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 ' -d iphenyl-p-(benzyl-1,3,4-oxadiazole)-2,2 ' -bipyridine (DPO-bpy), incorporat ing pi -conjugated oxadiazole linkage units and remote triphenylamine group s has been synthesized and characterized, as have the corresponding mononuc lear metal complexes, fac-(DPO-bpy)Re(CO)(3)Cl and fac-(DPO-bpy)Mn(CO)(3)Br . The rhenium complex was structurally characterized via X-ray crystallogra phy. The nature of the emitting state of each of the complexes was elucidat ed via transient DC photoconductivity (TDCP), a technique that reports on t he sign and magnitude of the difference in dipole moments between the emiss ive 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 stat e. 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 r esults, as well as extended Huckel calculations, support an intraligand cha rge-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. (C) 2001 Published by Elsevier Science B.V.