39 research outputs found

    Chemical and Photophysical Behaviour of π-Extended Tropyliums

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    Non-benzenoid annulenes have long fascinated organic chemists, due to their chemical properties, aromaticity, and photophysical behaviour. When incorporated into a polycyclic aromatic framework, such non-hexagonal rings can give rise to nonplanar structures with modified optical properties and improved solubility compared to planar, defect-free graphene. Among non-benzenoid annulenes, the tropylium cation has been of special interest, due to its unique blend of reactivity (owing to its positive charge) and stability (a result of its aromaticity). Indeed, this cation has found wide utility as a versatile ligand , stimulus-responsive dye , and an organocatalyst. Yet, reports of polycyclic aromatics featuring this heptagonal annulene are sparse. This Thesis employs tropylium and its neutral, nonaromatic homologue, cycloheptatriene (Figure 1) as key structural motifs within an extended π-framework to gain fundamental insights into the electronic and optical properties of non-benzenoid and charged polycyclic aromatics. We find that judicious engineering of strain into the framework of sterically overcrowded tropyliums can cause its aromaticity to rupture, forming an “aromatic-to-nonaromatic” equilibrium at room temperature. Moreover, modifying the conjugation length in a series of cycloheptatriene-rotors was found to vastly alter their photoluminescence properties, allowing for new modes of chemical reactivity. We also report our synthetic forays toward a highly warped redox-active warped nanographene, as well as preliminary findings on the excited-state proton transfer dynamics in a series of hydroxybenzotropyliums. 1. T. Murahashi, M. Fujimoto, M. A. Oka, Y. Hashimoto, T. Uemura, Y. Tatsumi, Y. Nakao, A. Ikeda, S. Sakaki and H. Kurosawa, Science, 2006, 313, 1104–1107 2. U. P. N. Tran, G. Oss, D. P. Pace, J. Ho and T. V. Nguyen, Chem. Sci., 2018, 9, 5145–5151. 3. D. J. M. Lyons, R. D. Crocker and T. V. Nguyen, Chem.—Eur. J., 2018, 24, 10959–10965. 4. P. K. Saha, A. Mallick, A. T. Turley, A. N. Bismillah, A. Danos, A. P. Monkman, A.- J. Avestro, D. S. Yufit and P. R. McGonigal, Nature Chem., 2023 15, 516–525. 5. A. T. Turley, P. K. Saha, A. Danos, Aisha N. Bismillah, A. P. Monkman, D. S. Yufit, B. F. E. Curchod, M. K. Etherington, and Paul R. McGonigal, Angew. Chem. Int. Ed., 2022, 61, e202202193

    Tartarus: A Benchmarking Platform for Realistic And Practical Inverse Molecular Design

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    The efficient exploration of chemical space to design molecules with intended properties enables the accelerated discovery of drugs, materials, and catalysts, and is one of the most important outstanding challenges in chemistry. Encouraged by the recent surge in computer power and artificial intelligence development, many algorithms have been developed to tackle this problem. However, despite the emergence of many new approaches in recent years, comparatively little progress has been made in developing realistic benchmarks that reflect the complexity of molecular design for real-world applications. In this work, we develop a set of practical benchmark tasks relying on physical simulation of molecular systems mimicking real-life molecular design problems for materials, drugs, and chemical reactions. Additionally, we demonstrate the utility and ease of use of our new benchmark set by demonstrating how to compare the performance of several well-established families of algorithms. Surprisingly, we find that model performance can strongly depend on the benchmark domain. We believe that our benchmark suite will help move the field towards more realistic molecular design benchmarks, and move the development of inverse molecular design algorithms closer to designing molecules that solve existing problems in both academia and industry alike.Comment: 29+21 pages, 6+19 figures, 6+2 table

    Nanopuntos de carbono : composición, estructura y fotofísica

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    Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Químicas, leída el 16-12-2022The main part of this thesis focuses on the production and characterisation of carbon nanodots. Regarding the production methods of the CNDs chosen in this work, CNDs produced by two clearly different bottom-up methods have been studied in detail. On the one hand, the solvothermal method has been used. This method consists of heating a liquid, in a closed container, above its boiling point, generating a pressure higher than atmospheric pressure, and thus inducing the condensation of CNDs. The obvious advantages of this synthetic method are: it is easy, cheap and environmentally friendly (provided that the solvent used in the solvothermal method is an environmentally friendly solvent, in our case we used water and ethanol). All these advantages provided a great opportunity to easily prepare and investigate CNDs, which, when prepared by this method, were found to have promising properties such as solubility in aqueous media, biocompatibility, photocatalytic activity in heterostructures with TiO2 and biocidal properties that can be employed to prepare biocidal polymer surfaces. The combination of the advantages proposed by the synthetic method and the promising properties of the products obtained have made this method the most common synthetic procedure for the production of CNDs...La parte principal de esta tesis se centra en la producción y caracterización de los nanopuntos de carbono. En cuanto a los métodos de producción de los CNDs elegidos en este trabajo, se han estudiado en detalle los CNDs producidos por dos métodos ascendentes claramente diferentes. Por un lado, se ha utilizado el método solvotérmico. Este método consiste en calentar un líquido, en un recipiente cerrado, por encima de su punto de ebullición, generando una presión superior a la atmosférica, e induciendo así la condensación delos CNDs. Las ventajas evidentes de este método sintético son: es fácil, barato y respetuoso con el medio ambiente (siempre que el disolvente utilizado en el método solvotérmico sea un disolvente respetuoso con el medio ambiente, en nuestro caso hemos utilizado agua y etanol). Todas estas ventajas proporcionaron una gran oportunidad para preparar e investigar fácilmente los CNDs, los cuales, al ser preparados por este método, resultaron tener propiedades prometedoras como la solubilidad en medios acuosos, la biocompatibilidad, la actividad fotocatalítica en heteroestructuras con TiO2 y propiedades biocidas que pueden ser empleadas para preparar superficies poliméricas biocidas. La combinación de las ventajas propuestas por el método sintético y las prometedoras propiedades de los productos obtenidos han hecho de este método el procedimiento sintético más común para la producción de CNDs...Fac. de Ciencias QuímicasTRUEunpu

    Neuartige Funktionsmaterialien auf Basis von funktionalisierten Poly(siloxanen) und Poly(isonitrilen) zum Einsatz in flüssigphasenprozessierten OLEDs

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    Diese Dissertation behandelt die Entwicklung von Funktionsmaterialien auf Basis von Poly(siloxanen) und Poly(isonitrilen) für den Einsatz in flüssigphasenprozessieren OLEDs. Die Poly(siloxane) wurden dabei polymeranalog funktionalisiert mit verschiedenen Lochleitern und Emittern sowie einem Quervernetzer. Die erhaltenen quervernetzbaren Copolymer wurden dann in ihren Eigenschaften untersucht vor allem im Bezug auf ihre Lösungsmittelresistenz und die Performance in einem OLED-Bauteil. Die Oxetane als Quervernetzer zeigten dabei eine bessere Lösungsmittelresistenz als das Diels-Alder-System. Das OLED-Bauteil zeigte für eine erste proof-of-concept OLED eine gute Performance. Für die Poly(isonitrile) konnten die entsprechenden Monomere und Copolymere mit Lochleitern und Quervernetzern hergestellt werden, jedoch gestalteten sich eine weitere Untersuchung schwierig, da die Löslichkeit der Polymere stark eingeschränkt war

    A review of fused-ring carbazole derivatives as emitter and/or host materials in organic light emitting diode (OLED) applications

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    This review focuses on fused-ring carbazole derivatives, their molecular design, electronic and photophysical properties, and in particular their applications as the emitter and/or the host material in the emitting layer of organic light emitting diodes (OLEDs), with emphasis on recent developments. This review is timely because of the rapidly expanding research into fused-ring carbazoles, predominantly indolocarbazole, indenocarbazole, benzofurocarbazole, benzothienocarbazole and diindolocarbazole derivatives. To our knowledge this class of materials has not been reviewed previously. The appeal of fused-ring carbazoles is their extended π-electron systems with good thermal stability, tunable frontier orbital energies that enable a wide gamut (red, green, blue and white) emission colour, high photoluminescence quantum yields, and versatility for chemical functionalisation at different sites, leading to outstanding OLED efficiencies. This review is divided into sections according to the molecules’ role in OLEDs: namely, as conventional luminescent emitters – especially in the deep-blue region; as state-of-the-art hosts for phosphorescent iridium-based emitters; as thermally activated delayed fluorescence (TADF) emitters with high external quantum efficiency; and as multiresonance (MR) emitters with unprecedented high colour purity. We conclude by highlighting the challenges and the great opportunities for fused-ring carbazole derivatives in OLEDs and other optoelectronic applications

    Investigating Novel Luminescent Materials Towards Applications in Light Emitting Electrochemical Cells

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    The search for new and better luminescent materials is becoming increasingly important, as there are significant cost-savings in using luminophores that are brighter and more efficient. Carbon quantum dots (CQDs) and other luminescent materials such as Pt-Ag nanoclusters and TADF compounds are an extremely appealing alternative to existing light-emitting materials, as they are low-cost, easy to synthesize, and non-toxic. This thesis explores the properties and performance of different luminescent materials to be used in light-emitting electrochemical cells (LECs). In this work, we focused on LECs as their low cost and ease of fabrication aligns well with the ethos of CQDs and carbon-based nanomaterials. Firstly, we carried out some foundational work: developing a method to accurately determine the absolute quantum efficiency of the materials we tested. This method was tested using a variety of photodetectors to increase the analytical applicability, including photomultiplier tubes and spectrograph/CCD camera setups, and addresses many of the problems with reporting relative ECL efficiency. We also looked at several different novel materials – Pt-Ag bimetallic nanoclusters, and organic compounds that exhibit thermally activated delayed fluorescence (TADF) to enhance quantum efficiency – to evaluate their feasibility for use in LECs. In addition, we developed a set of computer simulations using COMSOL Multiphysics to model these light-emitting reactions. Using these models, we are able to learn important parameters, such as the bimolecular annihilation rate constant in electrochemiluminescence reactions, gain more knowledge about how these materials can emit light, and further optimize their luminous performance and efficiency. In the final chapter of this thesis, we demonstrate our CQD-based LEC devices. These devices are the first example of CQDs being used in this fashion, and they exhibited bright white emission under electrical excitation. These CQDs, and, optimistically, derivatives of the material that are inspired by this work, are expected to be a substantial advancement in the research of next-generation, high performance light-emitting devices

    Unusual Excimer/Dimer Behavior of a Highly Soluble C,N Platinum(II) Complex with a Spiro-Fluorene Motif.

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    In this work, we introduce a spiro-fluorene unit into a phenylpyridine (CN)-type ligand as a simple way to deplanarize the structure and increase the solubility of the final platinum(II)···complex. Using a spiro-fluorene unit, orthogonal to the main coordination plane of the complex, reduces intermolecular interactions, leading to increased solubility but without significantly affecting the ability of the complex to form Pt···Pt dimers and excimers. This approach is highly important in the design of platinum(II) complexes, which often suffer from low solubility due to their mainly planar structure, and offers an alternative to the use of bulky alkyl groups. The nonplanar structure is also beneficial for vacuum-deposition techniques as it lowers the sublimation temperature. Importantly, there are no sp hybridized carbon atoms in the cyclometalating ligand that contain hydrogens, the undesired feature that is associated with the low stability of the materials in OLEDs. The complex displays high solubility in toluene, ∼10 mg mL , at room temperature, which allows producing solution-processed OLEDs in a wide range of doping concentrations, 5-100%, and EQE up to 5.9%, with a maximum luminance of 7400 cd m . Concurrently, we have also produced vacuum-deposited OLEDs, which display luminance up to 32 500 cd m and a maximum EQE of 11.8%

    Photophysical and Photocatalytic Properties of Covalent Organic Frameworks

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    This dissertation is most interested in how a class of materials known as covalent organic frameworks (COFs) can be designed to capture photon energy to initiate chemical reactions. Different COF designs change how long the energy is held, how it migrates, and how it is dispersed – and these differences can be used to change their performance as artificial photosynthesis platforms. Thus, it is helpful to have an informative discussion about the processes behind natural photosynthesis, that is, nature’s light harvesting strategies and photocatalytic schemes (Section 1.2) and will lead into an introduction of COFs and why they possess unique potential as artificial photosynthesis platforms (Section 1.3). Their beneficial physical qualities are complemented by understanding their electronic structures from theoretically predicted properties with specific focus on topological symmetry (Section 1.4). Synthesizing and characterizing COF systems then becomes an important consideration (Section 1.5) along with how their excited state behaviors are probed and interpreted at reaction timescales by ultrafast spectroscopic techniques (Section 1.6). Finally, a look is taken at how COF structure versatility adds unique potential in catalyst engineering (Section 1.7). The main body of this dissertation will present five main research projects that seek to test theoretical predictions, assess the impact of COF planarity, or fine tune electronic structures. To test theoretical predictions, “Tuning Photoexcited Charge Transfer in Imine-Linked Two-Dimensional Covalent Organic Frameworks, which involves exploring nodal symmetry in topologically similar COFs by varying monomers, is reported. This work has implications on charge separation characteristics of COFs which is important to retain activated catalytic sites for chemical reactions. The second project, “Impact of πConjugation Length on the Excited-State Dynamics of Star-Shaped Carbazole-π-Triazine Organic Chromophores,” doesn’t directly probe COF systems, but looks at the role of dihedral angles on intersystem crossing (ISC) rates in organic chromophores with similar star-shaped motifs like those often found in COFs. Another study on planarity is “Conjugation- and Aggregation-Directed Design of Covalent Organic Frameworks as White-Light-Emitting Diodes” that explores planar and non-planar COFs and the how this affects the deactivation of their photoexcited states. “Wavelength Dependent Excitonic Properties of Imine-Linked Covalent Organic Frameworks,” explores how subtle changes in donor-acceptor arrangements can lead to differences in excited state populations. Finally, the seminal work in this dissertation, “Imine Reversal Mediates Charge Separation and CO2 Photoreduction in Covalent Organic Frameworks,” explores the effect of the imine bond on photophysical and photocatalytic properties

    Curved nanographenes: Multiple emission, thermally activated delayed fluorescence, and non-radiative decay

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    The intriguing and rich photophysical properties of three curved nanographenes (CNG 6, 7, and 8) are investigated by time-resolved and temperature-dependent photoluminescence (PL) spectroscopy. CNG 7 and 8 exhibit dual fluorescence, as well as dual phosphorescence at low temperature in the main PL bands. In addition, hot bands are detected in fluorescence as well as phosphorescence, and, in the narrow temperature range of 100–140 K, thermally activated delayed fluorescence (TADF) with lifetimes on the millisecond time-scale is observed. These findings are rationalized by quantum-chemical simulations, which predict a single minimum of the S1 potential of CNG 6, but two S1 minima for CNG 7 and CNG 8, with considerable geometric reorganization between them, in agreement with the experimental findings. Additionally, a higher-lying S2 minimum close to S1 is optimized for the three CNG, from where emission is also possible due to thermal activation and, hence, non-Kasha behavior. The presence of higher-lying dark triplet states close to the S1 minima provides mechanistic evidence for the TADF phenomena observed. Non-radiative decay of the T1 state appears to be thermally activated with activation energies of roughly 100 meV and leads to disappearance of phosphorescence and TADF at T > 140
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