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 ((IL)-I-3) 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 (IL)-I-1 or (MLCT)-M-1 absorption bands results in the formation of both (MLCT)-M-3 and (IL)-I-3 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 (IL)-I-3 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 (PNI)-P-3 states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 mus 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.