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(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.