[n]-Polyenovanillins (n = 1−6) as New Push−Pull Polyenes for Nonlinear Optics:  Synthesis, Structural Studies, and Experimental and Theoretical Investigation of Their Spectroscopic Properties, Electronic Structures, and Quadratic Hyperpolarizabilities

We describe the synthesis and quadratic hyperpolarizability properties of [ n ]-polyenovanillins, a new family of push−pull polyenes deriving from vanillin, thiovanillin, and isothiovanillin. Twenty-five [ n ]-polyenovanillins of structure 3,4-(RO;R‘S)C 6 H 4 (CH CH) n CH O [with n = 1−6 and R, R‘ = Me or n -Bu] were synthesized by using a Wittig−Horner polyvinylogation methodology, and the structure of one of these was established by single-crystal X-ray crystallography. The UV−vis absorption and fluorescence spectra of these polyenovanillins were recorded and interpreted in light of CNDO/S calculations that allowed the identification of the different transitions. EFISH measurements provided up to 350 × 10 - 30 esu (for 14, n = 6) with an exponential increase of the quadratic hyperpolarizability with the conjugation length. These data were satisfactorily interpreted by means of a CNDO/S/NLO method which provided information as to the relative orientation of the dipole moment μ and β vec, the vector part of the hyperpolarizability tensor. In the sulfur-substituted series, there is a significant divergence between the orientation of μ and β vec, which in turn leads to an underestimation of β in the EFISH method. The calculation also revealed that when the length of the polyenic chain increases, the charge transfer from the donor toward the acceptor actually tends to concentrate on the polyenic chain rather than on the aldehyde group.