Dioxapyrene-Based Organic Semiconductors for Organic Field Effect

Jul 16, 2009 - Here, we report synthesis and charge transport properties of peri-condensed heteropyrenes in organic field effect transistors (OFETs). ...
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Dioxapyrene-Based Organic Semiconductors for Organic Field Effect Transistors Deepak Shukla,* Thomas R. Welter, Douglas R. Robello, David J. Giesen, Jerome R. Lenhard, Wendy G. Ahearn, Dianne M. Meyer, and Manju Rajeswaran Kodak Research Laboratories, Eastman Kodak Company, Rochester, New York 14650-2109 ReceiVed: April 15, 2009; ReVised Manuscript ReceiVed: June 15, 2009

Compared to linear acenes, only a few examples of peri-condensed organic semiconductors are known. Here, we report synthesis and charge transport properties of peri-condensed heteropyrenes in organic field effect transistors (OFETs). We show that 1,6-dioxapyrene derivatives are easily accessible and stable semiconductors that form well-ordered polycrystalline films, which results in moderately large hole mobility (ca. 0.25 cm2 V-1 s-1) in OFET devices. Introduction

internal standard. Because of their limited solubility, 1H and C NMR spectra of 1 and 2 were recorded in CDCl3 solvent. MS spectra (MALDI-MS) were obtained on a TofSpec2E Laser TOF mass spectrometer (Micromass, Inc., U.K.). Absorption spectra were recorded on a Lambda 900 (Perkin-Elmer) UV-vis-NIR spectrometer. Synthesis. Naphtho[1,8-bc:5,4-b′c′]dipyran-2,7-diyl-di(2-cyclohexylmethanone) (1). A suspension of 1,5-dimethoxy-4,8dibromonaphthalene6a (4.90 g, 10 mmol) in 50 mL of dry dichloromethane was cooled in a dry ice/isopropyl alcohol bath and treated dropwise with 90 mL of a 1.0 M solution of boron tribromide in the same solvent under Ar. The reaction mixture was allowed to warm slowly to ambient and then quenched cautiously in a slurry of 20 g of NaHCO3 in 200 g of ice. The resulting mixture was extracted with ethyl acetate and filtered. The precipitate was further extracted with warm ethyl acetate, and the combined extracts were dried (MgSO4), filtered, and concentrated to deposit a brown solid. The intermediate product, 1,5-dihydroxynaphthalene-4,8-dicarboxaldehyde, was purified by column chromatography on silica gel, eluting with a mixture of heptane and ethyl acetate, and isolated as a greenish-yellow solid. Yield: 2.75 g (64%). A mixture of 1,5-dihydroxynaphthalene-4,8-dicarboxaldehyde (3.07 g, 14 mmol), cesium carbonate (11.57 g, 36 mmol), 2-bromo-1-cyclohexylethanone,6b and tetrahydrofuran (50 mL) was stirred under Ar at ambient temperature for 48 h. The resulting red suspension was filtered though a pad of Celite, and the precipitate was washed exhaustively with hot chloroform until the filtrate lacked the red color characteristic of the product. The filtrate was concentrated to deposit a red paste. The product was purified by column chromatography on silica gel, eluting with a mixture of heptane and dichloromethane, followed by recrystallization from a mixture of heptane and toluene to deposit deep maroon crystals. Yield: 2.80 g (46%). mp 261-263 °C. 1 H NMR (CDCl3) δ: 1.2-1.4 (m, 10 H), 1.7-1.9 (m, 10 H), 2.82 (m, 2 H), 6.25 (d, J ) 7.9 Hz, 2 H), 6.40 (s, 2 H), 6.45 (d, J ) 7.9 Hz, 2 H). 13C NMR (CDCl3) δ: 25.68, 25.81, 28.56, 44.70, 109.65, 113.39, 121.59, 122.38, 127.30, 148.80, 153.61, 196.89. HR-MS (electrospray TOF): Calcd for C28H28O4 + H+ (protonated molecular ion), 429.2042; found, 429.2066. Naphtho[1,8-bc:5,4-b′c′]dipyran-2,7-diyl-di(2-phenylmethanone) (2). The previous procedure was followed, but substituting R-bromoacetophenone for 2-bromo-1-cyclohexylethanone. The product was recrystallized from toluene and obtained in 26% 13

Among p-type organic semiconductors, oligoacenes and oligothiophenes have attracted considerable attention as the most viable materials for organic thin film transistor devices.1 In such acene materials, increasing the conjugation by the linear annulation of benzene rings usually affords an increase in carrier mobility, but unfortunately, this also leads to increased chemical instability. Similar effects, albeit to a lesser degree, are seen upon analogous heterocyclic linear annulation.2 In contrast to these linearly conjugated systems, less is known about the effect of peri-annulation on the properties of organic semiconductors. Among peri-annulated hydrocarbons, pyrene is one of the smallest that exhibits pronounced π-stacking both in solution and in the solid state.3 This structural motif is known to be a critical parameter for efficient charge hopping within the solid state material. Although charge transport properties of the parent pyrene are not known, substituted pyrene derivatives have been synthesized; these compounds show low carrier mobility in organic field effect transistors (OFETs).4 Heterocyclic analogues of pyrenes are good electron donors and have been demonstrated to form conducting salts with 7,7′,8,8′-tetracyanoquinodimethane (TCNQ).5 However, charge transport properties of this class of materials in OFETs are not known. In the present work, we report the synthesis and charge transport behavior of two dioxapyrene derivatives 1 and 2.

Experimental Methods General. Chemicals were purchased from Aldrich and were used as received. 1H NMR (500 MHz) and 13C NMR (500 MHz) spectra were obtained on a Varian Inova narrow-bore multinuclear NMR spectrometer using tetramethylsilane as the * E-mail: [email protected].

10.1021/jp903472q CCC: $40.75  2009 American Chemical Society Published on Web 07/16/2009

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TABLE 1: Raw Energies in Hartrees for 1 and Pentacene pentacene 1

E1

E4

E3

E2

-841.852 776 1 -1376.748 667

-841.851 058 5 -1376.743 414

-841.632 673 -1376.520 381

-841.630 896 6 -1376.515 349

yield. 1H NMR (CDCl3) δ: 6.25 (s, 2 H), 6.36 (d, 2 H), 6.48 (d, 2 H), 7.98 (t, 4 H), 7.59 (m, 2 H), 7.79 (m, 4 H). ES-MS: 417+ (M + H+). Cyclic Voltammetry (CV) Measurements. The measurements were performed in acetonitrile/toluene (2:1) solution containing 1 × 10-4 mol/L of 1 and 2. Before measurements, the solution was deoxygenated by argon bubbling for about 5 min. The cyclic voltammetry (Figure S1 of the Supporting Information) indicated the first oxidation and reduction reactions for 1 and 2 to be one-electron processes. The oxidation was considered reversible (ipa/ipc ≈ 1, ∆Ep ≈ 60 mV), whereas the reductions were quasi-reversible with a potential scan-rate dependent ipc/ipa ratio. The formal potentials were measured relative to in situ ferrocene and are listed in Table S1 of the Supporting Information. The highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) values are calculated assuming a ferrocene HOMO value of -4.8 eV relative to vacuum. Thin Film Characterization. The X-ray diffraction (XRD) data were collected in the reflection mode geometry using a Bragg-Brentano diffractometer equipped with a copper rotating anode, diffracted beam graphite monochromator tuned to Cu KR radiation, and a scintillation detector. Data were collected in continuous mode at a scanning rate of 2° 2θ/min. Powder patterns of 1 were also taken on the same system. Crystals of 1 were grown by the train sublimation method. We were unable to grow good quality crystals of 2 either by sublimation or from solution. Single crystal diffraction data were collected at room temperature using a Nonius KappaCCD diffractometer with graphite monochromated Mo KR radiation (λ ) 0.710 73 Å) using a fine-focus sealed tube. Diffraction frames were collected using φ + ω scans to fill the asymmetric unit. The first 10 frames were used for indexing reflections using the DENZO package and refined to obtain final cell parameters. Data reductions were performed using DENZO-SMN. The structures were solved by using the direct methods and refined by full-matrix, least-squares on F2 with anisotropic displacement parameters for the non-hydrogen atoms using SHELXTL. The hydrogen atoms were incorporated into idealized positions. Material Studio software by Accelrys was used to generate structural diagrams. Crystallographic data (excluding structure factors) for the structure(s) reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-727749. Copies of the data can be obtained free of charge from www.ccdc.cam.ac.uk/ conts/retrieving.html. Film grain structure and morphology were examined by atomic force microscopy (AFM) using a NanoscopeR Dimension 3100 scanning probe microscope instrument in tapping mode. All experiments were carried out in an environmentally controlled laboratory (relative humidity set at 20%) using singlecrystal silicon Nanoprobes tips with a spring constant of ∼17-43 N m-1 and resonance frequencies in the 262-359 kHz. Calculations. HOMO and LUMO energies were calculated in the following manner: The geometries of 1 and 2 were optimized using the PQS program with the B3LYP density functional theory method and the MIDI! basis set. HOMO and LUMO energies of parent dioxapyrene, pyrene, and 1,4naphthalene diprpenoic acid ester were calculated similarly. Raw

HOMO and LUMO energies in Hartrees were obtained at the converged geometry. These HOMO and LUMO energies were modified by empirical equations developed in our laboratory for comparing B3LYP/MIDI! vacuum HOMO and LUMO energies in Hartrees with HOMO and LUMO energies in eV obtained from experimental solution oxidation and reduction potentials. These equations are

HOMO ) -2.13 + Raw HOMO × 17.5 LUMO ) -1.09 + Raw LUMO × 22.5 These equations have shown good agreement with experimentally obtained orbital energies for a wide range of organic electronic materials. Reorganization energies were calculated by optimizing the geometries of the neutral and cationic species of 1 and pentacene using B3LYP/MIDI!. The following energies were obtained using B3LYP/MIDI!: E1: energy of the neutral species at the geometry of the neutral species, E2: energy of the cation at the geometry of the neutral species, E3: energy of the cation at the geometry of the cation, and E4: energy of the neutral at the geometry of the cation. The reorganization energy, λ, was then calculated as

λ ) (E2 - E3) + (E4 - E1) A value of 27.21 eV/Hartree was used to convert Hartrees to electron volts. Raw energies in Hartrees for 1 and pentacene are given in Table 1. Fabrication and Evaluation of OFET Devices. To probe carrier transport properties, a bottom-gate, p-type OTFT device incorporating 1 and 2 were made by vacuum deposition (175-200 Å at a rate of 0.1 Å/s) on octadecyltrichlorosilane (OTS)-modified7 SiO2/Si and SiO2/Si substrate held at temperatures (Tdep) of 25 and 75 °C. The devices were fabricated in “top contact” geometry where the heavily doped silicon substrate served as the gate electrode, and the gold (Au) source and drain electrodes (50 nm thick, 200 µm × 650 µm wide) were thermally evaporated through a shadow mask on the organic semiconductor layer. The OFET devices had a variable channel length (L) of 50, 100, and 150 µm, a constant width (W) of 650 µm, and a SiO2 gate dielectric thickness of 190 nm. Electrical characterization of the fabricated devices was performed with a Hewlett-Packard HP 4145b parameter analyzer. All measurements were done under ambient atmosphere. The devices were exposed to air prior to testing. For each experiment performed, between 4 and 12 individual devices were tested on each sample prepared, and the results were averaged. For each device, the source-drain current (ISD) was measured as a function of source-drain voltage (VSD) for various values of gate voltage (VG). For most devices, VSD was swept from 0 to -100 V for each of the gate voltages measured, typically -5, -35, -55, and -80 V. In these measurements, the gate current (IG) was also recorded to detect any leakage current through the device. Furthermore, for each device, the

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SCHEME 1

drain current was measured as a function of gate voltage for various values of source-drain voltage. For most devices, VG was swept from 0 to -100 V for each of the drain voltages measured, typically -5, -35, -55, and -80 V. The saturation field effect mobility, µsat, was extracted from a straight-line fit to the linear portion of the ID-versus-VG curve according to the following equation:

ISD ) Ci µ(W/2L)(Vgs-VT)2 where ISD is the drain current, Ci is the capacitance per unit area of the gate dielectric layer, and VGS and VT are the gate voltage and threshold voltage, respectively. The threshold of the device was determined from the relationship between the square root of ID at the saturated regime, and VGS was determined by extrapolating the measured data to ISD ) 0. Results and Discussion The syntheses of 1 and 2 were realized by the procedure outlined in Scheme 1. Both 1 and 2 are thermally stable red crystals, which are readily purified by recrystallization or vacuum sublimation. The electronic absorption spectra of 1 and 2 (Figure 1A) are qualitatively similar to related derivates reported earlier.8 The absorption spectrum of vacuum-deposited thin film of 1, in addition to displaying a slight bathochromic shift, resemble that obtained in solution. Extended irradiation of a thin film of 1 in air did not lead to any observable change (Figure 1B), indicating that this class of materials is more photostable than the aforementioned linear acenes. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of 1 and 2 were estimated from cyclic voltammetry (CV) measurements (Figure S1). From the reversible oxidation and reduction potentials obtained from cyclic voltammetry, the HOMO and LUMO levels of 1 were calculated to be -5.09 and -2.75 eV, respectively, below the vacuum level, which corresponds to a HOMO-LUMO gap of 2.34 eV. Similarly, the HOMO and LUMO levels for 2 were calculated to be -5.13 and -2.94 eV with a HOMO-LUMO gap of 2.19 eV (Table S1). These experimental HOMO and LUMO levels as well as HOMOLUMO gaps are quantitatively consistent with the results derived from B3LYP/MIDI!9 density functional theory (DFT) calculations that give -5.20 and -2.76 for 1 (Figure 3) and -5.26 and -2.97 for 2 (the derivation of the DFT values is given in the Experimental Methods section). Although the molecular shape of dioxapyrenes 1 and 2 are pyrene-like, the conjugated system is somewhat different. The calculated HOMO and LUMO energies of pyrene are -5.62 and -2.34 eV, respectively, whereas the HOMO and LUMO levels of parent 1,6-dioxapyrene are -4.95 and -1.68 eV, respectively. Oxygens in 1,6-dioxapyrene act as electron-

Figure 1. (a) Absorption spectra of 1 and 2 CH3CN (black); (b) absorption spectra of a thin film of 1 on glass before and after irradiation in air (∼2 h).

Figure 2. (a) Pictorial presentation of HOMO and LUMO of 1 calculated at the B3LYP/6-31g* level in vacuum.

donating groups and raise the HOMO and LUMO energies and also make it a better electron donor (hole transporter) as compared to pyrene. Interestingly, electron-withdrawing keto groups in 1 and 2 lower the HOMO-LUMO levels and make them much closer to pyrene energy levels. It is interesting to note here that the conjugated chain in dioxapyrenes 1 and 2 is roughly equivalent to an ester of naphthalene dipropenoic acid, and such a structure might have been expected to lead to electron transport behavior; however, the calculated LUMO level of -2.84 eV in 1,4-naphthalene dipropenoic acid ester is similar to the LUMO energy of 1, whereas the HOMO level of -5.92 eV in 1,4-naphthalene dipropenoic acid ester is 0.6 eV more negative that that of 1. This is consistent with its observed hole transport properties (vide infra).

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Figure 3. (a) Crystal structure of 1. (b) Crystalline packing motif of 1 (cyclohexane rings omitted for clarity).

The reorganization energy plays an important role in charge mobility and can be calculated as the difference between the adiabatic and vertical ionization potential of the neutral compound and the difference between the adiabatic and vertical electron affinity of the cation.2b,10 B3LYP/MIDI! calculations give a reorganization energy of 0.28 eV for 1 and 0.10 eV for pentacene. This high reorganization energy is similar to those of the more flexible thiophene oligamers,2b despite the fact that the π system of 1 remains planar upon ionization. The crystal structure of 1 shows that the dioxapyrene core is planar and, within the unit cell, the molecules are slip-stacked (Figure 3B). The unit cell of 1 is triclinic (P1 space group) with the following parameters: Z ) 1; a ) 4.9240(2) Å, b ) 6.5425(3) Å, c ) 18.2042(9) Å; R ) 93.135(2)°, β ) 92.418(2)°, γ ) 107.550(2)°. It is evident that π-π-type stacking between neighboring dioxapyrene moieties is not completely cofacial, but is staggered with the shortest distance between two cores being of approximately 3.936 Å. Since good orbital overlap between neighboring molecules is required for strong electronic coupling2b and good charge mobility, the observed intermolecular distance in 1 is significantly larger than 3.4-3.5 Å π-π stacking distance commonly observed in cofacially packed linear acenes.1 The charge transport properties of these dioxapyrenes were examined via their incorporation in OFET devices. OFETs were fabricated in the top-contact configuration by vapor-depositing 1 and 2 on Si/SiO2 substrates that were surface-modified with octyltrichlorosilane (OTS). Gold served as the source and drain electrodes with W/L ≈ 4.5 in these experiments. Both 1 and 2 showed typical p-type field effect operation with excellent output and transfer characteristics. Representative output characteristics (drain current (IDS) versus source-drain voltage (VDS) curves at different gate voltages (VGS)) and transfer characteristics (ISD vs VGS plotted on a logarithmic scale and (ISD)1/2 vs VGS, at VDS ) 100 V) of the transistors based on 1 at 25 °C are shown in Figure 4. For a device with 150 µm channel length and 650 µm channel width, a saturation regime hole mobility of 0.25 cm2 V-1 s-1 and Vth 9.8 V and current on/off ratio of 4.5 × 105 was extracted for VDS ) -80 V (Table 2). The electrical performance did not

Figure 4. I-V characteristics of exemplary OFET devices based on 1 coated on OTS-treated SiO2/Si substrate with 150 µm channel length and 650 µm channel width: (a) The output curves at gate voltages VG ) -5, -30, -55, and -80 V. (b) Transfer characteristics for the same device at VDS ) -80 V.

TABLE 2: Electrical Performance of OFET Devices Incorporating 1 and 2 Dielectric 1 2

OTS/SiO2 OTS/SiO2 SiO2 OTS/SiO2 OTS/SiO2 SiO2

a

Tdep (°C)

b avg µ (σ)c (cm2 V-1 s-1)

22 75 22 22 75 22

0.26 (0.05) 0.15 (0.03) 1.2 × 10-3 0.07 (0.007) 0.1 (0.03) 5 × 10-5

d

Ion/Ioff

4.5 × 105 7.3 × 106 8.8 × 103 1.8 × 105 1.3 × 106 6 × 102

e

Vth (V) 9.8 8.08 6.81 14 14 6

a Tdep, substrate temperature during deposition of 1. b µavg, field effect mobility. c Standard deviation, n ) 6. d Ion/Ioff, current on to off ratio. e Vth, threshold voltage.

degrade when devices were left in ambient for over 48 h. These devices exhibit low contact resistance, and as a consequence, saturation mobility does not decrease too much with decreasing channel length. Accordingly, for devices with W/L ) 650/50, saturation mobility µ ) 0.19 ((0.02) cm2 V-1 s-1 was not significantly lower than for devices with W/L ) 650/150. Furthermore, as is the case with other OFETs, the mobility of 1 is gate-voltage-dependent and increases with gate voltage and then saturates for gate biases in excess of around -35 V (Figure S2). The gate voltage dependence of mobility is usually indicative of the presence of defects and grain boundaries in the polycrystalline thin film.7 The device based on 2 exhibits a hole mobility of 0.07 at 25 °C, but mobility was 0.1 cm2 V-1 s-1 at 75 °C (Table 2, Figure S3). OFET device performances of 1 and 2 as a function of substrate temperature and SiO2 dielectric treatment are summarized in Table 2. OFET devices of both 1 and 2 on Si/SiO2/OTS exhibit considerable hysteresis in their transfer characteristics (Figure

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Shukla et al. Conclusion In summary, we have demonstrated that 1,6-dioxapyrene derivatives are chemically and photochemically stable, potentially useful semiconductor molecules. X-ray diffraction and AFM analyses of thin films of 1,6-dioxapyrene derivatives shows that these molecules form well-ordered polycrystalline films, which results in moderately large hole mobilities ∼0.25 cm2 V-1 s-1 in OFET devices. Relatively easy synthetic accessibility and stability of these materials allows for further functionalization of the dioxapyrene core and access to a variety of new derivatives, which would further aid in the understanding of charge transport in these novel semiconducting materials. Note Added after ASAP Publication. This article was released ASAP on July 16, 2009. A change was made to the caption of Figure 5, and the article was reposted on July 20, 2009. Supporting Information Available: Crystallographic information file (CIF), gate dependence of saturation mobility of 1, OFET device data for 2, and cyclic voltammograms and redox data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 5. (a) 1 × 1 µm2 AFM image of a thin film (18 nm) of 1 vapor-deposited on OTS-modified SiO2/Si wafer. (b) X-ray diffraction pattern of a vapor-deposited thin film (18 nm) on OTS-modified SiO2/ Si (curve a) and powder pattern of 1 (curve b).

S4) which is indicative of charge trapping/detrapping at or near the organic semiconductor/dielectric interface. Although the SiO2/OTS interface is free of polar defects, however, since all our devices were exposed to the ambient before testing, the presence of O2 and/or H2O at the semiconductor-dielectric interface could act as a charge trap, leading to the observed hysteresis. The morphology of 1 in thin film was probed by XRD and AFM. The XRD of a thin film of 1 on OTS/SiO2/Si shows narrow, intense peaks, which is consistent with the polycrystalline nature of the film (Figure 5b). The AFM image of the thin film of 1 also shows a polycrystalline film with significantly larger grain size (0.5-1 µm) (Figure 5a). On the basis of the indexing from single crystal data, the peak at 2θ ) 5° is assigned as the first-order reflection, (001), and the remaining peaks are successive orders of reflections (Figure 5b). The match between single crystal peaks and the thin film peaks suggests that the thin film grows in the same crystal habit as the thick film. Furthermore, close correspondence between the molecular length of 1 of 18.2 Å, and d-spacing of 18.12 Å suggests that thin films of 1 on OTS/SiO2 grow with the crystallite ab faces parallel to the substrate surface.

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