Synthesis and photophysical properties of luminescent rhenium(I) and manganese(I) polypyridine complexes containing pendant 1,3,4-oxadiazole/triarylamine assemblies

A strongly absorbing and significantly luminescing diimine ligand, N, N′-diphenyl-p-(benzyl-1,3,4-oxadiazole)-2,2′-bipyridine (DPO-bpy), incorporating π-conjugated oxadiazole linkage units and remote triphenylamine groups has been synthesized and characterized, as have the corresponding mononuclear metal complexes, fac-(DPO-bpy)Re(CO)₃Cl and fac-(DPO-bpy)Mn(CO)₃Br. The rhenium complex was structurally characterized via X-ray crystallography. The nature of the emitting state of each of the complexes was elucidated via transient DC photoconductivity (TDCP), a technique that reports on the sign and magnitude of the difference in dipole moments between the emissive state and ground state. The TDCP signal is negative for the Re complex and is consistent with emission from a metal-to-ligand charge-transfer state. On the other hand, the TDCP signals for the Mn complex and for the free ligand are positive, with the complex yielding the larger value. The TDCP results, as well as extended Hückel calculations, support an intraligand charge-transfer assignment for the emissive states of both the Mn complex and the free ligand, with the former displaying a significantly higher emission quantum yield.