Novel Chiral Bis-dipolar 6,6‘-Disubstituted Binaphthol Derivatives for Second-Order Nonlinear Optics:  Synthesis and Linear and Nonlinear Optical Properties

A number of thermally and optically stable, bis-dipolar chiral molecules based on two geometries of the binaphthol (BN) system with different acceptors/substituents have been synthesized for the first time, and the synthetic routes are reported: optically pure 6,6‘-disubstituted 2,2‘-diethoxy-1,1‘-binaphthyls {R, R‘= −Br, −CHO, −CHC(CN)(COOEt), −CHC(CN)2, −CHCHCN, −CHCH(p-NO2Ph)} and optically pure 9,14-disubstituted dinaphtho[2,1-d:1‘,2‘-f][1,3]dioxepins {R, R‘ = −Br, −CHO, −CH=C(CN)(COOEt), −CHC(CN)2, −CHCHCN, −CHCHSO2CH3, −CHCHCHO, −CHCHCHC(CN)2}. All molecules possess two equal donor−acceptor systems linked together to give a bis-dipolar system. Two mono-dipolar 6-substituted 2-butoxynaphthalene (R = −CHC(CN)2, −CHC(CN)(COOEt)) donor−acceptor systems were prepared as references. The linear optical properties including solvatochromic shifts of absorption and fluorescence revealed strong charge transfer excitations in the new dipolar systems. The molecules show a high first hyperpolarizability β (up to (344−364) × 10-30 esu) as measured by electric-field-induced second-harmonic generation (EFISHG) and hyper-Rayleigh scattering (HRS). A model is developed to express the first hyperpolarizability of the bis-dipolar molecules in terms of the molecular geometry and the β of the monomeric donor−acceptor units. The tensor components were determined experimentally using this model and data from HRS and EFISHG. These techniques probe different combinations of the components of the molecular hyperpolarizability tensor. The results obtained are found to be in excellent mutual agreement.