Organic Luminescent Materials Research Articles

Organic Near‐Infrared Luminescent Materials Based on Excited State Intramolecular Proton Transfer Process†

Comprehensive SummaryOrganic near‐infrared (NIR) luminescent materials have captured intense research interest owing to their potential applications in optical communication, data storage, bioimaging, sensing and night vision. Excited state intramolecular proton transfer (ESIPT) process with absorption in normal form while emission in tautomer form can lead to a distinct redshift emission, based on which, a lot of organic NIR luminescent materials were designed. Because of attractive features such as ultrahigh sensitivity to the surroundings, large Stokes shift, and inherent four level system, ESIPT based NIR luminescent materials are supposed to be ideal fluorescent probes and gain materials. In this review, first, organic near‐infrared luminescent materials based on ESIPT process are summarized according to the core structures. Second, recent advances of ESIPT‐based organic near‐infrared fluorescent probes and organic NIR lasers are reviewed. Finally, the current challenges and prospects of ESIPT‐based organic NIR luminescent materials are introduced.

Building hybrid luminescent materials: Understanding stacking modes in aggregate structure to emission modulation

Two charge transfer (CT) complexes A1 and A2 with tunable emissions were explored by diluting anthracene functionalized stilbene as donors with 1,2,4,5-tetracyanobenzene (TCNB) as the acceptor unit. The effect of substituent tailoring on the supramolecular packing and optical properties of the two CT products was accomplished by various solid-state characterization and theoretical calculations. It was found that these binary CT cocrystals exhibit two types of alternatively sandwiched face-to-face patterns (A1 (DADA), A2 (DAD··DAD), and red-shifted emissions relative to their monomers through π-π interactions accompanied by hydrogen bonds as indicated by the structural analysis. The two crystalline aggregates feature molar ratios of 1:1 or 1:2 and molecular packing-dependent emissions, induced by different dihedral angles attributed to the substituent effect, thereby leading to diverse π-π interactions and charge transfer effects respectively, which further results in tunable photophysical properties. These results suggest that the marriage of substituent engineering and cocrystal engineering is a promising molecular design strategy that will help to understand the relationship between emission property and molecular packing with the aim to guide further the development of novel organic solid luminescent materials.

Mechanoresponsive luminogen (MRL)-based real-time and visible detection method for biaxial fatigue crack propagation

Fatigue cracks under complex stress state is crucial for safety assessment. Owing to the characteristics of visualization, rapid response and nondestructive measurement, mechanoresponsive luminogen (MRL) based methods have a great potential for stress sensing and damage detection. In this study, fatigue crack propagation in aluminum alloys under biaxial loadings were detected using a pure organic mechanochromic luminescent material. Both fatigue crack propagation behavior and luminescence emission were investigated. Real-time fatigue crack length and crack propagation path can be successfully detected using the fluorescence distribution. The MRL-based methods may open up new opportunities for nondestructive evaluation and structural damage evaluation.

Synthesis and Mechanistic Investigation of Bipyrazolo[1,5-a]pyridines via Palladium-Catalyzed Cross-Dehydrogenative Coupling of Pyrazolo[1,5-a]pyridines.

The synthesis of a range of 3,3'-bipyrazolo[1,5-a]pyridine derivatives via direct cross-dehydrogenative coupling of pyrazolo[1,5-a]pyridine precursors is herein presented. This simple and efficient methodology involving palladium(II)-catalyzed C-H bond activation showed good functional group tolerance and product yield (up to 94%). Through the mechanistic insights gained from both kinetic isotope effect experimental studies and density functional theory calculations, a plausible reaction mechanism was outlined. Furthermore, subsequent derivatizations of the resulting 7,7'-diaryl-3,3'-bipyrazolo[1,5-a]pyridines, executed by performing palladium-mediated ortho C-H bond activation followed by hypervalent iodine-induced chlorination, rendered this series of compounds more extended π-conjugation and twisted conformations. Our study on these bipyrazolo[1,5-a]pyridine-based luminogens provides new opportunities for tailor-made organic luminescent materials.

Fluorescent Covalent Organic Frameworks: A Promising Material Platform for Explosive Sensing.

Covalent organic frameworks (COFs) are a novel class of porous crystalline organic materials with organic small molecule units connected by strong covalent bonds and extending in two- or three-dimension in an ordered mode. The tunability, porosity, and crystallinity have endowed covalent organic frameworks the capability of multi-faceted functionality. Introduction of fluorophores into their backbones or side-chains creates emissive covalent organic frameworks. Compared with common fluorescent organic solid materials, COFs possess several intrinsic advantages being as a type of irreplaceable fluorescence materials mainly because its highly developed pore structures can accommodate various types of guest analytes by specific or non-specific chemical bonding and non-bonding interaction. Developments in fluorescent COFs have provided opportunities to enhance sensing performance. Moreover, due to its inherent rigidified structures and fixed conformations, the intramolecular rotation, vibration, and motion occurred in common organic small molecules, and organic solid systems can be greatly inhibited. This inhibition decreases the decay of excited-state energy as heat and blocks the non-radiative quenching channel. Thus, fluorescent COFs can be designed, synthesized, and precisely tuned to exhibit optimal luminescence properties in comparison with common homogeneous dissolved organic small molecule dyes and can even compete with the currently mainstream organic solid semiconductor-based luminescence materials. This mini-review discusses the major design principle and the state-of-the-art paragon examples of fluorescent COFs and their typical applications in the detection and monitoring of some key explosive chemicals by fluorescence analysis. The challenges and the future direction of fluorescent COFs are also covered in detail in the concluding section.

α-Cyanostilbene functionalized carbazole derivatives exhibiting dual-state emission and multi-stimuli responsive fluorescent switching

Interest in organic luminescent materials that show strong dual state emission (DSE) in both solution and the solid state is rapidly growing due to their widespread applications. The development of DSE luminogens (DSEgens) with multi-stimuli responsive properties capable of achieving multi-parameter detection in complicated environments remains challenging due to the lack of an explicit molecular design strategy. In this work, we devised and prepared two D-π-A-π-D-structured carbazole derivatives, PY-CZ-Pa and PY-CZ-Ta, with intramolecular charge transfer (ICT) characteristics and excellent DSE properties. Both the compounds as fluorescent probes are highly sensitive to trace amount of water and picric acid (PA) in organic solvents. The limits of detection are determined to be 0.016% (PY-CZ-Pa) or 0.040% (PY-CZ-Ta) for water and 320 nM (PY-CZ-Pa) or 538 nM (PY-CZ-Ta) for PA. Moreover, the two compounds exhibit reversible mechanofluorochromism, with spectral shifts of over 24 nm. The results presented in this work suggest that bright DSEgens with multi-stimuli responsive properties are promising materials for applications at both the molecular and aggregated levels.

Synthesis of an Acidochromic and Nitroaromatic Responsive Hydrazone‐Linked Pillararene Framework by a Macrocycle‐To‐Framework Strategy

AbstractTailoring the properties of solid‐state organic luminescent materials using a bottom‐up design principle is highly desirable for many applications. Herein, we present a “macrocycle‐to‐framework” strategy to construct macrocycle‐functionalized and hydrazone‐linked functional organic polymers with bright yellowish‐green luminescence and unique solvatochromism behaviors by the condensation of a diacylhydrazine‐functionalized pillar[5]arene with tris(4‐formylbiphenyl)amine. Outperforming their non‐macrocycle‐incorporated counterparts, the pillar[5]arene‐containing materials display amplified sensitivity to acidic conditions with luminescent and colorimetric dual‐modal patterns assisted by the enhanced intramolecular charge transfer (ICT), and exhibit satisfactory responsiveness to nitrobenzene compounds through rapid luminescence quenching with high selectivity and a low detection limit, where the sensing process proceeds through multiple dynamic quenching pathways.

Synthesis of an Acidochromic and Nitroaromatic Responsive Hydrazone-Linked Pillararene Framework by a Macrocycle-To-Framework Strategy.

Tailoring the properties of solid-state organic luminescent materials using a bottom-up design principle is highly desirable for many applications. Herein, we present a "macrocycle-to-framework" strategy to construct macrocycle-functionalized and hydrazone-linked functional organic polymers with bright yellowish-green luminescence and unique solvatochromism behaviors by the condensation of a diacylhydrazine-functionalized pillar[5]arene with tris(4-formylbiphenyl)amine. Outperforming their non-macrocycle-incorporated counterparts, the pillar[5]arene-containing materials display amplified sensitivity to acidic conditions with luminescent and colorimetric dual-modal patterns assisted by the enhanced intramolecular charge transfer (ICT), and exhibit satisfactory responsiveness to nitrobenzene compounds through rapid luminescence quenching with high selectivity and a low detection limit, where the sensing process proceeds through multiple dynamic quenching pathways.

Intermolecular Hydrogen-Bond-Assisted Solid-State Dual-Emission Molecules with Mechanical Force-Induced Enhanced Emission.

Hydrogen bonds not only play a crucial role in the life sciences but also endow molecules with fantastic physical and chemical properties, which help in the realization of their high-tech applications. This work presents an efficient strategy for achieving highly efficient solid-state dual-emission blue emitters with mechanical force-induced enhanced emission properties via intermolecular hydrogen bonds via novel pyrene-based intermediates, namely, 1,3,6,8-tetrabromo-2,7-dihydroxypyrene (1) and 1,3,6,8-tetrabromo-2-hydroxypyrene (2), prepared via hydroxylation and bromination of pyrene in high yields. Moreover, further use of a classical Pd-catalyzed coupling reaction affords new pyrene-based luminescent materials 3-5, which display high thermal stability (in range of 336-447 °C), blue emission (<463 nm), and high quantum yields in solution. Interestingly, with the monosubstituted hydroxyl (OH) or methoxy (OMe) group located at position 2 of pyrene, compounds 4a and 5 display exciting dual emission with mechanical force-induced enhanced emission properties, due to the presence of several hydrogen-bond interactions. Moreover, this series of compounds exhibits numerous advantages, for example, deeper blue emission with a narrower full width at half-maximum, a stronger steric effect, and higher hydrophilicity. Thus, these novel bromopyrene intermediates and related pyrene-based luminescent materials will pave the way for further exploration of novel organic solid-state luminescent materials for potential application in organic electronics, bioimaging, chemosensors, etc.

Highly Stable Metal-Free Long-Persistent Luminescent Copolymer for Low Flicker AC-LEDs.

Organic room-temperature long-persistent luminescent materials are promising light-emitting materials for encryption, architectural decoration, organic solar cells, and biomedical applications. However, their unstable structures and thermal- and humidity-induced emission quenching have greatly limited their utility and reliability. Here, we report a metal-free nonconjugated copolymer that possesses stable photoluminescence at both high temperature and humidity. The room-temperature long-persistent luminescence (LPL) of this copolymer lasts for more than 15 s and can be recovered in high humidity conditions by heating to remove moisture. Copolymer LPL can be achieved with various excitation wavelengths, ranging from ultraviolet to near-infrared, and the LPL color can be adjusted accordingly. The high initial LPL intensity and ultrafast filling time of the copolymer makes it suitable for low flicker alternating current-driven light-emitting diodes (AC-LEDs).

Synthesis and Photophysical Properties of Benzothiadiazole-Based Luminescence Materials with Tunable Emission Properties

Inspired by the advantages of long-wavelength emission of organic luminescence materials, this article presents two D-A-D type benzothiadiazole-based molecules with color-tunable emission from yellow to red via a suitable choice of the donor groups (phenyl/thiophenyl units). The optimized molecular structures indicated that the benzothiadiazole units are almost lying in a co-planer configuration with the donor groups, which would trend to strengthen the intermolecular interaction, resulting in a reasonable red-shifted emission with slightly fluorescence quenching. Thus, although the typical aggregation-induced emission (AIE) unit tetraphenylethene was introduced into those molecular skeletons, both compounds still display ACQ characteristic, due to the molecular aggregation via the strengthen intramolecular interaction (such as π-π stacking) could overwhelm the AIE effect. Thus, the large π-conjugated with planar molecular skeletons is not beneficial for constructing highly efficient AIE molecules.

Dual‐Stimulus Supramolecular Luminescent Switch Based on Cyanostilbene‐Bridged Bis(Dibenzo‐24‐Crown‐8) and a Diarylethene Derivative

AbstractThe development of multifunctional photo‐controlled luminescent switch, which provides versatile tools for smart systems and advanced organic luminescent materials, is of paramount significance. Herein, a photoswitchable supramolecular assembly is constructed from the noncovalent interaction of cyanostilbene‐bridged bis(dibenzo‐24‐crown‐8) with a diarylethene derivative. Possessing the reversible photoisomerization property and a high Förster resonance energy transfer (FRET) efficiency, the resultant supramolecular assembly can reversibly switch on/off the luminescence of the cyanostilbene group upon irradiation by different wavelength light and the addition of acid/base both in solution and in the solid state. Significantly, the characters written with the supramolecular assembly as ink can be reversibly and repeatedly erased and recovered by altering the wavelength of irradiation light.

Through-Space Interaction of Tetraphenylethylene: What, Where, and How.

Electronic conjugation through covalent bonds is generally considered as the basis for the electronic transition of organic luminescent materials. Tetraphenylethylene (TPE), an efficient fluorophore with aggregation-induced emission character, fluoresces blue emission in the aggregate state, and such photoluminescence is always ascribed to the through-bond conjugation (TBC) among the four phenyl rings and the central C═C bond. However, in this work, systematic spectroscopic studies and DFT theoretical simulation reveal that the intramolecular through-space interaction (TSI) between two vicinal phenyl rings generates the bright blue emission in TPE but not the TBC effect. Furthermore, the evaluation of excited-state decay dynamics suggests the significance of photoinduced isomerization in the nonradiative decay of TPE in the solution state. More importantly, different from the traditional qualitative description for TSI, the quantitative elucidation of the TSI is realized through the atoms-in-molecules analysis; meanwhile, a theoretical solid-state model for TPE and other multirotor systems for studying the electronic configuration is preliminarily established. The mechanistic model of TSI delineated in this work provides a new strategy to design luminescent materials beyond the traditional theory of TBC and expands the quantum understanding of molecular behavior to the aggregate level.

Photoluminescence behaviors of small aliphatic ketimines

<p indent="0mm">Non-traditional organic luminescent materials have attracted more and more attention in recent years because of their unique luminescent behaviors and mechanisms. Some aromatic imines (Schiff bases) have been found to exhibit aggregation-induced emission behaviors, but there is no report on the luminescence behavior of small aliphatic ketimines yet. This work reports that aliphatic primary amines easily react with acetone at room temperature, producing products with very strong fluorescence emissions. The fluorescence intensity increases with the increasing reaction time, and the maximum excitation and emission wavelengths red-shift significantly, with the maximum emission wavelengths more than <sc>600 nm.</sc> Structural characterizations prove that the fluorescent products are aliphatic ketimines. These compounds exhibit concentration-enhanced emission behaviors. Their fluorescence intensity increases and the emission red-shifts with the increasing concentration. The aliphatic ketimines show aggregation-enhanced emissions, and the aggregate particle size increases with the increase of concentration. The fluorescence emission of the ketimines is also sensitive to solvents. Theoretical calculation results show that the HOMO–LUMO energy gap of ketimine aggregates decreases with the increase of the number of the aggregated molecules. This study expands the scope of non-traditional luminescent materials and may have potential practical applications.

Alkyl-Engineered Dual-State Luminogens with Pronounced Odd-Even Effects: Quantum Yields with up to 48% Difference and Crystallochromy with up to 22 nm Difference.

Alkyl chain-resulted odd-even effects in fluorescence quantum yield (FLQY) have also been reported in organic luminescent materials (OLMs). However, the odd-even effects in FLQY caused by the alkyl substitutes in OLMs are generally very weak, with only single-digit differences. Here, we report a series of alkyl-substituted dual-state luminogens (DSEgens) showing extremely high solid-state FLQY in even-numbered analogues (>90% FLQY) and a dramatically pronounced odd-even effect in FLQY. The odd-even effect in FLQY is over 26% alternation, and a maximum of 48% difference in FLQY was observed between the compounds C1 and C2 with a methyl and ethyl substitution, respectively. C1 and C2 also displayed a crystallochromy with a 22 nm difference in emission wavelength. In addition, odd-even effects in the melting point and decomposition temperature were also observed. With these bright DSEgens, applications such as specific recognition of picric acid and ultrasensitive trace water detection have been demonstrated.

Combining π-conjugated groups by flexible alkyl chains for ultralong organic phosphorescence by photo-activation

Purely organic luminescent materials with ultralong room temperature phosphorescence by photo-activation have attracted great interest owing to their advantages of clean, safe, easy-to-use and control. Herein, we designed and synthesized a series of ultralong room temperature phosphorescence carbazole derivatives using flexible alkyl with different lengths to adjust the distance between two conjugated carbazole groups. Only, CzC3Cz crystal has the dynamic ultralong organic phosphorescence with increasing the excitation time of 365 nm UV light. By prolonging the UV excitation time (from 5 s to 3 min), room temperature phosphorescence intensity was improved about 10 times and phosphorescence lifetime increased from 48 ms to 898 ms. Additionally, it exhibited favorable stability and repeatability. Comparing the single crystal structure before and after photo-activation, intermolecular interaction of π···π and C–H···π bond strengthens indicating that metastable state appears after long-time excitation of UV light. Considering the photo-activation characteristic, encryption pattern was prepared using the sifted compounds with similar phosphorescence performance for the application of anti-counterfeiting.

Donor-Acceptor structured phenylmethylene pyridineacetonitrile derivative with aggregation-induced emission characteristics: photophysical, mechanofluorochromic and electroluminescent properties

Development of organic luminescent materials which are highly emissive in aggregation state is of great significance in practical applications. In this work, a phenylmethylene pyridineacetonitrile derivative fluorophore p-DBCNPy was synthesized via Knoevenagel condensation in good yield and its structure was characterized. p-DBCNPy exhibited evident aggregation-induced emission (AIE) feature and remarkable solvatochromic effect. The solid powder of p-DBCNPy emitted green fluorescence with a quantum yield of 0.59. Taking the advantages of the high emission efficiency and thermal stability, p-DBCNPy was employed as the light emitting material in an organic light-emitting diode (OLED) device and showed good potential in electroluminescent applications. Moreover, the fluorophore demonstrated reversible mechanochromism behavior which was ascribed to the transformation between the crystalline and amorphous phase. The greenish-yellow emission of p-DBCNPy powder turned orange with a red shift of over 40 nm upon grinding. The results demonstrated the versatilities of phenylmethylene pyridineacetonitrile derivatives in the applications in mechanochromism and optoelectronics.

Carbazole-functionalized V-shaped bis-coumarin derivatives as organic luminescent materials

Two novel functionalized V-shaped bis-coumarin derivatives, namely CC-Cz and CC-2Cz, were successfully synthesized and characterized. The thermal stability, photophysical and electrochemical properties of the compounds were investigated systematically. By combining theoretical and experimental investigation, to reveal the structure-property relationships of different substituted coumarin-fused coumarins. These compounds possess good thermal stability with decomposition temperature over 360 °C, and exhibit strong green emission in solution and solid thin films. The solution-processed doped devices using the blend of PVK:PBD with the V-shaped bis-coumarin derivatives as emitting layers emit green emission with maximum external quantum efficiency (EQEmax) of 2.20% for CC-2Cz, 1.98% for CC-Cz.

White light emission from AIE‐active luminescent organic materials

AbstractFluorescent organic materials that exhibit unique aggregation‐induced emission have been utilized for various applications, including white light emission (WLE). The materials typically rely on composite designs involving a mixture of compounds emitting at different wavelengths of light. To overcome the limitations offered by the design of such composite mixtures, tremendous efforts were also emphasized in developing single‐molecule substrates that emit white light. The white light by either method is typically controlled by the solvent polarity, energy and charge transfer, host‐guest interactions, gelation, or embedding in the polymer or organic frameworks. This article briefly reviews various molecules exhibiting aggregation‐induced emission behavior and emitting white light.

Assembly and Applications of Macrocyclic-Confinement-Derived Supramolecular Organic Luminescent Emissions from Cucurbiturils.

Cucurbit[n]urils (Q[n]s or CB[n]s), as a classical of artificial organic macrocyclic hosts, were found to have excellent advantages in the fabricating of tunable and smart organic luminescent materials in aqueous media and the solid state with high emitting efficiency under the rigid pumpkin-shaped structure-derived macrocyclic-confinement effect in recent years. This review aims to give a systematically up-to-date overview of the Q[n]-based supramolecular organic luminescent emissions from the confined spaces triggered host-guest complexes, including the assembly fashions and the mechanisms of the macrocycle-based luminescent complexes, as well as their applications. Finally, challenges and outlook are provided. Since this class of Q[n]-based supramolecular organic luminescent emissions, which have essentially derived from the cavity-dependent confinement effect and the resulting assembly fashions, emerged only a few years ago, we hope this review will provide valuable information for the further development of macrocycle-based light-emitting materials and other related research fields.