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Treball Final de Grau en Química. Codi: QU0943. Curs: 2014/2015 Two years ago, the solid state chemistry research group at Jaume I University studied the possibility of developing new lanthanide-based inorganic pigments. The synthesis of M (II) (Ca or Ba) and Tb (III) co-doped Y2O3 ceramics by a modified Pechini method was reported to obtain new environmental friendly yellow pigments as an alternative to the toxic existing ones Different solid solutions were prepared to accomplish the best composition. The general structure was Y1.86−xMxTb0.14O3−x/2 where M represents calcium or barium ions and x ranges from 0.06 to 0.64. Spectroscopic measurements and thermal treatments showed that all samples incorporated terbium (IV) and terbium (III) ions into the Y2O3 structure. However, samples with calcium showed more intense yellow colouration due to the higher amount of terbium (IV) ions, whereas samples with barium were pale beige. It was concluded that the presence of calcium (rCa (II) (IC=6) = 114 pm) or barium (rBa (II) (IC=6)= 149 pm) alters the structure of the Y2O3 lattice varying the distance terbium-oxygen. A possible explanation to those results was that calcium, with a similar radius to yttrium (rY(III) (IC=6)= 104 pm) allows the charge transfer between terbium and oxygen ions and thus the oxidation of terbium ions, but barium is bigger than yttrium and the charge transfer transitions are hindered. Additional studies on the same structure were performed substituting terbium (IV) ions for other chromophores such as chromium (III) or iron (III). Results also showed a major deviation in colour for the samples containing calcium in the matrix. The present project deepens into the reasons behind the change in colour between samples doped with barium and calcium.

Treball Final de Grau en Química. Codi: QU0943. Curs: 2014/2015 Thus, a gel can be defined as a two-component colloidal dispersion which has a continuous structure with macroscopic dimensions. Gels are composed mainly with a small amount of solid agent (minor component) named as gelator that is able to form a three dimensional network (3D) that can immobilise a large volume of solvent (major component). This network provides viscoelastic properties and the formation of a gel can be tested easily by the so-called “tube inversion test” where a gel is considered if it remains immobilized upon turning up side down the recipient. Molecular gels have attracted an enormous amount of interest by their potential applications proposed up to now in fields of great social impact like biomedicine or materials science. Molecular gels are nanostructured soft materials formed by low-molecular-weight organic compounds by the sole action of non-covalent intermolecular interactions (solvophobic, ionic, H-bonding, π-π stacking, van der Waals, metal coordination, etc.). These molecular interactions are reversible and can be modulated by stimuli. Amazingly, very simple and easy to synthesize molecules are capable of selfassembly into nanoscopic aggregates that form a 3D network that immobilises a large amount of solvent. Molecular gels are hierarchically built-up from the molecular to supramolecular level.The molecules are organised into 1D objects that bundle into fibrillar architectures, and after physical cross-linking, will form a network that percolates the solvent. The gel is formed in a super-saturated solution, and depending on the solubility constant and assembly mechanism, a given amount of free gelator could remain in solution.4 In many cases gels are formed by dissolution of gelator in hot solvent and subsequent cooling to a given temperature. The supramolecular approach to the field of catalysis has attracted the attention of many researchers since the pioneering work of Pedersen or Lehn in that field.5 Self-assembly is a ubiquitous phenomenon for the bottom-up construction of supramolecular catalysts that aim to emulate the efficiency and selectivity of natural enzymes due to the construction of a suprastructure capable of performing a task that the individual components cannot achieve independently.

Treball Final de Grau en Química. Codi: QU0943. Curs: 2014/2015 Supramolecular chemistry may be defined as the “chemistry beyond the molecule”. It involves the study of new molecular systems in which the components are linked by noncovalent interactions. These non-covalent interactions include electrostatic interactions, hydrogen bonding, π-π interactions, van der Waals forces or hydrophobic effects. Originally supramolecular chemistry was defined in terms of the non-covalent interactions between a “host” and a “guest” molecule as highlighted in Figure 1, which illustrates the relationship between molecular and supramolecular chemistry in terms of structures. In biology, such interactions play an important role, being responsible for many processes, as are for example, signal transduction, selective transport of ions and small molecules across membranes, enzymatic reactions, etc. This area of chemistry was born four decades ago with Charles J. Pedersen, Jean-Marie Lehn and Donald J. Cram who received the Nobel Prize in Chemistry in 1987. This award was granted "for their development and use of molecules with structur-especific interactions of high selectivity". However, before the name of Supramolecular Chemistry was introduced by Lehn, some other authors had already worked and made interesting contributions to this field of chemistry. In 1873, Johannes van der Waals Diderik postulated the existence of intermolecular forces. Later, the Nobel prize, Hermann Emil Fischer suggested that enzyme-substrate interactions take the form of a "lock and key", the fundamental principle of molecular recognition and host-guest chemistry.