Smart materials can respond to external signals such as light, electricity, heat, pressure, and magnetism, and output various signals such as color, light, electricity, and heat. It is an important carrier of high-tech fields such as the core of smart devices and Internet of Things / robots. In recent years, the use of color-changing molecules to design new types of optical, electrical, magnetic, biological and other smart materials and smart devices has become an important branch of the discipline of chemistry and materials. It is an inherent law of semiconductors that conductivity increases with increasing temperature. If this rule is broken, the conductivity of the semiconductor will suddenly drop around the upper limit of the operating temperature (about 80-120oC), which may produce interesting new applications, such as circuit overload and overtemperature protection, to prevent circuit damage and fire. At the same time, during the decline in conductivity, if the color of the semiconductor also changes, it can also indicate which specific component in the circuit has a problem, which is particularly meaningful for circuit detection and maintenance.
Supported by the National Natural Science Foundation of China, the Frontier Science Key Research Project of the Chinese Academy of Sciences, and the Excellent Membership Project of the Youth Innovation Promotion Association of the Chinese Academy of Sciences, based on the previous scientific discoveries in violet / metal violet-based photochromic compounds and intelligent color-changing semiconductors, The research team of Wang Mingsheng from Guo Guocong's research group of the State Key Laboratory of Structural Chemistry at the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences recently applied the characteristics of photochromic properties to the modulation of high-temperature electrical properties of semiconductors, breaking the traditional thinking. In order to achieve this goal, the research team led by Guo Guocong and Wang Mingsheng previously proposed the academic idea of ​​reducing the carrier concentration through thermal color development to reduce the conductivity, but unfortunately, due to the inherent law of conductivity increasing with temperature, it has an advantage. The discoloration of the studied color-changing semiconductors at high temperature differs greatly from the conductivity of the color-developing state (Sun C, Wang MS, * Guo GC * et al. Angew Chem Int Ed 2017, 56, 554). To solve this problem, the research team led by Wang Mingsheng recently proposed the academic idea of ​​"throttling" through the reverse thermal fading process of T-type photochromic semiconductors: looking for a color-changing semiconductor capable of photoelectron transfer After the transfer, the carrier (electron and hole) mobility is higher, but reverse electron transfer can occur quickly at high temperature, and the carrier (electron and hole) mobility becomes lower while fading. It is reported in the literature that viologen-based photochromic materials are usually capable of thermal fading, and viologen ions can construct organic semiconductors through cation-π interactions. In the previous work, Guo Guocong and Wang Mingsheng and others found that if electrons are transferred to the viologen ion π aggregates, the electron / hole coupling interaction between the π aggregates can be improved and the density of electronic states at the top of the valence state can be increased. The conductivity is greatly enhanced (Sun C, Wang MS, * Guo GC * et al. J. Am. Chem. Soc. 2018, 140, 2805–2811). Therefore, the viologen-based T-type photochromic semiconductor is a good example of the realization of the above academic ideas.
In order to prove the feasibility of the above strategy, the research team selected T-type photochromic compounds (H2bipy) (Hox) 2 (H2bipy = 4,4'-bipyridin-1,1 'from the previously studied photochromic compounds -dium; ox = oxalate; Cryst. Growth Des. 2014, 14, 2527-2531; see picture above). The selection of this compound mainly considers the following three aspects: 1) The compound has infinite π accumulation of viologen ions; 2) Its fading temperature (80oC) is close to the upper limit temperature of general electronic components. The results of the study show that (H2bipy) (Hox) 2 is an organic semiconductor whose conductivity is reduced by 81% at a high temperature of 100 oC, successfully verifying the above strategy. In the course of the research, for the first time, an electrical method was used to explore the mechanism of the bleaching kinetics of the photochromic material, and at least two electron return paths were found. This method has reference significance for the kinetic study of other photochromic semiconductors.
The results of this study were recently reported in the journal "German Applied Chemistry" (Sun C, Yu XQ, Wang MS * et al. Angew Chem Int Ed, 2019, 10.1002 / anie.201904121). Sun Cai and Yu Xiaoqing are co-first authors, and Wang Mingsheng is a corresponding author.
Previously, Guo Guocong and Wang Mingsheng's research team also discovered a metal viologen-based photochromic compound system, which overcomes the shortcomings of the traditional viologen-based photochromic compound, such as the high toxicity of synthetic raw materials, the difficulty of expanding the system, and the need for oxygen for discoloration. X-ray detection temperature of radiochromic materials is too low, detection band is too narrow and other issues (Angew 2008, 47, 3565, Angew 2012, 51, 3432, CC 2018, 12349 etc.), and can be adjusted based on light / thermal electron transfer The characteristics of the electronic / molecular structure, using viologen / metal viologen functional primitives to design and synthesize a series of intelligent materials that can respond to and modulate optical, magnetic, electrical and other input signals (Angew 2014, 53, 11529, JACS 2015, 137, 10882, Angew 2017, 56, 554, JACS 2018, 140, 2805, Angew 2019, 58, 2692).
T-type photochromic semiconductor (H2bipy) (Hox) 2 breaks the inherent law of increasing conductivity with increasing temperature
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