Matthew P. Aldred

Recent advances in mechano-responsive luminescence of tetraphenylethylene derivatives with aggregation-induced emission properties

Since the realization in 2011 that most aggregation-induced emission (AIE) molecules exhibit mechano-responsive luminescence (MRL), research regarding the MRL of AIE molecules has drawn much attention, and this area has been expanding tremendously. As one of the most extensively studied AIE cores, tetraphenylethylene (TPE) has been widely used to construct MRL molecules. This review will focus on recent advances in MRL of TPE derivatives with AIE properties, including a brief history of mechano-responsive AIE-active materials, mechanistic studies on MRL, mechano-responsive luminogens based on TPE, mechano-responsive luminogens containing multiple AIE-active units, mechano-memory chromism and mechanoluminescence of TPE derivatives. Moreover, this review will give a perspective on the possible opportunities and future challenges that exist in this research area.

An efficient yellow thermally activated delayed fluorescence emitter with universal applications in both doped and non-doped organic light-emitting diodes

Thermally activated delayed fluorescence (TADF) emitters have aroused serious attention for their use in organic light-emitting diodes (OLEDs). However, TADF emitters providing both doped and non-doped OLEDs with high device performance are rare. In this work, we present a yellow TADF emitter (OPDPO) meeting the above requirements. The OPDPO-based doped OLEDs show a maximum external quantum efficiency (EQE) of 26.7%, which is amongst the highest ever reported for doped yellow TADF-based OLEDs. The EQE remains as high as 22.1% at a luminance of 1000 cd m−2. Meanwhile, the OPDPO-based non-doped OLEDs afford a maximum EQE of 16.6% at a high luminance of 3600 cd m−2, which is amongst the best performances of non-doped yellow TADF-based OLEDs. Moreover, simple and efficient white OLEDs are implemented by combining a blue conventional fluorescent emitter with either a doped or non-doped OPDPO emissive layer, confirming the universal applications of OPDPO in high-performance OLEDs.

Hydrogen bonding-assisted loosely packed crystals of a diaminomaleonitrile-modified tetraphenylethene compound and their photo- and mechano-responsive properties

In this study, loosely packed crystals with the building blocks of aggregation induced emission (AIE)-active TPE-MN molecules were constructed with the assistance of intermolecular hydrogen bonding. The AIE properties of the compound TPE-MN were studied. From single crystal analyses, a sandwich-like crystal structure with tightly packed areas (TPE moieties) and loosely packed areas was observed. Nevertheless, the TPE-MN crystals display non-emissive properties in the loosely packed crystalline state. Differential scanning calorimetry (DSC) studies, powder X-ray diffraction analyses and various photophysical studies further reveal that the loosely packed crystal structure can provide enough space for significant molecular rotations and vibrations to occur, which leads to emission quenching. By breaking the loosely packed crystal structure with mechanical force or UV irradiation, the emission band with a maximum at ca. 562 nm can be turned on. These results can serve as a basis for constructing multi-responsive materials with turn-on emissive properties.

Highly-efficient fully non-doped white organic light-emitting diodes consisting entirely of thermally activated delayed fluorescence emitters

Thermally activated delayed fluorescence (TADF) emitters can harvest singlet and triplet excitons for light emission to afford highly efficient organic light-emitting diode (OLED) devices, especially for all-TADF white OLEDs (WOLEDs). However, the majority of TADF emitters suffer from concentration quenching effects and require complicated doping techniques for device fabrication. Herein, we demonstrate a yellow TADF emitter that enables non-doped OLED systems, which is beneficial for the simplification of the OLED device structure and fabrication. The non-doped yellow OLED achieves a maximum external quantum efficiency (EQE) of 16.7% at a high luminance of 3600 cd m−2, in which such a high magnitude is rarely reported. More significantly, by the combination of the yellow TADF emitter with a blue TADF emitter, a fully non-doped emissive layer (EML) strategy has been demonstrated for the fabrication of an all-TADF WOLED, in which the non-doped yellow and blue TADF emissive layers are closely stacked together without any interlayers. The fully non-doped WOLEDs show a maximum EQE of up to 19.8%, along with high quality white-light emission. This is the first report regarding the fully non-doped all-TADF WOLEDs, and the presented design strategy provides a universal route towards the fabrication of simple and efficient WOLEDs.

Efficient triplet harvesting in fluorescence–TADF hybrid warm-white organic light-emitting diodes with a fully non-doped device configuration

Warm-white organic light-emitting diodes (WOLEDs) fabricated with hybrid fluorescence-based emitters, consisting of traditional fluorescence and thermally activated delayed fluorescence (TADF) emitters can achieve high efficiency without the use of expensive organometallic phosphors. However, most of these types of OLEDs require complicated fabrication techniques, especially host–guest doping systems and multiple interlayers. Herein, an efficient design strategy for hybrid fluorescence WOLEDs is presented, in which a traditional blue fluorescence emitter is simply stacked together with a yellow TADF emitter without any interlayers. Due to the high triplet excited state of the chosen blue emitter, its intrinsically non-radiative triplet excitons can be efficiently utilized for light emission, by exoergic transfer to the adjacent yellow TADF emitter. Following this idea, a maximum external quantum efficiency of 20.8% at a high luminance of 400 cd m−2 is achieved for a fully non-doped WOLED when using a blue fluorescent emitter with the aggregation-induced emission property. This strategy provides a universal approach for the fabrication of highly-efficient fluorescence–TADF hybrid WOLEDs based on the fully non-doped device configuration.

Alkyl Chain Introduction: In Situ Solar‐Renewable Colorful Organic Mechanoluminescence Materials

Mechanoluminescence (ML) materials are environmentally friendly and emit light by utilizing mechanical energy. This has been utilized in light sources, displays, bioimaging, and advanced sensors. Organic ML materials are strongly limited to application by inu2005situ unrepeatable ML. Now, inu2005situ solar-renewable organic ML materials can be formed by introducing a soft alkyl chain into an ML unit. For the first time, the ML from these polycrystalline thin films can be iteratively produced by simply recrystallizing the fractured crystal inu2005situ after a contactless exposure to sunlight within a short time (≤60u2005s). Additionally, their ML color and lifetime can be also easily tuned by doping with organic luminescent dyes. Therefore, large-area sandwich-type organic ML devices can be fabricated, which can be repeatedly used in a colorful piezo-display, visual handwriting monitor, and sensitive optical sensor, showing a lowest pressure threshold for ML of about 5u2005kPa.

Intrinsic low dielectric constant polyimides: relationship between molecular structure and dielectric properties

Low dielectric organic materials have been studied extensively, however, the relationship and regularity between the molecular structure and the dielectric properties is still not clear and barely reported. In this study, we report the synthesis of three polyimides (PPy6F, mBPPy6F and mTPPy6F) containing the same diphenylpyridine core structure but different side-chains with varied number of benzene rings. The polyimides PPy6F, mBPPy6F and mTPPy6F which contain one, two and three benzene rings in the pendant group, respectively, show intrinsic dielectric constant k values of 2.81 (PPy6F), 2.61 (mBPPy6F) and 2.44 (mTPPy6F). The reason for the reduced k value with increasing number of benzene rings in the pendant group is discussed through the investigation of morphology, density and water absorption properties of these polyimides. This discovery provides us with a new and easy strategy to obtain lower dielectric polymers. Polyimide mTPPy6F shows excellent comprehensive properties, with a high glass transition temperature (Tg) of 342 °C, a 5 wt% loss temperature (Td,5%) of 551 °C, tensile strength of 105 MPa, low moisture absorption of only 0.61% and good solubility in common organic solvents.

Design, synthesis and photochromism studies of thienyl containing triarylethylene derivatives and their applications in real-time photoresponsive surfaces

Thienyl containing triarylethylene derivatives, namely 2ThDpF and 3ThDpF, with photochromic properties have been designed and synthesized. These new photochromic molecules with simple chemical structures show fast-response and striking photochromic behaviors in the solution-state, in the solid-state and in polymer films. Based on 2ThDpF, a real-time and repeatable photoresponsive surface was fabricated. Upon UV-light irradiation for 1 minute, most of the 2ThDpF nano-aggregates on the SiO2 substrate changed from cone-shaped to hump-shaped and the contact angles of a water droplet drastically increased from 43° to 95° simultaneously. The surface morphology and wettability can easily be reverted by white-light irradiation for 5 minutes. The key issues to affect the real-time morphology and wettability changes are also discussed in-depth based on single-crystal analyses and TDDFT calculations. Therefore, these triarylethylene derivatives with simple molecular structures are attractive in the areas of photochromism and photoresponsive surfaces.

Flexible Multifunctional Aromatic Polyimide Film: Highly Efficient Photoluminescence, Resistive Switching Characteristic, and Electroluminescence

We report a flexible multifunctional aromatic polyimide (BTDBPI) that shows yellow-green fluorescence with high photoluminescence quantum yield (PLQY) of 30% in the film state. The nonvolatile write once-read many (WORM) characteristic in a memory device with the configuration of ITO/BTDBPI/Au indicates that BTDBPI possesses organic semiconductor behavior. Moreover, polymer light-emitting diodes (PLEDs) with the structure of ITO/PEDOT:PSS/BTDBPI/TPBI/Mg-Ag exhibits an interesting dual-emission phenomenon that originates from the electroluminescence (EL) of the BTDBPI nanometer film (yellow-green, 525 nm) and TPBI (deep blue, 380 nm), demonstrating that BTDBPI shows both the charge-transporting and EL properties.