Packing Principles for DonorâAcceptor Oligomers ... - ACS Publications

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Packing Principles for Donor−Acceptor Oligomers from Analysis of Single Crystals† Chi-Feng Huang,§ San-Lien Wu,§ Yi-Fan Huang,§ Ying-Cheng Chen,§ Shu-Ting Chang,§ Tzu-Yi Wu,§ Kuan-Yi Wu,§ Wei-Tsung Chuang,‡ and Chien-Lung Wang*,§ §

Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30010, Taiwan National Synchrotron Radiation Research Center, 101 Hsin-Ann Road, Hsinchu Science Park, Hsinchu 300, Taiwan

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Chem. Mater. 2016.28:5175-5190. Downloaded from pubs.acs.org by DUQUESNE UNIV on 09/28/18. For personal use only.

S Supporting Information *

ABSTRACT: D−A conjugated molecules are complicated in both their molecular and their packing structures. In this perspective, we summarize more than 40 crystal lattices of conjugated oligomers to identify the morphological influence of each building block on the D−A molecules. These lattice structures reveal not only the packing preferences of the conjugated oligomers but also the conformational disorder in the lattices. The presence of this disorder in slowly grown crystals implies that attaining total long-range conformational order is challenging for D−A oligomers, which are structurally complicated and readily distorted and which have building blocks of incommensurate packing dimensions. In optoelectronic applications, a decreased duration of processing can prevent ordering and trap the thin films of D−A oligomers from becoming crystalline phases. Although D−A oligomers conform to packing principles in the formation of a single crystal, their phase behaviors in the formation of active thin films are much more difficult to comprehend. Continuous advances in methods of characterization are still strongly required for the steps of attaining a true structure−property relation of D−A oligomers in active films for optoelectronic applications.

1. INTRODUCTION The developments of conjugated molecules and of their applications in organic optoelectronics have been remarkable. Classified as third-generation semiconducting polymers,1 donor−acceptor (D−A) conjugated polymers play a particularly important role in progress toward an increased power conversion efficiency (PCE) of organic photovoltaics (OPV) and charge mobility (μ) of organic field-effect transistors (OFET). As illustrated in Figure 1a, D−A conjugated polymers are generally constructed with polymer backbones that have alternating D and A aromatic units, with lateral solubilizing side chains (R1 and R2). This anisotropic structure allows researchers to modulate the optoelectronic properties and the processing of the conjugated polymers separately.2−15 In seeking the best backbone D−A combinations,2,16−21 the energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the band gap (Eg) of D−A polymers were fine-tuned to offer ambient stability22 and to approach the theoretical limits of the device performance.23−25 Side-chain engineering was applied to adjust the processing, selforganization behavior,26−28 and solid-state morphology of D− A polymers.11,12,29−31 The molecular design of conjugated polymers as “quasi-onedimensional (quasi-1D)” materials can provide excellent anisotropic optical and electronic properties1 because charges can be transported through both the intrachain delocalization

Figure 1. Schematic illustrations of (a) a repeat unit of D−A conjugated polymers and (b) D−A conjugated oligomers; R: terminal alkyl chains, R1, R2: lateral alkyl chains.

and the interchain hopping in their condensed phases. When the interchain phase coherence is disturbed, the dimension of the charge transport becomes decreased from quasi-1D to 1D. The conformational disorder can hence disrupt the periodic Received: February 15, 2016 Revised: June 30, 2016 Published: July 8, 2016

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This Perspective is part of the Up-and-Coming series. © 2016 American Chemical Society

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DOI: 10.1021/acs.chemmater.6b00671 Chem. Mater. 2016, 28, 5175−5190

Perspective

Chemistry of Materials

influences of the internal angles of aryl units (θAr in Figure 2) and the interaryl torsional angles (φAr−Ar in Figure 3) on the

arrangements of the conjugated polymers and localize the charges onto individual chains, which increases the activation energy or the trap breadth for charge transport.32 Phase morphology, which includes the lattice structure, domain size, crystallinity, molecular orientation, etc., is consequently proved to be critical for the material performance in the OPV and OFET.33−39 PCE greater than 10% and hole and electron mobilities (μh and μe) greater than 10 cm2 V−1 s−1 have been attained in OPV and OFET devices.11,12,40−49 However, to articulate the relation between the molecular structure and the phase morphology of D−A polymers remains challenging. The main reasons are that the polydispersity and the long-chain structure of D−A polymers obscure the morphological influences of their building blocks. As the activation energies for the interaryl bond rotation are small,50,51 the many rotatable single bonds on the backbone also result in a complicated conformational isomerism of the polymers.52−55 Thus, the periodic lamellar packing and πstacking, as indicated by the (h00) and the board (0k0) diffractions, are generally obtained from X-ray diffraction (XRD),9,48,56−58 but the periodic packing along the mainchain axis are seldom reported. So far, only few conjugated polymers have had their lattice structures solved;59−70 the phase-separation morphologies of few representative polymer/ fullerene bulk heterojunction layers have been investigated in detail.71−74 Because the phase formation of the conjugated molecules is driven by weak physical interactions, polymorphism75,76 and diverse packing disorders77 can also be introduced into bulk and thin-film samples of the conjugated polymers. X-ray crystallography provides important information about the phase morphology, including the three-dimensional atomic arrangement of molecules, the retrieved molecular shapes, and intermolecular packing schemes.78−80 In this respect, the recently developed D−A conjugated oligomers provide an improved platform to investigate the morphological influences of the D and A building blocks.81−88 Compared with the D−A polymers, although the truncated conjugated backbone breaks the intrachain charge delocalization, the well-defined chemical structures of the D−A oligomers facilitate the formation of an ordered phase, and the single-crystal structures of the D−A oligomers are particularly informative about the packing schemes and the packing disorders of the D−A molecules. Molecules that have physicochemical properties sensitive to structure perform properly only when their molecular and solid-state structures are carefully engineered. Drug molecules likely represent the best example. The great molecular flexibility (5.6 rotatable bonds per molecule on average) causes conformational polymorphism,75 making the control of the solid-state structure essential for a pharmaceutical application. Similarly, the performance of D−A molecules is also highly sensitive to structure; they have even more numerous rotatable bonds on their backbones and on the alkyl substituents, but the morphological influences of the building blocks of D−A molecules have not been systematically summarized. Aiming to clarify the morphological influences of these building blocks, we organize this perspective in the following way. First, because the rotation of the interaryl and alkyl single bonds alters the molecular shape and brings conformational disorder to the phase structure, the definition of conformers and the classification of the condensed phases are introduced in Section 2, so that subsequent discussion of the consequences of conformational isomerism can proceed. Next, we discuss the

Figure 2. Schematic diagram of θAr of (a) p-phenylene (−pPh−), (b) thiophene-2,5-diyl (−Th−), (c) m-phenylene (−mPh−), and (d) dithieno(3,2-b;2′,3′-d)silole (−DTS−).

Figure 3. Schematic diagram of φAr−Ar, (a) φTh−Th for a bithiophene, (b) φPh−Ph for p-biphenyls, and (c) φPh−Ph for m-terphenyls.

backbone geometry and define the backbone curvature as θMol = (180° − bending angle of a molecule) as shown in Figure 4.

Figure 4. Schematic diagram of backbone curvatures (θMol) of (a) poligophenyls, (b) oligothiophene, (c) U-shaped m-oligophenyls, and (d) helical m-oligophenyls.

With θMol, the unsubstituted conjugated backbones are separable into linear and curved ones; their packing preferences are discussed in Section 3. As the terminal and lateral alkyl chains are also key components of conjugated molecules (Figure 1b), their influences on the packing schemes are discussed also in Section 3, using the crystal structures of conjugated oligomers based on thienyl (−Th−) and phenyl (−Ph−). In section 4, the packing principles summarized from the Th/Ph oligomers are applied to discuss the packing schemes of D−A oligomers. As the length of the building block of an oligomer increases, conformational disorder starts to appear in the crystal lattice; the conformational disorder of the conjugated backbone and in the alkyl-chain domain is discussed in Section 5.

2. CONFORMERS AND CONDENSED PHASES IUPAC defines a conformer as “one of a set of stereo-isomers, each of which is characterized by a conformation corresponding to a distinct potential-energy minimum”.89 In the dilute phases 5176


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backbone curvature; φAr−Ar, illustrated in Figure 3, determines the coplanarity of a conjugated molecule. The conformational isomerization can alter the molecular shape via altering φAr−Ar. In Figure 2, θAr of p-phenylene (−pPh−), thiophene-2,5-diyl (−Th−), m-phenylene (−mPh−), and dithieno(3,2-b;2′,3′d)silole (−DTS−) are 180°, 152°, 120°, and 116 o, respectively. In the crystalline phases, φTh−Th of oligo(thiophene) and φPh−Ph of p-oligophenyls are ∼0°,100 whereas φPh−Ph of m-oligophenyls are between 30° and 55°,103 as illustrated in Figure 3. As the large φPh−Ph of m-oligophenyls breaks backbone conjugation, m-oligophenyls are not considered to be conjugated molecules.104 Both oligophenyls103,105 and oligo(thiophene)s106−108 have nonplanar structures (φAr−Ar > 0°) in dilute phases. The oligomers have smaller φAr−Ar in a crystalline phase, because in the crystalline environment, distorting oligomers from a coplanar geometry imposes a large energetic penalty.107 Whereas θAr is for an aryl unit and φAr−Ar is for an Ar−Ar pair, the backbone curvature (θMol) is an angle resulting from the overall contribution from all θAr and φAr−Ar of a conjugated molecule. As shown in Figure 4a,b, although θAr of −pPh− and −Th− differ, p-oligophenyls and oligo(thiophene-2,5-diyl)s both have linear backbones (θMol = 0°), because θTh not equal to 180° can be compensated on adjusting the −Th− units of the oligo(thiophene-2,5-diyl)s into the transoid conformation. To describe the θMol of m-oligophenyls is difficult, because moligophenyls are not planar and can form several conformers. For example, the alternating signs of φPh−Ph (36°, 31°, −32°, and 36°) result into the U-shaped conformer (Figure 4c), whereas all positive φPh−Ph in a sequence (53°, 34°, 44°, 44°, 34°, etc.) produce a helical conformer (Figure 4d).101 To facilitate a more straightforward discussion about the relation between the backbone curvature and the packing scheme, we consider θMol = 180° for the U-shaped conformer and θMol = 120° for the helical conformer of m-oligophenyls. Their packing schemes are shown in Section 3.2. These molecular angles not only alter the backbone shapes of conjugated oligomers, either linear or curved, but also determine their packing approach. The packing schemes of crystal phases in conjugated oligomers are illustrated in Figure 5. The linear backbone of conjugated oligomers typically adopts a perpendicular or tilted arrangement within a layer structure with respect to the layer planes. In the tilted case, there are two possible correlations between adjacent layers; one is a uniformly tilted direction for synclinic (Figure 5b) and another is anticlinic (Figure 5c). The layer structures in a herringbone arrangement are due to attractive interactions for neighboring molecules and minimization of the excluded volume to decrease the Gibbs energy of crystals. The curved directions of the oligomers in adjacent layers can form parallel or antiparallel packing, as shown in Figures 5d,e, whereas an intercalated packing (Figure 5f) can also be stabilized with a favorable dipolar pair interaction and packing efficiency. 3.2. Packing Schemes of Unsubstituted Backbones. Figure 6 shows two lattice projections of rod-like conjugated oligomers. Along the long-axis projection, both p-oligophenyls98,100 and oligo(thiophene-2,5-diyl)s93 pack into 2D layers with a herringbone arrangement. The lateral projections show that the number of repeat units, although altering the molecular length and affecting the tilt angle of the molecule with respect to the normal of the 2D layer, does not alter the 2D herringbone arrangement. p-Terphenyl (p-Ph3) exhibits a packing different from the other three oligomers in Figure 6; it has a larger torsional angle with a synclinic arrangement.

(the gaseous or solution phase), a molecule with rotatable bonds can undergo conformational isomerization and be partitioned into the potential-energy wells of separate conformers.75 In a condensed phase, molecules must balance the intra- and intermolecular energy; solidification, such as deposition, crystallization, and vitrification, hence results in solid phases with either a unified conformation or conformational disorder. Wunderlich classified condensed phases into three classical phasesmelt, crystal, glassand six mesophasesliquid crystal (LC), plastic crystal (PC), conformationally disordered crystal (CC), LC glass, PC glass, and CC glass.77,90 The crystalline phase is defined as a phase with long-range positional, molecular orientational, and bond orientational orders. The positional order describes the periodic arrangement of the centers of masses of molecules; the molecular orientational order describes the alignment of the molecules, and the bond orientational order describes the alignment of bond vectors with respect to a lattice axis. The complete loss of order of the three types produces an amorphous melt or a glassy phase. These two amorphous phases are differentiated by the presence or absence of the cooperative molecular motions of large amplitude. According to this concept, mesophases involve a loss of only one or two particular long-range orders. LC, PC, and CC phases are thus mesophases without longrange positional order, molecular orientational order ,and bondorientational order, respectively. Cooling these mesophases to a temperature below their glass-transition temperatures (Tg) vitrifies the disordered domains and results in the remaining three glassy mesophasesi.e., LC glass, PC glass, and CC glass. Beyond the six main mesophases classified by Wunderlich, calamitic LC have many subphases. Details of the subphases of calamitic LC were provided by Pershan.91 However, the packing schemes summarized in this work were replotted from the single-crystal structures of conjugated molecules. The molecules thus possess long-range positional order in the lattice and cannot be classified as LC phases. In the following sections, via a systematic showing of the packing schemes of conjugated oligomers, it becomes clear that, as the backbone curvature, alkyl substituents, and electronwithdrawing groups gradually increase the structural complication of conjugated molecules, it becomes increasingly difficult for conjugated molecules to attain long-range conformational and positional orders in the solid phases. Although some conjugated oligomers have solved single-crystal structures, the solid phases of these oligomers should be considered as the CC phase, because conformational disorder is found in the alkyl domains or along the backbones. In thin films of D−A oligomers, the cumulative (paracrystalline) disorder causes a broadening or a lack of high-order diffractions in their grazing incidence X-ray diffraction (GI XRD) patterns. The long-range positional order is lost because of the cumulative disorder. With only their molecular orientational order, the thin films of the D−A oligomers are thus in the LC phases.

3. PACKING SCHEMES OF CONJUGATED OLIGOMERS 3.1. Geometries of Conjugated Backbones. Molecular shape is explicitly described with bond lengths, angles between bonds, and interaryl torsional angles of a molecule. The singlecrystal structures of linear oligothiophenes92−96 and linear poligophenyls97−103 show that the molecular angles have significant influences on the molecular geometry of a conjugated molecule. θAr, defined in Figure 2, affects the 5177


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Figure 5. Linear backbone of conjugated oligomers in (a) perpendicular arrangement and tilted arrangement within (b) tilted direction for synclinic and (c) anticlinic; curved backbone in (d) antiparallel scaly packing, (e) parallel scaly packing, and (f) intercalated packing.

Figure 7. Packing schemes of U-shaped conformer103 of (a) and helical conformer of105 (b) m-oligophenyls.

3.3. Packing Schemes of Thiophene/Phenylene CoOligomers. Conjugated oligomers constructed with both −Th− and −Ph− units have nonlinear backbones. Figure 8

Figure 6. Long-axis projection (up) and lateral projection (down) of crystal lattices of (a) p-terphenyl (p-Ph3), (b) p-quinquephenyl (pPh5), (c) terthiophene (Th3), and (d) sexithiophene (Th6).

Figure 8. Backbone geometries and conformations of Th/Ph cooligomers.

Oligothiophenes have smaller potential barriers for planarity (∼0.4 kJ mol−1)106,108 than p-oligophenyls. Terthiophene is hence completely planar, but p-Ph3 has φPh−Ph = 15° in a crystalline environment. Comparing the crystal structures of pPh3 and p-quinquephenyl (p-Ph5), we find that an increased length of conjugation improved the planarity of p-oligophenyls in the crystalline environment. The linear shapes of backbones (θMol = 0°) favor a 2D herringbone packing, whereas the packing complication increases when θMol and φAr−Ar deviate from 0°. For example, the U-shaped conformers of m-quinquephenyl stack antiparallel to form 2D sheets, as shown in Figure 7a,103 whereas the 5-fold symmetric helical conformers of m-deciphenyl (Figure 7b) align their long axes along the crystallographic c-axis and pack into a tetragonal lattice.109 These results thus provide a first glimpse into how θMol affects the packing preferences of conjugated backbones. In addition to the packing preference, moligophenyls formed only tiny crystals during slow crystallization, because of its distorted backbone geometry.109

shows the chemical structures of three Th/Ph co-oligomers. As the −Th− unit acts as the bending point, the oligomers have curved or S-shaped backbones. Furthermore, a backbone can switch between the two geometries through a conformational isomerization. Although these co-oligomers have nonlinear backbones, when crystallizing, they adjust their conformations to minimize θMol. For co-oligomers comprising −Th− units of even number, θMol can decrease to nearly 0° on arranging the −Th− units antiparallel. These co-oligomers forming 2D herringbone layers (Figure 9) are thus similar to those of the −Ph− and −Th− oligomers (Figure 6),110,111 but, for those cooligomers containing −Th− units of odd number, conformational isomerization cannot decrease θMol to ∼0°. In this case, the parallel or antiparallel scaly packing of the curved backbones is discernible in the lateral views of Figure 10a,b.112 When two cyano substituents (−CN) are placed onto the α,ω-positions of curved 2,5-di(biphenyl-4-yl)thiophene, an intercalated packing was formed because of a favorable interaction between the central −Th− and the terminal −CN units (Figure 10c).113 5178


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Figure 11. Long-axis (up) and lateral projections (down) of crystal lattices of TT oligomers; (a) 2,2′-bi(thieno[3,2-b]thiophenyl),116 (b) 2,5-bis(4-biphenylyl)thieno(3,2-b)thiophene,117 and (c) 2,5-bis(9Hfluoren-2-yl)thieno(3,2-b)thiophene.117 The conformational disorder along the backbone is emphasized with a red circle.

Figure 9. Long-axis (up) and lateral projections (down) of crystal lattices of Th/Ph co-oligomers, which have −Th− units of even number: (a) 5,5′-bis(4-biphenyl)-2,2′-bithiophene,110 (b) 5,5‴-bis(4biphenyl)-quaterthiophene,110 and (c) 1,4-bis(5-phenylthiophen-2yl)benzene.114

schemes of polycyclic arenes and heteroarenes are not summarized in this contribution,118−128 but, as long as chemical fusion of the neighboring aryl units does not significantly alter θMol, most nearly linear polycyclic arenes and heteroarenes form 2D herringbone layers. 3.4. Solubilizing Side Chains and Packing Schemes. The flexible terminal and lateral substituents show distinct influences on the packing schemes of conjugated molecules. The terminal groups, such as aliphatic chains or silane groups, affect the in-plane tilt angles of the conjugated backbones featuring anticlinic (Figure 12a,d) and synclinic (Figures 12b,c)

Figure 10. Long-axis (up) and lateral projections (down) of the crystal lattices of Th/Ph co-oligomers, which have −Th− units of odd number: (a) 2,5-bis(4′-methoxybiphenyl-4-yl)thiophene,112 (b) 5,5″di(biphenyl-4-yl)-terthiophene,115 and (c) 4′,4‴-(thiophene-2,5-diyl)bis([1,1′-biphenyl]-4-carbonitrile).113 Figure 12. Long-axis (up) and lateral projections (down) of crystal lattices of terminally substituted quaterthiophenes: (a) 5,5‴-dimethylquaterthiophene,129 (b) 5,5‴-dibutyl-quaterthiophene,130 (c) 5,5‴di(trimethylsilane)-quaterthiophene,133 and (d) 5,5′-bis(4′-methoxybiphenyl-4-yl)-2,2′- bithiophene.111

On the basis of the packing schemes of the −Ph− oligomers, the −Th− oligomers, and the Th/Ph co-oligomers, a rule of thumb for the packing of conjugated backbones is perceptible. First, a herringbone packing is favorable for the close packing of the unsubstituted conjugated backbones. Second, for close packing of the curved ones, within the 2D herringbone layer, the molecules adopt a parallel or antiparallel scaly packing shown in Figure 10a,b, respectively. A favorable interaction between central and end functional groups can induce intercalated packing (Figure 10c) of curved conjugated backbones. Some fused aromatic units such as thienothiophene (−TT−) (Figure 11) have θAr = 180°; these units do not bend the backbone. Examples such as the −TT− oligomers and the TT/ Ph co-oligomers thus also have θMol = 0° and pack into a 2D herringbone layer with a tilted arrangement as expected.116,117 Because of a lack of rotatable interaryl bonds, the packing

forms but have little influence on the 2D herringbone packing.111,129−134 The electron-withdrawing trifluoromethyl group and the electron-donating methoxyl group on two Th/ Ph co-oligomers5,5′-bis[4-(trifluoromethyl)phenyl-1-yl]2,2′-bithiophene133 and 5,5′-bis(4′-methoxybiphenyl-4-yl)2,2′-bithiophenealso do not alter the 2D herringbone packing (Figure 12d).111 The segregation of these terminal groups form 2D aliphatic layers between 2D herringbone layers of the backbones; an alternating aromatic and aliphatic lamellar structure along the long axis of the molecule is thus obtained. For the terminally alkylated polycyclic heteroarenes,135 similar effects on packing schemes were observed. 5179


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Chemistry of Materials Lateral alkyl chains have more severe influences than terminal alkyl chains on the backbone shape and the packing scheme, because they impose intra- and intermolecular steric hindrance on neighboring aryl units. Barbarella et al. showed that the two central β-methyl substituents on 4,4′,3″,4‴tetramethyl-quaterthiophene not only induce large φTh−Th (>40°) in solution136,137 but also distort the neighboring thiophene rings in the crystalline state.138 The steric effect of the lateral methyl substituents thus prevents the planarity of the conjugated backbone. In the Cambridge Crystallographic Data Centre (CCDC) database,139 only few lateral alkylated oligothiophenes and no lateral alkylated p-oligophenyl have resolved single crystal structure, indicating that lateral alkyl chains also hinder the crystallization of the conjugated oligomers. Regarding the packing schemes, because the lateral chains prevent edge-to-face packing of the backbones,140 the (brickwall or staircase) face-to-face π-stacking was found in crystals of the laterally alkylated oligothiophenes. For example, in the long-axis projections of Figure 13, the 4,4′,3″,4‴-tetramethyl-

Figure 14. Packing scheme of hexakis (dodecyl)-dodecithiophene (12T).141

Figure 13. Long-axis (up) and lateral projections (down) of crystal lattices of the lateral alkylated quaterthiophenes: (a) 4,4′,3″,4‴tetramethyl-quaterthiophene, 138 (b) 3,3‴-didodecyl-quaterthiophene,140 and (c) 3,3‴-dihexyl-quaterthiophene.140

quaterthiophenes adopt a brickwall packing with π-stacking distance dπ−π = 3.61 Å; the 3,3‴-didodecyl and 3,3‴-hexyl quaterthiophenes.140 adopt a staircase packing with dπ−π = 3.41 and 3.62 Å, respectively. The length of the alkyl chain affects the lateral segregation between the conjugated backbones. As shown in the lateral projections of Figure 13b,c, the dodecyl chains create a lateral distance 2.0 nm; the hexyl chains have a distance of 1.5 nm for the intercalated arrangement. 3,3‴Dipropyl-quaterthiophenes do not form a parallel-displaced π−π stacking, because the propyl chains are too short to provide sufficient physical interactions between themselves. Azumi et al.141 reported the crystal structure of a lateral alkylated dodecithiophene (12T in Figure 14). The much elongated conjugated backbone does not alter the face-to-face π-stacking but induces a superior alignment for the conjugated backbones in the lamellar structure, as shown in the lateral projection in Figure 14. 12T thus has a lattice structure that resembles the form II packing of poly(alkylthiophene)s.69 3,4-Dialkylthiophene and 9,9-dialkylfluorene are also lateral alkylated aryl units, but they differ from the transoid lateral alkylated oligothiopenes discussed above because their lateral alkyl chains are distributed on only one side of the aryl unit. In the crystal structures of 3″,4″-dibutylpentathiophene (Figure 15a)142 and 5,5″-bis(9,9-dioctyl-9H-fluoren-2-yl)-2,2′;5′,2″-terthiophene (Figure 15b),143 the lamellar structure is still found,

Figure 15. Long-axis (up) and lateral projections (down) of crystal lattices of (a) 3″,4″-dibutylpentathione142 and (b) 5,5″-bis(9,9-dioctyl9H-fluoren-2-yl)-terthiophene.143

but the conjugated backbones adopt an edge-to-face arrangement (Figure 15a) rather than the face-to-face arrangement in the aromatic domain. For comparison with face-to-face πstacking, the aromatic rings of conjugated backbones in edgeto-face stacking have larger φAr−Ar. In the more extreme cases, the lateral chains even intercalate into the stacking of the conjugated backbones and completely degrade the close πstacking of the backbones, as is visible in the crystal structures of 2,7-di(2-thienyl)-9,9-dihexylfluorene (Figure 16a),144 3′,3⁗,4,4⁗-tetrabutylhexathiophene,142 (Figure 16b), and 2,6di([2,2′-bithiophen]-5-yl)-4,4-dihexyl-4H-silolo[3,2-b:4,5-b′]dithiophene (Th2−DTS−Th2).145

4. PACKING SCHEMES OF THE D−A OLIGOMERS 2,1,3-Benzothiadiazole (BT), 5,6-difluoro-4,7-di(thiophen-2yl)benzo-2,1,3-thiadiazole (FBT), naphtho[1,2-c:5,6-c′]bis[1,2,5]thiadiazole (NT), thieno[3,4-c]pyrrole-4,6-dione 5180


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Figure 18. (a) 4,7-Di(thiophen-2-yl)benzo-2,1,3-thiadiazole146 (Th− BT−Th) and (b) 5,6-difluoro-4,7-di(thiophen-2-yl)benzo-2,1,3-thiadiazole52 (Th−FBT−Th); the conformational disorder on the backbone is emphasized with a red circle.

Figure 16. Packing schemes of (a) 2,7-bis(2-thienyl)-9,9-dihexylfluorene144 and (b) 3′,3⁗,4′,4⁗-tetrabutylhexathione.142 The conformational disorder on the backbone is emphasized with a red circle.

(TPD), and 1,4-diketo-pyrolo[3,4-c]pyrrole (DPP) in Figure 17 are widely used acceptors in D−A conjugated molecules.

Figure 17. Representative acceptor units used in D−A conjugated molecules.

Figure 19. (a) 5,6-Difluoro-4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole147 and (4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole;147 the conformational disorder on the backbone is emphasized with a red circle.

Because of additional electron-withdrawing groups, i.e., the pendant thiadiazole, fluorine, pyrrole-2,5-dione units, and the main-chain 1,4-diketo-pyrolo[3,4-c]pyrrole unit, the A units have more complicated chemical structures than the −Ph− and −Th− units. To understand the influences of these electronwithdrawing groups, we classified these acceptors based on their θAr and whether they have lateral side chains. θAr of BT, FBT, NT, and DPP units are hence all 180°, but θAr of TPD is 163°. TPD and DPP differ from the other acceptors because of their lateral alkyl groups. The influences of these structural factors are discussed based on the single crystal structures of the reported D−A molecules. As shown in Figure 18, 4,7-di(thiophen-2-yl)benzo-2,1,3thiadiazole (Th−BT−Th)146 and 5,6-difluoro-4,7-di(thiophen2-yl)benzo-2,1,3-thiadiazole (Th−FBT−Th)52 stack into a roof top arrangement that differs from the herringbone arrangement of p-Ph3 and Th3 in Figure 6. In the herringbone packing, the edge of one oligomer interacts with the face of the other, whereas in the rooftop packing, the end of one oligomer points to the face of the terminal thienyl group of the other. In Figure 19, the terminal hexyl groups do not alter the rooftop packing of the Th−FBT−Th backbones, but they turn Th−BT−Th backbones from the rooftop packing into face-to-face πstacking.146 The single-crystal structure of 5,10-di(thiophen-2-

yl)naphtho[1,2-c:5,6-c′]bis([1,2,5]thiadiazole) (Th−NT−Th) is not reported, but its alkylate analogue, 5,10-bis(4-methyl-2thienyl)[1,2,5] thiadiazolo[3′,4′:5,6]naphtho[1,2-c][1,2,5]thiadiazole, adopts face-to-face π-stacking as shown in Figure 20a.148 The influences of the lateral side chains are found also in the crystal structures of 5,5′-dioctyl-4H,4′H-1,1′-bithieno[3,4-c]pyrrole-4,4′,6,6′(5H,5′H)-tetrone (TPD2)149 and the laterally alkylated DPP oligomer, 3,6-bis(4-(2,2′-bithiophen-5yl)phenyl)-2,5-dihexyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4dione) (C6PT2),150 as shown in Figure 20b,a. TPD2 has a packing scheme similar to that of 3,3‴-dialkyl-quaterthiophenes (Figure 13b,c), indicating that the π-stacking is favored by the laterally alkylated conjugated oligomers. In Figure 21a, C6PT2 also adopts face-to-face π-stacking, but in the π-stacks the neighboring molecules shift along their long-axis ∼8 Å to avoid the steric hindrance among the lateral alkyl chains. Comparing the packing scheme of C6PT2 with those of C6PT2C6 (Figure 21b), C6PT1C6 (Figure 21c), and C6PT3C6 (Figure 21d), we found that adding the terminal alkyl chains improves the order of the herringbone packing, but 5181


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conformational disorder is located at the −TT− (Figure 11) and −FBT− units (Figures 19a and 22d). At the ends of the backbones, conformational disorder is found at the unalkylated terminal −Th− unit (Figures 16b, 18a,b, 21a, and 22a). Because −TT−, −BT−, and −FBT− units have θAr = 180°, flipping these units along the long axis of the conjugated molecules for 180° alters the orientation of the aromatic units but does not affect θMol and the potential energy of the conjugated molecules.50,51 Although θAr of a −Th− unit is not 180°, flipping the unalkylated −Th− unit at the ends of the backbone also has little influence on the backbone. These aromatic units were thus found to have two orientations in their crystal lattices. The conformational disorder in the alkyl-chain domain is perceptible on the lateral −EH groups of the DPP and DTS D−A oligomers. The conformational disorders in the crystal lattice of 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2- b:4,5-b′]dithiophene-2,6-diyl)bis(5-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) are illustrated in Figure 23. The formation of the conformational disorder in the alkyl chain domains can contribute to the fact that the lattice dimensions of the n-alkanes154 are incommensurate with the space created with a close stack of the conjugated backbones. The branched structure of the −EH unit makes long-range conformational order even more difficult to attain and generally results in amorphous alkyl domains. In the case of the D−A− DTS−A−D oligomers, the differences in the potential energy of the banana, twisted bithiophene, zigzag, and banana-withf lipped-bithiophenes conformations are less than 4 kJ mol−1.84,151,155 The thermal energy near 300 K is hence sufficient to distribute the molecules into the various conformations. If the crystallization cannot unify the conformations of the molecules before their molecular motions are restricted in the solid phases, the conformational disorders are trapped in the crystal lattices, resulting in the CC phase.

Figure 20. Long-axis (up) and lateral projections (down) of crystal lattices of (a) 5,10-bis(4-methyl-2-thienyl)[1,2,5]thiadiazolo[3′,4′:5,6]naphtho[1,2-c][1,2,5]thiadiazole148 and (b) 5,5′-dioctyl4H,4′H-1,1′-bithieno[3,4-c]pyrrole-4,4′,6,6′(5H,5′H)-tetrone.149

varying the conjugation length does not significantly affect the herringbone packing of the DPP oligomers.150 The replacement of the linear lateral hexyl chains with branched ethylhexyl (−EH) chains (Figure 21e) pushes the neighboring DPP units away from each other. The distance center to center between neighboring EHPT2C6 molecules increases to 5.7 Å; the face-to-face π-stacking is broken. Without the close π-stacking, the coplanarity of the EHPT2C6 backbone is degraded; larger φAr−Ar are observed. As a −Ph− unit generally causes larger φAr−Ar than a −Th− unit, wondering whether close π-stacking becomes restored on replacing the −Ph− units of EHPT2C6 with −Th− units, we synthesized 2,5-bis(2-ethylhexyl)-3,6-bis(5″-hexyl-[2,2′:5′,2″terthiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (EHT3C6). The crystal structure in Figure 21f shows that EHT3C6 forms an ordered π-stack. Although both have branched −EH units, EHT3C6 can attain a coplanar molecular structure and ordered π- stacking more readily than EHPT2C6, which contains the −Ph− units. The DTS centered D′−A−D−A−D′ oligomers developed by Bazan and co-workers are the best performing donor materials in OPV.81−85 To have sufficient solubility, these oligomers have curved backbones, lateral alkyl chains, and, in several cases, also terminal alkyl chains. The backbones of the oligomers are curved because the center −DTS− unit has θAr = 68° and also because the backbones are made with both −Th− and −Ph− units. In the single-crystal structures of the D−A−DTS−A−D oligomers,85,151−153 the conjugated backbones have effective coplanarity and adopt close face-to-face π-stacking with dπ−π between 3.4 and 3.8 Å, as shown in the long-axis projections of Figure 22. From the lateral projections of Figure 22, the antiparallel scaly packing of the curved backbones is found. Although the D−A oligomers have more complicated chemical structures, if one considers the electron-withdrawing groups of the A units also as the lateral substituents, the packing principles summarized from the Th/Ph oligomers are still applicable to a certain extent.

6. CONCLUSIONS AND OUTLOOK In this review, we summarize the morphological influences of the backbone geometries and the alkyl substituents of conjugated oligomers. The crystal lattices of the unsubstituted Th/Ph oligomers reveal the packing preferences of the conjugated backbones; the crystal lattices of the alkylated Th/Ph and D−A oligomers show how the alkyl substituents affect the packing of the conjugated molecules. In general, the unsubstituted conjugated backbones favor stacking into 2D herringbone layers; the backbone curvature, i.e., θMol, affects the lateral arrangements (parallel or scaly) of the backbones. The alkylated oligomers form lamellar structures with alternating 2D aliphatic or aromatic layers. For the terminal alkylated oligomers, the normal direction of the lamellar structure points along the long axis of the molecule, whereas, for the lateral alkylated oligomers, the lamellar normal is generally perpendicular to the long axis of the molecule. The lateral alkyl chains are more powerful to degrade the backbone coplanarity, altering the packing schemes (from herringbone to face-to-face π-stacking), and breaking the intimate π-stacking among the backbones. If we consider the electron-withdrawing groups of the A units also as the lateral substituents, some packing preferences of D−A oligomers are similar to those of the Th/ Ph oligomers. In addition to the packing principles, the summarized lattice structures reveal the conformational disorder in single crystals. On conjugated backbones, conformational disorder is found at

5. CONFORMATIONAL DISORDER IN LATTICES OF D−A OLIGOMERS Inspecting the packing schemes from Figures 5−22, one finds conformational disorder of the conjugated backbone in Figures 11, 16b, 18a,b, 19a, 21a, 22a, and 22d and those of alkyl chains in Figures 21e and 22a−d. In the conjugated backbones, the 5182


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Figure 21. Long-axis (up) and lateral projections (down) of crystal lattices of (a) 3,6-bis(4-(2,2′-bithiophen-5-yl)phenyl)-2,5-dihexyl-2,5dihydropyrrolo[3,4-c]pyrrole-1,4-dione (C6PT2),150 (b) 2,5-dihexyl-3,6-bis(4-(5′-hexyl-2,2′-bithiophen-5-yl)phenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (C6PT2C6),150 (c) 2,5-dihexyl-3,6-bis(4-(5-hexyl-2-thienyl)phenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (C6PT1C6),150 (d) 2,5-dihexyl-3,6-bis(4-(5″-hexyl-2,2′:5′,2″-terthiophen-5-yl)phenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (C6PT3C6),150 (e) 2,5-bis(2ethylhexyl)-3,6-bis(4-(5′-hexyl-2,2′-bithiophen-5-yl)phenyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (EHPT2C6),150 and (f) 2,5-bis(2-ethylhexyl)-3,6-bis(5″-hexyl-[2,2′:5′,2″-terthiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (EHT3C6); the conformational disorder along the backbone (in the alkyl domain) is emphasized with a red circle.

the −TT−, −BT−, −FBT−, or the unalkylated terminal −Th− unit; in the alkyl chain domain, the disorder is perceptible on

the lateral −EH groups of −DPP− and −DTS− conjugated oligomers. The small energy differences among the conforma5183


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Figure 22. (a) 4′-(4,4-Bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(7-(thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine),151 (b) 4,4′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(7-(benzofuran-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine),83 (c) 4,4′-(4,4bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(7-(5′-hexyl-[2,2′-bithiophen]-5-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine),152 and (d) 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(5-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole);153 the conformational disorder along the backbone (in the alkyl domain) is emphasized with a red circle.

blocks and rotatable bonds in a D−A oligomer make it more difficult for the oligomers to unify their conformations during crystallization. Except for the conjugated oligomers summarized here, many D−A oligomers, which have curved D−A backbones and lateral and terminal alkyl substituents, do not form single crystals of suitable size and quality,84,87,155−157 indicating that, besides the long-range conformational order, the positional and molecular orientational order are also challenging to build for many D−A oligomers. In particular, when D−A oligomers serve as active layers of OFET and OPV devices, in solution, they have only a limited period (few minutes, compared to few weeks for the growth of a single crystal) to adjust the conformations and intermolecular packing. In addition to conformational disorder, a positional disorder evident according to the absence or broadening of high-order diffraction signals in the GI XRD measurements56 was also commonly observed in active films of D−A oligomers.83,155,158,159 Without long-range conformational and positional order, many active films are in the LC phase with only molecular orientational order. Conformational disorder is known to affect the effective conjugation length, and, consequently, the optical and electronic properties of conjugated molecules.160,161 Molecules in the meso- or amorphous phases also have electronic properties and phase stability different from those in the crystalline phases. Although the packing principles of D−A oligomers in the crystalline phase are summarized in this work, the packing structure and packing disorder in active films remain a complicated topic to investigate. Developments of methods of characterization to

Figure 23. Illustration of conformational disorder in the lattice of 7,7′(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(5-fluoro-4-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole).153

tional isomers and the incommensurate packing dimensions of alkyl chains and D−A backbones result in a lack of long-range conformational order in the crystal lattices. According to Wunderlich’s classification of condensed phases,77 these oligomers are thus in a CC phase rather than a crystalline phase. The observation of the packing disorder in these single crystals thus indicates that the increased number of building 5184


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Chemistry of Materials Notes

show the molecular packing and to quantify the degree of structural disorder in the active films will yield structure− property relations of D−A oligomers for optoelectronic applications, so that the exact bottlenecks of device performance might be better identified in the future.

The authors declare no competing financial interest. Biographies Chi-Feng Huang received his B.S. degree in Chemical Engineering from National Taiwan University and his M.S. degree in Applied Chemistry from National Chiao-Tung University (NCTU). He worked as a research assistant at National Synchrotron Radiation Research Center for diffraction imaging from 2010. He is currently pursuing a Ph.D. degree under the supervision of Prof. Chien-Lung Wang at Department of Applied Chemistry, NCTU, and Prof. KengSan Liang at Institute of Physics, Academia Sinica. His research is focused on the structural characterization of conjugated molecules by X-ray scattering and diffraction.

7. EXPERIMENTS Materials and General Characterizations. All chemicals (from Aldrich, Acros or TCI) were used as received unless specified in a synthetic scheme. 1H and 13C NMR spectra were recorded (Varian, 400 MHz) for samples in deuterium-substituted chloroform CDCl3 as referenced with TMS (0.5 mass %) using spectrometers. Synthesis and Characterization of 2,5-Bis(2-ethylhexyl)-3,6bis(5″-hexyl-[2,2′:5′,2″-terthiophen]-5-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (EHT3C6). To a stirred solution of 3,6bis(5-bromothiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole1,4(2H,5H)-dione (400 mg, 0.6 mmol), 2-(5′-hexyl-[2,2′-bithiophen]5-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (428 mg, 1.5 mmol), and potassium carbonate (486 mg, 3.5 mmol) in degassed toluene/H2O (20/5 mL) containing Pd(PPh3)4 (68 mg, 0.06 mmol) was added a drop of Aliquat 336. The reaction mixture was stirred at 80 °C under a dinitrogen atmosphere for 16 h. After cooling to 295 K, a saturated aqueous NH4Cl solution was added. The reaction mixture was extracted with dichloromethane. The collected organic layer was dried over MgSO4, filtered, and concentrated under diminished pressure. The residue was purified on a column chromatograph (SiO2, hexane/ ethyl acetate = 30/1 (v/v)) and reprecipitated with methanol to give a navy blue solid EHT3C6 (110 mg, 20%). 1H NMR (400 MHz, CDCl3): δ/ppm 8.94 (d, J = 4.1 Hz, 2H), 7.29 (d, J = 4.2 Hz, 2H), 7.21 (d, J = 3.8 Hz, 2H), 7.04 (dd, J = 6.6, 3.7 Hz, 4H), 6.71 (d, J = 3.5 Hz, 2H), 4.05 (t, J = 6.9 Hz, 4H), 2.81 (t, J = 7.5 Hz, 4H), 1.94 (s, 2H), 1.83−1.63 (m, 4H), 1.44−1.22 (m, 30H), 0.96−0.84 (m, 16H). 13 C NMR (151 MHz, CDCl3): δ/ppm 161.46, 146.76, 139.42, 137.22, 134.13, 128.07, 125.10, 124.27, 124.20, 108.78,46.13, 39.43, 31.71, 30.53, 30.34, 29.84, 28.91, 28.73, 23.85, 23.29, 22.72, 14.25, 14.22, 10.74. EI-MS C58H72N2O2S689 calcd for [M]+: m/z = 1021.59, found [M]+: m/z = 1021.7 Single-Crystal X-ray Diffraction and Analysis of the Structure of EHT3C6. EHT3C6 (1 mg mL−1) was dissolved in a mixed solvent (dichloromethane and methanol, 2/1, v/v). A single crystal of DPP6T formed during slow evaporation of the mixed solvent near 295 K. The diffraction was measured (Oxford Gemini Duo system, single-crystal X-ray diffractometer equipped with Cryojet at 200 K). Parameters of the unit cell were determined on least-squares refinement of the three-dimensional centroids of several thousand reflections. The structure was solved by direct method (SHELXS-97) and refined using full-matrix least-squares on F2. Preparation of a Lattice Model. All CIF files of conjugated molecules were downloaded from the Cambridge Crystallographic Data Centre (CCDC).139 The files were processed (Cerius,2 Accelrys) to generate the projections of the packing schemes of the molecules.

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San-Lien Wu received his B. S. degree from the Department of Chemistry, National Sun Yat-Sen University, Taiwan; in 2012 he joined the group of Prof. Chien-Lung Wang at Department of Applied Chemistry, National Chiao Tung University, Taiwan as a Ph.D. graduate student. His research topic is the synthesis of self-assembling functional materials. Kuan-Yi Wu is a Ph.D. candidate in the Department of Applied Chemistry of National Chiao Tung University (NCTU) in Taiwan. He obtained his B.S. degree in Applied Chemistry at NCTU in 2011. He is currently conducting research under the guidance of Dr. Chien-Lung Wang. His research interests focus on structural characterization by diffraction and crystal engineering of conjugated molecules for an organic field-effect transistor. Dr. Wei-Tsung Chuang, born in 1975, studied polymer science and completed a Ph.D. program, focusing on polymer crystallization, in Department of Polymer Engineering at National Taiwan University of Science and Technology (Taiwan). He subsequently worked as beamline manager of the X-ray diffraction end station at National Synchrotron Radiation Research Center (NSRRC), Taiwan. In 2010, he was appointed Assistant Research Scientist of the Material Science Group at NSRRC, followed recently by promotion to Associate Research Scientist at the same organization. His research interest focuses on the hierarchical structural analysis and phase transitions of soft materials using synchrotron-based infrared spectra, X-ray microscope, and X-ray scattering and diffraction. Dr. Chien-Lung Wang received his Ph.D. degree in Polymer Science at the University of Akron in 2011. His Ph.D. research focused on the supermolecular chemistry and optoelectronic properties of the C60− porphyrin derivatives. After spending six months as a postdoctoral assistant in Prof. Xiong Gong’s research group at the Department of Polymer Engineering, University of Akron, he joined the Department of Applied Chemistry, National Chiao Tung University, in 2011 as an assistant professor. In 2015, he was promoted to associate professor. His current research focuses on the structural characterizations, the structure−property relationship, and the crystal engineering of conjugated molecules and self-organized functional materials.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b00671. Crystallographic information files of EHT3C6 (CIF)

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ACKNOWLEDGMENTS We thank Ministry of Science and Technology, Taiwan, ATP, of National Chiao Tung University and Ministry of Education, Taiwan, for support.

AUTHOR INFORMATION

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Corresponding Author

*Chien-Lung Wang. E-mail: [email protected].

REFERENCES

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Funding

Ministry of Science and Technology, Taiwan (MOST 1032221-E-009-213-MY3, MOST 104-2628-E-009-007-MY3) supported this work. 5185


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