Photophysical characterization of new osmium (II) photocatalysts for hydrohalic acid splitting.

Two osmium(II) photocatalysts bearing a dicationic 4,4′-bis-(trimethylaminomethyl)-2,2′-bipyridine (tmam) ligand and 2,2′-bipyridine {[Os(bpy)2(tmam)]4+} or 4,4′-(CF3)2-2,2′-bipyridine {[Os((CF3)2bpy)2(tmam)]4+} ancillary ligands were synthesized and characterized for application in HX splitting. Iodide titration studies in acetone solutions provided evidence for an in situ formed terionic complex with two iodide ions as evidenced by 1H NMR and UV-visible absorption spectroscopies, as well as by density functional theory calculations and natural bond order analysis. The photocatalyst [Os(bpy)2(tmam)]4+ was shown to be inefficient in iodide oxidation. In contrast, visible light excitation of [Os((CF3)2bpy)2(tmam)]4+ led to rapid iodide oxidation, kq = 2.8 × 1011 M−1 s−1. The data reveal that Os(II) photocatalysts can be fine-tuned for application in HX splitting. [ABSTRACT FROM AUTHOR]