The novel dendritic-shaped, heptanuclear compound 1 - the first mixed Os-Ru dendrimer with tris-chelating bridging ligands - has been synthesized and its photophysics studied by fs pump-probe transient spectroscopy. In 1, all light energy absorbed by six identical Ru(ii) chromophores is funnelled to the luminescent central Os(ii) core, with a time constant of 11 ps, by Dexter energy transfer.