Chemistry of Materials
Article
Phos (0.3 g, 0.73 mmol) were dissolved in toluene and distilled water
under bubbling with nitrogen for 30 min. Pd2(dba)3 (0.14 g, 0.15
mmol) was added to the solution, and the resulting solution was
refluxed for 24 h under nitrogen atmosphere. The mixture was diluted
with ethyl acetate and washed with distilled water. The organic layer
was dried over anhydrous MgSO4 and evaporated in vacuo to give the
crude product. The extract was evaporated to dryness, affording a
white solid, which was further purified by column chromatography by
ethyl acetate/n-hexane.
RESULTS AND DISCUSSION
■
The 4CzIPN emitter possesses a singlet energy of 2.45 eV,
triplet energy of 2.40 eV, the highest occupied molecular orbital
(HOMO) of −5.80 eV, the lowest unoccupied molecular
orbital (LUMO) of −3.40 eV, and ultraviolet−visible (UV−vis)
absorption edge of 472 nm.19 Therefore, the host materials for
the 4CzIPN emitter should have higher singlet and triplet
energy, deeper HOMO level, and shallower LUMO level than
4CzIPN for energy transfer and little exciton quenching. In
addition, the photoluminescence (PL) emission wavelength of
the host materials should be shorter than the UV−vis
absorption edge of 4CzIPN for efficient energy transfer from
the host materials to the 4CzIPN dopant. Moreover, bipolar
host materials may be effective to balance holes and electrons in
the 4CzIPN device.
The 3TPAPFP and 4TPAPFP host materials were designed
to show bipolar charge transport properties, high singlet and
triplet energy, appropriate HOMO/LUMO level, and PL
emission for energy transfer by combining a triphenylamine
moiety with a furodipyridine moiety. The furodipyridine moiety
is an electron transport unit because of high electron deficiency
by the two electron deficient pyridine units and possesses a
high triplet energy.18 The triphenylamine moiety is a hole
transport unit because of strong electron donating character of
aromatic amine and has a high triplet energy. Therefore, the
combination of triphenylamine with furodipyridine may
produce bipolar host materials with bipolar charge transport
properties and high triplet energy. In addition, the donor−
acceptor structure of the host materials may reduce the singlet
energy while keeping the high triplet energy of the host
materials, which may be beneficial for energy transfer to the
4CzIPN dopant.
The 3TPAPFP and 4TPAPFP host materials were
synthesized by Suzuki coupling reaction between boronic
ester of diphenylaniline and 3-chlorofuro[2,3-b:5,4-b′]-
dipyridine as shown in Scheme 1. The 3-chlorofuro[2,3-b:5,4-
b′]dipyridine intermediate was prepared according to the
synthetic procedure reported in other work.19 Synthetic yields
of 3TPAPFP and 4TPAPFP were 74% and 62%, respectively,
after vacuum train sublimation. The chemical structure of the
host materials was confirmed by chemical analysis, and highly
pure compound with a purity of 99.9% was used for
characterization and device fabrication.
Molecular simulation of 3TPAPFP and 4TPAPFP was
carried out to study the HOMO and LUMO distribution of
the host materials. Figure 1 shows the HOMO and LUMO
distribution of 3TPAPFP and 4TPAPFP calculated by Gaussian
09 program using density functional theory of B3LYP with 6-
1
Yield = 74%. Tg = 67 °C. Tm = 219 °C. H NMR (500 MHz,
CDCl3): δ 8.61 (s, 1H, J = 2.5 Hz), 8.51 (d, 1H, J = 3.25 Hz), 8.33 (s,
1H, J = 2.5 Hz), 8.28 (d, 1H, J = 4.75 Hz), 7.37 (m, 3H), 7.27 (m,
5H), 7.16 (d, 4H, J = 3.75 Hz), 7.12 (d, 1H, J = 4.75 Hz), 7.05 (t, 2H,
J = 4.83 Hz). 13C NMR (500 MHz, CDCl3): δ 162.1, 161.1, 148.8,
147.7, 147.6, 146.5, 138.7, 133.6, 130.5, 130.0, 129.4, 128.7, 124.5,
123.2, 123.1, 122.3, 121.3, 119.8, 115.6, 115.4. MS (FAB) m/z: 414
[(M + H)+]. Anal. Calcd for C28H19N3O: C, 81.34; H, 4.63; N, 10.16;
O, 3.87. Found: C, 81.03; H, 4.64; N, 10.02; O, 3.94.
4-(Furo[2,3-b:5,4-b′]dipyridin-3-yl)-N,N-diphenylaniline
(4TPAPFP). N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)aniline (2) (2.18 g, 5.87 mmol), 3-chlorofuro[2,3-b:5,4-b′]-
dipyridine (3) (1 g, 4.89 mmol), K3PO4 (3.11 g, 14.67 mmol), and S-
Phos (0.3 g, 0.73 mmol) were dissolved in toluene and distilled water
while bubbling with nitrogen for 30 min. Pd2(dba)3 (0.14 g, 0.15
mmol) was added to the solution, and the resulting solution was
refluxed for 24 h under nitrogen atmosphere. The mixture was diluted
with ethyl acetate and washed with distilled water. The organic layer
was dried over anhydrous MgSO4 and evaporated in vacuo to give the
crude product. The extract was evaporated to dryness, affording a
white solid, which was further purified by column chromatography by
ethyl acetate/n-hexane.
1
Yield = 62%. Tg = 74 °C. Tm = 242 °C. H NMR (500 MHz,
CDCl3): δ 8.68 (s, 1H, J = 2.5 Hz), 8.51 (d, 1H, J = 3.25 Hz), 8.38 (s,
1H, J = 2.5 Hz), 8.29 (d, 1H, J = 4.50 Hz), 7.49 (d, 2H, J = 4.25 Hz),
7.39 (t, 1H, J = 4.17 Hz), 7.29 (t, 4H, J = 5.33 Hz), 7.19 (d, 2H, J =
4.25 Hz), 7.15 (d, 4H, J = 4 Hz), 7.06 (t, 2H, J = 5 Hz). 13C NMR
(500 MHz, CDCl3): δ 162.1, 160.8, 148, 147.6, 147.4, 146.1, 133.5,
131, 130.4, 129.4, 128.2, 128, 124.7, 123.6, 123.3, 119.8, 115.7, 115.4.
MS (FAB) m/z: 414 [(M + H)+]. Anal. Calcd for C28H19N3O: C,
81.34; H, 4.63; N, 10.16; O, 3.87. Found: C, 81.35; H, 4.69; N, 10.10;
O, 3.86.
Device Fabrication and Measurements. The device structure of
the TADF OLEDs was indium tin oxide (ITO, 50 nm)/poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, 60
nm)/4,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)aniline]
(TAPC, 20 nm)/1,3-bis(N-carbazolyl)benzene (mCP, 10 nm)/
3TPAPFP or 4TPAPFP:4CzIPN (25 nm)/diphenylphosphine oxide-
4-(triphenylsilyl)phenyl (TSPO1, 35 nm)/LiF (1 nm)/Al (200 nm).
Hole-only device structure was ITO (50 nm)/PEDOT:PSS (10 nm)/
3TPAPFP or 4TPAPFP (100 nm)/TAPC (10 nm)/Al, and electron-
only device structure was ITO (50 nm)/Ca (10 nm)/3TPAPFP or
4TPAPFP (100 nm)/LiF (1 nm)/Al (100 nm). TADF OLED with
CBP as a host was also fabricated as a standard device, and the device
structure was ITO (50 nm)/PEDOT:PSS (10 nm)/TAPC (20 nm)/
mCP (10 nm)/CBP:4CzIPN (25 nm, 5%)/TSPO1 (35 nm)/LiF (1
nm)/Al (200 nm). PL measurement of the 4CzIPN-doped 3TPAPFP
and 4TPAPFP films was carried out using fluorescence spectropho-
tometer (HITACHI, F-7000), and transient PL measurement was
performed using a pulsed Nd−YAG laser (355 nm) as the excitation
light source and an intensified charge-coupled device (ICCD) as a
detector. Current density−voltage characteristics of the hole-only and
electron-only devices were measured using Keithley 2400 source
measurement unit, and the device performances of the TADF OLEDs
were characterized by Keithley 2400 source measurement unit and CS
1000 spectroradiometer. Lambertian distribution of light emission was
confirmed in all quantum efficiency measurement.
Figure 1. HOMO and LUMO distribution of 3TPAPFP and
4TPAPFP.
C
dx.doi.org/10.1021/cm403358h | Chem. Mater. XXXX, XXX, XXX−XXX