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Study of the properties of LB films of a new cellulose derivative bearing terthienyl side groups
Colloids and Surfaces A: Physicochemical and Engineering Aspects 171 (2000) 65 – 73 www.elsevier.nl/locate/colsurfa
Study of the properties of LB films of a new cellulose derivative bearing terthienyl side groups Nicola Ranieri , Gerhard Wegner * Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany Dedicated to Professor Dietmar Mo¨bius on occasion of his 60th birthday.
Abstract An electrochemically active derivative of cellulose bearing terthienyl side chains has been prepared starting from butylcellulose (degree of substitution = 1.9) and 5(2-chloroethyl)-2,2%:5%,2%%-terthiophene (CETT). This polymer was suitable for application of the Langmuir–Blodgett (LB) technique in order to obtain organized films. The degree of order of such films was investigated by X-ray reflectometry and by UV-Vis and fluorescence spectroscopy, that showed a small but measurable alignment of the side groups. The TT side groups could not be addressed by cyclovoltammetry when the LB-architecture was probed; however, the same polymer in solution showed an irreversible signal in cyclovoltammetry. When the LB-architecture on gold substrate was contacted with a solution of 3-penthylthiophene electropolymerization of the latter could be achieved and islands of the poly(3-penthylthiophene) were observed with concomitant disappearance of the UV-signal of the grafted TT moieties. © 2000 Elsevier Science B.V. All rights reserved. Keywords: LB films; Terthienyl side groups; Polymer
1. Introduction 2,2%:5%,2%%-terthiophene (TT) can be regarded as an oligomer of polythiophene whose electrochemical properties arise from the p-conjugation of the three thiophene rings. Although the conjugation length of the p-electrons is only 3 monomer units, TT and its derivatives show features very similar to polythiophenes, like thermochromic [1], solvatochromic [2] and electrochromic behavior [3]. In * Corresponding author. E-mail addresses: [email protected] (N. Ranieri), [email protected] (G. Wegner)
fact, such conjugation is responsible for the stability of the oxidized state of TT, that can be obtained both via chemical and electrochemical oxidation [4]. Hence TT will undergo redox cycles, in a totally reversible way if its a-positions are prevented to react [5]. Otherwise sexithiophene is obtained by TT oxidation, if only one a-position bears a substituent, or TT itself can polymerize and a homopolymer is obtained. The UV absorption of TTs falls in the visible range [6], both in the neutral and oxidized states, so that any change in the extent of conjugation or of the redox state gives rise to a visible change of
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N. Ranieri, G. Wegner / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 65–73
the light absorption. This, together with the reversibility of the redox processes, makes TTs suitable for all applications where a visible response is linked to a change of the potential applied [7]. In the present work the behavior of a TT derivative in matrix characterized by supramolecular order is studied. For this aim, it was chosen to prepare Langmuir – Blodgett (LB) films of a
cellulose derivative carrying terthienyl groups as side chains. The accessibility for substitution of the hydroxyl groups of cellulose allows the synthesis of derivatives, which show the required features for LB film deposition. The preparation of ultrathin films of cellulosic materials has been frequently described in literature using cellulose derivatives,
Scheme 1. [NR1]SSynthesis of 5(2-chloroethyl)-2,2%:5%,2%%-terthiophene (CETT).
Scheme 2. Preparation of butylcellulose carrying terthienyl substituents.
N. Ranieri, G. Wegner / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 65–73
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Fig. 1. Pressure vs. molecular area diagram at 12°C. Table 1 Conditions of Langmuir–Blodgett (LB) transfers of butylcellulose derivative (BCTTE)a Sample
Substrate
NRSi4 NRAu32 NRAu33 NRAu36 NRAu37 NRQu1 NITO1 NITO3
Au-covered Au-covered Au-covered Au-covered Au-covered Quartz ITO ITO
a
T (°C) Si glass glass glass glass
12 5 5 12 12 5 12 12
p (mN/m)
No. layers
14.5 17 17 14.5 14.5 16 14.5 14.5
50 40 40 40 40 40 50 120
Dipping rate =6 mm/min.
such as alkyl- [8 – 10] and alkylsilyl-substituted [11,12] cellulose. Films of cellulose homopolymer were obtained as well by the regeneration, after the LB transfer, of a precursor polymer, i.e. trimetylsilyl-cellulose [13]. Such films present a well defined supramolecular architecture, which can be referred to as an assembly of hairy rigid rods [14,15], as the rigid cellulose backbone is dispersed in the flexible side chains at a molecular level. Several TT derivatives have been prepared in recent works, bearing different substituents in its a positions, such as for instance methyl [16,17] or
methoxyl [18]. Thus a synthetic route was found to provide TT with the a suitable functionality in order to obtain a cellulose derivative, which can be used to construct LB films. So far, copolymers with a non-conjugated main chain and TT side chains [19,20], or block copolymers containing the oligomer in the backbone [21], were studied only in solution or as a free standing, amorphous film. In this case the feature of modified cellulose to be assembled in ordered films, could be used to study how TT was organized in such matrices. The objective was to study whether the structural order that is observed in
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the LB films of cellulose derivatives could induce supramolecular order among the short rigid rodlike terthienyl side chains during the LB deposition of the TT-carrying cellulose, in other words, the authors were looking for effects of orientation of the electrochemically active species with regard to the plane of the substrate.
Fig. 2. UV polarized spectrum of sample NITO1 (Table 1), where Ax and Ay are the absorbance polarized perpendicular and parallel to the Langmuir–Blodgett (LB) dipping direction y.
2. Results With the aim to prepare a cellulose polymer bearing terthienyl side chains, we first synthesized the 5(2-chloroethyl)-2,2%:5%,2%%-terthiophene (CETT, Scheme 1), following a strategy of synthesis similar to that described in the literature [22], in order to react the –OH groups of cellulose with the primary chloride to achieve a Williamson ether synthesis. A butylcellulose with a degree of substitution (DS) of the –OH groups of 1.9 (synthesized as in ref.[23]) as the cellulose derivative was used; the free OH-groups on the cellulose backbone were activated by NaH (Scheme 2). The H1-NMR analysis of the obtained butylcellulose derivative (BCTTE) showed the grafting of the TT side groups by the presence of a broad signal in the region 6.8–7.3 ppm, corresponding to the aromatic protons of TT. The presence of the heteroaromatic conjugated rings was confirmed also in the UV spectrum by a band at 375 nm. Elemental analysis gave a DS= 0.35 for the TT groups.
2.1. Deposition of LB layers Stable monolayers at the water–air interface were obtained when BCTTE was spread to the surface of a LB trough. As an example the pressure versus molecular area diagram recorded at 12°C is shown in Fig. 1. The principal features seen in this diagram are totally in agreement with the reported data in literature on the spreading behavior of cellulose ethers [9]. The substrates used for the depositions, together with the conditions of transfer, are shown in Table 1. All the substrates were hydrophobized by exposure to hexametyldisilaxane vapors before LB transfers. The transfer ratio was on average 70%.
2.2. Properties of the LB layers Fig. 3. polarized fluorescence of sample NRQu1 (Table 1), where xx, xy, yx, yy mean the orientations of the polarizer for the excitation and for the emission with respect to the Langmuir – Blodgett (LB) dipping direction y. (i.e. for xx mean both polarizers are perpendicular to the y direction, for xy the first polarizer is perpendicular and the second is parallel, etc.).
The thickness of the films obtained was monitored by X-ray reflectometry. The reflectogram showed a set of fringes, which were interpreted as Bragg reflections of the layered structure. The
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Fig. 4. Cyclic voltammetry (CV) of CH3CN solution of butylcellulose derivative (BCTTE) (0.5 g/l, corresponding to 5 × 10 − 4mol/l TT).
thickness per monolayer was estimated as roughly 0.8–0.9 nm. This value is supported by similar data reported in the literature [8 – 10]. The UV-Vis spectra observed with polarized light allow to draw conclusions on orientation phenomena concerning the chromophores in the x, y plane (substrate plane) of the sample. The reason is that the transition moment of the optical absorption of the TT residue is strongly polarized along the long axis of the molecule. Only a slightly stronger absorbance was observed along the LB dipping direction (Ay, Fig. 2) with respect to the one along the perpendicular direction (Ax ). The dichroic ratio R =Ay /Ax was on average only 1.05, as demonstrated in Fig. 2, that is the alignment of the TT chromophores as a consequence of possible shear alignment during the transfer is almost negligible. Fluorescence spectra with polarized light of the sample on quartz substrate was also recorded. The excitation wavelength was lexc =390 nm, and the maximum of the emission occurred at lmax = 495 nm. Fig. 3 shows the four spectra recorded in the polarized mode, where xx, xy, yx and yy mean the respective orientations of the excitation
polarizer and the emission polarizer referred to the x, y plane of the substrate. The polarization ratio was calculated as follows: p= (I// − IÞ)/(I// + IÞ) where I// is the emission intensity when the excitation and emission polarizers are parallel and IÞ is the emission intensity when the two polarizers are
Fig. 5. UV spectrum of sample NRAu37 (Table 1) before and after P3pT formation.
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Fig. 6. Islands of P3pT on sample NRAu32 (Table 1) by optical microscope.
of the layered structure does not establish an electrochemical equilibrium with the electrode surface — a mediator not being present — the cyclic voltammogram of a solution of BCTTE was recorded (Fig. 4). In this case of an unconfined geometry it was possible to observe an oxidation wave at about 1.1 V/SCE: this irreversible oxidation was interpreted as the formation of the radical of the terthienyl side groups, which was then likely to undergo side reactions at the free a-position. Indeed, because of the small DS of the terthienyl side groups, the coupling between two side groups has a very low probability to occur. The coupling would have been otherwise detected by the appearance of another oxidation wave at lower potential in the CV (presence of a sexithiophene moiety).
2.3. Grafting of thiophene onto modified cellulose
Fig. 7. UV polarized spectrum of sample NRAu32 before 3pT polymerization.
perpendicular [24]. A value for p of about 20% was found, confirming the small alignment of the terthienyl side chains observed in the UV spectra. The LB films of BCTTE on ITO substrates were submitted to cyclic voltammetry (CV) in order to study the electrochemical response of the LB architecture, and to compare the results with data of a work dealing with the electrochromic properties of polymer bound TT moieties in homogeneous solution [16]. None of the samples prepared showed an electrochemical response: as this could depend on the situation that the small amount of the electroactive moieties embedded in the confined geometry
In a recent work of our group [25] the electrochemical polymerization of 3-pentylthiophene (3pT) was carried out on electrodes covered by LB films of butylcellulose. The polythiophene ‘islands’ so formed showed a certain degree of anisotropy, observed by polarized optical microscopy. Consequently, substrates covered first by gold then by a number of LB layers of BCTTE as anodes for the oxidative polymerization of 3pT were used. The terthienyl moiety can work as the initiator of such oxidative polymerization, as its oxidative potential is lower than the one of the alkylthiophene monomer: 0.94 V/SCE [26] for TT versus 1.84 V/SCE of 3pT [27]. So TT is the first monomer in the reaction medium to be oxidized and it can initiate therefore the polymerization of the 3pT monomers. Such feature of terthienyl groups was already observed in a previous work dealing with the grafting in solution of thiophene onto methacrylate copolymers bearing terthienyl side rings [28]. The aim was to study whether the presence of the partially oriented terthienyl side groups, that initiate the polymerization in the LB matrix, could improve the macroscopic anisotropy of the formed P3pT phase, already observed in the previous work [25]. Further, the authors were inter-
N. Ranieri, G. Wegner / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 65–73
ested to see whether they could induce an anisotropy by chain alignment in the polythiophene domains, the growth of which takes place by grafting onto the rigid and partially ordered TT segments of the modified cellulose. The electropolymerization conditions of 3pT are summarized in Section 4. After the polymerization the substrates were washed with CHCN3 and kept without further treatment in air. When the LB film of BCTTE on the gold covered substrate was used as the anode in contact with a 0.2 M solution of 3pT under the conditions of electrochemical polymerization as described in Section 4, the formation of the expected polymer could be readily observed by UVVis spectroscopy as demonstrated by Fig. 5. In the spectra, the band due to the terthienyl moiety with maximum at 375 nm disappears after the polymerization of 3pT, while another band appears, with maximum at 500 nm, due to poly(3pentylthiophene) [29]. The islands of P3pT obtained, observed through an optical microscope (Fig. 6), did not show any anisotropy at a macroscopic level. On average, the islands had a diameter of ca. 75 mm. It is worth mentioning that the formation of islands is suppressed and continuos films are rather formed if the electropolymerization is carried out on plane gold electrodes. On the other hand, by the analysis with UV polarized light, a stronger absorbance of the poly(3pT) chains could be detected along the LB dipping direction (Ay, Fig. 7) rather than along the perpendicular one (Ax ). The degree of anisotropy can be estimated by the value R measured, that was on average 1.23. This preferential alignment of the poly(3pT) chains can be due either to the TT side chains, even if only partially oriented along the y axis, or to the aligning properties of the cellulose backbones, which are highly oriented [11,12].
3. Discussion A new cellulose derivative carrying terthienyl side groups, BCTTE, was synthesized. LB films were obtained on various substrates which were
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studied in order to observe the role of the ordered matrix on the electroactivity of the p-conjugated side chains. The side groups did not show an electrochemical response while embedded in the LB architecture and probed by cyclic voltammetry. However, the same polymer showed an irreversible voltammogram when probed in solution. This may be explained by the assumption that the sterically hindered terthiophene moieties do not have sufficient mobility in the layered architecture of the LB-film to make the necessary contact with the electrode surface and with each other. However, in solution, the contact of the terthiophene groups with the electrode to form the radical cation followed by rapid deactivation of the intermediate by side reactions is straightforward. A radical– radical coupling reaction, that is formation of sexithiophene moieties, is prevented by comparatively high dilution of the TT-species. Thus, electrochemical crosslinking in solution was not observed. The fact that the 375 nm band of the TT species disappears as the electrochemical polymerization of 3pT is induced in the matrix of the LB architecture indicates the involvement of the grafted TT moieties in the electrochemical deposition. Whether the island formation in terms of number density and distribution bears any relation to the occurrence of the TT species needs further investigations. So far it can not be substantiated that TT-species grafted to the cellulose backbone serve as nucleation sites for islands of poly(3pT).
4. Experimental section
4.1. Materials Thionyl chloride (SOCl2), buyllitium (BuLi), ethylene oxide (EO), acetonitrile (CH3CN), tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) were used without purification. Tetrahydrofuran (THF), sodium hydride (NaH), triethylamine (Et3N) were purified by conventional methods. TT was synthesized according to Ref. [30]
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4.2. Synthesis of 5(2 -hydroxyethyl) -2,2 %:5 %,2 %%-terthiophene (TTE) Terthiophene (57 mg, 0.230 mmol) was added, under an argon blanket, to a solution of BuLi (0.07 ml× 1.6 M, 0.112 mmol) and diisopropylamine (0.016 ml, 0.113 mmol) in 5 ml THF at −78°C. After stirring for 2 h, a solution of EO in THF (1 ml, 0.116 mmol) was slowly dropped in the reaction medium, that was then kept stirring for another 16 h. The mixture was hydrolyzed with diluted HCl, added of CHCl3 and then separated in the two phases. The organic one was dried over anhydrous Na2SO4 and the crude product was purified by silica gel column chromatography, using an ethylacetate/n-hexane 3:1 mixture as eluent. A total of 50 mg (0.171 mmol, 74% yield) of TTE were collected. The H1-NMR (CDCl3) showed signals at 1.65 ppm (s, 1H), 3.15 ppm (m, 2H), 3.80 ppm (m, 2H), 7.00–7.20 ppm (m, 7H). The UV-Vis (THF) spectrum had a broad peak with maximum at l= 404 nm.
4.3. Synthesis of TTE A solution of 50 mg TTE (0.171 mmol) in 2 ml THF was added to an excess of SOCl2 (3 ml, 41.57 mmol) and Et3N (0.1 ml, 0.72 mmol) at 0°C. After 20 h of stirring at RT, the mixture was refluxed for 1 h and then the solvent and the SOCl2 excess were removed under vacuum. Water and CHCl3 were added to the crude product, and the organic phase was washed 3 times with water and then dried over anhydrous Na2SO4. After filtration, 45 mg (0.15 mmol, 88% yield) of 5(2chloroethyl)-2,2%:5%,2%%-terthiophene (CETT) were collected. The H1-NMR (CDCl3) showed signals at 3.35 ppm (m, 2H), 3.45 ppm (m, 2H), 6.80 – 7.50 ppm (m, 7H). The UV-Vis (THF) spectrum had a broad peak with maximum at l= 401 nm.
4.4. Synthesis of (2,2 %:5 %,2 %%-terthien-5 -yl)ethyl-butyl cellulose Butylcellulose with DSOH =1.9 (44 mg, 0.18
mmol – OH) and purified NaH (7 mg, 0.29 mmol) were suspended in 6 ml THF and stirred for 2 h under argon atmosphere. A solution of CETT (45 mg, 0.15 mmol) in 2 ml THF was then dropped and the suspension was kept stirring at RT for 6 days. The mixture was then hydrolyzed with ice, filtered and rinsed with CHCl3. After three precipitation steps in MeOH, 25 mg of polymer with DSterthienyl = 0.35 was collected. The H1-NMR (CDCl3) showed broad signals at 0.80–0.95 ppm (m), 1.15–1.65 ppm (m), 2.80– 4.00 ppm (m), 4.10–4.30 ppm (m), 6.80–7.45 ppm (m). The UV-Vis (THF) spectrum had a broad peak with maximum at l= 404 nm.
4.5. Deposition of films by the LB technique BCTTE was spread onto a LB trough from a solution of 5 mg of polymer in 10 ml CHCl3. The deposition pressure was 14.5 mN/m at 12°C and 16–17 mN/m at 5°C. The substrates used were normal glass and silicon wafers sputtered with 25 A, of chrome and 100 A, of gold on top.
4.6. Electrochemical oxidation Cyclic voltammetry was carried out in a CH3CN solution containing Bu4NPF6 (0.1 M) as a supporting electrolyte with a Ag/Ag+ (0.01M) reference electrode. Sweep rate: 100 mV/s. Electrochemical polymerization of 3pT in contact with a gold electrode covered by BCTTE films was carried out under galvanostatic conditions (current density= 5 mA/cm2) in a CH3CN solution with a concentration of the monomer 0.2 M and Bu4NPF6 (0.1 M) as supporting electrolyte, using gold covered substrates with LB films as working electrode and a Platinum foil as counter electrode.
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N. Ranieri, G. Wegner / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 65–73 [2] F. Effenberger, F. Wurthner, F. Steybe, J. Org. Chem. 60 (1995) 2082. [3] K. Nawa, K. Miyawaki, I. Imae, N. Noma, Y. Shirota, J. Mater. Chem. 3 (1) (1993) 113. [4] J. Guay, P. Kasay, A. Diaz, R. Wu, J.M. Tour, L.H. Dao, Chem. Mater. 4 (1992) 1097. [5] M.G. Hill, J.-F. Penneau, B. Zinger, K.R. Mann, L.L. Miller, Chem. Mater. 4 (1992) 1106. [6] P. Garcia, J.M. Pernaut, P. Hapiot, V. Wintgens, P. Valat, F. Garnier, D. Delabouglise, J. Phys. Chem. 97 (1993) 513. [7] F. Geiger, M. Stoldt, H. Schweizer, P. Bauerle, E. Umbach, Adv. Mater. 5 (1993) 922. [8] G. Wegner, Ber. Bunsenges. Phys. Chem. 11 (1991) 95. [9] M. Schaub, C. Fakirov, A. Schmidt, G. Lieser, G. Wenz, G. Wegner, P.-A. Albony, H. Wu, M.D. Foster, C. Majrkzak, S. Satija, Macromolecules 28 (1995) 1221. [10] T. Kawaguchi, H. Nakahara, Thin Solid Films 113 (1985) 29. [11] G. Wiegand, T. Jaworek, G. Wegner, E. Sackmann, Langmuir 13 (1997) 3563. [12] M. Schaub, G. Wenz, G. Wegner, A. Steim, D. Klemm, Adv. Mater. 5 (1993) 919. [13] V. Buchholz, P. Adler, M. Ba¨cker, W. Ho¨lle, A. Simon, G. Wegner, Langmuir 13 (1997) 3206. [14] G. Wegner, Ber. Bunsenges. Phys. Chem. 95 (1993) 919.
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[15] G. Wegner, Mol. Cryst. Liq. Cryst. 216 (1992) 7. [16] G. Zotti, G. Schiavon, A. Berlin, G. Pagani, Chem. Mater. 5 (1993) 430. [17] P. Bauerle, Adv. Mater. 4 (1992) 102. [18] F. Effenberger, F. Wurthner, F. Steybe, J. Org. Chem. 60 (1995) 2082. [19] K. Nawa, I. Imae, N. Noma, Y. Shirota, Macromolecules 28 (1995) 723. [20] R.K. Khanna, H. Cui, Macromolecules 26 (1993) 7076. [21] S.A. Jenekhe, Macromolecules 23 (1990) 2848. [22] J. Kagan, S.K. Arora, A. Ustunol, J. Org. Chem. 48 (1983) 4076. [23] H. Tebbe, Ph.D. Thesis, Johannes-Gutenberg University Mainz (1996). [24] Joseph R. Lakowicz, Principles of Fluorescence Spectroscopy, Plenum Press, New York, 1983. [25] C.D. Henry, H. Tebbe, G. Wegner, F. Armand, A. Ruaudel-Teixier, Adv. Mater. 9 (1997) 805. [26] J.P. Ferraris, G.D. Skiles, Polymer 28 (1987) 179. [27] J. Roncali, R. Garreau, A. Yassar, P. Marque, F. Garnier, M. Lemaire, J. Phys. Chem. 91 (1987) 6706. [28] N. Ranieri, G. Ruggeri, Polymer International 48 (1991) 1091. [29] J. Roncali, Chem. Rev. 92 (1992) 711. [30] A. Carpita, R. Rossi, Gazz. Chim. Ital. 115 (1985) 575.
Study of the properties of LB films of a new cellulose derivative bearing terthienyl side groups Nicola Ranieri , Gerhard Wegner * Max-Planck-Institut fu¨r Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany Dedicated to Professor Dietmar Mo¨bius on occasion of his 60th birthday.
Abstract An electrochemically active derivative of cellulose bearing terthienyl side chains has been prepared starting from butylcellulose (degree of substitution = 1.9) and 5(2-chloroethyl)-2,2%:5%,2%%-terthiophene (CETT). This polymer was suitable for application of the Langmuir–Blodgett (LB) technique in order to obtain organized films. The degree of order of such films was investigated by X-ray reflectometry and by UV-Vis and fluorescence spectroscopy, that showed a small but measurable alignment of the side groups. The TT side groups could not be addressed by cyclovoltammetry when the LB-architecture was probed; however, the same polymer in solution showed an irreversible signal in cyclovoltammetry. When the LB-architecture on gold substrate was contacted with a solution of 3-penthylthiophene electropolymerization of the latter could be achieved and islands of the poly(3-penthylthiophene) were observed with concomitant disappearance of the UV-signal of the grafted TT moieties. © 2000 Elsevier Science B.V. All rights reserved. Keywords: LB films; Terthienyl side groups; Polymer
1. Introduction 2,2%:5%,2%%-terthiophene (TT) can be regarded as an oligomer of polythiophene whose electrochemical properties arise from the p-conjugation of the three thiophene rings. Although the conjugation length of the p-electrons is only 3 monomer units, TT and its derivatives show features very similar to polythiophenes, like thermochromic [1], solvatochromic [2] and electrochromic behavior [3]. In * Corresponding author. E-mail addresses: [email protected] (N. Ranieri), [email protected] (G. Wegner)
fact, such conjugation is responsible for the stability of the oxidized state of TT, that can be obtained both via chemical and electrochemical oxidation [4]. Hence TT will undergo redox cycles, in a totally reversible way if its a-positions are prevented to react [5]. Otherwise sexithiophene is obtained by TT oxidation, if only one a-position bears a substituent, or TT itself can polymerize and a homopolymer is obtained. The UV absorption of TTs falls in the visible range [6], both in the neutral and oxidized states, so that any change in the extent of conjugation or of the redox state gives rise to a visible change of
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the light absorption. This, together with the reversibility of the redox processes, makes TTs suitable for all applications where a visible response is linked to a change of the potential applied [7]. In the present work the behavior of a TT derivative in matrix characterized by supramolecular order is studied. For this aim, it was chosen to prepare Langmuir – Blodgett (LB) films of a
cellulose derivative carrying terthienyl groups as side chains. The accessibility for substitution of the hydroxyl groups of cellulose allows the synthesis of derivatives, which show the required features for LB film deposition. The preparation of ultrathin films of cellulosic materials has been frequently described in literature using cellulose derivatives,
Scheme 1. [NR1]SSynthesis of 5(2-chloroethyl)-2,2%:5%,2%%-terthiophene (CETT).
Scheme 2. Preparation of butylcellulose carrying terthienyl substituents.
N. Ranieri, G. Wegner / Colloids and Surfaces A: Physicochem. Eng. Aspects 171 (2000) 65–73
67
Fig. 1. Pressure vs. molecular area diagram at 12°C. Table 1 Conditions of Langmuir–Blodgett (LB) transfers of butylcellulose derivative (BCTTE)a Sample
Substrate
NRSi4 NRAu32 NRAu33 NRAu36 NRAu37 NRQu1 NITO1 NITO3
Au-covered Au-covered Au-covered Au-covered Au-covered Quartz ITO ITO
a
T (°C) Si glass glass glass glass
12 5 5 12 12 5 12 12
p (mN/m)
No. layers
14.5 17 17 14.5 14.5 16 14.5 14.5
50 40 40 40 40 40 50 120
Dipping rate =6 mm/min.
such as alkyl- [8 – 10] and alkylsilyl-substituted [11,12] cellulose. Films of cellulose homopolymer were obtained as well by the regeneration, after the LB transfer, of a precursor polymer, i.e. trimetylsilyl-cellulose [13]. Such films present a well defined supramolecular architecture, which can be referred to as an assembly of hairy rigid rods [14,15], as the rigid cellulose backbone is dispersed in the flexible side chains at a molecular level. Several TT derivatives have been prepared in recent works, bearing different substituents in its a positions, such as for instance methyl [16,17] or
methoxyl [18]. Thus a synthetic route was found to provide TT with the a suitable functionality in order to obtain a cellulose derivative, which can be used to construct LB films. So far, copolymers with a non-conjugated main chain and TT side chains [19,20], or block copolymers containing the oligomer in the backbone [21], were studied only in solution or as a free standing, amorphous film. In this case the feature of modified cellulose to be assembled in ordered films, could be used to study how TT was organized in such matrices. The objective was to study whether the structural order that is observed in
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the LB films of cellulose derivatives could induce supramolecular order among the short rigid rodlike terthienyl side chains during the LB deposition of the TT-carrying cellulose, in other words, the authors were looking for effects of orientation of the electrochemically active species with regard to the plane of the substrate.
Fig. 2. UV polarized spectrum of sample NITO1 (Table 1), where Ax and Ay are the absorbance polarized perpendicular and parallel to the Langmuir–Blodgett (LB) dipping direction y.
2. Results With the aim to prepare a cellulose polymer bearing terthienyl side chains, we first synthesized the 5(2-chloroethyl)-2,2%:5%,2%%-terthiophene (CETT, Scheme 1), following a strategy of synthesis similar to that described in the literature [22], in order to react the –OH groups of cellulose with the primary chloride to achieve a Williamson ether synthesis. A butylcellulose with a degree of substitution (DS) of the –OH groups of 1.9 (synthesized as in ref.[23]) as the cellulose derivative was used; the free OH-groups on the cellulose backbone were activated by NaH (Scheme 2). The H1-NMR analysis of the obtained butylcellulose derivative (BCTTE) showed the grafting of the TT side groups by the presence of a broad signal in the region 6.8–7.3 ppm, corresponding to the aromatic protons of TT. The presence of the heteroaromatic conjugated rings was confirmed also in the UV spectrum by a band at 375 nm. Elemental analysis gave a DS= 0.35 for the TT groups.
2.1. Deposition of LB layers Stable monolayers at the water–air interface were obtained when BCTTE was spread to the surface of a LB trough. As an example the pressure versus molecular area diagram recorded at 12°C is shown in Fig. 1. The principal features seen in this diagram are totally in agreement with the reported data in literature on the spreading behavior of cellulose ethers [9]. The substrates used for the depositions, together with the conditions of transfer, are shown in Table 1. All the substrates were hydrophobized by exposure to hexametyldisilaxane vapors before LB transfers. The transfer ratio was on average 70%.
2.2. Properties of the LB layers Fig. 3. polarized fluorescence of sample NRQu1 (Table 1), where xx, xy, yx, yy mean the orientations of the polarizer for the excitation and for the emission with respect to the Langmuir – Blodgett (LB) dipping direction y. (i.e. for xx mean both polarizers are perpendicular to the y direction, for xy the first polarizer is perpendicular and the second is parallel, etc.).
The thickness of the films obtained was monitored by X-ray reflectometry. The reflectogram showed a set of fringes, which were interpreted as Bragg reflections of the layered structure. The
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Fig. 4. Cyclic voltammetry (CV) of CH3CN solution of butylcellulose derivative (BCTTE) (0.5 g/l, corresponding to 5 × 10 − 4mol/l TT).
thickness per monolayer was estimated as roughly 0.8–0.9 nm. This value is supported by similar data reported in the literature [8 – 10]. The UV-Vis spectra observed with polarized light allow to draw conclusions on orientation phenomena concerning the chromophores in the x, y plane (substrate plane) of the sample. The reason is that the transition moment of the optical absorption of the TT residue is strongly polarized along the long axis of the molecule. Only a slightly stronger absorbance was observed along the LB dipping direction (Ay, Fig. 2) with respect to the one along the perpendicular direction (Ax ). The dichroic ratio R =Ay /Ax was on average only 1.05, as demonstrated in Fig. 2, that is the alignment of the TT chromophores as a consequence of possible shear alignment during the transfer is almost negligible. Fluorescence spectra with polarized light of the sample on quartz substrate was also recorded. The excitation wavelength was lexc =390 nm, and the maximum of the emission occurred at lmax = 495 nm. Fig. 3 shows the four spectra recorded in the polarized mode, where xx, xy, yx and yy mean the respective orientations of the excitation
polarizer and the emission polarizer referred to the x, y plane of the substrate. The polarization ratio was calculated as follows: p= (I// − IÞ)/(I// + IÞ) where I// is the emission intensity when the excitation and emission polarizers are parallel and IÞ is the emission intensity when the two polarizers are
Fig. 5. UV spectrum of sample NRAu37 (Table 1) before and after P3pT formation.
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Fig. 6. Islands of P3pT on sample NRAu32 (Table 1) by optical microscope.
of the layered structure does not establish an electrochemical equilibrium with the electrode surface — a mediator not being present — the cyclic voltammogram of a solution of BCTTE was recorded (Fig. 4). In this case of an unconfined geometry it was possible to observe an oxidation wave at about 1.1 V/SCE: this irreversible oxidation was interpreted as the formation of the radical of the terthienyl side groups, which was then likely to undergo side reactions at the free a-position. Indeed, because of the small DS of the terthienyl side groups, the coupling between two side groups has a very low probability to occur. The coupling would have been otherwise detected by the appearance of another oxidation wave at lower potential in the CV (presence of a sexithiophene moiety).
2.3. Grafting of thiophene onto modified cellulose
Fig. 7. UV polarized spectrum of sample NRAu32 before 3pT polymerization.
perpendicular [24]. A value for p of about 20% was found, confirming the small alignment of the terthienyl side chains observed in the UV spectra. The LB films of BCTTE on ITO substrates were submitted to cyclic voltammetry (CV) in order to study the electrochemical response of the LB architecture, and to compare the results with data of a work dealing with the electrochromic properties of polymer bound TT moieties in homogeneous solution [16]. None of the samples prepared showed an electrochemical response: as this could depend on the situation that the small amount of the electroactive moieties embedded in the confined geometry
In a recent work of our group [25] the electrochemical polymerization of 3-pentylthiophene (3pT) was carried out on electrodes covered by LB films of butylcellulose. The polythiophene ‘islands’ so formed showed a certain degree of anisotropy, observed by polarized optical microscopy. Consequently, substrates covered first by gold then by a number of LB layers of BCTTE as anodes for the oxidative polymerization of 3pT were used. The terthienyl moiety can work as the initiator of such oxidative polymerization, as its oxidative potential is lower than the one of the alkylthiophene monomer: 0.94 V/SCE [26] for TT versus 1.84 V/SCE of 3pT [27]. So TT is the first monomer in the reaction medium to be oxidized and it can initiate therefore the polymerization of the 3pT monomers. Such feature of terthienyl groups was already observed in a previous work dealing with the grafting in solution of thiophene onto methacrylate copolymers bearing terthienyl side rings [28]. The aim was to study whether the presence of the partially oriented terthienyl side groups, that initiate the polymerization in the LB matrix, could improve the macroscopic anisotropy of the formed P3pT phase, already observed in the previous work [25]. Further, the authors were inter-
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ested to see whether they could induce an anisotropy by chain alignment in the polythiophene domains, the growth of which takes place by grafting onto the rigid and partially ordered TT segments of the modified cellulose. The electropolymerization conditions of 3pT are summarized in Section 4. After the polymerization the substrates were washed with CHCN3 and kept without further treatment in air. When the LB film of BCTTE on the gold covered substrate was used as the anode in contact with a 0.2 M solution of 3pT under the conditions of electrochemical polymerization as described in Section 4, the formation of the expected polymer could be readily observed by UVVis spectroscopy as demonstrated by Fig. 5. In the spectra, the band due to the terthienyl moiety with maximum at 375 nm disappears after the polymerization of 3pT, while another band appears, with maximum at 500 nm, due to poly(3pentylthiophene) [29]. The islands of P3pT obtained, observed through an optical microscope (Fig. 6), did not show any anisotropy at a macroscopic level. On average, the islands had a diameter of ca. 75 mm. It is worth mentioning that the formation of islands is suppressed and continuos films are rather formed if the electropolymerization is carried out on plane gold electrodes. On the other hand, by the analysis with UV polarized light, a stronger absorbance of the poly(3pT) chains could be detected along the LB dipping direction (Ay, Fig. 7) rather than along the perpendicular one (Ax ). The degree of anisotropy can be estimated by the value R measured, that was on average 1.23. This preferential alignment of the poly(3pT) chains can be due either to the TT side chains, even if only partially oriented along the y axis, or to the aligning properties of the cellulose backbones, which are highly oriented [11,12].
3. Discussion A new cellulose derivative carrying terthienyl side groups, BCTTE, was synthesized. LB films were obtained on various substrates which were
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studied in order to observe the role of the ordered matrix on the electroactivity of the p-conjugated side chains. The side groups did not show an electrochemical response while embedded in the LB architecture and probed by cyclic voltammetry. However, the same polymer showed an irreversible voltammogram when probed in solution. This may be explained by the assumption that the sterically hindered terthiophene moieties do not have sufficient mobility in the layered architecture of the LB-film to make the necessary contact with the electrode surface and with each other. However, in solution, the contact of the terthiophene groups with the electrode to form the radical cation followed by rapid deactivation of the intermediate by side reactions is straightforward. A radical– radical coupling reaction, that is formation of sexithiophene moieties, is prevented by comparatively high dilution of the TT-species. Thus, electrochemical crosslinking in solution was not observed. The fact that the 375 nm band of the TT species disappears as the electrochemical polymerization of 3pT is induced in the matrix of the LB architecture indicates the involvement of the grafted TT moieties in the electrochemical deposition. Whether the island formation in terms of number density and distribution bears any relation to the occurrence of the TT species needs further investigations. So far it can not be substantiated that TT-species grafted to the cellulose backbone serve as nucleation sites for islands of poly(3pT).
4. Experimental section
4.1. Materials Thionyl chloride (SOCl2), buyllitium (BuLi), ethylene oxide (EO), acetonitrile (CH3CN), tetra-n-butylammonium hexafluorophosphate (Bu4NPF6) were used without purification. Tetrahydrofuran (THF), sodium hydride (NaH), triethylamine (Et3N) were purified by conventional methods. TT was synthesized according to Ref. [30]
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4.2. Synthesis of 5(2 -hydroxyethyl) -2,2 %:5 %,2 %%-terthiophene (TTE) Terthiophene (57 mg, 0.230 mmol) was added, under an argon blanket, to a solution of BuLi (0.07 ml× 1.6 M, 0.112 mmol) and diisopropylamine (0.016 ml, 0.113 mmol) in 5 ml THF at −78°C. After stirring for 2 h, a solution of EO in THF (1 ml, 0.116 mmol) was slowly dropped in the reaction medium, that was then kept stirring for another 16 h. The mixture was hydrolyzed with diluted HCl, added of CHCl3 and then separated in the two phases. The organic one was dried over anhydrous Na2SO4 and the crude product was purified by silica gel column chromatography, using an ethylacetate/n-hexane 3:1 mixture as eluent. A total of 50 mg (0.171 mmol, 74% yield) of TTE were collected. The H1-NMR (CDCl3) showed signals at 1.65 ppm (s, 1H), 3.15 ppm (m, 2H), 3.80 ppm (m, 2H), 7.00–7.20 ppm (m, 7H). The UV-Vis (THF) spectrum had a broad peak with maximum at l= 404 nm.
4.3. Synthesis of TTE A solution of 50 mg TTE (0.171 mmol) in 2 ml THF was added to an excess of SOCl2 (3 ml, 41.57 mmol) and Et3N (0.1 ml, 0.72 mmol) at 0°C. After 20 h of stirring at RT, the mixture was refluxed for 1 h and then the solvent and the SOCl2 excess were removed under vacuum. Water and CHCl3 were added to the crude product, and the organic phase was washed 3 times with water and then dried over anhydrous Na2SO4. After filtration, 45 mg (0.15 mmol, 88% yield) of 5(2chloroethyl)-2,2%:5%,2%%-terthiophene (CETT) were collected. The H1-NMR (CDCl3) showed signals at 3.35 ppm (m, 2H), 3.45 ppm (m, 2H), 6.80 – 7.50 ppm (m, 7H). The UV-Vis (THF) spectrum had a broad peak with maximum at l= 401 nm.
4.4. Synthesis of (2,2 %:5 %,2 %%-terthien-5 -yl)ethyl-butyl cellulose Butylcellulose with DSOH =1.9 (44 mg, 0.18
mmol – OH) and purified NaH (7 mg, 0.29 mmol) were suspended in 6 ml THF and stirred for 2 h under argon atmosphere. A solution of CETT (45 mg, 0.15 mmol) in 2 ml THF was then dropped and the suspension was kept stirring at RT for 6 days. The mixture was then hydrolyzed with ice, filtered and rinsed with CHCl3. After three precipitation steps in MeOH, 25 mg of polymer with DSterthienyl = 0.35 was collected. The H1-NMR (CDCl3) showed broad signals at 0.80–0.95 ppm (m), 1.15–1.65 ppm (m), 2.80– 4.00 ppm (m), 4.10–4.30 ppm (m), 6.80–7.45 ppm (m). The UV-Vis (THF) spectrum had a broad peak with maximum at l= 404 nm.
4.5. Deposition of films by the LB technique BCTTE was spread onto a LB trough from a solution of 5 mg of polymer in 10 ml CHCl3. The deposition pressure was 14.5 mN/m at 12°C and 16–17 mN/m at 5°C. The substrates used were normal glass and silicon wafers sputtered with 25 A, of chrome and 100 A, of gold on top.
4.6. Electrochemical oxidation Cyclic voltammetry was carried out in a CH3CN solution containing Bu4NPF6 (0.1 M) as a supporting electrolyte with a Ag/Ag+ (0.01M) reference electrode. Sweep rate: 100 mV/s. Electrochemical polymerization of 3pT in contact with a gold electrode covered by BCTTE films was carried out under galvanostatic conditions (current density= 5 mA/cm2) in a CH3CN solution with a concentration of the monomer 0.2 M and Bu4NPF6 (0.1 M) as supporting electrolyte, using gold covered substrates with LB films as working electrode and a Platinum foil as counter electrode.
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