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

Abstract

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)](PF(6))(2) and [Ru(PNI-phen)(3)](PF(6))(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 ((3)IL) 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 (1)IL or (1)MLCT absorption bands results in the formation of both (3)MLCT and (3)IL 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 (3)MLCT excited states, whereas the millisecond lifetime lower energy band was attributed to (3)IL phosphorescence of the PNI chromophore. At room temperature the data are consistent with an excited-state equilibrium between the higher energy (3)MLCT states and the lower energy (3)PNI states. Both complexes display MLCT-based emission with room-temperature lifetimes that range from 16 to 115 micros 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.

Photophysical and biological investigation of novel luminescent Ru(ii)-polypyridyl-1,8-naphthalimide Tröger's bases as cellular imaging agents

“…Excitation into all of the aforementioned absorption bands of 1 and 2 resulted in the appearance of 3 MLCT based emission centred at ca. 615 nm; the absence of any 1,8-naphthalimide associated emission suggesting that sensitisation of the MLCT excited state occurs through some mixing of low-lying isoenergetic triplet states, 3 MLCT and 3 Nap, as described by Castellano et al 24 as the ICT emission centred at 505 nm overlaps with the 1 MLCT absorption of the Ru(II) centre. Similarly, the fluorescence quantum yields measured for 1 and 2 were found to be identical with F MLCT = 0.015 (AE10%) but significantly lower than that measured for the parent [Ru(bpy) 3 ] 2+ (F MLCT = 0.028) again suggesting an interaction between the 1,8-naphthalimide and the 3 MLCT triplet state of the Ru(II) centres.…”

Unexpected DNA binding properties with correlated downstream biological applications in mono vs. bis-1,8-naphthalimide Ru(ii)-polypyridyl conjugates

“…[39][40][41][42][43][44] In addition to being effective DNA intercalators or groove binders, the 1,8-naphthalimides can also act as sensitising antenna for the Ru(II)-based metal-to-ligand charge transfer (MLCT) emission, allowing for the population of the excited state using two excitation channels. 45 Herein, we present two new DNA targeting Ru(II) com- DNA in a cooperative manner, 38 and potentially enhance cellular uptake. 36 To fully explore the activity of the complexes a detailed in vitro photophysical study was completed in the absence and presence of DNA.…”

So-called “dual emission” for 3MLCT luminescence in ruthenium complex ions: What is really happening?

“…Particularly prominent have been efforts from the laboratory of Prof. Castellano, of which we content ourselves with just two citations. One discusses multiple emissions at low temperature, and also reports thermal equilibrium among multiple states at room temperature [34]. Another raises the question whether such situations are best described as being independent states in equilibrium or as mixed states, with wave functions being intermediate and having characteristics of both [35].…”