Impact of Nitrogen Substitution and Molecular Orientation on the

ARTICLE pubs.acs.org/JPCC

Impact of Nitrogen Substitution and Molecular Orientation on the Energy-Level Alignment of Heteroacene Films Qian Xin,*,†,‡ Steffen Duhm,† Shunsuke Hosoumi,† Nobuo Ueno,† Xu-tang Tao,‡ and Satoshi Kera† † ‡

Graduate School of Advanced Integration Science, Chiba University, Chiba, 263-8522, Japan State Key Laboratory of Crystal Materials, Shandong University, Jinan, 250100, China

bS Supporting Information ABSTRACT: The impact of imine nitrogen (dN—) substitution in hydrocarbon oligoacenes on the charge-transport type has been carefully probed. A N-heteroacene containing one dN— and 7-fused rings, dianthrano [1,2-a:10 ,20 -j]pyridine (DAP), has been synthesized as a model molecule for the study. The electronic structure, energy-level alignment (ELA), and the molecular orientation of the molecule on two prototypical substrates, namely, highly oriented pyrolytic graphite (HOPG) and Ag(111), have been investigated using angle-resolved ultraviolet photoelectron spectroscopy. Combining crystal structure, optical property and electronic structure investigations, our results indicate that by introduction of only one dN— into a large oligoacene, the ELA can be effectively tuned and a potential of n-type conductivity can be realized. Furthermore, the permanent electric dipole induced by the intramolecular polar bonds (C—N and C—H) allows tuning the ELA by altering the molecular orientation. Tilted molecular orientations can impose smaller electron injection barriers and ionization energies to DAP thin films than a flat-lying orientation.

’ INTRODUCTION Organic electronic devices have become increasingly important because of their advantages such as low cost, high efficiency, large area, and flexible processability. In the past two decades, there has been great progress in both synthesis of organic functional materials and device fabrication techniques.1 However, basic characteristics that distinguish organic semiconductors (OSCs) from their inorganic counterparts are still not fully understood. Three main aspects for the improvement of organic electronic devices should be considered: (1) developing material functionalities, such as charge-transport properties and chemical and physical stabilities under ambient conditions; (2) unraveling phenomena at interfaces between dissimilar materials, including organic/organic and organic/inorganic interfaces; and (3) developing devices with improved performances. In addition to the component materials, interfaces are inherent and play a critical role in organic electronics, because they control the charge injection and transport. The synergic interplay between these two aspects determines the essential promotion of the performance of organic electronics. As a result, progress in organic electronics will be directed toward a deeper understanding of both materials chemistry and interface physics. OSCs are usually described as p-type (hole-transport) and n-type (electron-transport) materials. For a given OSC, however, the p or n type is not absolute, but is mainly determined by the energy-level alignment (ELA),2 which can be dominated by the energy distribution of the density of states (DOS) that r 2011 American Chemical Society

originates from the imperfectness of molecular packing and the spread of frontier molecular orbitals, namely, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).3 The well-developed and extensively studied p-type OSCs are already able to reach hole mobilities larger than that of amorphous silicon.4 However, n-type OSCs are still far less developed. Achieving high electron mobilities, by understanding the fundamental chemical structure versus electronic structure relationships, is essential for realizing organic p
n junctions, bipolar transistors, and organic complementary circuits.5 One approach is to alter chemically the ionization energy (IE) and electron affinity (EA), which are essential parameters for the ELA of a given material.6 Introducing electron-withdrawing side groups, such as fluorine (F)7 or cyano (CtN)8 groups, into π-conjugated systems can induce π-electron deficiency and thus increase EA. This might result in n-type transport in model devices. Moreover, the collective electrostatic impact of the intramolecular polar bonds inherent to such side groups leads to strongly molecular-orientationdependent IEs and EAs of thin OSC films.2c,9 As a result, for thin films of polar9a and nonpolar9b
e OSCs, the charge injection barriers can differ by up to 1 eV depending on the molecular orientation to a substrate. Thus, electron-withdrawing groups Received: May 8, 2011 Revised: June 26, 2011 Published: June 29, 2011 15502

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terms of intrinsic properties such as electronic structures or ELAs. In this work, we synthesized a N-heteroacene, dianthrano [1,2-a:10 ,20 -j]pyridine (DAP, Figure 1a), with seven nonlinearly fused rings, in which only one heteroatom N is included. Here, the nonlinear molecular shape can provide a larger polarization effect and electric dipole moment than in a molecule with a linear shape, and as a result an effective dependence of the ELA on molecular orientation is expected. We have studied the single-crystal structure, optical properties, and electronic structure, especially the ELA and molecular orientation of DAP on two prototypical substrates, highly oriented pyrolytic graphite (HOPG) and Ag(111), by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS).

Figure 1. (a) Chemical structure of DAP with a permanent electric dipole (blue arrow, μ = 2.41 D). (b) Two enatiomers with opposite chirality in the single-crystal structure: (S,S)-DAP and (R,R)-DAP. (c) Unit cell of the single-crystal structure projected onto the ab plane. (d) Part of the crystal packing structure, showing molecules associated by cofacial OFF π
π interactions (black double arrow line), uncofacial EF C—H 3 3 3 π interactions (pink dashed ellipse), and weak C—H 3 3 3 N hydrogen bonds (red dashed line). Atom code: C, green [dark green for (S,S)-DAP and light green for (N,N)-DAP]; N, blue; H, gray.

allow tuning of the IE and EA of an OSC film chemically by altering the IE and EA of a single molecule and physically by controlling the molecular orientation relative to a substrate. However, the actual application of many of such n-type OSCs is limited by their poor stability in ambient atmosphere.5b,10 Recently, another class of n-type mobility materials, heteroacenes containing imine nitrogen groups (dN—), have attracted increasing attention because of their high electron mobility together with good chemical and thermal stability.11 Replacement of dCH— groups in aromatic hydrocarbons by dN— groups can induce π-electron deficiency and, thus, increase the EA and impart electron injection and transport properties. The introduced dN— groups can enhance the intermolecular interactions and improve the dense packing structure because of heteroatom contacts incorporated within π
π interactions. This is highly beneficial to high charge-carrier mobilities and to stability to oxygen and moisture.12 One can also expect high stability toward photooxidation or Diels
Alder dimerization (two major degradation pathways in oligoacenes13) in these N-heteroacenes.12 Furthermore, the introduction of dN — groups supports the creation of intramolecular C—N polar bonds and a permanent electric dipole (in the case of unsymmetric N-substitution), thereby allowing the ELA to be well tuned by controlling the molecular orientation. However, fundamental research with reliable techniques on such N-heteroacenes is still very limited. Few such molecules have been studied in

’ EXPERIMENTAL METHODS DAP was first synthesized as a synthetic dyestuff in 1912,14 and very recently, it was reported as a byproduct during the synthesis of anthracene containing Tr€oger’s base analogue.15 Following the method of the latter, DAP was synthesized in relatively high yield by condensation of 2-aminoanthracene and hexamethylenetetraamine (HMTA) in trifluoroacetic acid (TFA) at ∼60 °C. The synthesis and verification methods are fully described in the Supporting Information. DAP thin films for ARUPS measurements were prepared by step-by-step sublimation on clean HOPG and Ag(111) surfaces at rates of ∼0.25 Å/ min in an ultra-high-vacuum (UHV) preparation chamber (base pressure of 2  10
8 Pa). Annealing of the bulk films on HOPG and Ag(111) was done by heating both samples at 135 °C for 1 h under UHV. Mainly p-polarized HeI radiation (21.22 eV) was used for the excitation light source. The incident angle was fixed to 65°. The spectra were measured at photoelectron emission angles (θ) of 0° (normal emission) and 45° (off-normal emission) with an acceptance angle of (10°. The energy resolution was set to 80 meV. The statistical error of UPS binding energy (BE) values is estimated to be 0.05 eV. All preparation steps and measurements were performed at room temperature. Detailed experimental setup conditions can be found in the Supporting Information. ’ RESULTS AND DISCUSSION Crystal Structure and Electric Dipole Moment. Studies of the molecular packing structure and intermolecular interactions can provide a better understanding of many physical phenomena. It has already been revealed by Tatar et al.15 that (1) the DAP molecule has acceptable planarity in the crystalline state, because the dihedral angle between the two anthracene moieties of the molecule is 16°, and (2) DAP enatiomers (Figure 1b) exhibit excellent packing through intermolecular C—H 3 3 3 π and π
π interactions in the single crystals. We prepared yellow needleshaped single crystals by slowly evaporating a solution of DAP in mixed methylene chloride and isopropanol. The single-crystal structure obtained by X-ray diffraction analysis at room temperature (Figure 1c,d) is similar to Tatar et al.’s results at 150 K.15 Our study shows that, in the racemic single crystals, the molecules form a herringbone-like arrangement with helicity: In the b direction, enantiomers with same chirality form chiral molecular arrays by offset face-to-face (OFF) π
π interactions operating on the molecular backbone, with a perpendicular distance of 3.35 Å, which is the same as the π
π stacking 15503

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Figure 2. UV
vis absorption (Abs) and photoluminescence (PL) spectra of DAP in dilute solution (s, 1  10
5 mol/L in n-hexane) and vacuum-sublimated thin films on quartz glass substrate (f, thickness ≈ 100 nm).

distance in graphite.16 In the a direction, the chiral molecular arrays form chiral helical columns by uncofacial edge-to-face (EF) C—H 3 3 3 π interactions with H 3 3 3 π distances in the range of 2.70
2.98 Å. The nonlinear configuration of this N-heteroacene is especially propitious to forming EF C—H 3 3 3 π interactions. In the c direction, molecular columns bearing opposite chirality and helicity arrange alternately by EF C—H 3 3 3 π interactions with H 3 3 3 π distances in the range of 2.70
2.98 Å and weak C—H 3 3 3 N hydrogen bonds with a H 3 3 3 N distance of 2.88 Å. These results illustrate that, in addition to relatively strong cofacial π
π and uncofacial C—H 3 3 3 π interactions, the weak C—H 3 3 3 N hydrogen bonds also play an important role that might make these π
π and C—H 3 3 3 π intermolecular interactions more effective than in hydrocarbon oligoacenes.17 As a result, a combination of these interactions leads to a close-packed structure and probably makes significant contributions to the charge-transport properties through molecular orbital overlap. The crystal data and structure refinement for the compound are summarized in Table 1 of the Supporting Information. Because of the intramolecular C—N and C—H polar bonds, the DAP molecule has a permanent electric dipole moment (μ). μ was evaluated to be 2.41 D by density functional theory (DFT) calculations [B3LYP/6-311+G(d,p)] using the molecular geometry of the single-crystal structure.18 We expect that the dipole moment affects the ELA especially in a thin film.9a Optical Properties. The optical properties of DAP dilute solution and thin films (thickness ≈ 100 nm) were investigated by UV
vis absorption and photoluminescence measurements (Figure 2). Aggregation of DAP induces broader and red-shifted absorption and fluorescence and changes the emission color from blue in the dilute solution to yellow in the thin films. The dilute solution shows a strong absorption maximum at 337 nm (ε = 2.22  104 L mol
1 cm
1) that probably originates from the s0
s2 transition of π electrons and a moderately strong absorption band at 412
437 nm (ε ≈ 4.2  103 L mol
1 cm
1) that originates from the s0
s1 transition with vibrational progressions. From the dilute solution to the thin films, the absorption bands become broader with a red shift of ∼7 nm owing to the intermolecular interactions. In addition, the optical energy gap

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(Eopt) was estimated to be 2.67 eV from the absorption edge of the thin films. This relatively large optical gap suggests that the compound is stable toward photooxidation. Photoluminescence was recorded on the dilute solution and the thin films under their optimized exciting conditions. The dilute solution shows three emission peaks at 426, 464, and 494 nm (upon excitation at 337 nm), while the thin films display an obviously red-shifted emission band with a maximum peak at 547 nm (upon excitation at 344 nm) (Figure 2). This red shift is a reflection of relatively strong intermolecular interactions in the aggregated state, whereas from the crystal structure analysis, it is also clear that there are efficient π
π and C—H 3 3 3 π intermolecular interactions and weak C—H 3 3 3 N hydrogen bonds in the crystalline state. Energy-Level Alignment and Molecular Orientation. Parts a and b of Figure 3 show the secondary-electron cutoff (SECO) and valence-band ARUPS spectra of DAP on HOPG and Ag(111), respectively, as functions of the nominal coverage (δ). BE was measured from the Fermi level (EF) of the substrate. DAP on HOPG. On HOPG, almost no vacuum level (VL, determined by the onset of SECO) shift could be observed from the interface to the bulk film (50 Å) (Figure 3a), and thus, the sample work function (Φ, measured as the energy difference from EF to VL) was nearly constant (4.50 ( 0.05 eV). With increasing δ, the intensity of σ* peak (in the SECO spectra), which is derived from the conduction band of the graphite,19 first increased to a maximum at 3 Å and then decreased. Thus, around 3 Å corresponds to a monolayer (ML), because up to the ML coverage, electrons excited in the DAP layer can move to the substrate, populate σ*, and increase its spectral intensity, whereas the intensity is attenuated with further increases in δ because of the short electron mean free path. The σ* peak could still be observed even for a 50-Å bulk film, indicating island growth on HOPG. The peak centered at 1.90 eV BE for ML coverage (3 Å) (Figure 3a) is assigned to the DAP HOMO. This peak showed enhanced intensity in off-normal emission (θ = 45°) compared to normal emission (θ = 0°) (Figure S1 of the Supporting Information). The ratio of this peak’s intensity in off-normal emission to that in normal emission, defined as RHOMO = I45/I0, is 3.7. (For more detailed information on RHOMO, see Figure S2 of the Supporting Information.) Such a dominance of the offnormal intensity is commonly observed for OSCs with relatively planar configurations if the molecules are flat-lying on an inert substrate.20 The flat-lying orientation is also the common growth mode for ML coverage of planar OSCs on HOPG.21 From 3 Å (∼1 ML) to 6 Å and then to 12 Å, the HOMO peak, as well as the whole spectrum, shifted about 0.16 eV to higher BE and underwent a broadening, whereas nearly no change occurred from 12 to 50 Å. [The variation of the HOMO is more clearly shown in the negative second derivative curves of the measured ARUPS spectra (see Figure S3, Supporting Information).] We suggest that the changes in the interlayer interactions play an important role in the HOMO shift from 3 to 12 Å,21d
f whereas other factors such as the screening effect by the substrate22 or molecular conformation changes23 might also influence this shift slightly. From 6 to 50 Å, the HOMO-derived peak always showed a tail at the lower-BE side. [The tail is more clearly shown as a peak in the negative second derivative curve (see Figure S3, Supporting Information).] As the tails are located at lower BE than the ML HOMO, they originate not from the ML HOMO, but most probably from an increase in an imperfect molecular 15504

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Figure 3. Secondary electron cutoff (SECO) and valence-band regions of ARUPS spectra for increasing coverage (δ) of DAP on (a) HOPG and (b) Ag(111). Spectra corresponding to substrate, about a monolayer (ML), and a bilayer (BL) are marked dark gray, red, and blue, respectively. The uppermost pink spectra are of the annealed films corresponding to ML coverage. Simplified energy-level alignment and suggested thin-film geometric structure of (c) DAP/HOPG and (d) DAP/Ag(111) interfaces. (Abbreviations: IE, ionization energy; HIB, hole-injection barrier; Eopt, optical band gap of the film.)

packing structure. When the 50-Å film was annealed at 135 °C for 1 h, a ML (defined as annealed ML) was obtained by sublimation of the multilayers, because the photoemission features of the annealed film were nearly the same as those of the as-deposited 3-Å film. Using these ARUPS data, we obtained the energy-level diagram of the DAP/HOPG system (Figure 3c), where the hole injection barriers (HIBs) are derived from the HOMO onset (neglecting the tails) and the IEs are defined as the energy difference between the HOMO maximum (HOMOmax) and the VL. Here, to evaluate the IE of the ML more accurately, the possible screening effect (0
0.1 eV)22c from the HOPG substrate was considered, and thus the IE of the ML (6.45 eV) was corrected to around 6.45
6.55 eV. It was found that the IEs for the as-grown MLs (∼6.45
6.55 eV) and thicker films (6.53
6.60 eV for 6
50 Å) were nearly constant. Both the absence of a VL shift (at the DAP/HOPG interface and also for increased δ) and the nearly constant IE upon further deposition confirm a flat-lying molecular orientation in both the monolayer and multilayers, because the molecule has a permanent electric dipole moment (μ = 2.41 D) parallel to the molecular

plane. Flat-lying DAP has a relatively large IE (∼6.6 eV) compared with common hydrocarbon oligoacenes (e.g., flat-lying pentacene on HOPG has an IE value of around 5.8 eV9b). These results provide evidence that the π-electron-deficient effect of introducing dN— lowers the upper occupied levels. Considering the optical band gap (Eopt = 2.67 eV) of the DAP films and assuming a typical exciton BE of ∼0.4 eV,24 we estimate the LUMO to be around 1.35 eV above EF. DFT calculations of isolated DAP molecule show that the energy difference between the LUMO and LUMO + 1 is only 0.14 eV;18 thus, DAP can easily produce a quasi-doubly degenerate LUMO, leading to a rather high DOS for the LUMO. The large IE value, as well as an electron injection barrier (∼1.35 eV) smaller than its hole injection barrier (1.72 eV), together with a probable large density of unoccupied states close to the Fermi-level, indicates that introduction of dN— can tune the electronic structure effectively, increasing IE and the possibility of n-type conductivity. Thus, a N-heteroacene with only one dN— group shows the potential to realize the n-type property in device applications. This appears even more pronounced in DAP on Ag(111), as discussed in the next section. 15505

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The Journal of Physical Chemistry C DAP on Ag(111). Subsequent studies were conducted on Ag(111). Also in this case, a 64-Å DAP film after annealing at 135 °C (1 h) showed ARUPS features similar to those of the asdeposited 3-Å film (Figure 3b); thus, the 3-Å film can be estimated as 1 ML coverage. The dependence of the ML HOMO intensity on θ (RHOMO = 3.2; see Figures S1 and S2 in the Supporting Information) also suggests a flat-lying ML on Ag(111), which is the preferred orientation of OSC MLs on metals.20a,22b,25 From 3 to 8 Å, the ARUPS spectra changed dramatically, and the intermediate spectra consisted of a mixture of features derived from the 3- and 8-Å spectra. The HOMOmax shifted by 0.38 eV toward higher BE from 3 to 8 Å, which is in the range of commonly observed screening effects at organic/metal interfaces.22 From 8 to 64 Å, the shift very slowly reached 0.41 eV. Thus, the 8-Å film might correspond to a bilayer (BL) coverage. Here, the HOMOmax shifts from ML to BL, which are caused by the different degrees of screening effects, on HOPG (shift ≈ 0
0.1 eV) and on Ag(111) (shift ≈ 0.38 eV) can be mainly attributed to the different optical dielectric constants of the substrates.22c Note that the screening effect at organic/metal interfaces makes the assignment of the ML IE difficult and that the situation is further complicated by the push-back effect (vide infra). The interplay of these two effects might even mimic “band-bending” (i.e., the rigid shift of VL and valence electron levels as function of coverage)3c,26 in the ultrathin region, even in the case of a change in IE (without appropriate corrections). Here, to evaluate accurately the IE of the ML (3 Å) on Ag(111), ∼0.38 eV due to the screening effect and ∼0.68 eV due to the push-back effect must be considered; as a result, the ML IE was estimated to be ∼6.59 eV, which corresponds well to the IE of flat-lying DAP on HOPG. From 8 to 64 Å, the HOMO always showed a tail to the lower-BE side. This tail feature originates from the ML HOMO, because it is located at the same BE as the ML HOMO [confirmed by the negative second derivative curve of the ARUPS spectra, Figure S3 (Supporting Information)] and the substrate Fermi-level is still visible for 64-Å films. Therefore, an island growth fashion on Ag(111) is suggested. For the multilayer films, RHOMO is only 1.5 and thus significantly smaller than the 2.4 value for the almost-flat-lying multilayer on HOPG. Therefore, a tilting of the molecules in multilayers is probable on Ag(111). Such orientational transitions, from flat-lying monolayer to tilting multilayers, in OSC films on metal substrates have been observed in other systems.25b,d,27 The tilted molecular orientation of multilayers leads to a large VL shift upon deposition of the molecule. Up to ML coverage, the VL decreased by 0.68 eV, followed by a 0.28 eV decrease from ML to BL, and finally, for the 64-Å film, the total VL shift reached 1.12 eV (Figure 3b,d). Such a large VL shift cannot be caused by only the push-back effect that is observed at organic/metal interfaces.6a,b,22b For typical OSCs on Ag(111), such a VL shift is in the range of 0.3
0.7 eV upon adsorption of 1 ML of the respective OSC.9c,25d,28 The VL shift of the annealed DAP/ Ag(111) film, which might represent a better monolayer, is 0.59 eV. In a tilted molecular orientation, however, the relatively large permanent electric dipole has a component pointing upward in the surface normal direction (Figure 3d), leading to a continuous VL shift even after ML formation. The 0.53 eV shift of the VL between the annealed ML and the 64-Å bulk film on Ag(111) might be mainly caused by the impact of the permanent dipole. Moreover, the IEs of the DAP multilayers on HOPG (50 Å, almost flat-lying) and on Ag(111) (64 Å, tilted orientation) are

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6.56 and 6.19 eV, respectively. The difference (ΔIE) of 0.37 eV can be ascribed to the vertical orientation of the molecular electric dipole in multilayers on Ag(111) and is in the range of energy level shifts in thin films of other polar molecules.9b,29 Consequently, the LUMO position estimated from the HOMO BE and Eopt24 is very close to the Fermi-level (Figure 3d). As a result, the small electron injection barrier (0.78 eV), together with a rather high DOS for the LUMO expected from the probable quasi-double degeneracy, suggests that this N-hereroacene has an excellent n-type conductivity on the Ag(111) substrate. Our results support the conclusion of previous theoretical study suggesting that nitrogen-rich oligoacenes are promising candidates for n-type OSCs because of their chemical properties.12 The origin of the different predominant molecular orientations of DAP multilayers on HOPG and Ag(111) might be due to the different ML structure. The growth of OSC thin films depends significantly on the subtle competition between the intermolecular and molecule
substrate interactions.30 The molecule
substrate interactions are usually much weaker for OSCs on HOPG than on Ag(111), so a more highly ordered ML on HOPG [with a smaller HOMO width of ∼0.29 eV (Figure S2, Supporting Information)] and different crystalline phases for the monolayers on the two substrates might be realized. This could lead to a larger HOMO width because of more disordered structures in thicker DAP films on Ag(111) [∼0.45 eV for 64-Å bulk film (Figure S2, Supporting Information)], as well as different growths and orientations of the multilayers. These results were recently confirmed by scanning tunneling microscopy (STM) experiments.31

’ CONCLUSIONS We have carried out systematic studies on a heteroacene containing one dN— group, DAP, including the single-crystal structure, optical properties, and electronic structure of the thin films on HOPG and Ag(111) substrates. In the single crystals, DAP molecules form a herringbone-like arrangement driven by substantive cofacial π
π interactions, uncofacial C—H 3 3 3 π interactions, and weak C—H 3 3 3 N hydrogen bonds. These intermolecular interactions lead to a rather dense packing, which can make significant contributions to the charge-carrier mobility. Aggregation induces obvious red shifts in both the UV
vis absorption and photoluminescence spectra of DAP because of relatively strong intermolecular interactions in the thin films. A relatively large optical band gap (2.67 eV) in general suggests that the molecule has good photooxidation stability. An ARUPS study of DAP thin films on HOPG and Ag(111) revealed that the bulk films (50
64 Å) on both substrates are n-type, but bear different molecular orientations. On HOPG, the molecules are oriented almost flat from monolayer to multilayers with a large IE (6.56 eV for 50 Å), whereas on Ag(111), an orientational transition from flat-lying monolayer to inclined multilayers occurs with a smaller IE (6.19 eV for 64 Å). The permanent molecular dipole leads to a larger VL shift with the decrease in IE upon tilting of the molecule, so that the LUMO is located close to the the Fermi-level. With only one dN— replacement of a dCH— group in a large hydrocarbon oligoacene, DAP exhibits the potential for n-type conductivity, especially in films where the molecules have an inclined orientation. Our results demonstrate that introducing dN— groups into oligoacenes should be an 15506

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’ ASSOCIATED CONTENT

bS

Supporting Information. Detailed systhesis methods, crystal data (including CIF) and structure refinement for DAP single crystals, experimental details for measurement of optical properties and electronic structures, and additional ARUPS information. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: xin@restaff.chiba-u.jp. Fax: +81-43-207-3896. Tel.: +81-43-290-3447.

’ ACKNOWLEDGMENT The authors thank Dr. Jiong Jia (Shandong University) for the DFT calculations and Mr. Fajun Wang (Shandong University) for the thin-film preparation for optical property measurements. The authors gratefully acknowledge financial support from the Global-COE program of MEXT (G3: Advanced School for Organic Electronics, Chiba University) and the State National Natural Science Foundation of China (Grant 51021062). S.D. gratefully acknowledges support from JSPS for foreign researchers, and S.K. gratefully acknowledges financial support from a Grantin-Aid for Young Scientists (A). ’ REFERENCES (1) (a) Forrest, S. R. Nature 2004, 428, 911–918. (b) D’Andrade, B. W.; Forrest, S. R. Adv. Mater. 2004, 16, 1585–1595. (c) Braga, D.; Horowitz, G. Adv. Mater. 2009, 21, 1473–1486. (d) Helgesen, M.; Søndergaard, R.; Krebs, F. C. J. Mater. Chem. 2010, 20, 36–60. (2) (a) Braun, S.; Salaneck, W. R.; Fahlman, M. Adv. Mater. 2009, 21, 1450–1472. (b) Miozzo, L.; Yassar, A.; Horowitz, G. J. Mater. Chem. 2010, 20, 2513–2538. (c) Heimel, G.; Salzmann, I.; Duhm, S.; Koch, N. Chem. Mater. 2011, 23, 359–377. (3) (a) Sueyoshi, T.; Fukagawa, H.; Ono, M.; Kera, S.; Ueno, N. Appl. Phys. Lett. 2009, 95, 183303/1
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