Supramolecular chemistry

Obiettivo

A.GENERAL BACKGROUND

A1.Why a COST Action for this topic?

Supramolecular Chemistry is a novel area of multi-disciplinary chemical research that has developed at an astonishing rate during the past two decades. Supramolecular chemistry focuses on complex structures formed by the association (covalent or not) of several moieties and on the novel chemical properties resulting from this higher complexity.

This principle, by which an increase of structural complexity gives rise to the emergence of properties which cannot be foreseen on the basis of the single constituting moieties, is the most important architectonic principle in nature, but only in the last decade has our technical expertise advanced to the point of bringing this domain securely into the realm of preparative chemistry.

The origin of Supramolecular Chemistry dates back to the late 1960's, when the pioneering works by Pedersen on crown-ethers and by Lehn on cryptands appeared. Significantly, Professors Pedersen and Lehn, with Professor Cram, were awarded the Nobel Prize in 1987, when Supramolecular Chemistry was already a well-established branch of chemistry, pursued in hundreds of laboratories all over the world. Present day supramolecular chemistry has two facets: one based upon the synthesis of complex organic structures by synthetic chemistry, and one in which the structural complexity is achieved by self-assembly and self-organization of smaller units without the help of covalent bonds.

A2.In which respect is Supramolecular Chemistry different from the other fields of chemistry?

Recognizing that the COST Chemistry Actions are mostly directed towards basic science, it is important to consider the following questions: how can our new action contribute to the development of the chemical science? Which novel and particular chemical concepts which are not included in other chemical actions hold centre stage here? Although the core of these questions is already outlined in the preceding introduction, we would like to reinforce the point by giving the following summary of key words, which in our opinion makes clear that indeed a COST Action on Supramolecular Chemistry would cover areas of basic science in a unique way.

1.Mesoscopic structures. In the continuum of molecular organization which, departing from the molecular and microscopic level, moves up to reach the macroscopic level, supramolecular chemistry is situated in the intermediate level of structural complexity (which is often referred to as mesoscopic level). Thus, the observation range of supramolecular chemistry is somewhat different from that of classic organic chemistry (interested primarily in single molecules or, at the most, in linear polymers) and therefore our new action will develop in an area which is not well represented in any other COST Action.

2.Notion of emergence. As already mentioned, Supramolecular Chemistry focuses on the emergence of novel chemical properties resulting from the progression of architectonic complexity. This gives a new dimension to the study of the relation between structure and function (or properties), and this may contribute to the understanding of the meaning of structural complexity in nature as well as to the development of novel synthetic materials.

3.Micro heterogeneity: reactions involving supramolecular structures as large as catenanes or of liposomes cannot be seen as classic homogeneous reactions, as they involve relatively large surface areas. It is a chemistry which is a combination of surface chemistry and homogeneous reaction, really a novel domain of chemical inquiry.

4.Supramolecular engineering: the possibility namely of constructing large and ordered molecular complexes composed by successive layers of molecules and tailored to specific goals.

5.Compartimentation: synthetic supramolecular structures may build cavities, and even more so surfactant aggregates such as micelles and liposomes: whether and to what extent the chemistry in the restricted volume of a micro cavity is different from that of bulk chemistry is a fascinating novel area of investigation. The observation that all biochemical reactions in vivo take place in a well-compartmented micro environment bears testimony to the relevance of this typical domain of supramolecular chemistry.

6.Self-assembly and self-organization. These are real central concepts and again unique to the field of supramolecular chemistry. This notion, which concerns primarily surfactant aggregates (bilayers, cubic and hexagonal phases, micelles and vesicles, but also, very importantly, liquid crystals...) is connected to the very important question of the spontaneous creation of order in complex chemical structures, which in turn is germane to the chemistry of life.

A3.Relation with other European programmes

Supramolecular Chemistry is of great importance in all European countries, as proved by the number of national programmes devoted to this field. The significant European interest in this area can be exemplified by the increasing number of meetings which take place in every country. Only a few selected examples are given below:

-13th International Symposium on Macrocyclic Chemistry, Hamburg, Germany, September 1988

-Meeting on Organized Molecular Systems, Parma, Italy, September 1989
-First International Summer School on Supramolecular Chemistry, Strasbourg, France, September 1990
-Supramolecular Chemistry, Towards Self-Organization, Le Bischenberg, Obernai, France, July 1991
-EUCHEM Conference on Supramolecular Reactivity and Catalysis, Padua, Italy, 1991
-Micelles and Liposomes Research in Switzerland and Europe, Z_rich, Switzerland, May 1992
-Supramolecular structures and self-replication, Maratea, Italy, June 1994
-Research Conference on Supramolecular Chemistry, Mainz, Germany, August 1994
-Liposomes: State of the Art, Freiburg, Germany, September 1995
-Nanogels and sol-gel processing: new approaches towards better materials, Z_rich, Switzerland, March 1996
-Convegno Nazionale GICI, scienza e tecnologia dei sistemi organizzati, Bari, Italy, September 1996
-11th international symposium on Surfactant in solution, Jerusalem, June 1996
-9th International Conference on surface and colloid science, Sofia, Bulgaria, July 1997.

As already mentioned, supramolecular chemistry is already well established in Europe. Many different research laboratories have started activity in this field, and our new Action comes at a point in which coordination among all this research is particularly important and needed. A list of research active in the area in given in Annex I. This is taken, in large part, from the Evaluation of European Supramolecular Science and Technology, which was prepared by Professor Stoddart in Birmingham a few years ago.

It should be noted that this new Action can be seen as a development of the previous D7 Action "Recognition Chemistry", and in fact it contains a few elements of it. Other elements of the recognition Chemistry D7 Action, in particular those dealing with more enzymatic and biological aspects, are not concerned and will be incorporated into another COST Action dealing with Bioorganic Chemistry.

At the moment, close coordination between these two Actions, Supramolecular Chemistry and Bioorganic Chemistry, is planned - for example, we will take into consideration the possibility of having joint meetings.

B.OBJECTIVES OF THE ACTION AND SCIENTIFIC CONTENT

B1.Main Objective

The main objective of this COST Action is to stimulate research activities in the field of supramolecular chemistry in general.

The scientific programme will depend on the projects submitted by individual research teams. The successful projects will be selected according to the objectives outlined above.

B2.Subtopics

1.Synthesis of novel supramolecular structures and study of their properties

This basic branch of Supramolecular Chemistry, bearing great future potential, is at present restricted to only a few countries. In this first objective we wish to emphasize the development of novel synthetic methods which are specific for the construction of large covalent structures, the development of novel analytical separation techniques, as well as the study of the basic properties of the compounds (mechanical bonding, spectroscopy, reactivity, acidity, etc.).

Catenanes, rotaxanes, knots and other topologically new molecules are the subject of this area: these interlocked species can only be synthesized with the help of a template (or catalytic) strategies - methods which are just being expressed in the literature. Dendrimers are becoming more and more interesting in this respect too. Other inorganic superstructures (rack-like, ladder-like, m x n-grid-like etc.) can be envisaged, including helical and double helical complexes (in the family of the already described polypiridine ligands), and others with increasing complexity and functionality - e.g. the formation of channel-like architectures formed from components containing polyether rings or various polyassociated microstructures (tubules, sheets, fluid and solid fibres, etc.).

2.Host-Guest complexation and molecular recognition

Mimicking nature, supramolecular chemists may design receptors able to recognize, i.e. to interact selectively with a given substrate. This has been partly already accomplished in the literature with crown-ethers and cryptands (viewed for example as simple prototype receptors for ammonium cations), and this Action should rather emphasize new construction principles. Rigid, possibly cyclic or polycyclic host molecules take advantage of their preorganization to enhance the efficiency and selectivity of the recognition process. The selectivity of the receptor-substrate interaction may be modulated through synthetic modifications made on the framework of the receptor molecule. Mention should be made here of cyclic receptors, clefts, modified cyclodextrins, cyclophanes, dendrimers, etc. for the selective transport of ions and neutral molecules. Biologically active compounds based on molecular recognition can also be developed. To the molecular recognition of guest-host complexation belongs also the principle of complementarity (which can be stereochemical or template directed).

There is of course a note of caution to be added here: the chemistry of macrocyclic compounds is traditionally a very important part of supramolecular chemistry, and it has now well established research groups all over Europe. The aim of the COST Action is not to support already established and, in a way, routine research, but to encourage the creativity in the area. Thus, we will tend not to encourage "work by analogy", but only original novel developments.

3.Supramolecular reactivity and catalysis

The obvious extension of the principle of host-guest interaction and molecular recognition is catalysis. In particular, a supramolecular system of appropriate topology may host two or more molecules and force them to react with each other according to a controlled pathway. In this sense, supramolecular systems behave as artificial enzymes. In this way, with the proper synthesis of supramolecular host system, this old and often frustrating field of artificial enzymes may see a new start, also because by using synthetic supramolecular systems it may be possible quantitatively to study the factors which determine catalysis. This catalysis may become important for applications, i.e. supramolecular catalysis represents a promising tool for the synthesis of fine chemicals.

The study of supramolecular design may be extended to inorganic supramolecular systems, by steered organization or self-organization processes or by design of connected fragments. Examples in this area are inorganic host guest species such as clathrate, poly-oxometallates, clusters, template-steered syntheses, as well as aspects of heterogeneous catalysis.

4.Transport processes and carrier design

Membrane processes are up-to-date convenient procedures in separation science and technology. Very selective separations may in principle be achieved by interfacing two aqueous phases by a liquid membrane in which a supramolecular carrier has been dissolved. The carrier should have such chemical and structural features to interact selectively with a given chemical entity dissolved in the donor phase and to transport it to the receptor phase - possibly making use of a pH gradient or a gradient of redox potential. It should also be noticed that all the chemistry developed in transport studies can be transferred to the science and technology of membrane-based sensors, a rapidly growing field of analytical and bioanalytical chemistry. Another area in which supramolecular designs can find application (and partly they already do) is in drug delivery.

5.Supramolecular Assemblies and Self-Organization

Several compounds, most notably surfactants, when present in solution above a certain critical concentration are capable of self-assembling into geometrically well defined supramolecular structures, such as micelles, bilayers, monolayers, vesicles, cubic and hexagonal phases.

The relation between structure of the monomer and structure of the supramolecular aggregate is still very poorly understood and the study may be part of this Action, particularly when novel aspects of the reactivity and recognition of these supramolecular structures are emphasized. The dynamic transformation from one type of supramolecular structure to another is another area which is now acquiring much interest. Also particular gels, although amorphous in nature, may acquire particular importance as structures which are intermediate between the solid and the liquid phase.

To this area belong also the study of liquid crystals, smectic phases, etc. as created both by low and high molecular weight compounds.

The novel chemical reactivity and physical properties of these supramolecular assemblies, as arising from the self-assembly, should be particularly emphasized. To be mentioned are also those studies which relate self-organized assemblies to the chemistry of life (e.g. liposomes as models of protocells), in particular self-reproduction and autocatalysis.

6.Molecular evolution and the origin of life

The process of self-assembly and self-organization acquires a particular meaning in the field of the chemistry of life, as transition from the inanimate to the living must have occurred throughout an increase of molecular complexity. The term "molecular evolution" is used to express this notion. It is commonly accepted that "minimal life" resulted as an emergent property as a consequence of this molecular evolution from simple molecules to supramolecular structures and finally to the cell. Several research groups all over the world, particularly in the United States, but also in France, Italy, Germany and Switzerland are studying the possible supramolecular processes which may have led to the transition to life. In this case, contrary to the other projects listed here, the chemistry must be such, to mimic the prebiotic (or early biochemical) chemistry, under the conditions which were presumably operating at that early time.

7.Supramolecular engineering and molecular devices

The ordered assembling of several different macromolecules or molecular fragments may give rise to a mesoscopic or macroscopic structure that exhibits useful physical and chemical properties in the condensed phase. This kind of molecular tinkering - often referred to as chemical engineering - is the basis for the design of molecular devices, namely structures which perform a very specific function.

For example, sophisticated and efficient molecular devices for the conversion of light into chemical energy may be designed (a research area defined as Supramolecular Photochemistry); for the control of the electron transfer between an electrode and a redox active species in solution (Supramolecular Electrochemistry); for the generation of signals and for the storage and transmission of chemical information processing and storage; for non-linear optics materials and reactive microgels.

8.Theoretical studies and molecular modelling

Most of the studies mentioned hitherto are experimental in nature. The field of supramolecular chemistry necessitates, however, also the development of a strong theoretical background, which is still missing. This Action is also meant to encourage theoretical studies, for example about the kinetics and the thermodynamics of the processes leading to the formation of supramolecular structures, as well as the processes governing the reactivity and selectivity of such structures towards their substrates.

Also, molecular modelling on these new systems should be encouraged. The first mechanical theoretical treatments of organized assemblies have just appeared in the literature, and it is clear that computer calculation and simulation will contribute to the development of novel structures with novel properties.

C.SCIENTIFIC PROGRAMME

The scientific programme will depend on the projects submitted by individual research teams. The successful projects will be selected according to the objectives outlined above. At this stage there is no specific scientific programme suggested for this Action in order to place no limitations on the invited proposals. The selection will strictly occur according to the outlined objectives.

D.ORGANIZATION AND TIMETABLE

D1.Organization

Research projects fitting in the sub-topics described in section C will be submitted by scientists to the Management Committee members. This Committee will establish contacts between scientists.

The Management Committee has responsibilities for:

1.Drawing up the inventory during the first year, organization of workshops and start of the activity; existing contacts will be used which should greatly facilitate this task.

2.The coordination of the joint activities with other COST Actions; joint meetings are likely to result from this activity.

3.Exploration of wider participation and exchange of information with EC-specific programmes, ESF, etc.

4.The planning of the intermediate report, the final report and the concluding symposium.

Progress in each of the projects will also be reported by the respective participants in their own countries within the framework of existing programmes.

D2.Reports

The progress of the programme will be monitored by brief annual reports from each of the participating scientists which will describe the results of research obtained through concertation. A milestone report will be prepared by the Management Committee after 3 years of joint activities. The report will be presented to the COST Technical Committee for Chemistry for their review.

A final report will be published to inform non-participating scientists and research workers interested in the results about the scientific achievements of the Action. It is expected that some reviews by participants which describe the progress made and state of the field will be published in International Journals. To conclude the COST Action, a symposium will be held after 5 years which will be accessible to other scientists.

D3.Timetable

The Action will last five years and comprise the following four stages:

Stage 1:After the first meeting of the Management Committee, a detailed inventory of ongoing research and existing plans of the participating groups to begin joint projects will be made. This will result in a discussion document which will allow further planning to occur.

Stage 2:It will be evident which projects are closely related and would benefit from joint activities. Researchers (and co-workers) will set up (and continue) joint collaborative projects and exchange their recent research results. It may be appropriate to explore wider collaboration with other European countries during this stage.

Stage 3:An intermediate progress report will be prepared after 3 years for review by the COST Technical Committee for Chemistry and by the COST Senior Officials Committee.

Stage 4:This final phase will begin after 4 years and will involve the evaluation of the results obtained. It may include the organization of a symposium for all the participants and co-workers.

E.ECONOMIC DIMENSIONS

The Supramolecular Chemistry Action was prepared under cooperation by scientists from 13 European countries (Belgium, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Netherlands, Spain, Sweden, Switzerland and the United Kingdom).

The total amount of research funding within these countries may be estimated according to the following scheme (after Stoddart: Evaluation of Supramolecular Science and Technology):

University25%
National Science Foundations45%
non-profit15%
industry15%.

From these data, the personnel costs (research plus administration) in those European countries for a total of 700 man-years amounts to about ECU 40 million.

The breakdown for the eight subprojects listed before is the following:

-Subproject 1 (Synthesis and properties of new supramolecular structures) in 11 countries for a total of 100 man-years, in total ECU 6,5 million;

-Subproject 2 (Host guest complexation) in 13 countries for a total of 110 man-years, in total ECU 7 million;

-Subproject 3 (Reactivity and catalysis) in 9 countries for a total of 80 man-years, in total ECU 5 million;

-Subproject 4 (Transport and carrier) in 7 countries for a total of 70 man-years, in total ECU 4,5 million;

-Subproject 5 (Self-assembly and self-organization) in 11 countries for a total of 110 man-years, in total ECU 6,5 million;

-Subproject 6 (Molecular evolution) in 5 countries for a total of 50 man-years, in total ECU 3,5 million;

-Subproject 7 (Supramolecular devices) in 5 countries for a total of 50 man-years, in total ECU 3,5 million;

-Subproject 8 (Molecular modelling) in 5 countries for a total of 50 man-years, in total ECU 3,5 million.

Coordination costs

The costs for coordination and travelling expenses, as well as for workshop and seminar organization and publication, are estimated at 400 kECU for the whole period of 5 years.

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N/A

Contributo UE

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Indirizzo

Francia

Costo totale

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