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

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• 作者：Hyeong-Mook Kim ; Jaeheung Park ; Young Tak Lee ; Manho Lim ; Young Keun Chung ; Youn K. Kang
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：November 17, 2011
• 年：2011
• 卷：115
• 期：45
• 页码：22557-22562
• 全文大小：866K
• 年卷期：v.115,no.45(November 17, 2011)
• ISSN：1932-7455

文摘

New 蟺-stacked [Ru(tpy)₂]²⁺ (T_T)-benzoquinone (Q) donor鈥揳cceptor (D鈥揂) systems, [Ru(6-(2-cyclohexa-2鈥?5鈥?diene-1,4-dione)-2,2鈥?6鈥?2鈥?terpyridine)(2,2鈥?6鈥?2鈥?terpyridine)][PF₆]₂ (TQ_T), and [Ru(6-(2-cyclohexa-2鈥?5鈥?diene-1,4-dione)-2,2鈥?6鈥?2鈥?terpyridine)(4鈥?phenyl-2,2鈥?6鈥?2鈥?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 H_DA between the ground and the lowest energy MQCT states are not available through conventional Mulliken鈥揌ush analysis. For such systems, H_DA values were successfully evaluated from the relative difference (尉) 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. 鈭?.97e, and the corresponding H_DA was 1600 cm^鈥?. This value was in good agreement with that estimated by the Mulliken population analysis of the ground-state geometry.