Electronic energy transfer and collection in luminescent molecular rods containing ruthenium(II) and osmium(II) 2,2 ': 6 ',2 ''-terpyridine complexes linked by thiophene-2,5-diyl spacers

The electronic absorption spectra, luminescence spectra and lifetimes (in MeCN at room temperature and in frozen n-C3H7CN at 77K). and electrochemical potentials (in MeCN) of the novel dinuclear [(tpy)Ru(3)Os(tpy)](4+) and trinuclear [(tpy)Ru(3)Os(3)Ru(tpy)](6+) complexes (3=2,5-bis(2,2`:6`,2``-terpyridin-4-yl)thiophene) have been obtained and are compared with those of model mononuclear complexes and homometallic [(tpy)Ru(3)Ru(tpy)](4+), [(tpy)Os(3)Os(tpy)](4+) and [(tpy)Ru(3)Ru(3)Ru(tpy)](6+) complexes. The bridging ligand 3 is nearly planar in the complexes, as seen from a preliminary X-ray determination of [(tpy)Ru(3)Ru(tpy)][PF6](4), and confers a high degree of rigidity upon the polynuclear species. The trinuclear species are rod-shaped with a distance of about 3 nm between the terminal metal centres. For the polynuclear complexes, the spectroscopic and electrochemical data are in accord with a significant intermetal interaction. All of the complexes are luminescent (Phi in the range 10(-4) - 10(-2) and tau in the range 6-340 ns, at room temperature), and ruthenium- or osmium-based luminescence properties can be identified. Due to the excited state properties of the various components and to the geometric and electronic properties of the bridge, Ru -> Os directional transfer of excitation energy takes place in the complexes [(tpy)Ru(3)Os(tpy)](4+) (end-to-end) and [(tpy)Ru(3)Os(3)Ru(tpy)](6+) (periphery-to-centre). With respect to the homometallic case. for [(tpy)Ru(3)Os(3)Ru(tpy)](6+) excitation trapping at the central position is accompanied by a fivefold enhancement of luminescence intensity.