Tetrathiafulvalene-Tetracyanoquinodimethane Charge-Transfer Complexes Wired to Carbon Surfaces: Tuning of the Degree of Charge Transfer

Charge-transfer complexes involving tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) derivatives are engineered in a 2D arrangement onto a carbon surface through the exposure of immobilized TTF units to TCNQ compounds. TTF molecules were immobilized as robust monolayers on carbon surfaces using the electrografting method followed by a click chemistry coupling. When the TTF monolayer is exposed to TCNQ, TCNQF 2 (2,5-difluoro-TCNQ), and TCNQF 4 (2,3,5,6-tetrafluoro-TCNQ), strong donor–acceptor complexes are formed onto the surface. A considerable decrease of the electrochemical response accompanies the formation of the charge-transfer complex. This observation is rationalized by the analysis of original crystal samples using an ultramicroelectrode cavity, confirming that charge-transfer complexes are electrochemically silent. A fine control of the degree of charge transfer with the judicious choice of different acceptors is evidenced through electrochemical and X-ray photoelectron spectroscopy (XPS) measurements. Thus, donor–acceptor complexes of different natures are formed. A fully ionic charge-transfer complex is formed upon exposure of the TTF monolayer to the more oxidizing TCNQF 4, while a neutral complex is obtained after exposure to the less oxidizing TCNQ. Exposition of the TTF monolayers to the intermediate TCNQF 2 yields a mixture of neutral-ionic systems. These donor–acceptor interactions that fully mimic those described in the solid state are rarely described in such a 2D arrangement, with systems being directly wired to an electrode.