An effective multipurpose building block for 3D electropolymerisation: 2,2′-Bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene

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Electrochimica Acta 55 (2010) 8352–8364

Contents lists available at ScienceDirect

Electrochimica Acta

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n effective multipurpose building block for 3D electropolymerisation:,2′-Bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene

rancesco Sannicolòa,b,∗, Simona Rizzoc, Tiziana Benincorid,c, Włodzimierz Kutnere,f,1,rzysztof Noworytae, Janusz W. Sobczake, Valentina Bonomettig,b,1, Luigi Falciolag,b,1,atrizia R. Mussinig,b,∗∗,1, Marco Pierinih

Dipartimento di Chimica Organica e Industriale, Università degli Studi di Milano, via Venezian 21, 20133 Milano, ItalyCentro Interdisciplinare Materiali e Interfacce Nanostrutturati (CIMaINa), Università degli Studi di Milano, Via Celoria 16, 20133 Milano, ItalyISTM-CNR, via Venezian 21, 20133 Milano, ItalyDipartimento di Scienze Chimiche e Ambientali, Università dell’Insubria, via Valleggio 11, 22100 Como, ItalyInstitute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, PolandFaculty of Mathematics and Natural Sciences, School of Science, Cardinal Stefan Wyszynski University in Warsaw, Dewajtis 5, 01-815 Warsaw, PolandDipartimento di Chimica Fisica ed Elettrochimica, Università degli Studi di Milano, via Golgi 19, 20133 Milano, ItalyDipartimento di Chimica e Tecnologie del Farmaco, Università degli Studi di Roma “La Sapienza”, piazzale Aldo Moro 5, 00185 Roma, Italy

r t i c l e i n f o

rticle history:eceived 23 December 2009eceived in revised form 18 April 2010ccepted 19 April 2010vailable online 27 April 2010

a b s t r a c t

Novel thiophene-based oligomer, 2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene (TX), wasdesigned and synthesized, and its electrochemical and spectral properties characterised. TX was read-ily polymerised electrochemically to form well organized conducting homopolymer films on varioussolid electrode substrates. Moreover, it was successfully used for deposition by electropolymerisation

eywords:hiophene-based oligomerslectropolymerisationo-polymersrosslinking

of electrochemically active thin films of co-polymers with three different monomers of functionalisedbis(2,2′-bithienyl)methane derivatives. It appeared that TX was an effective crosslinker and 3D promoterin these electropolymerisations involving co-monomers intrinsically showing limited aptitude for theelectropolymerisation or forming polymer films of low conductivity. This attractive TX ability stemsfrom combination of its (i) high conjugation efficiency in each of the two planar moieties, (ii) intrin-sic 3D structure on account of the presence of the central node, and (iii) intrinsic regioselectivity in

acco

D electroactive polymer functional films electropolymerisation on

. Introduction

Thiophene-based oligomers are a virtually boundless class ofrganic semiconductors and starting materials for preparation oflectroactive polymer films [1–11]. Current strategies of molec-lar design of these oligomers address a trade off between two

ey requirements [1,12], that is branching, to achieve the solubil-ty required for industrial processing, and planarity, for the mostfficient conjugation to achieve the highest conductivity.

∗ Corresponding author at: Dipartimento di Chimica Organica e Industriale, Uni-ersità degli Studi di Milano, via Venezian 21, 20133 Milano, Italy.el.: +39 02 50314173; fax: +39 02 50314139.∗∗ Corresponding author at: Dipartimento di Chimica Fisica ed Electrochimica, Uni-ersità degli Studi di Milano, via Golgi 19, 20133 Milano, Italy. Tel.: +39 02 50314211;ax: +39 02 50314300.

E-mail addresses: [email protected] (F. Sannicolò),[email protected] (P.R. Mussini).1 ISE members.

013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2010.04.070

unt of the positions of the two available free �-thiophene sites.© 2010 Elsevier Ltd. All rights reserved.

Actually, the “effective conjugation” is a determining factor ofthe electronic properties of a given molecule, directly accountingfor the HOMO–LUMO energy gaps, both the “spectroscopic” one (i.e.the energy required to promote an electron from the fundamen-tal to the excited state) and that “electrochemical” (i.e. the energyassociated with the potential difference between the first electronabstraction and the first electron injection). Usually, the spectro-scopic and the electrochemical energy gaps do not coincide sincethey correspond to different processes and are differently affectedby solvation. However, they often are very similar, and alwaysmutually linked. They both steadily decrease with the increase ofeffective conjugation. That is, the spectroscopic gap decreases withthe decrease of the energy difference between the fundamental andexcited state and the electrochemical one with the increasing abil-ity of the conjugated system to stabilize both a cation radical and

an anion radical, which therefore are formed at both less positiveoxidation potentials and less negative reduction potentials.

While the width of the HOMO–LUMO band gap is a function ofthe conjugation efficiency in the main conjugated system of themolecule, the absolute HOMO and LUMO energy levels, or, from the

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3,3′-bithianaphthene, TX (Fig. 2), which we have purposelydesigned and tested to act as an effective 3D electropolymeri-sation promoter. Beside showing an intrinsically high abilityfor electropolymerisation, we have proved this monomer to be

Fig. 2. (a) Structural formula of 2,2′-bis(2,2′-bithiophene-5-yl)-3,3′-

ig. 1. Structural formulas of two oligothiophene isomers, T146 [9] and T′146 [10], bNodes” are indicated with full circles.

lectrochemical perspective, the absolute values of the first reductionnd first oxidation potential depend upon inductive effects, eitheromogeneously distributed, as in all-thiophene systems, or asym-etric and localized, when non-thiophene substituents or building

locks are also present in the molecule.Within these general guidelines, the structural complexity

nd virtually unlimited variety of branched oligothiophenesequire identification of specific rationalisation criteria con-erning structure–reactivity relationships characteristic for theseligomers in order to achieve target-oriented design and optimisa-ion of the synthetic effort.

In this context, our recent investigations on spider-like oligoth-ophenes [10,11] have shown that the electronic properties of theseligomers do not depend upon the overall number of thiophenenits, but (i) on the number of conjugated thiophene units in the

ongest �-linked chain in the molecule, n�, and (ii) on the numberf “nodes”, i.e. bonds between thiophene rings capable of distor-ions from planarity of these rings along the same chain. Theseodes are present whenever two �-interconnected thiophene ringsarry facing thiophene substituents in � positions (as in the well-nown case of the biphenyl molecule, having a 44◦ angle betweenhe two phenyl rings as a consequence of facing H· · ·H interactions).n particular, the conjugation efficiency regularly increases, and thenergy gap decreases, with the increase of the number of �-linkedhiophene rings in the main backbone, although the gaps tend toatten for chains longer than five or six conjugated units, makinghe effort to synthesize more complex structures actually useless10]. On the other hand, at constant length of the �-thiophene con-ugated backbone the conjugation is increasingly impaired by anncreasing node density along the conjugated chain.

Comparison of our recently presented two oligothiophene iso-ers T146 [10] and T′146 [11] (Fig. 1) illustrates this property. That

s, both isomers contain the same total number of thiophene ringsn = 14) and the same number of thiophene rings in the main �-inked thiophene backbone (n = 6). However, T146 has a smallerumber of nodes in its backbone. Accordingly, both the opti-al and the electrochemical energy gaps, determined either fromhe CV peak maxima or from peak onsets, are significantly loweror T146 (optical: Eg,UV–vis max = 2.96 eV, Eg,UV–vis onset = 2.41 eV; elec-rochemical: Eg,EC max = 2.74 eV, Eg,EC onset = 2.44 eV) than for T′146optical: Eg,UV–vis max = 3.35 eV, Eg,UV–vis onset = 2.48 eV; electrochem-cal: Eg,EC max = 3.08 eV, Eg,EC onset = 2.74 eV).

Now, considering that most applications of branched oligothio-henes require low energy gaps, one should regard the presence

f nodes in most cases as an undesirable feature. This is becausehe higher the number of nodes the shorter is the effectiveonjugation length. In effect, gaps are larger even in cases of syn-hetically demanding molecules, which contain many thiopheneings. However, we recently realized that the intrinsic 3D charac-

ontaining 14 thiophene rings, six of which are in the main �-conjugated backbone.

ter of the nodes can also be advantageously exploited, particularlyin application to a typically three-dimensional process involvingoligothiophene molecules with free terminal � positions, i.e. elec-tropolymerisation.

In the present paper, we characterize in detail the electro-chemical properties of the intrinsically non-planar, asymmetricalmultithiophene-based monomer, 2,2′-bis(2,2′-bithiophene-5-yl)-

bithianaphthene, TX. The full circle indicates the central “node”, while fullsquares mark the free � terminal thiophene positions, i.e. the only two sitesavailable for oligomerisation. Dihedral angles � and � are the torsionals varied toperform the theoretical estimation of the enantiomerisation energy barrier. (b) Thepotential energy surface with the enantiomerisation pathway indicated with theblack arrow. (c) Structural formulas of the (R)- and (S)-TX enantiomers.

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354 F. Sannicolò et al. / Electroch

n extremely efficient building block affording fast and reg-lar electrooxidative growth of films of conducting polymersith co-monomers revealing important functional properties but

orming polymers of limited conductivity. As an example, wexplored an important applicative case, the preparation of aelamine-templated molecularly imprinted polymer (MIP) film,ell-performing as a recognition element of a selective piezomi-

rogravimetric chemosensor [13].

. Experimental

.1. Chemicals

Solvents were of HPLC grade and used as received from the sup-liers (Merck and Sigma–Aldrich). Supporting electrolytes wereigh-purity reagent grade (>99%) and used as received from theupplier (Fluka).

.2. Organic syntheses

.2.1. 2,2′-Dibromo-3,3′-bithianaphthene (TY)A solution of Br2 (0.77 cm3, 15 mmol) in CH2Cl2 (5 cm3) was

dded to a mixture of 3,3′-bithianaphthene (2.0 g, 7.5 mmol) andcOK (1.5 g, 15 mmol) in CH2Cl2 (15 cm3), and the reaction mixtureas stirred for 24 h. Next, water (5 cm3) was added and the organic

ayer was separated, and then dried over Na2SO4. The solvent wasvaporated to dryness to give a residue, which was purified by crys-allization from di-i-propyl ether (1.7 g, 55%). M.p. 171 ◦C; 1H NMRCDCl3): ı = 7.81 (d, 3J (H,H) = 8 Hz, 2 H, H7,7′ ), 7.40–7.30 (m, 2 H,

6,6′ ), 7.30–7.20 (m, 4 H, H4,4′ and H5,5′ ); 13C NMR-APT (CDCl3)= 140.29 (C), 138.59 (C), 130.55 (C), 125.43 (CH), 125.35 (CH),23.43 (CH), 122.28 (CH), 117.33 (C). MS (EI): 422, 424 (77%), 426t, M+), 344, 342 (d, M+-Br, 18%), 264 (M+-2Br, 100%).

.2.2. 2,2′-Bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene (TX)5-Tri-n-butylstannyl-2,2′-bithiophene (3.27 g, 7.2 mmol) and

etrakis(triphenylphosphine) palladium (0.42 g, 0.36 mmol) weredded under nitrogen atmosphere to a warm stirred solutionf 2,2′-dibromo-3,3′-bithianaphthene (TY) (1.0 g, 0.24 mmol) inoluene (40 cm3). The mixture was refluxed overnight, then addi-ional amounts of 5-tri-n-butylstannyl-2,2′-bithiophene (1.1 g,.4 mmol) and tetrakis(triphenylphosphine) palladium (0.28 g,.24 mmol) were added. The mixture was again refluxed for 4 h,hen the solvent was evaporated to dryness to give a residue,hich was column chromatographed (Merck silica gel 70–230esh; eluant – hexane:CH2Cl2, 7:3) to give the title compound

0.14 g, 0.236 mmol, 98%) as a white solid. M.p. 86 ◦C; 1H NMRCDCl3) ı = 7.87 (d, 3J(H, H) = 8 Hz, 2H, H7,7′ ), 7.38–7.30 (m, 2H,

6,6′ ), 7.23–7.17 (m, 4H, H4,4′ and H5,5′ ), 7.12 (dd, 3J(H, H) = 4.9 Hz,J(H, H) = 1.0 Hz, 2H, H5′ bithiophene), 7.09 (d, 3J(H,H) = 4 Hz, 2H, H4ithiophene), 6.98–6.87 (m, 6H, H3,3′ ,4′ bithiophene); 13C NMR-APTCDCl3) ı = 140.44, 138.90, 138.13, 136.81, 136.75, 134.45, 127.70CH), 127.05 (CH), 125.19 (CH), 125.00 (CH), 124.64 (CH), 123.94CH), 123.70 (CH), 122.90 (CH), 122.00 (CH). MS (EI): 594 (M+, 18%),30 (100%), 330 (95%).

.3. 1H NMR and 13C NMR spectroscopy measurements

The NMR spectra were recorded on a Bruker AC 300 spectrom-ter. Chemical shifts are given in ppm and the coupling constantsn Hz; d = doublet, dd = double doublet, m = multiplet.

.4. MS measurements

Mass spectra were recorded on a VG AUTOSPEC M246 spectrom-ter (EI).

Acta 55 (2010) 8352–8364

2.5. Quantum chemistry calculations for TX

The enantiomerisation energy barrier for TX was estimated byusing the following computer software packages: (i) PC Model 4.0(Serena Software, Bloomington, IN, USA) employed in molecularmechanics (MM) calculations based on MM2, MM3 and AMBERforce fields (f.f.); (ii) PC Spartan’04 (Wavefunction, Inc., Irvine,CA, USA) employed in calculations based on both semiempiri-cal (Hamiltonians AM1 and PM3) and molecular mechanics (f.f.FFMM94) methods. Variations of the conformational potentialenergy were computed as a function of both the single torsionangle � as well as the couple of torsion angles � and � drivenin steps of 6◦, according to a well established and effective pro-cedure [14,15]. Definitions of these dihedral angles are given inFig. 2.

2.6. UV–vis spectroscopy measurements

The UV–vis spectroscopy measurements were performed withthe UV2501 PC UV–vis recording spectrophotometer of Shimadzu(Kyoto, Japan) operating either in a transmission or reflection mode.

2.7. AFM imaging

The polymer films were imaged with atomic force microscopy(AFM) in a tappingTM mode using the Multimode NS3D microscope,equipped with the 10 �m × 10 �m or 125 �m × 125 �m scanner,of the Digital Instruments/Veeco Metrology Group (Woodbury, NY,USA).

2.8. X-ray photoelectron spectroscopy measurements

X-ray photoelectron spectroscopy (XPS) spectra of all sam-ples were acquired at room temperature using the ESCALAB-210spectrometer of VG Scientific (East Grinstead, UK) and non-monochromated Al K� (h� = 1486.6 eV) X-ray radiation source.Data were processed using the Avantage Data System softwareof Thermo Electron (East Grinstead, UK). For data smoothing,nonlinear Shirley background was subtracted. Peak synthesis bysymmetric Gaussian–Lorentzian product function, with the L/Gratio of 0.3 ± 0.05, was used to approximate line shapes of the fit-ting components. For quantification, the Scofield sensitivity factorsand determined transmission function of the spectrometer wereused.

2.9. Electrochemical characterization

TX was characterised by cyclic voltammetry, CV, at poten-tial scan rates typically ranging from 0.02 to 2.00 V s−1, in0.25–5.00 mM solutions of the CH2Cl2 solvent (in prelimi-nary experiments also ortho-dichlorobenzene, ODB, dimethylfor-mamide, DMF, and acetonitrile, ACN were used), deaerated byN2 purging before each experiment. 0.1 M tetrabutylammoniumhexafluorophosphate, TBAHFP, or tetrabutylammonium perchlo-rate, TBAP, was used as the supporting electrolyte, in a 4 cm3

cell equipped with a presaturator. The ohmic potential drop wascompensated by the positive feedback technique. The experimentswere carried out using an Autolab PGSTAT 12 potentiostat of Eco-Chemie (Utrecht, The Netherlands), run by a PC with the GPES 4.9software of the same manufacturer. The working electrode was

either (i) a 0.071 cm2 glassy carbon GC disk embedded in Teflon®

of Amel (Milan, Italy), (ii) a 0.0314 cm2 gold disk embedded inTeflon® of Amel, (iii) a 0.00785 cm2 platinum disk embedded inglass, (iv) a 1.44 cm2 platinum foil for bulk depositions, (v) anindium-doped tin oxide (ITO) coated glass slide of Aldrich (sheet

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esistance 9–12 �/sq.), or (vi) Au/Cr-coated glass slides for spec-roscopic characterizations. The counter electrode was a platinumire, or a platinum grid to afford higher symmetry of the elec-

ric field. The operating reference electrode was either an Ag|AgClseudo-reference one, or an aqueous saturated calomel one (SCE) ofredox potential of −0.495 V in our CH2Cl2 working solution vs. thatf the Fc+|Fc couple (the intersolvental redox potential referenceurrently recommended by IUPAC [16,17]) and +0.052 V vs. that ofhe Me10Fc+|Me10Fc couple (an improved intersolvental referencender investigation [18–20]). The optimised finishing procedureor the working disk electrodes consisted in surface polishing withdiamond powder of 1 �m in diameter of Aldrich on a wet DP-Naploth of Struers.

Electrochemical oligomerisation of the investigatedco)monomers by repeated CV cycling in the potential rangef the first anodic peak (or anodic peak system), at the 0.20 V s−1

otential scan rate, resulted in deposition of the conducting poly-er films on the working electrodes. The CV stability of the filmsas tested by repeated cycling over the same potential range atifferent scan rates, in the monomer-free CH2Cl2 solutions of 0.1 Mupporting electrolytes having the same cation but different anionsTBAHFP, TBATFB, or TBAPTS, were HFP = hexafluorophosphate,FB = tetrafluoroborate, PTS = p-toluenesulfonate); finally, theotential range was progressively extended negatively in search forhe first reduction potential and possible related charge-trappinghenomena.

Electropolymerisation and the polymer stability tests (withhe relevant ingress and egress of counter anions of the sup-orting electrolyte) have also been monitored by piezoelectricicrogravimetry using an EQCM 5710 electrochemical quartz

rystal microbalance (Institute of Physical Chemistry of theolish Academy of Sciences, Warsaw, Poland), equipped withhe gold-over-titanium coated AT-cut quartz crystal resonators5 MHz), connected to the Autolab PGSTAT 12 and run by theQCM 5710-S2 software of the Institute of Physical Chemistryf the Polish Academy of Sciences. In order to test rigidityhanges of the deposited films and check applicability of theauerbray equation, the electropolymerisation was also partiallytudied with the EQCM 5710 microbalance using the 10-MHzu/Ti-coated quartz crystal resonators, connected to the EP21otentiostat of ELPAN (Lubawa, Poland) and driven by the EQCM710-S2 software. This system allows for simultaneous measure-ents of the current, resonant frequency and dynamic resistance

hanges.

. Results and discussion

.1. Structural peculiarities of the TX monomer

As shown in Fig. 2, TX bears two identical and homotopic moi-

ties, each consisting of an �-linked T–T–TN system (T = thiophene,N = thianaphtene) that are nearly planar (in particular, approx-mately equivalent to an �-linear terthiophene) and, therefore, ofigh effective conjugation. The structure of this monomer was pur-osely designed to be endowed with the following properties.

f the (R, S)-TX preparation.

a) Intrinsic 3D character. Consistently with the typical oligothio-phene features described in Introduction, bulky substituentson both � positions of the TN thiophene rings induce forma-tion of a central node between the two moieties (full circle inFig. 2). Preliminary MM calculations have also shown that thedihedral angle between the interconnected TN units is ∼70◦.Therefore, the formation of a tridimensional structure should beexpected.

b) Intrinsic regioregularity in polymerisation. Despite its com-plexity, the TX molecule has only two sites available forpolymerisation, i.e. one free � position on the terminal T ringof each T–T–TN moiety (full squares in Fig. 2), largely sepa-rated one from the other, symmetrical, and easily accessible.This structure implies regular polymerisation along the direc-tions of the two planar systems, with a strong 3D character asa consequence of high angle between these planes.

(c) Inherent asymmetry. In spite of the absence of stereogenic cen-tres, the whole molecule is chiral, exhibiting a C2 symmetryaxis. The MM calculations, performed by progressively vary-ing the torsional angle � between the thianaphtene units,suggested that the enantiomerisation barrier was comprisedbetween 50 and 65 kcal mol−1 depending on the employedforce field (MMFF94: 49.5 kcal mol−1; MM2: 65.3 kcal−1; MM3:62.6 kcal−1; AMBER: 54.5 kcal mol−1). If the additional dihe-dral angle � was also taken into account then the barriervalue decreased to 47.7 kcal mol−1 (f.f. MM2). Semiempiri-cal methods, applied in calculations of the conformationalpotential energy pathway that leads to the enantiomericform of TX by progressive variation of the single dihe-dral angle �, suggested an even lower barrier value (AM1:32.5 kcal mol−1; PM3: 38.3 kcal mol−1). In any case, the calcu-lated barriers are sufficiently high to guarantee a racemisationhalf-lifetime of several centuries. Therefore, stable enan-tiomers of TX can be separated and stored. This chiralityis a very attractive feature in the perspective of obtainingoptically active conducting polymer films without stereocen-tres. Moreover, inherently asymmetric molecules (such as,e.g. thiahelicenes [21]) open up a new strategy to prepareasymmetric superior aggregates for the nonlinear optics (NLO)applications.

3.2. Synthesis of the TX monomer

TX was unknown in literature. It was synthesized inthe present work by reaction of the 2,2′-dibromo-3,3′-bithianaphthene (TY), unknown in literature as well, with2-tributylstannyl-5,2′-bithiophene in the presence of palladiumtetrakis(triphenylphosphine) in the refluxing toluene solution,according to the Stille coupling method. The dibromoderiva-tive was synthesized, in turn, by direct bromination of known

3,3′-bithianaphthene [22] (Scheme 1).

Hitherto unknown compounds were fully characterised by 1HNMR and 13C NMR spectroscopy as well as MS spectrometry.

The present electrochemical investigations were performed ona racemic TX mixture, since the resolution into antipodes of non-

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Fig. 5. (a) Electropolymerisation (10 CV cycles) of 1 mM TX in the 0.1 M TBAHFP

ig. 3. The CV curve for 0.5 mM TX in the 0.1 M TBAHFP solution of CH2Cl2 on thelassy carbon electrode, at 0.2 V s−1. Inset shows the potential scan rate effect onhe first anodic peak, each voltammogram being recorded on a freshly polishedlectrode surface. Potential scan rate is indicated for each curve.

unctionalised arenes is not trivial and, generally, requires veryccurate screening to individuate the most suitable procedure.

.3. Electrochemical activity of the TX monomer

A CV pattern for 0.5 mM TX in CH2Cl2 is shown in Fig. 3.he electrochemical energy gaps, determined from either the dif-erence of the first anodic and the first cathodic peak potential

ig. 4. UV–vis spectra for the TX monomer (in the absorbance mode, left ordinate)nd for the TX polymer deposited by electropolymerisation onto the Au-coated glasslide (in the reflectance mode, right ordinate). Values of the absorption wavelengthaxima for linear oligothiophenes T3 and T4 are indicated with vertical dash–dot

ines, for comparison.

solution of CH2Cl2 on the Au/quartz resonator of EQCM, at 0.2 V s−1. Solid curve, left-hand ordinate: CV curve; dash–dot curves, right-hand ordinate: frequency changes.(b) Variation with time of the working mass (left-hand ordinate) or monomer unitsper geometric surface unit (right-hand ordinate).

(Eg,EC max = 3.20 eV) or onsets (Eg,EC onset = 2.93 eV) are similar, albeitnot coincident, with the spectroscopic gaps, determined from theabsorption wavelength maximum, (Fig. 4; Eg,UV–vis max = 3.32 eV) oronset (Eg,UV–vis onset = 2.79 eV). The obtained values are intermedi-ate between the literature values for linear �-terthiophene (�T3:Eg,UV–vis max = 3.50 eV, Eg,UV–vis onset = 3.04 eV [23], Eg,EC = 3.10 eV[2]) and linear �-tetrathiophene (�T4; Eg,UV–vis max = 3.28 eV,Eg,UV–vis onset = 2.71 eV [23]; Eg,EC = 2.77 eV [2]).

These gap values, together with the anodic CV pattern consist-ing of two equal nearly merging peaks (with Epa,I = 0.62 V vs. Fc+|Fcand Epa,II = 0.78 V vs. Fc+|Fc) are well consistent with the aboveMM calculations. They all point to two equal, slightly interact-ing conjugated systems, resembling �-terthiophenes, with someenhancement in the effective conjugation. This enhancement is aconsequence of the presence of the condensed benzene ring and/orof some residual interaction with the adjacent twin � system (thedihedral angle at this node being significantly less than 90◦).

The absolute HOMO and LUMO energy levels can also be esti-mated from the first anodic and first cathodic peak potentials, usingthe following equations [24,25]:E (eV) ≈ −1e− ×

[(E /V

(vs. Fc+|Fc

))+ 4.8 V (Fc+|Fc vs. zero)

](1)

HOMO p,Ia

ELUMO (eV) ≈ −1e− ×[(

Ep,I c/V(

vs. Fc+|Fc))

+ 4.8 V (Fc+|Fc vs. zero)]

(2)

They yield in our case

EHOMO,max = −5.46 eV and ELUMO,max = −2.26 eV

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F thea d 0.90

w

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ig. 6. AFM images of the TX polymer film deposited by electropolymerisation fromfter 10 potential CV scans between (a) −0.45 and 0.93 V vs. Fc+|Fc and (b) −0.35 an

ith the maxima criterion, or

HOMO,onset = −5.35 eV and ELUMO,onset = −2.42 eV

ith the onset criterion.The potential of the first anodic peak in the monomer CV pattern

s only slightly shifted positively with the increase of the potentialcan rate (inset in Fig. 3), slope of the Ep vs. log v dependence being10 mV. This feature, together with the delayed ill-defined and low

athodic peak, recorded upon potential scan reversal, points to (i)eposition of a solid, electrochemically active product on the elec-rode surface, (ii) a chemically irreversible process initiated by aelatively fast electron transfer and followed by coupling of theation radical (some trace peak on the potential return, i.e. chemical

eversibility, might also be perceived at the higher scan rates).

Actually, consistently with its intrinsic 3D structure, TX elec-ropolymerises very rapidly and regularly, a behaviour steadily

aintained even after several consecutive CV cycles. This behaviours well accounted for by the piezoelectric microgravimetry at

1.12 mM TX in the 0.1 M TBAHFP solution of CH2Cl2 onto the Au-coated glass slideV vs. Fc+|Fc.

EQCM monitoring of the electropolymerisation (Fig. 5). The massof the polymer deposited that way increases steadily and regularly(dash–dot curve in Fig. 5b). The superimposed pulses account forthe supporting electrolyte anion ingress and egress thus balanc-ing the cation radical formation and reduction to its neutral form.These regular pulses increase with the increase of the polymermass, indicating an increasing number of counterions being ableto enter the film on the experiment timescale with the increaseof the film thickness. Moreover, the pulses feature a fine structurewith two maxima indicating multiple subsequent charge-balancingcycles during potential cycling.

Importantly, changes in the dynamic resistance, recorded dur-ing film deposition by electropolymerisation and its CV stabilitytesting (not shown), are very small not exceeding few ohms. As

the dynamic resistance changes are linked to the changes of theviscoelastic properties of the polymer [26], one may conclude thatthe growing polymer film does not change its viscoelasticity andwell adheres to the electrode surface. Moreover, ingress and egressof counterions does not change viscoelastic properties of the film.

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ig. 7. Stability of the TX polymer film, deposited by electropolymerisation, withesonator of EQCM, at 0.2 V s−1, with 0.1 M (a) TBAHFP, (b) TBATFB, and (c) TBAPTrequency changes. Variation with time of the working electrode mass or counterio

herefore, the use of the Sauerbrey equation [27] for calculationf the mass changes accompanying electrochemical processes isustified.

The AFM studies allowed for high resolution imaging of theX films deposited by electropolymerisation. Fig. 6 shows elon-ated domains in an ordered TX polymer structure, consistent withhe film stability and reproducibility. The estimated film thicknessrown during ten CV cycles, is ∼90 nm.

ect to anodic CV cycling in the monomer-free CH2Cl2 solution, on the Au/quartzid curves, left-hand ordinate: CV patterns; dash–dot curves, right-hand ordinate:s per geometric surface unit for (a′) TBAHFP, (b′) TBATFB, and (c′) TBAPTS.

The reflectance UV–vis spectra show Eg,UV–vis max ≈ 2.5 eV andEg,UV–vis onset ≈ 2.0 eV. These values point to a considerable increasein conjugation of the � system with respect to the starting

monomer. This increase can be due both to the increasing numberof �-linked thiophene rings and/or to the �–� stacking interac-tions.

In the fresh TBAHFP solution of CH2Cl2 without monomer, thepreviously deposited polymer appears very stable with respect to

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F yl)-5,b , MT.

Cbi7

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ig. 8. Bis(2,2′-bithienyl)methane-based functional monomers, vis. bis(2,2’-bithienenzo-[18-crown-6]methane, CT, and bis(2,2′-bithienyl)-6-methoxybenzomethane

V cycling in the potential range of the first anodic peak. This sta-ility is manifested by the EQCM monitoring, which shows a steady

ngress and egress of the charge-balancing counterions (Fig. 7a anda′).

Upon changing the hexafluorophosphate for tetrafluoroborateounterion (Fig. 7b and 7b′), the film remains still stable. How-ver, it shows a slight tendency to degrade despite that the anion ofetrafluoroborate is even smaller than that of hexafluorophosphate,n effect presumably linked to some “imprinting” of the former inhe polymer being formed.

Much more retarded is the dynamics at the polymer-solutionnterface of a much bulkier and very different anion, such as para-oluenesulphonate (PTS). In the presence of this counterion the firstnodic peak is progressively displaced remarkably to higher poten-ials (Fig. 7c and 7c′). At the potential window kept constant, theulses corresponding to the anion ingress and egress progressivelyecrease and the overall polymer mass asymptotically increases, as

f the counterion remained trapped in the polymer network. Thisntrapment may correspond to some irreversible polymer degra-ation, accounted for by the increasingly more positive oxidationotentials required for the cation radical formation.

Notably, the TX electropolymerisation appears to be influencedy the nature of the working electrode material; with that respect,he Pt and Au electrodes are generally more efficient than the GClectrode.

.4. Co-electropolymerisations

The ability of the TX monomer to promote formation of a 3D

lm in co-electropolymerisation was tested by using the BT, CT, andT co-monomers (Fig. 8). All of them contain a moiety capable of

lectropolymerisation, i.e. the bis-(2,2′-bithiophene-5-yl)methanehain, with two �-thiophene terminal positions free. Impor-antly, the methinic sp3 carbon does not interrupt the conjugation

5-dimethyl-2-phenyl-[1,3,2]-benzodioxaborinanemethane, BT, bis(2,2′-bithienyl)-

between the bithiophene units since the electropolymerisationinvolves the oxidative removal of a benzylic hydrogen atom pro-ducing a low-gap delocalized system. In this system the thiophenerings display the quinoid structure [28–30]. On this chain, both BTand CT molecules, synthesized by the D’Souza group of the WichitaState University [13], carry a functional group with ligating proper-ties (1,3,2-dioxaborinane “B” for BT, benzo-(18-crown-6) ether “C”for CT). These functional monomers are intended for the prepa-ration of molecularly imprinted polymers (MIPs) to be appliedas recognition materials in chemosensors for selective determi-nation of biorelevant analytes. The functional monomers weredesigned and synthesized specifically for application in MIP-basedchemosensors. Compounds containing dioxoborinane group areused for optical and electrochemical detection of sugars [31–33],fluorides [31,34,35], and amines [36,37]. Moreover, different com-pounds bearing crown ether substituents are well known for theirability of cation binding and, therefore, were extensively applied inchemosensors for determination of different cations [38–40].

In spite of featuring the bis-(2,2′-bithiophene-5-yl)methanechain, both BT and CT form polymers of limited conductivity underdifferent solution conditions (some of which are shown in panelsb, b′ and c, c′ in Fig. 9).

To investigate whether the borinane or crown ether moietycould cause this problem, a control MT compound (Fig. 8) wasprepared according to the literature procedure [41]. The anisylsubstituent in MT was chosen to mimic the electron-releasing prop-erties of the crown and dioxoborinane ring while being far lessdemanding from the sterical point of view.

The normalized CV curves for 0.5 mM and 5 mM MT in the 0.1 M

TBAP solution of CH2Cl2 are shown in Fig. 10a. A single first anodicpeak is observed at significantly higher potentials (Epa = 0.82 V vs.Fc+|Fc or Epa,onset = 0.68 V vs. Fc+|Fc) than that for TX, while the firstreduction process is again perceived as a small cathodic peak atthe same potential as that for TX. As a consequence, the electro-

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hemical energy gaps are significantly higher (Eg,EC max = 3.40 eV,g,EC onset = 3.11 eV) than those for TX, more resembling those for theinear �-terthiophene than those for the �-tetrathiophene. How-ver, they are much lower than that for bithiophene in any case.his result confirms that some communication, albeit hampered, is

ig. 9. CV Curves for (a) 1 mM BT and (a′) 1.22 mM CT on the Pt electrode (0.00785 cm2

ttempts with the use of ODB, in the positive potential range, under the same conditionhe same conditions but in the 0.1 M TBAHFP solution of ACN.

Acta 55 (2010) 8352–8364

maintained between the two bithiophene half chains through thecentral CH bridge. Apparently, this bridge plays a similar role asthat of the node in a “spider” oligothiophene structure.

Again, the observed potential scan rate effect on the first anodicpeak is modest, pointing to rather fast electron transfer. Like in the

), in the 0.1 M TBAP solution of ODB, at 0.1 V s−1; (b and b′) electropolymerisations; (c and c′) electropolymerisation attempts, in the positive potential range, under

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T and CT cases (Fig. 9a and 9a′), one or more chemical steps fol-owing the first oxidation lead to the deposition of an electroactiveroduct on the electrode surface in the MT case (Fig. 10a). Notewor-hy, this product is irreversibly reduced in the MT case showing aomplex CV pattern. The shape of this pattern strongly dependspon the substrate concentration. In contrast, both BT and CT dis-lay chemically reversible redox processes, although they mightrise from the different solvents used.

While electropolymerisation of BT and CT results in relativelyeakly conducting polymer films under any solution conditions,

he electropolymerisation of MT does not even commences, unlikehat of TX, at our typical 0.5 mM test concentration (curve 1 inig. 10b). However, upon increasing the monomer concentrationo, e.g. 5 mM, MT can electropolymerise (curve 2 in Fig. 10b).his behaviour may confirm that formation of the weakly con-ucting polymer film prepared by electropolymerisation of BT orT is mostly a consequence of the presence of the ligating sub-tituents. The less sterically hindered MT does electropolymeriset higher monomer concentrations, but requires more drasticonditions compared to those used for electropolymerisation ofX.

Maintaining the same anodic potential scan range (within whichoth TX and any of the three co-monomers can be oxidized to theation radical intermediate), but adding TX as a co-monomer inhe 1:1 mole ratio to a solution of either of the functional monomeresults in remarkable changes of its CV behaviour.

In the MT case, not only co-polymerisation with TX proceedsven at lower MT concentration (curve 3 in Fig. 10b, to be com-ared with curve 1 without TX) but at this low concentration itpparently proceeds even faster than that at the tenfold higher MToncentration in the absence of TX (curve 2 in Fig. 10b). Impor-antly, according to its CV and UV–vis spectroscopy features, theesulting polymer appears to be neatly different from both the MT

nd the TX homopolymer. Therefore, it can reasonably be assumedo be a co-polymer, as evidenced by our XPS studies (see below).

In the case of BT and CT (Fig. 11), intrinsically affording for-ation of weakly conducting polymers, conductivity is improved

ven more dramatically. The addition of equimolar TX affords fast

ig. 10. (a) CV curves for (1) 0.5 mM and (2) 5 mM MT on the GC electrode (0.071 cm2

erformed under the same conditions using 0.5 mM MT (curve 1′), 5 mM MT (curve 2′),ndicated at each CV curve.

Acta 55 (2010) 8352–8364 8361

and regular co-polymerisation under all solution conditions. Theobtained co-polymers are very stable with respect to potentialcycling and, again, both their CV and UV–vis spectroscopy fea-tures are significantly different from those of the TX homopolymer(Fig. 11a and b).

We also performed a co-electropolymerisation involving all theBT, CT and TX monomers in equimolar amounts (Fig. 11c). Theprocess was, again, very fast and regular. The CV and UV–vis spec-troscopy features of the resulting polymer were quite differentfrom those of both the homopolymers and the binary co-polymers,presumably indicating formation of a ternary co-polymer.

In order to obtain both qualitative and quantitative informationon the chemical composition of the deposited co-polymer films,XPS studies of binary BT–TX and CT–TX co-polymers as well as theternary BT–CT–TX co-polymer were performed. For that purpose,the films were deposited by electropolymerisation onto the Au-coated glass slides by multiple CV cycling, then washed with CH2Cl2and dried.

The obtained XPS spectra in the energy range characteristic forbinding energy of sulphur 2p, boron 1s and oxygen 1s electronsas well as deconvoluted peaks for these elements are shown inFig. 12. Subsequent analysis of the deconvoluted spectra resulted indetermination of stoichiometry of the polymers. For all depositedfilms, the XPS spectra reveal the presence of a peak at 164.1 eVcharacteristic for the binding energy of the 2p3/2 electrons of sul-phur, similarly as that observed for thiophene and polythiophene(curves 1–3 in Fig. 12) [42,43]. As expected, films of the BT–TX andBT–CT–TX co-polymers also contain boron indicating that, indeed,BT monomer is present in these films (curves 1′ and 3′ in Fig. 12).Taking into account that the TX monomer molecule contains 6 sul-phur atoms and the BT molecule 4 sulphur atoms and 1 boron atom,one can estimate the mole ratio of the BT to TX monomers in theobtained BT–TX co-polymer as 1:0.85, which is only slightly lower

than that of 1:0.92 expected on the basis of the composition of thesolution used for the electropolymerisation.

The CT–TX co-polymer does not contain a clear marker atom,since there is only a crown ether functional moiety in the CTmonomer. Therefore, estimation of the monomer mole ratio using

), in the 0.1 M TBAP solution of CH2Cl2, at 0.2 V s−1; (b) polymerisation attempts,0.5 mM MT, and 0.5 mM TX (co-electropolymerisation, curve 3′). Scan number is

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8362 F. Sannicolò et al. / Electrochimica Acta 55 (2010) 8352–8364

Fig. 11. Multiscan CV curves for co-electropolymerisation of 1 mM (a) BT, (b) CT or (c) BT and CT in the presence of 1 mM TX on the Pt electrode (0.00785 cm2), in the 0.1 MTBAP solution of CH2Cl2, at 0.1 V s−1. Scan number is indicated at each curve.

Fig. 12. X-ray photoelectron spectra in the region of (1, 2, 3) sulphur 2p, (1′ , 2′ , 3′) boron 1 s and (1′′ , 2′′ , 3′′) oxygen 1 s electrons for (1, 1′ , 1′′) BT–TX, (2, 2′ , 2′′) CT–TX and(3, 3′ , 3′′) BT–CT–TX co-polymers deposited onto the Au-coated glass slides during 36 CV cycles at the 0.2 V s−1 potential scan rate in the 0.1 M (TBA)PF6 in CH2Cl2 solutionof mixed monomers. Deconvoluted peaks are indicated by dashed curves.

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PS is more difficult. This is because the oxygen signal in the spec-rum of this monomer may not only originate from the crown ether

oiety but also from different other sources, such as the quartz sub-trate, gold oxides, overoxidized thiophene units, etc. All of themay introduce an error to the stoichiometry calculations. Neverthe-

ess, taking for the calculations only intensities of peaks at 532.7 and33.5 eV, which can be ascribed to binding energy of 1s electronsf oxygen in ethers (curves 2′′ and 3′′ in Fig. 12) [44] and comparinghe resulting atomic ratio of oxygen to sulphur, one can estimatehe CT-to-TX mole ratio as 1:0.45. This ratio is much smaller thanhat of 1:1.07, which follows from the composition of solution usedor electropolymerisation indicating that the CT monomers mayreferentially bind among themselves rather than with TX.

Moreover, the XPS spectra for all studied films exhibit a peakt ∼402.5 eV. This peak is characteristic for binding energy of 1slectrons of nitrogen in quarternary amines [45]. Apparently, TBA+

ation is also present in the film. Furthermore, there are peaks char-cteristic for fluorine and phosphorus indicating the presence ofhe PF6

− anions. This XPS behaviour indicates that a small amountf the supporting electrolyte still remains in the film even afterxtensive washing. In all studied samples an excess of PF6

− overBA+ can be surmised from the atomic content of nitrogen andhosphorus. Evidently, the deposited polymers are slightly posi-ively charged and they retain some supporting electrolyte anionso preserve electoneutrality.

Our results demonstrate the impressive effectiveness of the TXonomer as a crosslinker and 3D promoter in electropolymerisa-

ion and conducting polymer film formation with co-monomersntrinsically showing low aptitude for the elecropolymerisa-ion process, or forming polymer films of low conductivity.his property is even more attractive since it appears thathe film resulting from polymerisation fully retains (if it doesot improve) the functional properties of the target-orientedo-monomer. In the last months, while the present electrochem-cal characterization of TX was still in progress, a co-polymer,eposited by electropolymerisation of CT and TX, molecularly

mprinted with melamine, has outstandingly performed as theecognition element of a piezomicrogravimetric chemosensor forelective determination of melamine, in terms of linear concen-ration range (from 5 nM to at least 1 mM melamine), detectionimit (5 nM melamine), and selectivity (discriminating melaminerom the cyanuric acid, cyromazine, and ammeline interferingompounds) [13]. The chemosensor is currently being patented46].

. Conclusions

2,2′-Bis(2,2′-bithiophene-5-yl)-3,3′-bithianaphthene hasroven to be an effective crosslinker and 3D promoter in the elec-ropolymerisation involving co-monomers intrinsically showingimited aptitude for the process or forming films of low conduc-ivity. This attractive ability stems from combination of its (i)igh conjugation efficiency in each planar moiety, (ii) intrinsicD structure on account of the presence of the central node,nd (iii) intrinsic regioselectivity in electropolymerisation onccount of the positions of the two available free �-thiopheneites.

After the first successful analytical application for the develop-ent of a piezomicrogravimetric melamine chemosensor, we are

ooking forward to testing our co-monomer in a wider series ofpplicative cases. Moreover, we are also designing and character-

zing alternative 3D promoters featuring similar properties.

Finally, a further interesting feature of the new monomer is itsnherent asymmetry. In this light, separation of the TX enantiomersould afford the preparation of inherently chiral multithiopheneolymer films with high 3D character and outstanding mechan-

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Acta 55 (2010) 8352–8364 8363

ical properties, an attractive perspective for both analytical andoptoelectronic applications.

Acknowledgements

This work was financially supported by the Ministry of Sci-ence and Higher Education of Poland through the Project 548/6.PRUE/2008/7 (to W.K.). W.K. and P.M. gratefully acknowledge thefinancial support of the researcher’s exchange through the project“Novel conducting polythiophene polymers for electroanalyticaland optoelectronic applications” in the framework of the XIXExecutive Programme of Scientific and Technological Co-operationbetween Italy and Poland for the Years 2007–2009.

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