Modulating the Light Switch by [superscript 3]MLCT-[superscript 3]pi pi State Interconversion

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: {lambda}{sub max} = 393 nm) upon substitution at the terminal phenyl groups (2: {lambda}{sub 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 {lambda}{sub max} = 450 nm are observed for [Ru(phen){sub 2}bptt]{sup 2+}(3) and [Ru(phen){sub 2}tbptt]{sup 2+}(4). The photophysical characteristics of 3 and 4 in ethanol closely parallel those observed for [Ru(bpy){sub 3}]{sup 2+} and [Ru(phen){sub 3}]{sup 2+}, indicating that the MLCT excited state is primarily localized within the [Ru(phen){sub 3}]{sup 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: {phi}H{sub 2}O = 0.005, 4: {phi}H{sub 2}O = 0.011) and biexponential decay kinetics (3: {tau}{sub 1} = 40 ns, {tau}{sub 2} = 230 ns; 4: {tau}{sub 1} {approx} 26 ns, {tau}{sub 2} = 150 ns). Addition of CT-DNA to an aqueous solution of 3 causes a significant increase in the luminescence quantum yield ({phi}DNA = 0.045), while the quantum yield of 4 is relatively unaffected ({phi}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{sub b} = 3.3 x 10{sup 4} M{sup -1}). The origin of this light switch behavior involves two competing {sup 3}MLCT states similar to that of the extensively studied light switch molecule [Ru(phen){sub 2}dppz]{sup 2+}. The solvent- and temperature-dependence of the luminescence of 3 reveal that the extended ligand aromaticity lowers the energy of the {sup 3}{pi}{pi}* excited state into competition with the emitting {sup 3}MLCT state. Interconversion between these two states plays a significan