Liquid Crystalline Materials Based on Copper(I) Complexes: Comparison
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ThLiquis paper provides insight into the various studid crystals are generally referred to as substances that have already been carried out onblend the structure and properties of solid and liquid crystalline materials based on copper(I) complexes. Even though the study of copper(I) complexstates; they share with liquids the ability to flow but also exhibit some structural arrangement similarities with respect to theirsolids. With so many compounds synthesized, liquid crystalline property is quite few,s containing metals, also known as metallomesogens prepared with different structural components and ligands from groups such as aza macrocycles, alkyl thiolates, ethers, isocyanides, phenanthroline, Schiff bases, pyrazoles, phosphine, biquinoline, and benzoyl thiourea have, have become a major subject of study. The incorporation of metal into organic matrices enhances and induces unique magnetic, spectroscopic, and redox properties of the resulting materials. At least one liquid crystalline complex has been reported. A special section is dedicated to the discussion of the emission properties of copper(I) metallomesogen in the literature for most metals.

  • copper(I)
  • metallomesogens
  • luminescence
  • complex
  • ligand
  1. Introduction

1. Introduction

Liquid crystals are generally referred tCo as substances that blend the structure and properties of solid and liquid states, they share with liquids, the ability to flow but also exhibit some structural arrangement similarities with the solids. This intriguing combination of both the properties of the liquid and solid states gives liquid crystals the ability to induce certain properties that have gained useful applications in the displays of devices such as wristwatches, calculators, portable computers, and flat-screen televisions as well as in sensors, smart windows, optical switches, etc.[1].

With so many compounds synthpper(I) complexes have not been studied to a great extent as luminescesized, liquid crystals containing metals, also known as metallomesogens, have become a major subject of study [2]. Incorporation of the metal into the organic matrix enhances and induces unique magnetic, spectroscopic, and redox properties of the resulting materials [3–9]. At least one liquid- materials in the liquid crystalline complex has been reported in the literature for most of the metals. However, issues often encountered with these complexes are those relating to the high transition temperatures usually >100 C,state, but they and the low thermal stability associated with the metallomesogens at elevated temperatures, which are major drawbacks that hinder the study of the physical properties of these materials. Consequently, the preceding challenges make it puzzling to observe light emission (luminescence) at elevated temperatures due to the strong tendencies of the excited states to undergo de-activation through nonradiative transitions. Therefore, luminescence studies on metallomesogens in many cases were performed on samples in the solid state or those dissolved in organic solvents [10].

  1. Copper(I) metallomesogens with sulfur-containing ligands

A sero possess luminescence properties with great potenties of cationic macrocyclic copper(I) complexes based on non-mesogenic bisls [4-(n-alkyloxy) benzamide derivatives of 113,14,15,16,17,18,10-diaza-419,720,1321,16-tetrathiacyclooctadecane were reported in 1994 by Ghedini et al. [1122]. The cComplexes studied had transition temperatures from the solid to liquid crystalline states ranging from 93-123oC. XRD mpper, which is somewhasurements for Complex 1 (Figt abure 1) in the series and the study revealed an X-ray diffraction pattern consistent with a disordered layered structure associated with a smectic phase of A or C type.

Figure 1: Copper(I) compdant and affordable, is a suitablexes from azamacrocycle deriv alternative ligand. Transition temps. are in (OC) [11]

Anto nother class in the Sulphur containing ligand group is the benzoyl thiourea. An important class of copper(I) metallomesogens based on copper(I) halide cole metal complexes with thiourea-based ligands having long chain alkoxy groups and a perfluorooctyl group was reported in 2018 by Ilis and Circu [1223,24] (Figure 2). They found no liquid crystalline property for the ligand but ratiobserved a hexagonal columnar phase for both complexes 2a and 2b oveof tr a high temperature above 100 C via a combination study of POM, DSC, and XRD, whiple the thermal stability studied by TGA indicated a higher stability (160oC) f tor the corresponding copper(I) complexes compared to that of the BTU (180oC) ligand.

Figure 2: Copper(I) singlet excomplexes with benzoyl thiourea ligand. Transition temperatures are in (OC) [ons is 3:12].

  1. Copper(I) metallomesogens with N-donor ligands

Copper(I) metallomesogens with; three-coordinate geometry were first reported by Lin et al. in 2001 [13]. The complexes were derived from bis{2-[3’-(3’’,4’’-dialkoxyphenyl)-5’-meonsequenthyl-1’-pyrazolyl]ethyl} ethers and from bis{2-[3’-(3’’,4’,5’-trialkoxyphenyl)-5’-methyl-1’-pyrazolyl]ethyl} ethers. These novel complexes were obtained by complexing the ethers with [Cu(MeCN)4]BF4. An example (y, for luminescomplex 3) is given in Figure 3.

Figure 3. Copper(I) complex with ether-type ligand. All temperatures are in (oC) [13]

Mmateriany copper(I) complexes with N-donor ligands in the form of Schiff bases are known. These compounds show varying liquid crystalline properties with modification of their structural units. Although non mesogenic, the free ligands L4, upon compls to be used in OLEDs, they should exation with copper(I) showed mesogenic character as described by the DSC and POM experiments. The results indicated that complex 4 showed high stabilitsentially after several heating cycles as against those for some of the other Schiff base complexes, this attribute is explained to be due to the lack of substituents at position 6 (as seen in figure 3). In addition, the optical textures observed for complex 4 during be able to harveslow cooling from the isotropic melt are typical of a columnar phase (with pseudo-focal-conic textures [14].

Figure 4: Copper(I) metallomesogens with S all the exchiff base. All transition temperatures are in (oC) [14]tons.

A study by Ziessel et al. (2004) [15] presBented mesomorphic materials based on ause copper(I) complexes with phenanthroline-based ligands. The study aimed to engineer a structural framework with additional supramolecular binding factors (hydrogen bonding) so as to stabilize the mesophase both as a free ligand and within the complex. The thermotropic properties of the ligands and complexes of the phenanthroline derivatives were investigated via a combination of POM, DSC, and XRD. The ligand used for the complexes showed distinct cubic and disordered lamellar mesophases at the related copper(I) complexes with longer chains showing mesomorphic behaviors displaying mesophases characteristic of the oblique columnar phase. An example of such compounds is described in Figure 5.

Figure 5: Copperexhibit various metal-to-ligand charge-transfer (I) complex with phenanthroline-based ligand. Temperatures are in (OMLC) [15]

  1. Copper(I) metallomesogens with isocyanide ligands

The reaction of isocyanides with CuX (X is halide) gives a mononuclear complex but, on the contrary, the reaction of two equivalents of the isocyanide derivatives with CuX yields binuclear copper(I) complexes, an example of metallomesogens from the aforementioned ligand is shown in Figures 6.

Figure 6: Mononu behaviors, they clear and dinuclear copper(I) Isocyanide complexes. Transition temperatures are in bracket (OC) [16] [17]

The fir induce st set of liquid crystals based on copper(I) isocyanide complexes was reported in 2001 by Benouazzane et al [16]. Some of the in orbisocyanide ligands were reported to display nematic and/or smectic A phases. The mononuclear copper complex 6 (figal coure 6) shows SmA and SmC mesophases.

Chico et al. [17] reported two isocyanate-triphenylene copper(I) complexes both of which display good thermal stability in the range of study. The free isocyanide ligand appeared not to be mesomorphic as observed by POM. The identification of the columnar mesophase for the dinuclear complex 7 (figure 6) wag of the triplet and s achieved by small-angle X-ray scattering on powder samples which was measured as a function of temperature, consistent with the DSC and POM experiments. Furthermore, the columnar mesophase was stable in the temperature range of 46 to 79OC.

  1. Luminescent metallomesogens based on copper(I) complexes

A major drawback in the nglet states, leading to study of the physical properties of metallomesogens is mostly due to issues relating to their high transition temperature and stability at those elevated temperatures. It becomes increasingly difficult to study the emissions at those temperatures due to the strong tendencies of the excited electrons to undergo deactivation via nonradiative transitions [10]. Few attempts have been made in this regard and interesting findings have been made.

Kishimura et al. were the first to describe the emission properties of copper(I) compall energy separations between the energy levelexes in their liquid crystalline phase [1825] in 2005. They reported a number of dendritic Cu(I) pyrazolate complexes which were used to produce some thermally rewritable phosphorescent papers useful for security purposes.

Figure 7: Ligands and ris allows for revepresentative structure of the copper pyrazole trinuclear complex 7 & 8 se intereported in [18] [19]

The luminescence inveystigation of complexes from the pyrazole ligand [18] revealed dichroism at room temperature for the solid of complex 7. Cooliem crossing of the hot melt (which emitted a red luminescence at λmax650 nm) naturally or by slow coolRISC), i.e., sing leads to blue shift emission at 640 or 610 nm respectively. In essence, the red and yellow luminescence observed for 7 cot harvesting, resuld be thermally changed from one form to the other, depending on the manner and rate of cooling and this was also found to be the case for its liquid crystalline properties. Both the aged and non-aged complex 7 thing in thermally activated delat was analyzed appeared to be phosphoed fluorescent which is thought to be a result of Cu(I) to Cu(I) interactions ce (TADF) [1826].

Furt Thermore, XRD analysis of the aged sample of complex 7 showed diffforaction patterns synonymous with a one-dimensional columnar phase and the same, 7 viewed in a, polarized optical microscope showed a fan-shaped texture that is characteristic of Discotic liiquid crystals and on the basis of this, was concluded that aged complex 7 is composed of lobased ong-range discotic columnar assembly. XRD pattern of the non-aged complex after natural cooling also indicated the presence of a columnar structure. DSC measurements carried out on the aged and non-aged complex 7 rcopper(I) complexevealed patterns, upon which it was concluded that the discotic columnar assembly which is believed to involve metallophilic interactions of the Cu(I) to Cu(I) units having long alkyl chains, is formed in the aging process at about 40–50 °C. The stability of the dichroic luminescence was therefore found to be dependent on the pyrazole ligand structure.

Build have considerable promise for producing upon the aforementioned work by Kishimura and co-workers, Gimenez et al. (2020) [19] recently reported a series off liquid crystals achieved with cyclic trinuclear copper(I) complexes (the most important in the series is given in Figure 7 as complex 8), prepared ctive lusing 3,5-dimethyl-4-(trialkoxyphenyl)pyrazolate ligands.

Complex 8 was reported to have orange-red inescolored emissions at room temperature and the photoluminescence exhibited a broad band centered at around 661-664 nm. A landmark reported in this study [19] is the high Quantum yield (QY) value of 42%, measured in the liquid crystalline state which is the highest recorded so far for any copper(I) metallomesogens. The QY obtained for complex 7 in its liquid crystalline state nt materials for a was higher than those obtained in the crystalline state of complexes for the rest of the series. Thus, the work carried out by Giminez et al.[19] have shown that de range of opticalow-temperature phosphorescent metallomesogens can be obtained from the more affordable and abundant copper metal.

Camerel et al. (2016) [20] reporor electro-opted a new class of copper(I) liquid crystals with a cubane core and based on phosphine ligands functionalized with pro mesogenic gallate-based moieties bearing either long alkyl chains of C8, C12, and C16 or cyanobiphenyl (CBP) fragments. The copper(I) cubanes have already been known for being able to display both luminescence mechanochromism and thermochromism behavior [21–25]. This study is an inventive example of integrating luminescence characteristics of copper iodide clusters [Cu4I4(L24)4] wcal applications due to the large diversity of possith the flexible self-assembly of liquid crystals. Only the compound functionalized with cyano biphenyl group (CBP) i.e., Complex 8 (figutructure 8) showed liquid crystalline behaviour displaying a Sm A from room temperature to about 1000C. 

Figure 8. Ge, including moneral structure of the functionalized [Cu4I4(L24)4] copper iodide clusters with phosphine ligands [20].

All the complexes fearom the work by Camerel et al. [20] revealed luminescence thermochromism for all the studied comor pounds however, complex 8 displayed an unclassical behavior which was attributed to the intrinsic luminescence properties of the cyano biphenyl moiety itself, complex 8 uclear complexes, andisplayed a dual-emissive system which presented interesting emission properties. And also, in addition to its liquid crystalline properties, the compound 8 dthe great potentisplayed luminescence mechanochromism with a modification of the el of emission wavelength in response to grindingbehavior.

Cretu et al. (2018) [26] reported a Inew class of Cu(I) coordination complexes having 4,4’-bisubstituted-2,2’-biquinolines which shows low-temperature lamella-columnar and columnar hexagonal thermotropic liquid crystalline phases. The highest deduction from the luminescence study is the presence, at 578 and 560 nm of a medium-low intensity band, with a series of shoulders, believed to be due to metal-to-ligand charge-transfer (MLCT) electronic transitions. By heating the solid samples, they move towards the liquid-crystalline phases and still retain the luminescence displayed in the solid state but by increasing the temperature, the intensity of the luminescence band decreases, and when over 120°C, luminescence is completely quenched; but returns with subsequent cooling of the samples. This behavior is attributed to the gain of the non-radiative kinetic constants when vibrational modes are enhanced by heating the samples [26]. An example of this class’s structure is given in Figure 9.

Figure 9. Copper(Iaddition, the range of coordination geometries (such as linear, plan-trigonal, or tetrahedral) complex with bisubstituted biquinoline ligand [26]

Conclusion

The aned wim of this article was to make an overview of the various liquid crystalline materials based on copper(I) complexes with an emphasis on their luminescent properties. Examples of copper(I) metallomesogens based on isocyanide ligands are by far the most prevalent. By utilizing the suitable mesogenic groups, their LC characteristics may be simply modified. While a few other isocyanides produced hexagonal and rectangular columnar phases as well as a cubic phase for the dendritic isocyanide supermolecules, the bulk of these complexes exhibit calamitic behavior with either SmA or SmC phases. The reported copper(I) complexes of Schiff base type ligands of bipyridine imine, 2-aminopyridine, and picoline substituted imino ligand and those of alkyl thiolates all displayed varying columnar mesophases with different transition temperature ranges. Both the ether type and benzoyl thiourea ligands displayed characteristics of a hexagonal columnar mesophase for the corresponding metallomesogens, while the copper(I) complexes from phenanthroline ligands had an oblique columnar m the ligands’ structural design provide a significant benefit for controlling the LC properties, including their enhanced thermal stability and mesophase. Furthermore, interesting luminescent properties in liquid crystalline states were observed for copper(I) complexes from ligands with pyrazole derivatives, phosphine ligands functionalized with copper iodide and substituted biquinoline ligands. The complex from the functionalized phosphine ligand displayed interesting mechanochromic luminescence properties. A high quantum yield of 42% in the liquid crystalline phase of the copper(I) metallomesogens from a pyrazole ligand was reported. This work provides a pillar to build on for the preparation of many more copper(I) complexes having liquid crystalline behavior with improved properties type related to both calamitic and discotic materials.

R

2. Copper(I) Metallomesogens with Sulfur-Containing Ligands

A series of cationic macrocyclic copper(I) complexes based on non-mesogenic bis[4-(n-alkyloxy) benzamide derivatives of 1,10-diaza-4,7,13,16-tetrathiacyclooctadecane was reported in 1994 by Ghedini et al. [32]. The studied complexes had transition temperatures from the solid to liquid crystalline states ranging from 93 to 123 °C. Complex 1e (Figure 1) demonstrated the clearest mesomorphism among the series, with sufficient thermal stability, so the research group focused their X-ray analysis on it, and the study revealed an X-ray diffraction pattern consistent with a disordered layered structure associated with a smectic phase of A or C type.
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Figure 1. Copper(I) complexes from azamacrocycle derivatives. Transition temperatures are in °C [32].
Alkyl thiolates (R-SH) are strong ligands that can bind to metals via the donor S atom to obtain metal thiolates. In 1999, Espinet et al. [33] reported the preparation of a series of copper(I) thiolates, [CuSCnH2n+1], where n = 4, 6, 8, 10, 12, 14, 16, and 18. X-ray diffraction analysis of the polycrystalline sample revealed a layered structure in the solid state, contrary to previously reported work by Dance et al. (1991) [34]. However, the authors suggested that the discrepancies might have been due to the methodology used, indicating that the preparative method of the complex influences its stability towards oxidation. The textures of the complexes show a columnar mesophase based on stacking of cyclic [Cu4(µ2-SCnH2n+1)4] aggregates, with transition temperatures ranging from 56 to 210 °C.
A new class of copper(I) metallomesogens based on copper(I) halide complexes with thiourea-based ligands with long-chain alkoxy groups and a perfluorooctyl group were reported in 2018 by Ilis and Cîrcu [35] (Figure 2). They found no liquid crystalline behavior for the ligand but observed a hexagonal columnar phase for both complexes 2a and 2b at high temperatures above 100 °C via a combined study of POM, DSC, and XRD, while the thermal stability studied by TGA indicated a higher stability (180 °C) for the corresponding copper(I) complexes compared to that of the BTU (160 °C) ligand.
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Figuref 2. Copper(I) complexes with benzoylthiourea ligands. Transition temperatures are in °C. Inset: POM image of the Colh phase of 2a at 85 °C [35].

3. Copper(I) Metallomesogens with N-Donor Ligands

Many copper(I) complexes with N-donor ligands integrating pyrazole, 2,2’-bypiridine, 1,10-phenantroline moieties, or Schiff bases have been described to date. Copper(I) metallomesogens with three-coordinate geometry were first reported by Lin et al. in 2001 [36]. The complexes were derived from bis{2-[3’-(3’’,4’’-dialkoxyphenyl)-5’-methyl-1’-pyrazolyl]ethyl} ethers and from bis{2-[3’-(3’’,4’,5’-trialkoxyphenyl)-5’-methyl-1’-pyrazolyl]ethyl} ethers. These novel complexes were obtained by complexing the ethers with [Cu(MeCN)4]BF4 (Figure 3). The reported copper(I) complexes with four or six alkoxy chains exhibited liquid crystalline behavior and are characteristic of columnar discotics. DSC analysis results indicated a higher enthalpy for the melting transitions at lower temperatures and a relatively lower enthalpy for the isotropization transitions at higher temperatures. The attachment of an additional alkoxy chain on the terminal benzene ring resulted in lower transition temperatures, while preserving the mesophase type (Colhd).
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Figurenc 3. Copper(I) complexes with ether-type ligands. All temperatures are in °C [36].
Schiff bases are a diverse group of compounds formed by the nucleophilic substitution reaction of an aldehyde or ketone with an amine. These compounds are characterized by the presence of a double bond linking carbon and nitrogen atoms, the functionalities of which are generated in many ways to combine a variety of alkyl or aryl substituents useful for the design of liquid crystalline materials, either organic or metallomesogens [37,38,39,40,41,42]. The lone pair of electrons on nitrogen provides a basis for making complexes with metals, including copper(I). Diverse liquid crystalline copper(I) complexes have been prepared with these ligands; the resulting complexes are indicated in Figure 4. Dinuclear copper(I) complexes (4a and 4b) with Schiff bases based on the α,α’-imino-substituted 2,2’-bipyridine unit were developed by El-ghayoury et al. [43]. The two ligands adopt a rather unusual coordination mode in which the central unit bridges the two copper(I) centers. The tetrahedral surrounding is completed by the coordination of the two imino groups of each Schiff base ligand with the iminopyridine fragment chelated in a cis fashion to provide a symmetric structure. Complex 4a is non-mesomorphic, owing to the unsuitable balance between the aliphatic chains and the aromatic core. The optical textures observed upon slow cooling of the isotropic melt of complex 4b clearly indicate the existence of a viscous columnar phase at temperatures as low as 25 °C.
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Figures 4. Copper(I) metallomesogens with Schiff bases. All transition temperatures are in °C (transition temperature for 4a not reported) [43,44,45].
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Although non-mesogenic, upon complexation with copper(I), free ligands L5 and L6 showed mesogenic character, as described by the DSC and POM experiments. The results indicated that complex 6 showed high stability after several heating cycles relative to complex 5, which was thought to be due to the lack of substituents at position 6. In addition, the optical textures observed for complex 6 during slow cooling from the isotropic melt are typical of a columnar phase (with pseudo-focal-conic textures [45]. Similarly, Douce and coworkers prepared new complexes (7a–c) by modifying the organic ligands to enable scrutiny of the nature of the packing in the liquid crystalline phase. In their work, the authors reported the preparation of new ligands with various chain lengths (n = 8, 12, and 16) bearing an additional methyl fragment in the α-position of the pyridine ring in order to protect the copper complex (5) against oxidation and decomplexation. The prepared complexes displayed hexagonal columnar mesophases with a transition temperature range of 30 to 56 °C [44].

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A study by Ziessel et al. (2004) [46] presented mesomorphic materials based on copper(I) complexes with phenanthroline-based ligands (Figure 5). The authors aimed to engineer a structural framework with additional supramolecular binding factors (hydrogen bonding) so as to stabilize the mesophase both as a free ligand and within the complex. The thermotropic properties of the ligands and complexes of the phenanthroline derivatives were investigated via a combination of POM, DSC, and XRD methods. The ligand used for complexes 8b and 8c showed distinct cubic and disordered lamellar mesophases at different temperature regions in the reported XRD measurements. Neither the ligand with the shortest chain (n = 8) nor the corresponding copper(I) complex (8a) displayed any mesomorphic behavior in the investigated temperature region, but as expected, the related copper(I) complexes with longer chains (8b (n = 12) and

8, 6, 10073–10082, doi:10.1039/C8TC02999G.

c (n = 16)) showed mesomorphic behaviors, displaying mesophases characteristic of an oblique columnar phase. In effect, the complexation of the metal center with the organic ligand resulted in a distinct change in the mesomorphic properties. It was observed that the mesophase stability was enhanced upon coordination with copper(I), as reflected by the large increase in the clearing temperature by nearly 50 °C, whereas the melting temperature remained almost the same for the ligands, as well as for the corresponding complexes.

 

Chemistry 05 00046 g005 550
Figure 5. Copper(I) complexes with phenanthroline-based ligands. Temperatures are in (°C) [46].
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