Abstract
The valence isomerization of quadricyclanes to norbornadienes, the reverse conversion of energy-storing photoisomerization of norbornadienes, proceeds via a redox chain reaction initiated by one-electron oxidation. Substituents on quadricyclanes were screened in order to find out optimal molecule for the hole-catalyzed chain reaction, and quadricyclanes with electron-donating and electron-accepting substituents, which attracted attention as solar energy storage systems, were shown to have excellent properties. Typically, quite high efficiency for the isomerization and hole-transfer reacion could be achieved for quadricyclanes with phenyl as donating and COOR as accepting substituents, respectively. The electrochemical oxidation of such donor-acceptor quadricyclanes proceeded with very high current efficiency of > 1000 mol/F. The quadricyclane oxidized at electrode immediately affords the radical cation of norbornadiene. Since the oxidaion potential of norbornadiene is higher than that of quadricyclane, an electron is instantaneously moved back to the radical cation of norbornadiene, and hence no net current was consumed. Thus, the valence isomerization was catalyzed by the one-electron exchange at the electrode surface.
