Effect of ion–ligand binding on ion pairing dynamics studied by two-dimensional infrared spectroscopy

Abstract

Cation-specific ion pairing dynamics between M (M = Ag or Cu) and SCN in N,N-dimethylthioformamide (DMTF) are studied by probing the nitrile (CN) stretching vibration. The SCN ion, which is an ambidentate ligand, readily associates with cations to form two different types of contact ion pairs (CIPs) (i.e., M-SCN or M-NCS) and its CN stretching frequency is significantly blue-shifted so that free SCN and CIPs can be well-distinguished in the FTIR spectra. Interestingly, Ag ions prefer the formation of Ag-SCN in DMTF (Ag + SCN ⇋ Ag-SCN) but Cu ions form Cu-NCS (Cu + SCN ⇋ Cu-NCS). We have studied the effect of ion-ligand binding on the ion pairing equilibria and dynamics in great detail by using FTIR, IR pump-probe (IR PP), and two-dimensional infrared (2DIR) spectroscopy combined with quantum chemical calculations. First, our quantum chemical calculations corroborate that Ag-SCN and Cu-NCS of the two possible CIP configurations (M-SCN or M-NCS) are energetically stable and favored in DMTF. Second, the thermodynamic properties (ΔH and ΔS) of ion pairing equilibria are determined by temperature-dependent FTIR experiments. Finally, IR PP and 2DIR experiments are used to measure the association and dissociation rate constants. The ion pairing dynamics between Cu and SCN are found to occur on much faster timescales than those between Ag and SCN. Our current results provide important insights into understanding the effect of ion-ligand binding on the ion pairing equilibria and dynamics in polar solvents.

Vibrational Relaxation Dynamics of a Semiconductor Copper(I) Thiocyanate (CuSCN) Film as a Hole-Transporting Layer

“…As a result, they can vibrate in unison and thus form delocalized states, called vibrational excitons . Therefore, it is reasonable to assign the relatively small and broad peak centered at around 2150 cm –1 to the delocalized vibrational mode formed by CN stretching because it disappears in the FTIR spectrum of the CuSCN solution . The UV–vis spectrum of the CuSCN film on the CaF 2 substrate is shown in Figure C.…”

“…However, all of the SCN groups are parallel to each other in the CuSCN film, which is different from the case in the reverse micelle confined system. The distinct observation shown in Figure A indicated that the rotational dynamics of the CN stretching mode in the CuSCN semiconductor film should be very different from that observed in the CuSCN solution or the KSCN film …”

“…The ultrafast thermalization process in the CuSCN film followed by the photoinduced carrier dynamics is clearly observed, which should affect the charge-transport property in the HTL mediated by the thermal activation. On the other side, the orientation of the CuSCN and defects in the surface should also play an important role, controlling the path of the vibrational relaxation dissipation channel of SCN stretching in the CuSCN film. , The vibrational relaxation dynamics of CuSCN dissolved in the organic solvent did not show any abnormal anisotropy decay . Thus, the unusual anisotropy decay of CN stretching in CuSCN should be a characteristic behavior in the semiconductor.…”

“…30 Therefore, it is reasonable to assign the relatively small and broad peak centered at around 2150 cm −1 to the delocalized vibrational mode formed by CN stretching because it disappears in the FTIR spectrum of the CuSCN solution. 31 The UV−vis spectrum of the CuSCN film on the CaF 2 substrate is shown in Figure 1C. It is clear that the absorption peak is located at around 300 nm.…”

Oligomer Electrolytes for Light-Emitting Electrochemical Cells: Influence of the End Groups on Ion Coordination, Ion Binding, and Turn-on Kinetics

“…We also observe that the free anion peak is located ∼2 cm –1 lower for the electrolyte with TMPE-OH than for the other electrolytes because of interactions between the anions and the hydroxyl end groups of the ion transporter, as previously reported for other hydrogen-bonding systems . We emphasize that the different ionic species detected with FTIR exist in a dynamic equilibrium and that the same ion can be expected to rapidly shift from being free to aggregated on a short time scale, as also indicated by the single 7 Li signal in the NMR experiments.…”