Conjugated Semiconducting Polymer with Thymine Groups in the Side

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Conjugated Semiconducting Polymer with Thymine Groups in the Side Chains: Charge Mobility Enhancement and Application for Selective Field-Effect Transistor Sensors toward CO and H2S Yizhou Yang, Zitong Liu, Liangliang Chen, Jingjing Yao, Gaobo Lin, Xisha Zhang, Guanxin Zhang, and Deqing Zhang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00106 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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Chemistry of Materials

Conjugated Semiconducting Polymer with Thymine Groups in the Side Chains: Charge Mobility Enhancement and Application for Selective Field-Effect Transistor Sensors toward CO and H2S Yizhou Yang,†‡ Zitong Liu,*† Liangliang Chen,†‡ Jingjing Yao,† Gaobo Lin,†‡ Xisha Zhang,†‡ Guanxin Zhang,† Deqing Zhang*†‡ † Beijing National Laboratories for Molecular Sciences, CAS Key Laboratories of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China E-mail: [email protected]; [email protected] ‡ University of Chinese Academy of Sciences Beijing 100049, P. R. China ABSTRACT: Conjugated polymers with both high charge mobilities and responsive functionality have received increasing attentions because of their promising applications in organic electronic devices and chemo-/biosensors. We successfully incorporated thymine groups in the side chains of DPP-based conjugated polymer PDPP4T-T. Our studies reveal that thin film of PDPP4T-T exhibits improved crystallinity and higher charge mobility compared with analogous polymers with pure alkyl chains. This is attributed to the formation of H-bonding among thymine groups, which could strengthen interchain interactions and thus facilitate ordered packing of polymer chains. Importantly, thin films of PDPP4T-T containing either Pd (II) or Hg (II) ions were successfully fabricated via air-water interface coordination reactions of thymine groups with ions. Thin film FETs of PDPP4T-T containing Pd (II) ions exhibit sensitive and selective response toward CO even at low concentration of 10 ppb. Such FET-based CO sensor with high sensitivity and good selectivity is achieved for the first time by using polymeric semiconductors. Alternatively, FETs with PDPP4T-T-Hg (II) thin films can be utilized to detect H2S with good selectivity and high sensitivity (down to 1 ppb). Therefore, incorporating thymine groups into conjugated polymers can not only improve semiconducting mobilities, but also endow semiconducting thin films with sensing functionality.

INTRODUCTION The past decades have witnessed the rapid development of organic and polymeric semiconductors with various conjugated backbones, aiming to boost charge mobilities.1-6 Organic and polymeric semiconductors with high charge mobilities have been used to fabricate field-effect transistors (FETs) and organic circuits for various applications.7-18 One of important FETs applications is the chemo-/biosensing.19-21 FETs-based sensors for gas and chemical vapors were fabricated by using organic and polymeric semiconductors. The FET characteristics endow these gas/chemical vapor sensors with good sensitivity, but the sensitivity is affected by the morphologies of the semiconducting thin films. The sensing is usually operated through the physical adsorption of analytes (gases/chemical vapors) onto semiconducting layers and the analytes can trap charge carrier.22-26 Thus, selectivity of

these gas/chemical vapor sensors need to be improved. Efforts were made to improve the sensing selectivity by incorporating functional groups, which can interact or react with the analytes specifically, into organic and polymeric semiconductors.27-29 For instance, Torsi and coworkers synthesized organic semiconductors with chiral side chains with which differential detection of optical isomers was achieved.27 Katz and coworkers reported the detection of phosphonate vapor by using FETs with hydroxyl-terminated organic semiconductor.28 Some of us employed conjugated D-A polymer with –COOH groups in the side chains to construct a sensitive and selective FET sensor for ammonia and amines by taking the advantage of the reaction between carboxylic acid and ammonia/amines.29 In this paper, we report a diketopyrrolopyrrole (DPP)based D-A conjugated polymer PDPP4T-T entailing thymine groups (one of the four nucleobases in the nucleic

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Scheme 1. a) Chemical structure of PDPP4T-T; thymine groups are randomly connected to the conjugated backbone; b) Illustration of T-T intermolecular hydrogen bonding, and the coordination with Pd (II) and Hg (II).

acid of DNA) in the side chains (Scheme 1). The incorporation of thymine groups in the side chains is based on the fact that thymine can bind with certain metal ions30-33 as depicted in Scheme 1, and as a result metal ions can be incorporated into semiconducting thin films uniformly. We show that thin films of PDPP4T-T after binding with Pd (II) and Hg (II) can be utilized to construct FETs sensors for CO and H2S, respectively, with high selectivity and sensitivity. In comparison, FETs with similar conjugated polymers with pure alkyl chains display negligible responses toward CO and H2S under the same conditions. In addition, H-bonding can be formed among thymine groups as illustrated in Scheme 1, and such H-bonding can strengthen the interchain interactions. To our delight, thin film charge mobility of PDPP4T-T can reach 9.1 cm2V-1s-1, being higher than those of the analogous conjugated polymers with pure alkyl chains.

RESULTS AND DISCUSSION Synthesis and Characterizations. The synthesis of PDPP4T-T is outlined in Scheme S1. The synthetic details were provided in Supporting Information. The copolymerization of 2, 3 and 4 (Scheme S1) in a molar ratio of 1:20:21 led to PDPP4T-T in 82% yield after Soxhlet extraction with methanol, acetone, hexane and chloroform sequentially to remove the catalysts, and remaining monomers and oligomers. In principle, conjugated polymers with different contents of thymine can be obtained by varying the molar ratio between 2 and 3. But, the resulting conjugated polymer showed low solubility in organic solvents (e.g. chloroform, dichlorobenzene) due to the presence of more thymine groups in the side chains, when the molar ratio between 2 and 3 was higher than 1:20. PDPP4T-T was characterized with 1H NMR, solid state 13C NMR and elemental analysis (see Supporting Information). The presence of thymine groups in PDPP4T-T

was verified by the FT-IR spectrum. As shown in Figure S2, typical stretching IR absorption around 3500 cm-1 due to the stretching vibration signal of N-H was detected for PDPP4T-T. The molar ratio of 1:20 between the side chains with thymine groups and the branching alkyl chains in PDPP4T-T agrees well with the elemental analysis and 1HNMR data (see Figure S3 in Supporting Information). The formation of H-bonding among the thymine groups was corroborated by recording the 1H NMR of compound 2 in 1,1,2,2-tetrachloroethane at different temperatures. The chemical shift at 8.78 ppm due to the -NH of thymine was gradually up-field shifted by increasing the solution temperature (see Figure S4a). Alternatively, the chemical shift at 8.73 ppm due to the -NH of thymine was gradually up-field shifted by decreasing the concentrations (Figure S4b), which could reduce the formation of intermolecular H-bonding. Such variation of chemical shift was attributed to the intermolecular H-bonding of thymine groups according to previous studies.34 As expected, this 1H NMR signal disappeared after adding D2O. The average molecular weight (Mw) of PDPP4T-T was measured to be 99.5 kDa with a PDI of 3.3 (Figure S5). Its thermal decomposition temperature (measured at 5% weight loss) is higher than 350 °C on the basis of thermogravimetric analysis data (Figure S6). Thus, the presence of thymine groups does not affect the thermal stability of the semiconducting polymer. The incorporation of thymine groups into side chains has little effect on the electronic structure of polymer backbone. As depicted in Figure S7, the solution of PDPP4T-T shows absorption around 780 nm and the thin film absorbs strongly at 790 nm (0-0 vibrational absorption) and 728 nm (0-1 vibrational absorption). Compared to thin film absorptions of PDPP4T-A without pure alkyl groups, the maxima absorptions are slightly red-shifted for PDPP4T-T and the 0−1/0−0 intensity ratio of PDPP4T-T is slightly reduced. According to previous

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Figure 1. a) Transfer (left) and output (right) characteristics of FETs with thin films of PDPP4T-T after thermal annealing at 120 oC. The channel width (W) and length (L) were 1400 and 50 μm, respectively, VDS = -100 V. b) Out-of-plane (left) and in-plane (right) 1D GIWAXS of PDPP4T-T thin films. studies35-36, the low 0−1/0−0 intensity ratio is indicative of more ordered interchain packing for PDPP4T-T. Based on the thin film onset absorption, the optical bandgap was estimated to be 1.40 eV. Based on the onset oxidation and reduction potentials of PDPP4T-T (Figure S8), its HOMO/LUMO energies were estimated to be -5.36/-3.64 eV, which are also similar to those of the analogous polymers. 34 Charge Mobility Enhancement. The semiconducting property of PDPP4T-T was evaluated by fabricating the bottom-gate/bottom-contact (BGBC) FETs (for details, see Supporting Information) with the spin-coated thin films of PDPP4T-T entailing fiber-like aggregates (see AFM image in Figure S9). Figure 1 shows the transfer and output curves for FETs with thermally-annealed (120 oC) thin films of PDPP4T-T. The semiconducting performance data including saturated and linear mobilities, Ion/off ratio, VTh, subthreshold slope (SS) are listed in Table 1. As expected, thin film FETs of PDPP4T-T exhibit typical p-type semiconducting behavior in ambient atmosphere. Average/maximum hole mobilities were extracted to be 4.8/5.5 and 7.8/9.1 cm2V-1s-1 for the asprepared and thermally thin films of PDPP4T-T, respectively.

In order to demonstrate the effect of thymine groups in PDPP4T-T on the thin film charge mobility, BGBC devices with PDPP4T-A (with branching alkyl chains, Mw = 186.1 kDa, PDI = 3.2, see Table 1) and PDPP4T-B (with both linear and branching alkyl chains, Mw = 191.1 kDa, PDI = 2.1, see Table 1), which were prepared by following the reported procedures, 34 were fabricated and measured under the similar conditions. As listed in Table 1, charge mobilities of the thin films of PDPP4T-T are obviously higher than those of the respective thin films of PDPP4TA and PDPP4T-B before and after thermal annealing. Reliability factor, which was calculated by following the recent report, 37 is used to assess the deviation degree of the transporting behavior of FETs from the ideal one. When the reliability factor is higher, the charge mobility extracted by fitting the linear part of the plot of IDS1/2 vs VG becomes more reliable. As listed in Table 1, reliability factors for the fittings of the respective plots of IDS vs VG are all higher than 60%. Therefore, the comparison of charge mobility among PDPP4T-T, PDPP4T-A and PDPP4T-B is justified. In addition, thin film of PDPP4TT also shows higher linear charge mobility than those of PDPP4T-A and PDPP4T-B (see Table 1). These results clearly manifest that incorporation of thymine groups in



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Table 1. Hole mobilitiesa (highest (average)), threshold voltages, Ion/Ioff ratios, subthreshold slopes (SS), reliability factors of BGBC FETs with PDPP4T-T, PDPP4T-A and PDPP4T-B.

polymer

aThe

Temp (oC)

μh (cm2V-1s-1)

RT

5.5 (4.8)

120

9.1 (7.8)

RT

1.1 (0.8)

120

2.4 (2.0)

RT

2.6 (2.4)

100

5.1 (4.8)

μlin (cm2V-1s-1)

2.0 (1.7)

0.7 (0.6)

1.7 (1.2)

VTh (V)

Ion/Ioff

1 to 5

105-106

2 to 5

105-106

0 to 4

106-107

-1 to 2

105-106

-2 to 6

106-107

-1 to 9

106-107

SS (V dec-1)

r factor

1.1~1.9

74%

0.5~0.8

60%

0.9~1.6

67%

average mobilities are based on more than 10 devices on different OTS-modified SiO2/Si substrates.

the side chains of PDPP4T-T can effectively boost charge mobilities. Moreover, other semiconducting data including Ion/off, VTh and subthreshold slope (see Table 1) for FETs with PDPP4T-T are comparable to those with PDPP4T-A and PDPP4T-B. Such mobility enhancement is attributed to the fact that thin film crystallinity of PDPP4T-T is enhanced in comparison with those of the analogous conjugated polymers with pure alkyl chains. This is likely due to the formation of H-bonding among thymine groups, which induces the polymer chains to pack more orderly. Figure 1b shows the out-of-plane and in-plane GIWAXS profiles for thin film of PDPP4T-T on OTS modified SiO2/Si substrate after annealing at 120 oC. Scattering signals up to 4th order, owing to the lamellar stacking of side chains, were observed for thin films of PDPP4T-T in the out-of-plane direction. In comparison, the corresponding (100) and (200) signals were found to be weak and broad for thin film of PDPP4T-A (see Figure S10). Moreover, the lamellar stacking signals for thin film of PDPP4T-T are sharper with smaller FWHMs (full width at half maxima, see Table S1) than those of the respective ones of PDPP4T-A. These GIWAXS data indicate that PDPP4T-T shows improved lamellar stacking order of alkyl chains upon incorporation of thymine groups in the side chains. In addition, the scattering signal due to interchain π-π stacking was also detected for PDPP4T-T in the in-plane direction (see Table S1). The π-π stacking distance was estimated

to be 3.85 Å for PDPP4T-T, being shorter than that of PDPP4T-A (3.92 Å ). FET Sensor for CO and H2S and Mechanism Investigation. Thymine can not only form intermolecular Hbonding, but also coordinate with certain metal ions such as Pd (II)30 and Hg (II)31. In this way, metal ions can be incorporated into thin film of PDPP4T-T. After the incorporation of metal ions into semiconducting polymers, the resulting FETs show sensing functionality by utilizing the specific reactions of the metal ions such as Pd (II) and Hg (II) with gaseous analytes (e.g. H2S and CO). In the following, we report the semiconducting performance of PDPP4T-T after incorporation of Pd (II) and Hg (II) metal ions, and the application of the resulting FETs for sensing H2S and CO with high sensitivity and selectivity. Because of the hydrophobic feature of PDPP4T-T, the incorporation of metal ions into its thin film was achieved through the air-water interfacial coordination. As shown in Scheme 2, thin films of PDPP4T-T with either Pd (II) and Hg (II) ions were separately prepared by dropping a chloroform solution of PDPP4T-T (0.1 mg/mL) onto the surface of the aqueous solution containing K2PdCl4 or Hg(ClO4)2, followed by transferring the respective films to the substrates (for details, see Experimental Section). According to this procedure, the thin film surface, which was formed at the air/film interface, was contacted to the device substrate (OTS-modified SiO2/Si). This thin film transfer method was used by some of us recently to pre-


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Scheme 2. Schematic illustration of fabrication of FETs with thin films of PDPP4T-T-Pd (II) or PDPP4T-T-Hg (II) for sensing CO and H2S, respectively. 50 L of the CHCl3 solution of PDPP4T-T was dropped onto the aqueous solution surface containing either K2PdCl4 or Hg(ClO4)2. After evaporation of solvent, the respective thin film was transferred onto substrate to construct FETs with incorporation of Pd (II)/Hg (II), which act as active sites for sensing.

pare semiconducting polymer thin films with high crystallinity and thus high charge mobilities.38 Thin films of PDPP4T-T containing Pd (II) or Hg (II) ions were characterized with XPS. As shown in Figure S11, binding energies of Pd 3d3/2 and Pd 3d5/2 at 342.9 eV and 337.6 eV, respectively, were detected for the thin film prepared with PDPP4T-T and Pd (II) as described above, indicating that Pd (II) was successfully incorporated into the thin film. In comparison with those of K2PdCl4 (343.4 eV for Pd 3d3/2 and 338.2 eV for Pd 3d5/2), the binding energies of Pd (II) (3d3/2 and 3d5/2) for the PDPP4T-T-Pd (II) thin film are reduced. This can be ascribed to the coordination of Pd (II) with thymine groups. This assumption is augmented by the control experiment with polymer PDPP4T-A (see Table 1) without thymine groups in the side chains. No obvious peaks of Pd 3d were detected for the thin film (see Figure S12), which was fabricated with polymer PDPP4T-A solution and Pd (II) aqueous solution in the same way as that for PDPP4T-T solution and Pd (II) aqueous solution. This results provide additional evidence that the Pd (II) ions in the PDPP4T-T-Pd (II) thin film is induced by the coordination of thymine groups in PDPP4T-T with Pd (II) ions from the aqueous solution. In addition, the PDPP4T-T-Pd (II) thin film was analyzed with inductively coupled plasma mass spectrometry (ICP-MS). The mass content of Pd (II) in the thin film was measured to be 0.12%. On the basis of the mass content of Pd (II), the molar percentage of thymine groups in PDPP4T-T, which were bound with Pd(II), was calculated (see Supporting Information) by assuming that the coordination ratio of thymine groups with Pd (II) is usually 2:1 according to previous reports.39 The result shows that ca. 49% thymine groups in PDPP4T-T are involved in the

coordination with Pd (II) ions within PDPP4T-T-Pd (II) thin film. Similarly, the PDPP4T-T-Hg (II) thin film was prepared with PDPP4T-T solution and Hg(ClO4)2 aqueous solution. The appearance of binding energies of 101.6 eV (Hg 4f7/2) and 105.7 eV (Hg 4f5/2) indicates the presence of Hg (II) ions in the thin film. The following observations prove the coordination of Hg (II) with thymine groups: i) in comparison with those of Hg(ClO4)2 at 102.0 eV (Hg 4f7/2) and 106.0 eV (Hg 4f5/2), the binding energies of Hg 4f7/2 and Hg 4f5/2 for the PDPP4T-T-Hg (II) thin film were shifted to low binding energy region, and ii) the presence of Hg (II) ions in the thin film which was prepared with PDPP4T-A and Hg(ClO4)2 is hardly detected. According to the fact that thymine groups are usually coordinated with Hg (II) in 2:1 mode,40 30% thymine groups in PDPP4T-T are involved in the coordination with Hg (II) ions within PDPP4T-T-Hg (II) thin film on the basis of the ICP-MS data of the PDPP4T-T-Hg (II) thin film, in which the mass content of Hg (II) was measured to be 0.14%. BGBC FETs with thin films of PDPP4T-T with either Hg (II) or Pd (II) ions were fabricated conveniently. As shown in Figure S13, p-type semiconducting behavior of PDPP4T-T is remained, but the hole mobility of PDPP4T-T is reduced upon incorporation of either Pd (II) or Hg (II) ions by comparing with that of the neat thin film of PDPP4T-T. Hole mobilities of PDPP4T-T-Pd (II) and PDPP4T-T-Hg (II) were extracted to be 0.13 and 0.51 cm2V-1s-1, respectively, after incorporating the Pd (II) and Hg (II) ions based on the respective transfer curves. Such mobility reduction can be attributed to the formation of coordination complexes between thymine groups and Pd (II)/Hg (II) ions, which are expected to act as traps for



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Figure 2. Thin film transistor-based sensors for CO and H2S. a) Transfer curves for the FET with PDPP4T-T-Pd (II) after exposure to different concentrations of CO (10 ppb-100 ppm); VDS = -60 V. b) Variation of IDS for FET with PDPP4T-T-Pd (II) under different concentrations of CO (10 ppb-1 ppm); VDS = VG = -20 V. c) Variation of IDS after exposure to different solvent vapors and gases: hexane (52000 ppm), DCM (301000 ppm), acetone (1000 ppm), ethanol (1200 ppm), H 2 (pure), CO2 (pure), NO2 (100 ppm), H2S (100 ppm), CO (1 ppm). d) Transfer curves for the FET with PDPP4T-T-Hg (II) after exposure to different concentrations of H2S (1 ppb-1000 ppm); VDS = -60 V. e) Variation of IDS for FET with PDPP4T-T-Hg (II) under different concentrations of H2S (1 ppb-100 ppb); VDS = VG = -20 V. f) Variation of IDS after exposure to different solvent vapors and gases: hexane (52000 ppm), DCM (301000 ppm), acetone (1000 ppm), ethanol (1200 ppm), H 2 (pure), CO2 (pure), NO2 (100 ppm), H2S (1 ppm), CO (100 ppm). hole carriers. In addition, pinholes with sizes around 150 nm and cracks were observed within thin films of PDPP4T-T-Pd (II) and PDPP4T-T-Hg (II) (Figure S14), respectively. The presence of these pinholes and cracks, which were not observable in PDPP4T-T thin film, may also make contributions to the low charge mobilities of thin films of PDPP4T-T-Pd (II) and PDPP4T-T-Hg (II). But, FETs with such high charge mobilities are enough for construction of FET-based sensors. Moreover, as it will be discussed below, the existence of these pinholes and cracks is beneficial for improving the sensitivity of FETs sensors for CO and H2S. The FETs with thin films of PDPP4T-T containing Pd (II) and Hg (II) ions show sensitive and selective responses toward CO and H2S, respectively. As depicted in Figure 2, the transfer curve for the FET with PDPP4T-T-Pd (II) thin film is varied after exposure to CO of different concentrations. As shown in Figure 2a, both Ion and Ioff were found to decrease after exposure of CO. Figure 2b shows the variation of IDS after exposure of the device to different concentrations of CO. Obviously, IDS (VG = VDS = -20 V) decreases gradually by increasing the concentration of CO, which is still detectable even when the concentration of CO is as low as 10 ppb (Figure 2b). We also

examined the response of FETs with the neat thin films of PDPP4T-T toward CO. As depicted in Figure S15, the variation of the transfer curve is negligible for that of neat PDPP4T-T thin films, which were prepared either with spin-coating method or in the same way as for PDPP4TT-Pd(II) thin films without Pd(II) ion in water, after exposure to CO of different concentrations. Therefore, it can be concluded that the presence of Pd (II) ions in the thin film endows the sensing functionality for FETs with PDPP4T-T-Pd (II) thin films as it will be discussed below. FETs with PDPP4T-T-Pd (II) thin films were also exposed to H2, CO2, NO2, H2S and vapors of hexane, dichloromethane (DCM), acetone, ethanol. As shown in Figure 2c, the decreases of IDS are rather small41 or even negligible after exposure to these gases and solvent vapors, by comparing with that after treatment with 1 ppm of CO. Therefore, FET with PDPP4T-T-Pd (II) thin film shows good selectivity toward CO. Such selectivity is ascribed to the reduction reaction of Pd (II) with CO.42-44 Figure S11a shows XPS signals for thin film of PDPP4TT-Pd (II) before and after exposure to CO. The binding energies at 342.9 eV and 337.6 eV due to Pd (II) are shifted to 341.0 eV and 335.7 eV, which are characteristic of Pd (0) according to previous reports.45 It is expected that the


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Pd (0) species can function as new carrier traps, leading to reduction of IDS for FET with PDPP4T-T-Pd (II) thin film after exposure to CO. Alternatively, pinholes within PDPP4T-T-Pd (II) thin film can facilitate the diffusion and reaction of CO molecules with Pd (II) species from the semiconducting layer, leading to high sensitivity for the FET sensor. It is noted that the sensitive and selective detection of CO with OFETs was never reported before.4648

Similarly, FETs with the PDPP4T-T-Hg (II) thin film show sensitive and selective response toward H2S. As depicted in Figure 2d, the transfer curves were gradually varied after exposing the FET to H2S of different concentrations. Both Ion and Ioff decreased after exposure to H2S. Moreover, the variation of Vth was also noticed (see Figure 2d). Figure 2e shows the decrease of IDS for the FETs vs the concentration of H2S. Clearly, the decrease degree of Ion increased by enhancing the concentration of H2S, and the variation of Ion was still observable even the concentration of H2S was 1 ppb. Thus, the sensitive detection of H2S can be achieved by using FETs with PDPP4T-T-Hg (II) thin films. FETs with PDPP4T-T-Hg (II) thin films were also treated with other gaseous analytes (H2, CO2, NO2, CO) and solvent vapors (hexane, dichloromethane (DCM), acetone, ethanol) apart from H2S. As depicted in Figure 2f, the decrease of IDS is much smaller than that after exposure to H2S under the same conditions. Therefore, the selective detection of H2S is achieved with FETs of PDPP4T-T after incorporation of Hg (II). Figure S11b displays the XPS spectra of PDPP4T-T-Hg (II) thin film before and after exposure to H2S. The signals at 101.6 eV and 105.7 eV owing to Hg (II) in PDPP4T-T-Hg (II) shifted to 101.0 eV and 105.0 eV, which are attributed to Hg (II) in HgS on the basis of previous report.49 Accordingly, such selectivity can be ascribed to the reaction of H 2S with Hg (II) to form HgS. Alternatively, cracks within PDPP4T-T-Hg (II) thin film can facilitate the diffusion and reaction of H2S molecules with Hg (II) species from the semiconducting layer, leading to high sensitivity for the FET sensor. It is noted that the FETs sensors for CO or H2S with PDPP4T-T-Pd (II) or PDPP4T-T-Hg (II) show good repeatability. Firstly, the semiconducting performances of FETs with PDPP4T-T-Pd (II) or PDPP4T-T-Hg (II) are stable on the basis the fact that the transfer curves of FETs that were fabricated under the same conditions were almost overlapped. Secondly, the variation of the respective transfer curves after exposure of the devices to the same concentrations of CO or H2S can be well reproduced (see Figure S16). As discussed above, the mechanism of sensing is due to the selective chemical reactions between Pd (II) and CO or Hg (II) and H2S based on XPS data. Therefore, these FETs-based sensors show high selectivities in comparison with those with physical adsorption mechanism.50 Since the reactions between Pd (II) and CO or Hg (II) and H2S are irreversible, these FETs-based sensors are not reuseable. By considering the low costs of these FETs with polymeric semiconductors, such selective and sensitive sen-

sors for CO and H2S can be used as disposable devices in the future.

CONCLUSIONS We report conjugated D-A polymer PDPP4T-T, in which part of alkyl chains contain thymine groups that can form H-bonding and coordinate with certain metal ions. In comparison with that of polymers PDPP4T-A and PDPP4T-B with just alkyl chains, thin film of PDPP4T-T exhibits better semiconducting performance with hole mobility as high as 9.1 cm2V-1s-1 because of the improvement of thin film crystallinity based on the GIWAXS data. It is likely that the H-bonding among thymine groups facilitates the ordered interchain packing. Interestingly, Pd (II) and Hg (II) ions can be separately incorporated into thin films of PDPP4T-T via air-water interface coordination. FETs with the PDPP4T-T-Pd (II) and PDPP4T-T-Hg (II) thin films exhibit sensitive and selective responses toward CO and H2S, respectively. In fact, CO with low concentration of 10 ppb can be detected with PDPP4T-T-Pd (II)-based FETs, while H2S down to 1 ppb is detectable with FETs with PDPP4T-T-Hg (II) thin films. It is the first time that sensitive and selective detection of CO is achieved with polymeric field effect transistors. Therefore, the incorporation of thymine groups into side chains of conjugated polymers cannot only improve interchain packing order and charge carrier mobilities, but also endow the semiconducting thin films with sensing functionality. These results provide a new design strategy for charge mobility enhancement of semiconducting polymers, and the resulting polymers can also be utilized for multi-functional devices.

EXPERIMENTAL SECTION Synthesis and characterization of PDPP4T-T was provided in Supporting Information. The fabrication and characterization of thin film FETs were also detailed in Supporting Information. Preparation of PDPP4T-T-Pd (II) and PDPP4T-T-Hg (II) thin films for fabricating FETs and sensing CO/H2S: To a vessel (diameter 60 mm, height 30 mm) 40 mL of deionized water dissolving K2PdCl4 (8.0 mM) or Hg(ClO4)2 (2.0 mM) was added. It should be noted that Hg(ClO4)2 is an acute toxin and a potent and dangerous oxidizer. Be careful when using it! The content of metal ions in the film can be controlled by the concentrations of the K2PdCl4 or Hg(ClO4)2 solutions. High concentrations of K2PdCl4 or Hg(ClO4)2 can induce decrease of FET performance largely, while low concentrations are expected to lower the sensitivity of FET-based sensors. The concentrations of K2PdCl4 or Hg(ClO4)2 were optimized for the sensing studies. 50 μL of the CHCl3 solution of PDPP4T-T (0.1 mg/mL) was dropped onto the water surface with a micro-syringe. The formation of thin film was completed during several minutes after evaporation of solvents. Then, a substrate (glass or OTS-modified SiO2/Si with gold electrodes) was inserted at an angle into the water to transfer thin films on air-water interface to the substrate. After thorough washing with deionized water, the thin film was blown by dry nitrogen to remove residual water.



The details of device fabrication were provided in Supporting Information. The sensing experiments were conducted by following the reported procedures.51 To prove the reproducibility for the sensing performance data presented in Figure 2, the measurements for sensing either CO or H2S were conducted with at least five devices with either PDPP4T-T-Pd(II) or PDPP4T-T-Hg(II) thin films and the data can be repeated successfully.

ASSOCIATED CONTENT Supporting Information. Synthesis of materials, FT-IR, TGA, DSC, Uv-vis spectra, cyclic voltammograms, AFM, GIWAXS, fabrication of OFETs, XPS, OFETs characterization of sensing devices, 1H NMR and 13C NMR data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Present Addresses † Beijing National Laboratories for Molecular Sciences, CAS Key Laboratories of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, P. R. China ‡ University of Chinese Academy of Sciences Beijing 100049, P. R. China

Author Contributions D. Z. proposed the study and designed the polymer structures. Z. L. and G. Z. analyzed the data. Y. Y. and Z. L. carried out the experiments. G. L., X. Z. and J. Y. did structural characterization. Y. Y. and G. L. carried out the GIWAXS experiments. D.Z. and Z.L. prepared the manuscript. All authors discussed, revised, and approved the manuscript.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT We thank the financial support of NSFC (21661132006, 21790360), the Youth Innovation Promotion Association CAS (No. 2015024), the National Key R&D Program of China (2017YFA0204701), the Strategic Priority Research Program of the CAS (XDB12010300). We also thank 1W1A of Beijing Synchrotron Radiation Facility for GIWAXS measurements.

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Conjugated Semiconducting Polymer with Thymine Groups in the Side

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