# Origin of Stabilization of Ligand-Centered Mixed Valence Ruthenium Azopyridine Complexes: DFT Insights for Neuromorphic Applications

## TL;DR

This paper uses computational methods to understand how electron transfer and stabilization occur in a ruthenium complex, which could help improve neuromorphic devices.

## Contribution

The study provides new insights into the role of azo ligands in stabilizing mixed-valence states for neuromorphic applications.

## Key findings

- Azo groups in the ligand are key for electronic transport and stabilization of asymmetric states.
- Counterion mobility increases during charge disproportionation, linked to electron redistribution.
- The HOMO–LUMO gap decreases with redox states, indicating enhanced conductivity.

## Abstract

Redox-driven conductance changes are critical processes
in molecular-
and coordination-complex-based memristive thin films and devices that
are envisioned for neuromorphic technologies, but fundamental mechanisms
of conductance switching are not fully understood. Here, we explore
charge disproportionation (CD) processes in [RuIIL2]­(PF6)2 molecular systems that intrinsically
involve interfragment charge transfer (IFCT). Using a combination
of ab initio molecular dynamics simulation (AIMD),
time-dependent density functional theory (TD-DFT), and density functional
theory (DFT) calculations, we investigate the electron transfer mechanisms
and the roles of temperature and cell volumetric expansion in facilitating
the counterion movements and electronic transitions required for low-cost
IFCT and charge redistribution. A detailed analysis of the density
of states and TD-DFT calculations highlights that unpaired electrons
play a crucial role in low-energy transitions, with the azo (NN)
groups of the ligand serving as the primary sites for electronic transport
between molecular fragments, further stabilizing the asymmetric state.
Localization of added electrons on azo ligands occurs with negligible
change at the Ru centers, supported by atomic volume expansions up
to +4.74 bohr3, and goes along with a progressive reduction
of the HOMO–LUMO gap across redox states, suggesting enhanced
conductivity. The TD-DFT analysis reveals a dominant IFCT excitation
at 2082.76 nm in the doubly reduced (22) state, while a stabilization
energy of 1.20 eV of the asymmetric (13) state relative to the symmetric
(22) state is predicted by constrained DFT. Periodic DFT and AIMD
simulations emulating a molecular film show that the stabilization
of the asymmetric state, relative to a symmetric one, translates in
net charge separation values (order of ∼0.33 e) that are strongly
linked to increased counterion mobility (average counterion displacements
exceeding 0.7 Å per atom during CD events) and the involvement
of azo groups in electron redistribution. These findings, which align
with previously reported experimental and computational data, provide
key insights into the IFCT mechanisms and electronic transport facilitated
by azo groups, with important implications for redox-driven memristive
and neuromorphic technologies.