Radical-to-radical push–pull effect enhances single-molecule conductance in asymmetric diradicals

Abstract

Asymmetry produces a unique modulation of diradical character through the appearance of a push–pull electron spin effect when the two unpaired electrons reside in radical moieties of different electron donor (benzo-1,3-dithiol-2-ylidene)/acceptor (phenoxyl) character. Herein, asymmetric diradical substructures are perpendicularly appended to a linear bisphenyl-thiophene segment whose thiomethyl ends are linked to metal electrodes. Using a scanning tunneling microscope break junction technique, we measured the conductance of these diradicals. In the non-radical parents, the whole conductance is investigated through the linear conjugation path between the thiomethyls. In the diradicals, conductance increases compared to that of their non-radical parents, which is taken as fine-tuning by the presence of unpaired electrons of the primordial linear-path conductance. From a molecular orbital perspective, such enhancement is due to the inclusion of new intragap states (i.e., near the Fermi energy level). From a valence bond perspective, the increase is due to the additional linear conjugation channel provided by the hypervalent state of sulfur resulting from bonding of the two unpaired electrons by a push–pull electron spin effect. Also, there is a reduction of the conductance by enlarging the donor–acceptor moiety with more phenyl groups as a result of the variable weights of the hypervalent sulfur form and of the zwitterionic form. Therefore, the single-molecule junction technique can allow the establishment of relationships between intramolecular radical–radical coupling in off-site diradicals and single-molecule conductance tuning.