FTIR Spectroelectrochemistry of F4TCNQ Reduction Products and Their Protonated Forms

The tetrafluorinated derivative of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is of interest for charge transfer complex formation and as a p-dopant in organic electronic materials. Fourier transform infrared (FTIR) spectroscopy is commonly employed to understand the redox properties of F4TCNQ in the matrix of interest; specifically, the ν(C≡N) region of the F4TCNQ spectrum is exquisitely sensitive to the nature of the charge transfer between F4TCNQ and its matrix. However, little work has been done to understand how these vibrational modes change in the presence of possible acid/base chemistry. Here, FTIR spectroelectrochemistry is coupled with density functional theory spectral simulation for study of the electrochemically generated F4TCNQ radical anion and dianion species and their protonation products with acids. Vibrational modes of HF4TCNQ^-, formed by proton-coupled electron transfer, are identified, and we demonstrate that this species is readily formed by strong acids, such as trifluoroacetic acid, and to a lesser extent, by weak acids, such as water. The implications of this chemistry for use of F4TCNQ as a p-dopant in organic electronic materials is discussed.