Synthesis and evaluation of thermal, electrochemical and nonlinear optical properties of dicyano- and tricyanovinyl-bithienylpyrroles

Abstract

Materials exhibiting large optical nonlinearities continue to be the subject of intense studies because of their potential applications in modern communication technologies involving optical data processing, transmission, or storage, where they are gradually replacing classical electronic devices.[1] Earlier we reported on the synthesis and evaluation of thermal, redox and NLO properties of donor-acceptor substituted thienylpyrrole derivatives. Our studies have shown that replacing the benzene ring of a chromophore bridge with heterocycles (thiophene, pyrrole, (benzo)thiazole), results in an enhanced molecular hyperpolarizability of conjugated donor–acceptor compounds.[2,3,4,5,6]

As part of an on-going program to develop strong charge transfer donor-acceptor pyrrole systems with enhanced nonlinear optical (NLO) properties we have explored the potential of bithienyl-pyrrole derivatives 1-4 as efficient π-conjugated systems functionalized with dicyanovinyl- or tricyanovinyl- acceptor groups substituted at thiophene or pyrrole rings. Compounds 3a-b were prepared, in good yields, through Knoevenagel condensation of the precursor aldehydes 2a-b with malononitrile, and the tricyano derivative 4 were synthesized through direct tricyanovinylation reaction of 1 with tetracyanoethylene. On the other hand, bithienyl-pyrroles 1 and 2b were prepared through palladium-catalyzed decarboxylative cross-coupling reaction between 1-methyl-1H-pyrrole-2-carboxylic acid and the appropriate bithienyl bromides, whereas formyl-precursor 2a were prepared through Vilsmeier-Haack formylation from the corresponding bithienyl-pyrrole 1.

The molecular structure, especially the degree of planarity and the relative orientations of the auxiliary donor and acceptor entities, is found to strongly influence their electrochemical and optical (linear and nonlinear) properties. Here we report on the synthesis of compounds 1-4 and evaluate their potential as efficient and thermally stable NLO materials by experimentally characterizing their redox properties, thermal stability (TGA) and second order nonlinear optical response (HRS). These results indicate that these compounds have good potential for a variety of NLO applications.

Acknowledgments Thanks are due to the Fundação para a Ciência e Tecnologia (Portugal) and FEDER-COMPETE for financial support through the Centro de Química - Universidade do Minho, Projects PTDC/QUI/66251/2006 (FCOMP-01-0124-FEDER-007429), PTDC/CTM/105597/2008 (FCOMP-01-0124-FEDER-009457), PEst-C/QUI/UI0686/2011 (F-COMP-01-0124-FEDER-022716) and a PhD grant to M. C. R. Castro (SFRH/BD/78037/2011). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network and was purchased within the framework of the National Program for Scientific Re-equipment, contract REDE/1517/RMN/2005 with funds from POCI 2010 (FEDER) and FCT. [1] G.S. He, L.-S. Tan, Q. Zheng, P. N. Prasad Chem. Rev. 2008, 108, 1245-1330. [2] M. M. M. Raposo, A. M. R. C. Sousa, G. Kirsch, F. Ferreira, M. Belsley, E. Matos Gomes, A. M. C. Fonseca Tetrahedron 2005, 61, 11991-11998. [3] M. M. M. Raposo, A. M. R. C. Sousa, G. Kirsch, P. Cardoso, M. Belsley, E. Matos Gomes, A. M. C. Fonseca Org. Lett. 2006, 8, 3681-3684. [4] R. M. F. Batista, S. P. G. Costa, E. L. Malheiro, M. Belsley, M. M. M. Raposo Tetrahedron 2007, 63, 4258-4265. [5] M. M. M. Raposo, M. C. R. Castro, P. Schellenberg, A. M. C. Fonseca, M. Belsley Tetrahedron 2011, 67, 5189-5198. [6] M. M. M. Raposo, A. M. C. Fonseca, M. C. R. Castro, M. Belsley, M. F. S. Cardoso, L. M. Carvalho, P. J. Coelho Dyes Pigments 2011, 91, 62-73.