Design, synthesis and evaluation of redox, second order nonlinear optical properties and theoretical DFT studies of novel bithiophene azo dyes functionalized with thiadiazole acceptor groups M.Cidália R. Castro a , Peter Schellenberg b , M. Belsley b , A.Maurício C. Fonseca a , Sara S.M. Fernandes a , M.Manuela M. Raposo a, * a Center of Chemistry, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal b Center of Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal article info Article history: Received 10 April 2012 Received in revised form 10 May 2012 Accepted 15 May 2012 Available online 23 May 2012 Keywords: Heterocyclic azo dyes Thiadiazole Auxiliary donor/acceptor heterocycles Hyper-Rayleigh scattering (HRS) Density functional theory (DFT) Nonlinear optical (NLO) materials abstract Two series of novel thermally stable second-order nonlinear optical (NLO) heterocyclic azo dyes 4e5 have been designed and synthesized. The two series of compounds were based on different combina- tions of acceptor groups (thiadiazole or arylthiadiazole electron-deficient heterocycles) linked to bithiophene which acts at the same time as a donor group and as a p-conjugated bridge. The sol- vatochromic behavior of azo dyes 4e5 was investigated in several solvents of different polarity, while their thermal stability was evaluated using thermogravimetric analysis. Optimized ground-state molecular geometries and an estimation of the lowest energy single electron vertical excitation ener- gies in DMF solutions were obtained using density functional theory (DFT). Their redox properties were studied by cyclic voltammetry, while hyper-Rayleigh scattering (HRS) was employed to evaluate their second-order nonlinear optical properties. The measured molecular first hyperpolarizabilities and the observed electrochemical behavior showed variations for the different acceptor systems used (thiadia- zole or arylthiadiazole) and were also sensitive to the electronic acceptor strength of the substituents (R) linked to thiadiazole or arylthiadiazole heterocycles. Donoreacceptor arylthiadiazole-bithienyl diazenes exhibit the most promising thermal (T d ¼ 237e305  C) and solvatochromic (Dn ¼ 1117e2503 cm 1 ) properties and second order nonlinear optical response (136e226  10 30 esu). Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Heterocyclic azo dyes are versatile chromophores that can provide bright strong shades tunable for absorption over all of the visible spectrum [1]. For decades, they have been widely investi- gated mostly due to their application for coloring textiles and plastics or in general as dispersed dyes. More recently azo dyes bearing heterocyclic moieties such as thiophene, pyrrole and azoles have been used for multiple optical and electronic applications such as second harmonic generation, optical switching, chemo- sensing, and organic sensitized solar cells [2]. Due to their well established synthetic routes, they are well suited candidates for a systematic investigation of how NLO prop- erties are affected by variations within their p-conjugated system. In the past years we have been exploring the ways in which the second order nonlinear response of these pushepull organic molecules can be improved by inclusion of different heterocycles within the p-electron bridge, using them both as a means of modulating the conjugation in the p-electron network and to change the electronic character of the charge transfer by acting as auxiliary donors or acceptors [3,4]. Previously we reported also other NLO-active heterocyclic chromophores bearing electron rich groups (thiophene, pyrole) linked to electron deficient heterocycles (benzothiazole, benzimidazole and phenanthroline) [5]. However, despite intensive theoretical and experimental activity it remains at present unclear what the right strategy will be to optimize NLO properties. On the other hand, it is well known that pushepull systems bearing azole electron-deficient heterocycles can increase the quadratic molecular nonlinear optical response due to the electro- negative nitrogen and sulfur atoms [6]. In this respect, five- membered diazoles, in particular thiadiazole, seems to be an appropriate acceptor moiety. It possesses two nitrogen atoms and * Corresponding author. Tel.: þ351 253 604381; fax: þ351 253 604382. E-mail address: mfox@quimica.uminho.pt (M.M.M. Raposo). Contents lists available at SciVerse ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2012.05.014 Dyes and Pigments 95 (2012) 392e399