The redox- and photoactive mononuclear complex [Ru(PCP)(tpy-DTTANa4)]Cl {PCP = [C6H3(CH2PPh2)2-2,6]-; tpy-DTTA4- = 4'-(2,2':6',2''-terpyridine)-diethylenetriamine-N,N,N'',N''-tetraacetate} possesses an externally directed, vacant N3O4 polyaminocarboxylate-type binding site that coordinates to lanthanide(III) ions to give the neutral heterodinuclear RuII-LnIII complexes [Ru(PCP)(tpy-DTTA)Ln(H2O)2] (Ln = Gd3+, Eu3+). The photophysical properties of solutions of the mononuclear complex [Ru(PCP)(tpy-DTTANa4)]Cl were investigated in MeOH/EtOH (1:4) and compared to those of the solutions of heterodinuclear complexes [Ru(PCP)(tpy-DTTA)Ln(H2O)2] (Ln = Gd3+, Eu3+). Rigid matrix excitation at 77 K of the pi-pi* level of the ruthenium chromophore in the [Ru(PCP)(tpy-DTTA)Eu(H2O)2] complex results in a weak europium(III) emission pointing to a transfer of energy from RuEu as a result of the metal-to-ligand charge-transfer (MLCT) excited state of the ruthenium component to the luminescent lanthanide ion. The excited state lifetime of the europium complex is 0.2 ms in methanol solution. In deuterated solvents, the lifetime increases to 0.4 ms, which indicates that the process is solvent-dependent as a result of the strongly coordinated molecules of water that are responsible for the quenching in nondeuterated solvents.