Synthesis of Hypervalent Gallium(III) Complexes and Their Ability to Turn‐On Luminescent Sensors for Lewis Bases

The introduction of main‐group elements into the π‐conjugated systems enables the modulation of optical properties based on changes in the coordinated state of the elements, and the use of heavy elements with high Lewis acidity is expected to broaden the range of coordination response. In this study, focus is placed on hypervalent gallium azobenzene complexes with variable coordination motifs because of a wide accessible space around gallium. The six‐, five‐, and four‐coordinated gallium azobenzene complexes are isolated with pyridine ligands, and it is observed that the coordination of pyridine can dynamically change in the solution. Interestingly, the enhancement of the absolute fluorescence quantum yields is observed by the suppressing structural relaxation as increasing the coordination number. Furthermore, it is found that the emission efficiency is enhanced as increasing donor number which represents the strength of Lewis basicity. Finally, it is demonstrated that the filter paper soaked by the synthesized gallium complex is prepared, and it worked as a turn‐on luminescent sensor to easily visualize strength of Lewis basicity of solvents by simply dropping the solvent on the paper. The findings are valuable for the development of designable and controllable stimuli‐responsive materials focusing on the inherent properties of the main‐group elements and π‐conjugated systems.