Charge-transfer excited states in transition metal complexes are characterized by a redistribution of charge density between the central metal core and the ligand(s). We are interested in understanding the interplay between solvent and solute dynamics in the evolution of MLCT excited states triggered by this process. [Ru(dpb)3]2+ (where dpb = 4,4’-diphenyl-2,2’-bipyridine) displays a long-lived 3MLCT excited state that vibrationally relaxes via a ~44° rotation of the peripheral phenyl ring of the ligand toward coplanarity with the bpy fragment. Femtosecond time-resolved data probing the absorption assigned to the radical anion of the ligand at 530 nm reveals a biphasic increase in intensity associated with this rotation. The shorter kinetic feature exhibits a time constant of ~250 fs that is independent of solvent and excitation wavelength, but the slower component exhibits more complex dynamics. When exciting at 400 nm, a 2 ps time constant was observed in CH3OH solution; this increased to 8 ps in ethanol and remained invariant in linear-chain alcohols up through 1-octanol. In contrast, excitation at 480 and 550 nm (corresponding to the lowest energy 1MLCT and 3MLCT absorptions, respectively) revealed dynamics that increased linearly with greater alkyl chain length on the alcohol. These observations are being attributed to a dual mechanism attached to MLCT-state evolution. Specifically, the 400 nm data is consistent with a dielectric frictional response associated with rotation about the C-O bond of the alcohol and thus represents dynamics driven and/or coupled to local (as opposed to bulk) solvent-solute interactions. Excitation at lower energies on the other hand, produces dynamics more consistent with a viscoelastic response of the solvent and is therefore dominated by bulk solvent properties. The origin of this effect as it relates to charge density distribution in the MLCT excited state(s) of this system will be discussed, as well as its implications for potentially leveraging solvent- and excitation wavelength-dependent ultrafast processes in order to manipulate MLCT-state evolution in condensed phase.

Solvent and excitation wavelength-dependent dynamics in the excited-state evolution of 3MLCT states: The role of charge distribution in solvent-solute coupling

254th National Meeting (2017)

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