Structure-Based Model for Solvatochromism: Charge Transfer Driven by Ligand-Solvent Interactions

A general approach to model the influence of solvation on the structure and charge-transfer properties of solvatochromic complexes is reported. For validation, this approach was applied to known solvatochromic Mo-dithiolene complexes containing both reduced (Dt2-) and oxidized (Dt0) dithiolene ligands. Large structure libraries were generated using PCM-DFT optimizations, utilizing all implicit solvent options in the Gaussian16 package, and then PCM-TD-DFT calculations were performed on optimized structures to relate changes in conformation to experimental LL'CT bands. Experimental and theoretical data were combined to model the structure of the solvated minima in each solvent. Geometry optimizations revealed a dielectric constant (ε)-dependent Dt0 conformation in which more polar implicit solvents favored structures where the Dt0 ligand folded away from the Mo center, and excited-state calculations show that such structures produced lower-energy ligand-to-ligand charge-transfer bands (affording a mechanism for solvatochromism). Polar solvents stabilize the electron-deficient Dt0 ligand to fold away from the Mo dxy orbital, which better aligns donor and acceptor orbitals to facilitate interligand charge transfer. The solvent-dependent Dt0 conformations are also supported experimentally as solvent-responsive 13C NMR thione resonances. This approach can generally be applied to more accurately calculate solvated minima for complexes with solvent-dependent structural features.