Most of the solar energy collection systems presently in use act to convert solar radiation into heat or electricity, neither of which can be stored for later use as such. One solution to this problem involves the use of solar radiation to drive photochemical processes which result in the production of stable, high energy fuels. For example, the reactions involved in photosynthesis lead to the storage of solar energy in organic fuels such as wood, which can later be burned to produce heat and light. However, the solar production of biomass is not an ideal energy-storage system for man, except with respect to its use as food, since its cultivation requires large amounts of land, water, fertilizers, and is labor-intensive.
While the efficiency of solar energy storage by the complex reactions involved in photosynthesis is about 6-8%, the thermodynamic limitation for photochemical energy storage is about 15-20%. Therefore, considerable research has been directed to the synthesis of compounds which can form energy-rich products upon absorption of solar radiation. The energy thus stored must be regainable at a later date, with simultaneous recovery of the starting material. In recent years, the photochemical storage of solar energy in highly-strained organic molecules has been widely investigated, both theoretically and experimentally. The energy-storing photochemical reaction A.fwdarw.B should proceed in a high chemical yield; preferably greater than 99%, and exhibit an enthalpy of reaction of greater than about 500 J/g. With or without a sensitizer, chemical A should absorb a large percentage of the photochemically effective solar spectrum range (300-700 nm). Compound B should be stable over long periods of time and be reconvertable back to A in high yield.
The high energy storage capacity required by this process has lead to the investigation of isomerizations of A-type molecules which lead to small rings. The valence isomerization between norbornadiene (NBD) and quadricyclane (Q) is one of the most promising of these systems. ##STR1##
Although the enthalpy of isomerization of norbornadiene to quadricyclane is moderate (110 kJ/mol), the low molecular weight (92) means that an exceptionally high storage capacity of 1200 J/g results. Furthermore, Q is extremely stable, the half-life for conversion back to NBD at 140.degree. C. being at least 14 hours. However, either the utilization of an appropriate sensitizer or the introduction of substituents is required to realize a facile NBD.fwdarw.Q isomerization under sunlight, since norbornadiene itself does not absorb solar radiation of greater wavelengths than 300 nm. For example, K. Maruyama et al., in Chemistry Letters, 839 (1981) have reported that 3-phenylcarbamoyl-2,5-norbornadiene-2-carboxylic acid readily undergoes valence isomerization into the corresponding quadricyclane derivative upon exposure to sunlight. Extensive studies on the metal complex-catalyst photoisomerization of norbornadienes to quadricyclanes have also been carried out. See K. Maruyama et al., Chemistry Letters, 1259 (1980) K. Maruyama, et al., Chemistry Letters, 743 (1984), and H.-D. Scharf et al., Angew. Chem. Int. Ed. Engl., 18, 652 (1979). While most of the problems associated with the photochemical conversion of norbornadienes to quadricyclanes have apparently been solved, attempts to develop a simple method for the controlled release of energy from quadricyclanes have been less successful.
For example, R. W. Hoffmann et al., in J. Chem. Soc., Chem. Commun., 345 (1983) have disclosed that the addition of small amounts of stable tris-(p-bromophenyl)aminium salts to quadricyclane solutions catalyzes the Q.fwdarw.NBD isomerization. These salts presumably generate the cation radical of Q (Q.multidot..sup.+) which spontaneously isomerizes to the cation radical of NBD. However, once initiated, the conversion of Q to NBD continues to completion or until the quadricyclane cation radical is deactivated by side reactions. Furthermore, the high cost of the aminium salts, which are consumed during the reaction, also presents a commercial obstacle to the large scale use of this method to release the energy stored in quadricyclanes.
Therefore, a need exists for a method to convert quadricyclanes to the corresponding norbornadienes in high yield. A need also exists for a method of converting quadricyclanes to norbornadienes which can be interrupted and restarted. A further need exists for a method to convert quadricyclanes to norbornadienes which is energetically-favorable with respect to the total energy balance of the system.