Site-specific electronic couplings in dyads with MLCT excited states. Intramolecular energy transfer in isomeric Ru(II)-Ru(II) cyclometalated complexes

The rod-like binuclear complexes [(ttpy)Ru(tpy-ph(2)-phbpy) Ru(ttpy)](4+) and [(ttpy) Ru(tpy-ph(2)-tpy) Ru(phtbpy)](4+) (for abbreviations, see text) have been synthesized and characterized. In both complexes, the polypyridine Ru(II) centers have (N boolean AND N boolean AND N)Ru(N boolean AND N boolean AND N) and (N boolean AND N boolean AND N)Ru(C boolean AND N boolean AND N) coordination environment. The two isomeric species differ in whether the cyclometalating carbon resides on the bridging or on the terminal ligand. The two complexes have virtually identical energy levels, but MLCT excited states of different (bridging or terminal) ligand localization. They are thus ideally suited to investigate possible effects of excited-state localization on intramolecular energy transfer kinetics. In fact, ultrafast spectroscopic measurements yield different energy transfer time constants for the two isomers, with the bridge-cyclometalated complex (2.7 ps) being faster than the terminal-cyclometalated one (8.0 ps). This difference can be explained in terms of different electronic factors for Dexter energy transfer. The study highlights the peculiar intricacies of intramolecular energy transfer in inorganic dyads involving MLCT excited states.