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A donor–acceptor substituted derivative 2,6-diamino-4-(3,4,5-trimethoxy-phenyl)-pyridine-3,5-dicarbonitrile (DTPP) has been synthesized and its photophysical properties have been studied. Effect of solvent on the photophysical features of DTPP has been undertaken by steady state absorption and emission techniques. Strong solvatochromic emission has been observed due to intramolecular charge transfer characteristics, upon changing the solvent polarity, revealing the highly polar character of the excited state. Dipole moment changes between the excited and ground state have been estimated by using the theory of solvatochromism from Lippert–Mataga and Reichardt’s correlations. The geometrical parameters for ground and excited states, conformational flexibility, and NLO behavior of the molecule have been theoretically investigated. The electronic distributions of DTPP in HOMO and LUMO were also investigated using density functional theory (DFT) methods at B3LYP/6-31 G** level. The correlation of experimental results with theoretical predictions obtained via DFT substantiates the presence of polarity dependent of the emission spectra.

The halogen substitution in an organic fluorophore exerted significant influence on the molecular organization via halogen intermolecular interactions and solid-state fluorescence. The carbazole core unit has been extensively utilized for developing optoelectronic materials because of its excellent fluorescence and phosphorescence properties. Herein, we have synthesized dihalogen substituted carbazoles (Cz-Cl, Cz-Br, Cz-I) and investigated their role in the supramolecular organization and solid-state fluorescence. Solid-state structural analysis revealed that Cz and halogen derivatives produced a herringbone structure but with varied intermolecular interactions and molecular orientation. Cz showed N–H⋯π intermolecular interactions between the amine hydrogen and middle ring of carbazole. Cz-Cl and Cz-Br also showed N–H⋯π intermolecular interactions but between the amine hydrogen and halogen decorated phenyl ring of carbazole. Cz-I showed only weak halogen–halogen and halogen–hydrogen intermolecular interactions. Cz showed strong fluorescence in solution as well as the solid-state (Φf = 55.9%). However, halogen substituted carbazole derivatives showed only solid-state fluorescence with strong reduction of fluorescence efficiency (Φf = 6.5% (Cz-Cl), 1.2% (Cz-Br)). Further, Cz-I did not show any fluorescence even in the solid-state. The significant reduction of fluorescence might be attributed to the heavy atom effect. Halogen substitution also caused varied electron density distribution in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Thus, halogen substitution influenced the molecular organization via halogen intermolecular interactions and solid-state fluorescence due to the heavy atom effect. [ABSTRACT FROM AUTHOR]

The condensation reaction between naphthylacetonitrile isomers and anthracene aldehyde produced unexpected highly twisted AIEgens (2-(naphthalen-1-yl)-2-(10-oxo-9,10-dihydroanthracen-9-yl)acetonitrile (1) and (Z)-3-(anthracen-9-yl)-2-(naphthalen-2-yl)acrylonitrile (2)). 1 and 2 exhibited strong solid-state fluorescence and mechanical stimuli-induced reversible fluorescence switching. Crystallization of 2 in acidic CHCl3–CH3OH produced Cl substituted anthracene (2-Cl), whereas in CH2Cl2 pure crystals of 2 were produced. Single crystal analysis of 1 revealed a twisted conformation in the middle ring of anthracene due to carbonyl introduction. The carbonyl oxygen and cyano nitrogen are involved in the H-bonding interactions in the crystal lattice. 2 showed slipped stacking between naphthyl units whereas 2-Cl exhibited intermolecular H-bonding between Cl and anthracene hydrogens. 1 exhibited strong solid-state fluorescence (λmax = 541 nm, quantum yield (Φf) = 18.2%), whereas 2 and 2-Cl displayed tunable and relatively low fluorescence efficiency (λmax = 547 (2), 489 nm (2-Cl), Φf = 6.3 (2) and 3.6% (2-Cl)). Computational studies suggested clear intramolecular charge transfer (ICT) in 1 compared to 2 and 2-Cl. Further, 1 and 2-Cl showed mechanical crushing and heating induced reversible fluorescence switching but 2 did not show any fluorescence switching. Powder X-ray diffraction indicated a reversible phase transformation upon crushing and heating that caused reversible fluorescence switching. Hence, the present study provides insight into the subtle structural impact on intermolecular interactions and solid-state fluorescence. [ABSTRACT FROM AUTHOR]

Terpyridine (TP) integrated with tetraphenylethylene (TPE-TP), pyrene (Py-TP) and triphenylamine (TPA-TP) exhibited structure controlled molecular packing and distinct fluorescence in the solid state. TPE-TP exhibited infinite aromatic π⋯π stacking between terpyridine units, Py-TP produced an isolated pyrene dimer, and TPA-TP displayed helical assembly via weak intermolecular H-bonding in the crystal lattice. The different molecular assembly of TPE-TP, TPA-TP and Py-TP produced a locally excited (LE) state, charge transfer (CT) state and excimeric emission at 460, 520 and 568 nm, respectively, in the solid state. Interestingly, the different molecular packing of TPE-TP, Py-TP and TPA-TP produced varied mechanofluorochromism. TPA-TP exhibited on–off reversible fluorescence switching upon mechanical crushing and heating, whereas TPE-TP and Py-TP displayed switching between two fluorescence states but in opposite directions. Crushing caused red shift of emission in TPE-TP and blue shift in Py-TP. Both TPA-TP and Py-TP showed strong fluorescence in solution and crystalline states, but TPE-TP showed typical aggregation induced emission (AIE). Further, Py-TP and TPE-TP showed selective fluorescence sensing of picric acid in solution and solid matrix, respectively. TPA-TP exhibited sensing of nitroaromatics both in solution as well as in the solid state. Solid state fluorescence of TPA-TP and TPE-TP was further utilized for fabricating PVA polymer composite thin films and demonstrated reusable fluorescence sensing of nitroaromatics in aqueous medium. The PVA–fluorophore composite film exhibited clear fluorescence quenching upon immersing the thin film into 10−7 M picric acid. Thus, the present study demonstrates the role of molecular structure in developing stimuli-responsive fluorescence materials and reusable sensors. [ABSTRACT FROM AUTHOR]

The structural assembly of organic fluorescent molecules exerts a significant influence on solid-state fluorescence properties, including fluorescence tuning and switching. Herein, we have synthesized a Schiff base based organic fluorophore, 2-((E)-((E)-(4-hydroxybenzylidene)hydrazono)(phenyl)methyl)phenol (1), with intra- and intermolecular H-bonding hydroxyl functionalities, and studied its solid state structural arrangement and fluorescence properties. 1 exhibited aggregation induced enhanced emission (AIEE) in the solid state. The crystallization of 1 produced geometrical (E/Z) isomers (1a (E-isomer) and 1b (Z-isomer)) with completely different intra- and intermolecular H-bonding. 1a showed typical intramolecular H-bonding with an adjacent nitrogen and showed six membered planar rings while 1b revealed unusual intramolecular H-bonding with seven membered non-planar rings. The intermolecular H-bonding of the additional hydroxy group in 1a produced a hexagon arrangement with the inclusion of a water molecule whereas a 2D network structure is formed in 1b. However, both 1a and 1b isomers showed large Stokes shifted solid state fluorescence at 570 and 565 nm, respectively, due to the intramolecular H-bonding facilitated excited state intramolecular proton transfer (ESIPT) process. Interestingly, both 1a and 1b exhibited self-reversible fluorescence switching after crushing without showing any structural transformation. 1a exhibited a relatively slow fluorescence recovery compared to 1b. Computational studies showed that intramolecular H-bonding/conformational twisting controlled the electron density distribution in the HOMO. PXRD studies confirmed the structural stability and complete regeneration of the crystalline phase after mechanical crushing. Thus, the simple interplay of intra/intermolecular H-bonding in 1 led to the formation of geometrical isomers with distinct self-reversibility. [ABSTRACT FROM AUTHOR]

Integrating an intramolecular H-bonding functionality in an organic fluorophore produces an excited state intramolecular proton transfer (ESIPT) mechanism facilitating large Stokes shifted fluorescent materials. The tunable and environmental sensitive fluorescence of ESIPT molecules makes them potential candidates for white light emitting, chemo-sensing and bioimaging applications. Herein, we designed organic fluorophores (2-((E)-((E)-(2-hydroxybenzylidene)hydrazono)(phenyl)methyl)phenol (1), 2-((E)-(((E)-(2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl)-4-methoxyphenol (2), 2-((E)-((E)-((2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl)-5-methoxyphenol (3) and 5-(diethylamino)-2-((E)-((E)-((2-hydroxy phenyl)(phenyl)methylene)hydrazono)methyl)phenol (4)) with two intramolecular H-bonding functionalities and unsymmetrical structures (salicylaldehyde-imine and benzophenone-imine) and explored their substituent-dependent structural assembly and solid-state fluorescence. 1–4 showed tunable solid-state fluorescence between 536 and 645 nm dependent on the inter/intramolecular H-bonding. 3 produced yellow (3-Y) and red (3-R) fluorescent E/Z geometrical isomers. Solid-state structural studies revealed the formation of intramolecular H-bonding at both functionalities in 1, 2a and 3-Y, whereas 2b, 3-R and 4 displayed one intra- and another intermolecular H-bonding. The formation of intra- and intermolecular H-bonding significantly influenced the solid-state fluorescence. Mechanofluorochromic (MFC) studies showed self-reversible fluorescence switching for 1–3 and stimuli-induced reversible fluorescence switching for 4. Importantly, the geometrical isomers 3-Y and 3-R exhibited distinct fluorescence switching without showing any structural transformation upon crushing/heating. Computational studies suggested that the 3-R isomer is more stable by 9.1 kcal mol−1 compared to 3-Y. Powder X-ray diffraction studies further confirmed its reversible phase transition and structural stability. Overall, the present work explored the structural versatility of ESIPT molecules and its impact on fluorescence tuning and switching. [ABSTRACT FROM AUTHOR]

Interlinked aggregation induced emissive (AIE) fluorophores (triphenylamine (TPA) donor (D) and indanedione acceptor (A)) via non-conjugated spacers (1–3) showed concentration-dependent tunable fluorescence from blue to deep red/NIR in solution and deep red emission in the solid-state. 1–3 exhibited weak broad solid-state fluorescence covering the blue to deep red region (433 to 706 nm, ΦF = 2.3 to 5.3%). It is noted that the non-interlinked TPA-indanedione fluorophore was non-emissive in solution but showed strong solid-state fluorescence at 611 nm (ΦF = 17.7%). In contrast, 1–3 showed strong fluorescence in the solution state (ΦF = 0.001 to 0.09 compared to fluorescein). Importantly, 1–3 exhibited concentration-dependent tunable fluorescence from blue (457 nm) to deep red/NIR (720 nm) in CHCl3 due to varied aggregation of fluorophores. Dynamic light scattering (DLS) studies confirmed the molecular aggregation even at very low concentrations (10−7 M). The increasing concentration lowered the solubility and produced bigger aggregates with wide size distribution that caused tunable fluorescence. The deep red fluorescence of 1 was utilized for live-cell imaging applications using E. coli cells. Fluorescence microscopic images revealed clear red fluorescent E. coli cells with a rod morphology. Furthermore, the non-toxic nature of 1 was confirmed by in vitro as well as in vivo studies. Thus, deep red/NIR emissive fluorophores were achieved by interlinking two AIEgens using a non-conjugated spacer and utilized for live cell imaging. [ABSTRACT FROM AUTHOR]

The fluorophore structure and supramolecular interactions play important roles in the molecular conformation and packing in the solid state that strongly influence the solid-state fluorescence properties. Herein, we report the synthesis of pyridylacetonitrile integrated anthracene fluorophores ((Z)-3-(anthracen-9-yl)-2-(pyridin-2-yl)acrylonitrile (2-APy), (Z)-3-(anthracen-9-yl)-2-(pyridin-3-yl)acrylonitrile (3-APy) and (Z)-3-(anthracen-9-yl)-2-(pyridin-4-yl)acrylonitrile (4-APy)) and pyridine nitrogen position controlled stimuli-responsive fluorescence switching. 3-APy exhibited strongly enhanced solid state fluorescence (ΦF = 45.83%) compared to 2-APy (ΦF = 1.68%) and 4-APy (ΦF = 6.05%). Moreover, 3-APy displayed reversible pressure/heating induced off–on fluorescence switching whereas the quenched fluorescence of 2-APy and 4-APy by crushing did not recover after heating. Solid state structural analysis revealed a twisted molecular conformation between anthracene and pyridylacetonitrile in all three molecules. However, the pyridine nitrogen position significantly influenced the extent of twisting and molecular packing. The highly twisted molecular conformation and intermolecular interactions facilitated good separation of anthracene molecules without π–π stacking in the crystal lattice of 3-APy. In contrast, 2-APy showed a face-to-face arrangement of pyridine rings whereas anthracene and pyridine rings were overlapped in the 4-APy crystal lattice contributing to weak fluorescence. Powder X-ray diffraction (PXRD) of 3-APy showed reversible transformation between crystalline and amorphous phases upon crushing and heating, causing reversible off–on fluorescence switching. The strong solid-state fluorescence and pyridine unit of 3-APy were used to demonstrate halochromic reversible fluorescence switching. Further the weak fluorescence of 2-APy and 4-APy was strongly enhanced via supramolecular co-crystal formation with p-toluenesulfonic acid (PTSA) which hindered aromatic overlapping and close packing in the crystal lattice. Overall, the present work studied the impact of subtle positional change of nitrogen on the molecular aggregation and solid-state fluorescence properties. [ABSTRACT FROM AUTHOR]

pH-responsive fluorescence molecules are interesting candidates for achieving tunable as well as white light emission from a single molecule. Herein, we have synthesized pyridine-integrated carbazole and triphenylamine donor–acceptor isomers, (Z)-3-(4-(9H-carbazol-9-yl)phenyl)-2-(pyridinyl)acrylonitrile (1–3) and (Z)-3-(4-(diphenylamino)phenyl)-2-(pyridinyl)acrylonitrile (4–6), and studied the effects of the structure and solvent on acid-responsive fluorescence modulation in the solution state. Both carbazole (1–3) and triphenylamine (4–6) isomers produced twisted intramolecular charge transfer (TICT) states and showed tunable fluorescence depending on the solvent polarity. Highly blue shifted fluorescence was observed in non-polar toluene, while polar DMF showed large red-shifted fluorescence. Further, 1–3 isomers exhibited relatively strong fluorescence compared to 4–6. Interestingly, 1–6 exhibited organic acid-dependent (trifluoroacetic acid (TFA) and p-toluene sulphonic acid (PTSA)) and solvent-dependent fluorescence tuning. Carbazole isomers (1–3) showed wide fluorescence colour tuning from blue to red as well as white light emission in toluene, whereas green to red transition was observed in CHCl3. Addition of TFA, a stronger acid, to 1 and 3 afforded blue, green and red emission along with white emission in toluene, whereas weak PTSA produced blue and red emission along with white emission. In contrast, addition of TFA or PTSA to the triphenylamine isomers (4–6) exhibited complete quenching of fluorescence in all solvents. The meta-isomers did not show significant fluorescence tuning with acids. The wide fluorescence tuning of 1 in toluene with TFA has also been successfully transformed into a PMMA polymer matrix, which also exhibited clear fluorescence tuning from blue to red. Density functional theoretical (DFT) studies indicated high molecular twisting in the excited state and clear separation of charge transfer from the donor carbazole to the acceptor unit compared to the propeller-shaped triphenylamine isomer; this may contribute to the wide fluorescence tuning in the carbazole isomers. The present studies could be useful for choosing appropriate molecular structures and solvent systems for achieving wide fluorescence tuning and white light emission from a single donor–acceptor molecule. [ABSTRACT FROM AUTHOR]

Stimuli-responsive organic solid state fluorescent materials are considered as potential candidates for optoelectronic application as well as in the biomedical field. Molecular design and supramolecular interaction controlled organization in the solid state played an important role in producing switchable and tunable fluorescent materials. Excited state intramolecular proton transfer (ESIPT) mechanism-based solid state fluorescent materials showed unique photophysical properties such as a large Stokes shift and local environment (pH, polarity, ions and viscosity) responsive fluorescence modulation. The unique photophysical properties of ESIPT molecules made them interesting for various fields including laser dyes, molecular probes, optoelectronics, white emissive materials and optical information storage materials. Systematic fluorophore structural engineering has been performed over the years to gain insight on the ESIPT mechanism in order to improve the quantum efficiency and introduce desirable material attributes for functional applications. The substitutional unit, molecular conformation and supramolecular interactions played a significant role in transforming planar ESIPT fluorophores to stimuli-induced fluorescence switching materials either between two states or off–on states. In this review article, we have presented the recent developments in ESIPT-based solid state fluorescent materials and external stimuli-induced fluorescence switching. [ABSTRACT FROM AUTHOR]

Smart fluorescent materials exhibit a controlled fluorescence response to different external stimuli such as pressure, heat, light, pH, etc. Molecular structure and supramolecular assembly via weak intermolecular interactions strongly influence the fluorescence efficiency and colour. In particular, non-planar molecular structures play a significant role in developing solid state fluorescent materials. Triphenylamine (TPA), a typical non-planar propeller molecule with interesting optoelectronic properties, provides an excellent opportunity for developing a variety of molecular fluorescent materials by taking advantage of synthetic feasibility. For example, a donor–acceptor (D–A) aggregation induced emissive (AIE) fluorophore can be developed by integrating an acceptor group into the phenyl unit of a TPA donor. In this review, we focus exclusively on TPA-based smart fluorescent materials that exhibit distinct and reversible fluorescence switching towards different stimuli. Molecular engineering of the TPA fluorophore resulted in the development of different types of stimuli-responsive materials and the conformational flexibility of non-planar phenyl groups often produced polymorphism induced fluorescence tuning. A detailed discussion of the structure–property relationships of TPA derivatives could provide insights for developing new TPA-based smart fluorescent materials with versatile properties. [ABSTRACT FROM AUTHOR]

Schiff base molecules of 1–4 ((E)‐N′‐benzylidene‐2‐hydroxybenzohydrazide derivatives) showed structure controlled solid state fluorescence tuning, reversible stimuli induced fluorescence switching and highly selective Co2+ sensing. The strong intramolecular H‐bonding between imine and amine facilitates excited state intramolecular proton transfer (ESIPT) and large red shifted solid state fluorescence. Positional isomers and substitutional change tuned the solid state fluorescence (460 to 605 nm). The substituents and positions control stimuli‐induced fluorescence switching. ESIPT fluorophores showed strong fluorescence in DMF (Φf=0.057 to 0.35 compared to quinine sulphate). ESIPT fluorophores (2–4) exhibited highly selective fluorescence sensing of Co2+ ions without significant interference from other metal ions. Concentration dependent studies of fluorophores show the detection limit (LOD) of 54 nM that is much lower than the Environmental Protection Agency (EPA) guideline value (1.7 μM) in drinking water. Solid state fluorescence of ESIPT fluorophores have been made use to fabricate solid fluorescence assay (PVA polymer composite thin films) and demonstrated highly selective fluorescence sensing of Co2+ upon dipping into aqueous metal ions solution. Thus, the present studies demonstrate the versatility of Schiff base molecules for developing ESIPT fluorophores for stimuli responsive materials and highly selective fluorescence sensing both in solution and solid state. [ABSTRACT FROM AUTHOR]

Carbazole-based donor–acceptor derivatives of halogen-substituted positional isomers were synthesised and the effect of the halogen and position on the solid-state fluorescence and mechanofluorochromism was explored. All compounds showed good solid-state fluorescence (Φf = 15.67 to 31.32%). Interestingly, the crystallization of the positional isomers produced concomitant blue and green fluorescent polymorphs and tunable solid-state fluorescence. Solid-state structural analysis revealed a subtle conformational change in the acceptor unit, which led to polymorphism and tunable fluorescence. Halogen substitution induced a higher conformational twist and a large fluorescence shift between the polymorphs. The supramolecular interactions (H-bonding, C–H⋯π and π⋯π) observed in the crystal lattice rigidified the fluorophores and enhanced the solid-state fluorescence. Computational studies were performed to gain insights into the electronic energy level of the fluorophore and fluorescence modulation. All fluorophores showed mechanofluorochromism (MFC), reversible fluorescence switching, upon crushing and heating. The crushed solids also showed self-reversible fluorescence switching with a longer time. Interestingly, OFF–ON reversible fluorescence switching was also demonstrated by melting and warming/scratching. The powder X-ray diffraction studies supported self-reversible and crystallization-induced fluorescence switching. [ABSTRACT FROM AUTHOR]

The current study demonstrates a facile one-pot pyrolysis route to develop photoluminescent graphitic carbon nitride nanosheets (g-C3N4) for the fluorescence detection of hydrazine. The morphological images show that MnO2 nanotubes are decorated over the surface of g-C3N4 nanosheets, and the addition of hydrazine completely topples the MnO2 nanotubes; this uncovers the g-C3N4 nanosheets. The turn-off fluorescence was achieved with the decoration of MnO2 nanostructures on the g-C3N4-nanosheets via the fluorescence resonance energy transfer mechanism. Morphological influences of MnO2 nanostructures on the dynamic quenching of the fluorescence behavior of g-C3N4 nanosheets were analyzed. The fluorescence energy transfer between g-C3N4 and MnO2 was restricted with the addition of hydrazine; this led to the recovery of the fluorescence behavior of g-C3N4. The proposed sensor exhibits high sensitivity, the wide linear range of 2–55 μM, and the low detection limit of 0.12 μM. The developed nanocomposite exhibits excellent selectivity towards hydrazine over common metal ions and molecules with analogous structures. Furthermore, the prepared nanocomposite exhibits good recovery in environmental water samples; this unveils its practical competence in real samples. Conspicuously, this study provides an excellent platform for the development of a competent fluorescent probe and dynamic opportunities for the effectual monitoring of hydrazine in environmental water samples. [ABSTRACT FROM AUTHOR]

Abstract A simple hydrazine carboxamide derivative with intramolecular charge transfer and aggregation induced emission (AIE) properties have been designed and synthesized. The derivative (Probe 1) shows typical AIE characteristic emission in THF-water mixtures and emits yellowish orange color on reaching the water fraction 98%. The probe molecule show selective sensing response for Cu2+ ions via fluorescence turn-off mechanism in aqueous media over other metal ions. The emission changes towards Cu2+ ions that could be clearly observed by the naked eyes under 365 nm UV lamp promote the probe molecule as a promising candidate for practical utilization. Furthermore, from the fluorescence titration data, the detection limit for Cu2+ ions was found to be 0.44 μM. Graphical abstract Unlabelled Image Highlights • A simple caboxamide derivative displaying aggregation induced emission has been synthesized. • It can act as fluorescent probe for Cu2+ions. • The sensing ability of the derivative could be visualized using 365 nm UV lamp. [ABSTRACT FROM AUTHOR]

A novel pyrazolyl chromene derivative (Probe 1 ) displaying aggregation induced emission (AIE) properties that capable of sensing of multiple metal ions has been designed and synthesized. The multi analyte probe exhibits selective sensing for Cu 2+ and Ni 2+ ions via fluorescence turn-off mechanism and ratiometric selectivity for Hg 2+ ions in aqueous media. The extent of binding of the probe with sensitive metal ions has been demonstrated. The experimental results were further investigated by computational means by optimizing the ground state geometries of Probe 1 and its various metal complexes for Probe 1 -Ni, Probe 1 -Hg and Probe 1 -Cu using density functional theory (DFT) at B3LYP/6-31+g(d,p) (LANL2DZ) level. On the basis of binding energies, the stability of metal complexes has been studied. In Probe 1 -Ni and Probe 1 -Cu complexes, charge transfer has been observed from Probe 1 to metal ions revealing ligand to metal charge transfer (LMCT) while in Probe1-Hg complex LMCT as well as intra-molecular charge tranfer (ICT) within Probe 1 . [ABSTRACT FROM AUTHOR]

New carbazole-imidazole derivatives, (3-(4,5-diphenyl-1H-imidazole-2-yl)-9-(3-methoxyphenyl)-9H-carbazole (Cz-DPIM)) and 9-(3-methoxyphenyl)-3-(1,4,5-triphenyl-1H-imidazole-2-yl)-9H-carbazole (Cz-TPIM) are synthesized and investigated the effect of phenyl substitution on the fluorescence and stimuli-induced fluorescence properties. Cz-DPIM exhibited relatively strong fluorescence in solution compared to Cz-TPIM (Quantum yield (Φ f , compared to quinine sulphate standard) = 0.28 (Cz-DPIM) and 0.09 (Cz-TPIM)). In contrast, Cz-DPIM is weakly fluorescence in solid-state but Cz-TPIM showed strong fluorescence (Φ f = 1.3% (Cz-DPIM) and 18.2% (Cz-TPIM)). Cz-TPIM displayed mechanoresponsive reversible fluorescence switching upon crushing and solvent fuming. Both Cz-DPIM and Cz-TPIM exhibited reversible halofluorochromsim upon exposure of trifluoroacetic acid (TFA) and ammonia (NH 3). Interestingly, Cz-DPIM and Cz-TPIM exhibited photocleavage upon UV light irradiation in presence of oxygen in CH 3 CN and produced green emissive fluorescence product. The controlled studies revealed that the presence of oxygen is important for exhibiting photocleavage by UV light exposure and suggest potential use for oxygen sensing application. The UV light and oxygen induced photocleavage of Cz-DPIM and Cz-TPIM also demonstrated in thin films. Carbazole-imaidazole derivatives displayed substitutional group dependent fluorescence efficiency and mechano-responsive and acid-base controlled fluorescence switching and photochemical conversion in presence of oxygen. [Display omitted] • Substitutional group dependent fluorescence efficiency in solution and solid-state. • Acid-base controlled off-on and on-on reversible fluorescence switching. • Mechanical stimuli-induced on-off fluorescence switching. • UV light induced photocleavage in presence of oxygen in solution and thin film. • Substitutional group dependent photochemical conversion rate and fluorescence oxygen sensor. [ABSTRACT FROM AUTHOR]

Integrating twisted triphenylamine with urea functionality resulted dual state emissive fluorophore with reversible/self-reversible stimuli-induced fluorescence switching and water/methanol sensing. [Display omitted] • Twisted triphenylamine with urea functionality resulted strong dual state emission. • Deep blue emissive dual state fluorescence molecule. • Multi-stimuli responsive reversible/self-reversible fluorescence switching. • Water sensitive fluorescence modulation. • Distinguishing methanol and ethanol by showing different fluorescence. A new deep blue emissive organic fluorophore (N-cyclohexyl-N-(cyclohexylcarbamoyl)-4-(diphenylamino)benzamide (NCDPB)) was designed and synthesized, which showed strong fluorescence both in solution and solid-state. Solid-state structural analysis of NCDPB revealed non-planar twisted molecular conformation with extended hydrogen bonding between the amide functionalities. The propeller shaped triphenylamine (TPA) and non-planar cyclohexyl unit prevented close π...π stacking and produced strong deep blue emission in the solid state (λ max = 400 nm, quantum yield (Φ f) = 12.6 %). NCDPB also exhibited strong solvent polarity dependent tunable emission in solution (λ max = 402–462 nm, Φ f = 1.15 (compared to quinine sulphate)). NCDPB showed reversible fluorescence switching between two fluorescence states upon mechanical crushing and heating/solvent exposure. Mechanical crushing caused red shifting of fluorescence from 400 to 447 nm and heating/solvent exposure reversed the fluorescence. Further, NCDPB also displayed off–on reversible/self-reversible fluorescence switching upon exposure to trifluoracetic acid (TFA) and NH 3. The repeated fluorescence switching cycles indicated high reversibility without any significant change of fluorescence intensity. The drastically different fluorescence of NCDPB in CH 3 OH and EtOH was utilized to distinguish them and monitor CH 3 OH contamination in ethanol and benzene. It showed limit of detection (LOD) of methanol up to 0.25 % and 7 % in benzene and ethanol, respectively. The water sensitive fluorescence modulation of NCDPB in organic solvents was used to sensing water contamination in common organic solvents. Thus, integration of twisted TPA with H-bonding urea produced dual state emitting organic fluorophore with multi-responsive fluorescence switching and solvent sensing. [ABSTRACT FROM AUTHOR]

[Display omitted] • New fluorinated polyether-based organogels were prepared and charcaterized. • The photophysical properties showed strong emission and solvatochromism. • Polyethers organogels showed high ability to gelate solvents into nanofibers. • Supramolecular organogels showed thermal stability and high reversibility. • The polyethers displayed nematic and smectic mesophases. Supramolecular organogels that respond to external stimuli have been extensively investigated; however, assembling a liquid crystal-based fluorescent gel with an efficient thermoreversible activity is a difficult challenge. Various pentafluorobenzene-containing aromatic adducts have been synthesized via nucleophilic aromatic substitution (S N Ar), which is highly regio-selective to para positions. In order to prepare calamitic liquid crystals, those pentafluorobenzene-functionalized 1, 3, 4-oxadiazole aromatic adducts are a perfect choice. Novel fluoroaryl(FA)-containing 1, 3, 4-oxadiazole (Ox) main-chain polyether (PE) (FAOx@PE) were synthesized employing in situ S N Ar as a practical technique for the synthesis of fluorescent liquid crystals with perfluoroaryl groups and ether bonds. The perfluoroaryl group was inserted into the 1, 3, 4-oxadiazole compound by the reaction of potassium pentafluorobenzoate and tetrazole-containing biphenyl using a DCC-assisted decarboxylating coupling. Using 1H/13C/19F NMR and FTIR spectroscopic techniques, the chemical structure of FAOx@PE was determined. Differential scan calorimeter (DSC), X-ray diffraction (XRD) and polarization microscope (POM) were utilized to examine the liquid crystal mesophases. The absorption and emission spectral patterns were detected in the ultraviolet–visible range demonstrating solvatochromic activity. Scanning electron microscope (SEM) was employed to inspect the nanofibrous morphology of FAOx@PE organogel. [ABSTRACT FROM AUTHOR]

Dual state emissive TPA-C fluorophore showed reversible/self-reversible fluorescence switching, water sesning in organic solvents and discriminating propanol isomers. [Display omitted] • Mechanical stimuli-induced reversible/self-reversible fluorescence switching. • Fluorescence water sensing in organic solvents. • Sensing water in organic solvents up to less than 1%. • Distinguishing propanol isomers by exhibiting different fluorescence. • Ultrasensitive detection of isopropanol in n-propanol. The photophysical properties of conformationally flexible (TPA-C) and partially rigidified (Cz-C) triarylamine acids were explored in solid as well as solution state and correlated with the structure. TPA-C and Cz-C exhibited moderate solid-state fluorescence (Φ f = 6.2 % (TPA-C) and 5.6 % (Cz-C)) and self-reversible mechanofluorochromism. TPA-C produced fluorescent polymorphs (TPA-C-1 and TPA-C-2) with tunable fluorescence. TPA-C-1 showed unusual carboxylic acid intermolecular interactions whereas TPA-C-2 and Cz-C showed usual carboxylic acid dimer. TPA-C exhibited strong solvent polarity dependent tunable fluorescence (Φ f = 0.01 to 0.11 compared to quinine sulphate standard) but Cz-C was non-emissive in the solution state. The dual emissive TPA-C showed highly sensitive fluorescence changes in organic solvents (CH 3 CN, THF, DMF, EtOH) when trace amount of water was added. In CH 3 CN, TPA-C showed weak fluorescence at 474 nm and addition of water (1 %) exhibited significant blue shift (λ max = 416 nm). The fluorescence intensity was gradually decreased with blue shifting in DMF, THF and EtOH with water addition. Importantly, TPA-C showed drastically different fluorescence in n-propanol (n-PA) and iso -propanol (IPA). TPA-C in n-PA showed fluorescence at 408 nm that was clearly red shifted to 438 nm with 0.1 % addition of IPA. The limit of detection (LOD) of water in CH 3 CN, DMF, THF and EtOH by TPA-C revealed 0.02, 0.7, 0.08 and 0.77 %, respectively. The LOD of IPA sensing in n-PA is 0.05 % and indicated the very efficient sensing and distinguishing propanol isomers. Thus, simple triphenylamine acid showed excellent water sensing and propanol isomers discrimination that could be attributed to the twisted intramolecular charge transfer (TICT) formation. [ABSTRACT FROM AUTHOR]

Organic fluorophores with propellor/partially planar donor unit and naphthylacetonitrile positional isomer were synthesized and explored their impact on the solid-state structural assembly, fluorescence properties and stimuli-induced fluorescence switching. Partially planar phenyl carbazole (Cz) and propeller shaped triphenylamine (TPA) donor units integrated with 1-naphthylacetonitrile/2-naphthylacetonitrile acceptors (Cz-1-Naph, Cz-2-Naph and TPA-2-Naph) by simple condensation reaction. All three compounds showed weak and tunable fluorescence in solution (λ max = 452–520 nm, Φ f = 0.24 compared to quinine sulphate standard). However, Cz-Naph isomers showed relatively strong solid-state fluorescence compared to TPA-Naph (Φ f = 5.9% (Cz-1-Naph), 13.8% (Cz-2-Naph) and 1.2% (TPA-2-Naph)). Further, Cz-Naph isomers showed relatively blue shifted emission (λ max = 517 (Cz-1-Naph) and 498 nm (Cz-2-Naph)) compared to TPA-2-Naph isomer (λ max 543 nm). Solid-state structural analysis revealed varied molecular conformation and arrangement depending on the naphthyl isomerism and donor units. The modulation of molecular conformation and packing contributed for fluorescence tuning. Computational studies indicated a clear intramolecular electron transfer from TPA/Cz donor to naphthylacetonitrile acceptor unit. Interestingly, Cz-2-Naph exhibited reversible mechanical crushing and heating induced fluorescence switching that was attributed to the reversible phase transformation between crystalline and amorphous phase and vice versa. Naphthylacetonitrile positional isomerism and conformation of donor significantly impacted molecular organization, fluorescence properties and stimuli-induced fluorescence switching. [Display omitted] • Substitutional group dependent fluorescence efficiency in solution and solid-state. • Acid-base controlled off-on and on-on reversible fluorescence switching. • Mechanical stimuli-induced on-off fluorescence switching. • UV light induced photocleavage in presence of oxygen in solution and thin film. • Substitutional group dependent photochemical conversion rate and fluorescence oxygen sensor. [ABSTRACT FROM AUTHOR]

• Synthesis of AIEgens with unsymmetrical intramolecular H-bonding functionalities. • Large Stoke shifted emission in the solid-state by ESIPT process. • Molecular structure dependent self-assembly, fluorescence efficiency and tunable emission. • Highly reversible mechanofluorochromism. We have synthesized two new ESIPT fluorophores (1-((E)-((E)-((2-hydroxyphenyl)(phenyl)methylene)hydrazono)methyl) naphthalen-2-ol (Naph-2-OH) and 5-(diphenylamino)-2-((E)-((E)-((2-hydroxyphenyl)(phenyl) methylene)hydrazono)methyl) phenol (TPA-2-OH)) with two unsymmetrical intramolecular H-bonding functionalities and investigated the solid-state structural assembly and fluorescence properties. The crystal structure analysis confirmed the formation of strong intramolecular H-bonding and twisted molecular conformation. The nonplanar twisted molecular conformation produced well separated molecules in the crystal lattice that lead to strongly enhanced emission ((λ max = 538 nm, Φ f = 5.2% (Naph-2-OH), λ max = 562 nm, Φ f = 13.4% (TPA-2-OH)). Density functional theoretical (DFT) calculation supported the fluorescence tuning and intramolecular charge transfer (ICT) from TPA donor to imine acceptor. Both Naph-2-OH and TPA-2-OH showed mechanical crushing and heating induced reversible fluorescence switching due to the reversible phase transformation between crystalline and amorphous state. ESIPT fluorophores with two unsymmetrical intramolecular H-bonding exhibited structure dependent molecular packing, solid state fluorescence and mechanofluorochromism. [Display omitted] [ABSTRACT FROM AUTHOR]

Triphenylamine (TPA) – acetophenone donor-π-acceptor fluorophore with halogen (F, Cl and Br) substitution was synthesized and explored the influence of halogen atom on the solution as well as solid-state fluorescence properties. The unsubstituted 1 ((E)-3-(4-(diphenylamino) phenyl)-1-phenylprop-2-en-1-one) exhibited strong fluorescence in non-polar solvents but weak fluorescence in polar solvents. The halogen substitution (4F ((E)-3-(4-(diphenyl amino)phenyl)-1-(4-fluorophenyl)prop-2-en-1-one), 4Cl ((E)-3-(4-(diphenylamino)phenyl)-1-(4-chlorophenyl)prop-2-en-1-one) and 4Br ((E)-3-(4-(diphenylamino)phenyl)-1-(4-bromo phenyl)prop-2-en-1-one)) significantly influenced on the fluorescence both in solution as well as solid-state. 1 showed white light emission in polar methanol but 4F and 4Cl exhibited white light emission in polar as well as non-polar solvents. 4Br showed white light emission in CH 3 CN. Solid-state structural studies of 1 revealed the formation of interconnected dimers via intermolecular H-bonding between carbonyl oxygen and TPA phenyl hydrogens. Though 4F also exhibited dimer via H-bonding of carbonyl oxygen, the dimers are interconnected by π ... π interaction of TPA phenyls. The Br ... π interaction produced 1D structure in the crystal lattice of 4Br that was further interconnected by multiple intermolecular interactions. The molecular conformational and packing modification of 1 and halogen isomers produced tunable fluorescence between 522 and 593 nm. Interestingly, only 4Br showed clear external stimuli-induced reversible/self-reversible fluorescence switching between two fluorescence states. The fluorescence was red shifted with slight reduction of intensity while crushing and heating/solvent exposure caused recovering of fluorescence. Powder X-ray diffraction of 4Br indicated the reversible phase transition was responsible for stimuli-induced reversible fluorescence switching. Thus, the present studies utilized the halogen atom to tune the fluorescence and studied its role on the fluorescence switching and white light emission. • Halogen atom controlled solid-state fluorescence efficiency. • Single molecule white light emission in polar to non-polar solvents by halogen substitution. • Tunable solid-state fluorescence and molecular packing in the crystal lattice. • Halogen atom dependent mechano-responsive reversible fluorescence switching. [ABSTRACT FROM AUTHOR]

Methoxy substitution at acceptor end of TPA-acetophenone fluorophore showed white light emission, tunable emission by morphological transformation and mechano-responsive reversible fluorescence switching. [Display omitted] • White light emission from single fluorophore by methoxy substitution. • Tunable fluorescence by methoxy substitution and morphological transformation. • Fluorescent polymorphs with tunable emission. • Stimuli-responsive reversible and self-reversible fluorescence switching. Introducing methoxy substituent into triphenylamine-acetophenone based donor-π-acceptor fluorophore, 3-(4-(diphenylamino)phenyl)-1-phenylprop-2-en-1-one (1), produced strong solvatofluorochromism including white light emission, fluorescent polymorphs and mechano-responsive fluorescence switching. The unsubstituted and methoxy substituted compounds displayed strong solvent polarity mediated tunable emission in the solution. Interestingly, 3-(4-(diphenylamino)phenyl)-1-(4-methoxyphenyl)prop-2-en-1-one (2) and 3-(4-(diphenylamino)-2-methoxyphenyl)-1-(4-methoxyphenyl)prop-2-en-1-one (3) showed single molecule white light emission in DMSO and ethanol, respectively. 1-3 exhibited strong green/yellow fluorescence in the solid-state (Quantum yield (Φ f) = 10 to 23%). 2 produced fluorescent polymorphs (green (2-G) and yellow (2-Y). Single crystal structural analysis revealed that donor and acceptor phenyl units adopted coplanar conformation in 2-G and 3 whereas twisted molecular conformation in 1 and 2-Y. Further, 2-G exhibited π...π interactions facilitated isolated dimers whereas 2-Y showed well separated molecules in the crystal lattice. Aggregation induced emission (AIE) studies showed morphological transformation induced fluorescence tuning for 2. The intramolecular charge transfer (ICT) from TPA to acetophenone was confirmed by computational studies. Mechanofluorochromic (MFC) studies of 1 showed only slight reduction of intensity without modulating fluorescence wavelength significantly but 2 and 3 exhibited visible emissive colour change from yellow to green and vice versa by crushing and heating. Both 2 and 3 also exhibited self-reversible fluorescence switching that was confirmed by PXRD pattern. Thus, methoxy group introduction resulted in obtaining white light emitting fluorescence molecules in the solution state and self-reversible fluorescence switching materials. [ABSTRACT FROM AUTHOR]

• The Schiff base senses Al(III) ions with a fluorescent enhancement. • The detection mechanism involves formation of an octahedral Al(III) complex. • Sensing mechanism was delineated using spectral and DFT/TD-DFT studies. • The probe detected Al(III) successfully in different natural matrices. Spectroscopic investigation and Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations have been carried out to delineate the mechanism of fluorescent detection of Al(III) by a new Schiff base (CN3) in an aqueous solution. CN3 was synthesized and characterized using 1H and 13C NMR and mass spectral techniques. Upon adding Al(III), the weak fluorescence of CN3 (512 nm) became enhanced with a slight blue shift to 502 nm. CN3 served as an efficient probe for highly selective and sensitive Al(III) detection without the interference of environmentally and biologically significant cations and anions. The limit of detection (LOD) was determined to be 1.3 µM. Job's continuous variation method of analysis suggested the formation of a 1:1 complex between CN3 and Al(III), which was confirmed as an octahedral complex by 27Al NMR and mass spectral data. The binding constant of the complex was found to be 7×104 M−1. The mode of coordination of CN3 with Al(III) was ascertained by 1H NMR titration experiments. CN3 served as an OONN-type tetradentate ligand to form a chelate, which is responsible for the enhancement of fluorescence of CN3 upon binding with Al(III) due to chelation enhanced fluorescence (CHEF) mechanism. The optical properties of CN3 and its Al(III) complex were corroborated well by DFT and TD-DFT calculations. [ABSTRACT FROM AUTHOR]

A new highly selective fluorescent turn-on probe (RS4) possessing indolinium moiety has been designed and synthesized for the detection of cyanide ion. The probe RS4 displays instantaneous, highly selective and sensitive recognition of cyanide ion over other common anions via change in color and enhancement in fluorescence. Nucleophilic addition of cyanide to the indolinium C-atom leads to the enhancement in fluorescence (λ em 481 nm; Blueish-green emission) and decolourization due to different intramolecular charge transfer (ICT) process. The binding constant of the 1:2 RS4 /CN¯ adduct is determined to be 2.3 × 104 M−1. The limit of detection of cyanide by RS4 is estimated to be 21 nM. Density functional theory (DFT) based calculations strongly substantiate the results of the experimental investigations. Importantly, the probe detects cyanide ion in agro-products (cassava powder and bitter almond), which demonstrates its value to practical application. • The probe senses cyanide with color change and fluorescent enhancement. • The detection mechanism involves nucleophilic addition of CN− at indolinium C-atom. • Sensing mechanism was delineated using spectral and DFT studies. • The probe successfully sensed CN− of cyanogenic glycoside in agro-products. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • The probe displays a rapid fluorescent response for HSO 3 − ion. • Addition of HSO 3 − exhibits enhancement in blue fluorescence via nucleophilic addition mechanism. • DFT and TD-DFT methods have shed light on the sensing mechanism. • The probe was successfully applied to detect HSO 3 − in food samples. A new pyrene-based chemodosimeter (RS3) has been designed and synthesized, which is found to be a promising fluorescent probe for detection of bisulphite ion in DMSO:Na 2 HPO 4 /citric acid buffer of pH 5. 1H NMR titration indicates that nucleophilic addition of HSO 3 − to vinylic C-atom of RS3 terminates the π-conjugation and consequently enhanced the fluorescence significantly. The sensing mechanism is further confirmed by density functional theory (DFT) and time dependent density functional theory (TD-DFT) methods. The binding constant of the 1:1 RS3 -HSO 3 adduct is found to be 0.7 × 103 M−1. The probe RS3 exhibits high selectivity towards bisulphite ion with a limit of detection of 0.15 µM, which is lower than the maximum permissible limit of bisulfite ion (1.25 µM) in food products required by the European Union Regulations. Further, the probe RS3 has been used for the determination of HSO 3 − in real samples such as distilled water, tap water, red wine, jaggery and white sugar and also successfully applied for the bio-imaging of HSO 3 − in live cells. [ABSTRACT FROM AUTHOR]

[Display omitted] • The probe senses SO 2 derivatives with fluorescent enhancement. • The detection mechanism involves nucleophilic addition at vinylic C-atom. • Sensing mechanism was delineated using DFT and TD-DFT calculations. • The probe successfully sensed bisulphite ion in real samples. The selective detection of sulfite and bisulfite ions with anthracene-based compounds (CN1 and CN2) as chemo-dosimeters is reported using fluorescence changes in DMSO:HEPES (70:30%), v/v buffer solution (pH 7.4). Upon treatment with sulfite and bisulfite ions, the fluorescence of the probes gets enhanced significantly at 424 nm. The detection of these ions occurs through the nucleophilic addition at the vinylic C-atom of the probes, which terminates the photo-induced electron transfer (PET) process and consequently enhances the fluorescence. This detection mechanism is well supported by 1H NMR titration, HR-MS and DFT/TD-DFT calculations. The pseudo-first-order rate constants for the addition sulfite (at pH 7.4) and bisulfite (at pH 5) to CN1 are determined to be 3.22×10−3 and 5.02×10−3 s−1, respectively. The limits of detection for sulfite and bisulfite are found to be 1.85×10−7 and 1.56×10−7 M, respectively. The probe CN1 was successfully applied to the detection of bisulfite ion in real samples. [ABSTRACT FROM AUTHOR]