The enthalpies of isomerization of norbornadiene and of several substituted norbornadienes to the corresponding quadricyclenes have been determined by the method of photocalorimetry. This method also gives f the percent of absorbed light energy of a given wavelength that is stored chemically as a consequence of the photoinduced isomerization. The results for 25 °C are as follows (compound, solvent, AH in kcal mol™1, /waVeiength,nm): norbornadiene, cyclohexane, 14.0 ± 1,/313 = 15.0 ± 1%; 2,3-bis(carbomethoxy)norbornadiene, acetonitrile, 25.2 ± 3.5,/313 = 12.4 ± 2.2%; 2,3-dicyanonorbornadiene, acetonitrile, 22.0 ± 2.3,/313 = 11.2 ± 1.9%; l,2,3-trimethyl-5,6-dicyanonorbornadiene, acetonitrile, 21.0 ± 2.0,/366 = 15.5 ± 1.7%. The position of the first absorption band shifts progressively to longer wavelengths in the above sequence, and the results show that such shifting toward the solar insolation region does not impair the energy storage ability of the system. For the parent, norbornadiene system, it was necessary to use acetophenone as sensitizer, and it is possible that minor secondary photolysis effects led to the relatively low AH value; the value has the merit that it is the practical one for conditions relevant to actual solar energy storage.
