Galvanic replacement of anions in metal-organic semiconductors: a spontaneous redox reaction between TCNQ1− and TCNQF40 (TCNQ = 7,7,8,8-tetracyanoquinodimethane)

Abstract

Galvanic replacement reactions (GRRs) are spontaneous electroless reactions that proceed due to the favorable difference in the electrochemical potentials of the two chemical species participating in the reaction. To date, the reacting species in GRRs almost exclusively have been limited to a metal cation. In this work, for the first time, we provide a rare example of a redox mediated GRR where an anion in the original template is replaced by another anion. Extensive use of spectroscopies, microscopies and electrochemical techniques reveals that the exposure of metal-organic semiconductor of CuTCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) to neutral TCNQF40 (TCNQF4 = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) dissolved in acetonitrile results in the spontaneous oxidation of the TCNQ1– anion in CuTCNQ along with the concomitant reduction of the TCNQF40 to TCNQF41− anion. In this case, the exposure of phase I CuTCNQ grown on a Cu foil (Cu(foil)│CuTCNQ(solid)) to TCNQF40 in acetonitrile results in a hybrid material containing Cu(foil)│CuTCNQx(TCNQF4)y(solid), where x + y = 1. The favorable difference in the redox potentials of the TCNQF41−/0 and TCNQ1−/0 processes in acetonitrile provides a strong driving force to facilitate the GRR between TCNQ− and TCNQF4. The resulting hybrids show superior redox catalytic behavior in the reduction of [Fe(CN)6]3- to [Fe(CN)6]4- by S2O32− in comparison to CuTCNQ and CuTCNQF4 where the TCNQF4 content in the hybrid plays an important role in enhancing the catalytic rate. Mechanistic details related to the role of the Cu(foil)|CuTCNQ(F4) interface, and differences in the solubility of CuTCNQ and CuTCNQF4 in acetonitrile are presented.