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(tp y-tpy)Ru(ttp)(4+) and (ttp)Ru(tpy-ph-tpy)Ru(ttp)(4+), we have obtained ground-state absorption spectra and transient-absorption difference s pectra at room temperature and luminescence spectra and lifetimes in t he temperature interval from room temperature to the rigid matrix (90 K); the solvent was acetonitrile or butyronitrile (tpy is 2,2':6',2 '' -terpyridine, ttp is 4'-p-tolyl-2,2':6',2 ''-tpy, and ph is 1,4-phenyl ene). The gathered spectroscopic data indicate that after absorption o f visible light, formation of the luminescent metal-to-ligand charge t ransfer (MLCT) excited states takes place, which involves the bridging ligand (BL). Since we found that (ttp)Ru(tpy-tpy)Ru(ttp)(4+) is a goo d luminophore (lambda(max) = 720 nm, Phi = 4.7 x 10(-3), and tau = 570 ns) while both (ttp)Ru(tpy-ph-tpy)Ru(ttp)(4+) (lambda(max) = 656 nm, Phi = 1.1 x 10(-4), and tau = 4 ns) and the reference mononuclear comp lex Ru(ttp)(2)(2+) (lambda(max) = 640 nm, Phi = 3.2 x 10(-5), and tau = 0.9 ns) are not, we have explored the effects brought about by the d elocalization and energy content of the luminescent state. The study o f the temperature dependence of the luminescence lifetimes indicates t hat two main nonradiative paths, i and ii, are responsible for deactiv ation of the luminescent state. Path i directly connects the luminesce nt and ground states; within the frame of the ''energy-gap law'', vibr onic analysis of low-temperature luminescence profiles enables one to correlate the delocalization of the M --> BL CT state and the extent o f structural distortions occurring at the accepting ligand. Thermally activated decay via a metal-centered (MC, of dd orbital origin) excite d state characterizes path ii, with a MLCT-MC energy separation Delta E = 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)(2)(2+), respectively. At room t emperature, for this limited series of complexes it is found that nonr adiative processes governed by the ''energy-gap law'' play a minor rol e as compared to thermally activated processes, the ratios of the rate constants being k(nr)act/k(nr)(dir) approximate to 16, 1900, and 7000 for (ttp)Ru(tpy-tpy)Ru(ttp)(4+), (ttp)Ru(tpy-ph-tpy)Ru(ttp)(4+), and Ru(ttp)(2)(2+), respectively.