liquid crystal chemistry

8/3/2019 Liquid Crystal Chemistry

1/63

Effects of Chiral Dopants on Liquid

Crystals in the Nematic Phase

Kyle Troutman


2/63

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


3/63

Liquid Crystals?

Used in display devices

LCD Televisions, digital watch displays, mood rings

Biological liquid crystals

Phospholipids in the plasma membrane

Lyotropic liquid crystals


4/63

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


5/63

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


6/63


7/63

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


8/63

Temperature Effect on parameter S

Tc = Clearing Temperature

At temperature Tc

substance loses all

order

Anisotropic liquid

crystal becomes

isotropic liquid


9/63

Liquid Crystal Classification

Calamitic Discotic


10/63

Liquid Crystal Phases

1. Nematic

2. Smectic A

3. Chiral Smectic or Smectic C

4. Chiral Nematic or Cholesteric


11/63

Nematic Phase (N)

Molecules exhibit long

range orientational order

Mesogen aligns long axis

with a common director n


12/63

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


13/63

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


14/63

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


15/63

Chiral Nematic

Pitch (p) is the

distance in which thedirector rotates 360

n


16/63

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


17/63

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


18/63

Chirality Transfer

Chirality is amplified and transferred throughout the whole system


19/63

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


20/63


21/63

Shape Persistent Dopants

Mesogenic functionalism

Chiral coordination complexes

TADDOL and Dioxolane compounds

Atropisomer-based dopants


22/63

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


23/63

Amine Functionalization with

Anthraquinone

Intramolecular hydrogen bonding locks dopant

conformation

amine molecule

Amine molecule attached to anthraquinone


24/63

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


25/63

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)


26/63


27/63

Metal coordination improves helical twisting

power Reduced conformation flexibility

Chiral propeller shape of metal complex


28/63

TADDOL and Dioxolane Dopants

Tartaric acid

derivative


29/63

TADDOL and Dioxolane Dopants


30/63

Methyl groups on 31 and silane bridge on 32

prevent intramolecular hydrogen bondingbetween the diols


31/63

Helical twisting poweris greater in guest-hostcombinations wherearyl core distances are

ofsimilar length

-stacking interactionsbetween dopant and

biphenyl LC core


32/63

- interactions

Non-covalent intermolecular interaction

Arise from a combination of van der Waals

and electrostatic forces Forms supramolecular structure

- interactions are still actively debated


33/63

- interactions


34/63

Proposed Dopant-Mesogen Interaction


35/63

Atropisomer Dopants

Atropisomer considered chiral because

rotation around bond is sterically restricted


36/63

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


37/63

Atropisomer Dopants


38/63

Proposed model for chirality transfer between binaphthyl

dopant and liquid crystal mesogen


39/63

Bistable Switchable Dopants

Allows control of supramolecular chirality by

external stimuli

Can change helical: Sign

Pitch length (p)

Photochemical switchable dopants


40/63

Control of supramolecular chirality using

external stimuli


41/63


External Stimuli

pH

Pressure

Electric field

Heat

Magnetic field

Light


42/63


Potential use as molecular switch Molecular equivalent of transistor

Nanotechnology

Light versus electricity Less heat produced

Light more efficient and abundant

Faster data processing


43/63

Optical Switches

Type 1

Dopant switches enantiomeric state by irradiation

with circularly polarized light

Switch between P and M helicities


44/63

Optical Switches

Type 2

Dopant switches between diastereomers

Chirality of dopant is inversed

Dopant states are not enantiomers


45/63

Optical Switches

Type 3

Switch and chiral auxiliary are separate units

Chirality change is not as strong


46/63

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


47/63



48/63


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


49/63

Type 2 dopants: overcrowded alkenes

2-(2,6-dimethylphenyl)-9-(2,3-dihydro-1H-naphtho[2,1-b]thiopyran-1-ylidene)-9H-thioxanthene


50/63

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


51/63

Psuedoequatorial position with high

steric strain


52/63

Flourene Molecular Motor

MBBA could not be used as mesogen because

it absorbs


53/63

Polygonal Surface Structure

polyimide-coated glass surface

Surface topography of polygonal nematic surface


54/63

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


55/63

Thermal isomeration at 20 C

Reaction half-life of 9.9 min in

toluene

P

rotation

1b 1c


56/63

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


57/63

Thermal step 0.22 r.p.m.1b 1c


58/63

X8 speed


59/63


60/63

Type 3 dopants: azobenzene derived

molecular motor

High helical twisting power

allows for color switching

UV light causes 73 to change

helicity


61/63

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


62/63

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


63/63

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

liquid crystal chemistry

Documents