The cyanide anion is well known due to its toxicity to the environment and to mammals, leading to convulsions, loss of consciousness, and eventual death. It is lethal to humans for concentrations in the range of 0.5-3.5 mg per Kg of body weight. In addition to being found in many foods and plants, cyanides are used industrially in the synthesis of organic chemicals, polymers, metallurgy as well as in gold mining [1].

Consequently, selective detection and quantification of cyanide is very important and it has been the object of increasing investigation. A large number of fluorimetric and/or colorimetric chemosensors as well as dosimeters, capable of detecting this anion in organic solvent as well as in aqueous mixtures have been reported during the last decade. Even so, the majority suffer from several drawbacks such as difficult synthesis, poor selectivity, only work in an organic media and the use of instrumentation is required [2]. Therefore, the research on versatile and tunable chemosensors capable of selective and sensitive colorimetric sensing of the cyanide anion, especially in mixed aqueous solutions, is still a challenge [3].

In this communication, we report the synthesis, characterization and evaluation of the photophysical properties and the chemosensory ability of novel receptors based on benzofuran and benzoindole systems functionalized with the dicyanovinyl group. The new compounds were synthesized in good yields through Knoevenagel reaction between the precursor aldehydes and malononitrile.

The photophysical properties of the new push-pull systems were studied by UV-vis and fluorescence spectroscopy in acetonitrile. The evaluation of the compounds as colorimetric chemosensors was carried out by performing spectrophotometric titrations in acetonitrile and acetonitrile/water in the presence of relevant organic and inorganic anions, and of alkaline, alkaline-earth and transition metal cations. The benzoindole derivative exhibited great selectivity for the cyanide anion over other anions in acetonitrile/water (8:2) solution showing a distinct color change from colorless to yellow.

Acknowledgements: Thank are due to Fundação para a Ciência e Tecnologia (Portugal) and FEDER-COMPETE for financial support through Centro de Química (PEst-C/QUI/UI0686/2013 (FCOMP-01-0124-FEDER-037302)), and a PhD grant to R.C.M. Ferreira (SFRH/BD/86408/2012). The NMR spectrometer Bruker Avance III 400 is part of the National NMR Network and was purchased with funds from FCT and FEDER.

References:

1. a) Vennesland, B.; Comm, E. E.; Knownles, C. J.; Westly, J.; Wissing, F. in Cyanide in Biology, Academic Press, London, 1981. b) Ullmann’s Encyclopedia of Industrial Chemistry, 6th edn, Wiley-VCH, New York, 1999. c) Muir, G. Hazards in The Chemical laboratory, Royal Chemical Society, London 1977. d) Baskin, S. I.; Brewer, T. G. in Medical Aspects of Chemical and Biological Warfare, Eds. Sidel, F.; Takafuji, E. T.; Franz, D. R. TMM Publications, Washington DC, 1997, pp. 271.
2. Xu, Z.; Chen, X.; Kim, H. N.; Yoon, J. Chem. Soc. Rev. 2010, 39, 127.
3. For some recent examples reported by our group see: a) Batista, R. M. F.; Costa, S. P. G.; Raposo, M. M. M. Sensors Actuators B 2014, 191, 791. (b) Batista, R. M. F.; Oliveira, E.; Costa, S. P. G.; Lodeiro, C.; Raposo, M. M. M. Supramol. Chem. 2014, 26, 71. (c) Batista, R. M. F.; Costa, S. P. G.; Raposo, M. M. M. J. Photochem. Photobiol. Chem. A 2013, 259, 733. (d) Santos-Figueroa, L. E.; Moragues, M. E.; Raposo, M. M. M., Batista, R. M. F.; Ferreira, R. C. M.; Costa, S. P. G.; Sancenón, F.; Martínez-Máñez, R.; Ros-Lis, J. V.; Soto, J. Org. Biomol. Chem. 2012, 10, 7418. (e) Santos-Figueroa, L. E.; Moragues, M. E.; Raposo, M. M. M., Batista, R. M. F.; Ferreira, R. C. M.; Costa, S. P. G.;  Sancenón, F.; Martínez-Máñez, R.; Ros-Lis, J. V.; Soto, J. Tetrahedron 2012, 68, 7179. (f) Raposo, M. M. M.; García-Acosta, B.; Ábalos, T.; Calero; P.; Martínez-Manez, R.; Ros-Lis, J. V.; Soto, J. J. Org. Chem. 2010, 75, 2922.