Investigation of the Redox and Acid-Base properties of TCNQF and TCNQF(2): Electrochemistry, Vibrational Spectroscopy, and Substituent Effects

The electrochemistry and acid-base chemistry of TCNQF and TCNQF(2) (TCNQ=7,7,8,8-tetracyanoquinodimethane) are described, and the results are compared to those reported previously for TCNQ and TCNQF(4). 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 TCNQF(2)(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 H(2)TCNQF(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 H(2)TCNQF(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 TCNQF(2) and TCNQF(2) to produce a spectroscopic library of characteristic IR and Raman bands for these species. These data are used for comparisons with earlier calculations for TCNQF(4), and we report the experimental and calculated infrared and Raman spectra for TCNQF and TCNQF(2). The vibrational frequencies of the trifluorinated species, TCNQF(3), were also calculated to complete the series.