Probing Ground-to-CT State Electronic Coupling for the System with No Apparent Charge Transfer Absorption Intensity by Ultrafast Visible-Pump/Mid-IR-Probe Spectroscopy

Abstract

New π-stacked [Ru(tpy)2]2+ (T_T)-benzoquinone (Q) donor–acceptor (D–A) systems, [Ru(6-(2-cyclohexa-2′,5′-diene-1,4-dione)-2,2′:6′,2″-terpyridine)(2,2′:6′,2″-terpyridine)][PF6]2 (TQ_T), and [Ru(6-(2-cyclohexa-2′,5′-diene-1,4-dione)-2,2′:6′,2″-terpyridine)(4′-phenyl-2,2′:6′,2″-terpyridine)][PF6]2 (TQ_TPh) have been synthesized and characterized. Orthogonal alignment of Q to the tpy ligand imposes this unit juxtaposed cofacially on the central pyridyl ring in another tpy with a typical van der Waals distance. The low-energy electronic absorptions of these complexes are mainly metal-to-ligand charge transfer (MLCT) in nature, similar to that observed in T_T benchmark system, and do not exhibit distinguishable metal-to-Q charge transfer (MQCT) absorption in spite of the proximal location of the electron acceptor unit (Q) to the electron donor unit (T_T). TD-DFT calculation supports the experimental results that the collective oscillator strength of MQCT bands remains ∼0.002. Due to the negligible intensity of MQCT bands, evaluation of HDA between the ground and the lowest energy MQCT states are not available through conventional Mulliken–Hush analysis. For such systems, HDA values were successfully evaluated from the relative difference (ξ) of the carbonyl stretching frequency between the neutral Q and its one-electron radical anion, which was determined by an ultrafast visible-pump/mid-IR-probe (TrIR) spectroscopic method. TrIR results showed that the partial charge localized on the Q moiety in the MQCT state was ca. −0.97e, and the corresponding HDA was ∼1600 cm–1. This value was in good agreement with that estimated by the Mulliken population analysis of the ground-state geometry.