A Combined Experimental and DFT/TD-DFT Investigation of Structural, Electronic, and Cation-Induced Switching of Photophysical Properties of Bimetallic Ru(II) and Os(II) Complexes Derived from Imidazole-4,5-Dicarboxylic Acid and 2,2′-Bipyridine

Experimental results coupled with
computational studies were utilized to investigate the structural
and electronic properties of mixed-ligand bimetallic ruthenium­(II)
and osmium­(II) complexes of composition [(bpy)2M­(Imdc)­M­(bpy)2]+ [M = RuII (1) and M
= OsII (2)], where H3Imdc = imidazole-4,5-dicarboxylic
acid and bpy = 2,2′-bipyridine. The X-ray crystal structures
of both the bimetallic complexes were determined which showed that
compound 1 crystallizes in monoclinic form with space
group P2­(1)/c, while 2 is obtained in orthorhombic form with the space group Pca2­(1). The optimized geometrical parameters for the complexes computed
both in the gas phase and in solution are reported and compared with
the single-crystal X-ray data. The absorption spectra, redox behaviors,
and luminescence properties of the complexes were thoroughly investigated.
The complexes display very intense, ligand-centered absorption bands
in the UV and moderately intense MLCT bands in the visible regions.
While the Ru­(II) complex displays moderately strong luminescence,
the corresponding Os­(II) complex does not luminesce at room temperature.
Both the bimetallic complexes show two successive one-electron reversible
metal-centered oxidations. The effect of alkali, alkaline earth, and
transition metal cations on the absorption and emission spectral behavior
of the complexes has also been studied in detail. As compared to the
luminescence intensities and the quantum yields of the free complexes,
those of the complexes were enhanced substantially in the presence
of selective cations showing cation-induced molecular switching behaviors.
Density functional theory (DFT) and time-dependent DFT (TD-DFT) studies
provide insight into the nature of the ground and excited states with
resulting detailed assignments of the orbitals involved in absorption
and emission transitions. In particular, the blue-shifts of the absorption
and emission bands in the presence of cations are also reproduced
by our calculations.