Quantum Yield of Photolysis

The quantum yields for the photolysis of compounds 4, 6 and 8 were determined by an HPLC-based method [28]. The frequency-tripled (355 nm) output of a pulsed Nd:YAG laser (QuantaRay GCR-18S, Spectra Physics Lasers, Santa Clara, CA) was isolated from the fundamental and second harmonic emissions by a pair of harmonic separators (BSR-35, CVI Laser, LLC, Albuquerque, NM) and used for photolysis; the energy at the sample was ~100 mJ per pulse. The light intensity was determined by potassium ferrioxalate actinometry [29], and showed the laser power to be highly stable and the integrated energy output to be a linear function of time. The photorelease reactions were characterized by simple 1:1 stoichiometry between the caged starting material and the released product. To quantitate disappearance of starting caged compound, equal amounts of 4-nitrophenol were added as an internal standard to irradiated samples and to unirradiated controls. In the HPLC traces of these samples, the ratio of the areas of the peaks corresponding to the starting material and the internal standard were determined. From this, the loss of the starting material on photolysis was calculated. In all cases, 0.1 M sodium phosphate buffer (pH 7.41) was used as solvent, and the extent of photolytic conversion was kept at ~10%. Each photolysis experiment was performed in triplicate, and each of the triplicate samples was analyzed two or three times by HPLC.