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

New pi-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 ''-telpyridine)(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 similar to 0.002. Due to the negligible intensity of MQCT bands, evaluation of H-DA between the ground and the lowest energy MQCT states are not available through conventional Mulliken-Hush analysis. For such systems, H-DA values were successfully evaluated from the relative difference (xi) 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 H-DA was similar to 1600 cm(-1). This value was in good agreement with that estimated by the Mulliken population analysis of the ground-state geometry.