PHOTOCYCLIZATION OF CIS-DI(2-NAPHTHYL)-ETHYLENE AND ITS FLUORINE DERIVATIVES - FORMATION OF 2 COLORED INTERCONVERTIBLE MODIFICATIONS OF DIBENZO-4A,4B-DIHYDROPHENANTHRENE DERIVATIVES

UV irradiation of cis-di(2-naphthyl)-ethylene and several fluorinated derivatives results in photocyclization to analogues of 4a,4b-dihydrophenanthrene, but only in those cases where the expected product is not fluorinated in the 4a and/or the 4b positions. In solutions of the fluorinated naphthyl-ethylenes, the photocyclization is much less attenuated on cooling than in the parent compound. Photocyclization products of both rotamers are observed under the appropriate conditions. At room temperature, the primary coloured cyclization product L of the A rotamer rapidly converts into the more stable coloured S modification. In the temperature range -40 to -150-degrees-C, L is the sole photoproduct. It undergoes facile photoconversion into the corresponding cis isomer even at -150-degrees-C, while the photoconversion S --> cis is an activated process, making S practically light stable and fluorescent at and below -30-degrees-C. The energy barrier for the thermal conversion L --> S is 15-17 kcal mol-1. No photoconversion L reversible S is observed.