Development of Visible-to-NIR Light Absorbing and Emitting Polymers Incorporating Hypervalent Germafluorene-Fused π-Conjugated Systems

Abstract

Novel molecular designs for creating near-infrared (NIR)-light absorbing and emitting materials are required to meet growing demands for NIR light-based technologies. Herein, we demonstrate that the five-coordinate hypervalent germanium (Ge) compounds composed of germafluorene-fused azomethine and azo scaffolds with efficient light absorption and emission in the visible-to-NIR region (λ abs = 505-902 nm, λ PL = 667-1014 nm, Φ PL up to 18.1%). These excellent optical properties are attributed to the hypervalent Ge-centered spiroconjugation and their planar π-conjugated systems.Experimental and theoretical data indicate that the spiroconjugation plays a significant role in the elevation of the HOMO (highest occupied molecular orbital) energy level.Moreover, the introduction of bulky substituents at the germafluorene moiety contributes to enhancing planarity of the π-conjugated systems, which further narrows their energy gaps. Since the bulky substituents not only improved processability but also suppressed intermolecular interactions, efficient NIR emission was also observed in polymer films.These results suggest that the integration of hypervalent states with π-conjugated systems can be one of effective strategies for tuning electronic properties of NIR-light absorbing and emitting materials.