Article pubs.acs.org/Macromolecules
Tetrathiafulvalene (TTF)-Functionalized Thiophene Copolymerized with 3,3‴-Didodecylquaterthiophene: Synthesis, TTF Trapping Activity, and Response to Trinitrotoluene Jasmine Sinha,† Stephen J. Lee,† Hoyoul Kong,†,§ Thomas W. Swift,† and Howard E. Katz*,†,‡ †
Department of Materials Science and Engineering and ‡Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States S Supporting Information *
ABSTRACT: We report a synthesis route to a thiophene polymer where the repeat unit consists of 3,3‴didodecylquaterthiophene (as in PQT12) plus an additional thiophene ring from which other functional groups may be projected. The hydroxymethyl form of this polymer, while only a poor semiconductor in its own right, serves as a vehicle for compatibilizing PQT12 itself with arbitrary functional groups. In this article, we focus on tetrathiafulvalene (TTF) as the functionality. As expected, the TTF group acts as a hole trap, as shown by loss of hole mobility and a surprising negative Seebeck coefficient, but this enables a current-increase response to trinitrotoluene as an analyte and confirms a similar observation we recently reported for a dissolved TTF. Added dopants also fill the trap states, restoring hole mobility and the typical positive Seebeck coefficient.
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INTRODUCTION Solution-processable organic and polymeric semiconductors are being considered for low-cost electronic devices due to their modulated electronic conductivity combined with other functionality.1−4 Applications include light-emitting diodes (OLEDs),5,6 organic field effect transistors (OFETs),7,8 organic photovoltaic cells (OPV),9,10 chemical sensors,11 and thermoelectrics.12 Polythiophenes are frequently used as processable semiconducting polymers. Their substitution chemistry allows attachment of functional groups for activity beyond simple electronic switching. For example, side-chain functionalization of polythiophenes has been reported to influence the optical and electronic properties due to the interaction of the polymer backbone with the functional group.13−16 Oligothiophenes with thiol,17,18 bisulfide,19 thioacetate,20 and thiocyanate21 terminated functionality have been reported to show a rapid selfassembling behavior. Furthermore, polymer aggregation in form of hollow spheres, lamella, and hollow cylinders have been observed from the self-organization of rod−coil polymers based on hole-transporting units (carbazole) or electron-transporting units (fluorene/thiophene) on the side chain.22−25 Alkyl ether,26−29 crown ether,30 calixarene,31 and porphyrin32 © XXXX American Chemical Society
functionalized polythiophenes have been widely investigated for the recognition of metal cations. Also, postfunctionalization has been demonstrated on polythiophene wherein the substitution with phenyl, p-tolyl, 2- and 4-methoxyphenyl, biphenyl, 1-naphthyl, 2-thienyl, and phenylethylenyl groups caused red shifts in the absorption and emission.33 In this article, we consider the tetrathiafulvalene (TTF) group as a side chain from a poly(alkylthiophene) holetransporting main chain. Even though TTF, as a strong electron donor, would quench holes in the thiophene polymer, we had a particular motivation for preparing a prototypical TTFsubstituted polythiophene. We had recently discovered that polythiophene-based OFETs with an added TTF small molecule in a sensor element displayed a unique spectrum of responses to trinitrotoluene (TNT) exposures, including many cases of induced current increases.34 This is contrary to what is typically observed from chemical interactions with organic semiconductor devices, where currents decrease.35−40 Having Received: September 18, 2012 Revised: January 3, 2013
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Scheme 1. Synthesis of TTF-Based Side-Chain Unit
Scheme 2. Synthesis of Hydroxymethyl Copolymer P1
with side-chain functionality of ω-iodo-alkyl and ω-iodopolyether has been synthesized by electropolymerization of respective monomer and was subjected to postfunctionalization with a functional block bearing a thiolate derivative of TTF.71 The desired polymer was substituted with two polyether chains and was shown that the binding of Pb2+ could be driven electrochemically.72 Our synthetic approach begins with a hydroxymethylsubstituted copolymer, which was then substituted with TTFderivatized monomer as side-chain functionality. This TTF side-chain polymer was active for TNT detection in a manner similar to what we had observed for the small molecule.34 This demonstrates the new polymer as a vehicle for compatibilizing the often-insoluble TTF with an electron-rich polymer and also shows that the response we reported previously was not due to a special solid-state effect of the small molecule. We also explored the effect of F4TCNQ and NOPF6 (p-type dopant) on the hole transport properties and Seebeck coefficients of the synthesized side-chain-functionalized copolymers, including the TTF derivative as well as hydroxymethyl and propargyloxy derivatives for comparison, to verify the significant effects conferred by the TTF groups.
both response signs available allows fabrication of more specific, binary, and ratiometric sensing circuits.41 We had additionally observed that thiophene polymers mixed with small amounts of stronger electron donors can show simultaneous increases in Seebeck coefficient and electrical conductivity for thermoelectric applications.42 To date, thermoelectric properties have been studied on a few conjugated polymers43,44 including polyacetylenes,45−48 polyanilines,49−52 polypyrroles,53−55 poly(p-phenylvinylene),56,57 poly(2,7-carbazole)s,58−61 and polythiophene.42,62−65 Doping of a semiconductor66 generally results in increase in the electrical conductivity with decrease in the Seebeck coefficient. The decrease in Seebeck coefficient with increasing charge carriers is attributed to the reduction of the entropy change associated with the addition of carriers.43 Conversely, we expected that the added lower energy hole states arising from the TTF would increase the positive Seebeck coefficient. However, as we will later discuss, this did not occur in our undoped polymer, and instead, a negative Seebeck coefficient was conferred by the TTF. The first reported example of a poly(thiophene)−TTF was an electropolymerized product reported by Bryce and coworkers.67 Roncali and co-worker synthesized the monomer 3(10-tetrathiafulvalenyl-9-oxadec-1-yl)thiophene and electropolymerized it successfully in nitrobenzene.68 Further, the group electropolymerized TTF-derivatized polythiophene based on bithiophene or ethylenedioxythiophene (EDOT), resulting in an extensively π-conjugated TTF-derivatized polythiophene. The group investigated the behavior of TTF in the polymeric backbone, wherein they reported the presence of different monomeric and dimeric oxidized TTF species.69,70 PEDOT
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RESULTS AND DISCUSSION Synthesis of Monomers and Polymers. The syntheses of the initial hydroxymethyl polymer and O-alkyl derivatives are outlined in Schemes 1 and 2, respectively. The synthesis of the monomer 3,3‴-didodecylquaterthiophene was obtained as reported in the literature in 75% yield by Stille coupling of 2 equiv of 2-bromododecylthiophene with 5,5′-bis(trimethyltin)2,2′-bithiophene.73 2,2′-Dibromo-3,3‴-didodecylquaterthioB
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Scheme 3. Synthesis of Side-Chain-Functionalized Copolymers
finally with chloroform. The chloroform layer was reprecipitated thrice in methanol, which resulted in polymers in good yield. The copolymers P1−P3 were found to be highly soluble in chloroform, chlorobenzene, methylene chloride, THF, and toluene. The equimolar ratio of the monomer in the copolymer was confirmed by 1H NMR, wherein the CH2 proton of methylene group and the CH2 of alkyl chain integration ratio corresponded to 1:1. In addition, the disappearance of the OH peak at 2.1 ppm confirmed the desired polymer. The appearance of a broad peak at 4.2 and 3.5 ppm corresponding to methylene protons of TTF and OCH2, respectively, for the copolymer with TTF as the side-chain functionality confirmed the desired copolymer, whereas the appearance of a peak at 2.5 ppm corresponding to acetylene proton confirmed the acetylenic functionalized copolymer. While not essential to the present study, the acetylenyl group provides a route to further derivitization via “click” cycloaddition chemistry with azides. GPC studies indicated a weight-average molecular weight of 10 kDa (PDI 2.89) for P1, 11 kDa (PDI 2.2) for P2, and 12 kDa (PDI 2.41) for P3. Thermal Analysis. Figure 1 shows the thermal equilibration behavior of P1, P2, and P3 investigated with differential scanning calorimetry (DSC). The samples were dried at room temperature under high vacuum before testing.
phene was then subjected to bromination using 2 equiv of Nbromosuccinimide (NBS) in chloroform/acetic acid. The product was confirmed by the appearance of a singlet peak at 6.89 ppm in the 1H NMR corresponding to the C−H proton at the 2-position of the thiophene ring. Thiophen-3-ylmethyl acetate was synthesized from thiophen-3-ylmethanol, which when subjected to acetylation using acetyl chloride in presence of DMAP as a catalyst resulted in the desired compound in good yield.74 Thien-3-ylmethanol was obtained from the reduction of thiophene-3-carboxaldehyde using LiAlH4. 1H NMR confirmed the product. A functionalized TTF derivative was synthesized by subjecting TTF to monolithiation using lithium diisoproplyamide as reported in the literature. The monolithiated product hence formed was formylated using Nmethyl-N-phenylformamide, which then upon reduction with NaBH4 in methanol resulted in 4-formyltetrathiafulvalene.75 4Formyltetrathiafulvalene was then substituted with 1,8dibromooctane in a THF/DMF solvent mixture, which resulted in 4-(((8-bromooctyl)oxy)methyl)-2,2′-bi(1,3-dithiolylidene) in 65% yield. The product was confirmed by the disappearance of the −OH peak at 1.9 ppm and the appearance of two different types of triplet at 3.40 and 3.42 ppm. The above-mentioned monomers were then used for the synthesis of copolymers. An alternating copolymer containing 3,3‴-didodecylquaterthiophene and thiophen-3-ylmethyl acetate was polymerized via direct arylation. The polycondensation reaction of a 1:1 ratio of the two monomers was carried out at 100 °C in the presence of Pd(OAc)2 as a catalyst with pivalic acid and K2CO3 in dimethylacetamide.76 The copolymer hence obtained was then subjected to the deprotection of the acetyl group by treating it with 0.5 N HCl, resulting in −OH-functionalized copolymer (P1). The copolymer was then subjected to O-alkylation with 4-(((8bromooctyl)oxy)methyl)-2,2′-bi(1,3-dithiolylidene) or propargyl bromide in the presence of NaH in THF with DBU as the catalyst, resulting in copolymer with TTF derivative (P2) and propargyl side chains (P3) (Scheme 3). Polymers were isolated by extracting from chloroform. The concentrated chloroform solution was added dropwise to methanol, where red precipitation was observed. The red precipitate was subjected to Soxhlet extraction using methanol, hexane, and
Figure 1. DSC thermogram of P1−P3 with heating rate of 5 °C/min under an inert atmosphere. C
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Features were observed for copolymers P1, P2, and P3 at 54.4, 65.2, and, 61.9 °C, respectively, on initial heating. In addition, P2 showed a second peak at 134.5 °C. These transitions may reflect loss of the last traces of solvent or conformational stabilization. The peaks are not observed on cooling or reheating, as was also the case for poly(3hexylthienylene vinylene).77 Optical Properties of the Polymers. Figure 2 shows the absorption and emission spectra of the polymer solutions in
Table 2. Electrochemical Onset Potential and Band Gap of the Polymer Films
a Eopt (eV) g
P1 P2 P3
446 449 456
565 570 565
2.21 2.23 2.23
Egec a (eV)
P1 P2 P3
0.69/−5.39 0.80/−5.50 0.44/−5.14
−1.39/−3.31 −1.32/−3.38 −1.42/−3.28
2.08 2.12 1.86
10−5 cm2/(V s), an on/off ratio of 103, and curves displaced because gate leakage current at low drain voltage was comparable to source-drain current at high drain voltage, as shown in Figure 3. P2 and P3 did not show any OFET behavior at all. The P1 was less ordered than the parent PQT12 polymer, as shown by X-ray diffraction indicating amorphous structure. For P2, the lack of OFET activity was expected, as the TTF group should act as a powerful hole trapping site.34 The propargyl group might be reactive in the presence of holes or may disrupt the morphology even more than the hydroxymethyl group because of unproductive π-interactions. When P1 was blended with just 20 wt % PQT12, the OFET performance significantly improved (Figure 3). The drain current was observed to increase 8 times at Vg = −100 V, significantly exceeding gate leakage and coming closer to quadratic Id−Vg dependence, with mobility of 5.1 × 10−5 cm2/ (V s) and on/off ratio of 102. This shows that P1 can be a compatible additive for introducing OH groups into active PQT12 films, even though P1 is a poor semiconductor in its own right. OFETs with F4TCNQ as the Dopant. Tetrafluorotetracyanoquinodimethane (F4TCNQ), an electron acceptor, is an effective p-type dopant to increase polythiophene conductivity.78 We made doped films by spincoating different ratios of 10 mg/mL of P1−P3 with 0.5 mg/mL of F4TCNQ in chlorobenzene solution under ambient conditions at 1500 rpm for 60 s, followed by storage in a vacuum desiccator for 30 min prior to measurement. We investigated doping of P1 with 0.125, 0.25, 1.0, 3, and 7 wt % of F4TCNQ by checking the output and transfer characteristics of bottom contact OFETs. Normal p-type OFET behavior was observed with 0.125 and 0.25 wt % F4TCNQ, with slight increase in the current. When P1 was doped with 1.0 wt % of F4TCNQ, the current increased by 2 orders of magnitude but was only slightly adjusted by the gate voltage, indicating continued low mobility. The device was “ON” at zero applied gate voltage, with drain current of 2.6 μA (Figure 4 and Table 3). When F4TCNQ wt % was increased to 3% and 7%, respectively, a further increase in current was observed and apparent mobility increased by an order of magnitude, though gate modulation was small compared to bulk current . F4TCNQ did not confer OFET behavior on P2 or P3. TNT Sensing of P2. As previously demonstrated by our group, PQT12, blended with tetrakis(pentylthio)TTF (TPTTTF) in an OFET geometry and not showing OFET behavior, when exposed to dilute TNT analyte solution, resulted in “turnon” response of the current, which we attribute to complexation between TPT−TTF and TNT. Pure P2, and P2 with 5 or 10% PQT12 added, showed negligible OFET activity. However, a 50−50 wt % blend of P2 and PQT12 showed clearer OFET behavior (Figure 5). All the blend devices were then exposed to
Table 1. Absorption and Emission Spectral Peaks λflu max (nm)
φox (V vs Ag/Ag+)/ ELUMO (eV)
HOMO and LUMO levels are calculated from the onset of the first peak of the corresponding redox wave and are referenced to ferrocene, which has a HOMO of −4.8 eV.
chloroform. The absorption spectra of P1−P3 showed an absorption maximum at 446, 449, and 456 nm, respectively, which is typical of the π−π* transition in chloroform. Emission maxima of P1−P3 were observed at 565, 570, and 565 nm, respectively, in chloroform. The side substituents seem to have little effect on the emission wavelengths of the polythiophene backbone. However, the fluorescence intensity was observed to decrease by a factor of 2 with the substitution of the TTF group into the side chains, showing possible electron transfer quenching activity by the TTF. Data are listed in Table 1.
λabs max (nm)
φox (V vs Ag/Ag+)/ EHOMO (eV)
a
Figure 2. Absorption and emission spectra of the polymers in chloroform.
polymer
polymers
a
Optical band gap was obtained from empirical formula Eopt g = 1241/ λedge, where λedge is the onset wavelength of its absorption peak in the longer wavelength direction.
Electrochemical Properties. Figure S1 shows the cyclic voltammograms of the polymer films on the Pt electrode by scanning the potential from −2.0 to 1.8 V vs Ag/Ag+ at the scan rate of 100 mV/s. Platinum wire was used as the working electrode, solution cast P1−P3 on a Pt button electrode was used as the counter electrode, Ag/Ag+ (0.01 M AgNO3) was used as the reference electrode, and 0.1 M TBAP in acetonitrile was used as the supporting electrolyte. While a separate TTF oxidation peak was not apparent in the cyclic voltammogram of the P2 film, the peak was discernible near 0.36 V in a solutionphase cyclic voltammogram using the same supporting electrolyte and reference electrode, shown as the lower chart in Figure S1. Data are listed in Table 2. Field-Effect Transistor Properties of the Polymers. P1 showed a clear field effect but with low mobility of up to 1.4 × D
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Figure 3. (a) OFET output curves for P1 measured under ambient condition. (b) Output curve for P1 blended with 20 wt % PQT12. (c) Comparison of logarithmic plot of transfer curve at Vd = −100 V for P1 and P1 blended with 20 wt % PQT12.
Figure 4. (a) Logarithmic transfer curve of P1 doped with different wt % of F4TCNQ at Vd = −100 V. (b) Change in mobility of P1 with the increase in the dopant concentration.
the blended P2 with 5 wt % PQT12 devices exposed to 10−3 mg TNT/mL IPA solution, output currents were significantly increased, and a similar observation was made when P2 with 10 wt % PQT12 was exposed to 10−3 mg TNT/mL IPA solution. The P2:5% PQT12 blend device showed increased current on exposure to more dilute TNT solution (10−4 mg TNT/mL IPA), whereas the P2:10% PQT12 showed decrease in the current on exposure to 10−4 mg TNT/mL IPA, as the TTF activity was relatively less. The P2:50% PQT12 blend device showed lower response to ≥10−4 mg TNT/mL IPA (Figure 6). Thus, as was observed with TPT-TTF, the current increase was only observed for relatively dilute TNT solutions. Also, as was previously observed, a PQT12 film with essentially no TTF activity (the 95% PQT12 sample) showed current decreases in response to TNT at all the concentrations examined. Thermoelectric Characterization. We measured the Seebeck coefficient S and electrical conductivity σ of polymers
Table 3. Dopants and Properties for P1 Deposited from 10 mg/mL Solution F4TCNQ (wt %) 0 0.125 0.25 1.0 3.0 7.0
μ (cm2/(V s)) ± std dev 1.4 1.6 1.6 4.7 4.2 2.5
× × × × × ×
−5
10 10−5 10−5 10−5 10−4 10−3
± ± ± ± ± ±
3.1 4.7 5.3 1.6 9.9 1.4
× × × × × ×
−6
10 10−6 10−6 10−5 10−5 10−3
Ion/Ioff 2 × 103 22 20 2.6 1.8 1.4
≥10−4 mg TNT/mL in 2-propanol (IPA). The TNT solution was dropped on 0.81 cm2 of Novec fluoropolymer-bounded channel area with several OFET channels. When pure IPA solvent was dropped and dried on various blend ratio of P2:PQT12 films, no significant current changes were observed. The pure P2 devices when exposed to ≥10−4 mg TNT/mL in 2-propanol (IPA) did not show any transistor behavior. With E
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Figure 5. (a) Comparison of output curve at Vg = −100 V for different blend ratio of P2:PQT12 measured under ambient conditions. (b) Comparison of logarithmic plot of transfer curve at Vd = −100 V for different blend ratio of P2:PQT12 measured under ambient conditions.
Figure 6. Output curve current change ΔI/I (%) at Vd = Vg = −100 V of blend system (a) 95−5% (P2:PQT12) devices, (b) 90−10% (P2:PQT12) devices, (c) 50−50% (P2:PQT12) devices, and (d) 5−95% (P2:PQT12) devices when exposed to the TNT solutions with various concentrations, c (to 10−4 mg TNT/mL IPA). Square shows mean value, and bar shows standard deviation.
P1−P3 in the presence of two different p-type dopants: nitrosonium hexafluorophosphate (NOPF6) and F4TCNQ. P1 when doped with various wt % (3−9) of F4TCNQ was observed to show increase in Seebeck coefficient, and with 8 wt % of F4TCNQ, the value was found to be positive 75 μV/K (Figure 7), as a low-lying hole state was presumably filled. Further increasing the dopant concentration decreases the Seebeck coefficient, as expected. A similar result was observed when P3 was doped with various wt % (3−9) of F4TCNQ (Figure 8b,c). Undoped P2 showed contrasting behavior, in that its Seebeck coefficient was observed to be negative. While not
anticipated, this would be consistent with holes being so thoroughly trapped or consumed by chemical reaction that electrons in the highest occupied TTF orbitals could be excited on heating into impurity states that dominate the Seebeck coefficient. Even if this occurs to a very small degree, while there are no mobile holes present, a negative Seebeck coefficient would result. Once a sufficient amount of dopant is added, the material again becomes hole-dominated, and the Seebeck coefficient reverts to a positive sign (Figure 8a). While conductivity increased with dopant level, the power factor remained low (
