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

Question 19

What is the difference in transition between the Cu(I) and Cu(I) complexes in UV-Vis spectroscopy

Answer 19

• The Cu(II) is at a longer wavelength, that means it’s a correspondingly lower energy transition
• It is less intense because D2D transitions are formally forbidden within the selection rules

Question 20

Overtime the absorption at 670 nm from Cu¹¹N₄ is found to change
The solution changes colour from a pine tree green to a pale olive green, why?

Answer 20

• Corresponds to the rearrangement of the rings by rotation
• The rearrangement is driven by the tetrahedral copper(II) Cu¹¹N₄ preferring to be 5-coordinate Cu¹¹N₅
• The new Cu¹¹N₅ complex still possess a d→d transition, but the change in coordination geometry alters this transition wavelength and intensity

Question 21

How long does the electrolysis (oxidation) vs the rotation take relative to another?

Answer 21

• The electrolysis takes place over 1.5 hr
• The rotation takes place over 70 hr

Question 22

The catenate system is a molecular switch rather than a molecular motor
What is the difference?

Answer 22

• A molecular motor is capable of unidirection motion
• This means it can continuously rotate in the same direction (i.e., always clockwise, or always anticlockwise) and each cycle of rotation can be harnessed to do work

Question 23

Why is a Catenate system not a molecular motor?

Answer 23

• The rotation is driven by Brownian motion moving the rings around
• They have an equal likelihood or rotating clockwise and anticlockwise
• This means that over many cycles, they cannot be harnessed to do the work of a motor, as any useful work down will equally likely to be undone