Modification of Side Chains of Conjugated Molecules and Polymers

May 17, 2018 - performance metric for organic semiconductors is charge carrier mobility, ... performance.14,15 The influences of alkyl chains on semic...
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Article Cite This: Acc. Chem. Res. 2018, 51, 1422−1432

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Modification of Side Chains of Conjugated Molecules and Polymers for Charge Mobility Enhancement and Sensing Functionality Zitong Liu, Guanxin Zhang, and Deqing Zhang*

Acc. Chem. Res. 2018.51:1422-1432. Downloaded from pubs.acs.org by NAGOYA UNIV on 06/19/18. For personal use only.

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Organic Solids, CAS Center of Excellence in Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China CONSPECTUS: Organic semiconductors have received increasing attentions in recent years because of their promising applications in various optoelectronic devices. The key performance metric for organic semiconductors is charge carrier mobility, which is governed by the electronic structures of conjugated backbones and intermolecular/ interchain π−π interactions and packing in both microscopic and macroscopic levels. For this reason, more efforts have been paid to the design and synthesis of conjugated frameworks for organic semiconductors with high charge mobilities. However, recent studies manifest that appropriate modifications of side chains that are linked to conjugated frameworks can improve the intermolecular/interchain packing order and boost charge mobilities. In this Account, we discuss our research results in context of modification of side chains in organic semiconductors for charge mobility enhancement. These include the following: (i) The lengths of alkyl chains in sulfur-rich thiepin-fused heteroacences can dramatically influence the intermolecular arrangements and orbital overlaps, ushering in different hole mobilities. Inversely, the lamellar stacking modes of alkyl chains in naphthalene diimide (NDI) derivatives with tetrathiafulvalene (TTF) units are affected by the structures of conjugated cores. (ii) The steric hindrances owing to the bulky branching chains can be weakened by partial replacement of the branching alkyl chains with linear ones for diketopyrrolopyrrole (DPP)-based D (donor)−A (acceptor) conjugated polymers. Such modification of side chains makes the polymer backbones more planar and thus interchain packing order and charge mobilities are improved. The incorporation of hydrophilic tri(ethylene glycol) (TEG) chains into the polymers also leads to improved interchain packing order. In particular, the polymer in which TEG side chains are distributed uniformly exhibits relatively high charge mobility without thermal annealing. (iii) The incorporation of urea groups in the side chains induces the polymer chains to pack more orderly and form large domains because of the additional H-bonding among urea groups. Accordingly, thin film mobilities of the conjugated D−A polymers with side chains entailing urea groups are largely boosted in comparison with those of polymers of the same backbones with either branching alkyl chains or branching/ linear alkyl chains. (iv) The torsions of branching alkyl chains in conjugated D−A polymers can be inhibited to some extent upon incorporation of tiny amount of NMe4I in the thin film. As a result, the polymer thin films with NMe4I exhibit improved crystallinity, and charge mobilities can be boosted by more than 20 times. (v) Side chains with functional groups in the conjugated polymers can endow the thin film field-effect transistors (FETs) with sensing functionality. FETs with the conjugated polymer with −COOH groups in the side chains show sensitive, selective, and fast responses toward ammonia and amines, while FETs with the ultrathin films of the polymer containing tetra(ethylene glycol) (TEEG) in the side chains can sense alcohol vapors (in particular ethanol vapor) sensitively and selectively with fast response.

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INTRODUCTION Organic semiconductors, including conjugated small molecules and polymers, have been intensively investigated because of their promising applications in various optoelectronic devices.1−6 Charge carrier mobility is the key performance metric for organic semiconductors. In principle, charge carrier mobility is affected by the electronic structures of conjugated molecules and polymers and intermolecular or interchain π−π interactions and packing at both microscopic and macroscopic levels.7−10 In this sense, the structures of conjugated frameworks play a vital role in determining the charge transporting properties.11−13 Conjugated molecules and polymers usually contain alkyl chains, which enable them to be soluble in organic solvents and thus to become solution-processable, which is one of the prominent advantages of organic semiconductors in comparison with the inorganic counterparts. However, recent studies reveal that © 2018 American Chemical Society

these alkyl chains not only endow solubilities, but also affect the intermolecular/interchain packing and thus the optoelectronic performance.14,15 The influences of alkyl chains on semiconducting properties of conjugated molecules and polymers were explored in terms of the lengths of alkyl chains, odd−even effect, and branching points and chirality of branching alkyl chains.14−16 Alkyl chains containing heteroatoms, siloxane-terminated side chains, fluoroalkyl chains, oligo(ethylene glycol) chains, and ionic side chains, were also incorporated into conjugated systems to enhance charge mobilities.14,15,17−19 The influences of side chains on the semiconducting and photovoltaic performances have been discussed in a few Perspectives and Accounts.14,15,20,21 In this Account, we will highlight our Received: February 10, 2018 Published: May 17, 2018 1422

DOI: 10.1021/acs.accounts.8b00069 Acc. Chem. Res. 2018, 51, 1422−1432

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Accounts of Chemical Research

Figure 1. Chemical structures (a), intermolecular stacking (b), and microcrystals (c) for T1 and T6. Reproduced with permission from ref 24. Copyright 2013 Wiley-VCH Verlag GmbH&Co. KGaA.

intermolecular electronic coupling among HOMO − 1 orbitals is observed for T1. Thus, HOMO − 1 orbitals play a dominant role in the charge transport within the crystal of T1. Moreover, the calculations also indicate that T1 exhibits two-dimensional charge transport with an average mobility of 0.72 cm2 V−1 s−1. In comparison, weak intermolecular electronic coupling among HOMO − 1 orbitals is observed for T6, and thus poor carrier transport is anticipated with an estimated mobility of 0.07 cm2 V−1 s−1. These calculations agree well with the hole mobilities of field-effect transistors (FETs) from the respective microcrystals and thin films. Hole mobilities of microplates of T1 were measured in three directions (Figure 1c), and they are on the same order with highest mobility of 0.26 cm2 V−1 s−1. In comparison, the microrod of T6 exhibits much lower hole mobility (9 × 10−4 cm2 V−1 s−1). In order to tune the HOMO/LUMO levels and increase intermolecular interactions, NT-1−NT-3 (Figure 2a), in which two tetrathiafulvalene (TTF) moieties were incorporated into the naphthalene diimide (NDI) core, were synthesized. Besides TTF moieties, electron-withdrawing 2-(1,3-dithiol-2-ylidene)malonitrile groups were connected to NDI cores as in NTCN-1− NTCN-3.25 HOMO/LUMO level variations by incorporating electron donor or acceptor moieties result in different chargetransport behaviors. The unsubstituted NDI is an n-type semiconductor. However, thin films of NT-1−NT-3 behave as typical p-type semiconductors, while thin films of NTCN-1−NTCN-3 exhibit ambipolar semiconducting properties. Among the NDI derivatives, NTCN-3 shows highest hole and electron mobility up to 0.03 and 0.003 cm2 V−1 s−1, respectively, while the hole mobility of NT-2 can reach 0.31 cm2 V−1 s−1 after thermal annealing. There are two types of alkyl chains within them, which are substituted at N,N′-positions of the NDI core and TTF moieties, respectively. While the alkyl chains that are connected to the N,N′-positions are the same, the lengths of those that are linked to the TTF moieties are varied. Both types of alkyl chains can influence the intermolecular packing. Figure 2b shows thin film

recent studies on the modification of side chains of conjugated molecules and polymers for charge mobility enhancement and sensing functionality. These include (i) new aspects of side-chain effect on the intermolecular packing and charge transporting property for conjugated small molecules, (ii) replacing branching alkyl chains with linear ones partially in conjugated D−A polymers for weakening the steric crowding of alkyl chains, (iii) incorporating urea groups in side chains of conjugated polymers for enhancing charge mobilities, (iv) the effect of ionic additive (e.g., tetramethylammonium iodide, NMe4I) on the inhibition of side-chain rotations in conjugated D−A polymers for enhancing charge mobilities, and (v) side chains with functional groups in conjugated D−A polymers for sensing functionality.

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SIDE CHAINS OF DIFFERENT LENGTHS IN CONJUGATED MOLECULES It is known that tight and multidimensional intermolecular stacking arrangements are beneficial for enhancing semiconducting properties of organic semiconductors.3 The alkyl chain effect on the intermolecular arrangements and semiconducting properties have been reported for several conjugated molecular systems.22,23 We reported sulfur-rich thiepine-fused heteroacenes T1 and T6 (Figure 1a) with alkyl chains of different lengths.24 Their energy levels and bandgaps are almost the same, and thus the alkyl chains exert no effect on their electronic structures. However, the intermolecular orientation and stacking within the crystals are totally different. As shown in Figure 1b, molecules of T1 adopt a herringbone arrangement with the herringbone angle of 55.49°, and neighboring molecules are connected by multiple interactions. For T6, however, the conjugated backbones of two neighboring molecules with a separation of 3.55 Å are arranged antiparallel, and the dimers are further interconnected through short S···S interatomic interactions. On the basis of their crystal structures, intermolecular electronic coupling was calculated for T1 and T6. Significant 1423

DOI: 10.1021/acs.accounts.8b00069 Acc. Chem. Res. 2018, 51, 1422−1432

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Figure 2. Chemical structures (a) and XRD patterns (b) of NT-1−NT-3 and NTCN-1−NTCN-3. Reproduced with permission from ref 25. Copyright 2012 The Royal Society of Chemistry.

groups along the polymer backbone can affect the planarity of the conjugated backbone and prevent dense packing of polymer chains due to steric crowding of branching alkyl chains. In order to reduce such steric hindrance, new branching alkyl chains and siloxane-terminated side chains, in which the branching points of these side chains are away from the polymer backbones, have been devised and connected to the conjugated backbones.16,19,27 Such side-chain engineering improves the orderly packing of polymer chains with short π−π stacking distance. We investigated conjugated D−A polymers entailing the Pechmann dye framework, and the polymers bearing siloxane-terminated side chains exhibited more balanced hole and electron mobilities than the respective ones with branching alkyl chains.28 However, it should be noted that the respective alkyl halogens, precursors to these modified side chains, are not commercially available, and their syntheses involve three or more reaction steps and purifications.29 This will surely increase the costs for the scalable preparation of these conjugated D−A polymers for future device applications. We addressed this issue differently by partially replacing bulky branching alkyl chains with linear ones.30 The linear alkyl chains not only can weaken the steric hindrance but also can promote interchain interactions through interdigitation. According to this idea, we prepared conjugated terpolymers pDPP4T-1− pDPP4T-3 (Figure 3a), in which the backbones are composed of diketopyrrolopyrrole (DPP) and quaterthiophene units. The ratios of linear (n-dodecane)/branching (2-decyltetradecyl) chains in pDPP4T-1, pDPP4T-2, and pDPP4T-3 are 1:20, 1:10 and 1:5, respectively. This side-chain modification improves interchain packing order based on grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns shown in Figure 3b. Thermally annealed thin films of all polymers displayed up to fourth order scattering signals that were attributed to the alkyl chain lamellar packing. The lamellar stacking d-spacing is shortened by introducing linear chains, and moreover it further decreases by increasing the linear/branching ratios. This is because the presence of more linear alkyl chains is favorable for interdigitation of linear

XRD patterns of NT-1−NT-3 and NTCN-1−NTCN-3 after thermal annealing. Diffraction peaks at 3.7° and 7.4° were detected for NT-1, 3.6° and 7.2° for NT-2, and 2.9°, 5.6°, and 8.4° for NT-3. These diffractions are due to the lamellar stacking of alkyl chains, and the corresponding d-spacing increases from NT-1 to NT-3. As the lengths of alkyl chains connected to TTF moieties increase from NT-1 to NT-3, such lamellar structures are determined by the interactions among alkyl chains connected to TTF moieties as illustrated in Figure 2b. However, all thin films of NTCN-1−NTCN-3 exhibit diffractions at 4.3° and 8.6° after thermal annealing. Thus, they own similar lamellar structures with the same d-spacing, indicating that lamellar structures for thin films of NTCN-1−NTCN-3 are dictated by the alkyl chain interactions at N,N′-positions. Interestingly, new diffraction peaks at 3.6° and 7.2° appear for NTCN-3 after further thermal annealing at 160 °C. These new diffractions are the same as those for NT-2, for which the lamellar structure is attributed to the stacking of −n-C6H13 chains linked to TTF moieties. Therefore, two lamellar structures owing to the separate stacking of two types of alkyl chains coexist within the thin film of NTCN3 after thermal annealing. The orderly stacking of two types of alkyl chains can facilitate the molecules to form large domains, which are beneficial for enhancing the carrier mobilities. Our studies demonstrate that side alkyl chains can largely influence the intermolecular interactions and π−π stacking for conjugated molecules and thus their charge mobilities can be tuned by varying the alkyl chains. Although it is still challenging to predict the exact influences of alkyl chains on the intermolecular packing, the features of alkyl chains do provide us a molecular design dimension for boosting charge mobilities. In addition, long alkyl chains can also improve the stabilities of organic semiconductors by excluding moisture and oxygen from air.26

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LINEAR VERSUS BRANCHING ALKYL CHAINS IN CONJUGATED D−A POLYMERS Bulky branching chains are usually linked to either donor or acceptor moieties in conjugated D−A polymers to endow solubilities in organic solvents. The presence of bulky branching 1424

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Figure 3. (a) The structures and charge mobilities of pDPP4T, and pDPP4T-1−pDPP4T-3; (b) GIWAXS patterns of the polymers. Reproduced with permission from ref 30. Copyright 2017 The Royal Society of Chemistry.

Figure 4. Structures of pDPP2TSe-12, pDPP2TSe-10, and pDPP2TSe and their packing modes based on GIWAXS.

Clearly, semiconducting performance can be enhanced by introducing linear alkyl chains. The polymers discussed above are terpolymers in which the branching and linear alkyl chains are randomly arranged. Accordingly, this may lead to batch-to-batch variations.2,9 Alternatively, we have designed and synthesized regioregular DPP-based conjugated polymers pDPP2TSe-10 and pDPP2TSe-12 (Figure 4) in which each DPP unit entails one branching and one linear chain.31 On the basis of absorption, Raman, and 1H NMR spectra, it can be concluded that pDPP2TSe-10 and pDPP2TSe-12 entailing linear alkyl chains possess more planar conjugated backbones in comparison with pDPP2TSe with bulky branching chains. GIWAXS data indicate that the replacement of one bulky branching chain with a linear one at each DPP unit alters the polymer chain packing mode and improves polymer thin film crystallinity. Polymer chains of pDPP2TSe-10 and pDPP2TSe-12 are packed in edge-on mode on the substrate, whereas polymer chains

alkyl chains, which enables the polymer chains to pack more closely. Interchain π−π stacking signals were also detected in the inplane direction for these polymers. The corresponding π−π packing distances were estimated to be 3.81, 3.79, 3.76, and 3.73 Å, for pDPP4T, pDPP4T-1, pDPP4T-2, and pDPP4T-3, respectively. Clearly, the π−π packing distance becomes shorter after introducing more linear alkyl chains. Furthermore, polymer chains of pDPP4T-3 are packed in edge-on mode, while face-on and edge-on packing modes coexist in other polymers. It is noted that the π−π stacking direction for conjugated polymers with edge-on packing mode is favorable for lateral charge transport between source and drain electrodes of transistors. The respective hole mobilities of pDPP4T-1−pDPP4T-3 are higher than that of pDPP4T, and the hole mobilities are incremented with increasing linear chain contents. The maxima thin film hole mobility of pDPP4T-3 can reach 6.1 cm2 V−1 s−1, which is ca. 2 times of that of pDPP4T after thermal annealing. 1425

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Figure 5. (a) The structures and charge mobilities of pDPP3TG-1 and pDPP3TG-2; (b) GIWAXS patterns of the as-prepared thin films of the polymers. Reproduced with permission from ref 32. Copyright 2017 Wiley-VCH Verlag GmbH&Co. KGaA.

low charge mobility. It is obvious that the effect of TEG chain on the semiconducting properties of conjugated D−A polymers is dictated by how TEG chains are arranged along the conjugated backbones. It is noted that thermal annealing usually ushers in boosting charge mobilities for most conjugated polymers. For pDPP3TG-1, its charge mobility remains almost unaltered after thermal annealing and this feature will enable rapid device manufacturing and reduce thermal degradation.

of pDPP2TSe adopt mainly face-on mode. The correlation lengths (LC) along the π−π stacking directions for pDPP2TSe-10 and pDPP2TSe-12 are longer than that of pDPP2TSe, hinting that thin films of pDPP2TSe-10 and pDPP2TSe-12 entail longer ordering of interchain π−π stacking. The improvement of interchain packing is beneficial for boosting charge mobilities. Hole mobilities measured in air of thin films of pDPP2TSe-10 and pDPP2TSe-12 are ca. 6 and 7 times higher than that of pDPP2TSe. In addition, they exhibit ambipolar semiconducting behavior under nitrogen atmosphere, and hole/electron mobilities of pDPP2TSe-10 and pDPP2TSe-12 are 5/2 and 6/3 times higher than those of pDPP2TSe. Apart from hydrophobic linear alkyl chain, we also incorporated hydrophilic tri(ethylene glycol) (TEG) into DPP-based conjugated polymers including pDPP3TG-1 and pDPP3TG-2 (Figure 5a).32 Each DPP unit in pDPP3TG-1 bears a branching alkyl chain and a TEG chain, and TEG side chains are distributed uniformly, whereas TEG chains are randomly arranged in terpolymer pDPP3TG-2. According to GIWAXS data (Figure 5b), polymer chains of pDPP3TG-1 adopt the predominant edge-on packing mode on the substrate. Both lamellar stacking of side chains up to fourth order and interchain π−π stacking are observed for as-prepared thin film of pDPP3TG-1. Similar GIWAXS patterns were detected for thin film of pDPP3TG-2, but the corresponding signal intensities are much weaker than those of pDPP3TG-1, indicating lower packing order compared to pDPP3TG-1. In comparison, the analogous polymer (with the same backbone as for pDPP3TG-1) with branching alkyl chains shows both face-on and edge-on packing modes on the substrate. The as-prepared thin film of pDPP3TG-1 exhibits high hole mobility up to 2.6 cm2 V−1 s−1 without thermal annealing. In comparison, pDPP3TG-2 and the polymer with branching alkyl chains under the same conditions possess comparatively

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SIDE CHAINS WITH UREA GROUPS By considering that urea groups can form hydrogen bonds and such additional interchain interactions may induce the polymer chains to form ordered and large domains, we introduced side chains with urea groups in pDPP4TU-1−pDPP4TU-3 (Figure 6a).33 The molar ratios between the urea-containing alkyl chains and branching alkyl chains are 1:30, 1:20, and 1:10 for pDPP4TU-1, pDPP4TU-2, and pDPP4TU-3, respectively. On the basis of AFM images shown in Figure 6b, thin films of pDPP4TU-1−pDPP4TU-3 entail relatively long and wide nanofibers that are assembled from the polymer chains and well interconnected. The average widths of nanofibers in thin films of pDPP4TU-1, pDPP4TU-2, and pDPP4TU-3 are ca. 35, 45, and 50 nm, respectively, after thermal annealing at 100 °C. In comparison, the width of nanofibers in thin film of pDPP4TU-0 (without urea groups in the side chains) was ca. 30 nm. The formation of thick nanofibers can be attributed to the interconnection of polymer domains via H-bonding of urea groups in side chains of these polymers. The formation of H-bonding is verified by IR and 1H NMR spectra. According to GIWAXS patterns (Figure 6c), thin films of pDPP4TU-1−pDPP4TU-3 show improved crystallinity and interchain packing order in comparison with that of pDPP4TU-0. First, 1426

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Figure 6. (a) The structures and charge mobilities of pDPP4TU-1−pDPP4TU-3 and pDPP4TU-0; (b, c) AFM height images and GIWAXS patterns of the polymers. Reproduced with permission from ref 33. Copyright 2016 American Chemistry Society.

improve charge transport performance of conjugated D−A polymers.

lamellar stacking of alkyl chains up to the fourth order was observed for pDPP4TU-1−pDPP4TU-3, while only the first order and weak second order diffractions were detected for neat thin film of pDPP4TU-0. Second, the respective π−π stacking signals were detected for the thermally annealed pDPP4TU-2 and pDPP4TU-3 thin films. Therefore, the presence of urea groups not only improves the lamellar stacking of alkyl chains but also ushers in the π−π stacking of neighboring conjugated backbones. Average hole mobilities of the thermally annealed thin films of pDPP4TU-1, pDPP4TU-2, and pDPP4TU-3 were estimated to be 5.1, 7.1, and 11.4 cm2 V−1 s−1, respectively, being higher than that of pDPP4TU-0 (3.0 cm2 V−1 s−1).35 Moreover, hole mobilities are boosted gradually by increasing the urea group contents in the side chains. These results clearly demonstrate that the introduction of urea groups in the side chains can

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CHARGE MOBILITY ENHANCEMENT UPON INCORPORATION OF IONIC ADDITIVES Not only steric hindrance due to branching alkyl chains but also their own torsions affect the interchain packing for conjugated polymers. We invented a simple and facile approach to inhibit the torsions of branching alkyl chains and thus facilitate more ordered interchain packing, leading to charge mobility enhancement, by incorporating ammonium salts into certain conjugated D−A polymers.34 We investigated the semiconducting performance of thin films of pDPPTTT (Figure 7a) containing different amounts of NMe4I with molar ratios of the repeat unit of pDPPTTT against NMe4I being 7.5:1, 15:1, 30:1, and 45:1. As an example, the 1427

DOI: 10.1021/acs.accounts.8b00069 Acc. Chem. Res. 2018, 51, 1422−1432

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Figure 7. (a) Structure of pDPPTTT; (b) transfer and output curves; (c) GIWAXS patterns of pDPPTTT and pDPPTTT-NMe4I (30/1) thin films; (d) ESP map of pDPPTTT and illustration of the positions of I− and NMe4+; (e) variation of the calculated torsion potential vs the dihedral angle in the absence and presence of either NMe4+ or I−. Reproduced with permission from ref 34. Copyright 2016 AAAS.

transfer and output curves for the pDPPTTT-NMe4I thin film (30:1) are depicted in Figure 7b. The average hole mobility, which was extracted by fitting the linear part of the plot of IDS1/2 versus VGS at low voltages, is 19.5 cm2 V−1 s−1, being 24 times higher than that of the neat pDPPTTT film, and the maximum hole mobility can reach 26.2 cm2 V−1 s−1. The average and maximum hole mobilities of pDPPTTT-NMe4I thin films at molar ratios of 7.5:1, 15:1, and 45:1 were estimated to be 8.1 and 12.9, 15.4 and 20.1 and 13.6 and 17.3 cm2 V−1 s−1. It is obvious that hole mobilities are remarkably boosted by incorporating NMe4I, and the highest mobility enhancement is achieved for the pDPPTTT-NMe4I thin film at a molar ratio of 30:1. In addition, FETs with pDPPTTT-NMe4I thin films show good operational and air stabilities. Such charge mobility enhancement observed for pDPPTTTNMe4I thin films can be ascribed to several effects. The possible contributions of solvent effect, ionic conductivity, and chemical doping can be excluded on the basis of FET performance and

characterizations with UV−vis absorption, IR, Raman, and ESR, as well as photothermal deflection spectroscopy measurements. The characterizations with scanning kelvin probe microscopy indicate that the contact resistance between the thin film and Au electrode can be greatly reduced in the presence of NMe4I. Such contact resistance reduction is estimated to enhance charge mobility by factors of 1.3 to 1.9. In fact, the beneficial effect of the additive on the microstructure of the polymer chain packing and the thin-film morphology is mainly responsible for the mobility enhancement. GIWAXS patterns and the respective profiles along the out-ofplane (qz) and in-plane (qy) directions for the pDPPTTT-NMe4I (at a molar ratio of 30:1) and neat pDPPTTT films are shown in Figure 7c. In the out-of-plane direction, four orders of scattering signals owing to the lamellar stacking of side alkyl chains, were detected for pDPPTTT-NMe4I thin film. In comparison, neat pDPPTTT thin film just displayed the (100) scattering peak and the broad (200) signal along the out-of-plane direction. Thus, the 1428

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Figure 8. (a) Structures of pDPPBU-BT and pDPPCOOH-BT; (b) AFM images; (c) variation of IDS vs time for FET of pDPPCOOH-BT after exposure to different concentrations of NH3; (d) variation of IDS after exposure to different gases and solvent vapors. Reproduced with permission from ref 38. Copyright 2016 American Chemistry Society.

shown that the incorporation of side chains with functional groups in will confer FET-based sensors with good selectivity by taking advantage of the specific interactions of the functional groups with the analytes.36,37 We designed and synthesized conjugated D−A polymer pDPPBU-BT (Figure 8a) including tert-butoxycarbonyl (−COOtBu) groups in the side chains.38 Thermal annealing of thin film of pDPPBU-BT at 240 °C under vacuum results in the release of gaseous isobutylene and transformation into polymer pDPPCOOH-BT with −COOH groups in the side chains. This is based on the observation that the IR signal at 1745 cm−1 due to tert-butoxycarbonyl group became gradually weak, while the signal at 1729 cm−1 due to −COOH emerged. AFM images shown in Figure 8b clearly indicate that the formation of pinholes within the heated thin films, accompanying the transformation of −COOtBu into −COOH. Although thin film of pDPPCOOH-BT possesses low charge mobility, the resulting FET exhibits selective, sensitive, and fast response toward ammonia and amines. As shown in Figure 8c, the source−drain current (IDS) decreases after exposure to ammonia with different concentrations. The variation of IDS can be detected when the concentration of ammonia is as low as 10 ppb. Notably, the decrease of IDS becomes saturated within a few seconds and thus sensing response is rather fast. The FET with pDPPCOOH-BT also exhibits sensitive response to vapors of amines such as triethylamine, piperidine, and putrescine as to ammonia. But, the decrease of IDS is negligible after exposure to vapors of dichloromethane, ethanol, ethyl acetate, hexane, acetone, and hydrochloric acid with concentrations higher than 1000 ppm. The selective response can be attributed to the −COOH groups, which can selectively react with ammonia (and other amines) to form ammonium carboxylate. It is expected that ammonium carboxylate can act as additional traps for charge carriers. This agrees well with the observation that hole mobility of pDPPCOOH-BT thin film decreases after exposure to ammonia. In particular, the sensitive and fast response of this FET toward ammonia and amines can be ascribed to the presence of nanopores, which are formed because of the release of

lamellar stacking order of alkyl chains is improved for pDPPTTT thin film after incorporation of NMe4I. Moreover, scattering signals owing to the interchain π−π stacking were detected for pDPPTTT-NMe4I thin film in both directions, which were not observed for the neat thin film. Therefore, the incorporation of NMe4I can improve the interchain packing order. In order to understand this beneficial effect of NMe4I, theoretical calculations were performed for two repeat units of pDPPTTT. On the basis of the electrostatic potential (ESP) map of pDPPTTT alone, NMe4+ and I− were put close to the carbonyl (with negative ESP) and thienothiophene (with positive ESP) groups, respectively (Figure 7d), followed by optimizing the conformation. Then, the torsion potentials at different dihedral angles between the polymer backbone and side chain were calculated (Figure 7e). The presence of either I− or NMe4+ results in much steeper variation of torsion potential versus the dihedral angle with respect to the neat pDPPTTT. Therefore, the rotation of the side chain can be inhibited to some extent in the presence of I− or NMe4+, and the number of accessible conformations for the side chains is reduced. The inhibition of the torsion of the alkyl side chains by the ionic species will facilitate a more ordered interchain packing. This straightforward approach is also effective for a range of conjugated D−A polymers for enhancing charge mobilities. But, this approach cannot be extended to homopolymers like P3HT. Furthermore, it is interesting to note that other ionic additives such as NMe4Br can also be utilized to improve semiconducting performance of conjugated D−A polymers with branching alkyl chains.

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SIDE CHAINS WITH FUNCTIONAL GROUPS IN CONJUGATED D−A POLYMERS FOR SENSING FUNCTIONALITY OFETs have shown promising applications as chemical sensors.35 The sensing mechanism is in principle based on the interactions of analytes with the semiconducting layer close to the dielectric layer. However, sensitivity and selectivity are still open issues for FET-based sensors. Repeatable and fast responses are also required for practical applications. Recent studies have 1429

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Figure 9. (a−c) Structure of pDPP3TG, surface pressure (π) versus mean monomeric area , and AFM image; (d) variation of transfer characteristics for the monolayer FET upon exposure to ethanol vapor; (e) decrease of IDS after exposure to ethanol vapor (10 ppb) for FETs with monolayer, five layer, and spin-coated thin films. Reproduced with permission from ref 39. Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA.

reduction was detectable even at low concentration of 1 ppb. Moreover, the current reduction was completed within seconds. Thus, this monolayer FET shows ultrasensitive and fast response toward ethanol vapor. This is attributed to the nanopores within the monolayer ultrathin film, which may facilitate penetration of the ethanol molecules into the semiconducting layer more easily. The monolayer FET can also detect methanol and isopropanol vapors at low concentration. But, its response to other solvents including CH2Cl2, ethyl acetate, and acetone is negligible. The selective response toward alcohol vapors, for the monolayer FET, can be attributed to the hydrophilic TEEG chains in pDPP3TG, which are expected to interact with alcohols through hydroxyl groups.

isobutylene during the thermal annealing of pDPPBU-BT thin film. These nanopores can facilitate the penetration of analytes within the semiconducting layers and their reactions with the −COOH groups. As a result, high sensitivity and fast response can be achieved simultaneously. In addition, this FET sensor with pDPPCOOH-BT is reusable by heating the device at 80 °C under vacuum to transform the ammonium carboxylate back into −COOH again. Ultrathin semiconducting layers in FET sensors are beneficial for improving the sensitivities.35 One way to prepare ultrathin films is to employ Langmuir−Schaefer method. In order to facilitate monolayer formation at the air−water interface, we introduced a long hydrophilic tetra(ethylene glycol) (TEEG) chain into each DPP unit of pDPP3TG (Figure 9a) to endow it with amphiphilic nature.39 The surface pressure (π) versus mean monomeric area isotherm shown in Figure 9b indicates that monolayer of pDPP3TG can be formed at the air−water interface. Moreover, a monolayer and multilayers (up to five layers) of pDPP3TG can be deposited onto the substrate. AFM images (Figure 9c) indicate the formation of pores with sizes in the range of 50−100 nm within the monolayer thin film, which is beneficial for sensing gaseous analytes. These ultrathin films were successfully utilized to fabricate FETs. For the monolayer FET, the hole mobility can reach 0.014 cm2 V−1 s−1 with relatively high Ion/off ratio. The hole mobilities are slightly enhanced to 0.015 and 0.02 cm2 V−1 s−1 for FETs with bilayer and three layer ultrathin films, respectively. Such mobility enhancement can be ascribed to the fact that the bilayer and three-layer thin films entail fewer nanopores than the monolayer ultrathin film. FETs with these ultrathin films of pDPP3TG show good operational stability. The hydrophilic feature of TEEG chains endows the monolayer FET with sensing functionality. The transfer characteristics were obviously altered for the monolayer FET upon exposure to ethanol vapor (1 ppb to 1 ppm); the reduction of both off-current and on-current were detected (Figure 9d). The on-current

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SUMMARY AND OUTLOOK We and others have demonstrated that side chains in conjugated molecules and polymers are not only soluble groups but also influence the intermolecular/interchain packing and their optoelectronic properties. In this Account, we highlight our recent research progress with regard to the modification of side chains of conjugated molecules and polymers for charge mobility enhancement. These include (i) dramatic effects of the lengths of alkyl chains on the intermolecular orientations and packing for sulfur-rich thiepine-fused heteroacenes and NDI derivatives with TTF units, (ii) improvement of interchain packing order and charge mobilities by replacement of the bulky branching chains with the linear alkyl chains or tri(ethylene glycol) chains for conjugated D−A polymers, (iii) formation of large domains with improved crystallinity and significant charge mobility improvement for conjugated D−A polymers upon incorporating urea groups in the side chains, (iv) boosting charge mobilities for conjugated D−A polymers containing branching alkyl chains upon incorporation of a tiny amount of NMe4I, and (v) applications of FETs with conjugated polymers containing functional groups (e.g., −COOH and tetra(ethylene glycol)) in the side chains for sensing with high sensitivity and selectivity. 1430

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These studies indicate that appropriate combination of conjugated backbones and side chains can maximize the semiconducting performance of conjugated molecules and polymers. Linear chains and those with functional groups are beneficial for conjugated backbones to pack orderly and form thin films with improved crystallinity in comparison with branching alkyl chains. Moreover, incorporation of functional groups in the side chains can endow sensing functionality for organic and polymeric semiconductors. Side-chain effects on the intermolecular or interchain interactions and packing of conjugated frameworks have received increasing attention in designing high-performance organic optoelectronic materials. For future investigations the following aspects deserve further attentions: (i) applications of new self-assembly and processing methods to organic semiconductors with functional side chains to further improve their semiconducting properties, (ii) incorporation of photoresponsive groups in side chains of conjugated systems to achieve photoregulation of their semiconducting properties, and (iii) improvement of the biosensing performance of FETs toward biomarkers in real samples by incorporating targeting groups in the side chains of conjugated systems.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zitong Liu: 0000-0003-1185-9219 Guanxin Zhang: 0000-0002-1417-6985 Deqing Zhang: 0000-0002-5709-6088 Notes

The authors declare no competing financial interest. Biographies Zitong Liu received his Ph.D. in Chemistry from ICCAS in 2008. He is now an associate professor at the Institute. Guanxin Zhang received his Ph.D. in Chemistry from ICCAS in 2005. He was a JST Research Fellow at the University of Tokyo from 2005 to 2007. He is now a project professor at ICCAS. Deqing Zhang received his Ph.D. from Ruprecht-Karls University Heidelberg in 1996 under the supervision of Prof. Dr. H. A. Staab. He is currently a Research Professor at ICCAS.

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ACKNOWLEDGMENTS We thank the Strategic Priority Research Program of the CAS (XDB12010300) and NSFC (21661132006) for financial support.

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