Determination of the energy storage efficiency of the photoisomerization of norbornadiene to quadricyclane as a potential means for the trapping of solar energy

Abstract

The fraction of incident radiant energy stored in chemical form by the photosensitized isomerization of norbornadiene to quadricyclane has been estimated. This reaction has been widely viewed as a potential means for the conversion of solar energy to useful heat. Using a xenon lamp as a radiant energy source in lieu of natural sunlight, 1-M acetophenone-norbornadiene and benzophenone-norbornadiene solutions were irradiated for 2-h periods with 1–5-mW radiant power levels delivered in 4-nm bandwidths centered on 300 nm, 350 nm, 400 nm and 450 nm. In each case the reaction was carried out batchwise at four temperatures in the liquid-phase range of the solutions, namely, 278 K, 298 K, 318 K and 333 K, controlled to within ±2 K. The results indicate that about 8.8% of the total available incident light contained in the region from 300 nm (lower end of the solar spectrum that impinges upon the earth's surface) to 425 nm (approximate absorption threshold of the sensitizers acetophenone and benzophenone; maximum wavelength capable of promoting the reaction under the conditions of this study) can be stored in the chemical bonds of quadricyclane in the acetophenone-sensitized isomerization and about 14.5% when benzophenone is utilized. Based on the entire solar spectrum, these energy storage efficiencies translate into 0.9% and 1.5%, respectively. Efficiency values were estimated by determining the quantum yield of the reaction as a function of wavelength and temperature. While the data show no definitive temperature dependence, it has been established that the functional relationship with wavelength follows the same pattern as the absorption spectra of the sensitizers themselves.