Academic literature on the topic 'Photoinduced charge accumulation'

Titania is a widely used photocatalytic material possessing such advantages as low cost and high reactivity under the ultraviolet light illumination. However, the fast recombination of photoexcited charge carriers limits its application. Herein, we have synthesized original nanomaterials with mosaic structures that exhibited well-defined heterojunctions and new properties. Using SEM, XRD, EPR spectroscopy, photocatalytic measurements, and photoinduced pathphysiological activity of these photocatalysts, we studied the processes of charge carrier accumulation in TiO2/MoO3, TiO2/WO3, and TiO2/V2O5 under in situ UV illumination with emphasis on the charge exchange between energy levels of these nanosized semiconductors. It is shown that the accumulation of photoinduced charges occurs in two forms (i) filled electron traps corresponding to Ti4+/Ti3+ levels and (ii) Mo5+ centers, both forms contributing to the photoinduced biocide activity of the samples. This work demonstrates that light exposure of heterostructure photocatalysts with mosaic surfaces produces different types of charge-trapping centers capable of interacting with molecular oxygen yielding peroxo species, which provide long-life light-induced ”self-cleaning” behavior. Such photoaccumulating materials open new opportunities in developing light-driven self-sterilization structures exhibiting a prolonged bactericidal effect up to 10 h after stopping light exposure.

Strong Lewis acid/Lewis base interactions between Sc³⁺and superoxide anions permit the light-driven accumulation of two positive charges on oligotriarylamine units with appended Ru(ii) photosensitizers.

Semiconductor­metal nanocomposites provide a simple and convenient way to tailor the properties of photocatalysts. Modification of semiconductor surface improves charge separation and promotes interfacial charge-transfer processes in nanocomposite systems. Charge accumulation in the metal layer results in Fermi-level equilibration raising the quasi-Fermi level of the composite close to the conduction band level of the oxide semiconductor. Phototransformations of such compositesincluding morphological changes, interfacial charge-transfer processes and photocurrent generation of TiO2-capped gold colloidsare presented in this review article.

Under visible light irradiation [Cr(ttpy)2]3+ can be reduced twice by a tertiary amine; the photoreduction processes are accelerated in the presence of [Ru(bpy)₃]²⁺ acting as an antenna thanks to an efficient electron transfer reaction from [Ru(bpy)₃]^2+* to [Cr(ttpy)₂]³⁺.

Photoinduced electron transfer processes in self-assembled zinc porphyrin ( ZnP ) or zinc phthalocyanine ( ZnPc ) with semiconducting (7,6)- and (6,5)-enriched SWCNTs were investigated. To bind photosensitizers to SWCNTs, first, pyrene covalently functionalized with a phenylimidazole (Im-Pyr) entity was treated with SWCNTs. Exfoliation of SWCNTs occurred due to π–π stacking of pyrene with nanotubes walls leaving the imidazole entity that was subsequently used to coordinate ZnP or ZnPc in o-dichlorobenzene (DCB). The donor-acceptor nanohybrids thus formed were characterized by TEM imaging, steady-state UV-visible-near IR absorption and fluorescence spectra. Free-energy calculations suggested possibility of electron transfer from the photoexcited ZnP or ZnPc to Im-Pyr/SWCNT(n,m) in the nanohybrids. Consequently, steady-state and time-resolved fluorescence studies revealed efficient quenching of the singlet excited state of ZnP or ZnPc with the rate constants of charge separation (k CS ) in the range of (3–6) × 109 s-1. Nanosecond transient absorption technique confirmed the electron transfer products, ZnP·+←Im-Pyr/SWCNT·- and ZnPc·+←Im-Pyr/SWCNT·- (and opposite charged pairs) having characteristic absorptions with the decay rate constants due to charge recombination (k CR ) in the range of (1.4–2.4) × 107 s-1, corresponding to lifetimes of radical ion-pairs in the 70–100 ns range. The SWCNT·- was further utilized to mediate electrons to hexyl-viologen dication (HV2+) resulting in an electron-accumulation process in the presence of sacrificial electron donor, offering additional proof for the occurrence of photoinduced charge-separation and potential utilization of these materials in light energy harvesting applications. Further, photoelectrochemical cells have been constructed on FTO/ SnO2 electrodes to verify their ability to directly convert light into electricity. An IPCE efficiency of up to 7% has been achieved in case of ZnP←Im-Pyr/SWCNT modified electrode.

Inspirée de la nature, la conversion de l’énergie solaire par photosynthèse artificielle est l’une des solutions les plus prometteuses à la crise énergétique mondiale actuelle. Cependent, déployer des systèmes artificiels fonctionnels nécessite une compréhension approfondie des processus intégrés dans le fonctionnement des systèmes naturels car ils fournissent les directions pour réaliser des dispositifs de photosynthèse artificielle. Ces processus incluent l’absorption de la lumière, la séparation des charges, plusieurs étapes de transfert de charges menant à leur accumulation et, enfin, la catalyse. Dans ce travail, nous étudions toutes ces étapes élémentaires en utilisant des approches spectroscopiques résolues en temps, dans le but d’explorer la photophysique de différents systèmes moléculaires biomimétiques dédiés à la photoréduction du dioxyde de carbone (CO₂) pour produire des carburants solaires. Nous commençons par le développement d’un nouveau dispositif expérimental pompe-pompe-sonde, capable de déclencher et de détecter l’accumulation progressive de charges grâce à une sonde Raman résonante. Un système modèle contenant le dication méthylviologène (MV²⁺) comme double accepteur d’électrons, le complexe prototypique [Ru(bpy)₃]²⁺ comme photosensibilisateur, et l’ascorbate comme donneur réversible d’électron est utilisé pour une preuve de concept de la technique. En effet, avec la première pompe, MV•⁺ est formé et détecté grâce à son mode vibrationnel caractéristique à 1356 cm⁻¹. Lorsque la concentration transitoire de , MV•⁺ atteint son maximum, nous déclenchons la deuxième pompe laser pour montrer la possibilité de suivre la formation réversible du MV⁰ à travers d’un mode vibrationnel unique à 992 cm⁻¹. Nous passons ensuite à l’étude de systèmes catalytiquement actifs composés de dérivés de porphyrine de fer en tant que catalyseurs pour la réaction de réduction du CO₂. Ces porphyrines sont intégrées dans des systèmes biomimétiques multicomposants contenant du [Ru(bpy)₃]²⁺ et de l’ascorbate comme photosensibilisateur et donneur réversible d’électron, respectivement. Pour le dérivé fonctionnalisé avec des groupements urées (FeUr), un catalyseur contenant un réseau de liaisons hydrogène logé dans sa seconde sphère de coordination, nous fournissons une description mécanistique complète de tous les processus photoinduits conduisant à l’accumulation de charges et son activation vers le CO₂. En atmosphère inerte, en partant de l’état d’oxydation Feˡˡˡ, nous rapportons l’accumulation de deux électrons vers la formation de l’état Feˡ à la suite de la stratégie pompe-pompe-sonde. Sous conditions catalytiques en présence de CO₂, notre approche fournit des preuves convaincantes que l’état d’oxydation Feˡ, produit de deux étapes consécutives de transfert d’électron, est déjà catalytiquement actif, comme en témoigne l’accumulation de l’intermédiaire stable Feˡˡ‒CO caractéristique du cycle de réduction du CO₂. D’une façon générale, nous montrons également que Feˡ est catalytiquement actif indépendamment de la stratégie de fonctionnalisation du macrocycle de la porphyrine, remettant en cause l’interprétation classique de la catalyse de réduction du CO₂ promue par les porphyrines de fer. Enfin, nous nous éloignons du complexe prototypique Ru(bpy)₃]²⁺ pour étudier la photophysique de différents photosensibilisateurs basés sur des éléments abondants sur terre, y compris des complexes à base de cuivre(I), un dérivé porphyrine de zinc (ZnF₂₀), et un colorant carbocationique entièrement organique (TATA⁺). Notamment, nous montrons que le TATA⁺ est capable de photosensibiliser l’accumulation de charges sur le système actif à base de FeUr, activant ainsi la réaction de réduction du CO₂. La caractérisation de nouveaux photosensibilisateurs basés sur des éléments abondants sur terre est fondamentale pour le développement de photosystèmes artificiels avec des applications concrètes dans le monde réel
Inspired by nature’s masterpiece of evolution, the conversion of solar energy through artificial photosynthesis is one of the most promising solutions to the ongoing global energy crisis. Deploying functional artificial mimics of the photosynthetic apparatus, however, requires a deep understanding of the processes embedded in the functioning of naturally photosensitizing organisms as they provide the roadmap to realize artificial photosynthetic devices. These processes include light harvesting, charge separation, multiple charge transfer steps leading to effective charge accumulation and, finally, efficient catalysis. In this work, we investigate all of these elementary steps by employing state-of-the-art time-resolved spectroscopic approaches with the goal of exploring the photophysics of different biomimetic molecular systems devoted to the photoreduction of carbon dioxide (CO₂) to produce energy-rich solar fuels. We start with the development of a novel pump-pump-probe experimental setup that is capable of triggering and detecting the stepwise accumulation of charge through the powerful lens of a resonance-enhanced Raman scattering probe. A model system containing the methyl viologen dication (MV²⁺) as a dual electron acceptor, the prototypical [Ru(bpy)₃]²⁺ complex as a photosensitizer, and ascorbate as a reversible electron donor is used for a proof-of-concept of the technique. Indeed, with the first pump, MV•⁺ is formed and detected through its fingerprint vibrational mode at 1356 cm⁻¹. When the transient concentration of MV•⁺ peaks, we fire the second laser pump and show the possibility of tracking the reversible formation of the two-electron accumulated MV⁰ species through a unique vibrational mode at 992 cm⁻¹. We then move on to investigating catalytically active systems featuring iron porphyrin derivatives as CO₂ reduction catalysts. These porphyrins are integrated into multicomponent biomimetic systems that similarly contain [Ru(bpy)₃]²⁺ and ascorbate as photosensitizer and reversible electron donor, respectively. For the urea-functionalized derivative (FeUr), a catalyst with a hydrogen-bonding network lodged in its second coordination sphere, we provide a full mechanistic depiction of all photoinduced processes leading to charge accumulation and its activation towards CO₂. In inert atmosphere, starting from Feˡˡˡ, we report the stepwise formation of the formal Feˡ species as a result of the double pump excitation strategy. Remarkably, under catalytic conditions in the presence of CO₂, our spectroscopy-based approach provides compelling evidence that the Feˡ oxidation state of FeUr, product of two consecutive electron transfer steps, is already catalytically active, evidenced by the accumulation of the stable Feˡˡ‒CO intermediate of the CO₂ reduction cycle. Going beyond FeUr, we show that Feˡ is catalytically active irrespective of the design strategy used in the functionalization of the porphyrin macrocycle, challenging the classical picture of CO₂ reduction catalysis promoted by iron porphyrins. Finally, we move away from the prototypical [Ru(bpy)₃]²⁺ complex and dive into the photophysics of different photosensitizers based on earth-abundant elements, including copper(I)-based complexes, a perfluorinated zinc porphyrin derivative (ZnF₂₀), and a fully organic triazatriangulenium carbocationic dye (TATA⁺). Importantly, we show that the TATA⁺ dye is capable of photosensitizing charge accumulation on the active FeUr-based system, activating it towards the reduction of CO₂. The characterization of new photosensitizing units based on abundant elements is fundamental for the development of artificial photosystems with real-world applications

Photoinduced electron transfer is involved in a number of photochemical and photobiological processes. One example of this is photosynthesis, where the absorption of sunlight leads to the formation of charge-separated states by electron transfer. The redox equivalents built up by successive photoabsorption and electron transfer is further used for the oxidation of water and reduction of carbon dioxide to sugars. The work presented in this thesis is part of an interdisciplinary effort aiming at a functional mimic of photosynthesis. The goal of this project is to utilize sunlight to produce renewable fuels from sun and water. Specifically, this thesis concerns photoinduced electron transfer in donor(D)-photosensitizer(P)-acceptor(A) systems, in mimic of the primary events of photosynthesis. The absorption of a photon typically leads to transfer of a single electron, i.e., charge separation to produce a single electron-hole pair. This fundamental process was studied in several molecular systems. The purpose of these studies was optimization of single electron transfer as to obtain charge separation in high yields, with minimum losses to competing photoreactions such as energy transfer.Also, the lifetime of the charge separated state and the confinement of the electron and hole in three-dimensional space are important in practical applications. This led us to explore molecular motifs for linear arrays based on Ru(II)bis-tridentate and Ru(II)tris-bidentate complexes. The target multi-electron catalytic reactions of water-splitting and fuel production require a build-up of redox equivalents upon successive photoexcitation and electron transfer events. The possibilities and challenges associated with such processes in molecular systems were investigated. One of the studied systems was shown to accumulate two electrons and two holes upon two successive excitations, without sacrificial redox agents and with minimum yield losses. From these studies, we have gained better understanding of the obstacles associated with step-wise photoaccumulation of charge and how to overcome them.