ARAMENDIA Pedro Francisco
congresos y reuniones científicas
Photolysis of asymmetric diazenes. from solid state to supercritical fluids: a cage effect study
P. F. ARAMENDIA
Congreso; VIII Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2004
The aim of the project is the study of the cage effect in radical reactions that initiate photochemically from the singlet state. The experiments, performed in reaction media that span from low density supercritical fluids to high density matrices, i.e. solids, will show how the environment influences the fate of the reaction products and will also let us to obtain structural and dynamic information of the systems under study. Photolysis of asymmetric diazenes provides an efficient and irreversible way to produce a pair of geminate radicals from the first singlet excited state. The precursor (1-Biphenyl-4-yl-1-methyl-ethyl)-tert-butyl-diazene) and its expected photolysis products have been synthesized. The compound meets the basic requirements: adequate solubility in near critical CO2, thermal stability, high melting point, and absorption of UV light exclusively. The recombination inside the solvent cage with respect to escape and diffusion recombination in this type of experiments is evaluated by the distribution of reaction products. Disproportionation products are observed, mainly at low temperature (-75°C) photolysis in the solid state. The comparison of behavior in supercritical carbon dioxide (scCO2) and in an inert compressed gas (argon or nitrogen) will let us appreciate the real difference between the plain effect of pressure (through its normal effect on density and viscosity) and the critical phenomenon. The photolysis at high pressure is carried out in a thermostatized stainless steel cell with two sapphire windows, connections for reactant load and purge, a capacitive pressure transductor and a thermistor. A low-pressure mercury lamp is used for photolysis. After irradiation time at a temperature near 35ºC (for scCO2 and inert gases), distillation of the mixture and collection on hexane for further injection in HPLC is performed to evaluate the distribution of the products. Solid photolysis over a wide temperature range has been conducted. Computational calculations aid in the analysis and understanding of the initial steps of the reaction.