Synthesis and characterisation of electrical conducting polymers co polymers based on omega functionalised 3 alkylthiophenes 2

3 To effect chemical polymerisation of the monomers synthesised by FeCl3 oxidative method to form the following polymers: 4 To characterise all polymers formed by structural techniques

Chapter 2

Syntheses and Characterisation of Electrically

Conductive and Fluorescent Poly[3(

-bromoalkyl)thiophenes]

1 Introduction

1.1 Conducting polythiophene

Amongst the many families of electrically conductive materials, functionalised polythiophenes have always occupied an important position Apart from their good electrical properties, processibility, environmental stability and amenity to wide-ranging potential technological applications, their optical properties [1], thermochromistic [2], piezochromistic [3], luminescence [4] behavior are all attracting increasing research interests Chemical modification of monomeric building blocks in these conducting polymers allows for the structural control and tailoring of the respective polymer properties In particular, pendant functionalisation offers an attractive possibility of developing materials that, in addition to particular polymer electronic properties, incorporate specific properties of the pendant functionality [5] It was also reported that the semiconducting conjugated polymers exhibit fluorescent characteristics, which made them potential materials for application as polymer light-emitting diodes (PLEDs) [6]

1.2 Functionalised poly(3-alkylthiophene)

A variety of chemically modified structures have been developed through substitution on the thiophene ring [7] Attachment of pendant alkyl chains of suitable lengths at the 3-position of thiophene has been shown to afford a series of solvent processible and fusible poly(3-alkylthiophenes) [8] Amongst the many poly(3-alkylthiophenes) formed, poly[3-( -substituted alkyl)thiophene] is of particular interest It is anticipated that the functionalisation of the alkyl pendant chain with reactive -moieties in polythiophenes will produce novel kinds of practically useful polymers These polymers would lend themselves to post-polymerisation treatment processes, particularly in the generation of electrically conductive composite materials for anti-static applications Previous efforts in this direction include poly[3-( -hydroxyalkyl)thiophene] of different chain lengths [9], which demonstrated comparable conductivity to those of poly(3-alkylthiophene) upon doping but reduced solubility due to the hydroxyl moieties

It has also been proven that the hydroxyl group is still an active functional group after polymerisation [10] This creates the possibility of post-polymerisation treatment to generate new material As the first step of developing new material that graft commodity polymer and conductive polymer, this chapter report the findings on the preparation and properties of functionalised polythiophene with

-bromoalkyl substituents [viz Poly(Th-Cn-Br) n=4, 6, 8, 10, 12] These polymers are anticipated to be amenable to facile synthetic transformation to a range of other functional materials via the reactive halide moiety

The syntheses of the monomeric 3-( -bromoalkyl)thiophenes have been reported

briefly by Bäuerle et al [11] In this work, a straightforward FeCl3 polymerisation approach was utilised in order for preliminary data on the polymer properties to

be evaluated These functionalised polymers themselves may also be applicable in the field of sensors, microelectronic devices and/or electro-catalysis [12]

1.3 Scope of the work in this chapter

1) To synthesise the following monomers:

3-( -bromobutyl) thiophene (THC4Br) 3-( -bromohexyl) thiophene (THC6Br) 3-( -bromooctyl) thiophene (THC8Br) 3-( -bromodecyl) thiophene (THC10Br) 3-( -bromododecyl) thiophene (THC12Br) 2) To characterise all monomers synthesised by structural techniques using analytical instruments such as Fourier Transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), mass spectrometry (MS) and elemental analysis

3) To effect chemical polymerisation of the monomers synthesised by FeCl3

oxidative method to form the following polymers:

4) To characterise all polymers formed by structural techniques using analytical instruments such as Fourier Transform infrared spectroscopy (FT-IR), nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC) and elemental analysis

5) To study the conductivity of the polymers and the effects of dopants on conductivity

6) To study the electro-optical properties of the polymers through ultraviolet absorption spectrometry (UV) and fluorescence emission spectrometry

7) To determine the thermal properties of the polymers by thermogravimetry 8) To analyse the surface characteristics of the polymers by X-ray photoelectron spectroscopy (XPS)

9) To test the possibility of effecting functional group transformation via the reactive -bromo moiety

2 Experiment

2.1 Syntheses of monomers

2.1.1 Overview

Monomers were synthesised in accordance to the method of Bäuerle et al [11],

which is depicted in Scheme 2.1

Scheme 2.1 Monomer synthesis Reagents and conditions: (i) KOH/MeOH,

acetone, reflux; (ii) Mg, I 2 , anhydrous ether, reflux; (iii) Ni(dppp)Cl 2 , anhydrous Ether; (iv) HBr/Ac 2 O, 100 C

The -(p-methoxyphenoxy)alkyl bromides, easily prepared from , dihaloalkanes and hydroquinone monomethyl ether (HCM), react readily with magnesium to give the corresponding Grignard compounds The -(p-

-methoxyphenoxy)alkyl bromides (n = 4, 6, 8, 10, 12) undergo nearly quantitative Grignard reaction to afford its alkylmagnesium bromides Grignard reagent (n = 4,

BrMg(CH2)nO OCH3

S

Br+

S(CH2)nO OCH3

the alkyl magnesium bromides (n = 4, 6, 8, 10, 12) (0.1 – 1 mol % Ni(dppp)Cl2 as

catalyst in refluxing ether or THF*) leads to the terminally protected 3-[

-(p-methoxyphenoxy)alkyl] thiophenes, which after a single recrystallisation, can be obtained in 63 – 81% yield as a colourless, analytically pure solid

Of the numerous ether cleavage methods, that with hydrogen halide/acetic anhydride proved to be best suited for the cleavage of the HCM protecting group

in 3-[ -(p-methoxyphenoxy)alkyl] thiophenes These 3-substituted thiophenes

were thus converted directly into 3-( -bromoalkyl)thiophenes The best yields were obtained using HBr/Ac2O at 100 C/20-25 hr for 3-( -bromoalkyl)

thiophenes After separation of the simultaneously formed hydroquinone followed

by chromatographic and distillative purification, halides were obtained as analytically pure compounds in 51 –76% yield

In an attempt to achieve a higher yield for the key monomer, the methoxyphenoxy protecting groups were replaced with p-methylphenoxy and p-

p-t

butylphenoxy protecting groups for , -dibromodecane The same procedure

was followed except HCM was replaced with p-methylphenol and p-tbutylphenol respectively in the first step The yields were compared

* For n = 8, 10, 12, THF was used due to the decreasing solubility of the alkymagnesium Grignard

reagent in ether which caused practical difficulties in the transferring process

Polymerisation was carried out using the method of Casa et al [13] Poly[3(

-bromoalkyl)thiophenes] were obtained by dropwise addition of FeCl3 in nitromethane to a solution of the monomer in carbon tetrachloride Since FeCl3 is insoluble in CCl4, FeCl3 (the active state of the oxidant) gradually precipitates and reacts with the monomer This is a uniformed polymerisation method with the formation of a very fine powder suspension of the polymers The as-synthesised polymers, i.e complexed (doped) with FeCl3, could be completely changed to the neutral (undoped) state by soxhlet extraction with methanol and acetone in turn The dedoping was evidenced from the disappearance of characteristic doping bands in the FT-IR spectrum, at 1150 cm-1 and 1350 cm-1

As a comparison, the method of Sugimoto et al [14], the usual method for

oxidative polymerisation using a suspension of FeCl3 in dry chloroform was also used for 3-( -bromohexyl)thiophene This was effected by either rapid addition

of the monomer solution in one portion (PBHT2) or adding the monomer solution dropwise generated the polymers (PBHT3)

3 Results and Discussion

3.1 Experiments on different protecting group

According to the method of Bäuerle et al (scheme 2.1), the p-methoxyphenoxy

group was used as a protecting group and was later cleaved using hydrogen halide/acetic anhydride to afford the monomer One of the by-products of the

cleavage reaction was found to be 3-[ -(p-phenoxy)alkyl] thiophenes, which

indicated that cleavage could also occur on the methoxy group In the hope of

eliminating this side reaction, p-methylphenoxy and p-tbutylphenoxy protecting groups were tested It was found that the yields of the monomers were 32% and 20% respectively for these two protecting groups The lower yield indicated that the presence of the methoxy group did help the cleavage of the protecting group

in the original reaction The electron donating effect of the methoxy group makes the protecting group electron rich, hence function as a better leaving group under

acidic conditions Replacing this methoxy group with methyl or p-tbutylphenoxy group could not compensate for this electron donating effect thus the yield was lower, even though the side reaction was eliminated

3.2 Structural characterization of synthesised monomers

All 3-( -bromoalkyl)thiophenes and their predecessors were characterised by NMR The monomers were also characterised by MS, FT-IR and elemental analysis

In the NMR spectra of all monomers, multiplets at around 7.2 ppm indicated aromatic Hs on position 4 of the thiophene ring Multiplets at around 6.9 ppm indicated aromatic Hs on positions 2 & 5 of the thiophene ring The presence of the Br functional group was confirmed by the triplet at around 3.4 ppm, which showed methylene protons being deshielded by the adjacent bromine group The NMR analyses results is in good accordance with the literature reported value [11]

An overlaid IR spectrum of all monomers is shown in Figure 2.1

Bands at around 3110 cm-1 and 3050 cm-1 were results of C-H stretching for H(Hs at positions 2 and 5 on the thiophene ring) and H (H on position 4 of the thiophene ring) The longer the polythiophene chain the more extensive the delocalisation As a result the bond order reduced and the intensities of these peaks were getting weaker Band absorptions appearing at 1250 cm-1, 645 cm-1 and 565 cm-1 were attributed to the addition of the bromo group Attention should

be drawn to C-H stretching (3110 cm-1) and bending bands, since these should

change in the polymer IR spectrum and give structural information of the polymers

Fig 2.1 Overlaid FT-IR spectra of all monomers From top to bottom:

THC4Br, THC6Br, THC8Br, THC10Br, and THC12Br

3.3 Physical properties of polymers

The neutral polymers synthesised using the approach by Casa et al [13] in carbon

tetrachloride solvent were all black powders which were largely soluble in moderately polar organic solvents such as chloroform, tetrahydrofuran and benzene In addition, for the polymer series with n-( -bromohexyl) pendant, the

polymers obtained through the method of Casa et al [13] were found to be more

soluble than the polymers yielded through the Sugimoto approach [14] Further,

of the two polymers formed using the Sugimoto approach, the polymer (named PBHT3) synthesised by the dropwise addition of the monomer was found to have higher solubility compared to the polymer formed by adding the monomer in one portion (named PBHT2) The solubilised polymers were extracted using soxhlet extraction in CHCl3 and adopted for subsequent UV-Vis, fluorescence, NMR and GPC characterisation studies

The bulk stoichiometry of the polymers was determined from elemental microanalysis results Upon normalisation of the sulphur contents, the empirical formulae of the polymers displayed good agreement between the expected and calculated carbon and hydrogen values, although the bromine content was found

to be lower A possible rationale for this may be functional group degradation or substitution during polymerisation on account of the presence of a high concentration of HCl in the solution [15, 16]

Table 2.1 summarises the number average molecular weight (Mn) for the neutral polymers as determined from GPC The data was compared to poly(3-

hexyl)thiophene (PHT) that was obtained using the method of Casa et al [13]

These polymers with n-( -bromoalkyl) pendants have a polydispersity index

(PDI) ranging from ca 1.7 to 2.0 The Mn of the polymers ranged from 12,300 to 29,400, which corresponds to a degree of polymerisation (DP) comprising 57-93 monomeric units While pTHC4Br, pTHC6Br and pTHC8Br were totally soluble

in THF, pTHC10Br and pTHC12Br were only partially soluble Thus the molecular weights obtained from the soluble portion of the bulk of these two polymers possibly represent only the lower molecular weight fractions, the insoluble fractions were expected to possess higher molecular weights When the polymers generated under different conditions were compared, a relatively higher

DP was achieved by using the Sugimoto approach, which invariably resulted in polymers with lower solubility

Table 2.1 Summarised data of polymers formed

Polymer GPC results Conductivity HT dyads HH dyads

DPn Mn PDI I2 doped

(Scm-1)

pTHC4Br 57 12,300 1.8 4.7 68 32 pTHC6Br 82 19,900 2.0 7.9 73 27 pTHC8Br 93 25,200 1.8 10.4 87 13 pTHC10Br 73 22,000 2.0 8.4 62 38 pTHC12Br 90 29,400 1.7 7.5 60 40 PHT 138 23,100 2.7 9.3 71 29 PBHT2 95 23,400 1.9 8.2 70 30 PBHT3 92 22,600 2.0 7.9 68 32

DPn is the average degree of polymerisation

3.4 FT-IR and 1 H NMR characterisation

Figure 2.2 depicts the FT-IR spectra of pTHC4Br, pTHC6Br, pTHC10Br, pTHC6Br after I2 doping and 3-( -bromohexyl)thiophene In the spectra of neutral polymers, for example in the spectrum of pTHC6Br, the presence of the bromide moiety afforded absorption bands at 1254 (-CH2- deformation), 645 and

556 cm-1 (C-Br stretching) [17]

In addition, the polymers depicted bands at 3052 cm-1 which are ascribable to thienylene C-H stretching and 829 cm-1 due to C-H bending of a 2,3,5-trisubstituted thienylene moiety [18] Although the thienylene C-H stretching and bending modes at 3114 and 768 cm-1 respectively are clearly evident in the monomers, these bands are very much attenuated in the polymer This indicates that there is a predominant - coupling of thiophenes upon polymerisation

Other characteristic bands in the polymers include the alkyl-CH2 asymmetric and symmetric stretching at 2929 and 2856 cm-1, as of aromatic ring-vibrations at

1466 cm-1 Upon doping with I2, the polymers, as exemplified in pTHC6Br, took

on a noticeable change in their IR spectra Thus, strong and broad doping-induced bands at 500 - 1500 cm-1 became evident, which are due to the presence of free charge carriers typical in organic semiconductors [19]

Fig 2.2 FT-IR spectra of polymers before and after soxhlet extraction

(doped and undoped) From top to bottom: (a) undoped pTHC4Br; (b) pTHC6Br; (c) pTHC10Br; (d) doped pTHC6Br and (e) 3-( - bromohexyl)thiophene

A representative solution 1H NMR spectrum of these materials is shown in Figure

2.3 for the polymer pTHC6Br The band at ca 3.4 ppm is assigned to methylene

-CH2-Br proton resonance whilst that at ca 2.8 and 2.5 ppm are ascribed to the methylene protons on the pendant chain immediately adjacent to the thienylene rings which are arranged respectively in a head-to-tail (HT) and head-to-head (HH) dyad configuration [20] From the ratio of the integration of these signals, the relative proportions of HT : HH dyads in the polymer can be determined to be approximately 2.72: 1 corresponding to a 73 / 27 ratio of the respective dyads in pTHC6Br Proton resonance in the range 6.8-7.2 ppm is typical of hydrogen on polymer thienylene rings The four sets of singlets in the aromatic region can be attributed to the contribution in the polymer of the triads HT-HT ( 6.98), HT-HH ( 7.00), TT-HT ( 7.02) and TT-HH ( 7.05) linked thiophene rings [21]

The 1H NMR spectra of the other four polymers were assigned in a similar way to that of pTHC6Br In these, the estimated ratios of HT to HH dyads are 68/32 and 62/38 respectively for pTHC4Br and pTHC10Br (Table 2.1) Although increasing proportions of HT dyads are expected with increase in pendant alkyl chain lengths [31], the obtained experimental results proved otherwise This inconsistency may have arisen from the steric effects exerted by the bulky bromine group From Table 2.1, it is also evident that poly[3-( -bromohexyl)thiophene] prepared by

the method of Casa et al has a similar degree of regioregularity compared to that

of the Sugimoto approach [14]

Fig 2.3 NMR spectrum of pTHC6Br

3.5 Electrical conductivity of the doped polymers

The maximum conductivity of the polymers upon doping with I2 is shown in Table 2.1 Here, the maximum conductivity was attained with an I2 weight-uptake

of ca 100 % w/w The other two polymers with different chain lengths displayed

similar conductivity trends The conductivity of I2-doped polymers PHT, PBHT2 and PBHT3, were also examined for comparative purposes In these investigations, it was found that the maximum conductivity for the series of poly[3-( -bromoalkyl)thiophene]s and PHT are all within the same order of magnitude

The maximum conductivity of I2-doped pTHC6Br was obtained only after several repeated attempts on account of the brittle nature of the polymer upon doping It can be seen that the conductivity of the -bromine substituted polymers were only slightly lower than that of the PHT polymer This suggests that the presence

of the -bromo moiety has only marginal effects on the polymers’ macroscopic conductivity Also, the varying alkyl chain length did not affect the polymers’ conductivity drastically The result is in contrast to earlier observations by

Kaeriyama et al [22] In their study of polyalkylthiophenes where it was found

that the introduction of longer n-alkyl chains decreases the resulting conductivity

in the doped state Furthermore, with our polymers, different polymerisation conditions appear to have only a slight influence on electrical conductivity

3.6 UV-Vis and fluorescence spectroscopy

Standard polymer solutions of 10-5 M in chloroform were used for studies in Vis absorption and fluorescence spectroscopy Dilute solutions were used in order

UV-to avoid the possibility of concentration quenching or re-absorption / re-emission with all solutions being degassed prior to the measurements The condensed-phase UV-Vis absorption spectra of polymers were obtained from thin films on

ITO glass prepared by spin-casting from chloroform solutions of ca 10-3 M Fluorescent quantum yields ( s) were evaluated by comparison with quinine sulfate solutions [32] in 0.1 M H2SO4 using the following equation [23]:

r s

s

r r s

I

I A

A

where A s and A r are the absorbances of the sample and reference solutions, I r and

I s are the corresponding relative integrated fluorescent intensities Table 2.2 summarises the electronic absorption, emission maxima and the quantum yields relative to quinine sulphate (Spectra not shown here) Measurements on PHT were also conducted under identical conditions for comparative purposes

Table2.2 Optical properties of the polymer

b : cast from 10 -3 M CHCl 3 solution

c : relative to 10 -5 M quinine sulfate in 0.1 M H SO

All polymers showed similar characteristics with two absorption maxima: a less

intense band at ca 260 nm attributable to localized - * transitions of the

thienylene rings and a second maxima at higher wavelength ascribable to the - * transition of the -conjugated system of the polymer backbone [24] Whilst the first absorption maxima ( max) was found to be less dependent on the extent of conjugation, the position of the latter band is bathochromatically shifted with increasing effective conjugation along the polymer chain

However, the differences in max of these polymers are rather small as summarised in Table 2.2, This is most likely due to the presence of the bulky -bromo moiety, which should enhance the rigidity of the backbone structure The effect should be more important to polymers with a shorter chain pendant group, since those with a bulky longer chain pendant group already have a rigid backbone structure Therefore the max of these polymers are not very far apart Further, when their max are compared to that of PHT, we can see that the -bromo moiety only have a marginal effect on the polymers’ electro-optical property This is also in agreement with the observation of their macroscopic conductivity

When polymers pTHC4Br, pTHC6Br and pTHC10Br were cast from chloroform

solutions of ca 10-3 M to afford thin films on ITO glass, a red shift in the max of

ca 52 - 61 nm with respect to the polymer solution was observed in the UV-Vis

a higher degree of planarity and hence a higher degree of conjugation in the polymers’ condensed phase [25]

Table 2.2 also summarises the electronic emission maxima and the solution quantum yields with respect to quinine sulphate The polymers’ fluorescence

quantum yields, as calculated by the approach of Davey et al [26], ranged from

8.4 to 11.1 % with pTHC6Br having a higher value compared to PHT measured under identical conditions However, the differences in quantum yields of these three polymers are small relative to quinine sulphate Once again this could be attributed to the effect of the bulky -bromo moiety enhancing the rigidity of the polymer backbone structure As was discussed in the previous section, the effect

is more pronounced for the polymers with shorter pendant chains Since quantum yield is related to the rigidity of the polymer backbone structure, the similar rigidity of this series of polymers resulted in their having similar quantum yields

3.7 Thermal stability of the neutral polymers

The thermal properties of the poly[3-( -bromoalkyl)thiophenes], with different alkyl pendants in their neutral and doped states were investigated Degradation of neutral polymers occurred through three distinct weight-loss steps in air

n-Fig.2.4 illustrates the % weight loss versus temperature of neutral pTHC6Br in air Here, thermal oxidative degradation of pTHC6Br occurs over a temperature range from 180 to 700oC, with three maximum rates of degradation at 330, 455 and 580oC, leaving residues of less than 2% upon complete degradation This three-step degradation involved the sequential elimination of the bromine moiety, cleavage of the n-alkyl pendant group and backbone decomposition of the polymer The change in weight percentages of this three-step degradation correlates well with the above decomposition modes

In N2, thermal degradation started at 180oC with maximum rates of degradation occurring at 320 and 480oC These first two steps are similar to the degradation pattern of pTHC6Br in air However, unlike in air, the degradation rate of the polymer was much slower in N2 at temperatures above 600oC When the temperature reached 1000oC about 16% of the residue remained