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GENCY USE ONLY (Leave bln)2. REPORT DATE 3REPORT TYPE AND DATES COVERED

LEo AN SUBT-TLE . 20 Sep 93 FINAL_ .POR_' _15 Dec 93-14 Aug 93SLE AND SUBTITLE 5. FUNDING NUMBERS

Complex and Stereoregular Nonlinear OpticalMaterials

nj OR(S) - ___________63218C 1602/01 ,

Dr Brian G. DixonWEAF 4 R-t .A R J R 5 7

-ORMING ORGANIZATION NAME(S) AND ADDRESS(ES) - S=. PERFORMING ORGANIZATION

Cape Code Research, Inc REPORT NUMBERP 0 Box 600Buzzards Bay, MA 02532

9. SPONSORING/ MONITORING AGE NeYA/NAME(S) AND -. . r 10. SPON(SORING / MONITORINGAFOSR/NC , AGENCY

110 DUNCAN AVENUE SUITE B115BOLLING AFB DC 20332-0001 U'J. 9620-93-C-0006

11. SUPPLEMENTARY NOTES

12a. DISTRIBUTION,/ AVAILABILITY STATEMENT .12b. DISTRIBUTION CODE

Approved for public release; distribution is unlimite"d"

13. ABSTRACT (Maximum 200 words)

This Phase I research program resulted in the synthesis and identificationof a previously unknown set of aromatic phosphonic acid compounds, whichpossessed interesting thermal properties. There are very few phosphonatecompounds reported that are liquid crystalline in nature. However, thenon-linear optical properties of these compounds were determined.to beunremarkable. Very probably, this is due to a relatively small dipolebetween the phosphonate moiety and a distal cyano group.

93-24265"-IIIIIlI 1111 11 11111Im llI( I!i 'II

14, SUBJECT TERMS 15. NUMBER OF PAt.fS

non-linear optics, phosphonate 16. PRICE CODE

17. SECURITY CLASSIFICATION 18. SECUITY CLASSIFICATION 19. SECURIT.' CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF REPORT OF TH(S PAGE 01 AOBSiRACT

UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED"i'SN 7540-01-280-5500 Standard Form 298 (Rev 2-89)

Precf0Dcd my WNSi St'ld ý .9.

I (E COD REEARCH19 RESEARCH ROAD-) TELEPHONE (508) 540-4400EAST FALMOUTH, MA 02536 FAX (508) 540.4428

I

I COMPLEX & STEREOREGULARNONLINEAR OPTICAL MATERIALS

* Final Report

3 September 7, 1993U

Contractor: Cape Cod Research, Inc.19 Research RoadEast Falmouth, MA 02536

3 Principal Investigator: Dr. Brian G. DixonContract Number: F49620-93-C-0006 Contract Dollar Amount: $59,942

Contract Award: CompetitiveISponsor: Dr. Charles Y-C Lee (202) 767-4960

AFOSRDirectorate of Chemistry & Materials ScienceBuilding 410Boiling AFB DC 20332-6448

II

III

I EXECUTIVE SUMMARYI

This Phase I research program resulted in the synthesis and identification of a3 previously unknown set of aromatic phosphonic acid compounds, whichpossessed interesting thermal properties. There are very few phosphonatecompounds have been reported that are liquid crystalline in nature.

However, the non-linear optical properties of these compounds weredetermined to be unremarkable. Very probably, this is due to a relatively

small dipole between the phosphonate moiety and a distal cyano group.More specifically, the research yielded the following results: (1) A previously

unknown set of phosphono-substituted biphenyl compounds was

synthesized, and characterized. (2) It was established that this new set of3 compounds possessed liquid crystalline properties. (3) Corona poling of thenew compounds in polymer films was carried out to align the liquid

crystalline materials, in order to improve their non-linear optical properties.

(4) The synthesized phosphonates did not exhibit non-linear opticalproperties.

I

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DTIC TAB ElS,~~Ju.; t i f 1c:. o n

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Ij'i" £and/orDist Special

U

U TABLU OF CONTENTSi PAGE

I 1.0 STATEMENT OF THE OPPORTUNITY 4I 2.0 TECHNICAL APPROACH & PHASE I SUMMARY 5

2.1 Technical Approach 5

2.2 Technical Objectives 6I3.0 PHASE I RESEARCH CARRIED OUT 7

I 3.1 Phosphonatobiphenyl Syntheses 73.2 Liquid Crystallinity - Thermal Properties 10

3.3 Nonlinear Optical Results 15

3.4 Experimental 17

3.5 Analytical 20

4.0 RESEARCH RESULTS & ASSESSMENTS 23

5.0 CONCLUSIONS & RECOMMENDATIONS 24

1 6.0 POTENTIAL APPLICATIONS 24I 7.0 REFERENCES 24III

confidential proprietary informationI,1.0"' STATEMENT OF THE OPPORTUNITY

I This Phase I SBIR project was carried out in response to Topic SDIO92-011 under theI heading of Optical Computing and Optical Signal and involved research concerning

innovative organic-inorganic hybrid materials that possess superior nonlinear

optical properties.

It is widely recognized that materials possessing large and fast nonlinear optical3 (NLO) susceptibilities have great potential for use in the emerging photonicsindustry. 1-8 In fact, materials (for example LiNbO 3 ) which possess useful secondorder nonlinearities, p(2), are already being applied to optical switching and

frequency mixing applications. It is also clear that if the potential of NLO materials

is to be realized, significant advancements in the magnitude of the observed

nonlinearity will be required. A second area requiring progress is NLO materialsprocessing.

Much of the NLO research carried out to date has focused upon organic materials,and there are very good reasons why this is so. Compared to inorganic crystalline

materials, like LiNbO3, organic materials offer the distinct advantages of largesusceptibilities, high laser damage thresholds, faster response times, and a versatilityof molecular modifications. It would also seem clear that the key to obtaining alarge second order hyperpolarizability, D3, is to synthesize materials with specificstereochemistries. In essence the problem distills down to one of stereochemicalcontrol during the synthetic process. A number of interesting and partiallysuccessful attempts to achieve this goal have been carried out, including the use ofliquid crystalline solvents to prepare organic NLO materials, the use of orientational3 fields, Langmuir-Blogett films, and others. The use of templates for thestereochemical induction of NLO materials is a research area that is just beginning

i to be explored.

The study of inorganic-organic hybrid materials as template-NLO materials is anarea that is in its infancy, but it is also an area that has great potential. The primaryobjective of this Phase I research program was to demonstrate the feasibility of3 preparing organic-inorganic hybrid materials that possess interesting non-linearoptical properties. More specifically, this Phase I SBIR program investigated the

I 4

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1 feasibility of preparing lamellar liquid crystalline-phosphonate metal oxidecomposites with superior nonlinear optical properties.

2.0 TECHNICAL APPROACH & SUMMARY OF THE PHASE I WORK

2.1 Underlying Basis of the Technical Approach

The technical approach was to use the known high structural order of intercalating* metal oxides as templates for synthesizing inorganic-organic hybrid liquid

crystalline nonlinear optical materials. Basically the intercalating metal oxideserved as a crystalline framework which forced the liquid crystalline component

into a very highly organized and defect free state. The basic concept is showngraphically by the following artwork:

Metallo-oxy j Backbone

Liquid Crystal Z 7 7 71 metal oxidePhosphonate layer

LayerI

Metallo-oxy BackboneLiquid Crystal Cmetal oxidePhosphonate layer3 Layer

U Connection to 0 OxygenPhosphorous

Phosphorus Mesogen

IThe liquid crystalline portion of this complex molecule is closely related, chemically,3 to materials that are known to have useful and interesting NLO properties. Whathad not been heretofore investigated was making this type of hybrid organic-

inorganic species. In fact, the vast majority of research to date has focused upon

I5

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organic materials, and very little work had been done upon hybrid inorganic-

I organic materials. The proposed liquid crystalline-phosphonate metal oxidecomposites were previously unknown.

I There were two fundamental questions which had to be answered. First, couldcomplex and intermixed organic-inorganic composites be readily synthesized?

I Second, did they have useful NLO properties, and how did they compare with theNLO properties of simple liquid crystalline materials? The answer to the first

I question is yes. Although the specific intermixed materials to be synthesized hadnot previously been prepared, this research has demonstrated that the proposed3 layered inorganic-organic complexes are accessible.Unfortunately, the answer to the second question appears to be no, although the

chemical similarity between the basic liquid crystalline phosphonate moiety to beused in this research, structure 1, and the known liquid crystalline species, 2, is

apparent.

OH*PN 1

OH

i 0

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*Can the previously unknown liquid crystalline phosphonate monomer, 1,

be readily synthesized?

* Does the phosphonate monomer 1 form an intercalating lamellar

I composite within a metal oxide structure?* What are the physical and liquid crystalline properties of the novel

materials?

I�* What are the optical propertiLs of liquid crystal 1 and the final composite,and how do they compare with established nonlinear optical materials?

Answers to all of these questions have been obtained during the Phase I program.

The results are described in Section 3.0, which follows.

I 3.0 PHASE I RESEARCH CARRIED OUT

The Phase I research was split into two three fundamental parts. The first part dealt

with the synthesis and characterization of phosphonate 1, and its derivatives.

Section 3.2 reports on the liquid crystalline properties of the synthesized materials,

while Section 3.3 describes the non-linear optical experiments, and results. Sections3.4 and 3.5 detail the experimental and analytical procedures used throughout the

program.

3.1 Synthesis of 4-Cyano-4'-Phosphonato Biphenyls

A couple of different synthetic routes to the desired phosphonato compounds were

evaluated during the feasibility test program. As described in the original proposalthe structural similarities between 1 and known liquid crystals suggested that 1

* might be liquid crystalline as well.

NC O-\OH1 • OH

I 7

IIconfidential proprietary information

Both routes to 1 began with 1,4-dibromobiphenyl, a low cost starting material. One

route went through a sequence of conversion to the Grignard reagent, followed by a

sequential conversion to first the corresponding carboxylic acid, then the amide, and

finally to the desired nitrile. This route was ý bandoned for a number of reasons. A

four step synthesis from the dibromo compound to the bromo-cyano compound

was prohibitive. In addition, the yields were poor. An alternative route took the

dibromobiphenyl directly to the bromocyanobiphenyl in one step.

I The synthetic route is shown below, and involved simply taking 4,4'-dibromobiphenyl directly to the bromocyanobiphenyl in one step. 4,4'-

dibrombiphenyl was reacted, in dimethylformamide, with copper cyanide at reflux.

Separation of the products was accomplished with a combination of

recrystallization, column chromatography, and/or sublimation.

- - CuCN

*Br-c Br D Br -fO-f -CNDMF

U Compound 1 results directly from the reaction of the 4-bromo-4'-cyanobiphenyl inan Arbuzov type of reaction as shown in the first step in the scheme below.

Although in general aromatic compounds do not undergo the Arbuzov reaction at

all, the presence of a catalytic amount of nickel (II) chloride facilitates the

* transformation.

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NC Br + P(OEt)3 1700CIV NC K N OEt-- L I-Si(Me)3

NCP . H ot H2 0 NC

N OH --,J NC POSi(Me)3

ZnCI2

NC F Zn+21 -0-0--

The reaction conditions are relatively harsh, however, and require that the nickel

(II) chloride be pre-dried, and that the reaction be run neat. A number ofexperiments were run in which solvents were used, however, these reactions

mm simply did not go to completion.

The diester product was converted into the corresponding trimethylsilyl

phosphonate ester by reaction with iodotrimethyl silane, which in turn, is easily

converted to the phosphonic acid by hydrolysis with water to yield the desired

product 1. The synthesis of the metal ion salt derivatives of 1 was carried out by

simply doing the hydrolysis step with aqueous solutions of the desired salt, shown

above for the example of zinc. Once again standard recrystallization and/or column

chromatography procedures were used to purify the products.

I

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I 3.2 Liquid Crystallinity & Thermal Properties

I As hoped, the new phosphonate compounds were found to be liquid crystallinematerials. It was also found that the various phosphonate compounds gave thesame thermal scans, suggesting that the observed thermal properties were due to thesubstituted biphenyl groups and were virtually independent of the P-O-X moiety.

Differential Scanning Calorimetry (DSC, run on a Perkin Elmer Delta Series DSC7,

see Section 3.5 for the procedural details) measurements showed multipletransitions, as expected for liquid crystals. Figures 1-6 demonstrate results which are

representative of the experimental program.

I2 r

II I

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Figure 1: DSC Scan of 4-cyano-4'-diethylphosphonate biphenyl - Heating

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Scanning Ratu, -- 10.0 C/min

Somple Wt, 2.670 mg DiekACEB -- -7YHI--;I5-2. 67

Figure 2: DSC Scan of 4-cyano-4'-diethylphosphonate biphenyl - Cooling

Figures 1 and 2 are the heating and cooling curves for ester compound, 4-cyano-4'-

diethylphosphonate biphenyl, respectively. Figures 3 and 4 are the corresponding

curves for the phosphonic acid, compound 1. Figures 5 and 6 are the results for the

I zinc salt. The three sets of figures are very similar in that they show a series of small

thermal transitions, which typical of liquid crystalline behavior. An examination of

the cooling curves shows three transitions between 106-110, 148-153, and 170-1800 C.

I

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kJ'19, 51111 HX'-105-J. .77YG L.. .' 7

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IFigure 3: DSC Scan of 4-cyano-4'-phosphonic acid biphenyl - Heating

IUIII 12

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UFigure 4: DSC Scan of 4-cyano-4'-phosphonic acid biphenyl - Cooling

IIIII 13

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- - -/', 'r - . .zJ-L •.

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II Figure 5: DSC Scan of 4-cyano-4-phosphonic acid biphenyl, zinc salt - Heating

IIII3 14

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Data: Aug 10. 1993 it 20prm Perr.(E 4Scoanning Ratat -10. 0 C/mtnSample WItt 3. 9 40 -rS i 6k, A'Ci_ 1

IFigure 6: DSC Scan of 4-cyano-4'-phosphonic acid biphenyl, zinc salt - Cooling

SThe corresponding heating curves also indicate liquid crystalline activity, and asexpected, the peaks are shifted significantly from those of the cooling curves.

The associated energies of phase transition were between about 0.3 Joules/gram and3 20 Joules/gram. These values are typical for liquid crystalline transitions.3.3 Nonlinear Optical Measurements & Results

SHG is a special case of frequency mixing which occurs when light waves offrequency co passing through an array of molecules interact with them in such a wayas to produce coherent light waves at 2 co. Nonlinear optical (NLO) effects occur atthe molecular level in the dopants due to a deviation from a harmonic electronic

potential energy. Typical dopants have a conjugated n-electron system with an3 electron donor and electron acceptor on either end, creating a large molecular15

confidential proprietary informatioi,

hyperpolarizability, f3, along the transition dipole axis. Second-order NLOproperties, including SHG, are described by the second-order macroscopic

susceptibility, X (2), tensor in the relationship for the bulk material polarization, P:

P = (1 • E(co) + X2 : E()E() + X3 : E(co)E(o)E() + ...... (1)

where E(o) is the optical field incident on the sample, and the susceptibility tensors

X(n) measure the macroscopic compliance of the electrons. Since the second-order

polarization is proportional to the square of the optical field, SHG is zero in a

centrosymmetric system. Thus, to induce SHG, NLO dopants must be oriented

noncentrosymmetrically in the polymer matrix; this is typically achieved by electric

field poling.

U Under normal poling conditions gEz fz° is small compared to kT, where g is thedipole moment of the dopant, Ez is the applied dc-poling-field, fz is a local field

factor, k is Boltzmann's constant and T is absolute temperature)

X(2 ) - NEz°[p43/5kT + y] (2)Iwhere N is the number density of chromophores, P is the molecular second-ordernonlinear hyperpolarizability and y is one-fifth of the molecular third-order

nonlinear hyperpolarizability. The first term in brackets in equation (2) is due to

orientation of the NLO chromophores while the second term, Y, is due to electric-field-indLced third-order effects which appear instantaneously (

Iconfidential proprietary informationI

As measured relative to our quartz reference, the variously prepared biphenyl

phosphonates, including the diethyl ester and the free acid, showed no SHG

intensity, either with the electrical field applied or immediately after its removal,

I beyond the background noise of the photomultiplier. For the conditions tested (3wt.% dopant, etc), the value of X(2) achievable in these samples is approximately afactor of 200 or more times smaller than that achievable with Disperse Red 1 (also at

3 wt.%).

* 3.4 Experimental Procedures

All chemicals were purchased from the Aldrich Chemical Co., Milwaukee, WI.

i Synthesis of 4-bromo-4'cyanobiphenyl

4,4'-dibromobiphenyl (3.5g, 10.2mmole) and copper (I) cyanide (0.98g, 10.2mmole)

were dissolved in 120mL of dimethylformamide (DMF), and refluxed for 24 hours.A stock solution of 200g FeC13.6H 20, 50mL HCl(concentrated), and 300mL H 20 wasprepared. The latter solution was added to the cooled DMF reaction solution, and

the total was heated to 650 C for twenty minutes. This solution was then extractedwith successive 200mL aliquots of toluene. The combined toluene extracts werewashed successively with 300mL 10% HC1, 150mL water, 300mL 5% NaOH, 300mLwater (twice), and then dried over MgSO4. The toluene was removed in vacuoleaving a yellow solid. The melting point of the crude solid was determined to be

150-1520 C. FTIR analysis showed a definitive absorption, due to the -CN moiety, atE 2227cm- 1. The IR 9f this product is shown in Figure 7. Purification of the crudeproduct was performed by recrystallization from dichloromethane or chloroform, or

* column chromatography.

IIII* 1

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62.354%TI *1,

'; CM

W Cu 0 0 -"

- I ° -CU 10

4000 3500 3000 2500 2000 1500 1000 cm-' 500

Figure 7: FTIR of 4-bromo-4'cyano biphenyl

Synthesis of 4-cyano-4'-diethylphosphonatobiphenyl

Nickel chloride was dried for 24hrs at 1500C. 4-bromo-4'-cyanobiphenyl (2g,

S7.75mmole) and NiC12 (0.05g, 0.2mmole), were placed in a three neck round bottomflask and purged with N 2. The mixture was then heated, under N 2 , to a bathtemperature of 1700C, at which point the biphenyl compound was liquified.Triethyl phosphite (1 .33g, 8.Ommole) was then added dropwise very slowly with

stirring. The reaction was observed to have an incubation period of =30 seconds,

and the reaction was quite exothermic.

After the phosphite addition was completed, the mixture was kept at the sametemperature for thirty minutes. The crude product was purified by recrystallizationfrom chloroform. Definitive IR (see Figure 8) absorptions were: 2227 cm-1 (-CN),1248 cm-1 (-P=O), 966-1024 cm-1 (-P-O-).I

I

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107.25-I

7 IC, ,

C, , M; i

60.65 I I

4000 3500 3000 2500 2000 1500 1!000 cme-1 500

I Figure 8: FTIR of 4-cyano-4'-diethylphosphonatobiphenyl

IL I

Synthesis of 4-cyano-4"-phosphonic acid biphenyl & the Metal Ion Salts

I

The reaction procedure was carried out under an inert atmosphere. 4-cyano-4'-I diethylphosphonatobiphenyl (0.5g, 1.59mmole) was dissolved into 20mL CC14 (pre-

distilled from P205). Iodotrimethylsilane (1.28g, 6.34mmole) was slowly added toi the CCL4 solution, with stirring. After 30 minutes, the reaction mixture was heated

to drive off the volatiles. 4 mL of H20 was then added resulting in a change from an

orange to a yellow color. The solution was stirred for an hour, and the liquidsI removed in vacuo. The yellow residue was recrystallized from

dichloromethane/heptane. The IR spectrum, Figure 9, gave the expected somewhatI ~broad abs•,i.,,ion at =2900-3050cm-1 typical of phosphonic acids, as well as the CN

absorption at 2227cm-1.

I ~19

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3.5 Analytical & Testing

Infrared Analyses

The progress of the various reactions described, as well as all liquid product

characterizations, were performed using conventional infrared analysis between

polished KBr plates or liquid cells. Solid samples were characterized by Diffuse

Reflectance using Fourier Transform Spectroscopy (DRIFTS), with a Perkin Elmer

3 20

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Model 1600 FTIR equipped with MIR, Diffuse Reflectance and Fixed Angle Specular

attachments. DRIFTS is an excellent technique for samples which are in a powder

form. The technique involves dispersing the sample as a 3-5% mixture in

spectroscopic grade KBr. When radiation impinges on the sample, the scattered

I radiation contains absorption information characteristic of the sample. Thistechnique has the advantage of rapid sample preparation and high sensitivity. For

purposes of this Phase I research approximately 256 scans were taken on each sample

and averaged to insure that a high signal to noise ratio was obtained, resulting in

smooth, well resolved spectra.

I Latent Heats

Differential Scanning Calorimetry (DSC) analyses were carried out using a Perkin-

Elmer model Delta Series DSC7 at a scanning rate of 10 or 20oC/min. All heating

and cooling curves were normalized to a sample weight of 1mg. DSC data analysis

was accomplished on a Perkin-Elmer model 3700 Data Station equipped with a

Perkin-Elmer TAC 7/3 Instrument Controller and a Perkin-Elmer Graphics Plotter 2.

U Corona Poling & Nonlinear Optical MeasurementsI PMMA (Scientific Polymer Products) was used as received. Disperse Red 1 (Aldrich)

was recrystallized using toluene. (Structure of Disperse Red 1 is given in Figure 10.)

Other dopants provided to us were used as received. Polymer + 3 wt.% dopant was

dissolved in spectroscopic grade chloroform and spin coated onto a quartz substrate

* which was patterned with planar chrome electrodes using standard

photolithographic techniques. The gap between the two electrodes was 800 gtm. Thefilms were dried below the glass transition temperature, Tg, of the polymer (-1000

C) for 24 hrs and above Tg for 12 hrs under vacuum. The final film thickness was

0.85 pim. Films were poled using either 1200 or 2400 volts across the 800 ptm gapresulting in an electric field of 15 or 30 kV/cm. Steady-state SHG intensities were

obtained by poling the samples above Tg (where steady-state conditions are reached

in a matter of seconds) and then cooling to 35*C. Measurements are reported at

35 °C.

I3!2

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-Ia "CH 2 CH2 OHI NO 2 -Q N=N-Q.CH2CH3

3 Figure 10: Structure of Red Disperse 1Figure 11 shows a schematic of the SHG measurement apparatus. The laser light isgenerated by a q-switched Nd:YAG laser at 1.064 pgm. The beam is p-polarized andsplit so that a y-cut quartz reference, used for monitoring laser power, and the

sample are measured simultaneously. A photomultiplher and integrator collect andanalyze the emergent SHG light (at 532nm), and the optical signal is ratioed againstthe quartz reference in order to eliminate any issue concerning fluctuations in laserpower.

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I

ILI

beam attenuator

beam splitter polarizer mirror

I ~lens C•• lens

Sample "Quartz

Sample Beam 4 - Reference Beam

IIR filter an IRfilter

I [ Integrator, Computer & Chart Recorder

I Figure 11: Laser System used to Make the SHG Measurements

SI 4.0 RESEARCH RESULTS & ASSESSMENTS

The Phase I research has resulted in the synthesis and identification of a previouslyunknown set of aromatic phosphonic acid compounds, which possessed interesting

thermal properties. More specifically, the research yielded the following results:

* A previously unknown set of phosphono-substituted biphenyl compounds

was synthesized, and characterized.

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I*It was established that this new set of compounds possessed liquid

crystalline properties.

SCorona poling of the new com pounds in polym er film s w as carried out toalign the liquid crystalline materials, in order to improve their non-linear opticalproperties.

* The synthesized phosphonates did not possess non-linear optical

properties.

5.0 CONCLUSIONS & RECOMMENDATIONS

The Phase I research resulted in the synthesis and characterization of a previouslyunknown, and interesting set of aromatic phosphonic acid compounds that possess

liquid crystalline properties. There are very liquid crystals that have been reportedthat are liquid crystalline in nature. However, the non-linear optical properties ofthese compounds were determined to be unremarkable. Very probably, this is dueto a relatively small dipole between the phosphonate moiety and the cyano group.Based upon these results a Phase II program is not warranted, and therefore is notrecommended. Nevertheless, interesting ideas and research work suggest

themselves. Some of these ideas will undoubtedly be pursued in the future.

6.0 POTENTIAL APPLICATIONS

There is a great potential, both commercially and within the Federal Government,

for high performance nonlinear optical materials. Potential applications include,but are not limited to, switches, modulators, guided wave devices, oscillators andharmonic generators. However, recent years have witnessed a plateau in theimprovement of these materials.

7.0 REFERENCES

1. Heeger, A.J.; Orenstein, J.; Ulrich, D.R.; Nonlinear Optical Properties of Polymers,Materials Research Society; Pittsburgh, PA, Symposium Series # 109 1988.

2. Chiang, L.Y.; Chaikin, P.M.; Cowan, D.O.; Advanced Organic Solid State Materials,Materials Research Society; Pittsburgh, PA, Symposium Series # 173 1990.

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I3. Williams, D.J.; Angewante Chemie hit. Ed.; 23 690 1984.I 4. Chen, M. et al.; Macromolecules; 24 5421 1991.I 5. Yu, L.; Dalton, L.R.; J. Amer. Chem. Soc. 111 8699 1989.E 6. Koide, N.; et al.; Mol. Cryst. Liq. Cryst.; 198 323 1991.

7. Kanis, D.R.; Ratner, M.A.; Marks, T.J.; 1. Amer. Chem. Soc. 112 8203 1990.

I8. Green, M.L.H.; Qin, J.; O'Hare, D.; Bunting, H.E.; Pure & Applied Chem. 61 817I 1989.

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