A colorful detector: Crystalline material reversibly changes color when absorbing water

Date:
August 25, 2020
Source:
University of Tsukuba
Summary:
Scientists unveil a new crystalline material that reversibly changes
color from yellow to red when absorbing water, paving the way for
chemically sensitive detectors that can work without electricity.



FULL STORY ========================================================================== Researchers at the University of Tsukuba have developed a new kind of
color- shifting crystalline material that can be used to indicate the
presence of water. The change in hue is dramatic enough to be gauged by
the unaided human eye. This work could lead to the creation of highly
sensitive "vapochromic" sensors that can show if a particular gas or
water vapor is present without the need for external power.


========================================================================== Chemical sensors are important to many industrial processes. To
ensure safety and efficiency, factories often need to be monitored for potentially toxic gasses or even excess humidity. Sensors for water
vapor are particularly important, but may have limited lifetimes or
require external power. To address this, scientists at the University
of Tsukuba have invented a new crystalline material that changes color
when exposed to water vapor. Inside the crystal, long branching molecules called dendrimers are held together by van der Waals forces.

"The aromatic carbazole dendrimers containing carbon rings are anchored to
a dibenzophenazine core," explains senior author Professor Yohei Yamamoto.

"Interestingly, even though van der Waals forces are usually considered
to be relatively weak, the crystal stays together during operation." The research team also extensively characterized the new material. In addition
to studying the color in both the hydrated and dehydrated states using spectroscopy, the scientists used techniques including single-crystal
and powder X-ray diffraction analysis, as well as thermogravimetric
analysis. On the basis of the experimental results and theoretical
density functional theory calculations, they were able to determine
the molecular mechanism responsible for the different appearances under different water concentrations. The color- shifting properties of the
crystal come from conformation changes in the dendrimers. Upon exposure to water vapor, the planes of the outermost carbazole units in the crystal
twist simultaneously. This motion changes the energies of the electronic orbitals, which causes the electrons to absorb different colors of light.

"We believe that our findings will lead to the further exploration of
van der Waals porous crystals, much like metal-organic frameworks that
have found a place in chemistry," Professor Yamamoto says. "This work
can lead to a new class of gas sensors that can work in difficult to
reach locations, because they do not require external power."

========================================================================== Story Source: Materials provided by University_of_Tsukuba. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Hiroshi Yamagishi, Sae Nakajima, Jooyoung Yoo, Masato Okazaki,
Youhei
Takeda, Satoshi Minakata, Ken Albrecht, Kimihisa Yamamoto, Irene
Badi'a- Domi'nguez, Maria Moreno Oliva, M. Carmen Ruiz Delgado,
Yuka Ikemoto, Hiroyasu Sato, Kenta Imoto, Kosuke Nakagawa, Hiroko
Tokoro, Shin-ichi Ohkoshi, Yohei Yamamoto. Sigmoidally hydrochromic
molecular porous crystal with rotatable dendrons. Communications
Chemistry, 2020; 3 (1) DOI: 10.1038/s42004-020-00364-3 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200825110725.htm

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