Frederik W. M. Vanhelmont, Melath V. Rajasekharan, Hans U. Güdel, Silvia C. Capelli, Jürg Hauser and Hans-Beat Bürgi
Influence of electron donor and acceptor substituents on the excited state properties of rhenium(I) tetracarbonyl chelate complexes
J. Chem. Soc., Dalton Trans. 2893-2900 (1998)                 

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 ³p-p* 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 s-donor group on the energy of the excited states and on the mixing of ¹MLCT in the ³LC state is negligibly small. p-Electron donor substituents increase the energy of the excited states and increase the singlet-triplet splittings; p-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.