High-triplet-level phthalimide based acceptors for exciplexes with multicolor emission

Article abstract of DOI:10.1016/j.dyepig.2018.11.022

To provide high exciton utilization in organic light emitting diodes, phthalimide derivatives were designed and synthesized as exciplex-forming materials. Due to high triplet levels (2.92–3.11 eV) and ionization potentials (7.18–7.29 eV), the developed phthalimide derivatives were found to be not only appropriate accepting materials for the formation of different color exciplexes but also as bifunctional materials with a satisfactory hole and exciton-blocking abilities. Solid-state blends of the synthesized phthalimides as acceptors and a carbazole containing donors showed exciplex emission. Bimolecular blends exhibited multicolor exciplex emission which covered a visible spectrum from sky-blue to red colors, depending on the donor used. However, the photoluminescence quantum efficiencies of the studied exciplex-forming systems were found to be sensitive to the molecular design of the phthalimides. Acceptor with para-substituted phthalimide showed better exciplex-forming properties in comparison to other compounds. Exciplex-forming blend of (2-(4-benzoylphenyl)isoindoline-1,3-dione) as an acceptor and 1,3-di(9H-carbazol-9-yl)benzene (mCP) as a donor showed the most efficient sky-blue emission with small singlet-triplet splitting (0.06 ± 0.03eV). Such exciplex-forming molecular mixture was implemented as the light-emitting material in the sky-blue organic light emitting diodes which showed the brightness of 2500 cd m^?2 and maximum external quantum efficiency of 2.9% due to the employment of both singlet and triplet excitons.

Full text of DOI:10.1016/j.dyepig.2018.11.022

Accepted Manuscript
High-triplet-level phthalimide based acceptors for exciplexes with multicolor emission
Marian Chapran, Roman Lytvyn, Corentin Begel, Gabriela Wiosna-Salyga, Jacek
Ulanski, Marharyta Vasylieva, Dmytro Volyniuk, Przemyslaw Data, Juozas Vidas
Grazulevicius
PII:
S0143-7208(18)32178-8
https://doi.org/10.1016/j.dyepig.2018.11.022
DYPI 7166
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 1 October 2018
Revised Date: 6 November 2018
Accepted Date: 11 November 2018
Please cite this article as: Chapran M, Lytvyn R, Begel C, Wiosna-Salyga G, Ulanski J, Vasylieva M,
Volyniuk D, Data P, Grazulevicius JV, High-triplet-level phthalimide based acceptors for exciplexes with
multicolor emission, Dyes and Pigments (2018), doi: https://doi.org/10.1016/j.dyepig.2018.11.022.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Full Length Article
High-triplet-level phthalimide based acceptors for exciplexes with multicolor
emission
Marian Chaprana, Roman Lytvynb,c, Corentin Begelb, Gabriela Wiosna-Salygaa, Jacek
Ulanskia, Marharyta Vasylievad, Dmytro Volyniukb, Przemyslaw Datad,e and Juozas Vidas
Grazuleviciusb*
aLodz University of Technology, Department of Molecular Physics, Zeromskiego 116, 90-924 Lodz, Poland
b Department of Polymer Chemistry and Technology, Kaunas University of Technology,
Radvilenu Plentas 19, LT-50254 Kaunas, Lithuania
cDepartment of Organic Chemistry, Ivan Franko National University of Lviv, Kyryla i Mefodiya St. 6, Lviv
79005, Ukraine
dFaculty of Chemistry, Silesian University of Technology, M. Strzody 9, 44-100, Gliwice, Poland
eDepartment of Physics, Durham University, South Road, Durham DH1 3LE, United Kingdom
Keywords: exciplex, phthalimide, acceptors, thermally activated delayed fluorescence
(TADF), organic light emitting diode (OLED)
Highlights:
- Five novel phthalimide based acceptors with high triplet levels (2.92-3.11 eV)
- Exciplex forming properties of acceptors were sensitive to the molecules design
- A para- substituted phthalimide-benzophenone in blend with carbazole containing donor
showed sky-blue exciplex emission with small singlet-triplet splitting (0.06±0.03eV).
Abstract
To provide high exciton utilization in organic light emitting diodes, phthalimide derivatives
were designed and synthesized as exciplex-forming materials. Due to high triplet levels
(2.92-3.11 eV) and ionization potentials (7.18-7.29 eV), the developed phthalimide
derivatives were found to be not only appropriate accepting materials for the formation of
different color exciplexes but also as bifunctional materials with a satisfactory hole and
exciton-blocking abilities. Solid-state blends of the synthesized phthalimides as acceptors and
a carbazole containing donors showed exciplex emission. Bimolecular blends exhibited
multicolor exciplex emission which covered a visible spectrum from sky-blue to red colors,
depending on the donor used. However, the photoluminescence quantum efficiencies of the
studied exciplex-forming systems were found to be sensitive to the molecular design of the
phthalimides. Acceptor with para- substituted phthalimide showed better exciplex-forming
properties in comparison to other compounds. Exciplex-forming blend of (2-(4-
benzoylphenyl)isoindoline-1,3-dione) as an acceptor and 1,3-di(9H-carbazol-9-yl)benzene
(mCP) as a donor showed the most efficient sky-blue emission with small singlet-triplet
splitting (0.06±0.03eV). Such exciplex-forming molecular mixture was implemented as the
light-emitting material in the sky-blue organic light emitting diodes which showed the
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brightness of 2500 cd m-2 and maximum external quantum efficiency of 2.9 % due to the
employment of both singlet and triplet excitons.
1. Introduction
Organic light emitting diodes (OLEDs) have attracted the attention due to their application in
displays[1] and in solid-state lighting technology [2]. Organic molecules exhibiting thermally
activated delayed fluorescence (TADF) [3] are of great interest due to their applications in
OLEDs as efficient emitters and hosts [4]. Such devices can reach efficiency close to that of
phosphorescent OLEDs. A key factor to generate efficient TADF is small singlet-triplet
energy gap (∆EST <kT) [5]. Chemical obstacles arising during synthesis of such type of
molecules, often limit the realization of the TADF phenomenon via excited state of a single
molecule [4].
As an alternative, TADF can be achieved by using emission from bimolecular excited
species, in particular, exciplexes [6,7]. Such mechanism can operate in appropriately selected
donor (D) and acceptor (A) fluorescent low molecular semiconductor systems, with the
subsequent formation of excited intermolecular charge transfer (CT) complexes (exciplexes).
An exciplex can be formed as a result of electron transfer from donor to acceptor molecules
under Coulomb interactions [8]. In such systems, HOMO and LUMO energy orbitals are
separated by a relatively long distance leading to small exchange energy and efficient reverse
intersystem crossing (RISC) process from the triplet excited state to the singlet one. Owing to
harvesting of dark triplets excitons internal quantum efficiency close to 100% can be
achieved [9]. Bimolecular charge-transfer (CT) systems (exciplexes) find application in white
electroluminescence devices either as emitters due to broad emission spectrum [10,11,12] or
as hosts for fluorescent, phosphorescent or TADF dopants [13,12]. Employing exciplex
forming pair as a host in OLEDs provided good operation stability and long lifetime of
devices as a result of charge carrier balance in OLED structure [14]. Moreover, multicolor
OLEDs can be designed using efficient exciplex-forming systems with one acceptor and
several donors to achieve simultaneously multicolor luminescence and efficient triplet
harvesting [10]. However, application of exciplex-forming systems as emitters is still
challenging due to the difficulties in developing of efficient CT systems with high
photoluminescence (PL) and electroluminescence (EL) yields [7]. To obtain TADF the
charge-transfer singlet (1CT) and the charge-transfer triplet (3CT) states of exciplexes should
lie lower or close to the triplet levels of the corresponding exciplex forming units.[15,16 ,17].
A progress in the field of high-efficiency exciplex emission based OLEDs can be reached by
the design and synthesis of new suitable electron donating and electron accepting molecules
with high triplet levels. Different types of compounds were tested as the electron donating
units, among them triphenylamine and a carbazole containing derivatives due to their good
electronic and optical properties and commercial availability [4]. Also numerous accepting
molecules were developed as the electron accepting counterparts such as derivatives of
triazine, phthalimide, benzimidazole, oxadiazole, pyridine [18,19].
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One of the well-known accepting unit is phthalimide moiety, derivatives of which are used as
synthetic precursors for the synthesis of organic semiconductors. Phthalimide derivatives
have found applications in optoelectronic and electronic devices such as solar cells [20,21],
organic thin-film transistors [22,23] and organic light emitting diodes [24,25]. The main
advantages of phthalimides are cheap and simple synthetic synthesis, strong accepting
properties (low lying LUMO levels), high triplet energy levels and high thermal stability
[20,25,26]. Recently application of phthalimide moiety as the acceptor building block in
TADF molecules was reported [27,28,29]. However, to our best knowledge, exciplex-
forming properties of phthalimide containing materials have not been studied yet.
In this work, we unclosed potential of phthalimide derivatives as accepting exciplex-forming
materials for OLEDs application and studied the effect their molecular structure
modifications on exciplex emission efficiency. Five new phthalimide derivatives 2,2'-
(oxybis(4,1-phenylene))bis(isoindoline-1,3-dione)(DPOPht2), 2,2'-(pentane-1,5-diylbis(4,1-
phenylene))bis(isoindoline-1,3-dione) (CyPh2Pht2), 2-(2-benzoylphenyl)isoindoline-1,3-
dione
(2-BpPht), 2-(3-benzoylphenyl)isoindoline-1,3-dione
(3-BpPht), 2-(4-
benzoylphenyl)isoindoline-1,3-dione (4-BpPht) (Figure 1) were designed, synthesized and
characterized.
Figure 1. Chemical structures of the studied new phthalimide derivatives: DPOPht2,
CyPh2Pht2, 2-BpPht, 3-BpPht and 4-BpPht
Theoretical investigations by means of density functional theory (DFT) were performed to
determine the geometry and HOMO and LUMO distribution of the synthesized compounds.
Their photophysical properties were studied to elucidate exciplex-forming ability. All
phthalimide derivatives showed high triplet levels and low-lying HOMO levels. Moreover,
using DPOPht2, CyPh2Pht2 , 2-BpPht, 3-BpPht, 4-BpPht acceptors and 1,3-di(9H-carbazol-
9-yl)benzene (mCP), 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP), poly(9-vinylcarbazole)
(PVK), tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as electron donors, different exciplex-
forming systems emitting in sky-blue, green and red regions were developed. The detailed
steady-state and time-resolved spectroscopic studies were carried out to disclose the benefits
of the current design strategy of efficient exciplex-forming acceptors.
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2. Experimental section
2.1. Materials. All reactions do not require special precautions (excluding moisture
influence) and were performed under air atmosphere. Dry solvents and all the other reagents
were purchased from Sigma Aldrich or Alfa Aesar and used without further purification.
General procedure for the synthesis of phthalimide acceptors
An equimolecular mixture of phthalic anhydride (5 mmol) and corresponding amine (5
mmol) (in case of compounds DPOPht2 and CyPh2Pht2 the ratio of reagents were 2:1) were
stirred under refluxing in glacial acetic acid (35 ml) during 12 hr. Then acetic anhydride (15
ml) was added in one portion and mixture was stirred under refluxing for additional 5 hr. The
cold reaction mixture was poured into distilled water (150 ml). Precipitated sediment was
filtered off and washed several times with distilled water. The obtained crude material was
recrystallized from i-PrOH/DMF mixture of solvents.
2,2'-[Oxybis(4,1-phenylene)]bis(1H-isoindole-1,3(2H)-dione) DPOPht2
White crystals, yield 70 %, mp 289-290°C (mp 291°C[30]). 1H NMR (250 MHz, CDCl3), δ:
7.98 – 7.93 (m, 4H), 7.83 – 7.77 (m, 4H), 7.44 (dt, J = 9.0, 2.3 Hz, 4H), 7.20 (dt, J = 9.0, 2.3
13
Hz, 4H). C NMR (175 MHz, CDCl3), δ: 167.37, 156.46, 134.47, 131.79, 128.21, 127.07,
123.81, 119.56. MS (MALDI-TOF) m/z [M]- calcd for C28H16N2O5: 460.45; found, 460.65.
Elemental analysis cal. for C28H16N2O5 C, 73.04; H, 3.50; N, 6.08. Found (%): C, 73.25; H,
3.61; N, 6.13.
2,2'-[Cyclohexane-1,1-diylbis(4,1-phenylene)]bis(1H-isoindole-1,3(2H)-dione) CyPh2Pht2
Off-white crystals, yield 54 %, mp 283-284°C 1H NMR (250 MHz, CDCl3), δ: 8.00 – 7.91
(m, 4H), 7.83 – 7.74 (m, 4H), 7.45 (dt, J = 9.0, 2.3 Hz, 4H), 7.38 (dt, J = 9.0, 2.3 Hz, 4H),
2.38 – 2.31 (m, 4H), 1.66 – 1.49 (m, 6H). 13C NMR (175 MHz, CDCl3), δ: 167.35, 147.95,
134.35, 131.85, 129.20, 128.04, 126.11, 123.74, 46.26, 37.24, 26.30, 22.85. MS (MALDI-
TOF) m/z [M+K]- calcd for C34H26N2O4: 565.70; found, 565.75 Elemental analysis cal. for
C34H26N2O4 C, 77.55; H, 4.98; N, 5.32. Found (%): C, 77.69; H, 4.65; N, 5.42.
2-(2-Benzoylphenyl)-1H-isoindole-1,3(2H)-dione 2-BpPht
White crystals, yield 68 %, mp 198-199°C (mp 198-199°C[30]). 1H NMR (250 MHz, CDCl3)
δ 7.85 – 7.63 (m, 8H), 7.58 – 7.30 (m, 5H). 13C NMR (175 MHz, CDCl3), δ: 195.22, 167.14,
137.12, 136.07, 134.28, 132.78, 132.04, 131.72, 130.87, 130.54, 129.91, 129.33, 128.33,
128.22, 123.72. MS (MALDI-TOF) m/z [M+H]- calcd for C21H14NO3: 328.35; found,
328.51. Elemental analysis cal. for C21H13NO3: C, 77.05; H, 4.00; N, 4.28. Found (%): C,
77.17; H, 4.09; N, 4.37.
2-(3-Benzoylphenyl)-1H-isoindole-1,3(2H)-dione 3-BpPht
White crystals, yield 76 %, mp 179-180°C (mp 177-179°C[30]). H NMR (250 MHz,
1
CDCl3), δ: 8.01 – 7.93 (m, 2H), 7.93 – 7.84 (m, 4H), 7.84 – 7.77 (m, 2H), 7.74 – 7.63 (m,
2H), 7.59 (dt, J = 6.2, 1.4 Hz, 1H), 7.55 – 7.47 (m, 2H). 13C NMR (175 MHz, CDCl3) δ
195.42, 167.00, 138.47, 137.17, 134.63, 132.72, 131.80, 131.67, 130.25, 130.19, 129.46,
129.25, 128.45, 128.23, 123.93. MS (MALDI-TOF) m/z [M+H]- calcd for C21H14NO3:
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328.35; found, 328.55. Elemental analysis cal. for C21H13NO3: C, 77.05; H, 4.00; N, 4.28.
Found (%):C , 77.20; H, 4.11; N, 4.23.
2-(4-Benzoylphenyl)-1H-isoindole-1,3(2H)-dione 4-BpPht
White crystals, yield 72 %, mp 182-183°C (mp 183°C[30]). 1H NMR (250 MHz, CDCl3), δ:
8.02 – 7.92 (m, 4H), 7.88 – 7.80 (m, 4H), 7.69 – 7.57 (m, 3H), 7.56 – 7.46 (m, 2H). 13C NMR
(175 MHz, CDCl3), δ: 195.68, 166.88, 137.40, 136.69, 135.45, 134.73, 132.64, 131.65,
130.89, 130.09, 128.42, 125.91, 124.01. MS (MALDI-TOF) m/z [M+H]- calcd for
C21H14NO3: 328.35; found, 328.51. Elemental analysis cal. for C21H13NO3: C, 77.05; H, 4.00;
N, 4.28. Found (%): C, 77.15; H, 4.07; N, 4.17.
1
13
2.2. Methods. H spectra were recorded on a Bruker 250 MHz NMR spectrometer and C
NMR spectra were recorded on a Bruker Ultra Shield 700 MHz NMR spectrometer using
CDCl3 as the solvent. Chemical shifts (δ) are reported in ppm referenced to tetramethylsilane
(TMS) as internal reference and coupling constants (J) in Hz. Melting points were measured
using the Electrothermal MEL-TEMP melting point apparatus. The target compounds were
characterized by matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF)
using an Axima-Performance TOF spectrometer (Shimadzu Biotech, Manchester, UK),
equipped with a nitrogen laser (337 nm). The pulsed extraction ion source accelerated the
ions to a kinetic energy of 20 keV. Elemental analysis was performed on an Exeter Analytical
CE-440 elemental analyser.
2.3. Electrochemistry. Electrochemical measurements of 1.0 mM concentrations of
phthalimide derivatives were conducted in 0.1 M tetrabutylammonium hexafluorophosphate
(Bu4NPF6) (99%, Sigma Aldrich, dried) as the electrolyte and in dichloromethane
(CHROMASOLV®, 99.9% Sigma Aldrich) as the solvent. All solutions were degassed using
argon before measurement. The electrochemical cell comprised: platinum electrode with a 1
mm diameter of working area as a working electrode, an Ag/AgCl electrode as a reference
electrode and a platinum coil as an auxiliary electrode. Cyclic voltammetry measurements
were conducted at room temperature with a scan rate of 50 mV s-1 were calibrated with a
ferrocene/ferrocenium redox couple. Electron affinity (EA) was calculated from reduction
(Ered) potential, respectively, using following equation: EA = Ered + 5.1[31,32].
2.4. Photophysics. Absorption spectra were recorded with a Carry 5000 (Varian) spectrometer.
Photoluminescence spectra of thin films and solutions were performed at room temperature
with an Edinburgh Instruments FLS980 fluorescence spectrometer with Xe-lamp as an
excitation source and R-928 photomultiplier detector. The photoluminescence quantum yields
(PLQY) of the blends were measured using a calibrated integrating sphere from Edinburgh
Instruments. Phosphorescence, prompt fluorescence (PF), and delayed fluorescence (DF)
spectra and decays were acquired using nanosecond gated luminescence and lifetime
measurements (from 400 ps to 1 s) using either a high energy pulsed Nd:YAG laser emitting
at 355 nm (EKSPLA) or a N2 laser emitting at 337 nm. Sample emission was focused onto a
spectrograph and gated iCCD camera (Stanford Computer Optics). PF/DF time-resolved
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measurements were performed by exponentially increasing gate and integration times. Low-
temperature measurements were conducted using a liquid nitrogen cryostat (Janis Research)
under a nitrogen atmosphere.
2.5. Devices. OLEDs have been fabricated on pre-cleaned, patterned indium-tin-oxide (ITO)
coated glass substrates with a sheet resistance of 20 Ω/sq and ITO thickness of 100 nm. All
small molecules and cathode layers were thermally evaporated in Kurt J. Lesker Spectros II
evaporation system under pressure of 10-6 mbar without breaking the vacuum. The sizes of
the pixels were 8 mm2 and 16 mm2. NPB (N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-
4,4′-diamine), TCTA (Tris(4-carbazoyl-9-ylphenyl)amine) were used as a Hole Injection
Layer (HIL) and Hole Transport Layer (HTL), respectively. TmPyPB (3,3'-(5'-(3-(pyridin-3-
yl)phenyl)-[1,1':3',1''-terphenyl]-3,3''-diyl)dipyridine) was introduced as a Hole Blocking
Layer (HBL) and Electron Transport Layer (ETL). Lithium fluoride (LiF) and aluminium
were used as the cathode. Organic semiconductors and aluminium were deposited at a rate of
1 Ås-1, and the LiF layer was deposited at 0.1 Ås-1. The characteristics of the devices were
recorded using 10-inch integrating sphere (Labsphere) connected to a Source Meter Unit and
Ocean Optics USB4000 spectrometer. All materials were purchased from Sigma Aldrich or
Lumtec and were purified by temperature-gradient sublimation in a vacuum.
3. Results and discussion
3.1. Synthesis
Exciplex-forming acceptors were obtained in high yields by the simple one-pot condensation
reaction of phthalic anhydride with commercially available amines (Figure 2). The
compounds were obtained as white crystals, with good solubility in polar organic solvents
(e.g., dichloromethane, chloroform, tetrahydrofuran). The synthesized compounds were
characterized by 1H, 13C NMR spectroscopy, MALDI-TOF mass spectrometry and elemental
analysis (details are described in the Supporting Information). Isomeric benzophenone
containing compounds (2-BpPht, 3-BpPht, 4-BpPht) were prepared in order to check the
influence of substitution pattern on electro-optical and exciplex-forming properties. On the
other hand, disubstituted compounds with symmetric structure (DPOPht2, CyPh2Pht2) were
designed to increase the number of phthalimide groups per molecule.
Figure 2. Synthesis of 2-BpPht, 3-BpPht, 4-BpPht, DPOPht2, CyPh2Pht2
3.2. Theoretical calculations
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To analyze the geometry and the values of energies of the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) quantum chemical
calculations were performed for the molecules designed using density functional theory
(DFT). All the calculations were performed with the software package Gaussian09 (Revision
A.02)[33] with B3LYP/6-311G(d,p) level of theory.
The geometries of the molecules determined theoretically are presented in Figure 3.
Different linkage topologies of the developed phthalimide derivatives result in different
molecular structures from very twisted in the case of 2-BpPht to completely liner in the case
of 4-BpPht (Figure 3, Figure S11). The theoretical HOMO and LUMO energy values of
compounds are given in Table 1. For almost all the molecules LUMO orbitals predominantly
are located on phthalimide structural units, while HOMO frontier orbitals are located on N-
aryl substituents. For compound 2-BpPht due to orto-substitution of benzophenone and
phthalimide moieties, LUMO orbitals are located on phthalimide and closely situated benzoyl
unit of benzophenone moieties, while HOMO is located on phthalimide and N-phenyl units.
For all the synthesized compounds LUMO values are in the range of 2.4-2.6 eV, proving their
strong accepting properties.
Figure 3. Fully optimized structures of studied phthalimide derivatives and graphical
representation of the HOMO and LUMO calculated at the B3LYP/6-311G (d,p) level of
theory.
3.3. Photophysical properties
The normalized optical absorption spectra of the dilute solutions and thin films of the
acceptors are shown in Figure 4. Three main regions can be identified in the absorption
spectra: the lower-energy bands in the range of 275–310 nm, and the higher-energy bands in
the ranges of 240-260 nm and 200–250 nm.
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The higher energy absorption band corresponds to n-π* transition and the lower energy
absorption band is related to the main π-π* transition[34]. The observed absorption spectrum
originated mainly from the N-aryl-substituted phthalimide moieties. Absorption maxima of
the phthalimide derivatives of the thin films are slightly red-shifted in comparison to those of
dilute solutions. Such a shift can be attributed to the intermolecular interactions between
molecules that are more likely to occur in the solid state[35]. The optical band gaps Eg were
estimated from the longest wavelength absorption edge of the UV−VIS spectra. The Eg values
are given in Table 1. 4-BpPht showed the longest wavelength absorption centered at 280 nm,
with higher intensity than the other acceptors. This observation can be explained by more
effective conjugation in this para-substituted phthalimide derivative.
Figure 4. UV-VIS absorption spectra of the dilute solutions (10−5 M in THF) (a) and of spin-
coated thin layers (b) of the synthesized phthalimide derivatives.
Photoluminescence spectra (Figure 5) revealed the main band, with a small Stokes shift,
centered at 305 nm. Fluorescence bands of all the molecules studied have very similar shapes
with an identical position of maximum intensity. This observation indicates that the main
transitions originate from phthalimide moiety. PL spectra of neat films of DPOPht2,
CyPh2Pht2, 2-BpPht, 3-BpPht and 4-BpPht were not analysed due to a very weak
photoluminescence. One of the crucial parameters of exciplex forming acceptor is relatively
high triplet levels where locally excited states of donor or acceptor (3LED or 3LEA) lie close to
1
1
the CT of exciplex or visibly stay above the CT because of efficient RISC[36,37,38]
Molecules 2-BpPht, 3-BpPht, 4-BpPht showed similarly structured (with 5 peaks)
3
phosphorescence which can be assigned to the LE triplet emission. On another hand, two
symmetric molecules DPOPht2 and CyPh2Pht2 exhibited broad, not structured CT
phosphorescence (Figure 5b). The values of triplet levels were estimated from the onsets of
phosphorescence spectra (Table 2). The substituents attached at orto-, meta- and para-
positions of benzophenone moiety in 2-BpPht, 3-BpPht, 4-BpPht, respectively, do not have
any substantial influence on the phosphorescence and all triplet levels were found to be
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around 3.1eV. The onset of phosphorescence emission of 4-BpPht was slightly red-shifted in
comparison to those of other two benzophenone phthalimide acceptors. Symmetric
phthalimide acceptors DPOPht2 and CyPh2Pht2 exhibited triplet levels at 2.98 and 2.92 eV
which are slightly lower than those of 2-BpPht, 3-BpPht, 4-BpPht. This effect can be
explained by the presence of additional phthalimide moiety.
Figure 5. Photoluminescence spectra of THF solutions (10−5M) of the phthalimide derivatives
(a) and phosphorescence spectra of their molecular dispersions in the zeonex matrix at 80K
(b).
3.4. Electrochemical properties
Electrochemical properties of the solutions of the synthesized phthalimide derivatives in
dichloromethane (DCM) with 0.1 M tetrabutylammonium hexafluorophosphate as the
electrolyte were studied by cyclic voltammetry (CV). The CV curves of compounds are
shown in Figure 6. The curves had similar shapes, and the compounds exhibited close
reduction potentials, and as a consequence comparable values of electron affinity (EA).
(Table 1). All the compounds were characterized by the reversible one-electron process. The
positions of the main peaks of CV curves (Figure 6a) were conditioned by reduction of
phthalimide group, and small shifts were related to the different substituents. The position of
the second peak (the second step of reduction) (Figure 6b) was found to be close to the limit
of the scanning range, which was restricted due to the solvent (DCM) instability within the
negative potential range. Non-halogenated solvents could not be used due to poor solubility
of the compounds. DPOPht2, CyPh2Pht2 as well as 3-BpPht and 4-BpPht showed the
similar positions of the 2nd peaks of reduction. These peaks corresponded to the reduction of
phthalimide and benzophenone groups[39,40]. Small differences between positions of peaks
can be attributed to the different environment of redox groups. Two parts of CyPh2Pht2 are
linked by a non-conjugated bridge and therefore behave as separate species. The absence of
the 2nd peak in the CV curve of compound 2-BpPht can apparently be explained by the steric
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effect. Oxidation peaks were not observed in the CV curves, and consequently, the ionization
potentials (IP) were estimated using the UV-Vis spectroscopic data.
Figure 6. CV curves of phthalimide derivatives with the first (a) and the second (b) steps of
reduction.
Table 1. Summary of electrochemical and optical data
3
4
6
Eg1, Ered 2 vs EA
,
IP , LUMO5, HOMO5, T1 ,
eV Eferro, V
eV
eV
eV
eV
eV
DPOPht2 3.84
CyPh2Pht2 3.91
-1.73
-1.74
-1.75
-1.7
3.37 7.21
3.36 7.27
3.35 7.18
3.40 7.29
3.37 7.27
-2.48
-2.42
-2.52
-2.64
-2.64
-6.02
-6.20
-6.85
-6.82
-6.73
2.98
2.92
3.12
3.11
3.08
2-BpPht
3-BpPht
4-BpPht
3.83
3.89
3.90
-1.73
1Optical band gap estimated from the edges of electronic absorption spectra
2Ered are measured vs. ferrocene/ferrocenium
3Electron affinities measured by the electrochemical method
4Ionization potentials calculated from the band gap: IP = EA + Eg
5HOMO and LUMO energies calculated at the B3LYP/6-311G (d,p) level “in a vacuum”.
6T1 calculated from onset of phosphorescence spectra
3.5. Exciplex-forming properties
The main advantages of phthalimide acceptors are exciplex-forming properties which have
been detected for solid-state samples of molecular mixtures with carbazole-containing donors
such as 1,3-bis(N-carbazolyl)benzene (mCP), 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP),
tris(4-carbazoyl-9-ylphenyl)amine (TCTA) and poly(9-vynylcarbazole) (PVK). The
molecular mixtures formed with the mass ratio of the components 1:1 of all the synthesized
materials with appropriate donors showed emission bands red shifted in comparison with
either the donor or acceptor pristine emission spectra, and such broad Gaussian-type emission
bands are typical for exciplexes (Figure S12) [8,41,42]. Figure 7a (as well as Figure S13
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and Figure S14) show PL spectra of exciplex-forming blends containing phthalimide
acceptors and mCP, CBP, PVK and TCTA as donors. The acceptors have the similar electron
affinity (EA) values in the range 3.3-3.4 eV and exhibit sky-blue emission with mCP (Ip=6.1
eV), blue-greenish emission with CBP (Ip=6.0 eV), green emission with PVK (Ip=5.8 eV) and
orange emission with TCTA (Ip=5.7 eV) (Figure 7c). Such colour tuning can be explained by
CT exciplex emission which is a function of the difference between ionization potential (IP)
of donors and electron affinity (EA) of accepting molecules.[42,43,44]. This is one of the
advantages of exciplex emission. By selection of appropriate materials (donor or acceptor),
CT emission can be tuned. Such bimolecular exciplex-forming materials can be adopted in
OLEDs for emissive layers or as hosts for a wide range of phosphorescent or TADF emitters.
Figure 7. PL spectra (a) and PL decay curves (b) of the layers of blends: 4-BpPht:mCP, 4-BpPht:CBP,
4-BpPht:PVK and 4-BpPht:TCTA. Schematic representation of energy band diagrams of exciplex-
forming carbazole-based donors and phthalimide acceptors (c).
Considering the exciplex-forming properties, the PL decay dynamics and the
photoluminescence quantum efficiencies (PLQY) of all twenty exciplex-forming systems
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were characterized at room temperature (Table 2, Figure S13). The PL decay curves showed
biexpotential character. The life times (τ1, τ2) of the layers as well as the PLQY values of the
mixtures are given in Table 2. The exciplex-forming blends of studied compounds with mCP
exhibited sky blue emission with a maximum peak in the range of 494-497 nm (Figure S13,
Figure S14). The mCP:DPOPht2 and mCP:CyPh2Pht2 exciplexes exhibited low PLQY of
0.8% and 2 % respectively, and short-lived prompt (τ1 = 30 – 40 ns) and delayed emission (τ2
= ca.140ns). Bimolecular blends mCP:2-BpPht, mCP:3-BpPht, mCP:4-BpPht exhibited
much higher PLQY values of 18%, 20% and 26% respectively with longer decay times equal
around 80 ns (τ1) and 420 ns (τ2) in comparison with mCP:DPOPht2 and mCP:CyPh2Pht2
exciplexes. Bimolecular blends with CBP as a donor showed emission with a maximum peak
in the range of 504-510 nm (Figure S13, Figure S14). The values of PLQY were found to be
1.6% for CBP:DPOPht2, 4% for CBP: CyPh2Pht2, 17% for CBP:2-BpPht, 19% CBP:3-
BpPht and 22% for CBP:4-BpPht. The lifetimes (τ1, τ2) of the emission of the exciplexes
based on 2-BpPht, 3-BpPht and 4-BpPht were found to be longer than those observed for
the exciplexes based on symmetric acceptors (DPOPht2, CyPh2Pht2). Bimolecular excited
states of the molecular mixtures containing PVK displayed fluorescence maxima at 530-540
nm, i.e. in the green part of the visible spectrum. Photoluminescence quantum yield of thin
films were found to be 2.5% and 3% for PVK:DPOPht2 and PVK:CyPh2Pht2 exciplexes,
respectively. 2-BpPht, 3-BpPht and 4-BpPht dispersed in PVK showed slightly higher
values of PLQY: 6%, 6% and 8%, respectively. PL decays of the blends with PVK showed
similar behavior, but a longer-lived component of PVK:2-BpPht, PVK:3-BpPht and PVK:4-
BpPht exhibited twice longer emission lifetime than the molecular mixtures of PVK with
another acceptor studied (Figure S13, Figure S14). Indeed, the blends with PVK as a donor
showed less efficient exciplex emission than the molecular mixtures with mCP and CBP.
This observation can be explained by the weak interaction between the acceptor molecules
and the carbazole units of the polymer due to longer distance between the donor and the
acceptor molecules. The last group of materials exhibiting lower energy exciplex emission
contain TCTA as a donor. They emit light in an orange part of the spectrum with peaks at
565-575 nm (Figure S13, Figure S14). The solid films of the systems containing TCTA
showed low PLQY of exciplex emission. Taking all these observations into account, one can
predict, that introducing of stronger accepting (with lower HOMO) connectors between the
phthalimide units should prevent the formation of low energy intramolecular CT-state. As a
result, significant enhancement of photophysical and exciplex forming properties could be
expected.
The results described above show that the compounds with phthalimide moiety attached to
orto-, meta- and para position of benzophenone moiety exhibit good exciplex-forming
properties, and in particular the para isomer 4-BpPht. This compound in combination with
mCP forms exciplex with PLQY of 26 %, the highest observed in this work.
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Table 2. Photopysical propreties of exciplex-forming blends
Eciplexes
PL
λmax
(nm)
498
496
494
495
497
PLQY
(%)
τ1 ns (%)
τ2 ns (%)
χ2
mCP:DPOPht2
mCP:CyPh2Pht2
mCP:2-BpPht
mCP:3-BpPht
mCP:4-BpPht
0.8
2
38.1 (65) 138.9 (35) 1.23
36.8 (69) 145.8 (31) 1.28
82.5 (53) 420.6 (47) 1.28
80.3 (66) 420.2 (34) 1.23
77.5 (63) 418.9 (37) 1.28
18
20
26
CBP:DPOPht2
CBP:CyPh2Pht2
CBP:2-BpPht
510
503
504
1.6
4
24.1 (56)
98.1 (44)
1.25
40.3 (71) 131.9 (29) 1.25
63.6 (44) 193.9 (56) 1.15
17
CBP:3-BpPht
CBP:4-BpPht
503
510
19
22
64.3 (52) 178.3 (48) 1.17
64.1 (49) 173.5 (51) 1.13
PVK:DPOPht2
PVK:CyPh2Pht2
PVK:2-BpPht
540
530
525
2.5
3
54.1 (59) 204.7 (41) 1.25
69.7 (59) 255.9 (41) 1.19
86.1 (38) 348.1 (62) 1.07
6
PVK:3-BpPht
PVK:4-BpPht
528
533
6
8
99.2 (58) 389.4 (42) 1.22
71.2 (54) 291.7 (46) 1.20
TCTA:DPOPht2
TCTA:CyPh2Pht2
TCTA:2-BpPht
570
566
569
2
2
2
34.4 (84) 123.9 (16) 1.22
43.1 (79) 143.2 (21) 1.27
65.1 (79) 226.5 (21) 1.18
TCTA:3-BpPht
TCTA:4-BpPht
563
575
3
5
45.6 (79) 170.9 (21) 1.27
45.8 (83) 184.1 (17) 1.29
In general, photophysics of exciplex-forming systems is more complex than that of individual
compounds exhibiting intramolecular CT. We have noticed, that the biexponential PL decay
curves with short- and long-lived components are characteristic of the studied molecular
mixtures. Long-lived components can be of different origin and require deeper investigations.
To characterize the exciplex between mCP and 4-BpPht and to study the driving mechanisms
of it, the time-resolved PL measurements have been performed.
Figure 8a shows PL decay of mCP:4-BpPht recorded at 295K and 80K. At both
temperatures, the PL decays of the blend related to the short-lived prompt fluorescence and
the long-lived TADF clearly follow exponential decay laws. Delayed emission of bimolecular
CT at 80K showed longer lifetime (up to 100 ms), than at room temperature (up to 200 µs)
which is typical behavior for the TADF materials. This observation is consistent with the fact
that at high temperature due to thermal activation the RISC become more efficient causing
shorter-lived delayed emission[9]. To confirm the TADF exciplex mechanism, the
dependence of the delayed fluorescence intensity vs laser pulse energy was plotted (Figure
8d). The linear relationship with the slope close to 1 (0.89) confirms that the delayed
fluorescent mechanism in the studied exciplex is dominated by TADF process rather than by
triplet-triplet annihilation[45].
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Figure 8. Time-resolved fluorescence decay curves of the layer of mCP:4-BpPht blend
recorded at 295K and 80K (a). Time-resolved normalized emission spectra recorded at 80K
(b). Prompt and delayed fluorescence spectra of exciplex-forming blend mCP:4-BpPht
recorded at 295 K and 80 K and phosphorescence spectra of donor and acceptor (c). Laser
power dependence of delayed fluorescence of mCP:4-BpPht recorded at room temperature
(d).
Figure 8b shows the time-resolved photoluminescence spectra of mCP:4-BpPht at 80K. At
1
the short delay time (3.4ns) the CT state emission with a maximum peak at 475 nm was
1
detected. With the increase of delay time, a redshift of the CT emission band was observed
up to 495 nm at a delay time of 12.6 µs. The further increase of delay time did not affect the
1
1CT band. At low temperature, the CT-3CT states live longer due to the slower RISC.
Concerning time-resolved emission spectra recorded at 295K (Figure S15) the similar
1
behavior of CT relaxation was observed but with stronger redshift. This observation can be
1
explained by the more efficient relaxation of the CT at room temperature due to the
increased vibrational energy.
From the energy of onset of prompt fluorescence of mCP:4-BpPht at 295K (Figure 8c) it is
1
possible to determine the CT energy level, which was found to be 2.98± 0.02eV. From the
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ACCEPTED MANUSCRIPT
phosphorescence spectra of the donor and acceptor molecules shown in the Figure 8c the
energies of locally excited state of the donor (3LED) and locally excited state of the acceptor
(3LEA) were determined as 2.92±0.02eV and 3.08± 0.02eV, respectively. In our system, 3LEA
1
3
1
1
is above CT, but LED is located close to the CT state (Figure 8c). In such situation, CT
3
can be in resonance with LED, which allows spin-orbit coupling interaction of these states.
This case of triplet harvesting is in good agreement with exciplexes that reported earlier[46].
3
The singlet-triplet splitting could be calculated as a difference between CT energy and LED
energy and was found to be of 0.06±0.03eV. It is worth to note that at long delay time (ms
range) at 80K, in the spectrum of the blend also the phosphorescence of mCP could be
observed, as a result of the significant singlet-triplet gap. In addition, the exciplex-forming
molecular mixture at 80K showed enhanced prompt and delayed emission in comparison with
the signals observed at 295 K. This observation can be explained by a high contribution of
non-radiative transitions from singlet and triplet states[45].
To sum up, sky-blue mCP:4-BpPht exciplex-forming blend showed singlet-triplet splitting
around 0.06±0.03eV and 26% of PLQY as a result of the decreased contribution of TADF in
exciplex blend.
3.6. Device fabrication.
Since the layers of mCP:4-BpPht showed the highest PLQY value, this blend was used as an
emitter to fabricate OLEDs. The device structure was as follows: ITO/ (NPB)(20nm)/
TCTA(10nm)/ mCP(5nm)/ mCP:4-BpPht(20nm)/ TmPyPB(50nm)/ LiF(1nm)/ Al(100nm). A
thin layer of mCP was introduced into the device to prevent lower energy exciplex formation
between TCTA and 4-BpPht and triplet exciton quenching[47]. An emitting layer of mCP:4-
BpPht with the mass ratio of the components of 1:1 was deposited by co-deposition of donor
(mCP) and acceptor (4-BpPht) from different evaporators at the identical rates [48]. A 50 nm
thick layer of TmPyPB was used as an ETL layer to improve electron transport since
TmPyPB shows high electron mobility[49].
The exemplary output characteristics of OLED are presented in Figure 9; the energy levels
diagram of the device is presented in Figure 9c. The wavelength of maximum intensity of
electroluminescence (EL) spectrum of the device is almost the same as that of PL spectrum of
molecular mixture mCP:4-BpPht (Figure 9a). Commission Internationale de l’Eclairage
(CIE 1931) chromaticity coordinates (x, y) of OLED were determined as (0.24;0.41). Sky-
blue OLED showed a turn-on voltage of 5 V (Von) and maximum brightness (Lmax) of 2500
cd m-2 at 12 V (Figure 9b). Maximum EQE (ηext,max), power (ηP,max) and current efficiencies
(ηL,max) were found to be 2.9%, 2.2 Lm W-1 and 4.9 cd A-1 respectively.
15

ACCEPTED MANUSCRIPT
Figure 9. EL spectra of OLED and PL spectra of mCP:4-BpPht blend (a), Current
density−voltage and luminance−voltage characteristics (b), current efficiency(energy band
diagram of OLED) (c), power efficiency and quantum efficiency of the device based on
CB16:mCP exciplex (d).
In addition, the maximum theoretical external quantum efficiency of the fabricated
device based on the exciplex-forming light-emitting layer of mCP:4-BpPht was analyzed
using the equation[50]:
ηext=γ×ϕPL×χ×ηout
(Eq. 1)
where γ corresponds to the charge-balance factor, ϕPL is the photoluminescence quantum
efficiency (ϕPL=0.26 for the mCP:4-BpPht), χ is the efficiency of exciton production (χ=0.25
or χ=1 in case of the fluorescence or TADF type devices, respectively), and ηout corresponds
to the outcoupling efficiency (ηout is usually from 0.2 to 0.3). Assuming γ=1, χ=0.25 (in case
of only singlet exciton utilization) and ηout =0.3, the maximum theoretical external quantum
efficiency for the fabricated device is only 1.95 % as PLQY of 26 % was obtained for the
solid layer of mCP:4-BpPh. Since the experimentally obtained max. external quantum
efficiency is much higher - 2.9 % - utilization of both singlet and triplet excitons in
electroluminescence of the studied device is clearly demonstrated. We have to note, however,
16

ACCEPTED MANUSCRIPT
that the maximum theoretical external quantum efficiency for the fabricated device is 7.8 %
assuming γ=1, χ=1 (in case of the singlet and triplet exciton utilization) and ηout =0.3. This
maximum theoretical EQE is much higher than the experimental one. The efficiency of this
OLED can be limited due to significant singlet-triplet energy gap and increased amount of
non-radioactive losses in exciplex. We demonstrated the potential of phthalimide based
derivatives as exciplex-forming materials for efficient devices exploiting utilization of both
singlet and triplet excitons.
4. Conclusions
Synthesis and characterization of five phthalimide derivatives are reported. For the first time,
exciplex forming properties of phthalimides based acceptors were described with carbazole
containing donors The difference in the molecular architecture of molecules did not effect for
.
values of electron affinity and triplet levels but had an influence on exciplex forming
properties. The compounds showed high triplet levels (2.92-3.11 eV) and similar electron
affinities (3.3-3.4 eV). Phthalimide derivatives were suitable to achieve multicolor exciplex
emission with carbazole containing donors, which covered a large part of the visible spectrum
from sky-blue to orange colors. On the basis of photophysical investigation best performance
showed phthalimide-benzophenone acceptors rather than two phthalimide moieties
containing acceptors. Exciplxes containing non-symmetric phthalimide-benzophenone
acceptors showed highest photoluminescence quantum yield in the solid state in comparison
with symmetric phthalimide derivatives. The main reason is intramolecular charge transfer
state which is indicated by the broad non-structured phosphorescence spectra of these
materials and lower value of triplet levels. A para- substituted phthalimide-benzophenone (4-
BpPht) showed better exciplex-forming properties in comparison to other materials and in
the mixture with mCP exhibited sky blue exciplex emission with the highest among studied
systems photoluminescence quantum yield of 26%. From time-resolved PL measurements at
80K and 295K a singlet-triplet energy splitting of 0.06±0.03eV was determined. OLED based
on this emitting system showed the maximum external quantum efficiency of 2.9% and
brightness of 2500 cd m-2 at 12V with triplet harvesting.
5. Notes
The authors declare no competing financial interest.
Acknowledgements
The authors would like to acknowledge the EXCILIGHT project funded by the European
Union’s Horizon 2020 Research and Innovation Programme under grant agreement No.
674990. This work was also supported by Research Council of Lithuania (Project “OWEX”
No S-MIP-17-101). Special acknowledgement to prof. Tadeusz Biela for mass spectrometry
analysis.
Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version, at
https://doi.org/10.1016/j.dyepig.XXXX.
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