Synthesis and Characterization of Pyrrolo[3,4-d]Pyridazine-5,7-Dione-Based Conjugated Polymers for Organic Thin-Film Transistors

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Synthesis and Characterization of Pyrrolo[3,4-d]Pyridazine-5,7-Dione-Based Conjugated Polymers for Organic Thin-Film Transistors
  Copyright © 2017 American Scientific PublishersAll rights reservedPrinted in the United States of America Article Journal of Nanoscience and Nanotechnology  Vol. 17, 1–6, 2017  Synthesis and Characterization ofPyrrolo[3,4-d]Pyridazine-5,7-Dione-Based ConjugatedPolymers for Organic Thin-Film Transistors Hee Su Kim 1 , Gunel Huseynova 2 , Hoyoul Kong 3 , Jong Mok Park 3 ,Yong-Young Noh 2  ∗ , and Do-Hoon Hwang 1  ∗ 1 Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea  2 Department of Energy and Materials Engineering, Dongguk University 30 Pildong-ro, 1 gil, Jung-gu, Seoul 04620, Republic of Korea  3 Division of Convergence Chemistry, Research Center for Green Fine Chemicals, Korea Research Institute of Chemical Technology,Ulsan 44412, Republic of Korea  Two new conjugated polymers, poly{6-(2-dodecylhexadecyl)-1,4-di(thiophen-2-yl)-5H-pyrrolo[3,4-d]pyridazine-5,7(6H)-dione-alt-2,2  -bithiophene} (PPz-BT) and poly{6-(2-dodecylhexadecyl)-1,4- di(thiophen-2-yl)-5H-pyrrolo[3,4-d]pyridazine-5,7(6H)-dione-alt-3,3  -dimethoxy-2,2’-bithiophene } (PPz-MeOBT), which containmethoxy-substituted and unsubstituted bithiophene units, respectively, were designed and synthe-sized. Experimental results revealed that the introduction of methoxy groups increases the highestoccupied molecular orbital and lowest unoccupied molecular orbital energy levels of the polymerbecause of the electron donating ability of the methoxy group and improves the planarity of theconjugated polymer backbone. However, the two-dimensional grazing incidence X-ray diffractionpattern of the PPz-MeOBT thin film reveals a bimodal structure, whereas the PPz-BT thin filmshows a predominant edge-on orientation. As confirmed by X-ray analysis, after thermal annealingat 280   C, top-gate/bottom-contact organic thin-film transistors fabricated with PPz-BT show ahigher field-effect mobility (2 × 10 − 3 cm 2 V − 1 s − 1   compared to those fabricated with PPz-MeOBT(4 × 10 − 4 cm 2 V − 1 s − 1  . Keywords:1. INTRODUCTION Over the past decade, organic thin-film transistors (OTFTs)have attracted much attention because of their poten-tial applications in integrated circuits for large-area, low-cost, and flexible electronics. 1–3 Conjugated polymer-basedOTFTs with remarkably improved performance have beendeveloped using several electron-deficient    -conjugatedbuilding blocks. For example, diketopyrrolopyrrole (DPP)-and isoindigo (IID)-based polymers with high mobilitiesexceeding 10 cm 2 V − 1 s − 1 have been achieved throughmolecular engineering and device process optimization. 4  5 Moreover, phthalimide (Ph) derivatives have been usedas electron acceptors in OTFTs because of their facile ∗ Authors to whom correspondence should be addressed. synthesis, strong electron-withdrawing ability, and copla-nar structure. 6  7 Recently, the pyrrolo[3,4-d]pyridazine-5,7-dione (Pz) moiety, obtained by replacing the phenylring of the Ph group with the highly electron-deficientpyridazine, has been introduced as the electron-acceptingunit in conjugated polymers. 8 Zhang et al. reported theapplication of Pz-based donor–acceptor-type polymers toorganic photovoltaics (OPVs), achieving a power con-version efficiency of 3.66%. 9 Although Pz derivativesare rarely used as electron-accepting moieties, Pz-basedpolymers have great potential as electron-deficient    -conjugated building blocks for organic semiconductors.In this study, we designed and synthesizednovel Pz-based polymers for OTFTs. Pyrrolo[3,4-d]pyridazine-5,7-dione was copolymerized with 5,5  -bis(trimethylstannyl)-2,2  -bithiophene and (3,3  -dime- thoxy-[2,2  -bithiophene]-5,5  -diyl)bis(trimethylstannane) J. Nanosci. Nanotechnol. 2017, Vol. 17, No. xx   1533-4880/2017/17/001/006 doi:10.1166/jnn.2017.14734  1  Synthesis and Characterization of Pyrrolo[3,4-d]Pyridazine-5,7-Dione-Based Conjugated Polymers for OTFTs  Kim et al. SNNN NSBrBrNO OC 12 H 25 C 12 H 25 C 12 H 25 C 12 H 25 C 14 H 29 C 14 H 29 C 14 H 29 C 14 H 29 diphenyl etherN NNOOSSBrBr+SS n Pd 2 dba 3 , PPh 3 TolueneNNNOOSSNNNOOSSS nnS S n SS n  SS n OOPPz-BTPPz-MeOBTSSSOO 1342 Scheme 1.  Synthetic routes and chemical structures of the pyrrolo [3,4-d]pyridazine-5,7-dione derivatives and polymers. through Stille coupling polymerization. The introductionof methoxy groups affected the optical, electrochemical,crystallinity, and transistor properties of the polymers. Thesynthetic routes and chemical structures of the monomersand polymers are shown in Scheme 1. 2. EXPERIMENTAL DETAILS 2.1. Materials Solvents were dried and purified by fractional distillationover sodium/benzophenone and handled in a moisture-free atmosphere. 3,6-Bis(5-bromothiophen-2-yl)-1,2,4,5-tetrazine (1), 5,5  -bis(trimethylstannyl)-2,2  -bithiophene( 3 ) and (3,3  -dimethoxy-[2,2  -bithiophene]-5,5  -diyl)bis(trimethylstannane) ( 4 ) were synthesized accordingto literature procedures. 8  10  11 2.2. Measurements 1 H and  13 C NMR spectra were measured using a Var-ian Mercury Plus 300 MHz spectrometer. Thermal anal-yses were carried out on a TA Instrument Q600 (PH407PUSAN, Korea Basic Science Institute (KBSI)) undera N 2  atmosphere, with a heating and cooling rate of 10   C min − 1 . The UV-visible absorption spectra weremeasured using a JASCO JP/V-570 model. The number-average ( M  n   and weight-average ( M  w   molecular weightswere determined by gel permeation chromatography(GPC) (Waters model M590) using polystyrene as stan-dard. Cyclic voltammetry experiments were performedwith a CH Instruments Electrochemical Analyzer in ace-tonitrile solutions containing 0.1 M tetrabutylammoniumtetrafluoroborate (Bu 4 NBF 4   as the supporting electrolyte,using Ag/AgCl as the reference electrode, a platinum wireas the counter electrode, and a platinum working electrode.Density functional theory (DFT) calculations were per-formed using the Gaussian 09W package with the Beckethree-parameter Lee-Yang-Parr (B3LYP) function and the6–31 G(d) basis set to elucidate the HOMO and LUMOlevels. Two-dimensional grazing incidence X-ray diffrac-tion (2D GIXD) experiments were performed at the 3Cbeamlines of the Pohang Accelerator Laboratory (PAL),Korea. 2.3. Device Fabrication and Characterization Top-gate/bottom-contact (TG/BC) OTFTs were fabricatedwith PPz-BT and PPz-MeOBT, and were characterizedin a N 2 -filled glove box. The two polymers were spin-coated at 2000 rpm from chloroform (CF) solutions(5 mg ml − 1   on glass substrates with pre-patterned Ausource/drain electrodes (ca. 20 nm thick), and wereannealed at 280   C for 30 min before gate dielectric depo-sition. Next, poly(methyl methacrylate) (PMMA) (Sigma-Aldrich,  M  n  = 120 kg mol − 1   was spin-coated at 2000 rpmfrom an 80 mg ml − 1 n -butyl acetate solution onto the semi-conductor layers to form a dielectric layer with a thick-ness of ca. 500 nm, as confirmed using a surface profiler(Alpha-Step IQ, Tencor). Finally, 50-nm-thick Al was ther-mally evaporated on top of the PMMA layer within thechannel regions as gate electrodes of top-gate transistors,using a shadow mask. The devices were characterized ina N 2 -filled glove box using a Keithley 4200 parameteranalyzer. 2.4. Synthesis of Monomers and Polymers  2.4.1. Synthesis of 1,4-Bis(5-bromothiophen-2-yl)-6-(2- dodecylhexadecyl)-5H-Pyrrolo[3,4-d]Pyridazine- 5,7(6H)-Dione(2) Compound  1  (1.2 g, 2.97 mmol) and 1-(2-dodecylhexadecyl)-1H-pyrrole-2,5-dione (1.5 g,3.06 mmol) were dissolved in 40 ml diphenyl ether andheated to 160   C under nitrogen for 16 h. The mixture wasallowed to cool down to room temperature and purifiedby silica column chromatography. The organic layer was 2  J. Nanosci. Nanotechnol. 17, 1–6  ,  2017   Kim et al.  Synthesis and Characterization of Pyrrolo[3,4-d]Pyridazine-5,7-Dione-Based Conjugated Polymers for OTFTs Figure 1.  TGA (outer) and DSC (inner) thermograms for the polymers. extracted with EtOAc, dried over MgSO 4 , concentrated,and purified by silica column chromatography using ahexane/chloroform (1:1) mixture as eluent to give yellowsolid compound  2  (1.4 g, 54%)  1 H-NMR (300 MHz,CDCl 3  : 8.64 ( d , 2 H), 7.20 ( d , 2 H), 3.67 ( d , 2 H),1.91 ( m , 1 H), 1.24 ( br  , 48 H), 0.87–0.83 ( m , 6 H).  2.4.2. Polymerization of poly{6-(2-dodecylhexadecyl)-1,4-di(thiophen-2-yl)-5H-pyrrolo[3,4-d]pyridazine- 5,7(6H)-Dione-Alt-2,2  -Bithiophene} (PPz-BT) A Mixture of   2  (250 mg, 0.289 mmol),  3  (142 mg,0.289 mmol), Pd 2 dba 3  (5.3 mg, 5.78   mol), and PPh 3 (6.1 mg, 23.1   mol) in anhydrous toluene (5 ml) wasstirred at 110   C for 2 days. Excess amounts of 2-bromothiophene and tributyl(thiophen-2-yl)stannane wereadded for end-capping, and the mixture was stirred for12 h. Next, the solution was slowly added into methanol.The resulting precipitate was collected by filtration andwashed by Soxhlet extraction with acetone for 1 day toremove the oligomers and the Pd catalyst. Flash columnchromatography was then performed in order to removeany residue of Pd catalyst, and the solvent was evap-orated. The polymer was dissolved in chloroform andreprecipitated in methanol. The precipitate was collectedby filtration and dried at 60   C. The resulting polymerswere soluble in common organic solvents (195 mg, 77%).  2.4.3. Polymerization of poly{6-(2-dodecylhexadecyl)-1,4-di(thiophen-2-yl)-5H-pyrrolo[3,4-d]pyridazine- 5,7(6H)-dione-alt-3,3  -dimethoxy-2,2  - bithiophene} (PPz-MeOBT). The same procedure used for the polymerization of PPz-BT was followed, using a mixture of   2  (250 mg,0.289 mmol),  4  (159 mg, 0.289 mmol), Pd 2 dba 3  (5.3 mg,5.78   mol), and PPh 3  (6.1 mg, 23.1   mol) in anhydroustoluene (5 ml), to give PPz-MeOBT (187 mg, 69%). 3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization PPz-BT and PPz-MeOBT were synthesized via Stillecoupling reaction as shown in Scheme 1. The two poly-mers showed good solubility in common organic sol-vents such as CF, toluene, tetrahydrofuran, chlorobenzene,and 1,2-dichlorobenzene, and formed uniform thin films.The number-average molecular weights of PPz-BT andPPz-MeOBT, measured by gel permeation chromatogra-phy (GPC) using polystyrene as standard, were 48,000(PDI = 1  2) and 36,000 (PDI = 1  4), respectively. The ther-mal stabilities of the polymers were evaluated by ther-mogravimetric analysis (TGA). PPz-BT and PPz-MeOBTshowed good thermal stabilities, losing less than 5% of their weight upon heating up to 367 and 366   C, respec-tively, under a nitrogen atmosphere. The DSC thermo-grams did not showed any transition up to 300   C. TheTGA and DSC thermograms of the polymers are shown inFigure 1. 3.2. Optical and Electrochemical Properties The absorption spectra of the polymers in chloroformand in the thin-film state are shown in Figure 2. In bothcases, broad absorption bands are observed from 400 to800 nm; however, the absorption of PPz-MeOBT is red-shifted compared with that of PPz-BT. This indicates thatthe electron-donating effect of the methoxy group results Figure 2.  UV-visible absorption spectra of the polymers in chloroform and in the thin-film state. J. Nanosci. Nanotechnol. 17, 1–6,  2017   3  Synthesis and Characterization of Pyrrolo[3,4-d]Pyridazine-5,7-Dione-Based Conjugated Polymers for OTFTs  Kim et al. Figure 3.  Cyclic voltammograms of the polymers. in effective intramolecular charge transfer (ICT) interac-tions between donor and acceptor moieties, thus leadingto a decrease of the optical band gap. The optical bandgap energies ( E  opt g    of PPz-BT and PPz-MeOBT were cal-culated from the absorption edge in the absorption spec-trum of the film and were found to be 1.63 and 1.40 eV,respectively.The cyclic voltammetry of the polymers are shownFigure 3. The HOMO energy levels of PPz-BT and PPz-MeOBT, determined by measuring the oxidation onsets( E  ox   of the polymer films by cyclic voltammetry, were − 5.28 and  − 4.91 eV, respectively. These results demon-strated that the introduction of methoxy groups intothe bithiophene unit effectively increased the electron-donating ability, narrowed the band gap, and raised theHOMO energy level. The LUMO energy levels of PPZ-BTand PPz-MeOBT, determined by combination of the mea-sured HOMO energy values and the optical band gaps of the polymers, were  − 3.65 and  − 3.51 eV, respectively. Theoptical and electrochemical properties of the polymers aresummarized in Table I. 3.3. DFT Calculations DFT calculations were performed with the B3LYP func-tional and 6-31G(d) basis sets using simplified alkyl chainsto reduce the calculation time. As shown in Figure 4, thecharge density of PPz-BT and PPz-MeOBT in HOMO isdelocalized along the conjugated backbone, whereas theLUMO is localized on the pyrrolo[3,4-d]pyridazine-5,7-dione moiety. The dihedral angles were calculated to be3.95  , 13.03  , and 0.58  for PPz-BT, and 1.64  , 0.01  , and0.74  for PPz-MeOBT. This result can be explained by the Table I.  Optical and electrochemical properties of the synthesizedpolymers.  max  abs  (nm)   max  (nm)   edge  (nm)Optical HOMO LUMOPolymers Solution Film Film  E  opt g  (eV) (eV) (eV)PPz-BT 570 581 758 1.63  − 5.28  − 3.65PPz-MeOBT 608 618 886 1.40  − 4.91  − 3.51 Figure 4.  The most stable conformations and frontier orbitals of PPz-BT and PPz-MeOBT. noncovalent S · O attractive interactions, which improvedthe conjugated backbone planarity. 3.4. 2D GIXD Analysis In order to reveal the crystallinity of the thin films, 2DGIXD experiments were performed. At 280   C, the 2DGIXD pattern of PPz-BT revealed a highly ordered edge-on structure, whereas the PPz-MeOBT showed a bimodalstructure in which the edge-on and face-on orientationscoexisted (Fig. 5). The lamellar spacing and the    –   stacking distance are, respectively, 31.89 Å and 3.74 Å forPPz-BT and 32.22 Å and 3.65 Å for PPz-MeOBT. Thesmaller    –    stacking distance in PPz-MeOBT as com-pared with that in PPz-BT can be attributed to its moreplanar structure. In order to gain further insight into thepolymer orientation, azimuthal scans were conducted at q   =  1  6894 Å − 1 , at which (010) diffraction peaks wereobserved (Fig. 5(e)). 3.5. OTFT Performance To investigate the charge transport characteristicsof pyrrolo[3,4-d]pyridazine-5,7-dione-based polymers, Figure 5.  Two-dimensional grazing incidence X-ray diffraction (a, b),out-of-plane and in-plane X-ray scattering profiles, (c, d) and azimuthangle scan, (e) of PPz-BT and PPz-MeOBT. 4  J. Nanosci. Nanotechnol. 17, 1–6  ,  2017   Kim et al.  Synthesis and Characterization of Pyrrolo[3,4-d]Pyridazine-5,7-Dione-Based Conjugated Polymers for OTFTs Figure 6.  Transfer and output characteristics of the PPZ-BT (a, b) and PPz-MeOBT-based (c, d) OTFTs.  I  D ,  V  D ,  V  G -drain current, drain voltage andgate voltage, respectively and  I  D1 ,  I  D2  drain currents in linear and saturation regimes, respectively Applied  V  D  = − 80 V. TG/BC OTFTs were fabricated. Figure 6 shows the rep-resentative transfer and output curves at various gatevoltages. The fabricated OTFTs exhibited typical  p -typetransistor characteristics. The field-effect mobility (  ) wascalculated in the saturation regime using the followingequation: I  D  = W  2 LC  i V  GS − V  th  2 (1)where  W   and  L  are the channel width and length, respec-tively ( W   =  1 mm,  L  =  20   m),  C  i  is the capacitanceof the gate dielectric,  V  th  is the threshold voltage, and   is the field-effect mobility. 12 The maximum hole    of PPz-BT OTFTs (2  0 ×  10 − 3   was one order of magni-tude higher than that of the PPz-MeOBT OTFTs (4  0 × 10 − 4 cm 2 V − 1 s − 1  , as summarized in Table II. Althoughthe backbone of polymer PPz-MeOBT showed improvedplanarity as confirmed by DFT calculations and 2D GIXDanalysis, the PPz-MeOBT OTFTs exhibited lower charge Table II.  Fundamental parameters of the fabricated OTFTs.AnnealingSolvent temperature Mobility  V  th Semiconductor (concentration) (  C) (cm 2  /vs) (V)  I  on / off  PPZ-BT CF (5 mg/ml) 280/30 2 × 10 − 3 − 42.9 10 3 PPZ-MeOBT CF (5 mg/ml) 280/30 4 × 10 − 4 − 63.3  < 10 2 transport characteristics than the PPz-BT OTFTs. This canbe attributed to the bimodal orientation of the PPz-MeOBTmolecules in thin film, which is less favorable for lateralcharge transport through channel as compared with thehighly ordered edge-on orientation exhibited by PPz-BTmolecules after thermal annealing at 280   C. 4. CONCLUSION In conclusion, two polymers (PPz-BT and PPz-MeOBT)containing pyrrolo[3,4-d]pyridazine-5,7-dione were syn-thesized by the Stille coupling reaction. The introductionof methoxy groups into the bithiophene units affected theiroptical, electrochemical, charge transport, and crystallinityproperties. PPz-MeOBT showed a higher HOMO energylevel, improved backbone planarity, and bimodal orienta-tion compared to PPz-BT, which exhibited predominantedge-on orientation. These findings were in good agree-ment with the X-ray data of PPz-BT and PPz-MeOBTOTFTs. Acknowledgments:  This work was supported by theNational Research Foundation of Korea (NRF) grantfunded by the Korean Government (MSIP) (NRF-2014R1A2A2A01007159)and the R&D Convergence Pro-gram of NST (National Research Council of Science andTechnology) of Republic of Korea (CAP-15-04-KITECH). J. Nanosci. Nanotechnol. 17, 1–6,  2017   5
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