Effect of Methoxy Substituents on the Properties of the Derivatives of

ARTICLE pubs.acs.org/JPCC

Effect of Methoxy Substituents on the Properties of the Derivatives of Carbazole and Diphenylamine Asta Sakalyte,†,|| Jurate Simokaitiene,† Ausra Tomkeviciene,† Jonas Keruckas,† Gintaras Buika,† Juozas V. Grazulevicius,*,† Vygintas Jankauskas,‡ Chao-Ping Hsu,§ and Chou-Hsun Yang^,z †

Department of Organic Technology, Kaunas University of Technology, Radvilenu pl. 19, LT-50254, Kaunas, Lithuania Department of Solid State Electronics, Vilnius University, Sauletekio Aleja 9, LT-2040, Vilnius, Lithuania § Institute of Chemistry, Academia Sinica, 128 Sec. 2 Academia Road, Taipei115, Taiwan ^ Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan z Molecular Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, 128 Sec. 2 Academia Road, Taipei 115, Taiwan ‡

ABSTRACT: The synthesis and properties of carbazole and diphenylamine derivatives with different numbers and positions of methoxy groups in the diphenylamino moiety are reported. A comparative study on their thermal, optical, and photoelectrical properties is presented. All the synthesized compounds are found to form glasses with the glass transition temperatures in the range of 39-69 C, as characterized by differential scanning calorimetry. The ionization potentials of the synthesized compounds were estimated both theoretically by quantum chemical calculations and experimentally by an electron photoemission in air technique. The trends observed by theoretical calculations are confirmed by experimental results. The experimental values of ionization potentials range from 5.10 to 5.56 eV. Compounds containing monomethoxy-substituted phenyl rings exhibited lower ionization potentials than compounds containing di- and trimethoxy-substituted phenyl moieties. The highest ionization potential was observed for the compound containing trimethoxysubstituted phenyl rings. The best charge transport properties were observed for the compounds containing one methoxy group in para and ortho positions of phenyl rings of the diphenylamino moiety. Room-temperature hole drift mobility in the amorphous film of 3-[N,N-(bis-4-methoxyphenyl)]amino-9-ethylcarbazole established by a xerographic time-of-flight technique was found to be 1.2 10-4 V/cm at an electric field of 6.4 105 V/cm.

’ INTRODUCTION Much attention is recently paid to organic low-molar-mass compounds that form glasses above room temperature. Such compounds are named as molecular glasses or amorphous molecular materials. Among these materials, the most widely studied are electroactive molecular glasses and charge-transporting molecular glasses in particular. The main fields of application of these materials are organic light-emitting diodes and photovoltaic devices. Depending on the structure of a device, chargetransporting materials with different ionization potentials are required. Since the first application of poly(9-vinylcarbazole) in the photoreceptors of copying machines by IBM in 1970,1 many different polymeric and low-molar-mass hole-transporting carbazole-based materials were reported.2,3 Over 40 years, the hole drift mobilities increased from 10-7 cm2/V 3 s for poly(9-vinylcarbazole)2 to 10-2 cm2/V 3 s for molecular glasses of carbazolebased bis(enamines).4 The ionization potential of carbazolebased compounds depends on the substituents and can be varied r 2011 American Chemical Society

from 5.04 to 5.8 eV.5 In this work, we studied the influence of the number and position of methoxy groups in the diphenylamino moiety on the ionization potentials, charge transport properties, and other characteristics of carbazole and diphenylamine derivatives.

’ EXPERIMENTAL METHODS The 1H NMR and 13C NMR spectra were recorded using a Varian Unity Inova [300 MHz (1H), 75.4 MHz (13C)] spectrometer. All the data are given as chemical shifts δ (ppm) downfield from Si(CH3)4. IR spectra were recorded using a PerkinElmer Spectrum GX II FT-IR System. The spectra of the solid compounds were performed in the form of KBr pellets. Mass spectra were obtained on a Waters ZQ 2000. UV spectra of 10-5 M solutions of the synthesized compounds in tetrahydrofuran Received: October 8, 2010 Revised: January 26, 2011 Published: March 02, 2011 4856

dx.doi.org/10.1021/jp109643r | J. Phys. Chem. C 2011, 115, 4856–4862

The Journal of Physical Chemistry C (THF) were recorded on a PerkinElmer Lambda 35 spectrometer using a microcell with an internal width of 1 mm. Steadystate fluorescence spectra were recorded with a Hitachi MPF-4 spectrometer. Differential scanning calorimetry (DSC) measurements were carried out in a nitrogen atmosphere with a PerkinElmer Pyris Diamond calorimeter at a heating rate of 10 C/min. Thermogravimetric analysis (TGA) was performed on a Mettler TGA/SDTA851e/LF/1100 in a nitrogen atmosphere at a heating rate of 20 C/min. Melting points were measured on an Electrothermal MEL-TEMP melting point apparatus. Cyclic voltammetry (CV) measurements were carried out by a threeelectrode assembly cell from Bio-Logic SAS and a micro-AUTOLAB Type III potentiostat-galvanostat. The measurements were carried out with a glassy carbon electrode in dichloromethane solutions containing 0.1 M tetrabutylammonium perchlorate as the electrolyte, Ag/AgNO3 as the reference electrode, and a Pt wire counter electrode. The EHOMO energy values of the compounds were determined from oxidation potentials, taking the value of -4.8 eV as the EHOMO energy level for ferrocene (Fc) with respect to zero vacuum level.6 Ionization potentials (Ip) of the films of the synthesized compounds were measured by an electron photoemission in air method, as described before.7 The samples for ionization potential measurements were prepared as described previously.8 Hole drift mobilities were measured by a xerographic time-of-flight (XTOF) method.9-11 The samples for the charge carrier mobility measurements were prepared by a procedure described earlier.12 The thickness of the charge-transporting layers was 2-12 μm. The structures of the molecules were optimized using B3LYP density functionals and 6-31G* basis sets. The Ip were calculated by the difference in total energies of the ground state in neutral or cationic molecules, and they were obtained with both Hartree-Fock (HF) and density functional theory (DFT) using the B3LYP functional, with 6-31G* basis sets for both calculations. All calculations were performed with Q-Chem 3.2.13 The arrangement of molecules was modeled by obtaining geometry optimization using the AMBER force field (rms gradient, 0.001 kcal/Å mol).

’ MATERIALS 3-Amino-9-ethylcarbazole, diphenylamine, 4-iodoanisole, 2-iodoanisole, 3-iodoanisole, 1-bromo-3,5-dimethoxybenzene, 5-bromo-1,2,3-trimethoxybenzene, potassium carbonate, 18crown-6, copper powder, potassium tert-butoxide, tris(dibenzylideneacetone)dipalladium, and tri-tert-butylphosphine were purchased from Aldrich and used as received. 3-Iodo-9ethylcarbazole was prepared by alkylation of 3-iodo-9H-carbazole in the presence of a phase-transfer catalyst by the procedure described earlier.14 ’ GENERAL PROCEDURE A A mixture of 3-amino-9-ethylcarbazole (1 g, 4.76 mmol), iodoanisole (3.34 g, 14.28 mmol), 18-crown-6 (0.25 g, 0.95 mmol), potassium carbonate (4.47 g, 32.37 mmol), and copper powder (1.21 g, 19.04 mmol) in o-dichlorobenzene (15 mL) was refluxed under argon at 180 C. The reaction was monitored by thin-layer chromotography (TLC) (eluent: n-hexane/ethylacetate = 2:1). After 24 h, the reaction was stopped, and copper powder and inorganic salts were removed by filtration of the hot reaction mixture. The solvent was distilled under reduced pressure, and the crude product was purified by column chromatography (eluent: n-hexane/ethyl acetate = 2:1).

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3-[N,N-(Bis-4-methoxyphenyl)]amino-9-ethylcarbazole (1). Compound 1 was prepared according to the general proce-

dure A from 4-iodoanisole. The yield of the yellow crystals was 0.76 g (76%). mp = 139-140 C. Elemental analysis for C28H26N2O2: % calc. C 79.59, H 6.20, N 6.63, O 7.57; % found C 79.57, H 6.22, N 6.64. 1H NMR (300 MHz, DMSO, δ, ppm): 1.31 (t, 3H, J = 7.1 Hz, -CH2CH3), 3.71 (s, 6H, -OCH3), 4.39 (q, 2H, J = 7.1 Hz, -CH2CH3), 6.83 (d, 4H, J = 9.0 Hz, Ar), 6.92 (d, 4H, J = 9.0 Hz, Ar), 7.07-7.17 (m, 2H, Ar), 7.42 (t, 1H, J = 7.1 Hz, Ar), 7.49-7.58 (m, 2H, Ar), 7.80 (s, 1H, Ar), 7.98 (d, 1H, J = 7.7 Hz, Ar). 13C NMR (75.4 MHz, DMSO, δ, ppm): 14.54 (-CH2CH3), 37.65 (-CH2CH3), 56.44 (-O-CH3), 109.66, 111.46, 114.39, 118.80, 120.44, 120.89, 121.87, 122.58, 122.99, 125.60, 126.11, 127.64, 135.60, 137.75, 140.61, 142.02, 155.03. IR (KBr), (cm-1): 3034, 3000 (CHar); 2975, 2931 (CHaliphatic); 2833 (-OCH3); 1605, 1583, 1502, 1483, 1469 (CdCar); 1348, 1320 (C-N). MS (APClþ, 20 V), m/z (%): 423 ([M þ H]þ, 100). 3-[N,N-(Bis-2-methoxyphenyl)]amino-9-ethylcarbazole (2). Compound 2 was prepared according to the general procedure A from 2-iodoanisole. The yield of the white cystals was 0.7 g (35%). mp = 172-173 C. Elemental analysis for C28H26N2O2: % calc. C 79.59, H 6.20, N 6.63, O 7.57; % found C 79.56, H 6.21, N 6.63. 1H NMR (300 MHz, DMSO, δ, ppm): 1.31 (t, 3H, J = 7.1 Hz, -CH2CH3), 3.56 (s, 6H, -OCH3), 4.37 (q, 2H, J = 7.1 Hz, -CH2CH3), 6.80-7.15 (m, 10H, Ar), 7.36 (s, 1H, Ar), 7.39 (d, 2H, J = 8.8 Hz, Ar), 7.53 (d, 1H, J = 8.2 Hz, Ar), 7.88 (d, 1H, J = 7.7 Hz, Ar). 13C NMR (75.4 MHz, DMSO, δ, ppm): 14.50 (-CH2CH3), 37.70 (-CH2CH3), 55.88 (-OCH3), 109.77, 110.69, 115.35, 117.27, 119.19, 121.21, 122.53, 123.68, 124.53, 124.77, 126.49, 136.83, 140.70, 142.87, 154.93. IR (KBr), (cm-1): 3054 (CHar); 2962, 2932 (CHaliphatic); 2830 (-OCH3); 1582, 1497, 1469, 1457 (CdCar); 1346, 1322 (C-N). MS (APClþ, 20 V), m/z (%): 423 ([M þ H]þ, 100). 3-[N,N-(Bis-3-methoxyphenyl)]amino-9-ethylcarbazole (3). Compound 3 was prepared according to the general procedure A from 3-iodoanisole. The yield of the white powder was 1.1 g (55%). Elemental analysis for C28H26N2O2: % calc. C 79.59, H 6.20, N 6.63, O 7.57; % found C 79.81, H 6.37, N 6.47. 1H NMR (300 MHz, DMSO δ, ppm): 1.34 (t, 3H, J = 7.0 Hz, -CH2CH3), 3.64 (s, 6H, -OCH3), 4.43 (q, 2H, J = 7.0 Hz, -CH2CH3), 6.50-6.60 (m, 6H, Ar), 7.11-7.28 (m, 4H, Ar), 7.45 (t, 1H, J = 7.2 Hz, Ar), 7.60 (d, 2H, J = 7.2 Hz, Ar), 7.98 (d, 1H, J = 2.1 Hz, Ar), 8.09 (d, 1H, J = 7.7 Hz, Ar). 13C NMR (75.4 MHz, DMSO, δ, ppm): 14.50 (-CH2CH3), 37.78 (-CH2CH3), 55.65 (-OCH3), 107.44, 108.79, 109.88, 110.95, 115.34, 119.43, 119.94, 121.41, 122.56, 123.84, 126.51, 126.68, 130.64, 137.85, 139.04, 140.75, 149.97, 160.71. IR (KBr), (cm-1): 3049 (CHar); 2971, 2934 (CHaliphatic); 2832 (-OCH3); 1593, 1483, 1460 (CdCar); 1380, 1317 (C-N). MS (APClþ, 20 V), m/z (%): 423 ([M þ H]þ, 20).

’ GENERAL PROCEDURE B The mixture of tris(dibenzylideneacetone)dipalladium (0.09 g, 0.10 mmol) and tri-tert-butylphosphine (0.02 g, 0.10 mmol) was dissolved under argon in 5 mL of dry toluene and stirred for 10 min at room temperature. The mixture of 3-amino-9-ethylcarbazole (0.5 g, 2.38 mmol), bromocompound (7.13 mmol), and potassium tert-butoxide (1.60 g, 14.27 mmol) in 10 mL of dry toluene was added. The reaction mixture was heated at 90 C 4857

dx.doi.org/10.1021/jp109643r |J. Phys. Chem. C 2011, 115, 4856–4862

The Journal of Physical Chemistry C

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Scheme 1

for 3 h. After cooling, the reaction mixture was diluted with ethyl acetate and the organic phase was washed with water and brine. After being dried over anhydrous Na2SO4 and filtered, the solvent was removed and the residue was purified by column chromatography and recrystallized from the eluent mixture of solvents. 3-[N,N-(Bis-3,5-dimethoxyphenyl)]amino-9-ethylcarbazole (4). Compound 4 was prepared according to the general procedure B using 1-bromo-3,5-dimethoxybenzene (1.55 g, 7.13 mmol) as a bromocompound. The reaction was monitored by TLC (eluent: n-hexane/ethylacetate = 2:1). The resulting solid product was purified by column chromotography (eluent: n-hexane/ethylacetate = 2:1). The yield of the white cystals was 0.58 g (51%). mp = 128-130 C. Elemental analysis for C30H30N2O4: % calc. C 74.67, H 6.27, N 5.81, O 13.26; % found C 74.66, H 6.29, N 5.81. 1H NMR (300 MHz, CDCl3, δ, ppm): 1.49 (t, 3H, J = 7.2 Hz, -CH2CH3), 3.70-3.79 (m, 12H, -OCH3), 4.40 (q, 2H, J = 7.2 Hz, -CH2CH3), 6.15-6.16 (m, 2H, Ar), 6.31-6.32 (m, 3H, Ar), 7.20-7.53 (m, 6H, Ar), 7.95-8.04 (m, 2H, Ar). 13C NMR (75.4 MHz, CDCl3, δ, ppm): 14.22 (-CH2CH3), 37.92 (-CH2CH3), 55.59 (-O-CH3), 94.42, 101.76, 108.86, 109.53, 119.07, 119.71, 120.93, 122.95, 124.02, 126.16, 137.82, 139.15, 140.67, 150.55, 161.41. IR (KBr), (cm-1): 3051, 3007 (CHar); 2961, 2934 (CHaliphatic); 2837 (-OCH3); 1595, 1470, 1460, 1444 (CdCar); 1348, 1326 (C-N). MS (APClþ, 20 V), m/z (%): 483 ([M þ H]þ, 100). 3-[N,N-(Bis-3,4,5-trimethoxyphenyl)]amino-9-ethylcarbazole (5). Compound 5 was prepared according to the general procedure B using 5-bromo-1,2,3-trimethoxybenzene as a bromocompound (1.76 g, 7.13 mmol). The reaction was monitored by TLC (eluent: n-hexane/ethylacetate = 4:1). The resulting solid product was purified by column chromotography (eluent: n-hexane/ethylacetate = 4:1). The yield of the white cystals was 0.70 g (54%). mp = 199-200 C. Elemental analysis for C32H34N2O6: % calc. C 70.84, H 6.32, N 5.16, O 17.69; % found

C 70.81, H 6.33, N 5.18. 1H NMR (300 MHz, CDCl3, δ, ppm): 1.51 (t, 3H, J = 7.2 Hz, -CH2CH3), 3.72-3.97 (m, 18H, -OCH3), 4.41 (q, 2H, J = 7.5 Hz, -CH2CH3), 6.36 (s, 3H, Ar), 7.21-7.53 (m, 6H, Ar), 7.93-8.06 (m, 2H, Ar). 13C NMR (75.4 MHz, CDCl3, δ, ppm): 14.25 (-CH2CH3), 37.96 (-CH2CH3), 56.35 (-O-CH3), 61.33 (-O-CH3), 100.81, 108.91, 109.43, 118.73, 119.08, 120.87, 122.84, 123.91, 125.54, 126.17, 133.25, 137.43, 139.56, 140.65, 145.09, 153.72. IR (KBr), (cm-1): 3087, 2995 (CHar); 2965, 2935 (CHaliphatic); 2834 (-OCH3); 1499, 1482, 1459, 1446 (CdCar); 1349, 1334 (C-N). MS (APClþ, 20 V), m/z (%): 543 ([M þ H]þ, 100). 3-[N,N-(Bisphenyl)]amino-9-ethylcarbazole (6). A mixture of 3-iodo-9-ethylcarbazole (1 g, 3.11 mmol), diphenylamine (0.63 g, 3.74 mmol), 18-crown-6 (0.08 g, 0.03 mmol), potassium carbonate (1.46 g, 10.59 mmol), and copper powder (0.40 g, 6.23 mmol) in o-dichlorobenzene (15 mL) was refluxed under argon at 180 C. The reaction was monitored by TLC (eluent: n-hexane/chloroform = 5:1). After 24 h, the reaction was stopped and copper powder and inorganic salts were removed by filtration of the hot reaction mixture. The solvent was distilled under reduced pressure, and the crude product was purified by column chromatography (eluent: n-hexane/ethyl acetate = 5:1) and recrystallized from the eluent mixture of solvents. The yield of the green cystals was 0.82 g (73%). mp = 133-134 C. Elemental analysis for C28H26N2O2: % calc. C 86.15, H 6.12, N 7.73; % found C 86.75, H 6.25, N 7.82. 1H NMR (300 MHz, CDCl3, δ, ppm): 1.52 (t, 3H, J = 7.2 Hz, -CH2CH3), 4.42 (q, 2H, J = 7.0 Hz, -CH2CH3), 7.02 (t, 2H, J = 7.2 Hz, Ar), 7.15-7.56 (m, 13H, Ar), 8.04-8.09 (m, 2H, Ar). 13C NMR (75.4 MHz, CDCl3, δ, ppm): 14.24 (-CH2CH3), 37.94 (-CH2CH3), 108.87, 109.63, 119.07, 120.94, 121.75, 122.89, 124.13, 125.96, 126.14, 129.34, 137.56, 139.71, 140.71, 148.99. IR (KBr), (cm-1): 3036 (CHar); 2975, 2932 (CHaliphatic); 1586, 1488, 1478, 1453 (Cd Car); 1345, 1325 (C-N). MS (APClþ, 20 V), m/z (%): 322 ([M þ H]þ, 25). 4858



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Table 1. Thermal Characteristics of Compounds 1-6 compounds

Tm (C), first heating

Tg (C), second heating

TID (C)

1

143

51

402

2

174

50

372

39a

375

3

a

4

136

41

354

5

206

69

332

6

137

47

358

First and second heating.

Figure 1. DSC thermograms of compound 2 (scan rate of 10 C/min, N2 atmosphere).

’ RESULTS AND DISCUSSION The derivatives of 9-ethylcarbazole and diphenylamine with different amounts and positions of methoxy groups in diphenylamino moieties were synthesized as described in Scheme 1 from 3-amino-9-ethylcarbazole and the corresponding iodo- or bromophenyl derivatives. Compounds 1-3 and 6 were prepared by Ullmann coupling reactions15 of amino and iodo derivatives. Compounds 4 and 5 were prepared by palladium-catalyzed aromatic C-N coupling16,17 reactions of 3-amino-9-ethylcarbazole with 1-bromo-3,5-dimethoxybenzene or 5-bromo-1,2,3-trimethoxybenzene, respectively. All the compounds (1-6) were purified by column chromatography. They were identified by elemental analysis and IR, 1H NMR, 13C NMR, and mass spectrometries. Compounds 1-6 were readily soluble in common organic solvents, such as chloroform, dichloromethane, and tetrahydrofuran (THF), at room temperature. The behavior under heating of compounds 1-6 was studied by DSC and TGA under a nitrogen atmosphere. The values of glass transition temperatures (Tg), melting points (Tm), and temperatures at which 5% loss of mass was observed (TID) are summarized in Table 1. All the newly synthesized compounds demonstrate high thermal stability. The values of TID are higher than 332 C, as confirmed by TGA with a heating rate of 20 C/ min. Compounds 1-3 containing monomethoxy-substituted phenyl groups exhibit higher thermal stabilities as compared with compounds 4 and 5 containing di- and trimethoxy-substituted phenyl groups.

Compounds 1, 2, and 4-6 were isolated after the synthesis as crystalline materials. In the first DSC heating scans, they showed endothermic melting signals with the melting points in the range of 136-206 C. These compounds formed glasses upon cooling from the melts. In the second heating scans, they showed glass transitions in the range from 41 to 69 C. The illustration of the above-stated DSC curves of compound 2 are shown in Figure 1. In the first heating scan, compound 2 showed melting at 174 C. Upon cooling, it did not crystallize and showed a glass transition at 50 C in the second heating scan. The highest glass transition temperature and melting point were observed for compound 5 containing six methoxy groups in the molecule. This observation can apparently be explained by the stronger dipole-dipole intermolecular interaction in the solid amorphous state and by the higher molecular weight of 5. Compound 3 was isolated as an amorphous substance. When its sample was heated, a glass-transition was observed at 39 C and no peaks due to crystallization and melting appeared. Cooling down and the following repeated heating revealed only a glass transition. The amorphous nature and low glass transition temperature of 3 can be explained by the relatively loose packing of the molecules. Figure 2 shows the optimal arrangement of molecules of compounds 1-3 obtained by geometry optimization using the AMBER force field. The least close arrangement is observed for molecules of compound 3. The UV absorption and fluorescence spectra of dilute solutions of compounds 1-6 in THF are shown in Figure 3. Wavelengths of the lowest-energy absorption maxima and emission maxima are summarized in Table 2. UV spectra of these compounds are similar, and the maxima wavelength values are in the range of 211-390 nm. The number and position of methoxy groups have no significant effect on the character of UV spectra; however, they influence the position of absorption edges and consequently band gaps. The lowest band gap is observed for compound 1 (Table 2). Dilute solutions of compounds 1-6 exhibit intensive fluorescence. Their fluorescence intensity maxima appear in the region of wavelengths from 413 to 443 nm. The Stokes shifts of compounds 1-6 are comparable and range from 43 to 58 nm (Table 2). To elucidate the energetic conditions for energy and electron transfer in dilute solutions, the HOMO/LUMO values were established by cyclic voltamperometry. The cyclovoltammperometric curves of the solutions of the synthesized compounds in dichloromethane show one reversible oxidation couple in the region of 0-0.8 V. The electrochemical data are summarized in Table 3. The HOMO values are in the range from -5.02 to -4.81 eV. Compounds 1 and 2, having methoxy groups in para and ortho positions, respectively, show increased energy of the HOMO band. The LUMO levels were determined from optical energy band gaps and EHOMO values. Compounds 1-6 exhibit close LUMO energy levels ranging from -1.96 to -1.85 eV. The EHOMO values determined by cyclic voltammetry do not represent any absolute solid-state or gas-phase ionization energies; they can be used only for the comparison of different compounds. It was, therefore, of interest to estimate the ionization potentials of the amorphous layers of the synthesized compounds. The values of the ionization potential measured by an electron photoemission technique are presented in Table 3. The ionization potentials of the synthesized materials range from 5.10 to 5.56 eV. The lowest ionization potentials of 5.10 eV were observed for compounds 1 and 2 having monomethoxy phenyl 4859



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Figure 2. Optimal arrangement of molecules of compounds 1-3 obtained by geometry optimization using the AMBER force field (rms gradient, 0.001 kcal/Å mol).

Figure 3. UV absorption and fluorescence (λex = 360 nm) spectra of dilute THF solutions (10-4 mol l -1) of compounds 1-6.

Table 2. Absorption and Emission Characteristics of Compounds 1-6 b compound UV, λmax (nm)a Eopt (eV) FL, λmax (nm) Stokes shift (nm) g,

Table 3. HOMO, LUMO, Band Gap Energies, Ionization Potentials, and Electrochemical Characteristicsa of Compounds 1-6 compound

E1/2 vs Fcb (V)

Ip (eV)

EHOMOc (eV)

ELUMOd (eV)

1 2

387 374

2.90 3.03

443 417

56 43

3

362

3.07

415

53

4

361

3.08

413

51

3

0.08

5.30

-5.01

-1.94

5

372

3.00

427

55

6

360

3.04

418

58

4 5

0.22 0.11

5.31 5.56

-5.02 -4.91

-1.94 -1.91

6

0.20

5.36

-5.00

-1.96

a

Wavelenths of lowest-energy absorption maxima of solutions in THF with the concentration of 10-4 mol l -1. b The optical band gaps, Eopt g , estimated from the edges of electronic absorption spectra.

groups. This observation correlates with the UV spectrometry results. Compounds 1 and 2 exhibit low band gaps. The ionization potentials of compounds 1 and 2 are lower than that of compound 6 containing no methoxy groups. This observation shows that introduction of methoxy groups into the structures of aromatic amines enables one to decrease their ionization potentials. However, it seems that the positions of methoxy groups in aromatic rings influence the photoelectrical properties of the compounds. Compound 3 having a methoxy group in the meta position exhibits slightly higher ionization potentials than compounds 1 and 2 having methoxy groups in para and ortho

1

0.01

5.10

-4.81

-1.91

2

0.21

5.10

-4.88

-1.85

a

The measurements were carried out at a glassy carbon electrode in dichloromethane solutions containing 0.1 M tetrabutylammonium perchlorate as the electrolyte and Ag/AgNO3 as the reference electrode. Each measurement was calibrated with ferrocene (Fc), with the meab sured EFc 1/2 = 0.27 V vs Ag/AgNO3. E1/2 = (Epa þ Epc)/2; Epa and Epc are peak anodic and peak cathodic potentials, respectively. E1/2 vs Fc = Fc opt c d E1/2 - EFc 1/2. EHOMO = 4.8 þ (E1/2 - E1/2). ELUMO = EHOMO - Eg .

Table 4. Ip Values of Compounds 1-6 Calculated by HF and DFT Compound energy (eV)

4860

HF DFT

1

2

3

4

5

6

4.68 5.58

4.58 5.68

4.88 5.83

4.98 5.75

4.94 5.79

4.77 5.94



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Figure 4. Isosurfaces of the HOMO for molecules 1-3 calculated with HF/6-31G*.

Table 5. Hole Mobility Data for the Amorphous Layers of Net Compounds 1-3 and Compounds 1-6 Doped in PC-Z (50%) μ0a (cm2/V 3 s)a

μb (cm2/V 3 s)

4.7 10-6

1.2 10-4

7.7 10

-6

1.2 10-4

3.2 10

-7

3.2 10-5

4.5 10

-7

7.9 10-6

-7

1.2 10-5

3 þ (PC-Z), 1:1 4 þ (PC-Z), 1:1

-8

8.3 10 9.3 10-9

1.8 10-6 4.7 10-7

5 þ (PC-Z), 1:1

6.5 10-9

3.9 10-7

6 þ (PC-Z), 1:1

-7

3.9 10-6

transport material, host polymer 1 2 3 1 þ (PC-Z), 1:1 2 þ (PC-Z), 1:1

a

Figure 5. Electric field dependencies of hole drift mobility in charge transport layers of compounds 1-3 and compounds 1-6 doped in PC-Z (50%).

positions. The increase of the number of methoxy groups in phenyl rings does not lead to the further decrease of ionization potentials. Compound 5 having trimethoxy-substituted phenyl rings shows the highest value of Ip. To gain further physical insights of the molecules, we have performed quantum chemistry calculations. The Ip values were calculated as the energy difference for the neutral and cationic molecules. As seen in Table 4, the Ip values of para- and orthosubstituted molecules (compounds 1 and 2) are smaller than that of the meta-substituted case, and this trend is similar to experimental observation. We have also plotted the isosurface of their highest occupied molecular orbital (HOMO), a representation for the distribution of the removed electron in ionization. In the HOMO distribution shown in Figure 4, there exist contributions from the long pair electron of oxygen atoms in para- and orthosubstituted molecules (1 and 2) but not for the meta-substituted one (3). We further analyzed the population on the O atoms in the HOMOs. There are larger populations from O atoms in the para(1) (B3LYP, 5.82%; HF, 3.07%) and ortho- (2) (B3LYP, 2.44%; HF, 1.04%) substituted molecules, and smaller numbers are seen in the meta case (3) (B3LYP, 0.12%; HF; 0.15%). This result is consistent with the commonly accepted idea of electronic resonance in the π and lone-pair electrons. For the para- and ortho-substituted molecules, there is a resonance involving lonepair electrons on the oxygen atom, as seen in the HOMO population. This is an electron-donating effect from oxygen. For the meta-substituted molecule, it is not possible to draw the electronic resonance structure that involves the lone pairs of the oxygen, and the contribution to the HOMO of oxygen is much

6.5 10

2.7 10 b

The zero-field hole drift mobility. The hole drift mobility at an electric field of 6.4 105 V/cm.

smaller, leading to a less electron-donating effect. Therefore, the cationic state for the para- and ortho-substituted molecules are more stable than that of the meta case. For dimethoxy- and trimethoxy-substituted molecules (4 and 5), the Ip values are larger than for para- and ortho-substituted compounds, and these results also generally agree with experiments. In these compounds, we believe that the effect of methoxy substitution is mainly due to the electron-withdrawing effect arising from the electron negativity of the oxygen atoms. Time-of-flight measurements were used to characterize charge-transporting properties of the synthesized compounds. Figure 5 shows electric field dependencies of hole drift mobilities (μ) in the amorphous layers of compounds 1-3 and in the films of bisphenol Z polycarbonate molecularly doped with compounds 1-6. At room temperature, μ showed linear dependencies on the square root of the electric field. The hole drift mobility values are summarized in Table 5. The layers of compounds 1 and 2 showed higher charge carrier mobilities than those of compound 3. Hole drift mobility values of the glassy layers of compounds 1 and 2 were 1.2 10-4 cm2/V 3 s at an electric field of 6.4 105 V/cm at room temperature, and that observed for the solid amorphous layer of 3 was 3.2 10-5 cm2/V 3 s at the same conditions. The solid solutions of 1-6 in PC-Z demonstrated hole drift mobility values in the range from 4.7 10-7 to 1.2 10-5 cm2/V 3 s at electric fields of 6.4 105 V/cm. The best charge transport properties among molecularly doped polymers were observed for the systems containing compounds 1 and 2. These compounds exhibited superior charge transport properties relative to compound 3, having methoxy groups in the meta position, and relative to compound 6, having no methoxy groups. The lowest charge mobilities were observed for the systems containing compounds 4 and 5 with dimethoxy- and 4861


The Journal of Physical Chemistry C trimethoxy-substituted phenyl rings. Inferior charge-transporting properties of compounds 4 and 5 can apparently be explained by higher polarity of the molecules. Borsenberger and Fitzgerald18 in 1993 reported about the sharp decrease in mobility of organic semiconductors with increasing dipole moment. The increasing dipole moment results in the increase of the width of the hoping site manifold. The origin of this effect was attributed to random internal fields associated with the dipole moments.

’ CONCLUSIONS In conclusion, we have synthesized a series of carbazole and diphenylamine derivatives with different numbers and positions of methoxy groups in the diphenylamino moieties and studied the influence of methoxy groups on the thermal, optical, and photoelectrical properties of the synthesized materials. The ionization potentials of the synthesized compounds range from 5.10 to 5.56 eV, as established by an electron photoemission in air technique. Compounds containing monomethoxy-substituted phenyl rings exhibited lower ionization potentials than compounds containing di- and trimethoxy-substituted phenyl moieties. Among the derivatives containing monomethoxysubstituted phenyl groups, the lowest ionization potentials were observed for para- and ortho-substituted molecules. The highest ionization potential was observed for the compound containing trimethoxy-substituted phenyl rings. The best charge transport properties were observed for the compounds containing one methoxy group in para and ortho positions of phenyl rings of the diphenylamino moiety. Room-temperature hole drift mobility in the amouphous film of 3-[N,N-(bis-4-methoxyphenyl)]amino-9ethylcarbazole established by a xerographic time-of-flight technique was found to be 1.2 10-4 cm2/V 3 s at an electric field of 6.4 105 cm2/V 3 s.

ARTICLE

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. )

Present Addresses

Departament d’Enginyeria Química, Universitat Rovira i Virgili, Av. Països Catalans 26, Campus Sescelades, 43007 Tarragona, Spain.

’ ACKNOWLEDGMENT Financial support from the Research Council of Lithuania and from National Science Council of Taiwan is gratefully acknowledged. ’ REFERENCES (1) Schattuk, M. D.; Vahtra, U. US Patent 3,484,327. (2) Grazulevicius, J. V. Polym. Adv. Technol. 2006, 17, 694–696. (3) Morin, J. F.; Ades, D.; Siove, A.; Leclerc, M. Macromol. Rapid Commun. 2005, 26, 761–778. (4) Puodziukynaite, E.; Burbulis, E.; Grazulevicius, J. V.; Getautis, V.; Jankauskas, V. Synth. Met. 2008, 158, 993–998. (5) Blazys, G.; Grigalevicius, S.; Grazulevicius, J. V.; Gaidelis, V.; Jankauskas, V.; Kampars, V. J. Photochem. Photobiol., A 2005, 174, 1–6. (6) Thelakkat, M.; Ostrauskaite, J.; Leopold, A.; Bausinger, R.; Haarer, D. Chem. Phys. 2002, 285, 133–147. (7) Kirkus, M.; Lygaitis, R.; Tsai, M. H.; Grazulevicius, J. V.; Wu, C. C. Synth. Met. 2008, 158, 226–232. 4862


Effect of Methoxy Substituents on the Properties of the Derivatives of

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