Aromatic rings with rotated adjacent chromophores

A compound having nonlinear optical properties of the general formula: EQU Ar.sup.1 CR.sup.1 .dbd.CR.sup.2 (Ar.sup.2 CR.sup.3 .dbd.CR.sup.4).sub.n Ar.sup.3 (I) wherein, n represents a whole number; R.sup.1, R.sup.1, R.sup.3 and R.sup.4 groups are independently selected from H, branched aliphatic, straight chain aliphatic, branched silane or straight chain silane; Ar.sup.1 and Ar.sup.3 are aromatic radicals having a substituent in at least one position adjacent to a site of attachment of the C.dbd.C double bond and having at least one electron withdrawing or electron donating substituent in at least one of the remaining positions; Ar.sup.2 is an aromatic radical having a substituent in at least one position adjacent to a site of attachment of the C.dbd.C double bond; with the proviso that the substituent in at least one position adjacent to the site of attachment of the C.dbd.C double bond and R.sup.1, R.sup.2, R.sup.3 and R.sup.4 groups are sufficiently large to cause Ar.sup.1, Ar.sup.2 and Ar.sup.3 to rotate approximately perpendicular to a plane containing each adjacent C.dbd.C double bond and its substituents and a nonlinear optical device including a compound of the general formula (I).

FIELD OF THE INVENTION 
The present invention relates to aromatic rings with rotated adjacent 
chromophores. More particularly, the present invention relates to aromatic 
rings with adjacent chromophores rotated out of conjugation to provide 
unique nonlinear optical properties. 
BACKGROUND OF THE INVENTION 
In the presence of an electric field a molecule's dipole moment can be 
described by the following expansion: 
EQU .mu.=.mu..sub.0 +.alpha.E+.beta.EE+.gamma.EEE+ 
where .mu. is the induced dipole moment plus .mu..sub.0, the permanent 
dipole moment of the compound; .alpha., .beta., and .gamma. are the 
linear, second order and third order polarizabilities, respectively; and E 
is the applied electric field. The nonlinear response is reflected in the 
magnitude of .beta. and .gamma., etc. To describe an ensemble of molecules 
such as a crystal, the macroscopic relationship for the polarization can 
be described as the following expansion: 
EQU P=P.sub.0 +.chi..sup.(1) E+.chi..sup.(2) EE+.chi..sup.(3) EEE+ 
where P is the induced polarization plus P.sub.0, the permanent 
polarization; and .chi..sup.(1), .chi..sup.(2) and .chi..sup.(3) are the 
linear, second order and third order susceptibility, respectively. Second 
order nonlinear phenomena such as second harmonic generation, sum and 
difference frequency generation, parametric processes and electro-optical 
effects all arise from the .chi..sup.(2) term. For crystals of a material 
to a large .chi..sup.(2), the molecules making up the material should 
possess both a large .beta. and crystallize in a noncentrosymmetric 
structure. Third order nonlinear phenomena, such as those useful in 
photonics, arise from the .chi..sup.(3) term. To have a large 
.chi..sup.(3) a molecule should possess a large .gamma.. 
For a more detailed discussion of nonlinear optics, reference is made to 
U.S. Pat. Nos. 4,807,968; 4,887,889 and 4757,130, incorporated herein by 
reference. 
Materials with new, novel nonlinear optical properties are continually 
being sought in order to invent new devices. For example, inorganic 
nonlinear optical materials have been developed into a variety of useful 
devices such as frequency doublers that are used in laser systems to 
generate additional frequencies of light and as electro-optical switches. 
Organic materials are known to possess superior nonlinear optical 
properties but the development of suitable useful applications has been 
limited by some of the other properties of the materials. For example, 
absorption of light at the frequency of frequency doubled radiation, low 
thermal stability and poor chemical stability. 
Some of the classes of organic materials that have been explored as 
nonlinear optical materials are typified by a conjugated .pi. system 
supporting an electron withdrawing and an electron donating group. They 
function by possessing a significant transfer of electron density when 
light is absorbed, a charge transfer, from the electron rich to electron 
poor center through the intervention of the conjugated .pi. system. 
Another class is typified by very long symmetrical conjugated .pi. 
systems, for example poly(diacetylenes). 
It will be appreciated from the foregoing that there is still a significant 
need for organic materials with fundamentally new structural motifs 
possessing nonlinear optical properties. Accordingly, it is an object of 
the present invention to provide novel compounds having nonlinear optical 
properties. It is a further object of the present invention to provide 
optical devices having a nonlinear optical component comprising a 
transparent solid medium of a compound having nonlinear optical 
properties. 
SUMMARY OF THE INVENTION 
Briefly, according to the present invention there is provided a compound 
having nonlinear optical properties of the general formula: 
EQU Ar.sup.1 CR.sup.1 .dbd.CR.sup.2 (Ar.sup.2 CR.sup.3.dbd.CR.sup.4).sub.n 
Ar.sup.3 (I) 
wherein, n represents a whole number; R.sup.1, R.sup.2, R.sup.3 and R.sup.4 
groups are independently selected from H, branched aliphatic, straight 
chain aliphatic, branched silane or straight chain silane; Ar.sup.1 and 
Ar.sup.3 are aromatic radicals having a substituent independently selected 
from H, branched aliphatic, straight chain aliphatic, branched silane or 
straight chain silane in at least one position adjacent to a site of 
attachment of the C.dbd.C double bond and having at least one electron 
withdrawing or electron donating substituent in at least one of the 
remaining positions; Ar.sup.2 is an aromatic radical having a substituent 
independently selected from H, branched aliphatic, straight chain 
aliphatic, branched silane or straight chain silane in at least one 
position adjacent to a site of attachment of the C.dbd.C double bond; with 
the proviso that the substituent in at least one position adjacent to the 
site of attachment of the C.dbd.C double bond and R.sup.1, R.sup.2, 
R.sup.3 and R.sup.4 groups are sufficiently large to cause Ar.sup.1, 
Ar.sup.2 and Ar.sup.3 to rotate approximately perpendicular to a plane 
containing each adjacent C.dbd.C double bond and its substituents. 
The present invention also includes a nonlinear optical device comprising a 
nonlinear optical element, a source of coherent radiation, and means for 
directing the radiation into the element. The nonlinear optical element 
includes a crystalline solid or poled solid containing a compound of the 
general formula: 
EQU Ar.sup.1 CR.sup.1 .dbd.CR.sup.2 (Ar.sup.2 CR.sup.3 .dbd.CR.sup.4).sub.n 
Ar.sup.3 (I) 
wherein, n represents a whole number; R.sup.l, R.sup.2, R.sup.3 and R.sup.4 
groups are independently selected from H, branched aliphatic, straight 
chain aliphatic, branched silane or straight chain silane; Ar.sup.1 and 
Ar.sup.3 are aromatic radicals having a substituent independently selected 
from H, branched aliphatic, straight chain aliphatic, branched silane or 
straight chain silane in at least one position adjacent to a site of 
attachment of the C.dbd.C double bond and having at least one electron 
withdrawing or electron donating substituent in at least one of the 
remaining positions; Ar.sup.2 is an aromatic radical having a substituent 
independently selected from H, branched aliphatic, straight chain 
aliphatic, branched silane or straight chain silane in at least one 
position adjacent to a site of attachment of the C.dbd.C double bond; with 
the proviso that the substituent in at least one position adjacent to the 
site of attachment of the C.dbd.C double bond and R.sup.1, R.sup.2, 
R.sup.3 and R.sup.4 groups are sufficiently large to cause Ar.sup.1, 
Ar.sup.2 and Ar.sup.3 to rotate approximately perpendicular to a plane 
containing each adjacent C.dbd.C double bond and its substituents. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention is directed to compounds having nonlinear optical 
properties represented by the general formula: 
EQU Ar.sup.1 CR.sup.1 .dbd.CR.sup.2 (Ar.sup.2 CR.sup.3 .dbd.CR.sup.4).sub.n 
Ar.sup.3 (I) 
wherein, n represents a whole number. 
In the above general formula, Ar.sup.1 and Ar.sup.3 are aromatic radicals 
having a substituent independently selected from H, branched aliphatic, 
straight chain aliphatic, branched silane or straight chain silane group 
in at least one position adjacent to a site of attachment of the C.dbd.C 
double bond and having at least one electron withdrawing or electron 
donating substituent in at least one of the remaining positions. Ar.sup.2 
is an aromatic radical having a substituent independently selected from H, 
branched aliphatic, straight chain aliphatic, branched silane or straight 
chain silane group in at least one position adjacent to a site of 
attachment of the C.dbd.C double bond. 
In a preferred embodiment, Ar.sup.1, Ar.sup.2 and Ar.sup.3 are 
independently selected from aromatic radicals and heterocyclic aromatic 
radicals. The aromatic radicals include radicals of benzene, naphthalene, 
anthracene and biphenyl. The heterocyclic aromatic radicals contain 
nitrogen, oxygen, phosphorous, sulfur or selenium. Examples include 
radicals of heterocyclic ethers and derivatives thereof such as furan, 
thiophene, heterocyclic amines and derivatives thereof such as pyridine 
and quinoline. 
In accordance with the present invention, the substituent in at least one 
position adjacent to the site of attachment of the C.dbd.C double bond and 
R.sup.1, R.sup.2, R.sup.3 and R.sup.4 groups are sufficiently large to 
cause the aromatic radicals Ar.sup.1, Ar.sup.2 and Ar.sup.3 to rotate 
approximately perpendicular to a plane containing each adjacent C.dbd.C 
double bond and its substituents. 
The substituents of the aromatic radical Ar.sup.2 and on the positions 
adjacent to the site of attachment of the C.dbd.C double bond on the 
aromatic radicals Ar.sup.1 and Ar.sup.3 may be independently selected from 
H, branched aliphatic, straight chain aliphatic, branched silane or 
straight chain silane group. In a preferred embodiment, the substituents 
of the aromatic radicals may be independently selected from H, alkyl, 
branched alkyl, silyl, alkylsilyl, dialkylsilyl, and trialkylsilyl. Most 
preferably, the substituents of the aromatic radicals may be independently 
selected from H and tertiary butyl. 
The aromatic radicals Ar.sup.1 and Ar.sup.3 also each include at least one 
electron donating and/or electron withdrawing substituent. 
The term "electron donating" refers to organic substituents which 
contribute electron density to the .pi. electron system when the 
conjugated electronic structure is polarized by the input of 
electromagnetic energy. The electron donating substituent may be 
--O.sup.-, --COO.sup.-, --OR, --CR.sub.3, --OCOR, --NR.sub.2, or --SR 
where R group is any alkyl group or H. 
The term "electron withdrawing" refers to electronegative organic 
substituents which attract electron density from the .pi. electron system 
when the conjugated electronic structure is polarized by the input of 
electromagnetic energy. The electron withdrawing substituent may be 
--NR.sub.3.sup.+, --SR.sub.2.sup.+, --NO.sub.2, --SO.sub.2 R, --SO.sub.3 
R, --CN, --SO.sub.2 Ar, --COOH, --F, --Cl, --Br, --I, --COOR, --COR, 
--CCR, --Ar, --CH.dbd.CR.sub.2, or --C(CN).dbd.C(CN).sub.2 where R group 
is any alkyl group or H. 
The electron donating substituent and electron withdrawing substituent are 
positioned in at least one of the remaining positions of the aromatic 
radicals Ar.sup.1 and Ar.sup.3. The electron withdrawing properties may be 
varied by attaching, for example, acidic or basic substituents on the 
aromatic radicals as well known in the art. In a preferred embodiment, for 
a benzene ring, the electron donating substituent or electron withdrawing 
substituent may be at the meta or para position of the aromatic ring. 
R.sup.1, R.sup.2, R.sup.3 and R.sup.4 groups may be independently selected 
from H, branched aliphatic, straight chain aliphatic, branched silane or 
straight chain silane. In a preferred embodiment, R.sup.1, R.sup.2, 
R.sup.3 and R.sup.4 groups may be independently selected from H, alkyl, 
branched alkyl, silyl, alkylsilyl, dialkylsilyl, and trialkylsilyl. In a 
most preferred embodiment, R.sup.1, R.sup.2, R.sup.3 and R.sup.4 groups 
may be independently selected from H and tertiary butyl. 
The alkyl or silyl substituents of the aromatic radicals Ar.sup.1, Ar.sup.2 
and Ar.sup.3 or of R.sup.1, R.sup.2, R.sup.3 and R.sup.4 groups preferably 
contain from 1-5 carbon atoms to effectively control the rotation of the 
aromatic rings. However, it will be appreciated that it is believed that 
groups of more than 5 carbon atoms will also effectively control the 
rotation of the aromatic rings and improve the solubility properties of 
the compound having the general formula (I) in accordance with the present 
invention. 
In one embodiment of the present invention, wherein n=0, the compound (I) 
having nonlinear optical properties is Z-stilbene with adjacent 
chromophores twisted out of conjugation. The substituted Z-stilbene 
includes cofacial, locked arene rings. The substituted Z-stilbene has a 
system of chromophores which are connected together, but due to out of 
plane twisting, are not conjugated. Large groups, tertiary butyl groups, 
are substituted at R.sup.1 group and at R.sup.2 group to force the arene 
rings out of conjugation as they rotate so that their planes are 
perpendicular to the plane of the central C.dbd.C double bond and its 
substituents. Consequently, the phenyl rings are directly adjacent to a 
.pi. bond but not conjugated to the .pi. bond. Thus, it will be 
appreciated that the three .pi. systems are not formally conjugated. In 
addition, two faces of the phenyl rings are protected from complexation by 
the tertiary butyl groups but the other two faces form a unique pocket or 
cleft. At the base of the pocket, the phenyl rings are forced to interact 
at the ipso positions by a very short 2.65 .ANG. internuclear distance. 
This close contact facilitates charge transfer in suitably substituted 
derivatives. In other words, the system is arranged for a charge transfer 
ring hopping transition through space rather than through the .pi. bond as 
previously understood. The relative electronegativity of the two phenyl 
rings determines the degree of charge transfer. The control of the charge 
transfer activity can be achieved as a function of whether the chromophore 
substituents on the Z-stilbenes have the same or different electron 
donating or withdrawing properties. In a preferred embodiment, the 
chromophore substituents have a para nitro group on one ring and a para 
amino group on the other ring. All other positions on the rings are 
occupied by H. 
The electron withdrawing or donating properties may be made variable by 
attaching, for example, acidic or basic substituents on the Z-stilbene in 
the para position. Substituents in the para positions predominantly effect 
the molecular orbital which interacts most strongly at the ipso positions. 
In one embodiment, under strongly acidic or basic conditions the 
substituents are identical such that there is no charge transfer activity. 
Increasing or decreasing the pH reversibly provides a different electron 
density on the phenyl rings and it is expected to show a charge transfer 
band in its absorption spectrum. In a preferred embodiment, the 
chromophore substituents have hydroxyl groups in the para position of both 
rings or amino groups in the para position of both rings. 
When the materials as further described herein absorb light or are exposed 
to high temperatures, the Z-stilbene can isomerize about the C.dbd.C 
double bond to the E-configuration. It is recognized that if this 
isomerization is undesirable, a link can be established between the R 
groups as well known in the art, e.g. R.sup.1 and R.sup.2, which 
effectively prevents such isomerization. 
The orientation of the molecules in solids is critical in designing 
nonlinear optical materials for certain applications. It will be 
appreciated that the Z-configuration of these molecules can pack together 
into interlocking doublets which then interact with adjacent doublets to 
form long interacting systems which can exhibit charge transfer 
transitions between adjacent units. 
In another specific embodiment of the present invention, the compound (I) 
is an extended version of the embodiment previously described above. In 
this specific embodiment, n&gt;0 and all C.dbd.C double bonds are in the 
Z-configuration and Ar.sup.1 and Ar.sup.3 have an electron withdrawing and 
an electron donating substituent, respectively. These materials with 
extended chromophores frequently show high values of .chi..sup.2 and 
.chi..sup.3. In a preferred embodiment, the compound having the general 
formula (I) given previously has a length of n=5. It will be appreciated 
that with very large values of n, materials may become difficult to 
manipulate due to reduced solubility. However, it will also be appreciated 
that using large alkyl chains on the R groups will alleviate this problem. 
In yet another specific embodiment of the present invention, wherein n&gt;0, 
all of the C.dbd.C double bonds are in the Z-configuration and Ar.sup.1 
and Ar.sup.3 have identical electron withdrawing or electron donating 
substituents. It will be appreciated that these materials with extended 
chromophores frequently show high values of .chi..sup.3. In a preferred 
embodiment, the compound having the general formula (I) given previously 
has a length of n=5. As previously stated, it will be appreciated that 
with very large values of n, materials may become difficult to manipulate 
due to reduced solubility. However, it will also be appreciated that using 
large alkyl chains on the R groups will alleviate this problem. 
In another specific embodiment of the present invention, n=0, and the 
stilbene unit previously described is in the E-configuration and R.sup.1 
group and R.sup.2 group are tertiary butyl. An electron withdrawing group 
is attached to one end of the system and an electron donating group is 
attached to the other end of the system for a charge transfer through a 
weakly coupled system of chromophores. It will be appreciated that because 
the chromophores are weakly coupled, the molecule is only slightly 
polarized in the ground state. In the excited state, a very large degree 
of polarization is expected. It is believed that a very small difference 
in substituents may provide a .pi. system which is very polarized in the 
excited state although almost unpolarized in the ground state. Again, the 
prerequisite for nonlinear optical properties is achieved without the 
presence of a conjugated .pi. system. In this preferred embodiment, the 
chromophore substituents have a nitro group in the para position of one 
ring and an amino group in the para position of the other ring. 
When the E-configuration is exposed to high temperatures, the molecule will 
not isomerize as in the case of the Z-isomer because of the increased 
stability of the E-configuration. 
In another embodiment of the present invention, the compound (I) is an 
extended version of the embodiment described above. In this embodiment, 
n&gt;0 and all C.dbd.C double bonds have the E-configuration and Ar.sup.1 and 
Ar.sup.3 have an electron withdrawing and an electron donating 
substituent, respectively. Materials with extended chromophores frequently 
show high values of .chi..sup.2 and .chi..sup.3. Preferably, in accordance 
with this embodiment of the present invention the compound having the 
general formula (I) given previously has a length of n=5. It will be 
appreciated that with very large values of n, materials may become 
difficult to manipulate due to reduced solubility. However, it will also 
be appreciated that using large alkyl chains on the R groups will 
alleviate this problem. 
In yet another embodiment of the present invention, wherein n&gt;0, all double 
bonds are in the E-configuration and Ar.sup.1 and Ar.sup.3 have identical 
electron withdrawing or electron donating substituents. It will be 
appreciated that materials with extended chromophores frequently show high 
values of .chi..sup.3. In a preferred embodiment, the compound having the 
general formula (I) given previously has a length of n=5. It will be 
appreciated that with very large values of n, materials may become 
difficult to manipulate due to reduced solubility. However, it will also 
be appreciated that using large alkyl chains on the R groups will 
alleviate this problem. 
Another embodiment of the present invention is an extended version wherein 
n&gt;0, but some C.dbd.C double bonds have the E-configuration and some 
C.dbd.C double bonds are locked into the Z-configuration. Ar.sup.1 and 
Ar.sup.3 have identical or different electron withdrawing or electron 
donating substituents. In a preferred embodiment of the invention, the 
compound having the general formula (I) given previously has a length of 
n=4 and the central C.dbd.C double bond is in the E-configuration and the 
remaining C.dbd.C double bonds are in the Z-configuration. The materials 
in accordance with this aspect of the present invention with extended 
chromophores frequently show high values of .chi..sup.2 and .chi..sup.3. 
It will be appreciated that with very large values of n, materials may 
become difficult to manipulate due to reduced solubility. It will also be 
appreciated that using long alkyl chains on the R groups will alleviate 
this problem. 
A nonlinear optical device in accordance with the present invention 
includes means to direct at least one incident beam of electromagnetic 
radiation into an optical element having nonlinear optical properties 
whereby electromagnetic radiation emerging from the element contains at 
least one frequency different from the frequency of any incident beam of 
radiation, the different frequency being an even multiple of the frequency 
of one incident beam of electromagnetic radiation. The optical element 
includes a compound which is captured in a noncentrosymmetric material, 
the compound having the general formula (I) given previously. 
Preferably, the emerging radiation of a different frequency is doubled 
(second order) (SHG). Preferably, the electromagnetic radiation is from 
one of a number of common lasers, such as Nd-YAG, Raman-shifted Nd-YAG, 
and Ar or Kr ion and semiconductor diode. 
The optical element is oriented in one of a potentially infinite number of 
crystal orientations which achieve partially maximized SHG conversion by 
virtue of phase matching. The specific orientation is chosen for reasons 
of noncriticality, maximum nonlinearity, increased angular acceptance, 
etc. For example, polarized light of wavelength 1.06.mu. from a Nd-YAG 
laser is incident on the optical element along the optical path. A lens 
focuses the light into the optical element, light emerging from the 
optical element is collimated by a similar lens and passed through a 
filter adapted to remove light of wavelength 1.06.mu. while passing light 
of wavelength 0,532.mu.. 
The optical element is preferably a single crystal or a poled material of a 
type well known in the art having at least one dimension of about 0.5 mm 
or greater but can be of one or more substantially smaller crystals 
embedded in a film of polymer or in glass. The smaller crystals can be 
randomly oriented or aligned with the same orientation, but are preferably 
aligned. For the smaller crystals, if their size is small enough to 
minimize light scattering, they can be dispersed in the polymeric binder 
and pressed, molded or shaped into an optically clear element capable of 
SHG. It will be appreciated that the polymer binder should be chosen to be 
a non-solvent for the aromatic compound. For larger crystallites, similar 
elements can be prepared and it is preferred that the binder used has an 
index of refraction matched to the complex, so as to minimize light 
scatter yet remain transparent. 
Another device in accordance with the present invention is an 
electrooptical modulator. An electrooptical modulator is an optical 
modulator in which a Kerr cell, an electrooptical crystal or other signal 
controlled electrooptical device is used to modulate the amplitude, phase, 
frequency or direction of a light beam. The electrooptical modulator 
includes a means to direct a coherent light beam into an optical element, 
and means to apply an electric field to the element in a direction to 
modify the transmission property of the light beam, the optical element 
meeting the description given above for the optical element. The preferred 
optical elements for the nonlinear optical device and electrooptical 
modulator of the invention are those embodiments set forth earlier herein 
for the nonlinear optical element. More particularly, an electrooptical 
modulator embodying the present invention utilizes an optical element. A 
pair of electrodes are attached to the upper and lower surfaces of the 
element, across which a modulating electric field is applied from a 
conventional voltage source. The optical element is placed between two 
polarizers. A light beam, such as that from a Nd-YAG laser, is polarized 
by a polarizer focused on the optical element propagated through the 
crystal or crystals and subjected to modulation by the electric field. The 
modulated light beam is led out through an analyzer polarizer. Linear 
polarized light traversing the optical element is rendered elliptically 
polarized by action of the applied modulating voltage. A polarizer renders 
the polarization linear again. Application of the modulating voltage 
alters the birefringence of the optical element and consequently is 
ellipticity impressed on the beam. The polarizer then passes a greater or 
lesser fraction of the light beam as more or less of the elliptically 
polarized light projects onto its nonblocking polarization direction. 
Another device, in accordance with the invention, is a device to reduce the 
size of a coherent light beam. The device increases the density of 
information on an optical read-write storage device. An optical element as 
described above for the optical element of the nonlinear optical device is 
affixed to the surface of an optical information storage medium of a type 
well known in the art. As the coherent optical beam travels through the 
element to the medium, the beam size is reduced because the center of the 
beam, being of higher light intensity, experiences a different refractive 
index than the periphery of the beam. It will be appreciated that this 
effect originates from large .chi..sup.3 in the optical element. 
Accordingly, the smaller beam size allows more information to be stored in 
a fixed surface. 
Another device, in accordance with the present invention, is an all optical 
switch. In an optical switch, two light guides composing the optical 
element lie in close proximity. A light pulse moving through this region 
passes from one guide to the other and back. As the pulse tracks down the 
switch the number and frequency of passes is determined by the switch 
geometry and the refractive index of the medium. Ultimately, the pulse 
exits through one of the two guides at the end of the switch creating an 
"on" or "off" state. Since the refractive index of the optical element is 
a function of light intensity, a .chi..sup.3 effect, the switch is 
controlled "on" or "off" by the intensity of the pulse entering the 
switch. Such switching devices can be the basis for all optical computers. 
See, for example, Service, R. F. "Nonlinear Competition Heats Up", 
Science, 1995, 267, 1918-21, incorporated herein by reference. 
Yet another device in accordance with the present invention is a three 
dimensional information storage device. The device requires a 
photorefractive material of a type well known in the art. It is well known 
that photorefractive organic materials can be prepared and caused to 
function in this manner. See Dagan, "Chemical Engineering News", Feb. 20, 
1995, p. 28-32 and Mar. 6, p. 22-27 and Vu, L.; Chan, W. K.; Pong, Z.; 
Gharari, A., "Multifunctional Polymers Exhibiting Photorefractive Effects" 
Accts. Chem. Res. 1996, 29, 13-21, each document incorporated herein by 
reference. In this instance, compound (I) described above constitutes the 
primary component in the photorefractive material, i.e., the nonlinear 
optical material. 
Compounds of the type having nonlinear optical properties represented by 
the general formula (I) as provided above may be synthesized by direct 
electrophilic aromatic substitution of diphenylethylene units and/or 
McMurry type coupling reactions as well known in the art. Provided below 
are examples representative of the preparation of compounds in accordance 
with the present invention.

EXAMPLE 1 
A solution of 0.518 g 2,2,5,5-tetramethyl-3,4- diphenylhex-3-ene and 0.608 
g Hg(NO.sub.3).sub.2 H.sub.2 O was refluxed in trifluoroacetic acid (TFA) 
for 30 min. The TFA was removed under reduced pressure and the residue was 
dissolved in CH.sub.2 Cl.sub.2. The solution was washed with aqueous 
sodium bicarbonate and the solvent evaporated to give the crude product, 
0,691 g. This was purified by chromatography on silica gel (25% CH.sub.2 
Cl.sub.2 -hexane) to give 0.50 g pure 
2,2,5,5-tetramethyl-3,4-di(4-nitrophenyl)hex-3-ene. This showed .sup.1 H 
NMR (CDCl.sub.3): .delta.0.69 (s, 18H), 7.34-7.39 (d, 4H), 8.16-8.21 (d, 
4H). 
EXAMPLE 2 
A solution of 0.62 g 2,2,5,5-tetramethyl-3,4-di(4-nitrophenyl)hex-3-ene,2.6 
g cyclohexene and 1.4 g 10% Pd/C in THF was refluxed for 2 hrs. The Pd/C 
was removed by filtration and the residue after evaporation of the solvent 
was purified by chromatography on silica gel (25% CH.sub.2 Cl.sub.2 
-hexane) to give 0.285 g (50%) 
2,2,5,5-tetramethyl-3-(4-nitrophenyl)-4-(4-aminophenyl)hex-3-ene: .sup.1 H 
NMR (CDCl.sub.3): .delta.0.66 (s, 8H), 0.70 (s, 8H), 3.61 (s, 2H), 
6.61-6.63 (d, 2H), 7.30-7.34 (d, 2H), 8.12-8.14 (d, 2H): .sup.13 C NMR 
(CDCl.sub.3): .delta.32.81, 32.84, 37.59, 38.16, 113.58, 121.93, 131.34, 
131.48, 133.12, 143.78, 144.58, 146.22, 146.88, 152.11. 
For a more detailed discussion of the preparation of compounds of the type 
having nonlinear optical properties represented by the general formula (I) 
as provided above reference is made to Gano, J. E., "Effects of Phenyl 
Substitution on the Fragmentation of Sterically Congested 
Di-tert-butylstilbene Radical Cations", Journal of Mass Spectrometry, 
1996, 31, 363-366, incorporated herein by reference. 
Having described presently preferred embodiments of the invention, it is to 
be understood that it may be otherwise embodied within the scope of the 
appended claims.