1
Self-assembly for high NLO susceptibilities
William Thomas*, Benjamin Olbricht†
*Department of Physics and †Department of Chemistry, University of
Washington, Seattle, Washington
(7 March 2006)
The concept of self-assembly (SA) is explored for maximizing the second order
macroscopic nonlinearity of both inorganic and organic non-linear optic (NLO)
chromophores in terms of the relevant intermolecular forces driving the SA
mechanism. The importance of noncentrosymmetic order as dipole alignment in
2nd order NLO is reviewed, and a variety of methods for manipulating this model
via SA from the literature are highlighted and potentially viable SA routes are
proposed. Highlighted are bulk formation mechanisms from poled and spin
coated films and layer-by-layer technique to afford practical properties in devices.
Introduction
The first observation of non-linear optical process was published by J. Kerr in
1875 as the dependence of incident light and induced electric field in a sample of carbon
disulfide. [J. Kerr, Phil. Mag. 50(4), 337 (1875).] Franken et al. reopened the field in
1961 with the advent of the laser. This phenomenon was the frequency doubling and
spatial shift of an incident ruby laser beam on a photographic plate that coincided with
generation of the second harmonic of the laser’s fundamental output frequency and the
spatial division of the beam via interaction with a birefringent medium into incident and
extraordinary beams, also known as the electro-optic (EO) effect, or Kerr effect for its
original namesake. [P.A. Franken, A.E. Hill, C.W. Peters and G. Weinreich, Phys. Rev.
Lett. 7, 118 (1961).] Since that time the field of non-linear optics has been of interest to
physicists and chemists alike and more recently has achieved unprecedented advances
under the umbrella of photonics—that is, as a method of processing and storage of
information in the form of light.
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Nonlinear materials are fundamentally characterized by their non-steady-state
interaction with incident electromagnetic (EM) radiation. When the electric field
component of an EM wave interacts with the electric structure of a nonlinear media a
nascent light wave is produced that has a perturbed phase or wavelength. The oscillatory
behavior of EM waves is described by the asymmetry of the polarization oscillation about
the nodal axis, where all interactions of polarization and electric fields satisfy Maxwell’s
equations. In order to understand how this effect applies to NLO films we can consider a
media acted on by an EM wave. There is a polarization induced into the media by the EM
field, E (Ulman, 339). The polarization induced, P, is:
P = (1) E + (2) E E+ (3) E E E + ……..
(Eq. 1)
Source: Ubachs, W. Nonlinear Optics, Lecture Notes. Laser centre Vrije Universiteit Amsterdam,2001, 4.
Where, (1) is the linear susceptibility tensor and (2) and(3) are the higher order
nonlinear susceptibilities tensors in the power series expansion of the electric field. If the
nonlinear susceptibility tensors are nonzero the material is considered nonlinear (Shah,
70). Moreover, an important condition to note is the relation of induced polarization to
inherent properties of the material being manipulated, foremost of which is the symmetry
of the material’s lattice. In centrosymmetric lattices, i.e. lattices that contain inversion
symmetry, an important distinction from the above summation is required. Use of the
inversion symmetry operator, Î, on the induced polarization makes this distinction
obvious:
Î E = -E
(Eq. 2)
Î P = -P = -χ(1) . E + χ(2) . E . E - χ(3) . E . E . E ± Ö
Pcentrosymmetric(n)= χ(1) . E + χ(3) . E . E . E + Ö(2n-1)
(Eq. 3)
(Eq. 4)
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Thus second and successive even-ordered nonlinear optical properties are only
observed in noncentrosymmetric lattices. Furthermore, the more noncentrosymmetric the
medium is, the more powerful the contribution of the even-ordered NLO terms. The
second important material-dependant features neglected in the first macroscopic
summation are the microscopic tensors associated with the electronic structure of the
molecule, given by:
P = . E + . E . E + . E . E . E + Ö
(Eq. 5)
Where P and E remain analogous to the macroscopic relation above, and is the
so-called linear polarizability for obvious reasons,is the second-order molecular
polarizability (first-order molecular hyperpolarizability), andis the third-order
molecular polarizability (second-order molecular hyperpolarizability). The physical
interpretation of these terms arises from their relation to P and E, which is they signify
the ability of the electrons to interact with incident electromagnetic radiation according to
the Lorentz ‘push-pull’ model where electrons are delocalized by polarized light and
relaxed via a restoring force with the removal of the incident perturbation. The chemical
structure of push-pull chromophores elucidates this concept.
Before the introduction of organic push-pull chromophores, it is necessary to
discuss the motivation for organic materials as NLO chromophores over the current
inorganic crystalline technology. To avoid repetition, a chart below highlights the
strengths and weaknesses of these materials. It is important, however, to note a few
details. Firstly, very large scale integration (VSLI) has not been realized for devices
based on organic materials, thus a production cost per unit has not been established and
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estimates vary drastically: in nearly every case below 6000USD, however. The phase
relaxation time of π-electrons in organic materials is on the order of femtoseconds,
producing ultrafast modulation in devices. It is also important to highlight the superiority
of organics in EO coefficients and their inferiority for thermal stability. Lastly, it is
important to note that consensus in the field indicates that inorganic NLO materials are
reaching a limit, with doping being explored for miniscule gains. Organic chromophores,
on the other hand, have nearly unlimited design and engineering possibilities.
Criteria
Cost/unit (USD)
Bandwidth (GHz)
Refractive index (n)
EO Coefficient (pm/V)
Operating Voltage (V)
Optical Loss (dB/cm)
Thermal Stability (deg C)
Dielectric Constant
LiNbO3
Organic
Note:
6000
30
2.15-2.22
31
6
~0.2
~1500
<<6000
200
1.5-1.7
~400
1
0.7
200+
no VSLI/organics
29-85
2.5-4
@ 1.3 microns
@ 100 kHz
Highlights of Organic Chromophores vs. LiNbO3 crystals
Another crucial topic in NLO is the devices and how they operate. Many
platforms for utilizing NLO materials have been developed, including micro-ring
resonators (especially for wave division multiplexing applications), frequency-doubling
crystals for OPO laser systems, phase array radar, and Mach-Zehnder (MZ) type devices.
For the purpose of clarifying the purpose of materials in devices, the latter will be
emphasized. The MZ modulator is simply SiO2 coated Si chip with arms etched via
electron beam lithography. A fiber optic cable couples into the modulator and the light is
divided equally into two arms, both containing a film of NLO material typically
deposited by spin coating. One arm is poled and switched with a digital electrical signal,
which switches the permittivity of that arm to EM radiation. The light then recouples
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with the unbiased arm and produces either coherent or destructive interference,
effectively interconnecting optical and electronic digital signals.
Another feature of 2nd order NLO materials is second harmonic generation (SHG).
SHG and phase matching are characteristic features demonstrated by NLO materials.
Second harmonic generation is when two light waves of frequency and  combine to
form a nascent light wave of frequency  in a nonlinear media as shown in Figure 1.
Figure 1. Second Harmonic Generation.
Source: Ulman, Abraham. An Introduction to Ultrathin Organic Films From
Langmuir-Blodgett to Self-Assembly. San Diego: Academic Press, 1991, 341.
Phase matching is the characteristic of the nonlinear media to match the phases of the
initial and second harmonic wave. These characteristics and others make the nonlinear
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materials very useful for different electronic and photonic devices such as repeaters
(Ulman, 342). The usefulness of these characteristics demands inexpensive and reliable
techniques for the generation of the materials that exhibit them.
Returning to the concept of push-pull chromophores, such an EO material is
simply a molecule with an electron-rich donor, typically an amine or other nitrogen-based
functional group, an electron-deficient electron acceptor, typically a heavily
heteroatomically substituted cyclic hydrocarbon. These moieties are separated by a πconjugated bridge, the motive for which is to facilitate efficient internal charge transfer
(ICT) between the donor and acceptor.
Structure of a typical standard organic 2nd order NLO chromophore: FTC.
π-conjugation creates a state of intrinsically low bandgap due to the narrowing density of
states in the π molecular orbitals. The “conduction” band forming creates an efficient
ICT mechanism. These structural features are combined producing a molecule with a
highest occupied molecular orbital (HOMO) consisting of electron density localized in
the donor and a lowest unoccupied molecular orbital (LUMO) whose electron density is
localized at the acceptor; this is the fundamental concept of the push-pull chromophore
model.
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Density functional theory computation results of a typical push-pull chromophore
showing the electron density of the HOMO (left) and LUMO (right).
[http://stc-mditr.org/outputs/annual_report2005/Ch-2.pdf]
Moreover, these molecules produce a strong dipole that is highly polarizable to
favorable satisfy the microscopic relation of induced polarization and electric field—
better stated, chromophores designed by this concept have a large dipole moment and I
high first-order molecular hyperpolarizability. To satisfy the analogous macroscopic
formulation, these dipoles must be arranged noncentrosymmetrically, that is, “pointing”
in the same direction—otherwise known as acentric ordering. All these terms combined,
the canonical equation resulting is:
(Eq. 6)
whereis the second-order molecular polarizability from above, n is the index of
refraction of the material, cos3(> is a product of the average dipole order and the N is
number density of the material. The vague relation of chi and r is simple best noted as
tensorial, that the effective r of a material is the dominant component the susceptibility
term per the definition of the axis of alignment related to the axis of incident EM
radiation.
As is obvious by now, alignment and order, terms used interchangeably in the
field, are essential to producing highly nonlinear materials, that is, N must be maximized
and  minimized in equation (6) above, signifying a high degree of long-range order.
Macroscopic alignment is conventionally obtained by electrical field poling of the
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materials, either via electrode contact poling or corona poling. In both techniques, the
idea is to heat the molecules to allow enhanced mobilities, specifically rotational
mobility, that the molecules may align to the applied electric field. The lattice is then
cooled down while still under the electric field, thereby reducing the mobility and
phenomenologically producing ordered arrays of chromophores even when the electric
field is removed. This process, however, requires fields typically on the order of MV/cm
and temperatures that can decompose sensitive parts of the chromophore. An ideal
process would involve assembly of aligned lattices without electric fields or elevated
temperatures, such as SA.
Self-assembly by layer
Self-assembly offers many potentially viable options for the inexpensive
and efficient means of creating ultrathin films for use in nonlinear optics. In order to
understand the many self-assembly techniques it is necessary we understand the
components of the self-assembly system. These components include the molecules and
atoms in the system as well as the interactions and forces that are acting between them.
The self-assembly process is driven by the intermolecular interactions between the atoms
and molecules. These interactions have been ordered into three groups: Coulomb
interactions, van der Waals interactions, and short-range repulsions. (Zhang, et al., 7)
The intermolecular interactions due to charged particles are called
Coulomb interactions. These interactions include the ion-permanent dipole interaction
(Eq. 2), ion-ion interaction (Eq. 3), and the permanent dipole-permanent dipole
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interaction (Eq. 4). They can be attractive or repulsive depending on the charge of the
particle. (Zhang, et al., 8)
Source: Zhang, Jin., et al., Self-Assembled Nanostructures. New York: Kluwer Academic/ Plenum Publishers, 2003, 8.
Induced polarization by local molecules into other molecules causes the
interactions known as van der Waals forces. The Debye, Keesom, and the London
interaction are all examples of van der Waals forces. The Debye interaction is a
permanent dipole-induced dipole interaction that stems from free and rotation dipoles
(Eq. 5). The London interaction is an induced dipole-induced dipole interaction and are a
result of shifts in the electron cloud (Eq. 6). The Keesom interaction is a permanent
dipole-permanent dipole interaction and is caused by fixed or average angled dipoles
(Eq. 7). (Zhang, et al., 8)
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Source: Zhang, Jin., et al., Self-Assembled Nanostructures. New York: Kluwer Academic/ Plenum Publishers, 2003, 8.
The short range repulsion (Eq. 8) arises as a result of the Pauli exclusion principle
which states that two fermions can not occupy the same state. For fermions, if the two
individual particle wave functions were equal then by the two particle wave function for
fermions (Eq. 9) , the resultant two particle wave function would be zero (Griffiths, 204).
The short range repulsion increases dramatically with a decrease in separation is usually
summed together with the attractive van der Waals forces in the Lennard-Jones potential
(Eq. 10). Other intermolecular forces are at work in the self-assembly system but these
interactions contribute the most at close distances. The left hand of the Lennard-Jones
potential represents the contribution of the short range repulsive force and the right hand
is from the van der Waals contribution to the interaction. Figure 2 shows graphically the
strength of the interaction energies with respect to the distance of separation between the
particles. (Zhang, et al., 9)
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Source: Zhang, Jin., et al., Self-Assembled Nanostructures. New York: Kluwer Academic/ Plenum Publishers, 2003, 9.
Figure 2: Graphical representation of the strength of the interaction energies as a
function of distance
Source: Zhang, Jin., et al., Self-Assembled Nanostructures. New York: Kluwer Academic/ Plenum Publishers, 2003, 9.
The hydrogen bond is a type of polar covalent bond and is another interaction that
must be considered in the self-assembly process. The hydrogen bond is a directional bond
due the hydrogen atom being a positive charge. When the hydrogen atom is near a
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negatively charged atom there is a dipole-dipole interaction which is attractive (Zhang
11). Another characteristic of the hydrogen bond is its relative weakness compared to
metallic and covalent bonds but its unusual strength compared to the van der Waals
interactions. This adds an element of flexibility to the self-assembly process and can be
helpful in terms of creating the dipolar order we want. There is an issue with the thermal
stability of the hydrogen bond in that the orientation held by the hydrogen bond is often
broken or relaxed when excessive heat is applied into films.
Amphiphiles are a type of molecule that consists of both hydrophobic and
hydrophilic groups. Typical amphiphiles include detergents, dispersive agents for paints,
and emulsifiers. The hydrophobic interaction is a result of non-hydrogen bonding
molecules, such as fluorocarbons and alkanes, coming into contact with water. The
hydrophobic part of the molecule, in order to attain a lower energy state, will reorient
itself so that the hydrophobic element of the molecule will point away from the water
interface. Hydrophilic molecules repel one another in water due to the fact they prefer
contact with water. When hydrophilic molecules are in water they are apt to scatter which
has an effect of disordering the water system (Zhang, 13). These properties make
amphiphiles desirable molecules for the self-assembly process because of their
organizational nature.
Surfactants are molecules that can be described as being cationic, anionic,
zwitterionic, or nonionic. Surfactants have a single hydrophobic tail unit and a least one
hydrophilic head unit. Positively charged head units are considered cationic surfactants
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which are usually made from long chains of ammonium salts or amines. Negatively
charged head units are considered anionic surfactants which are usually made from
sulfonic or carboxylic acid salts. Nonionic surfactants have a neutral head unit while
zwitterionic surfactants have both positively and negatively charged head units (Zhang,
13). These different types of surfactants allow for a multitude of different interactions
that can be used to bond molecules to substrates and each other. These interactions can be
used to arrange the molecules into the preferred noncentrosymmetric polar ordered
needed for NLO phenomena such as in ISAMs.
One thin film deposition technique that enables the generation of NLO materials
is the dip coating process. The dip coating process involves dipping an activated substrate
in to a solution or water subphase with a layer of amphiphilic molecules on the surface.
The solute or amphiphiles thereby bond to the substrate either ionically or covalently
typically with energies of 40-45 kcal/mol (Ulman, 237). Multiple immersions into the
subphase or solution build up successive layers to the film.
There are three regions of film thickness in the dip coating process; the start up
region, the entrainment region and the meniscus region. In the meniscus region there are
several forces acting on the film. Gravitational forces which have the effect of draining
the solution off the substrate. Viscous drag forces which are proportional to solution
viscosity and the withdrawal velocity of the substrate. We can neglect inertial forces if
the withdrawal velocity is slow enough but if the withdrawal is fast then the force is
proportional to the square root of the substrate length, velocity, and solution density and
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viscosity. There is a force in the downward direction due to capillary forces that put
pressure on the convex side of the meniscus. Disjoining pressure also becomes an issue
with sufficiently thin films which is proportional to the inverse cube of the thickness of
the film. (Kim, 8)
Kim, Jae Hoon. Organic Thin Film Deposition from Liquid or Supercritical Carbon Dioxide. Phd Requirement. North Carolina State
University, 2003.
Langmuir-Blodgett (LB) films are an example of the dip coating process that can
be used for the generation of NLO materials. There are X, Y, and Z-type LB films. Ytype LB films are made by first inserting a hydrophilic substrate into a water subphase
covered with a layer of amphiphiles. Upon removal of the substrate, the hydrophilic head
group of the amphiphiles bond to the substrate and then upon immersion a second layer
bonds. This process is not desirable for NLO materials because of the centric order it
produces. Also, Y-type LB films are more stable than X or Y type due to strong
hydrophobic and hydrophilic interactions. X-type and Z-type films are layered onto the
substrate only during removal from or immersion into the subphase. These type films
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produce the noncentrosymmetric order desirable in NLO materials. Due to weak
hydrophobic and hydrophilic interactions of the head and tail groups of the X and Z type
films, they often degenerate into Y type films. Polyamides have been included into some
LB films in order to stabilize the film structure and increase thermal stability. Problems
with the LB films process arise during the insertion and removal of the substrate from the
subphase because a constant layer of amphiphiles on the surface of the subphase must be
maintained in order to ensure an unrelaxed orientation of the amphiphiles. This is an
expensive process and it is easily disturbed by system contaminants and conditions.
(Neyman,
22)
Self-assembled monolayers are another type of thin film that can be generated by
depositing surfactants onto a substrate by dip coating. Self-assembling surfactants are a
type of amphiphile that have three groups; a head group which typically binds to the
surface of the substrate ionically or covalently with bonding energies around ~ 40-45
kcal/mol, alkyl chains with interchain interaction energies of about 10 kcal/mol, and a
surface group with typical energies around a few kT’s. Where k is the Boltzmann
constant and T is the absolute temperature. Figure 3 is a schematic representation of the
interactions of the surfactant. In some chemical designs, polar bulky groups were
attached to the surfactants alkyl chains. In these cases long range electrostatic interactions
become important due to the existence of two local energy minima. Long range surfactant
order, as successive monolayers were deposited on the film, was shown to degenerate
rapidly with increasing distance from the substrate (Ulman, 237).
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Figure 3
Spin coating is another process that enables the generation of thin films usable for
nonlinear optic materials. This is a process by which an excess amount of solution on a
substrate is spun at a high rate of speed about an axis whereby the centrifugal force
spreads the solution over the substrate. Stage one of the spin coat process is the
deposition of the solution on to substrate surface. In stage two the substrate is rotated to
speed, called spin-up, where viscous shear drag forces and forces due to rotational
acceleration reach equilibrium. Stage three is characterized by a steadily decreasing layer
thickness, called spin-off, where if the fluid flow can be described as Newtonian and is at
some time uniformly thick it will be uniform at anytime thereafter. In stage 4 the
thickness of the film reaches a point where solvent evaporation becomes the dominant
process further thinning the film. Thermal heat treatment is then applied in many
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applications to relieve radial symmetry and remove any remaining solvent. Figure.4 give
a graphically representation of the spin coat process.
Figure 4. Spin Coat Process
One self-assembly technique to generate NLOs that utilizes the spin coat process
is asymmetrically substituted polydiacetylenes. Upon spin coating poly(BPOD)
polymer chains on a polydiacetylene backbone were found to order themselves in
acentric polar order. The proposed chemical structure of the hydrogen bonded
poly(BPOD) is shown in Figure 5. (Tripathy)
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Figure 5
The intermolecular hydrogen bonding of the urethane moiety to the side groups of
the polymer chains appears to be the cause of this order. Even in the absence of
classical second order NLO chromophores and without the help electric field poling
substantial SHG activity was observed. At high temperatures relaxation of the polar
order occurs and appears to be due to the disruption of the intermolecular hydrogen
bonds. Figure 6 is a graphically representation of second harmonic intensity as a
function of temperature. (Tripathy)
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Figure 6
Self-assembly by molecular engineering
Alex Jen’s research group has pioneered one example of synthesizing molecular
architectures to tailor intermolecular forces affording long-range ordered systems. This
is done by functionalizing a conventional push-pull chromophore at the bridge and donor
with dendrons containing two benzene rings. The dendron at the donor contains a pair of
phenols and the dendron at the bridge contains a pair of perfluorinated benzenes; one
example structure is show below.
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Molecular structure of an HDFD functionalized NLO chromophore.
The mechanism of “self-assembly” reported suggests that the electron rich
fluorine atoms shift the electron density radially outward from the center of the ring in
one dendron type (FD), and the phenols exhibit electron density localized about the ring
itself in the other dendron type (HD). The perfluorinated benzenes then are approximated
as resembling rings or “sockets,” and the normal phenol moiety represents a spherical
shape or “ball.” Thus, electrostatically, the steric interactions of these benzenes find an
energy minimum as the “ball and socket” self-assembles (HDFD). This is due, according
to the literature, to the “face-to-face” stacking of phenol moieties with complementary
quadruple moments and van der Waals attractions of the differently substituted phenols.
Upon assembly at elevated temperatures and cooling, the chromophores tethered to these
assembled dendrons are restricted in mobility primary tailored to produce materials with
long-range noncentrosymmetry, as shown in the schematic below.
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Schematic of macroscopic alignment due to HDFD assembly.
This example is the closest realization to true self-assembly in the literature.
Firstly, the mechanism uses rationally tailored weak forces to align molecular dipoles.
Secondly, the mechanism uses a favorable balance of attractive and repulsive forces to
produce this order. Although the electrostatics at work here do not have a clear meaning
in the tense of “attractive and repulsive” it is obvious that the steric hindrance of the
moieties is at a medium when the chromophores can rotate to repel steric bulk of the FD
dendrons as a repulsive force, and align when the forces are balanced with the HDFD
ensemble producing an attractive force. Because the dendrons are paired in a 1:1 mole
ratio per chromophore, this same argument suggests the assembled acentric material is at
a global energy minimum.
A number of questions still are left unanswered about this supposed mechanism.
No direct measurements support this claim, such XRD/SAXS data that could confirm this
as the dominating structure of an assembled lattice. Still, the challenge remains to
explain the extraordinary results, being r33 values in excess of 370 pm/V and increased
thermal stability, using traditional explanations of this system. Initially the thought that
comes to mind is resonance enhancement of the EO coefficient. For a long time
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researchers in the field have been hesitant to publish values greater than 100 pm/V in
peer-reviewed sources because of the resonance enhancement problem. Moreover, one
can suppose that these values are reported not because the issue of resonance
enhancement has been addressed, but because one can phenomenologically attribute the
mechanism to be self-assembly. On the other hand, the EO coefficients of an organic
glass of the non-dendritic active chromophore are higher although the field applied to
pole the material was also higher. Additionally, it is important to note that at a lower
weight percent (decrease in N, the number density) the poling slope is higher for the
dendritic chromophores. This, combined with a grocery list of attractive forces including
van der Waals, quadruple moments, and π-π stacking of the phenols and inner dendron
benzenes, leads to a modestly convincing mechanism for assembly. Not surprisingly, the
perfluorinated phenol is a conventional way to restrict mobilities and also the higher
molecular weight of this material due to the dendrons will increase its viscosity and thus
producing higher quality films, which is crucial to obtaining optimum Teng-man
ellipsometry measurements.
Another recent development in materials consideration in the field of organic 2nd
order NLO materials is the discotic mesomorphic chromophores of Larry Dalton et al.
[Bhatambrekar, N., Hammond, S., Sinness, J., Clot, O., Rommel, H., Chen, A., Robinson,
B., Jen, Alex K.Y., Dalton, L.R. A novel approach to achieve higher order using pseudo
discotic chromophores in electro-optic materials and devices. Proc. Of SPIE. 5724, 322327. (2005).] The primary motivation for this work was the concept of centric ordering of
chromophore dipoles as the primary mode of degradation of EO materials. The Keesom
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potential indicates that the attractive force of dipoles is related to their distance by a
cubed inverse power law relationship. Monte Carlo statistical mechanics simulations [J.
Phys. Chem. B; 108(25); 8659-8667. (2004).] have indicated that for a specific
chromophore, such as the structure below,
the dipole attractive force of the Keesom potential is rendered ineffective at a distance of
1.4 times the length of the chromophore itself, placing this distance in the nanoscale
regime, typically about 4 nm. The synthetic challenge was then to produce a molecule
with equatorial (bridge functionalized) steric bulk to prevent centric ordering.
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Disk-like acentric stacking of bridge-functionalized NLO chromophores
The reason the potential to describe this system as self-assembled is the tradeoff
of side-to-side (centric) dipole attractions for head-to-tail interactions. This combination
of forces fosters long-range noncentrosymmetric order because of the head-to-tail
interactions might cause induce columnar stacking. Moreover, the complex and poorly
understood rheology of these modified mesomorphic discotic systems produces many
complications in the poling process and introduces a wide variety of relaxations.
Fundamentally limiting was first the glass transition temperature, Tg, for the
material being below room temperature, making the traditional poling process impossible
without retooling. As more proprietary structures were synthesized, the Tg for the
materials increased along with the optimum poling temperatures as observed via in situ
Teng-Man simple reflectivity measurements. A thorough comparison of the glass
transitions measured via differential scanning calorimetry (DSC) and the optimum poling
temperature has not yet been completed for these systems. The literature [Gray, T.,
Overney, René M., et al. Applied Physics Letters 86, 2119081-3. (2005).] suggests the
primary mode of EO activity degradation is the formation of centric dipole aggregates. If
the mobilities of the dendrons are largely equivalent to the mobility of the molecule as a
whole there should be much less loss of EO activity at the Tg due to aggregation and the
extra mobility granted by thermal relaxations can then be used to enhance order without
detrimental effects. Although this sounds promising, however, it is important to note that
the higher temperatures in the poling process also increase chromophore damage,
Thomas, Olbricht
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photobleaching, electrolysis, charge injection, ITO/Au migration, and other processspecific modes of decay in the material.
To understand the thermal properties of such a system more sensitive and
reproducible techniques than the conventional DSC and in situ Teng-Man simple
reflection ellipsometry are required. A nanorheological approach to study the relaxations
and probe intermolecular interactions would greatly aide the efforts in this project.
Conclusion
The field of NLO has been reviewed in the context of self-assembly to afford high
long-range noncentrosymmetric lattice, thereby enhancing the N <cos3(θ)> and the
electro-optic coefficient. Self-assembly mechanisms include layer-by-layer techniques
such as dip coating and langmuir-blodgett film preparation, and proposed molecular
engineering possibilities such as the HDFD and discotic chromophore assembly
mechanisms, which largely have yet to undergo peer review. In conclusion, more
fundamental scientific data is needed to understand the assembly mechanisms and better
molecular engineering is needed to refine them.
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References
1.
Ubachs, W. Nonlinear Optics, Lecture Notes. Laser centre Vrije Universiteit Amsterdam,2001, 4.
2.
Ulman, Abraham. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly. San
Diego: Academic Press, 1991, 341.
3.
Zhang, Jin., et al., Self-Assembled Nanostructures. New York: Kluwer Academic/ Plenum Publishers, 2003, 8.
4.
Kim, Jae Hoon. Organic Thin Film Deposition from Liquid or Supercritical Carbon Dioxide. Phd Requirement. North
Carolina State University, 2003.
5.
Neyman, Patrick J. Nonlinear Optical Properties and Structural Characteristics of Ionically Self-Assembled
Nanoscale Polymer Films Influenced by Ionic Concentration and Incorporation of Monomer Chromophores. Thesis,
Virginia Polytechnic Institute and State University, 2002.
6.
Tripathy, S. K., et al. Self Assembled Asymmetrically Substituted Polydiacetylene as a Novel Second Order
NLO Material. University of Massachusetts, 1995.
Thomas, Olbricht