The 77 K emission spectra of a series of [Ru(Am)6-2n(bpy)n]2+ complexes (n = 1-3) have been determined in order to evaluate the effects of appreciable excited state (e)/ground state (g) configurational mixing on the properties of simple electron-transfer systems. The principal focus is on the vibronic contributions, and the correlated distortions of the bipyridine ligand in the emitting MLCT excited state. To address the issues that are involved, the emission band shape at 77 K is interpreted as the sum of a fundamental component, corresponding to the {e,0'} --> {g,0} transition, and progressions in the ground-state vibrational modes that correlate with the excited-state distortion. Literature values of the vibrational parameters determined from the resonance-Raman (rR) for [Ru(NH3)4bpy]2+ and [Ru(bpy)3]2+ are used to model the emission spectra and to evaluate the spectral analysis. The Gaussian fundamental component with an energy Ef and bandwidth Deltanu1/2 is deconvoluted from the observed emission spectrum. The first-, second-, and third-order terms in the progressions of the vibrational modes that contribute to the band shape are evaluated as the sums of Gaussian-shaped contributions of width Deltanu1/2. The fundamental and the rR parameters give an excellent fit of the observed emission spectrum of [Ru(NH3)4bpy]2+, but not as good for the [Ru(bpy)3]2+ emission spectrum probably because the Franck-Condon excited state probed by the rR is different in symmetry from the emitting MLCT excited state. Variations in vibronic contributions for the series of complexes are evaluated in terms of reorganizational energy profiles (emreps, Lambdax) derived from the observed spectra, and modeled using the rR parameters. This modeling demonstrates that most of the intensity of the vibronic envelopes obtained from the frozen solution emission spectra arises from the overlapping of first-order vibronic contributions of significant bandwidth with additional convoluted contributions of higher order vibronic terms. The emrep amplitudes of these complexes have their maxima at about 1500 cm(-1) in frozen solution, and Lambdax(max) decreases systematically by approximately 2-fold as Ef decreases from 17,220 for [Ru(bpy)3]2+ to 12,040 cm(-1) for [Ru(NH3)4bpy]2+ through the series of complexes. Corrections for higher order contributions and bandwidth differences based on the modeling with rR parameters indicate that the variations in Lambdax(max) imply somewhat larger decreases in first-order bpy vibrational reorganizational energies. The large attenuation of vibrational reorganizational energies of the [Ru(Am)6-2n(bpy)n]2+ complexes contrasts with the apparent similarity of reorganizational energy amplitudes for the absorption and emission of [Ru(NH3)4bpy]2+. These observations are consistent with increasing and very substantial excited-state/ground-state configurational mixing and decreasing excited-state distortion as Ef decreases, but more severe attenuation for singlet/singlet than triplet/singlet mixing (alphage > alphaeg for ...

The variations in the nonchromophoric ligands of [Ru(L)4bpy]2+ complexes are shown to result in large changes in emission band shapes, even when the emission energies are similar. These changes in band shape are systematically examined by means of the generation of empirical reorganizational energy profiles (emreps) from the observed emission spectra (Xie, P.; et al. J. Phys. Chem. A 2005, 109, 4671), where these profiles provide convenient probes of the differences in distortions from the ground-state structures of the 2,2-bipyridine (bpy) ligands (for distortion modes near 1500 cm(-1)) in the metal-to-ligand charge-transfer (MLCT) excited states for a series of complexes with the same ruthenium(II) bipyridine chromophore. The bpy ligand is nearly planar in the X-ray structures of the complexes with (L)4 = (NH3)4, triethylenetetraamine (trien), and 1,4,7,10-tetraazacyclododecane ([12]aneN4). However, for (L)4 = 5,12-rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane, the X-ray crystal structure shows that the bpy ligand is twisted in the ground state (a result of methyl/bpy stereochemical repulsion) and the emrep amplitude at about 1500 cm(-1) is significantly larger for this structure than for the complex with (L)4 = 1,4,8,11-tetraazacyclotetradecane, consistent with larger reorganizational energies of the bpy distortion modes in order to form a planar (bpy(-)) moiety in the excited state of the former. The trien and [12]aneN4 complexes have very nearly the same emission energies, yet the 40% smaller vibronic sideband intensity of the latter indicates that the MLCT excited state is significantly less distorted; this smaller distortion and the related shift in the distribution of distortion mode reorganizational energy amplitudes is apparently related to the 36-fold longer lifetime for (L)4 = [12]aneN4 than for (L)4 = trien. For the majority (77%) of the [Ru(L)4bpy]2+ complexes examined, there is a systematic decrease in emrep amplitudes near 1500 cm(-1), consistent with decreasing excited-state distortion, with the excited-state energy as is expected for ground state-excited state configurational mixing in a simple two-state model. However, the complexes with L = [12]aneN4, 1,4,7,10-tetraazacyclododeca-1-ene, and (py)4 all have smaller emrep amplitudes and thus less distorted excited states than related complexes with the same emission energy. The observations are not consistent with simple two-state models and seem to require an additional distortion induced by excited state-excited state configurational mixing in most complexes. Because the stereochemical constraints of the coordinated [12]aneN4 ligand restrict tetragonal distortions around the metal, configurational mixing of the 3MLCT excited state with a triplet ligand-field excited state of Ru(II) could account for some of the variations in excited-state distortion. The large number of vibrational distortion modes and their small vibrational reorganizational energies in these complexes indicate that a very large number of relaxation channels ...

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A photoelectrochemical solar cell based on porous ZnO-covered TiO 2 film has been fabricated with ruthenium bipyridyl complex as the sensitizer. The cell generated a shortcircuit photocurrent of 21.3 mA cm -2 and an open-circuit voltage of 712 mV under irradiation of 81.0 mW cm -2 white light from a xenon lamp with an overall conversion efficiency of 9.8%. Compared with the pure TiO 2 (anatase) film, the ZnO-covered TiO 2 film possesses more outstanding ability to transport electrons with an overall power conversion efficiency increase by 27.3%. Optical elctrochemical studies show that surface modification of TiO 2 with ZnO can increase the concentration of free electrons in the conduction band of TiO 2 . This result implies that the charge recombination is reduced in the process of electron transport through the porous network, which can decrease the photocurrent loss and hence improve both short-circuit photocurrent and open-circuit photovoltage.

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The 77 K emission spectra of 21 [Ru(L) 4bpy] ( m+ ) complexes for which the Ru/bpy metal-to-ligand-charge-transfer ( (3)MLCT) excited-state energies vary from 12 500 to 18 500 cm (-1) have vibronic contributions to their bandshapes that implicate excited-state distortions in low frequency ( lf; hnu lf < 1000 cm (-1)), largely metal-ligand vibrational modes which most likely result from configurational mixing between the (3)MLCT and a higher energy metal centered ( (3)LF) excited state. The amplitudes of the lf vibronic contributions are often comparable to, or sometimes greater than those of medium frequency ( mf; hnu mf > 1000 cm (-1)), largely bipyridine (bpy) vibrational modes, and for the [Ru(bpy) 3] (2+) and [Ru(NH 3) 4bpy] (2+) complexes they are consistent with previously reported resonance-Raman (rR) parameters. However, far smaller lf vibronic amplitudes in the rR parameters have been reported for [Os(bpy) 3 ] (2+), and this leads to a group frequency approach for interpreting the 77 K emission bandshapes of [Ru(L) 4bpy] ( m+ ) complexes with the vibronic contributions from mf vibrational modes referenced to the [Os(bpy) 3] (2+) rR parameters (OB3 model) and the envelope of lf vibronic components represented by a "progression" in an "equivalent" single vibrational mode ( lf1 model). The lf1 model is referenced to rR parameters reported for [Ru(NH 3) 4bpy] (2+). The observation of lf vibronic components indicates that the MLCT excited-state potential energy surfaces of Ru-bpy complexes are distorted by LF/MLCT excited-state/excited-state configurational mixing, but the emission spectra only probe the region near the (3)MLCT potential energy minimum, and the mixing can lead to larger distortions elsewhere with potential photochemical implications: (a) such distortions may labilize the (3)MLCT excited state; and (b) the lf vibrational modes may contribute to a temperature dependent pathway for nonradiative relaxation.

RecQ family helicases play a key role in chromosome maintenance. Despite extensive biochemical, biophysical, and structural studies, the mechanism by which helicase unwinds double-stranded DNA remains to be elucidated. Using a wide array of biochemical and biophysical approaches, we have previously shown that the Escherichia coli RecQ helicase functions as a monomer. In this study, we have further characterized the kinetic mechanism of the RecQ-catalyzed unwinding of duplex DNA using the fluorometric stopped-flow method based on fluorescence resonance energy transfer. Our results show that RecQ helicase binds preferentially to 3-flanking duplex DNA. Under the pre-steady-state conditions, the burst amplitude reveals a 1:1 ratio between RecQ and DNA substrate, suggesting that an active monomeric form of RecQ helicase is involved in the catalysis. Under the single-turnover conditions, the RecQ-catalyzed unwinding is independent of the 3-tail length, indicating that functional interactions between RecQ molecules are not implicated in the DNA unwinding. It was further determined that RecQ unwinds DNA rapidly with a step size of 4 bp and a rate of ϳ21 steps/s. These kinetic results not only further support our previous conclusion that E. coli RecQ functions as a monomer but also suggest that some of the Superfamily 2 helicases may function through an "inchworm" mechanism.Helicases are molecular motor proteins that use the energy of nucleotide triphosphate hydrolysis to translocate along and separate the complementary strands of a nucleic acid duplex. These enzymes play essential roles in most aspects of the DNA metabolic pathway, such as replication, repair, recombination, and transcription (1-5). A large number of helicases have been identified; however, the mechanisms by which helicases unwind double-stranded DNA (dsDNA) 3 remain obscure.One of the major concerns in studying the unwinding mechanism of a DNA helicase is its oligomeric state. This is because an oligomeric structure could provide multiple potential binding sites for the DNA substrate and nucleotide cofactors, which are absolutely required for the helicase to translocate along the DNA track during the processive DNA unwinding. Indeed, the biochemical and structural studies have shown that some helicases assemble into stable cooperative hexameric rings (3). These helicases include the Escherichia coli DnaB (6, 7) and Rho (8, 9), bacteriophage T4 gp41 (10, 11), and T7 gp4 (12)(13)(14). DNA passes through such a central channel and is unwound with a high processivity. For the non-ring helicases, the enzymes could be assembled into oligomers, and the DNA binding sites may be located on the separate subunits. On the basis of quantitative analyses of DNA binding properties, a "rolling model" was proposed to explain factorial DNA unwinding (15). This model suggests that each monomer of the Rep dimer binds alternatively to ssDNA and dsDNA. This process is regulated by repeated binding and hydrolysis of ATP and release of ADP. In this way, Rep dimer rolls along ...

A high-molecular-weight and well-defined ladder polyphenylsilsesquioxane (Ph-LPSQ) was synthesized via a new three-step approach: monomer self-organization in solution, lyophilization, and surface-confined polycondensation. A ladder superstructure, which served as a template to direct the polycondensation, was self-assembled from the 1,3-diphenyl-tetrahydroxy-disiloxane monomer (M) in acetonitrile solution. Following that, it was lyophilized to form a thin layer on the inner surface of a flask. Subsequently, polycondensation of the ordered monomeric thin layer was performed under a triethylamine (TEA) atmosphere. This strategy increased the ladder regularity of the Ph-LPSQ by preventing common complications faced in solution polycondensation of silanol-containing monomers, such as cyclization and gelation side reactions. 29Si NMR analysis showed a very narrow peak (peak width at half-height, w 1/2 = 2.5 ppm) at δ = –78.5 (corresponding to a Ph-SiO3/2unit), indicating a high degree of regularity of the polymer structure.