Reversible photodimerization: a new type of photochromism.

The photodimers formed by polycyclic aromatic hydrocarbons, such as anthracene, and their derivatives can be photodissociated to the original monomers. When the photodimer is held in a rigid matrix, the dimer can be remade efficiently because the two monomers are held in the proper relationship for photodimerization. The absorption spectra of monomer and photodimer differ substantially, and associated with this is a difference in refractive index. These characteristics as well as the stability of both states at room temperature make such a system attractive for reversible phase holographic recording with nondestructive readout. We have investigated several photodimer systems as solutions in rigid transparent polymer matrices, as well as some single crystals of photodimers. We have been able to write and erase diffraction gratings repeatedly. The photosensitivities ( 0.1 J cm(-2)), which are close to the maximum possible for a system with no gain, are high enough to be useful. Grating scattering efficiencies in the range of 0.01-5% have been observed. The physical, chemical, and photochemical characteristics of these systems, as well as some basic general considerations, are discussed.