The reversible [4 + 4] photocycloaddition of acridizinium derivatives

Several aspects of the photoinduced dimerization of acridizinium ions, also known as benzo[b]quinolizinium ions or 4a-azoniaanthracenes, are reviewed. In particular, the photochemical and photophysical studies of the [4 + 4] photocycloaddition and the cycloreversion of the corresponding acridizinium dimers in solution, in the solid state and in other organized media are presented and discussed. Examples are given for the possible application of this photochromic system for data storage or for photoinduced switching of material properties.