a b s t r a c t A series of merocyanines containing isophorone or pyran rings in the spacer have been synthesized. For a given pair of donor and acceptor groups, the isophorone derivatives show higher second-order optical nonlinearities, but lower thermal stabilities. The first NLO-chromophores endowed with a dihydropyran fragment in the spacer have also been prepared and studied. Current achievements in the field of second-order nonlinear optical (NLO) materials are the result of the efforts carried out to establish structure/NLO–activity relationships, ultimately seeking to optimize the microscopic molecular nonlinearity (b) and the macroscopic (bulk) NLO response of such materials. 1–3 To this end, conjugated push–pull compounds, where an electron donor and an electron acceptor are linked through a conjugated spacer (D–p–A), have been extensively studied. While a great deal of re-search has been devoted in the last two decades to the develop-ment of new donors, acceptors and spacers, the effect of structural perturbations within these subunits (such as the modifi-cation of atoms or substituents) on the optical properties of D–p–A chromophores is much less known, there being only scant reports in the literature addressing this interesting question, from both the theoretical 4,5 and the experimental 6,7 points of view. Most of the hitherto carried out work along this line has focused on the replacement of acyclic polyenic spacers, suffering from limited thermal or photochemical stabilities, by carbo-or heterocyclic fragments. To this end, isophorone-derived moieties have been widely used as carbocyclic spacers since their introduction gener-ally leads to increased thermal stabilities 8–10 of the corresponding chromophores (although exceptions are known). 11 A structurally related six-membered heterocyclic ring which has enjoyed wide-spread popularity is 4H-pyran-4-ylidene, since it has afforded not only one-and two-dimensional NLO-chromophores, 12–14 but also red dopants for organic light-emitting diodes (OLEDs). 15 Comparisons between isophorone-and pyran-containing chro-mophores have usually focused on synthetic advantages or lumi-nescence properties, 16–18 but their relative merits as spacers in second-order NLO-chromophores have not been previously dis-cussed, except for one report where only two compounds (one of each family) were compared. 19 In this Letter, we disclose the effect that the presence of isophorone or 4H-pyran-4-ylidene rings has on the optical properties and thermal stabilities of D–p–A com-pounds endowed with aromatic or proaromatic donor groups and provide a rationale for the observed trends based on theoretical calculations. Moreover, we report the first merocyanines with a 2,3-dihydro-4H-pyran-4-ylidene moiety which, to the best of our knowledge, has not been exploited in the NLO field. Five different isophorone derivatives (12 and 13) and their ex-actly comparable pyran counterparts (14 and 15) have been chosen for this study, as shown in Figure 1. Compounds 13c, 20 14a 21 and 14b 14 are known and were pre-pared as previously described. The remaining compounds were synthesized as shown in Scheme 1. Thus, Knoevenagel reactions of isophorone derivative 1 22,23 with aldehydes 2, 3 14,24 and imini-um salt 4 25 afforded compounds 12a, 26 12b and 12c, respectively. Similarly, compound 14c was prepared by condensation of pyran derivative 7 27 with aldehyde 8. 28 Target thiobarbituric derivatives (13a 26 and 15a, c) were analogously synthesized starting from acceptors 5 29 and 6, 14 respectively. The experimentally determined linear and nonlinear optical properties of the studied compounds are collected in Table 1, inspection of which reveals several trends. Thus, in their UV–vis spectra pyran derivatives show blue-shifted absorptions relative 0040-4039/$ -see front matter Ó