Mechanically Induced Multicolor Change of Luminescent Materials.

Minireviews

DOI: 10.1002/cphc.201500181

Mechanically Induced Multicolor Change of Luminescent Materials Zhiyong Ma, Zhijian Wang, Mingjun Teng, Zejun Xu, and Xinru Jia*[a] Mechanofluorochromic or piezochromic fluorescence chemistry involves the switching and tuning of the luminescent properties of solid-state materials induced by exogenous forces, such as grinding, shearing, compression, tension, and so forth. Up until now, most reported mechanochromic systems, including liquid crystals, organic molecules, organometallic compounds, polymers, and dye-doped polymers, have displayed reversible two-color changes, which arise from either supramolecular or

chemical structure transformations. However, fluorescent materials that undergo mechanically induced multicolor changes remain rare; this Minireview is focused on such materials. Topics are categorized according to the different applied forces that are required to induce the multicolor change, including mechanical control of either the supramolecular structures or the chemical structures, and mechanical control of both the supramolecular structures and chemical structures.

1. Introduction Mechanochemistry refers to physiochemical and chemical transformations of materials induced by mechanical energy. It has had a recent renaissance and is now a burgeoning field.[1] In the fourth century B.C., Theophrastus, a philosopher and natural scientist of ancient Greece, obtained quicksilver by grinding cinnabar in a copper mortar with acetic acid;[2] this is the earliest example of a mechanochemical process. In recent years, mechanochemistry has attracted increasing interest and has become a well-established research field.[3] The advantages of mechanochemical technology are: 1) mechanical force is a credible power source available in our daily life; 2) the operation process is simple; 3) mechanical energy is environmentally clean and safe, because it can avoid the use of solvents or other media; 4) the reactions can be accomplished in a precise way, because compared with conventional energy supply paths, such as heat, light, electronic, and so forth, the external force can act directly on the covalent bonds, thus limiting undesirable reactions and enhancing the rate of some prohibitive reactions; and most interestingly, 5) it can produce a substance in a metastable state, which is difficult to obtain by other methods.[4] Mechanochromism or piezochromism are terms used to describe the sensitivity of materials to a mechanical stimulus. Mechanofluorochromic materials can reversibly switch between emission colors (spectra) under applied mechanical force, such as grinding, crushing, or rubbing,[5] and piezochromic materials show a force-induced color change, but not the emission colors’ variation.

[a] Z. Ma,+ Z. Wang,+ Dr. M. Teng, Dr. Z. Xu, Prof. X. Jia Department of Polymer Science and Engineering Chemistry College Peking University, Beijing, 100871 (China) E-mail: [email protected] [+] These authors contributed equally to this work

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Until now, various compounds have been reported to show force-induced luminescent color changes in the solid state, including liquid crystals,[6] organic molecules,[7] organometallic compounds,[8] polymers,[9] dye-doped polymers,[10] and so forth. However, most reported mechanochromic materials display a reversible change between two colors, which arises from either a change in the molecular arrangement or a transformation in the chemical structure.[5b, d] Several mechanisms have been proposed to explain the mechanochromic behavior, such as liquid-crystal-phase variation, transformation of polymorphism in crystals, transitions of assembled structures, metal–metal interactions, conformational folding or twisting, intramolecular ring-opening reactions, and so forth. A number of reviews have described the development of mechanochemistry, for example, Xu et al. summarized the recent mechanochromic luminescent materials, including metal complexes and aggregation-induced emission (AIE) molecules.[5b,d] Also, Moore et al. reviewed the color change of mechanophores in a polymer matrix[1b, 11] and Nasser and Mingelgrin studied mechanochemically induced transformations of pollutants, including pesticides and other organic substances.[12] The colors red, green, and blue can be combined together in various ways to make a colorful world. In this sense, fluorescent materials that switch between multiple colors upon a mechanical trigger are of great importance in fundamental research and in practical applications; however, compared with materials that switch between two emission colors, there have been a limited number of reports in this area. This is because, 1) until now, the development of new fluorescent materials that switch between multiple colors, in most cases, relies on serendipitous and random screening, as rational molecular design in this area is still unknown, and 2) studies of such materials are deficient, owing to the difficulty to rationally explain the correlation between the structures and their corresponding emission properties.

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Minireviews Herein, we focus our attention on the recent advances in mechanofluorochromic chemistry. The present review is mainly concerned with materials that undergo mechanically induced multicolor changes.

2. Multicolor Changes Induced by Different Applied Mechanical Forces The molecular orientation and intermolecular interactions in mechanochromic systems can be perturbed by mechanical forces, including shearing, grinding, tension, and hydrostatic pressure, which lead to a dramatic change in the emission color. It is known that the various external forces and the power have a great influence on the properties of luminescent materials. However, only a few reports describe the responses of materials to different forces. The quantitative analysis of applied force, including the power and mode of force, remains a bottleneck problem in the study of mechanochemistry al-

Zhiyong Ma obtained his B.S. degree in 2011 from Peking University. He joined Jia’s group as a research assistant in 2009. He was admitted to be a PhD candidate in 2011 in the Department of Polymer Science and Engineering, College of Chemistry, Peking University. He majors in polymer science and his research interests are focused on the synthesis and modification of functional dendrimers and the design and preparation of mechanofluorochromic materials with multicolored switch.

Zhijian Wang received his B.S. degree in 2010 from Peking University. Currently, he is a PhD candidate under the supervision of Prof. Jia in the Department of Polymer Science and Engineering, College of Chemistry and Molecular Engineering, Peking University. His research interests concentrate on the design and synthesis of stimuli-responsive polymers, including mechano-, light- and pH-sensitive polymers.

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though it is an important factor when considering the practical applications. The following examples reported the distinct luminescence response to different applied mechanical forces, such as those of anisotropic grinding and isotropic compression, and ultrasonication, and its sensitivity to power. In 2013, Saito and Yamaguchi et al. demonstrated the distinct luminescent response of a new organic fluorophore, 2,3,4,5-tetra(2-thiazolyl)thiophene (1), to the stresses of anisotropic grinding and isotropic compression (Figure 1).[13] 1 exhibited a yellow emission at 556 nm. As a result of grinding, the emission was largely blueshifted to 490 nm with a green color and a slight enhancement of quantum yield. Moreover, 1 showed blue emission at 449 nm in a polymethylmethacrylate (PMMA) film (10 wt % of 1). Thus, the tricolored emission of 1 was achieved by changing the solid environment. Structure analysis suggested the yellow emission of 1 could be attributed to preformed excimers with a face-to-face dimeric structure that was deformed into a nonordered structure

Dr. Mingjun Teng received his B.S. Degree in 2008 and obtained his doctorate in 2013 from College of Chemistry and Molecular Engineering, Peking University. His research interests are centered on the design and synthesis of mechanochromic fluorescent materials.

Dr. Zejun Xu received his B.S. degree in 2009 from Wuhan Institute of Technology, and his doctorate in Materials Science and Engineering from Beijing University of Chemical Technology in 2014. Currently, he is a postdoctoral researcher at the College of Chemistry, Peking University. His research interests include multicolor fluorescent materials, and the design and synthesis of dendrimer-based dual-targeting carriers for treating brain tumors. Prof. Xinru Jia graduated from Department of Polymer Science, Peking University. She is now a full Professor of Chemistry at the Department of Polymer Science and Engineering, College of Chemistry in Peking University. Her research interests include dendrimer chemistry, stimuli-responsive polymers, biopolymers and mechanofluorochromic materials.

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Figure 1. a) The structure of 1 and its yellow-emissive crystals (left), a greenemissive powder of 1 formed upon mechanical grinding, and an orangeemissive crystal of 1 under hydrostatic pressure (right). b) Micrographs of a crystal under high pressure. A UV LED lamp was used for the excitation (lex = 365 nm). Adapted with permission from Ref. [13] Copyright 2013, American Chemical Society.

upon grinding. As a consequence, the intermolecular distances between the neighboring fluorophores varied to some extent, thus resulting in a broad fluorescence band with a green color. Remarkably, 1 showed a significant redshift in the fluorescent emission under hydrostatic pressure. As shown in Figure 1 b, the emission band was gradually redshifted from 556 nm to a strong emission at 609 nm with an orange color at 3.2 GPa during the compression process; this color change was reversible. When the hydrostatic pressure was gradually returned to ambient pressure, the original fluorescent emission was restored. High-pressure IR spectra analysis of 1 revealed that no significant phase transition occurred, and the corresponding hydrogen bonds were strengthened at the applied pressure of 3.2 GPa. High-pressure single-crystal XRD analysis of 1 (up to 2.8 GPa) and the optimized geometries under high pressure (up to 4.0 GPa) showed similar compression behaviors in the packing structure. The deformed hydrogen-bonded lattice reduced the void space and the interfacial distance between the face-to-face 2,5-dithiazolylthiophene moieties, and their mean planes became much shorter at 2.8 GPa. In addition, the two teraryl skeletons had moved along the long axis so that these units overlapped with each other to a greater extent, thus resulting in the formation of a highly overlapped excimer. This was likely the origin of redshifted emission under hydrostatic pressure. Their work revealed novel multichromism that covers the entire visible region from blue to orange by using only one fluorophore. The study discriminates between how the molecular orientation and the intermolecular interactions are perturbed, ChemPhysChem 2015, 16, 1811 – 1828


Figure 2. The chemical structure of 2. Photographic images and transformation in the molecular arrangement of a) 2 OC crystal before and after smashing, b) fragmental crystals before and after grinding, and c) 2 OC crystal under different isotropic pressures. Adapted with permission from Ref. [14]. Copyright 2015, John Wiley and Sons.

either through the anisotropic stress of mechanical grinding or through the isotropic stress of hydrostatic pressure, and the consequent emission characteristics. Very recently, Zhang and Zou et al. reported a novel boroncontaining compound (2), which showed multicolor emission that was dependent on the molecular arrangement (Figure 2). This compound had two polymorphs: 2 OC, an orange crystal with its main emission centered at 585 nm, and 2 RC, a red crystal with a broad emission band at 605 nm.[14] When the orange crystals of 2 OC were smashed with a stainless steel spoon, the fluorescence intensity of the sample was significantly enhanced with the emission band blueshifted from 585 to 565 nm. The molecular arrangements of the main bulk were unchanged by the molecular perturbations; this indicates that the tensile force disrupted the weak intermolecular interaction between the dimers (Figure 2 a), thus resulting in blueshifted emission and enhanced luminescent intensity. When the fragmental crystals, which were generated by smashing, were further ground with a mortar, a weakly orange–red luminescent powder with a broad emission band centered at 605 nm was obtained. The fragmental crystals and original 2 OC crystals were converted into an amorphous powder, which showed redshifted luminescence (Figure 2 b), due to the different molecular arrangements such as p-stacked dimers induced by grinding.

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Minireviews When 2 OC crystals were compressed by using isotropic pressure, a significant redshifted emission, from 585 to 760 nm, was observed. This emission change could be reversed upon decompression (Figure 2 c), thus indicating that the packing mode of the molecules did not undergo a substantial change during the high-pressure experiment. Compression may shorten the distance between the p planes in a dimer composed of two molecules, and hence lead to redshifted luminescence. The unusual molecular packing of 2 OC could be readily perturbed by an external force, including tensile, anisotropic, and isotropic pressures. The applied isotropic and anisotropic pressures had different effects on the 2 OC crystals. This was the first example of a luminescent material that displays distinct mechanoluminescence triggered by applying different mechanical forces. Zhang, Xu et. al reported the novel ultrasonic-sensitive mechanofluorochromic compound 3 (Figure 3). The fluorescent properties of suspensions of 3 and its aggregation morphologies were greatly influenced by ultrasonic treatment and were extremely sensitive to the power of the ultrasonication.[15] In contrast to most reported mechanofluorochromic compounds, which display redshifted emission and reduced or even quenched intensity, aggregates of 3 suspended in the THF/H2O (90 % water) had a maximum emission wavelength that was remarkably blueshifted from 540 to about 470 nm and the fluorescent intensity was also greatly enhanced by ultrasonication. The fluorescence of suspensions of 3 showed a strong ultrasonic-power-dependent behavior. The aggregates of 3 in THF/ H2O (90 % water) showed weak fluorescence centered at about

540 nm before treatment by ultrasonication. After ultrasonication at a power of 80 W and a frequency of 40 kHz, the fluorescent emission was blueshifted to 513 nm and the intensity was enhanced by 10 times. When the ultrasonic power was increased to 120 W, the color changed to bluish green with a wavelength of about 480 nm. Upon a further increase of the ultrasonic power to 200 W, the fluorescent color became pure blue and the intensity was enhanced by 25 times (Figure 3 a–f). Pristine aggregates of 3 were spherical particles with yellow fluorescence, owing to the surface tension. At an ultrasonic power of 80 W, the spherical particles changed to regular foursided prism crystals with green emission. When the ultrasonic power was increased to 120 W, the four-sided prisms became thinner and most of them became thin slices. Upon further increases in the ultrasonic power to 160 W or 200 W, the slices became thinner and turned into transparent rhombic nanosheets. The formation of the crystalline nanosheet structure greatly enhanced the fluorescence quantum yield and the blueshift effect (Figure 3 g). Theoretical studies involving systematic X-ray diffraction and DFT calculations showed that the imide and the amino groups of 3 played a significant role. Special intermolecular interactions and their crystalline growth mode made the aggregates of 3 extremely sensitive to ultrasound. The strong interactions affected the structure geometry and spatial electron distributions of the molecules, leading to different fluorescent emissions. The results reported in this article showed us that the fluorescent properties can be tuned by changing the assembled structures of 3 from an amorphous form into the regular crystals upon sonication at different powers. Particularly, nanosheets with perfect crystalline structure, special fluorescent color, and high purity could be obtained simply by a facile ultrasonic process with the appropriate power supply.

3. Multicolor Switch Owing to Mechanical Control of Supramolecular Structures

Figure 3. The structure of 3, and the influence of ultrasonic power on the morphology and fluorescence color of suspensions of 3 in THF/H2O mixtures (90 % content of water) with a frequency of 40 kHz: a) non-ultrasonic (scale bar: 1.0 mm), b) 80 W (scale bar: 10.0 mm), c) 120 W (scale bar: 5.0 mm), d) 160 W (scale bar: 5.0 mm), e) 200 W (scale bar: 5.0 mm). f) The luminescent spectra of 3 in THF/H2O mixtures (90 % of water) with a frequency of 40 kHz and different ultrasonic powers. g) Powder XRD patterns of 3 with and without ultrasonic treatment and a simulated powder XRD pattern of a single crystal. Adapted with permission from Ref. [15]. Copyright 2014, Royal Society of Chemistry.

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An efficient approach to fluorescence modulation is to construct fluorescent materials through molecular self-assembly with noncovalent interactions of hydrogen bonds, p–p stacking, van der Waals forces, and so forth, because the fluorescent properties of materials closely depend on the packing modes of molecules, conformational flexibility, and intermolecular interactions. Moreover, the dynamic nature of noncovalent interactions allows the light-emitting properties to be tuned, to obtain materials with highly efficient emissions. In addition, nonbonding interactions determine the flexible arrangement of fluorophores, either in polymorphs or in assemblies, thus enabling reversible emission transformations to be achieved. 3.1. Multicolor Switch Owing to Different Phases The first example of a brightly tricolored mechanochromic material was reported in 2011 by Sagara and Kato, who combined a dumbbell-shaped compound bearing a luminophore of 9,10bis(phenylethynyl)anthracene (4) with a branched molecule (5)

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Minireviews in equimolar amounts to form a luminescent liquid crystal (LC; Figure 4).[16] The reversible tricolor change between reddishorange, yellow, and green was observed directly by the naked eye, and could be switched by mechanical and thermal stimuli. Upon heating the mixture from room temperature to 146 8C, a luminescent LC with a reddish-orange color (cubic phase) was observed under UV light. Application of mechanical shearing to the reddish-orange sample at 90 8C triggered a color change to green (columnar phase). A subsequent color transformation to yellow (mesomorphic phase) occurred upon further shearing at room temperature. The spectral features of the emission in these states were obviously different. The cubic phase showed a broad and structureless emission band at 630 nm and a longer lifetime component at room temperature, owing to the excimer formation of 9,10-bis(phenylethynyl)anthracene moieties. The emission band of the mixture in the columnar phase was observed at the shorter wavelength of 540 nm, and no component with a longer lifetime was detected. As with the mesomorphic phase, two peaks were displayed

in the emission spectra. A broad peak at 583 nm was attributed to the partial-overlap excimers of the emission cores and another peak at 541 nm was identical to the peak in the emission spectrum of the mixture in the columnar phase. The different photoluminescence arose from a shear-induced LC phase transition. The micellar cubic phase could change to a more thermodynamically stable green columnar phase upon mechanical shearing. Moreover, when the mixtures, either in the micellar cubic phase or in the columnar phase, were sheared at room temperature, a thermodynamically metastable unidentified mesomorphic phase was obtained. The color change was reversible. Upon heating the mixture in the mesomorphic phase, it transformed into the isotropic phase at 145 8C and the reddish-orange emission could be observed again from the mixture in the micellar cubic phase after subsequent cooling. The different LC phases correlated to the self-assembled structures. The micellar cubic phase was composed of an equal number of compounds 4 and 5, leading to the formation of

Figure 4. a) The structures of 4 and 5. b) Procedures for writing and erasing tricolor luminescent images. Top panel shows: mechanically sheared sections, a and b. c) Schematic illustration of the assembled structures of compounds 4 and 5: cubic phase (red), columnar phase (green), and mesomorphic phase (yellow). The blue spheres represent the amide groups of 4. Adapted with permission from Ref. [16]. Copyright 2011, John Wiley and Sons.

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Minireviews a stable segmented columnar assembly with a reddish-orange excimer emission. In the green columnar phase, a linear hydrogen-bonding array from compound 4 associated with compound 5, as a stabilizer, was observed. The luminescent cores were spaced in order to hinder the excimer formation. It was assumed that less-ordered columnar structures were present in the yellow mesomorphic phase, although no clear peaks were found in the X-ray diffraction pattern. Some partially overlapped cores gave the partial-overlap excimer emission. Energy migration and energy transfer might occur from nonoverlapped luminescent cores to the partial-overlap excimer sites, thus leading to the mixture having a yellow emission. This work is was the first example of a tricolor switch with only by a single luminophore in the LC state. The study evidences that the fluorescent emission of materials depends closely on the packing modes of molecules and intermolecular interactions, and that control of assembled structure is one of the most promising ways to obtain mechanostimuli-responsive luminescent materials. Yagai et al. reported a p-conjugated compound (6) that contained three different segments: a polarized luminophoric core of oligo(p-phenylenevinylene), an electron-donating amino group with hydrophobic dodecyl chains (C12H25), and an electron-withdrawing ester group with a hydrophilic tri(ethyleneglycol) (TEG) chain (Figure 5). Such an amphiphilic molecule could form several metastable self-assembled phases with distinct packing states of the p-conjugated units. The metastable self-assembled phases could transform into the more stable structures, from a p–p stacked aggregate to a liquid crystal and further to a crystalline phase with variable luminescence upon mechanical stimulus.[17]

Figure 5. a) The structure of 6; b) Schematic representation of the segregated packing structure of amphiphilic dipolar molecule; c) Schematic representation of stimuli responsive phase transition of 6; d) Photographs of a thin film of (~ 0.8 X 0.8 cm2) showing stimuli-responsive change in the luminescence under UV(365 nm) irradiation. Adapted with permission from Ref. [17]. Copyright 2014, Macmillan Publishers Limited.

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The drop-cast thin film of 6 exhibited yellow luminescence (6 Y). After pressing or gently grinding, a waxy, fluidic, and highly birefringent mesophase with an orange color (6 O) was formed. Continuous, vigorous grinding of 6 O triggered a further change in the luminescence to yellowish green (6 G#). 6 G# was a less crystalline form that transformed to bluish-green 6 G upon thermal annealing at 50 8C. The H-aggregated phase 6 Y had a maximum emission at 545 nm, owing to an imperfect H-type stacking with a rotational displacement around the stacking axis, thereby allowing the radiative decay to originate from the lower-lying exciton state. The mesomorphic phase 6 O exhibited a more redshifted emission with a maximum at 587 nm and was unchanged after transformation into the isotropic phase. It had the longest lifetime, but the same radiative rate constant, thus indicating that the intermolecular energy transfer was suppressed in 6 O. In addition, geometric distortion of the p-conjugated systems in this fluid phase’s excited state facilitated intramolecular charge-transfer interactions to generate a relatively long-lived, redshifted emission. As for the crystalline phase 6 G, it exhibited an emission maximum at the shortest wavelength of 498 nm and the shortest lifetime. The bluish-green emission from 6 G could be derived from a monomeric luminophore packed in a rigid crystalline phase. The variable luminescence originated from alteration of intermolecular interactions and the conformation of the p-conjugated luminophores upon a mechanical stimulus. 6 Y was a long-range ordered, multilamellar structure. The in-plane molecular ordering in the 6 Y microsheet was assigned to an oblique lattice containing cofacially p–p-stacked p-conjugated moieties. 6 O was a lamellar mesophase without in-plane molecular ordering. The crystalline phase 6 G exhibited the most complicated diffraction pattern, which indicated a 3D molecular order induced by crystallization. The results demonstrated that the mechanochromic luminescence from 6 Y to 6 O was caused by the loss of excitonic interactions between the dipolar p-conjugated systems, whereas that from 6 O to 6 G was due to increased rigidity of the molecular structure upon crystallization. Yagai et al. reports a new mechanochromic material created by using a molecular-level design strategy of generating metastable, kinetically trapped states in the luminophoric compounds with emission properties that are sensitive to a mechanical stimulus. The designed amphiphilic dipolar p-conjugated systems can form several metastable self-assembled phases with distinct molecular packing modes and emission properties. This work provides a basic molecular strategy for the design of new mechanoresponsive compounds with various p-conjugated luminophores. Park et al. reported high-contrast red–green–blue (RGB) tricolor fluorescence switching of a bicomponent molecular film consisting of (2Z,2’Z)-2,2’-(1,4-phenylene)bis(3-(4-butoxyphenyl)acrylonitrile) (DBDCS; 7) and (2Z,2’Z)-3,3’-(2,5-bis(6-(9H-carbazol-9-yl)hexyloxy)1,4-phenylene)bis(2-(3,5-bis(trifluoromethyl)phenyl)acrylonitrile) (m-BHCDC; 8; Figure 6 a).[18] Reversible red (emission at 594 nm), green (emission at 527 nm) and blue (emission at 458 nm; RGB) luminescent switching with a high

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Minireviews by exposing the patterned sample to SVA again (CH2Cl2 for 5 min). This color switching was due to the selective self-assembly of 7 and 8, driven by their crystalline nature. When 7 formed a self-assembled structure with a radius larger than the Fçster radius, and 8 was in the crystal state, the reversible blue and green colors were observed. When 8 became amorphous and 7 was disassembled, Fçster resonance energy transfer (FRET) from 7 to 8 occurred, which quenched the blue and green luminescence and enhanced the red emission. Thus, FRET between 7 and 8 was suppressed. In this report a new strategy for supramolecular FRET control Figure 6. a) The chemical structures of fluorescence-switching molecules 7 and 8. b) Photoluminescent spectra of to achieve tricolor switching a bicomponent film, 7/8 = 10:3 w/w lex = 275 nm), comparing the thermally annealed (140 8C, 3 min; B(on) state), with a bicomponent mixture solvent-vapor annealed (using dichloromethane; G(on) state), and smeared (R(on) state) films. c) RGB fluorescence system was proposed. In this switching in the 7/8 bicomponent film: fluorescence changes upon various stimuli including SVA, TA, and SM system, the switching of FRET (2.5 cm Õ 2.5 cm quartz plate, illuminated with a handheld UV lamp of 254 nm wavelength). Adapted with permission from Ref. [18]. Copyright 2015, John Wiley and Sons. between 7 and 8 by supramolecular structure control was key to suppress the crosstalk and to achieve high-contrast RGB ratiometric color contrast was realized with different external switching. stimuli, such as heat, solvent vapor exposure, and mechanical force. 7 exhibited two distinguishable and reversibly switchable lu3.2. Multicolor Switch in Crystals minescent phases of blue (458 nm, generated by thermal anSˇket et al. prepared a BF2 complex of 1-phenyl-3-(3,5-dimethoxynealing, TA) and green (533 nm, generated by solvent vapor, SVA, or mechanical force, SM) in the solid state. The red emisphenyl)propane-1,3-dione (9), which contains two meta-mesion (600 nm) of 8 was turned on in its amorphous solid state, thoxy groups on one of the phenyl rings (Figure 7).[19] 9 is triggered by SM, whereas it was completely quenched upon a yellow solid, termed solid A when it is prepared with BF3·OEt2 crystallization with TA or SVA, owing to the fast intramolecular in benzene. Upon recrystallization from CH2Cl2/hexane, comand intermolecular photoinduced electron transfer in the cryspound 9 formed two different types of crystals, greenishtalline state. yellow-emitting prismlike crystals (polymorph A) and yellowBy using the multistimuli-responsive characteristics of the 7/ emitting platelike crystals (polymorph B). XRD analysis of 8 mixture, RGB fluorescence patterning was demonstrated solid A was in good agreement with that of the greenish(Figure 6 c). After SVA treatment of the blue or red films, yellow-emitting prismlike crystals. Solid A showed pronounced a green emission film (527 nm) was observed, which coincided chromic effects, due to its better organizability or crystallinity. well with the green luminescent phase of 7. The green film The fluorescence band of pristine solid A was centered at became blue, with an emission at 458 nm, upon TA; this corre490 nm, but redshifted to 526 nm when the solid was thorsponded to the blue luminescent phase of 7. Interestingly, oughly ground in an agate mortar for 10 min; this redshift was upon smearing, the TA-treated blue film became red, correattributed to the effect of a loose packing pattern in the amorsponding to the red luminescent phase of 8 with a wavelength phous solid. of 594 nm; this change was also observed upon mechanical Interestingly, upon heating the ground pastel-yellow powder perturbation of the green film. However, upon smearing, crysof solid A (form 1) with a heat gun for approximately 5–7 s, the tals of 8 were transformed into an amorphous powder. Owing resulting solid exhibited blue emission under UV light (form 2), to the high contrast crosstalk-free switching characteristic, with the corresponding emission shifting from 523 to 474 nm, three luminescent colors could be inscribed into a single film which closely matched the emission of the unground solid A. with well-defined RGB color patterning (Figure 6 c, center). The The ground sample was in a metastable amorphous state and blue and red colors could be restored to the initial green color could convert into the original crystal state, thus indicating that amorphization and crystallization of solid A are reversible ChemPhysChem 2015, 16, 1811 – 1828


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Figure 7. a) The molecular structure of 9. b, c) Reversible switching emission of solid A by repeated grinding–heating cycles: cycle A (black), cycle B (red). Excitation wavelength = 370 nm. Adapted with permission from Ref. [19]. Copyright 2014, American Chemical Society.

processes in the grinding–heating cycle, and there is almost no loss of emitting color and intensity (cycle A; Figure 7).[19] When the blue powder was heated above its melting point, the color disappeared and a virtually nonemissive dark-yellow solid was observed after a few seconds at room temperature (form 3). Upon gently touching the dark-yellow powder with a pestle, a bright yellow color was obtained (form 4). The associated spectral band was significantly intensified with little fluctuation in its emission maxima at about 534 nm in comparison with form 3. Solid form 4 could be converted back into the blue-emitting form 2 (cycle B), after intensive grinding and subsequent heating. Notably, the emissions could be reversibly tuned between cycles A and B by the grinding–heating process without exhibiting any degradation in luminescence. This report describes that solid A emits strongly in the crystalline phase, but only faintly in the amorphous phase. Reversible switching of emissions was observed upon repeated grinding–heating processes, which result in different molecular arrangements of the same fluorophore. Kanbara et al. reported a luminescent pincer PtII complex with the hexanoylamide amide group (10; Figure 8).[20] Crystals of 10, recrystallized from N, N-dimethylformamide (DMF), displayed green luminescence at 512 nm under UV light. These green luminescent crystals became an orange powder, which emitted at 635 nm, after grinding in a ceramic mortar and could be reformed by recrystallization. In addition, exposure of the orange powder to methanol vapors induced a color change to yellow (574 nm); the orange luminescence could be recovered by grinding or heating the yellow sample. The green, orange, and yellow samples were further investigated by powder X-ray diffraction (XRD) to explore the difference in molecular orientation. The green luminescent crystals exhibited sharp diffraction. In contrast, the orange powder did not exhibit clear diffraction, indicating the regular arrangements of 10 in the green sample were disturbed by grinding. Interestingly, Kanbara et al. found that methanol vapors could induce crystallization of the orange powder and, upon exposure of the orange powder to methanol vapors for several minutes, methanol molecules became inserted into the crystal lattice.

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Kanbara et al. further explored the difference between the crystals of 10 formed by recrystallization from DMF or by exposure to methanol vapors by single crystal X-ray diffraction studies. They found that the molecular structures of 10 obtained from DMF and methanol were similar, however, their molecular orientations and hydrogen-bonding networks were different, thus indicating that the hydrogenbonding networks affected the Pt–Pt distance in the crystals

Figure 8. a) The structure of 10. b) Two-step changes in the photoluminescence of 10 upon external stimuli. The photographs were taken during irradiation with UV light (365 nm). Adapted with permission from Ref. [20]. Copyright 2014, Chemical Society of Japan.

and played an important role in determining the luminescence colors. To confirm the effect of Pt–Pt interactions on luminescence properties, luminescence spectra were measured for various concentrations of 10 in DMF. At a high concentration of 10 mm, a long wavelength emission at 651 nm was observed, which might be caused by excimer formation through metal– metal interactions. Thermogravimetric analysis evidenced that the loss of solvent molecules upon heating had the same effect as grinding, which induced formation of an amorphous and orange luminescent solid. The hydrogen-bonding capacity of the amide groups with DMF played a crucial role in keeping the solid crystalline and maintaining its green luminescence. This study shows that the luminescent colors are strongly affected by the Pt–Pt distance in molecular packing, which depends on the hydrogen-bonding networks. It demonstrates that hydrogen-bonding capability is a key factor for tuning luminescent colors of materials in the solid state.

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Minireviews Tian’s group reported an excellent example of piezochromic luminescence for molecule of 9,10-bis((E)-2-(pyrid-2-yl)vinyl)anthracene (11). The largest piezochromic shift, from 528 to 652 nm, was achieved upon applying external pressure to a powder of 11 (Figure 9).[21]

aggregation states of the three crystals. In this process, external pressure impelled the molecular aggregation state of 11 to transform from J-type aggregation (such as that of C1), to Htype aggregation (as found for C2), and further to aggregated dimers stacked in a more tightly bound face-to-face arrangement (as in C3). Meanwhile, the intermolecular p–p interactions gradually strengthened, and thus, induced the luminescence of 11 to change from green to orange emission and then to a red emission. This article describes a novel mechanochromic molecule with three crystal forms that exhibit distinctly different fluorescence emission. The study evidences that the three crystal polymorphs of this molecule with gradually enhanced p–p interactions upon grinding or under external pressure, and that higher pressure may lead to a molecule with a much greater density, thus resulting in transition between molecular aggregation states. 3.3. Multicolor Switch Induced by Conformational Twisting

Figure 9. a) The molecular structure of 11. b) Luminescent spectrum of a powder of 11 under external pressure. c) Stacking modes and corresponding emission colors for the various molecular aggregation states of 11. Adapted with permission from Ref. [21]. Copyright 2012, John Wiley and Sons.

To gain more insight into the origin of the piezochromic properties of 11, Tian et al. prepared single crystals that exhibited three polymorphs: C1, C2, and C3. The molecules adopted a stacking mode of J-type aggregation along the y axis in C1, whereas in C2, 11 formed H-type aggregation along the x axis, and in C3, dimers in a tight face-to-face stacking mode along the x axis were found. The different stacking modes of C1, C2, and C3 resulted in different fluorescent colors. In C1, the anthracene moieties of two adjacent molecules had no p–p interaction, as there were no overlapping dimeric units; therefore, C1 exhibited green emission at 527 nm. In C2, adjacent anthracene planes formed weak p–p interactions by overlapping with each other about 40 % and the vertical distance between them was measured to be approximately 3.65 æ. Thus, C2 was an orange crystal with an emission wavelength of 579 nm. For C3, adjacent anthracene planes overlapped almost in a face-toface arrangement with a distance of approximately 3.52 æ. The resulting strong p–p interactions caused the red emission of C3 at 618 nm. As shown in Figure 9 b, the luminescent spectra of the powder of 11 under external pressure underwent a large red shift of approximately 125 nm, from 528 nm at 0 GPa to 652 nm at 8 GPa. The range of this shift totally covers those of the corresponding spectra of the three crystals, C1, C2, and C3. The density of the three crystal polymorphs increased from C1 to C3, and under increasing pressure the equilibrium would be expected to shift to the densest form. The observed piezochromic luminescence indicated that when pressure was applied, the powder underwent transformations between the molecular ChemPhysChem 2015, 16, 1811 – 1828


Very recently, Zhang et al. reported a new type of mechanoresponsive fluorophore, (Z)-4-(2-cyano-2-(4’-(diphenylamino)-[1,1’biphenyl]-4-yl)-vinyl)benzonitrile (12), which is a donor–acceptor-type fluorophore containing twisted diphenylacrylonitrile and triphenylamine structures. This fluorophore showed intramolecular charge-transfer (ICT) properties and high fluorescence efficiency in the aggregated state (Figure 10).[22] A crystalline powder of 12 (G form) emitted green fluorescence at 507 nm under UV illumination. Upon gentle grinding, the fluorescence color changed immediately to yellow (Y form) that converted to an orange solid (O form), which emitted at 608 nm, with further mechanical action. Interestingly, after extensive grinding, the powder ultimately showed a red color at 618 nm (R form). The fluorescence color of the ground powder could be fully recovered by recrystallization. G form crystals showed noticeable emission color transitions from green to yellow then to red during the compression process. Notably, the emission at 550 nm under a pressure of 1.49 GPa was similar to that of the Y form powders. Up to 6.09 GPa, the crystal emitted faint luminescence at 608 nm, which was blueshifted by 20 nm relative to that of the R form powder. Interestingly, the fluorescence spectra and colors returned to the original state after the pressure was released. Differential scanning calorimetry (DSC) measurement revealed that in the (partial) amorphous phase 12 was in a metastable state; upon annealing, a phase transition to the crystalline phase occurred. NMR and UV/Vis analysis revealed that the crystalline and amorphous phases coexisted in the powders of the O and Y forms, due to the uneven grinding force. Upon extensive grinding, the powders became entirely amorphous. Clearly, the phase transition from the crystalline to amorphous phase was stepwise, which accounted for the multicolor fluorescence. The synergistic effect of the conformational change and the different molecular packing modes endowed the resulting dye with high-contrast mechanochromic fluorescent behavior with large emission color/wavelength changes from green to red and up to 111 nm.

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Minireviews 3.4. Multicolor Switching in AIE Molecules AIE materials with more efficient emission in the aggregated state than in the dissolved form were reported by Tang et al. in 2001.[23] Since then, a number of AIE compounds including triphenylethylene,[24] tetraphenylethylene (TPE), silole, cyano distyrylbenzene,[25] and distyrylanthracene derivatives were synthesized and most of them were found to show mechanochromic luminescence. Dong et al. reported a series of TPE-based AIE molecules (13, 14, and 15) that underwent a force-induced multicolor change upon the introduction of alkoxy groups (methoxy groups in 13, ethyoxy groups in 14, and propoxy group in 15) to the para positions of the phenyl Figure 10. a) The molecular structure of 12. b) Fluorescence spectra of pristine crystals under hydrostatic pressure rings in the molecules during compression. c) Decompression. d) Digital images of 12 in different phases recorded under UV light (365 nm): A) after slight grinding (Y form), B) pristine powder (G form), and C) after extended grinding (fully (Figure 11).[26] ground, R form). The letters HZU were written with a spatula on filter paper coated with 12: D) pristine aggreDeep-blue emissive crystals of gates without force perturbation, E) full shearing of the aggregates, F) slight shearing of the aggregates, and 14 (14 CA; 448 nm) were obG) digital image of sample (E) under natural light. H) Fluorescence images of the crystal under hydrostatic prestained by slowly evaporating the sure (from compression to decompression. Adapted with permission from Ref. [22]. Copyright 2014, John Wiley and Sons. solvent. Upon grinding, the deep-blue emission turned to green (491 nm), which overlapped with the emission of the pure amorphous solid form of Zhang et al. further investigated the lifetime of the excited 14 (14Am), obtained by quenching its melt. Interestingly, state of the pristine powders upon gentle grinding and ex14Am did not be convert back into 14 CA upon heating at tended grinding. From Stokes shift analysis, they concluded 115 8C, but instead transformed into a sky-blue crystal (14 CB), that for powders of 12, the fluorescence of the G form was which emitted at 462 nm. Upon grinding, 14 CB transformed mainly ascribed to the formation of a local excited (LE) state, into 14Am with the emission changing from sky blue to green. and the yellow fluorescence of the Y form could be attributed Thus, the emissions of 14 could be reversibly switched: 1) beto the coexistence of a LE state and a charge-transfer (CT) tween green and deep-blue by grinding at room temperature state, which resulted in a broad and redshifted emission. In the and annealing at 90 8C and 2) between green and sky-blue by O/R forms, the CT state mainly dominated the luminescence, grinding at room temperature and annealing at 115 8C.[26a] Simwhich showed further redshifts. ilar to 14, 13 and 15 also emitted three colors at different agIn this system, the multicolor transitions were related to two gregation states (Figure 12). The mechanism of the tricolor different kinds of excited state: 1) the conversion from green switch was dependent on the different molecule packing patinto yellow fluorescence resulted from the transformation of terns (two types of crystals and one amorphous phase) in the a small amount of the original powder from a LE state into solid state.[26b] a CT state, and 2) an additional color change to red occurred as a result of further conversion into this CT state making it the Many AIE-active molecules take propeller-like conformations, predominant state. Thus, the large shift in fluorescent waveleading to loose of packing patterns. Hence, the introduction length was due to the transformation from a LE to a CT state, of a weak interaction to such propeller-like molecules may which was accompanied by a change of intramolecular conforafford the materials showing morphology-dependent reversimation and intermolecular packing modes. ble multicolor emission, because the loose packing enables Employing an ICT dye that undergoes excited-state transieasy transformation between different molecular arrangetions upon grinding may be a novel design strategy for explorments. ing high-contrast mechanochromic materials. When the skeleton of TPE was replaced with dibenzofulvene, it displayed more molecular packing styles. The molecule di(4ChemPhysChem 2015, 16, 1811 – 1828


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Figure 11. The molecular structure s of 13, 14, and 15. Photos of a) 14 CA, b) 14Am fumed by acetone, c) 14Am, d) 14 CB, and e, f) 14Am annealed at 90 8C (e) and 115 8C (f). g) PL spectra (excitation wavelength: 370 nm). The photos were taken under UV illumination. Conditions: I) 115 8C, 1 min; II) 90 8C, 1 min; III) fuming with acetone vapor, 5 min; IV) heating to melt and quickly cooling. Adapted with permission from Ref. [26a]. Copyright 2012, American Chemical Society.

Figure 13. a) The chemical structure of 16. b) Photos of ground solid of 16 in a heap (A1 and A3) or as a dispersed powder on a quartz plate (A2 and A4). B–E) Photos of the ground solid after treatment by process I or III are placed in corresponding places. F) PL spectra of the samples in the photos: a) Sample A1, b) Sample B, c) Sample C, d) Sample D, and e) Sample E. Excitation wavelength: 400 nm for a, and 350 nm for (b)–(e). Photos were taken under UV illumination. Conditions: I) heating at 120 8C, 10 min, under ambient condition; II) grinding; III) fuming with chloroform, 5 min. Adapted with permission from Ref. [26c]. Copyright 2013, Science China Press and Springer-Verlag Berlin Heidelberg 2013.

Figure 12. Photos of a) 13 CA, b) 13Am, c) 13 CB, and d) 13Am fumed with acetone. e) PL spectra of (a)–(d) (excitation wavelength: 338 nm). f) Powder XRD patterns of samples (a)–(d) in the images. Photos were taken under UV illumination. Conditions: I) Heating to melt and quickly cooling; II) 110 8C for 10 min; III) fuming with acetone vapor, 5 min. Adapted with permission from Ref. [26b]. Copyright 2013, Royal Society of Chemistry.

ethoxyphenyl)dibenzofulvene (16) exhibited tetracolor switching in the solid state (Figure 13).Green emissive single crystals of 16 (16 GSC) were obtained through slow evaporation of a chloroform/petroleum ether mixture; upon heating at 120 8C for 30 min, these crystals turned a sky-blue color. The fluorescent spectrum of annealed 16 GSC agreed well with that of the crystals grown from methanol, thus indicating that the color change from green to sky blue was due to a change in the crystalline phase upon heating. The amorphous solid of 16 ChemPhysChem 2015, 16, 1811 – 1828


(16Am), obtained through quickly quenching its melt with liquid nitrogen, emitted a weak orange color under UV light. The emission of 16Am could transform to a blue color when it was heated or fumigated with the solvent, which resulted in a crystalline structure change.[26c] All of the crystalline structures of 16 showed mechanochromic properties with a transformation from crystals to amorphous solids upon grinding. Interestingly, the ground sample could be returned to different colors by controlling the heating rate. The green emissive solid was obtained when the sample was heated in a heap, and the sky-blue solid was formed when heating the dispersed powder in an oven. The authors explained that in the dispersed state the ground powder could be heated quickly and the sample in a heap could not. Recently, Ma et al. developed another remarkable tetracolor luminophore 17 (TPENSOH), which combined tetraphenylethene and 6-hydroxylbenzothiazole in the molecular structure. The aim of introducing 6-hydroxylbenzothiazole was to achieve a site specific in response to acid (Figure 14).[27] 17 was AIE active and showed a tetracolor change based on piezo and protonation–deprotonation control. As shown in Figure 14, an as-prepared powder of 17 (B1) exhibited strong blue-light emission centered at 454 nm. When B1 was exposed to concentrated hydrochloric acid (37 %) vapors, the fluorescence shifted to 550 nm resulting in a yellow color (Yfa1). After

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Minireviews method. The crystals constituted of unprotonated 17, 17 protonated with HCl and tetrahydrofuran, and 17 protonated with HCl and, emitted intense blue (454 nm), green (521 nm), and yellow (553 nm) colors with quantum yields of 64 %, 34 % and 29 %, respectively. Notably, the blue and yellow crystals agreed well with the powders before and after protonation of initial 17. Ma et al. proposed that the acid-stimulus response of 17 was a two-step transformation, that is, protonation of the benzothiazole moiety, and then planarization, as well as solvent relaxation of Yfa1. The results of wide-angle X-ray diffraction (WXRD) and DSC, as well as theoretical calculations showed that the mechanochromic luminescence of pristine 17 powder upon grinding, originated from amorphization of microcrystals and the extension of molecular conjugation. This study described the structural relationship between AIE compounds and mechanochromism. AIE compounds are expected to become a rich source of mechanochromic luminescent materials. In addition, molecular-design strategies based on AIE molecules may be widely applicable to the preparation of multicolor mechanochromic luminescent materials in the future.

4. Multicolor Change Owing to Mechanical Control of Chemical Structures

Figure 14. a) Molecular structure of 17 and fluorescence images of the powders: B1 = as-prepared 17, G1 = ground sample, Yfa1 = B1 exposed to HCl vapor for 10 min, Ofa1 = G1 exposed to HCl vapor for 10 min (excitation wavelength: 365 nm). b) PL spectra of the three single crystals. The insets depict the fluorescence microscope images of the single crystals (excitation wavelength: 365 nm.) Adapted with permission from Ref. [27]. Copyright 2014, Royal Society of Chemistry.

subsequent exposure to concentrated ammonium hydroxide (25 %) vapors, Yfa1 fully recovered the initial blue emission. After grinding with a pestle or shearing with a spatula, the luminescence of the B1 powder shifted to 492 nm (G1), which could be entirely restored to B1 upon heating or solvent fumigation. Fumigation with acid vapors also significantly affected the fluorescence of the G1 powder, turning it from a bluishgreen color to orange (565 nm; Ofa1). Acting in a similar manner to the Yfa1 powder, when the deprotonated sample, Ofa1, was treated with NH3, G1 was recovered. Also, when the Yfa1 powder was ground, its fluorescence changed from yellow to orange (GY), with the same emission as the Ofa1 powder. Both Yfa1 and GY could be restored to the initial form (B1) by heating. The fluorescence change induced by force and fumigation was due to the different packing modes of 17. Ma et al. prepared three single crystals by using the solvent evaporation ChemPhysChem 2015, 16, 1811 – 1828


Compared with mechanochromic systems, for which the color change results from variation of the supramolecular structures, force-induced switching of fluorescent emission through chemical transformation is extremely limited to date. The reported examples show that mechanophores are usually covalently bonded to a polymer matrix that is used as an actuator, in which certain chemical bonds, usually the weakest ones, are ruptured by a mechanical force, such as sonication, stretching, compression, and so forth.[9a,b, d, i, 28] Based on this strategy, a number of mechanophores including gem-dihalocyclopropanes,[9f, 29] benzocyclobutene, force-induced electrocyclic-ringopening and cycloreversion reactions have been studied. The history of the study of mechanically induced chemical transformations in polymer materials has been summarized by Moore et al.,[1b, 11] who uncovered the remarkable example of spiropyran, which changed from a colorless spirolactam to a merocyanine form with a reddish color upon mechanical stimulus. Craig and co-workers prepared a poly(dimethylsiloxane) (PDMS) film chemically embedded with spiropyran groups (18; Figure 15).[30] Interestingly, they found that the film underwent a color change from colorless to a deep blue under applied tension, and turned purple upon relaxation. The reversible blue and purple color changes of the film were associated with the subsequent change in the tension/relaxation state. Both the absorption and fluorescence emission spectra were redshifted. However, the redshift wavelength was about 25 nm for the relaxation process and 50 nm for the stretching process, respectively. They identified that the purple color was generated by a transient state of the merocyanine form, which was formed by isomerization about the methine bridge of the activated merocyanine (Figure 16).

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Figure 15. a) Platinum-catalyzed hydrosilylation covalently incorporates a bis(alkene)-functionalized molecule into a vinyl-terminated PDMS and a hydrosilylated PDMS copolymer. b) Ring-opening of spiropyran leads to the activated colored merocyanine compound. c) The mechanically activated film is blue under strain but purple when relaxed, and the transition between the colors is instantaneous and repeatable over multiple cycles. The purple and blue states show different characteristics in both d) their absorption and e) emission spectra. Adapted with permission from Ref. [30]. Copyright 2014, American Chemical Society.

Weng et al. has also observed similar phenomena. They incorporated the hydrogen-bonding group of ureidopyrimidinone (UPy) and mechanophore spiropyran (18) into a polyurethane chain (Figure 17).[9h, 31] The UPy dimers afforded polymer materials with good mechanical properties, and the spiropyran could change its form before catastrophic failure, thus acting as a stress sensor. The color changed from yellow to blue, as stretching produced higher strain, and eventually to purple red upon catastrophic failure. They also studied the mechanical activation of spiropyran in a doubly cross-linked polyurethane,

Figure 16. Isomerization about the methine bridge of the activated merocyanine. The arrows indicate the attachment points to the poly(dimethylsiloxane) film. Adapted with permission from Ref. [30]. Copyright 2014, American Chemical Society.

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which contained chemical cross-linking and hydrogen bonding, by using UPy as the hard segments (Figure 18). The purple color always appeared when the specimen was in the relaxed state, which was consistent with the work of Craig et al.[30] When the specimen was fractured, and the broken pieces relaxed, the purple color appeared. They further used the twocolor transition of spiropyran to study the fracture of the elastomer during crack propagation. Along the crack line, two colors, purple and blue, were found. The blue region indicated where the stress was highly concentrated, whereas the purple color represented the stress-free area. Thus, the crack could be observed by the naked eye through the different colors. However, there is little direct experimental data to explain how the purple color was generated and why it appeared in the relaxation process, let alone how to control the color change. The presence of different states has provided the possibility of obtaining multicolor change in polymer materials, but the mechanism is not clear. Recently, Craig and his co-workers reported a remote stereochemical lever arm effect in the polymer chain.[32] They observed that the force-induced electrocyclic ring opening of gem-dichlorocyclopropanes (gDCC) was sensitive to the stereochemistry of the a-alkene substituent. The force required to accelerate the ring-opening reaction of an E-alkene substituent was about 800 pN and the chemical bonds of a Z-alkene substituent would break under the force of 1200 pN. These two chemical bonds could be broken sequentially by increasing the applied force. The strategy of embedding two mechanophores in a polymer chain offers the possibility to construct novel polymeric fluorescent materials. In the future, this strategy may be a significant way to develop multicolored materials.

5. Multicolor Switch through Mechanical Control of Both the Supramolecular Structures and the Chemical Structures As mentioned above, one promising approach for realizing mechanochromic materials with reversible multicolor transitions is to control the microstructures of the molecular assemblies and another one is to change the chemical structures. The combination of these two distinct approaches to bridge supramolecular structures and mechanochemical reactions has proved effective, and has the advantages of easy operation, good color stability and significant color transformations. In the past three years, our group has reported several mechanochromic molecules. We firstly prepared a dendritic polypeptide (19, Figure 19) with a fluorescent pyrene moiety at the focal point. The results revealed an excimer-to-excimer (E1 to E2) transition mechanism of pyrene, partially overlapped packing to give a sandwich arrangement, and evidenced that the polypeptide was an effective framework for preparing mechanochromic materials.[33] Afterwards, we found a dipeptidebased mechanophore (20, Figure 19) with two lactam rhodamine B moieties in the structure that could act as a mechanical light switch in the solid state.[34] The as-prepared powder showed low emission intensity, whereas a bright red emission centered at 583 nm emerged after heavily shearing it in situ

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Figure 17. Design of a polymeric structure with covalent mechanophore SP (spiropyran) units and noncovalent UPy motifs linked by soft polymer chains. a) The expected microscopic morphology and the response under force. The UPy aggregates and the dimers can be broken, and the SP units can be activated to MC (merocyanine) form upon mechanical loading. b) Chemical structure of the mechanoresponsive modular polymer. Adapted with permission from Ref. [31a]. Copyright 2013, American Chemical Society.

Figure 19. Chemical structures of 19 and 20.

Figure 18. Design of a mechanochromic elastomer containing both chemical and physical cross-links. Adapted with permission from Ref. [31a]. Copyright 2014, American Chemical Society.

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with a pestle, which contributed to the emission of the ringopening isomer of rhodamine B with a more conjugated structure. Taking the advantages of the different mechanochromic behaviors of 19 and 20, we tried to incorporate 20 into the microstructure of 19 to develop a new mechanochromic material through a self-assembly process (hereafter referred to as 19/ 20).[34] 19/20 displayed reversible multicolor mechanochromic properties with an initial blue color (410 nm) and a green color

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Minireviews (480 nm) after grinding in situ with a spatula; upon subsequent mechanical action the green emission switched to a reddish color (580 nm; Figure 20). This is the first example to correlate luminescence properties with molecular assembly and mechanochemical reaction. Such a binary system not only displays the unique advantages in the multiple outputs, but also in the facile preparation method by exploiting already available dyes, which avoids tedious molecular synthesis and is of great importance in practical applications

Figure 21. Chemical structures of 21 and 22. Adapted with permission from Ref. [35]. Copyright 2013, Elsevier.

Figure 20. a) 19/20 aggregates showing different emission colors upon force-induced perturbation (irradiated by 365 nm UV light) and their color restoration upon heating. Blue: as-prepared aggregates without force perturbation, green: slight shearing of the aggregates with a spatula, reddish: further shearing of the green aggregates; b) Fluorescence images of 19/20 on filter paper with blue luminescence. c) Force-induced green luminescence pattern. d) Force-induced reddish luminescence pattern and three different emission colors on one piece of paper. e) Fluorescence (FL) spectra of the samples with blue, green, and reddish emission colors and the sheared sample after annealing, and f) the tunable fluorescence property of the samples under force. Adapted with permission from Ref. [34]. Copyright 2012, John Wiley and Sons.

How to make the multicolor switch in a more simple and convenient way? To achieve this, our earliest idea was to link two chromophores by an appropriate spacer to give a single molecule. Initially, we synthesized compound 21 by using a branched amidoamine structure as the skeleton to bridge two chromophores of pyrene and rhodamine B at each side (21, Figure 21).[35] However, this molecule only changed between two colors, from bright blue (under UV) to a reddish color (under UV and visible light), which contributed to the ChemPhysChem 2015, 16, 1811 – 1828


pyrene dimers with a sandwich packing and the ring-opening reaction of rhodamine B from a spirolactam to a ring-opened amide. We also synthesized another molecule (22, Figure 21) with the chromophores anthracene and rhodamine 6G linked by the same spacer (branched amidoamine). A similar result was achieved with a color change from green (496 nm) to orange (560 nm). These two unsuccessful examples, led us to realize that the spacer between the two chromophores plays a key role in the achievement of a tricolor switch. To restrict the pyrenes to a partly overlapped packing mode with a deep blue color, a rigid spacer may be required, because E1 usually exists in a restricted environment, such as crystals and viscous liquids. With this in mind, we synthesized a series of organic molecules with different numbers of phenylalanine molecules as the spacers to link the chromophores of pyrene and rhodamine B. Interestingly, the molecules with tetra- or pentaphenylalanine (23 and 24) as the spacers exhibited a reversible tricolor switch upon the sequential increase of external forces (Figure 22).[36] The initial 23 and 24 emitted blue luminescence at 440 nm, 435 nm, respectively, because of the partially overlapped packing of excimer I in a helical-like hexagonal column. Upon slight grinding, the assembled structures were destroyed, which led to a sandwich stacking of the pyrene moieties and resulted in a change in fluorescence from blue to bluish-green (23: 480 , 24: 465 nm). Extreme force perturbation induces the ringopening reaction of rhodamine B from a spirolactam to a ringopened amide with a reddish color (23: 583, 24: 580 nm). This result correlates to the molecular structure, because more phenylalanine fragments in the linker restricts the movement of the pyrene units. That means the spacers of tetra- and pentaphenylalanine constrain the pyrene units in the molecules to be partially overlapping.

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Minireviews We prepared a single organic molecule that undergoes multicolor fluorescence switching by combining the force-induced transformation of assembled structures and chemical reactivity mechanisms: “one molecule, two mechanisms, three colors”. The crucial point for controlling and tuning the tricolor fluorescent switch of this system is to constrict the pyrene excimer in an overlapped packing mode, which can be achieved 1) by controlling the molecular structure; and 2) by confining the excimers of pyrene in a restricted environment.

6. Summary

Figure 22. a) Chemical structure of 23 and 24. b) Different emission colors (upon irradiation with 365 nm UV light) of 23 on an agate mortar upon grinding: original blue (center), reddish (middle ring), and bluish-green (external ring). c) Fluorescence spectra of the initial powder and the same sample upon grinding (lex = 365 nm). Adapted with permission from Ref. [36a]. Copyright 2013, John Wiley and Sons.

We also found that the aforementioned molecules with less phenylalanine molecules in the spacer,[36b] such as the molecule with two phenylalanine fragments to link pyrene and rhodamine B (25), could show a tricolor change when the pyrene excimers were confined in the E1 state by gelling 25 in an organic solvent (Figure 23). 25 emitted a deep blue color in the gel phase with lamellar packing, upon removing the solvent the assembled lamellar structure slipped, which led to sandwich stacking of the pyrene units and resulted in the transition of excimers from E1 to E2 with the bluish green color. Subsequent extreme force perturbation induced a reddish color.

We have summarized the recent development of mechanochromic fluorescent materials with multicolor change, which may open new ways for discovering new solid fluorescent materials and improving existing ones. The study and applications of such materials should continue to grow, as mechanochemistry is green and has the added advantages of ease of recovery and high efficiency. The related study is full of challenges, but also opportunities. A major focus may be the design and discovery of more mechanochromic compounds, complexes, and systems, which should further extend and enrich investigations in this field and advance our knowledge of mechanochemistry. Finding structure–property relationships is crucial, because these can enable properties to be predicted and reveal feasible directions for constructing structural features of mechanochromic materials with relevant solid-state properties. The quantitative analysis of applied forces, rather than the qualitative description of the phenomenon, remains unresolved, especially with regards to the force of the grinding, and multiple techniques and new approaches are needed. Sequential multicolor change is of great significance for practical applications and may give us more useful information about mechanofluorochromic materials. A notable and promising way to achieve a sequential multicolor switch is by the combination of mechanical control of supramolecular structures and chemical structures. The study of mechanochemistry is an interesting but challenging research area. It shows new directions for making functional materials and is of great importance for both fundamental research and practical applications in the future.

Acknowledgements This work is financially supported by the National Natural Science Foundation of China (21174005) to X.-R. Jia and partially supported by National Basic Research Program of China (No.2011CB933300). Keywords: fluorescence · forces multicolor switch · piezochromism


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Figure 23. Chemical structure of 25 and pictures of a) 25 gel in ethyl acetate, b) 25 xerogel, and c) 25 xerogel after grinding. Adapted with permission from Ref. [36b]. Copyright 2015, Royal Society of Chemistry.

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1828


Preparation and microscopic visualization of multicolor luminescent immunophosphors.

Upconversion luminescent materials: advances and applications.

Luminescent materials for solid-state lighting.

Luminescent hybrid materials based on laponite clay.

Luminescent materials containing multiple benzoxaphosphole units.

Synthesis of dithienogermole-containing oligo- and polysilsesquioxanes as luminescent materials.

Surface Sensitive Techniques for Advanced Characterization of Luminescent Materials.

Thermochromic Luminescent Materials and Multi-Emission Bands in d10 Clusters.

New cyanopyridone based luminescent liquid crystalline materials: synthesis and characterization.

Biologic-free mechanically induced muscle regeneration.

Non-enzymatic-browning-reaction: a versatile route for production of nitrogen-doped carbon dots with tunable multicolor luminescent display.

Programmable mechanically assisted geometric deformations of glassy two-stage reactive polymeric materials.

Highly stable and luminescent layered hybrid materials for sensitive detection of TNT explosives.

Long-lived luminescent soft materials of hexanitratosamarate(III) complexes with orange visible emission.

inorganic hybrid materials.

Novel chitosan-ZnO based nanocomposites as luminescent tags for cellulosic materials.

Luminescent materials: locking π-conjugated and heterocyclic ligands with boron(III).

Application of phase-change materials in memory taxonomy.

Luminescent liquid crystalline materials based on palladium(II) imine derivatives containing the 2-phenylpyridine core.

Interactions of mechanically induced coughing and sneezing in cat.

Filter-Dense Multicolor Microscopy.

Recent Advances on Neuromorphic Systems Using Phase-Change Materials.

On-chip photonic memory elements employing phase-change materials.

Chalcogenide phase-change thin films used as grayscale photolithography materials.