Electronic and optical properties of polypyridylruthenium derivatized polystyrenes: multi-level computational analysis of metallo-polymeric chromophore assemblies

Abstract

Great effort is geared toward investigation of new materials for solar energy conversion in recent years. Polymeric chromophore assemblies consisting of [Ru(bpy)₃]²⁺ complexes attached to a polystyrene backbone have gained considerable interest in recent years because of their structural flexibility combined with their ability to efficiently capture solar energy and transport the captured energy in the form of exciton or charges. We employ a combination of computational methods to examine how opto-electronic properties of [Ru(bpy)₃]²⁺ complexes are influenced by the polymer dynamics in these polymeric chromophore assemblies. The covalent linker between the polymer and the light-absorbing Ru complex is thought to play an important role in optimizing the assemblies for solar energy conversion and transport. We find that the presence of –CH₂– groups in the linker has a significant impact on the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) energies of the pendants. Generally speaking, a longer linker leads to higher HOMO energies. Without the presence of –CH₂– groups, a mixture of cis and trans amide bond in the covalent linker leads to a bimodal distribution for both HOMO and LUMO energies. Importantly, we find that distributions of orbital energies from individual [Ru(bpy)₃]²⁺ pendants have the maximum overlap when there is only one –CH₂– group in the linker. Such an isotropic energy distribution is likely to be important for charge transport within the assemblies. We also find that in contrast to the isolated [Ru(bpy)₃]²⁺ complex, the HOMO is generally found on the linker rather than on Ru atom. This does not change the character of the metal-to-ligand charge transfer (MLCT) excited states, as these excitations in the pendants do not derive from HOMO/LUMO transitions but rather from HOMO − 2/LUMO transition since HOMO − 2 is located on the Ru atom.