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

Yang, Xingzhou and Hou, Songjun and Su, Meiling and Zhan, Qian and Zhang, Hanjun and Quintero, Sergio M. and Liu, Xiaodong and Liu, Junyang and Hong, Wenjing and Casado, Juan and Wu, Qingqing and Lambert, Colin J. and Zheng, Yonghao (2023) Quasi-Free Electron States Responsible for Single-Molecule Conductance Enhancement in Stable Radical. The Journal of Physical Chemistry Letters, 14 (17). pp. 4004-4010. ISSN 1948-7185

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Abstract

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 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 ( ). In other words, conversion from a closed-shell fluorene derivative to the free radical in 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.