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

Three homoleptic Al(III) complexes (<b>Al1</b>-<b>Al3</b>) 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 (¹O₂), and second, in the isomerization of (<i>E</i>)- to (<i>Z</i>)-stilbene. ¹O₂ 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 <i>in situ</i> UV/vis spectroscopy. In a previous study, the presence of additional methyl groups was found to be beneficial for the photocatalytic reduction of CO₂ to CO, but here <b>Al1</b> 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, <b>Al1</b> 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, <b>Al1</b> provided the highest rate constant (0.043 min^-1) for the photocatalytic oxygenation of DPF, outperforming even noble metal-based competitors such as [Ru(bpy)₃]²⁺. Finally, its superior photostability enabled a long-term test (7 h), in which <b>Al1</b> was successfully recycled seven times, underlining the high potential of this new class of earth-abundant PSs.