Excited State Dynamics of a Conformationally Fluxional Copper Coordination Complex.

The conversion of solar energy into chemical fuel represents a capstone goal of the 21st century and has the potential to supply terawatts of power in a globally distributed manner. However, the disparate time scales of photodriven charge separation (∼fs) and steps in chemical reactions (∼µs) represent an inherent bottleneck in solar-to-fuels technology. To address this discrepancy, we are developing earth-abundant coordination complexes that undergo light-induced conformational rearrangements such that charge separation (CS) is hastened, while charge recombination (CR) is slowed. To these ends, we report the preparation and characterization of a new series of conformationally fluxional copper coordination complexes that contain a twisted intramolecular charge transfer (TICT) fluorophore as part of their ligand scaffold. Structural and spectroscopic characterization of the Cu(I) and Cu(II) complexes formed with these ligands in their ground states establish oxidation state-dependent conformational dynamicity, while time-resolved emission and transient absorption spectroscopies define the photophysical parameters of photo-induced excited states. Building on initial reports with a related set of molecules, the improved ligand design presented here greatly simplifies the observed photophysics, effectively shutting down unwanted ligand-centered excited states previously observed. Time-dependent density functional theory (TDDFT) analyses reveal an unusual metal-to-TICT electronic transition only reported once before, and though the formation of a CS state is not observed directly through experiments, TDDFT geometry optimizations in the excited states support the formation of transient Cu(II) CS species, lending credence to the potential success of our approach. These studies establish a clear model for the excited state dynamics at play in proof-of-concept systems and clarify key design parameters for future optimizations toward achieving long-lived CS via photoinduced conformational gating.

A shock tube study of OH + H(2)O(2) --> H(2)O + HO(2) and H(2)O(2) + M --> 2OH + M using laser absorption of H(2)O and OH.

The rate constants of the reactions: (1) H2O2+M-->2OH+M, (2) OH+H2O2-->H2O+HO2 were measured in shock-heated H(2)O(2)/Ar mixtures using laser absorption diagnostics for H(2)O and OH. Time-histories of H(2)O were monitored using tunable diode laser absorption at 2550.96 nm, and time-histories of OH were achieved using ring dye laser absorption at 306 nm. Initial H(2)O(2) concentrations were also determined utilizing the H(2)O diagnostic. On the basis of simultaneous time-history measurements of OH and H(2)O, k(2) was found to be 4.6 x 10(13) exp(-2630 K/T) [cm(3) mol(-1) s(-1)] over the temperature range 1020-1460 K at 1.8 atm; additional measurements of k(2) near 1 atm showed no significant pressure dependence. Similarly, k(1) was found to be 9.5 x 10(15) exp(-21 250 K/T) [cm(3) mol(-1) s(-1)] over the same temperature and pressure range.

High-temperature shock tube measurements of dimethyl ether decomposition and the reaction of dimethyl ether with OH.

We measured the first high-temperature rate measurements of two dimethyl ether (DME) reactions, (1) DME + Ar --> CH3O + CH3 + Ar and (2) DME + OH --> CH3OCH2 + H2O, in a shock tube by monitoring OH radicals. OH was measured with a narrow-line width laser absorption diagnostic using the well-known R1(5) line of the A-X(0,0) transition at 306.7 nm. The rate k1 is in the falloff regime at high temperatures, so it was measured at several pressures from 0.6 to 11.5 atm and temperatures from 1349 to 1790 K. OH radicals were formed by shock-heating mixtures of DME and O2 in Ar. These mixtures take advantage of the rapid decomposition of the product CH3O, forming H-atoms, which react with O2 to form OH. In carefully chosen mixtures, OH concentration is primarily sensitive to k1 and the well-known rate of H + O2 --> OH + O. Uncertainty in the k1 measurements was estimated to be +/-35%. The rate measurements were then modeled using RRKM theory, which describes the data quite well. Both the rate measurements and the RRKM model were fit from 1000 to 1800 K using the Troe falloff form: k(1,infinity)(T) = (4.38 x 10(21))T(-1.57) exp(-42,220 K/T) s(-1), k(1,o) = 7.52 x 10(15) exp(-21,537 K/T) cm3 mol(-1) s(-1), and F(cent) = 0.454 exp(-T/2510). The rate of k2 was measured at pressures near 1.6 atm and temperatures from 923 to 1423 K. OH radicals were generated by the thermal decomposition of the OH precursor tert-butyl hydroperoxide (TBHP), and k2 was inferred from the observed decay of OH with an estimated uncertainty of +/-40%. The high-temperature measurements were compared with several rate evaluations and previous low-temperature measurements. The rate evaluation by Curran et al. of k2 = (6.32 x 10(6))T2 exp(328 K/T) (cm3 mol(-1) s(-1)) was found to be an excellent fit to both the previous low-temperature measurements and this work.

Isolation and growth of a cytopathic agent from multiple sclerosis brain tissue.

Although many studies support a role for viruses in multiple sclerosis (MS) etiopathology, no specific agent has been consistently associated with significant numbers of MS patients without concomitant detection in non-MS controls. Previous studies have shown the presence of viral-like structures in MS plaques, although the specificity of these structures for MS has been questioned. The present study describes the use of polyclonal antisera against feline and human brain-derived cytopathic agents and immunoaffinity chromatography to purify and partially characterize possible virus-like structures from MS brain tissue. Chromatography eluates from 4 MS brains contained pleomorphic particles up to 350 nm in diameter and tubular/filamentous-like structures approximately 10-18 nm in thickness. Inoculation of primary rat glial cell cultures with chromatography eluates from MS brain tissue resulted in a reproducible pattern of cytopathic effects in the form of multinucleation in cells identified immunocytochemically as oligodendrocytes. Antisera raised against the feline and MS-derived cytopathic agents were used to successfully immunolabel infected oligodendrocyte-like cells and syncytia and to detect a 66,000 M(r) protein on Western blots of inoculated cultures or concentrated MS brain eluates. Similar structures, cytopathic effects (CPE) and protein expression were not observed in eluates from 5 control brains or in cultures inoculated with control brain eluates. These studies demonstrate that cytopathic, virus-like structures can be isolated from MS brain tissue using antisera raised against a cytopathic agent rescued from demyelinating brain lesions in cats. The identity of this agent and its possible role in MS aetio-pathology remains unknown.