INSTITUTO DE QUIMICA, FISICA DE LOS MATERIALES, MEDIOAMBIENTE Y ENERGIA
Unidad Ejecutora - UE
congresos y reuniones científicas
Cage effect in supercritical fluids and compressed gases in the photolysis of an asymmetrically substituted diazene.
PABLO A. HOIJEMBERG; JOCHEN ZERBS; MARÍA LAURA JAPAS; CARLOS ALBERTO CHESTA; JÖRG SCHRÖDER; PEDRO F. ARAMENDÍA
Cubatao, San Pablo, Brasil
Congreso; 9º Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2008
An asymmetrically substituted diazene was synthesized: (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl diazene. The compound has ,* ( = 2.05 ± 0.05 104 M-1cm-1) and diazo n,* ( = 70 ± 10 M-1cm-1) absorption bands, which have their maxima at 254 nm and at 365 nm. It dissociates from the 1n,* state with 0.5 quantum yield in cyclohexane. It has a good solubility in supercritical fluids (SCF) and compressed gases, allowing the photolysis measurements to be performed at 1.4 K above Tc and at pressures as low as 70 bar. Picosecond photolysis of the diazene was carried out in cyclohexane and supercritical CO2 and Xe, as well as in compressed Kr, at different temperature and pressure conditions. It showed the build up of the 1-biphenyl-4-yl-1-methyl-ethyl (BME·) radical by nitrogen loss of the diazene with a risetime of 0.7 ps either in cyclohexane or SCF CO2 at 155 bar and 308 K. Relative cage effect was monitored by measuring the absorbance at 320 nm (At0) corresponding to the total amount of out of cage BME· radical produced after nanosecond laser pulses at 266 nm along different isotherms. In supercritical CO2 and Xe, isothermal values of At0 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. On the other hand, the temperature dependence of At0 at densities close to the critical value evidences an activated process previous to bond scission. The comparison of the behavior of At0 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 analysed.