A study of the optical properties of metal-doped polyoxotitanium cages and the relationship to metal-doped titania

To what extent the presence of transition metal ions can affect the optical properties of structurally well-defined, metal-doped polyoxotitanium (POT) cages is a key question in respect to how closely these species model technologically important metal-doped TiO₂. This also has direct implications to the potential applications of these organically-soluble inorganic cages as photocatalytic redox systems in chemical transformations. Measurement of the band gaps of the series of closely related polyoxotitanium cages [MnTi₁₄(OEt)₂₈O₁₄(OH)₂] (1), [FeTi₁₄(OEt)₂₈O₁₄(OH)₂] (2) and [GaTi₁₄(OEt)₂₈O₁₅(OH)] (3), containing interstitial Mn(ii), Fe(ii) and Ga(iii) dopant ions, shows that transition metal doping alone does not lower the band gaps below that of TiO₂ or the corresponding metal-doped TiO₂. Instead, the band gaps of these cages are within the range of values found previously for transition metal-doped TiO₂ nanoparticles. The low band gaps previously reported for 1 and for a recently reported related Mn-doped POT cage appear to be the result of low band gap impurities (most likely amorphous Mn-doped TiO₂).