Two-photon luminescence from polar bis-terpyridyl-stilbene derivatives of Ir(III) and Ru(II)

Four structurally related iridium(III) and ruthenium(II) complexes bearing two polar terpyridyl-stilbene derived chromophores 4-(4-{2-[4-(methoxy) phenyl] ethenyl} phenyl)-2,2 '-6 ', 2 ''-terpyridine (ttpyeneanisole) and 4-(4-{2-[phenyl] ethenyl} phenyl)-2,2 '-6 ', 2 ''-terpyridine (tpystilbene) have been synthesised and characterised in the solid state and in solution. In the solid state, the dihedral angle subtending the pyridyl and tolyl groups of 27.1 degrees in the Ir(III) complex [Ir(ttpyeneanisole)(2)]center dot 3PF(6) is more acute than in the Ru(II) derivative [Ru(tpystilbene)(2)]center dot 2PF(6) (35.5 degrees), indicating the presence of a greater degree of pi-delocalisation across the terpyridine unit in the former compound. Their luminescence properties in fluid solution have been investigated following both resonant and non-resonant excitation. We have shown that each of the complexes undergoes two-photon excitation when excited in the near infrared (740 to 820 nm), with two-photon absorption cross sections in the range 11-67 x 10(-50) cm(4) s photon(-1). The larger cross sections for the Ir(III) complexes reflect the differences observed in the solid state. This work therefore demonstrates that such complexes are promising as luminescent markers for 3D imaging and illustrates that simple functionalisation of the chromophores and the choice of metal can lead to marked enhancements in the two-photon cross sections (sigma(2)) compared to those of simpler heteroleptic polypyridyl based derivatives.