Study of the order-disorder transitions in methoxy-functionalized polyalkylthiophenes

European Polymer Journal 44 (2008) 3987–3996

Contents lists available at ScienceDirect

European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Study of the order-disorder transitions in methoxy-functionalized polyalkylthiophenes Massimiliano Lanzi *, Luisa Paganin Dipartimento di Chimica Industriale e dei Materiali, University of Bologna, Viale del Risorgimento, 4 I-40136 Bologna, Italy

a r t i c l e

i n f o

Article history: Received 23 June 2008 Received in revised form 14 September 2008 Accepted 22 September 2008 Available online 4 October 2008

Keywords: Chromism Crystallinity Electrical conductivity Functionalized polyalkylthiophenes High-temperature organic semiconductors

a b s t r a c t This article describes the solvatochromic properties of two polyalkylthiophene (PAT) samples functionalized at the end of the hexamethylenic side-chains with a methoxy group, which is able to strongly enhance the solubility, workability and filmability of this kind of polymers. The latter are obtained using either a regioselective or a regiospecific polymerization procedure, thus leading to a different configurational order in the final polymer. The optical features of the synthesized samples—which are very interesting for chemosensor and electrooptical applications—are observed in many solvent/ non-solvent systems and derive from the conformational modification of the conjugated backbone induced by side-chain order-disorder transitions. These transitions strongly depend on the content of HT dyads; a fact which undeniably shows the importance of the polymer configuration, directly deriving from the adopted polymerization method, on the final electrical and electronic properties of the obtained material. The low sensitivity of the regioregular sample towards the temperature changes together with its higher tendency to give thick, semicrystalline and self-consisting films makes it very promising for the obtainment of organic semiconductors for electronic devices subjected to high temperature variations. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Regioregular polythiophenes are a very important class of conjugated polymers characterized by interesting features for practical applications. Their air-stable conducting property makes them suitable as chemoresistive sensing materials while their solubility in a variety of organic solvents enables them to be easily processed and filmed for device fabrication [1,2]. The backbone of the polymer is made of thiophenic rings at a variable degree of planarization; an alkylic side-chain is generally attached to each thiophenic ring along the polymer in order to enhance its solubility, leading to the polyalkylthiophenes (PAT) class. A functional group can also be added to the end of the PAT side-chains to obtain a plethora of possible structures, * Corresponding author. Tel.: +39 51 2093689; fax: +39 51 2093669. E-mail addresses: [email protected], [email protected] (M. Lanzi). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.09.034

which combine polythiophene properties and those typical of the chosen substituent. The introduction of substituents on the thiophene rings is, in fact, the simplest way of tuning the chemical and physical properties of the polymers; for example, the use of long alkyl side-chains increases solubility in organic solvents [3–5] while hydrophilic substituents produce water-soluble polythiophenes [6,7]. Generally, PAT functionalized with ester [8–11] or ether [12–14] groups proved to be soluble in a wide range of solvents in spite of their molecular weights and percentage of regioregularity. The configurational regularity of the final polymer is fundamental in order to obtain high electronic delocalization and mobility. In fact, only Head-toTail (HT) linkages among repeating units make it possible to reach the maximum degree of planarization of the aromatic rings, thus leading to the most effective superposition of the pz orbitals of the thiophenic carbons. In this work, the structural, optical and electrical properties of a regioregular all-HT poly[3-(6-methoxyhexyl)thi-

3988

M. Lanzi, L. Paganin / European Polymer Journal 44 (2008) 3987–3996

ophene] are examined and compared with those of a non-regioregular sample, with the objective to investigate how far the regiochemistry of thiophenic linkages can affect polymer characteristics. 2. Experimental 2.1. Materials and instruments All the solvents and reagents used for the synthesis were purchased from Aldrich Chemical Company. NMR spectra were run on a Varian Gemini 300 (300 MHz) FT-NMR spectrometer using TMS as reference. FT-IR spectra were carried out using a Perkin Elmer 1750 and Perkin Elmer Spectrum One FT-IR spectrophotometers. Molecular weights were determined by size exclusion chromatography (SEC) relative to polystyrene standards by using a HPLC Lab Flow 2000 apparatus equipped with a Phenogel mixed MXM column and a Linear Instrument UV–vis detector (model UVIS-200) working at 263 nm. THF was used as an eluent at a flow rate of 1.0 ml/min. The UV–vis spectra were recorded with a Perkin Elmer Lambda 19 using spectroquality solvents. Thermal analysis was performed by means of a TA Instruments DSC 2920 at a heating rate of 10 °C/min. Microanalyses were carried out by Redox Laboratories (Monza, Italy). X-ray diffraction data were recorded at room temperature by using a CuKa (k = 1.5406 Å) radiation source (Philips PW1050) and a Bragg–Brentano diffractometer (Philips PW1710) equipped with a graphite monochromator in the diffracted beam. The 2h range between 2.0 and 90.0° was scanned by 881 steps of 0.1° with a counting time of 15 s for each step. The XRD investigation was carried out on films on glass slides obtained by casting the polymer dissolved in CHCl3. Electrical measurements were made by means of a Keithley Picoammeter 6485, an Agilent Technologies 6110C source voltage monitor and a four-probe system made by AI Alessi Instruments and equipped with four Os conic-shape tips, with an applied load of 50 g per tip, a probe spacing of 1.5 mm and a probe tip radius of 0.1 mm. 2.2. Monomer synthesis 2.2.1. 3-(6-methoxyhexyl)thiophene (M) M was prepared by reacting 12.4 mmol of 3-(6-bromohexyl)thiophene with 37.1 mmol of sodium methoxide 30% m/v in methanol, as reported in Ref. [5]. The crude M was purified by column chromatography (silica gel, n-heptane/diethyl ether 8:2) to give 1.96 g (9.88 mmol, 80% yield) of pure product. 1 H-NMR (CDCl3, ppm): d7.21 (d, 1H); 6.95 (m, 2H); 3.35 (t, 2H); 3.28 (s, 3H); 2.62 (t, 2H); 1.59 (m, 2H), 1.50 (m, 2H), 1.37 (m, 4H). 13 C-NMR (CDCl3, ppm): d143.8; 128.9; 125.7; 120.5; 73.5; 59.2; 31.1; 30.8; 30.2; 29.8; 26.6. IR (KBr disk, cm 1): 3102, 3050, 2977, 2931, 2857, 2826, 2807, 1537, 1478, 1459, 1409, 1387, 1199, 1119, 856, 773, 730, 684, 633.

2.2.2. 2,5-dibromo-3-(6-methoxyhexyl)thiophene (2,5BM) 1.76 g (9.88 mmol) of N-bromosuccinimide (NBS) in 10 ml of N,N-dimethylformamide (DMF) were added to a solution of 1.96 g (9.88 mmol) of M in 10 ml of DMF. The mixture was reacted for 6 h at room temperature, in the dark and under inert atmosphere, afterwards a second amount of brominating agent (2.64 g of NBS in 15 ml of DMF) was added. After stirring for 24 h at 20 °C in the dark and in an inert atmosphere, the mixture was poured in 280 ml of distilled water and extracted several times with petroleum benzine. After anhydrification and concentration of the collected organic phases 3.23 g (9.07 mmol) of crude 2,5BM were obtained. The product was purified by column chromatography (SiO2, n-heptane/diethyl ether 8:2) giving 2.82 g (7.92 mmol) of colourless oil (80% yield). 1 H-NMR (CDCl3, ppm): d6.78 (s, 1H); 3.38 (t, 2H); 3.30 (s, 3H); 2.50 (t, 2H); 1.63 (m, 2H), 1.54 (m, 2H), 1.30 (m, 4H). 13 C-NMR (CDCl3, ppm): d143.5; 131.6; 111.0; 108.6; 73.4; 59.2; 30.2; 30.1; 30.0; 29.6; 26.5. IR (KBr disk, cm 1): 3046, 2977, 2930, 2857, 2825, 2806, 1541, 1478, 1459, 1418, 1391, 1189, 1119, 1001, 825, 727. 2.3. Polymers synthesis 2.3.1. Poly[3-(6-methoxyhexyl)thiophene] at high HT dyads content (P1) 1.40 g (3.93 mmol) of 2,5BM in 22 ml of anhydrous THF was poured into a three-neck round bottom flask and 4.0 ml (4 mmol) of a 1 M solution of methylmagnesium bromide in n-butyl ether were added. After refluxing for 1 h under stirring and in an anhydrous atmosphere, 0.0227 g (0.041 mmol) of Ni(dppp)Cl2 were added. The mixture was refluxed for further 2 h under Argon and poured in 250 ml of methanol; then the polymer was recovered by filtration on a PTFE septum (0.45 lm pore size). The crude P1 was redissolved in 10 ml of CHCl3, precipitated in 100 ml of methanol and recovered using the kind of filter described above. 0.54 g (2.75 mmol) of P1 were obtained (66% yield). 1 H-NMR (CDCl3, ppm): d6.98 (s, 1H); 3.39 (t, 2H); 3.33 (s, 3H); 2.90, 2.50 (2m, 2H); 1.75 (m, 2H), 1.50 (m, 2H), 1.28 (m, 4H). 13 C-NMR (CDCl3, ppm): d140.5; 134.4; 131.2; 129.3; 73.5; 59.2; 31.2; 30.3; 30.1; 29.8; 26.7. IR (Ge disk, cm 1): 3056, 2975, 2930, 2856, 2820, 2804, 1509, 1477, 1454, 1387, 1120, 823, 748, 727, 647. Elemental analysis: (C11H16OS)n (196.31)n: Calcd. C 67.30, H 8.22, O 8.15, S 16.33 Found C 66.91, H 7.93, O 8.55, S 16.61. 2.3.2. Poly[3-(6-methoxyhexyl)thiophene] at low HT dyads content (P2) A solution of 0.97 g (6.0 mmol) of FeCl3 in 5.8 ml of CH3NO2 was added dropwise, in 20’, to 0.30 g (0.15 mmol) of the monomer M in 17 ml of anhydrous CCl4 under stirring and under a gentle flux of Ar. After 1 h at room temperature the mixture was added with 50 ml of CHCl3 and the organic phase was washed several times with HCl 5%. After the anhydrification and evaporation of the solvent at reduced pressure, the polymer was dissolved in CH2Cl2

3989

M. Lanzi, L. Paganin / European Polymer Journal 44 (2008) 3987–3996

and fractionated with MeOH, giving 0.08 g of P2 (80% yield). 1 H-NMR (CDCl3, ppm): d7.05, 7.02, 7.00, 6.98 (4s, 1H); 3.40 (t, 2H); 3.30 (s, 3H); 2.82, 2.78 (2m, 1H); 2.56, 2.52 (2m, 1H); 1.78 (m, 2H), 1.50 (m, 2H), 1.25 (m, 4H). 13 C-NMR (CDCl3, ppm): d139.2; 139.0; 137.4; 137.1; 136.5; 134.3; 134.0; 132.4; 132.1; 131.3; 130.3; 129.7; 129.2; 128.9; 128.3; 127.3; 73.3; 59.4; 31.0; 30.8; 30.4; 30.1; 29.8; 29.5; 26.6. IR (Ge disk, cm 1): 3055, 2975, 2930, 2856, 2820, 2803, 1511, 1460, 1387, 1120, 829, 728. Elemental analysis: (C11H16OS)n (196.31)n: Calcd. C 67.30, H 8.22, O 8.15, S 16.33 Found C 66.80, H 8.05, O 8.64, S 15.99, Fe 0.52.

means of the magnesium-halogen exchange (metathesis reaction) with a preformed Grignard reactive and subsequent Ni(II) catalyzed cross-coupling reaction (Scheme 2). After fractionation using the CHCl3/CH3OH system, a dark red polymer, P1, at high molecular weight and HT dyads content (see Table 1) was recovered in a good yield. The intermediate M was also subjected to an oxidative polymerization reaction using an accurately set-up procedure entailing the use of a CCl4/CH3NO2 solvent mixture, which was able to generate the oxidant (FeCl3) in a highly dispersed and active microcrystalline form, thus preventing the formation of insoluble polymeric fractions [17]. The obtained polymer, P2, was accurately fractionated using the CH2Cl2/MeOH system in order to obtain a fraction with ponderal characteristics similar to those showed by P1. P1 and P2 were fully characterized by different techniques, i.e. 1H- and 13C-NMR, FT-IR and elemental analysis which confirmed the chemical structure of the two samples.

3. Results and discussion The intermediate 3-(6-bromohexyl)thiophene (T6Br, Scheme 1)—synthesized as reported in Ref. [15]—was subjected to a SN2 reaction with sodium methoxide in methanol, which involved the bromine atom at the end of the side-chain. 3-(6-methoxyhexyl)thiophene (M) was obtained in a good yield (80%) and it was selectively dibrominated in the 2,5-positions of the thiophenic ring by means of N-bromosuccinimide (NBS) in anhydrous N,N-dimethylformamide (DMF). The optimization of the reaction conditions, providing for the addition of the NBS in two distinct amounts, led to a high yield (80%) in 2,5-dibromo-3-(6methoxyhexyl)thiophene (2,5BM), which was used for the subsequent polymerization without further purification. The two bromine atoms in the 2,5-positions of the thiophene ring were exploited for the organometallic coupling reaction using the McCullough method [16], which is a useful and straightforward way to synthesize regioregular Head-to-Tail (HT) coupled poly(3-alkylthiophene)s by

(CH2)6

S

Br

3.1. FT-IR characterization The characteristic IR bands of the synthesized monomers and polymers are shown in Table 2.

Table 1 Yields, regioregularity and molecular weights of the synthesized polymers. Sample

Yield (%)a

%HTb

Mn (KDa)

Mw/Mn

DPn

P1 P2

66 55

98 70

28.7 29.5

1.2 1.3

146 150

a

In fractionated polymer. HT (Head-to-Tail) dyads percentage as determined by spectroscopy. b

CH3ONa

NBS

MeOH

DMF

T6Br

(CH2)6 OMe

(CH2)6 OMe

S

Br

Br

S 2,5BM

M Scheme 1. Monomers synthesis.

(CH2)6 OMe

(CH2)6 OMe 1) CH3MgBr

Br

S

Br

2) Ni(dppp)Cl2

P1 S

THF

2,5BM

n

(CH2)6 OMe

(CH2)6 OMe FeCl3

S

CH3NO2 / CCl4

M Scheme 2. Polymers synthesis.

P2 S

n

1

H-NMR

3990 Table 2 IR absorption bands (cm

M. Lanzi, L. Paganin / European Polymer Journal 44 (2008) 3987–3996

1

) and relative assignments for the synthesized monomers and polymers.

Assignment

M

2,5BM

P1

P2

C–H stretching (thiophene, a-hydrogen) C–H stretching (thiophene, b-hydrogen) C–H stretching (antisymmetric, methyl) C–H stretching (antisymmetric, methylenes) C–H stretching (symmetric, methylenes) C=C stretching (antisymmetric, thiophene) C=C stretching (symmetric, thiophene) CH3 deformation C–O–C stretching C–Br stretching C–H bending out-of-plane (2,3,5-trisubstituted thiophene) C–H bending out-of-plane (3-substituted thiophene) CH2 rocking

3102 3050 2977 2931 2857 1537 1459 1387 1119 — — 773, 684 730

— 3046 2977 2930 2857 1541 1459 1391 1119 1001 825 — 727

— 3056 2975 2930 2856 1509 1454 1387 1120 — 823 — 727

— 3055 2975 2930 2856 1511 1460 1387 1120 — 829 — 728

P1 and P2 show very similar spectral profiles. The absorption at 3102 cm 1, which is ascribable to the stretching of the thiophene a-hydrogens, is absent in the P2 sample while the band at 1001 cm 1, due to the thiophenic C–Br stretching, is not found in the P1 spectrum, according with the high degree of polymerization found for both products via GPC (Table 1). Passing from P2 to P1, the band relative to the C–H bending out-of-plane in 2,3,5-trisubstituted thiophenes shows a shift from 829 to 823 cm 1, according to the higher regioregularity of the latter [18]. Moreover, the intensity ratio Isym/Iasym of the IR bands at 1454 and 1509 cm 1 for P1 and at 1460 and 1511 cm 1 for P2 is lower for the former (3.2 vs. 5.5) depending on its more extended conjugation in the film state, which is also confirmed by the values of the absorption maxima (518 vs. 500 nm). In the neutral state, the two poly[3-(6-methoxyhexyl)thiophene]s are dark red and easily obtainable as selfconsisting films with copper-bronze lusters. Despite their high molecular weight, the two samples are very soluble in a wide range of organic solvents owing to the presence of the x-methoxyterminated hexamethylenic side-chains, thus facilitating their further characterization. 3.2. NMR characterization In order to assign P1 and P2 signals, the spectra of the monomers M and 2,5BM were carefully examined. Their 1 H and 13C-NMR data are shown in Table 3, together with the corresponding assignments obtained by using bidimensional techniques, i.e. COSY [19], HSQC [20], and HMBC [21] and by comparison with some selected references [22–24]. Polymers resonances are reported in Table 4 by following the atoms numbering shown in Fig. 1. The monomer 2,5BM is devoided of H atoms in the 2,5-positions of the aromatic ring; therefore, its polymerization is only confirmed by the shift of the signal ascribable to thiophenic H-4 atom to a lower field and by the splitting of the thiophene a-CH2 signal in two new resonances, ascribable to the different kind of diads linkages. In fact, the degree of regioregularity (98% HT) was evaluated on the basis of the intensity ratio of the aforementioned two signals at 2.90 and 2.50 ppm.

Table 3 1 H and 13C-NMR signals and relative assignments for monomers M and 2,5BM. Atom noa

2 3 4 5 6 7 8 9 10 11 12 a b

M

2,5BM

d 1H (ppm)b

d

6.95 — 6.95 7.21 2.62 1.59 1.37 1.37 1.50 3.35 3.28

120.5 143.8 128.9 125.7 29.8 31.1 30.8 26.6 30.2 73.5 59.2

(m) (m) (d) (t) (m) (m) (m) (m) (t) (s)

13

C (ppm)

d 1H (ppm)b

d

— — 6.78 — 2.50 1.63 1.30 1.30 1.54 3.38 3.30

111.0 143.5 131.6 108.6 29.6 30.2 30.1 26.5 30.0 73.4 59.2

(s) (t) (m) (m) (m) (m) (t) (s)

13

C (ppm)

See Fig. 1. s, singlet; d, doublet; m, multiplet.

The signal at a lower field is assigned to the HT diads while the signal at a higher field, at very low intensity (see Fig. 2a), is assigned to the HH and TT diads. In the aromatic region, the absorption of the proton in the 4-position of the thiophenic ring is shown as a singlet at 6.98 ppm, thus confirming the almost exclusive presence of HT–HT triads. The disappearance of the signal at 7.21 ppm and the reduced intensity of the peak at 6.95 ppm, which are registered in the spectrum of M, prove the obtainment of the polymer P2 at high molecular weights. The degree of regioregularity (70% HT) of this polymer was estimated by considering the intensity ratio of the two signals at 2.80 and 2.54 ppm, as in the previous case (see Fig. 2b). The configurational regularity of P2 is also confirmed on the basis of the relative intensity of the signals ascribable to the different kinds of possible triads, i.e. 6.98 (HT–HT triad), 7.00 (TT–HT), 7.02 (HT–HH), 7.05 (TT–HH) [25]. As expected, the regioregularity of P2 is in line with the values usually obtained using the regioselective oxidative polymerization with iron trichloride [26] and the value obtained for P1 is in good agreement with those obtained using the Ni(II) catalyzed coupling of organometallic 3-alkylthiophene derivatives [27].

3991

M. Lanzi, L. Paganin / European Polymer Journal 44 (2008) 3987–3996 Table 4 1 H and 13C-NMR signals and relative assignments for polymers P1 and P2. Atom noa

P1

2 3 4 5 6 7 8 9 10 11 12 Atom noa

d 1H (ppm)b

d

13

— — 6.98 (s) — 2.90, 2.50 (2m) 1.75 (m) 1.28 (m) 1.28 (m) 1.50 (m) 3.39 (t) 3.33 (s)

131.2 140.5 129.3 134.4 29.8 31.2 30.3 26.7 30.1 73.5 59.2

C (ppm)

P2 d 1H (ppm)b

d

13

C (ppm)

Triad

HT–HT

TT–HT

HT–HH

TT–HH

HT–HT

TT–HT

HT–HH

TT–HH

2 3 4 5 6 7 8 9 10 11 12

— — 6.98 — 2.78 1.78 1.25 1.25 1.50 3.40 3.30

— — 7.00 — 2.82 1.78 1.25 1.25 1.50 3.40 3.30

— — 7.02 — 2.52 1.78 1.25 1.25 1.50 3.40 3.30

— — 7.05 — 2.56 1.78 1.25 1.25 1.50 3.40 3.30

131.3 139.2 128.9 134.0 29.8 31.0 30.4 26.6 30.1 73.3 59.4

132.1 139.0 130.3 132.4 29.8 31.0 30.4 26.6 30.1 73.3 59.4

128.3 137.4 127.3 136.5 29.5 30.8 30.4 26.6 30.1 73.3 59.4

129.7 137.1 129.2 134.3 29.5 30.8 30.4 26.6 30.1 73.3 59.4

a b

(s) (m) (m) (m) (m) (m) (t) (s)

(s) (m) (m) (m) (m) (m) (t) (s)

(s) (m) (m) (m) (m) (m) (t) (s)

(s) (m) (m) (m) (m) (m) (t) (s)

See Fig. 1. s, singlet; d, doublet; m, multiplet.

10 8 6 3

4 5

O

12

11 9

7 2

S 1

Fig. 1. Adopted atoms numbering for NMR analysis.

3.3. Thermal analysis The main transition temperatures recorded by DSC analysis of P1 and P2 polymers are collected in Table 5. The reported values were obtained in the second run, with a heating and cooling rate of 10 °C/min. P1 shows both a weak second order transition ascribable to a glass transition at 25 °C and a sharp exothermic peak at 189 °C, which originates from the melting of the main chain crystalline domains (Fig. 3). The latter transition also determines the endothermic crystallization peak found at 136 °C in the cooling run. During this step, there is a noteworthy near-complete recovery of the initial cristallinity of P1, as testified by the similar integrated intensity of the exo- and endo-thermic peaks. Both the very high melting temperature and the narrow interval in this phase transition are related to the high crystallinity degree of this sample and are mainly determined by its high degree of regioregularity. In fact, P2 shows broader thermal transi-

Fig. 2. 1H-NMR spectrum of polymer P1 (a) and P2 (b).

3992

M. Lanzi, L. Paganin / European Polymer Journal 44 (2008) 3987–3996

Table 5 Thermal characteristics of the synthesized polymers. Sample

Tga (°C)

Tcb (°C)

Tmc (°C)

DHc (J/g)

DHm (J/g)

P1 P2

25 9

136 —

189 55–85

11.4 —

11.1 8.1

a b c

Glass-transition temperature (heating run). Crystallization temperature (cooling run). Melting temperature (heating run).

Fig. 3. DSC thermograms of polymers P1 and P2

tions at temperatures that are remarkably lower than P1 (see Table 5), even though the chemical structure and ponderal characteristics of the former are the same as those of the regioregular sample. The obtainment of P1, a soluble, highly filmable and processable functionalized polyalkylthiophenic sample with particularly enhanced thermal stability is indeed strategical for practical applications. Moreover, as far as we know, this is the first type of x-functionalized PAT which does not have any relevant thermal transition before 189 °C, apart from a negligible very weak second order transition [28,29]. In fact, only thin films of regioregular and not functionalized poly(3-hexyl)thiophene showed phase transitions at comparable temperatures [30]. It is well-known that the absence of relevant conformational changes of the polymeric backbones and, consequently, of important variations of the mean conjugation length in the temperature range used by the final device (0– 120 °C) is a fundamental characteristic for the direct application of conjugated polymers in electronic and microelectronic fields, ensuring the steadiness of the electrical, optical and electronic features of the employed materials. 3.4. UV–vis analysis The chromic behaviour of the two polymers has been examined in many different solvent/non-solvent systems owing to their very good solubility. The first examined system was CHCl3 (solvent) and CH3OH (non-solvent). In Fig. 4 the spectra of P1 and P2 are depicted at increasing non-solvent molar fractions. In both cases the solvatochromic transition from the solvated to the less solvated conformation is clearly visible, which results in the solu-

Fig. 4. UV–vis spectra of the synthesized polymers in CHCl3/MeOH solutions at increasing methanol molar fractions.

tion changing from a yellow-orange to violet colour without any trace of aggregation being detected even after many days. The effect of the methanol is particularly evident for P1, as confirmed by the Dkmax of 108 nm which is notably higher than for P2 (only 27 nm). This indicates that, in the examined polymers, the precise balance required between repulsive inter and/or intrachain steric interactions and attractive electrostatic forces is driven not only by the nature of the substituents [31,32] but also by the percentage of regioregularity. This is not surprising because configurational characteristics become important when highly orderable polymers are considered, in which the solvatochromic transition is the result of a cooperative twisting of relatively long sequences of thiophenic units [33]. These cooperative interactions can be compared to a set of dominoes where the flip of a repeating unit induces the twisting of the neighbouring one and so on and have been mainly observed in polyalkylthiophenes able to crystallize [34]. The chromism of the more sensitive polymer, i.e. P1, has been examined also in solutions of other solvents (see Fig. 5).

M. Lanzi, L. Paganin / European Polymer Journal 44 (2008) 3987–3996

3993

Fig. 5. UV–vis spectra of P1 in Dimethylpropileneurea (DMPU)/MeOH (A). Dioxane/MeOH (B). THF/MeOH (C) and CHCl3/HCOOH (D) at increasing nonsolvent molar fractions (v).

Different solvents can affect the shape and position of the spectral profiles. In particular, they act on the position and intensity of the pure electronic transition p–p* (0–0), on the relative intensity of the first vibronic quanta (0–1, 0–2), and on the threshold value, which corresponds to the lowest molar fraction of methanol necessary for the formation of the more conjugated conformer (Fig. 5A–C). However, a large amount of the latter is generated in all the examined systems thus indicating the ability of P1 to self assemble in a very ordered conformation in dilute solutions of different solvents. A different behaviour is obtained in CHCl3/HCOOH (Fig. 5 D). In this case a new absorption appears on the lowest energy side of the spectrum and can be ascribed to the formation of polaronic species along the backbones induced by the protic acid. It is distinct from the two assigned to the solvated and unsolvated polymeric chains. The possibility of P1 reaching high ordered yet soluble conformations in a wide range of solvent systems is particularly intriguing especially for the new technologies of polymer applications such as

electrospinning. In fact, high molecular weights, good solubility, and the possibility of having pre-ordered polymeric chains in a nematic fashion are the most important requirements to obtain high-quality electrospun nanofibers of semiconductor polymers easily [35,36]. Moreover, P1 can be easily applied as a chromic sensor, sensitive to specific solvents or analytes (e.g. carboxylic or dilute mineral acids) since its dilute solutions have different colours depending on the kind of the employed solvent mixture (i.e. yellow–orange in good solvents, red–purple in marginal solvents and blue–violet in presence of protic acids). These chromatic transitions also occur when the polymer is in solid state (adsorbed on microcrystalline cellulose or silica) and exposed to vapours of different substances. The spectral behaviour of P1 and P2 has been examined also using films cast from their chloroform solutions on quartz slides (Fig. 6). The P1 spectrum is well structured and clearly shows the electronic pure transition at 604 nm and the first three vibronic quanta respectively at 553, 518 (kmax) and

3994

M. Lanzi, L. Paganin / European Polymer Journal 44 (2008) 3987–3996

Fig. 6. UV–vis spectra of P1 and P2 films on quartz slides.

478 nm. The spectrum of P2 is blue-shifted if compared to that of P1 and it is completely devoid of any kind of vibronic structure, even if the wavelength of the absorption maximum of P2 (500 nm) is not very far from that of P1. The appearance of a fine structure in PAT film spectra indicates the presence of segments with stronger p-electron orbital overlapping between thiophenic rings, namely the existence of more planarized segments in the main chain [37]. This confirms the possibility of P1 reaching very high ordered conformations also in the film state, thanks to both its high regioregularity and the suitably tailored kind of substituent. In fact, these x-methoxy functionalized alkyl side-chains, in spite of being more sterically-demanding if compared to the alkylic ones, make it possible to reach the same ordered states and conjugation lengths of the most ordered thin films of poly(3-hexyl)thiophene (P3HT). In detail, P3HT film has the same optical bandgap as that for P1, i.e. 650 nm (1.9 eV) and a similar vibronic progression, since the former shows the first two vibronic quanta at 549 and 521 nm, and the 0–0 transition at 600 nm [38,39]. These observations clearly indicate that the electronic performance of synthesized polymers is not decreased by the presence of the alkyleteric substituent which, conversely, strongly enhances their solubility, filmability and workability with respect to the alkylsubstituted PATs [40]. 3.5. X-ray diffraction analysis X-ray diffraction (XRD) patterns of the polymer films cast on glass slides from their chloroform solutions are depicted in Fig. 7, which shows that P1 is more planar and more crystalline than its non-regioregular analogue. In fact, P1 diffractogram exhibits typical features of homogeneous semicrystalline polyalkylthiophenes [41], while P2 shows a pattern ascribable to an essentially amorphous polymer. The XRD of a 10 lm film of P1 clearly shows first, second and third order reflections at 2h = 5.0, 9.9 and 14.9°, respectively, corresponding to an intermolecular spacing of polymeric chains on the XY plane of 18.0 Å (interchain distance). The small peak at 2h = 22.1° (4 Å) can be

Fig. 7. X-ray diffractograms of P1 and P2.

ascribed to the p–p stacking distance of P1 lamellae. In agreement with its essentially amorphous structure already evidenced in the Thermal analysis section, P2 only provides for a weak diffraction peak at 2h = 5.1° (18.4 Å), a second one at 10.1°, and a broad amorphous halo at wider angles. Films morphology was also examined by means of an atomic force microscope (AFM) Burleigh Vista 100 operating in a non-contact mode. The topography study—carried out on a 40  40 lm area of polymeric films used for XRD experiments—substantially agrees with the aforementioned results (see Fig. 8). In fact, while P2 denotes a homogeneous surface typical of amorphous materials, P2 shows a more irregular profile, where crystallites clearly emerges from the amorphous zones. 3.6. Electrical properties The electrical behaviour of P1, the more promising polymeric sample, was carefully monitored using a suitably set-up apparatus. Low temperatures were measured on a 150 nm thick film of polymer cast on an optical glass slide and held in contact with a cold finger cooled by means of a temperature-controlled liquid nitrogen flow cryostat. The sample and the measurement probes were hermetically sealed at a pressure of about 1.5 Pa to avoid any interference from the atmospheric oxygen or moisture. The

M. Lanzi, L. Paganin / European Polymer Journal 44 (2008) 3987–3996

3995

Fig. 8. AFM analysis of P1 and P2.

Table 6 Electrical conductivity of P1 at different temperatures. T (K)

r (S  cm 1)

203 223 243 263 283 303 323 343 363 383 403

2.03  10 2.07  10 2.12  10 2.16  10 2.19  10 2.23  10 2.26  10 2.30  10 2.33  10 2.38  10 2.39  10

5 5 5 5 5 5 5 5 5 5 5

stability performances can be collected into a unique polythiophene system, without any detriment to its solubility, workability and filmability. To our knowledge, P1 is the first intrinsically conducting polymer that has exhibited all these strategical features under fabrication and utilization conditions, thus leading to important developments towards the obtainment of widely and easily applicable electronic organic materials. In detail, the synthesized polymers could be effectively employed to prepare Schottky diodes with a reduced dependence on the temperature using aluminium and gold-tin electrodes cast on their FeCl3 doped films, or field-effect transistors. Acknowledgment

sample was then heated to higher temperatures using a hot-plate controlled by a Pt 100 thermocouple. The current-voltage curves at different temperatures were recorded using an Agilent 6110C source voltage monitor and a Keithley Picoammeter Model 6485 by means of an AI Alessi Instruments four-probe system. At the examined low voltages ( 1.5  +1.5 V) the dependence of I from V was nearly linear, thus making it possible to easily obtain the specific conductivities (r) at the different temperatures as reported in Table 6. In the 203–403 K (from –70 to +130 °C) the specific conductivity r is subjected to low changes, thus reflecting the enhanced thermal stability of P1 which had already been observed by DSC measurements. In the examined temperature and applied voltage interval, the non-doped (neutral) polymer denotes the typical behaviour of the semiconductors used in electronic devices subjected to high temperature variations [42]. 4. Conclusions In this work we demonstrated that morphological, optical, electrical properties of functionalized polyalkylthiophenes strongly depend on the conjugation length of the polymer backbone, which is mainly influenced by both the type of substituent and the configurational regularity. Better planar polymers have a higher degree of cristallinity, a more detailed vibrational structure in the optical spectrum and a reduced energy gap, thus ensuring higher electrical conductivities in film state. Moreover, through the structural design for proper self assembling capabilities and enhanced p-conjugation, optical, electrical and thermal

Dr. G. Cesari of the University of Ferrara is gratefully acknowledged for the help in the polymers synthesis. References [1] Bao Z, Dodabalapur A, Lovinger AJ. Appl Phys Lett 1996;69:4108. [2] Sirringhaus H, Tessler N, Friend RH. Science 1998;280:1741. [3] Souto Maior RM, Hinkelman K, Eckert H, Wudl F. Macromolecules 1990;23:1268. [4] Barlett PN, Dawson DH. J Mater Chem 1994;4:1805. [5] Demanze F, Yassar A, Garnier F. Adv Mater 1995;7:907. [6] Chayer M, Faid K, Leclerc M. Chem Mater 1997;9:2902. [7] Ewbank PC, Lowe RS, Zhai L, Reddinger J, Sauvé G, McCullough RD. Tetrahedron 2004;49:11269. [8] Rasmussen SC, Pickens JC, Hutchinson JE. Macromolecules 1998;31:933. [9] Pomerantz M. Tetrahedron Lett 2003;44:1563. [10] Casanovas J, Zanuy D, Aleman C. Polymer 2005;46:9452. [11] Lanzi M, Costa Bizzarri P, Paganin L, Cesari G. Eur Polym J 2007;43:72. [12] Bauerle P, Scheib S. Adv Mater 1993;5(11):848. [13] Goldenberg LM, Lévesque I, Leclerc M, Petty MC. J Electroanal Chem 1998;447:1. [14] Wang Y, Euler WB, Lucht BL. Chem Commun 2004;6:686. [15] Bauerle P, Wurthner F, Heid S. Angew Chem Int Ed Engl 1990;29:419. [16] Lowe RS, Ewbank PC, Lin J, Zhai L, McCullough RD. Macromolecules 2001;34:4324. [17] Costa Bizzarri P, Andreani F, Della Casa C, Lanzi M, Salatelli E. Synth Met 1995;75:141. [18] Chen TA, Wu X, Rieke RD. J Am Chem Soc 1995;117:233. [19] Bax A, Davis DG. J Magn Reson 1985;65:355. [20] Bax A, Summers MF. J Am Chem Soc 1986;108:2093. [21] Costa Bizzarri P, Lanzi M, Paganin L, Caretti D, Parenti F. Polymer 2004;45:8629. [22] Goldoni F, Iarossi D, Mucci A, Schenetti L, Costa Bizzarri P, Della Casa C, et al. Polymer 1997;38:1297. [23] Iarossi D, Mucci A, Schenetti L, Costa Bizzarri P, Della Casa C, Lanzi M. Magn Reson Chem 1999;37:182.

3996

M. Lanzi, L. Paganin / European Polymer Journal 44 (2008) 3987–3996

[24] New Advances in Analytical Chemistry, Ed. Atta.ur-Rahamn Taylor & Francis, Inc., vol. 3 2002, p. 1–40. [25] Barbarella G, Bongini A, Zambianchi M. Macromolecules 1994;27:3039. [26] Niemi VM, Knuuttila P, Osterholm JE, Sorvola J. Polymer 1992;33:1559. [27] Lowe RS, Khersonsky SM, McCullough RD. Adv Mater 1999;11:250. [28] Zhao Y, Keroack D, Yuan G, Massicotte A, Hanna R, Leclerc M. Macromol Chem Phys 1997;198:1035. [29] Chen SA, Ni JM. Macromolecules 1992;25:6081. [30] Hugger S, Thomann R, Heinzel T, Thurn-Albrecht T. Colloid Polym Sci 2004;282:932. [31] Leclerc M, Daoust G. J Chem Soc Chem Commun 1990:273. [32] Roux C, Bergeron JY, Leclerc M. Macromol Chem 1993;194:869. [33] Leclerc M, Frèchette M, Bergeron JY, Ranger M, Lévesque I, Faid K. Macromol Chem 1996;197:2077.

[34] Roux C, Faid K, Leclerc M. Macromol Chem Rapid Commun 1993;14:461. [35] Li D, Xia Y. Adv Mater 2004;16:1151. [36] Kim S, Cook SM, Tuladhar SA, Choulis J, Nelson JR, Durrant DDC, Bradley M, et al. Nat Mater 2006;5:197. [37] Rughooputh SDDV, Hotta S, Heeger AJ, Wudl F. J Polym Sci Polym Phys Ed 1987;25:1071. [38] Yu G, Phillips SD, Tomozawa H, Heeger AJ. Phys Rev B 1990;42: 3004. [39] Tomozawa H, Braun D, Phillips SD, Worland R, Heeger AJ, Kroemer H. Synthetic Met 1989;28:C687. [40] Nishino H, Yu G, Heeger AJ, Chen TA, Rieke RD. Synthetic Met 1995;68:243. [41] Luzny W, Trznadel M, Pron A. Synthetic Met 1996;81:71. [42] Transistor, Teoria e applicazioni, Philips Semiconduttori, Milano, Italy, 1965.