J O U R N A L O F C H E M I S T R Y Materials Communication Rational design of new acid-sensitive organogelators Jean-Luc Pozzo,* Gilles Michel Clavier and Jean-Pierre Desvergne Photochimie Organique et Supramole�culaire, UMR 5802, Universite� Bordeaux I, 351 cours de la Libe�ration, 33405 Talence Cedex, France. E-mail: jl.pozzo@lcoo.u-bordeaux.fr Received 16th September 1998, Accepted 7th October 1998 2,3-Di-n-alkoxyphenazines were shown to act as acid- are reported in Table 1.The data revealed the trend of phenazine derivatives to fairly easily gelify polar solvents; it sensitive organogelators at ambient temperature in acetonitrile; the protonated yellow species formed in the also indicates that 2b, possessing two longer alkyl side-chains, acts as a more eYcient gelator than 2a.These compounds are presence of acid displays stronger aggregative properties and higher temperature resistance than the colourless neu- soluble in low polarity solvents (halogenated or aromatic)11 and hence cannot rigidify such solutions, with the exception tral phenazine; the protonation/deprotonation process is reversible. of compound 2b, which can gelify n-heptane at low temperature (Tgel=3 °C).Under the same conditions, the parent molecules (DAOA, 1) displayed, in general, similar behaviour and a Thermoreversible physical gels generated from low molecular mass organic compounds are an active field of research from slightly higher capacity to encage the alcoholic solvents.4 Thus the replacement of CH groups by nitrogen atoms on the both an academic viewpoint and because of their practical applications.1–3 Organogelators not only rigidify the solvents anthracene moiety does not deprive the dialkoxyaromatic from its gelling capacities.but also create supramolecular networks which could induce specific and unique properties to the resulting materials.4 Owing to the presence of basic sites on the phenazine moiety, addition of trifluoroacetic acid (TFA) to solutions of Although hydrogen-bonding donor and acceptor groups are necessary for most organogelators which self-aggregate in 2a and 2b strongly modifies both their 1H and 13C NMR signals, which are shifted upfield, and also the UV-visible water and organic solvents, it has been recently shown by us5 and others6,7 that small non-hydrogen bonding molecules absorption spectra, the maxima being bathochromically shifted from 249 and 390 nm to 260 and 413 nm, respectively.The could also exhibit gelling properties. Amongst the compounds investigated, 2,3-di-n-alkoxyanthracenes (DAOA, 1) were absorption spectra of 2a and 2b are superimposable both in the presence or absence of TFA. Taking into account the range reported to display, at very low concentrations, exceptional aggregative properties in various organic fluids.5 For these of acid concentration used (0 to 2×10-2 M) and according to the reported data for neutral, mono- and di-protonated phena- self-assembling systems, van der Waals interactions, dipole– dipole contributions and p-stacking are the main driving forces zine, 12 the yellow coloured species obtained upon acid addition is consistent with formation of the monoprotonated moiety.involved for building the fibrous aggregates. The gel-to-sol phase transition temperature of an More importantly, the gel-forming ability of 2a and 2b is significantly enhanced in the presence of acid, resulting in an organogelator is usually found to depend on the fluid and the gelator concentration, but it can also be influenced by external increase of Tgel by much as 60 °C in acetonitrile, as illustrated in Fig. 1. Reinforcement of the gelling properties could be stimuli ( light, pH etc.) as recently observed for some polymer containing hydrogels.8 Apart from light and cation com- due, in addition to other aggregative factors, to hydrogenbonding between nitrogen atoms and ammonium centres.As plexation, which were found to slightly modulate gelling abilities of cholesteric derivatives,7 such external controls observed by Weiss et al., formation of ammonium groups was shown to influence the gelation in alkyl substrates.6 The remain quite rare for low molecular mass gelling agents. It occurred to us that the introduction of basic sites on DAOA strongest eVect for a 10-2 M acetonitrile solution of 2b was recorded using one equivalent of TFA, as displayed in Fig. 2. might provide a new family of pH-sensitive organogelators. From our previous studies, it emerged that long alkyl chains It was observed that supplementary addition of acid decreases the gelling ability of the material, and that gel formation was and the oblong shape of the molecule characterized by the anthracene ring system were necessary for gel formation.We totally suppressed in strongly acidic medium; the strengthening of the aggregation through hydrogen-bonding is limited therefore decided to investigate the gel-forming abilities of the analogous phenazine derivatives 2a and 2b. by solubility of the gelling species. The role of the anion, which could be of importance in the construction of the In this communication, we report the straightforward synthesis9 of novel organogelators 2a and 2b whose gelling properties and colouration are significantly and reversibly modified by altering the acidity of the solution.Table 1 Gel-forming abilities and gel-to-sol phase transition temperatures (Tgel/°C) of phenazines 2a and 2b at 2×10-2 Ma Solvent 2a 2b MeOH 15 31 EtOH 14 (9) 33 (28) EtOH–H2O (451) pg 12 N N OR OR OR OR 1 2a R = n-C9H19 b R = n-C11H23 Acetonitrile -13 (-18) 44 (30) DMF p 9 The gelating capacity of 2a and 2b was screened at 2×10-2 M Acetone pg 9 by means of the inverted test-tube method.Phenazine (2a: n-Heptane s 3 9.2 mg; 2b: 10.2 mg) and solvent (1 ml ) were warmed in a as=soluble at ambient temperature, pg=partial gel, p=precipitate; septum-capped tube until complete dissolution of the solid.values in parentheses refer to Tgel determined with 1% wt of The solution was immersed in a thermocontrolled bath and organogelator (ca. 1.75×10-2 M and 1.50×10-2 M for 2a and 2b, slowly cooled at 2 °Cmin-1 until the gel formation occurred. respectively).The observed gel-to-sol phase transition temperatures (Tgel) J. Mater. Chem., 1998, 8, 2575–2577 2575Fig. 1 Tgel for 2b in acetonitrile as a function of organogelator concentration with or without trifluoroacetic acid: (%) [TFA]=0, (,) [TFA]=[2b], (#) [TFA]=10-2 M. Fig. 2 Tgel for 2b in acetonitrile as a function of TFA concentration ([2b]=10-2 M). tridimensional supramolecular network, is under current investigation and will be discussed in the forthcoming full paper. Moreover, it was also shown that the gel-forming ability of the system can be fine controlled by the reversible protonation of the phenazine ring.For example, bubbling ammonia through the gel provoked fading of the coloured protonated species and led to an optically translucent gel, the process being reversible upon addition of acid.The magnitude of the increase in Tgel obtained by addition of acid was found to depend on both fluid and concentration of organogelator. As shown in Fig. 1 for acetonitrile solution, the eVect is less pronounced at higher concentrations and most dramatic at Fig. 3 Transmission electron micrographs of a dried acetonitrile gel concentrations below 10-2 M.of 2b (a) in the absence and (b) in the presence of TFA. Direct evidence of the microscopic organization of the gel formed in acetonitrile was obtained from transmission electron fibres. Presumably, this latter is necessary to accommodate micrographs (TEM, Fig. 3). Numerous juxtaposed, fused, and hydrogen-bonding interactions formed upon protonation of intertwined thin straight fibres (several microns length) are the phenazine ring.formed by entanglement of long, slender aggregates with a In conclusion, the present study has demonstrated that the width of ca. 150 nm woven in a three-dimensional architecgelling abilities and colouration of low molecular mass organic ture. The diameter of the smallest fibres represents several compounds can be fine-tuned by variation in pH. Further gelator molecular l.Of note is the formation of nodes studies are in progress, devoted to the elucidation of the corresponding to areas of large gelator concentration where microscopic arrangement of these supramolecular assemblies. no diVraction pattern could be recorded, suggesting a non- We believe that these and other systems would open up new microcrystalline environment. These intertwined fibres are able prospects for signal-responsive chemistry in molecular to encage the solvent molecules.At 10-2 M, with one equivalent assembly systems. of TFA, TEM [Fig. 3(b)] shows the occurrence of longer We are indebted to Michel Chambon and Christine Corra elongated fibre-like structures without nodes, indicating that (Universite� Bordeaux I ) for TEM experiments. the molecules are apparently more eYciently packed.Compounds 2a and 2b diVer from other hydrogen-bond based organogelators in that the neutral species already form gels Notes and references through self-aggregation via dipole–dipole and van der Waals interactions, and that addition of acid induces a structural 1 R. Dagani, Chem.Eng. News, 1997, 75(23), 26. 2 P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133. reorganization of the molecular rearrangement within the 2576 J. Mater. Chem., 1998, 8, 2575–25773 J. van Esch, R. Kellogg and B. Feringa, Tetrahedron Lett., 1997, 7 K. Murata, M. Aoki, T. 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