Article pubs.acs.org/cm
H-Bonding Control of Supramolecular Ordering of Diketopyrrolopyrroles Chaoying Fu, Patrick J. Beldon, and Dmitrii F. Perepichka* Department of Chemistry and Center for Self-Assembled Chemical Structures, McGill University, 801 Sherbrooke Street West, Montréal, Quebec, Canada H3A 0B8 S Supporting Information *
ABSTRACT: Diketopyrrolopyrrole (DPP) is a widely used building block for high-mobility ambipolar semiconductors. Hydrogen bonding of N-unsubstituted DPPs has recently been identified as a tool for controlling their solid state structure and properties of semiconducting films, yet little is known about supramolecular packing of H-bonded DPP derivatives. Here we report a comparative study of three archetypical DPP derivatives, difurylDPP (DFDPP), diphenylDPP (DPDPP), and dithienylDPP (DTDPP), at the interface and in bulk crystals. Using scanning tunneling microscopy (STM) combined with X-ray crystallographic analysis, we demonstrate how the interactions of the (hetero)aromatic substituents interplay with H-bonding, causing dramatic differences in the supramolecular ordering of these structurally similar building blocks. Under all explored conditions, DPDPP exclusively forms H-bonded homoassemblies; DFDPP strongly prefers to co-assemble with alkanoic acids, through a rare lactam···carboxylic acid H-bonded complex, and DTDPP, depending on conditions, either co-assembles with alkanoic acids or self-assembles in one of two H-bonded polymorphs. One of these polymorphs suggests an out-of-plane twist of thiophene rings that form π-stacks running along the surface plane; this is unexpected considering the large energetic penalty of DTDPP deplanarization. The results are explained in terms of inter- versus intramolecular interactions, which are quantified with density functional theory calculations. This work shows that aryl substituents can strongly influence H-bonding assembly of DPP derivatives that is likely to affect their charge-transport properties.
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polymers,17−19 in OFET and OPV devices. Some comparative studies show that H-bonded DPP semiconductors have charge mobility higher than that of their N,N′-dialkylated homologues as a result of enforcing cofacial π−π stacking via Hbonding.13,14 All of the data suggest that H-bonding lends DPP polymers higher solvent resistance, morphological stability, and possibly chemical stability. However, the crucial structural information connecting Hbonding with supramolecular organization of DPP semiconductors is rarely available and is limited to a few singlecrystal X-ray analyses.14,15,20 As a result, the unique ability of high-fidelity H-bonding interactions to control the structure and properties of organic semiconductors21,22 has not been realized in DPP, as some key questions critical for rational design of such materials have not yet been addressed. Does the H-bonding in bulk DPP materials persist unaltered at the interfaces? How do the aromatic substituents on DPP affect the H-bonding assembly? Does the H-bonding affect the intramolecular structure of DPP-containing conjugated chains? Scanning tunneling microscopy (STM) is particularly well suited to the study of supramolecular ordering on surfaces with high, submolecular resolution.23−26 STM has been widely used
INTRODUCTION Diketopyrrolopyrrole (DPP) derivatives have long been used as high-performance industrial pigments in many patented applications in paints, plastics, and inks.1 Since DPP was first introduced as a building block in organic semiconducting materials,2 an enormous research effort has been dedicated to developing a broad variety of DPP-based molecular and polymer semiconductors for organic field-effect transistor (OFET)3−6 and photovoltaic (OPV) applications.7 The structural optimization of DPP-based materials involves tuning their electronic properties with various (hetero)aromatic substituents, on the carbon atoms, and their processability and solid state morphology with solubilizing chains, on the nitrogen atoms. DPP polymers currently rank among the best organic materials for high charge-carrier mobility.8 Lately, H-bonded organic dyes, including perylene bisimide,9 indigo,10 and quinacridone,11 have shown remarkable potential as organic semiconductors. In this context, N-unsubstituted DPP pigments that possess free NH group(s) for H-bonding are being revisited as materials for OFET devices.12−14 Strong aggregation of such H-bonding molecules poses a significant obstacle to their application in devices, which can be overcome by employing labile solubilizing groups that undergo in situ thermal or acid-induced cleavage after the semiconductor films have been processed. This approach has allowed the use of Hbonding DPP semiconductors, both small molecules15,16 and © 2017 American Chemical Society
Received: December 17, 2016 Revised: February 21, 2017 Published: February 24, 2017 2979
DOI: 10.1021/acs.chemmater.6b05327 Chem. Mater. 2017, 29, 2979−2987
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Chemistry of Materials to probe H-bonded molecular networks, where a remarkable control over symmetry, periodicity, porosity, disorder, and molecular orientation has been demonstrated.27−32 While a number of these studies include π-functional molecules as supramolecular building blocks,28,30,33 only a few involve active organic semiconductors, and the great potential of STM to probe dynamically growing semiconducting films has received limited attention in the field of organic electronics.21,34 Herein, we report the first systematic study of the supramolecular orderings of H-bonded diaryl-DPP derivatives (DFDPP, DPDPP, and DTDPP) both in monolayers at a solid−liquid interface (by STM) and in bulk crystalline solids (by single-crystal X-ray diffraction). The three (hetero)aromatic substituents were chosen for the H-bonding studies, because they present common monomeric units used in the design of DPP-based semiconducting polymers.3 We demonstrate the versatility of H-bonding of the DPP lactam group that, depending on the (hetero)aromatic substituents, can lead to homoassembly, enforcing intermolecular π−π interactions, or to previously unknown heteroassembly with alkanoic acids, permitting further control of the supramolecular structure in these materials. Exploring these experimental results with density functional theory (DFT) calculations, we show how the supramolecular structure at the interface is controlled by a fine interplay between the π−π and H-bonding interactions. We also discuss the manifestations of DPP chirality in surfaceconfined layers and correlate the monolayer structure at the interface, revealed by STM, with X-ray crystal packing and the gas-phase calculated structures of individual molecules and twodimensional (2D) networks.
Figure 1. STM images of DFDPP at the solution−HOPG interface in (a) octanoic acid (Iset = 200 pA; Vbias = −700 mV), unit cell of a = 1.0 ± 0.1 nm, b = 2.2 ± 0.1 nm, and α = 78 ± 2°; (b) nonanoic acid (Iset = 220 pA; Vbias = −600 mV), unit cell of a = 1.0 ± 0.1 nm, b = 2.3 ± 0.1 nm, and α = 78 ± 2°; (c) 1:4.7 dodecanoic acid/TCB (Iset = 220 pA; Vbias = −600 mV), unit cell of a = 1.0 ± 0.1 nm, b = 2.7 ± 0.1 nm, and α = 78 ± 2°; and (d) 1:4.7 octadecanoic acid/TCB (Iset = 260 pA; Vbias = −550 mV), unit cell of a = 1.0 ± 0.1 nm, b = 3.4 ± 0.1 nm, and α = 78 ± 2°. (e) Two enantiomeric domains of C8COOH···DFDPP coassembly, oriented 60° with respect to each other. The insets show the enlarged bright lamellar regions with the superimposed molecular models. Iset = 280 pA; Vbias = −500 mV. The arrows show the crystallographic axes of the HOPG substrate, and the red dotted lines indicate the long molecular axis of DFDPP, oriented approximately ±40° vs one HOPG crystallographic axis. (f) X-ray structure of a C8COOH···DFDPP cocrystal (pseudo-2D unit cell of a′ = 1.00 nm, b′ = 2.30 nm, and α′ = 78.5°).
of the molecular networks to the underlying HOPG, we see that the alkyl chains of C11COOH and C17COOH coassemblies lie in register with HOPG, while for shorter alkanoic acids (C7COOH and C8COOH), the aromatic lamellae are aligned with the main crystallographic axis of the HOPG and the corresponding alkyl chains are not (Figure 1a− d). This difference can be understood as an interplay of the surface interaction with aromatic versus aliphatic moieties: for longer alkyl chains, the surface−network interactions are dominated by adsorption of alkyl chains, while the weakened binding of shorter chain acids increases the relative importance of the DFDPP···HOPG interactions. We also note that no odd−even effect that leads to a different orientation of alkyl chains in the lamella in other H-bonded co-assemblies36 is observed in this case. DFDPP is a prochiral molecule and becomes chiral when adsorbed on a surface. The CnCOOH···DFDPP networks form homochiral domains, where the chiral information is transmitted along the lamellae by H-bonding and across the lamellae by interdigitation of alkyl chains (vide infra). The enantiomeric domains can be easily differentiated by the relative orientations of the molecular three-dot features in the bright lamellae (Figure 1e). Although amides are commonly observed to form H-bonds with carboxylic acids in bulk crystals, only a few examples of such have been recorded for lactams (cyclic amides).37 To better understand this particular H-bonding motif observed in the monolayers as well as to probe the similarities between 2D and three-dimensional (3D) assemblies, we grew DFDPP single
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RESULTS Self-Assembly of DFDPP. Applying a solution of DFDPP in octanoic acid (C7COOH) to HOPG leads to immediate formation of a periodically structured molecular network. STM imaging shows alternating bright/dark lamellae with the following unit cell dimensions: a = 1.0 ± 0.1 nm, b = 2.2 ± 0.1 nm, and α = 78 ± 2° (Figure 1a). The bright lamellae have the width and contrast appearance expected for the πconjugated DFDPP molecules, while the dark lamellae are more typical for interdigitated aliphatic chains.24,31,35 Thus, we hypothesized that DFDPP co-assembles with the solvent molecules (octanoic acid) and tested this hypothesis by studying assembly of DFDPP in a series of longer alkanoic acids [C8COOH, C11COOH, and C17COOH (Figure 1b−d, respectively)]. In each case, lamellar patterns are formed with the unit cell having the same vector a and angle α, but the vector b varying with the length of the acid, as expected (Figure 1a−d). High-resolution STM images of DFDPP assemblies in the presence of higher alkanoic acids (C11COOH and C17COOH) show submolecular features (CH2 fragments) of the alkyl chains, which are oriented almost parallel to unit cell vector b (Figure 1c,d). Hence, spatial separation of DFDPP lamellae can be periodically modulated by choosing an alkanoic acid with a different chain length. Correlating the STM images 2980
DOI: 10.1021/acs.chemmater.6b05327 Chem. Mater. 2017, 29, 2979−2987
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Figure 2. (a) Large scale STM image of the DPDPP homoassembly at the octanoic acid−HOPG interface. Iset = 250 pA; Vbias = −550 mV. (b) Crystal plane (side and top views) in a DPDPP single crystal40 that resembles the 2D self-assembled network (pseudo-2D unit cell a′ = 0.73 nm, b′ = 1.51 nm, and α′ = 76.4°). (c) High resolution STM image of a racemic domain of DPDPP homoassembly at the octanoic acid−HOPG interface with the following unit cell dimensions: a = 0.7 ± 0.1 nm, b = 1.6 ± 0.1 nm, and α = 75 ± 2°. Iset = 250 pA; Vbias = −550 mV.
crystals from a nonanoic acid solution. X-ray diffraction analysis reveals that DFDPP cocrystallizes with C8COOH, forming triclinic crystals (space group P1̅, a = 5.26 Å, b = 8.26 Å, c = 18.35 Å, α = 79.0°, β = 85.4°, and γ = 86.0°) with one DFDPP and two nonanoic acid molecules in the unit cell (Table S2). In the crystal structure, each DFDPP is H-bonded with two C8COOH molecules via R22(8)38 synthons. These complexes form continuous sheets via furan···furan dipolar interactions and multiple van der Waals (vdW) contacts of interdigitated alkyl chains, affording a pseudo-2D unit cell almost matching that found by STM (a′ = 1.00 nm, b′ = 2.30 nm, and α′ = 78.5°, Figure 1f). Self-Assembly of DPDPP. Replacing furans with less polar benzene rings leads to a completely different self-assembly. At both alkanoic acid−HOPG (C7COOH, C8COOH, and C 11 COOH) and TCB39−HOPG interfaces, DPDPP is consistently observed to form a structure of densely packed bright lamellae with the following unit cell dimensions: a = 0.7 ± 0.1 nm, b = 1.6 ± 0.1 nm, and α = 75 ± 2° (Figure 2 and Figure S2); this is consistent with a homoassembly model. We have also attempted to cocrystallize DPDPP with nonanoic acid but obtained only a polycrystalline powder with an X-ray diffraction pattern matching that of pure DPDPP homocrystals40 (Figure S1). The crystal structure of the latter matches the STM data: both show that DPDPP assembles into an Hbonded polymer chain via R22(8) interaction of lactam groups (Figure 2b). The large-scale STM image (Figure 2a; see also the 2D-FFT analysis in Figure S2d) shows that DPDPP homoassembles into columns with a unit cell (a = 0.7 ± 0.1 nm, b = 1.6 ± 0.1 nm, and α = 75 ± 2°) similar to that of the H-bonded layers of the 3D crystal (a′ = 0.73 nm, b′ = 1.51 nm, and α = 76.4°). However, higher-resolution STM of smaller areas reveals orientational disorder of DPDPP: the long molecular axes align at approximately ±80° with respect to the column axis (a direction) (Figure 2c). Molecular modeling suggests that the different tilting results from the prochiral DPDPP adopting two enantiomeric states (R and S) on the surface (see superimposed molecular models in Figure 2c). Statistical analysis of the DPDPP orientations in the homoassembled domains shows an approximately equal occurrence of 80° and −80° tilted molecules, indicating that DPDPP forms randomly intermixed
achiral domains. It can also be seen from STM images that the tilt (chirality) of the molecules is maintained along the Hbonded lamellae; this is expected because the R22(8) bonding cannot occur between the opposite enantiomers (see Figure S3). On the other hand, the enantiomeric H-bonded lamellae interact with each other via nondirectional weak vdW contacts of the Ph rings, and thus, the chiral information in not transmitted along this direction. Indeed, a structurally related bis(p-chlorophenyl)DPP, in which phenyl rings expeirence stronger dipole−dipole interactions, forms homochiral 2D domains on HOPG.41 Self-Assembly of DTDPP. Isostructural to DFDPP, DTDPP forms a similar co-assembled lamellar structure (a = 1.0 ± 0.1 nm, b = 2.2 ± 0.1 nm, and α = 80 ± 2°) at the octanoic acid−HOPG interface (Figure 3). As observed for
Figure 3. (a) High-resolution STM image of the DTDPP···C7COOH network on HOPG (a = 2.2 ± 0.1 nm, b = 1.0 ± 0.1 nm, and α = 80 ± 2°). Iset = 260 pA; Vbias = −500 mV. (b) Crystal plane in the DTDPP··· C11COOH cocrystal that resembles the 2D self-assembled network (pseudo-2D unit cell of a′ = 1.04 nm, b′ = 2.62 nm, and α′ = 78.0°).
DFDPP, changing the solvent from C7COOH to C11COOH leads to a unit cell expansion along the b direction (Figure S4). X-ray diffraction analysis reveals that DTDPP forms triclinic cocrystals (space group P1̅, a = 5.35 Å, b = 8.45 Å, c = 20.82 Å, α = 99.5°, β = 90.1°, and γ = 94.6°) with dodecanoic acid. The lengths of NH···OC and CO···HO H-bonds in these are 2.03 and 1.84 Å, respectively, similar to those observed in DFDPP− nonanoic acid cocrystals (2.01 and 1.84 Å, respectively). Deposition of a hot (∼80 °C) DTDPP suspension in TCB (nominal concentration of ∼10−5 M) onto HOPG leads to 2981
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homoassembly and, likewise, is attributed to the presence of the two enantiomeric DTDPP lamellae. A different arrangement of R and S enantiomers leads to multiple diastereomeric clusters, differentiated by the mutual orientation of the (highcontrast) S atoms. Mapping the STM images using these diastereomeric models (Figure 4c and Figure S8) allows us to identify the chirality of each molecule and provides further support for the proposed supramolecular model.
formation of two polymorphs, I and II, distinguished by different intercolumnar spacings [1.6 and 2.0 nm, respectively (Figure 4a)]. Phase I has the same unit cell (a = 0.7 ± 0.1 nm, b
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DISCUSSION The three DPP derivatives have shown very different selfassembly behavior that can be explained by different rigidity and intermolecular interactions of these structural motifs. Gasphase DFT calculations (Table 1) of unsubstituted DPP Table 1. DFT-Calculated [M06-2X/6-31G(d)] Enthalpies of ArDPP···ArDPP and C7COOH···ArDPP···C7COOH HBonding Complexes EHb [per R22(8)]b(kcal/mol)
ArDPP
conformation
molecular straina (kcal/mol)
DPP DFDPP
n/a Z E n/a Z E
0 0 9.04 0 0 2.95
DPDPP DTDPP
Figure 4. (a) STM image of two coexisting DTDPP polymorphs at the TCB−HOPG interface. Iset = 250 pA; Vbias = −600 mV. (b) Highresolution STM image of polymorph I with the following unit cell dimensions: a = 1.6 ± 0.1 nm, b = 0.7 ± 0.1 nm, and α = 75 ± 2°. Iset = 240 pA; Vbias = −650 mV. (c) High-resolution STM image of polymorph II with a spacing of 2.0 ± 0.1 nm along x and 0.7 ± 0.1 nm along y. Iset = 260 pA; Vbias = −600 mV. The superimposed grids indicate molecular chirality. The R′ and S′ notation refers to the DTDPP diastereomers with sulfur atoms facing the HOPG substrate. (d) Molecular models of six DTDPP diastereomeric trimers for phase II. For the sake of illustration, the protruding S atoms in DTDPP are assigned to the vertices of parallelograms.
homoassembly
heteroassembly with acid
−18.34 −15.93 −23.29 −21.03 −18.55 −22.32
−18.92 −18.09 −20.67 −20.45 −19.11 −20.50
a The enthalpy difference between the calculated and the most stable conformations. bThe H-bonding energy is calculated as EHb = Edimeropt − 2E molecule opt for homoassembly and E Hb = (E complex opt − ΣEmoleculesopt)/2 for heteroassembly.
indicate a very similar strength of dimeric DPP···DPP (18.3 kcal/mol) and DPP···COOH (18.9 kcal/mol) R22(8) Hbonding. To investigate the effect of aryl substituents on Hbonding assembly of DPP, we have analyzed the intra- and intermolecular binding energies of the three diarylDPP derivatives as single molecules, H-bonded complexes, and 2D crystals [periodic boundary conditions (PBC)]. Trans versus Cis Conformation. Both DFDPP and DTDPP have polarized heteroaromatic rings, and thus, their s-cis (Z) and s-trans (E) conformers (with respect to the DPP− aryl inter-ring bond) are energetically inequivalent. DFT calculations predict that the Z conformation is preferred for both DFDPP (9.0 kcal/mol) and DTDPP (3.0 kcal/mol) (Figure 5a). The reason for this difference is apparent from an electrostatic potential map (Figure 5b) that shows electrostatic repulsion between the negatively charged heteroatom (oxygen or sulfur) and the carbonyl oxygen in the E conformer, but electrostatic attraction (weak H-bonding) between the heteroaromatic proton and carbonyl oxygen in the Z conformer. In accord with the DFT analysis described above, the Z conformation is the only one observed in the reported crystal structures of N-alkylated DFDPP43 and DTDPP derivatives.,14,20 It was thus surprising to see the E conformation of the furan rings in the DFDPP···C8COOH cocrystals44 (Figure 1f). To understand this discrepancy and gain a deeper insight into the observed self-ssembly, we performed detailed DFT calculations of enthalpy of H-bonded assembly in the DPP
= 1.6 ± 0.1 nm, and α = 75 ± 2°) as DPDPP, suggesting a similar homoassembly arrangement (Figure 4b). However, the intercolumnar distance of phase II (2.0 nm) exceeds the vdW length of DTDPP (1.6 nm), which challenged its structural interpretation. Multilayer formation is not likely as the STM contrast does not indicate any height differences between the two phases. After considering different scenarios,42 we concluded that DTDPP must adopt a nonplanar geometry to achieve such packing (Figure 4d). The periodicity within the Hbonded lamella (a = 0.7 nm) is approximately twice that of a typical π−π stacking distance, which allows thiophene rings from the neighboring lamellae to interdigitate, forming continuous π-stacks. One should expect antiparallel alignment of the adjacent thiophene rings (which belong to different lamellae), to maximize the electrostatic interactions within the π-stack. In the proposed model, the highest STM contrast is expected for the protruding S atoms. Because of the antiparallel orientation of neighboring thiophene rings, every second DTDPP column must have low contrast and thus is not visible to STM, explaining the experimental 2.0 nm periodicity (Figure 4d). The high-resolution STM image of phase II reveals a liquidcrystal-like disorder, in which the translational symmetry between the H-bonded lamellae is absent (Figure 4c). This behavior is similar to that observed in the DPDPP 2982
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Figure 5. (a) Calculated [M062X/6-31G(d)] relative energy vs twist angle of one aryl substituent in diarylDPP derivatives. (b) Electrostatic potential map of DFDPP, DTDPP, and DPDPP in Z and E conformations. The isosurface value is 0.0004. ESP extrema: red for −0.05 eV and blue for 0.05 eV.
calculated total binding energies of the Z conformers of both DFDPP (15.9 kcal/mol) and DTDPP (18.7 kcal/mol) are very similar to those of the isolated H-bonded dimers (Table 1), suggesting that pairwise H-bonding is the main driving force for these assemblies. Indeed, the only additional interactions observed in these homoassemblies are a few H···H vdW contacts between the aromatic substituents (Figure 6 and Figure S11). In contrast, heteroassembled structures allow for strong secondary intermolecular interactions of neighboring furan [A1, A2 (Figure 6a,b, left panel)] and thiophenes (B1, B2), in addition to multiple vdW contacts of interdigitated alkyl chains. In particular, the head-to-head oriented furans form specific CHF···OF pairwise interactions [A1 (Figure 6a, left panel)] that behave as a weak R22(6) H-bonding synthon. The gained aryl··· aryl interactions (12.3 kcal mol−1 for DFDPP, 9.7 kcal mol−1 for DTDPP) are more than sufficient to overcome the intrinsic molecular strain of the E conformation and significantly enhance the total binding energies. However, these interactions cannot be realized in heteroassemblies of (Z)-DFDPP and (Z)DTDPP (Figure S12),45 thus explaining the prevalence of the E conformation in DFDPP/DTDPP H-bonding assembly with alkanoic acids. Heteroassembly versus Homoassembly. We performed a quantitative comparison of the stabilization energies of the two types of self-assemblies (Table 2). In the DFDPP··· C7COOH co-assembly structure, the total intermolecular interactions per unit cell amount to 62.8 kcal/mol. This originates from H-bonding of DFDPP with two octanoic acids (EHb = 2 × 22.8 kcal/mol), furan···furan interactions (EAr = 12.3 kcal/mol), and vdW interactions of alkyl chains (EAlk = 5.0 kcal/mol) in the aliphatic lamellae (see Methods for the derivation of each energy term). In comparison, the DFDPP homoassembly is stabilized only by the lactam R22(8) Hbonding (15.9 kcal/mol) (not counting the surface adsorption energy). This H-bond is destabilized by the repulsions of negatively polarized carbonyl and furan oxygens (Figure 6a, contact A′1) and thus is weaker than the acid−lactam H-bond (22.8 kcal/mol). When normalized by the occupied surface area [Eb/A (Table 2)], the DFDPP···C7COOH phase appears to be more stable than the homoassembly by 10.8 kcal mol−1 nm−2. According to molecular mechanics calculations, the adsorption energy of the DFDPP···C7COOH co-assembled structure (25.8
series. Gas-phase calculations of homoassemblies (ArDPP··· ArDPP) and heteroassemblies (ArDPP···C7COOH) predict a stronger H-bonding in E conformer of DFDPP and DTDPP (Table 1). This can be attributed to secondary intermolecular interactions between the carbonyl oxygen and the heteroaromatic ring, which is attractive in the E conformation (C O···H−C/CS···H−C, contacts A3/B3 in Figure 6) and
Figure 6. DFT molecular models of the observed and hypothetical polymorphs of (a) DFDPP, (b) DPDPP, and (c) DTDPP derivatives, in homoassemblies (right) and heteroassemblies (left). Labels A1−A3, B1−B4, C1, B′2, and C′1 indicate stabilizing intermolecular contacts. Labels A′1 and B′1 indicate repulsive interactions.
repulsive in the Z conformation (CO···O/CO···S, contacts A′1/B′1 in Figure 6, right panel). However, on its own, this increased stabilization (2 × 2.58 kcal/mol for DFDPP···C7COOH) seems insufficient to overcome the intrinsic strain of the E conformation (9.04 kcal/mol for DFDPP). To account for all intermolecular interactions, we performed PBC calculations for the 2D crystals of Ar2DPP homo- and heterocomplexes (Table 2). In the homoassemblies, the 2983
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Table 2. DFT PBC [M06-2X/6-31G(d)] Models of 2D Homo- and Heteroassemblies of DFDPP, DTDPP, and DPDPP and Derived Interaction Energies (experimentally observed networks shown in brackets) network E-DFDPP···C7COOH E-DPDPP···C7COOHh E-DTDPP···C7COOH Z-DFDPP···DFDPP Z-DPDPP···DPDPP Z-DTDPP···DTDPP
unit cell, a(nm), b (nm), α (deg) 0.99, 2.13, 76.4 [1.0 ± 0.1, 2.2 ± 1.06, 2.26, 85 [not observed]g 1.03, 2.16, 74.7 [1.0 ± 0.1, 2.2 ± 0.73, 1.47, 72.9 [not observed]g 0.73, 1.60, 71.3 [0.7 ± 0.1, 1.6 ± 0.73, 1.51, 79.6 [0.7 ± 0.1, 1.6 ±
Aa (nm2)
ETOTb (kcal/mol)
EHbc [per R22(8)] (kcal/mol)
EArd (kcal/mol)
Eb/Ae (kcal mol−1 nm−2)
Eads/Af (kcal mol−1 nm−2)
2.05
−62.8
−45.6 (22.8)
−12.3
−26.2
−25.8
2.39
−43.3
−42.0 (21.0)
−2.8
−18.1
−23.1
2.15
−57.1
−42.6 (21.3)
−9.7
−25.2
−25.5
1.03
−15.9
−15.9
−
−15.4
−21.2
1.11
−21.5
−21.0
−
−19.4
−24.5
1.08
−18.7
−18.6
−
−17.3
−23.3
0.1, 78 ± 2]g
0.1, 80 ± 2]g
0.1, 75 ± 2]g 0.1, 75 ± 2]g
a
Unit cell area. bEnergy of all intermolecular interactions. cThe H-bonding energy per unit cell (or per H-bonding synthon, in parentheses); the derivation of energies of individual interactions is explained in Methods. dEnergy of Ar···Ar interactions. eAreal density of energy of intermolecular binding Eb (accounting for the internal strain of the Z conformation). fAreal density of adsorption energy determined with the MM+ force field. g Experimental unit cell. hThis PBC calculation has not converged; the network was modeled via cluster calculations (Figure 6b, left panel).
kcal mol−1 nm−2) is also higher than that of its homoassembled phase (21.1 kcal mol−1 nm−2). Thus, our calculations predict that the DFDPP co-assembly with alkanoic acids should predominate over its self-assembly, in agreement with the experiment. The same arguments can be applied to the comparison between hetero- and homoassemblies of DTDPP, although the energetic preference for the former is somewhat weaker than that of DFDPP (Table 2). In contrast to the heteroaryl-DPP derivatives, DPDPP does not co-assemble with alkanoic acids but forms only homoassemblies. Such behavior is predicted by calculations that show a slightly higher intermolecular binding density Eb/A for homoassembly (19.4 kcal mol−1 nm−2) versus heteroassembly (18.1 kcal mol−1 nm−2) as well as a higher adsorption energy for homoassembly (24.5 kcal mol−1 nm−2) versus heteroassembly (23.1 kcal mol−1 nm−2) (Table 2). This is attributed to the steric repulsion of larger phenyl rings (compared with furan and thiophene), preventing DPDPP from achieving a closely packed lamella found in the coassembled DFDPP···CnCOOH and DTDPPP···CnCOOH structures (Figure 6b and Table 2). Overall, in the presence of alkanoic acids, both DFDPP and DTDPP favor co-assembly with the acids, while DPDPP prefers homoassembly. The secondary interactions of aryl substitutents are those that tip the balance between the formation of homoand heteroassemblies. The preference for hetero- versus homoassembly correlates well with the strength of the aryl··· aryl interactions (EAr) permitted in the former. DFDPP with the highest EAr (12.3 kcal/mol) forms exclusively heteroassemblies; DPDPP with the lowest EAr (2.8 kcal/mol) forms exclusively homoassemblies, and DTDPP with an intermediate EAr (9.7 kcal/mol) can assemble in both homo- and heterostructures, depending on the conditions. Planar versus Twisted. All three Ar2DPPs possess highly rigid conjugated structures. As depicted in Figure 5, the calculated twisting potential is the steepest for DFDPP (12.0 kcal/mol per ring), followed by DTDPP (9.0 kcal/mol) and DPDPP (7.5 kcal/mol). The higher rigidity of oligofurans versus oligothiophenes was previously attributed to the more quinoidal character of the former.46 This is also in agreement with the length of inter-ring C−C bonds for DFDPP (1.43 Å),
DTDPP (1.44 Å), and DPDPP (1.46 Å); therefore, the least aromatic furan moiety provides the most extended πconjugation in its oligomers and/or polymers. The question is whether these (significant) calculated twisting barriers in the gas phase reliably predict the molecular structure in a condensed phase. All reported X-ray single-crystal structures of DFDPP and DTDPP derivatives indeed show a completely planar Ar2DPP core. In thin films and interfacial layers, diffraction methods do not provide such structural details, but the planarity of the Ar2DPP moiety is commonly presumed. Nevertheless, our high-resolution STM images (Figure 4) strongly suggest a twisted conformation in DTDPP polymorphs II. Molecular modeling of this polymorph requires an ∼65° twist of the thiophene···DPP bond (Figure 6c) that, according to DFT, results in an ∼7 kcal/mol intramolecular strain (per thiophene ring). This geometry is stabilized by π-stacking of the thiophene rings47 (oriented edge on vs the surface plane) and weak H-bonding between the αhydrogen of the thiophene and the amide oxygen (Figure 6c, contact C′2). Furthermore, polymorph II provides a much higher molecular density on HOPG (1.45 molecules/nm2 vs 0.92 molecules/nm2 for polymorph I) and, possibly, better solvation of the adsorbed molecules (because of polar heteroatoms pointing away from the surface). While an accurate quantitative assessment of these interactions is challenging,47 the STM observations suggest that their collective contribution may outweight the strain of the twisted conjugated chain. We speculate that for absorption on gold (a common electrode in orgain electronics), the specific S···Au interaction can further stabilize the twisted conformation of DTDPP-based materials.
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CONCLUSION This paper documents the first comparative study of the Hbonding assembly of DPP model derivatives, in single crystals and surface-supported monolayers, providing new insights into supramolecular interactions within the growing field of Hbonded semiconductors. While H-bonding is the main and strongest stabilizing interaction in these asssemblies, their structures vary dramatically depending on the nature of the pendant heteroaromatic substituents (furan, phenyl, and 2984
DOI: 10.1021/acs.chemmater.6b05327 Chem. Mater. 2017, 29, 2979−2987
Article
Chemistry of Materials
clusters (four to eight molecules, as shown in Figure 6 and Figure S11), using the experimentally observed unit cell as a starting geometry before the optimization. For cluster calculations, plane symmetry Cs was imposed to mimic the on-surface assembly. The comparisons of intermolecular interactions of homo- and heteroassemblies of DFDPP and DTDPP in the Z and E conformations are given in Figures S11 and S12. The periodic structures of the three Ar2DPP homo- and heteroassemblies were optimized by PBC DFT calculations. The total enthalpy of intermolecular interactions (ETOT) and the intermolecular binding energy (Eb) were calculated as
thiophene). We demonstrate a new co-assembly through Hbonding of the DPP lactam group with a carboxylic acid, which persists in both single crystals and interfacial layers. The strong preference of DFDPP to adopt such a heteroassembly, the exclusive homoassembly of DPDPP, and the ambivalence of DTDPP are analyzed using DFT calculations that explain the observed difference in terms of the balance between the secondary (Ar···Ar, alkyl···alkyl, and specific heteroaryl··· carbonyl) interactions. In particular, this work emphasizes the significant role of furan’s dipolarity allowing specific furan··· furan R22(6) H-bond-like interactions. Our results suggest an unexpected tendency of DTDPP derivatives to assemble on graphite in a twisted conformation that provides for stronger intermolecular interactions and higher molecular density. To the best of our knowledge, this is the first case in which surface adsorption of a planar molecule enforces the less favorable (in the gas phase) twisted geometry. This finding is contrary to DFT analysis predicting a rigid planar structure in the gas phase of DTDPP and to X-ray crystallographic structures, emphasizing the need for submolecular-resolution studies in thin films for the design of organic electronic materials. Inter-ring twisting in DPP-based semiconductors is particularly relevant for their application in OFETs, where the structure of the interfacial layer can dominate the charge-transport properties of the device. Last but not least, the discovered co-assembly of DFDPP or DTDPP with alkanoic acids can potentially serve as a new tool for supramolecular control of DPP-based materials, aiding in processing of poorly soluble H-bonded semiconductors or templating a desired morphology on their films.
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E TOT = E PBCopt − ΣEconst opt , E b = E TOT − ΣEstrain where EPBCopt is the energy of the optimized PBC structure, Econstopt is the energy of the gas-phase optimized individual molecules constituting the unit cell, and Estrain is the energy difference between the given and the most stable conformers of DFDPP and DTDPP molecules. Individual energy terms listed in Table 2 were derived as illustrated for DFDPP···C7COOH in Figure 7 and eqs 1 and 2. The molecular
Figure 7. Schematic representation of the clusters used in the calculations of the different types of intermolecular interactions (EHb, EAr, and EAlk).
METHODS
STM Measurements. STM experiments were performed on a Nanoscope Multimode 8 with A-scanner and standard STM scanning head at room temperature using a mechanically cut Pt/Ir tip. All solution samples (6 μL) were applied to the basal (0001) plane of freshly cleaved HOPG. The self-assembled molecular network (SAMN) was visualized immediately by STM using constant current mode at the liquid−solid interface. Molecular resolution was achieved at negative tip biases. Image analysis and calibration were performed using WSxM software.48 Sample Preparation. DFDPP49 and DPDPP50 were synthesized as described previously. DTDPP (97%), octanoic acid (≥98%), nonanoic acid (≥97%), dodecanoic acid (≥98%), and 1,2,4trichlorobenzene (≥99.0%) were purchased from Sigma-Aldrich and used without further purification. For all three diarylDPPs, 10−4 M solutions in alkanoic acid or 1,2,4-trichlorobenzene were employed. For co-assembly of diaryl-DPP with solid dodecanoic acid, 1,2,4trichlorobenzene solutions containing a nominal concentration of 10−4 M diarylDPP and 1.73 M dodecanoic acid were used. Single-Crystal Analysis. Needlelike single crystals of DFDPP··· C8COOH and DTDPP···C11COOH were grown by slow evaporation from saturated solutions in C8COOH and a C11COOH/TCB (1:4.7) mixture, respectively, at room temperature. Suitable crystals were set on a Cryoloop and mounted on a Bruker Venture Metaljet diffractometer. The crystals were kept at 105 K during data collection. Powder X-ray Diffraction (PXRD). Data were collected at room temperature using a Bruker D2 phaser X-ray diffractometer equipped with a Ni-filtered Cu Kα source operated at 30 kV and 10 mA and a LYNXEYE position-sensitive detector. See the Supporting Information for the PXRD pattern and interpretation. Molecular Modeling. All DFT calculations were performed using the Gaussian 09 program package.51 The M06-2x functional with the 6-31G(d) basis set was used to account for short-range dispersion interactions.52,53 The supramolecular structures were modeled as infinite 2D crystals [periodic boundary conditions (Figure 6)] and as
cluster was built from four optimized (in PBC calculations) unit cells. Then single-point energy calculations with the same functional and basis set as PBC calculations were performed on this cluster (E1SP) and the other three clusters with selectively removed molecules (E2SP, E3SP, and E4SP). This approach retrives the intermolecular interactions experienced by the removed molecules and allows mathematical derivation of individual energy terms, using eq 1.
⎡ ⎤ E1SP − E2 SP − E DFDPPopt ⎥ ⎡ 2 2 0 ⎤⎡ E Hb ⎤ ⎢ ⎥ ⎢ opt ⎢ E SP − E SP − 2E ⎥ ⎥ E ⎢ = 1 3 C7COOH ⎥ ⎢ 2 0 2 ⎥⎢ Ar ⎥ ⎢ ⎣ 0 2 2 ⎦⎢⎣ E ⎥⎦ ⎢ SP ⎥ SP opt Alk ⎢⎣ E1 − E4 − E DFDPP‐2C7COOH ⎥⎦
(1)
Counterpoise correction was not applied as it often overcompensates the basis set superposition error. The inverse of the matrix in eq 1 gives the matrix in eq 2, from which EHb, EAr, and EAlk are derived. The sum of these three terms corresponds to the ETOT mentioned above. ⎡ ⎤ E1SP − E2 SP − E DFDPPopt ⎡ E Hb ⎤ ⎥ ⎡ 1 1 −1⎤⎢ ⎢ ⎥ SP SP opt ⎥ ⎢ ⎥⎢ ⎢ EAr ⎥ = 1/4⎢ 1 −1 1 ⎥⎢ E1 − E3 − 2EC7COOH ⎥ ⎢ ⎥ ⎣−1 1 1 ⎦⎢ SP SP opt ⎥ ⎣ EAlk ⎦ − − E E E 4 DFDPP ‐ 2C7COOH ⎦ ⎣ 1
(2) The surface adsorption energy of molecular adlayers was evaluated with the MM+ force field (HyperChem 8) by placing individual molecules on a graphene cluster (120 atoms) and performing geometry optimization until a root-mean-square energy gradient of
