One Material, Three Colors

Researchers have developed a material that can switch its color between yellow, red and orange in response to heating, fuming and grinding.

AsianScientist (Mar. 9, 2017) – Chemists at Osaka University and physicists at Durham University have developed a multicolored environmental sensing material. Their results, published in Chemical Science, also showed that the material has efficient thermally activated delayed fluorescence, a quality required for high performance organic light emitting diode (OLED) devices.

Mechanochromic luminescent (MCL) materials change their color in response to a change in their environment, like pressure and temperature. To date, most MCL materials only change between two colors, limiting their applications.

“Most MCL materials generate two colors by switching between a stable state and one metastable state. To realize multicolor MCL, more metastable states are necessary,” explained Professors Youhei Takeda and Satoshi Minakata at the Department of Applied Chemistry, Graduate School of Engineering of Osaka University.

To create these states, the chemist team led by Takeda and Minakata designed a new molecule by using a conformationally-switchable phenothiazine (PTZ) as the donor.

“By making the use of a promising and unique acceptor, dibenzophenazine (DBPHZ), which we previously developed, we made a PTZ-DBPHZ-PTZ triad,” said Takeda. “In this structure, the PTZ moiety could take two distinct conformers, which therefore in principle creates in total four metastable states as a whole molecule.”

In response to heating, fuming and grinding, the molecule switched its color between yellow, red and orange. The team found that the three colors derive from different conformers in which each PTZ takes either an equatorial or axial conformation relative to the DBPHZ core.

“For red, both of PTZ units take an equatorial-equatorial conformer, for orange, PTZ had an equatorial-axial conformer, and for yellow, PTZ had an axial-axial conformer,” Takeda said.

Most OLED devices with high energy conversion efficiencies depend on expensive precious metals. Thermally activated delayed fluorescence light emitting devices, on the other hand, can achieve equal or better efficiency at much lower cost, which is why they have gained popularity for the design of displays in daily electronics like smart phones.

In collaboration with the physicists team at Durham University, the Osaka researchrs successfully made highly efficient OLED devices by applying the newly developed molecule as an emissive material. Incorporating the PTZ-DBPHZ-PTZ triad into a light emitting device resulted in an efficiency three times higher than the theoretical maximum of conventional fluorescent materials.

“Our molecule could become a basis for efficient light-emitting devices and pressure- and temperature-responsive sensors in the future,” Takeda adds.

The article can be found at: Okazaki et al. (2017) Thermally Activated Delayed Fluorescent Phenothiazine–dibenzo[a,j]phenazine–phenothiazine Triads Exhibiting Tricolor-changing Mechanochromic Luminescence.


Source: Osaka University; Photo: Shutterstock.
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