Sensitization of nanocrystalline TiO₂ with 3,4,9,10-perylene tetracarboxylic acid

Optical properties of 3,4,9,10-perylene tetracarboxylic acid (PTCA) in the presence of nanocrystalline TiO₂ (nominal diameter 15 nm) have been investigated in ethanol solution and in the solid state by UV–Vis absorption spectroscopy and steady-state and time-resolved fluorescence. Experimental results show that, in ethanol, the presence of TiO₂ nanoparticles causes significant changes in the typical absorption and fluorescence bands of PTCA and in the fluorescence relaxation time. Similar effects are also detected in solid samples, obtained by electrospray deposition technique. The nonlinear optical properties of the PTCA–TiO₂ in ethanol solutions were investigated using the single-beam Z-scan techniques at 532 nm. It was found that, at high TiO₂ nanoparticles content, PTCA exhibits a reverse saturable absorption with significant nonlinear absorption and intensity-dependent negative nonlinear refractive coefficient, indicating self-defocusing behaviour. The third-order nonlinear susceptibility and optical limiting threshold were also determined. Data analysis leads consistently to attribute the observed behaviours to chemical adsorption of PTCA to the TiO₂ surface both in liquid and solid samples. Potentialities of these novel luminescent nanostructured composites as components of dye-sensitized solar cells or components where the nonlinear optical behaviour is requested have been highlighted.