Excited State Processes in Ruthenium(II)/Pyrenyl Complexes Displaying Extended Lifetimes

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Abstract

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 CH3CN, 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 CH3CN 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 3pyrene absorption was directly monitored in both complexes and ranged from 2.8 × 1010 s-1 in [Ru(bpy)2(py-phen)]2+ to 2.4 × 1011 s-1 in [Ru(py-phen)3]2+. In both cases, the transient absorption spectra contain features of 3pyrene 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 1O2 luminescence at 1270 nm.

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