Properties of novel azodyes containing powerful acceptor groups and thiophene moiety

An important goal in the development of materials for quadratic nonlinear optics is to find chromophore molecules, which have a large β value. In the area of electrooptical applications, the early attempts included improvement of the electronic biasing strengths by increased donor-acceptor strengths and optimization of conjugation. Replacement of benzene moieties by five-membered conjugated heterocycles increases the electron transmission between donor and acceptor groups, owing to lower resonance energy, determining an improvement of β value. Our approach to design improved nonlinear optical chromophores is based on the employment of both strong acceptor group and thiophene ring instead of benzene in a mixed azobenzene–stilbene like chromogen. The synthesis was performed in two steps: (1) diazonium salts from p-aminobenzaldeide or 2-amino-5-formylthiophene was coupled on the 2-(N-ethylanilino)ethanol; (2) aldehydic group reacted with carbanion of 1,3-indandione, malonitrile, 3-dicyanovinylindan-1-one, and benzo[b]thiophen-3-(2H)-one-1,1-dioxo by Knoevenagel procedure. μβ(0) values, inferred from EFISH measurements at 1.34 and 1.9 μm using the two-level model, show that the substitution of a benzene ring by a thiophene one determines a general enhancement of the hyperpolarizability up to three times than the DR1 (450×10−48 esu). Solvatochromic data are in agreement. Analogous effect is observable in benzene series inserting the vinyldicyano group on indandione moiety. Thermal stability was evaluated by TG-DTA. Decomposition temperatures appear lower than DR1 (260–299°C) and, in general, chromophores with improved nonlinearity exhibit decreased thermal stability, but again exceeding 200°C, limit value as far as the processing and device fabrication is concerned.