We describe the synthesis and quadratic hyperpolarizabilityproperties of [
n]-polyenovanillins, a new family of push-pull polyenes deriving from vanillin,thiovanillin, andisothiovanillin. Twenty-five [
n]-polyenovanillins ofstructure3,4-(RO;R'S)C
6H
4(CH=CH)
nCH=O [with
n = 1-6 and R, R' = Me or
n-Bu] were synthesized by using aWittig-Hornerpolyvinylogation methodology, and the structure of one of these wasestablished by single-crystal X-ray crystallography. The UV-vis absorption andfluorescence spectra of thesepolyenovanillins were recorded and interpreted in light of CNDO/Scalculations that allowedthe identification of the different transitions. EFISHmeasurements provided
up to350 × 10
-30 esu (for
14,
n = 6) with an exponential increase of the quadratichyperpolarizability with the conjugation length. These data weresatisfactorily interpreted by meansof a CNDO/S/NLO method which provided information as to the relativeorientation of thedipole moment
and
vec, the vector part of thehyperpolarizability tensor. In the sulfur-substituted series, there is a significant divergence between theorientation of
and
vec,which in turn leads to an underestimation of
in the EFISH method.The calculation alsorevealed that when the length of the polyenic chain increases, thecharge transfer from thedonor toward the acceptor actually tends to concentrate on the polyenicchain rather thanon the aldehyde group.