Photolytic Studies of Norbornadiene Derivatives under High-Intensity Light Conditions.

The photoconversion of a norbornadiene (NBD) derivative was studied under high-intensity mono- and polychromatic light conditions at high concentrations. The photoisomerization quantum yield (ϕ), proceeding from NBD to its quadricyclane (QC) isomer, was determined using a tunable OPO laser and a solar simulator light source. The solar simulator was designed to mimic the AM1.5G solar spectrum between 300 and 900 nm. Using the OPO laser, ϕ was measured at discrete values between 310 and 350 nm in steps of 10 nm, and a variation between 0.81 and 0.96 was observed. Weighting these values of ϕ with the spectral profile of the solar simulator, an averaged value of 0.87 ± 0.03 was obtained. Determination of ϕ was also performed directly in the solar simulator providing a value of 0.97 ± 0.14, in good agreement with the weighted values from the OPO. Photoisomerization quantum yields were found to decrease slightly at higher concentrations. At high concentrations, we found that correcting for the presence of QC was important due to similar absorption coefficients of the NBD and QC isomers at the absorption tail. Cyclability of the forward and backward NBD/QC conversion was studied over several cycles. The NBD/QC couple exhibited excellent thermal stability, but a slight photodegradation per cycle was observed, increasing with the concentration of the sample. This result indicates that the molecules undergo some intermolecular reactions.