Synthesis and photophysical properties of a highly fluorescent azo derivative

A highly fluorescent and photochromic azo derivative has been obtained by linking in a covalent manner a red-emitting squaraine unit to an azo dye. The bifunctional molecule exhibits uncoupled chromophores in the ground state allowing for the specific addressing of the azo photoisomerization and squaraine emission. Photosensitization of the squaraine unit by exciting the azo moiety in its UV absorption band has been obtained. Such an unusual energy transfer has been modulated by changing the solvent from chloroform to toluene. This led to a dramatic enhancement of the emission quantum yield Phi(f) (from 0.35 up to 0.71) without affecting the photoisomerization process. Strong competitive pi-stacking interactions between toluene and squaraine have been put forward to interpret the emission changes.