Study on chrysazin–aluminium(III) interaction in solution by spectroscopy and quantum chemical calculations Stéphanie Say-Liang-Fat, Jean-Paul Cornard ⇑ , Aurélien Moncomble LASIR, CNRS UMR 8516, Université des Sciences et Technologies de Lille, Bât C5 – 59655 Villeneuve d’Ascq Cedex, France article info Article history: Received 30 July 2012 Accepted 28 August 2012 Available online 7 September 2012 Keywords: Chrysazin Al(III) complexation TD-DFT UV–Vis Fluorescence abstract Spectroscopic (UV–Vis and fluorescence) and theoretical studies were used to assess relevant interaction of aluminium(III) and chrysazin (1,8-dihydroxyanthraquinone) in methanol solution. The complexation reaction has been followed by electronic absorption spectroscopy and chemometric methods of the data set have highlighted the formation of two complexes of stoichiometry 1:1 and 2:1 (metal:ligand) with stability constants of log b 1:1 = 3.51 ± 0.01 and log b 2:1 = 5.71 ± 0.01, respectively. Chrysazin has a lower complexing power towards Al(III) than its isomer, alizarin, meaning a significantly lower fixation power of the hydroxy-keto function than that of the catechol function. The theoretical spectra obtained from hypothetical structures by TD-DFT calculations were compared to the experimental ones. This allowed demonstrating, for the free ligand and the two complexes, the absence of tautomeric form in solution. The complexes structures have been elucidated and notably the non-symmetric geometry of the 2:1 complex. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Chrysazine (Chz) (1,8-dihydroxyanthraquinone), also known as Danthron and commonly extracted from plants [1], and its deriva- tives have been known for many years due to their therapeutic, pharmacologic [2,3] and anticancer properties [4]. The potential role of Chz as a retinoic X receptor antagonist has been recently investigated [5]. However health risks, as genotoxic and cytotoxic effects, or DNA damage, could be attributed to its use [6–8]. The structure of Chz, illustrated in Fig. 1, shows the possibility of intramolecular hydrogen bonds that led to study the possible exis- tence of tautomer, conformer and rotamer forms [9,10]. Indeed, two forms of Chz were characterized by Smulevich and Marzocchi [11], with a quasi-irreversible phase transition between both [12]. The first form, with two unequivalent hydrogen bonds, is stable at room temperature in polar solvents, the second one, with two equivalent hydrogen bonds, above 145 °C in apolar matrices. For both, fluores- cence emission observed involves a p–p ⁄ charge transfer transition and an excited state intramolecular proton transfer (ESIPT) [13–15]. The ESIPT has been the object of different studies [16–18]. Bloom et al. suggested that an intramolecular proton transfer was possible in molecules presenting carbonyl and hydroxyl functions in a-posi- tion [19]. The transfer is proved by the dual fluorescence excitation and emission, and resonance Raman spectra [20] and explained by a model based on Lippincott–Schroeder double minimum potentials along the proton transfer coordinate [14]. Quantum chemical calculations were used to obtain a better knowledge of these systems. Above all, density functional theory based methods allowed the study of both fundamental (in a station- ary formalism) and excited states (in the time-dependent formal- ism, TD-DFT). Among many properties were especially studied the determination of the equilibrium geometries (in fundamental and first excited state) of neutral and ionized forms, tautomers and con- formers, vibronic couplings, the involved electronic transitions, or the understanding of the ESIPT [21–29]. However, for some workers the existence of tautomers, conformers or rotamers is not clear [30]. Chz is often used in the detection and determination of metal ions [31] such as Li(I) [32], Mg(II) [33–36], Ca(II) [37,38], Cu(II) [37,39,40], Pb(II) and Cd(II) [41–43], Zn(II) [41,31], Ni(II), Mn(II), Fe(II), Pd(II) and Pt(II) [39,31] Co(II) and Fe(III) [37]. Other applications have been investigated, as mediator for indirect dye reduction [44]. If Chz has been the subject of many studies, another interesting application should be pointed out. Such molecule could be used as model to clarify phenomena as metal detoxification in plants [45] or ion metal retention or transport in soils [46,47]. Indeed, Chz has high complexing ability due to the presence of hydroxyl and carbonyl functions in its structure. However, to our knowledge, the Al(III) complexation by Chz has never been reported. Previ- ously, we studied the Al(III) complexation by alizarin (1,2-dihy- droxyanthraquinone) [48], an isomer of Chz, presenting two complexing sites (a catechol and a hydroxy-keto groups) in compe- tition. The aim of the present work is to compare to this system, 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.08.067 ⇑ Corresponding author. Tel.: +33 3 20 43 69 26; fax: +33 3 20 43 67 55. E-mail address: jean-paul.cornard@univ-lille1.fr (J.-P. Cornard). Polyhedron 48 (2012) 237–244 Contents lists available at SciVerse ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly