Development of NIR Emissive Fully-Fused Bisboron Complexes with π-Conjugated Systems Including Multiple Azo Groups

Development of novel near-infrared (NIR) emitters is essential for satisfying the growing demands of improving optical telecommunication and medical technology. We synthesized elemental skeletons composed of robust π-conjugated systems including two boron-fused azo groups, which showed intense emission in red or near-infrared (NIR) region both in solution and solid states. Two types of bisboron complexes with different aromatic linkers showed emission properties with larger bathochromic shifts and emission efficiencies in solution than the corresponding monoboron complex. Transient absorption spectroscopy disclosed that the inferior optical properties of the monoboron complex should be attributed to fast nonradiative deactivation accompanied by large structural relaxation after photoexcitation. The expanded π-conjugated system through multiple boron-fused azo groups can contribute to rigid molecular skeletons followed by improvement of emission properties. Moreover, the anti-form of bisboron complex having fluorine groups in the opposite directions to the π-plane exhibited crystallization-induced emission enhancement in the NIR region. The molecular design by using multiple boron-fused azo groups is expected to be a critical strategy for creating novel NIR emitters.