Frankfurt 2006 – scientific programme
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MO: Molekülphysik
MO 23: Photochemistry
MO 23.1: Talk
Tuesday, March 14, 2006, 14:00–14:15, H12
Ultrafast dissociation and recombination of substituted diarylmethylchlorides — •Cord Elsner1, Uli Schmidhammer1, Eberhard Riedle1, and Herbert Mayr2 — 1LS für BioMolekulare Optik, Ludwig-Maximilians-Universität München — 2Department Chemie und Biochemie, Ludwig-Maximilians-Universität München
Derivatives of diarylmethane Ar2CH-X are well proven precursors for the investigation of the reactivity of diarylcarbenium ions produced by the photoinitiated dissociation to Ar2CH and X. On the microsecond timescale both the radicals and the ions are detected optically. It has, however, been reported that the relative yield of the two product pairs depends strongly on the substitution and the solvent. We have now investigated the primary dissociation process and the subsequent sub-nanosecond kinetics with tuneable UV excitation and broadband UV to visible detection with sub-100 fs resolution. At 330 nm we can observe the concentration of the radicals and at 430 nm the diarylcarbenium ions. We find that the dissociation proceeds in just a few hundred femtoseconds. In about 10 ps part of the radicals convert to ions, presumably by electron transfer between the closely spaced radical pair. Within the next 100 to 200 ps a sizable fraction of the ions recombines to the diarylmethyl compound. This means that only after this time the ions separate sufficiently to avoid geminate recombination and further recombination will occur with diffusion limited speed. The specifics of the processes can be tuned by suitable substitution of the basic diarylmethylchloride. This allows the comparison of the geminate recombination probability with the known “macroscopic“ reactivity.