Zwitterionic Cyanine-Cyanine Salt: Structure and Optical Properties

Cyanines with long conjugation length such as heptamethines are well known to have strong intramolecular and/or intermolecular interactions (i.e., ion paring and aggregation), which can affect their structures (i.e., symmetry breaking) and optical properties remarkably. In this paper, we report a covalently linked complementary cyanine complex of cationic and anionic heptamethines forming a highly polarizable zwitterionic cyanine-cyanine salt. The effect of the modification was studied in detail on both its electronic structure and its optical properties. This novel zwitterionic salt was found to exhibit a decreased ion-pairing-induced charge localization but with an increased electronic coupling between the excited states of the cyanine cation and the anion, resulting in unusual optical properties compared to closely related noncovalent complementary cyanine salts. The dual-arm Z-scan technique was used to study their third-order NLO properties, suggesting the electronic coupling in such preorganized zwitterionic system has a weak impact on NLO properties, and both cyanines contribute to the large third-order molecular polarizabilities exceeding 4 × 10^-32 esu for potential all-optical photonic applications.