Covalent Attachment Strategies of Molecular Electrochromes for Enhancing Electrochromic Performance.

Electrochromes are emerging materials for enabling sustainable energy devices such as smart windows and low power-consuming displays along with automotive mirrors. This is owing to their electrochemical activity that results in unique optical transmission, modulating with applied potential. The molecular design rules of electrochromes are well established, consisting of electroactive components such as viologens, rhodamines, and transition metal complexes. While molecular electrochromes offer the advantage of establishing accurate structure/property relationships for tuning the optical transmission contingent on molecular structure, their physisorption on the electrodes limits the performance of electrochromic devices. Indeed, molecular electrochromes suffer from poor performance compared to their polymer counterparts in operating electrochromic devices. This perspective presents approaches to overcome these challenges. Focus is given to various strategies of covalently attaching molecular electrochromes to the device electrode for improving key electrochromic properties of contrast ratio and coloration efficiency. These operating device metrics are improved compared with the physisorption of molecular electrochromes via noncovalent interactions. The overarching goal is to provide useful insight that can be leveraged for the rational design of molecular electrochromes for their covalent attachment to electrodes toward matching device metrics of their polymer counterparts.