Cage Effect in Supercritical Fluids and Compressed Gases in the Photolysis of an Asymmetrically Substituted Diazene

We studied the photolysis of (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene in supercritical CO2 and Xe, as well as in compressed Kr. The compound has good solubility in the mentioned fluids, allowing the photolysis measurements to be performed in CO2 at 1.4 K above T c and at pressures as low as 70 bar. We monitored relative cage effect after nanosecond laser pulses by measuring the absorbance at 320 nm (ΔA t→0) corresponding to the total amount of out-of-cage 1-biphenyl-4-yl-1-methyl-ethyl radical (BME·) produced after nitrogen loss of the diazene. In supercritical CO2 and Xe, isothermal values of ΔA t→0 showed an increase−decrease behavior with increasing pressure at constant temperature, a typical feature of the transition from the solvent energy transfer to the friction controlled regimes. The comparison of the behavior of ΔA t→0 in CO2 at reduced temperatures between 1.004 and 1.027, in Xe, and in Kr points to an absence of enhanced cage effect near the critical point. Compatibility with spectroscopic data is analyzed.