Linear tris-terpyridines and their trinuclear Ru(II) complexes: Synthesis, absorption spectra, and excited state properties

The absorption and luminescence spectra and lifetimes (at room temperature and 77 K) of the linear tris-terpyridine bridging ligands 1 and 2 and of their trinuclear Ru(ll) complexes 3 and 4, with 2,2':6',2''-terpyridine (tpy) as terminal ligands, have been investigated. The bridging ligand 1, where the spacers separating the three tpy groups are -CH2-CH2- units, shows fluorescence in dichloromethane solution at room temperature (lambda(max) = 345 nm, Phi = 0.055, tau < 1 ns) and fluorescence (lambda(max) = 342 nm, tau < 1 ns) and phosphorescence (lambda(max) = 444 nm, tau = 1.7 s) in a rigid butyronitrile matrix at 77 K. The bridging ligand 2, where the three tpy units are linked by -CH=CH- spacers, shows a red-shifted, very strong fluorescence at room temperature (lambda(max) = 404 nm, Phi = 0.79, tau < 1 ns) and at 77 K (lambda(max) = 429 nm, tau < 1 ns) but no phosphorescence. The luminescence behavior of the trinuclear Ru(ll) complex 3 (based on the bridging ligand 1) is almost identical to that of Ru(tpy)(2)(2+) (no room temperature emission, strong (3)MLCT phosphorescence at 77 K with lambda(max) = 608 nm, tau = 9.4 mu s. The trinuclear Ru(II) complex 4 (based on the bridging ligand 2), besides a strong 3MLCT phosphorescence at 77 K (lambda(max) = 620 nm, tau = 1.6 mu s), shows a 3MLCT emission even at room temperature (lambda(max) = 690 nm, tau = 11 ns). The results obtained are discussed in light of the different spacers (-CH2-CH2- or -CH=CH-) between the tpy subunits of 1 and 2 and the different coordination environment of the various RU(2+) ions in the trinuclear complexes.