Chromophores assembled together, such as in molecular crystals or nanoscale assemblies, give rise to formation of molecular excitons -electronically excited states of molecular aggregates. The molecular excitons are key players in operation of organic optoelectronic devices as well as in the natural process of photosynthesis. A particularly interesting situation arises if the monomer itself -the building block of a molecular aggregate -is a photochromic system. In this case, aggregation may affect not only photophysics of the chromophore (e.g., electronic spectra 6 ) but also its photochemistry (e.g., hinder isomerization .