Magnetic Properties of Metallomesogens. Part II. Phase Behaviours and Luminescent Properties

Ministry of Science and Higher Education of the Russian Federation «Kazan National Research Technological Univesity»



Y. Galyametdinov, N. Selivanova, A. Knyazev




                MAGNETIC PROPERTIES OF METALLOMESOGENS

                Part II

                PHASE BEHAVIOURS
                AND LUMINESCENT PROPERTIES






Kazan
KNRTU Press
2020

        UDC 546.443.03
        LBC G 122-26


It is published by decision of the Editorial and Publishing Council of Kazan National Research Technological University

Reviewers:
PhD in Chemical Sciences, Professor, I. S. Antipin
PhD in Chemical Sciences, Professor, L. Y. Zakharova



         Galyametdinov Y.

         Magnetic Properties of Metallomesogens : monograph : in 2 p. Part II. Phase Behaviours and Luminescent Properties / Y. Gal-yametdinov, N. Selivanova, A. Knyazev; Ministry of Education and Science of Russia, Kazan National Research Technological University. - Kazan : KNRTU Press, 2020. - 292 p.

         ISBN 978-5-7882-2796-2
         ISBN 978-5-7882-2798-6 (ч. II)

     The monograph presents the published works results on the creation and study of metal-containing liquid crystals - metallomesogens. The liquid crystal complexes of transition metals with giant magnetic anisotropy, spin-crossover properties at room temperature and the magnetic behavior of mesogenic metal-polymers are considered. Especial attention is devoted to the luminescence properties of lanthanide derivatives.
     It is intended for Bachelors in the field of study 18.03.01 «Chemical Technol-ogy» with the subjects «Physical Chemistry» and «Additional chapters of physical chemistry», for Masters in the field of study 18.04.01 «Chemical Technology» with the subjects «Theoretical and experimental research methods in chemistry». The monograph can be used by PhD students studying in the field of «Chemical Sciences» with the postgraduate program «Physical Chemistry».
    The monograph is prepared at the Department of Physical and Colloidal Chemistry.

        UDC 546.443.03
        LBC G122-26



ISBN 978-5-7882-2798-6 (ч. II)
ISBN 978-5-7882-2796-2

   © Galyametdinov Y., Selivanova N., Knyazev A., 2020
   © Kazan National Research Technological University, 2020

        CONTENTS


INTRODUCTION......................................................5
1. SYNTHESIS AND MESOPHASE IDENTIFICATION OF D- AND F-ELEMENTS CONTAINING METALLOMESOGENS...............6
    1.1. Paramagnetic liquid crystalline nickel(II) compounds.....6
    1.2. Spectroscopic and thermodynamic properties of Er³⁺, Nd³⁺
and Tb³⁺ containing liquid crystals..............................10
    1.3.    Arrangement of trace metal contaminations in thin films of liquid crystals studied by X-ray standing wave technique...14
    1.4. Tris(e-diketonates) lanthanum nematic adducts...........22
2. FERROCENE DERIVATIVES LIQUID CRYSTALS.........................31
    2.1.    Synthesis, computational modelling and liquid crystalline properties of some [3]ferrocenophane-containing Schiff’s bases and в—aminovinylketone: Molecular geometry phase behaviour relationship.....................................................31
    2.2.    A Novel Series of Heteropolynuclear Metallomesogens: Organopalladium Complexes with Ferrocenophane-Containing Ligands  43
3. LYOTROPIC METALLOMESOGENS.....................................51
    3.1.    Mesogenic and Luminescent Properties of Lyotropic Liquid Crystals Containing Eu(III) and Tb(III) ions.51
    3.2.    Hybrid silica luminescent materials based on lanthanide-containing lyotropic liquid crystal with polarized emission...64
    3.3.    Lyotropic La-containing lamellar liquid crystals: phase behaviour, thermal and structural properties...............73
    3.4.    Modification of nonionic vesicles by adding decanol and functional lanthanide ions...................................84
    3.5.   Evaluation of Interactions between Liquid Crystal Films
and Silane Monolayers by Atomic Force Microscopy.................99
    3.6.    Phase behaviour, structural properties and intermolecular interactions of systems based on substituted thiacalix [4] arene and nonionic surfactants....................................106
    3.7.    Lyotropic mesomorphism of rare-earth trisalkylsulphates in the water-ethylene glycol system.........................115

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4. LUMINESCENT PROPERTIES OF MESOGENES.............................119
    4.1.    Polarized Luminescence from Aligned Samples of Nematogenic Lanthanide Complexes................................119
    4.2.    Mesomorphic behaviour and luminescent properties of mesogenic в-diketonate lanthanide adducts with 5,5‘-di(heptadecyl)- 2,2‘-bipyridine..........................127
    4.3.    Ab Initio Study of Energy Transfer Pathways in Dinuclear Lanthanide Complex of Europium(III) And Terbium(III) Ions..........134
    4.4.    Influence of Structural Anisotropy on Mesogenity of Eu(III) Adducts and Optical Properties of Vitrified Films Formed on their Base................................................148
    4.5.    Changes in luminescent properties of vitrified films
of terbium(III) в—diketonate complex upon UV laser irradiation.....158
    4.6.    Influence of Lewis bases on mesogenic and luminescent properties of Eu(III) tris(e-diketonates) adducts homogenous films.169
    4.7.    Influence of Eu(III) Complexes Structural Anisotropy on Luminescence of Doped Conjugated Polymer Blends.................178
    4.8.    Controlled polarized luminescence of smectic lanthanide complexes...............................................191
    4.9.    A photostable vitrified film based on a terbium(III)-diketonate complex as a sensing element for reusable luminescent thermometers.................................................204
    4.10.    Optical and structural characteristics of PMMA films doped with a new anisometric Eu(III) complex.................215
    4.11.    Photostable Anisometric Lanthanide Complexes as Promising Materials for Optical Applications..............226
CONCLUSION...................................................237
REFERENCES...................................................238
ABOUT AUTHORS................................................290

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        INTRODUCTION


      Metallomesogens or metal-containing liquid crystals - a class of compounds combining the physical characteristics exhibited by metal coordination complexes with those of organic molecules which give liquid crystals. Such compounds were first synthesized over 110 years ago but the topic has achieved a distinctive character only thirty years ago. Today, the development of nanotechnology leads to new areas of metallomesogens application in biomedicine and diagnostics, optical material designs for electronic and optoelectronic.
      The scope for metallomesogens is great, as there are some 60 metals which, in principle, can be coordinated to organic ligands. Metals also show a remarkable variety of geometries and the incorporation of a metal immediately opens up a wide choice of new geometric shapes. Further important effects arise from the large and polarizable electron density which is a feature of every metal atom. Many metal ions of the d- and f-block elements have unpaired electrons and are colored, their inclusion opens up the capabilities of new physical properties in liquid crystals. Because mesophase formation depends on intermolecular forces and the space around the metal is occupied by the ligand, the liquid crystal properties of metallomesogens are dominated by the ligands and their arrangement, in other words by the overall shape of the molecule. Thus, for example, long monodentate ligands (or bidentate ones with small bite angles) will tend to give rodlike nematics and smectics. The flat, disklike polydentate ligands (like macrocycles) will give discotics. Specific physicochemical properties will be determined by a metal ion.
      This monograph is devoted to the study of metallomesogens by Russian group of scientists led by Galyametdinov Y. G. In the first part, covering the English versions of publications, the results of the synthesis and experimental studies of some theoretical works in the field of magnetism and optics of metallomesogens were considered. Some results of the practical application of metal-containing liquid crystals in promising fields of science and technology are considered.
      Present book presents synthesis and mesophase identification of d- and f-elements containing metallomesogens. Synthesis, computational modelling and liquid crystalline properties of some [3]ferrocenophane-containing Schiff’s bases and в—aminovinylketone are described. Mesogenic and luminescent properties of lyotropic rare-earth containing system are devoted. These systems were the base for creation of hybrid silica luminescent material with polarized emission. Luminescent properties of thermotropic lantha-nidomesogens and their complexes in polymer matrices are considered.

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1.      SYNTHESIS AND MESOPHASE IDENTIFICATION OF D- AND F-ELEMENTS CONTAINING METALLOMESOGENS

        1.1. Paramagnetic liquid crystalline nickel(II) compounds

      Copper(II), nickel(II) and oxovanadium(IV) complexes of 4-(4-hep-tyloxybenzoyloxy)-N-(S)-2-methylbutylsalicylaldimine, as well as the parent ligand, were studied by optical and DSC methods. For the first two complexes, there exist between a tightly twisted chiral nematic phase and the isotropic liquid either some blue phases or novel-type amorphous phases, with their temperature ranges depending significantly on the metal centre. For the third complex, direct isotropization takes place.
      It is a well known property of liquid crystals that strong disymmetry of molecules, resulting in their high twisting ability, can generate novel chiral mesophases [1-2]. Among them there are blue phases, existing between the chiral nematic (N*) phase and the isotropic liquid [2]. Most probably, a similar situation arises also in systems having a twist-grain-boundary smectic A* phase [3]. Recently, an intermediate phase (IP) formed by paramagnetic copper metallomesogens was reported and tentatively identified as a blue phase [4]. In this communication we confirm the existence of such paramagnetic IPs and compare their properties for different chelated metal ions.
      Since complexation of a divalent transition metal ion by bidentate chiral ligands doubles the number of chiral centres within the resulting complex molecules, the twisting power of such complexes should become much higher than that of the parent ligand, and the helical pitch much tighter. The helical pitch is a major factor which controls the presence and temperature range of particular blue phases [5, 6]. For complexes, therefore, we expect an enhanced stability of these phases.
      Complexes 7ML2 (fig. 1.1), as well as their parent ligand, 7HL = 4-(4-heptyloxybenzoyloxy)-N-(S)-2 methylbutylsalicylaldimine, were chosen for our studies. Their syntheses were performed similarly to those reported in [7]. As a chiral substrate we used S(-)-2-methylbutylamine, [a]D²⁰-5,9°, obtained from Aldrich. Synthetic details, analytical data and magnetic properties will be described elsewhere. Besides an obvious paramagnetism for the copper and oxovanadium complexes, a weak paramagnetism is observed for the nickel analogue (cf. [7] for properties of some related compounds).


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Fig. 1.1. Chemical structure of investigated compounds nML2, where M = Cu, Ni, Vo, Pd

       All compounds, when studied microscopically, reveal the presence of a chiral nematic phase; no other mesophases were detected. For the ligand, a selective light reflection was observed, and so a helical pitch of 0,30,4 pm, in the temperature range 50-40 °C, was estimated. For the complexes, selective light reflection was observed only for samples diluted by achiral metallo-mesogens or conventional nematics, which proves that the pure chiral complexes are rather tightly twisted. Their helix was found to be right-handed by applying the contact method, using cholesteryl chloride (RH) and propionate (LH) as standards. Such handedness is in agreement with the empirical SED rule [8].
       Since a high twist can generate blue phases with a short lattice constant, the question arises as to whether such phases are present in the complexes under study. Our approaches to detecting them by means of observation of selective light reflection or a transmission dip in the UV spectra have failed. On the other hand, the strong optical absorption of the samples, and not the absence of these phases, might be the decisive factor here. To elucidate the situation, we used differential scanning calorimetry (DSC). This method, if carefully applied, sometimes detects blue phases unobservable in the visible range [9].
       Using a Perkin-Elmer DSC-7 and optimizing all the experimental parameters involved, we obtained good quality thermograms for all the compounds. For the ligand, above its melting temperature, a typical A-shaped isotropization peak is observed, confirming the lack of blue phases. The thermograms of the complexes are distinctly different (fig. 1.2).

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Fig. 1.2. DSC curves for 4-(4-heptyloxybenzoyloxy)-N-(S)--2-methylbutylsalicylaldimine complexes: а — 7CuL2 (sample 6,1 mg, scan 1 °C min⁻¹); b — 7NiL2 (6,1 mg, 1 °C min⁻¹); c — 7VOL2 (4,9 mg, 2 °C min⁻¹)

       The most simple occurs in the case of 7VOL2, for which only a signal for a phase transition to an isotropic liquid is present. However, the absence of pre-transitional anomalies in the specific heat, as well as an extremely low enthalpy change for the phase transition, should be noticed. The corresponding entropy change is only 0.095 R. Such behaviour would correspond, in terms of the Landau-de Gennes model, to a small value of the pre-transitional coefficient, a, in the free energy expansion for the system. There is also a simple interpretation of the DSC data for 7NiL2, for which the thermogram corresponds fully to the response of a system having a chiral nematic phase and blue phases. Two small peaks corresponditig to transitions relating to intermediate phases are overlapped on one side of the isotropization peak. Furthermore, typical for blue phases are the temperature ranges of the phases (0-28 °C and 0-81 °C) and the transition enthalpies (table 1.1).

Table 1.1

Phase transition temperatures (°C) and, inparentheses, enthalpies (J^g⁻¹) for the ligand, 7HL = 4-(4-heptgloxybenzoyloxy)-N-(S)-2-methylbutylsalicylaldimine, and its Cu(II), Ni(II) and VO(IV) complexes

Compound c                 N* IP,               IP,                              I
7HL      •      57-4(64-7) •                --- ---    510(1-1)                  •
7CuL2    •     156 0(59-6) •  154-2(0-27)     • ---    156-9(1-3)                •
7NiL2    •     154-7(52-8) •  187-6(0-20)     • 187-9(0-03)     •     188-7(2-5) •
7VOL2    •    131-6(441)   •                --- ---    142-0(0-36)               •

Phases: C, crystal; N, nematic; I, isotropic liquid; IP, intermediate phase; •, the phase exists; -, the phase does not exist.

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       In contrast to these complexes, for the copper anaiogue, some nontypical, but nevertheless reproducible, thermograms were recorded. The iso-tropization peak is diffuse over a broad temperature range. This broadening is more likely to result from specific heat pre-transitional anomalies and is certainly not connected with contamination of the samples. The thermograms were confirmed for samples synthesized independently in two laboratories and they do not change after subsequent recrystallizations. Moreover, the low temperature peak, corresponding to the N*-IP transition, is very sharp. This phase transition is obviously first order (a small hysteresis in the phase transition temperature is also observed) with an unexpectedly high transitional enthalpy and no pre-transitional effects. In general, the thermogram is, toutes proportions gardtes, similar to those observed for some smectic A* systems with an amorphous phase of unknown structure [3].
       Although the presence of the BP(III) alone is expected for short pitched chiral nematogens [10], the existing experimental data do not suffice to prove that the intermediate, 2,7 °C wide, mesophase of 7CuL2 is this blue phase. Besides optical studies (difficult to perform with an inherent light absorption), phase diagram studies also appeared unrealistic. Construction of detailed binary phase diagrams, with reference substances, was too subtle a task, bearing in mind both the narrow temperature range involved and the low enthalpy of the N*-BP(III) transition for the standards. The intermediate phase, because of its unusual thermal properties, might even be an entirely novel phase which exhibits a near-continuous phase transition to an isotropic liquid. To study the molecular organization of the phase, NMR analysis is inappropriate because of its paramagnetic properties. EPR studies could be more promising; this method has proved to be applicable to some chiral systems.
       A tentative DSC examination of the diamagnetic analogues, 12NiL2 and 12PdL2 also reveals the presence of intermediate phases. In addition, the phase for the Palladium(II)-complex is detectable optically; it appears as a fog-like texture.
       In comparing the thermal stabilities of the complexes, we find that both the isotropization temperatures and the enthalpies are ranged in the sequence NIL2 > CuL2 > VOL2. We checked that this sequence holds for Shiff’s base complexes having normal as well as a- and в-branched N-alkyl substituents. This fact points to the role of the metal centre as a factor determining molecular shape (through the coordination geometry) and molecular interactions. In contrast, it is difficult to explain why the temperature range of the intermediate phases is wider for CuL2 than for NIL2. Phase transitions in chiral

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systems are determined by the chirality, k = qo(Kia/b²)1/² (2a)], rather than simpiy by the inverse helical pitch, p⁻¹o = qo/2n. As yet, not much is known about the additional elastic (K) and free energy (a, b) constants for complexes. However, in view of our DSC results, a relatively low chirality, resulting from a small value of the a factor, might be responsible for the absence of any intermediate phases for the 7VOL2 compound. This effect might be connected with the marked non-planarity of oxovanadium complexes.
      To summarize, for these metal complexes specified and having two chiral centres, mesophases intermediate between the nematic and isotropic liquids exist, in contrast to the parent, chiral ligand. The role of the metal centre, which influences the occurrence, temperature range and type of these phases, is not clear at present.

        1.2. Spectroscopic and thermodynamic properties of Er³⁺, Nd³⁺ and Tb³⁺ containing liquid crystals

      A ligand and three metallo-organic complexes containing Nd³⁺, Th³⁺ and Er³⁺ ions were synthesized. Absorption, linear dichroism spectroscopy and domain structures investigations plus X-ray difractometry measurements at heating-cooling cycles were performed. An influence of a rare earth metals on LC thermodynamic properties have been discussed.
      Metal-containing liquid ciystals have been known for long time since Vorlander in 1910 discovered thermotropic phase in alkali metal carboxylates. Nevertheless, only after an publication of an pioneer work on mesogemc nickel and platinum complexes by Giroud-Godquin and Muller-Westenhoff in l977 an interest to such materials grew up. These new liquid crystal materials were intensively investigated during last decade. Mostly they are aromatic compounds containing heteroatoms, which are able to bind metals and create stable metal complexes as liquid ciystals. In broad terms this means that the dipole-dipole and dispersion forces which hold LC in anisotropic supramolecular arrays, are not destroyed by an introduction of the metal ions. Indeed, in some cases the mesophases found are identical to those exhibited by the organic ligands themselves [11, 12]. The aim of this work is to establish a contribution of rare-earth ions to macroscopic properties of LC.
      A ligand and three metallo-organic complexes containing Nd³⁺, Tb³⁺ and Er³⁺ ions were synthesized.
      Absorption, linear dichroism spectroscopy in visible region and X-ray diffractometry measurements were performed plus microscopic investigation of the domain structures in heating-cooling cycles. Linear dichroism


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