Optoelectronic Properties of Quaternary Phosphine Oxide Hosts
FULL PAPER
evaporation on glass substrates under the same condition as for device
fabrication. The morphological characteristics of these films were mea-
AHCTUNGERTGsNNUN ured with an atom force microscope (AFM) Agilent 5100 under the tap-
ping mode and a scanning electron microscope (SEM) HITACHI S-4800
(spraying, accelerating voltage: 5.0 kV, current: 10 mA and observed alti-
tude: 8 mm).
Through the mixed indirect and multi-insulating linkages,
high T1 energy levels about 2.9 eV were achieved for all of
them with a molecular weight about 1000 gmol–1, which
should be a result of suppressed intramolecular interplays
between the functional groups. The S1 energy levels and the
electrical properties of these hosts were conveniently and
accurately modulated by the variation of the substitution po-
sition, the number and the ratio of DPPO, diphenylether
and carbazolyl. As a result, two quaternary hosts DPES-
POFPhCz and DPEPOFPhCz with very similar optoelec-
tronic properties excluding their different T1 states contrib-
uted by fluorenyl and carbazolyl, respectively, were selected
to investigate the impact of the T1 state location on the EL
performance of the hosts. Their full-colour PHOLEDs
showed that DPESPOFPhCz endowed its devices with a
lower driving voltages and higher efficiencies compared
with DPEPOFPhCz. This discrepancy reflects that the inde-
pendent contribution of the functional groups to the optoe-
lectronic properties is beneficial to restrain the polaron–ex-
citon quenching, and that the embedded T1 state by periph-
eral carrier transporting groups can suppress the collision-in-
duced triplet quenching. This work indicated the significance
of controlling the T1 state location for complicated host sys-
tems, which may further guide the molecular design of
promising multi-functional host materials.
Synthesis
2,2’-[4,4’-(9H-Fluorene-9,9-diyl)bis(4,1-phenylene)]bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolane) (FPhB2): nBuLi (30 mL, 75 mmol) was added into
a solution of FPhBr2 (12 g, 25 mmol) in THF (50 mL) at ꢀ788C. The
mixture was stirred for 1 h at ꢀ788C, followed by adding triisopropyl
borate (35 mL, 150 mmol) dropwise at ꢀ788C. The solution was allowed
to warm to room temperature after 1 h and was stirred for another 24 h
at room temperature. A saturated aqueous solution of NH4Cl (16 g,
300 mmol) was added to the mixture and the resulting solution was stir-
red for 1 h at 08C. The mixture was allowed to warm to room tempera-
ture and was then stirred for 12 h. The reaction solution was extracted
with diethyl ether (3ꢁ50 mL). The combined organic layer was dried
with anhydride Na2SO4. Benzene (150 mL) and pinacol (17.76 g,
150 mmol) were added after removal the solvent. The reaction solution
was extracted by dichloromethane (3ꢁ50 mL). The organic layer was
dried with anhydrous Na2SO4, and then the solvent was removed by dis-
tillation in vacuo. The residue was purified by recrystallisation from ace-
tone to afforded the product as white needle crystals (7.1 g, 50%).
1H NMR (TMS, CDCl3, 400 MHz): d=7.75 (d, J=7.6 Hz, 2H), 7.65 (d,
J=8.4 Hz, 4H), 7.34 (t, J=7.2 Hz, 4H), 7.23–7.17 (m, 6H), 1.3 ppm (s,
24H); LDI-TOF: m/z (%): 570 [M+] (100); elemental analysis (%) for
C37H40B2O4: C 77.92, H 7.07, O 11.22; found C 78.01, H 7.08, O 11.34.
9-(4’-{9-[4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-9H-fluo-
ren-9-yl}biphenyl-4-yl)-9H-carbazole
(FPhCzB):nBuLi
(0.6 mL
,1.5 mmol) was added into a solution of FPhCzBr (0.6386 g, 1 mmol) in
THF (10 mL) at ꢀ788C. The mixture was stirred for 1 h at ꢀ788C, fol-
lowed by adding triisopropyl borate (0.7 mL, 3 mmol) dropwise at
ꢀ788C. The solution was allowed to warm to room temperature after 1 h
and was stirred for another 24 h at RT. A saturated aqueous solution of
NH4Cl (0.321 g, 6 mmol) was added into the mixture and the resulting
solution was stirred for 1 h at 08C. The mixture was allowed to warm to
room temperature and was then stirred for 12 h. The reaction solution
was extracted with diethyl ether (3ꢁ50 mL). The combined organic
layers were dried with anhydrous Na2SO4. Benzene (10 mL) and pinacol
(0.3545 g, 3 mmol) were added after removal of the solvent. The reaction
solution was extracted with dichloromethane (3ꢁ50 mL). The dichloro-
methane extract was dried over anhydrous Na2SO4. The solvent was re-
moved in vacuum and the residue was purified by recrystallisation by
using acetone. The product was obtained as a white powder (0.41 g,
60%). 1H NMR (TMS, CDCl3, 400 MHz): d=8.13 (d, J=7.6 Hz, 2H),
7.79 (d, J=7.6 Hz, 2H), 7.73–7.71 (q, J1 =2.8, J2 =8.4 Hz, 4H), 7.57 (d,
J=8.4 Hz, 2H), 7.50 (d, J=8.4 Hz, 2H), 7.46–7.35 (m, 8H), 7.31–7.26 (m,
8H), 1.31 ppm (s, 12H); LDI-TOF: m/z (%): 685 [M+] (100); elemental
analysis (%) for C49H40BNO2: C 85.83, H 5.88, N 2.04, O 4.67; found C
85.78, H 5.91, N 2.11, O 4.72.
Experimental Section
Materials and instruments: All reagents and solvents used for the synthe-
sis of the compounds were purchased from Aldrich and Acros and used
without further purification. 1-(4-Bromophenoxy)-1-[2-(diphenylphosphi-
noyl)phenoxy]benzene (DPESPOBr),[10d] 4-bromo-2-(diphenylphosphino-
yl)-1-[2-(diphenylphosphinoyl)phenoxy]benzene (DPEPOBr),[10d] 9,9-
bis(4-bromophenyl)-9H-fluorene (FPhBr2)[8n] and 9-{4-[9-(4-bromophen-
yl)-9H-fluoren-9-yl]phenyl}-9H-carbazole (FPhCzBr)[10b] were synthesised
according to previous reports.
1H NMR spectra were recorded by using a Varian Mercury plus 400NB
spectrometer relative to tetramethylsilane (TMS) as internal standard.
The molecular mass was determined by a FINNIGAN LCQ by using
electro-spraying ionisation-mass spectrometry (ESI-MS) or MALDI-
TOF-MS. Elemental analyses were performed on a Vario EL III elemen-
tal analyzer. Absorption and photoluminescence (PL) emission spectra of
the target compound were measured by using a SHIMADZU UV-3150
spectrophotometer and a SHIMADZU RF-5301PC spectrophotometer,
respectively. Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were performed on Shimadzu DSC-60A and DTG-
60A thermal analyzers under a nitrogen atmosphere at a heating rate of
108Cminꢀ1. Cyclic voltammetric (CV) studies were conducted by using
an Eco Chemie B.V. AUTOLAB potentiostat in a typical three-electrode
cell with a platinum sheet working electrode, a platinum wire counter
electrode and a silver/silver nitrate (Ag/Ag+) reference electrode. All
electrochemical experiments were carried out under a nitrogen atmos-
phere at room temperature in CH2Cl2 for oxidation and THF for reduc-
tion. Phosphorescence spectra were measured in dichloromethane by
using an Edinburgh FPLS 920 fluorescence spectrophotometer at 77 K
cooling by liquid nitrogen with a delay of 300ms by using a time-correlat-
ed single photon counting (TCSPC) method with a microsecond-pulsed
xenon light source for 10 ms–10 s lifetime measurement, the synchronisa-
tion photomultiplier for signal collection and the multi-channel scaling
mode of the PCS900 fast counter PC plug-in card for data processing.
The thin films of the PO compounds were prepared through vacuum
9,9-Bis-{4’-[2-(diphenylphosphinoyl)phenoxy]biphenyl-4-yl}-9H-fluorene
(DDPESPOF): A mixture of FPhB2 (0.5704 g, 1 mmol), DPESPOBr
(1.3478 g, 3 mmol), TBAB (0.0323 g, 0.1 mmol), [PdACHTNUTRGNE(UNG PPh3)4] (0.1156 g,
0.1 mmol) and 2m NaOH (3 mL,6 mmol) in THF (20 mL) was stirred at
858C for 24 h. The organic layer was separated and washed with an am-
monium chloride solution. The extract phase was dried with anhydrous
Na2SO4 and the solvent was removed by distillation in vacuo. The crude
product was further purified by flash column chromatography by using
methanol/ethyl acetate (1:30) as eluent to give the product as a white
powder (0.6 g, 57%). 1H NMR (TMS, CDCl3, 400 MHz): d=8.10–8.05
(dd, J1 =J2 =7.6 Hz, 2H), 7.81–7.75 (m, 10H), 7.47–7.21ACTHNUTRGNE(UNG m, 34H), 6.81–
6.78 (dd, J1 =5.2, J2 =8 Hz, 2H), 6.62 ppm (d, J=8.8 Hz, 4H); LDI-TOF:
m/z (%): 1054 [M+] (100); elemental analysis (%) for C73H52O4P2: C
83.10, H 4.97, O 6.07; found C 83.07, H 5.00, O 6.13.
9,9-Bis-{3’-(diphenylphosphinoyl)-4’-[2-(diphenylphosphinoyl)phenoxy]bi-
phenyl-4-yl}-9H-fluorene (DDPEPOF): The procedure was similar to
that for DDPESPOF except for the use of DPEPOBr (1.9408 g, 3 mmol)
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zhang, Zhen
