Light harvesting and charge separation in a π-conjugated antenna polymer bound to TiO₂

This paper describes the photophysical and photoelectrochemical characterization of a light harvesting polychromophore array featuring a polyfluorene backbone with covalently attached Ru(II) polypyridyl complexes (PF-Ru-A), adsorbed on the surface of mesostructured TiO₂ (PF-Ru-A//TiO₂). The surface adsorbed polymer is characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, providing evidence for the morphology of the surface adsorbed polymer and the mode of binding. Photoexcitation of the Ru(II) complexes bound to the metal oxide surface (proximal) results in electron injection into the conduction band of TiO₂, which is then followed by ultrafast hole transfer to the polymer to form oxidized polyfluorene (PF⁺). More interestingly, chromophores that are not directly bound to the TiO₂ interface (distal) that are excited participate in site-to-site energy transfer processes that transport the excited state to surface bound chromophores where charge injection occurs, underscoring the antenna-like nature of the polymer assembly. The charge separated state is long-lived and persists for >100 μs, a consequence of the increased separation between the hole and injected electron.