Metal salen coordination compounds: A new type of ambipolar charge transport materials

Synthetic Metals 160 (2010) 2299–2305

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Metal salen coordination compounds: A new type of ambipolar charge transport materials Li Qu a , Dan Wang b , Cheng Zhong a , Yinjun Zou a , Jun Li a , Dechun Zou b,∗ , Jingui Qin a,∗∗ a b

Department of Chemistry, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072, PR China BNLMS, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China

a r t i c l e

i n f o

Article history: Received 16 April 2010 Received in revised form 29 May 2010 Accepted 31 August 2010 Available online 24 October 2010 Keywords: Metal salen compounds Mobility TOF Ambipolar charge transport DFT calculation

a b s t r a c t A series of metal (Ni or Cu) salen coordination compounds were designed, synthesized and characterized. Their photophysical properties, electrochemical properties and thin film morphologies were studied. Their charge transport properties were measured by time of flight (TOF) technique. Compounds 1a and 1b showed hole transport properties, while compounds 2 and 3 exhibited balanced ambipolar mobilities. DFT calculation results were in good agreement with the experimental results and provided an explanation for the unexpected TOF results. Compound 3 showed the highest mobilities (10−5 cm2 /Vs) for both holes and electrons at ambient temperature, suggesting a promising candidate of ambipolar charge transport materials. © 2010 Elsevier B.V. All rights reserved.

1. Introduction With the development of organic electronics and optoelectronics, the active materials used in such devices as organic field-effect transistors (OFET), organic solar cells (OSC) and organic light-emitting diodes (OLED) have received great attention [1]. Charge transport property of these materials is a key factor in their performance [2]. Although many compounds have been reported to show high hole or electron mobilities [1], materials with balanced hole and electron mobilities are rare. Ambipolar materials have been exploited to fabricate multi-functional devices such as light-emitting field-effect transistors and complementary-like logic circuits [3]. Incorporating a single layer of an ambipolar organic semiconductive material makes device fabrication process simplified and cost-effective. However, it remains a great challenge due to the poor environmental stability and unbalanced mobilities for holes and electrons of most organic materials [4]. Organometallic and coordination compounds have attracted considerate interest on their charge transport properties due to the following advantages [5]: (i) Metal ions with d8 (or sometimes d9 ) electron structure often form a square planar molecular configuration with four coordination atoms of the ligands, and the dz2 orbital of the metal can overlap with suitable orbitals of the lig-

∗ Corresponding author. ∗∗ Corresponding author. E-mail addresses: [email protected] (D. Zou), [email protected] (J. Qin). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.08.024

ands to form a delocalized electron system, which will effectively increase the conjugation length for charge transport. (ii) Interactions of the d orbital of the metals with the HOMO and/or LUMO of the ligands will provide a wide range of redox potentials and tunable energy levels and bandgaps. For example, Ni bis(dithiolene) complexes (II), as a kind of electron or ambipolar transport materials, have been used to fabricate air-stable ambipolar transistors and complementary-like circuits [5a,3b]. N,N -bis(salicylidene)ethylenediamine (salen) ligands and their coordination compounds are among the most studied substances in chemistry. They have been used in the fields of homogeneous catalysis, photo-/electro-luminescence, all optical switching, etc. [6]. Thiophene derivative functionalized metal salen compounds can undergo electro-polymerization process to obtain conductive polymers [7,9]. To the best of our knowledge, their charge carrier transport properties are remaining unexploited. Recently, we have synthesized and studied a series of new square planar metal-salen coordination compounds. Three types of salen derivatives were designed, and all consist of thiophene moieties so as to enlarge and enrich the electron conjugation system (Scheme 1). N,N -bis(salicylidene)-1,2-phenlenediamine (salphen) and N,N -bis(salicylidene)-3,4-diaminothiophene (saloth) have more rigid skeletons with larger conjugated system, while N,N bis(salicylidene)-butylamine (salbu) is more flexible and its square planar coordination compounds should have a centrosymmetric structure. Ni(II) or Cu(II) were chosen as the central ions since they normally form square planar coordination compounds with extended conjugated system. Medium sized (C4 or C6) alkyl chains

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Scheme 1. Synthesis of the metal-salen compounds.

were attached to the skeleton to improve the solubility. Interestingly, two of the compounds have shown comparable mobilities for both holes and electrons. Here we report the synthesis, physiochemical properties and charge transport properties of these new metal salen coordination compounds. 2. Experimental 2.1. Synthesis The synthetic route and chemical structures of the coordination compounds are shown in Scheme 1. Thiophene-3, 4-diamine, 5bromo-2-hydroxybenzaldehyde (4), 5-hexylthiophen-2-ylboronic acid (5) and thiophen-2-ylboronic acid (6) were synthesized as described in the literature [7c,8]. 5-(5-hexylthiophen-2-yl)2-hydroxybenzaldehyde (7) and 2-hydroxy-5-(thiophen-2-yl)benzaldehyde (8) were synthesized by palladium-catalyzed Suzuki coupling of 4 and corresponding boronic acid, respectively [7b,10]. 2.1.1. HTsalphen-Ni (1a) Compound 7 (1.24 g, 4.30 mmol) and 1,2-phenylenediamine (0.23 g, 2.20 mmol) were dissolved in degassed ethanol (40 ml) and heated to reflux overnight. The orange solution was then cooled to room temperature, filtered and washed with cold ethanol. The orange residue 9 (1.1 g, yield 79.1%) was dried in vacuo and used directly for the next step. 1 H NMR (300 Hz, CDCl3 ) ı 0.89 (6H, t), 1.22–1.41 (12H, m), 1.68 (4H, m), 2.80 (4H, t), 6.71 (2H, d), 7.01 (2H, d), 7.05(2H, d), 7.26 (2H, d), 7.37 (2H, d), 7.55–7.58 (4H, m), 8.67 (2H, s), 13.10 (2H, s). The intermediate 9 (0.32 g, 0.5 mmol) and nickel(II) acetate tetrahydrate (0.13 g, 0.52 mmol) were dissolved in 50 ml degassed methanol and 10 ml dry THF under nitrogen atmosphere and heated to reflux for 5 h. The mixture was cooled to room temperature, filtered and washed with methanol and diethyl ether, then recrystallized from THF/EtOH to afford a dark red solid (0.25 g, yield 71.4%). Mp = 294–297 ◦ C. 1 H NMR (300 Hz, CDCl3 ) ı 0.89 (6H, t), 1.24–1.41 (12H, m), 1.67 (4H, m), 2.77 (4H, t), 6.67 (2H, d), 6.92 (2H, d), 7.13 (2H, d), 7.49 (4H, d), 8.20 (2H, s). IR (KBr):  = 3437, 2925, 1615, 1577, 1524, 1491, 1454, 1384, 1337, 1180, 825, 799, 743 cm−1 . MS (ESI) m/z: 705.1 [(M+H)+ ]. Anal. Calcd. for

C40 H42 N2 O2 S2 Ni: C, 68.09; H, 6.00; N, 3.97. Found: C, 68.01; H, 5.55; N, 3.59 (%). 2.1.2. HTsalphen-Cu (1b) Compound 1b was prepared similarly to 1a using copper(II) acetate monohydrate instead of nickel(II) acetate tetrahydrate to afford a red brown solid (yield 66.9%). Mp = 275–277 ◦ C. IR (KBr):  = 3439, 2924, 1615, 1579, 1522, 1490, 1455, 1384, 1334, 1173, 829, 799, 747 cm−1 . ESI–MS m/z 710.2 [(M+H)+ ]. Anal. Calcd. for C40 H42 N2 O2 S2 Cu: C, 67.62; H, 5.96; N, 3.94. Found: C, 67.15; H, 5.75; N, 3.62 (%). 2.1.3. HTsaloth-Ni (2) Compound 7 (0.15 g, 1.21 mmol) and nickel(II) acetate tetrahydrate (0.16 g, 0.66 mmol) were dissolved in 60 ml degassed methanol and 10 ml dry THF, stirred for 1 h. Then thiophene3,4-diamine (0.15 g, 1.31 mmol) was added and heated to reflux for 20 h. The mixture was cooled to room temperature, filtered and washed with methanol and diethyl ether, then recrystallized from THF/EtOH to afford a dark red solid (0.32 g, yield 72.4%). Mp = 274–279 ◦ C. 1 H NMR (300 Hz, CDCl3 ) ı 0.89 (6H, t), 1.26–1.41 (12H, m), 1.67 (4H, m), 2.75 (4H, t), 6.65 (2H, d), 6.91 (2H, d), 7.04 (2H, s), 7.08 (2H, d), 7.43 (2H, s), 7.44 (2H, d), 7.73 (2H, s). IR (KBr):  = 3446, 2927, 1615, 1597, 1526, 1491, 1457, 1384, 1324, 1268, 1178, 825, 794 cm−1 . MS (ESI) m/z: 710.2 (M+ ). Anal. Calcd. for C38 H40 N2 O2 S3 Ni: C, 64.14; H, 5.67; N, 3.94. Found: C, 64.18; H, 5.29; N, 3.71 (%) 2.1.4. Tsalbu-Ni (3) Compound 8 (0.25 g, 1.22 mmol) and n-butylamine (0.25 ml, 2.5 mmol) were dissolved in 60 ml degassed methanol and heated to reflux for 2 h. The orange solution was then cooled to room temperature, nickel(II) acetate tetrahydrate (0.16 g, 0.66 mmol) dissolved in 10 ml methanol was added. The mixture was heated to reflux again for 20 h, then cooled and filtered. The residue was washed with methanol, and recrystallized from acetone/EtOH to afford a green solid (0.22 g, yield 62.8%). Mp = 228–231 ◦ C. 1 H NMR (300 Hz, CDCl3 ) ı 1.01 (6H, t), 1.47 (4H, m), 1.88 (4H, m), 4.01 (4H, b), 6.53 (2H, d), 7.01 (2H, t), 7.05 (2H, dd), 7.13 (2H, d), 7.37 (2H, d), 7.50 (2H, d), 9.32 (2H, b). 13 C NMR (CDCl3 ) ı 14.33, 20.58,

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35.49, 59.24, 121.58, 122.75, 123.31, 127.79, 128.80, 131.68, 144.25, 159.60. IR (KBr):  = 3435, 2956, 1615, 1542, 1475, 1430, 1392, 1334, 1184, 817, 693 cm−1 . MS (ESI) m/z: 575.1 [(M+H)+ ]. Anal. Calcd. for C30 H32 N2 O2 S2 Ni: C, 62.62; H, 5.61; N, 4.87. Found: C, 62.60; H, 5.16; N, 4.58 (%)

1 H and 13 C NMR spectroscopies were conducted with a Varian Mercury300 spectrometer using tetramethylsilane (TMS; ı = 0 ppm) as an internal standard. Thermogravimetric analysis (TGA) were performed on a Netzsch STA449C thermal analyzer at a heating rate of 10 ◦ C/min in nitrogen at a flow rate of 50 cm3 /min. UV–visible spectra were obtained using a Schimadzu UV-2550 spectrometer. Cyclic voltammetry (CV) was carried out on a CHI voltammetric analyzer in a three-electrode cell with a Pt counter electrode, a Ag/AgCl reference electrode, and a glassy carbon working electrode at a scan rate of 10 mV/s. The potential values obtained were converted to values versus the ferrocenium/ferrocene (Fc+ /Fc) standard.

2.3. TOF measurements Time of flight (TOF) technique was used to determine charge carrier mobilities of the metal salen derivatives. The organic thin films were prepared by vacuum deposition on ITO precoated glass substrates for 1–10 ␮m, then Al was deposited for 200 nm as electrodes. The laser excitation light source passed through the transparent ITO substrate to induce a thin sheet of excess carriers in the interface of ITO substrate and organic thin film. Under an applied dc bias, the carriers drifted and swept across the bulk of the organic film to reach the electrode (Al), then the transient photocurrent was recorded and the charge transit times (Ttr ) were measured. The mobilities of different carriers (holes or electrons) were determined by switching the polarity of the applied dc bias. The measurements were carried out at room temperature in vacuum condition, and the electric fields ranged from 30 to 300 V.

1.0

9 1a 1b 2 3

0.8

Abs.

2.2. Characterization

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0.6 0.4 0.2 0.0 300

400

500

600

Wavelength(nm) Fig. 1. UV–vis absorption spectra in CH2 Cl2 solution.

3.2. Absorption spectra Fig. 1 shows the absorption spectra of four metal salen compounds and the ligand 9. Ligand 9 exhibits its strongest absorption band at ca. 300 nm, due to ␲–␲* transition of the thienylsubstituted phenyl unit [10]. After coordinated with metal, the coordination compounds (1a and 1b) have their max red-shifted about 35 nm, which is the consequence of expansion in conjugation. 2 and 3 have also shown the same max at ca. 335 nm. Four coordination compounds exhibit broad bands at 400–600 nm corresponding to charge transfer transitions, which have been previously observed in other metal-Schiff base compounds [10,11]. 3.3. Electrochemical properties Fig. 2 shows the cyclic voltammograms of the ligand 9, compounds 1a, 1b, 2 and 3. Each compound can undergo two oxidation and one reduction process. According to the following equations (Eqs. (1) and (2)) [12], we could calculate their HOMO and LUMO levels:





OX + 4.8 (eV) HOMO = − Eonset

(1)

3. Results and discussion 3.1. Synthesis and thermal stabilities The target products were synthesized in two methods [9]. Compounds 1a and 1b were prepared by two steps from salicylaldehyde intermediate: the ligand 9 was first synthesized via condensation salicylaldehyde 5 with 1, 2-diaminobenzene in hot ethanol overnight and purified by filtration to remove the unreacted material in solution, then dissolved in mixture of methanol and minimum THF and treated with M(OAc)2 ·xH2 O in reflux to obtain the target products. 2 and 3 were synthesized by one pot reaction with corresponding amines and Ni(OAc)2 ·4H2 O in refluxed methanol. The target products were purified by simply filtration and recrystallization. Both methods lead to desired products in good yields. 1a, 1b and 2 are still sparingly soluble in almost all solvents in spite of introducing hexyl chains, thus preventing characterization by 13 C NMR spectroscopy; meanwhile, compound 3 demonstrates good solubility in common solvents such as THF, chloroform, acetone, etc. Thermogravimetric analysis (TGA) shows the onset decomposition temperatures were 410 ◦ C, 398 ◦ C, 383 ◦ C and 371 ◦ C for 1a, 1b, 2 and 3 respectively, confirming the high thermal stability of salen derivatives.

Fig. 2. Cyclic voltammograms in CH2 Cl2 solution containing 0.1 mol/L Bu4 NPF6 , Fc+ /Fc couple as internal references.

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Table 1 Frontier orbital energy levels calculated from electrochemical data.

9 1a 1b 2 3

HOMO (eV)

LUMO (eV)

El Eeg (eV)

−5.17 −5.07 −5.07 −5.06 −5.04

−2.86 −3.05 −3.15 −3.02 −2.85

2.31 2.02 1.92 2.04 2.19





red LUMO = − Eonset + 4.8 (eV)

3.5. Thin film morphology (2)

where the unit of potential is versus the ferrocenium/ferrocene (Fc+ /Fc) standard. Their electrochemical properties are listed in Table 1. From the table, we can see the HOMO values of four coordination compounds are close to each other, in the range of −5.04 to −5.06 eV. However, their LUMO levels are somewhat different. Copper salen compound 1b has the lowest LUMO level while compound 3 has the highest LUMO level, related to the differences of their metal atoms and ligand substitutions. Nevertheless, among the four coordination compounds, only compound 3 exhibits reversible redox processes at both positive and reductive potential ranges, indicating 3 can form cation and anion radicals with better stability, which might be more beneficial for charge transport [2]. 3.4. Mobility measurements From the TOF measurements, mobilities of metal salen compounds were calculated according to the equation [13]: =

(3)

where V is the applied bias and d is the thickness of the organic layer. The results (Table 2) demonstrate something interesting. 1a and 1b exhibit only hole-transport properties, and cooper salen compound 1b exhibits the lowest mobility than three nickel coordination compounds. However, 2 and 3 have ambipolar transport properties. 2 has electron mobilities more than twice higher as its hole mobilities, representing a little better electron transport property. Among four compounds, 3 obtains the best performance with comparable hole and electron mobilities to 10−5 (cm2 /Vs) when applied dc bias of 35 V, in the order of the mobility of typical charge transport materials as Alq3 , PVK, etc. Mobilities of 1b and 3 are negative field dependence while mobilities of 1a and 2 are almost independent over a wide range of applied voltages (Figs. S1–S4, supplementary data), which have been observed in some disordered conjugated systems [14]. Such a relationship can be described by Bässler formalism (Eq. (4)) [15]:

    2 2 −

3kB T

exp

    2 C

kB T



− ˙2

√ E



(4)

where  is the values of the disorder-free mobility, /kB T is the parameter of energetic disorder, ˙ is the parameter of spatial disorder, kB is the Boltzmann constant and C is an empirical Table 2 Maximum mobilities measured by TOF technique. E1/2 (V/cm)1/2

1a 1b 2 3

The morphology of vacuum deposited films of metal salen derivatives on ITO substrates were investigated by AFM and XRD experiments. Fig. 3 shows AFM images of the deposited films of the four compounds. Relatively small grains are obtained for 1a and 1b (ca. 50–100 nm), and it is difficult to find large domains. From the AFM image of 2, no obvious crystalline domains are observed, and the film seems much smoother. Compound 3 forms more densely packed and large crystalline grains, which are helpful for charge transport. XRD were taken to gain insight the crystallinity of the thin films (Fig. 4). The film of 1a shows a relatively strong diffraction peak at 2 = 8.7◦ and several weak peaks, while 1b exhibits only two weak peaks 2 = 20.9◦ and 29.8◦ , indicating some crystalline phases in their films. On the contrary, from the XRD pattern of 2 no sharp peaks can be observed, suggesting that the film is amorphous and the molecules are randomly oriented in this thin film. Compound 3 exhibits a very strong peak with intensity up to 2000 at 2 = 8.1◦ and a series of sharp reflections at 2 = 19.4◦ , 21.3◦ and 30.2◦ , indicating high crystallinity and ordering of the film, consistent with AFM results. 3.6. DFT calculations

 d2 = E V × Ttr

 = 0 exp

constant. When the spatial disorder parameter ˙ is large enough to be comparable or exceeded the energetic disorder parameter /kB T, mobility will no longer be positive field dependence. Thus the field-dependence results imply the intermolecular interactions in metal salen systems may be so positional disorder that charge carriers are apt to hop in disordered ways.

338 519 311 241

Mobility (cm2 /Vs) Electrons

Holes

None None 3.5 × 10−6 1.1 × 10−5

1.5 × 10−6 1.4 × 10−7 1.2 × 10−6 1.1 × 10−5

To understand the intrinsic electronic structures and unexpected results of mobility measurements, optimum geometries of the salen compounds were optimized at B3LYP/6-31g(d) (with Lanl2dz pseudopotential basis set for Ni) level. Then the electronstate-density distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were calculated at B3LYP level with Lanl2dz pseudopotential basis set for Ni and 6-311+g(d) for all other atoms (Fig. 5, large figures of their HOMO and LUMO in Figs. S5–S8, supplementary data). All the calculations were carried out using Gaussian 03 program [16]. Good correlations between experimental results (calculated from cyclic voltammograms) and theoretical energy levels can be established. The correlation coefficient of the HOMO level is not so good (R = 0.73, Fig. 6a), but the energy difference between experimental and theoretical results is relatively small. The correlation of the LUMO level is excellent with its R about 0.99 (Fig. 6b). When excluding 1b which has a different metal centers, the correlation factors of the nickel coordination compounds rise largely to above 0.92 for HOMO levels and 0.99 for LUMO levels (Fig. 6c and d). In general, DFT calculations can provide good estimations of the electronstate-density distributions of salen coordination derivatives, thus allowing investigating their electronic properties. From the DFT results, it is noted that HOMOs of four compounds are similar, mainly located on the ␲ orbital of thiophene and salicaldehyde moieties, overlapping the dz2 orbital of metal atoms to some extent. Among the four compounds, 1b shows less delocalized HOMO because copper makes fewer contribution to its HOMO than nickel salen complex, thus 1b has relatively poorer hole transport property which are well matched with TOF results. Nevertheless, their LUMOs represent different from each other. For 1a, the LUMO is only located on the ␲ orbital of salphen ligand, not including nickel atom; but for 1b, the LUMO is located on the d orbital of copper atom, probably due to the copper(II) is more easily to be reduced than salphen ligand. For 2, the LUMO involves not

L. Qu et al. / Synthetic Metals 160 (2010) 2299–2305

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Fig. 3. AFM images of vacuum deposited films of the four metal salen compounds on ITO substrates.

only the ␲ orbital of salphen ligand but also the dz2 orbital of nickel atom. For 3, the LUMO is mainly located on the d orbital of metal atom just like 1b because the aromatic imino group is replaced by N-alkyl chain which will little affect its frontier orbital. Their calculated electron-state-density distribution of LUMO provides an explanation for their electron charge transport behav-

ior. The LUMO of 2 incorporates both nickel atom and saloth-type ligand, indicating more efficient delocalization and intermolecular orbital overlap for electron transporting, thus 2 has some n-type behavior. However, for nickel salphen complex 1a, the nickel atom is not involved in its LUMO, and therefore nickel atom with large atomic radius but no contribution to 1a’s LUMO hinders suffi-

Fig. 4. ARD patterns of vacuum deposited films of the four metal salen compounds on ITO substrates.

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Fig. 5. Theoretical and experimental HOMO and LUMO levels.

Fig. 6. Correlation of theoretical and experimental energy levels: (a) HOMO levels of four metal salen compounds; (b) LUMO levels of four metal salen compounds; (c) HOMO levels of three nickel salen compounds; (d) LUMO levels of three nickel salen compounds.

cient intermolecular overlap of LUMOs; in addition, the LUMO on diiminobenzene ring (1a) is less extensive than diiminothiophene moiety (2). Consequently, 1a exhibits poor electron transport property [17]. For 1b and 3, their electron transport properties are more relevant to the intrinsic electronic properties of metal(II) atom. It can be concluded from their cyclic voltammograms that Ni(II) in salbu coordination system can be able to form stable anion radical, thus 3 has better electron transport property; meanwhile, Cu(II) to Cu(I) process here is unstable and irreversible, so 1b exhibits no electron mobility. On the whole, in salen coordination system,

a change in ligand substitution and/or metal atom will affect the electron transport property to a large extent.

4. Conclusions A series of new metal-salen coordination compounds were synthesized and characterized, their optical and electrochemical properties as well as thin film structures were investigated, and charge carrier mobilities were measured by TOF technique.

L. Qu et al. / Synthetic Metals 160 (2010) 2299–2305

1a and 1b exhibit only hole transport properties, while 2 and 3 have balanced mobilities for both holes and electrons. DFT calculations are used to investigate their electronic structures and interesting charge transport properties. The highest mobility of 3 reached 10−5 (cm2 /Vs) for both holes and electrons at room temperature, suggesting an interesting candidate for ambipolar charge transport materials. Study on more design, synthesis, properties and application in opto-electronic devices are under way. Acknowledgements

[6]

[7] [8] [9] [10] [11]

The authors thank Dr. Yi Lin and Qingying Luo for AFM discussions. This work was financially supported by the National Natural Science Foundation of China (No. 20772094). Appendix A. Supplementary data

[12] [13]

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.synthmet.2010.08.024. [14]

References [1] (a) S. Allard, M. Forster, B. Souharce, H. Thiem, U. Scherf, Angew. Chem. Int. Ed. 47 (2008) 2; (b) M.L. Tang, A.D. Reichardt, P. Wei, Z. Bao, J. Am. Chem. Soc. 131 (2009) 5264; (c) C. Di, G. Yu, Y. Liu, D. Zhu, J. Phys. Chem. B 111 (2007) 14083; (d) J. Roncali, P. Leriche, A. Cravino, Adv. Mater. 19 (2007) 2045. [2] Y. Shirota, H. Kageyama, Chem. Rev. 107 (2007) 953. [3] (a) J. Zaumzeil, R.H. Friend, H. Sirringhaus, Nat. Mater. 5 (2006) 69; (b) T.D. Anthopoulos, S. Setayesh, E. Smits, M. Cölle, E. Cantatore, B. de Boer, P.W.M. Blom, D.M. de Leeuw, Adv. Mater. 18 (2006) 1900. [4] J. Zaumseil, H. Sirringhaus, Chem. Rev. 107 (2007) 1296. [5] (a) T.D. Anthopoulos, G.C. Anyfantis, G.C. Papavassiliou, D.M. Leeuw, Appl. Phys. Lett. 90 (2007) 122105; (b) S. Noro, H.C. Chang, T. Takenobu, Y. Murayama, T. Kanbara, T. Aoyama, T. Sassa, T. Wada, D. Tanaka, S. Kitagawa, Y. Iwasa, T. Akutagawa, T. Nakamura, J. Am. Chem. Soc. 127 (2005) 10012; (c) J. Qin, D. Liu, C. Dai, C. Chen, B. Wu, C. Yang, C. Zhan, Coord. Chem. Rev. 188 (1999) 23; (d) T. Taguchi, H. Wada, T. Kambayashi, B. Noda, M. Goto, T. Mori, K. Ishikawa, H. Takezoe, Chem. Phys. Lett. 421 (2006) 395;

[15]

[16]

[17]

2305

(e) J.-Y. Cho, B. Domercq, S.C. Jones, J. Yu, X. Zhang, Z. An, M. Bishop, S. Barlow, S.R. Marder, B. Kippelen, J. Mater. Chem. 17 (2007) 2642; (f) H. Wada, T. Taguchi, B. Noda, T. Kambayashi, T. Mori, K. Ishikawa, H. Takezoe, Org. Electron. 8 (2007) 759. (a) S.J. Wezenberg, A.W. Kleij, Angew. Chem. Int. Ed. 47 (2008) 2354; (b) W.-L. Tong, L.-M. Lai, M.C.W. Chan, Dalton Trans. (2008) 1412; (c) T. Huang, Z. Hao, H. Gong, Z. Liu, S. Xiao, S. Li, Y. Zhai, S. You, Q. Wang, J. Qin, Chem. Phys. Lett. 451 (2008) 213. (a) R.P. Kingsborough, T.M. Swager, J. Am. Chem. Soc. 121 (1999) 8825; (b) A. Voituriez, M. Mellah, E. Schulz, Synthetic Met. 156 (2006) 166. A.N. Cammidge, V.H.M. Goddard, H. Gopee, N.L. Harrison, D.L. Hughes, C.J. Schubert, B.M. Sutton, G.L. Watts, A.J. Whitehead, Org. Lett. 18 (2006) 4071. J.L. Reddinger, J.R. Reynolds, Chem. Mater. 10 (1998) 1236. A. Pietrangelo, B.C. Sih, B.N. Boden, Z. Wang, Q. Li, K.C. Chou, M.J. MacLachlan, M.O. Wolf, Adv. Mater. 20 (2008) 2280. (a) L. Rigamonti, F. Demartin, A. Forni, S. Righetto, A. Pasini, Inorg. Chem. 45 (2006) 10976; (b) M. Asadi, K.A. Jamshid, A.H. Kyanfar, Transit. Met. Chem. 32 (2007) 822; (c) A.B.P. Lever, Inorganic Electronic Spectroscopy, Elsevier, New York, 1984. D.W. De Leeuw, M.M.J. Simenon, A.R. Brown, R.E.F. Einerhand, Synthetic Met. 87 (1997) 53. (a) M.A. Lampert, P. Mark, Current Injection in Solids, Academic Press, New York, 1970; (b) J. Mort, D.M. Pai (Eds.), Photoconductivity Related Phenomena, Elsevier, New York, 1976; (c) P.M. Borsenberger, D.S. Weiss, Organic Photoreceptors for Imaging Systems, Marcel Dekker, New York, 1993. (a) A.J. Mozer, N.S. Sariciftci, Chem. Phys. Lett. 389 (2004) 438; (b) A. Kokil, I. Shiyanovskaya, K.D. Singera, C. Weder, Synthetic Met. 138 (2003) 513. (a) H. Bässler, Phys. Status Solidi B 15 (1993) 175; (b) F. Laquai, G. Wegner, C. Im, H. Bässler, S. Heun, J. Appl. Phys. 99 (2006) 203712. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M. W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision E.01, Gaussian, Inc., Wallingford CT, 2004. (a) S. Handa, E. Miyazakia, K. Takimiya, Chem. Commun. (2009) 3919; (b) X. Yang, L. Wang, C. Wang, W. Long, Z. Shuai, Chem. Soc. Rev. 39 (2010) 423.