Francesco Zerbetto

Unidirectional rotation in a mechanically interlocked molecular rotor

Molecular motor proteins are ubiquitous in nature and have inspired attempts to create artificial machines that mimic their ability to produce controlled motion on the molecular level. A recent example of an artificial molecular rotor is a molecule undergoing a unidirectional 120° intramolecular rotation around a single bond; another is a molecule capable of repetitive unimolecular rotation driven by multiple and successive isomerization of its central double bond. Here we show that sequential and unidirectional rotation can also be induced in mechanically interlocked assemblies comprised of one or two small rings moving around one larger ring. The small rings in these [2]- and [3]catenanes move in discrete steps between different binding sites located on the larger ring, with the movement driven by light, heat or chemical stimuli that change the relative affinity of the small rings for the different binding sites. We find that the small ring in the [2]catenane moves with high positional integrity but without control over its direction of motion, while the two rings in the [3]catenane mutually block each others movement to ensure an overall stimuli-induced unidirectional motion around the larger ring.

Quantum-chemical investigation of Franck-Condon and Jahn-Teller activity in the electronic spectra of Buckminsterfullerene

Abstract Quantum-chemical results are reported which indicate that the absorption, fluorescence and phosphorescence spectra of Buckminsterfullerene are governed by a 1 T 1u ← 1 A g , 1 T 2g → 1 A g and 3 T 2g → 1 A g transition, respectively. Normal modes are calculated and their Franck-Condon and Jahn-Teller activity in these spectra are evaluated, along with the Jahn-Teller distortion of the radical anion. Vibrational progressions are expected to be short. A high phosphorescence quantum yield is predicted.

Supramolecular self-assembled fullerene nanostructures

Four ionic fullerene derivatives, which are relatively soluble in polar solvents, are shown to organize into morphologically different nanoscale structures. Spheres, nanorods, and nanotubules form in water depending on the side chain appendage of the fullerene spheroid. Images at different nanoscale structures were obtained via transmission electron microscopy. Also, computer simulations were used for investigating the relative spatial arrangements. The efficient method to fabricate almost perfect and uniformly shaped nanotubular crystals, which order spontaneously by self-assembly, opens the way to the possibility of exploiting the fullerene properties at the nanometer scale.

Interpretation of the vibrational structure of the emission and absorption spectra of C60

The vibronic intensity borrowing activity of the lowest electronically excited singlet states of C60 has been obtained through quantum chemical calculations. The vibrational structure of the UV–visible spectra is found to be dominated by false origins. The calculated intensities of the false origins of the T1g state agree with the vibrational structure observed in the fluorescence spectrum. The same false origins are recognized to be responsible for the vibrational structure of the red edge portion of the absorption spectrum. Only two bands in the spectra are assigned as combination bands involving an ag or a Jahn–Teller active mode. Absorption bands that may be associated with false origins of the states T2g and Gg which are quasidegenerate with S1 are tentatively assigned.

Pentagon adjacency as a determinant of fullerene stability

Optimisation of geometries of all 40 fullerene isomers of C40, using methods from molecular mechanics and tight-binding to full abinitio SCF and DFT approaches, confirms minimisation of pentagon adjacency as a major factor in relative stability. The consensus predictions of 11 out of 12 methods are that the isomer of lowest total energy is the D2 cage with the smallest possible adjacency count, and that energies rise linearly with the number of adjacencies. Quantum mechanical methods predict a slope of 80–100 kJ mol-1 per adjacency. Molecular mechanics methods are outliers, with the Tersoff potential giving a different minimum and its Brenner modification a poor correlation and much smaller penalty.

Increasing cost of pentagon adjacency for larger fullerenes

The cost of a pentagon adjacency in a fullerene cage grows linearly from 72 kJ mol−1 for C30 to 111 kJ mol−1 for C60, according to systematic QCFF/PI model calculations on a set of 2624 structural isomers.

Mechanochemistry: targeted delivery of single molecules:

The use of scanning probe microscopy-based techniques to manipulate single molecules1 and deliver them in a precisely controlled manner to a specific target represents a significant nanotechnological challenge2,3. The ultimate physical limit in the design and fabrication of organic surfaces can be reached using this approach. Here we show that the atomic force microscope (AFM), which has been used extensively to investigate the stretching of individual molecules4,5,6,7,8,9,10,11,12, can deliver and immobilize single molecules, one at a time, on a surface. Reactive polymer molecules, attached at one end to an AFM tip, are brought into contact with a modified silicon substrate to which they become linked by a chemical reaction. When the AFM tip is pulled away from the surface, the resulting mechanical force causes the weakest bond — the one between the tip and polymer — to break. This process transfers the polymer molecule to the substrate where it can be modified by further chemical reactions.

Theoretical study of the force fields of the three lowest singlet electronic states of linear polyenes

The potential energy surfaces of all trans hexatriene and octatetraene are investigated within the harmonic approximation in the diabatic and adiabatic representations for the 1A−g, 2A−g, and 1B+u electronic states by an extended Pople–Pariser–Parr (PPP/CI) model. The effect of excitation and of vibronic coupling on the molecular force fields of the three states is examined. While electronic excitation affects only diagonal force constants of local oscillators, vibronic coupling changes drastically the couplings between local oscillators. The calculations reproduce well the observed increase of the frequency of the in‐phase ag C=C stretch upon excitation to the 2A−g state.

Singling out the Electrochemistry of Individual Single-Walled Carbon Nanotubes in Solution

Bandgap fluorescence spectroscopy of aqueous, micelle-like suspensions of SWNTs has given access to the electronic energies of individual semiconducting SWNTs, while substantially lower is the success achieved in the determination of the redox properties of SWNTs as individual entities. Here we report an extensive voltammetric and vis-NIR spectroelectrochemical investigation of true solutions of unfunctionalized SWNTs and determine the standard electrochemical potentials of reduction and oxidation as a function of the tube diameter of a large number of semiconducting SWNTs. We also establish the Fermi energy and the exciton binding energy for individual tubes in solution. The linear correlation found between the potentials and the optical transition energies is quantified in two simple equations that allow one to calculate the redox potentials of SWNTs that are insufficiently abundant or absent in the samples.

Photoisomerization of a rotaxane hydrogen bonding template: Light-induced acceleration of a large amplitude rotational motion

Establishing methods for controlling aspects of large amplitude submolecular movements is a prerequisite for the development of artificial devices that function through rotary motion at the molecular level. Here we demonstrate that the rate of rotation of the interlocked components of fumaramide-derived [2]rotaxanes can be accelerated, by >6 orders of magnitude, by isomerizing them to the corresponding maleamide [2]rotaxanes by using light.