INVESTIGADORES
ARAMENDIA Pedro Francisco
congresos y reuniones científicas
Título:
Photolysis of isopropylbiphenyl containing -diazene and -cetone. Radical precursors for probing confinement effects in supercritical fluids.
Autor/es:
PABLO A. HOIJEMBERG; JOCHEN ZERBS; PHILIP WAGENER; JÖRG SCHRÖDER; CARLOS ALBERTO CHESTA; MARÍA LAURA JAPAS; PEDRO ARAMENDÍA
Lugar:
Salvador, Brasil
Reunión:
Congreso; 17 IAPS meeting; 2006
Resumen:
Diazenes are extensively used to initiate radical polymerization because they render a pair of radicals by nitrogen loss very efficiently in fluid media. Studies were mainly centered in the photolysis of symmetric diazenes. Many ketones also show the same dissociation pattern with loss of CO, but this loss in the primarily formed acyl radical is normally slower than the loss of nitrogen from the corresponding azenyl radical. Asymmetric ketones were used as probes for confinement effects from the solid state to supercritical fluids (SCF). We undertook the study of asymmetric diazene photolysis as a priori adequate probes for confinement effects in SCF. We used the symmetric ketone as reference precursor of the isopropylbiphenyl radical. In this work, we report on nanosecond and picosecond flash photolysis, as well as on steady state irradiation experiments of (1-Biphenyl-4-yl-1-methyl-ethyl)-tert-butyl-diazene (AZ) and bis-(1-Biphenyl-4-yl-1-methyl-ethyl)-ketone (PCK) in the near and far UV in hexane, in cyclohexane, and in SCF CO2. For AZ in normal liquid solution, there are different absorbing species in the first 2ps after irradiation in the nð* or in the ðð* absorption bands. Irradiation in the nð* band shows the formation of the benzyl radical with ca.1 ps lifetime. This means that the first bond breaking is faster than the resolution of the experiment (<200 fs). The photolysis of the ketone shows the formation of the first benzyl radical but not of the second at t<50ps. Photolysis in the nanosecond to microsecond time range shows out of cage benzyl radicals and biphenyl centered triplet for both substances. The benzyl radical decays with a second order kinetics with k/å = 2.55 105 cm/s at 325 nm. The triplet decays with a first order rate constant of 2.2 105 s-1. AZ shows good solubility in SCF CO2. Flash photolysis of AZ in the picosecond time range also shows the build up of the benzyl radical in 2 ps. The diazene system seems adequate to probe confinement effects in SCF.2. For AZ in normal liquid solution, there are different absorbing species in the first 2ps after irradiation in the nð* or in the ðð* absorption bands. Irradiation in the nð* band shows the formation of the benzyl radical with ca.1 ps lifetime. This means that the first bond breaking is faster than the resolution of the experiment (<200 fs). The photolysis of the ketone shows the formation of the first benzyl radical but not of the second at t<50ps. Photolysis in the nanosecond to microsecond time range shows out of cage benzyl radicals and biphenyl centered triplet for both substances. The benzyl radical decays with a second order kinetics with k/å = 2.55 105 cm/s at 325 nm. The triplet decays with a first order rate constant of 2.2 105 s-1. AZ shows good solubility in SCF CO2. Flash photolysis of AZ in the picosecond time range also shows the build up of the benzyl radical in 2 ps. The diazene system seems adequate to probe confinement effects in SCF.
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