Light driven proton pumps

Abstract

Biological systems efficiently use photon powered proton pumps and protonic-electrical circuits for energy conversion. There is no fundamental reason why light driven proton based technical, photoelectrochemical, photovoltaic, and photographic devices should not be feasible. Furthermore, they may be attractive because they could also provide a convenient storage mechanism for solar energy. The biggest challenge is the choice of appropriate materials. Can materials be found which are cheaper, or easier to prepare than electronic photovoltaic materials? Can such systems be tailored to become highly efficient and can they be made to be photochemically stable? The experience with two model systems studied in electrochemical two-compartment cells is discussed. The permeation rate for proton insertion and hydrogen transport through FeS2 (pyrite) with a diffusion coefficient of D=10−5 cm2/s could be increased by illumination from 0.45 to 0.8 parallel with a generation of 150 mV of photopotential. The second system studied was the semiconducting thiophene sexithiophene in which illumination could also increase the permeation rate of protons by a factor of two. An effort was made to learn from a biological model system like the proton pumping bacteriorhodopsin. The light utilization here is based on a photoinduced molecular structure change which, via an aminoacid chain, energizes an array of protonation sites which alter their pK-value. By utilizing the stored chemical energy an autocatalytic process then moves the protons against the existing membrane electrical field. Such biomolecular mechanisms may technically become accessible by genetic engineering only but presently provide interesting model systems for learning about choices in letting protons utilize solar light to perform work and store energy.