The Frenkel exciton Hamiltonian for functionalized Ru(II)–bpy complexes

Electronic excitation energies and wavefunctions are calculated for Ru ( bpy ) 3 + 2 ( bpy = 2 , 2 ′ ‐ bipyridine ) and its two derivatives, where one or two bpy ligands are functionalized with carboxyl and methyl groups. We show that the structure of these molecules allows one to express their excitations in terms of wavefunctions localized on individual ligands via the Frenkel exciton model. The model is based on three parameters – effective single-ligand excitation energy, inter-ligand interaction coupling, and energy shift brought by the ligand functionalization – that are extracted from time-dependent density functional theory (TDDFT). This simple model is able to accurately explain the optical intensity, localization properties, and splitting patterns of the low-energy excited states not only in molecules with a high degree of symmetry, but also in non-symmetrically functionalized Ru(II) complexes in vacuum and in solvent. Such reduced description of the excited states provides better understanding and interpretation of experimental data on Ru–polypyridine complexes, and allows for description of excited-state structure in a large ensemble of interacting molecules and treatment of possible charge and energy transfer phenomena in the material.