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

Abstract

The transition metal complexes [Ru(dmb) 3 ] 2+ and [Ru(dpb) 3 ] 2+ , 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 ] 2+ 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 CH 3 CN solution at room temperature following metal-to-ligand charge transfer (MLCT) excitation at 400 nm. Data on [Ru(dmb) 3 ] 2+ at 532 nm, a region of net excited-state absorption, revealed biphasic decay kinetics (∼120 fs and 5 ps) attributed to a combination of 1 MLCT f 3 MLCT intersystem crossing and vibrational cooling dynamics. Dynamics for [Ru(dpb) 3 ] 2+ under identical conditions revealed biphasic rise times in the region of the ligand-based π* r π* absorption at λ probe ) 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.

Femtosecond Absorption Spectroscopy of Transition Metal Charge‐Transfer Complexes

“…19 Whereas ground-state recovery dynamics are independent of the spectral region monitored, we have found that, in general, the kinetics of excited-state formation are extremely sensitive to the probe wavelength chosen (Figure 7a). The utility of spectroelectrochemical data now becomes clear, insofar as it provides us with assignments for the various features being examined.…”

“…Figure 10 shows femtosecond transient absorption data for [Ru(dpb) 3 ] 2+ in CH 3 CN solution; the compound's spectroelectrochemical properties were described previously ( Figure 6). 19 As with the simpler polypyridyls discussed above, the MLCT bleach region is relatively uninformative, as is the LMCT absorption toward the red. However, wavelengths probing the dpb -radical reveal a significant qualitative difference in the nature of the signal as compared to [Ru(dmb) 3 ] 2+ : instead of a decay in the initial absorbance at 532 nm, the data for [Ru(dpb) 3 ] 2+ correspond to an increase in the absorption cross section of the excited state as a function of time.…”

“…For example, in other work we have shown that charge-transfer to ligand-field state conversion following 1 A 1 f 1 MLCT excitation of an Fe II polypyridyl complex occurs with a time constant of 80 ( 11 whereas vibrational cooling is observed on a time scale of several picoseconds. 19 The point we wish to emphasize is that ascribing the origin of kinetic features on the basis of a presumption of what the relative magnitudes of various processes "should" be is, in our opinion, wholly unjustified when dealing with excitedstate evolution in transition-metal chromophores. This raises the bar in terms of the amount of information one must acquire in order to construct a viable model for dynamics on ultrafast time scales, but it is critically important that these sorts of criteria be applied to efforts now while protocols for this type of research are still being established.…”

Femtosecond Absorption Spectroscopy of Transition Metal Charge-Transfer Complexes

“…To illustrate, in Figure 6 are shown data for [Ru(dpb) 3 ] 2+ , where dpb is 4,4′-diphenyl-2,2′-bipyridine. 19 Oxidation of the molecule eliminates the Ru II species, hence the…”

Origin of electronic absorption spectra of MLCT-excited and one-electron reduced 2,2′-bipyridine and 1,10-phenanthroline complexes

“…Indeed, comparison of UV-Vis absorption as well as resonance Raman and IR spectra of reduced and excited states is used to assign and characterize the lowest excited state as MLCT [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. This kind of experiments helped to establish that both reduction and MLCT excitation of [ …”