From Photochemistry to Supramolecolar Chemistry, Molecular Nanotechnology, and Societal Commitment.

Auguri! On the occasion of the 85th birthday of Vincenzo Balzani, Chemistry - A European Journal would like to celebrate this great chemist. His excellent scientific activity and his deep commitment to the societal aspects of science and to the ethical behavior of scientists have opened several new fields of research and inspired entire generations of scientists. This special collection gathers the contributions of chemists, co-workers and/or friends of Balzani, who have the pleasure to honor him. Happy birthday!

Hierarchical Growth of Supramolecular Structures Driven by Pimerization of Tetrahedrally Arranged Bipyridinium Units.

A shape-persistent molecule, featuring four bipyridinium units, has been synthesized that upon reduction undergoes intermolecular pimerization because of the rigid architecture of the molecule. The pimerization process has been investigated by a variety of techniques, such as absorption measurements, EPR spectroscopy, as well as gamma and pulse radiolysis, and compared with the behavior of a model compound. Computational studies have also been performed to support the experimental data. The most interesting feature of the tetramer is that pimerization occurs only above a threshold concentration of monoreduced species, on the contrary to the model compound. Furthermore, there is an increase of the apparent pimerization constant by increasing the concentration of reduced bipyridinium units. These results have been interpreted by the fact that pimerization is favored in the tetrahedrally shaped molecule because of a cooperative mechanism. Each multiply reduced molecule can indeed undergo multiple intermolecular interactions that enhance the stabilization of the system, also leading to hierarchical supramolecular growth. The resulting supramolecular system formed by such intermolecular pimerization should exhibit a diamond-like structure, as suggested by a simplified modeling approach. The intermolecular nature of the pimerization process occurring in the tetramer has been demonstrated by measuring the corresponding bimolecular rate constant by pulsed radiolysis experiments.

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

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.

Dethreading of a Photoactive Azobenzene-Containing Molecular Axle from a Crown Ether Ring: A Computational Investigation.

Pseudorotaxanes formed by a dibenzo[24]crown-8 ring (R) and a dialkylammonium axle bearing either two E- or two Z-azobenzene units (EE-A or ZZ-A) revealed useful for the construction of light-powered molecular machines and motors, as they provide the opportunity of photocontrolling self-assembly/disassembly processes. The potential energies profiles for the dethreading of these complexes have been investigated by adopting a combination of first-principles molecular dynamics, metadynamics and quantum-chemical geometry optimization approaches. While the dethreading of the EE-A axle is associated with a monotonic energy increase, for that of the ZZ-A axle a transition state and an intermediate structure, in which the components are still threaded together, are found. The rate determining step for the dethreading of the ZZ axle has a higher energy barrier than that of the EE axle, in agreement with the experimental kinetic data. Moreover, the results suggest that the elliptic shape of the ring cavity is important for discriminating between the E and Z terminal azobenzene during dethreading.

Light to investigate (read) and operate (write) molecular devices and machines.

The development of multicomponent (supramolecular) systems that can perform predetermined functions under external control - i.e., molecular devices - is a challenging task in chemistry and a fascinating objective in the frame of a bottom-up approach to nanostructures. In this context light signals can be conveniently used both for supplying energy to the system and for probing its states and transformations. The aim of this tutorial review is to recall a few basic aspects of light-induced processes that can be used to "write" and "read" onto molecular and supramolecular systems. These principles are illustrated through some examples of artificial molecular devices and machines taken from our work, which provide a flavour of current research. They are molecular and supramolecular systems that operate and/or perform valuable functions by exploiting photoinduced energy- or electron-transfer processes, photoisomerization reactions, or photoinduced proton transfer. The choice of these examples was based on both their intrinsic importance for the referred topic and their educational value. In the last section of the review potential applications, limitations and future directions of the research in the field of artificial molecular devices and machines are also discussed.

Light-operated machines based on threaded molecular structures.

Rotaxanes and related species represent the most common implementation of the concept of artificial molecular machines, because the supramolecular nature of the interactions between the components and their interlocked architecture allow a precise control on the position and movement of the molecular units. The use of light to power artificial molecular machines is particularly valuable because it can play the dual role of "writing" and "reading" the system. Moreover, light-driven machines can operate without accumulation of waste products, and photons are the ideal inputs to enable autonomous operation mechanisms. In appropriately designed molecular machines, light can be used to control not only the stability of the system, which affects the relative position of the molecular components but also the kinetics of the mechanical processes, thereby enabling control on the direction of the movements. This step forward is necessary in order to make a leap from molecular machines to molecular motors.

Light control of stoichiometry and motion in pseudorotaxanes comprising a cucurbit[7]uril wheel and an azobenzene-bipyridinium axle.

Pseudorotaxanes are the simplest prototypes for the construction of molecular machines based on threaded species. Investigation on molecular motions in these model systems is a necessary action for an efficient design of working molecular machines and motors. Herein we report on photoactive pseudorotaxanes based on the interaction between bipyridinium and cucurbit[7]uril (CB7). The molecular axle is composed of a central bipyridinium unit and two azobenzene moieties at the extremities. CB7 can form two different complexes with this molecule: a [2]pseudorotaxane, in which the macrocycle shuttles fast along the length of the axle, and a [3]pseudorotaxane, in which two CB7 s are confined at the extremities of the axle. Upon trans to cis isomerization of the azobenzene moieties, the [3]pseudorotaxane is destabilized, and only one CB7 resides on the axle, surrounding the bipyridinium unit. The system was successfully inserted into the core of liposomes, and preliminary investigations confirmed that it maintains its switching ability.

A highly luminescent tetramer from a weakly emitting monomer: acid- and redox-controlled multiple complexation by cucurbit[7]uril.

The tetrahedral, shape-persistent molecule 1(4+), containing four pyridylpyridinium units connected through a central carbon atom, exhibits unexpected photophysical properties including a substantially redshifted absorption (2350 cm(-1)) and a very strong fluorescence (Φem = 40 %), compared with the monomer 2(+) (Φem = 0.4 %). Density functional theory calculations on the structure and spectroscopic properties of 1(4+) and 2(+) show that exciton interactions, homoconjugation, and orbital nature account for the observed differences in their photophysical properties. The protonated tetramer binds four cucurbit[7]uril molecules and the host/guest interactions can be controlled by chemical (acid/base) as well as redox stimuli.

Toward directionally controlled molecular motions and kinetic intra- and intermolecular self-sorting: threading processes of nonsymmetric wheel and axle components.

We have investigated the self-assembly of pseudorotaxanes composed of viologen-type axle and calix[6]arene wheel components. The distinctive feature of this system is that both components are structurally nonsymmetric; hence, their self-assembly can follow four distinct pathways and eventually give rise to two different orientational pseudorotaxane isomers. We found that the alkyl side chains of the viologen recognition site on the molecular axle act as strict kinetic control elements in the self-assembly, thereby dictating which side of the axle pierces the calixarene cavity. Specifically, nonsymmetric axles with alkyl side chains of different length thread the wheel with the shorter chain. Such a selectivity, in combination with the face-selective threading of viologen-type axles afforded by tris(N-phenylureido)calix[6]arenes, enables a strict directional control of the self-assembly process for both the face of the wheel and the side of the axle. This kinetic selectivity allows both intramolecular self-sorting between two different side chains in a nonsymmetric axle and intermolecolar self-sorting among symmetric axles with alkyl substituents of different length.

Photophysical properties and conformational effects on the circular dichroism of an azobenzene-cyclodextrin [1]rotaxane and its molecular components.

The photophysical properties of a multicomponent [1]rotaxane bearing a ß-cyclodextrin ring covalently connected to an axle comprising an azobenzene photoisomerisable moiety and a naphthalimide-type fluorescent stopper are investigated by a combined experimental and computational study. The absorption and fluorescence spectra, and particularly the induced circular dichroism (ICD) signal, are determined. The latter shows a sign relation that cannot be rationalised in terms of the simple general rules commonly employed to analyse the ICD spectra of achiral guests encircled by chiral hosts. To assist the interpretation of experimental results, DFT and time-dependent (TD) DFT calculations are performed to explore the availability of low-energy conformations and to model their spectroscopic response. Molecular dynamics simulations performed in water show the interconversion of a number of conformers, the contribution of which to the ICD signal is in agreement with the observation.

Hierarchical self-assembly of amphiphilic calix[6]arene wheels and viologen axles in water.

We have designed and synthesized two amphiphilic calix[6]arene derivatives, CA8 and CA18, that combine the potential to act as wheel components for pseudorotaxane structures with the self-assembly features typical of surfactant molecules in aqueous solution. Their endo-cavity recognition and selfaggregation properties were compared with those of a non-amphiphilic analogue, C8. TEM, DLS, and fluorescence experiments show that in water the amphiphilic calixarenes form vesicle- and micelle-like aggregates. The size, nature and properties of such aggregates depend on the length of the alkyl chain anchored at the lower rim of the calix[6]arene skeleton, as well as on the inclusion of a molecular guest into the wheel. Specifically, the release of a fluorescent guest entrapped inside the CA8 vesicles is accelerated in the presence of dioctylviologen axles that can pierce the calixarene cavity.

The beauty of chemistry in the words of writers and in the hands of scientists.

Chemistry is a central science because all the processes that sustain life are based on chemical reactions, and all things that we use in everyday life are natural or artificial chemical compounds. Chemistry is also a fantastic world populated by an unbelievable number of nanometric objects called molecules, the smallest entities that have distinct shapes, sizes, and properties. Molecules are the words of matter. Indeed, most of the other sciences have been permeated by the concepts of chemistry and the language of molecules. Like words, molecules contain specific pieces of information that are revealed when they interact with one another or when they are stimulated by photons or electrons. In the hands of chemists, molecules, particularly when they are suitably combined or assembled to create supramolecular systems, can play a variety of functions, even more complex and more clever than those invented by nature. The wonderful world of chemistry has inspired scientists not only to prepare new molecules or investigate new chemical processes, but also to create masterpieces. Some nice stories based on chemical concepts (1) show that there cannot be borders on the Earth, (2) underline that there is a tight connection among all forms of matter, and (3) emphasize the irreplaceable role of sunlight.

Chiral supramolecular switches based on (R)-binaphthalene-bipyridinium guests and cucurbituril hosts.

We designed and synthesized the three molecular tweezers 1 a-c(4+) containing an electron acceptor 4,4'-bipyridinium (BPY(2+)) unit in each of the two arms and an (R)-2,2'-dioxy-1,1'-binaphthyl (BIN) unit that plays the role of chiral centre and the hinge of the structure. Each BPY(2+) unit is connected to the BIN hinge by an alkyl chain formed by two- (1 a(4+)), four- (1 b(4+)), or six-CH(2) (1 c(4+)) groups. The behavior of 1 a-c(4+) upon chemical or photochemical reduction in the absence and in the presence of cucurbit[8]uril (CB[8]) or cucurbit[7]uril (CB[7]) as macrocyclic hosts for the bipyridinium units has been studied in aqueous solution. A detailed analysis of the UV/Vis absorption and circular dichroism (CD) spectra shows that the helicity of the BIN unit can be reversibly modulated by reduction of the BPY(2+) units, or by association with cucurbiturils. Upon reduction of 1 a-c(4+) compounds, the formed BPY(+·) units undergo intramolecular dimerization with a concomitant change in the BIN dihedral angle, which depends on the length of the alkyl spacers. The alkyl linkers also play an important role in association to cucurbiturils. Compound 1 a(4+), because of its short carbon chain, associates to the bulky CB[8] in a 1:1 ratio, whereas in the case of the smaller host compound CB[7] a 1:2 complex is obtained. Compounds 1 b(4+) and 1 c(4+), which have longer linkers, associate to two cucurbiturils regardless of their sizes. In all cases, association with CB[8] causes an increase of the BIN dihedral angle, whereas the formation of CB[7] complexes causes an angle decrease. Reduction of the CB[8] complexes results in an enhancement of the BPY(+·) dimerization with respect to free 1 a-c(4+) and causes a noticeable decrease of the BIN dihedral angle, because the BPY(+·) units of the two arms have to enter into the same macrocycle. The dimer formation in the CB[8] complexes characterized by a 1:2 ratio implies the release of one macrocycle showing that the binding stoichiometry of these host-guest complexes can be switched from 1:2 to 1:1 by changing the redox state of the guest. When the reduction is performed on the CB[7] complexes, dimer formation is totally inhibited, as expected because the CB[7] cavity cannot host two BPY(+·) units.

Solvent- and light-controlled unidirectional transit of a nonsymmetric molecular axle through a nonsymmetric molecular wheel.

The development of a pseudorotaxane motif capable of performing unidirectional threading and dethreading processes under control of external stimuli is particularly important for the construction of processive linear motors based on rotaxanes and, at least in principle, it discloses the possibility to access to rotary motors based on catenanes. Here, we report a strategy to obtain the solvent-controlled unidirectional transit of a molecular axle through a molecular wheel. It is based on the use of appropriately designed molecular components, the essential feature of which is their non-symmetric structure. Specifically they are an axle containing a central electron-acceptor 4,4'-bipyridinium core functionalized with a hexanol chain at one side, and a stilbene unit connected through a C6 chain at the other side, and a heteroditopic tris(phenylureido)-calix[6]arene wheel. In apolar solvents the axle threads into the wheel from its upper rim and with the end carrying the OH group, giving an oriented pseudorotaxane structure. After a stoppering reaction, which replaces the small hydroxy group with a bulky diphenylacetyl moiety, and replacement of the apolar solvent with a polar one, dethreading occurs through the slippage of the stilbene unit from the lower rim of the wheel, that is, in the same direction of the threading process. The essential role played by the stilbene unit to achieve the unidirectional transit of the axle through the wheel, and to tune the dethreading rate by light is also demonstrated.

Probing donor-acceptor interactions and co-conformational changes in redox active desymmetrized [2]catenanes.

We describe the synthesis and characterization of a series of desymmetrized donor-acceptor [2]catenanes where different donor and acceptor units are assembled within a confined catenated geometry. Remarkable translational selectivity is maintained in all cases, including two fully desymmetrized [2]catenanes where both donors and acceptors are different, as revealed by X-ray crystallography in the solid state, and by (1)H NMR spectroscopy and electrochemistry in solution. In all desymmetrized [2]catenanes the co-conformation is dominated by the strongest donor and acceptor pairs, whose charge-transfer interactions also determine the visible absorption properties. Voltammetric and spectroelectrochemical experiments show that the catenanes can be reversibly switched among as many as seven states, characterized by distinct electronic and optical properties, by electrochemical stimulation in a relatively narrow and easily accessible potential window. Moreover in some of these compounds the oxidation of the electron donor units or the reduction of the electron acceptor ones causes the circumrotation of one molecular ring with respect to the other. These features make these compounds appealing for the development of molecular electronic devices and mechanical machines.

Reversible photoswitching of rotaxane character and interplay of thermodynamic stability and kinetic lability in a self-assembling ring-axle molecular system.

We have designed, synthesized, and investigated a self-assembling system that can be reversibly interconverted between thermodynamically stable (pseudorotaxane) and kinetically inert (rotaxane) forms by light irradiation. The system is composed of a dibenzo[24]crown-8 ring and an axle comprised of a dibenzylammonium recognition site and two azobenzene end groups. The isomeric form of the azobenzene units of the axle has a little influence on the stability constants of the respective pseudorotaxanes but greatly affects the threading-dethreading rate constants. In fact, equilibration of the ring and the axle in its EE isomeric form occurs within seconds in acetonitrile at room temperature, whereas the ZZ axle threads-dethreads the ring at least four orders of magnitude slower. Moreover, we show that a change in the stability of the complex, achieved by deprotonating the dibenzylammonium recognition site on the axle, affects its kinetic behavior. We compare the results of these experiments with those observed upon dethreading the (pseudo)rotaxane by using a competitive guest for the ring, an approach which does not inherently destabilize the ring-axle interaction. This study outlines a general strategy for the reversible photochemical control of motion kinetics in threaded and interlocked compounds and constitutes a starting point for the construction of multicomponent structures that can behave as photochemically driven nanomachines.

Self-assembly of calix[6]arene-diazapyrenium pseudorotaxanes: interplay of molecular recognition and ion-pairing effects.

The calix[6]arene wheel CX forms pseudorotaxane species with the diazapyrenium-based axle 1.2PF(6) in CH(2)Cl(2) solution. The macrocyclic component is a heteroditopic receptor, which can complex the electron-acceptor moiety of the axle inside its cavity and the counterions with the ureidic groups on the upper rim. The self-assembled supramolecular species is a complex structure, which involves three components--the wheel, the axle and its counterions--that can mutually interact and affect. The stoichiometry of the resulting supramolecular complex depends on the nature and concentration of the counterions. Namely, it is observed that in dilute solution and with low-coordinating anions the axle takes two wheels, whereas with highly coordinating anions or in concentrated solutions the complex has a 1:1 stoichiometry.

Light-powered molecular devices and machines.

One century ago Giacomo Ciamician predicted that photochemistry would have had a wealth of useful applications, starting from the conversion of solar energy into fuels. Most of Ciamician's predictions have not yet been achieved, but in the last decade outstanding progress concerning the interaction between light and molecules has led to the creation of artificial photochemical molecular devices and machines capable of using light as an energy supply (to sustain energy-expensive functions) or as an input signal (to be processed and/or stored). This paper illustrates (i) the principles of photochemical molecular devices for information processing, with a few examples of memories, logic functions, and encoding/decoding systems; (ii) the operational mechanisms of light-powered molecular machines, with some examples of rotary motors, shuttles, valves, and switchable boxes; and (iii) the recent progress made in the design and construction of the components of artificial photosynthetic systems. The use of photons to convert abundant low energy molecules into high energy valuable compounds, and to read, write, and erase smart molecular and supramolecular systems for information processing is likely to play a fundamental role for the progress of mankind.

Towards controlling the threading direction of a calix[6]arene wheel by using nonsymmetric axles.

Traffic control: By exploiting the interplay of kinetic and thermodynamic effects, the direction of threading/dethreading in a nonsymmetric calixarene wheel can be selected by an appropriate choice of the head group incorporated in the molecular axle (see figure).The possibility of obtaining full control on the direction of axle threading in calix[6]arene wheel 1 either from its upper or lower rim was evaluated in solution. To this aim, we prepared nonsymmetric axles characterised by a 4,4'-bipyridinium recognition unit with two alkyl side chains, one of which terminates with a stopper, and the other with either ammonium (2), hydroxy (3) or methyl (4 and 5) head groups. When the axles were mixed with 1 in apolar solvents at room temperature, the formation of oriented pseudorotaxanes derived from the threading of the axles from the upper rim was observed. The stability constants of such complexes are in the order of 10(7) m(-1) and are almost independent of the type of axle. A detailed thermodynamic and kinetic study revealed that stability constants and activation parameters for complex formation between 1 and axles 2 and 3 are of the same order of magnitude, suggesting a common threading process. However, upon heating a solution of 1 and 2 in benzene at 340 K, the formation of another supramolecular complex was observed, the structure of which is consistent with an oriented pseudorotaxane derived from the threading of axle 2 from the lower rim of the calixarene wheel. By carrying out the threading-stoppering reaction sequence between 1 and 2 in the presence of an excess of diphenylacetyl chloride, the orientational rotaxane isomers R1 and R2, derived from lower- and upper-rim threading, respectively, were collected in about a ratio of 7:3 as the unique chromatographic fraction. Our results suggest that at room temperature the threading process is under kinetic control for all axles. On increasing the temperature only the threading behaviour of axle 2 is substantially modified, most likely because the process becomes thermodynamically controlled owing to the peculiar recognition properties of the ammonium head of this axle.