PHOTOCOLOURATION OF 2,4,4,6-TETRAARYL-4H-PYRANS AND THEIR HETEROANALOGUES - IMPORTANCE OF HYPERVALENT PHOTOISOMERS

Molecular geometries of 2,4,4,6-tetraphenyl-4H-pyran (la), 4,4-(biphen yl-2,2'- diyl)-2,6-diphenyl -4H-pyran (Ib) and their heterocyclic isom ers Il-V were optimized by the PM3 method and used for the calculation of electronic absorption spectra by the CNDO/S-CI procedure. Comparis on of the theoretical data with experimental UV-VIS absorption spectra made possible to select hypervalent molecules IIIa, IIIb, IVa and IVb being responsible for the photocolouration of 4H-pyrans la, Ib, while compounds Va, Vb, VI and VII come into account as possible photodegra dation products. The bleaching process of the UV illuminated compound la is analyzed in terms of dispersive first-order reaction kinetics.