Creation of metal-to-ligand charge transfer excited states with two-photon excitation

We examined the steady state and time-resolved emission spectral properties of [Ru(bpy)(3)](2+) and [Ru(bpy)(2)(dcb)](2+), where bpy is 2,2'-bipyridine and dcb is 2,2'-bipyridine-4,4'-dicarboxylic acid, in fluid solution when excited with 90 fs pulses from a mode-locked Ti/sapphire laser. Over the wavelength range 820-900 nm, both complexes displayed two-photon excitation as observed by a quadratic dependence of the emission intensity on incident power. Steady state emission and time-resolved frequency-domain intensity decay measurements revealed that two-photon excitation of each complex resulted in the same emission spectra and single-exponential decays as observed for one-photon excitation at a variety of temperatures in different solvents. The two-photon excitation cross section of [Ru(bpy)(3)](2+) measured at 880 nm was determined to be 4.3 x 10(-50) cm(4) s/photon. These results clearly show that metal-to-ligand charge transfer (MLCT) excited states can in fact be obtained through multiphoton processes.