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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]

Molecular structural tailoring is one of the versatile approaches for developing functional organic fluorescent molecules with desired properties for practical applications. In this manuscript, we report the synthesis of triphenylamine based boron complexes (TPA-BF2) with unusual thermofluorochromism and alkyl chain length controlled molecular fluorescent liquids/solids. TPA-BF2 complexes with ethyl, propyl, butyl and pentyl groups produced fluorescent solids whereas with hexyl, heptyl and octyl groups yielded fluorescent liquids. The longer-alkyl chain compounds significantly improved the fluorescence efficiency (quantum yield (Φf = 12.6%)) by inhibiting close arrangement of fluorophores. TPA-BF2-2 and TPA-BF2-3 showed very weak fluorescence (Φf = 1.9%). On the other hand, all complexes exhibited strong solvent polarity dependent tunable fluorescence in the solution state. Importantly, the ability to form multiple excited states (locally excited (LE), charge transfer (CT) and twisted intramolecular charge transfer (TICT) states) resulted in unusual fluorescence tuning and reversible thermofluorochromism. The conversion of CT emission at RT to LE state emission at 77 K (under liquid nitrogen) of TPA-BF2 complexes in CHCl3/toluene resulted in strong blue shifting, while warming towards RT, the TPA-BF2 CHCl3 solution showed unusual fluorescence tuning. The fluorescence λmax of TPA-BF2-2 was red shifted from 476 to 600 nm and then blue shifted to RT fluorescence (550 nm). Interestingly, TPA-BF2 complexes in solution and in the PMMA matrix showed clear phosphorescence at 77 K. The unusual fluorescence tuning while warming from 77 K is attributed to the phosphorescence emission coupled with fluorescence at room temperature. Notably, the fluorescence intensity at RT was relatively stronger compared to red shifted fluorescence intensity while warming. TPA-BF2 complexes in toluene showed strong linear enhancement of fluorescence intensity (1.9 to 2.5 times) with a blue shifted peak position upon increasing the temperature from 0 to 100 °C. The alkyl chain length exhibited a strong influence on the rate and extent of fluorescence tuning with temperature. Thus, alkyl group variations in simple Schiff base–boron complexes produced unusual thermofluorochromism and molecular fluorescent liquids. [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]

Excited state intramolecular proton transfer (ESIPT) process-based organic fluorophores provide an opportunity to develop large Stokes-shifted multifunctional fluorescence systems for light emitting, chemosensing and bioimaging applications. In this manuscript, ESIPT molecules with two thiazole ring fused symmetrical (1) structures and one thiazole and a Schiff base functional unsymmetrical (2) structure were synthesized, and the impact of their extended π-conjugation was investigated on the fluorescence properties in the solid and solution state as well as metal ions sensing. The strong intramolecular H-bonding and multiple intermolecular H-bonds in the crystal lattice of 1 and 2 lead to ESIPT-induced solid state fluorescence (λmax = 549 (1) and 580 nm (2); ΦF = 7.6 (1) and 6.1% (2)). Interestingly, the molecular assembly of 2 in the solid state facilitated mechanical stimuli-induced reversible fluorescence switching, whereas the flat molecular arrangement in 1 did not show recovery of fluorescence. In contrast, 1 showed strong fluorescence in solution (ΦF = 0.59 in DMF compared to quinine sulphate) due to its symmetrical structure, with strong intramolecular H-bonding, but the PET effect contributed to the very weak fluorescence of 2. The strong solution and solid state fluorescence coupled with the metal chelating functionality of 1 and 2 showed highly selective fluorescence sensing of Cu2+ ions in aqueous solution. 1 and 2 revealed detection limits of 130 and 10 nM, respectively, for Cu2+ ions, which are significantly lower compared to the WHO-recommended level (1.3 mg L−1). The strong solid state fluorescence of 1 and 2 was further used for fabricating filter paper-based fluorescence sensors for the selective sensing of Cu2+ ions. [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]

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]

Triphenylamine (TPA) and N-methylbarbituric acid/indanedione-based donor–acceptor derivatives were synthesized and demonstrated molecular conformation and packing dependent tunable fluorescence from yellow to red in the solid state. TPA with N-methylbarbituric acid (BA-1) showed bright yellow fluorescence (λmax = 550 nm, Φf = 22.8%) whereas OCH3 substitution at the phenyl rings of TPA, (BA-2 and BA-3) produced strong red (λmax = 602 nm, Φf = 41.1%) and orange fluorescent solids (λmax = 582 nm, Φf = 19.1%). Indanedione acceptor dyes exhibited red to deep red fluorescence. ID-1 showed red fluorescence at 604 nm (Φf = 17.7%) whereas ID-2 and ID-3 showed fluorescence at 611 (Φf = 19.4%) and 636 nm (Φf = 14.1%) in the solid state. Solid state structural analysis revealed alteration of the molecular conformation and packing by OCH3 substitution which led to tunable fluorescence. BA and ID compounds showed turn-off fluorescence/substantially reduced fluorescence intensity upon hard crushing (Φf = 1.2 to 3.1%). Interestingly, heating of BA and ID crushed powders led to turn-on fluorescence/significant enhancement of fluorescence intensity (Φf = 5.6 to 22.5%). Powder X-ray diffraction (PXRD) studies indicated the conversion of the crystalline phase to amorphous and the amorphous phase to crystalline by hard crushing and heating. The reversible conversion of the crystalline phase to amorphous phase was responsible for the fluorescence switching of BA and ID. Computational studies have been performed to get an insight into the energy level modulation upon the change of the molecular conformation. Thus, we have presented the facile preparation of strong red fluorescent dyes exhibiting high contrast stimuli-responsive reversible dark and bright fluorescence switching in the solid state. [ABSTRACT FROM AUTHOR]