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First luminescent molecular system with a lower critical solution temperature

Date:
May 1, 2017
Source:
Osaka University
Summary:
Researchers have developed a luminescent small-molecule system that changes from a solution to a suspension when heated. The solubility change is accompanied by a color change in the photoemission from blue to green. The transition is mediated by oleic acid and methylamine, which aid the reversible reorganization from molecular wires in solution through an intermediate co-crystal and finally into nanoparticles at higher temperatures. This new system could aid the development of new-generation thermo-responsive materials.
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These are pictures of LCST system under UV light irradiation at low temperature (left), high temperature (center), and during the course of decreasing temperature (right).
Credit: Osaka University

Depending on their solubility, solids can completely dissolve in liquids to form clear solutions, or form suspensions that still contain undissolved solid. Solutions of polymers often have a lower critical solution temperature; only below this temperature is the polymer completely soluble at all concentrations.

However, it is rare for non-polymeric mixtures to have a lower critical solution temperature because small molecules usually become more soluble as they are heated.

Osaka University researchers have now created a mixture of small organic and inorganic molecules that has a lower critical solution temperature. Their luminescent mixture is easily switched from a solution to a suspension and back again, simply by changing the temperature. The system, which has a different emission color depending on whether it is in the solution or suspension state, will be useful for the development of new thermo-responsive materials that change color when heated. The study was recently published in the journal Advanced Materials.

"This behavior is usually only observed in polymer systems," says Associate Professor Akinori Saeki, corresponding author of the study, "because they undergo structural changes at high temperatures that reduce their solubility. This is the first example of a luminescent molecule/ion-based lower critical solution temperature system."

The researchers based their system on methyl ammonium lead bromide nanoparticles, which have been used to develop new-generation LEDs and lasers. Noting that these nanoparticles are reversibly broken apart into their molecular components in the presence of certain amines, the researchers prepared a mixture of the nanoparticles with methylamine and other organic molecules.

At room temperature, the mixture was a clear solution that emitted blue light when it was irradiated under UV light. When the researchers heated this clear solution, however, it became white and cloudy, and then formed a yellow suspension above a critical temperature. The yellow suspension emitted green light when irradiated with UV light.

"Using X-ray diffraction, we found that the clear solution contained soluble 1D wires made up of lead bromide, methylamine and oleic acid," Dr Saeki says. "As the solution was heated, these wires rearranged into a co-crystal containing lead bromide and methylamine, which was insoluble in the solvent."

The intermediate co-crystal was an essential step before formation of the yellow nanoparticles at higher temperatures, and its assembly and fragmentation were mediated by the organic molecules oleic acid and methylamine.

Tuning the system by varying the concentrations of the organic molecules or adjusting the ratio of halide ions (chloride, bromide and iodide) in the nanoparticles, the researchers have developed a series of multicolored systems with the same luminescent behavior, and hope to use them in new-generation photomaterials.


Story Source:

Materials provided by Osaka University. Note: Content may be edited for style and length.


Journal Reference:

  1. Ryosuke Nishikubo, Norimitsu Tohnai, Ichiro Hisaki, Akinori Saeki. Thermoresponsive Emission Switching via Lower Critical Solution Temperature Behavior of Organic-Inorganic Perovskite Nanoparticles. Advanced Materials, 2017; 1700047 DOI: 10.1002/adma.201700047

Cite This Page:

Osaka University. "First luminescent molecular system with a lower critical solution temperature." ScienceDaily. ScienceDaily, 1 May 2017. <www.sciencedaily.com/releases/2017/05/170501094322.htm>.
Osaka University. (2017, May 1). First luminescent molecular system with a lower critical solution temperature. ScienceDaily. Retrieved May 24, 2017 from www.sciencedaily.com/releases/2017/05/170501094322.htm
Osaka University. "First luminescent molecular system with a lower critical solution temperature." ScienceDaily. www.sciencedaily.com/releases/2017/05/170501094322.htm (accessed May 24, 2017).

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