Faculty of Sciences
Department of Organic Chemistry
European PhD in Chemistry Science
DESIGN AND SYNTHESIS OF ORGANIC-INORGANIC HYBRID MATERIALS
EXHIBITING OPTICAL AND LUMINESCENT PROPERTIES
IN THE SOLID STATE
LAURA JIMÉNEZ GARCÍA-PATRÓN
Madrid, June 2012
This Doctoral Thesis has been realized in the Department of Organic Chemistry of Faculty of Sciences at the Universidad Autónoma de Madrid (UAM), under supervision of Professor Ernesto Brunet Romero, to whom I am greatly indebted for his continuing support.
This research project has been led with the collaboration of Dr. Gilles Muller at the San José State University to whom I do wish to express my most sincere gratefulness.
This work has been also developed with the support of the Servicio Interdepartamental de Investigación (SIdI) at the UAM, specifically of Ajo, Maite, Pascual, Luis, Mario, Noemí, José, Mª Jesús and Ramón, who performed an excellent work to characterize my materials. To all of them I want to express my deeper thankfulness as well as the good moments shared.
The development of this work has been financed by a fellowship F.P.I.
granted by the Ministerio de Ciencia e Innovación (MICINN) which let me realize two short placements in the laboratories of Dr. Gareth Williams in Durham (UK) and Dr. Gilles Muller in California (USA), to whom I appreciate the kindness and attention both academic and personal. And I desire to extend my appreciation to FYSE-ERCROS S.A. for indirect funding during the PhD.
Brunet, por haberme permitido formar parte en su grupo de investigación brindándome su confianza y ayudándome a crecer sus con consejos y su dedicación.
Y a la Dra. Olga Juanes por su apoyo y compresión, y muy especialmente por su cercanía que día a día ha ido dejando paso a una especial amistad. También quiero agradecer al Dr. Juan Carlos Rodríguez-Úbis toda la ayuda prestada. Este ha sido un largo viaje en el que nunca he dejado de aprender, me siento afortunada de haber podido llevarlo a cabo y vosotros lo habéis hecho posible. Os estaré eternamente agradecida y ojalá podamos seguir compartiendo pequeños momentos y alguna que otra comida.
Quiero agradecer a todos los compañeros que han pasado por el laboratorio: Álvaro, Jorge, Antonio, Carlos, María C. y Marichu, y a los que siguen aquí: Elenis, María (Flor), Laura, Arturo, Emili, Fer y Angeles, especialmente su comprensión y su ayuda. No es fácil convivir tanto tiempo en esta que a veces parece nuestra primera casa y entre todos hemos solventado las dificultades, superándonos día a día, disfrutando y sobre todo viviendo esta increíble experiencia con mucho humor y muchos cafés. Quería agradecer también a Ajo y Rosa, sus buenos consejos y su ánimo, sobre todo en los momentos difíciles.
También a mis compañeros de los cursos de doctorado, con quienes compartí una experiencia inolvidable, a Clara, David, Mario, Andrés, Nacho y Julián. Likewise I would like to thanks Dr. Roger Terril and all my lab-mates in California, Vinh, Jamie, Truman, Andrew, Brian, Eliseo and Khan, for all the help and kindness that made me feel like at home. And my lab-mates in Durham Pier Paolo, Nick, and Jhon with whom I had a great time in Durham, and to my African family, Marvin, Precious and the little princesses to whom I am specially grateful.
Agradecer también a mis otros compañeros, mis vecis, los que he tenido la oportunidad de ir conociendo y con los que he crecido y compartido tantos y tan especiales momentos dentro y fuera de la universidad. Empezando por mis “chicas carreño”, Mercedes, las Silvis B.y V., Leti, Glori, Mati, Anita, Jajaime, Yoli & Carolain, a su califa Alfonso LT, a DJ Markos, Somoza y Luis M., a los que han vivido un poco más arriba, Jorge, Alberto, Tati, Alfon, Abraham, Alvariño, Ire Hypi, Juanjito, Andrés,
Rebe, Rita, Ruti, Manu, Alicia y a Vilas Phapale que recuerdo con mucho cariño.
Gracias a todos por vuestra ayuda, apoyo y amistad, por las risas compartidas y por las que están por llegar.
Deseo expresar especialmente mi agradecimiento a mis amigos, a Mercedes y a Andrés con quienes he compartido este viaje sideral compartiendo todo tipo de emociones, a Manute, Tomi, Diego y Clarita, a los Tanos, a Amelia, a Chui &Laura, Paloma, Diego, Julito, Dani, Roberto, Peter, Yubia y Elihú, por lo bien que lo hemos pasado arreglando el mundo y porque sigamos disfrutando de esta vida que se hace mucho más llevadera con vuestra compañía. Y como no a los no químicos por aguantar las charlas de químicos y a todos gracias por vuestro apoyo y vuestro cariño.
Por supuesto quería dar las gracias a toda mi familia incluidos los que no lo sois como tal, pero si en mi corazón. A Badara por tanto cariño, ayuda y apoyo en este momento tan complicado, y en general por todo lo compartido siempre entre risas y momentos de desestress, pero sobre todo por hacerme comprender que las limitaciones están sólo en mis pensamientos. A JAL, a Montse y a mis “Valdericeda”
Noe, Alba, Olalla y Dévora, porque siempre estáis a mi lado cuando os necesito.
Porque cuando estoy de bajón, vuestra lucha me hace levantarme y seguir luchando y me traspasáis con esa energía positiva que os caracteriza. Albita no me salen las palabras para agradecerte todo lo que siento. Me siento afortunada de tenerte siempre tan cerca, me encanta como eres y lo que me transmites porque pase lo que pase me llena de felicidad compartir esta vida contigo. Y a Olalla… idem de idem, ya tu sabes mi sista.
Por último, a mis padres agradecerles todo lo que me han enseñado.
Gracias por el apoyo incondicional, la paciencia, el cariño y la comprensión que habéis tenido conmigo día a día. Pero especialmente quería daros mil gracias por estar ahí siempre pendientes de mí y tener las palabras adecuadas en los momentos más difíciles porque me han dado la energía necesaria para seguir adelante. Y por haber superado el pasado dando paso a un futuro prometedor.
1. INTRODUCTION
1.1. Materials: Brief history 1
1.2. Materials, molecules and atoms: The unresolved problem of
structure predictability 3
1.3. The idea of confinement 6
1.4. Porous and layered materials 7
1.5. Gamma Zirconium phosphate (γγγγ-ZrP): Structure and reactivity 8
1.5.1. Intercalation properties of γ-ZrP 12
1.5.2. Topotactic exchange of the surface phosphates 14
1.5.3. Pillaring of γ-ZrP 15
1.6. Our experience with γγγγ-ZrP based materials: a case of serendipity 19 1.6.1. Supramolecular chirality an related phenomena 19
1.6.2. Confined luminescence of lanthanides 21
1.7. Merging the two worlds: supramolecular chirality and lanthanide
luminescence 30
1.7.1. Polarized light 30
1.7.2. Materials exhibiting polarized light 34 1.7.3. Chiroptical techniques Circular Dichroism (CD) and Circular
Polarized Light (CPL) 39
2. OBJECTIVES 45
3. RESULTS AND DISCUSSION 51 3.1. Synthesis and characterization of the organic chromophores 51
3.1.1. Bisphosphonic acid derived from the bistriazolylpyridine
(BTP) unit 51
3.1.2. Introducing chirality in the bisphosphonic acid derived from the
bistriazolylpyridine unit (CBTP) 69
3.2. Synthesis and photophysical properties of the lanthanide
complexes in solution 81
3.2.1. Luminescence 81
3.2.2. Circular polarized luminescence (CPL) and Circular
Dichroism (CD) 91
3.3. Preparation and characterization of the porous hybrid materials 96 3.3.1. Topotactic exchange of BTP and CBTP into γ-ZrP 109 3.3.2. Intercalation of 1-phenylethylamine in γ-ZrP- BTP 121 3.3.3. Topotactic exchange of the chiral phosphonic acids CBTP
in γ-ZrP 128
3.4. Preparation and characterization of the luminescent porous
hybrid materials 134
3.5. Circularly polarized luminescence: CPL 153
4. CONCLUSIONS AND FUTURE 175
5. EXPERIMENTAL PART 181
5.1. General 181
5.1.1. Reagents and spectroscopic techniques 181
5.1.2. Determination of the quantum yields 184
5.1.3. CPL spectroscopy 186
5.2. Synthesis of the organic chromophores 188
5.2.1. Synthesis of BTP 188
5.2.2. Synthesis of CBTP 192
5.3. Formation of the lanthanides complexes 198
5.4. Synthesis and characterization of the porous hybrid materials 200
5.4.1. Preparation of γ-ZrP 200
5.4.2. General procedure for the cross-linking of γ-ZrP with the
phosphonic acids 202
5.5. General procedure for the intercalation of 1-phenylethylamine (PEA) in
the γγγγ-ZrP based materials 251
5.6. Topotactic exchange of γγγγ-ZrP with a preformed complex of
terbium (III) 263
5.7. Methods for the intercalation of Tb (III) in γγγγ-ZrP-BTP 265 5.8. Intercalation of Eu (III) and Tb (III) in γγγγ-ZrP and derived materials 275
INTRODUCTION
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1. INTRODUCTION
1.1. Materials: Brief history
Along the history, the development and evolution of civilization has been tightly related to the production and use of a large diversity of materials which have progressively been adapted to the fast growing needs concerning survival, transport, building, communication, clothing, lighting, warfare, etc. The starting was quite slow (stone and metal ages) but in the last centuries an exponential rising has taken place in the development of new technologies.
The importance of materials for humankind can be easily envisioned in simple facts like for instance the names given to the first ages of humanity: Stone (until ca. 3,000 BC), Copper (from ca. 7,500 to 1,500 BC), Bronze (from ca. 3,300 BC to 1,100 BC) and Iron (from ca. 1,300 BC 50 ca. 500 AC) Ages, showing the shift of tool making towards harder and longer lasting materials which, at the beginning, were those supplied by Nature without much transformation (Figure 1). However, even at those early stages, chemistry came into play because the metals were not found as such and their source minerals needed some complex processing. The way the latter was discovered is still much of a mystery.
Figure 1. Viktor Mikhailovich Vasnetsov Moscow, Kiev (St. Vladimir Cathedral).
One milestone for the advancement of science and dissemination of culture in general was the invention of the press in 1,450. Therefore, at the end of
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medieval times, the new discoveries in all branches of science and technology started spreading fast all over Europe and the New World.
The English Industrial Revolution in the XVIII century deeply transformed the economy previously based in handcraft into a completely different concept relaying in the massive production of goods that set the foundations for the explosive advance of science, technology and engineering we are witnessing nowadays. Important discoveries in the field of new materials are barely one century old, namely the use of aluminum, steel, plastics and polymers, synthetic textiles, artificial dyes, fertilizers, concrete, etc. The development of new transforming techniques like electrolysis is very recent too. These new technologies heavily depend on reliable and potent power sources and the production of cheap electricity becomes a must. The massive use of coal and petroleum furnished those needs. Petroleum itself has become the raw material from which many new materials have been developed for the general and pharmaceutical industry.
The disasters of WWII paved the way for a new scientific and technical advancement which has a lot to do with a revolution in the fields of automation, electronics and artificial intelligence. Research and competitiveness have led in the last 50 years to an unprecedented technological expansion easily felt in the everyday living chores of mankind. Concerning materials one just has to have a look at the immediate surroundings to discover new polymer composites, tiny semiconductors, complex magnetic devices, biomaterials and a long etc that we simply take for granted these days.
In a few words, Scientists have put together their profound knowledge of Physics and Chemistry and have been able to comprehend the relationship between the structural elements of materials and their properties, in order to get new, a-la-carte ones. The name of Material Science and Engineering has been given to this important activity.1
1 a) Desiraju, G.R. (Ed.), “The Crystal as a Supramolecular Entity”. Perspectives in Supramolecular Chemistry, Vol. 2, Wiley, 1996, Chichester, UK; b) Lehn, J.M.; Atwood, J.L.; Davies, J.E.D.; McNicol, D.D.; Voegtle, F. (Eds.), Comprehensive Supramolecular Chemistry, vol. 11, Pergamon 1996, Elsevier Science: Oxford; c) Callister, W.D., Introducción a la Ciencia e Ingeniería de los Materiales,
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However, in the high tide of these huge advances, some challenging comments have been uttered out loud from highly reputed voices like that of Feynman’s2 who, in 1959 in his famous lecture “There’s plenty of room at the bottom” made this important query: “What would the properties of materials be if we could really arrange the atoms (molecules) the way we want them?” More recently, in 1988, Maddox’s3 reddening grievance: “One of the continuing scandals in the physical sciences is that it remains in general impossible to predict the structure of even the simplest crystalline solids from knowledge of their chemical composition.” These questions and complaints still hold today because it results very difficult to predict the structure of crystalline assemblies.
1.2. Materials, molecules and atoms: The unresolved problem of structure predictability
The design of porous solids with properties that change when their wide enough cavities are accessed by external molecular species is a challenge of great scientific and technological importance. Yet, the building of solid matrices with prefixed structure is even today quite difficult a task to be achieved.
The relatively new concept of crystal engineering4 has come to the rescue.
It lays a bridge between Material Sciences1a,b and Supramolecular Chemistry,5 the
Reverté, S.A. 1997, Barcelona; d) Shackelford, J.F., Introducción a la Ciencia de Materiales para Ingenieros, Pearson Alhambra, Prentice Hall. 2005.
2 Feynman, R. “There's plenty of room at the bottom” Eng. Sci. 1960, 23, 22.
3 Maddox, J. “Crystals from first principles” Nature 1988, 335, 201.
4 a) Garcia-Garibay, M.A. “Crystalline molecular machines: Encoding supramolecular dynamics into molecular structure” Proc. Natl. Acad. Sciences USA 2005, 102 (31), 10771; b) Miller, J.S. “Crystal engineering or crystal mysticism? A case of study” CrystEngComm. 2005, 7, 458; c) Braga, D.;
Grepioni, F. “Making crystals from crystals: a green route to crystal engineering and polymorphism” Chem. Comm. 2005, 3635; d) Trask, A.V.; Jones, W. Top. “Crystal engineering of organic cocrystals by the solid-state grinding approach” Curr. Chem. 2005, 254, 41; e) Braga, D.;
Brammer, L.; Champness, N.R. “New trends in crystal engineering” CrystEngComm. 2005, 7, 1; f) Reichenbaecher, K.; Suess, H.I.; Hulliger, J. “Fluorine in crystal engineering-the little atom that could” Chem. Soc. Rev. 2005, 34, 22; Erk, P.; g) Hengelsberg, H.; Haddow, M.F.; Van Gelder, R.
“The innovative momentum of crystal engineering” CrystEngComm. 2004, 6, 474; h) Desiraju, G.R., Crystal Engineering: The Design of Organic Solids, 1989, Elsevier, Amsterdam; i) Braga, D.;
Desiraju, G. R.; Miller, J. S.; Orpen, A. G.; Price, S. L. “Innovation in crystal engineering”
CrystEngComm. 2002, 4, 500.
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latter being a new conceptual realm in Chemistry established not long ago and worth a Nobel Price (Figure 2).
Donald J. Cram Jean-Marie Lehn Charles J. Pedersen
Figure 2. Scientists awarded with the 1987 Nobel Prize in Chemistry.
The concept of Supramolecular Chemistry is a new way of thinking that goes “beyond” the classical molecule. It started from quite simple arrangements like that between a crown-ether and a cation. Yet, these simple assemblies, not ruled by the classical covalent bonds reigning within discrete molecules, led to quite complex scaffolds sustained by “weak forces” 6 that, even though they were well known for decades, were not considered as central in classical chemistry.
Therefore, Supramolecular Chemistry made chemists look at old stuff in a different, enlightening way, as if one were playing around with the building blocks of a Lego®
5 a) Lehn, J.M. “Perspectives in supramolecular chemistry – From molecular recognition towards molecular information processing and self-organization” Angew. Chem. Int. Ed. Engl. 1990, 29, 1304; b) Lehn, J.M., Supramolecular Chemistry: Concepts and Perspectives, VCH, 1995, Weinheim.
6 a) Costisor, O.; Linert, W. “Metal directed self-assembled systems” Rev. Inorg. Chem. 2003, 23, 289; b) Moulton, B.; Zaworotko, M.J. “From molecules to crystal engineering. Supramolecular isomerism and polymorphism in network solids” Chem. Rev. 2001, 101, 1629; c) Braga, D.;
Greponi, F. “Organometallic crystal engineering: prospects for a systematic design” Coord. Chem.
Rev. 1999, 183, 19; d) Desiraju, G.R. “Hydrogen Bridges in Crystal Engineering: Interactions without Borders” Acc. Chem. Res. 2002, 35, 565; e) Aakeroy, C. B.; Beatty, A. M. “Crystal engineering of hydrogen-bonded assemblies. A progress report” Aust. J. Chem. 2001, 54, 409; f) Strauch, H.C.; Rinderknecht, T.; Erker, G.; Frohlich, R.; Wegelius, E; Zippel, F.; Hoppener, S.; Fuchs, H.; Chi, L.F. “Substituent-dependent formation of supramolecular aggregates of 6-hydroxy-trans- 3-hexenoic acids in the solid state” Eur. J. Org. Chem. 2000, 187; g) Ranganathan, A.; Pedireddi, V.R.; Sanjayan, G.; Ganesh, K.N.; Rao, C.N.R. “Sensitive dependence of the hydrogen-bonded assemblies in cyanuric acid-4,4'-bipyridyl adducts on the solvent and the structure of the parent acid” J. Mol. Struct., 2000, 522, 87.
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construction set. Taking advantage of metal-ligand interactions,7,6b,c hydrogen bonding8,6e,f or dispersive van der Waals forces, and carefully considering the geometry of the molecules, large new assemblies can be built displaying outstanding new properties which are not the simple addition of those of their individual components. Nevertheless, the weakness of the interactions makes the supramolecular association to be ruled by thermodynamics and self-assembly which many times introduce again the aforementioned problem of lack of predictability. Despite that, it cannot be argued that molecular self-assembly9 is at present one of the most attractive approaches to produce new materials. The scope of their applications involves quite interesting fields as optoelectronics,10 conductivity and superconductivity,11 charge transfer,12 and magnetism,13 nano- and biomimetic materials,14 etc.
In contrast to all these predictive deficiencies, the art or science of designing and synthesizing organic molecules has reached very high levels of
7 Chisholm, M.H.; Folting, K.; Huffman, J.C.; Li, H.; Macintosh, A.M.; Wu, D.D., “d3-d3 diolates of dimolybdenum and ditungsten. Part 4. Molybdenum derivatives of S(-)-1,1-diphenyl-1,2- propanediol, S(-)-1,1,2-triphenyl-1,2-ethanediol and (1R, 2R)-(-)-1,2-dicyclohexyl-1,2-ethanediol”
Polyhedron, 2000, 19, 375.
8 a) Strauch, H.C.; Rinderknecht, T.; Erker, G.; Frohlich, R.; Wegelius, E; Zippel, F.; Hoppener, S.;
Fuchs, H.; Chi, L.F. “Substituent-dependent formation of supramolecular aggregates of 6-hydroxy- trans-3-hexenoic acids in the solid state” Eur. J. Org. Chem. 2000, 187; b) Ranganathan, A.;
Pedireddi, V.R.; Sanjayan, G.; Ganesh, K.N.; Rao, C.N.R. “Sensitive dependence of the hydrogen- bonded assemblies in cyanuric acid-4,4'-bipyridyl adducts on the solvent and the structure of the parent acid” J. Mol. Struct., 2000, 522, 87.
9 Lehn, J.M. “Perspectives in supramolecular chemistry: from molecular recognition towards self organization” Pure Appl. Chem. 1994, 66, 1961.
10 Marks, T.J.; Ratner, M.A. “Design, synthesis and properties of molecule-based assemblies with large second-order optical nonlinearities” Angew. Chem. Int. Ed. 1995, 34, 155.
11 Miller, J.S.; Epstein, A.J. “Organic and organometallic molecular magnetic materials: designer magnets” Angew. Chem. Int. Ed., 1994, 33, 385.
12 a)Segura, J.L.; Martin, N. “New concepts in tetrathiafulvalene chemistry” Angew. Chem. Int. Ed.
2001, 40, 1372; b) Breu, J.; Kratzer, C.; Yersin, H. “Crystal Engineering as a Tool for Directed Radiationless Energy Transfer in Layered {Λ-[Ru(bpy)3] ∆-[Os(bpy)3]}(PF6)4” J. Am. Chem. Soc.
2000, 122, 2548.
13 a) Gatteschi, D. “Molecular magnetism. A basis for new materials” Adv. Mater. 1994, 6, 635; b) Braga, D.; Maini, L.; Prodi, L.; Caneschi, A.; Sessoli, R.; Grepioni, F. “Anions derived from squaric acid form interionic π-stack and layered, hydrogen-bonded superstructures with organometallic sandwich cations: the magnetic behavior of crystalline [(η6-6H6)2Cr]+[HC4O4]-” Chem. Eur. J.
2000, 6, 1310.
14 Aizemberg, J.; Hanson, J.; Koetzle, T.F.; Weiner, S.; Addadi, L. “Control of Macromolecule Distribution within Synthetic and Biogenic Single Calcite Crystals” J. Am. Chem. Soc. 1997, 119, 881.
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sophistication, based upon a relatively simple set of rules that guide both the invention and synthesis of new compounds. This set of rules is construed as the rational synthetic method of organic chemistry. We, as organic chemists confronted to the task of building new solid structures with tailored chemical properties, find it necessary to develop some rational approach and to establish the corresponding set of rules allowing for a realistic level of predictive knowledge in the construction of solid scaffolds. We found these conditions reasonably accomplished by the use of layered salts of tetravalent transition metals, namely zirconium phosphate.
1.3. The idea of confinement
It should be noted that Feynman also conjectured “What could we do with layered structures with just the right layers? The placing of organic molecules between the layers of certain inorganic salts leads to sturdy, solid materials where the act of confinement makes the organic molecules change their properties or even display new ones at the supramolecular level. The development of what one might call classical chemistry has been dominated by the running of reactions in liquid phase, where molecules are relatively free to move in any direction, their movement and orientation being controlled by essentially non-selective processes as those of diffusion and salvation. It is thus fairly easy to infer that the inherent rigidity conferred by a solid-state environment of layers would condition a high variety of chemical and physical processes such us chirality, sensitization and light emission. In fact, many of these processes performed in solution require molecular pre-association in order to limit the movement of molecules and to condition their relative orientation, inasmuch as it would be in a rigid, solid environment. The technological advances experienced by solid-state NMR and x-ray diffraction techniques and the development of relatively cheap thermal and surface analysis equipment have made it possible to routinely carry out the characterization of solid structures with a degree of certainty similar to that achieved in solution. Processes occurring in the solid state or at the solid-liquid interphase, i.e. under conditions of heavy confinement, are nowadays much easier to study than in the recent past.
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Along the years we and others have found that zirconium phosphate is a versatile porous, layered inorganic salt that acts as a “carving board” where organic compounds can be attached, either covalently or by ionic forces, by mild hydrothermal reactions, to form 3D structures with properties and potential applications only limited by the imagination of the researcher.
1.4. Porous and layered materials
The area of chemistry that studies the porous materials has received a great impulse in the last 30 years. In addition to the classic porous materials (activated charcoal, silica, alumina, zeolites), synthetic materials have been developed with analogous structures to those observed in Nature. An important landmark in this field has been the preparation of synthetic zeolites. The term zeolite derives from Greek and refers to their capacity to release the water coordinated in its pores when they are warmed up. The specific design of these pores has allowed its use as catalysts, adsorbents and ion exchangers.
An important effort has been dedicated to the development of other porous materials with analogous three-dimensional structures. To this effect, the chemistry of metallic phosphates15 with laminar structure has received a great impulse, because they are very versatile materials whose handling fits very well with what we have termed rational synthetic method, i.e. the development of a set of relatively simple rules that confers sufficient predictive knowledge to the building of crystalline materials. The synthetic rationale elaborated by us through a number of years16 is modular because it comprises the design and synthesis of appropriate organic molecules in one hand and on the other, their stepwise
15 Clearfield, A., “Metal phosphate chemistry”. Progress in Inorganic Chemistry, Vol. 47. Karlin, K.D.
(Ed.), Wiley, 1998, New York.
16 a) Brunet, E.; Huelva, M.; Rodríguez-Ubis, J. C. “Covalent bonding of aza-18-crown-6 to γ- zirconium phosphate. A new layered ion-exchanger with potential recognition capabilities”
Tetrahedron lett. 1994, 35, 8697; b) Brunet, E.; Huelva, M.; Vazquez, R.; Juanes, O.; Rodríguez- Ubis, J.C. “Covalent bonding of crown ethers to γ-zirconium phosphate - new layered ion exchangers showing selective recognition” Chem. Eur. J. 1996, 2, 1578; c) Alberti, G.; Brunet, E.;
Dionigi, C.; Juanes, O.; de la Mata, M.-J.; Rodríguez-Ubis, J. C.; Vivani, R. “Shaping solid-state supramolecular cavities: chemically induced accordion-like movement of γ-zirconium phosphate containing polyethylenoxide pillars” Angew. Chem. Int. Ed. 1999, 38, 3351.
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introduction into the inorganic framework. Common characteristics of this synthetic approach to solid materials based on layered metallic phosphates are as follows:
• Their reactions proceed at low temperatures.
• They can be functionalized either with organic or inorganic molecules, by ionic or covalent bonds, what confers them very specific properties and a broad field of application.
Their functionalization is accomplished by means of topotactic reactions that do not affect the fundamental structure of the material, so that it is possible to predict the arrangement of the final products.
When the introduced organic moieties have two points of anchorage, it is possible to join the consecutive lamellae of these materials, and three-dimensional, pillared structures can be thus easily created.
In the following sections we will describe the general structural properties of these layered organic-inorganic salts and the major accomplishments performed in our laboratory.
1.5. Gamma-Zirconium phosphate (γγγγ-ZrP): Structure and reactivity It is assumed that a solid exhibits a layered structure only when the bonds among atoms of the same plane are much stronger than the interactions among atoms of adjacent planes. In the majority of typical layered solids (e.g., graphite, clays, MIV phosphates and phosphonates, etc) there are covalent bonds between atoms of the same layer and weak forces (van der Waals, hydrogen bonding, etc.) between adjacent layers.
A single layer can be seen as a giant, planar macromolecule and can be called lamella. The distance between the barycenters of two adjacent lamellae (Figure 3) is called the interlayer distance (d). When the thickness of the lamella is subtracted from the interlayer distance, the distance marking the free space
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between adjacent layers (df) is obtained. The available space between two adjacent lamellae is usually called the interlayer region.17
d-spacing Layer thickness
Interlayer region df
Figure 3. Schematic illustration of the meaning of typical terms used to characterize a layered solid.
Layered structures formed by zirconium phosphate are produced by octahedra-tetrahedra combinations in three different formats, named α-, γ- and λ- phases (Figure 4).
Figure 4. Schematic representation of the three typical layered structures of zirconium phosphate by octahedra-tetrahedra building-block combinations: (a) α, (b) γ and (c)
λ.
17 Alberti, G., “Layered metal phosphonates and covalently pillared diphosphonates”. In Comprehensive Supramolecular Chemistry, Vol.7. Lehn, J.M. (Ed.), Pergamon, 1996, Oxford.
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In α and γ structures the Zr octahedrons are formed by six oxygens of six surrounding tetrahedral phosphates, the difference between α and γ being that in the former all phosphates are equivalent (they all use three oxygens to bond Zr) whereas in the γ-phase one phosphate uses its four oxygens to metal coordination and another one only two of them. The molecular formulas for α- and γ-ZrP are thus Zr(HPO4)2·nH2O and Zr(PO4)(H2PO4)·nH2O, respectively. In the recently discovered λ-phase,18 Zr coordinates to four phosphate oxygens and to two other additional species, one negatively charged (halides, alcoxydes, carboxylates, etc.) and another one neutral (amines, sulfoxides, etc.).
The organic-inorganic structures presented in this work are based on the γ- phase (γγγγ-ZrP from now on) because of its unique property: surface phosphates (blue tetrahedra in Figure 4b) can be smoothly replaced (see below) by phosphonates carrying different organic functions.
Figure 5. Molecular model of two consecutive layers of γ-zirconium phosphate (γγγγ-ZrP). There are two water molecules per Zr atom in the interlayer region that have been omitted for the sake of clarity (right). Schematic representation of the distance
between adjacent phosphates and accessible area
18 Poojary, D.M.; Zhang, B.; Clearfield, A. “Synthesis and crystal structure of a new layered zirconium phosphate compound, Zr(PO4)F(OSMe2)” J. Chem. Soc., Dalton Trans. 1994, 16, 2453.
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The internal and surface phosphates of γγγγ-ZrP give different signals in the solid-state
31P-NMR spectrum and its exact crystal structure has been deduced by Clearfield et al. by x-ray powder diffraction data (Figure 5),19 corresponding to a molecular formula Zr(PO4)(H2PO4)·2H2O. The thick layers of γγγγ-ZrP are relatively rigid and the interlayer distance results to be 1.22 nm and layer thickness 0.9 nm, leaving an interlayer space of 0.32 nm where two water molecules per Zr atom reside.
The most important reactions of γγγγ-ZrP take place because of the surface phosphates:
1. Ionic processes (intercalation): The reversible insertion of guest species into the interlayer region of a layered host is called the intercalation process (Scheme 1). These reactions are caused by the acidic character of the surface phosphates.
Scheme 1. Representation of the intercalation reaction in γ -ZrP.
2. Formation of covalent bonds (esterification and exchange reactions): The surface phosphates may act as nucleophiles rendering organic phosphates as shown in Scheme 2. This method has been used in the literature20 to obtain esters of ethylenglycol by the use of ethyleneoxide as electrophile.
In general, these derivatives are relatively unstable due to the sensitivity of the P-O bonds to hydrolysis reactions.
19 Poojary, D.M.; Shpeizer, B.; Clearfield, A. “X-ray powder structure and Rietveld refinement of γ-zirconium phosphate, Zr(PO4)(H2PO4)·2H2O” J. Chem. Soc., Dalton Trans. 1995, 111
20 Yamanaka, S. “Synthesis and characterization of the organic derivatives of zirconium phosphate”
Inorg. Chem., 1976, 15, 2811.
12
Scheme 2. Esterification of surface phosphates of γ-ZrP.
The most interesting reaction of the surface phosphates of γγγγ-ZrP is that in which they are replaced by other phosphorous functions such as phosphonates (Scheme 3). For this reaction to take place, the layers of γγγγ-ZrP has to be separated first in a process named exfoliation. The exchange reaction takes place in a topotactic manner, i.e. it occurs without affecting the integrity and rigidity of the inorganic layers.
Scheme 3. Exchange reaction of surface phosphates of γ -ZrP.
1.5.1. Intercalation properties of γγγγ-ZrP
The presence of the two acidic OH groups pointing to the interlayer region (Figure 5) accounts for the high acidity of γγγγ-ZrP. The two molecules of water per acidic phosphate present in the interlayer region, favor the diffusion of guest species. Therefore γγγγ-ZrP easily reacts with alkaline bases and amines and intercalating the corresponding cations into the layered structure. In the important case of alkylamines, the intercalation process has been extensively studied.21
21 a) Yamanaka, S.; Horibe, Y.; Tanaka, M. “Uptake of pyridine and n-butylamine by crystalline zirconium phosphate” J. Inorg. Nucl. Chem. 1976, 38, 323; b) Clearfield, A.; Tindwa, R.M. “On the mechanism of ion exchange in zirconium phosphates. XXI. Intercalation of amines by α- zirconium
13
It may be formally regarded as an ionic exchange in which one acidic proton of each surface phosphate is replaced by the corresponding ammonium group. Protonation of the amine usually generates an exothermic reaction and the energy required for the expansion of the amine is provided by the formation of these new bindings. The available area around each surface phosphate (0.36 nm2; cf. Figure 4) only allows for the presence, per reacting phosphate, of a single alkylamine since its cross section is ca. 0.20 nm2. Figure 6 shows interlayer distances of γγγγ-ZrP intercalated of alkylamines of increasing chain length.
Experimental data are only compatible with the formation of a double layer of alkylamines in zig-zag conformation as shown in Figure 6. The relationship between interlayer distance and chain length is linear and indicates that the longitudinal axis of the alkyl group is inclined by 55° relative to the inorganic layers.
The intercalation of alkylamines in the materials formed may allow us elucidate the possible rearrangement of the compounds when are placed within the γγγγ-ZrP as we will see later.
phosphate” J. Inorg. Nucl. Chem. 1979, 41, 871; c) Nowell, D.V.; Gupta, J.P. “Mechanism of some alkylammonium-ion exchanges by α-zirconium bis-monohydrogenphosphate) monohydrate” J.
Chem. Soc., Dalton Trans. 1979, 7, 1178; d) Alberti, G.; Costantino, U. “Recent progress in the intercalation chemistry of layered α-zirconium phosphate and its derivatives, and future perspectives for their use in catalysis” J. Mol. Catal. 1984, 27, 235; e) Clearfield, A.; Tindwa, R.M.;
Ellis, D.K.; Peng, G.Z. “Intercalation of n-alkylamines by α-zirconium phosphate” J. Chem. Soc., Faraday Trans. 1985, 81, 545; f) Costantino, U.; Casciola, M.; Di Croce, L.; Marmottini, F.
“Intercalation of α-ω-alkyldiamines in layered α-zirconium phosphate and the inclusion behaviour of some of the intercalates obtained” J. Incl. Phenom. 1988, 6, 291; g) García, J.R.; Trobajo, C.;
Espina, A.; Khainakov, S.A. “Direct synthesis of bis(n-alkylammonium) monohydrogen phosphates by solid-vapour reaction” Inorg. Chem. Commun. 2002, 5, 372; h) Kijima, T.; Ohe, K.; Sasaki, F.;
Yada, M.; Machida, M. “Intercalation of dendritic polyamines by α- and γ- zirconium phosphates”
Bull. Chem. Soc. Jpn. 1998, 71, 141; i) Chang, T.C.; Ho, S.Y.; Chao, K.J. “Polyaniline intercalated into zeolites, zirconium phosphate and zirconium arsenate” J. Phys. Org. Chem. 1994, 7, 371; j) Hasewaga, Y.; Matsuda, R.; Kisa, M.; Iso, M. “Intercalation of N,N-dimethyl-1-phenylethylamine into α-zirconium phosphate” J. Incl. Phenom. Mol. Macroc. Chem. 2002, 42, 33; k) Hasewaga, Y.;
Seki, H.; Tomita, I. “Intercalation of phenylethylamines into α-zirconium phosphate and characterization of intercalates” J. Incl. Phenom. Mol. Recognit. Chem. 1991, 10, 313; l) Matsubayashi, G.; Okuno, S. “Direct intercalation of pyridinium-derivative cations into α-zirconium phosphate interlayer by a redox reaction and fluorescence behavior of the intercalation compounds” Inorg. Chem. Acta 1996, 245, 101; m) Costantino, U.; Casciola, M.; Peraio, A.
“Intercalation compounds of α-zirconium hydrogen phosphate with heterocyclic bases and their ac conductivity” Solid State Ionics 1991, 46, 53.
14
Figure 6. Intercalation of alkylamines of increasing chain length into γγγγ -ZrP. An example of the double layer arrangement is presented for hexylamine.
1.5.2. Topotactic exchange of the surface phosphates
Topotactic replacement of the H2PO4 groups originally present in the interlayer region occurs gradually with the initial formation of a partially converted derivative as it shown in the Equation 1:
γ-ZrPO4.O2P(OH)2→ γ-ZrPO4 [O2P(OH)2]1-2x(O2PRR´)x→ γ-ZrPO4.O2PRR´ Equation 1.
Mixed organic derivatives of γγγγ-ZrP such as γγγγ-ZrP /phosphonate, γγγγ-ZrP /phosphite and γγγγ-ZrP /phosphinate compounds are possible. Thus γγγγ-ZrP derivatives can be represented by the formula ZrPO4.O2PRR´·nS, where R and R´ can be H, OH or an organic group and S is the intercalated solvent.
For this reaction to proceed at a reasonable rate, γγγγ-ZrP layers have to be separated or exfoliated, either by intercalation of an alkylamine (see above) of sufficient chain length or by solvation of the inorganic layers in a 1:1 mixture of water:acetone at 80°C.22
22 Alberti, G.; Dionigi, C.; Giontella, E.; Murcia-Mascarós, S.; Vivani, R. “Formation of colloidal dispersions of layered γ-zirconium phosphate in water/acetone mixtures” J. Coll. Interf. Sci. 1997, 188, 27.
15
The mechanism of this reaction was proposed by Alberti.23 The reaction takes place in various steps (Figure 7). Firstly, one Zr-O bond to a surface H2PO4 is broken by a molecule of water. In the next step, water is replaced by one oxygen atom of the phosphonate group, for instance, thus forming its first bond with the layer. The reaction is completed when the second oxygen of the phosphonate group expels (directly of water-assisted) the surface phosphate.
Figure 7. Proposed mechanism by Aberti et Al. for the topotactic exchange reaction.
When the topotactic phosphate/phosphonate exchange reactions occur at the same time on the facing surfaces of two adjacent layers by a molecule with two phosphonate groups, pillared γ-ZrP is formed. If the pillars have a suitable height and are sufficiently spaced, microporous solids are thus obtained.
1.5.3. Pillaring of γγγγ-ZrP
One can obtain “pillared” structures when the topotactic exchange reactions are performed with a diphosphonate and the two phosphorus functions
23 Alberti, G.; Giontella, E.; Murcia-Mascarós, S. “Mechanism of the Formation of Organic Derivatives of γ-Zirconium Phosphate by Topotactic Reactions with Phosphonic Acids in Water and Water- Acetone Media” Inorg. Chem. 1997, 36, 2844.
16
react with two different layers. The lamellae of γγγγ-ZrP are thus covalently bonded to each other24 (Figure 8A). The resulting material will bear different properties (porosity for example) depending on the structure and the number of exchanged molecules. The literature contains various examples of pillared structures25 using different diphosphonates, rigid and flexible,26 the latter being the most versatile ones because they enable drastic changes in the porous structure by means of simple chemical methods.
Fórmula molecular:
ZrPO4(H2PO4)1-2x(HO3P-R-PO3H)x
Fórmula molecular:
ZrPO4(H2PO4)1-x(HO3P-R-PO3H)x
= bisfosfonato
A B
= H2PO4
Figure 8. Representation of the two possible products of exchange when using bisphosphonic acids: A) pillared, B) laminar.
Alberti’s example is quite interesting.27 His research group performed the exchange with alkyldiphosphonates leading to materials of molecular formula ZrPO4(H2PO4)1-2x(HO3P-(CH2)m-PO3H)x·nH2O where the exchange level (x = 0.1 to 0.9) and chain length (m = 4 to 12) were amply varied. It was observed that the interlayer distance was initially independent of the latter but, in the presence of water it reached a maximum value coherent with the length of the involved chain.
24 Alberti, G.; Bein, T. (Eds.), “Solid-state Supramolecular Chemistry: Two- and Three-dimensional Inorganic Networks”. In Comprehensive Supramolecular Chemistry, vol. 7. Pergamon, 1996, Oxford, UK.
25 Alberti, G.; Marmottini, F.; Murcia-Mascarós, S.; Vivani, R. “Synthesis and preliminary characterization of a covalently strained zirconium phosphate diphosphonate with interlayer microporosity” Angew. Chem. Int. Ed. Engl. 1994, 33, 1594.
26 Alberti, A.; Murcia-Mascarós, S.; Vivani, R. “Preparation and characterization of zirconium phosphate diphosphonates with the γ-structure: a new class of covalently pillared compounds”
Mater. Chem. Phys. 1993, 35, 187.
27 Alberti, G.; Murcia-Mascarós, S.; Vivani, R. “Pillared derivatives of γ-zirconium phosphate containing nonrigid alkyl chain pillars” J. Am. Chem. Soc. 1998, 120, 9291.
17
The observed variation of porosity exerted by the presence/absence of solvent was coined as “accordion effect” (Scheme 4).
S
- S
+S S = disolvente
Scheme 4. Representation of the "accordion effect".
We have a very important example of our own. Diphosphonates derived from polyethylenglycol chains of different lengths (from di- to hexa-ethylenglycol) were prepared and exchanged at 25% level into γγγγ-ZrP.28 The corresponding pillared materials (Figure 9) were thus formed, bearing the general molecular formula of Zr(PO4)(H2PO4)0.75[HO3P(CH2)2(OCH2CH2O)n (CH2)2PO3H]0.125 (n = 2 to 6). The remaining surface phosphates [(H2PO4)0.75] were quantitatively replaced by hypophosphite [(H2PO2)0.75] leading to two sets of materials, polar/polar (polar columns and polar surface phosphates) and polar/non-polar (polar columns and non-polar surface hypophosphites). Figure 9 summarizes the observed variation of interlayer distance (as measured by XRD) when the materials containing pentaethylenglycol diphosphonate (n = 5), as a representative example, were treated with methylamine in aqueous dispersion. It may be seen that the polar/polar material steadily augmented its interlayer distance with the increasing amount of intercalating amine. When no amine is present, the polar columns interact by hydrogen bonding with the surface phosphates and the layers are compressed to one another (Figure 9A). The amine reacts with the surface phosphates and progressively disrupts the web of hydrogen bonding interactions, thus making the columns to stand up (Figure 9B).
28 Alberti, G.; Brunet, E.; Dionigi, C.; Juanes, O.; De la Mata, M.J.; Rodriguez-Ubis, J.C.; Vivani, R.
“Shaping solid-state supramolecular cavities: chemically induced accordion-like movement of γ- zirconium phosphate containing polyethylenoxide pillars” Angew. Chem. Int. Ed. 1999, 38, 3351.
18 A
B
Figure 9. Molecular models of γγγγ-ZrP exchanged with pentaethylenglycol diphosphonate with variable interlayer distance (see text); plot of interlayer distance variation with pH
of the indicated materials (see text).
The overall porosity of the material is thus heavily increased by the mild acid-base reaction. In the case of the polar/non-polar material the increase of interlayer distance occurs all of a sudden, within a very narrow pH range (see plot in Figure 9). The replacement of the surface phosphates by hypophosphite greatly diminishes the overall acidity of the material, the only acidic OH groups being those of the phosphonates. In the absence of amine, the polar columns still can establish O…H-P hydrogen bonds with the surface hypophosphite groups. When sufficient amount of amine is present, the few remaining acidic OH groups are quickly neutralized and the methylammonium ions act as wedges that pull up the columns to suddenly rise. Therefore, the interlayer distance is doubled and the porosity is on the whole profoundly changed. To the best of our knowledge, this odd supramolecular behaviour in the solid state, responding to a simple acid-base reaction in the solid-liquid interface, has never been observed before.29
29 a) Brunet, E.; De la Mata, M.J.; Juanes,O.; Rodriguez-Ubis, J.C. “Solid-state reshaping of crystals:
Flash increase in porosity of zirconium phosphate-hypophosphite that contains polyethylenoxa diphosphonate pillars” Angew. Chem. Int. Ed. 2004, 43, 619; b) Brunet, E.; De la Mata, M.J.;
Alhendawi, H.M.H; Cerro, C; Alonso, M.; Juanes, O.; Rodriguez-Ubis, J.C. “Engineering of
19
1.6. Our experience with γγγγ-ZrP based materials: a case of serendipity 1.6.1. Supramolecular chirality and related phenomena
Chirality is a property that can be found at different levels of molecular hierarchy. Chirality of single molecules is a well known consequence of dissymmetry and handedness.30 But chirality may arise from the assembly of symmetric units forming a dissymmetric assemblage under the influence of an external or internal chiral species.31 Nature is an overwhelmingly well-doer of this superstructures and nucleic acids and proteins clearly outclass man-made materials.
The term supramolecular chirality was used for the first time in the title of a paper as recently as only twenty years ago.32 On the other hand, the literature search with the keywords “supramolecular chirality” renders a few references from which almost all are concerned with this property in solution,33 not in the solid state.
Microcrystalline Solid-State Networks Using Cross-Linked γ-Zirconium Phosphate/Hypophosphite with Nonrigid Polyethylenoxadiphosphonates. Easy Access to Porously Dynamic Solids with Polar/Nonpolar Pores” Chem. Mat. 2005, 17, 1424.
30 Eliel, E.L.; Wilen, S.H. Stereochemistry of Organic Compounds. Wiley, 1994, Chichester.
31 For some reviews on related topics see for example: a) Harmata, M. “Chiral Molecular Tweezers”
Acc. Chem. Res. 2004, 37, 862; Cataldo, F.; b) Keheyan, Y. “An essay on asymmetric polymerization and the origin of chirality in the biologically active macromolecules” Int. Perspectives Chem.
Biochem. Res. 2003, 43; c) Tsukube, H.; Shinoda, S. “Armed cyclen receptors: from three dimensional cation recognition to supramolecular architecture” Bull. Chem. Soc. Japan 2004, 77, 453; d) Yashima, E.; Maeda, K.; Nishimura, T. “Detection and amplification of chirality by helical polymers” Chem. Eur. J. 2004, 10, 42; e) Voegtle, F.; Pawlitzki, G. “Cyclophanes: from planar chirality and helicity to cyclochirality” Cyclophane Chemistry for the 21st Century 2002, 55; f) Verbiest, T.; Persoons, A. “Nonlinear optics and chirality” Top. Stereochem. 2003, 24, 519; g) Spector, M.S.; Selinger, J.V.; Schnur, J.M. “Chiral molecular self-assembly” Top. Stereochem. 2003, 24, 281; h) Kawabata, T.; Fuji, K. “Memory of chirality: asymmetric induction based on the dynamic chirality of enolates” Top. Stereochem. 2003, 23, 175; i) Thilgen, C.; Gosse, I.; Diederich, F. “Chirality in fullerene chemistry” Top. Stereochem. 2003, 23, 1; j) Barlow, S.M.; Raval, R.
“Complex organic molecules at metal surfaces: bonding, organization and chirality” Surf. Sci. Rep.
2003, 50, 201.
32 Green, M.M.; Weng, D.; Noguchi, J.; Okamoto, Y. “Liquid crystals and the question of the interplay of intramolecular and supramolecular chirality” Polym. Prep. 1993, 34, 166.
33 Mateos-Timoneda, M.A.; Crego-Calama, M.; Reinhoudt, D.N. “Supramolecular chirality of self-assembled systems in solution” Chem. Soc. Rev. 2004, 33, 363.
20
The pursuit of chiral assemblages is an area of basic research that may lead to important applications in artificial molecular recognition and the creation of molecular devices with chiral memory.34
A relevant finding of our work, concerning the study of the chiroptical features in the solid state, comprises the study of the influence that the layered γ- ZrP exerts in the physical properties of the materials built from it. The structure of every lamella in γ-ZrP is intrinsically dissymmetric35 and in the absence of any chiral influence must not display any optical activity inasmuch as either racemic mixtures or meso forms do not.
We have been working in our laboratory on this problem. We induced supramolecular chirality in three-dimensional organic-inorganic composites by means of the preparation of polyethylenoxa pillared zirconium phosphate.36
First we found that the intercalation of optically pure phenylethylamine (PEA) within the γ-ZrP gave rise to the material named as γ-ZrP-(+)-PEA displaying a huge value of specific optical rotation ([α]D
25 = + 54000 ± 6000), more than 3 orders of magnitude higher than that of pure (+)-PEA in solution. The intercalated chiral amine breaks the symmetry of the meso-type supramolecular structure of γ-ZrP.37
In a second step of this research we prepared a new material named as ZrP- H25 that contains 12 organic chains of hexaethylenglycol diphosphonate per 100 Zr atoms. We found that this material became optically active when intercalated with (+)-PEA. The resulting pillared-intercalated material (ZrP-H25-PEA+) was treated with hexylamine in order to smoothly replace the chiral (+)-PEA with the non- disymmetrical amine, leading to the material named after ZrP-H25-C6H13N2. Astoundingly, the optical activity of the material was almost completely retained in absence of chiral (+)-PEA. It should be noted that either pristine γ-ZrP or its
34 a) Purrello, R. “Supramolecular chemistry: Lasting chiral memory” Nature Mat. 2003, 2, 216; b) Ziegler, M.; Davis, A.V.; Johnson, D.W.; Raymond, K.N. “Supramolecular chirality: A reporter of structural memory” Angew. Chem. Int. Ed. 2003, 42, 665.
35Poojary, M.; Shpeizer, B.; Clearfield, A. “X-ray powder structure and Rietveld refinement of γ- zirconium phosphate, Zr(PO4)(H2PO4)·2H2O” J. Chem. Soc., Dalton Trans. 1995, 111.
36 Brunet, E. “Asymmetric Induction under Confinement” Chirality, 2002, 14, 135.
37Brunet, E.; De la Mata, M. J.; Juanes, O.; Alhendawi, H. M. H.; Cerro, C.; Rodríguez-Ubis, J. C.
“Solid-state reshaping of nanostructured crystals: supramolecular chirality of layered materials derived from polyethylenoxa-pillared zirconium phosphate” Tetrahedron: Asymm. 2006, 17, 347.
21
derivatives with racemic PEA or hexylamine did not display appreciable average optical rotation (Figure 10). The only plausible explanation to this odd behaviour is that intercalated (+)-PEA induced the homochirality of the helicity of the polyethoxy chains which was retained when the disymmetric amine was replaced by a symmetric one.
Figure 10. Optical rotation measurement of the indicated samples (see text).
This serendipity-driven phenomenon made us plan new research goals, related to the idea of supramolecular quirality, many of wich are the subjetc of this work.
1.6.2. Confined luminescence of lanthanides
The ability of zirconium phosphates to coordinate different cations and neutral organic molecules, allows its use as a matrix in which an organized and selective metal complexes may be inserted. The incorporation of these complexes can follow two paths:
• Direct introduction of ligand-metal complex structures.
22
• Sequential intercalation of species, introducing the ligand in the first place and then incorporating the desired metal,38 or viceversa.
There are many examples of successfull direct incorporation of metal complex into laminar materials, among which can be highlighted the additions of Ru(bpy)3+,39 porphyrinic derivates,40 different transition metal complexes41 and complexes with lanthanide ions.42 These last products, usually associated with Europium and Terbium, which present interesting properties as luminescent materials,43 have been not much studied in spite of their relevance in materials science as they are associated with high technology applications such as unimolecular spectroscopy,44 fluorescent solar collectors,45 switches, sensors and optical nanodevices,46 among others.
Europium (Eu) and Terbium (Tb) are elements that belong to the lanthanide or rare earth metals (atomic numbers 63 and 65 respectively), contain partially filled 4f layer and are therefore paramagnetic. Generally, the oxidation state is 3+,
38 a) Ferragina, C.; Massucci, M.; La Ginestra, A.; Patrono, P.; Tomlinson, A.A.G. “Diffusion of large amine ligands into layered α-Zr(PO4)·2H2O. Access to solid-state coordination chemistry” J. Chem.
Soc., Chem. Commun. 1984, 1024. b) Ferragina, C.; La Ginestra, A.; Massucci, M.A.; Patrono, P.;
Tomlinson, A.A.G., “ Intercalation of 2,2´-bipyridyl into α-zirconium phosphate and in situ formation of Co2+, Ni2+ and Cu2+/2,2´-bipyridyl complex pillars” J. Phys. Chem. 1985, 89, 4762.
39 a) Alonso, M. “Nuevos materiales organo-inorgánicos: separación de cargas fotoinducidas en fosfatos de zirconio”, DEA 2001, UAM. c) Brunet, E.; Alonso, M.; De la Mata, M.J.; Fernández, S.;
Juanes, O.; Chavanes, O.; Rodríguez-Ubis, J.C. “Covalent bonding of phosphonates of fullerene and Ru complexes to γ-zirconium phosphate as a template for building chemical devices in the solid state” Chem. Mater. 2003, 15, 1232.
40 Thompson, M.E.; Kim, R.M.; Pillion, J.E.; Burwell, D.A.; Groves, J.T. “Intercalation of aminophenyl and pyridinium substituted porphyrins into zirconium hydrogen phosphate: evidence for subtituent derived orientational selectivity” Inorg. Chem. 1993, 32, 4509.
41 Rosenthal, G.L.; Caruso, J. “Photochemical behavior of metal complexes intercalated in zirconium phosphate” J. Solid State Chem. 1991, 93, 128.
42 a) Kumar, Ch. V.; Chaudhary, A. “Photon antennas: self-assembly of donor and acceptor metal ions at the galleries of layered α-zirconium phosphonates” Micropor. Mesopor. Mater. 1999, 32, 75. b) Xu, R.; Xu, Q.; Fu, L.; Li, L.; Zhang, H. “Preparation, characterization and photophysical properties of layered zirconium bis(monohydrogenphosphate) intercalated with rare earth complexes” J. Mat. Chem. 2000, 10, 2532.
43 Vogler, A.; Kunkely, H. “Luminescent metal complexes: diversity of excited status” Top. Curr.
Chem. 2001, 213, 143.
44 Basché, T; Moerner, W.E.; Orrit, M.; Wild,U.P. (Eds.), In Single-Molecule Optical Detection, Imaging and Spectroscopy, VCH, 1997, Weinheim, and references herein.
45 a) G. Seybold, G. Wagenblast, Dyes and Pigments, 1989, 11, 303.
46 O’Neil, M.P.; Niemczyk, M.P.; Svec, W.A.; Gosztola,D.; Gaines, G.L.III; Wasielewski, M.R.
“Picosecond optical switching based on biphotonic excitation of an electron donor-acceptor- donor molecule” Science, 1992, 257, 63.
