Interfacial ion pairing at the interface between water and a room-temperature ionic liquid, N-tetradecylisoquinolinium bis(pentafluoroethylsulfonyl)imide

Potential dependence of the ionic structure at the ionic liquid/water interface studied using MD simulation

The structure at the electrochemical liquid/liquid interface between water (W) and trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, a hydrophobic ionic liquid (IL), was studied using molecular dynamics (MD) simulation in which the interfacial potential difference was controlled. On the IL side of the IL/W interface, ionic multilayers were found in the number density distribution of IL ions whereas monolayer-thick charge accumulation was found in the charge density distribution. This suggests that the potential screening is completed within the first ionic layer and the effect of overlayers on the potential is marginal. The W side of the interface showed the diffuse electric double layer as expected, and unexpectedly unveiled a density depletion layer, indicating that the IL surface is hydrophobic enough to be repelled by water. The IL ions in the first ionic layer showed anisotropic orientation even at the potential of zero charge, in which the polar moieties were oriented to the W side and the non-polar moieties preferred parallel orientation to the interface. When an electric field is applied across the interface so that the IL ions are more accumulated, the non-polar moieties changed the parallel preference to more oriented to the IL side due to the dipolar nature of the IL ions. The ionic orientations at the IL/W interface were compared with those at other two IL interfaces, the vacuum and graphene interfaces of the IL. The parallel preference of the non-polar moieties was similar at the IL/graphene interface but different from the perpendicular orientation toward the vacuum side at the IL/vacuum interface. The comparison suggests that water behaves like a wall that repels IL ions like a solid electrode.

Kinetics of Excimer Electrogenerated Chemiluminescence of Pyrene and 1-Pyrenebutyricacid 2-Ethylhexylester in Acetonitrile and an Ionic Liquid, Triethylpentylphosphonium Bis(trifluoromethanesulfonyl)imide

We describe the kinetics of excimer electrogenerated chemiluminescence (ECL) of a liquid pyrene derivative, 1-pyrenebutyricacid 2-ethylhexylester (PLQ) dissolved in a molecular solvent, acetonitrile (MeCN), and an ionic liquid, triethylpentylphosphonium bis(trifluoromethanesulfonyl)imide ([P₂₂₂₅][TFSI]). Pyrene was also used for comparison. To discuss the kinetics of the excimer ECLs, the photophysical and electrochemical properties and electronic states of PLQ and pyrene were revealed. The photoluminescence (PL) spectra, rate constants for the radiative transitions, and redox potentials of PLQ and pyrene dissolved in MeCN and [P₂₂₂₅][TFSI] suggest that as a solvent, [P₂₂₂₅][TFSI] behaves more polar than MeCN. By analyzing the PL decay curves, the rate constants to form the excimer were determined to be on the order of 10⁹ and 10⁷ M^-1 s^-1 in MeCN and [P₂₂₂₅][TFSI], respectively, which were limited by the diffusion. For neat PLQ (1.6 M), a delay of 0.3-0.4 ns for the excimer emission compared to the monomer emission was observed. It is likely that the delay corresponds to the timescale for arranging the conformation to form the excimer. The ECL of PLQ was generated by applying a square wave voltage to produce the radical anion and cation, and on the ECL spectra, the excimer emission was more prevailed compared to the PL spectra. Kinetic analysis for the electron transfer reaction between the radical ions based on Marcus theory indicates that the electron transfer is limited by the diffusion of the radical ions. Moreover, the electron transfer distance (d_et) between the radical cation and anion to generate excited states was calculated with a framework of the theory. Kinetically, the electron transfer can take place at d_et < ∼11 Å in MeCN and d_et < ∼12 Å in [P₂₂₂₅][TFSI]. The density functional theory (DFT) and time-dependent DFT calculations show that the potential energy curve of the excimer against the distance between the pyrene rings reaches a minimum at 3.50 Å. This suggests that through the electron transfer, the process of the direct formation of the monomer S₁ state followed by the excimer formation is more prevailed than that of the direct excimer formation.