Exploring the Potential of Al(III) Photosensitizers for Energy Transfer Reactions.

Three homoleptic Al(III) complexes (-) with different degrees of methylation at the 2-pyridylpyrrolide ligand were systematically tested for their function as photosensitizers (PS) in two types of energy transfer reactions. First, in the generation of reactive singlet oxygen (O), and second, in the isomerization of ()- to ()-stilbene. O was directly evidenced by its characteristic NIR emission at around 1276 nm and indirectly by the reaction with an organic substrate [e.g. 2,5-diphenylfuran (DPF)] using UV/vis spectroscopy. In a previous study, the presence of additional methyl groups was found to be beneficial for the photocatalytic reduction of CO to CO, but here without any methyl groups exhibits superior performance. To rationalize this behavior, a combination of photophysical experiments (absorption, emission and excited state lifetimes) together with photostability measurements and scalar-relativistic time-dependent density functional theory calculations was applied. As a result, exhibited the highest emission quantum yield (64%), the longest emission lifetime (8.7 ns) and the best photostability under the reaction conditions required for the energy transfer reactions (e.g. in aerated chloroform). Moreover, provided the highest rate constant (0.043 min) for the photocatalytic oxygenation of DPF, outperforming even noble metal-based competitors such as [Ru(bpy)]. Finally, its superior photostability enabled a long-term test (7 h), in which was successfully recycled seven times, underlining the high potential of this new class of earth-abundant PSs.