Exploring\nthe Potential of Al(III) Photosensitizers\nfor Energy Transfer Reactions

Three homoleptic Al(III) complexes (<b>Al1</b>–<b>Al3</b>) with different degrees of methylation at the 2-pyridylpyrrolide\nligand were systematically tested for their function as photosensitizers\n(PS) in two types of energy transfer reactions. First, in the generation\nof reactive singlet oxygen (<sup>1</sup>O<sub>2</sub>), and second,\nin the isomerization of (<i>E</i>)- to (<i>Z</i>)-stilbene. <sup>1</sup>O<sub>2</sub> was directly evidenced by its\ncharacteristic NIR emission at around 1276 nm and indirectly by the\nreaction with an organic substrate [e.g. 2,5-diphenylfuran (DPF)]\nusing <i>in situ</i> UV/vis spectroscopy. In a previous\nstudy, the presence of additional methyl groups was found to be beneficial\nfor the photocatalytic reduction of CO<sub>2</sub> to CO, but here <b>Al1</b> without any methyl groups exhibits superior performance.\nTo rationalize this behavior, a combination of photophysical experiments\n(absorption, emission and excited state lifetimes) together with photostability\nmeasurements and scalar-relativistic time-dependent density functional\ntheory calculations was applied. As a result, <b>Al1</b> exhibited\nthe highest emission quantum yield (64%), the longest emission lifetime\n(8.7 ns) and the best photostability under the reaction conditions\nrequired for the energy transfer reactions (e.g. in aerated chloroform).\nMoreover, <b>Al1</b> provided the highest rate constant (0.043\nmin<sup>–1</sup>) for the photocatalytic oxygenation of DPF,\noutperforming even noble metal-based competitors such as [Ru(bpy)<sub>3</sub>]<sup>2+</sup>. Finally, its superior photostability enabled\na long-term test (7 h), in which <b>Al1</b> was successfully\nrecycled seven times, underlining the high potential of this new class\nof earth-abundant PSs.