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⁺ (<b>T_T</b>)-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 (<b>TQ_T</b>), 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 (<b>TQ_TPh</b>) 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 <b>T_T</b> 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 (<b>T_T</b>). 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 <i>H</i>_DA between the ground and the lowest energy MQCT states are not available through conventional Mulliken–Hush analysis. For such systems, <i>H</i>_DA 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. −0.97<i>e</i>, and the corresponding <i>H</i>_DA was ∼1600 cm^–1. This value was in good agreement with that estimated by the Mulliken population analysis of the ground-state geometry