Synthesis and electrochemical properties of tetrathienyl-linked branched polymers with various aromatic cores

Electrochimica Acta 79 (2012) 154–161

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Synthesis and electrochemical properties of tetrathienyl-linked branched polymers with various aromatic cores Krzysztof R. Idzik a,b,g,∗ , Jaroslaw Frydel c , Rainer Beckert a , Przemyslaw Ledwon d , Mieczyslaw Lapkowski d,e , Carlo Fasting b , Carsten Müller f , Tobias Licha g a

Institute of Organic and Macromolecular Chemistry, Friedrich-Schiller University Jena, Humboldstraße 10, D-07743 Jena, Germany Institut für Chemie und Biochemie, Organische Chemie, Freie Universität Berlin, Takustr. 3, D-14195 Berlin, Germany c VENITUR Sp. z o.o., ul Wawozowa 34 B, 31-752 Krakow, Poland d Silesian University of Technology, Faculty of Chemistry, 44-100 Gliwice, Poland e Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 44-121 Gliwice, Poland f Institut für Chemie und Biochemie, Physikalische und Theoretische Chemie, Freie Universität Berlin, Takustr. 3, D-14195 Berlin, Germany g Geoscience Centre of the University of Göttingen, Department of Applied Geology, Goldschmidtstr. 3, 37077 Göttingen, Germany b

a r t i c l e

i n f o

Article history: Received 4 June 2012 Received in revised form 26 June 2012 Accepted 27 June 2012 Available online 4 July 2012 Keywords: Stille cross-coupling procedure Electrochemical polymerization Tetrathienyl-cross-linked polymers Cyclic voltammetry Fluorescence spectroscopy

a b s t r a c t A series of various tris(2,2 -bithiophen-5-yl)-aromatic derivatives were synthesized by Stille crosscoupling procedure. Their structures were characterized by 1 H NMR, 13 C NMR, and elemental analysis. DFT calculations for monomers were also performed. The optical properties of the synthesized materials as well as their energy levels were investigated by UV–vis absorption supported by fluorescence spectra and CV analysis. Oligomers obtained in the process of electropolymerization, possess a tetrathienyl bond with various aromatic and heteroaromatic cores. Electrochemical results confirm that the gained materials can apply successfully for a diversity of organic–electronic devices like organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic solar cells. © 2012 Elsevier Ltd. All rights reserved.

1. Introduction Organic ␲-conjugated structures containing thiophene units undergo both oxidation and reduction processes and play an important role in the search for new materials and their novel applications [1]. Thiophene-based polymers are among the most widely used -conjugated systems for organic electronic devices. For example, linear polythiophenes have both an excellent performance when acting as a building material of organic solar cells [2,3] and high hole mobility in organic field effect transistors [4]. Oligothiophenes with well defined structures have recently received a great deal of attention not only as an example of model compounds for conducting polythiophenes, but also as a new class of functional -electron systems. According to the literature, a variety of oligothiophenes have recently been synthesized [5–11]. Their molecular and crystal structures, as well as their electrochemical, optical, and self ordering properties have been studied.

∗ Corresponding author. Tel.: +49 15779202076; fax: +49 3641948212. E-mail addresses: [email protected] (K.R. Idzik), [email protected] (R. Beckert). 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.06.101

In addition, their potential application to field-effect transistors, photovoltaic (PV) systems, and organic electroluminescent devices has been investigated. Branched oligothiophenes, based on central phenyl core, have come to the fore over the last several years, acting both as monomers in cross-linked semiconducting polymers [12–15] and as components of conjugated dendrimers [16–18]. Star-shaped systems with donor–acceptor interactions based on an electron-withdrawing triazine core substituted at the 2, 4 and 6 positions with various electron-donating bithiophene branches, find applications in organic solar cells, crystal engineering [19–27] due to their broad internal charge transfer band at low energy. In this context, we present here also the synthesis of a series of planar donor–acceptor systems. Following Stille cross-coupling procedure we obtained various tris(2,2 -bithiophen-5-yl)-aromatic (8–12) derivatives in high yield (80–90%). We report also characterization of a whole series of synthesized compounds. Furthermore, we describe electropolymerization process of monomers substituted with various aromatic and heteroaromatic cores. The oxidative polymerization of the monomers 8–12 led to the formation of tetrathiophene-linked networks (Fig. 1). We expect better conductivity from system enriched with four thiophenes.

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155

S S R Br

Br

1

1

S R

R

R

S

R

1

R

1

S

Br

3, 4, 5

3, 8: R = R 1 = H 4, 9: R = OMe, R 1 = H 5, 10: R = R 1 = OMe

S

Cl

N

Cl N

8, 90% 9, 85% 10, 87%

N

S

Cl

N S

S

1

N

S

S

S

S

S

6 Sn(Bu)3

Br

Br

2

N

S

N N

N

S

11, 80% S Br

7

S

S S N N

N

S

12, 82%

S

Scheme 1. Synthesis of tris(2,2 -bithiophen-5-yl)-aromatic derivatives.

2. Experimental

were characterized by 1 H and 13 C NMR spectroscopy, supported by elemental analysis.

2.1. Materials All chemicals, reagents, and solvents were used as received from commercial sources without further purification, except tetrahydrofuran (THF) and toluene, which were distilled over sodium/benzophenone. 1 H NMR and 13 C NMR spectra were recorded in CDCl3 on 250 MHz liquid state Bruker spectrometer. Chemical shifts are denoted in ı unit (ppm) and referenced to the internal standard: TMS (tetramethylsilane) at 0.0 ppm. The splitting patterns are annotated as follows: s (singlet), d (doublet), t (triplet) and m (multiplet). Preparative column chromatography was carried out on glass columns of different sizes packed with silica gel: Merck 60 (0.035–0.070 mm). We present reaction routes for the synthesis of tris(2,2 bithiophen-5-yl)-aromatic derivatives 8–12 (Scheme 1). Stille cross-coupling reaction of compounds 3–7 with 5-(tri-n-butyltin)2,2 -bithiophene followed by palladium-catalyst resulted in the desired products 8–12 at yield of 80–90%. All obtained monomers

2.2. Preparation of 5-(tri-n-butyltin)-2,2 -bithiophene (2) 2,2 -Bithiophene (1.10 g, 6.6 mmol) was dissolved in dry THF (50 mL). The resulting mixture was cooled to −78 ◦ C and n-BuLi (1.4 M, 5.2 mL, 7.3 mmol) was slowly added. After the reaction had been stirred intensively for 2 h at −78 ◦ C, tributyltin chloride (2.38 g, 7.3 mmol) was added. The mixture was stirred overnight and then the solvent was removed under reduced pressure until a brown oil appeared. The crude product 2 was used without further purification. (Warning: 5-(tri-n-butyltin)-2,2 -bithiophene decomposes to the starting material during purification with water and it is also unstable under chromatography column). 1 H NMR (250 MHz, CDCl ): ı = 7.30 (d, J = 3.4 Hz, 1H); 7.22 (d, 3 J = 3.5 Hz, 1H); 7.20 (d, J = 4.6 Hz, 1H); 7.07 (d, J = 3.4 Hz, 1H); 7.02 (dd, J = 4.6, 3.4 Hz, 1H); 1.58 (m, 6H); 1.33 (m, 6H); 1.13 (t, J = 8.0 Hz, 6H); 0.91 (t, J = 7.2 Hz, 9H).

156

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R S

R1

R

S

S

S

S

R1

R1

a3: R = R1 = OMe

m

S

S

R1

N

S

S

N

S

S

S

N

N

N

S

S n

n

m

S

S

S S

S

N

S

S

S a1: R = R1 = H a2: R = OMe, R1 = H

S

S

n

n a

b

S S

S S S

S

S

N

N N

m

S

N

N

N

S

S

S

S n

c

n

Fig. 1. Polymers containing (a) phenyl derivatives core. (b) Triazine core. (c) Triphenyltriazine core.

2.3. General procedure for the preparation of tris(2,2 -bithiophen-5-yl)-aromatic derivatives (8–12) To compound 3 or 4 or 5 or 6 or 7 (2.0 mmol) dissolved in 150 mL of anhydrous toluene, under argon in 250 mL round two-bottom flask were added (5-tributylstannyl)- 2,2 -bithiophen (6.6 mmol) and Pd(PPh3 )4 (0.46 g, 0.4 mmol). The resulting mixture was stirred for 4 days at 110 ◦ C. Subsequently, the mixture was cooled down to room temperature. Water (100 mL) was added and the resulting solution was extracted three times with 50 mL of CHCl3 . The combined organic layers were washed with 50 mL brine, dried over MgSO4 and evaporated until brown oil appeared. The crude product (8–12) was purified by column chromatography (hexane/AcOEt, 10:1). 2.3.1. 1,3,5-Tris(2,2 -bithiophen-5-yl)-benzene (8) Yellow solid, 90% yield, mp 179–180 ◦ C. 1 H NMR (250 MHz, CDCl3 ): ı = 7.70 (s, 3H); 7.34 (d, J = 3.8 Hz, 3H); 7.26–7.24 (m, 6H); 7.20 (d, J = 3.8 Hz, 3H); 7.06 (dd, J = 4.8, 3.8 Hz, 3H). 13 C NMR (250 MHz, CDCl3 ): ı = 141.9; 137.4; 137.2; 135.4; 127.9; 124.6; 124.5; 124.4; 123.8; 121.8. Elemental analysis for: C30 H18 S6 Calc.: C, 63.12; H, 3.18. Found: C, 63.42; H, 3.23. 2.3.2. 2,4,6-Tris(2,2 -bithiophen-5-yl)-1-methoxybenzene (9) Yellow solid, 85% yield, mp 146–147 ◦ C. 1 H NMR (250 MHz, CDCl3 ): ı = 7.74 (s, 2H); 7.48 (d, J = 3.9 Hz, 2H); 7.27–7.17 (m, 10H); 7.08–7.03 (m, 3H); 3.66 (s, 3H). 13 C NMR (250 MHz, CDCl3 ): ı = 152.5; 141.7; 138.6; 137.4; 137.3; 137.1; 137.0; 130.9; 129.0; 127.9; 127.0; 124.8; 124.6; 124.5; 124.1; 123.7; 123.6; 123.5; 60.5. Elemental analysis for: C31 H20 OS6 Calc.: C, 61.96; H, 3.35. Found: C, 61.65; H, 3.53. 2.3.3. 2,4,6-Tris(2,2 -bithiophen-5-yl)-1,3,5-trimethoxybenzene (10) Deep yellow solid, 87% yield, mp 173–174 ◦ C. 1 H NMR (250 MHz, CDCl3 ): ı = 7.40 (d, J = 3.8 Hz, 3H); 7.26–7.21 (m, 9H); 7.04 (dd, J = 4.9, 3.8 Hz, 3H); 3.46 (s, 9H). 13 C NMR (250 MHz, CDCl3 ): ı = 156.4; 138.2; 137.5; 132.1; 129.7; 127.8; 124.3; 123.5; 123.3; 119.3; 60.6.

Elemental analysis for: C33 H24 O3 S6 Calc.: C, 59.97; H, 3.66. Found: C, 59.72; H, 3.46. 2.3.4. 2,4,6-Tris(2,2 -bithiophen-5-yl)-1,3,5-triazine (11) Lightly green solid, 80% yield, mp 247–248 ◦ C. 1 H NMR (250 MHz, CDCl3 ): ı = 8.18 (d, J = 3.9 Hz, 3H); 7.39 (d, J = 3.6 Hz, 3H); 7.34 (d, J = 5.1 Hz, 3H); 7.28 (d, J = 3.9 Hz, 3H); 7.10 (dd, J = 5.1, 3.6 Hz, 3H). 13 C NMR (250 MHz, CDCl3 ): ı = 167.2; 144.1; 139.6; 137.1; 132.5; 128.1; 125.8; 125.1; 124.7. Elemental analysis for: C27 H15 N3 S6 Calc.: C, 56.51; H, 2.63; N, 7.32. Found: C, 56.68; H, 2.44; N, 7.53. 2.3.5. 2,4,6-Tris[p-(2,2 -bithiophen-5-yl)-phenyl]-1,3,5-triazine (12) Yellow solid, 82% yield, mp 238–239 ◦ C. 1 H NMR (250 MHz, CDCl3 ): ı = 8.78 (d, J = 8.5 Hz, 6H); 7.77 (d, J = 8.4 Hz, 6H); 7.61–7.58 (m, 3H); 7.39 (d, J = 3.8 Hz, 3H); 7.27–7.25 (m, 3H); 7.20 (d, J = 3.8 Hz, 3H); 7.06 (dd, J = 4.8, 3.9 Hz, 3H). 13 C NMR (250 MHz, CDCl3 ): ı = 171.0; 142.2; 138.0; 137.9; 136.3; 135.2; 132.5; 129.6; 129.0; 128.6; 125.5; 125.0; 124.8. Elemental analysis for: C45 H27 N3 S6 Calc.: C, 67.38; H, 3.39; N, 5.24. Found: C, 67.58; H, 3.26; N, 5.56.

3. Measurements Electrosynthesis and studies on polymer films were performed in dichloromethane (POCH 99.8%) containing 0.1 M tetrabutylammonium tetrafluoroborate (Aldrich) as a supporting electrolyte, using Electrochemical Analyzer model 600 (CH Instruments). Polymer films were synthesized on a platinum wire at a scan rate of 50 mV/s. An Ag pseudo-reference electrode was used and its exact potential depending on ferrocene was measured. The platinum wire served as a counter electrode. Spectral measurements were carried out at UV–vis Hewlett Packard spectrophotometer 8453, while fluorescence measurements were performed on Hitachi F-2500 fluorescence spectrometer. The target polymer was synthesized on the indium tin oxide (ITO) coated quartz electrode.

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157

0.03

(a)

(b) 0.02

current / mA

current / mA

0.10

0.01

0.05

0.00

0.00 -0.01 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

-0.5

potential vs. ferrocene / V

0.5

potential vs. ferrocene / V

0.02

0.7

(c)

0.6

Absorbance

0.01

current / mA

0.0

0.00

(d)

0.5 0.4 0.3 0.2 0.1

-0.01

0.0

-0.5

0.0

300

0.5

400

500

600

700

800

Wavelength / nm

potential vs. ferrocene / V

Fig. 2. (a) CV of 0.3 mM 1,3,5-tri(2,2 -bithiophen-5-yl)benzene; (b) CV during polymerisation; (c) CV of poly[1,3,5-tri(2,2 -bithiophen-5-yl)benzene]; (d) UV–vis of poly[1,3,5tri(2,2 -bithiophen-5-yl)benzene].

3.1. Electrochemical properties Cyclic voltammograms of 1,3,5-tri(bithienyl)benzenes 8–10 and their polymers obtained during electropolymerization process were recorded to study the impact of methoxy substituents in meta positions. The anodic voltammetric curve of monomer 8 reveals at least three peaks (Fig. 2a), the first located at 0.66 V, and subsequent ones at 1.27 V and 1.58 V. The monomer with methoxy substituent attached to the central benzene ring 9 has almost the same first oxidation potential, while the monomer 10 with three methoxy groups has a higher oxidation potential equal to 0.72 V (Table 1). This could be explained as resulting from the spherical interactions, which are associated with the substitution at all positions in the benzene core. This substitution induces larger torsion angles between thiophene moieties and the benzene core and hence decreases effective conjugation length. Redox cycling of 8 up to the first oxidation potential resulted in an appearance of additional voltammetric waves with a maximum in the range of 0.2–0.6 V (Fig. 2b). Both an increase of the current in every subsequent scan and covering of the working electrode with an orange film indicate the formation of a conductive polymer with tetrathiophene moieties. The cycling of poly(8) film performed in the polymerisation potential range in a monomer free

electrolyte solution reveals an oxidation onset at approximately 0.17 V and a peak maximum at about 0.53 V (Fig. 2c). In the cases of poly(9) and poly(10) the presence of methoxy substituents slightly increased the oxidation onset to 0.18 V and 0.24 V, respectively. The UV–vis spectroscopy of poly(8) reveals a complex absorption spectrum with low peak at 260 nm, and higher wide band from approximately 300 nm up to 550 nm consisting of at last two peaks (Fig. 2d). Moreover, a low band is observed at the vicinity of 550 nm owing to an incomplete dedoping of the polymer obtained by electropolymerization. The cyclic voltammogram of the monomer 11 reveals the first oxidation peak at 0.9 V and the second one at 1.28 V (Fig. 3a). This indicates that the presence of a triazine core increases oxidation potential. An incorporation of additional phenyl units extends the effective conjugation length, therefore the monomer 12 has much lower oxidation potential at 0.68 V. The cyclic oxidation of 11 results in the appearance of a new peak within the second scan with an oxidation onset at 0.09 V (Fig. 3b) and formation of a deep-red film on the electrode surface. The anodic cycle of the poly(11) film reveals an unclear oxidation peak at approximately 0.6 V (Fig. 3c). The UV–vis spectrum of poly(11) is red shifted in comparison to the analogical one of poly(8) with a low peak at 284 nm, and higher

Table 1 ox is the monomer oxidation potential; Eponset is the polymer onset oxidation potential; Eg is the polymer HOMO–LUMO gap Electrochemical and optical results where Em estimated from equation Eg = 1240/␭onset where ␭onset is an absorption onset. Compound 

1,3,5-Tri(2,2 -bithiophen-5-yl)benzene (8) 2,4,6-Tri(2,2 -bithiophen-5-yl)-1methoxybenzene (9) 2,4,6-Tri(2,2 -bithiophen-5-yl)-1,3,5-trimethoxybenzene (10) 2,4,6-Tri(2,2 -bithiophen-5-yl)-1,3,5-triazine (11) 2,4,6-Tri[p-(2,2 -bithiophen-5-yl)-phenyl]-1,3,5-triazine (12)

ox I Em [V]

Eponset [V]

Eg [eV]

0.66 0.67 0.72 0.90 0.68

0.17 0.18 0.24 0.26 0.09

2.21 2.21 2.19 2.07 2.10

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0.03

0.10

current / mA

current / mA

(a)

0.05

(b)

0.02

0.01

0.00

0.00 -1.0

-0.5

0.0

0.5

1.0

1.5

2.0

-0.5

potential vs. ferrocene / V

0.0

0.5

potential vs. ferrocene / V 0.4

0.010

(d)

(c) Absorbance

current / mA

0.3

0.005

0.000

-0.005

0.2

0.1

0.0

-0.5

0.0

300

0.5

400

500

600

700

800

Wavelength / nm

potential vs. ferrocene / V

Fig. 3. (a) CV of 0.3 mM 2,4,6-tri(2,2 -bithiophen-5-yl)-1,3,5-triazine; (b) CV during polymerisation; (c) CV of poly[2,4,6-tri(2,2 -bithiophen-5-yl)-1,3,5-triazine]; (d) UV–vis of poly[2,4,6-tri(2,2 -bithiophen-5-yl)-1,3,5-triazine].

Fluorescence/Absorbance

Fluorescence/Absorbance

1.0 Absorbance Excitation Emmision

0.8

0.6

0.4

0.2

1.0

Absorbance Excitation Emmision

0.8 0.6 0.4 0.2 0.0 300

400

500

600

700

Wavelength / nm

0.0 300

400

500

600

700

Wavelength / nm Fig. 4. Absorbance, fluorescence excitation and emission of 1,3,5-tri(2,2 bithiophen-5-yl)benzene (8) in CH2 Cl2 .

wide band from approximately 315–590 nm. Also energy gap (Eg) is lower and amounts to 2.07 eV while for poly(8) Eg takes the value of 2.21 eV. Electrochemical oxidation of bisthiophene units leads to the formation of dimer with tetrathiophene linkers (Scheme 2). The other bisthiophene units can be also oxidized and form tetrathiophene linkers leading to crosslinked polymer, according to the

Fig. 5. Absorbance, fluorescence excitation and emission of 2,4,6-tri(2,2 bithiophen-5-yl)-1,3,5-triazine (11) in CH2 Cl2 .

well-known thiophene polymerization mechanism [28]. Obtained polymers are not soluble in any solvent. 3.2. Optical properties Compounds 8–12 were studied by optical spectroscopy. In all cases fluorescence activity was observed. The excitation spectra performed on compounds 8–12 reveal a lot of similarities. Fig. 4 shows absorbance, fluorescence excitation, and emission spectra of compound 8 dissolved in dichloromethane, excitation at

Table 2 Optical results of compounds dissolved in CH2 Cl2 where ab is absorption maxima, em fluorescence maxima. Compound

ab [nm]

em [nm]

1,3,5-Tri(2,2 -bithiophen-5-yl)benzene (8) 2,4,6-Tri(2,2 -bithiophen-5-yl)-1methoxybenzene (9) 2,4,6-Tri(2,2 -bithiophen-5-yl)-1,3,5-trimethoxybenzene (10) 2,4,6-Tri(2,2 -bithiophen-5-yl)-1,3,5-triazine (11) 2,4,6-Tri[p-(2,2 -bithiophen-5-yl)-phenyl]-1,3,5-triazine (12)

245, 358 243, 356 233, 344 252, 394 267, 388

410, 426 420 402, 419 460 492

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159

Table 3 Calculated (B3LYP/6-31G*) highest occupied (HOMO), lowest unoccupied molecular orbitals (LUMO) and HOMO–LUMO gaps for neutral states of monomers 8–12. Compound

HOMO [eV]

LUMO [eV]

E [eV]

1,3,5-Tri(2,2 -bithiophen-5-yl)benzene (8) 2,4,6-Tri(2,2 -bithiophen-5-yl)-1methoxybenzene (9) 2,4,6-Tri(2,2 -bithiophen-5-yl)-1,3,5-trimethoxybenzene (10) 2,4,6-Tri(2,2 -bithiophen-5-yl)-1,3,5-triazine (11) 2,4,6-Tri[p-(2,2 -bithiophen-5-yl)-phenyl]-1,3,5-triazine (12)

−5.328 −5.245 −5.112 −5.586 −5.361

−1.675 −1.650 −1.544 −2.159 −2.135

3.65 3.60 3.57 3.43 3.23

Table 4 Isosurfaces (±0.02 a.u.) for the HOMO and LUMO orbitals of monomers 8–12. Compound

8

9

10

11

12

HOMO

LUMO

160

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S S

S

S

S

S S

S

S

S

-2e-

S S

2

S

-2H+

S

2

S

S

+

S

CH

S

+

S

H

S

S

S +

S

S

H

S

S

S

S

S

S

S

S

S

S

S

S

Scheme 2. Mechanism of electrochemical oxidation of bisthiophene units leading to formation of tetrathiophene linker.

358 nm led to the emission with maxima at: 410 nm and 426 nm. A replacement of the central benzene core by triazine results in a bathochromic shift. The excitation spectrum of compound 11 has two maxima at: 272 nm and 382 nm, whereas the emission spectrum has only one at 460 nm (Fig. 5). Fluorescence maxima are collected in Table 2. 4. Theoretical calculations In theoretical calculations using the Gaussian09 program, the structures of molecules 8–12 were optimized at the B3LYP level with the 6-31G* basis set [29,30], and the energies of the highest occupied (HOMO) and lowest unoccupied molecular orbitals (LUMO) were determined, in order to calculate the HOMO–LUMO gaps shown in Table 3. This computational level was mainly chosen to enable comparison of our results with those previously published by Hehre et al. [29] However, we did additional calculations in order to verify our results. With more sophisticated basis sets, namely Dunnings cc-pVDZ and cc-PVTZ, [31,32] the size of the HOMO–LUMO gap did not change by more than 0.04 eV, and also the absolute HOMO–LUMO gaps only show narrow differences compared to calculations with other methods (RHF, PW91 and M06). Additionally, the relative order of the molecules with respect to their HOMO–LUMO gap is preserved with errors never exceeding 0.02 eV. For compounds 11 and 12 containing triazine ring a slightly decreased HOMO–LUMO gap was calculated, which correlates with optical results. In the equilibrium structure none of the molecules stay planar or adopt a propeller shape with a C3 rotational axis perpendicular to the molecule’s plane. Instead, two substituents are rotated in one and the third substituent in the opposite direction. Due to the different torsion angles between the central core and the directly bounded aromatic ring in the range of 15–39◦ , all monomers show minor differences in size and shape of their molecular orbitals (Table 4). 5. Conclusion All star-shaped bithienylbenzene and bithienyltriazine derivatives are electroactive and undergo electropolymerization. Tetrathienyl moieties between central aromatic cores were created during electrochemical polymerisation. All momonomers were synthesized using a Stille cross-coupling reaction and fully characterized. Their electrochemical properties were studied by cyclic voltammetry and consequently involved into electropolymerization studies. During electropolymerization under potentiodynamic conditions it was possible to obtain thicker and much more stable

polymer films than those built of analogues monomers with monothiophene moieties. Here, the term: ‘stability’ is understood as the qualitative (shape) and quantitative (current value) reproducibility of current–voltage curves. The stability of polymers based on tetrathienyl bridges is much higher when comparing them to those based on dithienyl ones [33–35]. Moreover, fluorescence activity was detected in the cases of all synthesized compounds. The electrochemical properties and energy gap of both monomers and their polymers are slightly affected by a methoxy group, which are attached to the central benzene core. The replacement of the benzene core by triazine increases oxidation potential while simultaneously lowering the energy gap. The electrochemical analysis supported by spectroscopic methods confirms that the compounds described in this paper fulfil the technological requirements for materials which can be successfully employed for constructing organic–electronic devices. Theoretical calculation provided values for HOMO–LUMO gaps for neutral states of monomers, while electrochemical and optical methods resulted in experimental values of polymer HOMO–LUMO gaps. Acknowledgements This work was realized within the European Union Project (SNIB, MTKD-CT-2005-029554). This support is gratefully acknowledged. This work was supported by grant of Ministry of Science and Higher Education NN205106935, and by the European Community from the European Social Fund within the RFSD 2 project. The presented study was partially funded by the German Federal Ministry of Environment (promotional reference No. 0325417, Reaktherm). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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