The closo-Si₁₂C₁₂ molecule from cluster to crystal: Effects of hydrogenation and oligomerization on excited states

Excited state properties of chain and cyclic oligomers of closo-Si₁₂C₁₂ moieties are calculated using time-dependent density functional theory methods. Ultraviolet, visible, and near-infrared photo-absorption properties are described for oligomers that form by linking closo-Si₁₂C₁₂ monomer moieties through Si-Si bonds. Natural transition orbitals for electron and hole states of stationary-state excitons in oligomers were compared to understand how exciton states are influenced by oligomer structure. Depending on the structure, some prominent excited states have large electron-hole charge separation while others do not; some exhibit exciton delocalization while others do not. With increasing oligomer length, the character of the transition between silicon and carbon regions tends to be maintained. And the extent of exciton delocalization and charge separation for an excitation is strongly influenced by the number and types of Si-Si links between oligomer units. We find that cyclic quadramers have spectroscopy properties akin to those of J-aggregates, including the tendency to collapse oligomer excitation transition energies into a narrow single peak. Hydrogenation influences some excited state distributions and energies. Phase behaviors reveal electron state or hole state equivalence in certain molecules that are differently hydrogenated, illustrating the potential for near-resonant exciton transfer between adjacent donor and acceptor species.