Creation of flexible heteroatom-fused π-conjugated systems and their application to optical functional materials

Abstract The properties of π-conjugated compounds have traditionally been enhanced by introducing structural “rigidity” into their π-conjugated systems through heteroatom-fused ring structures. In this review, we describe that excellent optical properties and stimuli-responsiveness can also be achieved by loading structural “flexibility” onto π-conjugated compounds with heteroatom-fused ring systems. We employed 2 main strategies to impart such flexibility to heteroatom-fused π-conjugated systems. First, we designed π-conjugated ligands incorporating boron-fused ring structures to induce distortion within the π-conjugated systems. As a result, these systems exhibit bending motions upon photoexcitation, leading to environment-sensitive optical behaviors such as aggregation-induced emission and crystallization-induced emission. Furthermore, by extending the π-conjugation, molecular motions are partially suppressed, and efficient solid-state luminescence and sensitive stimuli-responsiveness can be realized. Second, we constructed π-conjugated systems including hypervalent tin-fused ring structures which are capable of adopting coordination states beyond the conventional Lewis octet rule. The unique electronic interactions originating from these hypervalent bonds allow for distinct modulation of the optical properties through the reversible coordination number changes with attachment and detachment of Lewis bases. By utilizing other hypervalent elements, we further developed a novel class of stimuli-responsive compounds exhibiting element-specific flexibility, such as variations in structural diversity and oxidation states. Through these approaches, we establish a new design strategy for developing advanced optical functional materials based on flexible and heteroatom-fused π-conjugated systems.