Molecular conformation was measured by electron paramagnetic resonance technique

Molecular semiconductor materials have a very long room temperature spin lifetime and have great potential in realizing efficient room temperature spin transport and regulation. Their structural diversity, designability and abundant photoelectric properties also provide a broad space for the development of molecular spintronics. The structure-activity relationship between chemical structure and spin transport properties of molecular semiconductor materials is an important basis for the development of highly efficient spin transport molecular semiconductor materials and the construction of efficient spin devices. The application of electron paramagnetic resonance (ESR) technology in the detection of spin lifetime of molecular materials provides an effective measurement method for the development of this research direction.

Recently, Sun Xiangnan's research group has made new progress in the study of the structure-activity relationship between the molecular conformation of isomers and the spin lifetime of materials by using electron paramagnetic resonance technology. The results were published online in Advanced Materials under the title Structural isomeric effect on spin transport in molecular semiconductors. DOI: 10.1002/adma.202402001.

Molecular semiconductors are typically composed of light elements with low atomic numbers and therefore have weak spin-orbit coupling strength and long spin lifetime, which holds great potential for room-temperature spin transport and applications. The spin-orbit coupling effect dominated by elemental composition is generally considered to be the main factor leading to spin relaxation in molecular semiconductors, which in turn affects the spin lifetime and spin diffusion length of materials. Isomerism is a typical phenomenon of organic semiconductor materials. Since the elemental composition of isomers is exactly the same, it is generally assumed that the spin lifetime and transport performance of isomers should not differ greatly. ITIC and BDTIC are classical commercial small molecule semiconductor materials that are structural isomers of each other in molecular electronics research, with a definite chemical structure and high purity. Based on the study of the spin transport properties of the isomers of ITIC and BDTIC, Sun's group demonstrated for the first time that although the charge transport and molecular packing properties of the two films of ITIC and its structural isomer BDTIC are very similar, their spin transport properties are completely different. Through further electron paramagnetic resonance experiments and density functional theory calculations, it is found that the non-covalent conformation locks formed in BDTIC can increase the spin-orbit coupling on the spin transport path, thereby reducing the spin lifetime. Therefore, this study shows that the influence of structural isomerism must be considered in the development of efficient spin-transport molecular semiconductor materials, which also provides a reliable theoretical basis for solving the great challenge of quantitative measurement of conformational locks in future thin films. In addition, this method also has great potential to be extended to a wider range of molecular science applications, such as molecular phase transitions, aggregation structures and other research fields.

Tingting Yang, PhD, Qin Yang, Special Research Assistant, and Meng Wu, a master's student jointly trained by the National Nanoscience Center and China University of Petroleum (Beijing), are co-first authors of the paper. Lidan Guo, Associate researcher and Xiangnan Sun, Associate researcher of the National Nanoscience Center, are corresponding authors. The research results have been funded by the National Natural Science Foundation of China and the Strategic Pilot Science and Technology Project B of the Chinese Academy of Sciences.