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

ABSTRACT

A new generation UV absorber is obtained by microwave-heating-assisted hydrothermal synthesis: [Hgua](2)·(Ti(5)O(5)F(12)). 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 (∞)(Ti(5)O(5)F(12)) 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 TiO(2) and, consequently, [Hgua](2)·(Ti(5)O(5)F(12)) 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.