Nondoped deep blue OLEDs based on Bis-(4-benzenesulfonyl-phenyl)-9-phenyl-9H-carbazoles

Nondoped deep blue OLEDs based on Bis-(4-benzenesulfonyl-phenyl)-9-phenyl-9H-carbazoles

Author’s Accepted Manuscript Nondoped Deep Blue OLEDs Based on Bis–(4– benzenesulfonyl –phenyl)–9–phenyl–9H– carbazoles Bin Huang, Zhihui Yin, Xinxin ...

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Author’s Accepted Manuscript Nondoped Deep Blue OLEDs Based on Bis–(4– benzenesulfonyl –phenyl)–9–phenyl–9H– carbazoles Bin Huang, Zhihui Yin, Xinxin Ban, Zhongming Ma, Wei Jiang, Wenwen Tian, Min Yang, Shanghui Ye, Baoping Lin, Yueming Sun www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(15)30353-7 http://dx.doi.org/10.1016/j.jlumin.2015.11.012 LUMIN13711

To appear in: Journal of Luminescence Received date: 5 August 2015 Revised date: 5 November 2015 Accepted date: 7 November 2015 Cite this article as: Bin Huang, Zhihui Yin, Xinxin Ban, Zhongming Ma, Wei Jiang, Wenwen Tian, Min Yang, Shanghui Ye, Baoping Lin and Yueming Sun, Nondoped Deep Blue OLEDs Based on Bis–(4–benzenesulfonyl –phenyl)–9– p h e n y l – 9 H – c a r b a z o l e s , Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.11.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nondoped Deep Blue OLEDs Based on Bis–(4–benzenesulfonyl –phenyl)–9–phenyl–9H–carbazoles Bin Huang a,b, Zhihui Yin b, Xinxin Ban a, Zhongming Ma b, Wei Jiang a,*, Wenwen Tian a, Min Yang c, Shanghui Ye c,*, Baoping Lin a, Yueming Sun a,* a

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu, P. R.

China 211189 b

Department of Chemical and Pharmaceutical Engineering of Southeast University Chengxian

College, Nanjing, Jiangsu, P. R. China 210088 c

Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications

(NUPT), Nanjing, Jiangsu, P. R. China 210023 * Corresponding author: Tel/fax: +86 25 52090621 E-mail address:[email protected][email protected][email protected] Abstract: Two bipolar materials based on 9–phenylcarbazole and diphenyl sulfone for nondoped deep blue OLEDs, namely bis–(4–benzenesulfonyl–phenyl)–9–phenyl–9H–carbazoles, have been designed and synthesized by Suzuki coupling reactions. Their thermal, photophysical, and electrochemical properties

have

been

systematically

investigated.

The

3,6–bis–(4–benzenesulfonyl–phenyl)–9–phenyl–9H–carbazoles

and

nondoped

devices

using

2,7–bis–(4–benzenesulfonyl

–phenyl)–9–phenyl–9H–carbazoles as the emitters show deep blue emission color with peaks at 424 and 444 nm, and the Commission Internationale de l’Eclairage (CIE) coordinates of (0.177, 0.117) and (0.160, 0.117), respectively. Furthermore, these materials based devices have high color–purity with small width at half maximum (FWHM) of 65 and 73 nm, respectively. The results provide a novel approach for the design of deep blue emitter for nondoped OLEDs. Keywords: carbazole, sulfone, color–purity, nondoped, deep blue, OLEDs 1. Introduction Since the first high–efficiency organic light emitting diodes (OLEDs) reported by Tang [1] et al in 1

1987, tremendous interest has been attracted in the development of OLEDs because of their potentials to replace traditional light sources and displays.[2–6] Despite great progress made in the development of materials for OLEDs, the performance of deep blue OLEDs is still a challenge to achieve full color displays due to poor carrier injection in deep blue emitters which possess the intrinsic wide bandgap.[7] Therefore, many researchers have devoted to the development of blue emitters to realize high–efficiency deep blue OLEDs. Recently, a number of excellent deep blue emitters based on pyrene [8], anthracene [9–13], fluorine [14–16], and phenanthrene [17] have been developed. However, these emitters based OLEDs usually show wide emission spectra (> 75 nm) for the full width at half maxima (FWHM), which may affect the emission color purity. There is still a clear need of deep blue emitters in terms of efficiency and color purity. Recently, sulfone–containing materials show great potential in high–efficiency OLEDs as hosts [18–23], electron transporters [24,25], fluorescence emitters [26–30], thermally activated delayed fluorescence (TADF) emitters [31–34] and aggregation–induced emission(AIE) luminogens [35–37]. In which, Zhang et al reported two novel blue–violet emitting materials based on electron–donor N–phenylcarbazole and electron–acceptor sulfone for high–effciency nondoped devices with the Commission Internationale de l’Eclairage (CIE) below 0.06 and small emission spectra (<7 0 nm) for FWHM. [28] Howerver, new classes of sulfone–based deep blue emitters are still to be explored. In this contribution, we report two bipolar materials based on 9–phenylcarbazole and diphenyl sulfone, namely 3,6–bis–(4–benzenesulfonyl–phenyl)–9–phenyl–9H–carbazoles (2a) and 2,7–bis– (4–benzenesulfonyl–phenyl)–9–phenyl–9H–carbazoles (2b), as blue emitter for high–color–purity OLEDs. Their thermal, photophysical, electrochemical and properties have been systematically investigated. The nondoped devices using 2a and 2b as the emitters show deep blue emission color with peaks at 424 and 444 nm, and CIE coordinates of (0.177, 0.117) and (0.160, 0.117), respectively. Furthermore, 2a and 2b based devices have high color purity with small FWHM of 65 and 73 nm, respectively.

2. Experimental 2.1 General All reactants and solvents were used as received from commercial sources without further 2

purification. 1H NMR and

13

C NMR spectra were measured with a Bruker ARX300 NMR

spectrometer with Si(CH 3)4 as the internal standard using CDCl 3 as solvent in all cases. Elemental analysis was performed on an Elementar Vario EL CHN elemental analyzer. Mass spectra were recorded on a Thermo Electron Corporation Finnigan LTQ mass spectrometer. UV–vis absorption and

PL spectra

were

recorded

on

a

spectrophotometer

(SHIMADZU

2450)

and

a

fluorospectrophotometer (HORIBA, Fluoromax-4), respectively. The absolute fluorescence quantum yields (Φf) were measured in an integrating sphere system. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were recorded on a Netzsch simultaneous thermal analyzer (STA) system (STA 409PC) and DSC 2910 modulated calorimetry under dry nitrogen atmosphere at a heating rate of 10 oC min–1. Cyclic voltammetry (CV) measurements were performed on a Princeton Applied Research potentiostat/galvanostat model 283 voltammetric analyzer in CH2Cl2 solutions (10–3 M) at a scan rate of 100 mV s –1 with a platinum plate as the working electrode, a silver wire as the pseudo–reference electrode, and a platinum wire as the counter electrode. Tetra–n–butylammonium hexafluorophosphate (TBAPF 6, 0.1M) was used as supporting electrolyte and ferrocene was selected as the internal standard, respectively. The solutions were bubbled with a constant argon flow for 10 min before measurements. All the calculations were performed using the Gaussian 09 program package.[38] Density fuctional theory (DFT) was used to optimize the structures of bis–(4–benzenesulfonyl–phenyl)–9– phenyl–9H–carbazoles at the B3LYP/6–31G(d) basis sets. The molecular orbitals were visualized using Gaussview. Device fabrication and measurement are as following. In a general procedure, indium-tin oxide (ITO)-coated glass substrates were pre-cleaned carefully and treated by UV ozone for 4 min. The substrate was then transferred into an evaporation chamber. The devices were fabricated by evaporating organic layers at evaporation rates of 1-2 Å/s under a pressure of 1×10 -3 Pa. Finally, the Ca/Ag

bilayer

cathode

was

evaporated

at

evaporation

rates

of

0.1-10

Å/s.

The

current-voltage-brightness characteristics of the devices were characterized with Keithley 2602 semiconductor characterization system. The electroluminescent spectra were collected with a Photo Research PR655 Spectrophotometer. All measurements of the devices were carried out in ambient atmosphere without further encapsulations. 3

2.2 Synthesis of bis–(4–benzenesulfonyl–phenyl)–9–phenyl–9H–carbazoles Synthesis of 4–bromo–diphenylsulfide (1a) To a solution of iodobenzene (4.08 g, 20.0 mmol), 4–bromothiophenol (3.78 g, 20 mmol) 1,10–phenanthroline monohydrate (0.8 g, 4 mmol) in 40 mL of DMSO was added K 2CO3 (5.52 g, 40 mmol).The reaction mixture was then purged with nitrogen for 10 min before adding Cu 2O (0.144 g, 1 mmol). After stirring at 100 oC for 24 h under nitrogen, the resulting mixture was cooled to room temperature and then poured into 20 mL water. The mixture was extracted with 60 mL (3×2 0 mL) dichloromethane. The combined organic phase was then washed with 20 mL (2×10 mL) saturated aqueous NaCl solution and dried with anhydrous Na 2SO4. After removal of the solvent by rotary evaporation, the residue was purified by silica gel column chromatography to afford 1a as an oil. Yield: 82.1%. 1H NMR (300 MHz, CDCl3) δ (ppm): 7.43 (d, J=6.0 Hz 2H), 7.39-7.28 (m, 5H), 7.19 (d, J=6.0 Hz, 2H).

13

C NMR (75 MHz, CDCl 3) δ (ppm):120.83, 127.58, 129.39, 130.87, 131.57,

132.24, 134.85, 135.52. Synthesis of 4–bromo–diphenylsulfone (1b) To a solution of 30 mL H 2O2 (30%, g/g) and 30 mL of HOAc was added 1a (3.44 g, 10 mmol). After refluxing for 24 h, the resulting mixture was cooled to room temperature and then poured into water. The white precipitate was filtered and dried. The crude product was recrystallized from ethanol to produce 1b as a white solid. Yield: 84.6%. M.p.:109–110 oC. 1H NMR (300 MHz, CDCl3):δ (ppm) 7.91 (t, J=7.2, 7.2Hz, 2H), 7.80 (d, J=5.7Hz, 2H), 7.65–7.48 (m, 5H). Synthesis of 2–(4–benzenesulfonyl–phenyl)–4,4,5,5–tetramethyl–[1,3,2]dioxaborolane (1c) A mixture of 1b (2.97 g,10 mmol), bis (pinacoloto)diboron (2.80 g, 11 mmol),anhydrous potassium acetate (2.93 g, 30 mmol) and Pd(dppf)Cl 2 (0.4 g, 0.53 mmol) in dioxane (60 mL) was refluxed at 110 °C for 24 h under nitrogen. After cooled to room temperature, the reaction was filtrated. The filtrate was added H2O (60 mL), extracted with CH 2Cl2 (2×60 mL), washed by saturated aqueous NaCl solution (2×20 mL), and dried over anhydrous Na 2SO4. After removal of the solvent by rotary evaporation, the residue was purified by silica gel column chromatography to afford 1c as a white solid. Yield: 82.3%. 1H NMR (300 MHz, CDCl3) δ (ppm):7.94–7.92 (m, 6H), 7.57–7.45 (m, 3H), 1.32(s, 12H). Synthesis 3,6–bis–(4–benzenesulfonyl–phenyl)–9–phenyl–9H–carbazole (2a) 4

To a solution of 1c (1.21 g, 3.5 mmol) and 3,6–dibromo–9–phenyl–9H–carbazole (0.66 g, 1.6 mmol) in toluene (20 mL) and ethanol (4 mL) was added aqueous K 2CO3 solution (2.0 M, 3 mL). The reaction mixture was then purged with nitrogen for 10 min before adding Pd[PPh 3]4 (0.055 g, 0.048 mmol). The reaction mixture was heated under reflux for 24 h under nitrogen. The resulting mixture was cooled to room temperature and then poured into water (20 mL) and extracted with CH 2Cl2 (3×20 mL). The combined organic phase was washed with saturated aqueous NaCl solution (2×10 mL) and dried with anhydrous Na 2SO4. After removal of the solvent by rotary evaporation, the residue was purified by silica gel column chromatography to afford 2a as a white solid. Yield: 45.2%. M.p. 311~314 °C. 1H NMR (300 MHz, CDCl3) δ (ppm):8.38(s, 2H), 8.06–8.00 (m, 8H), 7.83 (d, J=8.4Hz, 4H), 7.67–7.46 (m, 15H).

13

C NMR (75 MHz, CDCl 3) δ (ppm):110.69, 119.34,

123.92, 125.93, 126.99, 127.62, 127.89, 128.10, 128.30, 129.31, 130.13, 131.64, 133.12, 137.01, 139.42, 141.59, 141.94, 146.73. MS (MALDI–TOF) [m/z]:calcd for C 42H29NO4S2, 675.83; found, 675.46. Anal. Calcd. for C42H29NO4S2 (%): C, 74.64; H, 4.33; N, 2.07;O, 9.47;S, 9.49. Found: C, 76.48; H, 4.38; N, 2.05; S, 9.53. Synthesis 2,7–bis–(4–benzenesulfonyl–phenyl)–9–phenyl–9H–carbazole (2b) A procedure similar to that used for 2a was followed but with 2,7–dibromo–9–phenyl–9H–carbazole instead of 3,6–dibromo–9–phenyl–9H–carbazole.Yield: 48.8%. M.p. 257~259 °C. 1H NMR (300 MHz, CDCl3) δ (ppm):8.23 (d, J=8.7 Hz, 2H), 8.01 (t, J=8.1 Hz, 4.5 Hz, 8H), 7.76 (d, J=8.4Hz, 4H), 7.69–7.53 (m, 15H).

13

C NMR (75 MHz, CDCl 3) δ (ppm):108.69, 119.95, 121.13, 123.05, 127.29,

127.62, 127.91, 127.99, 128.18, 128.27, 129.30, 130.29, 133.15, 136.99, 137.72, 139.98, 141.82, 142.17, 146.75. MS (MALDI–TOF) [m/z]:calcd for C 42H29NO4S2, 675.83; found, 675.54. Anal. Calcd. for C42H29NO4S2 (%): C, 74.64; H, 4.33; N, 2.07;O, 9.47;S, 9.49. Found: C, 76.54; H, 4.36; N, 2.05; S, 9.51. 3 Results and discussion 3.1 Synthesis and characterization Scheme 1 outlines the synthetic routes of the two new compounds 2a and 2b. The start materials 2,7–dibromo–9–phenyl–9H–carbazole and 3,6–dibromo–9–phenyl–9H–carbazole were synthesized according to the literature procedure.[39] Firstly, 1b was obtained in a two–step procedure from the start of 4–bromo–thiophenol: C–S coupling with iodobenzene and oxidization with H 2O2. Secondly, 5

1b was converted to the arylboronic ester (1c) catalyzed by Pd(dppf)Cl2 in the presence of KOAc in 1,4–dioxane. Finally, Suzuki coupling reaction of 1c with 2,7–dibromo–9–phenyl–9H–carbazole and 3,6–dibromo–9–phenyl–9H–carbazole led to 2a and 2b with yields of 45.21% and 48.82%, respectively. The products were purified by the silica column method and vacuum sublimation, yielding the very pure white powders. 1H NMR,

13

C NMR, mass spectrometry, and elemental

analysis were employed to confirm the chemical structure of 2a and 2b in the experimental section. 3.2 Thermal properties of 2a and 2b The thermal properties of 2a and 2b were investigated by TGA and DSC under nitrogen atmosphere. As shown in Fig.1, high decomposition temperatures (Td, corresponding to a weight –loss of 5% ) are 430 oC (2a) and 427 oC(2b), respectively. High glass transition temperatures (T g) of 96 oC (2a) and 95 oC (2b) were also obtained from Fig.1. The similarly excellent thermal stability of 2a and 2b may be attributed to their similar structures and would benefit the forming of amorphous films. 3.3 Photophysical properties of 2a and 2b The UV–vis absorption and photoluminescence spectra of 2a and 2b in CH2Cl2 were depicted in Fig. 2(a). Three major absorption peaks locate at 224, 308, 343 nm (2a), and 221, 242, 339 nm (2b). The absorption peaks located at 224 , 221 and 242 nm are π–π* transition of diphenyl sulfone, and the absorption peak (308 nm) is assigned to N-phenyl, and the absorption peaks (343 and 339 nm) are attributing to the intramolecular charge transfer (ICT) transition. From the absorption edge of the UV–vis absorption, the optical bandgaps (E g) of 2a and 2b can be estimated to be 3.22 and 3.14 eV, respectively. The positions of the acceptor have an effect on the E g values. 2a exhibits a higher E g value than 2b, this results confirm that the connections at 2,7–positions of N–phenylcarbazole give more extended π–conjugation than at the 3,6–positions.[40,41] 2a and 2b exhibit broad and structureless emission bands with peaks at 395 and 411 nm in CH 2Cl2, which can be ascribed to ICT. Changing the position of diphenyl sulfone group from 3,6–substitutions to 2,7–substitutions induces a shift of the emission maximum to longer wavelengths, the results are in agreement with the extension of π–conjugation. We measured the fluorescence spectra in various solvents with different polarities. The results are summarized in Table 1. In the polar solvent methanol, the emission maximum of 2a and 2b red–shifts to 434 nm. Conversely, in the nonpolar solvent hexane, the emission maximum of 2a and 2b blue–shifts to 377 6

and 409 nm, respectively. The Stocks shifts for 2a and 2b are very large in polar solvents as compared to non–polar solvents indicating a considerable energetic stabilization of the excited state in polar solvents. To obtain more information about the change in the dipole moment upon excitation, we use the Lipperte–Mataga equation [42,43], the dipole moment between the excited state and the ground state (Δµ) were estimated to be 37 and 23 D for 2a and 2b. The large change in dipole moment upon excitation is typical for ICT processes. In addition, we determined the absolute fluorescence quantum yields (Φf) of 2a and 2b in neat film, 0.38 for 2a and 0.59 for 2b, suggesting that the connections at 2,7–substitutions have higher Φf values than at 3,6–substitutions.[40] The phosphorescence spectra of 2a and 2b measured from a frozen 2–methyltetrahydrofuran matrix at 77K were depicted in Fig. 2(b). From the phosphorescence spectra maximum, the triplet energy of 2a and 2b can be estimated to be 3.00 and 2.86 eV, respectively. 3.4 Electrochemical analysis The electrochemical properties of 2a and 2b were studied in solution through CV using ferrocene as the internal standard. As shown in Fig. 3, the HOMO energy levels of 2a and 2b can be estimated to be –5.62 and –5.72 eV, respectively, which are matched with the hole–injecting layer mCP(–5.80 eV). From the equation E LUMO =EHOMO+Eg, the LUMO energy levels of 2a and 2b were determined to be –2.40 and –2.58 eV, respectively. Compared with 2a, the LUMO energy level of 2b is shallower, favoring the electron injection and transporting. 3.5 DFT calculations To gain insights into the physical properties of 2a and 2b, DFT calculations were performed at he B3LYP/6–31G(d) basis sets. The results are listed in Table 2, and the HOMOs and LUMOs of 2a and 2b are shown in Fig. 4. The HOMOs of 2a and 2b are mainly delocalized over the central N–phenylcarbazole. In contrast, the LUMOs of 2a and 2b are delocalized over the outer diphenyl sulfone. The large HOMO–LUMO overlaps of 2a and 2b leads to energy gaps between the singlet and triplet (△EST) of 0.69 and 0.73 eV, respectively. In comparison with typical TADF materials based on sulfone [31–34], the larger △EST values of 2a and 2b indicate that these materials may be not suitable for TADF materials. As can be seen from Table 2, 2b has lower LUMO energy level than 2a. In comparison, the HOMO energy level of 2a and 2b are slightly influenced by the change of the substitution positions of 7

diphenyl sulfone. The trends of the theoretically calculated energy levels are consistent with the experimental ones, although the exact values are somewhat different, probably reflecting that the calculations are employed under the gas–phase conditions. 3.6 Electroluminescent properties In order to investigate the electroluminescent properties of 2a and 2b, we fabricated nondoped OLEDs with a configuration of ITO/NPB (30 nm)/mCP(10 nm)/ 2a or 2b (30 nm)/TPBI (30 nm)/ Ca/Ag, NPB (N,N–bis(naphthylphenyl)–4,4’–biphenyldiamine) served as hole–transporting layer (HTL), mCP (1,3–di(9H–carbazol–9–yl)benzene) served as exciton–blocking layer, and TPBi (1,3,5–tris(1–phenyl–1H–benzo[d]imidazol–2–yl)benzene) is used as electron–transporting and hole–blocking material, respectively. Fig. 5 present the energy levels and configurations of 2a and 2b based OLED devices. As shown in Fig.6, the electroluminescence (EL) spectra of 2a and 2b based devices are identical with peaks at 424 nm and 444 nm, and the CIE coordinates of (0.177, 0.117) and (0.160, 0.117), respectively. Furthermore, 2a and 2b based devices have high color purity with small width at half maximum of 65 and 73 nm, respectively. As revealed in Fig.6, 2a and 2b based devices exhibited excellent EL performance with high current efficiency (CE) values of 1.36 and 1.62 cd A-1, maximum EQE of 1.9% and 2.4%, luminance of 2684 and 3703 cd m-2, respectively. 2b based device exhibited higher brightness and larger EL efficiency than 2a based device. The results show that substitutions at 2,7–positions of N–phenylcarbazole may be beneficial for the reduction of the aggregation and π–π* stacking in film, thereby improving the EL performance of the OLEDs, which are in good agreement with the observations in the Φf in film. As shown in Fig. 7, the EL spectra of 2a and 2b based devices show little change under different driving voltages. The operating voltages of non–doped blue devices based on 2a and 2b were 5.6 and 6.3 V (1 cd m −2), respectively. In comparison with 2b, the 2a based device has low turn–on voltage due to its high–lying HOMO energy levels, which better matched with the HOMO energy level of mCP, thereby facilitated holes injection of the device.

4. Conclusions In summary, we have designed and synthesized a series of deep blue emitters based on N–phenylcarbazole and diphenyl sulfone for nondoped deep blue OLEDs. Their thermal, 8

electrochemical and photo–physical properties have been investigated in detail by TGA, DSC, UV–vis, CV, fluorescence spectroscopic measurements, and DFT calculations. The EL spectra of 2a and 2b based devices are identical with peaks at 424 and 444 nm, and the CIE coordinates of (0.177, 0.117) and (0.160, 0.117), respectively. Furthermore, 2a and 2b based devices have high color purity with small width at half maximum of 65 and 73 nm, respectively. Based on this study, we present a novel strategy for the design of deep blue emitter for high–color–purity nondoped OLEDs.

Acknowledgments. This work was supported by the National Natural Science Foundation of China (Grant No.51103023 and 21173042), the National Basic Research Program of China (Grant No.2013CB932900), the science and technology support program (industry) project of Jiangsu province (BE 2013118), the Research Fund for the Practice Innovation Training Program Projects of Jiangsu College Students (NO.SCX1519), the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (No.14KJB150003) and Jiangsu Planned Projects for Postdoctoral Research Funds (No. 1501011A).

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11

Figure Captions Scheme 1.

Synthesis of bis-(4-benzenesulfonyl-phenyl)-9-phenyl-9H-carbazoles.

Table 1.

Photophysical data of 2a and 2b in different solvents at room temperature.

Table 2.

Cyclic voltammetry, photoluminescence properties and theoretically calculated energy levels of 2a and 2b.

Table 3.

EL characteristics of nondoped deep blue OLEDs based on 2a and 2b.

Fig. 1.

TGA traces and DSC thermograms of the 2a and 2b.

Fig. 2.

(a) UV-Vis absorption and PL spectra of 2a and 2b in CH2Cl2; (b)The phosphorescence spectra of 2a and 2b measured from a frozen 2-methyltetrahydrofuran matrix at 77K.

Fig. 3.

CV curves of 2a and 2b in CH2Cl2 solutions (10 -3M).

Fig. 4.

Optimized geometries and calculated HOMO and LUMO density maps for 2a and 2b.

Fig. 5.

Energy level diagram of the devices based on 2a and 2b.

Fig. 6.

(a) Current-voltage, (b) Luminance-voltage characteristics, and (c) Current- efficiencyluminance of the devices. (d) The normalized EL spectra of devices at a driving voltage of 10 V.

Fig. 7.

EL spectra of 2a and 2b based devices at different voltages.

Table 1 Photophysical data of 2a and 2b in different solvents at room temperature. compound

solvent

Δf

UV (nm)

PL (nm)

Δν (cm-1)

2a

n-hexane

0

343

377

2629

Toluene

0.013

340

381

3165

CH2Cl2

0.217

343

395

3838

DMF

0.274

345

431

5784

12

2b

MeOH

0.307

343

434

6113

n-hexane

0

334

409

5490

Toluene

0.013

338

413

5373

CH2Cl2

0.217

339

411

5168

DMF

0.274

341

437

6442

MeOH

0.307

336

434

6720

Table 2 Cyclic voltammetry, photoluminescence properties and theoretically calculated energy levels of 2a and 2b.

a

2a

2b

LUMO(eV) a

-1.58

-1.88

LUMO(eV)

-2.40

-2.58

HOMO(eV)a

-5.74

-5.88

HOMO(eV)

-5.62

-5.72

Eg(eV) a

4.16

4.00

Eg(eV)

3.22

3.14

Φf

0.38

0.59

Singlet(eV)a

3.63

3.44

Triplet(eV)a

2.94

2.71

Triplet(eV)

3.00

2.86

ΔEST (eV)a

0.69

0.73

Theoretically calculated energy levels of 2a and 2b.

Table 3 EL characteristics of nondoped deep blue OLEDs based on 2a and 2b. V (v)

LE (cd A-1)

Max luminance (cd m-2)

Max EQE

λEL (nm)

CIE (x, y)

2a

5.6

1.36

2684

1.9%

424

(0.177, 0.117)

2b

6.3

1.62

3703

2.4%

444

(0.160, 0.117)

13

1.0 2a 2b

2a 2b

0.6 0.4

Tg

Exothermic

Weight loss

0.8

0.2

Tg 90

0.0

100

100

110 120 o Temperature ( C)

200

300

130

400

500

600

o

Temparature ( C)

Fig. 1. TGA traces and DSC thermograms of the 2a and 2b.

(a)

1.0

2a UV-vis 2b UV-vis 2a PL 2b PL

Normalized intensity

0.8 0.6 0.4 0.2 0.0 250

300

350 400 450 500 Wavelength (nm)

14

550

600

(b)

1.0

Normalized intensity

0.8

2a 2b

0.6 0.4 0.2 0.0 400

450

500

550

600

Wavelength (nm)

Fig. 2. (a) UV-Vis absorption and PL spectra of 2a and 2b in CH2Cl2; (b)The phosphorescence spectra of 2a and 2b measured from a frozen 2-methyltetrahydrofuran matrix at 77K.

Current

2a 2b

0.8

1.0

1.2

1.4 +

Potential (V vs Fc/Fc )

Fig. 3. CV curves of 2a and 2b in CH2Cl2 solutions (10-3 M).

15

1.6

Fig. 4. Optimized geometries and calculated HOMO and LUMO density maps for 2a and 2b.

-2.30 eV

-2.40 eV-2.40 eV

-2.58eV -2.80 eV

Ca/Ag NBP

mCP mCP

2a

2b

TPBI

ITO

-5.50 eV

-5.62 eV -5.72 eV -5.90 eV -6.20 eV

Fig. 5. Energy level diagram of the devices based on 2a and 2b. (b)

500

2a

1000

2a 2b

2b

Luminance (cd/m )

2

Current Density (mA/cm )

(a) 600

2

400 300 200 100 0 0

4

8

12

16

100

10

1

0

4

Voltage (V)

8 Voltage (V)

16

12

16

(d) 1.0 2a 2b

EL intensity(a.u.)

-1

Current Efficiency (cd A )

(c) 2

1

0.6 0.4 0.2

0 0

1000

2000

3000

0.0 300

4000

2

Luminance (cd/m )

Fig.

2a 2b

0.8

6.

(a)

Current-voltage,

(b)

400 500 Wavelength (nm)

Luminance-voltage

600

characteristics,

700

and

(c)

Current

efficiency-luminance of the devices. (d) The normalized EL spectra of devices at a driving voltage of 10 V. (b) 1.0

(a) 1.0 EL of 2a at 12 V EL of 2a at 10 V EL of 2a at 8 V

0.6 0.4 0.2 0.0 300

EL of 2b at 12 V EL of 2b at 10 V EL of 2b at 8 V

0.8

EL intensity (a.u.)

EL intensity (a.u.)

0.8

0.6 0.4 0.2 0.0

400

500 600 Wavelength (nm)

700

300

400

500 600 Wavelength (nm)

700

Fig. 7. EL spectra of 2a and 2b based devices at different voltages.

17

82.3% O O B B O O

82.1% 84.6% I Br

S

SH

Cu2O K2CO3 1,10-phenroline

Br

H2O2 HOAc reflux

1a

O S O

Br

Pd(dppf)Cl2 KOAc 1,4-dioxane N2, reflux

1b

DMSO N2, 100 oC

O B O

1c

O

O S

S

O Br

O S O

O

Br N

N

1c

45.2%

Pd(PPh3)4 K2CO3 Br

Br N

toluene/ethanol/H2O N2, reflux

2a O S O

N

O S O 48.8%

2b

Scheme 1. Synthesis of bis-(4-benzenesulfonyl-phenyl)-9-phenyl-9H-carbazoles.

18