PNNL

Abstract

Linear cyanide-bridged polymetallic complexes, which undergo photoinduced metalto- metal charge transfer (MMCT), represent prototypical systems for studying long-range electron-transfer reactions and understanding the role played by specific solute–solvent interactions in modulating the excited state dynamics. To tackle this problem, while achieving a statistically meaningful description of the solvent and of its relaxation, one needs a computational approach capable of handling large polynuclear transition metal complexes, both in their ground and excited states, as well as the ability to follow their dynamics in several environments up to nanosecond timescales. Here, we present a mixed quantum classical approach, which combines large-scale molecular dynamics (MD) simulations, based on an accurate quantum mechanically derived force field (QMD-FF) and self-consistent QMD polarized point charges, with IR and UV-Vis spectral calculations to model the solvation dynamics and optical properties of a cyano-bridged trinuclear mixed valence compound (trans-[(NC)5FeIII(µ-CN)RuII(pyridine)4(µ-NC)FeIII(CN)5]4-). We demonstrate the reliability of the QMD-FF/MD approach in sampling the solute conformational space and capturing the local solute-solvent interactions by comparing the results with higher-level QM-MM/MD reference data. The IR spectra calculated along the classical MD trajectories in different solvents correctly predict the red-shift of the CN stretching band in the aprotic medium (acetonitrile) and the subtle differences measured in water and methanol, respectively. By explicitly including the solvent molecules around the cyanide ligands and calculating the thermal averaged absorption spectra using TD-DFT/TDA calculations, the experimental solvatochromic shift is quantitatively reproduced going from water to methanol, while it is overestimated for acetonitrile. This discrepancy can likely be traced back to the lack of important dispersion interactions between the solvent cyano groups and the pyridine substituents in our microsolvation model. The proposed protocol is applied to the ground state in water, methanol, and acetonitrile and can be flexibly generalized to study excited state non-equilibrium solvation dynamics.