AIE gel exhibiting continuous gradient fluorescence based on a polar‐responsive AIE‐gen

Traditional multicolor fluorescent hydrogels are generated through the assembly of discrete fluorescent hydrogels, which is not a complete integration much distinct from living organisms. On the basis of aggregation‐induced emission (AIE), a special solvent polar‐responsive AIE molecule possessing a twisted intramolecular charge transfer (TICT) effect was noticed. By incorporating it into the gel network, an AIE gel that displays continuous gradient fluorescence was fabricated. First, hydrogel A containing the solvent polar‐responsive AIE‐gen was prepared to show orange fluorescence. After soaking in the organic solvents, the fluorescence color transition of hydrogel A ranging from orange to green occurred when being immersed in high‐polarity organic solvents ascribed to the embedded AIE‐gen owning TICT effect. Then, hydrogel A was successively lifted up from organic solvents. Due to the different immersion time of each section for the hydrogel, the polarity difference occurred. Then, the produced gel B showed continuous gradient fluorescence ranging from orange to green under the irradiation of UV light.

Huazhong University of Science and Technology, Grant/Award Number: 2020MCF08

| INTRODUCTION
In nature, some enchanting multicolor phenomena, like an iridescent rainbow that one color melts into another color, are coming into people's sight, turning into a new spotlight for researchers generally.Over a long period of evolution, a great many living creatures have evolved into showing the appearance with abundant colors.As the most representative example, the chameleon is recognized as owning colorful skins, which takes on the color to match its surroundings for the sake of adapting to the new environment. 1Also, this particular multicolor characteristic of skins ensures itself getting rid of the attack from enemies.Apart from that, another fascinating creature, the peacock is acquainted for its splendid colorful tails, which can be employed as a sort of mating signal; more to the point, it is supposed to be the protective color to disturb the vision of predators, fulfilling to prevent the potential offensive threat. 2 At the bottom of the sea that down to thousands of feet, the clownfish wear distinctive white stripes on their skins.This special colorful pattern is conducive to build a mutualistic relationship with anemones, where external surfaces are in constant contacting with one another. 3dditionally, Lagopus lagopus is a typical kind of arctic bird, known for the bewitching feather colors that enable itself to be a perfect pretender for defending enemies.All these vivid colorful patterns of organisms have a specific purpose, mainly regarded as cryptic coloring for sheltering themselves.
][6] Among these reported studies, fluorescent hydrogel, 5,[7][8][9][10][11] emerging as a brilliant candidate for constructing biomimetic multicolor materials, has received widespread attention.3][14] Moreover, the remarkable fluorescence property is the key approach to mimic the multiple colors exhibited by creatures. 15,168][19] For example, inspired by the chameleon, Chen, Zhou, Lu, and coworkers 20 prepared distinct hydrogel blocks that exhibited diverse emission colors by introducing multiple fluorophores (blue and red/green lanthanide coordinated ones).After tailoring into specific shapes by a laser cutting machine, these discrete hydrogel blocks could be assembled to produce an integrated hydrogel showing diverse fluorescence colors, successfully fabricating the biomimetic colorful skins.Besides, Liao and coworkers 21 reported a poly (vinyl alcohol) hydrogel exhibiting dynamic fluorescence colors via incorporating dual-color fluorescent nanoparticles.Additionally, this kind of hydrogel is capable of being employed as the "building block" to construct different 2D or 3D structures owning various fluorescence.These fluorescent hydrogels are allowed to be assembled together to generate a holistic multicolor system, serving as a smart strategy for biorobots.These pioneering advances surely prompt the development of biomimetic multicolor materials; nevertheless, the obtained multicolor pattern results from the assembly of discrete fluorescent hydrogels but not a single integrated fluorescent hydrogel system (Scheme 1A, left).
][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] Electron-donating (D) and -accepting (A) units are often embedded into AIE-gens for triggering polarityresponsive emission changes assisted by the twisted intramolecular charge transfer (TICT) effect. 43,44On the basis of this special effect, we prepared a kind of AIE hydrogel, which then displayed continuous gradient fluorescence after being immersed in organic solvents based on a polar-responsive AIE-gen (Scheme 1A, right).First, hydrogel A consisting of N-isopropylacrylamide (NIPAM), triphenylamine vinyl pyridine acrylate (TVPA), and N,N′-methylenebisacrylamide (MBA) was fabricated, which showed orange fluorescence under the irradiation of UV light (Scheme 1B, left).Through immersing into organic solvents, hydrogel A was capable of showing variable fluorescence colors upon increasing the time.The immersion time of diverse regions for hydrogel was different thereby leading to distinct concentrations of the organic solvent entering the gel networks, thus causing the difference in polarity.Owing to the incorporation of polar-responsive TVPA assisted by the TICT effect, the resulting gel B displayed continuous gradient fluorescence colors ranging from orange to green under UV light (Scheme 1B, right).So, to manage the immersion time, hydrogel A was constantly being lifted up from mixed organic solvents at different time intervals (Scheme 1C).

| Synthesis and characterization of TVPA and hydrogel A
A water-soluable AIE-gen TVPA was synthesized; therein, the ethylene cores are linked to electron-donating triphenylamine and the electron-withdrawing pyridinium units, which enables TVPA owning molecular rotors, a D−A structure, and ionic character.The three-step synthetic route is presented in Supporting Information: Scheme S1.The reaction intermediates and TVPA were fully characterized by nuclear magnetic resonance as well as mass spectroscopies.As for the preparation of hydrogel, the synthesis process was conducted in water.Hydrogel A was generated via free radical copolymerization of NIPAM and AIE-gen TVPA, and MBA was utilized as the chemical crosslinker using redox initiation at room temperature with potassium peroxodisulfate and Thermo Scientific pierce tetramethylethylenediamine.The rheological experiment was conducted, as shown in Figure 1A.Hydrogel A exhibited little independence on frequency, demonstrating its solid-like properties.Meanwhile, the obtained results apparently revealed that the values of storage modulus (G′) of hydrogel A were higher than that of loss modulus (G″), consistent with the anticipated results of hydrogel formation. 45Also, the obtained scanning electron microscopeimage suggested that dense pores were generated, which gave direct support for the cross-linked structures of the hydrogel (Figure 1B).Then, the strain behavior of the hydrogel was investigated via tensile stress-strain measurements (Figure 1C).It could be clearly seen that hydrogel A could be stretched up with a higher strain value, and the corresponding photographs reflecting the tensile test were taken, as shown in Figure 1D.

| Characterization of hydrogel A immersed in a single kind of organic solvents
For the sake of exploring the fluorescent color transition, fluorescent photographs of hydrogel A immersed in different types of organic solvents were recorded (Figure 2).Meanwhile, the quantum yield of hydrogel A before absorbing the organic solvent was measured as 11.63%.First, hydrogel A was cut into small pieces of 1 cm × 1 cm.Then, these hydrogel patches were soaked into distinct kinds of organic solvents including dimethyl sulfoxide (DMSO), ethanol (EtOH), dimethylformamide (DMF), acetonitrile (ACN), tetrahydrofuran (THF), 1, 4-dioxane, and isopropyl alcohol (IPA).Apart from this, hydrogel A was soaked in nonpolar toluene (Supporting Information: Figure S8).With time increasing, fluorescent photographs of hydrogels were taken every 20 min.It could be clearly seen that the hydrogels soaked in DMSO, EtOH, DMF, as well as ACN, exhibited obvious fluorescence color transition.In DMSO, hydrogel A presented a transition trend ranging from orange to green.Meanwhile, the fluorescent colors of hydrogel A immersed in DMF also transformed from orange to green.Besides, as for immersing in EtOH, the photographs indicated that fluorescence colors changed from orange to yellow; likewise, in ACN, fluorescence colors turned yellow as well.With regard to other organic solvents, the fluorescence colors were almost unchanged.These obvious fluorescent color transitions of hydrogels occurred due to the embedded TVPA possessing polarityresponsive emission changes assisted by the TICT effect.In organic solvents of high polarity, TVPA exhibited a greater fluorescence color transition roughly ranging from orange to green.To further verify the fluorescence color changes, the fluorescent spectra of hydrogel A that was immersed in distinct kinds of organic solvents were recorded along with time increasing (500-700 nm).The recorded excitation wavelength is 365 nm.As shown in Figure 3, it could be found that the blue shift of 61 nm in maximum emission wavelength (λ max ) from 620 (orange) to 559 nm (green) was observed when hydrogel A was immersed in DMSO, which was consistent with the taken fluorescent photographs.Concerning about soaking in EtOH, λ max shifted from 620 (orange) to 578 nm (yellow).When hydrogel A was immersed in DMF, the blue shift of λ max ranging from 620 (orange) to 549 nm (green) was recorded.When the organic solvent changed to THF, λ max shifted from 620 (orange) to 578 nm (light orange).Regarding the other two types of organic solvents, in 1,4-dioxane, the blue shift of λ max was found to change from 620 to 600 nm, while in IPA, λ max shifted from 620 to 588 nm, both confirming that the fluorescence color merely remained orange.In addition, when the hydrogel patch was immersed in toluene, λ max merely unchanged with time increasing (Supporting Information: Figure S8).These obtained spectrum data were all in line with the photographs taken under UV light, further proving that the fluorescence color transformation in high-polarity solvents embodying DMSO, DMF, EtOH, and ACN occurred on account of the remarkable effect of TVPA.
For surveying the weight change, the weight of hydrogel A before and after being immersed in distinct organic solvents was measured (Figure 4); meanwhile, photographs of the hydrogel patches before and after being immersed were recorded (Supporting Information: Figure S9).As time increased, the weight change rate of hydrogel A soaked in DMSO increased up to 195.2%, demonstrating that the hydrogel patch was getting larger on account of more solvents infiltrating the hydrogel networks.With respect to being steeped in EtOH, the weight of the hydrogel patch after being immersed reached a value of nearly fourfold higher than that of before being immersed.As for soaking in DMF, it could be clearly found that the weight of the hydrogel patch after being immersed went up dramatically with the weight change rate reaching 388.1%.When the hydrogel patch was immersed in ACN, the weight change rate was relatively low compared with other organic solvents, and the value was around 91.1%.In THF, the weight change rate rose to 332.7%; likewise, in 1,4-dioxane and IPA, the values of the weight change rate climbed to 316.3% and 391.5%, respectively.Besides, in toluene, the weight of the hydrogel patch nearly remained the same (Supporting Information: Figure S8).These obtained results indicated that the polymeric molecular frameworks exhibited diverse affinity toward these respective organic solvents of distinct polarity, proving that some kinds of organic solvents including DMF are favored to get into the hydrogel networks, which provided possibilities to produce fluorescence transition.

| Characterization of hydrogel A immersed in the mixed organic solvents
A series of above-mentioned experiments concerning hydrogel A soaked in different single kinds of organic solvents were conducted.Then, it clearly suggested that some hydrogel patches that were immersed in highpolarity organic solvents like DMSO, DMF, EtOH, and ACN exhibited an obvious fluorescence transition, while the less noticeable fluorescence transition was detected as hydrogel patches were soaked in other types of organic solvents with relatively low polarity.In this way, four groups of mixed organic solvents were picked out including DMF and IPA, DMSO and IPA, DMF and 1,4-dioxane, and DMSO and 1,4-dioxane, for which the volume ratio of these mixed solvents is 1:1.Similarly, hydrogel A was cut up into patches with the size of 1 cm × 1 cm and subsequently soaked into these mixed organic solvents.Fluorescent photographs were taken for exploring the fluorescence color transition of hydrogel patches.As shown in the photographs, the fluorescence color transformed from orange to green when the hydrogel patch was immersed in DMF and IPA with time going by.With regard to being soaked in DMSO and IPA, the fluorescence color transition ranged from orange, then yellow-green, and finally to green.When the hydrogel patch was immersed in DMF and 1, 4-dioxane, the original orange fluorescence gradually transformed to green.As for soaking in DMSO and 1, 4-dioxane, likewise, the photographs indicated the fluorescence color transition ranging from orange to green.These obtained experimental data also gave direct evidence for proving the solvent polar-responsive property of TVPA (Figure 5).
As for further verifying the fluorescence color transition, the fluorescent spectra with regard to the hydrogel A immersed in these four groups of mixed organic solvents were recorded over time, for which the recorded excitation wavelength is 365 nm.From Figure 6A, it could be apparently seen that λ max shifted from 620 (orange) to 568 nm (green) when the hydrogel patch was immersed in DMF and IPA.The blue shift of 81 nm in λ max ranging from 620 (orange), then 574 (yellow), and finally, to 539 nm (green) was detected as the hydrogel patch was being soaked in DMSO and IPA.As for being immersed in DMF and 1,4-dioaxane, λ max shifted from 620 (orange) to 549 nm (green) with time going by.When hydrogel patches were soaked in DMSO and 1,4-dioaxane, the blue shift of 70 nm in λ max changing from 620 (orange) to 550 nm (green) could be obtained from the fluorescent spectra as well.These obtained data also proved that the fluorescence colors of hydrogel patches immersing in four groups of mixed organic solvents had changed within a certain range from orange to green.Due to the high polarity of mixed organic solvents, TVPA was capable of showing fluorescence responses, resulting in the fluorescence transition of hydrogel patches.
To investigate the weight change of the hydrogel immersed in mixed organic solvents, the weight of the hydrogel before and after being immersed was recorded (Figure 7), and the corresponding photographs were captured (Supporting Information: Figure S10).As immersed in DMF and IPA, the weight change rate reached 267.9%, confirming that the soaked hydrogel absorbed more organic solvents.When the hydrogel patch was soaked in DMSO and IPA, the weight change rate is up to 158.6%.With regard to being soaked in DMF and 1,4-dioxane, the graph suggested that the weight of the hydrogel patch after being immersed rose to a value of nearly threefold higher than that of before being immersed.As for soaking in DMSO and 1,4-dioxane, the weight change rate of the hydrogel patch grew up to 202.2%.These obtained results apparently suggested that upon immersing in mixed organic solvents, the polymeric molecular frameworks showed various affinity toward solvents; in this way, different kinds of mixed solvents have the inclination to penetrate into the network to generate polarity difference, thereby leading to fluorescence transition.

| Fabrication process of gel B via lifting up hydrogel A slowly from mixed organic solvents
These mentioned experiments confirmed that the fluorescence color of the hydrogel patch was able to change from orange to green when immersed in high-polarity solvents.Herein, four groups of mixed organic solvents were selected to prepare a gel.Therefore, hydrogel A was tailored into narrow strips with the shape of 7 cm long and 1 cm wide.Then, the hydrogel strip was fixed on the iron stand by using a clamp.At first, the hydrogel strip was soaked in mixed organic solvents of DMF and IPA around 6 cm long, and then, the timer starts; after 2 min passed, the hydrogel strip was lifted up to 1 cm high.Subsequently, when time passed for another 2 min, thus the hydrogel strip was again raised upward 1 cm.This step was repeated six times until the hydrogel strip was totally out of the mixed organic solvents.Then, the produced gel was irradiated with UV light of 365 nm and the fluorescent photograph was taken, as shown in Figure 8.The whole gel showed continuous gradient fluorescence ranging from orange, yellow to green, which was induced by the distinct immersion time of every corresponding hydrogel part.Due to the different soaking span time of the corresponding part, each section of the gel contained different amounts of solvent molecules, leading to the difference in polarity.The top of the gel had been immersed in solvents for the shortest time, while the bottom of the gel was immersed for the longest time.Thereupon, TVPA exhibited polarity responses, causing the generation of gradient fluorescence colors.In addition to that, other kinds of mixed organic solvents were also employed for fabricating this special AIE gel with continuous gradient fluorescence (Supporting Information: Figures S11-S13).As for immersing in DMSO and IPA, the displayed gradient fluorescence colors began with orange, then bright yellow, ending with yellow green.Apart from that, when the hydrogel was soaked in DMF and 1,4-dioxane, the fluorescent photographs exhibited consecutive gradient fluorescence colors transiting from orange to green.Different from these kinds of solvents, when immersed in DMSO and 1,4-dioxane, the obtained gel exhibited a complete fluorescence transformation from orange to yellow.

| CONCLUSION
In conclusion, a special AIE gel displaying continuous gradient fluorescence colors was prepared.First, hydrogel A was generated through free radical copolymerization of NIPAM, TVPA, as well as MBA.This generated hydrogel A was cut into pieces with the same specific shapes, being soaked into different types of organic solvents for an hour, embodying DMSO, EtOH, DMF, ACN, THF, 1,4-dioxane, and IPA.The fluorescent spectra as well as the photographs indicated that the transformation of fluorescent colors of hydrogel A varied on account of distinct polarity for organic solvents.In solvents of high polarity like DMSO and DMF, the fluorescent colors changed from orange to green.Along with the immersion time increasing, the hydrogel network was filling with more high-polarity solvents, for which TVPA was able to sensitively detect the polarity changes, displaying diverse fluorescence colors.As for the solvents of relatively low polarity like IPA and 1,4-dioxane, the fluorescent color transformation is not such obvious.Four groups of mixed organic solvents were selected, including DMF and IPA, DMSO and IPA, DMF and 1,4-dioxane, and DMSO and 1,4-dioxane.Likewise, hydrogel A was immersed into mixed organic solvents, and the taken fluorescent photos proved that the fluorescence color transitions present a trend from orange to green or yellow.Meanwhile, the recorded fluorescent spectra are consistent with these obtained results.In addition to that, the weight change rate of the hydrogel before and after being immersed in organic solvents was calculated, demonstrating that polymeric molecular frameworks showed distinct affinity toward organic solvents.In this way, hydrogel A was fixed on the clamp soaking in the mixed organic solvents, which had been pulled up from the solvents at the same speed until the whole gel was completely lifted out of the solvent.Under the irradiation of UV light, this generated gel B exhibited continuous gradient fluorescence within the scope of orange to green due to the TICT effect owned by TVPA.This strategy of preparing an AIE gel exhibiting continuous gradient fluorescence surmounted the barrier brought by the disunity of the traditional assembled multicolor materials.Furthermore, this special type of AIE gel offered more valuable insights into the application of smart materials, speeding up the development of vast fields embodying hydrogel materials, fluorescent materials, polymeric materials, as well as biomimetic materials.

S
C H E M E 1 (A) Preparation process comparations of multicolor fluorescent hydrogels between previous studies and this study, (B) chemical composition and cartoon representation of hydrogel A and the formation of gel B after hydrogel A is immersed in organic solvents to exhibit diverse fluorescence colors, and (C) cartoon representation of the fabrication process of gel B via lifting up hydrogel A slowly from mixed organic solvents, for which the lifting time lasted for 12 min.

F I G U R E 8
Photographs of the lifting process of hydrogel A from mixed organic solvents (dimethylformamide and isopropyl alcohol) at 0 min (A), 2 min (B), 4 min (C), 6 min (D), 8 min (E), 10 min (F), and 12 min (G).(H) Fluorescent photograh of the completely lifted gel.