A group of researchers from Osaka Metropolitan University led by Takuya Ogaki and Hiroshi Ikeda has created two new organic semiconductors. A photochemical reaction - a chemical transformation triggered by light - was used for the synthesis. The results were published in Chemistry-A European Journal.
Organic semiconductors are materials based on carbon compounds that can conduct electric charge while remaining flexible and lightweight. Charge in them is carried by "holes" - conventional positive charges that arise when electrons are lacking. For holes to move quickly, the molecules must be packed densely and in an ordered way.
In the experiment, light caused parts of the molecule to form new carbon-carbon bonds and fold into a compact ring structure. The compound contains sulfur and nitrogen. By changing the lighting conditions, the scientists obtained either a mixture of two isomers - molecules of the same composition but different spatial structure - or only one of them. The isomers were designated as b-DNT (bent) and l-DNT (linear).
Tracing the formation of the structures was not easy: some intermediate particles existed for fractions of a second. To capture them, laser flash photolysis and nanosecond absorption spectroscopy were used - methods that make it possible to "see" short-lived particles by their absorption of light. In addition, quantum-chemical calculations (density functional theory) were performed to explain the structure of the intermediate compounds.
Analysis of the crystal structure showed that both molecules form densely packed crystals held together by sulfur-sulfur and sulfur-nitrogen interactions. However, in the linear isomer l-DNT, neighboring molecules overlap more strongly, meaning the pathways for charge transfer are more efficient. According to the authors' estimates, the predicted hole mobility of l-DNT is about five times higher than that of b-DNT and is comparable to the performance of the best organic semiconductors.
"The combination of clean photochemical synthesis with molecular design that includes a thiazole fragment makes it possible to obtain promising candidates for high-performance organic semiconductors," the university press release quotes the first author of the work, Masato Uenaga, as saying.
However, these are laboratory samples: before commercial application, for example in flexible displays or wearable sensors, the materials still have to undergo testing. The conclusion about the fivefold advantage is based on theoretical calculations, not on direct measurements of finished devices. The study only shows that the linear structure creates better conditions for charge movement.