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Supramolecular-Architecture and SelfAssembly

towards NANOTECHNOLOGY

Prof. L Cronin Lecture 1

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NO PAPER HANDOUTS WITH THIS COURSE

To get a PDF go to:

www.chem.gla.ac.uk/staff/lee/lectures.htm

Look for the 4th year supramolecular architecturecolumn and then right click to download the pdf for

lecture 1.

Username = 3rdyear

Password = cronin3a

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Nanotechnology is:

The science and engineering of creating materials,functional structures and devices on the nanometerscale. Examples include carbon nanotubes,nanocrystals, quantum dots, molecular clusters.

The top down manufacturing paradigm; Molecularengineering, molecular electronics, nano- fabrication,

bionanotechnology. Imaging and manipulating at theatomic scale.

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Nanotechnology

The essence of nanotechnology is the ability to workat the molecular level, atom by atom, to create largestructures with fundamentally new molecular

organization.

Compared to the behavior of isolated molecules ofabout 1 nm or of bulk materials, behavior of structuralfeatures in the range of about 10-9 to 10-7 m (1 to 100

nm) exhibit important differences that are not yet fullyexplained by explained by theory.

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Topics to be covered Fundamental interactions utilized in molecular assemblies; coordinate

bonds, hydrogen bonding, electrostatic interactions, hydrophobicinteractions etc.

Cooperative interactions in the formation of supra-molecules; concepts ofkinetic and thermodynamic control in assembly and template effects.

The building block concept in the formation of supramolecular molecules;the interplay of molecular shape, symmetry and topology, discussion ofvirtual combinatorial libraries.

Biological self assembly in the context of protein folding, virus assembly andsome examination of the assembly of DNA and other networks

Understand the requirements to design building blocks that can formsupramolecular architectures

The application of supramolecular molecules as molecular devices,molecular machines and functional materials and as drugs

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Nano-scale

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What is Supramolecular Chemistry ?

Supramolecular chemistry is the chemistry of theintermolecular bond, covering the structures andfunctions of the entities formed by the associationof two or more chemical species

J.-M- Lehn

"Supramolecular chemistryis defined as chemistry

beyond the Molecule, as chemistry of tailorshaped inter-molecular interaction.

F. Vgtle

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What is supramolecular chemistry?Where did it come from?

Why does it deserve to be a field of study all its own?

the study on non-covalent & intermolecular forces and the

structures created by these forces: chemistry beyond themolecule

inspired from biology and built on the shoulders oftraditional synthetic organic chemistry

the next logical step in synthetic chemistry; understandingand interface with the biological world; nanotechnology

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Metallosupramolecular chemistry

The ultimate goal of metallosupramolecular chemistry, as in thesupramolecular chemistry of organic systems, is to assemble metalcomplexes that may be applied to various tasks at hand, e.g., lightharvesting, cation and anion sensing, and the photosplitting of water. Thekey to any supramolecular system is that the properties of the individualsub-units are still found in the final structure, i.e., that we still have asupramolecular system and not a supermolecule

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Supermolecules

A supermolecule may still possess interestingproperties or functions, but is not composed ofindependent subunits that work in together to deliversupramolecular properties.

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Watson & Crick 1953

Information Storage

The ultimate supramolecular material?

Encodes gigabytes of data

Can Self-Replicate

Built-in Error Correction

Is the basis of life

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Catalysis

Enzymes: Biological Catalysts with remarkable

efficiency and selectivity.

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Tobacco Mosaic Virus (TMV)

Smart Nanostructured Materials

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Molecular Data Storage & Data Transmission

Nano-Machines

High efficiency, high selectivity, green catalysts

Molecular Sensors

Separation Devices

Drug & Gene delivery

Next generation medicine (replacement organs, etc.)

The potential payoff:

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Emil Fischer: The Lock & Key

1894: Recognized even before molecular structure waswell understood that shape matters.

In order for two species to interact in a specific fashion,they must have complementaryshape & chemistry like a

key fitting into a lock.

Wins Nobel Prize in 1902.

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1987 Nobel Prize

1987 Nobel prize in chemistry goes to:

* Donald J. Cram (1919-2001), UCLA, USA

* Jean-Marie Lehn (1939-), U. Louis Pasteur,France

* Charles J. Pedersen (1904-1989), Du Pont, USA

for their development and use of molecules withstructure-specific interactions of high selectivity

http://www.nobel.se/chemistry/laureates/1987/index.html

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Charles J. Pedersen: Crown Ethers

O

O

O

O

O

O

Dibenzo-18-Crown-6

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Charles J. Pedersen: Crown Ethers

O

O

O

OO

O

O

O

O

O

O

O

O

O

O O

O O

Oxygen atoms donatea fraction of theirelectron density toelectron deficient

species such as alkalications: Li, Na, K

Q: Why did he get selectivityfor different ions?

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Charles J. Pedersen

Discovered essentially by accident Think about structure. 3D shape. Geometry.

Computer simulations and CPK models Prepared an entire series of crown-ethers each withtheir own particular selectivity for various cations

Selectivity base primarily on size:Li 1.36 14-crown-4 1.2-1.5Na 1.94 15-crown-5 1.7-2.2K 2.66 18-crown-6 2.6-3.2

Its easy: some fit some dont

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Donald J. Cram: Pre-organization

ORRO

RO

OR

OR

OR

Spherand - preorganized binding siteSelectivity for Li+ > Na+ >> K+

Q: What did he mean by preorganized?

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Donald J. Cram

Synthesized a huge series of increasingly complicatedhost-guestsystems that bind molecules instead of just ions

and eventually tries to emulate an enzyme catalytic site.

Introduces the idea of a carcerand - synthetic molecular

(jail) cells. Q: What other carcerands can you think of?

Primary contribution is to show how importantpreorganizationis to binding strength.

Shows the application of some hosts for practicalapplication: resolution of racemic mixtures

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Host

Guest

Complex

How do you tell which is a host & which is a guest?

binding sites face inward, or converge

binding sites face outward, or diverge

Two or more molecules held togetherin a unique structural relationship bynon-covalent(weak) forces.

Self Assembly

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Self Assembly

Molecular Self Assembly

the spontaneous assembly of supramolecular molecules

and networks

Dependent or thermodynamic and kinetic considerations

Building blocks

Templates

X

X X

XX

X

Forces involved in self assembly

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Forces involved in self assembly

Hydrogen bonding Strong N-HF, moderate N-HOH and weak C-HCl-

Pi-stacking interactions

Electrostatic (ion-ion, ion-dipole and dipole-diple)

Dispersion and induction forces (van der Waals)

Coordinate bonding

Hydrophobic or solvatophobic effects

Interaction Energies

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Interaction Energies

Bond energy of a single covalent bond X-Y 350 kJ mol-1

Bond energy of a triple bond e.g N2 942 kJ mol-1

Non-covalent interactions can range from 2 kJ mol-1

to 20 kJ mol-1

for a hydrogen bond and 250 kJ mol-1

for an ion-ion interaction

POWER OF SUPRAMOLECULAR CHEMISTRY is inthe combination of a number of these interactions

Example of electrostatic interactions

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Example of electrostatic interactions- Valinomycin

Valinomycin (VM) is a dodecadepsipeptide,that is it is made of twelve alternating aminoacids and hydroxy acids to form a

macrocyclic molecule. It is a member of thegroup of natural neutral ionophores becauseit doesn't have a residual charge.

It is a naturally occurring macrocylic antibioticthat selectively transports potassium cationsacross mitochondiral membranes in thePRESENCE of sodium cations

Confromation of Valinomycin is stabilised byNCOHN hydrogen bonds. K+ is bound bythe O= atoms of the ester groups

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Valinomycin

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Valinomycin

K+ is complexed by the ester group oxygen atoms

Once complexed it can be efficiently transportedthroughthe hydrophobic membrane.

The membrane is lipophilic and so will not normallyallow charged species to pass through

The exterior has greasy alkyl groups providing thecomplex with solubility

Metallosupramolecular chemistry

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Metallosupramolecular chemistry

The term metallosupramolecular chemistry wascoined in the early 90s to describe supramolecularc o o r d i n a t i o n c h e m i s t r y . T h i s s u b - f i e l d o f

supramolecular chemistry started off by examining themetal-directed self-assembly of discrete structures andsoon expanded to infinite structures. Although metal

directed self-assembly proved to be a powerful tool tobuild up large supramolecular structures, the true goalhas always been the assembly of architectures withspecific functions.