PPV-type π-conjugated polymers based on hypervalent tin(IV)-fused azobenzene complexes showing near-infrared absorption and emission

We demonstrate near-infrared (NIR) absorptive and emissive poly(p-phenylene vinylene) (PPV)-type pi-conjugated polymers based on hypervalent tin-fused azobenzene (TAz) complexes. Taking advantage of the inherent narrow-energy gap of TAz complexes originating from the three-center four-electron (3c-4e) bond and nitrogen-tin (N-Sn) coordination, the synthesized polymers, TAz-PPVs, showed absorption, and emission in wavelength regions of >750 and 810 nm in diluted solution, respectively. From the experimental and theoretical investigations, the elevation of the highest occupied molecular orbital (HOMO) and the reduction of the lowest unoccupied molecular orbital (LUMO) were simultaneously shown to be caused by the extension of pi-conjugation. The effective conjugation length was calculated to be n > 10 (n: degree of polymerization), and the value was comparable to conventional PPV systems. Through this research, we revealed that pi-conjugated systems including hypervalent bonds were able to expand pi-conjugation. According to the concept of "element blocks", the development of heteroatom-containing narrow-energy-gap monomers should be a novel approach for the construction of new NIR-absorptive and emissive bland materials.