New Ru(II) Chromophores with Extended Excited-State Lifetimes

We describe the synthesis, electrochemical, and photophysical properties of two new luminescent Ru(II) diimine complexes covalently attached to one and three 4-piperidinyl-1,8-naphthalimide (PNI) chromophores, [Ru(bpy)2(PNI−phen)](PF6)2 and [Ru(PNI−phen)3](PF6)2, respectively. These compounds represent a new class of visible light-harvesting Ru(II) chromophores that exhibit greatly enhanced room-temperature metal-to-ligand charge transfer (MLCT) emission lifetimes as a result of intervening intraligand triplet states (3IL) present on the pendant naphthalimide chromophore(s). In both Ru(II) complexes, the intense singlet fluorescence of the pendant PNI chromophore(s) is nearly quantitatively quenched and was found to sensitize the MLCT-based photoluminescence. Excitation into either the 1IL or 1MLCT absorption bands results in the formation of both 3MLCT and 3IL excited states, conveniently monitored by transient absorption and fluorescence spectroscopy. The relative energy ordering of these triplet states was determined using time-resolved emission spectra at 77 K in an EtOH/MeOH glass where dual emission from both Ru(II) complexes was observed. Here, the shorter-lived higher energy emission has a spectral profile consistent with that typically observed from 3MLCT excited states, whereas the millisecond lifetime lower energy band was attributed to 3IL phosphorescence of the PNI chromophore. At room temperature the data are consistent with an excited-state equilibrium between the higher energy 3MLCT states and the lower energy 3PNI states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 μs depending upon solvent and the number of PNI chromophores present. At 77 K it is apparent that the two triplet states are no longer in thermal equilibrium and independently decay to the ground state.