Inorganic Chemistry, Vol.49, No.24, 11333-11345, 2010
Modulating the Light Switch by (MLCT)-M-3-(3)pi pi(star) State lnterconversion
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-dialkryl portion of the free ligand, a property that is reflected in a red shift of the lowest energy electronic transition (1: lambda(max) = 393 nm) upon substitution at the terminal phenyl groups (2: lambda(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(max) = 450 nm are observed for [Ru(phen)(2)bptt](2+) (3) and [Ru(phen)(2)tbptt](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: phi(H2O) = 0.005,4: phi(H2O) = 0.011) and biexponential decay kinetics (3: tau(1) = 40 ns, tau(2) = 230 ns; 4: tau(1) similar to 26 ns, tau(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-b = 3.3 x 10(4) M-1). The origin of this light switch behavior involves two competing (MLCT)-M-3 states similar to that of the extensively studied light switch molecule [Ru(phen)(2)dppz](2+). The solvent- and temperature-dependence of the luminescence of 3 reveal that the extended ligand aromaticity lowers the energy of the (3)pi pi(star) excited state into competition with the emitting (MLCT)-M-3 state. lnterconversion between these two states plays a significant role in the observed photophysics and is responsible for the dual emission in aqueous environments.