A hybridized local and charge transfer excited state for solution-processed non-doped green electroluminescence based on oligo(: p -phenyleneethynylene)

Article abstract of DOI:10.1039/d0tc01266a

We herein report a new highly efficient green emissive hot-exciton molecule, 1,4-bis((4′-diphenylamino-3-cyano-[1,1′-biphenyl]-4-yl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-TPA-CNPE) that consists of an extended D′-π′-A-π-D-π-A-π′-D′ molecular π-sys

Full text of DOI:10.1039/d0tc01266a

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Can, J. Mater. Chem. C, 2020, DOI: 10.1039/D0TC01266A.
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DOI: 10.1039/D0TC01266A
A Hybridized Local and Charge Transfer Excited State for Solution-Processed Non-
doped Green Electroluminescence Based on Oligo(p-phenyleneethynylene)
Hakan Usta1*, Dilek Alimli2, Resul Ozdemir1, Emine Tekin3, Fahri Alkan1, Rifat Kacar3, Ahu
Galen Altas3, Salih Dabak3, Ayşe Gül Gürek2, Evren Mutlugun4, Ahmet Faruk Yazici1,
Ayse Can1
1 Department of Materials Science and Nanotechnology Engineering, Abdullah Gül University,
38080 Kayseri, Turkey.
2 Department of Chemistry, Gebze Technical University, 41400 Gebze, Kocaeli, Turkey.
3
The Scientific and Technological Research Council of Turkey (TUBITAK)-Marmara
Research Center (MAM), 41470 Gebze, Kocaeli, Turkey.
4
Department of Electrical and Electronics Engineering, Abdullah Gül University, 38080
Kayseri, Turkey.
*Address correspondence to: hakan.usta@agu.edu.tr
1

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DOI: 10.1039/D0TC01266A
ABSTRACT
We herein report a new highly efficient green emissive hot-exciton molecule, 1,4-
bis((4'-diphenylamino-3-cyano-[1,1'-biphenyl]-4-yl)ethynyl)-2,5-bis(2-
ethylhexyloxy)benzene (2EHO-TPA-CNPE) that consists of an extended D’-π’-A-π-D-π-A-π’-
D’ molecular π-system with diphenylamino end units (D’) and ethynylene/phenylene spacers
(π/π’). The new molecule exhibits high photoluminescence (PL) quantum efficiencies (ΦPL
=
0.95 (solution) and 0.45 (spin-coated neat thin-film)), and a strong PL solvatochromic behavior
revealing significant changes in excited state energies/characteristics (locally excited (LE) →
hybridized local and charge transfer (HLCT) → charge-transfer (CT)) depending on solvent
polarity. Highly efficient (radiative exciton yield (ηr) = 50-59% >> 25%) green-emitting
OLEDs were fabricated in a conventional device architecture by employing (non-)doped thin-
films reaching a maximum current efficiency (CEmax) of 12.0 cd/A and a maximum external
quantum efficiency (EQEmax) of 5.5%. The emission profile of the non-doped OLED has CIE
1976 (u',v') chromaticity coordinate of (0.10, 0.55) corresponding to a night vision imaging
systems (NVIS) compatible Green A region. 2EHO-TPA-CNPE-based OLED devices of
industrial relevance were also fabricated by ink-jet printing the emissive layer and by
fabricating an inverted architecture, which possessed respectable device performances of 2.4-
6.1 cd/A. The solid-state solvation effect in OLED devices yields HLCT electronic behavior
resulting in high ηr’s, which is confirmed by TDDFT to originate from energetically/spatially
favorable reverse intersystem crossings (RISCs) (T2/3→S1). As a unique observation, delayed
fluorescence due to this RISC was evident in the PL decay lifetime measurement with a ns-
scale lifetime of ~10 ns. These results clearly allow a better understanding of the structure-
photophysical property-electroluminescence relationships in this new class of oligo(p-
phenyleneethynylene)-based hot-exciton molecules, and it could open up new opportunities for
high-performance solution-processed optoelectronic/sensing applications.
2

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DOI: 10.1039/D0TC01266A
INTRODUCTION
The development of rare-metal-free emissive materials in organic light-emitting diode
(OLED) applications for flat-panel displays and solid-state lightings has greatly focused on
fluorescent small molecules, which offers all the intrinsic advantages (i.e., well-defined
structure, high purity, expensive metal-free, low-cost, good solubility, and synthetic
reproducibility) of molecular materials for use in new generation optoelectronics.1–8 In
particular, fluorescent small molecules having proper molecular sizes and solution rheological
properties could be printed into pixelated emissive layers, which is of critical importance for
future solution processed and printed optoelectronic technologies.9–11 However, as controlled
by spin statistics of electrically generated excitons and spin-allowed electronic transitions,12
conventional fluorescent small molecules generally suffer from a limited emission efficiency
(radiative exciton yields (ηr-max) = 25%) during electroluminescence, which makes them less
attractive as compared to their inorganic and phosphorescent counterparts.13,14 Since only the
singlet excited state of a molecule could decay radiatively to the singlet ground state,
developing properly designed unconventional molecular π-systems that could increase singlet
excited state population through triplet-to-singlet excited state conversions is particularly
important.1 This has become an attractive avenue in recent years to achieving high (>>25%)
ηr‘s. In this regard, much progress has been made in triplet-triplet annihilation (TTA) and
thermally activated delayed-fluorescence (TADF) approaches.15,16 In these systems, while TTA
generates an extra singlet exciton through fusion of two T1 excitons intrinsically limiting ηr-max
to 62.5%,17 TADF is based on thermal energy assisted reverse intersystem crossing (RISC) of
T1 excitons to energetically close (typically within ~0.1-0.3 eV) S1 excitons yielding very high
ηr-max’s of more than ~80-90%.18 Although both mechanisms enhance the overall
electroluminescence performance in fluorescent OLEDs, accumulation of relatively long-lived
T1 excitons could cause serious efficiency roll-off at high current densities owing to undesired
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triplet-triplet annihilation and singlet-triplet quenchings.19,20 Activation of hot-exciton RISC
DOI: 10.1039/D0TC01266A
channels in the upper excited states (Tn (n > 1) → Sm (m ≥ 1)), which is as an exception to the
classic Kasha’s rule in photochemistry,21 has recently been unveiled as an effective third
approach (Figure 1A). When the proper electronic conditions are met, RISC occurring from
high-lying Tn to Sm was shown to be fast process (several nanoseconds)22 and could compete
with the internal conversions down to T1 state.23 From an energy-gap-law standpoint, for hot-
exciton channels to operate with a fast RISC rate, the energy of the high-lying triplet state
should be close to that of the corresponding singlet excited state and far from T1 to deactivate
low-lying triplet internal conversions (ΔESm-Tn << ΔETn-T1 for kRISC >> kIC(T)). For triplet and
singlet excited states involved in the hot-exciton channels, the configurations of the natural
transition orbitals (NTOs) should possess both locally excited (LE) and charge-transfer (CT)
characteristics, namely hybridized local and charge transfer (HLCT) excited state (Figure 1A).
In a HLCT excited state, while LE character enables highly efficient fluorescence emission
(large transition moment) arising from large wave function overlap, CT character decreases the
binding energies of spatially seperated excitons yielding small energetic splitting
(quasidegenerate singlet and triplet excitons) and facilitates triplet-to-singlet excitonic
transitions.24–27 Combining both characteristics into a single HLCT excited state form is key to
developing highly emissive fluorescent molecules with high singlet exciton generation (ηr >>
25%).28
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DOI: 10.1039/D0TC01266A
Figure 1. A. The schematic of exciton recombination and radiative decay through hot-exciton
mechanism in an electroluminescence process indicating energy levels of molecular electronic
states and energetic/wave function requirements (kRISC, reverse intersystem crossing rate; kIC(T)
,
triplet internal conversion rate; kph/kfl, phosphorescence/fluorescence rate; ηr, radiative exciton
yield). (Note that if m > 1 (n > 2) additional internal conversions among singlet (triplet) states
would occur) The illustration of hybridized local and charge transfer (HLCT) excited state
formation on a molecular π-backbone with regards to hole/particle natural transition orbitals
indicating locally excited (LE) and charge transfer (CT) spatial contributions. B. Chemical
structures of HLCT hot-exciton small molecules reported in the literature with values of ηr >>
25% in the OLEDs.19,29–32
In the past decade, a number of molecules with HLCT and hot-exciton properties in
OLED devices have been developed following the first report (TPA-PPI in Figure 1B) by Ma
et al. in 2012 (See Table S1 for the full list).19,29–32 The general molecular design strategy
involves constructing π-systems with donor (D) and acceptor (A) moieties to yield extended π-
delocalization (LE character) with controlled frontier orbital wave function seperation (CT
character) for efficient charge-transport, radiative electronic transition, and large singlet exciton
generation. To this end, π-electron rich structures such as triphenylamine,24,32–34 carbazole,29
anthracene,19 phenothiazine,35 tetraphenylethene,36 and pyrene31 have been employed as donor
units; π-electron deficient structures such as phenanthro[9,10-d]imidazole,32,34 naphtho[2,3-
c][1,2,5]thiadiazole,24 triphenylphosphine oxide,19 benzothiadiazole, 29,35 and thioxanthene-S,S-
dioxide31 have been used as acceptor units (Figure 1B and Table S1). In these HLCT molecules,
5

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the hybridization of LE and CT is engineered by employing a number of design approaches
DOI: 10.1039/D0TC01266A
such as tuning D-A interring twists and intramolecular π-conjugation length,19,28 introducing π-
spacer between D and A units,37 and changing the electronic strength of D and A units.29,34 To
this end, we have recently demonstrated for the first time that D-A functionalized oligo(p-
phenyleneethynylene) π-framework offers an ideal electronic structure and optical properties to
form HLCT excited states and hot-exciton channels for high radiative exciton yields (ηr ~ 70%
>> 25%) in OLEDs.38 Record high external quantum efficiency (EQE) of ~7.1% has been
achieved with our previously developed 1,4-bis((2-cyanophenyl)ethynyl)-2,5-bis(2-
ethylhexyloxy)benzene (2EHO-CNPE, Figure 2) in solution-processed deep-blue
(Commission Internationale de l’Eclairage CIE(x,y) = 0.15, 0.09) emitting OLEDs. Ethynylene
(-C≡C-) π-spacers have played a critical role in this π-system between D and A units to yield
HLCT excited state properties and very effective hot exciton channels (T3/T2 → S1) (ΔET3/T2-S1
= 0.01-0.08 eV << ΔET2-T1 = 0.81 eV). Motivated with these promising results, we envision to
visit further π-extended oligo(p-phenyleneethynylene) π-systems with the goal of realizing
color coordinates in lower energy regions (i.e., green in this study) and elucidating the
formation of hot-exciton channels/HLCT characteristics. With regards to designing efficient
fluorescent molecules with a wide-color-gamut for high performance display and lighting
applications, it is still of utmost urgency to study and expand the library of π-extended HLCTs,
especially considering that the majority of hot-exciton molecules are high energy (deep-
blue/blue) emissive π-systems (Table S1). Going from high energy deep-blue to lower energy
emissive systems, energetic separations and HLCT characteristics of π-electronic states might
greatly change.19,28
Herein, we report the design, synthesis, and full characterization of a new rod-shaped
oligo(p-phenyleneethynylene) molecule, 1,4-bis((4'-diphenylamino-3-cyano-[1,1'-biphenyl]-4-
yl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-TPA-CNPE, Figure 2). In this new
6

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molecular π-system, an extended D’-π’-A-π-D-π-A-π’-D’ molecular architDeOctI:u10re.1039w/Di0thTC01266A
diphenylamino end units (D’) and ethynylene/phenylene π-spacers (π/π’) was employed to
achieve highly efficient green luminescence. The thermal, photophysical, electrochemical, and
electroluminescence device characteristics of the new molecule have been studied in detail.
max
High photoluminescence quantum efficiencies (Φ ’s) of 0.95 and 0.45 were measured (
λfl
PL
at 518-519 nm) in solution and as spin-coated neat thin-film, respectively, by using integrating-
sphere. Solvent-dependent photoluminescence study revealed a strong solvatochromic behavior
for the new molecule showing significant changes in the excited state energies (Δ max = +130
λfl
nm) and characteristics (LE → HLCT → CT) upon polarity increase. Similar polarity effect
was also evident in the electroluminescence of the emissive thin-film in OLEDs that solid-state
solvation effect tunes emission profile and radiative exciton yield. The (non-)doped emissive
thin-films were found to be in a medium polarity environment to facilitate HLCT
characteristics. Highly efficient (ηr = 50% >> 25%) green-emitting OLEDs were fabricated in
a conventional device architecture by employing spin-coated non-doped emissive 2EHO-TPA-
CNPE thin-film, which showed maximum current efficiency (CEmax) of 12.0 cd/A, a maximum
power efficiency (PEmax) of 5.4 lm/W, and a maximum external quantum efficiency (EQEmax
)
of 4.5%. The emission profile of the non-doped OLED has CIE 1976 (u',v') chromaticity
coordinate of (0.10, 0.55) corresponding to a night vision imaging systems (NVIS) compatible
Green A region. Additional OLEDs were fabricated by doping the new molecule in CBP host
matrix, which exhibits a sky-blue emission with improved EQEmax of 5.5% (ηr = 59% >> 25%).
2EHO-TPA-CNPE-based OLED devices of industrial relevance were also fabricated by ink-
jet printing the emissive layer and by fabricating an inverted architecture, which showed
respectable device performances of 2.4 cd/A and 6.1 cd/A, respectively. TDDFT calculations
reveal that the observed high radiative exciton yield originates from very effective RISC
processes between energetically/spatially suitable HLCT excited states (T2/3 → S1). A ns-scale
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delayed fluorescence (~10 ns) due to high-lying (R)ISCs was confirmed in the PL decay profile
DOI: 10.1039/D0TC01266A
following a prompt fluorescence decay (~1 ns). To the best of our knowledge, this result is one
of the very few examples in the hot-exciton literature that high-lying RISC-related delayed
fluorescence is directly observed in a PL decay profile without using a triplet sensitizer.
EXPERIMENTAL SECTION
Materials and Methods
Unless otherwise noted, all reagents were purchased from commercial sources and used as
received. Schlenk techniques were employed and all the reactions were carried out under N2
atmosphere. 1H/13C NMR spectra were recorded using a Bruker 400 spectrometer (for 1H: 400
MHz and for 13C: 100 MHz). Elemental analyses were done using a LecoTruspec Micro model
instrument. MALDI-TOF characterization was performed on a Bruker Microflex LT MALDI-
TOF-MS Instrument. Thermal characterizations including thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC) measurements were performed under nitrogen at a
heating rate of 10 °C/min using Mettler Toledo-TGA/STDA 851 and Mettler Toledo-DSC 822
model instruments, respectively. UV-Vis absorption spectra were recorded on a Shimadzu, UV-
1800 UV-Vis spectrophotometer. Photoluminescence (PL) characterizations were carried out
with Agilent-Cary Eclipse fluorescence spectrophotometer. Time-correlated single photon
counting measurements were performed using Pico Quant FluoTime 200 equipped with 375nm
pulsed laser diode. The PL quantum yields in solution and solid state were measured (λexc = 350
nm) on a Hamamatsu absolute PL Quantum Yield Spectrometer C11347 using the integrating-
sphere method. Electrochemical measurements were carried out on a BAS-Epsilon
potentiostat/galvanostat from Bioanalytical Systems Inc. (Lafayette, IN) equipped with a C3-
cell stand. Theoretical calculations except for the ones involving spin-orbit couplings were
performed by Gaussian 09 using density functional theory (DFT) and time-dependent density
functional theory (TDDFT) at the B3LYP/6-311G** level.39 For spin-orbit couplings between
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singlet and triplet states, we employed the perturbative approach of zeroth-oDrdOeI:r10.r10e3g9u/Dl0aTrC01266A
approximation (ZORA) Hamiltonian with B3LYP/TZP level of theory as implemented in
ADF2019 program package.40–42
Synthesis and Structural Characterization
Synthesis of 1,4-Bis((4-bromo-2-cyanophenyl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene
(2EHO-Br-CNPE). The reagents 5-bromo-2-iodobenzonitrile (0.507 g, 1.646 mmol), CuI
(0.007 g, 0.036 mmol), and Pd(PPh3)2Cl2 (0.048 g 0.068 mmol) in Et3N:THF (2:1) (15 mL)
were stirred for 5 minutes. Then, 1,4-bis(ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-
DEB) (0.263 g, 0.687 mmol) in 3 mL of THF was added, and the resulting reaction mixture
was heated at 80 °C under nitrogen for 24 h. The solution was cooled down to room temperature
and evaporated to dryness to yield a crude compound. The crude product was then purified by
column chromatography on silica gel using dichloromethane:n-hexane (3:2) solvent mixture to
afford 2EHO-Br-CNPE as a pure light yellow solid (0.362 g, 71% yield). 1H NMR (400 MHz,
CDCl3): δ 7.82 (d, 2H, J = 4.0 Hz), 7.72 (d, 1H, J = 4.0 Hz), 7.70 (d, 1H, J = 4.0 Hz), 7.49 (s,
1H), 7.47 (s, 1H), 7.08 (s, 2H), 3.93-3.95 (m, 4H), 1.79-1.85 (m, 2H), 1.54-1.59 (m, 8H), 1.27-
1.37 (m, 8H), 0.95 (t, 6H, J = 12.0 Hz), 0.87 (t, 6H, J = 16.0 Hz). 13C NMR (100 MHz, CDCl3):
δ 154.1, 135.7, 135.3, 133.5, 126.3, 121.9, 116.6, 116.4, 116.2, 113.6, 93.9, 90.3, 72.3, 38.0,
31.9, 30.1, 29.7, 26.9, 22.7, 14.1. Tm.p. = 68-69 °C. MS(MALDI-TOF) m/z (M+): calcd. for
C40H42Br2N2O2: 740.16, found: 515.987 for [M-2×(2EH)]+, 438.772 for [M-2×(2EH)-Br]+.
Anal. calcd. for C40H42Br2N2O2: C, 64.70; H, 5.70; N, 3.77; found: C, 64.38; H, 5.99; N, 3.44.
Synthesis of 1,4-bis((4'-diphenylamino-3-cyano-[1,1'-biphenyl]-4-yl)ethynyl)-2,5-bis(2-
ethylhexyloxy)benzene (2EHO-TPA-CNPE). The solution of 1,4-Bis((4-bromo-2-
cyanophenyl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-Br-CNPE) (0.315 g, 0.424
mmol) and 4-(dipheylamino)phenylboronic acid pinacol ester (0.347 g, 0.935 mmol) in 20 mL
of dimethoxyethane (DME) was stirred at room temperature for 5 min. Then, Pd(PPh3)4 (0.024
9

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g 0.021 mmol) and Na2CO3 (0.395 g 3.73 mmol) in 5 mL of water were added, and DthOeI: 1r0e.1s0u39lt/iDn0gTC01266A
mixture was heated at 80 °C under nitrogen for 24 h. After the reaction was completed, the
mixture was cooled down to room temperature and quenched with water. The reaction mixture
was then extracted with chloroform, and the organic phase was washed with water, dried over
Na2SO4, filtered, and evaporated to dryness to give a crude product. The crude product was then
purified by column chromatography on silica gel using dichloromethane:n-hexane (3:1) solvent
1
mixture to afford 2EHO-TPA-CNPE as a pure yellow solid (0.295 g, 65% yield). H NMR
(400 MHz, CDCl3): δ 7.88 (s, 2H), 7.75 (d, 2H, J = 8.0 Hz), 7.64 (d, 2H, J = 8.0 Hz), 7.45 (d,
4H, J = 8.0 Hz), 7.33 (s, 2H), 7.29 (d, 8H, J = 8.0 Hz), 7.07-7.17 (m, 16H), 3.95 (t, 4H, J = 8.0
Hz), 1.83-1.86 (m, 2H), 1.54-1.59 (m, 8H), 1.26-1.35 (m, 8H), 0.96 (t, 6H, J = 16.0 Hz), 0.87
(t, 6H, J = 16.0 Hz). 13C NMR (100 MHz, CDCl3): δ 154.1, 148.5, 147.3, 140.8, 132.8, 131.3,
130.5, 130.1, 129.5, 127.6, 124.9, 123.6, 123.1, 117.8, 116.7, 115.3, 113.7, 93.0, 91.3, 72.0,
39.5, 30.6, 29.1, 24.0, 23.1, 14.1, 11.3. Tm.p. = 193-194 °C. MS(MALDI-TOF) m/z (M+): calcd.
for C76H70N4O2: 1070.55, found: 1071.698 for [M+H]+. Anal. calcd. for C76H70N4O2: C, 85.20;
H, 6.59; N, 5.23; found: C, 85.37; H, 6.39; N, 5.41.
OLED Device Fabrication and Characterization
The green light-emitting diode devices studied here were fabricated based on the
architecture of ITO/PEDOT:PSS(60 nm)/2EHO-TPA-CNPE(55 nm)/TPBI(40 nm)/LiF(0.8
nm)/Al(100 nm) for conventional devices and ITO/TNC(50 nm)/2EHO-TPA-CNPE(55
nm)/TPD(50 nm)/V2O5(15 nm)/Al(100 nm) for inverted devices. First, patterned indium tin
oxide (ITO) (10 Ω square-1, 120 nm, Kintec Company) substrates were cleaned with distilled
water, and acetone-ethanol-isopropanol in an ultrasonic solvent bath, and then treated by
oxygen
plasma
for
5
min.
For
the
conventional
device,
poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Heraeus Clevios
GmbH) was spin-coated (3000 rpm for 60 s) on cleaned ITO-glass substrates as the hole-
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injecting layer (60 nm) and baked at 120 °C for 20 min. 10 mg of 2EHO-TPA-CNPE solid
DOI: 10.1039/D0TC01266A
was dissolved in 1 mL of chloroform and filtered through a filter (0.20 μm, Millipore, PTFE).
Then, 2EHO-TPA-CNPE solution was spin-coated on PEDOT:PSS at a speed of 800 rpm (40
s) and annealed at 80 °C for 10 min to remove residual solvent, which afforded the green
emissive layer (55 nm). For ink-jet-printed devices, 2EHO-TPA-CNPE thin-film (50 nm) was
printed from the ink formulation (1 mg/ml) prepared in o-xylene:phenoxytoluene solvent
mixture (50:50 v/v %). Then, electron-transport/hole-blocking 1,3,5-tris(phenyl-2-
benzimidazolyl)benzene (TPBI) layer was deposited by thermal evaporation at a pressure of
2×10−6 Pa. The thickness of the TPBI layer was 40 nm. Finally, a cathode composed of LiF (0.8
nm) and aluminum (100 nm) were sequentially deposited via thermal evaporation under high
vacuum (2×10−6 Pa). For inverted OLEDs, ZnO:PEI:TPBi ternary nano-composite (TNC) was
prepared as specified.43 TNC solution was spin coated at 2500 rpm for 30 seconds on cleaned
ITO glass substrates and cured at 120°C for 120 minutes in ambient air to give the electron-
injecting layer (50 nm). Then, 2EHO-TPA-CNPE solution was spin-coated on the TNC layer
(1500 rpm 40 s) and annealed at 80 °C for 10 min. Next, TPD (N,N′-Bis(3-methylphenyl)-N,N′-
diphenylbenzidine) (50 nm) was deposited as the hole transport layer by thermal evaporation at
a pressure of 2×10−6 Pa. V2O5 (15nm) and aluminum (100 nm) were evaporated as the hole-
injecting layer and the cathode, respectively. The size of the active OLED area is 9 mm2. The
electroluminescence spectra and current density–voltage-brightness of the devices were
measured by Hamamatsu PMA-12 C10027 Photonic Multichannel analyzer. The thicknesses
of the organic layers were measured by a stylus profiler (KLA Tencor P-6). The surface
morphology of the emissive 2EHO-TPA-CNPE thin-film spin-coated on ITO/PEDOT:PSS
was characterized by atomic force microscopy (FlexAFM-NanoSurf, Abdullah Gül University-
Central Research Facility(AGU-CRF)).
RESULTS AND DISCUSSION
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DOI: 10.1039/D0TC01266A
Molecular Design, Synthesis, and Characterization.
The D’-π’-A-π-D-π-A-π’-D’ molecular architecture of 2EHO-TPA-CNPE with
diphenylamino end units (D’) and ethynylene/phenylene π-spacers (π/π’) is envisioned to give
an extended π-system for green emission, facilitate hole injection/transport characteristics of
the emissive layer,44 and improve thermal/morphological stability of the deposited films. The
presence of highly twisted aryl substituents at molecular termini and swallow-tailed substituents
is expected to allow for good solubility and impede undesired aggregation and intermolecular
interactions in the solid-state (i.e., reduced excimer formation/non-radiative decays), which
might enable the solution-based fabrication of non-doped emissive layer (i.e., no host matrix).45
Considering the good solubility and the decent molecular size (1071.39 amu) (i.e., potentially
good solution rheology), fabrication of an emissive layer based on the new molecule could be
achieved even by using printing techniques such as ink-jet printing. Ethynylene and phenylene
π-spacers are employed between D and A units to control the optical band gap, and facilitate
efficient fluorescence emission by adjusting the formation of LE character in the HLCT excited
states. It’s noteworthy that, different than our initial design,38 the presence of additional π-
electron rich D end units in the new molecule is very likely to enhance the CT characteristics
of the HLCT excited states. Finally, the coexistence of donor and acceptor π-units in the same
molecular backbone could allow for ambipolar charge-transport, which is crucial for efficient
electroluminescence in organic thin-films.46
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DOI: 10.1039/D0TC01266A
Figure 2. The chemical structures of deep-blue emissive molecule 2EHO-CNPE developed in
our earlier study38 and green emissive molecule 2EHO-TPA-CNPE developed in this study,
and the energy diagrams showing theoretically calculated (light colors) and experimentally
estimated (dark colors) HOMO/LUMO energy levels, frontier orbital topographies, and
optical/electrochemical/theoretical band gaps.
The quantum mechanical modelling (DFT, B3LYP/6-311G**) shows that the
theoretical HOMO and LUMO energies of the new molecule are -5.21 eV and -2.39 eV,
respectively, which indicates a very similar LUMO and a significantly increased HOMO
(ΔEHOMO = +0.45 eV) as compared to the deep blue emissive 2EHO-CNPE (Figure 2). The
calculated absorption spectrum shows that S0 → S1 (HOMO → LUMO; f = 2.32) electronic
transition occurs at 2.48 eV (~500 nm), which is very likely to produce green molecular
emission (Figure S1). The HOMO of the new molecule is found to be delocalized along the
entire molecular π-system signifying the strong donor effect of the diphenylamino termini,
which is expected to benefit hole-injection/transport in the new emissive thin-film. On the other
hand, LUMO tends to delocalize on the central A-π-D-π-A part giving a LUMO topography
that is almost identical to that of 2EHO-CNPE. This explains the minimal change in the LUMO
energy level going from 2EHO-CNPE to 2EHO-TPA-CNPE, which is expected to retain a
good electron injection/transport character in the new emissive thin-film. Based on frontier
orbital topographies of the new molecule, it is reasonable to suggest that both LE and CT
characteristics would be utilized during electronic transitions that could yield an efficient green
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electroluminescence with favorable emission characteristics. These initial theoretical outcomes
DOI: 10.1039/D0TC01266A
are found to be in good agreement with the experimental characterizations (vide infra).
Scheme 1. The synthesis route for green emissive molecule 1,4-bis((4'-diphenylamino-3-
cyano-[1,1'-biphenyl]-4-yl)ethynyl)-2,5-bis(2-ethylhexyloxy)benzene (2EHO-TPA-CNPE).
Scheme 1 shows the synthesis route for the new green emitter 2EHO-TPA-CNPE,
which involves two successive Pd-catalyzed cross-coupling reactions starting from 2EHO-
DEB that was synthesized following our previously described procedure (Scheme S1,
Supporting Information). 2EHO-DEB was first reacted with 5-bromo-2-iodobenzonitrile in the
presence of Pd(PPh3)2Cl2 catalyst and CuI/Et3N co-catalyst/base in THF affording the
dibrominated intermediate 2EHO-Br-CNPE in 71% yield. This reaction involved very
selective Sonogashira cross-couplings to the iodo positions with a yield of 84% at each reaction
site. The selectivity undoubtedly arises from the better reactivity of iodo as compared to bromo
and the presence of electron-withdrawing cyano group ortho to the iodo substituent, both of
which facilitates rate-determining oxidative addition step in the catalytic cycle.47,48 In the final
step, Suzuki cross-coupling reaction between triphenylamine pinacol boronic ester and 2EHO-
Br-CNPE was performed in the presence of Pd(PPh3)4 catalyst and Na2CO3 base in
dimethoxyethane (DME), which yielded the target molecule 2EHO-TPA-CNPE in 65% yield.
Despite its large molecular weight (1071.39 amu), 2EHO-TPA-CNPE was found to be quite
soluble in common organic solvents such as chloroform and THF, as well as printing friendly
solvents such as toluene and xylene. 2EHO-TPA-CNPE was purified by column
chromatography on silica gel using CH2Cl2:n-hexane (3:1, v/v) solvent mixture. The chemical
structures and purities of the intermediate compounds and the final molecule were characterized
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1
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DOI: 10.1039/D0TC01266A
with H/13C NMR spectroscopies (Figures S2-S6, S8, and S9, Supporting Information),
MALDI-TOF mass spectrometry (Figures S7 and S10, Supporting Information), and elemental
analysis.
Thermal Properties.
Figure 3. Differential scanning calorimetry (DSC) measurement curves (A) and
thermogravimetric analysis (TGA) curve (B) of 2EHO-TPA-CNPE at a temperature ramp of
10 °C/min under N2.
Thermal properties of the new molecule were studied by thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) (Figure 3). The thermolysis onset
temperature (Tonset, 5% weight loss) of 380 °C indicates an excellent thermal stability for the
new molecule. In comparison to deep-blue emissive 2EHO-CNPE (Tonset = 346 °C),38 2EHO-
TPA-CNPE’s increased thermal stability (ΔTonset = +34 °C) against thermolysis is the result of
terminal diphenylamino moieties introducing additional aryl units (6 extra phenyls per
molecule) on the molecular backbone. The DSC characterization of the 2EHO-TPA-CNPE
solid, which was crystallized from the solvent removal after chromatography, shows a sharp
endothermic peak at 195 °C (enthalpy of 38.89 J/g) corresponding to the melting point of the
solid (Tm.p. = 193-194 °C). Prior to the isotropic transition, two endothermic peaks at 155 °C
(enthalpy of 49.56 J/g) and 164 °C (enthalpy of 39.12 J/g) were observed, which indicates the
presence of mesophase(s) at temperatures higher than ~150 °C for this molecule.49 The isotropic
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liquid phase of 2EHO-TPA-CNPE was found to be transformed to a completely amorphous
DOI: 10.1039/D0TC01266A
phase upon cooling since no crystallization peak was observed and the successive heating cycle
yields only a glass transition (Tg) at 82 °C with no melting. The amorphous morphology of the
2EHO-TPA-CNPE solid is consistent with those of previously reported arylamine terminated
emissive and hole-transport small molecules.44,50,51 Much higher melting temperature of
2EHO-TPA-CNPE (ΔTm.p. ~ 80 °C), in comparison to that of the deep-blue emissive 2EHO-
CNPE (Tm.p. = 116-117 °C), reflects its extended molecular π-backbone with increased donor-
acceptor characteristics.
Photophysical/Electrochemical Properties and Solvatochromic Effects
Photophysical properties of the new molecule were studied by optical absorption,
steady-state photoluminescence, and transient photoluminescence spectroscopies in different
solutions with varied polarities (1×10-5 M, hexane → acetonitrile) and as thin-films. As depicted
max
in Figure 4A, 2EHO-TPA-CNPE showed an absorption maximum
at 417 nm in
)
(λ
abs
dichloromethane solution that corresponds to π-π* (S0 → S1) transition of the molecular
backbone. The corresponding photoluminescence spectrum in dichloromethane solution
max
displays a green fluorescence emission with a peak maximum
at 519 nm. The
)
(λ
fl
photoluminescence quantum efficiency measured by using the integrating-sphere method was
found to be near unity (ΦPL = 0.95). This indicates a very efficient radiative S1 → S0 electronic
transition on the molecular backbone with minimal non-radiative losses. From the intersection
0 ― 0
0 ― 0
of low-energy absorption
and high-energy fluorescence
spectral band edges,
)
(λabs
)
(λfl
the optical band gap is measured as 2.68 eV, which is consistent with the TDDFT-calculated S0
→ S1 electronic transition (2.48 eV, HOMO → LUMO (f = 2.32), Figure S1). Both absorption
and fluorescence peaks, as well as the optical band gap, for the new molecule are found to be
max
max
shifted significantly to lower energies (Δ
(λabs
= +30 nm, Δ
(λfl
= +85 nm, ΔE = -0.3 eV)
)
g
)
relative to the deep-blue emissive 2EHO-CNPE.38 These spectral changes are obviously the
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result of extended π-conjugation of the new molecule and increased donor-acceptor nature of
DOI: 10.1039/D0TC01266A
max
the backbone. Going from solution to solid-state, slightly red-shifted absorption (Δ
λabs
= +19
nm) profile was obtained (Figure 4A) that most likely originates from intramolecular π-
planarization formed in the solid-state. The solid-state (neat spin-coated thin-film) fluorescence
max
spectrum of 2EHO-TPA-CNPE has
at 518 nm showing almost no change from that in
λfl
the dichloromethane solution. This indicates that the intermolecular interactions formed in the
solid-state does not change the excitonic properties of the new molecule and the polarity created
by the molecules itself in the solid-state (solid-state solvation effect)30,52,53 is similar to that of
the medium polarity solvent, dichloromethane. Because the thin-film fluorescence emission
remains in the green spectral region showing a favorable full width at half-maximum (fwhm
~70 nm), the desired green emitting OLEDs could be fabricated without needing a host matrix.
The thin-film photoluminescence quantum efficiency was measured as 0.45 by using the
integrating-sphere method. Electrochemical characterization reveals the coexistence of clear
(quasi)reversible oxidation and reduction peaks in the same cyclic voltammetry scan at +0.98
V and -1.65 V (vs. Ag/AgCl), respectively (Figure 4B). This reflects bipolar charge-transport
nature of the new emissive molecule, which potentially would allow for efficient hole and
electron injection/transport in the emissive layer. The LUMO and HOMO energies are
estimated to be -2.75 eV and -5.38 eV, respectively. In agreement with the optical and
theoretical band gap values, the electrochemical band gap (EgCV) is measured as 2.63 eV. As
compared with deep-blue emissive 2EHO-CNPE, LUMO of the new molecule remains almost
the same (ΔELUMO = +0.04 eV) while HOMO considerably increases (ΔEHOMO = +0.45 eV),
th
which is very consistent with the DFT results (ΔELUMOth/ΔEHOMO = +0.07/+0.40 eV) and the
corresponding orbital topographies (Figure 2).
In order to explore the excited state characteristics, optical absorption and
photoluminescence spectra of 2EHO-TPA-CNPE were studied in fourteen different solvents
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with wide-ranging orientational polarizabilities (f(ɛ,n)). As shown in Figures 4C, SD1O1I,: 1a0n.10d39S/D102T,C01266A
upon increasing solvent polarity (hexane (f (ɛ,n) = 0.001) → acetonitrile (f (ɛ,n) = 0.305)),
although the absorption profiles remain almost the same, photoluminescence spectra exhibit
extremely large bathochromic shifts of up to 130 nm. The complete data is provided on Table
S2. This clearly suggests that while the ground state (S0) of 2EHO-TPA-CNPE is not of any
CT character, the radiative excited state (S1) possesses significant intramolecular CT character
with a large excited state dipole moment. Interestingly, the spectral red shifts in the high polarity
region (481 nm (chloroform) → 582 nm (acetonitrile), Δf(ɛ,n) = 0.15) was found to be much
more pronounced than that (452 nm (hexane) → 481 nm (chloroform), Δf(ɛ,n) = 0.15) in the
low polarity region. This clearly suggests that there are two different excited state behaviors in
the low and high solvent polarity regions for 2EHO-TPA-CNPE π-system. The excited state
dipole moment (μe) of the S1 state could be estimated from the slope of the Lippert-Mataga plot
(Stokes shifts (νabs - νfl) against f(ɛ,n)) and using the DFT-calculated ground state dipole moment
(See Supporting Information for details). As shown in Figure 4D, two section linear fittings
were obtained in the low and high solvent polarity regions giving two different excitonic
characters. Based on the fitted line slopes, it is clear that S1 shows LE-dominated excitonic
character in the low-polarity region and CT-dominated excitonic character in the high-polarity
region. The LE-dominated excited state character was evident in the vibrational fine structure
of the photoluminescence spectra in this region. The emission peak intervals were found to be
in the order of 0.17-0.20 eV (~1400-1600 cm-1) corresponding to the vibrational frequencies of
aromatic C=C bond stretches.54,55 Going to high-polarity region, vibronic structure of the
fluorescence spectrum completely disappears and significant emission band broadening was
observed, surely reflecting the CT character dominance in the excited state. The fwhm becomes
~135 nm in acetonitrile while it is estimated to be as narrow as ~35-40 nm in hexane. The slope
of the curve in the high polarity region was found to be very large (slope ≈ 29,700 and R2 =
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0.97) yielding a μe of 35.3 D. Note that significantly large μe value of 2EHO-TPA-CNPE is in
DOI: 10.1039/D0TC01266A
the order of some of the highest excited state dipole moments predicted in the literature and it
refers to a very strong CT character in the high polarity region.33–35,56 The observed
solvatochromic behavior for 2EHO-TPA-CNPE reveals the presence of both LE and CT
characteristics in the excited state, which tends to hybridize nonequivalently in the low- and
high-polarity regions. However, (quasi)equivalent hybridization of LE and CT to generate a
HLCT excited state character is very likely to occur in a medium polarity environment (f =
0.15-0.20) (Figure 4D, green shaded area). The solution photoluminescence quantum
efficiencies were measured as low as 0.25 and 0.15 in high-polarity acetone and acetonitrile
solvents, respectively, as a result of strong CT character. Similarly, the quantum efficiency in
the LE region was also found to be low (ΦPL = 0.17 (in hexane)). The highest
photoluminescence quantum efficiencies for the new molecule were measured in the HLCT
region (ΦPL = 0.94 (in chloroform), ΦPL = 0.95 (in ethyl acetate), and ΦPL = 0.95 (in
dichloromethane)). When this solvatochromic behavior is compared with that of our previously
developed deep-blue emissive molecule, μe’s of 2EHO-CNPE in both low- and high-polarity
solvents are found to be undistinguishable yielding a single μe value of 8.4 D (R2 = 0.96) (Figure
S13). A good linearity retains across the whole solvent polarity region, which supports
sufficiently hybridized (strong interstate coupling) LE and CT components in 2EHO-CNPE’s
HLCT excited state. The observed much stronger CT behavior in the excited state of the new
molecule, as compared to 2EHO-CNPE, is undoubtedly the result of extended π-conjugation
with additional diphenylamino end units. Excitingly, the electroluminescence profile for
max
2EHO-TPA-CNPE-based thin-film in OLED exhibits
at 505 nm (Figure 4D-inset) that
λel
corresponds to the medium solvent polarity HLCT region in the Lippert-Mataga plot. This
indicates that the polar-environment created by the molecules itself in the neat thin-film (solid-
state solvation effect)30,52,53 could facilitate HLCT character in the excited state, which is highly
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beneficial to employ efficient radiative emission and triplet-to-singlet excitonDicOI:t1r0a.1n0s39it/iDo0nTC01266A
features at the same time during electroluminescence.
Figure 4. A. Optical absorption and photoluminescence spectra of 2EHO-TPA-CNPE in
dichloromethane (1.0 × 10−5 M) and as spin-coated neat thin-film on glass. B. Cyclic
voltammogram of 2EHO-TPA-CNPE showing oxidation and reduction peaks in 0.1 M
TBAPF6/CH2Cl2 solution vs. Ag/AgCl (3.0 M NaCl) at a scan rate of 100 mV/s. Inset shows
the experimental HOMO/LUMO energy levels, frontier orbital topographies, and
optical/electrochemical/theoretical band gaps. C. Solvatochromic optical absorption and
photoluminescence spectra of 2EHO-TPA-CNPE in different solvents with increasing polarity
(hexanes → acetonitrile). Inset shows the optical images of 2EHO-TPA-CNPE solutions in
different solvents under optical excitation. D. Solvatochromic Lippert-Mataga model showing
the fitted linear correlation (νabs-νfl vs. f (ɛ,n)), and LE-dominated, CT-dominated, and HLCT
excitonic
regions.
Inset
shows
the
device
architecture
and
solid-state
absorption/electroluminescence profiles for 2EHO-TPA-CNPE-based green emitting OLED
device.
Electroluminescence Properties and Conventional/Inverted/Printed Organic Light-
Emitting Diodes
Based on its good solubility in organic solvents, highly favorable photophysical and
frontier orbital (i.e., wave function topographies and energies) characteristics, and good thermal
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properties, 2EHO-TPA-CNPE stands as a very promising candidate for solution-processed
DOI: 10.1039/D0TC01266A
multilayered electroluminescent devices. In order to study the electroluminescence properties,
OLED devices were first fabricated in a conventional device architecture of
ITO/PEDOT:PSS(60 nm)/2EHO-TPA-CNPE(55 nm)/TPBI(40 nm)/LiF(0.8 nm)/Al(100 nm).
Because 2EHO-TPA-CNPE neat thin-film exhibits a favorable green fluorescence without a
sign of undesirable aggregation-induced excimer emission, the emissive layer in the current
OLED device was prepared by spin-coating 2EHO-TPA-CNPE solution (10 mg/ml) in
chloroform without using a host matrix. The atomic force microscopy (AFM) characterization
reveals that the new molecule forms a continuos amorphous morphology with highly
interconnected granular domains (~50-100 nm in sizes) and a smooth organic surface (root
mean square roughness < 0.5 nm) on ITO/PEDOT:PSS (Figure S14). Energy level diagram of
this device is shown in Figure 5B, in which PEDOT:PTSS and LiF serve as hole- and electron-
injecting layers, respectively; TPBI is used as electron-transport and hole-blocking layer. As
shown in Figures 5A and 6A, the device turns on at a voltage of 3.5 V (Von, 1 cd/m2), which is
in the order of the band gap value of the emissive layer, and strong electroluminescence with a
peak maximum at 505 nm (at 200 cd/m2) was observed that corresponds to green emission with
CIE 1976 (u',v') chromaticity coordinate of (0.10, 0.55). The electroluminescence profile of the
new molecule coincides with the solid-state photoluminescence indiating that the radiative
excitons formed under electrical bias are in the molecular S1 state and excimer formation is
effectively prevented. As shown in Figure 5B, the (u',v') chromaticity coordinate of the new
molecule occurs to be in the night vision imaging systems (NVIS) Green A region, which
indicates that the new molecule holds great promise for use in NVIS compatible displays,
especially for military and commerical avionics.57,58 As shown in Figure 6, 2EHO-TPA-
CNPE-based OLED device demonstrated good performance with a maximum current
efficiency (CEmax) of 12.0 cd/A, a maximum power efficiency (PEmax) of 5.4 lm/W, and a
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maximum external quantum efficiency (EQEmax) of 4.5% (all values at ~320 cd/m2). The device
DOI: 10.1039/D0TC01266A
maintains a chromaticity coordinate of (0.11, 0.55) and electroluminescence performance of
CE = 10.8 cd/A, PE = 4.3 lm/W, and EQE = 4.0 % under a high luminance value of ~1000
cd/m2. On the basis of measured ΦPL-neat film value of 0.45 and EQEmax value of 4.5% (assuming
ηout ~ 0.2 and γ ~ 1), it is evident that the radiative exciton yield (ηr = EQE/(γ × ΦPL × × ηout))
for the new molecule can reach as high as ~50%.
Figure 5. A. EL spectra and the corresponding CIE 1976 (u',v') chromaticity coordinates for
2EHO-TPA-CNPE-based green emitting conventional OLED device (ITO/PEDOT:PSS(60
nm)/2EHO-TPA-CNPE(55 nm)/TPBI(40 nm)/LiF(0.8 nm)/Al(100 nm)) operated at different
voltages. Inset shows the chemical structure of 2EHO-TPA-CNPE. B. Energy level diagram
and CIE 1976 (u',v') chromaticity diagram showing NVIS Green A region and (u',v')
coordinates of the green emitting OLED device at 200 cd/m2. The insets show optical images
of the OLEDs (9 mm2 active area) during device operation and the letters (on a glass substrate)
under UV light illumination based on ink-jet printed fluorescent 2EHO-TPA-CNPE thin-films.
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DOI: 10.1039/D0TC01266A
Figure 6. Luminance-voltage-current density (A) and current efficiency-current density-
external quantum efficiency (B) plots for 2EHO-TPA-CNPE-based OLED device
(ITO/PEDOT:PSS(60 nm)/2EHO-TPA-CNPE(55 nm)/TPBI(40 nm)/LiF(0.8 nm)/Al(100
nm)).
Since such a strong solvatochromic behavior was seen in the photoluminescence of the
new molecule that intensely tunes the excited state energy/dipole depending on the solvent
polarity, we were interested in exploring whether a similar solid-state solvation effect could be
observed on the electroluminescence properties of the OLED devices. For this purpose, OLEDs
employing a 2EHO-TPA-CNPE-based doped emissive layer (ITO/PEDOT:PSS(60
nm)/CBP:2EHO-TPA-CNPE(60 nm)/TPBI(40 nm)/LiF(0.8 nm)/Al(100 nm)) were fabricated
with the same device architecture. The emissive layer was deposited by spin-coating chloroform
solution of 2EHO-TPA-CNPE (4.0 wt %) and 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP)
(96.0 wt %) host matrix. CBP host was used based on its good charge-transport properties,
proper frontier orbital energies, and good spectral overlap between CBP’s emission and 2EHO-
TPA-CNPE’s absorption bands (Figures S15B and S15C). Also, in contrast to the neat thin-
film in which each 2EHO-TPA-CNPE emissive molecule is neighboring similar molecules of
strong donor-acceptor nature (high polarity), each emissive molecule in the doped thin-film is
surrounded by relatively less polar CBP host molecules.59 Consistent with the
photoluminescence profile of this doped CBP:2EHO-TPA-CNPE thin-film (Figure S15A), the
electroluminescence spectrum is found to show a sky-blue emission with a peak maximum at
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475 nm (at 200 cd/m2) and a (u',v') chromaticity coordinate of (0.09, 0.45) (Figure S15).
DOI: 10.1039/D0TC01266A
Therefore, it is evident that the electroluminescence of 2EHO-TPA-CNPE blue shifted as the
polarity of the adjacent molecules decreased, which indicates the destabilization of the radiative
S1 state. This is in close agreement with the observed photoluminescence solvatochromic
behavior, and the change in max (505 nm → 475 nm) corresponds to a polarity change of f (ɛ,n)
λel
= 0.21 → 0.15 in the HLCT excitonic region (Figure 4D). Indeed, close inspection of the EL
spectrum reveals that there is still a shoulder at ~505 nm, which might be due to residual
presence of self-aggregated 2EHO-TPA-CNPE molecules in the doped thin-film. The doped
device exhibited an improved EQEmax of 5.5% with CEmax of 10.1 cd/A and PEmax of 4 lm/W
(all values at 250 cd/m2), which corresponds to ηr = 59% (using ΦPL-doped film in CBP = 0.47) (Figure
S16). The ηr value for the doped CBP:2EHO-TPA-CNPE thin-film is ~9% higher than that of
the neat 2EHO-TPA-CNPE thin-film. Therefore, it is clear that the solid-state solvation effect
does not only change the exciton energy level but also affect the yield of radiative exciton
formation process.60 Similar effects of host matrix polarity on RISC efficiency during molecular
electroluminescence have been observed in previous studies.30,61,62 Therefore, it is very likely
that solid-state solvation effect for 2EHO-TPA-CNPE could be finely tuned by using different
host matrices to expand the scope of excitonic states (LE vs. HLCT vs. CT) for various
optoelectronic applications.
In order to further explore the industrial relevance of the new molecule for organic
optoelectronics, additional OLED devices were fabricated by ink-jet printing the emissive layer
and by fabricating an inverted architecture. Although these devices are not fully optimized and
show somewhat lower performances than conventional spin-coated devices, they are significant
to demonstrate that 2EHO-TPA-CNPE could potentially offer key manufacturing advantages.
The high precision deposition of OLED materials by ink-jet printing is of great technological
importance for making cost-effective flexible displays, because it could significantly reduce
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material waste and process complexity (less number of fabrication/lithographic steps).
DOI: 10.1039/D0TC01266A
However, the formulation of OLED materials into non-chlorinated printable inks is not
straightforward; a delicate balance between molecular weight/size, solubility, solvent boiling
point, viscosity, and surface tension is required for favorable printing and film drying processes
to yield an efficient electroluminescent thin-film.64 Initially, as a proof of concept, shapes in
letter characters, comprised of fluorescent 2EHO-TPA-CNPE-based thin-film, were deposited
onto a glass substrate via ink-jet printing (PIXDRO LP50) (Figure 5B). In this process, jettable
formulation of the new molecule was prepared in 1 mg/ml concentration in o-xylene:3-
phenoxytoluene (50:50 v/v %) solvent mixture (viscosity = 2.1 mPa·s at 22 °C). This non-
chlorinated high boiling-low boiling solvent mixture was selected based on preliminary drop-
casting experiments for an efficient printing process and to overcome any ring formation.65
Proper droplet generation was achieved using print head KM512 at a voltage of 14 V and pulse
width of 5 µs. The corresponding OLED devices were fabricated in a conventional device
architecture (ITO/PEDOT:PSS(60 nm)/2EHO-TPA-CNPE(50 nm/ink-jet printed)/TPBI(40
nm)/LiF(0.8 nm)/Al(100 nm)), in which the emissive layer is ink-jet printed as a non-doped
thin-film. The maximum current efficiency of this unoptimized printed device was 2.4 cd/A (at
250 cd/m2) and the maximum luminance (Lmax) was 550 cd/m2 ((u',v') = (0.16, 0.55)) (Figure
S17). These results show that 2EHO-TPA-CNPE has favorable molecular properties
(structural size/solubility and photo-/electro-luminescence) making itself a promising material
for the development of ink-jet printed display technologies. On the other hand, inverted OLED
architecture of ITO/TNC(50 nm)/2EHO-TPA-CNPE(55 nm)/TPD(50 nm)/V2O5(15
nm)/Al(100 nm) was also studied with the new molecule. Inverted OLEDs with bottom cathode
electrode are highly desirable for mass production of large area active-matrix displays as they
can be easily integrated to the drain electrode of n-channel TFT backplane.66 They also have
higher stabilities resulting from the fact that water-/oxygen-sensitive electron-injecting layer
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remains under the emissive organic layer and metal oxides are used instead PEDOT:PSS.
DOI: 10.1039/D0TC01266A
Since electron-injecting layer plays a critical role in the performance of inverted OLEDs, a
ternary nano-composite (TNC) of zinc oxide/polyethyleneimine/1,3,5-tris(N-
phenylbenzimidazol-2-yl)benzene (ZnO:PEI:TPBI) was used combining electron-injecting/-
transport and hole-blocking characteristics.43 In this inverted structure, Al (anode) and ITO
(cathode) are used as electrodes while N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine
(TPD) and V2O5 serve as hole-transport/electron-blocking and hole-injecting layers,
respectively. The emissive 2EHO-TPA-CNPE-based thin-film was deposited via spin-coating.
The inverted device demonstrated a good performance with CEmax value of 6.1 cd/A (at 1250
cd/m2) and Lmax value of 3600 cd/m2 ((u',v') = (0.18, 0.55)) (Figure S18).
Figure 7. A. Energy levels of the first ten singlet/triplet excited states for our previously
reported deep-blue emissive molecule 2EHO-CNPE and the new green emissive molecule
2EHO-TPA-CNPE calculated with TDDFT in S0 geometry, and the proposed exciton decay
processes (RISC, reverse intersystem crossing; IC, internal conversion; EL,
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electroluminescence). B. The transient photoluminescence decay profile measured at 469 nm
DOI: 10.1039/D0TC01266A
for 2EHO-TPA-CNPE solution in hexane (10-5 M) under nitrogen atmosphere upon excitation
at 375 nm and the corresponding lifetimes for prompt (PF) and delayed fluorescence (DF). The
inset shows the gradual decrease in PL intensity when the same solution is exposed to ambient
atmosphere. C. Natural transition orbitals (NTOs) for 2EHO-CNPE-TPA’s S1, T2 and T3
excited states calculated with TDDFT. The coefficients of each hole-particle pair are given in
parentheses.
The photophysical properties of the new molecular π-system are further studied via
TDDFT calculations (B3LYP/6-311G** level of theory), and time resolved and comparative
(ambient vs. nitrogen atmosphere) photoluminescence spectroscopies. On the basis of the EQEs
achieved, the radiative singlet exciton yields for 2EHO-CNPE-TPA–based OLEDs (ηr ~ 50-
59%) exceed the spin-statistical limit of 25% for singlet exciton. Therefore, in agreement with
our previously reported 2EHO-CNPE molecule, triplet-to-singlet excited state conversions via
RISC are likely to be effective during the electroluminescence process of the new molecule. To
this end, TADF process is predicted to be energetically inefficient as the energy gap between
S1 and T1 states (ES1-T1 = 0.64 eV) is much larger than that required for TADF-based molecular
electronic structures (typically ES1-T1 ≤ 100 meV).18,69 However, as shown in Figures 7A and
7C, hot-exciton channels employing RISC’s from high-lying triplet excited states to the singlet
excited state (T2 → S1 and T3 → S1) are identified to be quite favorable both from an energetic
(ES1-T2 = 0.11 eV and ES1-T3 = 0.04 eV) and wave function characteristics standpoints. As
clearly seen in the NTO wave functions (Figure 7B and Table S3), energetically close excited
states S1, T2 and T3 exhibit a good balance of spatial separation (CT) and orbital overlap (LE)
along the molecular π-backbone. The coexistence of LE and CT characteristics (HLCT
behavior) in the excited states is crucial to simultaneous realization of triplet-to-singlet
excitonic conversions and S1 → S0 radiative transition during electroluminescence. However,
as compared with 2EHO-CNPE, the CT character of the new molecule is found to be more
pronounced. The NTO analysis shows significant wave function contributions from
diphenylamino donor end units (D’) for the hole NTOs, whereas the particle NTOs are found
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Journal of Materials Chemistry C
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to be delocalized only on the central part (A-π-D-π-A). The increased CT characteDriOsIt:i1c0.s10o39f/Dth0TeC01266A
excited states in the new molecule show a good agreement with the observed strong PL
solvatochromic behavior (vide supra). As a result of extending -conjugation, the energetic
separations between the excited states of the same multiplicity become smaller in 2EHO-TPA-
CNPE. According to energy gap law, having a large ΔET2-T1 is particularly important for
efficient hot-exciton channel formation by minimizing undesired T2 → T1 internal conversion
(kRISC >> kIC).21,35 In the new molecule, although the ES1-T2/T3 is very similar to those of 2EHO-
CNPE, the energy difference between T2 and T1 excited states was found to be lower than that
of 2EHO-CNPE (ET1-T2 = 0.53 eV < 0.81 eV), which partially explains the observed lower
radiative exciton yield (ηr  70% → 50-59%). At this point, we speculate that hot-exciton RISCs
are likely to be more efficient with short π-conjugated systems since they usually tend to have
wider T1-T2 separation to prevent undesired triplet (T2 → T1) internal conversion. Indeed,
consistent with this thought, the highest performing hot-exciton molecules reported in the
literature are typically wide band gap (deep)-blue emissive π-systems.33,38,70 We also note that
while hot-exciton channels of T2/3 → S1 are very favorable to take place for the new molecule
since these states are expected to have relatively higher population densities among all excited
states, higher-energy hot-exciton channels (Tm (m ≥ 4) → Sn (n ≥ 2)) could not be excepted for
radiative singlet exciton formation.71,72 Indeed, our calculations with spin-orbit couplings
indicate that triplet-to-singlet conversions could be favorable between these high-lying excited
states when their energies match (Table S4 and Figure S19).The excited state properties of the
new molecule were further studied via time-resolved photoluminescence decay measurements
and comparative (ambient vs. nitrogen atmosphere) photoluminescence spectroscopy. The PL
decay profile of the new molecule in nonpolar hexane solution was recorded under nitrogen
atmosphere at the fluorescence maximum (λexc = 375 nm and 200 ps pulse width) by the time-
correlated single photon counting method. As shown in Figure 7B, double exponential decay
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Page 29 of 37
Journal of Materials Chemistry C
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profile was obtained showing first a prompt decay with a lifetime of 1.16 ns (~90%
DOI: 10.1039/D0TC01266A
contribution) and then a delayed component with a relatively longer lifetime of 9.14 ns (~10%
contribution). Note that, as expected, TADF mechanism-based fluorescence delays on a
microsecond (μs) timescale were not observed. The prompt decay is identified as the direct S1
→ So electronic transition (fluorescence) originating from the photoexcited S1 states. The longer
lifetime component could be attributed to delayed S1 → So fluorescence occurring through
(R)ISCs between energetically matching S1 and T2/T3 states because the PL spectrum of this
delayed component is identical to that of the prompt fluorescence. To the best of our knowledge,
this is one of the very few examples in the hot-exciton literature that high-energy RISC-related
delayed fluorescence is directly observed in a PL decay profile without using a triplet sensitizer.
Recently in the literature, Ma et al. reported similar lifetime scales for high-energy RISC
processes (~14-20 ns) by employing a triplet sensitization technique.22 A gradual decrease was
observed in the PL intensity when the nitrogen-flushed solution was exposed to ambient
atmosphere (Figure 7B-inset), which further warrants that high-lying triplet state(s) are
populated during the luminescence process, and they are somewhat quenched by triplet oxygen
upon exposure to ambient atmosphere.15,73,74
CONCLUSIONS
In summary, a new highly soluble rod-shaped oligo(p-phenyleneethynylene) molecule,
1,4-bis((4'-diphenylamino-3-cyano-[1,1'-biphenyl]-4-yl)ethynyl)-2,5-bis(2-ethylhexyloxy)
benzene (2EHO-TPA-CNPE) has been designed, synthesized, and characterized. The thermal,
photophysical, electrochemical, and electroluminescence properties have been studied in detail.
Highly efficient green luminescence was achieved with the new molecule yielding high
photoluminescence quantum efficiencies (ΦPL’s) of 0.95 (solution) and 0.45 (neat thin-film),
and high (>> 25%) radiative exciton yields of ηr = 50-59% in electroluminescence. A strong
solvatochromic behavior was observed in photoluminescence as evident in significant changes
29