Regioregular Synthesis of Azaborine Oligomers and a Polymer with a syn Conformation Stabilized by NH⋅⋅⋅π Interactions

The regioregular synthesis of the first azaborine oligomers and a corresponding conjugated polymer was accomplished by Suzuki-Miyaura coupling methods. An almost perfectly coplanar syn arrangement of the heterocycles was deduced from an X-ray crystal structure of the dimer, which also suggested that NH⋅⋅⋅π interactions play an important role. Computational studies further supported these experimental observations and indicated that the electronic structure of the longer azaborine oligomers and polymer resembles that of poly(cyclohexadiene) more than poly(p-phenylene). A comparison of the absorption and emission properties of the polymer with those of the oligomers revealed dramatic bathochromic shifts upon chain elongation, thus suggesting highly effective extension of conjugation.