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 β-C−C Cleavage in C₄H₄ Residue

ArF (193 nm) and KrF (248 nm) laser-induced photolysis of gaseous selenophene and tellurophene (C4H4M, M=Se and Te) has been examined. It is shown that, unlike thiophene and furan, selenophene and tellurophene cleave both M-C bonds and yield the elemental heteroatom (Se, Te), 1-buten-3-yne, and ethyne. The proposed mechanism involves an intermediate .HC=CH-CH=CH. diradical that decomposes via two competitive pathways, namely, 1,3-H shift to 1-buten-3-yne and beta-cleavage to two molecules of ethyne. It is shown that the relative importance of the channels depends both on the energy of the photon and on the heteroatom. Specifically, the 1,3-H shift/beta-cleavage ratios are 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 preference 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 mechanistic studies.