Studies on photophysical and electrochemical properties of synthesized hydroxy perylenediimides in nanostructured titania thin films

Four new bis- N-hydroxyalkyl perylenediimide derivatives (PDIs) have been synthesized and were characterized by UV–vis, IR, 1 H and 13 C NMR, FAB Mass, DTA/TGA and CV measurements. Their photophysical properties were determined in solution phase and in sol–gel matrix. The fluorescence life times were measured to be in the range of 2.2–4.7 ns. Both electron acceptor (Co 2+) and donor (anthracence) were used for solution phase fluorescence emission quenching experiments. The calculated quenching rate constants ( k q s) for fluorescence quenching between Co 2+, anthracence, iodide and synthesized hydroxy PDI derivatives were around 10 10 and 10 9 M −1 s −1 in solutions. Quenching experiment of PDI on titania thin film with the aqueous solution of Co 2+ and iodide ion gave the similar quenching rates. The free energies of exothermic photo-electron transfer (ET) processes between PDI and Co 2+ ions [−22.1−(−27.2) kcal mol −1] were approximately two fold of the corresponding value between anthracene and PDI derivatives [−10.9−(−16.4) kcal mol −1] and half of the value between PDI and KI [−44.0−(−46.5) kcal mol −1]. The LUMO energy values, determined by cyclic voltammetry were between −3.5 and −3.7, which have shown two reversible reduction steps (versus Fc) in solution. The precursors used for sol–gel matrix preparation were tetraisopropyl-orthotitanate in TP, and a mixture of GLYMO (30%) and tetraisopropyl-orthotitanate (70%) in TG. In TP type of coatings, the maximum single film thickness was 0.3 μm. TG type of coatings have no such limitation and more advantageous. An efficient ET is being observed between the PDI derivatives immobilised in TG and Co 2+ and KI in water with k q values of around 10 10 and 10 9 M −1 s −1, respectively. The substituted hydroxyl groups in synthesized PDI derivatives are found to increase the solubilities in sol–gel matrix. Overall results prove that bis- N-hydroxyalkyl substituted perylene diimides are suitable for applications in Graetzel type dye sensitized solar cells.