Design and Synthesis of Far-Red to Near-Infrared Chromophores with Pyrazine-Based Boron Complexes.

We synthesized new binuclear boron complexes based on pyrazine with ortho and para substitution patterns. It was demonstrated that the para-linked complexes possess a significantly narrow energy gap between highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), leading to their far-red to near-infrared emission properties. Meanwhile, the ortho-substituted complex showed orange emission. Considering the HOMO and LUMO distributions of pyrazine, the boron complexation to the nitrogen atoms would stabilize its LUMO more efficiently than its HOMO because a nodal plane in the HOMO passes through the two nitrogen atoms. The theoretical study suggests that the para-substitution would not significantly perturb such a characteristic HOMO distribution originating from pyrazine in stark contrast to the ortho-substituted one. As a result, the HOMO-LUMO gap of the para-linked complex is dramatically narrower than that of the ortho-linked one.