J O U R N A L O F C H E M I S T R Y Materials Poly[3,3¾-dialkyl-2,2¾-(ethyne-1,2-diyl )bis(thiophene)]: electrically conducting and fluorescent polymers incorporating a rigid acetylenic spacer Siu-Choon Ng,*a Teng-Teng Onga and Hardy S. O. Chana,b aDepartment of Chemistry, National University of Singapore, Singapore 119260. †E-mail: chmngsc@leonis.nus.edu.sg bDepartment of Material Science, National University of Singapore, Singapore 119260 Received 27th July 1998, Accepted 2nd September 1998 A series of poly[3,3¾-dialkyl-2,2¾-(ethyne-1,2-diyl )bis(thiophene)]s, comprising a rigid carbon–carbon triple bond between two bithiophene repeating units, were synthesized.The improved rigidity of the polymer backbone led to an increased fluorescent quantum yield in comparison to poly(3-alkylthiophene)s.A generic trend depicting decreasing Stokes shift in the fluorescence spectra with increasing pendant alkyl chain length was observed. The incorporation of the acetylenic spacer also resulted in a significant red shift in the absorption spectra in comparison to poly(3-alkylthiophene)s, corresponding to an increase in eVective conjugation over the entire series of polymers.These polymers, upon doping with iodine or ferric chloride, gave electrical conductivity in the range of 100 to 10-4 S cm-1. Thermochromism studies showed a blue shift in absorption peak as the temperature changes from 25 to 180 °C. The influence of alkyl chain length and the acetylenic spacer on the conductivity and UV–VIS absorption is also discussed.In situ electrochemical doping studies were monitored using UV–VIS–near infrared absorption spectroscopy and showed the evolution of polaron bands at around 1.4 eV. Poly(3-alkylthiophene)s remain attractive candidates for ophene) polymers. We report here our eVorts on the synthesis, characterization and properties of these materials. research studies on account of their good chemical stability, processability and high conductivity in the doped state.1–3 Recently, numerous reports on the light emitting properties of poly(3-alkylthiophene)s4–8 have aroused our interest in this burgeoning field of research.An ideal organic polymer light emitting diode (LED) should as a first requirement exhibit high fluorescence quantum yields, charge mobility, injection barriers and eVective p–conjugation.9–11 Whilst short chain oligomers are known to limit delocalization by diminishing the eVective conjugation, they are nevertheless more rigid, which can result in reducing relaxation from the excited states through non-radiative process with consequently enhanced S R S R R = C12H25 PEBT PDBEBT PDHEBT PDOEBT PDDEBT x R = H R = Bu R = C6H13 R = C8H17 fluorescence.12 Although numerous polythiophene derivatives with high fluorescent quantum yield (>50%)13 have been reported, there has been relatively little research into thio- Experimental phene-based polymers incorporating rigid acetylene spacers.14 In conjunction with our ongoing research on structure–prop- Synthesis of monomers erty correlation of functional and conducting polymers, we Monomer syntheses were carried out in accordance with the have synthesized a series of symmetrical 3,3¾-dialkyl-2,2¾- generic approach depicted in Scheme 1. 3-Alkylthiophenes II (ethyne-1,2-diyl )bis(thiophene) monomers which upon chemiwere synthesised from 3-bromothiophene I by a nickel cata- cal oxidative polymerization with FeCl3 aVorded polymers lysed Grignard cross-coupling approach.18 Bromination of II that exhibited both electrical conductivity and enhanced at the 2-position was eVected using 1 equiv.of N-bromosuc- fluorescence on comparison with polythiophenes. cinimide to aVord 2-bromo-3-alkylthiophene III in nearly The incorporation of the acetylenic spacers into the quantitative yield.19 Thereafter a one-pot reaction of III with polythiophene backbone is anticipated to oVer several distinct 2-methylbut-3-yn-2-ol in the presence of Pd(PPh3)4 as advantages.Thus, they can act as rigid conjugative spacers catalyst20 aVorded the symmetrical monomer IV. linking two bithiophene repeating units through the 2,2¾- positions on the same plane. The resulting polymer can be 2,2¾-( Ethyne-1,2-diyl )bis(thiophene) (EBT) expected to aVord a more planar conformation through diminished steric eVects so that a maximum degree of delocalization A mixture of 2-bromothiophene (2.05 g, 12.6 mmol), of the p-electrons is achieved.15 In addition, the rigid spacer 2-methylbut-3-yn-2-ol (1.06 g, 12.6 mmol), tetrakis(triphenylwhich helps to minimize neighboring ring interactions in this phosphine)palladium(0) (0.15 g, 0.333 mmol), benzyltriethylseries should result in a bathochromic shift in the UV–VIS ammonium bromide (0.099 g, 0.363 mmol) and cuprous iodide absorption maxima with correspondingly reduced bandgap (0.097 g, 0.509 mmol) in 10 ml of benzene was deareated with energy when compared to polythiophene or polyalkylthi- N2 for 15 min.Thereafter, aq. NaOH (5.5 M, 10 ml ) was ophene analogues.16,17 In regard to these favorable factors added.The resulting reaction mixture, which turned brown– which an acetylenic spacer oVers, we have successfully synthe- black, was heated at reflux under a nitrogen atmosphere for 18 h, whence a second portion of 2-bromothiophene (2.08 g, sized a series of 3,3¾-dialkyl-2,2¾-(ethyne-1,2-diyl )bis(thi- J. Mater. Chem., 1998, 8, 2663–2669 2663(24H, m), 0.86 (6H, t, J=7.0 Hz); m/z 414 (M+, 100%), 329 (20), 217 (55) (Found: C, 75.1; H, 8.7; S, 15.4.Calc. for C26H38S25C, 75.3; H, 9.2; S, 15.5%). 3,3¾-Didodecyl-2,2¾-(ethyne-1,2-diyl )bis(thiophene) (DDEBT) 31% yield; dH(300 MHz, CDCl3), 7.18 (2H, d, J=5.0 Hz), 6.88 (2H, d, J=5.0 Hz), 2.74 (4H, t, J=7.5 Hz), 1.66–1.25 (40H, m), 0.87 (6H, t, J=7.5 Hz); m/z 526 (M+, 100%), 385 (20), 217 (50) (Found: C, 78.0; H, 10.3; S, 11.7.Calc. for C34H54S25C, 77.5; H, 10.3; S, 12.1%). Electrochemistry Polymer films were grafted onto a platinum or an indium tin S Br S R S R Br S R S R S R S R I II III IV V iv iii ii i x oxide (ITO) glass electrode via spin-coating using chloroform Scheme 1 Reagents and conditions: i, RMgBr, Et2O, Ni(dppp)Cl2 as solvent. Cyclic voltammetry of the polymers was conducted (cat.); ii, N-bromosuccinimide, chloroform, acetic acid, 30 min, 0 °C; in a three-electrode single compartment electrochemical cell iii, Pd(PPh3)4, 2-methylbut-3-yn-2-ol, benzene, reflux, 48 h; iv, FeCl3, consisting of platinum foil as the working electrode, a platinum CHCl3, 0°C.wire as the counter electrode and Ag/AgNO3 (0.1 M using dry acetonitrile as solvent) as the reference electrode (0.34 V vs.SCE). CVs of polymers were studied under argon atmos- 12.8 mmol) in benzene (10 ml ) was added into the reaction phere using tetra-n-butylammonium fluoroborate (0.1 M) as mixture. After heating at reflux for a further 16 h, the reaction electrolyte. mixture was allowed to cool, followed by addition of aq. NH4Cl (5.5 M, 50 ml ); it was then stirred for 3 h at room Chemical polymerization temperature.The crude product was extracted with benzene and purified by flash chromatography to aVord white needle General procedure. A solution of the monomer (0.1 M) in shaped crystals in 54% yield; mp 99 °C ( lit.,20 99.5–101 °C); dry chloroform was added dropwise into a reaction vessel dH(300 MHz, CDCl3) 7.30 (2H, dd, J2,4=1.1 Hz, J3,4= containing 4 equiv.of anhydrous ferric chloride at 0 °C for 5.1 Hz), 7.26 (2H, dd, J2,4=1.1 Hz, J3,4=3.6 Hz), 7.00 (2H, 1 h. Thereafter, polymerisation was terminated by adding an dd, J3,4=3.6, J2,3=5.1); m/z 190 (M+, 100%), 145 (50) excess amount of methanol. The resulting polymer was sub- (Found: C, 63.3; H, 3.4; S, 33.0. Calc. for C10H6S25C, 63.1; jected to Soxhlet extraction with methanol and then acetone H, 3.2; S, 33.6%).in turn for 24 h each. The resulting polymer was dedoped by stirring the polymer powder in hydrazine hydrate–water (151 3,3¾-Dibutyl-2,2¾-(ethyne-1,2-diyl )bis(thiophene) (DBEBT) v/v) for 24 h to aVord a deep red polymer powder which was dried in vacuo. The dedoped polymer was further extracted Representative procedure.To a mixture of 2-methylbut-3-ynwith chloroform for 4 h to obtain the soluble portion (ca. 2-ol (4.20 g, 50 mmol), 2-bromo-3-butylthiophene (11.03 g, 10%) of the bulk polymer. The soluble portion was adopted 50 mmol), benzyltriethylammonium bromide (0.22 g, for characterisation studies such as solution UV–VIS, fluores- 1.02 mmol), cuprous iodide (0.22 g, 1.15 mmol) and tetrakis cence and nuclear magnetic resonance (NMR) spectroscopy (triphenylphosphine)palladium (1.50 g, 1.28 mmol) in benzene and gel permeation chromatography (GPC).(80 ml ) under a nitrogen atmosphere was added aq. sodium hydroxide (5.5 M, 80 ml ). The resulting black mixture was Chemical doping heated under reflux for 72 h whence a second portion of 2- bromo-3-butylthiophene (11.10 g, 50.0 mmol) in benzene Iodine doping of pressed pellets of dedoped polymers was (5 ml ) was added and heating continued for another 48 h.eVected by placing them in an iodine chamber for 1 week in Upon cooling, aq. ammonium chloride (100 ml ) was added the dark. The iodine uptake was monitored by progressive and the mixture stirred 3 h at room temperature. The organic weight gain and increasing electrical conductivity. Solution phase is separated whilst the aqueous phase is extracted with doping was eVected by stirring polymer powder (ca. 50 mg) in benzene (2×80 ml ). The combined organic phases were 0.1 M ferric chloride solution (ca. 50 ml ) in anhydrous nitrowashed with deionised water (3×100 ml ) and then dried methane under nitrogen for 1 h.Polymer pellets were observed (MgSO4), whereupon after removal of the solvent the crude to turn from deep red to deep green when doped. compound was obtained as a dark brown viscous liquid which was purified by vacuum distillation; [bp 168–170 °C Instrumentation (0.5 mmHg)] as a pale yellow liquid in 40% yield; dH(300 MHz, CDCl3) 7.18 (2H, d, J=5.1 Hz), 6.88 (2H, d, J=5.1 Hz), 2.75 Elemental analysis of all monomer and polymer samples was (4H, t, J=7.5 Hz), 1.69–1.32 (4H, m), 0.94 (6H, t, J=7.5 Hz); performed at the NUS Microanalytical Laboratory on a m/z 302 (M+, 100%), 217 (97), 273 (60) (Found: C, 71.2; H, Perkin-Elmer 240C elemental analyser for C, H, N and S 7.2; S, 21.2.Calc. for C18H22S25C, 71.5; H, 7.3; S, 21.2%). determination. Halogen determinations were done either by ion chromatography or the oxygen flask method. FT-IR 3,3¾-Dihexyl-2,2¾-(ethyne-1,2-diyl )bis(thiophene) (DHEBT) spectra were recorded for monomer and polymer dispersed in KBr disks using a Perkin-Elmer 1600 spectrometer.UV–VIS 35% yield; bp 180–185 °C (0.2 mmHg); dH(300 MHz, CDCl3) spectra were obtained for dilute solutions or thin polymer 7.20 (2H, d, J=5.2 Hz), 6.88 (2H, d, J=5.2 Hz), 2.74 (4H, t, films deposited onto indium tin oxide coated glass plates on a J=7.4 Hz), 1.70–1.24 (12H, m), 0.87 (6H, t, J=7.4 Hz); m/z Perkin-Elmer Lamda 900 spectrophotometer. 1HNMR spectra 358 (M+, 100%), 301 (50), 217 (85) (Found: C, 73.8; H, 8.1; were recorded on a Bruker ACF 300 FT-NMR spectrometer S, 18.4. Calc. for C22H30S25C, 73.7; H, 8.3; S, 17.8%). operating at 300 MHz, while 13C NMR spectra were recorded at 62.9 MHz.Deuterated solvents were used as indicated and 3,3¾-Dioctyl-2,2¾-(ethyne-1,2-diyl )bis(thiophene) (DOEBT) tetramethylsilane (TMS) was used as the internal reference. Mass spectra were obtained using a Micromass VG 7035E 40% yield; dH(300 MHz, CDCl3), 7.19 (2H, d, J=5.6 Hz), 6.89 (2H, d, J=5.6 Hz), 2.73 (4H, t, J=2.7 Hz), 1.67–1.25 mass spectrometer at a source temperature of 200 °C and an 2664 J.Mater. Chem., 1998, 8, 2663–2669ionising voltage of 70 eV. Thermogravimetric analyses (TGA) of polymer powders were conducted on a Du Pont Thermal Analyst 2100 system with a TGA 2950 thermogravimetric analyser. A heating rate of 10 °Cmin-1 with an air flow of 75 ml min-1 was used. The runs were conducted from room temperature to 800 °C.Conductivity measurements were carried out on polymer pellets of known thickness using a fourpoint probe connected to a Keithley constant current source. Conductivities were calculated from at least 10 pairs of consistent readings taken at diVerent points of the pressed pellet. Fluorescence measurements were conducted on a Shimadzu RF5000 spectrofluorophotometer using a xenon lamp as the light source.Standard polymer solutions dissolved in dry chloroform (10-5 M) were used for analysis and Coumarin (Aldrich) was used as the calibration standard. In situ electrochemical doping studies of polymers were carried out using an EG&G 263A potentiostat together with UV–VIS–near infrared spectrophotometer.GPC analyses were carried out using a Perkin-Elmer Model 200 HPLC system with PhenogelTM MXL and MXM columns (300 mm×4.6 mm ID) calibrated using polystyrene standards and THF as eluent. Results and discussion Physical properties and structural characterization Elemental composition of the neutral polymers as determined from microanalyses showed good agreement between the expected and calculated empirical formulae, with low iron and chloride contents (Table 1).Table 1 also summarizes the number average molecular weights (Mn) for the dedoped polymers as determined via GPC. These polymers have a polydispersity index (PDI) ranging from 1.3 to 1.8 and Mn values in the range 5400 to 6800, corresponding to the existence of 10–18 monomeric repeat units. As the molecular weights obtained from the soluble portion of the bulk polymers possibly represent only the lower molecular weight fractions, Fig. 1 FTIR of 3,3¾-dibutyl-2,2¾-(ethyne-1,2-diyl )bis(thiophene) and the insoluble fractions are expected to have higher molecular its corresponding polymer, PDBEBT: (a) monomer, (b) dedoped weights.21 This series of polymers was found to be partially PDBEBT and (c) I2 doped PDBEBT.soluble (ca. 10%) in common organic solvents like chloroform, THF, DMF and DMSO. Purification of these polymers was carried out by first dissolving the polymer in solvent followed CH2 groups. A weak rocking bend due to CH2 group at 718 cm-1 is also observed. The COC group manifests as a by precipitation from cold methanol. Although the soluble portion of these polymers only represents the lower molecular very low intensity stretching band at 2196 cm-1.The thiophene ring depicts C–H stretching at 3101 cm-1 and C–H in-plane weight fractions, uniform solid thin films could be grafted onto ITO glass plates easily from chloroform solution and deformation at 1085 and 1237 cm-1, whilst ring vibrational modes are seen at 1377, 1460, 1517 and 1598 cm-1.The used for UV–VIS absorption, cyclic voltammetry, fluorescence spectroscopy and electrochemical doping studies. Currently, monomer also depict vibrational bands at 721 and 837 cm-1 which are ascribable respectively to the C–Ha and C–Hb out- we are investigating ways of deriving a suitable polymerization route so as to synthesize higher molecular weight polymers of-plane bending modes of the thiophene rings.22 In contrast, neutral PDBEBT shows a strong b-CH out-of-plane bend at that have good solubility in common organic solvents.The chemical structure of the monomers and bulk polymers 826 cm-1, whilst the a-CH out-of-plane bend is relatively insignificant, suggesting that predominant a–a¾ coupling of the were studied in detail using FT-IR spectroscopy.The FT-IR spectra of the representative 3,3¾-dibutyl-2,2¾-(ethyne-1,2-diyl )- thiophene ring is inherent in this polymer. Thiophene ring stretches at 1376, 1429, 1528, 1654 and 1718 cm-1 remain bis(thiophene) and its corresponding polymer PDBEBT both in its neutral and doped forms are shown in Fig. 1. The unchanged with respect to the monomer. The presence of two intense bands at 2852 and 2920 cm-1 suggests that the alkyl presence of the butyl pendants in the monomer is evident from C–H stretching at ca. 2850 and 2950 cm-1 due to CH3 and chain remains intact after polymerization. A weak stretching Table 1 Physical properties of polymers PEBT, PDBEBT, PDHEBT, PDOEBT, PDDEBT including GPC, elemental analysis and conductivity results GPC results Elemental analysis Conductivity Polymer Pn PDI Mn (I2 doped)/S cm-1 Empirical formula Found PEBT 8 1.5 1523 1.5 C10.0H4.0S2.0 C10H4.1S2.0 PDBEBT 18 1.8 5500 0.03 C18.0H20.0S2.0 C18.0H20.4S2.1 PDHEBT 18 1.4 6500 0.002 C22.0H28.0S2.0 C22.0H27.9S2.1 PDOEBT 17 1.3 6900 0.001 C26.0H36.0S2.0 C26.0H36.5S2.1 PDDEBT 11 1.3 5400 0.0002 C34.0H52.0S2.0 C34.0H51.3S2.0 J.Mater. Chem., 1998, 8, 2663–2669 2665attributed to the COC group is also observed at 2210 cm-1.reference solutions and Ir and Is are the corresponding relative integrated fluorescence intensities. Table 2 summarises the The FT-IR spectrum of iodine-doped PDBEBT exhibits dramatic changes in the 1035 to 1438 cm-1 region. Here the IR electronic absorption and emission maxima and the respective polymer fluorescent quantum yields relative to Coumarin 334 bands are intensified and overlap to constitute a broad band.This broad band is assigned to doping-induced bands attri- laser dye. There is generally little diVerence in the UV absorption maximum between the parent polymer PEBT and the buted to the vibrational modes in the anion-doped (charged transferred) thiophene rings.23 The low intensity broad band alkyl-substituted polymers in their solutions UV–VIS spectra.All polymer solutions depicted absorption maxima at ca. at 3000 to 3800 cm-1 is characterized as weakly bonded O–H stretching due to absorbed moisture from the atmosphere. 476 nm, which is red-shifted in comparison to that of polythiophene at 450 nm.27 This observation is consistent with enhanced ring conjugation in the polymers attributed to Conductivity of doped polymers reduced steric eVects imposed by alkyl pendants with the The dedoped polymers upon doping with iodine or ferric introduction of an acetylenic spacer between the thiophene chloride yielded electrical conductivity ranging from 100 to rings.This assumption also correlates well with our experimen- 10-4 S cm-1.The graphical representation of the variation of tal data whereby the UV–VIS absorption of PEBT and the conductivity with iodine uptake for the typical polymer PEBT alkyl-substituted polymers remain fairly unchanged. is shown in Fig. 2. As summarised in Table 1, the conductivity When the alkyl-substituted polymers PDBEBT, PDHEBT, of the unsubstituted polymer PEBT is highest followed by a PDOEBT and PDDEBT are spin-coated on ITO glass from trend of diminishing conductivity with increasing pendant chloroform solutions to aVord thin polymer films, a red shift alkyl chain length on going from PDBEBT to PDHEBT, of only 20–60 nm with respect to the polymer solution resulted.PDOEBT and PDDEBT. These results are consistent with This bathochromic shift is somewhat less than that observed earlier reports by Kaeriyama et al.24 in their study of polyalkyl- for poly(3-alkylthiophene)s at between 60–100 nm28 on going thiophene.It is observed experimentally that with increasing from the solution to the condensed phase, suggesting that the alkyl chain length, the rate of iodine uptake reflected from acetylene spacer has imparted a significant amount of rigidity percentage weight change also decreases.This phenomenon is to the polymer backbone. The electrochemically synthesized attributed to the increasing size of the alkyl group, which unsubstituted polymer PEBT, on the other hand, is obtained takes up more of the weight of the polymer, therefore the as a deep red–violet film with an absorption maximum at amount of iodine absorbed is correspondingly smaller.The 519 nm, which is red-shifted from the 496 nm of rate of iodine uptake was found to be progressively slower polybithiophene.29 from PDBEBT to PDDEBT, although the doping period (4 The band gap energies of these polymers (Table 2) can be days) was kept consistent for all samples, implying that the deduced from the energy absorption edge of the UV–VIS doping eYciency was diminished.Elemental analysis of iodine- spectrum according to the approach of Johnson et al.30 A doped polymers showed that the concentration of I3- dopant reduced band gap energy of 1.7–1.9 eV is observed in this also decreased on going from PDBEBT to PFHEBT, PDOEBT series of polymers in comparison to polythiophene (2.1 eV).31 and PDDEBT. This reduction in rate of doping is largely Although PDBET (535 nm, 1.7 eV) depicted enhanced ring assigned to significant steric hindrance towards the dopant conjugation compared to PEBT (519 nm, 1.8 eV), the longer molecules, which inhibits charge carrier formation during the alkyl chain polymers PDHEBT, PDOEBT and PDDEBT on doping process.25 the contrary do not exhibit significant red shifts with respect to PEBT.Electronic (UV–VIS) and fluorescence spectroscopy The fluorescence excitation/emission studies of these polymers from PEBT to PDDEBT showed a green emission Standard polymer solutions of 10-5 M concentration in in chloroform solution (10-4 M) when the polymers were chloroform were used for UV–VIS and fluorescence spectroexposed to ultraviolet radiation. These polymers gave an scopic measurements.Fluorescence measurements were comemission peak between 540 and 573 nm with an accompanying pared with Coumarin 334 laser dye (Aldrich) which absorbs decreasing Stokes shift as the chain length of the alkyl substitu- at 450 nm and emits at 490 nm. The calculation of fluorescence ent increases. The progressively smaller Stokes shift from quantum yield of a solution sample (Ws) relative to a reference PEBT to PDDBET implies that the polymer backbone sample of known quantum yield (Wr) is related to eqn.(1),26 becomes more rigid32,33 as the alkyl chain length is increased. Ws=Wr [(Ar/As)×(Is/Ir)] (1) The increasing rigidity of these polymers must have contributed to the increase in overall fluorescent quantum yield by reducing where As and Ar are the absorbencies of the sample and the extent of non-radiative losses.As the Stokes shift diminishes, the quantum yield becomes progressively higher in the order PEBT<PDBEBT<PDHEBT but decreases again in going to PDDEBT. From the Mn values presented in Table 1, whilst the polymer chains of PDBEBT, PDHEBT and PDOEBT contain a fairly consistent number of monomeric repeat units (17–18) in their soluble fractions, PDDEBT has a significantly smaller degree of polymerisation (DPn) with only 11 repeat units.Previously, it has been shown with thiophene oligomers that a trend of diminishing fluorescent quantum yields can be correlated with a reduced number of conjugated repeat units in the oligomers.34 Consequently, arising from this, PDDEBT has a reduced fluorescence quantum yield even though its Stokes shift is the smallest among this series of polymer.Thermochromism eVects The temperature dependency in the optical absorption spectra Fig. 2 Conductivity plot of PEBT against uptake of iodine (wt%). of alkyl substituted polymers PDBEBT to PDDEBT were 2666 J. Mater. Chem., 1998, 8, 2663–2669Table 2 UV–VIS absorption and fluorescence emission data and band gap of various polymers in chloroform solution at 25 °C lmax/nm Fluorescence/nm Quantum Stokes Band Polymer Solution Film Excitation Emission yield shift gapa/eV PEBT 476 519 476 573 22 97 1.8 PDBEBT 475 535 478 568 27 90 1.7 PDHEBT 470 491 470 549 39 79 1.9 PDOEBT 475 492 475 550 37 75 1.9 PDDEBT 471 493 470 540 29 70 1.9 aBand gap is derived from UV–VIS spectrum of polymer film coated on ITO glass.studied. Solid polymer samples spin-coated from chloroform twisting of the polymer chain, reducing the extent of interring conjugation. However, the extent of twisting and conse- solution onto ITO-coated glass were used in these studies. These polymers undergo a colour change from red to orange quently the magnitude of blue shift is significantly reduced in comparison to poly(alkylthiophene)s,36 due to the eVect of upon heating. This is accompanied by a blue shift in the absorption maxima from 535 to 466 (PDBEBT), 491 to 452 the spacer unit on the polymer chain, which helps diminish the repulsive intrachain steric interactions exerted by the alkyl (PDHEBT), 493 to 458 (PDOEBT) and 493 to 468 nm (PDDEBT).It is diYcult to identify any intermediate phases pendant groups. Moreover, experimental fluorescence spectroscopy results also show decreasing Stokes shift values in formed in this series as no clear isosbestic point is observed in the UV–VIS spectra (Fig. 3).35 Only a gradual blue shift is the same order. This corroborates the assumption that substitution with longer alkyl pendants aVords polymers with a observed in these polymers upon heating, which is indicative of a decrease in conjugation. The magnitude of the blue shift more rigid structure.This thermochromic behaviour is fully reversible, regaining the initial absorption state upon cooling. diminishes with increasing chain length of the pendant alkyl group, with PDDEBT<PDOEBT<PDHEBT<PDBEBT.Electrochemistry The blue shift is ascribed to heat-induced disorder in the side chain leading to accentuated steric interaction and concomitant The cyclic voltammograms of various polymers are shown in Fig. 4. The electrochemical oxidation of PEBT is compared with polythiophene (PT) and polybithiophene (PBT). A generally lower monomer oxidation potential (1.43 V) in comparison to PT (1.65 V)38 is required for generation of PEBT when a current density of 1 mA cm-2 is used.PEBT is highly electroactive showing excellent reversibility of its p-doping redox states when subjected to repeated electrochemical cycling between -1.0 to 0.9 V (vs. SCE). The p-doped polymer PEBT has a deep green colour which turned red–brown upon electrochemical dedoping.The polymer oxidation potential of PEBT is comparable to polybithophene (1.0 V)37 but is significantly lower than pristine polythiophene (1.3 V).38 The eVect of lowering the oxidation potential is advantageous as this would prevent over-oxidation of the polymer film when a high oxidising (positive) potential was applied. The film formation process for alkyl-substituted polymers is diYcult to achieve as these polymers are slightly soluble in acetonitrile. Therefore electrochemical analysis of these polymers was conducted on chemically polymerized samples which were spin-coated onto platinium electrodes. The p-doping of PDBEBT, PDHEBT, PDOEBT and PDDEBT was observed to be stable, with clearly defined anodic and cathodic peaks.When these polymers were scanned repeatedly using cyclic voltammetry, no significant overoxidation or degradation of the polymer films was observed.The ratio of polymer oxidation potential (Epa) to reduction potential (Epc) showed a slight deviation from unity, suggesting that the doping/dedoping process of these polymers is not absolutely reversible. On varying the scan rates from 20 to 80 mV s-1, the peak current densities of these polymers were observed to scale linearly with increasing scan rates, implying that the doping/undoping processes are non-diVusion controlled reactions.39 The Epa values of alkyl-substituted polymers were found to increase with increasing alkyl chain length, whilst the corresponding Epc values shifted towards lower potential.This phenomenon was also observed by Tanaka et al.24 and Yamabe et al.40 in poly(3-alkylthiophene)s.Our findings indicate that the electron donating eVect attributed to the butyl groups in PDBEBT (Epa=0.92 V) leads to a lowering of Epa compared to PEBT (Epa=0.94 V) and PBT (Epa=1.01 V),37 as well as Fig. 3 Variation of absorption maxima in UV–VIS spectra of PT (Epa=1.30 V).38 In PDHEBT, PDOEBT and PDDEBT (a) PDBEBT and (b) PDOEBT upon heating in solid state from 25 to 180 °C.on the other hand, the Epa values shows a gradual increase J. Mater. Chem., 1998, 8, 2663–2669 2667Fig. 5 In situ electrochemical doping studies of various polymers: (a) PDBBT, (b) PDHEBT, (c) PDOEBT and (d) PDDEBT. infrared region. The position of this polaron band occurs at 880, 860, 840 and 840 nm for PEBT, PDHEBT, PDOEBT and PDDEBT, respectively, which correspond to about 1.4 eV.Therefore when polymers are in lightly doped states within the potential range 0.0 to 1.0 V, two electronic bands corresponding to the p–p* interband transition as well as the polaron band were observed. When the extent of doping was increased through applying a potential greater than 1.25 V, the polaron band continued to grow in intensity whilst the p–p* band diminished in intensity until a negative deviation formed.The polymer film appears to be deep blue in this heavily doped state. The calculated electrochemical bandgaps of these polymers were determined from the position where the isosbestic point occurs in the optical spectrum obtained during in situ electrochemical doping studies.It has been mentioned by several Fig. 4 Cyclic voltammograms of various polymers at scan rate of 20 mV s-1: (a) PEBT, (b) PDBEBT, (c)PDHEBT, (d) PDOEBT and authors41 that the relative electrochemical band gap energy (d)PDDEBT. can be estimated from the position of the isosbestic point in the optical spectrum using eqn. (2), with reference to PEBT.The electro-oxidation process involves E (eV )=hn=hc/l=1240/l (nm) (2) the removal of one electron from the neutral polymer and the resulting polymer aquires a single positive charge on the sulfur where h is Plank’s constant, l is wavelength in nm and c is the speed of light, and E denotes the band gap energy of the heteroatom. This positively charged species is stabilised electrochemically in the presence of solvated counter anions.It was polymer. From Table 3, the evaluated electrochemical band gap energies of PDBEBT, PDHEBT, PDOEBT and PDDEBT anticipated that with increasing alkyl chain length, the bulky alkyl group is likely to slow down the rate of mobility of are found to be 1.8, 1.9, 1.9 and 1.9 eV, respectively. These readings correlate well with the experimentally determined counter anions into and out of the polymer surface, thus resulting in higher oxidation potential.optical band gap energies. In summary, the polymer oxidation potentials for this series of polymers lie between 0.92 to 1.30 V, which is comparatively Thermal stability of neutral and doped polymers lower than the oxidation potential of polythiophene.Reductive The thermal properties of polymers in both their neutral and n-doping studies of these polymers were also examined in the doped states were studied in air over a temperature range of potential range 0 to -2.5 V vs. SCE. However, no significant 25 to 800 °C. The neutral polymer, PEBT, depicted a single n-doping peaks were observed. weight-loss step in the temperature range 300 to 500 °C, corresponding to the thermal oxidative degradation of the In situ electrochemical doping studies of polymers polymer backbone.All the alkyl-substituted polymers PDBEBT to PDDEBT depicted a two-step weight loss. The The UV–VIS-near infrared absorption shifts of doped polymers during electrochemical doping are depicted in Fig. 5. first step occurring in the temperature range 220 to 370 °C, corresponded to cleavage of the alkyl chain.42 The next weight Usually, a low potential of 0.0 V vs.SCE was applied to first obtain an undoped polymer spectrum. At this potential, only loss step, which took place in the temperature range 370 to 800 °C, was attributed to the degradation of the polymer one main potential corresponding to the p–p* interband transition is observed at 526, 495, 493 and 494 nm for chain.In most cases, a small residue content of less than 5% was left behind. PDBEBT, PDHEBT, PDOEBT and PDDEBT, respectively. As the applied potential was gradually increased from 0.00 to The iodine-doped polymers depict diVerent TGA spectra to those of the undoped polymers. The first step, occuring at 100 0.95 V, the intensity of the p–p* interband transition decreased slightly with evolution of a new polaronic band in the near to 200 °C, is attributed to the expulsion of molecular iodine 2668 J.Mater. Chem., 1998, 8, 2663–26699 M. Sato, S. Tanaka and S. Kaeriyama, Makromol. Chem., 1987, from the polymer surface. The second weight-loss step, from 188, 176. 200 to 800 °C, due to degradation of polymer chain adopts a 10 T.J. Kang, J. Y. Kim, C. Lee and S. B. Rhee, Synth. Met., 1995, gradual weight loss pattern. The thermal dedoping process 69, 377. occurring in the temperature range 100 to 200 °C in doped 11 M. Feldhues, G. Kampf, H. Litterer, T. Mecklenburg and PDBEBT was closely monitored using FT-IR spectroscopy. P. Wegener, Synth. Met., 1989, 28, C487. 12 M. T.Vala and J. Haebig, J. Chem. Phys., 1965, 43, 886. These results showed decreasing intensity of the doped induced 13 J. K. Tai, Y. K. Jae, J. K. Kyung, J. L. Chang and B. R. 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