Functionalized Picolinium Quinodimethane Chromophores for Electro-Optics: Synthesis, Aggregation Behavior, and Nonlinear Optical Properties

A new series of functionalized and thermally stable zwitterionic picolinium (dicyano)esterquinodimethane (PeQDM) chromophores have been synthesized in one step from 7,8-di(methoxycarbonyl)-7,8-dicyanoquinodimethane (DMCQ) and various picolinium salts in methanol in nearly 50% overall yields. Hyper-Rayleigh scattering (HRS) measurements at 1.07 μm show very large near-resonant molecular first hyperpolarizabilities β up to 1800 × 10 −30 esu, despite the fact that, compared to previously studied zwitterionic NLO chromophores, one of the strong CN acceptor groups is substituted by a weaker ester group to allow for further functionalization. The ester functionality can be further utilized for attachment of solubility-imparting groups, cross-linkers, and reactive groups for grafting the chromophore onto a polymer. To optimize the electro-optic (EO) activity in the polymer matrix, the aggregation behavior of the PeQDM chromophores is studied. X-ray crystallographic analysis confirms the charge-separated ground-state of PeQDM and reveals the formation of a face-to-face, antiparallel H-aggregation with a large slip angle (θ = 64.5°) between the chromophores. In a less polar polymer matrix, PeQDM chromophores can form H- and/or J-aggregates and only J-aggregates could contribute to the EO activity if the monomeric chromophores released during the thermally induced J−H transformation could be captured and effectively poled. In a more polar polymer host, chromophores are well dissolved as monomers and effectively poled. A large and stable EO activity ( r 33 = 110 pm/V) has been achieved with a polyethersulfone film doped with 5 wt % of N -benzyl picolinium (dicyano)esterquinodimethane (PeQDM-Ben).