Abstract
Cyanine dyes in dilute solution have demonstrated the required third-order nonlinear optical properties necessary for all-optical switching (AOS) applications. However, in the solid state, interactions between the cyanine and its counterion typically induce significant geometric and electronic changes. When the counterion localizes toward one end of the cyanine, symmetry is broken and the figure-of-merit for AOS is dramatically reduced. Here, we use quantum-chemical methods to assess zwitterionic cyanines based on the heptamethine thiopyrylium dye as a means to eliminate untethered counterion interactions. In these zwitterionic systems, rigid π-conjugated substituents oppositely charged to the cyanine backbone are added to the thiopyrylium central carbon atom. For substituents that are unable to retain their anionic charge, and thus have a lower degree of zwitterionic character, electronic coupling between the substituent and cyanine backbone leads to profound geometry modifications and the thiopyrylium excited-state properties advantageous for AOS are lost. The presence of strong electron withdrawing groups on the substituent allows it to retain more charge, which maintains the favorable cyanine character of the backbone. This improved understanding of the relationship between the zwitterionic cyanine chemical structure and molecular properties can pave the way to better materials for AOS devices.