Transition Metal Dithiolene Complexes as Near-IR Dyesfor Liquid Crystal Device Applications
K. L. Marshall, B. Schudel, and I. A. LippaUniversity of RochesterLaboratory for Laser Energetics
SPIE Annual MeetingLiquid Crystals VII
San Diego, CA3–8 August 2003
G5965
The unique physical and optical properties of transitionmetal dithiolenes make them excellent candidatesfor near-IR liquid crystal device applications
• Zerovalent dithiolene core affordshigh solubility in liquid crystal (LC) hostsand the largest wavelength-range ofany near-IR dye class (up to 1600 nm).
• Both thermal stability andphotochemical stability are excellentcompared to organic dyes.
• Positive or negative dichroismcan be produced by changing theligand structure.
• Mesogenic dithiolene complexes preserve both LC host order and device contrastwhen used at high concentrations.
• Nickel ditholenes with chiral ligands have low melting points and can induce bothchirality and optical absorption when added to a nematic LC host.
• Potential applications include optical modulation, switching, nonlinear optics, andsensor protection out to telecom wavelengths (1.5 mm).
Summary
G5966
The development of an ideal dye candidate for LC deviceapplications represents a formidable challengefor synthetic chemists
• High solubility in the host matrix
• Good long-term chemical, thermal, and optical stability
• Low impact on LC order parameter
• Large molar absorptivity
• Low conductivity
• Broad range of lmax
• Large dichroic ratio
G5967
The selection of available dyes for near-IR applications islimited to a handful of candidates in ten chemical “classes”
Transition metal dithiolenes stand out as the most promising and interestingcandidates by virtue of their solubility, stability, and wavelength range.
P. Gregory, High-Technology Applications of Organic Colorants (Plenum Press, New York, 1991).
Species
Cyanine 735–1100
Azulenium 728 —
748–879
Iminium 725–1090
700–845
600–1600 10
748–810 2–3
Phthalocyanines 630–830
Azo 700–900 2–3
Indoanilines 660–800
OrganiccationicOrganiccationicOrganiccationicOrganiccationicOrganiccationic
Dye Class
Organiczerovalent
Organometalliczerovalent
Organic or organometalliczerovalent
Organiczerovalent
Solventslmax Range(nm)
—
—
—
—
—
Pyrilium/Thiapyrilium
Quinones/Anthraquinones
Squarilium/Croconium
Transition metaldithiolenes
Organometallic(zerovalent or anionic)
0.01–0.05(Q-switch 5)
Nonpolar(hexane, toluene)
Polar(acetone, methanol)
Polar(acetone, methanol)
Polar(acetone, methanol)
Polar(acetone, methanol)
Polar(acetone, methanol)
Nonpolar(hexane, toluene)
Nonpolar(hexane, toluene)
Nonpolar(hexane, toluene)
Nonpolar(hexane, toluene)
Solubility in LCHosts (wt%)
G5968
Transition metal dithiolenes have been investigatedfor a wide variety of applications
Passiveabsorbers for
imagingapplications
Q-switchingsaturableabsorbers for lasers
“Redoxswitch” for
aliphaticolefins
Guest-host dyesfor liquid crystal
near-IR electro-opticalapplications
Unimolecularmetals exhibiting
metal-likeconductivity
down to 0.6∞K
Optical limitingand all-optical
switching
• Overlap of electron-rich sulfur orbitals with metal d-orbitals shifts the nearIR lmax to longer wavelengths (up to 1600 nm) and imparts exceptionalthermal and photochemical stability.
• Zerovalent oxidation state of the central metal allows high solubility innonpolar hosts—including liquid crystals.
• Change in oxidation state of the metal both eliminates the near IR lmaxand strengthens visible region absorption.
• The near-IR lmax shifts by 20 to 30 nm depending on the central metal.
G4948
The availability of metal d-orbitals is responsiblefor the unique physical and optical propertiesin transition metal dithiolene complexes
S
S
H
H
S
S
H
HM
Electron-poor
Electron-rich
Pt Ni Pd
+
–
G4947
Transition metal dithiolenes have a number of importantproperties for near-IR LC device applications
• High solubility in LC hosts (up to 10% wt/wt)†
• Can be mesogenic – preserves the LC order parameter at higherconcentrations
• Alkylthio terminal groups impart higher solubility and lower melting pointsto their parent complexes than do phenyl-containing substituents.
• Complexes containing chiral ligands have low melting points and caninduce both chirality and optical absorption when added to a nematic host.
† K. L. Marshall and S. D. Jacobs, “Near-Infrared Dichroism of a Mesogenic Transition MetalComplex and Its Solubility in Nematic Hosts,” Mol. Cryst. Liq. Cryst. 159, 181–196 (1988).
Ph =
where X = HPhPhRSR
and R = CnH2n + 1OCnH2n + 1N(CH3)2
XS
SX
M
S
S
X
XM = Ni, Pd, Pt
G4954
Symmetrical zerovalent alkythio-substituted transitionmetal dithiolenes are synthesized in six steps
Yields: 27%–68% (simple—SR groups); 5%–10% (bulky chiral—SR groups)
NiCl2 • 6H2OMeOHAr
RBrCH3CN
Reflux
SS
S
S
SS
S
S– Na+
S– Na+S
S4 CS2 + 4 Na
RS
RS
SR
SR
RS
RS
SR
SR
Na+ S–
Na+ S–
NaOMeMeOH
ArNBu4BrReflux
SR
SRS
S– Na+
+ SS– Na+
DMF
Ar
ZnCl2Et4NBr
S
S
2[NBu4+]
FeCl3 or l2CH3COCH3
O2
2–
S
SNi
S
S
S
SZn
S
S
S
SNi
S
S
SR
SR
G5969
The solubility of nickel dithiolene complexesin Merck E7 was evaluated experimentallyand by computational methods
Experimental
• Guest-host mixtures prepared at concentrations ranging from0.3–1.0 wt% and filtered (0.5-mm membrane pore size).
• Samples were checked microscopically for signs of dye precipitation.
Computational
• Semi-empirical structural energy minimization calculations wereconducted using Spartan 5.0 (Wavefunction, Inc.).
• Ab initio calculations of nickel dithiolene solvation energies in severalhosts† using Jaguar 3.5 (Schroedinger, Inc.) were also attempted.
• All calculations were done using a 32-processor SGI Origin 2000 server.
† “Design and Synthesis of Near-Infrared Absorbing Dyes for the LiquidCrystal Point Diffraction Interferometer (LCPDI),” Laboratory for Laser EnergeticsLLE Review 81, 37–47, NIS document No. DOE/SF/19460-335 (1999).
G5970
Computational chemistry software is not sufficientlyadvanced to conduct solubility calculations in eitheranisotropic hosts or host mixtures
• Fallback strategy: use single-component, isotropic host with molecularstructure similar to E7 to establish qualitative solubility trends (CB15).
• Jaguar uses the host dielectric constant, molecular weight, and densityto calculate a “probe radius” using its own Poisson–Boltzman solver.
• Assumes the solvent host molecules are rigid and spherical.
Cyclohexane CB15
Calculated probe radius for CB15 (3.647) exceedsJaguar’s maximum limit of 3.1!
Terminal Melting lmax in(X) Group Point (∞C) E7 (nm) Cyclohexane CB-15 Merck E7
Solubility Solubility Solubilitylimit DGsolv limit DGsolv limit(wt%) (wt%) (wt%)
–SC8H17 73 1020 0.5% 6.9435 1.0% –7.6725 0.50%
–SC7H15 81.5 1020 0.5% 6.2764 0.5%–1% –7.7164 0.50%
–SC4H9 101 1020 0.3%–0.5% 3.9248 0.5%–1% –7.7190 0.50%
–PhC4H9 228–230 910 0.05% 3.2985 0.3%–0.5% –14.4373 0.50%
–PhN(CH3)2 280–283 1056 0.3%–0.5% 0.0812 0.3%–0.5% –17.4080 0.05%
–PhOC9H19 184–189 970 0.025% 5.2212 0.1%–0.3% –21.6724 0.30%
–PhOC4H9 246–248 970 0.001% 0.9780 0.1%–0.3% –21.6950 0.30%
G5971
Alkylthio-substituted nickel dithiolenes have both lowermelting points and better general solubility than theirphenyl-containing counterparts
Larger positive value of DGsolv indicates better solubility.
G5972
Either positive or negative dichroism can be obtained froma common core by employing different terminal groups
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Ab
sorb
ance
(O
.D. u
nit
s)
“9-dye” mixture in E7Total dye concentration: 3.2%10-mm path length
Field off
Field on
H
Ni
H
H9C4
C4H9
BisBuSDNi
117∞C 178.6∞CC N I
S
S S
S
X
X
Ni
X
X
S
S
S
S
whereX = SR
Thioalkyl-substitutednickel dithiolene
3.0
2.4
1.8
1.2
0.6
0.0
Ab
sorb
ance
(O
D u
nit
s)
400 500 600 700 800 900 1000 1100 1200
Wavelength (nm)
No fieldapplied
Fieldapplied
1% BisBuSDNi in K-1524-mm path length
G5973
Bulky chiral terminal groups dramatically lower the meltingpoints of the resulting nickel dithiolene complexes
A new class of “liquid chiral dyes” for the near IR!
X-group Spacer(n)
MP(∞C)
lmax (nm)(a) E7(b) Toluene
Helicalpitch
length(E7, mm)
3
6
9
3
6
9
3
6
9
3
6
9
–46
–52
< 25
< 25
–46
–52
–45
–46
–45
–49
< 25
–50
–43
108.5 1020a
30
80
1020a
1009b
1009b
1009b
1008b
1008b
1008b
1005b
1005b
1010b
1011b
1011b
1011b
n[H2C]
S
SNi
S
SS
n[H2C]X
[CH2]n X
X
X
S
S
S[CH2]n
0
0
O
OH
O
OO
OO
H
CH3CH3
OO
OPh
O
H
G5939
Enantiomerically enriched nickel dithiolenes imparttheir chirality to a nematic LC host when addedas a dopant
• Samples viewed under polarizing microscope, crossed polarizers,100¥ magnification
E7 nematic host 0.5 wt% S2MeBu dye in E7
80 mm
G5039
Transition metal dithiolenes find application in a liquid crystalpoint-diffraction interferometer (LCPDI) for the 1-mm region
• Addition of a dye to the LC host is required to match the intensityof the sample and reference portions of the incident beam.
• Field-induced dichroism is undesirable; mixtures of positive andnegative dichroic dyes are required to maintain constant contrast.
C. Mercer and K. Creath, “Liquid-Crystal Point-Diffraction Interferometerfor Wave-Front Measurements,” Appl. Opt. 35, 1633 (1996).
LC guest–host mixture
Glass substrate
Reference portion
Phase-shiftedportion
Glass microsphere
ITO conductivecoating Cell spacer
Alignmentlayer
G5974
Chiral transition metal dithiolenes provide two modes oftunability when added as dopants to a nematic LC host
• Device concept for sensor protection from both fixed andfrequency-agile laser threats.
Selective reflection band tunableby electric field or temperature
Electronic absorption bandtunable by synthesis
200 400 600 1200 1400 1600800 1000
nm
RH
V2
LHl2
l1
l2 = np
RH
V1
LH
LH
LH chiraldye LC cell
BroadbandRH circular
polarizer
l2
l1
l2 π np
G5975
Transition metal dithiolenes offer exciting new researchopportunities in both materials chemistryand device applications
• Their high solubility, mesomorphic capability, excellent thermal andphotochemical stability, structure-dependent dichroism, and broadwavelength range are valuable attributes for near-IR LC device applications.
• Addition of enantiomerically enriched terminal groups to the dithiolenecore gives rise to a new family of “liquid chiral dyes” with a host ofapplication possibilities (nonlinear optics, sensor protection).
Research Topics
• Design and develop dithiolene dyes with both a lmax near 1.5 mm andgood LC host solubility for telecom and sensor protection applications.
• Prepare complexes based on other transition metals.
• Determine structure–property relationships in chiral dithiolene metalcomplexes to maximize helical twisting power for applications.
• Modify chiral dithiolene structures to induce liquid crystalline behavior.
Can transition metal dithiolenes show ferroelectric LC phases?
Conclusions
G5976
Acknowledgments
S. M. Corsello
S. Kinsella
A. Ayub
M. Moore
K. Bussey
E. Wolcott
