A study on delocalization of MLCT excited states by rigid bridging ligands in homometallic dinuclear complexes of ruthenium(II)

For the structurally rigid homometallic dinuclear complexes (ttp)Ru(tpy-tpy)Ru(ttp)⁴⁺ and (ttp)Ru(tpy-ph-tpy)Ru(ttp)⁴⁺, we have obtained ground-state absorption spectra and transient-absorption difference spectra at room temperature and luminescence spectra and lifetimes in the temperature interval from room temperature to the rigid matrix (90 K); the solvent was acetonitrile or butyronitrile (tpy is 2,27prime;:6′,2″-terpyridine, ttp is 4′-p-tolyl-2,2′:6′,2″-tpy, and ph is 1,4-phenylene). The gathered spectroscopic data indicate that after absorption of visible light, formation of the luminescent metal-to-ligand charge transfer (MLCT) excited states takes place, which involves the bridging ligand (BL). Since we found that (ttp)Ru(tpy-tpy)Ru(ttp)⁴⁺ is a good luminophore (λ_max = 720 nm, Φ = 4.7 x 10^-3, and τ = 570 ns) while both (ttp)Ru(tpy-ph-tpy)Ru(ttp)⁴⁺ (λ_max = 656 nm, Φ = 1.1 x 10^-4, and τ = 4 ns) and the reference mononuclear complex Ru(ttp)₂²⁺ (λ_max = 640 nm, Φ = 3.2 x 10^-5, and τ = 0.9 ns) are not, we have explored the effects brought about by the delocalization and energy content of the luminescent state. The study of the temperature dependence of the luminescence lifetimes indicates that two main nonradiative paths, i and ii, are responsible for deactivation of the luminescent state. Path i directly connects the luminescent and ground states; within the frame of the "energy-gap law", vibronic analysis of low-temperature luminescence profiles enables one to correlate the delocalization of the M → BL CT state and the extent of structural distortions occurring at the accepting ligand. Thermally activated decay via a metal-centered (MC, of dd orbital origin) excited state characterizes path ii, with a MLCT - MC energy separation ΔE = 3800, 2300, and 1600 cm^-1 for (ttp)Ru(tpy-tpy)Ru-(ttp)⁴⁺, (ttp)Ru(tpy-ph-tpy)Ru(ttp)⁴⁺, and Ru(ttp)₂²⁺, respectively. At room temperature, for this limited series of complexes it is found that nonradiative processes governed by the "energy-gap law" play a minor role as compared to thermally activated processes, the ratios of the rate constants being k_nr^act/k_nr^dir ≈ 16, 1900, and 7000 for (ttp)Ru(tpy-tpy)Ru(ttp)⁴⁺, (ttp)Ru(tpy-ph-tpy)Ru(ttp)⁴⁺, and Ru(Up)₂²⁺, respectively.