Red Phosphorescence Research Articles

A red-emitting Mn(II)-based coordination polymer build on 1,2,4,5-tetrakis(diphenylphosphinyl)benzene

The reaction of Mn(ClO4)2·6H2O with 1,2,4,5-tetrakis(diphenylphosphinyl)benzene (L) in DMF affords one-dimensional coordination polymer (CP), [MnL(DMF)2](ClO4)2·DMF, in 87% yield. The cationic chains of the CP are assembled from alternating Mn(II) ions and ligand molecules; each metal ion has a octahedral MnO6 environment composed of four oxygen atoms from two adjacent ligands and two DMF oxygen atoms. At the temperatures below 200 K, this CP demonstrates a red phosphorescence with λemmax~660 nm and lifetimes of 1.5–4.5 ms. The EPR study reveals the high-spin state of the Mn(II) ions and the absence of exchange interactions between them.

Two-Photon Image Tracking of Neural Stem Cells via Iridium Complexes Encapsulated in Polymeric Nanospheres.

Iridium(III) complexes have been shown to be promising probes in two-photon imaging to real-time track the transplanted cells in stem-cell-based therapy. Here, we report on polymeric nanocapsules loaded with red phosphorescence dye of bis(2-methyldibenzo[f,h]quinoxaline) (acetylacetonate) iridium(III) (Ir(MDQ)2acac) with excellent stability created by the double emulsion method. The Ir(MDQ)2acac nanocapsules present high biocompatibility and an efficient fluorescent labeling rate when incubated with cultured mouse neural stem cells (NSCs). More importantly, the Ir(MDQ)2acac nanocapsules had both one- and two-photon imaging properties with stable phosphorescence lasting for 72 h. Furthermore, data from in vivo tracking in nude mice demonstrated that the photoluminescence from Ir(MDQ)2acac nanocapsules in NSCs could be stably monitored for up to 21 days. Our data shed light on the potential clinical application of iridium complexes encapsulated in polymeric nanospheres for two-photon imaging in real-time tracking of the transplanted stem cells.

Tetradentate platinum(II) complexes: Synthesis, photophysical properties, liquid crystalline characteristics and charge transport behaviour

Three new tetradentate phosphorescent Pt(II) complexes (Pt6, Pt12 and Pt16) with half-disc molecule structure were designed and synthesized, which are based on tetradentate ligands with six alkoxyl chains on the periphery, (6,6′-bis((3′,4′,5′-alkoxy-[1,1′-biphenyl]-4-yl)oxy)-2,2′-bipyridine). All complexes show red phosphorescence in dichloromethane solution (λem ≈ 640 nm) with quantum yields in range of 4.3%–6.7% and phosphorescent lifetimes in range of 0.46–0.55 μs in degassed dichloromethane solution. Among these complexes, the complex, Pt12, with 12 carbons chain lengths shows columnar mesophase with liquid crystal temperature range of - 22.0 °C to 77.8 °C, which was confirmed via polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). The complex, Pt12, exhibits ambipolar carrier transport behaviors, which has been confirmed by the space charge limited current (SCLC) measurement. Moreover, the carrier mobility in mesogenic state is two orders of magnitudes greater than that in the amorphous state, showing ordering packing in liquid crystalline phase can facilitate the charge transport.

Luminescent cis-Iridium(III) Complex Based on a Bis(6,7-dimethoxy-3,4-dihydroisoquinoline) Platform Featuring an Unusual cis Orientation of the C∧N Ligands: From a Theoretical Approach to a Deep Red LEEC Device.

By pursuing the strategy of manipulating natural compounds to obtain functional materials, in this work, we report on the synthesis and characterization of a luminescent cationic iridium complex (cis-1), designed starting from the catecholic neurotransmitter dopamine, exhibiting the unusual cis arrangement of the C∧N ligands. Through an integrated experimental and theoretical approach, it was possible to delineate the optoelectronic properties of cis-1. In detail, (a) a series of absorption maxima in the range 300–400 nm was assigned to metal-to-ligand charge transfer and weak and broad absorption maxima at longer wavelengths (400–500 nm) were ascribable to spin-forbidden transitions with a mixed character; (b) there was an intense red phosphorescence with emission set in the range 580–710 nm; and (c) a highest occupied molecular orbital was mainly localized on the metal and the 2-phenylpiridine ligand and a lowest unoccupied molecular orbital was localized on the N∧N ligand, with a ΔH–L set at 2.20 eV. This investigation allowed the design of light-emitting electrochemical cell (LEEC) devices endowed with good performance. The poor literature reporting on the use of cis-iridium(III) complexes in LEECs prompted us to investigate the role played by the selected cathode and the thickness of the emitting layer, as well as the doping effect exerted by ionic liquids on the performance of the devices. All the devices exhibited a deep red emission, in some cases, quite near the pure color (devices #1, #4, and #8), expanding the panorama of the iridium-based red-to-near-infrared LEEC devices. The characteristics of the devices, such as the brightness reaching values of 162 cd/m2 for device #7, suggested that the performances of cis-1 are comparable to those of trans isomers, opening new perspective toward designing a new set of luminescent materials for optoelectronic devices.

Red Room-Temperature Phosphorescence of CDs@Zeolite Composites Triggered by Heteroatoms in Zeolite Frameworks.

Carbon dots (CDs) with red-emitting room-temperature phosphorescence (RTP) are rarely reported because of the increasing nonradiative decay of the excited states and the decreasing energy gap between the excited states and ground states. Herein, we demonstrate a facile strategy for modulating the RTP properties of CDs in terms of donor–acceptor energy transfer (EnT) in the CDs-in-zeolite system. Upon tuning of the heteroatoms (Zn2+, Mn2+) doped in the aluminophosphate zeolite frameworks, CDs@zeolite composites with green and red phosphorescence have been prepared via in situ hydrothermal synthesis. In such composites, the zeolite matrix provides an efficient confinement role in stabilizing the triplet states of CDs. Significantly, the Mn-doped zeolite could act as an energy acceptor allowing EnT from excitons of CDs to the dopant in the host matrix, generating the intriguing red RTP behavior. This work provides an effective strategy for developing CD-based composite materials with special RTP emissions as well as new fields for applications.

On the Nature of Luminescence Thermochromism of Multinuclear Copper(I) Benzoate Complexes in the Crystalline State

A model luminescent [(PhCO2)4Cu4] (Cu4) complex in the crystalline state was investigated via combined crystallographic and spectroscopic techniques contributed substantially by theoretical modelling. The complex appeared to exhibit luminescence thermochromism, i.e., red phosphorescence at room temperature which changes to green when lowering the temperature to 90 K. The low-energy emissive state was assigned as a cluster-centred triplet, 3CC. The emission from this state predicted in TDDFT (~635 nm) matches the experimental red band observed at 660–715 nm. In contrast, the nature of the high-energy “green” band was less straightforward. The next reached cluster-centred triplet excited state occurred to be energetically close to the experimental value of ~545 nm. The two excited states also exhibit significant metal-to-ligand and ligand-to-metal charge transfer characteristics, especially for solid-state distorted geometries. In both cases the cluster core was expected to become notably contracted when compared to the ground state. Time-resolved photocrystallographic results supported the computationally predicted core contraction upon excitation. Additionally, the differences between the spectroscopic behaviour of the related tetra- and hexanuclear copper(I) complexes, Cu4 and Cu6 (i.e., [(PhCO2)6Cu6]) in the crystalline state were discussed and examined. It appeared that crystal packing may constitute an important factor as far as the lack of luminescence thermochromism in the latter case is concerned. Synopsis: Structure–property relationships characterising a model luminescent [(PhCO2)4Cu4] (Cu4) complex in the crystalline state were investigated via combined crystallographic and spectroscopic techniques contributed by theoretical modelling, and compared with the properties of the related [(PhCO2)6Cu6] (Cu6) complex.

Crystallization-Induced Red Phosphorescence and Grinding-Induced Blue-Shifted Emission of a Benzobis(1,2,5-thiadiazole)-Thiophene Conjugate.

Mechanochromic luminogens are of significant importance in both academic and technical aspects. Thus far, most mechanochromic compounds exhibit bathochromically shifted emission upon grinding; the examples of those that exhibit blue-shifted emission still remain limited. Herein, a donor–acceptor–donor (D–A–D)-structured conjugate, namely 4,7-di(2-thienyl)-2,1,3-benzothiadiazole (DTBT), comprising benzobis(1,2,5-thiadiazole) and thiophene units, has been carefully synthesized and investigated. DTBT exhibits typical intramolecular charge transfer (ICT) characteristics, crystallization-induced phosphorescence (CIP), and remarkable mechanochromism. Although it merely emits fluorescence in solutions with distinct ICT features, its crystals demonstrate bright-red room-temperature phosphorescence (616 nm) with efficiency up to 25.0% and generate yellow excimer fluorescence (578 nm) upon mechanical grinding, accompanying decreased lifetimes from 10.9 μs to 3.5 ns and a blue-shifted emission of 38 nm. These results highly indicate the feasibility to fabricate novel CIP luminogens with blue-shifted mechanochromism.

An efficient heterodinuclear Ir(iii)/Pt(ii) complex: synthesis, photophysics and application in light-emitting electrochemical cells

A novel heterodinuclear Ir(iii)/Pt(ii) complex delivers vibrant red phosphorescence with high efficiency in a light-emitting electrochemical cell.

1D-helical platinum(ii) complexes bearing metal-induced chirality, aggregation-induced red phosphorescence, and circularly polarized luminescence.

A series of binuclear cyclometalated Pt(ii) complexes with the bridge ligand of binaphthyl quinoline were synthesized and characterized. Owing to the restriction of intramolecular motions in chiral binaphthyl linkers, the complexes exhibited two unusual metal-induced chiral components and aggregation-induced red phosphorescence. The (R,R,R)/(S,S,S) enantiopure complexes that were confirmed by X-ray diffraction, circular dichroism spectra, time-dependent density functional theory calculations, and circularly polarized luminescence did not require purification by chiral high-performance liquid chromatography and could yield unique 1D M or P hierarchical helices of helices via intermolecular noncovalent interactions. Moreover, it is the first time that racemization-induced phosphorescence enhancement has been observed.

Aqueous Phase Phosphorescence: Ambient Triplet Harvesting of Purely Organic Phosphors via Supramolecular Scaffolding

AbstractAmbient solution and amorphous state room temperature phosphorescence (RTP) from purely organic chromophores is rarely achieved. Remarkable stabilization of triplet excitons is realized to obtain deep red phosphorescence in water and in amorphous film state under ambient conditions by a unique supramolecular hybrid assembly between inorganic laponite clay and heavy atom core substituted naphthalene diimide (NDI) phosphor. Structural rigidity and oxygen tolerance of the inorganic template along with controlled molecular organization via supramolecular scaffolding are envisaged to alleviate the unprecedented aqueous phase phosphorescence.

Aqueous Phase Phosphorescence: Ambient Triplet Harvesting of Purely Organic Phosphors via Supramolecular Scaffolding

Ambient solution and amorphous state room temperature phosphorescence (RTP) from purely organic chromophores is rarely achieved. Remarkable stabilization of triplet excitons is realized to obtain deep red phosphorescence in water and in amorphous film state under ambient conditions by a unique supramolecular hybrid assembly between inorganic laponite clay and heavy atom core substituted naphthalene diimide (NDI) phosphor. Structural rigidity and oxygen tolerance of the inorganic template along with controlled molecular organization via supramolecular scaffolding are envisaged to alleviate the unprecedented aqueous phase phosphorescence.

Aerobic Solid State Red Phosphorescence from Benzobismole Monomers and Patternable Self‐Assembled Block Copolymers

AbstractThe synthesis of the first bismuth‐containing macromolecules that exhibit phosphorescence in the solid state and in the presence of oxygen is reported. These red emissive high molecular weight polymers (>300 kDa) feature benzobismoles appended to a hydrocarbon scaffold, and were built via an efficient ring‐opening metathesis (ROMP) protocol. Moreover, our general procedure readily allows for the formation of cross‐linked networks and block copolymers. Attaining stable red phosphorescence with non‐toxic elements remains a challenge and, thus, our new class of soluble (processable) polymeric phosphor is of great interest. Furthermore, the formation of bismuth‐rich cores within organic–inorganic block copolymer spherical micelles is possible, leading to patterned arrays of bismuth in the film state.

Aerobic Solid State Red Phosphorescence from Benzobismole Monomers and Patternable Self-Assembled Block Copolymers.

The synthesis of the first bismuth-containing macromolecules that exhibit phosphorescence in the solid state and in the presence of oxygen is reported. These red emissive high molecular weight polymers (>300 kDa) feature benzobismoles appended to a hydrocarbon scaffold, and were built via an efficient ring-opening metathesis (ROMP) protocol. Moreover, our general procedure readily allows for the formation of cross-linked networks and block copolymers. Attaining stable red phosphorescence with non-toxic elements remains a challenge and, thus, our new class of soluble (processable) polymeric phosphor is of great interest. Furthermore, the formation of bismuth-rich cores within organic-inorganic block copolymer spherical micelles is possible, leading to patterned arrays of bismuth in the film state.

Decisive Influence of the Metal in Multifunctional Gold, Silver, and Copper Metallacycles: High Quantum Yield Phosphorescence, Color Switching, and Liquid Crystalline Behavior.

Three cyclic trinuclear pyrazolate complexes with Au(I), Ag(I), or Cu(I) have been studied. These complexes have interesting and distinct optical and thermal properties depending on the metal, namely, liquid crystalline behavior, red or deep-red phosphorescence at room temperature, thermoluminochromism, and response to silver ions. The selected ligand, 4-hexyl-3,5-dimethylpyrazolate, maximizes the effect that the nature of the metals has on the properties of the complexes, thus allowing the intermolecular metallophilic interactions to be responsible for the optical properties. Moreover, the gold and silver complexes show columnar liquid crystal phases at high temperature. All of the complexes have good solubility properties for processing as poly(methyl methacrylate) (PMMA) doped films. Films of the gold, silver, and copper complexes show interesting optical behavior such as wide-range color switching or phosphorescence turn-on upon cooling. In addition, films of the gold complex show a bright color switching (red to blue) in the presence of silver ions. The gold and copper complexes are bright phosphors with phosphorescent quantum yields of 90% in PMMA films, the highest values reported for this class of compounds at room temperature.

Long-lasting green, yellow, and red phosphorescence of carbon dots embedded on ZnAl2O4 nanoparticles synthesized by a combustion method

This work presents the luminescent, structural and morphological properties of carbon dots (CDs) synthesized on ZnAl2O4 (ZAO) by a simple combustion method. The samples were annealed at different temperatures (600 °C, 700 °C, 800 °C, and 900 °C) in order to change the concentration and size of the CDs on ZAO. The XRD patterns indicated that the annealed samples present a cubic phase (spinel structure). According to the TEM analysis, the CDs on ZAO (CZAO) have a particle size in the range of 2.0–4.0 nm. The annealing treatment, allowed to tune their luminescence emission from green (517 nm) to red (650 nm) under excitation with blue light (455 nm). Yellow, orange, and red phosphorescence were observed for the first time from CDs. The green/yellow and orange/red phosphorescence lasted 5 min and 15 min, respectively, which are the longest phosphorescence times reported for CDs. It is proposed that CDs’ luminescence originates from the deep levels formed after the annealing treatment. Due to the color tunability presented by the CDs in fluorescence and phosphorescence, the CZAO samples can be useful for applications in biolabeling, bioimaging, lighting, and signaling for buildings and roads.

Room-Temperature Orange-Red Phosphorescence by Way of Intermolecular Charge Transfer in Single-Component Phenoxazine–Quinoline Conjugates and Chemical Sensing

Achieving red phosphorescence from purely organic system is a challenging feat due to the predominant thermal nonradiative decay pathways of the excited electrons. Here, we design single-component ...

A Single‐Anion‐Based Red‐Emitting Cationic Diiridium(III) Complex Bearing a Pyrimidine‐Based Bridging Ligand for Oxygen Sensing

Compared with double‐anion‐based cationic diiridium complexes, the study of single‐anion‐based cationic diiridium complexes as emitters is largely unexplored. In this work, two methods were used to prepare single‐anion‐based cationic diiridium complex Ir‐Ir‐1 bearing 2‐pyrimidine‐benzimidazole as bridging ligand, both sides of which are symmetrical structures comprising 2‐phenylpyridine as cyclometalated ligand. It shows efficient red phosphorescence emission with redshift of the emission wavelength by 85 nm relative to the corresponding monoiridium complex Ir‐1. Moreover, it exhibits long emission lifetime and high oxygen sensitivity, which were attributed to the intraligand charge‐transfer excited state. Hence, Ir‐Ir‐1 was used as a oxygen probe to monitor intracellular O2 content by time‐resolved luminescence imaging.

A theoretical design of some silole-based dibenzothiophene-S,S-dioxide semiconducting compounds for red phosphorescence

Abstract A series of molecular organic semiconductors constituting of two side groups R, R = silole or silole-benzene, attached symmetrically to a central dibenzothiophene-S,S-dioxide unit were theoretically designed as host materials in red phosphorescence. Their electronic and transport properties were determined using density functional theory methods. Structural modifications of the R groups and central units were performed with the aim of increasing both triplet energy and charge mobility. Calculated results suggested a few compounds to have sufficiently high triplet energies (>2.5 eV) to be used as host materials in PhOLED devices in combination with low-triplet-energy guest materials.

Bipolar carbazole/quinoxaline-based host materials for efficient red PhOLEDs

Three bipolar hosts, 9-(4-(2,3-diphenylquinoxalin-6-yl)phenyl)-9H-carbazole (p-PQPC), 9-(3-(2,3-diphenylquinoxalin-6-yl)phenyl)-9H-carbazole (m-PQPC) and 9-(2-(2,3-diphenylquinoxalin-6-yl)phenyl)-9H-carbazole (o-PQPC) were designed, synthesized and characterized for red phosphorescence organic light emitting devices (PhOLEDs). These three compounds exhibited excellent physical properties with high thermal stabilities, reasonable HOMO-LUMO energy levels and balanced charge transport. All of them were applied to fabricate red PhOLEDs as host materials, and the p-PQPC based device demonstrated outstanding electroluminescence performance with the maximum current efficiency, power efficiency and external quantum efficiency of 21.9 cd/A, 15.4 lm/W and 12.2%, respectively, which suggesting that p-PQPC is a promising host for red PhOLEDs.

Persistent luminescence nanothermometers

Persistent phosphorescence nanoparticles emitting in the red and near-infrared spectral regions are strongly demanded as contrast nanoprobes for autofluorescence free bioimaging and biosensing. In this work, we have developed Sr4Al14O25:Eu2+, Cr3+, Nd3+ nanopowders that produce persistent red phosphorescence peaking at 694 nm generated by Cr3+ ions. This emission displays temperature sensitivity in the physiological temperature range (20–60 °C), which makes these nanoparticles potentially useful as fluorescence (contactless) nanothermometers operating without requiring optical excitation. Nd3+ ions, which act as shallow electron traps for the red Cr3+ persistent emission, also display infrared emission bands, extending the fluorescence imaging capability to the second biological window. This unique combination of properties makes these nanoparticles multifunctional luminescent probes with great potential applications in nanomedicine.