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Winner of the IUPAC Prize
for Young Chemists - 2000

 

Alberto Credi wins one of the first 4 IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Molecular-Level Machines and Logic Gates"

Current address (at the time of application)

Dipartimento di Chimica "G. Ciamician"
UniversitÓ di Bologna
Via Selmi 2
I -40126 Bologna, Italy

Tel: +39 051 2099540
Fax: +39 051 2099456
E-mail: acredi@ciam.unibo.it
<http://www.ciam.unibo.it/photochem.html/credi.html>

Academic degrees

  • Ph.D. in Chemical Sciences, UniversitÓ di Bologna; February 1999.
  • "Laurea" in Chemistry, UniversitÓ di Bologna; March 1994.
  • "MaturitÓ" in Industrial Physics, Istituto Tecnico "O. Belluzzi", Bologna; July 1988.

Ph.D. Thesis

Title Molecular-Level Machines and Logic Gates
Adviser Prof. Vincenzo Balzani
Thesis Committee Prof. Goffredo Rosini (chairman), Dipartimento di Chimica Organica "A. Mangini", UniversitÓ di Bologna.; Prof. Walther Caminati, Dipartimento di Chimica "G. Ciamician", UniversitÓ di Bologna; Prof. Alberto Juris, Dipartimento di Chimica "G. Ciamician", UniversitÓ di Bologna.

Essay

ESSAY FROM THE THESIS
Molecular-Level Machines and Logic Gates

by Alberto Credi
Supervisor: Prof. V. Balzani

UNIVERSITY OF BOLOGNA, ITALY

 

 

From macroscopic devices
to molecular machines

In everyday life we make extensive use of macroscopic devices. A macroscopic device is an assembly of components designed to achieve a specific function. Each component of the device performs a simple act, while the entire device performs a more complex function, characteristic of the assembly.

In recent years it has been shown that the concept of device can be extended to the molecular level. A molecular-level device can be defined as an assembly of a discrete number of molecular components (that is, a supramolecular structure) designed to achieve a specific function. Each molecular component performs a single act, while the entire supramolecular structure performs a more complex function, which results from the cooperation of the various molecular components.

Molecular-level devices operate via electronic and/or nuclear rearrangements, and like macroscopic devices they need energy to operate and signals to communicate with the operator. Light is the most important answer to this dual requirement, as shown by Nature where photons are used as energy by the devices responsible for photosynthetic processes and as signals by the devices responsible for vision-related processes. In the same way, in artificial supramolecular systems photons can be used to cause (by photochemical reactions) and to monitor (by absorption and emission spectroscopy) the occurrence of electronic and nuclear rearrangements. Electrochemistry is also very useful to cause and monitor the occurrence of electronic and nuclear rearrangements in molecular and supramolecular systems.

Two kinds of macroscopic devices play a very important role in our civilization: mechanical machines and electronic computers. The aim of this Ph. D. work was to develop prototypes of molecular-level machines and components (in particular, molecular logic gates) related to the construction of molecular-based chemical computers.

Pseudorotaxanes, rotaxanes

 

and catenanes

   

as molecular-scale machines

Because of their structure, interlocked molecules such as catenanes and rotaxanes, and their precursor complexes, the pseudorotaxanes, are particularly attracting for the construction of molecular-level machines, as motions of their molecular components can be easily imagined (Figure 1). The preparation of these chemical systems, however, requires a deep understanding of self-assembly and self-organizational processes and their coupling with covalent synthetic methods. The present work was performed in the frame of an extremely fruitful, interactive and long-lasting collaboration with Professor J. Fraser Stoddart and his co-workers, who are responsible for the synthesis and structural characterization of nearly all the systems described in the thesis. The Candidate's credits in this collaborative work include the design of the systems (see below), the photophysical, photochemical and electrochemical studies in solution, and all the experiments aimed to operate the supramolecular species as molecular machines.


Figure 1. Schematic representation of the motions that can take place between the components of pseudorotaxanes (a), rotaxanes (b) and catenanes (c).

One of the main messages of this work is that a careful design is needed for the construction of a molecular-scale device. In other words, the structure and functions of supramolecular species has to be 'programmed' through the appropriate selection of the molecular components and of the molecular subunits they contain. During this Ph.D. work, the features of a number of groups or units suitable for incorporation in pseudorotaxanes, rotaxanes and catenanes have been investigated. Such units constitute a sort of 'molecular meccano set' from which the 'pieces' presenting suitable properties for the preparation of a supermolecule with a desired functionality were chosen by time to time. This is particularly true and immediate for pseudorotaxanes, as they were obtained simply by mixing in solution a macrocyclic and a thread-like component with complementary stereoelectronic properties.

A large part of the thesis deals with the photochemical and electrochemical behavior of families of catenanes and rotaxanes. Some of these systems have not been designed with the aim of producing molecular-scale devices, but rather to understand the intercomponent interaction that one must be able to control for making molecular machines work. Some other have revealed not to be good prototypes of molecular machines or information-processing systems, and may be considered as 'failures' on the way to the construction of such molecular devices. This testifies that, despite the progresses made, the design and preparation of a 'working' molecular device is still a difficult task. Also, the importance of photochemical and electrochemical methods in supramolecular science has been clearly demonstrated.

The major accomplishment of this work is certainly the development of many examples of chemically, photochemically or electrochemically-driven motion in pseudorotaxanes, rotaxanes and catenanes, namely (i) the threading/dethreading of pseudorotaxanes (Figure 1a), (ii) the shuttling of the ring component between two 'stations' in rotaxanes (Figure 1b), and (iii) the rotation of one ring with respect to the other in asymmetric catenanes (Figure 1c).

During these studies, it has become evident that suitably designed molecular-level machines can be employed to perform functions that go well beyond the mechanical movements. Such functions mimick those performed by the components of macroscopic electronic devices, and include logic operations, plug-in-socket, and multi-pole switching. Particularly worth of note are pseudorotaxanes which behave as an acid/base-operated plug-in-socket device for the photoinduced flow of energy, and as a three-pole supramolecular switch. Even more interesting are chemical systems, again based on pseudorotaxanes, which are responsive to acid/base or oxidative/reductive stimulation according to the XOR logic function.

Perspectives

The expertise gained with the present work has stimulated ideas on the development of new and more interesting molecular devices and on their possible applications. Some of these ideas are illustrated in the last part of the dissertation. Such systems could also play a role in the emerging field of nanoscience, with particular reference to the small-upward (bottom-up) approach to nanostructures.

It is very difficult to foresee if and when this kind of molecular-scale devices will come onto the scene and change our lives as macroscopic machines and computers did. It is evident, however, that the progresses made in the past few years are enormous; this Ph. D. work has contributed, of course to a little extent, to such advances.

Full-text PDF files can be downloaded from <http://www.ciam.unibo.it/photochem.html/acphd.html>


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