FULL PAPER DOI: 10.1002/ejoc.200900218 Push–Pull Azo-Chromophores Containing Two Fused Pentatomic Heterocycles and Their Nonlinear Optical Properties Roberto Centore,* [a] Sandra Fusco, [a] Andrea Peluso, [b] Amedeo Capobianco, [b] Matthias Stolte, [c] Graziano Archetti, [c] and Hans-Georg Kuball* [c] Keywords: Chromophores / Heterocycles / EOA spectroscopy / Isomerization / Nonlinear optics / Density functional calculations We have developed procedures for the synthesis of push–pull azo-chromophores containing the s-triazolo[3,4-b]thiadiazole heterocycle compatible with the presence of electron-ac- ceptor groups (nitrophenyl group) as substituents on both the triazole and thiadiazole rings. The linear and non-linear op- tical properties of the synthesized chromophores have been characterized by the EFISH technique, electro-optical ab- sorption spectroscopy and DFT calculations. The combined Introduction Heterocycles play a prominent role in modern chemistry: They are important building blocks in supramolecular chemistry, [1] in biomimetic compounds for advanced phar- maceutical/medical applications [2] and in advanced materi- als, such as conducting polymers, [3] OFETs, [4] organic solar cells [5] and materials that exhibit non-linear optical (NLO) response. The emergence of photonic technologies in telecommuni- cations has led to a growing demand for NLO materials. [6] In this field, organic compounds are technologically privi- leged in principle, compared with inorganics because they exhibit large non-linearities with ultra-fast responses, and their ease of handling makes possible their integration with existing silicon-based technology. [6] NLO effects depend on the ability of the molecular elec- tronic cloud to show an electric-field-dependent polarizabil- ity. Accordingly, attention has been mainly focused on or- ganic compounds possessing long conjugated domains (comprising polyene/phenylene backbones) end-capped with electron-donor and -acceptor groups to induce the necessary directional bias, the so-called push–pull chromo- phores. [6] Phenylene-based chromophores usually exhibit [a] Department of Chemistry “Paolo Corradini”, University of Naples “Federico II”, Via Cinthia, 80126 Naples, Italy [b] Department of Chemistry, University of Salerno, Via S. Allende, 84081 Salerno, Italy [c] Department of Chemistry-Physical Chemistry, Technische Universität Kaiserslautern, Erwin-Schrödinger Str., 67663 Kaiserslautern, Germany Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.200900218. Eur. J. Org. Chem. 2009, 3535–3543 © 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3535 results indicate a significant modulation of the properties of the chromophores depending on the substitution pattern on the heterobicycle. Fast UV-driven trans  cis and slower thermally driven cis  trans isomerizations around the N=N bond were observed and characterized for the chromo- phores. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) high electronic excitation energies, which, according to the well-known two-state model, [7] lowers the second-order NLO response. A simple way to obviate this shortcoming is to substitute phenylene with heteroaromatic rings due to the smaller aromatic stabilization energy of the heterocycle as compared with benzene. It has been experimentally shown that the presence of heterocycles significantly en- hances the non-linear optical response of conjugated push– pull chromophores. [8] The most studied chromophores contain single-ring aro- matic heterocycles with one or more heteroatoms (e.g., pyr- idine, [9] pyrrole, [10] thiophene, [11] imidazole, [12] thiazole, [13] and oxadiazole [14] ). Examples of chromophores containing two aromatic fused rings, one pentatomic heterocycle and one hexatomic non-heterocycle, have also been reported (benzimidazole, [15] benzoxazole [16] and benzothiazole [17] ). Chromophores containing two fused rings that are both heteroaromatic are more rare, in particular, those in which both rings are pentatomic. [18] In the search for new het- eroaromatic compounds suitable for quadratic NLO appli- cations we have started a general investigation of push–pull chromophores containing two fused rings that are both het- eroaromatic. In this paper we report a study of chromophores contain- ing the fused s-triazolo[3,4-b]thiadiazole heterocycle (Scheme 1). This heterocycle has been investigated recently in biochemistry and has a diverse biological activity. [19] Some of its features suggest that this heterocycle is interest- ing as a building block for NLO-active chromophores. a) The high number of electronegative nitrogen atoms and the sulfur atom are expected to increase the quadratic mo- lecular NLO response [8] and to provide the whole chromo-