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

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 [Ru^IIL₂](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.

1

Molecular structure of [Ru^IIL₂]­(PF₆)₂.

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a) Optimized gas-phase structures at the PBE/LANL2DZ Level and AIM Atomic Charge Distribution for the [Ru^IIL₂]­(PF₆)₂/State 0 complex during successive reduction processes and dimer formation. b) Color-coded representation of AIM atomic charge distribution for States 0, 1, and 2 of the [Ru^IIL₂]­(PF₆)₂ complex, illustrating the evolution of electron density as additional electrons are introduced. The azo (−NN−) groups in the ligand exhibit a pronounced increase in electron density (blue regions) during the transition between the states.

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Comparison of HOMO–LUMO energy levels in a) State (00), b) State (11) and c) State (22), showing the effects of counterions and electron addition. HOMO = highest occupied molecular orbital; LUMO = lowest unoccupied molecular orbital.

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a) Structure and spin density of States (22) represented with an isosurface value of 0.002 au (e⁴/bohr⁴) (dark pink). b) Theoretical TD-DFT UV–vis spectrum for a pair of doubly reduced molecules, M₁ and M₂, associated with the symmetric and uniform redox state (22). c) Relative contributions of each transition to the total excited state, providing insight into the symmetry breaking that leads to the formation of the asymmetric (13) state through charge disproportionation in the film.

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Transition atomic charges during interfragment charge transfer (IFCT) from State (22) to excited State (13), showing the redistribution of electron density during the initial step of charge disproportion. The heat maps display positive atomic charges in red and negative charges in blue. Each small rectangle represents an individual atom, while each row corresponds to all atoms of a specific element present in the system. At the bottom, an expanded view highlights the charge transfer regions within each molecular fragment.

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a) Cell structure and atomic distribution of four molecules M = [Ru^IIL₂]­(PF₆)₂ in State (22) at the key event time of 3527.0 fs. b) Average counterion displacement ΔP­(t) during AIMD: Evidence of IFCT between M1 and M3. Superimposed curves show the total AIM charges of ligands coordinated to M1 and M3, revealing a clear IFCT. The black arrow at 3527 fs highlights a point where counterion displacement correlates with charge disproportionation.