Indian JqurnaI of Chemi$tryVol. 33Al June 1994, pp. 506-523

Pohtmeric phthalocyanines anjd other electrically conducting polymers forelectronic and phptonic applications - A review t

S venkafchalam* & V N KrishnamurthyPropellants Polymers and Chemicals En ity, Vikram Sarabhai Space Centre, Trivandrum 695022, India

Re eived 19 November 1993

tion defects such as solitons, polarons and bi­polarons are associated with bulk l~lectrical con­ductivity which is enhanced by doping or photoexcitation 2.12-14.

Organic conjugated polymers are synthesizedeither by direct or by indirect approach. Thedirect approach involves the convlersion of themonomer to polymer by either addition or con­densation polymerisation. The indirect route in­

volves the preparation of the precursor polymer(either through addition or condensation polymer­ization), followed by a variety of reactions such aselimination, addition, isomerization, etc. The mostpopular conductive polymer, viz., polyacetylene(PA), is made by addition polymerization of ace­tylene gas, which is passed through a solutioncontaining Ziegler. catalyst, Ti(OBukAl(Et)3' orLuttingers catalyst, NaBH4 + Co(N03)2 (refs 15­17). Depending upon the temperatun~ of polymer­ization, cis or trans PA can be obtained. Film

Introduc ion

Orga . and metallo organic polymers h ve at­tracted .de attention for their electrical elec­tronic, onlinear optical, electrochemic andphotoch ..Cal applications 1-11.For poly rs toexhibit e ctrical conductivity, ordered conj gatedstructure with extended pi electrons an largecarrier c: ncentration are prerequisites. The eneralprocess . volves the synthesis of conjugate poly­mers 1'0 owed by incorporating the re uiredcharge ca rier in desired concentration.

The el ctrical conduction behaviour in1 these

polymers has been extensively studied. Nov I con­duction echanisms involving coupling 0 elec­tronic ex .tations to nonlinear conformation havebeen pro osed1.2.4.Dopant induced bond al em a-

Polymers are generally insulators an to exhibit electrical conductivity they must have orderedco jugation with extended pi electrons d large carrier concentrations. This review summarises theva 'ous methods of making conjugated polymers by either addition or condensation H:actions ofrn omers by chemical or electrochemic routes. These reactions can follow either a direct route oran . termediate precursor route, follow by elimination, addition or isomerization reactions. Conju­gat d polymers are made conductive ge erally by doping or by thermal treatment. The doped po­ly ers have limitations like poor stabili y, poor processibility and corrosivity. This review lists theva ous methods adopted for overcomin these drawbacks. The review also gives an account of var­iou types of phthalocyanine polymers t have been prepared in our laboratory, and their electricalan magnetic properties. The conductivi of these polymers increases by seven orders of magnitudeby ermal treatment. These polymers h ve suitable structures that facilitate solitonic, polaronic andbip laronic conformations leading to I ge, ultra-fast nonlinear optical processes. The peripheralfun tional groups can be used for struc ral modifications to improve processibility and allow fabri­cat' n of various device structures such as films, channel wave guides, planar wave guides, fibres,etc. The presence of macrocyclic structu es in these polymers imparts mechanical, thermal and envi­ro ental stabilities. Conjugated polym rs, including phthalocyanine polymers, also exhibit nonli­

optical properties because of their esponse to high intensity radiations like laser. The require-ts of organic molecules and polymer to have x(2) and X(3) properties, and the various methods

to study their behaviour as well as techniques employed to improve the nonlinear optical res­e are also discussed. The various ethods that have been used to synthesize ladder polymerssolubilisinggroups for processibility ve also been discussed.

t A part ias presented at the US-India workshpp on"Frontiers a Research on Polymers and Advanced Materials"held at Goa, anuary 5-10, 1992.

I

VENKATACHALAM et aL: ELECfRONIC & PHOTONIC APPUCATIONS OF POLYMERIC PHTIIAWCYANINES 507

Ar • %-{O)-, @,l10-@-0Ei, ~(II)

PolY-Jrphenylene has also been synthesized viapoly.merisation of soluble derivatives of 5,6-dihy­droxycyc1ohexa-l,3-diene, which is prepared bybacterial oxidation of benzene. These derivatives

are polymerised using free radical initiators togive soluble precursors, which on thermolysisyield polY-Jrphenylenes as shown in (ill) (ref. 25).

Methods of doping organic conducting polymersThe conjugated polymers are made conductive

by doping which is carried out through chemicalroute, electrochemical route, or photochemicalroute. Doping reactions are characterized bycharge transfer from the dopant to the polymer orvice versa. An ionic state of the polymer, deloca­lised along the backbone, is formed with a coun­terion derived from ,the dopant. Conductivity ofthe doped material is varied and controlled by theamount of dopant used. Polymers such as poly­acetylene (PA), polyphenylene (PPP), polypheny­lene sulphide (PPS), polypyrrole (PPy), polythio­phene (PI') and polyqpinolene (PQL) are dopedby either chemical or electrochemical means andconductivity ranging from 1 to 2000 SI cm hasbeen achievedI9.26-35.Stretch orientation of dopedpolyacetylene has improved the electrical conduc­tivity to values above 105 S/cm (ref. 36). Highlyconjugated polyparaphenylene (PPP) has beenshown to have a conductivity of 104 S/cm (ref.37).

In electrochemical doping, the polymer to bedoped is used as one of the electrodes in an elec­trolytic cell in which the electrolyte contains theion for doping. For p type doping, the polymer isused as anode, while for n type doping the polym­er acts as cathode. The degree of doping dependson the dopant concentration, voltage applied andthe amount of charge passed.

Polymers that become conductive upon doping

•

R.IoW, OMe, Ph

'''>0' + ~~."o,ClII)

FJ)-::{FJFJ c-C!!C-CFJ ®+ +

F:JC_ CFJ FJC_CFJ

01-~:-g~~N~.tio!,~- tf::rc I co, ••

\JVV(J)

thickness can be controlled by the pressure ofacetylene gas, time of reaction and amount of ca­talyst usedI8.19. The electrochemical route givesconjugated polymers as peelable films on an elec­trode surface. It offers precise control on thethickness and electrical conductivity of the film,which is effected by varying the current density,solvent, supporting electrolyte, nature of electro­lytes, etc.20. Other conjugated polymers like po­lY(Jrphenylenes) (PPP), polyphenylene sulphide(PPS), polyquinolenes (PQL), and ladder polymersof the benzimidazo benzophenanthrolene (BBL)type are prepared by condensation polymeriza­tion21. The polymers made by either addition orcondensation routes via chemical or electrochemi­

cal polymerization techniques are intractable, andhence efforts are now concentrated on the pre­paration of melt and solution processible poly­mers with minimum defects and a high degree ofchain alignment. One of the most innovative anduseful techniques for the preparation of conduct­ing polymers has been the synthesis of highly sol­uble precursor polymers, that can be easily hand­led in solution, purified and then later convertedto th6 less tractable conducting polymer. The firstexample was the dehydrohalogenation of poly­vinyl chloride (PVC)22. This reaction, like most el­imination reactions on polymers, rarely goes tocompletion. The Durham route to polyacetylene(PA) developed by Feast et aF3 involves meta­thesis polymerization of Diels Alder adduct ofcyclooctatetraene and substituted acetylene togive a soluble and thermally unstable polymerprecursor. This undergoes retro-Diels Alder reac­tion slowly at room temperature or more rapidlyat 80°C, yielding amorphous PA eliminating asubstituted benzene as shown in (I) (ref. 23).

By choosing the appropriate monomers, retro­Diels Alder reaction can be controlled.

Poly(phenylene vinylene) (PPV) has been synthes­ized24 via a soluble precursor route, yielding theproduct via the elimination of HCI and dimethylsulfide as shown in (II).

508 INDIAN J CHEM, SEe. A, JUNE 1994

can be lassified into two categories38. T e firstcat(:gory is that in which polymers form simplecharge t ansfer complexes with donors or ccep­tors, e.g. PA doped with Na forms n-typ semi­conduct r or PPP doped with AsFs forms p-typesemi con uctor. These polymers are useful or ap­plication where variable conductivity is eded,i.e., the: olymer is used both as a conduct r andas a no -conductor. The switching of n -con­ductive ate to conductive state is reversi Ie and

this is c ntrolled by the amount- of dopant . Thisphe:nom on is made use of in secondary atter­ies, vis displays and variable trans .ssionshields .. his category of polymers is charac erisedby broa valence band and low ionisation oten­tial for -type conductors and by broad c nduc­tion ban and high electron affinity for n-typeconduct rs. In the second category, the po ymersbecome highly conductive due to format on ofnew cov lent bonds either during the dop' g pro­cess or uring other treatment. When poly heny­lene sUlIde is doped with AsFs, the latter intro­duces iC alent bonds or crosslinking in a ditionto partia oxidation. The polymer undergo s irre­versible hanges during the doping proces . Thepolymeri ation energy decreases and the bandwidth in reases on doping. While these po ymers 'are not uitable where doping/dedoping rocessis requir d for specific applications, they an beeasillypr cessed.

Photo hemical doping calls for a co poundwhose p otoproducts are effective dopan , e.g.,diaryl ilo onium salts39, which on photolysi yielda proto acid which serves as a dopan. Themethod selective; specific parts of the p lymercan be oped. In most of the doped po mers,solitons ( adical ions) or polarons form the hemi­cal basi for their electrical conductivity as inother mo ecular metals 16.

Most f the doped polymers have cert' limi­tationsl,211.13,14.16:(i) they exhibit stable con uctiv-ity only s long as the dopants remain in t e sys­tem, (ii) opants often migrate out of the ulk ofthe poily er over a period of time and th con­ductivilty of the polymer varies with the v .ationin lthe d pant concentration, (iii) the dop d po­lymers ave poor stability against heat, light,moisture and air, (iv) some of the dopants "ke 12,AsFs, N , S03 are corrosive, and (v) the dopedpolymer have poor processibility and sho metalto semic nductor transition (Peierl's instabi ity) atlow tern ratures.

T hermall treatment of polymersThermkl treatment is also a method which IS

used for the improvement of electrical conductiv­ity of polymers4o-48. This method appears to beadvantageous, as conductivity can be controlledby varying the conditions of treatment. These sys­tems have excellent stability towards environmen­tal conditions such as heat, light, moisture and air.Improvement in electrical conductivity occurs dueto the build-up of ordered polycondensed rings,allowing charge transport through charge-hoppingand tunnelling between aromatized rings. Thesecharge carriers are generated through formationof radical defects during the heat treatment.These materials show good thermal stability andstable conductivity and may find potential applic­ations in the fabrication of devices and as matrix

materials for advanced composites49,so. Althoughthey show large improvement in thermal propert­ies and electrical conductivity, most of these po­lymers have poor processability.

Improvements in processibilities of conductingpolymers

Various attempts have been made to improvethe environmental stability of the doped polyacet­ylenes and its copolymers through external pro­tection of the polymer by encapsulation in oxygenbarriers such as glass, silicone rubber, polyethy­lene, polyurethanes, epoxy resins and poly-p-xy­lylenes1-ss. The stability of conducting polyacety­lenes and substituted polyacetylenes has alw beenimproved to a limited extent by the use of classi­cal antioxidants such as phenols and hinderedaminesS6-S8.

The choice of counter ion (anion or cation) hasa major effect on the stability of conducting poly­mersS9-63,in addition to its effect on mechanicalproperties. In air, the perchlorate and tetrafluoro­borate salts of polypyrrole decompose and loseconductivity above ISOaC, but polypyrrole-p­toluene sulfonate salts remain stable up to 280aC.The electro-polymerization of pyrrole in aqueoussolution, in the presence of sulfonated cobaltphthalocyanine, form highly tractable films whichexhibit better stability in air as compared toperchlorate doped material. The conductivity ofthe composite increased from 4 to 400 S/cm dur­ing 2 months of exposure to air64. Pyrrole hasbeen polymerized in the presence of the sodiumsalt of metallophthalocyanine polymer which ex­hibits stable electrical and electrochemical pro­perties6s.

Electrical conduction requires conjugated struc­ture, but this in turn makes the chain rigid and in­soluble. The insolubility makes the polymer diffi­cult to purify or properly characterize. Hence the

"

/

I

VENKATACHALAM et al.: ELECTRONIC & PHOTONIC APPUCATIONS OF POLYMERIC PHTIIALOCYANINES 509

interest is in making soluble conducting polym­ers6Sb.

The major requirement for making useful gad­gets out of conducting polymers is to make themtractable. Some of the approaches that are adopt­ed for this purpose are66:

(i) the preparation of blends, dispersions andcomposites, (ii) the synthesis of random, graft andblock copolymers, (iii) the use of precursor po­lymers that can be converted to conducting po­lymers by thermal and reactive treatments and,(iv) the use of solvents like molten iodine andAsFs-AsF3, that render some of the conductivepolymers soluble.

Polypyrrole films formed in the presence of p­toluene sulfonate in acetonitrile gave a flexiblepolymer. Pyrrole electro-polymerized in the pres­ence of a sulfonated styrene and hydrogenatedbutadiene yields a flexible polymer that can bemelt-pressed to form conducting polymerplaques67. Dodecylsulfate anion doped polypyr­role renders flexible conducting films with tensilestrength of the order of 10 MPa (ref. 68).

Polymer blending or alloying is yet another ap­proach to processible polymers. This can beachieved either by chemical or electrochemicalmeans. For example, suspensions of polypyrrolehave been prepared by chemical polymerizationusing ferric chloride in an aqueous solution of ahigh molecular weight polymer such as methylcellulose or polyethylene oxide. They are used formaking polymer composites used in all polymersecondary batteries69.

Another technique for preparing soluble deriva­tives of intractable polymers is to add solubilisingsubstituents to the monomer. Thus, insoluble pol­yphenylenes have been rendered soluble in com­mon solvents such as chloroform by suitable sub­stitution on the phenyl ring. They are renderedwater soluble by introducing carboxyl terminals 70.Chemical and electrochemical polymerization ofN-substituted pyrroles gave polymers of substan­tially lower conductivity as compared to that ofunsubstituted polypyrrole7l. Substituted pyrrole orthiophene having alkyl or alkoxyl group substitu­tions at 3 or 4 positions gave soluble and proces­sible polymers7l-73. But long alkyl groups at the 3or 4 positions reduces the conjugation length· andhence the electrical conductivity due to stericeffects74,7s. Dialkoxy substituted thiophene viny­lene copolymers in which the vinylene units act as"spacer" to reduce steric interactions of substi­tuents show improved processibility and electricalconductivity. The doped polymers are also trans­parent16,77. By incorporating sulphonic or car-

boxylic acids in the alkyl substituent of the 3-al­kylthiophenes, water soluble thiophenes havebeen prepared with excellent tractability and con­ductivity78. To obtain highly conducting substitut­ed polyheterocycles, an alkyl spacer is used tosuppress electronic inductive effect of the heteroelements and avoid the effects of steric hindrance

of the bulky groups on polymerization and conju­gation in the macromolecules79. Polypyrroles.withenhanced redox reversibility by substitution ofpoly ether chains at 3 position have been synthes­ized80. Bipolymer complexes of polyethers andother conjugated polymers like polypyrro1e andpoly 3-methylthiophene are useful in sensors andactivators lOb.

P hthalocyanines and their propertiesPhthalocyanines and their polymers are known

to have highly aromatic structures and excellentstability against heat, light, moisture and air.Moreover, they are soluble without decomposingin strong acids. Hence, they have attracted a greatdeal of attention in the search for environmentallystable electrically conducting materials8l-8S. Theelectrical conductivity in these materials is asso­ciated with thermal or optical excitation of mobilepi electrons from the valence band containing thehighest occupied molecular orbitals to the con­duction band containing the lowest unoccupiedmolecular orbitals86,87.

Simple monomeric phthalocyanines are insula­tors and they form stable and highly conductivecharge transfer salts when partially oxidised withiodine88. In these compounds the phthalocyani­nato ligand (dianionic ligand) forces the centralmetal atom in a square planar configuration. Thehighly conductive material is built up of stacks ofmetallophthalocyanine units and chains of polyha­lide anions, e.g., nickel phthalocyanine forms acomplex with iodine whose formula is written as(NiPc)O.33+(13)°.33(ref. 89).

The single crystals of this compound show me­tallic conductivity, i.e., dol df is negative. In mostof such compounds the electrical conductionoccurs mainly through overlap of pi orbitals alongthe stacks of the ligands88. Martinson et al.90 haveshown that in partially oxidised cobalt phthalocy­anine (CoPcIx), the conduction pathway is throughmetal-metal overlap, similar to that in Krogmansalts9l, where electron delocalisation occurs alongthe overlapping dz' orbitals of closely packed Ptatoms (Pt-Pt 2.9A) in the Pt(CN)~- chains . .TheCo-Co spacing is 3.12 A (ref. 89). This type ofone-dimensional conductors is different from sim­ple organic charge transfer complexes92, in which

510

--~-~-~---------~-- ~---- -._"c_co_-_---~~.~_- -----~ l'

INDIAN J CHEM, SEe. A, JUNE 1994

strong electron donors, such as tetrathiofulvalene(TTF), are combined with strong electron accep­tors like tetracyanoquinodimethane (TCNQ). Inthe latter, the transfer of electronic charge fromthe donor to the acceptor results in the formationof cation and anion radicals, which stack togetheras alternating columns within the crystal. Theoverlap of pi orbitals along these columns of radi­cal ions causes their electrical conductivity. Theelectrical conductivity is very sensitive to thepresence of impuritie~ in the system93. At lowtemperatures these charge transfer complexes in­cluding partially oxidised metal10 macrocycliccompounds and doped conjugated polymers un­dergo metal to semiconductor transition (Peierl'stransition)28. This phenomenon is common to allone dimensional conductors.

Phthalocyanine polymersMonomeric phthalocyanines are generally pre­

pared by heating aromatic ortho-dinitriles or aro­matic anhydrides in the presence of metal or me­tal salts, urea and catalysts85,94.When the reactionis carried out with aromatic tetranitriles or dian­

hydrides such as 1,2,4,5 tetracyanobenzene(TCNB) or pyromellitic dianhydride (PMDA) in­stead of the difunctional dinitriles or anhydrides,polymeric phthalocyanines are formed95-lOo• Thestructural uniformity of the polymers obtained de-

pends on the reaction conditions and the methodof recovery of the final polymer8!.

There are several types of phthalocyanine po­lymers (Fig. la-d). Some ofthem are listed below:

1. Polymers in which benzene rings of phthalocy­anine moieties are joined through one bond asin biphenyls!O! and biphenylenes (Fig. 1a & a').

2. Polymers in which monomeric units are joinedthrough substituents on benzene rings42,46,102.103(Fig.1b).

3. Those in which benzene rings of neighbouringphthalocyanine moieties are shared in com­mon94,95,98,99(Fig. 1c).

4. Those in which phthalocyanine rings are con­nected through atoms or ligands at the centralmetal atom, as washers threaded on to a shaft(bridge stacked polymers)!04-!08(Fig. 1d).

5. Those in which phthalocyanine compounds arebonded to polystyrene or its copolymers8!,!09.

The conductivity of different types of phthalo­cyanine polymers are given in Table 1. FromTable 1 it is obvious that the electrical conductiv­

ity of the polymeric phthalocyanines is higherthan that of unsubstituted monomeric phthalocya­nines. This is probably due to the increase in ext­ent of conjugation in the polymeric phthalocya­nines which causes intramolecular charge transfer.Polymers of Type 2, in which phthalocyanine mo-

I-V",-I

\

r~--r-'II

R' -C=N-C=N-

S -Sf0~H

R -@-o-@-N N

N ~ N R, -@-s-@--a)N¢cr =G¢N-~-N~

v & ~ ~N-M-N

, '-S:

"" t QR,~=CH -@-CH:NH-

NpN 1t0IIR.-c-

(a)

(.1') (b)

(c) (d)

Fig. I-Types ofphthalocyanine polymers [a. Type I; b. Type 2; c. Type 3; d. Type 4]

".<'",.'~.-

VENKATACHALAM et at.: EIECTRONIC & PHOTONIC APPLICATIONS OF POLYMERIC PHTHALOCYANINES 511

Compound

Table I-Electrical conductivity of polymeric phthalocyanines

Type Electricalconductivity

(S/cm)

Ref.

Copper phthalocyanine Nickel phthalocyanine

Polymeric copper phthalocyanine

Poly (copper octacyanophthalocyanine)

Polymer of bis-phthalonitriles (Thermally treated)

Poly copper phthalocyanine

(i) with cyano end groups

(ii) with COOH end groups

Monomeric (phthalocyaninato silicon) dihydroxide. PcSi(OHb

Polymeric (phthalocyaninato-l-siloxane). (PcSiO)n

,u-Cyano(phthalocyaninato cobalt III)

Octacyano copper phthalocyanine

Monomer

Polymer type 1

Type 2

Type 2

Type 3

Type 3Monomer

Type 4

Type IVMonomer

10-11_10-15

2 X lO-K

6xlO-7

102

10-2

10-3

6 x 1O-~

3xlO-7

4 X 10-2

2 X 10-2

88101

46

41.42

124

124

124

124

108

124

ieties are connected through groups such as-CN -COOH and -C-(N), , H

oo~ -CH~N-CH-

-@-s-@- -C-N-o.N-( IV)

show large improvement in conductivity on heat­ing41,42,46.110-112.

Poly(copper octacyanophthalocyanine )46 inwhich phthalocyanine moietiesl are connectedthrough conjugated - C =N - C =N - linkages(Type 2) has a conductivity of 10-7 S/cm. This in­creases by 8 orders of magnitude (10 S/cm) onheating, due to formation of conjugated networkstructures through opening of peripheral cyanogroups.

Polymers of Type '3, that are prepared by heat­ing pyromellitic dianhydride with metal salts, formsheet-like structures13 and have conductivitiesmore than those of monomeric ones. The conduc­

tivity in polymers prepared from tetracyanoben­zenell4 is due to simple thermally activated hopp­ing between adjacent sites and varies approxi­mately as the square root of the applied pressure.The metallised polymers have higher conductivitythan the unmetallised ones. Similar to monomeric

phthalocyanines, polymeric phthalocyanines showimprovement in electrical conductivity on dopingwith iodine115. In the latter case, however, the im­provement in conductivity is only by 2-3 ordersof magnitude as comparoo to monomeric phthalo­cyanines which show in conductivity of the orderoften on doping!l8.

The conductivity of doped phthalocyanine po-

lymers depends on the nature of stacking of don­or and acceptor units, their orientation and theirrepeat distances. Hence, the nature of stackingand the crosslinked structure of the polymericphthalocyanine system are likely to introdu~emore randomness leading to lower conductivi­ties 115.

Polymers in which neighbouring phthalocyaninerings are cofacially stacked through substituentatoms such as F, a or ligands such as - C == C - ,- C == N -, pyrazine, 4,4'-bipyridine, l,4-dicya­nobenzene attached to the central metal atom(Type 4), show large improvement in electricalconductivity on doping28.84,85,108.116.Hanack etat.108 have shown that cofacially stacked polymersof Fe, Co, Ru pthalocyanines through ligands suchas pyrazine, 4,4'-bipyridine and 1,4-dicyanoben­zene show both metal and phthalocyanine cen­tered electrical conductivity. These polymers inwhich phthalocyanine moieties are joined throughSi - a- Si linkages with metal-to-metal distanceof 3.33 A has a conductivity of 10-7 S/cm, whichimproves to 1 S/ cm on doping with iodine due toformation of segregated stacks of metallomacro­cyclic cations and parallel arrays of polyhalidecounteranions. In these metallomacrocyclicpolymeric conductors, the pathways of conduc­tion is through the pi system in the ligand co­lumns. The conduction band of the partially oxi­dised (PCSiO)n is composed of carbon P:n: orbitalslocated near the macrocyclic core117,1l8.It hasbeen shown that there occurs minimal mixing be­tween carbon p" orbitals and Si or 0 orbitals inthe highest occupied molecular orbital and theSi - a units do not play any direct role in theelectronic conduction 117-119.Variable temperatureconductivity data in these systems have been fit-

512 INDIANIJ CHEM, SEe. A, JUNE 1994

ted! to fluctuation induced tunnellin~ model

which is based on tunnelling between sm I metalparticle in an insulating mediuml20. The ode! isconsiste t with inhomogeneous nature of dopingin these ystems.

Theo tical studies on the bridge stacke metal­lopthal yanines have shown that when th bridg­ing gro p is an atom e.g., 0, F and the centralmetal i the macrocycle is a non-transitio metal,e.g., .AI, Si, the inter-ring spacing deter nes theband st cture. Both overlap and inter-ri g spac­ing ar,e so important for atom bridged p lymerswith a on-transition metal atom in the acrocy­cleo For diatomic and polyatomic bridging groups,e.g., == C - , - C == N, pyrazine, th bandstructur is determined by the overlap 0 the dxz

and dyz orbitals of the metal and the pi or itals ofthl~brid ing ligand 121.

Th(~s materials have been incorporate~in high

strengt polyaramide fibre, e.g., Kevil', anddoped efore and afteJ: extrusion to .eld air

stable onductive fibres which demonstr te con­ductivi es upto 5 S/cm with metal-like p operties(i.e., do. dTis negative)122,123.

Repl cement of the central silicon ato s by Geor Sn c uses increase in metal-metal dista ce, andhence ere is decrease in electrical con uctivitydue to diminishing of the intramolecula chargetransfe 120.124.These doped bridge-stack d poly­mers I' tain their electrical conductivities only aslong as the dopant molecules remain in the sys­tem. L e all other one-dimensional co ductive

polyme s these polymers also are predict d to un-

..•

t'

8.82x 10-5No signal8.0

Polymer IA

PolymerIB

Polymer II A

Polymer II B

Polymer III A

Polymer III B

Table 2- Electrical conductivity, ESR line'width and magneticsusceptibility Ofphthalocyanine polymers at 298 K (ref. 112)

Polymert Electrical Peak to pe:ak Magneticconductivity linewidth. susceptibility

(S/cm) (mT) (emu/g)

9.8 x 10-6 7.0 5.00 X 10-6

8.0xlO-' 15.0 1.80 x 10-5

4.0 x 10-7 3.0 - 3.12 X 10-7

2.33 144.0 3.12 x 10-4

2.0 X lO-H 90.0 4.6 X 10-5

il'l'I'" , '''''!Ilfll~I''illllll, II I"'ill

tPolymer lA, IIA, and IlIA contain Cu, Ni, and Co respect­

ively as the centra] metal atoms. Polymer IB, IIB and IIIB

represent the corresponding heated samples.

Thermal and chemical refinements of electricaland magnetic properties of polymeric phthalo­cyanines

In our laboratory different types of polymericphthalocyanines have been synthesised and theirelectrical conductivity improved by effecting re­finements in the polymers via thermal and reac­tive treatments 110, l12125,126.Polymeric phthalocya-nines containing Cu, Ni and Co in which benzenerings are shared in common with macro cyclicphthalocyanine rings (Type 3) containing peri­pheral COOH groups have been prepared(Fig. 2). Thermal treatment of these polymers re­sults in increase in the number of charge carriersand extended conjugated structures through de­carboxylative polymerization reactions of theterminal COOH groups. The Co polymer exhibitslarger conductivity (10 S/cm) than the Ni and Cuanalogues. The ESR line width, magnetic suscep­tibility and electrical conductivity of the phthalo­cyanine polymers before and after thermal treat­ment are given in Table 2 (ref. 112).

dergo metal to semiconductor transition at lowtemperatures 120.

The metal to semiconductor transition in mostof the one dimensional system can be overcomeby improving interchain interactions. Such inter­actions in phthalocyanine polymers would be fa­voured by introducing electron-withdrawing sub­stituents such as cyano group in the phthalocya­nine rings. These substituents improve interchaincharge transfer, e.g., low molecular weight octa­cyanophthalocyanines show high electrical con­ductivities of the order of 10 - 1 51/ em (Table 1)due to intermolecular charge transfe:r124.

eOOH

eOOH

eooH

eOOH

''''11'1 i""", I

Hooe eoOH Hooe eOOH

,~, ,-8.,N¢C¢N

t """ "'>: •• .....-",=:,

N-M-N N-M-N""'-~

N~~: # NS~11'#~ ;) ~

N N N" N

~ ),¢C¢""" ~ /"'"N-M-N N-M-N

,~>,h ,-&'1U UHooe eOOH Hooe eOOH

Fill!.2-Structure of metallophthalocyanine olhiomers[M = Cu, polymer IA; M = Ni, polymer II

M = Co, polymer IlIA]

Hooe

Hooe

Hooe

Hooe

VENKATACHALAM et of.: ELECTRONIC & PHOTONIC APPUCATIONS OF POLYMERIC PHTHALOCYANINES 513

In the heated polymersl1o.ll2, the ESR linewidth decreases with increase in temperature, asin the case of lightly doped polyacetyleneI27.128.The magnetic susceptibility of these polymers ex­hibits Curie-like behaviour 110.112,similar to polar­ons, bipolarons, solitons, etc.127-132in doped con­jugated polymers like PA and PPP. The variabletemperature study of ESR line width and intensityof the ESR signal width helps in understandingthe nature of the charge carriers and confirms thepresence of spinless dipolar charge carriers in thesysteml10-112.

The three-dimensional octacyanophthalocyani­nato polysiloxane polymer (Fig. 3), on thermaltr~atment causes both lateral overlap of pi orbi­tals through formation of poly condensed ringstructure via opening of terminal cyano groups,and, longitudinal overlap of phthalocyanine orbi­tals via formation of more Si - 0 - Si linkages 111.The bond-alternation structures are also shown in

Fig. 3. Polymers possessing locked-in ladder typestructures have also been prepared by condensingthe terminal carboxyl 'groups of polymeric phthal­ocyanines with aromatic orthlrsubstituted tetra­amines 125(Fig. 4). The electrical conductivity andmagnetic properties as well as ESR behaviour in­dicate the presence of spinless dipolar charge car­riers. ESCA measurements on the phthalocyaninepolymers before and after heat treatment also in-

JI

fi> ~ ~ ~ Phas~Bu"",,~hN N N N N N N

A

dicate the formation of extended conjugatedstructures. The binding energies for N and metalsshift to lower values in the heat-treated phthalocy­anine polymers indicating the formation of ex­tended conjugated structures in the latter.

Although thermal treatment imparts large stableconductivities, the rigid structure of the resultingpolymers makes them poorly processable. Henceattempts have been made to incorporate thesepolymers in a suitable polymer base such as poly­imide, polyamide, polypyrrole, etc., by chemicalor electrochemical means to obtain composites ofimproved electrical and electrochemical beha­viour64,65,SI.Li and Guarr133 have electrochemical­

ly prepared thin films of tetraamino metallophtha­locyanines. Since the prepolymer containingCOOH groups is soluble in polar solvents likedimethylacetamide, it can be incorporated intoprocessible polymers capable of forming films,e.g., polyamic acid of N,N'-p,p'-oxydiphenylenepyromellitimides2. The polymeric copper phthalo­cyanine containing peripheral carboxyl groups hasbeen converted into the corresponding polyimideby condensing the anhydride of the latter with4,4'-diaminodiphenyl ether in our laboratory 134.The electrical conductivity of the polymer im­proves (10 S/cm) on heating to 520°C in vacuum(10 mT). This method, thus yields coatings basedon polymeric phthalocyanine, which could find

Fig. 3-Bond alternation structures of (octacyanophthalocyamnato polysiloxane) polymers (C.j()HHNI6SiOln

514 INDIAN J CHEM, SEe. A, JUNE 1994

n

1,#

Fig. 4-Structure of metallophthalocyaniqe polymers containing pyrrone typc ladder moieties 1M= Cu, Ni J

..~

applicatlion in the prevention of elechostaticchargllnt; of spacecrafts and rockets.

NLO roperties of organic materials and con­ducting polymers

The evelopment of new nonlinear optic 1(NLO)materia s which allow frequency multiplica ion andmixing f UV-vis-IR radiation for laser a d elec­tro-opti s applications (optical processi glcom­puting, mage analysis, switches, modulat rs), hasbeen e tended to conjugated polymers -6,135-137,

HigWy elocalized pi electron systems ot onlyexhibit arge electrical conducivity but al 0 showlarge n nlinear optical effects, which ma e themuseful aterials for photonic application . Orga­nic co jugated polymers exhibit photo- nducedabsorpt on, bleaching and photolumi escencewhich ccur in picoseconds 18. This imp 'es thatphoton absorption by these polymers causesstructur I distortions in the system due t whichmajor ifts in oscillator strengths occu withinpicosec nds.

Nonli ear optical properties originateT p'marily

due to he response oJ a dielectric mate 'al to astrong lectromagnetic field. Polarizatio is in­duced the electromagnetic field. The lariza­tion res onse of a molecule is given by

~ = ajjEJ + /3ijkEJEk + YijklEjEkEJ +where a, /3, Y, etc. are tensor quantities relatingthe induced polarization vector components ~, tothe appropriate field components, and where Ej,Ek, etc. and i,j,k and I refer to the molecular co­ordinate system. a,/3, Y, etc., are also called mole­cular hyper polarizabilities. The odd-order rankedtensors such as /3 require that the molecule pro­viding the response has no centre of inversionsymmetry; otherwise its contribution to the in­duced polarization is zero in the dipole approxi­mationI37,l3H. Molecules with low-lying charge­transfer states of the appropriate symmetry cangive v~ry large /3. In general molecules exhibitinglarge /3have the structure (V).

(V)

where D and A are electron donating or electronaccepting substituents that provide a substantialcharge-transfer resonance interaction through thearomatic ring. Approaches to increasing the /3 ofcompounds, include placement of strong electrondonor and electron acceptor groups at oppositeends of a conjugated electron system and incteas-

VENKATACHALAM et al: ELECTRONIC & PHOTONIC APPliCATIONS OF POLYMERIC PHTHALOCYANINES 515

ing the conjugation length139-141.Examples are2-methyl,4-nitroaniline (MNA) (structure I) andazodye disperse Red (structure il) as shown in(VI).

Katz et al142 have introduced the use of cyano­vinyl electron acceptor groups and dithiolylidenemethyl electron donor group. Polarization inducedin bulk media (liquids and solids) can be ex­pressed by the following expansion:P,=x(1)E+X(2)EE +x(3)E·E E

1 ) ) k J k I

where the coefficients x(n) have meanings similarto their molecular counterparts but are scaled tothe properties of the medium. Construction of abulk material possessing a large bulk susceptibilityrequires not only molecular constituents withlarge microscopic susceptibilities, but also a non­centro symmetric system where the orientation ofthe molecular species results in constructive addi­tivity of p. A number of approaches to constructbulk materials with large nonlinearities areknown. These include crystal engineering, LB filmpreparation, electric field poling of polymeric sys­tems, development of molecular composites andaggregates and the preparation of fibres with crys­talline organic materials on the corel43. Crystalengineering approaches for NLO materials sufferfrom inherent difficulties. For example, symmetryand orientation of molecular units within the unit

cell affect the noncentrosymmetry of the system.Noncentrosymmetric organization of the mole­cules can be achieved by techniques such asLangmuir Blodgett (LB) technique, guest hosttechnique and side chain liquid crystalline polym­er techniqueI44-146.

The LB technique is one of the well-knownmethods available for manipulating the architec­ture of an assembly of organic molecules. It maybe regarded as an organic analogue of molecularbeam epitaxy, which is extremely important tosolid state electronics as a means of fabricatingprecise micro structures with novel transport pro­perties. The LB technique offers the means toconstruct one organic mono layer at a time withprecise geometries (molecular orientation andthickness). Very few materials are suitable for LBfilm formation aftd these molecules contain both

hydrophobic and hydrophilic end groups such aslong alkyl and carboxylic groupsI44-146.One prob-

lem with LB films in optical applications is thatthey are not ttuely single crystalIine materials andthis limits applications in nonlinear and guidedwave optics.

NLO of poled polymers and side chain liquidcrystalline· polymers

In the guestlhost method, the NW compound(guest) under investigation is dissolved in a poly­mer host and spm coated on a substrate. TheNLO polarizable molecules are oriented by apply­ing a DC electric field across the polymer just be­

low its glass transition tempe.ratu~~ (1) to givelarge molecular hyper polanzability14 -149. Thepoling can be effected by contact or corona pol­ing. Stability of the polar orientation after coolingthe polymer well below the I;, depends on thepolymer used and the methods used for increas­ing the degree of polar orientation. NLO effectsare obtained when the medium is illuminated by avery intense laser beam. The advantage 'of such asystem is that it can be manufactured in differentshapes-cylinder, fibres or films. A typical exam­ple is the field induced alignment of the azo dyedisperse Red I in PMMA. Using disperse redmolecule number density of 2.74 x 102°/cm3, thesystem gives a X(2) of 3 x 10-6 esu at a polingfield of 0.6 x 106 VIcm. This value agrees with thetheoretically predicted valuel45. The guest-hosttechnique suffers frtlm limited solubility of activespecies in host polymer which lowers the achiev­able level of NLO properties. In addition to these,problems such as phase separation, nonuniformityof dispersion and evaporation of the active spe­cies also exist. Many of the above difficulties areremoved when the NLO active species are chemi­cally attached to polymer chains, e.g. the guestmolecule Red dye I is attached to the monomerMMA and then copolymerized with MMA145.Theresulting polymer is poled just above its I;, andthen cooled in its polar orientation. This tech­nique allows incorporation of high concentrationof active moiety in the system. If the concentra­tion is increased, higher X(2) values areobtainedl45. It can be manipulated by various pro­cessing techniques into different shapes, sizes anddegree of orientation and would enable the use ofthese materials for many devices and applicationsof second order NLO phenomena. Moreover,their long relaxation times can be utilized tomaintain the desired structure at ambient tempe­rature for an indefinite period 145,150,151.

Due to the tensorial character of NLO coeffi­

cients, alignment and the extent of noncentrosym­metry of NLO moieties in NLO materials are cri-

In additi n, as polymers these materials als pos­sess glas transition temperature and hen e themolecuIa order for a given phase may be rozenfor use vel' a wide range of temperatur . ForNLO ap lications side chain polymers a e fa­voured b cause of their high orientational order

in the melt phase. Utilizing slow relaxation char­acteristics of polymers, different NLO characteris­tics, viz., unoriented X(3) material, oriented X(3)

material by cooling an aligned smectic A structureand oriented X(2) material by poling under a highelectric field, of the sample can be obtained. Lowconcentration of dopant is dispersed in a nematicliquid crystalline thermoplastic polymerl52. Thepolymer should show nematic behaviour above ~in addition to it being homogeneous and alignablein an external applied field 150,151.The sample isheated to nematic regions and an electric field isapplied producing axial alignment of the nematicmedium. The dopant molecule is now in an aniso­tropic environment and its degree of alignmentwill be determined by the superposition of thealignment field Ed on the anisotropic potential ofthe mediumI50,152. In yet another approach, largenonlinear optical properties are found in mole­cules covalently bound in a polar polymer chainin an otherwise isotropic mediuml5o. In this ap­proach, extended polymer chains are treated withnonlinear chromophores as part of the repeatunit. For example, polybenzyl-y-glutamate (PBG)dissolved in a helix forming solvent and subjectedto an electric field forms a rigid extended helixand has a large dipole moment owing to theasymmetric placement of repeat units in the rigidpolymer chain. Hence it shows total field inducedpolar orientation at modest field strength. The f3

for the monomer repeat unit is 2 x 10 - 31 esuwhile" for the individual chains it is 500 x 10- 30esu. It is therefore possible to have a moleculardesign approach in which polar polymer chainswould contain a repeat unit of a chromophorewith large and permanent field induced polaralignment of polymer chains.

Importance of ladder polymers and phthalo­cyanine polymers for NLO applications

In view of the large electron displacement inconjugated electron system, -the conjugated

H polymers offer better nonlinear properties thanc 3 the current inorganic systems like LiNbO 3 KH 2

P04 (KHP) and III-V semiconductors GaAs orInSb (ref. 5). In addition to their large architec­tural flexibility, conjugated organic polymers havebeen valued for their ultra fast switching times,third order NLO susceptibilities and high opticaldamage thresholds4-6. Among the conjugatedpolymers, polydiacetylenes (PDA) are one of themost extensively studied polymeric systems be­cause of the processibility and capability to exhi­bit phase transition in the solution stateI53-156.Since various changes are possible in its backbone

INDIAN rrCHEM, SEe. A, JUNE 1994

( VII)n.2.3.5.qa.l'.12

516

tically . portant for maximizing the N 0 re­sponse .. or example, the value of X(3) for an un­oriented polymer containing long axially s mmet­ric NLO unit is increased by a factor 0 5, byorienting all the NLO units along the same .s152.Unorient d polymers are by definition cent osym­metric d have no X(2) properties, here asorie:nting high f3 moieties in the same di ectionproduces high X(2) values. Thus, by con troll ng theorientati n and symmetry of polymeric m terialsdifferent pes of NLO responses can be I' alized.The liqu' crystalline polymer chains are pic allycompose of three parts: backbone, side chainspacer g oups and mesogens. Intermolec ar in­teraction between the mesogens, and to a lesserextent, tween spacer groups results in meso­phase j[o mation in the melts of these po mers.The: fun ion of the spacer group is to de ouplethe side hain motion from that of the bac bone,since ot rwise the latter will affect phase stabi­lity. The requirements are the same as t at re­quired f I' mesogens in a liquid crystallin poly­mer for highly active NLO molecule. Hen e onecan use LO molecules as mesogens while main­taining t e other molecular structure unc ged.By ap1'r priate manipulation of the bac bone,spacer a d NLO group, a variety of meso hasescan be I' alized in these polymers. Nemati , cho­lesteric, ectic A and smectic C have be n ob­served in side chain liquid crystalline poly ers152.The folio ing types of side chain liquid cry tallinepolymers represent some of the systems th t havebeen stud ed151,152(VII).

"iI! , I i! 'I,1111I 'I~ HUllIIll.m liIIllill I kdilill

VENKATACHALAM et al.: ELECTRONIC & PHOTONIC APPLICATIONS OF POLYMERIC PHTHALOCYANINFS 517

structure a number of substituted PDAs have

been studied 157.Recently a number of other con­jugated polymers including poly thiophene, poly­aniline, and poly(phenylene vinylenes) have alsobeen examinedI58-161.

The X(3) values for the present day electro activepolymers are too small for applications such asreceiver protection and for a variety of compo­nents necessary for optical information process­ing4-6. As optical nonlinearity increases with in­crease in conjugation or electron delocalisation,ladder polymers give maximum overlap and delo­calisation, and hence are best candidates for largeX(3) (refs 162, 163). Ladder polymers are alsocharacterized by excellent thermal and chemicalstabilities and frequently exhibit exceptional me­chanical strength. However, they are well-knownfor their poor solubility and hence are difficult toprocess. The present day efforts164-168focus uponthe preparation of derivatized monomers and theeffects of solvent and temperature upon the ratesof the various condensation reactions. Schluter et

al.169 have described a Diels-Alder route for syn­thesizing soluble ladder polymers. The generalmethodology for the production of high symmetryladder polymers by condensation of derivatizedquinones with. aromatic di- and tetra-substitutedamines as mentioned by Dalton165 is given in thefollowing Scheme:

:ur ·

not increase with increasing polymer length aftera certain chain length.

Odd parity susceptibilities such as x(l}, ,,(3) arealways present in any material. In pi electron con­jugated polymers which exhibit large x(3), thepresence or otherwise of centre of symmetry doesnot pose any problems. Theoretical calculationssuggest4-6,164,170,171that the extent of electron delo-calisation influences third order optical susceptib­ility X(3).

Although electrical conduction property andnonlinear optical properties are based on ex­tended pi conjugation (or even a conjugation insilanes), conductivity is found only in doped po­lymers and is influenced by interchain and intrac­hain charge transfer while large X(3) is found onlyin neutral polymers. Based on these observationsit has been realised that conducting polymers mayfind application as fast nonlinear optical materials.The NLO property in conjugated polymers is pri­marily a microscopic one. As oxidation/reductionis expected to affect optical properties, it isworthwhile to direct future research on the NLOof doped polymers.

Another class of compounds that exhibits NLObehaviour is the transition metal organic com­pound with extended pi electron delocalisa­tion172,173as shown in (VIll).

M = Pd, PI

X = H,CH3,NH2,OC~,c2H5

The results of Dalton's work suggest that the sub­stituent does not strongly influence the electrondelocalisatlon and this is not expected to influ­ence x(3) (ref. 165). It also indicates that x(3) will

R • alkyl, ••tkoxy, phonoocy

Their behaviour is analogous to nonlinear pheno­mena observed in purely organic compounds hav­ing similar electronic characteristics. Third har­moni~ generation and power limiting measure­ments corroborate the presence of large third or­der susceptibility in these metallopolyynes. Theextent of orbital overlap is determined by theproximity and relative energies of the combiningorbitals of the correct symmetryl74. In addition tothese parameters, conformational issues also exist.For the pi electrons of· aromatic ring to overlapeffectively with 'the pi electrons of acetylenemoiety, which interact in turn with two of the dz'orbitals of the metal, the plane of the benzenering must be perpendicular to the metal phospho­rous bonding axis 172.The arene plane however,rotates about the bond between one of the acety­lene carbons and the connecting carbon of thebenzene rings so that at any given moment only afraction of the benzene rings' in the polymer chain

X. O,S,NH

~R

z = a,s

518 INDlANIJ CHEM, SEe. A, JUNE 1994

y oriented to provide good orbit I over­negates the expected hyperpolar' ability

enhance ent due to extended delocalisat on. To

prevent rotation, polyynes with substitue t aro­matic 'gs are required. Free standin purepolymer . ms of good optical quality are e pectedto be of nsiderable use in NLO applicatio .

Phthal cyanines and their polymers po sessing- CN a d - COOH terminals have suita Ie ex­tended s ructures that facilitate solitonic, olaro­nic and bipolaronic conformational d forma­tions110,1l,126 and hence are expected t showlarge ult afast nonlinear optical propert~ . Theperipher functional groups can be us d forstfUlCtur modifications to improve procesabilityand allo fabrication of various device st cturessuch as film, channel waveguides and planarwavegui s, etc. The presence of macr cyclicstructure in these polymers provide mec anical,thermal, nd environmental stabilities. Mo eover,the optic I damage threshold of these poly ers isalso exp ted to be much higher than that f sim­ple, low olecular weight materials. The t rminalanhydriid s of phthalocyanine polymers ar con­densed ith aromatic and aliphatic dia . es togive pol 'c acids which are imidized th rmallyto form ectroactive films134.Condensation of the

anhydrid units with tetrarnines gives semi ladderor ladde type polymers possessing pyrro e typestructure 126.The combination of a and . elec­tron syst ms' in polymeric phthalocyanine struc­tures ca stabilize the excited electronic state,which al ws these polymers to undergo redoxprocesse in a wide potential range65,8 ,175,176.These Ipr perties make them useful as se 'con­ductors, lectrochemical catalysts and m terialsfor nonli ear optics. The excitation of p lymeI'chains is trongly coupled to local bond ord I' andtherefore a strong nonlinear response is hown.These p ymers can be easily incorporate .e1ec­trochemi ally into other conducting polym I' ma­trices64,65 so that polymeric thin films pos essingboth pro essibility and electrochemical pro ertiescan be re .zed.

In vie of the similarities in the reqUir~ments

for stmc ral and NLO applications, one m st at­tempt sy thesis of the heterocyclic ladder poly­mers wit extensive aromatic rings to incre e theconjugati n, and with suitable backbone su stitu­ents" to i rease the processibility. To obtai

imurn t,e sile properties and stability in ~light

weight terial for structural application, the

clear choi es are organic, conjugated, arom tic 0I:hetero ar matic and rigid structures to gi e or­dered or sotropic materials 177.The poly-c den-

sation of substituted diquinones andl diamines in asolvent yields a soluble precursor which on heat­ing is converted to fully cyclized materiap65. It ispossible to enhance X(3) by more than an order ofmagnitude by chemical and electrochemical dop­ing and values up to 10 -7 esu has been achievedin ladder polymers. Polymers posse:ssing phthalo­cyanine structures are shown to have large X(3)

values I58,170,178,179.Cyclic voltammetry shows elec­tro-chromic and redox properties of thesystem64,65,175,176,Magnetic and optical measure­ments show that the charges are associated withgeneration of polaron and bipolaron states 110,112.

Semiconductor devices from phthalocyaninesand other conducting polymers

Organic conducting polymers have very differ­ent conductivity in the reduced and oxidised stateand their conductivities are controlled by the do­pant concentration, applied potential, etc,1.2.4.Thismakes them useful for a wide ranging applicationssuch as electrochemical insertion electrodes, highconductivity low density metals, materials forNLO applications and semiconductors. The ex­tended pi conjugated structure provides the selflocalised states, which determine the operation ofa wide variety of semiconductor devices, metal-in­sulator semiconductor (MIS) devices, metal-insu­lator-semiconductor field effect transistors (MIS­FET), etc. In these devices the charge can be in­troduced into the region of surface charge at thesemiconductor/insulator interface by the applica­tion of a strong electric field across the insulator,for example two .terminal devices such as Schott­ky diode and MIS structure. FETs based on poly­acetylene, polypyrrole, polythiophenesand metal­lophthalocyaninesl80-186 have been fabricated .andanalysed. These FETs operate through formationof accumulation layers generated Ithrough bandbending at the insulator semiconductor interfacewhich is controlled by the applied voltage. Thesedevices require fabrication as a selies of layers.For example, to construct a MIS structure a sili­con wafer is needed as a substrate with a dopedlayer to act as gate and the native oxide layer ontop as insulator. This structure has to be coatedwith the polymer and finally capped with an evap­()rated thin gold top contact. For MISFET struc­tures, it is convenient to put the source and draincontacts into the insulator layer and the polymerlayer on top as the last stage of fabrication.Source and drain contacts may be poly-n-siliconor Au structures . .The mode of operation of or­ganic conjugated polymer based FET showsl87that in addition to carrier mobility, the ohmic

, I "1"1" II""I!' i! illllll' 'IMlHII!IIII.~ IIII1III,j 1~~llil~I

VENKATACHALAM et al.: ELECTRONIC & PHOTONIC APPLICATIONS OF POLYMERIC PHTHALOCYANINES 519

"

contribution to the drain current must also be

taken into account. The mobility/conductivity ra­tio appears as the determining factor and hence,short conjugated oligomers such as polysesqui­thienylene188 which possess a high carrier mobilityand low conductivity would make them potential­ly interesting. In these systems, charge trapping bystructural defects may playa prominent role in li­miting the charge transport propertiesl89. Petit etal. have made insulated gate FETs with metal­lophthalocyanine based active semiconductorswhose performance is comparable to that of am­orphous'silicon based devicesl86. Schottky barrierdiodes have been made and formation of spacecharge regions in M /PcM/M 2 cells have beenproved by Andre et aL 190

Conclusion

The non-resonant X(3) values observed in conju·gated polymers are less than 10-9 esu (ref. 170).For practical purposes, the values has to be in­creased so that devices using small switching en­ergies can be fabricated. In order to enhance theX(3) values, suitable structural modifications andspectral responses of NLO coefficients areneeded. In order to make use of organic materialsin NLO devices such as wave guides (used to con­fine intense light over sizable distances), process­ability and optical clarity are as important as largenonline(ir optical response. For example, thoughpolyacetylene has large X(3) it is practically uselessbecause of high scattering losses. Optical qualityof the materials is important for integrated opticalapplications which will involve wave guide confi­gurations. The best optical media is provided bysilica although they have low x(3). A compositestructure of glass and polyphenylene vinylene(PPV) with optical clarity has been obtained bychemical processing of oxide glass using a sol-gelprocess. Up to 50% of PPV has been incorporat­ed into the composite191• Similar approaches arepossible with phthalocyanine polymers possessingfunctional terminals.

Crystalline materials offer a natural way to re­ducing scattering losses but their birefringencemay hinder bending of wave guide structures. Itseems that glassy materials .would make a work­able compromise. But chemical reactivity of theorganic materials could pose problems. In allthese aspects metallophthalocyanine polymersseem to offer good thermal, chemical and photo­chemical stabilities for use in a wide range of tem­perature and environments. Hence, in order torealize useful and viable materials for electronic

and photonic applications, the theoretical under-

standing of third order optical nonlinearity andstability of poled X(2) .molecules dispersed in po­lymer matrices and a thorough study uf these ef­fects in metallophthalyanine based systems haveto be undertaken. In the organic conductingpolymer based MISFET structures, the accumula­tion or inversion layer is confined to the interfacebetween the polymer and the insulator and is verysensitive to the surface structure of the polymer.Hence, control of the electronic structure of theconjugated polymer on which the accumulationlayer or inversion layer is formed is very import­ant for the improved performance of the devices.

Acknowledgement.The authors are thankful to Dr S. C. Gupta, Di·

rector, VSSC, for his keen interest in the work.They are also thankful to Dr K N. Ninan and DrR. Ramasamy for their encouragement. They arethankful to their colleagues at PEPF, PPC, at var­ious stages.

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