Enhancement of Near-Infrared Emission Based on Hypervalent Germanium(IV)-Fused Azobenzene Compounds with Electron-Donating Groups

Abstract NIR-light emitting materials are significant as a source for NIR-light technology. Previously, we have found that hypervalent germanium-fused azobenzene (GAz) compounds showed NIR emissions despite the small π-conjugated systems owing to trigonal bipyramidal geometry with three-center four-electron (3c-4e) bonds. Herein, we succeed in enhancing their luminescence properties by introduction of electron-donating groups. It is revealed that the frontier molecular orbitals of the π-conjugated systems with the hypervalent bonds can be varied depending on the type of substituents. As a result, a GAz compound with diphenylamine groups shows efficient NIR emission at room temperature (λPL = 761 nm and ΦPL = 0.07) and at 77 K (λPL = 749 nm and ΦPL = 0.20). Furthermore, data from theoretical calculations support the experimental results and provide additional information on the producibility of the hypervalent compounds. Our findings afford strategies to expand the library of the fine-tunable NIR-luminescent materials involving heteroatoms.