AIEEgens: Recent Advances and


Nov 24, 2017 - Functional 1,8-Naphthalimide AIE/AIEEgens: Recent Advances and Prospects. Peddaboodi Gopikrishna†‡, Niranjan Meher†§ , and Param...
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Functional 1,8-Naphthalimide AIE/AIEEgens: Recent Advances and Prospects Peddaboodi Gopikrishna, Niranjan Meher, and Parameswar Krishnan Iyer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14473 • Publication Date (Web): 24 Nov 2017 Downloaded from http://pubs.acs.org on November 25, 2017

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Functional 1,8-Naphthalimide AIE/AIEEgens: Recent Advances and Prospects Peddaboodi Gopikrishna,#§ Niranjan Meher ‡§ and Parameswar Krishnan Iyer*#,‡ #

Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati-781039,

India. ‡

Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781039,

India. FAX: +913612582349; E-mail: [email protected] §

Equal contribution

KEYWORDS: Naphthalimide; Aggregation induced emission; Nanoparticles; Bio and chemo- sensing; Drug delivery; Cell imaging; OLEDs

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ABSTRACT This comprehensive review surveys the up to date development of aggregation induced emission/aggregation induced emission enhancement (AIE/AIEE) active naphthalimide (NI) based smart materials with potential for wide and real-world applications and serving as a highly versatile building block with tunable absorption and emission in the complete visible region. The review article commences with a precise description of the importance of NI moiety and its several restricted area of applications owing to its aggregation caused quenching (ACQ) properties, followed by the discovery and importance of AIE/AIEE-active NIs. The introduction section is tracked by an overview of the state of the art in NI luminogens in multiple applications. It also includes few mechanistic studies on the structureproperty correlation of NIs and provides more insights into the condensed-state photophysical properties of small aggregation-prone systems. The review aims to ultimately accomplish current and forthcoming views comprising the use of the NIs for the detection of biologically active molecules, such as amino acids and proteins; recognition of toxic analytes, fabrication of light emitting diodes, and their potential in therapeutics and diagnostics.

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1. INTRODUCTION Naphthalimide (NI) core is considered as one of the most versatile fluorophore unit owing to its unique photophysical properties that has been explored extensively in various real-world applications.1 The aromatic core along with the N-imide site can be modified through easy and cost effective synthetic route that allows for varieties of structural units and functional groups to be fused. As the NI core is electron acceptor in nature, the aromatic core can be functionalized with donor moieties like amine or hydroxyl groups to have a red-shifted intramolecular charge transfer (ICT) band with marked solvatochromic effect. Its UV-visible absorption and fluorescence emission energy falls within the visible region and can be tuned up to near-IR region which could provide an excellent platform to probe the microenvironment of biological systems. Besides, the NI dyes have been used in the construction of novel therapeutics and in the dye industry as a powerful absorber for colorful dyes.2 The NIs and its derivatives have immense potential in the area of optoelectronic materials, fluorescent sensors, laser dyes, and bio-imaging etc. due to their extraordinary thermal and chemical stability with high fluorescence quantum yields.1,3 The NIs are also known to have high antitumor activity towards various murine and human cells.4 Again, extension of the aromatic ring with thiazole and polythiazole groups have been manipulated in the construction of several photo-nucleases and anticancer bleomycin related antibiotics.5 Sulfonated NI derivatives were also reported to have antiviral activity with selective in vitro activity towards the human immunodeficiency virus (HIV-1).6 Brominated NIs at the 3rd and 4th place could act as the photochemotherapeutic inhibitory materials in blood.7 NI derivatives are also powerful photo-reagents and can be photoactivated to kill tumor cells.8 This unlocks their wide probable applications towards photo-therapy. The NI core could also be extended for anticancer agents and DNA-binding motifs.

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In the development of versatile fluorescent materials, a notorious photophysical phenomenon called ACQ frequently comes into the fore and restricts their potential use.9 In solid state or in poor solvent, fluorescent properties of the classical fluorophores are destroyed through the formation of aggregates. Most of the conventional organic fluorophores exhibited planar structures that have strong affinity to pack closely in their condensed state through intermolecular π-π interactions. Due to the strong packing, these fluorophores relax via nonradiative pathways and the emission quenches effectively. This is mainly due to the H-type aggregation of the fluorophores and thus the phenomenon is termed as ACQ. However, in solution state the fluorophores remain in the molecularly dispersed state and display high fluorescence emission due to lack of aggregation. The ACQ effect has restricted many realworld technological applications, because, practically a fluorophore is frequently used as a whole (aggregate), rather than in its monomeric form. Also, considering the practical applications, the sensory materials (chemo- or biosensors) ought to work in physiological conditions. Although polar functional moieties like carboxylic and sulfonic acid groups can be integrated with the hydrophobic aromatic cores, the subsequent fluorophores still display aggregation behavior in water owing to the high hydrophobicity of the aromatic cores and the alkyl chains through which they are usually functionalized. The ACQ effect has a huge adverse impact on the fabrication organic light-emitting diodes (OLEDs) devices, where the active materials are generally applied in the aggregate state as thin films. These observations indicate that ACQ is a negative optical effect preventing their wide and versatile application as light emitting materials. Tang et al. in 2001 discovered an interesting phenomenon based on propeller shaped silole derivatives which were found to have exactly opposite photophysical properties to that of ACQ properties and this phenomena was termed as aggregation induced emission (AIE).10 These luminogens are highly fluorescent in condensed state but they became non-emissive in

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solution state. Following the discovery, Park et al. in 2002 introduced another phenomenon called as aggregation induced emission enhancement (AIEE), a sub-group of AIE.11 AIEEactive molecules exhibit emission in all physical states like solution, solid and aggregated state. To get into the mechanistic insight of this interesting phenomenon, many research groups hypothesized several mechanistic pathways including J-aggregate formation, conformational planarization, twisted intramolecular charge transfer (TICT), excited state intramolecular proton transfer (ESIPT) and E/Z isomerization.12 However, none of these mechanisms could be universalized due to their inapplicability to all the reported AIE and AIEE systems. In 2015, Tang and co-workers once again tried to generalize its working mechanism and had drawn a strong aspect regarding the AIE/AIEE process.9 In that review article, with solid theoretical and experimental proof, restriction of intramolecular motions were hypothesized to be the most reliable and effective working mechanism for their condensed state emission. Applying fundamental physics to the organic molecules having rigid rotators, upon excitation, could consume maximum amount of energy through dynamic intramolecular rotation and vibrations. Due to this they became less emissive or non-emissive in solution state. Whereas, in aggregated or solid state, the molecules interlock themselves that restrict the intramolecular rotation (RIR) and vibration and relax through radiative pathways. It is also noteworthy to mention that the molecules should not acquire a dense face-to-face packing which favours the strong π−π stacking interaction and subsequently the radiationless relaxations and bothachromic shifts, often seen in the crystals of the conventional luminophores or ACQphores. Considering these prospects, the well-known AIEgens (TPE, HPS, etc.) were designed in such a way that their propeller shaped molecular conformations restrict intermolecular π−π stacking interaction and make them highly emissive in their condensed state. This general concept can be applied to any planar ACQ

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system to induce AIE properties through structural manipulation (by disturbing their planarity which will prevent the dense face-to-face packing).

Figure 1. Graphical representation of ACQ NI and AIE/AIEE active NI in solution and aggregated state. Till date, a wide variety of cores with the AIE or AIEE properties have been reported. They include tetraphenylethenes (TPE), siloles, diphenyldibenzofulvenes, anthracene derivatives, pyrrole, cyanostilbenes, phenothiazine derivatives and diketopyrrolo-pyrrole derivatives, ESIPT molecules, phosphole and organoborons, o-carborane dyes phosphindole oxides, pyrazine derivatives, metal complexes, NIs etc.9,12,13 In the past decades, although N-imide functionalized NI probes have been widely used as a classical fluorophores, their ACQ properties screen their potential applications. To the best of our knowledge in 2011 Chang and co-workers reported AIEE active NIs for the first time by extended the π-conjugation by core substitution with rigid aromatic units.14 These V-shaped non-planar and flexible NIs were reported to form fluorescence organic nanoparticles with AIEE characteristic. As shown in Figure 1, the highly fluorescent planar NI molecules (in solution state) were extensively affected by dense face-to-face packing and got quenched in aggregated state through H-

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aggregation. However, their core substitution with rigid aromatic core restrict their H-type aggregation (non-fluorescent or weakly fluorescent in solution state due to dynamic rotation and vibration), and became highly fluorescent in aggregated and solid state (Figure 1). This example established that an ACQ fluorophore can be easily transformed into an AIE-active material by breaking its planarity through rigid aromatic core substitution. There are also some other methods, such as appending ACQ units on a polymeric chain to synthesize AIEgens. The basic aim of such structural transformation is to restrict the molecules to form H-aggregates through π-π stacking in poor solvent or solid state.

Figure 2. Possible technological applications of the AIE/AIEE active NIs. Subsequently, this ACQ hurdle was potentially resolved by well-designed structural modifications by various research groups. To date number of NI derivatives have been reported with high fluorescence quantum yields in solid or aggregated state by introducing different organic moieties at 4th position or by N-functionalization, and opened up various restricted areas with potential applications (Figure 2). These transformed luminogens could be efficiently employed in OLEDs along with brightening material to stain living cells. Most

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of the luminogens are usually insoluble in aqueous media yet can form highly stable fluorescent organic nanoparticles. These nanoparticles with higher surface area along with their inherent molecular properties can be used for various biological activities both in vitro and in vivo. Thus, it is evident that the NI derivatives have found wide applications, in the past few years and many outstanding examples of NI based AIE/AIEEgens have been reported, that demonstrate the high versatility of this NI core by integrating them with the young concept of AIE/AIEE.

Figure 3. Representation of NI core indicating the most reactive positions (red star). Due to the easy N-functionalization of the 1,8-Naphthalic anhydride precursor with simple Gabriel phthalimide synthesis, most literatures have reported substituting the non-polar alkyl groups to form imide linkages which enhanced the solubility in organic solvents. Apart from this, some other reactive positions are available on naphthalene moiety, as shown in the Figure 3, which could be easily functionalized by rigid aromatic core to transform the ACQ fluorophore into AIE/AIEEgens. In particular, the 4th position has been manipulated in most of the examples due to their easy and cost-effective synthetic route. The various facile synthetic routes that have evolved in past decades are summarized in Scheme 1.

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Scheme 1. Possible synthetic routes for producing the AIE/AIEE active NIs. This review concentrates on the importance and versatility of NI core integrated with AIE properties for the development of various smart materials; beginning with a short introduction on the significance of the NI core, a discussion on the hurdles for their wide application as a result of ACQ phenomenon, followed by the introduction and mechanism of AIE in NI derivatives. The discussion part is tracked by an overview of the upto date report of NI AIE/AIEEgens. The instances summarized herein are mainly grouped according to the bond connectivity to the NI core (Figure 4), which mainly involved the N-functionalization and the 4th position substitutions.

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Figure 4. Classified model structures of NIs based on their bond connectivity.

2. NIs with N-functionalization (I) Four NI (1a, 1b, 1c and 1d) based water soluble fluorophores with carboxylic groups were developed by Sun and co-workers and were successfully utilized for turn-on sensor of casein through the AIE phenomenon.15-18 These NIs were applied for its quantification in milk powder samples. The molecules possess donor and acceptor units that can modulate its photophysical properties by varying solvent polarity. Self-association and TICT played a significant role that caused to decrease the fluorescence intensity in solution state. These molecules bind efficiently with Tyrosine and Tryptophan residues present in the hydrophobic cavity of casein that were consequently responsible for the aggregation of molecules on the casein surface with enhanced emission. Accordingly, a novel casein assay method was developed with a detection limit of 5.6 ng/mL for the water soluble NI fluorophore 1a. The highly reversible and reproducible result achieved from this method was found to be in well agreement with the Biuret technique result for casein quantification in milk powder. However, the NI fluorophores with single electron-donating group affected the binding of 1a with casein due to the inhibition of PET in the presence of transition metal ions Fe3+ or Cu2+ and showed high fluorescence enhancement. The sensitivity and selectivity was improved for 1b, 1c and 1d fluorophores, which were developed by changing the core substitution at 4th position. The sensitivity was increased up to a limit of detection of 2 ng/mL, (LOD for 1a, 5.6 ng/mL; 1b, 3.0 ng/mL; 1c, 2.8 ng/mL) but the interference of transition metal ions like

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Fe3+ and/or Cu2+ remained as a drawback for these AIEgen systems. The highest sensitivity was achieved with fluorophore 1d due to the presence of four carboxylic acid groups which were bound to casein more compactly and became more hydrophobic (Figure 5). Between casein and fluorophores, a new complex formation was responsible for the AIE phenomenon, which reduced the solubility in water.

Figure 5. Possible mechanisms of AIE/AIEE of 1a, 1b, 1c and 1d with casein. Reproduced by permission from ref 17 and 18. Copyright 2013 Elsevier. NIs based simple donor-acceptor (Figure 6) AIEE luminogen 2 with positive solvatochromism was reported and applied for proton sensor by Yang et al.19 The absorption and emission wavelengths were red shifted and the emission intensities were reduced from nonpolar to polar solvents. The fluorescence spectra were recorded under different pH conditions and a gradual increase in the blue shifted emission intensity was observed by changing the pH range from 7.6 to 5.0. This was mainly due to the protonation on the imide moiety that restricted the photoinduced electron transfer (PET) in the molecules and served as a proton sensor.

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Figure 6. Chemical structure of 2, suggested mechanisms for AIEE and TICT processes and DFT optimized structures of protonated compound 2 (DONH+) with electron distribution in HOMO and LUMO. Reproduced by permission from ref 19. Copyright 2012 Elsevier. Sarkar et al. reported a series of 4-bromonaphthalimide (3-Cn) congeners with different alkyl side chains (3-Cn; n=4, 6, 10, 12 and 16) to fabricate organic fluorescent micro materials (Figure 7).20 In this study, the effect of alkyl chains that played the character analogous to that of stabilizing additives on the aggregation behavior was investigated by spectroscopic and microscopic techniques. To investigate the AIE phenomenon, they performed fluorescence study for 3-C12 by changing water fraction (fw) in DMSO, clearly indicating that the emission intensity gradually increased up to 70% on increasing the fw due to the formation of nano-aggregates (which were confirmed by FESEM and DLS studies). Further increase in the fw caused to decrease the fluorescence intensity due to the formation of agglomeration. Microscopic investigation exposed the decline in the aggregates size with an increase in the length of alkyl side chain. The microscopic investigation revealed that with increasing length of the alkyl chain the nano-aggregate shapes changed from rod-like to spherical. It was concluded that the alkyl chain length dependent aggregation properties could be due to the

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interplay between the intermolecular forces. The photophysical properties clearly indicated that the emission and absorption spectra were red-shifted for all fluorophores from molecular forms to aggregates owing to the J-aggregates formation. A confocal fluorescence microscopic examination was also conducted and the long chain systems were found to have the cell permeability that could be applicable in live cells imaging.

Figure 7. Chemical structure of 3 and probable aggregation arrangement of molecules with different alkyl chain length. Reproduced by permission from ref 20. Copyright 2013 American Chemical Society. Xu et al. reported (Figure 8) highly sensitive and selective “turn-on” fluorescent probe to detect Fe3+ in aqueous media based on novel NI-diethylenetriamine-quinoline (4) derivative.21 The AIEE activity of the probe was analyzed in acetonitrile and water mixtures where 4 formed fluorescent nanoparticles in water, which were confirmed by the TEM analysis. These nanoparticles were successfully utilized for the imaging studies of the Fe3+ in the living cells. The nanoparticles exhibited super optical properties with excellent biocompatibility and could endorse a possible convenient assay for disease-related studies.

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Figure 8. Graphical representation of the synthesis of 4 FONs and the strategy for Fe3+ detection and imaging. Reproduced by permission from ref 21. Copyright 2014 The Royal Society of Chemistry. In this study, Liu et al. demonstrated an easy synthetic and structurally simple NI luminogen with intense blue emission.22 The novel NI (5) displayed significant luminescence vapochromic and thermochromic characteristic owing to the interconversion in the dynamic self-assembly from J-type aggregate to H-type excimer state (Figure 9). The absolute quantum yield of 5 exhibited 82.33% in solid state. Through a comparative study, the prime role of bromine atom in the molecular self-assembly was confirmed through the single crystal analysis. The strong blue emission could be due to J-type aggregation as confirmed from the single crystal analysis. However, it was assumed that the poor intermolecular C-H…O hydrogen bonding could disappear with heating or exposing to CH2Cl2 and CHCl3 vapors. This resulted in the transition of the crystal packing from J-type aggregation to H-type excimer formation, similar to the crystal packing of 6 (Figure 10). Again the molecular packing transition was reversible that makes the simple NIs potentially effective candidates to develop stimuli-responsive fluorescent materials.

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Figure 9. (a) The chemical structure of 5 and (b) the single crystal structure of 5. (c) A sketch of the probable thermochromism and vapochromism mechanisms of 5. Reproduced by permission from ref 22. Copyright 2014 The Royal Society of Chemistry.

Figure 10. (a) The chemical structure, (b) The single crystal structure, (c) the structural arrangement showing π—π interactions, (d) the fluorescence image of 6 crystals under 365 nm UV illumination. Reproduced by permission from ref 22. Copyright 2014 The Royal Society of Chemistry. Cho and co-workers investigated the effect of solvent polarity on the nature of aggregation. They reported two NI AIEgens to describe the reason of AIE phenomenon in different types of NI derivatives (7 and 8).23 The chemical structures of luminogens are shown in Figure 11. Theoretical calculation suggested the lowering of T2 state compared to the S1 state in nonpolar solvents that facilitated the intersystem crossing. Whereas, the emission intensity got increased by increasing the solvent polarity (or Protic polar solvent fraction like water)

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and could be attributed to the S1 and T2 states of energy ordering (Figure 12). As shown in Figure 13 the luminogen 7 showed red-shifted AIE in highly polar or nonpolar solvents and can be correlated with the formation of excimer through intermolecular π-π interaction. However, in the case of 8, with D-A combination, AIE could be due to intramolecular charge transfer complex (exciplex) formation.

Figure 11. Chemical structures of 7 and 8. Schematic diagram showing solubility trends versus aggregation behavior. Reproduced by permission from ref 23. Copyright 2014 The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS) and the RSC.

Figure 12. Representative energy diagram for NI derivatives in (a) highly nonpolar and (b) highly polar solvents. Reproduced by permission from ref 23. Copyright 2014 The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS) and the RSC.

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Figure 13. (a) Emission spectra of 7 with 15 µM concentration (at different fw), and (c) 25 µM concentration of 7 in H2O-CH3CN. λex = 334 nm. Inset: photographs of vials under 365 nm UV illumination, and (c) fluorescence spectra of 8 in CH3CN solutions having fw in the range of 0-70%. (b and d) Fluorescence quantum yields of 7 and 8 in CH3CN solutions at different fw in the range of 0-95%. Reproduced by permission from ref 23. Copyright 2014 The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS) and the RSC. Smet et al. reported for the first time an AIE-active hyperbranched polymer nanoparticle (Figure 14).24 The NI based AIE polymer nanoparticles were ratiometrically utilized for intracellular pH sensor. These polymers showed excellent biocompatibility and can also be selectively accumulated in acidic organelles of living cells. Before synthesizing the polymers, the AIE activity of the 9 and 10 monomers were examined in different DMF/Water and both

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Figure 14. Chemical structure of 9 and 10. Representation of hyperbranched polylactide polymers. (a) Normalized PL spectra of P1-P5 in K2HPO4/KH2PO4 buffer solutions (pH 7.0), (b) Fluorescence images of P1-P5 observed under 365 nm UV illumination, (c) & (d) SEM images of P2, P3 polymer nanoparticles, (e) normalized (I450

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PL spectra of P3 (20 µg

mL−1) in 0.1 M K2HPO4/KH2PO4 buffer at various pH, (f) study of pH reversibility of P3 between pH 5.0 and 8.1. λex = 365 nm. Reproduced by permission from ref 24. Copyright 2015 American Chemical Society.

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the monomers were found to exhibit AIE properties in water with blue and green emissions respectively. To have a better understanding on the aggregation properties of the hyperbranched polymer along with the role of individual 9 and 10 monomers, five polymers (P1-P5) were synthesized by varying the ratio (the molar seed ratios of 9 and 10 are 1:0, 3.3:1, 1:1, 1:2.5, and 0:1 for P1-P5 respectively) of 9 and 10. Among all polymers the P1 polymer exhibited the AIE activity efficiently. In addition, polymer P6 was also synthesized for the comparison without using any 9 and 10 units, which showed strong fluorescence in DMF but gets quenched in water. The polymer nano-particles were prepared by nanoprecipitation method. The polymers showed ratiometric color change with pH. The pH responsive polymers were studied by the PL spectra (Figure 14a) at different pH ranges from 4.7 to 8.0. Among all the polymers, P3 demonstrated huge tuning of emission color from blue (~ 450 nm) to green (~520 nm) while moving from pH 8.1 to 5 respectively. The pH response is due to the intramolecular PET from the donor to the NI acceptor moiety. The quantitative investigations of intracellular pH in HeLa cells were performed with this ratiometric nanosensor that gave the first example to construct intracellular pH sensor with AIE hyperbranched polymer nanoparticles. Confocal fluorescence microscopy photographs of HeLa cells are given in Figure 15.

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Figure 15. Confocal fluorescence microscopy photographs of HeLa cells incubated with (a, b) P2, (e, f) P3, and (i, j) P4-based nano-sensors and (c, g, k) Lyso Tracker Deep Red. The fluorescence images of first, second, and third column were obtained in blue channel (430490 nm, λex = 405 nm), green channel (500-550 nm, λex = 405 nm), and red channel (650-700 nm, λex = 635 nm). (d, h, l) Merged images of blue, green, red channels. Reproduced by permission from ref 24. Copyright 2015 American Chemical Society. The effect of core substituted halogens on the self-aggregation behavior of NI derivatives (11, 12, 13 and 14) was explored by Bhattacharya et al.25 These luminogens exhibited the unusual AIEE behavior in aqueous solution. In their work the mechanistic impact of chlorosubstitution toward intermolecular electrostatic communication to shape up nanorings via Jtype aggregation was emphasized. The J-aggregation formation via in-plane oblique angle conformation was also justified by the angle restriction rule proposed by Kasha. According to Kasha’s rule, J- or H-aggregates can be defined by the angle (φ) between the adsorption surface and the longitudinal axis of the molecules. φ >54.7° signified the formation of Haggregates, whereas for φ 50%, the emission intensity decreased, due to the nanoparticle formation (as confirmed by the SEM and DLS analysis). This process was attributed to the TICT. Apart from good optical properties, these molecules showed sensitive fluorescence response towards nitroaromatic explosives. It was observed that the congener with SiMe3 group exhibited enhanced optical properties and higher sensitivity towards nitroaromatic detection.

Figure 51. (a) Normalized fluorescence spectra of 71 (a) and 72 (b) in various solvents and digital photographs of 71 in various solvents under 365 nm UV illumination. Reproduced by permission from ref 50. Copyright 2016 The Royal Society of Chemistry.

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Figure 52. Emission spectra of 71 (a) and 72 (b) in the CH3OH–H2O mixture with varying fw. (c) Plots of fluorescent areas of 71 and 72 versus the fw in the CH3OH–H2O mixture (d) Digital photographs of 71 and 72 with different fw under UV illumination at 365 nm. Reproduced by permission from ref 50. Copyright 2016 The Royal Society of Chemistry. Similarly, Zhang and co-workers designed another AIEE active NI based D-A fluorescent system (73) by substituting anthracene unit and employed them to detect picric acid in aqueous solution (LOD = 0.47 µM).51 The AIEE activity of the fluorophore was investigated in THF and water mixtures (Figure 53a). The fluorophore became non emissive when fw reached 50%, due to the dipole-dipole interaction between the fluorescent molecule with solvent molecules in excited state. After fw exceeded >50%, the fluorescence intensity increased up to 100%. The typical intramolecular charge transfer (ICT) emission was blocked due to the proton transfer from picric acid to 73 as confirmed by NMR analysis and contributed towards the sensing mechanism (Figure 53b).

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Figure 53. (a) The emission spectra of 73 measured in water/THF mixture solvent with varying fw. The inset: the variation of emission with different fw. (b) Schematic illustration of the proposed intermolecular interaction mode and 1H NMR spectra of 73 (10 µM) is with and without TNP in CDCl3. Reproduced by permission from ref 51. Copyright 2016 Elsevier. Xu and co-workers designed smart material based on bisnaphthalimide (74) where two naphthalimide cores were conjugated at the 4th position via a twisted C—C single bond.52 The molecule behaved as a double-channel rotor due to the presence of two naphthalimide planes. The bisnaphthalimide derivative showed emission at the shorter wavelength in its twisted state whereas emission at longer wavelength was observed in its planar state (Figure 54). In DMSO solution, due to dynamic rotation of the naphthalimide units, the molecule was almost non-emissive, but in case of solvents with high viscosity or in poor solvent where it formed nanoaggregates, it showed strong emission character owing to the restriction of intramolecular rotation (RIR) effects. The AIE activity was investigated in three different solvent systems, including THF/water, DMSO/water (Figure 54), and acetonitrile/water in varying ratios. It was observed that, formation of a more polar non-radiative ICT state occurred in DMSO with weak ICT fluorescence, whereas ICT fluorescence was quite high in THF and acetonitrile. But, the synergetic effects of intramolecular planarization and RIR led the molecule to emit intensely at longer wavelength by the addition of water due to aggregate formation. Thus, in this study, a simple fluorometric method for the detection of viscosity has been developed by using the bisnaphthalimide with dramatic variation in both emission

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intensity and wavelength. The fluorogen produced a logarithmic response to the viscosity of a water/glycerol system with varying ratios. Additionally, the fluorogen demonstrated high sensitivity towards temperature-related viscosity disparities that could also work in the presence of common fluorescent dyes. Thus the property could be of high significance in many biological and mechanical systems as temperature always alters the surrounding viscosity.

Figure 54. Mechanistic overview of the AIE-based fluorescent rotor (74). AIE properties. Emission spectra (A) and emission intensity (B) of 74 with change in the fw from 0 to 99% in DMSO solution. [74] = 10 µM. Reproduced by permission from ref 52. Copyright 2016 The Royal Society of Chemistry. Recently, Iyer et al designed and synthesized four D-π-A novel organic molecules (75-76) based on NIs and monosubstituted dibenzofulvenes and studied the effect of heavy atom and other photophysical properties (Figure 55).53 Despite the very similar structural building of the four NIs, two compounds exhibited the AIEE phenomena whereas remaining two

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compounds displayed weak AIEE and ACQ property (Figure 56). The four luminogens displayed different photophysical properties based on the bridge (thiophene and phenyl) between D and A. The AIEE active luminogens displayed orange to red emission (575 nm and 602 nm) with large bathochromic shifts (35 nm & 112 nm) in the aggregated state owing to ladder type J-aggregate formation with high quantum yields of 84.10% & 65.65% for 75a and 75b respectively. The nanoaggregate formation was confirmed by the FESEM and DLS analysis. In contrast, the phenyl bridged derivatives showed poor AIEE characteristic (76a) in blue region and ACQ nature (76b) due to the strong C-H···π intermolecular interactions (confirmed by the single crystal X-ray study). Additionally, theoretical analyses were carried out to support the photophysical characteristic of the AIEEgens. Also the HOMO and LUMO energy levels were calculated by theoretical study and cyclic voltammetry. The thermal behavior of the NIs were studied by TGA and DSC and the results showed good thermal stabilities.

Figure 55. Chemical structures of 75 and 76. ORTEP diagrams a) & b) and crystal packing (c) & (d) of 76a and 76b. Reproduced by permission from ref 53. Copyright 2017 The Royal Society of Chemistry.

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Figure 56. PL spectra and relative emission intensity of 75a (a) & (b) 75b (c) & (d) 76a (e) & (f) and 76b (g) & (h) in THF at different fw. The photographs were taken under 365 nm UV illumination; the number specifies the computed quantum yields using an integrating sphere. Reproduced by permission from ref 53. Copyright 2017 The Royal Society of Chemistry.

5. NIs with C—C double and triple bond substitutions (V and VI) A series of NI-rhodamine dye (77-81) was designed and synthesized by Qian et al. by varying the terminal substituent at the 4th position of NI core.54,55 The rhodamine moiety was used as a rigid and bulky substituent to boost the AIE characteristic. All the NI-rhodamine congeners exhibited fluorescence emission in the condensed state signifying the AIEE phenomena (Figure 57). Although the dyes showed fluorescence in their aggregated state, their poor solubility in pure water makes their application limited. Thus, the as-prepared dye-doped fluorescent silica nanoparticles (Si-NPs) exhibited good solubility in pure water. It was also observed that the dye 77 and 78 displayed selective recognition towards Hg2+ ions due to the presence of terminal N,N-dimethylethylenediamino unit whereas 81 showed selective recognition towards both Fe3+ along with Hg2+ due to the presence of dipicolylamine moiety. The congeners showed high fluorescence emission in their solid state with tunability. Both the photo-induced electron transfer (PET) and fluorescence resonance energy transfer (FRET) endorsed a ratiometric detection of Hg2+/Fe3+ in 81 with a detection limit of 2.72 × 10-6 M

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and 5.7 ×10-7 M in EtOH/PBS buffer (1:1, v/v, pH = 7.4) solution respectively (Figure 58). Apart from the detection of Hg2+, the dye 77 and 78 were also employed to image MCF-7 breast cancer cells and Hela cells and certified their potential in biology (Figure 59).

Figure 57. Chemical structures of 77-81. The normalized emission spectra (a) and the CIE chromaticity coordinate (b) of dye 77-80 in solid state. Inset: solid state fluorescence images under the UV lamp. Reproduced by permission from ref 54. Copyright 2016 Elsevier.

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Figure 58. The suggested dual-switch sensing mechanism of probe 81 for the detection of (a) Hg2+ and (b) Fe3+.. Reproduced by permission from ref 55. Copyright 2017 Elsevier.

Figure 59. Fluorescence photographs of dye 77 in MCF-7 breast cancer cells (a) and dye 78 in Hela cells (b). Reproduced by permission from ref 54. Copyright 2016 Elsevier. A series of novel NI derivatives (82-86) with D-A architecture were reported Chang et al. by substituting 4-methoxystyrene, 4-(N,N-diphenylamino)styrene, 4-nitrostyrene and 4hydroxystyrene moieties and presented their absorption and PL properties.14 Their AIEE characteristics were explored and their practicability was illustrated through cellular imaging. The styrene derivatives substituted 83 and 84 displayed low fluorescence in dilute solutions but proficiently emitted in aggregated state (Figure 60a-b). The emission intensity of 84 amplified drastically when aqueous acid solution was added due to the formation of selfassembled fluorescent nanoparticles by the protonated 84 (84-3H+). All the derivatives were well dispersed in the cytoplasm with good cell penetrability. Owing to their AIEE properties, bright cellular imaging of Human breast adenocarcinoma cell line (MCF7) was achieved by these probes at different emission wavelengths at a fixed excitation wavelength (Figure 60c). 82 and 83 (Figure 61) were co-cultured with MRC-5 normal lung fibroblast cells and CL1-0 lung cancer cells and the results indicated that, although 82 was not capable to differentiate normal cells and cancer cells, molecule 83 was found to disperse in the cytoplasm of normal living cells whereas they get accumulated in cancer cells resulting in the bright imaging

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capability. These different cellular staining arrays signified that 83 could be employed to discriminate cancer cells from normal cells.

Figure 60. Chemical structures of 82-86. Plots of FONs for 84; a) plots of the fluorescence quantum yields of different solvent fractions versus [H+] values. b) Fluorescence changes for 84 under 0, 25, 50, 75 and 100 percentages of 10 mM HCl aqueous solutions in THF. (λex = 400 nm.) Insets: digital pictures and TEM images of nanoparticles achieved at 10 mM HCl aqueous solutions in THF, 25/75 v/v, and scale bar: 50 nm. (c) The corresponding bright field picture (left) and fluorescence photographs (right, light path through a 380/10 nm bp filter and fluorescence was measured and filtered through a 450-nm lp filter) of MCF-7 breast cancer cells incubated with 10 µM 82 (NIM-1), 83 (NIM-2) and 84 (NIM-3) compounds for 4 h respectively. Reproduced by permission from ref 14. Copyright 2011 The Royal Society of Chemistry.

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Figure 61. Confocal microscopy photographs of co-cultured CL1-0 lung cancer and MRC-5 normal lung fibroblast cell lines incubated with 10 mM probe for 4 h, and 405 nm light source was applied to visualize the cells. Compound 82 (NIM-1): a) Bright field image; b) spectra measured at 450–560 nm in PMT 1; c) 530–650 nm in PMT 2; d) merger of b with c; i.) λ scanning from 460 to 650 nm with 5 nm integration. Compound 83 (NIM-2): e) Bright field image; f) spectra measured at 450–590 nm in PMT 1; g) 560–740 nm in PMT 2; h) merger of f with g; j) λ scanning from 450 to 780 nm with 10 nm integration, insert photo was the zoom in of h but filter cut at 560 nm. 50 individual cancer and normal cells were taken and the average intensities were plotted to get the spectra. Reproduced by permission from ref 14. Copyright 2011 The Royal Society of Chemistry.

O

N

OCH3

O Ar= 87

NMe2

OCH3

OCH3 88

Ar

Figure 62. Chemical structures of 87-89.

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Panchenko and co-workers synthesized a series of styryl NI derivatives (87-89) and investigated their photophysical properties in detail considering their pronounced potential of being used as imaging agents in vivo (Figure 62).56 The key relaxation pathways of the local excited singlet state of the NIs were found to be fluorescence, E,Z-photoisomerization and formation of the TICT state that could be controlled through solvent polarity or structural modulation. All congeners showed a positive solvatochromism in the steady-state fluorescence and absorption spectra. In non-polar solvents like cyclohexane and toluene, isomerization took place effectively where the planar local excited (LE) states were dominant. The introduction of high polar solvent was favored by the twisting between styryl and NI fragments leading to the formation of a TICT state and was confirmed through quantum-chemical calculations and NMR spectroscopy analysis (Figure 63).

Figure 63. Aromatic and aliphatic region of the 1H NMR spectrum of compound 89 in toluene-d8 before (top) and after (bottom) illumination at 436 nm. The Z-isomer signals were marked with color. [89] = 2 × 10-2 M. Reproduced by permission from ref 56. Copyright 2017 the PCCP Owner Societies.

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N O O

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O O N

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Figure 64. Chemical structures of fluorophores 90-94. Organic light emitting diodes (OLEDs) have been considered as the ‘‘next generation of lighting technology’’ and the ‘‘third generation of flat panel displays’’, that have attracted much attention in both industrial and scientific societies. Extremely proficient fluorescent OLEDs are highly favorable for display applications owing to their quick response time, longer operational lifetime and comparatively low efficiency roll-off at high current densities. In this respect, Yu et al. developed AIE active fluorophores for various roles in OLED applications (90-94) (Figure 64).57 Using this AIEgen they fabricated the non-doped and heavily-doped red OLEDs. They designed two guest fluorophores with NI and fluorene moiety bridged through an ethenylene bond to form a red D-π-A luminogens. One of them (91) was found to be AIE-active, which makes it a potential nominee to be a guest to fabricate heavily-doped red OLEDs (Figure 65). To procedure an effective energy transfer guest/host pair, a structurally similar green host material 92 was synthesized. The device configuration was: ITO/NPB (30 nm)/CBP (2 nm)/92:91 (x wt%) (20 nm)/Bphen (40 nm)/Mg:Ag (200 nm). As a result of the AIE behavior of the guest molecule, and the effective energy-transfer between the structurally similar host and guest as well, the 6 wt% heavily-doped NI based devices showed good performance, with maximum brightness of 6250 cd m-2 and current efficiency of 3.13 cd A-1. To prove that 92 was a better host for 91 they have also fabricated OLED device by using Alq3 instead of 92 since the host was having much inferior EL properties despite its comparatively efficient spectral match, which could

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be attributed to the insufficient energy transfer between Alq3 and 91 and achieved maximum brightness whereas the luminous efficiency was 1160 cdm-2 and 0.75cd A-1 respectively. Thus, it could be concluded that the design of suitable host/guest pair for efficient energy transfer is of extreme importance for the enhanced device performance and also certified the naphthalimide derivatives as promising host/guest material for fabrication of heavily-doped red OLED. They have also fabricated non-doped OLEDs with ITO/NPB (30 nm)/91 (20 nm)/Bphen (40 nm)/Mg:Ag (200 nm) device configuration. These devices gave maximum brightness and luminous efficiency of 3520 cdm-2 and 0.55 cd A-1 respectively.

Figure 65. a) Normalized absorption and emission spectra of 90 and 91 in polycrystalline powder state and in THF solution (10 µM). Insets: images of the polycrystalline powder of 90 (A, B), 91 (C, D) under white light (top) and UV light at 365 nm (bottom). b) Emission spectra of 91 in acetonitrile at different fw. (Concentration of 91: 10 µM, λex = 460 nm). Insets: digital photographs of 91 in acetonitrile/water mixture under UV light at 365 nm. Reproduced by permission from ref 57. Copyright 2014 Elsevier. According to the electroluminescence (EL) mechanism, OLEDs are of two types, namely fluorescent (FOLEDs) which is generally limited to 25% internal quantum efficiency (IQEmax) and phosphorescent ones (PhOLEDs), although a high IQEmax of 100% could be attained, but the use of rare metal complexes make them costly.58-60 However, through several mechanisms like thermally activated delayed fluorescence (TADF), triplet fusion delayed

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fluorescence (TFDF), and hybridized local and charge-transfer excited states (HLCT), FOLEDs can be harvested via both singlet and triplet excitons. For TADF-OLEDs, design of small singlet–triplet energy gap (DEST) with high radiative rate within TADF is quite hard. While for HLCT materials, the internal conversion from Tn to Tm states (m ˂ n) should be blocked by disobeying Kasha’s rule.61,62 Thus, the balanced molecular design of HLCT compounds are still highly challenging.

Figure 66. (a) Current density-voltage-luminance characteristics curves, (b) current efficiency-current density curves for Device III (Inset: EL spectra of Device III), and (c) Representation of the proposed TTA and energy transfer (ET) processes in Device II through energy level diagram. The energy levels were for 94 (host) and 91 (guest). Reproduced by permission from ref 58. Copyright 2014 The Royal Society of Chemistry. However, for OLEDs constructed on the basis of TFDF compounds, their IQEmax could reach up to 62.5% by the up-conversion of triplet into singlet efficiently through the TTA process.63-66 The triplet excitons could harvest efficiently by the host compounds via energy transfer to emitting guest materials. However, small molecule based TF compounds are limited to anthracene derivatives, Alq3, and rubrene, whose S1 and T1 states are both of 3ππ* character. In 2014, Lu et al. reported 94 as a promising TFDF-OLED material with ICT ACS Paragon Plus Environment

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properties and small exchange energy but a lower lying 3ππ* state than the 3CT state (Figure 66).79 By using 94 as host material with the same guest material 91, they fabricated device following the previous configuration as: ITO/NPB (30 nm)/CBP (2 nm)/91:94 (6 wt%, 20 nm)/Bphen (40 nm)/Mg:Ag. The orange-red emitting electroluminescent device with a brightness of 16840 cdm-2 and a highest current efficiency of 7.2 cd A-1 achieved was more than 2-fold as compared to their previous report using the same guest material (Figure 66). Lu et al. further reported newly designed 93, an ICT-featured red D-π-A TFDF fluorophore, and used it as a guest material with the 94 host material to fabricate OLED devices (Figure 67).67 As both 93 and 94 were NI derivatives, they could generate a proficient host/guest energy transfer pair and the presence of a large diphenylamine D-moiety in 93 could be equipped with less intermolecular interactions. To find out that 93 was a TADF material capable of harvesting triplet excitons via the TTA process, they fabricated the non-doped OLEDs (Device-I) with device configuration ITO/MoO3 (1 nm)/TCTA (40 nm)/CBP (2 nm)/93 (20 nm)/ TPBI (45 nm)/ LiF (1 nm)/Al (80 nm). The device showed the highest brightness and current efficiency was 5100 cd m-2 and 0.53 cd A-1 respectively. The maximum EQE obtained for this device was 0.59 %, which is quite higher than that predicted from the 25% singlet production limit (0.34%). Further, the heavily doped OLEDs were fabricated with two different doping ratios of 93 (1.4 wt% and 6.0 wt%). The 1.4 wt% doped device (Device-II) exhibited the yellow color emission at 570 nm, with maximum brightness, EQE and current efficiency of 24900 cd m-2, 2.49 % and 3.19 cd A-1 respectively. Another device (Device-III) which was made using 6.0 wt% exhibited the emission at 583 nm with a highest brightness of 31940 cd m-2 and maximum current efficiency of 7.73 cd A-1, and the maximum EQE was 5.83%. Compared to 1.4 wt%, the 6.0 wt% device performance had improved due to the triplet excitons that could harvest effectively not only by the host materials, but also by the guests through the TTA process. These results certified the importance of TFDF in generating

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proficient OLEDs, and may shed light on the molecular design strategy for host materials to achieve high performance OLED devices to match the market demand.

Figure 67. (a) Current density-luminance-voltage (J-L-V) curves of devices from II to IV and (b) EL spectra of devices from II to V (at 10 V) and (c) Graphical representation for the TTA and energy transfer processes in device III. ). Reproduced by permission from ref 67. Copyright 2015 The Royal Society of Chemistry. AIE materials with intramolecular charge transfer properties are gaining significant attention from both theoretical and experimental researchers owing to their possible applications in optoelectronics and sensors. However, the non-radiative deactivation of AIEgens with the DA combination in polar solvents was generally attributed to the TICT mechanism. The same group, investigated the non-radiative deactivation mechanism of the pre-reported AIE-active NI (91) in the polar solution combining the theoretical and experimental observations.68 It was observed that, 91 emits green color (530 nm) and exhibited 75% quantum yield in cyclohexane, while the emission became invisible in dichloromethane solution. The thorough and systematic investigation on solvent polarity indicated that, in the deactivation process, conformational planarization mechanism would be favored by 91. The geometric

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conformation and the potential energy surface curves of the studied molecule did not support the general TICT phenomena suggested for numerous classical D-π-A systems. The greater reorganization energy in the S1 state and the small energy gap between HOMO and LUMO suggested that changing the solvent polarity from non-polar to polar could encourage the compound to achieve the planarization. As seen, the substantial red-shift in fluorescence, the reduction in the radiative decay rate and the enhancement in the nonradiative decay rate were witnessed by changing the solvent polarity from non-polar to polar. These findings could provide better insights into the earlier studies to justify the debate on the deactivation phenomena of the AIE ICT featured luminogens in solution state and could motivate further studies on the decay mechanism of the other AIE systems. Yi et al. reported two NI gelators based on organometallic terpyridyl platinum, which have hydrophobic cholesterol units by varying the alkyl substituents (Figure 68).69 The complex with three tert-butyl units (95a) was found to form stable gel in various solvent (ethyl acetate, alcohol and toluene) with ultrasound irradiation. These gels showed different color and fluoroscence properties than that of solutions and precipitates. The photoluminescence enhancement of 95a in gel state was due to aggregate formation. The morphology and the surface wettability was tuned through sonication and heating (Figure 69). However, the 95b complex which is not having tert-butyl units displayed inferior gelation capability and solubility along with lower fluorescence quantum yield due to metal–metal and intermolecular π—π interaction. The self-assembled behavior indicated that the gelation and PL properties of complexes were mainly dependent on the steric hindrance of tert-butyl units (Figure 68). Sonication could modify the molecular arrangement of 95a and encourage the hydrophobic and ionic dipolar interactions for gelation. Hence, it changes the photophysical properties of the complex in dense state. This reversible sonication prompted gelation and the PL enhancement could generate new strategies for the application in microfluidic systems in

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therapeutics. Furthermore, these metallogels could emerge as ideal frameworks for the development of supramolecular emitting materials for displaying information with tunable optical properties.

Figure 68. Chemical structure of 95a and 95b with the schematic representation of the gelation mechanism in 95a. (a) heating-cooling and (b) sonication. Reproduced by permission from ref 69. Copyright 2013 The Royal Society of Chemistry.

Figure 69. The SEM (a & b) and TEM (c & d) photographs of 95a in gelated condition made by sonication method (a & c) (0.16 W cm-2, 40 kHz, 30 min, 40 °C), and by cooling down

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from hot solution to room temperature (b & d); inset of (b) is the zoomed view of the nanoparticle of 95a in n-propanol; SEM photographs of 95b after sonication for 60 min (e) and after heating-cooling process (f) in acetonitrile solution; scale bars are 5.0 µm for (a), 2.0 µm for (b), 0.2 µm for (c), 0.1 µm, for (d) 2.0 µm for (e) and 20 µm for (f). Concentration: 40 mg mL-1. Reproduced by permission from ref 69. Copyright 2013 The Royal Society of Chemistry. Generally, a gel exhibits ACQ phenomenon which is undesirable for their light-emitting properties. Yi and co-workers reported AIE-active 4-ethynyl-1,8-naphthalimide based gelators (96 and 97).70 With very high fluorescence quantum yields both in solution as well as in aggregated state, the NI gelator exhibited reversible sol-gel transition and the fluorescent emission had a large red-shift of 80 nm in gel state to that of the solution state (Figures 70 and 73). The NI was able to gelate the polar solvents like EtOH, DMF, ethyl acetate, DMSO, and acetone that were stable at room temperature for more than a week. The hot solution of the NI was blue emitting and became yellow emitting after it was cooled naturally at room temperature for 9 minutes due to the transition of sol to gel. The aggregation behavior was confirmed by the addition of water that showed similar type of red-shift. The optimized models endorsed the presence of highly planar bulky conjugated structure in 96 and 97, which favored strong intermolecular π−π interactions and played the key role in the formation of organogel. The morphology study showed the formation of microbelts in its gelated state. The organogels also exhibited thermally reversible fluorescence from blue to yellow between sol and gel state (Figure 71). Thus, the variation of the fluorescence color between sol and gel could act as a switch for various purposes such as switchable molecular devices and visual recognition sensors.

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Figure 70. Chemical structure of 96 and 97. Photographs of sol−gel transition of 96 in DMF at 0.6% concentration (16 mM): (a, c) the solution and gel states under white light, respectively; (b, d) the solution and gel states under UV irradiation of 365 nm, respectively. Reproduced by permission from ref 70. Copyright 2014 American Chemical Society.

Figure 71. (a) Emission spectra of 96 during the sol-gel transition through natural cooling from 120 to 59 °C (Csol = 16 mM; λex = 390 nm). (b) Emission spectra of 96 in DMF (1.0 × 10-5 M) at various fw (λex = 365 nm) and (c) The corresponding photographs of (b) (the top row was under white light, and the bottom row was under UV irradiation at 365 nm). Reproduced by permission from ref 70. Copyright 2014 American Chemical Society.

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6. CONCLUSION AND PERSPECTIVES Naphthalimide (NI) compounds are imide group containing well-known acceptors and their derivatives have been developed to realize multiple applications since the past several decades. Due to their accepting capacity and high PL quantum yields, NIs has received considerable attention among the scientific community. The AIE activity in NIs opens a new class of AIE active materials with immense possibilities for various real-world applications both in solution and condensed states. Although TPE, TPP and HPS derivatives have been established as an efficient and standard luminogenic cores, a large number of research groups have been enthusiastically participating in this area and have designed many new AIE/AIEEactive derivatives. After the first report on NI luminogens, exploration of the aggregation properties of NIs, owing to their wide potential and easy synthetic route, has increased over the years. Multiple research groups have devoted careful efforts to expand the scope of this core as a possible alternative to existing systems by adding crucial research and information to the existing literature. In this review, we have summarized the up to date progress made with the NI core that demonstrates the versatility of this chromophore. We have presented typical examples of different types of NI AIE/AIEEgens, discussed their high-tech applications as advanced materials along with structure-property relationships. The highly attractive applications of NI luminogens are in the areas of bio and chemosensors and selfassembly studies. Apart from this, various groups are also exploring the NIs in the field of organic light emitting diodes, stimuli response materials, in-vitro and in-vivo imaging of cells and biogenic molecules. Due to the good biocompatibility, photostability and high condense state emission behavior, it makes the NIs promising for in-depth and long-term bioimaging and tracking applications. Apart from the discussed examples, there are plenty of reported NI derivatives that may show AIE/AIEE characteristic and could be extended towards many unexplored possibilities. In this article, we have attempted to include all those results where

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authors have mentioned the aggregation properties of the NIs and their derivatives. Hence, the future of the exponentially spreading research on the AIE/AIEE-active NI family in this decade seems very rich, bright and multicolored. We eagerly look forward to new findings and developments with these potentially amazing NIs derivatives. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (P.K.I.). Authors Contributions §

P.G. and N.M. contributed equally.

Notes The authors declare no competing financial interest. ORCID P. K. Iyer: 0000-0003-4126-3774 P. Gopikrishna: 0000-0002-4408-2701 N. Meher: 0000-0003-3558-3712 ACKNOWLEDGMENTS The authors thank the Department of Science and Technology (DST), New Delhi, (No. DST/SERB/EMR/2014/000034), (No. DST/TSG/PT/2009/23), DST-Max Planck Society, Germany (No. IGSTC/MPG/PG(PKI)/2011A/48) and Department of Electronics & Information Technology, (DeitY) No. 5(9)/2012-NANO (Vol. II), for financial support.

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