Massimo Baroncini

A Simple Unimolecular Multiplexer/Demultiplexer

Here we show that the reversible acid/base switching of the absorption and photoluminescence properties of a fluorophore as simple as 8-methoxyquinoline in solution can form the basis for molecular 2:1 multiplexing and 1:2 demultiplexing with a clear-cut digital response.

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.

Polyviologen dendrimers as hosts and charge-storing devices

Two dendrimers were designed and synthesized that contain a 1,3,5-trisubstituted benzenoid core and incorporate 9 and 21 viologen (4,4-bipyridinium) units in their branches in addition to hydrophilic (aryloxy) terminal groups. For comparison purposes, model compounds containing one and two viologen units were also studied. These polycationic dendrimers form strong host-guest complexes with the dianionic form of the red dye eosin in dilute CH(2)Cl(2) solutions. Titration experiments, based on fluorescence measurements, showed that each viologen unit in the dendritic structures becomes associated with an eosin dianion. Electrochemical (in MeCN) and photosensitization (in CH(2)Cl(2)) experiments revealed that only a fraction of the viologen units present in the dendritic structures can be reduced. This fraction corresponds to the number of viologen units present in the outer shells of the dendrimers. The reasons for incomplete charge pooling are discussed. Comparison with the behavior of polyviologen dendrimers that are terminated with bulky tetraarylmethane groups and were studied previously enabled the role played by the terminal groups in the redox and hosting properties to be elucidated.

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.

Tweezering the Core of Dendrimers: Medium Effect on the Kinetic and Thermodynamic Properties

We have investigated the complex formation between dendritic guests and a molecular tweezer host by NMR, absorption, and emission spectroscopy as well as electrochemical techniques. The dendrimers are constituted by an electron-acceptor 4,4-bipyridinium core appended with one (DnB(2+)) or two (Dn(2)B(2+)) polyaryl-ether dendrons. Tweezer T comprises a naphthalene and four benzene components bridged by four methylene groups. Medium effects on molecular recognition phenomena are discussed and provide insight into the conformation of dendrimers: change in solvent polarity from pure CH(2)Cl(2) to CH(2)Cl(2)/CH(3)CN mixtures and addition of tetrabutylammonium hexafluorophosphate (NBu(4)PF(6), up to 0.15 M), the supporting electrolyte used in the electrochemical measurements, have been investigated. The association constants measured in different media show the following trend: (i) they decrease upon increasing polarity of the solvent, as expected for host-guest complexes stabilized by electron donor-acceptor interactions; (ii) no effect of generation and number of dendrons (one for the DnB(2+) family and two for the Dn(2)B(2+) family) appended to the core is observed in higher polarity media; and (iii) in a low-polarity solvent, like CH(2)Cl(2), the stability of the inclusion complexes is higher for DnB(2+) dendrimers than for Dn(2)B(2+) ones, while within each dendrimer family it increases by decreasing dendron generation, and upon addition of NBu(4)PF(6). The last result has been ascribed to a partial dendron unfolding. Kinetic investigations performed in lower polarity media evidence that the rate constants of complex formation are slower for symmetric Dn(2)B(2+) dendrimers than for the nonsymmetric DnB(2+) ones, and that within the Dn(2)B(2+) family, they decrease by increasing dendron generation. The dependence of the rate constants for the formation and dissociation of the complexes upon addition of NBu(4)PF(6) has also been investigated and discussed.

Liposome Destabilization by a 2,7‐Diazapyrenium Derivative Through Formation of Transient Pores in the Lipid Bilayer

The effect of the luminescent heteroaromatic electron acceptor N,N-dimethyl-2,7-diazapyrenium dichloride (DM-DAP(2+)) on the stability of 1-palmitoyl-2-oleoylphosphatydilcholine (POPC) liposomes is determined on the basis of the rate of release of different fluorescent probes entrapped within the liposome. The experiments show that DM-DAP(2+) exerts a substantial destabilizing action on the liposomal bilayer, particularly at low concentrations. Molecular dynamics simulations suggest that the activity of DM-DAP(2+) is related to its tendency to surround itself with water molecules, conceivably favoring the formation of transient pores across the bilayer.

Making and operating molecular machines: a multidisciplinary challenge

Abstract 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 Minireview, we discuss, with the help of a few examples, the multidisciplinary aspects of research on artificial molecular machines and highlight its translational character.

Azobenzene photoisomerization: an old reaction for activating new molecular devices and materials

The E–Z photoisomerization of azobenzene – owing to its high efficiency, excellent reversibility and significant physico-chemical differences between the two forms – is highly interesting for the construction of photocontrollable molecular systems and materials. In this chapter we illustrate how this outstanding photoreactivity can be utilized to develop (supra)molecular systems and materials with valuable light-induced functionalities by presenting some investigations carried out in our laboratory.The E–Z photoisomerization of azobenzene – owing to its high efficiency, excellent reversibility and significant physico-chemical differences between the two forms – is highly interesting for the construction of photocontrollable molecular systems and materials. In this chapter we illustrate how this outstanding photoreactivity can be utilized to develop (supra)molecular systems and materials with valuable light-induced functionalities by presenting some investigations carried out in our laboratory.

Dendronised diazapyrenium derivatives: host–guest complexes in aqueous solution

Dendronised diazapyrenium derivatives were synthesised with glycol-appended dendrons to confer water solubility. These dendrimers form self-assembled supramolecular structures with cucurbituril hosts. The self-assembly process could be monitored by changes in the absorption spectra and redox properties of the dendrimers due to electronic interactions of the electron-accepting diazapyrenium core with the electron-rich inner cavity of cucurbiturils.

Solution and Solid-State Emission Toggling of a Photochromic Hydrazone

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.