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Three new polycatenated mixed-ligand MOFs containing fluorene and naphthalene scaffolds have been solvothermally synthesized. Their solid-state luminescence has been investigated showing a Förster resonance energy transfer involving the two linkers.

Natural and anthropogenic gas radionuclides such as radon, xenon, hydrogen and krypton isotopes must be monitored to be managed as pathogenic agents, radioactive diagnostic agents or nuclear activity indicators. State-of-the-art detectors based on liquid scintillators suffer from laborious preparation and limited solubility for gases, which affect the accuracy of the measurements. The actual challenge is to find solid scintillating materials simultaneously capable of concentrating radioactive gases and efficiently producing visible light revealed with high sensitivity. The high porosity, combined with the use of scintillating building blocks in metal–organic frameworks (MOFs), offers the possibility to satisfy these requisites. We demonstrate the capability of a hafnium-based MOF incorporating dicarboxy-9,10-diphenylanthracene as a scintillating conjugated ligand to detect gas radionuclides. Metal–organic frameworks show fast scintillation, a fluorescence yield of ∼40%, and accessible porosity suitable for hosting noble gas atoms and ions. Adsorption and detection of 85Kr, 222Rn and 3H radionuclides are explored through a newly developed device that is based on a time coincidence technique. Metal–organic framework crystalline powder demonstrated an improved sensitivity, showing a linear response down to a radioactivity value below 1 kBq m−3 for 85Kr, which outperforms commercial devices. These results support the possible use of scintillating porous MOFs to fabricate sensitive detectors of natural and anthropogenic radionuclides.

Fluorinated Metal-Organic Frameworks (MOFs), comprising a wheel-shaped ligand with geminal rotating fluorine atoms, produced benchmark mobility of correlated dipolar rotors at 2 K, with practically null activation energy (Ea=17 cal mol−1). 1H T1 NMR revealed multiple relaxation phenomena due to the exchange among correlated dipole-rotor configurations. Synchrotron radiation X-ray diffraction at 4 K, Density Functional Theory, Molecular Dynamics and phonon calculations showed the fluid landscape and pointed out a cascade mechanism converting dipole configurations into each other. Gas accessibility, shown by hyperpolarized-Xe NMR, allowed for chemical stimuli intervention: CO2 triggered dipole reorientation, reducing their collective dynamics and stimulating a dipole configuration change in the crystal. Dynamic materials under limited thermal noise and high responsiveness enable the fabrication of molecular machines with low energy dissipation and controllable dynamics.

International audience; Natural radioactive gases and anthropogenic radionuclides such as radon, xenon, hydrogen and krypton isotopes, need to be carefully monitored to be properly managed as pathogenic agents, radioactive diagnostic agents or indicators of nuclear activity. State-of-the-art gas detectors based on liquid scintillators suffer from many drawbacks such as lengthy sample preparation procedures and limited solubility of gaseous radionuclides, which produces a detrimental effect on measurement sensitivity. A potential breakthrough solution to this problem is the use of solid porous scintillators that act as gas concentrators and therefore could increase detection sensitivity. Highly porous scintillating metal-organic frameworks (MOFs) stand out as relevant materials for the realization of these devices. We demonstrate the capability of porous hafnium-based MOF nanocrystals exploiting dicarboxy-9,10-diphenylanthracene (DPA) as a scintillating conjugated linker to detect gas radionuclides. The nanocrystals show fast scintillation properties in the nanosecond domain, a fluorescence quantum yield of ~40% and an accessible porosity suitable to host noble gas atoms and ions. The MOFs have been tested for the adsorption and detection of the radionuclides 85Kr, 222Rn and 3H in a customized newly developed device based on a time coincidence technique. For all of them, MOF nanocrystals demonstrate an improved sensitivity with respect to a reference detector, showing an excellent linear response down to an activity value lower than 1 kBq·m-3 that outperforms that of commercial devices. The results obtained strongly support the possible use of scintillating porous MOF nanocrystals as the building block of ultrasensitive sensors for the detection of natural and anthropogenic radioactive gases.

Achieving sophisticated juxtaposition of geared molecular rotors with negligible energy-requirements in solids enables fast yet controllable and correlated rotary motion to construct switches and motors. Our endeavor was to realize multiple rotors operating in a MOF architecture capable of supporting fast motional regimes, even at extremely cold temperatures. Two distinct ligands, 4,4′-bipyridine (bipy) and bicyclo[1.1.1]pentanedicarboxylate (BCP), coordinated to Zn clusters fabricated a pillar-and-layer 3D array of orthogonal rotors. Variable temperature XRD, 2H solid-echo, and 1H T1 relaxation NMR, collected down to a temperature of 2 K revealed the hyperfast mobility of BCP and an unprecedented cascade mechanism modulated by distinct energy barriers starting from values as low as 100 J mol–1 (24 cal mol–1), a real benchmark for complex arrays of rotors. These rotors explored multiple configurations of conrotary and disrotary relationships, switched on and off by thermal energy, a scenario supported by DFT modeling. Furthermore, the collective bipy-ring rotation was concerted with the framework, which underwent controllable swinging between two arrangements in a dynamical structure. A second way to manipulate rotors by external stimuli was the use of CO2, which diffused through the open pores, dramatically changing the global rotation mechanism. Collectively, the intriguing gymnastics of multiple rotors, devised cooperatively and integrated into the same framework, gave the opportunity to engineer hypermobile rotors (107 Hz at 4 K) in machine-like double ligand MOF crystals.

An anionic mechanism is used to create polymers and copolymers as confined to, or anchored to, high-surface-area porous nanoparticles. Linear polymers with soft and glassy chains, such as polyisoprene and polymethylmethacrylate, were produced by confined anionic polymerization in 3D networks of porous aromatic frameworks. Alternatively, multiple anions were generated on the designed frameworks which bear removal protons at selected positions, and initiate chain propagation, resulting in chains covalently connected to the 3D network. Such growth can continue outside the pores to produce polymer-matrix nanoparticles coated with anchored chains. Sequential reactions were promoted by the living character of this anionic propagation, yielding nanoparticles that were covered by a second polymer anchored by anionic block copolymerization. The intimacy of the matrix and the grown-in polymers was demonstrated by magnetization transfer across the interfaces in 2D 1 H-13 C-HETCOR NMR spectra.

The solid state is typically not well suited to sustaining fast molecular motion, but in recent years a variety of molecular machines, switches and rotors have been successfully engineered within porous crystals and on surfaces. Here we show a fast-rotating molecular rotor within the bicyclopentane–dicarboxylate struts of a zinc-based metal–organic framework—the carboxylate groups anchored to the metal clusters act as an axle while the bicyclic unit is free to rotate. The three-fold bipyramidal symmetry of the rotator conflicts with the four-fold symmetry of the struts within the cubic crystal cell of the zinc metal–organic framework. This frustrates the formation of stable conformations, allowing for the continuous, unidirectional, hyperfast rotation of the bicyclic units with an energy barrier of 6.2 cal mol−1 and a high frequency persistent for several turns even at very low temperatures (1010 Hz below 2 K). Using zirconium instead of zinc led to a different metal cluster–carboxylate coordination arrangement in the resulting metal–organic framework, and much slower rotation of the bicyclic units. Molecular rotors have been engineered within the bicyclopentane–dicarboxylate struts of a metal–organic framework—the bicyclic unit is the rotator and the carboxylate groups serve as the stator. In a zinc-based metal–organic framework, the crossed conformation of the strut–metal nodes enables fast rotation of the bicyclic moiety, but in the corresponding zirconium metal–organic framework a change in the conformation results in much slower rotation.

The incorporation of photoswitchable molecules into solid-state materials holds promise for the fabrication of responsive materials, the properties of which can be controlled on-demand. However, the possible applications of these materials are limited due to the restrictions imposed by the solid-state environment on the incorporated photoswitches, which render the photoisomerization inefficient. Here we present responsive porous switchable framework materials based on a bistable chiroptical overcrowded alkene incorporated in the backbone of a rigid aromatic framework. As a consequence of the high intrinsic porosity, the resulting framework readily responds to a light stimulus, as demonstrated by solid-state Raman and reflectance spectroscopies. Solid-state 13C NMR spectroscopy highlights an efficient and quantitative bulk photoisomerization of the incorporated light-responsive overcrowded olefins in the solid material. Taking advantage of the quantitative photoisomerization, the porosity of the framework and the consequent gas adsorption can be reversibly modulated in response to light and heat. Despite numerous potential applications, the development of light-responsive solid materials based on molecular photoswitches is impeded by the low efficiency of photoisomerization in the solid environment. Now a robust, solid porous material made from tetraphenylmethane and a photoswitchable overcrowded alkene exhibits nearly quantitative photoisomerization in the bulk and in photomodulation of gas uptake.

The design and engineering of precise molecular building blocks expand the scope of porous materials and boost the development of innovative photonic, dynamic, and adsorptive materials. Light-responsive Porous Switching Frameworks (PSFs) with BET surface area as high as 3948 m2 g-1 were generated by co-polymerization of molecular motors with rigid tetrahedral building blocks.[1] The quantitative molecular photoisomerization in the solid state promotes the modulation of gas adsorption properties, generating light-responsive materials suitable for actively switching devices. Self-standing sensitized triplet-triplet annihilation (s-TTA) upconverting nanoparticles were developed by inclusion of a porphyrin unit (sensitizer) in the cavities of Porous Emitting Frameworks (PEFs) comprising highly luminescent diphenyl anthracene (DPA) units.[2] The proximity between the guest sensitizer and DPA emitters promotes effective energy transfer and an up-conversion quantum yield as high as 0.15, a record value for solid-state materials. Porous Aromatic frameworks (PAFs) provide a stable platform with ultra-high pore volume. Highly reactive anionic species were generated on the framework or diffused in the pores to promote anchored and confined polymerization.[3] The “ship-in-a-bottle” polymerization process generates intimately interdigitated nanocomposites proved by 2D 13C solid-state NMR. Moreover, the living nature of anionic polymerization allowed the fabrication of polymer-coated nanoparticles with sequential architecture. The adsorptive properties of POPs can be tailored for selective gas adsorption: calixarene-based POPs provide great opportunity for gas storage and separation proved by high-pressure sorption, adsorption-coupled calorimetry, and breakthrough measurements under flow condition.[4] References [1] F. Castiglioni, W. Danowski, J. Perego, F. K.-C. Leung, P. Sozzani, S. Bracco, S. J. Wezenberg, A. Comotti, B. L. Feringa, Nat. Chem. 2020, 12, 595–602. [2] J. Perego, J. Pedrini, C. X. Bezuidenhout, P. E. Sozzani, F. Meinardi, S. Bracco, A. Comotti, A. Monguzzi, Adv. Mater. 2019, 31, 1903309. [3] J. Perego, S. Bracco, A. Comotti, D. Piga, I. Bassanetti, P. Sozzani, Angew. Chem. Int. Ed. 2021, 60, 6117–6123. [4] A. Pedrini, J. Perego, S. Bracco, C. X. Bezuidenhout, P. Sozzani, A. Comotti, J. Mater. Chem. A 2021, 9, 27353–27360.

Radiation detection is of utmost importance in fundamental scientific research, as well as medical diagnostics, homeland security, environmental monitoring and industrial control. Lead halide perovskites (LHPs) are attracting growing attention as high-atomic-number materials for next-generation scintillators and photoconductors for ionizing radiation detection. To unlock their full potential as reliable and cost-effective alternatives to conventional materials, it is necessary for LHPs to conjugate high scintillation yields with emission stability under high doses of ionizing radiation. To date, no definitive solution has been devised to optimize the scintillation efficiency and kinetics of LHPs and nothing is known of their radiation hardness for doses above a few kilograys, to the best of our knowledge. Here we demonstrate that CsPbBr3 nanocrystals exhibit exceptional radiation hardness for γ-radiation doses as high as 1 MGy. Spectroscopic and radiometric experiments highlight that despite their defect tolerance, standard CsPbBr3 nanocrystals suffer from electron trapping in dense surface defects that are eliminated by post-synthesis fluorination. This results in >500% enhancement in scintillation efficiency, which becomes comparable to commercial scintillators, and still retaining exceptional levels of radiation hardness. These results have important implications for the widespread use of LHPs in ultrastable and efficient radiation detectors.

Typically, the solid state is not well suited to sustaining fast molecular motion - however, in recent years a variety of molecular machines, switches and rotors have been successfully engineered within porous crystals and on surfaces. Here, we report on a combined 1H-NMR [1] and SR [2] study of fast-rotating molecular rotors within the bicyclopentane-dicarboxylate struts of a zinc-based metal-organic framework. Here, the carboxylate groups anchored to the metal clusters act as an axle while the bicyclic units are free to rotate. The three-fold bipyramidal symmetry of the rotator conflicts with the four-fold symmetry of the struts, frustrating the formation of stable conformations and favouring the continuous, unidirectional, ultrafast rotation of the bicyclic units down to cryogenic temperatures. As a remarkable consequence, the fast-motions regime for the 1H-NMR spin-lattice relaxation rate is maintained down to at least 2 K, as confirmed by its dependence on temperature and magnetic field. These results are confirmed by zero-field and longitudinal-field SR experiments and, in particular, by the dependence of the longitudinal relaxation rate on temperature. At the same time, the experimental evidences suggest several implantation sites for the muons, among which one directly onto the rotating moiety. Muons thermalized in this latter site generate clear oscillations in the depolarization (shown in the picture) resulting from the dipolar interaction with the 1H nuclear moments on the rotors. We evidence a highly unusual dependence of these oscillations on temperature, suggesting a complex influence of the rotations on the muon implantation and diffusion. [1] J. Perego et al., Nature Chemistry 12 845 (2020). [2] G. Prando et al., Nano Letters 20 7613 (2020).

Molecular rotors offer a platform to realize controlled dynamics and modulate the functions of solids. The motional mechanisms in arrays of rotors have not been explored in depth. Crystal-like porous organosilicas, comprising p-phenylene rotators pivoted onto a siloxane scaffold, were modelled using molecular dynamics (MD) simulations. Long simulations, on a microsecond scale, allowed to follow the reorientation statistics of rotor collections and single out group configurations and frequency distributions as a function of temperature. The motions observed in the MD simulations support a multiple-site model for rotor reorientations. Computed motional frequencies revealed a complex rotatory phenomenon combining an ultra-fast libration motion (oscillation up to 30°) with a slow and fast 180° flip reorientation. Adopting a multiple-site model provides a more accurate simulation of the 2H-NMR spectra and a rationalization of their temperature dependence. In particular, rotators endowed with distinct rates could be explained by the presence of slower rings locked in a T-shaped conformation.

Molecular rotors offer a platform to realize controlled dynamics and modulate the functions of solids. The motional mechanisms in arrays of rotors have not been explored in depth. Crystal-like porous organosilicas, comprising

A high efficiency emission with a massive Stokes shift is obtained by fluorescent conjugated acene building blocks arranged in nanocrystals. The two ligands of equal molecular length and connectivity, yet complementary electronic properties, are co-assembled by zirconium oxy-hydroxy clusters, generating highly crystalline hetero-MOF nanoparticles The fast diffusion of singlet molecular excitons in the framework, coupled with the fine matching of ligands absorption and emission properties, enables to achieve an ultrafast activation of the low energy emission by diffusion-mediated non-radiative energy transfer in the 100 ps time scale, by using a low amount of co-ligands. This allow to obtain MOF nanocrystals with a fluorescence quantum efficiency of ̴ 70% and an actual Stokes shift as large as 750 meV. This large Stokes shift suppresses the reabsorption of fast emission issues in bulk devices, pivotal for a plethora of applications in photonics and photon managing spacing from solar technologies, imaging, and detection of high energy radiation. These features allowed to realize a prototypal fast nanocomposite scintillator that shows an enhanced performance with respect to the homo-ligand nanocrystals, achieving benchmark. values which compete with those of some inorganic and organic commercial systems.

Suitably functionalized porous matrices represent versatile platforms to support well-dispersed catalytic centers. In the present study, porous organic polymers (POPs) containing phosphine oxide groups were fabricated to bind transition metals and to be investigated for potential electrocatalytic applications. Cross-linking of mono- and di-phosphine monomers with multiple phenyl substituents was subject to the Friedel-Crafts (F-C) reaction and the oxidation process, which generated phosphine oxide porous polymers with pore capacity up to 0.92 cm

Rotors, motors and switches in the solid state find a favorable playground in porous materials, such as Metal Organic Frameworks (MOFs) and Porous Aromatic Frameworks (PAFs), thanks to their large free volume, which allows for fast dynamics. We have realized a fast molecular rotor in the solid state whose rotation speed approaches that of unhindered rotations in organic moieties even at very low temperatures (2 K). Continuos, unidirectional hyperfast rotation with an energy barrier of 6.2 cal/mol and a high frequency persistent for several turns are achieved (10 GHz below 2 K).[1] Responsive porous switchable framework materials endowed with light-responsive overcrowded olefins, took advantage of both the quantitative photoisomerization in the solid state and the porosity of the framework to reversibly modulate the gas adsorption in response to light. [2] Motors were inserted into metal-organic frameworks wherein two linkers with complementary absorption-emission properties were integrated into the same materials. Unidirectional motion was achieved by simple exposure to sun-light of the solid particles, which thus behave as autonomous nanodevices.[3] MOF nanocrystals comprising high-Z linking nodes interacting with the ionizing radiation, arranged in an orderly fashion at a nanometric distance from diphenylanthracene ligand emitters showed ultrafast sensitization of the ligand fluorescence, thus supporting the development of new engineered scintillators.[4,5] References 1. A. Comotti, P. Sozzani et al Nature Chem. 2020, 12, 845. 2. A. Comotti, B. L. Feringa et al Nature Chem. 2020, 12, 595. 3. A. Comotti, B. L. Feringa et al J. Am. Chem. Soc. 2020, 142, 9048. 4.P. E. Sozzani, A. Comotti, A. Monguzzi et al Adv. Mater. 2019, 31, 1903309. 5. A. Comotti, A. Monguzzi et al Nature Photonics 2021, doi 10.1038/s41566-021-00769-z.

Metal organic frameworks provide a versatile platform that can generate intriguing behaviours and innovative properties. Specifically, this contribution highlights our recent results related to the installation of highly dynamic molecular rotors in MOFs[1] and the development of fast scintillating MOFs and MOF/polymer composites for fast detection of high-energy radiations[2]. MOFs provide precise spatial disposition of organic struts and enough free volume to preserve the dynamic properties of molecular motors and rotors even in condensed matter. Molecular rotor bicyclo[1.1.1]pentane–dicarboxylate was installed in the 3D cubic structure of a highly porous zinc MOF[1] (figure A,B,C,D). Its dynamic behaviour was investigated with solid state NMR relaxation and muon-spin spectroscopy performed at temperatures as low as 2 K and molecular dynamic simulations, providing clear evidence of very fast molecular reorientation in the GHz regime even at the lowest temperatures, consistent with a low activation energy for rotational motion of 6.2 cal mol-1. High-Z MOFs were generated by coordination of zirconium-based clusters and highly emissive 9,10-bis(4-carboxyphenyl)anthracene (DPA) ligands allowing for the efficient sensitization of the linker fluorescence under high-energy radiation excitation (figure E,F). MOF nanocrystals were embedded in a continuous polymer matrix producing ultra-fast scintillators with rise time of ~ 50 ps and high light yields suitable for application as detectors for time-of-flight positron emission tomography (TOF-PET). [1] J. Perego, S. Bracco, M. Negroni, C. X. Bezuidenhout, G. Prando, P. Carretta, A. Comotti, and P. Sozzani, Nat. Chem. (2020), 12, 845. [2] J. Perego, I. Villa, A. Pedrini, E. C. Padovani, R. Crapanzano, A. Vedda, C. Dujardin, C. X. Bezuidenhout, S. Bracco, P. E. Sozzani, A. Comotti, L. Gironi, M. Beretta, M. Salomoni, N. Kratochwil, S. Gundacker, E. Auffray, F. Meinardi, A. Monguzzi, Nat. Photonics (2021) https://doi.org/10.1038/s41566-021-00769-z.

Rotors, motors and switches in the solid state find a favorable playground in porous materials, such as Metal Organic Frameworks (MOFs) and Porous Aromatic Frameworks (PAFs), thanks to their large free volume, which allows for fast dynamics. We fabricated MOFs and PAFs with reorientable linkers and benchmark mobility also at very low temperature, to reduce the energy demand for motion-activation and light stimulus-response.[1] A fast molecular rotor in a Zn-MOF endowed with continuous, unidirectional hyperfast rotation with energy barrier of 6.2 cal/mol and high frequency persistent for several turns is achieved (Figure 1). Ligh responsive porous switchable frameworks took advantage of quantitative photoisomerization in the solid state and porosity of the framework to modulate the gas adsorption in response to light.[2] MOF nanocrystals comprising high-Z linking nodes interacting with the ionizing radiation, arranged in an orderly fashion at a nanometric distance from ligand emitters showed ultrafast sensitization of the ligand fluorescence, thus supporting the development of new engineered scintillators.[3] References [1] A. Comotti, P. Sozzani et al. Nat. Chem. 12 (2020) 845; [2] A. Comotti, B. Feringa, et al. Nature Chem. 12 (2020) 595; [3] A. Comotti, A. Vedda, A. Monguzzi, et al. Nature Photonics (2021), https://doi.org/10.1038/s41566-021-00769-z. Financial support from the Italian Ministry of University and Research (MIUR) through the grant ‘Dipartimenti di Eccellenza-2017 Materials For Energy’ is acknowledged. This research was funded by the PRIN-2015CTEBBA-003.

The search for new sophisticated functions integrated in porous solids prompted us to engineer switches and motors in metal-organic and covalent architectures for the construction of working nanodevices. 3D Porous Aromatic Frameworks were built with photo-active monomer units, and could be switched on command between two permanent states, entailing structure breathing and gas sorption switching. A fast molecular rotor in the solid state whose rotation speed approaches that of unhindered rotations in organic moieties even at very low temperatures (10 GHz below 2 K) was realized generating continuous, unidirectional hyperfast rotation with an energy barrier of 6.2 cal/mol. The energy transfer effect with radiation up-grading was also realized in Porous Emitting Frameworks (PEFs): porphyrin (red absorber) and diphenyl-antracene (blue emitter) were integrated in a 3D network showing very effective convertion of low energy light into photons of higher energy. With a similar concept, motors were inserted into metal-organic frameworks (MOFS), wherein two linkers with complementary absorption-emission properties were integrated in the same materials. The linkers were put in action by the visible light up-grading to UV-radiation, for fuelling the motors. MOF nanocrystals comprising high-Z linking nodes interacting with the ionizing radiation, arranged in an orderly fashion at a nanometric distance from ligand emitters showed ultrafast sensitization of the ligand fluorescence, thus supporting the development of new engineered scintillators. A. Comotti, B. L. Feringa et al Nature Chem. 2020, 12, 595; A. Comotti, P. Sozzani et al Nature Chem. 2020, 12, 845; P. Sozzani, A. Comotti, A. Monguzzi et al Adv. Mater. 2019, 31, 1903309; A. Comotti, B. L. Feringa et al J. Am. Chem. Soc. 2020, 142, 9048; A. Comotti, A. Monguzzi et al Nature Photonics 2021, doi 10.1038/s41566-021-00769-z.

It has been over half a century since polyacrylonitrile (PAN)-based carbon fibers were first developed. However, the mechanism of the carbonization reaction remains largely unknown. Structural evolution of PAN during the preoxidation reaction, a stabilization reaction, is one of the most complicated stages because many chemical reactions, including cyclization, dehydration, and cross-linking reactions, simultaneously take place. Here, we report the stabilization reaction of single PAN chains within the one-dimensional nanochannels of metal–organic frameworks (MOFs) to study an effect of interchain interactions on the stabilization process as well as the structure of the resulting ladder polymer (LP). The stabilization reaction of PAN within the MOFs could suppress the rapid generation of heat that initiates the self-catalyzed reaction and inevitably provokes many side-reactions and scission of PAN chains in the bulk state. Consequently, LP prepared within the MOFs had a more extended conjugated backbone than the bulk condition., Accommodation of polyacrylonitrile in MOFs facilitated and regulated the transformation to ladder polymer in the carbonization process.

The realization of tunable and functionalized MOFs is a winning strategy for CO2 capture. Here we report on a series of robust Fe-MOFs with triangular channels constructed by rod-like fluorinated pyrazolate ligands, comprising an increasing number of fluorine atoms on the central p-phenylene core (F = 1, 2, and 4). This yielded a series of isoreticular frameworks, engineered with orientational flexibility of the fluorinated aryl rings pivoted on ethynyl groups with an sp2–sp soft rotary barrier, providing a stable axel, which supported reorientable C–F dipoles. A combined approach, including powder X-ray diffraction, multinuclear solid-state NMR (2D 1H–13C, 19F, hyperpolarized 129Xe NMR and distance measurements by paramagnetic shift), gas-adsorption and microcalorimetry, enabled the exhaustive description of the fluorinated ring arrangement and the organization of functionalized sites for accommodating CO2. In the tetrafluoro-aryl-derivative MOF, protrusion of perfluorinated rings towards the channel space plays a major role in CO2 capture. Partially fluorinated aryl rings of mono- and di-fluoro MOFs turn to retract into the channel-walls to form continuous ribbons of inter-strut supramolecular interactions, contributing to the robustness of the overall architecture. Detailed computational models obtained using GCMC and DFT of CO2 diffusion and interactions in MOFs showed how the gas molecules approach the channel walls. The highly occupied sites are aligned at the corners of the triangular channels, wherein fluorine atoms participate in host–CO2 interactions. A CO2–matrix adsorption enthalpy of 33 kJ mol−1, suitable for capture/delivery cycles, was accurately measured in situ by simultaneous acquisition of microcalorimetric and volumetric-isotherm data. Thus, the designed advantages of rotationally flexible fluorinated moieties were successfully explored.

Scintillators, materials that produce light pulses upon interaction with ionizing radiation, are widely employed in radiation detectors. In advanced medical-imaging technologies, fast scintillators enabling a time resolution of tens of picoseconds are required to achieve high-resolution imaging at the millimetre length scale. Here we demonstrate that composite materials based on fluorescent metal–organic framework (MOF) nanocrystals can work as fast scintillators. We present a prototype scintillator fabricated by embedding MOF nanocrystals in a polymer. The MOF comprises zirconium oxo-hydroxy clusters, high-Z linking nodes interacting with the ionizing radiation, arranged in an orderly fashion at a nanometric distance from 9,10-diphenylanthracene ligand emitters. Their incorporation in the framework enables fast sensitization of the ligand fluorescence, thus avoiding issues typically arising from the intimate mixing of complementary elements. This proof-of-concept prototype device shows an ultrafast scintillation rise time of ~50 ps, thus supporting the development of new scintillators based on engineered fluorescent MOF nanocrystals. Composites of fluorescent metal–organic framework nanocrystals in a polymer are exploited to create fast scintillators with a rise time of about 50 ps.

Microporous materials offer synthetic versatility allowing the generation of advanced materials with unique photonic and scintillating properties. Highly luminescent diphenylanthracene (DPA) moieties can be framed into Porous Aromatic Frameworks (PAFs) generating porous nanoparticles with high quantum yield in the solid state[1]. After the diffusion and tethering of sensitizer molecules in the accessible voids inside the materials, each nanoparticle operated as self-standing solid state upconverting materials with potential applications in bio-imaging and photovoltaics. Scintillating materials emit light under excitation with ionizing radiations. They are fundamental for particle physics detectors and for medical imaging. State of the art technologies based on inorganic or polymeric materials produce high light yields or fast time responses, but neither of these standard approaches provide both properties within the same material. An innovative approach based on Metal-Organic Frameworks nanocrystals embedded in polymer matrixes successfully produced composite scintillators with promising light yield and fast rise and scintillation times[2]. High-Z MOFs were generated by coordination of Zirconium-based oxo-hydroxy clusters and highly emissive DPA ligands allowing for the efficient sensitization of the ligand fluorescence. Modulated synthesis produced nanocrystalline MOFs with controlled particle sizes and shapes that can be easily embedded in a continuous polymer matrix to generate self-standing monoliths. The scintillating properties of these composites were characterized showing high light yields and scintillation rise time of ~ 50 ps. These outstanding properties provides fast detection of high-energy radiations and made them suitable for application in detectors for time-of-flight positron emission tomography (TOF-PET). [1] Perego, J.; Pedrini, J.; Bezuidenhout, C. X.; Sozzani, P. E.; Meinardi, F.; Bracco, S.; Comotti, A. and Monguzzi, A. Advanced Materials, 2019, 31 (40), 1903309. [2] Perego, J.; Villa, I.; Pedrini, A.; Padovani, E. C.; Crapanzano, R.; Vedda, A.; Dujardin, C.; Bezuidenhout, C. X.; Bracco, S.; Sozzani, P. E.; Comotti, A.; Gironi, L.; Beretta, M.; Salomoni, M.; Kratochwil, N.; Gundacker, S.; Auffray, E.; Meinardi, F. and Monguzzi, A. Nature Photonics, 2021, https://doi.org/10.1038/s41566-021-00769-z.

The supramolecular capacity of calixarenes towards guests is largely consolidated; in contrast, the synthesis of porous calixarene-based frameworks by covalent bond formation is still a challenge. Our target was to yield 3D polymers and copolymers based on calixarenes for selective gas-capture, endowed with easy pore accessibility and specific sites, and built via a straightforward synthetic route. The covalent calixarene frameworks (CXFs) were prepared by the Yamamoto coupling reaction starting from tetrabromo calixarene propoxy- and methoxy-monomers of three stable calixarene (partial cone, effective cone, and 1,3-alternate) conformers and complete post-synthetic deprotection to achieve polar phenolic calixarene derivatives. Moreover, the copolymer of calixarene-based monomers with tetrabromo-tetraphenylmethane exhibited remarkable surface area up to about 3000 m2 g−1. Smart architectures endowed with hierarchical porosity from micro- to meso-porosity showed notable sponge-like swellability by CO2, which was captured effectively at room temperature, even in competition with N2, yielding CO2 removal in column breakthrough experiments. Indeed, CXFs displayed excellent CO2 and CH4 energy binding of 35 and 24 kJ mol−1, respectively. Ultramicropore sites were highlighted by Xe capture and in situ detection after a xenon diffusion time of a few milliseconds, by laser-assisted hyperpolarized 129Xe NMR, revealing the accessibility of calixarene capsules and the available space. This synthetic route demonstrated the possibility to modulate at will the pore capacity and selectivity, displaying porous frameworks with two distinct pore families, wherein calixarene moieties play the role of small and selective sites. A contractile behavior of the frameworks was observed upon deprotection which produced more polar sites, due to the formation of hydrogen bond networks.

Porous organic frameworks (PAFs) and Metal-organic frameworks (MOFs) were extensively studied in the past 20 years, widening the landscape of microporous materials. Responsive porous frameworks can be manipulated by means of external stimuli such as applied electric field or light irradiation, thus controlling their textural properties at will. Molecular photoswitches were co-polymerized with tetraphenylmethane generating 3D PAFs with high surface area (up to 4800 m2/g) and photo-responsive properties that quantitatively switch between stable and metastable state under U.V. light irradiation[1]. The localized and reversible photoisomerization reaction modified the bulk adsorptive properties of the porous materials with a 20% modulation of the adsorption capacity. These materials can be engineered to provide “on demand” adsorption properties for gas separation and gas storage/release. Scintillating materials are widely employed in high-energy particles detection and medical imaging. Innovative composite scintillators with high light yield and fast response time were developed embedding luminescent MOFs in a polymer matrix[2]. Highly emissive MOFs nanocrystals (ZrDPA) were synthetized by the assembly of 9,10-bis(4-carboxyphenyl)anthracene (DPA) and zirconium oxo-hydroxy cluster and dispersed in polymer matrixes to obtain self-standing monoliths. The MOF/polymer composites showed outstanding radioluminescence and scintillating properties with high light yields and scintillation rise time of ~ 50 ps, making them suitable for application in detectors for time-of-flight positron emission tomography (TOF-PET). [1] F. Castiglioni, W. Danowski, J. Perego, F. K.-C. Leung, P. Sozzani, S. Bracco, S. J. Wezenberg, A. Comotti, B. L. Feringa, Nature Chemistry. 2020, 12, 595. [2] J. Perego, I. Villa, A. Pedrini, E. C. Padovani, R. Crapanzano, A. Vedda, C. Dujardin, C. X. Bezuidenhout, S. Bracco, P. E. Sozzani, A. Comotti, L. Gironi, M. Beretta, M. Salomoni, N. Kratochwil, S. Gundacker, E. Auffray, F. Meinardi, A. Monguzzi, Nature Photonics, 2021, https://doi.org/10.1038/s41566-021-00769-z.

The present study focuses on the assembly of organo-cations with organo-anions in water. The anions, characterized by symmetric moieties (carbon-, adamantane-, or calixarene-based) functionalized with directional hydrogen bond (HB) acceptor functions (tetra-sulfonate moieties), are combined with planar guanidinium or terephtalimidamide cations as hydrogen bond donors, the purpose being to integrate water molecules into the lattice. The imbalance between the charge on the two components, and the considerable number of HB donor and acceptor sites, promotes the insertion of water into the structures. In the reported structures, a part of the water molecules serves as a structural linker between the anions and cations, while the remaining molecules cluster into channels and cavities in a loose association with the supramolecular matrix framework.

In the search for flexible molecular crystals endowed with porosity, we achieved the fabrication of expandable crystalline prototypal structures, which allow the absorption of gases, without modifying the crystal architecture. The design brings together highly symmetrical tetrahedral elements to construct swellable porous adamantoid frameworks through co-operation of eight surrounding hydrogen bonds mounted on conformationally flexible groups. The flexibility of the porous crystals manifests itself in response to stimuli of selected gases, which promote reversible conformational changes, inducing breathing in the molecular structure. The backbone of the reticular construction is based on the formation of the carboxylic dimers, which project outwards from the tetrahedral molecular core to consolidate the 3D framework. Contact with proper gases such as CO2, Xe and hexane triggers a 56-70% enlargement of the channel cross-section. The accommodation of CO2 and Xe in the channel chambers was revealed by synchrotron-light X-ray diffraction, combined with molecular dynamics and density functional theory (DFT) theoretical calculations. Rare experimental observations of xenon dynamics, in which Xe diffuses along the channels and experiences different chamber orientations in the crystal, were gathered by analysing 129Xe NMR chemical shift anisotropy profiles, which encode the shape and orientation of each visited cavity along the channel. The jump rate and activation energy experienced was uniquely established by exploring Xe atoms in their diffusional path. Nitrogen showed a low affinity to the matrix and was unable to enlarge the pores, thus it was excluded from the restrictive pores of the empty crystal. Given the properties of molecular crystals, it is possible to outline some advantageous aspects, such as simple design, easy self-assembly, solubility, reversible gas uptake and absence of metal ions, and they can thus be considered for eco-friendly gas capture and separation.

Porous aromatic frameworks (PAFs) were engineered to generate solid-state upconverting materials that emit higher energy photons under a suitable light stimulus [1]. Fluorescent PAFs were generated by the inclusion of diphenylanthracene moieties in a low-density 3D porous frameworks that maintained the optical properties of the emitting chromophores in the solid-state. Upon inclusion of a suitable sensitizer (a metallo-porphyrin) inside the nanometer-sized pores, the copolymer displayed sensitized photon upconversion with a quantum yield as high as 15%, a record value for solid-state materials. Moreover, it was possible to tether the sensitizer to the porous matrix through a stable covalent bond, generating self-standing upconverting nanoparticles that can be possibly applied in photovoltaics and bio-imaging. PAFs can also be engineered as light-responsive materials. The co-polymerization of a photoswitch with tetraphenylmethane generated porous networks that provided the free volume for the photoisomerization of the overcrowded alkene [2]. Under UV light irradiation, the quantitative photoisomerization led to structural changes and modulated the CO2 adsorptive properties of the material. The process is reversible by irradiation or heating leading to a cyclable material.

Three new fluorinated bis(pyrazoles), namely: 1,4-bis(1H-pyrazol-4-ylethynyl)-2-fluorobenzene (H2BPEFB), 1,4-bis(1H-pyrazol-4-ylethynyl)-2,3-difluorobenzene (H2BPEF2B) and 1,4-bis(1H-pyrazol-4-ylethynyl)-tetrafluorobenzene (H2BPEF4B), have been synthesized taking advantage of Sonogashira coupling reactions, and characterized as per their crystal and molecular structure, spectroscopic and dielectric properties, and hydrophobicity. In the crystal structures, the three molecules, whose deviation from planarity increases on increasing the fluorination degree, interact by means of hydrogen bonds, forming 2D supramolecular layers. Notably, the absorption and fluorescence emission properties are only slightly affected by the fluorination degree in both the solid state and solution. Furthermore, the spectral line-shapes are weakly dependent on the environment when dissolved in a number of solvents of different polarity and hydrogen-bonding affinity. On the other hand, the dielectric constant monotonically increases on increasing the number of fluorine atoms. In silico molecular modeling with time-dependent density functional theory has offered a valuable means to rationalize the above mentioned behaviors and has shed some light on the ligand affinity towards representative gases – H2O and CO2 – and organic solvents – toluene.

The conversion of low-energy light into photons of higher energy based on sensitized triplet-triplet annihilation (sTTA) upconversion is emerging as the most promising wavelength-shifting methodology because it operates efficiently at excitation powers as low as the solar irradiance. However, the production of solid-state upconverters suited for direct integration in devices is still an ongoing challenge owing to the difficulties concerning the organization of two complementary moieties, the triplet sensitizer, and the annihilator, which must interact efficiently. This problem is solved by fabricating porous fluorescent nanoparticles wherein the emitters are integrated into robust covalent architectures. These emitting porous aromatic framework (ePAF) nanoparticles allow intimate interaction with the included metallo-porphyrin as triplet sensitizers. Remarkably, the high concentration of framed chromophores ensures hopping-mediated triplet diffusion required for TTA, yet the low density of the framework promotes their high optical features without quenching effects, typical of the solid state. A green-to-blue photon upconversion yield as high as 15% is achieved: a record performance among annihilators in a condensed phase. Furthermore, the engineered ePAF architecture containing covalently linked sensitizers produces full-fledge solid-state bicomponent particles that behave as autonomous nanodevices.

A cyclic hexapeptide with three pyridyl moieties connected to its backbone forms a hydrogen-bonded dimer, which tightly encapsulates a single xenon atom, like a pearl in its shell. The dimer imprints its shape and symmetry to the captured xenon atom, as demonstrated by 129 Xe NMR spectroscopy, single-crystal X-ray diffraction, and computational studies. The dimers self-assemble hierarchically into tubular structures to form a porous supramolecular architecture, whose cavities are filled by small molecules and gases.

Porous molecular materials represent a new front in the endeavor to achieve high-performance sorptive properties and gas transport. Self-assembly of polyfunctional molecules containing multiple charges, namely, tetrahedral tetra-sulfonate anions and bifunctional linear cations, resulted in a permanently porous crystalline material exhibiting tailored sub-nanometer channels with double helices of electrostatic charges that governed the association and transport of CO2 molecules. The charged channels were consolidated by robust hydrogen bonds. Guest recognition by electrostatic interactions remind us of the role played by the dipolar helical channels in regulatory biological membranes. The systematic electrostatic sites provided the perfectly fitting loci of complementary charges in the channels that proved to be extremely selective with respect to N2 (S = 690), a benchmark in the field of porous molecular materials. The unique screwing dynamics of CO2 travelling along the ultramicropores with a step-wise reorientation mechanism was driven by specific host–guest interactions encountered along the helical track. The unusual dynamics with a single-file transport rate of more than 106 steps per second and an energy barrier for the jump to the next site as low as 2.9 kcal mol−1 was revealed unconventionally by complementing in situ13C NMR anisotropic line-shape analysis with DFT modelling of CO2 diffusing in the crystal channels. The peculiar sorption performances and the extraordinary thermal stability up to 450 °C, combined with the ease of preparation and regeneration, highlight the perspective of applying these materials for selective removal of CO2 from other gases.

Dipeptides with two hydrophobic side chains have proved to be an exceptional source of microporous organic materials, but since previous structures were limited to the incorporation of only proteinogenic residues, their full potential as adsorbents has remained unexplored. Single-crystal XRD data for ten new compounds with non-proteinogenic L-2-aminobutanoic acid and/or L-2-amino-pentanoic acid are presented. The gas-phase accessibility of their crystal pores, with cross-sections of 2.3 to 5.1 Å, was monitored by CO2 and CH4 adsorption isotherms. Included CO2 was also detected spectroscopically by 2D MAS NMR. An extensive conformational analysis reveals that the use of linear rather than branched side chains (such as L-valine and L-isoleucine) affords peptides with a greater degree of conformational freedom and yields more-flexible channel surfaces that may easily adapt to a series of potential guest molecules.

The synthesis of porous organic 3D frameworks, wherein amine, hydroxyl and Li-alkoxide functions were built directly on the monomer-unit carbon core, realizes improved interactions with target gases. CO2 was retained by the amine group with a remarkable energy of 54 kJ mol-1, while 2D MAS NMR provided rare evidence of amine-to-gas short-distance interactions. Frameworks containing hydroxyl and Li-alkoxide functions show optimal interaction energies with CH4 of up to 25 kJ mol-1. The light network of 3-branch building units ensures the expandability of the nano-sponges.

The new class of periodic mesoporous organosilica materials (PMOs), due to the peculiar features, has attracted growing interest from several research areas. We present an atomistic model of a p-phenylenesilica crystalline mesoporous structure with a hexagonal framework, explicitly including channels on nanoscale. OPLS-AA force-field optimization, to get a suitable PMO structure compared with the experimental one, is described. In particular, DFT calculations have been performed to calculate torsional energy barrier of phenyl rings connected to the silicon atoms belonging to inorganic layers and to improve the OPLS-AA force field performances for these materials. Finally, inclusion of small molecules and their interactions with PMO walls have been investigated for CO2 and H2O.

Two novel pillared Zn(II)-based Metal–Organic Frameworks were de novo synthesized exploiting N,N′-(1,1′-biphenyl)-4,4′-diylbis-4-pyridinecarboxamide (bpba) and its fluorinated analogous N,N′-(perfluoro-1,1′-biphenyl-4,4′-diyl)diisonicotinamide (F-bpba) as suitable pillar linkers and 2,6-naphthalene dicarboxylic acid as carboxylic ligand. The resulting heteroleptic MOFs, namely, PUM210, [Zn4(bpba)1.5·(ndc)4·(H2O)]n and PUM210F, [Zn3(F-bpba)1·(ndc)3·(DMF)]n, feature an uncommon truncation of the Zn(II) paddle-wheel nodes along the pillaring direction. PUM210 and PUM210F exhibit a polycatenated architecture, resulting in microporous channels decorated by amide moieties. The activated forms show a permanent porosity and a selective adsorption of CO2 over N2. Moreover, the partially transmetalated Cu-PUM210 and Cu-PUM210F were obtained by convenient transmetalation protocol and their adsorption propriety toward CO2 were subsequently investigated.