Solid-state Luminescence Research Articles

Effect of Substituents with the Different Electron-Donating Abilities on Optoelectronic Properties of Bipolar Thioxanthone Derivatives.

The synthesis and optoelectronic properties of four simple-structure thioxanthone derivatives employing thioxanthone as an acceptor unit, coupled with moieties having very different electron-donating abilities such as phenoxazine, 3,6-di-tert-butylcarbazole, 3,7-di-tert-butylphenothiazine, or 2,7-di-tert-butyl-9,9-dimethylacridane, are reported. The compounds form molecular glasses with glass transition temperatures reaching 116 °C. Ionization potentials of the compounds estimated by photoelectron emission method range from 5.42 to 5.74 eV. Thioxanthone derivatives containing 3,6-tert-butylcarbazole or 2,7-di-tert-butyl-9,9-dimethylacridane moieties with weak electron-donating strengths were characterized by bipolar charge transport with relatively close hole and electron mobility values of 6.8 × 10-5/2.4 × 10-5 and 3.1 × 10-5/4.6 × 10-6 cm2/(V s) recorded at 3.6 × 105 V/cm. The other compounds demonstrated hole-transporting properties. The films of thioxanthones containing phenoxazine or 2,7-di-tert-butyl-9,9-dimethylacridane moieties showed efficient thermally activated delayed fluorescence with a photoluminescence quantum yield of up to 50% due to the solid-state luminescence enhancement. Organic-light-emitting diodes containing the synthesized compounds as emitters showed very different external quantum efficiencies (0.9-10.3%) and blue, sky blue, green, or yellow electroluminescence colors, thus reflecting the effects of donor substituents.

Remarkable solution and solid-state color-tunable luminescence of diethylamino-coumarin derivatives and application in warm-white LEDs

Organic materials with solution and solid dual efficient, color-tunable luminescence are potentially useful for sensors and optical devices. Herein, a series of donor-π-acceptor luminophores, named BMs, with different phenyl substituents at the 3-positions of 7-diethylamino-coumarin was developed. All four compounds with slightly twisted structure rendering BMs have bright emission in solution and solid state. BMs not only possess solvatochromic properties in solution but also realize color tuning from cyan (480 nm) to deep red (661 nm) in solid states. Among them, the fluorescence quantum yield of compound BM1 in solid state reached 98.2%, and the fluorescence quantum yield in solution was 77.1%. Time dependent density functional theory calculation and single crystal analysis show that with the increase in the electron-withdrawing ability of 4-phenyl moiety at the 3-position of coumarin, the charge transfer effect was strengthened gradually, and the photoluminescence mechanism changed from intramolecular charge transfer (ICT) to twisted ICT on account of the electron acceptor changed accordingly. Moreover, the cyan phosphor BM1 and the deep-red phosphor BM4 were laminated to fabricate an impressive warm-white light-emitting diode device with a maximum brightness of 52,090 cd/m2 at an operating voltage of 4.0 V.

Investigating the AIE and water sensing properties of a concise naphthalimide fluorophore

A simple naphthalimide fluorophore NAP-H2O was designed and synthesized. Basic photophysical properties were investigated, especially found that the probe showed robust green fluorescence in water compared with that in various organic solvents, and the specific mechanism was conformed to be the aggregation induced emission (AIE) through dynamic light scattering (DLS) analysis, solid-state luminescence and fluorescence imaging. Accordingly, the capability of NAP-H2O for water sensing was examined, and good linear relationships between fluorescence intensities at the green emission band and the water content were obtained, enabling quantitative detection of water in organic solvents. The detection limits were calculated to be 0.004 % (v/v) in ACN, 0.117 % (v/v) in 1,4–dioxane, 0.028 % (v/v) in THF, 0.022 % (v/v) in DMF and 0.146 % (v/v) in DMSO, respectively. In addition, the probe presented fast response time within 5 s to water and good photostability. Furthermore, the probe was successfully applied for fast and naked-eye detection of water in organic solvents via test papers. This work provides a rapid, sensitive and naked-eye method for trace amount detection of water in organic solvents and has potential for practical applications.

Structural diversity and luminescence of zinc coordination polymers constructed by flexible ligands

Three new zinc coordination polymers based on the flexible ligand L (5,5′-((1,3-phenylenebis(methylene))bis(oxy))diisophthalic acid) have been prepared by solvothermal synthesis, which was named for [Zn4(L)2(pdp)(H2O)5]n (1), {[Zn(L)0.5(pdp)]·(NMP)0.5}n (2) and {[Zn(L)0.5(bpp)]·3H2O}n (3) (pdp=trans-1,2-bis(4-pyridyl)ethylene), bpp = 4,4′-(1,3-propanediyl)bis-pyridine) respectively. By controlling the length of auxiliary ligands, compounds 1 and 3 can be assembled under similar reaction conditions with different 3D structures. Although the ligands are all achiral components, it is interesting to note that compound 1 crystallizes in P21 chiral space group. The two Zn ions are in an asymmetric coordination environment resulting in symmetric breaking, which might be the reason compound 1 crystallizes in the chiral space group. While, compound 3 is composed of two identical 2D layers interpenetrating with each other, displaying a 2D + 2D → 3D polycatenation network. By changing the solvent and temperature, compound 2 can be available which presents a 3D structure with a two-fold interpenetrating framework, and the guest accessible volume (2414 Å3 per unit cell) accounts for 16.8% of the unit cell volume. In addition, compounds 1–3 have also had their solid-state luminescence characteristics, powder X-ray diffraction, and thermal stabilities examined.

Hydrogen bonds and space restriction promoting long-lived room-temperature phosphorescence and its application for white light-emitting diodes

Achieving the long-lived and strong room-temperature phosphorescence (RTP) is challengeable but desirable, especially for the enhanced phosphorescence and metal-free nanomaterials. Herein, we initially synthesized the green-fluorescence carbon dots (pm-CDs), and further obtained the composite of pm-CDs@DCDA with a long RTP lifetime of 1.01 s through embedding pm-CDs in dicyandiamide (DCDA). And the bright and long-lived afterglow of pm-CDs@DCDA with 365 nm of UV light excitation was observed by the naked eyes for more than 17 s either emerging as the dry solid or in water. Importantly, the phosphorescence intensity and lifetime of pm-CDs@DCDA were remarkably promoted owing to the intermolecular hydrogen bonds and the rigid environment, hence facilitating the intersystem crossing (ISC) process and restricting the non-radiative transition of triplet excitons. Taking advantage of the superior solid-state luminescence of pm-CDs@DCDA, we further innovatively prepared the white light-emitting diodes (WLEDs) with the tunable color temperatures by regulating the mass of pm-CDs@DCDA coated on the chips. This proposed study originally employed DCDA as a matrix to separate and immobilize pm-CDs, which built up a new avenue to improve the RTP property and offered a promising application in WLEDs.

Crystal structure, fluorescence properties, and biological activity of three Butterfly-shaped Ln4 compounds

Three tetranuclear lantharide-based compounds: [Ln4(dbm)4(L)6(μ3-OH)2]·CH3CN (LnIII = Eu (1), Tb (2) and Ho (3), HL = 5-(4-methylbenzylidene-8- hydroxyquinoline, dbm- = dibenzoylmethane) were fabricated and structurally characterized. X-ray diffraction studies reveal that all the compounds 1–3 belong to monoclinic crystal system with space group P21/n, and compounds 1–3 are isostructural and possess a rhombic Ln2O2 core. The solid-state luminescence property measurements indicated that the compounds 1 and 2 displayed the characteristic lanthanide luminescence at room temperature. Furthermore, biological activity researches revealed that the compounds 1–3 had better antibacterial activity against Escherichia coli, Bacillus subtilis, Staphylococcus aureus and Candida albicans than the ligand HL and Ln(dbm)3·2H2O (Ln = Eu, Tb, and Ho). The interaction between compounds 1–3 and calf thymus DNA was studied and the results revealed that the compounds were mainly intercalated with calf thymus DNA.

Halogen bonding with carbon: directional assembly of non-derivatised aromatic carbon systems into robust supramolecular ladder architectures.

Carbon, although the central element in organic chemistry, has been traditionally neglected as a target for directional supramolecular interactions. The design of supramolecular structures involving carbon-rich molecules, such as arene hydrocarbons, has been limited almost exclusively to non-directional π-stacking, or derivatisation with heteroatoms to introduce molecular assembly recognition sites. As a result, the predictable assembly of non-derivatised, carbon-only π-systems using directional non-covalent interactions remains an unsolved fundamental challenge of solid-state supramolecular chemistry. Here, we propose and validate a different paradigm for the reliable assembly of carbon-only aromatic systems into predictable supramolecular architectures: not through non-directional π-stacking, but via specific and directional halogen bonding. We present a systematic experimental, theoretical and database study of halogen bonds to carbon-only π-systems (C-I⋯πC bonds), focusing on the synthesis and structural analysis of cocrystals with diversely-sized and -shaped non-derivatised arenes, from one-ring (benzene) to 15-ring (dicoronylene) polycyclic atomatic hydrocarbons (PAHs), and fullerene C60, along with theoretical calculations and a systematic analysis of the Cambridge Structural Database. This study establishes C-I⋯πC bonds as directional interactions to arrange planar and curved carbon-only aromatic systems into predictable supramolecular motifs. In >90% of herein presented structures, the C-I⋯πC bonds to PAHs lead to a general ladder motif, in which the arenes act as the rungs and halogen bond donors as the rails, establishing a unique example of a supramolecular synthon based on carbon-only molecules. Besides fundamental importance in the solid-state and supramolecular chemistry of arenes, this synthon enables access to materials with exciting properties based on simple, non-derivatised aromatic systems, as seen from large red and blue shifts in solid-state luminescence and room-temperature phosphorescence upon cocrystallisation.

Luminescence properties of mixed-ligand MOFs containing fluorene scaffolds functionalized with isonicotinoyl arms

Three new polycatenated mixed-ligand MOFs containing fluorene and naphthalene scaffolds have been solvothermally synthesized. Their solid-state luminescence has been investigated showing a Förster resonance energy transfer involving the two linkers.

Carboxylate BODIPY integrated in MOF-5: easy preparation and solid-state luminescence

This study presents a facile room-temperature route to prepare new MOF@BODIPY hybrid materials. Their luminescence was associated with an efficient dispersion allowing high photoluminescence quantum yield in the solid state.

Polymeric copper(I)-NHC complexes with bulky bidentate (N^C) ligands: synthesis and solid-state luminescence.

Starting from imidazolium chlorides bearing bulky nitrogen donors, a series of four complexes, mainly [Cu(C^N)Cl]n coordination polymers were obtained directly as luminescent species by simple filtration from the aqueous reaction medium, highlighting a simple, eco-friendly, robust and reproducible synthetic procedure. Additionally, we have shown on the most efficient example that chloride could be exchanged very easily by other halides/pseudohalides (Br-, I-, NCS-, N3-) allowing to slightly modulate the emitted colour while conserving the polymeric structure, except for azide for which a dimer was obtained. The combination of chemical analyses, of photoluminescence studies in the solid state including quantum yield measurement and X-ray diffraction on single crystals and as-synthesized microcrystalline powders highlighted that the polymeric luminescent species was indeed obtained directly by simple filtration and that no major alteration of the structure was observed upon recrystallisation. Samples of all polymeric complexes displayed remarkable stability towards air oxidation remaining unchanged upon storage for several months and partially retaining their photoluminescence properties even after a thermal treatment at 100 °C for 24 h.

Perylene diimide with solid-state NIR luminescence for imaging of latent fingerprints (sweat pores)

PDI 2 showed near-IR emission at 683 nm, 99.53% red colour purity and a thick film morphology with holes for daylight and fluorescence imaging of fingerprints (levels 1–3).

Organic compounds for solid state luminescence enhancement/aggregation induced emission: a theoretical perspective.

Organic luminophores displaying one or more forms of luminescence enhancement in solid state are extremely promising for the development and performance optimization of functional materials essential to many modern key technologies. Yet, the effort to harness their huge potential is riddled with hurdles that ultimately come down to a limited understanding of the interactions that result in the diverse molecular environments responsible for the macroscopic response. In this context, the benefits of a theoretical framework able to provide mechanistic explanations to observations, supported by quantitative predictions of the phenomenon, are rather apparent. In this perspective, we review some of the established facts and recent developments about the current theoretical understanding of solid-state luminescence enhancement (SLE) with an accent on aggregation-induced emission (AIE). A description of the macroscopic phenomenon and the questions it raises is accompanied by a discussion of the approaches and quantum chemistry methods that are more apt to model these molecular systems with the inclusion of an accurate yet efficient simulation of the local environment. A sketch of a general framework, building from the current available knowledge, is then attempted via the analysis of a few varied SLE/AIE molecular systems from literature. A number of fundamental elements are identified offering the basis for outlining design rules for molecular architectures exhibiting SLE that involve specific structural features with the double role of modulating the optical response of the luminophores and defining the environment they experience in solid state.

Solid state luminescence of phosphine-EWO ligands with fluorinated chalcone skeletons and their PdX2 complexes: metal-promoted phosphorescence enhancement.

Complexes trans-[PdX2L2] (X = Cl and Br), where L is 1-(PR2),2-(CHCH-C(O)Ph)-C6F4 (R = Ph, Cy, and iPr), display phosphorescent emission in the solid state, whereas due to their substantially lower lifetimes, the free ligands exhibit fluorescent behaviour. Alternatively, structurally identical derivatives with halide replaced by CN- or Pd replaced by Pt are non-emissive. DFT calculations explain this diverse behaviour, showing that the hybridization of orbitals of the MX2 moiety with those of the chalcone fragment of ligands is significant only for the LUMO of the emissive compounds. In other words, in our complexes, only MLMCT processes (LM = Metal-perturbed Ligand-centered orbital) lead to observable luminescence.

Bright solid-state luminescence and high-temperature resistance of Ga-doped carbon dots with ultra-wideband white emission for light-emitting diodes.

Fluorescent CDs tend to undergo solid-state aggregation quenching in powder form. This is caused by the stacking of π-π conjugate structures and excessive resonant energy transfer. Moreover, various forms of N play an important role in white CDs suitable for LED applications. White, single-component, non-N-doped CDs have never been reported for LED application. In this study, to overcome this limitation, we developed Ga-doped CD powders containing no N element that exhibit ultra-wideband white emission in the range of 420-800 nm for LED applications and were able to resist solid-state aggregation quenching. Furthermore, the Ga-doped CD powders demonstrated excellent luminescence stability under high temperatures. Another strength of the Ga-doped CD powders is their large Stokes shift, where the peak center of white emission shifts from 550 nm to 650 nm under 365 nm excitation as the Ga doping concentration is adjusted from 0.05 to 0.6 (Ga : H2O, mass ratio). The full width at half-maximum can reach 262 nm. Additionally, the Ga-doped CD powders exhibit good luminescence stability under long-time exposure to an air atmosphere. Their luminescent intensity retained 70%-74% of the initial values even after being left in natural placement for 100 days. Moreover, the Ga-doped CDs demonstrate afterglow features.

Solid-state red-emissive (cyano)vinylene heteroaromatics via Pd-catalysed C-H homocoupling.

Thiophene-based π-conjugated systems are important materials for organic electronics; thus, their synthesis is of topical interest. We report fluorescent thiophene/furan-based vinylene and cyanovinylene systems via Pd-catalysed homocoupling [Pd(OAc)2, pivalic acid, KOAc, DMAc, 140 °C]. The methodology is versatile and allows the development of a variety of π-conjugated systems without the need for pre-functionalized building units. The reaction tolerates electron-rich, electron-deficient and large π-conjugated substrates. The developed compounds absorb in the visible region (400-515 nm) and emit green to orange fluorescence in the solution state (510-600 nm). Most importantly, the compounds exhibit strong aggregation-induced emission (AIE) in the NIR region (λem = 650 nm), with quantum yields reaching up to 10%. Steric hindrance imparted by vinylene/cyanovinylene units is responsible for the strong solid-state luminescence. DFT-optimized structures reveal an apparent twist of 20-40° in the molecular backbone of the compounds, supporting the AIE behaviour of the compounds.

Ratiometric sensing of β-galactosidase based on excited-state intramolecular proton transfer (ESIPT) and solid-state luminescence enhancement

An ESIPT-based ratiometric probe with solid-state luminescence enhancement is developed for the sensitive detection of glycosidase activity.

SYNTHESIS, CRYSTAL STRUCTURE, SPECTROSCOPIC, LUMINESCENCE STUDIES AND HIRSHFELD SURFACE ANALYSIS OF BLUE LIGHT EMITTING HETEROMETALLIC MNIII-NIII COMPLEX

The synthesis, crystal structure, spectroscopic, luminescence properties and Hirshfeld surface analysis of a new heterometallic complex, [Mn(L)(H2O)(CH3CH2OH)]2[Ni(CN)4], (H2L=N,N՛-bis(3,5-di-t-butylsalicylidene)-1,2-propanediamine) was reported. Single crystal diffraction measurements showed that the heterometallic MnIII-NiII complex consists of two [(Mn(L)(H2O)(CH3CH2OH)]+ cations and one [Ni(CN)4 ]2- anion. The Mn1 and Mn2 ions are hexacoordinate by N2O4 donor sets and have a distorted octahedral geometry. The 3-D Hirshfeld surface and 2-D fingerprint plots showed that the main interactions were the H∙∙∙H intermolecular interactions with relative contributions of about 73.8%. According to solid-state luminescence measurements, the new heterometallic MnIII-NiII complex showed weak blue light emission.

Carborane-thiol protected copper nanoclusters: stimuli-responsive materials with tunable phosphorescence.

Atomically precise nanomaterials with tunable solid-state luminescence attract global interest. In this work, we present a new class of thermally stable isostructural tetranuclear copper nanoclusters (NCs), shortly Cu4@oCBT, Cu4@mCBT and Cu4@ICBT, protected by nearly isomeric carborane thiols: ortho-carborane-9-thiol, meta-carborane-9-thiol and ortho-carborane 12-iodo 9-thiol, respectively. They have a square planar Cu4 core and a butterfly-shaped Cu4S4 staple, which is appended with four respective carboranes. For Cu4@ICBT, strain generated by the bulky iodine substituents on the carboranes makes the Cu4S4 staple flatter in comparison to other clusters. High-resolution electrospray ionization mass spectrometry (HR ESI-MS) and collision energy-dependent fragmentation, along with other spectroscopic and microscopic studies, confirm their molecular structure. Although none of these clusters show any visible luminescence in solution, bright μs-long phosphorescence is observed in their crystalline forms. The Cu4@oCBT and Cu4@mCBT NCs are green emitting with quantum yields (Φ) of 81 and 59%, respectively, whereas Cu4@ICBT is orange emitting with a Φ of 18%. Density functional theory (DFT) calculations reveal the nature of their respective electronic transitions. The green luminescence of Cu4@oCBT and Cu4@mCBT clusters gets shifted to yellow after mechanical grinding, but it is regenerated after exposure to solvent vapour, whereas the orange emission of Cu4@ICBT is not affected by mechanical grinding. Structurally flattened Cu4@ICBT didn't show mechanoresponsive luminescence in contrast to other clusters, having bent Cu4S4 structures. Cu4@oCBT and Cu4@mCBT are thermally stable up to 400 °C. Cu4@oCBT retained green emission even upon heating to 200 °C under ambient conditions, while Cu4@mCBT changed from green to yellow in the same window. This is the first report on structurally flexible carborane thiol appended Cu4 NCs having stimuli-responsive tunable solid-state phosphorescence.

Using a diphenyl-bi-(1,2,4-triazole) tricarbonylrhenium(I) complex with intramolecular π-π stacking interaction for efficient solid-state luminescence enhancement.

Since intramolecular π-π stacking interactions can modify the geometry, crystal packing mode, or even the electronic properties of transition metal complexes, they are also likely to influence the solid-state luminescence properties. Following this concept, a new tricarbonylrhenium(I) complex (Re-BPTA) was designed, based on a simple symmetrical 5,5'-dimethyl-4,4'-diphenyl-3,3'-bi-(1,2,4-triazole) organic ligand. The complex was prepared in good yield using a three-step procedure. The crystallographic study revealed that both phenyl rings are located on the same side of the molecule, and twisted by 71° and 62°, respectively, with respect to the bi-(1,2,4-triazole) unit. They overlap significantly, although they are slipped parallel to each other to minimize the intramolecular interaction energy. The π-π stacking interaction was also revealed by 1H NMR spectroscopy, in good agreement with the results of theoretical calculations. In organic solutions, a peculiar electrochemical signature was observed compared to closely-related pyridyl-triazole (pyta)-based complexes. With regard to the optical properties, the stiffness of the Re-BPTA complex led to the stabilization of the 3MLCT state, and thus to an enhancement of the red phosphorescence emission compared to the more flexible pyta complexes. However, an increased sensitivity to quenching by oxygen appeared. In the microcrystalline phase, the Re-BPTA complex showed strong photoluminescence (PL) emission in the green-yellow wavelength range (λPL = 548 nm, ΦPL = 0.52, 〈τPL〉 = 713 ns), and thus a dramatic solid-state luminescence enhancement (SLE) effect. These attractive emission properties can be attributed to the fact that the molecule undergoes little distortion between the ground state and the triplet excited state, as well as to a favorable intermolecular arrangement that minimizes detrimental interactions in the crystal lattice. The aggregation-induced phosphorescence emission (AIPE) effect was clear, with a 7-fold increase in emission intensity at 546 nm, although the aggregates formed in aqueous medium were much less emissive than the native microcrystalline powder. In this work, the rigidity of the Re-BPTA complex is reinforced by the intramolecular π-π stacking interaction of the phenyl rings. This original concept provides a rhenium tricarbonyl compound with very good SLE properties, and could be used more widely to successfully develop this area of research.

Luminescent Pt(II) Complexes Containing (1-Aza-15-crown-5)dithiocarbamate and (1-Aza-18-crown-6)dithiocarbamate: Mechanochromic and Solvent-Induced Luminescence.

The strong tendency to stack in the solid state and rich luminescence for the Pt(II) complexes makes them potential candidates as new mechanochromic materials and sensing applications. Six mononuclear complexes [Pt(ppy)(O4NCS2)] (1), [Pt(bpy)(O4NCS2)]ClO4 (2), [Pt(ppy)(O5NCS2)] (3), [Pt(phen)(O4NCS2)]ClO4·CH3OH (5a), [Pt(phen)(O4NCS2)]ClO4 (5b), and [Pt(phen)(O5NCS2)]ClO4 (6a), one dinuclear complex [Pt2(phen)2(NaO5NCS2)2(ClO4)3]ClO4 (6b), and one one-dimensional (1-D) coordination polymer {[Pt2(bpy)2(NaO5NCS2)2(ClO4)2](ClO4)2}n (4) were synthesized by reacting [Pt(ppy)Cl]2, Pt(bpy)Cl2, and Pt(phen)Cl2 (ppy = 2-phenylpyridine, bpy = 2,2'-bipyridine, and phen = 1,10-phenanthroline) with (1-aza-15-crown-5)dithiocarbamate (O4NCS2) or (1-aza-18-crown-6)dithiocarbamate (O5NCS2), respectively, which have been isolated and structurally characterized by X-ray diffraction. Neutral complexes 1 and 3 contain no intermolecular Pt(II)···Pt(II) contact, whereas cationic complexes 2, 5a, 5b, and 6a with ClO4- as counteranions show alternative intermolecular Pt(II)···Pt(II) contacts of 3.535/4.091, 3.480/5.001, 3.527/4.571, and 3.446/4.987 Å in the solid state, respectively. Interestingly, complex 4 forms a 1-D coordination polymer through coordination between the encapsulated Na+ ions inside the azacrown ether rings of O5NCS2 and ClO4- anions with respective intra- and intermolecular Pt(II)···Pt(II) contacts of 3.402 and 3.847 Å in crystal lattices, whereas a dinuclear complex 6b was surprisingly formed and also connected by the encapsulated Na+ ions and ClO4- anions with alternative intra- and intermolecular Pt(II)···Pt(II) contacts of 3.650 and 3.677/4.4.372 Å, respectively. Upon excitation, complexes 1 and 3 showed similar vibronic luminescence at 507, 534, and 502, 532 nm, respectively, and the other complexes 2 and 4-6 showed broad luminescence with maxima at 537-567 nm. The B3LYP/LanL2DZ calculation was carried out and used to clarify their excited-state properties. In addition, the powder samples for complexes 1-4 almost showed no energy shift for the luminescence and significantly those of complexes 5-6 exhibited the mechanochromic luminescence upon grinding. It is noted that complexes 5a and 6a only showed minor red shifts (i.e., from 544 to 556 nm for complex 5a and from 551 to 565 nm for complex 6a), whereas complex 6b exhibited a remarkable red shift from 558 to 603 nm upon grinding. Besides, their luminescence reversibility was also examined toward various solvents.