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Iodide-Centered Cuprous Octatomic Ring: A Luminescent Molecular Thermometer Exhibiting Dual-Emission Character Cheng-Yang Yue,†,‡,§ Fu-Ling Liu,† Wen-Ting Deng,† Jun Tao,*,†,# and Mao-Chun Hong*,†,§ †
Collaborative Innovation Centre of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, P. R. China ‡ Department of Chemistry and Chemical Engineering, Jining University, Qufu 273155, Shandong, P. R. China § State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, Fujian, P. R. China # Key Laboratory of Cluster Science of Ministry of Education, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China S Supporting Information *
ABSTRACT: Luminescent molecular thermometers have attracted great attention due to their potential application for temperature detection in special environments. By a very simple one-step method, we have synthesized one novel cuprous compound, [I@Cu8(HMTZ)(MTZ)7]·MeCN·2H2O (I@Cu8), which consists of an infrequent octatomic [I@Cu8S8] ring. Importantly, compound I@Cu8 features dualemitting behavior and shows successive visual luminescent emissions varying from green, to yellow, to orange, and then to red in the temperature range of 80−300 K, suggesting potential application for a luminescent molecular thermometer.
I
luminescent molecular thermometers (LMTs) with clear crystal structures and thermochromic dual-emission characteristics from the molecular level. Until now, transition metal coordination compounds have been proven successful in LMT applications due to their multiple luminescent centers and excellent luminescence properties.13,14 Typically, copper(I) clusters, especially copper(I) halide clusters, have become excellent candidates since they possess fascinating structural diversities and distinctive photophysical properties, such as long lifetimes, visible-range absorption, moderate brightness, large Stokes shifts, etc.15−30 Currently, a large amount of Cu(I) clusters built with halides, chalcogens, or other elements have been reported, and their abundant structural motifs include rhomboid dimers,31,32 rhomboid33,34 and cubane Cu4X4 tetramers,35−46 hexagonal prisms,47 cubic Cu8 clusters,23 stairstep (CuIL)∞ clusters,48 hexahedral Cu6 clusters19,23,25 and other complicated polyhedra.37−52 Notably, the luminescent behaviors of copper(I) halide clusters are able to vary remarkably along with very slight structural differences.53−57 For example, a series of onedimensional (1D) CuI(L) hybrid phosphors based on similar pyridine derivative ligands reported by the Li group emitted
n recent years, great efforts have been contributed to luminescent thermometers because of their advantages of intrinsic visual read-out, noncontact temperature detection, fast response, and high sensitivity; such thermometers are potentially used in biological fluids, strong electromagnetic fields, fast moving objects, etc.1−5 Usually, to obtain advanced thermochromic luminescent materials, a temperature-dependent readable response of emission including the variation of intensity and/or wavelength must be realized.6 Obviously, a dual-emitting system could be considered as an effective candidate for its extraordinarily accurate and facile temperature sensing, in which the variation of intensity ratio of the two emissions under single excitation will bring various visible luminescent emissions. To date, several dual-emitting materials with multichromophores or luminophores have been reported, such as MOF@dye composition, dual-emitting quantum dot clusters and rare earth doped substances, etc.7−9 For example, a series of mixed-rare earth metal complexes have been explored as luminescent ratiometric thermometers and demonstrate the linear correlation between the intensity ratio of characteristic emissions from two rare earth metal ions and temperature, such as Tb0.8Eu0.2BPDA, Nd0.866Yb0.134BTB, Eu0.0089Tb0.9911L [L = 2,6-di(2′,4′-dicarboxylpheny)pyridine], etc.10−12 However, these mixed composites lack structural designabilities due to the nanostructures without accurate crystal structures. For this reason, it is an attractive and great challenging goal to build new © XXXX American Chemical Society
Received: September 5, 2017 Revised: November 30, 2017
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over the entire red-to-blue visible spectrum;53 a high-nuclearity cluster [Cu6L3(Cu2I2)Cu6L3] [L = 3,5-bis((3,5-dimethylpyrazol-4-yl)methyl)-2,6-dimethylpyridine] showed blue/red dual emission in a wide temperature range (120−450 K),12 while two clusters of [Cu4I4(dmimpr)2] [dmimpr = 1,3-di(2methyl-imidazol-1-yl)-propane] and [Cu6I6(dimb)3] [dimb = 1,4-di(imidazol-1-yl)butane] displayed fascinating thermochromic phenomena from bright yellow to orange-red or red luminescence with temperature-dependent wavelength variations, respectively.54 Intrigued by the rich structural flexibilities and interesting LMT properties, we have recently undertaken systematic studies in exploring new types of hybrid cuprous halide compounds based on crystal engineering and structure decoration strategy. To this end, we successfully synthesized a novel cuprous compound, namely, [I@Cu8(HMTZ)(MTZ) 7 ]·MeCN·2H 2 O [I@Cu8, HMTZ = 1-{2-(N,Ndimethylamino)ethyl}-5-mercapto-1H-tetrazole], by a simple one-step reaction of CuI and multidentate HMTZ ligand under alkalescent conditions. The title compound features a new structural motif of octatomic [I@Cu8S8] ring, which to our knowledge is one of the highest nuclearity in cuprous LMT compounds until now. Most interesting is that this compound exhibits excellent dual-emitting LMT behavior (512 and 724 nm) ranging from visible red, orange, yellow, to green luminescence along with a temperature decrease from 300 to 80 K, solely based on relative variation of the two-emission intensity. The title compound was synthesized via a very simple procedure, by adding HMTZ and triethylamine to an acetonitrile solution of CuI. After being stirred for about 6 h, the resulting clear solution was left to evaporate under ambient atmosphere until the crystallization of colorless crystals, which were first determined as [I@Cu8(MTZ)8]·MeCN·2H2O. However, by considering the electrovalence principle, eight dehydrogenated MTZ− ligands can just balance the positive charges of eight Cu(I) cations, and the existence of an iodide ion brings one more negative charge into the molecule. To balance the negative charge, there are three possibilities: a copper ion is divalent, a ligand is protonated, or a proton is attached to the iodide ion. In order to find out the exact formula, we measured its electron paramagnetic resonance (EPR) spectrum and further characterized the sample by an electrospray ionization time-of-flight mass spectrometer with an electrospray ionization source in the positive mode (Figure S1). The EPR spectrum indicates the absence of divalent copper ion. In the mass spectrum, a peak at m/z equal to 2014.8590 was observed, which is consistent with the calculated value (2014.8771) of positive species [H2I@Cu8(MTZ)8]+, revealing an extra proton in the molecule. While in the negative-mode mass spectrum, a peak at m/z equal to 2012.8500 was observed, this was consistent with the calculated value (2012.8620) of [I@Cu8(MTZ)8]−. Hence, we can speculate that one hydrogen atom should be located on the [I@Cu8(MTZ)8] unit. Considering the abundant exposed N atoms possessing stronger basic properties and binding abilities than iodine atom, it is impossible that the protonated hydrogen atom locates on the iodine atom. We further study the IR spectrum of title compound and found that there are some dense and weak peaks in the range of 2778−2940 cm−1, which may belong to the vibration absorption peak of H−N+(CH3)2(CH2) bonds in MTZ ligand. So we consider that one single protonated hydrogen atom is located on one N atom in MTZ ligands. As a
result, the title compound was determined as an electroneutral [I@Cu8(HMTZ)(MTZ)7] ring with lattice acetonitrile and water molecules. It should be mentioned that a similar cuprous octatomic ring has only been reported, which was positively charged and formulated as [Cu8I(MTZ)4(HMTZ)4]3+·2I−· ClO4−.58 Single-crystal X-ray diffraction analyses revealed that I@Cu8 crystallizes in the monoclinic space group P2/n. As shown in Figure 1a, the centrosymmetric unit of [I@Cu8(HMTZ)-
Figure 1. Perspective view of the [I@Cu8(HMTZ)(MTZ)7] ring (a) and compound I@Cu8 viewed along the a axis (b).
(MTZ)7] consists of four different Cu(I) ions, which are bridged by four different MTZ molecules via the Cu−S and Cu−N bonds to form a crown-like octatomic ring. At the center of [I@Cu8(HMTZ)(MTZ)7] ring, an iodide ion is bonded to the eight Cu(I) ions through weak Cu···I interactions with contact distances of 2.915−3.418 Å, which is close to that of [PyH][{TpMo(μ3-S)4Cu3}4(μ12-I)] (3.195 Å).59 The iodide ion may play an important role (e.g., behaving as template) in the formation of such an eight-nuclear ring structure. In I@ Cu8, all Cu(I) ions are surrounded by two sulfur atoms and one nitrogen atom and thus adopt similar, slightly distorted trigonal pyramidal geometries instead of the common planar triangular geometries. The Cu(I) atoms are located 0.317− 0.529 Å off the plane formed by nitrogen and sulfur atoms and weakly interact with the central iodide ion. Within the ring structure of I@Cu8, the Cu···Cu distances from 2.709 to 2.811 Å are near the sum of van der Waals radii of two Cu(I) ions (2.80 Å), indicating the existence of weak Cu···Cu interactions in the molecule.60 B
DOI: 10.1021/acs.cgd.7b01261 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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these similar results and the theoretical calculations by the Ford group, the higher energy green emission (514 nm) of compound I@Cu8 is possibly assigned to the halogen to cuprous charge transfer (XMCT), and the lower energy red emission (724 nm) may be attributed to the ligand-to-metal charge transfer (LMCT) and/or metal-to-ligand charge-transfer (MLCT) characters, mixed with metal-centered transfer (3MC: d10 → d9s1Cu) modified by Cu−Cu interactions within Cu8 clusters.63−65 Furthermore, time-resolved emission measurements also reveal that the solid-state lifetime for 514 and 724 emissions are in the range of microseconds (32.3 and 268.1 ns) at 300 K following triexponential decay, respectively, indicating both emissions are fluorescence (Figure S7).34 More interesting, I@Cu8 shows thermochromic behavior, which indicates the occurrence of temperature-dependent photoinduced luminescence. At the liquid-nitrogen temperature, the white powder sample shows shining green emission at excitation of 365 nm. While the sample is warmed to room temperature, the emission color varies from green to red, as shown in Figure 2c (inset photographs). This temperaturedependent behavior implies that the luminescent color of I@ Cu8 can be controlled in the temperature range of 80−300 K, so we tried to observe its temperature-dependent luminescent colors under man-made rough temperature gradients. A long test tube was filled with compound I@Cu8, and one end of the tube was put into liquid nitrogen in a bowl-shaped Dewar and the other end was left outside in the air. Under this circumstance, the sample showed gradually varied emission colors from bottom to top of the tube, i.e., from shining green to yellow-green, to yellow, to orange, and then to red (Figure 2b). Because of the highly sensitive temperature-dependent luminescent behavior, compound I@Cu8 may be a good candidate material for low-temperature thermometer. Further temperature-dependent emission spectra have been measured between 300 and 80 K for the investigation of the nature of luminescent thermochromism (Figure 2c). Upon temperature decreasing, the intensity of the red emission at about 724 nm increases first and reaches its maximum at 130 K and then turns to slow decrease until 80 K, whereas the intensity of the green emission at about 512 nm features a constant increase from 300 to 80 K. As a result, the solid sample at 80 K shows two more intense emissions of green (512 nm) and red (724 nm), which can be ascribed to the effective reduction of energy loss by nonradiation decay at low
Cuprous complexes usually show fascinating photophysical properties; we also investigated the luminescent properties of compound I@Cu8. The solid state optical absorption spectrum of I@Cu8 was measured at room temperature, and the results show that this compound features complete absorption in the UV light range with an absorption edge of about 400 nm (Figure S6). Subsequently, under the UV excitation at 365 nm, solid-state I@Cu8 shows visible red emission at room temperature. As shown in Figure 2a, the photoinduced
Figure 2. (a) Solid-state emission spectra of I@Cu8 at 300 K; (b) the CIE chromaticity diagram, coincident with the part-in-liquid-nitrogen emission color under 365 nm UV lamb (inset); (c) solid-state emission spectra measured between 80 and 300 K; (d) temperaturedependent emission integrated intensity from 80 to 300 K.
luminescent emission maximum of I@Cu8 locates at about 724 nm with photoluminescence quantum yield of 3.72%; meanwhile, a weak shoulder at 514 nm is observed that corresponds to green emission. The green luminescent emission of I@Cu8 is similar to some cuprous complexes, such as [CuI(Hdppa)]4·H2O·C2H5OH (540 nm), [Cu2(μ3-I)(μ5-Cpta)]n (525 nm), [Cu2(μ-I)2(dpppy)2] (497 nm), and the red one is close to those of [Cu3(pymt)3]n (752 nm) and [Cu2(μ-I)2(dpppyz)2] (638 nm), etc.27,42,61,62 On the basis of
Figure 3. (a) Temperature-dependent emission maximum of 724 nm and the fitted curve at different temperatures from 130 to 270 K; (b) working curves and equations of I1/I2 to temperature in the range of 80−250 K. C
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temperature. Such dual-emission has generated various emission colors strongly coupled with temperature, which makes I@Cu8 a wonderful luminescent material for a molecule thermometer. The rational design of novel dual-emitting cuprous complex systems will lead to new progress in the field of luminescent molecular thermometers. Further studies on the structural modification of I@Cu8 by using various templates (such as Cl− and Br−) and on the theoretical structure−luminescence relationship are in progress in our group.
temperature. More importantly, the relative intensities of red and green emission undergo a reversal in the cooling process with the critical temperature being 130 K; that is, above 130 K the intensity of red emission is higher than that of green emission, while below 130 K the green emission is stronger than the red one (Figure 2d). The different change rates of the two emissions exactly explain why the emission color changes are dependent on temperature: the mixture of two emissions with variable intensities of different change rates results in various colors from red, orange, yellow, to green upon temperature decreasing. The CIE chromaticity diagram based on the emission spectra at different temperatures exhibits the exact emission color at a certain temperature, which demonstrates the opportunity to explore I@Cu8 as a photoluminescent molecular thermometer (Figure 2b). Prompted by the unique temperature-dependent luminescent properties of compound I@Cu8, we have examined the emission intensities of the two emissions with narrow intervals (10 K) and try to correlate these emissions with temperature for the purpose of evaluating its possibility as a luminescent thermometer. The results show that a good linear relationship between the maximum of emission intensity at 724 nm and the temperature in the range of 130−270 K can be deduced. This linear relationship can be fitted as a function of I = −1.19 × 104T + 3.32 × 106 with a correlation coefficient of R2 = 0.9979 (Figure 3a). This emission intensity decreases on average by 6.69% per 1 K and 93.7% in total from 130 to 270 K. Furthermore, we also calculate the relationship between the I1/ I2 value (I1 and I2 represent the maximum emission intensity of 512 and 724 nm, respectively) and the temperature in the range of 80−250 K. The correlation can be divided into two segments: linear correlation in the 80−150 K temperature range with the function of I1/I2 = −0.014T + 2.731 (R2 = 0.9997), and power function relation with the function of I1/I2 = 3.88 × 106 T−3.15 (R2 = 0.9994) in the 150−250 K temperature range (Figure 3b). The relative emission intensity decreases on average by 8.98% and 8.32% per 1 K in two temperature ranges, respectively. Therefore, the emission intensity of compound I@Cu8 is sensitive to temperature with exact digital relationship, suggesting that it can be used as an intensity-dependent luminescent thermometer. To investigate the thermal stability, thermogravimetric analysis on I@Cu8 was performed in air atmosphere at the rate of 10 °C/min (Figure S4). Compound I@Cu8 is thermally stable in the temperature range where thermochromic luminescence occurs. The sample shows a mass loss of 2% at 66.9 °C, corresponding to the removal of the lattice acetonitrile molecule (calculated 1.98%). Further heating to 176.8 °C results in the loss of the two water molecules, suggesting a total mass loss 3.69% (calculated 3.69%). Between 176.8 and 228.3 °C, a rapid mass loss happens that should be assigned to the loss of the functional group −CH2CH2N(CH3)2 (experimental 31.4%, calculated 31.3%), revealing the break-off of all ligands. Heating to above 455 °C results in a complete collapse of the structure of I@Cu8, which is accompanied by a rapid mass change corresponding to the burning of organic components with CuO as the residue (experimental 30.8%, calculated 30.4%). In conclusion, a cuprous halide compound (I@Cu8) with a new structural motif has been successfully synthesized. This compound exhibits a dual-emitting photoluminescence at 512 and 724 nm, respectively, while the intensities of the dual emissions show different variation rates influenced only by
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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.cgd.7b01261. Synthesis; crystallographic data; structural diagrams; ESIMS spectra; PXRD data (PDF) Accession Codes
CCDC 1549815 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 data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*(J.T.) E-mail:
[email protected]. *(M.-C.H.) E-mail:
[email protected]. ORCID
Jun Tao: 0000-0003-0610-4305 Funding
This work was financially supported by the National Natural Science Foundation of China (Grants 21571081, 21325103, and 21671161), the Fund of State Key Laboratory of Structural Chemistry (Grants 20150005 and 20170017), and the Cultivating Project for Talent Team and Ascendant Subject of University in Shangdong Province. Notes
The authors declare no competing financial interest.
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REFERENCES
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