Communication pubs.acs.org/crystal
Multistimuli-Responsive Luminescent Material Reversible Switching Colors via Temperature and Mechanical Force Xiao-Chen Shan,†,‡ Hua-Bin Zhang,†,‡ Lian Chen,† Ming-Yan Wu,† Fei-Long Jiang,*,† and Mao-Chun Hong*,† †
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China ‡ Graduate University of Chinese Academy of Sciences, Beijing 100039, P. R. China S Supporting Information *
ABSTRACT: A new multistimuli-responsive luminescent material, [Cu8I8(4-dpda)6]n (1) (4-dpda=4-(diphenylphosphino)-N,N-dimethylaniline), has been obtained by combining inorganic Cu8I8 clusters and organic ligands 4-dpda, providing a novel in-/organic hybrid approach as an interesting alternative to all-organic approaches. Compound 1 responds well to thermo and mechanical force independently with color change due to the fact that the stimuli affect frontier molecular orbital and molecular packing modes, respectively.
D
inorganic hybrid materials, since they combine organic and inorganic components covalently as building blocks, sharing the advantages of stability of inorganic moieties and flexibility of organic groups that improve functionalities and widen applications.6 Inspired by the easy synthesis, low cost, robustness, and simple approach to formation of organic− inorganic hybrid materials, our intention here is to introduce the hybrid system into multistimuli-responsive luminescent materials. Herein, we envisage the probability of designing a dualstimuli sensitive system by incorporating a temperature sensitive functionality on one block of inorganic iodide copper cluster, a mechanical force sensitive functionality on the other block of organic ligand 4-(diphenylphosphino)-N,N-dimethylaniline (4-dpda), and connecting the two by metal−ligand coordination bonds. Copper iodide clusters are chosen as inorganic moieties, not only for their thermochromic performances which display two distinct emission bands (high-energy emission and low-energy emission) that would vary their relative intensities at different temperatures3e−g but also for the lower price and larger abundance properties of Cu,7 which make it possible for wider applications and larger-scale productions. As for the 4-dpda ligand, the presence of its multiphenyl moieties will be a key of mechanochromic properties, according to some reports.8 We have now successfully obtained [Cu8I8(4-dpda)6]n (1), which crystallized in ethanol solvent of mixed CuI and 4-dpda and showed thermo- and mechanical-responsive luminescence independently, as we expected.
riven by the requirements of color-tunable luminescent materials in terms of scientific research and practical applications like sensing, detection, memories and display devices,1 the stimuli-responsive luminescent materials have served as an international focal point recently. Generally, the stimuli-responsive luminescent materials utilizing their dynamic molecular architectures varied with exposure to an external stimulus (e.g., light, pH values, temperature, mechanical force or electric and magnetic fields),2 and this molecular level response is translated to the macroscopic scale associated with the change of photophysical properties. Particularly, there are various stimuli-responsive materials, such as mechanochromic, thermochromic, vapochromic, and acidchromic materials,3 which adopt different mechanisms to read the input stimulus and response. Currently, it is worth noting that most of the reported stimuli-responsive luminescent materials tune their colors only in response to a single stimulus.3 However, in nature, the changes in behavior of responsive materials often result from a combination of environmental stimuli. Consequently, formulation of materials that independently respond to multiple stimuli in a divinable way is very important in practical applications and has aroused significant interest. In consideration of the key design concept that the nature of the dynamic molecular architecture decides which stimulus the materials will respond to, it is possible to realize multistimuliresponsive materials by tailoring dynamic molecular moieties that can sense specific external change. Nevertheless, the most current candidates of multistimuli-responsive luminescent materials are sophisticated organic systems,4 and their high cost of extreme reaction conditions and time-consuming postprocessing limits their applications.5 Hence, alternative multistimuli-responsive luminescent materials are required. Recently, increasing attention has been given to organic− © 2013 American Chemical Society
Received: January 6, 2013 Revised: February 21, 2013 Published: February 22, 2013 1377
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Figure 1. (a) The monomer structure of 1 with a Cu8I8 cluster. (b) The monomers in the crystal structure stack to form a two-dimensional (2D) molecular sheet by the π−π interactions between adjacent molecule’s phenyl moieties with the distance of (c) 3.94 Å and (d) 3.59 Å.
Table 1. π−π Interactions in Compound 1a
The X-ray crystallographic diffraction shows that compound 1 crystallizes in the triclinic P1̅ space group and contains a discrete Cu8I8 cluster which can be seen as a double-cubane Cu4I4 cluster linked together through two Cu−I bonds (Figure 1a). The double-cubane clusters have been reported in a few Fe−S and Mo−O series,9 but they were limited in copper halides and as a discrete cluster like 1 has, to the best of our knowledge, never been reported. In the crystal of 1, the π−π interactions between adjacent molecular planes (Figure 1, panels c and d) give rise to stacking of the monomer cluster to form the planar “molecular sheet” that each Cu8I8 cluster monomer contacts with along with four other clusters as shown in Figure 1b. The detailed π−π interactions are listed in Table 1. Temperature-dependent luminescence of 1 is investigated to track its response to thermo (Figure 2). Compound 1 shows green emission in solid state at room temperature, emitting a single emission band with the maximum peak at about 530 nm [low-energy emission (LE)]. As the temperature cools to about 170 K, a new emission band appears at a higher energy around 460 nm [high-energy emission (HE)]. As further cooling occurs to 10 K, the intensity of this band increases gradually with the concomitant decreasing of the LE band and a slight blue shift to 447 nm. When the sample is gradually warmed up to room temperature, the green emission of the LE band is recovered, showing reversible luminescent thermochromic
π−π interactions (face-to-face) R(i) → R(j)
distance between the barycenters (Å)
R(1) → R(1)i R(2) → R(3)ii R(3) → R(2)ii
3.59 3.94 3.94
a
R(i)/R(j) denotes the centroids of the ith/jth ring; R(1) C1−C2− C3−C4−C5−C6; R(2) C7−C8−C9−C10−C11−C12; and R(3) C41−C42−C43−C44−C45−C46. Symmetry codes: (i) 1 − x, 1 − y, 2 − z and (ii) 1 − x, 1 − y, 1 − z.
behaviors; detailed data are listed in Table 2. As compound 1 can be considered as two Cu4I4 clusters combined and the distances of Cu−Cu or Cu−I bonds are quite the same as those previous reports of the [Cu4I4L4] copper iodide clusters,3e−g a reasonable thermochromic mechanism of 1 can be offered based on the Cu4I4 clusters where the above LE bands are attributed to the triplet cluster-centered (3CC) excited state and the above HE bands to an iodide to phosphine ligand charge transfer (XLCT). In accordance with the systematic analysis of the thermochromism Cu4I4 cluster,10 the lowest energy excited state is relevant to geometrical configuration. A report studied by Kim et al.11 directly pointed out that the Cu− Cu distances were changed as the temperature cools, leading to geometrical variations of Cu4I4 cluster. As a result, we conclude 1378
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Figure 2. Emission spectra of 1 at varied temperatures (left). CIE (Commission International de L’Eclairage) 1931 chromaticity diagram showing the tunable colors from green emission to blue step-by-step with different temperatures (right). CIE coordinates are calculated using the software GoCIE, obtained from http://www.geocities.com/krjustin/gocie.html, accessed in 2010.
molecular orbital, leading to a display of different colors at different temperatures. Besides the temperature, the emission color of 1 can also be switched by mechanical forces (Figure 3). When solid samples of 1 are ground in a mortar, the white powder obtained exhibits orange luminescence which can be reverted to the original green luminescence after treating it with drops of ethanol and drying. The dynamic recovery of the emission color between green and orange is attested with repeating cycles of grinding and treating the sample with solvent. With the uncertain mechanisms of mechanochromism, further investigations of spectroscopy and powder X-ray diffraction (PXRD) are performed to elucidate the mechanochromic property of 1. As shown in Figure 4, both of the unground and recovered samples of 1 display almost the same spectra, with a broad absorption ranging from 260 to 340 nm and green emission at 530 nm. As being ground, the sample displays a slight red shift in the absorption spectrum and an obvious red shift in the emission spectrum of about 100 nm, which is quite different from the previous reports that always blue-shift after grinding.13 The PXRD pattern of the unground sample of 1 exhibits diffraction peaks ascribable to well-defined microcrystalline structures, which are in good agreement with the simulated
Table 2. PL Data of Unground and Ground Samples of Compound 1 at Room Temperature and 10 K compound 1
T (K)
λex (nm)
λem (nm)
unground
300
340
530
10
340
447
300
355
620
10
355
460
ground
a
τ (μs) 8.82 (51%a), 2.48 (36%a), 0.56 (13%a) 1120 (79%b), 6438 (21%b) 3.70 (50%a), 1.41 (41%a), 0.31 (9%a) 1340 (48%a), 42.2 (22%a), 227 (30%a)
Φ (%)
band attribution
1.83
LE
6.19
HE
1.01
LE
2.14
HE
Triexponential decays. bBiexponential decays.
that the temperature stimulus affects the frontier molecular orbital as the sizable variations of configuration determine the energy of which type of charge transfer possess the minima. Actually, the thermal activation process between these two coexisting excited states of 3CC and XLCT gives rise to the population conversion,12 leading to the thermochromic result. As a result, the temperature stimulus affects the frontier
Figure 3. Photographs showing compound 1 on an agate mortar under UV irradiation with black light (365 nm). (a) the powder of 1 after grinding the right-half with a pestle, (b) the same sample under ambient light, (c) entirely ground powder of 1, (d) partial reversion to the green luminescence by dropwise addition of ethanol onto the center of the ground powder, (e) powder after treatment with ethanol, and (f) repetition of the green emission by grinding the powder with a pestle. 1379
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reversible mechanochromism process exactly matches those of the amorphous and crystalline phase, and the ground sample also shows thermochromic behavior, which is the character of the Cu8I8 monomer. The emission bands of previous reports of mechanochromic materials always had the blue shift after grinding,13 due to the face that they broke the π−π interactions and caused the unoccupied π* orbital to rise because of the decrease of delocalization, causing an energy gap increase. However, as the LE band of 1 originates from 3CC, its position has been directly related to the Cu−Cu distance in Cu(I) clusters. As the molecular packing changes, the molecules get more random and closer to each other so as to extrude the monomer clusters and make the Cu−Cu distance shorter. As a result, the Cu−Cu interactions inside the Cu8I8 monomer increase, which leads to an ∼100 nm bathochromic shift for LE in the amorphous state compared with the crystalline state. Ultimately, it is rational to explain that the uncommon red-shift color emission of the sample under a mechanical stimulus is induced by the cuprophilic interactions within the molecular materials, where the monomer moieties adopt a different packing mode. In summary, we have established a design strategy of the multistimuli-responsive materials in which luminescent colors can be smartly switched by different environmental stimuli simultaneously, including temperature and mechanical force. We prove that compound 1 is responsive to these two external factors independently, one affecting the frontier molecular orbital and the other one tuning the molecular packing modes. The thermochromic and mechanochromic properties of compound 1 demonstrate that the organic−inorganic hybrid system can be introduced into multistimuli-responsive materials, providing a promising approach to develop smart photofunctional materials.
Figure 4. UV−vis absorption spectra of the unground, ground, and recovered solid (left). Their emission spectra (right).
peaks calculated from the X-ray data of the single crystal of 1. In contrast, the PXRD pattern of the ground sample shows only weak and ambiguous reflections, indicating that the crystal lattice is significantly disrupted and there is a crystal-toamorphous phase conversion caused by grinding. Upon treatment of the ground sample with ethanol, the reflection peaks restore, which means reversion from the amorphous to the crystalline phase (Figure 5). Furthermore, elemental
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis and crystal data of 1, X-ray crystallographic files in CIF format, selected bond lengths (Å) and bond angles (deg) for 1, and some additional figures. Crystallographic data for the structures reported in this article have been deposited in the Cambridge Crystallographic Data Center with CCDC reference number 880630 for complex 1. This material is available free of charge via the Internet at http://pubs.acs.org.
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Figure 5. Simulated XRD pattern of compound 1 (black), observed data of the unground sample (red) and ground sample (green), and recovered sample by ethanol (orange).
AUTHOR INFORMATION
Corresponding Author
*M.-C.H.: e-mail,
[email protected]. F.-L.J.: e-mail, fjiang@ fjirsm.ac.cn; Tel: +86-591-83792460; Fax: +86-591-83794946.
analyses of the unground, ground, and solvent-recovered samples show the expected values corresponding to calculated data on the basis of formula 1, excluding the involvment of solvents in the vaporchromic system. As being monitored by PXRD analysis, the mechanochromism transition representing a switch between the crystal phase and amorphous is likely to be driven by the stacking mode changes. Figure 1b shows the molecular packing structures of the single crystal 1 as determined by single-crystal X-ray analysis. When 1 is in the crystalline state, the monomer clusters are likely to stack and form a planar “molecular sheet” with the help of π−π interactions. After crystalline 1 is ground, the regular structure is disrupted by pressure, which leads to the arrangement shifts and the molecules move disorderly. This hypothesis is proven by the fact that the emission spectra of the
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are thankful for financial support from the 973 Program (Grants 2011CB932504 and 2011CBA00507), National Nature Science Foundation of China (Grant 21131006), Knowledge Innovation Program (Grant KJCX2-EW-H01) of the Chinese Academy of Sciences and Nature Science Foundation of Fujian Province.
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