Modulating the Light Switch by 3MLCT-3ππ* State Interconversion

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• 作者：Brigitte R. Spencer ; Brian J. Kraft ; Chris G. Hughes ; Maren Pink ; Jeffrey M. Zaleski
• 刊名：Inorganic Chemistry
• 出版年：2010
• 出版时间：December 20, 2010
• 年：2010
• 卷：49
• 期：24
• 页码：11333-11345
• 全文大小：1126K
• 年卷期：v.49,no.24(December 20, 2010)
• ISSN：1520-510X

文摘

The spectroscopic, electronic, and DNA-binding characteristics of two novel ruthenium complexes based on the dialkynyl ligands 2,3-bis(phenylethynyl)-1,4,8,9-tetraaza-triphenylene (bptt, 1) and 2,3-bis(4-tert-butyl-phenylethynyl)-1,4,8,9-tetraaza-triphenylene (tbptt, 2) have been investigated. Electronic structure calculations of bptt reveal that the frontier molecular orbitals are localized on the pyrazine-dialkynyl portion of the free ligand, a property that is reflected in a red shift of the lowest energy electronic transition (1: λ_max = 393 nm) upon substitution at the terminal phenyl groups (2: λ_max = 398 nm). Upon coordination to ruthenium, the low-energy ligand-centered transitions of 1 and 2 are retained, and metal-to-ligand charge transfer transitions (MLCT) centered at λ_max = 450 nm are observed for [Ru(phen)₂bptt]²⁺(3) and [Ru(phen)₂tbptt]²⁺(4). The photophysical characteristics of 3 and 4 in ethanol closely parallel those observed for [Ru(bpy)₃]²⁺ and [Ru(phen)₃]²⁺, indicating that the MLCT excited state is primarily localized within the [Ru(phen)₃]²⁺ manifold of 3 and 4, and is only sparingly affected by the extended conjugation of the bptt framework. In an aqueous environment, 3 and 4 possess notably small luminescence quantum yields (3: _H2O = 0.005, 4: _H2O = 0.011) and biexponential decay kinetics (3: τ₁ = 40 ns, τ₂ = 230 ns; 4: τ₁ 26 ns, τ₂ = 150 ns). Addition of CT-DNA to an aqueous solution of 3 causes a significant increase in the luminescence quantum yield (_DNA = 0.045), while the quantum yield of 4 is relatively unaffected (_DNA = 0.013). The differential behavior demonstrates that tert-butyl substitution on the terminal phenyl groups inhibits the ability of 4 to intercalate with DNA. Such changes in intrinsic luminescence demonstrate that 3 binds to DNA via intercalation (K_b = 3.3 × 10⁴ M⁻¹). The origin of this light switch behavior involves two competing ³MLCT states similar to that of the extensively studied light switch molecule [Ru(phen)₂dppz]²⁺. The solvent- and temperature-dependence of the luminescence of 3 reveal that the extended ligand aromaticity lowers the energy of the ³ππ* excited state into competition with the emitting ³MLCT state. Interconversion between these two states plays a significant role in the observed photophysics and is responsible for the dual emission in aqueous environments.