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DESIGN AND STUDY OF NEW

ORGANIC POLYMER CRYSTALS AND THIN FILMS

Defense Advanced Research Projects Agenc MATS

Order Numbe DAAK7i -C-00 5/ AAK70-83-C-0074

Monitored by Army Night Vision and Electroop ics Laboratory

Principal Investigator: Position:

Prof. A.F. Garito Professor of Physics(215)898-5810

School: Department: Physics

Faculty of Arts and Sciences DTICAPR 2 4 1986

Starting Date: Expiration Date: UOctober 1, 1976 December 31, 1983 A

- Reporting Period: October 1, 1976 - December 31, 1983

L Corporate Name of University:The Trustees of the University of Pennsylvania(a Pennsylvania Non-Profit Corporation)

*Contracting Office:

Office of Research Administration3451 Walnut Street [ Th-is document has been approved

for public release and sale; itsPhiladelphia, PA 19104 Ldsrbto su~~e

L ~ribution is unlimited.

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Technical Report

Nonlinear Optical Properties of

Organic and Polymer Crystals and Films

A.F. Garito, C.C Teng, K.Y. Wong and 0. Zammanai'Khamiri

Department of Physics

University of Pennsylvania

Philadelphia, PA 19104-3859

Spring 1984

LO

The work reported in this document was performed at the

University of Pennsylvania, Department of Physics. This work was

sponsored by the Defence Advanced Research Projects Agency under

Contract DAAK-700-77-C-0045. This report may be reproduced to

satisfy needs of U.S. Government agencies.

The views and conclusions contained in this document are

those of the contractor and should not be interpreted as

necessarily representing the official policies, either expressed

or implied, of the United States Government.

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\ ABSTRACT

Physical studies of nonlinear optical properties of organic

and polymer structures have demonstrated exceptionally large

second and third order nonlinear optical responses that are

important to the fields of nonlinear optics and optical device

technologies. These unusual responses have been exhibited by a

1 "large number of structures, phases, and states that include

organic solids and films, single crystal polymers, Langmuir-

Blodgett films, liquid crystals, and liquid crystal polymers.

Experimental and theoretical studies of such systems have

achieved significant advances in the understanding of these

exceptional macroscopic nonlinear optical responses based on

theoretically calculated microscopic electronic mechanisms,

especially the role of electron-electron correlations and highly

charge correlated electron excited states. Combined theory and

experiment has led to the recent discovery of liquid crystal

polymerization of divinyldiacetylene monomers to form highly

conjugated liquid crystal polymers exhibiting large second and

third order nonlinear optical responses.

This report has served as a major review article in the field of nonlinear

optics and was published in Molecular Crystals and Liquid Crystals as part

of an international symposium proceedings.

lI

4

I. INTRODUCTION

This article concerns the macroscopic non-linear optical responses of organic and poly-meric structures and the fundamental understand-ing of the relations and origins of nonlinearprocesses to the nature of electronic excita-tions and their interactions. As natural de-velopments in studies of nonlinear optical phe-nomena, the recent demonstrations of phase con-jugateS wave generation , optical bistablestates, and exceptionally large nonlinear op-tical susqeptibilities in organic and polymericmaterials"' have stimulated considerable growthin research and development activities in cen-ters throughout the world. These intriguingphenomena suggest a wide variety of potentialapplications in future optical telecommunica-tions, image reconstruction, integrated optics,optical signal switching and processing, datastorage, and optical memory and logic technol-ogies. We believe the recent advances in non-linear optics research can be rapidly acceler-ated and enlarged through fundamental studies oforganic and polymer crystals and films.

In recent years, theoretical and experimen-tal interest has ntered on the origin of theexceptional second and third or er nonlinearoptical responses observed for certain organicand polymeric structures, particularly as exhib-ited by the second order processes of secondharmonic generation (SHG) and linear electro-optic effect (LEO) properties. The case of 2-methyl-4-nitroaniline /VNA) (Figure 1) serves asan important example % MNA crystals possessSHG and LEO figures of merit 50 times those ofcommon dielectric insulators such as potassiumdihydrogenphosphate (KDP) and 200 times those qfsemiconductors such as gallium arsenide (GaAs) .The origin of such unusual nonlinear opticalresponses resides in the r-electron structure(Figure 2), and the current view is that the r -electron molecular structure represents thefinite chain limit to the infinite conjugatedpolymer chain.

2

II. NONLINEAR OPTICAL PROPERTIES AND ORGANICNONLINEAR MEDIA

Nonlinear optical responses are expressedthrough the constitutive relation for thedielectric polarization P(t) of a nonlinearmedium ir an intense optical field E(t) =Re (Eelwt) 10

P(t) = X(')E(t) + )(2)E 2 (t) + X(3)E 3 (t) +(1)

The first and third order terms in odd a9 wers ofE(t) are common for all materials. X") repre-sents linear optics, and X15) third order non-linear processes. Important examples includethird harmonic generation (w + w + w = 3w ) andself-focusin ( w-w + w - ). The second orderresponse X' ) occurs only in noncentrosymmetricmedia; that is, those lacking a natural centerof inversion symmetry. In general for secondorder processes, fundamental frequencies at wiand w combine to create a third frequency at

j (w3lf w2 - w 3 ) For example, having wlan 2 the same, the sum leads to second har-monic generation (SHG) (w + w - 2 w ), and thedifference, optical rectification ( w- w = 0).Moving one frequency down to dc results in thelinear electrooptic effect (LEO) (w +0 = w ).Thus, there are a large number of importantnonlinear optical processes over the entire

3

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-- m,

' .'.'.'. '.% ." ' ' ' 2 ; 1, , ' .'...-. ,"." " -.-- ' , ',

frequency range, and X frequenq y dependencesor dispersions, of 'and x, are funda-mentally important properties of the nonlinearmedium.

There are numerous ways to induce a non-linear optical polarization in a medium. Theygenerally involve either absorptive, or reac-tive, nonlinear responses. In an absorptiveresponse, the absorbed photon causes an actualtransformation in the physical state of thenonlinear medium. There is a net populationchange in the statistical average of the excitedcenters, and the response is accompanied by realoptical loss of the propagating waves. Examplesinclude photorefractive migration of charge,resonant saturable absorption, and high orderfield alignment in liquid crystals.

In contrast, a reactive nonlinear responseis due to lossless virtual optical excitationsin the nonlinear medium that do not involve netpopulation changes or any material transforma-tions. Examples include non-resonant harmonicgeneration, frequency mixing, and optical Kerreffects. By far the shortest response timesoccur in reactive nonlinear media since reactiveresponses do not involve relatively slow mater-ial changes of the nonlinear medium. Moreover,among the possible virtual excitations of reac-tiv 5 responses, electron excitations of order10- seconds are intrinsically faster thanphonon excitations which involve much slowernuc Lar displacements and vibrations of order1o- T seconds.

For large classes of conjugated moleculesand polymer structures, the remarkable propertyis that the nonlinear optical responses aredominated by lossless virtual excitations of the7 -electron states, especially those possessinglarge charge correlations. This property is mostreadily observable in second order responses.For example, in crystalline solids, only elec-tronic excitations contribute to second harmonicgeneration, whereas both electron and phonon

4

4"-" %

excitations contribute to the linear electro-optic effect. For MNA (Figure 1), for exa iu e,the large second harmoniclusceptibility X

w2;,w) of 500 + 125 x 10 m/V was shown toethe same as the linear electrooptic suscepkibil-ity x1 (- W ;w '0) - 500 + 150 x l0- r/Vwithin experimental error, demonstrating thatthe second order response is primarily purelyelectronic with little or no lattice contribu-tion, and thus intrinsically fast . Examples atthe molecular level include the frequency depen-dence of the microscopic second order responseof organic molecular sjtructures, such as MNA,that is described below '

III. ORIGIN OF X (2 ) AND HIGHLY CHARGECORRELATED r -ELECTRON STATES

Currently, the 7T-electron excitations areviewed as occurring on individual molecular, orpolymer, sites and providing macroscopic sourcesof nonlinear response through the correspondingon-site microscopic nonlinear optical suscepti-bility. Since intramolecular bonding interac-tions (100 kcal/mole) are much stronger thanrelatively weak van der Waals intermolecularinteractions (1 kcal/mole), each molecular, orpolymer, unit is essentially an independentsource of nonlinear response. Each term in themacroscopic polarization of eqn. (1) can thus beexpressed as comprised of the correspondingmicroscopic nonlinear optical response

p(t) = cE(t) + E2 (t) + y E3 (t) + -" (2)

where a is the linear polarizability, and Band Y are the second and third order nonlinearelectronic susceptibilities, respectively. Inthe rigid 1attice gas approximation, the macro-scopic X(2 for example, is expressed as

5

)

X 1jk(-W3; q- 1 W2 )

Nf Wi fW2 fW < 8ijk(-w3 WI W2) > (3)

w ere N is the number of sites per unit volume,

f! represent small local field corrections,

and 8 ik is averaged over the unit cell. A7analogols expression is obtained for the Xresponse. Thus, unlike inorganic semiconductorsand dielectric insulators, the problem of under-st ding th 3,rigin of the large macroscopicX or X ,is reduced to experimental and

theoretical studies of the corresponding micro-scopic a , or Y, of single molecular, or poly-mer, units, making up the optically nonlinearorganic solid.

The quantum-field theory treatment1 2 ofSik(-2w;w,w) provides a convenient diagram-matic representation of the three-wave-mixingprocess as shown by the space-time diagrams inFigure 3. A ground state Ig> molecule witheignestates In> and In'> and eigenvalues W nan electromagnetic field E (t) = Re(I erspondt to the perturbing Hamiltonian H' =eEw(t).r in the dipole approximation that in-duces an electronic polarization in the mole-

-' cular structure,

p2w: a (-2w; w. w)EwEw (4)1 j k i j kf k

where both the fundamental w and createdharmonic 2 w frequencies are below electronicresonances but well above vibrational and rota-tional modes.

The quantized field is given by the

6

expression

c at (5)

V ka

where a+(a) is the photon creation (annihila-tion) operator, e, the polarization unit vectorwith a= 1,2; k, the unit direction vector; andV, the normalization volume. The resultingquantum-field theory expression for Bijk is

+ (r ,'r. .r, "0 (6)) w(w~4 ,,+,

sse. Th qunite suc a ,i an r

F .(rr.',,.,,r,.. - ,') ,,a.'1,. ,+ ,',. 4 (g2)( wwW)I (6)

1 w ' 1 1( , ' - ' )

where summations are over the complete sets ofeigenstates In> and In'> of th9 unperturbedsystem. The quantities such as r'~ and r1 are

matrix elements of the ith comionents thedipole operator for the molecule between theunperturbed ground (g) and excited states (n andn') r _ =<Ir-In>, and between two excitedstate% ri. <nlr'In'", respectively; Arlrn _ - rC1 is the difference between the excited

ana grotd state dipole moments; w is the fre-quency of the applied optical field; and wWa - W is the difference between the excItedand grouid state frequencies.

7

0.%

The 7t-electron states In> and In'> aretrue many-body states because electron-electroncorrelations due to natural repulsive Coulombinteractions tend to localize the otherwisedelocalized electrons. In calculating a andits dispersion from eqn. (6) , these many-bodystates can be obtained by a self-consistentfield procedure (SCF) including configurationinteractions (CI) for electron-electron correla-tions (Figure 4) . Detailed theoreticalanalyses of the contributions to a for severalmolecular structures have demonstrated that thenature of the electron-electron correlationsdetermiges the nonlinear response of the organic

* medium

Of the many IT-electron excited states inconjugated structures, only those statespossessing highly asymmetric charge correlationsare principally responsible for the measuredmagnitude, sign and dispersion of a • Thesestates characteristically possess relativelylarge, values for both the transition moment '

= -gr1 and the dipole moment dif erence Al n =

-erl Important in eqn. (6) for a For MNA, forexample, of the nine lowest energy singlet ex-cited states, only two highly charge correlated7T -electron states uniquely determine . Theelectron wavefunction contour diagrams of theMNA ground state and one of the principal ex-cited states are given in Figure 5 along withthe corresponding values for x x and -+ x

Upon virtual excitation, the many-body excitedstate clearly exhibits redistribution of elec-tron density from the region of the NH 2 donorgroup to the NO2 acceptor group and its neigh-boring sites, giving the excited state a highlycharge correlated feature along the dipolar xaxis of the molecule. Similar results have beenobtained for a wide raj e of molecular struc-tures such as quinoids', stil nes, polyenes,acetylenes, and diacetylenes . The basicmechanism of charge correlations always appearsto determine the nonlinear optical response.

|.B

IV. DISPERSION OF a FOR ORGANIC SYSTEMS

The microscopic susceptibility a and itsdispersion is experimentally determined by elec-tric field induced second harmonic genernion(DCSHG) measurements of liquid solutions asillustrated in Figure 6. An applied DC field E°removes the natural center of inversion symmetryof the solution, and the second harmonic signalis measured using the wedge Maker fringe method.The measured polarization at the second harmonicfrequency 2w produced by applied fields at wand zero is given by

p 2w E 0 Ew (7)

with statistical averaging over the Boltzmanndistribution of molecules and all laboratoryfields havin% a common linear polarization. Theterm i El is the effective second harmonicsusceptiblllity of the liquid solution. When theground state dipole moment + is aligned alongthe molecular x axis, 1 1111 is given by

r,,,,=r,=Nf0 (f'0)21 1 (8)

where y is the microscopic third order suscept-ibility, which is negligibly small for con-jugated molecules ' < < a x U /5kBT; ax is thevector part of B ijk'

ax = axxx + 1/3(e xyy + xzz + 2 yyx +

2 a zzx ) (9)

9

I;

I,< '" ""V " "" " " " " ' " .. .

However, local field effects and solutioninterac Ions can lead to widely differing valuesfor S. By combined experimental studies ofDCSHG, dielectric constant, index of refraction,and specific volume, an infinite dilution ex-trapolation method for B was developed thataccounts for local fields and minimizes solutg-solute and solvent-solvent interactions".Theoretical gas phase values of a can then becompared to infinite dilution values if solventinduced shifts of the singlet-singlet electronicexcitations of the measured system areincluded

From the DCSHG measurements of 5, the fre-quency dependent experimental values a expt(Figure 7) for MNA in dioxane are observed toincrease smoothly as the fundamental frequencyw is increased and begin to diverge as 2w

approaches the excitation frequency Kwng - 3.5eV), which corresponds to the first majoroptical excitation to the charge correlatedexcittd states of MNA dissolved in dioxane. Theaexp values are red shifted to lower energiescompared to the theoretical gas phase dispersioncurve due to solvent induced (solvatochromic)shifts A w n& to lower energies of the gas phasesinglet-sin let excitation energies. Theseshifts are easily observable in spectra for MNAmeasured in the gas pase and in dilute dioxanesolutions (Figure 8)f .

It has lonv been known that solvent-inducedshifts in i - r excitation energies are princi-pally caused by dipole mediated interactionsthat change the difference between the soluteground state and excited state dipole moments.The shifts in excitation ener' -iwng in thedipole approximation is given by

RAW =AAq(1.+1g)+BASq ~ (1

10

where

A" 1 n2- 1I

a3 2n 2 +I

B2 1E-I n2-I1a 3 2E+1I2n2+ 1

with n the index of refraction of the solution;tEe the dielectric constant of the solution; a,the effective cavity radius of the molecularsite; p and V the solute dipole moments forthe excited and ground states, respectively.The first and second terms, respectively, arethe reaction field experienced by the permanentdipole moment of the solute due to interactionwith (1) the induced dipole moments of the sur-rounding molecules, and (2) the permanent dipolemoments of the solvent and of other solute mole-cules at relatively high concentrations.

New values for the state energies tyng ofthe excited states for MNA in dioxane were ob-tained using eqn. (10) and the SCF-CI calcula-tions for the frequency dependent 8 of MNAwere repeated. The resulting theoretical dis-persion curve for MNA is directly compared withthe frequency dependent experimental values" expt in Figure 7. Within experimental error,the agreement between experiment and theory isquite satisfactory. One can conclude, there-fore, that the microscopic electronic mechanismwhereby virtual excitations to highly chargecorrelated Tr-electron excited states result inexceptional second order nonlinear optical re-sponses is essentially correct and that thetheoretical SCF-CI calculation represents asuitable procedure for determining and analyzingthe intrinsic 8 components. Moreover, in re-gard to experimental methodology, these resultsdemonstrate that DCSHG liquid solutions deter-

IIi

!.

minations of 8 must account for solvent inducedchanges in the electronic .excitations of themeasured system. For T - 7 excitations, clas-sical reaction field methods appear to adequate-ly account for these solvent effects as dipolemediated interactions.

Finally, it is important to point out thatthere is a direct relationship between the elec-tronic second order nonlinear optical response(eqn. (6)) and dipole mediated solvent shifts(eqn. (10)) of optical excitations, or solvato-chromism. Important to the development of newoptically nonlinear structures, one can initial-ly identify by simple independent measurementsof the solvent shift _iw suitable structurespossessing optically nonlinear responses opti-mized for experimental studies, or opticaldevice applications. These structures wouldinclude organics, charge-transfer salts, dyes,liquid crystals, polymer{ and even organictransition-metal complexes.

..

V. SECOND ORDER NONLINEAR OPTICAL PROCESSES ANDSTRUCTURALLY ORDERED POYMERS: DISUBSTITUTEDDIACETYELNE POLYMERS

The same highly charge correlated r -elec-tron states can be systematically incorporatedinto macromolecular polymer structures by intro-ducing optically nonlinear molecular units aseither (i) pendant side groups, or (ii) monomerrepeat units, of the polymer main chain. Theimportant case of the second order nonlinear

* optical process of second harmonic generationwill be discussed although many of the samefeatures apply to third order processes. Along

-.[. with the symmetry requirement of noncentrosym-metry, several basic conditions are required ofthe nonlinear polymeric medium for efficientsecond order processes. The nonlinear mediumshould exhibit (i) optical transparency over thefrequency range of the fundamental and second

I 12

- ..*E

harmonic waves to avoid optical loss; (ii) phasematching where the optical birefringence cancelsthe natural dispersion in order to obtain equalindices of refraction at the fundamental and

" second harmonic frequencies for efficient secondharmonic generation; and (iii) long range struc-tural order so that the microscopic 8 compon-ents are not diminished, or cancelled entirely,over large regions of the nonlinear medium.

Phase matched second harmonic generation insingle crystal polymers was first demonstratedin noncentrosymmetric diacetylene polymers sub-stituted with MNA molecular sites as pendant

*. side groups of the repeaN units of the conjug-. ated polymer main chain. Diacetylene polymer

structures were selected to satisfy the basicconditions for second order nonlinear opticalprocesses but also to allow systematic develop-ment and understanding of the fundamentalphysics through combined theory and experiment.

Optically nonlinear single crystal diacet-ylene polymers are formed by polymerization ofdisubstituted dicetylene monomers in the solidstate (Figure 9). With R as the acentric, oreven chiral, optically nonlinear side group,diacetylene monomers crystallize as linear ar-rays that can be initiated to polymerize bytherml annealing, by irradiation with uv, x-rays or electron beams, and in some cases,even mechanically - The resulting diacetylenepolymer chain possesses three dimensional longrange order as illustrated, for example, by theI" unit cell polymer structure of DNP [1,6-bis(2,4-dinitrophenoxy)-2,4-hexadyneI (Figure 10) fromour early work of incorporating optically non-linear dinitrophenoxy units as pendant sidegroups2l.

In addition to the v-electron system con-tained in the R group substituents, the polymerconjugated chain possesses a 7r-electron bandstructure that, in general, exhibits opticaltransparency from approximately the mid-visibleregion into the near infrared, leading to in-

13

dices of refraction in the range 1.6 to 2. Thenatural structural anisotropy of the orderedpolymer chains leads to large optical birefrin-gence important to phase matching. Under polar-ized visible light, single crystals exhibit alustrous metal-like reflectance for polarizationalong the chain axis direction and a rich blue,or green, reflectance for polarization perpen-dicular to the chain axis. Diacetylene polymercrystals also possess exceptional radiation andmechanical damage resistance; for instance,radiation damage thresholds have been observedas high as 1 gig watt/cm for 25 nanosecondpulses at 1.89 im .

Theoretical SCF-CI calculations reveal thatthe highly charge correlated w -electron excitedstates of MNA molecular units are present in MNAsubstituted diacetylene monomers such as MNADAand NTDA (Figure 11) . DCSHG measurements ofthe MNADA monomer, for example, show the s8elarge value of a as MNA alone (Figure 11)In the solid state, the phase matched secondharmonic signal increases as the diacetylenepolymer is formed. Figure 12 shows early dataof the second harmonic intensity for NTDA micro-crystals polymerized under x-ray radiation. Asthe polymer forms, the second harmonic intensitysteadily increases to 10-15 times the lithiumiodate reference signal. The constant secondharmonic intensity at high x-ray dosages beyondthat required for complete polymer formationconfirms that the observed signal is intrin icand not due to x-ray induced defect formation" .

In addition to MNA and DNP pendant groups,other molecular units intrinsically possessinglarger microscopic susceptibilities can be sub-stituted on diacetylene monomers (Figure 13).Development of the new quinoid containing strutures is based on earlier theoretical studies7.An exceptionally large $ value of -170 x 10-30esu for DCNQA (7,7-diamino-8,8-dicyanoquinodi-methane) resulting from charge correlations inthe r -electron excited states was predicted andsubsequently confirmed by DCSHG experiments on

14

.

the related derivative DCNQI [2-(4-dicyanometh-ylenecyclohexa-2,5-dienyli.Re)-imidazolidine)with $ of -240 + 60 x 10- esu. These classesof structures continue under study particularlyas optical guided wave structures fabricated byLangmuir-Blodgett methods 3 .

VI LIQUID CRYSTAL POLYMERIZATION ANDLIQUID CRYSTAL POLYMERS: DIVINYLDIACETYLENES

In the diacetylene monomers and polymersexhibiting phase matched second harmonic genera-tion, such as MNADA, both theory and experimentsuggest that the charge correlated states remainon the MNA side group because the connectingmethylene carbon group essentially acts as a w -electron blocking group. Our SCF-CI calcula-tions of the electronic structures of alternatediacetylene monomer structures show that removalof the methylene carbon group, or replacement byan ethylene group causes extensive delocaliza-tion of the r -electron states over the entiremonomer structure. The simplest structures thatillustrate the delocalization are centrosymmet-ric divinyldiacetylene structures (Figure 14).

The contour diagrams (Figure 15) of theprincipal ground state and low lying singlet( B ) n-electron excited state for bis(p-meth-ylpgenyl)-divinyldiacetylene (DV3I) summarizesa number of remarkable features . Upon opticalexcitation, the w-electron distributionthoroughly reorganizes over every carbon sitecomprising the DVDA1 structure. There is nodistinction between the aromatic ring and diace-tylene states; entirely new DVDA n-electronstates are formed. Second, charge forrelationsare present in the lowest lying Bu excitedstate with the most pronounced charge correla-tions occuring on the first vinylic carbon atomadjacent to the phenyl ring structure. Theassociated optical transition moment x forthis state is greater than 10D compared to 3.6D,for example, for the principal 7r-electron state

15

* 15

of MNA described earlier.

Extensive molecular synthesis of DVDA mono-mer structures has been completed with varietiesof enhancing substituents yielding both centro-symmetric Sd noncentrosymmetric structures(Figure 14) . Additionally, the theoreticalcalculations show that the DVDA structures areextended rigid rod structures possessing highlyanisotropic polarizabilities reminiscent of mes-ogenic, or liquid crystal, molecular structures.Indeed, calorimetric measurements demonstratethat bis(substituted phenyl)-divinyldiacetylenemonomer crystals undergo liquid crystal phasetransitions above room temperature as illus-trated by the DSC data for DVDA3[l,8-bis(p-propylphenyl)-octa-l,7-diene-3,5-diyneI inFigure 16. Upon heating, DVDA3 undergoes a com-pletely reversible liquid crystal phase trans-ition at 1600 C to a stable nematic phase that ishighly viscous and optically birefringent. Byoptical microscopy, the disinclination texturecorresponds to a nematic phase. With furtherheating, irreversible polymerization spontan-eously takes place near 180°C with exothermic

*heat evolved in the liquid crystal state,resulting in a highly conjugated polymer struc-ture in a glassy, or liquid crystal, polymer

.state depending on the thermal annealing con-ditions. The resulting liquid crystal polymer isyellow-orange in color with high opticalquality, insoluble in common organic solvents,and thermally stable above 210 0 C.

X-ray crystallographic studies26 reveal thatDVDA monomers condense at room temperature asmesogenic crystals containing linear arrays andplanar sheets of DVDA sites aligned parallelwith their long molecular axes. Such structuralpacking features are usually low temperatureprecursors to liquid crystal states. As anexample, Figure 17 shows a typical x-ray projec-tion of the monoclinic (C2 1/c) mesogenic pack-ing structure of DVDA3. As to the final DVDAconjugated polymer produced by liquid crystalstate polymerization, the structure need not be

16

the same as the common linear chain polymerstructure obtained from topotactic solid statepolymerization of disubstituted diacetylenes ofFigure 9. In fact, there is accummulatingexperimental evidence for the liquid crystalpolymerization of DVDAs to involve some forms ofextended conjugated ring structures.

Figure 18 shows typical optical transmissiondata for a DVDA1 polymer film contained betweentwo glass slides. Optical transparency with lowoptical loss extends from the near infraredregion at 2 'p m to the visible red region at 0.6Pm where there is sharp onset of electronicexcitations of the conjugated polymer struc-tures. Our knowledge of three dimensionallylong range ordered structures can be mapped tothese liquid crystal DVDA systems (Figure 19) bymaximally orienting the number N of orderedunits with orienting electromagnetic fields(poling) and then locking the preferred struc-ture in place by subsequent polymerization.

To date the new DVDA structures haveexhibited several major properties:

(i) noncentrosymmetric DVDA structures exhibitlarge phase matched second harmonic generation;

(ii) both centrosymmetric and noncentrosymmetricDVDA monomer crystals undergo liquid crystalphase transitions;

(iii) both centrosymmetric and noncentrosym-metric DVDA monomers spontaneously react byliquid crystal state polymerization to formconjugated liquid crystal polymers displayinghigh optical transparency; and

(iv) by third harmonic generation measurements,unoriented centrosymmetric DVDA liquid crystalpolymers exhibit unusually large thi 1j)ordernonlinear optical susceptibilities X thatare 10 -10 times larger than reference glass orquartz27.

17~4

71

. ,.'

I,

In summary, DVDA monomers provide crystals,liquid crystals, and glassy and liquid crystalconjugated polymer structures for studying ex-ceptional second and third order nonlinearoptical processes in organic and polymericstructures. In general, as we have summarizedabove, the magnitudes of the macroscopic secondX(2) and third X order nonlinear opticalresponses critically depend on the nature of themicroscopic charge correlated w-electron statesand also on the symmetry and degree of struc-tural order. Such symmetry and order commonlyresult by virtually accidental crystal packingduring single crystal growth processes. As iswell known, however, liquid crystal phases pos-sess 'cooperative molecular alignment that canbe changed and thereby controlled by externallyapplied electric and magnetic fields to increasethe nonlinear optical response z u. Addition-ally theoretical results such as those from theSCF-CI calculations can be combined with stan-dard guidelines and moelcular criteria thatwidely exist for providing liquid crystallinephases. The spontaneous formation of liquidcrystalline conjugated polymer structures fromorientable liquid crystal monomer phases, asexemplified by the DVDA systems, appears toprovide high potential both scientifically andtechnologically. Having now been demonstratedfor the DVDA case, there likely exist many otheroptically nonlinear structures possessingsimilar behavior and properties. Finally, thestudy of important nonlinear optical phenomena,processes, and device applications of organicand polymeric media comprises a new field whichwe refer to as Molecular Optics.

ACKNOWLEDGEMENTSThis research was supported by the Defence

Advanced Research Projects Agency under GrantNo. DAAK-700-77-C-0045 and the National ScienceFoundation Materials Research LaboratoriesProgram under Grant No. DMR-79-23647. Wegratefully thank K.N. Desai, A. Panakal, K.D.Singer, M. Kuzyk, C. Grossman, A McGhie, and P.Carroll.

Is~18

4_

FIGURE CAPTIONS

Figure 1. The molecular structure of 2-methyl-4-nitroaniline (MNA) as determined by x-raycrystallography. Key: C, carbon; N, nitrogen;0, oxygen; and H, hydrogen. Ref. 8.

Figure 2. w -electron structures.

Figure 3. Diagrammatic representation of secondharmonic generation.

Figure 4. SCF-CI calculation procedure for BRef. 5.

Figure 5. Contour diagrams of the wavefunctions for the MNA ground state (bottom) anda principle excited state (top) showing chargecorrelations. Ref. 14.

Figure 6. The sample arrangement in the DCSHGexperiment where the solution is insertedbetween two glass slips (a) and DC field E° isapplied (b). Ref. 12.

Figure 7. Frequency dependence of B for MNA.Key: +, experi ental exPdata points; and-

theretca x ~aapit;ad-theoretical $ curve accounting for solventshift effect. Ref. 12.

Figure 8. Optical absorption spectra of MNAdissolved in dioxane (solid line) and in gasphase (dashed line) showing solvent shift. Ref.12.

Figure 9. Solid state polymerization ofdisubstituted diacetylene monomers to formcrystalline polymer chains.

Figure 10. Monoclinic unit cell of thediacetylene polymer of i,6-bis(2,4-dinitrophenoxy)-2,4-hexadiyne (DNP). Ref. 21.

Figure 11. Comparison of MNA (a) and MNADA (b)Bx values as measured by DCSHG. Ref. 22.

19

* 0 I . -+ . .. * * . - ... *-*, , ** : m , ,+, • - ,. • + . . . .

Figure 12. The second harmonic 1 2w of NTDAmicrocrystals relative to the second harmonicintensity of lithium iodate (LiIIO3) powder I wwith increasing x-ray induced polymerization.Ref. 22.

Figure 13. Classes of disubstituteddiacetylene structures exhibiting phase matchedsecond harmonic generation.

Figure 14. Bis(phenyl)-divinyldiacetylenemonomers with various substituent groups.

Figure 15. Contour diagrams of the wavefunctionfor the DVDAI grounq state (bottom) and theprinciple singlet ( Bu) excited state (top)showing delocalization and charge correlations.Ref. 24.

Figure 16. DSC calorimetric thermogram of DVDA3

showing liquid crystal phase transition followedby liquid crystal polymerization. Ref. 25.

Figure 17. Mesogenic crystal structure of DVDA3showing linear arrays and planar sheets aspossible precursors to liquid crystal state.Ref. 26.

Figure 18. Optical transmission of DVDAIpolymer film over the near infrared-visiblerange. Ref. 27.

* Figure 19. Experimental poling configurationfor orienting DVDA monomers and polymers in anapplied electromagnetic field while monitoringthe nonlinear optical signal w 3 derived fromthe inputs wI and w 2 . Ref. 27.

20

I,

REFERENCES

1. R.W. Hellwarth, J. Opt. Soc. Amer. 67, 1(1977).

2. H.M. Gibbs, S.L. McCall and T.N.C.Venkatesan, Phys. Rev. Lett. 36, 1135 (1976).

3. See, for example, A.F. Garito and K.D.Singer, Laser Focus 18, 59 (1982).

4. C. Sauteret, J.P. Hermann, R. Frey, F.Padere, J. Ducuing, R. Baughman, and R. Chance,Phys. Rev. Lett. 36, 956 (1976).

5. S.J. Lalama and A.F. Garito, Phys. Rev.A20, 1179 (1979).

6. C. Flytzanis in Nonlinear OpticalProperties of 'Organic and Polymer Materials, D.Williams, ed., ACS Symp. Ser. No. 233 (1983).

*7. B.F. Levine, C.G. Bethea, C.D. Thurmond,R.T. Lynch, and J.L. Bernstein, J. Appi. Phys.50, 2523 (1979).

8. G.F. Lipscomb, A.F. Garito and R.S. Narang,J. Chem. Phys. 75, 1509 (1981).

9. Nonlinear Optical Properties of Organic andPolymer Materials, D. Williams, ed., ACS Symp.Ser. No. 233 (1983).

10. N. Bloembergen, Nonlinear Optics (W.A.Benjamin Inc., New York 165)

11. C.C. Teng and A.F. Garito, Phys. Rev. Lett.50, 350 (1983).

12. C.C. Teng and A.F. Garito, Phys. Rev. B28,6766 (1983).

13. 0. Zammani'Khariri and A.F. Garito, to bepublished.

14. 0. ZammaniKhamiri, C.C. Teng and A.F.

21

L=1No.,

Garito, to be published.

15. S.J. Lalama, K.D. Singer, A.F. Garito andK.N. D-sai, Appl. Phys. Lett. 39, 940 (1981).

16. K.D. Singer and A.F. Garito, J. Chem. Phys.75, 3572 (1981).

17. See, for example, A.T. Amos and B.L.*1Burrows, Adv. Quantum Chem. 7, 289 (1973) and

references therein.

18. A.F. Garito, K.D. Singer, K. Hayes, G.F.Lipscomb, S.J. Lalama, and K.N. Desai, J. Opt.Soc. Amer. 70, 1399 (1980).

19. See, for example, G. Wegner in MolecularMetals, W. Hatfield, ed., Plenum (197779y andreferences therein.

20. J.E. Sohn, A.F. Garito, K.N. Desai, R.S.Narang and M. Kuzyk, Makromol. Chem. 180, 2975(1979).

21. A.R. McGhie, G.F. Lipscomb, A.F. Garito,K.N. Desai, and P. Kalyanaraman, Makromol. Chem.182, 965 (1981).

22. K.D. Singer and A.F. Garito, to bepublished.

23. A.F. Garito, K.D. Singer and C.C. Teng, inRef. 9.

24. 0. Zammani'Khamiri and A.F. Garito, to bepublished.

25. A. Panakal, K.N. Desai, A.R. McGhie andA.F. Garito, to be published.

26. K.Y. Wong,, K.N. Desai, P. Carroll and A.F.Garito, to be published.

27. K.Y. Wong, C.C. Teng and A.F. Garito, J.Opt. Soc. Amer, in press.

22

28. See, for example, G. Meredith, J.G. Van

references therein.Dusen and D.J. Williams, in ref. 9 and

1

23

H H

4..N

:4H

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Fig 1

-H H--

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7"- ELECTRON ORGANIC7' 7r SYSTEMS

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DIACETYLENEPOLYMERS .

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SCF -Cl CALCULATION PROCEDURE FORMOLECULAR SECOND ORDER OPTICAL

SUSCEPTIBI LIT IES.

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ONE- ELECTRON LEVEL.

* MOLECULAR HARTREE-FOCK CALCULATION

0 EXPERIMENTAL PHOTOEMISSION DATA USEDFOR VERIFICATION OF I-ELECTRON LEVEL.

21 CALCULATION OF MANY-BODY EXCITED STATE

ENERGIES AND TRANSITION MOMENTS

e CONFIGURATION INTERACTION CALCULATIONFOR EXCITED STATES

e ULTRAVIOLET ABSORPTION DATA USED FOR

,VERIFICATION OF EXCITED STATES

I. CALCULATION OF MOLECULAR SECOND ORDER

SUSCEPTIBILITY

0 SUSCEPTIBILITY DETERMINED DIRECTY FROM

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% COMPARISON OF CALCULATION WITH RESULTSOF D.C. INDUCED SECOND HARMONIC GENERATION

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Fig. 9

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Fig. 12

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SECOND HARMONIC GENERATION

R -(CH)- C-=C-C=EC-(CH2)m-R'

R PENDANT SIDE GROUP

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