Materials Chemistry Frontiers Blog

Supramolecular self-assembly can be described as a spontaneous process of association of individual molecules to construct complex and large architectures with distinct features from the corresponding monomers. Encouraged by the natural self-assembly processes, creating new artificial self-assembled systems with desired properties has become a pressing research area of interest. Accordingly, during the last few decades, the scientific community has invested enormous effort in searching and constructing novel self-organized organic architectures with diverse functionalities, which have registered their potential in a wide range of applications, such as biosensing, drug delivery, tissue engineering, organic electronics, catalysis, and others.

During the last few decades, pyrene, a small polyaromatic hydrocarbon, has captured an immense attention to the scientific community as a unique blue emissive fluorophore. Pyrene has been an outstanding choice in organic functional materials because of its superb photophysical characteristics and versatile applications, and the possibility of synthetic modifications on the pyrene core. In comparison to systems that only enable self-assembly, the examples of pyrene-based transient assembly and further disassembly remain elusive in the past literature. Transient assembly is an out-of-equilibrium process which is thermodynamically unstable, and hence it dissociates into monomers via the disassembly process.

In the search of such assembly–disassembly system, the group of Prof. Apurba Lal Koner of the IISER Bhopal, India, in collaboration with Prof. Anup Pramanik at Sidho-Kanho-Birsha University, have developed a new biogenic amine (BA)-induced pyrene-based transient assembly and spontaneous disassembly system to access blue emissive Py-BA conjugated monomers which exhibited solid-state emissive property in addition to lysosomal targeting application (Figure 1).

Figure 1. Representation of reaction-based transient assembly and disassembly and demonstration of solid-state emission and bioimaging application of the monomers.

The group has demonstrated that the nucleophilic reaction between BA and pyrene-anhydride led to form open polar conjugates which created the transiently assembled network driven by strong π–π and multiple H-bonding interactions. The evolution of transient assembly via ground-state pre-association was fully established by a combination of spectroscopic and microscopic techniques in addition to computational study. Addition of BA such as 1,4-DAB in probe solution governed to generate an intense excimer band near 550 nm (yellow emission) in fluorescence spectra which further showed quenching with BA concentration as well as time (Figure 2a–b).

Figure 2. (a) UV-Vis. and (b) fluorescence spectra of Py-DA (5 µM) with increasing concentration of 1,4-DAB (0–50 µM), inset showing the change of O.D. and fluorescence intensity with concentration. (c) representation of temporal intensity changes in excimer and monomer band in CHCl₃. (d) plot of change of I₅₄₅/ I₄₀₅ ratio with time; inset cuvette images showing transformation of fluorescence color from excimer to monomer species.

Moreover, time-dependent quenching of excimer band with concomitant enhancement of monomer emission near 400 nm signified dissociation of yellow-emissive excimer/ aggregation to blue-fluorescent monomers in solution (Figure 2c–d). SEM study for Py-1,4-DAB conjugate unravelled temporal evolution of larger bulk aggregates at initial time which were transformed to interconnected network morphology due to disassembly (Figure 3a–c). Two types of products, i.e., Py-BA monomers and dimer were mainly responsible for demonstrating such assembly–disassembly phenomenon in solution as evident from NMR and mass spectrometry. The blue-emissive monomer conjugates exhibited yellowish-orange emission in solid-state (Figure 3d) which can be highly useful for fabricating light-emitting devices in the future. Interestingly, the newly formed water-soluble selective Py-BA conjugates were found to be of potential significance as lysosome targeting fluorophores (Figure 3e–f). Their lysosomal staining property provided a great advantage to monitor the lysosomal membrane degradation in a time-dependent manner.

Figure 3. (a)–(c) Time-dependent SEM images of Py-1,4-DAB conjugate in CHCl₃ showing morphological change with time. (d) CIE-coordinate of Py-1,4-DAB solid-state emitter; inset shows fluorescence image of thin-film captured upon illuminating at 365 nm. CLSM images of live BHK-21 cells: (e) merge image of Py-Spermine and lysotracker red (f) scatter plot with Pearson’s correlation coefficient 0.96 ± 0.02 (scale bars: 20 µm).

The work represents an intriguing example of harvesting water-soluble fluorophores with multifunctional features from reaction-induced transient assembly–disassembly processes.

Read the full article in Materials Chemistry Frontiers:

Prof. Apurba Lal Koner (Indian Institute of Science Education and Research Bhopal, India)

Apurba Lal Koner is an associate professor in the Department of Chemistry at the Indian Institute of Science Education and Research Bhopal (India). He received his PhD in Chemistry from Jacobs University Bremen (Germany) in 2009. From 2009 to 2012 he did postdoctoral studies at the University of Oxford (UK). Professor Apurba Lal Koner’s research team is working at the interface of chemistry, biology, and emissive materials and their applications in various research domains. He is the author of more than 95 articles of international repute and cited more than 2600 times with an index H = 25.

Web Page link: https://bionanolab.wixsite.com/akoner-iiserb

Emerging Investigator: Jintao Zhang

Position         Professor Postdoc         2013–2015  Case Western Reserve University (USA)                       2012–2013  Nanyang Technological University (Singapore) Education      2008–2012  University of Singapore                      Ph.D. ORCID          0000-0002-1029-3404

Read Jintao Zhang’s Emerging Investigator Series article in Materials Chemistry Frontiers and learn more about him.

	Hollow CoOX nanoparticle-embedded N-doped porous carbon as an efficient oxygen electrocatalyst for rechargeable zinc–air batteries
	Maomao Yang,‡ Siyu Ding,‡ Xinxin Shu, Wei Pan and Jintao Zhang *
	The porous carbon embedded with hollow cobalt oxide nanoparticles was prepared via a spray-drying method followed by carbonization, which endows rechargeable zinc–air batteries with the improved bifunctional catalytic activity.
	From the themed collection: Frontiers Emerging Investigator Series
	The article was first published on 24 Oct 2022
	Mater. Chem. Front., 2022, Advance Article
	https://doi.org/10.1039/D2QM00858K

My research interests

Key words: interface electrochemistry, electrocatalysis, energy conversion and storage

My research interests include the rational design and the synthesis of advanced electrode materials for electrocatalysis, electrochemical energy storage and conversion. Typically, with the fundamental understanding of interface electrochemical reactions in combination with in situ electrochemical methods, model electrode materials are rationally designed for advanced electrocatalysis and energy storage devices, such as carbon dioxide reduction reaction, oxygen reduction and iodine oxidation reactions for advanced rechargeable batteries.

10 Facts about me

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When a molecule absorbs light, electrons can be excited to higher energy states. Multiple ways exist for the extra energy to be released, including emitting photons in radiative processes. The competition between radiative and nonradiative relaxation pathways dictates that the faster the process is, the more likely it is to occur. As such, for light emitting devices, the radiative decay needs to be fast to decrease losses in non-radiative relaxation. On the other hand, for light harvesting devices, it is important that the radiation is suppressed. Hence, it is crucial to control the radiative rate to achieve higher efficiency in light emitting or harvesting devices. Besides altering the chemical structures of fluorophores, the design of their dielectric environment provides a straightforward means of altering their fluorescent properties. Control of radiative rate has been thoroughly achieved and understood for inorganic photonic structures, whereas it is less so for organic ones. However, for the future of flexible devices, it is important to explore this effect in polymer structures so that they can be easily integrated in device design. A promising polymer photonic structure is a microcavity formed by two polymer dielectric mirrors sandwiching a very thin layer of fluorescent material as schematized in Fig. 1a. The dielectric mirrors consist of alternating thin layers of materials with high and low refractive index, which form a photonic crystal with high reflectance that can be engineered in the desired spectral range, making them more efficient than metallic ones. Additionally, they are easy to fabricate from solution.

Recently, researchers at the Rely photonics group at the University of Genoa and their collaborators at the National Research Council in Genoa demonstrated radiative rate change in microcavities incorporating an NIR emitting dye (Fig. 1b). The cavities employ dielectric mirrors consisting of the low-index fluorinated polymer Aquivion and a high-index polyvinyl carbazole, which together present the highest dielectric contrast reported so far for polymer mirrors. The samples are almost completely transparent to visible light (Fig. 1c), but strongly reflect light from 750–870 nm (Fig. 1d). This has been designed to reflect the fluorescence of a stable dye of choice. However, there is an allowed microcavity mode that has lower reflectance intensity at 850 nm.

Figure 1. (a) Schematic of the microcavity structure. (b) Structure of the NIR dye used. (c) Photo of an NIR-Reflecting microcavity. (d) Transmittance spectrum of microcavity.

The dielectric mirrors change the density of states available for photons, making it highly intensified at the microcavity mode (red arrow in Fig. 1d at 850 nm), and change the fluorescence intensity and kinetics. A series of references were fabricated in order to compare the effects of the microcavity. Compared to a film of the dye in polyacrylic acid matrix, the fluorescence intensity in the microcavity is nearly completely suppressed except at the cavity mode where it is amplified 15 times as strongly. (Fig. 2a).

Figure 2. Comparison between fluorescence of the dye/polymer film and the microcavity regarding (a) spectral intensity and (b) fluorescence lifetime.

Fluorescence intensity amplification at the microcavity wavelength is a commonly reported phenomenon. On the other hand, change in the fluorescence lifetime is rarely reported. However, it has been observed in this work (Fig. 2b), where lifetime decreased in the cavity compared to the reference film. Further measurements exclude any unintended effects and confirm that indeed the radiative rate does decrease due to cavity effects.

This significant change in the lifetime was achievable thanks to the high refractive index contrast achieved by the researchers. The system is highly promising for future implementation in devices. Further development of structures achieving higher contrast and using sharper emitters is needed to increase the radiative rate in order to improve performance of light emitting devices.

Corresponding author:

Prof. Davide Comoretto
University of Genoa

Davide Comoretto was born in Milano, Italy, in 1963. He graduated with a major in Physics in 1988. In 1993 he obtained a PhD in Chemical Sciences and was then enrolled as Research Scientist at the Department of Chemistry and Industrial Chemistry of the University of Genoa where he is now Full Professor of Industrial Chemistry. During his research activity, he worked at the Institute of Macromolecular Chemistry – CNR (Milan, Italy), the Department of Physics “A. Volta” (University of Pavia, Italy) and at the Institute for Polymers and Organic Solids at the University of California Santa Barbara (USA) in the group chaired by Prof. A. J. Heeger. He is the Team Leader of the Rely Photonics research group focusing on design and fabrication of functional solution-processed photonic crystals and spectroscopy. The main topics are related to emission control, sensing, thermal shielding, engineering and fabrication of metamaterials, light harvesting enhancement in photovoltaics, and photocatalysis.

Website: https://www.rely-photonics.it/people

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.