Thermal stability of cis-dithiocyanato(2,2′-bipyridyl4,4′dicarboxylate) ruthenium(II) photosensitizer in the free form and on nanocrystalline TiO2 films

Thermal stability of ruthenium (II) complexes of the type [Ru(H2dcbpy)2(NCS)2] 1, (Bu4N)2[Ru(Hdcbpy)2(NCS)2] 2, (Bu4N)4[Ru(dcbpy)2(NCS)2] 3, and (Im)4[Ru(dcbpy)2(NCS)2] 4, where dcbpy=2,2′-bipyridyl-4,4′dicarboxylate, Bu4N+=tetrabutylammonium, and Im=dimethylethylimidazolium, has been studied using thermoanalytical techniques, IR, UV–VIS, and 1H NMR spectroscopic methods. These complexes show remarkable stability in both nitrogen and air atmospheres at high temperatures, ranging from 180°C for 2 to 250°C for 1. The only process that is observed at lower temperatures is the dehydration which occurs between 40 and 110°C. High temperature processes including deamination of the counterion as well as decarboxylation and decomposition of the complex occur between 200 and 400°C with different characteristics in air and in nitrogen. The decarboxylation reaction is an endothermic process in nitrogen atmosphere and overlaps with decomposition of the complexes. In air, on the other hand, it is an exothermic process distinctively separated from decomposition. Higher thermal stability is observed for 1 and 2 when anchored onto nanocrystaline TiO2 films. The activation energy of decarboxylation is estimated for 1 in the free form (ca. 103 kJ mol−1) and on TiO2 (ca. 126 kJ mol−1).