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

In recent years, near-infrared (NIR) dyes, exhibiting absorption in the NIR region (750-2500 nm), have been applied to various optical applications such as security marking, photovoltaic cells and chemotherapy of deep tissues in vivo. Electrochromic systems capable of switching NIR absorption are attractive from the viewpoint of applications for material and life science, and thus several examples have been reported to date. The development of organic-based materials is needed to reduce the environmental impact and improve biocompatibility, however, 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, several studies on organic electrochromic systems that allow ON/OFF switching of NIR absorption have been developed in recent years. In this review, we focus on redox-active well-defined small molecules that enable ON/OFF switching of NIR absorption, and present recent studies on their intrinsic electrochemical and spectroscopic properties and/or structural characterization of their charged states. We also address more sophisticated electrochromic systems that can modulate their properties in response to external stimuli such as light, heat, and electric potential.