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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 (C 60 ) in a mesoporous tetrathiafulvalene (TTF)-based MOF. The CT process between the electron-acceptor C 60 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 C 60 and the TTF-based framework.

Understanding how molecular systems self-assemble to form well-organized superstructures governed by noncovalent interactions is essential in the field of supramolecular chemistry. In the nanoscience context, the self-assembly of different carbon-based nanoforms (fullerenes, carbon nanotubes and graphene) with, in general, electron-donor molecular systems, has received increasing attention as a means of generating potential candidates for technological applications. In these carbon-based systems, a deep characterization of the supramolecular organization is crucial to establish an intimate relation between supramolecular structure and functionality. Detailed structural information on the self-assembly of these carbon-based nanoforms is however not always accessible from experimental techniques. In this regard, quantum chemistry has demonstrated to be key to gain a deep insight into the supramolecular organization of molecular systems of high interest. In this review, we intend to highlight the fundamental role that quantum-chemical calculations can play to understand the supramolecular self-assembly of carbon-based nanoforms through a limited selection of supramolecular assemblies involving fullerene, fullerene fragments, nanotubes and graphene with several electron-rich π-conjugated systems.

Donor–acceptor‐substituted alkynes, endowed with 11,11,12,12‐tetracyano‐9,10‐anthraquinodimethane (TCAQ) and N,N‐dimethylaniline (DMA) units, have been further functionalized by a [2+2] cycloaddition with tetracyanoethylene (TCNE) followed by a subsequent retro‐electrocyclization to form distorted nonplanar structures with bridging 1,1,4,4‐tetracyanobuta‐1,3‐diene (TCBD) units. Comprehensive spectroscopic, electrochemical, and computational studies have been carried out to compare the electronic communication in planar (alkyne bridges) and nonplanar (TCBD bridges) TCAQ‐based push–pull chromophores. Cyclic voltammetry and UV/Vis absorption measurements confirm the electronic communication between the TCAQ and DMA units despite the nonplanarity of the TCBD group. The experimental trends are strongly supported by density functional theory calculations, which further support the active electron‐withdrawing role of the TCBD bridges. The novel push–pull TCAQ‐based derivatives incorporating the TCBD bridge show a broad absorption in the whole visible range while the structures are highly distorted from planarity. Twisting conjugation: Cycloaddition–retro‐electrocyclization reactions between tetracyanoethylene and push–pull chromophores based on 11,11,12,12‐tetracyano‐9,10‐anthraquinodimethane (TCAQ) and one or two dimethylaniline units provide new chromophores with significant electronic communication despite the largely bent geometry promoted by the formed 1,1,4,4‐tetracyanobuta‐1,3‐diene bridge (see figure; TCNE=tetracyanoethylene).

Two novel and simple donor‐π‐bridge‐donor (D‐π‐D) hole‐transporting materials (HTMs) containing two units of the p‐methoxytriphenylamine (TPA) electron donor group covalently bridged by means of the 3,4‐dimethoxyselenophene spacer through single and triple bonds are reported. The optoelectronic and thermal properties of the new selenium‐containing HTMs have been determined using standard experimental techniques and theoretical density functional theory (DFT) calculations. The selenium‐based HTMs have been incorporated in mesoporous perovskite solar cells (PSCs) in combination with the triple‐cation perovskite [(FAPbI3)0.87(MAPbBr3)0.13]0.92 [CsPbI3]0.08. Limited values of power conversion efficiencies, up to 13.4 %, in comparison with the archetype spiro‐OMeTAD (17.8 %), were obtained. The reduced efficiencies showed by the new HTMs are attributed to their poor film‐forming ability, which constrains their photovoltaic performance due to the appearance of structural defects (pinholes). Two novel donor‐p‐bridge‐donor (D‐π‐D) selenophene‐based HTMs are reported, named TPASe‐1 and TPASe‐2, where a methoxy‐substituted selenophene moiety is used as π‐conjugated linker between two electron‐donor triphenylamine (TPA) units through single and triple bonds, respectively. The incorporation of these new systems in perovskite solar cells leads to modest power conversion efficiencies (PCE) of up to 13.7 %, which has been accounted for by their low hole‐mobility along with their poor film‐forming ability.

Hybrid halide perovskites materials have the potential for both photovoltaic and light-emitting devices. Relatively little has been reported on the kinetics of charge relaxation upon intense excitation. In order to evaluate the illumination power density dependence on the charge recombination mechanism, we have applied a femtosecond transient mid-IR absorption spectroscopy with strong excitation to directly measure the charge kinetics via electron absorption. The irradiance-dependent relaxation processes of the excited, photo-generated charge pairs were quantified in polycrystalline MAPbI3, MAPbBr3, and (FAPbI3)0.97(MAPbBr3)0.03 thin films that contain either methylamonium (MA) or formamidinium (FA). This report identifies the laser-generated charge species and provides the kinetics of Auger, bimolecular and excitonic decay components. The inter-band electron-hole (bimolecular) recombination was found to dominate over Auger recombination at very high pump irradiances, up to the damage threshold. The kinetic analysis further provides direct evidence for the carrier field origin of the vibrational Stark effect in a formamidinium containing perovskite material. The results suggest that radiative excitonic and bimolecular recombination in MAPbI3 at high excitation densities could support light-emitting applications.

A series of donor–acceptor (D‐A) small molecules based on electron‐deficient benzothiadiazole (BTD) and electron‐rich benzodithiophene (BDT) featuring an A‐D‐A structure is presented. Exhaustive spectroscopic, electrochemical, and computational studies evidence their electroactive nature and their ability to form well‐ordered thin films with broad visible absorptions and low band gaps (ca. 2 eV). Time‐resolved microwave conductivity (TRMC) studies unveil unexpected n‐type charge transport displaying high electron mobilities around 0.1 cm2 V−1 s−1. Efficient electron transport properties are consistent with the low electron reorganization energies (0.11–0.17 eV) theoretically predicted. Getting electrons on the move: Acceptor‐donor‐acceptor triads based on benzothiadiazole and benzodithiophene moieties show unprecedented high electron mobilities (0.1 cm2 V−1 s−1) for these sort of low band‐gap (2.0 eV) materials (see figure). These experimental findings are explained by the calculated low reorganization energies and the good quality of the film.

Fullerene receptors prepared by a twofold CuI‐catalyzed azide‐alkyne cycloaddition reaction with π‐extended tetrathiafulvalene (exTTF) have been covalently linked to single‐walled carbon nanotubes and multi‐walled carbon nanotubes. The nanoconjugates obtained were characterized by several analytical, spectroscopic and microscopic techniques (TEM, FTIR, Raman, TGA and XPS), and evaluated as C60 receptors by using UV‐Vis spectroscopy. The complexation between the exTTF‐triazole receptor in the free state and C60 was also studied by UV‐Vis and 1H NMR titrations, and compared with analogous triazole‐based tweezer‐type receptors containing the electron‐acceptor 11,11,12,12‐tetracyano‐9,10‐anthraquinodimethane and benzene rings instead of exTTF motifs, providing in all cases very similar values for the association constant (log Ka≈3.0−3.1). Theoretical density functional theory calculations demonstrated that the enhanced interaction between the host and the guest upon increasing the size of the π‐conjugated arms of the tweezer is compensated by an increase in the energy penalty needed to distort the geometry of the host to wrap C60. Catching the ball: The intermolecular forces contributing to the overall stability of complexes formed by triazole tweezer‐type receptors and C60 are analyzed by 1H NMR titrations and theoretical methods. These forces allow the construction of sophisticated three‐component assemblies where receptors anchored to carbon nanotubes sidewalls recognize C60 units.

Azoarenes remain privileged photoswitches – molecules that can be interconverted between two states using light – enabling a huge range of light addressable multifunctional systems and materials. Two key innovations to improve the addressability and Z -isomer stability of the azoarenes have been ortho -substitution of the benzene ring(s) or replacement of one of the benzenes for a pyrazole (to give arylazopyrazole switches). Here we study the combination of such high-performance features within a single switch architecture. Through computational analysis and experimental measurements of representative examples, we demonstrate that ortho -benzene substitution of the arylazopyrazoles drastically increases the Z -isomer stability and allows further tuning of their addressability. This includes the discovery of new azopyrazoles with a Z -isomer thermal half-life of ≈46 years. Such results therefore define improved designs for high performance azo switches, which will allow for high precision optically addressable applications using such components.

A thorough density functional theory study is performed for the three carboxyl-based derivatives of the exTTF-TCF chromophore, where the π-extended tetrathiafulvalene (exTTF) electron-donor is linked to the tricyanofuran (TCF) electron-acceptor through an ethylene bridge, as dyes for dye-sensitized solar cells. Calculations predict that the carboxyl group in the acceptor moiety adopts an adequate orientation for an efficient anchoring on the semiconductor TiO 2 surface. The carboxylic acid group holds a negative charge twice larger than the cyano moiety that favors the electron injection to the semiconductor. Time-dependent calculations allow for the assignment of the absorption bands in the UV–vis spectrum of exTTF-TCF and confirm the presence of two low-lying charge-transfer electronic transitions that account for the moderately-intense absorption in the 450–800 nm range. The striking optical absorption properties of exTTF-TCF are preserved for the carboxylic analogues. Finally, periodic calculations show relevant topological differences between the carboxylic derivatives anchored on the TiO 2 surface, which would notably influence in the power conversion efficiency of a dye-sensitized solar cell.

Fullerene receptors prepared by a twofold Cu I ‐catalyzed azide‐alkyne cycloaddition reaction with π‐extended tetrathiafulvalene (exTTF) have been covalently linked to single‐walled carbon nanotubes and multi‐walled carbon nanotubes. The nanoconjugates obtained were characterized by several analytical, spectroscopic and microscopic techniques (TEM, FTIR, Raman, TGA and XPS), and evaluated as C 60 receptors by using UV‐Vis spectroscopy. The complexation between the exTTF‐triazole receptor in the free state and C 60 was also studied by UV‐Vis and 1 H NMR titrations, and compared with analogous triazole‐based tweezer‐type receptors containing the electron‐acceptor 11,11,12,12‐tetracyano‐9,10‐anthraquinodimethane and benzene rings instead of exTTF motifs, providing in all cases very similar values for the association constant (log K a ≈3.0−3.1). Theoretical density functional theory calculations demonstrated that the enhanced interaction between the host and the guest upon increasing the size of the π‐conjugated arms of the tweezer is compensated by an increase in the energy penalty needed to distort the geometry of the host to wrap C 60 .