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Effect of heterocycles on field-effect transistor performances of donor-acceptor-donor type small molecules
Chemical Physics Letters 661 (2016) 107–113
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Effect of heterocycles on field-effect transistor performances of donor-acceptor-donor type small molecules Mrinmoy Kumar Chini ⇑, Rajashree Y. Mahale, Shyambo Chatterjee Polymers and Advanced Materials Laboratory, CSIR-National Chemical Laboratory, CSIR-Network of Institutes for Solar Energy, Pune 411008, India
a r t i c l e
i n f o
Article history: Received 12 July 2016 In final form 27 August 2016 Available online 29 August 2016 Keywords: Small molecules Donor-acceptor-donor Opto-electronic properties Quadrupole Mobility Field-effect transistor
a b s t r a c t Two D–A–D small molecules comprising triphenylamine and diketopyrrolopyrrole were synthesized having either furan or thiophene connected to the fused lactam ring. In this design, furan/thiophene diketopyrrolopyrrole acts as an acceptor and triphenylamine acts as a donor. Propeller shaped triphenylamine has its effect on packing, processability and plays a vital role in determining the p-p molecular orbital stacking in such compounds and thus the mobility of charge carriers. With TDPPT and FDPPT, maximum hole carrier mobility obtained is 2.88 103 cm2 V1 s1 and 1.60 103 cm2 V1 s1, respectively using bottom gate bottom contact field-effect transistor. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Conjugated polymers (CPs) have been potentially used as an active layer of organic electronic devices such as organic light emitting diodes (OLEDs) [1], organic field-effect transistors (OFETs) [2] and organic photovoltaics [3]. The advantages of CPs are easy solution processability, controllable band gap and excellent charge transport properties [4–7]. The major drawbacks of the CPs have been complex purification process, reproducibility [8], polydispersity, and end group contaminations. Thus, p-conjugated small molecules have attracted a lot of attention owing to their synthetic purity and reproducibility [9–11]. The main issue with the small molecules is solution processability. To overcome this problem, different methods have been reported [12]. These systems have the limited chain to chain contacts that unfavourably impact the device efficiency. In order to enhance the contact between chains, small molecules containing p-stacking functionalities have been synthesized. Lactam containing diketopyrrolopyrrole (DPP) has shown strong tendency to stack, driven by quadrupolequadrupole interaction [13]. Conjugated molecules having strong electron-donating substituent can sufficiently increase the charge carrier mobility [14,15]. Comprising of electron deficient units with an electron rich conjugated moiety is widely reported as the Donor–Acceptor–Do nor (D–A–D) system [16–19]. Thus, a solution processable D–A–D ⇑ Corresponding author. E-mail address: [email protected] (M.K. Chini). http://dx.doi.org/10.1016/j.cplett.2016.08.073 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
small molecule is an attractive material as it incorporates the advantages of both polymers and small molecules and show excellent charge carrier properties despite having less ordered microstructures. DPP is known to be a versatile building block [20,21] and also an outstanding acceptor owing to its strong electron withdrawing nature and reasonably good photochemical stability [22–26]. We have chosen triphenylamine (TPA) which is an excellent optoelectronic organic material [27–29] owing to its good electron-donating and hole-transporting abilities, as a donor to have D-A-D type structure with DPP (acceptor). We observed that the solubility of TPA containing molecules usually enhanced due to its propeller shaped structure. Here, we have connected different heterocyclic molecules (such as furan and thiophene) as a bridge in between TPA and DPP functionality in the molecular backbone of the small molecule. We are interested in studying the impact of the changes in the bridging heterocyclic part of the molecular backbone on the OFET performances of the small D-A-D type molecules. We report the synthesis, electrochemical, optical and charge transport properties of two D-A-D molecules FDPPT and TDPPT. 2. Experimental section 2.1. General methods and materials Instruments and Materials: 1H and 13C NMR spectra were recorded on a Bruker arx 200 MHz and 100 MHz AVANS spectrometer respectively using CDCl3 as the solvent unless otherwise
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noted. Chemical shifts are reported in parts per million (ppm), chemical shifts in 1H NMR were referenced to TMS (0.0 ppm) and 13 C NMR spectra were referenced to CDCl3 (77.0 ppm). MALDITOF mass spectra were recorded on Voyager-De-STR MALDI-TOF (Applied Bio systems, Framingham, MA, USA) equipped with 337nm pulsed nitrogen laser. Sample solution (1 lM) was mixed with DHB (2,5 dihydroxy benzoic acid) matrix in CHCl3 and sonicated before casting on 96-well stainless steel MALDI plate. Perkin Elmer Lambda-35 spectrophotometer was used for UV/vis spectra. PerkinElmer STA 6000 thermogravimetric analyzer was used for Thermogravimetric analysis (TGA). The X-ray diffraction data were obtained on X’PertPro Panalytical Diffractometer at a wavelength of 1.5406 Å. Atomic force microscopy studies were performed with a Nanoscope IIIa microscope and carried out in tapping mode at ambient temperature. All the electrochemical studies were done using CH-Instruments. Agilent 4156 C semiconductor probe analyzer and semi probe station had been used for all the FET measurements. Thiophene-2-carbonitrile, Furan-2-carbonitrile, Potassium tertbutoxide, Diethyl succinate, Triphenylamine (TPA), Potassium acetate, Bis(pinacolato)diboron (B2pin2), were purchased from Sigma Aldrich and used without further purification. NBromosuccinimide (NBS) were purchased from Sigma Aldrich and used after recrystallization. 1,10 -Bis [(diphenylphosphino)ferro cene] dichloropalladium (II) [Pd(dppf)Cl2], Tris(dibenzylideneace tone)dipalladium (0) [Pd2(dba)3], Tri (o-tolyl)phosphine [P(otolyl)3] were purchased from Alfa Aesar chemicals. Sodium metal, tert-amyl alcohol, Acetic acid, Chloroform (CHCl3), N, Ndimethylformamide (DMF), Toluene, Methanol (MeOH), Potassium carbonate (K2CO3) were purchased from Loba Chemie. All the solvents were dried by following reported procedures. 2.2. Synthetic procedures 2.2.1. 3,6-Di (thiophen-2-yl)-2,5-dihydropyrrolo [3,4-c] pyrrole-1,4dione (S-DPP) (1) To argon filled oven-dried three-neck round-bottom flask equipped with a magnetic stir bar, a dropping funnel and a reflux condenser, potassium tert-butoxide (7.72 g, 68.9 mmol) and tertamyl alcohol (35 mL) were added. The mixture was heated to 100–110 °C for 1.5 h. To this mixture 2-thiophenenitrile (5.0 g, 45.8 mmol) was injected in one portion and the stirring continued at 105 °C for 30 min. A mixture of diethyl succinate (4.00 g, 22.9 mmol) in tert-amyl alcohol (10 mL) was added drop wise over a period of 1 h with rapid stirring. The mixture was then stirred at 100–110 °C for a further 4 h, and then cooled to 50 °C. Then the mixture was diluted with of methanol (30 mL) and neutralized with acetic acid (5 ml). The reaction mixture was then heated to reflux for 45 min before cooling to room temperature. The suspension was filtered over a Buchner funnel and the solid was washed with hot methanol and water several times and dried under vacuum at 80 °C for 16 h to give the product, 3,6-di (thiophen-2-yl)2,5-dihydropyrrolo [3,4-c] pyrrole-1,4-dione (S-DPP) (1). Yield: 3.6 g (26%) as a red solid. This compound was used without further purification. 1H NMR (DMSO-d6, 400 MHz) dppm: 7.30 (dd, 2H,), 7.95 (d, 2H), 8.22 (d, 2H), 11.21 (s, 2H); 13C NMR (DMSO-d6, 400 MHz) dppm: 108.53, 128.65, 130.76, 131.23, 132.58, 136.11, and 161.58. 2.2.2. 3,6-Di (furan-2-yl)-2,5-dihydropyrrolo [3,4-c] pyrrole-1,4-dione (O-DPP) (2) Compound 2 was synthesized following same procedure for compound 1. Yield: 3.1 g (21%). 1H NMR (DMSO-d6, 400 MHz) dppm: 6.83 (dd, 2H), 7.65 (d, 2H), 8.04 (d, 2H), 11.17 (s, 2H). 13C NMR (DMSO-d6, 400 MHz) dppm: 107.57, 113.71, 116.79, 131.27, 143.75, 146.91, and 161.71.
2.2.3. 2,5-Dioctyl-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c] pyrrole-1,4-dione (3) In a three-necked, oven-dried 250 mL round-bottom flask, 3,6dithiophen-2-yl-2,5-dihydro pyrrolo[3,4-c]pyrrole-1,4-dione (1) (3.00 g, 10.0 mmol) and anhydrous K2CO3 (5.52 g, 30.0 mmol) were dissolved in 100 mL of anhydrous N,N-dimethylformamide (DMF) and heated to 120 °C under argon for 1 h. n-octylbromide (5.76 g, 30.0 mmol) was then added dropwise, and the reaction mixture was further stirred and heated overnight at 130 °C. The reaction mixture was allowed to cool down to room temperature; after that it was poured into 400 mL of distilled water, and the resulting suspension was stirred at room temperature for 1 h. The solid was collected by vacuum filtration, washed with several portions of distilled water, methanol, and then air-dried. The crude product was purified by flash chromatography using dichloromethanehexane as eluent, and the solvent was removed in vacuo to obtain a pure product, 2,5-dioctyl-3,6-di(thiophen-2-yl)-2,5-dihydropyr rolo[3,4-c]pyrrole-1,4-dione (3) as a purple brown shiny crystalline powder (yield: 75.4%). 1H NMR (400 MHz, CDCl3) dppm: 8.94 (d, 2H), 7.64 (d, 2H), 7.29 (t, 2H), 4.08 (t, 4H), 1.75 (m, 4H), 1.27– 1.44 (m, 28H), 0.88 (t, 6H). 13C NMR (400 MHz, CDCl3) dppm: 161.04, 139.69, 134.92, 130.36, 129.44, 128.27, 107.35, 41.89, 31.43, 29.62, 28.87, 26.54, 22.30, 13.76.
2.2.4. 3,6-Di(furan-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4-c] pyrrole-1,4-dione (4) Compound 4 was synthesized following same procedure for compound 3. By using compound O-DPP (2) (3 g, 11.18 mmol) along with n-octylbromide (6.44 g, 33.55 mmol) and K2CO3 (6.17 g 44.73 mmol) (yield: 81.6%). 1H NMR (400 MHz, CDCl3) dppm: 8.31 (d, 2H), 7.64 (d, 2H), 6.70 (t, 2H), 4.11 (t, 4H), 1.68 (m, 4H), 1.27–1.39 (m, 28H), 0.87 (t, 6H). 13C NMR (400 MHz, CDCl3) dppm: 160.54, 144.83, 133.33, 120.18, 117.71, 113.60, 106.10, 42.09, 31.47, 29.87, 28.85, 26.51, 22.30, 13.76.
2.2.5. 3,6-bis(5-Bromothiophen-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione (5) A 100 mL single-neck round-bottom flask was charged with a stir bar, compound 3 (1 g, 1.72 mmol) was added to a solution in chloroform (50 mL) under ambient conditions. Flask was wrapped in aluminum foil to avoid the exposure of the reaction to light. After the reaction mixture was stirred in an ice bath at 0 °C for 20 min, N-bromosuccinimide (NBS) (0.76 g, 4.30 mmol) was added in three portions to it, the solution stirred at room temperature for 48 h. Resulting crude product was extracted with chloroform, washed with water, and dried over anhydrous Na2SO4. Solvent was removed under reduced pressure and the product was purified using silica gel chromatography eluting with a mixture of hexane and DCM to get dark red purple solid. Yield: (1.2 g, 72%). 1H NMR (CDCl3, 400 MHz) dppm: 8.69 (d, 2H), 7.25 (d, 2H), 3.99 (t, 4H), 1.72 (m, 4H), 1.42–1.28 (m, 20H), 0.89 (t, 6H). 13C NMR (CDCl3, 400 MHz) dppm: 160.77,138.74,135.09, 131.38, 130.86, 118.88, 107.55, 42.03, 31.50, 29.71, 28.88, 26.56, 22.37, 13.82.
2.2.6. 3,6-bis(5-Bromofuran-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4c]pyrrole-1,4-dione (6) Compound 6 was synthesized by following the same procedure used for the synthesis of compound 5. By using compound 4 (1.5 g, 3.05 mmol) and NBS (1.35 g, 7.61 mmol). Yield: (1.35 g, 78%). 1H NMR (CDCl3, 400 MHz) dppm: 8.26 (d, 2H), 6.64 (d, 2H), 4.06 (t, 4H), 1.70 (m, 4H), 1.41–1.28 (m, 20H), 0.88 (t, 6H). 13C NMR (CDCl3, 400 MHz) dppm: 160.65, 143.54, 132.63, 122.03, 115.26, 110.25, 105.97, 45.94, 39.83, 30.23, 28.45, 22.93, 22.89, 13.81.
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2.2.7. N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) aniline (8) A 3-neck 250 mL round bottom flask was charged with a stir bar, 4-bromotriphenylamine (4.00 g, 12.3 mmol) (7), bis(pinacolato)diboron (B2pin2) (3.26 g, 12.8 mmol), anhydrous potassium acetate (KOAc) (3.28 g, 33.41 mmol), dichloro[1,10 -bis(diphenylpho sphino)-ferrocene]palladium(II) dichloromethane adduct (Pd (dppf)Cl2CH2Cl2) (0.243 g, 0.334 mmol) and degassed dioxane (120 mL). After the reaction mixture was heated at 85 °C for 16 h, it was extracted with diethyl ether and washed with distilled water. The organic extract was dried over MgSO4, and solvent was removal under reduced pressure yielded brown viscous oil. Purification with flash chromatography (25% hexanes in CH2Cl2) yielded 4.30 g of compound 8 as tacky off-white solid (95%). 1H NMR (400 MHz, CDCl3) dppm: 7.69 (d, 2 H), 7.27 (t, 4 H), 7.23 (d, 4 H), 7.13–7.02 (m, 4 H), 1.34 (s, 12 H). 13C (400 MHz, CDCl3) dppm: 150.65, 147.54, 136.01, 129.45, 125.26, 123.87, 121.79, 83.76, 25.26. 2.2.8. 3,6-bis(5-(4-(Diphenylamino)phenyl)thiophen-2-yl)-2,5dioctyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (9) A 50 mL Schlenk tube was charged with materials, 5 (0.800 g, 1.17 mmol), 8 (0.958 g, 2.58 mmol), bis(dibenzylideneacetone) palladium (0) (Pd2(dba)3) (0.021 g, 0.023 mmol), tri-otolylphosphine (P(o-tol)3) (0.028 g, 0.094 mmol), anhydrous K2CO3 (1.54 g, 11.13 mmol), 2 drops of aliquat 336, freeze-pumpthawed toluene (20 mL) and freeze-pump-thawed distilled water (6.0 mL). The reaction mixture was heated at 90 °C for 16 h before being precipitated into 250 mL of MeOH. The precipitates were filtered through a 20 lm nylon membrane. Purification by flash chromatography (10% hexanes in chloroform) yielded compound 9 as a metallic purple solid (56%). 1H NMR (400 MHz, CDCl3) dppm: 8.96 (s, 2H), 7.52–7.06 (m, 30H), 4.09 (s, 4 H), 1.76 (m, 4H), 1.55–1.24 (m, 20H), 0.82 (t, 6H). 13C NMR (400 MHz, CDCl3) dppm: 161.02, 152.75, 149.57, 148.24, 146.76, 141.78, 138.90, 136.58, 129.12, 127.48, 124.67, 123.36, 107.41, 41.95, 31.48, 29.69, 28.90, 26.59, 22.30, 13.76 ESI–HRMS calcd for C66H66N4O2S2 1011.40, found 1011.87. 2.2.9. 3,6-bis(5-(4-(Diphenylamino)phenyl)furan-2-yl)-2,5-dioctyl2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (10) Compound 10 was synthesized by following the procedure used for the synthesis of compound 9. Herein, 6 (0.800 g, 1.23 mmol), 8 (1.01 g, 2.71 mmol), bis(dibenzylideneacetone)palladium (0) (Pd2(dba)3) (0.023 mg, 0.0245 mmol), tri-o-tolylphosphine (P(o-tol)3) (0.030 g, 0.098 mmol), anhydrous K2CO3 (1.60 g, 11.68 mmol), 2 drops of aliquat 336 were used, (Yield = 62.5%). 1 H NMR (400 MHz, CDCl3) dppm: 8.37 (s, 2 H), 7.55–7.57 (d, 4 H), 7.29–7.24 (t, 8 H), 7.12–7.05 (m, 16 H), 4.17 (t, 4 H), 1.76 (m, 4H), 1.57–1.18 (m, 20H), 0.78 (t, 6H). 13C NMR (400 MHz, CDCl3) dppm: 160.81, 157.04, 148.55, 147.02, 143.50, 132.18, 129.44, 125.44, 125.09, 123.76, 122.69, 122.50, 107.97, 42.56, 31.72, 30.35, 29.43, 29.22, 27.13, 22.56, 14.04. ESI–HRMS calcd for C66H66N4O4 979.28, found 979.35. 3. Results and discussion The synthetic steps for the synthesis of the DPP molecules are shown in Scheme 1. The structure of all the key intermediates and target D-A-D molecules has been confirmed by 1H NMR, 13C NMR and ESI–HRMS. The O-DPP and S-DPP were synthesized by Stobbe condensation between diethyl succinate and 2-furonitrile or 2thiophenecarbonitrile, respectively. The O-DPP and S-DPP were subjected to alkylation using 1-bromo octane, which was then
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reacted with NBS to obtain compound 5 and 6 according to reported procedure [30]. Then borylation of compound 7 was carried out by Miyaura borylation reaction using Pd(dppf)Cl2 catalyst and anhydrous dioxane as solvent to get compound 8. The final compounds 9 and 10 were obtained with the Suzuki-Miyaura coupling using the following reagents, bis(dibenzylideneacetone)palla dium (0) (Pd2(dba)3), tri-o-tolylphosphine (P(o-tol)3) and anhydrous K2CO3 in anhydrous toluene solvent. The UV–vis absorption spectra were recorded in chloroform (CHCl3) solution. It can be noticed that both the FDPPT and TDPPT molecules exhibits welldefined two peaks both in solution and in thin film. The lower wavelength absorption bands ascribed to the p–p⁄ transitions and the strongest absorption peak results from the intramolecular charge transfer (ICT) [31]. Absorption spectra of compound FDPPT in chloroform shows two peaks at kmax 587.2 and 635.1 nm and TDPPT in chloroform shows two peaks at kmax 593.2 and 633.9 nm (Fig. 1a). Both the molecules have shown similar characteristic in their UV/vis absorption spectra possibly due to their similar molecular structures. The maximum absorption wavelength (kmax) of both the TDPPT and FDPPT are red-shifted as much as 15 to 45 nm compared to their absorption in solution. This can attributed to the better packing (p–p stacking) of the small molecules in solid thin film state. The absorption spectrum of TDPPT becomes remarkably broad compared to the FDPPT spectrum in solid state which indicates that the different heterocycle has its influence on molecular packing and p–p stacking in film. The optical band gaps are estimated from the absorption edges of thin films using the following formula Eopt g = 1240/konset. The band gaps of FDPPT and TDPPT are 1.82 eV and 1.79 eV, respectively. The Egopt of FDPPT is noticeably larger than that of TDPPT. The frontier orbital energy levels (HOMO-LUMO) of these materials were determined by cyclic voltammetry (CV) techniques. Platinum wire and platinum foil were used as working and counter electrode, respectively. Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) electrode was used as reference electrode. The cyclic voltammetry were performed using tetra butyl ammonium perchlorate (0.01 M TBAP in acetonitrile) solution as supporting electrolyte at room temperature with a scan rate of 50 mV/s and ferrocenium/ferrocene (Fc+/Fc) redox couple as external reference. The CVs of TDPPT and FDPPT are shown in Fig. 1b. HOMO values were calculated using the formula, HOMO = [Eox (onset) + 4.8] eV assuming that the absolute energy level of Fc is 4.8 eV [32]. The LUMO level was estimated from the optical band gap values. The HOMO and the LUMO level of TDPPT (5.2 eV and 3.4 eV) is both deeper than FDPPT (5.0 eV and 3.2 eV), while both the small molecules are identical because they have same electron donating (TPA) and accepting unit (DPP) except two different heterocyclic units (furan/ thiophene). The sulphur atom in thiophene unit is responsible for lower HOMO/LUMO level for TDPPT. It is quite often seen in oxygen vs sulphur systems. The optical band gap is weakly affected by this. The energy level diagram of the TDPPT and FDPPT is shown in Fig. 1d. DFT studies were done using B3LYP functional and polarized 631g⁄ basis set. The surface plots of both the molecules show that the wave function is well delocalized within the p-backbone for HOMO since both TPA and furan/thiophene DPP is contributing. However, the wave function is delocalized preferably over the DPP part in case of LUMO for both the compounds. This may be attributed to its electron accepting nature. The surface energy plots are shown in Fig. S18 (ESI). The optimized ground state geometry of FDPPT and TDPPT were taken for determining the dihedral angle at the junction of the furan/thiophene part of the DPP and phenyl part of the TPA connected to furan/thiophene. The dihedral angle was 175.49° for FDPPT whereas the angle was found to be 165.41° for TDPPT. Optimized ground state structures of FDPPT and TDPPT and dihedral angle are shown in Fig. S19. The dihedral
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OH K+ButO-, CN
X
H N
X
Br-C8H17
100-110 °C , 4-6 hrs O
X = S/O
R N
O
O
O
O O
X
N H
K2CO3, DMF,120 °C
X N R
Br
O
X
RT, 48 hrs
O
X N R
Br
5 S-DPPR-Br 6 O-DPPR-Br
O 5 S-DPPR-Br 6 O-DPPR-Br
O
Pd(dppf)Cl2.CH2Cl2 Br
O
R N
3 S-DPPR 4 O-DPPR
B B
N
O
1 S-DPP 2 O-DPP
O O
X
NBS, CHCl3
KOAc
R
Pd2(dba)3, P(o-tolyl)3 O N
B
K2CO3, aliquat 336
O
Dioxane, 85 °C, 16h
N N
H2O/Toluene 90 °C, 16h
X O
O X
N
N R
7 Br-TPA
8 TPA-BE
9 S-DPPT (TDPPT) 10 O-DPPT (FDPPT) R= n-Octyl
Scheme 1. Synthesis scheme of FDPPT and TDPPT.
Fig. 1. (a) UV/vis spectra in solution and in thin film state, (b) cyclic voltammograms showing oxidation, (c) the schematic diagram of FET device and (d) energy levels of FDPPT and TDPPT.
angle between the planes indicates that the DPP and specified phenyl part of the TPA are in plane with a deviation of 5° for FDPPT which is higher for TDPPT (15°). The thermal stability of the molecules was examined using thermogravimetric analysis. Significant weight loss was not observed until 230 °C in the thermogram, indicating good thermal stability of the molecules. The X-ray diffraction (XRD) measurement on the thin films deposited at 80 °C was also carried out and the diffraction pattern was given in Fig. 4b. This suggests that the thin film molecules are amorphous in nature due to the
presence of propeller structured TPA moiety. The thin film morphologies of deposited FDPPT and TDPPT were investigated by atomic force microscopy (AFM) before and after annealing under similar conditions, used for FET device fabrication. The morphology of FDPPT was found to be a non uniform film with appearance of aggregation of small rods at random places. They become more pronounced upon annealing (Fig. 2b). On the other hand, TDPPT also showed a film with nano rods (Fig. 2c). The thin films of FDPPT and TDPPT deposited at room temperature showed a root mean square (rms) roughness value (Rq) of 18.5 nm and 14.9 nm after
M.K. Chini et al. / Chemical Physics Letters 661 (2016) 107–113
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Fig. 2. (a and b) Represents AFM amplitude images of FDPPT, (c and d) represents AFM amplitude images of TDPPT before and after annealing of the thin films, respectively.
annealing showed the Rq values of 23.8 nm and 18.1 nm respectively. The roughness had increased after annealing. The interaction between alkyl chains of the modified substrates and the small molecules is majorly responsible for the change in the surface roughness [33]. This phenomenon is known for conjugated small molecules. Finally after all the molecular characterization, we have done the transistor measurements using prefabricated field effect transistors having bottom gate (SiO2) and bottom contact (Au) configuration on top of the silicon wafer substrates. The thickness of the gate dielectric, SiO2 was 210 nm. The substrates were cleaned using ultrasonication in chloroform, acetone and isopropanol respectively prior to use. The substrates were modified by dipping them into 5% (V/V) hexamethylenedisilazane (HMDS) solution of CHCl3 followed by annealing at 100 °C for 10 min under N2 atmosphere since devices based on unmodified substrates showed no measurable current (ID) upon sweeping the drain voltage (VD) at various gate voltages (VG) due to the possible poor interaction between the SiO2 and the studied molecules [34]. The FDPPT and TDPPT molecules were then spin coated from chloroform solution (10 mg/mL) on top of the HMDS modified substrates and the device characterization was carried out under inert conditions. The annealing temperature could enhance the device performances. But beyond certain annealing temperature mobility decreased rapidly. Thus, beyond that particular annealing temperature, we had not annealed the thin film devices. To check the change in FET performances for the FDPPT and TDPPT, the prepared devices were further annealed at temperatures such as 80 °C and 100 °C for 10 min each inside glove box. The VD was swept between 0 and 100 V while holding constant negative VG. The output characteristics of the devices showed obvious linear and saturation regimes for FDPPT (Fig. 3a) and TDPPT (Fig. 3c) and indicated that both of the molecules are hole transporting (p type) in nature. The transfer characteristics were obtained by sweeping the VG
while holding the VD at 40 V and 60 V for FDPPT and TDPPT, respectively (determined from the output characteristics). The charge carrier mobility (l) in the saturation regime was calculated using the following relationship, ID = (lCW/2L) [(VG VT)2]. W and L are the semiconductor channel width and length, respectively. Ci (Ci = 14.9 nF) is the capacitance per unit area of the gate dielectric layer. VT is the threshold voltage. The device parameters are summarized in Table 1. The hole mobility of FDPPT and TDPPT based transistors was 3.47 104 cm2 V1 s1 and 6.29 104 cm2 V1 s1 measured for unannealed thin films, respectively. Upon annealing at 80 °C, the mobility for FDPPT and TDPPT was increased to 1.59 103 cm2 V1 s1 and 7.79 104 cm2 V1 s1, respectively. This is attributed to the better packing upon annealing. Furthermore, on annealing at 100 °C the hole mobility was reduced to 5 104 cm2 V1 s1 and 7 105 cm2 V1 s1, respectively. The maximum mobility obtained for the annealed FDPPT and TDPPT thin film was 2.88 103 cm2 V1 s1 and 1.60 103 cm2 V1 s1 respectively. The hole carrier mobility is comparable for both FDPPT and TDPPT. This is likely due to the TPA donor units nullifying the possible packing facilitated furan and thiophene. In such cases, morphology can impact the charge carrier mobility [35]. However, both FDPPT and TDPPT showed similar morphology, hence the charge carrier mobility is not significantly different. We observed the contact resistance (RC) varied with annealing temperature. The RC was calculated using the relationship, RT = RC + RCh. Where, RT and RCh are the total and the channel resistance, respectively. The RC for the FDPPT unannealed thin film device is 2 109 O at RT whereas upon annealing RC was reduced little bit to 1 109 O at 80 °C (Fig. 4c). The RC for the TDPPT as such spin coated thin film device is 5 109 O whereas on annealing RC was reduced by order to 1 108 O at 80 °C (Fig. 4d). This can be correlated to the increase in mobility values. On further annealing at higher temperature, the RC values increase which in turn correlates to the lower charge carrier mobility obtained.
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Fig. 3. Output (a) and transfer characteristics curve (b) of FDPPT. Output (c) and transfer characteristics (d) of TDPPT after annealing, respectively.
Table 1 OFET parameters at various temperatures. Molecules
T (°C)
lh,max (cm2 V1 s1)
lh,avg (cm2 V1 s1)
VT (V)
Ion/off
FDPPT
As-cast 80 100
8.56 104 2.88 103 4.15 104
3.47 104 1.59 103 5.04 104
46 62 58
5.5 102 6.5 101 4.5 101
TDPPT
As-cast 80 100
1.53 103 1.60 103 1.04 104
6.29 104 7.79 104 6.94 105
22 18 15
4.0 102 1.0 103 5.3 102
Fig. 4. (a) The TGA curve, (b) thin film XRD of both FDPPT and TDPPT annealed at 80 °C shows amorphous nature, (c and d) represents the RC vs VG plot for FDPPT and TDPPT respectively.
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4. Conclusion In summary, we have synthesized and characterized DPP and TPA based small molecules with octyl side chains. The synthesized molecules are solution processable and form good films on HMDS modified substrates. Before annealing, the HMDS modified devices have shown proper FET characteristics and the obtained charge carrier mobility values for both FDDPT and TDPPT were low but comparable. On annealing the modified thin films at 80 °C, the hole carrier mobility values for both D-A-D molecules have increased by approx. one order to give relatively high values due to better packing and improved quadrupole-quadrupole interaction arising from the DPP units enhanced the interchain contacts. The maximum mobility obtained for FDPPT and TDPPT thin film was 2.88 103 cm2 V1 s1 and 1.60 103 cm2 V1 s1 while annealed at 80 °C, respectively. Overall, we can conclude that heterocyclic moieties have little impact on charge carrier mobility of these small molecules while connected to TPA moiety. Acknowledgements MKC and RYM thank CSIR for fellowship. We are grateful to Dr. K. Krishnamoorthy for his support. We also thank CSIR for financial support through NWP 54 (TAPSUN). We thank Mrs. Pooja Muddellu for AFM imaging. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.08. 073. References [1] J. Pei, W.L. Yu, W. Huang, A.J. Heeger, A novel series of efficient thiophenebased light-emitting conjugated polymers and application in polymer lightemitting diodes, Macromolecules 33 (2000) 2462. [2] Y. Huang, X. Guo, F. Liu, L. Huo, Y. Chen, T.P. Russell, C.C. Han, Y. Li, J. Hou, Improving the ordering and photovoltaic properties by extending p– conjugated area of electron-donating units in polymers with D-A structure, Adv. Mater. 24 (2012) 3383. [3] F. Liu, C. Wang, J. Baral, W. Zhao, L. Zhang, J.J. Watkins, A.L. Briseno, T.P. Russell, Relating chemical structure to device performance via morphology control in diketopyrrolopyrrole-based low band gap polymers, J. Am. Chem. Soc. 35 (2013) 19248. [4] A. Facchetti, P-Conjugated polymers for organic electronics and photovoltaic cell applications, Chem. Mater. 23 (2011) 733. [5] X. Zhao, X. Zhan, Electron transporting semiconducting polymers in organic electronics, Chem. Soc. Rev. 40 (2011) 3728. [6] L. Biniek, B.C. Schroeder, C.B. Nielsen, I. McCulloch, Recent advances in high mobility donor–acceptor semiconducting polymers, J. Mater. Chem. 22 (2012) 14803. [7] F. Huang, K.S. Chen, H.L. Yip, S.K. Hau, O. Acton, Y. Zhang, J.D. Luo, A.K.Y. Jen, Development of new conjugated polymers with donor-p-bridge-acceptor side chains for high performance solar cells, J. Am. Chem. Soc. 131 (2009) 13886. [8] W.R. Saaneck, R.H. Friend, J.L. Bredas, Electronic structure of conjugated polymers: consequences of electron–lattice coupling, Phys. Rep. 319 (1999) 231. [9] B. Walker, C. Kim, T.Q. Nguyen, Small molecule solution-processed bulk heterojunction solar cells, Chem. Mater. 23 (2011) 470. [10] R. Fitzner, E. Mena-Osteritz, A. Mishra, G. Schulz, E. Reinold, M. Weil, C. Körner, H. Ziehlke, C. Elschner, K. Leo, M. Riede, M. Pfeiffer, C. Uhrich, P. Bäuerle, Correlation of p-conjugated oligomer structure with film morphology and organic solar cell performance, J. Am. Chem. Soc. 134 (2012) 11064. [11] Y. Lin, Y. Li, X. Zhan, Small molecule semiconductors for high-efficiency organic photovoltaics, Chem. Soc. Rev. 41 (2012) 4245.
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Research paper
Effect of heterocycles on field-effect transistor performances of donor-acceptor-donor type small molecules Mrinmoy Kumar Chini ⇑, Rajashree Y. Mahale, Shyambo Chatterjee Polymers and Advanced Materials Laboratory, CSIR-National Chemical Laboratory, CSIR-Network of Institutes for Solar Energy, Pune 411008, India
a r t i c l e
i n f o
Article history: Received 12 July 2016 In final form 27 August 2016 Available online 29 August 2016 Keywords: Small molecules Donor-acceptor-donor Opto-electronic properties Quadrupole Mobility Field-effect transistor
a b s t r a c t Two D–A–D small molecules comprising triphenylamine and diketopyrrolopyrrole were synthesized having either furan or thiophene connected to the fused lactam ring. In this design, furan/thiophene diketopyrrolopyrrole acts as an acceptor and triphenylamine acts as a donor. Propeller shaped triphenylamine has its effect on packing, processability and plays a vital role in determining the p-p molecular orbital stacking in such compounds and thus the mobility of charge carriers. With TDPPT and FDPPT, maximum hole carrier mobility obtained is 2.88 103 cm2 V1 s1 and 1.60 103 cm2 V1 s1, respectively using bottom gate bottom contact field-effect transistor. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Conjugated polymers (CPs) have been potentially used as an active layer of organic electronic devices such as organic light emitting diodes (OLEDs) [1], organic field-effect transistors (OFETs) [2] and organic photovoltaics [3]. The advantages of CPs are easy solution processability, controllable band gap and excellent charge transport properties [4–7]. The major drawbacks of the CPs have been complex purification process, reproducibility [8], polydispersity, and end group contaminations. Thus, p-conjugated small molecules have attracted a lot of attention owing to their synthetic purity and reproducibility [9–11]. The main issue with the small molecules is solution processability. To overcome this problem, different methods have been reported [12]. These systems have the limited chain to chain contacts that unfavourably impact the device efficiency. In order to enhance the contact between chains, small molecules containing p-stacking functionalities have been synthesized. Lactam containing diketopyrrolopyrrole (DPP) has shown strong tendency to stack, driven by quadrupolequadrupole interaction [13]. Conjugated molecules having strong electron-donating substituent can sufficiently increase the charge carrier mobility [14,15]. Comprising of electron deficient units with an electron rich conjugated moiety is widely reported as the Donor–Acceptor–Do nor (D–A–D) system [16–19]. Thus, a solution processable D–A–D ⇑ Corresponding author. E-mail address: [email protected] (M.K. Chini). http://dx.doi.org/10.1016/j.cplett.2016.08.073 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
small molecule is an attractive material as it incorporates the advantages of both polymers and small molecules and show excellent charge carrier properties despite having less ordered microstructures. DPP is known to be a versatile building block [20,21] and also an outstanding acceptor owing to its strong electron withdrawing nature and reasonably good photochemical stability [22–26]. We have chosen triphenylamine (TPA) which is an excellent optoelectronic organic material [27–29] owing to its good electron-donating and hole-transporting abilities, as a donor to have D-A-D type structure with DPP (acceptor). We observed that the solubility of TPA containing molecules usually enhanced due to its propeller shaped structure. Here, we have connected different heterocyclic molecules (such as furan and thiophene) as a bridge in between TPA and DPP functionality in the molecular backbone of the small molecule. We are interested in studying the impact of the changes in the bridging heterocyclic part of the molecular backbone on the OFET performances of the small D-A-D type molecules. We report the synthesis, electrochemical, optical and charge transport properties of two D-A-D molecules FDPPT and TDPPT. 2. Experimental section 2.1. General methods and materials Instruments and Materials: 1H and 13C NMR spectra were recorded on a Bruker arx 200 MHz and 100 MHz AVANS spectrometer respectively using CDCl3 as the solvent unless otherwise
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noted. Chemical shifts are reported in parts per million (ppm), chemical shifts in 1H NMR were referenced to TMS (0.0 ppm) and 13 C NMR spectra were referenced to CDCl3 (77.0 ppm). MALDITOF mass spectra were recorded on Voyager-De-STR MALDI-TOF (Applied Bio systems, Framingham, MA, USA) equipped with 337nm pulsed nitrogen laser. Sample solution (1 lM) was mixed with DHB (2,5 dihydroxy benzoic acid) matrix in CHCl3 and sonicated before casting on 96-well stainless steel MALDI plate. Perkin Elmer Lambda-35 spectrophotometer was used for UV/vis spectra. PerkinElmer STA 6000 thermogravimetric analyzer was used for Thermogravimetric analysis (TGA). The X-ray diffraction data were obtained on X’PertPro Panalytical Diffractometer at a wavelength of 1.5406 Å. Atomic force microscopy studies were performed with a Nanoscope IIIa microscope and carried out in tapping mode at ambient temperature. All the electrochemical studies were done using CH-Instruments. Agilent 4156 C semiconductor probe analyzer and semi probe station had been used for all the FET measurements. Thiophene-2-carbonitrile, Furan-2-carbonitrile, Potassium tertbutoxide, Diethyl succinate, Triphenylamine (TPA), Potassium acetate, Bis(pinacolato)diboron (B2pin2), were purchased from Sigma Aldrich and used without further purification. NBromosuccinimide (NBS) were purchased from Sigma Aldrich and used after recrystallization. 1,10 -Bis [(diphenylphosphino)ferro cene] dichloropalladium (II) [Pd(dppf)Cl2], Tris(dibenzylideneace tone)dipalladium (0) [Pd2(dba)3], Tri (o-tolyl)phosphine [P(otolyl)3] were purchased from Alfa Aesar chemicals. Sodium metal, tert-amyl alcohol, Acetic acid, Chloroform (CHCl3), N, Ndimethylformamide (DMF), Toluene, Methanol (MeOH), Potassium carbonate (K2CO3) were purchased from Loba Chemie. All the solvents were dried by following reported procedures. 2.2. Synthetic procedures 2.2.1. 3,6-Di (thiophen-2-yl)-2,5-dihydropyrrolo [3,4-c] pyrrole-1,4dione (S-DPP) (1) To argon filled oven-dried three-neck round-bottom flask equipped with a magnetic stir bar, a dropping funnel and a reflux condenser, potassium tert-butoxide (7.72 g, 68.9 mmol) and tertamyl alcohol (35 mL) were added. The mixture was heated to 100–110 °C for 1.5 h. To this mixture 2-thiophenenitrile (5.0 g, 45.8 mmol) was injected in one portion and the stirring continued at 105 °C for 30 min. A mixture of diethyl succinate (4.00 g, 22.9 mmol) in tert-amyl alcohol (10 mL) was added drop wise over a period of 1 h with rapid stirring. The mixture was then stirred at 100–110 °C for a further 4 h, and then cooled to 50 °C. Then the mixture was diluted with of methanol (30 mL) and neutralized with acetic acid (5 ml). The reaction mixture was then heated to reflux for 45 min before cooling to room temperature. The suspension was filtered over a Buchner funnel and the solid was washed with hot methanol and water several times and dried under vacuum at 80 °C for 16 h to give the product, 3,6-di (thiophen-2-yl)2,5-dihydropyrrolo [3,4-c] pyrrole-1,4-dione (S-DPP) (1). Yield: 3.6 g (26%) as a red solid. This compound was used without further purification. 1H NMR (DMSO-d6, 400 MHz) dppm: 7.30 (dd, 2H,), 7.95 (d, 2H), 8.22 (d, 2H), 11.21 (s, 2H); 13C NMR (DMSO-d6, 400 MHz) dppm: 108.53, 128.65, 130.76, 131.23, 132.58, 136.11, and 161.58. 2.2.2. 3,6-Di (furan-2-yl)-2,5-dihydropyrrolo [3,4-c] pyrrole-1,4-dione (O-DPP) (2) Compound 2 was synthesized following same procedure for compound 1. Yield: 3.1 g (21%). 1H NMR (DMSO-d6, 400 MHz) dppm: 6.83 (dd, 2H), 7.65 (d, 2H), 8.04 (d, 2H), 11.17 (s, 2H). 13C NMR (DMSO-d6, 400 MHz) dppm: 107.57, 113.71, 116.79, 131.27, 143.75, 146.91, and 161.71.
2.2.3. 2,5-Dioctyl-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c] pyrrole-1,4-dione (3) In a three-necked, oven-dried 250 mL round-bottom flask, 3,6dithiophen-2-yl-2,5-dihydro pyrrolo[3,4-c]pyrrole-1,4-dione (1) (3.00 g, 10.0 mmol) and anhydrous K2CO3 (5.52 g, 30.0 mmol) were dissolved in 100 mL of anhydrous N,N-dimethylformamide (DMF) and heated to 120 °C under argon for 1 h. n-octylbromide (5.76 g, 30.0 mmol) was then added dropwise, and the reaction mixture was further stirred and heated overnight at 130 °C. The reaction mixture was allowed to cool down to room temperature; after that it was poured into 400 mL of distilled water, and the resulting suspension was stirred at room temperature for 1 h. The solid was collected by vacuum filtration, washed with several portions of distilled water, methanol, and then air-dried. The crude product was purified by flash chromatography using dichloromethanehexane as eluent, and the solvent was removed in vacuo to obtain a pure product, 2,5-dioctyl-3,6-di(thiophen-2-yl)-2,5-dihydropyr rolo[3,4-c]pyrrole-1,4-dione (3) as a purple brown shiny crystalline powder (yield: 75.4%). 1H NMR (400 MHz, CDCl3) dppm: 8.94 (d, 2H), 7.64 (d, 2H), 7.29 (t, 2H), 4.08 (t, 4H), 1.75 (m, 4H), 1.27– 1.44 (m, 28H), 0.88 (t, 6H). 13C NMR (400 MHz, CDCl3) dppm: 161.04, 139.69, 134.92, 130.36, 129.44, 128.27, 107.35, 41.89, 31.43, 29.62, 28.87, 26.54, 22.30, 13.76.
2.2.4. 3,6-Di(furan-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4-c] pyrrole-1,4-dione (4) Compound 4 was synthesized following same procedure for compound 3. By using compound O-DPP (2) (3 g, 11.18 mmol) along with n-octylbromide (6.44 g, 33.55 mmol) and K2CO3 (6.17 g 44.73 mmol) (yield: 81.6%). 1H NMR (400 MHz, CDCl3) dppm: 8.31 (d, 2H), 7.64 (d, 2H), 6.70 (t, 2H), 4.11 (t, 4H), 1.68 (m, 4H), 1.27–1.39 (m, 28H), 0.87 (t, 6H). 13C NMR (400 MHz, CDCl3) dppm: 160.54, 144.83, 133.33, 120.18, 117.71, 113.60, 106.10, 42.09, 31.47, 29.87, 28.85, 26.51, 22.30, 13.76.
2.2.5. 3,6-bis(5-Bromothiophen-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo [3,4-c]pyrrole-1,4-dione (5) A 100 mL single-neck round-bottom flask was charged with a stir bar, compound 3 (1 g, 1.72 mmol) was added to a solution in chloroform (50 mL) under ambient conditions. Flask was wrapped in aluminum foil to avoid the exposure of the reaction to light. After the reaction mixture was stirred in an ice bath at 0 °C for 20 min, N-bromosuccinimide (NBS) (0.76 g, 4.30 mmol) was added in three portions to it, the solution stirred at room temperature for 48 h. Resulting crude product was extracted with chloroform, washed with water, and dried over anhydrous Na2SO4. Solvent was removed under reduced pressure and the product was purified using silica gel chromatography eluting with a mixture of hexane and DCM to get dark red purple solid. Yield: (1.2 g, 72%). 1H NMR (CDCl3, 400 MHz) dppm: 8.69 (d, 2H), 7.25 (d, 2H), 3.99 (t, 4H), 1.72 (m, 4H), 1.42–1.28 (m, 20H), 0.89 (t, 6H). 13C NMR (CDCl3, 400 MHz) dppm: 160.77,138.74,135.09, 131.38, 130.86, 118.88, 107.55, 42.03, 31.50, 29.71, 28.88, 26.56, 22.37, 13.82.
2.2.6. 3,6-bis(5-Bromofuran-2-yl)-2,5-dioctyl-2,5-dihydropyrrolo[3,4c]pyrrole-1,4-dione (6) Compound 6 was synthesized by following the same procedure used for the synthesis of compound 5. By using compound 4 (1.5 g, 3.05 mmol) and NBS (1.35 g, 7.61 mmol). Yield: (1.35 g, 78%). 1H NMR (CDCl3, 400 MHz) dppm: 8.26 (d, 2H), 6.64 (d, 2H), 4.06 (t, 4H), 1.70 (m, 4H), 1.41–1.28 (m, 20H), 0.88 (t, 6H). 13C NMR (CDCl3, 400 MHz) dppm: 160.65, 143.54, 132.63, 122.03, 115.26, 110.25, 105.97, 45.94, 39.83, 30.23, 28.45, 22.93, 22.89, 13.81.
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2.2.7. N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) aniline (8) A 3-neck 250 mL round bottom flask was charged with a stir bar, 4-bromotriphenylamine (4.00 g, 12.3 mmol) (7), bis(pinacolato)diboron (B2pin2) (3.26 g, 12.8 mmol), anhydrous potassium acetate (KOAc) (3.28 g, 33.41 mmol), dichloro[1,10 -bis(diphenylpho sphino)-ferrocene]palladium(II) dichloromethane adduct (Pd (dppf)Cl2CH2Cl2) (0.243 g, 0.334 mmol) and degassed dioxane (120 mL). After the reaction mixture was heated at 85 °C for 16 h, it was extracted with diethyl ether and washed with distilled water. The organic extract was dried over MgSO4, and solvent was removal under reduced pressure yielded brown viscous oil. Purification with flash chromatography (25% hexanes in CH2Cl2) yielded 4.30 g of compound 8 as tacky off-white solid (95%). 1H NMR (400 MHz, CDCl3) dppm: 7.69 (d, 2 H), 7.27 (t, 4 H), 7.23 (d, 4 H), 7.13–7.02 (m, 4 H), 1.34 (s, 12 H). 13C (400 MHz, CDCl3) dppm: 150.65, 147.54, 136.01, 129.45, 125.26, 123.87, 121.79, 83.76, 25.26. 2.2.8. 3,6-bis(5-(4-(Diphenylamino)phenyl)thiophen-2-yl)-2,5dioctyl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (9) A 50 mL Schlenk tube was charged with materials, 5 (0.800 g, 1.17 mmol), 8 (0.958 g, 2.58 mmol), bis(dibenzylideneacetone) palladium (0) (Pd2(dba)3) (0.021 g, 0.023 mmol), tri-otolylphosphine (P(o-tol)3) (0.028 g, 0.094 mmol), anhydrous K2CO3 (1.54 g, 11.13 mmol), 2 drops of aliquat 336, freeze-pumpthawed toluene (20 mL) and freeze-pump-thawed distilled water (6.0 mL). The reaction mixture was heated at 90 °C for 16 h before being precipitated into 250 mL of MeOH. The precipitates were filtered through a 20 lm nylon membrane. Purification by flash chromatography (10% hexanes in chloroform) yielded compound 9 as a metallic purple solid (56%). 1H NMR (400 MHz, CDCl3) dppm: 8.96 (s, 2H), 7.52–7.06 (m, 30H), 4.09 (s, 4 H), 1.76 (m, 4H), 1.55–1.24 (m, 20H), 0.82 (t, 6H). 13C NMR (400 MHz, CDCl3) dppm: 161.02, 152.75, 149.57, 148.24, 146.76, 141.78, 138.90, 136.58, 129.12, 127.48, 124.67, 123.36, 107.41, 41.95, 31.48, 29.69, 28.90, 26.59, 22.30, 13.76 ESI–HRMS calcd for C66H66N4O2S2 1011.40, found 1011.87. 2.2.9. 3,6-bis(5-(4-(Diphenylamino)phenyl)furan-2-yl)-2,5-dioctyl2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (10) Compound 10 was synthesized by following the procedure used for the synthesis of compound 9. Herein, 6 (0.800 g, 1.23 mmol), 8 (1.01 g, 2.71 mmol), bis(dibenzylideneacetone)palladium (0) (Pd2(dba)3) (0.023 mg, 0.0245 mmol), tri-o-tolylphosphine (P(o-tol)3) (0.030 g, 0.098 mmol), anhydrous K2CO3 (1.60 g, 11.68 mmol), 2 drops of aliquat 336 were used, (Yield = 62.5%). 1 H NMR (400 MHz, CDCl3) dppm: 8.37 (s, 2 H), 7.55–7.57 (d, 4 H), 7.29–7.24 (t, 8 H), 7.12–7.05 (m, 16 H), 4.17 (t, 4 H), 1.76 (m, 4H), 1.57–1.18 (m, 20H), 0.78 (t, 6H). 13C NMR (400 MHz, CDCl3) dppm: 160.81, 157.04, 148.55, 147.02, 143.50, 132.18, 129.44, 125.44, 125.09, 123.76, 122.69, 122.50, 107.97, 42.56, 31.72, 30.35, 29.43, 29.22, 27.13, 22.56, 14.04. ESI–HRMS calcd for C66H66N4O4 979.28, found 979.35. 3. Results and discussion The synthetic steps for the synthesis of the DPP molecules are shown in Scheme 1. The structure of all the key intermediates and target D-A-D molecules has been confirmed by 1H NMR, 13C NMR and ESI–HRMS. The O-DPP and S-DPP were synthesized by Stobbe condensation between diethyl succinate and 2-furonitrile or 2thiophenecarbonitrile, respectively. The O-DPP and S-DPP were subjected to alkylation using 1-bromo octane, which was then
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reacted with NBS to obtain compound 5 and 6 according to reported procedure [30]. Then borylation of compound 7 was carried out by Miyaura borylation reaction using Pd(dppf)Cl2 catalyst and anhydrous dioxane as solvent to get compound 8. The final compounds 9 and 10 were obtained with the Suzuki-Miyaura coupling using the following reagents, bis(dibenzylideneacetone)palla dium (0) (Pd2(dba)3), tri-o-tolylphosphine (P(o-tol)3) and anhydrous K2CO3 in anhydrous toluene solvent. The UV–vis absorption spectra were recorded in chloroform (CHCl3) solution. It can be noticed that both the FDPPT and TDPPT molecules exhibits welldefined two peaks both in solution and in thin film. The lower wavelength absorption bands ascribed to the p–p⁄ transitions and the strongest absorption peak results from the intramolecular charge transfer (ICT) [31]. Absorption spectra of compound FDPPT in chloroform shows two peaks at kmax 587.2 and 635.1 nm and TDPPT in chloroform shows two peaks at kmax 593.2 and 633.9 nm (Fig. 1a). Both the molecules have shown similar characteristic in their UV/vis absorption spectra possibly due to their similar molecular structures. The maximum absorption wavelength (kmax) of both the TDPPT and FDPPT are red-shifted as much as 15 to 45 nm compared to their absorption in solution. This can attributed to the better packing (p–p stacking) of the small molecules in solid thin film state. The absorption spectrum of TDPPT becomes remarkably broad compared to the FDPPT spectrum in solid state which indicates that the different heterocycle has its influence on molecular packing and p–p stacking in film. The optical band gaps are estimated from the absorption edges of thin films using the following formula Eopt g = 1240/konset. The band gaps of FDPPT and TDPPT are 1.82 eV and 1.79 eV, respectively. The Egopt of FDPPT is noticeably larger than that of TDPPT. The frontier orbital energy levels (HOMO-LUMO) of these materials were determined by cyclic voltammetry (CV) techniques. Platinum wire and platinum foil were used as working and counter electrode, respectively. Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) electrode was used as reference electrode. The cyclic voltammetry were performed using tetra butyl ammonium perchlorate (0.01 M TBAP in acetonitrile) solution as supporting electrolyte at room temperature with a scan rate of 50 mV/s and ferrocenium/ferrocene (Fc+/Fc) redox couple as external reference. The CVs of TDPPT and FDPPT are shown in Fig. 1b. HOMO values were calculated using the formula, HOMO = [Eox (onset) + 4.8] eV assuming that the absolute energy level of Fc is 4.8 eV [32]. The LUMO level was estimated from the optical band gap values. The HOMO and the LUMO level of TDPPT (5.2 eV and 3.4 eV) is both deeper than FDPPT (5.0 eV and 3.2 eV), while both the small molecules are identical because they have same electron donating (TPA) and accepting unit (DPP) except two different heterocyclic units (furan/ thiophene). The sulphur atom in thiophene unit is responsible for lower HOMO/LUMO level for TDPPT. It is quite often seen in oxygen vs sulphur systems. The optical band gap is weakly affected by this. The energy level diagram of the TDPPT and FDPPT is shown in Fig. 1d. DFT studies were done using B3LYP functional and polarized 631g⁄ basis set. The surface plots of both the molecules show that the wave function is well delocalized within the p-backbone for HOMO since both TPA and furan/thiophene DPP is contributing. However, the wave function is delocalized preferably over the DPP part in case of LUMO for both the compounds. This may be attributed to its electron accepting nature. The surface energy plots are shown in Fig. S18 (ESI). The optimized ground state geometry of FDPPT and TDPPT were taken for determining the dihedral angle at the junction of the furan/thiophene part of the DPP and phenyl part of the TPA connected to furan/thiophene. The dihedral angle was 175.49° for FDPPT whereas the angle was found to be 165.41° for TDPPT. Optimized ground state structures of FDPPT and TDPPT and dihedral angle are shown in Fig. S19. The dihedral
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OH K+ButO-, CN
X
H N
X
Br-C8H17
100-110 °C , 4-6 hrs O
X = S/O
R N
O
O
O
O O
X
N H
K2CO3, DMF,120 °C
X N R
Br
O
X
RT, 48 hrs
O
X N R
Br
5 S-DPPR-Br 6 O-DPPR-Br
O 5 S-DPPR-Br 6 O-DPPR-Br
O
Pd(dppf)Cl2.CH2Cl2 Br
O
R N
3 S-DPPR 4 O-DPPR
B B
N
O
1 S-DPP 2 O-DPP
O O
X
NBS, CHCl3
KOAc
R
Pd2(dba)3, P(o-tolyl)3 O N
B
K2CO3, aliquat 336
O
Dioxane, 85 °C, 16h
N N
H2O/Toluene 90 °C, 16h
X O
O X
N
N R
7 Br-TPA
8 TPA-BE
9 S-DPPT (TDPPT) 10 O-DPPT (FDPPT) R= n-Octyl
Scheme 1. Synthesis scheme of FDPPT and TDPPT.
Fig. 1. (a) UV/vis spectra in solution and in thin film state, (b) cyclic voltammograms showing oxidation, (c) the schematic diagram of FET device and (d) energy levels of FDPPT and TDPPT.
angle between the planes indicates that the DPP and specified phenyl part of the TPA are in plane with a deviation of 5° for FDPPT which is higher for TDPPT (15°). The thermal stability of the molecules was examined using thermogravimetric analysis. Significant weight loss was not observed until 230 °C in the thermogram, indicating good thermal stability of the molecules. The X-ray diffraction (XRD) measurement on the thin films deposited at 80 °C was also carried out and the diffraction pattern was given in Fig. 4b. This suggests that the thin film molecules are amorphous in nature due to the
presence of propeller structured TPA moiety. The thin film morphologies of deposited FDPPT and TDPPT were investigated by atomic force microscopy (AFM) before and after annealing under similar conditions, used for FET device fabrication. The morphology of FDPPT was found to be a non uniform film with appearance of aggregation of small rods at random places. They become more pronounced upon annealing (Fig. 2b). On the other hand, TDPPT also showed a film with nano rods (Fig. 2c). The thin films of FDPPT and TDPPT deposited at room temperature showed a root mean square (rms) roughness value (Rq) of 18.5 nm and 14.9 nm after
M.K. Chini et al. / Chemical Physics Letters 661 (2016) 107–113
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Fig. 2. (a and b) Represents AFM amplitude images of FDPPT, (c and d) represents AFM amplitude images of TDPPT before and after annealing of the thin films, respectively.
annealing showed the Rq values of 23.8 nm and 18.1 nm respectively. The roughness had increased after annealing. The interaction between alkyl chains of the modified substrates and the small molecules is majorly responsible for the change in the surface roughness [33]. This phenomenon is known for conjugated small molecules. Finally after all the molecular characterization, we have done the transistor measurements using prefabricated field effect transistors having bottom gate (SiO2) and bottom contact (Au) configuration on top of the silicon wafer substrates. The thickness of the gate dielectric, SiO2 was 210 nm. The substrates were cleaned using ultrasonication in chloroform, acetone and isopropanol respectively prior to use. The substrates were modified by dipping them into 5% (V/V) hexamethylenedisilazane (HMDS) solution of CHCl3 followed by annealing at 100 °C for 10 min under N2 atmosphere since devices based on unmodified substrates showed no measurable current (ID) upon sweeping the drain voltage (VD) at various gate voltages (VG) due to the possible poor interaction between the SiO2 and the studied molecules [34]. The FDPPT and TDPPT molecules were then spin coated from chloroform solution (10 mg/mL) on top of the HMDS modified substrates and the device characterization was carried out under inert conditions. The annealing temperature could enhance the device performances. But beyond certain annealing temperature mobility decreased rapidly. Thus, beyond that particular annealing temperature, we had not annealed the thin film devices. To check the change in FET performances for the FDPPT and TDPPT, the prepared devices were further annealed at temperatures such as 80 °C and 100 °C for 10 min each inside glove box. The VD was swept between 0 and 100 V while holding constant negative VG. The output characteristics of the devices showed obvious linear and saturation regimes for FDPPT (Fig. 3a) and TDPPT (Fig. 3c) and indicated that both of the molecules are hole transporting (p type) in nature. The transfer characteristics were obtained by sweeping the VG
while holding the VD at 40 V and 60 V for FDPPT and TDPPT, respectively (determined from the output characteristics). The charge carrier mobility (l) in the saturation regime was calculated using the following relationship, ID = (lCW/2L) [(VG VT)2]. W and L are the semiconductor channel width and length, respectively. Ci (Ci = 14.9 nF) is the capacitance per unit area of the gate dielectric layer. VT is the threshold voltage. The device parameters are summarized in Table 1. The hole mobility of FDPPT and TDPPT based transistors was 3.47 104 cm2 V1 s1 and 6.29 104 cm2 V1 s1 measured for unannealed thin films, respectively. Upon annealing at 80 °C, the mobility for FDPPT and TDPPT was increased to 1.59 103 cm2 V1 s1 and 7.79 104 cm2 V1 s1, respectively. This is attributed to the better packing upon annealing. Furthermore, on annealing at 100 °C the hole mobility was reduced to 5 104 cm2 V1 s1 and 7 105 cm2 V1 s1, respectively. The maximum mobility obtained for the annealed FDPPT and TDPPT thin film was 2.88 103 cm2 V1 s1 and 1.60 103 cm2 V1 s1 respectively. The hole carrier mobility is comparable for both FDPPT and TDPPT. This is likely due to the TPA donor units nullifying the possible packing facilitated furan and thiophene. In such cases, morphology can impact the charge carrier mobility [35]. However, both FDPPT and TDPPT showed similar morphology, hence the charge carrier mobility is not significantly different. We observed the contact resistance (RC) varied with annealing temperature. The RC was calculated using the relationship, RT = RC + RCh. Where, RT and RCh are the total and the channel resistance, respectively. The RC for the FDPPT unannealed thin film device is 2 109 O at RT whereas upon annealing RC was reduced little bit to 1 109 O at 80 °C (Fig. 4c). The RC for the TDPPT as such spin coated thin film device is 5 109 O whereas on annealing RC was reduced by order to 1 108 O at 80 °C (Fig. 4d). This can be correlated to the increase in mobility values. On further annealing at higher temperature, the RC values increase which in turn correlates to the lower charge carrier mobility obtained.
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Fig. 3. Output (a) and transfer characteristics curve (b) of FDPPT. Output (c) and transfer characteristics (d) of TDPPT after annealing, respectively.
Table 1 OFET parameters at various temperatures. Molecules
T (°C)
lh,max (cm2 V1 s1)
lh,avg (cm2 V1 s1)
VT (V)
Ion/off
FDPPT
As-cast 80 100
8.56 104 2.88 103 4.15 104
3.47 104 1.59 103 5.04 104
46 62 58
5.5 102 6.5 101 4.5 101
TDPPT
As-cast 80 100
1.53 103 1.60 103 1.04 104
6.29 104 7.79 104 6.94 105
22 18 15
4.0 102 1.0 103 5.3 102
Fig. 4. (a) The TGA curve, (b) thin film XRD of both FDPPT and TDPPT annealed at 80 °C shows amorphous nature, (c and d) represents the RC vs VG plot for FDPPT and TDPPT respectively.
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4. Conclusion In summary, we have synthesized and characterized DPP and TPA based small molecules with octyl side chains. The synthesized molecules are solution processable and form good films on HMDS modified substrates. Before annealing, the HMDS modified devices have shown proper FET characteristics and the obtained charge carrier mobility values for both FDDPT and TDPPT were low but comparable. On annealing the modified thin films at 80 °C, the hole carrier mobility values for both D-A-D molecules have increased by approx. one order to give relatively high values due to better packing and improved quadrupole-quadrupole interaction arising from the DPP units enhanced the interchain contacts. The maximum mobility obtained for FDPPT and TDPPT thin film was 2.88 103 cm2 V1 s1 and 1.60 103 cm2 V1 s1 while annealed at 80 °C, respectively. Overall, we can conclude that heterocyclic moieties have little impact on charge carrier mobility of these small molecules while connected to TPA moiety. Acknowledgements MKC and RYM thank CSIR for fellowship. We are grateful to Dr. K. Krishnamoorthy for his support. We also thank CSIR for financial support through NWP 54 (TAPSUN). We thank Mrs. Pooja Muddellu for AFM imaging. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cplett.2016.08. 073. References [1] J. Pei, W.L. Yu, W. Huang, A.J. Heeger, A novel series of efficient thiophenebased light-emitting conjugated polymers and application in polymer lightemitting diodes, Macromolecules 33 (2000) 2462. [2] Y. Huang, X. Guo, F. Liu, L. Huo, Y. Chen, T.P. Russell, C.C. Han, Y. Li, J. Hou, Improving the ordering and photovoltaic properties by extending p– conjugated area of electron-donating units in polymers with D-A structure, Adv. Mater. 24 (2012) 3383. [3] F. Liu, C. Wang, J. Baral, W. Zhao, L. Zhang, J.J. Watkins, A.L. Briseno, T.P. Russell, Relating chemical structure to device performance via morphology control in diketopyrrolopyrrole-based low band gap polymers, J. Am. Chem. Soc. 35 (2013) 19248. [4] A. Facchetti, P-Conjugated polymers for organic electronics and photovoltaic cell applications, Chem. Mater. 23 (2011) 733. [5] X. Zhao, X. Zhan, Electron transporting semiconducting polymers in organic electronics, Chem. Soc. Rev. 40 (2011) 3728. [6] L. Biniek, B.C. Schroeder, C.B. Nielsen, I. McCulloch, Recent advances in high mobility donor–acceptor semiconducting polymers, J. Mater. Chem. 22 (2012) 14803. [7] F. Huang, K.S. Chen, H.L. Yip, S.K. Hau, O. Acton, Y. Zhang, J.D. Luo, A.K.Y. Jen, Development of new conjugated polymers with donor-p-bridge-acceptor side chains for high performance solar cells, J. Am. Chem. Soc. 131 (2009) 13886. [8] W.R. Saaneck, R.H. Friend, J.L. Bredas, Electronic structure of conjugated polymers: consequences of electron–lattice coupling, Phys. Rep. 319 (1999) 231. [9] B. Walker, C. Kim, T.Q. Nguyen, Small molecule solution-processed bulk heterojunction solar cells, Chem. Mater. 23 (2011) 470. [10] R. Fitzner, E. Mena-Osteritz, A. Mishra, G. Schulz, E. Reinold, M. Weil, C. Körner, H. Ziehlke, C. Elschner, K. Leo, M. Riede, M. Pfeiffer, C. Uhrich, P. Bäuerle, Correlation of p-conjugated oligomer structure with film morphology and organic solar cell performance, J. Am. Chem. Soc. 134 (2012) 11064. [11] Y. Lin, Y. Li, X. Zhan, Small molecule semiconductors for high-efficiency organic photovoltaics, Chem. Soc. Rev. 41 (2012) 4245.
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