The synthesis of chiral, cationic nonlinear optical dyes based on the 1,1′-binaphthalenyl unit

The R enantiomers of three, novel, 2D dipolar cations with 2,2′-bis(dimethylamino)-1,1′-binaphthalenyl electron donor groups connected to N-R-pyridinium acceptors were synthesised. The compounds displayed intense intramolecular charge-transfer bands in the visible region, the maximal energy of which decreased as R changed in the order Me > 2,4-dinitrophenyl > 2-pyrimidyl. Molecular quadratic nonlinear optical responses were determined for the chloride salts in methanol using femtosecond, hyper-Rayleigh scattering with 1300 and 800 nm lasers. The measured first hyperpolarizabilities β HRS were in the range 34–155 × 10 −30 esu and increased at both wavelengths, showing the expected inverse relationship with respect to the charge-transfer energies. Single crystal X-ray structures were determined for the precursor compounds ( R)-6,6′-dibromo-2,2′-bis(dimethylamino)-1,1′-binaphthalenyl and ( R)-2,2′-bis(dimethylamino)-1,1′-binaphthalenyl-6,6′-dicarbaldehyde, revealing an average dihedral angle between the naphthalenyl units of 73°. Making the assumption that this angle was similar to that found in the chromophores studies in solution allowed the determination of two independent tensor components, β zzz and β zxx . For these molecules the greatest proportion of the total first hyperpolarizability lay along the charge-transfer axis ( β zzz ), but the “off-diagonal” β zxx values were still substantial (31–141 × 10 −30 esu) and larger when compared with those determined by hyper-Rayleigh scattering for other 2D dipoles in the literature.