(isoEpi) Crystal for Organic Field-Effect Transistor

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Hydrogen-Bonding Mediated Solid-State Self-Assembled Isoepindolidiones (isoEpi) Crystal for Organic Field-Effect Transistor Haichang Zhang, Kewei Liu, Kuan-Yi Wu, Yu-Ming Chen, Ruonan Deng, Xiang Li, Hailiang Jin, Si Li, Steven S.C. Chuang, Chien-Lung Wang, and Yu Zhu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11992 • Publication Date (Web): 19 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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The Journal of Physical Chemistry

Hydrogen-Bonding Mediated Solid-State Self-Assembled Isoepindolidiones (isoEpi) Crystal for Organic Field-Effect Transistor

Haichang Zhang,† Kewei Liu,† Kuan-Yi Wu,§ Yu-Ming Chen,† Ruonan Deng,† Xiang Li,† Hailiang Jin,† Si Li,† Steven S. C. Chuang,† Chien-Lung Wang,*§ Yu Zhu*† †

Department of Polymer Science, College of Polymer Science and Polymer Engineering, The University of Akron, 170 University Circle, Akron, Ohio 44325-3909, United States §

Department of Applied Chemistry, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 30010, Taiwan

ABSTRACT: Novel pigments isoepindolidiones (isoEpi) and di-tert-butyl quinolino[8,7h]quinoline-1,7-diyl

bis(carbonate)

(isoEpi-Boc)

were

synthesized

and

characterized.

Spectroscopic characterizations indicated that the tert-butyloxycarbonyl (t-Boc) units could be removed from soluble isoEpi-Boc by thermal annealing, forming insoluble isoEpi with hydrogen-bonding. A solid-state isoEpi-Boc crystal -to- isoEpi crystal transition was observed during the annealing process and the molecular packing was significantly changed. With the emergence of hydrogen bonds, the isoEpi molecules were arranged into a brick-in-wall structure with π-stacking along the crystal growth axis, leading to a significant enhancement of charge mobility along the crystal growth direction (the hole mobility from 3.4×10-4 cm2/V·s to 0.32 cm2/V·s, and the electron mobility from non-detectable to 5.6×10-3cm2/V·s). The results indicate that isoEpi is a promising chromophore for organic field-effect transistor (OFET). The crystal-tocrystal transition driven by the formation of hydrogen bonds is a unique method for modulating the charge transport properties in organic semiconductive materials for OFET devices.

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INTRODUCTION Small-molecule based organic field-effect transistors (OFETs) have been developed rapidly in the past two decades.1-2 Rubrene3-6 and pentacene,7-9 which formed perfect single crystals when grown using vapor phase growth technique, exhibited mobility up to ~ 40 cm2/V·s. However, those unsubstituted polyacenes also showed poor solubility in common organic solvents due to the strong π–π interactions. To improve the solution processability, soluble conjugated small molecules were developed by introducing flexible side-chains. Many successful soluble molecules, with comparable performance as insoluble rubrene and pentacene, were reported in recent years as p-channel10-13 and n-channel14-16 materials. However, the soluble small-molecule materials also brought several challenges to OFET: First, the flexibility of the side-chains made the molecular packing sensitive to processing variables, such as temperature, concentration, casting methods and solvent nature. Thus, the prediction and control of molecular packing in those materials were challenging. Second, the thermal stability of the soluble small molecule is generally lower since the glass transition temperature and melting temperature of conjugated molecules decreased when the flexible side-chains were introduced. Finally, the crystallization of the small molecules during the film formation process could be challenging for some molecules. It has been widely accepted that the charge carriers in small-molecule organic semiconductors need to travel across individual molecules, thus the molecular packing and crystal size are crucial for efficient charge transport in device.17-18 Under this scenario, the design of molecules with a precise control of intermolecular interactions to achieve the desired molecular packing, crystal structure and alignment/interconnection of the crystalline grains is crucial for small-molecule OFETs. The use of strong intermolecular interaction such as hydrogen-bonding to control selfassembly of molecules has been widely adopted to design new conjugated polymer and small


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molecules.19-23 Among those compounds, the molecules with hydrogen-bonding sites fused with conjugated units, often observed in organic pigment molecules, demonstrated interesting OFET performance.22, 24-29 Similar to rubrene and pentacene, those side chain-free pigment molecules had stable crystal structure but poor solubility. However, these pigment molecules can be solution processable if the hydrogen bonds were blocked.30 Following removal of the blocking units, hydrogen bonds were reformed, regenerating the original pigments. This hydrogen bonding mediated self-assembly provided a potential way to prepare crystalline organic semiconductive materials in the solid state. In this report, a novel pigment semiconductor, isoepindolidione (isoEpi) (Scheme 1) was synthesized, and subsequently investigated as a small-molecule active material in OFET. As shown in Scheme 1, isoEpi was prepared by reaction of Meldrum’s acid and 1,5diaminonaphthalene, followed by a double cyclization reaction. The pigment contained lactam units that could form intermolecular hydrogen-bonding pairs (N-H…O=C, Supporting Information, Figure S1). The strong hydrogen-bonding and π-π stacking resulted in solid materials with strong intermolecular interaction and high crystal-lattice energy, which were essential for the thermal- and photostability of similar organic pigment molecules.31-32 Fusing hydrogen-bonding and π-π stacking of isoEpi, on the other hand, also led to very poor solubility, which prohibited the growth of large pigment crystals in solution. Since crystallinity played a key role for small-molecule OFET device, a crystal-to-crystal transition method was used to fabricate crystalline isoEpi OFET device.


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Scheme 1. The synthesis scheme of soluble isoEpi (isoEpi-Boc) and transition between isoEpi and isoEpi-Boc.

RESULTS AND DISCUSSION Synthesis and Characterization of isoEpi and isoEpi-Boc The tert-butyloxycarbonyl (t-Boc) substituted isoEpi (isoEpi-Boc) was synthesized by following the previous reported method for similar molecules.33-34 The two t-Boc units on isoEpi-Boc blocked the hydrogen-bonding, and thus rendered the molecule soluble in common organic solvents. The side-chain of isoEpi-Boc could be removed to generate original pigment, releasing isoprene and carbon dioxide (Supporting Information, Figure S2). It is worthy to note that the tBoc group was connected with the O atom instead of the N atom, which was confirmed by the single crystal structure of isoEpi-Boc. (Supporting Information) To verify the formation of isoEpi with fused hydrogen-bonding during thermal annealing, TGA (Thermogravimetric Analysis), NMR (Nuclear Magnetic Resonance spectroscopy), DSC (Differential Scanning Calorimetry) and FTIR (Fourier Transform Infrared spectroscopy) characterizations were conducted. TGA (Figure 1a) showed that there was a weight loss starting between 115 oC and 155 oC, depending on the heating rate (1 oC/min to 50 oC/min) for isoEpiBoc. The experimental weight loss was 42.84 % at 220 oC, which matched well with the theoretical value (42.85 %). To verify the isoEpi-Boc conversion to isoEpi during t-Boc elimination, solid-state

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C-NMR spectra were collected before and after thermal annealing. In


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Figure 2b, the chemical shift at 27.6 ppm and 85.9 ppm (t-Boc butyl) disappeared after annealing (5 min at 200 oC). Meanwhile the peaks between 100 ppm and 180 ppm were rearranged and a single peak with chemical shift at 178.1 ppm appeared (O=C). The NMR spectrum of the original isoEpi pigment exhibited identical signals as the 23observations validated the conversion from isoEpi-Boc chromophore to isoEpi pigment after thermal annealing. DSC of isoEpi-Boc (Figure 1c) exhibited an endothermic transition in the temperature range between 160 o

C and 200 oC. This broad endothermic transition was consistent with the thermal cleavage of the

t-Boc units from the isoEpi core, which was also observed in TGA experiments. There was no endothermic or exothermic transition in the temperature range of 30 oC to 300 oC in the second DSC cycle, suggesting the formation of stable, hydrogen-bonded isoEpi.

Figure 1. (a) TGA spectra of isoEpi-Boc with different heating rate. (b) Solid-state

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C-NMR

spectra of isoEpi-Boc before (black curve) and after thermal annealing (AN) at 200 oC for 5 min (red curve), and isoEpi (blue curve). (c) DSC spectra of isoEpi-Boc with heating rate of 10 o

C/min (two heating-cooling cycles). (d) Kinetic FTIR spectra of isoEpi-Boc during thermal

annealing process. (e) Kinetic FTIR spectra of isoEpi (from annealed isoEpi-Boc) during cooling.


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Inset: IR difference spectra of isoEpi during cooling, which was obtained by applying spectrum at 280°C as background.

Kinetic FTIR experiments were conducted to further verify the formation of hydrogen-bonding. As shown in Figure 1d, the kinetic FTIR spectra clearly indicated that the strong hydrogenbonding-associated isoEpi was formed during the thermal annealing, based on the following observations: 1) the absorption peak at 1756 cm-1 (C=O stretching of t-Boc, red band) disappeared after thermal annealing, indicating the removal of t-Boc unit; 2) the emerging peaks at 1609 cm-1 (O=C, blue band) and 1569 cm-1 (N-H bending, purple band) confirmed the formation of O=C and N-H structure on a conjugated core, indicating the rearrangement of the molecule to isoEpi structure; 3) a broad band absorption between 2600 cm-1 and 3500 cm-1 emerged (green band), which is attributed to the hydrogen-bonding N-H stretching vibration. Figure 1e shows the kinetic FTIR results during the cooling process after the first thermal annealing. The spectra in the hydrogen-bonding region have subtle changes as indicated from red curve to green curve. The inset in Figure 1e shows the difference spectra in the N-H region which gave the negative band at higher wavenumbers and the positive band at lower wavenumber, reflecting a gradual downward shift of the N-H stretching peak with decreasing temperature. This downward shift is a manifestation of increasing hydrogen-bonding interaction between N-H and C=O when temperature is decreased.35-37 The specific step detail of difference absorbance spectra of N-H stretching peak shift is shown in the Supporting Information (, Figure S3). The kinetic FTIR results agreed well with well-known hydrogen-bonding based systems.3840


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Figure 2. (a) UV/Vis absorption spectra of isoEpi-Boc (black curve) in solution (dichloromethane), as thin film before (red curve) and after (blue curve) thermal annealing (forming isoEpi). AN (annealed). (b) Images of isoEpi-Boc and isoEpi in solid state. (c) Cyclic voltammograms of isoEpi-Boc film before (black curve) and after thermal annealing (red curve).

The UV/Vis absorption spectra of the pigments are shown in Figure 2a. There is a sequential bathochromic shift on the UV/Vis spectra following the order of isoEpi-Boc solution (362 nm), isoEpi-Boc thin film (364 nm), and annealed isoEpi-Boc (converted to isoEpi) film (384 nm). In addition, the absorption peak of the annealed film is significantly broadened, suggesting the formation of new intermolecular-packing modes. The bathochromic shift of isoEpi-Boc to isoEpi can be clearly evidenced by the images of isoEpi-Boc (white/colorless) and isoEpi (dark yellow) in solid state (Figure 2b). The electrochemical properties of pigments were studied using CV (Cyclic Voltammetry) in an Argon-filled glove box. The results are shown in Figure 2c. From the onset of anodic oxidation and cathodic reduction, the electrochemical band gap of isoEpi-Boc and isoEpi are calculated to be 2.43 eV and 2.12 eV, respectively. (Support Information, Table S1) As expected, the band gap of hydrogen-bonded isoEpi is smaller than the corresponding isoEpi-Boc.


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Crystal-to-Crystal transition of IsoEpi-Boc to IsoEpi IsoEpi-Boc crystals were grown on the surface of glass slide by a slow evaporation method (the details are described in Support Information). The morphological change of the crystal sample during the heating process was observed under an optical microscope. Figure 3a shows the optical microscopy (OM) and polarized optical microscopy (POM) micrographs of the crystals. In the OM micrographs, it can be found that the shape of the crystals remains largely unchanged during the heating process. Nevertheless, in the POM micrographs, the variation in the brightness of the crystals suggests that within the crystals, the molecular orientation of the isoEpi molecules could rearrange as resultant of the thermal treatment. In Figure 3b, a 1st-order retardation plate helps to reveal rough molecular orientation in the crystals. In general, crystals containing molecules aligned parallel to the slow axis of the retardation plate (white arrow in Figure 3b) appear in blue color, whereas those containing molecules aligned perpendicular to the slow axis appear yellow. Therefore, the color change in Figure 3b gives a first indication that the isoEpi molecules rearrange their orientation from perpendicular to parallel to the long axis of the crystal, due to the removal of t-Boc group. Figure 3c shows the 1D XRD patterns of isoEpi-Boc crystal before (black curve) and after (red curve) thermal annealing. The thermally treated crystals gave a diffraction pattern that is different from the untreated isoEpi-Boc crystal, further indicating the change of crystal structures. To identify the structure of annealed isoEpi-Boc, physical vapor transport (PVT) grown isoEpi crystals were prepared for structural characterization. The XRD pattern of the insoluble isoEpi crystal (blue curve) prepared from the PVT method are the same as the XRD pattern of annealed isoEpi-Boc (red curve).


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Figure 3. (a) Optical microscopy (OM) micrographs (first row) and polarized optical microscopy (POM) micrographs (second row) of isoEpi-Boc crystals obtained through drop-casting/slow evaporation method. The series of micrographs depict the crystal-to-crystal transition during the heating process. (b) POM micrographs of isoEpi-Boc before (first row) and after (second row) thermal annealing under the 1st-order retardation plate. The inset white arrow shows the slow axis of retardation plate. (c) 1D XRD patterns of isoEpi-Boc and isoEpi. black curve: soluble isoEpi-Boc crystal prepared by slow evaporation in solution; red curve: isoEpi-Boc crystal after thermal annealing (converted insoluble pigment crystal); blue curve: insoluble isoEpi pigment crystal prepared by physical vapor transport (PVT) technique. (d) The ab projection of the crystal lattice of isoEpi-Boc, and (e) the bc projection of the crystal lattice of isoEpi. Hydrogen bonds are formed between the adjacent isoEpi molecules. The length and the angle of the hydrogen bonds are illustrated in the figure.

The evidences of structural change and hydrogen-bonding formation were further obtained by a single-crystal diffraction method using solution growth isoEpi-Boc single crystal and PVT isoEpi single crystal. The representative structure information of isoEpi-Boc and isoEpi are shown in Figure 3d and 3e. In Figure 3d, the single crystal structure of isoEpi-Boc shows that


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two t-Boc units are on the isoEpi chromophore. In contrary, Figure 3e shows the PVT-grown isoEpi crystal consists of isoEpi molecules without side-chains. More importantly, hydrogen bonds (N-H…O=C) were found among the isoEpi molecules at the bc plane with N-H distance of 0.91 Å, C=O distance of 1.25 Å, N-H…O=C distance of 1.83 Å, and N-H-O angle of 154 o. It is worthy to mention that every isoEpi molecule forms two hydrogen bonds with two adjacent molecules, rather than form a pair of hydrogen bonds with one adjacent molecule. Therefore, the hydrogen-bonding chained molecules can still rotate slightly to accommodate an optimized πstacking configuration. Since the thermally treated crystals of isoEpi-Boc show identical diffraction pattern with the PVT-grown isoEpi crystal (shown in Figure 3c), it can be affirmed that the thermal treatment had caused the t-Boc elimination and triggered a subsequent structural transformation that establishes inter-molecular hydrogen-bonding and rearrangement of the isoEpi molecules in the crystals. Based on the characterization results, the transition from isoEpi-Boc crystals to isoEpi crystals could be described by the following two procedures: i) during the thermal treatment, the t-Boc units were removed from isoEpi-Boc molecules, generating the hydrogen-bonding donor units (N-H) and acceptor units (C=O). In this process, isoEpi-Boc molecules were converted to isoEpi molecules; ii) due to the elevated temperature, the as-formed isoEpi molecules were still mobile; therefore, isoEpi molecules self-assembled to associate the free donor units (N-H) with the acceptor units (C=O). The assembly process formed hydrogen bonds (N-H…O=C) among neighboring isoEpi molecules and rearranged the molecules into crystal structure identical to that of the PVT grown crystals. The packing of isoEpi-Boc is mainly governed by Van der Waals force and π-π stacking, resulting in a herringbone structure (Figure 3d). In contrast, there is strong hydrogen bond (N-H…O=C) in isoEpi. The hydrogen bonds lock the neighboring isoEpi


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molecules on the same plane and result in isoEpi with a brick-in-wall packing as a layered structure (Figure 3e). The hydrogen bonds significantly elevate the melting temperature of the isoEpi crystal. The thermally treated sample thus showed no phase transition in the subsequent cooling and heating DSC scan (Figure 1c). Under a higher magnification, scanning electron microscopy (SEM) further revealed that the t-Boc elimination, and the crystal-to-crystal transition caused significant crystal shrinkage (Supporting Information, Figure S4).

OFET Devices and Structure-Charge Transport Analysis As the ribbon like isoEpi-Boc crystal can be formed on the surface of silicon wafer and converted to isoEpi pigment crystal, it could serve as good example to study the effect of hydrogen-bonding mediated self-assembly on the charge transport behavior of the semiconductive pigment molecules. The isoEpi-Boc crystal-to-isoEpi crystal transition in solid state allowed the fabrication of crystalline isoEpi OFET from the solution-processable isoEpiBoc crystal. The details of device fabrication are described in the Supporting Information. Figure 4a and 4b are the POM micrographs of the OFET devices before and after thermal treatment. The SEM images of the OFET device after thermal treatment is shown in Figure 4c, in which the size of the crystal is accurately measured. The device performances before and after thermal treatment are shown in Figure 4d to 4i and Table S2. The isoEpi-Boc crystals exhibited p-type behavior with hole mobility of 3.4×10-4 cm2/V·s. After converting the isoEpi-Boc crystal to hydrogen-bonded isoEpi crystal, the device exhibited ambipolar behavior with hole mobility up to 0.32 cm2/V·s and electron mobility of 5.6×10-3 cm2/V·s. Compared to the isoEpi-Boc device, the isoEpi device have three orders of magnitude higher mobility value. The on/off ratio of the device was enhanced during this transition. The detailed transistor performance data are


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summarized in Table S2. It is worth to notify that the threshold voltages (Vth) of p-channel for both isoepindolidiones show almost the same values irrespective of remarkable deference of HOMO levels (0.5 eV). This is an indication that the crystal transistors could eliminate grain boundaries generating carrier trapping sites.

Figure 4. Images and the characteristics of the OFET device. (a,b) Polarized optical image of isoEpi-Boc device before and after thermal annealing. (c) SEM image of isoEpi-Boc device after thermal annealing (converted to isoEpi device). (d,e) The output characteristics and transfer characteristics of isoEpi-Boc device (VSD = 40 V). (f-i) The output characteristics and transfer characteristics (VSD = 40 V) of annealed isoEpi-Boc (converted to isoEpi) devices.


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To understand the origin of this drastic change in OFET device, electron diffraction (ED) patterns of the isoEpi-Boc and isoEpi crystals were collected to reveal the molecular packing along the charge-transport pathway. Using the lattice parameters provided from the single crystal structures (Table S3), it was found that the ED pattern in Figure 5a showed the b*c* reciprocal lattice (b* = 0.23 Å-1, c* = 0.97 Å-1, α* = 90o) of the isoEpi-Boc crystal, whereas the one in Figure 5b projected the a*b* reciprocal lattice (a* = 1.65 Å-1, b* = 0.48 Å-1, γ* = 90o) of the isoEpi crystal. Moreover, by relating the defocus images of the two crystals with the ED patterns, it was confirmed that the long axis of the crystal (i.e the charge-transport direction in the transistors) was along the c axis of the isoEpi-Boc lattice and the a axis of the isoEpi lattice. The relative orientations of the crystal and the corresponding lattice are illustrated in Figure 5c-1 (for isoEpi-Boc) and 5c-4 (for isoEpi), respectively. After knowing the lattice orientation in the crystal, the molecular packing in the isoEpi-Boc crystal or the isoEpi crystal were further identified and shown in Figure 5c. In the isoEpi-Boc crystal, Although the isoEpi-Boc molecules form an ordered structure with dπ-π of 3.46 Å (Figure 5c-3) in the isoEpi-Boc crystal, the direction of π-stacking is not along the long-axis of the crystal. On the contrary, a continuous charge-transport pathway with molecules stacked with a dπ-π of 3.36 Å (Figure 5c-6) can be clearly identified in the isoEpi crystal. As a result, the isoEpi crystal provided a more effective charge-transport pathway in device, which delivered the higher charge mobilities in the OFET devices than the isoEPi-Boc crystal did. More importantly, the results also confirmed that the well-aligned π-π stacks of the isoEpi molecules can be prepared in-situ in the solution-processed isoEPi-Boc crystals via the structural transformation triggered by hydrogen-bonding formation.


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Figure 5. Transmission electron microscopy (TEM) micrographs and electron diffraction (ED) patterns of (a) isoEpi-Boc crystal (grown from solution process) and (b) isoEpi crystal (b, grown from PVT process). (c) The projection view (1, 4), top view (2, 5), and lateral view (3, 6) of the molecular packing in the isoEpi-Boc crystal and the isoEpi crystal. The charge-transport direction, i.e. the long axis of the crystal, is along the c-axis of the isoEpi-Boc lattice, whereas it is along the a-axis of the isoEpi lattice.


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CONCLUSIONS In summary, novel pigments isoEpi and isoEpi-Boc were synthesized in this work. A series of spectroscopic techniques including TGA, NMR, FTIR and DSC characterizations were conducted to confirm that the isoEpi-Boc could be decarboxylated, forming hydrogen-bonded isoEpi during thermal annealing. An isoEpi-Boc-crystal-to-isoEpi-crystal transition was observed during the thermal annealing process, supported by both the microscopic and structural characterization results. The thermal-induced t-Boc elimination significantly changed the molecular packing within the solution-processed isoEpi-Boc crystals. Triggered by the t-Boc elimination and hydrogen bond formation, the in-situ formation of the well-aligned π-π stacks of the isoEpi molecules significantly improved the quality of the charge transport pathway, and enhanced the hole mobility from 3.4×10-4 cm2/V·s to 0.32 cm2/V·s, and the electron mobility from non-detectable to 5.6×10-3 cm2/V·s, respectively. The results showed that isoEpi is a promising chromophore for OFET applications. The crystal-to-crystal transition technique for OFET device is also suitable for other t-Boc substituted pigments and even oligomers that cannot form crystal easily using PVT technique. This work will open up a new route to study a variety of conjugated pigment molecules for organic semiconductors.


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SUPPORTING INFORMATION AVAILABLE Additional NMR, TGA, DSC and materials synthesis procedures. These materials are available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *[email protected] or *[email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work is supported by the National Science Foundation (DMR-1554851). Part of the work is supported by the CBET-1706681, CBET-1505943 and CBET-1336057 and ACS Petroleum Research Fund (PRF# 53560 -DNI 10).


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TOC Graphic


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(isoEpi) Crystal for Organic Field-Effect Transistor

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