Origin Of Second-Order Nonlinear Optical Effects Of Nonpoled Donor-Acceptor Chromophores On Surface

Thin films of alkyl conjugated donor-acceptor (D-A) molecules, prepared by a high-yielding addition reaction between electron-rich alkynes and tetracyanoethylene (TCNE) or 7,7,8,8-tetracyanoquinodimethane (TCNQ), were investigated by ultraviolet-visible-near infrared (UV-vis-NIR) spectroscopy, in-plane and out-of-plane X-ray diffraction (XRD), and second-harmonic generation (SHG) measurements. The variable self-assembling ability of D-A chromophores produced different molecular orientations in the thin films. The TCNQ-adducted D-A molecule formed a centrosymmetric bilayer structure based on the strong interactions between the chromophores, while the film of the TCNE-adducted counter molecule was composed of interdigitated alkyl layers and randomly oriented chromophores due to their weak interactions. The interfacial interactions between a substrate and a TCNE-adducted molecular layer were expected to induce a noncentrosymmetric chromophore orientation in the thin film, which produced an effective SHG coefficient (d(eff)) of ca. 21pmV(-1) without electric poling. (C) 2011 The Japan Society of Applied Physics