Synthesis and field-effect properties of α,ω -disubstituted sexithiophenes bearing polar groups

Synthesis and field-effect properties of a,v -disubstituted sexithiophenesbearing polar groups

Antonio Dell’Aquila,ac Piero Mastrorilli,a Cosimo Francesco Nobile,*a Giuseppe Romanazzi,b

Gian Paolo Suranna,a Luisa Torsi,*c Maria Cristina Tanese,c Domenico Acierno,d Eugenio Amendolae andPiero Moralesf

Received 2nd November 2005, Accepted 5th January 2006

First published as an Advance Article on the web 23rd January 2006

DOI: 10.1039/b515583e

The synthesis of sexithiophenes bearing amide or ester groups in the a,v-terminal positions is

described, along with their characterization in the solid state. The influence of the functional

group on mobilities and on/off ratios of the organic FET devices was investigated. The oligomer

bearing the ester functional group separated from the sexithiophene core by an ethylene spacer

showed a hole field-effect mobility as high as 0.012 cm2 V21 s21, which is among the highest

reported so far for organic FETs using sexithiophenes modified with polar groups.

Introduction

An ever increasing effort is being successfully directed towards

the synthesis of organic materials based on p-conjugated

oligomers, as they hold easily controllable properties that

render them suitable as conducting or semiconducting active

layers in devices such as photovoltaic cells,1 light emitting

diodes2 and organic field effect transistors (OFET).3,4 The first

reports on OFET date back to the mid-eighties,5 but the idea

that these devices could reach mobilities and on/off ratios

comparable to those of amorphous silicon was consolidated

only after the charge transport properties of a-sexithiophene

were discovered.6 Since then on, continuously increasing con-

sideration has been given to OFET based on both unsub-

stituted and substituted thiophene oligomers.7 In particular,

after the seminal work by Garnier et al.,8 several reports have

dealt with the synthesis and characterization of well-defined

a,v-disubstituted sexithiophenes aiming at obtaining more

soluble and processable derivatives, while preserving or even

improving their electronic properties: for instance, field-effect

mobilities as high as 1.0 cm2 V21 s21 have been achieved using

a,v-dihexyl-sexithiophene.9

An intriguing and fast developing application of organic

thin-film transistors is their use as chemical sensors.10,11 In this

area, the role of functional groups attached to the polymer/

oligomer chain is very important, as it allows modulation of

the device selectivity towards chemically homologous

species.12 On the other hand, the key role of the side chains

in promoting the organic active layer adhesion/orientation on

the gate dielectric surface or in guiding the oligomer stacking

is well known.8 In this respect the presence of suitable side

groups might critically influence the molecular packing by

means, for instance, of hydrogen-type interactions. Despite

the scientific and technological interest towards oligothio-

phenes, to date only a few studies have dealt with the use

of thiophene oligomers bearing polar side groups in p-type

OFET devices , reporting hole mobilities not higher than

1024–1023 cm2 V21 s21.13 Comparable mobilities have been

recently reported also for OFET comprising a carbonyl sub-

stituted quaterthiophene as channel material. This figure of

merit became as high as 1022 cm2 V21 s21 when the substrate

was held at 70 uC during the organic thin-film evaporation.14

In this paper we report on our studies on the synthesis of

new a,v-disubstituted sexithiophenes, bearing polar functional

groups as amides or esters. The influence of these moieties on

the field effect properties of sexithiophenes was also addressed.

Experimental

Materials

The reactants were purchased by Aldrich, Acros or Fluka and

used without further purification. All manipulations were

carried out under inert nitrogen atmosphere using Schlenk

techniques unless otherwise specified and the solvents

used were carefully dried and freshly distilled according to

common laboratory practice. Flash chromatography was

performed using Merck Kieselgel 60 (230–400 mesh) silica

gel or Macherey-Nagel polygoprep 60-50 C18 silica gel

reversed phase.

Methods

1H NMR and 13C{1H} NMR spectra were recorded on a

Bruker Avance 400 and are reported in ppm using tetra-

methylsilane as standard. FT-IR spectra were recorded in KBr

aDepartment of Water Engineering and Chemistry (DIAC), Polytechnicof Bari, via Orabona 4, I-70125, Bari, Italy. E-mail: [email protected];Fax: +39 080 5963611; Tel: +39 080 5963608bDepartment of Civil and Environmental Engineering (DICA),Polytechnic of Bari, via Orabona 4, I-70125, Bari, ItalycDepartment of Chemistry, T.I.R.E.S. Centre of Excellence, Universityof Bari, via Orabona-4, I-70126, Bari, Italy.E-mail: [email protected]; Fax: +39 080 5442026;Tel: +39 080 5442092dDepartment of Materials and Production Engineering (DIMP),University of Naples Federico II, p.le Tecchio 80, I-80125, Naples, ItalyeInstitute of Composite and Biomedical Materials (IMCB), Italy’sNational Research Council, p.le Tecchio 80, I-80125, Naples, ItalyfENEA, Unita Materiali e Nuove Tecnologie, Centro Ricerche dellaCasaccia, 00060 S. Maria di Galeria, Roma, Italy

PAPER www.rsc.org/materials | Journal of Materials Chemistry

This journal is � The Royal Society of Chemistry 2006 J. Mater. Chem., 2006, 16, 1183–1191 | 1183

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http://dx.doi.org/10.1039/b515583e

http://pubs.rsc.org/en/journals/journal/JM

http://pubs.rsc.org/en/journals/journal/JM?issueid=JM016012

on a Bruker Vector 22 spectrometer. UV-Vis spectra were

measured with a Kontron Uvikon 942. GC-MS data (EI,

70 eV) were acquired on a HP 6890 instrument equipped with a

HP-5MS capillary column (crosslinked 5% Ph Me siloxane)

30.0 m 6 250 mm 6 0.25 mm coupled with a HP 5973 mass

spectrometer. LC-MS analyses were performed on selected

compounds by direct injection on an Agilent HPLC system

equipped with DAD, autosampler and MS system (Agilent

1100 LC-MS SL series) using an atmospheric pressure

chemical ionization interface (APCI). APCI conditions:

positive ion mode, flow rate 0.5 mL min21, nitrogen as

nebulizing and drying gas, nebulizer pressure 60 psi, vaporizer

temperature 350 uC, corona current 4.0 mA, drying-gas flow

5 L min21, drying gas temperature 350 uC, capillary voltage

4000 V. Mass spectrometry data were acquired in the scan

mode (mass range m/z = 200–3000). C, H, N elemental

analyses were carried out with a Eurovector CHNS-O

elemental analyser.

Thermogravimetric analyses (TGA) were carried out under

a nitrogen flow with a TA Instruments 2950 thermobalance at a

scanning rate of 10 uC min21; differential scanning calorimetry

(DSC) analyses were carried out with a TA Instruments 2920

thermosystem at a scanning rate of 10 uC min21. Polarised

optical microscopy (POM) investigations were performed

using a Reichert-Jung Polyvar II polarizing microscope

equipped with a Linkam TH600 heating stage.

An atomic force microscope (AFM, Quesant Nomad) was

used to study the surface morphology of the sublimed 6T films.

Surface images were collected in intermittent contact mode,

with typically 20 N m21 silicon cantilever probes. Care was

taken to minimize the cantilever oscillation amplitude, in order

to avoid surface damage. This was checked by subsequent

imaging of the same area by first smaller and then larger scans.

Device fabrication

Field effect transistors were fabricated in a bottom gate top

contact configuration. A cross-sectional schematic of the

device configuration is reported in Fig. 1.

The devices comprised a heavily n-doped silicon substrate

(resistivity: 0.02–1 V cm21) coated by a 300 nm thick SiO2

thermal oxide layer (Ci = 10 nF cm2). A gold ohmic contact

created directly on the silicon substrate functions as the gate

contact (G). The organic 6T layers were deposited by vacuum

sublimation at room temperature (1026 Torr) onto the SiO2

layer until a 200–300 nm film was formed. The film thickness

was measured by a KLA Tencor T10 profilometer. No

patterning of the active layer was performed in order to

confine it to the channel region. This is a very convenient and

easy thin-film transistor fabrication procedure as no litho-

graphic pattering is required. It has, however, the disadvantage

that a small leakage current can add to the source–drain

current (Ids) flowing in the channel region. The gold source (S)

and drain (D) contacts were defined by thermal evaporation

on the organic layer through a shadow mask. The resulting

channel length is L = 200 mm and the width is W = 4 mm. The

current–voltage (Ids–Vds) characteristics were evaluated with

an Agilent 4155B semiconductor parameter analyzer operating

the device in a common source configuration and in the

accumulation mode and the potentials (both Vds and Vg) were

ranged between 0 V and 2100 V. Cyclic voltammetry

measurements were carried out under inert nitrogen atmo-

sphere with an Autolab potentiostat PGSTAT 10 using a

three-electrode cell (a glassy carbon as working electrode, an

Ag wire as reference electrode, and a platinum counter

electrode). The rate scan was 200 mV s21 and the supporting

electrolyte was a n-Bu4NClO4 (0.1 M) solution in CHCl3.

Potentials are referenced to the Fc/Fc+ couple.

Synthesis

Some of the precursors were synthesized following literature

procedures, namely: 59-bromo-2,29-bithiophene-5-carbonyl

chloride (1),15 5,59-bis-trimethylstannyl-2,29-bithiophene (8),16

5-bromo-2,29-bithiophene (9),17 decylmethylamine,18 and

Pd0(PPh3)4.19 The syntheses of the other compounds were

carried out as reported below.

N-Decyl-59-bromo-2,29-bithiophene-5-carboxamide (2). A

solution of 1 (720 mg, 2.34 mmol) in methylene chloride

(180 cm3) was added dropwise over 1 h to an ice-cooled

solution of decylamine (369 mg, 2.34 mmol) in CH2Cl2 (20 cm3)

containing triethylamine (710 mg, 7.02 mmol). After the

addition, the reaction was allowed to reach room temperature

and kept under stirring overnight. The reaction mixture was

subsequently washed with 100 cm3 of 5%w HCl, 100 cm3 of

saturated NaHCO3 solution, and 100 cm3 of brine. The

organic layer was dried over anhydrous Na2SO4, and the

solvent removed in vacuo. The crude was purified by flash

chromatography on silica gel using methylene chloride as

eluent yielding 2 (802 mg, 80%) as a white solid. Mp 135.7–

136.2 uC; nmax(KBr pellet)/cm21 3331, 3096, 3094, 2956, 2930,

2916, 2850, 1628, 1545, 1534, 1510, 1478, 1470, 1454, 1416,

1319, 1293, 1268, 809, 793 and 742; dH(400 MHz; CDCl3;

Me4Si) 0.87 (t, J = 7.0 Hz, 3H, CH3), 1.21–1.38 (m, 14H), 1.60

(pseudoquintet, J = 7.0 Hz, 2H, -CH2CH2CH2NHCO), 3.42

(m, 2H, -CH2NHCO), 5.90 (br t, J = 5.7 Hz, 1H, -NH-), 6.98–

7.00 (m, 2H), 7.04 (d, J = 4.0 Hz, 1H) and 7.34 (d, J = 4.0 Hz,

1 H); dC(100.6 MHz; CDCl3; Me4Si) 14.12, 22.67, 26.95, 29.30,

29.53, 29.68, 31.87, 40.12, 112.43, 123.93, 124.95, 128.26,

130.85, 137.66, 137.86, 140.42 and 161.32; m/z (EI, 70 eV) 429

(M+, 29), 289 (M+ 2 CH2LCH(CH2)7CH3, 41), 273 (M+ 2

NH(CH2)9CH3, 100) and 245 (M+ 2 NH(CH2)9CH3 2 CO, 8).

N-Decyl-N-methyl-59-bromo-2,29-bithiophene-5-carboxamide

(3). The synthesis of 3 was carried out by the same procedure

outlined for 2 using 697 mg (2.26 mmol) of 1, 388 mg

(2.26 mmol) of decylmethylamine, and triethylamine (686 mg,

6.78 mmol) respectively. The product 3 was obtained as a white

solid (850 mg, 85%). Mp 53.0–54.0 uC; nmax(KBr pellet)/cm21Fig. 1 Bottom gate top contact OFET device structure.

1184 | J. Mater. Chem., 2006, 16, 1183–1191 This journal is � The Royal Society of Chemistry 2006

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3074, 2952, 2914, 2850, 1589, 1515, 1495, 1459, 1428, 1404,

1315, 1260, 1199, 1109, 1082, 1048, 971, 864, 785, 735 and 462;

dH(400 MHz; CDCl3; Me4Si) 0.88 (t, J = 7.0 Hz, 3H, CH3),

1.23–1.33 (m, 14H), 1.65 (pseudoquintet, J = 7.0 Hz, 2H,

-CH2CH2CH2N-), 3.16 (br s, 3H, N-CH3), 3.52 (m, 2H,

-CH2N-), 6.96–6.99(m, 2H), 7.02 (d, J = 4.0 Hz, 1H) and 7.22

(br s, 1H); dC(100.6 MHz; CDCl3; Me4Si) 14.10, 22.67, 26.69,

29.28, 29.32, 29.52, 31.87, 112.10, 123.29, 124.76, 129.48,

130.85, 137.90, 138.16, 139.45 and 163.50; m/z (EI, 70 eV)

443 (M+, 15), 303 (M+2 CH2LCH(CH2)7CH3, 14), 273 (M+ 2

CH3N(CH2)9CH3, 100) and 245 (M+ 2 CH3N(CH2)9CH3 2

CMO, 10).

N-Butyl-59-bromo-2,29-bithiophene-5-carboxamide (4). The

synthesis of 4 was carried out by the same procedure outlined

for 2 using 922 mg (3.00 mmol) of 1, 227 mg (3.10 mmol) of

n-butylamine, and triethylamine (910 mg, 9.00 mmol) respec-

tively. The product 4 was obtained as a white solid (692 mg,

67%). Mp 113.2–115.8 uC; nmax(KBr pellet)/cm21 3332, 3095,

3078, 2955, 2929, 2901, 2870 , 1629, 1536, 1506, 1455, 1302,

1256, 809, 794, 742 and 595; dH(400 MHz; CDCl3; Me4Si) 0.95

(t, J = 7.4 Hz, 3H, CH3), 1.40 (m, 2H, CH2CH2CH3), 1.59 (m,

2H, CH3CH2CH2CH2N-), 3.43 (td, J1 = 7.1 Hz, J2 = 5.8 Hz,

2H, CH3CH2CH2CH2NH-), 5.96 (br s, 1H, -NH-), 6.97–6.99

(m, 2H), 7.03 (d, J = 4.0 Hz, 1H) and 7.35 (d, J = 4.0 Hz, 1H);

dC(100.6 MHz; CDCl3; Me4Si) 13.77, 20.12, 31.75, 39.72,

112.41, 123.91, 124.93, 128.27, 130.8, 137.65, 137.85, 140.42

and 161.36; m/z (EI, 70 eV) 345 (M+, 38), 289 (M+ 2

CH2LCHCH2CH3, 42), 273 (M+ 2 NH(CH2)3CH3, 100) and

245 (M+ 2 NH(CH2)3CH3 2 CMO, 10).

N-Butyl-N-methyl-59-bromo-2,29-bithiophene-5-carboxamide

(5). The synthesis of 5 was carried out by the same procedure

reported for 2 using 671 mg (2.18 mmol) of 1, 200 mg

(2.29 mmol) of butylmethylamine, and triethylamine (662 mg,

6.54 mmol) respectively. The product 5 was obtained as a white

solid (494 mg, 63%). Mp 48.1–49.3 uC; nmax(KBr pellet)/cm21

3102, 3074, 2949, 2925, 2869, 2857, 1597, 1513, 1486, 1452,

1424, 1401, 795 and 731; dH(400 MHz; CDCl3; Me4Si) 0.95 (t,

J = 7.4 Hz, 3H, CH3), 1.35 (m, 2H, CH2CH2CH3), 1.64

(pseudoquintet, J = 7 Hz, 2H, CH3CH2CH2CH2N-), 3.3 (br s,

3H, N-CH3), 3.53 (t, J = 7.6 Hz, 2H, CH3CH2CH2CH2N-),

6.97–6.99 (m, 2H), 7.02 (d, J = 4.0 Hz, 1H) and 7.22 (br d, J =

3.8 Hz, 1H); dC(100.6 MHz; CDCl3; Me4Si) 13.83, 19.94,

22.68, 29.68, 37.35, 49.20, 112.11, 123.31, 124.79, 129.56,

130.81, 137.17, 137.88, 139.49 and 163.56; m/z (EI, 70 eV)

359 (M+, 20), 303 (M+ 2 CH2LCHCH2CH3, 9), 273 (M+ 2

CH3N(CH2)3CH3, 100) and 245 (M+ 2 CH3N(CH2)3CH3 2

CMO, 12).

Butyl-59-bromo-2,29-bithiophene-5-carboxylate (6). A toluene

solution (17 cm3) containing butan-1-ol (246 mg, 3.32 mmol), 1

(1.022 g, 3.32 mmol), and pyridine (295 mg, 3.73 mmol) was

refluxed under stirring overnight. After cooling to room

temperature, the solution was poured into water and extracted

with diethyl ether (3 6 25 cm3). The collected organic layers

were dried over anhydrous Na2SO4, and the solvent removed

in vacuo. The crude residue was then purified by flash chro-

matography on silica gel using methylene chloride as eluent

yielding 6 (1.126 g, 98%) as a pale yellow solid. Mp 26–28 uC;

nmax(KBr pellet)/cm21 3101, 3069, 2956, 2938, 2874, 1705,

1514, 1446, 1419, 1295, 1281, 1251, 1097, 783 and 748;


1.46 (m, 2H, CH2CH2CH3), 1.73 (m, 2H, CH3CH2CH2CH2O-),

4.3 (t, J = 6.7 Hz, 2H, CH3CH2CH2CH2O-), 6.99–7.02 (m,

2H), 7.06 (d, J = 3.7 Hz, 1H) and 7.67 (d, J = 3.9 Hz, 1H);

dC(100.6 MHz; CDCl3; Me4Si) 13.73, 19.18, 30.71, 65.16,

112.85, 124.05, 125.21, 130.91, 132.25, 133.98, 137.80, 142.74

and 162.06; m/z (EI, 70 eV) 346 (M+, 32), 290 (M+ 2

CH2LCHCH2CH3, 100), 273 (M+ 2 O(CH2)3CH3, 35) and

245 (M+ 2 O(CH2)3CH3 2 CMO, 14).

Decyl-59-bromo-2,29-bithiophene-5-carboxylate (7). The

synthesis of 7 was carried out by the same procedure reported

for 6 using 1.012 g (3.29 mmol) of 1, 522 mg (3.29 mmol) of

decan-1-ol, and pyridine (300 mg, 3.79 mmol) respectively.

Product 7 was obtained as a pale yellow solid (1.174 g, 83%).

Mp 47–48.5 uC; nmax(KBr pellet)/cm21 3093, 3076, 2960, 2927,

2853, 1695, 1510, 1454, 1420, 1286, 1106, 787 and 744;


1.27–1.45 (m, 14H), 1.74 (pseudoquintet, J = 6.7 Hz, 2H,

CH3(CH2)7CH2CH2O-), 4.28 (t, J = 6.4 Hz, 2H, CH3(CH2)7-

CH2CH2O-), 6.99–7.03 (m, 2H), 7.07 (d, J = 4.0 Hz, 1H) and

7.67 (d, J = 4.0 Hz, 1H); dC(100.6 MHz; CDCl3; Me4Si) 14.11,

22.67, 25.93, 28.65, 29.23, 29.29, 29.50, 29.52, 31.88, 65.43,

112.85, 124.05, 125.20, 130.91, 132.27, 133.98, 137.81, 142.73

and 162.01; m/z (EI, 70 eV) 430 (M+, 27), 290 (M+ 2

CH2LCH(CH2)7CH3, 100), 273 (M+ 2 O(CH2)9CH3, 34), 245

(M+ 2 O(CH2)9CH3 2 CMO, 10).

N,N9-Bis-decyl-2,29;59,20;50,2-;5-,29-;59-,20--sexithiophene-

5,50--dicarboxamide (6Ta). To a 25 cm3 three-necked round-

bottomed flask were added 754 mg (1.76 mmol) of 2, 431 mg

(0.88 mmol) of 5,59-bis-trimethylstannyl-2,29-bithiophene (8),

and dry dimethylacetamide (DMA, 7.0 cm3). The solution was

purged from residual traces of oxygen by three freeze–pump–

thaw cycles and subsequently 20 mg (1.7 6 1022 mmol) of

Pd(PPh3)4 were added. The mixture was then heated at 120 uCand kept under stirring overnight. After cooling to room

temperature, the product formed as a red precipitate was

collected and washed on a Soxhlet using hot methanol,

acetone, chloroform and hexane to afford 6Ta as a red powder

(455 mg, 60%). Further purification was achieved by vacuum

sublimation. nmax(KBr pellet)/cm21 3360, 3073, 3061, 2955,

2918, 2872, 2850, 1627, 1556, 1537, 1252, 1496, 1467, 1439,

1322, 1266, 1074, 841, 818, 793 and 741; Found: C, 64.12; H,

6.30; N, 3.14%. Calc. for C46H56N2O2S6: C, 64.14; H, 6.55; N,

3.25%.

N,N9-Bis-decyl-N,N9-bis-methyl-2,29:59,20:50,2-:5-,29-:59-,

20--sexithiophene-5,50--dicarboxamide (6Tb). To a 25 cm3

three-necked round-bottomed flask was added 800 mg

(1.81 mmol) of 3, 443 mg (0.90 mmol) of 5,59-bis-trimethyl-

stannyl-2,29-bithiophene (8), and dry DMA (7.5 cm3). The

solution was purged from residual traces of oxygen by

three freeze–pump–thaw cycles and subsequently 21 mg

(1.8 6 1022 mmol) of Pd(PPh3)4 were added. The mixture

was then heated at 120 uC and kept under stirring overnight.


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After cooling to room temperature, the solid was collected

and washed several times on a Soxhlet with hot methanol,

acetone, and hexane to afford 6Tb as a red powder (576 mg,

72%). Further purification was achieved by vacuum sublima-

tion. nmax(KBr pellet)/cm21 3075, 3057, 2957, 2919, 2873,

2850, 1600, 1529, 1502, 1485, 1454, 1437, 1404, 1322,

1306, 1080, 1069, 840, 791 and 735; lmax(CHCl3)/nm 443;

m/z(APCI) 889.3 ([M + H]+ requires 889.31); Found: C, 64.59;

H, 6.76; N, 3.14%. Calc. for C48H60N2O2S6: C, 64.82; H, 6.80;

N, 3.15%.

N,N9-Bis-butyl-2,29:59,20:50,2-:5-,29-:59-,20--sexithiophene-

5,50--dicarboxamide (6Tc). The synthesis of 6Tc was carried

out by the same procedure reported for 6Ta using 650 mg

(1.89 mmol) of 4, 462 mg (0.94 mmol) of 8, 22 mg (1.9 61022 mmol) of Pd(PPh3)4, and 8.0 cm3 of dry DMA. The

title product was obtained as a red powder (423 mg, 65%).

Further purification was achieved by vacuum sublimation.


1628 , 1538, 1495, 1438, 1295, 841, 792 and 742; Found: C,

58.84; H, 4.56; N, 3.97%. Calcd for C34H32N2O2S6: C, 58.92;

H, 4.65; N, 4.04%.

N,N9-Bis-butyl-N,N9-bis-methyl-2,29:59,20:50,2-:5-,29-:59-,

20--sexithiophene-5,50--dicarboxamide (6Td). The synthesis of

6Td was carried out by the same procedure reported for 6Tb

using 452 mg (1.26 mmol) of 5, 310 mg (0.63 mmol) of 8, 15 mg

(1.3 6 1022 mmol) of Pd(PPh3)4, and 5.0 cm3 of dry DMA.

The title product was obtained as a red powder (363 mg, 80%).



1599, 1528, 1501, 1485, 1454, 1403, 1078, 841, 791 and 734;


H, 4.98; N, 3.87%. Calc. for C36H36N2O2S6: C, 59.96; H, 5.03;

N, 3.88; O, 4.44%.

Bis-butyl 2,29:59,20:50,2-:5-,29-:59-,20--sexithiophene-5,50--

dicarboxylate (6Te). The synthesis of 6Te was carried out by

the same procedure reported for 6Tb using 1.020 g (2.95 mmol)

of 6, 726 mg (1.47 mmol) of 8, 34 mg (2.9 6 1022 mmol) of

Pd(PPh3)4, and 12.0 cm3 of dry DMA. The title product was

obtained as a red powder (766 mg, 75%). Further purification

was achieved by vacuum sublimation. nmax(KBr pellet)/cm21

3081, 3063, 2960, 2930, 2900, 2870, 1691, 1527, 1454, 1437,

1286, 1093, 835, 787 and 744; lmax(CHCl3)/nm 451; m/z

(APCI) 695.1 ([M + H]+ requires 695.05); Found: C, 58.45; H,

4.32%. Calc. for C34H30O4S6: C, 58.76; H, 4.35%.

Bis-decyl 2,29:59,20:50,2-:5-,29-:59-,20--sexithiophene-5,50--

dicarboxylate (6Tf). The synthesis of 6Tf was carried out by

the same procedure reported for 6Tb using 974 mg (2.27 mmol)

of 7, 558 mg (1.13 mmol) of 8, 27 mg (2.3 6 1022 mmol) of

Pd(PPh3)4, and 10.0 cm3 of dry DMA. The title product was

obtained as a red powder (878 mg, 90%). Further purification

was achieved by vacuum sublimation. nmax(KBr pellet)/cm21

3071, 3035, 2951, 2917, 2844, 1712, 1532, 1458, 1441, 1321,

1278, 1248, 1097, 839, 796 and 749; lmax(CHCl3)/nm 451;


H, 6.25%. Calc. for C46H54O4S6: C, 64.00; H, 6.30%.

Ethyl (2E)-3-(2,29-bithien-5-yl)acrylate (10). To a solution of

5-bromo-2,29-bithiophene 9 (2.000 g, 8.16 mmol) in DMF

(12 cm3), ethyl acrylate (1.144 g, 11.42 mmol), triethylamine

(2.32 g, 22.84 mmol) and Pd(PPh3)4 (94.3 mg, 8.2 61022 mmol) were added respectively. The mixture was heated

at 120 uC and kept under stirring overnight. After cooling to

room temperature, the solution was poured into water and

extracted with diethyl ether. The organic layers were collected,

washed with water and dried over anhydrous Na2SO4.

Evaporation of the solvent and flash chromatography on

silica gel using petroleum ether–methylene chloride (60 : 40) as

eluent afforded pure 10 as a bright orange solid (1.51 g, 70%).

Mp 60.0–61.0 uC; nmax(KBr pellet)/cm21 3108, 3076, 3048,

3023, 2970, 2922, 2901, 2863, 1763, 1752, 1707, 1617, 1547,

1476, 1464, 1453, 1441, 1281, 1274, 1263, 1209, 1164, 1112,

1099, 1044, 1032, 915, 873, 800 and 695; dH(400 MHz; CDCl3;

Me4Si) 1.32 (t, J = 7.2 Hz, 3H, CH3), 4.25 (q, J = 7.2 Hz, 2H,

OCH2CH3), 6.18 (d, J = 15.7 Hz, 1H, LCH), 7.03 (dd, J1 =

5.1 Hz, J2 = 3.7 Hz, 1H), 7.11 (d, J = 3.8 Hz, 1H), 7.15 (d,

J = 3.8 Hz, 1H), 7.22 (dd, J1 = 3.7 Hz, J2 = 1.1 Hz), 7.26 (dd ,

J1 = 5.1 Hz, J2 = 1.1 Hz, 1H) and 7.71 (d, J = 15.7 Hz, 1H,

LCH); dC(100.6 MHz; CDCl3; Me4Si) 14.32, 60.49, 116.61,

124.30, 124.74, 125.56, 128.06, 132.05, 136.69, 136.81, 138.16,

140.25 and 166.83; m/z(EI, 70 eV) 264 (M+, 100), 236 (15), 219

(M+ 2 OCH2CH3, 58) and 192 (74).

Ethyl 3-(2,29-bithien-5-yl)propanoate (11). To a solution of

10 (661 mg, 2.5 mmol) in MeOH (50 cm3) was added 266 mg of

supported Pd (10 wt% on activated charcoal) and hydro-

genated in a stainless-steel autoclave at 25 bar of pressure; the

substrate conversion was monitored by GC-MS. After reaction

completion, the solution was filtered through a short plug of

Celite and the solvent evaporated off. The residue was purified

by flash chromatography on silica gel using petroleum ether–

methylene chloride (50 : 50) as eluent affording 11 as a

yellowish oil (653 mg, 98%). nmax(KBr pellet)/cm21 3107, 3069,

2980, 2960, 2928, 2871, 2855, 1732, 1638; 1519, 1465, 1444,

1427, 1374, 1351, 1298, 1256, 1176, 1122, 1095, 1081, 1037,

941, 910, 887, 872, 838, 800 and 695; dH(400 MHz; CDCl3;

Me4Si) 1.26 (t, J = 7.2 Hz, 3H, CH3), 2.68 (t, J = 7.5 Hz, 2H,

CH2CH2), 3.13 (t, J = 7.5 Hz, 2H, CH2CH2), 4.16 (q, J =

7.2 Hz, 2H, OCH2CH3), 6.72 (br d, J = 3.7 Hz, 1H), 6.97–6.99

(m, 2H), 7.09 (dd, J1 = 3.7 Hz, J2 = 1.1 Hz, 1H) and 7.17 (dd,

J1 = 5.1 Hz, J2 = 1.1 Hz, 1H); dC(400 MHz; CDCl3; Me4Si)

14.21, 25.37, 35.94, 60.63, 123.26, 123.45, 125.42, 127.69,

135.54, 137.60, 142.45 and 172.28; m/z(EI, 70 eV) 266 (M+, 98),

237 (M+ 2 CH2CH3, 20), 192 (81) and 179 (100).

Ethyl 3-(59-bromo-2,29-bithien-5-yl)propanoate (12). A solu-

tion of NBS (392 mg, 2,2 mmol) in CH2Cl2 (30 cm3) was added

dropwise over a period of 2 h to a solution of 11 (586 mg,

2.2 mmol) in CH2Cl2 (25 cm3) kept at 0 uC and in the dark.

After the addition the solution was allowed to reach room

temperature and was held overnight. The mixture was then

poured onto ice and extracted several times with CH2Cl2. The

organic phases were collected, washed with water, and dried

over Na2SO4. After solvent evaporation, the crude residue was

purified by chromatography on reverse phase (RP-18) silica

using methanol as eluent affording pure 12 as a pale yellow


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solid (722 mg, 95%). Mp 49–50 uC; nmax(KBr pellet)/cm21

3072, 3056, 2980, 2960, 2928, 2871, 2855, 1725, 1520, 1463,

1446, 1428, 1396, 1380, 1345, 1314, 1277, 1199, 1059, 1037,

969, 873, 803 and 789; dH(400 MHz; CDCl3; Me4Si) 1.25 (t, J =

7.2 Hz, 3H, CH3), 2.67 (t, J = 7.5, 2H, CH2CH2), 3.11 (t, J =

7.5, 2H, CH2CH2), 4.15 (q, J = 7.2 Hz, 2H, OCH2CH3), 6.75

(d, J = 3.7 Hz, 1H), 6.83 (d, J = 3.8 Hz, 1H), 6.90 (d, J =

3.7 Hz, 1H) and 6.93 (d, J = 3.8 Hz, 1H); dC(100.6 MHz;

CDCl3; Me4Si) 14.19, 25.34, 35.86, 60.65, 110.45, 123.31,

123.75, 125.50, 130.49, 134.50, 139.08, 143.03 and 172.18;

m/z(EI, 70 eV) 344 (M+, 58), 315 (7), 270 (30) and 257 (100).

Bis-ethyl 3-(2,29:59,20:50,2-:5-,29-:59-,20--sexithien-5,50--

diyl)dipropanoate (6Tg). The synthesis of 6Tg was carried out

by the same procedure reported for 6Tb using 656 mg

(1.90 mmol) of 12, 467 mg (0.95 mmol) of 8, 22 mg (1.9 61022 mmol) of Pd(PPh3)4, and 8 cm3 of dry DMA. The

product 6Tg was obtained as a red powder (462 mg, 70%).



1441, 1380, 1.352, 1.321, 1298, 1.179, 1097, 1070, 1034, 858,

838 and 793; lmax(CHCl3)/nm 439; m/z(APCI) 695.1 ([M + H]+

requires 695.05); Found: C, 58.43; H, 4.38%. Calc. for

C34H30O4S6: C, 58.76; H, 4.35%.

Results and discussion

Synthesis

The structure of the novel sexithiophenes synthesized in this

study are reported in Fig. 2.

The sexithiophene 6Ta is functionalized in the a,v-positions

with two amide groups bearing decyl alkyl chains. In order to

investigate the possible effect of hydrogen bonding between

amide groups of different sexithiophene functionalities on the

properties of the materials, compound 6Tb, bearing a methyl

group instead of the amide hydrogen, was synthesized. The

next target was to synthesise the oligomers 6Tc and 6Td

bearing a shorter (butyl) alkyl chain. The synthesis of 6Te

and 6Tf was carried out in order to compare the effect of the

ester group with respect to the amide group. Eventually, the

effect of a spacer between the ester functional group and

the sexithiophene core was investigated with the synthesis

of 6Tg.

The synthetic approach to the preparation of the 6Ta-g

sexithiophenes comprised a Stille coupling (vide infra) between

5,59-bis-trimethylstannyl-2,29-bithiophene (8) and suitable

bithiophene derivatives. The synthetic approach followed

for the 2-bromo-bithiophene precursors 2–7 is reported in

Scheme 1.

The bithiophene derivatives 2–7 were obtained in yields

ranging from 63 to 98% by reacting 59-bromo-2,29-bithio-

phene-5-carbonyl chloride (1)15 with the corresponding amines

or alcohols under Schotten–Baumann conditions. The synth-

esis of the 2-bromo-bithiophene derivative 12 required a

different approach, depicted in Scheme 2.

5-Bromo-2,29-bithiophene (9)17 was submitted to a Heck

coupling with ethyl acrylate affording ester 10 in 70% yield.

Hydrogenation of 10 was carried out in the presence of

supported palladium under a constant hydrogen pressure

(25 bar) yielding 11 in 98% yield. Bromination of 11 with

stoichiometric NBS afforded 12 in 95% yield. The structures of

1–12 were confirmed by IR, 1H and 13C NMR spectroscopy as

well as GC-MS.

The target sexithiophenes were obtained by means of a Stille

coupling, due to its versatility and its compatibility towards

amide and ester functional groups.20 The synthesis of the

functionalized sexithiophenes 6Ta-g is sketched in Scheme 3

along with the product yields (60–90%).

Fig. 2 Structures of the synthesised a,v-sexithiophenes.

Scheme 1 (i) Conditions for 2–5: reaction with 1.0 eq. of the

corresponding amines in the presence of triethylamine in CH2Cl2 at

rt. (ii) For 6 and 7: reaction with 1.0 eq. of the corresponding alcohols

in the presence of pyridine in toluene at reflux.

Scheme 2 (i) 1% mol/mol Pd(PPh3)4 with respect to the bromide 9 in

the presence of triethylamine in DMF at 120 uC; (ii) P(H2) = 25 bar,

10% mol/mol Pd on active charcoal in MeOH at rt; (iii) NBS (1 eq) in

CH2Cl2.


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The products precipitated from the reaction medium as

orange or red powders and were purified from soluble

byproducts by subsequent Soxhlet washings with methanol,

chloroform, acetone and n-hexane.

Compounds 6Tb, 6Td, 6Te, 6Tf and 6Tg were characterised

by APCI-mass spectrometry that confirmed the achievement

of the desired products. Elemental analyses of all oligomers

were in accordance with the proposed structures and

confirmed the high purity of all sexithiophenes. As expected,

all synthesised sexithiophenes are poorly soluble in common

organic solvents except hot DMSO, chlorobenzene and

o-dichlorobenzene. However, the solubility of 6Tb, 6Te, 6Tf,

and 6Tg in chloroform was good enough to measure their

UV-Vis spectra. The lmax are 443 (6Tb), 451 (6Te), 451 (6Tf)

and 439 (6Tg) nm. These absorptions are red-shifted with

respect to the lmax of unsubstituted sexithiophene (lit.,21

432 nm) because of the increase of conjugation length brought

about by the carbonyl groups directly attached to the

sexithiophene core in the case of 6Tb, 6Te, 6Tf. In the case

of 6Tg the inductive effect of the substituent may account for

the observed slight red shift.

Thermal behaviour

The thermal properties of 6Ta-g are summarized in Table 1.

The stability of all sexithiophenes was investigated by TGA,

that revealed high decomposition temperatures (at 5% weight

loss) between 333 and 399 uC. It can be noted, comparing the

Tdec of 6Ta,6Tb and those of 6Tc,6Td, that the substitution of

a hydrogen atom with a methyl group increased the thermal

stability of the sexithiophenes. On the other hand the

introduction of the ester groups lowered the decomposition

temperature. The thermal behaviour of all obtained sexithio-

phenes was also investigated by means of polarizing optical

microscopy (POM) and differential scanning calorimetry

(DSC) methods, and the main results are also summarized in

Table 1.

6Tb is the only compound that presented an evident fusion

point at 295.9 uC without passing through mesophases. For all

the other compounds, isotropization was concomitant to

partial decomposition. POM observation permitted the charac-

terisation of several observed thermal events as transitions to

unidentified smectic phases (SmX). In other cases, in particular

for 6Tg, the thermal events could not be further characterised

because the POM texture did not change in the course of

the transition. This behaviour can be interpreted either by

invoking a solid–solid transition or with the fusion of small

portions of crystalline sample deriving from the synthesis.

The difference in thermal behaviour between 6Ta and 6Tb is

noteworthy. The possibility of hydrogen bonding in solid 6Ta

seems to be responsible for the more complex thermal

behaviour probably related to a more structured organization.

Comparing the behaviour of 6Ta to that of 6Tc it can be

inferred that the shorter alkyl chain prevents the occurrence of

mesophases in the latter. Similar behaviour was observed also

for the ester substituted compounds, namely 6Te and 6Tf: both

sexithiophenes showed multiple transitions to smectic phases,

before isotropization occurs concomitantly to decomposition.

Increasing the ester chain length led to a lowering of the

transition temperatures. In Fig. 3 is displayed the change in

optical texture of 6Tf passing from the three observed SmX

phases.

The observation of such smectic mesophases indicates a

degree of order of the materials that reflects an ordered solid

state.22

Field-effect properties and surface morphology

The OFET devices have been fabricated as reported in the

Experimental section. The main figures of merit, namely the

on/off ratio and the field effect mobility (mFET), for the devices

are reported in Table 2. The transistor characteristics were

measured by polarizing the devices up to 2100 V. Negative

source–drain and gate voltage biases were applied as p-type

behavior was expected. All sexithiophene films were evapo-

rated on two sets of substrates, one of which was pre-treated

with hexamethyldisilazane (HMDS), aiming at improving the

adhesion of the organic layer to the SiO2 dielectric surface.4

No field-effect modulation on either treated or untreated

Scheme 3 (i) 2% mol/mol Pd(PPh3)4 with respect to the organo-

distannane (8) in dimethylacetamide at 120 uC.

Table 1 Thermal data for sexithiophenes 6Ta-6Tg

Compound Transition temperatures (uC) and enthalpies of transitions (in parentheses, J g21) TGAa/uC

6Ta 203.5b (6.1) 285.0b (3.2) 340.9c (68.9) 371.7d (dec.) 368.16Tb 295.9d (109.3) 398.76Tc 367.0d (dec.) 372.56Td 311.2c (111.2) 329.0d 384.36Te 210.5c (18.6) 267.9c (32.5) 343.4c (dec.) 341.26Tf 177.0c (26.9) 239.5c (27.2) 300.0c (10.8) 344.56Tg 159.2b (7.1) 178.9b (10.3) 332.9a At 5% weight loss. b The transitions observed cannot be evidenced in the POM, they may be solid–solid transitions or the fusion of a smallportion of the crystalline phases obtained in the course of the synthesis of the material. c Transition to a SmX mesophase. d Transition toisotropic phase.


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substrates could be observed for OFET obtained with 6Ta

and 6Tb bearing n-decylamide groups in positions a,v to the

sexithiophene unit. The use of shorter chain amides did not

result in better device performances: 6Tc exhibited a field

effect modulation, but mobilities (1026 cm2 V21 s21) and

on/off ratios (,10) were poor and were not improved by

surface pre-treatment with HMDS. No field effect modulation

could be observed in the case of the OFET obtained with

6Td either. Better results were obtained with sexithiophenes

bearing ester side chains (6Te,f). As a matter of fact mFET

increased by two orders of magnitude. Better on/off ratios

could be obtained for 6Te. Aiming at improving the OFET

characteristics, the synthesis of 6Tg was devised. The rationale

for the choice was to separate the sexithiophene oligomer from

the ester functionality. We expected better performances due

to the structural similarity of 6Tg to a dialkylsexithiophene,

that should consequently lead to a better solid state organiza-

tion. The electrical characteristics of the OFET fabricated

using 6Tg as active layer on an HMDS untreated SiO2

substrate are reported in Fig. 4.

Fairly well shaped transistor characteristics can be seen,

exhibiting clear linear and saturation regions along with a

significant increase in mobility with mFET reaching the value of

1.2 1022 cm2 V21 s21. The on/off ratio was also improved,

reaching the order of magnitude of 103. To the best of our

knowledge, such a mobility is among the highest reported so

far for organic FET using sexithiophenes modified with polar

groups.13,14 No improvement of the figures of merit could be

obtained by Si/SiO2 pretreatment with HMDS. In order to

gain insight into the different behaviour of esters 6Te and 6Tg,

we studied the electrochemical behaviour of these compounds

by carrying out cyclic voltammetry experiments aiming at

evaluating the influence of the electron withdrawing group on

the energy levels of the sexithiophenes: the estimate of the

HOMO level was carried out by measuring the half-wave

oxidation potential while the onset of UV-Vis absorption was

used for the calculation of the energy gaps. The results point

out that the HOMO levels are almost identical (6Te =

25.57 eV, 6Tg = 25.48 eV), while the LUMO level (6Te =

23.20 eV, 6Tg = 23.03 eV) is more stabilised by the direct

linkage to the carbonyl groups. This is a further confirmation

that the better performance of 6Tg may be related to the solid

state arrangement rather than to electronic effects.

AFM micrographs of the deposited films of esters 6Te, 6Tf,

and 6Tg deposited onto the Si/SiO2 substrate were taken. The

5 mm 6 5 mm micrographs reported in Fig. 5 revealed in all

cases a grain-like morphology, although in the cases of 6Te

and of 6Tg the morphology was considerably more uniform.

On the other hand, 6Tf exhibited a more dishomogeneous

morphology, presumably due to the presence of the longer

alkyl chains giving rise to more elongated grains, as previously

Fig. 3 POM images for compound 6Tf at different transition

temperatures: (a) 200 uC, (b) 250 uC and (c) 312 uC.

Table 2 Field effect properties of sexithiophenes 6Ta–6Tg

mFET/cm2 V21 s21 Ion/Ioff mFET/cm2 V21 s21 a Ion/Ioffa

6Ta — — — —6Tb — — — —6Tc 6 6 1026 — 5 6 1026 —6Td — — — —6Te 5 6 1024 102 7 6 1024 102

6Tf 4 6 1024 102 6 6 1024 106Tg 1.2 6 1022 103 1.4 6 1022 103

a Data obtained for OFET device treated with HMDS.

Fig. 4 I–V characteristics for the OFET with 6Tg as active layer.


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reported.11 The root mean square (RMS) deviations of the

measured topography are 0.87 nm for 6Te, 1.7 nm for 6Tg and

42 nm for 6Tf. AFM observations were also carried out on the

other 6T films, that showed a microstructured morphology as

well, the RMS values being 14.96 (6Ta), 2.266 (6Tb), 1.681

(6Tc), 0.6 nm (6Td). No morphological differences were

observed for thin films deposited on treated or untreated

Si/SiO2.

Conclusions

Seven new sexithiophenes bearing amide or ester function-

alities in the a,v-positions were synthesised and characterised,

and their properties as OFET active layers were investigated.

The amide substituted materials resulted in poor charge

transport features. Use of sexithiophenes functionalised with

ester moieties permitted the construction of OFET with

moderate hole mobilities. Isolation of the ester functionalities

from the sexithiophene unit by an ethylene spacer led to a

device with good modulation, which showed hole mobilities of

1.2 6 1022 cm2 V21 s21 and on/off ratios of 103. The latter

sexithiophene holds interesting properties for future use as an

active layer in chemically selective OFET sensors.

Acknowledgements

The FIRB project Micropolys (Italian MIUR) is gratefully

acknowledged for funding. We also thank Dr Giuseppe

Ciccarella for the APCI-mass spectrometric measurements,

Dr Francesco Marinelli for laboratory assistance and Dr

Roberto Grisorio for helpful discussions. We are indebted to

Professor Pynalisa Cosma (University of Bari) for carrying out

the cyclic voltammetry experiments.

References

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Synthesis and field-effect properties of α,ω -disubstituted sexithiophenes bearing polar groups