UNIVERSITA’ DEGLI STUDI DI PADOVA
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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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 tweezers, doors and
boxes
 Rotary motors
 Piston/cylinder systems
 Molecular shuttles and muscles
 Sliding molecular rings
Molecular wires
Molecular switches
Molecular antennas
Charge-separation devices
Molecular memories
Molecular sensors
Molecular logic gates
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Switching of electron or energy transfer processes
e- or E
e- or E
A
C
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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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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Example: diarylethene switches
 First described by Irie in 1988.
 M. Irie, M. Mohri, J. Org. Chem. 53 (1988), 803-808
 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),
1685-1716
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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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Mechanism: increase of the conjugation
Open ring, parallel conformation
Closed ring
Open ring, antiparallel conformation
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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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Optical memories by holographic recording
 Holographic optical storage is a unique method for achieving highdensity 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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Methacrylic backbone
Conjugated trans-azoaromatic chromophore
CH3
CH2 C
n
O C
O
*
N
N
N
Optically active cyclic group
R
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
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Absorption coefficient/nm
-1
0.010
poly[(S)-MAP-C], film 120 nm
poly[(S)-MAP-N], film 140 nm
488.0 nm
632.8 nm
0.008
0.006
0.004
0.002
0.000
200
300
400
500
600
700
Wavelength/nm
Absorption in the visible:
azo-dyes n *, * and CT el. trans.
Pump @ 488 nm
Probe @ 633 nm
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Photoinduced trans  cis  trans isomerization cycles
trans
N
cis
h
N
STOP
N
N
Ē
rotational
diffusion
N
N
D
h
N
N
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Reversible photoinduced orientation
of azobenzene groups
Linear Pol.
Circular Pol.
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Linear Dichroism
Irradiation with CP light
90
120
150
180
Time (s)
210
240
270
0.14
0.12
n2
0.10
0.08
0.06
0.04
0.02
0.00
30
Birefringence
60
with CP light
60
n1
Irradiation
0.005
30
K //
with LP light
0.006
Irradiation
Irradiation with LP light
0.007
n
K
-1
Abs. coeff. K (nm )
0.008
90 120 150 180 210 240 270 300 330
Time (s)
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Native
irr. LP light
irr. CP-L light
irr. CP-R light
0.15
Ellipticity [mdeg/nm]
0.10
0.05
0.00
Reversible inversion
of the CD signal by
irradiation with CP-L light
-0.05
-0.10
-0.15
-0.20
-0.25
250
300
350
400 450 500
Wavelength [nm]
550
600
650
Native
Irr. CP-L light
Irr. CP-R light
0.15
poly[(S)-MAP-N] Tg = 208 C
thin films 100  300 nm
I  160 mW/cm2 x 60 s
Ellipticity [mdeg/nm]
0.10
0.05
0.00
-0.05
-0.10
-0.15
-0.20
300
L. Angiolini et al., Chem. Eur. J., 8 (2002) 4241
350
400
450
500
Wavelength [nm]
550
600
650
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PHOTORESPONSIVE
PROPERTIES
CONVENTIONAL
MATERIALS
CHIRAL
PHOTOCHROMIC
POLYMERS
Photomodulation of
linear
birefringence and
dichroism
OPTICAL
STORAGE
OPTICAL
STORAGE
AND
CHIROPTICAL
SWITCHES
Photomodulation of
chiroptical
properties
CHIROPTICAL
SWITCHES
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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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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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Towards molecular logic gates
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Electronic vs. ‘chemical’ computers
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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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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, 412-420
Copernicus, Springer - Verlag, New York,
1996
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Natural Molecular Machines and Motors - 2
Rotary motor proteins
ATP Synthase
Bacterial flagellar motor
Linear motor proteins
Myosin
Kinesin
Dynein
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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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Photocontrolled Opening-Closing of Molecular Cavities
Azobenzene containing macrotricyclic
receptors
Azobenzene capped
β-cyclodextrin
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A Photochemically Driven Molecular Rotary Motor
B.L. Feringa et al., Nature, 1999, 401, 152
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Photochemically Driven Threading-Dethreading
Motions in Pseudorotaxanes
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Undirectional Circumrotation of Macrocycles in Catenanes
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