Equilibrium Solvation, Electron-Transfer Reactions, and Stokes-Shift Dynamics in Ionic Liquids

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Room-temperature ionic liquids (RTILs) exhibit complex solvent response crucial for chemical reactions.
• Existing dielectric theories of solvation often fail to capture the unique dynamics of RTILs.
• Understanding charge density fluctuations is key to developing accurate solvation theories for RTILs.

Purpose of the Study:

• To formulate a microscopic theory of solvent response in RTILs based on dynamic longitudinal susceptibility.
• To investigate the relationship between charge-density fluctuations, solvation dynamics, and electron transfer.
• To challenge and refine existing solvation theories for ionic liquids.

Main Methods:

• Formulation of a microscopic theory using dynamic longitudinal susceptibility of liquid's charge density.
• Analysis of charge-density structure factors and intermediate scattering functions via molecular dynamics simulations.
• Calculation of Stokes-shift dynamics for coumarin-153 and nonergodic reorganization energy for electron transfer.

Main Results:

• Identified two distinct timescales for charge-density fluctuations: fast stretched-exponential and slow exponential dynamics.
• Observed de Gennes narrowing phenomenon for slower relaxation, with relaxation time peaking at the structure factor's first peak.
• Demonstrated that charge density structure significantly contributes to solvation free energy, invalidating purely dielectric approaches.
• Showed that fast electron-transfer reactions are unaffected by slow charge dynamics, but reorganization energy is reaction-rate dependent.

Conclusions:

• The new theory accurately describes solvent response and solvation dynamics in RTILs, incorporating structural and dynamic aspects.
• The sharp peak in charge density structure significantly influences solvation free energy, necessitating microscopic theories.
• The reaction-rate dependence of reorganization energy impacts the energy-gap law for electron transfer, potentially altering inverted regions.