Hydrogen-bonding-mediated J-aggregation and white-light emission from a remarkably simple, single-component, naphthalenediimide chromophore

Fluorescence of Aqueous Assemblies Regulated by Integral Water

Water is an integral and functional component for aqueous assemblies of POSS-NDI-POSS molecules. Water serves not only as a solvent but also as an integral component (interstitial water) in aqueous colloids self-assembled from POSS-NDI-POSS (POSS = polyhedral oligomeric silsesquioxanes, NDI = naphthalene diimide). The interstitial water confined within these colloids is attributed to the steric hindrance of the bulky POSS groups, which dissipates energy and quenches the fluorescence (FL). Control experiments with NDI molecules bearing less steric side groups (propyl or octyl), namely C3-NDI-C3 and C8-NDI-C8, confirm this role of water in structural assembly and optical function. This discovery opens new possibilities for the innovation of aqueous materials.

Drug-Assisted White Light Generation Via Self-Assembly

White-light (WL) generation using small organic molecules has gained significant attention from researchers working on the interface of supramolecular chemistry and organic materials. Self-assembled multi-chromophoric materials utilizing a drug molecule and microenvironment-sensitive intramolecular charge transfer dye as an emitter offer the possibility of tunable emission. In this investigation, we focused on WL generation via the combination of a polarity-sensitive red-emitting styryl chromone (SC) and a blue-emitting anticancer and psychotherapeutic drug Norharmane (NHM) in a self-assembled micellar system. A detailed spectroscopic investigation allows us to understand the premicellar aggregation process of different types of surfactants with varying charges using the SC dye. Encapsulation of SC and NHM emitters inside the micellar system offers an improved fluorescent behavior, resulting in WL emission due to complementary wavelength overlap. The generated WL is highly photostable and thermally reversible in the self-assembled system. This investigation highlights the significance of the co-assembly of SC dye and NHM drug for the generation of a highly stable WL.

Light Harvesting Materials: A Study on Förster Resonance Energy Transfer and Optoelectronic Properties of Potential Nerium oleander Flowers

The present study focused on extracting the anthocyanin dyes in ethanol, acidic ethanol, methanol, and acidic methanol solvents from Nerium oleander flowers by a simple maceration extraction process. FTIR spectroscopy and vibrational studies have confirmed the existence of polyphenolic groups in 2-phenyl chromenylium (anthocyanin) dyes. The optoelectronic results show the least direct bandgap (2.89 eV), indirect bandgap (1.98 eV), Urbach energy (0.120 eV), high refractive index (1.654), dielectric constant (3.294) and high optical conductivity (1.813 103 S/m) for the anthocyanin dye extracted found in acidic ethanol solvent. The photoluminescence properties such as Stokes' shift, quantum yield, and lifetime results show that anthocyanin dyes are promising candidates for red-LEDs and optical materials. The excellent correspondence between the absorption and emission spectra reinforces that the anthocyanins are efficient (89%) FRET probes. Further, the donor and acceptor undergo redshift in excitation and emission spectra in all studied solvents. The photometric properties such as CIE, CRI, CCT and color purity results of anthocyanins in all studied solvents revealed that this material exhibits pink to red shades (x = 0.40 → 0.50 and y = 0.46 → 0.39) and is well suitable for have great potential in the manufacturing of Organic-LEDs and other optoelectronic device applications.

Resonant energy transfer among naturally available bio materials for white light emission

In this work we have studied the fluorescence of natural dyes and generated nearly pure white light with chromaticity intensity (CIE) coordinate (0.35,0.35). The colour rendering index (CRI) and colour temperature corresponding to the CIE coordinate are calculated and these dyes are ideal for cool white light emission. It was observed that a broadband and simultaneous emission involving anthocyanin and polyphenol ellergic acid from jamun, curcumin and chlorophyll from spinach leaves played a vital role in obtaining a CIE index close to that of pure white light. The white light emission is due to the Förster resonance energy transfer (FRET) from curcumin to anthocyanin and ellergic acid to curcumin. Efficiency of FRET is calculated and different possibilities studied. For the polyphenol ellergic acid, curcumin FRET pair the spectral overlap integral and the efficiency are 3.29 × 10-24 m2, 99.97% and for the curcumin, anthocyanin pair, they are 4.03 × 10-24 m2, 76%, respectively.

Anthocyanins of Delonix Regia Floral Petals: A Novel Approach on Fluorescence Enhancement, Forster Resonance Energy Transfer Mechanism and Photostability Studies for Optoelectronic Applications

In this work, we focused on extracting the anthocyanin dye in acetone, butanol, ethanol, and water solvents from Delonix regia flowers by a simple maceration extraction process. The identification of functional group analysis, vibrational studies, energy transfer mechanisms, optoelectronic properties, photostability studies, FRET-assisted potential light emissions and photometric properties of the anthocyanin dyes are successively investigated. FTIR spectroscopy and vibrational studies have confirmed the existence of polyphenolic groups in 2-phenyl chromenylium (anthocyanin) dyes. The optoelectronic results show the least direct bandgap (2.04 eV), indirect bandgap (1.55 eV), Urbach energy (0.380 eV), high refractive index (1.20), dielectric constant (2.794), and high optical conductivity (1.954 × 103 S/m) for the anthocyanin dye extracted found in water solvent. The photoluminescence properties such as Stoke's shift, high quantum yield, and lifetime results show that anthocyanin dyes are promising candidates for red-LEDs and optical materials. The absorption and emission spectra of the anthocyanin dyes follow the mirror image rule and the Franck-Condon factor exists between vibrational energy levels corresponding to all the electronic transitions. The excellent correspondence between the absorption and emission spectra reinforces that the anthocyanins are efficient (46%) FRET probes. Further, photometric properties such as CIE, CRI, CCT and colour purity results of anthocyanins in all studied solvents revealed that this material exhibits orange to red shades (x = 0.48 → 0.54 and y = 0.36 →0.45) and is well suitable for have great potential in the manufacturing of Organic-LEDs and other optoelectronic device applications.

Natural Pigments-Based Two-Component White Light Emitting Systems

In this paper, a new class of two component white light emitting systems viz, JaB (java plum + beetroot) {I}, and CaB (carrot + beetroot) {II} were developed through resonance energy transfer (RET) phenomenon by using a fruit (java plum) and two vegetable (carrot and beetroot) extracts. In these white light emitting systems, java plum and carrot are the donors while beetroot is the acceptor. The primary fluorescent pigments present in the natural extracts (i.e., anthocyanin in java plum, β-carotene in carrot, and betanin in beetroot) were responsible for the white light emission. The CIE (Commission Internationale d'Eclairage) coordinates for I and II were {0.32, 0.34} and {0.33, 0.33}, respectively, in solution phase. Interestingly, the white light emission (WLE) was also achieved in agar-agar gel medium. In gel medium, the CIE values were {0.31, 0.34} and {0.33, 0.32} for I and II, respectively. The donor-acceptor distance (r) for I and II were found to be 0.5 and 0.4 nm, respectively. Furthermore, the rate of energy transfer was also quantified with the values of 2.78 × 109 s-1 for JaB (I) and 1.02 × 108 s-1 for CaB (II) systems. The mechanistic investigation of the two white light systems was further supported by DFT studies.

Self-Assembly of Discrete Oligomers of Naphthalenediimides in Bulk and on Surfaces

Here, we report on the synthesis of discrete oligomers of alkyl-bridged naphthalenediimides (NDIs) and study their molecular nanostructures both in bulk, in solution, and at the liquid-solid interface. Via an iterative synthesis method, multiple NDI cores were bridged with short and saturated alkyl-diamines (C3 and C12 ) or long and unsaturated alkyl-diamines (u2 C33 to u8 C100 ) at their imide termini. The strong intermolecular interaction between the NDI cores was observed by probing their photophysical properties in solution. In bulk, the discrete NDI oligomers preferentially ordered in lamellar morphologies, irrespective of whether a saturated or unsaturated spacer was employed. Moreover, both the molecular architecture as well as the crystallization conditions play a significant role in the nanoscale ordering. The long unsaturated alkyl chains lead preferably to folded-chain conformations while their saturated analogues form stretched arrangements. At the solution-solid interface, well-defined lamellar regions were observed. These results show that precision in chemical structure alone is not sufficient to reach well-defined structures of discrete oligomers, but that it must be combined with precision in processing conditions.

Citric Acid-Based Intrinsic Band-Shifting Photoluminescent Materials

Citric acid, an important metabolite with abundant reactive groups, has been demonstrated as a promising starting material to synthesize diverse photoluminescent materials including small molecules, polymers, and carbon dots. The unique citrate chemistry enables the development of a series of citric acid-based molecules and nanomaterials with intriguing intrinsic band-shifting behavior, where the emission wavelength shifts as the excitation wavelength increases, ideal for chromatic imaging and many other applications. In this review, we discuss the concept of "intrinsic band-shifting photoluminescent materials", introduce the recent advances in citric acid-based intrinsic band-shifting materials, and discuss their potential applications such as chromatic imaging and multimodal sensing. It is our hope that the insightful and forward-thinking discussion in this review will spur the innovation and applications of the unique band-shifting photoluminescent materials.

Synthesis and Properties of Electron-Deficient and Electron-Rich Redox-Active Ionic π-Systems

There is growing interest towards the design and synthesis of organic redox-active systems, which exist in ionic form. Multi- redox systems entail life-sustaining processes like photosynthesis and cellular respiration. The significant challenge for material scientists is to rationally design complex molecular materials that can store and transfer multiple electrons at low operational potentials and are stable under ambient conditions. Also, important are the designed ionic π-systems that combine efficient electron and ion transport. Here, we discuss the synthesis of ionic π-systems which exist in the closed-shell form. Firstly, different classes of ionic arylenediimides and viologens with different π-linkers are discussed from the synthetic, structural and redox perspective. These ionic π-systems are based on the electron deficient π-scaffolds, and are shown to accumulate upto six electrons. We then discuss electron-rich ionic arylenediimides which can exist in anionic form or zwitterionic form. The anionic electron donors have absorption extending to the near Infrared (NIR) region and can be stabilized in aqueous solution. We also discuss the effect of the electron accumulation on the aromaticity and non-aromaticity of the naphthalene and the imide rings of the naphthalenediimides. We finally discuss in brief, the applications related to the organic mixed ionic-electronic conductors.

New white light-emitting halochromic stilbenes with remarkable quantum yields and aggregation-induced emission

Highly efficient single-component white light emitters (SWLEs), are attractive candidates for the simple and cost-effective fabrication of high-performance lighting devices. This study introduced a donor-π-acceptor and a donor-π-donor stilbene-based chromophores, representing pH-responsive fluorescence. The emitters showed yellow and green fluorescence in their neutral form. At the same time, protonation of the chromophores caused blue fluorescence color with a strong hypsochromic shift. The white light emission (WLE) for these chromophores was observed at approximately pH  3 due to the simultaneous presence of the neutral and protonated forms of the chromophores, covering almost all the emission spectra in the visible region (400-700 nm). These chromophores presented exceptional white light quantum yields (Φ) between 31 and 54%, which was desirable for producing white light-emitting devices. Density functional theory (DFT) and time-dependent (TD)-DFT were applied to study the structural and electronic properties of the chromophores.

Nanotechnology-Assisted, Single-Chromophore-Based White-Light-Emitting Organic Materials with Bioimaging Properties

White-light-emitting (WLE) organic materials, especially small molecules comprising a single chromophoric unit, have received much attention due to their tremendous use in modern-day electronic devices and biomaterials. They can increase the efficiency and lifetime of devices compared to the currently used combination approach. Herein, we explored a small symmetric push-pull organic molecule Hexyl-TCBD with a single 1,1,4,4-tetracyanobuta-1,3-diene (TCBD) chromophoric unit containing urea as a key functional group on an acceptor-donor∼donor-acceptor (A-D∼D-A) backbone for its ability to show white-light emission in solution as well as in the solid state. The luminescence was absent in the solid state due to the H-bonding- and π-stacking-driven quenching processes, while emission behavior in solution was tunable with variable CIE chromaticity index values via hydrogen (H)-bonding-governed disaggregation phenomena. Translation of WLE from the Hexyl-TCBD solution to a solid state was demonstrated by utilizing nonemissive polystyrene (80 wt % with respect to the chromophore) as the matrix to obtain WLE nanofibers (made by the electrospun technique) via segregating the molecules. The optical microscopy study validated the WLE nanofibers. The presence of multicolor photoluminescence, including white light, could be fine-tuned through various excitation wavelengths, solvent polarities, and polystyrene matrices. Furthermore, the detailed photophysical studies, including lifetime measurements, indicated that the inherent intramolecular charge transfer (ICT) bands of Hexyl-TCBD exhibit better ICT state stabilization by space charge distribution through the modulation of H-bonding between urea groups. Finally, a cytotoxicity study was performed for Hexyl-TCBD on normal and cancer cell lines using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay to explore bioimaging applications in biosystems. MTT results revealed significant toxicity toward cancer cells, whereas normal cells exhibited good biocompatibility.

Luminescence property switching in 1D supramolecular polymerization of organic donor–π-acceptor chromophores

The naphthalene monoimide building block endows with amide functionality undergoes supramolecular polymerization in a J type fashion in a particular co-solvent composition. This leads to luminescent property switching as a result of PET effect.

White light employing luminescent engineered large (mega) Stokes shift molecules: a review

Large (mega) Stokes shift molecules have shown great potential in white light emission for optoelectronic applications, such as flat panel display technology, light-emitting diodes, photosensitizers, molecular probes, cellular and bioimaging, and other applications. This review aims to summarize recent developments of white light generation that incorporate a large Stokes shift component, key approaches to designing large Stokes shift molecules, perspectives on future opportunities, and remaining challenges confronting this emerging research field. After a brief introduction of feasible pathways in generating white light, exemplifications of large Stokes shift molecules as white light candidates from organic and inorganic-based materials are illustrated. Various possible ways to design such molecules have been revealed by integrating the photophysical mechanisms that are essential to produce red-shifted emission upon photoexcitation, such as excited state intramolecular proton transfer (ESIPT), intramolecular charge transfer (ICT), excited state geometrical relaxation or structural deformation, aggregation-induced emission (AIE) alongside the different formations of aggregates, interplay between monomer and excimer emission, host-guest interaction, and lastly metal to ligand charge transfer (MLCT) via harvesting triplet state. Furthermore, previously reported fluorescent materials are described and categorized based on luminescence behaviors on account of the Stokes shifts value. This review will serve as a rationalized introduction and reference for researchers who are interested in exploring large or mega Stokes shift molecules, and will motivate new strategies along with instigation of persistent efforts in this prominent subject area with great avenues.

Continuous color tuning of single-fluorophore emission via polymerization-mediated through-space charge transfer

Tuning emission color of molecular fluorophores is of fundamental interest as it directly reflects the manipulation of excited states at the quantum mechanical level. Despite recent progress in molecular design and engineering on single fluorophores, a systematic methodology to obtain multicolor emission in aggregated or solid states, which gives rise to practical implications, remains scarce. In this study, we present a general strategy to continuously tune the emission color of a single-fluorophore aggregate by polymerization-mediated through-space charge transfer (TSCT). Using a library of well-defined styrenic donor (D) polymers grown from an acceptor (A) fluorophore by controlled radical polymerization, we found that the solid-state emission color can be fine-tuned by varying three molecular parameters: (i) the monomer substituent, (ii) the end groups of the polymer, and (iii) the polymer chain length. Experimental and theoretical investigations reveal that the color tunability originates from the structurally dependent TSCT process that regulates charge transfer energy.

Naphthalenediimides with High Fluorescence Quantum Yield: Bright-Red, Stable, and Responsive Fluorescent Dyes

The naphthalenediimide (NDI) scaffold in contrast to its higher congeners possess low-fluorescence. In spite of elegant synthetic developments, a highly emissive NDI is quite rare to find, as well as, a green-light-emitting NDI is yet to be explored. Herein, we report a novel class of symmetric and asymmetric NH₂ -substituted core-NDIs (1-5) with tunable fluorescence in the visible region and extending to the NIR frontier. Importantly, the bis-NH₂ -substituted NDI 2 revealed quantum yield, Φ f of ≈81 and ≈68 % in toluene and DMSO, respectively, suggesting versatility of the fluorophore in a wide range of solvent polarity. The dye 1 is shown to be the first NDI-based green-light emitter. The donor piperidine group in 5 diminish the Φ f by 40-fold providing a lever to modulate the excited-state intramolecular proton transfer (ESIPT) process. Our synthetic protocol applies a Pd catalyst and a benign hydride source simplifying the non-trivial -NH₂ group integration at the NDI-core. TD-DFT calculations predicted strong intramolecular hydrogen bonds in the excited state in the bulk nonpolar medium and responsiveness to solvent polarity. The maximization of the NDI emission outlined here would further boost the burgeoning repertoire of applications of the NDI scaffold.

Supramolecular Assembly-Driven Color-Tuning and White-Light Emission Based on Crown-Ether-Functionalized Dihydrophenazine

The development of color-tunable white-light-emitting systems is significant for artificial smart materials. Recently, a set of conformational dependent fluorophores N,N'-diaryl-dihydrodibenzo[a,c]phenazines (DPACs) have been developed with unique photoluminescence mechanism vibration-induced emission (VIE). DPACs can emit intrinsical blue emission at a bent excited state and abnormal orange-red emission at a planar excited state, which are due to the varied π-conjugation via excited-state configuration transformation along the N-N' axis from bent to planar form. Herein, a novel VIE-active compound DPAC-[B15C5]₂ is designed and synthesized with two wings of benzo-15-crown-5. The excited-state vibration of the DPAC moiety can be modulated by tuning the supramolecular assembly and disassembly via chelation competition of K⁺ between 15-crown-5 and 18-crown-6, and hence, a wide-color-tuning emission is achieved from blue to orange-red including white. Dynamic light scattering and transmission electron microscopy experiments were conducted to exhibit the supramolecular assembling process. Additionally, the moisture detection in organic solvents is realized since the water could dissociate the supramolecular assembly.

Single-Molecular White-Light Emitters and Their Potential WOLED Applications

White organic light-emitting diodes (WOLEDs) are superior to traditional incandescent light bulbs and compact fluorescent lamps in terms of their merits in ensuring pure white-light emission, low-energy consumption, large-area thin-film fabrication, etc. Unfortunately, WOLEDs based on multilayered or multicomponent (red, green, and blue (RGB)) emissive layers can suffer from some remarkable disadvantages, such as intricate device fabrication and voltage-dependent emission color, etc. Single molecules, which can emit white light, can be used to replace multiple emitters, leading to a simplified fabrication process, stable and reproducible WOLEDs. Recently, the performance of WOLEDs by using single molecules is catching up with that of the state-of-the-art devices fabricated by multicomponent emitters. Therefore, an increasing attention has been paid on single white-light-emitting materials for efficient WOLEDs. In this review, different mechanisms of white-light emission from a single molecule and the performance of single-molecule-based WOLEDs are collected and expounded, hoping to light up the interesting subject on single-molecule white-light-emitting materials, which have great potential as white-light emitters for illumination and lighting applications in the world.

White Light Emission from a Simple Mixture of Fluorescent Organic Compounds

Three fluorescent organic compounds—furocoumarin (FC), dansyl aniline (DA), and 7-hydroxycoumarin-3-carboxylic acid (CC)—are mixed to produce almost pure white light emission (WLE). This novel mixture is immobilised in silica aerogel and applied as a coating to a UV LED to demonstrate its applicability as a low-cost, organic coating for WLE via simultaneous emission. In ethanol solution and when immobilised in silica aerogel, the mixture exhibits a Commission Internationale d’Eclairage (CIE) chromaticity index of (0.27, 0.33). It was observed that a broadband and simultaneous emission involving coumarin carboxylic acid, furocoumarin and dansyl aniline played a vital role in obtaining a CIE index close to that of pure white light.

Carbon Dots-Stimulated Amplification of Aggregation-Induced Emission of Size-Tunable Organic Nanoparticles

Carbon dots (CDs)-induced microstructural modulation and amplification of emission intensity of self-aggregated fluorescent organic nanoparticles (FONPs) is a challenging task since CD is a well-known fluorescence quencher. In the present study, we have designed l-tyrosine-tagged hydrophobically (C-10) tailored naphthalene diimide derivative (NDI-i), which formed FONPs in tetrahydrofuran (THF)-water binary solvent mixture. NDI-i exhibited aggregation-induced emission (AIE) at 580 nm (orange) up to f_w = 70 vol % of water in THF via excimer formation in combination with intramolecular charge transfer (ICT) upon excitation at 350 nm. Beyond f_w = 70 vol %, the emission intensity gradually reduced up to f_w = 99 vol % due to poor water dispersibility of NDI-i FONPs. Doping of hydrophobically (C-2 to C-11 alkyl chain) surface-functionalized CDs (CD-i-iii) within self-aggregates of NDI-i FONPs at f_w = 99 vol % resulted in the modulation of both morphology and emission intensity of resulting self-assembled nanoconjugate. In the presence of C-2 alkyl chain tethered CD, the emission intensity of FONP-CD nanohybrid got quenched compared to that of native NDI-i FONPs. The emission intensity of NDI-i FONPs markedly enhanced by 3.6- to 5.0-fold upon inclusion of C-6 and C-11 alkyl chain containing CDs, respectively. Increasing the alkyl chain length on CD surface facilitated the interchain hydrophobic interaction between the organic nanoparticles and surface-functionalized CDs to form larger CD-doped fused FONPs. The extent of ICT between π-donor and π-acceptor residues became more efficient to exhibit enhanced AIE due to the accumulation of more NDI-i around CD surface through interchain hydrophobic interaction. The C-11 alkyl chain containing CD-integrated FONPs showed the brightest orange emission with superior aqueous stability. These water-dispersible, orange-emitting, cytocompatible NDI-i-CD-iii FONPs were explored for long-term bioimaging of mammalian cells.

Thermodynamically versus Kinetically Controlled Self-Assembly of a Naphthalenediimide-Thiophene Derivative: From Crystalline, Fluorescent, n-Type Semiconducting 1D Needles to Nanofibers

The control over aggregation pathways is a key requirement for present and future technologies, as it can provide access to a variety of sophisticated structures with unique functional properties. In this work, we demonstrate an unprecedented control over the supramolecular self-assembly of a semiconductive material, based on a naphthalenediimide core functionalized with phenyl-thiophene moieties at the imide termini, by trapping the molecules into different arrangements depending on the crystallization conditions. The control of the solvent evaporation rate enables the growth of highly elaborated hierarchical self-assembled structures: either in an energy-minimum thermodynamic state when the solvent is slowly evaporated forming needle-shaped crystals (polymorph α) or in a local energy-minimum state when the solvent is rapidly evaporated leading to the formation of nanofibers (polymorph β). The exceptional persistence of the kinetically trapped β form allowed the study and comparison of its characteristics with that of the stable α form, revealing the importance of molecular aggregation geometry in functional properties. Intriguingly, we found that compared to the thermodynamically stable α phase, characterized by a J-type aggregation, the β phase exhibits (i) an unusual strong blue shift of the emission from the charge-transfer state responsible for the solid-state luminescent enhancement, (ii) a higher work function with a "rigid shift" of the electronic levels, as shown by Kelvin probe force microscopy and cyclic voltammetry measurements, and (iii) a superior field-effect transistor mobility in agreement with an H-type aggregation as indicated by X-ray analysis and theoretical calculations.

Self-assembly of surface functionalized amphiphilic carbon dots: tuning in morphological manifestations

Despite the continuous surge of interest in supramolecular chemistry, the design and synthesis of building blocks to develop diverse examples of self-assemblies is still challenging. During the past decades, formation of self-assemblies such as micelles, vesicles, and gels with a fibril network using amphiphiles has been investigated at length. Considering the increasing applications of these self-aggregates across the scientific domain, it is crucial to adopt an alternative strategy for the preparation of self-aggregates using a new building block that has been applied in diverse domains. With this aim, surface functionalized carbon dots (CDs) with varying aliphatic/aromatic (cholesteryl, palmitoyl, naphthyl) substitutions linked with spacers such as ethylenediamine, p-phenylenediamine, 2,2'-(ethylenedioxy)bis(ethylamine) were developed. The surface passivated CDs formed self-assemblies in dimethylsulfoxide-water (DMSO-H₂O, 2 : 1, v/v). The roles of surface functionalities and spacer units in the formation of self-assemblies using the synthesized CDs were investigated by microscopic and spectroscopic studies. Progressive morphological transition was found from vesicle-to-fiber in DMSO-H₂O (2 : 1, v/v) which was dependent on surface passivating substitutions of the CDs from cholesteryl to naphthyl to palmitoyl. Together with the exclusive formation of self-assemblies using amphiphilic CDs, the present study enabled the tuning of self-organization behaviour of the CD by alteration of its surface functionality.

White-Light-Emitting Melamine-Formaldehyde Microspheres through Polymer-Mediated Aggregation and Encapsulation of Graphene Quantum Dots

Graphene quantum dot (GQD) encapsulated melamine-formaldehyde (MF) polymer microspheres with uniform particle size and tunable high-quality white-light emissions are prepared via a polymer-mediated GQD assembly and encapsulation strategy. In solution, GQDs are first aggregated with MF prepolymer through electrostatic interaction and further encapsulated inside the microspheres formed by polymerization of MF prepolymer under acid catalysis and heating. During this process, the aggregated GQDs are fixed in the MF polymer matrix with their emission extended from blue to full visible range, presenting bright white luminescence under ultraviolet excitation. The prepared white-light-emitting GQD-MF microspheres exhibit uniform morphology with an average particle size of 2.0 ± 0.08 µm and their luminescence properties are effectively regulated by the doping concentration of GQDs in the MF polymer matrix. A series of white-light-emitting GQD-MF microspheres with quantum yields from 0.83 to 0.43, Commission Internationale de L'Eclairage coordinates from (0.28, 0.28) to (0.33, 0.32), and color rendering index from 0.75 to 0.88 are obtained with excellent photostability and thermal stability. By dispersing the GQD-MF microspheres in cross-linked polydimethylsiloxane matrix, flexible film with dual functions of high-quality white-light-emitting and light diffusion is obtained and successfully applied for white light-emitting diode fabrication.

Tunable Aggregation-Induced Multicolor Emission of Organic Nanoparticles by Varying the Substituent in Naphthalene Diimide

In this article, we have designed l-aspartic acid-linked naphthalene diimide (NDI)-based amphiphilic molecules having a benzyl ester group at both the terminals with varying substituents (NAB-1-5). The substituent was judiciously modified from an electron-withdrawing group (EWG) like nitrobenzene to an electron-donating group (EDG), methoxybenzene, and finally to an extended aromatic residue (naphthalene) to regulate the π-electron density at the terminal of NDI derivatives. All of the synthesized NDI derivatives were molecularly dissolved in dimethyl sulfoxide (DMSO), and with an increase in the water content within the DMSO solution, the NDI derivative starts to get self-assembled through J-aggregation at and above 40% water content. Self-assembled spherical organic nanoparticles formed in 99% water in DMSO ( f_w = 99%) were characterized by microscopic studies. All of the NDI derivatives showed very weak emission in the molecularly dissolved state (DMSO). Aggregation-induced emission (AIE) was observed for the NDI derivatives (except NAB-1) at the self-assembled state through excimer formation. Upon excitation at 350 nm, the emission maxima of these NDI-based AIE luminogens (AIE-gens) (NAB-2-5) get red shifted from 463 to 588 nm upon altering the substitution from EWG to EDG at the donor site. Inclusion of proper donor-acceptor moieties in the molecular backbone of the self-assembling unit can govern the AIE in combination with the intramolecular charge-transfer process. Consequently, the emission color of these AIE-gens (NAB-2-5) gets tuned from cyan blue to faint green to strong green and finally to bright orange. The tunable aggregation-induced multicolor emission was investigated by different spectroscopic techniques. These cytocompatible, multicolor-emitting fluorescent organic nanoparticles were utilized for bioimaging applications.

Aggregation-Induced Emission and White Luminescence from a Combination of π-Conjugated Donor-Acceptor Organic Luminogens

Two new star-shaped phenyl- and triazine-core based donor-acceptor (D-A) type conjugated molecules bearing triphenylamine end-capped arms were synthesized and characterized as imminent organic optoelectronic materials. Photophysical properties of the compounds were explored systematically via spectroscopic and theoretical methods. Because of the presence of donor-acceptor interactions, these luminogens display multifunctional properties, for instance, high extinction coefficient, large stokes shift, and pronounced solvatochromic effect. The compounds also exhibited phenomenon known as aggregation-induced emission on formation of nano-aggregates in the tetrahydrofuran-water mixture. The aggregate formation was confirmed by transmission electron microscopy, scanning electron microscopy, and dynamic light scattering analyses. Moreover, by controlling the electron withdrawing ability of the acceptor, complementary emissive fluorophores (blue and yellow) were achieved. These two complementary colors together span the entire range of visible spectrum (400-800 nm) and therefore when mixed in a requisite proportion generate white light in solution phase. These findings have potential for the progress of new organic white light radiating materials for applications in lighting and display devices.

White-light emission from a single organic compound with unique self-folded conformation and multistimuli responsiveness

White-light emitting organic materials attract broad attention which are ascribed to their potential for applications in lighting devices and display media. Most reported organic white-light emitters rely on the combination of several components that emit different colors of light (red/green/blue or orange/blue), which may cause problems to stability, reproducibility and device fabrication. By contrast, white-light emission from single-molecule systems offers opportunities to overcome these disadvantages, meanwhile engendering white-light with high quality. Nevertheless, limited cases of white-light emission at the molecular scale reported principally concentrate on organic solvents. Herein, we designed and synthesized new bi-functional organic molecules with a symmetric donor-acceptor-donor (D-A-D) type structure with the aim to construct a single-molecule white-light emitting system in aqueous solution. Further experiments and calculations demonstrate the possibility of stacking between the pyridinium-naphthalene (PN) core and coumarin groups in the designed molecules, ascribed to hydrophobic effects, π-π stacking and donor-acceptor interactions, which could dramatically enhance the intramolecular charge transfer (ICT) efficiency along with remarkable charge transfer (CT) emission. Based on this, multicolor photoluminescence including white-light can be finely tuned in various modes including excitation wavelength, solvent polarity, temperature, and host-guest interactions. A white-light emitting (WLE) hydrogel was also facilely prepared through the dispersion of one of the compounds in a commercial agarose gelator. This innovative study helps enrich the strategies to construct single-molecule organic white-light emitting materials in aqueous medium using the self-folding behavior.

Facile preparation and investigation of the properties of single molecular POSS-based white-light-emitting hybrid materials using click chemistry

Recently, organic white light emitting devices (OWLEDs) have attracted great interest in flexible displays and solid state lighting devices.

Secondary inner filter effect allows extremely efficient pure white light emission by spatially separated organic fluorophores

White light emission with 70 ± 3% efficiency under dilute conditions was obtained via the secondary inner filter effect between 2-(1-pyrenyl)benzimidazole (PyBIM) and pyrelene monoimide (PMI).

H-Aggregated π-Systems Based on Disulfide-Linked Dimers of Dipyrrolyldiketone Boron Complexes

Dipyrrolyldiketone boron complexes linked by a disulfide bond were synthesized, forming H-aggregated dimers assisted by intramolecular π-π and hydrogen-bonding interactions. The conformations of the dimers, with small C-S-S-C dihedral angles, were examined by UV-vis absorption and ¹H NMR spectra as well as single-crystal X-ray analysis and theoretical studies.

l-Phenylalanine-Tethered, Naphthalene Diimide-Based, Aggregation-Induced, Green-Emitting Organic Nanoparticles

The present article delineates the formation of green fluorescent organic nanoparticle through supramolecular aggregation of naphthalene diimide (NDI)-based, carboxybenzyl-protected, l-phenylalanine-appended bola-amphiphile, NDI-1. The amphiphilic molecule is soluble in DMSO, and, with gradual addition of water within the DMSO solution, the amphiphile starts to self-assemble via H-type aggregation to form spherical nanoparticles. These self-assembly of NDI-1 in the presence of a high amount of water exhibited aggregation-induced emission (AIE) through excimer formation. Notably, in the presence of 99% water content, the amphiphile forms spherical aggregated nanoparticles as confirmed from microscopic investigations and dynamic light scattering study. Interestingly, the emission maxima of molecularly dissolved NDI-1 (weak blue fluorescence) red-shifted upon aggregation with increase in water concentration and led to the formation of green-emitting fluorescent organic nanoparticles (FONPs) at 99% water content. These green-emitting FONPs were utilized in cell imaging as well as for efficient transportation of anticancer drug curcumin inside mammalian cells.

Selective Binding of Genomic Escherichia coli DNA with ZnO Leads to White Light Emission: A New Aspect of Nano-Bio Interaction and Interface

Here, we report for the first time, a novel and intriguing application of deoxyribonucleic acid (DNA) in the area of optics by demonstrating white light emission by tuning the emission of a nanomaterial, ZnO rods, exhibiting surface defects, in the presence of genomic Escherichia coli DNA with a comparatively high quantum efficiency. In order to understand the DNA specificity, we have also studied the interaction of ZnO with CT, and ML DNA, ss EC DNA, synthetic polynucleotides and different mononucleosides and bases. Further, in order to understand the effect of particle shape and defects present in ZnO, we have also extended our study with ZnO rods prepared at higher temperature exhibiting red emission and ZnO particles exhibiting yellow emission. Interestingly, none of the above studies resulted in white light emission from ZnO-DNA complex. Our studies unequivocally confirmed that the concentration and the nature of DNA and ZnO together plays a crucial role in obtaining CIE coordinates (0.33, 0.33) close to white light. The much enhanced melting temperature (T_m) of EC DNA and the energetics factors confirm enhanced hydrogen bonding of ZnO with EC DNA leading to a new emission band. Our experimental observations not only confirm the selective binding of ZnO to EC DNA but also open a new perspective for developing energy saving light emitting materials through nano-bio interactions.

Supramolecular Assemblies of Ferrocene-Hinged Naphthalenediimides: Multiple Conformational Changes in Film States

We design a new naphthalenediimide (NDI) π-system, NDI-Fc-NDI, having a ferrocene linker as a hinge unit and long alkyl chains as supramolecular assembling units. The NDI units are "directionally flexible" in concert with the pivoting motion of the ferrocene unit with a small rotational barrier. The NDI units rotate around the ferrocene unit faster than the NMR time scale in solution at room temperature. UV-vis absorption, synchrotron X-ray diffraction, and atomic force microscope studies reveal that NDI-Fc-NDI forms a fibrous supramolecular assembly in solution (methylcyclohexane and highly concentrated chloroform) and film states, wherein the NDI units are in the slipped-stack conformation. The NDI-Fc-NDI supramolecular assembly in the film state exhibits multiple phase transitions associated with conformational changes at different temperatures, which are confirmed by differential scanning calorimetry, polarized optical microscopy, and temperature-dependent X-ray diffraction. Such thermal transitions of NDI-Fc-NDI films also induce changes in the optical and electronic properties as revealed by UV-vis absorption and photoelectron yield spectroscopies, respectively. The thermal behaviors of NDI-Fc-NDI, realized by the unique molecular design, are considerably different from the reference compounds such as an NDI dimer connected with a flexible 1,4-butylene linker. These results provide us with a plausible strategy to propagate the molecular dynamics of the π-system into macroscopic properties in film states; the key factors are (i) the supramolecular alignment of molecular switching units and (ii) the directional motion of the switching units perpendicular to the supramolecular axis.

H-bonding directed programmed supramolecular assembly of naphthalene-diimide (NDI) derivatives

In this review we have collated various supramolecular designs, all surrounding H-bonding among well-known functional groups (peptides, nucleic acids, amides, ureas, carboxylic acids, pyridine-hydroxyls, urethanes, imides and others), to dictate self-assembly of naphthalenediimide (NDI) π-systems (both small molecules and polymeric building blocks) that exhibit several exciting features including strong propensity for π-π interactions, π-acidity, excellent n-type semiconductivity, CT-complexation, ion-π interactions, ring-substitution dependent redox properties and photophysical properties. This article reveals that H-bonding can indeed serve as a very powerful and versatile tool to programmed self-assembly of a single or multiple dye system producing a wide range of tailored soft materials, including fibrillar gels, chromonic mesophases, foldamers, nanotubes, vesicles, reverse micelles and polymersomes, both in water and organic medium with distinct photophysical properties, charge transport properties, conductivity properties and functional group displays that are highly relevant in the fields of biology and organic electronics.