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Pyrene based conjugated materials: synthesis, characterization and electroluminescent properties† Jagadish K. Salunke,a Prashant Sonar,‡*b F. L. Wong,c V. A. L. Roy,*c C. S. Leec and Prakash P. Wadgaonkar*a In this work, three novel pyrene cored small conjugated molecules, namely 1,3,6,8-tetrakis(6-(octyloxy)naphthalene-2-yl)pyrene (PY-1), 1,3,6,8-tetrakis((E)-2-(6-(n-octyloxy)naphthalene-2-yl)vinyl)pyrene (PY-2)

Received 18th August 2014, Accepted 3rd September 2014

and 1,3,6,8-tetrakis((6-(n-octyloxy)naphthalene-2-yl)ethynyl)pyrene (PY-3) have been synthesized by Suzuki, heck and Sonogashira organometallic coupling reactions, respectively. The effects of single,

DOI: 10.1039/c4cp03693j

double and triple bonds on their optical, electrochemical, and thermal properties are studied in detail. These all materials are fluorescent and they have been used in organic light-emitting diodes (OLEDs) and

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their electroluminescent properties have been studied.

Introduction Since the discovery of conjugated materials and their exciting applications in organic light emitting diodes (OLEDs), organic photovoltaic (OPVs) and organic field effect transistor (OFETs) devices,1–12 the field of conjugated material is evolving with several classes of new p-conjugated structures. In the last decade, the branch called ‘‘organic electronics’’ has made significant progress using such materials and achieved several major milestones including comparable high charge carrier mobility in OFET (similar or even higher than amorphous silicon) and power conversion efficiency of more than 10% in OPV devices.13 p-Conjugated materials are interesting systems because of their tunable opto-electronic and charge transport properties.14,15 These materials can be either polymers or small molecules based on their polydisperse or uniform nature. Small molecules are a better choice in terms of batch to batch purity and easy purification. In addition, their structures can be precisely controlled by using specific building blocks with exact a

Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Dr. HomiBhabha Road, Pune 411008, India. E-mail: [email protected] b Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology, and Research (A*STAR), 3 Research Link, Singapore 117602 c Center of Super Diamond and Advanced Films, and Department of Physics and Materials Science City, University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong † Electronic supplementary information (ESI) available: NMR, MALDI-TOF MS, UV-vis, TGA and theoretical modelling data of compounds j1, j2, j3, PY-1, PY-2 and PY-3. See DOI: 10.1039/c4cp03693j ‡ Current address: School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), GPO Box 2434, Brisbane, QLD 4001, Australia. E-mail: [email protected]

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molecular masses. Among various applications, in OLEDs, the electroluminescence properties of light-emitting materials remains challenging, particularly in terms of stability, efficiency, and color purity for OLED devices.16,17 There is a continuous search for new classes of materials, which can fulfill the above described requirements. For OLED devices, the emission of conjugated materials in a visible region can be systematically tuned on the basis of their optical band gaps. This systematic optical band gap tuning from blue, green to red (primary colors) can be tailored by using an appropriate class of conjugated building blocks in the backbone. Again, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the final materials can be modulated with respect to the ionization potential (IP) and electron affinity of the conjugated building blocks used, while designing and synthesizing the new structures. Pyrene has proved to be a promising chromophore because of its excellent emission properties, strong absorption cross section and a long excited state lifetime. These features make pyrene a promising building block for making new emitting materials for OLED devices.18–22 Additionally, pyrene and its derivatives have shown several important properties for actual device fabrications, and those are: (a) solution processability, (b) intense luminescence efficiency, (c) high thermal stability, and (d) enhanced charge carrier mobility. In order to enhance the stability of the materials for OLED devices, it is essential to synthesize lightemitting materials with a high-energy level lying HOMO. From the large area fabrication point of view, an easy fabrication process is expected with fewer layers. Thus, in order to avoid deposition of both a hole-transporting layer and an emitting layer, it is imperative to use organic materials with both

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Fig. 1 (a) Chemical structures of pyrene cored dendritic conjugated materials PY1, PY2 and PY3 and (b) their emission under a UV-vis lamp.

light-emission and hole-transporting properties. In this way, pyrene, as a blue-light-emitting chromophore with good chemical stability and good charge carrier mobility appears to be a very attractive building block for light-emitting devices.23–28 The most simplistic approach to tune the absorption and emission characteristics of any new material is by introducing functional groups, such as pyrene, to the core of the fluorophore. In this paper, we report the synthesis and characterization (photophysical, electrochemical and thermal properties) of single, double and triple bond incorporated pyrene cored naphthalene substituted materials as shown in Fig. 1. These materials possess decent solution processability in common organic solvents, such as THF, chloroform and DCM, and exhibit deep blue to green emission under a UV lamp (see Fig. 1). Previously, there have been reports on the synthesis of tetrafunctionalized pyrene materials, but most of them are either single or triple bonded materials.18–22 To the best of our knowledge, there is no report wherein all single, double and triple bond based pyrene cored materials have been reported and systematically studied.

Experimental and procedure Materials Bis(pinacolato)diborane, all palladium catalysts, triethoxyvinylsilane, trimethylsilylacetylene, triphenylphosphine and solvents were purchased from Sigma Aldrich and were used without further purification. All the reactions were carried out using a round bottom flasks or Schlenk tubes under an argon or nitrogen atmosphere in anhydrous solvents. Synthesis of 6-bromo-2-naphthol. 2-Naphthol (10 g, 1 mmol), and acetic acid (30 mL) were added to a round bottom flask (250 mL) fitted with a dropping funnel and a reflux condenser on a magnetic stirrer bar. A solution of 22.22 g (2 mmol) of bromine in (7 mL) acetic acid was added dropwise to the above reaction mixture over a period of 45 min. The reaction mixture was heated for reflux and then cooled to room temperature. Tin was added in three portions of 1.7 g, 1.7 g and 7 g at a time intervals of 15 min each. The reaction mixture was refluxed for 3 h, cooled to room temperature, and filtered to remove tin

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salts. The filtrate was poured into cold water (200 mL) to precipitate the product. The product was filtered with suction, and further washed with cold water to obtain 6-bromo-2naphthol (15 g, 94% yield) as a white solid. M.P.: 122–124 1C (123–127 1C). Synthesis of 2-bromo-6-(n-octyloxy)naphthalene (j-1). 6-Bromo2-naphthol (4 g, 1 mmol), potassium carbonate (4.98 g, 2 mmol), and N,N-dimethylformamide (DMF) (100 mL) were added to a round bottom flask fitted with a dropping funnel and a reflux condenser on a magnetic stirrer bar. The mixture was stirred for half an hour. 1-Bromooctane (3.7 mL, 1.1 mmol) was added dropwise into the reaction mixture. After the addition, the reaction mixture was stirred at 80 1C for 18 h. The reaction mixture was extracted with ethyl acetate (100 mL), and then washed with water and brine. The organic layer was separated, dried over anhydrous sodium sulfate and filtered. Removal of solvent via rotary evaporator afforded a crude product, which was then chromatographed on a silica gel column with petroleum ether as eluent to obtain the title compound (5 g, 90% yield), as a white solid. M.P.: 50–52 1C. 1H NMR (400 MHz, CDCl3, d ppm): 7.90 (1H, s), 7.60–7.48 (3H, m), 7.11 (2H, d), 4.05 (2H, t), 1.86 (2H, quin), 0.82 (3H, t) 13C NMR (100 MHz, CDCl3, d ppm): 157.40, 133.10, 129.51, 128.39, 128.31, 120.08, 116.85, 106.52, 68.11, 31.82, 29.36, 29.23, 26.09, 22.65, 14.09. Synthesis of 4,4,5,5-tetramethyl-2-(6-(n-octyloxy)naphthalene2-yl)-1,3,2-dioxaborolane (j-2). 2-(Bromo-6-octyloxy)naphthalene (3 g, 1 mmol), bis(pinacolato)diboron (2.72 g, 1.2 mmol), PdCl2(dppf) (0.08 g, 0.1 mmol), and potassium acetate (2.63 g, 3 mmol) were added to a Schlenk flask under argon flow and then evacuated for 10 min. Under an argon flow anhydrous dioxane (25 mL) was added to the reaction mixture. The solution was first stirred at room temperature for 30 min and then at 80 1C for 24 h. The reaction mixture was quenched by the addition of water (200 mL) and extracted with ethyl acetate (200 mL). The combined organic layer was washed with water, dried over anhydrous sodium sulfate and filtered. Removal of solvent via rotary evaporator afforded a crude product, which was chromatographed on neutral alumina with 2% ethyl acetate in petroleum ether as an eluent to obtain the title compound (2.60 g, 76% yield) as a white solid. 1H NMR (400 MHz, CDCl3, d ppm): 8.3 (s, 1H), 7.8 (d, 1H, J = 8.1 Hz), 7.75 (d, 1H, J = 9.2 Hz), 7.7 (d, 1H J = 8.1), 7.8 (d, 1H, 8.1 Hz), 7.08–7.03 (m, 2H), 4.1 (t, 2H, J = 6.6 Hz), 1.85 (quin, 2H, J = 6.6 Hz, J = 7.6 Hz), 1.5 (quin, 2H, J = 8.1 Hz, J = 7.1 Hz), 1.4 (s, 12H), 1.3 (t, 3H) ppm. 13C NMR (100 MHz, d ppm): 158.5, 136.5, 136.0, 131.0, 130.1, 128.3, 125.8, 124.2, 119.0, 83.7, 68.0, 31.8, 29.4, 29.2, 29.2, 26.1, 24. Synthesis of (E)-triethoxy(2-(6-(n-octyloxy)naphthalene-2-yl)vinyl)silane (j-3). 2-(Bromo-6-n-octyloxy)naphthalene (1 g, 1 mmol), N-cyclohexyl-N-ethylcyclohexanamine Cy2NMe (0.9 mL, 1.5 mmol), toluene (30 mL), triethoxyvinylsilane (0.8 mL, 1.5 mmol) and Pd[P(tBu)3]2 (30 mg, 0.02 mmol) were added to a Schlenk flask under nitrogen. The reaction mixture was stirred at 90 1C for 48 h, and then extracted three times with 5% HCl at 5 1C and once with water (300 mL). After drying over anhydrous magnesium sulphate, the solvent was removed by rotary evaporator. The crude product was obtained as a sticky colorless solid (1.1 g, 83% yield) and used

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without further purification for the next reaction step. 1H NMR (400 MHz, CDCl3,): 7.75–7.66 (m, 4H), 7.39–7.10 (m, 2H of Nph 1H olefin), 6.22 (d, 2H olefin), 4.06 (t, 2H OCH2), 3.91 (q, 6H OCH2CH3), 1.74 (t, 2H), 1.35 (t, 21H), 1.03 (t, 3H). 13C NMR (100 MHz, d ppm): 158.30, 149.91, 135.58, 133.59, 130.36, 129.33, 128.02, 127.60, 124.38, 120.00, 117.10, 107.26, 68.69, 59.23, 32.43, 29.99, 29.86, 26.73, 23.27, 18.91, 14.71. Synthesis of 1,3,6,8-tetrakis(6-(octyloxy)naphthalene-2-yl)pyrene (PY-1). 1,3,6,8-Tetrabromopyrene (90 mg, 1.0 mmol), 2-octyloxy-6yl-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)naphthalene (398 mg, 6.0 mmol, 6 equiv.), tetrakis(triphenylphosphine)Pd(0) (20 mg, 0.10 mmol) and potassium carbonate (2 M aqueous 5 mL) were added to a flame dried Schlenk flask and placed under vacuum for 15 min. Under argon flow, anhydrous THF (15 mL) was added dropwise to the above reaction mixture. The reaction mixture was subjected to vacuum/argon refill cycles three times and then it was heated to reflux at 85 1C with vigorous stirring for 48 h under argon. Reaction progress was monitored by TLC. The mixture was then poured into water (200 mL) and extracted with dichloromethane (250 mL). The organic layer was washed with water (400 mL) and dried over anhydrous magnesium sulphate. Removal of solvent via rotary evaporator afforded a crude product, which was further purified using column chromatography (silica gel, 2% ethyl acetate in hexane as eluent) to obtain PY-1 (130 mg, 61% yield) as a green solid. 1 H NMR (400 MHz, CDCl3): 8.24 (s, 4H, pyrene), 8.18 (s, 2H, pyrene), 8.06 (dd, 4H, Nph), 7.88 (dd, 4H, Nph), 7.82 (dd, 8H, Nph), 7.22 (dd, 8H, Nph), 4.11 (t, 8H, 4OCH2), 1.87 (quin, 8H, 4CH2), 1.30 (t, 40H, 10CH2), 0.88 (t, 12H, 4CH3). 13C NMR (100 MHz, CDCl3): 152.52, 137.41, 136.33, 133.85, 129.66, 129.48, 129.34, 129.48, 128.98, 128.38, 126.70, 125.54, 119.65, 106.52, 68.21, 31.94, 29.80, 29.37, 26.24, 25.01, 22.78, 4.23. MALDI-TOF Spectroscopy calculated for C88H98O4: 1218.7465; found: 1218.8081. Synthesis of 1,3,6,8-tetrakis((E)-2-(6-(n-octyloxy)naphthalene2-yl)vinyl)pyrene (PY-2). 1,3,6,8-Tetrabromopyrene (100 mg, 1.0 mmol), (E)-triethoxy(2-(6-(octyloxy)naphthalene-2-yl)vinyl)silane (455 mg, 6.0 mmol, 6 equiv.), Pd(dba)2 (15 mg, 0.16 mmol), and P(o-tol)3 (50 mg, 0.47 mmol) were added to a Schlenk tube. The tube was evacuated and then refilled with N2 three times. Toluene (25 mL) was added, and the reaction mixture stirred at 80 1C for 20 min. Tetrabutyl ammonium fluoride (1.2 mL, 1.0 M in THF, 12 equiv.) was then added, and the reaction mixture was stirred at 80 1C for 2 days. The reaction mixture was extracted with dichloromethane (250 mL), washed with water (500 mL), and dried over anhydrous magnesium sulphate. Removal of solvent via rotary evaporator afforded a crude product. The crude product was purified using column chromatography (silica gel, 2% hexane/ dichloromethane as eluent) to obtain PY-2 (200 mg, 75% yield) as a yellow colored sticky solid. 1 H NMR (400 MHz, CDCl3): 7.74 (s, 2H, pyrene), 7.71–7.62 (m, 12H, Pyrene and Nph) 7.16 (dd, 4H, Nph), 6.9 (dd, 8H, Nph), 5.82 (d, 4H, olefinic), 5.28 (d, 4H, olefinic), 4.07 (t, 8H, 4OCH2), 1.86 (quin, 8H, 4CH2), 1.45 (t, 40H, 10CH2), 1.84 (t, 12H, 4CH3), 13 C NMR (100 MHz, CDCl3): 157.30, 136.97, 134.36, 132.80, 129.46, 128.82, 126.95, 126.17, 123.64, 119.27, 112.97, 106.60,

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68.04, 31.84, 29.26, 26.12, 22.67, 14.12. MALDI-TOF Spectroscopy calculated for C96H106O4: 1322.8091; found: 1361.23 (M + K), 1385.7582 (M + Na + K adduct). Synthesis of 1,3,6,8-tetrakis((6-(n-octyloxy)naphthalene-2yl)ethynyl)pyrene (PY-3). 1,3,6,8-Tetraethynylpyrene (125 mg, 0.6 mmol), 2-bromo-6-(octyloxy)naphthalene (1.117 g, 4.8 mmol, 8 equiv.), PdCl2(PPh3)2 (84 mg, 0.18 mmol), CuI (30 mg, 0.24 mmol) and PPh3 (42 mg, 0.24 mmol) were added to a Schlenk flask. The flask was evacuated and then refilled three times with N2. Toluene (5 mL) and triethylamine (15 mL) were added and the reaction mixture was stirred at 80 1C for 48 h. The reaction mixture was extracted with water (300 mL) and dichloromethane (300 mL) and dried over anhydrous magnesium sulphate. Removal of solvent via rotary evaporator afforded a crude product; the crude product was purified by column chromatography (silica gel, 2% hexane/dichloromethane as eluent). Precipitation was carried out in methanol and the brown colored solid (400 mg, 72% yield) PY-3 was collected by vacuum filtration and dried overnight. 1 H NMR (400 MHz, CDCl3): 7.92 (s, 4H, pyrene), 7.67 (s, 2H pyrene), 7.62–7.46 (m, 12H of Nph), 7.20–7.09 (m, 12H, Nph), 4.06 (t, 8H, 4OCH2), 1.84 (qui, 8H, 4CH2), 1.4 (t, 40H, 10CH2), 0.90 (t, 12H, 4CH3), 13C NMR (100 MHz, CDCl3): 157.07, 132.76, 129.56, 129.28, 129.17, 128.07, 127.99, 119.74, 116.51, 106.13, 67.76, 31.49, 29.04, 28.92, 25.76, 22.34, 13.79. MALDI-TOF Spectroscopy calculated for C96H98O4: 1314.7465; found: 1314.8032. OLED device fabrication details Patterned ITO glass substrates were routinely cleaned using Decon 90 detergent and deionized water. Then, they were blown dry by nitrogen gas and maintained in an 110 1C oven for 3 h before 25 min surface treatment in an ultra-violet ozone (UVO3) cleaner. After the UVO3 treatment, the ITO glass substrates were loaded into a vacuum deposition system with a base pressure of B7  107 Torr. Organic and metal layers were sequentially deposited on the ITO glass substrates by thermal evaporation. The device configuration is shown in Fig. 1, where dipyrazino[2,3-f:20 ,3 0 -h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN), N,N 0 -bis(naphthalen-1-yl)-N,N 0 -bis(phenyl)-benzidine (NPB), 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) doped with compound 1 or compound 2, MADN, 2,2 0 ,200 (1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) and 4,7-diphenyl-1,10-phenanthroline doped with lithium (BPhen:Li) were responsible for the hole injection layer (HIL), hole transport layer (HTL), emitting layer (B-EML), exciton confining layer, electron transport layer (ETL) and electron injection layer (EIL), respectively. Compound 1 or compound 3 were doped on the MADN host with various dopant concentrations. After device fabrication, the devices were maintained at ambient condition for electroluminescence (EL) measurement. In the EL measurement, the voltage–current–brightness (I–V–B) characteristics, the CIE coordinates and the EL emission spectra were measured by a Spectra PR650 CCD camera with a computer controller power supply. The current and power efficiencies were calculated from the I–V–B data of the devices.

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Results and discussion In this study, we have selected naphthalene as a substituent, which is attached to a pyrene core as a chromophore through single, double and triple bonds via various organometallic coupling reactions. In order to make a more electron rich system, we selected naphthalene acene as a functional group. First, the octyl chain was introduced to the 2-position of naphthalene using 6-bromonaphthalen-2-ol starting material via Williamson etherification in the presence of 1-bromooctane and strong base, which resulted in the product 2-bromo-6(octyloxy) naphthalene (j-1). 2-Bromo-6-(octyloxy)naphthalene (j-1) was converted into 4,4,5,5-tetramethyl-2-(6-(octyloxy)naphthalen-2-yl)-1,3,2-dioxaborolane (j-2) using potassium acetate, bis(pinacolato)diboron and PdCl2(dppf) in the presence of 1,4-dioxane solvent in good yield. The compound (E)-triethoxy(2-(6-(octyloxy) naphthalen-2-yl)vinyl)silane (j-3) was synthesized in 83% yield from 2-bromo-6-(octyloxy)naphthalene using Cy2NMe, triethoxyvinylsilane and Pd[P(tBu)3]2 in the presence of toluene solvent29 (see Scheme 1 for j-1, j-2 and j-3 synthesis). As shown in Scheme 2, 1,3,6,8-tetrakis(6-(octyloxy)naphthalen-2-yl)pyrene (PY-1) and 1,3,6,8-tetrakis((E)-2-(6-(octyloxy)naphthalen-2-yl)vinyl)pyrene (PY-2) were synthesized and purified as green and yellow powders by Suzuki–Miyura and Heck cross-coupling reactions, respectively using 4,4,5,5-tetramethyl-2-(6-(octyloxy)naphthalen-2-yl)-1,3,2dioxaborolane (j-2) and (E)-triethoxy(2-(6-(octyloxy) naphthalen-2yl)vinyl)silane (j-3) with common 1,3,6,8-tetrabromopyrene (6a) starting material.31 The third compound of the series, 1,3,6,8tetrakis((6-(octyloxy)naphthalen-2-yl)ethynyl)pyrene (PY-3) was synthesized by a Sonogashira cross-coupling reaction using 2-bromo-6-(octyloxy)naphthalene (j-1) and tetraethynylpyrene30,31 (converted from 1,3,6,8-tetrabromopyrene in situ and used as such without purifying because of stability issues). In order to induce solution processability, we attached an alkoxy chain to the naphthalene. 1H, 13C NMR spectra and MALDI-TOF were performed in order to confirm the structure and purity of all the three compounds (see ESI† for further details). Among the three compounds, PY-1 synthesis was considerably facile than PY-2 and PY-3. The ease of PY-1 synthesis is attributed to straightforward

Scheme 1

Suzuki coupling conditions whereas for the other two compounds, incorporation of double and triple bonds for PY-2 and PY-3 via Heck and Sonogashira coupling was synthetically challenging because of the electron rich system. Thermogravimetric analysis (TGA) of compounds PY-1, PY-2 and PY-3 clearly shows slight variation in the thermal decomposition (Td) temperatures (5% weight loss in nitrogen atmosphere) because of similarity in the structure. All the three compounds are thermally stable up to 200 1C, which is appropriate for device fabrication. The single bond compound PY-1 exhibits the highest Td of 270 1C, whereas compounds PY-2 and PY-3 show Td at 255 1C and 245 1C, respectively. Insertion of double and triple bonds slightly reduced the Td values because of the electron rich double and triple bonds. The optical properties of PY-1, PY-2 and PY-3 were measured in chloroform and in thin films by UV-vis absorption and photoluminescence (PL) spectroscopy. All the three compounds exhibit absorption in the range of 250 nm to 450 nm indicating a wide optical band gap (see Fig. 2 and Table 1). Compound PY-1 shows wavelength absorption maxima (lmax) at 400 nm, whereas compounds PY-2 and PY-3 exhibit absorption maxima (lmax) at 350 nm and 337 nm, respectively. PY-1 and PY-2 exhibited red shifts of 62 nm and 12 nm, respectively, when compared to molecular pyrene (lmax = 338 nm). PY-3 exhibited an almost identical lmax to that of pyrene. The absorbance observed for PY-1 is 28 nm red shifted compared to the earlier reported PY-PhOC4 molecule because of the extended conjugated length of naphthalene compared to phenyl.18 The solid state absorption of PY-1, PY-2 and PY-3 were measured by spin coating the thin film on glass. Among all the three compounds, only PY-1 exhibits a red shift of 24 nm compared to its solution counterpart, whereas the other two compounds (PY-2 and PY-3) demonstrate similar absorptions, indicating the overall amorphous nature of the materials. Two absorption bands arise from the naphthalene and pyrene moieties. The 50 nm and 63 nm blue shifts of PY-2 and PY-3 compared to PY-1 are because of the distortion of the naphthalene moiety from the pyrene core arising from double and triple bonds. The photoluminescence spectra of all the these compounds were recorded both in solution and in the solid state as shown in Fig. 3. The luminescence spectra were measured for all the

Synthesis of starting precursors j-1, j-2 and j-3 for making pyrene cored semiconductors.

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Scheme 2

Paper

Synthesis of naphthalene substituted pyrene cored single (PY1), double bond (PY2) and triple bond (PY3) containing organic semiconductors.

Table 1 Optical, thermal properties and energy levels calculated by PESA of compounds PY-1, PY-2 and PY-3

UV-vis

PL

lmaxa

lmaxb

Compound (nm)

(nm)

PY-1 PY-2 PY-3

lmaxa lmaxb Bandgapc Tde (nm) (nm) (eV) HOMOd (1C)

315, 400 337, 424 520 335, 350 335, 352 505 324, 337 324, 340 535

540 530 546

2.83 2.91 3.44

5.59 5.71 5.45

212 206 211

Measured in chloroform (105 M) solution. b Measured from thin films on glass. c Measured from the absorption onset of the UV-vis spectrum. d Measured from the onset of the photoelectron spectroscopy in air (PESA). e Obtained from TGA measurement (temperature at 5% weight loss under nitrogen, 10 1C min1 ramp rate). a

Fig. 2 Normalized absorption spectrum of pyrene cored dendritic conjugated materials PY-1, PY-2 and PY-3 in chloroform (upper) and solid state (lower).

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three compounds using an excitation wavelength of 370 nm. In solution, PY-1 exhibits emission at 520 nm, whereas PY-2 and PY-3 show emission at 505 nm and 535 nm, respectively. The observed PL emission for PY-1 and PY-3 is in good agreement with earlier reported analogous compounds.18,30 The 127 nm, 112 nm and 142 nm PL lmax red shifts have been observed in solution compared to the pyrene core (PL lmax = 393). The red shifts in PY-1, PY-2 and PY-3 have been observed because of the effect of conjugation length arising from both the naphthalene moiety and the introduction of various bonds. The thin film PL emission of all the three compounds is shown in Fig. 3. The luminescence of all the compounds shows a typical excimer

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Fig. 3 Normalized photoluminescence spectrum of pyrene cored dendritic conjugated materials PY-1, PY-2 and PY-3 in solution (upper) and solid state (lower).

emission of the pyrene due to the formation of p–p stacking between different pyrene units. The PL spectra of these three compounds, PY-1, PY-2 and PY-3, show red shifts of 20 nm, 25 nm and 11 nm, respectively compared to their corresponding solution state measurements. This is possibly because of the solid state aggregation versus the dilute solution state. The observed PL of PY-1 is 58 nm red shifted compared to the PL (462 nm) of earlier reported butoxy-phenyl substituted PY-PhOC4.18 This is because of the extended conjugation of the naphthalene group compared to the phenylene group. The observed optical transition for compounds PY-1, PY-2 and PY-3 compared to the pyrene core clearly indicates that the naphthalene moiety and single, double and triple bonds are responsible for the optical shifts. The observed PL spectra for single and triple bond compounds PY-1 and PY-3 are more or less identical, whereas compound PY-2 exhibits blue shift in the PL emission. This observation is consistent with the absorbance data. The double and triple bonds cause slight distortion and make the material more non-coplanar than the single bond compound, which causes tuning in optical band gaps. The energy levels of PY-1, PY-2 and PY-3 were investigated using photoelectron spectroscopy in air (PESA). When a surface is bombarded with a slowly increasing amount of ultraviolet energy, photoelectrons start to emit at a certain energy level. This energy level is called the ‘‘Photoelectron Work Function’’ or ‘‘Work Function’’. When the photoelectron output is plotted on an X/Y axis, with the horizontal axis as the UV energy applied and the vertical axis as the standardized photoelectron

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yield ratio (Y), the result is a line with a specific slope of degree (Y/eV). HOMO values of the PY-1, PY-2 and PY-3 were calculated from the onset energy levels recorded in PESA. The PESA data for the compounds PY-1, PY-2 and PY-3 are shown in Fig. 4. The HOMO values for PY-1, PY-2 and PY-3 are 5.59 eV, 5.71 eV and 5.45 eV, respectively, indicating the lower HOMO and highly air stable nature of the compounds. For PY-1 and PY-2, the HOMO values are comparable whereas for PY-3, the HOMO value is higher, which might be attributed to the incorporation of electron rich triple bonds between the pyrene core and naphthalene. In order to test the performance of the pyrene based materials in the devices, we used all of these materials as OLED devices. As an initial trial, PY-2 and PY-3 were selected to fabricate non-doped devices. PY-1 was selected as a dopant emitting material in combination with the host matrix material because of its blue-green emission and deep HOMO. The respective recorded electroluminescence data and the specific OLED device structure are given in Fig. 5. In order to further enhance the device performance, PY-1 was doped to the MADN host with various dopant concentrations. After the device fabrication, the devices were maintained at ambient condition for electroluminescence (EL) measurements. The device configurations of compounds PY-2 and PY-3 are: ITO/HATCN/NPB/PY-2/TPBi/BPhen:Li/Al and ITO/HATCN/NPB/ PY-3/TPBi/BPhen:Li/Al, respectively, whereas dipyrazino[2,3f:2 0 ,3 0 -h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (HATCN), N,N0 -bis(naphthalen-1-yl)-N,N0 -bis(phenyl)-benzidine (NPB), 2,20 ,200 (1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi), lithium doped 4,7-diphenyl-1,10-phenanthroline BPhen:Li and aluminum (Al) are responsible for the hole injection layer (HIL), hole transporting layer (HTL), electron transport layer (ETL), electron injection layer (EIL) and cathode, respectively. As shown in Fig. 6 and 7, the EL maxima of the PY-2 and PY-3 devices are 525 and 545 nm, respectively. The PY-2 device exhibited a power efficiency of 0.06 lm W1 at 8 V, turn on voltage of 5 V and C.I.E. coordinates of (0.31, 0.54) at 7 V. Whereas the PY-3 device exhibited a power efficiency of 0.27 lm W1, turn on voltage of 3 V and C.I.E. coordinates of (0.36, 0.56) at 7 V. Obviously, the superior EL performance of compound PY-3 makes it a more practical green emitting material. The EL peaks, current efficiencies, turn on voltage and color coordinates of the devices are shown in Tables 2 and 3, respectively. The electroluminescence quantum yield (ELQY) of PY-1, PY-2 and PY-3 based OLED devices were measured (see Table 1 in the ESI† for further details). The voltage–current– brightness (I–V–B) characteristics, the C.I.E. coordinates and the EL emission spectra were measured by a Spectra PR650 CCD camera with a computer controller power supply. The current and power efficiencies were calculated from the I–V–B data of the devices. The EL (electroluminescence) maximum of the OLED devices was recorded at 438 nm for compound PY-1 that confirms that PY-1 is a blue emitting material, which is a very important colour component of OLED displays and lighting panels. The detailed OLED device summary is given

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Fig. 4 Photoelectron spectroscopy in air (PESA) for compounds PY-1, PY-2 and PY-3 using a thin film spin coated on the glass substrates.

Fig. 6

Fig. 5 (a) PY-1 based OLED device electroluminescence spectrum and (b) OLED device structure.

in Table 4. The 10% doped PY-1 device exhibited the strongest EL emission peak as shown in Fig. 8. This is a suitable dopant

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PY-2 based OLED device electroluminescence spectrum.

concentration in the present device configuration. For the 10% doped device, a power efficiency of 1.1 lm W1 was achieved at a drive voltage of approximately 4 V. The C.I.E. coordinates were maintained at (0.17, 0.15) to (0.17, 0.16), respectively, as the voltage changed from 5 V to 11 V. The stable colour output further proved that PY-1 has high potential to be applied as a reliable blue emitting dopant in OLED applications. With a device turn on voltage of 3 V, it was also found to be an efficient blue dopant.

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Conclusion

Fig. 7 PY-3 based OLED device electroluminescence spectrum.

Table 2 devices

Electroluminescent performance summary of the PY-2 OLED

Thickness Turn Current efficiency of PY-2 on (nm) (V) (cd A1) 40 45 50

Table 3 devices

4.5 5 5.5

CIE @ 7V

Max. brightness (cd m2)

0.1 @ 8V 0.04 @ 8 V 0.31, 0.55 105 0.15 @ 8.5 V 0.06 @ 8.5 V 0.31, 0.54 170 0.06 @ 8.5 V 0.02 @ 8.5 V 0.31, 0.54 160

Electroluminescent performance summary of the PY-3 OLED

Thickness Turn Current of PY-3 on efficiency (nm) (V) (cd A1) 40 45 50

Power efficiency (lm W1)

3.0 3.5 4.0

Power efficiency (lm W1)

CIE @ 7V

Max. brightness (cd m2)

0.3 @ 3.5 V 0.27 @ 3.5 V 0.36, 0.56 115 0.27 @ 3.5 V 0.23 @ 3.5 V 0.36, 0.56 89 0.19 @ 3.5 V 0.17 @ 3.5 V 0.36, 0.57 66

Table 4 Electroluminescence performance summary of the 10% PY-1 doped OLED device

C.I.E. co-ordinates (x, y)

Maximum current efficiency (cd A1)

Turn on voltage (V)

At 5 V

At 8 V

At 11 V

1.5

3

0.17, 0.15

0.17, 0.16

0.17, 0.16

Fig. 8 Power efficiency of the PY-1 doped OLEDs in different dopant concentrations.

Phys. Chem. Chem. Phys.

In conclusion, single, double and triple bond incorporated pyrene cored dendritic molecules were designed and synthesized via Suzuki, Heck and Sonogashira organometallic coupling reactions. The optical, electrochemical, and thermal properties of these materials were studied. All the three materials were used for OLED device applications and their electroluminescence properties were studied. The turn on voltages for all of the devices were calculated in the range of 3 to 4.5 V. One of the molecules (PY-1) was used as a dopant in OLED devices. The devices exhibited a low turn on voltage of 3 V, an EL peak of 438 nm, and power efficiency of 1.1 lm W1. Most importantly, a stable C.I.E. co-ordinate was observed throughout the device operation with a voltage range of 5 to 11 V. These performances indicate high potential as OLED materials. Further improvements are expected to be possible to obtain better performance with optimized device structure to suit commercial needs for OLED displays.

Acknowledgements A grant from the Research Grants Council of the Hong Kong Special Administrative Region (Project No. T23-713/11) and Visiting Investigatorship Program (VIP), A*STAR is acknowledged.

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Pyrene based conjugated materials: synthesis, characterization and electroluminescent properties.

In this work, three novel pyrene cored small conjugated molecules, namely 1,3,6,8-tetrakis(6-(octyloxy)naphthalene-2-yl)pyrene (PY-1), 1,3,6,8-tetraki...
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