160 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



pointed out in his contribution ^ our complete dependence on the proc- 

 ess known as photosynthesis : the use by green plants of solar energy 

 in the visible spectrum to produce carbohydrates out of carbon doxide 

 and water. He also emphasized the extreme complexity of the proc- 

 ess — the fact that no one has been able to extract the essential 

 chlorophyll and carotenoids from a plant leaf and make the reaction 

 go in a test tube. By some process, which we have hardly begun to 

 imderstand, the leaf structure succeeds in capturing the energy of 

 sunlight and transferring it to the reaction: carbon dioxide + 

 water = carbohydrate + oxygen, a reaction absorbing 112 kilocal- 

 ories per gram atom of carbon. But to store solar energy chemically 

 one does not have to carry out the same reaction that nature does; 

 any chemical reaction which absorbs energy and produces a fuellike 

 product capable of later combustion to return the energy for use at 

 the proper time would be acceptable. Chemical industry has often 

 succeeded in competing with nature in the production of a material 

 of desired characteristics, not by attempting a complete imitation of 

 nature, but by focusing attention on those properties of the natural 

 material important to its use and imitating them with a synthetic 

 product, perhaps chemically quite different from nature's product. 



In the photochemical field, then, a combination of sensitizers and 

 catalysts might be attempted that would allow us to perform some 

 relatively simple energy-storing reaction such as the decomposition 

 of water. A major problem woulH be to provide suitable inter- 

 mediate steps in the process in order that the relatively small energy 

 quanta, which constitute visible light, could be used in stepwise fash- 

 ion such as nature apparently uses them in the photosynthetic ap- 

 paratus of green plants. The photochemical system would probably 

 have one of the characteristics of the photochemical system of the 

 plant, namly heterogeneity. But the heterogeneity might be accom- 

 plished not by constructing some sort of imitation leaf, but rather, 

 for example, by a colloidal solution. 



Another approach to the problem is possible. We may renounce 

 the production of metastable products or mixtures with a high con- 

 tent of chemical energy — fuels or explosives — and turn our attention 

 to the utilization of the energy of the unstable intermediate products 

 obtained in almost every photochemical reaction. Among the ways 

 of utilizing these products is to convert their high energy content 

 into electrical energy. A reaction must be found in which passage 

 from the unstable to the stable state can be made to proceed as an 

 electrode reaction in a galvanic cell. Examples of this kind are 

 oxidation-reduction reactions in electrolytes. The properties of such 

 a reaction, carried out in what is known as a photogalvanic cell, are 



»Thlmann, Kenneth V., The action of light on organisms. Sigma XI Quart., vol. 29, 

 No. 1, pp. 23-35, April 1941. 



