Quadratic nonlinear optical properties of correlated chromophores: cyclic 6,6'-dinitro- 1, l'-binaphthyl-2,2'-ethers

The first hyperpolarizability, /3, of a series of cyclic 6,6'-dinitro-l,l'-binaphthyl-2,2'-ethers has been analyzed with hyper-Rayleigh scattering and electric-field-induced second-harmonic-generation and compared with the absorption data and semi-empirical calculations. The results show the critical dependence of /3 on the conformation of the alkoxy donor and the dihedral angle of the binaphthyl unit. The search for new organic molecules with large first hyperpolarizabilities,/3, is of considerable interest [1]. In contrast to their (mono-)dipolar and octopolar counterparts, the multi-dipolar molecules combine several charge-transfer units within a single molecule and hence can provide larger hyperpolarizabilities, without the classic bathochromic shift of the charge transfer band. Apart from the calixarenes [2], the 6,6'-disubstituted derivatives of binaphthol are emerging as useful multi-dipolar molecules [3,4]. In this Letter we present new experimental results on hyper-Rayleigh scattering (HRS) and electric-fieldinduced second-harmonic-generation (EFISHG) measurements on chiral 6,6'-disubstituted binaphthyl ethers in which the dihedral angle is varied by bridging the two oxygen atoms with alkyl chains of different lengths (Fig. 1). The synthesis of these molecules will be published elsewhere [5]. An important advantage of the binaphthyl derivatives is their chirality, which may provide a non-centrosymmetric structure of the active dipolar subunits in the crystalline phase. As these molecules are composed of two dipolar subunits, the EFISHG and HRS measurements have to be analyzed using a vector model for the hyperpolarizability tensor of the bis-dipolar molecule. We have shown the validity of this approach for bis-dipolar binaphthyl derivatives [6] poly(isocyanide)s [7], and bacteriorhodopsin [8]. Quantum-chemical calculations and depolarized HRS measurements have shown that for dipolar molecules the hyperpolarizability tensor is dominated by the tensor component along the direction of the charge-transfer [9,10]. Thus we start from the assumption that the hyperpolarizability tensor of the dipolar subunits can be approximated by a single tensor component (/3mo,o = /3 : . z, with Z along the (mono)molecular chargetransfer axis). If we assume that the two dipolar 0009-2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All fights reserved. PH S0009-261 4(97)00325-4 E. Hendrickx et al. / Chemical Physics Letters 270 (1997) 241-244 243