A New [5] Helicene Derivative as Novel Emissive Material for Organic Light-Emitting Diode Siriporn Kamtonwong , Somboon Sahasithiwat , Waraporn Panchan, Laongdao Menbangpung, and Thanasat Sooksimuang National Metal and Materials Technology Center (MTEC) , Thailand Thank you chairman for your kind introduction. Good morning, I would like to thanks organizer for give me a chance to present our work here. Today I’m gladly present our research on topic of A New penta- Helicene Derivative as Novel Emissive Material for Organic Light-Emitting Diode Pure and Applied Chemistry International Conference 2013 The Tide Resort, January 25, 2013 1

Outline Introduction Experimental Results and discussion Conclusion I will start my talk with -Indroduction -Experimental -Results and discussion -and conclusion 2

Introduction OLED Device Operation Principle - Light - Light + Cathode Anode ETL HTL Hole + - EML Excitation Light + Cathode EML/ETL - HTL Anode Substrate Organic light emitting diode or “OLED” is the electronic device that emit light when applied voltage. The OLED can be used as display or light source. The basic structure for an OLED consists of four thin film layers deposited on substrate. 1 The first layer is the anode, which is Indium-tin-oxide. 2. The hole transport layer is the second. 3. The third layer is an organic emitter. 4. Lastly, fourth layer is the cathode, which is made of metal. The substrate is made of either a glass or transparent plastic. When a voltage is applied across the two inner layers holes and electrons combine forming excitons. When an exciton decays, a photon is emitted. The light is created. OLED displays are a promising new display technology that posses advantage such as -thinner -fiexible display -lower power consumption - a wider viewing angle OLED Structure Basic for light generation via OLED 3

Helicenes 3,12-Dimethoxy-7,8-dicyano-[5]helicene (DDH) [5] Helicene [6] Helicene [7] Helicene Φf = 0.02-0.04 3,12-Dimethoxy-7,8-dicyano-[5]helicene (DDH) 2 9 3 4 5 6 8 10 11 12 14 1 7 13 Electron donating withdrawing Under UV DDH in CHCl3 Helicenes are polycyclic aromatic hydrocarbons (PAHs) containing numbers of benzene ring connected at ortho- position. Fluorescence quantum yields of them were very low. Our research, we study a new penta-helicene derivative. Recently, DDH was reported by our team. It consists of two electron donating groups, methoxy, and electron withdrawing groups , cyano, in a specific location to allow and enhance electron flow in the molecule which enhances its optical properties. DDH in CHCl3: Φf = 0.27 S. Sahasithiwat, T. Mophuang, L. Menbangpung, S. Kamtonwong and T. Sooksimuang. Synthetic Metals 160(2010)1148-1152. 4

3,12-Dimethoxy-[i]furan-1,3-diono-[5]helicene (DFH) as novel Emissive Material for Organic light-Emitting Diode  In this work, we designed a new penta-helicene derivative which has the electron withdrawing group as furan-diono (anhydrie group). Synthesis of DFH was easy and very cheap cost. We will hope that it has good thermal and optical properties. 5

Experimental  The experimental composed of two parts of a new penta-helicene derivative and the fabrication of the OLED. 6 6 6

Preparation of DFH we prepare DFH as following, 1 2 3 70-80% 75% 70% we prepare DFH as following, Diels-Alder cycloaddition between diene and maleic anhydride gave adduct 2, then treated with DDQ gave the partially aromatized product 3 in good yield. The fully aromatized penta- helicene (DFH) was synthesed by bromination of compound 3 with NBS and follow by dehydrobromination with sodium acetate in acetic acid. 7

J-V-L Characteristics OLED fabrication Configuration: ITO/PEDOT:PSS(35nm)/DFH/Ca(10nm)/Al(100nm) Ca/Al DFH J-V-L Characteristics Measured PEDOT:PSS ITO The OLED with ITO/PEDOT:PSS/DFH/Ca/Al configuration were fabricated. - The patterned ITO coated- glass slide was cleaned. Subsequently, PEDOT:PSS was spin coated onto ITO coated-glass, then this was baked at 140 oC for 10 min to remove residue water. -DFH was deposited by thermal evaporation. - The metal cathode layer was also prepared by thermal evaporation of calcium. -Finally, Aluminum was coated to protect calcium form moisture and oxygen. We studied the effect of DFH thickness on characteristic device. Glass 8

Results and discussions 9

Electron withdrawing group Electron withdrawing group The optical properties of DFH UV in CHCl3, λmax : 340, 437, 451 nm PL, λmax : 535 nm FWHM : 78 nm Φf = 0.13 This slide shows the optical properties of DFH solution in chloroform. - Red line present uv absorption spectra. And Blue line present photoluminescence spectra. (UV) The absorbtion band at three hundred forty nano-meter (340 nm) is associated with Pi to pi star transition. A broad absorption form four hundred to five hundred nano-meter (400-500 nm) is attributed to the intramolecular charge transfer (ICT). (PL) when a dilute solution of DFH in chloroform was excited at three hundred sixty nano-meter (360 nm), photoluminescence exhibited a peak at five hundred and thirty-five nano-meter (535 nm). A full width at half maximum is seventy-eight nano-meter (78 nm). And quantum yield is point one three (0.13), measured using harmane and harmine as references. Under UV Normalized UV/Vis absorption and fluorescence spectra of DFH in dilute (3x10-6 M) CHCl3 solution. DFH in dilute CHCl3 solution. 10 Electron withdrawing group Electron withdrawing group Electron donating group Electron donating group

Thermal properties of DFH DSC: Tm 315 oC TGA: Td,5 360 oC The thermal properties of DFH were investigated by DSC and TGA (differential scanning calorimeter) (thermal gravimetric analyzer). The heating scan of DSC indicated that DFH has a high melting point at three hundred and fifteen degree-Celsius (315 oC). The TGA thermogram shows that DFH starts to decompose at high temperature about three hundred thirty degree- Celsius( 330 oC). These thermal properties point out that DFH is very stable at high temperature. DSC thermogram of DFH TGA thermogram of DFH 11

Energy levels of DFH Cyclic voltammetric technique HOMO: Ip = -(E'ox + 4.4) eV (1) LUMO: Ea= -(E‘red + 4.4) eV (2) The energy levels was studied by using cyclic voltammetric technique. HOMO is five point nine electron volt. (5.9 eV) LUMO is three point six electron volt. (3.6 eV) And energy band gap is two point three electron volt. (2.3 eV) The lowest unoccupied molecular obital: (LUMO) The highest occupied molecular orbital: (HOMO) Cyclic voltammogram of DFH in 0.1 M Bu4NBF4 in acetonitrile solution performed in a three-electrode cell (a glassy carbon working, a platinum counter and a Ag/Ag+, 0.1 M AgNO3, non-aqueous reference electrode) at a scan rate of 50 mV/s. 12

Structure and energy level diagram of the OLED ITO/PEDOT:PSS(35nm)/DFH/Ca(10nm)/Al(100nm) Vacuum Level HIL EML Anode Cathode h+ e- ITO PEDOT:PSS DFH Ca(10 nm)/Al (100 nm) Glass Light ITO PEDOT:PSS DFH Ca -4.9 eV -5.2 eV -5.9 eV -3.6 eV -2.8 eV LUMO This slide shows that structure and energy level diagram of the OLED device. PEDOT:PSS layer is utilized as a hole injection layer while DFH acts as an emissive material in this device. HOMO Structure of the OLED Energy level diagram 13

Current density-voltage-brightness characteristics of the OLED ITO/PEDOT:PSS(35nm)/DFH/Ca(10nm)/Al(100nm) ILV Characteristic of OLED with DFH We study the electroluminescent characteristic of the OLED. Example of I-L-V- characteristics of one of the device, it shows the diode characteristic. -blue line present current density -and red line present brightness. Plots of current density-voltage-brightness characteristic of ITO/PEDOT:PSS/DFH(80 nm)/Ca/Al device 14

ITO/PEDOT:PSS(35nm)/DFH/Ca(10nm)/Al(100nm) thickness (nm) Turn on voltage (V) Current Eff. (cd/A) Power Eff. (lm/W) Max. Brightness (cd/m2) CIE Coordinate (x,y) 60 3.4 0.16 0.09 696 (0.44,0.58)-(0.46,0.54) 80 4.9 0.33 0.15 881 (0.41,0.55)-(0.46,0.52) 100 4.4 0.30 0.13 760 (0.44,0.53)-(0.47,0.52) (0.41,0.55)-(0.46,0.52) CIE Coordinate increasing applied voltage OLED Excimer formation Green yellow 5.5 V 6.5 V 8.5 V We vary thickness of DFH layer to optimized the efficiency of device. The results are shown in the table. The best device composed of DFH, eighty nano-meter thickness (80 nm), which gave -turn on voltage is four point nine volt (4.9 V) -current efficiency is point three three candela per ampar (0.33 cd/A) -power efficiency is point one five lumen per watt (0.15 lm/w) -Maximum brightness is eight hundred and eighty-one candela per square meter (881 cd/m2) We also observe that increasing applied voltages, the emitted light changes from green to yellow color. The phenomenon implied that DFH molecules tend to undergo an excimer formation at high applied voltages. 15

Conclusion Easy to synthesized with good yield Good thermal stability DFH Easy to synthesized with good yield Good thermal stability Excellent optical properties In conclusion, penta-helicene (DFH) is -easy synthesized with good yield, -good thermal stability -excellent optical properties and DFH was successfully utilized as a new emitter for OLED. These results have opened new windows for molecular structural design and synthesis of other new penta-helicence derivatives as a new class for small organic emissive material. It offers longer absorption and fluoresces in the greenish region. DFH was successfully utilized as a new emitter for OLED. 16

Acknowledgments National Metal and Materials Technology Center (MTEC) National Science and Technology Development Agency (NSTDA) Ministry of Science and Technology (MOST), Thailand under grant number MT-B-55-POL-07-523-I Finally, I would like to thank MTEC, Nstda and Ministry of science and technology for financial support and facilities. Facilities = fac-ci-li-ties 17

Thank you for your kind attention 18

19 19 19

The formation of excimer in organic molecule Excited monomer Excimer Monomer emission Ground State Energy High Low LUMO LUMO HOMO HOMO Donor molecule Acceptor Excimer emission Light emission from excimer is red shifted from the excited monomer Excimer: a single excited molecule can interact with an unexcited neighbor to produce the excited dimer, or excimer 20

The multi-layered device Cathode EIL ETL EML HTL HIL Anode Light + Cathode Anode EIL ETL EML Excitation HIL HTL - Hole Light Fig. Basis for light generation via OLED 21

22 22 22

OLED fabrication ITO/PEDOT:PSS/DFH/Ca/Al Configuration: ITO coated-glass slide was cleaned. PEDOT:PSS was spin-coated onto ITO substrate. then it was baked at 140 oC for 10 min. DFH was deposited by thermal evaporation onto the PEDOT:PSS coated substrate. Calcium was evaporated onto the organic layer. Aluminum was coated onto calcium layer. 23

NMR 24

FT-IR spectrum 1827, 1762 25

Enantiomers Helical structure (M) [5] Helicene (P) [5] Helicene This slide show the stucture of pentahelicene Since helicenes are twisted molecules which able to form helical structures. They generally have two enantiomers. (M)-isomer for clockwise helix And (P)-isomer for anticlockwise helix Especially, Chiral helicenes exhibit large specific rotation and a nonlinear optical property. (M) [5] Helicene (P) [5] Helicene Clockwise helix Anticlockwise helix 26

3,12-Dimethoxy-7,8-dicyano-[5]helicene (DDH) under UV 365 nm In this work, we interested in the substituted helicene, 3,12-Dimethoxy-7,8-dicyano-pentahelicene (DDH) Because the direction of electron donating and electro withdrawing group in the molecule could contribute to give higher efficiency of fluorescence emission. DDH in CH2Cl2 S. Sahasithiwat, T. Sooksimuang, T. Mophuang, L. Menbangpung and S. Kamtonwong Synthetic Metals 160(2010)1148-1152 27

OLED displays are a promising new display technology that posses advantage such as -thinner -fiexible display -lower power consumption - a wider viewing angle Fig. 3. OLED Research 28 28 28

29 29 29

Vacuum Level -2.8 eV -3.6 eV -4.9 eV -5.2 eV -5.9 eV ITO PEDOT:PSS DFH Ca -4.9 eV -5.2 eV -5.9 eV -3.6 eV -2.8 eV

ศูนย์เทคโนโลยีโลหะและวัสดุแห่งชาติ 114 อุทยานวิทยาศาสตร์ประเทศไทย ถนนพหลโยธิน ตำบลคลองหนึ่ง อำเภอคลองหลวง จังหวัดปทุมธานี 12120 โทร. +(66)2564-6500 โทรสาร +(66)2564-6501-5   National Metal and Materials Technology Center 114 Thailand Science Park Paholyothin Rd., Klong 1, Klong Luang, Pathumthani 12120 Thailand Tel. +(66)2564-6500 FAX +(66)2564-6501-5 31

32 32 32