synthesis and electrochemical characterization of new linear conjugated arylamine copolymers

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Synthesis and electrochemical characterization of new linear conjugated arylamine copolymers

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Synthesis and electrochemicalcharacterization of new linearconjugated arylamine copolymers

Loredana Vacareanu and Mircea Grigoras

AbstractNew conjugated copolymers having linear structures were synthesized by palladium catalyzed cross-coupling reactions ofbis(4-bromophenyl)phenylamine or 4-hydroxymethyl-N,N’-bis(4-bromophenyl)aniline and two bisboronic acids of fluor-ene or thiophene. The copolymers were obtained as fully or partially soluble materials, in chlorinated and aprotic polarsolvents. Having a hydroxymethyl group attached to the para position of the triphenylamine units, the synthesized copo-lymers can be viewed as functional polymers which were transformed into new ones by employing suitable chemical reac-tions. A new copolymer was obtained by coupling the hydroxymethyl group of triphenylamine of P35 copolymer, withphenyl isocyanate. The chemical structures and the properties of copolymers were investigated by spectroscopic methodssuch as 1H nuclear magnetic resonance, Fourier transform infrared, ultraviolet-Vis and photoluminescence spectroscopy.Cyclic voltammetry was performed in order to obtain information about the electrochemical stability and reversibility ofthe redox processes of copolymers. Electrochemical studies were carried out on films cast on glassy carbon electrode(GCE) or In-Tin oxide (ITO) glass-coated electrode.

KeywordsTriphenylamine-based copolymers, postfunctionalization reaction, spectroscopic characterization, cyclic voltammetry

Introduction

So far, organic semiconductors have been continuously

studied as active layers in various devices, such as organic

light-emitting diodes (OLEDs), organic field effect transis-

tors (OFET), solar cells, etc.1–5 Increased developments

carried out in the field of conducting polymers behaving

like semiconductors, have encouraged researchers to incor-

porate these polymers as basic materials in various electri-

cal devices, due to their unique electrical and optical

properties. Indeed, the conjugated structure with alternat-

ing single and double bonds provides a conducting pathway

along the backbone through the p-orbital overlapping gen-

erating an electrical conductivity comparable to that of the

undoped conventional semiconductors. Unfortunately,

owing to their intrinsic structure, conjugated polymers are

non-moldable materials, having poor solubility, poor

mechanical properties and environmental and thermal

instability. Therefore their technological applications are

limited by processing difficulties, and in most cases they

cannot be transformed into usable forms after their synth-

esis.6 These drawbacks can be removed by tuning the con-

ducting polymers structure and thus, the optical and

electrical properties can be controlled by synthesis or

post-polymerization reactions.

In the last decade, electron-rich triphenylamine deriva-

tives have been widely studied and used as hole transport-

ing materials7–12 due to the easy oxidizability of the

nitrogen center and the ability to transport positive charges

via the radical-cation species. These interesting properties

are associated with the presence of nitrogen atom (electro-

active site) linked to three electron rich phenyl groups in a

three-dimensional propeller-like shape.13 The electro-

oxidation process which ends, in the most cases, with the

formation of stable radical-cations, reveals that the triphe-

nylamine derivatives exhibit excellent reversible electro-

chromism upon redox switching.14–17 On the other hand,

Petru Poni, Institute of Macromolecular Chemistry, Electroactive

Polymers Department, Iasi, Romania

Corresponding Author:

Loredana Vacareanu, Petru Poni, Institute of Macromolecular Chemistry,

Electroactive Polymers Department, 41A Gr. Ghica Voda Alley, Iasi –

700487, Romania

Email: [email protected]

High Performance Polymers23(2) 112–124ª The Author(s) 2011Reprints and permission:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0954008310395413hip.sagepub.com

at TRENT UNIV on October 15, 2014hip.sagepub.comDownloaded from


fluorene unit can be understood in the way that the pair of

phenylene rings is locked into a coplanar arrangement,

which improves the solubility and processability of the

polymers without significantly increasing the steric interac-

tions in the polymer backbone.18–20

Based on these assumptions, the incorporation of triphe-

nylamine and fluorene units in a copolymer structure might

be an easy way to prepare amorphous materials with

increased solubility, and balanced hole and electron trans-

port properties. Encouraged by this idea in this article we

report the synthesis and electrochemical characterization

of functional copolymers having alternating triphenyla-

mine and fluorene or thiophene units in their backbones.

These copolymers were synthesized under Suzuki polycon-

densation conditions. The electronic properties of the new

copolymers were analyzed by UV-Vis absorption and

fluorescence emission spectroscopy and cyclic voltamme-

try (CV).

The presence of the pendant groups (–CH2OH) attached

to the para position of the triphenylamine units, allows the

modification of the copolymer properties on a wide range,

by post-reactions with properly selecting the chemical

compounds or even biological molecule.

Experimental

Instruments

Fourier transform infrared (FT-IR) spectra were recorded

in KBr pellets on a DIGILAB-FTS 2000 spectrometer and

UV-Vis and fluorescence measurements were carried out in

CHCl3 solutions (spectrophotometric grade), on a Specord

200 spectrophotometer and Perkin Elmer LS 55 apparatus,

respectively. 1H-NMR spectra were recorded at room tem-

perature on a Bruker Avance DRX-400 spectrometer

(400 MHz) as solutions in CDCl3, and chemical shifts are

reported in ppm and referenced to TMS as internal stan-

dard. The relative molecular weights were determined by

gel permeation chromatography (GPC) using a PL-EMD

instrument and polystyrene standards for the calibration

plot, and tetrahydrofuran (THF) as solvent.

The cyclic voltammograms (CV) reported in this article

were recorded using a Bioanalytical System, Potentiostat–

Galvanostat (BAS 100B/W). The electrochemical cell was

equipped with three electrodes: a working electrode (disk-

shape glass carbon electrode (GCE, F ¼ 3 mm) or In-Tin

oxide (ITO)-coated glass with 2.5 cm � 2.5 cm area), an

auxiliary electrode (platinum wire), and a reference elec-

trode (consisted of a silver wire coated with AgCl).

Before experiments, the ITO-coated glass electrode was

sonicated in a mixture of detergent and methanol for 5 min

and then rinsed with a large amount of doubly distilled

water. The glassy carbon electrode was polished between

each set of experiments with aluminum oxide powder on

a polishing cloth. The reference electrode (Ag/Agþ) was

calibrated at the beginning of the experiments by running

the CV of ferrocene as the internal standard in an identical

cell without any compound in the system (E1/2 ¼ 0.425 V

versus the Ag/AgCl). Prior to the each experiment, the

Bu4NBF4 solutions were deoxygenated by passing dry

argon gas for 10 min. All measurements were performed

at room temperature (25 �C) under argon atmosphere.

Materials

Triphenylamine (97%), 9,9-dioctylfluorene-2,7-diboronic

acid, 2,5-thiophenediboronic acid, tetrakis(triphenylpho-

sphine) palladium(0) and phenyl isocyanate were pur-

chased from Aldrich and were used as received.

Phosphorus oxychloride was purchased from Fulka and

used as received. N-bromosuccinimide (NBS) used in the

bromination reaction of triphenylamine was purchased

from Aldrich, and was recrystallized from DMF before

used. All solvents are commercially products and purified

before use by distillation.

Tetrabutylammonium tetrafluoroborate (Bu4NBF4) was

synthesized by neutralization reaction of tetrabutylammo-

nium hydroxide solution (40%) with fluoroboric acid

(40% solution) (both from Fluka) and recrystallized twice

from ethyl acetate and then dried in vacuo prior to use.21

This salt was used as supporting electrolyte in electroche-

mical studies.

Synthesis of bis(4-bromophenyl)phenylamine (1). A portion of

14.51 g (81.52 mmol) of N-bromosuccinimide (NBS) was

added dropwise to a solution consisting of 10 g (40.76 mmol)

of triphenylamine and 80 mL of DMF, under argon atmo-

sphere. The resulting mixture was stirred at room tempera-

ture for 12 h, and then poured into water and extracted

several times with methylene chloride. The organic layer

was concentrated by solvent evaporation and light-brown oil

was formed. The obtained crude product was consisting of a

mixture of mono- and dibromo-triphenylamine. bis(4-bro-

mophenyl)phenylamine (1) was purified by column chro-

matography using silica gel and a gradient of solvent

mixture, methylene chloride/hexane. Finally, the desired

compund was obtained as slight brown viscous oil.1H-NMR (400 MHz, ppm, d CDCl3): 7.36–7.30 (d, 4H,

J ¼ 8.8 Hz), 7.28–7.23 (d, 2H, J ¼ 8.0 Hz), 7.08–7.00 (m,

3H, J ¼ 8.8 Hz), 6.94–6.91 (d, 4H, J ¼ 8.8 Hz).

Synthesis of 4-formyl-N,N’-bis(4-bromophenyl)aniline (2). An

aliquot of 20.16 mL of phosphorus oxychloride

(21.8 mmol) was added dropwise to 16.83 mL of DMF,

under argon at 0 �C, and the reaction mixture was stirred

for 1 h. After removing the ice bath, the reaction mixture

was stirred for another 30 min, until it reached at the room

temperature. 8.7 g of bis(4-bromophenyl)phenylamine

(1) (21.8 mmol) was added, and the resulting mixture was

stirred at 95 �C. After 32 h, the mixture was cooled at room

temperature, and then poured into ice-water (200 mL), and

Vacareanu and Grigoras 113



neutralized at pH ¼ 7 using NaHCO3 powdery. The

precipitate was filtered off and washed several times with

water and dried. A mixture of desired compound and starting

material was formed. 4-Formyl-N,N’-bis(4-bromophenyl)a-

niline (2) was purified by column chromatography using

silica gel and a mixture of methylene chloride and hexane

(3 : 1) as eluent. After removing the solvent, the product

(2) was obtained as yellow powder with 61% yield (5.76 g).1H-NMR (400 MHz, ppm, d CDCl3): 9.83 (s, 1H), 7.72–

7.00 (d, 2H, J ¼ 8.4 Hz), 7.45–7.43 (d, 4H, J ¼ 8.4 Hz),

7.027–7.00 (t, 6H, J ¼ 8.4 Hz).

Synthesis of 4-hydroxymethyl-N,N’-bis(4-bromophenyl)aniline(3). To a solution containing of 1.5 g (3.5 mmol) of 4-for-

myl-N,N’-bis (4-bromophenyl) aniline (2) and a mixture of

benzene and ethanol (20 mL, 1 : 1 / v: v), 0.132 g

(3.5 mmol) of NaBH4 was added in small portions, under

argon atmosphere. The resulting mixture was stirred for

48 h at room temperature, and then was poured into

150 mL of water, and extracted with methylene chloride.

The crude product was obtained as light-green oil and the

purification process was performed using an chromato-

graphic column filled with silica gel, and a mixture of

methylene chloride and hexane (4 : 1/ v:v). 4-hydroxy-

methyl-N,N’-bis(4-bromophenyl)aniline (3) was obtained

as pure light-green powder, in 93.9% (1.42 g) yield.1H-NMR (400 MHz, ppm, d CDCl3): 7.36–7.32 (d, 4H),

7.28–7.25 (d, 2H), 7.06–7.03 (d, 2H), 6.94–6.90 (d, 4H),

4.65 (s, 2H).

General procedure for Suzuki polycondensationSynthesis of poly (9,9-dioctylfluorene-co-4,4’-triphenylamine)

(P15). In a Schlenk tube, equipped with a condenser and a

magnetic stirrer, 0.793 g (1.97 mmol) of (1), 0.942 g

(1.97 mmol) of (5), and 8 mL of degassed THF, were added

under argon atmosphere. Then, 1 ml of 2 mol L�1 K2CO3

aqueous solution (degassed by bubbling argon) and the cat-

alyst Pd (PPh3)4 (0.009 g), were added to the reaction mix-

ture. The mixture was stirred for 48 h, at the refluxing

temperature of the solvent. At the end of the

polycondensation the copolymer was precipitated in

methanol, washed many times with 1 N HCl (aqueous solu-

tion) and dried. P15 was obtained as a green powder. The

copolymer was fractioned by precipitation in methanol

from chloroform solution and two fractions were separated

with corresponding yields: 81% (1.216 g) of chloroform-

soluble fraction and 14% (0.220 g) of chloroform-

insoluble fraction. The other three copolymers were

synthesized using similar procedure as for P15 synthesis.

Results and discussion

In order to obtain new linear conjugated polymers based on

triphenylamine units, as a first step the synthesis of two bis-

bromine derivatives of triphenylamine was performed. The

synthesis of triphenylamine monomers is presented in

Scheme 1. These triphenylamine derivatives were further

used in one-pot Suzuki cross-coupling reactions.

Analyzing the structures of bis(4-bromophenyl)pheny-

lamine (1) and 4-formyl-N,N’-bis(4-bromophenyl)aniline

(2), it can be clearly seen that the bromination and formyla-

tion reactions of triphenylamine led to the para-position

substituted products. Electrophilic substitution at the

ortho-position is sterically inhibited.22 Using the proper

ratio between triphenylamine and NBS, a mixture of mono-

and di-bromo triphenylamine was always present in the

final reaction product, and the single pure components can

be obtained by flash chromatography using silicagel as sta-

tionary phase and a mixture of hexane and methylene chlor-

ide as eluent. The reduction of aldehyde moieties was

performed using NaBH4, and the 4-hydroxymethyl-N,N’-

bis(4-bromophenyl)aniline (3) was obtained as pure light-

green powder (Scheme 1).

The Pd-catalyzed cross-coupling via Suzuki reactions of

monomers pairs (1) or (3) and (4) or (5), was carried out

according to the procedure reported in the literature.23,24

These reactions are depicted in Scheme 2.

The palladium-catalyzed cross-coupling via Suzuki

reaction of bis(4-bromophenyl)phenylamine (1) or

Scheme 1. Synthesis of triphenylamine monomers.

114 High Performance Polymers 23(2)



4-hydroxymethyl-N,N’-bis(4-bromophenyl)aniline (3) with

thiophene- and fluorene-based bis-boronic acids (4, 5) was

carried out in a two-phase system of THF and 2 mol L�1

aqueous potassium carbonate at reflux temperature using

a feed ratio of 1 : 1 between the components.

The copolymers were partially soluble in chloroform;

however, the exception was made by the P14, for which

an insoluble fraction as majority part was obtained.

In case of copolymers P14 and P34, the increased solubi-

lity is due to two long alkyl substituent attached to each

fluorene units, comparative with P15 and P35. The pres-

ence of the hydroxymethyl substituent attached to the

para position of the triphenylamine units can induces a

further increase in the solubility of the copolymers P34

and P35. This can be observed in case of P34, for which

we obtained 21% soluble fraction compared to 6% for

P14. The results of the polycondensations are given in

Table 1.

The number-average molecular weights (Mn) of these

copolymers were determined according to polystyrene

standards by GPC technique, and the values are in the

range of 11704–1587, with corresponding polydispersity

degree of 1.088–4.290. The low values of the Mn and PDI

can be due to a degradation process that occurs during the

polycondensation reactions. Thus, the bromine atoms

are removed from the triphenylamine compounds, process

which is responsible for the stopping of the coupling reac-

tion. This is the reason why it was obtained a mixture of oli-

gomers with high polydispersity degree and low molecular

weights.

The chemical structures of the triphenylamine-based

copolymers containing fluorene or thiophene were

determined by 1H-NMR (Figures 1 and 2) and FT-IR

(Figure 3). 1H-NMR spectra of soluble fractions (Figures

1 and 2) confirm the copolymer structures and display sig-

nals that are assigned to aliphatic protons from alkyl groups

Table 1. The experimental data of the copolymer synthesis.

Monomers (mol L�1)

Copolymer

Yield (%)

Mn/Mw (PDI) Observations(1) (3) (4) (5) a) b)

0.275 0.275 P14 6 43 117040/123130 (1.052) the reaction mixture become viscous;0.058 0.058 P34 21 76 – the polymer precipitated from the solution;

0.246 0.246 P15 81 14 22560/ 53610 (2.37) after 24 h, the reaction mixture became more viscous;0.058 0.058 P35 61 30 15870/ 68100 (4.29) the reaction mixture become viscous

CHCl3 soluble fraction.CHCl3 insoluble fraction.

Scheme 2. Synthesis of triphenylamine copolymers by Pd catalyzed cross-coupling reaction.




substituent (copolymer P15 and P35) and aromatic protons

from triphenylamine and fluorene/thiophene rings. The aro-

matic protons of triphenylamine, fluorene or thiophene

appear in 7.80–7.00 ppm region. The dioctyl groups

attached to the fluorene units appear in the aliphatic range,

2.05–0.5 ppm.

Figure 3 shows FTIR spectra of fluorene and thiophene-

based linear copolymers containing absorption peaks

assigned to triphenylamine and thiophene or fluorene units,

but their relative intensities can differ due to their different

molecular weights

Thiophene-containing copolymers show absorption

bands at 800 cm�1 (aromatic C�H bending), 1485 and

1579–1583 cm�1 (aromatic ring C¼C vibration). The

absorption bands which are located at 814 cm�1 (aromatic

C–H bending), 1464 and 1509–1600 cm�1 (aromatic ring

C¼C vibration) 1894 cm�1 (overtone band, disubstituted

benzene ring), 2852 and 2924 cm�1 (assigned with alkyl

stretching due to incorporation of 9,9-dioctylfluorenyl

group) and 3030 cm�1 (aromatic C–H stretching), are

observed in IR spectra of both fluorene copolymers. The

bands positioned at 1262–1263 cm�1 and 1314–1320 cm�1

are assigned to the stretching vibration of tertiary amine

from triphenylamine ring and are present in all copolymer

IR spectra, while the absorption bands situated at 521 cm�1,

can be assigned to C–Br vibration.

The copolymers having hydroxymethyl groups, attached

to the triphenylamine units, can be viewed as functional

polymers that can be quantitatively transformed into new

polymers. Therefore, their physical properties can be

adjusted by chemical transformation of the reactive group.

To prove this assumption, a new copolymer was obtained

by coupling the hydroxymethyl group of triphenylamine

of the P35 copolymer, with phenyl isocyanate, in dry

CHCl3. The hydroxyl functional group reacts with phenyl

isocyanate to form a urethane linkage (Scheme 3).

The reaction took place quantitatively and the chemical

structure of the modified copolymer was evidenced by FT-

IR and UV-Vis spectroscopy. Comparing the IR spectra of the

copolymer P35 and the postfunctionalized copolymer P35a,

significant changes are noticeable in the 1736 cm�1 (–N–

C¼O) and 1680 cm�1 (C¼O) region. FT-IR analysis showed

that urethane linkage was formed by the reactions between –

NCO groups of isocyanate and –OH groups (Figure 4).

It is known that the polymers structure dictates its elec-

tronic properties. It is reported in the literature, that the poly

(9,9-dioctylfluorene) displays, in THF solution, an absorp-

tion band with the maximum located at about 392 nm.25

Figure 1. 1H-NMR spectra of poly (thiophene-co-4,4’-triphenylamine) (P14).




The synthesized triphenylamine-fluorene copolymers,

exhibit two distinct absorption bands in the wavelength

region of 293–380 nm. The UV-Vis and photoluminescence

(PL) spectra were recorded in CH2Cl2 solutions,

and the absorption and emission data are summarized

in Table 2.

Figure 2. 1H-NMR spectra of poly (9,9-dioctylfluorene–co-N-(4-hydroxymethylphenyl) diphenylamine (P35).

Figure 3. IR-spectra of P14, P34 and P15 copolymers.




From the UV-Vis data of the copolymers, can be

observed that the maximum of the absorption bands are

blue-shifted with 12 nm (for P15) and 16 nm (for P35), rela-

tive to that of polyfluorene, and this is the effect caused by

the incorporation of the triphenylamine units in the

polyfluorene backbone.23 The decrease in the p–p* transi-

tion energy is attributed to an increase in the effective con-

jugation length (increased delocalization).26 These results

Scheme 3. Post-reaction of hydroxymethyl substituents with phenyl isocyanate.

Figure 4. IR spectra of P35 and P35a copolymers.




indicate that triphenylamine is significantly conjugated

with two fluorene rings in the polymer chain.

The UV-Vis spectra of the fluorene-triphenylamine

copolymers in methylene chloride solution exhibit by two

absorption bands, but only one is well defined and this is

located at 380 nm (P15) and 376 nm (P35), being responsi-

ble for p–p* transition in the triphenylamine group. The red

shift of the absorption maximum from 380 nm (P15) and

376 nm (P35) to 407 nm (P14) and 388 nm (P34) can be due

to an extension of the conjugation length of triphenylamine-

thiophene backbone. Thus, despite of its planar conformation,

fluorene group doesn’t lead to an increase of the planarity

of the polymer backbone and this may be due to the steric

interactions in the polymer backbone.

In case of triphenylamine-fluorene copolymers (P15

and P35) the smaller Stokes shift (51 and 65 nm) between

the absorption and emission maxima, compared to

triphenylamine-thiophene copolymers indicates a smaller

structural differences between the ground and excited

states.27

The electronic properties of copolymer P15 are affected

by the presence of the new structure Ph-NH-CO-O-

attached at the para position of triphenylamine units, and

this can be observed in the UV-Vis spectra. The maximum

absorption band is blue-shifted at 370 nm from 376 nm, as

it is for P35. This blue-shift phenomenon can be explained

by the appearance of the sterical hindrances between the

long alkyl groups attached to the fluorene units and the

structure Ph–NH–CO–O–attached to triphenylamine units.

Thus, by modifying the functional group of a polymer with

proper molecule (chemical or even biological one) can be

generated new polymers with new properties.

Electrochemical characterization

The anodic oxidation of triphenylamine derivatives (either,

as small molecules or incorporated into a macromolecule

backbone) had been extensively studied, starting from

1966.28–34 In general, the redox processes of triphenyla-

mine derivatives can be subscribed in a multi-step process

category, involving successive electrochemical–chemical–

electrochemical (ECE) reactions. Triphenylamine deriva-

tives undergo reversible one-electron oxidation process and

form radical-cations, which are not stable and undergo

chemical reactions involving reactions with parent

molecule or dimerization reactions; the chemical reactions

follow up to produce tetraphenylbenzidine by tail-to-tail

coupling with the loss of two protons per dimer.

Using cyclic voltammetry (CV) technique, we investi-

gated the electrochemical behavior of three copolymer

films (P14, P15 and P35) deposited on a glassy carbon

electrode (GCE). Making the correlation between redox

process and the energy levels, it was possible to determine

the band gap energy of the copolymers. The CVs were

recorded in Bu4NBF4-acetonitrile solutions (Figure 5).

The deposition of copolymers on GCE surface was done

layer by layer. Both TPA-fluorene and thiophene copoly-

mers are redox-active compounds exhibiting a quasi-

reversible redox process. The electro-oxidation process of

P14 and P15 starts at 0.839 V, and for P35 the oxidation

process starts at 0.812 V versus Ag/AgCl.

The copolymer P14 exhibits, in anodic range, three

broad oxidation peaks at 1.290, 1.940 and 2.343 V and

one reduction peak at 0.719 V (versus Ag/AgCl). P35

is more easily oxidable compound exhibiting three oxida-

tion peaks at 0.998, 1.452 and 1.805 V versus Ag/AgCl.

In the ase of P15 copolymer, the CVs exhibit reversible

curves which are characterized by two oxidation peaks

located at 1.086 and 1.582 V and two reduction peaks

located at 1.399 and 0.894 V. The anodic peaks which

appeared at 1.290 and 1.086 V are assigned to the oxida-

tion of triphenylamine units, whereas the peaks at 1.940

and 1.582 V are assigned to oxidation of thiophene and

fluorene units of copolymers.

Thus, the first oxidation potential is in the following

order: P14 (1.290 V) > P15 (1.086 V) > P35 (0.998 V).

The possible anodic oxidation pathway of the conju-

gated copolymer based on triphenylamine and fluorene

units was sketched postulated as in Scheme 4.

The introduction of the hydroxymethyl group, not only

prevented the coupling reaction by blocking the para posi-

tions of the triphenylamine moieties, but also lowered the

oxidation potentials of the electroactive conjugated copoly-

mer P35 (0.812 V), as compared with the corresponding

copolymer P15 (0.839 V).

In order to detect the color changes of the copolymer

films during the oxidation processes, an ITO-coated glass

was used as working electrode. For this, a thin layer of the

copolymer P15 was deposited on ITO, from toluene solu-

tion by spin coating technique at room temperature, in

nitrogen atmosphere. The values of the rotational speed and

time of turn were set on 1500 turns per minute by 40 s. In

toluene solution, P15 emits strong violet light under UV

excitation.

The repetitive CVs of the P15 film cast on ITO

were recorded by sweeping the electrode potential in the

range from 0.0 to 1.5 V, in Bu4NBF4-acetonitril solution

and are represented in Figure 6.

In the first scan on the anodic range of potentials, the

copolymer P15 gives only two oxidation peaks, which are

Table 2. The UV-Vis and PL characteristics of arylaminecopolymers.

CopolymerAbsorption l

max (nm)Emission l

max (nm)Eg

(eV)Stokes shift

(nm)

P14 312; 407 454 3.14 124P15 293; 380 431 2.90 51P34 312; 388 437 3.32 125P35 295; 376 440 2.92 65




slightly shifted to less positive values, and one reduction

peak at 0.954 V (versus Ag/AgCl). After the fourth cycle

it could be observed that the current peak intensity was

diminished, and only one redox couple appears. However,

the redox switching color of the film is irreversible because

the initial color did not come back to the original (yellow)

after cycling sweep.

In order to investigate the electrochemical behavior

of copolymers in solution, was analyzed only the

triphenylamine-fluorene copolymers (P15 and P35), due

Figure 5. Cyclic voltammograms of P14 (a), P15 (b) and P35 (c) copolymer films cast on glassy carbon electrode ( ¼ 3 mm), inacetonitrile solution containing 0.1 mol L�1 Bu4NBF4, as support electrolyte; scan rate ¼ 50 mV s�1 versus Ag/AgCl.

Scheme 4. Possible mechanism for electrochemical oxidative reaction of triphenylamine-fluorene copolymers (P15).




to their good solubility in organic solvents. The typical CVs

of the triphenylamine-fluorene copolymers are shown in

Figure 7. Both triphenylamine-fluorene copolymers are

electrochemical active exhibiting reversible oxidation-

reduction processes with corresponding half-wave peaks

located at 0.978 V for P15 and 0.976 V for P35.

P35 exhibits one anodic peak located at 1.077 V and the

corresponding catodic peak at 0.875 V and these peaks are

related to the oxidation and reduction processes of triphe-

nylamine units. Having the hydroxymethyl substituent, tri-

phenylamine units follow one-electron reversible process

and forms an cation-radical which is stable and thus, the

further chemical reaction do not take place.

By increasing the number of the scans, the current peak

intensities decreased. The oxidation-reduction processes

are reversible, but we did not observe any product depos-

ited on the glassy carbon electrode, even at repetitive scan-

ning up to the twentieth scan. The absence of

electrodeposited product on the GCE can be due to the two

long octyl side chain substituents attached to the fluorene

units of the copolymer backbone, which enhances the solu-

bility of the resulting product.

Cycling the electrode potential between 0.0 and 1.5 V, a

red product appeared at the electrode–solution interface,

and when the potential reached the 1.5 V value, the result-

ing product became dark red. Further, this dark-red product

dissipated into the methylene chloride solution. The P35

copolymer underwent an identical electrochemical beha-

vior. All the electrochemical data of copolymers cast on

two types of electrodes (GCE and ITO electrode) are sum-

marized in Tables 3 and 4. All the potential values are

recorded versus Ag/AgCl.

According to the correlations between HOMO and

LUMO energy levels and redox potentials, which can be

done, the value of the energy levels can be calculated from

CVs using the onset values of the oxidation and reduction

peaks. The onset values were estimated from the intersec-

tion of the two tangents drawn at the rising oxidation (or

reduction) current and the background current in the CVs.

According to Li et al.,35 the E½ of the redox couple Fc/Fcþ,

measured in Bu4NBF4 methylene chloride solution is equal

to 0.425 V versus Ag/AgCl and the energy levels of

oligomers (in electron volts, eV) can be obtained by

adding 4.37 V to the values of the redox potential. Thus,

EHOMO ¼ �e (Eoxonset þ 4.37) and ELUMO ¼ �e (Ered

onset

þ 4.37) and the value of gap energy, Eg¼EHOMO� ELUMO.

The energy levels values and the energy gap characteristics

of linear copolymers having triphenylamine and thiophene

or fluorene units in the main chain are summarized in

the Table 4.

The HOMO and LUMO energy levels of the materials

are very crucial parameters for electronic devices config-

uration. The ionization potential of the copolymers was

determinate in films coated on electrode or in solution, and

gave estimated high molecular orbital (HOMO) levels with

values in the range of 5.184–5.209 eV (Figure 8). These

Figure 7. Cyclic voltammograms of P15 (dotted line) and P35 (fullline), 2 � 10�3 mol L�1 in CH2Cl2 solution, and 2 � 10�1 mol L�1

Bu4NBF4, as support electrolyte; scan rate ¼ 50 mV s�1.

Figure 8. The distribution of the HOMO and LUMO energylevels for triphenylamine copolymers.

Figure 6. Repetitive cyclic voltammograms (4th cycles) in aceto-nitril solution (containing Bu4NBF4, as support electrolyte) of theP15 thin layer spin-coated on ITO glass; scan rate ¼ 50 mV s�1.




values are comparable to those reported in the literature for

fluorene copolymers.36,37 The ionization potential is

smaller than in the polyfluorene (5.8 eV).

The electron affinities of the copolymers were mea-

sured in film, and gave estimated lowest molecular orbital

(LUMO) levels with values in the range of 3.556–

2.645 eV. The LUMO level values are smaller compared

to LUMO level of the polyfluorene, for which the

single-particle HOMO–LUMO energy gap is known to

be 3.6 eV, from X-ray photoelectron-spectroscopy and

ultraviolet photoelectron-spectroscopy on poly (9, 9-

dioctylfluorene).38

From the distribution of the HOMO and LUMO energy

levels (Figure 8) it can be observed that the incorporation

of fluorene units instead of tiophene, the Eg value is

increased from 1.94 to 2.564 eV. Furthermore, it can be

observed that the effect of –CH2–OH group attached to the

para position of triphenylamine units decreased the Eg

values at 1.628 eV.

The large energy difference in P15 arises from the

strong steric repulsion between hydrogen atoms in the fully

planarized structure of the triphenylamine unit. It is well

known that, when two rings of the triphenylamine are fixed

to be coplanar, the third ring is rotated with 90� out of the

plane defined by the other two rings.39 The copolymer P15

adopts a kinked structure, due to the presence of tripheny-

lamine unit, which has a propeller-like shape in the bulk

(Figure 9).

Conclusions

The synthesis and chemical and electrochemical character-

ization of two series of triphenylamine copolymers, with

linear structure, and fluorene or thiophene units is

Figure 9. The optimized geometry considered for the triphenylamine-dioctylfluorene dimer (P15).

Table 4. The HOMO and LUMO energy levels and the energy gap characteristics of linear copolymers.

Copolymer

GCE electrode CH2Cl2 solution ITO glass electrode Eg (e V)

EHOMO

(e V)ELUMO

(e V)EHOMO

(e V)ELUMO

(e V)EHOMO

(e V)ELUMO

(e V)GCE

electrodeCH2Cl2solution

ITO glasselectrode

P14 �5.209 �3.265 – – – – 1.94 – –P15 �5.209 �2.645 �5.189 �3.247 �5.141 �3.229 2.564 1.94 1.91P35 �5.182 – �5.184 �3.556 – – 1.628 –

–, indeterminate.

Table 3. Electrochemical characteristics of copolymers, employing two type of electrodes: glassy carbon electrode (GCE) and ITOelectrode.

Copolymer

GCEelectrode CH2Cl2solution ITO glasselectrode

E oxonset (V) E red

onset (V) E oxonset (V) E red

onset (V) E oxonset (V) E red

onset (V)

P14 0.839 1.105 – – – –P15 0.839 1.725 0.891 1.123 0.771 1.141P35 0.812 – 0.814 1.093 – –

–, indeterminate.




presented. These copolymers were obtained by Pd(0) cata-

lyzed cross-coupling reaction as fully or partially soluble

materials, in chlorinated and aprotic solvents.

The presence of the hydroxymethyl group allows mod-

ifying the copolymer structure and properties by reacting

with different chemical or biological compounds. Thus, the

physicochemical properties of the functional copolymers

could be tuned on a wide range, enabling applications in

electronic or biomedical fields.

Using cyclic voltametry (CV) technique, we investigate

the electrochemical behavior of the three copolymer films,

deposited on glassy carbon electrode (GCE) and ITO, and

from the correlation between redox process and the energy

levels it was possible to determine the band gap energy of

the copolymers. The electrochemical studies show that

these copolymers have good redox activities, the oxidation

(doping) process being accompanied by changes in the

color of the films. The yellow color turns to red. However,

the redox switching color of the film is irreversible because

the initial color did not come back to the original (yellow)

after cycling sweep.

Acknowledgments

One of the authors (L.V.) acknowledges the financial support of

European Social Fund – Cristofor I. Simionescu’’ Postdoctoral

Fellowship Programme (ID POSDRU/89/1.5/S/55216), Sectoral

Operational Programme Human Resources Development 2007 –

2013.

References

1. Brutting W Berleb S and Muckl AG. Device physics of

organic light emitting diodes based on molecular materials.

Org Electron 2001; 2: 1–36.

2. Forrest SR. The road to high efficiency organic light emitting

devices. Org Electron 2003; 4: 45–48.

3. Dimitrakopoulos CD, Purushothaman S, Kymissis J,

Callegari A and Shaw JM. Low -voltage organic transistors

on plastic comprising high-dielectric constant gate insulators.

Science 1999; 283: 822–824.

4. Peumans P, Uchida S and Forrest SR. Efficient bulk hetero-

junction photovoltaic cells using small-molecular-weight

organic thin films. Nature 2003; 425: 158–162.

5. Gratzel M. Mesoscopic solar cells for electricity and hydro-

gen production from sunlight. Chem Lett 2005; 34: 8–13.

6. Bendrea AD, Vacareanu L and Grigoras M. Synthesis,

characterization and (electro)chemical polymerization of

triphenylamine-end-functionalized poly(e-caprolactone).

Polym Int 2010; 59: 624–629.

7. Thelakkat M. Star-shaped, dendrimeric and polymeric triary-

lamines as photoconductors and hole transport materials.

Macromol Mater Eng 2002; 287: 442–461.

8. Lee CC, Yeh KM and Chen Y. New host homopolymers con-

taining pendant triphenylamine derivatives: synthesis, opti-

cal, electrochemical properties and its blend with Ir(ppy)(3)

for green phosphorescent organic light-emitting devices.

J Polym Sci Par. A: Polym Chem 2008; 46: 7960–7971.

9. Vellis PD, Mikroyannidis JA, Cho MJ and Choi DH. Carba-

zolevinylene-based polymers and model compounds with

oxadiazole and triphenylamine segments: Synthesis, photo-

physics and electroluminescene. J Polym Sci Part A: Polym

Chem 2008; 46: 5592–5603.

10. Shirota YJ. Photo- and electroactive amorphous molecular

materials - molecular design, syntheses, reactions, properties

and applications. Mater Chem 2005; 15: 75–93.

11. (a) Liou GS, Lin HY, Hsieh YL and Yang YL. Synthesis

and characterization of Wholly Aromatic Poly(azo-

methine)s Containing Donor-Acceptor Triphenylamine

Moieties. J Polym Sci Part A: Polym Chem 2007; 45:

4921–4932.

12. Liou GS, Yang YL, Chen WC and Su YO. 4-methoxy-

substituted poly(triphenylamine): a p-type polymer with

highly photoluminescent and reversible oxidative electro-

chromic characteristics. J Polym Sci Part A: Polym Chem

2007; 45: 3292–3302.

13. Pan JH, Chiu HL, Chen L and Wang BC. Theoretical inves-

tigations of triphenylamine derivatives as hole transporting

materials in OLEDs: correlation of Hammett parameter of the

substituent to ionization potential and reorganization energy

level. Comput Mater Sci 2006; 38: 105–112.

14. Beaupre S, Dumas J and Leclerc M. Toward the development of

new textile/plastic electrochromic cells using triphenylamine-

based copolymers. Chem Mater 2006; 18: 4011–4018.

15. Choi K, Yoo SJ, Sung YE and Zentel R. High contrast ratio

and rapid switching organic polymeric electrochromic thin

films based on triarylamine derivatives from layer-by-layer

assembly. Chem Mater 2006; 18: 5823–5825.

16. Otero L, Sereno L, Fungo F, Liao YL, Lin CY and Wong KT.

Synthesis and properties of a novel electrochromic polymer

obtained from the electropolymerization of a 9,9’-

spirobifluorene-bridged donor�acceptor (D�A) bichromo-

phore system. Chem Mater 2006; 18: 3495–3502.

17. Natera J, Otero L, Sereno L, Fungo F, Wang NS, Tsai YM, Hwu

TY and Wong KT. A novel electrochromic polymer synthesized

through electropolymerization of a new donor�acceptor

bipolar system. Macromolecules (2007; 40: 4456–4463.

18. Yu ZQ, Tan ST, Zou YP, Fan BH, Yuan ZL and Li YF.

Synthesis, characterization, and optoelectronic properties of

two new polyfluorenes/poly(p-phenylenevinylene)s copoly-

mers. J Appl Polym Sci 2006; 102: 3955–3962.

19. Kim JH, You NH and Lee HS. Electroluminescent copoly-

mers based on dihexylfluorene and 2-f2,6-bis[2-(4-dipheny-

laminophenyl)vinyl]pyran-4-ylidenegmalononitrile units.

J Polym Sci Part A: Polym Chem 2006; 44: 3729–3737.

20. ItoY, Shimada T, Ha J, Vacha M and Sato H. Synthesis and

characterization of a novel electroluminescent polymer based

on a phenoxazine derivative. J Polym Sci Par. A: Polym

Chem 2006; 44: 4338–4345.

21. Vacareanu L and Grigoras M. Electrochemical characteriza-

tion of arylene vinylene oligomers containing triphenylamine




and carbazole units. J Appl Electrochem 2010; DOI: 10.1007/

s10800-010-0173-z.

22. Xue M, Liu Y, Huang D and Gong B. Crystal structure of

p-formylphenyl, di(p-methylphenyl)amine, and p-bromophenyl,

di(o-bromo-p-methylphenyl)amine. J Chem Cryst 2000; 30:

749–753.

23. Grigoras M and Stafie L. Synthesis of hyperbranched conju-

gated copolymers containing triphenylamine and fluorene or

thiophene moieties. High Perform Polym 2009; 21: 304–314.

24. Fang Q and Yamamoto T. New alternative copolymer consti-

tuted of fluorene and triphenylamine units with a tunable

�CHO group in the side chain. quantitative transformation

of the �CHO group to –CH¼CHAr groups and optical and

electrochemical properties of the polymers. Macromolecules

2004; 37: 5894–5899.

25. Li Y, Ding J, Tao Y, Lu J and D’Iorio M. Synthesis and prop-

erties of random and alternating fluorene/carbazole copoly-

mers for use in blue light-emitting devices. Chem Mater

2004; 16: 2165–2173.

26. Charas A, Morgado J, Martinho.MG, Alc’cer L, Lim SF and

Friend RH. Synthesis and luminescence properties of three

novel polyfluorene copolymers. Polymer 2003; 44: 1843–1850.

27. Iraqi A, Simmance TG, Yi HN, Stevenson M and Lidzey DG.

Preparation and properties of 4-dialkylamino-phenyl N-func-

tionalized 2,7-linked carbazole polymers. Chem Mater 2006;

18: 5789–5797.

28. Marcoux LS, Adams RN and Feldbeng SW. Dimerization of tri-

phenylamine cation radicals. evaluation of kinetics using the

rotating disk electrode. J Phys Chem 1969; 73: 2611–2623.

29. Nelson RF and Feldbeng SW. Chronoamperometric determi-

nation of the rate of dimerization of some substituted triphe-

nylamine cation radicals. J Phys Chem 1969; 73: 2623–2626.

30. Seo ET, Nelson RF, Fritsch JM, Marcoux LS, Leedy DW and

Adams RN. Anodic oxidation pathways of aromatic amines.

electrochemical and electron paramagnetic resonance studies.

J Am Chem Soc 1966; 88: 3498–3503.

31. Creason SC, Wheeler J and Nelson RF. Electrochemical and

spectroscopic studies of cation radicals i. coupling rates of 4-

substituted triphenylaminium ion. J Org Chem 1972: 37:

4440–4446.

32. Chiu KY, Su TX, Li JH, Lin TH, Liou GS and Cheng SH.

Novel trends of electrochemical oxidation of amino-

substituted triphenylamine derivatives. J Electroanal Chem

2005; 575: 95–101.

33. Faber R, Mielke GF, Rapta P, Stasko A and Nuyken O. Ano-

dic oxidation of novel hole-transporting materials derived

from tetraarylbenzidines. electrochemical and spectroscopic

characterization. Coll Czech Chem Commun 2000; 65:

1403–1418.

34. Yamamoto K, Higuchi M, Uchida K and Kojima Y. Excellent

redox properties of poly(thienylphenylamine)s. Macromol

Rapid Commun 2001; 22: 266–270.

35. Li Y, Cao Y, Gao J, Wang D, Yu G and Heeger A.

Electrochemical properties of luminescent polymers and

polymer light-emitting electrochemical cells. Synth Met

1999; 99: 243–248.

36. Grizzi I, Foden C, Goddard S and Towns C. Electrochemical

characterization of blue-emitting polyfluorene LEP. Mater

Res Soc Symp Proc 2003; 771: L1.7.1.

37. Kanicki J, Lee SJ, Hong Y and Su CC. Optoelectronic prop-

erties of poly(florene) co-polymer light-emitting devices on a

plastic substrate. J SID 2005; 13: 993–1002.

38. Salanek WR, Seki K, Kahn A and Pireause JJ. Conjugated

Polymers and Molecular Interface Science and Technol-

ogy For Photonic and Optoelectronic Application. Chap-

ter 12: Metal-Polyfluorene Interface and Surface. pp:

401–442.

39. Sancho-Garcıa JC, Foden CL, Grizzi I, et al. Joint theore-

tical and experimental characterization of the structural

and electronic properties of poly(dioctylfluorene-alt-N-

butylphenyl diphenylamine). J Phys Chem B 2004; 108:

5594–5599.




synthesis and electrochemical characterization of new linear conjugated arylamine copolymers

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