Photophysical properties of ruthenium(II) polyazaaromatic compounds: a theoretical insight

Quantum-chemical methods are applied to study the nature of the excited states relevant in the photophysical processes (absorption and emission) of a series of polyazaaromatic-ligand-based ruthenium(II) complexes. The electronic and optical properties of the free polyazaaromatic ligands and their corresponding ruthenium(II) complexes are determined on the basis of correlated Hartree-Fock semiempirical approaches. While the emission of complexes containing small-size ligands, such as 1,10-phenanthroline or 2,2'-bipyridine, arises from a manifold of metal-to-ligand charge-transfer triplet states ((3)MLCTs), an additional ligand-centered triplet state ((3)L) is identified in the triplet manifold of complexes containing a pi-extended ligand such as dipyrido[3,2-a:2',3'-c]phenazine, tetrapyrido[3,2-a:2',3'-c:3',2'-h:2',3'-j]phenazine, and 1,10-phenanthrolino[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene. Recent experimental data are interpreted in light of these theoretical results; namely, the origin for the abnormal solvent- and temperature-dependent emission measured in pi-extended Ru complexes is revisited.     

Novel DNA sensor for guanine content

The bifunctional complex [Ru(TAP)(2)POQ-Nmet](2+), 1, formed with a [Ru(TAP)(2)Phen](2+) metallic unit linked to a quinoline moiety, and [Ru(TAP)(2)Phen](2+), 2, as reference, have been tested as photoprobes of DNA. Interestingly, 1 exhibits an emission enhancement of a factor of 16-17 upon binding to calf thymus DNA. Moreover, this emission is modulated by the nucleic base content of the polynucleotide. It varies by almost an order of magnitude from a polynucleotide containing 100% of G-C to a guanine-free nucleic acid where the excited-state lifetime reaches about 2 micros. The origins of these interesting properties are analyzed by comparing 1 with reference 2 in the presence of different polynucleotides.     

Dinuclear RuIIPHEHAT and -TPAC complexes: effects of the second RuII center on their spectroelectrochemical properties

Four novel dinuclear RuII compounds and, for comparison purposes, two corresponding mononuclear complexes containing the PHEHAT or TPAC ligand (PHEHAT=1,10-phenanthrolino[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene and TPAC=tetrapyrido[3,2-a:2',3'-c:3' ',2' '-h:2' ',3' '-j]acridine) have been synthesized and characterized. Conclusions on the effects of dinucleation of these two bridging ligands can be drawn only for the compounds for which the results demonstrate that the bridging ligand is involved in the first electrochemical reduction and lowest emission energy. The behavior of these complexes, which is not always predictable, is discussed, and the differences are highlighted in this work. Interestingly, all of the compounds are luminescent except one dinuclear species, [(phen)2Ru-mu-PHEHAT-Ru(TAP)2]4+, which does not luminesce in MeCN and BuCN at room temperature.     

Synthesis, characterization, electrochemical and photophysical properties of elbow-shaped trinuclear Ru(II) complexes, building blocks for novel supramolecular constructions

Three trinuclear elbow-shaped Ru(II) complexes based on the non symmetrical bridging PHEHAT ligand (PHEHAT = 1,10-phenanthrolino[5,6-b]-1,4,5,8,9,12-hexaazatriphenylene) have been prepared and characterized by NMR, electrochemistry, absorption and emission spectroscopy. It is shown that the dichloro trinuclear complex 1 should behave as an excellent precursor for the synthesis of larger species. Indeed, it reacts easily with 1,10-phenanthroline (phen) and 1,4,5,8-tetraazaphenanthrene (TAP) and leads to the trinuclear compounds 2 and 3, respectively. The electrochemical and emission studies indicate that for 2 and 3, there is an intramolecular energy transfer from the center to the periphery of the elbow-shaped trinuclear complex, whereas for complex 1 the energy transfer takes place in the other direction.     

Total Synthesis and Biological Evaluation of Zealactone 1a/b

Strigolactones are plant hormones, which play pivotal roles in plant growth and development with potential application in sustainable agriculture. Recently, zealactone 1a/b has been identified as the major strigolactone from the root exudates of corn. Although zealactone is a promising molecule affecting signaling in the rhizosphere as well as in planta, evaluating its biological activities has been hampered by its low natural abundance and its relative chemical instability. Herein, we present the total synthesis of zealactone 1a/b based on our studies employing a [2+2]-cycloaddition strategy and a chemoselective Baeyer-Villiger oxidation to forge the γ-butyrolactone fragment. Furthermore, we disclose the biological activities of zealactone 1a/b on corn and in soil in comparison with related synthetic analogues.     

Ultrasonic study of molecular interaction in binary liquid mixtures at 30°C

Densities ρ and ultrasonic speeds u of the binary mixtures of tetrahydrofuran (THF) with 1-butanol and tert-butanol, at 30°C, over the entire composition range were measured. From these data isentropic compressibility, K _s, intermolecular free length L _f, relative association R _A, acoustic impedance Z, molar sound speed R _m, deviations in isentropic compressibility ΔK _s, and excess volume V ^E were calculated. The variation of these parameters with composition of the mixture helps us in understanding the nature and extent of interaction between unlike molecules in the mixtures. Further, theoretical values of ultrasonic speed were evaluated using theories and empirical relations. The relative merits of these theories and relations were discussed.     

Rheology of semi-dilute solutions of calf-thymus DNA

We study the rheology of semi-dilute solutions of the sodium salt of calf-thymus DNA in the linear and nonlinear regimes. The frequency response data can be fitted very well to the hybrid model with two dominant relaxation times τ₀ and τ₁. The ratio (τ₀/τ₁) ∼5 is seen to be fairly constant on changing the temperature from 20 to 30°C. The shear rate dependence of viscosity can be fitted to the Carreau model.     

PHOTOINDUCED ELECTRON TRANSFER QUENCHING OF EXCITED Ru(II) POLYPYRIDYLS BOUND TO DNA: THE ROLE OF THE NUCLEIC ACID DOUBLE HELIX

In the presence of double helical polynucleotides (sodium poly(dA-dT).poly(dA-dT) or calf thymus DNA), the efficiency of oxidative or reductive electron transfer between photoexcited ruthenium(II) chelates Ru(tap)2(hat)2+ or Ru(phen)2+(3) (where tap = 1,4,5,8-tetraazaphenanthrene, hat = 1,4,5,8,9,12-hexaazatriphenylene, and phen = 1,10-phenanthroline) and appropriate cationic quenchers (ethidium, Ru(NH3)3+(6), methyl viologen, or M(phen)3+(3), where M = Co, Rh, Cr) increases 1-2 orders of magnitude compared to the efficiency of the same quenching in microhomogeneous aqueous medium (kq = 0.3-1.8 x 10(9) M-1 s-1). The enhancement is more pronounced when the binding constant of the quencher (10(3) less than Kb less than 10(6) M-1) is large. Similar trends are found when the biopolymers are replaced by sodium poly(styrenesulfonate) (PSS). The accelerated electron transfer process is proposed to be due mainly to the effect of accumulation of the reagents in the electrostatic field of the polymer; if corrections for this effect are introduced (e.g. ratioing [quencher]/[polynucleotide]), the reaction rate becomes essentially independent of the polymer concentration. Based upon a model for electron transfer reaction of the complexes within a small cylindrical interface around the DNA helix, calculations of the bimolecular electron transfer rate constants are computed to be 10(3) times smaller when the reactants are bound to the double-stranded polynucleotides and decreased mobility of the cationic species is apparent. The effect is less pronounced if a simpler polyelectrolyte (PSS) is employed. Emission lifetimes of the Ru(II) polypyridyls bound to the DNA (0.32-2 microseconds, double exponential decays) are discussed as well.