Hole transfer equilibrium in rigidly linked bichromophoric molecules

Two bichromophoric molecules consisting of anthracene and diphenylpolyene moieties linked by two fused norbornyl bridges undergo photoionization upon ultraviolet (UV) pulsed laser irradiation. The simultaneous observation of the cation radicals of both anthracene and polyene groups points to a rapid (nanosecond or faster) intramolecular hole transfer equilibrium between the two chromophores. The existence of an equilibrium is supported by the results of one- and two-laser transient absorption and electrochemical experiments. Equilibrium constants (293 K) were determined by both transient absorption and cyclic voltammetry measurements and were independent of the method used within experimental error. For A-sp-VB, which contains anthracene and vinyldiphenylbutadiene chromophores, Keq = 4.0 ? 2 (transient absorption) and 3.2 ? 2 (electrochemical), favoring the anthracene cation radical. For A-sp-VS, containing anthracene and vinylstilbene groups, Keq = 70 ? 30 (transient absorption) and 105 ? 50 (electrochemical), favoring the anthracene cation radical.Peer reviewed: YesNRC publication: Ye

Radical-trapping antioxidants in vitro and in vivo

The contributions from urate (58% ? 18%), plasma proteins (21% ? 10%), ascorbate (14% ? 8%) and vitamin E (7% ? 2%) to the Total Radical-trapping Antioxidant Parameter (TRAP) of plasma account for all of the peroxyl radical-trapping antioxidant activity in the majority of samples. Time dependent studies show that vitamin E is regenerated or spared during peroxyl radical attack by the water soluble antioxidants. The rates of uptake of dietary RRR- and SRR-[alpha]-tocopherols have been determined using deuterium labelled [alpha]-tocopherol acetates. There is a preferential uptake of the natural, RRR-[alpha]-tocopherol by all tissues (except the liver in the first month). These studies have potential implications for human nutrition.NRC publication: Ye

Redox properties

NRC publication: Ye

Electrochemistry of some organic and organometallic radicals and their applications in thermochemical cycles

A simple experiment is described (photomodulation voltammetry) that allows the electrochemical oxidation and reduction potentials of short lived intermediates to be measured directly. These data are used in thermochemical cycles to determine a number of radical cation a-bond energies, DO(R-R+'), in acetonitrile solution. By comparison with gas phase ionization potentials it is shown these bond energies are generally similar in the gas phase implying that the solvation energies of the radical cations and the product cations are the same. Only in those cases where a there is a significant change in the charge distribution of R+ compared to R-R+' are large differences between gas phase and solution observed. A comparison of hydrazine and diphosphine radical cation bond energies also is made. In contrast to hydrazines which form strong two-center three electron bonds upon removal of one electron, there appears to be very little three electron bonding interaction between the phosphorous atoms of tetraaryldiphosphine radical cations. This results in a modest weakening of the P-P bond relative to the neutral species. The standard potentials, EO, for the oxidation of the tributyl- and triphenylstannyl radicals were estimated from a combination of electrochemical and kinetic measurements. These data were combined with reduction potentials of nitroalkanes to estimate the thermochemistry for electron transfer between these species. The free energy change for the electron transfer reaction (outer sphere) must be > 12 kcal mol-I. The rates of these reactions, measured by laser flash photolysis, were on the order of 108 M-Is-I showing, unequivocally, that mechanism of the reaction of stannyl radicals with nitroalkanes does not proceed via outer sphere electron transfer.NRC publication: Ye

Photosolvolysis of a carbon-carbon bond. The photolysis of n,n-dimethyl-2,2-diphenylethylamine in methanol.1

The title compound is photolyzed in methanol to give good yields (ca. 50%) of diphenylmethane indicating that the C-C bond undergoes a formal heterolytic cleavage from the excited state as predicted from the measured redox potentials of the two ions.NRC publication: Ye

Radical thermochemistry and organic reactions

Most free radicals are transient species and it is therefore difficult to measure their thermochemical properties. We have used two new techniques that address this problem. Photoacoustic calorimetry was used to measureheats of formation of free radicals and thus homolytic bond strengths, while photomodulated voltammetry was used to measure their oxidation and reduction potentials. The data offer fresh insights into the thermochemistry of free radical processes and can therefore be used to design new reactions.NRC publication: Ye

Electrochemically directed micropatterning of a conducting polymer covalently bound to silicon

NRC publication: Ye

Bond energies in solution from electrode potentials and thermochemical cycles. A simplified and general approach

NRC publication: Ye