Theoretical investigation of solvent and oxidation/deprotonation effects on the electronic structure of a mononuclear Ru-aqua-polypyridine complex in aqueous solution

Author(s)

Franco, Leandro Rezende ; Toledo, Kalil Cristhian Figueiredo ; Matias, Tiago Araujo ; Araujo, C. Moyses ; Araki, Koiti ; Coutinho, Kaline

• Document type: Journal article
• Source: Physical Chemistry Chemical Physics; v. N/A, p. 20-pg., 2023-08-22.

Abstract

Mononuclear polypyridine ruthenium (Ru) complexes can catalyze various reactions, including water splitting, and can also serve as photosensitizers in solar cells. Despite recent progress in their synthesis, accurately modeling their physicochemical properties, particularly in solution, remains challenging. Herein, we conduct a theoretical investigation of the structural and electronic properties of a mononuclear Ru-aqua polypyridine complex in aqueous solution, considering five of its possible oxidation/protonation states species: [RuII(H2O)(py)(bpy)2]2+, [RuII(OH)(py)(bpy)2]+, [RuIII(H2O)(py)(bpy)2]3+, [RuIII(OH)(py)(bpy)2]2+ and [RuIV(O)(py)(bpy)2]2+, where py = pyridine and bpy = 2,2 & PRIME;-bipyridine. At first, we investigate the impact of proton-coupled and non-coupled electron transfer reactions on the geometry and electronic structure of the complexes in vacuum and in solution, using an implicit solvent model. Then, using a sequential multiscale approach that combines quantum mechanics and molecular mechanics (S-QM/MM), we examine the explicit solvent effects on the electronic excitations of the complexes, and compare them with the experimental results. The complexes were synthesized, and their absorption spectra measured in aqueous solution. To accurately describe the QM interactions between the metal center and the aqueous ligand in the MM simulations, we developed new force field parameters for the Ru atom. We analyze the solvent structure around the complexes and account for its explicit influence on the polarization and electronic excitations of the complexes. Notably, accounting for the explicit solvent polarization effects of the first solvation shells is essential to correctly describe the energy of the electronic transitions, and the explicit treatment of the hydrogen bonds at the QM level in the excitation calculations improves the accuracy of the description of the metal-to-ligand charge-transfer bands. Transition density matrix analysis is used to characterize all electronic transitions in the visible and ultraviolet ranges according to their charge-transfer (CT) character. This study elucidates the electronic structure of those ruthenium polypyridyl complexes in aqueous solution and underscores the importance of precisely describing solvent effects, which can be achieved employing the S-QM/MM method. Ru-aqua complex in water, showcasing Ru atom, coordinated water, and hydrogen bonds on left; UV-Vis spectrum and comparison to experiment on right. QM/MM approach emphasized. (AU)

• FAPESP's process: 17/11631-2 - CINE: computational materials design based on atomistic simulations, meso-scale, multi-physics, and artificial intelligence for energy applications
• Grantee: Juarez Lopes Ferreira da Silva
• Support Opportunities: Research Grants - Research Centers in Engineering Program
• FAPESP's process: 18/21489-1 - Supramolecular nanotechnology: design, materials and devices
• Grantee: Henrique Eisi Toma
• Support Opportunities: Research Projects - Thematic Grants
• FAPESP's process: 18/04523-1 - Activation of OER catalysts based on binuclear ruthenium complexes by hydrogen bonding groups
• Grantee: Kalil Cristhian Figueiredo Toledo
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 21/09016-3 - Structural and electronic characterization of biogenic molecules emitted by the Amazon flora
• Grantee: Kaline Rabelo Coutinho
• Support Opportunities: Regular Research Grants
• FAPESP's process: 17/50129-0 - Hybrid nanomaterials and nanosystems for solar fuel production
• Grantee: Koiti Araki
• Support Opportunities: Regular Research Grants