Photophysical and electrochemical properties of Ru(II) complexes containing tridentate bisphosphino-oligothiophene ligands

Nine Ru(II) complexes containing the conjugated oligothiophene ligands 3,3 `'-bis(diphenylphosphino)-2,2': 5',2 `'-terthiophene (P2T3) and 4', 3'''-bis(diphenylphosphino)-3,3''''-dihexyl-2,2': 5',2 `':5 `',2''':5''',2''''-pentathiophene (P2T5) were prepared and characterized. P2T3 and P2T5 bond as tridentate ligands and three of the complexes (1, 2 and 5) form green five-coordinate Ru(II) complexes in solution. Cyclic voltammetry, variable temperature UV-vis spectroscopy and time-resolved transient absorption spectroscopy were used to characterize the electronic properties of the complexes. Increased conjugation in the complexes containing the P2T5 ligand resulted in a lowering of the oxidation potential of the oligothiophene, but electropolymerization was not observed. The electronic spectra were dominated by pi-pi{*} transitions. All of the complexes were non-emissive both at room temperature and low temperature, indicating the excited state decays by other, non-radiative pathways. The transient absorption spectrum of complex 7 shows a species with a band at 475 nm and a lifetime of similar to 100 ns, assigned to a ligand-based triplet state.