Voltammetry and Spectroelectrochemistry of TCNQ in Acetonitrile/RTIL Mixtures

Abstract

Understanding the solvation and ion-pairing interactions of anionic substrates in room-temperature ionic liquids (RTIL) is key for the electrochemical applications of these new classes of solvents. In this work, cyclic voltammetry and visible and infrared spectroelectrochemistry of tetracyanoquinodimethane (TCNQ) was examined in molecular (acetonitrile) and RTIL solvents, as well as mixtures of these solvents. The overall results were consistent with the formation of RTIL/acetonitrile nanodomains. The voltammetry indicated that the first electrogenerated product, TCNQ−, was not incorporated into the RTIL nanodomain, while the second electrogenerated product, TCNQ2−, was strongly attracted to the RTIL nanodomain. The visible spectroelectrochemistry was also consistent with these observations. Infrared spectroelectrochemistry showed no discrete ion pairing between the cation and TCNQ− in either the acetonitrile or RTIL solutions. Discrete ion pairing was, however, observed in the acetonitrile domain between the tetrabutylammonium ion and TCNQ2−. On the other hand, no discrete ion pairing was observed in BMImPF6 or BMImBF4 solutions with TCNQ2−. In BMImNTf2, however, discrete ion pairs were formed with BMIm+ and TCNQ2−. Density function theory (DFT) calculations showed that the cations paired above and below the aromatic ring. The results of this work support the understanding of the redox chemistry in RTIL solutions.

Ion Pairing versus Solvation of Dinitrobenzene Anions in Room-Temperature Ionic Liquids (RTILs): Vibrational Signatures of RTIL–Substrate Interactions

“…This shoulder at 1596 cm –1 disappeared at high concentrations of the RTIL, which would be consistent with the 1589 cm –1 band due to the dication ion pair. These results are consistent with the infrared spectroelectrochemistry of TCNQ 2– in acetonitrile/RTIL, where general solvation and tight ion pairs were observed depending on the RTIL . Unfortunately, one cannot calculate quantitatively the concentration ratios of general solvation to ion pairs based on intensity ratios because the Raman enhancements for each species may be different.…”

“…For catalysis, one would expect tight ion pairs to stabilize high-energy intermediates (e.g., CO 2 −• ) better than general solvation. This work and our previous work 21 show that RTILs with larger, asymmetric anions (e.g., NTf 2 − ) are better able to form tight ion pairs with anionic substrates than smaller symmetrical anions (e.g., BF 4 − or PF 6 − ).…”

“…Recently, Atifi and Ryan 21 used infrared spectroelectrochemistry to study the reduction of tetracyanoquinodimethane (TCNQ) in mixed molecular/RTIL solutions. The voltammetry showed the presence of ion pairing, especially for the dianion.…”

“…These results are consistent with the infrared spectroelectrochemistry of TCNQ 2− in acetonitrile/RTIL, where general solvation and tight ion pairs were observed depending on the RTIL. 21 Unfortunately, one cannot calculate quantitatively the concentration ratios of general solvation to ion pairs based on intensity ratios because the Raman enhancements for each species may be different. Tian and Jin 37 also examined the second reduction of DNB in acetonitrile using infrared spectroelectrochemistry.…”

Tetraphenylethene Derivatives Bearing Alkylammonium Substituents: Synthesis, Chemical Properties, and Application as BSA, Telomere DNA, and Hydroxyl Radical Sensors

“…The Epc(4) and Epc( 5) values of TPEC(m)N + Me3TCNQ −• (m = 2 and 6) were Epc(4) = -0.20 V and Epc(5) = -0.74 V (vs. Ag + /Ag) and Epc(4) = -0.18 V and Epc(5) = -0.75 V (vs. Ag + /Ag), respectively. These values are lower than the corresponding Epc values of TCNQ (Epc = -0.094 and -0.64 V (vs. Ag + /Ag) [29]) in a solution of tetrabutylammonium…”