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

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posted on 20.12.2010, 00:00 by Brigitte R. Spencer, Brian J. Kraft, Chris G. Hughes, Maren Pink, Jeffrey M. Zaleski
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: λmax = 393 nm) upon substitution at the terminal phenyl groups (2: λmax = 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 λmax = 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: ϕH2O = 0.005, 4: ϕH2O = 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 (ϕDNA = 0.045), while the quantum yield of 4 is relatively unaffected (ϕDNA = 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 (Kb = 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.