Electronic Properties and Supramolecular Organization of Terminal

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Electronic Properties and Supramolecular Organization of Terminal Bis(alkylethynyl)-Substituted Benzodithiophenes† Mark A. M. Leenen,‡,§ Fabio Cucinotta,§ Lucas Viani,| Alexey Mavrinskiy,⊥ Wojciech Pisula,⊥,# Johannes Gierschner,*,|,∇ Je´roˆme Cornil,| Anna Prodi-Schwab,‡ Heiko Thiem,‡ Klaus Mu¨llen,⊥ and Luisa De Cola*,§ EVonik Degussa GmbH, CreaVis-Technologies and InnoVation, Paul-Baumann-Strasse 1, D-45764 Marl, Germany, Physikalisches Institut and Center for Nanotechnology, CeNTech, UniVersita¨t Mu¨nster, Mendelstrasse 7, D-48149 Mu¨nster, Germany, Laboratory for Chemistry of NoVel Materials, UniVersity of Mons, Place du Parc 20, B-7000 Mons, Belgium, Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany, EVonik Degussa GmbH, Process Technology & Engineering, Process Technology-New Processes, Rodenbacher Chaussee 4, D-63457 Hanau-Wolfgang, Germany, and Faculty of Science, Madrid Institute of AdVanced Studies (IMDEA-Nanoscience), UAM, Module C-IX, Third LeVel, AVenida To´mas y Valiente, 7 Campus Cantoblanco, E-28049 Madrid, Spain ReceiVed: March 15, 2010; ReVised Manuscript ReceiVed: June 20, 2010

Benzodithiophene (BDT) was symmetrically bisubstituted in the terminal positions with five different alkynes CtC-(CnH2n+1) with n ) 4, 6, 8, 10, 12. The materials were characterized as potential materials for fieldeffect transistor applications. Electrochemical measurements in solution and photophysical measurements in solution and in the solid state, together with UV photoelectron spectroscopy in air and quantum-chemical calculations, elucidate the nature of the frontier orbitals and of the excited states as well as their deactivation pathways. Structural information on the molecular assembly in the solid state, both at room temperature and at elevated temperatures, is obtained by a combination of DSC, polarized optical microscopy, and 2D-WAXS, which point to the crystallinity of the compounds in all phases and reveal π-stacking arrangements independently of the length of the alkyl side chains. 1. Introduction Due to the huge potential of the emerging printed electronics market,1 research activity into organic electronics has increased tremendously in the past decade. In addition to organic lightemitting diodes and organic photovoltaics (OPV),2 organic fieldeffect transistors (OFETs)3,4 are in an advanced stage of development. Material properties of organic semiconductors are being optimized for application in these devices. In addition, solution processing of organic semiconductors allows for costefficient production by techniques, such as roll-to-roll printing in comparison to vacuum deposition. Recently developed solution-processable organic p-type semiconductors already show suitable physical properties for OFET application.5-11 In many cases, fused (hetero)acenes are an essential part of the molecular structure of these semiconductors. A frequently used fused-ring system is benzo[1,2-b;4,5b′]dithiophene (BDT). BDT has been already used as conjugated core in small molecules12-14 and (co)polymers,15-17 mostly in OFETs but also in OPVs.18-20 In the molecules studied in this work, the BDT core is symmetrically 2,6-substituted with alkylethynyl groups with varying lengths of the alkyl chains (compounds 1-5, Scheme 1), as synthesized by similar procedures as those reported in the literature.14,21 †

Part of the “Michael R. Wasielewski Festschrift”. * To whom correspondence should be addressed. E-mail: (J.G.) [email protected], (L.D.C.) [email protected]. ‡ Evonik Degussa GmbH, Creavis-Technologies and Innovation. § Universita¨t Mu¨nster. | University of Mons. ⊥ Max Planck Institute for Polymer Research. # Evonik Degussa GmbH, Process Technology & Engineering. ∇ Madrid Institute of Advanced Studies.

SCHEME 1: Structure of the BDT molecules 1 (n ) 4), 2 (n ) 6), 3 (n ) 8), 4 (n ) 10) and 5 (n ) 12)

Semiconductor performance depends on the HOMO (H) and LUMO (L) energy levels, which control hole or electron injection efficiency. Moreover, the energy levels also govern the environmental stability of the material in both the undoped and doped states.22 Finally, if the band gap of the system is large enough, the material can be transparent. Transparency can be an advantageous material property, since an emerging technology of transparent, “invisible” electronics is taking shape. Here, the introduction of the ethynyl functionality might significantly tune the electronic levels characteristic of the BDT core. To analyze the evolution of the (optical) bandgaps as well as of the frontier orbitals going from BDT to the derivatives 1-5, a full electrochemical and photophysical characterization of the molecules is reported and is supported by quantumchemical calculations. The rigid acetylene bond extends not only the conjugation length but also the size of the rigid core of the molecules. The alkyl chains increase the solubility of the molecules and, via intermolecular interactions with alkyl chains of adjacent molecules, could facilitate a tight molecular packing.23 The different alkyl chain lengths might also critically influence the phase behavior. In previous reports, similar materials have revealed strongly phase-dependent charge carrier mobilities.24,25 Accordingly, the phase behavior and supramolecular organization of these five molecules have been investigated by differential

10.1021/jp102360v  2010 American Chemical Society Published on Web 07/26/2010

Bis(alkylethynyl)-Substituted Benzodithiophenes scanning calorimetry (DSC), polarized optical microscopy (POM), powder X-ray diffraction (XRD) and 2-dimensional wide-angle X-ray scattering (2D-WAXS). 2. Experimental Section 2.1. Materials, Electrochemistry, and Photoelectron Spectroscopy. All chemicals were commercially available (Sigma Aldrich) and were used as received without further purification. Electrochemistry was performed with an Autolab Potentiostat in a three-electrode, single-compartment cell. As electrolyte, 0.1 M tetrabutylammoniumhexafluorophosphate (TBAPF6) in anhydrous dichloromethane (DCM) was used; the compound concentration was 10-3 M in the electrolyte. A platinum working electrode, stainless steel counter electrode, and Ag/AgCl pseudoreference electrode were used. UV photoelectron spectroscopy in air (PESA) was performed on a Riken Keiki AC2 photoelectron spectrometer. Thin films (drop-cast from chloroform) of the materials were irradiated with a beam of UV light (deuterium UV lamp), ranging in energy from 3.4 to 6.2 eV in steps of 0.05 eV (grating type monochromator). Through the photoelectric effect, electrons are ejected from the HOMO level of the (grounded) sample by UV light of sufficient energy. These ejected electrons ionize oxygen molecules which are counted by a detector. The UV spot area is 4 mm2, and the spectra were corrected for relative light intensity at each wavelength. Repeated measurements on the same sample did not show a drift in ionization potential after at least three repetitions. 2.2. Spectroscopy. UV-vis absorption spectra were recorded on a Varian Cary 5000 double-beam UV-vis-NIR spectrophotometer; all spectra were measured in quartz cuvettes (1 cm, Hellma). Steady-state emission spectra in the spectral range 300-800 nm were recorded on a HORIBA Jobin-Yvon IBH FL-322 Fluorolog 3 spectrometer equipped with a 450 W xenon arc lamp, double grating excitation, emission monochromators (2.1 nm/mm dispersion; 1200 grooves/mm), and a Hamamatsu R928 photomultiplier tube or a TBX-4-X single-photon-counting detector. Emission and excitation spectra were corrected for source intensity (lamp and grating) and emission spectral response (detector and grating) by standard correction curves. Time-resolved measurements up to ∼5 µs were performed using the time-correlated single-photon counting (TCSPC) option on an Edinburgh Life Spec II equipped with MCP-PMT. A picosecond laser diode (378 nm; pulse width 70 mW) with maximum repetition rate of 20 MHz was used to excite the samples. For excited-state lifetimes >10 µs, a different experimental setup was used by equipping the Fluorolog 3 with the FL-1040 phosphorescence module with a 70-W xenon flash tube (fullwidth at half-maximum, fwhm ) 3 ms) with a variable flash rate (0.05-25 Hz). The signals were recorded on the TBX-4-X single-photon-counting detector and collected with a multichannel scaling card in the IBH DataStation Hub photon-counting module, and data analysis was performed as described above. Emission quantum yields were determined with a Hamamatsu Photonics absolute PL quantum yield integration sphere system (C9920-02) equipped with a L9799-01 CW Xenon light source (150 W), monochromator, and C7473 photonic multichannel analyzer and employing U6039-05 PLQY measurement software (Hamamatsu Photonics, Ltd., Shizuoka, Japan). 2.3. Calculations. Quantum-chemical calculations of the geometric, electronic, and optical properties were carried out on BDT and compound 1; all geometries were optimized by imposing the highest possible symmetry. The ground-state

J. Phys. Chem. B, Vol. 114, No. 45, 2010 14615 geometries and electronic structures were obtained at the density functional theory (DFT) level and vertical transition energies (Emax) to singlet and triplet excited states with the timedependent (TD-) DFT formalism. Adiabatic ionization potentials were obtained via the relaxed geometries of the neutral and charged species. Properties in solution were calculated using the polarizable continuum model (PCM) using the overlap index between two interlocking spheres set as 0.8 and a minimum radius of 0.5 for the solvent-excluded surface spheres. All calculations were carried out using the B3LYP functional and the 6-311G* basis within the Gaussian03 program package.26 2.4. Thermal and Structural Characterization. Thermogravimetric analysis (TGA) was performed on a Netzsch TG 209 thermo microbalance. DSC measurements were performed on a Mettler Toledo DSC820 calorimeter on open (hole in lid) aluminum sample pans. In addition, the transition temperatures of the compounds were investigated with a ZEISS West Germany POM equipped with a digital temperature control system, UNKAM TMS 591. Each sample was sandwiched between glass slides to form a thin film and was afterward heated above the melting point under nitrogen atmosphere using a heating stage. It was then cooled down to 25 °C at a rate of 1 K/min. The images were recorded at different temperatures between cross-polarizers. Both powder X-ray diffraction and 2D-WAXS temperaturedependent experiments were performed to investigate the supramolecular structure of all compounds. A Siemens D500 Kristalloflex with a graphite-monochromized CuKR X-ray beam was used for X-ray powder diffraction experiments. The diffraction patterns were recorded in the 2θ range from 1° to 40° and are presented as a function of the scattering angle. The 2D-WAXS measurements were performed with a rotating anode (Rigaku 18 kW), an X-ray beam with a pinhole collimation, and a 2D Siemens detector. A double graphite monochromator for the CuKR radiation (λ ) 0.154 nm) was used. The samples were prepared by filament extrusion at elevated temperatures.27 3. Results and Discussion BDT derivatives 1-5 were synthesized in good yields according to literature procedures.14,21 Analytical data on new compounds 1 and 5 can be found in the Supporting Information. 3.1. Electronic Properties. Electrochemistry and Photoelectron Spectroscopy. To estimate the HOMO and LUMO energy levels of the molecules, redox potentials were determined by cyclic voltammetry using 0.1 M TBAPF6 in anhydrous dichloromethane as electrolyte. The oxidation potentials, Eox, are summarized in Table 1; the reduction potentials Ered could not be measured due to the large band gap of the materials. The same Eox was measured for 1-5, since the conjugated core is identical for all molecules. The monoelectronic oxidation process is irreversible (see Figure S1 of the Supporting Information) so that Eox is prone to a relatively large error. This error occurs in general when the onset (the definition of which is also not standardized) of oxidation/reduction is used to determine Eox/Ered. Since this is a common way of determining redox potentials in the field of organic electronics, 14,21,28 the error margin is a systematic one in this entire field of materials research. To make a more reliable determination of Eox, we used differential pulse voltammetry (DPV, see Figure 1), which is more sensitive than CV. The DPV measurements support the qualitative conclusions from CV. The values of the oxidation potentials are +0.88 V estimated against ferrocene; 0.07 eV lower than those obtained from CV (Eox ) +0.81 V) due to

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TABLE 1: Electronic and Optical Properties of BDT and Compounds 1-5a BDT exptl

calcd

vacuo

DCM

-7.26 -5.66 -1.31 4.35 3.95 (0.14) 4.40 (0.02) 5.07 (0.55) 2.65 3.65 3.81 3.84

-5.66 -5.73 -5.6c 3.64d -5.63 -5.66 -1.28 4.38 3.93 (0.23) 4.38 (0.02) 4.94 (0.77) 2.67e 3.68 3.81 3.86

Ti states

vacuo

DCM

-6.69 -5.47 -1.77 3.69 3.44 (0.64) 3.78 (0.63) 4.53 (0.32) 2.17 3.08 3.11 3.70

-5.61 -5.68 -5.6c 3.35d -5.52 -5.62 -1.92 3.70 3.37 (0.96) 3.75 (0.58) 4.50 (0.32) 2.18 3.09 3.14 3.70

b

CV

EOx EOxDPV IP Eg,opt IPad EHOMO ELUMO EH-L Si states

compounds 1-5

1 1B u 2 1B u 3 1B u 1 3B u 2 3B u 3 3B u 2 3A g

1 1 Bu 2 1 Bu 3 1 Bu 13 Bu 2 3 Ag 2 3 Bu 3 3 Bu

a

Experimental (exptl) oxidation potentials from electrochemistry (Eox, reported against vacuum, under the premise that ferrocene has an Eox at -4.80 eV vs vacuum),21,29-31 ionization potentials (IP) from PESA on thin films, and optical bandgap (Eg,opt). Quantum-chemical results (DFT B3LYP/6-311G* in vacuo and in DCM using the PCM model) for the adiabatic IPs, frontier orbital energies (EHOMO, ELUMO), energy gap (EHL), and vertical transition from the ground state S0 (11Ag) to the lowest singlet (Si, only 1Bu states, oscillator strengths in parentheses) and triplet (Ti) states. All values are given in eV. b Literature value is -5.6 eV (ref 21). c Measured on thin films. d Determined from the intersection of the absorption and emission spectra, vide infra. e Literature value is 2.59 eV in 5/5/2 diethyl ether/isopentane/ethyl alcohol (ref 32).

Figure 1. Normalized differential pulse voltammograms of 4 as representative (black line) and BDT (red line) in solution (0.1 M TBAPF6 in anhydrous dichloromethane), estimated versus ferrocene. Scan rate 25 mV/s.

the larger CV error margin. In parallel with CV, Eox of BDT is 0.05 eV lower (Eox ) +0.93 V) than those of 1-5, due to its smaller conjugation length. The HOMO energy levels in thin film have been estimated from the ionization potentials (IP) of drop-cast films, determined by UV PESA. A typical spectrum of 5 is depicted in Figure 2. Within experimental error, compounds 1- 5 exhibit the same IP in thin films in air (see Table 1). Remarkably, BDT has about the same IP in thin film as compounds 1-5, thus pointing to similar HOMO energies. The expected decrease of the band gap of 1- 5 (since the acetylene moieties extend the conjugation of BDT) thus appears to be largely attributable to a decrease (more negative) of the LUMO energy level. The calculation of the IP at the DFT level (see Table 1) demonstrates that even in a nonpolar solvent such as DCM, solvent contributions to the energy of charged species can easily exceed 1 eV and, thus, have to be included to correctly reproduce the experimental results. They also indicate that a simple estimation of HOMO energies from experimental IPs might be biased by very different solvent

Figure 2. Typical PESA spectrum of a drop cast film of 5 on glass. The onset of oxidation corresponds to the ionization potential of the material.

contributions (see Table 1). Quantum-chemical calculations thus prove of fundamental importance to elucidate the changes of the electronic nature of the frontier orbitals going from BDT to 1. The changes can be easily understood from a frontier orbital correlation diagram of the BDT and acetylene units. According to Figure 3, the energy difference between the LUMOs of the constituting subunits is much smaller than for the HOMOs. Therefore, the resulting LUMO of compounds 1-5 is, indeed, stabilized in a stronger way than the HOMO is destabilized compared to BDT. 3.2. Optical and Photophysical Properties. UV-vis absorption together with fluorescence emission and excitation spectra of compounds 1-5 in solution are almost identical to each other, reflecting the lack of π-electrons in the alkyl side chains;14 representative spectra of compound 1 are shown in Figure 4. The absorption spectrum consists of three main bands, with maxima located at 3.39 (366 nm), 3.69 (336 nm), and 4.37 eV (284 nm). According to the TD-DFT calculations, they can be assigned to the lowest three Bu states formed by π-π*

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Figure 5. Normalized fluorescence and phosphorescence spectra of compound 1 (red line) at 77 K in DMF rigid matrix compared to BDT (black line). Figure 3. Orbital correlation diagram of the HOMO (H) and LUMO (L) levels of compound 1 and its constituting subunits (BDT and acetylene) based on DFT calculations.

Figure 6. Normalized absorption and emission spectra of compounds 1 (solid line) and 5 (dashed line) from drop-cast films (λexc ) 322 nm).

Figure 4. Absorption (red), excitation (black), and emission (blue) spectra of 1 in dimethylformamide solution.

transitions, calculated for 1 in DCM at 3.37 (368 nm, mainly described by HOMOfLUMO electronic excitation), 3.75 (331 nm), and 4.50 eV (276 nm) (see Table 1). The satellite absorptions (*) can be assigned to vibronic side bands, showing an “effective mode” of 1390 cm-1 also encountered in other fused aromatic systems formed by normal modes with main contributions from carbon-carbon stretching/deformations along the long axis of the π-conjugated backbone. The optical band gap, determined from the intersection of the absorption and emission spectra,21 amounts to 3.35 eV. According to the TDDFT calculations, the introduction of two acetylene groups decreases the band gap by ∼0.5 eV; an experimental estimation gives ∼ 0.3 eV (Table 1), essentially due to the decrease in the LUMO energy by the insertion of the acetylene unit (vide supra). The well-structured emission spectrum shows maxima at 3.32 (373 nm) and 3.16 eV (392 nm) and a small Stokes shift of 8 nm between absorption and emission, as usually found for rigid, polycyclic aromatic systems,32-34 which further reduces upon band narrowing in spectra recorded at 77 K (see the Supporting Information). The fluorescence excitation spectrum, recorded at a fixed emission wavelength of 392 nm, is very similar to the absorption profile, indicating that the conversion of higher excited states to the lowest emitting one is complete. Nevertheless, the low fluorescence quantum yield (ΦF ) 0.05) suggests that the emitting state undergoes effective radiationless pathways, such as fast internal S1 f S0 conversion and possibly S1 f Tn intersystem crossing (ISC), as observed for the parent compound BDT.32 However, although phosphorescence can be recorded at 77 K for BDT (τ ) 107.3 ms; literature: τ ) 110 ms),32 this is possible for compound 1 (τ ) 9.34 ms) only upon addition of methyl iodide (MeI) which facilitates the triplet state population (Figure 5).35 The different photophysical pathways can be attributed to the difference in the triplet manifolds of BDT and 1 as supported by DFT calculations (see Table 1).

Whereas the closest accessible triplet state of proper symmetry (33Bu) is only 0.1 eV below S1 for BDT, the energy difference in compound 1 is considerably larger, thus significantly reducing the probability for ISC, according to the energy gap law. It is worth noting that the experimental data (Figure 5) for the fluorescence and phosphorescence spectra of compound 1 correspond very nicely to the calculated S1 and T1 states (Table 1). In drop-cast films (from DCM solutions), the emission spectra of compounds 2-5 are structured similarly to in solution (the high energy subband is partly suppressed by reabsorption) but are red-shifted by about 0.2 eV due to the interaction between the π-electron systems.36,37 A quite different situation is found for compound 1 carrying the shortest alkyl chains of the series, where the emission is almost completely unstructured and centered at 2.78 eV (446 nm); concomitantly, the absorption suffers a slight red shift against solution by about 0.25 eV (see Figure 6). The loss of structure and pronounced red shift are usually attributed to a stronger overlap of the π-systems of adjacent molecules, leading to an “excimer-like” arrangement.36,38 The fluorescence lifetime of 1 in film (826 ps) is about half as long as the lifetime of 5 (1.6 ns). BDT (synthesized by literature procedures)21 also displays red-shifted absorption and emission peaks in film, as compared with the spectra in solution. The optical band gap in film is reduced by ∼0.18 eV, amounting to 3.50 eV, which is larger than the Eg of compounds 1-5 (Figures S2 and S3, Supporting Information). 3.3. Solid-State Morphology. Thermal Characterization and Optical Textures. To check the thermal stability of the materials, TGA was performed on 1 and 5, both in air and in inert atmosphere (N2). In all cases, 10 kJ/mol; see Table S2 of the Supporting Information),39,40 which is indicative of rather large changes during those phase transitions. Remarkably, none of the compounds exhibit liquid-crystalline phases. Structural Characterization. To gain insight into the changes in the molecular packing at the phase transitions, powder X-ray diffraction was performed at different temperatures. As a representative example, the X-ray diffractograms of 1 at three different temperatures (Figure S8, Supporting Information) reveal a high number of reflections, which is characteristic of a crystalline material, in agreement with the POM observations. The reflections in the scattering range between 15 and 30° are

related to the packing of the molecules; however, the data do not allow for a quantitative assignment. Heating from room temperature to 90 and 140 °C changes the reflection distribution, indicating a modification in the molecular packing between the phases. The smaller-angle reflection only shifts from a d spacing of 1.7 nm at 30 °C to 1.92 nm at 140 °C. The d spacing is attributed to the distance between packed layers of 1 (illustration in Figure 9c, top view), with the layers separated by the alkyl side chains. The shift in d spacing upon heating is most probably due to a thermally induced side chain mobility’s preventing a close interdigitation. Further information about the organization of small molecules as building blocks within complex superstructures can be obtained by 2D-WAXS on macroscopically aligned fibers.41-43 For such X-ray scattering experiments, the extruded fibers were positioned vertically with respect to the 2D detector. The scattering patterns reveal a similar characteristic distribution of the reflections for all compounds, pointing to an identical supramolecular organization. The pattern of compound 1 is presented in Figure 9a, b. The equatorial reflection (I) in the small-angle area is related to a d spacing of 1.63 nm which is close to the value determined for the powder (1.7 nm). The theoretical size of 2.25 nm for the long axis of 1 (as determined by quantum-chemical calculations) also suggests that the side chains are interdigitated. Furthermore, the equatorial position of the corresponding reflection points to well-aligned layers along the fiber axis. As expected, the substitution of the rigid BDT core by longer alkyl chains leads to an increased interlayer spacing up to 3.04 nm for 5 (Figure 10). The almost linear dependence between the interlayer distance and the alkyl chain length suggests that the substituents are interdigitated in the same fashion and with the same degree among the substituents. Heating the sample over the phase transitions increases slightly the layer distance, but does not

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Figure 9. 2D-WAXS patterns of 1 as an extruded filament at (a) 30 and (b) 90 °C. (c) Schematic illustration of the layered organization (distances are given for 1; the arrow indicates the fiber alignment). The BDT core is illustrated by the yellow block; the alkynyl chains by the gray screws (bond angles omitted for clarity).

spectra of 2-5. In this case, the 2D-WAXS results also do not allow one to distinguish between a herringbone versus 1D layer ordering; the latter has been reported for the analogous 2,6bis(alkyl)benzo[1,2-b:4,5-b′]dithiophenes.14 4. Conclusions

Figure 10. Relationship between layer distance (at 30 °C) and side chain length for 1- 5.

change the overall organization (Figure 9b). The alkyl side chain length has a negligible effect on the intralayer period and the π-stacking distance. The intermolecular interactions between neighboring alkyl chains are thus apparently not strong enough to have a decisive influence on the supramolecular packing. The meridional and off-meridional reflections are attributed to correlations within the crystalline layered structures (Figure 9c). The meridional scattering intensity (II) is related to a d spacing of 0.54 nm along the stack made of molecules tilted by an angle of ∼45°, and the π-stacking distance of 0.37 nm is determined from the off-meridional reflections (III). Such a layer organization was also found for other crystalline rigid rods carrying terminal alkyl chains.44 Since the π-stacking distance is the same for compounds 1-5, the 2D-WAXS measurements on extruded fibers do not explain the occurrence of the “excimer-like” emission spectrum of 1 in the thin film, as compared with the structured emission

Bis(1-alkynyl)benzodithiophenes with different alkyl chain lengths have been synthesized as materials for organic fieldeffect transistors. Charge transport properties of these materials are expected to be influenced both by their intrinsic electronic properties as well as by their supramolecular arrangement. We thus carried out a full electrochemical and optical characterization in solution and in the solid state and elucidated the supramolecular architecture by thermal, optical, and X-ray investigations. The electronic and optical properties were further calculated by DFT to gain a deeper understanding of the experimental observations. Compared with the parent BDT molecule, the introduction of the ethynyl linkage strongly stabilizes the LUMO level, whereas the HOMO level is only slightly changed; this can be rationalized by the asymmetric energetic offset of the frontier levels of the ethynyl unit with respect to BDT. The overall reduction of the bandgap is recovered in the absorption spectra, where the different transitions were assigned with the help of quantum-chemical calculations. Compared with BDT, which shows significant intersystem crossing, the novel compounds deactivate mainly nonradiatively, with little fluorescence and only phosphorescence when methyl iodide is added. Although the length of the side chain has no significant influence on the properties in solution, it governs the thermal behavior of the materials, as revealed by DSC. Polarized microscopy shows that the phases at elevated temperatures are crystalline in nature, but no liquid crystalline phases were

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detected, independently of the alkyl chain length. The observation of LC phases might thus require branched or more flexible alkoxy side chains. In extruded fibers, all compounds arrange in very similar π-stacked layers, with the interlayer distance increasing linearly with the length of the alkyl side chains. First OFET results, fabricated by simple spin-coating on standard silicon substrates, give hole mobilities in the 10-3 cm2/(V s) range. Because of the highly crystalline nature of the materials, crystal grain size has to be carefully controlled to prevent charge trapping at grain boundaries. Such grain boundaries are currently responsible for hysteresis and large threshold voltages in transfer characteristics. A fabrication technique such as zone-casting is able to increase crystal grain size as well as to produce highly ordered thin films, with the molecules aligned along the OFET channel, which will greatly improve OFET performance of these materials. Zone casting experiments are currently in progress and will be published in due course. Supporting Information Available: Analytical data on new compounds 1 and 5; absorption, excitation, and emission spectra of BDT and 1 in solution (room temperature and frozen matrix) and of all compounds in thin film; TGA plot and DSC data; powder XRD plots of 1 at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgment. The authors thank the European Commission for financing through the Human Potential Programme (Marie-Curie RTN NANOMATCH, Grant no. MRTN-CT-2006035884) and the BMBF for funding through the MaDriX project. J.G. is a Ramo´n y Cajal Research Fellow of the Spanish Ministry for Science and Innovation. J.C. is an FNRS research fellow. References and Notes (1) Leenen, M. A. M.; Arning, V.; Thiem, H.; Steiger, J.; Anselmann, R. Phys. Status Solidi A 2009, 206, 588–597. (2) See Handbook of Organic Electronics and Photonics; Nalwa, H. S., Ed; American Scientific Publishing: Stevenson Ranch, CA, 2008; ISBN: 978-1-58883-095-5, for a complete overview of the state-of-the-art of current research in all areas of organic electronics. (3) Zaumseil, J.; Sirringhaus, H. Chem. ReV. 2007, 107, 1296–1323. (4) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070–4098. (5) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Macdonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. J. Nat. Mater. 2006, 5, 328–333. (6) Milia´n Medina, B.; Van Vooren, A.; Brocorens, P.; Gierschner, J.; Shkunov, M.; Heeney, M.; McCulloch, I.; Lazzaroni, R.; Cornil, J. Chem. Mater. 2007, 19, 4949–4956. (7) Ebata, H.; Izawa, T.; Miyazaki, E.; Takimiya, K.; Ikeda, M.; Kuwabara, H.; Yui, T. J. Am. Chem. Soc. 2007, 129, 15732–15733. (8) Park, S. K.; Jackson, T. N.; Anthony, J. E.; Mourey, D. A. Appl. Phys. Lett. 2007, 91, 063514. (9) Subramanian, S.; Park, S. K.; Parkin, S. R.; Podzorov, V.; Jackson, T. N.; Anthony, J. E. J. Am. Chem. Soc. 2008, 130, 2706–2707. (10) Tsao, H. N.; Cho, D.; Andreasen, J. W.; Rouhanipour, A.; Breiby, D. W.; Pisula, W.; Mu¨llen, K. AdV. Mater. 2009, 21, 209–212. (11) Gao, P.; Beckmann, D.; Tsao, H. N.; Feng, X.; Enkelmann, V.; Baumgarten, M.; Pisula, W.; Mu¨llen, K. AdV. Mater. 2009, 21, 213–216. (12) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359– 369. (13) Takimiya, K.; Kunugi, Y.; Otsubo, T. Chem. Lett. 2007, 36, 578– 583. (14) Kashiki, T.; Miyazaki, E.; Takimiya, K. Chem. Lett. 2008, 37, 284– 285. (15) Pan, H.; Li, Y.; Wu, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G. Chem. Mater. 2006, 18, 3237–3241. (16) Pan, H.; Wu, Y.; Li, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G. AdV. Funct. Mater. 2007, 17, 3574–3579. (17) Pan, H.; Li, Y.; Wu, Y.; Liu, P.; Ong, B. S.; Zhu, S.; Xu, G. J. Am. Chem. Soc. 2007, 129, 4112–4113. (18) Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-Z.; Son, H.-J.; Li, G.; Yu, L. J. Am. Chem. Soc. 2009, 131, 56–57.

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