Quasi-Free Electron States Responsible for Single-Molecule Conductance Enhancement in Stable Radical

• Published in: The journal of physical chemistry letters 2023-05, Vol.14 (17), p.4004-4010
• Main Authors: Yang, Xingzhou, Hou, Songjun, Su, Meiling, Zhan, Qian, Zhang, Hanjun, Quintero, Sergio M., Liu, Xiaodong, Liu, Junyang, Hong, Wenjing, Casado, Juan, Wu, Qingqing, Lambert, Colin J., Zheng, Yonghao
• Format: Article
• Language: English
• Subjects: Physical Insights into Materials and Molecular Properties
• ISSN: 1948-7185; 1948-7185
• DOI: 10.1021/acs.jpclett.3c00536

Stable organic radicals, which possess half-filled orbitals in the vicinity of the Fermi energy, are promising candidates for electronic devices. In this Letter, using a combination of scanning-tunneling-microscopy-based break junction (STM-BJ) experiments and quantum transport theory, a stable fluorene-based radical is investigated. We demonstrate that the transport properties of a series of fluorene derivatives can be tuned by controlling the degree of localization of certain orbitals. More specifically, radical 36-FR has a delocalized half-filled orbital resulting in Breit–Wigner resonances, leading to an unprecedented conductance enhancement of 2 orders of magnitude larger than the neutral nonradical counterpart (36-FOH). In other words, conversion from a closed-shell fluorene derivative to the free radical in 36-FR opens an electron transport path which massively enhances the conductance. This new understanding of the role of radicals in single-molecule junctions opens up a novel design strategy for single-molecule-based spintronic devices.