Materials Chemistry Frontiers Blog

Organic molecule-based crystalline dielectric materials have attracted broad attention from chemists in recent years to develop new organic electronic devices. In their designing strategy, the molecular motion induced by the external dielectric field is required to maximize the polarization effect in the materials to realize a large dielectric constant.

When people pay attention to the molecular motion in curved-π aromatics and their supramolecular complexes, it is assumed that the “curve-to-curve” contact in the curved-π aromatics will afford the smooth molecular motion in the solid state. Sumanene (1) is one of the representative buckybowls and is known to show unique properties such as bowl inversion behaviour derived from its unique bowl shape and is recognised as the potential molecular switch (Figure 1a). However, another feature of 1 to form unidirectionally arraigned π-stacking columns in the solid state makes the bowl inversion behaviour of 1 useless for the switching applications due to the large bowl-inversion energy in the 1D-column formation.

Figure 1. a) Molecular structures of 1 and 2 and schematic diagrams of the features of 1. b) Pendulum-like in-plane motion of 2 exhibiting a dielectric response in an electric field.

Recently, the group of Osaka University and collaborators of Tohoku University, Tokyo Institute of Technology and Kyoto University have demonstrated that the in-plane motion of disfluorinated sumanene 2 in the solid state is applicable as the source of stimuli responsive function instead of the bowl inversion to bring out the dielectric response (Figure 1b).

The group has focused that 2 possesses two fluorine atoms on the same benzylic carbon on pristine sumanene to possess a large dipole moment along the in-plane direction and gives the isostructural crystalline packing to 1. Thermal analyses, variable temperature X-ray diffraction and IR measurements indicated the presence of pendulum-like in-plane motion of 1 at high temperature region although no clear phase transition was involved. Indeed, the dielectric measurement using its both powder and single crystal clearly showed that both real (ε₁) and imaginary (ε₂) parts of the dielectric constant were enhanced above ~360 K at 1 MHz with a Debye-type dielectric relaxation, confirming the in-plane motion of 2 induced by the external electric field (Figure 2).

Figure 2. a) Schematic model of the relationship between the single-crystal shape of 2 and the applied electric field. b) to e) Temperature dependence of b), d) the real part (ε1) and c), e) the imaginary part (ε2) of the dielectric constant of 2 in a single-crystalline form measured at various frequencies. The direction of the electric field applied for b) and c): parallel to the c axis; for d) and e): orthogonal to the c axis.

These results, focusing on the in-plane molecular motion in the π-stacking column of a buckybowl, will help to provide a better understanding of the dynamics of solid-state curved-π systems and will accelerate their application in functional materials.

Corresponding author:

Yumi Yakiyama is an associate professor in the Division of Applied Chemistry at Osaka University (Japan). She got a PhD degree in Chemistry at Osaka University in 2010. From 2010 to 2015 she did postdoctoral studies at POSTECH (Korea). The research field of Professor Yumi Yakiyama is physical organic chemistry especially about functional organic crystals as well as metal-organic and pure organic frameworks.

https://researchmap.jp/yumiyakiyama

https://www-chem.eng.osaka-u.ac.jp/~sakurai-lab/

Deep blue electroluminescence is highly required for organic light-emitting diode (OLED) technology. However, designing fluorophores displaying adequate CIE coordinates and particularly a low CIEy is far from an easy task. We report in this work the synthesis, the physico-chemical properties and the application in OLED of deep blue emitters constructed on the dibenzosuberene (DBS) molecular fragment. Three emitters, SPA-DBS, SIA-DBS and SQPTZ-DBS, have been constructed following a similar molecular design strategy that is the spiro connection of an electron rich unit, namely N-phenylacridine (PA), indoloacridine (IA) or quinolinophenothiazine (QPTZ) to the DBS core. The PA, IA and QPTZ fragments are known to be efficient hole injecters due to their strong electron-rich character. Through a structure/properties relationship study, the group of Prof Cyril Poriel (Institut des Sciences Chimiques de Rennes- UMR 6226, Rennes) reports the electrochemical, photophysical and thermal behaviours of these three emitters.

The resulting organic materials display similar LUMO levels lying at ca -2.30 eV and different HOMO levels driven by the donor unit comprised between -5.22 and -5.48 eV. The spiro-configuration allows maintaining high T_g and T_d in accordance with OLED application. SPA-DBS displays a deep-blue emission with CIE of (0.16, 0.04), reaching an EQE of ca 1% and possessing a very low CIEy coordinate of 0.04. This CIEy coordinate fits the NSTC, ITU and EBU standards. This work not only reports a deep blue emitter for OLED but also shed light on interesting properties displayed by the DBS fragment, such as its low LUMO energy level, ca -2.3 eV, which is significantly decreased compared to its counterpart fluorene. This particularity can be advantageously used in further designs to favour the electron injection in electronic devices.

Corresponding Author:

Cyril Poriel received his PhD in 2003 from the University of Rennes 1. After a postdoctoral stay at the University of Exeter (UK), he joined the CNRS (Institut des Sciences Chimiques de Rennes) in 2005, where he is currently CNRS Research Director. His main research interest deals with the design of π-conjugated architectures for Organic Electronics. He is author/co-author of more than 120 publications, reviews and book chapters.

There are a number of natural product extracts that contain pharmaceutically active ingredients or APIs. These APIs are extracted alongside a host of other compounds such as terpenes, fatty acids, sterols and waxes that naturally occur within the plant. A very topical example are the family of cannabinoids found in hemp extract. Isolating these cannabinoids is a multi-step, energy and solvent intensive process.

Recent work at the Green Chemistry Centre of Excellence, University of York, has shown that mesoporous materials made from naturally derived alginic acid, known as Starbons, work incredibly well at isolating cannabinoids in a single step. Here the Starbon is used in solid phase extraction (SPE) to purify a host of cannabinoids by solvent selection (Figure 1).

Figure 1 – Simple depiction of isolation of cannabinoids from hemp extract

Starbons are able to act in highly selective SPE for two reasons, the first being that their mesopores are large enough (2-50 nm) for complex compounds to enter and leave. The second is the temperature at which they are pyrolysed allows tuning of surface chemistry from hydrophilic to hydrophobic. By screen Starbons produced at 300, 450 and 800 °C, adsorption and desorption solvents (hexane and ethanol respectively) and contact time (30 seconds), we were able to isolate 93% of cannabinoids (by GC-FID) from the extract in a single step (Figure 2). The process was also validated using in line extraction and isolation in supercritical carbon dioxide, where the feedstock was hemp dust. This was especially interesting as the cannabinoid content in this industrial by-product is significantly lower than in hemp top flowers.

Figure 2 – GCchromatograph of hemptops; Crude C. sativa extract, Desorption phase (ethanol)after passing through A300.

Corresponding Author:

Dr Con Robert Mcelroy

Dr Con Robert McElroy (Rob) is a senior researcher at the Green Chemistry Centre of Excellence, Department of Chemistry, University of York. Rob gained his Ph.D in 2007 at Keele University working on the production of composite materials from copolymers incorporating renewable resources. He was a postdoc working on carbonate chemistry for two years at Ca Foscari University of Venice and joined the Green Chemistry Centre of Excellence, University of York as a PDRA in 2011. His current role is as Deputy Director of the Circa Renewable Carbon Institute which focuses on reactions and/or applications of levoglucosenone and the bio-derived solvent Cyrene (developed at the University of York and which he is a co-inventor of). He has published 3 book chapters, 39 papers, 4 reviews and 3 patents and has a H index of 20.

In 2019 he became chief technical officer of Starbons Ltd in which time it has become revenue generating, award winning and has won funding/made sales in relation to projects he managed.

https://www.researchgate.net/profile/Con-Mcelroy

We are delighted to announce the Best Materials Chemistry Frontiers Covers of 2021!

Read below the scientific papers.

Achieving metal-free phosphorescence in dilute solutions for imaging hypoxia in cells and tumors

Peng Li, Yurong Guo, Yan Jia, Hongwei Guan, Chao Wang, Zibo Wu, Shuqing Sun, Zongjin Qu, Panwang Zhou and Guangjiu Zhao

Mater. Chem. Front., 2021, 5, 7170-7175

https://doi.org/10.1039/D1QM00733E

Issue 19 Vol. 5 Outside Front Cover

Biologically-derived nanoparticles for chemo-ferroptosis combination therapy

Haifeng Sun, Zhonghe Zhang, Xiaoyu Kang, Qiong Dai, Aixin Song, Jingcheng Hao and Jiwei Cui

Mater. Chem. Front., 2021, 5, 3813-3822

https://doi.org/10.1039/D1QM00295C

Issue 10 Vol. 5 Outside Front Cover

Synergistic improvements in the performance and stability of inverted planar MAPbI₃-based perovskite solar cells incorporating benzylammonium halide salt additives

Hung-Cheng Chen, Jie-Min Lan, Hsiang-Lin Hsu, Chia-Wei Li, Tien-Shou Shieh, Ken-Tsung Wong and Chih-Ping Chen

Mater. Chem. Front., 2021, 5, 3378-3387

https://doi.org/10.1039/D0QM00983K

Issue 08 Vol. 5 Outside Front Cover

Congratulations to the winners of Best Materials Chemistry Frontiers Covers of 2021!

We expressed our sincere appreciation for all the support and contributions from our authors, reviewers, and readers in the past 2021.

Looking forward to receiving your high-quality work in 2022.

Happy Lunar New Year!

In the emerging technologies of organic electronics, positional isomerism has appeared in recent years as an efficient molecular tool to tune the electronic and physical properties of organic semi-con­­ductors (OSCs), which are at the heart of the devices. For example, modifying the position of the linkages (ortho, meta, para), allows to extend or restrict the π-con­jugation between two molecular fragments and, all the key electronic parameters of an OSC can be easily modified (HOMO / LUMO energy levels, single and triplet state energies, charge carriers mobilities…) leading to substantial different performances in electronic devices.

The indacenobisthiophene (IDT) fragment, which is an association of two thienyl cores to a central phe­nyl ring, has appeared in the last ten years as an important building unit to construct efficient or­ganic materials mainly for organic photovoltaics but also for Organic Field-Effect Transistors (OFETs). However, with a few exceptions, nearly all the IDT-based molecules described to date in the literature are cons­tructed on a central para-linked phenyl ring with the two thienyl sulfur atoms in an anti-configuration (see para-IDT core in pink in Chart 1). Re­cent­ly, the study of IDT positional isomers possessing a central meta-linked phenyl unit with the two sulfur atoms in a syn configuration (see meta-IDT core in blue in Chart 1) has allowed to show the impact of positional isomerism on this family of compounds.

Continuing this systematic approach, the authors investigate herein the incorporation of the widely known electron rich phe­nyl­­acridine (PA) fragment spiro-linked to the meta- or para-IDT core. Thus, Di­Spiro­Phe­nyl­A­cri­dine-IDT isomers, para-DSPA-IDT and meta-DSPA-IDT, constructed on the so-called “3π-2spiro” ar­chi­te­ctu­re, have been investigated through a detailed structure-properties relationship study.

Chart 1. Positional isomers investigated (para-DSPA-IDT and meta-DSPA-IDT)

In the present work, the group of Prof Cyril Poriel and Prof Joëlle Rault-Berthelot (Institut des Sciences Chimiques de Rennes- UMR 6226, Rennes) reports that the phenylacridine fragment significantly modifies the elec­tronic, physical and charge transport properties compared to structurally related DiSpiro­Fluo­­re­ne-IDT analogues (para-DSF-IDT, meta-DSF-IDT). This finding is different to what was reported in literature for other couples of IDT-based isomers and shows the key role played by the spiro-connected frag­ments on the charge transport properties of these molecular systems and open new avenues for meta-substituted oligomers.

Despite that the properties are mainly driven by the IDT core, this work shows that the bridges (herein spiro-connected phenylacridine) allow a fine tuning of all the properties. Particularly, the electrochemical studies have revealed the strong differences observed in the oxidation of the two isomers.

Figure 1. Left, CVs of para-DSPA-IDT (1.5 × 10^-3 M) and of meta-DSPA-IDT (2.15 × 10^-3 M) recorded between 0.25 and 1.8 V, sweep-rate 100 mV.s^-1. Right: DPVs normalized on the first oxidation wave for para-DSPA-IDT and on the second oxidation wave for meta-DSPA-IDT. Pulse Height: 25 mV, sweep-rate 50 mV.s^-1, t: 50 ms. Working platinum disk electrode (Ø 1 mm).

This work also shows that the mobilities of charge carriers are higher for the meta isomer than for the para isomer. This finding is different to what was reported in literature for other couples of IDT-based isomers and shows the key role played by the spiro-connected frag­ments on the charge transport properties of these molecular systems.

Figure 2. Thickness-scaled current-voltage characteristic of the para– and meta-DSPA-IDT and para– and meta-DSF-IDT hole-only SCLC devices (Left), SCLC mobility and SCLC device (right). In left, the dotted lines indicate the SCLC regime and the continuous ones the Ohmic regime.

BIOGRAPHICAL INFORMATIONS

Cyril Poriel received his PhD in 2003 from the University of Rennes 1. After a postdoctoral stay at the University of Exeter (UK), he joined the CNRS (Institut des Sciences Chimiques de Rennes) in 2005, where he is currently CNRS Research Director. His main research interest deals with the design of π-conjugated architectures for Organic Electronics. He is author/co-author of more than 120 publications, reviews and book chapters.

Joëlle Rault-Berthelot received her PhD in 1986 from the University of Rennes 1, where she is currently CNRS research director (Institut des Sciences Chimiques de Rennes). She has been working for 40 years in electrochemistry. Since 2005, she is involved in the design of π-conjugated systems for Organic Electronics and is author/co-author of more than 160 publications, reviews and book chapters.

Carbon dots (CDs) are nanosized carbon particles that have attracted the attention of researchers from different fields for their potential applications due to their high photostability, tunable excitation and emission, low toxicity and high biocompatibility. Reports on CDs that are soluble in non-aqueous solutions are still scarce, despite being recognized as promising materials and their compatibility with biological membranes facilitates their traversing. This feature is attractive for biomedical applications, in particular in cancer, where drug resistance and low specificity (i.e. side effects) urge the development of new drugs whose fate may be compromised by solubility, stability and clearance rate.

Recently the group of Prof. Santoyo and collaborators at Universidad of Granada (Spain) have demonstrated that the thermolysis of citric acid in DMSO and then reaction with alkyl amines yields CDs (LCDs) that bear both hydrophobic alkyl chains and carboxylic groups, and that the former make them suitable for hosting hydrophobic guests and the latter allow the modulation of their hydrosolubility. As a proof of concept the hydrophobic drug camptothecin (CPT) and the NIR fluorescent hydrophobic dye IR780 were assayed. The clinical implementation of both molecules is limited by their poor solubility, although CPT is a potent chemotherapeutic agent and IR786, besides the emission in the 807–823 nm wavelength range that makes it suitable for bioimaging, shows an absorption peak at 792 nm that yields a temperature increase and a production of ROS upon illumination with NIR light, enabling its use in photothermal and photodynamic therapies.

When LCD-2Na was loaded with CPT to yield LCD-2Na@CPT and the toxicity on a battery of cell lines was compared with an equivalent amount of free fresh CPT (Fig. 1), results demonstrated that the interaction between LCD-2Na and CPT is reversible and that the released drug is functional, despite it underwent a processing incompatible with the stability of free CPT.

Fig.1 Comparison of the cytotoxicity of LCD-2Na@CPT (green) and free CPT (red) on different cancer cell lines at different equivalent concentrations of CPT. Cell lines were incubated for 24 h with suitable amounts of either free CPT or LCD-2Na@CPT and cytotoxicity was assayed by the MTT method. Results are means ±1 standard deviation.

Additionally, the system LCD-2Na@IR780 was found to provoke an increase in the temperature of the solution up to 68℃ upon illumination with a 808 nm laser and yielded the formation of oxygen singlet with the concomitant destruction of IR780. At this point, it is important to recall that temperatures above 48℃ for minutes provoke irreversible injury that is enhanced by the reactive species produced during the destruction of IR780 by the laser.

Fig. 2 Temperature increase as a function of time of 200 μL of a water solutions containing 133 μM IR783 (red), 72 μM LCD-2Na@IR780 (blue) and 285 μM LCD-2Na@IR780 (magenta) irradiated with a 808 nm NIR laser at a power of 1.2 W cm−2 for 5 min (A) or 10 min (B). Inset B: Thermal image of the epperdorf containing the LCD-2Na@IR780 solution and the sample holder during the illumination. The temperature was recorded in real time with a high-resolution infrared camera.

It is important to highlight that LCDs are well tolerated by cells and, using a suitable length of the alkyl chains, they form inclusion complexes with hydrophobic guests of complementary size. The values of log P and the results obtained from the model molecules CPT and IR780 support the biotechnological potential of LCDs as drug carriers and in photothermal therapy.

Francisco Santoyo-Gonzalez is a Full Professor in Organic Chemistry at Universidad de Granada (Spain), full member of the Academy of Mathematics, Physical-Chemistry and Natural Sciences and  founder and leader of the group Glycochemistry & Bioconjugation. His research focuses on the development of new synthetic methodologies, with a particular emphasis in those related with the click-chemistry concept, and their application in a variety of diverse (bio)fields including cyclodextrins, glycochemistry, bioconjugation, non catalytic and catalytic hybrid-materials, nano-materials, targeted drug delivery, gene-transfection and (bio)sensors.

Supramolecular hydrogels arise from the self-assembly of low molecular weight gelators (LMWG) and a large amount of water (typically higher than 99% by mass) using non-covalent interactions.  Bolaamphiphilic molecules comprise of at least two parts including a hydrophobic skeleton such as alkyl chains, a steroid, or a porphyrin, and two hydrophilic groups on both ends which to give either symmetric or asymmetric end groups.  Such bolaamphiphiles with hydrophobic spacers can act as low molecular mass gelators.

Non-chemically crosslinked hydrogels have been extensively studied in the context of enzyme encapsulation and stabilization. However, a flexible and porous structure is required to facilitate full penetration of the enzyme into the substrate.  The immobilization of amphiphilic graphene quantum dots (GQDs) and enzymes into hydrogels remains of great interest for amplifying a signal to give high sensitivity for sensing applications and maintaining enzymatic activity in biological applications.

Graphene quantum dot nanozymes (GQDzymes) with peroxidase mimic catalytic activity for analytical and biomedical applications, for example of glucose, organophosphate detection, are of great interest.  Graphene quantum dots embedded within porous hydrogels exhibit useful physical properties. For instance, mediated through hydrogel encapsulation they show advantageous high fluorescent intensity compared to those in solution. This can result in high sensitivity for fluorescent sensing applications.

Recently, the groups of Boosayarat Tomapatanaget (Chulalongkorn University, Thailand) and Jonathan W. Steed (Durham University, UK) have collaborated to design novel fluorescent hybrid materials comprising graphene quantum dots and enzymes supported in L–phenylalanine derived bis(urea) supramolecular hydrogels (GQDs/Enz/Gels) for detection of organophosphates such as pesticides. Determination of organophosphates (OPs) using this hybrid material arises from the turn-on photoluminescent responses observed in the presence of the pesticide and is caused by inhibition of the enzymatic production of hydrogen peroxide, which quenches luminescence as shown in Scheme 1.

Scheme 1. Hybrid hydrogels of GQDs/Enz/Gels and proposed mechanism of organophosphate pesticide detection.

The enzyme-loaded photoluminescent sensing hybrid gel materials (GQD/Enz/Gel) were prepared in two stages (Figure 2, S1 – S2). The microstructures of materials were examined by SEM with the comparison of GQDs/Gels and GQDs/Enz/Gels with different gelation time. Moreover, the rheological properties of gels demonstrated large G¢ values by the addition of GQDs or by using phosphate buffer as the solvent. GQD/Enz/Gel materials have been investigated for their organophosphate sensing ability. Gratifyingly, the visual changes showed that the photoluminescence intensity of the GQDs/Enz/Gels depended on the concentration of organophosphate pesticide dichlorvos, which inhibited the activity of the acetylcholine esterase enzyme (AChE) in the hybrid hydrogels.  The inhibition efficiency of AChE by OPs in oxo-form of the phosphate group (methyl-paraoxon and dichlorvos) are higher than those of OPs in thio-form (malathion and parathion).  Interestingly, the naked-eye fluorescence images obtained by the GQDs/Enz/Gels with dichlorvos, under UV irradiation at 365 nm displayed a significant brightness change upon increasing the analyte concentration.

Figure 2. (A1) Preparation of partial GQDs/Gels (stage1) and GQDs/Enz/Gels hybrid hydrogels (stage2). (A2) SEM images of the dried partial gels (xerogels) from stage 1 prepared for 1 min and the hybrid hydrogels from stage 2 prepared for 30 min followed by drying under ambient conditions for 2 days. (B) frequency-sweep rheology of gelator 4b and GQDs/Gels from 4b in different conditions. (C1) visual changes of GQDs/Enz/Gels with various number of OPs and ACh. (C2) Comparison of inhibition efficiency (%) of AChE in hybrid hydrogels after incubation in 1.25 x10^-6 and 12.5 x10^-6 M of four organophosphate pesticides. (D) Photographs of hybrid GQDs/Enz/Gels sensory chips (Glass slide 7.6 x 2.6 cm and circle with diameter of 0.5 cm) in the presence of various concentration of dichlorvos (DV) and time of measurement.

This work also provides new insight into improving the performance of low molecular weight hybrid hydrogels through combination with GQDs, enzyme and phosphate buffer. The result is a highly effective and potentially versatile sensing system, with a high sensitivity and stability.  This is the candidate sensing system regarding to the enzymatic turn-on sensing in a small molecule gel medium. This efficient enzymatic sensing candidate offers opportunities for the development of a myriad of specific and highly sensitive hybrid nanomaterial-based (bio)sensors.

Prof. Jonathan W. Steed
University of Durham

Assoc. Prof. Boosayarat Tomapatanaget
Chulalongkorn University

Jonathan W. Steed obtained his B.Sc. and Ph.D. degrees at University College London, working with Derek Tocher on organometallic and coordination chemistry. He graduated in 1993 winning the Ramsay Medal for his Ph.D. work. Between 1993 and 1995 he was a NATO postdoctoral fellow at the University of Alabama and University of Missouri, working with Jerry Atwood. In 1995 he was appointed as a Lecturer at King’s College London. In 2004 he joined Durham University where he is currently Professor of Inorganic Chemistry. Professor Steed is co-author of the textbooks Supramolecular Chemistry (2000, 2009 & 2021) Core Concepts in Supramolecular Chemistry and Nanochemistry (2007) and around 350 research papers. He has edited various books including the 8-volume Supramolecular Chemistry from Molecules to Nanomaterials (2012). He is the recipient of a number of awards, most recently the 2021 RSC Tilden Prize. He is Editor-in-Chief of the American Chemical Society journal Crystal Growth & Design. His interests are in crystallization, supramolecular gels and crystalline solids particularly pharmaceutical solids, co-crystals and hydrates. See personal web pages for full details.

Boosayarat Tomapatanaget is associate professor in the Department of Chemistry, Faculty of Science, Chulalongkorn University (Thailand). She received her PhD in Chemistry at the Faculty of Science, Chulalongkorn University in 2003. The research field of Professor Boosayarat Tomapatanaget encompasses many aspects of supramolecular chemistry, particularly in applications such as the design and synthesis of chemosensors. The molecular recognition of biological targets has formed a major part of host-guest chemistry including the hybrid organic/inorganic based on nanomaterials for sensing purpose.

The development of efficient organic host matrices for the emissive layer of Phosphorescent Organic Light-Emitting Diodes (PhOLEDs) is at the origin of the fantastic progresses made by this technology in the last twenty years. The role played by the host matrix is crucial as it should prevent energy back transfers from the guest emitter to the host and favour the confinement of excitons. The rational design of host materials for guest phosphors has allowed to reach very high-efficiency red, green and blue PhOLEDs (with external quantum efficiency EQE>25%). However, all these devices are multi-layer PhOLEDs, which are constituted of a stack of organic layers in order to improve the injection, transport and recombination of charges within the emissive layer. To reduce the cost and the environmental footprint of the OLED technology, simplifying the multi-layer structure is one interesting direction for the future. The so-called Single-Layer PhOLEDs (SL-PhOLEDs, Figure 1), the simplest device only made of the electrodes and the emissive layer, have thus stimulated a strong interest for the last fifteen years. However, high efficiency SL-PhOLEDs (especially for the blue emission) are very rarely reported in literature due to the difficulty to design an efficient host material. To reach high-performance SL-PhOLEDs, the host material should fulfil several precise criteria: (i) a high triplet state energy E_T˃ 2.7 eV to confine the triplet excitons within phosphorescent guest, (ii) HOMO/LUMO energy levels well adapted to the electrode Fermi levels allowing efficient charge injection, (iii) good and well balanced mobilities of electrons and holes (ambipolar character) in order to compensate for the absence of electron/hole transporting layers, and (iv) thermal and morphological stabilities to extend the lifetime of the devices. These four criteria can be fulfilled by the careful association of an electron-rich and an electron-deficient unit within a single molecule. 

Figure 1. Schematic representation of the architecture of a Single-Layer PhOLED (Left) and a Multi-Layer PhOLED (Right). Hole injection layer (HIL), hole transporting layer (HTL), electron injection layer (EIL), electron transporting layer (ETL), hole blocking layer (HBL), electron blocking layer (EBL), Emissive Layer (EML).

In the present work, the group of Prof Cyril Poriel (Institut des Sciences Chimiques de Rennes- UMR 6226, Rennes) reports a high-efficiency host material constructed on a barely studied electron rich fragment, namely quinolinophenothiazine (QPTZ). The QPTZ fragment is a phenylacridine bridged by a sulphur atom, Figure 2. Despite appealing properties induced by this bridging (e.g. strong electron rich character), this fragment remains almost unexplored to date in organic electronics. Herein, the strong potential of the QPTZ fragment as host for the new generation of simplified SL-PhOLEDs is demonstrated. Red, green and blue SL-PhOLEDs were successfully fabricated and yielded an average external quantum efficiency of ca 10% (Figure 3). High luminances of ca 10000 cd/m² for red and blue devices and 40000 cd/m² for green devices were obtained. These luminances are higher than the best reported to date with structurally related analogues and highlight the strong interest of the QPTZ fragment in such devices. Thanks to its high HOMO energy level, the QPTZ unit also allows to decrease the threshold voltage of the corresponding devices, which is a key point in ‘single-layer’ technology.

Figure 2. Phenylacridine and Quinolinophenothiazine molecular fragment

This work demonstrates the potential of the QPTZ fragment in the design of host materials for high performance single-layer PhOLEDs. QPTZ appears as a promising building unit and can advance the field of organic semi-conducting materials. We are convinced that the future development of QPTZ-based materials would also be appealing for other organic devices.

Figure 3. SL-PhOLEDs characteristics using SQPTZ-2,7-F(POPh2)2 as host material. A) Current density (mA/cm²) and luminance (cd/m²) as a function of the voltage; B) Current efficiency (cd/A, filled symbols) and power efficiency (lm/W, empty symbol) as a function of the current density (mA/cm²) and C) Normalized EL spectra.

BIOGRAPHICAL INFORMATIONS

Cyril Poriel received his PhD in 2003 from the University of Rennes 1. After a postdoctoral stay at the University of Exeter (UK), he joined the CNRS (Institut des Sciences Chimiques de Rennes) in 2005, where he is currently CNRS Research Director. His main research interest deals with the design of π-conjugated architectures for Organic Electronics. He is author/co-author of more than 120 publications, reviews and book chapters. twitter: @CyrilPoriel

Silk fibroin (SF) is a biopolymer from Bombyx mori mulberry silkworm that has been utilized as textile for millennia. Recent advancement has emphasized to the biodegradation and biocompatibility of silk for medical applications. 

Due to the hydrogen bonding in silk fibroin, SF interacts with water molecules through the random coil conformation, while the β-sheet conformation provides mechanical strength to the entire spherical structure. 

Figure 1. Optical (a), fluorescence (b), and SEM (c) images of the SF microspheres. Scale bars: 5 µm. (d) PL spectra of a single SF microsphere upon excitation with cw laser (λex = 450 nm). Each WGM peak is assigned as shown on the bottom. (e) PL spectra of a single SF microsphere upon excitation with fs pulsed laser.

Recently, the group of Prof. Yohei Yamamoto and collaborators of University of Tsukuba have demonstrated that self-assembled SF microspheres, doped with ionic fluorescent dye, display resonant luminescence, which shifts in response to the humidity change. The resonant peaks result from the total internal reflection of the fluorescence, causing interference at the circumference of the microspheres. The SF microspheres display lasing property upon femto-second laser pumping (Figure 1).

The WGM peaks respond to the change of the ambient humidity. When the humidity is low, SF microsphere release water molecules, leading to shrinkage of the SF microsphere and thereby causing the WGM peaks shift to the lower wavelength. In the reversal process where ambient humidity is high, SF microsphere is hydrated, causing the shift WGM of the WGM peaks to the higher wavelength (Figure 2).

Figure 2. (a) Humidity-dependent PL spectra of a single SF microsphere upon excitation with cw laser (λex = 450 nm). (b) Plot of the wavelength of the resonant peak of TE26 upon increasing (filled circle) and decreasing (open circle) the surrounding humidity. (c) Plot of the wavelength of the resonant peak of TE26 upon 6 cycles of hydration (red) and dehydration (blue) between 93 and 25 %RH.

It is interesting to highlight the secondary structure of silk fibroin. Upon treated with alcohol, β-sheet conformation is increased, providing rigid structure for the SF microspheres while the remaining random coil structure in the SF microspheres is interacting with ambient moisture. This combination contributes to the SF microsphere for obtaining a high responsivity and high sensing range toward humidity.

These properties have given a new prospect or direction for designing next generation microresonators for optical sensing or lasing applications.

Prof. Yohei Yamamoto, University of Tsukuba

Yohei Yamamoto is a professor in the department of materials Science, faculty of Pure and applied sciences, University of Tsukuba. He received his doctor degree in 2003 from Osaka University. After post-doctoral researcher term, he was appointed as an associate professor in University of Tsukuba at 2010. on 2018, he promoted to full professor in University of Tsukuba. His research interests are self-assembly of π-conjugated molecules, polymers and biomolecules to construct electronically and optically active nano/micrometer-scale materials. He is the author of more than 90 articles and cited more than 2900 times with an index H = 28.

https://publons.com/researcher/1757952//