Pure White Light Emission Research Articles

A 2-Styryl-1,8-naphthyridine derivative as a versatile fluorescent probe for the selective recognition of Hg2+, Ag+ and F− ions by tuning the solvent

A novel fluorescent probe 1 has been synthesized by a microwave reaction, and its ion-binding and fluorescence-sensing properties have been investigated under different solvent conditions. The analysis results indicated that probe 1 can act as a multiple analysis probe by simply tuning the solvent. Probe 1 exhibited high selectively toward Hg2+ through fluorescence quenching in H2O/DMF. In H2O/1,4-dioxane solution, probe 1 selectively recognized and discriminated between Ag+ and Hg2+ displaying ratiometric behaviour. Moreover, probe 1 readily recognized the anion F− via the ratiometric fluorescent mode in CH3CN. Furthermore, distinct colour changes were observed under UV light, which can be seen by the naked eye and thus used for distinguishing Hg2+, Ag+ and F− from the other ions screened herein using probe 1. Interestingly, almost pure white light emission was evident by simply tuning the F− anion-concentration, which makes this system a potential candidate for smart and tunable luminescent materials.

Rationally Designed Carbon Nanodots towards Pure White-Light Emission.

We report a rational synthesis of carbon nanodots (CNDs) aimed at tailoring their emission, starting from a reasoned choice of organic precursors. To showcase the potential of this approach in a field such as optoelectronics, we designed experiments aimed at preparing materials that emit across the entire visible spectrum. Specifically, using precursors such as arginine, ethylenediamine, naphthalene dianhydride, and 2,6-dibromonaphtalene dianhydride, in appropriate ratios, it was possible to obtain pure white-light (0.33, 0.33; CIE coordinates) emitting carbon nanodots (WCNDs) through a one-step microwave-assisted synthesis and facile purification. The characterization and properties of this novel nanomaterial is discussed.

Efficient white-light emission from a single polymer system with “spring-like” self-assemblies induced emission enhancement and intramolecular charge transfer characteristics

A new strategy of “spring-like” self-assemblies induced efficient and pure white-light emission from a single-layer device architecture by using a dopant–host copolymer system.

A semi-conductive organic–inorganic hybrid emits pure white light with an ultrahigh color rendering index

A single-component semiconductor emits pure white light with CIE chromaticity coordinates of (0.33, 0.33) and an ultrahigh color rendering index of 92.5.

Multicolor Photoluminescence Including White-Light Emission by a Single Host-Guest Complex.

Achieving multicolor photoluminescence, especially white-light emission, under mild conditions based on a single fluorescent compound is a great challenge. Herein, we report a novel colorful-emission host-guest complex BPCY, which is composed of a two-arm fluorescent guest molecule (BPC) and γ-cyclodextrin (γ-CD) as the host molecule. BPC bears a unique asymmetrical donor-acceptor-donor (D1-A+∼D2)-type structure, where D1, A+, and D2 stand for the binaphthol electron donor, pyridinium electron acceptor, and coumarin electron donor, respectively. The luminescence property of BPC shows dual-sensitivity, i.e., toward the excitation wavelength and the cyclodextrin host molecule. Under certain conditions, the complex shows three different emission wavelengths, allowing the realization of multicolor photoluminescence, including red (R), green (G), and blue (B) as well as various intermediate colors by orthogonally modulating these two stimuli. In this way, nearly pure white-light emission with CIE coordinates (0.33, 0.34) could be generated. A combination of structural, spectroscopic, and computational simulation studies revealed the occurrence of synergetic mechanistic processes for the stimuli-responsive multicolor luminescence of the BPCY complex, namely, host-enhanced intramolecular charge-transfer (ICT) and host-induced restriction of intramolecular rotation (RIR). This new supramolecular complex with superior multicolor emission abilities may find wide applications in the fields of information processing and display media. Furthermore, the molecular design rationale presented here may provide a new design strategy for the development of high performance optical materials using a single supramolecular platform.

Photophysics of Diphenylbutadiynes in Water, Acetonitrile-Water, and Acetonitrile Solvent Systems: Application to Single Component White Light Emission.

Diacetylenes have been the subject of current research because of their interesting optoelectronic properties. Herein, we report that substituted diphenylbutadiynes exhibit locally excited (LE) and excimer emissions in water and multiple emissions from the LE, excimer, and intramolecular charge transfer (ICT) states in acetonitrile-water solvent systems. The LE, excimer, and ICT emissions are clearly distinguishable for a diphenylbutadiynyl derivative with push (-NMe2)-pull (-CN) substituents and those are closely overlapped for non-push-pull analogues. In neat acetonitrile, the excimer emission disappears and the LE and ICT emissions predominate. In the case of the push (-NMe2)-pull (-CN) diphenylbutadiyne, the intensity of the ICT emission increases with increasing the fluorophore concentration. This suggests that the ICT emission accompanies with intermolecular CT emission which is of exciplex type. As the LE and exciplex emissions of the push-pull diphenylbutadiyne together cover the visible region (400-700 nm) in acetonitrile, a control of the fluorophore concentration makes the relative intensities of the LE and exciplex emissions such that pure white light emission is achieved. The white light emission is not observed in those diphenylbutadiynyl analogues in which the peripheral substituents of the phenyl rings do not possess strong push-pull character.

Pure white light emission from organic molecules using solvent induced selective self-assembly

Organic materials generating pure white light emission have attracted considerable attention in recent times due to their potential applications in lighting and display devices. We have used anthracene carboxylic acid (AnA) and anthracene carbaldehyde (AnC) to generate white light using solvent induced selective self-assembly (SISSA). Mechanistically, SISSA prevents a fast intersystem crossing of the AnC monomer and generates an emissive state from the self-assembled AnC. While AnA emits in the blue region, the self-assembled AnC emits in the greenish yellow region with identical emission intensity. CIE coordinates (0.33, 0.33) of pure white light have been achieved by incorporating orange emitting rhodamine B. More importantly, the pure white light formation has been demonstrated at desired temperatures in both aqueous medium as well as in an agar-agar hydrogel.

White light emission and optical gains from a Si nanocrystal thin film

We report a Si nanocrystal thin film consisting of free-standing Si nanocrystals, which can emit white light and show positive optical gains for its red, green and blue (RGB) components under ultraviolet excitation. Si nanocrystals with ϕ = 2.31 ± 0.35 nm were prepared by chemical etching of Si powder, followed by filtering. After being mixed with SiO2 sol-gel and thermally annealed, a broadband photoluminescence (PL) from the thin film was observed. The RGB ratio of the PL can be tuned by changing the annealing temperature or atmosphere, which is 1.00/3.26/4.59 for the pure white light emission. The origins of the PL components could be due to differences in oxygen-passivation degree for Si nanocrystals. The results may find applications in white-light Si lasing and Si lighting.

Enhancement Performances in White Organic Light-Emitting Diode (WOLED) by Formation of Charge-Transfer (CT) Complex.

This study elucidates the optoelectronic properties of high efficiency white organic light-emitting diodes (WOLED) with molybdenum oxide (MoO3) doping into N, N0-di (naphthalene-1-yl)-N, N0-diphenyl-benzidine (NPB) as a p-doping hole-transport layer (p-HTL). The device with a MoO3-doping NPB layer shows a turn-on voltage of 2.01 V and the maximum power efficiency of 4.6 lm/W. The X-ray photoelectron spectroscopy (XPS) and UV-vis-NIR absorption spectra of MoO3-doping NPB layer revealed that the MoO3-doping NPB p-HTL had an improvement on holes injection. The improvement is caused by the formation of the charge transfer (CT) complex (NPB(+)-MoO3-) that is generated by doping MoO3 into NPB, markedly increasing the number of holes carrier, improving the balance of the electrons and holes in recombination zone. The pure white light emission with Commissions Internationale De L'Eclairage (CIE) coordinates of (0.335, 0.321) was achieved at the operating voltage of 6 V. This device shows the maximum luminance of 12230 cd/cm2 and the maximum luminous efficiency of 7.01 cd/A at an operating voltage of 7 V.

White Light Emission from Vegetable Extracts.

A mixture of extracts from two common vegetables, red pomegranate and turmeric, when photoexcited at 380 nm, produced almost pure white light emission (WLE) with Commission Internationale d’Eclairage (CIE) chromaticity index (0.35, 0.33) in acidic ethanol. It was also possible to obtain WLE in polyvinyl alcohol film (0.32, 0.25), and in gelatin gel (0.26, 0.33) using the same extract mixture. The colour temperature of the WLE was conveniently tunable by simply adjusting the concentrations of the component emitters. The primary emitting pigments responsible for contributing to WLE were polyphenols and anthocyanins from pomegranate, and curcumin from turmeric. It was observed that a cascade of Forster resonance energy transfer involving polyphenolics, curcumin and anthocyanins played a crucial role in obtaining a CIE index close to pure white light. The optimized methods of extraction of the two primary emitting pigments from their corresponding plant sources are simple, cheap and fairly green.

White-Light Emission from Unmodified Graphene Oxide Quantum Dots

We report herein the synthesis and characterization of unmodified graphene oxide quantum dots (GOQDs) with white-light-emitting properties, upon photoexcitation at 340 nm. The Commission International de l’Eclairage (CIE) 1931 chromaticity coordinates for GOQDs (x = 0.29, y = 0.34) suggest that highly pure white-light emission was achieved. A detailed mechanistic study was carried out utilizing UV–visible absorption, steady-state and time-resolved fluorescence spectroscopy, and dynamic light scattering (DLS) techniques to understand the origin of the white-light emission. The results taken together suggest that GOQDs could self-assemble in solution and thus transform the luminescence behavior. Furthermore, the results indicate that the pH of the medium also plays a crucial role in assisting the aggregation to generate the white-light emission. The concentration-dependent DLS measurements support a cooperative mechanism for the aggregation kinetics in the system. More importantly, the study suggests that w...

New heterometallic Ir(III)2-Eu(III) complexes: white light emission from a single molecule.

The design of a relatively simple bifunctional ligand led to the obtainment of three new Ir(III)2-Eu(III) heterometallic complexes. Variation of the degree of fluorination in the cyclometalating ligands coordinated to the Ir(III) centres allows tuning of the photophysical properties and pure white light emission from a single complex.

Improved performance of flexible white hybrid light emitting diodes by adjusting quantum dots distribution in polymer matrix

A flexible hybrid white light emitting diode (LED), in which the CdSe/ZnS quantum dots (QDs) acted as an orange emitter and small organic molecules as the blue emitter in a multilayered structure was fabricated by spin-coating. The poly (N-vinylcarbazole) (PVK) was added in the emitting layer to adjust QDs distribution in polymer matrix. By increasing the concentration of PVK, the electroluminescent intensity at 610 nm emission was enhanced since QDs had a better dispersibility in polymer matrix with the presence of PVK, and a pure white light emission was obtained due to the optimized architecture of the hybrid active layer. Our study provides a novel method for improving the device performance of hybrid white LEDs by controlling the QDs distribution in the active layer.

A single 2-(2'-hydroxyphenyl)benzothiazole derivative can achieve pure white-light emission.

The synthesis and photophysics of two novel 2-(2'-hydroxyphenyl)benzothiazole (HBT) derivatives are presented. The electron-withdrawing trifluoromethyl (CF3) group in compound 1 facilitates the deprotonation of the phenolic hydroxy group. Well-resolved triple fluorescence from the enol, keto, and phenolic anion, which ranges from 350 to 600 nm, was detected for 1 in ethanol, which marks the first time triple fluorescence from an excited-state intramolecular proton transfer (ESIPT) molecule has been reported. Both triphenylamine and CF3 were introduced into derivative 2. Intramolecular charge transfer and the "red-edge effect" resulted in the bathochromic shift of dual fluorescence of 2. Triple fluorescence was also observed for 2 in ethanol. In mixed acetonitrile and ethanol, pure white-light emission with CIE coordinates of (0.33, 0.33) and a quantum yield of 0.25 was achieved for 2. This work provides a new avenue for the rational design of an ESIPT molecule to achieve white-light generation under mild conditions.

Blue, green, and orange-red emission from polystyrene microbeads for solid-state white-light and multicolor emission.

Solid-state white-light emission was achieved from polystyrene (PS) microbeads incorporated with fluorophores based on perylene bisimide (PBITEG) and oligo(p-phenylenevinylene) (OPV) as acrylic cross-linkers. The PS beads incorporated with only PBITEG gave intense orange-red emission; PS incorporated with OPV exhibited blue-emission, whereas a series of polymers incorporating both cross-linkers exhibited varying shades of white-light emission. One of the PS samples, PS-PBITEG-6.25-OPV-4.28 (PBITEG incorporation: 6.25 × 10(-7) mole; OPV incorporation: 4.28 × 10(-7) mol), exhibited pure white-light emission in the powder form with CIE coordinates (0.33, 0.32). The rigid aromatic cross-linkers were incorporated into the PS backbone in a two-stage dispersion polymerization to afford PS beads in the size range 2 to 3 μm. The incorporation of fluorophores as cross-linkers enabled covalent attachment of the dye to the polymer backbone, avoiding dye leakage besides avoiding aggregation-induced fluorescence quenching.

A carbazole–oxadiazole diad molecule for single-emitting-component white organic light-emitting devices (WOLEDs)

The synthesis is reported of a new molecule consisting of N-phenylcarbazole linked through a butadiyne bridge to 2,5-diphenyl-1,3,4-oxadiazole. The compound shows blue photoluminescence with positive solvatochromism: e.g., λmax 401 nm in cyclohexane, 461 nm in chloroform solution (quantum yield Φf 0.55) and 428 nm in thin film, with a low intensity band extending to ca. 700 nm in films. OLEDs with a device configuration of [ITO/PEDOT:PSS (50 nm)/molecule:PVK:OXD-7 (80 nm)/Bphen (20 nm)/LiF (0.8 nm)/Al (120 nm)] were fabricated. Pure white-light emission with Commission Internationale De L'Eclairage (CIE) coordinates of (0.313, 0.330) at 13 V and good colour stability with a very small offset of coordinates (0.034, 0.033) under biases of 9–15 V has been achieved.

Flexible white phosphorescent organic light emitting diodes based on multilayered graphene/PEDOT:PSS transparent conducting film

A double-layered graphene/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) conductive film was prepared, in which the PEDOT:PSS layer was obtained by using spray-coating technique. A flexible white phosphorescent organic light-emitting devices based on the graphene/PEDOT:PSS conductive film was fabricated. Phosphorescent material tris(2-phenylpyridine) iridium (Ir(ppy)3) and the fluorescent dye 5,6,11,12-tetraphenylnapthacene (Rubrene) were co-doped into 4,4′-N,N′-dicarbazole-biphenyl (CBP) host. N,N′-diphenyl-N,N′-bis(1-naphthyl)-(1,1′-biphenyl)-4,4′-diamine (NPB) and 4,7-diphenyl-1,10-phenanthroline (Bphen) were used as hole-transporting and electron-transporting layer, respectively, and 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) was used as blue light-emitting layer. The device presented pure white light emission with a Commission Internationale De I’Eclairage coordinates of (0.31, 0.33) and exhibited an excellent light-emitting stability during the bending cycle test with a radius of curvature of 10mm.

Self‐Assembly of Carboxylic Acid Appended Naphthalene Diimide Derivatives with Tunable Luminescent Color and Electrical Conductivity

Self-assembly of a series of carboxylic acid-functionalized naphthalene diimide (NDI) chromophores with a varying number (n=1-4) of methylene spacers between the NDI ring and the carboxylic acid group has been studied. The derivatives show pronounced aggregation due to the synergistic effects of H-bonding between the carboxylic acid groups in a syn-syn catemer motif and π stacking between the NDI chromophores. Solvent-dependent UV/Vis studies reveal the existence of monomeric dye molecules in a "good" solvent such as chloroform and self-assembly in "bad" solvents such as methylcyclohexane. The propensity of self-assembly is comparable for all samples. Temperature-dependent spectroscopic studies show high thermal stability of the H-bonding-mediated self-assembled structures. In the presence of a protic solvent such as MeOH, self-assembly can be suppressed, suggesting a decisive role of H-bonding, whereas π stacking is more a consequence of than a cause for self-assembly. Syn-syn catemer-type H-bonding is supported by powder XRD studies and the results corroborate well with DFT calculations. The morphology as determined by AFM is found to be dependent on the value of n; with increasing n, the morphology gradually shifts from 2D nanosheets to 1D nanofibers. Emission spectra show sharp emission bands with relatively small Stokes shifts. In addition, a rather broad emission band is observed at longer wavelengths because of the in situ formation of excimer-type species. Due to such a heterogeneous nature, the emission spectrum spans almost the entire red-green-blue region. Depending on the value of n, the ratio of intensities of the two emission bands is changed, which results in a tunable luminescent color. Furthermore, in the case of n=1 and 3, almost pure white light emission is observed. Time-resolved photoluminescence spectra show a very short lifetime (a few picoseconds) of monomeric dye molecules and biexponential decays with longer lifetimes (on the order of nanoseconds) for aggregated species. Current-voltage measurements show electrical conductivity in the range of 10(-4) S cm(-1) for the aggregated chromophores, which is four orders of magnitude higher than the value for a structurally similar NDI control molecule lacking the H-bonding functionality.

Color-Stable White-Light-Emitting Diodes Doped with Phosphorescent Dopants via Enhanced Energy Transfer Through Homogeneous Morphology

We demonstrate that white light-emitting diodes using polyfluorene-based blue-emitting conjugated copolymer and small molecule as co-host doped with green and red-emitting phosphorescent dyes. The energy transfer from co-host to phosphorescent dyes is enhanced by blending with small molecule due to the suppressed phase separation between conjugated polymer and Ir complexes as well as the high triplet state of co-host. We obtained voltage-independent pure white light emission with homogeneous morphology in the blended single active layer system by reducing charge trapping in Ir complexes and enhancing the efficient Förster energy transfer.

Single-component polyfluorene electrolytes bearing different counterions for white light-emitting electrochemical cells

A series of polyfluorene (PF) electrolytes bearing Br−, BF4-, or PF6- counterions were synthesized and characterized. 2,1,3-Benzoselenadiazole moieties were incorporated into polymer main chains to produce single-component white light-emitting polymers. The thermal stability of Br-containing ionic PF was decreased because of the Hofmann elimination occurred at higher temperature. By replacing Br− with BF4- or PF6- counterions, the thermal stability of polymers was significantly improved. UV–vis absorption and photoluminescence spectra revealed that Br-containing ionic PF generates blue shift in methanol, while BF4 and PF6-containing ionic PFs show blue shift in acetonitrile. Electrochemical analysis revealed that oxidation potentials of BF4 and PF6-containing ionic PFs were decreased, resulting in increase in HOMO and LUMO levels. The optimized spin-coated light-emitting electrochemical cell (LEC) with the configuration of ITO/PEDOT/polymer/Ag showed a maximum luminescence efficiency of 1.56lm/W at a low operation bias of 3V. The single-component LEC device exhibited pure white light emission with CIE’1931 coordinates approaching (0.33, 0.33) and high color rendering index (CRI>85), referring to its potential use in solid-state-lighting application.