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8/3/2019 Liquid Crystal Chemistry
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Effects of Chiral Dopants on Liquid
Crystals in the Nematic Phase
Kyle Troutman
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Outline of Discussion
Introduce liquid crystal science
Explain the role ofchirality in liquid crystals
Discuss the ability of various dopants to induce
chiral phases in liquid crystal hosts
Delve into the significance and development of
optically switchable dopants
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Liquid Crystals?
Used in display devices
LCD Televisions, digital watch displays, mood rings
Biological liquid crystals
Phospholipids in the plasma membrane
Lyotropic liquid crystals
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Discovery!
First observed by an Austrian chemist, FriedrichReinitzer, in 1888
The substance seemed to have two distinct meliting
points
cloudy, white liquid at 145.5 C
transparent, clear liquid 178.5 C
Molecular structure of
Cholesteryl benzoate
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Liquid Crystal Mesophase
The liquid crystal state is between the solidand liquid phases
Refered to as mesophase
Display properties of both crystal and liquid
Sensitive to temperature, electric andmagnetic fields, solvent, and dopant
molecules Self-Assemble into unique supramolecular
structures
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Order Parameter S
Mesogens align along a common axis called a
director (n)
Can align with directional or positionalorientation
Describes different liquid crystal phases
Degree of order expressed by parameter S
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Temperature Effect on parameter S
Tc = Clearing Temperature
At temperature Tc
substance loses all
order
Anisotropic liquid
crystal becomes
isotropic liquid
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Liquid Crystal Classification
Calamitic Discotic
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Liquid Crystal Phases
1. Nematic
2. Smectic A
3. Chiral Smectic or Smectic C
4. Chiral Nematic or Cholesteric
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Nematic Phase (N)
Molecules exhibit long
range orientational order
Mesogen aligns long axis
with a common director n
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Smectic Phases
In the smectic phases molecules flow freely
within a layer and align with a director n
In Smectic C* phase the molecules align at
an angle to the normal z
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Chirality
A molecule can only be considered chiral if itlacks an axis of improper rotation
Lacks an Sn axis
Rotation by 360/n followed by a reflection perpendicular tothe axis of rotation
Structure is non-superimposable on its mirrorimage
Chirality can be induced upon a completelyachiral molecule
Chirality transfer
Supramolecular chirality
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Chiral Nematic
Also known as cholesteric phase or twisted
nematic
Molecules in this phase are chiral
Helical change in orientation of the director
Director has helical twist perpendicular to the
long axis of helix Helical propagation of the director is non-
superimposable on its mirror image
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Chiral Nematic
Pitch (p) is the
distance in which thedirector rotates 360
n
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Doped Liquid Crystals
Magnitude of cholestericpitch
Sign of cholesteric pitch
Right handed helix is
positive (P)
Left handed helix isnegative (M)
P (+) M (-)
Chiral dopant
Chiral guest molecule is dissolved into a nematic
host
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The efficiency of a dopant to induce helicalorganization
Helical twisting power ()
The pitch is inversely proportional to:
Concentration (c)
Helical twisting power ()
Enantiomeric excess (ee)
Helical Twisting Power
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Chirality Transfer
Chirality is amplified and transferred throughout the whole system
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Advantages of Doped LCs
Doped liquid crystals have advantages over
liquid crystals with chiral mesogens
Pitch of helix is tunable by changing host-guest
ratio
Colors can be generated by a single dopant
Host and dopant self assemble
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Shape Persistent Dopants
Mesogenic functionalism
Chiral coordination complexes
TADDOL and Dioxolane compounds
Atropisomer-based dopants
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Mesogenic Functionalism
A chiral molecule is
functionalized to liquid
crystal mesogen
Functionalization enhances
solubility and molecularinteraction with LC host
Chiral alcohol dopant
Chiral alcohol attached to mesogen
(R)-octan-2-ol
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Amine Functionalization with
Anthraquinone
Intramolecular hydrogen bonding locks dopant
conformation
amine molecule
Amine molecule attached to anthraquinone
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Color test for enantiomeric excess
High helical twisting power of 18 caused
liquid crystal to reflect light in the
visible range
The color of the liquid crystal is
dependent on the enantiomeric excess
of the dopant
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Chiral Coordination Complexes as
Dopants
Chirality of a coordination compound can be
from chiral ligand or chiral metal center
Chiral metal center can be (Dextro) or (Levo)
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Metal coordination improves helical twisting
power Reduced conformation flexibility
Chiral propeller shape of metal complex
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TADDOL and Dioxolane Dopants
Tartaric acid
derivative
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TADDOL and Dioxolane Dopants
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Methyl groups on 31 and silane bridge on 32
prevent intramolecular hydrogen bondingbetween the diols
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Helical twisting poweris greater in guest-hostcombinations wherearyl core distances are
ofsimilar length
-stacking interactionsbetween dopant and
biphenyl LC core
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- interactions
Non-covalent intermolecular interaction
Arise from a combination of van der Waals
and electrostatic forces Forms supramolecular structure
- interactions are still actively debated
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- interactions
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Proposed Dopant-Mesogen Interaction
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Atropisomer Dopants
Atropisomer considered chiral because
rotation around bond is sterically restricted
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Atropisomer Dopants
If substitution at the 2 and 2 positions arecovalently or hydrogen bonded
Cisoid conformation is preferred
Substitution by large unlinked constituents Transoid conformation preferred
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Atropisomer Dopants
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Proposed model for chirality transfer between binaphthyl
dopant and liquid crystal mesogen
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Bistable Switchable Dopants
Allows control of supramolecular chirality by
external stimuli
Can change helical: Sign
Pitch length (p)
Photochemical switchable dopants
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Control of supramolecular chirality using
external stimuli
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Bistable Switchable Dopants
External Stimuli
pH
Pressure
Electric field
Heat
Magnetic field
Light
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Bistable Switchable Dopants
Potential use as molecular switch Molecular equivalent of transistor
Nanotechnology
Light versus electricity Less heat produced
Light more efficient and abundant
Faster data processing
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Optical Switches
Type 1
Dopant switches enantiomeric state by irradiation
with circularly polarized light
Switch between P and M helicities
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Optical Switches
Type 2
Dopant switches between diastereomers
Chirality of dopant is inversed
Dopant states are not enantiomers
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Optical Switches
Type 3
Switch and chiral auxiliary are separate units
Chirality change is not as strong
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Type 1 dopants: bicyclic ketones
Circularly polarized light induces deracemization with
enantiomeric excess of 0.4%
Not strong enough to induce helical change due to low
helical twisting power
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Type 1 dopants: bicyclic ketones
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Type 1 dopants: bicyclic ketones
Mesogen resembling group incorporated that matches LC ZLI-
1167 aided in solubility and helical twisting power 13 mol%
racemic 83
ZLI-1167 Liquid Crystal Host
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Type 2 dopants: overcrowded alkenes
2-(2,6-dimethylphenyl)-9-(2,3-dihydro-1H-naphtho[2,1-b]thiopyran-1-ylidene)-9H-thioxanthene
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Type 2 dopants: flourene derived
molecular motor
High helical twisting power possibly due to structural similarity of dopant
and biphenyl core of host mesogen
Thermal isomerisation from unstable to stable form is possible at room
temperature = allows for reversible color change
Induces change in
helical twist sign P-MRotar
Axle
Stator
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Psuedoequatorial position with high
steric strain
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Flourene Molecular Motor
MBBA could not be used as mesogen because
it absorbs
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Polygonal Surface Structure
polyimide-coated glass surface
Surface topography of polygonal nematic surface
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Transition from 1a 1b
White bar is 50 m
E7 with 1 wt % of1a dopant
Irradiated with 365 nm light
Each square is 15 second interval
Clockwise rotation
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Thermal isomeration at 20 C
Reaction half-life of 9.9 min in
toluene
P
rotation
1b 1c
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Work created by molecular motor was used to move 5 X 28 m glass rod
Photo chemical step 0.67 r.p.m. 1a 1b
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Thermal step 0.22 r.p.m.1b 1c
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X8 speed
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Type 3 dopants: azobenzene derived
molecular motor
High helical twisting power
allows for color switching
UV light causes 73 to change
helicity
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Liquid crystals are self assembling molecules
Have both crystalline and liquid properties
Chiral dopants can be added to change from thenematic to the cholesteric phase
Dopants vary in their ability to induce cholestericchange described by the helical twisting power ()
Current research is focused on developing opticallyswitchable dopants with high helical twisting powers
Switchable LCs could be used for nanotechnology, fasterdata processing, or molecular motors
Conclusion
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References
1. Shri Singh. Liquid Crystal Fundamentals, Banaras Hindu University World Scientific PublishingCo. Pte. Ltd. 2002; 72, 43-49.
2. Demus, D., Goodby, J. W., Gray, G. W., Spiess, H.-W., Vill, V., Eds. Handbook of Liquid CrystalsWiley-VCH:Weinheim, 1998; Vol. 2A, pp 3-23.
3.Mioskowski, C.; Bourguignon, J.; Candau, S.; Solladie , G. Photochemically Induced CholestericNematic Transition in LiquidCrystals. Chem. Phys. Lett. 1996, 38, 456-459.
4. I-Hsin Lin, Gary M. Koenig Jr., Juan J. de Pablo, Nicholas L. Abbott. Ordering of SolidMicroparticles at Liquid CrystalWater Interfaces. The Journal of Physical ChemistryB 2008 112 (51), 16552-16558
5. Joon-Seo Park,, Sarah Teren,, William H. Tepp,, David J. Beebe,, Eric A. Johnson, and, Nicholas L.
Abbott.,
Formation ofOligopeptide-Based Polymeric Membranes at Interfaces betweenAqueous Phases and Thermotropic Liquid Crystals Chemistry of Materials 2006 18 (26), 6147-6151
6. Siok Lian Lai, Shisheng Huang, Xinyan Bi, Kun-Lin Yang Optical Imaging of Surface-ImmobilizedOligonucleotide Probes on DNA Microarrays Using Liquid Crystals Langmuir2009 25 (1), 311-316
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References
Special thanks to research by Ben L. Feringa andRienk Eelkema at the University of Groningen,Netherlands
Rienk Eelkema, Michael M. Pollard, Nathalie Katsonis, Javier Vicario, Dirk J. Broer,, and,
Ben L. Feringa.
Rotational ReorganizationofDoped Cholesteric Liquid Crystalline FilmsJournal of the American Chemical Society2006 128 (44), 14397-14407
Rienk Eelkema and, Ben L. Feringa;Macroscopic Expressionofthe ChiralityofAmino
Alcohols by a Double AmplificationMechanism in Liquid CrystallineMedia. Journal of
the American Chemical Society2005 127(39), 13480-13481
Eelkema R, Pollard M, Vicario J, et al.Molecular machines: Nanomotorrotatesmicroscaleobjects Nature [serial online]. March 09, 2006;440(7081):163-163.
Alessandro Bosco, Mahthild G. M. Jongejan, Rienk Eelkema, Nathalie Katsonis,
Emmanuelle Lacaze, Alberta Ferrarini, Ben L. Feringa. PhotoinducedReorganizationof
Motor-Doped Chiral Liquid Crystals: BridgingMolecular Isomerization and Texture
RotationJournal of the American Chemical Society2008 130 (44), 14615-14624