Diseño y aplicación de receptores moleculares bioinspirado en la naturaleza anfótera del anillo del imidazol

Zusammenfassung

The development of specific chemosensors for the efficient detection of anions and cations is an important subject in the field of Supramolecular Chemistry due to their fundamental roles in biological, environmental and chemical processes. Simultaneously, an emerging field within this area is based on the design and application of preorganized heteroditopic receptors for the simultaneous sensing both cationic and anionic guest species. In this context, the main objectives of this PhD thesis are focused on the design, synthesis and study of sensor behavior of several kinds of azaheterocyclic receptors containing a signal unit and, at least, a covalently linked amphoteric imidazole ring as binding unit. In addition, the angular annelation of additional azaheterocyclic units to the imidazole ring have been also carried out in order to improve the binding ability of the resulting poly-azaheterocycle receptors. Another common structural motif in the design of these receptors is the ferrocene unit that displaying remarkable electrochemical-sensing properties. Therefore, first objetive of this PhD Thesis is related to the design and study of a ferrocene-based heteroditopic receptor in which the ferrocene moiety is attached to an imidazo[4,5-f]quinoxaline. The presence of a quinoxaline ring, angularly fused to a imidazole unit, create a cavity where the N atoms present could synergistically cooperate in recognizing cationic species, while the NH group of the imidazole ring would be available for the recognition of anionic. In fact, this receptor behaves as a dual electrochemical and optical chemosensor molecule for Zn2+, Cd2+, Hg2+ y Pb2+ metal cations The second objetive is based on the design and study of 7,8-disubstituted 2-ferrocenyl-1H-imidazo [4,5-f]quinoxalines containing phenyl, pyridyl, furanyl or thienyl units. The 7,8-diphenyl substituted receptor behaves as a highly selective redox, chromogenic and fluorescent chemosensor molecule for Pb2+ cations in MeCN solution, while the receptor bearing two additional pyridine rings as substituents, exhibits ability for sensing Hg2+ cations in the same medium. On the other hand, the functionalization of the quinoxaline ring with furane o thiophene rings contributes to improve the efficiency of the recognition processes. Thus, both receptors show a dramatic enhancement in the binding of HSO4- and H2PO4- anions when are co-bound with Zn2+, Pb2+, Cd2+, Mg2+ o Ni2+ cations, whereas no affinity of the free receptors by HSO4- anion and Ni2+ or Mg2+ cations individually is observed. These receptors behave as ion-pair chemosensors by strong perturbation of the redox potential of the ferrocene unit and a remarkable enhancement of the fluorescence in the presence of an anionic and cationic species. Another objetive of this PhD Thesis is based on the annulation of an additional policyclic ring, such as phenanthrene, phenanthroline or pyrene. to the imidazoquinoxaline core, in order to improve the selectivity and sensitivity of this set of receptors. Thus, the dipyrido-imidazo-phenazine and imidazo-phenanthro-phenazine based receptors act as a selective molecular probe of Hg2+ cation through three different channels: electrochemical, colorimetric and fluorescent. However, dibenzo-imidazo-phenazine based receptor behaves as a highly selective redox/chromogenic/fluorescent chemosensor molecule for Pb2+ cations in MeCN/H2O (9/1). In addition, we have synthesized other type of multichannel chemosensor molecules based on the 2-ferrocenil-7-substituted benzobisimidazol system, decorated with another ferrocene, 2,4-dinitrobenzene or pyrene unit. The bisferrocene-benzobisimidazole acts as a dual highly selective redox and fluorescent molecular sensor for Hg2+ cation and HSO4- anion probably through initial proton transfer followed by hydrogen bond formation and subsequent anion coordination. The ferroceyl-2,4-dinitrophenyl-bisimidazole behaves as a redox and chromogenic chemosensor molecule for AcO-, H2PO4- and SO42- anions and Zn2+, Hg2+ and Pb2+ cations the recognition process being accompanied by a colour change which allows their "naked eye" detection. On the other hand, the ferroceyl-pyrenyl-bisimidazole shows a strong increase of the monomer emission band only in the presence of H2PO4-. In the same way, we have also described the synthesis and binding properties of a receptor in which the redox activity of the ferrocene group, the fluorogenic behaviour of pyrene and the binding ability of the imidazole ring are combined in a highly preorganized system. This ferrocenil-imidazopryrene dyad behaves as a host separated ion pair sensor which is able to simultaneously recognize an anion and a cation through two different channels: electrochemical and fluorescent. Finally, a set of imidazo[4,5-e]-2,1,3-benzothiadiazole multifunctional receptors, differently functionalized with other heterocyclic rings to improve the sensing properties (pyrrol, pyridine, imidazole) or the luminescent character (7-azaindole, benzo[g]indole) have also been synthesized. These receptors display solvatofluorochromism and intense fluorescence both in solution and in the solid state. Moreover, the receptor that contain a pyrrol unit as substituent, acts as luminescent molecular chemosensors for the detection of nitroaromatic compounds, particularly exhibiting a selective response towards picric acid. On the other hand, the receptor functionalized with a pyridine unit behaves as an ion-pair receptor of Cd(AcO)2 and Zn(AcO)2 either in solutions or in the solid state. Interestingly, the formation of the ion-pair complexes in solution, allows the selective extraction of the Zn2+ in the presence of the Cd2+ salt, either by a chloroform or diethyl ether solution of the receptor.