diethynylbenzene copolymers

Polymer 53 (2012) 1072e1078

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Synthesis and characterization of naphthalene diimide/diethynylbenzene copolymers Tissa Sajoto a, Shree Prakash Tiwari b, Huifang Li a, Chad Risko a, Stephen Barlow a, Qing Zhang a, Jian-Yang Cho a, Jean-Luc Brédas a, Bernard Kippelen b, Seth R. Marder a, * a b

School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, 901 Atlantic Drive NW, Atlanta, GA 30332-0400, USA School of Electrical and Computer Engineering and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, GA 30332-0250, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 October 2011 Received in revised form 4 January 2012 Accepted 11 January 2012 Available online 18 January 2012

A series of conjugated polymers has been synthesized by Sonogashira coupling of N,N0 -bis(2octyldodecyl)-2,6-dibromonaphthalene-1,4,5,8-bis(dicarboximide) and four para-diethynylbenzene derivatives: 1,4-diethynyl-2,5-dihexadecyloxybenzene, 1,4-diethynyl-2,5-bis(2-octyldodecyloxy) benzene, 1,4-bis(2-ethylhexyl)-2,5-diethynylbenzene, 1,4-diethynyl-2,5-bis(trifluoromethyl)benzene. The polymers display absorption maxima at wavelengths ranging from 530 nm to 654 nm with molar absorptivities ranging from ca. 4 to 7  104 M1 cm1. The peak reduction potentials, determined by differential pulse voltammetry, for polymer films varied from 0.93 to 1.14 V vs. ferrocenium/ferrocene with the trifluoromethyl-substituted derivative being the most readily reduced. All four polymers exhibited electron transport characteristics in bottom-gate/top-contact field-effect transistors, showing average electron mobility values ranging from 1.4  104 to 3.7  103 cm2 V1 s1. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Naphthalene diimide polymers Electron transport Conjugated polymers

1. Introduction Processable conjugated polymers with large electron mobility values and large electron affinities are of interest as the active layer of n-channel organic field-effect transistors (OFETs) and as acceptors in organic photovoltaic (OPV) cells [1e4]. Recently conjugated polymers based on rylene diimides have shown promising properties for both of these applications [5e12]. For example, alternating dithienothiophene/perylene diimide polymer Ia (Fig. 1) has shown an electron mobility value of 1.3  102 cm2 V1 s1 in an OFET [5], while OPV cells using polymer Ib (Fig. 1) as an acceptor in combination with a polythiophene-based donor have shown power conversion efficiencies of up to 1.48% [6]. The bithiophene/naphthalene diimide (NDI) polymer II (Fig. 1) has recently been shown to exhibit even higher mobility values of up to 0.45e0.85 cm2 V1 s1 [7]. Although a recent report describes arylene-bridged NDIs and their conversion to ladder polymers [13], the conjugated NDI polymers reported to date have generally incorporated thiophenebased bridging groups. We were interested in the extent to which the spectroscopic, electrochemical, and charge-transport properties of NDI-based conjugated polymers with less strongly electrondonating bridging groups differ from those of the previously

* Corresponding author. Tel.: þ1 404 385 6048; fax: þ1 404 894 5909. E-mail address: [email protected] (S.R. Marder). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.01.016

reported systems with thiophene-based bridges; here, we report the synthesis and properties of a range of NDI polymers with diethynylbenzene bridging groups (Fig. 2). Whereas NDI-thiophene steric interactions are likely to lead to significant distortion from planarity, reduced twisting in the alkynyl-based polymers may lead to enhanced NDIeNDI or NDI-bridging group electronic coupling. 2. Experimental section 2.1. Materials Triethylamine was distilled from calcium hydride, while THF was dried by passage through an MBraun solvent purification system and was deoxygenated with three freeze-pump-thaw cycles before use in polymerizations. The monomers were made according to the literature [8,14e20] or by minor variations of literature procedures (see Supplementary Information for details). Other materials used in the polymerizations were purchased from commercial sources and were used without further purification. 2.2. Characterization 1 H NMR spectra were acquired using a Varian 400 MHz spectrometer. Gel-permeation chromatography (GPC) was performed at 35  C with THF at the mobile phase at flow rate of 0.3 mL min1 using a Waters 1515 isocratic HPLC pump, a 4.6  300 mm StyragelÒ

T. Sajoto et al. / Polymer 53 (2012) 1072e1078

1073

Fig. 1. Examples of previously reported rylene diimide conjugated polymers.

HR 5E THF column, and a Waters 2489 UV/Visible detector; molecular weights were estimated against polystyrene standards. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA 449C analyzer under a nitrogen flow of 40 mL min1 with a heating rate of 5  C min1. Differential scanning calorimetry (DSC) measurements were performed on a TA Instruments DSC Q200 analyzer under a nitrogen flow of 50 mL min1 with a heating rate of 5  C min1. Elemental analyses were carried out by Atlantic Microlab using a LECO 932 CHNS elemental analyzer. UVeVis absorption spectra were obtained using a CARY 5E UVeViseNIR spectrophotometer. Solution spectra were acquired in 1 cm quartz cuvettes. For the thin-film spectra, solutions in chlorobenzene (ca. 3 mg in 0.3 mL) were filtered through a 0.2 mm filter syringe and spin-coated onto glass substrates (previously sonicated in absolute EtOH for 20 min and blown dry with N2) at 2000 rpm for 2 min. Differential pulse voltammetry was carried out using polymer films drop cast from THF solutions (ca. 1 mg mL1) onto a Pt working electrode. A Pt wire was used as the auxiliary electrode, an Ag wire anodized in 1 M aqueous potassium chloride solution was used as a pseudo-reference electrode, and

a deoxygenated 0.1 M acetonitrile solution of nBu4NPF6 was used as the electrolyte. Differential pulse voltammetry was recorded at 20 mV s1 using a CH Instruments 620D potentiostat under þ=0 by use of an internal nitrogen and were referenced to FeCp2 FeCp2 reference in the electrolyte solution. 2.3. Synthesis 2.3.1. P1 In a nitrogen-filled glove box, M5 (1.00 g, 1.02 mmol), M1 (0.616 g, 1.02 mmol), Pd(PPh3)2Cl2 (14.3 mg, 0.020 mmol), and CuI (3.9 mg, 0.020 mmol) were loaded into a pressure vessel containing a stir bar. Dry THF (5 mL) and triethylamine (5 mL) were added and the pressure vessel was sealed and heated at 80  C with stirring for 2.5 d during which time a dark blue coloration developed. After cooling to room temperature, the reaction mixture was transferred by pipette (in air) into MeOH (200 mL) and a blue precipitate formed. The dark blue solid were filtered, washed with MeOH (2  50 mL), air-dried, and further purified by Soxhlet extraction with methanol and acetone for 24 h each. The remaining solid in the thimble was then Soxhlet extracted with chloroform for 24 h. The chloroform extracts were concentrated under reduced pressure (to about 50 mL), and then transferred by pipette into MeOH solution to give a precipitate, which was filtered, washed with MeOH (2  50 mL), and air-dried to give P1 as a dark blue solid (1.13 g, 78%). 1H NMR (400 MHz, CD2Cl2): d 6.50e9.00 (m, br, 4H), 3.50e5.00 (br, 2H), 2.00e2.50 (br, 8H), 1.10e1.50 (s, br, 120H), 0.87 (s, br, 18H). UVeVis: lmax 646 nm (in CH2Cl2), 646 nm (in THF). Anal. calcd for (C96H152N2O6)n: C, 80.62; H, 10.71; N, 1.96. Found: C, 80.51; H, 10.72; N, 2.08. P2 and P3 were synthesized by the same procedure as P1 using M2 and M3, respectively, in place of M1. 2.3.2. P2 Dark blue solid (1.59 g, 95%). 1H NMR (400 MHz, CD2Cl2): d 8.65e8.95 (m, br, 2H), 7.05e7.40 (m, br, 2H), 3.50e4.50 (m, br, 4H), 1.95e2.50 (s, br, 8H), 1.00e1.60 (s, br, 128H), 0.85 (s, br, 24H). UVeVis: lmax 637 nm (in CH2Cl2), 652 nm (in THF). Anal. calcd for (C104H168N2O6)n: C, 80.98; H, 10.98; N, 1.82. Found: C, 80.91; H, 10.92; N, 1.83.

Fig. 2. NDI polymers with diethynylbenzene bridging groups (P1eP4).

2.3.3. P3 Red-purple solid (1.38 g, 89%). 1H NMR (400 MHz, CD2Cl2): d 8.40e8.70 (m, br, 2H), 6.75e8.00 (m, br, 2H), 4.00e4.75 (m, br, 2H), 2.50e3.50 (m, br, 2H), 1.90e2.40 (m, br, 2H), 1.00e1.50 (s, br,

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80H), 0.86 (s, br, 24H). UVeVis: lmax 584 nm (in CH2Cl2), 584 nm (in THF). Anal. calcd for (C80H120N2O4)n: C, 81.86; H, 10.30; N, 2.39. Found: C, 81.36; H, 10.38; N, 2.52.

dielectric/semiconductor interface, which is a primary limiting factor for n-channel conduction [21]. The BCB was diluted in trimethylbenzene (TMB) with the ratio 1:20, and spin-coated at 3000 rpm for 60 s to provide a thin uniform layer (thickness was not measured, final capacitance density was measured). The samples were annealed at 250  C for 1 h inside a N2 glove box for crosslinking. The total capacitance density (COX) was measured from the slope of capacitance versus area plot for 12 parallel-plate capacitors with varying area, and it was w13.9 nF/cm2. A thin layer of organic materials was formed on the substrates by spin coating with solution prepared from chlorobenzene or chloroform (10e20 mg/mL) at 1000 rpm for 60 s. Devices were never exposed to normal ambient in the process. Ca covered by Au (Ca/Au w 40 nm/60 nm) electrodes were deposited by thermal evaporation through a shadow mask to define the source and drain electrodes. The samples were transferred in a vacuum-tight vessel without being exposed to atmospheric conditions into another N2-filled glove box (O2, H2O < 0.1 ppm) for electrical characterization. The electrical measurements were performed using an Agilent E5272A source/monitor unit. Output (IDS vs. VDS) and transfer (IDS vs. VGS) characteristics were measured for the devices, and field-effect mobility (m) values and threshold voltages (VT) were measured in the saturation regime from the highest slope of jIDS j1=2 vs. VGS plots using the saturation-region current equation for standard transistors as discussed in section 3.3.

2.3.4. P4 In a nitrogen-filled glove box, M5 (1.00 g, 1.02 mmol), M4 (0.266 g, 1.02 mmol), Pd(PPh3)2Cl2 (14.3 mg, 0.020 mmol), and CuI (3.9 mg, 0.020 mmol) were loaded into a pressure vessel containing a stir bar. Dry triethylamine and THF (5 mL each) were added. The pressure vessel was sealed and the reaction mixture was heated at 80  C with stirring for 2 d. After cooling to room temperature, MeOH (50 mL) was added (in air) into the reaction mixture; the resulting dark-colored precipitates were filtered, washed with more MeOH (2  50 mL), air-dried, and further purified by Soxhlet extraction with methanol, acetone, chloroform, and hexane for 24 h each. Finally, the remaining material was extracted with chlorobenzene for 3 d and the resulting dark solution was evaporated using rotary evaporation. The residue was redissolved in chlorobenzene and the solution was transferred by pipette into MeOH (150 mL) to give a precipitate, which was filtered and washed with MeOH to give P4 as a dark golden-green solid (0.31 g, 28%). 1H NMR (400 MHz, THF-d8): d 9.00e8.70 (m, br, 1H), 8.50e8.20 (m, br, 1H), 7.90e7.60 (m, br, 1H), 7.25e6.95 (m, br, 1H), 4.80e3.80 (s, br, 4H), 1.73 (s, br, 2H), 1.70e1.00 (m, br, 64H), 0.88 (s, br, 12H). UVeVis: lmax 535 nm (THF). Anal. calcd for (C66H86F6N2O4)n: C, 73.03; H, 7.99; N, 2.58. Found: C, 72.78; H, 8.12; N, 2.67.

2.5. Quantum-chemical calculations 2.4. Fabrication and characterization of thin-film transistors NDI-diethynylbenzene oligomers with n ¼ 1e4 repeat units were evaluated with density functional theory using the B3LYP [22,23] functional and a 6-31G** basis set [24,25]. Geometry optimizations of the ground-state (S0) were performed for the entire oligomer series. All alkyl chains appended to the conjugated backbone were truncated to methyl groups to reduce the computational requirements. Vertical ionization potentials (IPs) and electron affinities (EAs) were obtained by means of the DSCF method on the basis of the optimized S0 structures [26]. Timedependent DFT (TDDFT) calculations were performed to assess the excited-state vertical transition energies, oscillator strengths, and electronic configurations. All calculations were performed with Gaussian 09 (Revision A.02) [27].

OFETs were fabricated on heavily doped n-type silicon substrates (resistivity < 0.005 U cm, with wafer thickness 525  15 mm from Silicon Quest Int., also serving as gate electrodes) with 200 nm thick thermally grown SiO2 as the gate dielectric, in a topcontact configuration. Ti/Au (10 nm/100 nm) metallization on the backside of the substrate was done to enhance the gate electrical contact. First, the substrates were cleaned by O2 plasma for 3 min, to increase the hydrophilicity of the SiO2 surface. The SiO2 dielectric surface was then passivated with a thin buffer layer divinyltetramethyl-disiloxanebis(benzocyclobutene) (BCB) on top (CycloteneÔ, Dow Chemical), to provide a high-quality hydroxylfree interface. BCB was chosen to minimize electron trapping at the

n-C10H21

n-C10H21

n-C8H17

n-C8H17 O

O

N

O

Br

X X

+

O n-C8H17

N

X

Pd(PPh3)2Cl2 THF, 80oC, 2-3 d

O

N

n-C10H21 M1-M4

P1-P4 n-C10H21

O-n-C16H33 1

O n-C8H17 n-C10H21

M5

X=

O

CuI NEt3

Br O

X N

n-C8H17

O 2

C2H5 CH2 3

n-C4H9

Scheme 1. Synthesis of the NDI-diethynylbenzene copolymers.

CF3 4

n

T. Sajoto et al. / Polymer 53 (2012) 1072e1078

3. Results and discussion

1075

1x10 FeCp

3.1. Synthesis, characterization, and thermal properties While all previous examples of conjugated NDI polymers have been synthesized using Stille couplings [7e12], small-molecule arylalkynyl-substituted NDIs have been obtained from bromo-NDI derivatives by both Stille coupling with ethynylstannane derivatives [28], and Sonogashira coupling with terminal alkynes [29]. The four NDI-diethynylbenzene copolymers P1eP4 were synthesized by Sonogashira coupling [30] of N,N0 -bis(2-octyldodecyl)-2,6dibromonaphthalene-1,4,5,8-bis(dicarboximide) [8,14] and paradiethynylbenzene derivatives [15e20] using a Pd(PPh3)2Cl2 catalyst in THF in the presence of triethylamine and CuI over 2e3 days (Scheme 1). The monomers were synthesized by minor modifications of literature procedures [8,14e20]. Each polymer was precipitated using methanol and purified by Soxhlet extractions with methanol, acetone, and chloroform. In the case of P1eP3 the chloroform extracts were then concentrated under reduced pressure; the polymers were then reprecipitated in methanol and collected by filtration. P4 was barely soluble in chloroform, even at reflux, and so was extracted into chlorobenzene prior to reprecipitation. All four polymers were fully characterized by 1H NMR spectroscopy, GPC, UVeVis spectroscopy, differential pulse voltammetry, and elemental analysis. The molecular weight distributions of the polymers were investigated using gel-permeation chromatography (GPC) relative to polystyrene standards using THF as the eluent; estimates of the number- and weight-average molecular weights, Mn and Mw respectively, and polydispersities (Mw/Mn) of P1eP4 are shown in Table 1. The estimated molecular weights are reasonably large, corresponding to degrees of polymerization of ca. 16e49, and fall within the range previously reported for conjugated NDI polymers synthesized by Stille couplings [7e12,31]. The thermal stabilities of P1eP4 were investigated using thermal gravimetric analysis (see Supplementary Information for TGA curves); the decomposition temperatures, defined as those at which 5% weight loss occurs, are higher than 350  C (Table 1), comparable to that of II (450  C). In contrast to II and other thiophene-bridged NDI polymers, which show melting points varying from 220 to 300  C [7,31], the differential scanning calorimetry (DSC, see Supplementary Information for DSC scans) of P1eP4 showed no evidence for any phase transitions or other processes in the temperature range examined.

3.2. Electrochemical and optical properties The electrochemical properties of NDI-diethynylbenzene copolymer thin films were investigated on a Pt electrode using differential pulse voltammetry (DPV). Two reductive peaks were observed in each case at peak potentials of 0.93 to 1.14 V þ=0 and 1.30 to 1.44 V vs. ferrocenium/ferrocene ðFeCp2 Þ; DPV Table 1 Average molecular weights and thermal properties of the NDI-based copolymers. Polymer

Mna/kDa

Mwa/kDa

Mw/Mna

Tdb/ C

P1 P2 P3 P4

49 75 19 49

143 184 48 138

2.9 2.5 2.5 2.8

370 385 360 377

a

Average molecular weights (Mn and Mw) and polydispersity indexes (Mw/Mn) were determined by GPC vs. polystyrene standards with THF as eluent. b Temperature at which 5% weight loss was observed by TGA under N2 using a heating rate of 5  C/min.

Current (A)

1x10 red.

5x10 0

-5x10

ox.

-1x10

0.0

-0.4

-0.8

-1.2

Potential vs. FeCp

-1.6

(V)

Fig. 3. Reductive (solid) and oxidative (broken line) differential pulse voltammograms (50 mV s1) of a film of P1 in a 0.1 M acetonitrile solution of nBu4NPF6 referenced to þ=0 internal FeCp2 (which is responsible for the peak at 0 V).

traces for P1 are shown in Fig. 3 as a representative example, while the qualitatively similar traces for P2eP4 are shown in the SI. The potentials are anodically shifted with decreasing electron-richness of the benzene moiety, with the CF3-substituted compound, P4, being most easily reduced. The potentials are close to half-wave potentials determined by cyclic voltammetry for other NDI polymers (ca. 1.0 V for films of thiophene-bridged NDIs in acetonitrile [31]) and to solution half-wave potentials for small-molecule N,N0 -dialkyl-NDIs (1.0 to 1.1 V) [28,29,31]. No electrochemical oxidation of the polymers was observed within the solvent window. Density functional theory (DFT) calculations were performed for oligomeric models (n ¼ 1e4, with R ¼ CH3 in all cases) for P1eP4 to gain further insight into the redox and electronic properties. Calculated ionization potentials, electronic affinities, and frontier molecular orbital energies for tetrameric model compounds are provided in Table 2. The vertical electron affinity (EA) trends of P1eP4 are consistent with the anodic shifts of the first reduction potentials, and follow decreased electron-richness of the benzene moiety. The calculated EA range is somewhat larger (0.4 eV), however, than the potential range (0.2 V) of the first reduction peak. The vertical ionization potentials (IP) also reveal a dependence on the electron-richness of the benzene ring. The strongly electron-donating alkoxy groups of P1/P2 lead to an IP of 5.90 eV, while moving to the less electron-donating alkyl substituent leads to a w0.3 eV IP increase (6.24 eV). The strongly electronwithdrawing CF3 groups in P4 further increase the IP by w0.5 eV (6.77 eV). Table 2 Electrochemical and electronic properties of NDI-diethynylbenzene copolymers. Polymer

DFT (B3LYP/6-31G**)b

DPV a

P1 P2 P3 P4

a

Ered (1) /V

Ered (2) /V

IPc/eV

EAc,d/eV

EHOMO/eV

ELUMO/eV

1.14 1.11 1.08 0.93

1.36 1.44 1.34 1.30

5.90 e 6.24 6.77

2.95 e 3.04 3.34

5.34 e 5.67 6.20

3.49 e 3.59 3.90

þ=0

a Peak potentials vs. FeCp2 observed in reductive differential pulse voltammograms of films in MeCN/0.1 M nBu4NPF6. b DFT results are for model tetramers. c Vertical, i.e. at ground-state neutral geometry. d Defined here as the energy change for M þ e / M.

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Fig. 4. Representations of the DFT wave functions for the frontier molecular orbitals of tetrameric models for P1/P2 (X ¼ OCH3), P3 (CH3), and P4 (CF3).

B 7x10

P1 P2 P3 P4

6x10 5x10 4x10 3x10 2x10 1x10 0 300

400

500 600 Wavelength (nm)

700

800

1.0 Normalized Absorbance

-1

-1

Molar Absorptivity (M cm )

A

P1 P2 P3 P4

0.8 0.6 0.4 0.2 0.0 400

600 800 Wavelength (nm)

Fig. 5. UVeVis absorption spectra of P1eP4 in (A) chlorobenzene and (B) thin films.

1000

T. Sajoto et al. / Polymer 53 (2012) 1072e1078

1077

Table 3 Solution, thin-film, and TDDFT (B3LYP/6-31G**) optical properties of NDI-diethynylbenzene copolymers. Polymer

Solution

P1 P2 P3 P4 a

Thin-film

TDDFTa

lmax[C6H5Cl]/nm (εmax/104 M1 cm1)

lmax[THF]/nm

lmax[film]/nm

Evert (lmax)/eV (nm)

f

654 615 585 530

646 652 584 535

654 664 585 530

1.59 (781) e 1.80 (688) 2.00 (619)

3.53 e 3.98 4.48

(5.00) (3.91) (6.71) (4.82)

TDDFT calculated S0eS1 transition energies and oscillator strengths (f) for model tetramers.

Table 4 OFET characteristics for P1eP4. Polymer

me/cm2 V1 s1

P1 P2 P3 P4

3.7 4.7 1.4 2.3

(0.4) (0.7) (0.5) (0.2)

   

103 104 104 104

VT/V 5.2 1.8 1.3 1.8

Ion/off (1.2) (1.1) (0.9) (0.2)

5 2 4 2

   

103 103 102 102

The frontier orbitals of these tetrameric model compounds are shown in Fig. 4; the HOMO coefficients are largest on the diethynylbenzene moieties while the LUMOs are principally NDI-based, although the localization of HOMO and LUMO onto bridge and rylene diimide respectively is less extreme than calculated for a monomeric model for Ia (where the bridge and diimide are also less coplanar) [32]. As the electron-richness of diethynyl-bridging groups decreases from P1/P2 to P3 to P4, both HOMO and LUMO become increasingly delocalized over both NDI and diethynylbenzene units. Consistent with the greater localization of HOMO and LUMO coefficients on bridges and NDIs respectively, the HOMO energies and vertical ionization potentials (IPs) are calculated to be much more strongly substituent-dependent than the LUMO energies and electron affinities (EAs). However, the substituents are still predicted to affect the LUMO energy/EA, which is in agreement with the electrochemical data that show the CF3-substituted derivative (P4) being significantly more readily reduced than P1eP3. The UVeVis spectra of P1eP4 in chlorobenzene are shown in Fig. 5, with relevant data collected in Table 3. The low-energy absorption maxima are in all cases hypsochromically shifted with

respect to that seen in high molecular weight bithiophene- and dialkoxybithiophene-bridged NDI polymers (690e985 nm, 1.80e1.26 eV) [7,8,31], falling in the same range as examples with highly non-planar dialkylbithiophene bridges (534e598 nm, 2.32e2.07 eV) [31] and a single thiophene bridge (568 nm, 2.18 eV) [31]. It has previously been suggested that the low-energy absorptions of compounds of type I have significant bridge-torylene-diimide charge-transfer character [33] and the low-energy band of II and other thiophene-bridged NDI polymers presumably have a similar origin. Accordingly, this hypsochromic shift is consistent with the anticipated poorer donor character of the diethynylbenzene bridging groups compared to that of bithiophene, although the greater overall planarity possible in ethynylbridged systems is presumably an opposing factor. A partial bridge-to-NDI charge-transfer assignment is also consistent with the observation that the position of the maxima in the present polymers is increasingly hypsochromically shifted from P1/P2 to P3 to P4 as the benzene group becomes increasingly electron poor. It should be noted that P1 and P2, which are expected to be electronically similar since both contain a dialkoxy-substituted diethynylbenzene moiety, actually show similar absorption onsets to one another, the different maxima in Table 3 reflecting differences in overall band shapes; indeed the maxima in THF are much closer (646 [1.92] and 652 nm [1.90 eV], respectively). Time-dependent DFT (TDDFT, B3LYP/6-31G**) calculations were carried to gain further insight into the nature of these transitions. While the S0eS1 transition energies obtained from the TDDFT calculations are underestimated with respect to experiment (even though the calculations refer to low molecular weight oligomers), the trend in

Fig. 6. Output (left) and transfer characteristics (right) of OFET devices based on P2 (top-contact with Ca/Au S/D, W/L ¼ 1200 mm/100 mm, n-channel (on SiO2/BCB)).

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transition energies is consistent with experiment. The calculations indicate that these transitions are predominantly HOMO-to-LUMO (70e90%) in character, consistent with the variation in transition energies following that in the HOMOeLUMO energy difference and, importantly, confirms that these transitions have considerable bridge-to-NDI charge-transfer character. There is little change in the spectra of the thin films compared to those in solution, although a change in the band shape of the absorption of P2 leads to a bathochromic shift in the maximum from chlorobenzene solution to thin-film absorption (Fig. 4b).

in organic photovoltaics in which the strong tunable visible absorption may play a useful role. Acknowledgements We would like to acknowledge Benjamin Wunsch, Chun Huang, and Sergio Paniagua for acquiring TGA and DSC data. This work was funded by Solvay S.A., the Office of Naval Research, and the STC Program of the National Science Foundation (DMR-0120967).

3.3. Field-effect transistor (FET) characteristics

Appendix. Supplementary information

Charge-transport in P1eP4 was investigated by fabricating OFETs with polymer active layers, top-contact Ca/Au (40 nm/ 60 nm) source and drain electrodes, and a benzocyclobutene (BCB)-treated SiO2 gate dielectric layer. Field-effect mobility values, m, and threshold voltages, VT, were measured in the saturation regime from the highest slopes of plots of jIDS j1=2 vs. VGS according to the saturation-region current equation for a standard MOSFET:

Supplementary data related to this article can be found online at doi:10.1016/j.polymer.2012.01.016.

IDS ¼

1 W mC ðV  VT Þ2 2 i L GS

where Ci is the capacitance per unit area of the gate dielectric, and W and L are respectively the width and length of the semiconductor channel defined by the source and drain electrodes. In all four cases n-channel behavior was observed. Table 4 summarizes the device behavior; output and transfer characteristics for one example is shown in Fig. 6 (data for P1, P3 and P4 are given in the SI). The electron mobility values are moderate, ranging from 1.4 (0.5)  104 to 3.7 (0.4)  103 cm2 V1 s1, compared to the reported electron mobility values of II. Bottom-gate top-contact thinfilm transistor (TFT) performance of II on conventional Si/SiO2-OTS substrates with Au electrode yield electron mobility values in the range of 0.01e0.08 cm2 V1 s1 [8], while the top-gate bottomcontact TFT architecture having glass or polyethylene terephthalate (PET) substrate/Au (source-drain contacts)/II/polymer dielectric/Au (gate contact) yielded electron mobility values ranging from 0.2 to 0.85 cm2 V1 s1 and current on-off ratios Ion/Ioff > 106 (polymeric dielectric materials included CYTOP (a poly perfluoroalkenylvinyl ether), polystyrene, poly(t-butylstyrene, D2200 (a polyolefinpolyacrylate), and poly(methylmethacrylate)) [7]. 4. Conclusion Four new NDI-diethynylbenzene copolymers have been synthesized with reasonably high molecular weights using Sonogashira coupling. Variation of the substituents on the diethynylbenzene moieties from alkoxy to trifluoromethyl groups allows the absorption maxima to be varied over ca. 0.45 eV, while the variation in reduction potentials is somewhat smaller (ca. 0.2 V). DFT and TDDFT calculations on oligomeric model compounds are consistent with the observed trends in optical and electrochemical properties and show the LUMOs to be largely NDI-localized, while the largest HOMO coefficients are on the diethynylbenzene groups. The polymers exhibit only moderate electron mobility values in organic field-effect transistors; however, these mobility values may be sufficient for use, in combination with suitable donor materials,

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