POLITECNICO DI MILANO
Tesi di Dottorato di
FRANCISCO FERNANDEZ PALACIO
Matricola 802404
DESIGN AND APPLICATIONS OF HALOGEN-BONDED RESPONSIVE MATERIALS DIPARTIMENTO DI CHIMICA, MATERIALI E INGEGNERIA CHIMICA “Giulio Natta” Dottorato di Ricerca in Chimica Industriale e Ingegneria Chimica (CII)
XXVIII cicle 2012 - 2015
Coordinatore: Prof. Tiziano Faravelli Tutor: Prof. Luca Lietti
2
To my parents, Esther and Carlos.
To my brother Pablo.
A mis padres Esther y Carlos.
A mi hermano Pablo.
3
Acknowledgements.
Thanks to everyone who has supported me throughout my doctoral studies
over these three years.
A very special thanks to the most important person, Priscilla.
¿Qué te voy a decir?
4
List of Publications
Manuscripts in preparation
“Zinc(II) coordination networks decorated with azobenzene halogen bonding donors”
Francisco Fernandez-Palacio, Marco Saccone, Luca Catalano, Tullio Pilati, Giancarlo Terraneo, Pierangelo Metrangolo, Giuseppe Resnati
Ready for publication, intention to submit to CrystEngComm
“Photoinduced phase transitions in halogen-bonded liquid crystals”
Francisco Fernandez-Palacio, Mikko Poutanen, Marco Saccone, Antti Siiskonen, Giancarlo Terraneo, Giuseppe Resnati, Olli Ikkala, Pierangelo Metrangolo and Arri Priimagi.
Ready for publication, intention to submit to Angew. Chem. Int. Ed.
“Halogen bond-directed nanostructuring of block copolymers”
Francisco Fernandez-Palacio, Roberto Milani, Marco Saccone, Alessandro Luzio, Gabriella Cavallo, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi, and Olli Ikkala
Ready for publication, intention to submit to J. Am. Chem. Soc.
“Halogen bond in ionic liquid crystals”
Francisco Fernandez-Palacio, Gabriella Cavallo, Giancarlo Terraneo, Marco Saccone, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
“Halogen bonding in Molecular Clips Co-Crystal with I2”
Luca Catalano, Francisco Fernandez-Palacio, Giancarlo Terraneo, Lyle Isaacs, Giuseppe Resnati, Pierangelo Metrangolo
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Conferences
“Photoinduced Phase Transitions in Halogen-Bonded Liquid Crystals” (Presenting Author)
F. Fernandez-Palacio, M. Poutanen, M. Saccone, G. Terraneo, A. Siiskonen, G. Resnati, P. Metrangolo, A. Priimagi.
Abstract and Poster
RSC Macrocylic and Supramolecular Chemistry Group. Durham (United Kingdom)
December, 2015
“Zn(II) Coordination Networks based on an Azobenzene-containing Halogen Bond-Donor Ligand” (Presenting Author)
F. Fernandez-Palacio, M. Saccone, L. Catalano, T. Pilati, G. Terraneo, G. Resnati, P. Metrangolo.
Abstract and Poster
RSC Macrocylic and Supramolecular Chemistry Group. Durham (United Kingdom)
December, 2015
“Fast and Efficient Photoinduced Phase Transitions in Halogen-Bonded Liquid Crystals” (Presenting Author)
Francisco Fernandez-Palacio, Mikko Poutanen, Marco Saccone, Giancarlo Terraneo, Pierangelo Metrangolo, Giuseppe Resnati, Arri Priimagi.
Abstract and Poster. POSTER PRIZE, by Crystal Growth and Design
2nd ICSU/IUPAC Workshop on Crystal Engineering. Como (Italy)
August-September 2015
“Zinc(II) Coordination Networks based on an Azobenzene-containing Halogen-Bond Donor Ligand” (Presenting Author)
Francisco Fernandez-Palacio, Marco Saccone, Tullio Pilati, Pierangelo Metrangolo, Giuseppe Resnati
Abstract and Poster
2nd ICSU/IUPAC Workshop on Crystal Engineering. Como (Italy)
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“Halogen-Bond Driven Self-Assembly of Borromean Systems” (Co-Author)
Gabriella Cavallo, Francisco Fernandez-Palacio, Vijith Kumar, Frank Meyer, Tullio Pilati, Pierangelo Metrangolo, Giuseppe Resnati, Giancarlo Terraneo
Abstract and Poster
2nd ICSU/IUPAC Workshop on Crystal Engineering. Como (Italy)
August-September 2015
“Self-Assembly of Photo-Switchable Fluorinated Liquid Crystals through Halogen Bonding” (Presenting Author)
Francisco Fernandez-Palacio, Marco Saccone, Mikko Poutanen, Tullio Pilati, Gabriella Cavallo, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
Oral Presentation
21st International Symposium on Fluorine Chemistry and 6th International Symposium In Fluorous Technologies. Como (Italy)
August 2015
“Photoinduced Phase Transitions in Halogen-Bonded Liquid Crystals” (Presenting Author)
Francisco Fernandez-Palacio, Mikko Poutanen, Marco Saccone, Tullio Pilati, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
Abstract and Poster
10th The International Symposium on Macrocyclic and Supramolecular Chemistry. Strasbourg (France)
June, 2015
“Tunable halogen-bonded responsive systems: A closer look at solid state” (Co-Author)
Giancarlo Terraneo, Francisco Fernandez-Palacio, Gabriella Cavallo, Valentina Dichiarante, Marco Saccone, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
Poster
Gordon Research Conference. Artificial Molecular Switches & Motors. Easton, Massachusetts, (United States)
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“Photoinduced Phase Transitions in Halogen-Bonded Liquid Crystals” (Co-Author)
Mikko Poutanen, Francisco Fernandez-Palacio, Marco Saccone, Tullio Pilati, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
Poster
Gordon Research Conference. Artificial Molecular Switches & Motors. Easton, Massachusetts, (United States)
June, 2015
“Halogen-Bonded Photoresponsive Liquid Crystals” (Co-Author)
Marco Saccone, Francisco Fernandez-Palacio, Mikko Poutanen, Tullio Pilati, Giuseppe Resnati, Pierangelo Metrangolo, Arri Priimagi.
Abstract and Poster
Gordon Research Conference. Self-Assembly and Supramolecular Chemistry. Lucca (Italy)
May, 2015
“Borromean Systems via Anion Driven Self-assembly: Topology Invariance and Metric Tuning on Anion Change” (Presenting Author)
F. Fernandez-Palacio, F. Meyer, T. Pilati, P. Metrangolo, G. Resnati Abstract and Poster
First International symposium on Halogen Bonding. Porto Cesareo (Italy)
June, 2014
“Organic frameworks formed via hydrogen and halogen bonding orthogonal self-assembly.” (Presenting Author)
F. Fernandez-Palacio, L. Colombo, J. Martí-Rujas, G. Terraneo, T. Pilati, P. Metrangolo, G. Resnati.
Abstract and Poster
Past, Present, and Future of Crystallography@Politecnico di Milano. Milan (Italy)
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Index
Index of figures 11
Index of tables 16
State of the art 17
1. Introduction 18
1.1. What is Halogen Bonding?
18
1.1.1. Halogen Bonding: History
21
1.1.2.Halogen Bonding in Supramolecular Chemistry 22
1.2. Responsive materials 24
1.2.1. Photoresponsive materials 24
1.2.2. Azobenzene system 25
1.2.3. Responsive does not only mean Photo- 26
1.3. Liquid Crystals 27
1.3.1. Halogen-Bonded Liquid Crystals 31
1.3.2. Photoresponsive Halogen-Bonded Liquid Crystals 32
1.3.3. Ionic Liquid Crystals 33
1.4. Metal Organic Frameworks 35
1.4.1. Photoswitchable Metal Organic Frameworks 36
9
2. Neutral and Ionic Halogen-Bonded Liquid Crystals 45
2.1. Objectives 45
2.2. Materials 48
2.3. Results and discussion 50
2.3.1. Structural analysis 50
2.3.2. Mesophase characterization 54
2.3.3. Photochemical studies 57
2.4. Experimental part 63
2.4.1. General procedure for the synthesis of AZO molecules 63
2.4.2. Crystal structure determination 64
2.5. Conclusions 68
2.6. Halogen-bonded ionic liquid crystals 69
2.6.1. Objectives 69
2.6.2. Materials, results and discussion 69
2.6.3. Conductivity studies 73
2.6.4. Conclusions 76
3. Metal Organic Frameworks 77
3.1. Objectives 77
3.2. Materials 80
3.3. Results and discussion 81
3.3.1. {[Zn(1)(Py)2](2-propanol)}n (3) 81
3.3.2. {[Zn(1)2(4,4’-bipyridyl)2](DMF)2}n (4) 83
3.3.3. {[Zn(2)(1,2-di(4-pyridyl)ethylene](DMF)1.81}n (5) 85
3.3.4. Photochemical studies 86
3.4. Experimental part 88
3.4.1. Synthesis of AZO molecules 88
3.4.2. Synthesis of MOFs 89
10
4. Block Co-Polymers 92
4.1. Objectives 92
4.2. Materials 94
4.3. Results and discussion 96
4.4. Experimental part 106
4.4.1. Preparation of samples for AFM 106
4.4.2. Removal of DIPFO in the complex 106
4.4.3. Preparation of samples for IR 106
4.4.4. Preparation of samples for TGA 107
4.4.5. Metalation 107
4.5. Conclusions 108
General conclusions and future perspectives 109
11
Index of figures
Figure 1. General scheme for the formation of halogen bonds. Y is a carbon,
nitrogen or halogen atom, X is the electrophilic halogen atom (XB donor,
Lewis acid) and D is a donor of electron density (XB acceptor, Lewis base). ... 18
Figure 2. Molecular electrostatic potential, in Hartrees, mapped on the 0.001 electrons Bohr-3 isodensity surface. Color code: positive electrostatic potential in red and negative electrostatic potential in blue11 ... 20
Figure 3. Photoisomerization of trans azobenzene to cis azobenzene and viceversa. ... 25
Figure 4. Examples of calamitic mesogens.12 ... 28
Figure 5. Smectic phases, showing layered structure: (left) Smectic A, and (right) Smectic C (tilted). ... 29
Figure 6. On the left Smectic phase. On the right Nematic phase. On the top schematic views of the different phases. On the bottom, POM images for the two phases... 29
Figure 7. Examples of discotic liquid crystals.12 ... 30
Figure 8. The three kinds of bent-core liquid crystals... 31
Figure 9. First example of Liquid Crystal formed through Halogen Bonding.71 ... 32
Figure 10. Simple scheme of Metal Organic Framework Synthesis. ... 35
Figure 11. The azobenzene forming the MOF structure can be switched from the trans to the cis state by UV light (ν1) and vice versa by visible light (ν2). 98 ... 37
Figure 12. Structure of the azobenzene-containing linkers and MOFs.104 ... 38
Figure 13. Schematic representation of supramolecular assembly formation through hydrogen bonding between the P4VP block of PS-b-P4VP copolymer and any halogen-bond donor molecule. ... 40
Figure 14. Mean-field prediction of the thermodynamic equilibrium phase structures for conformationally symmetric diblock melts. fA is the volume fraction. ... 40
12 Figure 15. A theoretical phase diagram (depending on temperature) for a
conformationally symmetric block co-polymer melt. ... 41
Figure 16. On the left perpendicular and on the left parallel orientation between
cylindrical order and the film. ... 43
Figure 17. Schematic sketch of the fabrication process. ... 43 Figure 18. Photoinduced phase transition of LC-azobenzene mixtures. Tcc refers
to the LC-isotropic phase transition temperature of the mixture with the cis form, while Tct refers to that of the mixture with the trans form.
134
... 46
Figure 19. The complexes prepared in this study are assembled by halogen
bonding between promesogenic stilbazole molecules (left) and photoresponsive
halogen-bond donors (right). ... 47
Figure 20. On the left, crystal packing of cis-AZO12. On the right, crystal
packing of trans-AZO12.144 ... 51
Figure 21. X-ray spacefill model of the AZO10-ST1 complex showing both the
N···I halogen bonding and arene-perfluoroarene quadrupolar interactions.144 ... 52
Figure 22. The crystal structure of the complex AZO12-ST1 reveals the XB
interaction and the segregation of the aliphatic chains from the aromatic cores. ... 52
Figure 23. Crystal structure of the complex AZO8-ST2.144 ... 53
Figure 24. X-ray Powder diffraction for sample AZO12-ST1. In red calculated,
in black simulated. ... 53
Figure 25. X-ray Powder diffraction for sample AZO10-ST1. In blue calculated,
in black simulated. ... 54
Figure 26. POM images for a typical Nematic phase (left) for the complex AZO8-ST8 and Smectic A phase (right) in the complex AZO12-AZO8-ST8. ... 56 Figure 27. Chart of the thermal behavior of the studied complexes. Crystal phase
is in blue, Smectic phase in red and Nematic phase in green. All the transitions are reported on heating, with the exception of the AZO12-ST12 complex. ... 57
Figure 28. The normalized absorbance spectra of the AZOm and STn molecules,
13
photostationary spectra under illumination wavelengths of 365 nm, 395 nm and 457 nm, and the calculated spectrum of the cis-isomer for the AZO10
molecule. ... 58
Figure 29. The photoinduced nematic-to-isotropic transition and the reverse transition of AZO10-ST8 observed under POM. ... 60
Figure 30. On the left the birefringence and absorbance measurements of the photoinduced nematic-to-isotropic transition. On the right the absorbance behavior under different illumination conditions. ... 61
Figure 31. Transition crystal-to-isotropic liquid of AZO10-ST8 at 95 °C (10 °C below melting point) with 395 nm light. The initial crystal (1) melts into isotropic liquid (2) after ca. 30 s of illumination. The recrystallization (3) occurs through a partial nematic phase formation approximately 3 minutes after the UV light illumination. The recrystallization proceeds as a front due to uneven illumination conditions under the microscope. The end state is fully crystalline material (4) without phase separation. ... 62
Figure 32. Crystal structure of the compounds (left to right, top to bottom): trans-AZO8, trans-AZO10, trans-AZO12 and cis-AZO12.144 ... 64
Figure 33. From the top to bottom: Crystal structure of the AZO12; ST1-AZO10 and ST2-AZO8.144 ... 65
Figure 34. The complexes prepared in this study are assembled by halogen bonding between imidazolium iodides “IMIn” (left) and photoresponsive halogen-bond donors AZOm (right). ... 69
Figure 35. POM image of the 2AZO12-IMI2 at 70°C (cooling). ... 70
Figure 36. DSC of IMI2, AZO12 and 2AZO-IMI2 ... 71
Figure 37. Crystal structure of 2AZO12-IMI12.144 ... 72
Figure 38. Crystal structure of 2-stilbeneC8-IMI10.144 ... 72
Figure 39. Crystal structure of N,N-azo halogen-bond donor and IMI8 iodide.144 ... 73
Figure 40. On the left IMI12 conductivity. On the right 2AZO12-IMI2 conductivity. ... 74
14 Figure 41. Comparison between conductivity of 2AZO12-IMI2 and IMI12 ... 74 Figure 42. On the left compound 1, on the right compound 2 ... 78 Figure 43. Organic commercial linkers used in this work. ... 81 Figure 44. On the left the coordination polymer 3, projected along its main
axis. On the right projected approximately orthogonal to main axis.144 ... 82
Figure 45. A single complete net projected along the a axis, showing the cage
that contains two DMF molecules (larger balls).144 ... 83
Figure 46. A single complete net projected along the b axis, where halogen
bond (I•••O) is evident.144
... 84
Figure 47. Interdigitating of three 2D layers projected along a axis using
different colors. ... 85
Figure 48. Left: packing view along a showing the bidimensional pipe network
containing DMF in Mercury style. Right: the same viewed along c. In all the plots, the DMF is omitted and only one of the disordered conformers is reported, for clarity.144 ... 86
Figure 49. Left, UV-vis spectra of 1 before (black curve) and after (coloured
curves) irradiation using 457 nm light. Right, time development of the absorbance of 1. ... 87
Figure 50. Left, UV-vis spectra of 2 before (black curve) and after (coloured
curves) irradiation using 457 nm light. Right, time development of the absorbance of 2. ... 87
Figure 51. On the top, a dual height (left) and phase (right) AFM images for the
polymer (blank). On the bottom, a dual height (left) and phase (right) AFM images for the polymer plus DIPFO (complex) ... 96
Figure 52. A dual height (left) and phase (right) AFM images for the polymer
with DIPFO showing tetragonal order. ... 97
Figure 53. Distribution parameters of diameter bulk (left) and distance
15 Figure 54. A dual height (left) and phase (right) AFM images for the
complex at different speeds (from the top to the bottom, 500, 1000 and 2000 rpm respectively) ... 98
Figure 55. TEM images ... 99 Figure 56. SAXS patterns of PS-b-P4VP (trace 1) and PS-b-P4VP(DIPFO)
(trace 2) samples prepared by drop casting. ... 100
Figure 57. Comparison of IR spectrum of DIPFO, PS-b-P4VP and complex ... 101 Figure 58. Comparison of TGA for PS-b-P4VP, complex and DIPFO. ... 102 Figure 59. Comparison of Raman spectrum for PS-b-P4VP, complex
and DIPFO. ... 103
Figure 60. A dual height (left) and phase (right) AFM images of a thin complex
film after washing with ethanol. ... 104
Figure 61. ATR-FTIR spectra of PS-b-P4VP, DIPFO and of a thin spin-coated
complex film, both as prepared and after ethanol washing. ... 104
Figure 62. AFM topographic micrograph on the left and section profile on the
right of gold nanostructures prepared by metalation of the hollow template left after washing the thin complex film in ethanol, and subsequent removal of the
polymer template by acetone. ... 105
16
Index of tables
Table 1. Thermal data for the LC complexes ... 55 Table 2. Crystallographic data for the compounds trans-AZO12, trans-AZO10, trans-AZO8 and cis-AZO12 ... 66 Table 3. Crystallographic data for the complexes AZO12-ST1, AZO10-ST1 and AZO8-ST2. ... 67 Table 4. Thermal data for the LC complexes ... 70 Table 5. Crystallographic data for the complexes 2AZO12-IMI12,
2stilbeneC8-IMI10 and 2N,N-azo-IMI8. ... 75 Table 6. Crystallographic data for the compounds 3, 4, 5. ... 90
17
State of the art
From the prehistory, humans have been creating materials with the goal of helping their daily chores. Therefore, knowing the problem, one can design apparatus to overcome common drawbacks and it is particularly attractive to layout materials from smaller things. Applying this concept to chemistry, the design
of materials from small molecules and the interaction between them, give us the definition of the “Supramolecular Chemistry”.
Recently, Supramolecular Chemistry has been deeply studied for many groups on every side of the world, in which molecules respond to stimuli applied from diverse
non-internal sources by undergoing reversible transformations
between two different states. The importance of these phenomena rises from the molecules undergoing changes in their
noted characteristics (e.g. electronic and topological) and then acting as switching different elements in functional materials. Thus, designing a functional system responding to external stimuli, one can create a material which provides us with predictable response.
In the recent years, several studies related to halogen bonding have been developed by numerous groups. The remarkable interest in this non-covalent interaction may find explanation in its properties (specificity, robustness and high directionality), which present more advantages than others non-covalent interactions frequently used in Supramolecular Chemistry.
Therefore, highly motivated from the discussion above mentioned, I considered three well known topics in supramolecular chemistry to review the studies developed until now, adding halogen bonding properties in order to improve their characteristics. These topics are responsive Liquid Crystals (LC), Metal Organic Frameworks (MOFs) and block co-polymers.
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CHAPTER 1
Introduction
1.1.
What is Halogen Bonding?
Recently, IUPAC has defined Halogen Bonding (XB)1 as “an attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another or the same, molecules entity”. To clarify the general scheme Y-X…D, illustrated in figure 1has been introduced2 to define a halogen bond.
Figure 1. General scheme for the formation of halogen bonds. Y is a carbon, nitrogen or halogen
atom, X is the electrophilic halogen atom (XB donor, Lewis acid) and D is a donor of electron density (XB acceptor, Lewis base).3
19
In this scheme, the Lewis acid X (halogen atom) is covalently bound to Y (carbon, nitrogen or halogen atom) which non-covalently interacts with the Lewis Base D (N, O, S, Se, Cl, Br, I, Halides…). Halogens participating in halogen bonding are iodine (I), bromine (Br), chlorine (Cl) and, rarely,4 fluorine (F). All four halogens act as halogen-bond donors (as proven through theoretical and experimental data) following the general trend: F < Cl < Br < I, with iodine normally forming the strongest interactions since it is most likely polarizable.5 Y can be any atom (e.g., C, N or halogen). Of particular interest is the case where Y is a halogen because it is a dihalogen. Dihalogens (I2, Br2, etc.) tend to form strong halogen bonds.
6-7
In the same way, the presence of electron-withdrawing groups (for example fluorine atoms) near the atom covalently bound to the halogen, increases the strength of halogen bond considerably. 8-9-10
In organic halides, the electron density is anisotropically distributed around halogen atoms (as you can see in figure 2) where the molecular electrostatic potential for every halogen atom is represented.11 According to this scheme, it is possible to note that the positive potential along the C-X axis increases on moving from F to I, in the same way as the polarizability of the halogen atom. Therefore, an important area of positive charge is shown for I, while for the F atom, the electrostatic potential remains negative.12 Thus, a negatively charged belt surrounds the smaller halogens element and, indeed, this anisotropic distribution of the electron density makes possible for halogen atoms to work at the same time as XB donors and acceptors. Nevertheless, the magnitude of the positive area increases as the electron-withdrawing effect of the neighboring groups increases and, therefore, theoretical calculations prove it is possible to carefully tune the strength of the halogen bond in a given halocarbon by modifying the substituents on the carbon skeleton.8-10
20
Figure 2. Molecular electrostatic potential, in Hartrees, mapped on the 0.001 electrons Bohr-3
isodensity surface. Color code: positive electrostatic potential in red and negative electrostatic potential in blue11
The well-localized electron positive region described above, permits to dispose the electron pair of other, or the same, molecule towards the halogen atom, and it plays a crucial role for the orientation of this interaction. For this reason, halogen bonding is markedly directional, and the angle between the atom covalently bound to the halogen, the halogen and the electron rich atom usually approximates to 180°.2-13
Another important point is that the presence of halogen atoms in a molecule decreases its hydrophilic character. It has been reported that polar solvents have low interaction in the energies and geometries of halogen bond in solution.14 However, since it does not happen with hydrogen bond, halogen bond is considered as a hydrophobic equivalent of the hydrogen bonding.
All the benefits of halogen bonding described above, particularly the high directionality, linearity and robustness, have been recognized and much used in crystal engineering,15 medicinal chemistry16 and, more recently, in the design of functional materials.17 Thus, halogen bond is presented as an attracting interaction for supramolecular chemistry. Furthermore, halogen bonding has been used for other applications in different topics as anion recognition,18 polymers,19 nanoparticles,20 catalysis21 among many others.
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1.1.1. Halogen Bonding: History.
In the mid-19th century the first systems involving halogen atoms as electrophilic “sticky” sites in self-organization processes due to NH3
.
I2 and pyridine-alkyl iodides
adducts being isolated, was described.22-23 About sixty years later, Benesi and Hildrebrand24 published a paper where they described the UV-Vis changes that accompany the spontaneous complexation in non-polar solvents of various aromatic hydrocarbons with I2. In the 1950s, Robert S. Mulliken developed a detailed theory of
electron donor-acceptor complexes, classifying them as being outer or inner complexes.25-26-27 The Mulliken theory has been used to describe the mechanism through which halogen bond formation occurs. Outer complexes were those in which the intermolecular interaction between the electron donor and acceptor were weak and had very little charge transfer. Inner complexes have extensive charge redistribution.
In 1969, Odd Hassel won the Nobel Prize in Chemistry (shared with Derek H. R. Barton) for “his contributions to the development of the concept of conformation and its applications in chemistry”. These contributions were based on the studies about X-ray crystallographic measurements of Br2 complexes with benzene and dioxane in
which it was provided evidence that these intermolecular complexes involve close contacts between electron-donor and acceptor molecules (with interatomic separation significantly shorter than the sum of their Van der Waals radii).28-29 Therefore, in his Nobel lecture,30 he highlighted the importance of intermolecular interactions involving halogen atoms to direct supramolecular self-assembly.31
Nowadays, there are many groups around the world working on Halogen Bonding from many different topics. In graphic 1, it is possible to note how the number of papers, which have “halogen bond” as topic, has been raising to reach more than 200 papers in 2014. Therefore, recently, the IUPAC introduced the definition of Halogen Bonding.1 In June 2014, the first “International Symposium On Halogen Bonding” was celebrated. It was held on Porto Cesareo (Italy)32
22
200 participants. The Second Symposium will be held on Gotherburg (Sweden) in June 2016.33 For all these reasons, I can quote Metrangolo and co-workers2 saying “the halogen bonding concept is still in its infancy”
Graphic 1. Number of papers including “halogen bond” in the title. Source: Web of Science
1.1.2. Halogen Bonding in Supramolecular Chemistry.
In his Nobel Price Lecture, Professor Jean-Marie Lehn defined Supramolecular Chemistry as “the chemistry of the intermolecular bond, covering the structures and functions of the entities formed by association of two or more chemical species”.34 The forces, which permit to assemble these domains, may vary from strong covalent bonds into every single molecules up to weak interactions, (including electrostatic interactions, ion-dipole interaction, dipole-dipole interaction, hydrogen bonding, halogen bonding…). This allows individual molecules to held together with non-covalent intermolecular forces to form a bigger unit called supramolecule,35 where individuals have its own organization, their stability and tendency to associate or isolate.
23
Therefore, the main advantage of supramolecular chemistry is to provide structures in which the properties are given by the cooperation between the small constituents. Thus, supramolecular chemistry is playing an important role in concepts as molecular self-assembly, host-guest chemistry, folding, molecular recognition, dynamic covalent chemistry.36 In addition, supramolecular chemistry is crucial to the understanding of many biological processes from cell structure to vision which relies on these forces for structure and function.
As it was discussed above, the weak interactions play a critical role in the final structure of the supramolecules. In organic molecules, the halogen atoms are usually located at the periphery of them and prone to be involved in intermolecular interactions. For example, halogen bonding is also presented as a functional, effective and reliable interaction to direct intermolecular recognition processes in gas, liquid and solid phases. In fact, in the last decade many examples have been reported using halogen bonding to direct the self-assembly of non-mesomorphic components into supramolecular liquid crystals,37 to afford and tune second-order nonlinear optical responses,38 to control the structural and physical properties of conducting and magnetic molecular materials,39 to separate mixtures of enantiomers and other isomers,40 to exert supramolecular control on the reactivity in the solid state,41 to optimize the binding of ligands in a receptor, molecular folding, and other biopharmacological properties,42 as well as to bind anions in solution and in the solid state.43
One of the most important advantages working with supramolecular structures is to design and predetermine these macro-domains in order to obtain specific properties. This has a great potential for developing useful materials and, in this context, halogen bonding has been deeply studied for self-assembled systems.19-44
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1.2. Responsive materials
1.2.1. Photoresponsive materials.
Several studies on material sciences have been developed, where molecules (or materials) respond to the stimuli of an applied external source by undergoing reversible transformations between two different conformations or isomers.45 The importance of this fact is that molecules undergo changes in their characteristics (e.g. electronic and topological) and can act as switching elements in functional materials.46 Therefore, this relationship among material-answer opens to a plethora of combinations.
Particularly, modern lasers are used in order to achieve fast response times and focus on fine-tuned light stimulus of a specific wavelength on localized areas. Photochromism46 is the process by which a large number of compounds, for instance
azobenzenes, interconvert between two different isomers when they are stimulated in
the absorption spectra and, for this reason, azobenezenes are the most studied molecule that gives photoresponsive properties to the materials. The variation in the electronic structure entails that this materials are useful because of their particular properties in many areas, such as refractive index,47 luminescence,48 electrical conductance49 and optical rotation.50
In this Doctoral Thesis photoresponsive materials designed through halogen bonding will be discussed. This combination is particularly attractive and commonly used in the recent years,37 due to the well-known and above highlighted properties of this interaction as, for example, the high directionality,51 which can enhance the optical performance52 and the ability to tune the interactive strength between the building blocks via single halogen atom mutation at the binding site.
25
1.2.2. Azobenzenes system.
Azobenzenes are molecules containing two aromatic rings held together by a nitrogen-nitrogen double bond. Due to the N=N link, they exist in two stereoisomeric forms, trans and cis (Fig. 3). Actually, 'azobenzene' is the term used to refer to the generic molecule, but it is extended to the molecules with different chemical groups which replaced the aromatic hydrogen.
Figure 3.Photoisomerization of trans azobenzene to cis azobenzene and viceversa.
At room temperature, the trans form is predominant because it is more stable than the cis form due to thermodynamically reasons, mainly because of close proximity of the rings in the latter that leads to steric repulsion. Through the absorption of UV-Vis radiation, trans form can be switched to the cis and the process depends on the wavelength of the absorbed photon and on the transition involved (π → π* for the trans azobenzene). The effect produced on the absorption spectrum on azobenzenes molecules attaching functional groups to the rings has been deeply studied.
The wavelengths at which azobenzene isomerization occurs depend on the particular structure of each azomolecule and, for this reason, azobenzenes have been divided into three different classes,53 according to the absorption spectra. The first ones (yellow) are azobenzene type materials, which are electronically and chemically similar to the azobenzene. These compounds show a higher intensity π-π* absorption in the ultraviolet region but low-intensity n-π* absorption in the visible. Aminobenzenes are the second type (orange) and they are characterized by a weak
26 pull-push character because they are ortho- or para- substituted with electron-donor
groups and tend to closely spacedn-π* and π-π* bands in the visible. The last ones are the pseudostilbenes molecules (red), which present a particularly hard push-pull character because they are substituted on the 4 and 4’ position by functional groups which are strong electron donors or acceptors and, for this reason, the strong π-π* band is red shifted and overlaps with the n-π* one.54
1.2.3. Responsive does not only mean
Photo-In addition to the photoresponsive materials explained above, many materials can be sensitive to a different number of factors, such as washing solvent, temperature, humidity, pH or the intensity of light and can undergo changes in many different ways, like altering color or transparency, becoming permeable to water or changing shape among others. Therefore, in the last years, many studies also focused on these “changes” in order to design systems which provide an answer required when they are stimulated not only with light. Particularly, as in medical applications, the polymers are often used so that the answer cannot be induced by light and, for this reason, other stimuli, as the change of temperature, have been studied.55
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1.3. Liquid Crystals
The liquid crystalline state is often quoted as the fourth state of matter56-57 because of its ability to form the so-called mesophases. Their properties are intermediate between those of the crystalline solid state and those of the liquid state. Therefore, liquid crystals flow like liquids, but they are anisotropic compounds.58-59 In the late 1880s, liquid crystals were discovered by Friedrich Richard Reinitzer,60 when he was experimenting with cholesteryl benzoate. However, it was one year later when Otto Lehmann defined what liquid crystals are for the first time.61 For almost a hundred years, the interest on liquid crystals remained entirely academic up to the 1970s, when they were used and exploited in flat-panel displays.62 After that moment, several new applications for liquid crystals have been found, such as optical imaging,63 medical applications64 and erasable optical disks65 among many others.
Depending on the order of molecules within the liquid crystal phase, we can distinguish several different kinds of them. Therefore, the first classification depends on how the liquid crystal phase is reached. If liquid crystallinity is induced by temperature or solvent they are called thermotropic or lyotropic liquid crystals respectively.
For a better understanding, we need to introduce the concept of anisotropy, because it helps us distinguish liquid-crystalline molecules (mesogens) from those that are not liquid crystalline. Anisotropy is the property of being directionally dependent. Isotropy is the opposite because it implies identical properties in all directions. Usually, liquid crystals systems have one axis, which is very different from the other two. When there are one short and two long axes we have disc-like
molecules while when there are one long and short axes we have rod-sharped molecules.
Attending to their molar mass, liquid crystals can be classified in low molar
28
mass liquid crystals have been studied in this Doctoral Thesis, while the high molecular mass ones will not be consider in the following discussion.
Low molar mass liquid crystals are divided into three major categories depending on the kind of molecule which forms the liquid crystal phase (calamitic,
discotic and bent-core). Calamitic mesogens is characterized by a rod-shaped rigid
core formed by two or more rings and one flexible chain at least (Fig. 4).
Figure 4. Examples of calamitic mesogens.12
The liquid crystal phases of calamitic mesogens may be classified into two types: Smectic (Sm) and Nematic (N). In the Smetic phase, molecules are arranged in layers, with the long molecular axis approximately perpendicular to the laminar planes. The only long-range order extends along this axis, with the result that individual layers can slip over each other (in a manner similar to that observed in graphite). Within one layer there is a certain amount of short-range order. Although there are many categories of smectic phases depending on the angle between the layer and the direction of the molecules, in this Doctoral Thesis I will only consider the
Smectic A (molecules orthogonal to the layers) and Smectic C (the molecules are
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Figure 5. Smectic phases, showing layered structure: (left) Smectic A, and (right) Smectic C
(tilted).66
The Nematic phase is the most disordered of the liquid crystal phases and molecules are aligned in the same direction but are free to randomly drift around, as it similarly happens in an ordinary liquid (Fig. 6).
Figure 6. On the left Smectic phase. On the right Nematic phase. On the top schematic views of
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On the other hand, discotic liquid crystals are disposed around a fairly flat core structure and are usually surrounded by six or eight peripheral alkyl(oxy) chains (Fig. 7). Calamitics molecules tend to form smectic mesophases whereas discotics molecules self-organize into columnar mesophases.
Figure 7. Examples of discotic liquid crystals.12
The third major category is the bent-core (also called banana liquid crystals) and it is characterized by the angle between the two parts of the molecule. Each of these two parts is formed by at least two aromatic rings and, in addition, by one long chain. In some case, one part contains the aromatic rings and another contains the long chain. Despite the fact that constituent molecules of these mesophases are not chiral, they show polar order and chiral superstructures in their LC mesophases. As it is possible to see in figure 8, three types of bent-core liquid crystals are noted.
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Figure 8. The three kinds of bent-core liquid crystals.68
1.3.1. Halogen-Bonded Liquid Crystals
The use of non-covalent interactions for the induction of liquid-crystalline order is particularly attractive, because it is possible to reach liquid crystal phases by mixing molecules which do not show liquid crystal phase by themselves. Hydrogen bonding has been deeply used in this area.69 For instance, in one of the first cases studied,70 several liquid crystals were formed through hydrogen bonding between alkoxystilbazoles and various substituted phenols where neither component was liquid crystalline. Thus, hydrogen bonding represents an example of a non-covalent intermolecular interaction capable of inducing mesomorphism from non-mesomorphic species.
The first example of non-mesomorphic tectons forming liquid-crystal phases induced by halogen bonding was reported in 2004.71 It was demonstrated that liquid crystal phase was reached by mixing 4-alkoxystilbazole and iodopentafluorobenzene,
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and halogen-bonded interaction was confirmed by X-ray single-crystal. Thermotropic smectic A and nematic phases were detected by microscope (Fig. 9).
Figure 9. First example of Liquid Crystal formed through Halogen Bonding.71
In order to compare different halogen-bonded liquid crystals, the non-mesogenic halogen bonding acceptor was also mixed to bromopentafluorobenzene but no evidence was found that the thermal behavior of the free stilbazole changed. As mentioned above, this fact can be explained because the strength of the halogen bonding interaction depends on the polarizability of the halogen atom.
Right after these studies, the versatility of halogen bonding in liquid crystals in trimers formed (2:1) between stilbazoles with diiodoperfluoroalkanes72 and diiodotetrafluorobenzene, was also confirmed.73 These were the first studies on a topic which is still in its first years.
1.3.2. Photoresponsive Halogen-Bonded Liquid Crystals
As I anticipated in the introduction, halogen bonding is a high directional non-covalent interaction which shares many features with the much better-known hydrogen bonding.74 In 2002, Ikeda et al.75 have published a seminal work for doped
covalent liquid crystals. A photoinduced phase transition in liquid crystals doped with azobenzene derivatives was studied earlier under the polarized microscope. In order to induce the nematic-to-isotropic transition phase, a uniform UV light of wavelength
33
(365 nm) was used. It was assumed that the isotropic phase appears as a microscopic domain, formed around the cis isomer of azobenzene derivative molecules. Their model is based on randomly positioned formation and growth of circularly shaped isotropic domains, characterized by the constant growth rate of their radius during the irradiation.
In a paper published in 2012 by Priimagi, Metrangolo, Resnati et al.37 it was demonstrated that liquid crystals assembled by halogen bonding had an unique light-responsive properties. In particular, they studied the photoaligneament of halogen bonding liquid crystals and the efficiency of surface-relief-grating formation of a complex between a mesogenic halogen-bond donor with an azo-group and a non-mesogenic alkoxystilbazole that acts as a halogen boning acceptor. Regarding the photoaligneament, after a film done by spin-coating was irradiated, the absorbance changes demonstrating that the system was alienated perpendicularly to the sample with time. The second thing verified in this paper was the efficient surface-relief-grating formation of the halogen-bonded supramolecular liquid crystals. Starting with the initial film thickness of 250 nm after irradiation, the depth of the sample was 600 nm increasing the modulation 2.4 times.
Recently, Yinjie Chen et al.76 published a study on photoresponsive liquid crystals based on the formation of halogen bonding interaction between azopyridines and molecular iodine and bromine. Although photochemical phase transition was induced by UV irradiation with iodine molecule, no changes were still observed in brominated compounds due to the different polarizability of the halogen atoms.
1.3.3. Ionic Liquid Crystals
Ionic liquid crystals (ILC) are a class of liquid-crystalline compounds which contain anions and cations. The ionic character means that some of the properties of the ionic liquid crystals significantly differ from those of conventional liquid crystals. One of the most important features of ionic liquid crystals is the ion conductivity. Another significant characteristic is the ionic interaction that stabilizes lamellar
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mesophases. In addition, ionic liquid crystals show uncommon mesophases (nematic columnar phase).77 From an application point of view, due to the fact that ionic liquid crystals combine the properties of ionic liquids and liquid crystals, the interest on this topic has been growing in the last years.78-79-80
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1.4. Metal Organic Frameworks
In recent years, the use of metals in crystal engineering has attracted considerable attention and lead to the development of concepts defined by IUPAC, such as coordination polymers81 (CP) and Metal Organic Frameworks (MOF),81-82-83 crystalline materials composed of self-assembled organic ligands and metal cations.84 The understanding of molecular recognition and intermolecular interactions that drives the crystal packing in solids is a hot topic in present-day research in view of the design and synthesis of new materials.85
In this kind of materials, single metal ions or metal clusters are used as rigid nodes, while multidentate organic molecules possessing diverging coordination sites are used as linkers in order to build one, two or three-dimensional architectures (Fig. 10).
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Since a large variety of organic ligands and metal centers can be used for the construction of non-porous and porous networks, the possible combination between them, and in consequence, the number of MOF that are possible to obtain are infinite. Therefore, the features of the metal and organic ligand play a key role in the properties of the final network. Moreover, the periodical arrangement of the atoms in the crystalline structure might introduce additional advantages for specific applications (e.g. an ordered arrangement of identical catalytic sites).87-88-89 For this reason, an important field in the studies of MOFs and CPs is the introduction of different functional groups in order to obtain specific properties. Due to these characteristics, MOFs and CPs are interesting because of their structural and functional tunability which allows plethora of applications in many topics, such as biology and medicine,90 sensor techniques,91 luminescent and magnetic materials,92-93 gas storage and separation94 or even catalysis.95
The synthesis of MOFs is an important field. There are mainly two different techniques used to obtain the crystalline structures. The first one is the hydrothermal
reaction96 where the synthesis of single crystal is performed in an apparatus
consisting of a steel pressure vessel (autoclave) where a nutrient is supplied along with water. Keeping the volume constant and supplying temperature of ramp, the reaction within the autoclave is developed. The main disadvantage of the method is the impossibility of observing the crystals as they grow. On the other hand,
isothermal techniques are based on growing slowly (even for weeks) single crystals
from a hot solution.
1.4.1. Photoswitchable Metal Organic Frameworks
As explained above, Metal Organic Frameworks are usually pre-designed in order to obtain specific properties that permit us to use them for many applications. In recent years, MOFs have been pre-designed introducing photoswitchable molecules (Fig. 11) which provide the system with photoresponsive properties.97 Some of these
37
studies are oriented to demonstrate the remote-controlled release of guest molecules thanks to the photoswitching of the azobenzene in the MOF structure.98-99 On the other hand, the goal of any other studies is to demonstrate how the adsorption capacity of gas molecules, for example carbon dioxide, can be changed.100-101-102-103
Figure 11. The azobenzene forming the MOF structure can be switched from the trans to the cis
state by UV light (ν1) and vice versa by visible light (ν2).98
Recently an interesting work has been published104 where two similar MOFs have been compared as both “a priori” photoswitchables. As it is possible to see in figure 12, both MOFs are similar in their structure.The first one is formed by copper which is the metal center, to which both 4,4’-bipyridyl and 3-azobenzene-4,4-biphenyldicarboxylic as photoswitchable molecule, are linked. Instead, the photoswitchable molecule on the second MOF is the 3-azobenzene-4,4-bipyridine which is also linked to the copper atoms. As shown in figure 12, both azobenzenes can be switched to the bent cis state by UV light. The azobenezenes present a reversible behavior. They go back to trans state when they are irradiated with visible light. However, when these azobenzenes are placed on the MOF, the behavior is different. While in MOF a) the photoisomerizations is enabled, the photoisomerization in MOF b) is sterically hindered.
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1.5. Block Co-polymers
A polymer is a large molecule composed of many repeated subunits (monomers). In 1963 Karl Ziegler and Giulio Natta, Professor from the Politecnico di Milano, were awarded with the Nobel Prize for "for their discoveries in the field of
the chemistry and technology of high polymers".105 After this moment, a whole new topic on polymers has been opened and therefore, due to their broad range of properties, both synthetic and natural polymers play an essential role in everyday life.
When two or more different of these monomers are united together to polymerize, their result is called polymer. Of particular interest is a kind of co-polymer called block co-co-polymer, which are made up of blocks of different polymerized monomers. Since the term polymer represents an entered world, only block co-polymers will be discussed in this Doctoral Thesis, along with their applications.
I focused my studies on the ability of block co-polymers which remain one of the most extensively studied and utilized classes of macromolecules,106 due to their high capacity to induce microphase separation, which has generated significant interest in their application.107 Therefore, the formation of ultrathin films of predetermined morphology with well-defined order and well-known dimensions has been a hot-topic in recent years. Particularly interesting is the combination of these organic self-assembled molecules with inorganic components having electronic properties because it permits the fabrication of electronic materials.108 Additionally, they may be used for microfiltration, but they are also widely used as templates to obtain functional materials.109
For instance, phase separation is generated by binding (usually one) polymer, which forms the co-polymer block, with small molecule surfactants. Generally, the most widely studied polymer is poly(4-vinylpyridine-co-styrene)110-111 because of its facility to form hydrogen bonding through its pyridine (Fig. 13). In fact, there are
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many examples in literature using hydrogen bonding to achieve phase separation.
112-113-114
Phase separation leads to multiscale structured assemblies with features at length scales of the order of ten to hundred nanometres.115-116
Figure 13. Schematic representation of supramolecular assembly formation through hydrogen
bonding between the P4VP block of PS-b-P4VP copolymer and any halogen-bond donor molecule.
These co-polymers tend to self-assembly in many different possible ways. Depending on the organization and the volume fraction of the space occupied (or the free space) we can mainly distinguish four categories: spheres (s), cylinders (c), bicontinous cubic (g) and lamellae (l) as is reported in figure 14.117
Figure 14. Mean-field prediction of the thermodynamic equilibrium phase structures for
41
Nevertheless, to exploit these morphologies, it is essential to know and understand the control factors and the transitions between them. For instance, depending on the Flory–Huggins interaction parameter between the monomer units, the length of the block copolymers (N) and the composition (f), different structures are formed due to the balancing of the enthalpic interfacial energy between the blocks and the entropic chain stretching energy of the individual blocks.
Figure 15. A theoretical phase diagram (depending on temperature) for a conformationally
symmetric block co-polymer melt.119
The deposition of the sample in the substrate plays a critical role in the ordering of block co-polymers thin films.120 Therefore, there are mainly two methodologies to deposit the sample: spin-coating and drop-cast. The first one, a polymer solution with the linker molecule, is deposited on a substrate undergoing rapid rotation. The centrifugal forces play a critical role and the solution flows off the rotating substrate forming a uniform film while simultaneously evaporating and, consequently, when the solvent is completely evaporated the thin film is formed. The second one consists of dropping a low volume of the solution containing both the polymer and the linker and, after the evaporation of the solvent, thin film is obtained. In this case, the process is far less violent in comparison to the previous one. In addition, less volatile solvents are conventionally more utilized than chloroform. In
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this methodology the formation of the polymer film allows the solvent to evaporate more slowly.121
After the deposition, in the case that order is not detected (something very common), it is possible to create (or improve) order through the annealing. Annealing of the sample is obtained through high temperature or by using vapors of another (or the same) solvent. In the first case, the temperature has a remarkable influence on the final arrangement of the block co-polymer. Indeed, as shown in figure 15 (in the diagram of the different phases) it was emphasized that every temperature has a different influence in the final disposition. This change in the order has a particular importance when you are closed to the glass transition temperature of the polymer. On the second hand, solvent annealing is used to improve the order in the films but, unfortunately, the thin film behavior becomes even more complicated in comparison to the temperature annealing. However, the main advantages of using solvent annealing are that there is no danger for polymer degradation and the time required to achieve the order is remarkably reduced.122 In some cases, the long range order can even be greatly improved.123 Unfortunately, there are many more important aspects to consider such as, for instance, the solvent evaporation rate, the selectivity of the solvent and the vapor pressure. In fact, depending on the speed at which the solvent evaporates, the final morphology can change dramatically. For example, by using a fast solvent evaporation, kinetically (non-equilibrium) non-ordered structures are favored while, with a slow solvent evaporation, the thermodynamic equilibrium is reached and the structures are more ordered.124 Therefore, the election of the solvent used highly contributes to the final result and, for example, it is possible to reach non-thermodynamic equilibrium structures but well ordered.
Since we are studying polymers formed by more than one component, it is obvious that solvents do not equally interact with both constituents. Therefore, by changing the composition of the solvents, the interaction with the block co-polymer will be more (or less) efficient and, consequently, it is possible, for instance, to change the system morphology from lamellar to cylindrical to spherical....125
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Certainly, the interaction between the surface where the sample is placed and the thickness of the sample is highly important in the final structuration of the polymer. Usually, these films are parallel alienated to the surface,126 but actually, looking from an applicative point of view (fabrication of nanomaterials), the most attractive orientation is the perpendicular one (Fig. 16). Indeed, many forms on how to redirect the preferred microdomain orientation have been reported,127 such as, for example, electric fields, solvent interactions and by using roughsubstrates.128
Figure 16. On the left perpendicular and on the left parallel orientation between cylindrical order
and the film.
The perpendicular orientation is the most attractive one since it is possible to pre-design a space made of cylinders, which can work as templates for the creation of films of ordered nanoparticles. Therefore, the controlled incorporation of nanoparticles into self-assembled block copolymers has attracted a great deal of interest in recent years.129-130 Additionally, this methodology is a well-known way to improve the properties of materials at the nanometer scale.
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For a better understanding of the process, in figure 17 the fabrication process is schematically shown. As widely highlighted in this discussion, the first important point is to obtain perpendicular cylinders. Thus, in the second step it is possible to deposit pre-synthesized inorganic nanoparticles selectivity into the P4VP domains. The red part in the representation of the polymer concerns the pyridine domains, which obviously have interaction with the deposited nanoparticles. When these inorganic materials have reached the organization, the last step of the process is to remove the polymer used as template. There are mainly three methodologies to remove the polymer. The first one is by heating the sample in air furnace at high temperature (pyrolysis)132 the second one is by oxygen plasma etching,133 and the last one is by washing the sample with an organic solvent. These methodologies leave behind arrays of metallic nanodots on the surface.
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CHAPTER 2
Neutral and Ionic Halogen-Bonded
Liquid Crystals
2.1. Objectives
Based on the works already mentioned in the introduction,37-72-75-76 we considered that the recently developed supramolecular Low-Molecular Weight Liquid Crystalline Actuators (LMWLCA) have not been investigated in depth yet. However, it has been demonstrated134 that the main option to assemble these LMWLCA consists of doping a fraction of azobenzene molecules in nematic liquid crystals as 4-pentyl-4’-cyanobiphenyl. Therefore, in order to destroy the liquid crystal alignment by inducing a transition liquid crystal-to-isotropic phase, the isomerization of small quantity of azobenzene molecules from the trans to cis is enough. This change on even small amount of it, permits the propagation of isotropic phase on the whole sample thought an efficient cooperative molecular motions (Fig. 18).
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Figure 18. Photoinduced phase transition of LC-azobenzene mixtures. Tcc refers to the LC-isotropic
phase transition temperature of the mixture with the cis form, while Tct refers to that of the
mixture with the trans form.134
Unfortunately, this approach mainly presents two problems. On the first hand, sometimes the correspondent cis-azobeneze studied presents problems in the solubility on the liquid crystal phase causing phase separation on the sample. To overcome this problem, the azobenzene quantity should be reduced.135 On the other hand, the nematic-to-isotropic phase transition cannot be induced at an arbitrary temperature in the whole nematic phase range of the mixture, as it is shown in figure 18. The phase transition can be induced only at temperatures higher than the phase transition temperature of the mixture with the azobenzene-derivative fully as cis-isomer.
It was considered that the versatility of the supramolecular approach to LMWLCAs should provide the possibility to overcome the problems described above. In order to assemble the molecules which form our tectons, halogen bonding was chosen because, as I anticipated many times in this Doctoral Thesis, it has been proven particularly reliable and robust in self-assembly.
The key of this work136 was to assemble dimeric liquid crystals where the photoactive alkoxyazobenzene molecule containing an iodoperfluoroarene ring acting as the halogen-bond donor moiety with the halogen bond acceptor is a 4,4’-alkoxystilbazole derivative. Therefore, three photoresponsive halogen bond137 donors,
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synthetized for the first time in this research project, and five promesogenic stilbazole molecules were used. Thus, in this work, fifteen new supramolecular liquid crystals are presented. Their general scheme is depicted in figure 19.
Figure 19.The complexes prepared in this study are assembled by halogen bonding between
promesogenic stilbazole molecules (left) and photoresponsive halogen-bond donors (right).
These compounds were labeled as STn and AZOm, where n and m are the numbers of carbon atoms in the alkyl chain. All the liquid crystals described in this work feature enantiotropic mesophases, except for the complex with the longest alkyl chain, which exhibited a monotropic LC phase.
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2.2. Materials
The starting materials were purchased from Sigma-Aldrich. Commercial HPLC-grade solvents were used without further purification, except for acetonitrile, used as solvent for the synthesis of azobenzenes, which was dried over CaH2 and distilled
prior use. 1H, 13C and 19F NMR spectra were recorded at room temperature on a Bruker AV 400 or AV500 spectrometer, using CDCl3 as solvent.
1
H NMR and 13C NMR spectroscopy chemical shifts were referenced to tetramethylsilane (TMS) using residual proton or carbon impurities of the deuterated solvents as standard reference, while 19F NMR spectroscopy chemical shifts were referenced to an internal CFCl3
standard.
The LC textures were studied with a Leica DM2700M polarized light optical microscope equipped with a Linkam Scientific LTS 350 heating stage and a Canon EOS 6D camera. The melting points were also determined on a Reichert instrument by observing the melting process through an optical microscope. The attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were obtained using a Nicolet Nexus FTIR spectrometer. The values, given in wave numbers, were rounded to 1 cm-1 using automatic peak assignment. Mass spectra were recorded on a Bruker Esquire 3000 PLUS. X-ray powder diffraction experiments were carried out on a Bruker D8 Advance diffractometer operating in reflection mode with Ge-monochromated Cu Kα radiation (λ=1.5406 Å) and a linear position-sensitive detector. Powder X-ray diffraction data were recorded at ambient temperature, with a 2θ range of 5−40°, a step size 0.016°, and exposure time of 1.5 s per step.
The photoresponsivity in the liquid crystal phase was studied in planar liquid crystal cells with a gap of 2 μm. The temperature control was done with Linkam Scientific LTS 350 heating stage, and the spectra were measured with USB 2000+ spectrometer (Ocean Optics, Inc.) with a deuterium-halogen light source. The birefringence data was measured using 820 nm wavelength laser and crossed
49
polarizers. The sample was placed in between the polarizers at a 45° angle, and the laser intensity was measured with a photodiode. The illumination of the samples was done with 365 nm and 395 nm high power LEDs (Thorlabs), equipped with 10 nm, OD 4.0, band pass filters (Edmund Optics), or then with a 457 nm laser.
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2.3. Results and discussion
2.3.1. Structural analysis
In order to demonstrate the halogen bonding interaction in our complexes, single crystals of some combinations were grown. In addition, the single crystals of the three new trans-azobenzenes (trans-AZO8, trans-AZO10 and trans-AZO12) and the isomer cis-AZO12 were also obtained (Fig. 20). For this reason, I will take the opportunity to describe all these single crystals structures in detail in order to determinate, besides the halogen bonding, which interactions are involved in the crystal packing.
It is remarkably interesting to compare both isomers cis- and trans- of the
AZO12. Therefore, studying the crystal packing of cis-AZO12, it is possible to
conclude that the nitrogen lone pairs plays a crucial role in the packing, since it makes more accessible to interact with Lewis acids, transforming the azo unit in a good halogen-bond acceptor, as shown in figure 20. The distance between the iodine and the nitrogen of the neighbor molecule is 2.995 Å and C-I···N angle 170.2°. The overall crystal packing nicely illustrates strong segregation between aromatic and aliphatic parts of the molecule. The aromatic moieties of cis-AZO12 interact to form a columnar arrangement, running through the crystallographic c-axis, thanks to the tilted-off set stacking interactions occurring between the fluorinated surfaces. On the other hand, the crystal structure of trans-AZO12 allows to determinate that halogen bonding is not detected and, in addition, there is not a very clear segregation between the aliphatic and aromatic parts. However, a weak interaction between perfluoroarene-perfluoroarene systems is shown, as you can also see in figure 20 (distance between centroids 4.693 Å). Additionally, weak F···H contacts occur between neighboring molecules.
