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Add a twist to π-molecules: A new design strategy for organic semiconductor materials

A research team has synthesized three-dimensionally shaped molecules containing an internal twist and shown that they possess the properties of organic semiconductors. By introducing methyl groups into a planar molecule containing several thiophene units and forcing it into a twisted conformation, the team created a solid-state structure in which electricity can flow three-dimensionally.

The molecule was verified to act as an organic semiconductor in an organic field-effect transistor, paving the way for next-generation electronic devices.

The results were online in Chemical Communications.

Electronic devices based on organic materials are lightweight, flexible, and can display a wide variety of properties by subtle changes in molecular structure, making them promising environmentally friendly next-generation devices. Most organic electronic materials developed to date are made of planar molecules, so charge transport is confined to limited directions; as a result, devices require strict control of molecular orientation.

The team, led by Associate Professor Yasushi Segawa, graduate students Mai Nagase (at the time of the research) and Rui Yoshida, and technical staff member Sachiko Nakano of the Institute for Molecular Science (IMS) and SOKENDAI (The Graduate University for Advanced Studies), together with Associate Professor Takashi Hirose of Kyoto University's Institute for Chemical Research, wondered whether "twisting" molecules could yield a new material architecture in which charge carriers move easily in three dimensions.

The researchers attached methyl groups to molecules containing multiple thiophene units, thereby synthesizing twisted molecules.

X-ray crystallography confirmed the twisted geometry and revealed that, in the solid state, the molecules stack in a three-dimensional fashion. Computational analysis of charge-transport pathways predicted an aggregated structure in which holes can migrate in several directions.

When the molecule was fabricated into an organic field-effect transistor, it exhibited a hole mobility of 1.85 × 10^-4 cm² V^-1 s^-1, experimentally confirming its behavior as an organic semiconductor.

This work offers a new perspective on designing organic electronic materials: in addition to flat molecules, twisted molecules can also be exploited. The strategy may solve the long-standing problem of orientation control in devices. The findings are expected to spur the development of higher-performance organic semiconductors that employ such three-dimensional molecular architectures.