High melting point stilbazolium salts

Stilbazolium salts that retain high second harmonic generation properties as well as high melting points are disclosed herein. More particularly, novel 4'-dimethylamino-N-methylstilbazolium cyclopentanesulfonate and homologs thereof which are expected to be useful in optoelectric devices and systems are disclosed.

FIELD OF THE INVENTION 
This invention relates to new compositions of matter and more particularly 
to stilbazolium salts that unexpectedly retain high second harmonic 
generation properties (nonlinear optical activity) as well as high melting 
points. 
BACKGROUND OF THE INVENTION 
The optics and electronics industries rely upon inorganic compounds for 
fabrication of various components. However, these industries may benefit 
largely from the plethora of organic compounds, both known and unknown. Of 
the many potential applications of organic compounds to the 
above-mentioned industries, many relate to the electro-optic effect as 
described by Kerr (1875) and Pockels (1906). Additionally, frequency 
doubling by second harmonic generation (SHG) is often considered. SHG may 
be defined as the doubling of light's fundamental frequency. 
A test to study SHG has been developed (Kurtz and Perry, 1968) which 
analyzes, for instance, the noncentrosymmetric crystal structure of 
organic compounds. Organic compounds which posses a noncentrosymmetric 
structure exhibit optical nonlinearity and are generally said to be 
nonlinear. 
Organic nonlinear optical materials displaying high SHG properties are 
potentially useful in applications which require high speed optical 
modulators. Such applications include high speed long distance data links 
and electric field sensors for use in electromagnetically noisy 
environments. In addition, such materials provide efficient wavelength 
shifting capability for optical and infrared remote sensing (e.g., of 
pollutant particulate concentration) and diode laser frequency doubling 
for optical data storage. 
It has been of increasing interest to prepare organic nonlinear optical 
materials, such as stilbazolium salts, that posses high melting points 
without adversely affecting their SHG properties. High melting point 
stilbazolium salts are desirable because many processing steps involving 
such materials are conducted at temperatures that are near or greater than 
their conventional melting points. This often causes molecular breakdown 
or molecular restructuring of the materials which inevitably results in 
loss of nonlinear optical properties. 
The present invention, therefore, is based on the discovery of stilbazolium 
salts that display high SHG properties as well as high melting points. 
High SHG properties may be defined as a second harmonic generation powder 
efficiency of at least about 1000 as compared to a urea standard which is 
assigned a value of 1 (SHG powder efficiency as described by Marder et 
al., Science, 245, 626-628 (1989). High melting points may be defined as 
at least about 230.degree. C. 
Description of the Prior Art 
Accordingly, attempts have been made to prepare stilbazolium salts with 
high melting points. In commonly assigned U.S. Pat. No. 5,094,553, 
4'-dimethylamino-4-methylstilbazolium p-toluenesulfonate (DAST) is 
disclosed. Said DAST has a favorable melting point range. However, as a 
result of its toluenesulfonate anion, the molecular dipoles within the 
crystals of the compound form a herringbone arrangement which 
characteristically reduces their SHG properties. 
Other investigators have focused their attention on stilbazolium salts that 
posses favorable SHG properties. In commonly assigned U.S. Pat. No. 
5,194,984, 4'-dimethylamino-4-methylstilbazolium methanesulfonate (DASMS) 
is disclosed. Crystals of said DASMS possess favorable SHG properties. 
However, since DASMS crystals contain a tetrahydrated methanesulfonate 
anion, they melt at lower temperatures and are more difficult to process. 
Efforts to produce stilbazolium salts that unexpectedly retain high SHG 
properties and high melting points have not been disclosed. 
The instant invention, therefore, is patentably distinguishable from the 
above-mentioned patents, since among other reasons, it is based on the 
discovery of stilbazolium salts that unexpectedly display high second 
harmonic generation properties as well as high melting points. 
SUMMARY OF THE INVENTION 
The instant invention is based on the discovery of stilbazolium salts that 
unexpectedly retain high second harmonic generation properties as well as 
high melting points. Said stilbazolium salts are represented by the 
formula 
##STR1## 
wherein each R.sup.1 is independently hydrogen or deuterium, R.sup.2 is 
HO--, H.sub.3 CO-- or (R.sup.3).sub.2 N--. Each R.sup.3 is independently 
an aliphatic, alicyclic or aromatic radical; however, methyl groups in 
which from 0 to 3 of the hydrogens are replaced by deuterium are 
preferred. 
It is preferred that the stilbazolium salt of the instant invention is 
4'-dimethylamino-N-methylstilbazolium cyclopentanesulfonate (DASCp) and 
represented by the formula 
##STR2## 
=p The structures depicted hereinabove are not limited to any 
sterioisomeric (cis or trans) arrangement. However, the trans-isomer is 
often preferred in nonlinear optical applications. The cis- and 
trans-isomers may be separated by conventional methods such as fractional 
crystallization or flash column chromatography. 
The additional features and advantages of the invention will be made 
evident upon reference to the following detailed description. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Materials that display high SHG properties have been prepared. The SHG 
properties of these materials are obtained in part by synthesizing and 
incorporating therein a salt in which the cation portion exhibits large 
molecular hyperpolarizability. Coupling said cation portion with a 
specific counterion (anion) can lead to materials that demonstrate 
favorable SHG properties. 
No efforts, however, have been disclosed which address stilbazolium salts 
that retain high second harmonic generation properties as well as high 
melting points. In the case of DASMS (described above), its 
methanesulfonate anion exists as a tetrahydrate. Said anion is pentagonal 
and can be represented by the formula 
##STR3## 
Because of this anion, the molecular dipole in DASMS crystals fully align 
along a common axis which results in a maximum bulk crystalline dipole. 
This maximum bulk crystalline dipole plays a critical role in giving DASMS 
its favorable SHG properties. 
Nonetheless, the DASMS anion comprises hydrogen bonds which are susceptible 
to heat since their bond energies are on the order of only about 1.5 
kcal/mol. As a result, DASM melts at temperatures that are lower than 
conventional processing (e.g., metal deposition) temperatures. 
Conversely, DAST (described above) possesses a toluenesulfonate anion. Said 
toluenesulfonate anion comprises C--C and C--H covalent bonds which have 
bond energies on the order of about 100 kcal/mol. As a result, DAST has a 
melting point range on the order of about 40.degree. C. greater than that 
of DASMS. Hence, it is readily processed. 
Notwithstanding this fact, the SHG properties of DAST are approximately 20% 
less efficient than those of DASMS. This is true because DAST crystals do 
not fully align along a common axis as in the case of DASMS. In fact, the 
molecular dipoles within the crystals of DAST form a herringbone 
arrangement which leads to the above-mentioned decrease in SHG efficiency. 
The instant invention therefore is based on the discovery of DASCp and 
homologs thereof. Said DASCp and its homologs contain anions as depicted 
by formulae I and II. 
As a result of said anions (which are pentagonal as in the case of DASMS), 
the molecular dipole of novel DASCp crystals (or crystals of DASCp 
homologs) fully align along a common axis yielding SHG properties which 
are expected to be at least about 1000 as compared to a urea standard 
which is assigned a value of 1. Moreover, since said ions consist of 
carbon to carbon and/or carbon to hydrogen and/or carbon to deuterium 
covalent bonds, the melting points of DASCp and homologs thereof are high. 
The stilbazolium salts described in the instant invention are expected to 
be useful in the fabrication of optical materials. Any light transmitting 
properties that may arise from said compounds are expected to be based on 
the different crystalline structures and degrees of hydration they may 
obtain. Moreover, it is expected that electro-optic modulators may be 
prepared from the compounds produced via the instant invention since they 
possess light transmitting properties which can possibly be varied by 
application of an electric field. 
Additionally, it is anticipated that optical waveguides, such as those 
described in U.S. Pat. Nos. 5,094,553 and 5,194,984, may be prepared from 
the salts prepared via the instant invention since they may possess light 
transmitting properties resulting from predictable crystalline structures. 
The stilbazolium salts of the instant invention may be prepared, for 
instance, by synthesizing cyclopentane sulfonic acid via the oxidation of 
cyclopentyl mercaptan with peroxy acetic acid. The sulfonic acid may 
subsequently be treated with silver oxide in order to produce a silver 
salt of the acid. Said salt may then be treated with a solution of 
methanol and dimethylamino N-methylstilbazolium iodide to produce 
dimethylamino stilbazolium cyclopentane sulfonate and a silver iodide 
precipitate. The precipitate can be recovered by conventional filtration 
methods and solid 4'-dimethylamino N-methylstilbazolium cyclopentane 
sulfonate may be obtained after methanol removal. 
It should be noted that all reactants described herein may be deuterated or 
perdeuterated in order to produce a deuterium containing stilbazolium 
salt. 
The following examples and table are provided to further facilitate the 
understanding of the invention, and they are not intended to limit the 
instant invention. 
Moreover, all stilbazolium salts produced can be confirmed by conventional 
techniques such as proton and carbon 13 nuclear magnetic resonance 
spectroscopy as well as x-ray crystallographic techniques.

EXAMPLE 1 
A one liter three-neck flask was charged with 300 mL of methylene chloride 
and 25 grams (245 mmole) of cyclopentyl mercaptan to produce a reaction 
solution. The reaction solution was chilled to 15.degree. C. in an 
ice-water bath and followed by the addition of 190 mL of 35% peroxy acetic 
acid. Said peroxy acetic acid was added over a period of about 3 hours and 
the reaction solution was maintained at a temperature of about 
18.degree.-26.degree. C. After the reaction solution was stirred overnight 
at room temperature, 100 mL of water and 25 grams of sodium bisulfite were 
added followed by 2 additional hours of stirring. The reaction solution 
was then reduced on a rotovap and the pH was adjusted with dilute 
hydrochloric acid until a white solid precipitate formed. Said precipitate 
was isolated via filtration and dried in vacuo (50% yield of cyclopentane 
sulfonic acid). 
EXAMPLE 2 
Equimolar amounts of 4-picoline and methyl iodide were combined with 
methanol and refluxed in a reaction vessel to produce a solution of 
N-methylpicolinium iodide. To the resulting solution was added one 
equivalent of 4-dimethylamino benzaldehyde and 100 mL of piperidine to 
produce a mixture. Said mixture was refluxed for 3 hours and then cooled 
to about 5.degree.-10.degree. C. Subsequent to filtration, 4-dimethylamino 
N-methylstilbazolium iodide was recovered (greater than 90% yield). 
EXAMPLE 3 
120 milligrams (1.2 mmole) cyclopentane sulfonic acid (as prepared in 
Example 1) and 213 milligrams (1.0 mmole) of silver(I) oxide were combined 
with 30 mL of acetonitrile solvent in a flask and stirred for 1 hour in 
the dark to produce a reaction mixture. The reaction mixture was filtered 
through a Celite pad and the solvent was removed. 310 milligrams (0.85 
mmole) of 3-dimethylamino N-methylstilbazolium iodide (as prepared in 
Example 2) and 20 ml of methanol were added to the flask to produce a 
second mixture which was stirred overnight at room temperature. The second 
mixture was filtered through a Celite pad to remove the methanol solvent 
and a red solid precipitate. Said solid was dried in vacuo to yield 230 
milligrams of 4'-dimethylamino N-methylstilbazolium cyclopentylsulfonate 
(90% yield). 
The data in the table below has been compiled to confirm the high melting 
point of the compounds of the instant invention. 
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Entry 1 Stilbazolium Salt 
Melting Point (.degree.C.) 
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1 DASMS &lt;219 
2 DAST 259 
3 DASCp 256 
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