We report the synthesis and the photophysical properties of UV light-harvesting arrays constructed around a [Ru(bpy)3]2+ core (bpy = 2,2‘-bipyridine) bearing one and three pyrenyl units in the periphery. The free ligand containing the pyrenyl unit, 4-methyl-4‘-(2-hydroxyethylpyrenyl)-2,2‘-bipyridine, displays an intense emission band centered near 400 nm with a lifetime of 264 ns, characteristic of singlet pyrene emission. The complexes [Ru(dmb)2(4-methyl-4‘-(2-hydroxyethylpyrenyl)-2,2‘-bipyridine)](PF6)2, where dmb is 4,4‘-dimethyl-2,2‘-bipyridine, and [Ru(4-methyl-4‘-(2-hydroxyethylpyrenyl)-2,2‘-bipyridine)3](PF6)2 exhibit visible emission characteristic of the [Ru(bpy)3]2+ unit, regardless of excitation wavelength. The singlet emission from the pyrene chromophores is almost quantitatively quenched by the metal-to-ligand charge transfer (MLCT) states of each respective Ru(II) complex resulting in the observation of sensitized MLCT-based emission. On account of the energetic proximity between the 3MLCT states and the 3pyrene states, a long-lived 3MLCT emission is observed which decays with the same first-order lifetime as the pyrene triplet states in deaerated CH3CN. In deaerated CH3CN, the bichromophoric system displays a lifetime of 2.96 μs, whereas the tetrad complex exhibits a lifetime of 9.0 μs. The results are indicative of excited-state equilibrium between the 3MLCT and 3pyrene states. Our findings demonstrate rapid and efficient singlet−singlet energy transfer through the antenna effect whereas the reversible triplet−triplet energy transfer processes help sustain long-lived MLCT excited states.

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.

A new Ru(II) complex is described which serves as a luminescence lifetime-based sensor for fluoride and cyanide anions (KF = 640 000 mol-1, KCN = 430 000 mol-1). This chromophore displays observable changes in its UV-vis and steady-state luminescence spectra upon cyanide binding. Prior to cyanide addition, this complex exhibits a single-exponential lifetime (tau = 377 +/- 20 ns). With increasing cyanide concentrations, the intensity decays are composed of two exponentials: long tau (320-370 ns) and short tau (13-17 ns). The average lifetimes shorten as a function of cyanide concentration since the fractional intensity shifts from an initial dominant long lifetime component to the short lifetime component. This work represents the first example of a direct method for the luminescence lifetime-based sensing of anions.

The synthesis and photophysical properties of two Ru(II) diimine complexes bearing one (dyad) and three (tetrad) pyrenyl units, respectively, are presented. The pyrene chromophore in each metal complex is tethered through a single C-C bond in the 5-position of 1,10-phenanthroline (py-phen). Both Ru(II) complexes display increased absorption cross sections near 340 nm largely due to the presence of the pyrenyl chromophore(s). Excitation from 300 to 540 nm results exclusively in the observation of metal-to-ligand charge transfer (MLCT) based emission that is exceptionally long lived, 23.7 µs and 148 µs in deaerated CH 3 CN, respectively. This luminescence was analyzed using steady-state and time-resolved techniques at room temperature and 77 K. The tetrad complex, [Ru(py-phen) 3 ] 2+ , displays a dynamic self-quenching reaction at room temperature in dilute CH 3 CN solutions that is well modeled by a Stern-Volmer expression. The excited-state processes occurring between the MLCT core and the pyrenyl units were further evaluated with ultrafast transient absorption spectroscopy and conventional flash photolysis. Formation of the 3 pyrene absorption was directly monitored in both complexes and ranged from 2.8 × 10 10 s -1 in [Ru(bpy) 2 (py-phen)] 2+ to 2.4 × 10 11 s -1 in [Ru(py-phen) 3 ] 2+ . In both cases, the transient absorption spectra contain features of 3 pyrene excited states, whereas the room-temperature luminescence is MLCT-based, both decaying with the same kinetics. This is consistent with the formation of a thermal excited-state equilibrium between the two triplet states at room temperature. Both Ru(II) complexes were found to sensitize the production of molecular singlet oxygen with a quantum efficiency of 0.69, measured by observing the characteristic 1 O 2 luminescence at 1270 nm.

We describe the synthesis, electrochemistry, and photophysical properties of several Ru(II) complexes bearing different numbers of pyrenylethynylene substituents in either the 5- or 5,5'-positions of 2,2'-bipyridine, along with the appropriate Ru(II) model complexes bearing either bromo- or ethynyltoluene functionalities. In addition, we prepared and studied the photophysical behavior of the diimine ligands 5-pyrenylethynylene-2,2'-bipyridine and 5,5'-dipyrenylethynylene-2,2'-bipyridine. Static and dynamic absorption and luminescence measurements reveal the nature of the lowest excited states in each molecule. All model Ru(II) complexes are photoluminescent at room temperature and exhibit excited-state behavior consistent with metal-to-ligand charge transfer (MLCT) characteristics. In the three Ru(II) molecules bearing multiple pyrenylethynylene substituents, there is clear evidence that the lowest excited state is triplet intraligand (3IL)-based, yielding long-lived room temperature phosphorescence in the red and near IR. This phosphorescence emanates from either 5-pyrenylethynylene-2,2'-bipyridine or 5,5'-dipyrenylethynylene-2,2'-bipyridine, depending upon the composition of the coordination compound. In the former case, the excited-state absorption difference spectra that were measured for the free ligand are easily superimposed with those obtained for the metal complexes coordinated to either one or two of these species. The latter instance is slightly complicated since coordination of the 5,5'-ligand to the Ru(II) center planarizes the diimine structure, leading to an extended conjugation on the long axis with a concomitant red shift of the singlet pi-pi absorption transitions and the observed room temperature phosphorescence. As a result, transient absorption measurements obtained using free 5,5'-dipyrenylethynylene-2,2'-bipyridine show a marked blue shift relative to its Ru(II) complex, and this extended pi-conjugation effect was confirmed by coordinating this ligand to Zn(II) at room temperature. In essence, all three pyrenylethynylene-containing Ru(II) complexes are unique in this genre of chromophores since the lowest excited state is 3IL-based at room temperature and at 77 K, and there is no compelling evidence of interacting or equilibrated excited states.