A new organic-inorganic hybrid oxyfluorotitanate [Hgua]₂• (Ti₅O₅F₁₂) as a transparent UV filter

A new generation UV absorber is obtained by microwave-heating-assisted hydrothermal synthesis: [Hgua]₂•(Ti₅O ₅F₁₂). The structure of this hybrid titanium(IV) oxyfluoride is ab initio determined from powder X-ray data by combining a direct space method, Rietveld refinement [orthorhombic, Cmm2, a = 22.410(1) Å, b = 11.191(1) Å, c = 3.802(1) Å], and density functional theory geometry optimization. The three-dimensional network is built up from infinite inorganic layers _∞(Ti₅O₅F₁₂) separated by guanidinium cations. The theoretical optical gap (3.2 eV) estimated from density of state calculations is in good agreement with the experimental gap (3.3 eV) obtained by UV-vis diffuse reflectivity. The optical absorption is mainly due to O(2p) → Ti(3d) and F(2p) → Ti(3d) transitions at higher energies. The refraction index is low in the visible range (n ≈ 1.9) compared to that of TiO₂ and, consequently, [Hgua]₂•(Ti ₅O₅F₁₂) shows a good transparency adapted to UV shielding. Under UV irradiation at 254 nm for 40 h, the white microcrystalline powder turns to light purple-gray. This color change is caused by the reduction of Ti^IV to Ti^III, confirmed by magnetic measurements.