Sm

EFa

b

a

ARRA

KEOTPE

1

r[tsidnOpt4Eha

((f(

0h

Synthetic Metals 171 (2013) 23– 31

Contents lists available at SciVerse ScienceDirect

Synthetic Metals

journa l h o me page: www.elsev ier .com/ locate /synmet

ynthesis and characterization of �-substituted 3,4-ethylenedioxy terthiopheneonomers for conducting polymer applications

lodie Pardieua, Ali Saada, Laurence Dallerya, Francis Garniera, Christophe Vedrinea,anny Hauquiera,∗, Peter Dalkob, Christine Pernellea

MAQIM/SITI, Conservatoire National des Arts et Métiers, 292 rue Saint Martin, 75141 Paris Cedex 03, FranceLaboratoire de Chimie et Biochimie Pharmacologiques et toxicologiques-CNRS UMR 8601, Université Paris Descartes, 45, rue des Saints Pères, 75006 Paris, France

r t i c l e i n f o

rticle history:eceived 14 January 2013eceived in revised form 28 February 2013ccepted 12 March 2013

eywords:thylenedioxythiophenerganic synthesis

a b s t r a c t

2,5-Bis[(3,4-ethylenedioxy)thien-2-yl] thiophene monomers bearing 3′-hydroxymethylene (2), 3′-hydroxyethylene (3), 3′-acetoxyethyl (4) and 3′-acetoxyl (5) functions were prepared in a shortbidirectional synthesis using Kumada–Corriu or Stille chemistry. The newly prepared monomers arestable enough in organic solvents at RT in the absence of acids and can be safely stored withoutexclusion of oxygen, at 4 ◦C in the dark. The chloroform solutions of 2–5 exhibit a blue-shifted �max

in the range of 345–352 nm, when compared to 2,5-bis[(3,4-ethylenedioxy)thien-2-yl] thiophene (1).Smaller shifts were observed in the fluorescence spectra where all �max values were centered in the

erthienylhotoluminescent characteristicslectrochemistry

range of 438–440 nm, similar to the parent unsubstituted 1 (439 nm) with fluorescence quantum yields�f = 2.2–2.8%, that was roughly half of the monomer 1 reference value (4.8%). Cyclic voltammetry wasemployed to investigate the electrochemical behaviour of the monomers. Polymer films deposited byrepeated potential cycling show oxidation potentials ranging from 320 to 540 mV. AFM analysis showsthat these polymers display large active surface because of their roughness. All the polymers exhibit a

m wi

broad absorption spectru

. Introduction

Conjugated polymers (CPs) found broad applications in mate-ial sciences as organic-based conductors and optical devices1]. They are used in thin film transistor fabrication [2] pho-ovoltaics [3], electrochromic [4], light emitting diodes [5] andensors [6]. A particularly attractive field of sensor developments related to human health, such as the diagnosis of infectiousiseases, genetic mutations, drug design, forensics and food tech-ology, due to their simplicity and relatively high sensitivity [7].ne of the most promising groups of conducting polymers isoly(3,4-ethylenedioxythiophene) (PEDOT) and its derivatives, dueo their attractive properties: high stability, high conductivity (up to00–600 S/cm) high transparency and solubility in aqueous media.

DOT analogs such as sulfur [8] and selenophene derivatives [9]ave been used as building block in several polymers showing valu-ble electrochromic [10–12] and low-band-gap electro-conducting

∗ Corresponding author. Tel.: +33 1 40 27 28 44; fax: +33 1 40 27 28 44.E-mail addresses: elodie.pardieu@gmail.com (E. Pardieu), saad82.ali@gmail.com

A. Saad), laurence.dallery@cnam.fr (L. Dallery), francis.garnier@cnam.frF. Garnier), christophe.vedrine@cnam.fr (C. Vedrine), fanny.hauquier@cnam.fr,annyhauquier@hotmail.com (F. Hauquier), peter.dalko@parisdescartes.frP. Dalko), christine.pernelle@cnam.fr (C. Pernelle).

379-6779/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.synthmet.2013.03.012

th a �max ranging from 464 nm to 533 nm.© 2013 Elsevier B.V. All rights reserved.

properties [13]. Conducting polymer films were prepared bythin-film electropolymerization, or, by anodic oxidation of mixedsolutions of (poly)EDOT and carbazole [14], vinyl, phenyl, biphenyl,furan, thiophene [11], bipyridine [15], 4-dicyanomethylene-4H-cyclopenta[2,1-b,3,4-b′]dithiophene [15], cyanovinylene [13],thieno[3,4-b]pyrazine [16]. Co-polymerization with thiophenesbearing functionalities at C(3) and/or C(4) positions allowed aflexible tailoring of application oriented properties and tuning sol-ubility, conductivity, electroactivity and electrochromic properties[12,17]. The covalent modification of the C(3) and/or C(4) positionsresulted not only in the appearance of desired chemical function-alities but they also influenced considerably the physico-chemicalproperties of the whole material by upsetting the conformation ofthe oligothiophene units. The modulation of the planarity of theconjugated system resulted in the modified conducting and pho-toelectronic properties that were also exploited in a number ofapplications.

As the chemical structure of these materials is highly depend-ent on the polymerization conditions, more regular polymers canbe obtained by using monomers where the components are alreadylinked constitutively together [18]. Moreover the oxidation poten-

tial of these monomers is also significantly lower, allowing theintroduction of more sensitive functions prior to polymerization.While monomers having functionalized terthiophene units arewell known [19] and the unfunctionalized EDOT-thiophene-EDOT

24 E. Pardieu et al. / Synthetic Metals 171 (2013) 23– 31

SS S

O O O O

SS S

O O O O

SS S

O O O O

SS S

O O O O

HOOH

RO2C

R = Et ; 4

R = H; 5

1 2 3

iophe

(3fiictIpE

u[tpos

t3pei

2

2

2

wAtbwm(taaasya

y

1916

Fig. 1. 3,4-Ethylenedioxythiophene-3′-substituted th

E-T-E) 1 has been already prepared and characterized [11], the′-functionalized E-T-E terthiophenes are less known [20]. The dif-culty in preparing 3′-functionalized E-T-E terthiophenes analogs

s due to the lower reactivity of the sterically crowded couplingenter of both thiophene reagents and to the high sensitivity ofhe prepared monomer toward aerobic oxidation, acids and light.n fact, the synthetic chemistry of 3′-functionalized E-T-E terthio-henes is rather more straightforward than that of 3′-substitutedDOT derivatives [21].

In a previous work, we showed that these polymers can besed as matrix for memory shaped conducting polymer (MSCP)22]. The conjugated MSCP, poly(3,4-ethylenedioxythiophene-co-hiophene-acetic acid), has been electrochemically synthesized byolymerization of co-monomers. Nevertheless, the exact structuresf these copolymers obtained during anodic oxidation of mixedolutions of two different monomers have not been elucidated.

In this frame, we report here the chemical synthesis andhe photo- and electrochemical characterization of four novel′-functionalized E-T-E derivatives 2–5 (Fig. 1) as well as theirolymers, prepared by anodic polymerization on Pt, gold or ITOlectrodes, as future functional monomers in molecularly imprint-ng conducting polymer.

. Experimental

.1. Synthesis

.1.1. Method A: Kumada–Corriu couplingGeneral procedure of the Kumada–Corriu coupling:Under inert atmosphere at −78 ◦C n-butyllithium (3.3 equiv)

as added via syringe to a solution of EDOT 6 (3.0 equiv) in THF.fter stirring the mixture 45 min at this temperature the reac-

ion was allowed to warm to 0 ◦C by replacing the dry ice bathy ice bath, then a solution of MgBr2 in Et2O (1 M, 3.3 equiv)as introduced via syringe. After 45 min TBS protected dibro-othiophene alcohols (8a or 8b, 1.0 equiv) and NiCl2dppe catalyst

1.5 mol%) were added and the reaction was allowed to reach roomemperature resulting a color change from red clear to black-redfter 2 h. The reaction was stirred for 2 days more, then filtered on

cake of silica and the solid washed with a mixture of cyclohex-ne/ethyl acetate (4/1). The residual oil was purified on column ofilica gel by using cyclohexane/ethyl acetate gradient (4/1–2/1) toield the desired terthienyls 9a and 9b respectively (1.50 g (76%)nd 1.46 g (75%).

2,5-Bis(2-(3,4-ethylenedioxy)thienyl)-(thiophen-3-l)methoxy)(t-butyl)dimethylsilane (9a)

1H NMR (250 MHz, CDCl3) � 7.26 (s, 1H), 6.25 (s, 1H), 6.19 (s,

H), 4.73 (s, 2H), 4.30 (m, 2H), 4.26 (m, 2H). 4.21 (m, 4H), 0.91 (s,H), 0.14 (s, 6H). 13C NMR (63 MHz, CDCl3) � 141.8, 141.5, 139.9,37.8, 137.3, 134.0, 126.1, 124.5, 112.5, 110.2, 97.8, 97.1, 65.2, 65.2,4.8, 65.7, 60.6, 26.2, 18.6, −5.0. MS (ESI) m/z 509 [M + H]+.

ne-3,4-ethylenedioxythiophene (E-T-E) monomers.

2,5-Bis(2-(3,4-ethylenedioxy)thienyl)-(thiophen-3-yl)ethoxy)(tert-butyl)dimethylsilane (9b)

1H NMR (250 MHz, CDCl3) � 7.27 (s, 1H), 6.46 (s, 1H), 6.30 (s, 1H),4.63 (s, 2H) 4.31 (m, 4H), 4.12 (M, 4H), 3.73 (t, J = 7 Hz, 2H), 2.88 (t,J = 7.2 Hz, 2H), 0.91 (s, 9H), 0.14 (s, 6H). 13C NMR (63 MHz, CDCl3) �141.8, 141.5, 139.9, 138.2, 137.4, 133.7, 126.7, 125.6, 112.1, 109.9,99.0, 96.7, 66.0, 64.8, 64.5, 65.3, 63.3, 32.9, 26.0 18.3, −5.2. MS (ESI)m/z 523 [M + H]+.

General procedure for the TBS cleveage:To a solution of terthienyl 9a, or, 9b (1.0 equiv) in THF, tetra-

n-butylammonium fluoride (1 M in THF, 5 equiv) was added. Thesolution was stirred overnight at RT then concentrated underreduced pressure. The crude residue was dissolved in ethyl acetate,poured on saturated NaCl, separated, and the aqueous phase waswashed by ethyl acetate. The pooled organic layers were dried overNa2SO4, concentrated and the residual oil was purified on columnof silica gel by using cyclohexane/AcOEt gradient (9/1–2/1) to yieldthe desired terthienyl 2 or 3 respectively as a pale yellow solid (1.2 g(90%) and 0.83 g (75%)).

2,5-Bis(2-(3,4-ethylenedioxy)thienyl)thiophen-3-yl-methanol(2)

1H NMR (250 MHz, CDCl3) � 7.2 (s, 1H), 6.29 (s, 1H), 6.14(s, 1H), 4.55 (s, 2H), 4.20 (M, 2H), 4.13 (m, 2H). 4.11 (m, 4H),2.92 (s, 1H). 13C NMR (63 MHz, CDCl3) � 141.8, 141.5, 139.1,138.0, 137.7, 134.4, 127.6, 124.5, 111.7, 109.5, 99.6, 97.1, 64.9,64.5, 64.3, 63.7, 59.1. MS (ESI) m/z 395 [M + H]+. Element. Anal.:Calc. C(51.76%); H(3.58%); O(20.28%); S(24.38%); Found C(52.64%);H(4.12%); O(20.08%); S(23.07%). Melting point: 51–52 ◦C.

2,5-Bis(2-(3,4-ethylenedioxy)thienyl)thiophen-3-yl-ethanol(3)

1H NMR (250 MHz, CDCl3) � 7.08 (s, 1H), 6.34 (s, 1H), 6.19(s, 1H), 4.28 (m, 4H), 4.24 (m, 4H), 3.83 (t, J = 7 Hz, 2H), 2.92 (t,J = 7.2 Hz, 2H), 1,94 (s, 1H). 13C NMR (63 MHz, CDCl3) � 142.2, 142.0,138.8, 138.1, 137.1, 135.0, 127.6, 126.3, 112.3, 110.1, 99.9, 99.6,65.3, 65.0, 63.2, 33.2. MS (ESI) m/z 409 [M + H]+. Element. Anal.:Calc. C(52.92%); H(3.95%); O(19.58%); S(23.55%); Found C(53.04%);H(4.20%); O(20.86%); S(21.92%). Melting point: 33–34 ◦C.

2.1.2. Method B: the Stille coupling protocolEthyl 2,5-Dibromothiophene-3-acetate (11)A total of 8.15 g (45.7 mmol) of N-Bromosuccinimide was added

to a solution of 3.12 g (18.3 mmol) of ethyl thiophene-3-acetatein 70 mL of THF. The solution was stirred overnight. The solventwas removed under vacuum, and the residue was dissolved inCH2Cl2, washed with water, and dried with MgSO4. The crudeproduct was purified by chromatography on silica gel with cyclo-hexane/ethyl acetate (9/1) as eluent. A total of 5.7 g of ethyl

2,5-Dibromothiophene-3-acetate as obtained (95%).

1H NMR (63 MHz, CDCl3) � 6.9 (s, lH), 4.14 (q, 2H, J = 7.15 Hz),3.52 (s, 2H), 1.24 (t, 3H, J = 7.14 Hz). 13C NMR (63 MHz, CDCl3) �169.4, 134.6, 131.4, 110.9, 110.6, 61.1, 34.9, 14.2.

etic M

(

5(fmCipu

(s2(sCtus

1N16EF1

a

o(TfapMba0

1216EF1

2

sPsws

�

wnma

E. Pardieu et al. / Synth

2,5-Bis-(2-(3,4-ethylenedioxy)thienyl)thiophen-3-yl-acetate4)

To a solution of 3,4-ethylenedioxythiophene (EDOT, 6) (0.8 g,.62 mmol) in dry THF (25 mL) at −78 ◦C, 2.5 M n-buthyllithium2.5 mL, 6.2 mmol, in hexane) was added dropwise. After stirringor 1 h, Bu3SnCl (1.84 mL, 6.8 mmol) was added, and the reaction

ixture was allowed to warm up to RT and stirred for overnight.oncentrated under reduced pressure, the residue was dissolved

n pentane and filtered. The solvent was removed under reducedressure to afford a crude product (10) as a brown oil that wassed without further purification.

Into a solution of dichlorobis(triphenylphosphine)palladium90 mg, 2 mol%) in a dry DMF (8 mL) under inert atmo-phere the prepared crude tributhyl(3,4-ethylenedioxythienyl-)stannane 10 and ethyl 2-(2,5-dibromothiophen-3-yl)acetate 110.92 g, 2.8 mmol) were added. The mixture was heated to 100 ◦C,tirred for 2 days then poured into water (100 mL). Extracted withH2Cl2, dried on Na2SO4, concentrated under reduced pressure andhe crude product was purified by chromatography on silica gelsing cyclohexane/ethyl acetate gradient providing 4 as pale yellowolid (0.65 g, 52%, two steps overall).

1H NMR (250 MHz, CDCl3) � 7.19 (s, 1H), 6.39 (s, 1H), 6.23 (s,H), 4.37–4.10 (m, 10H), 3.72 (s, 2H), 1.27 (t, J = 7.1 Hz, 3H). 13CMR (63 MHz, CDCl3) � 171.02, 141.87, 141.65, 138.36, 137.79,34.26, 131.53, 128.25, 125.52, 111.88, 109.37, 99.57, 97.12, 65.03,4.86, 64.59, 64.47, 60.83, 35.14, 14.26. MS (ESI) m/z 451 [M + H]+.lement. Anal.: Calc. C(53.31%); H(4.03%); O(21.31%); S(21.35%);ound C(53.32%); H(3.97%); O(21.09%); S(21.22%). Melting point:09–110 ◦C.

2,5-Bis-(2-(3,4-ethylenedioxy)thienyl)thiophen-3-yl-aceticcid (5)

Sodium hydroxide (20% aq, 30 mL) was added to a solutionf 2,5-bis-(2-(3,4-ethylenedioxy)thienyl)thiophen-3-yl-acetate 40.4 g, 0.88 mmol) in 1/1 mixture of methanol/acetonitrile (4 mL).he mixture was heated to reflux and stirred at this temperatureor 2 h. Cooled to RT the aqueous solution was washed with CH2Cl2,cidified with HCl (cc) to pH 1 and extracted with CH2Cl2. Theooled organic phases were washed by brine up to pH 7, dried ongSO4, filtered and concentrated. The crude product was purified

y chromatography on silica gel using a gradient eluent of cyclohex-ne/ethyl acetate (9/1) and ethyl acetate/methanol (2/1) yielding.32 g (85%) of acid 5.

1H NMR (250 MHz, CDCl3) � 11.27 (s, 1H), 7.18 (s, 1H), 6.40 (s,H), 6.25 (s, 1H), 4.40–4.30 (m, 2H), 4.26–4.22 (m, 6H), 3.75 (s,H). 13C NMR (63 MHz, CDCl3) � 177.36, 141.82, 141.63, 138.35,37.85, 134.62, 130.59, 128.74, 125.55, 111.75, 109.12, 99.81, 97.28,5.01, 64.81, 64.58, 64.46, 34.80. MS (ESI) m/z 421 [M − H]−.lement. Anal.: Calc. C(51.17%); H(3.34%); O(22.72%); S(22.77%);ound C(51.03%); H(3.48%); O(22.62%); S(22.90%). Melting point:81–182 ◦C.

.2. Spectroscopic measurements

Absorbance spectra are obtained on a Shimadzu UV-160Apectrometer using a slow scanning speed and 1-nm slit width.hotoluminescence spectra are obtained on a Shimadzu RF-5301PCeries. The excitation slit width was 1.5 nm, while the emission slitidth was 1.5 nm. By comparison to a standard reference of quinine

ulfa, �f is calculated by Eq. (1):

f = �rI

Ir

Ar

A

n2

n2r

(1)

here I is the integrated emission intensity, A is the absorbance, and is the refractive index. The subscript “r” refers to the referenceaterial, in this case quinine sulfate in 0.05 M H2SO4 (�r = 0.546

t 20 ◦C for excitation at 350 nm). Emission spectra for quantum

etals 171 (2013) 23– 31 25

yield measurements were obtained at the same temperature andexcitation wavelength as the standard.

2.3. Electrochemical studies

All electrochemical experiments were performed using a Golddisk electrode (diameter: 1.6 mm, BASi), Gold coated silicon wafer(Aldrich) or Indium Tin Oxide glass slide (Aldrich) as workingelectrode. The counter electrode was a Pt electrode and thequasi-reference electrode was a silver wire coated with AgNO3.Dichloromethane was purchased from Aldrich and was distilledunder P2O5 before use. The three-electrode cell was connected toa CHI2400 potentiostat from CHInstruments. Cyclic voltamperom-etry is used for the electrosynthesis of the polymers. The polymersare obtained by repeated cycles between two potentials. The char-acterization of these polymers was performed in free monomersolution.

2.4. Surface characterization

AFM experiments were carried out using gold coated siliconwafers (Aldrich) as working electrode. AFM images were recordedwith a PicoSPM II microscope from Molecular Imaging using sili-con nitride cantilevers with a cantilever length of 125 �m (tappingmode) from Scientec.

3. Results and discussion

3.1. Synthesis

A short synthetic sequence was elaborated toward the3′-functionalized E-T-E terthiophenes 2–5 having � function-alized thiophene central element either by using bidirectionalKumada–Corriu (Scheme 1), or, Stille approaches (Scheme 2). Thesynthesis of the hydroxymethyl and hydroxyethyl analogs, 2 and3 was realized from EDOT 6 and �-substituted dibromothiophenes8a and 8b under modified Reynolds’ conditions [11] as depictedin Scheme 1. The coupling reaction was realized with the tran-sient formation of the organomagnesium intermediate 7 formedby transmetallation of the lithiated thiophene using a stoichiomet-ric amount of MgBr2·Et2O. We were pleased to observe that theputative Grignard intermediate 7 undergo coupling with the pro-tected dibromo thiophene derivatives 8a and 8b respectively in thepresence of a catalytic amount of NiCl2dppe (1.5 mol%) at RT in thedark. These mild reaction conditions allowed the “one pot” assem-bly of the tertienyl scaffold having �-methyleneoxy-TBS (9a) and�-ethyleneoxy-TBS (9b) central elements in 76% and 75% yields,respectively. The free alcohols 2 and 3 were isolated after standarddeprotection, by using TBAF.

As the strongly basic conditions of the Kumada–Corriu reactionare incompatible with the acetoxy substituent of 11, tristhiopheneanalogs 4 and 5 were prepared by the more conventional Stille cou-pling (Scheme 2). The in situ generated stannylated EDOT derivative10, previously described [23], was coupled to the dibromo ester 11in the presence of PdCl2(PPh3)2 (2 mol%) giving raise to the desiredterthiophenyl ethyl acetate 4 in 52% yield. The free acid 5 was iso-lated after saponification under standard conditions (85%).

3.2. Characterization of the terthienyls

The absorption and emission spectra of the 3′-functionalizedE-T-E terthiophenes derivatives 2–5 are shown in Fig. 2 and are

compared to E-T-E reference having unsubstituted central thio-phene unit (1).

The DCM solution of ETE exhibits a �max value at 377 nm,whereas the DCM solutions of 2–5 exhibit a blue-shifted �maxin

26 E. Pardieu et al. / Synthetic Metals 171 (2013) 23– 31

S

OO

S

OO

MgBr

S

R

Br Br

S

S

R

O O

S

O O

i) nBuL i, THF -78°C, 45 min

ii) MgBr2.Et2O

0°C, 45 min

8a, b

2 R=CH 2OH

3 R=(CH 2)2OH

TBA F in THF RT, 12H

6 7

NiCl2(dppe) (1 .5 mol%)

-78°C, 48H

9a R=CH 2OTBDMS

9b R=(CH 2)2OTBDMS

Scheme 1. The preparation of E-T-E monomers 2 and 3 by using Kumada–Corriu coupling.

S

OO

S

OO

SnBu3

SBr Br

O

OEt

SS

O O

O

OR

S

O O

i) nBuL i, THF -78°C, 45 min

ii) Bu3SnCl

11

NaO H (20%a q)MeOH/CH3CN

6 10

Pd( PPh3)2Cl2(2 mol%) DMF,100° C, 48H

4 R=E t

5 R=H

Scheme 2. The preparation of E-T-E monom

Fig. 2. Absorbance (–) and emission (–) spectra of 1–5 in dichloromethane (DCM).

(1/1)

ers 4 and 5 by using Stille coupling.

the range of 345–352 nm. Smaller shifts were observed in the fluo-rescence spectra where all �max values were centered in the rangeof 438–446 nm.

The quantum yield �f of the 3′-functionalized E-T-E terthio-phenes was estimated by actinometry, by comparison to astandard reference of quinine sulfate [22,24]. Quantum yield mea-surements were carried out in dilute dichloromethane solution(c = 10−5 mol L−1). These results are summarized in Table 1.

All materials have quantum yields ranging from 2.2% to 4.8%.Under the same conditions, the unsubstituted thiophene deriva-tive 1 showed the highest quantum yield. These quantum yieldsare consistent with some previously described EDOT derivatives[25]. For conjugated aromatic oligomers and polymers, a sourcefor nonradiative deactivation concerns low frequency interannu-lar torsional modes, which are dominant for twisted interannularconfigurations. Torsional vibrations are generally expected to bedominant for polymers in solution, particularly when coplanarity isprevented by steric factors. The twisted nature of these conjugated

segments thus results in lower quantum yields than those pos-sessing considerably higher rigidity. Polymers possessing higherdegrees of coplanarity and rigidity are less susceptible to deac-tivation via torsional vibrations. Another source for nonradiative

Table 1Spectroscopic data for 3′-functionalized E-T-E terthiophenes compounds.

Absorption �max(nm) ε (L mol−1 cm−1) Emission �max (nm) �f (%)

1 377 17,600 439 4.82 346 12,910 440 2.23 345 12,972 437 2.54 351 23,738 436 2.85 352 25,010 436 2.6

E. Pardieu et al. / Synthetic Metals 171 (2013) 23– 31 27

F E-T-E terthiophenes: (A) [1] = 2.5 mM, (B) [2] = 2.5 mM, (C) [3] = 2.5 mM, (D) [4] = 2.5 mMa

dpyftatbsNnc

tt(B

lp

ig. 3. Cyclic voltammograms in dry CH2Cl2 + 0.1 M Bu4NPF6 for 3′-functionalized

nd (E) [5] = 2.5 mM. Scan rate: 100 mV s−1.

eactivation is the rotation of the substituents. In fact, for 2–5 com-ounds in DCM, this phenomenon can explain the lowest quantumield. For the development of biosensors based on MSCP, during theormation of template–monomer complex, these optical proper-ies may provide information on template–monomer interactionsnd binding equilibrium. For example, Berlin et al. have analyzedhe alkali metal coordination ability of PEDOT-based polymer filmsearing the 18-crown-6-ether substituents [26]. The pronouncedpectral shifts of these monomer absorption maximum caused bya+ and K+ coordination allowed the spectrophotometric determi-ation of the coordination constants and the stoichiometry of theoordination reaction.

The electrochemical behaviors of all the 3′-functionalized E-T-Eerthiophenes were studied by cyclic voltammetry. Fig. 3 depictshe repeated potential scan electropolymerization of 1 (Fig. 3A), 2Fig. 3B), 3 (Fig. 3C), 4 (Fig. 3D) and 5 (Fig. 3E) in CH2Cl2 + 0.1 M

u4NPF6 at a scan rate of 100 mV s−1.

All new compounds 2–5 are irreversibly oxidized at a relativelyower potential than the previously described E-T-E terthio-hene 1 (0.72 V vs Ag/AgNO3), from 0.53 V to 0.69 V vs that of

Fig. 4. Absorbance spectra for the undoped polymer on ITO surface. (–) P1, (�-) P2,(©-) P3, (�-) P4 and (�-) P5.

28 E. Pardieu et al. / Synthetic Metals 171 (2013) 23– 31

ction p

Att

tmlidwE

Fig. 5. Tapping-mode (2 �m × 2 �m) AFM images and cross-se

g/AgNO3. As expected, increasing the electron-donating ability ofhe substituent resulted in a lower oxidation potential than E-T-Eerthiophene 1 under the same conditions.

Application of recurrent potential scans of each monomer solu-ion with a positive limit set at the first oxidation wave of the

onomer leads to the development of a new redox process at aower potential. The current of this new redox process increases

n intensity upon additional cycling, which is consistent with theeposition of an electroactive polymer onto the surface of theorking electrode. Nevertheless, the behaviour of 3′-functionalized

-T-E terthiophene 2 is different from the other monomer. Indeed,

rofiles for (A) bare gold (B) P1, (C) P2, (D) P3, (E) P4 and (F) P5.

the threshold of the increase lies at higher potential for thismonomer. The phenomenon may be explained by the formationof shorter polymer chains.

3.3. Polymer characterization

Tin-doped indium oxide (ITO)-coated surfaces have been inten-

sively used in the literature in various technologically importantapplications, e.g. such as solar cells, owing to their excellentelectrical conductivity and optical transparency. Thin-film of thecorresponding polymer of 1–5, respectively P1, P2, P3, P4 and

E. Pardieu et al. / Synthetic Metals 171 (2013) 23– 31 29

for: (A

Pffi

w�Pma�tttoumpiaap

the gold surface. Furthermore, the films seem to form a continuouscoverage, without individual nuclei. Quantitative measurementscan be extracted from individual cross sections [27]. Surface

Table 2Peak-to-valley measurements and surface roughness.

Fig. 6. Cyclic voltammograms in dry CH2Cl2 + 0.1 M Bu4NPF6

5, were deposited on ITO surface by electropolymerization andurther washed with CH2Cl2 to remove excess monomer. The thin-lm absorption spectra of these polymers are shown in Fig. 4.

All undoped polymers exhibit a broad absorption spectrumith a �max varying from 464 nm to 653 nm ((�max P1 = 653 nm,

max P2 = 533 nm, �max P3 = 486 nm, �max P4 = 464 nm, �max

5 = 466 nm). While P1 shows the highest �max, the other poly-ers are blue-shifted from 464 nm to 533 nm. The shifts in the

bsorption spectra can be attributed to differences in energy of the–�* transitions, directly related to the extent of conjugation in

hese materials, which appear to diminish by the substitution inhe 3′-position. Furthermore, the shift in the solid-state absorp-ion spectra also illustrates the ability of the polymer to form morerdered solid-state structures with reduced band gaps. For thensubstituted derivative P1, the optical bandgap is about 1.6 eV anday be explained by the absence of chemical modification in the 3′-

osition, and consequently by a more-ordered structure. Due to the

ntroduction of a substituent, P2–P5 are less well-ordered materi-ls, with optical bandgap ranging from 1.7 eV to 1.9 eV. As electronicbsorbance bands are sensitive to conformational changes in theolymer chains, optical properties of P2–5 can be used for the

) P1, (B) P2, (C) P3, (D) P4 and (E) P5. Scan rate: 100 mV s−1.

studies of template rebinding in the development of new memoryshaped conducting polymer.

In order to investigate the polymer film morphology, AFM mea-surements were made on the polymer modified gold surfaces intapping mode. Fig. 5A shows the initial morphology of non-treatedgold surface.

AFM images recorded after polymerization (Fig. 5B–F) show thatthe overall morphology of the initial gold surface has completelydisappeared and was replaced by small spheroids. These impressivechanges in the morphology reflect the growth of the polymer onto

Au P1 P2 P3 P4 P5

Peak-to-valley distance (nm) 12 262 434 507 492 534RMS (nm) 2 38 64 99 75 84

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tatistics such as peak-to-valley measurements and surface rough-ess are gathered and summarized in Table 2.

In the case of untreated gold surface, the surface is smooth andts average roughness is of the order of 2 nm. After polymeriza-ion, the RMS increases to reach a value of 99 nm (i.e. for P3).or sensor applications, this parameter is crucial, as the rough-ess gives an indication on the active surface area able to interactith the medium. Moreover, the size of the grains for the unsub-

tituted P1 is completely different from the other 3′-substitutedolymer. In fact grains are bigger, both in vertical and lateralimensions (200–300 nm and 800–1200 nm respectively). In thease of P2, the coverage is uniform and the grains are clearlymaller in size (i.e. 80–100 nm). For P3–P5, the grains vary in sizerom 100 to 400 nm and it can be seen that the bigger grains aressentially conglomerates of smaller grains. In addition, the sizef these smaller grains is quite uniform. Semenikhin suggestedn his work [28] that this mesoscopic homogeneity of conductingolymer films is governed by a molecular weight distribution. Theolymers P1 and P2 appear more uniform in the variation of thescillations of the cantilever, which confirms a more homogenousoating.

The P1–P5 cyclic voltammetry studies were performed inonomer free electrolyte. Fig. 6 shows the resulting cyclic voltam-ograms of each polymer in CH2Cl2 + 0.1 M Bu4NPF6 at a scan rate

f 100 mV s−1.The overall electrochemical response of the deposited polymers

resents various shapes. For the well-known P1, Fig. 6A, the wavexhibits a shoulder at −0.38 V, a main anodic wave peak at −0.05 V,nd a shoulder at 0.23 V. For P3 (Fig. 6C) and P4 (Fig. 6D) the sameehaviour is observed. The oxidation wave exhibits a shoulder at

ow potential, ranging from −0.07 V to −0.41 V, and a main anodiceak at higher potential, respectively 0.36 V and 0.10 V. Previousork on electrogenerated poly(DiEDOT) has revealed the occur-

ence of a similar additional anodic wave at positive potentialsf the main oxidation wave, which has been related to the oxi-ation of substrate molecules or short oligomer chains trapped

n the film during its deposition [29]. In the case of P2 and P5Fig. 6B and E), a single peak is observed at 0.54 V and 0.10 Vespectively. For each polymer, the scan rate dependence wasnvestigated, and the main peak current varies linearly with thecan rate as expected for electroactive species that are surfaceound.

. Conclusion

This work reports the synthesis of four new compounds basedn 3,4-ethylenedioxythiophene. The three thiophene-based ringsis-EDOT derivatives was synthesized by Kumada or Stille couplingnd exhibited blue emission characteristics with quantum yieldsanging from 2.2% to 2.8%. Cyclic voltammetry was also employedo characterize the electrochemical behavior of these compounds.he onset of the oxidation wave was found at lower potentials thanhe corresponding non-functional compound, i.e. EDOT-thiophene-DOT. These monomers can be electrochemically polymerized byepeated potential cycling, giving well-adhered films on gold orTO electrodes. The AFM analysis of the polymers material con-rms the roughness of the electropolymerized films, which is anttractive property for sensing development. The electrochemicalharacterizations of the polymer films show single oxidation waveor P2 and P5 whereas multiple oxidation waves are present for3 and P4, maybe due to the different conjugation length. Optical

and-gap, from 1.7 eV to 1.9 eV, has been extracted from absorptionpectra of the undoped polymer. Spectroelectrochemistry studiesf all compounds 2–5 and the corresponding polymers P2–P5 arender investigation. Optimization of memory shaped conducting

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polymer (MSCP) biosensors detection/quantification perform-ances, based on absorption and emission spectra, electrochemicaland AFM measurements of electropolymerized films, is currentlyunderway.

Acknowledgements

The authors gratefully thank Jalal Ghilane and Jean-ChristopheLacroix from ITODYS for help for the acquisition of the AFM images.

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