Chinese Chemical Letters  2016, Vol. 27 Issue (8): 1155-1165   PDF    
π-Conjugated cyanostilbene-based optoelectric functional materials
Hang Cheng, Wu Hong-Wei, Zhu Liang-Liang     
State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, China
Abstract: π-Conjugated organic luminescent materials are essential components for modern optical and photoelectric research. This review mainly probes into the recent work in the progress of smart π-conjugated organic systems in the form of cyanostilbene and its derivatives, which can regulate its excellent features in response to a variety of physical or chemical stimuli (e.g. viscosity, light, magnetism, electric field, polarity, pH and solvent environment). As a result of its extensive applicability and adaptability, cyanostilbene and its derivatives have been planted into different structural architectures such as polymers, functional nanoparticles, solid membranes, supramolecular systems and so on. This review will first give a general description of the preparation and characterization of cyanostilbenebased optoelectric luminophores and then focus on their peculiar functional properties in the need for advanced material applications, such as AIEE (aggregation-induced enhanced emission effect), solidstate emission, photovoltaics, photolithography and photochromism to be further processed afterwards. The purpose of this review is to give a platform of practical organic materials, mostly cyanostilbene and its derivatives, based on stable aromatic derivatives, to contribute to the booming of modern π-conjugated photoelectric materials that integrate with contemporary physics, material chemistry, bioengineering, medical science and aerospace altogether.
Key words: Cyanostilbene     Multi-mode stimuli-response     Photoisomerization     Intelligent materials     Conjugated polymers     Supramolecular electronics    
1. Introduction

π-Conjugated organic luminescent materials have become a hot area of research in organic optoelectronics for years. With the efforts in recent decades, this research field has become gradually mature with much progress in their structural multiformity, thermal stability and photoelectric peculiarity. These excellent properties and tunable characteristics of them have been applied to the fabrication of OLED (Organic Light-Emitting Diode) [1], OFET (organic field effect transistor), chemosensors [2], bioimaging detectors [3], medical treatment products and so forth. In terms of their certain functional moieties which are necessary in their structures, the conventional π-conjugated organic/polymer species meet with relatively simple optoelectronic behavior. In order to strengthen its tunable properties and sensitivity to environmental changes, molecular organizations [4] and device fabrication techniques [5] with different luminophores have been utilized to acquire more specific photophysical properties. Moreover, numerous π-conjugated stimuli-responsive materials have also been emerging prosperously now and innovation comes in that these stimuli-responsive luminescent compounds [6] can be well tuned in a controlled form, for which the smart responsive optoelectronic substances have been regarded as seeded candidates for the development of new classes of intelligent optoelectric materials in recent few decades.

Scientists have been dedicated to the improvement of the stimuli-responsive functions of these luminophores to make them more advanced and pragmatic, in which the multi-responsive characteristic is one of the goals. Several tuning methods within a designed chemical system to realize different purposes are exploited, and the luminescent core structures can then be facilely adapted to be functionalized to construct macro-objects for diverse photophysical applications. As a result, in this short review, we will explore the structure characteristics and formation process of the cyanostilbene-based chemical material, and then discuss the versatile uses in α-cyanostilbene, which is the core structure for the construction of a series of high-efficient π-conjugated luminescent materials. To conclude, the subsequent prospects of the intelligent cyanostilbene-based organic materials will be further investigated to excavate its particular improvement possibility finally.

2. Cyanostilbene

The stilbene (1, 2-diphenylethylene), which is a kind of typical optical active symmetrical π-conjugated molecules, has been delved into sufficiently in recent decades for its diverse application in various areas. Its trans-cis [7] photoisomerization features like azobenzene are highlighted for the tunable competence, and with a unique fluorescent chromophore [8] it becomes very appealing luminophore studies. However, the quantum yield for the stilbene is relatively low as its emission band is mostly located in shortwave spectral region [9], which in the end confines its further application space, so it has been mostly regarded as a model structure in the research ofluminophore constitution. Fig. 1 gives us the chemical structures and the isomeric change of stilbene and its derivatives. To overcome its existing shortcomings in optical properties, the structural modification and pathway optimization of this luminogen have been employed to meet the plentiful needs in modern materials science.

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Figure 1. Chemical structures and isomeric change of (1) stilbene, (2) 4-cyanostilbene and (3) α-cyanostilbene and their corresponding trans-cis isomerization.

Cyanostilbene, a kind of derivative molecules of stilbene, is composed of a well-stretched π-conjugated molecular plane with a covalently linked -CN group in the para-position of the aromatic ring, which is shown in the chemical structures in Fig. 1 (2) and (3). The existence of cyano-group and other side-chain groups extends the π-electron cloud distribution of the stilbene and makes the conjugation system broader in the 4-cyanostilbene. Hence, the light emission of this molecule can efficiently cover the range of the visible region in spectrum and be monitored directly by the naked eyes. Many studies have focused on the improvement and utilization of this phenomenon, which is called twisted intramolecular charge transfer (TICT) [10], mostly in the core structure of 4-cyanostilbene containing an electron donor (i.e., amino group, julolidinyl group, dimethylamino group) in the opposite position of the cyano-group acceptor. The TICT effect can be significantly enhanced in those cyanostilbene compounds through the intramolecular rotation phenomenon in the central covalent bond when photoexcited, which is indicated in Fig. 1. Consequently, these compounds can exhibit dual fluorescent emission enhancement in many polar solvents, including local-excitation fluorescence (LE) and TICT fluorescence. This effect will produce a much higher dipole moment and result in controllable sensitivity to the external environmental changes like temperature and solvent polarity, and the 4-cyanostilbene derivatives have been applied to the indication of tiny environmental variation consequently as fluorescent probes.

Unfortunately, the 4-cyanostilbene compounds often suffer from the lack of active sites in its chemical structure, and these defects can lead to low possibility in the modification of the luminescent molecules for improvement in its post-synthetic properties. As an alternative, the α-cyanostilbene is created and prepared when the cyano-group is positioned at the α-position of the ethylenic linkage (chemical structure 3 in Fig. 1) [11]. This π-conjugated molecule still maintains the eminent properties from TICT effect and environmental sensitivity, and furthermore, it is light-active and can be easily modified with other chemical groups or systems to be adapted to the multi-functionality necessity of high-performance intelligent materials world. In this way, the α-cyanostilbene-based system shows more application potential than 4-cyanostilbene in its tunable chemical structures and diverse functionalizations. We will first categorize them into their versatile uses according to recent research progress and then demonstrate their potential of optoelectronic application development in contemporary research of advanced materials.

3. Optoelectronic functionality studies on α-cyanostilbene- based optoelectronic materials and its derivatives 3.1. Aggregation-induced enhanced emission (AIEE)

Many organic luminophores often show different optical emissive behaviors in dilute or condensed condition. The mechanism behind it is that their luminescent intensity is often weakened or quenched when the concentration rises because of the widely known phenomenon named concentration quenching or aggregation-caused quenching (ACQ) [12]. Contrary to the ACQ effect which lowers the practical luminescent efficiency, the aggregation-induced enhanced emission (AIEE) in some specific structures of organic compounds changed this situation. It was first discovered by Tang's group in their research work on pentaphe- nylsilole derivatives [13]. Afterwards, Park and his co-workers also observed the AIEE phenomenon in an α-cyanostilbene-based luminophore [14]. As the mechanism that the flat-dimensional molecular configuration brings about the restriction of spontaneous intramolecular covalent bond rotation in poor solvent has been a dominate cause of AIEE enhancement, many subsequent studies using the well-designed α-cyanostilbene prototype have been emerging with various synthetic and modification approaches [15].

It is worth mentioning that Park et al. developed a superior α- cyanostilbene-based compound which is a kind of low-molecular- mass organic gelator called CN-TFMBE showing strong AIEE phenomenon [16]. Although this molecule cannot gelate 1, 2- dichloroethane at relatively low concentration or high temperature, and the consequent solution shows little fluorescence emission (shown in the left vials of Fig. 2a and b), the emission can be significantly enhanced over 170 times in the gel state, which is indicated in the right vials of Fig. 2a and b.

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Figure 2. Strong fluorescence emission in organogel system based on the cyanostilbene derivative CN-TFMBE [16]. (a) Image of CN-TFMBE (0.8 wt/vol%) dissolved in 1, 2- dichloroethane at 60 ℃ (left vial) and the corresponding organogel at 20 ℃ (right vial). (b) Fluorescence emission of CN-TFMBE solution (left vial) and CN-TFMBE gel (right vial) under UV light (365 nm), the same vials as in (a). (c) SEM images of dried CN-TFMBE gel in 1, 2-dichloroethane solution. (d) Fluorescence microscopy images of CN-TFMBE organogel. The arrows in the inset photo indicate the node where the fibrous structures assembled from CN-TFMBE are bundled and knotted. (e) Photoluminescent (PL) spectra of CN-TFMBE (0.02 mmol L-1) in THF and its nanoparticle suspension. The PL intensities were normalized by the corresponding UV absorbance.

This phenomenon can be further verified in the fluorescence microscopy image in Fig. 2c and d. Furthermore, as a result of the cooperative influence of π-π stacking between the rigid cyanos- tilbene core structures, the CN-TFMBE luminescent compounds can be used as a gelator without the assistance of the side alkyl chains and steroidal groups. The molecular dipole moments which are induced by the four -CF3 groups at the terminal position and produce strong intermolecular interaction can also help distinguish the AIEE effect likewise. As the remarkable AIEE efficiency in this CN-TFMBE compounds is well accepted, the α-cyanostilbene model structure has been a research highlight for the design and fabrication of both multi-responsive soft intelligent materials and organic light-emitting materials.

The AIEE effect can also be found pervasively in other cyanostilbene analogs with new molecular structures and selfassembly forms [17]. In 2011, Neckers et at.reported a carbazolyl decorated cyanostilbene derivative molecule named CN-CPE [18]. It was observed that the weak luminescence of the dilute compound solution can be greatly enhanced when the water content increased from 60% to 70% in acetonitrile-water mixture solution, upon the aggregation and formation of about 2-3 nm sized microcrystals (Fig. 3). It is interesting that the resulting luminescence emission has undergone a blueshift when observed in the PL spectra graph (Fig. 3c). The conformational twisting in the crystal packing process has become a significant factor for the abnormal blueshift phenomenon of CN-CPE in aggregated condition. In general, the AIEE effect of this compound and its analog can be developed to be applied in highly sensitive and selective chemosensors for the detection of ions in a mixed system, and provides a feasible solution for ACQ in condensed condition.

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Figure 3. Carbazole-containing cyanostilbene microcrystals with high fluorescence emission [18]. (a) Chemical structure of CN-CPE. (b) Single crystal structure of CN-CPE. (c) PL spectra of CN-CPE in acetonitrile-water mixture solution, concentration = 10 mmol L-1, λex = 360 nm. (d) Variation in the integrated PL intensity of CN-CPE upon increasing water ratio in acetonitrile. (e) Picture of CN-CPE in acetone-water (40:60, v/v) mixture (left) and in pure acetone (right) under 365 nm UV light, concentration of CN- CPE =10 mmol L-1. (f) SEM image of CN-CPE in acetone-water (40:60, v/v) mixture. (g) Fluorescence microscopy image of CN-CPE microcrystals in acetone-water (40:60, v/v) mixture.

3.2. Near infrared bioimaging

Optical bioimaging is an important application direction in both the biological and the biomedical research fields. Near infrared light spectrum at around 650-900 nm is very beneficial to the biodetection usages of the AIEE model molecule for its low susceptibility to the self-interference of fluorescence from organisms, low damage to the living body, minimal interference absorption, low scattering rate and autofluorescence [19]. Hence, the construction of large π-conjugated system, which can shift the light emission to the near infrared section, has been an optimal pathway for the high-quality bioimaging technique.

Until now, many organic AIE fluorophores with highly twisted structures have been synthesized and their various applications in optoelectronic device and bioimaging have been explored. Park and his co-workers have synthesized a new kind of biopolymeric amphiphiles which were composed of the hydrophilic glycol chitosan (GC) as a backbone chain and the densely conjugated tricyanostilbene derivative (3CN) as a dipolar hydrophobic pendant [20]. The molecular design of 3CN has a specially designed dipolar electronic structure which enabled the emission band of the α-cyanostilbene skeleton to be shifted to near infrared range in the spectral region, and this advantage made it more adapted to bioimaging usage. The amphiphiles can also be self-assembled to prepare core-shell nanomicelles (GC3CNx) in water (Fig. 4a). It is found that if the ratio content of the 3CN assembled nanomicelles was increased in the amphiphilic biopolymer, the fluorescent emission can be redshifted and intensified correspondingly (Fig. 4c). Through in vitro and in vivo imaging experiments in Fig. 4b, we can conclude that this GC3CNx has a great potential in the NIR bioimaging.

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Figure 4. Tricyanostilbene-based biopolymeric hybrids for near-infrared bioimaging [20]. (a) Schematic representation of GC3CNx hybrid nanoparticles, where the feed ratio (x) varies from 5 mol% to 83 mol%. (b) Overlaid views of optical (gray) and pseudo-colored NIR fluorescence (colored) images of a live mouse subcutaneously injected with nanoparticle dispersions of (i) GC3CN20 and (ii) GC3CN50. (c) Normalized FL spectra of GC3CNx nanoparticles dispersed in water (1 mg mL-1). The 3CN feed ratio in GC3CNx is indicated by numbers in mol% (x = 5, 10, 20 and 50 mol%).

Another case is that if copolymerized with other organic skeleton chains, the tricyanostilbene-based material can reveal improved optical or biological properties. It is also reported by Tang et al. that AIE fluorophores with a red-shifted fluorescence emission could be obtained by conjugation between a normal AIE molecule (e.g. tetraphenylethylene) and ACQ fluorophores [21] . Shimizu et al.demonstrated that 1, 4-bis(diarylamino)-2, 5- bis(4-cyanophenylethenyl) benzenes can exhibit efficient NIR emissions in the solid state with a high quantum yield of 0.33 [22] . Most recently, Hui Gao and his group members have reported a new AIE fluorophore with efficient AIE emission and high quantum yield in NIR region (Fig. 5) [23].

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Figure 5. Structures of the A-π-D-π-A fluorophores 1-5 with distinct AIE attributes [23].

This highly-twisted molecular conformation is realized by combining the diphenylamine and cyano moieties inclusively in cyanostilbene-based skeleton molecular structure. Due to the unique AIE property for bioimaging, and the hindrance in ACQeffect of organic FL probes, they all show distinct characters in bioimaging with high electron transfer ability. In vitro and in vivoimaging experiments have also been taken by them to further demonstrate the potential of this NIR FL probes for bioimaging applications and biological treatment. These examples above give us a reminder that the modified α-cyanostilbene derivatives with steric hindrance moieties or functional polymer backbones can have more possibility for application in organic optoelectric functional materials.

3.3. Bulk heterojunction polymer solar cells

Polymer solar cells (PSCs) is a hot research topic in energy conversion and optical supramolecular materials today, distinguished for their mechanism in operation process and benefiting from their microscopic structures and compositions. Soluble fullerene derivatives are quite suitable materials for the design and fabrication of n-type semiconductors used in solution- processed optoelectric device [24]. Moreover, such n-type electron acceptor can also be coordinated with an organic π-type donor for the formation of bulk heterojunction (BHJ) active layer [25]. Now the bottleneck of high-efficiency PSCs lies in the creation of a new component which can heighten the LUMO energy to increase the open circuit voltage of PSCs.

A realistic example is that Mikroyannidis et al.prepared a modified fullerene F, with the grafting of nitro-α-cyanostilbene onto the [6, 6]-phenyl-C-61-butyric acid methyl ester (PCBM) (Fig. 6a) [26]. The F compound was more soluble in many organic solvents than its precursor PCBM thanks to its increased organic fragments in the structure. Therefore, F in both solutions or thin films condition showed stronger absorption than PCBM in the range of400-800 nm (indicated in Fig. 6b). From the analysis of the current-voltage measurements (Fig. 6c), we can conclude that through the participation of α-cyanostilbene region, the energy level of LUMO in F was 0.25 eV higher than the parent compound PCBM, which can strengthen the energy conversion efficiency of PSCs. In their further characterization work, they found the F-based BHJ-PSCs have maximum open circuit voltage of 0.86 V and minimum circuit current of 8.5 mA cm-2, which greatly increased the overall energy conversion efficiency (5.25%) of the fabricated PSCs. This excellent result has been constantly improved by many other cyanostilbene-based compounds with different modification approaches [27].

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Figure 6. Cyanostilbene modified acceptor in bulk heterojunction solar cells [26]. (a) Structures of PCBM and its cyanostilbene-modified derivative F. (b) UV-vis absorption spectra of PCBM and F in chloroform. (c) Current-voltage characteristics of the polymer solar cells based on P3HT as donor and PCBM or F as acceptor with a weight ratio of 1:1 deposited from chloroform solution.

3.4. Photoisomerization characterization of cyanostilbene derivatives 3.4.1. Switchable viscosity sensors

Viscosity, measurement of the resistance to gradual deformation by shear stress or tensile stress, is an important rheological parameter of a fluid, especially for those organic polymeric materials with relatively high molecular weight or in a biological environment for the reason that the unusual changes in the fluid viscosity of organisms or other biological systems can reflect their perturbations or diseases to some degree [28]. As the rheological data tend to become an important medical detection means for metabolic disturbance, the design of molecule-scale viscometer device has attracted the attention of many scientists and researchers globally. As we have talked above, the TICT fluorescence which can be used to serve as sensitive environmental indicators in complex systems has great potential in the precise tracking of viscosity. For the relation between florescence intensity and viscosity to be more practical and credible, the inner quantitative correlation modeled between the TICT fluorescence output and the viscosity statistic is required.

The π-(dialkylamino)-benzylidenemalonitrile derivative is a typical molecular rotor used in optical measurement, in which the fluorescence emission intensity can be increased dramatically according to the rise of the viscosity in solution [29]. The good news is that, in accordance with the calculation in Förster- Hoffmann equation, the logarithm of fluorescence intensity in this designed compound has a linear relation with the logarithm of viscosity [30]. More efforts have been dedicated by Tian and his coworkers in combining this viscosity-sensitive molecular rotor with the concept of molecular photo-switches through developing a new pathway using cyclodextrin-based polypseudorotaxane containing the traditional photo-responsive tetramethyl julolidinyl cyanostilbene unit (Fig. 7) [31]. Taking advantages of the reversible photoisomerization from the trans-cis conversion in the cyanos- tilbene framework, the viscosity-sensitive function can be either switched on or switched off with the induction from luminescence emission accordingly, and can also be effectively distinguished through different output mode of fluorescence signal.

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Figure 7. Julolidinyl cyanostilbene functionalized cyclodextrin polypseudorotaxane as a photo-switchable ratiometric viscosity sensor. (a) Structures of the polypseudorotaxane PRCD in its trans- and cis-form upon photoisomerization [31]. (b) Optimized conformations of (left) the trans- and (right) cis-form of the cyanostilbene moiety by B3LYP/6- 31G(d, p). (c) Emission spectra ofPRCD (6 μmol L-1, 298 K) in alkaline aqueous solutions (pH 11) under different concentrations (left) before and (right) after irradiation at 254 nm to photostationary state. The inset in the left spectrum is a double-logarithmic plot, demonstrating the ratio between the fluorescent intensity of PRCD and the corresponding viscosity.

The following computational studies using the B3LYP/6- 31G(d, p) in Fig. 7 indicate that the switchable control performance of the isomer pair comes from the existent TICT effect, which works within the photo-sensitive cyanostilbene moiety through the intramolecular rotation interaction of the pendant phenyl group. The transconformation in Fig. 7a was in a relatively free state with a large dihedral angle of 177.6°. Such rotation process in organic solvents was tightly related to the instant viscosity inside the system where the linear correlation was demonstrated clearly. Contrary to this result, when the photoisomer has been transformed to the cis-configuration, the dihedral angle would be reduced to only 9.2°, where the conformation of pendant group was stiffened in space and cannot be sensitive to the changes in viscosity any longer (Fig. 7b). This α-cyanostilbene-based unit was adapted to be bridged onto β-cyclodextrin (RCD), and then connected to be fabricated into a polypseudorotaxane (PRCD), resulting in a photo-switchable instant viscosity sensor in molecular scale. To sum up, this supramolecular material has two basic styles of sensitive channels, one is the free rotate conformation arising from the cyanostilbene itself which is efficiently susceptible to viscosity difference in solvents, and the other is the intramolecular excimer in steric rigid situation that cannot be sensitive to the outside viscosity. By means of photo excitation, a dual switchable viscosity-sensitive machine was synthesized and verified afterwards [32].

3.4.2. Photoconversion of multicolored luminescence for unimolecular selective imaging and labeling

While hybrid materials or heterogeneous systems have been applied pervasively in the in situtuning of multicolor emission [6], the realization of the effective switching and combination of luminescent color on a unimolecular scaffold still remains a great challenge because of the tough integration of several tunable luminophores with different color expressions [6]. Many researchers have been dedicated to the improvement of advanced bioimaging and labeling nowadays, and the design and synthesis of multicolored luminophores in hierarchically assembled unimolecular system have been one of the feasible solutions in this area since this integrated compound can realize the facile tuning of photophysical properties and is highly desirable for boosting the performance and versatility of in vivo detection and therapy.

As cyanostilbene and its derivatives are a group of light- responsive luminophores where the mutual converting of Z/E- isomerization in its phenyl group can usually be utilized for adjusting their emission frequency and intensity. In 2013, Zhao and his group members designed an α-cyanostilbene-based dyad on which the hierarchical self-assembly conformation and the disordered self-assembly conformation can be realized through the conversion between Z- and E-isomerization process through photoirradiation and high temperature (Fig. 8) [33]. Luminescent spectral studies revealed that compound 1 was able to exhibit a dual-fluorescence phenomenon, showing the appearance of a hypsochromically shifted and significantly intensified emission in the blue spectral region induced by Z-to-E photoisomerization of the core cyanostilbene unit. It was also found that in the mixed solvent of DMSO with 90% water, all molecules of compound 1 began to form aggregates and assembled superstructure, thus the hierarchical self-assembly of Z-formed 1 in the mixed solvent results in a significant bathochromic shift from green to yellow in the emission band. Continuous photoirradiation led to an increase in the ratio of the E- to Z-isomers, resulting in further enhancement of the emission intensity of this E-cyanostilbene unit and the corresponding changes of luminescent color from green to blue. The following in vitro cell experience with the assistance of fluorescence spectra and confocal microscopy further confirmed that the signal intensity and resolution of this modified cyanostilbene-based unimolecular platform can well accommodate the complex environment in body fluid. Above all, such spectrally tunable unimolecular entity presented a promising potential for the application of advanced selective bioimaging and labeling at intracellular level with relatively low cytotoxicity.

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Figure 8. (a) Chemical structures of compounds R1, R2, and 1, Z/E isomerization of the cyanostilbene unit in compound 1, and a schematic representation of the corresponding helical and disordered assemblies of the Z-isomer and E-isomer, assisted by solvent polarity. (b) Optimized conformational (i) Z- and (ii) E-forms of 1 at the B3LYP/6-31G* level of theory [35].

3.5. In situ photo-tunable white-light emission

White-light emissive materials are very promising candidates for display and screening in electronic facility due to their high color fidelity and low distortion [34]. To acquire the white light emission for practical application, innovative methodologies are demanded in its structure design and preparation technique. The in situ tuning for the white-light emission can simulate the formation of natural light conversion and advantageously substitute the conventional approaches of white-light acquisition process. So far, a few research work on the white-light formation using π- functional heterogeneous materials through in situ approaches have been reported [35]. In 2012, Zhao and his coworkers synthesized a luminescent chemical entity combining the thiol- containing dimethylamino α-cyanostilbene luminogen onto the surface of CdSe quantum dots through self-assembly approach (Fig. 9) [36]. Through photochemical and thermal regulation in the in situ controlling, the luminescent colors of the entity are capable of being tunable in intensity and efficiency reversibly, especially in the white light emission section.

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Figure 9. In situ photo-tunable luminescent color conversion of cyanostilbene-functionalized quantum dots [38]. (a) Representation of stimuli-responsive trans-cis isomerization of the cyanostilbene unit and its functionalized CdSe nanocrystal (CdSe570@1). (b) Absorption spectra and (c) emission spectra (λex = 365nm) of thiol- containing dimethylamino α-cyanostilbene (7.5 μmol L-1, 298 K) in MeOH at the initial state (curve 1) and after sufficient photoirradiation (curve 2). (d) Emission spectra (λex = 365nm)ofCdSe570@1(0.2mg mL-1 in ethylene glycol)at (1) the initial state and after photoirradiation at 254 nm for (2) 1h, (3) 3h, (4) 4.5 h, and (5) 6 h, and images of the corresponding states under a UV light (λex = 365 nm). (e) CIE 1931 chromaticity diagram illustrated from (d). The black dots signify the luminescent color coordinates for the corresponding states 1 (0.39, 0.48), 2 (0.37, 0.44), 3 (0.30, 0.35), 4 (0.26, 0.28), and 5 (0.23, 0.23).

We can see from Fig. 9c that the transform of the α- cyanostilbene-based unit suffered from a rigid and low- fluorescence condition. The quenching phenomenon in the structure resulted from its intramolecular rotation of the aromatic ring, which was embodied dynamically in the twisted conformation in organic solvents like the ethylene glycol or in the polymer-doped film state. In this case, the hybrid compound emitted the fluorescence in a greenish-yellow color, which was typically generated from the CdSe core inside (Fig. 9d). After photoirradiation at 254 nm for 3 h, the transconformation in this α-cyanostilbene-based entity would be partially inversed into cis-form, and this change would further make the emission of purplish-blue light become possible as the intramolecular rotation of the aromatic ring was restricted (Fig. 9c). The complementarity of the emission light from the nanoparticle core and the peripheral cyanostilbene group can create the stabilized white light in the end (Fig. 9d and e). If irradiated by the 254 nm UV for another 3 h, the transconformation of cyanostilbene-based isomers would be completely inversed and its fluorescent emission intensity would overwhelm that of the QD cores. The generation of the purplish-blue color was the result of this photo-conversion conformation. Through this method, in different ultraviolet radiation time, the luminescent color was changed from yellow, white to blue in a single entity by the in situ tuning manner. Moreover, the cyanostilbene-QD hybrid materials can be further modified with other organic fluorophores when serving as a switchable chromic center [37].

3.6. Mechanochromism tuned by molecular conformation

Mechanochromism refers to the change of color which occurs when chemicals are put under stress or high pressure in the solid state by mechanical grinding, crushing and milling. This kind of force-induced luminescence change has broad application in mechanical sensors, security papers, optical memory and opto- electric device [38]. It is interesting that most mechanochromic luminescent materials are endowed with AIE or AIEE effect, which can greatly restrict the ACQ in condensed phase [39].

It is reported that Tang et al. found the special mechanochromic behavior in tetraphenylethene and 9, 10-divinylanthra-cene derivatives, which was originated from the loose molecular stacking in the crystal states and would be destroyed under mechanical stimuli with the transformation of emitting color [40]. Their possible loose packing characteristic has made them one of the candidates for intelligent mechanochromic materials. Moreover, recently, Ran Lu and his group synthesized three D-π- A type phenothiazine modified triphenylacrylonitrile derivatives called PVTPAN, P3TPAN and P10TPAN (chemical structures seen in Fig. 10) [41]. When in their single crystal structure, the multiple intermolecular interactions (hydrogen bonding, π-π interactions, etc.) strongly locked the molecular conformation variation, which reduced the energy loss and led to enhanced light emission.

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Figure 10. Synthetic routes and chemical structures of PVTPAN, P3TPAN and P10TPAN [43].

Mechanochromism was embodied in different solid states under UV luminescence (indicated in the scheme of Fig. 11), and was also reversible upon the treatment of grinding and heating/ fuming with DCM. This mechanism between crystalline and amorphous states provides a new possibility for the design of controllable mechanochromic materials with tunable molecular conformations.

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Figure 11. Normalized fluorescent emission spectra of PVTPAN (a), P3TPAN (b) and P10TPAN (c) (λex = 400 nm) and XRD patterns of PVTPAN (d), P3TPAN (e) and P10TPAN (f) in different solid states [43]. Insets are photographs of PVTPAN, P3TPAN and P10TPAN in different solid states under UV radiation.

3.7. Cyanostilbene derivative molecular recognition through host- guest complexation

Host-guest complexation with macrocyclic structures is an important interaction within biological systems and the formation of supramolecular particles through incorporating a host part and a guest part in a single molecule [42]. Among them the water- soluble organic macrocycles and fluorescent guests have been used broadly for potential biological and environmental applications in the areas of sensing and signaling [43]. These self-assembled systems have various utilization in both topology and dynamic supramolecular functional materials. Similar to diverse process, most host-guest systems are constituted by weak interaction like hydrogen bonding and π-π stacking, and often possess capabilities like molecular recognition and complexing power.

Pillararene is a sort of fluorescence excitation molecule which is linked by the methylene groups in the para-positions of 2, 5- dialkoxybenzene rings, and is mainly divided into pillar[5]arenes [44] and pillar[6]arenes [45]. Because of the pillar architecture, they have been a renascent type of macrocyclic hosts after cyclodextrins [46], crown ethers [47], cavitands [48], calixarenes [49] and cucurbiturils [50]. It is foreseeable that the symmetrical structures and easy functionalization characters endow them with excellent adaptation in host-guest chemistry, especially with the complexation reaction with cyanostilbene-based materials [51].

Most recently, the preparation of nanoparticles with NIR emission enhanced by host-guest complexation between a hydrosoluble pillar[5]arene (WP5) and a cyanostilbene derivative in water has been reported [52]. As been said above, cyanostilbene derivatives absorb strongly in the visible region and can emit light brightly in the red to NIR range in its fluorescent spectrum, so it has been a compatible and suitable component for the fabrication of NIR AIE-active nanostructures. The nanoparticles also formed a dimer through host-guest complexation and greatly increased the luminescent intensity by aggregation and crystallization (the design and assembly process are shown in Fig. 12). Although the amphiphiles 1 self-assembled into nanoribbons with relatively weak fluorescent emission in water, driven by the interactions of π-π stacking between cyanostilbene groups and hydrophobic force, these nanoribbons can be transformed into nanoparticles after addition of water-soluble pillar[5]arene WP5 because of the formation of the supramolecular amphiphile WP51 and the following lower solubility of nanoparticles in water.

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Figure 12. (a) Structures and cartoon representations of 1, 1H, 2, WP5, and WP5H, and (b) Cartoon representations of self-assemblies of 1 and WP51 and pH responsiveness of nanoparticles prepared from WP51 [54].

The interaction can be assigned into an aggregation pathway. This result enhanced the NIR emission due to the strong complexation system induced by pillar[5]arene and modified cyanostilbene compounds. These fluorescent nanoparticles showed obvious pH-responsiveness because the coordinated system would collapse after treatment with acid. Furthermore, through the experiment in HeLa cells, minimal influence on cell viability and proliferation was observed, which proved that these nanoparticles can be safely utilized as an imaging agent for living cells due to their harmless NIR emission range. Other supramolec- ular host-guest interaction systems such as calixarene and viologen were also reported with controllable electrochemical and pH stimulus-responsive properties [53], and this multiresponsive function can also be extended between a viologen dication (MV2D) connector and a cyanostilbene fluorophore by means of host-stabilized charge-transfer (CT) interactions [54] . Enormous application potentials will be excavated in biological and pharmaceutical fields making use of these nanoscale aggregates, including cell imaging, drug and gene delivery, and biosensors.

4. Conclusion

Besides the above-mentioned responsive physical quantities (i.e., heat, solvent, viscosity, electric field and irradiation), the π- conjugated cyanostilbene-based organic materials with other functional groups also have application realization in many other chemosensing ensemble (ions, magnetism, etc.) through rational modulation process. With the matching of its steric conformation and size, cyanostilbene can also be inserted in the cavity of cyclodextrin and crown ethers to form a host-guest complexation, integrated as a functional supramolecular architecture and system [55] . Considering its distinct structural and electrical properties, the cyanostilbene compounds have also been explored and exploited in many synthetic methodology studies [2].

In this short review, we summarize a group of π-conjugated cyanostilbene-skeleton molecules, biopolymers, nanoaggregates and liquid crystals which are distinguished for their multi-responsibility, intensified luminescence, modifiability and tunable characteristics in the fluorescence emission process. Over the last few decades, large amounts of intelligent cyanostilbene-based materials and their derivatives have been developed for broad application in energy, biology, pharmacy, sensing and optoelectronics. These numerous achievements have brought new vigor and vitality into different research fields and covered many popular research areas in contemporary materials engineering world. Currently, scientists are committed to conducting more indepth work in the transformation of these multi-responsive materials so as to make them more mature and easier to operate in modern applicable devices. It is believable that all the efforts and devotion in the transformation of the π-conjugated luminescent systems to the high-tech responsive intelligent components can promote the well-being of us mankind and the whole environment in the future.

Acknowledgments We thank the Research Grant for Talent Introduction of Fudan University (No. JIH1717006) and National Program for Thousand Young Talents of China for financial support.
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