Molecular Machines

Question 1

For an object too be a machine it must be able to…

Answer 1

…perform work
This implies a mechanical movement that accomplishes a useful taks

Question 2

This movement is typically because of an…

Answer 2

…external stimuli or input being supplied

Question 3

Machines must be designed for the environment that they operate in - the molecular scale is very different to the macroscopic scale
Explain…

Answer 3

• It is difficult to power molecular machines thermally, due to the rapid dissipation of heat on the molecular scale
• The medium molecular machines are in, is realtively viscous, This means that the forces of friction and gravity are irrelevant

Question 4

What is Brownian motion and how does it relate to how molecular machines are powered?

Answer 4

• The Brownian motion of all particles in a liquid ensure the mobility of individual parts of a molecular machine
• Molecular machines are often powered by chemical changes, i.e., the breaking and making of chemical bonds

Question 5

How can we affect the Brownian motion of molecules?

Answer 5

• Brownian motion means that all molecules in a liquid continously moving anyway, and so if we wan to control the motion, we need to put constraints on molecules
• This is where interlocked molecules such as rotaxanes and catenanes come in
• The mechanical bond present stops the pieces coming apart but allow the pieces to move past each other

Question 6

What are the ways the molecules in a [2]rotaxane can move around another?

Answer 6

• Translational motion of ring along axle
• Rotational motion of ring along axle

Question 7

what are the ways the molecules in [2]catenane can move around another?

Answer 7

Rotational motion of rings around each other

Question 8

• A [2]rotaxane can be synthesised with two different binding stations
• In the absence of any metal ion, this results in…

Answer 8

• … the macrocycle having free movement along the axle
• This free moment is random and driven by Brownian motion and so does not yet qualify this system as a machine

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Question 9

How can we use copper and redox chemistry we can make this system undergo translational or linear motion in response to external stimuli
How does this work?

Answer 9

• The copper(I) ions are oxidised electrochemically
• the ring and the metal ion undergo translational motion from binding site A to B. This moves the newly formed copper(II) ion from a less favourable environment to a more favourable 5-coordinate environment
• The copper(II) ions are reduced electrochemically
• the ring and the metal ion undergo translational motion from binding site B to A. This moves the newly formed copper(I) ion from a less favourable 5-coordinate environment to a more favourable tetrahedral environment

Question 10

This system runs in a…
And can be described as…

Answer 10

…4-step cycle
… a molecular shuttle (its kinda like a lift)

Question 11

Rotational motion is often studied in catenates
What does the structure involve?

Answer 11

• A macrocycle is designed containing two different binding stations
• Macrocycle C is slightly larger with one bidentate binding station
• Macrocycle D has two binding stations
• It has a phenanthroline binding station that is bidentate and a terpyridine binding station that is tridentate

Question 12

What happens when we remove the metal ions from the [2]catenane?

Answer 12

• In the absence of metal ions, the [2]catenane has flexibility to adopt a range of different conformations and both rings can rotate freely through each other
• However, upon addition of a metal ion to make the [2]catenate, this fixed the preferred conformation to satify the binding preferences of the metal ion

Question 13

What is the difference in binding for the copper(I) vs copper(II) metls in [2]catenate

Answer 13

• In the presence of copper(I), both bidentate sites come together so that the metal ion is bound in an approximately tetrahedral geometry
• In the presence of copper(II), the terpyridine site on D combines with the phenanthroline site on C to bind the metal ion in a

Question 14

Using redox chemistry, this [2]catenate system is capable of…
This type of system is referred to as a…

Answer 14

…rotational motion in a four-step cycle
…molecular switch

Question 15

Describe how the [2]catenate system has rotational motion

Answer 15

• The copper(I) ion is oxidised electrochemically
• The ring undergoes rotational motion relative to each other. This enables the newly formed copper(II) to bind in a more favourable 5-coordinate environment
• The copper(II) is reduced electrochemically
• The rings undergo rotational motion relative to each other. This enables the newly formed copper(I) to bind in a more favourable tetrahedral environment

Question 16

The switching of this [2]catenate can be characterised by…

Answer 16

UV-Vis Spectroscopy

Question 17

Before the copper(I) is oxidised, it is in a tetrahedral geometry (abbreviated Cu¹N₄).
What is the d-electron configuration?
The solution is an intense red-brown colour due to?

Answer 17

• d¹⁰ electron configuration
• The solution is an intense red-brown colour due to a Metal to Ligand Charge Transfer (MLCT) from d orbitals to π star on ligand

Question 18

After the copper is oxidised, copper(II) ion is in a tetrahedral geometry (Abbreviated as Cu¹¹N₄)
What is the d-electron configuation?
The solution becomes a pine tree green colour due to?

Answer 18

• d⁹ electron configuration
• d→d transition