The synthesis of chiral, cationic nonlinear optical dyes based on the 1,1 0 -binaphthalenyl unit Benjamin J. Coe a, * , Elizabeth C. Harper a , Koen Clays b , Edith Franz b a School of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, UK b Department of Chemistry, University of Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium article info Article history: Received 14 December 2009 Received in revised form 22 January 2010 Accepted 25 January 2010 Available online 18 February 2010 Keywords: Organic salts Nonlinear optics Hyper-Rayleigh scattering Chirality abstract The R enantiomers of three, novel, 2D dipolar cations with 2,2 0 -bis(dimethylamino)-1,1 0 -binaphthalenyl electron donor groups connected to N-R-pyridinium acceptors were synthesised. The compounds dis- played 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 b HRS were in the range 34e155  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 deter- mined for the precursor compounds (R)-6,6 0 -dibromo-2,2 0 -bis(dimethylamino)-1,1 0 -binaphthalenyl and (R)-2,2 0 -bis(dimethylamino)-1,1 0 -binaphthalenyl-6,6 0 -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, b zzz and b zxx . For these molecules the greatest proportion of the total first hyper- polarizability lay along the charge-transfer axis (b zzz ), but the “off-diagonal” b zxx values were still substantial (31e 141  10 30 esu) and larger when compared with those determined by hyper-Rayleigh scattering for other 2D dipoles in the literature. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction There is continued interest in the synthesis and study of molecular nonlinear optical (NLO) materials, primarily inspired by various potential applications such as optical data processing and biological imaging [1]. Although most organic NLO compounds are charge neutral, salts are especially promising due to their often high stabilities and synthetic tailorability. By varying the counterion accompanying an active charged chromophore, it is possible to modify crystal packing arrangements to produce polar structures showing bulk quadratic NLO effects such as second harmonic generation (SHG) and electro-optic (EO) behaviour. Much attention has focused on stilbazolium species, especially (E)-4 0 -(dimethyla- mino)-N-methyl-4-stilbazolium tosylate (DAST) [2] and closely related compounds [3]. Because of their large EO coefficients, such materials are useful for terahertz (THz) wave generation via nonlinear frequency mixing, of relevance to a range of applications including security scanning, biomedical analysis and space communications [4]. One approach to producing polar materials with some degree of control is to incorporate chirality into either the active chromophore or the accompanying ions if the compound is a salt. Resolved chiral species must inevitably crystallise noncentrosymmetrically, although it is noteworthy that this does not guarantee that the molecular dipoles will align favourably. This strategy has been adopted successfully with stilbazolium compounds by Andreu et al. who produced a chromo- phore bearing a 2-(methoxymethyl)pyrrolidinyl substituent which packs in the space group P2 1 and gives a large powder SHG response of 80 times a urea standard (measured using a 1907 nm laser) [5]. The 1,1 0 -binaphthalenyl unit is a convenient chiral motif that has been incorporated into various neutral NLO compounds [6], including polymeric materials [6fek] and also some charged organometallic species [7]. Very recently, Yan et al. reported a dicationic 1,1 0 - binaphthalenyl chromophore for use as a probe for imaging cell membranes via resonance-enhanced SHG [8], although the molecular- level NLO properties of this species were not described. Previous studies in our laboratory have shown that the static first hyper- polarizabilities b 0 (the origin of quadratic NLO effects) of stilbazolium * Corresponding author. Tel.: þ44 161 275 4601; fax: þ44 161 275 4598. E-mail address: b.coe@manchester.ac.uk (B.J. Coe). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2010.01.018 Dyes and Pigments 87 (2010) 22e29