An Ultra‐Long‐Lived Triplet Excited State in Water at Room Temperature: Insights on the Molecular Design of Tridecafullerenes

Suitably engineered molecular systems exhibiting triplet excited states with very long lifetimes are important for high-end applications in nonlinear optics, photocatalysis, or biomedicine. We report the finding of an ultra-long-lived triplet state with a mean lifetime of 93 ms in an aqueous phase at room temperature, measured for a globular tridecafullerene with a highly compact glycodendrimeric structure. A series of three tridecafullerenes bearing different glycodendrons and spacers to the C60 units have been synthesized and characterized. UV/Vis spectra and DLS experiments confirm their aggregation in water. Steady-state and time-resolved fluorescence experiments suggest a different degree of inner solvation of the multifullerenes depending on their molecular design. Efficient quenching of the triplet states by O2 but not by waterborne azide anions has been observed. Molecular modelling reveals dissimilar access of the aqueous phase to the internal structure of the tridecafullerenes, differently shielded by the glycodendrimeric shell

Efficient catalysts for water oxidation: synthesis, characterization and computational study

L'emmagatzematge d'energia en enllaços químics ha esdevingut avui dia clau principal per fer possible la transició de la dependència del carboni a l'energia renovable (per exemple, solar o eòlica) resolent-ne la intermitència. La divisió de l’aigua implica l’emmagatzematge de l’energia en H2 i O2. Tot i això, la reacció d'oxidació de l'aigua a O2 suposa un coll d'ampolla d'aquest procés. Aquesta Tesi s'uneix al repte internacional de comprendre l'oxidació de l'aigua cercant catalitzadors heterogenis eficients, robustos i econòmics. Ens hem centrat en l'ús de polioxometalatos (POMs) descrits com a catalitzadors d'oxidació de l'aigua a diferents pH. Hem sintetitzat, caracteritzat i avaluat la seva activitat catalítica. Hem confirmat l'estabilitat i la integritat dels catalitzadors caracterització pre i post catàlisi. En primer lloc, hem estudiat els polioxometalatos que contenen Co en medi àcid explorant l'estructura, la composició i la nuclearitat a través d'una sèrie de cobalt-POM com a sals insolubles de Cs+ i Ba2+ incorporats en elèctrodes de pasta de carboni (CP). Després passem a avaluar el POM compost amb el metall de transició més abundant i més barat (ferro). Ens hem centrat en el sandvitx Weakley de tetraferro [(FeIIIOH2)2FeIII2(PW9O34)2]6- (Fe4P2W18) com a isoestructural del sandvitx Weakley de tetracobalt, un dels Co-POM més estudiats. Hem començat a avaluar l'activitat catalítica en medi neutre confirmant el catalitzador i l'eficiència faradaica. Després, investiguem el mecanisme de reacció emprant mètodes computacionals per comprendre el diferent rendiment catalític. Finalment, provem el caràcter robust de POM en medi àcid a l'entorn de CP, avaluant el comportament catalític Fe4P2W18 explorant l'efecte pH. Efectivament, confirmem l'estabilitat del Fe-POM. Sorprenentment, observem una baixa eficiència faradaica que relacionem amb una reacció competitiva basada en l'oxidació del carboni.El almacenamiento de energía en enlaces químicos se ha convertido hoy en día en la clave principal para hacer posible la transición de la dependencia del carbono a la energía renovable (por ejemplo, solar o eólica) resolviendo la intermitencia de estos. La división del agua implica el almacenamiento de la energía en H2 y O2. Sin embargo, la reacción de oxidación del agua a O2 supone un cuello de botella de este proceso. Esta Tesis se une al desafío internacional de comprender la oxidación del agua buscando catalizadores heterogéneos eficientes, robustos y económicos. Nos hemos centrado en el uso de polioxometalatos (POMs) descritos como catalizadores de oxidación del agua a diferentes pH. Hemos sintetizado, caracterizado y evaluado su actividad catalítica. Hemos confirmado la estabilidad e integridad de los catalizadores caracterización pre y post catálisis. En primer lugar, hemos estudiado los polioxometalatos que contiene Co en medios ácidos explorando la estructura, la composición y la nuclearidad a través de una serie de cobalto-POM como sales insolubles de Cs+ y Ba2+ incorporados en electrodos de pasta de carbono (CP). Luego pasamos a evaluar el POM en función del metal de transición más abundante y más barato (hierro). Nos hemos centrado en el sándwich Weakley de tetrahierro [(FeIIIOH2)2FeIII2(PW9O34)2]6- (Fe4P2W18) como isoestructural del sándwich Weakley de tetracobalto, uno de los Co-POM más estudiados. Hemos empezado a evaluar la actividad catalítica en medio neutro confirmando el catalizador y la eficiencia faradaica. Luego, investigamos el mecanismo de reacción empleando métodos computacionales para comprender el diferente rendimiento catalítico. Finalmente, probamos el carácter robusto de POM en medios ácidos en el entorno de CP, evaluando el comportamiento catalítico Fe4P2W18 explorando el efecto pH. Efectivamente, confirmamos la estabilidad del Fe-POM. Sorprendentemente, observamos una baja eficiencia faradaica que relacionamos con una reacción competitiva basada en la oxidación del carbono.Storage energy into chemical bonds has become nowadays the primary key to make possible the transition from carbon dependency to renewable (i.e., solar or wind) energy solving the intermittency of this. Water splitting involves storing of the energy in H2 and O2. However, the oxidation reaction of water to O2 is a bottleneck in this process. This Thesis cover the international challenge of deeply understanding of water oxidation and search for an efficient, robust, and inexpensive heterogeneous catalysts. We have focused on the use of already described polyoxometalates (POMs) as water oxidation catalysts at different pH. We have synthesized, characterized, and evaluate their catalytical activity. We have confirmed the stability and integrity of the catalysts by performing a pre and post catalytic characterization. First, we have deeply studied the Co-containing polyoxometalate in acidic media by exploring structure, composition and nuclearity through a series of Cobalt-POM incorporated into carbon paste electrodes as insoluble salts of Cs+ and Ba2+. Then we have moved to evaluate POM based on the most abundant and cheapest transition metal (Iron). We have focused on the tetrairon Weakley sandwich [(FeIIIOH2)2FeIII2(PW9O34)2]6- (Fe4P2W18) as is isostructural of tetracobalt Weakley sandwich one of the most extensively studied Co-POMs. We have started evaluating the catalytic activity in the neutral media confirming true catalyst and faradaic efficiency. Then we have investigated the reaction mechanism employing computational methods to understand the different catalytic performance of Co-POM. Finally, we want to test the robust character of POM in acidic media in the carbon paste environment, we have evaluated Fe4P2W18 as water oxidation catalyst in acidic media to determine the pH effect. Effectively, we could confirm the stability of the Fe-POM. Surprisingly, our results show a low faradaic efficiency which we could relate to a competitive reaction to water oxidation based on carbon oxidation

Understanding polyoxometalates as water oxidation catalysts through iron vs. cobalt reactivity

International audienceCobalt polyoxometalates (Co-POMs) have emerged as promising water oxidation catalysts (WOCs), with the added advantage of their molecular nature despite being metal oxide fragments. In comparison with metal oxides, that do not offer well-defined active surfaces, POMs have a controlled, discrete structure that allows for precise correlations between experiment and computational analyses. Thus, beyond highly active WOCs, POMs are also model systems to gain deeper mechanistic understanding on the oxygen evolution reaction (OER). The tetracobalt Weakley sandwich [CoII4(H2O)2(B-α-PW9O34)2]10− (Co4-WS) has been one of the most extensively studied. We have compared its activity with that of the iron analog [FeIII4(H2O)2(B-α-PW9O34)2]6− (Fe4-WS) looking for the electronic effects determining their activity. Furthermore, the effect of POM nuclearity was also investigated by comparison with the iron- and cobalt-monosubstituted Keggin clusters. Electrocatalytic experiments employing solid state electrodes containing the POMs and the corresponding computational calculations demonstrate that CoII-POMs display better WOC activity than the FeIII derivatives. Moreover, the activity of POMs is less influenced by their nuclearity, thus Weakley sandwich moieties show slightly improved WOC characteristics than Keggin clusters. In good agreement with the experimental data, computational methods, including pKa values, confirm that the resting state for Fe-POMs in neutral media corresponds to the S1 (FeIII–OH) species. Overall, the proposed reaction mechanism for Fe4-WS is analogous to that found for Co4-WS, despite their electronic differences. The potential limiting step is a proton-coupled electron transfer event yielding the active S2 (FeIV[double bond, length as m-dash]O) species, which receives a water nucleophilic attack to form the O–O bond. The latter has activation energies slightly higher than those computed for the Co-POMs, in good agreement with experimental observations. These results provide new insights for the accurate understanding of the structure–reactivity relationships of polyoxometalates in particular, and or metal oxides in general, which are of utmost importance for the development of new bottom-up synthetic approaches to design efficient, robust and non-expensive earth-abundant water oxidation catalyst