Materials Chemistry Frontiers Blog

Photochromic dyes are a specific class of molecules that can undergo reversible transformations under exposure to light between isomers that possess different optical properties. This peculiar feature makes them appealing for a myriad of applications, ranging from bioimaging, data storage, sensing or optical lenses.

Very recently, they have been employed in photovoltaics. Few photochromic dyes have been investigated as photosensitizers in dye-sensitized solar cells (DSSCs) and the only photochromes that have demonstrated a fully reversible photochromic process with polychromatic light once embedded in solar cells are diphenyl-naphthopyran derivatives, as reported by the group of Renaud Demadrille of CEA Grenoble in France and collaborators of the University Pablo de Olavide in Spain. Using this class of dyes, they demonstrated the fabrication of semi-transparent smart photovoltaic devices capable to self-adapt their transparency to the ambient light. Recently, they have designed and synthesized a series of spiro-indoline naphthoxazine (SINO) and spiro-indoline naphthopyran (NIPS) with a donor-photochrome-acceptor (D-p-A) chemical structure for their use in DSSCs.

Figure 1. General structure of the new dyes and the interconversion reaction between their close (CF), merocyanine (MC) and protonated merocyanine forms (MCH).

The interconversion process and optical properties of the two SINO and two NIPS dyes synthesized in this work have been proven more complex to unravel than the ones of diphenyl-naphthopyrans, as they are not only photochromic but also acidochromic. The interactions of the molecules with both stimuli, i.e. H⁺ and light, have been studied separately or simultaneously, in solution or after grafting onto the surface of TiO₂. A positive photochromism in solution characterized by extremely fast thermal discoloration kinetics has been observed for some of the compounds. All the dyes demonstrated acidochromic properties, but in the presence of acid, only NIPS derivatives showed a negative photochromism, i.e. a fast bleaching under illumination.

Figure 2. (a) Normalized discoloration curves of SINO-1, SINO-2 and NIPS-1 and (b) coloration curve of NIPS-1 after the addition of HCl and bleaching with light (2 x 10^-5 solutions at 25 °C, irradiation source: 200-600 nm/200 W xenon lamp).

After unraveling the photochromic and acidochromic properties of the new molecules, they were employed in the fabrication of dye-sensitized solar cells. The choice of the electrolyte was found to be critical, due to the pH-driven optical behavior of the dyes. If a low-pH electrolyte is used, a non-photochromic protonated open form was mainly produced, which was mostly avoided in the case of a neutral electrolyte. This work highlights that the photochromic properties of the dyes can be conserved when going from the solution to the devices for some of the dyes.

Figure 3.(a) J-V curves (dashed lines recorded in the dark, solid lines recorded under illumination) of opaque NIPS-2-based DSSCs using our acidic homemade electrolyte (black) and Iodolyte (red) and (b) transmittance spectrum of the transparent device together with a picture of the NIPS-2/Iodolyte device (standard irradiation conditions AM 1.5 G, 1000 W m⁻²; 25 °C; active area= 0.36 cm²).

This study is the first to investigate in detail the complex relationships between photochromic, acidochromic and photovoltaic properties for these classes of dyes. The structure-property relationships established will undoubtedly be useful for the development of new photochromic compounds with optimized optoelectronic properties for applications in various fields, including photovoltaics.

Corresponding authors:

Dr. José María Andrés Castán (Interdisciplinary Research Institute of Grenoble, CEA-Grenoble)

José María Andrés Castán is a postdoctoral fellow in the Molecular Systems and nanoMaterials for Energy and Health (SyMMES) at the Atomic and Alternative Energies Commission (CEA) in Grenoble, France. He received his PhD in Materials Science in 2018 at the Université d’Angers in France. His research is focused on the synthesis of photochromic dyes for their use in Dye-Sensitized Solar Cells. He is the author of more than 20 indexed publications.

Dr. Renaud Demadrille (Interdisciplinary Research Institute of Grenoble, CEA-Grenoble)

R. Demadrille is a team leader at the Atomic and Alternative Energies Commission (CEA) in France. He received his PhD in organic chemistry in 2000 from the University of Aix-Marseille with a grant from PPG Industries and Essilor International before to join the R&D department of an international chemical company to work on functional polymer materials. Then he moved to CEA as a postdoctoral fellow to develop semiconducting polymers for organic photovoltaics before being appointed in 2005 as a permanent researcher. His research focuses on the synthesis and the characterization of new pi-conjugated molecules and polymers for organic and hybrid photovoltaics and electronics. In 2018, he was recipient of the “Chemistry Energy” prize of the French Society of Chemistry, and in 2019, he was awarded of an ERC Advanced Grant to develop photochromic solar cells. Since 2020, he is Associate Editor of Journal of Materials Chemistry C and Materials Advances. He is the author of 8 patents, 2 book chapters and more than 90 articles indexed by SCI.

Controllable defects and interface engineering are perceived as promising routes to develop efficient noble-metal-free electrocatalysts for oxygen evolution reaction (OER), the bottleneck of overall water splitting. Recently Metal-organic frameworks (MOFs) have received particular attention for their remarkable OER performance because of their vast surface area, ability to tune porosity, and functionalization with mixed metals/ligands. Among various MOFs, Prussian blue analogs (PBAs) have been extensively studied for their potential in catalyzing the OER process. However, pristine PBA cubes suffer from low conductivity and exhibit insufficient OER activity, resulting in high overpotential and limiting their OER electrocatalytic applicability.

PBAs’ components, topologies, and surface engineering can be modified to improve their catalytic properties to address these limitations. One common structural alteration is the introduction of vacancies (defects) within the PBA cubes to facilitate ion diffusion, often accomplished through chemical etching that can change the local electronic configuration to boost the OER kinetics. There are only a few works on the selective etching of PBAs to enhance their performance; therefore, modification of the PBAs by the chemical etching process is a trending topic in OER electrocatalyst designing.

Recently, the Zidki’s research group at Ariel University highlighted the significance of combining the PBA etching effect with the decoration of molybdenum disulfide (MoS₂) on the edges and surfaces of Co-Fe PBA for catalyzing the OER kinetics. They proposed using an ethanol-water mixture as a mild etchant, eliminating the need for a capping or stabilizing agent.

Figure 1. (i) Synthetic scheme of Etched-PBA-MoS2 nanoframes. (ii) HR-SEM images of PBA nanocubes (a), Etched-PBA nanocages (c), and Etched-PBA-MoS2 nanoframes (e), and their corresponding TEM images (b,d, and f).

The Etched-PBA-MoS₂ nanoframes presented superior OER performance, requiring an overpotential as low as 260 mV on carbon cloth substrate to obtain the current density of 10 mA cm⁻² with a corresponding Tafel slope of 55 mV dec^-1 (Fig. 2a and b). The Etched-PBA-MoS₂ nanoframes also outperform with a lower charge transfer resistance (R_ct) value (smallest semicircle), indicating its faster charge-transfer kinetics (Fig. 2c). The catalyst retains its catalytic activity for a long-term stability test, proving its OER in a real-time application (Fig. 2d).

Figure 2. (a) LSV curves; (b) Tafel plots; (c) EIS-Nyquist plots measured at 1.5 V vs. RHE in 1.0 M KOH; (d) OER stability test of Etched-PBA-MoS2/CC.

This work demonstrates that the excellent electrocatalytic activity arises from two primary factors: (1) hollowing PBA nanocubes by the etching process increases the density of active sites to promote mass transport; (2) binding MoS₂ on the surface of PBA nanocages induces a synergistic effect – the electronic interactions among the active components tune the electronic structures of Co, Fe, and Mo sites. Their work renders a feasible pathway to optimize the etching effect and fasten different metal sulfide heterostructures on PBAs to achieve an excellent OER performance.

Corresponding author:

Dr. Tomer Zidki received his Ph.D. degree in 2010 from the Chemistry Department, Ben-Gurion University, Beer Sheva, Israel, in the field of radical reactions with nanoparticle catalysts. He pursued postdoctoral research at the Brookhaven National Laboratory, NY, USA, where he gained experience in redox catalytic processes. Dr. Zidki is an Assistant Professor in the Chemical Sciences Department, Ariel University, Israel, where he leads the Nanoparticle Catalysts group. He is also the Head of the Linear Electron Accelerator Facility for fast chemical reactions. Dr. Zidki has guided ten Ph.D. and three M.Sc. students. His research focuses on redox catalyzed reactions by nanoparticles and photo- & electrocatalytic water splitting reactions using non-precious catalysts. In addition, Dr. Zidki’s group studies the kinetic mechanisms of redox reactions and radical reactions using the electron accelerator. Another field of interest of Dr. Zidki is Environmental Chemistry, in which he wrote two patents on nitrogen and sulfur oxides removal from flue gases.

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