Efficient Solid-state Emission Research Articles

Single Solid-State Emissive Carbon Quantum Dots for Multicolor, Bright and Efficient Electroluminescent Light-Emitting Diodes.

Color-tunable electroluminescent light-emitting diodes (LEDs) based on quantum dots (QDs) are rapidly emerging as a key technology for next-generation full-color displays and solid-state lighting. However, achieving broad color tunability in LEDs that utilize a single QD emissive material continues to pose significant challenges. Here, we present the first example of bright, multicolor electroluminescent LEDs with tunable emission peaks spanning from 535 to 640 nm, utilizing a new type of single red solid-state emissive carbon quantum dots (R-SSCQDs). Unlike conventional CQDs, which often exhibit weak or negligible emission in solid-state form, R-SSCQDs demonstrate bright red emission in solid state and green emission in diluted state. Experimental investigations and theoretical calculations reveal that the unique non-planar spatial structure and significant steric hindrance of R-SSCQDs effectively suppress π-π stacking, resulting in efficient solid-state fluorescence emission. By adjusting the weight ratios of R-SSCQDs doped in host materials, we showcase bright, efficient and multicolor CQD-based LEDs that emit green, yellow, orange, and red light, achieving a record-high luminance of 15,834 cd m-2 and current efficiency of 10.3 cd A-1. This work presents a straightforward and universal strategy for creating efficient solid-state emissive CQDs, showcasing significant potential for color-tunable LED applications.

Suzuki-Miyaura coupling reaction: Blue-yellow emitting AIE active dyes for organic electronics

A series of eight novel donor‒acceptor (D─A) based 2,3-di(thiophen-2-yl)quinoxaline derivatives 2–9 were prepared by modulating donor species on quinoxaline core with Palladium catalyzed Suzuki–Miyaura ‘C–C bond’ coupling reaction. The synthesized molecules were fully characterized and studied for impact of D–A interaction on opto-electrochemical properties. Absorption spectra of 2–9 display intramolecular charge transfer (ICT) transitions in the range of 388–414 nm. Dyes shows positive solvatochromism and emit in blue‒yellow region with emission maxima 444−550 nm on excitation at their respective ICT maxima in toluene, chloroform, DCM, DMSO and neat solid film. Further, solid state emission was studied for aggregation‒induced emission (AIE) effect in THF–water system due to the nanoparticles formation, as confirmed by FEG‒SEM technique. The HOMO and LUMO energy level for compounds 2–9 were measured by cyclic voltammetry and found in the range of −5.15 to −5.94 eV and −2.81 to −3.39 eV. Theoretical studies of molecules were also carried out by using DFT and TD‒DFT calculations. The comparable HOMO and LUMO energy levels with reported ambipolar materials and efficient solid-state emission make synthesized compounds potential candidate for solid state emissive, ambipolar charge transporting materials in organic electronics.

Mechanistic insights into the AIE properties of BF2 complexes of N-acetyl 2-aminobenzothiazoles as rotorless polyheteroaromatics

A series of four novel BF2 complexes of N-acetyl 2-aminobenzothiazoles featuring rigid rotorless polyheteroaromatic structures have been designed and synthesized. All of these compounds exhibited AIE properties with excellent solid-state fluorescence quantum yields up to 81.7%. Theoretical calculations revealed the characteristics of their ground-state structures and excitation processes. Their seemingly abnormal nonradiative decay in solutions was investigated from both direct vibrational relaxation and S0/S1 surface crossing perspectives by the two-channel model. The minimum energy conical intersection (MECI) located by SF-TDDFT method indicated that the CI is mainly induced by the deformation of the BF2 complex ring, leads to elevated energy level, and thus is not responsible for the fluorescence quenching. The potential energy curve (PEC) constructed along the distortion angle θ of BF2 complex ring showed that when θ reaches 50° (the dark-state region approaching MECI), the energy barrier is only 0.11 eV, suggesting that the deformed dark state could serve as an effective deactivation channel via ultrafast internal conversion (IC). Crystallographic analysis illustrated that the F atoms perpendicular to the polyheterocycle could effectively prohibit detrimental π-π stacking and enhance intermolecular interactions, thereby resulting in efficient solid-state emission.

Efficient blue-light-excitable copper(I) coordination network phosphors for high-performance white LEDs

The blue-light-excitable phosphors play a crucial role in the high-performance white LEDs. Here, we report on two new Cu(I) coordination network materials as yellow-emitting phosphors prepared by suitably expanded π-conjugated triazole ligands. Upon blue-light irradiation, these complexes exhibit efficient solid-state emission and enhanced photostability. Through incorporating the yellow phosphor and a commercial blue-green powder (BaSi2N2O2:Eu2+) with a blue LED chip, the phosphor-converted LED devices display remarkable white emission properties. The experimental results demonstrate that the Cu(I) coordination network materials function as promising blue-light excitable phosphors with great application potential for full-spectrum white LEDs.

Side-chain engineering of highly efficient solid-state red fluorophore based on diaminoterephthalate with steric ortho-substituents

A series of 2,5-diaminoterephthalate-based fluorophores (Me, Et, iPr, and tBu) with steric ortho-substituents were designed and synthesized. There is an optical changing tendency of the four crystals with distinct red-shifted emissions. The crystal characteristics suggest that the enhanced anti-aromaticity of the central benzene rings of four compounds in the ground state might be an important reason for achieving long-wavelength fluorescence emission, which matches well with the changing tendency of C–N bond lengths. Moreover, all compounds exhibit highly efficient solid-state emission with fluorescence quantum efficiency reaching up to 69%.

Synthesis, structures and thermal dependence of photoluminescence of copper(I) halide metallacycle and 1-D chain complexes

In this work, we report the synthesis, X-ray crystal structures, and temperature dependence of solid-state photoluminescence properties of two cuprous iodide complexes, namely, [(mpbi)CuI]n (1) and [(bimb)CuI]n (2) (mpbi = 1,1′-(5-methyl-1,3-phenylene)bis (1H-imidazole), bimb = 1,2-bis((imidazole-1-yl)methyl)benzene). The complexes exhibit efficient solid-state emission at room temperature, with peak emission wavelengths at 580 and 650 nm, respectively. Two complexes display outstanding thermal stability. The X-ray diffraction analysis revealed that the structure of 1 is a discrete metallacycle with the copper atom in the structure adopting a less common triangular configuration, while the structure of 2 is a one-dimensional chain structure of Cu2I2 binuclear rhomboid. The temperature dependence of photoluminescence in the wide temperature range is investigated, which demonstrates that the thermal quenching of luminescence of copper complexes can be regulated by cuprophilic interaction.

Synthesis and characterization of tris(5,7-diphenyl-8-quinolinolato) aluminum(III), gallium(III), and indium(III) complexes: Effect of metal ions on the structural, photoluminescence, thermal and electrochemical properties

The 5,7-diphenyl-8-hydroxyquinoline (5,7-Phq) organic ligand was successfully synthesized via Suzuki cross-coupling reaction followed by synthesis of the novel organometallic complexes tris(5,7-diphenyl-8-quinolinolato) metal(III) [M(5,7-Phq)3; M = Al, Ga, or In]. All synthesized M(5,7-Phq)3 complexes were characterized using various analytical techniques. The molecular structure of In(5,7-Phq)3 was confirmed using single-crystal X-ray crystallography, which indicates that the three 5,7-Phq ligands are arranged in a meridional configuration around the central metal In3+ ion, and the complex is distorted from ideal octahedral geometry. There are no significant differences in the UV–visible absorption spectra of the complexes in the tetrahydrofuran (THF) solution and they are all red-shifted compared to that of the parent Mq3 (M = Al3+, Ga3+, In3+) compounds. The broad absorption band at around 417 nm is assignable to the π-π* transition. The synthesized M(5,7-Phq)3 complexes in the solution and in the solid-state show red-shifted emission compared to that of the parent unsubstituted compounds. The decrease in the solution and the solid-state emission efficiency and intensity from Al to In complexes is attributed to the heavy atom effect. Despite the electrochemical band gap energies (Egec) of the complexes determined from cyclic voltammetry being in good agreement with those reported for the unsubstituted complexes, the optical band gap energies (Egopt) determined from the Tauc plot method are smaller. The results of the fluorescence decay of the M(5,7-Phq)3 complexes both in CHCl3 solution and in solid state at 298 K show that the decays fit a bi-exponential model with two lifetimes (τ1 and τ2). The average values of fluorescence lifetimes of the complexes in CHCl3 are significantly greater than those of the complexes in the solid state.

Aggregation-induced emission of cyclometalated rhodium(III) and iridium(III) phenylpyridine complexes with ancillary 1,3-diketones.

A joint structural and spectroscopic study of simple bis-cyclometataled rhodium(III) and iridium(III) complexes with 2-phenylpyridine and aromatic β-diketones (dibenzoylmethane, benzoylacetone, benzoyltrifluoroacetone, and 2-thenoyltrifluoroacetone) reveals an interplay between the solid-state emission efficiency and crystal packing peculiarities of the complexes. Although the prepared rhodium(III) cyclometalates are isostructural with iridium(III) analogues, different types of π-π interactions are responsible for the aggregation-induced emission (AIE) of the complexes depending on the metal ion. For iridium(III) complexes, pyridyl-pyridyl contacts are essential for AIE because they lower the energy of the emissive metal-to-ligand charge transfer state below that of the non-emissive state located at the ancillary ligand. Enabled by phenyl-pyridyl interactions partially blocking the population of non-emissive d-d states, solid-state phosphorescence enhancement is successfully achieved in a rhodium(III) complex with ancillary benzoyltrifluoroacetone, which is the first example of a rhodium complex exhibiting AIE.

Facile synthesis of ultrabright luminogens with specific lipid droplets targeting feature for in vivo two-photon fluorescence retina imaging

The application of fluorescent probes for in vivo retinal imaging is of great importance, which could provide direct and crucial imaging evidence for a better understanding of common eye diseases. Herein, a group of bright organic luminogens with typical electron-donating (D) and electron-accepting (A) structures (abbreviated as LDs-BDM, LDs-BTM, and LDs-BHM) was synthesized through a simple single-step reaction. They were found to be efficient solid-state emitters with high fluorescence quantum yields of above 70% (e.g., 83.7% for LDs-BTM). Their light-emission properties could be tuned by the modulation of π-conjugation effect with methoxy groups at different substituent positions. Their resulting fluorescent nanoparticles (NPs) were demonstrated as specific lipid droplets (LDs) targeting probes with high brightness, good biocompatibility, and satisfactory photostability. LDs-BTM NPs with a large two-photon absorption cross section (σ2 = 249 GM) were further utilized as ultrabright two-photon fluorescence (2PF) nanoprobes for in vivo retina imaging of live zebrafish by NIR excitation at an ultralow concentration (0.5 µmol/L). Integrated histological structures at the tissue level and corresponding fine details at the cellular level of the embryonic retina of live zebrafish were clearly demonstrated. This is the first report of using ultrabright LDs-targeting nanoprobes to accurately measure fine details in the retina with 2PF microscopic technique. These good results are anticipated to open up a new avenue in the development of efficient 2PF emitters for non-invasive bioimaging of living animals.

Reduced Graphene Oxide Quantum Dot Light Emitting Diodes Fabricated Using an Ultraviolet Light Emitting Diode Photolithography Technique.

Graphene quantum dots usually suffer from serious fluorescence quenching in aggregates and the solid state due to easy agglomeration and aggregation-induced quenching, which seriously restrict their practical applications. An ingenious strategy to kill three birds with one stone, the ultraviolet (UV) photolithography technique, was studied, and blue-emitting reduced graphene oxide quantum dot (rGOQD)-based light emitting diodes (LEDs) with efficient solid state emission were first fabricated using UV photolithography. First, rGOQDs were prepared by the in situ photoreduction of GOQDs by using the photoinitiator phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide with 395 nm UV LED exposure. Furthermore, rGOQD/photoresist patterns were prepared under the same conditions. Meanwhile, the in situ photoreduction of GO in the aforementioned photoresist to rGO was realized by UV photolithography to improve the conductivity of the rGOQD/photoresist films. Additionally, the in situ photoreduction of GOQDs in different surroundings was studied, with the results showing that GOQDs are more easily photoreduced in ionic liquids and that the photoluminescence spectrum obtained for rGOQDs exhibits a 70 nm blueshift with a narrow full-width at half-maximum compared to GOQDs.

2,2′,3,3′-Tetrakis(4-bromophenyl)-6,6′-biquinoxaline based novel Donor–Acceptor–Acceptor−Donor (DAAD) Blue−Orange emitting molecules: Opto-electrochemical, AIE and theoretical investigation

A series of six novel donor‒acceptor‒acceptor‒donor (DAAD) based biquinoxaline amine derivatives 2–7 were obtained by employing palladium catalyzed Buchwald‒Hartwig coupling amination reaction. The synthesized compounds were characterized by spectroscopic and electrochemical techniques. The characteristic absorption spectra of dyes 2–7 display intramolecular charge transfer (ICT) transitions in the range of 415–461 nm. Dyes 2–7 show positive solvatochromism in emission and emit in blue−Orange region with emission maxima 492–598 nm in various solvents and neat solid film. Herein, dyes 2, 3 and 4 which possess intense emission in solid state were further studied for aggregation‒induced emission (AIE) effect in THF–water system due to the formation of nanoparticles, as confirmed by FEG‒SEM technique. The HOMO and LUMO energy levels for compounds 2–7 were measured by cyclic voltammetry (CV) and are found in the range of −4.96 to −5.35 eV and −3.33 to −3.46 eV which further correlated by density functional theory calculations. On the basis of comparable HOMO and LUMO energy levels to reported ambipolar materials, efficient solid state emission with good AIE and AIEE properties of the synthesized dyes signify their potential to be used as solid state emitters and ambipolar materials in opto−electronics.

Regulating Crystal Packing by Terminal tert-Butylation for Enhanced Solid-State Emission and Efficacious Charge Transport in an Anthracene-Based Molecular Crystal.

Organic semiconductors with combinative high carrier mobility and efficient solid-state emission are full of challenges but urgently pursued for developing new emerging optoelectronics. Herein, by delicately regulating the crystal packing of an anthracene-based molecular crystal via terminal tert-butylation, we developed a superior high mobility emissive molecule, 2,6-di(6-tert-butylnaphthyl)anthracene (TBU-DNA). The unique "slipped herringbone" packing motif of TBU-DNA enables its appropriate exciton-exciton coupling and electron-phonon coupling, thus resulting in remarkably high solid-state emission (photoluminescence quantum yield, ΦF ≈74.9 %) and efficacious charge transport (carrier mobility, μ=5.0 cm2 V-1 s-1 ). Furthermore, OLETs based on TBU-DNA show an external quantum efficiency (EQE) of 1.8 %, which is among the highest EQE values for single component OLETs reported till now. This work presents a crystal engineering strategy via exquisite molecular design to realize high mobility emissive organic semiconductors.

High-Performance Organic Laser Semiconductor Enabling Efficient Light-Emitting Transistors and Low-Threshold Microcavity Lasers.

An organic light-emitting transistor (OLET) is a candidate device architecture for developing electrically pumped organic solid-state lasers, but it remains a critical challenge because of the lack of organic semiconductors that simultaneously possess a high solid-state emission efficiency (Φs), a high and balanced ambipolar mobility (μh,e), and a large stimulated emission cross-section. Here, we designed a molecule of 4,4'-bis(2-dibenzothiophenyl-vinyl)-biphenyl (DBTVB) and prepared its ultrathin single-crystal microplates with herringbone packing arrangements, which achieve balanced mobilities of μh = 3.55 ± 0.5 and μe = 2.37 ± 0.5 cm2 V-1 s-1, a high Φs of 85 ± 3%, and striking low-threshold laser characteristics. Theoretical and experimental investigations reveal that a strong electronic coupling and a small reorganization energy ensure efficient charge transport; meanwhile, the exciton-vibration effect and negligible π-π orbital overlap give rise to highly emissive H-aggregates and facilitate laser emission. Furthermore, OLET-based DBTVB crystals offer an internal quantum efficiency approaching 100% and a record-high electroluminescence external quantum efficiency of 4.03%.

Pressure Tuning of Optical Properties and Structures in All-Inorganic Halide Perovskite Rb7Sb3Cl16.

All lead-free inorganic halide perovskites, as efficient solid-state light emission materials, have become ideal green optoelectronic materials to replace lead halide perovskites for diversified lighting and display applications with their excellent stability. Here, we investigated the pressure-derived optical and structural response of a zero-dimensional lead-free perovskite Rb7Sb3Cl16 through applying controllable pressure. A pressure-induced blue shift of the broadband emission was achieved, and it was followed by the emission color transformation from yellow to green, which was ascribed to the electron-phonon coupling weakening and the suppression of structural deformation upon lattice contraction. In parallel, the band gap was narrowed by about 0.5 eV as a result of enhanced metal halide orbital overlap under high pressure. This work provides a fundamental understanding for modulating the optical properties of the low-dimensional metal halide perovskites.

Effects of molecular geometry on the efficiency of intramolecular charge transfer-based luminescence in o-carboranyl-substituted 1H-phenanthro[9,10-d]imidazoles

The distinct difference in solid-state emission efficiency between 1H-phenanthro[9,10-d]imidazole–o-carboranyl luminophores showed that the geometric orientation is a key factor for controlling intramolecular charge transfer-based radiative decay.

Dimerization-Induced Solid-State Excimer Emission Showing Consecutive Thermochromic Luminescence Based on Acridine-Modified o-Carboranes.

Although excimer emission is a useful luminescent phenomenon for fabricating optical sensors and probes, it is difficult to apply excimer emission for film sensors due to critical concentration quenching in the solid state. Therefore, robust molecular designs for solid-state excimer emission are still being explored. One of the key examples is the previously reported acridine-ethynyl-o-carborane AcE1, which showed a bright solid-state excimer emission assisted by characteristic CcageH···N interactions. In this paper, we report the newly synthesized acridine-diehynyl-o-carborane AcE2 and comprehensively compare it to AcE1. Both compounds had the same crystalline packing mode based on dimer formation, resulting in an efficient π-overlapping area and solid-state excimer emission. Variable-temperature photoluminescence (VT-PL) measurements revealed the consecutive thermochromic luminescence of these compounds. Finally, on the basis of the easily accessible spray-coating method, we constructed the thermochromic luminescent sensors on quartz substrates. According to the mechanistic studies, it is demonstrated that the design strategy based on a dimer-induced solid-state excimer should have great potential for applications as a molecular thermometer.

ESIPT fluorophores derived from 2,3-dichloro-5,6-dicyano-p-benzoquinone based carbon dots for dual emission and multiple anti-counterfeiting.

Fluorophores and hydrogen bonding interactions play key roles in the fluorescence properties of bottom-up carbon dots. In this work, an excited-state intramolecular proton-transfer (ESIPT) active fluorophore, 5-chloro-6-ethoxy-4,7-dihydroxyisoindoline-1,3-dione (CEDD) and a non-ESIPT 7-cyano-5,8-dihydroxyquinoxaline-6-carboxamide (CDQC) are extracted from 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) based carbon dots. The enol form of CEDD shows a weak blue, small Stokes shift and short lifetime emission under the aprotic or alkali conditions, but the keto form exhibits a strong green, large Stokes shift and long lifetime emission in a protic or an acidic environment. Due to the lack of the ESIPT process, CDQC has no dual emission characteristics, but shows efficient solid-state emission. By virtue of the ESIPT ability of CEDD, multiple anti-counterfeiting methods are achieved by using hydrogen chloride, ammonia, and fluorescence lifetime imaging, as well as dimethyl sulfoxide as the encryption/decryption tools.

Aggregation-induced emission characteristics and distinct fluorescent responses to external pressure stimuli based on dumbbell D-π-A-π-D cyanostyrene derivatives

Abstract Two dumbbell cyanostyrene derivatives PBCAN and PBTAN have been designed and prepared. Quantum chemical calculations and optical spectra confirm that two compounds are D-π-A-π-D structures. Both of them show aggregation-induced emission (AIE) characteristic, giving high emission efficiency in solid state. Interestingly, two molecules exhibit distinct fluorescent response to external pressure stimuli. Upon the treatment of grinding, PBCAN exhibits unusual hypsochromic shift of the emission wavelength of 10 nm, while PBTAN shows bathochromic shift of 18 nm. We think that the former might be due to close molecular packing in the initial state, while the latter might be derived from the transformation of π-packing mode from H-aggregate to J-aggregate. The current result is valuable for exploiting structurally similar molecules with distinct MFC characteristics.

Pillar[5]arene-Based Solid-State Supramolecular Polymers with Suppressed Aggregation-Caused Quenching Effects and Two-Photon Excited Emission.

Herein we develop a new pillar[5]arene-mediated supramolecular polymerization strategy to control the assembly of dyes 4,7-di-2-thienyl-2,1,3-benzothiadiazole and 4,7-di-2-thienyl-2,1,3-benzoselenadiazole in the solid state. The resulting supramolecular polymeric structures drive the dye aggregates in the solid state from parallel alignments to highly ordered "head-to-tail" structures, which effectively suppresses excimer formation and thus aggregation-caused quenching effects and enhances the solid-state emission efficiency by almost 1 order of magnitude. This, together with two-photon excited emission, endows these solid-state polymers with exciting potential in optoelectronic, photonic, and bioimaging applications.

Design, Synthesis and Opto-electrochemical Properties of Novel Donor–Acceptor Based 2,3-di(hetero-2-yl)pyrido[2,3-b]pyrazine amine derivatives as Blue-Orange Fluorescent Materials

We herein report the design and synthesis of novel donor‒acceptor (D‒A) type 2,3-di(thiophene-2-yl)pyrido[2,3-b]pyrazine and 2,3-di(furan-2-yl)pyrido[2,3-b]pyrazine amine derivatives by employing palladium catalyzed Buchwald‒Hartwig coupling reaction. The synthesized compounds have been characterized by spectroscopic, electrochemical and thermal methods. Absorption spectra of all dyes showed an intramolecular charge transfer (ICT) transitions (λmax = 438–473 nm) in solution and positive solvatochromism with emission maxima in blue‒orange region (λemm = 489–617 nm). Further, solid state emission was studied for aggregation‒induced emission (AIE) effect in THF–water system due to the nanoparticles formation, as confirmed by FEG‒SEM technique. The HOMO (−5.22 to −5.75 eV) and LUMO (−3.35 to −3.45 eV) energy level of these molecules were measured by cyclic voltammetry and correlated by DFT calculations. On the basis of comparable HOMO and LUMO energy level with reported ambipolar materials and efficient solid state emission warrants the application of synthesized compounds as emitter and ambipolar charge transport materials in organic electronics.