Dual Emission Properties Research Articles

Rational design for thermochromic luminescence in amorphous polystyrene films with bis‐o‐carborane‐substituted enhanced conjugated molecule having aggregation‐induced luminochromism

AbstractWe designed the triad molecule, bis‐o‐carborane‐substituted bis(thienylethynyl)benzene, as a filler for realizing thermochromic luminescent behaviors based on conventional polymer films, such as polystyrene. From the optical measurements, it was found that the triad can show solid‐state emission and dual‐luminescent properties with variable intensity ratios depending on media. From the mechanistic studies including the experiments with the methyl‐substituted model compound, it was revealed that dual emission should be originated from the locally excited and twisted intramolecular charge transfer states, and the latter emission band is significantly enhanced in the solid states. We prepared amorphous films containing variable concentrations of the triad with the spin‐coating method and investigated optical properties. It was found that intensity ratios were drastically changed by altering the concentration of the triad. By increasing the proportion of the triad, aggregation occurred, and emission color was apparently varied through the changes in intensity ratios of the dual emission property. Based on the aggregation‐induced luminochromic property of the triad, thermochromic luminescence was finally realized by heating the amorphous films. The rational design for obtaining thermochromic luminescent amorphous films is illustrated in this paper.

White Light Luminescence from a Homo-conjugated Molecule with Thermally Activated Delayed Fluorescence.

Luminophores with tunable emission properties are appealing due to various applications. Among those properties, thermally activated delayed fluorescence (TADF) has been attracting enormous research interests. Herein, we synthesized a 9,9'-spirobifluorene based homo-conjugated molecule 1, which connects a diphenylamino moiety as electron donor and a naphthalimide group as electron acceptor via 2,2'-positions of spirofluorene. Compound 1 displays dual emission behaviour with both blue and orange fluorescence. The one orange fluorescence around 555 nmshows sensitivity to oxygen and a prolonged lifetime of 284 ns in degassed toluene. Such characteristics imply TADF nature for this emission from a charge-transfer excited state. The other emission at 440 nm with blue colour displayed resistance to oxygen quenching and a normal fluorescence lifetime of 1.5 ns. Compared with control molecule, this emission band is assigned as conventional fluorescence from a localized excited state. In addition, dual emission property allows molecule 1 to be modulated to emit white photoluminescence in thin film with a CIE color coordinate of (0.25, 0.33).

Gold nanoparticles-mediated ratiometric fluorescence aptasensor for ultra-sensitive detection of Abscisic Acid

Herein, a novel ratiometric aptasensor based on carbon quantum dots@2-Methylimidazole zinc salt (CQDs@ZIF-8) and aptamer-functionalized gold nanoparticles (Apt-AuNPs) was developed for highly sensitive detection of ABA by fluorescence spectrometry. The CQDs@ZIF-8 nanomaterials displayed dual-emission properties at 490 nm and 657 nm with excitation at 420 nm were synthesized for the first time. ZIF-8 not only served as an anchor point for CQDs but also acted as a modulator to regulate fluorescence signals of CQDs. Interestingly, introduction of ZIF-8 changed the quenching properties of the AuNPs on CQDs. The AuNPs quenched the fluorescence of CQDs@ZIF-8 at 490 nm but not at the second peak of 657 nm. Few studies have been reported on the ineffectiveness of AuNPs in fluorescence quenching as far as we know. In this study, we found that incorporation of ABA triggered the aggregation of AuNPs due to the specific ABA-aptamer recognition and this changed the fluorescence intensity of the ratiometric probe (CQDs@ZIF-8@Apt-AuNPs). The proposed probe increased the sensitivity and selectivity of determining ABA levels in rice seeds in the range of 0.100–150 ng/mL with an LOD of 30.0 ng/L. Importantly, the method proposed here offers a new unique strategy for the construction of ratiometric probes and ultra-sensitive measurement of biomolecules.

Porphyrin derived dual-emissive carbon quantum dots: Customizable synthesis and application for intracellular Cu2+ quantification

Exploring sensory systems from metal ion receptors enables the continuous development of ion sensing technologies. Herein we report the synthesis of carbon quantum dots (CQDs) from 5,10,15,20-tetrakis(4-sulfophenyl)porphyrin (TSPP) and additional carbon source, and its application in Cu2+ imaging and quantifying in living cells. The CQDs are synthesized via a hydrothermal method in a successive polymerization and carbonization process. Due to the surface traps introduced by TSPP residue, the CQDs are featured with dual-emissive property of green (505 nm) and red (682 nm) fluorescence. Moreover, the red-to-green fluorescence ratio is readily customizable by tuning the molar fraction of TSPP. Benefited from Cu2+ complexation induced red fluorescence quenching, the CQDs can be served as the ratiometric fluorescent nanosensor for Cu2+ detection with ultra-high sensitivity and excellent specificity. The nanosensor is demonstrated to linearly detect Cu2+ in the range of 0−50 nM, with an extremely low limit of detection (LOD) of 37 pM. More importantly, interference from other biological compositions, especially Fe3+, is successfully excluded via the site-selective coordination and specific electron transfer path. Based on the excellent analytical performance of the nanosensor, quantifying Cu2+ in living cells is realized according to the linear correlation between fluorescence ratio and intracellular Cu2+ content, which is parallelly detected by inductively coupled plasma mass spectrometry (ICP-MS). To demonstrate its practicality, the nanosensor is challenged to measure Cu2+ uptake kinetics for cell lines derived from different tissues.

Achieving long lifetime of pure organic room-temperature phosphorescence via constructing hydrogen-bonded organic frameworks

Metal-free pure organic room-temperature phosphorescence (ORTP) materials have attracted widespread attention in the fields of information security, optoelectronics and displays. Hydrogen-bonded organic frameworks (HOFs) can potentially stabilize the triplet excitons and achieve highly efficient RTP. However, it is still a tough challenge to prepare HOFs with ultra-long lifetime RTP. Herein, we report a facile and green hydrothermal method to synthesize rigid melamine-cyanuric acid (M-CA) supramolecular frameworks by a melamine hydrothermal reaction. The luminescence properties of M-CA supramolecular HOFs have been explored under ambient conditions for the first time. There are three modes of luminescence under ambient conditions, including prompt fluorescence, delayed fluorescence (DF) and RTP. The M-CA material shows a long RTP lifetime of 864.1 ms, which is over 7000 times longer than that of melamine itself. The effective intermolecular interactions suppress nonradiative transition of the triplet excitons, enhance the intersystem crossing and provide an oxygen-insulated environment, thus boosting RTP emission. In addition, due to dual emission properties of RTP and fluorescence, the obtained M-CA supramolecular HOFs can be potentially applied in the field of information encryption.

Multi‐Stimuli Responsive and Multicolor Adjustable Pure Organic Room Temperature Fluorescence‐Phosphorescent Dual‐Emission Materials

AbstractA simple bromine and cyanogen substituents positional isomeric 9‐phenyl‐9H‐carbazole (PhCz) donor–acceptor system (namely p‐NN‐Br and m‐NN‐Br) displays ultralong organic phosphorescence, and exhibits unique dual emission properties (fluorescence and phosphorescence). These molecules are found to display distinct responses toward multiple external stimuli including temperature (T), excitation light intensity (I) and pressure (P), and show multicolor tunable behaviors (including the white light, the Commission Internationale d'Eclairage (CIE) = 0.33, 0.34). The unique stimuli‐triggered proportion between singlet and triplet excitons for p‐NN‐Br and m‐NN‐Br is demonstrated systematically by investigating the photophysical spectrum, scanning electron microscope (SEM) imaging and X‐ray analysis, coupled with theoretical calculations. They reveal that the simultaneous introduction of halogens (Br) and pseudohalogens (CN) to the PhCz skeleton can improve the intermolecular interaction and thermal stability. Single crystal analysis shows that there are many more types of dimers and J‐aggregates, thereby stabilizing the excitation of triplet states. Moreover, these isomers have “latent” fingerprint recognition and anti‐background interference performance, which is expected to provide a new method for fingerprint identification. All in all, this strategy paves the way to a multifunctional platform for the development of multi‐stimuli responsive, multicolor regulation and smart luminescent materials with long‐lived emission at room temperature.

Dual channel mitochondria-targeted fluorescent probe for detection of nitric oxide in living cells and zebrafish

A novel dual channel mitochondria-targeted fluorescent probe has been developed to detect nitric oxide (NO), an important and short-life reactive nitrogen species (RNS) which mainly generates in the mitochondria. The new probe Mi-NO has exhibited dual emission property as well as unique spectrum responses towards NO. Upon the addition of NO, the fluorescence of Mi-NO quenched significantly. Molecular docking calculations suggest a potential molecular target for Mi-NO in the mitochondria protein adenine nucleotide translocator (ANT). High colocalization coefficient also suggested that Mi-NO can effectively target mitochondria. Moreover, Mi-NO has been successfully employed in imaging NO in living cells and zebrafish, affording a potential for studying the biological functions of NO in living systems.

Paintable Hybrids with Thermally Stable Dual Emission Composed of Tetraphenylethene-Integrated POSS and MEH-PPV for Heat-Resistant White-Light Luminophores.

Thermally stable dual emission followed by white-light luminescence from hybrid materials is reported. Hybrid films were prepared with a spin-coating method with the mixture solution containing tetraphenylethene (TPE)-integrated polyhedral oligomeric silsesquioxane (POSS) and poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV). TPE-tethered POSS (TPE-POSS) showed high compatibility with MEH-PPV. Therefore, homogeneous films with variable concentrations of TPE-POSS were obtained. Owing to good dispersion of rigid silica cubes into matrices, POSS-containing films demonstrated high thermal stability toward molecular rearrangement by annealing as well as pyrolysis, similar to conventional polymer hybrids. Furthermore, it was found that TPE-POSS was able to enhance emission efficiencies, probably by suppressing chain aggregation. By modulating introduction ratios of TPE-POSS, dual-emission properties followed by white-light luminescence composed of cyan and orange emissions from TPE-POSS and MEH-PPV, respectively, were accomplished. It should be noted that these color balances can be preserved even in the high-temperature region (425 K). Finally, white-light luminescent materials with thermal durability were obtained.

High-Pressure Synthesis of Rare-Earth Borate-Nitrate Crystals for Second Harmonic Generation.

The crystals of a novel family of rare-earth borate-nitrate compounds, Ln7(BO3)3(NO3)N3O (Ln = Pr, Nd), were grown at high-pressure in KAs flux and their crystal structure was determined. The new type of the crystalline structure consists of parallel chains of Ln6 octahedra connected by common faces and forming the channels with the NO3 triangular planar motifs in the center, and isolated OLn4 tetrahedra separated from each other by N3 triangular motifs. Each NO3 triangle is in fact a part of rather unusual (NB3O12) block consisting of 3 distorted BO4 tetrahedra around central nitrogen atom. Under near-infrared (NIR) (λex = 1064 nm) excitation, both compounds revealed a strong signal of second harmonic generation (SHG) at half the excitation wavelength (λem = 532 nm), which is in agreement with their noncentrosymmetric structure. In addition, a photon up-conversion (UC) emission at λem = 880 nm was observed for microcrystals of Nd7(BO3)3(NO3)N3O, which was assigned to the UC process occurring within the 4f electronic manifold of Nd3+ ions. The dual-emission (SHG/UC) properties of Nd7(BO3)3(NO3)N3O microcrystals, concomitant with the absence of photobleaching, makes them prospective candidates for microscopic probes in biological studies.

Fluorescence resonance energy transfer fluorescent polymer dots without conventional chromophores: Synthesis, emission mechanism and applications as Cu2+ probe and fluorescent ink

Recently, the non-aromatic fluorescent materials without benzene rings in their structures have aroused widespread interest. In this work, a citric acid functionalized polyethyleneimine oligomer, CA-PEI, was synthesized through acyl chloride method. In CA-PEI, the polyethyleneimine was bonded to citric acid molecule with a molar ratio of 1:1 through an amide bond, which was confirmed by NMR, FTIR, GPC and XPS. The results indicated that CA-PEI had dual excitation and dual emission properties, and the two emission centers were derived from n–σ* and n–π* transition, respectively. CA-PEI was used to synthesize the light-emitting polymer dots N-PDs through a hydrothermal reaction. Interestingly, compared with CA-PEI, N-PDs had dual excitation and single emission properties, and its Stoke's shift was as high as 193 nm. This can be attributed to the compact structure of N-PDs, which promoted the fluorescence resonance energy transfer (FRET) process. Importantly, N-PDs can serve as excellent probes in response to Cu2+, with a detection limit as low as 2.5 nmol L−1. The response mechanism involves the formation of a copper amine complex between Cu2+ and the PEI part of N-PDs, which then quenches the fluorescence emission of N-PDs through the fluorescence inner-filter effect. N-PDs as well were applied as an encryptable fluorescent ink.

Boron regulated dual emission in B, N doped graphene quantum dots

Abstract Graphene quantum dots are promising candidates for LED and solar cell studies with their low toxicity, inexpensive production cost and high luminescence capacity. However, controlling optical properties of graphene quantum dots is a challenge, which can be partially done by heteroatom doping. In this study, we synthesized dual emissive boron and nitrogen doped graphene quantum dots through a bottom-up synthesis method. Dual emission properties of boron and nitrogen doped quantum dots appeared in the presence of high amount of boron precursor in synthesis medium. Dual emissive features of quantum dots were controlled via controlling amount of boron content in quantum dot structure. Boron and nitrogen doped graphene quantum dots showed dual emissive properties at different excitation wavelengths, therefore they were suitable for applications concerning white – LED and solar cells. With their low toxicity, low production cost and controllable dual emission properties, dual emissive boron and nitrogen doped graphene quantum dots can be considered as a powerful alternative to cadmium and indium based quantum dots.

1,8-Naphthalimide-Based Polymers Exhibiting Deep-Red Thermally Activated Delayed Fluorescence and Their Application in Ratiometric Temperature Sensing.

A series of naphthalimide (NAI)-based red-emissive thermally activated delayed fluorescence (TADF) acrylic monomers has been designed and synthesized. When copolymerized with a host material by Cu(0)-reversible deactivation radical polymerization (Cu(0)-RDRP), polymers exhibiting orange to deep-red TADF were obtained with quantum yields of up to 58% in solution and 31% in the solid state. These emitters exhibit dual emission consisting of high-energy prompt fluorescence from the NAI acceptor (λmax = 340 nm in toluene) and red-delayed fluorescence from the charge-transfer process (λmax = 633-711 nm in toluene). This dual emissive property was utilized to create red-to-blue temperature-responsive polymers by copolymerization of NAI-DMAC with N-isopropylacrylamide and a blue fluorescent dopant. These polymers exhibit red TADF at room temperature and blue fluorescence at 70 °C, with a high ratiometric fluorescent thermal response of 32 ± 4% K-1. Such systems are anticipated to have utility in bioimaging, drug delivery, and temperature sensing, further expanding the range of applications for red TADF materials.

Manipulating the AIE and low-temperature phosphorescence properties of o-carborane-imidazole derivatives via fine tuning their structural features

Four D−π−A dyads based on various imidazole derivatives donor and o-carborane acceptor moiety have been designed and synthesized for efficient tunable solid-state fluorescence and low-temperature phosphorescence. The results showed that four dyads are all AIEgens with dual emission properties in solution, highly solid-state emission, and low-temperature phosphorescence. The dual emissions were attributed to the locally excited (LE) and twisted intramolecular charge transfer (TICT) emission. The crystallographic measurements and theoretical calculations demonstrated that the strong phosphorescence with a long lifetime up to 2.18 s at low temperature may be attributed to the C–H⋅⋅⋅N, C–H⋅⋅⋅π interactions, strong TICT effect and the reduced vibration coupling by attaching a bulky carborane moiety to the chromophore.

Two‐Coordinate Copper(I)/NHC Complexes: Dual Emission Properties and Ultralong Room‐Temperature Phosphorescence

AbstractAs a kind of photoluminescent material, CuI complexes have many advantages such as adjustable emission, variable structures, and low cost, attracting attention in many fields. In this work, two novel two‐coordinate CuI‐N‐heterocyclic carbene complexes were synthesized, and they exhibit unique dual emission properties, fluorescence and phosphorescence. The crystal structure, packing mode, and photophysical properties under different conditions were systematically studied, proving the emissive mechanism to be the locally excited state of the carbazole group. Based on this mechanism, ultralong room‐temperature phosphorescence (RTP) with a lifetime of 140 ms is achieved by selective deuteration of the carbazole group. These results deepen the understanding of the luminescence mechanism and design strategy for two‐coordinate CuI complexes, and prove their potential in applications as ultralong RTP materials.

Two-Coordinate Copper(I)/NHC Complexes: Dual Emission Properties and Ultralong Room-Temperature Phosphorescence.

As a kind of photoluminescent material, CuI complexes have many advantages such as adjustable emission, variable structures, and low cost, attracting attention in many fields. In this work, two novel two-coordinate CuI -N-heterocyclic carbene complexes were synthesized, and they exhibit unique dual emission properties, fluorescence and phosphorescence. The crystal structure, packing mode, and photophysical properties under different conditions were systematically studied, proving the emissive mechanism to be the locally excited state of the carbazole group. Based on this mechanism, ultralong room-temperature phosphorescence (RTP) with a lifetime of 140 ms is achieved by selective deuteration of the carbazole group. These results deepen the understanding of the luminescence mechanism and design strategy for two-coordinate CuI complexes, and prove their potential in applications as ultralong RTP materials.

Dual Emission in a Ligand and Metal Co-Doped Lanthanide-Organic Framework: Color Tuning and Temperature Dependent Luminescence.

In this study, we report the luminescence color tuning in the lanthanide metal-organic framework (LnMOF) ([La(bpdc)Cl(DMF)] (1); bpdc2− = [1,1′-biphenyl]-4,4′-dicarboxylate, DMF = N,N-dimethylformamide) by introducing dual emission properties in a La3+ MOF scaffold through doping with the blue fluorescent 2,2′-diamino-[1,1′-biphenyl]-4,4′-dicarboxylate (dabpdc2−) and the red emissive Eu3+. With a careful adjustment of the relative doping levels of the lanthanide ions and bridging ligands, the color of the luminescence was modulated, while at the same time the photophysical characteristics of the two chromophores were retained. In addition, the photophysical properties of the parent MOF (1) and its doped counterparts with various dabpdc2−/bpdc2− and Eu3+/La3+ ratios and the photoinduced energy transfer pathways that are possible within these materials are discussed. Finally, the temperature dependence study on the emission profile of a doped analogue containing 10% dabpdc2− and 2.5% Eu3+ (7) is presented, highlighting the potential of this family of materials to behave as temperature sensors.

Unusual dual-emissive heteroleptic iridium complexes incorporating TADF cyclometalating ligands.

Five new neutral heteroleptic iridium(iii) complexes IrL2(pic) (2-6) based on the archetypical blue emitter FIrpic have been synthesised. The cyclometallating ligands L are derived from 2-(2,6-F2-3-pyridyl)-4-mesitylpyridine (7), 2-(3-cyano-2,6-F2-phenyl)-4-mesitylpyridine (8), 2-(2,6-F2-phenyl)-4-[2,7-(HexO)2-9H-carbazol-9-yl]pyridine (9), 2-(2,6-F2-3-pyridyl)-4-[2,7-(HexO)2-9H-carbazol-9-yl]pyridine (10) and 2-(3-cyano-2,6-F2-phenyl)-4-[2,7-(HexO)2-9H-carbazol-9-yl]pyridine (11) for complexes 2, 3, 4, 5 and 6, respectively. The carbazole-functionalised ligands 9-11 show weak thermally activated delayed fluorescence (TADF) in solution. Complexes 5 and 6 reveal dual emission in polar solvents. A broad charge transfer (CT) band appears and increases in intensity relative to the higher energy emission band as solvent polarity is increased. The dual emission occurs when the energy of the ligand 3CT state is comparable to that of the 3MLCT state of the complex, resulting in fast interconversion between the two. Assignment of the ligand TADF and dual emission properties is supported by hybrid density functional theory (DFT) and time dependent DFT (TD-DFT) calculations. Phosphorescent organic light emitting devices (PhOLEDs) have been fabricated using these complexes as sky-blue emitters, and their performance is compared to devices using FIrpic and the previously reported complex IrL2(pic) 1 (L from the 2-(2,6-F2-phenyl)-4-mesitylpyridine ligand). For identical device structures, the device containing the carbazole complex 4 performs best out of the seven complexes. The dual emission observed in solution for complexes 5 and 6 is not observed in their devices.

Dual emitting Ag35 nanocluster protected by 2-pyrene imine thiol

In this communication, we present the synthesis of 2-pyrene imine thiol (2-PIT)-protected Ag35 nanoclusters using a ligand exchange-induced structural transformation reaction. The formation of the nanocluster and its composition were confirmed through several spectroscopic and electron microscopic studies. The UV-vis absorption spectrum showed a set of characteristic features of the nanocluster. This nanocluster showed blue emission under UV light due to pyrene to metal core charge-transfer, and NIR emission due to charge-transfer within the metal core. This is the first report on dual emitting pyrene protected atomically precise silver nanoclusters.

A built-in self-calibrating luminescence sensor based on RhB@Zr-MOF for detection of cations, nitro explosives and pesticides.

A RhB@Zr-MOF composite with dual-emission properties was successfully constructed, which comprises a zirconium-based metal–organic framework and the luminescent dye molecule, Rhodamine B (RhB), embedded via the encapsulation method. The fluorescence intensity ratio of the two emissions was found to be ca. 370 nm/590 nm for RhB@Zr-MOF. The fluorescence intensity values of the two emissions of RhB@Zr-MOF can also be affected by the structures of analytes containing different organic groups. Due to the existence of the dual-emission properties in RhB@Zr-MOF, the relative fluorescence intensity of the emission peaks was introduced as a detection index instead of absolute fluorescence intensity. RhB@Zr-MOF, which possesses the characteristics of a built-in self-calibrating fluorescence sensor, was investigated for detecting cations, nitroaromatics and pesticides. Aside from high sensitivity and selectivity, recyclability is the most important property for sensing pesticides. This work shows that RhB@Zr-MOF can maintain its stability after 5 cycles of detecting nitenpyram, with LOD of 0.2 μM. These results demonstrate that dye@MOFs with dual-emission properties can be employed as multifunctional fluorescence sensors for different types of analytes, and that RhB@Zr-MOF provides a new paradigm for analyte sensing.

Interparticle distance as a key factor for controlling the dual-emission properties of carbon dots.

Due to their distinct and useful features, multi-color-emissive carbon dots (CDs) have gained much attention, however, their lack of red emission has so far limited their practical use for various applications, thus understanding of their emission origins is required. Concentration-dependent photoluminescence (PL) is a unique feature of CDs and is regarded as one way to realize tunable PL, providing information on their PL mechanism. In this work, we synthesized concentration-dependent blue/red dual-color-emissive CDs under single excitation at 365 nm. With the increase in concentration, the blue emission band showed a large red-shift from 431 to 500 nm, while the red emission band at around 650 nm showed only a relative intensity change, without a significant spectral shift. We performed spectroscopic analysis by changing the solution environment including the solvent type and pH to investigate the origins of the two emission bands. These results, combined with the zeta potential and fluorescence microscopy results, confirmed that the interparticle distance of CDs is a key factor for their dual-emissive PL behavior. With the advantage of unique PL features, we showed the potential of the CDs for white light-emitting diode (WLED) and ratiometric pH sensing applications.