In the continuation of this project we will focus on the redox chemistry of quinones to reach a complete understanding of how the quinone redox chemistry is affected by 1) the cycling ion, 2) substitutions on the quinone ring and specific solvent interactions.
The first part, understanding effects of the cycling ion, is motivated by the large difference in experimentally derived reduction potentials of quinones when protons, lithium and non-coordination ions are cycled. We have also seen, experimentally, that effects of substitution on the aromatic ring are quite different for different cycling chemistries. In addition the well-known Hammet correlation for aromatic substitution breaks down in water solution, but not in protic organic solvents, in quinones with electron withdrawing character. These effects need to be understood on a fundamental level in order to allow for rational design of quinone-based conducting redox polymers (CRPs) for battery applications. To that end we will use first-principles theory based on density functional theory (DFT) with specific solvent interactions treated explicitly.
For the second part of the proposal, i.e. to understand charge transport in CRPs, we will now turn our attention towards, the largely overlooked, field of n-type conducting polymers. As a first step we will investigate why phenyl-thiophene, as one of the few examples, show stable n-type conductivity. The working hypothesis is that the phenyl ring energies match the conduction band energy of the native polythiophene. Variations of the substitution pattern on the phenyl ring will allow variation of the energy levels of the phenyl group. The computational results will be compared with experimental results, including data from in-situ UV-vis-IR absorption UV protoelectron spectroscopy and in-situ conductance measurements during electrochemical redox conversion. The results are expected to provide the community with an understanding of the requirements for efficient charge transport in n-doped conducting polymers and for the basis for development of n-type CRPs.
By employing first-principles theory based on density functional theory (DFT), we will investigate how the nature of substituents, cucling ion and solvent affects the redox potential of quinones. The redox potentials in solution will be calculated by combining DFT and self-consistent reaction field methods where the free energies (including all internal energy and entropic contribution) are calculated using the Born–Haber thermodynamic cycle. The explicit solvent effect will be assessed by carrying out ab-initio molecular dynamics (MD) simulations. A sequential MD/DFT scheme will be used to determine the electronic structure at a given temperature. In this approach, snapshots of the MD simulations are selected to carry out high-accurate single point DFT calculations, and subsequently, the obtained electronic structures are averaged. We will explore the Born-Oppenheimer molecular dynamics as implemented in the Vienna Ab-initio Simulation Package (VASP).