Preparation of acridine-modified benzofuropyridine and benzothienopyridine derivatives and their thermally activated delayed fluorescence properties

Full text of DOI:10.1002/bkcs.10976

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DOI: 10.1002/bkcs.10976
BULLETIN OF THE
H. Kim et al.
KOREAN CHEMICAL SOCIETY
Preparation of Acridine-modified Benzofuropyridine and
Benzothienopyridine Derivatives and Their Thermally Activated
Delayed Fluorescence Properties
Hyojeong Kim,† Kyung Yeon Lee,† Jun Yun Kim,‡ HyoJin Noh,‡ Dae-Wi Yoon,‡
Joong Hwan Yang,‡ Jinook Kim,‡ InByeong Kang,‡ and Suk Joong Lee†,
*
†Department of Chemistry, Research Institute for Natural Sciences, Korea University, Seoul 136-701,
Republic of Korea. *E-mail: slee1@korea.ac.kr
‡LG Display R&D Center, Paju-si 413-811, Republic of Korea
Received July 6, 2016, Accepted September 3, 2016
Keywords: Thermally activated delayed fluorescence, Fluorescence, Phosphorescent, Organic light-
emitting diodes, Host–dopant, Electroluminescence
Recently, organic light-emitting diodes (OLEDs) have
received great deal of attention due to their increased
demand in the fabrication of high-resolution displays,1 and
a lot of researches are concentrated on the development of
new type of materials with high efficiency of electrolumi-
nescence properties.2 Due to their long-lifetime, the devel-
opment of new phosphorescent materials have become one
of the most widely studied research fields, along with the
development of appropriate host materials. The quantum
efficiency of luminescence materials may increase when they
are used together with appropriate hosts, because the host–
dopant system may overwhelm the electroluminescence
instability originated from using the dopant exclusively.3,4
Therefore, it is well established that the high-performance
phosphorescent materials have been considered as one of the
most critical components for current high-resolution displays
that depend highly on the materials with outstanding
electroluminescence efficiency5 and the most developed
high-pitched materials are generally containing expensive
rare metals such as iridium or platinum.6
Meanwhile, the thermally activated delayed fluorescence
(TADF) has been recently suggested as an alternate system
for the rare metal-based high-cost phosphorescent materials,
because they are usually made of organic compounds and
often show outstanding internal quantum efficiency.7 In
fact, the TADF uses both singlet and triplet excitons, and
their energy gap (ΔEst, the energy gap between singlet and
triplet states) is relatively very small, which facilitates the
reverse intersystem crossing (RISC).8,9 And the small ΔEst
can be achieved by the restriction of orbital overlapping
between the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO).9,10
For the restricted overlap of orbitals, a twisted structure
between donor and acceptor units within the molecular sys-
tem is highly preferred.9,11 Therefore, the internal quantum
efficiency of a properly designed TADF system is theoreti-
cally expected to be over 90%.12 Consequently, the
materials employing TADF mechanism are exhibiting great
potentials as alternatives to the conventional luminescence
materials.
In here, we like to describe the synthesis and the photo-
luminescence characteristics of acridine-modified benzofur-
opyridine (1) and benzothienopyridine (2) derivatives,
employing TADF mechanism with aforesaid heavily dis-
torted molecular structures between acceptor parts (benzo-
furopyridine and benzothienopyridine) and donor parts
(dimethylacridines) to promote a small ΔEst between S1
and T1 states (Scheme 1). Furthermore, the heterocyclic
acceptors are decorated by two dimethylacridine groups
expected to display high T1 state.
The photophysical properties of the compounds 1 and
2 were carefully studied by UV–vis and photoluminescence
(PL) spectra (Figure 1). Their photophysical data are sum-
marized in Table S1, Supporting Information and theoreti-
cal energy levels calculated using their electrochemical data
are demonstrated in Table S2. As described in Figures 1
and S1, the peaks showing around 300 nm are related to
the π–π* transition of the dimethylacridine units.13 Figure 1
also shows the PL spectra taken in various degassed sol-
vents including hexane, toluene, chloroform, and methylene
chloride, displaying all the materials with good solvato-
chromic effect in wide range of solvent polarities.14
In order to study their excited state properties, the theo-
retical calculations of low-lying excited states for com-
pounds 1 and 2 were carried out using the density
functional theory (DFT). The Gaussian09 software15 was
used to achieve the ground-state geometry with DFT
method (B3LYP/6-31G[d] level).16 Using these functional
and basis set, the singlet- and triplet-excited states with
lowest energy states were further investigated with their
best ground-state molecular structures. Moreover, cyclic
voltammetry (CV) were used to calculate the HOMO levels
of the compounds 1 and 2, exhibited to be −5.15 and
−5.30 eV, respectively (Figure S2). The experimental
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Figure 2. Frontier molecular orbitals for (a) 1 and (b) 2 from DFT
calculations.
Scheme 1. Preparation of 1 and 2.
oxygen-free environment. On the other hand, a solution
sample of compound 2 in toluene shows the emission max-
ima of 462 nm with a decay time of 21 ns and the delayed
fluorescence components with a lifetime of 1.14 μs under
the oxygen-free environment (Figure 3). These delayed
fluorescence components found in both samples of com-
pounds 1 and 2 were only observed in oxygen-free environ-
ment. It is well-known that oxygen is very strong quencher
of the triplet state and would reduce the fluorescence life-
time dramatically.19 Furthermore, the significant elongation
of fluorescence lifetime found in sulfur-containing com-
pound 2 over oxygen-containing compound 1 is presuma-
bly attributed to different energy gaps (ΔEst) between
singlet and triplet states (Table S2).20 Consequently, the
small ΔEst facilitates RISC process and improves TADF
characteristics dramatically, as described by Van Stryland
and coworkers.20 They have successfully demonstrated
with the direct comparison of a series of sulfur- vs. -
oxygen-containing squaraine systems, and verified that the
replacement of oxygen to sulfur atom in a squaraine ring
conducts a significant reduction of ΔEst.19,20
HOMOs show a great similarity to the simulated HOMOs
as shown in Table S2. On the other hand, the energy levels
of LUMOs for both molecules were obtained from the cor-
responding HOMOs and the resulting optical band gaps
were 3.90 and 3.94 eV for compounds 1 and 2, respec-
tively. The values are quite comparable to the band gaps
obtained from DFT calculations, as indicated in Table S3.
From both CV observations and DFT calculations for
the compounds 1 and 2, the well-separation of electron
densities of HOMO and LUMO in the compounds 1 and
2 is confirmed. For example, the electron density in
HOMO is mostly localized on the acridine units and the
LUMO’s electrons are restricted on the electronegative
heterocycle group (Figure 2). Clearly, perpendicularly
oriented acridine groups with strong electron-donating
capabilities17 are providing small electron density overlap-
ping between HOMO and LUMO, which may expedite
the RISC process.12,18
In order to measure the emission lifetime of compounds
1 and 2, a transient PL study was carried out at room tem-
perature. The transient PL decay profiles of the samples
were obtained at their emission maxima after the excitation
by the He-Cd laser (λ = 355 nm and time duration = 7 ns).
For the oxygen-free experiments, the solutions were
degassed successfully by three consecutive freeze-pump-
thaw rounds. A solution sample of compound 1 in toluene
exhibits the emission maxima of 462 nm with a decay time
of 20 ns (Figure 3). Clearly, it shows the delayed fluores-
cence components with a lifetime of 197 ns under the
Moreover, the photoluminescence quantum yield
(PLQY) of compound 2 improved considerably from 0.045
to 0.36 after bubbling with nitrogen whereas the PLQY of
compound 1 enhanced from 0.017 to 0.15 after this degas-
sing process. The small PLQY and poor fluorescence life-
time in air-saturated environment found in compounds
1 and 2 are apparently due to the presence of oxygen that
quenches their singlet excited states.21
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Hex
Hex
Tol
Tol
CHCl3
DCM
CHCl3
DCM
300
400
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 1. Absorption and emission spectra of (a) 1 and (b) 2 in various solvents (C = 1.0 × 10−6 M) at room temperature.
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Figure 3. Transient PL decay profiles of (a) 1 and (b) 2 in toluene at their emission maxima using 355 nm excitation.
Another important factor for the good performing
increase of PLQY from 0.045 to 0.36 after the removal of
dopants is the high thermal stability when they are fabri-
cated in the OLED devices. The luminescence efficiency of
OLED devices may be decreased dramatically when they
are not able to endure the device operation temperature
(70–90ꢀC), because the decomposition of dopants produces
dark spots on the electrodes which prompt the degradation
of devices.22 Therefore, thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) analysis were
performed to investigate the thermal stabilities of com-
pounds 1 and 2 (Figure S3). The TGA analysis shows that
compound 2 has a high thermal stability and the decompo-
sition temperature is turned out to be 370ꢀC, while com-
oxygen, whereas compound 1 changed from 0.017 to 0.15.
Experimental
Bisdimethylacridylbenzofuropyridine (1). To a degassed
solution of L6 (1 g, 4.20 mmol) and dimethylacridine
(1.76 g, 8.40 mmol) were added a solution of NaO-t-Bu
(1.21 g, 12.6 mmol), [Pd(Π-allyl)Cl]2 (0.154 g, 0.420
mmol), and SPhos (0.345 g, 0.840 mmol) in o-xylene
(20.0 mL) under N2. The resulting mixture was allowed to
stir at 120ꢀC for 12 h. After cooling to room temperature,
the insoluble was filtered off and the filtrate was extracted
with DCM and washed with water and dried over anhy-
drous Na2SO4. After filtration, volatiles were removed
under reduced pressure and the residue was purified by
silica-gel column chromatography (DCM:hexane, 1:3 v/v)
to afford pure 1 as a light green solid (2.00 g, 81.6% yield).
pound
1
exhibits
a
two-step decomposition with
decomposition temperatures of 160 and 305ꢀC. In the ini-
tial step, compound 1 started decomposing slowly at 160ꢀC
with corresponding mass of 2.7% approximately. The sec-
ond decomposition appeared above 305ꢀC. Meanwhile, the
glass transition temperature (Tg) of compound 1 was not
able to analyze, whereas compound 2 showed Tg of 135ꢀC,
showing that 2 would be suitable for further applications.
In summary, we have described the preparation of new
series of highly efficient emitting materials and their PL
properties. Particularly, compound 2 showed effective
TADF with a lifetime of 1.14 μs at 462 nm, HOMO–
LUMO band gap energy of 3.94 eV and calculated energy
gap (ΔEst) of 0.04 eV between S1 and T1 states, while com-
pound 1 showed 197 ns at 462 nm, 3.94 and 0.06 eV,
respectively. Both compounds 1 and 2 exhibit well-
separated HOMO–LUMO energy levels, small ΔEst
between singlet and triplet states as well as long fluores-
cence lifetime with excellent TADF properties, because of
their distorted structures. Furthermore, the significant elon-
gation of fluorescence lifetime found in sulfur-containing
compound 2 is presumably attributed to smaller singlet-
triplet energy difference (ΔEst), facilitating the RISC proc-
ess and improves TADF characteristics. However, these
delayed fluorescence components exhibited in compounds
1 and 2 were not observed in air-equilibrated condition,
due to the dissolved oxygen quenching the excited states
effectively. Meanwhile, compound 2 showed dramatic
4
1H NMR (CDCl3): δ (ppm) 8.50 (d, JH-H = 2.4 Hz, 1H),
4
3
8.30 (d, JH-H = 2.4 Hz, 1H), 7.99 (d, JH-H = 8.6 Hz, 1H),
4
3
7.97 (d, JH-H = 2.4 Hz, 1H), 7.58 (dd, JH-H = 8.6 Hz,
1H), 7.52–7.48 (m, 4H), 7.00–6.95 (m, 8H), 6.30–6.26
(m, 4H), 1.73 (d, JH-H = 3.2 Hz, 12H). 13C NMR
4
(CDCl3): δ (ppm) 162.8, 154.8, 150.5, 141.3, 141.2, 137.3,
134.3, 133.8, 132.7, 130.7, 130.4, 126.8, 126.6, 125.7,
125.6, 125.0, 124.7, 121.5, 121.1, 118.9, 115.2, 114.20,
114.17, 77.5, 36.2, 31.5, 31.3. MS (MALDI-TOF) m/z:
Calcd C41H33N3O [M+]; 583.74 found 584.14. Anal. Calcd
for C41H33N3O: C, 84.36; H, 5.70; N, 7.20; O, 2.74.
Found: C, 86.07; H, 5.67; N: 6.94.
Bisdimethylacridylbenzothienopyridine (2). A procedure
used for 1 was followed except L7 (0.100 g, 0.394 mmol)
in place of L6. The product was obtained as a light ivory
1
solid (0.190 g, 80.5% yield). H NMR (CDCl3): δ (ppm)
4
4
8.69 (d, JH-H = 2.4 Hz, 1H), 8.38 (d, JH-H = 2.4 Hz, 1H),
3
4
8.22 (d, JH-H = 8.4 Hz, 1H), 8.11 (d, JH-H = 1.6 Hz, 1H),
3
7.57 (dd, JH-H = 8.4 Hz, 1H), 7.50–7.47 (m, 4H),
4
7.00–6.91 (m, 8H), 6.31–6.30 (m, 4H), 1.71 (d, JH-
= 3.2 Hz, 12H). 13C NMR (CDCl3): δ (ppm) 161.8,
H
152.3, 141.01, 140.95, 138.9, 138.8, 135.0, 134.8, 132.8,
131.7, 131.3, 130.6, 130.2, 126.6, 126.5, 125.9, 125.6,
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82.10; H, 5.55; N, 7.01; S, 5.35. Found: C, 82.02; H,
5.66; N, 6.91; S, 5.19.
Acknowledgments. The authors acknowledge the financial
supports from the Research Foundation Program
(NRF2013R1A1A2007429, NRF20110020033) and Key
Research Institute Program (NRF20100020209) through
the National Research Foundation of Korea (NRF) and LG
Display Co. Limited (2015–2016).
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pounds and complete characterizations, UV–vis spectra,
additional fluorescence microscopy, CV, TGA, and DSC
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