Department of Chemistry, Faculty of Science and Graduate School of Science, Tohoku University

　Near-infrared light (wavelength: 700~2500 nm) -based technologies have been utilized in various fields such as telecommunications, sensors, and bioanalysis due to its invisibility and high transparency through biological tissues. Today, in most of these applications, silicon-based inorganic semiconductors are used for near-infrared photodetectors, which can respond to light at wavelengths below 1100 nm. Beyond 1100 nm, expensive inorganic materials such as indium gallium arsenide are required. For this reason, organic semiconductors that can absorb near-infrared light are attracting attention as inexpensive alternatives with tunable and easy processability.
　Our laboratory has previously reported that compound 2 (X = S) in Figure (a) functions as an organic semiconductor that absorbs near-infrared light exceeding 1100 nm, in which the central fused-ring substructure highlighted by the bold line is the key to the near-infrared absorption. However, compound 2 showed an electron mobility of about 0.04 cm² V⁻¹ s⁻¹, which is one order of magnitude lower than that of amorphous silicon (0.5 ~ 1 cm² V⁻¹ s⁻¹). Recently, we have newly synthesized analogues 1 and 3, which have oxygen and selenium atoms instead of sulfur atoms in the central part of 2, maintaining the essential skeleton. We found that both 1 and 3 exhibited near-infrared absorption above 1100 nm, similar to 2 (Figure b), and that 1 showed a high electron mobility of 0.33 cm² V⁻¹ s⁻¹, comparable to that of amorphous silicon. Through thorough investigation of the series of compounds, we revealed that the significantly shorter carbon-oxygen bond lengths in the central structure of 1 than the carbon-sulfur and carbon-selenium bond lengths of 2 and 3, respectively, result in higher planarity and rigidity of the molecule, which leads to the improvement in the electron mobility. Based on these findings, we are currently working on further development of near-infrared absorbing organic semiconductors based on these molecular substructures for higher carrier mobility and longer absorption wavelengths.