Luminescent Compounds Research Articles

Aggregation-induced emission meets magnetic bistability: Synergy between spin crossover and fluorescence in iron(II) complexes

Luminescent spin crossover (SCO) materials have attracted significant interest owing to their potential applications in magneto-optical switches. However, the majority of previously reported FeII-based SCO complexes are adversely affected by fluorescence quenching in the solid-state. Here, we have constructed the first mononuclear FeII complex decorated with an aggregation-induced emission (AIE) luminophore (i.e., tetraphenylethylene) that exhibits synergistic SCO and fluorescence behavior. Intriguingly, we obtained two types of crystals in different solvent systems, both displaying distinct magnetic bistability and fluorescence properties. The fluorescence intensity was observed to track the magnetic susceptibility, which confirmed that SCO and solid-state fluorescence operate synergistically. We introduce a novel approach for the construction of luminescent SCO compounds using an AIEgen as a luminophore, which leads to fluorescence emission in the solid-state, thus allowing us to study the synergy between SCO and fluorescence.

Post-synthesis of Lewis acid-base adducts as thermally activated delayed fluorescence radical emitters for color-tunable displays and encryption via inkjet printing

Post-synthesis of color-tunable luminescent compounds, from either solid-state mechanical grinding or solution blending, was demonstrated with a triazine-based fluorescent host as Lewis base and a typical Lewis acid tris(pentafluorophenyl)borane (BCF). The nitrogen atoms on the triazine group of the Lewis base provide lone pairs of electrons for binding BCF which resulted in charge transfer emission and radical nature, due to a dramatic reduction of the singlet–triplet energy gap from 0.59 to 0.08 eV. This allows triplet exciton up-conversion via reverse intersystem crossing (RISC) process, proven by a high external quantum efficiency of 6.7% measured from the spin-coated organic light emitting diodes (OLEDs), consisting of the Lewis acid-base adducts. By inkjet printing BCF ink on the seeding layer of the fluorescent Lewis base, we precisely regulated the emission ranging from deep-blue to orange-red.

Ligand Control of Structural Diversity in Luminescent Hybrid Copper(I) Iodides

Copper(I) iodide hybrids are of interest for next-generation lighting technologies because of their efficient luminescence in the absence of rare-earth elements. Here, we report 10 structurally diverse hybrid copper(I) iodides that emit in the green–red region with quantum yields reaching 67%. The compounds display a diversity of structures including ones with one-dimensional (1D) Cu–1 chains, Cu2I2 rhomboid dimers, and structures with two different arrangements of Cu4I4 tetramers. The compounds with Cu2I2 rhomboid dimers or Cu4I4 cubane tetramers have higher photoluminescence quantum yields than those with Cu–I 1D chains and octahedral Cu4I4 tetramers, owing to the optimal degree of condensation of the inorganic motifs, which suppresses nonradiative processes. Electronic structure calculations on these compounds point out the critical influence of the inorganic motif and organic ligand on the nature of the band gaps and thus the excitation characteristics. Temperature-dependent photoluminescence spectra are presented to better understand the nature of luminescence in compounds with different inorganic motifs. The emerging understanding of composition–structure–property correlations in this family provides inspiration for the rational design of hybrid phosphors for general lighting applications.

Clustering emission of cucurbit[n]urils in the solid- and solution-state induced by the outer surface interactions of cucurbit[n]urils

Atypical luminescent compounds that do not contain conventional chromophores emit light due to clustering and have important basic research value and a broad range of potential applications. To date, most atypical luminescent compounds are small molecules or polymers containing groups such as cyano, carbonyl and hydroxyl. In this work, driven by some sporadic and accidental luminescence phenomena observed for cucurbit[n]urils (Q[n]s), the luminescent properties and mechanism of Q[n]s in the solid- and solution-state were systematically studied and the clustering emission of Q[n]s confirmed. Our experiments have revealed that the self-induced outer-surface interactions of Q[n]s (OSIQ) are the most important driving force resulting in the clustering emission of Q[n]s. Substances that can weaken the effect of self-induced OSIQ, such as the presence of various aromatic compounds and anions, may weaken or quench the clustering emission of Q[n]s. This not only reveals the new characteristics and mechanism of the clustering emission of Q[n]s, but also provides new insights on how to utilize the clustering emission of Q[n]s and construct new types of macrocyclic luminescence systems.

Halogen-induced core structural evolution of four dinuclear copper(i) luminescent coordination compounds

Four luminescent di-copper(i) coordination complexes have been prepared and characterized, which reveal two kinds of Cu2-core structural configurations induced by differently sized halogen anions.

Recent advances in the synthesis of luminescent tetra-coordinated boron compounds.

Tetra-coordinated boron compounds offer a plethora of luminescent materials. Different chelation around the boron center (O,O-, N,C-, N,O-, and N,N-) has been explored to tune the electronic and photophysical properties of tetra-coordinated boron compounds. A number of fascinating molecules with interesting properties such as aggregation induced emission, mechanochromism and tunable emission by changing the solvent polarity were realised. Owing to their rich and unique properties, some of the molecules have shown applications in making optoelectronic devices, probes and so on. This perspective provides an overview of the recent developments of tetra-coordinated boron compounds and their potential applications.

Ditriphenylenothiophene butterfly-shape liquid crystals. The influence of polyarene core topology on self-organization, fluorescence and photoconductivity

Two series of regio-isomeric mesomorphous, luminescent and conductive compounds, based on a ditriphenylenothiophene core (α/β-DTPT), were successfully synthesized by the Suzuki–Miyaura cross-coupling/Scholl cyclo-dehydrogenation reactions tandem.

TADF: Enabling luminescent copper(i) coordination compounds for light-emitting electrochemical cells.

The last decade has seen a surge of interest in the emissive behaviour of copper(i) coordination compounds, both neutral compounds that may have applications in organic light-emitting doides (OLEDs) and copper-based ionic transition metal complexes (Cu-iTMCs) with potential use in light-emitting electrochemical cells (LECs). One of the most exciting features of copper(i) coordination compounds is their possibility to exhibit thermally activated delayed fluorescence (TADF) in which the energy separation of the excited singlet (S1) and excited triplet (T1) states is very small, permitting intersystem crossing (ISC) and reverse intersystem crossing (RISC) to occur at room temperature without the requirement for the large spin–orbit coupling inferred by the presence of a heavy metal such as iridium. In this review, we focus mainly in Cu-iTMCs, and illustrate how the field of luminescent compounds and those exhibiting TADF has developed. Copper(i) coordination compounds that class as Cu-iTMCs include those containing four-coordinate [Cu(P^P)(N^N)]+ (P^P = large-bite angle bisphosphane, and N^N is typically a diimine), [Cu(P)2(N^N)]+ (P = monodentate phosphane ligand), [Cu(P)(tripodal-N3)]+, [Cu(P)(N^N)(N)]+ (N = monodentate N-donor ligand), [Cu(P^P)(N^S)]+ (N^S = chelating N,S-donor ligand), [Cu(P^P)(P^S)]+ (P^S = chelating P,S-donor ligand), [Cu(P^P)(NHC)]+ (NHC = N-heterocyclic carbene) coordination domains, dinuclear complexes with P^P and N^N ligands, three-coordinate [Cu(N^N)(NHC)]+ and two-coordinate [Cu(N)(NHC)]+ complexes. We pay particular attention to solid-state structural features, e.g. π-stacking interactions and other inter-ligand interactions, which may impact on photoluminescence quantum yields. Where emissive Cu-iTMCs have been tested in LECs, we detail the device architectures, and this emphasizes differences which make it difficult to compare LEC performances from different investigations.

Introducing Distinct Structural and Optical Properties into Organotin Sulfide Clusters by the Attachment of Perylenyl and Corannulenyl Groups.

We report the introduction of distinct optical properties into organotin sulfide clusters by the attachment of extended polycyclic aromatic organic molecules. This was realized by the reactions of [(RNSn)4S6] (RN = CMe2CH2CMeNNH2) with 3-perylenecarbaldehyde and corannulenecarbaldehyde, respectively. The reaction with the first reactant leads to the formation of two products [(RperylSn)3S4][SnCl3] [1a; Rperyl = CMe2CH2CMeNNCH(C20H11)] and [(RperylSn)3S4Cl] (1b). Structural differences between these two compounds are reflected in their different optical absorption and luminescence behavior, yet in both cases, the main emission is red-shifted relative to 3-perylenecarbaldehyde. The second organic molecule affords the compound [(RcorSn)4Sn2S10] [2; Rcor = CMe2CH2CMeNNCH(C20H9)] with intriguing optical properties, including a broad emission with essentially no shift in λmax compared to corannulenecarbaldehyde. All compounds were obtained as single crystals, and their structures were determined by means of single-crystal X-ray diffraction. The optical properties of the highly luminescent compounds were investigated by means of emission and time-resolved photoluminescence spectroscopy measurements.

Molecular design for organic luminogens with efficient emission in solution and solid-state

Design and synthesis of organic luminogens with efficient emission in both solution and solid-state (namely, dual-state emission: DSE) still face enormous challenges, a majority of luminescent compounds are always either aggregation-caused quenching or aggregation-induced emission luminogens. The development of dual-state emissive materials is extremely desirable for advanced applications. However, up to now, how to design this kind of materials remains a big challenge. Thus, a further in-depth understanding of the dual-state emission still needs to be established. In this work, a rational molecular design strategy has been presented based on triphenylamine via theoretical calculation and practical verification. First, steric hindrance effect of triphenylamine molecule can render the intramolecular rotation restricted in solution state, which could reduce the non-radiative transition; second, the twisted structure prevents or alleviates detrimental close molecular packing in the solid state; third, rational chemical modification forbids or inhibits the Sn – Tn intersystem crossing, which prevents the formation of triplet excitons and decrease the non-radiative transition rate. Once the above conditions are satisfied, the triphenylamine derivatives can exhibit bright emission in both solution and solid-state. This efficient strategy has been strongly confirmed by TPA-BP, TPA-BT, TPA-BBT and several molecules previously reported. We believe that this work will help to design novel luminescent materials with efficient emission in both solution and solid-state, which is of benefit to organic light-emitting diodes (OLEDs), chemical sensors, smart optoelectronic materials, and other high-tech areas.

Fabrication of Alq3/PMMA nanocomposite sheet and its potential applications as radiation dosimeter

Tris (8-hydroxyquinoline) aluminum (Alq3) is a highly luminescent organometallic compound, and therefore has numerous applications in electronic devices. Its unique optical property is suitable for organic light-emitting diodes, optoelectronic, and photodiodes. Despite its numerous potentials, no study investigated its response to ionizing radiation, particularly in its nanostructure form. In this work Alq3 were incorporated in a highly transparent polymethyl methacrylate (PMMA) polymer sheet using chloroform as a proper solvent for both materials. The ratio of Alq3 to PMMA powders was fixed at 1:10. The resulting composite showed recrystallized fine nanoparticles of Alq3, uniformly embedded in the PMMA sheet. Then small pieces of this nanocomposite sheet were exposed to various doses of X-ray and electron beam (E-beam) radiation doses in the range of 5–20 Gy. The molecular fingerprint and optical emission properties of the pristine and irradiated nanocomposite sheets were evaluated using Raman and PL spectroscopy. A systematic increase in the PL and Raman signals with increasing radiation doses were recorded. In addition, blueshift in the PL peak positions of nearly 6 and 9 nm for X-ray and E-beam irradiated sheets, respectively were also observed. These increments were attributed to the formation of radiative vacancies in the Alq3 molecules while the blueshift might be related to possible weakening of some of the Alq3 bonds such as C-O, C-C, C-H and Al-O, respectively. Therefore, the systematic changes in the optical properties might serve as preliminary parameters to investigate its potential use in radiation dosimetry following its nearly excellent linear dose dependence on signal intensities, which are quite encouraging.

Voltage Oscillation in PEFCs during Power Generation Analyzed By Operando Multi-Probe Monitoring System of Oxygen Partial Pressure

The stability of polymer electrolyte fuel cells (PEFCs) during power generation is one of the most important issues. We found a new oscillation phenomenon during power generation using a GDL without a micro porous layer (MPL) at 60 oC and 60% RH. [1] The differential pressure between the cathode inlet and outlet was 0 kPa during the oscillation, which was very different from the cell voltage fluctuation previously reported to be synchronized with the differential pressure caused by the plugging of flow channels [2]. An operando system was developed for detecting the oxygen partial pressure (p(O2)) using optical probes for analyzing this phenomenon. [1]A PEFC with straight channels, 30 mm in length and 1 mm in width, was used. To measure p(O2), we created five 100-μm holes through the GDL, into which we inserted glass-fiber probes of 50 μm in diameter (Fig. 1(a)). As an oxygen sensor, a luminescent dye compound was coated on the apex of each probe (Fig. 1(a)), which absorbed 530-nm blue light and emitted 650-nm red light; the emission intensity lowered as p(O2) increased. [1,3-5] The excitation light was introduced into each probe, the emission from the apex returned through the same probe, and the intensity was recorded. The emission intensities were converted to p(O2) by using the calibration curves obtained prior to the measurements. The cell was operated at 60 oC with air and H2 humidified at 60% RH.Fig. 1(b) shows the cell voltage (black line) and p(O2) near the inlet (blue line) and the outlet (red line) under the flow channel and rib. A GDL without an MPL was used. The apexes of the probes were located 10 μm from the catalyst-layer surface. Cell voltage was seen oscillated at the period of approximately 4 s with the amplitude of approximately 0.03 V at the constant current density of 0.6 A cm-2. p(O2) also oscillated in phase, and the amplitude became larger toward the outlet. The average p(O2) under the rib became higher near the outlet (red line in Fig. 1(b)). For this phenomenon, the amount and the distribution of water in the catalyst layer are expected to be involved: the repetition of the overpotential brought about by the excess water in the catalyst layer, periodically evaporated to vapor out to the GDL by the heat generated by the overpotential itself eventually to lower p(O2). Fig. 1(c) shows the results using a GDL with an MPL. In this case, voltage oscillation was not observed and the cell voltage was higher than that using a GDL without an MPL at 0.6A cm-2. These effects are understood by considering a capillary phenomenon of an MPL.

Cover Feature: Hetero‐bimetallic Lanthanide‐Coinage Metal Compounds Featuring Possible Metal‐Metal Interactions in the Excited State (Chem. Eur. J. 61/2021)

The excited coinage metal catch, or how to form luminescent hetero-bimetallic lanthanide coinage metal compounds. The cover picture depicts how the ligand system dpfam, represented by an octopus, can form preformed lanthanide scaffolds that can catch silver or gold cations. Upon excitation of the system, the octopus can force the two different metal centres to interact. The new deep-sea dwellers obtained illuminate the dark ocean and reveal a new class of interesting hetero-bimetallic compounds. More information can be found in the Full Paper by P. W. Roesky et al. (DOI: 10.1002/chem.202102430).

Photoluminescence of pefloxacindi-ium manganese(II) and zinc(II) tetrahalides

Mn2+-based hybrid materials have become the hotspot of current research studies owing to their high photoluminescence quantum yield (PLQY), low-cost, environmental friendliness and stability. For the first time, we report the hydrothermal synthesis of two lead-free zero-dimensional luminescent organic-inorganic hybrid compounds, PefH2[MnBr4] (1) and PefH2[MnCl4] (2) (Pef = pefloxacin). They were characterized by elemental analysis, TG-DSC, single-crystal and powder XRD. Compounds 1–2 exhibit a distorted tetrahedral geometry around the manganese(II) metal center, which is isolated from the same centers by bulky pefloxacindi-ium (PefH22+) ions with a Mn···Mn distance of 7.3 Å. Their structures are stabilized by N—H···O, O—H···X (X = Br, Cl), C—H···O and C—H···X hydrogen bands and π–π stacking interaction. Thermal decomposition starts at T > 230°С for 1 and T > 210°С for 2 and proceeds for several stages. Upon UV excitation compounds exhibit a bright green emission with a moderate PLQY of 45% for 1 and 30% for 2. The influence of the halide ion and metal ion on the photoluminescence properties of isostructural compounds PefH2[MX4] (M = Mn, Zn and X = Br, Cl) is discussed.

Electronic Structure of Bi-Activated Luminescent Compounds and Pure Bismuth Photocatalytic Compounds

The trends in electronic structure of Bi3+ and Bi2+ as luminescent dopant in wide bandgap inorganic compounds and that of pure Bi-compounds for photocatalytic splitting of water are explored by determination of vacuum referred electron binding energies. Spectroscopic data combined with the chemical shift model from the luminescence field and data on flatband potentials in electrochemistry provide most of the needed data. Occasionally data from photoelectron spectroscopy is used. The trends in VRBE of Bi3+ and Bi2+ ground state and excited state levels resemble that of Ce3+ and Eu2+ lanthanides. To some extend a pure Bi-compound can be regarded as a 100% Bi3+ doped La-compound. This all will be demonstrated and an overview on Bi electronic structure is presented. Comparison of electronic structure for Bi as luminescent dopant in inorganic compounds with pure Bi-compounds entails also a comparison of two disciplines in electro-chemistry, that of luminescence materials and of photo-catalytic compounds.

Dramatically Enhanced and Red-shifted Photoluminescence Achieved by Introducing an Electron-withdrawing Group into a Non-traditional Luminescent Small Organic Compound.

Small organic compounds without any traditional fluorescent chromophores are generally non-emissive, and only very few are reported to emit weak blue fluorescence. Here we synthesized a non-traditional luminescent small organic compound N-(2,2,2-trifluoroethyl)acrylamide (TFAM) with dramatically enhanced and red-shifted photoluminescence by introducing a strong electron-withdrawing group into acrylamide (AM). Very impressively, TFAM emits cyan (472 nm) and yellow-green (560 nm) fluorescence in solutions and solid state, respectively. TFAM also shows aggregation-induced emission enhancement (AIEE) and excitation-dependent fluorescence (EDF) characteristics, as well as temperature and metal cations-responsive fluorescence. Theoretical calculations show that the introduction of electron-withdrawing group leads to a lower energy gap between the HOMO-LUMO energy levels in TFAM than in AM. And strong cooperative hydrogen bonds are formed in TFAM molecules, resulting in rigidification of molecular conformations. The study provides a strategy for preparing non-traditional luminescent compounds with enhanced and red-shifted photoluminescence.

Dithiocarbamate Complexes of Platinum Group Metals: Structural Aspects and Applications

The incorporation of dithiocarbamate ligands in the preparation of metal complexes is largely prompted by the versatility of this molecule. Fascinating coordination chemistry can be obtained from the study of such metal complexes ranging from their preparation, the solid-state properties, solution behavior as well as their applications as bioactive materials and luminescent compounds, to name a few. In this overview, the dithiocarbamate complexes of platinum-group elements form the focus of the discussion. The structural aspects of these complexes will be discussed based upon the intriguing findings obtained from their solid- (crystallographic) and solution-state (NMR) studies. At the end of this review, the applications of platinum-group metal complexes will be discussed.

Optical and Dynamic Properties of ZnGa2S4

ZnGa2S4 luminescent compound is prepared by solid‐state reaction, which is confirmed by X‐ray diffraction to have a tetragonal structure. The photoluminescence (PL) emission and excitation spectra, infrared (IR) reflectivity and Raman scattering of the defective ZnGa2S4 compound are studied experimentally. High stability of the shape and peak positions of the PL spectra is observed in a wide range temperature range (100–290 K). It was determined that PL intensity increases with decreasing temperature. Higher correlated color temperature (CCT) value >6000 K implies the suitability of using this material for light emitting diodes. Five modes of optical phonons are found in both IR reflectance and Raman spectra of ZnGa2S4 and identified from point group symmetry analysis. The calculations of the dynamic properties are carried out within the framework of the density functional theory (DFT) with full optimization of the lattice parameters. Results of calculations show that, the phonon‐dispersion curve is sensitive to S–Se replacement in ZnGa2S4 and gives rise to clearly pronounced energy gaps in the phonon spectrum.

Efficient Intramolecular Charge‐Transfer Fluorophores Based on Substituted Triphenylphosphine Donors

AbstractTriphenylphosphine (TPP)‐based luminescent compounds are rarely investigated because of the low photoluminescence quantum yield (PLQY). Here, we demonstrate that introducing steric hindrance groups to the TPP moiety and separating the orbitals involved in the transition can drastically suppress the non‐radiative decay induced by structural distortion of TPP in the excited state. High PLQY up to 0.89 as well as thermally activated delayed fluorescence are observed from the intramolecular charge‐transfer (ICT) molecules with substituted TPP donors (sTPPs) in doped films. The red organic light‐emitting diodes employing these emitters achieve comparable external quantum efficiencies to the control device containing a classical phosphorescent dye, revealing the great potential of the ICT emitters based on electrochemically stable sTPPs.

Temperature-Triggered Structural Dynamics of Non-Coordinating Guest Moieties in a Fluorescent Actinide Polyrotaxane Framework.

We present here the synthesis of a novel fluorescent actinide polyrotaxane compound URCP1 through the utilization of an end-cutting pseudorotaxane precursor with only the cucurbit[6]uril (CB[6]) macrocyclic components acting as linking struts. The non-coordinating guest motif in the obtained polyrotaxane, with increased freedom and structural flexibility, can display intriguing temperature-triggered conformational variations inside the cavity of CB[6], which was clearly evidenced by crystallographic snapshots at different temperatures. Notably, this observation of temperature-triggered structural dynamics in URCP1 represents the first report of actinide polyrotaxane with such feature in solid-state. Moreover, URCP1 has a high photoluminescence quantum yield (PLQY) of 49.8 %, comparable to other luminescent uranyl compounds, and can work as a fluorescent probe to selectively detect Fe3+ over other eight competing cations in aqueous solution, with the limit of detection being as low as 4.4×10-3 ppm.