Organic and Polymeric Semiconductors Enhanced by Noncovalent

Review pubs.acs.org/CR

Organic and Polymeric Semiconductors Enhanced by Noncovalent Conformational Locks Hui Huang,*,† Lei Yang,† Antonio Facchetti,*,‡,§ and Tobin J. Marks*,‡ †

College of Materials Science and Optoelectronic Technology and Chinese Academy of Sciences Key Laboratory of Vacuum Physics, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ‡ Department of Chemistry and Materials Research Center, Northwestern University, Evanston, Illinois 60208, United States § Flexterra Corporation, 8025 Lamon Avenue, Skokie, Illinois 60077, United States ABSTRACT: Constructing highly planar, extended π-electron systems is an important strategy for achieving high-mobility organic semiconductors. In general, there are two synthetic strategies for achieving π-conjugated systems with high planarity. The conventional strategy connects neighboring aromatic rings through covalent bonds to restrict the rotation about single bonds. However, this usually requires a complex sequence of synthetic steps to achieve this target, which can be costly and labor-intensive. More recently, noncovalent through-space intramolecular interactions, which are defined here as noncovalent conformational locks, have been employed with great success to increase the planarity and rigidity of extended π-electron systems; this has become a well-known and important strategy to design and synthesize highly planar π-conjugated systems for organic electronics. This review offers a comprehensive and general summary of conjugated systems with such noncovalent conformational locks, including O···S, N···S, X···S (where X = Cl, Br, F), and H···S through-space interactions, together with analysis by density functional theory computation, X-ray diffraction, and microstructural characterization, as well as by evaluation of charge transport in organic thin-film transistors and solar cells.

CONTENTS 1. Introduction 2. Oxygen···Sulfur Noncovalent Conformational Locks 2.1. Theory of O···S Conformational Locks 2.2. Small Molecules with O···S Conformational Locks 2.3. Conjugated Polymers with O···S Conformational Locks 3. Nitrogen···Sulfur Noncovalent Conformational Locks 3.1. Theory of N···S Conformational Locks 3.2. Small Molecules with N···S Conformational Locks 3.3. Conjugated Polymers with N···S Conformational Locks 4. Halogen···Sulfur Noncovalent Conformational Locks 4.1. Theory of X···S Conformational Locks 4.2. Conjugated Systems with Intermolecular X··· S Interactions 4.3. Conjugated Systems with Intramolecular X··· S Conformational Locks 5. Hydrogen-Bonded Conformational Locks 5.1. Small Molecules with Hydrogen-Bonded Conformational Locks 5.2. Conjugated Polymers with HydrogenBonded Conformational Locks 6. Summary and Outlook © 2017 American Chemical Society

Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments References

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1. INTRODUCTION During the past decade, organic semiconductors have attracted much attention due to their possible applications in organic optoelectronic devices such as organic field-effect transistors (OFETs)1 and organic photovoltaics (OPVs),2 to cite the most investigated device types (Figure 1). Compared to traditional Siand GaAs-based technologies, organic semiconductors offer unique features including tunable electronic structure and physicochemical properties, core functionalization, light weight, mechanical flexibility, and variable optical band gaps.3 To date, numerous small-molecule and polymeric π-conjugated organic semiconductors having diverse core/backbone architectures and pendant groups have been realized, with the goals of manipulating device-specific optoelectronic properties, solidstate packing, solution processability, and the resulting thin-film morphology and microstructure.4−8

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DOI: 10.1021/acs.chemrev.7b00084 Chem. Rev. 2017, 117, 10291−10318

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Figure 1. (a) Structure of a top-contact/bottom-gate organic field-effect transistor (OFET), where L is the channel length and W is the channel width. (b) Conventional organic solar cell device architecture, where HTL is the hole-transporting layer and ETL is the electron-transporting layer. (c) Typical current−voltage (I−V) transfer plot of a OFET device. (d) Typical current density−voltage (J−V) curve of an organic solar cell.

essential to achieve optimal electronic properties. (4) Esub represents the effect of substituents on the conjugated backbone. Grafting electron-donating and electron-withdrawing substituents on the conjugated backbone (Esub) also contributes to Eg reduction. This approach has been widely employed by synthetic chemists to tune the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the corresponding structures (vide infra). (5) Eint represents solid-state interactions between individual molecules/ polymer chains, which are also important to tune the band gap (Eint).11 This intermolecular or interchain coupling can also influence the properties of a single conjugated molecule or chain, such as Eδr. The influence of intermolecular interactions on supramolecular polymers has been thoroughly investigated by Meijer and co-workers.12 Through understanding those factors determining the band gap, various synthetic strategies have been employed to realize high-performing small molecules and conjugated polymers. One of the most effective approaches is the so-called donor−acceptor design strategy.13 Here, alternating electron-rich (or donor) and electron-poor (or acceptor) units promotes electron density delocalization along the conjugated backbone and enables lowenergy charge-transfer transitions, influencing both Eδr and Eres and compressing the band gap. Incorporating alkyl chains or conjugated moieties as backbone substituents is also widely employed to tune band gap, solubility, and solid-state electronic structure, since it also affects Eint.14 Thus, suitable alkyl chains can facilitate the self-assembly of conjugated systems into closely packed supramolecular structures.15 Conjugated backbone substituents have also been shown to promote two-dimensional conjugation for creating novel electron-donor polymers for highefficiency OPVs.16 Both alkyl chains and conjugated groups are typically donor groups, which elevate both the HOMOs and LUMOs of organic/polymeric semiconductors. Furthermore, tuning the aromatic and quinoidal contributions to the description of conjugated systems is another important way to affect the core electronic structure (Figure 3).17 In most cases the quinoid form has a smaller band gap.

Band-gap (Eg) engineering in a semiconductor is an important approach to achieving desired physical and electronic properties and can be ascribed to five parameters (Figure 2 and eq 1).9,10 Eg = E δr + E θ + Eres + E sub + Eint

(1)

Figure 2. Schematic representation of five parameters (Eδ, Eθ, Eres, Esub, and Eint) affecting the band gap (Eg) of an organic semiconductor. Reprinted with permission from ref 9. Copyright 1997 American Chemical Society.

(1) Eδr represents bond-length alternation (BLA), which depends on the difference between single- and double-bond lengths of the π-conjugated core. This is typically the major contribution to Eg. One should note that increased BLA is correlated to increased band gap, but the increased BLA may be a consequence, not a cause, of the increased band gap. (2) Eθ represents the degree of interannular rotations, which relates to different conformations of the conjugated core. Any distortion from core/backbone planarity due to interannular rotations about single bonds will increase Eg, because the orbital overlap varies with the cosine of the torsional angle (θ). (3) Eres, the aromatic resonance energy of cyclic π-systems, plays a major role in Eg. There is a competition between π-electron confinement within the aromatic rings and delocalization along the conjugated backbone chain. Typically, highly delocalized π-electrons are 10292


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molecular interactions to enhance the planarity and rigidity of conjugated backbones, implementing O···S, N···S, X···S (where X = halide), and hydrogen-bonding interactions.27,28 In 1993, McCullough er al.29 first reported the influence of conformation on the electric conductivity of conjugated poly(3-alkylthiophene) (PAT), which is the first investigation of structure− property relationships in these materials. In 2012, the concept of noncovalent conformational lock was first introduced to construct highly planar π-structures with improved OFET charge carrier mobilities.30 Since then, this concept has been widely employed to construct planar and rigid conjugated organic semiconductor structures and has significantly impacted the fundamental chemical design parameters of organic semiconductors and their resulting performance in diverse optoelectronic devices. One should note that the conformational locks may result in redder absorbance, increased thermal excitation of carriers, and higher charge carrier mobility. Mobility in general can be the result of absolute band energies, crystal sizes, purity, etc. This review summarizes the structural characteristics of molecular building blocks and polymers having noncovalent conformational locks that have been specifically utilized for the realization of π-conjugated semiconductors. Since many molecular units may have undetected noncovalent conformational locks, this review focuses on those systems where researchers have provided strong evidence for these interactions via density functional theory (DFT) computations, crystallography, and microstructural data. Furthermore, the influence of this conformational locking on small molecule and polymer optoelectronic properties, as well as selected OFET and OPV device performance, will be compared and contrasted.

Figure 3. Representative aromatic and quinoid electronic structures.

Last but not least, controlling the molecular/backbone conformation of organic semiconductors to achieve high core planarity is critical to precisely tune the band-gap and chargetransport/transfer properties, since the backbone conformation contributes to Eθ. Two general methods have been employed to achieve highly planar conformations. The first is to connect the neighboring aromatic rings via covalent bonds to restrict the rotation about single bonds. Milián-Medina and co-workers18 employed quantum-chemical calculations to investigate the impact of backbone rigidity on the optical properties of thiophene-based compounds by analyzing in detail the geometrical, electronic, optical, and vibronic features of a family of oligothienoacenes in comparison to nonfused α-oligothiophenes. For polymers, use of this strategy to rigidify polybithiophene (P1),19 polydithienylethylene (P2),20−22 and polyterthiophene (P3) affords ladder-type conjugated polymers P4, P5, and P6, respectively (Figure 4).23 Swager and co-

2. OXYGEN···SULFUR NONCOVALENT CONFORMATIONAL LOCKS 2.1. Theory of O···S Conformational Locks

Since Rosenfield et al.31 first analyzed the structural features of noncovalent intra- and intermolecular O···S interactions, these weak through-space forces have been recognized to play important roles in a number of phenomena including selfassembly, charge transport, and molecular recognition.32 It is believed that these interactions can influence the biological activity of organosulfur compounds, possibly regulating enzymatic functions and stabilizing folded protein structures.33,34 For example, the intramolecular O···S noncovalent interactions in (acylimino)thiadiazoline derivatives affect not only the planar conformation of the (acylimino)thiadiazoline moiety in these molecules but also their antagonistic activities (Figure 5).35 Note that there is a zwitterionic resonance in these structures that can form anion−cation pairs, inducing the noncovalent conformational locks.

Figure 4. Chemical structures of conjugated polymers P1−P6.

workers24,25 and Roncali and co-workers26 have used poly(ethylene oxide) (PEO)-tethered polythiophene and poly(3,4ethylenedioxythiophene) (PEDOT) derivatives for applications in sensors. However, multiple synthetic steps are necessary to reach this target, which is costly and tedious. The second, very different synthetic strategy to maximize πsystem delocalization utilizes noncovalent through-space intra-

Figure 5. Chemical structures of antagonists with (acylimino)thiadiazoline moieties. 10293


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Figure 6. Chemical and single-crystal structures of 1−4 and 6 and DFT-computed structure of 5. Red labels indicate the maximum torsional angle, whereas blue dots and lettering indicate the O···S distance.

2.2. Small Molecules with O···S Conformational Locks

Recently, noncovalent O···S interactions of the general type shown in Figure 5 were utilized to lock the conformations of πconjugated semiconductors, influencing important electronic and physical properties. The O···S noncovalent interactions can involve a thiophene sulfur and either an alkoxy oxygen (-OR) or a carbonyl oxygen (-CO-). In theoretical studies, the role of these noncovalent conformational locks on backbone rigidification is still under debate. Studies based on Hartree−Fock calculations and Mulliken partial charge data suggest that a large electrostatic interaction is operative between a partially charged negative carboxyl oxygen atom and a partially charged positive thiophene sulfur atom, which is also consistent with crystal structure data.36,37 For example, Pomerantz and Cheng36 employed ab initio calculations (3-21G*) on dimethyl 2,2′-bithiophene-3,4′dicarboxylate to reveal that the dihedral angles between the thiophene rings are extremely small and that the rotational barriers about the thiophene−thiophene single bonds are also small (see Figure 6). This is ascribed to a favorable Coulombic interaction between the negative carbonyl oxygen atom and the positive sulfur atom of the adjacent ring, flattening and bending the ground-state structure and lowering the rotational barrier. In contrast, Gierschner and co-workers39 employed a quantumchemical method to investigate the room-temperature optical absorption spectrum of the 4-ethylenedioxythiophene (4EDOT) molecule by a thorough comparison with the unsubstituted species four thiophene molecules. Their calculations revealed that EDOT-type materials are not rigid systems38 but have to be considered as soft materials, similar to the unsubstituted species. Furthermore, Ratner and co-workers28 pointed out that DFT computations may not be suitable for investigating these interactions due to self-interaction errors, which lead to incorrect overdelocalization of the wave function. It was proposed that resolution of identity second-order Møller−Plesset perturbation theory (RI-MP2) was a more accurate method to analyze O···S noncovalent interactions and conformational locks. The corresponding O···S binding energy was estimated to be ∼0.5 kcal/mol, suggesting that the O···S interactions are not dominant stabilizing forces in this particular class of conjugated molecules.

Experimental evidence for noncovalent O···S conformational locks has been presented in a variety of conjugated systems, although the magnitude and nature of such interactions is still under discussion among computational scientists. Alkyl and alkoxy chains have been introduced into poly(thiophenes) to enable sufficient solubility for film processing and to tune the energy levels.16 At the same time, it was recognized that these substituents also play an important role in influencing the conformation of the conjugated backbone. For example, the crystal structure of terthiophene 1 with two n-butyl substituents displays an all-anti configuration with ∼33° mean dihedral angle between adjacent thiophene rings (Figure 7).40 Furthermore, the crystal structure of tetramethyl-substituted quaterthiophene 2 indicates that all neighboring rings are not coplanar (Figure 6),41 but have dihedral angles of 11.8°, 28.8°, and 21.3°. The major reason for these severe inter-ring torsions is the steric demands of the α-CH2-R substituents. Replacement of the alkyl chains with alkoxy chains is an effective way to alleviate steric demands as well as to induce intramolecular contacts. Thus, building blocks 4,4′-dipentoxy-2,2′-bithienyl (3) and 3,3′-dipentoxy-2,2′-bithienyl (4) were designed and synthesized to probe the impact of alkoxy chains on these bithiophene conformations (Figure 6).42 The crystal structures reveal that both molecules possess planar or nearly planar anti conformations in the solid state with a maximum torsional angle of 1.1°. Furthermore, the intramolecular contacts between the alkoxy oxygen and thiophene sulfur atoms are clearly detectible in 4, with O···S distances of 2.881(4) and 2.835(2) Å. Based on DFT calculations, the thiazole analogue of 4, molecule 5, also possesses short O···S contacts, resulting in a highly planar structure (Figure 6).43 Interestingly, replacement of the (thiophene) C−H group with (thiazole) N removes the steric hindrance between the thiazole moiety and the neighboring aromatic rings by eliminating C− H···H−C nonbonded repulsion. Furthermore, thiazole is an electron-poor heterocycle which compensates for the alkoxy electron-donating characteristics, lowering the HOMO energy of 5 (5.07 eV) versus that in 4 (4.67 eV) (Table 1). Another 10294


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Table 1. Chemical Structures, HOMO and LUMO Energies, and Band Gaps of Thiophene Derivatives 4, 5, 7−13, and 15− 19a

Figure 7. Chemical structures of (a) 7 and (b) TVT. (c) Single-crystal structure of 7. (d) Energy levels of 7 and TVT, based on DFT//B3LYP/ 6-31G** calculations.

materials with low HOMO energies and good oxidative stability. Subsequently, this conformational lock concept was employed by the community to explore the rigidifying effects of noncovalent interactions. Thus, Heeger and co-workers46 utilized this concept to show that N···S noncovalent interactions alter conformer energetic distributions. Furthermore, Yum et al.47 also employed multiple conformational locks, including F··· S, F···H, and O···S interactions, to achieve excellent semiconducting polymers with hole mobilities (μh) as high as 1.9 cm2/(V·s). Note that EDOT is an important π-electron building block since it is used in the widely employed polymeric conductor poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PEDOT−PSS), which has found application in antistatic coatings, OFET electrode materials and supercapacitors, and as an interfacial layer in light-emitting diodes and solar cells.48−51 Recently, EDOT was employed to construct other functional conjugated systems due to, among other chemical and physical characteristics, unique O···S noncovalent interactions, enforcing structural planarity and optimal band gaps. There are several excellent reviews on EDOT-based materials.52−56 Here we focus on the EDOT-based molecules for which single-crystal diffraction studies have been reported. Crystal structure of the bis(EDOT) derivative 8 clearly reveals strong intramolecular noncovalent interactions.57 Structural analysis (Figure 8) shows that the distances between oxygen and sulfur (2.92 Å) are significantly shorter than the sum of the van der Waals radii of the two atoms (3.25 Å), and locks the πconjugated structure into a fully planar anti conformation with a torsion angle of 6.9°. The authors suggested that this molecular self-rigidification is the principal reason for the excellent charge-transport properties of 8-based polymers compared to bithiophene-based polymers. Furthermore, bis(EDOT) was employed as a building block to construct novel molecules. The crystal structure of molecule 9 reveals the existence of noncovalent O···S and S···S interactions, acting as conformational locks to rigidify the conjugated structure (Figure 8).58 The O···S distance of 2.914(5) Å is far shorter than the sum of the van der Waals radii of the two atoms (3.25 Å), and the torsional angle between the two adjacent EDOT rings is only 0.2°. Furthermore, the S···S distance between the dithiafulvalene groups and the thiophene units

a

DFT//B3LYP/6-31G computed quantity. bFirst anodic peak potential. cOptical band gap. dElectrochemical band gap.

bithiophene molecule 6 also exhibits strong O···S noncovalent interactions between the alkoxy chains and the thiophene rings according to crystal structure analysis, which shows that the O···S distance is 2.783(3) Å, remarkably shorter than the sum of the van der Waals radii of sulfur and oxygen (3.25 Å) (Figure 6).44 As a result, the two thiophene rings are essentially coplanar with a small torsion angle of 2.1°. Our laboratory also contributed to the design of new building blocks having intramolecular O···S conformational locks. In 2012, a new building block molecule, diethoxy-substituted thienylvinylene (TVT) 7, was designed and synthesized via olefin metathesis methodology.45 The crystal structure of 7 shows that the distance between oxygen and sulfur atoms is only 2.690(2) Å, indicative of strong O···S interactions (Figure 7). Interestingly, functionalization with alkoxy chains at the TVT core double-bond positions, as in 7, only slightly raises the HOMO energies (−5.60 eV) in comparison to the TVT building block (−5.70 eV) (Figure 7), thus offering opportunities to synthesize alkoxy-functionalized molecular and polymeric 10295


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Figure 8. Chemical and single-crystal structures of 8−10.

Figure 9. Chemical structures of EDOT-based molecules 11−14.

Figure 10. Chemical and single-crystal structures of 15−17.

[3.098(4) Å] is far shorter than the sum of the van der Waals radii of two sulfur atoms (3.68 Å). Interestingly, the solution UV−vis spectrum exhibits clearly resolved vibronic fine structure, in accord with conformational locking and rigidification/planarization in solution.58 Due to the extended π-conjugation, the HOMO energy of 9 (−4.78 eV) is considerably higher than that

of 8 (−5.28 eV) (Table 1). Another example of an EDOT molecule in this series is 10, a trimer of EDOT end-capped with two n-hexyl chains.59 Both the crystal structure and optical spectroscopy reveal that O···S interactions operate both in the solid state and in solution (Figure 8). The intramolecular O···S 10296


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Figure 11. Chemical structures of 18 and 19.

distance between the neighboring EDOT moieties is ∼2.9−3.0 Å. Another effective strategy for realizing highly conjugated building blocks for polymeric conductors/semiconductors is to insert aromatic rings between EDOT units. Thus, the triblock systems 11,60 12,61 13,61 and 1461 evidence O···S conformational locks as indicated by both crystal structure and optical absorption data (Figure 9). Furthermore, the absorption maxima of these molecules (450−578 nm) are considerably red-shifted versus that of terthiophene (3T) (350 nm),62 indicating planarization and strong intramolecular coupling. Thus, the band gaps of 11 (1.84 eV),63 12 (2.33 eV), and 13 (2.19 eV) are considerably smaller than that of 3T (3.1 eV).62 Not surprisingly, electropolymerized homopolymers based on these molecules exhibit very small band gaps in the range 1.1−1.3 eV. Another remarkable example emphasizing the conformational lock function of O···S noncovalent interactions can be seen by comparing structures 15a and 15b (Figure 10).64,65 The crystal structure of 15a reveals that the molecule deviates significantly from planarity, with torsional angles of 28.2° between the mean planes of the central aryl ring and neighboring thiophenes. However, when alkoxy chains are introduced on the central aryl ring, the aromatic rings in 15b are nearly coplanar, featuring only a small twisting of 6.8°. Furthermore, the O (alkoxy)···S (thiophene) distance is 2.626 Å, significantly shorter than the sum of the van der Waals radii of the two atoms (3.25 Å). Furthermore, the HOMO of 15b (−4.94 eV)66 lies higher than that of 15a (−5.09 eV) due to the alkoxy electron-donating properties (Table 1). Another instructive feature is seen by comparing the two hybrid quaterthiophenes 16 and 17 (Figure 10).67 The crystal structure of 16, having EDOT groups in the outer positions, indicates that the two inner thiophenes adopt a syn conformation with a 14.6° torsion angle. However, insertion of the bis(EDOT) block in the middle of the molecule affords 17, featuring an all-anti and highly planar conformation stabilized by the O···S conformational locks. Due to the increased planarity of the conjugated backbone, the HOMO of 17 (−4.92 eV) lies higher than that of 16 (−5.01 eV) (Table 1). Building blocks with O···S interactions have also been used to construct molecular semiconductors, since they usually possess highly planar conformations. Building block 7 was used to synthesize molecular semiconductor 18 with HOMO of −5.40 eV and LUMO of −3.10 eV. OFETs based on 18 afford a mobility of 3 × 10−3 cm2/(V·s).45 Bis(EDOT) 8 is also an excellent building block for constructing molecular semiconductors. For example, 8 was employed to synthesize 19 (Figure 11), a semiconductor exhibiting OFET mobility of 5 × 10−5 cm2/(V·s).68 Undoubtedly the most attractive characteristics of bis(EDOT) or EDOT are as precursors for PEDOT,69−71 which commercially is one of the most extensively used polymeric semiconducting materials for organic electronics. The conformational locks of conjugated systems are usually determined by several factors. Thiophene-based isomers 20 and 21 are excellent examples of the intrinsic factors governing

conformational locks (Figure 12).37 The crystal structure of 20 indicates that this molecule is planar, featuring a small torsional

Figure 12. Chemical and single-crystal structures of 20 and 21.

angle of 2.7° and a sulfur−oxygen distance of only 2.668 Å. Note that plausible resonance structures of 20 shown in Figure 13

Figure 13. Plausible resonance forms of isomers 20 and 21.

suggest Coulombic stabilization of the positive charge on the sulfur atom by the negative carbonyl oxygen atom on the opposite ring. In contrast, the two thiophene rings of molecule 21 are substantially twisted with respect to each other, even though the aforementioned Coulombic stabilization may also be operative. This appears to reflect substantially greater steric congestion between sulfur and oxygen in isomer 21. 2.3. Conjugated Polymers with O···S Conformational Locks

Conjugated polymers with O···S conformational locks have been employed for organic electronics including OFETs and OPVs.72 In view of the previously mentioned advantages, several πconjugated molecular and polymer semiconductors with O···S 10297


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Figure 14. Chemical structures of polymers P7−P20.

derivative of 4 with N-dodecyl-3,6-dibromephthalimide follow-

conformational locks have been designed and synthesized. Figure 14 depicts representative polymers, and Table 2 summarizes selected physical properties. P7a is the first polymeric semiconductor based on dialkoxydithiophene molecule 4 (Figure 14). This polymer was synthesized by coupling the distannylated

ing a Stille polymerization protocol. OFET measurements indicate that P7a is a p-type (hole-transporting) semiconductor with an OFET mobility of 0.28 cm2/(V·s).73 10298


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Table 2. Chemical Structures, HOMO and LUMO Energies, Band Gaps, Carrier Mobilities, and PCE of Polymers P7−P20a

a

Optical band gap. bElectrochemical band gap.

difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) afford the unusual polymers P9.77 Upon varying the number of BODIPY core methyl substituents from four (P9b) to two (P9a), Egap decreases from 1.67 to 1.23 and HOMO rises from −5.22 to −5.04 eV (Table 2), reflecting the enhanced polymer backbone planarity. As a result, the hole mobility increases from 1 × 10−3 cm2/(V·s) (P9b) to 7 × 10−3 cm2/(V·s) (P9a). Similarly, the thiazole analogue of 4, compound 5, was used as a building block to construct p-type conjugated polymers P10 (Figure 14).43 The HOMO energies of P10 (−5.18 and −5.16 eV) are lower than those of P7 (−5.07 and −5.09 eV) because of the electron-poor thiazole group. DFT computation indicates that reduced steric hindrance of the P10 thiazole N atom versus the CH of thiophene in P7 leads to a more planar backbone, affording higher OFET hole mobility [μh = 0.25 cm2/(V·s)].

Due to its excellent charge carrier mobility and optimal band gap (Table 2), P7a was also used as the donor semiconductor in solar cells to achieve a power conversion efficiency (PCE) of 2.0%.74 Furthermore, upon tuning the alkyl chain substitution at the imide nitrogen, polymer P7b-based solar cells achieve a PCE of 4.1%.75 Copolymerization of an analogue unit of 4, a strong electron-donating building block, with the strongly electrondeficient building block naphthalene diimide (NDI) affords polymeric semiconductor P8. This polymer transports both positive and negative charges (ambipolar transport) and exhibits an OFET electron mobility (μe) of 0.04 cm2/(V·s) and a hole mobility of 0.003 cm2/(V·s).76 Energetically, this result is consistent with the high-lying HOMO (−5.10 eV) and low-lying LUMO (−3.70 eV) of this polymer, estimated by cyclic voltammetry (Table 2). In addition, copolymers of 4 with 4,4′10299


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Furthermore, P10-based devices exhibit high Ion/Ioff ratios and excellent ambient stability due to the weak electron-donating characteristics of 5. Structure 7 was employed as a neutral planar building block to realize a series of p- and n-type semiconducting copolymers with optimal electronic properties (Figure 14).45 For example, copolymerization of 7 with benzodithiophene (BDT), a typical weak electron donor unit, affords the p-type polymeric semiconductor P11, exhibiting a hole mobility of 0.05 cm2/(V·s). In contrast, the copolymer of 7 with the strong electron acceptor naphthalene diimide (NDI) is an n-type polymeric semiconductor (P12). In comparison, 4-based polymer P8 is an ambipolar semiconductor. This result demonstrates that 7 is a weaker donating group than 4, which is supported by the lowerlying HOMO of P12 (−5.40 eV) in comparison to P8 (−5.10 eV). Remarkably, the electron mobility of P12-based OFETs approaches 0.5 cm2/(V·s) in a top-gate bottom-contact device. Since P12 exhibits a similar LUMO energy as the fullerene acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), P12 was also used as a non-fullerene acceptor for all-polymer solar cells using the polymer donors poly(3-hexylthiophene) (P3HT) and poly{4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5b′]dithiophene-2,6-diyl-alt-3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophene-4,6-diyl} (PTB7). PCEs of 1.7% were achieved with PTB7 as donor, which was the highest PCE for solar cells based on NDI-type acceptors in 2014.78 Recently, Yu and co-workers79 used 7 as a building block to copolymerize with diketopyrrolopyrroles (DPP) to afford a donor−acceptor copolymer (P13) with multiple O···H−C and O···S conformational locks (Figure 14). P13 possesses highly planar backbones with optimal HOMO energy levels (ca. −5.20 eV) and strong light absorption. Through tuning of the alkyl chains, the TFT mobilities increased from 2.07 cm2/(V·s) for P13a to 5.37 cm2/ (V·s) for P13c (Table 2). In order to tune the conformational and electronic properties of the building block simultaneously, Guo and co-workers80 recently reported an interesting building block, 3-alkyl-3′-alkoxy2,2′-bithiophene (TRTOR), with a single O···S group. The calculated HOMO (−4.92 eV) and LUMO (−0.86 eV) energy levels lie lower than those of small molecule 4 (−4.67 and −0.73 eV, respectively), since one alkoxy chain is replaced by a weaker electron-donating alkyl chain. Based on this building block, a series of conjugated polymers (P14) were synthesized for OFETs and OPVs (Figure 14). Through varying the alkyl side chains, a highest hole mobility of 0.18 cm2/(V·s) (P14a) and a maximum PCE of 6.31% (P14b) were achieved. Yu and co-workers81 reported two conjugated polymers (P15a,b) based on a new building block, vinylidenedithiophenmethyleneoxindole, that possesses a noncovalent O···S conformational lock (Figure 14). The HOMO energy levels of the polymers (ca. −5.40 eV) are suitable for the work function (5.13 eV) of gold, suggesting effective hole injection from the electrode into the polymer films. As a result, the highest hole mobility for P15a-based TFTs reaches 0.35 cm2/(V·s) (Table 2). Wei and co-workers82 investigated the influence of O···S conformational locks on photovoltaic performance by replacing the alkyl chains with alkoxy chains in P16 (Figure 14). Interestingly, in comparison to P16a, the new polymer P16c exhibits enhanced planarity, optical absorbance, and processability. As a result, the PCE of P16c-based solar cells increases up to 8.18% with increased JSC and fill factor but decreased VOC. The net PCE is greater than that of P16a-based cells (7.59%). It is not surprising that there are several kinds of conformational locks in one conjugated systems. P17c has a N−H

hydrogen-bonded conformational lock with a HOMO energy of −5.24 eV and a moderate hole mobility of 0.007 cm2/(V·s) (Table 2). In addition to N−H hydrogen bonding, P17a also has O···S noncovalent interactions between thiophene and 5,6bis(octyloxy)-1H-benzo[d[1,2,3]triazole, leading to a small dihedral angle of 10° based on DFT calculations.47 As a result, the HOMO rises slightly to −5.20 eV, affording a respectable OFET hole mobility of 0.019 m2/V·s. In another example, replacing a H atom with a methoxyl group in the aryl ring promotes O···S conformational locking in the backbone of polymer P18 (Figure 14).83 Therefore, the dihedral angle between the thiophene and the aryl rings decreases from 21.3° (P18a) to 12.4° (P18b) in DFT calculations. Moreover, the HOMO rises from −5.11 eV (P18a) to −5.03 eV (P18b) due to the strong methoxyl group donating characteristics. Surprisingly, the average hole mobilities also decrease from 0.21 to 0.11 cm2/ (V·s), which is ascribed to the π-face-on electrode orientation of the P18b chains in the film, in contrast to face-on/edge-on bimodal orientation of P18a. This observation was supported by grazing-incidence X-ray diffraction (GIXD) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). Another donor−acceptor copolymer, P19, is proposed to have a weak O···S interaction between the carbonyl oxygen and the thiophene sulfur atom.28 After tuning of the alkyl chains, excellent donor materials for OPVs are achieved with an efficiency as high as 5.21%.84,85 A similar O···S conformational lock is believed to be operative in P20, resulting in an extremely small dihedral angle of ∼0° between the dithienosilole and thieno[3,4-c]pyrrole-4,6-dione mean planes in DFT calculations.86 With an OFET mobility of 3 × 10−3 cm2/(V·s), P20based solar cells reach a PCE as high as 6.83%.

3. NITROGEN···SULFUR NONCOVALENT CONFORMATIONAL LOCKS 3.1. Theory of N···S Conformational Locks

Nitrogen···sulfur noncovalent conformational locks have been observed in a large number of organosulfur compounds, controlling the conformations of small and large molecules.87 These N···S conformational locks can play an important role in the reaction mechanisms of organosulfur compounds as well as in biological functions. For example, noncovalent N···S conformational locks are believed to affect the cyclization reactions of amide-substituted benzenesulfonic acids, supported by theoretical calculations and crystal structure analysis, which show that the N···S distance is ∼2.760 Å, significantly shorter than the sum of the van der Waals radii of the two atoms (3.35 Å).88 Also, a unique N···S conformational lock is observed to stabilize the conformations of a 2-aminothiazol-5-ylpyrimidine series required for binding to the p38α enzyme active site. X-ray crystallographic studies established the binding mode of this class of inhibitors in p38α and provided evidence for N···S interactions.89 In the past decade, noncovalent N···S interactions have been employed to construct a variety of planar conjugated compounds. The roles of N···S interactions in the conformations of conjugated π-systems have also been of theoretical interest. Ö zen et al.90 employed DFT calculations and atoms-inmolecules (AIM) techniques to identify N···S interactions in conjugated systems by the existence of topological bond critical points along the bonding path. Furthermore, the polymer bandgap values extrapolated from the time-dependent (TD) DFT excitation energies and the computed DFT HOMO−LUMO 10300


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Figure 15. Variation of TDDFT excitation energy and DFT HOMO−LUMO gap with respect to the inverse number of monomer units, n, for polymers (a) P21 and (b) P22. Reprinted with permission from ref 91. Copyright 1996 American Chemical Society.

Waals radii of these atoms, indicative of the existence of a through-space noncovalent interaction, which leads to a small torsion angle (3.7°) between the two conjugated moieties. Furthermore, Bazan and co-workers93 utilized a dithieno(3,2b;2′3′-d)silole (DTS) donor and pyridal[2,1,3]thiadiazole (PT) acceptor to synthesize a series of molecular semiconductors with a donor/acceptor/donor/acceptor/donor (D/A/D/A/D) cores (Figure 17). Combining DTS and PT affords semiconducting molecules 23, 24, and 25, which are excellent molecular donors for OPVs (Figure 17). After tuning of the end groups from thiophene to bithiophene, the band gaps decrease from 1.68 eV (23) to 1.51 eV (25) (Table 3). As a result, the organic solar cells based on 25 reach PCEs as high as 3.2%.93 To understand the correlation between molecular structure, dipole moment, selfassembly, and solar cell performance, Bazan and co-workers46 designed and synthesized D/A/D/A/D molecules 26, 27, and 28. These three molecules have similar band gaps: 1.50 eV for 26, 1.52 eV for 27, and 1.58 eV for 28 (Table 3). The authors speculate that dipole moments and conformational stability subtly influence the crystallization behavior, leading to significant differences in OFET and OPV performance. For example, OFETs based on 26 have a high hole mobility of 0.2 cm2/(V·s), and 26/PCBM blends have a highly ordered structure, yielding a PCE of 7.0%, ascribed to the symmetrically disposed dipole moment and N···S locked conformation. In contrast, OFETs based on 28 exhibit a low mobility of 0.01 cm2/(V·s), and the corresponding 28/PCBM blends are highly disordered and exhibit a low PCE of 0.2%. This result was rationalized by considering that 28 has a smaller dipole moment and no conformational locks. Bazan and co-workers94 next extended this N···S conformational lock investigation to the impact of regiochemistry and isoelectronic bridgehead substitution on the molecular shapes and bulk organization of conjugated DTS−PT-based molecules (Figure 18). The subtle regiochemical differences between 29 and 30 lead to major differences in optical spectroscopy, thermal transitions, solubility, bulk morphology, and molecular structures/packing within the crystalline lattice, which was attributed

gaps are consistent with the experimental band-gap trends. For example, the P21 and P22 band gaps were calculated from Figure 15 by extrapolating to infinite chain size and were found to be in close agreement with the experimental values. For P21, the Eg value (1.03 eV) determined from TDDFT excitation energies is consistent with the experimental band gap (1.0 eV) characterized by cyclic voltammetry experiments. Similarly, the TDDFTcomputed Eg (1.26 eV) of P22 (Figure 15) is in agreement with the experimentally determined band gap (1.2 eV).91 Bronstein et al.92 employed Aug-cc-pVDZ with Hartree−Fock (HF), HF with single-point MP2 perturbative corrections, and B3LYP hybrid DFT to study the noncovalent N···S interactions in thiophene (T)−thazole (Tz) dimers as model compounds and suggested that the N lone pair in the T−Tz dimers may interact with the adjacent thiophene antibonding orbitals. Furthermore, the strength of these interactions was estimated to be 0.72 kcal/ mol from a natural bond orbital (NBO) analysis. Additionally, Ratner and co-workers28 employed RI-MP2 to study the noncovalent N···S interactions in similar compounds and estimated the bond strength to be 0.46 kcal/mol, suggesting that N···S interactions are not conformationally dominant in many of these conjugated polymer and small-molecule systems. 3.2. Small Molecules with N···S Conformational Locks

The existence of N···S conformational locks in small molecules has been shown with single-crystal structures. The X-ray structure of the model molecule 22 (Figure 16) points to a highly planar core with a distance of only 2.89 Å between proximate N and S atoms,93 smaller than the sum of the van der

Figure 16. Chemical and single-crystal structures of 22. 10301


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Figure 17. Chemical structures of 23−28.

λmax = 400 nm. In contrast, 29 shows fine structure within the lowest energy band centered at 615 nm (λmax1,2 = 632 and 598 nm). This fine structure in solution argues that 30 has restricted intramolecular rotation due to the S···N conformational locks.95 Furthermore, differential scanning calorimetry (DSC) shows that the melting point and crystallization temperature of 29 are greater by 59 and 66 °C, respectively, than those in 30. These increases in thermal transition temperatures imply that 29 has more and/or stronger, more rigid intermolecular interactions in the solid state. Consistently, the solubility of 29 in organic solvents is poor versus that of 30 (2 vs 15 mg/mL). Also, polarized optical spectroscopy studies show that thin films of 29 have larger crystal domains than 30, consistent with the higher crystallization temperatures and lower solubility. The singlecrystal structures of 30 and 29 are shown in Figure 18. Even though both molecules adopt crescent-type geometries with the sulfur atoms of the [1,2,5]thiadiazolo[3,4-c]pyridine (PT) and dithienosilole (DTS) fragments trans to one another, the degrees of bending within the molecules are very different. The bend angle is defined between the Si bridgehead atom and centroids of the PT rings. Molecule 30 has a bend angle of 116.1°, while that of 29 is 112.0°. This difference was attributed to two effects: an attractive intramolecular N···S interaction in 29 and a possibly repulsive C−H···S interaction in 30.

Table 3. Chemical Structures, HOMO and LUMO Energy Levels, Band Gaps, and Carrier Mobilities of Small Molecules and Polymers 23−38a

3.3. Conjugated Polymers with N···S Conformational Locks

a

In addition to conjugated molecules, noncovalent N···S conformational locks have also been utilized in polymeric systems (Figure 19). Tian and Kertesz96 proposed a ladderlike polymer, P23, having N···S conformational locks and a low band gap (1.65 eV). The DFT-computed noncovalent N···S distance is 2.554 Å, far shorter than the sum of the S and N van der Waals radii. Polymers P24 and P25 are closely analogous to P3HT by replacement of thiophene with thiazole. In addition to lowering the MO energy levels by 0.45 eV, intramolecular N···S noncovalent conformational locks were also proposed on the basis of DFT calculations. These conformational locks enhance backbone planarity and thus increase the crystallinity of the thin films and propensity for solution aggregation. Furthermore, P25-

Optical band gap. bNumber from DFT//B3LYP/6-31G calculations.

to the intramolecular N···S conformational locks in 29.94 The optical spectra of 29 and 30 solutions reveal similar absorption onsets but different spectral line shapes. The dual-band absorption profile of 30 has a broad and featureless low-energy transition at λmax = 600 nm and a less intense high-energy band at 10302


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Figure 18. Chemical and single-crystal structures of 29 and 30.

Figure 19. Chemical structures of polymers P23−P26, showing conformational locking.

fluorine is the most electronegative atom, and as a substituent, it typically lowers π-frontier MO energies with little change in the band gap, yielding enhanced chemical and air stability, tuned charge-transport characteristics in OFETs, and higher Voc values in OPVs. Furthermore, the weak noncovalent interactions between F and various other atoms such as H, S, and O can also fine-tune semiconducting core structural and electronic characteristics. As an example, perfluorination of organic semiconducting cores such as pentacene or copper phthalocyanine, or introduction of perfluoroarene or perfluoroalkyl substituents in oligothiophenes, can convert p-type OFET materials to n-type OFET materials having good carrier mobilities and environmental stability.99−106 Furthermore, partial fluorination of several semiconducting polymer classes has yielded a large number of high-performing OPV materials.107−110 Concurrently, Ratner and co-workers99 computationally investigated F···S noncovalent interactions and proposed that the interactions should be rather weak, with binding energies of ∼0.44 kcal/mol, yet sufficient to impart unusual properties. In contrast, Chen and co-workers111 employed DFT calculations at the B3LYP/6-31G(d) level to investigate six noncovalent conformational locksF···S, F···N, F···H, N···S, S···H, and

based solar cells afforded good efficiencies (∼4.5%) with higher VOC versus P3HT.97 Recently, Pei and co-workers98 successfully introduced N···S noncovalent conformational locks into the backbone of conjugated polymers through substitution of the CH of an aryl ring with an sp2 N atom. The electron-deficient N atom produces two effects. First, the N···S noncovalent conformational locks increase the planarity of the backbone. The dihedral angle between thiophene and benzene rings is 21.5° in P26a, while the dihedral angle between thiophene and pyridine significantly decreased to 0.5° in P26b (Figure 19). Second, the electrondeficient N significantly lowers the LUMO: the LUMO of P26a is −4.15 eV, while that of P26b is −4.37 eV. As a result, the TFT electron mobility increases from 1.74 cm2/(V·s) in P26a to 3.22 cm2/(V·s) in P26b (see Table 5).

4. HALOGEN···SULFUR NONCOVALENT CONFORMATIONAL LOCKS 4.1. Theory of X···S Conformational Locks

Halogens have been incorporated into organic π-semiconductors to capitalize on their unique electronic properties. For example, 10303


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Figure 20. Conjugated molecules conformationally locked by intramolecular noncovalent conformational locks (highlighted in the area within the red dashed line), rigid-conjugated molecules covalently locked by CH2, and nonplanar molecules sterically hindered by CH3 in (a) 5-5, (b) 5-6, and (c) 6-6 aromatic ring architectures. Reprinted with permission from ref 111, Copyright 2016 American Chemical Society.

Figure 21. Chemical and crystal structures and packing diagrams for (a−c) 31a and (d−f) 31b, showing the effects of fluorine substituents.

N···Hin three types of molecular structures containing 5-5, 56, and 6-6 aromatic rings (Figure 20). This comprehensive DFT study suggests that F···S, S···N, and N···H interactions strength are strong enough to overcome repulsive interactions and thermal fluctuations, and thus to conformationally lock the neighboring aromatic rings in the solid state, while F···N, F···H, and S···H are too weak to lock the molecular conformations. Furthermore, noncovalent conformational locking can enhance

the aromaticity and other electronic properties of such compounds in comparison to the nonplanar analogues. 4.2. Conjugated Systems with Intermolecular X···S Interactions

Intermolecular F···S noncovalent interactions have been utilized to tune the solid-state packing that accelerates crystallization, which may facilitate charge transport in organic semiconductors (Figure 21). For example, compared to 31a,112 the fluorinated analogue 31b shows significantly larger hole mobility [1 cm2/(V· 10304


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Figure 22. Chemical and crystal structures and packing diagrams for 32a−e.

Figure 23. Chemical and crystal structures and packing diagrams for 33. Conformational locks are shown by dotted lines.

s)] since the film has enhanced crystalline characteristics. Furthermore, this difference is ascribed to subtle discrepancies, including a 0.3 Å long-axis shift between neighboring molecules in the π-stacks as shown in Figure 21.113 Another example is the fluorinated molecules 32a−e. On the basis of crystallographic studies, the intermolecular F···S noncovalent interactions influence crystalline packing to generate different packing modes (Figure 22).114

bithiophene and that the optimal geometry of 34 should be more stable than that of bithiophene (Figure 24).

4.3. Conjugated Systems with Intramolecular X···S Conformational Locks

Intramolecular F···S conformational locks have been employed to construct highly planar conjugated molecules and polymers. Molecule 33 was reported by Suzuki and co-workers115 as the first perfluorinated oligothiophene. As shown in Figure 23, the crystal structure clearly reveals that the molecule has a highly planar structure with small inter-ring torsion angles (0.7−3.6°) and with distances between neighboring S and F atoms in the 2.90−2.93 Å range, far shorter than the sums of the S and F van der Waals radii (3.17 Å).115 These observations are consistent with the existence of noncovalent F···S interactions. Difluorodithiophene (34) is a small molecule with the F···S intramolecular interactions. Hou and co-workers116 carried out theoretical calculations to compare the thermodynamic stabilities of rotational conformers of 34 and bithiophene, which indicate that 34 has a far more planar conformation than

Figure 24. Chemical structures of 34 and bithiophene and DFT calculations at the B3LYP/6-31G(d,p) level of their torsional energy profiles. Adapted with permission from ref 116. Copyright 2016 American Chemical Society.

Building block 34 was used to construct p-type conjugated polymers by several groups (Figure 25 and Table 4). Hou and coworkers116 synthesized conjugated polymers 27 to investigate the influence of F···S conformational locks on electronic and photovoltaic properties. Not surprisingly, copolymer P27b (based on 34) has a more stable backbone conformation and 10305


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P28a possesses relatively low-lying HOMO/LUMO levels and stronger interchain interactions in thin films. As a result, P28abased solar cells afford a PCE of 7.8%, much higher than that of 28b (2.8%), as seen in Table 4). Furthermore, replacing difluorinated benzotriazole with benzothiazole generates copolymer P28c, which further enhances the solar cell performance, resulting in a PCE of 10.4% by using TC71BM as the acceptor with an inverted structure of ITO/ZnO/active layer/ MoO3/Ag.118 Jo et al.119 also synthesized P28c and P28d to understand the influence of fluorinating bithiophene on polymer optoelectronic performance. The P28d-based solar cells afford a low efficiency of 1.6%, while the P28c-based solar cells provide a higher efficiency of 7.1% with PC71BM as the acceptor by using a standard structure of ITO/PEDOT−PSS/active layer/Ca/Al. The authors argued that the dramatic PCE efficiency enhancement is due to the deeper HOMO and stronger interchain interaction resulting from fluorination of the bithiophene unit. Yan and co-workers124 used isoindigo-based conjugated polymers P29 (Figure 25) as models to investigate the effect of fluorinating bithiophene. Bithiophene-based copolymer P29a has HOMO/LUMO energies of −5.18/−3.67 eV, and OPVs based on P29a produce a moderate efficiency of 3.5%. However, replacement of bithiophene with its difluoro anologue yields copolymer P29b with lower HOMO/LUMO energies (−5.30/ −3.78 eV), reflecting the F electron-withdrawing properties (Table 4). Remarkably, P29b-based solar cells afford an enhanced efficiency of 6.7%, which is ascribed to stronger aggregation and more ordered/crystalline film characteristics. Furthermore, the building block 34 was employed by Li and coworkers126 to construct copolymer P30 (Figure 25) with different alkyl chains. OFET studies showed that the hole mobilities of P30a, P30b, and P30c are 0.79, 0.091, and 0.022 cm2/(V·s), respectively. In combination with ITIC as the

Table 4. Chemical Structures, HOMO and LUMO Energy Levels, Band Gaps, and PCE of Polymers P27−P33a

a

Optical band gap.

stronger π−π stacking in the solid state in comparison to the bithiophene analogue P27a. As a result, the P27b-based OPV generates a PCE of 9.04%, much higher than that of P27a (6.53%), as seen in Table 4. Small molecule 34 was additionally utilized as a building block to construct high-performance conjugated polymers.117−129 Yan and co-workers122 synthesized P28a and P28b (Figure 25) by combining 34 or bithiophene, respectively, with difluorobenzotriazole. Compared to P28b, 10306


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acceptor, P30a−P30c-based non-fullerene solar cells afforded efficiencies of 8.7%, 8.3%, and 8.0%, respectively (Table 4). This efficiency discrepancy is reasonably attributed to the different charge-transport properties of the polymers. As a highly planar building block, 34 was also employed to construct high-performance n-type conjugated polymers. Thus, Jen and co-workers121 synthesized P31 by combining 34 with NDI building blocks. Since P31 possesses a low-lying LUMO (−3.91 eV), the authors investigated it as an acceptor for allpolymer solar cells. In combination with p-type conjugated polymer PBDTTT, the P31-based solar cells deliver an excellent efficiency of 6.28%, which is higher that of its bithiophene anologue (5.28%). The authors reasonably attributed the increased efficiency to the preferential π-face-on electrode orientation and increased crystallinity in bulk heterojunction (BHJ) blend films. Woo and co-workers129 also employed P31 as an acceptor for nonfullerene solar cells to combine with poly{4,8bis[5-(2-ethylhexyl)thiophen-2-yl]benzo[1,2-b:4,5-b′]dithiophene-alt-5-octyl-4H-thieno[3,4-c]pyrrole-4,6(5H)dione} (PBDTTTPD). As a result, the solar cells afforded an efficiency of 6.0%. Liu and co-workers125 employed 34 to copolymerize another electron-deficient building block, B←Nbridged bipyridine, to achieve an n-type conjugated polymer P32 (Figure 25). DFT calculations showed that the F···S distance in the building block is 2.93 Å, far shorter than the combined van der Waals radii of sulfur and fluorine (3.17 Å). Furthermore, P32 was employed to construct solar cells as acceptors with PTB7-Th as the donor to achieve an excellent efficiency of 6.26%. Choi and co-workers130 also copolymerized isoindigo with bisthiophene and difluorodithiophene to afford copolymers P33a and P33b, respectively (Figure 25). Substitution of H with F successfully increases the coplanarity through F···S noncovalent conformational locks. Furthermore, the electronegative properties of F also depress the HOMO levels of the copolymers. Thus, the photovoltaic performance of P33b is enhanced due to its increased VOC, affording a PCE of 6.21%, higher than that of P33a (4.58%). Another conjugated polymer that shows intramolecular F···S conformational locks is P17b (see Figure 14). First, fluorine incorporation into the backbone decreases both HOMO and LUMO energies (−5.36 and −3.44 eV) versus those of P17c (−5.24 and −3.37 eV) (see Table 2). Also, with both N···H bonding and F···S conformational locks, the torsional angle between the thiophene and benzotriazole ring mean planes is 0° according to DFT calculations. As a result, tighter interchain ordering with an edge-on orientation of the film polymer πplanes with respect to the substrate plane was confirmed by X-ray diffraction for P17b but not for P17c. Furthermore, P17b-based OFETs exhibit an enhanced hole mobility of 1.9 cm2/(V·s), much higher than that of P17c, 0.007 cm2/(V·s).47 Intramolecular X···S interactions are observed in other conjugated molecules and polymers (Figure 26). For example, compounds 35a and 35b show that the presence of bromine at the 3-position of the thiophene ring results in a noncovalent intramolecular Br···S interaction in addition to the noncovalent O···S interactions in these compounds.131 Crystallographic analysis of 35b shows that the distance between S and Br is 3.220(1) Å, considerably shorter than the sum of the S and Br van der Waals radii (3.80 Å), and that there is only a small torsion angle of 1.0° between the two π-planes. The UV−vis absorption spectrum of 35b in solution shows better resolved fine structure in comparison to 35a, evidence of a more rigidified π-conjugated system and that Br···S conformational locking takes place even in

Figure 26. Chemical and single-crystal structures of 35 and chemical structures of polymers P34a and P34b.

solution. Furthermore, the presence of the electron-withdrawing Br atom in 35b lowers the HOMO and LUMO energy levels to −5.1 and −1.0 eV, respectively, in comparison to those in 35a (−5.3 eV and −1.2 eV, respectively) based on DFT calculations, and yielding similar band gaps (4.1 eV) (see Table 3). In contrast, the electrochemically polymerized homopolymers P34a and P34b, also having Br···S conformational locks, exhibit different band gaps of 1.82 and 1.72 eV, respectively (see Table 5).131

5. HYDROGEN-BONDED CONFORMATIONAL LOCKS 5.1. Small Molecules with Hydrogen-Bonded Conformational Locks

Since Pauling first identified electrostatic hydrogen bonding between hydrogen and other electronegative atoms,132 this noncovalent interaction has been recognized to operate in numerous chemical and biological systems, including proteins, polynucleotides, dyes and pigments, and organic semiconductors, directing diverse intramolecular properties and selfassembly.133 Note that DPP,134 quinacridones,135 perylenediimides (PDIs),136 and indigos137 are the four major industrial colorants having intramolecular hydrogen bonds between the -NH protons and carbonyl oxygens. Classical organic semiconductors with intramolecular hydrogen bonds include indigo (36a),137 the oldest and probably bestknown natural organic pigment, and the dibrominated derivative Tyrian purple (36b)138,139 (Figure 27). In most indigo derivatives, individual molecules have intramolecular hydrogen bonds between adjacent N···H O groups, and thus the dihedral angle between two indole units is close to 0°. For

Figure 27. Chemical and single-crystal structures of 36a and 36b. 10307


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tail.143−150 Here we focus only on DPP-based molecules for which the single-crystal structures are reported and some selected copolymers. Small molecule 38a is an n-type semiconductor adopting a highly planar structure with a torsional angle of 2.2° as shown in Figure 29.151 Due to the strong electron-withdrawing capacity of the cyano groups, 38a has an extremely low LUMO energy level (4.51 eV). Furthermore, the intramolecular hydrogen bonds (C−H···O, 2.167 Å) enforce a cis conformation of the double bond linking the thiophene ring to the DPP unit with a torsional angle of 2.2°, where the S atoms of the thiophene rings and the N atoms of the DPP unit are located at the same side.. The highly planar skeleton doubtless promotes the substantial electron mobility of 0.6 cm2/(V·s). Another analogue, 38b, functionalized with different N-alkyl substituents and featuring intramolecular H-bonds (C−H···O, 2.214 Å) was independently reported by Heeney and coworkers.152 The torsional angle between the DPP moiety and the neighboring thiophene moiety is 9.6°, somewhat greater than that in 38a. However, the linear alkyl chains in 38b enable strong interdigitation between adjacent molecules. Nevertheless, the electron mobility of 38b-based OFETs is 0.5 cm2/(V·s), slightly smaller than that of 38a. Other DPP-based molecules with interesting/instructive intramolecular hydrogen bonding are 39a−c (Figure 30).153,154 Molecule 39a has C−H···O hydrogen bonds with a H···O distance of 2.3 Å. Thus, the DPP ring and the thiophene planes are nearly coplanar with a small torsional angle of 8.7°. Furthermore, the π−π interplanar distance between two neighboring molecules of 39a is 3.3 Å (Figure 30a). Despite the planarity and relatively close π−π stacking distance, OFETs based on 39a have only a modest holetransport mobility of 5.3 × 10−4 cm2/(V·s). Replacement of the O atom in the furanyl ring with an S atom yields 39b. Here the C−H···O hydrogen bonds have an H···O distance of distance of 2.2 Å, enforcing a coplanar conformation with a small torsional angle of 5.6° between the DPP and thiophene moieties. Compared to 39a, the π−π interplanar distance between the neighboring molecular planes of 39b is slightly increased to 3.5 Å, due to the overlapping location of 39b, where the benzothienyl groups contain large sulfur atoms. Not surprisingly, the OFET hole-transport mobility falls to 4.3 × 10−5 cm2/(V·s). Molecule 39c, with an N-H replacing the S, adopts a twisted conformation due to the significant steric repulsion. The DPP−thiophene

example, the crystal structures of 36a and 36b indicate that both molecules are planar with small torsional angles of 5.8° and 2.1°, respectively.140,141 Furthermore, 36a has a remarkably small band gap of 1.7 eV and exhibits excellent ambipolar transport with both electron and hole OFET mobilities of ∼0.01 cm2/(V· s). Similarly, 36b is also ambipolar with comparable hole and electron mobilities of ∼0.4 cm2/(V·s).138,139 Other indigo derivatives include 5,5′-dibromoindigo (37a) and 5,5′-diphenylindigo (37b).142 As shown in Figure 28, 37a adopts a planar

Figure 28. Chemical and single-crystal structures of 37a and 37b.

structure with a torsion angle of 2.9°, but in 37b the phenyl planes are twisted by 29° from the central indigo core. Furthermore, the packing mode of 37a indicates that along the c-axis the adjacent molecules are alternately tilted in opposite directions with respect to the molecular long axes, while the packing pattern of 37b adopts a hybrid herringbone/brickwork structure. Molecules 37a and 37b have slightly different HOMO and LUMO energies but identical optical band gaps (1.7 eV, Table 3). Both 37a and 37b are ambipolar, with the electron and hole mobilities of 37b [μh = 0.6 and μe = 1.0 cm2/(V·s)] being greater than those of 37a [μh = 0.2 and μe = 0.4 cm2/(V·s)]. This discrepancy may be due to the more extended π-system of 37b and the different packing pattern versus 37a. DPP is another important moiety featuring intramolecular hydrogen-bonded conformational locks. There are several recent review articles describing DPP-based semiconductors in de-

Figure 29. Chemical and single-crystal structures and packing diagrams for (a) 38a and (b) 38b. Alkyl chains were simplified to -CH3. 10308


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Figure 30. Chemical and single-crystal structures and packing diagrams for 39a−c.

Figure 31. Chemical and single-crystal structures and packing diagram for 40. Alkyl substituents are omitted for clarity.

Figure 32. Optimized structures according to DFT calculations at B3LYP/6-31G* level. Reprinted with permission from ref 156. Copyright 2016 American Chemical Society.

Yu and co-workers156 recently screened computationally the influence of introducing N atoms into the isoindigo framework to create diazaisoindigo (Figure 32). By varying the position of the N atom in the pyridyl rings, intramolecular hydrogen bonds C− H···O were created that significantly enhance the planarity of the 7,7-diazaisoindigo framework, as supported by DFT analysis and 1 H NMR. DFT analysis shows that the H···O distances are ca. 2.02 Å, close to a classic hydrogen-bond distance. Furthermore, the 1H NMR reveals that the Ha signal is significantly shifted to low field (δ 9.29 ppm), attributable to the deshielding effects of the CO group.

torsional angle is 17.4°, and the torsion angle between thiophene and terminal indole planes is 25.3°, leading to diminished π−π orbital overlap. Not surprisingly, the hole mobility of 39c OFETs is only 1.5 × 10−5 cm2/(V·s), smaller than those of 39a and 39b. Fréchet and co-workers155 reported DPP molecule 40 with pyrene as the terminal conjugated group, as shown in Figure 31. The molecule has an intramolecular hydrogen bond (C−H···O, 2.3 Å) with a torsional angle of 5.8° between the DPP moiety and neighboring thiophene moiety. The introduced pyrene groups form close packing with intermolecular distances of 3.5 Å. As a result, the space charge limit current (SCLC) mobility of 40 is 5 × 10−4 cm2/(V·s). Furthermore, the PCE of OPVs based on 40 reaches 2.7%. 10309


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due to strong interchain π−π stacking and a highly crystalline morphology, supported by X-ray diffraction and atomic force microscopic studies. Li et al.159 reported wide-band-gap copolymer P37 based on DPP and thieno[3,2-b]thiophene (TT). This polymer has a deep HOMO (−5.25 eV) and a shallow LUMO (−3.4 eV), and exhibits a substantial hole mobility [0.94 cm2/(V·s)] as measured in OFETs (Table 5). Again, this high mobility is attributed to close π−π stacking of the fused DPP and TT rings and substantial intermolecular π-overlap of the fused ring structures. Wudl and co-workers160 prepared a series of donor−acceptor copolymers P38−P40, consisting of DPP and various accepting units, to better understand donor− acceptor interactions (Figure 33). The electron-withdrawing strength of the accepting unit was systematically varied by use of electronically neutral aryl (phenylene), weakly accepting benzothiadiazole (BT), and strongly accepting benzobisthiadiazole (BBT) units. Both P38 and P39 exhibit excellent transistor performance with hole mobilities greater than 0.2 cm2/(V·s). By introducing the strong BBT accepting unit, Wudl and co-workers also tuned P40 to be ambipolar with HOMO/LUMO energies of −4.55/−3.9 eV. The strong accepting characteristics of BBT also

5.2. Conjugated Polymers with Hydrogen-Bonded Conformational Locks

DPP-based building blocks have been employed to construct various conjugated polymers for OFETs and OPVs. Selected DPP-based copolymers are shown in Figure 33, and their OFET and OPV device performances are summarized in Tables 5 and 6. In 2008, Winnewisser and co-workers157 reported the synthesis of copolymer P35 by copolymerization of DPP and bis(thiophene) building blocks; this is one of the earliest DPPbased polymers. P35-based OFETs with bottom-gate topcontact structures have ambipolar characteristics with balanced electron [0.09 cm2/(V·s)] and hole [0.1 cm2/(V·s)] mobilities. Furthermore, P35 was used to fabricate light-emitting transistors (LETs), which emit near-infrared light due to the narrow band gap. Sonar et al.158 synthesized similar low-band-gap conjugated polymers P36 based on DPP and thiophene−benzothiadiazole− thiophene (TBT) with suitable HOMO (−5.2 eV) and LUMO (−4.0 eV) energies for application to ambipolar transistors (Table 5). The hole and electron mobilities are as high as 0.35 and 0.40 cm2/(V·s), respectively, in OFETs annealed at 200 °C.158 The authors reasonably argue that the high mobilities are 10310


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DPP and (E)-1,2-(thiophene-2-yl)vinylthiophene (TVT). The authors tuned the film-forming ability, interchain interactions, and charge carrier transport characteristics of the polymers through changing the alkyl chains. They suggested that P41b, with longer alkyl chains, can form more uniform thin films and closer π−π stacking distances and thus exhibit a higher carrier mobility of 8.2 cm2/(V·s). Gao et al.162 copolymerized (E)-1,2bis(3,4-difluorothien-2-yl) ether with a DPP unit to synthesize copolymer 41c via direct arylation polycondensation. By introduction of F atoms in the thiophene ring β-position, the reactivity of the thiophene rings toward direct arylation is remarkably enhanced, and undesired C−H activation pathways are suppressed. Furthermore, the authors argued that intramolecular weak F···S interactions may enhance the planarity of the conjugated backbone. Finally, the F electronegativity can lower the HOMO/LUMO energies, thereby enhancing the polymer environmental stability and facilitating electron injection from the electrodes. As a result, OFETs based on 41c exhibit ambipolar transport in air, with hole and electron mobilities up to 3.40 and 5.86 cm2/(V·s), respectively (Table 5). Selenophene is an important building block for constructing conjugated systems due to the delocalized π-orbitals and strong interchain Se···Se interactions. Kim and co-workers163,164 employed (E)-1,2-(selenophen-2-yl)vinylselenophene (SVS) to replace TVT in copolymerization with DPP, yielding conjugated polymer P42 (Figure 33). P42a has a highly ordered structure and remarkable hole mobility of 4.97 cm2/(V·s), measured in OFETs. The authors proposed that the excellent performance can be ascribed to strong intermolecular interactions of the selenophene units. Furthermore, moving the branching position of the alkyl chains away from the backbone of the DPP-based polymers enhances intermolecular interactions and affords extremely short π−π stacking distances. As a result, the P42bbased OFETs exhibit a remarkably high mobility of 12 cm2/(V·s) (Table 5). As a consequence of their highly planar structures, DPP units have been used in conjugated polymers for organic solar cells. In 2008, Janssen and co-workers165 synthesized polymer P35b through Stille coupling (Figure 33). The polymer has a narrow band gap (1.4 eV) with HOMO/LUMO energies of −5.1/−3.7 eV (Table 6), and it was used the donor for BHJ solar cells, affording a moderate efficiency of 4.0%. Bijleveld et al.166 reported easily accessible polymer P43 (Figure 33) via Suzuki coupling of DPP and benzene units. P43-based solar cells yield a high PCE of 5.5% with PC71BM as the acceptor. The authors suggested that, in addition to optimal morphology, high carrier mobilities are critical for achieving high efficiency. Yang and coworkers167 synthesized P44 (Figure 33) by copolymerizing DPP and benzodithiophene (BDT) units. By tuning the side chains of the BDT moiety from phenyl to thienyl, the efficiency of solar cells was increased from 6.2% (P44) to 6.6% (P45a) (Table 6), which was ascribed to the higher mobility of P45a. Furthermore, Yang and co-workers168 used selenophene to replace thiophene in the DPP unit to synthesize copolymer P45b (Figure 33). P45b-based solar cells afford a higher efficiency of 7.2% with VOC = 0.69 V, lower than that of P45a (0.73 V). This increased efficiency was ascribed to contraction of the band gap and enhancement of the polymer charge-transport properties due to replacement of thiophene with selenophene. McCulloch and coworkers169 reported a family of DPP-based copolymers (P46; Figure 33) with different chalcogenophene comonomers (thiophene, selenophene, tellurophene) for organic solar cells. The results show that the band gap of the polymers decreases

Table 5. Chemical Structures, HOMO and LUMO Energy Levels, Band Gaps, and Charge Carrier Mobilities for Polymers P23−P26, P34−P42, and P47−P52a

a

DFT//B3LYP/6-31G calculated data. bOptical band gap. cElectrochemical band gap.

strengthen the interchain interactions. As a result, P40b exhibits the most impressive charge transport in the series, with μh = 1.17 and μe = 1.32 cm2/(V·s) (Table 5). Liu and co-workers161 synthesized copolymers P41a,b (Figure 33) by copolymerizing 10311


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high planarity to tune electronic characteristics. Polymers P47− P51 are good examples of this strategy (Figure 34).96 Based on DFT calculations, the dihedral angles in P47, P48, and P49 are 26.37°, 13.78°, and 24.54°, and as a result, the calculated band gaps are 3.04, 2.55, and 2.30 eV, respectively. In comparison, the proposed polymers P50 and P51 have much smaller computed band gaps (1.37 and 1.55 eV, respectively) because of three factors: (1) Both polymers adopt a highly planar structure due to the conformational locking double N···H hydrogen bonds. (2) Incorporation of another pyrazine unit into the backbone lowers the LUMO values. (3) The electron-donating characteristics of amino and hydroxyl groups raise the HOMO values. Another series of polymeric semiconductors with intramolecular conformational locking hydrogen bonds is P17 (see Figure 14).47 P17c is a semiconducting polymer with intramolecular hydrogen bonds, exhibiting a moderate hole mobility of 7 × 10−3 cm2/(V·s). In comparison, P17a has both hydrogen bonds and O···S noncovalent conformational locks, enhancing the hole mobility to 0.019 cm 2 /(V·s) (see Table 2). Furthermore, P17b has both hydrogen-bond and F···S noncovalent conformational locks, leading to a substantial hole mobility of 1.9 cm2/(V·s).47 In another example, 7,7-diazaisoindigo was copolymerized with bithiophene to yield conjugated polymers P52 (Figure 34).156 Interestingly, in addition to the C−H···O hydrogen bonds, the backbone of the conjugated polymers also possesses an N···S interaction between the diazaisoindigo and thiophene moieties, resulting in a highly planar conformation. Through tuning of the alkyl chains, P52b-based TFTs afford a very high hole mobility of 7 cm2/(V·s).

Table 6. Chemical Structures, HOMO and LUMO Energy Levels, Band Gaps, and PCE of Polymers P24, P25, P35b, and P43−P46a

a

Optical band gap. bElectrochemical band gap.

6. SUMMARY AND OUTLOOK Noncovalent conformational locks provide a driving force to planarize and rigidify π-conjugated molecular and macromolecular backbones and to tune the band gaps of diverse classes of organic semiconductors. In this review we have summarized and analyzed the development of several classes of noncovalent conformational locks and their applications to the design and synthesis of molecular and polymeric semiconductors with enhanced optoelectronic performance. Important conclusions from the present discussion include the following: (i)

upon increasing the chalcogen atom size (S < Se < Te). Furthermore, the larger heteroatom size enhances intermolecular heteroatom···heteroatom interactions and raises the HOMO values, resulting in increased solar cell mobilities and decreased VOC values. As a result, the P46a-based solar cells afford a high PCE of 8.8% (Table 6). Poly(p-phenylene) (PPP) and corresponding nitrogen heterocyclic polymers are a class of semiconducting polymers that have attracted great interest. The intramolecular hydrogen bonds have been employed to create ladder-type structures with

Figure 34. Chemical structures of semiconducting polymers P47−P51. 10312


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Theoretical analyses of noncovalent conformational locks are currently incomplete in terms of quantifying the relatively weak bonding energetics involved; however, most studies concur that there are weak but nonnegligible through-space interactions between O···S, N···S, and various hydrogen bonds. (ii) Experimentally, noncovalent O···S, N···S, and X···S (X = Br, F) interactions and hydrogen bonding can serve as conformational locks to rigidify the backbones and tune the physical and electronic characteristics of molecular or polymeric π-systems. (iii) In some instances, combinations of two or three types of conformational locks can operate in, and modify the electronic properties of, the same molecular or polymeric π-system. Noncovalent conformational locks represent an important emerging strategy to tune the electronic structures and architectures of molecular and macromolecular conjugated electroactive systems, and they offer attractive alternatives to strategies requiring the tedious and costly installation of covalent bonding. Conjugated systems with multiple noncovalent conformational locks per molecular unit have barely been investigated and would appear to offer new opportunities in structural control. Furthermore, beyond noncovalent O···S, N··· S, X···S, and hydrogen-bonded conformational locks, other systems offering weak linkages, such as O···Se, N···Se, and X···Se, would be attractive to explore in novel conjugated materials. Finally, theoretical methods to analyze noncovalent conformational locks must be refined and tested against experiment, with the goal of developing predictive tools to guide synthetic design and to shed light on the properties of novel prospective semiconducting systems with optimal optoelectronic characteristics.

engineering from Shenyang University of Chemical Technology in 2014. He is now a Ph.D. student at the University of Chinese Academy of Sciences (UCAS). He has published over six peer-reviewed papers. His research interests include the synthesis of conjugated materials for organic electronics. Antonio Facchetti obtained a Laurea degree in chemistry, cum laude, and a Ph.D in chemical sciences from the University of Milan. In 2002 he joined Northwestern University, where he is currently an adjunct professor of chemistry. He is a cofounder and currently the Chief Technology Officer of Flexterra Corporation. Dr. Facchetti has published more than 400 research articles and 11 book chapters and holds more than 120 patents. He received the 2009 Italian Chemical Society Research Prize, the team IDTechEx Printed Electronics Europe 2010 Award, and the corporate 2011 Flextech Award. In 2010 was elected a Kavli fellow, in 2012 a fellow of the American Association for the Advanced of Science (AAAS), and in 2013 a fellow of the Materials Research Society. In 2010 he was selected among the Top 100 Materials Scientists of the Past Decade (2000−2010) by Thomson Reuters, and in 2015 he was recognized as a Highly Cited Scientist. In 2015 he became a fellow of the Royal Society of Chemistry. Recently he has been elected a fellow of the National Academy of Inventors, and he was awarded the 2016 ACS Award for Creative Invention. Tobin J. Marks is the Vladimir N. Ipatieff Professor of Chemistry and Professor of Materials Sciences and Engineering at Northwestern University. He received his B.S. degree from the University of Maryland (1966) and Ph.D. from Massachusetts Institute of Technology (1971) and came to Northwestern immediately thereafter. He is a fellow of the American Academy of Arts and Sciences (1993), a member of the U.S. National Academy of Sciences (1993), a member of the U.S. National Academy of Engineering (2012), a member of the German National Academy of Sciences (2005), a member of the India National Academy of Sciences (2011), a fellow of the Materials Research Society (2009), a fellow of the Royal Society of Chemistry (2005), a honorary fellow of the Chemical Research Society of India (2009), a honorary member of the Israel Chemical Society (2012), and a honorary member of the Chinese Chemical Society (2015). Among other recognitions, he was awarded the National Medal of Sciences (2005), the Spanish Principe de Asturias Prize for Scientific Research (2008), the Von Hippel Award of the Materials Research Society (2009), the Dreyfus Prize in the Chemical Sciences (2011), the U.S. National Academy of Science Award in the Chemical Sciences (2012), and the Priestley Award of the American Chemical Society (2017).

AUTHOR INFORMATION Corresponding Authors

*E-mail [email protected] (H.H.). *E-mail [email protected] (A.F.). *E-mail [email protected] (T.J.M.). ORCID

Hui Huang: 0000-0002-6102-2815 Tobin J. Marks: 0000-0001-8771-0141 Notes

The authors declare no competing financial interest. Biographies

ACKNOWLEDGMENTS We acknowledge the NSFC (51303180 and 21574135), Beijing Natural Science Foundation (2162043), One Hundred Talents Program of Chinese Academy of Sciences, University of Chinese Academy of Sciences, U.S. National Science Foundation Materials Research Science and Engineering Centers program through the Northwestern University Materials Research Center (Grant DMR-1121262), and U.S. Air Force Office of Scientific Research (Grant FA9550-15-1-0044) for financial support.

Hui Huang is currently a professor at the College of Optoelectronic Engineering and Materials Science, University of Chinese Academy of Sciences (UCAS). He obtained a B.A. in chemistry from Beijing Normal University, M.A. from Institute of Chemistry, Chinese Academy of Sciences, and Ph.D. from Dartmouth College under the supervision of Professor Russell P. Hughes in 2008. From 2008 to 2010 he carried out his postdoctoral training with Professors Tobin J. Marks and Antonio Facchetti at Northwestern University. In 2010, he joined the Renewable Energy Group at Research Center of ConocoPhillips as a research scientist. He moved back to China to join UCAS as a member of the Hundred Talents Program in 2013. He has published over 40 peerreviewed papers and holds over 10 patents. He was named as an Emerging Investigator by the Journal of Materials Chemistry A, Royal Society of Chemistry, in 2017. His research interests include synthetic methodology and applications of organic/polymeric semiconductors for photovoltaics, thin-film transistors, photodetectors, and biosensors.

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Lei Yang received a B.S. from the College of Chemistry at Liaoning University in 2011. He obtained his M.A. in material processing 10313


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