PHOTO-CHEMICAL ELECTRON-TRANSFER IN MONOLAYER ASSEMBLIES .2. PHOTO-ELECTRIC BEHAVIOR IN CHLOROPHYLL-A-ACCEPTOR SYSTEMS

A photoelectric effect has been observed in monolayer assemblies of chlorophyll and acceptor molecules deposited on a semitransparent aluminum electrode. The counterelectrode was mercury. With acceptors containing saturated side chains an open-circuit photovoltage of 150 mV and a short-circuit photocurrent of 34 nA cm-2were detected across a load resistor. The quantum yield of electrons per photon was ~2 X 10-4and the maximum power conversion efficiency was ~4 X 10-5. The current-voltage behavior was rectifying with a dark conductivity of ~1.5 X 10-13Ω-1cm-1. If acceptors with unsaturated side chains were used such as ubiquinone or plastoquinone (10 mol % in stearic acid), the conductivity improved by a factor of 10. The open-circuit photovoltage now was 270 mV and the short-circuit photocurrent detected was 140 nA cm-2. The quantum yield was ~2 X 10-3and the maximum power conversion efficiency was ~4 X 10-4. The reduction of the quinone group left the photoresponse unchanged. Similarly, the use of the polyisopropene squalene instead of plastoquinone left the photoresponse unchanged. The suggestion is made that the polyisopropene chain may be acting as “nature's molecular wire” making possible the tunnelling of electrons through lipid membranes. © 1979, American Chemical Society. All rights reserved.