a b s t r a c t Dipolar, V-shaped compounds derived from 4H-pyranylidene-linked acceptors have been synthesized, and their linear and nonlinear optical properties (displaying lb values up to 3000 Â 10 À48 esu) have been compared to those of analogous one-dimensional derivatives. The pyranylidene ring behaves strictly as a spacer, and not as a donor group. Second-order nonlinear optical (NLO) materials are at the fore-front of research in the fields of optoelectronics and photonics which, for the last two decades, have undergone a rapid growth, in both academia and industry, with applications in telecommuni-cation and optical information processing, to name just a few. To that end, donor–p–acceptor (D–p–A) compounds with one-dimen-sional charge transfer (1D-CT) character have been widely studied, leading to the establishment of useful structure/property relation-ships. 1 Alternatively, dipolar two-dimensional (2D) molecules with multiple donor/acceptor arrangements, such as 'X-shaped' 2 and 'V-shaped' (also called 'K-shaped') 3 NLO-phores, have attracted a great deal of attention since they show improved hyperpolarizabil-ities (b) when compared to their 1D counterparts and also offer the possibility of circumventing the nonlinearity-transparency trade-off. 4 Polyenic spacers are very effective at promoting the intramolec-ular charge transfer (ICT) that is necessary for the second-order NLO response, but NLO-phores with such subunits often show de-creased thermal or (photo)chemical stability, 5 a drawback that can be overcome by (partly) incorporating the p system into alicyclic, aromatic, or heterocyclic structures. 1b,d Thus, it is not surprising that 2,6-dialkyl-4H-pyran-4-one derivatives had been studied to prepare both 1D and 2D (D–A–D) NLO-phores, since the incorpora-tion of that fragment allows the introduction of (i) one C@C bond of the spacer into a ring, (ii) strong acceptor groups on position 4 of that ring and (iii) the linking of one or two donor groups through Knoevenagel reactions of the 2-(and 6-) alkyl groups. 6 In this Letter, we report the synthesis and characterization of two new 2,6-dimethyl-4H-pyran-4-ylidene derivatives bearing strong acceptors, 1e and 1f, and their reactions with p-diethylami-nobenzaldehyde. The electrochemical, linear, and nonlinear optical properties of the resulting push-pull V-shaped compounds (6e–f) are compared to those of similar derivatives (6a–d) prepared from previously known pyranylidene acceptors. Moreover, a comparison between the properties of some 1D and 2D compounds is made thanks to the use of closely related derivatives. Compounds 1a (commercially available), 7 1b, 8 1c, 6d,9 and 1d 6d,9 were prepared as previously described. The hitherto unreported acceptors 1e and 1f were prepared by Ac 2 O-mediated reaction of commercially available pyrone 2 with 3 10 and 4, 11 respectively (Scheme 1). Target compounds 6a–f were synthesized by the Knoevenagel reaction of p-diethylaminobenzaldehyde 5 (2 equiv) with accep-tors 1a–f (Scheme 2). The design of linear chromophores comparable to V-shaped molecules 6 must take into account the reactivity of the methyl groups at positions 2 and 6 of the pyran ring, which may give rise to mixtures of mono-and disubstituted derivatives, 6b,6d,12 even when equimolar amounts of 1 and an aldehyde are used. Moreover, nonsymmetrical acceptors such as 1d–f are expected to yield two different monosubstitution products, very difficult to separate. To avoid these problems we have chosen to prepare model com-pounds 8a–b, bearing only one reactive position and a symmetric 0040-4039/$ -see front matter Ó