Communication pubs.acs.org/IC
Simultaneous Luminescent Thermochromism, Vapochromism, Solvatochromism, and Mechanochromism in a C3‑Symmetric Cubane [Cu4I4P4] Cluster without Cu−Cu Interaction Kai Yang,† Shi-Li Li,† Fu-Qiang Zhang,† and Xian-Ming Zhang*,†,‡ †
School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, China Institute of Crystalline Materials, Shanxi University, Taiyuan 030006 P. R. China
‡
S Supporting Information *
and they proposed that intermolecular interactions, the geometry of the [Cu4I4] cluster core, and modification of the Cu−Cu interactions are responsible for mechanochromic luminescence.9b,c Strangely, no mechanochromic luminescence was described in the analogous cubane complex [Cu4I4(PPh3)4].7a Vapochromism and interconversion between two [Cu4I4] cluster-based metal−organic frameworks were also observed.11 This indicates that there is no general rule for the origin of various chromisms except for thermochrosim in copper complexes, not to mention simultaneous multiply stimuliresponsive luminescent chromism. For a multiply stimuli-responsive chromic luminescent complex, the molecular crystal and packing of its molecules should be relaxed.12 The application of a mechanical force, vapor, and/or solvation force to crystals can produce some remarkable changes. On the basis of the above deduction, tris(3methylphenyl)phosphine (TMP), a bulky methyl derivative of PPh3, was selected to combine the cubane [Cu4I4] cores to construct a multiply stimuli-responsive chromic luminescent complex. Herein, we present a chiral C3-symmetric cubane [Cu4I4(TMP)4] complex (1) with enough long Cu−Cu distances to eliminate the presence of Cu−Cu interaction and that simultaneously shows luminescent thermochromism, solvatochromism, vapochromism, and mechanochromism. This work makes us reconsider the role of Cu−Cu interaction on the chromic luminescence of copper complexes due to the apparent absence of Cu−Cu interaction in 1. Compound 1 features a chiral C3-symmetric cubane structure. It crystallizes in cubic space group I4̅3d and has a large cell volume of 32482.4(8) Å3 at 293 K. There are two crystallographically independent copper atoms, and both show tetrahedral geometry, coordinated by one phosphorus atom from TMP and three iodine atoms. The molecular structure of 1 (Figure 1) presents the classical cubane Cu4I4 cluster. The Cu−P bond lengths of 2.245(3) and 2.256(4) Å at 293 K are comparable with those in related clusters.9b,c,13 The average Cu−I bond distance of 2.737 Å is a little longer than the reported values for Cu4I4 clusters with phosphine derivatives. The Cu−Cu distances (average 3.470 Å, range 3.6242−3.3157 Å at 293 K) are about 0.5 Å longer than those (average 2.968 Å, range 2.839− 3.165 Å at 298 K) in similar cubane clusters with phosphine ligands,7a,9b and they are much longer than those (ca.2.60−2.79
ABSTRACT: A chiral C3-symmetric cubane cluster, [Cu4I4(TMP)4], with enough long Cu−Cu distances to eliminate the presence of Cu−Cu interaction has been synthesized and characterized, which shows simultaneous luminescent thermochromism, solvatochromism, vapochromism, and mechanochromism and is a multiply stimuli-responsive chromic luminescent material. This complex could partly transform into a yellow-emissive bicapped cubane cluster, [Cu6I6(TMP)4(MeCN)2], in acetonitrile (MeCN) vapor and solution, which provides some insight into vapochromism and solvatiochromism. This work challenges and makes us reconsider the conventional viewpoint that Cu−Cu interaction is involved in thermochromism and mechanochromism of copper complexes.
C
hromism in chemistry is a process in which various external stimuli induce a change in the colors of the compounds, which generally can be classified by thermochromism, photochromism, electrochromism, vapochromism, solvatochromism, mechanochromism, and acidochromism.1,2 In most cases, chromism may be based on a change in the electron state of organic molecules,3 the coordination geometry and number in metal complexes, the band gap in semiconductors, the crystal field strength in ruby, the tautomeric equilibrium, the molecular rearrangement, the interchange between different colored stereoisomers, and the layer gratings in liquid crystals.4 An interesting subclass in the field is luminescent chromic materials, which describe a phenomenon of color changes of materials influenced by luminescence.5 Complexes with d10 metals are well-known for their structural diversity,6b,g,7 rich photophysical behavior,8,9 and high luminance efficiency.6 However, it is worth noting that the majority of known chromic luminescent materials change their colors only in response to a single external stimulus. The development of doubly and multiply stimuli-responsive chromic luminescent materials is quite meaningful.8,9b In the context, tetranuclear cubane [Cu4I4] complexes are good candidates for multiply stimuli-responsive chromic luminescent materials because they have been wellknown for thermochromic luminescence, characterized by two emission bands of different energy whose relative intensities vary in temperature.6f,10 In 2010, Perruchas reported doubly stimuliresponsive luminescence in [Cu4I4(PPh2(CH2CHCH2))4], © XXXX American Chemical Society
Received: April 13, 2016
A
DOI: 10.1021/acs.inorgchem.6b00922 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
502 nm with τ = 14.3 μs. When the temperature is lowered, the LE emission band is progressively blue-shifted to 484 nm with τ = 52.9 μs at 5 K. With a decrease of the temperature from room temperature to 100 K, Cu−Cu is shorted and the emission band is progressively blue-shifted. Different from other Cu 4 I4 complexes,5a the intensity of the LE band in the single-crystal sample slightly increases with a decrease of the temperature. The HE emission band in a single crystal shows a slight blue shift, and its emission intensity is enhanced at low temperature, similar to known Cu4I4 complexes. In contrast to the single-crystal sample, the intensities of the LE and HE emission bands remarkably increase with a decrease of the temperature (Figure S6). Vapor adsorption experiments were conducted. The solidstate emission spectra of 1 greatly changed upon adsorption and desorption of MeCN vapor, as shown in Figure S7. As can be seen, the MeCN-adsorbed sample shows an orange emission band with a maximum at 583 nm, while the CH3CN-desorbed sample shows a khaki emission with a maximum at 533 nm (Figure S7). Complex 1 also displays significant solvatochromic luminescence behavior (Figure 3). Compared to the solid state, the
Figure 1. View of the structure (left) and Cu4I4P4 core (middle) in 1.
Å at 298 K) in cubane clusters with pyridine- or amine-derived ligands.6a,10b,11 The Cu−Cu distances are also greatly longer than the sum of the van der Waals radii (2.800 Å). All of these eliminate the existence of so-called cuprophilic interaction in 1.14 Compared with isostructural [Cu4I4(PPh3)4],7a the simple introduction of methyl in each phenyl ring resulted in an increase of the cell volume from 24631.5 to 32482.4(8) Å3, namely, 31.9%. The overall structure is a three-dimensional supramolecular array formed via face cubic packing of the molecular clusters via weak van der Waals interactions (Figure S4). A calculation by PLATON shows that there is a potential solvent-accessible void volume of 871.7 Å3 in the unit cell, but no solvent molecule was included in the crystalline structure. We measured the low-temperature structure of 1 at 100 K, which also crystallizes in the same cubic space group I4̅3d. Compared with room temperature, the cell volume of 31583.3(3) Å3 is decreased by 2.8%, and the Cu−Cu distances of 3.5889 and 3.2393 Å are a little contracted. Compound 1 features quadruply stimuli-responsive luminescent chromism, namely, thermochromism, solvatochromism, vapochromism, and mechanochromism. To measure thermochromic luminescence, both a solid-state single crystal and a ground powder sample of 1 were packed in a sample tube and then immersed in liquid nitrogen for a few minutes. When the tube was exposed to irradiation by a 365 nm UV lamp, the reversible thermochromic luminescence for both the single crystal and powder sample can be easily distinguished by the naked eye (Figure 2, inset). Solid-state luminescence performed from 5 to 300 K is in accordance with thermochromic luminescence. For the single-crystal sample (Figure 2), the maximum of the LE emission at room temperature is found at
Figure 3. (a) Images for the emulsion solution of 1 irradiated by ambient light (top) and 365 nm UV light (bottom). (b) Emission spectra of 1 in different solvents after 50 min of ultrasonication.
emission bands in these solvents remarkably change: in BuOH and acetone solutions, both the HE and LE bands are blueshifted and the dominant band is the HE band; in MeOH, EtOH, and EtOAc solutions, the dominant band is the LE band and the HE band is almost invisible. Quite strangely, the emissions in MeCN consist of three bands with maxima at 436, 469, and 577 nm (Figure 3b) seemingly incomprehensible. However, two different crystals were obtained and they show apparently different colors (Figure S11). The majority of khaki-emissive crystals are recrystallized 1, while the minority of orange-emissive crystals are bicapped cubane [Cu6I6(PPh3)4(MeCN)2] (2; Figure S10), which confirms that minor 1 was transformed into 2 during solvation. On the basis of the isolation of orangeemissive bicapped cubane 2, the above emission spectra in MeCN could be rationalized as follows: the bands with maxima at 436 and 469 nm come from 1, while the band at 577 nm originates from the bicapped cubane 2. Actually, the suspension could be slowly dissolved, which is also revealed from timedependent emission spectra (Figure 3b and Figure S8). The final product in a MeCN solution should be 2, which comes from MeCN-induced recombination of 1. Generally, vapochromic and solvatochromic luminescence originates from reversible structural changes which were induced by vapor adsorption and desorption.8,15 Vapochromic and solvatochromic luminescence of 1 in MeCN involves structural transformation.
Figure 2. Temperature-dependent luminescence spectra of 1 in a singlecrystal sample from 5 to 300 K. Inset in part a: Images for single crystals under 365 nm UV lamp irradiation at room temperature (left) and in liquid nitrogen (right). B
DOI: 10.1021/acs.inorgchem.6b00922 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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Surprisingly, mechanochromism luminescence was also revealed (Figure S9). The blue-green emission excited at 365 nm in the crystalline sample is converted to a khaki emission. After treatment of ground powder 1 with drops of CH3CN, the emission color changes to an intense yellow luminescence. PXRD recorded before and after grinding showed diffraction peaks largely in agreement with those calculated from the singlecrystal X-ray structure of 1, confirming no apparent phase transition during grinding (Figure S3). Weakened intensity and a slight shift of some peaks in the ground powder sample indicated that the crystallinity of 1 is a little lost during grinding, which may be why the geometry of the cubane clusters and/or their relative arrangement are subjected to some changes. In contrast to grinding, the PXRD patterns of the MeCN-treated sample clearly show the presence of characterized peaks of bicapped cubane 2 (Figure S3, pink and deep-blue lines). The simultaneous presence of luminescent thermochromism, solvatochromism, vapochromism, and mechanochromism in 1 raises the question of the possible reason for the unprecedented multiply stimuli-responsive chromic luminescent complex. Because the complication in this system and current ambiguous knowledge in the copper(I) complexes on the origin of chromisms except for thermochromism, no clear explanation could be given. However, on the basis of the structure of 1, related to chromic references and isolation of orange-emissive bicapped cubane 2, we attempt to provide some clues that may be helpful in revealing new multiply stimuli-responsive chromic luminescent complexes. First, the molecular crystal and loose packing of molecules are necessary so that the movement of atoms or molecules cannot be severely constrained. Compared with isostructural [Cu4I4(PPh3)4], bulky TMP ligands result in not only bulky 1 with long Cu−Cu distances but also less condensed packing of molecules due to decreased π−π stacking of phenyl rings. Simultaneous quadruply stimuli-responsive chromic luminescence in 1 is mainly due to the flexible cubane molecular structure and relaxed packing of the cubane molecules. Second, enough weak intermolecular interactions are important so that they can be changed or broken by solvation and mechanical force. Third, the existence of a potential solventaccessible void volume is possibly important. Although no solvent molecule is included in 1, the potential solvent-accessible void volume may provide space for approaching guests. Finally, the synthesis and full characterization of 1 may impel us to reconsider the role of Cu−Cu interactions on chromism. Further investigations on the syntheses of new members of multiply stimuli-responsive chromic luminescent complexes are necessary to fully clear up the origin on quadruply stimuli-responsive chromic luminescence.
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Communication
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +86 357 2051402. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the 973 Program (Grant 2012CB821701), Plan for 10000 Talents in China, National Science Fund for Distinguished Young Scholars (Grant 20925101), and Sanjin Scholars.
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REFERENCES
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00922. Selected bond distances, packing arrays, PXRD patterns, TGA, and additional figures (PDF) Structural data at various temperatures in CIF format (CIF) Structural data at various temperatures in CIF format (CIF) C
DOI: 10.1021/acs.inorgchem.6b00922 Inorg. Chem. XXXX, XXX, XXX−XXX
