Bistable Systems
and
Molecular Switching
Supervisor: Prof.Davor Boghai
By: Seyyed Mohammad Reza Sanavi
Hosseini
Bistable Systems
Any molecular-level system that can be reversibly switched
between two different states by use of an external stimulus
can be taken as a basis for storing information, i.e. for
memory purposes. An ideal molecular-level memory should
be stable and easy to write, and its switched form should be
stable, easy to read, and erasable when necessary. Systems
that undergo an irreversible change can be used as
permanent memories (e.g. photography, dosimetry) . In
more complex systems switching can be performed among
more than two states. This possibility can be exploited to
obtain memories which are permanent unless they are
erased on purpose, or for performing logic operations.
Important types of stimulus
1) Light energy (photons)
2) Electrical energy (electrons/holes)
3)Chemical energy ( in the form of protons, metal
ions, etc.)
Note:
In photochemical stimulation the most common
switching processes are related to photoisomerization
or photoinduced redox reactions; for electrochemical
inputs, the induced processes are, of course, redox
reactions.
Comparison between different
types of stimulation
Compared whit chemical stimulation, photochemical and, to
some extent, electrochemical stimuli can be switched on
and off easily and rapidly. A further advantage of the use of
photo chemical and electro chemical techniques is that
photons and electrons, besides supplying the stimulus to
make a switch work (i.e. ”to write” the information bit),
can also be useful “to read” the state of system and thus to
control and monitor its operation. It should be also noted
that photochemical and electrochemical inputs and outputs
are among the easiest to interface to microscopic systems,
making them amenable to the multi-scale engineering
required for the eventual creation of real devices
Significant switching systems
1)Photochromic systems:
The term “photochromic” is applied to molecules
that can be reversibly inter converted, with at
least one of the reactions being induced by light
excitation, between two forms whit different
absorption spectra. The two forms, differ not
only in their absorption spectra, but also in
several other properties such as redox potentials,
dielectric constant, etc.
Representation of a photochromic system (a) and of
its energy profile (b)
Some important
families of
photochromic
compounds:
a) Diarylethenes
b) Flugides
c) Spiropyrans
d) Azobenzenes
e) dihydroazulenes
2) Modulation
interactions:
of
Host-Guest
Switching host-guest interactions by means of
photochromic reactions might lead to a variety
of sensors and to transport of guest molecules
across a membrane. An example is shown next
page.
Photoswitchable recognition of saccharides by
diarylethene derivative 1
3)Fluorescent Switches:
Several photochromic compounds have different
fluorescence properties in their two forms.
Occasionally on/off switching is observed. Two
examples is shown next page
On/off switching of fluorescence in (a) fulgide
derivative 3 and (b) diaryiethene derivative
4) Chiroptical Switches:
Chiral photobistable molecules are a particularly interesting class of
photochromic compounds. In such molecules
reversible
photochemical trans formation can lead to a change in chirality.
The importance of chiroptical switches is further emphasized
since chirality controls most natural chemical processes,
including molecular recognition, transport, information storage,
catalysis, assembly, and replication.
Chiral switches based on photochromic molecules can be
subdivided as follows:
a) Switching of enantiomers
b) Switching of diastereoisomers
c) Functional chiral switches
d) Switching of macromolecules or supramolecular
organization
Besides the general requirements needed for
photochromic switches, chiroptical switches must be
stable towards thermal racemization.
Thermal and photochemical isomerization
processes of chiroptical switch
5)Photochemical Biomolecular Switches:
It is well known that many biological processes are
triggered by light signals. Such systems consist of a
biological material or an environment the innate
function of which can be activated/deactivated by
artificial photoresponsive units.
6) Electrochromic Systems:
Electrochromic is applied to compounds that can
be inter converted, by reversible redox
processes, between two forms whit different
absorption spectra.
In electrochromic systems the interconverting
species are not isomers, because they have
different number of electrons. In these systems
several successive switching processes can occur.
7) Redox Switches:
an example of redox switch is shown below
Multistate-Multifunctional Systems
1)Biphotochromic Supermolecular Systems:
Many attempts have been made to couple two
photochromic units in the same supramolecular
species. The objective of these studies was to
obtain a synergistic effect between the
properties of the two units to create materials
with novel photokinetic properties.
2)Photochemical Inputs Coupled whit
Other Stimuli:
a) Three-state Systems. Write-Lock-Read-Unlock-Erase
Cycle
b) Orthogonal photochemical-Electrochemical
Stimulation
c) Orthogonal Photochemical-(Acid-Base) Stimulation
d) Molecular Shift Register
Schematic representation of the behavior
of three types of photochromic systems
3) Multielectron Redox Processes:
a) System whit Equivalent redox Units
b) System whit Nonequivalent Redox Units
4) Electrochemical inputs Coupled
whit Chemical Inputs
5) Multiple Chemical Inputs
Photochemical and electrochemical switching
of a diarylethene derivative to perform a writelock-read-unlock-erase cycle
Schematic representation of the write-lockread-unlock-erase cycle of a system
Light-and redoxdriven
switching of
the DHA-VHF
system
Photo-switching materials
in following parts we can see some examples and
applications of molecular switching.
Photo-magnetic molecular solids
1)Prussian blue analogues and CN-bridging
compounds
a) Fe–Co Prussian blue
b) CN-bridging compounds
(Right) Structure and (left)
field-cooled
magnetization curves at
5G
(Top)
View
of
the
molecular structure, a
projection of the threedimensional
crystal
packing, and (bottom)
magnetic susceptibilities
(MT) vs. temperature
before and after UV light
illumination
for
Nd(DMF)4(H2O)3(CN)Fe(CN)5·H2O
2)Light-induced excited spin state trapping
compounds
(Top) View of
molecular structure
and (bottom)
temperature
dependence
of the magnetization
before and after light
illumination for
[FeIII(pap)2]·ClO4
Photo-chromic magnetic
materials
Photo-controllable magnetic vesicles:
the first successful example of
the preparation of photo-chromic magnetic (composite)
materials. A commercially available double-chain ammonium
amphiphile, didodecyldimethylammonium bromide
([CH3(CH2)11]2(CH3)2N+Br-), which forms a vesicle
structure in aqueous solution by sonication, and an azobenzenecontaining
amphiphile (C12AzoC5N+Br-), which was
synthesized according to a reported procedure, were mixed
with an aqueous solution of poly(vinyl alcohol) (PVA). The
photo-chromic vesicle films were prepared by casting the above
solution on a clean glass plate at room temperature
Design of photoresponsive magnetic
vesicles containing
Prussian blue
and azobenzene
Photo-switchable magnetic films
1)Prussian blue intercalated in Langmuir–Blodgett
films consisting of an amphiphilic azobenzene and a clay
Mineral.
2) Anisotropic photo-induced magnetization effect in
ultrathin Fe–Co Prussian blue films
Preparation of
the hybrid
multilayered
films
Schematic representation of photo-switchable
magnetic layer-by-layer films.
(Bottom)
Changes in
the
magnetizatio
n for the
films induced
by alternating
illumination
with UV and
visible light
at 300K at
10G.
Molecular models of
the gemini peptide
lipid 1 evaluated
by molecular
mechanism
calculation using
the Cerius2
software based on
DREIDING
force field: (a)
trans-form; (b) cisform bound a
metal ion.
An example of data storage
Memory operation of the single-molecule
system. The blue line
shows the write, read, and erase pulse
pattern applied and the red
line demonstrates the resulting switching
between "off" and "on"
states of the molecular system. As can be
seen in the picture, the
system is initially in the "off" state. Then a
write pulse of +1.6 V is
applied and the molecule switches to the
"on" state. This state can be
read out using a voltage of +1.1 V. The
molecule can be switched
back to the "off" state by another pulse
(erase pulse) of -1.6 V.
( Source: IBM)
References:
1)Journal of Photochemistry and Photobiology C:
Photochemistry Reviews7 (2006) 69–88
2)Electrochemistry Communications 9 (2007) 173–179
3)Journal of Photochemistry and Photobiology A:
Chemistry 183 (2006) 309–314
4)Journal of Luminescence 119–120 (2006) 478–481
5)Composites: Part A 38 (2007) 747–754
6)www.physorg.com
7)Molecular Devices and Machines, a Journey into the
Nanoworld,V.Balzani, M.Venyuri, A.Credi
The End
Thanks for your attention