Publications

 

 

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2021

Thioureidocalix[6]arenes pseudorotaxanes

Gianpiero Cera, Margherita Bazzoni, Leonardo Andreoni, Federica Cester Bonati, Chiara Massera, Serena Silvi, Alberto Credi, Andrea Secchi, Arturo Arduini
EUROPEAN JOURNAL OF ORGANIC CHEMISTRY
Published online: 24 September 2021
2021, in press

DOI: 10.1002/ejoc.202101080

Heteroditopic calix[6]arenes are extensively exploited as synthetic receptors and as molecular components of supramolecular threaded and interlocked structures. We describe the synthesis of two novel calix[6]arene macrocycles, the upper rim of which is functionalized with either two or three phenylthioureido groups. These moieties are excellent hydrogen-bond donors and largely affect the conformation of the macrocycle, both in solution and in the solid state. Alternate conformers are observed in solution, and they give rise to supramolecular chains in the solid state. Moreover, and because of this conformational freedom, also the recognition properties toward the formation of pseudorotaxanes with bipyridinium-based axles are strongly affected. The number of phenylthioureido units of the macrocyclic host and the nature of the counterions of the bipyridinium guest determine the stability of the complexes. NMR, UV-Vis and electrochemical measurements revealed that very stable 1 : 1 inclusion complexes are formed between the triphenylthioureido calix[6]arene and a bipyridinium guest with tosylate counterions, whereas no complex is observed between the diphenylthioureido macrocycle and a hexafluorophosphate bipyridinium salt.

Second-generation light-fueled supramolecular pump

Martina Canton, Jessica Groppi, Lorenzo Casimiro, Stefano Corra, Massimo Baroncini, Serena Silvi, Alberto Credi
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Published online: 20 July 2021
2021, Vol. 143, pp. 10890-10894

DOI: 10.1021/jacs.1c06027

We describe the modular design of a pseudorotaxane-based supramolecular pump and its photochemically driven autonomous nonequilibrium operation in a dissipative regime. These properties derive from careful engineering of the energy maxima and minima along the threading coordinate and their light-triggered modulation. Unlike its precursor, this second-generation system is amenable to functionalization for integration into more complex devices.

Direct synthetic routes to functionalised crown ethers

Federico Nicoli, Massimo Baroncini, Serena Silvi, Jessica Groppi, Alberto Credi
ORGANIC CHEMISTRY FRONTIERS
Published online: 1 July 2021
2021, Vol. 8, pp. 5531-5549

DOI: 10.1039/D1QO00699A

Crown ethers are macrocyclic hosts that can complex a wide range of inorganic and organic cations as well as neutral guest species. Their widespread utilization in several areas of fundamental and applied chemistry strongly relies on strategies for their functionalisation, in order to obtain compounds that could carry out multiple functions and could be incorporated in sophisticated systems. Although functionalised crown ethers are normally synthesised by templated macrocyclisation using appropriately substituted starting materials, the direct addition of functional groups onto a pre-formed macrocyclic framework is a valuable yet underexplored alternative. Here we review the methodologies for the direct functionalisation of aliphatic and aromatic crown ethers sporadically reported in the literature over a period of four decades. The general approach for the introduction of moieties on aliphatic crown ethers involves a radical mediated cross dehydrogenative coupling initiated either by photochemical or thermal/chemical activation, while aromatic crown ethers are commonly derivatised via electrophilic aromatic substitution. Direct functionalization routes can reduce synthetic effort, allow the later modification of crown ether-based architectures, and disclose new ways to exploit these versatile macrocycles in contemporary supramolecular science and technology.

Space charge behaviour of quantum dot-doped polystyrene polymers

Zhipegn Lei, Davide Fabiani, Tommaso Bray, Chuanyang Li, Xinyu Wang, Thomas Andritsch, Alberto Credi, Marcello La Rosa
IEEE TRANSACTIONS ON DIELECTRICS AND ELECTRICAL INSULATION
Published online: June 2021
2021, Vol. 28, Issue 3, pp. 753-761

DOI: 10.1109/TDEI.2021.009223

This paper deals with the role played by the interface and bulk volume of the nanofiller about affecting the electrical properties of a nanocomposite material. For this purpose, a simple and completely amorphous matrix, polystyrene (PS), is used as base material, and core-shell quantum dots are exploited for simulating the structure of nanocomposites: CdSe core and CdSe-ZnS core-shell semiconductor quantum dots (QDs) are added into a PS matrix. The latter is to highlight the effect of the ZnS interface and as contrast to the core material. Dispersion and distribution of QDs are first microscopically observed and optimized, by including isopropyl alcohol in the manufacturing phase as an additional solvent. Among electrical properties the focus is on space charge accumulation, tested by means of the pulsed electroacoustic technique at 10 kV/mm and 50 kV/mm on CdSe and CdSe-ZnS doped PS composites. Results are then compared with a reference PS without QDs. Trap depth and density are also obtained by space charge measurement results. When CdSe QDs are added to PS, the trap density increases with respect to the baseline values measured on the unfilled polymer. In contrast, the ZnS shell around the CdSe core creates an additional trap level with lower trap depth, which increases charge mobility, thus turning homocharge into heterocharge accumulation. Therefore, the surface shell-structure of QD nanocrystals appears to significantly influence the space charge behavior of the nanocomposite, independently of the polymer.

Artificial supramolecular pumps powered by light

Stefano Corra, Lorenzo Casimiro, Massimo Baroncini, Jessica Groppi, Marcello La Rosa, Marina Tranfic Bakic, Serena Silvi, Alberto Credi
CHEMISTRY - A EUROPEAN JOURNAL
Published online: 5 May 2021
2021, Vol. 27, pp. 11076-11083

DOI: 10.1002/chem.202101163

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The development of artificial nanoscale motors that can use energy from a source to perform tasks requires systems capable of performing directionally controlled molecular movements and operating away from chemical equilibrium. Here we describe the design, synthesis and properties of pseudorotaxanes in which a photon input triggers the unidirectional motion of a macrocyclic ring with respect to a non‐symmetric molecular axle. The photoinduced energy ratcheting at the basis of the pumping mechanism is validated by measuring the relevant thermodynamic and kinetic parameters. Owing to the photochemical behaviour of the azobenzene moiety embedded in the axle, the pump can repeat its operation cycle autonomously under continuous illumination. We use NMR spectroscopy to observe the dissipative non‐equilibrium state generated in situ by light irradiation. We also show that fine changes in the axle structure lead to an improvement in the performance of the motor. Such results highlight the modularity and versatility of this minimalist pump design, which provides facile access to dynamic systems that operate under photoinduced non‐equilibrium regimes.

Stereodynamics of E/Z isomerization in rotaxanes through mechanical shuttling and covalent bond rotation

Stefano Corra, Christiaan de Vet, Massimo Baroncini, Alberto Credi, Serena Silvi
CHEM
Published online: 17 May 2021
2021, Vol. 7, pp. 2137-2150

DOI: 10.1016/j.chempr.2021.04.010

The mechanical bond has opened a new world for structural and dynamic stereochemistry, which is still largely underexplored and whose significance for various applications is becoming increasingly evident. We demonstrate that designed rearrangements involving both covalent and mechanical bonds can be integrated in [2]rotaxanes, leading to interesting consequences in terms of E/Z isomerization mechanisms. Two entirely distinct and concomitant stereomutations, pertaining to the same stereogenic element but involving different kinds of linkages within the molecule, are observed and are thoroughly characterized. The rate of the two processes is affected in opposite ways upon changing solvent polarity; such a phenomenon can be used to selectively modify the rate of each motion and adjust the relative contribution of the two mechanisms to the isomerization. Although the movements are not synchronized, an analysis of the intriguing fundamental implications for transition state theory, reaction pathway bifurcation, and microscopic reversibility was triggered by our experimental observations.

Chemically induced mismatch of rings and stations in [3]rotaxanes

Massimiliano Curcio, Federico Nicoli, Erica Paltrinieri, Ettore Fois, Gloria Tabacchi, Luigi Cavallo, Serena Silvi, Massimo Baroncini, Alberto Credi
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Published online: 29 April 2021
2021, Vol. 143, pp. 8046-8055

DOI: 10.1021/jacs.1c02230

The mechanical interlocking of molecular components can lead to the appearance of novel and unconventional properties and processes, with potential relevance for applications in nanoscience, sensing, catalysis, and materials science. We describe a [3]rotaxane in which the number of recognition sites available on the axle component can be changed by acid–base inputs, encompassing cases in which this number is larger, equal to, or smaller than the number of interlocked macrocycles. These species exhibit very different properties and give rise to a unique network of acid–base reactions that leads to a fine pKa tuning of chemically equivalent acidic sites. The rotaxane where only one station is available for two rings exhibits a rich coconformational dynamics, unveiled by an integrated experimental and computational approach. In this compound, the two crown ethers compete for the sole recognition site, but can also come together to share it, driven by the need to minimize free energy without evident inter-ring interactions.

Selective access to constitutionally identical, orientationally isomeric calix[6]arene-based [3]rotaxanes by an active template approach

Margherita Bazzoni, Leonardo Andreoni, Serena Silvi, Alberto Credi, Gianpiero Cera, Andrea Secchi, Arturo Arduini
CHEMICAL SCIENCE
Published online: 1 April 2021
2021, Vol. 12, pp. 6419-6428

DOI: 10.1039/d1sc00279a

Tris(phenylureido)calix[6]arene is endowed with unique properties that make it a valuable macrocyclic component for the synthesis of mechanically interlocked molecules. Its three-dimensional and intrinsically nonsymmetric structure is kinetically selective toward two processes: (i) in apolar media, the threading of bipyridinium based axle-like components takes place exclusively from the upper rim; (ii) SN2 alkylation reactions of a pyridylpyridinium precursor engulfed in the cavity occur selectively at pyridylpyridinium nitrogen atom located at the macrocycle upper rim (active template synthesis). Here we exploit such properties to prepare two series of [3]rotaxanes, each consisting of three sequence isomers that arise from the threading of two identical but nonsymmetric wheels on a symmetric thread differing only for the reciprocal orientation of the macrocycles. The features of the calix[6]arene and the active template synthetic approach, together with a careful selection of the precursors, enabled us to selectively synthesise the [3]rotaxane sequence isomers of each series with fast kinetics and high yields.

Heteroditopic calix[6]arene based intervowen and interlocked molecular devices

Gianpiero Cera, Arturo Arduini, Andrea Secchi, Alberto Credi, Serena Silvi
THE CHEMICAL RECORD
Published online: 3 March 2021
2021, Vol. 21, pp. 1161-1181

DOI: 10.1002/tcr.202100012

This personal account summarizes the achievement gained during the last two decades on the use of the calix[6]arene as a platform to build‐up interwoven and interlocked structures for the synthesis of oriented (pseudo)rotaxanes. We also account on how these calix[6]arene hosts affect the reactivity or spectroscopic properties of their bound guests.

Ruthenium tris(bipyridine) complexes: Interchange between photons and electrons in molecular-scale devices and machines

COORDINATION CHEMISTRY REVIEWS
Vincenzo Balzani, Paola Ceroni, Alberto Credi, Margherita Venturi
Published online: 15 January 2021
2021, Vol. 433, art. n. 213758

DOI: 10.1016/j.ccr.2020.213758


This review deals with a very peculiar molecule, [Ru(bpy)3]2+ (bpy: 2,2′-bipyridine), and its interaction with photons and electrons. We summarize the properties that make [Ru(bpy)3]2+ and related compounds a unique family of “clever” molecules which find application as components of chemical species capable of processing optical and electrical signals to perform a variety of interesting functions. Examples discussed in the article, that showcase the realization of these concepts with molecular and supramolecular systems, include molecular wires, switches, antennas and mechanical machines.

Out-of-equilibrium threaded and interlocked molecular structures

Massimo Baroncini, Serena Silvi, Alberto Credi
in OUT-OF-EQUILIBRIUM (SUPRA)MOLECULAR SYSTEMS AND MATERIALS (Eds: N. Giuseppone, A. Walther), Wiley-VCH, Weinheim
Published online: April 2021
Chapter 11, pp. 305-336

ISBN: 978-3-527-34615-8

The bottom-up design, construction, and operation of artificial mechanical devices and machines on the molecular scale is a topic of great interest in nanoscience and a fascinating challenge of nanotechnology. Biomolecular motors, executing crucial functions within living cells, demonstrate the utility and feasibility of systems of this kind. In recent years, the progress in several areas of chemistry, together with an improved understanding of biomolecular machines, has led to the development of a large variety of wholly synthetic molecular devices and machines that, in some cases, are able to perform specific tasks. These systems have the potential to bring about radical innovations in several areas of technology and medicine.

Hierarchical self-assembly and controlled disassembly of a cavitand-based host-guest supramolecular polymer

POLYMER CHEMISTRY
Daniele Zuccaccia, Roberta Pinalli, Rita De Zorzi, Monica Semeraro, Alberto Credi, Cristiano Zuccaccia, Alceo Macchioni, Silvano Geremia, Enrico Dalcanale
Published online: 26 November 2020
2021, Vol. 12, pp. 389-401

DOI: 10.1039/D0PY01483D

There is considerable interest in dynamic materials featuring modular components with nano-scale dimensions and controlled responsiveness to external stimuli. Supramolecular polymers are a class of materials that fulfil all these conditions well. Here, we present a family of host–guest supramolecular polymers that combine the outstanding complexing properties of tetraphosphonate cavitands toward N-methylpyridinium guests with molecular switching. The designed monomer is a cavitand featuring four inward facing P[double bond, length as m-dash]O groups at the upper rim and a single N-methylpyridinium unit at the lower rim, forming instantaneously a polymeric species in solution, thanks to the high complexation constants measured for these host–guest interactions. This system has been analyzed by NMR spectroscopy and electrochemical techniques. In order to interpret the results of diffusion-sensitive experiments, we took advantage of the X-ray crystal structure obtained for the polymeric species and developed an original treatment for the PGSE data by non-linear fitting. The analysis of the experimental data identified an isodesmic polymerization model at a monomer concentration below 20 mM, driven by intrachain host–guest interactions, and an additional level of tetrameric bundle aggregation above 20 mM, due to interchain dipolar and quadrupolar interactions. Two orthogonal disassembly procedures have been implemented: electrochemical reduction for the linear chains and solvent-driven dissolution for the bundles.

Binary logic operations with artificial molecular machines

COORDINATION CHEMISTRY REVIEWS
Federico Nicoli, Erica Paltrinieri, Marina Tranfic Bakic, Massimo Baroncini, Serena Silvi, Alberto Credi
Published online: 13 October 2020
2021, Vol. 428, art. n. 213589

DOI: 10.1016/j.ccr.2020.213589


Living systems rely on ions and molecules for gathering, elaborating and storing information, as well as for controlling motion across different length scales. Artificial mimics of such biomolecular systems, i.e., logic devices and mechanical machines, have been developed in the past decades. Here we discuss their combination to produce logic-controlled nanomachines, a research that holds great potential for basic science and prospective applications. Systems of this kind may be forerunners of ‘molecular robots’ that can execute specific functions as a result of gathering and processing multiple environmental stimuli.

Light-controlled regioselective synthesis of fullerene bis-adducts

ANGEWANDTE CHEMIE INTERNATIONAL EDITION
Luka Ðorđević, Lorenzo Casimiro, Nicola Demitri, Massimo Baroncini, Serena Silvi, Francesca Arcudi, Alberto Credi, Maurizio Prato
Published online: 28 July 2020
2021, Vol. 60, pp. 313-320

DOI: 10.1002/anie.202009235

Multi-functionalization and isomer-purity of fullerenes are crucial tasks for the development of fullerene chemistry in various fields. In both current main approaches – tether-directed covalent functionalization and supramolecular masks – the control of regioselectivity requires multi-step synthetic procedures to prepare the desired tether or mask. Herein, we describe light-responsive tethers, containing an azobenzene photoswitch and two malonate groups, in the double cyclopropanation of [60]fullerene. The formation of the bis-adducts and their spectroscopic and photochemical properties, as well as the effect of azobenzene photoswitching on the regiochemistry of the bis-addition, have been studied. The behavior of the tethers depends on the geometry of the connection between the photoactive core and the malonate moieties. One tether lead to strikingly different adduct distribution for the E and Z isomers, indicating that the covalent bis-functionalization of C60 can be controlled by light.

2020

Improving fatigue resistance of dihydropyrene by encapsulation within a coordination cage

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Martina Canton, Angela B. Grommet, Luca Pesce, Julius Gemen, Shiming Li, Yael Diskin-Posner, Alberto Credi, Giovanni M. Pavan, Joakim Andréasson, Rafal Klajn
Published online: 14 August 2020
2020, Vol. 142, pp. 14557-14565

DOI: 10.1021/jacs.0c06146

Photochromic molecules undergo reversible isomerization upon irradiation with light at different wavelengths, a process that can alter their physical and chemical properties. For instance, dihydropyrene (DHP) is a deep-colored compound that isomerizes to light-brown cyclophanediene (CPD) upon irradiation with visible light. CPD can then isomerize back to DHP upon irradiation with UV light or thermally in the dark. Conversion between DHP and CPD is thought to proceed via a biradical intermediate; bimolecular events involving this unstable intermediate thus result in rapid decomposition and poor cycling performance. Here, we show that the reversible isomerization of DHP can be stabilized upon confinement within a PdII6L4 coordination cage. By protecting this reactive intermediate using the cage, each isomerization reaction proceeds to higher yield, which significantly decreases the fatigue experienced by the system upon repeated photocycling. Although molecular confinement is known to help stabilize reactive species, this effect is not typically employed to protect reactive intermediates and thus improve reaction yields. We envisage that performing reactions under confinement will not only improve the cyclic performance of photochromic molecules, but may also increase the amount of product obtainable from traditionally low-yielding organic reactions.

Photophysical properties of 4‐dicyanomethylene‐2‐methyl‐6‐(p‐dimethylamino‐styryl)‐4H‐pyran revisited: fluorescence versus photoisomerization

CHEMISTRY - A EUROPEAN JOURNAL
Lorenzo Casimiro, Stéphane Maisonneuve, Pascal Retailleau. Serena Silvi, Juan Xie, Rémi Métivier
Published online: 11 July 2020
2020, Vol. 26, pp. 14341-14350

DOI: 10.1002/chem.202002828


Although 4‐dicyanomethylene‐2‐methyl‐6‐(p‐dimethylamino‐styryl)‐4H‐pyran (DCM) has been known for many decades as a bright and photostable fluorophore, used for a wide variety of applications in chemistry, biology and physics, only little attention has been paid so far to the presence of multiple isomers and conformers, namely s‐trans‐(E), s‐cis‐(E), s‐trans‐(Z), and s‐cis‐(Z). In particular, light‐induced E–Z isomerization plays a great role on the overall photophysical properties of DCM. Herein, we give a full description of a photoswitchable DCM derivative by a combination of structural, theoretical and spectroscopic methods. The main s‐trans‐(E) isomer is responsible for most of the fluorescence features, whereas the s‐cis‐(E) conformer only contributes marginally. The non‐emitting Z isomers are generated in large conversion yields upon illumination with visible light (e.g., 485 or 514 nm) and converted back to the E forms by UV irradiation (e.g., 365 nm). Such photoswitching is efficient and reversible, with high fatigue resistance. The E→Z and Z→E photoisomerization quantum yields were determined in different solvents and at different irradiation wavelengths. Interestingly, the fluorescence and photoisomerization properties are strongly influenced by the solvent polarity: the fluorescence is predominant at higher polarity, whereas photoisomerization becomes more efficient at lower polarity. Intermediate medium (THF) represents an optimized situation with a good balance between these two features.

Precision molecular threading/dethreading

ANGEWANDTE CHEMIE INTERNATIONAL EDITION
Jessica Groppi, Lorenzo Casimiro, Martina Canton, Stefano Corra, Mina Jafari-Nasab, Gloria Tabacchi, Luigi Cavallo, Massimo Baroncini, Serena Silvi, Ettore Fois, Alberto Credi
Published online: 12 May 2020
2020, Vol. 59, pp. 14825-14834

DOI: 10.1002/anie.202003064

The general principles guiding the design of molecular machines based on interlocked structures are well known. Nonetheless, the identification of suitable molecular components for a precise tuning of the energetic parameters that determine the mechanical link is still challenging. Indeed, what are the reasons of the “all‐or‐nothing” effect, which turns a molecular “speed‐bump” into a stopper in pseudorotaxane‐based architectures? Here we investigate the threading and dethreading processes for a representative class of molecular components, based on symmetric dibenzylammonium axles and dibenzo[24]crown‐8 ether, with a joint experimental‐computational strategy. From the analysis of quantitative data and an atomistic insight, we derive simple rules correlating the kinetic behaviour with the substitution pattern, and provide rational guidelines for the design of modules to be integrated in molecular switches and motors with sophisticated dynamic features.

Manufacturing at nanoscale: from molecular machines to transducers

in ADVANCES IN NANOSTRUCTURED MATERIALS AND NANOPATTERNING TECHNOLOGIES (Eds: M. L. Focarete, V. Guarino, D. Pisignano) Elsevier, Amsterdam
Marcello La Rosa, Massimo Baroncini, Serena Silvi, Alberto Credi
Published online: 2020
Chapter 2, pp. 41-63

DOI: 10.1016/B978-0-12-816865-3.00002-0

The bottom-up design, construction, and operation of artificial mechanical devices and machines on the molecular scale is a topic of great interest in nanoscience and a fascinating challenge of nanotechnology. Biomolecular motors, executing crucial functions within living cells, demonstrate the utility and feasibility of systems of this kind. In recent years, the progress in several areas of chemistry, together with an improved understanding of biomolecular machines, has led to the development of a large variety of wholly synthetic molecular devices and machines that, in some cases, are able to perform specific tasks. These systems have the potential to bring about radical innovations in several areas of technology and medicine.

Semiconductor quantum dots as components of photoactive supramolecular architectures

CHEMISTRYOPEN
Marcello La Rosa, Emily H. Payne, Alberto Credi
Published online: 10 February 2020
2020, Vol. 9, Issue 2, pp. 200-213

DOI: 10.1002/open.201900336

Luminescent quantum dots (QDs) are colloidal semiconductor nanocrystals consisting of an inorganic core covered by a molecular layer of organic surfactants. Although QDs have been known for more than thirty years, they are still attracting the interest of researchers because of their unique size‐tunable optical and electrical properties arising from quantum confinement. Moreover, the controlled decoration of the QD surface with suitable molecular species enables the rational design of inorganic‐organic multicomponent architectures that can show a vast array of functionalities. This minireview highlights the recent progress in the use of surface‐modified QDs – in particular, those based on cadmium chalcogenides – as supramolecular platforms for light‐related applications such as optical sensing, triplet photosensitization, photocatalysis and phototherapy.

Synthesis and properties of a redox-switchable calix[6]arene-based molecular lasso

ORGANIC CHEMISTRY FRONTIERS
Guido Orlandini, Lorenzo Casimiro, Margherita Bazzoni, Beatrice Cogliati, Alberto Credi, Marco Lucarini, Serena Silvi, Arturo Arduini, Andrea Secchi
Published online: 30 December 2019
2020, Vol. 7, pp. 648-659

DOI: 10.1039/c9qo01379b

The synthesis and characterisation of calix[6]arene-based lasso-like molecular structures is described. These interwoven structures consist of an electrochemical responsive N,N′-dialkylviologen arm covalently anchored at the upper rim of a triphenylureido calix[6]arene-based wheel. Upon reduction of the viologen core, a hollow tridimensional macrocyclic structure can be generated. This process is reversible, and the original lasso-like structure can be regenerated by oxidizing the viologen arm to its original dicationic form. Electrochemical and EPR techniques investigated the ability of the system to perform threading/dethreading movements upon redox switching. The functionalisation of the arm ω-hydroxy ending with a bulky diphenylacetyl group converts the self-threaded structure in a blocked interwoven molecular compound belonging to the class of [1]rotaxanes. The ability to form dimeric structures in the shape of a [c2]daisy chain was also demonstrated, an unprecedented result for calixarene macrocycles.

Macchine molecolari

ITHACA - VIAGGIO NELLA SCIENZA
Alberto Credi, Vincenzo Balzani
Published online: 1 January 2020
2020, Vol. XIV, pp. 5-17

La costruzione di macchine di dimensioni molecolari è uno straordinario risultato scientifico e un obiettivo primario della nanotecnologia. Nel corso degli ultimi trent’anni, i chimici in varie parti del mondo hanno imparato a realizzare delle semplici macchine nanometriche. Anche se questi minuscoli dispositivi non sono ancora divenuti parte integrante della nostra vita quotidiana, siamo ormai alle soglie di una nuova rivoluzione industriale, capace di cambiare il nostro futuro con applicazioni innovative nella tecnologia dei materiali, nell’informatica, nella robotica e nella medicina.

Photo-activated artificial molecular machines that can perform tasks

ADVANCED MATERIALS
Stefano Corra, Massimiliano Curcio, Massimo Baroncini, Serena Silvi, Alberto Credi
Published online: 20 January 2020
2020, Vol. 32, Issue 20, art. n. 1906064, pp. 1-22

DOI: 10.1002/adma.201906064

Research on artificial photoactivated molecular machines has moved in recent years from a basic scientific endeavor toward a more applicative effort. Nowadays, the prospect of reproducing the operation of natural nanomachines with artificial counterparts is no longer a dream but a concrete possibility. The progress toward the construction of molecular‐machine‐based devices and materials in which light irradiation results in the execution of a task as a result of nanoscale movements is illustrated here. After a brief description of a few basic types of photoactivated molecular machines, significant examples of their exploitation to perform predetermined functions are presented. These include switchable catalysts, nanoactuators that interact with cellular membranes, transporters of small molecular cargos, and active joints capable of mechanically coupling molecular‐scale movements. Investigations aimed at harnessing the collective operation of a multitude of molecular machines organized in arrays to perform tasks at the microscale and macroscale in hard and soft materials are also reviewed. Surfaces, gels, liquid crystals, polymers, and self‐assembled nanostructures are described wherein the nanoscale movement of embedded molecular machines is amplified, allowing the realization of muscle‐like actuators, microfluidic devices, and polymeric materials for light energy transduction and storage.

Photoinduced electron transfer involving a naphthalimide chromophore in switchable and flexible [2]rotaxanes

CHEMISTRY - A EUROPEAN JOURNAL
Benoit Colasson, Alberto Credi, Barbara Ventura
Published online: 22 October 2019
2020, Vol. 26, Issue 2, pp. 534-542

DOI: 10.1002/chem.201904155

The interlocking of ring and axle molecular components in rotaxanes provides a way to combine chromophoric, electron‐donor and electron‐acceptor moieties in the same molecular entity, in order to reproduce the features of photosynthetic reaction centers. To this aim, the photoinduced electron transfer processes involving a 1,8‐naphthalimide chromophore, embedded in several rotaxane‐based dyads, were investigated by steady‐state and time‐resolved absorption and luminescence spectroscopic experiments in the 300 fs–10 ns time window. Different rotaxanes built around the dialkylammonium/ dibenzo[24]crown‐8 ether supramolecular motif were designed and synthesized to decipher the relevance of key structural factors, such as the chemical deactivation of the ammonium‐crown ether recognition, the presence of a secondary site for the ring along the axle, and the covalent functionalization of the macrocycle with a phenothiazine electron donor. Indeed, the conformational freedom of these compounds gives rise to a rich dynamic behavior induced by light and may provide opportunities for investigating and understanding phenomena that take place in complex (bio)molecular architectures.

Photo- and redox-driven artificial molecular motors

CHEMICAL REVIEWS
Massimo Baroncini, Serena Silvi, Alberto Credi
Published online: 15 August 2019
2020, Vol.120, pp. 200-268

DOI: 10.1021/acs.chemrev.9b00291

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature’s choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed.

2019

Ultrafast processes triggered by one- and two-photon excitation of a photochromic and luminescent hydrazone

BEILSTEIN JOURNAL OF ORGANIC CHEMISTRY
Alessandro Iagatti, Baihao Shao, Alberto Credi, Barbara Ventura, Ivan Aprahamian, Mariangela Di Donato
Published online: 15 October 2019
2019, Vol. 15, pp. 2438-2446

DOI: 10.3762/bjoc.15.236

In this work we apply a combination of steady state and time resolved luminescence and absorption spectroscopies to investigate the excited-state dynamics of a recently developed molecular photoswitch, belonging to the hydrazone family. The outstanding properties of this molecule, involving fluorescence toggling, bistability, high isomerization quantum yield and non-negligible two-photon absorption cross section, make it very promising for numerous applications. Here we show that the light induced Z/E isomerization occurs on a fast <1 ps timescale in both toluene and acetonitrile, while the excited state lifetime of the Z-form depends on solvent polarity, suggesting a partial charge transfer nature of its low lying excited state. Time-resolved luminescence measurements evidence the presence of a main emission component in the 500–520 nm spectral range, attributed to the Z-isomer, and a very short living blue-shifted emission, attributed to the E-isomer. Finally, transient absorption measurements performed upon far-red excitation are employed as an alternative method to determine the two-photon absorption cross-section of the molecule.

Design of photo-activated molecular machines: highlights from the past ten years

CHEMICAL COMMUNICATIONS
Jessica Groppi, Massimo Baroncini, Margherita Venturi, Serena Silvi, Alberto Credi
Published online: 25 September 2019
2019, Vol. 55, pp. 12595-12602

DOI: 10.1039/c9cc06516d

Molecular machines powered by light are appropriately designed multicomponent molecular structures that exploit photochemical processes to perform controlled movements. We discuss the advances in the design and investigation of light-driven molecular machines by presenting a selection of examples from the past decade.

Individual‐molecule perspective analysis of chemical reaction networks: the case of a light‐driven supramolecular pump

ANGEWANDTE CHEMIE INTERNATIONAL EDITION
Andrea Sabatino, Emanuele Penocchio, Giulio Ragazzon, Alberto Credi, Diego Frezzato
Published online: 4 August 2019
2019, Vol. 58, pp. 14341-14348

DOI: 10.1002/anie.201908026

The first study in which stochastic simulations of a two‐component molecular machine are performed in the mass‐action regime is presented. This system is an autonomous molecular pump consisting of a photoactive axle that creates a directed flow of rings through it by exploiting light energy away from equilibrium. The investigation demonstrates that the pump can operate in two regimes, both experimentally accessible, in which light‐driven steps can be rate‐determining or not. The number of photons exploited by an individual molecular pump, as well as the precision of cycling and the overall efficiency, critically rely on the operating regime of the machine. This approach provides useful information not only to guide the chemical design of a self‐assembling molecular device with desired features, but also to elucidate the effect of the environment on its performance, thus facilitating its experimental investigation.

Light on molecular devices

CHEMPHOTOCHEM
Alberto Credi
Published online: 24 June 2019
2019, Vol. 3, pp. 578-579

DOI: 10.1002/cptc.201900172

Meet the board: Alberto Credi is a Professor of Chemistry at the Università di Bologna (Italy) and an Associate Research Director at the Consiglio Nazionale delle Ricerche. He is a current member of the ChemPhotoChem Editorial Board.

Chemical on/off switching of mechanically planar chirality and chiral anion recognition in a [2]rotaxane molecular shuttle

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Stefano Corra, Christiaan de Vet, Jessica Groppi, Marcello La Rosa, Serena Silvi, Massimo Baroncini, Alberto Credi
Published online: 25 May 2019
2019, Vol. 141, pp. 9129-9133

DOI: 10.1021/jacs.9b00941

We exploit a reversible acid-base triggered molecular shuttling process to switch an appropriately designed rotaxane between prochiral and mechanically planar chiral forms. The mechanically planar enantiomers and their interconversion, arising from ring shuttling, have been characterized by NMR spectroscopy. We also show that the supramolecular interaction of the positively charged rotaxane with optically active anions causes an imbalance in the population of the two enantiomeric coconformations. This result represents an unprecedented example of chiral molecular recognition and can disclose innovative approaches to enantioselective sensing and catalysis.

Light-responsive (supra)molecular architectures: recent advances

ADVANCED OPTICAL MATERIALS
Massimo Baroncini, Jessica Groppi, Stefano Corra, Serena Silvi, Alberto Credi
First published: 17 May 2019
2019, Vol. 7, Issue 16, art. n.1900392, pp. 1-16

DOI: 10.1002/adom.201900392

The development and investigation of (supra)molecular‐based architectures characterized by light‐activated functionalities is a highly relevant topic of chemical research. The interest on photo‐controlled systems arises not only from their potential applications in different fields of technology but also from their scientific significance related to the understanding of light–matter interactions at the nanoscale. Indeed, light is a peculiar and unique tool as it can be conveniently applied to supply the energy required to affect and operate a system and, at the same time, to probe its state and investigate its transformations. Some basic aspects of light‐induced processes in (supra)molecular architectures are discussed here in the frame of their use to implement novel functionalities in nanostructured systems and materials. In this perspective, a few recent examples from our own work will be illustrated which are meant to provide an overview of the current directions in this highly cross‐disciplinary field of research.

Threading-gated photochromism in [2]pseudorotaxanes

CHEMICAL SCIENCE
Giorgio Baggi, Lorenzo Casimiro, Massimo Baroncini, Serena Silvi, Alberto Credi, Stephen J. Loeb
Published online: 23 April 2019
2019, Vol. 10, pp. 5104-5113

DOI: 10.1039/c9sc00913b

Rigid, Y-shaped imidazole compounds containing the bis(thienyl)ethene moiety were designed and synthesized. The 4,5-bis(benzothienyl)-2-phenylimidazolium cations were then used as axles for [2]pseudorotaxane formation with 24-membered crown ether wheels. It was demonstrated using 1H NMR spectroscopy, UV-Vis absorption and emission spectroscopies that this host-guest interaction results in significant changes in the photochromic properties of the imidazolium axles. This is a rare example of gated photochromism, which exploits the recognition event of an interpenetrated molecular system to tune the photochromic properties in one of the components.

Solution and solid state photochromism in a family of shape persistent azobenzene tetramers functionalized with alkyloxy substituents

PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES
Jacopo Vialetto, Jessica Groppi, Marcello La Rosa, Serena Silvi, Alberto Credi, Massimo Baroncini
Published online: 10 April 2019
2019, Vol. 18, Issue 9, pp. 2281-2286

DOI: 10.1039/c9pp00022d

Shape-persistent azobenzene tetramers functionalized at the periphery with alkyloxy substituents of different length have been synthesized, and their photochemical behaviour has been investigated. Efficient E → Z photoisomerization of the azobenzene units takes place both in solution and in the solid state, a highly desirable yet uncommon property for azobenzene-type photochromic compounds. The solid state E → Z photoisomerization is accompanied by an isothermal crystal–amorphous phase transformation; successively, anisotropic crystals can be grown upon promoting the Z → E isomerization by thermal annealing of the irradiated samples. These results validate the strategy of engineering multiphotochromic architectures with a rigid star-shaped geometry to preserve the solution-based photoreactivity also in the solid state. The observed unexpected photoinduced alignment makes these materials potentially attractive for the development of photo-patternable and photo-responsive surfaces.

New geometries for calix[6]arene-based rotaxanes

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY
Margherita Bazzoni, Valeria Zanichelli, Lorenzo Casimiro, Chiara Massera, Alberto Credi, Andrea Secchi, Serena Silvi, Arturo Arduini
Published online: 22 March 2019
2019, pp. 3513-3124

DOI: 10.1002/ejoc.201900211

Understanding the role played by the nature, number and arrangement of binding sites anchored to a macrocycle remains a topic of current interest for the synthesis of new interwoven species. We report the synthesis and detailed structural characterisation of a new calix[6]arene derivative decorated with two phenylureido groups anchored at the diametrical position of the calix upper rim that adopts a 1,2,3-alternate conformation in solution and in the solid state. Preliminary data on the ability of this host to form redox-active pseudorotaxanes and rotaxanes are reported.

A molecular cable car for transmembrane ion transport

ANGEWANDTE CHEMIE INTERNATIONAL EDITION
Alberto Credi
Published online: 20 February 2019
2019, Vol. 58, pp. 4108-4110

DOI: 10.1002/anie.201814333

The controlled transport of molecular and ionic substrates across bilayer membranes is a fundamental task for the operation of living organisms. It is also a highly fascinating and demanding challenge for artificial molecular machines. The recent report of a synthetic transmembrane molecular shuttle that can transport potassium ions selectively down a gradient in a liposomal system makes a small but significant step forward towards this goal.

EPR sensing of metal and organic cations using a novel spin-labelled dibenzo-24-crown-8-ether

PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Lorenzo Gualandi, Paola Franchi, Alberto Credi, Elisabetta Mezzina, Marco Lucarini
Published online: 7 September 2018
2019, Vol. 21, pp. 3558-3563

DOI: 10.1039/C8CP04362K

The synthesis of novel dibenzo-24-crown ether substituted nitroxides and their use as spin probes for the detection of cation guests by EPR are reported. Formation of a host–guest complex between the proposed spin probes and several cations, both organic and inorganic, was evidenced by a significant change in the value of the benzylic and nitrogen EPR hyperfine splittings upon complexation. This favorable feature provided a reliable EPR sensor that is able to selectively distinguish different cationic guests.

2018

Solution and solid-state emission toggling of a photochromic hydrazone

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Baihao Shao, Massimo Baroncini, Hai Qian, Laura Bussotti, Mariangela Di Donato, Alberto Credi, Ivan Aprahamian
Published online: 25 September 2018
2018, Vol. 140, pp. 12323-12327

DOI: 10.1021/jacs.8b07108

The proliferation of light-activated switches in recent years has enabled their use in a broad range of applications encompassing an array of research fields and disciplines. All current systems, however, have limitations (e.g., from complicated synthesis to incompatibility in biologically relevant media and lack of switching in the solid-state) that can stifle their real-life application. Here we report on a system that packs most, if not all, the desired, targeted and sought-after traits from photochromic compounds (bistability, switching in various media ranging from serum to solid-state, while exhibiting ON/OFF fluorescence emission switching, and two-photon assisted near infrared light toggling) in an easily accessible structure.

Photoactive molecular-based devices, machines and materials: recent advances

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY
Massimo Baroncini, Martina Canton, Lorenzo Casimiro, Stefano Corra, Jessica Groppi, Marcello La Rosa, Serena Silvi, Alberto Credi
Published online: 14 September 2018
2018, pp. 4589-4603

DOI: 10.1002/ejic.201800923

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Molecular and supramolecular-based systems and materials that can perform predetermined functions in response to light stimulation have been extensively studied in the past three decades. Their investigation continues to be a highly stimulating topic of chemical research, not only because of the inherent scientific value related to a bottom-up approach to functional nanostructures, but also for the prospective applications in diverse fields of technology and medicine. Light is an important tool in this context, as it can be conveniently used both for supplying energy to the system and for probing its states and transformations. In this microreview we recall some basic aspects of light-induced processes in (supra)molecular assemblies, and discuss their exploitation to implement novel functionalities with nanostructured devices, machines and materials. To this aim we illustrate a few examples from our own recent work, which are meant to illustrate the trends of current research in the field.

Among the top 10% most downloaded articles of the journal in the 12 months following publication

Reversible photoswitching and isomer-dependent diffusion of single azobenzene tetramers on a metal surface

ANGEWANDTE CHEMIE INTERNATIONAL EDITION
Christophe Nacci, Massimo Baroncini, Alberto Credi, Leonhard Grill
Published online: 6 September 2018
2018, Vol. 57, pp 15034-15039

DOI: 10.1002/anie.201806536

Azobenzene is a prototypical molecular switch that can be reversibly photoisomerized between the nearly planar and apolar trans form, and the distorted, polar cis form. Thus far, most studies related to azobenzene derivatives have focused on planar adsorbed molecules. We present here the study of a three‐dimensional shape‐persistent molecular architecture consisting of four tetrahedrally arranged azobenzene units and adsorbed on a Ag(111) surface. While the azobenzenes of the tripod in contact with the surface lose their switching ability, different isomers of the upright standing arm of the tetramer are obtained reversibly and efficiently by illumination at different wavelengths, revealing time constants of only a few minutes. Diffusion on the surface turns out to be dependent on the isomeric state ‐ trans or cis ‐ of the upright oriented azobenzene group. Hence, molecular mobility can be modulated via their isomeric state, which suggests that for instance molecular growth processes could be controlled by external stimuli.

Electrochemically triggered co-conformational switching in a [2]catenane comprising a non-symmetric calix[6]arene wheel and a two-station oriented macrocycle

MOLECULES
Valeria Zanichelli, Luca Dallacasagrande, Arturo Arduini, Andrea Secchi, Giulio Ragazzon, Serena Silvi, Alberto Credi
Published online: 11 May 2018
2018, Vol. 23, Issue 5, art. n. 1156, pp. 1-12

DOI: 10.3390/molecules23051156

Catenanes with desymmetrized ring components can undergo co-conformational rearrangements upon external stimulation and can form the basis for the development of molecular rotary motors. We describe the design, synthesis and properties of a [2]catenane consisting of a macrocycle—the ‘track’ ring—endowed with two distinct recognition sites (a bipyridinium and an ammonium) for a calix[6]arene—the ‘shuttle’ ring. By exploiting the ability of the calixarene to thread appropriate non-symmetric axles with directional selectivity, we assembled an oriented pseudorotaxane and converted it into the corresponding oriented catenane by intramolecular ring closing metathesis. Cyclic voltammetric experiments indicate that the calixarene wheel initially surrounds the bipyridinium site, moves away from it when it is reduced, and returns in the original position upon reoxidation. A comparison with appropriate model compounds shows that the presence of the ammonium station is necessary for the calixarene to leave the reduced bipyridinium site.

Synthesis of densely-packaged, ultrasmall Pt(0)2 clusters within a thioether functionalised MOF and their catalytic activity in industrial reactions at low–temperature

ANGEWANDTE CHEMIE INTERNATIONAL EDITION
Marta Mon, Miguel A. Rivero-Crespo, Jesús Ferrando-Soria, Alejandro Vidal-Moya, Mercedes Boronat, Antonio Leyva-Pérez, Avelino Corma, Juan C. Hernández-Garrido, Miguel López-Haro, José J. Calvino, Giulio Ragazzon, Alberto Credi, Donatella Armentano, Emilio Pardo
Published online: 30 March 2018
2018, Vol. 57, pp. 6186-6191

DOI: 10.1002/anie.201801957

The gram-scale synthesis, stabilization, and characterization of well-defined ultrasmall subnanometric catalytic clusters on solids is a challenge. The chemical synthesis and X-ray snapshots of Pt02 clusters, homogenously distributed and densely packaged within the channels of a metal–organic framework, is presented. This hybrid material catalyzes efficiently, and even more importantly from an economic and environmental viewpoint, at low temperature (25 to 140 °C), energetically costly industrial reactions in the gas phase such as HCN production, CO2 methanation, and alkene hydrogenations. These results open the way for the design of precisely defined catalytically active ultrasmall metal clusters in solids for technically easier, cheaper, and dramatically less-dangerous industrial reactions.

Photochemical investigation of cyanoazobenzene derivatives as components of artificial supramolecular pumps

PHOTOCHEMICAL & PHOTOBIOLOGICAL SCIENCES
Lorenzo Casimiro, Jessica Groppi, Massimo Baroncini, Marcello La Rosa, Alberto Credi, Serena Silvi
Published online: 3 May 2018
2018, Vol. 17, Issue 6, pp. 734-740

DOI: 10.1039/c8pp00062j

Among the plethora of photochromes reported so far, azobenzene has been proven to be the most suitable photoswitch for molecular systems and materials, due to its highly efficient and clean E–Z photoisomerization. Here we report two ammonium-based molecular axles bearing one or two p-cyanoazobenzene units at the extremities, able to form pseudorotaxanes with a crown ether macrocycle. The photochemistry of these compounds was studied in the isolated forms and in the pseudorotaxanes, showing that the functionalization speeds up the threading process without affecting the photochemical properties of the system. These results suggest that the investigated pseudorotaxanes can form the basis of new prototypes of artificial molecular-level pumps.

Designed long lived emission from CdSe quantum dots by reversible electronic energy transfer with a surface bound chromophore

ANGEWANDTE CHEMIE INTERNATIONAL EDITION
Marcello La Rosa, Sergey A. Denisov, Gediminas Jonusauskas, Nathan D. McClenaghan, Alberto Credi
Published online: 2 February 2018
2018, Vol. 57, pp. 3104-3107

DOI: 10.1002/anie.201712403

The size-tunable emission of luminescent quantum dots (QDs) makes them highly interesting for applications that range from bioimaging to optoelectronics. For the same applications, engineering their luminescence lifetime – in particular, making it longer – would be as important; however, no rational methodology to reach this goal is available to date. Here we describe a strategy to prolong the emission lifetime of QDs by electronic energy shuttling to the triplet excited state of a surface-bound molecular chromophore. To implement this idea we made CdSe QDs of different sizes, and we self-assembled them with a pyrene derivative. We observed that the conjugates exhibit delayed luminescence, with emission decays that are prolonged by more than 3 orders of magnitude compared to the parent CdSe QDs (lifetimes up to 330 us). The mechanism invokes unprecedented reversible quantum dot-to-organic chromophore electronic energy transfer.

Making and operating molecular machines: a multidisciplinary challenge

CHEMISTRYOPEN
Massimo Baroncini, Lorenzo Casimiro, Christiaan de Vet, Jessica Groppi, Serena Silvi, Alberto Credi
Published online: 2 February 2018
2018, Vol. 7, pp. 169-179

DOI: 10.1002/open.201700181

Movement is one of the central attributes of life, and a key feature in many technological processes. While artificial motion is typically provided by macroscopic engines powered by internal combustion or electrical energy, movement in living organisms is produced by machines and motors of molecular size that typically exploit the energy of chemical fuels at ambient temperature to generate forces and ultimately execute functions. The progress in several areas of chemistry, together with an improved understanding of biomolecular machines, has led to the development of a large variety of wholly synthetic molecular machines. These systems have the potential to bring about radical innovations in several areas of technology and medicine. In this article we discuss, with the help of a few examples, the multidisciplinary aspects of research on artificial molecular machines, and highlight its translational character.

Among the top 10% most downloaded articles of the journal in the 12 months following publication

2017

Remote electrochemical modulation of pKa in a rotaxane by co-conformational allostery

PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Giulio Ragazzon, Christian Schäfer, Paola Franchi, Serena Silvi, Benoit Colasson, Marco Lucarini, Alberto Credi
Published online: 27 November 2017
2018, Vol. 115, pp. 9385-9390

DOI: 10.1073/pnas.1712783115

Allosteric control, one of Nature’s most effective ways to regulate functions in biomolecular machinery, involves the transfer of information between distant sites. The mechanistic details of such a transfer are still an object of intensive investigation and debate, and the idea that intramolecular communication could be enabled by dynamic processes is gaining attention as a complement to traditional explanations. Mechanically interlocked molecules, owing to the particular kind of connection between their components and the resulting dynamic behavior, are attractive systems to investigate allosteric mechanisms and exploit them to develop functionalities with artificial species. We show that the pKa of an ammonium site located on the axle component of a [2]rotaxane can be reversibly modulated by changing the affinity of a remote recognition site for the interlocked crown ether ring through electrochemical stimulation. The use of a reversible ternary redox switch enables us to set the pKa to three different values, encompassing more than seven units. Our results demonstrate that in the axle the two sites do not communicate, and that in the rotaxane the transfer of information between them is made possible by the shuttling of the ring, that is, by a dynamic intramolecular process. The investigated coupling of electron- and proton-transfer reactions is reminiscent of the operation of the protein complex I of the respiratory chain.

An efficient method for the surface functionalization of luminescent quantum dots with lipoic acid-based ligands

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY
Marcello La Rosa, Tommaso Avellini, Christophe Lincheneau, Serena Silvi, Iain A. Wright, Edwin C. Constable, Alberto Credi
Published online: 9 October 2017
2017, pp. 5143-5151

DOI: 10.1002/ejic.201700781

We describe herein an operationally advantageous general methodology for efficiently activating lipoic acid based compounds, a family of popular surface ligands for semiconductor nanocrystals, through the use of a borohydride exchange resin, and the use of the activated species to replace the native surface ligands of quantum dots. The procedure enabled phase transfer of the nanocrystals between polar and aqueous media and, if unsubstituted lipoic acid was used, a facile adjustment of their solubility in a wide range of solvents with varying polarity (from hexane to water). We show that the protocol is applicable to different types of nanocrystals and a variety of lipoic acid based ligands, and that the resulting quantum dots maintain their optical properties, in particular, an intense luminescence, and long-term colloidal stability.

Efficient active-template synthesis of calix[6]arene-based oriented pseudorotaxanes and rotaxanes

ORGANIC & BIOMOLECULAR CHEMISTRY
Valeria Zanichelli, Giulio Ragazzon, Guido Orlandini, Margherita Venturi, Alberto Credi, Serena Silvi, Arturo Arduini, Andrea Secchi
Published online: 25 July 2017
2017, Vol. 15, pp. 6753-6763

DOI: 10.1039/C7OB01642E

A substrate can modify its chemical features, including a change of its reactivity, as a consequence of non-covalent interactions upon inclusion within a molecular host. Since the rise of supramolecular chemistry, this phenomenon has stimulated the ingenuity of scientists to emulate the function of enzymes by designing supramolecular systems in which the energetics and selectivity of reactions can be manipulated through programmed host–guest interactions and/or steric confinement. In this paper we investigate how the engulfment of a positively charged pyridinium-based guest inside the π-rich cavity of a tris-(N-phenylureido)calix[6]arene host affects its reactivity towards a SN2 reaction. We found that the alkylation of complexed substrates leads to the formation of pseudorotaxanes and rotaxanes with faster kinetics and higher yields with respect to the standard procedures exploited so far. More importantly, the strategy described here expands the range of efficient synthetic routes for the formation of mechanically interlocked species with a strict control of the mutual orientation of their non-symmetric molecular components.

Processing light signals by molecular and supramolecular devices, machines and materials

EPA NEWSLETTER
Massimo Baroncini, Alberto Credi, Serena Silvi
June 2017, No. 92, pp. 30-34

Research in the past two decades has shown that chemical substrates can operate as input/output devices and are capable of handling chemical, electrical and optical signals. The conversion of the input(s) into the output(s) by the substrates, which include molecules, supramolecular systems, nanostructured and bulk materials, can give rise to functions spanning from chemosensing to smart drug delivery and mechanical actuation. Among the possible types of signals that can be processed by chemical substrates, light is certainly very attractive because it can be used both to write and read the system, it can be finely tuned in energy, time and space domains, and it is a potentially clean stimulus.

Gearing up molecular rotary motors

SCIENCE
Massimo Baroncini, Alberto Credi
Published online: 2 June 2017
2017, Vol. 356, Issue 6341, pp. 906-907

DOI: 10.1126/science.aan4353

Machines and motors based on synthetic small molecules are realized by a “bottom-up” approach to nanotechnology and could exploit molecular motion in one of two ways. The first generates macroscopic work by collecting the actions of many nanodevices organized in an array that provides spatial and temporal control of the motion activated by an energy supply. This approach mimics myosin motor proteins in skeletal muscles. The second route uses the energy-consuming directed movement of individual molecular machines to perform a task at the nanoscale, mimicking kinesin-based transport. Both cases mechanically couple an active component (the molecular machine) with nearby passive components and, ultimately, with the surrounding environment. Štacko et al. report the synchronous transmission of a photoactivated directional motion from a synthetic molecular motor to a coupled rotor. This demonstration takes an important step forward toward more complex mechanical functions with artificial nanoscale devices.