Radical-induced single-molecule conductance tuning in 9,9'-bifluorenylidene derivatives

Single-molecule techniques provide new perspectives for understanding the relationship between spin delocalization of organic radicals and the intramolecular electronic structure. In this study, a series of 9,9'-bifluorenylidene (9,9'-BF) derivatives with four functionalization sites were synthesized, showcasing the orthogonalization of non-conducting (between radical sites) and conducting (between thiomethyl groups) paths. By precisely controlling the amount of radicals, or radical injection, ranging from mono-radical (Mono-PFPR) to diradicals, and among these, varying from medium (Di-PFPRy ₀ = 0.66) to small (Di-PFNRy ₀ = 0.11) to vanishing (Di-NFNRy ₀ = 0) diradical characters (y ₀ represents the diradical index), the influence of organic radical spin delocalization on the conducting path can be gradually modulated, transforming linear conjugated conducting channels into cross-conjugated channels and significantly reducing single-molecule conductance. This discovery provides an in-depth understanding of the complex relationship between radicals, spin delocalization, and molecular conductance, which is rather unique in the area of functional stable radical compounds. Ultimately, it provides forward-looking guidance for research on these materials in the field of organic electronic materials.