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 ($d_{\text{eff}}$) of ca. 21 pm V -1 without electric poling.