The photophysical properties of mono- and dinuclear complexes based on the bridging ligand tpphz (tpphz = tetrapyrido [3,2-a:2',3'-c:3 ",2 "-h:2"'-3"'-j]phenazine) were investigated. The complexes are of general formula [M(bpy)(2)(tpphz)](2+) [M = Ru(II), Os(II)] and [(bpy)(2)M-1(tpphz)M-2(bpy)(2)](n+) [M-1 = M-2 = Ru(II), n = 4; M-1 = M-2 Os(II), n = 4; M-1 = Ru(II), M-2 = Os(II), n = 4; M-1 = Ru(II), M-2 = Os(III), n = 5]. The tpphz bridging ligand, being aromatic, rigid, and planar, has interesting structural features for the design of covalently linked donor-acceptor systems. In this work particular attention was devoted to the electronic properties of this bridge and their effect on the photophysical behavior. All of the results are consistent with direct involvement of the tpphz bridge in the photophysically active, lowest MLCT excited states. Relevant findings are as follows: (i) in mononuclear complexes, MLCT excited-state energies are highly sensitive to interactions at the free bpy-like end of the tpphz ligand, such as metalation and protonation; (ii) in thp dinuclear complexes, the electronic ground state behaves as a valence-localized, supramolecular system, while a substantial amount of intercomponent electronic coupling is indicated by MLCT excited-state behavior; (iii) in the heterodinuclear complex, fast (k > 10(9) s(-1)) energy and/or electron transfer processes take place across the tpphz bridge.