Investigation of the Redox and Acid‐Base properties of TCNQF and TCNQF2: Electrochemistry, Vibrational Spectroscopy, and Substituent Effects

Abstract

AbstractThe electrochemistry and acid‐base chemistry of TCNQF and TCNQF2 (TCNQ=7,7,8,8‐tetracyanoquinodimethane) are described, and the results are compared to those reported previously for TCNQ and TCNQF4. In acetonitrile solution, both mono‐ and di‐fluorinated TCNQ derivatives show two well‐resolved, diffusion‐controlled chemically and electrochemically reversible one‐electron‐transfer processes under the conditions of cyclic voltammetry. The reversible potentials of the mono‐ and difluoro‐TCNQ were determined for both the monoanionic and dianionic processes, that is, TCNQF0/1−/2− and TCNQF20/1−/2−. A Hammett plot shows a linear relationship of the potential differences between the three fluorinated derivatives and TCNQ with the sum of the Hammett constants for fluoro substitution. A significant positive shift in the reversible potential is found by increasing the number of fluoro substituents. Although the first TCNQFn0/1− (n=1,2) reduction process is not affected by addition of trifluoroacetic acid (TFA), the dianions are more basic and rapidly protonate to form H2TCNQFn. This explains the significant change detected in the electrochemistry of the second TCNQFn1−/2− redox process. The addition of TFA to solutions of the monoanionic form also gives rise to the disproportionation of TCNQFn1− to TCNQFn and H2TCNQFn, as proven electrochemically and spectroscopically. Furthermore, a higher number of fluoro substituents results in the reduced form of the molecule being more stable, as evidenced by both UV/Vis spectra and voltammetric measurements. Finally, we performed DFT calculations for TCNQF2 and TCNQF2 to produce a spectroscopic library of characteristic IR and Raman bands for these species. These data are used for comparisons with earlier calculations for TCNQF4, and we report the experimental and calculated infrared and Raman spectra for TCNQF and TCNQF2. The vibrational frequencies of the trifluorinated species, TCNQF3, were also calculated to complete the series.