention theoretically and experimentally.18 Rational modulation and delicate balance between CT and ET play a crucial role in functional diversity and are still at an initial stage, which is largely dependent on the design and introduction of new electron accepting/donating units and further elucidation of matching rules. As a difunctional material, the title compound {[MNH][Cu2I3]2}n can exhibit excellent ET photochromism and CT thermochromism, which is ascribed mainly to the employment of a new electron acceptor MNH2+ in the iodocuprate(I) hybrid and demonstrates the unique modulating effect of component variation on the internal electronic behavior and chromic performance of organic−inorganic hybrids. Especially, its high photocoloration contrast, fast response to UV irradiation, and wide spectral response from UV to visible light are likely to take advantage of simultaneous excitation for organic acceptors and inorganic donors5 as well as a synergetic effect of valence change of copper ions. In summary, component variation and suitable matches can be used as an effective strategy for the design and preparation of new multifunctional hybrid materials. The relevant work is underway.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01599. Details of the synthesis and characterization; crystallographic data and structure refinement; the asymmetric unit; PXRD patterns; TGA curve; UV−vis diffuse reflectance spectra under visible light illumination; N 1s, C 1s, and O 1s XPS core-level spectra; and IR spectrum (PDF) X-ray crystallographic data for 1 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 21171110) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20131404110001).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b01599 Inorg. Chem. 2016, 55, 8271−8273
