Crystal structure characterization of some π-conjugated ionic crystals toward electronic applications

Crystal Structures and Optical Properties of Cyanine Dyes Depending on Various Counter Anions

“…Ionic materials constructed using charged π-conjugated systems have attracted much attention because of their ability to control the properties by a combination of cations and anions. − By using monovalent ions, cations and anions are crystallized in a 1:1 ratio based on electrostatic interactions, and crystals with high melting point and hardness are provided with the strong Coulomb force. Moreover, the crystal structures and electronic states can be influenced by the location and structure of the counter ions as cations and anions are sufficiently close to each other. − Cyanine cations, one of the polymethines, have recently been studied as photoswitching materials with photoisomerization properties and as near-infrared light-emitting materials with an increased Stokes shift. − A symmetrically N-substituted cyanine cation has a long-conjugated resonance hybrid structure expressed using two asymmetrical resonance forms. However, the electronic state can be asymmetrically modified by varying the location of the counter anion (Figure a). − Such asymmetric polar π-electronic states are useful for piezoelectric, pyroelectric, and second-order nonlinear optical materials.…”