4. Copper(I) metallomesogens with isocyanide ligands

Isocyanides are a class of organic compounds of the type R-NC, where R is a combination of groups obtained by the removal of a hydrogen atom from an organic compound and the carbon therein is triply bonded to nitrogen whose site is also capable of coordinating with metals. The isocyanides are isomers of the nitriles [45]. These kinds of ligands have been used to design a large variety of transition metal complexes [46] and copper(I) metallomesogens are also well known for this. In general, the reaction of isocyanides with CuX (X is halide) gave a mononuclear complex but, on the contrary, the reaction of two equivalents of the isocyanide derivatives with CuX yielded the binuclear copper(I) complexes as depicted in Figure 6.

Figure 6. Mononuclear and dinuclear copper(I) isocyanide complexes. Transition temperatures are in °C [49–53].

The first set of liquid crystals based on copper(I) isocyanide complexes was reported in 2001 by Benouazzane et al. [52]. Some of the isocyanide ligands (L⁹⁻¹¹) were reported to display nematic and/or smectic A phases. The copper complexes (9a-e) showed SmA and SmC mesophases, while complexes 10b-e and 11a-d were found to display only SmA phases [52]. Further study indicated that the range of the SmC phase increases and that of the SmA phase decreases as the length of the chain increases. All the copper isocyanides reported in Figure 6 are mesogenic, except complexes 10a, 12a, 12b, 13a, 14a-c, 20, 17a-b, and 20b. The isocyanide ligands L¹² and L¹³, with a single aromatic ring, did not appear to be mesogenic, but upon complexation with copper(I), all the complexes (except 13a with the shortest chain (n = 4)) showed liquid crystalline behavior.

After the successful preparation of stable linear copper(I) liquid crystals (complexes 9–13) with isocyanide ligands reported in [52], in 2002, the same research group subsequently reported another series of binuclear copper(I) complexes (14–16) with mesogenic properties. The free isocyanide ligands were reported as promesogenic, with nematic and/or smectic A phases. Copper complexes 14a–c lack mesogenic properties, whereas, copper complexes 15a-i were found to be mesogenic. On the other hand, although the uncomplexed isocyanide ligands (L¹³) are not liquid crystals, all their dinuclear copper isocyanide complexes (16a-i) had liquid crystalline properties, displaying columnar mesophases [53]. These liquid crystalline binuclear copper(I) complexes with isocyanide ligands reported by Benouazzane et al. were the first examples of liquid crystals with a core formed by two tetrahedral structures sharing an edge. Dendrimers based on isocyanides were first reported by Coco et al. (2008) [51]. The authors found that whereas all the free, highly branched isocyanide ligands (L¹⁷ and L¹⁸) and the metal complexes (17a and 17b) were not liquid crystals, complexes 18a and 18b showed a cubic mesophase.

Chico et al. [50] reported two isocyano-triphenylene copper(I) complexes (19a and 19b), both of which displayed 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 19a and 19b was 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. For these complexes, the columnar mesophases were stable in the temperature range of 46 to 79 °C.

The effect of incorporating a fluorinated chain in an isocyanide ligand with respect to its mesomorphic behavior when compared to its hydrocarbon derivative was studied by Dembinski and coworkers [49]. Previously, several studies [54–56] had investigated the fluorophobic effect of single aromatic ring-containing organic molecules containing a perfluoroalkyl chain; as such, a study was carried out on fluorinated analogs of hydrocarbon complexes in which mesomorphic behaviors had not been previously observed. The semi-perfluorinated isocyanide ligand (L²⁰), in contrast to its alkyl analog, exhibited liquid crystalline properties, showing a smectic A mesophase upon both heating and cooling. This was attributed to the fluorophobic effect, allowing for the mesogenicity of the isocyanide compound with a single benzene unit. The corresponding mononuclear copper complex retained the mesophase but with a high crystalline–mesophase transition temperature (152 °C), while the dinuclear analog did not show liquid crystalline properties [49].

5. Luminescent Metallomesogens Based on Copper(I) Complexes

A major drawback in the study of the physical properties of metallomesogens is associated with issues relating to their high transition temperature and stability at elevated temperatures. It becomes increasingly difficult to study the emissions at such high temperatures due to the strong tendencies of the excited electrons to undergo deactivation via non-radiative transitions [12]. Few attempts have been made in this regard, and interesting findings have been reported. The luminescence data of the copper(I) complexes discussed in this section are summarized in Table 1.

Table 1. Summary of luminescence data for copper(I) metallomesogens reported in [57–60].

t = averaged lifetime; QY = absolute quantum yield; λ_em = wavelength of maximum emission; λ_exc = excitation wavelength; na = non-aged; a = aged; Pg = ground sample; SmA = smectic A phase; g = glassy state; Cr = crystalline phase; Iso = isotropic liquid; L_col = lamello-columnar phase; Col_h = columnar hexagonal phase; L_colg = frozen lamello-columnar phase

Kishimura et al. were the first to describe the emission properties of copper(I) complexes in their liquid crystalline phase [57] in 2005. They reported a number of dendritic copper(I) pyrazolate complexes 21 and 22(Figure 7), which were used to produce some thermally rewritable phosphorescent papers useful for security purposes.

Figure 7. Ligands and representative structure of the copper pyrazolate complexes reported in [57,58].

The luminescence investigation of complexes from the pyrazolate ligands L²¹ [57] revealed dichroism at room temperature for the solid form of complex 21a. Cooling of the hot melt (which emitted a red luminescence at λ_max650 nm) naturally or by slow cooling led to blue-shift emission 640 and 610 nm. In essence, the red and yellow luminescence observed for 21a could be thermally changed from one form to the other, depending on the manner and rate of cooling, which was also found to be the case for its liquid crystalline properties. Both the analyze complex (21a), both in aged and non-aged form, appeared to be phosphorescent, which is thought to be a result of Cu(I) to Cu(I) interactions [57]. The photoluminescent data are summarized in Table 1.

Furthermore, XRD analysis of the aged sample of complex 21a showed diffraction patterns synonymous with a one-dimensional columnar phase, and the same complex (21a) viewed under a polarized optical microscope showed a fan-shaped texture characteristic of discotic liquid crystals; accordingly, it was concluded that the aged complex (21a) was composed of long-range discotic columnar assembly. The 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 (21a) revealed patterns based on which it was concluded that the discotic columnar assembly, which is believed to involve metallophilic interactions of Cu(I) to Cu(I) units with 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 pyrazolate ligand structure. Complex 21b exhibited similar phosphorescent properties as complex 21b upon rapid and slow cooling of its hot melt; however, the red luminescence turned yellow spontaneously, even at very low temperatures. Complex 22 (Figure 7), which has more dendritic units than the other studied complexes, did not show clear luminescence dichroism, and aging of the complex (22) by slow cooling of its hot melt resulted in only a 10 nm red shift of the luminescence [57].

Building on the aforementioned work by Kishimura and coworkers, Gimenez et al. (2020) [58] recently reported a series of liquid crystals achieved with cyclic trinuclear copper(I) complexes (23a and 23b) prepared using 3,5-dimethyl-4-(trialkoxyphenyl) pyrazolate ligands (Figure 7). The compounds displayed well-organized hexagonal columnar mesophases, which were found to be stable at room temperature or near room temperature (Figure 8).

Figure 8. (a) Microphotograph of the region between crossed polarizers observed for the texture of the Col_h phase of 23a upon cooling of the isotropic liquid at 126 °C. (b) Microphotograph of the region between crossed polarizers observed for the texture of the Col_h phase of 23b upon cooling of the isotropic liquid at 98 °C. (c) DSC thermogram of 23a. (d) XRD patterns of the Col_h phase of 23a. The arrow indicates the halo corresponding to the stacking distance. (e) Self-assembly in the hexagonal columnar mesophase. (Reproduced from ref. [58] with permission from the Royal Society of Chemistry).

Both complexes 23a and 23b were reported to have orange–red-colored emissions at room temperature, and the photoluminescence exhibited a broad band centered around 661–664 nm (Figure 9). A landmark reported in this study [58] is the high quantum yield (QY) value of 42% measured in the liquid crystalline state, which is the highest recorded to date for any copper(I) metallomesogen. The QY obtained for 23a in its liquid crystalline state was higher than that obtained in the crystalline states of complexes 23b and 23c. Thus, the work carried out by Giminez et al. [58] showed that low-temperature phosphorescent metallomesogens can be obtained from the more affordable and abundant copper metal.

Figure 9. Photoluminescence spectrum in the columnar mesophase for 23a at 25 °C (excitation wavelength, 290 nm) and emission of the film observed under irradiation with a 254 nm handheld lamp (reproduced from ref. [58 with permission from the Royal Society of Chemistry).

Camerel et al. (2016) [59] reported a new class of copper(I) liquid crystals with a cubane core and based on phosphine ligands functionalized with promesogenic gallate-based moieties bearing either long alkyl chains of C8, C12, and C16 or cyanobiphenyl (CBP) fragments (Figure 10). Copper(I) cubanes are known for their ability to display both luminescence mechanochromism and thermochromism behavior [61–65].

This study is an inventive example of integrating luminescence characteristics of copper iodide clusters [Cu₄I₄(L²⁴)₄] with the flexible self-assembly of liquid crystals. Only the compound functionalized with a cyanobiphenyl group (CBP), i.e., 24, showed liquid crystalline behavior, displaying an SmA mesophase from room temperature to about 100 °C.

Figure 10. General structure of the functionalized [Cu₄I₄(L²⁴)₄] copper iodide clusters with phosphine ligands [59].

All the complexes reported in the work by Camerel et al. [59] revealed luminescence thermochromism for all the studied compounds; however, complex 24 displayed an unclassical behavior, which was attributed to the intrinsic luminescence properties of the cyanobiphenyl moiety itself, with a dual-emissive system that presented interesting emission properties. In addition to its liquid crystalline properties, compound 24 displayed luminescence mechanochromism with a modification of the emission wavelength in response to grinding (Figure 11).

Figure 11. Photos of 24 under 365 nm (UV lamp) before and after grinding at 298 and 77 K. Reprinted (adapted) with permission from Chem. Mater. 2016, 28, 22, 8190–8200 (ref. [59]). Copyright 2016 American Chemical Society.

Cretu et al. (2018) [60] reported a new class of Cu(I) coordination complexes (25a, 25b, 26a, and 26b) with 4,4’-bisubstituted-2,2’-biquinolines as depicted in figure 12, which showed low-temperature lamello-columnar and columnar hexagonal thermotropic liquid crystalline phases. The highest deduction from the luminescence study is the presence of a medium–low-intensity band with a series of shoulders at 578 nm for 25a and 25b and at 560 nm for 26a and 26b, which is believed to be due to metal-to-ligand charge-transfer (MLCT) electronic transitions. When the solid samples were heated, they moved towards the liquid crystalline phases and retained the luminescence displayed in the solid state; however, with increased temperature, the intensity of the luminescence band decreased, and at temperatures over 120 °C, luminescence was completely quenched, although reversed with subsequent cooling of the samples. This behavior is attributed to the gain of the non-radiative kinetic constants when vibrational modes were enhanced by heating the samples [60].

Figure 12. Copper(I) complexes with bisubstituted biquinoline ligands [60].

6.Conclusions

This review provides an overview of the different strategies imagined by chemists to control the LC properties of copper(I) complexes, focusing on their luminescent properties. Examples of copper(I) metallomesogens based on isocyanide ligands are, by far, the most prevalent. By utilizing suitable mesogenic groups, their LC characteristics can 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-iminopyridin-, and picoline-substituted imino ligands 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 mesophase. Furthermore, interesting luminescent properties in liquid crystalline states were observed for copper(I) complexes from ligands with pyrazolate derivatives, phosphine ligands functionalized with copper iodide, and those of bisubstituted biquinoline ligands. The complex from the functionalized phosphine ligand displayed interesting mechanochromic luminescence characteristics. A high quantum yield of 42% in the liquid crystalline phase of the copper(I) metallomesogens from a pyrazolate ligand was reported. This work provides a basis for the design and preparation of many new multifunctional materials based on more copper(I) complexes with liquid crystalline behavior and improved luminescent properties.