Aromatic S-Heterocycle and Fluorene Derivatives as Solution

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Aromatic S-Heterocycle and Fluorene Derivatives as Solution-Processed Blue Fluorescent Emitters: StructureProperty Relationships for Different Sulfur Oxidation States Liang Yao, Shuheng Sun, Shanfeng Xue, Shitong Zhang, Xiaoyan Wu, Huanhuan Zhang, Yuyu Pan, Cheng Gu, Fenghong Li, and Yuguang Ma J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp403463k • Publication Date (Web): 10 Jun 2013 Downloaded from http://pubs.acs.org on June 14, 2013

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The Journal of Physical Chemistry

Aromatic S-Heterocycle and Fluorene Derivatives as Solution-Processed Blue Fluorescent Emitters: Structure-Property Relationships for Different Sulfur Oxidation States Liang Yao, Shuheng Sun, Shanfeng Xue, Shitong Zhang, Xiaoyan Wu, Huanhuan Zhang, Yuyu Pan, Cheng Gu, Fenghong Li and Yuguang Ma* State Key Lab of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun, 130012, P. R. China

* To whom correspondence should be addressed. E-mail: [email protected].

ACS Paragon Plus Environment

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ABSTRACT: Based on the two systems of dibenzothiophene (DBT) and phenothiazine (PTZ), blue fluorescent small molecules (D1, D2, P1 and P2) constructed by aromatic S-heterocycles with different sulfur oxidation states (sulfides or sulfones) were synthesized. The property discrepancies caused by the difference of sulfur oxidation states were investigated, including that of thermal stability, photophysical property, electrochemistry behavior, carrier transport property and electroluminescence performance. Higher sulfur oxidation states induced significantly better electron injection and transport abilities, which is potentially beneficial to enhance the performance of solution-processed (SP) small-molecule (SM) devices. Single-layer SP OLEDs with the structures of ITO/PEDOT:PSS (40 nm)/emitters (80 nm)/CsF (1.5 nm)/Al (120 nm) were fabricated. DBT system exhibited high electroluminescence performances with maximum external quantum efficiency (EQE) of 1.7 % (D1) and 2.6 % (D2), respectively, which are among the best of the undoped deep blue and blue SP SM single-layer devices reported so far.

KEYWORDS: aromatic S-heterocycle, structure-property relationships, solution-processed, blue fluorescent small-molecule


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■Introduction Organic light-emitting diodes (OLEDs) are being developed extensively because of their promising applications in full-color flat panel displays and solid-state lighting technologies.1-7 Considerable attention have been focused on solution-processed small-molecule OLEDs (SP SM OLEDs), since they have the potential to combine the advantage of small molecules and solution processing. Small molecules possess well-defined structure and could be purified facilely, and solution processing affords simple fabrication and the possibility of low-cost large area electronics.8-11 Nevertheless, highly-efficient blue small-molecule OLEDs based merely on solution processing are rarely reported. One of the reasons can be ascribed that the intrinsically wide band-gap of blue emitters hampers charge injection and balance and consequently impairs the efficiency of blue OLEDs.12-20 Moreover, solution processing limits fabrication of composite device structures because the solvent used for subsequent layer can dissolve or damage the previous layers.10 An effective approach toward highly efficient blue SP SM OLEDs is improving the carrier injection and transport abilities through rational molecular design. It is relatively easy to reduce hole-injection barriers through introducing typical hole-transport moieties such as carbazole and triphenylamine.21,22 As for decreasing the lowest unoccupied molecular orbital (LUMO) levels of the materials and matching the function of the cathode, one of the common methods is to incorporate strong electron-withdrawing substituents in the chromophores, such as cyano or fluoro group.23-25 However, in most cases, the introduction of the strong electron-withdrawing groups always causes the charge-transfer (CT) emissions and results in a large red-shift of the fluorescent spectra. In addition, sometimes with the LUMO levels decreasing the highest occupied molecular orbital (HOMO) levels reduce simultaneously, which may increase the barrier of hole-injection. Thus, how to reduce the LUMO levels and maintain efficient blue emissions simultaneously is an attractive challenge.


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Aromatic S-heterocycles have been widely investigated and explored in organic electronics for their diverse chemical structures and properties.26-32 Among them, dibenzothiophene (DBT) and phenothiazine (PTZ) have attracted considerable research interests due to their fundamental optoelectronic properties. DBT has a rigid and planar molecular structure with wide band-gap, and the sulfur atom in DBT offers the potential for short S-S contacts that could facilitate effective charge transport.33,34 Compared to DBT, PTZ contains an additional sp3-hybridized nitrogen atom, which provides PTZ with powerful electron-donating ability and certain flexibility.35,36 Through reduction and oxidation reactions, aromatic S-heterocycles with various sulfur oxidation states (such as sulfide, sulphoxide and sulfone) could be easily achieved. In the meanwhile, their chemical properties including the energy levels and carrier-transport abilities are adjusted with the sulfur oxidation state changing.37,38 In our previous work, we have developed a bipolar molecular system for highly efficient SP SM OLEDs, and all the materials exhibited favorable electroluminescent performance.39-41 In this paper, aromatic S-heterocycles (DBT and PTZ) and their sulfone forms were used to construct solution-processed blue fluorescent small molecules (D1, D2, P1 and P2). The rigid backbones based on the aromatic S-heterocycles and fluorene oligomers endow the molecules with high luminous efficiency and good thermal stability. The introduction of the peripheral alkyl-linked carbazole groups was based on following considerations: 1) the flexible alkyl chains ensure excellent solubility and film-forming ability so that the final compounds could be fabricated by solution-processed technique; 2) peripheral carbazole groups could guarantee acceptable hole injection and transport abilities when changing the sulfur oxidation state leads to the reducing of the HOMO levels of the chromophores. The property discrepancies caused by different sulfur oxidation states have been investigated in detail. Compared to sulfides, the sulfones with higher sulfur oxidation states possess better thermal stabilities and significantly lower LUMO levels


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which endow better electron injection and transport abilities. With the sulfur oxidation state increasing, the emission spectra exhibited a red-shift in DBT system while a blue-shift in PTZ system. Single-layer devices were fabricated by solution-processing. DBT system showed high electroluminescent performance with maximum external quantum efficiency (EQE) of 1.7 % and 2.6 %, reaching the peak level of the undoped deep blue and blue SP SM single-layer OLEDs.

■Experimental Section General information. All the reagents and solvents used for the synthesis were purchased from Aldrich and Acros companies and used without further purification. 1H and

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C NMR spectra

were recorded on a Bruker AVANCE 500 spectrometer at 500 MHz and 125 MHz respectively, using tetramethylsilane (TMS) as the internal standard. The compounds were characterized by a Flash EA 1112, CHNS-O elemental analysis instrument. The MALDI-TOF-MS mass spectra were recorded using an AXIMA-CFRTM plus instrument. UV-vis and fluorescence spectra were recorded on an AXIMA-CFRTM plus instrument. Shimadzu UV-3100 spectro-photometer and a Shimadzu RF-5301PC spectro-photometer used 1 cm path length quartz cells, respectively. Thermal gravimetric analysis (TGA) was undertaken on a PerkinElmer thermal analysis system at a heating rate of 10 °C/min and a nitrogen flow rate of 80 mL/min. Differential scanning calorimetry (DSC) analysis was carried out using a NETZSCH (DSC-204) instrument at 10 °C min-1 while flushing with nitrogen. Electrochemical measurements were performed using a BAS 100W Bioanalytical System: a glass-carbon disk electrode was used as the working electrode, a Pt wire as the counter electrode, Ag/Ag+ as the reference electrode and Bu4NPF6 (0.1 M) in N,Ndimethylformamide (DMF) or CH2Cl2 as the electrolyte. Device Fabrication. Indium-tin oxide (ITO) coated glass with a sheet resistance of 15-20 Ω/square was used as the substrate. The substrate was prepatterned by photolithography to give


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an effective device size of 19 mm2. Then cleaned in an ultrasonic bath with acetone, detergent, deionized water, and isopropanol in sequence, and dried in oven. After oxygen plasma cleaning for 4 min, a 40 nm-thick PEDOT:PSS layer was first spin-coated on the ITO substrate from water solution and then dried by baking in a vacuum oven at 80 °C overnight. The emissive layers from P-xylene solutions (20 mg/mL) were spin-coated onto the PEDOT:PSS and annealed at 60 °C for 20 min in the glovebox. The emissive layers were about ~80 nm. The thicknesses of these organic films were determined by the surface profiler (Tencor Alfa-Step 500). Finally a 1.5 nm thick the Cesium fluoride (CsF) film and a 120 nm thick aluminum (Al) film were evaporated with a shadow mask to form the top electrode, at a base pressure of 3 × 10−4 Pa. The preparation of ZnO thin films and ZnO sol-gel solution with ZnO nanoparticles were spin-coated and heated. For the preparation of ZnO sol-gel solutions, zinc acetate [Zn(CH3COO)2·2H2O] was dissolved in 2-methoxyethanol solution containing ethanolamine as a stabilizer. Concentration of zinc acetate was 0.75 M. This solution was stirred at 60 °C for 30 min to yield a clear and homogeneous solution, which served as the coating solution. The surface of spin-coated ZnO thin films using zinc acetate solution on ITO could be converted into a ridge structure by heating to 350 °C with a constant heating rate (11 °C /min). The device fabrication, except PEDOT:PSS coating, was carried out in a nitrogen atmosphere glove-box (Vacuum Atmosphere Co.) containing less than 10 ppm oxygen and moisture. The current density-luminance-voltage characteristics were measured by a Keithley 236 source measurement unit. The EL spectra and CIE coordination characteristics were measured by using a PR-705 Spectroscan spectrometer. Synthesis. M1, M3 and M5 were synthesized according to the reported procedure.39-43 The synthesis of M2 and M4 is depicted in supporting information S1. Described below are the synthesis and purification procedures for final products.


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3,7-bis(9,9,9',9'-tetrakis(6-(9H-carbazol-9-yl)hexyl)-9H,9'H-[2,2'-bifluoren]-7yl)dibenzo[b,d] thiophene (D1): 3,7-dibromo-dibenzothiophene (M2) (68 mg, 0.2 mmol) and 4,4,5,5-tetramethyl2-(9,9,9',9'-tetrakis(6-(9H-fluoren-9-yl)hexyl)-9H,9'H-[2,2'-bifluoren]-7-yl)-1,3,2-dioxaborolane (M5) (640 mg, 0.44 mmol) were added to a solution of Na2CO3 (2.0 M) in a 3:2 (V/V) mixture of toluene/water. Pd(Pph3)4 (14 mg) was used as the catalyst. The reaction mixture was stirred at 85-90 °C for 2 days under a nitrogen atmosphere. After cooling to room temperature, dichloromethane was added and the water phase was extracted. The organic phase was collected and dried with anhydrous magnesium sulfate. After evaporation of the solvent, the residue was purified via chromatography by petroleum ether/dichloromethane (1: 2). A white solid was obtained (363 mg, 64%). 1H NMR (500 MHz, CDCl3, δ): 8.25 (d, J = 8.2Hz, 2 H), 8.16 (s, 2H), 8.05 (m, 16H), 7.78 (m, 12H), 7.65 (m, 10H), 7.39 (m, 22H), 7.26 (m, 16H), 7.20 (m, 16H), 4.13 (m, 16H), 2.02 (m, 16H), 1.69 (m, 16H), 1.12 (m, 32H), 0.69 (m, 16H);

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C NMR (125 MHz,

CDCl3, δ): 151.5, 151.5, 151.2, 150.6, 140.7, 140.6, 140.5, 140.5, 140.3, 140.0, 139.7, 134.4, 127.2, 127.0, 126.5, 126.4, 126.2, 125.5, 124.2, 122.8, 122.8, 121.9, 121.6, 121.2, 121.1, 120.3, 120.1, 119.9, 118.7, 108.6, 55.3, 55.1, 42.8, 40.5, 40.3, 29.8, 29.7, 28.8, 28.7, 26.9, 26.8, 23.8, 23.7; MALDI-TOF MS (mass m/z): 2835.2 [M+]. Anal. calcd for C208H192N8S: C 88.09, H 6.82, N 3.95, S 1.13; found: C 88.03, H 6.70, N 3.68, S 1.25. 3,7-bis(9,9,9',9'-tetrakis(6-(9H-carbazol-9-yl)hexyl)-9H,9'H-[2,2'-bifluoren]-7-yl)dibenzo[b,d] thiophene5,5-dioxide (D2): Following the procedure described above, we obtained D2 as a light green solid (385 mg, 72%). 1H NMR (500 MHz, CDCl3, δ): 8.14 (s, 2H), 8.03 (m, 16H), 7.86 (d, J = 8.1 Hz, 2H), 7.77 (m, 10H), 7.60 (m, 12H), 7.37 (m, 18H), 7.23 (m, 20H), 7.15 (m, 16H), 4.10 (t, J = 7.1 Hz, 16H), 1.97 (m, 16H), 1.63 (m, 16H), 1.09 (m, 32H), 0.67 (m, 16H); 13C NMR (125 MHz, CDCl3, δ): 151.8, 151.6, 151.3, 150.6, 144.0, 141.5, 141.0, 140.7, 140.6, 140.4, 140.2, 139.6, 138.7, 137.6, 132.6, 130.0, 127.3, 127.0, 126.5, 126.2, 125.6, 122.9, 122.8, 122.0, 121.2,


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121.2, 120.6, 120.5, 120.3, 120.1, 119.9, 118.7, 108.6, 55.4, 55.1, 42.8, 40.5, 40.3, 29.8, 29.7, 28.8, 28.7, 26.9, 26.8, 23.8, 23.7; MALDI-TOF MS (mass m/z): 2868.2 [M+]. Anal. calcd for C208H192N8O2S: C 87.11, H 6.75, N 3.91, S 1.12; found: C 86.98, H 6.64, N 3.91, S 1.29. 10-hexyl-3,7-bis(9,9,9',9'-tetrakis(6-(9H-carbazol-9-yl)hexyl)-9H,9'H-[2,2'-bifluoren]-7-yl)10H-phenothiazine (P1): Following the procedure described above, we obtained P1 as a light green solid (218 mg, 58%). 1H NMR (500 MHz, CDCl3, δ): 8.05 (m, 16H), 7.76 (m, 6H), 7.59 (m, 16H), 7.39 (m, 20H), 7.26 (m, 18H), 7.19 (m, 18H), 6.97 (s, 2H), 4.10 (m, 16H), 3.95 (m, 2H), 1.98 (m, 18H;), 1.64 (m, 18H), 1.40 (m, 4H), 1.11 (m, 32H), 0.94 (m, 3H), 0.91 (m, 16H); 13

C NMR (125 MHz, CDCl3, δ): 151.2, 150.5, 140.7, 140.4, 140.3, 127.2, 127.0, 126.0, 125.5,

122.8, 122.7, 120.3, 120.0, 119.9, 118.7, 108.6, 55.2, 55.0, 42.8, 40.4, 40.3, 29.8, 29.7, 28.7, 26.9, 23.8, 23.6; MALDI-TOF MS (mass m/z): 2935.8 [M+]. Anal. calcd for C214H205N9S: C 87.57, H 7.04, N 4.30, S 1.09; found: C 87.78, H 6.84, N 4.04, S 1.29. 10-hexyl-3,7-bis(9,9,9',9'-tetrakis(6-(9H-carbazol-9-yl)hexyl)-9H,9'H-[2,2'-bifluoren]-7-yl)10H-phenothiazine 5,5-dioxide (P2): Following the procedure described above, we obtained P2 as a white solid (420 mg, 68%).1H NMR (500 MHz, CDCl3, δ): 8.49 (d, J = 2.0 Hz, 2H), 8.05 (m, 16H), 7.90 (d, J = 8.9 Hz, 2H), 7.78 (m, 8H), 7.63 (m, 12H), 7.39 (m, 24H), 7.26 (m, 16H), 7.17 (m, 16H), 4.12 (m, 16H), 2.01 (m, 16H), 1.67 (m, 16H), 1.11 (m, 32H), 0.67 (m, 16H); 13C NMR (125 MHz, CDCl3, δ): 151.7, 151.5, 151.2, 150.6, 140.7, 140.6, 140.5, 140.3, 139.8, 139.4, 137.6, 135.4, 131.9, 127.2, 127.0, 126.4, 126.2, 125.9, 125.5, 124.3, 122.8, 122.7, 121.5, 121.1, 121.0, 120.5, 120.3, 120.2, 120.1, 119.9, 118.7, 118.6, 116.5, 108.6, 55.3, 55.0, 42.8, 40.5, 40.3, 29.7, 29.7, 28.7, 26.8, 23.8, 23.6; MALDI-TOF MS (mass m/z): 2967.5 [M+]. Anal. calcd for C214H205N9O2S: C 86.63, H 6.96, N 4.25, S 1.08; found: C 86.42, H 6.70, N 4.37, S 1.15.


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■Results and Discussion Synthesis and Characterization. Scheme 1 illustrates the synthetic procedures for the compounds.

3,7-dibromo-dibenzothiophene-S,S-dioxide

(M1),

3,7-dibromo-10-n-

hexylphenothiazine (M3) were prepared according to literature procedures.42,43 The reduction reaction between M1 and LiAlH4 yielded 3,7-dibromo-dibenzothiophene (M2) in 59% yield. M3 was oxidized by excess of 3-chloroperoxybenzoic acid, and converted to its sulfone form, 3,7dibromo-10-n-hexylphenothiazine-S,S- dioxide (M4). 4,4,5,5-tetramethyl-2-(9,9,9’,9’-tetrakis(6(9H-fluoren-9-yl)hexyl)-9H,9’H-[2,2’-bifluoren]-7-yl)-1,3,2-dioxaborolane

(M5)

was

synthesized according to our previous reported procedure.39 The S-heterocyclic conjugated compounds were prepared through Suzuki cross-coupling reactions between M1-M4 and M5. 1H and 13C NMR, MS, and elemental analysis were employed to confirm the chemical structures of the final compounds. Thermal Properties. The thermal properties of the materials were evaluated by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Figure 1 and Table 1). The four compounds exhibited high thermal decomposition temperatures (Td, corresponding to 5% weight loss) in the range of 393-434 °C. DSC scans showed that no crystallization or melting point was observed for all the compounds from 0 to 300 °C. Their glass-transition temperatures (Tg) ranged from 101 to 114 °C, which is acceptable for the small molecules with long alkyl chains. Notably, Td and Tg of the sulfones were enhanced compared to that of the sulfide forms, indicating that the introduction of the oxygen atom increased the rigidity of the backbones. Photophysical Properties. Figure 3 exhibits the absorption and emission spectra of the compounds as thin films on quartz substrates at room temperature. The photophysical data are summarized in Table 1. In the absorption spectra, for all the compounds, there are three similar peaks at 266, 298 and 349 nm, which are attributed to the absorption of carbazole groups (266,


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298 nm) and fluorene oligomers (349 nm), respectively.44,45 The individual peaks at 372, 392, 384 and 372 nm are assigned to the π-π∗ transitions of the backbones containing different aromatic S-heterocyclic groups. As shown in Figure 3, the emission peaks of the thin films (D1, D2, P1, P2) are located at 443 nm, 458 nm, 491 nm, 418 nm, respectively. According to our previous work39, energy transfer from the separated carbazole to the conjugated backbones could be occurred. Otherwise, the emission of carbazole will be existed in the fluorescent spectra of the green or red compounds. In this work, since the band gap of separated carbazole is wider than difluorene connected to the four S-containing moieties, the fluorescence of the compounds should be originated from the backbones. The emission spectra of the two systems showed quite different changing trends with the sulfur oxidation state increasing. The emission peaks exhibited a red-shift of 15 nm in DBT system, while a blue-shift of 73 nm was observed in PTZ system. Generally, when an electron-withdrawing group is introduced in the chromophore, the emission spectra of the compound prefer to move to longer wavelengths because of the strong CT effects.24,25 The red-shift of the fluorescent emission in DBT system is mainly attributed to the CT effects induced by strong electron-withdrawing ability of the sulfone. Structural optimizations by using density functional theory (DFT) calculations at B3LYP/6-31G demonstrate that except CT effect a configural effect was existed in PTZ system. (Figure 2) At ground state both PTZ and PTZSO possessed the nonplanar "butterfly" structure with the dihedral angle of the N atom side (θN) of 140.3° and 147.5°, respectively, which played the role of disrupting the conjugation. At excited state the configuration of PTZ was rotated to a more planar structure with θN of 162.1° while PTZSO did not exhibit apparent configural changes, so the conjugated length was different at excited state. The blue-shift of the fluorescent emission in PTZ system may result from the configural distinction at excited state.


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Electrochemical Properties. Electrochemical properties of the materials were investigated by cyclic voltammetry. (Figure 4) The oxidation and reduction cycles were measured in acetonitrile/dichloromethane

(V/V=3/2)

and

dimethylformamide,

respectively.

Tetrabutylammonium hexafluorophosphate was utilized as the supporting electrolyte. The reductive and oxidative onset potentials were used to estimate the orbital energies and band gaps (Table 1). Compared to those of sulfides, the LUMO levels of sulfones (D2 and P2) were significantly reduced by 0.40 eV and 0.10 eV, respectively. The results indicated higher sulfur oxidation states are beneficial to decrease the electron injection barriers. P1 exhibited the best hole-injection ability with the HOMO level of -4.92 eV due to the strong electron-donating ability of PTZ.35,36 The HOMO levels of the other compounds were located at -5.43 eV, which could be assigned to the carbazole groups.38 It is obvious that the electron-donating ability of P2 has a significant decline compared to that of P1, and the reason could be ascribed that higher sulfur oxidation state of PTZSO reduces the oxidation potential. Thus, the peripheral carbazole groups could provide favourable hole injection ability when the HOMO level of the compounds decreased due to the increasing of the sulfur oxidation states. Charge Transport Properties. Charge transport properties were investigated by hole-only and electron-only devices, and the current density-voltage curves are shown in Figure 5. The device structures were ITO/PEDOT:PSS (40 nm)/emitters (110 nm)/Au (60 nm) for hole only and ITO/ZnO (30 nm)/emitters (110 nm)/Ca (20 nm)/Al (120 nm) for electron only.46 The hole and electron current densities depend on the energy barriers for charge injection and transport properties. Compared to sulfides, the sulfones exhibit higher electron current density and lower hole current density. The results indicate that higher sulfur oxidation states are beneficial to improve the electron injection and transport abilities. On the contrary, increasing the sulfur oxidation states has a negative effect on the hole injection and transport properties. In addition, it


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is obvious that the hole and electron current density of the DBT system is higher than that of the PTZ system. The origin of the better carrier transport ability is the more planar and rigid structure of DBT. Electroluminescent Devices. Single-layer devices with the configuration of ITO/PEDOT:PSS (40 nm)/emitters (80 nm)/CsF (1.5 nm)/Al (120 nm) were fabricated by spin-coating from the pxylene solution of the materials. The emission layers were annealed at 60 °C in the glovebox. All the small molecules showed the fairly smooth surface morphologies with root mean square (rms) roughness around 0.5 nm, and after the annealing no obvious morphology changes were observed. (Figure S6, S7) The EQE-current density characteristics and EL spectra under 10.5 mA cm-2 are presented in Figure 6. The device characteristics are summarized in Table 2. The devices based on D1 and D2 showed the maximum EQE of 1.7 % and 2.6 % with CIE coordinates of (0.17, 0.09) and (0.17, 0.22), respectively. The EQE declined slightly with the increase of current density, indicating that the materials and devices have good stabilities. At a high current density of 100 mA cm-2, EQE of 1.1 % and 1.7 % are still retained for the device based on D1 and D2, respectively. As shown in Figure 6, the performance of D2 device is much better, and the main reason for the improvement is that: 1) D2 device has a significantly lower electron injection barrier and higher electron mobility due to the stronger electron affinities; 2) D2 has higher photoluminescence quantum yield in the solid-state film (Table 1). To the best of our knowledge, the performance of D1 and D2 device is among the best undoped deep-blue and blue SP SM single-layer OLEDs respectively.14-18 In PTZ system, the devices of P1 and P2 displayed the EQE of 2.1 % and 1.2 % with CIE coordinates of (0.22, 0.51) and (0.21, 0.19), respectively. The EL spectrum of the device based on P2 was not stable (Figure S9), which may be due to the intermolecular excimer induced by the aggregation of emissive molecules. To explain the origin of the instable EL spectra and obtain stable blue emission, the non-doped film


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of P2 was replaced by the blending film of PVK: PBD: P2 (60%: 30%: 10%) as the emissive layer. The doped device exhibited a deep-blue emission with excellent stability at various current density (Figure S10), and the corresponding CIE coordinates were (0.17, 0.08). Although P2 showed unstable EL emission in non-doped device, the adjustment of oxidation states offers an rational approach to design deep blue emitters with the relatively lower LUMO levels and wide band-gap. In addition, the DBT system exhibited higher EL efficiency compared with the PTZ system. This phenomenon can mostly be attributed to the more rigid and fixed five-membered heterocyclic structure, which could suppress the intramolecular vibration and reduce the energy consumption. ■Conclusions Based on the two systems of DBT and PTZ, blue fluorescent small molecules (D1 and D2, P1 and P2) constructed by aromatic S-heterocyclic centre with different sulfur oxidation states were designed and synthesized. Compared to sulfides, the sulfones with higher sulfur oxidation states possess better thermal stabilities and significantly lower LUMO levels which endow better electron injection and transport abilities. With the sulfur oxidation state increasing the emission spectra exhibited a red-shift in DBT system while a blue-shift in PTZ system. The reason is probably due to the distinction of conjugation length between PTZ and PTZSO at excited state. The DBT system showed higher electroluminance efficiency based on the SP SM single-layer OLEDs because of the more planar and rigid molecular structures of DBT. The performances of the devices based on D1 and D2 with EQE of 1.7 % and 2.6 %, respectively, are among the best of the deep blue and blue devices with similar device structures reported so far. The studies on the adjustment of oxidation states provide an approach for the design of highly efficient blue materials with favorable carrier injection and transport properties.


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ASSOCIATED CONTENT Supporting Information. The synthesis and purification procedures for M2 and M4. Absorption and photoluminescence spectra in THF solutions. The luminance-current densityvoltage characteristics of the devices. The EQE-current density characteristics of the device with the emissive layer of PVK: PBD: P2 (60%: 30%: 10%). The EL spectra of the doping device under the different currents. This information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We are grateful for financial support from National Science Foundation of China (grant number 91233113), National Basic Research Program of China (973 Program grant number 2013CB834705), and Graduate Innovation Fund of Jilin University (grant number 20121044).


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(33) Ren, Z. J.; Sun, D. M.; Li, H. H.; Fu, Q.; Ma, D. G.; Zhang, J. M.; Yan, S. K. Synthesis of Dibenzothiophene-Containing Ladder Polysilsesquioxane as a Blue Phosphorescent Host Material. Chem. Eur. J. 2012, 18, 4115-4123. (34) Jin, E.; Du, C.; Wang, M.; Li, W. W.; Li, C. H.; Wei, H. D.; Bo, Z. S. DibenzothiopheneBased Planar Conjugated Polymers for High Efficiency Polymer Solar Cells. Macromolecules 2012, 45, 7843-7854. (35) Kulkarni, A. P.; Kong, X. X.; Jenekhe, S. A. High-Performance Organic Light-Emitting Diodes Based on Intramolecular Charge-Transfer Emission from Donor–Acceptor Molecules: Significance of Electron-Donor Strength and Molecular Geometry. Adv. Funct. Mater. 2006, 16, 1057-1066. (36) Sang, G. Y.; Zou, Y. P.; Li, Y. F. Two Polythiophene Derivatives Containing Phenothiazine Units: Synthesis and Photovoltaic Properties. J. Phys. Chem. C 2008, 112, 1205812064. (37) Liu, J.; Zou, J. H.; Yang, W.; Wu, H. B.; Li, C.; Zhang, B.; Peng, J. B.; Cao, Y. Highly Efficient

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Stable Deep-Blue Electroluminescence, Good Hole Injection, and Transporting Materials with Very High Tg. Adv. Funct. Mater. 2009, 19, 3987-3995. (46) Lu, L. P.; Kabra, D.; Johnson, K.; Friend, R. H. Charge-Carrier Balance and Color Purity in Polyfluorene Polymer Blends for Blue Light-Emitting Diodes. Adv. Funct. Mater. 2012, 22, 144-150.


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Table 1. Optical, thermal and electrochemistry properties of the materials. Compound

λabs (nm)

λPL,max (nm) Solutiona Filmb

QPLc

Td d

Tge

LUMOf

HOMOf

Eg

(°C)

(°C)

(eV)

(eV)

(eV)

Solutiona

Filmb

D1

369

372

410

443

0.93 (0.36)

422

106

-2.44

-5.43

2.99

D2

382

391

444

458

0.84 (0.53)

434

114

-2.84

-5.43

2.59

P1

378

386

493

491

0.38 (0.19)

393

101

-2.24

-4.92

2.68

P2

369

374

406

418

0.74 (0.28)

422

107

-2.34

-5.43

3.09

a

Measured in dilute THF solutions (ca. 1.0×10-4 M). bMeasured in thin films on quartz plate by spin-coating. cFluorescence quantum yields in THF were measured with quinine sulfate (F = 0.546) as a standard, and the fluorescence quantum yields in solid-state films were measured on a quartz plate with an integrating sphere (in parenthesis). dDecomposition temperature (5 wt.% loss). eGlass-transition temperature. fHOMO levels were calculated using the oxidation onset potentials measured in acetonitrile/CH2Cl2 (glass carbon electrode with 0.1 M nBu4NPF6 as the supporting electrolyte). LUMO levels were calculated using the reduction onset potentials measured in DMF. The voltages are referenced to an Ag/Ag+ standard.


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Table 2. Electroluminescence characteristics of the devices

Von

a

Device (V)

Lmaxb

LEmaxc

EQE

(cd m-2)

(cd A-1)

(%)

d

J=100 mA cm-2 LE

EQE

(cd A-1)

(%)

CIEe (x, y)

D1

3.4

840

0.9

1.7

0.6

1.1

0.17, 0.09

D2

3.8

2730

2.8

2.6

1.8

1.7

0.17, 0.22

P1

4.0

5140

4.5

2.1

3.0

1.4

0.22, 0.51

P2

3.6

1110

1.2

1.2

0.8

0.8

0.21, 0.19

PVK: PBD: P2 (60%: 30%: 10%)

4.5

220

0.3

0.7

0.2

0.5

0.17, 0.08

a

Calculated with a luminance of 1 cd m-2; bMaximum luminance; cMaximal front viewing luminous efficiency in cd A-1; dMaximum external quantum efficiency; eMeasured at 10.5 mA cm-2. Device structure: ITO/PEDOT:PSS (40 nm)/emitters (80 nm)/CsF (1.5 nm)/Al (120 nm).


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Figure 1. (a) TGA thermograms of the materials recorded at a heating rate of 10 °C min-1; (b) DSC measurements of the materials recorded under at a heating rate of 10 °C min-1 flowing nitrogen atmosphere.

Figure 2. Top views and side views of DFT-optimized geometries of PTZ and PTZSO. The dihedral angle displayed in the figure is θN.


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Figure 3. Absorption and photoluminescence spectra of a) D1 and D2, b) P1 and P2 in solid film.

Figure 4. (a) Cyclic voltammograms of the materials; (b) Energy level diagrams for the devices.


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Figure 5. Current density-voltage characteristics of ITO/PEDOT:PSS (40 nm)/emitters (110 nm)/Au(60 nm) for hole only and ITO/ZnO (30 nm)/emitters (110 nm)/Ca (20 nm)/Al (120 nm) for electron only.

Figure 6. EQE-current density curves of the devices. The inset graph is the EL spectra of the device.


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Scheme 1. Synthetic route to the compounds. Reagents and conditions: (i) Lithium aluminium hydride, dry diethyl ether, 2 h; (ii) 3-chloroperoxybenzoic acid, dichloromethane, 0 °C, 4 h; (iii) Pd(PPh3)4, Na2CO3 (aq), toluene, 90 °C, 48 h.


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SYNOPSIS TOC. Blue fluorescent small molecules constructed by aromatic S-heterocycles with different sulfur oxidation states were synthesized, and the property discrepancies caused by different sulfur oxidation states were investigated in detail.


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Aromatic S-Heterocycle and Fluorene Derivatives as Solution

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