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

The 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, TCNQF^0/1−/2− and TCNQF₂ ^0/1−/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 TCNQF_n ^0/1− (n=1,2) reduction process is not affected by addition of trifluoroacetic acid (TFA), the dianions are more basic and rapidly protonate to form H₂TCNQF_n. This explains the significant change detected in the electrochemistry of the second TCNQF_n ^1−/2− redox process. The addition of TFA to solutions of the monoanionic form also gives rise to the disproportionation of TCNQF_n ¹⁻ to TCNQF_n and H₂TCNQF_n, 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.