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Mechanochromic Luminescence Based on Crystal-to-Crystal Transformation Mediated by a Transient Amorphous State Shiki Yagai, Tomohiro Seki, Hiroaki Aonuma, Kohsuke Kawaguchi, Takashi Karatsu, Takuma Okura, Aya Sakon, Hidehiro Uekusa, and Hajime Ito Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03932 • Publication Date (Web): 04 Dec 2015 Downloaded from http://pubs.acs.org on December 4, 2015
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Chemistry of Materials
Mechanochromic Luminescence Based on Crystal-to-Crystal Transformation Mediated by a Transient Amorphous State Shiki Yagai,*† Tomohiro Seki,# Hiroaki Aonuma,† Kohsuke Kawaguchi,† Takashi Karatsu,† Takuma Okura,# Aya Sakon,= Hidehiro Uekusa,= Hajime Ito*# †
Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan #
Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
=
Department of Chemistry and Materials Science, Graduate School of Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8551, Japan ABSTRACT: Photoluminescent materials that exhibit tunable emission properties when subjected to mechanical stimuli have numerous potential applications. Although many organic/inorganic and organometallic compounds display this property, called mechanochromic luminescence, most of these materials undergo a crystalline-to-amorphous (C→A) phase transition; examples of crystalline-to-crystalline (C1→C2) transformation are rare. Single-crystal X-ray diffraction may allow direct analysis of the molecular packing of mechanochromic luminescence materials before and after C1→C2 transformation, which may help to understand the underlying mechanism of this transformation. Reported herein is a mechanochromic luminescence material that displays an unprecedented type of C1→C2 transformation mediated by a transient amorphous phase (C1→[A]→C2). This mechanochromic luminescence material was developed by introducing soft triethylene glycol side chains in a crystalline gold(I) complex that exhibits mechanochromic luminescence based on a C→A phase transition. When this new gold(I) complex bearing triethylene glycol chains was subjected to a mechanical or thermal stimulus, dynamic phase changes were observed with irreversible luminescence color changes from blue to yellow to green in both the cases. The crystallinity of the mechanically generated C2 phase was lower than that of the thermally generated C2 phase. This is because the mechanically induced C1→[A]→C2 process was finished within seconds, whereas the thermal C1→[A]→C2 process occurred over a few minutes. To control the C1→[A]→C2 transformation, we doped the complex with an inactive soft component. This successfully made the transformation reversible (from green to blue) upon thermal annealing of the mechanically obtained C2 phase. This approach allowed the development of an imaging process involving invisible information storage even under UV illumination.
amorphous (C→A) phase.24,31–37 Such materials show a change in emission color in response to mechanical stimuli.
INTRODUCTION Stimuli-responsive luminescent solid materials have potential applications as flexible, printable sensors and recording devices, and in security printing.1,2 Chemical substances, temperature, and light are typical stimuli used to induce changes in the luminescence properties of these materials. Unlike these widely exploited stimuli, use of mechanical force, such as grinding or pressing, as a stimulus to change luminescence behavior has recently attracted increasing interest.3–5 Numerous organic compounds have been observed to change their photoluminescence color following application of a mechanical force, and this property is called mechanochromic luminescence6–30 The majority of the reported compounds have multiple solid structures with different luminescent properties, and when interconversion of these different structures is induced by mechanical stimuli, such compounds show mechanochromic luminescence. Many of the reported systems were serendipitous discoveries of mechano-induced structural transitions from a crystalline to an
In contrast to mechanochromic luminescence derived from a C→A phase transition, crystalline-to-crystalline, i.e., C1→C2 (where C1 and C2 are different crystalline phases) transformation is rare.17,20,27,28,30,38–50 The molecular packing structures of mechanochromic luminescence materials that undergo C1→C2 transformation can be directly investigated before and after transformation using single-crystal X-ray diffraction (XRD) analysis, which is very informative for understanding the underlying mechanism of mechanochromic luminescence.20,28,30,44,47,48 However, mechano-induced C1→C2 transformations generally occur irreversibly; recovery of the “C1” phase from the resulting “C2” phase requires dissolution and recrystallization. We previously reported that the gold(I) isocyanide complex 1 (Figure 1a) exhibits mechanochromic luminescence that shifts from blue to yellow when its crystalline powder is ground (Figure 1b).31 The mechanochromic luminescence of 1 is attributed to the induction of aurophilic (Au⋅⋅⋅Au)
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interactions,6,51–57 accompanied by a mechanically driven C→A transformation. The ground yellow-emitting 1Ym amorphous phase (the subscript m in 1Ym denotes mechanical stimulation) is stable and neither changed to another phase nor reverted to the original blue-emitting phase (1B) even after storage under ambient conditions for several months. Although 1B could be recovered partially by heating 1Ym, complete recovery of the original phase required recrystallization from dichloromethane.
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type of mechanically induced phase change, although several materials have shown mechanochromic luminescence properties derived from C1→[A]→C1 transformation.58–60 In addition, we also achieve the reverse transformation from the mechanically formed C2 to the original C1 upon thermal treatment with the assistance of a non-emissive dopant. The unprecedented mechanochromic luminescence of 2 is investigated by single-crystal XRD analysis of the two crystalline structures as well as spectroscopic analysis. We also find that 2 exhibits a thermal phase transition with luminescence color changes almost parallel to that of its mechanochromic luminescence (Figure 1d). The mechanically and thermally generated C2 phases have the same crystal structure, but their degree of crystallinity is different. We develop a unique image processing system by taking advantage of this finding.
RESULTS AND DISCUSSION
Figure 1. (a) Molecular structures of gold(I) isocyanide complex 1 and 2. Schematic representations of mechanochromic luminescence of (b) 1 and (c) 2. (d) Schematic representation of thermochromic luminescence of 2.
Mechanochromic Luminescence Compound 2 was synthesized according to a reported method31 and characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis (Supporting Information). Complex 2 forms a mixture of multiple phases upon evaporation from 1,2-dichlorobenzene. Optical microscopy upon excitation with UV light revealed the formation of three phases exhibiting blue, green, and yellow luminescence (Figure 2a). Observation of the same sample by polarized optical microscopy (POM) unveiled the dramatically different crystallinity of these three phases (Figure 2b). The green-emitting phase was found in a strongly birefringent crystalline domain, whereas the blue-emitting phase emerged as spherulites with moderate birefringence, which is characteristic of a semicrystalline state. The yellow-emitting phase showed almost no birefringence, revealing an amorphous state. Luminescence microspectroscopic analysis showed that the blue-, green-, and yellow-emitting phases have emission maxima λmax at 454, 530, and 561 nm, respectively. The formation of multiple phases highlights the potential capability of 2 to show multicolor emission depending on its solid-state structure.
To further expand the functionality of this seminal mechanochromic luminescence material, in this study we introduce flexible triethylene glycol (TEG) side chains in 1 to form gold(I) complex 2 (Figure 1a). We expected that the segregated aggregation of the rigid luminophore unit and flexible TEG units of 2 would facilitate mutual phase transitions between crystalline and amorphous phases upon thermal treatment. Interestingly, the introduction of TEG chains induced a “transient” amorphous phase that continuously undergoes a phase transition to another crystalline phase. The overall phase transformation of 2 upon mechanical stimulation is thus denoted as “C1→[A]→C2”, in which [A] indicates a transient amorphous phase that forms between C1 and C2 phases. These multistep phase transitions are observed as a continuous change in luminescence from blue to yellow to green (Figure 1c). Thorough literature survey revealed that C1→[A]→C2 transformation is an unprecedented
When complex 2 was precipitated from a mixture of ethyl acetate and hexane, a white powder was obtained that exhibited only blue luminescence upon exposure to UV light, suggesting the selective formation of a crystalline state. When this blue-emitting phase, denoted as 2B, was ground with a spatula, the ground area temporarily exhibited yellow luminescence because of the formation of 2Ym, which automatically vanished to apparently display the original blue luminescence within seconds (Figure 3a and Supporting Movie). Thermographic measurements of 2B demonstrated that the temperature of the ground area increased only slightly (12 wt.%) spoiled the mechano-response of 2B#. (65) Lv, Y.; Liu, Y.; Ye, X.; Liu, G.; Tao, X. The Effect of Mechano-Stimuli on the Amorphous-to-Crystalline Transition of Mechanochromic Luminescent Materials. CrystEngComm 2015, 17, 526– 531.
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