Alicia Brune

Enzyme-assisted Reforming of Glucose to Hydrogen in a Photoelectrochemical Cell¶

Hydrogen gas has been produced by reforming glucose in a hybrid photoelectrochemical cell that couples a dye‐sensitized nanoparticulate wide band gap semiconductor photoanode to the enzyme‐based oxidation of glucose. A layer of porphyrin sensitizer is adsorbed to a TiO2 nanoparticulate aggregate sintered to a conducting glass substrate to form the photoanode. Excitation of the porphyrin results in electron injection into the TiO2, and migration to a microporous platinum cathode where hydrogen is produced by hydrogen ion reduction. The oxidized sensitizer dye is reduced by NADH, regenerating the dye and poising the NAD+/NADH redox couple oxidizing. The NAD+ is recycled to NADH by the enzyme glucose dehydrogenase, which obtains the necessary electrons from oxidation of glucose. The reforming of glucose produces gluconolactone, which hydrolyzes to gluconate; the electrochemical potential necessary to overcome thermodynamic and kinetic barriers to hydrogen production by NADH is provided by light. The quantum yield of hydrogen is ∼2.5%.

Artificial photosynthetic antenna-reaction center complexes based on a hexaphenylbenzene core

A hexaphenylbenzene scaffold has been used to organize the components of artificial photosynthetic antennas and antenna-reaction center mimics that feature bis(phenylethynyl)anthracene antenna moieties and porphyrin-fullerene charge-separation units. The five bis(phenylethynyl)anthracene chromophores absorb in the spectral region around 430-480 nm, where porphyrins have low extinction coefficients but solar irradiance is maximal. The hexaphenylbenzene core was built up by the well-known Diels-Alder reaction of diarylacetylenes with substituted tetraphenylcyclopentadienones. The latter were in turn prepared by condensation of substituted benzils and dibenzyl ketones, allowing flexibility in the design of the substitution pattern on the core. The spacing between the various chromophores is suitable for rapid singlet-singlet energy transfer among antenna moieties and the porphyrin, and the relatively rigid structure of the hexaphenylbenzene limits conformational heterogeneity that could reduce the efficiency of energy and electron transfer. NMR studies reveal a high barrier to rotation of the porphyirn plane relative to the hexaphenylbenzene.

Bioinspired energy conversion

Artificial photosynthetic antenna systems have been synthesized based on carotenoid polyenes and polymer-polyenes covalently attached to tetrapyrroles. Absorption of light in the blue/green region of the spectra excites the polyenes to their S2 state, and ultrafast singlet energy transfer to the tetrapyrroles occurs when the chromophores are in partial conjugation. The additional participation of other excited states of the polyene in the energy-transfer process is a requirement for perfect antenna function. Analogs of photosynthetic reaction centers consisting of tetrapyrrole chromophores covalently linked to electron acceptors and donors have been prepared. Excitation of these constructs results in a cascade of energy transfer/electron transfer which, in selected cases, forms a final charge-separated state characterized by a giant dipole moment (>150 D), a quantum yield approaching unity, a significant fraction of the photon energy stored as chemical potential, and a lifetime sufficient for reaction with secondary electron donors and acceptors. A new antenna-reaction center complex is described in which a carotenoid moiety is located in partial conjugation with the tetrapyrrole π-system allowing fast energy transfer (<100 fs) between the chromophores. In this assembly, the energy transduction process can be initiated by light absorbed by the polyene.