Photophysics and photochemistry of an asymmetrically substituted diazene: A suitable cage effect probe
The photophysics and photochemistry of (l-biphenyl-4-yl-l-methyl-ethyl)- tert-butyl diazene were thoroughly studied by laser flash photolysis from the picosecond to the microsecond time domain. The compound has favorable features as a radical photoinitiator and as a probe for cage effect studies in...
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| Autores principales: | , |
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2009
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| Acceso en línea: | https://bibliotecadigital.exactas.uba.ar/collection/paper/document/paper_10895639_v113_n19_p5531_Hoijemberg http://hdl.handle.net/20.500.12110/paper_10895639_v113_n19_p5531_Hoijemberg |
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| Sumario: | The photophysics and photochemistry of (l-biphenyl-4-yl-l-methyl-ethyl)- tert-butyl diazene were thoroughly studied by laser flash photolysis from the picosecond to the microsecond time domain. The compound has favorable features as a radical photoinitiator and as a probe for cage effect studies in liquids, supercritical fluids, and compressed gases. The biphenyl moiety acts as an antenna efficiently transferring electronic energy to the dissociative 1n,π* state centered on the azo moiety. By picosecond experiments irradiating at the biphenyl-and at the azo-centered transitions, we were able to demonstrate this fact as well as determine a lifetime of 0.7 ps for the buildup of 1-biphenyl-4-y1-1-methyl-ethyl radicals (BME ·). The sum of in-cage reaction rate constants of BME· radicals by combination and disproportionation is 5 × 1010 s-1. The free radical quantum yield in solution is 0.21 (ØBME·) in n-hexane at room temperature, whereas the dissociation quantum yield approaches 50%. The symmetric ketone, 2,4-bis-bipheny1-4-y1-2,4-dimethyl-pentan-2-one, was used as a reference compound for the production and reaction of BME· radicals. Transient IR measurements show CO stretching bands of the excited 3π, π* and 1n,π* states but no dissociation up to 0.5 ns. A fluorescence lifetime of 1 ns for this ketone is consistent with this observation. By transient actinometry and kinetic decays in the microsecond time range, we measured εBME· = (2.3 ± 0.2) × 104 M-1 cm-1 at 325 nm and a second-order rate constant of 5.8 × 109 M -1s-1 for the consumption of BME· radicals. © 2009 American Chemical Society. |
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