NLO 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 beta 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 beta 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. mu beta>(*) over bar * (0) values, inferred from EFISH measurements at 1.34 and 1.9 mum 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 x 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 DRI (260-299 degreesC) and, in general, chromophores with improved nonlinearity exhibit decreased thermal stability, but again exceeding 200 degreesC, limit value as far as the processing and device fabrication is concerned.