[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

Abstract

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)C6H4(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 βvec, the vector part of the hyperpolarizability tensor. In the sulfur-substituted series, there is a significant divergence between the orientation of μ and βvec, 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.

Iminium Salts of ω-Dithiafulvenylpolyenals: An Easy Entry to the Corresponding Aldehydes and Doubly Proaromatic Nonlinear Optic-phores

“…Therefore, the two-step synthesis of 1b from 3 herein reported constitutes a simpler, improved procedure, with an overall yield of 74%. We also want to point out that our attempts to prepare 1b by vinylogation of the easily available 1a with a variety of reagents ( A − F ) – did not meet with success. …”

Multipolar engineering of molecules and materials for quadratic nonlinear optics

“…Special attention has been paid to the nature and length of the conjugated path. Polyene-like 'push-pull' molecules have been widely synthesized and characterized as model systems towards the maximization of quadratic [20][21][22] and cubic [23] hyperpolarizabilities and the investigation of possible 'saturation' of hyperpolarizabilities for large values of the number N of carbon-carbon double bonds [24]. Such a saturation effect is related, according to various theoretical models, to the bond length alternation (BLA) δ corresponding to the difference between the lengths of two adjacent carbon-carbon bonds in the polyenic path [25].…”

Experimental and theoretical studies on solvent effects of amphiphilic conjugated polyenalsElectronic supplementary information (ESI) available: Table S1: dipole moments, energies of solvation, HOMO–LUMO band gaps and solvent effects on the first pi–pi transition of the studied alkoxyphenylpolyenals. See http://www.rsc.org/suppdata/cp/b2/b208125c/

“…The UV-Vis spectra of synthesized conjugated polyenals ROC 6 H 4 (CH=CH) n CHO (R ¼ CH 3 , C 6 H 13 , n ¼ 1-6, 8) were measured in seven various polarity solvents (i.e., 2,2,4trimethylpentane (TMP), CCl 4 , tetrahydrofuran (THF), CH 2 Cl 2 , C 2 H 5 OH, CH 3 OH, CH 3 CN and dimethylsulfoxide (DMSO) at 25 C. For the studied compounds the following attempts have been performed: effect of complete p-electron conjugation, n eff [n ¼ n + 2 (ref. 73)], solvent polarity and length of the alkyl chain upon the shape and position of the absorption band. The effective length parameter, n eff defines an equivalent contribution of the phenyl rings to the conjugated electronic system.…”