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Citation: Heo, Y.; Kwon, H.; Park, S.;
Dae, S.; Lee, H.; Lee, K.; Park, J. A
High-Efficiency Deep Blue Emitter
for OLEDs with a New Dual-Core
Structure Incorporating ETL
Characteristics. Molecules 2023,28,
7485. https://doi.org/10.3390/
molecules28227485
Academic Editors: Guijie Li and
Sidhanath V. Bhosale
Received: 17 October 2023
Revised: 4 November 2023
Accepted: 7 November 2023
Published: 8 November 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
molecules
Article
A High-Efficiency Deep Blue Emitter for OLEDs with a New
Dual-Core Structure Incorporating ETL Characteristics
Yeongjae Heo, Hyukmin Kwon, Sangwook Park, Sunwoo Dae, Hayoon Lee, Kiho Lee and Jongwook Park *
Integrated Engineering, Department of Chemical Engineering, Kyung Hee University,
Gyeonggi 17104, Republic of Korea; youngxi_0514@khu.ac.kr (Y.H.); hm531@khu.ac.kr (H.K.);
pswook@khu.ac.kr (S.P.); sunwoo4674@khu.ac.kr (S.D.); kssarang1@khu.ac.kr (H.L.); kiholee@khu.ac.kr (K.L.)
*Correspondence: jongpark@khu.ac.kr
Abstract:
In this study, we introduced the weak electron-accepting oxazole derivative 4,5-diphenyl-
2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)oxazole (TPO) into both anthracene and
pyrene moieties of a dual core structure. Ultimately, we developed 2-(4-(6-(anthracen-9-yl)pyren-
1-yl)phenyl)-4,5-diphenyloxazole (AP-TPO) as the substitution on the second core, pyrene, and
4,5-diphenyl-2-(4-(10-(pyren-1-yl)anthracen-9-yl)phenyl)oxazole (TPO-AP) as the substitution on the
first core, anthracene. Both materials exhibited maximum photoluminescence wavelengths at 433 and
443 nm in solution and emitted deep blue light with high photoluminescence quantum yields of 82%
and 88%, respectively. When used as the emitting layer in non-doped devices, TPO-AP outperformed
AP-TPO, achieving a current efficiency of 5.49 cd/A and an external quantum efficiency of 4.26% in
electroluminescence. These materials introduce a new category of deep blue emitters in the organic
light-emitting diodes field, combining characteristics related to the electron transport layer.
Keywords: OLED; blue; dual core; side group; oxazole; ETL
1. Introduction
The conventional blue-emitting materials for organic light-emitting diodes (OLEDs)
have primarily used single-core chromophores with an excellent photoluminescence quan-
tum yield (PLQY), such as anthracene, pyrene, and fluorene [
1
–
3
]. However, these chro-
mophores are planar, which increases the intermolecular packing in the film state and
promotes excimer formation, leading to reduced electroluminescence (EL) efficiency and
color purity [
4
–
6
]. Also, bulky side groups are typically added to the chromophore to
prevent molecular interaction. However, in our study, rather than using side groups, we
combined anthracene and pyrene to create a dual-core chromophore, the AP-Core chro-
mophore, maintaining a dihedral angle of approximately 90 degrees between the two
moieties. This strategy aimed to prevent excimer formation and suppress intermolecular
packing, leading to the development of a new chromophore. Our approach resulted in
more than twice the efficiency and device lifetime compared to materials using anthracene
and pyrene as single cores [
7
]. Subsequent research introduced various side groups, sym-
metrically or asymmetrically, based on the dual core to provide blue-emitting materials [
8
].
However, in this study, we incorporated the same side group on each of the dual cores,
anthracene and pyrene, using the 4,5-diphenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)phenyl)oxazole (TPO) moiety. TPO possesses a weak electron-accepting characteristic
that was not previously applied to the AP-Core, aiming to enhance charge balance [
9
,
10
].
This molecular design allowed for a detailed examination of the roles of the first core,
anthracene, and the second core, pyrene. Finally, we synthesized two new blue-emitting
materials, 2-(4-(6-(anthracen-9-yl)pyren-1-yl)phenyl)-4,5-diphenyloxazole (AP-TPO) and
4,5-diphenyl-2-(4-(10-(pyren-1-yl)anthracen-9-yl)phenyl)oxazole (TPO-AP). We also inves-
tigated the changes in the photophysical, thermal, and EL properties of the synthesized
compounds (Figure 1).
Molecules 2023,28, 7485. https://doi.org/10.3390/molecules28227485 https://www.mdpi.com/journal/molecules
Molecules 2023,28, 7485 2 of 12
Molecules2023,28,xFORPEERREVIEW2of13
yl)phenyl)oxazole(TPO-AP).Wealsoinvestigatedthechangesinthephotophysical,
thermal,andELpropertiesofthesynthesizedcompounds(Figure1).
Figure 1. ChemicalstructuresofAP-TPOandTPO-AP.
2. Results and Discussion
2.1.PhotophysicalPropertiesandTheoreticalCalculations
WepreparedTPO-APandAP-TPObysubstitutingsidegroupsonthefirstcore,
anthracene,andthesecondcore,pyrene,basedontheAP-Core.TheAP-Coremaintains
orthogonality,preventingtheextensionofmolecularconjugation,makingitasuitable
coreforablue-emiingmaterial.TheTPOexhibitsaweakelectron-accepting
characteristicwhichcanenhancetheoverallchargebalanceofthemolecule.Table 1and
Figure2provideacomprehensivesummaryoftheultraviolet-visible(UV-Vis)absorption
andthephotoluminescence(PL)spectraofthesynthesizedcompoundsinboththe
solutionandfilmstates.Inthesolutionstate,theAP-TPOshowsmainabsorptionpeaks
at371nmand390nm,whiletheTPO-APshowsmainabsorptionpeaksat328nmand
341nm.BothcompoundsshareacommonabsorptionaributedtotheTPOinthe300–
340nmrange,andtheabsorptioninthe340–400nmrangeisduetotheπ-π*transitionof
theanthraceneandpyrenecores[9,11,12].However,theAP-TPOshowsahighabsorption
intheAP-Coreregion,andtheTPO-APexhibitsahighabsorptionintheTPOabsorption
region.
Figure 1. Chemical structures of AP-TPO and TPO-AP.
2. Results and Discussion
2.1. Photophysical Properties and Theoretical Calculations
We prepared TPO-AP and AP-TPO by substituting side groups on the first core,
anthracene, and the second core, pyrene, based on the AP-Core. The AP-Core maintains
orthogonality, preventing the extension of molecular conjugation, making it a suitable core
for a blue-emitting material. The TPO exhibits a weak electron-accepting characteristic
which can enhance the overall charge balance of the molecule. Table 1and Figure 2
provide a comprehensive summary of the ultraviolet-visible (UV-Vis) absorption and the
photoluminescence (PL) spectra of the synthesized compounds in both the solution and
film states. In the solution state, the AP-TPO shows main absorption peaks at 371 nm and
390 nm, while the TPO-AP shows main absorption peaks at 328 nm and 341 nm. Both
compounds share a common absorption attributed to the TPO in the 300–340 nm range,
and the absorption in the 340–400 nm range is due to the
π
-
π
* transition of the anthracene
and pyrene cores [
9
,
11
,
12
]. However, the AP-TPO shows a high absorption in the AP-Core
region, and the TPO-AP exhibits a high absorption in the TPO absorption region.
Table 1. Photophysical properties of synthesized compounds.
Solution aFilm b
PLQY c
(%)
UVmax
(nm)
PLmax
(nm)
FWHM
(nm)
UVmax
(nm)
PLmax
(nm)
FWHM
(nm)
AP-TPO 371, 390 433 54 377, 396 464 80 82/20
TPO-AP 328, 341, 443 58 332, 345 462 72 88/24
a
Toluene solution (1.0
×
10
−5
M).
b
Film thickness: 50 nm on a glass substrate.
c
Absolute PLQY of solution
state/film state.
Molecules2023,28,xFORPEERREVIEW3of13
Figure 2. NormalizedUV-visibleabsorptionandPLspectraofnewlysynthesizedcompounds:(a)
insolutionstate(concentration:1×10
−5
Mintoluene),(b)inevaporatedfilmstate(thickness:50nm).
Figure 2.
Normalized UV-visible absorption and PL spectra of newly synthesized compounds: (
a
) in
solution state (concentration: 1
×
10
−5
M in toluene), (
b
) in evaporated film state (thickness: 50 nm).
Molecules 2023,28, 7485 3 of 12
Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were
performed using the B3LYP/6-311G(d) method and basis set with the ORCA program [
13
].
We calculated the optimized structures of the synthesized materials, as well as the energy
level values at highest occupied molecular orbital (HOMO), the lowest unoccupied molec-
ular orbital (LUMO), and the frontier orbitals using DFT (Figure 3). Furthermore, we used
the TD-DFT to compute the expected absorption wavelengths, the oscillator strengths at
those wavelengths, the transition states, and the contributions at each transition state for
each material (Table 2). In previous studies, it was noted that in the AP-Core, the electron
density distribution primarily resides in anthracene for the HOMO and LUMO, while
HOMO
−
1 and LUMO+1 are mainly distributed in pyrene. Based on this, anthracene was
denoted as the first core and pyrene as the second core. Additionally, previous studies
indicated that the transition with the highest oscillator strength occurs between HOMO and
LUMO at the longest wavelength [
8
]. However, AP-TPO and TPO-AP exhibited different
trends. In the case of the AP-TPO, the electron density is evenly spread across the molecule
in HOMO and HOMO
−
1, and in LUMO, it is predominantly distributed in pyrene and the
TPO side. At the wavelength of the highest oscillator strength, 386 nm, the most significant
contribution was observed in the transition from HOMO
−
1 to LUMO, accounting for
55%. On the other hand, the transition from HOMO to LUMO was calculated to be 21%.
For the TPO-AP, in both HOMO and LUMO, the electron density is primarily present in
anthracene, and in LUMO, it is partially present in the TPO. HOMO
−
1 and LUMO+1
mainly have electron density in pyrene, exhibiting a trend similar to previous AP-core
derivatives. However, at the wavelength where the oscillator strength is the highest,
382 nm, the transition from HOMO
−
2 to LUMO+2, with an 83% contribution, was iden-
tified as the primary transition. The electron density of HOMO
−
2 and LUMO+2 in the
TPO-AP is mainly distributed in the TPO moiety. Consequently, considering the major tran-
sition with a significant contribution at the wavelength with the highest oscillator strength,
it can be presumed that absorption occurs predominantly in the AP-Core for the AP-TPO
and in the TPO moiety for the TPO-AP. In the film state, the AP-TPO exhibited maximum
absorption at 377 nm and 396 nm, while the TPO-AP exhibited maximum absorption at
332 nm and 345 nm, with trends similar to those in the solution state. However, transi-
tioning from the solution state to the film state resulted in molecular interactions due to
the close proximity of molecules, causing a red-shift and broadening of the UV-Vis and
PL spectra. When compared with the solution state, the absorption wavelengths of the
AP-TPO and the TPO-AP red-shifted by 6 nm and 4 nm, respectively, in the film state, and
they showed a broader shape [14].
In the solution state, the PL maximum (PLmax) values for the AP-TPO and the TPO-AP
were 433 nm and 443 nm, respectively, indicating emission of light in the deep blue region
for both materials. In the film state, the PLmax values for the AP-TPO and the TPO-AP
were 464 nm and 462 nm, respectively, signifying a bathochromic shift of 32 nm and 19 nm,
respectively, compared to the solution state. Moreover, the full width at the half maximum
(FWHM) values were 80 nm and 72 nm for AP-TPO and TPO-AP, respectively, in the film
state, representing an increase of 26 nm and 17 nm, respectively, compared to the solution
state. The significant difference and broader FWHM in the PLmax between the solution
and film states of AP-TPO are believed to result from stronger molecular interactions and
packing compared to TPO-AP [
15
]. A detailed examination of the optimized structures
using molecular calculations revealed that the dihedral angle of the AP-Core exhibited the
anticipated orthogonal structure. However, in the AP-TPO, the dihedral angle between
the pyrene and the TPO was 56.5
◦
, and in the TPO-AP, the dihedral angle between the
anthracene and the TPO was approximately 81.7
◦
, which is close to 90
◦
. Therefore, it is
presumed that the TPO-AP, which effectively prevents intermolecular packing, contributes
to the change in PLmax. Both synthesized materials exhibited an emission in the blue
region in the film state (Figure 4).
Molecules 2023,28, 7485 4 of 12
Molecules2023,28,xFORPEERREVIEW5of13
Figure 3. ElectrondensitydistributionsofHOMO−2,HOMO−1,HOMO,LUMO,LUMO+1,and
LUMO+2calculatedwithB3LYP/6-311G(d).
Table 2. AbsorptionfrequenciesandoscillatorstrengthscalculatedwithB3LYP/6-311G(d)forAP-
TPOandTPO-AP.
Compound Absorption
Wavelength (nm)
Oscillator
Strength Characteristic of Transition Contribution
a
(%)
AP-TPO
412.90.011HOMO−1→ LUMO
HOMO→LUMO
23.5
70.0
403.20.014HOMO−1→LUMO+1
HOMO→LUMO+1
52.7
34.7
386.11.321
HOMO−1→LUMO
HOMO→LUMO+1
HOMO→LUMO
54.5
12.8
21.6
363.90.010HOMO−1→LUMO+1
HOMO→LUMO+1
25.1
51.0
TPO-AP399.40.005HOMO→LUMO93.4
Figure 3.
Electron density distributions of HOMO
−
2, HOMO
−
1, HOMO, LUMO, LUMO+1, and
LUMO+2 calculated with B3LYP/6-311G(d).
Table 2.
Absorption frequencies and oscillator strengths calculated with B3LYP/6-311G(d) for AP-
TPO and TPO-AP.
Compound Absorption
Wavelength (nm) Oscillator Strength Characteristic of
Transition Contribution a(%)
AP-TPO
412.9 0.011 HOMO−1→LUMO
HOMO →LUMO
23.5
70.0
403.2 0.014 HOMO−1→LUMO+1
HOMO →LUMO+1
52.7
34.7
386.1 1.321
HOMO−1→LUMO
HOMO →LUMO+1
HOMO →LUMO
54.5
12.8
21.6
363.9 0.010 HOMO−1→LUMO+1
HOMO →LUMO+1
25.1
51.0
TPO-AP
399.4 0.005 HOMO →LUMO 93.4
398.7 0.003 HOMO−1→LUMO+1 94.0
382.8 0.383 HOMO−2→LUMO+2 83.9
372.7 0.164 HOMO−1→LUMO
HOMO →LUMO+1
19.1
78.7
a
When the sum of contributions is less than 100%, the remaining contributions include various small portions of
transitions. Only the main process was indicated.
Molecules 2023,28, 7485 5 of 12
Molecules2023,28,xFORPEERREVIEW5of12
398.70.003HOMO−1→LUMO+194.0
382.80.383HOMO−2→LUMO+283.9
372.70.164HOMO−1→LUMO
HOMO→LUMO+1
19.1
78.7
a
Whenthesumofcontributionsislessthan100%,theremainingcontributionsincludevarioussmall
portionsoftransitions.Onlythemainprocesswasindicated.
Inthesolutionstate,thePLmaximum(PLmax)valuesfortheAP-TPOandtheTPO-
APwere433nmand443nm,respectively,indicatingemissionoflightinthedeepblue
regionforbothmaterials.Inthefilmstate,thePLmaxvaluesfortheAP-TPOandtheTPO-
APwere464nmand462nm,respectively,signifyingabathochromicshiftof32nmand
19nm,respectively,comparedtothesolutionstate.Moreover,thefullwidthatthehalf
maximum(FWHM)valueswere80nmand72nmforAP-TPOandTPO-AP,respectively,
inthefilmstate,representinganincreaseof26nmand17nm,respectively,comparedto
thesolutionstate.ThesignificantdifferenceandbroaderFWHMinthePLmaxbetween
thesolutionandfilmstatesofAP-TPOarebelievedtoresultfromstrongermolecular
interactionsandpackingcomparedtoTPO-AP[15].Adetailedexaminationofthe
optimizedstructuresusingmolecularcalculationsrevealedthatthedihedralangleofthe
AP-Coreexhibitedtheanticipatedorthogonalstructure.However,intheAP-TPO,the
dihedralanglebetweenthepyreneandtheTPOwas56.5°,andintheTPO-AP,the
dihedralanglebetweentheanthraceneandtheTPOwasapproximately81.7°,whichis
closeto90°.Therefore,itispresumedthattheTPO-AP,whicheffectivelyprevents
intermolecularpacking,contributestothechangeinPLmax.Bothsynthesizedmaterials
exhibitedanemissionintheblueregioninthefilmstate(Figure4).
Figure 4. OptimizedstructuresofAP-TPOandTPO-AP.
Inboththesolutionandfilmstates,theAP-TPOexhibitedaPLQYof82%and20%,
whiletheTPO-APshowedvaluesof88%and24%.Generally,luminescenceefficiency
decreasesinthefilmstatecomparedtothesolutionstateduetonumerousnon-radiative
pathwaysresultingfromcloserintermoleculardistances,whichleadstoquenching[16].
However,theTPO-APdemonstratedahigherPLQYcomparedtotheAP-TPOduetothe
substitutionofpyreneasthesecondcoreandtheTPOasthesidegrouponbothsidesof
theanthracene,resultinginsterichindranceandahighlytwistedstructure.Thishindered
intermolecularpackingandyieldedahigherPLQY.TheseincreasedPLQYvaluesare
beneficialforachievinganincreasedELefficiencyinOLEDdevices.Thetime-resolved
photoluminescence(TRPL)wasusedtomeasurefluorescencelifetime(τ
F
)whichallowed
forthecalculationoftheradiativerateconstant(k
r
)andthenon-radiativerateconstant
(k
nr
).Timedecaycurvesofthesecompoundsshowedamono-exponentialtypeanda
promptfluorescenceemissionwithnodelayedfluorescencecomponent(FigureS1).[17]
Theτ
F
valuesfortheAP-TPOandtheTPO-APweremeasuredas1.26and1.36ns,
respectively.TheAP-TPOandtheTPO-APbothexhibitedsimilarvaluesfork
r
at6.50and
6.48×10
8/
s,respectively.However,theAP-TPOhadahigherk
nr
of14.3×10
7
/scompared
totheTPO-AP’s8.83×10
7
/s,resultinginalowerPLQY.
Figure 4. Optimized structures of AP-TPO and TPO-AP.
In both the solution and film states, the AP-TPO exhibited a PLQY of 82% and 20%,
while the TPO-AP showed values of 88% and 24%. Generally, luminescence efficiency
decreases in the film state compared to the solution state due to numerous non-radiative
pathways resulting from closer intermolecular distances, which leads to quenching [
16
].
However, the TPO-AP demonstrated a higher PLQY compared to the AP-TPO due to the
substitution of pyrene as the second core and the TPO as the side group on both sides of
the anthracene, resulting in steric hindrance and a highly twisted structure. This hindered
intermolecular packing and yielded a higher PLQY. These increased PLQY values are
beneficial for achieving an increased EL efficiency in OLED devices. The time-resolved
photoluminescence (TRPL) was used to measure fluorescence lifetime (
τF
) which allowed
for the calculation of the radiative rate constant (k
r
) and the non-radiative rate constant (k
nr
).
Time decay curves of these compounds showed a mono-exponential type and a prompt
fluorescence emission with no delayed fluorescence component (Figure S1). [
17
] The
τF
values for the AP-TPO and the TPO-AP were measured as 1.26 and 1.36 ns, respectively.
The AP-TPO and the TPO-AP both exhibited similar values for k
r
at 6.50 and 6.48
×
10
8/
s,
respectively. However, the AP-TPO had a higher k
nr
of 14.3
×
10
7
/s compared to the
TPO-AP’s 8.83 ×107/s, resulting in a lower PLQY.
2.2. Thermal and Electrical Properties
The thermal properties of the synthesized materials were determined using ther-
mogravimetric analyzers (TGA) and differential scanning calorimetry (DSC). The results
showed that the AP-TPO had values of 448
◦
C, 151
◦
C, and 283
◦
C for degradation tem-
peratures (T
d
, 5% weight loss temperature), glass-transition temperature (T
g
), and melting
temperature (T
m
), respectively. The TPO-AP exhibited measurements of 395
◦
C, 162
◦
C,
and 307
◦
C for T
d
, T
g
, and T
m
, respectively (Table 3and Figure 5). The incorporation of
the TPO side group demonstrated superior thermal stability compared to the AP-Core.
The AP-TPO exhibited a higher Tdcompared to the TPO-AP. This can be attributed to the
highly twisted conformation between the anthracene and the TPO in the TPO-AP, leading
to an increased internal molecular instability and, consequently, a lower T
d
when com-
pared to the AP-TPO. However, the T
g
of the TPO-AP is higher than that of the AP-TPO.
Additionally, all the thermal properties of both materials are well above the temperatures
required for stable device operation, making them suitable for use in device processing
and operation [
18
]. Moreover, the measurements were conducted for device preparation,
determining the HOMO level, band gap, and the LUMO level. The HOMO energy level
was determined using AC-2, and the LUMO energy level was calculated by adding the
band gap energy to the measured HOMO energy level. The band gap energy was estimated
from the absorption edge in the thin film state using a plot of (hv) vs. (
α
hv)
2
, where
α
,
h, and vrepresented the absorbance, Planck’s constant, and frequency of light, respec-
tively. The measured HOMO levels for the AP-TPO and the TPO-AP were
−
5.80 eV and
−
5.84 eV, respectively, showing similarity. The measured band gaps were also similar, with
the AP-TPO at 2.92 eV and the TPO-AP at 2.88 eV, with the longer conjugation length of the
TPO-AP resulting in a slightly smaller band gap. To aid in understanding the experimental
data of the synthesized materials, we performed theoretical calculations to obtain the trend
of relative changes by comparing the theoretical values. Generally, the DFT calculation
methods commonly used for the computation of the HOMO and LUMO energy levels are
Molecules 2023,28, 7485 6 of 12
primarily designed for total energy calculations and the description of optimized structures.
They are not oriented towards achieving precise molecular orbital calculations which can
lead to discrepancies between the calculated and measured values [
19
,
20
]. To facilitate
the relative comparison between theoretical predictions and experimental measurements,
we conducted molecular calculations to determine the HOMO and LUMO energy lev-
els. The confirmed HOMO levels for the AP-TPO and the TPO-AP were
−
5.45 eV and
−
5.42 eV, respectively, while the LUMO levels were
−
2.00 eV and
−
1.95 eV, respectively.
These values may differ from the experimental measurements, but the trends were found to
be similar.
Table 3. Thermal and electrical properties of AP-TPO and TPO-AP.
Td(◦C) Tg(◦C) Tm(◦C) HOMO (eV) LUMO (eV) Band Gap (eV)
AP-TPO 448 151 283 −5.80 −2.88 2.92
TPO-AP 395 162 307 −5.84 −2.96 2.88
Molecules2023,28,xFORPEERREVIEW7of13
2.2.ThermalandElectricalProperties
Thethermalpropertiesofthesynthesizedmaterialsweredeterminedusing
thermogravimetricanalyzers(TGA)anddifferentialscanningcalorimetry(DSC).The
resultsshowedthattheAP-TPOhadvaluesof448°C,151°C,and283°Cfordegradation
temperatures(T
d
,
5%weightlosstemperature),glass-transitiontemperature(T
g
),and
meltingtemperature(T
m
),respectively.TheTPO-APexhibitedmeasurementsof395°C,
162°C,and307°CforT
d
,T
g
,andT
m
,respectively(Table3andFigure5).Theincorporation
oftheTPOsidegroupdemonstratedsuperiorthermalstabilitycomparedtotheAP-Core.
TheAP-TPOexhibitedahigherT
d
comparedtotheTPO-AP.Thiscanbeaributedtothe
highlytwistedconformationbetweentheanthraceneandtheTPOintheTPO-AP,leading
toanincreasedinternalmolecularinstabilityand,consequently,alowerT
d
when
comparedtotheAP-TPO.However,theT
g
oftheTPO-APishigherthanthatoftheAP-
TPO.Additionally,allthethermalpropertiesofbothmaterialsarewellabovethe
temperaturesrequiredforstabledeviceoperation,makingthemsuitableforuseindevice
processingandoperation[18].Moreover,themeasurementswereconductedfordevice
preparation,determiningtheHOMOlevel,bandgap,andtheLUMOlevel.TheHOMO
energylevelwasdeterminedusingAC-2,andtheLUMOenergylevelwascalculatedby
addingthebandgapenergytothemeasuredHOMOenergylevel.Thebandgapenergy
wasestimatedfromtheabsorptionedgeinthethinfilmstateusingaplotof(hv)vs.(αhv)
2
,
whereα,h,andvrepresentedtheabsorbance,Planck’sconstant,andfrequencyoflight,
respectively.ThemeasuredHOMOlevelsfortheAP-TPOandtheTPO-APwere−5.80eV
and−5.84eV,respectively,showingsimilarity.Themeasuredbandgapswerealsosimilar,
withtheAP-TPOat2.92eVandtheTPO-APat2.88eV,withthelongerconjugationlength
oftheTPO-APresultinginaslightlysmallerbandgap.Toaidinunderstandingthe
experimentaldataofthesynthesizedmaterials,weperformedtheoreticalcalculationsto
obtainthetrendofrelativechangesbycomparingthetheoreticalvalues.Generally,the
DFTcalculationmethodscommonlyusedforthecomputationoftheHOMOandLUMO
energylevelsareprimarilydesignedfortotalenergycalculationsandthedescriptionof
optimizedstructures.Theyarenotorientedtowardsachievingprecisemolecularorbital
calculationswhichcanleadtodiscrepanciesbetweenthecalculatedandmeasuredvalues
[19,20].Tofacilitatetherelativecomparisonbetweentheoreticalpredictionsand
experimentalmeasurements,weconductedmolecularcalculationstodeterminethe
HOMOandLUMOenergylevels.TheconfirmedHOMOlevelsfortheAP-TPOandthe
TPO-APwere−5.45eVand−5.42eV,respectively,whiletheLUMOlevelswere−2.00eV
and−1.95eV,respectively.Thesevaluesmaydifferfromtheexperimentalmeasurements,
butthetrendswerefoundtobesimilar.
Figure 5. DSCcurvesofthesynthesizedAP-TPOandTPO-AP.
Figure 5. DSC curves of the synthesized AP-TPO and TPO-AP.
2.3. Electroluminescence Properties
Using the measured energy levels, non-doped OLED devices were fabricated with AP-
TPO and TPO-AP as the EML using the following structure: ITO/2-TNATA (60 nm)/NPB
(15 nm)/emissive layer (EML) (35 nm)/Alq
3
(20 nm)/LiF (1 nm)/Al (200 nm). The proper-
ties of the OLED devices at 10 mA/cm
2
are summarized in Table 4. The operating voltages
for the fabricated AP-TPO and TPO-AP devices were measured at 5.44 V and 5.86 V at
10 mA/cm
2
, respectively. The two materials, having similar molecular structures and
energy levels, exhibited comparable operating voltages [
21
,
22
] (Figure 6a,b). The turn-on
voltage of the two devices, AP-TPO and TPO-AP, was observed to be relatively higher at
3.27 V and 3.73 V at 1 cd/m
2
, respectively, compared to the commercialized blue devices.
In particular, the TPO-AP exhibited a slightly higher turn-on voltage. This is believed
to be attributed to the interface properties between the hole transporting layer (HTL) of
the NPB and the EML. The interface characteristics appear to be fine for the AP-TPO but
relatively poorer for the TPO-AP. Further investigations involving dielectric constant and
contact angle studies of the NPB material and both materials will be discussed in future
research. The power efficiency (PE) for both devices was similar, with values of 2.52 lm/W
for the AP-TPO and 2.94 lm/W for the TPO-AP (Figure 6d). However, the current efficiency
(CE) for the AP-TPO and the TPO-AP was 4.33 cd/A and 5.49 cd/A, respectively, and the
external quantum efficiency (EQE) was 3.73% for the AP-TPO and 4.26% for the TPO-AP,
indicating that the TPO-AP exhibited a higher CE than the AP-TPO (Figure 6c,e). This is
attributed to the TPO substituents on the first core, anthracene, effectively suppressing
intermolecular packing, leading to a high PLQY. Furthermore, the measurement of transient
EL revealed delayed fluorescence in both synthesized materials (Figure S2). This delayed
Molecules 2023,28, 7485 7 of 12
fluorescence is attributed to the triplet-triplet annihilation (TTA) occurring in the anthracene
and the pyrene moieties during the operation of the EL device. It is known that anthracene
and pyrene are moieties where the TTA occurs efficiently during the operation of the EL
devices [
23
,
24
]. When measuring the LTPL of the two synthesized materials, the triplet
energy level (T
1
) calculated from the band edge of each spectrum for the AP-TPO and the
TPO-AP was found to be 2.61 and 2.23 eV, respectively (Figure S3). The singlet energy level
(S
1
) was determined to be 3.05 eV for the AP-TPO and 3.01 eV for the TPO-AP through
measurement of PL at room temperature. Due to the longer molecular conjugation in the
TPO-AP, its S
1
and T
1
values were lower than those of the AP-TPO. Both synthesized mate-
rials exhibited significant
∆
E
ST
values of 0.44 and 0.78 eV, indicating the difficulty of the
thermally activated delayed fluorescence process. The TPO-AP, with a higher occurrence of
the TTA, showed a longer transient EL response compared to the AP-TPO, which is believed
to contribute to its higher EQE [
25
]. Generally, polycyclic aromatic hydrocarbons tend to
exhibit hole-dominant properties with lower electron mobility, causing an imbalance in
the charge within the device and leading to an efficiency roll-off [
26
,
27
]. However, the two
materials synthesized in this study incorporate the TPO, which has weak electron-accepting
characteristics, into the AP-core, improving the overall carrier balance and minimizing
the roll-off, even at higher current densities. The EL maximum (ELmax) for the AP-TPO
and the TPO-AP were observed at 447 nm and 453 nm, respectively, with CIE coordinates
of (0.156, 0.134) and (0.168, 0.154), respectively, indicating blue emission (Figure 6f). The
ELmax of the AP-TPO showed an approximately 17 nm blue shift compared to the film
PL max, while the TPO-AP exhibited a 9 nm hypsochromic shift. However, the shape of
the PL and the EL spectra was similar. This can be attributed to the recombination zone
(RZ) existing within the EML due to the fast hole mobility, resulting in its formation closer
to the ETL. To address this, we aim to adjust the thickness of the HTL and ETL layers or
consider the substitution of a high electron mobility ETL in future research [
28
]. The device
lifetime (LT) of the AP-TPO and the TPO-AP was measured at 1000 nits, and the LT
50
for
each device was found to be 0.54 and 3.1 h, respectively (Figure S4). In the case of the
TPO-AP, where the side groups are substituted on both sides of the first core, anthracene,
it is believed that this molecular design efficiently hinders the intermolecular packing,
reduces the non-radiative pathways, and enables stable device operation, resulting in the
longer lifetime observed in the TPO-AP.
Molecules2023,28,xFORPEERREVIEW9of13
adjustthethicknessoftheHTLandETLlayersorconsiderthesubstitutionofahigh
electronmobilityETLinfutureresearch[28].Thedevicelifetime(LT)oftheAP-TPOand
theTPO-APwasmeasuredat1000nits,andtheLT
50
foreachdevicewasfoundtobe0.54
and3.1h,respectively(FigureS4).InthecaseoftheTPO-AP,wherethesidegroupsare
substitutedonbothsidesofthefirstcore,anthracene,itisbelievedthatthismolecular
designefficientlyhinderstheintermolecularpacking,reducesthenon-radiative
pathways,andenablesstabledeviceoperation,resultinginthelongerlifetimeobserved
intheTPO-AP.
Figure 6. ELperformancesofAP-TPOandTPO-AP:(a)energylevelsofthefabricateddevices,(b)
J-V-Lcurve,(c)CEcurves,(d)PEcurves,(e)EQEcurves,and(f)ELspectra.
Table 4. ELperformancesofnon-dopedOLEDdevicesat10mA/cm
2
.
Volt (V) CE (Cd/A) PE (lm/W) EQE (%) CIE (x, y) EL
max
(nm)
AP-TPO5.444.332.523.73(0.156,0.134)447
TPO-AP5.865.492.944.26(0.168,0.154)453
3. Experimental
3.1.Materials,Measurements,andDeviceFabrication
ThesynthesizedcompoundswereconfirmedusingaBrukerAva nce400forproton
NMRspectroscopy(Bruker,Billerica,MA,USA).Theopticalpropertieswereassessed
usinganHP8453UV-VISNIRspectrometer(Agilent,SantaClara,CA,USA)forUV-Vis
absorptionandaPerkin-ElmerluminescencespectrometerLS50(Perkin-Elmer,Walth a m,
MA,USA)withaXenonflashtubeforPL.TheT
d
ofmaterialsweredeterminedusingTGA
(SDTQ600,TAInstruments,NewCastle,DE,USA).TheT
g
andT
m
ofthecompoundswere
determinedusingDSCunderanitrogenatmospherewithaDSC4000(PerkinElmer,MA,
USA).TheHOMOenergylevelwasmeasuredbyaphotoelectronspectrometer(AC-2,
RikenKeiki,Tokyo,Japan).ThelowtemperaturePL(LTPL)wasobtainedbyaJASCOFP-
8500spectrofluorometer(JASCO,Tokyo,Japan).ThetransientELwascarriedoutat10
mA/cm
2
.Theaspectratioofthepulseappliedfromthe33600Afunctiongenerator
(KEYSIGHT,SantaRosa,CA,USA)is1:1,andthepulsewidthis500us.TheTRPLcurves
wereobtainedbyaquantaurus-taufluorescencelifetimespectrometerC11367-11
(HAMAMATSUPHOTONICSK.K,Hamamatsu,Japan).Non-dopedOLEDdevicesfor
Figure 6.
EL performances of AP-TPO and TPO-AP: (
a
) energy levels of the fabricated devices,
(b) J-V-L curve, (c) CE curves, (d) PE curves, (e) EQE curves, and (f) EL spectra.
Molecules 2023,28, 7485 8 of 12
Table 4. EL performances of non-doped OLED devices at 10 mA/cm2.
Volt (V) CE (Cd/A) PE (lm/W) EQE (%) CIE (x, y) ELmax (nm)
AP-TPO 5.44 4.33 2.52 3.73 (0.156, 0.134) 447
TPO-AP 5.86 5.49 2.94 4.26 (0.168, 0.154) 453
3. Experimental
3.1. Materials, Measurements, and Device Fabrication
The synthesized compounds were confirmed using a Bruker Avance 400 for proton
NMR spectroscopy (Bruker, Billerica, MA, USA). The optical properties were assessed
using an HP 8453 UV-VISNIR spectrometer (Agilent, Santa Clara, CA, USA) for UV-Vis
absorption and a Perkin-Elmer luminescence spectrometer LS50 (Perkin-Elmer, Waltham,
MA, USA) with a Xenon flash tube for PL. The T
d
of materials were determined using TGA
(SDT Q600, TA Instruments, New Castle, DE, USA). The T
g
and T
m
of the compounds
were determined using DSC under a nitrogen atmosphere with a DSC4000 (PerkinElmer,
MA, USA). The HOMO energy level was measured by a photoelectron spectrometer (AC-2,
Riken Keiki, Tokyo, Japan). The low temperature PL (LTPL) was obtained by a JASCO
FP-8500 spectrofluorometer (JASCO, Tokyo, Japan). The transient EL was carried out at
10 mA/cm
2
. The aspect ratio of the pulse applied from the 33600A function generator
(KEYSIGHT, Santa Rosa, CA, USA) is 1:1, and the pulse width is 500 us. The TRPL
curves were obtained by a quantaurus-tau fluorescence lifetime spectrometer C11367-11
(HAMAMATSU PHOTONICS K.K, Hamamatsu, Japan). Non-doped OLED devices for
blue emission were fabricated with the following structure: ITO/2-TNATA (60 nm)/NPB
(15 nm)/EML (35 nm)/Alq
3
(20 nm)/LiF (1 nm)/Al (200 nm). In this structure, the 2-
TNATA represents 4,4
0
,4
00
-tris(N-(2-naphthyl)-N-phenylamino)-triphenylamine and serves
as the hole injection layer (HIL); the N,N
0
-bis(naphthalen-1-yl)-N,N
0
-bis(phenyl)benzidine
(NPB) functions as the HTL; the Tris(8-hydroxyquinolinato)aluminium (Alq
3
) forms the
electron transporting layer (ETL); the electron injection layer (EIL) consists of lithium
fluoride (LiF); and the ITO is used as the anode, while the aluminum (Al) serves as the
cathode. The organic compounds were thermally evaporated at a vacuum pressure of
10
−6
torr and formed an emitting layer with an area of 4 mm
2
at a deposition rate of
0.1 nm/s. The aluminum layer was also deposited under a pressure of 10
−6
torr. The
current density-voltage (J-V) characteristics of the fabricated OLED devices were evaluated
using a Keithley 2400 Source Meter (Keithley, Cleveland, OH, USA). The EL spectrum of
the devices was measured with a Minolta CS-1000A spectroradiometer (Konica Minolta,
Tokyo, Japan).
3.2. Synthesis
The synthesis method for compound (
4
) was previously described in a prior paper [
7
].
Scheme 1illustrates the synthetic pathways for AP-TPO and TPO-AP.
Molecules 2023,28, 7485 9 of 12
Molecules2023,28,xFORPEERREVIEW10of13
blueemissionwerefabricatedwiththefollowingstructure:ITO/2-TNATA(60nm)/NPB
(15nm)/EML(35nm)/Alq
3
(20nm)/LiF(1nm)/Al(200nm).Inthisstructure,the2-TNATA
represents4,4′,4″-tris(N-(2-naphthyl)-N-phenylamino)-triphenylamineandservesasthe
holeinjectionlayer(HIL);theN,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine(NPB)
functionsastheHTL;theTris(8-hydroxyquinolinato)aluminium(Alq
3
)formstheelectron
transportinglayer(ETL);theelectroninjectionlayer(EIL)consistsoflithiumfluoride(LiF);
andtheITOisusedastheanode,whilethealuminum(Al)servesasthecathode.The
organiccompoundswerethermallyevaporatedatavacuumpressureof10
−6
torrand
formedanemiinglayerwithanareaof4mm
2
atadepositionrateof0.1nm/s.The
aluminumlayerwasalsodepositedunderapressureof10
−6
torr.Thecurrentdensity-
voltage(J-V)characteristicsofthefabricatedOLEDdeviceswereevaluatedusinga
Keithley2400SourceMeter(Keithley,Cleveland,OH,USA).TheELspectrumofthe
deviceswasmeasuredwithaMinoltaCS-1000Aspectroradiometer(KonicaMinolta,
Tokyo,Japan).
3.2.Synthesis
Thesynthesismethodforcompound(4)waspreviouslydescribedinapriorpaper
[7].Scheme1illustratesthesyntheticpathwaysforAP-TPOandTPO-AP.
Scheme 1. SyntheticroutesofAP-TPOandTPO-AP.
3.2.1.2-(4-Bromophenyl)-4,5-diphenyloxazole(1)
(4-Bromophenyl)methanamine(1.00g,5.37mmol),benzyl(1.13g,5.37mmol),
potassiumcarbonate(K
2
CO
3
)(2.23g,16.12mmol),andiodine(0.41g,1.61mmol)were
addedtoa250mLthree-neckedflask.50mLofwaterwasplacedinaround-boomflask,
andthemixturewasstirredat60°Cfor8h.Afterthereactionwascomplete,themixture
wasextractedwithethylacetate(EA)anddistilledwater(DIwater).Theorganiclayerwas
driedwithanhydrousmagnesiumsulfateanhydrous(MgSO
4
)andthenfiltered.The
solutionwasevaporated,andthenitwaspurifiedbycolumnchromatographyusingEA
andhexaneina1:19ratioastheeluent.(Yield:15%)
1
HNMR(400MHz,CDCl
3
,δ)8.03–
7.99(d,2H),7.72–7.61(m,6H),7.41–7.36(m,6H).
3.2.2.4,5-Diphenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)oxazole(2)
Compound(1)(0.50g,1.33mmol),bis(pinacolato)diboron(0.57g,2.26mmol),
potassiumacetate(KOAc)(0.39g,3.99mmol),and
bis(diphenylphosphino)ferrocene)palladium(II)dichloride(Pd(dppf)Cl
2
)(0.05g,0.07
mmol)wereaddedintoa100mLthree-neckedflaskunderanitrogenatmosphere.25mL
Scheme 1. Synthetic routes of AP-TPO and TPO-AP.
3.2.1. 2-(4-Bromophenyl)-4,5-diphenyloxazole (1)
(4-Bromophenyl)methanamine (1.00 g, 5.37 mmol), benzyl (1.13 g, 5.37 mmol), potas-
sium carbonate (K
2
CO
3
) (2.23 g, 16.12 mmol), and iodine (0.41 g, 1.61 mmol) were added
to a 250 mL three-necked flask. 50 mL of water was placed in a round-bottom flask, and
the mixture was stirred at 60
◦
C for 8 h. After the reaction was complete, the mixture was
extracted with ethyl acetate (EA) and distilled water (DI water). The organic layer was dried
with anhydrous magnesium sulfate anhydrous (MgSO
4
) and then filtered. The solution
was evaporated, and then it was purified by column chromatography using EA and hexane
in a 1:19 ratio as the eluent. (Yield: 15%)
1
H NMR (400 MHz, CDCl
3
,
δ
) 8.03–7.99 (d, 2H),
7.72–7.61 (m, 6H), 7.41–7.36 (m, 6H).
3.2.2. 4,5-Diphenyl-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)oxazole (2)
Compound (
1
) (0.50 g, 1.33 mmol), bis(pinacolato)diboron (0.57 g, 2.26 mmol), potas-
sium acetate (KOAc) (0.39 g, 3.99 mmol), and bis(diphenylphosphino)ferrocene)palladium(II)
dichloride (Pd(dppf)Cl
2
) (0.05 g, 0.07 mmol) were added into a 100 mL three-necked flask
under a nitrogen atmosphere. 25 mL of 1,4-dioxane was also added, and the mixture was
refluxed overnight at 60
◦
C. After the reaction was complete, the mixture was extracted
with EA and DI water. The organic layer was dried with anhydrous MgSO
4
and then
filtered. The solution was evaporated, and it was then purified by column chromatogra-
phy using EA and hexane in a 1:19 ratio as the eluent. (Yield: 74%)
1
H NMR (400 MHz,
CDCl
3
,
δ
) 8.16–8.12 (d, 2H), 7.93–7.89 (d, 2H), 7.73–7.67 (m, 4H), 7.42–7.34 (m, 6H), 1.38–1.35
(s, 12H).
3.2.3. 1-(10-Bromoanthracen-9-yl)pyrene (3)
1-(Anthracen-9-yl)pyrene (0.20 g, 0.53 mmol) and N-bromosuccinimide (0.10 g,
0.58 mmol) were added to 10 mL of methylene chloride (MC). Acetic acid (1.0 mL) was
then added into the reaction. The mixture was stirred at 25
◦
C for 3 h. After the reaction
was complete, the reaction mixture was extracted with MC and DI water. The organic layer
was dried with anhydrous MgSO
4
and then filtered. The solution was evaporated. Then,
it was purified by column chromatography using hexane and toluene in a 9:1 ratio as the
eluent. The precipitate was filtered, washed with ethanol, and a yellow compound was
obtained (Yield: 76%).
1
H NMR (400 MHz, CDCl
3
,
δ
) 8.61–8.57 (d, 2H), 8.54–8.51 (d, 1H),
Molecules 2023,28, 7485 10 of 12
8.39–8.34 (m, 2H), 8.33–8.30 (d, 1H), 8.24–8.20 (d, 1H), 8.11–8.04 (m, 2H), 7.92–7.89 (d, 1H),
7.74–7.68 (q, 2H), 7.38–7.32 (q, 2H), 7.25–7.20 (d, 2H), 7.11–7.06 (m, 1H).
3.2.4. 2-(4-(6-(Anthracen-9-yl)pyren-1-yl)phenyl)-4,5-diphenyloxazole (5) (AP-TPO)
Compound (
2
) (0.36 g, 0.85 mmol), Compound (4) (0.30 g, 0.66 mmol), palladium-
tetrakis(triphenylphosphine) (Pd(PPh
3
)
4
) (0.023 g, 0.02 mmol), and K
2
CO
3
(0.27 g,
1.97 mmol) were added to a 100 mL three-necked flask. Anhydrous THF (15 mL) was
added, followed by the consecutive addition of DI water (3 mL). The mixture was then
refluxed and stirred under nitrogen at 80
◦
C for 14 h. After the reaction was complete,
it was extracted with MC and DI water. The organic layer was dried with anhydrous
MgSO
4
and then filtered. The solution was evaporated. It was then purified by column
chromatography using a 3:7 ratio of hexane and MC as the eluent. (Yield: 57%)
1
H NMR
(400 MHz, DMSO,
δ
) 8.86–8.82 (s, 1H), 8.55–8.51 (d, 1H), 8.39–8.23 (m, 7H), 8.13–8.06
(q, 2H), 7.98–7.94 (d, 1H), 7.90–7.85 (d, 2H), 7.75–7.68 (m, 4H), 7.55–7.40 (m, 8H), 7.31–7.26
(t, 2H), 7.21–7.12 (q, 3H).
13
C NMR (101 Mhz, THF-d
8
)
δ
159.95, 145.83, 143.45, 137.13, 137.00,
135.16, 134.35, 132.84, 131.74, 131.35, 131.16, 131.10, 131.00, 129.62, 129.25, 128.73, 128.51,
128.40, 128.10, 127.94, 127.86, 127.75, 127.72, 127.14, 126.77, 126.73, 126.57, 126.40, 125.73,
125.64, 125.24, 125.21, 125.15, 125.07, 124.97, 124.82. m/z= 674.25; calcd. For C51H51NO:
673.24 [M+] Anal. calcd. for C51H51NO: C, 90.91; H, 4.64; N, 2.08; O, 2.37% found: C, 90.89;
H, 4.71; N, 2.08; O, 2.21%.
3.2.5. 4,5-Diphenyl-2-(4-(10-(pyren-1-yl)anthracen-9-yl)phenyl)oxazole (6) (TPO-AP)
Compound (
2
) (0.36 g, 0.85 mmol), Compound (
3
) (0.30 g, 0.66 mmol), Pd(PPh
3
)
4
(0.023 g, 0.02 mmol), and K
2
CO
3
(0.27 g, 1.97 mmol) were added to a 100 mL three-necked
flask. Anhydrous THF (15 mL) was added, followed by the consecutive addition of DI
water (3 mL). The mixture was then refluxed and stirred under nitrogen at 80
◦
C for
14 h. After the reaction was complete, the mixture was extracted with MC and DI water.
The organic layer was dried with anhydrous MgSO
4
and then filtered. The solution was
evaporated. It was then purified by column chromatography using a 1:1 ratio of hexane
and MC as the eluent. (Yield: 77%)
1
H NMR (400 MHz, THF,
δ
) 8.49–8.43 (m, 3H), 8.29–8.21
(q, 3H), 8.17–8.13 (d, 1H), 8.07–7.99 (m, 2H), 7.86–7.70 (m, 9H), 7.46–4.30 (m, 11H), 7.21–7.16
(t, 2H).
13
C NMR (101Mhz, THF-d
8
)
δ
159.98, 145.88, 141.60, 137.01, 136.68, 135.84, 134.08,
132.88, 131.97, 129.32, 128.66, 128.31, 128.02, 127.89, 127.36, 127.06, 126.79, 126.61, 126.48,
125.31. m/z= 674.25; calcd. For C51H51NO: 673.24 [M+] Anal. calcd. for C51H51NO: C,
90.91; H, 4.64; N, 2.08; O, 2.37% found: C, 90.27; H, 4.51; N, 2.09; O, 2.91%.
4. Conclusions
The AP-Core based molecules, AP-TPO and TPO-AP, were synthesized by substituting
the weak electron-accepting TPO as the second core and the first core, respectively. In
the solution state, both the AP-TPO and the TPO-AP exhibited PL maxima at 433 nm and
443 nm, respectively, with high PLQYs of 82% and 88%, respectively, indicating efficient
luminescence. When applied as the EML in OLED devices, the AP-TPO and the TPO-AP
demonstrated CE values of 4.33 cd/A and 5.49 cd/A, respectively, as well as EQE values of
3.73% and 4.26%, respectively. As a result, the TPO-AP showed a higher efficiency in OLED
devices. The introduction of side groups on the first core, anthracene, effectively suppresses
the intermolecular packing, allowing for high efficiency. Moreover, the incorporation of
oxazole side groups helps maintain a charge balance even at high current densities. These
results emphasize the potential of AP-TPO and TPO-AP for application in high-efficiency
OLED devices.
Supplementary Materials:
The following supporting information can be downloaded at:
https://www.mdpi.com/article/10.3390/molecules28227485/s1, Figure S1: TRPL curves of AP-
TPO and TPO-AP; Figure S2: Transient EL responses of AP-TPO and TPO-AP; Figure S3: RTPL and
LTPL (77K) in solution state AP-TPO and TPO-AP; Figure S4: Device lifetime at 1000 nits of AP-TPO
and TPO-AP.
Molecules 2023,28, 7485 11 of 12
Author Contributions:
Conceptualization, Y.H. and H.L.; methodology, H.L. and K.L.; validation,
H.K., S.P. and S.D.; formal analysis, Y.H., S.D. and K.L.; investigation, Y.H., H.K. and S.P.; resources,
J.P.; writing—original draft preparation, Y.H.; writing—review and editing, H.L. and J.P.; visualization,
H.K.; supervision, J.P.; project administration, J.P.; funding acquisition, J.P. All authors have read and
agreed to the published version of the manuscript.
Funding:
This work was partly supported by the GRRC program of Gyeonggi
province [(GRRCKYUNGHEE2023-B01), Development of ultra-fine process materials based on the
sub-nanometer class for the next-generation semiconductors]. This research was supported by the
Basic Science Research Program through the National Research Foundation of Korea (NRF) funded
by the Ministry of Education (2020R1A6A1A03048004). This research was supported by Basic Science
Research Capacity Enhancement Project through Korea Basic Science Institute (National research Fa-
cilities and Equipment Center) grant funded by the Ministry of Education (No. 2019R1A6C1010052).
This work was supported by the Technology Innovation Program (20017832, Development of TiN-
based electrode materials and ALD equipment for 10-nm DRAM capacitor electrode deposition
process) funded By the Ministry of Trade, Industry & Energy (MOTIE, Korea).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Data are contained within the article and Supplementary Materials.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Wu, C.-H.; Chen, C.H.; Hsu, F.M.; Shih, P.I.; Shu, C.-F. Efficient non-doped blue-light-emitting diodes incorporating an anthracene
derivative end-capped with fluorene groups. J. Mater. Chem. 2009,19, 1464. [CrossRef]
2.
Thomas, K.R.J.; Kapoor, N.; Prasad, M.N.K.; Jou, J.-H.; Chen, Y.-L.; Joun, Y.-C. Pyrene-Fluorene Hybrids Containing Acetylene
Linkage as Color-Tunable Emitting Materials for Organic Light-Emitting Diodes. J. Org. Chem. 2012,77, 3921. [CrossRef]
3.
Jeon, S.; Jeon, Y.; Kim, J.; Lee, C.; Gong, M. Blue organic light-emitting diode with improved color purity using 5-naphthyl-
spiro[fluorene-7,90-benzofluorene]. Org. Electron. 2008,9, 522. [CrossRef]
4.
Hayashi, T.; Mataga, N.; Sakata, Y.; Misumi, S.; Morita, M.; Tanaka, J. Excimer fluorescence and photodimerization of anthraceno-
phanes and 1,2-dianthrylethanes. J. Am. Chem. Soc. 1976,98, 5910. [CrossRef]
5.
Matsui, A.H. Excitonic processes in aromatic molecular crystals of strong exciton-phonon coupling. Pure Appl. Chem.
1995
,
67, 429. [CrossRef]
6.
Feng, X.; Xu, Z.; Hu, Z.; Qi, C.; Luo, D.; Zhao, X.; Mu, Z.; Redshaw, C.; Lam, J.W.Y.; Ma, D.; et al. Pyrene-based blue emitters
with aggregation-induced emission features for high-performance organic light-emitting diodes. J. Mater. Chem. C
2019
,7, 2283.
[CrossRef]
7.
Kim, B.; Park, Y.; Lee, J.; Yokohama, D.; Lee, J.; Kido, J.; Park, J. Synthesis and electroluminescence properties of highly efficient
blue fluorescence emitters using dual core chromophores. J. Mater. Chem. C 2013,1, 432. [CrossRef]
8.
Lee, H.; Kim, B.; Kim, S.; Kim, J.; Lee, J.; Shin, H.; Lee, J.H.; Park, J. Synthesis and electroluminescence properties of highly
efficient dual core chromophores with side groups for blue emission. J. Mater. Chem. C 2014,2, 4737. [CrossRef]
9. Xing, Z.H.; Zhuang, J.Y.; Xu, X.P.; Ji, S.J.; Su, W.-M.; Cui, Z. Novel oxazole-based emitters for high efficiency fluorescent OLEDs:
Synthesis, characterization, and optoelectronic properties. Tetrahedron 2017,73, 2036. [CrossRef]
10.
Zhang, C.; Li, L.; Wu, H.; Liu, Z.; Li, J.; Zhang, G.; Wen, G.; Shuang, S.; Dong, C.; Choi, M.M.F. Synthesis and photophysical
studies of oxazole rings containing compounds as electron accepting units. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.
2013
,
102, 256. [CrossRef]
11. Jones, R.N. The Ultraviolet Absorption Spectra of Anthracene Derivatives. Chem. Rev. 1947,41, 353. [CrossRef]
12.
Desai, N.K.; Mahajan, P.G.; Mali, S.S.; Kolekar, G.B.; Patil, S.R. Enhanced Exciplex Emission of Pyrene Thin Films Doped by
Perylene: Structural, Photophysical and Morphological Investigation. J. Fluoresc. 2018,28, 897–903. [CrossRef] [PubMed]
13.
Neese, F.; Wennmohs, F.; Becker, U.; Riplinger, C. The ORCA quantum chemistry program package. J. Chem. Phys.
2020
,
152, 224108. [CrossRef] [PubMed]
14.
Huang, Y.; Xing, J.; Gong, Q.; Chen, L.C.; Liu, G.; Yao, C.; Wang, Z.; Zhang, H.L.; Chen, Z.; Zhang, Q. Reducing aggregation caused
quenching effect through co-assembly of PAH chromophores and molecular barriers. Nat. Commun.
2019
,10, 169. [CrossRef]
[PubMed]
15.
Meng, G.; Zhang, D.; Wei, J.; Zhang, Y.; Huang, T.; Liu, Z.; Yin, C.; Hong, X.; Wang, X.; Zeng, X.; et al. Highly efficient and stable
deep-blue OLEDs based on narrowband emitters featuring an orthogonal spiro-configured indolo[3,2,1-de]acridine structure.
Chem. Sci. 2022,13, 5622. [CrossRef]
Molecules 2023,28, 7485 12 of 12
16.
Yuan, W.Z.; Lu, P.; Chen, S.; Lam, J.W.Y.; Wang, Z.; Liu, Y.; Kwok, H.S.; Ma, Y.; Tang, B.Z. Changing the Behavior of Chromophores
from Aggregation-Caused Quenching to Aggregation-Induced Emission: Development of Highly Efficient Light Emitters in the
Solid State. Adv. Mater. 2010,22, 2159. [CrossRef] [PubMed]
17.
Jiang, H.; Li, H.; Jin, J.; Bodedla, G.B.; Tao, P.; Ma, D.; Wong, W.Y. Orthogonal anthracene and pyrene derivatives for efficient pure
deep-blue organic light-emitting diodes. J. Mater. Chem. C 2023,11, 6438–6443. [CrossRef]
18.
Ki, M.S.; Sim, M.; Kwon, O.; Im, K.; Choi, B.; Cha, B.J.; Kim, Y.; Jin, T.Y.; Paeng, K. Improved Thermal Stability and Operational
Lifetime of Blue Fluorescent Organic Light-Emitting Diodes by Using a Mixed-Electron Transporting Layer. ACS Mat. Lett.
2022
,
4, 1676. [CrossRef]
19.
Zhang, G.; Musgrave., C.B. Comparison of DFT Methods for Molecular Orbital Eigenvalue Calculations. J. Phys. Chem. A
2007
,
111, 1554–1561. [CrossRef] [PubMed]
20.
Hall, D.; Sancho-García, J.C.; Pershin, A.; Beljonne, D.; Zysman-Colman, E.; Olivier, Y. Benchmarking DFT Functionals for
Excited-State Calculations of Donor–Acceptor TADF Emitters: Insights on the Key Parameters Determining Reverse Inter-System
Crossing. J. Phys. Chem. A 2023,127, 4743–4757. [CrossRef]
21.
Sasaki, T.; Hasegawa, M.; Inagaki, K.; Ito, H.; Suzuki, K.; Onno, T.; Morii, K.; Shimizu, T.; Fukagawa, H. Unravelling the electron
injection/transport mechanism in organic light-emitting diodes. Nat. Commun. 2021,12, 2706. [CrossRef]
22.
Sasaki, T.; Onno, T.; Shimizu, T.; Fukagawa, H. Effects of Energy-Level Alignment on Operating Voltages of Blue Organic
Light-Emitting Diodes. Adv. Mater. Interfaces 2023,10, 220192. [CrossRef]
23.
Shan, T.; Gao, Z.; Tang, X.; He, X.; Gao, Y.; Li, J.; Sun, X.; Liu, Y.; Liu, H.; Yang, B.; et al. Highly efficient and stable pure
blue nondoped organic light-emitting diodes at high luminance based on phenanthroimidazole-pyrene derivative enabled by
triplei-triplet annihilation. Dyes. Pigm. 2017,142, 189–197. [CrossRef]
24.
Lim, H.; Woo, S.J.; Ha, Y.H.; Kim, Y.H.; Kim, J.J. Breaking the Efficiency Limit of Deep-Blue Fluorescent OLEDs Based on
Anthracene Derivatives. Adv. Mater. 2022,34, 2100161. [CrossRef]
25.
Kondakov, D.Y.; Pawlik, T.D.; Hatwar, T.K.; Spindler, J.P. Triplet annihilation exceeding spin statistical limit in highly efficient
fluorescent organic light-emitting diodes. J. Appl. Phys. 2009,106, 124510. [CrossRef]
26.
Yin, C.; Zhang, Y.; Huang, T.; Liu, Z.; Duan, L.; Zhang, D. Highly efficient and nearly roll-off–free electrofluorescent devices via
multiple sensitizations. Sci. Adv. 2022,8, eabp9203. [CrossRef]
27.
Lee, S.; Koo, H.; Kwon, O.; Park, Y.J.; Choi, H.; Lee, K.; Ahn, B.; Park, Y.M. The Role of Charge Balance and Excited State Levels
on Device Performance of Exciplex-based Phosphorescent Organic Light Emitting Diodes. Sci. Rep.
2017
,7, 11995. [CrossRef]
[PubMed]
28.
Ahn, D.H.; Maeng, J.H.; Lee, H.; Yoo, H.; Lampande, R.; Lee, J.Y.; Kwon, J.H. Rigid Oxygen-Bridged Boron-Based Blue Thermally
Activated Delayed Fluorescence Emitter for Organic Light-Emitting Diode: Approach towards Satisfying High Efficiency and
Long Lifetime Together. Adv. Optical Mater. 2020,8, 2000102. [CrossRef]
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