ChemPhysChem 2000-12-15

Quantum yields lower than unity in photo- induced dissociative electron transfers: the reductive cleavage of carbon tetrachloride.

L Pause, M Robert, J M Savéant

Index: ChemPhysChem 1(4) , 199-205, (2000)

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Abstract

It has been shown recently that the electrochemical reduction of carbon tetrachloride in N,N'-dimethylformamide follows a mechanism in which electron transfer and bond cleavage are concerted. We report here results concerning photoinduced electron transfer from the singlet excited state of two aromatic molecules, 2-ethyl-9,10-dimethoxyanthracene and perylene, to CCl4 , which is characterised by a quantum yield of complete quenching fragmentation ranging from 0.7 to 0.8. It is shown that a quantum yield below unity is compatible with a dissociative mechanism and arises from partitioning of the system at the intersection of the product- and ground-state potential energy surfaces. This phenomenon predominates over back electron transfer from the clustered fragments state. The photoinduced reductive cleavage of CCl4 thus provides a clear illustration of the recent theoretical prediction, that photoinduced dissociative electron transfers are not necessarily endowed with a unity quantum yield. This offers an opportunity to estimate the magnitude of the electronic matrix element that couples the fragmented product state and the ground reactant state.© 2000 WILEY-VCH Verlag GmbH, Weinheim, Fed. Rep. of Germany.