Mesogens with aggregation‐induced emission properties: Materials with a bright future

Within the past two decades, chromophores, which show aggregation‐induced emission (AIE), have gained considerable attention with respect to the development of luminescent liquid crystals. In contrast to common luminogens, AIE emitters do not suffer from aggregation‐caused quenching of the emission in the solid state. In this review, we summarize the recent development in the field of AIE‐active liquid crystals and show first model devices, which already prove the application potential of these materials. Currently, three different approaches are followed, to get access to luminescent liquid crystals––namely the synthetic approach yielding luminescent mesogens, the doping approach, and the supramolecular approach, which will be described and discussed in detail in this review.


INTRODUCTION
The phenomenon of emission increase, observed for flexible luminophores upon binding to a specific target or when aggregated, is known since decades [1] and has been used for a variety of different purposes, such as DNA [2] or RNA [3] binding. Initial observations regarding this ability have been reported nearly 100 years ago by Schmidt for a series of dyes, which were embedded in solid matrices. [1] Ever since numerous studies have been published in this field. [4] A renaissance and boost of this interesting field has been experienced in 2001, when Tang and coworkers discovered that 1-methyl-1,2,3,4,5-pentaphenylsilole reveals striking emission properties, contrary to classic emitters, in the solid and aggregated state, but not when dissolved in an appropriate solvent. [5] They introduced the term aggregationinduced emission (AIE) for this phenomenon (Figure 1), which started a renaissance in the comprehensive investigation of the effect. Especially the mode of action appeared to be more complex than expected and is still the focus of controversial discussions. It was found that a restriction of the molecular motion, such as vibration or rotation, is one of the key steps in the emission induction. Besides that, secondary noncovalent interactions in the aggregated state, such as π-π interactions, CH-π interactions, hydrogen bonding, metal-metal contacts, as well as heteroatom interactions, play a crucial role in the emission wavelength, photoluminescence quantum yields (PLQYs), as well as theemission life-times (τ). [6] Especially π-π interactions are known to hinder emission efficiency due to the formation of nonradiative charge-transfer complexes, which are also the reason for classic emitters to lose their emission properties in the aggregated and solid state, since these planar aromatic systems (e.g., pyrene, fluorescein, and rhodamine) tend to stack in poor solvents (aggregation-caused quenching--ACQ) ( Figure 1). AIE emitters often show a twisted conformation of the aromatic units leading to a stabilization of the aggregates mainly via CH-π interactions, which is favorable for the PLQY. [7] AIE emitters were found to be excellent candidates for applications in fields where classic emitters fail, such as the monitoring of assembly processes, solid materials, and supramolecular recognition. The main advantage of AIE systems is the fluorescence or phosphorescence "on" behavior, which can be induced by steric hindrance, restriction of motions, or changes in the molecular environment, such as the viscosity of surrounding media. Especially applications in biomedical chemistry have attracted interest in recent years. Functionalization of AIE emitters with specific recognition units, such as carbohydrates, peptides, or drugs, leads to nonemissive compounds when dissolved in water, but emits upon binding to a specific target, such as cell compartments, [8] enzyme, and protein binding pockets [9] or bacteria. [10] Besides that, material scientists already used AIE emitters to construct novel materials with unique emission properties, such as organic light emitting diodes, [11] F I G U R E 1 Schematic representation of the aggregation-caused quenching (ACQ) mechanism of perylene and the aggregation-induced emission (AIE) phenomena of tetraphenylethene (TPE) S C H E M E 1 Schematic representation of the temperature dependency on the order of mesogens as well as its influence on mobility and potential emission enhancement of hosted AIE luminophores solar cell concentrators, [12] soft supramolecular systems, [13] as well as liquid crystalline systems.
Since the fluorescence behavior of AIE emitters strongly depends on their molecular packing and their interaction with their environment, the combination with liquid crystals appears highly attractive in the view of materials science. Liquid crystallinity, the state between the liquid and the crystalline phase (therefore, it is also called mesophase), features the characteristics of both phases--the order of the crystalline materials with the mobility of the liquid state. [14] Thermotropic liquid crystals change the degree of order as a function of the temperature (Scheme 1). These materials lose their order from the crystalline state to the smectic to the nematic phase with increasing temperature and in the same row, they gain more rotational freedom and mobility. The loss of order and the simultaneous gain of mobility has a direct impact on the fluorescence behavior an AIE emitter present in the liquid crystalline host material. Thus, the combination of mesogens and luminophores provides the opportunity to create materials with tunable fluorescence properties. Mesophases are usually investigated by complementary analytical techniques, such as differential scanning calorimetry (DSC), small and wide angle X-ray diffraction (WAXD), as well as polarized optical microscopy (POM).
In this review, we would like to focus on recent advances in the design and applications of luminescent liquid crys-tals using mesogens featuring AIE or emission enhancement properties. Here, it is noteworthy, that AIE is typically linked in its majority to aggregation processes, which would lead to heterogeneity in the investigated materials. This effect is unfavorable and might lead to phase separation in the liquid crystalline materials. In our review, we focus on the effect of AIE luminophores and related emitters in liquid crystalline materials, which cannot solely be attributed to molecular aggregation but also to the hindrance of molecular motion of single molecules, which leads to an emission "on" or an emission increase. In this scenario, the molecular order of the mesogen in dependence of the temperature, the molecular structure, as well as inter and intramolecular interactions are the main driving force for the emission or emission enhancement.
As mentioned before, the AIE properties are controlled by a complex interplay of noncovalent interactions and strongly depend on rotational freedom and intramolecular vibrations of the emitting moiety. Therefore, the combination of AIE emitters with the liquid crystalline state provides access to materials with temperature depending on fluorescence behavior with broad application potential. In general, there are three different routes toward the generation of luminescent liquid crystalline materials--the synthetic, the dopant, and the supramolecular approaches ( Figure 2). The synthetic approach yields mesogenic structures, which exhibit AIE behavior. Hereby, two different ways can be followed. The first one contains the synthetic modification of a luminogen by attaching alkyl chains to induce liquid crystalline behavior. The second approach couples a mesogenic structure with an AIE luminophore to yield the luminescent mesogens. The doping approach leads to luminescent liquid crystals by doping liquid crystals or mixtures of liquid crystals with an AIE emitter. Here, the mesogens act as host matrix for the AIE luminophore which starts to emit upon formation of the more ordered mesophase due to a restriction in the molecular motion. This approach is especially interesting with respect to display applications, since a wide range of liquid crystals or liquid crystal mixtures are commercially available.
The supramolecular access employs noncovalent interactions to obtain defined assemblies, which combine liquid crystallinity and AIE behavior. Synthetically, these systems are accessible by simple mixing the molecular building F I G U R E 2 Approaches toward luminescent liquid crystalline behavior blocks in a suitable solvent. Subsequent removal of the solvent yields a functional assembly, which combines the properties of the individual building blocks and shows in many cases additional functionality/properties as a result of the selfassembly process.
For all three approaches, the AIE emitter is embedded in a bulk material and exhibits a multitude of intermolecular interactions to adjacent molecules. The efficiency of its luminescence strongly depends on these underlying interactions and is a function of the restriction of intramolecular motion (RIM) by the surrounding entities, which can be either other AIE emitters, a liquid crystalline host material, or supramolecular assemblies. To sum up, we would like to shine light on different approaches toward emissive liquid crystalline materials, such as synthetic, doping, or the supramolecular approach.

SYNTHETIC APPROACH
As already mentioned, two synthetic approaches are followed to obtain AIE mesogens. The first approach decorates well-known AIE emitters, such as tetraphenylethene (TPE), tolanes (TOLs), cyanostilbenes (CSTs), aromatic thioethers thiophenes, siloles, as well as oxadiazoles or thiadiazoles ( Figure 3), with flexible alkyl chains to introduce liquid crystallinity. The second route yields emissive mesogens by coupling a mesogenic structure with AIE-active luminophores by covalent linkers. In both of these approaches, the molecular shape of the resulting compound is crucial for the liquid crystalline properties and the luminescent behavior. Commonly, mesogenic structure consists of a rigid unit with a rod-, disk-, or bentlike shape and flexible chains in the periphery. Depending on the molecular shape and the interplay of noncovalent interactions between adjacent mesogens, different mesophases with their characteristic molecular orders arise. The challenge of the synthetic approach is to introduce the luminescent moiety without disturbing the molecular order of the mesophase. The introduction of a new structural element, such as by the attachment of a luminophore, will have a crucial impact on the molecular packing, which in turn affects the liquid crystalline and photophysical properties of the resulting material. A general prediction of these effects is challenging; however, it is liable that the introduction of the luminophore structure will reduce the order and packing efficiency.

Emissive mesogens based on TPE
Most common AIE luminophores are based on the TPE core. In 2001, the first example of a liquid crystalline material based on TPE, however, the AIE behavior of these mesogens has not been discovered, was reported by Laschat and coworkers ( Figure 4). [15] They synthesized a TPE derivative coupled with four alkoxy-substituted benzoic acids (TPE-1) and observed hexagonal columnar mesophases as proven by POM, DSC, and X-ray diffraction. More than a decade later, Yuan et al. reported for the first time the AIE behavior of TPE-based mesogens and introduced a rational design for these materials. [16] Thereby, they combined the AIE-active TPE with mesogenic TOL units to obtain luminescent liquid crystals (TPE-2, Figure 5). The nature of the mesophase was investigated by one-and twodimensional WAXD.
The luminescence of the tetra-substituted TPE-2 was investigated in solution and in the solid state, proving high solid-state quantum yields up to 67.4 ± 5%. It was discovered that upon cooling from the isotropic phase to 190 • C, F I G U R E 4 (A) Molecular structure of a tetra-substituted tetraphenylethene mesogen (TPE-1) and (B) fan-shaped texture of TPE-1 as observed by light microscopy with cross polarizers at 63 • C upon cooling from the isotropic state. Adapted and reproduced with permission from Wiley-VCH, copyright 2001 [15] F I G U R E 5 (A) Molecular structure of a tetra-substituted tetraphenylethene-tolane mesogen (TPE-2). (B) Polarized optical microscopy of TPE-2 cooled to 190 • C and (C) to 100 • C. Adapted and reproduced with permission from the Royal Society of Chemistry, copyright 2012 [16] an anisotropic mesomorphic phase was obtained, which converted to an even higher ordered, but noncrystalline, mesomorphic phase at further cooling to 100 • C. Although high PLQY was observed in the solid state, no deeper investigation of the emission properties in the liquid crystalline phase was conducted, proving again the need of in-depth investigations of emissive liquid crystal (LC)'s featuring AIE properties.
In 2014, Cho and coworkers synthesized tetra-substituted TPE derivatives by copper catalyzed alkyne-azide cycloaddition (CuAAC) reaction (TPE-3, TPE-4, and TPE-5) showing columnar mesophases ( Figure 6). [17] TPE-4 showed a transition from a rectangular (Col rec ) to a hexagonal columnar (Col hex ) phase, which was accompanied by a reversible change in the emission color from blue to green ( Figure 6D). The authors claim the change in the emission to flattening of the propeller-like structure of the mesogens upon LC-to-LC phase transition ( Figure 6C), leading to a more extended πconjugated system with longer lifetime and a bathochromic shift. In 2016, the same group expanded their studies on the tetrasubstituted TPE systems by investigating the impact of nonpolar and polar exterior side chains on the liquid crystalline and AIE behavior ( Figure 6). [18] Their results showed that TPE derivatives with nonpolar peripheral chains exhibit columnar mesophases, while the polar substituted analogue was polycrystalline. These differences were also reflected by temperature-depending relative emission intensities. While the emission spectra of TPE-3 and TPE-4 are gradually redshifted and quenched upon heating. Similarly, the polar analogue TPE-5 showed quenching of the emission signal up to 140 • C; however, at higher temperatures, a new band emerges at 550 nm. This behavior indicates the crucial impact of the intermolecular forces between the entities on their molecular alignment in the bulk materials and thus on their emission behavior.
Just recently, Jiang et al. reported related aromatic acylhydrazone derivatives of TPE (TPE-6, 7, 8) with columnar mesophases as proven by POM, DSC, and XRD analyses (Figure 7). [19] It was observed that the number of the alkylchains drastically influences the mesomorphic behavior. A higher number of alkyl-chains decreased the phase-transition temperatures going along with a broadening of the mesophase range. For all compounds, distinct phase transitions upon cooling and heating were detected (Cr-Col and Col-Iso). The mesogens showed absolute fluorescence quantum yield up to 27.4% (for TPE-7) in the solid state and between 6.7%and 24.2% in the mesophase. Preliminary studies of the electroluminescence performance (EL) suggested application potential for AIE mesogen TPE-7 in OLED devices. Here, TPE-7 was used since it features the highest quantum yield and was embedded in a device composing of ITO/PEDOT/emissive layer (∼40 nm)/TPBI (20 nm). This OLED revealed bright yellow emission (λ em = 582 nm) with very good spectral stability over a wide voltage range.
The Tang group synthesized a monosubstituted TPE derivative combining the blue emission of the TPE core with the luminescent mesogenic TOL unit showing a monotropic smectic phase ( Figure 8). [20] The temperature-depending luminescence of TPE-9 showed a blue-shift of 17 nm in the emission wavelength upon cooling from the metastable mesophase to the crystalline state, which was attributed to the denser packing of the TPE cores in the latter.
Following the design concept of the Tang group, Wang et al. reported a mono and disubstituted tetraphenylenephenylpyridine (TPE-10, TPE-11) derivative coupled to 4′cyano-4-hydroxybiphenyl by an alkyl linker ( Figure 9). [21] POM, DSC, and temperature-dependent XRD studies revealed that both mesogens are liquid crystalline at high temperatures and showed blue/green emission with quantum efficiencies up to 83% in the crystalline phase. TPE-10 gave a birefringent fluid at 156 • C. The mesophase was described as a lamellar phase, which was not further characterized. The related TPE-11 showed an oblique mesophase with 2D arrangement at 174 • C.  [17,18] Since the fabrication of devices was the major goal of this contribution, conductivity measurements were performed. It was shown that the hole mobilities of thin films significantly increased up to 10 −4 cm 2 V −1 s −1 upon active layer annealing at 165 • C, which is an order of magnitude higher than for the pristine films. Furthermore, the electroluminescence of TPE-10 was tested in an OLED device. With a current efficiency of 6.2 cd A −1 and an external quantum efficiency of 4.1%, this material seems to be appealing for the development of optical devices.
In 2015, Lu and coworkers studied the circularly polarized luminescence (CPL) of TPE derivatives with two cholesterol pendants (TPE-12). [22] Materials showing CPL are highly promising with respect to their potential application in stereoscopic optical information storage and processing, [23] quantum computing, [24] and 3D display technology. [25] POM of TPE-12 displayed the characteristic focal conic texture of a cholesteric liquid crystal. Temperature-dependent circular dichroism and CPL spectra demonstrated the tuneability of the emission behavior by variation of the temperature. In the pristine state, the CPL showed a dissymmetry factor of ∼10 −2 and a fluorescence efficiency of 42% (Figure 10), which suggests potential application in optoelectronic devices.
Just recently, Yu et al. reported related systems combining the AIE properties of the TPE moiety with the reversible photoswitching capacity of azobenzenes and the liquid crystalline nature of cholesterol (TPE-13, Figure 11). [26] All obtained TPE derivatives showed liquid crystalline properties, while TPE-13(C 3 ) and TPE-13(C 5 ) additionally form gels in polar solvents, such as N,N-dimethylformamide (DMF). It was found that all compounds formed smectic A phases with slightly varying phase transitions in dependence to the linker lengths. Also, the emission behavior in the gel state is controlled by the spacer length. TPE-13 (C 5 ) showed a gradual decrease upon heating, whereas TPE-13 (C 3 ) showed an abrupt emission loss at 65 • C.
In the recent years, also a number of fluorescent liquid crystalline polymers based on TPE were reported. The Tang group synthesized highly fluorescent and liquid crystalline polymers by incorporating TPE units within the main chain of polytriazoles (TPE-14) ( Figure 12). [27] The polymeric material showed high solid-state emission efficiencies of up to 63.7%. Similar to the findings by Yu et al., [26] the authors described the spacer length as crucial structural element which controlled the packing of the mesogenic units and thereby the photophysical properties as well as the liquid crystalline behavior of the polymer. Increasing the length of the spacer yielded lower solid-state quantum yields, which can be attributed to a higher degree of rotational freedom. In addition, the higher flexibility of the derivatives with longer spacer lengths changes the mesophase from nematic to F I G U R E 7 (A) Molecular structure of the AIE-active hydrazone functionalized tetraphenylethenes (TPE-6-8) and (B) fluorescence spectra of TPE-6, 7, and 8 of solid films before and after heating at mesophase temperatures (150 • C for TPE-6, 120 • C for TPE-7, and 50 • C for TPE-8). Inset shows photographs of TPE-7 before and after heating. Adapted and reproduced with permission from Elsevier, copyright 2018 [19] smectic. In 2017, Guo et al. obtained a series of polystyrenebased polymers with two TPE units attached to the side chain (TPE-15) ( Figure 12). [28] The polymers were strongly luminescence in the mesophase as well as in the solid state. Similar to the previous example, the extension of the alkyl spacer length between the polymer main chain and the TPE unit lowered the solid-state quantum yields, while the mesomorphic behavior changes from smectic to hexagonal columnar ( Figure 12).
An AIE-active liquid crystal elastomer was designed by Liu et al. via crosslinking of a commercially available LC monomer with a four-alkenyl-armed TPE core (TPE-16, Figure 12). [29] The authors were able to show that the elastomer serves as an actuator with reversible thermally induced shape morphing accompanied by a significant lost in photoluminescence during heating. The change in the photophysical properties was mainly attributed to the enhanced rotational freedom of the AIE emitter, which yielded in a lower luminescence efficiency. The simultaneous contraction of the elastomer should have in principal a contrary effect; however, in this case, the effect was negligible.

Mesogens based on CST
Another widely used class of AIE luminogens is based on CSTs. [30] In 2012, the Park group reported the thermochromic luminescence of the dicyanodistyrylbenzenebased mesogen CST-1, which formed a hexagonal columnar phase at room temperature ( Figure 13). [31] These materials showed an intense yellow/green fluorescence in the solid state, which gradually decreases during heating and finally completely vanishes in the isotropic state. The authors were Adapted and reproduced with permission from the Royal Society of Chemistry, copyright 2017 [21] able to prove that the emission behavior is controlled by intra and intermolecular interactions of the CST unit in the corresponding phase, which restricts the molecular motion. The observed two-color luminescence was attributed to the packing of the discotic mesogens. Changes in the stacking yielded different excited-state dimeric coupling and resulted in changes in the emission color from green to yellow. In 2014, the same group used the reversible photoswitch capability of the CST derivatives CST-2 to generate highly fluorescent surface relief gratings by crystallization-induced mass flow. To this end, they produced a thin film of CST-2 by spin coating. To monitor the phase transition upon UV-light irradiation, the sample was observed under POM. At room temperature, the phase transition from crystalline film to isotropic Adapted and reproduced with permission from the Royal Society of Chemistry, copyright 2015 [22] occurred slowly and even after 30 min was not completed.
In contrast, at ∼39 • C, the optical texture of the sample vanished within 90 s, which was accompanied by the loss of fluorescence. This behavior was used to prepare a surface relief grating on thin films, which was investigated by atomic force microscopy ( Figure 13C and D). [32] Lu et al. synthesized the related dicyanodistyrylbenzene emissive mesogen CST-3 and investigated its multi-stimuli controlled emission behavior ( Figure 14). [33] The mesogens showed luminescence changes from green, to yellow, to orange, which was attributed to changes in the molecular alignment of the mesogens, whereby the changes were induced by shearing or thermal annealing of the sample. Heating CST-3 to its isotropic state (200 • C) yielded a complete loss of emissive behavior due to thermally activated nonradiative decay. Upon cooling to 150 • C, a smectic C phase was observed and the sample slightly emits yellow light, which intensified by fast cooling to room temperature by liquid nitrogen (Figure 14), which was attributed to a slim stratified structure, which was proven by scanning electron microscopy (SEM) and X-ray diffraction. Cooling the same sample from the isotropic state slowly to ∼70 • C yielded an emission change to green (smectic C*). This behavior was attributed to π-π stacking and multiple CH-N and CH-O hydrogen bonds, which led to a torsionally stratified structure. Fast cooling enhances the emission intensity under preservation of the structure. Applying a shearing force to this sam-ple at room temperature yielded an orange emission (Figure 14), which was also shown by applying a shear force by a ball pen using a PMMA/CST-3 composite. The obtained image could be erased by heating, which is in agreement with the transition from state A to C. The authors attribute the thermochromic and piezochromic behavior of CST-3 to changes in the intermolecular packing, which is supported by results from X-ray diffraction measurements and SEM, which reveals a sheet-like morphology. The green state (A) revealed distinct and well-defined peaks in the XRD with a layer spacing d smaller than the actual length l of the compound. This observation was attributed to a tilted conformation of CST-3. The orange (B) and yellow (C) state showed only weak and more amorphous peaks in XRD based on only marginal intermolecular interactions.
Very recently, Tang and coworkers reported two new CPLmesogens based on CST attached to cholesterol via ester (CST-4) or hydrazine linkages (CST-5) ( Figure 14). [34] The ester-linked derivative CST-4 showed a chiral smectic C* phase and CPL with high dissymmetry factors of g CD = -0.20 and g lum = +0.38, respectively. The dissymmetry factors remained high also in the isotropic state (g lum = +0.18), indicating the preservation of the chirality in the isotropic state. Interestingly, the hydrazine-based mesogens CST-5 showed two chiral, coexisting phases, a smectic and a hexagonal phase with CPL of opposite polarization. The coexistence of two phases as suggested by the authors and supposedly shown by the X-ray data should be regarded with caution, since it might be attributed to an incomplete data set or the recording of a transient state during the measurement. The hexagonal phase is attributed to the formation of discotic dimers of the mesogens via intermolecular hydrogen-bonding. The CPL signal of these assemblies, however, remains weak due to the nonperiodic molecular orientation of the mesogens.
The Yang group combined the mesogenic triphenylene core with luminescent CSTs (CST-6-11) and reported highly ordered hexagonal columnar mesophases ( Figure 15). [35] The fluorescence efficiencies were investigated in the solid state and the aggregated state in solution. Compounds CST-9-11 (PLQY aggregated = 18.8%, 19.0%, 19.1%, PLQY solid = 8.3%, 8.7%, 8.9%) revealed columnar phases with transition temperatures down to 10 • C (Cr-Col, compound CST-11). Similar behavior was observed for compounds CST-6-8 (PLQY solid = 15.9%, 7.5%, 5.6%) but with higher transition temperatures. Surprisingly, the temperature dependency of the emission properties has not been investigated in this report, which would have been helpful to gain a deeper understanding of the fundamental mechanism of the emission behavior. The same group also reported perylenebased mesogens (CST-12 and CST-13) decorated with CSTs, which exhibited hexagonal columnar phases down to 56 • C ( Figure 15). [36] In solution and in thin films, strong fluorescence was observed (CST-12 PLQY solution = 7% [λ ex = 330 nm], 79% [λ ex = 530 nm], PLQY film = 68% [λ ex = 380 nm], CST-13 PLQY solution = 4% [λ ex = 330 nm], 30% [λ ex = 530 nm]), which the authors attribute to cooperative mechanism of AIE and Förster resonance energy transfer (FRET) between the perylene unit and the diphenylacrylonitrile moiety ( Figure 15). This concept used the emission of the AIE-active CST core when aggregated in THF/water mixtures as excitation wavelength for the perylene core. This stepwise excitation pathway differs drastically from the other  [26] described systems, where the emission solely comes from the AIE-active core. For CST-12, this concept was also proven to work in the thin film state featuring a simulated Col h phase.
Recently, Zhang and coworkers designed a series of liquid crystalline polymers with CST moieties in the side chain (CST-14 and CST-15, Figure 16). [37] In-line with the results of Tang et al. on liquid crystalline polymers-based TPE, [27] a significant impact of the spacer length between the polymer backbone and the emissive CST on the mesomorphic properties was found and a decrease of the clearing point with increased spacer length was observed. Surprisingly, a contrary behavior concerning the emission properties, particularly the PLQY, was observed. Here, an increase in the spacer length yielded an increase in the fluorescence quantum yields, which is contrary to the results of Tang [27] and Xie, [28] who found a decrease of PLQY upon an increase of the spacer length for polymers containing TPE-luminophores, which was attributed to a higher degree of freedom of the TPE moieties. For CST-14 and CST-15, however, the increase of PLQY by increasing spacer lengths was explained by the more effective stacking of the CST moieties into J-aggregates and subsequently a stronger fluorescence due to the restriction of motion.

Mesogens based on TOL
Another class of liquid crystalline materials with AIE behavior is based on TOLs. The first example of luminescent liquid crystal based on TOLs was reported by Tang and coworkers, who attached an alkoxy group to a cyclohexyl TOL derivative (TOL-1, Figure 17). [38] Cooling from the isotropic phase yielded the phase sequence from nematic (160 • C), to smectic C (125 • C), to smectic B (100 • C) phase and finally crystalline state accompanied at 20 • C with an increase in the fluorescence. Later, the same group reported the fluorophenyl substituted TOL derivative TOL-2, which shows a smectic and a nematic phase. [39] The authors found a strong correlation between the order of the solids and the fluorescence quantum efficiencies. In the crystalline state, TOL-2 emits UV light (388 nm) with a quantum efficiency of 60%. In contrast, the quantum efficiency in the amorphous solid is reduced to 9% emitting deep-blue fluorescence (413 nm). Solids derived from liquid nitrogen quenching of samples in the nematic or smectic phase yielded Φ F values of 33% and 40%, respectively. These results clearly demonstrate the relevance of the molecular arrangement and mobility of the luminophors in the different phases, which is driven by a multitude of noncovalent interactions. Yamada et al. reported a series of fluorinated-TOL derivatives TOL-3, which displayed nematic and smectic A mesophases with a temperature range of up to 97 • C. [40] For TOL derivative TOL-3 (R = C 5 H 11 , Ar = pentafluorophenyl), temperature-dependent photoluminescence measurements showed that emission intensities and color can be tuned by temperature changes, which was attributed to changes in the molecular alignment in the corresponding phases. The authors showed, that upon heating, a Cr-Smectic A transition occurs leading to a bathochromic shift (140 • C), which reverts to the initial state upon further heating (160 • C).
Cheng et al. synthesized a chiral emissive mesogen based on the TOL structure (TOL-4), which exhibited twist-grain boundary A (TGB A *) phase and Blue phase II (BP-II). [41] The authors reported strong fluorescence of TOL-4 in the aggregated form in solution, the solid state, as well as in the LC phase. Furthermore, the samples showed structural coloration, which changed from blue to red with decreasing temperature.

Mesogens based on oxa and thiadiazoles
Achalkumar and coworkers synthesized a series of alkoxyfunctionalized oxadiazoles and thiadiazoles, and investigated F I G U R E 1 2 (A) Molecular structure of the polymeric TPE mesogens (TPE-14, 15, and 16) and (B) schematic representation of the smectic and columnar structure of TPE-15 with varying spacer lengths. Adapted and reproduced with permission from the American Chemical Society, copyright 2017 [28] their AIE behavior and mesomorphic properties. They found that the number alkoxy substituent controls the mesomorphic phase ranging from lamellar solid-state structures of the trisubstituted derivatives to exceptional broad hexagonal columnar phases for the nine-fold substituted oxadiazole OXA-1 ( Figure 18). [42] The fluorescence quantum yields in THF solution of the OXA-1 derivatives (PLQY = 43-48%) were found to be slightly higher than the TIA-1 derivatives (PLQY = 21-35%), but the PLQY was not explored in the solid or mesophase. In 2018, the same group reported related oxa-(OXA-2) and thiadiazole (TIA-2) systems with a central pyridine unit, which formed columnar mesophases and showed blue AIE and were also found to emit in the gel and xerogel state as well as thin film. [43] The related thiadiazole-based mesogens TIA-3 were tested with respect to application in OLEDs as single emissive material or as guest in a polymeric host. [44] The host guest OLED devices clearly showed a higher efficiency and brightness of the blue emission. In this regard, a device consisting of poly(9-vinylcarbazole) as host material with 5% of TIA-3 (R 1 = R 2 = R 3 = OC 10 H 21 ) as emitting layer together with PVK:3T/10 (5 wt%) as the emitting layer together with bathocuproine (hole blocking layer) and tris-(8-hydroxyquinoline)aluminum (electrontransporting layer) showed an external quantum efficiency of 0.63%.

Emissive mesogens based on other luminophores
In 2012, the Kato group designed a series of ionic liquid crystals based on tripodal pyridinium (PYR-1), pyrimidinium (PYM-1), and quinolinium (QUI-1) salts, which showed color tunable photoluminescence ( Figure 19). [47] In the mesophase, the molecules self-assembled into columnar structures driven by nanosegregation of the ionic moieties and the aliphatic side chains. The photophysical properties of the materials were comprehensively investigated in annealed films and solution, showing that the molecular design allows tuning of the emission from blue-green for the pyridiniumbased systems, to yellow-orange for the pyrimidiniumbased molecules to red for quinolinium-based mesogens. The formation of ionic donor (D)-acceptor (A) pairs led to an emission from an intramolecular charge transfer state in solution, which was confirmed by density-functional theory (DFT) calculations. As electron-donating moieties, 3,4-didodecyloxybenzene, 3,4,5-tridodecyloxybenzene, or 4-(N,N-didodecylamino)benzene were employed and combined with electron accepting pyridinium, pyrimidinium, and quinolinium salts. The PLQYs in solution were significantly higher (up to 41%) than in the annealed films (up to 12.6% (Cr)) and even further decrease when a columnar phase is entrapped in the films (up to 8.8% (Col h )).
Wan et al. reported two silole-based AIE molecules (TPS-1 and TPS-2, Figure 19), which tend to gelate in organic solvents and form liquid crystalline phase in the molten state as proven by POM, DSC, and X-ray diffraction. [48] The organogels (CH 2 Cl 2 ) showed characteristic photophysical properties of silole AIE emitters with an emission maximum of around 519 nm with a slight hypsochromic shift upon heating to 50 • C (λ em = 515 nm).
Very recently, the Yang group presented two classes of mesogens based on the AIE-active tetraphenylthiophene core (TPT-1-3 and TPT-4-6), which formed square, rectangular, or hexagonal columnar phases depending on the structure of the mesogenic core and the length of the attached alkyl chains. [49] Interestingly, TPT-4-6 just show hexagonal columnar phases with emission wavelengths around 410 nm. Besides the fluorescence emission in solution as well as in the bulk material, the mesogens showed narrow band gap energies and good gelation ability, which makes them appealing for applications in organic semiconductors or sensors  Figure 19). The results again prove the significance of the molecular packing on the emission behavior.
In 2017, Voskuhl and coworkers reported a new class of AIE emitters based on aromatic thioethers, which are synthetically easy to access and variation of the substitution pattern provides a simple way to tune the emission wavelength. [7,10] Together with the Giese group, the mesomorphic behavior of the alkylated derivatives of the thioethers ATE-1-4 was comprehensively investigated (Figure 19). [50] Exclusively thioethers of the meta-nitrile-para alkoxy series with even numbers of carbon atoms in the alkyl chain exhibited enantiotropic mesomorphism, while for derivative ATE-4 with a branched alkyl chain or ATE-2 with an odd number of carbon atoms in the alkyl chain, only monotropic LC phase transitions were observed ( Figure 19). The accompanying crystallographic study and an in-depth crystalline packing analysis supported by computational methods revealed that enhanced fluorescence emissions are correlated with a significant increase of C-H•••π and a decrease of π•••π interactions in the solid state, which is in line with the well-accepted AIE mechanism. [50] Here, an RIM via CH-π interactions, leading to a stabilization of the aggregates, is favorable, whereas π-π contacts lead to stacking with nonradiative decay pathways. Further analysis of the crystalline packing will be used to deduce design principles for related emissive mesogens with broader mesophases.
To summarize, the classic synthetic approach combines numerous advantages, such as the use of distinct molecules avoiding potential challenges, such as phase separation upon mesogen mixing. Besides that, due to well-known synthetic strategies, as well as known mesogens, the design of novel luminescent mesogens is well affordable but remains challenging in terms of time-consuming syntheses and purifications issues.

DOPING APPROACH
The doping approach is a promising approach toward application of AIE-active materials in optical devices. In this approach, usually commercially available liquid crystals or liquid crystal mixtures are doped with AIE-active dyes or mesogens to obtain thermoresponsive luminescent materials. A major challenge of this approach is to avoid phase In order to overcome this issue, the luminophore structure should closely related to the molecular structures of the liquid crystalline host. Therefore, most approaches use luminophores consisting of a rigid core, which is at the same time the emissive moiety and flexible alkyl chains in the periphery. Following the doping concept, the Tang group developed test devices by doping the TPE-17 ( Figure 20A) into the nematic liquid crystal PA0182. [51] The LC mixture was transferred into an LC-cell with homogeneously rubbed alignment. The LC system clearly showed unidirectional orientation of the TPE-17 molecules and exhibited polarized luminescence. In the field-off state, the molecules of the LC mixture were oriented parallel to the substrate, showing a dichroic ratio of 4.2. In the field-on state, however, the LC molecules aligned perpendicular to the substrate surface yielding a dichroic ratio of nearly 1:1. This results encouraged the authors to design a test device with a patterned ITO glass substrate. Applying an electrical field allowed switching from a dark state to a yellow-green fluorescence under UV-light ( Figure 20). The same group also used the TPE-17 mesogen to design a reflective photoluminescent chiral nematic liquid crystal display. [52] Here, they mixed TPE-17 with commercially available LC (SLC1717) and added different amounts of the right-handed chiral dopant CB-15 to the mixtures to obtain a photoluminescent chiral nematic liquid crystal mixture (PL-N*LC). Due to the chiral-nematic nature of these materials, they selectively reflected circular polarized light of one handedness (in this case, right-handed), when the pitch is within the range of the wavelength of visible light ( Figure 20B and C). Depending on the composition of the PL-N*LC mixtures, reflective colors ranging from blue to red were observed. The test device showed parallel alignment of the mesogens to the substrate with the helical axis of the N*LCs perpendicular to the surface of the substrate in the field off-state. In this state, the display device reflects yellow circular-polarized light at day light and appears green fluorescent under UV light. By applying an electrical field to the LC cell, the molecules align perpendicular to the substrate surface and the cell appears dark proving that no light is reflected and nonpolarized emission of UV-light was found ( Figure 20D-G). Lu et al. synthesized the α-cyanostilbenic CST-16 and investigated its photoisomerization-induced phase separation in E7 as liquid crystalline host ( Figure 21). [53] For this purpose, E7 was doped with 16.7% of (Z)-CST-16 and filled into a coated ITO test cell in which the molecules align parallel to the substrate. Upon irradiation with UV light, (E)-CST-16 is formed, which is, due to its bent shape, incompatible with the calamitic shape of the E7 host yielding a fluorescent-molecule dispersed liquid crystal (FMDLC, see Figure 21). The photophysical response of the FMDLC on electrical fields was tested by applying a voltage of 30 V. In the field-off state, the LC domains are not oriented leading to randomly scattering of the traversed light ( Figure 21). The scattered light had a higher chance to encounter the flu-orescent molecules, which enhanced the fluorescence signal in the off-state. In the field-on state, however, the mesogens were aligned along the electrical field and the incident light traversed the sample less hindered yielding a weaker fluorescence signal.
Based on the related AIE-active dye CST-17, Lu et al. fabricated a switchable light-emitting liquid crystal display (Figure 21). For this purpose, 2 wt% of CST-17 were mixed with E7 and 2 wt% of the commercially available chiral dopant R6N. The test display allowed fast and reversible switching of the photoluminescence and transmittance with high contrast.
The Cheng group doped E7 with chiral (R/S) Bin-1. [54a] In this case, the (R/S) BIN-1 acted as chiral dopant due to its axial chirality and at the same time as AIE-active dye, which emitted at ∼540 nm ( Figure 22). Thin films of the LC mixtures showed aggregation-induced polarized luminescence with dissymmetry factors of up to ±0.4, which was attributed to dipolar interactions between the cyano-groups and π-π interactions of the binaphthyl unit with the biphenyl moiety of the liquid crystalline guest. Recently, the same group reported the tuning of the emission wavelength by using binapthol derivative BIN-2 as chiral dopant and a series of AIE-active dyes (CST-18-21) (Figure 22) in the nematic host E7. [54b] Changing the AIE-active dye allowed tuning the fluorescence wavelength from 403 to 610 nm with quantum efficiencies ranging from 7.55% to 20.42% ( Figure 22E). The dissymmetry factor by using BINOL-based dopants of opposite handedness was found to be ±1.4.
In 2019, Duan, Li, and coworkers reported a very similar system for CPL liquid crystals using binol derivatives as chiral dopant and fluorescent chromophores in 5CB. [55] In 2015, Wang et al. investigated the lasing capability of cholesteric liquid crystals doped with an AIE-active dye. [56] They doped E7 with the commercially available chiral binaphthol dopant DIX-1 (4.84 wt%) and added 1 wt% of an AIE-active dye-based TPS-3 ( Figure 23). The LC mixture was filled into a polyimide-coated test cell and the amplified spontaneous emission and lasing performance was investigated ( Figure 23). Preliminary results suggest the capability of the AIE-active dye doped cholesteric LC mixtures for application in lasing devices and revealed a large Stokes shift of ∼150 nm combined with a moderate threshold of 600 J/mm 2 . In addition, the authors were able to prove that focused high-intensity laser pulses do not reduce the performance of the test devices proving the high photostability of the AIE dye.
More recently, Guo and coworkers described another doping approach using the chiral cycanostilbenoid (CST-22) as photoswitch together with the chiral dopant CD-1 inside the commercially available SLC1717 as nematic LC host (Figure 23). [57] They were able to control the handedness of the cholesteric phase by UV light irradiation. Therefore, the left-handed switch CST-22 was combined with right-handed CD-1 in the host matrix to yield a cholesteric liquid crystalline (CLC) suprastructure determined by right-handed CD-1. Upon light irradiation (450 nm), the handedness of the helical structures was switched from right to left and back (365 nm) based on the reversible trans/cis isomerization of CST-22.
In 2019, Li et al. presented two light switchable emissive α-cycanostilbenes linked to binapthyl derivatives as  [58] To this end, these CSTs were embedded in an achiral commercially available nematic host LC (SLC-1717) and its photoisomerization ((Z)→(E)→(Z)) was studied by using alternating light sources (450 and 365 nm). Upon low-intensity irradiation at 450 nm (5 mW cm −2 ), a transparent LC cell was obtained. It was possible to write letters in this cell using 365 nm, which led to changes in the helical twisting power, inducing emission at 518 nm, which can be erased by prolonged irradiation with 450 nm light. This process was also monitored by a change in reflection color, which was found to be in the red portion of the electromagnetic spectrum (655 nm). Hence, the authors reported a dual mode transparent platform based on fluorescence and reflection readout. Another approach using cycanostilbenes for dual mode detection and readout was recently reported by Quin et al., who used CLC microdroplets as platform for the CST. [59] Hence, the dual mode detection can be attributed to F I G U R E 1 9 Mesogens based on tripodal pyridinium (PYR-1), pyrimidinium (PYM-1), and quinilinium (QUI-1), as well as aromatic thioethers (ATE-1-4), tetraphenylsiloles (TPS-1-2), and tertraphenylthiophenes (TPT-1-6) two phenomena: (1) the emission color is determined by the molecular structure of the CST moiety and (2) the structural color is influenced by the overall pitch length of the CLCs, leading to a reflection color tuning, based on the helical suprastructures. [59] The doping approach represents a suitable method to achieve mesogenic materials by using a well-known, often commercially available host matrices, typically with LC properties, in combination with emissive compounds from the AIE family ( Figure 3). Here, the main challenge remains a complete mixing of the two independent components, and hence avoiding phase separations, which remains a hot topic for future researchers. . Inset: Photos of the cells with the electric field "off" and "on." Adapted and reproduced with permission from Wiley-VCH, copyright 2016 [52]

SUPRAMOLECULAR APPROACH
Another efficient route toward AIE-active liquid crystalline materials is the use of noncovalent interactions for the formation of defined assemblies, which combine the properties of the individual building blocks and may generate new properties as a result of the supramolecular assembly process. Thus, in some examples, the liquid crystalline properties arise upon the self-assembly process of the molecular building blocks. In contrast to the previous routes to luminescent liquid crystals, the supramolecular approach is by far less established and investigated. In this regard, noncovalent interactions, such as metal-ligand interactions, ionic attraction, hydrogen-and halogen bonding, as well as the formation of charge transfer complexes, have been facilitated to generate emissive mesogens based on a noncovalent supramolecular approach.

Metal complexes
A common design principle in supramolecular chemistry is the employment of metal complexes to generate functional assemblies. [60] Fuijisawa et al. synthesized a series of gold(I) complexes with 4-alkoxyphenylisocyanide (AU-1-5) (Figure 24), which show smectic mesophases. [61] Crystal structures of the complexes clearly proved the crucial role of aurophilic interactions (Au•••Au = 3.31-3.38 Å, Figure 24B) in this system. Based on powder XRD measurements, the authors suggest the formation of dimers via aurophilic interactions with head-to-tail arrangement in the smectic phase.
The luminescence behavior of these complexes is controlled by the aggregated structures, and a reversible "onoff" switching of luminescence was observed upon phase transition between the ordered LC phase and the disordered isotropic state ( Figure 24C). In addition, the color of luminescence was reversibly controlled by thermal phase transition. The smectic C and Cr 1 phase shows deep blue luminescence, while recrystallization from the isotropic melt yielded a different crystalline phase (Cr x ), which displayed greenbluish luminescence. The authors attributed the change in color to a different packing of the mesogens. Recently, the same group reported related gold complexes with terminal siloxane groups and investigated the effect of the molecular packing on the luminescence behavior. [62] In this case, the authors were able to prove that the changes in the luminescence behavior can be attributed to the CH•••Au interactions causing the emission bands above 500 nm. In the smectic phase, these interactions did not play a significant role and the related luminescence bands vanished.
The groups of Xie, Bruce, and Wang designed a series of platinum complexes showing polarized phosphorescence ( Figure 25). [63] While complex PT-1 showed an enantiotropic nematic phase, the TPE-based analogue PT-2 shows a monotropic smectic A phase, which remains persisted at room temperature. A test cell with a PT-1: polyimide composite film showed a broad electroluminescence spectra with a polarized ratio of 1.33 with external quantum efficiencies between 1.1% and 1.8% ( Figure 25B). The TPE-based analogue PT-2, however, had lower emission efficiencies and inhibited the mesophase.  [64] The authors took advantage of the varying emission in the aggregated (orange) and nonaggregated (green) state of the materials and investigated the mechanochromic and solvatochromic behaviors ( Figure 26).
Wu et al. recently reported two ionic iridium-based metallomesogens IR-1 and IR-2 with smectic mesophases and investigated the impact of the linker between the metal complex and the mesogenic core on the mesomorphic behavior and the photophysical properties ( Figure 25). [65] Both complexes showed enantiotropic mesophases, where IR-2 displayed the lower transition temperatures due to the triethyleneglycol linkage. In THF-water mixtures exclusively, IR-1 showed AIE behavior, whereas IR-2 did not, which was attributed to the good solubility of IR-2 in water leading to a molecularly dissolved, freely moving (rotation and vibration) compound. Based on the metallomesogens, nondoped polarized OLEDs were fabricated showing a polarization ratio of 4, which is the highest value for metallomesogens reported so far. However, the luminous efficiencies of the metal complexes were low and need further improvement before being applied in optical devices.

Ionic assemblies
The Ren group investigated the mesomorphic and photophysical properties of ionic TPE-surfactant assemblies ( Figure 27). [66] They synthesized the tetrabenzoatefunctionalized TPE derivative TPE-18 and combined it with dimethyldioecylammonium bromide (DOAB). After ionic self-assembly, liquid crystalline materials with AIE properties were obtained. The assemblies showed a low ordered columnar phase at room temperature, which converted to higher ordered columnar structure upon heating. The solid-state quantum yield was found to be as high as Adapted and reproduced with permission from the Royal Society of Chemistry, copyright 2014 [61] 46%, further investigations on the thermoresponse of the photophysical properties were not reported. The same authors reported in 2017 a related system based on 4′,4′′,4′′′,4′′′′-(ethane-1,1,2,2-tetrayl)tetrabiphenyl-4-carboxylic-acid (H 4 ETTC). [67] The self-assembly of H 4 ETTC with DOAB (diemethyldioctadecyl-ammoniumbromide) yielded a supramolecular mesogen with AIE behavior . Compared to the previously reported TPE-18 system, the greater degree of conjugation in the TPE-19 complexes yielded a slight shift of the emission wavelength to 494 nm (compared to 480 nm) and improved fluorescence quantum yield of 66%.
In addition, solutions of the ETTC•DOAB complexes in water/ethanol mixtures (1:1, v:v) were used to sense selectively Cu 2+ ions by fluorescence quenching. The limit of detection concentration was found to be as low as 12.6 nM. F I G U R E 2 5 (A) Molecular structures of the metal mesogens based on platinum and iridium. (B) Polarized electroluminescence spectra of the complex PT-1. Adapted and reproduced with permission from the Royal Society of Chemistry, copyright 2018 [63]

Hydrogen-bonded assemblies
In 2016, Pathak et al. synthesized a series of star-shaped liquid crystalline gelators based on the stilbene-structure showing AIE activity (STI-1-5) in the gel state as well as in the mesophase and the solid state ( Figure 28A). [68] A crucial structural element of the mesogens is the amide linkages, which induces columnar stacking of the mesogens by intermolecular hydrogen bonding. A comprehensive structure-property relationship study revealed that the nature and thermal stability of the mesophase strongly depend on the number and length of the peripheral alkyl chains. The derivatives with two trialkoxy styrene units (STI-3-STI-5) showed broad hexagonal columnar mesophases down to room temperature, whereas STI-1 and STI-2, with two dialkoxy styrene units, revealed a broad crystalline phase over F I G U R E 2 6 (A) Molecular structure of PT-3. (B) Stimuli responsive behavior of PT-3. Photographs of (a) as prepared, (b) upon partial grinding with a pestle, (c) after complete mechanical agitation, (d) after the addition of a drop of acetone, and (e) upon complete treatment with acetone. UV light images were taken by excitation at 365 nm. (f) Emission spectra of PT-3 during two grinding/fuming cycles with vapors of acetone. (g) Writing-erasing-rewriting process taking advantage of the reversible self-assembly behavior. Adapted and reproduced with permission from Wiley-VCH, copyright 2019 [64] F I G U R E 2 7 Molecular structure of the ionic AIE-active mesogen TPE-18 and TPE-19 F I G U R E 2 8 (A) Molecular structures of the stilbene-related mesogens STI-1-5. (B) Molecular structures of the π-conjugated imidazoles (IM-1-4) as well as schematic depiction of the self-assembly process with 3,4,5-trialkoxybenzoic acids to form discotic assemblies. Adapted and reproduced with permission from the Royal Society of Chemistry, copyright 2019 [69] a large temperature range. Time-dependent UV-Vis measurements displayed an increase in the emission intensities upon gelation from n-hexadecane solutions and a slight red-shift indicating the formation of J-aggregates in the gel state.
Tian et al. followed a different approach by combining a series of π-conjugated imidazole moieties (IM-1-4) with 3,4,5-trialkoxybenzoic acids to form columnar mesophases via intermolecular hydrogen bonding ( Figure 28B). [69] By varying the core moieties, the authors were able to tune the color of the fluorescence from blue to cyan, to green and to yellow.
The Giese group reported a hydrogen-bonded mesogens based on the AIE-active luminophores introduced by Voskuhl and coworkers (e.g., ATE-5, Figure 29A). [70] Here, they mixed solutions of AIE-active aromatic thioesters (hydrogen bond donor) with azopyridines and stilbazole derivatives (hydrogen bond acceptors) to yield a ratio of 1:2 (HB donor: HB acceptor). Upon slow evaporation of the solvent, a series of liquid crystalline assemblies was obtained showing a bathochromic shift compared to the starting materials ( Figure 29A). The formation of the hydrogen-bonded assemblies was proven by IR-spectroscopy, and single crystal X-ray diffraction, POM, and DSC investigations indicate the presence of nematic or smectic phases, and temperaturedependent fluorescence measurements prove the enhancement of the AIE depending on the order of the assemblies in the corresponding mesohases (iso < nematic < crystalline).
In 2020, the Giese group reported on hydrogen-bonded liquid crystals, based on stilbazoles and hydroxybenzoic acid derivatives, revealing nematic mesophases, which showed reversible emission "on-off" behavior. Here, photoinitiated proton transfer from a carboxylic acid of hydroxybenzoic acid derivatives (hydrogen bond donor), using UV-light (λ ex = 405 nm), to the stilbazole moiety (hydrogen bond acceptor) was observed, leading to an emission "on." This process was found to be reversible upon prolonged heating to 150 • C. [71]

Other inter and intramolecular forces
In Section 2.2 ( Figure 14), we already reported a series of mesogens combining the AIE-activity of the CST unit and the mesogenic character of the triphenylene core.  [72] emission behavior as well as their liquid crystalline properties. Although the compounds revealed the expected AIE behavior, no mesomorphism was observed. [72] Interestingly, the formation of a charge transfer complex of TPE-21 with 2,4,7-trinitrofluorenone (TNF) yielded a hexagonal columnar phase. Since TNF is known to be an efficient fluorescence quencher, the TPE-21•TNF complexes did not show AIE. Very recently, the Giese group reported the formation of luminescent mesogens based on nitro-substituted alkylated CSTs, which were able to form supramolecular assemblies via halogen bonding. These supramolecular mesogens revealed nematic mesophases over different temperature ranges in dependence to their substitution pattern. Furthermore, bright green to yellow emission was observed for the bulk materials, which lose their emission upon heating, due to increased molecular motion. [73] The supramolecular approach combines elegantly the advantages of both, the synthetic and the doping approach by using the benefits of both. Here, complicated, timeconsuming syntheses can be avoided by using molecular recognition via noncovalent interactions, by simple mixing of the individual compounds. Additionally, the supramolecular complexes behave as single component avoiding potential phase separations influencing negatively the mesogenic properties of the materials by unwanted crystallization processes. We believe that this approach will be the strategy of choice for designing the next generation of highly adaptable and tunable materials with mesogenic properties.

CONCLUSIONS AND OUTLOOK
Within the past decades, compounds showing AIE have attracted much attention with respect to luminescent func-tional materials. Since AIE dyes do not suffer from ACQ, such as classic emitters, yielding a nonemissive solid state, they are promising candidates for the development of luminescent liquid crystals. These materials may find application in sensing of analytes, lasing applications, as well as the fabrication of a new generation of LC displays. Currently, three approaches are followed to achieve AIE-active LC materials. First, the synthetic approach, which yields luminescent mesogens by covalent linkage of a mesogenic core to one of the common AIE emitters (e.g., tetraphenylethene [TPE], cyanostilbene [CST], and tolane [TOL]). Second, the doping approach, which simply mixes the AIE-active dye with a commercially available liquid crystal as host compound. Finally, the supramolecular approach, which employs noncovalent interactions to construct defined assemblies, which combine the properties of the individual building blocks and may show additional properties, such as liquid crystallinity, as a result of the self-assembly approach. With respect to applications, especially the doping and supramolecular approaches are promising, since they provide easy access to a plethora of new materials and a simple way to tune the LC and photophysical properties of the systems. Based on the doping approach, a series of model devices to prove the potential in display technology and lasing applications have already been reported. Future research has now to overcome low PLQYs in the liquid crystalline or molten phase, thermal instability, compatibility issues of host guest systems, as well as high fabrication costs. Especially the gain of a deeper understanding concerning the design of AIE emitters for the fabrication of highly emissive mesogens is needed. The mode of action in AIE luminophores needs to be comprehensively understood. [74] In this respect, the restriction of motion seems to be a key parameter for the emission enhancement, and closely related, noncovalent interactions in the solid, liquid crystalline, and molten state have a significant impact on the photophysical properties. [75] Hence π-π contacts, known to be beneficial for the formation of liquid crystals, lead to the formation of nonradiative charge transfer complexes between the mesogens, decreasing the emission efficiency. Therefore, the main challenge in the future design of luminescent mesogens will be the careful orchestration of numerous noncovalent interactions to overcome low PLQYs in the solid or molten phase, thermal instability, or compatibility issues of the components. Besides that, the position of the luminophore in the mesogens (e.g., as central core unit, as ligand of a mesogen, or in the backbone/sidechain of a mesogenic polymer) plays a crucial role and is strongly dependent on the molecular structure of the luminophore used (see Section 2.2). We are convinced that a supramolecular perspective will contribute to address these challenges.

A C K N O W L E D G M E N T S
Jun.-Prof. Jens Voskuhl acknowledges the "Fonds der Chemischen Industrie (FCI)" for financial support. Additionally, Jun.-Prof. Michael Giese is thankful for generous financial support by the Professor-Werdelmann Stiftung and the Boehringer Ingelheim Foundation. The authors thank the Center for Nanointegration Duisburg-Essen (CENIDE) for funding.

C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.