Photoinduced energy and electron transfer reactions in lamellar polyanion/polycation thin films: Toward an inorganic 'leaf'

Sequential adsorption of polyanions and polycations was used to make a five-component energy/electron-transfer cascade, which mimics some of the functions of natural photosynthetic assemblies. The photon antenna part of the system consists of coumarin- and fluorescein-derivatized poly(allylamine hydrochloride) (Coum-PAH and Fl-PAH), palladium(II)tetrakis(4-N,N,N- trimethylanilinium) porphyrin (PdTAPP⁴⁺ or palladium(II)tetrakis(4- sulfonatophenyl) porphyrin (PdTSPP^4-) layers, interleaved with anionic Zr(HPO₄)₂ · H₂O (α-ZrP) sheets. α-ZrP or HTiNbO₅ sheets separate the porphyrin electron donor from a polyviologen electron acceptor layer. Layer- by-layer growth of these thin film assemblies was characterized by atomic force microscopy (AFM) and ellipsometry on planar supports, and by elemental analysis, surface area measurements, and transmission electron microscopy high on surface area silica supports. UV-vis absorption and steady-state emission spectroscopies showed that the overall energy/electron-transfer reaction (Coum → Fl → PdTSPP^4- → viologen) occurs with approximate quantum yields of 0.47 and 0.61 for systems containing α-ZrP and HTiNbO₅ sheets, respectively. Transient diffuse reflectance spectroscopy established that a porphyrin-viologen charge separated state is formed in the reaction, and that it has an exceptionally long-lived component (τ ≃ 900 μs) with the HTiNbO₅ spacer. It is inferred that the semiconducting HTiNbO₅ sheets play an active role in relaying the electron from photoexcited PdTSPP^4- to the viologen electron acceptor.