Highly Light‐Sensitive Luminescent Cyanostilbene Flexible Dimers

Switchable luminescent organic materials with unusual high light-sensitivity in the bulk at room temperature are described. The compounds consist of two promesogenic photoactive cyanostilbene units linked by a flexible spacer with an odd number of carbon atoms, analogous to the structural design of a liquid crystal dimer. Experimental and computational studies support the formation of excimers in solution due to the characteristic geometry of the dimers. In their condensed phases they show a high tendency to pack in lamellar phases, exhibiting bent-core lamellar liquid crystal phases and crystalline polymorphs with different photoluminescence and high quantum yields. The compounds are highly photoreactive, and their photoactivity in terms of photoisomerization and/or photocycloaddition reactions in solution and in the bulk is different. By irradiation of the compounds in the condensed phases at room temperature a fast photochemical process takes place, identified as a photocycloaddition reaction favored by the lamellar packing. Due to this photoreaction a luminescence modulation with green/blue color switching and an increase in quantum yield takes place.