Ultrafast Dynamics in the Metal-to-Ligand Charge Transfer Excited-State Evolution of [Ru(4,4′-diphenyl-2,2′-bipyridine)3]

The transition metal complexes [Ru(dmb)3] and [Ru(dpb)3], where dmb is 4,4′-dimethyl-2,2′-bipyridine and dpb is 4,4′-diphenyl-2,2′-bipyridine, have been studied by femtosecond visible electronic absorption spectroscopy. Spectroelectrochemical measurements in conjunction with nanosecond time-resolved absorption spectroscopy allow for the assignment of various features in the excited-state differential absorption spectra as both ligand-based π* r π* and ligand-to-metal charge transfer (LMCT) in nature. A unique absorptive feature centered at ∼ 530 nm in [Ru(dpb)3] was identified as an optical marker for the thermalized (and hence fully intraligand delocalized) excited state. Single wavelength and full spectrum transient absorption data were obtained on both molecules in CH3CN solution at room temperature following metal-to-ligand charge transfer (MLCT) excitation at 400 nm. Data on [Ru(dmb)3] at 532 nm, a region of net excited-state absorption, revealed biphasic decay kinetics (∼120 fs and 5 ps) attributed to a combination of 1MLCT f 3MLCT intersystem crossing and vibrational cooling dynamics. Dynamics for [Ru(dpb)3] under identical conditions revealed biphasic rise times in the region of the ligand-based π* r π* absorption at λprobe ) 532 nm. Although the origin of the fast component (∼ 200 fs) is not yet clear, the ca. 2 ps rise is assigned to rotation of the peripheral aryl ring and thus corresponds to the time scale for intraligand electron delocalization.