Aggregation‐induced emission materials for nonlinear optics

Aggregation‐induced emission (AIE) is a vital photophysical phenomenon that the luminogens in the concentrated or aggregated cases will engender the dramatically boosted emission in comparison with the dispersive states. Given this extraordinary emitting capacity exactly resolves the aggregation‐caused quenching (ACQ) situations residing in the traditional luminophores, the booming AIE luminogens have drawn tremendous interest owing to their advanced performances and colossal potential applications in various areas. Further exploitations of AIE molecules also drive the research interests in the midst of these AIE materials toward the nonlinear optical (NLO) regime. The combination of AIE and NLO effects have nurtured some unforeseen properties of AIE materials and extended their application spheres. Therefore, some NLO‐active AIE materials have been wielded in many crucial applications, for example, optical limiting, laser, bioimaging, and photodynamic therapy. Meanwhile, the impacts of aggregate on the NLO effect also deserve deep considerations and pursuits, and the modifications of aggregates promise an easy, efficient, and prompt avenue to tune the NLO properties of materials. The recent achievements and progress in the NLO properties of AIE materials have been summarized in this review. The second‐order and third‐order NLOs of the AIE materials have been introduced and their correlative applications have been discussed.


INTRODUCTION
The proposed concept of "aggregation-induced emission (AIE)," which the originally weak or nonemissive luminescent chromogens are induced to emit intensively at the aggregate states, blazes a new trail for the advancements and innovations of luminescent materials. [1] Hereafter, the spring and advanced theories including crystallization-induced emission (CIE), [2] clusterization-triggered emission (CTE), [3] and polymerization-induced emission (PIE), [4] with the AIE as basis and core, have also cultivated and accelerated the buildup of diverse and versatile AIE luminogens (AIEgens) and the corresponding luminescent materials. Certainly, in pace with the booming flourish and thrive of luminescent materials, the numerous aggregates of AIE materials, e.g., nanoparticles (NPs), [5] crystals, [6] clusters, [7] polymers, [8] supermolecules, [9] gels [10] and composites, [11] endow them with astonishing flexibility, marvelous tailoring capabilities, and even exceptional robustness. [12] Thereby, AIE materials not only share extensive attentions in the scope of solidstate emission, [13] but are also highly favored in optoelec-This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2021 The Authors. Aggregate published by John Wiley & Sons Australia, Ltd on behalf of South China University of Technology and AIE Institute tronic systems, [14] stimuli responses, [15] anticounterfeiting security, [16] biomedicine, [17] and sensors. [18] Heretofore, in the developed AIEgens, a portion of such luminogens have been anticipated or determined to be nonlinear optical (NLO) active with the charming NLO coefficients. In particular, the AIEgens featuring the D-A or D-π-A analogous structures, the intramolecular charge transfer (ICT) characteristics, [19,20] and transition dipole moment [21,22] may arouse the conversions toward the NLO activity. The ICT states can be depicted by two forms, i.e., neutral and zwitterionic forms, and the high NLO hyperpolarizabilities of these molecules will be acquired if the impact of the two forms is optimized. [23,24] In other words, the strength of donor and acceptor, the π-conjugated distance, and the symmetry exert the manifest effect on the NLO properties of the D-A systems. [25,26] Nevertheless, the emissions of these D-A compounds are always weak in the aggregate state, limiting the application greatly, and the AIE effect can resolve the predicament. [27] In addition, the D-A configurations also afford the diversified electrical and photonic activities to the AIEgens. [28] Additionally, the substantial explorations and investigations on the NLO properties of AIEgens have authenticated the effects of aggregationinduced enhanced NLO. The NLO diversifications derived from the diverse inherent aggregates in materials open up the innovate approaches to tailoring the NLO properties of materials conveniently and efficiently. The researches in the NLO properties of AIEgens also contribute to the extension of their applications and supply the opportunities to dig out, design, and develop the comprehensive and versatile AIE materials. In this review, we have retrospected the developments and advancements of these significant AIE materials. The chief NLO effects of AIE materials and the associated NLO applications have been highlighted and discussed. Ultimately, the state of the art and the prospect futural development tendency have been clarified.

Pure organic AIE materials
In 2001, Tang group discovered the first AIE molecule 1methyl-1,2,3,4,5-pentaphenylsilole (MPPS) with the core of silole and explored its turn from the weak luminogen in dilute solution into the strong emitter in aggregates. [29] Afterward, a mass of organic molecules, polymers, and their corresponding derivatives with AIE trait surged out, including hexaphenylsilole (HPS), tetraphenylethene (TPE), and Schiff-bases. [30] These compounds are often innate with aromatic moieties for the efficient emission and associated with twisted molecular conformations to suppress the intermolecular π-π stacking interactions. [1,31] In the aggregation states, the AIE processes are closely linked to the theoretical channels such as restriction of intramolecular rotation (RIR), twisted intramolecular charge transfer (TICT), [32,33] J-aggregation, [34] and excited-state intramolecular proton transfer (ESIPT). [35] Actually, the RIR is the primary factor to arouse the AIE effect. [36] HPS, a well-known AIEgen, can be regarded as the derivative of MPPS. [37] The six peripheral benzene rings can revolve round the central silole when molecularly dispersed in good solvents, but be blocked in the aggregation states such as crystal, film, or nanostructures, impeding the nonradiative energy loss and brightening the emission. The numerous substituents can serve to modify the silole structures via facile chemical reactions, and offer them with the plentiful AIE activities and other unique photophysical properties. [38] Furthermore, the AIE nature affords the silole derivatives with σ*-π* conjugation bearing low-lying lowest unoccupied molecular orbital (LUMO) levels, which are appropriate to the light-emitting diodes (LEDs). Nie et al. conceived and prepared the simplified and high-performance organic light-emitting diodes (OLEDs) in the light of the AIE and photonic characters of siloles. [39] The four silole derivatives, (PBI) 2 DMTPS, (PBI) 2 MPPS, (PPI) 2 DMTPS, and (PPI) 2 MPPS, composing of 2,3,4,5-tetraphenylsilole core, 1-phenyl-1H-benzo[d]imidazole (PBI), and 1-phenyl-1H-phenanthro[9,10-d]-imidazole (PPI) substituent groups were synthesized (Figure 1(A)). In contrast with the watergoverning THF/H 2 O mixtures, a high photoluminescence quantum yield Φ PL of 49.5%-62.1% in solid states was revealed, implying the splendid AIE attributes. In virtue of the synergistic effect of the silole and PBI units, high elec-tron mobilities of (PBI) 2 DMTPS and (PBI) 2 MPPS conferred them with superior electroluminescence (EL) performance in the nondoped OLED (Figure 1(C)). Chen et al. synthesized another three silole derivatives, (MesB) 2 DMTPS, (MesB) 2 MPPS, and (MesB) 2 HPS (Figure 1(D)), all carrying the dimesitylboryl groups, to manufacture the efficient OLEDs. [40] The three AIEgens emitted very weakly in the good solvents, while they became highly bright emitters in solid film with the upgraded Φ PL (56%, 58%, and 62% for (MesB) 2 DMTPS, (MesB) 2 MPPS, and (MesB) 2 HPS, respectively). The double-layer OLED device ITO/NPB (60 nm)/(MesB) 2 HPS (60 nm)/LiF (1 nm)/Al (100 nm) was afforded with protruding EL performance: 13.9 cd /A (maximum luminance), 4.35% (maximum external quantum efficiency), and 11.6 lm/W (maximum power efficiency), respectively. Moreover, the siloles frequently act as the central skeleton to bridge the chiral groups in chiral AIEgens. [41] . Ng et al. reported the AIE compound 1 with thiourea linkers and chiral phenylethanamine groups, whose solid state could radiate strong green fluorescence with the ultrahigh quantum efficiency of 95% compared with the molecular dissolution. [42] Interestingly, it was not circular dichroism (CD)-active upon photoexcitation, yet the visible CD and circularly polarized luminescence (CPL) signals could be detected upon the complexation with the particular chiral acids such as mandelic acid (Figure 1(I)). Liu et al. introduced the double mannose side chain to the HPS core via Click chemistry. [43] The resultant chiral AIE siloles gave the large dissymmetry factors (g em ) of −0.32 at the self-assembly aggregates in the confined microchannel surrounding.
TPE, an archetypal hydrocarbon of AIEgens, which boasts of a propeller-like conformation composed of the central olefin stator and terminal phenyl rings, can be obtained readily from one-step reaction. [44,45] In its dilute solution, the relatively free circumstance furnishes the essential prerequisites for the phenyl rings rotating smoothly on the C-H axis, and the intramolecular rotations deplete the energy of excited state dramatically, resulting in the fluorescent quenching. [44] Contrarily, in the aggregation/solid state, the rotations and π-twist are obstructed by the dense intermolecular C-H•••π interactions, and thus the radioactive channels are rejuvenated, which provokes the enhanced luminescence. [46,47] Hitherto, TPE and its derivatives still account for the majority of AIE-active molecules. [44,48] Aside from the AIE behaviors, TPE-based AIEgens are often susceptible to the external stimuli. [49,50] Xie et al. acquired a simple TPE hydrocarbon TETPE (1,1,2,2tetrakis(4-ethynylphenyl) ethene) via McMurry Coupling reaction, [51] implying the mechanoluminescence (ML) analogous to other heteroatom TPE compounds. [49] Liu et al. coupled TPE with anthracene (AN) to gain the AIE molecule 9-(3-(1,2,2-triphenylvinyl)phenyl)-anthracene (mTPA-AN) via Suzuki reaction. [52] Distinguished from the pressure-induced red-shifted and subdued emission of the common organic piezochromic materials, its emission underwent an abnormal blue-shift (Figure 2(A)). In the crystals of mTPE-AN, the TPE units hindered the π-π interactions between the two adjacent AN dimer, the complete energy transfer (ET) generating the single emission band from the AN excimer. Once the pressure exceeded 1.23 GPa, the compact aggregate of TPE units not only inspired the AIE mechanism for the raised emission, but also suppressed the ET  [39] (D-F) Reproduced with permission: Copyright 2014, Wiley-VCH. [40] (H and I) Reproduced with permission: Copyright 2014, Royal Society of Chemistry [42] interaction, contributing to the high-energy emission wavelength. For the sake of realizing multidimensional anticounterfeiting, Huang et al. designed a TPE derivative compound 2, satisfying the requirements for multiresponses synchronously. [16] The compound 2 exhibited classical AIE attribute in the THF/H 2 O mixture. From its crystal structure, it could be found that the interactions between the four phenyl rings were very weak, encouraging its loose packing in the crystals and enabling its sensitivity to the external stimuli. After grounding the powder (2p-o) of compound 2, the emission (λ em = 450 nm, Φ F = 24.8%) of ground powder (2p-g) was unaltered. When 2p-p was fumed by dichloromethane vapor within 10 s (2p-f), the original cyan emission was transformed into the blue-emission. Annealing of 2p-g and 2p-f at 120 • C resulted in the immediate quenching of their bright fluorescence (Figure 2(F)). Both the annealed powder (2p-h) and the crystal of compound 2 experienced reversible color switch to deep red after the 365 nm irradiating within 1 min. The UV/Vis reflection spectra suggested that the photochromism of compound 2 might be ascribed to photocyclization of stilbene units (Figure 2(G)). Actually, the deeper explorations concerning AIE mechanism of TPE demonstrate the impact of E/Z isomerization (EZI) on AIE process. [53] Tian et al. reported several pairs of configuration-controllable AIE E/Z isomers revolving around TPE, and all the Z isomers showed obvious piezofluorochromic phenomena after grounding. [54] Wang et al. prepared the TPEcored luminogen 1,2-bis{4-[1-(6-phenoxyhexyl)-4-(1,2,3triazol)yl]phenyl}-1,2-diphenylethene (BPHTATPE) by the copper-catalyzed Click reaction and successfully isolated the two pure E/Z stereoisomers. [53] The E/Z-BPHTATPE expressed the astonishing AIE effect (α AIE ≥ 322) with the marked Φ F of 100% (Figure 2(H)). The EZI process of the two isomers could occur with their exposure to the UV light at the temperature excessing 200 • C. The crystals of the Z isomer assembled in the poor crystallization quality. In comparison, the E encounter could self-organize into the high-order microstructures. The grinding, exerting pressure and fuming all could cause the varieties of their emission spectra.
Polymerization is a seminal route to manipulate the light emitting properties of luminescent materials, and the polymer networks also furnish the chromophores with braced frames, mechanical and chemical protections to engineer the smart, stable and flexible luminescent materials. [55] Suleymanov et al. prepared the TPE-Tr (triphenylethenyl triazene) from the swift and versatile acid-induced coupling. [56] The TPE groups could be grafted to the polymers with arenes directly by the vinylation procedures, for instance, harvesting  [52] (F and G) Reproduced with permission: Copyright 2019, Wiley-VCH. [16] (H) Reproduced with permission: Copyright 2012, American Chemical Society. [53] (I) Reproduced with permission: Copyright 2019, Wiley-VCH. [59] (J) Reproduced with permission: Copyright 2019, Wiley-VCH [60] the TPE-polymers: TPE-PS (polystyrene), TPE-PC (parylene C), and TPE-PVK (polyvinylcarbazole) conveniently. The variant swelling solvents had the appreciable influence on the fluorescence intensity of polymer TPE-PS suspensions for the reason of the varied conformational freedom of the TPE groups in solvents. Imitating the preparation of graphdiyne, [57,58] Liu et al. polymerized the TPE groups through the diyne coupling to fashion the two-dimension (2D) fluorescent polymer (Figure 2(I)). [59] In this 2D structure, the sufficient acetylene bonds locked the TPE building blocks, leading to its highly luminescent property. Wang et al. built the supramolecular polymer networks (SPNs) for efficient AIE regulation with the TPE-based tetratopic guests and pillar [5]arene polymer hosts (Figure 2(J)). [60] The noncovalent cross-linking in the host-guest systems could adjust the fluorescence intensity by changing the guest binding sites, pillar [5]arene unit density, solvent, or temperature. Thus, the considerably high Φ F of 98.22% could be reached.
Schiff-bases AIE materials are affiliated to the heteroatomcontaining AIE family, and the superiorities including fairly simplified synthetic procedures, [61] convenient purification process, and variable AIE responses render them to be geared toward the diversified spheres. Especially, the easy coordination with metal ions enable them to be satisfactory candidates for developing fluorescent probes. Xie et al. developed the TPE-functionalized salicylaldehyde Schiff-base TPE-An-Py via the Knoevenagel condensation with the yield of 60%. [61] Its emission behaviors in the THF, THF/H 2 O, and solid state certificated the aggregation-induced enhanced emission (AIEE) and the typical ESIPT process. This TPE-An-Py fluorescent probe, whose solution turned yellow from colorless under sunlight and fluorescence was quenched evidently, signified the specificity and selectivity for the detection of Cu 2+ (Figure 3(A)). In the practical water samples, the TPE-An-Py could receive the content recoveries with the low detection limit of 2.36 × 10 -7 M. Chai et al. explored the optical properties of the Schiff-base compound 3 on the basis of its AIE effect. [62] In the crystal state, the Schiff-base 3 exhibited the reversible color switch from the colorless to the apparent yellow upon UV irradiation (Figure 3 [61] (B and C) Reproduced with permission: Copyright 2018, Royal Society of Chemistry. [62] (D) Reproduced with permission: Copyright 2019, Nature. [67] (E-G) Reproduced with permission: Copyright 2018, Royal Society of Chemistry [68] to the formation of the strong combination of Cu 2+ to the Schiff-base ( Figure 3(C)).
Generally, these traditional AIEgens are often devised with the single or multiple aromatic motifs considering the strong electron conjugation, [63,64] yet there are also some peculiar AIEgens without aromatic systems. [65,66] Zhao et al. unfolded the typical AIE effect form the extraordinarily nonaromatic annulene derivative cyclooctatetrathiophene (COTh). [67] The akin propeller-like conformations in TPE or HPS were not retained in the COTh. Beyond this, the COTh was a conformationally chiral structure. Thus, the CD and CPL spectroscopy could serve to reflect the dynamical change of the chiral properties. In the THF solution, the CD signals of the enantiomers gradually degraded and almost vanished after UV irradiation for 14 min (Figure 3(D)). However, in the solid state of COTh, the CD signals illustrated no obvious difference. Similar situation also occurred in the CPL spectroscopy, clarifying confined conformation inversion in the solid state.
The discovery of the nonaromatic AIEgen supplied a brand-new strategy to tune the molecular vibration in AIE systems. Fang et al. heeded the persistent room temperature phosphorescence (RTP) from the nonaromatic organic compound cyanoacetic acid (CAA) with the considerably low molecular weight of 85. [68] The green RTP emission with a long lifetime of 0.862 s and RTP quantum efficiency of 2.1% could be spotted from the CAA crystal ( Figure 3(G)). On the contrary, with the concentration quenching effect of the ordinary luminophores, the fluorescence and phosphorescence were both converted from the nonemission in dilute solution to the brilliant emission at high concentration, signifying that the aggregation played the key role in the formation of AIE and the persistent RTP. The analysis of CAA crystal confirmed the hydrogen bonds between carbonyl and hydroxyl groups in the packing mode facilitated the restrictions of intramolecular motions and the boosted phosphorescence and fluorescence in aggregates. Zheng et al. reported a highly twisted nonaromatic AIEgens with tunable optical behaviors, consisting of acylated succinimides. [69] At room temperature, the photoluminescence and afterglows of AIEgens N,N'-carbonylbissuc-cinimide (CBSI), and N,N'-oxalylbissuccinimide (OBSI) in crystal state varied Society. [88] (C and D) Reproduced with permission: Copyright 2020, Wiley-VCH. [91] (E and F) Reproduced with permission: Copyright 2019, American Chemical Society [93] along with the change of excitation wavelength (λ ex ), which was more apparently at cryogenic conditions. The crystal structure analyses disclosed that the effective through-space conjugation (TSC) from the n/π electrons facilitated the formation of diverse clusters, resulting in the tunable optical properties. In summary, there may not be integral π-conjugation residing in these irregulated AIEgens, but some substituted groups, e.g., carbonyl, imide, cyano with lone pair electrons will encourage the electron delocalization and establish the valid and stable TSC in the rigid molecular aggregates. [70,71] With regard to some other nonaromatic AIEgens, the vibration deriving from the aromaticity reversal [72] will be suppressed in the aggregates, [73] rendering the bright emission.

Organometallic AIE materials
Similar to the Schiff bases, some of the AIEgens have been incorporated into the constituents of organometallic materials such as the metal-organic frameworks (MOFs) [74][75][76][77] and metallosupramolecules, [78] to forge the luminescent materials with the tunable optical properties [79,80] owing to the tunable structures and variable compositions. [81,82] The insert of varied metal ions into organometallic AIE materials will draw forth some special electronic and optical properties. For example, the easily accessible transition from the S 1 to the T 1 leading to the phosphoresce, [83] which is usually tough to achieve for pure organic AIEgens. Furthermore, the sundry functional groups can be incorporated into the AIE organometallics due to the diversity of the metal ions and organic ligands, and thus tune the quantum yields, lifetime or emissive color more flexibly. [74] However, the high preparation cost and the inevitable toxicity [84] are also the challenges that cannot be ignored.
Different from the plenitude of organic AIE molecules, the AIE metallosupramolecules [9] are relatively rare owing to the finite combination modes of AIEgens with metallosupramolecular architectures. [92] Liu et al. assembled a string of organoplatinum isostructural metallaprisms consisting of the central Pt(II), linker 4,4′-bipyridine, and the triangular ligands [tris(4-pyridyl)benzene (abbreviated as tpb) and tris(4-pyridyl)triazine (abbreviated as tpt)]. [93] The different coordination sequences also induced to generate the varied complexes ( Figure 4(E)). The emission spectra of tpb-based cis-1, trans-1, and the tpt-based cis-2 complexes affirmed the AIE emission upon expanding the hexane content in CH 2 Cl 2 solutions traced to the phenyl rotation hampered at the metal corners. The trans-2 was not AIE-active either in the solution or in the solid, ascribed to the subtle structural alterations between cis-2 and trans-2, which gave rise to the emission quenching ( Figure 4(F)).

AIE MATERIALS FOR NLO
In the light of the novel and amusing effect, AIE materials have drawn numerous and continuing attention since the initial discovery in 2001. In the area of solid-state emission, [94,95] the emergence of AIE materials derives intriguing phenomena such as aggregation-induced phosphorescence [96,97] and aggregation-induced delayed fluorescence (AIDF). [98,99] Nay, it is not content with the original and single investigations concerning AIE phenomena, but puts sight on the researches of aggregates. [100] Thus, some new and intriguing properties have been certificated and exploited, vastly employed in the myriad domains, involving therapy, [101] imaging, [102] solar cell, [14] chemical probe, [103] photodetector, [104] and data storage, [105] and so forth. [106,107] However, the majority of these applications about AIEgens revolve round the linear optics regime, hampering the indepth explorations of their applications. [108,109] The NLO responses of materials can only be spotted on the occasion of the intense light such as laser. [110,111] While the intense light impinges upon the optical media, the interaction between light and matter will arouse the relative displacement of electrons, impacting the impinging light conversely. [112] The complicated interferences between them lead to numerous NLO effects, such as second-harmonic generation (SHG), [113] sum-frequency generation (SFG), [114] optical rectification, [115] third-harmonic generation (THG), [116,117] two-photon absorption (2PA), [118] two-photon luminescence (2PL), [119] four-wave mixing (FWG), [120] optical Kerr effect, [121] saturable absorption (SA), [58] and so forth. Actually, some typical AIEgens, such as TPE, [109] have been anticipated and manifested to possess the NLO properties. Furthermore, the transformations and modifications of aggregate states can also be a facile and accessible channel to manipulate the inherent NLO and other properties [100] of AIE materials. It has been demonstrated that the aggregate of AIE materials can elevate the relative NLO responses, and can also lead to the switch of the NLO effect. [122] The conversion of the orientation and strength of molecular dipoles in AIEgens might be the main inducement for the variation of NLO effects.

Second-order NLO in AIE materials
The quadratic optical susceptibility of materials, i.e., χ (2) , is tightly correlative to the molecular hyperpolarizability β. [123] Noted that the β converges toward zero for the centrosymmetric material systems whereby it demands the asymmetry or noncentrosymmetric space groups for second-order NLOs. [124][125][126] SHG is the most developed and vital second-order NLO process in which the output frequency is twofold of the incident photon field. [127] Some of AIE molecules have been deemed to have the potential for second-order NLO and forecasted the SHG activity. For instance, Liu et al. designed the helical TPE compound with the inherent chirality and fixed propeller-like conformation, and predicted its second-order NLO properties via density functional theory (DFT) and time-dependent DFT calculations. [128] Nevertheless, the realistic reports about AIE molecules exhibiting SHG responses are rare. The classical AIEgen TPE also has been certified to be the SHG active. Astonishingly, the TPE molecules can crystallize in a polar P 21 space group, [1] talented to manifest the exceptional even-order NLO effects. In this context, Xiong et al. explored the NLO behaviors of TPE AIEgen. Noteworthy that the TPE microcrystals expressed the fantastic wavelength-dependent NLO effects that the various NLO phenomena resided in the diverse wavelength ranges. [109] Upon femtosecond (fs) laser excitations between 840 nm and 970 nm, the explicit and sharp SHG signals were captured ( Figure 5(B)). While the incident and the two-photon excited fluorescence character arose as the beam wavelength was below 800 nm. Moreover, the halogen substituted TPE molecules, including 4Br-TPE and 4I-TPE, and the correlate high-quality crystals could be acquired through the ordinary solvent evaporation (THF/n-hexane). The resultant 4X-TPE crystals also formed with a noncentrosymmetric space group (P2 1 2 1 2 1 ) ( Figure 5(C)). Analogously, the NLO emissions of 4X-TPE also evinced the remarkable wavelengthdependent ( Figure 5(D)), power-dependent ( Figure 5(E)), and polarization-dependent ( Figure 5(F)) NLO properties. For the wavelength longer than 800 nm, the NLO emission primarily behaved as SHG, and meanwhile, the 2PF emission dominated as the wavelength was between 740 and 800 nm.
Apart from the traditional AIEgen TPE derivatives, the fluorenone and its derivatives also have been certified to be  [109] the SHG active and tunable optical properties. [129][130][131] For the sake of the SHG active, some push-pull π-conjugated molecules with the moderate dipole moments are utilized as organic NLO materials since the large magnitude of the molecular dipole moment might lead to the centrosymmetric packing modes. [132] Duan et al. devised and synthesized a hexaphenylene derivative 2,7-di([1,1′-biphenyl]-4-yl)-fluorenone (abbreviated to 4-DBpFO) via introducing the carbonyl group into the original p-hexaphenylene (p-6P) matrix ( Figure 6(A)). [108] In the dilute CHCl 3 solution, the 4-DBpFO radiated the yellow-red emission at around 595 nm with a short lifetime of 1.9 ns. It exhibited the brighter green-yellow emission in the aggregated state, indicating the AIE characteristics. The corresponding quantum yield of fluorescence was dramatically elevated from 3.9% to 35.8% under the excitation at 440 nm, ascribed to the restricted rotations of the phenyl rings. In contrast to the p-6P skeleton featuring centrosymmetric configuration, the added carbonyl group broke the limit of centrosymmetry and also introduced a permanent dipole perpendicular to the long axis of molecules. Thus, the minor variation resulted in the SHG responses. Moreover, the disparate morphologies (microplate and microbelt) in crystal aggregate also exhibited the distinct SHG responses at 400 nm and 2PF responses at around 550 nm (Figure 6(C)). Wavelength-dependent SHG spectra from 770 to 960 nm showcased the maximum SHG signal at 445 nm ( Figure 6(D)). The SHG response of 4-DBpFO microplates indicated twice or three times stronger than its analogue diphenylfluorenone (Figure 6(E)). In addition, the 4-DBpFO crystal exhibited excellent laser damage threshold of 4.8 × 10 5 W cm -2 at the 880 and 970 nm pump laser.
The push-pull structures or D-π-A systems are prevalent in AIE molecules owing to their standout properties, [133,134] e.g., strong and tunable intramolecular charge transfer, [,135] highly ordered crystal packings or assemblies. [136] Besides, being sensitive to polarity, the introduction of push-pull structures is an effective strategy to improve the molecular hyperpolarizability efficiently and induce the NLO hyperpolarizability. [137] Jiang et al. reported a push-pull compound 4 based on the diphenylamine (DPA) donor and dicyanovinyl acceptor. [138] The synergy mechanism between hydrophilic oligo-oxyethylene chain and hydrophobic dicyanovinyl groups conferred it amphiphilic properties ( Figure 6(F)), beneficial to the formation of mechanostimulable properties. The emitting fluctuation of this molecule in aggregation was examined in THF/H 2 O system, and the recorded spectra suggested the aggregation-caused quenching (ACQ) of the long-wavelength luminescence as well as the AIE of short-wavelength emission. Crystal structure analysis revealed the noncentrosymmetric arrangements from the highly dipolar push-pull structure, facilitating the conspicuous SHG. The more elaborate SHG effects of the D-π-A system indicated the mechano-or thermal-stimulated SHG responses (Figure 6(H)). In the D-π-A systems, the modification of organic groups always plays a key role in modulating the physical characteristics and shapes of molecules. David H (compound 5), NO 2 (compound 6)) launched dense fluorescence upon aggregation. [139] The PL spectra and the time-resolved luminescence spectra of ferrocenyl Schiff-bases in acetonitrile-water system disclosed that the  [108] (F-H) Reproduced with permission: Copyright 2015, WILEY-VCH [138] enhanced emission intensity and longer fluorescence lifetime at a water content of 90%. Kurtz-Perry powder method was utilized to confirm the SHG efficiency of the crystals of compound 5 and 6. Surprisingly, the centrosymmetric crystal 6 (P 21 /c space group) output the unexpected SHG response at 532 nm in virtue of the noncovalent (C-H•••π) interactions. In contrast to the Schiff-base 5 without SHG signals, the existence of strong electron-withdrawing group (-NO 2 ) extremely hoisted the NLO property for compound 6, 1.46 times higher than the standard urea material.
Actually, the introduction of chiral groups is a convenient avenue to manufacture the noncentrosymmetric organization of the molecules. Therefore, a mass of chiral AIEgens are most likely to manifest the second-order NLO behaviors. Besides, the chiral AIEgens are also the hot spots in the CPL, which is tightly bound to the optoelectrical devices, threedimensional (3D) display, and information storage. [140,141] The superior CPL not only need the chirality but also require the bright emission at the solid state, and the chiral AIEgens with large g em can stimulate the marked CPL signals assisted by the AIE effect, reflecting the aggregation-induced CPL. [41] Jiang et al. reported the generation of CPL using the natural chiral DNA templates. [142] The achiral carbazolebased biscyanine fluorophores were designed exquisitely, and could be assembled with the DNA chains. The coassembly with DNA templates restricted the intramolecular rotational of the biscyanine fluorophores, sparking the enhanced emission. Meanwhile, the chirality of DNAs transferred to the DNA-biscyanine complexes, resulting in the CPL activity. Shang et al. realized the multicolor CPL in the single AIE system. [143] They linked the achiral AIEgens with the chiral cholesterol group by the ester bond. The obtained molecule Chol-CN-Py could assemble into the nanohelix and form the gel (g em = −3.0 × 10 -2 ) and xerogel (g em = −1.7 × 10 -2 ) films. In the protonation of Chol-CN-Py, the initial blue CPL could display the obvious bathochromic shift ranging from the green to orange while the g em still remained constant.

Third-harmonic generation (THG)
THG and other odd-order NLO processes do not suffer from the symmetry constraint. [144] Analogue to SHG, THG has been applied in up-conversion fluorescence, [145] drug delivery, and imaging. [146] THG is beneficial to deep-tissue and high-resolution bioimaging since the intensity of THG rise with the cube of the excitation power. [147] A plethora of representative inorganic materials, e.g., transition metal dichalcogenides (TMDs), [148] black phosphorus (BP), [149] germanium selenide (GaSe), [150] and perovskites, [151] have revealed striking third-order susceptibilities for THG process. Latterly, some AIEgens also signify the THG behaviors, gifted to be the ideal alternative for inorganic materials owing to their better flexibility [152] and biocompatibility. [153] Zheng et al. designed an AIEgen DCCN with merging the DPA and dicyanomethylene-benzopyran unit via Knoevenagel condensation. [154] In the solution of acetone/water mixtures, DCCN displayed the obvious AIE characteristic. In the crystalline state, it also unfolded the CIE feature with NIR fluorescence. The nanocrystals attained from aqueous solution were encapsulated to yield crystalline dots (CDs), and the impressive THG responses centered at 520 nm of CDs were recorded under the excitation of a 1560 nm fs laser. THG signals of the corresponding amorphous dots (ADs) and dilute solution were also inspected under the same condition, and 366-fold and 1183-fold higher signals were detected in ADs and CDs than solution state, respectively. The transformation of aggregation state also touched off the alteration of the inherent NLO property, referred to as the aggregationinduced nonlinear optical (AINLO) effects. The strong THG effects springing form strong push-pull dipolar moment and the plenteous π-conjugation of DCCN were employed for the clear and ultradeep imaging of the mouse cerebral vasculature and complex vessels.
As mentioned, incorporating donor and acceptor components into the AIE backbone structure is an effective pathway to realize NLO effects [155,156] since this construction is vulnerable to the polarity of the surroundings. For the THG active AIEgens, the prototypical nonplanar DPA or triphenylamine (TPA) are adopted as strong donor along with α-cyanostilbene acting as strong acceptor, forming the patterns like D-π-A, D-A-D. Tian et al. synthetized the AIEgen 1,1-dicyano-2-phenyl-2-(4diphenylamino)phenylethylene, referred to as DCPE-TPA based on a D-π-A skeleton with a deep-red emission. [157] This luminogen could generate three various aggregations relying on the packing model, showing polymorphismdependent luminescence. The DCPE-TPA NPs fabricated by encapsulating the DCPE-TPA molecules within polymer produced marked THG signal at 517 nm under 1550 nm laser excitation. Similarly, Wang et al. explored the in vivo imaging of the intravital brain vasculature grounded on the TPATCN NPs, and the AIE essence of this D-A-D molecule was verified via THF/H 2 O system. [158] To achieve the deeper penetration depth, the multichannel NLO imaging covering both the THG and three-photon fluorescence (3PF) signals was operated. The responses of THG channel (495-540 nm) circumfused 517 nm. In view of the great coherence of THG signals, it was much convenient to track the flowing of NPs in blood vessels from THG imaging. Meanwhile, the 3PF images of blood vessels were consistent with the THG ones. Based on the original D-A-D structure, Qin et al. attached the tertbutyl (t-Bu) groups to the terminal benzene rings of DPA, impeding the generation of strong π-π stacking interactions (Figure 7(A)). [159] The obtained AIEgen, referred to as BTF, expressed the ultrabright far-red/near-infrared emission with quantum yields of 42.6%. The readily fabricated BTF dots via nanoprecipitation manifested the distinguishing THG peak at 517 nm and intense 3PF at 650 nm. The intrinsic THG signals could be detected at various penetration depths from 0 to 900 μm, offering additional structural information at superficial depths.
TPE is one of the most iconic AIE units, and the incorporation of TPE can not only reinforce the nonplanarity of molecule, but also extend the π-conjugation, aiming to form and foster the NLO effects for luminogens. Based on the 2-(2,6-bis((E)-4-(diphenylamino)styryl)-4Hpyran-4-ylidene)malononitrile (TPA-DCM), Nicol et al. manufactured the aniline derivate TPE-TETRAD with TPE groups. Stimulated by a femtosecond laser at 1560 nm, the spike of THG occurred at 515 nm. [160] The concurrent three-photon luminescence (3PL) was spotted at 668 nm, speculated that it was interrelated with the reabsorption of THG photons by TPE-TETRAD molecules. Qian et al. modified the 2,3-bis(4-(diphenylamino)phenyl)fumaronitrile with TPE moieties to generate the typical Dπ-A-π-D architecture 2,3-bis(4-(phenyl(4-(1,2,2triphenylvinyl)phenyl)amino)phenyl)fumaronitrile (TTF) (Figure 7(D)). [161] The associated explorations concerning the TTF focused on three different systems: chloroform/toluene solution, aqueous solution, and solid state. In the chloroform/toluene mixture, only 3PL and 4PL (four-photon luminescence) could be detected. However, it engendered the TTF nanoaggregates in the aqueous solution, and the observed THG intensity was boosted with the aggregation degree of TTF molecules increasing. As it was transformed into the solid state, the THG signals altered in pace with the wavelength of laser varying: while the laser excitation converted from 1320 to 1560 nm, the THG responses in spectrum was narrow (Figure 7(f)); when the wavelength of laser shifted from 1620 to 1860 nm, the THG signal peaks turn into the wide ones (Figure 7(G)).

Two-photon fluorescence (2PF)
In 1990, Webb et al. invented two-photon fluorescence microscopy (2PM), [162] and henceforth, this technology started to be exploited and capitalized on further. In comparison with the traditional one-photon fluorescence, the energies of absorbed photons in 2PF process are lower, and their excitation wavelengths concentrated on the near-infrared region rather than the visible or UV region, diminishing the autofluorescence severely. Furthermore, 2PF produces less photodamage to the specimens and display the deeper tissue penetration. [163,164] The 2PA action cross section (ησ 2 ), a quantitative criterion to estimate the two-photon-induced threshold, [165] could have been hoisted by the reasonable structure design. Diverse aggregates of luminogens, such as cocrystal, [166,167] NPs, polymers, have been demonstrated to hold the AIE and 2PF features synchronously. Besides, the boosted ησ 2 has also been realized upon the molecular aggregates owing to the higher quantum efficiency (η) of AIE effect. [168] Imaging [17] and fluorescence probe [169] are the two chief applications of the two-photon AIE molecules. Li et al. presented to the conversion from the ACQ to AIE via adjusting the molecular packing mode through regioisomerization. The basic framework of the molecules consisted of the Dithieno[2,3-a:3′,2′-c]benzo[i]phenazine (TBP) and the twisted molecular rotor TPA. [170] From the  [159] (D-G) Reproduced with permission: Copyright 2015, WILEY-VCH. [161] (H and I) Reproduced with permission: Copyright 2020, WILEY-VCH. [170] (J and K) Reproduced with permission: Copyright 2018, WILEY-VCH [173] TBP-e-TPA molecule with long-range cofacial packing mode to the TBP-b-TPA with discrete cross packing mode (Figure 7(H)), the minor modification of the positions of TBP units arose the switch from ACQ to AIE due to the generation of steric hindrance. The AIE active TBP-b-TPA also expressed the marvelous 2PA property (Figure 7(I)). In the NP state, it was noticed that the reinforced 2PF response in comparison with that in THF. From the 2PF spectra of TBPb-TPA, its 2PA cross section σ 2 was measured to be 608 ± 9 GM and 207 ± 7 GM (1 GM = 10 -50 cm 4 s photon -1 ) for NIR-I and NIR-II regions, respectively. The TBP-b-TPA NPs were executed to visualize the vascular architecture of the mouse brain, conveying the clear vasculature information even at 700 μm.
Varying from the generally elaborate synthetic route to two-photon AIEgens, Niu et al. utilized an atom-economic and handy synthetic method to attain the AIE-active acrylonitrile derivatives. [17] Two different acrylonitriles TPAT-AN-XF and 2TPAT-AN exposed the dissimilar AIEE effect in aqueous suspensions where the several-fold fluorescence enhancement of TPAT-AN-XF was attributed to the denser aggregate than 2TPAT-AN. In view of the favorable ICT effect as well as the beneficial π-conjunction structure, the two AIEgens both illustrated marked 2PF signals between 800 and 980 nm with the femtosecond pulsed laser excitation. The ησ 2 of 2TPAT-AN was higher than TPAT-AN-XF in the area of 800-880 nm, and the σ 2 of 2TPAT-AN and TPAT-AN-XF were calculated to be 508 GM and 366 GM at 880 nm, respectively. The vitro imaging implied that 2TPAT-AN NPs retained the high biocompatibility, great resolution, and deep tissue penetration, and meanwhile it could stain lysosome in live cells selectively. Wang et al. designed the highly bright fluorophore BTPETQ for the intravital 2PF imaging of the mouse brain and tumor vasculatures. [5] The uniform BTPETQ dots emerged in the process of nanoprecipitation, bearing the high quantum yield of 19% ± 1% in aqueous media. The 2PA cross section of BTPETQ dots was evidenced to be as large as 7.63 × 10 4 GM at 1200 nm, and the ultradeep tissue penetration of 924 μm was observed at the assistance of NIR-II excitation. Moreover, the AIE dots showcased the obviously distinct 2PF in the tumor vasculatures and normal blood vessels resulting from the aggregation of dots in the unique leaky structures of the tortuous tumor blood vessels.
2PF probes are the promising tools to identify the ions or small molecules, [171] and even to sense the fluctuations of the surroundings. [172] Li et al. reported a conjugated macrocycle polymer P[5]-TPE-CMP composed of TPE and pillararene with the strong 2PF property and gratifying stability against photobleaching. [173] The introduction of pillararene integrated the virtues of solid porous polymer and macrocyclic host, e.g., the recyclability and indissolubility in the common solvents. Under the excitation of both near-UV and NIR realms, the P[5]-TPE-CMP acted as a talented sensor (Figure 7(J)). When various ions such as Na + , Ca 2+ , F -, Cl -, Fe 2+ , Ce 3+ , Y 3+ were added into P[5]-TPE-CMP suspension, no noticeable changes in 2PF were discovered. However, the Fe 3+ ion could spark the prominent quenching of 2PF (92.9%) on account of the size-matching effect of P [5]-TPE-CMP. Similarly, the carcinogenic organic dye 4-amino azobenzene could also touch off the distinguished 2PF quenching (98.5%) selectively, and it remained the great repeatability after washing with water for several times followed by centrifugation. Zhang et al. devised a brand novel AIEgen MTPA-Cy through condensation reaction with 4-(bis(4-methoxyphenyl)amino)benzaldehyde and 3ethyl-1,1,2-trimethyl-1H-indolium iodide. [174] As a result of the TPA group, MTPA-Cy emitted weak fluorescence upon the 365 nm radiation, and the intrinsic color of MTPA-Cy solution faded and the weak fluorescence was replaced by the bright yellow-green fluorescence at 514 nm after the original ethylene bridge was oxidated by ClO -. Under the 730 nm excitation, it manifested the 2PF behaviors with the cross section of 15.3 GM, and the 2PF intensity was gradually raised with the concentration of ClOascended from 0 to 50 μM, revealing the astounding potential to be the 2PF probe.
Apart from 2PF, the 3PF pertaining to the fourth-order NLO effects also reside in the piles of AIE materials. [175] Compared to 2PF, 3PF will lessen the out-of-focus excitation efficiently and dramatically elevated the signal-tobackground ratio to the higher order levels. [176,177] Fang et al. tested and verified two new three-photon absorption (3PA) AIEgens Tpy and Tpy-Hex for detecting the trace amounts of silver ions quantitatively in organism. [178] The 3PA coefficient of Tpy-Hex could reach to 1.06 × 10 -22 cm 3 /W 2 with 3PA cross section σ 3 of 4.1 × 10 -78 cm 6 s 2 photon -2 , and the 3PF intensity also climbed with formation of aggregate. The Tpy-Hex contained two main constituents TPE and pyridine. The pyridine moiety could coordinate with Ag + easily, leading to the unexpected optical properties. In the aqueous solution of Tpy-Hex, it displayed significantly strengthened fluorescence at 435 nm in the presence of Ag + . Besides, the combination of the Tpy-Hex and Ag + (denoted as Tpy-Hex-Ag + ) upgraded the initial NLO activity of Tpy-Hex, holding the higher 3PA cross section σ 3 (6.9 × 10 -78 cm 6 s 2 photon -2 ) and 3PA coefficient (2.21 × 10 -22 cm 3 /W 2 ). Zong et al. incorporated the large isolation groups on the perylene diimide (PDI) core to realize the conversion from ACQ to AIE via suppressing the π-π stacking. Under the excitation of a 1550 nm fs laser, the bright 3PF was discovered at 650 nm. The DCzPDI (PDI with double carbazolyl moieties) NPs could offer the high-resolution brain blood vessels at varied vertical depths, and the maximum penetration depth could attain to 450 μm.
The restricted intramolecular rotation (RIR) mechanism of the AIE materials will also affect their NLO responses. Peng et al. prepared a new D-π-A compound 5,6-di(4-N,N-(dimethylamino)phenyl)pyrido [2,3-b]pyrazine (APPP) with the propeller-like construction. [122] The AIEE effect of APPP in the water-dominated mixture could be affirmed by the fluorescence spectra. APPP also demonstrated the viscosityinduced emission characteristic in the glycerin/EtOH solution owing to the conversion to the TICT state from the RIR. Expect for the outstanding AIE phenomena, the unique thirdorder NLO features were also demonstrated by open-aperture Z-scan technique. It was uncovered that APPP possessed the reverse saturable absorption (RSA) property in EtOH solution, yet it switched into the converse SA at the state of solid. Astonishingly, the conversion between RSA and SA could also come true as the viscosity changed, and the NLO absorption coefficients (β eff ) of APPP in pure EtOH and the glycerin/EtOH mixture (v/v = 9:1) were measured to be 0.54 × 10 -11 m/W and −2.95 × 10 -11 m/W, respectively. Moreover, the ascension of temperature also could shift the state of RSA and SA.

NLO APPLICATIONS OF AIE MATERIALS
AIE materials, especially those with D-A building blocks, are endowed with fortes involving large NLO coefficients, ultrafast NLO responses, fortified optical parameters, less thermal dissipations, high chemical and physical stability. These excellences enable the NLO-active AIE materials as the satisfactory platform to practice NLO applications. The alterations of aggregates draw forth the mutations of intermolecular interactions, [183] packing modes, [184] or the transition dipole moments, and then conquer the field of regulating the NLO properties of AIE materials. From the concentrated solutions to the polymers, crystals, NPs, gels, clusters, and so forth, the distinct aggregates of AIEgens may trigger the disparate NLO effects or the modifications of NLO strength, which are adopted to the different NLO domains. In this section, we will discuss the NLO applications of AIE materials in optical limiting, solid-state laser (SSL), and bioimaging.

Optical limiting
The emergence of optical limiting is grounded on the process of RSA or excited state absorption. [185] When the laser illuminates the optical medium with optical limiting, the high energy radiation can be filtered out whereas the low-intensity light can pass through the medium effortlessly. Thereby, the optical limiters can not only effectively protect human eyes or sensitive optical devices from the photodamage, but also have the capability of tuning the laser. Chan [186] (C and D) Reproduced with permission: Copyright 2012, Royal Society of Chemistry. [187] (E and F) Reproduced with permission: Copyright 2017, Elsevier. [188] (G) Reproduced with permission: Copyright 2019, American Chemical Society. [201] (H and I) Reproduced with permission: Copyright 2019, WILEY-VCH. [202] (J) Reproduced with permission: Copyright 2020, Royal Society of Chemistry. [203] (K and L) Reproduced with permission: Copyright 2016, Royal Society of Chemistry. [205] (M and N) Reproduced with permission: Copyright 2019, WILEY-VCH. [206] (O and P) Reproduced with permission: Copyright 2020, WILEY-VCH [207] or C 6 H 5 ] via polymerizations. [186] The resultant polymers P1 (R = C 8 H 17 ) and P2 (R = C 6 H 5 ) revealed the AIE (Figure 8(A)) and ACQ effect, respectively, indicating the impact of variation of molecular structure. In its solution of chloroform, transmitted fluence of the two polymers rose almost linearly with the enhancement of input fluence when the fluence was lower than 1 J/cm 2 . As the fluence outstepped 4 J/cm 2 , the transmitted fluence reached a plateau and saturated state, elucidating the optical limiting nature of P1 and P2 polymers (Figure 8(B)).
The diyne polymers like graphdiyne have been verified with unexpected NLO absorption, which may therefore qualify the optical limiting property. Hu et al. constructed the hyperbranched diyne polymer with TPE via the polycy-clotrimerization under the catalysis of TaBr 5 . [187] The hyperbranched poly(tetraphenylethene) hb-P1 could keep steady at the high temperature of over 400 • C and are highly soluble in the majority of organic solvents by virtue of the twisted conformation of TPE. The polymer hb-P1 inherited the impressive AIE feature from the TPE. Its THF solution emitted faint fluorescence, while its aggregates in THF/H 2 O mixture or the solid both gave out intensive fluorescence with the maximal quantum yields of 81%. The optical limiting characteristic of hb-P1 was inspected in the homologous THF solution, and the optical pulse at 532 nm could transport in the THF solution at the low incident fluence. Once the incident fluence outstripped 60 J/cm 2 , the transmitted fluence started to deviate from linearity growth.
Other than the AIE polymers with high molecular weight, some AIE-active small molecules have been also explored as the potential candidates of optical limiter (Figure 8(D)). Liu et al. have developed a series of Pt(II) complexes bearing difluoro-boron-dipyrromethene (Bodipy) acetylide ligands and the diverse 2,2′-bipyridyl derivate ligands. [188] The three disparate Pt(II) complexes, named as Pt-1, Pt-2, and Pt-3, have been explored for their photophysical properties. The Pt-1, Pt-2, and Pt-3 were gradually clustered with the ratio of water increased in CH 3 CN solutions with much brighter light synchronously, suggesting typical AIE characteristics. These polymers had the positive absorptions ranging from the 380 to 460 nm and after 530 nm, implying the possibility of RSA. The associated optical power limiting examinations were conducted by nanosecond laser pulser at 532 nm, and the outstanding output energy attenuation as the high-density incident imported, confirming the occurrence of optical limiting. Among the three polymers, Pt-2 demonstrated the optimal capability of optical limiting (Figure 8(F)).

Organic SSL
Laser is one of the indispensable elements in the NLO domains, [189,190] and the sustainable, jarless, and adjustable laser resources are the fundamentals of yielding and characterizing the diverse NLO effects. [191] For the moment, the solid-SSLs recline principally on the inorganic materials, such as TMDs, [192] perovskites, [193] MXene. [194] Although the lasing technologies counting on the inorganic materials have been growing maturity, some drawbacks still exist, containing the hard preparation, high cost, and tough tunability. [195] The organic lasers emerge with high robustness, [196] handy, time-saving and economical solution-processed fabrications, and admirable optical performance, grabbing the tremendous attention. [197] In addition, the manifold excited state processes can be fulfilled facilely among most of the organic luminogens. [198,199] Howbeit, the ACQ effect of organic gain materials seriously encumber the wider and further their employments in SSL due to the upper lasing threshold and the forbidden of the laser actions. [200] Direct at this issue, one of the appropriate approaches is to incorporate the AIEgens to shrink the nonradiative loss. Wei et al. devised and synthesized the organic molecule 1,4-bis((E)-4-(1,2,2-triphenylvinyl)styryl)-2,5-dimethoxybenzene (TPDSB), unveiling the protruding AIE behavior as high as ∼2500 times augmented. [201] TPDSB microribbons were gained by the solution-drying method at ambient temperature, and the spatial resolved PL spectra proved the appealing optical waveguiding property of the microribbon with the low optical loss of 0.012 dB μm -1 . At the excitation of 355 nm nanosecond pulse laser with varied pump energy, the related PL spectra witnessed the transition from the spontaneous emission to the amplified spontaneous emission (ASE) and the lasing threshold was identified to be as low as 653 nJ cm -2 (Figure 8(G)). Meanwhile, the discovery of spatial interference pattern at both terminuses also declared the existence of Fabry-Pérot (F-P) microcavity with a quality factor of 2565 within the microribbon, suggesting that the TPDSB was an attractive active medium for laser action of 520 nm. Analogously, Liu et al. realized the microlaser by adopting a typical TPE-modified AIEgen TPE-BODIPY. [202] In contrast to the organic laser with high-crystallinity crystals, this microlaser stemmed from the noncrystalline coaggregation of TPE-BODIPY and epoxy resin. The aggregation of TPE-BODIPY in THF/H 2 O system appeared with sevenfold enhancement of the fluorescence quantum yield. With the help of surface tension effects, the coaggregation of epoxy resin and TPE-BODIPY could be fashioned into noncrystalline microspheres, and the suitable addition of water and surfactant could guarantee the appropriate size of microspheres. The constructed microspheres were excited by the nanosecond laser (480 nm, 5 ns duration, 20 Hz), and the shining outer boundary could be found, symbolizing the formation of whispering gallery mode (WGM) above the lasing threshold (Figure 8(H)). From the lasing spectra of microlaser with distinct diameters, the singlemode lasing at 538.2 nm for 8.9 μm was detected accompanied with the low threshold of 2.24 mJ cm -2 and the full width at half maximum (FWHM) of 0.2 nm (Figure 8(I)). Apart from the TPE containing SSL, some new-style organic lasers also spring up. Lv et al. constructed a highly emissive AIEgen BPMT with the building block of TPA and benzo[c] [1,2,5]thiadiazole (BTA). [203] The acquired rod-like crystals could achieve the high PLQY of 48.7% with the fluorescence spanning the deep-red and NIR region. Besides, the crystals were susceptible to the alteration of the pressure, and the reversible decline of brightness along with the redshift of fluorescence wavelengths occurred when the pressure converted from 1 atm to 5.1 GPa. Dissolving the BPMT and epoxy resin with dichloromethane, and the BPMT-doped hemispherical microresonators assembled spontaneously on a distributed Bragg reflector (DBR) mirror surface (Figure 8(J)). Excited under the 10 × objective (NA = 0.3) system by 351 nm nanosecond laser, the fluorescence microscope images presented the shining outer boundary subordinated to the WGM microresonator for the pump power exceeding the lasing threshold of 22.3 kW cm -2 . At the lasing wavelength of 735.2 nm with the line width of 0.23 nm, the quality factor of WGM microcavity was determined to be 3200.
Aside from the complicated π-conjugated structures, [204] the AIE materials with the simple π-conjugated structures may also actualize the efficient laser. Tang et al. studied the optical properties of a series of (2-hydroxyphenyl)propanone derivatives with the oversimplified scaffolds. [205] All of the four derivatives including single-benzene emitted poorly in the solutions but displayed bright emission in the film and crystals. The four compounds could foster the high-quality crystals through the ordinary solvent diffusion, and the organic crystals had the similar and strong absorption peak at around 365 nm. They gave out the intensive green fluorescence with the high quantum yields of 0.72-0.84, suggesting the CIE feature. Additionally, the tip parts of these needlelike crystals showcased much more brilliant emission than their bodies, implying the self-waveguided properties. The PL spectrum of the one of the crystals collected from the edge side verified the ASE by the excitation of pulsed laser, revealing the lasing property of the simple AIE materials.

Bioimaging and photodynamic therapy (PDT)
AS expounded, bioimaging is one of the most vital and practical NLO implementations touching upon the AIE materials.
Extraordinarily higher resolution and larger signal-to-noise ratio, improved penetration depth and lower photodamage to the biological cells and tissues incline it to be the promising alternate to the convenient single-photon bioimaging technology. Wang et al. attained the 2PF composites by complexation of the AIEgen TPEPy with the biologic fetal bovine serum (FBS). [206] The intensity of TPEPy fluorescence acquired sixfold reinforcement with the 10% FBS aqueous solution, and the gained TPEPy-FBS was endued with the great biocompatibility, photostability, and low phototoxicity and cytotoxicity. More significantly, TPEPy-FBS behaved the fetching 2PF in the area of far-red and NIR upon the excitation of laser, which was fit for the in vivo bioimaging. The suspension of TPEPy-FBS nanocomposites was injected into the mouse brain model with craniotomy. The 840 nm femtosecond laser was applied for the 2PF imaging owing to the largest σ 2 value, and the clear and bright 2PF images at diverse tissue depths could be obtained with the largest depth of 656 μm (Figure 8(N)). The detailed information about the big blood vessels and small blood capillary vessels could be distinguished in the 2PF imaging, and the signal-to-background ratios of the 2PF images were measured to be as high as 234.
The majority of 2PF bioimaging mainly involves the widerange biological organs or tissues, but less attention has been paid to the cell organelles. Alam et al. put forward two zwitterionic AIEgens CDPP-3SO 3 , CDPP-4SO 3 with the sulfonate function group as organelle-targeting fluorophores. [207] The core molecules of CDPP-3SO 3 and CDPP-4SO 3 embraced the D-π-pyridine architecture, in which the pyridine moiety could be transformed into pyridinium, intensifying the initial push-pull effect (Figure 8(O)). Therefore, the two compounds were ideal for bioimaging. The special pyridiniumsulfonate group of the two AIEgens offered the possibility to allow for the endoplasmic reticulum (ER) targeting selectively as a result of ample existence of phosphocholine cytidylyltransferase (CCT) on the ER membrane. Clear signals of HeLa cells could be obtained for the three AIEgens from the two-photon microscope (Figure 8(P)).
Some AIE molecules are also propitious to 3PF since the AIE chromophores will amplify the 3PA cross sections at the aggregation. [175] Compared to the complex synthesis method in organics above, Feng et al. synthesized the novel thiophene terpyridine zinc complex (DZ1) to be awarded with remarkable 3PA property and AIE activities alongside the conspicuous response to RNA, totally varying from its organic ligand (DL1) without obvious NLO effect. [208] From the three-photon excitation fluorescence method under 1700 nm fs laser, the 3PA cross section σ 3 was determined to be 5.28 × 10 -82 cm 6 S 2 photon -2 . The σ 3 of DZ1 in aggregation (ethyl acetate/acetonitrile (70%)) was measured to be 1.11 × 10 -81 cm 6 S 2 photon -2 , rising 1.78-time higher than the molecular state. DZ1 was feasible to mobilize in the organisms by occasion of its low toxicity, wonderful chemical stability and admirable biocompatibility. The imaging experiments of DZ1 was operated in the HepG2 cell. Monitoring the motivations of DZ1 in the cells found that it could pass through the cell membrane and then target mitochondria like MitoTracker (a green fluorescent protein). The open-aperture Z-scanning and 3PF was performed to explore the variability of NLO properties in the case of the addition of RNA, and the σ 3 value move up to be 1.17 × 10 -81 cm 6 S 2 photon -2 (1.92-fold) because of the interaction between DZ1 and RNA.
PDT is a noninvasive strategy to fight the tormenting caners or tumor, in which the cytotoxic reactive oxygen species (ROS), [209] such as singlet oxygen ( 1 O 2 ), [210] from photosensitizer (PS) under the focused light will induce the apoptosis of cancer cells directly [211] or indirectly. [212] However, the ineffective generation of ROS, low tissue penetration, poor selectivity, and biocompatibility of common molecules hinder its application in biomedicine. [213] Some AIEgens with multiple-photon characteristics are recognized as the ideal alternatives, which can not only improve the extinction coefficients but also increase the light penetration in the therapeutic process. [214,215] Wang et al. exploited an AIE PS TQ-BTPE composed of [1,2,5]thiadiazolo [3,4-g]quinoxaline (TQ) for strong acceptor and TPE for the electron donor. [101] The rational design of this PS contributed to the 2PA upon NIR-II femtosecond laser excitation, red-shift and broad absorption in visible region. The σ 2 of TQ-BTPE was measured to be 168 GM at 920 nm and 49 GM at 1200 nm, revealing the remarkable two-photon activity. For the aggregate of TQ-BTPE in aqueous medium, it could engender much more 1 O 2 than the classical chlorin e 6 (Ce6) even under the general visible light excitation. Comparing with the radiation of NIR-I, the NIR-II light could induce the better ablation of HeLa cells in fresh pork tissue on occasion of the deeper penetration capability in the tissues.
Although some AIE PSs are capable to produce the ample ROS to kill the cancerous cells, they also present the high phototoxicity to the normal tissues. In order to settle the dilemma, some PS systems bearing the prominent selectivity to tumors have been exploited. Yang et al. encapsulated the typical PS bis(pyrene) (BP) with the liposomes to form the BP@liposomes complex. The BP was selected as PS for its considerable high σ 2 of 2.4 × 10 5 GM, and the coassembly of BP with liposomes elevated the accuracy of PDT. While the BP@liposomes complex was in touch with the normal tissues, the BP molecules still maintained the pristine dispersed state, appearing the negligible phototoxicity. As it reached tumor sites with passive and active mechanism, the liposomes were gradually decomposed by intracellular phospholipase, and the dispersed state of BP was converted into the aggregate state, leading to the eradication of tumor under the two-photon laser radiation. Apart from the challenge of target selectivity in two-photon PDT processes, the residual of PSs in vivo is also a hassle. Li et al. fashioned the AIE NPs 4-(5-(1-(4-(tert-Butyl)phenyl)-1Hphenanthro [9,10d]imidazol-2-yl)-thiophen-2-yl)-7-(4-(1,2,2triphenylvinyl)phenyl)benzo[c] [1,2,5]thiadiazole (TPE-PTB) via lipid-encapsulated method. The TPE-PTB exhibited the high quantum yield of 23% and the remarkable 2PA cross section of 560 GM (excitation at 800 nm). Benefiting from the penetration depth (500 μm) in tumor tissue, the TPE-PTB NPs could release the two ROS (O 2 and hydroxyl radicals) simultaneously under the NIR laser. More importantly, the PS could be eliminated naturally after five days of injection, dramatically reducing the side effect to the body.

CONCLUSIONS AND PERSPECTIVES
The emergence of AIEgens refreshes the cognition of the orthodox fluorescent materials. From the primitively distressful ACQ to the presently exciting AIE, the countless branches of AIE families involving HPS, TPE, Schiff-bases, fluorenone, and organometallics have sprung up over the past few decades. In the regimes of linear optics and optoelectronics, the numerous AIE materials yield the brilliant achievements and close concern. Meanwhile, the AIE materials start to be budding and evolve in the NLO fields, and the scads of AIEgens have been witnessed to share the impressive NLO effects, e.g., SHG attributed to the second-order NLOs, THG, 2PA, 2PL, SA, and RSA for the third-and higher order NLOs. Currently, the multiphoton fluorescence of AIE materials has been functioned into the bioimaging, biosensing, and biotherapy widely and maturely. Aside, the SA and RSA effects of AIEgens also have been progressively exerted in the laser technologies.
Amid these researches dealing with the NLOs of AIE materials, nearly all of them have been aware of the aggregation-induced enhanced NLOs and even triggering the switches between the varied NLOs. On the side of AIE, some mature and sound theories, involving the RIR, RIM (restriction of intramolecular motion), RIV (restriction of intramolecular vibration), ICT, and J-aggregation, and so forth, have explained the AIE behaviors satisfactorily and guided the devises of novel AIE compounds. However, there is still no well-established theoretical system to analyze and explicate the alterations of NLO rooting from the aggregation. Moreover, the researches about the NLOs in AIEgens are still in their infancy. The sterner challenge is how to develop and exploit more AIEgens suitable for NLOs. A mass of NLO features in developed AIE materials have not yet been uncovered, and there is ample scope for AIE materials utilizing in NLO domains. For instance, numerous AIEgens can crystalize in the noncentrosymmetrical space group, which are potential to the even-order NLOs such as SHG, SFG, and optical rectification. Moreover, these AIE compounds embodying the conspicuous push-pull structures also tend to the generation of molecular dipole, having the latent capacity in the appreciable NLO responses. In sum, the flourish of AIE materials provide a favorable moment for the developments of NLO, and the more flexible and stable AIE materials with optimized NLO responses are prospected to be employed into the photonics and optical devices.

A C K N O W L E D G M E N T S
Financial supports from National Natural Science Foundation of China (project numbers 21773168, 21531005, and 91622111) are gratefully acknowledged.