Photolysis Of Asymmetric Diazenes. From Solid State To Supercritical Fluids: A Cage Effect Study

HOIJEMBERG, PABLO ARIEL; SANRAMÉ, CARLOS NORBERTO; ZERBS, JOCHEN; KARLEN, STEVE; WETZLER, DIANA ELENA; SCHROEDER, JÖRG; CHESTA, CARLOS A.; GARCÍA-GARIBAY, MIGUEL ANGEL; JAPAS, MARÍA LAURA; ARAMENDÍA, PEDRO FRANCISCO

Los Cocos, Provincia de Córdoba, Argentina

Conferencia; 9ª Conferencia Latinoamericana de Fisicoquímica Orgánica (9 CLAFQO); 2007

Asociación Argentina de Investigaciones Físico Químicas, SAIQO

We investigated the photochemistry of an asymmetric azo compound: (1-biphenyl-4-yl-1-methyl-ethyl)-tert-butyl-diazene (AZ). AZ and its expected photolysis products were synthesized. AZ proved to have two interesting features, a high UV absorption and high radical production quantum yield, as well as suitability as a cage effect probe from the solid state to the gaseous phase. The experiments, performed in reaction media that span from low density supercritical fluids to high density matrices, i.e. solids, show how the environment influences the photochemical reaction and the fate of the produced radicals and also let us obtain structural and dynamic information of the systems under study. AZ photolysis in cyclohexane produces a pair of geminate radicals from the first singlet excited state with an efficiency of 45%. AZ has also other interesting features: adequate solubility in near critical CO2, thermal stability, high melting point, and absorption of UV light exclusively. In this work, we report on nanosecond and picosecond flash photolysis of AZ in the UVA and UVC in hexane, in cyclohexane, and in supercritical CO2 (scCO2), as well as on steady state irradiation experiments in the solid state, in hexane, and in cyclohexane. In some cases we used model compounds to provide supporting evidence. Photolysis in the solid state serves as a model for radical’s behavior “in-cage”. Computational calculations aid in the analysis and understanding of the initial steps of the reaction. The reactions inside and outside the solvent cage are evaluated by the distribution of reaction products and escape and diffusion is also evaluated by time-resolved experiments in the nanosecond regime. Solid photolysis over a wide temperature range has been conducted. AZ has a highly efficient intramolecular energy transfer, considering its lack of emission from the excited biphenyl center, when compared to model compounds. Picosecond photolysis shows differences in the early stages of excited AZ when irradiated at the pi,pi* band (biphenyl centered) or at the n,pi* band (diazo centered), being the n,pi* the dissociative state. Photolysis in the nanosecond to microsecond time range shows out of cage isopropylbiphenyl (IPB) radicals and biphenyl centered triplet. Photolysis of AZ in scCO2 gives a bell-shaped plot for IPB escape versus solvent density, showing energy transfer promoting effects at low densities and confinement effects at densities above the critical one. The comparison of behavior in scCO2 and in an inert compressed gas (N2) will let us eventually appreciate the real critical caging effect.