The squaramide motif with the rigid four-member ring structure that shows the aromaticity is a kind of important and ideal hydrogen bond donor/receptor unit, which can bond to many guest molecules as a double-hydrogen-bond receptor and donor. The squaramide derivatives are easy to be functionalized, which provides the convenient way for the design of the novel ion receptors. In this review, we summarize the structures of squaramide derivatives, synthetic methods, and their recent research progress in the anion, cation, and zwitterionic guest recognition, as well as in the organocatalysis. Finally, the prospects of squaramide derivatives in the future are described.

Contents
1 Introduction
2 Responsive mechanism of responsive photonic crystals
3 Device structure of electrically responsive photonic crystals
3.1 Conductive substrate and electrolyte
3.2 Opal and inverse opal photonic crystal electroactive materials
4 Classification of electrically responsive photonic crystals
4.1 Liquid crystal-based electrically responsive photonic crystals
4.2 Polyelectrolyte hydrogel-based electrically responsive photonic crystals
4.3 Organometallic polymer gel-based electrically responsive photonic crystals
4.4 Conductive polymer-based electrically responsive photonic crystals
4.5 Core-shell electrically responsive photonic crystals
5 Application of electrically responsive photonic crystals
6 Existing problems and outlook