ArF and KrF laser-induced gas-phase photolysis of selenophene and tellurophene: Extrusion of Te and Se and intramolecular 1,3-H shift competing with beta-C-C cleavage in C4H4 residue

ArF (193 nm) and KrF (248 nm) laser-induced photolysis of gaseous selenophe ne and tellurophene (C4H4M, M = Se and Te) has been examined. It is shown t hat, unlike thiophene and furan, selenophene and tellurophene cleave both M -C bonds and yield the elemental heteroatom (Se, Te), 1-buten-3-yne, and et hyne. The proposed mechanism involves an intermediate . HC=CH-CH= CH . dira dical that decomposes via two competitive pathways, namely, 1,3-H shift to 1-buten-3-yne and P-cleavage to two molecules of ethyne. It is shown that t he relative importance of the channels depends both on the energy of the ph oton and on the heteroatom Specifically, the 1,3-H shiftJPcleavage ratios a re 2.3 (193 nm, M = Se), 3.6 (248 nm, M = Se), 1.4 (193 nm, M = Te), and 10 .5 (248 nm, M = Te). The inertness of the Te residuum and the high preferen ce for the 1,3-H shift in KrF laser photolysis of tellurophene suggest that this photolysis can serve as a source of the C4H4 diradical for mechanisti c studies.