1. Laurea specialistica in Scienza e Ingegneria dei Materiali
Curriculum Scienza dei Materiali
Chimica Fisica dei Materiali Avanzati Part 8 – Molecular switches and molecular machines
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2. Molecular-level devices
Molecular-level electronic components
Molecular-level systems that play the same functions played by macroscopic components in electronic devices
Molecular-level mechanical machines
Molecular-level systems that perform mechanical movements analogous to those observed in macroscopic machines
Molecular-level
electronic components
Molecular-level
mechanical machines
Molecular wires
Molecular switches
Molecular antennas
Charge-separation devices
Molecular memories
Molecular sensors
Molecular logic gates
Molecular tweezers, doors and boxes
Rotary motors
Piston/cylinder systems
Molecular shuttles and muscles
Sliding molecular rings
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3. Switching of electron or energy transfer processes
e- or E A C
Switching requires an external stimulus which, at the molecular level, causes electronic and nuclear rearrangements. Usually one of the two types of rearrangements prevails or is more relevant to the performed function.
The three main types of stimuli that can be used to switch a chemical compound are:
Light energy (photons)
Electrical energy (electrons or holes)
Chemical energy (in the form of reactants)
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4. Molecular switches
A molecular switch is a system on the molecular scale which can be reversibly brought from one state into another by means of an external stimulus.
Important properties for practical applications:
thermal irreversibility
quick response
high efficiency
fatigue resistance
nondestructive readout
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5. Example: diarylethene switches
First described by Irie in 1988.
M. Irie, M. Mohri, J. Org. Chem. 53 (1988),
Reaction is thermally irreversible, response is quick, efficiency and fatigue resistance are high.
Many studies have been performed on diarylethenes, with as main goals improving the switching properties and achieving non-destructive read-out.
For an extensive review, see: M. Irie, Chem. Rev. 100 (2000),
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6. Photochromic response of diarylethene
A very broad absorption extending over most of the visible range appears upon UV irradiation and ring closure
The process can be reversed by irradiation with l > 500 nm
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7. Mechanism: increase of the conjugation
Open ring, parallel conformation
Open ring, antiparallel conformation
Closed ring
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8. Other photochromic switches
Cis-trans isomerization of azobenzenes
Switching of a spiropyran derivative
(c = closed, o = open)
Photochemical transformations of fulgides
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9. Optical memories by holographic recording
Holographic optical storage is a unique method for achieving high-density data storage in three dimensions.
Photopolymer systems are quite attractive because of their high sensitivity, ease of preparation, and self-development capability.
Rewritable holographic recording materials based on photochromic conversion have attracted strong interest for three-dimensional optical recording
Holographic writing set-up
A phase hologram with Dn = 1.15x10-3 and period dependent on the angle 2 is written.
Photochromic cyclability
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10. n * N R O C H Methacrylic backbone3 2
Methacrylic backbone
Optically active cyclic group
Conjugated trans-azoaromatic chromophore
R = CN, NO2
poly[(S)-MAP-C]
R = CN Mn=43,900Mw /Mn=1.4 Tg=192°C Td= 311°C
poly[(S)-MAP-N]
R = NO2 Mn=18,300Mw /Mn=1.4 Tg=208°C Td= 315°C

11. 488 nm
633 nm
Absorption in the visible:
azo-dyes n *, * and CT el. trans.

12. h  Photoinduced trans  cis  trans isomerization cycles Ē trans cis
rotational
diffusion h
STOP
trans
cis

13. Reversible photoinduced orientation of azobenzene groups

15. irradiation with CP-L light
Reversible inversion
of the CD signal by
irradiation with CP-L light
poly[(S)-MAP-N] Tg = 208 C
thin films 100  300 nm
I  160 mW/cm2 x 60 s
L. Angiolini et al., Chem. Eur. J., 8 (2002) 4241

16. OPTICAL STORAGE OPTICAL STORAGE AND CHIROPTICAL SWITCHES
PHOTORESPONSIVE PROPERTIES
CONVENTIONAL MATERIALS
CHIRAL
PHOTOCHROMIC POLYMERS
Photomodulation of linear birefringence and dichroism
OPTICAL STORAGE
AND
CHIROPTICAL SWITCHES
OPTICAL STORAGE
Photomodulation of chiroptical properties
CHIROPTICAL SWITCHES

17. Switching of electron transfer by a photon input
Light switching of electron transfer in multicomponent molecular systems based on the 1,2-bis-(3-thienyl)-ethene photochromic bridge
In the closed form energy is transferred to the conjugated diarylethene unit instead of electron transferred to the pyridinium ion
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18. Multistate-multifunctional supramolecular systems
Multistate systems : compounds that can be reversibly interconverted between more than two stable states
Multifunctional systems : compounds that can be reversibly interconverted between stable states by means of different stimuli
Two photochromic units can be coupled in the same supramolecular species, giving biphotochromic multistate systems
The photochemical inputs used to stimulate photochromic compounds can be couped with several other types of stimuli, leading to a variety of interesting multistate-multifunctional systems
On application of n independent stimuli, each related to 2 states, 2n different states of the system become available in principle
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19. Towards molecular logic gates
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20. Electronic vs. ‘chemical’ computers
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21. Molecular-level machines
A molecular machine is a particular type of a molecular device whose outcome is a mechanical motion, i.e., in which the component parts can be set in motion as a result of some external stimulus
What are the possibilities of small but movable machines? [...]
Lubrication might not be necessary. Bearings could run dry; they wouldn’t run hot because heat escapes from such a small device very, very rapidly. [...]
An internal combustion engine of that size is impossible. Other chemical reactions, liberating energy when cold, can be used instead. [...]
What would be the utility of such machines? Who knows? [...]
I cannot see exactly what would happen, but I can hardly doubt that when we have some control of things on a molecular scale we will get an enormously greater range of possible properties that substances can have, and of the different things we can do.
R. P. Feynman, Address to the American Physical Society, December 1959
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22. Natural Molecular Machines and Motors
Cells have hundreds of different types of molecular motors, each specialized for a particular function. Many biological motor-like proteins have been discovered and characterized in recent years.
(Natural) molecular motors come in a wide variety of designs. Some motors operate in a cyclic fashion, undergoing a number of steps that correspond to changes in conformation and/or in chemical state, and eventually resetting themselves to their initial configuration.
C. Bustamante, D. Keller, G. Oster, Acc. Chem. Res., 2001, 34,
Copernicus, Springer - Verlag, New York, 1996
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23. Natural Molecular Machines and Motors - 2
Rotary motor proteins
ATP Synthase
Bacterial flagellar motor
Linear motor proteins
Myosin
Kinesin
Dynein
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24. Artificial molecular machines: molecular tweezers
Macroscopic tweezers
Molecular tweezers (photochemically controlled)
S. Shinkai et al., J. Am. Chem. Soc., 1981, 103, 111
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25. Photocontrolled Opening-Closing of Molecular Cavities
Azobenzene containing macrotricyclic receptors
Azobenzene capped β-cyclodextrin
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26. A Photochemically Driven Molecular Rotary Motor
B.L. Feringa et al., Nature, 1999, 401, 152
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27. Photochemically Driven Threading-Dethreading Motions in Pseudorotaxanes
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28. Undirectional Circumrotation of Macrocycles in Catenanes
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