PHOTOPHYSICS AND REVERSIBLE PHOTOCHEMISTRY OF 1,2-DIARYLETHYLENES CONTAINING PERYLENYL OR BIPHENYLENYL AS AN ARYL GROUP

Several ethylene derivatives of perylene (I), and two of biphenylene (II), were synthesized and their photophysical and photochemical properties investigated. Class II compounds undergo facile photoisomerization to the respective cis isomers, whereas class I compounds do not. Class I compounds are strongly fluorescent even at room temperature, whereas class II compounds show only very weak fluorescence. Variation of the emission spectrum with excitation wavelength is observed in most cases where expected because of rotamerism. All compounds exhibit transient spectra assigned to triplet states. There is no evidence for the photocyclization of cis-IIA and cis-IIB to 4a,4b-dihydrophenanthrene-like products. It is concluded that the photochemical and photophysical properties of the investigated diarylethylenes are largely determined by the low energies of the lowest excited states.