CIS-TRANS-PHOTOISOMERIZATION OF BIS-STILBENES WITH ETHYLENE BRIDGES

The mechanisms for Z-E photoisomerization of two bisstilbenes, 4,4'-bis(3,5-di-tert-butylstyryl)bibenzyl, 1, and [2.2]-4,4'-stilbenophane, 2, have been studied. The isomerizations of ZZ-1 and EE-1 are similar to those of Z- and E-stilbene, respectively. On excitation of ZE-1 there is a competition between an energy transfer process from the E- to the Z-chromophore and isomerization. This results in a small quantum yield (<0.01) for isomerization of ZE-1 to ZZ-1, whereas that of ZE-1 to EE-1 is 0.19. The Z-E isomerizations of the stilbenophane, 2, are clearly different from those of 1. EE-2 is, to our knowledge, the first example of two stilbene units forming an excimer. In EZ-2 there is neither efficient energy transfer from EZ*-2 nor from E*Z-2 while for a higher excited state EZ**-2 efficient energy transfer yielding E*Z-2 takes place. The Z-E isomerization of ZZ-2 is again more similar to that of Z-stilbene.