Fluoride-responsive organogelator based on oxalamide-derived anthraquinone{ Zoran Dz ˇolic ´, a Massimo Cametti,* b Antonella Dalla Cort, b Luigi Mandolini* b and Mladen Z ˇ inic ´ * a Received (in Cambridge, UK) 15th May 2007, Accepted 23rd July 2007 First published as an Advance Article on the web 6th August 2007 DOI: 10.1039/b707466b Anthraquinone derived oxalamide gelator 1 forms with aromatic solvents and alcohols very stable gels which selectively respond to the presence of fluoride anion by colour change and/ or gel-to-sol transition. Low molecular weight organic compounds capable of forming gels with water and various organic solvents have recently received considerable attention. 1 Solvent gelation is generally a consequence of the formation of 3-dimensional gel networks constituted by the entanglement of many fibers. It is well documented that gel fibers are formed by the predominantely unidirectional self-assembly of gelator molecules through intermolecular hydrogen bonding, aromatic stacking, lipophilic and electrostatic interactions. Gels are dynamic soft materials in a state that lingers between solution and the solid state. For that reason gels are considered very promising for new applications in materials science. In particular, many efforts are devoted to the development of stimuli responsive gels, whose properties can be either switched on–off or tuned by an external or internal chemical or physical stimulus. 2 Such responsive systems are highly desirable for the development of sensor devices or in applications like drug delivery or catalysis. Fluoride anion is one of the most significant targets for sensing because of its importance in many biological and industrial systems. 3 Therefore, a large number of sensors for fluoride anion have been reported to date which, however, operate in solution. 4 We have recently described oxalamide-based gelators capable of forming various organo- as well as hydrogels. 5 This type of gelator self-assembles into fibrous aggregates through the cooperative and unidirectional hydrogen-bonding of oxalamide units. Here we report on the preparation of the anthraquinone based oxalamide gelator 1 (Fig. 1), which possesses pronounced gelling properties and exhibits selective binding of fluoride ion. This selectivity makes it the first example of a fluoride responsive gel system which allows naked-eye detection of fluoride by colour change or gel-to-sol transition. In addition the properties of its regioisomer 2, and the model compounds 3 and 4 have been investigated. Systems containing an anthraquinone group have previously been reported as colorimetric sensors for anions. 6 The gelation properties of 1 and 2 were assessed using various organic solvents. They are scarcely soluble and a co-solvent was required. Typically, the compounds were first dissolved in the minimal volume of DMF after gentle heating and then the selected solvent was added. After cooling to ambient temperature, the vessel was turned upside down. When the fluidity of the system was absent, it was denoted as a gel (Table 1). The results in Table 1 show that the anthraquinone derivative 1 possesses excellent gelation properties toward aromatic solvents and alcohols, which could be gelled at concentrations as low as 1.0 mM. All prepared gels are greenish yellow and transparent. Surprisingly, we have found that regioisomer 2 was incapable of forming gels with any of the tested solvents. This observation shows that the position of substitution at the anthraquinone ring has a pronounced influence. The morphology of the p-xylene gel was investigated by trans- mission electron microscopy (TEM). The presence of a gel network structure consisting of many entangled tape-like aggregates of widths between 20–90 nm and lengths of several mm is shown in Fig. 2a. Atomic force microscopy (AFM) images of diluted p-xylene and EtOH gels are in agreement with the tape-like morphology of the aggregates, as observed in the TEM images. a Department of Organic Chemistry and Biochemistry, Rud¯er Bos ˇkovic ´ Institute, Bijenic ˇka 54, 10000, Zagreb, Croatia. E-mail: zinic@irb.hr; Fax: +385 1 46 80 195; Tel: +385 1 46 80 217 b Dipartimento di Chimica and IMC-CNR Sezione Meccanismi di Reazione, Universita ` La Sapienza, 00185, Roma, Italy. E-mail: luigi.mandolini@uniroma1.it; Fax: +39 06490412; Tel: +39 064957808 { Electronic supplementary information (ESI) available: Experimental procedures for the preparation of 1 and 2, UV-Vis titration experiments. See DOI: 10.1039/b707466b Fig. 1 Structures of the anthraquinone derivatives 1–4. Table 1 Results of gelation tests for 1 and 2 a Solvent 1 2 DMSO S S DMF S P Ethanol G (1.0) P 1-Butanol G (1.2) P Acetonitrile G (2.6) I Benzene G (14.4) P Toluene G (4.2) P p-Xylene G (1.3) P Decalin G (1.4) P Cyclohexane S P a S, solution; P, precipitate; G, transparent gel; I, insoluble. Numbers in parentheses present the minimal gelation concentration (mgc) in mM. COMMUNICATION www.rsc.org/chemcomm | ChemComm This journal is ß The Royal Society of Chemistry 2007 Chem. Commun., 2007, 3535–3537 | 3535