Multichromophoric Bimetallic Ru(II) Terpyridine Complexes Based on Pyrenyl-bis-phenylimidazole Spacer: Synthesis, Photophysics, Spectroelectrochemistry, and TD-DFT Calculations

A symmetrical bridging ligand, 5,11-bis(4-([2,2':6',2?-terpyridine]-4'-yl)phenyl)-4,12-dihydropyreno[4,5-d:9,10-d']diimidazole (tpy-H2PhImzPy-tpy), containing terpyridyl coordinating units connected via a pyrenyl-bis-phenylimidazole spacer have been designed to synthesize a new class of light harvesting bimetallic Ru(II) complexes. The electronic properties of this complexes can be fine-tuned by varying tridentate terminal ligands. Full characterization of the compounds has been done with the help of (1)H NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. Geometry optimization of the complexes was also carried out with density functional theory (DFT). Electronic absorption spectra exhibit a number of very intense pi-pi* and n-pi* transitions in the UV and moderately intense MLCT and ILCT transitions in the visible region. Interestingly, the present bimetallic complexes exhibit moderately strong luminescence in the range between 657 and 703 nm and lifetimes (long component) between 5.8 and 67.0 ns at room temperature showing the dependence of the emission characteristics upon the type of terminal ligand and solvent. Detailed temperature-dependent emission measurements showed that an overall enhancement of photoluminescence intensity and lifetime occur in all three cases upon lowering of temperature. The redox behavior of the compounds is characterized by a single reversible anodic wave corresponding to two closely spaced one-electron processes. The appearance of intervalence charge transfer transition (IVCT) bands in the NIR region on electrochemical generation of (RuRuII)-Ru-II/(RuRuIII)-Ru-II species indicates the presence of substantial electronic communication among the two ruthenium centers in the bimetallic complexes. DFT and TDDFT calculations were also done for better understanding of the absorption and emission spectral characteristics of the complexes.