Synthesis and characterization of amphiphilic poly(p phenylene) based nanostructured materials 4

4.1 Introduction Ultrathin films of conjugated polymers have received tremendous interest during the past few decades owing to their diverse applications and interesting physico-chemical

Renu, R.;Ajikumar, P K.;Sheeja, B.;Hanafiah,N B M.; Baba, A.; Advincula, R C.; Knoll, W.; Valiyaveettil, S Ultrathin Conjugated Polymer Network Films of Carbazole

Functionalized Poly(p-phenylene)s via Electropolymerization J Phys Chem B (In

4.1 Introduction

Ultrathin films of conjugated polymers have received tremendous interest during the past few decades owing to their diverse applications and interesting physico-chemical properties.1-4 The intrinsic film forming abilities of polymers cast from solution using convenient wet coating techniques are an attractive advantage for practical applications.5 Polymers with a variety of tailored physico-chemical properties can be fabricated as ultrathin films with many different methods such as spin coating, Langmuir-Blodgett technique, layer-by-layer self-assembly, and surface-initiated polymerization.6-7 Thin films of conjugated polymers are expected to have wide range of applications in organic light-emitting diodes (OLED), field-effect transistors (FET), and bio- and chemosensors, and mostly fabricated by spin coating or electrochemistry through physisorption on the substrate Generally, the properties of conjugated polymers are the privileged domains of chemists who can incorporate functional groups with specific electroactive properties.8,9

It is well-known that a balanced and efficient charge injection/transport for both carrier types (electron and hole) is essential for high device efficiency.10 Polymers, however, are rarely good conductors for both electrons and holes In most cases, they transport holes better than electrons In order to facilitate the charge injection/transport, additional electron injection/transport layer between the emitter and cathode or/and a hole-transporting layer between the emitter and the anode needs to be introduced Polymer blends which contain a polymer matrix doped with the necessary components, usually small molecules, facilitate electron/hole transporting properties.11 In addition, a more robust approach which minimizes the conventional problems involves the design of new polymer containing both electron and hole transporting segments as well as emissive

units.10a,12 A hole transporting group such as oxadiazole or carbazole can be incorporated either in the main chain or in the side chains to improve the hole transporting ability of the polymer Even when these requirements are achieved, it is necessary to optimize the quality of the emitting layer by an appropriate deposition technique, to control the film morphology, the carrier mobility, and the emission yield for the device development.13 In this respect, the Langmuir-Blodgett Kuhn (LBK) has been a most useful technique to provide self-organized systems with good molecular order and molecular alignment.14The present study summarizes the preparation of two chemically distinct π-conjugated

polymers with a poly(p-phenylene) backbone and the incorporation of a hole transporting

polycarbazole as side chain The development of highly crosslinked functional thin films

is delineated

Poly(p-phenylene)s or PPPs are an interesting class of polymers which have

quantitative emission properties, interesting LC phases (anisotropic properties), and enhanced ordering at interfaces.15,16 Our group is focusing on the design and development

of homologous series of conjugated polymers and fabrication of micro-/nano architectures 17-18 to investigate the effectiveness of these polymers towards different film deposition techniques which lead to interesting morphologies and improved properties

Among the various polymers poly(p-phenylene) functionalized with six carbon alkoxy

chain and hydroxyl side-group (C6 PPPOH) provided the desired amphiphilicity It

displayed a three-phase region with interesting structural contrast along the polymer backbone, which is directly observable in a Langmuir film.18 The study of carbazole based conjugated polymers have gained tremendous interest for the construction of functional materials, such as photorefractive materials,19 photoconductors,20 nonlinear

optical materials,21 light-emitting,22 and hole-transporting materials.23 This is due to their inherent electron-donating nature, excellent photoconductivity, and unique nonlinear optical properties Among the various carbazole incorporated polymers, poly(N-vinylcarbazole), poly(3,6-N-vinylcarbazole) and polycarbazole have been extensively studied and are of great interest for electrical conductivity and electrochemical device applications.24-25 Among these, poly(N-vinylcarbazole) exhibit interesting electrical and optical properties as light emitting diode materials,26 and photovoltaic materials.27Applications in various electrochromic devices and amperometric chemical sensors should make carbazole based polymers attractive thin film materials.28 Thus carbazole incorporated polymers are potential candidates for tuning the optical and electrical properties of light emitting and semiconducting organic materials 29 The surface grafting

of carbazole-functionalized polyfluorenes to self-assembled monolayer (SAM) of carbazole on indium tin oxide (ITO) surfaces has been demonstrated to form network films.30 Recently, electropolymerization of a substituted polyacetylene such as poly(N- alkoxy-(p-ethynylphenyl)carbazole), with electropolymerizable carbazole resulted in the

formation of conjugated polymer network (CPN) films.29a In line with these previous studies towards combining the physico-chemical properties of a soluble amphiphilic

poly(p-phenylene) and polycarbazole in functional thin films, a PPP derivative with

alkoxy carbazole group (-O(CH 2 ) 5 Cb) incorporated on the polymer backbone

(C6 PPPC 5 Cb) was synthesized and fully characterized The polymer thin films were

prepared using the LBK and spin coating techniques and subsequently electropolymerized for the preparation of mixed π-conjugated polymer network films

O (CH 2 ) 5

n

(CH 2 ) 5 N

CH 3

Figure 4.1 Chemical structure of the polymer C 6 PPPC 5 Cb

4.2 Results and Discussion

4.2.1 Synthesis and Characterization of the polymer C6PPPC5Cb

The polymer C6 PPPOH was synthesized using Suzuki polycondensation of the

respective monomers and the details of the polymer synthesis and characterization is

described in the experimental section Chapter 6 The polymer C 6 PPPC 5 Cb was

characterized using NMR, FT-IR, and thermogravimetric analysis Molecular weight of the polymers were determined by gel permeation chromatography (GPC) with reference

to polystyrene standards using THF as eluent The number average molecular weight of

the hydroxyl protected precursor polymer C6 PPPOBn was 10400 (Da) and that of the

polymer C 6 PPPC 5 Cb was 13100 (Da) The thermogravimetric analysis, of the polymer

showed good stability in nitrogen up to 325 °C, where the mass loss is less than 2 %

(Figure 4.2)

Further, solution optical properties of the polymer was investigated and compared with the parent polymer The normalized UV-Vis and PL spectra of the polymer

C 6 PPPC 5 Cb and the parent polymer C 6 PPPOH are shown in Figure 4.3

0 200 400 600 800 1000 0

10 20 30 40 50 60 70 80 90 100 110

10 20 30 40 50 60 70 80 90 100 110

Figure 4.2 TGA traces of the polymer samples C 6 PPPC 5 Cb and C 6 PPPOH

The absorption maxima at 332 nm for the C 6 PPPC 5 Cb is slightly blue shifted

compared to the C6 PPPOH after the incorporation of alkoxy carbazole group Similarly,

the onset is also slightly blue-shifted compared to parent polymer, indicating a change in the conformation of polymer backbone owing to the presence of carbazole group The additional shoulder peaks below 300 nm was apparent which corresponds to the π-π*, and n-π* transitions of the carbazole peak and were absent in the case of the absorption

spectra of the parent polymer C6 PPPOH The calculated electrooptical band gap, Eg, of

the polymer C6 PPPC 5 Cb is 3.4 eV, slightly higher compared to the parent polymer (3.19

eV) Similar to the UV-Vis spectra, comparison of the PL spectra indicated that the

C 6 PPPC 5 Cb emission maxima (λemis = 400 nm) is blue shifted by 15 nm compared to the parent polymer (λemis = 415 nm) with a blue shift in the onset It may be due to a reduction in the persistence conjugation length of the PPP backbone due to the grafting of the alkoxy carbazole moiety

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

4.2.2 LB film deposition and characterization

In order to study the film deposition of the newly synthesized polymer, LBK technique was used This technique provides a way to fabricate self-organized systems with good molecular order and molecular alignment Previous studies about Langmuir-Schaefer (LS) monolayer and LBK multilayer film of a newly designed conjugated polymer,

poly(p-phenylene)s (CnPPPOH) bearing amphiphilic side chains showed that the

polymer with a short alkoxy group (C 6 PPPOH) forms a more uniform monolayer at the

air water interface and can be transferred to make multilayered polymeric films The

isotherm of the polymer C6 PPPOH, exhibited a liquid expanded region and similar

characteristic was observed for the C 6 PPPC 5 Cb The isotherm of C 6 PPPC 5 Cb showed a

small shift to a more condensed solid-state phase (Figure 4.4) The addition of the

carbazole group probably increases the visco-elastic component of the film but at the same time it retains amphiphilicity to form a good monolayer at the air water interface

Both polymers, C6 PPPOH and C 6 PPPC 5 Cb, have a collapse pressure of ~ 43 mN/m

The calculated area per repeat unit for both polymers is 0.20 ± 0.02 nm² The extrapolation of the solid region in the surface pressure-area isotherm to zero pressure, resulted in the area per repeat unit (A) = 0.20 nm2, which is close to the cross-sectional area of an alkyl-chain This confirms that the carbazole incorporated polymer,

C 6 PPPC 5 Cb, forms good monolayer at the air-water interface with close packed alkyl

chains

0 10 20 30 40 50

Mean Molecular Area (Å2)

Mean Molecular Area (Å2)

Figure 4.5 Absorption spectra of LB films of C 6 PPPC 5 Cb with different number of

layers (A) and the dependence of the film absorption on the number of transferred layers

(B)

0.0 0.1 0.2 0.3

Wavelength (nm)

5 10 15 20

0.0 0.1 0.2 0.3

Wavelength (nm)

5 10 15 20

0 0.05 0.1 0.15 0.2 0.25

Number of layers

0 0.05 0.1 0.15 0.2 0.25

In order to study the deposition of multilayers of C 6 PPPC 5 Cb, the monolayers were

transferred to different hydrophilic substrates using Z-type deposition at a surface pressure of 10 mN/m Increase in absorbance from UV-Vis studies of LBK films of

C 6 PPPC 5 Cb transferred to quartz substrates was linear to the number of layers deposited

(Figure 4.5) A similar result was observed for the parent C6 PPPOH polymer.18a The peak-shifts (Δθ) of angular scans of the plasmon curves of LBK multilayer assemblies on the Au surface relative to the bare gold increases linearly with the number of layers

(Figure 4.6A and B) This is also supported by our previous studies that the shorter alkoxy chain polymer, C6 PPPC 5 Cb is a better candidate for the preparation of LBK films

with layer-by-layer structure Multilayers of up to 20 were deposited with a uniform transfer and used for electropolymerization of the carbazole group for preparing a cross-

linked conducting polymer network film The comparison of the solution (Figure 4.3)

and film state UV and PL indicated that there is blue shift in emission maxima for the film with clear peak broadening at the higher wavelength region with appearance of a

shoulder around 530 nm (Figure 4.7) However there is no change in the observed UV

spectra in the solid-state film compared to the solution

Figure 4.6 SPR curves of the multilayers of C 6 PPPC 5 Cb (A) and plot of the shift of

the resonance minimum for LBK films of C6 PPPC 5 Cb obtained from the SPR angular

scan (B)

0.0 0.2 0.4 0.6 0.8

Wavelength (nm)Figure 4.7 Comparison of the emission spectrum of polymer in CHCl3 solution, 20 layers transferred to quartz at a surface pressure 10 mN/m and spin coated film

4.2.3 Electropolymerization of the LB and spin coated films of

C6PPPC5Cb

Recent studies about the electrochemical polymerization and cross-linking of

poly(vinyl-N-carbazole) (PVK), poly[9-[2-(4-vinylphenoxy)ethyl]-9H-carbazole] (PHC), and

poly(N-alkoxy-(p-ethynylphenyl)carbazole) (PAA-Cz-C6) through the carbazole units

demonstrated the ability to form thin films with unique optical and electrochemical properties with different morphologies.29c, 31 The cross-linked structures were formed through a three-electron transfer process with dimerization of pendant carbazole ring occurring via the 3,6-position leading to intermolecular cross-linking The intermediate is believed to be based on a carbazolylium radical cation which rapidly reacts via coupling-deprotonation to form the dimer.32 Subsequent cycles lead to higher oligomeric species

and further cross-linking as evidenced by a lowering of the oxidation potential and increase in charge density with each succeeding cycle Similarly, multilayer thin films of

carbazole incorporated polymer C6 PPPC 5 Cb on ITO were prepared using

Langmuir-Blodgett technique and spin coating The film was further electropolymerized to prepare the crosslinked film CV measurements were carried out using an electrolyte solution of 0.1 M tetrabutylammoniumpercholorate (Bu4NClO4) dissolved in acetonitrile, in which the precursor polymer was not soluble An undivided three electrode configuration cell was used with the thin films of the polymer on ITO or gold coated LaSFN9 as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode Different scan rates such as 100 mV/s and 20 mV/s were used to study the influence of scan rates on the stability of cross-linked polymers The polymerization with slow scan rate (20 mV/s) showed a reduction in the intensity of the oxidation and reduction peaks after two cycles which may be due to degradation of the crosslinked film after one cycle Thus all further experiments were solely performed at a scan rate of 100 mV/s This was further investigated using a combined SPR-cyclic voltammetry set up

The oxidation and reduction potentials of C6 PPPOH were not in the range of the applied

potentials for electropolymerization; therefore oxidation of only carbazole groups was expected within this potential range Cross linking of the carbazole monomer units occurs during the electropolymerization without affecting the PPP conjugated polymer backbone

Figure 4.8 CV for electrochemical cross-linking of 20 layers (A) 5 layers (B) of LB film

and spin coated (C) of C 6 PPPC 5 Cb at scan rate of 100 mv/s (D), (E)and (F) are the

corresponding precursor polymer free scan

Cyclic voltagram of the cross-linking of the LB multilayer and spin coated films

of C6 PPPC 5 Cb deposited on ITO substrates with a scan rate of 100 mV/s is shown in Figure 4.8 The oxidation onset for 20 layers of LB multilayers is 0.93 V and the

corresponding reduction peak is 0.78 V(vs Ag/AgCl) (Figure 4.8A) This oxidation peak

-0.00005 0.00000 0.00005 0.00010 0.00015

Potential E Vs Ag/AgCl

0.0 0.2 0.4 0.6 0.8 1.0 1.2 -0.00010

-0.00005 0.00000 0.00005 0.00010 0.00015

-0.00004 -0.00002 0.00000 0.00002 0.00004 0.00006 0.00008

Potential E Vs Ag/AgCl

0.0 0.2 0.4 0.6 0.8 1.0 1.2 -0.00006

-0.00004 -0.00002 0.00000 0.00002 0.00004 0.00006 0.00008

-0.000005 0.000000 0.000005 0.000010 0.000015 0.000020

-0.000005 0.000000 0.000005 0.000010 0.000015 0.000020

was absent in the first cycle with an appearance of another peak at 1V, which indicated that first cycle is different from the second cycle with the possibility of cross-linking of carbazole units at about 1.0 V Peaks due to doping and dedoping were found in the second to the subsequent cycles with doping at 0.93 V and 0.80 V for dedoping The peaks due to dedoping were slightly shifted to 0.78 V in the subsequent cycles The observed peak value for doping and dedoping was slightly higher than previously reported carbazole incorporated films The current increased with the number of cycles For carbazole grafted poly(phenylacetylene) polymer, the oxidative doping was observed

at about 0.7-0.8 V, followed by another current increase at about 1.0 V during the anodic scan of the cycle.29a In the case of the polyvinyl carbazole (PVK), the first cycle always showed an oxidation onset at 0.9 V and the appearance of the oxidation doping peak in the 0.6-0.7 V range in the subsequent cycles.25 The slightly higher value in the doping

and dedoping for new polymer C6 PPPC 5 Cb can be accounted for the rigid rod structure

of the polymer backbone with good chain-to-chain polymer packing in the film This generates a dense structure of the carbazole moiety compared to the previously reported

flexible polymers such as the copolymers of carbazole and thiophene, PVK or alkoxy-(p-ethynylphenyl)carbazole), but at the same time slows down the counter ion

poly(N-transport properties due to a less porous structure.29 The oxidation onset for 5 layers of

LB multilayers is at 0.93 V and the corresponding reduction peak is at 0.8 V There was a decrease in the peak area after four cycles which could indicate a slight degradation of the film In the case of the spin coated film the electrochemical behavior was almost the same as the 20 layer LB film The difference in behavior with the film thickness could be