Recent Advances in NIR-Switchable Multi-Redox Systems Based on Organic Molecules

Graphical Abstract

π-Conjugated organic systems with multi-step redox behavior often exhibit electrochromic properties. Among them, organic molecules that can exhibit ON/OFF switching of NIR absorption are rare but have been developed rapidly. Isolation, structural determination, and electrochemical and spectroscopic measurements of multiple redox states are essential for a better understanding of their natures.

Abstract

Electrochromic systems capable of switching absorption in the near-infrared (NIR) region (750–2500 nm) are attractive from the viewpoint of applications for material and life science, and thus several examples have been reported to date. In general, the development of organic-based systems is needed to reduce the environmental impact and improve biocompatibility. Although extending the switchable spectral range is crucial for the application of organic electrochromic molecules, the switching of NIR absorption based on redox interconversion is still a challenging issue regarding reversibility and durability during interconversion. To overcome this potential instability, the introduction of heteroatoms into the molecular backbone and/or π-extension could be useful strategies in terms of effective delocalization of charge and spin in the corresponding redox states. In this review, we focus on redox-active well-defined small molecules that enable ON/OFF switching of NIR absorption based on precise control of the redox states, and present recent studies on their intrinsic electrochemical and spectroscopic properties and/or structural characterization of their charged states.