Influence of electron donor and acceptor substituents on the excited state properties of rhenium(I) tetracarbonyl chelate complexes

The new complexes [Re(CO)4(meppy)], [Re(CO)4(dmdcb)]PF6 and [Re(CO)4(dmeob)][CF3SO3] [Hmeppy = 3-methyl-2-phenylpyridine, dmdcb = 4,4′-di(methoxycarbonyl)-2,2′-bipyridine and dmeob = 4,4′-dimethoxy-2,2′-bipyridine] have been synthesized. The crystal and molecular structure of [Re(CO)4(meppy)] was determined by X-ray diffraction. High-resolution optical absorption, luminescence and Raman spectroscopy on single crystals and glasses at cryogenic temperatures revealed the lowest excited state for all complexes as a nominally 3π–π* ligand centered state with some 1MLCT character mixed in by spin–orbit coupling. This leads to the occurrence of metal–ligand vibrational sidebands in the absorption and luminescence spectra. The lowest excited state has a charge-transfer character of 1.7, 1.3 and 1.0% for [Re(CO)4(meppy)], [Re(CO)4(dmdcb)]PF6 and [Re(CO)4(dmeob)][CF3SO3], respectively. These results are compared with those for the unsubstituted complexes [Re(CO)4(ppy)] and [Re(CO)4(bpy)]PF6 (Hppy = 2-phenylpyridine and bpy = 2,2′-bipyridine). The influence of the σ-donor group on the energy of the excited states and on the mixing of 1MLCT in the 3LC state is negligibly small. π-Electron donor substituents increase the energy of the excited states and increase the singlet–triplet splittings; π-electron withdrawing groups have the opposite effect and increase the 1MLCT character in the first excited state. The luminescence decay of [Re(CO)4(meppy)] is not single exponential due to a large inhomogeneous distribution of chromophores in the glass. This large distribution is due to the steric interaction of the methyl group with the phenyl part of the ligand.