Theoretical modelling of photoactive molecular systems: insights using the Density Functional Theory

Nous rendons compte ici des performances d'une approche moderne et opérante de la théorie de la fonctionnelle de la densité (DFT) pour la prédiction du comportement photophysique de composés du ruthénium (II) et de l'osmium(II). Pour interpréter leurs propriétés d'absorption électronique, l'approche DFT dépendante du temps a été utilisée. Nous illustrons notre propos par l'analyse des deux systèmes suivants : (1) un complexe qui présente une isomérisation photo-induite de la liaison de coordination impliquant l'un de ses ligands ; (2) une molécule bipartite composée d'un complexe photosensibilisateur lié à un accepteur d'électron, conçue pour être le siège d'un transfert d'électron photo-induit conduisant à la formation d'un état de « charges séparées » (CS). Au-delà du remarquable accord quantitatif constaté entre les caractéristiques calculées et expérimentales des spectres d'absorption, nos résultats ont permis de clarifier, ab initio, la nature des états excités impliqués, illustrant ainsi les vertus prédictives de l'approche théorique employée.An account of the performance of a modern and efficient approach to Density Functional Theory (DFT) for the prediction of the photophysical behavior of a series of Ru(II) and Os(II) complexes is given. The time-dependent-DFT method was used to interpret their electronic spectra. Two different types of compounds have been analyzed: (1) a complex undergoing a light induced isomerization of one of its coordination bonds; (2) an inorganic dyads expected to undergo intramolecular photoinduced electron transfer to form a charge separated (CS) sate. Besides the noticeable quantitative agreement between computed and experimental absorption spectra, our results allow to clarify, by first principles, both the nature of the excited states and the photochemical behavior of these complex systems, thus underlying the predictive character of the theoretical approach

Intramolecular spin alignment in photomagnetic molecular devices: a theoretical study

Ground- and excited-state magnetic properties of recently characterized π-conjugated photomagnetic organic molecules are analyzed by the means of density functional theory (DFT). The systems under investigation are made up of an anthracene (An) unit primarily acting as a photosensitizer (P), one or two iminonitroxyl (IN) or oxoverdazyl (OV) stable organic radical(s) as the dangling spin carrier(s) (SC), and intervening phenylene connector(s) (B). The magnetic behavior of these multicomponent systems, represented here by the Heisenberg-Dirac magnetic exchange coupling (J), as well as the EPR observables (g tensors and isotropic A values), are accurately modeled and rationalized by using our DFT approach. As the capability to quantitatively assess intramolecular exchange coupling J in the excited state makes it possible to undertake rational optimization of photomagnetic systems, DFT was subsequently used to model new compounds exhibiting different connection schemes for their functional components (P, B, SC). We show in the present work that it is worthwhile considering the triplet state of anthracene, that is, P when promoted in its lowest photoexcited state, as a full magnetic site in the same capacity as the remote SCs. This framework allows us to accurately account for the interplay between transient (³An) and persistent (IN, OV) spin carriers, which magnetically couple according to a sole polarization mechanism essentially supported by phenyl connector(s). From our theoretical investigations of photoinduced spin alignment, some general rules are proposed and validated. Relying on the analysis of spin-density maps, they allow us to predict the magnetic behavior of purely organic magnets in both the ground and the excited states. Finally, the notion of photomagnetic molecular devices (PMMDs) is derived and potential application towards molecular spintronics disclosed

Ab Initio Optimization of Dye-Sensitized Solar Cells: From Individual Components to Interfaces.

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Modeling Dye-Sensitized Solar Cells: From Theory to Experiment

International audienceDensity functional theory (DFT) and time-dependent DFT are useful computational approaches frequently used in the dye-sensitized solar cell (DSSC) community in order to analyze experimental results and to clarify the elementary processes involved in the working principles of these devices. Indeed, despite these significant contributions, these methods can provide insights that go well beyond a purely descriptive aim, especially when suitable computational approaches and methodologies for interpreting and validating the computational outcomes are developed. In the present contribution, the possibility of using recently developed computational approaches to design and interpret the macroscopic behavior of DSSCs is exemplified by the study of the performances of three new TiO2-based DSSCs making use of organic dyes, all belonging to the expanded pyridinium family

Challenging the [Ru(bpy) 3 ] 2+ Photosensitizer with a Triazatriangulenium Robust Organic Dye for Visible-Light-Driven Hydrogen Production in Water

Photosensitizers used in homogeneous photocatalytic systems for artificial photosynthesis such as hydrogen production are typically based on expensive transition metal complexes such as d 6 ruthenium(II) or iridium(III). In this work, we demonstrate efficient H2 production in acidic water by using an organic dye derived from the triazatriangulenium (TATA +) family as a visible-light-absorbing photosensitizer (PS). By associating the hydrosoluble tris(ethoxyethanol)triazatriangulenium with an efficient H2-evolving cobalt catalyst and ascorbic acid as sacrificial electron donor (SD), remarkable photocatalytic performances were reached in aqueous solution at pH 4.5, under visible light irradiation, with up to 8950 catalytic cycles versus catalyst. Noteworthy, the performances of this dye largely exceed those of the benchmark Ru tris-bipyridine in the same experimental conditions, when low concentrations of catalyst are used. This higher efficiency has been clearly ascribed to the remarkable robustness of the reduced form of the organic dye, TATA •. Indeed, the combination of the planar structure of TATA + together with the presence of the three electron-donating nitrogen atoms, promote the stabilization of TATA • by delocalization of the radical, thereby preventing its degradation in the course of photocatalysis. By contrast the reduced form of the Ru photosensitizer, [Ru II (bpy)2(bpy •-)] + ("Ru-"), is much less stable. Nanosecond transient absorption experiments confirm the formation of TATA • in the course of the photocatalytic process in accordance with the mechanism initiated by the reductive quenching of the singlet excited state of TATA + by ascorbate. The second electron transfer from TATA • to the catalyst has also been evidenced by this technique with the detection of the signature of the reduced Co(I) form of the catalyst. The present study establishes that certain organic dyes are to be considered as relevant alternatives to expensive metal-based PSs insofar as they can exhibit a high stability under prolonged irradiation, even in acidic water, thereby providing valuable insights for the development of robust molecular systems only based on earth-abundant elements for solar fuel generation