Organic Luminescent Materials Research Articles

Precise Synthesis of Organic Cocrystal Alloys with Full-Spectrum Emission Characteristics for the Stepless Color Changing Display.

Organic luminescent materials are indispensable in optoelectronic displays and solid-state luminescence applications. Compared with single-component, multi-component crystalline materials can improve optoelectronic characteristics. This work forms a series of full-spectrum tunable luminescent charge-transfer (CT) cocrystals ranging from 400 to 800nm through intermolecular collaborative self-assembly. What is even more interesting is that o-TCP-Cor(x)-Pe(1-x), p-TCP-Cor(x)-Pe(1-x), and o-TCP-AN(x)-TP(1-x) alloys are prepared based on cocrystals by doping strategies, which correspondingly achieve the stepless color change from blue (CIE [0.22, 0.44]) to green (CIE [0.16, 0.14]), from green (CIE [0.27, 0.56]) to orange (CIE [0.58, 0.42]), from yellow (CIE [0.40, 0.57]) to red (CIE [0.65, 0.35]). The work provides an efficient method for precisely synthesizing new luminescent organic semiconductor materials and lays a solid foundation for developing advanced organic solid-state displays.

Dibenzofulvene Derivatives as Promising Materials for Photovoltaic and Organic Electronics

This review aimed to summarize the current knowledge regarding dibenzofulvene derivatives (DBF) investigated for photovoltaics and organic electronics applications. The work begins with a detailed analysis of the synthesis and modification methods for dibenzofulvene derivatives’ structure. Then, the physicochemical properties (thermal, electrochemical, and optical) of the selected compounds are discussed in detail. Moreover, this article also presents the DFT calculations performed so far. Finally, the review presents the latest research on the applications of dibenzofulvene derivatives as dyes for DSSC cells, hole transport materials (HTMs) for perovskite solar cells (PSCs), organic light-emitting diodes (OLEDs), and luminescent and electrochromic materials. Considering the above, this review may be helpful when designing new organic compounds for photovoltaic and organic electronic applications.

Chromatically polarized Afterglow: Photo-controlled and anisotropic persistent luminescence from doped polymeric systems

Organic persistent luminescent (OPL) materials showing persistent luminescence at room temperature have caught significant research attention for decades. Integrating chromatic polarization into an OPL system with tunable emissions is attractive and challenging but rarely reported. Herein we propose a strategy to realize linear-polarization-dependent afterglow color with reversible photo-patterning capability. This is accomplished through the use of an acridone-based Förster-resonance energy transfer (FRET) system, which is coupled with aligned Au-nanorods. The composite shows reversible UV-lithographic capability attributed to the generation of radicals by the acridone luminophore. Importantly, this system allows for the variation of chromatic afterglow based on the linear polarized angles relative to the aligned direction of AuNRs. The multiple delayed emission peaks are absorbed in a polarized-dependent manner due to the anisotropic plasmonic resonance of the aligned Au-nanorods. With the photo-patterning capability, polarization- and humidity-dependent afterglow, this approach holds the potential to contribute to various applications of OPL materials, such as sensors, information encryption, and anti-counterfeiting.

Tailored Near‐Infrared Luminescent Materials with Fast Reverse Intersystem Crossing for Efficient Deep‐Red/Near‐Infrared Organic Light‐Emitting Diodes with High Efficiencies and Small Roll‐Offs

AbstractRobust organic luminescent materials radiating deep‐red/near‐infrared (DR/NIR) light are highly desired in organic light‐emitting diodes (OLEDs). However, most reported DR/NIR materials show only moderate electroluminescence (EL) efficiencies accompanied by rock‐ribbed efficiency roll‐offs, which immensely limit their practical use in OLEDs. In this work, two tailor‐made NIR luminescent materials with thermally activated delayed fluorescence (TADF) property are developed based on rigid and strong electron‐accepting core of 2,7‐bis(4‐cyanophenyl)dibenzo[a,c]phenazine‐11,12‐dicarbonitrile and electron donors of triphenylamine or N,N‐di([1,1′‐biphenyl]‐4‐yl)phenylamine. They exhibit high photoluminescence quantum yields, fast reverse intersystem crossing processes and large horizontal dipole ratios. Their nondoped OLEDs show NIR lights with maximum external quantum efficiencies (ηext,maxS) of 0.59% at 856 nm and 1.14% at 850 nm. And their doped OLEDs radiate DR to NIR lights (646–737 nm) and attain superb ηext,maxs of up to 30.3% at 646 nm with small efficiency roll‐offs. The state‐of‐the‐art EL performances of both NIR TADF materials demonstrate their high potential as emitters in OLEDs and the insights gained in this work will enlighten the further development of efficient NIR luminescent materials.

A multifunctional emitter with synergistical adjustment of rigidity and flexibility for high-performance data-recording and organic light-emitting devices with “hot exciton” channel

An intramolecular dual-locking design is proposed for organic luminescent materials, aiming at synergistically adjusting the molecular rigidity and flexibility for applications in data storage and light-emitting devices. According to this concept, a multifunctional donor–acceptor type emitter, LPCz-TRZ, is developed, featuring a “soft” diphenylmethane lock and a “hard” single-bond lock in its donor moiety. The highly locked structure results in appreciable rigidity that endows LPCz-TRZ with high luminescence efficiency, narrow emission, promoted charge conductivity and triplet harvesting “hot exciton” channels, allowing high-performance deep-blue organic light-emitting diodes (OLEDs) with an external quantum efficiency as high as 7.88%, which is among the best deep-blue “hot exciton” OLEDs. Notably, the structure of LPCz-TRZ is semi-flexible, leading to unique mechanochromic luminescence (MCL) behavior. Compared to the conventional MCL emitters, the orchestration of rigidity and flexibility in LPCz-TRZ affords high-contrast, fatigue-resistance MCL, as evidenced by a simple, rewritable data-recording demonstration with negligible fatigue after up to 15 wiring-erasing cycles.

A new insight into aggregation structure of organic solids and its relationship to room‐temperature phosphorescence effect

AbstractIn order to improve the performance of organic luminescent materials, lots of studies have been carried out at the molecular level. However, these materials are mostly applied as solids or aggregates in practical applications, in which the relationship between aggregation structure and luminescent property should be paid more attention. Here, we obtained five phenothiazine 5,5‐dioxide (O‐PTZ) derivatives with distinct molecular conformations by rational design of chemical structures, and systematically studied their room‐temperature phosphorescence (RTP) effect in solid state. It was found that O‐PTZ dimers with quasi‐equatorial (eq) conformation tended to show stronger π‐π interaction than quasi‐axial (ax) conformers in crystal state, which was more conducive to the generation of RTP. Based on this result, a multi‐level structural model of organic solids was proposed to draw the relationship between aggregation structure and RTP effect, just like the research for the structure‐property relationship of proteins. Using this structural model as the guide, boosted RTP efficiency from 1% to 20% was successfully achieved in the corresponding host‐guest doping system, showing its wide applicability.

A simple strategy for obtaining aggregation-induced delayed fluorescence material achieving nearly 20% external quantum efficiency for non-doped solution-processed OLEDs

The development of efficient non-doped organic luminescent materials is challenging work due to the existence of aggregation-caused quenching (ACQ) effect in most of emitters. Here, 1,10-phenanthroline (Phen) with a rigid structure was selected as acceptor unit, which could well limit intramolecular vibrations and rotations. According to the Ullmann coupling, the twisted tert-butyldiphenylamine-modified carbazole named N3,N3,N6,N6-tetrakis(4-(tert-butyl)phenyl)-9H-carbazole-3,6-diamine (tBuPhACz) was introduced into 2, 9-position of the Phen to afford tBuPhACz-Phen as target material, which not only can limit the π–π stacking of acceptor, but also could obtain the TADF and AIE effects due to the twisted D-A structure. The non-doped neat film of tBuPhACz-Phen shown high photoluminescence quantum yield (PLQY) of 76.5 %. The non-doped solution-processed electroluminescent devices obtained high performance with the maximum external quantum (EQEmax) up to 19.7 %. This result illustrated that the performance of this device is one of the best in the non-doped solution-processed OLEDs.

Thiophene derived sky-blue fluorescent probe for the selective recognition of mercuric ion through CHEQ mechanism and application in real time samples

A vibrant blue organic luminescent material with enhanced photophysical properties is in great demand for the generation of optoelectronic devices and luminescent sensors. In this context, the thiophene-benzimidazole probe TH-IMI was designed and synthesized by a simple condensation reaction. The synthesized probe has shown excellent photophysical properties like high FL intensity, a high quantum yield of 90% in the solution phase, a low optical bandgap of 2.84 eV, positive solvatochromic effect in emission spectra and Disaggregation Caused Quenching Effect (DCQE). Such a high luminescent probe was employed for the recognition of mercuric ions in the solution phase, solid state detection, and in tracking mercury in green gram sprouts. UV–visible absorption and emission spectra, 1H NMR titration, IR spectroscopic and ESI-MS techniques confirmed that the probe underwent a fluorescence quenching response via the CHEQ effect upon exposure to Hg2+. The stoichiometry was found to be 1:1 through Job’s plot and has a fast response rate and relatively low limit of detection of about 6.13 × 10-11 M in a linear range between 0 and 110 µL.

The second near-infrared fluorescence concealed imaging for identifying smuggled baggage

Smuggling methods are renovating with more diversified, complicated means, thereby developing concealed and non-sensitive luggage identification technology is of great significance. Herein, for the first time, the second near-infrared (NIR-II, 1000–2500 nm) concealed imaging for identifying smuggled baggage based on organic small molecule luminescent material was studied. A small molecule luminescent material (Y6) with the excitation in invisible near-infrared (NIR) light was used to generate fluorescence-emission in NIR-II region. The traceless Y6 sprayed on the surface of criminal luggage does not emit light under visible light irradiation, but shows hidden NIR-II signs under NIR excitation that can prevent the suspects to find abnormality and lose their luggage. The Y6 organic luminescent material has invisibility, low toxicity, long-term fluorescence stability and high transparency. This NIR-II fluorescence imaging technology can be used as a new type of concealed baggage marking, which is of great significance to help the customs combat smuggling crimes.

Solvation effect on photophysical properties and ESIPT behaviours for 2-benzooxazol-2-yl-6-thiophen-2-yl-phenol fluorophore: a theoretical study

Inspired by its potential applications of organic luminescence and fluorescence probe materials for 2-(2′-hydroxyphenyl)benzazoles (HBX) derivatives, this study mainly investigates the excited-state behaviours of a novel 2-benzooxazol-2-yl-6-thiophen-2-yl-phenol (BYTYP) fluorophore. Theoretical exploration has been conducted on the solvent-dependent interactions of excited-state intramolecular hydrogen bonding and the process of excited-state intramolecular proton transfer (ESIPT) for BYTYP. By combining optimised geometrical modifications, infrared (IR) vibrational spectra and the core-valence bifurcation (CVB) index, hydrogen bonding strengthening can be confirmed. Predicting the bond energy (E HB), we assert that nonpolar solvents are more conducive to enhancing hydrogen bonding interactions. The ESIPT tendency of BYTYP is further elucidated by charge reorganisation resulting from photoexcitation. By exploring potential energy surfaces (PESs) and identifying transition states (TS), we have uncovered the solvent-polarity-controlled ESIPT behaviours. We hope these insights into excited-state dynamics will facilitate the design and development of novel fluorescent sensors in future.

Boosted charge carrier concentration via luminescent quenching of photoexcited electrons in CdS/FI S-scheme heterostructures for efficient photocatalytic hydrogen production

One of the key strategies to go beyond the limited hydrogen production efficiency during photocatalytic process is to increase effective charge carrier concentration participating hydrogen evolution reaction (HER). Herein, considering organic luminescent materials as “photo-excited electron factory” with large numbers of exited carriers under light irradiation, 2-Fluorene-4,5-diphenylimidazole/CdS (FI/CdS) S-scheme heterojunction was constructed to modulate charge transfer via extracting abundant of photoexcited electrons in FI as luminescent quenching process. The impeded charge recombination in FI/CdS S-scheme heterojunction gave rise to 6 orders of magnitude higher in carriers concentration and 7.8 times higher in H2 evolution rate for CF20 (9.67 × 1026 cm−3, 5.28 mmol h−1 g−1) than that of pure CdS (2.69 × 1020 cm−3, 0.68 mmol h−1 g−1). It demonstrates that quenching photo-excited electrons in luminescent component through synergistic organic-luminescent/semiconductor interaction could be a creative and prominent strategy to enhance photocatalytic performance.

UV-curing-enhanced Organic Long-Persistent Luminescence Materials.

Amorphous organic long-persistent luminescence materials (OLPLMs) can realize simpler solution processing and large-area uniform luminescence, which luminescent properties are significantly influenced by the rigid environment. However, research on utilizing the rigidity to promote long-persistent luminescence (LPL) properties of amorphous OLPLMs is still relatively rare due to the lack of an unambiguous and effective strategy to construct the rigid environment. Here, we propose a universal strategy to enhance the LPL performance of organic host-guest doping systems by UV curing, which utilizes the rigid environment constructed by UV curing to promote the interaction between host and guest, thus inducing the generation of highly efficient LPL performance. This solution-processable, large-area and "easy to realize" material fabrication strategy can make amorphous OLPLMs show broader application prospects in some fields, such as anti-counterfeiting, non-destructive detection, and pattern marking or indication. This article is protected by copyright. All rights reserved.

Linkages Make a Difference in the Photoluminescence of Covalent Organic Frameworks

AbstractCovalent organic frameworks (COFs) with structural designability and tunability of photophysical properties enable them to be a promising class of organic luminescent materials by incorporating well‐designed fluorescent units directly into the periodic skeletons. The photophysical properties of COFs are mainly affected by the structural features, which determine the conjugation degree, charge delocalization ability, and exciton dynamics of COFs. To understand the relationship between COF structures and their photophysical properties, two COFs with the same pyrene chromophore units but different linkages (imine or vinylene) were designed and synthesized. Interestingly, different linkages endow COFs with huge differences in solid‐state photoluminescence quantum yield (PLQY) for imine‐ and vinylene‐linked pyrene‐based COFs, which possess PLQY values of 0.34 % and 15.43 %, respectively. The femtosecond‐transient absorption spectra and time‐dependent density functional theory reveal the different charge‐transfer pathways in imine‐ and vinylene‐linked COFs, which influence the exciton relaxation way and fluorescence intensity. In addition, an effective white‐light device was obtained by coating the vinylene‐linked COF on a light‐emitting diode strip.

Linkages Make a Difference in the Photoluminescence of Covalent Organic Frameworks.

Covalent organic frameworks (COFs) with structural designability and tunability of photophysical properties enable them to be a promising class of organic luminescent materials by incorporating well-designed fluorescent units directly into the periodic skeletons. The photophysical properties of COFs are mainly affected by the structural features, which determine the conjugation degree, charge delocalization ability, and exciton dynamics of COFs. To understand the relationship between COF structures and their photophysical properties, two COFs with the same pyrene chromophore units but different linkages (imine or vinylene) were designed and synthesized. Interestingly, different linkages endow COFs with huge differences in solid-state photoluminescence quantum yield (PLQY) for imine- and vinylene-linked pyrene-based COFs, which possess PLQY values of 0.34 % and 15.43 %, respectively. The femtosecond-transient absorption spectra and time-dependent density functional theory reveal the different charge-transfer pathways in imine- and vinylene-linked COFs, which influence the exciton relaxation way and fluorescence intensity. In addition, an effective white-light device was obtained by coating the vinylene-linked COF on a light-emitting diode strip.

Unraveling the substituent group effects on the emission properties of fluorenone derivatives

Elucidating the structure–property relationship is vital for developing organic luminescent materials with excellent properties and performance optimization. Herein, we designed and synthesized three types of fluorenone-based molecules named FO-Ph, FO-Na and FO-An, with similar structures but different sizes of substituent groups (benzene, naphthalene and anthracene). This slight modification resulted in significantly different optical properties of luminogens in dilute solutions and powders. FO-Ph as an aggregation-caused quenching (ACQ) emitter displayed strong emissions in solution; moreover, after the addition of a poor solution, the fluorescence intensity of FO-Ph gradually decreased. On the contrary, FO-Na displayed aggregation-induced emission enhancement properties, and FO-An was non-emissive in solutions but displayed enhanced emission owing to its molecular aggregation, indicating that FO-An comprises an AIE-active molecule. The characterization of the photophysical properties, theoretical calculations and an X-ray single crystal analysis showed that the emission properties of the three compounds could be affected by the size of substituent groups, twisted geometry and molecular packing modes. The three fluorenone luminogens exhibited different force-induced emission enhancement properties. Moreover, FO-Ph can be used as a water detector and showing high sensitivity. This study elucidates the AIE and ACQ phenomena of fluorenone complexes and provides effective ways for developing organic luminescent materials.

Manipulating the excited states from charge-transfer to hybridized local and charge-transfer towards high-performance blue electroluminescence

Manipulating the excited states of organic luminescent materials can efficiently improve the utilization of both singlet and triplet excitons for developing high-performance organic light-emitting diodes (OLEDs), but the issue remains difficult due to the lack of well-controlled ways. Here, we proposed a molecular design strategy of excited state manipulation from charge-transfer (CT) to hybridized local and charge-transfer (HLCT) via adjusting the cyano position on pyridine acceptor. The meta-substituted PyAn4CN is mainly composed of a CT component, while the para-substituted PyAn5CN is endowed with a HLCT component. On further extending the conjugation of PyAn5CN by inserting a benzene unit between the pyrene and anthracene core, the HLCT character is preserved in PyPhAn5CN, accompanied by a faster radiative decay. Consequently, the vacuum-evaporated OLEDs exhibit blue electroluminescence (EL) with the emission peaks in the range of 455–460 nm and high external quantum efficiency (EQE) up to 7.52%, together with well-suppressed efficiency roll-offs of 0.8% and 3.7% at the luminance of 100 and 1000 cd m-2, respectively. More importantly, the solution-processed device shows an excellent performance with EQE of 6.49%, which is one of the best results in the solution-processed HLCT OLEDs. Our results clearly indicate that the formation of HLCT state is an efficient way to realize high-efficiency blue electrofluorescence.

Synthesis and Photophysical, Electrochemical, and DFT Studies of Piperidyl and Pyrrolidinyl Chalcones.

Small organic molecules with interesting optical and electrochemical properties find applications as organic luminescent materials. In this work, we report the synthesis of novel chalcones with D-A-D and D-A-D-A architecture, followed by their optical, electrochemical, and computational studies. The absorption band of these compounds occurs at 360-480 nm with emission maxima appearing around 513-552 nm. The large Stokes shifts (Δλ) for all compounds (90-132 nm) suggest intramolecular charge transfer (ICT) in the excited states. The molar absorptivity and fluorescence quantum yields were found to be in the range of 1.7-4.26 × 104 M-1 cm-1 and 0.29-0.39, respectively. The electrochemical parameters were determined by using cyclic voltammetry (CV). Density functional theory (DFT) calculations of all compounds were made by using B3LYP/G (d,p) functionals in chloroform and were found to have a good correlation with experimental results. Preliminary studies of absorption, photoluminescence, CV, and their theoretical correlation suggest that these compounds may be optimized for their applications in optoelectronics, sensing, and bioimaging.

Aggregation‐induced delayed electrochemiluminescence of organic dots in aqueous media

AbstractFull utilization of the excited species at both singlet states (1R*) and triplet states (3R*) is crucial to improving electrochemiluminescence (ECL) efficiency but is challenging for organic luminescent materials. Here, an aggregation‐induced delayed ECL (AIDECL) active organic dot (OD) containing a benzophenone acceptor and dimethylacridine donor is reported, which shows high ECL efficiency via reverse intersystem crossing (RISC) of non‐emissive 3R* to emissive 1R*, overcoming the spin‐forbidden radiative decay from 3R*. By introducing dual donor‐acceptor pairs into luminophores, it is found that nonradiative pathway could be further suppressed via enhanced intermolecular weak interactions, and multiple spin‐up conversion channels could be activated. As a consequence, the obtained OD enjoys a 6.8‐fold higher ECL efficiency relative to the control AIDECL‐active OD. Single‐crystal studies and theoretical calculations reveal that the enhanced AIDECL behaviors come from the acceleration of both radiative transition and RISC. This work represents a major step towards purely organic, high‐efficiency ECL dyes and a direction for the design of next‐generation ECL dyes at the molecular level.

Electrochemiluminescence biosensing of spatially conjugated ester groups triggered by smart bridging between nano-silica and poly (ethylene maleate)

Organic luminescent materials with unique photoelectric properties are as a kind of emerging luminophore at the forefront of research in biological detection on account of their facilely varied functional groups. However, most organic luminophores are aromatic compounds with certain biological toxicity to limit their application. Herein, we envisioned a green organic electrochemiluminescencence (ECL) luminophore without aromatic ring structure but electron-rich functional groups via in-situ growth of poly (ethylene maleate) (PEM) on nano-silica (NS). The experimental and theoretical modelling results indicate that ECL signal of PEM-NSs was enhanced by the spatial conjugation of ester groups (ester clusters), due to the contribution of smart bridging of NS with different sizes. The smaller the NS size is, the more growth of ester groups on the NS surface is, and the closer individual ester groups are linked via “through-space interaction” to form stable ester clusters with smaller energy gaps, thus achieving high ECL performance. For further application, PEM-NS-1 was selected as meaningful ECL luminophore, and integrated with exonuclease III-assisted catalytic hairpin assembly amplification strategy, an efficient platform for biomarkers (microRNA-21) detection was constructed with the detection limit of 2.73 fM (S/N = 3). This work sets the stage for strengthening ECL of electron-rich functional groups, and provides a way of thinking for low-toxicity organic luminophores.

Aggregation‐induced narrowband isomeric fluorophores for ultraviolet nondoped OLEDs by engineering multiple nonbonding interactions

AbstractTraditional donor‐acceptor type organic luminescent materials usually suffer from unfavorable spectral broadening and fluorescence quenching problems arising from strong inter/intra‐chromophore interactions in aggregation state. Here, two ultraviolet carbazole‐pyrimidine isomers (named o‐DCz‐Pm and m‐DCz‐Pm) with novel aggregation‐induced narrowband phenomenon are constructed and systematic investigated by experiments and theoretical simulations. Benefitting from strengthened steric hindrance and multiple noncovalent interactions, the nonradiative decay, vibrational motion, and structural relaxation of singlet state can be effectively suppressed in aggregation state. Consequently, the electroluminescence peak of 397 nm, full width at half maximum of 21 nm and external quantum efficiency of 3.4% are achieved simultaneously in nondoped o‐DCz‐Pm‐based device. This work paves an avenue toward the development of high‐performance narrowband nondoped ultraviolet materials and organic light‐emitting diodes.