Highly Efficient Electrofluorescence Material Based on Pure Organic Phosphor Sensitization*.

Pure organic room-temperature phosphorescence (RTP) materials are considered as potential candidates for replacing precious metal complexes to fabricate highly efficient organic light-emitting devices (OLEDs). However, applications of the reported RTP materials in OLEDs are seriously impeded by their low photoluminescence quantum yields (PLQYs) in a thin film state. To overcome these obstacles, we established a new strategy to construct highly efficient OLEDs based on a pure organic RTP material sensitized fluorescence emitter by selecting benzimidazole-triazine molecules (PIM-TRZ), 2,6-di(phenothiazinyl)naphthalene (ß-DPTZN), and 5,6,11,12-tetraphenylnaphthacene (rubrene) as host, phosphor sensitizer, and fluorescent emitter, respectively. The perfect combination of host, phosphorescent sensitizer, and fluorescent emitter in the emitting layer ensure the outstanding performance of the devices with an external quantum efficiency (EQE) of 15.7 %.

An Electroactive Pure Organic Room-Temperature Phosphorescence Polymer Based on a Donor-Oxygen-Acceptor Geometry.

An electroactive room-temperature phosphorescence (RTP) polymer has been demonstrated based on a characteristic donor-oxygen-acceptor geometry. Compared with the donor-acceptor reference, the inserted oxygen atom between donor and acceptor can not only decrease hole-electron orbital overlap to suppress the charge transfer fluorescence, but also strengthen spin-orbital coupling effect to facilitate the intersystem crossing and subsequent phosphorescence channels. As a result, a significant RTP is observed in solid states under photo excitation. Most noticeably, the corresponding polymer light-emitting diodes (PLEDs) reveal a dominant electrophosphorescence with a record-high external quantum efficiency of 9.7 %. The performance goes well beyond the 5 % theoretical limit for typical fluors, opening a new door to the development of pure organic RTP polymers towards efficient PLEDs.

Pure Organic Persistent Room-Temperature Phosphorescence at both Crystalline and Amorphous States.

Persistent room-temperature phosphorescence (p-RTP) of pure organic materials is attracting increasing attention. The design of efficient phosphors and understanding the origin of p-RTP, however, remain challenging. Herein, to gain further insights into pure organic p-RTP, we prepared a group of carbazole (CZ) and methyl benzoate (MBA) conjugates with a methyl ester unit at the para (4-MBACZ), meta (3-MBACZ), and ortho (2-MBACZ) sites. These isomers merely produce prompt fluorescence in solutions, but generate blue prompt/delayed fluorescence (DF) and orange p-RTP with lifetimes up to 865.2 ms in the crystalline state. Lifetimes of p-RTP are in the order of 2-MBACZ>3-MBACZ>4-MBACZ, which might be mainly ascribed to the combined effect of packing density, intermolecular interactions and steric hindrance. Meanwhile, upon mechanical grinding, while the emission color and profile of the luminogens do not significantly change, no (4-MBACZ) or shortened (3-MBACZ, 2-MBACZ) p-RTP is observed, accompanying the change from crystalline to amorphous states. Such p-RTP at amorphous states without external hosts is rarely reported, which demonstrates important implications for the molecular design and mechanism understanding towards p-RTP. Furthermore, their p-RTP attribute and different emission colors before and after ceasing the UV irradiation endow them promising applications in encryption and anticounterfeiting fields.

Improved impurity determination of pure organic substances by differential scanning calorimetry with a dynamic method.

Differential scanning calorimetry can be used to measure the impurity contents of pure organic substances on the principle of freezing-point depression. Impurity determination by differential scanning calorimetry with a dynamic method, which has the advantages of speediness and convenience, remains to be explored. Here, a series of acetanilide and dibenzothiophene samples with various purities was prepared through zone melting, and the samples were then analyzed by gas chromatography-mass spectrometry. A modified dynamic method, including encapsulating the analyte in a volatile pan through cold welding, remelting the analyte with a low heating rate, calculating the melted fraction considering the area of the tailing under the heat-flow curve, and reducing the error from solid-solution formation, is proposed. Encapsulating with a volatile pan using a proper torque gave an accurate result. Remelting gave a lower impurity content and a more narrow and sooth peak of heat-flow compared with the first melting. The impurity-content results calculated by the modified method were usually higher than those calculated by the ASTM standard method. For acetanilide and dibenzothiophene with impurity contents of less than 0.30%, the modified dynamic method showed good accuracy. The proposed method is applicable to determination of reference materials of organic substances with high purity owing to its accuracy and convenience.

Efficient Room-Temperature Phosphorescence from Discrete Molecules Based on Thianthrene Derivatives for Oxygen Sensing and Detection.

In this work, two thianthrene (TA) derivatives, 1-phenylthianthrene (TA1P) and 2-phenylthianthrene (TA2P), were synthesized with single-phenyl modification for pure organic discrete-molecule room-temperature phosphorescence (RTP). They both show the dual emission of fluorescence and RTP in amorphous polymer matrix after deoxygenation, as a result of a new mechanism of folding-induced spin-orbit coupling (SOC) enhancement. Compared with TA1P, TA2P exhibits a higher RTP efficiency and a larger spectral separation between fluorescence and RTP, which is ascribed to the substituent effect of TA at the 2-position. With decreasing oxygen concentration from 1.61% to 0%, the discrete-molecule TA2P shows an about 18-fold increase in RTP intensity and an almost constant fluorescence intensity, which can make TA2P as a self-reference ratiometric optical oxygen sensing probe at low oxygen concentrations. The oxygen quenching constant (K SV) of TA2P is estimated as high as 10.22 KPa-1 for polymethyl methacrylate (PMMA)-doped film, and even reach up to 111.86 KPa-1 for Zeonex®-doped film, which demonstrates a very high sensitivity in oxygen sensing and detection. This work provides a new idea to design pure organic discrete-molecule RTP materials with high efficiency, and TA derivatives show a potential to be applied in quantitative detection of oxygen as a new-generation optical oxygen-sensing material.

Ultralong Organic Phosphorescent Nanocrystals with Long-Lived Triplet Excited States for Afterglow Imaging and Photodynamic Therapy.

The development of novel applications of ultralong organic phosphorescent (UOP) materials is highly desired. Herein, a series of UOP materials (EDCz, E = O, S, Se, and Te) for bacterial afterglow imaging and photodynamic therapy (PDT) is reported. By structurally bonding with the chalcogen atoms with π-conjugated scaffolds, EDCz not only absorbs visible light but also emits UOP with an efficiency of ca. 0.01-6.8% and a long lifetime of 0.08-0.318 s under ambient conditions. Benefiting from the long-lived triplet excited states, the SeDCz nanocrystals (NCs) possessed the best optical properties in the series, generating 1O2 under white light irradiation and performing as an agent for Staphylococcus aureus afterglow imaging and PDT at a low concentration (98 ng mL-1). The SeDCz NCs are also utilized as real-time UOP imaging agents and promoted healing of infected wounds in living mice. To the best of our knowledge, this study presents the first example of UOP-based bacterial photodynamic theranostic agents and creates a platform for the next-generation efficient UOP-based photosensitizers for bioimaging and skin regeneration.

Achieving Persistent, Efficient, and Robust Room-Temperature Phosphorescence from Pure Organics for Versatile Applications.

Pure organic persistent room-temperature phosphorescence (p-RTP) under ambient conditions is attractive but challenging due to the slow intersystem crossing process and susceptibility of triplet excitons. Fabrication of pure organic RTP luminogens with simultaneously high efficiency and ultralong lifetime still remains a daunting job, owing to their conflicting requirements for the T1 nature of (n,π*) and (π,π*) characteristics, respectively. Herein, a group of amide-based derivatives with efficient p-RTP is developed through the incorporation of spin-orbital-coupling-promoting groups of carbonyl and aromatic π units, giving impressive p-RTP with lifetime and efficiency of up to 710.6 ms and 10.2%, respectively. Furthermore, two of the luminogens demonstrate intense p-RTP after vigorous mechanical stimulation, indicating their robust nature, which is rarely encountered. Efficient and robust p-RTP even in the amorphous state endows them promising potential for encryption and bioimaging with facile fabrication processes. A bioimaging study with live mice indicates that such highly robust p-RTP is tremendously beneficial for in vivo afterglow imaging with an ultrahigh signal-to-background ratio of 428. These results strongly imply the possibility of realizing efficient and robust p-RTP from pure organics even without meticulous protection, thus paving the way to their promising and versatile applications.

High-Efficiency WOLEDs with High Color-Rendering Index based on a Chromaticity-Adjustable Yellow Thermally Activated Delayed Fluorescence Emitter.

A chromaticity-adjustable yellow thermally activated delayed fluorescence (TADF) material, PXZDSO2 as a triplet harvester provides a rational device concept, giving two-color and three-color pure organic white organic light-emitting diodes (WOLEDs) with unprecedented color-rendering index of 95 and external quantum efficiency of 19.2%.

Achieving Persistent Room Temperature Phosphorescence and Remarkable Mechanochromism from Pure Organic Luminogens.

Persistent room temperature phosphorescence (RTP) from pure organic luminogens can be rationally realized based on the crystallization-induced phosphorescence phenomenon and severe crystallization. A perfect crystal with dense molecular packing and effective inter-molecular interactions isolates the triplet excitons from quenching sites and significantly blocks the high-energy vibrational dissipation, thus yielding long-lasting RTP.