Long-Range Molecular Self-Assembly from Ï-Extended Pyrene

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Long-Range Molecular Self-Assembly from #Extended Pyrene-Functionalized Diketopyrrolopyrroles Ke Zhang, Philipp Wucher, Tomasz Marszalek, Maxime Babics, Andreas Ringk, Paul W. M. Blom, Pierre M. Beaujuge, and Wojciech Pisula Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b01276 • Publication Date (Web): 09 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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Chemistry of Materials

Long-Range Molecular Self-Assembly from π-Extended PyreneFunctionalized Diketopyrrolopyrroles Ke Zhang,a Philipp Wucher,b Tomasz Marszalek,a,c Maxime Babics,b Andreas Ringk,b Paul W. M. Blom,a Pierre M. Beaujuge*,b and Wojciech Pisula*,a,c a

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

b

Physical Sciences and Engineering Division, KAUST Solar Center (KSC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia

c

Department of Molecular Physics, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland ABSTRACT: End-group-directed molecular self-assembly can be an effective strategy in the design of “organic electronics” amenable to specific morphologies and charge transport patterns for device applications. In this study, we report on the design, selfordering and transistor characteristics of an analogous set of pyrene-functionalized diketopyrrolopyrroles (DPP; namely SM1-3) obtained by “successive incorporation” of DPP motifs. The well-defined pyrene-substituted DPP analogues are systematically examined in correlation of (i) the number of incorporated DPP motifs in the π-extended main-chain and (ii) the solution-processing conditions employed for the thin film formation. Solvent vapor enhanced drop-casting (SVED) from chloroform (CHCl3) and tetrahydrofuran (THF) are found to promote very distinct, long-range morphologies of SM1-3. During THF mediated SVED, the growth of one dimensional fiber of SM1-3 origins from the formation of initial aggregates in THF. In particular, extending the πconjugation of the DPP core and, concurrently, the number of alkyl side-chains involved, is found to mitigate the long-range selfassembly of SM1-3 and, in turn, to lower crystal size and fiber length. Field-effect transistors based on SM3 exhibit ambipolar behavior, demonstrating the relevance of pyrene-functionalized diketopyrrolopyrroles in the design and development of solutionprocessed ambipolar small-molecule semiconductors.

Introduction Solution-processed “organic electronics” designed from self-assembling materials with tunable optical and electronic properties offer a number of significant advantages over conventional inorganic semiconductors, including facile thin film depositions and greater mechanical conformability in electronic devices.1 Of all device applications, some have proven especially relevant to the integration of organic electronics: light-emitting diodes,2 photovoltaics,3 and fieldeffect transistors.4,5 Molecular semiconductors that can be solution-processed into thin-film devices may be the most promising class of organic materials being currently developed because of the well-defined molecular structures and high levels of purity achievable with those, whereas πconjugated polymers tend to suffer from batch-to-batch variations and more extensive purifications (lowering their yields). Most solution-processable π-extended small molecules with meaningful carrier mobilities transport only one type of carriers (holes or electrons), and organic field effect transistors (OFETs) made thereof are said to be unipolar. Some of

the best molecular organics have reached carrier mobilities >10 cm2 V-1 s-1, for both holes and electrons.6,7 In contrast, molecular semiconductors with ambipolar characteristics are not common, and in general their hole and electron mobilities have remained modest and/or unbalanced. Molecular semiconductors with established ambipolar behavior were in general vacuum-sublimed into films, and the few reported instances of solution-processable analogues have shown fairly low performance figures thus far.8-10 One of the main difficulties in achieving efficient charge transport with small molecules is related to the requirement for a high valence band edge and/or a low conduction band edge conducive to a sufficiently low band gap.11 In turn, rational molecular design principles geared to the development of solutionprocessable ambipolar semiconductors remain at the forefront of research on organic electronics. Interestingly, the performance of π-conjugated small molecules does not only depend on their energy band pattern, it also depends on their molecular packing characteristics, their structural order over various lengthscales and their overall morphology in thin films.12,13 To improve molecular ordering and induce specific morphologies, one strategy is to tune the

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intermolecular and molecule-solvent interactions14 That are prone to formation of intermolecular aggregation in solution.15 By selecting appropriate solution-processing techniques, such as solvent vapor enhanced drop-casting (SVED) and solvent vapor diffusion (SVD), it is possible to control the self-assembly of molecular systems.16,17 For instance, dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5b′]dithiophene18 can self-assemble and be grown into micrometer-size crystals, while cyclopentadithiophene-benzothiadiazole17 can form highly ordered single polymer fibers via the use of SVD and SVED techniques. In earlier studies, solution-processable DPP-based small molecules have been described as promising materials for organic photovoltaics (OPVs) and OFETs.19 Amongst those, C2-symmetric pyrene-functionalized DPPs have been reported as self-assembling via pyrene end-group interactions, promoting π−π intermolecular interactions and crystal packing alignment.20 In bulk-heterojunction (BHJ) solar cells, those same analogues afforded power conversion efficiencies of up to 4.1% and high device fill-factors of ca. 60%.20 Turning to OFETs, a few instances of DPP-based ambipolar small molecules with added strong electron-deficient units have been described, leading to balanced hole and electron mobilities of 0.01 cm2 V-1 s-1.21,22 In separate work, hydrogenbonding DPP-based analogues with pronounced intermolecular interactions and suppressed “lowest unoccupied molecular orbital” (LUMO) energy level exhibited ambipolar behavior as well, with hole and electron mobilities as high as 0.01 cm2 V-1 s-1.23 Therefore, DPP-based πconjugated small molecules are promising candidates for use in ambipolar OFETs, but an efficient, rational molecular design approach remains to be developed to further improve their ambipolar carrier transport characteristics towards higher mobility values and a controlled self-assembling process. In this report, we demonstrate that an incremental extension of π-conjugation in pyrene-functionalized diketopyrrolopyrroles (DPP; namely SM1-3 in Chart 1) obtained by “successive incorporation” of DPP motifs mitigates longrange self-assembly, while inducing a transition from unipolar to ambipolar charge transport characteristics in OFETs. Distinct crystal morphologies for SM1-3 can be obtained from SVED with CHCl3, whereas SVED with THF induces various fiber morphologies. The formation of one dimensional fibers results from initial aggregates of SM1-3 in THF. The high planarity of pyrene end-groups leads to π−π stacking interactions, while weak molecule-solvent interaction between pyrene and THF results in a reduced solubility. Extending the π-conjugation length by incorporating additional DPP motifs along the main-chain in SM1-3 suppresses the LUMO level, which in turn promotes electron injection from the transistor gold electrode into the active layer. Although this work focuses on the relation between molecular structure, processing and self-assembly and does not try to establish a new performance record for OFETs, SM3 exhibit hole and electron mobilities of ca. 0.06 cm2 V-1

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s-1 and ca. 0.02 cm2 V-1 s-1 for electrons, which are among the best ambipolar carrier mobility values reported thus far for solution-processed oligomeric systems.24 This transition from unipolar to ambipolar induced by the extension of πconjugation provides a generalizable route towards highmobility solution-processed ambipolar small-molecule semiconductors. Chart 1. Pyrene-functionalized diketopyrrolopyrrole (DPP) analogues SM1-3.

N S O

O S

N

SM1

N S O

O S

N

N S O

O S

N

SM2

N S O

N

O S

N S O

N

O S

N S O

O S

N

SM3

Results and Discussion Considering the importance of structurally ordered and interconnected domains on carrier transport and BHJ solar cell performance, the incorporation of functional end-groups that can promote π-stacking in SM derivatives and facilitate their long-range molecular self-assembly is a relevant approach deserving of further examinations.20,25-27 Chart 1 provides the molecular structures of the Pyrene-DPP analogues SM1-3 examined throughout this study, and Scheme S1 depicts the synthetic route to SM1-3 via incremental incorporation of DPP motifs and overall extension of π-conjugation (cf. experimental details in the SI). In our initial work, we showed that C2-symmetric pyrene end-groups appended to a DPP core (as in SM1) promotes molecular self-assembly via endgroup π-stacking interactions.20,27 In brief, Iridium(I)catalyzed C-H activation28, 29 was employed to synthesize the C2-symmetric pyrene boronate pinacol ester (5), and the functional pyrene end-group was subsequently reacted with mono- or bis-brominated DPP (4 and 3) by selective palladium-mediated Suzuki cross-coupling, yielding intermediate 6 and compound SM1, respectively. The palladium-catalyzed direct arylation reaction30-34 of the Pyrene-DPP intermediate 6 with 3 afforded the longest Pyrene-DPP analogue SM3, and the homocoupling product SM2 (Yields: ca. 20 %; onepot reaction). SM2 and SM3 were separated and purified by


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preparative size-exclusion chromatography (SEC), and the molecular structures were confirmed by 1H and 13C NMR spectroscopy. Further details are provided in the supporting information (SI), along with the optical and electrochemical parameters of SM1-3 (summarized in Table 1). Table 1. Optical and electrochemical parameters for SM1-3, estimated by PESA and thin-film optical absorption. PyreneDPPs

λsolua

λfilmb

Eoptd

IPc

EAe

[nm]

[nm]

[eV]

[eV]

[eV]

SM1

571, 610

600, 660

1.72

-5.08

-3.36

SM2

660, 692*

722, 801

1.39

-5.08

-3.69

SM3

694*, 730

778, 874

1.28

-5.15

-3.87

a

Optical absorption spectra in CHCl3 solution. b Optical absorption spectra in thin films. c Estimated from the optical absorption spectra (films). d Estimated by photoelectron spectroscopy in air (PESA). e Inferred from PESA-estimated IPs and Eopt values. *Peak of absorption shoulder.

In Figure 1a, the solution absorption of SM1 in CHCl3 shows two transition peaks (0-1 transition at ca. 571 nm and 0-0 transition at ca. 610 nm). The absorption peak of SM2 is observed at ca. 660 nm (0-1 transition), with a longwavelength shoulder at ca. 692 nm (0-0 transition). SM3 exhibits the absorption peak at ca. 730 nm (0-0 transition) and a short-wavelength absorption shoulder at ca. 694 nm (01 transition). The absence of defined spectral features in SM2 and SM3 suggests that higher side-chain density resulting from the incorporation of several DPP motifs reduces πaggregation. The thin-film optical absorption spectra of SM1-3 provided in Figures 1b-d show significant red-shifts as π-conjugation extends from SM1, to SM2 and to SM3: onsets of absorption shifting from ca. 700 nm, to ca. 900 nm, to ca. 1,000 nm in thin films, respectively. The appearance of

two vibronic peaks and significant red-shift for spin-coated SM1-3 films result from a much stronger aggregation in solid films than in solution. Planarization and π-electron delocalization effects are critically important parameters impacting the efficiency of charge transport in self-assembling π-conjugated systems.35 Figure 2 shows how delocalization effects are expected to occur in SM1-3; density functional theory (DFT) calculations using the gap-tuned, long-range corrected (LRC) hybrid functional ωB97XD/6-31G(d,p) (cf. computational details in SI, and in Tables S1, S2 and S3).36 In all three Pyrene-DPP analogues, the highest occupied molecular orbital (HOMO) and the lowest occupied molecular orbitals (LUMO) are well delocalized across the main-chains. As summarized in Table 1, the ionization potentials (IP) determined by photoelectron spectroscopy in air (PESA) for SM1-3 fall in the range (-)5.0-5.2 eV. The electron affinity (EA) estimates are provided from consideration of the IP and optical gap (Eopt) values inferred from the onset of thin-film absorption: here EA values increases from (-)3.36 eV to ()3.69 eV and to (-)3.87 eV upon extending the π-conjugation as in SM1, SM2 and SM3, respectively.

Figure 2. Representation of the frontier orbitals (HOMO, bottom; LUMO, top) for a) SM1, b) SM2, and c) SM3 obtained by density functional theory (DFT) modeling using the gap-tuned LRC-hybrid functional ωB97XD/6-31G(d,p).

Figure 1. a) Normalized solution (CHCl3) optical absorption spectra of SM1, SM2, SM3; solution and spin-coated film optical absorption spectra for b) SM1, c) SM2 and d) SM3.

To probe charge carrier transport in SM1-3, solutionprocessed OFETs were fabricated with a bottom-gate//topcontact geometry. Heavily doped silicon substrates with a 300 nm-thick thermally grown oxide dielectric modified by hexamethyldisilazane (HMDS) were used for the device fabrication. When SM1-3 were drop-cast from a CHCl3 solution at a concentration of 2 mg/ml, only inhomogeneous and disordered macroscopic patches could be obtained (Figure S1). Changing the concentration of SM1-3 in CHCl3 did not improve film quality. Turning to spin-casting, a homoge-



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Figure 3. POM images of images of (a,d) SM1, (b,e) SM2 and (c,f) SM3 layers obtained by solvent vapor enhanced drop-casting (SVED) from CHCl3 (top images) and THF (bottom images). Scale bars: 100 µm.

neous layers of SM1-3 could easily be achieved from the 2 mg/mL CHCl3 solution (Figure S2), and those were then subjected to thermal annealing at 150 °C to remove any residual solvent. Source and drain electrodes were deposited by gold evaporation. As shown in Figure S3 and Table S4, the hole mobility of spin-coated films of SM1 was found to be 5 × 10-4 cm2 V-1 s-1 and 2.5 - 4.0 × 10-3 cm2 V-1 s-1 for SM2 and SM3, respectively (Figure S3). These relatively modest mobility values in spin-coated films can be attributed to the limited molecular ordering occurring in the rapid spincoating and solidification steps (Figure S4). In order to improve the film-forming propensity and molecular organization of SM1-3 in solution-cast layers, a solvent vapor enhanced drop-casting (SVED) approach was followed to (i) minimize dewetting effects and (ii) mitigate the solvent evaporation rate.16 During SVED, the drop-cast solution, deposited on a HMDS-modified silicon substrate, was exposed to an atmosphere of saturated solvent vapor in a solvent container with a covered lid (nearly airtight). CHCl3 was selected as the solvent because of its relatively low boiling point of 61.2 °C and high partial pressure, and because of the good solubility the SM1-3 analogues in that solvent. And the solubility of SM1-3 gradually decreased with the increase of conjugation length. The Hansen solubility parameters of CHCl3, such as dispersion force, polar force and hydrogen bonding, are 17.8, 3.1 and 5.7, respectively.37 As SVED processing protocol, 0.15 mL CHCl3 solution of SM1-3 at a concentration of 2 mg/mL was cast on the substrate and was subsequently exposed to saturated CHCl3 vapor atmosphere. At the slow solvent evaporation rate during SVED, the concentration of SM1-3 increased leading to a gradual self-assembly of SM1-3. After 8 hours, SM1-3 solidified into semicrystalline films which were annealed at 150 °C for 60 min to remove residual solvent and to promote molecular ordering. In Figures 3a-c and 4a-c, the polarized optical microscopy (POM) and atomic force microscopy

(AFM) images of the thin films cast by SVED show the occurrence of very distinct morphologies and morphological lengthscales. In particular, crystal domain sizes are found to decrease on going from layers obtained with SM1, SM2 and with SM3, i.e. as π-conjugation length extends across the Pyrene-DPP analogues. CHCl3-mediated SVED leads to the formation of diamond-shaped crystals with SM1, with lengths and widths of 80-250 µm and 30-35 µm, respectively (thickness: 1.5-2 µm). In contrast, spherical-crystals are obtained with SM2, with diameters of 5-10 µm (thickness: 0.6-1 µm). Meanwhile, CHCl3-mediated SVED yields a grain-like morphology with SM3, with significantly smaller features of 1-2 µm in size. The large crystallites developed in SM1-based layers are characteristic of long-range macroscopic order and, as a general trend, microstructure size decreases for the more extended molecular analogues SM2 and SM3, pointing to an evident correlation between molecular length and crystal feature size. By increasing the number of DPP motifs along the main-chain of the SM analogues, the side-chain density also increases and, in turn, the selfassembly pattern of SM1-3 changes with an apparent mitigation of aggregation as the density of pyrene end-groups decreases on going from SM1 to SM2 and to SM3. The boiling point, polarity and solubility of the solvent greatly influence the thermodynamics and crystallization kinetics of π-conjugated, solution-processable molecules. To investigate the influence of solvent on the self-assembly of SM1-3, THF was taken as a second solvent for a direct comparison to CHCl3, considering that both solvents possess a similar boiling point (65.4 °C for THF and 61.2 °C for CHCl3) and close polarity (4.0 for THF and 4.1 for CHCl3). It is important to note however that the Hansen solubility parameter of THF is different in the dispersion force, polar force and hydrogen bonding values of 16.8, 5.7 and 8.0, respectively.37 In contrast to CHCl3, THF shows a lower


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Figure 4. Tapping-mode AFM images of (a,d) SM1, (b,e) SM2 and (c,f) SM3 layers obtained by solvent vapor enhanced drop-casting (SVED) from CHCl3 (top images) and THF (bottom images).

dispersion force, but a higher polar force and hydrogen bonding power, suggesting that the solubility of SM1-3 in THF may be lower than that in CHCl3. For this reason, a longer time is actually needed for the complete dissolution of SM1-3 in THF, compared to CHCl3. In addition, we observed a stronger propensity for SM1-3 to aggregate in THF during the SVED processing step, compared to what we observed for CHCl3-mediated SVED. Figures 3d-f and 4d-f show that in THF-mediated SVED protocols, SM1-3 self-order into various types of fibers in a wide range of lengthscale and aspect ratio. SM1 forms onedimensional wavelike-fibers, with lengths varying from 200 µm to 10 mm, while the widths of these fibers are 2-5 µm (thickness: 0.3-0.5 µm). As for SM2, needle-like fibers with smaller aspect ratios are formed that are 150 µm to 1 mm in length and 1-20 µm in width (thickness: 0.1-3 µm). In consistency with the outcome of the CHCl3-mediated SVED protocol, SM2 forms shorter fibers compared to SM1 (likely due to the same reasons), and SM3 forms the shortest fibers of 10-50 µm in length. These observations are in agreement with the correlation seen between molecular structure and selfassembly patterns for SM1-3 subjected to the CHCl3-mediated SVED protocol. The remarkable morphological differences observed comparing the self-assembly patterns of SM1-3 from CHCl3- and THF-mediated SVED is directly connected to the solubility and aggregation propensity differences of the SM1-3 analogues in the two solvents. As stated earlier, the solubility of SM1-3 in THF is slightly lower than in CHCl3, owing to the weaker molecule-solvent interactions between pyrene and THF. In Figure 5, the normalized solution optical absorption spectra of SM1-3 (0.01 mg/mL) in THF and CHCl3 imply that the aggregation of SM1-3 in different solvents governs the film morphology. The optical absorption of SM3 in THF

exhibits a long-wavelength shoulder at 810 nm in contrast to CHCl3, indicating SM3 tends to form initial aggregates during THF-mediated SVED. The A0-0/A0-1 ratios of SM1-2 in THF are slightly higher than in CHCl3, suggesting SM1-2 may be more prone to self-assembly into aggregates due to their reduced solubility in THF. As shown in Figure S5, the normalized concentration-dependent optical spectra in THF further confirms that raising the concentration of SM1-3 in THF from 0.0005 mg/mL to 0.01 g/mL increases the A0-0/A0-1 ratio of SM1-2 and results in a new shoulder at 810 nm of SM1-3, which is not observed in CHCl3. Therefore, during THFmediated SVED, the weaker molecule-solvent solvent and ππ stacking interactions contribute to the formation of initial aggregates of SM1-3 in dilute THF, favoring the growth of one-dimensional fibers.

Figure 5. Normalized optical absorption spectra of SM1-3 (0.01 mg/mL) in CHCl3 (dash line) and THF (solid line).


5


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Figure 6. GIWAXS patterns of (a,d) SM1, (b,e) SM2 and (c,f) SM3 layers obtained by solvent vapor enhanced drop-casting (SVED) from CHCl3 (top images) and THF (bottom images). Reflections related to the layer structure and π-stacking are indicated in the patterns.

The correlated effects of π-conjugation length and sidechain density on the self-organization of the SM analogues during the SVED treatment was further examined via grazing incidence wide-angle X-ray scattering (GIWAXS). The corresponding GIWAXS patterns are shown in Figure 6. The surface orientation of SM1-3 on the HMDS-modified SiO2 substrate, as well as the interlayer and π-stacking distance parameters are summarized in Table 2. The GIWAXS pattern for CHCl3-mediated SVED-processed SM1 films shown in Figure 6a possesses a notably high number of reflections characteristic of a crystalline film. The distinct out-of-plane scattering intensity at qz = 0.43 Å-1 for qxy = 0 Å-1 is assigned to an interlayer distance of 14.6 Å and a preferential edge-on organization where the long molecular axis is aligned in the in-plane direction, parallel to the surface. The surface arrangement of the molecules SM1-3 is derived from the azimuthal intensity integration of the main 100 interlayer peak – marked in Figure S6a, which can be quantified by the Rin/out parameter.38 This value is defined as a ratio between the in-plane and the out-ofplane intensity of a corresponding reflection. If Rin/out is equal 1, the azimuthal intensity distribution on the pattern is isotropic, indicating a random orientation of the crystals or domains on the surface. If the value is significantly higher or lower that 1, the film bears a preferential face-on or edge-on orientation of the molecules. For CHCl3-mediated SVEDprocessed SM1 films, a Rin/out = 0.02 points to an edge-on organization of the molecules relative to the substrate surface. The wide-angle off-equatorial reflection at qz = 0.47 Å-1 for qxy = 1.77 Å-1 is associated to a π-stacking distance of 3.5 Å between two pyrene units which are tilted by about 15° with respect to the out-of-plane direction. This angle can be directly derived from the position of the maximum peak intensity of the π-stacking reflection. The π-stacking interactions between

pyrene units of SM1 molecules are proven by recently published single crystal data.20 Taking into account the dihedral angle of 30° between pyrene and DPP units determined by DFT calculations before, the DPP core is also titled, but by an angle of -15° with respect to the out-of-plane (cf. schematic illustration of the surface organization in Figure S7). The inplane reflection at qz = 0 Å-1 and qxy = 0.85 Å-1 is associated to a distance of 0.74 nm and corresponds to two times the πstacking value, as well as to the a parameter of the single crystal unit cell, implying a lateral shift of the molecules along their long axis.20 The GIWAXS pattern for THF-mediated SVED-processed SM1 films shown in Figure 6d describes a larger interlayer and an identical π-stacking distance. The smearing out along the azimuthal direction set aside, the reflections are broader, suggesting slightly lower crystallinity compared to the films obtained by CHCl3-mediated SVED (Figure 6a). For THF-mediated SVED-processed SM1 films, the pyrene units are tilted by a larger angle of ca 45° with respect to the out-of-plane as evident from the corresponding off-reflections at qz = 1.37 Å-1; qxy = 1.23 Å-1 assigned to the π-stacking (see Figure 6d - π-π), while the DPP main-chain maintains an orientation tilt of -15°. The larger tilting angle of the pyrene motifs with respect to the DPP main-chain inferred from GIWAXS analyses is in agreement with the DFTconducted geometry optimizations described earlier, which indicated a dihedral out-of-plane rotation of the pyrene moieties relative to the DPP core by ca. 30°. The surface arrangement/orientation of SM1, processed from CHCl3 and THF, is schematically illustrated in Figure S7. The expansion of the interlayer distance from 14.6 Å to 16.5 Å by changing the solvent from changing the solvent from CHCl3 to THF may be explained by subtle variations in alkyl side-chain packing and inter-digitation thereof in the films obtained from two


6

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different solvents. The degree of the surface alignment of SM1 achieved from THF is slightly reduced in comparison to that seen in the crystals obtained from CHCl3, as indicated by the rise of the Rin/out parameter from 0.02 to 0.07. Contrasting with the highly crystalline patterns of SM1 obtained from CHCl3-mediated SVED, Figures 6b and 6c show that self-organization in SM2 and SM3 is shorter-ranged, as evident from the reduced number of reflections across the GIWAXS patterns. Both SM2-3 analogues also show a layered organization with an interlayer spacing of about 17.0 Å and some extent of edge-on arrangement as indicated by the maximum intensity of the out-of-plane interlayer reflections (Figure S6). However, the domains are found to be more randomly ordered on the surface, as the Rin/out parameter increases to 0.23 for SM2 and 0.70 for SM3. The larger interlayer distance found for SM2 and SM3, in comparison to SM1, can be explained by the greater number of DPP motifs in the former– motifs appended with sterically demanding branched sidechains.39,40 The near-identical interlayer distance estimated for SM2 and SM3 can be related to subtle changes in the in-plane organization relative to the surface. Both compounds are only weakly in-plane packed (Figure S6), as the corresponding πstacking peak shows a rather low intensity for SM2 (πstacking distance of 3.7 Å) or is absent in the case of SM3. The reduction in structural order on going from SM1 to SM2 and to SM3 is in agreement with the data obtained from optical spectra (discussed in earlier sections), from which it was noted that the more π-extended SM analogues aggregated more weakly in solution (likely due to the side-chain effect).

processing. This is reflected in the greater number of distinct higher-order interlayer and π-stacking reflections in the corresponding patterns in Figures 3e and f. In both cases, the molecules are organized preferentially edge-on (Rin/out = 0.02 for SM2 and SM3), with interlayer and π-stacking distances of 19.2 Å and 3.5 Å for SM2 and 19.2 Å and 3.5 Å for SM3. Although the X-ray results do not provide strong evidence about the molecular packing, it is assumed based on the structural data of SM1 that the π-stacking interactions occur between pyrene units also for SM2 and SM3 (Figure S8). Similar to SM1, the angle of the wide-angle off-reflections for SM2 also suggest a 45° tilt of the pyrene motifs with respect to the surface. For SM3, the molecular packing is significantly enhanced by using THF in comparison to CHCl3, yielding a distinct in-plane π-stacking reflection associated to a distance of 3.5 Å. The in-plane position of this peak is characteristic for a non-tilted arrangement of the pyrenes. While a close correlation between conjugation length in SM1-3 and thin-film crystallinity exists for films processed from CHCl3–mediated SVED, those are not as prominent for films processed from THF–mediated SVED. The films obtained from THF show a distinct layer organization for all three SM analogues, with only a modest suppression of molecular packing with increasing π-conjugation length, whereas the degree of surface alignment tends to drop.

Table 2. Summary of the interlayer and π-stacking distance parameters for SM1-3 as determined by GIWAXS. Rin/out for the interlayer peak is derived to gain information about the surface arrangement. Rin/out = 1 – random crystal orientation on the surface; Rin/out > 1 – face-on; Rin/out < 1 – edge-on orientation of the molecules. Interlayer

π-stacking

Rin/out

distance (Å)

distance(Å)

Interlayer (-)

CHCl3

14.6

3.5

0.02

SM2

CHCl3

17.0

3.7

0.23

SM3

CHCl3

17.0

-*

0.70

SM1

THF

16.5

3.5

0.07

SM2

THF

19.2

3.5

0.02

SM3

THF

19.2

3.5

0.02

PyreneDPPs

SVED♯

SM1

♯

Solvent vapor enhanced drop-casting, *no corresponding reflection.

The GIWAXS patterns of SM1-3 shown in Figures 6d-f and S6, and obtained from THF-mediated SVED, indicate a different organizational pattern in comparison to those discussed above for CHCl3-mediated SVED. In contrast to SM1, for which structural order was found to decrease slightly on changing the SVED solvent from CHCl3 to THF, the crystallinity of SM2 and SM3 improves by THF-mediated SVED

Figure 7. Transistor transfer and output curves of (a,c) SM2 and (b,d) SM3 films obtained by solvent vapor enhanced drop-casting (SVED) from CHCl3.

To examine the influence of morphology and molecular organization on the carrier transport in SM1-3, bottom-gate and gold top-contact OFETs were fabricated following the same protocol as that developed for the spin-coated films (see details provided earlier). As shown in Figure 7 and Table 3, transistors based on SM2 obtained from CHCl3-mediated SVED afford unipolar transport with a hole mobility of 0.12 cm2 V-1 s-1 and a threshold voltage VT of -5 V. Meanwhile, transistors based on SM3 achieve ambipolar transport with a hole mobility of 0.06 cm2 V-1 s-1 and a rather balanced electron


7


mobility of 0.02 cm2 V-1 s-1. In the forward drain mode for VDS > 0 V, the crossover point from hole- to electron-dominated current is at approximately Vg = 40 V (Figure 7b). On the other hand, the crossover point from electron- to holedominated current in the reverse drain mode is at around Vg = 4 V (Figure 7b). The output curve indicates contact resistance for SM2-3. This can be related partly to a mismatch between the HOMO of the compounds and work function of the gold electrodes. More importantly, the distinct crystalline morphologies of SM2 and SM3 also influence the semiconductor/electrode interface leading to different contact resistances. We assign the transition from unipolar transport in SM2 to ambipolar transport in SM3 to the suppressed LUMO level induced on extending the π-conjugation through successive incorporation of the DPP motifs. The decrease of the hole mobility from 0.12 cm2 V-1 s-1 in SM2 to 0.06 cm2 V-1 s-1 in SM3 origins from the less of π-stacking and the reduced grain domain size in CHCl3-mediated SVED-processed SM3. Surprisingly perhaps, transistors based on SM1 and fabricated from CHCl3-mediated SVED and those made with SM1-3 and fabricated from THF-mediated SVED afforded only modest hole mobilities of less than 0.001 cm2 V-1 s-1 (Figure S9 and Table 3). The random orientations of crystals and/or fibers leading to a high density of grain boundaries and the voids formed between crystals and/or fibers may be at the origin of those limitations. Therefore, SM1 diamond-like crystals and SM1-3 fibers obtained from THF processing tend to show higher molecular order, but lower carrier mobilities compared to CHCl3-mediated SVED processed OFETs with SM2 and SM3. Table 3. OFET characteristics of SM1-3 processed by CHCl3mediated and THF-mediated SVED. Silicon wafer substrates with a 300 nm thick SiO2 layer were treated with (HMDS). Bottom-gate and gold top-contact transistors are fabricated after annealing 60 min at 150 °C. PyreneDPPs

SVED♯

µhole / µelectron

VT [V]

[cm2 V-1 s-1]

CHCl3

(6.0 ± 1.2) × 10-4 / N/A

-3

THF

(2.1 ± 0.8) × 10-4 / N/A

-3

CHCl3

(1.2 ± 0.1) × 10-1 / N/A

-5

THF

(4.2 ± 1.4) × 10-4 / N/A

-7

SM1

SM2

CHCl3 SM3 THF

(6.1 ± 0.1) × 10

-2

/

(2.0 ± 0.2) × 10-2 (3.4 ± 0.9) × 10-4 / N/A

4 / 40* -3.0

♯solvent vapor enhanced drop-casting,*crossover point in forward and reverse drain mode. Average value and standard deviation are calculated from 10 individual devices.

To lower the injection barrier for electrons in OFETs processed from CHCl3-mediated SVED with SM2, we tentatively used aluminum source and drain electrodes. As shown in Figure S10 and Table S5, these transistors based on SM2

Page 8 of 11

achieve ambipolar behavior with a hole mobility of 0.04 cm2 V-1 s-1 and an electron mobility of 0.01 cm2 V-1 s-1. Here, it is worth noting that ambipolar transport is achieved via the low work-function of aluminum (ca. 4.2 eV), promoting electron injection into the semiconductor layer in spite of the shallower LUMO of SM2 compared to that of SM3. At the same time, the energy offset between the HOMO of SM2 and the metal work-function increases, resulting in reduced hole injection and hole mobility values following this device engineering approach.

Conclusion In summary, we have demonstrated that the extension of πconjugation length across the set of well-defined, analogous pyrene-functionnalized DPP molecules SM1-3 induces critical changes in their self-assembly when the molecules are subjected to SVED protocols. In particular, we found that the more extended pyrene-DPP analogues tend to form smaller crystal sizes and very distinct morphologies as a result. The self-assembly also depends on the nature of the solvent used in the SVED protocols. The formation of the one-dimensional fibers of SM1-3 in THF is attributed to initial aggregation in solution, owing to the reduced solubility and strong π−π stacking interactions. Furthermore, swapping CHCl3 for THF also improves structural order for the more π-extended molecular systems (SM2, SM3). Further incorporations of DPP motifs into the main-chain of the small molecules induces a significant narrowing of the energy gap, mainly through a suppression of the LUMO level on going from SM1, to SM2 and to SM3. While OFETs fabricated with SM2 show unipolar hole transport characteristics only, OFETs made with SM3 exhibit ambipolar field-effect behavior. This transition from unipolar to ambipolar OFET characteristics by incremental extension of the π-conjugation length in pyrene-functionalized DPPs paves the way to a general, rational design approach towards achieving solution-processable small-molecule ambipolar OFETs with high, balanced hole and electron transport.

Experimental section Optical absorption. Solution optical absorption measurements were performed using a Cary 5000 spectrometer (Varian). Optical absorption spectra were calculated from the transmission spectra using the following equation [A= 2– log(transmission)], where A is the absorbance at a certain wavelength (λ) and T is the transmitted radiation. Solvent vapor enhanced drop-casting (SVED). Silicon wafer substrates with a 300 nm thick SiO2 layer were treated with hexamethyldisilazane (HMDS) at 150 °C for 6 h to form a self-assembled monolayer. For SVED, 0.15 mL SM1-3 solution (concentration of 2 mg/mL in CHCl3 or THF) is drop-cast on a HMDS-modified silicon wafer (1.5 × 2 cm2), which is exposed to saturated CHCl3 (THF) vapor in a container with a covered lid (nearly airtight). The container volume of 40 mL contained 3 mL of CHCl3 (THF). In the saturated solvent vapor atmosphere, SM1-3 solidified into semicrystalline films of SM1-3 after around 8 hours. The estimated


8

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solubility of SM2 and SM3 is less than 20 mg/mL in CHCl3 and THF. With the solvent evaporation, SM1-3 pre-aggregates in THF. Morphology and molecular organization. The OM and POM images were recorded using a Zeiss Axiophoto microscope under polarized light equipped with a Hitachi KP-D50 color digital CCD camera. AFM images were obtained with a Digital Instruments Nanoscope IIIa AFM in tapping mode. GIWAXS experiments were performed by means of a solid anode X-ray tube (Siemens Kristalloflex X-ray source, copper anode X-ray tube operated at 35 kV and 40 mA), Osmic confocal MaxFlux optics, X-ray beam with pinhole collimation, and a MAR345 image plate detector. The beam size was 0.5 × 0.5 mm, and samples were irradiated just below the critical angle for total reflection with respect to the incoming X-ray beam (≈0.18°). OFET characterization. The bottom-gate top-contact configuration was employed for OFET devices. The heavily doped n-type Si wafers were used as gate electrode and the HMDS treated layer of 300 nm thick SiO2 (with a capacitance of 11 nF cm-2) was adopted as a gate dielectric layer. The source and drain electrodes with a 50 nm thickness were deposited by gold and aluminum evaporation, respectively. The channel length and width are 20 µm and 25µm. Before measurement, annealing at 150 °C was applied for 1 hour in order to remove residual solvents. A Keithley 4200-SCS was used for all standard electrical measurements in glove-box under nitrogen atmosphere.

ASSOCIATED CONTENT Supporting Information. The supporting information is available free of charge via the Internet at http://pubs.acs.org. The detailed synthetic routes to SM1-3, experimental methods, characterization, and additional figures and tables.

AUTHOR INFORMATION Corresponding Author Email: [email protected] Email: [email protected]

ORCID Ke Zhang: 0000-0002-8854-244X

Present Addresses Philipp Wucher: Merck in Germany, Darmstadt, 64293, Germany.

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT P. M. Beaujuge acknowledges concurrent support from the King Abdullah University of Science and Technology (KAUST) Office

of Sponsored Research (OSR) under Award No. CRG_R2_13_BEAU_KAUST_1 and under Baseline Research Funding. P. Wucher thanks the KAUST ACL for technical support in the mass spectrometry analyses. K. Zhang thanks the China Scholarship Council (CSC) for financial support. W. Pisula acknowledges National Science Centre, Poland, through the grant UMO-2015/18/E/ST3/00322.

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Long-Range Molecular Self-Assembly from Ï-Extended Pyrene

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