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

Abstract

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(dmecb)]-[CF3SO3], respectively. These results are compared with those for the unsubstituted complexes [Re(CO)4(ppy)] and [Re(CO)4(bpy J]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.