Synthesis and characterization of amphiphilic poly(p phenylene) based nanostructured materials 1

Since the discovery of conjugated polymers, many new areas of potential applications for these materials such as polymer light emitting diodes LEDs, photoconductors, nonlinear optical ma

Poly(p-phenylene)s and

Derivatives: Promising Blue

Light Emitting Conjugated

Polymers

Chapter 1

onjugated polymers are a new class of processible, film-forming semi-conducting or metallic organic macromolecules which consist of a backbone with alternating double-

alternating double and single bonds mesomerize more or less, i.e the single and double bonds becomes similar, double bonds overlap over the single bonds The essential

extending over a large number of recurring monomer units The π-electrons can be easily moved from one bond to the other, which makes conjugated polymers to be one- dimensional semiconductors Similar to inorganic semiconductors, they can be doped to drastically increase their conductivity For the discovery and development of such

conducting polymers, Alan J Heeger, Alan G MacDiarmid and Hideki Shirakawa

received the Nobel Prize in chemistry in the year 2000

1.1 Conjugated polymers-an overview

The history of conducting polymers began after the discovery of poly(sulfur nitride) [(SN)x] in 1975 which becomes superconducting at low temperatures.1 Research into the electronic, optical, and magnetic properties of conjugated polymers intensified after a number of seminal experimental achievements One of the first discovery in this direction is the polyacetylene (PA) thin films (initially discovered by Shirakawa et al., using a Ziegler Natta type polymerization catalyst) by MacDiarmid and Heeger.2 In their seminal work, they demonstrated that the electrical conductivity of PA (10−9 S cm−1) could be enhanced by several orders i.e 105 S cm−1 by simple doping with oxidizing agents e.g I2, AsF5, NOPF6 (p-doping) or reducing agents (n-doping) e.g sodium napthalide Later on, in 1990, Friend et al reported the synthesis of phenyl-based

C

polymers and discovery of electroluminescence under low voltages in these systems established the field of polymer optoelectronics.3 The later discovery of photoinduced

electron-transfer from a conducting polymer to buckminsterfullerene opened a new direction toward photo-detector and photovoltaic cells.4 In addition to this, work done by several other polymer and materials scientists around the world has generated renewed interest of the scientific community towards the study and discovery of new conducting polymeric systems The electronic and optical properties of conjugated polymers, coupled with their processability and interesting mechanical properties make these attractive materials for the electronics industry The attractiveness of using organic materials in semiconductor devices emerge from the desired properties such as low-cost processing, mechanical flexibility, light weight and color-tunability.5 The discovery and development

of conducting polymers was recognized by the award of the Nobel prize for chemistry in

2000 to Heeger, MacDiarmid, and Shirakawa.2

π-Conjugated polymers are often intrinsic semiconductors due to their delocalized π-electrons Most of the CPs studied today have alternating single and double bonds on the main chain Such one dimensional π-systems are often conceived, as having a two-band structure using the one electron model approximation.6 The highest occupied

molecular orbitals (HOMO) form the occupied π-band (valence band) of the polymer and the lowest unoccupied molecular orbitals (LUMO) form the π*-band (conduction band) of

the polymer As a consequence of the band alternation, band gap of the neutral polymer lies in the range of c.a 1.5 (near IR) to 4 eV (UV), resulting in semiconducting properties The high values of the electrical conductivity obtained with this organic macromolecules have led to the name ‘synthetic metals’.The alternating bonds provide the pathway for

charge transport along such a chain The chemical structures of these polymers have the intrinsic characteristics as materials with the electronic structure of semiconductors

Conductivities of conjugated polymers are summarized in Figure 1.1

Doped Polypyrrole and polythiophene

10 5

Polystyrene Polyethylene Nylon

Silicon

Doped Germanium TTF-TCNQ Graphite Mercury Iron Copper Silver

Non-doped PA Non-doped polythiophene

Doped PPP Doped PANI Doped PA Doped Polypyrrole and polythiophene

10 5

Polystyrene Polyethylene Nylon

Silicon

Doped Germanium TTF-TCNQ Graphite Mercury Iron Copper Silver

Non-doped PA Non-doped polythiophene

Doped PPP Doped PANI Doped PA Doped Polypyrrole and polythiophene

10 5

Polystyrene Polyethylene Nylon

Silicon

Doped Germanium TTF-TCNQ Graphite Mercury Iron Copper Silver

Non-doped PA Non-doped polythiophene

Doped PPP Doped PANI Doped PA

10 5

Polystyrene Polyethylene Nylon

Silicon

Doped Germanium TTF-TCNQ Graphite Mercury Iron Copper Silver

10 5

Polystyrene Polyethylene Nylon

Silicon

Doped Germanium TTF-TCNQ Graphite Mercury Iron Copper Silver

Non-doped PA Non-doped polythiophene

Doped PPP Doped PANI Doped PA

Non-doped PPP Non-doped PPV

Non-doped PA Non-doped polythiophene

Doped PPP Doped PANI Doped PA

Figure 1.1 Conductivities of conjugated polymers compared with other common

materials

From the beginning of 1980’s chemists started to synthesize new conducting polymers with improved/desired properties In conjugated polymers, the energy difference between the highest occupied state in the π band and the lowest unoccupied state in the π*

band is the π - π* energy gap, Eg, depends upon the molecular structure of the repeating unit This provided synthetic chemists opportunities to control the energy gap through molecular level design Since the discovery of conjugated polymers, many new areas of potential applications for these materials such as polymer light emitting diodes (LEDs), photoconductors, nonlinear optical materials, laser dyes, scintillators, piezoelectric and pyroelectric materials, optical data storage, optical switching and signal processing, solar energy conversion, transparent antistatic coating, molecular wires, and chemical and biosensors have been identified.7-8 This creates many commercial interests as well as academic research that led to the development of conjugated polymer derivatives having a base structure of alternating single and double/triple bonds.9-10 Some of the parent

structures of conjugated or conducting polymers such as polyacetylene, poly(p-phenylene)

(PPP), poly(p-phenylenevinylene) (PPV), polyaniline (PANI), polypyrrole (PPy) and

polythiophene (PT) are shown in Figure 1.2

N H

N H

n

PANI

Figure 1.2 Chemical structures of some of the important conjugated polymers PA = poly

acetylene, PDA = polydiacetylene, PPV = polyphenylenevinylene, PT = polythiophene,

PPP = poly(p-phenylene), PF = polyfluorene, PANI = polyaniline

1.2 Poly(p-phenylene)s the simplest aromatic conjugated polymer

1.2.1 A brief history of PPP

Poly(p-phenylene)s, PPPs, constitute the prototype of rigid-rod polymers.11 Over the past few decades, numerous chemists have been exploring the synthesis of this

simplest aromatic conjugated polymer, poly(p-phenylene) (PPP), with new and novel

molecular architectures.12 The key advantages of PPPs arise from their conceptually

simple and appealing molecular structure, high chemical stability, and interesting physical

properties In addition, PPP and its derivatives have the large HOMO-LUMO energy gaps

required for obtaining blue emission and have been used as blue-light emitting materials in LED devices.13 PPPs also showed high quantum yield and good charge transport

properties.14 However poor solubility of the unfunctionalized PPPs limited the

processability for device fabrications

n

Figure 1.3 Chemical structure of unsubstituted PPP

There are many attempts to improve the solubility of PPPs by the introduction of alkyl substituents on PPP backbone However substituents caused deplanarization of the

polymer backbone and reduced the extent of π- conjugation resulting in hypsochromic shift of the emission wavelength.15 The tilt angle and the reduction in effective conjugation length strongly influence the energy gap, which increases with the degree of substitution

To circumvent such limitations, the planarization of the PPP backbone was investigated

through various methods such as incorporation of additional covalent bonds, and weak interactions such as hydrogen bonds along the polymer backbone.99 Through the

planarisation of PPPs, it was possible to minimize the tilt angle between neighboring

phenyl rings leading to reduction of the energy gap and photoluminescence energy.93(b)

The details of the soluble substituted PPPs and further efforts to minimize the tilt angle by modification on the PPP backbone are summarized in the following section, which

describes various synthetic efforts reported

1.2.2 Synthetic strategies of PPP and its derivatives

The attempts to synthesize PPP may be classified as either direct or indirect

methods.11b In the direct method the monomers that contain the phenylene moiety will

polymer is first synthesized from which PPP is then released, e.g., by thermal treatment

Unfortunately both methods have serious drawbacks For most of the direct syntheses, the reaction conditions are too harsh for a regiospecific coupling reaction to take place Thus, linkages between wrong sites, cross linking, and other side reactions occur The molecular weights of the polymers synthesized are very low, which is specifically due to solubility problems Typically degrees of polymerization (DP) range from 5 to 15 owing to the low solubility of higher oligomers Precursor method is superior to all direct methods, in which high molecular weights are achieved A serious limitation of this method is that the structural irregularities contained in the precursor are inevitably transplanted into the final polyarylene The conversion of the precursor polymer does not proceed as cleanly as desired and it is either incomplete or leads to chain fracture There are only a small number of suitable precursor polymers available From this consideration it becomes evident that chemists had to develop solutions for the individual problems of both approaches The following section summarize different methodologies developed for the

synthesis of PPPs and its derivatives

1.2.2.1 Oxidative condensation of benzene derivatives

The first attempts to generate poly(p-phenylene) were undertaken in 1960s by

Kovacic et al.16 They reported that the oxidative treatment of benzene with copper (II) chloride in presence of strong Lewis acids (aluminium trichloride) led to condensation of the aromatic rings Benzene subunits are preferentially connected in the 1, 4-position, however, cross-linking and oxidative coupling to form polycyclic aromatic hydrocarbons

occur as side reactions Based on this initial procedures, other 1,4-sustituted benzene derivatives were coupled to poly(p-phenylene)s In another oxidative condensation

method, Katsuya et al,27 reported the oxidative coupling using (copper(II) chloride/ aluminum chloride) of electron-rich benzene derivatives such as 2,5-dimethoxy-benzene

to poly(2,5-dimethoxy-1,4-phenylene) The polymer obtained by this method also had limited solubility and only soluble in concentrated sulfuric acid, and is fusible at 320 οC

1.2.2.2 Transition metal-mediated couplings

One of the early and easy methods reported by Yamamoto et al.18 described the nickel(0)-catalyzed or -mediated coupling between dihaloaromatic compounds and Mg metal In a typical reaction, 1,4-dibromobenzene, 1 equivalent each of Mg and dichloro-Ni(bipyridyl) complex were refluxed in tetrahydofuran Even though, the coupling was mild and yielded exclusively para-linked polyphenylenes, the low molecular weight of the obtained polymer was a drawback of this method Later, the adoption of Pd (0)-catalyzed coupling of various bromobenzene derivatives with benzene boronic acid developed by Suzuki19 et al have been used for the synthesis of high molecular weight PPPs with

improved materials properties.20-21,23a Generally, Suzuki polycondensation (SPC) is a growth polymerization of bifunctional aromatic monomers to poly(arylene)s and related

step-polymers The general outline for SPC reaction is shown in Scheme 1.2 The mechanism

of the SPC involves oxidative addition, transmetallation, and reductive elimination

Reductive elimination Oxidative Addition

Ligand Substitution

Ar-Ar’

Reductive elimination Oxidative Addition

Ligand Substitution

Scheme 1.2 The general outline of Suzuki polycondensation (SPC) reaction

Since the development of Suzuki polycondensation (SPC) for the synthesis of

poly(p-phenylene), the methodology have been modified to incorporate flexible groups on

the polymer backbone One of the great advantages of this method is that during polymerization, the solubilizing groups keep the growing polymer chain in solution and therefore accessible for further growth Such synthetic methodologies led design of structurally diverse, processable PPP derivatives by the introduction of solubilizing side groups and of electro active groups.21-22 According to the structural features, the modified

PPPs which correlate with certain properties can be classified as (a) polymers with alkyl

or alkoxy chains, (b) amphiphilic PPPs, (c) polyelectrolytes, (d) PPP precursors for ladder polymers, (e) polymers with main-chain chirality, (f) dendronized PPPs, and (g)

poly(arylene vinylene)s and poly(arylene ethinylene)s During the same period of development of SPC, Kaeriyama et al.22 reported the synthesis of PPP using Ni(0)-

catalyzed coupling The strategy of Kaeriyama represents a so-called precursor route, and was developed in order to overcome the shortcomings (insolubility, lack of processability)

of previously synthesized PPP The condensation reaction was carried out with

solubilized monomers, leading to a soluble polymeric intermediate In the final reaction

step this intermediate was then converted in the solid state to form of homogenous PPP films or layers, into PPP or other poly(arylene)s

Br Br

n COOMe

n COOMe

-Scheme 1.3 Precursor route synthesis of PPP using Ni(0)-catalyzed couplings

Another approach on the synthesis of structurally homogeneous, processable PPP

derivatives started with the pioneering work of Schlüter and Wegner.23 The preparation of

soluble PPPs were achieved via introduction of solubilizing side groups to prepare poly

(2,5-dialkyl-1,4-phenylene)s The coupling of aromatic compounds containing aryl magnesium halide and aryl halide functions catalyzed by Ni(0) compounds was also reported The incorporation of solubilizing side-chains at the 2- and 5- positions of the benzene rings makes these compounds soluble in common organic solvents The products are characterized with 1,4-linked benzene rings on the main chain

Scheme 1.4 First soluble poly(2,5-dialkyl-1,4-phenylene)

Later on, repetitive Suzuki aryl-aryl cross-coupling method was adopted for the

synthesis of soluble PPPs with very high molecular weight (number average up to 100 phenylene units) 2,5-Dialkyl substituted PPPs (Scheme 1.4) were intensively studied as prototypes of so-called ‘hairy-rod’ macromolecules, composed of a linear, rigid PPP

main-chain and flexible, ‘hairy alkyl side-chains’.24 Poly(2,5-di-n-dodecyl-1,4-phenylene) (R:-C12H25) revealed a sandwich-type structure with layers of aliphatic side-chains

perpendicular and PPP main-chain parallel to the substrate surface

Poly(2,5-di-n-dodecyl-1,4-phenylene) of Mw 73000-94000 show a single anisotropic liquid crystalline mesophase in the molten state and macromolecules with Mw 44000-73000 gave coexisting isotropic/anisotroic phases.25

Br (HO) 2 B

R

R

R R

R

R

R R

R

n

R: -alkyl, -alkoxy Pd(0)

Scheme 1.5 Synthesis of 2,5-dialkyl-substituted soluble PPPs using Suzuki aryl-aryl

cross-coupling

Many other groups also reported the Ni(0)-mediated couplings to generate several

PPP derivatives. 18,26,27 The homocouplings of various 1,4-dihalobenzene derivatives using nickel(II)chloride/triphenylphosphine/zinc or the nickel(0)/cyclooctadiene complex was reported Ni(0)-mediated homocouplings of 2-substituted 1,4-phenylenebis(triflate)s have been reported by Percec et al.28 to provide substituted poly(p-phenylene)s (Scheme

1.6 ) containing alkyl, aryl or ester substituents in the 2- and 3-positions of the

Scheme 1.6 Ni(0)-mediated homocouplings for substituted poly(p-phenylene)s

In addition to alkyl-substituted derivatives, water soluble PPPs with ionic side

groups such as carboxy and sulfonic acid groups are also synthesized.21(d),29 Soluble PPPs

decorated with densely packed, sterically demanding dendrons were recently described by Schlüter’s group.30 In another approach towards planarized and soluble PPPs, our group

reported the preparation and characterization of homo and copolymers of amphiphilic

PPPs (C n PPPOH) containing alkoxy groups as well as hydroxyl groups on the polymer

backbone.31 Poly(2-hydroxy-5-alkoxy-p-phenylene) (Scheme 1.7) containing long alkyl

chains, were prepared using Suzuki polycondensation of alkoxybenzene and bis(boronic ester) monomers All polymers showed good solubility in common organic solvents such as tetrahydrofuran (THF), chloroform, toluene, and dimethylformamide (DMF) Copolymers were prepared by Suzuki coupling using 1-

2,5-dibromo-1-benzyloxy-4-benzyloxy-4-dodecyloxyphenyl-2,5-bis(boronic acid) and the corresponding dibromoro pyridyl monomers.32

Scheme 1.7 Amphiphilic PPPs synthesized using Suzuki polycondensation (A) Polymer

chains with phenolic groups and alkyl chains with varying length (B) Coplymer of

phenolic and pyridyl monomers

Bloom and Sheares reported the synthesis of another class of PPP

macromonomers via Ni(0) catalytic coupling of aryl chlorides for use as rigid-rod units in high-performance multiblock copolymers.33 Rigid-rod poly(4,9-methyl-2,5-benzophenone) macromonomers were synthesized using Ni(0) catalytic coupling of 2,5-dichloro-4,9-methylbenzophenone and end-capping agent 4-chloro-4,9-

fluorobenzophenone (Scheme 1.8) The macromonomers produced were labile to

nucleophilic aromatic substitution The molecular weight of poly(4,9-methyl-2,5- benzophenone) was controlled by varying the amount of the end-capping agent in the reaction mixture Poly(4'-fluoro-2,5-diphenyl sulfone) was synthesized using Ni(0)-

catalyzed coupling and was reacted with various nucleophiles via aromatic substitution

(SNAr) to produce derivatized poly(p-phenylene) (PPP) thermoplastics and thermosets. 34

Cl Cl

O

NiCl 2 , Zn, PPh 3 , BiPy NMP, 80°C, 2h

O

F F

n

Cl Cl

O

NiCl 2 , Zn, PPh 3 , BiPy NMP, 80°C, 2h

O

F F

n

Scheme 1.8 Multiblock copolymers of PPP synthesized by Bloom et al

The aforementioned PPP systems describe the synthetic demands for being able to prepare processable, structurally defined PPPs, in which the π-conjugation remains fully intact or is even increased as compared to that of the parent PPP system The π-orbital

overlap can be considered as a function of the cosine of the twist angle, at 23º in the unsubstituted parent PPPs Still system exists with a fair amount of conjugative interaction.35 The incorporation of the substituents along the PPP backbone (e.g at the 2-

and 5-positions), increased the solubility dramatically with a reduced π-overlap Twist angles of 60-80º are reported for PPPs with alkyl substituents in 2,5-positions.36 Thus, for poly(2,5-dialkyl-1,4-phenylene)s, only negligible optical absorption can be detected in the wavelength region above 300 nm, which is characteristic for delocalized π- π* transitions

The preparation of planar PPP was started from the work of Yoshino et al. 37

through the synthesis of poly(9,9-dialkylfluorene)s via oxidative coupling of fluorine derivatives Later, poly(9,9-dialkylfluorene) derivatives have been synthesized via nickel

and palladium catalyzed aryl-aryl homo-and cross-coupling reactions using substituted monomers such as 2,7-dibromofluorene and fluorene-2,7-diboronic acid derivatives.38 Yamamoto et al., coupled 2,7-dibromo-9,10-dihydrophenanthrene to give an ethano-

bridged poly(p-phenylene) derivative (Scheme 1.9) using low-valent nickel complexes as

catalysts.39 The complex used was either stoichiometric (Ni(COD) 2) or generated electrochemically in the reaction mixture Only the oligomer fraction with Mn <1000 was soluble in common organic solvents, the polymeric product was precipitated as an insoluble powder

Br Br

n

Ni(0) Br

Br

n Ni(0)

Scheme 1.9 Synthesis of poly(9,10-dihydrophenanthrene-2,7-diyl) from

2,7-dibromo-9,10-dihydrophenanthrene

The introduction of alkyl-substituted dihydrophenanthrenes or tetrahydropyrenes allowed the synthesis of soluble step-ladder PPPs using the monomer 2,7-dibromo-4,9-dialkyl-4,5,9,10-tetrahydropyrenes The difunctionalized tetrahydropyrene monomers were first prepared by Müllen et al.40 by adopting the Yamamoto coupling 2,7-Dibromo-4,9-dialkyl-4,5,9,10-tetrahydropyrenes in presence of catalyst Ni(COD)2 in DMF/toluene

yielded soluble poly(4,9-dialkyl-4,5,9,10-tetrahydropyrene-2,7-diyl) (PTHP) (Scheme

1.10) The solubilizing alkyl substituents were incorporated on the periphery of the

macromolecules, which minimizes the twisting of the main-chain The new polymer

PTHP possesses a relatively high number average molecular weight, up to Mn = 45000,

which corresponds to coupling of about 100 THP units One of the advantage of this new

system is that diastereomeric forms can be generated which are composed of cis- or

trans-configured monomeric THP building blocks The stereochemistry of the substituents at 4- and 9-position strongly influence the solid state packing behavior of the polymer PTHP

R: -alkyl

Br Br

R

R

n Ni(0)

R: -alkyl

Br Br

R

R

n Ni(0)

Scheme 1.10 Conversion of 2,7-dibromo-4,9-dialkyl-4,5,9,10-tetrahydropyrenes to

poly(4,9-dialkyl-4,5,9,10-tetrahydropyrene-2,7-diyl) (PTHP)

The incorporation of the parent PPP chromophore into the network of completely

planar ladder polymer, culminated by the logical continuation of the ‘step-ladder’ strategy, minimizes the mutual distortion of adjacent main chain phenylene units Maximum conjugation was achieved through flattening of the conjugated π-system by bridging all

subunits The synthesis of a soluble conjugated ladder polymer of the PPP-type (LPPP)

was reported by Scherf and Müllen in 1991 (Scheme 1.11).41 The polymer, LPPP,

obtained (number average molecular weight of 25 000) had no significant structural defects observed from NMR spectroscopy The absorption maximum (λmax) showed a bathochromic shift as a consequence of planarization of the chromophore to a value of ca

440 nm, which also depend on the substituents –R and –R’ on the polymer backbone In addition, the longest wavelength π-π* absorption band possesses an unusually sharp absorption peak as an indication of the fully planarized, geometrically rigid ladder structure

O R

H

n R'

R'

BF 3

R’: -alkyl R: -1,4-C 6 H 4 -alkyl

O R

H

n R'

R'

BF 3

R’: -alkyl R: -1,4-C 6 H 4 -alkyl

Scheme 1.11 Detailed synthetic scheme for substituted LPPP

Solution state photoluminescence studies showed that LPPP has a very intense

blue emission λemi of 460 nm In addition, the geometric fixation of the chromophore in the ladder structure led to very small Stokes shift (ca 150 cm-1)between absorption and

emission Compared to the unsubstituted PPP (4%), obtained by ICI-precursor route, the

LPPP has an extremely high PL quantum yield, between 60 and 90% (solution) and 40%

(solid state) making this materials an attractive candidate for further investigation as active component in LEDs. 42,43 Detailed studies of LED fabrication and characterization showed

that in addition to primary emission of the LPPP chromophore in the blue region, the PL

and EL spectra exhibited an additional and unstructured broad emission band in the yellow region (ca 600 nm).44 The yellow emission band was characterized as emission from polymer aggregates using photophysical evidences.45 In addition, the excimer formation was not observed EL experiments revealed that the yellow-emitting LEDs prepared from

LPPP i.e from single layer construction ITO/LPPP 12/Ca; showed quantum efficiency of

ca 1.0% at an applied voltage: 4-6 V.46

n R'

n R'

R'

R: -aryl, R’:-alkylScheme 1.12 Chemical structure of Me-LPPP

The preparation of blue LEDs from LPPP materials is still limited due to the

emission of the yellow light Thus further studies are necessary to efficiently mask out or suppress the dominant yellow emission.46 The suppression of the aggregate emission is

possible through chemical modification of the LPPP structure Introduction of an additional methyl group to methylene bridge of LPPP, yielded ladder polymers Me-LPPP

(Scheme 1.12).47 The suppression of aggregate formation dramatically increased the PL quantum efficiency >90% in solution and up to 40% in the solid state The efficiency of

Me-LPPP as solid state laser material in several configurations (waveguide configuration,

distributed feedback configuration) was later on demonstrated by two independent research groups.48 The high molecular weights of up to 50000 (Mn) allow for the fabrication of thick films and strips (up to 10 μm) The devices showed enhanced stability and lasing over a period of more than 107 pulses.48d

1.2.2.3 Other synthetic strategies for poly(p-phenylene)s

One of the very early reported methods for the synthesis of unsubstituted PPP by Marvel et al.49 involves the polymerization of 5,6-dibromocyclohexa-1,3-diene to poly(5,6-dibromo-1,4-cyclohex-2-ene) followed by a thermally induced, solid state

defect free and showed several types of structural defects (incomplete cyclization, cross linking etc.)

5,6-diacetoxycyclohexa-unit Unfortunately, the polymerization of the monomer did not proceed as a uniform addition, ca 10% of 1,2-linkages were also formed as result of a 1,2-polymerization of the monomer

Another improved precursor route to high molecular weight structurally regular

PPP, starting from the cyclohexa-1,3-diene derivative was also reported.51 Stereoregular precursor polymer was obtained via transition metal-catalyzed elimination of acetic acid to

convert precursor polymer into PPP and a free-standing PPP film was obtained However

these films contained large amounts of the acidic reagent, polyphosphoric acid Absorption maximum (λmax) of 336 nm was observed for new PPP materials Another class of phenylated PPPs were developed by Stille and co-workers using a set of suitable

monomers such as diethynylbenzene and

1,4-phenylenebis(triphenylcyclopentadienone) derivatives (Scheme 1.15.).52 By this method, a very high molecular weight (20 000 - 100 000) was achieved However the repetitive

polyaddition does not proceed in a regioselective way; polymers containing para- as well

as meta-phenylene units within the main-chain skeleton were formed

Ph Ph

Ph

n

Ph(H) Ph

Scheme 1.15 Synthesis of phenylated PPPs

In 1993 Tour et al 53 reported the Bergman cyclization to produce PPP derivatives

The details of the synthetic scheme involved substituted endiynes, e.g phenylene) starting from 1-phenyl-hex-3-en-1,5-diyne or the structurally related poly(2-

poly(2-phenyl-1,4-phenyl-1,4-naphthalene) starting from 1-phenylethynyl-2-ethynyl-benzene (Scheme 1.16)

n

n Ph

Scheme 1.16 Synthesis of poly(2-phenyl-1,4-phenylene)

1.2.2.4 Hyperbranched poly(phenylene) derivatives

The first report about hyperbranched PPP structures was established by Kim and

could be used to synthesize hyperbranched poly(phenylene)s structures The condensation of 1,3-dibromophenyl-5-boronic acid leads to the formation of soluble,

self-hyperbranched PPP-type macromolecule (Scheme 1.17) The transformation of the

hydrophobic periphery composed of bromosubstituents into hydrophilic carboxylic acid functional groups was achieved by reacting with (i) butyllithium and (ii) carbon dioxide The polymer analogous transformation provided water soluble, amphiphilic derivatives of

hyperbranched PPP with useful covalently bonded models for micellar structures

(Br)

(Br) (Br)

(Br)

(Br) (Br)

led to the synthesis of branched oligo(phenylene).56 The phenylated, two-dimensional arylene structures based on a tetrabenzoanthracene core with interesting topologies were

R R

R R

R R

R

R R= -C 6 H 5

(B) R

R

R R

R R

R R

R R

R

R R= -C 6 H 5

(B)

Scheme 1.18 Synthetic route to branched oligo phenylenes

An oxidative cyclization of the branched oligo(arylene)s A and B with copper(II)

chloride or triflate/aluminum chloride was performed and polycyclic aromatic

hydrocarbons (PAHs) were synthesized These large polycyclic aromatic hydrocarbons (PAHs) were characterized by their extremely high thermostability Under ultrahigh

vacuum (UHV) conditions it can be treated up to temperatures of 550 - 650 ºC This extremely high temperature sublimation step is crucial for the pure compounds due to the very poor solubility of the reaction products in organic solvents

In another novel approach, Müllen and co-workers developed repetitive Diels-Alder procedure for the generation of dendritic and hyperbranched poly(phenylene)s.57A wide

variety of disk-like PBAHs were generated based on the planarisation of suitable

oligophenylenes via oxidative cyclodehydrogenation of hexa-peri-hexabenzocoronene (HBC, C42, 4) derivatives as outlined in scheme 1.19.58 The carbon framework of the target molecule was assembled in the readily soluble and characterizable three-dimensional precursor molecule Prerequisites for such an approach involve precursor molecules that can be drawn in their 2D-projections without any overlapping phenyl rings

Monodisperse PBAHs with larger dimensions were prepared and characterized and they

were processable from solution due to the presence of flexible alkyl chains The length and the degree of branching of the alkyl chains plays a crucial role by strongly influencing the solubility of the molecule and the thermal transitions of the columnar phases.59

O

R R

R =

(a )

(b )

Scheme 1.19 The cyclotrimerization (a) or Diels-Alder cycloaddition (b) routes to a

variety of hexa-peri-hexabenzocoronene (HBC) derivative

The dendritic precursor molecules prepared via cyclotrimerisation of diphenylacetylenes (tolanes),60 yielded molecules with hexafold symmetry, or by utilizing the highly versatile Diels–Alder cycloaddition of phenyl-substituted alkynes with tetraphenyl-substituted cyclopentadienones.61 Different cyclopentadienones and alkyne derivatives can be combined in a modular way, leading to a diversity of substitution patterns and molecular topologies The synthesis of such large molecules with aromatic core diameters of up to 2.3 nm, make these new planar molecules promising candidates for new developments in nanochemistry.62 By the utilization of a building-block approach, different substituents or functional groups can be introduced at a very late stage

of the synthesis offering a high degree of flexibility.63 The modular synthetic approaches led to mono-, di-, tri- and even hexa-functional HBC derivatives, which can be further derivatised by transition metal-catalysed coupling reactions.64 The number and the nature

of the functional groups which can be introduced into the periphery of the PBAHs obviously have important implications for the supramolecular assembly of these disk-like molecules Depending on the requirements, well-defined, nano-sized ‘monomeric’ disks were made from components carrying the specific information for their self-assembly to highly ordered supramolecular aggregates

PPP constitutes rigid-rod linear structures with extended π- conjugation This

feature qualifies PPPs as electronic materials or active components in devices such as

field-effect transistors or light-emitting diodes Poly(p-phenylenevinylene)s which have

been studied as emitters for LEDs, gave yellow or yellow-green emission In the search

for blue light emitters, PPPs have attracted particular attention Thin emitting layers of

PPPs are mostly obtained by spin coating or casting

1.3 Thin films to devices

In order to fabricate devices, the preparation of nanostructured thin films of the conjugated polymer with optimum optoelectronic property is needed However, the development of methods for controlling the organization of polymer in the solid state has been limited because polymers often fail to assemble into organized structures due to their amorphous character and large molecular weight There are many methods for making ultrathin films For example, polymers may be spin coated, dipped from a concentrated solution or roll coated onto a substrate For the applications involving thin films of only a few molecules thick with high level of molecular order, the challenge of finding an appropriate coating technique is greater Fortunately in many potential applications, it is possible to use amphiphilic molecules, and make well-ordered films as thin as a single layer of molecules due to strong intermolecular interaction

One way of making these thin films is to self-assemble the molecules on to a solid substrate from a solution of the amphiphilic molecules To do this, a substrate is soaked in the solution of the molecule for a few minutes to a few hours and during this time molecules absorbs on to the substrate Because of the amphiphilic nature of the molecules they are attached to the substrate in an ordered way If the substrate is hydrophilic, then the hydrophilic part of the molecule will interact with it or vice versa In this way molecules organize side-by-side on the substrate In many cases, the molecules also form multilayers from solution A disadvantage of self-assembly is the difficulty in controlling continuity of the deposited layers Some areas of the substrate may not be covered with molecules whereas other areas may end up with multilayers If the amphiphilic molecule

is carefully chosen, these problems can be overcome Determining the criteria for choosing an appropriate molecule is an important area of current research

Another method for building amphiphilic molecules into ordered films is the Langmuir-Blodgett film deposition Again, one is limited in choice of amphiphilic molecules for which this technique can be used because the molecules must assemble into

a smooth, ordered monolayer at the air-water interface If the molecules are too soluble in water, the technique will not be useful Also, if the molecules do not form a monolayer at the interface but instead aggregate or form bulk crystals, then it will not be possible to make smooth films However, in contrast to the other techniques such as spin coating or drop casting, Langmuir-Blodgett-Kuhn (LBK) or Langmuir-Schaefer (LS) techniques have been proven to be powerful tools for the fabrication of ultrathin polymeric thin films with controlled structures and enhanced electronic and optical properties.65

1.3.1 The Langmuir- Blodgett process and LB films of substituted PPPs

In order to fabricate optoelectronic devices, the preparation of thin films of the conjugated polymers with a good optical quality is needed An organic thin film can be deposited on a solid substrate by various techniques such as thermal evaporation, sputtering, electrodeposition, molecular beam epitaxy, adsorption from solution, Langmuir-Blodgett (LB) technique, self-assembly, etc.66The LB-technique is one of the most promising techniques for preparing such thin films as it enables (i) the precise control of the monolayer thickness and molecular order or alignment with an enhanced nonlinear optical properties or electroluminescence along the direction of orientation (ii) homogeneous deposition of the monolayer over large areas and (iii) the possibility to make multilayer structures with varying layer composition An additional advantage of the

LB technique is that monolayers can be deposited on almost all types of solid substrates

Furthermore, the functional properties of materials are closely related to their structures

micro-The LB technique has been widely accepted as a practical method for fabricating highly ordered structures of ultrathin films Irving Langmuir first introduced this method

in the 1920,67 but it was a colleague of his, Katherine Blodgett, who developed and improved the technique in 193468 so that it could be used to obtain multilayer films Much

of the current interest in LB films derives inspiration from the pioneering work of Hans Kuhn in the 1960s who used LB methods to control the position and orientation of functional molecules within complex assemblies,65(a),69 which is currently being called

“supramolecular assembly” Monolayer forming materials are applied to the air-water interface by first dissolving them in a solvent The necessary properties for such a solvent are that it should be able to dissolve an adequate quantity of the material, it must not react chemically with the material or dissolve in the water and finally the solvent must evaporate within a reasonable period so that no trace remains in the condensed monolayer.70 Most materials which form monolayers at the air-liquid interface are amphiphilic, having both hydrophilic and hydrophobic moieties The separation of the polar and the nonpolar parts enables the amphiphilic molecule to stretch at the air/water interface forming ordered monolayer, when they are compressed by the barrier of the LB trough Traditionally, LB films have been prepared from low molecular-weight compounds, like fatty acids Amphiphilic polymers or hairy-rod polymers with long alkyl side-chains can also be used to form LB films at the air-water interface.71 The traditional hairy-rod polymers have macroscopically cylindrical shape on which the long alkyl chains are evenly distributed Therefore, the liquid-like alkyl chains effectively disperse the polymers at the air–water interface Conjugated polymers including

poly(phenylenevinylene), polypyrrole, poly(p-phenylene), poly thiophene and polyanilines

have been fabricated by LB approaches to form mono-or multilayers.72

Essentially all LB film work begins with the Langmuir-Blodgett trough, containing

an aqueous subphase, made from a solid piece of teflon (Figure 1.4(a)) Moveable

barriers that sit across the width of the trough permit control of the surface area available

to the floating monolayer In a typical experiment to form a Langmuir monolayer, the molecule of interest is dissolved in a volatile organic solvent such as chloroform, and a few drops of this solution are introduced to the bath surface These drops then spread, distributing the amphiphilic or surfactant molecules over the water surface After spreading the molecules, the surface area available for the monolayer can be decreased continuously by compressing the barrier Systematic two-dimensional (2D) phase transformations occur during this process as the molecules reorient Surface tension on the bath is lowered, resulting in an increase in surface pressure (П) and the state of the monolayer on the water surface is monitored by measuring the surface pressure, defined as

the difference between the surface tension of the monolayer (γ) and the pure subphase (γ o)

as a function of the lateral compression of the film

П = γ o – γ

1.3.2 Isotherm

As the monolayer on the bath surface is compressed, the surface pressure increases A plot of surface pressure versus surface area at constant temperature is called

an isotherm An idealized surface pressure-area isotherm is depicted in Figure 1.4(b) As

the surface pressure increases, the two dimensional monolayer goes through different phases that have some analogy with the three-dimensional gas, liquid, and solid states At

low pressure and large area, the molecules are separated from each other and neither

attractive nor repulsive forces are noticeable This is the two-dimensional gaseous phase

in which the molecules are thought to lie nearly flat at the water surface Upon lateral compression of the monolayer, the molecules come close to each other and the interaction between them becomes detectable The surface pressure begins to rise, followed by the

transformation of gaseous phase into an isotropic fluid phase called liquid expanded phase As the monolayer area is progressively reduced further, the pressure begins to rise

more steeply and the molecules orient in such a way that their hydrophilic portions interact with the water surface and their hydrophobic portions extend almost vertically

from the bath surface This results in a close packed anisotropic condensed phase (or a series of condensed phases) In all phases, repulsive electrostatic interactions between

hydrophilic groups tend to push the molecules apart, while attractive short-range van der Waals forces between hydrophobic groups tend to hold them together, the balance influences monolayer stability.73 Upon further compression, the monolayer loses their integrity and form irreversibly undefined multilayers either sliding over itself or folding under the subphase This phenomenon is referred to as collapse and the surface pressure at which this occurs is termed the collapse pressure (Пc) Extrapolating the steepest part of the surface pressure area isotherm prior to collapse to zero pressure corresponds to the surface area occupied per molecule

Figure 1.4 Compression of a Langmuir monolayer, (a) molecules spreading into gas

phase followed by expanded and condensed phase of the monolayer after compression of the barrier, (b) surface pressure versus area per molecule isotherm indicating phase transitions of the monolayer

1.3.3 Film deposition

In order to deposit the floating monolayers on the subphase onto a solid substrate, the surface pressure and temperature has to be well controlled so that the Langmuir films are in a condensed and stable/metastable state Vertical deposition and horizontal lifting are the two methods used for transferring films onto a substrate The former one is the most common method of LB transfer; however, horizontal lifting of Langmuir monolayers onto solid supports, called Langmuir-Schaeffer deposition, is also possible Either highly hydrophilic or highly hydrophobic substrates are desired After the monolayer has been spread and compressed to the desired transfer pressure, a hydrophobic substrate is dipped vertically through the monolayer which transfer the molecules from monolayer to the

Mean Molecular Area (Å 2 /molecule)

Surface Pressure (mN/m) (b)

Gaseous phase Liquid expanded phase Liquid condensed phase