PHOTOLYSIS OF TRANS-ALPHA,ALPHA'-DIHALOSTILBENES

The photochemical behaviour of trans isomers of the four alpha,alpha prime-di-X-stilbenes (X triple bond F,Cl,Br or I in the olefinic positions) was studied in solution at room temperature. The main photoreaction of F-2- or Cl-2-stilbene os trans right arrow cis isomerization, whereas diphenylacetylene (DPA) and X(2) are the photoproducts of Br-2- and I-2-stilbene. The quantum yields of Br-2 and I-2 elimination are typically 0.2 and 0.6 respectively and are essentially independent of the kind of solvent and the irradiation wavelength (222, 254 or 313 nm). The same transient (lambda(max) approximate to 410 nm), observed by laser flash photolysis (lambda(exe)=248 nm) of either Br-2- or I-2-stilbene, is assigned to the lowest triplet state of DPA which is formed by consecutive absorption of two photons; the first step generates DPA via the excited singlet state of trans-X(2)-stilbene by homolytic cleavage of the two C-C bonds and subsequent excitation of DPA within the 20 ns laser pulse yields (3)DPA* via intersystem crossing.