Computations on electroactive molecular materials: from donor–acceptor architectures to supramolecular polymers

En la presente tesis, se ha realizado un estudio teórico exhaustivo sobre una gran variedad de sistemas moleculares electroactivos con propiedades ópticas, electónicas y de autoensamblaje interesantes para ser explotadas en electrónica molecular y ciencia de nuevos materiales. Los resultados del trabajo de investigación se resumen en tres secciones de acuerdo con: Arquitecturas dador–acceptor covalentes Se han caracterizado teóricamente las propiedades electrónicas y ópticas de cromóforos dador–aceptor que absorben luz en todo el espectro visible. En particular, los cálculos DFT han proporcionado información sobre los regioisómeros más interesantes de exTTF-DCF para obtener un máximo beneficio en celdas solares sensibilizadas con colorantes (DSCs). Los tres regioisómeros posibles muestran características de absorción equivalentes en comparación con el análogo exTTF-TCF, con bandas de transferencia de carga intensas de baja energía calculadas en el intervalo visible. El derivado exTTF-DCF-A ha demostrado ser el colorante más prometedor debido a su anclaje más estable y su disposición perpendicular al unirse al semiconductor de TiO2. Asimismo, se han investigado cromóforos di-ramificados basados en el fragmento dador de hemiexTTF, que muestran bandas de transferencia de carga de baja energía y un desplazamiento al rojo de la absorción tras la inclusión del puente π-conjugado EDOT entre las unidades dador y aceptor. Los cálculos de primeros principios demuestran que el modo de adsorción más plausible de los colorantes di-ramificados sustituidos con carboxilo en dióxido de titanio es una coordinación monodentada del grupo ácido cianoacrílico en ambas ramas de anclaje. Esta coordinación cuenta con el anclaje más estable del colorante di-ramificado a la superficie del semiconductor, así como con dos amplias e intensas bandas LUMO que mejorarían la inyección de electrones en el semiconductor. Complejos dador–aceptor supramoleculares Se han estudiado complejos supramoleculares dador–aceptor bajo el marco DFT incluyendo correcciones de dispersión para ayudar a racionalizar el origen de las fuerzas que guían el autoensamblaje de nanoformas de carbono por receptores electroactivos. Los cálculos teóricos muestran que la incompatibilidad convexo–planar entre los anfitriones de porfirina y un derivado de fullereno C60 queda superada por fuertes interacciones no covalentes, que muestran una naturaleza electrostática predominante tras la sustitución metálica en la porfirina. Cálculos químico–cuánticos de receptores de porfirina ditópicos de fullereno C60 confirman una reducción de la densidad electrónica del receptor tras la inclusión del primer huésped de fullereno, de acuerdo con la cooperatividad negativa encontrada experimentalmente para estos sistemas. De forma interesante, la cinta de porfirina ditópica puede acomodar la segunda bola de C60 en una posición syn o anti. Los cálculos de primeros principios demuestran que la disposición syn es ~ 5 kcal mol−1 más estable, hecho que se explica por una interacción π–π estabilizante entre las dos bolas de fullereno que compensa parcialmente la cooperatividad negativa. Por otra parte, los agregados supramoleculares cóncavo-cóncavo inesperados se han calculado como los ensamblajes más estables para los complejos dador–aceptor de truxTTF•buckybowl. En estas configuraciones, no sólo interacciones π–π sino especialmente contactos CH···π son prominentes. Las estimaciones de energía libre para la asociación dador–aceptor indican que sólo los agregados escalonados (cóncavo–cóncavo) se forman en disolución, en buen acuerdo con la información de RMN experimental. Los cálculos teóricos confirman la existencia de bandas de transferencia de carga débiles a bajas energías descritas por la promoción de un electrón desde el dador de electrones truxTTF al buckybowl, que actúa como aceptor. La existencia de las correspondientes especies de separación de carga tras la fotoexcitación se ha confirmado mediante técnicas espectroscópicas en una fructífera colaboración con grupos de investigación experimental. Polímeros Supramoleculares Se han utilizado bloques de construcción electroactivos de tipo discótico para analizar teóricamente las propiedades de autoensamblaje en polímeros supramoleculares gobernados por interacciones no covalentes. En particular, se han calculado trisamidas N-centradas que sufren agregación supramolecular en forma de hélices columnares guiadas por interacciones π–π entre los núcleos aromáticos y, especialmente, por una red triple de enlaces de H entre las amidas. Este patrón de enlaces de H es el responsable de la cooperatividad del proceso de polimerización, que provoca un incremento en la energía de asociación y momento dipolar por unidad monomérica al aumentar el tamaño del polímero. El enfoque teórico multi-nivel empleado inequívocamente ha permitido la identificación del sentido helicoidal en función de la cadena alifática estereogénica, e indica un efecto insignificante de la conectividad de los grupos amida en la orientación helicoidal resultante. Por otro lado, se han estudiado derivados de pirenoimidazol que promueven el autoensamblaje en forma de organogeles con una emisión azul inesperada. Los cálculos teóricos demuestran, en sinergia con las evidencias experimentales, que el proceso de autoensamblaje se rige por las interacciones de los enlaces de H entre los grupos imidazol y amida junto con un gran número de interacciones CH···π entre la cadena alifática y los núcleos aromáticos de pireno. La ausencia de interacciones π–π entre los núcleos de pireneimidazol permite además explicar la preservación de las características de emisión de azul que exhiben dichos derivados tras la polimerización

Charge-transfer interactions between fullerenes and a mesoporous tetrathiafulvalene-based metal-organic framework

The design of metal-organic frameworks (MOFs) incorporating electroactive guest molecules in the pores has become a subject of great interest in order to obtain additional electrical functionalities within the framework while maintaining porosity. Understanding the charge-transfer (CT) process between the framework and the guest molecules is a crucial step towards the design of new electroactive MOFs. Herein, we present the encapsulation of fullerenes (C60) in a mesoporous tetrathiafulvalene (TTF)-based MOF. The CT process between the electron-acceptor C60 guest and the electron-donor TTF ligand is studied in detail by means of different spectroscopic techniques and density functional theory (DFT) calculations. Importantly, gas sorption measurements demonstrate that sorption capacity is maintained after encapsulation of fullerenes, whereas the electrical conductivity is increased by two orders of magnitude due to the CT interactions between C60 and the TTF-based framework

Selective CO2 sorption using a compartmentalized coordination polymers with discrete voids

Carbon capture and storage with porous materials is one of the most promising technologies to minimize CO2 release into the atmosphere. Here, we report a family of compartmentalized coordination polymers (CCPs) capable of capturing gas molecules in a selective manner based on two novel tetrazole-based ligands. Crystal structures have been modelled theoretically under the Density Functional Theory (DFT) revealing the presence of discrete voids of 380 Å3. Single gas adsorption isotherms of N2, CH4 and CO2 have been measured, obtaining a loading capacity of 0.6, 1.7 and 2.2 molecules/void at 10 bar and at 298 K for the best performing material. Moreover, they present excellent selectivity and regenerability for CO2 in mixtures with CH4 and N2 in comparison with other reported materials, as evidenced by dynamic breakthrough gas experiments. These frameworks are therefore great candidates for separation of gas mixtures in the chemical engineering industry

Spin-Crossover Grafted Monolayer of a Co(II) Terpyridine Derivative Functionalized with Carboxylic Acid Groups

The synthesis and characterization of a new Co(II) spin-crossover (SCO) complex based on 4′-(4-carboxyphenyl)−2,2′:6′,2″-terpyridine ligand are reported. This complex can be successfully grafted on silver surface maintaining the SCO behavior. Thus, atomic force microscopy (AFM), matrix assisted laser desorption ionization - time-of-flight mass spectrometry (MALDI-TOF MS), Raman spectroscopy, and XPS measurements, upon surface deposition, evidence the formation of a monolayer of intact molecules grafted through carboxylate groups to the Ag surface. Three different techniques: Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS), supported by first-principles calculations, confirm that the deposited molecules undergo a gradual spin transition with temperature. This phenomenon is unprecedented for a monolayer of molecules directly grafted onto a metallic surface from solution

Tuning the Optical Absorption of Sn-, Ge-, and Zn-Substituted Cs2AgBiBr6 Double Perovskites: Structural and Electronic Effects

Lead-free halide double perovskites (DPs) are highly tunable materials in terms of chemical composition and optical properties. One of the most widely reported DPs is Cs2AgBiBr6, which is envisaged as a promising absorber for photovoltaics. Nevertheless, its bandgap (around 1.9−2.3 eV) remains too large for common tandem solar cells. In this work, we report the mechanochemical synthesis of Sn-, Ge-, and Zn-substituted Cs2AgBiBr6 in powder form; their bandgaps reach 1.55, 1.80, and 2.02 eV, respectively. These differences are rationalized through density functional theory calculations, demonstrating combined electronic and structural (disorder) effects introduced by the divalent metal-cation substituents. Finally, we present the first vacuum-deposited thin films of the Sn-substituted DP, which also show a notable narrowing of the bandgap, and this paves the way toward its implementation in photovoltaic solar cells

Semiconductor Porous Hydrogen-Bonded Organic Frameworks Based on Tetrathiafulvalene Derivatives

Herein, we report on the use of tetrathiavulvalene-tetrabenzoic acid, H4TTFTB, to engender semiconductivity in porous hydrogen-bonded organic frameworks (HOFs). By tuning the synthetic conditions, three different polymorphs have been obtained, denoted MUV-20a, MUV-20b, and MUV-21, all of them presenting open structures (22, 15, and 27%, respectively) and suitable TTF stacking for efficient orbital overlap. Whereas MUV-21 collapses during the activation process, MUV-20a and MUV-20b offer high stability evacuation, with a CO2 sorption capacity of 1.91 and 1.71 mmol g-1, respectively, at 10 °C and 6 bar. Interestingly, both MUV-20a and MUV-20b present a zwitterionic character with a positively charged TTF core and a negatively charged carboxylate group. First-principles calculations predict the emergence of remarkable charge transport by means of a through-space hopping mechanism fostered by an efficient TTF π-π stacking and the spontaneous formation of persistent charge carriers in the form of radical TTF¿+ units. Transport measurements confirm the efficient charge transport in zwitterionic MUV-20a and MUV-20b with no need for postsynthetic treatment (e.g., electrochemical oxidation or doping), demonstrating the semiconductor nature of these HOFs with record experimental conductivities of 6.07 × 10-7 (MUV-20a) and 1.35 × 10-6 S cm-1 (MUV-20b)

Exploiting the Redox Activity of MIL-100(Fe) Carrier Enables Prolonged Carvacrol Antimicrobial Activity

The design of efficient food contact materials that maintain optimal levels of food safety is of paramount relevance to reduce the increasing number of foodborne illnesses. In this work, we develop a smart composite metal-organic framework (MOF)-based material that fosters a unique prolonged antibacterial activity. The composite is obtained by entrapping a natural food preserving molecule, carvacrol, into a mesoporous MIL-100(Fe) material following a direct and biocompatible impregnation method, and obtaining particularly high payloads. By exploiting the intrinsic redox nature of the MIL-100(Fe) material, it is possible to achieve a prolonged activity against Escherichia coli and Listeria innocua due to a triggered two-step carvacrol release from films containing the carvacrol@MOF composite. Essentially, it was discovered that based on the underlying chemical interaction between MIL-100(Fe) and carvacrol, it is possible to undergo a reversible charge-transfer process between the metallic MOF counterpart and carvacrol upon certain chemical stimuli. During this process, the preferred carvacrol binding site was monitored by infrared, Mössbauer, and electron paramagnetic resonance spectroscopies, and the results are supported by theoretical calculations

Chalcohalide Antiperovskite Thin Films with Visible Light Absorption and High Charge-Carrier Mobility Processed by Solvent-Free and Low-Temperature Methods

Silver chalcohalide antiperovskites represent a rather unexplored alternative to lead halide perovskites and other semiconductors based on toxic heavy metals. All synthetic approaches reported so far for Ag3SI and Ag3SBr require long synthesis times (typically days, weeks, or even months) and high temperatures. Herein, we report the synthesis of these materials using a fast and low-temperature method involving mechanochemistry. Structural and optical properties are examined experimentally and supported by first-principles calculations. Furthermore, we deposit Ag3SI as thin films by pulsed laser deposition and characterize its optoelectronic properties using optical-pump-terahertz-probe measurements, revealing a high charge-carrier mobility of 49 cm2 V-1 s-1. This work paves the way to the implementation of chalcohalide antiperovskites in various optoelectronic applications

Implementing Mesoporosity in Zeolitic Imidazolate Frameworks through Clip-Off Chemistry in Heterometallic Iron-Zinc ZIF-8

Bond breaking has emerged as a new tool to postsynthetically modify the pore structure in metal-organic frameworks since it allows us to obtain pore environments in structures that are inaccessible by other techniques. Here, we extend the concept of clip-off chemistry to archetypical ZIF-8, taking advantage of the different stabilities of the bonds between imidazolate and Zn and Fe metal atoms in heterometallic Fe-Zn-ZIF-8. We demonstrate that Fe centers can be removed selectively without affecting the backbone of the structure that is supported by the Zn atoms. This allows us to create mesopores within the highly stable ZIF-8 structure. The strategy presented, combined with control of the amount of iron centers incorporated into the structure, permits porosity engineering of ZIF materials and opens a new avenue for designing novel hierarchical porous frameworks

Multivariate sodalite zeolitic imidazolate frameworks: a direct solvent-free synthesis

Different mixed-ligand Zeolitic Imidazolate Frameworks (ZIFs) with sodalite topology, i.e. isoreticular to ZIF-8, unachievable by conventional synthetic routes, have been prepared using a solvent-free methodology. In particular, the versatility of this method is demonstrated with three different metal centres (Zn, Co and Fe) and binary combinations of three different ligands (2-methylimidazole, 2-ethylimidazole and 2-methylbenzimidazole). One combination of ligands, 2-ethylimidazole and 2-methylbenzimidazole, results in the formation of SOD frameworks for the three metal centres despite this topology not being obtained for the individual ligands. Theoretical calculations confirm that this topology is the lowest in energy upon ligand mixing