John S. Connolly

Limiting and realizable efficiencies of solar photolysis of water

A critical analysis of various schemes for the solar photolysis of water into hydrogen and oxygen places ideal limits on their efficiencies. Single-bandgap schemes will have practical efficiencies of up to 10%, but the prospects of higher efficiencies from dual-bandgap schemes are encouraging.

Synthesis and characterization of a directly linked porphyrin-anthraquinone molecule

Abstract We have synthesized a porphyrin-anthraquinone molecule (PAQ) in which the AQ moiety is attached directly at a meso -position of tritolylporphyrin. The absorption spectrum shows pronounced perturbations in the bands of both the P and AQ groups that are independent of solvent polarity; a charge-transfer band was not observed. In contrast, the spectrum of the porphyrin fluorescence in PAQ is only moderately red-shifted, but the lifetimes and intensities depend markedly on solvent dielectric constant: the fluorescence is quenched negligibly in solvents with ϵ s ≤ 4, moderately in solvents with 4 s ≤ 6 and strongly in solvents with ϵ s ≥ 7. In methylene chloride, the major emitting component has a lifetime of ∼30 ps as compared to ∼9.0 ns for both tetratolylporphyrin (TTP) and an ester-linked TTP-AQ molecule. An electron-transfer mechanism is implicated even though the energetics for net electron transfer do not appear to be favorable (the sum of the redox potentials being essentially isoenergetic with the porphyrin S 1 state in benzonitrile). We infer that the short distance between the P and AQ moieties (∼1.4 A edge-to-edge) compensates for the otherwise marginal energetics in accord with Marcus theory. However, calculations of the reorganization energy, based on a two-sphere dielectric continuum model, and estimates of the solvent-dependent reaction energetics, using the Weller equation, do not yield a meaningful correlation with the fluorescence data measured in 19 solvents and binary solvent mixtures. Electron transfer in this intimately linked donor-acceptor molecule may involve an inner-sphere and/or adiabatic mechanism.

A Diene Synthesis of Quinone Aldehydes

Abstract Cycloaddition of 2-(hydroxymethyl)-1,3-butadiene with representative quinones occurs readily in refluxing toluene. Oxidative dehydrogenation of the resulting cycloadduct with activated manganese dioxide in refluxing benzene affords quinone aldehydes in good to excellent overall yield.

Effects of temperature and intensity on thermodynamic limits for efficiencies of photochemical conversion of solar energy

The subject of thermodynamic limits on photochemical conversion of light to work has been of considerable interest for over twenty years. Recently, Ross and Hsiao calculated quantum conversion efficiencies for solar radiation at air mass zero (AMO). Bolton later extended this treatment to AM1.2 solar flux and also considered some kinetic as well as thermodynamic limitations. These methods are applied to a variety of solar intensities and absorber temperatures. Also, improvements in efficiency which can be obtained by using systems with several absorbers of different effective band-gap wavelengths are examined. The results, which are applicable to photovoltaic as well as to photochemical and photobiological conversion devices, represent absolute (i.e., ideal) upper limits on conversion efficiencies, analogous to Carnot efficiencies of heat engines. Also, calculations for two-photon processes (i.e., two discrete absorbers) are carried out over a limited range of temperature-intensity combinations. The treatment is extended to calculate the optimum wavelengths for multiphoton cases (3 less than or equal to n less than or equal to 8) at fixed temperature and intensity. (WHK)

Inter- and Intramolecular Quenching of Porphyrin Excited States by Quinones

We have measured the bimolecular rate constants for quenching of the excited singlet and triplet states of the free-base and zinc forms of tetraphenylporphine (TPP) by three quinones with greatly different reduction potentials in several solvents spanning a range of dielectric constant and viscosity. Where available, we have made comparisons with unimolecular rate data for some analogous linked systems. Our data indicate that the orientation of the donor and acceptor not only effects the rates of excited-state quenching, but can also determine whether such interactions result in net electron transfer. In bimolecular interactions, the geometry between the donor and acceptor is not restricted, and fluorescence quenching can be efficient even when net electron transfer is energetically unfavorable. However, in linked systems, when the geometries of the two π-ring systems are restricted, no fluorescence quenching is observed unless the energy requirements for net electron transfer are met. The rates of both inter- and intramolecular quenching of excited singlet states correlate with the energetics for electron transfer, but other mechanisms appear to be involved in intermolecular quenching of the triplet state of free-base TPP.