An approach to discovering novel exciplex supramolecular complex based on carbazole-containing 1,8-naphthalimide

Article abstract of DOI:10.1016/j.dyepig.2017.10.013

A new luminescent ambipolar semiconductor consisting of 1,8-naphthalimide as an electron-accepting moiety and carbazole as an electron-donating moiety has been synthesized and characterized. The synthesized compound as ambipolar semiconductor has been investigated by using density functional theory calculations coupled with the charge-hopping model and the time-of-flight experimental method. The time-of-flight hole and electron drift mobilities in the synthesized material's layer approached about 10^?4 cm² V^?1 s^?1 at high electric fields. The exciplex complex is stabilized by the intermolecular hydrogen bond and electron donor-acceptor interactions. The energy of interactions between the molecules in the exciplex complex has been estimated to be about ?100 kJ mol^?1 by DFT. Non-doped emissive layer organic light-emitting diode (OLED) using the synthesized compound as an emitter exhibited an orange color emission with the Commission International de L'Eclairage chromaticity coordinates of (0.47, 0.49).

Full text of DOI:10.1016/j.dyepig.2017.10.013

Accepted Manuscript
An approach to discovering novel exciplex supramolecular complex based on
carbazole-containing 1,8-naphthalimide
Mykola Bezuglyi, Khrystyna Ivaniuk, Dmytro Volyniuk, Juozas V. Gražulevičius,
Gintautas Bagdžiūnas
PII:
S0143-7208(17)31592-9
10.1016/j.dyepig.2017.10.013
DYPI 6311
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 23 July 2017
Revised Date: 9 October 2017
Accepted Date: 10 October 2017
Please cite this article as: Bezuglyi M, Ivaniuk K, Volyniuk D, Gražulevičius JV, Bagdžiūnas G, An
approach to discovering novel exciplex supramolecular complex based on carbazole-containing 1,8-
naphthalimide, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.10.013.
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Graphical abstract
Highlights
•
•
•
•
Semiconductor consisting of the 1,8-naphthalimide and carbazole moieties was
synthesized and characterized.
For this compound, the ambipolar semiconductor properties and recombination of exciton
in exciplex complex are indicated.
The exciplex complex is stabilized by the intermolecular hydrogen bond and electron
donor-acceptor interactions.
OLED from the compound as an emitter exhibited an orange color emission.
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An Approach to Discovering Novel Exciplex Supramolecular Complex based
on Carbazole-Containing 1,8-Naphthalimide
Mykola Bezuglyi1, 2, Khrystyna Ivaniuk1, 4, Dmytro Volyniuk1, Juozas V. Gražulevičius1,
Gintautas Bagdžiūnas1, 3*
1Santaka Valley, Kaunas University of Technology, K. Barsausko str. 59, LT-51423, Kaunas,
Lithuania
2R&D Department "T.M.M."-LCC, Volodymirska st. 49a, 01001, Kyiv, Ukraine
3Department of Material Science and Electrical Engineering, Center for Physical Sciences and
Technology, Sauletekio av. 3, Vilnius, LT-10257, Lithuania
4Lviv Polytechnic National University, S. Bandera 12, 79013 Lviv, Ukraine
Corresponding Author
*E-mail: gintautas.bagdziunas@ftmc.lt (G. Bagdžiūnas, ORCID ID: orcid.org/0000-0002-9924-
6902)
Abstract
A new luminescent ambipolar semiconductor consisting of 1,8-naphthalimide as an electron-
accepting moiety and carbazole as an electron-donating moiety has been synthesized and
characterized. The synthesized compound as ambipolar semiconductor has been investigated by
using density functional theory calculations coupled with the charge-hopping model and the
time-of-flight experimental method. The time-of-flight hole and electron drift mobilities in the
synthesized material’s layer approached about 10-4 cm2 V-1 s-1 at high electric fields. The
exciplex complex is stabilized by the intermolecular hydrogen bond and electron donor-acceptor
interactions. The energy of interactions between the molecules in the exciplex complex has been
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estimated to be about -100 kJ mol-1 by DFT. Non-doped emissive layer organic light-emitting
diode (OLED) using the synthesized compound as an emitter exhibited an orange color emission
with the Commission International de L’Eclairage chromaticity coordinates of (0.47, 0.49).
Keywords: carbazole, 1,8-naphthalimide, ambipolar semiconductor, exciplex, hydrogen
bonding, OLED.
1. Introduction
The specific intermolecular interactions in the bimolecular electron donor-acceptor systems are
broadly investigated and their versatile applications in the optoelectronic devices are extensively
studied.1 2 3It is well known that donor-acceptor systems can generate either charge-transfer (CT)
complexes in the electronic ground state or excited-state CT complexes (exciplexes) which are
formed after the excitation of one of the photoactive moieties.4 In particular, an exciplex
formation that occurs between the excited singlet state of one molecule or its fragment and the
ground state of another one has attracted much attention due to its unique potential in tailoring
emission wavelengths without altering the original absorption characteristics of the donor-
acceptor pair.5 Moreover, the exciplexes and their application in the organic light emitting diodes
(OLEDs) have attracted significant attention due to the observation of thermally activated
delayed fluorescence (TADF).6 On the other hand, molecular wires have been intensively studied
as electron and/or hole conducting materials.7 The molecular wire as a donor-acceptor system
has served as a suitable model compound for the elucidation of the electron and/or hole transfer
in molecular arrays.8 The 1,8-naphthalimide derivatives are known as the photochromic and
room-temperature-phosphorescent materials9 as well as n-type semiconductors.10,11 Moreover,
the following derivatives are known to form the various supramolecular assemblies through non-
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covalent interactions.12,13 At the same time, the carbazole derivatives are well known as p-type
semiconductors with the interesting luminescent properties14,15,16 and ability to form the
exciplexes with different acceptors.5 Nowadays, significant progress has been made in the
efficient orange-red or orange electroluminescent devices.17 Several red materials have been
developed, such as fluorescent emitter of 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-
tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)18 and phosphor materials of tris(1-
phenylisoquinoline) iridium(III) (Ir(piq)3)19 and (bis(2-(3,5-dimethylphenyl)quinoline acetyl-
acetonate) bis(2-phenylquinoline iridium(III) (Ir(mphq)2(acac)20 the most common red emission
materials. However, the high cost due to the presence of rare heavy metal iridium is a
disadvantage to a practical application. Orange-red OLEDs based on the organic materials are
still rare.21
The aim of this work was the synthesis of the donor-acceptor material based on the carbazole and
1,8-naphtalendicarboxylic acid imide active moieties and investigation of the optical,
electrochemical and photophysical properties of the synthesized compound.
2. Experimental Section
2.1. General procedures and instrumentation
Organic solvents were dried according to the standard methods and freshly distilled under argon
prior to use. Commercially available hydroxylamine hydrochloride, triethylamine and 1,8-
naphthalenedicarboxylic acid anhydride from Sigma-Aldrich were used without further
purification. 9-(Oxiran-2-ylmethyl)-9H-carbazole from Reahim was twice recrystallized from
isopropyl alcohol. Zinc acetate was obtained after reaction of zinc powder with excess of glacial
13
acetic acid, washed with acetone and dried. 1H and C NMR spectroscopy was carried out on a
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Bruker Avance 400 NMR spectrometer. The chemical shifts are reported in units ppm (parts per
million). The residue signals of the solvents were used as internal standards (1H NMR: standard
internal CDCl3, 7.26 ppm). Attenuated total reflection infrared (ATR IR) spectrum was recorded
using a Bruker VERTEX 70 spectrometer. Mass spectrometry (MS) data was recorded on
UPLC-MS Acquity Waters SQ Detector 2. The UV/Vis absorption spectra were recorded with
Avantes AvaSpec-2048XL spectrometer. The photoluminescence spectra and the fluorescence
decay curves were recorded by Edinburgh Instruments FLS980 spectrometer with TMS300
monochromators and a red cooled detector (Hamamatsu R928P). Fluorescence quantum yields
were measured using integrated sphere of a 120 mm inside diameter spherical cavity calibrated
with two standards: quinine sulfate in 0.1 M H2SO4 and rhodamine 6G in ethanol. The cyclic
voltammetry (CV) measurements were carried out with a glassy carbon working electrode in a
three electrode cell in the dry THF solutions containing 0.1 M tetrabutylammonium
hexafluorophosphate as the electrolyte at room temperature under argon atmosphere at 100 mV
s-1 potential rate. The electrochemical cell comprised platinum wire with 1 mm diameter of
working area as a working electrode, Ag wire calibrated versus ferrocene/ferrocinium redox
couple as a quasi-reference electrode and platinum coil as auxiliary electrode. Electron
photoemission spectrum were recorded from thin solid films of materials on indium tin oxide
(ITO) coated glass substrates under negative voltage of 300 V with illumination by deep UV
deuterium light source ASBN-D130-CM and CM110 1/8m monochromator and A 6517B
Keithley electrometer connected to the counter-electrode. The charge drift mobility
measurements by a time-of-flight (ToF) method were recorded from thin solid film (d=2.0 µm)
of material deposited in vacuum on indium tin oxide (ITO) coated glass substrates with delay
generator Tektronix AFG3011 used to generate square pulse voltage with a pulsed third-
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harmonic Nd:YAG laser EKSPLA PL2140 working at a pulse duration of 25 ps and the
wavelength of 355 nm and digital storage oscilloscope Tektronix DPO4032 to record the ToF
transients. The charge mobilities were evaluated from the equation µ=d2/(U×ttr), where ttr is a
transit time. Thermogravimetric analysis (TGA) was performed on
a
Mettler
TGA/SDTA851e/LF/1100 apparatus at a heating rate of 20°C min-1 under nitrogen atmosphere.
The differential scanning calorimetry (DSC) measurements were done on a DSC Q 100 TA
Instrument at a heating rate of 10 °C min-1 under nitrogen atmosphere. The monocrystal X-ray
measurements were made on a Rigaku XtaLAB mini diffractometer using graphite
monochromated Mo-Ka radiation (for further details please see in ESI).
2.2 Theoretical calculations
The geometry of CbzNI was optimized from X-ray structure as the starting point by B3LYP
functional and 6-31G(d,p) basis set with the Spartan’14 program22, followed by calculations of
their harmonic vibrational frequencies to verify their stability. All the calculated vibrational
frequencies are real, which indicates the true minimum of the total energy on the potential energy
hypersurface. A conformational search was performed applying the Monte–Carlo method and
MMFF94 force field. The minimum energy conformers found by molecular mechanics were
further optimized at DFT/B3LYP/6-31G(d,p) level using the polarizable continuum model
(PCM) in THF (ε = 7.6). This was followed by calculations of their harmonic vibrational
frequencies to verify their stability and to calculate the conformational free energies. The
population percentages were calculated using ∆Go and an applying Boltzmann statistics at T=
298 K. The Tamm-Dancoff approximation (TDA) scheme and the B3LYP/6-31+G(d) method
are used for all the calculations of singlet and triplet excitation energies because of its lower
computational cost and its avoidance of the triplet instability issue compared to full TD-DFT.23
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The intermolecular interaction energies between the molecules in dimers were estimated using
the basis set superposition effect (BSSE) concept24 and at wB97X-D functional with the London
dispersion corrections and 6-31G(d,p) basis set.The reorganization energies (λ) for the modelled
compounds were calculated by the adiabatic potential energy surface method25 and at the
B3LYP/6-31+G(d) level in vacuum. Values of λ were calculated according to the following
equation (1):
ꢁ
±ꢃ ꢅꢆ
ꢀ = ꢂ ꢄ − ꢂ
±ꢃ ±ꢆꢇ
ꢁ
ꢅꢃ ±ꢆ ꢅꢃ ꢅꢆꢇ
+ ꢂ ꢄ − ꢂ ꢄ
ꢄ
(1)
where E corresponds to the energy of neutral molecule (go) in the geometry of cationic/anionic
species (g±), respectively. The coupling integrals (Vi) for the pathways (see Figure 1) between
molecules m and n were obtained by the site-energy overlap correction method (2)26 at the long-
range corrected hybrid density functional wB97X-D and 6-311G(d) basis set in vacuum as
ꢏ
ꢐ
ꢍ
ꢊ
ꢎ ꢑ ꢃꢊ
ꢒꢊ
ꢆ
ꢋꢌ
ꢋꢌ ꢋꢋ
ꢌꢌ
ꢈ =
ꢉ
(2)
ꢐ
ꢓꢎꢑ
ꢋꢌ
where ꢈꢅ is the electronic coupling (transfer) matrix element, ꢖꢔꢕ is the overlap integral, and
ꢔꢕ
ꢈꢔꢔꢃꢕꢕꢆ are the energies of the neutral and charged dimer states.
2.3 Synthesis
2.3.1 2-Hydroxy-1H-benzo[de]isoquinoline-1,3(2H)-dione.
Dispersion
of
1,8-
naphthalenedicarboxylic acid anhydride (1.98 g, 10 mmol) in the mixture of hydroxylamine
hydrochloride (0.70 g, 10 mmol) and triethylamine (1.01 g,10 mmol) in tetraethylene glycol (20
mL) was heated in microwave oven to 150 °C during 30 min in the presence of zinc acetate (2
mg, 0.01 mmol) as catalyst. After reaction mixture cooling solution was poured into hydrochloric
acid (1% water solution of HCl, 300 mL). Formed precipitate was filtered, washed by water (100
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mL) and dried. Crude product was purified by recrystallization from methanol. Yield 85 %, m.p.
279-281 °C (lit. mp 280-281 °C). This compound was characterized in literature27.
2.3.2
2-(3-(9H-carbazol-9-yl)-2-hydroxypropoxy)-1H-benzo[de]isoquinoline-1,3(2H)-dione
(CbzNI). Solution of 2-Hydroxy-1H-benzo[de]isoquinoline-1,3(2H)-dione (0.43 g, 4.0 mmol) in
DMSO (20 mL) was heated with equimolar amount of 9-(oxiran-2-ylmethyl)-9H-carbazole
(0.89g, 4.0 mmol) in presence of triethylamine (1 mg, 0.01 mmol) to 120 °C during 180 min for
target material forming. After reaction finish (control via TLC) solution was poured into
hydrochloric acid (2% water solution of HCl, 200 mL). Formed precipitate was filtered, washed
by water (100 mL) and dried. The crude material was purified by flash-column chromatography
(silica, eluent-hexane:chloroform:ethylacetate 5:1:1, later - dichloromethane:methanol 10:1).
Yield 76 %, m.p. 193-195 °C, IR (neat), cm-1 3462 br., 3057 med. 2950 med. 1712 br., 1656 br.,
1625 w., 1581 br., 1512 w., 1485 br., 1455 br., 1402 med., 1375 br., 1356 br., 1326 br., 1260 w.,
1233 br., 1193 br., 1151 br., 1118 br., 1085 br., 1036 br., 965 br., 886 br., 841 br., 774 br., 753
1
br., 724 br., 693 med., 653 med., 614 med; H NMR (400 MHz, CDCl3) δ 8.62 (dd, J = 7.3, 1.1
Hz, 2H), 8.26 (dd, J = 7.4, 1.1 Hz, 2H), 8.08 (d, J = 7.7 Hz, 2H), 7.82 – 7.74 (m, 2H), 7.56 (d, J
= 8.2 Hz, 2H), 7.49 (ddd, J = 8.2, 7.1, 1.2 Hz, 2H), 7.25 (ddd, J = 6.9, 6.9, 1.1 Hz, 2H), 4.56 (d, J
13
= 5.2 Hz, 2H), 4.48 (d, J = 9.0 Hz, 2H), 4.30 (dd, J = 11.6, 7.8 Hz, 1H); C NMR (100 MHz,
CDCl3) δ 156.3, 149.9, 148.7, 147.6, 144.4, 142.7, 141.2, 139.9, 135.7, 135.2, 120.5, 68.0, 60.5,
58.2; Elemental analyses: found: C, 74.1; H, 4.7; N, 6.65 %; molecular formula C27H20N2O4
requires C, 74.3; H, 4.6; N, 6.4 %; MS (ES+) found: m/z [M+23: M+Na]+ 459.3; molecular
formula C27H20N2O4Na requires [M+Na]+ 459.1; UV-Vis (THF, 2⨯10-4 M): λmax, 332 nm
(23,000 mol-1dm3cm-1), 342 (22,000); fluorescence data (THF, 10-5 M), excitation λmax 290 nm,
emission λmax 345, 360 nm; quantum yield (QY) 0.45, excited state lifetime 7.2 ns.
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2.4 OLED fabrication and characterization
The electroluminescent device was fabricated by vacuum deposition of organic semiconductor
layers and metal electrodes onto pre-cleaned ITO coated glass substrate under a vacuum of about
10−3 Pa. The devices were fabricated by step-by-step deposition of different layers. MoO3 was
used for the preparation of hole-transporting layer. CbzNI and BCP were applied for the
fabrication of donor and acceptor layers respectively. The structure of the device was of
ITO/MoO3(5nm)/CbzNI(7nm)/BCP(7nm)/Ca(15nm))/Al(85nm). The active area of the obtained
devices was 3x6 mm2. The density-voltage and luminance-voltage characteristics were measured
using a Keithley 6517B meter in air without passivation immediately after the formation of the
device. The brightness measurements were done using a calibrated photodiode. The
electroluminescence spectrum was accomplished with an Avaspec-2048XL spectrometer.
3. Results and Discussion
3.1 Synthesis and identification
The synthesis of CbzNI was done from the commercially available reagents as shown at Scheme
1. Hydroxylimide was synthesized according to a previously published method.28 Cyclic
anhydride, hydroxylamine hydrochloride and triethylamine as base in tetraethylene glycol were
heated using microwave irradiation. Target compound CbzNI was synthesized by a nucleophilic
epoxy ring opening with hydroxylimide as nucleofile and triethylamine as base of the target. The
molecular structure of target compound CbzNI was confirmed employing the nuclear magnetic
resonance (NMR) (Fig. S1), infrared (IR) spectroscopy (Fig. S2a) and mass (MS) spectrometry
(Fig. S2b) methods.
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Scheme 1. Synthesis of target compound CbzNI.
In addition, single crystal X-ray analysis of CbzNI was conducted.29 Acetonitrile solvent was
employed to grow the CbzNI mono-crystals. The cell constants and an orientation matrix for
data collection corresponded to a primitive monoclinic cell with a space group of P2 /c. All data
1
are collected in Table S1. Fig. 1 shows the hydrogen bonded dimer structures of CbzNI.
Additionally, the intermolecular interaction energies (Ei) were estimated employing BSSE
method to be -99.3, -32.5 and -14.5 kJ mol-1, respectively for D1-D3 dimers. In D1 dimer with
highest Ei, the carbazole (donor) and 1,8-naphthalimide (acceptor) moieties are regularly
arranged one by one with the distance of 3.6 Å between them. The hydrogen bonds between
H4···O1 and H15A···π-carbazole system were determined. In D2 and D3 dimers, additional
hydrogen bonds between O2···H23 and O4···H5 as well as hidrogens of aliphatic moiety with
neighboring π-naphthalene imide and carbazole aromatic systems were found, respectively.
Moreover, the X-ray analysis showed that hydrogen bonds are not intramolecular but
intermolecular ones. Furthermore, the photoactive moieties planes in the D1 dimer are parallel
and interacted via strong H-bond and donor-acceptor π···π interactions in the solid state (Fig. 1).
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Figure 1. Hydrogen bonded dimer structures of CbzNI and theirs calculated interactions
energies.
3.2 Thermal properties
In optoelectronic devices, the materials are often incorporated as solid amorphous thin films.
Therefore, the favorable molecular optoelectronic properties and chemical stability of organic
electronic materials need to have desired the morphological and thermophysical properties in the
solid state because OLEDs are subject to thermal stress from Joule heating and the external
environment. Materials with low glass-transition temperature can be more easily diffused into
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other layers in OLEDs. Therefore, the interlayer diffusion significant reduces the brightness and
efficiency of OLEDs.30 For an investigation of thermal properties, differential scanning
calorimetry (DSC) was employed to probe the thermal transition of CbzNI compound. From first
heating scan, the compound exhibits a melting transition (Tm) at 194 °C, which disappears in the
second heating. Moreover, an endothermic baseline shift owing to glass transition (Tg) at 80 °C
was observed from second heating cycle. (Fig. 2b) This observation shows that the material can
form a stable glassy film. The thermogravimetric analysis (TGA) revealed the thermal stability
of CbzNI with 5% weight loss (T5d) temperature is above 295 °C (Fig. 2a). Therefore, this data
can provide the morphologically stable films for the electroluminescence (EL) applications.
Figure 2. Thermal properties of CbzNI. (a) TGA and (b) DSC curves.
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3.3 Photophysical and electrochemical properties
UV/Vis absorption and photoluminescence were investigated in solution, frozen to 77 K solution
and in a thin solid film. The dilute solution of CbzNI is transparent in the visible region and
characterized by intensive absorption below 300 nm and two maxima at 332 nm and 342 nm
(Fig. 3a). For the solid film the low-energy maxima become broader one with absorption up to
400 nm and with a weak absorbance "tail" in the region up to 500 nm. The low-energy
absorption bands are due to an intramolecular charge transfer (ICT) between the carbazole and
1,8-naphtalimide moieties. The optical band gaps were estimated to be 3.2 and 2.6 eV in solution
and solid state, respectively. Moreover, the absorption spectrum in the solid state is wider than in
solution due to an interaction between molecules in bulk. Therefore, an exciton is localized on
more than one molecule in the aggregated state. Therefore, a difference between the optical band
gaps in the solution and solid state is indicated an intermolecular CT between the associated dyes
in the solid state. Small red shift of the absorption maximum of ca. 3 nm of the solid sample as
compared to that of the dilute solution indicate that a polarity difference of media (the dielectric
permittivities of THF and organic solids are of 7.6 and 3-4, respectively) does not seriously
affect because the charge transfer transition between the donor and acceptor moieties is spatially
forbidden.
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Figure 3. Photopgysical properties of CbzNI. (a) Absorbance spectra of CbzNI in THF solution
and in thin solid film, (b) photoluminescence spectra of the dilute (0.1 mg·ml-1) solution and
solid film (λex= 290 nm), (c) time-correlated single-photon counting curves of (a) fluorescence
(λmax=390 nm) and (b) phosphophorescence (λmax=550 nm).
Photoluminescence (PL) spectrum of dilute CbzNI solution in THF characterized by emission in
UV region with the intensity maxima at 345 nm and 360 nm corresponded to the carbazole
moiety emission and shoulder-type low intensive fluorescence signal near 375-390 nm
corresponded to 1,8-naphthalimide fluorescence.31 In the PL spectrum of the solution recorded at
77 K and with a delay time of 1 µs the additional maxima corresponding to the emission from
excited triplet state (i.e., phosphorescence) of the carbazole moiety in the region of 400-500 nm
were observed (Fig. 3b).32 In addition, the intensive photoluminescence maxima at 539 nm and
584 nm corresponding to emission from the singlet and triplet ICT states of a 1,8-naphthalimide
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moiety were recorded, respectively.33 Due to close distance between the donor and acceptor
moieties in the crystal it was expected that the exciplex emission can appear in the solid state.
The maxima in a range of 350-400 nm corresponding to the singlet emission of the carbazole and
naphthalene imide moieties were observed in the photoluminescence spectrum of the solid
sample of CbzNI. (Fig. 3b) In addition, the broad band in the region of 500-670 nm (maximum
at 560 nm) corresponding to the triplet intermolecular CT emission of exciplex appeared in the
spectrum of the CbzNI layer. The emission lifetime measurements confirmed that the emission
bands appearing in the UV region were due to fluorescence and were characterized with the
lifetime of 7.2 ns. (Fig. 3c) The emission band appearing in the visible region of the spectrum
was characterized with the lifetime of 0.30 µs which confirms the phosphophorescence character
of emission due to the exciplex formation (Fig.3d). Photoluminescence chromaticity of CbzNI
was found to be yellow with the CIE 1931 coordinates of (0.41, 0.46), see Fig. S3. The
photoluminescence quantum yield of the solution in THF was found to be 45±2 % and that of the
solid film was of 6.0±2 %.
The possibility of an intramolecular exciplex formation was estimated from the conformational
analysis. However, the energy of the conformer with these properties is higher by 32.5 kJ mol-1
than that of the most energetically favorable conformer Conf7 (for the details see in ESI, Fig.
S4). Taking into account this information, we can conclude that the intramolecular exciplex
formation is not possible. On the other hand, for proving stability of exciplex supramolecular
complex, the intermolecular interaction energy between the molecules in the complex was
estimated to be -99.3 kJ mol-1 using the basis set superposition effect (BSSE) concept34 and at
the wB97X-D/6-31G(d,p) method. The structure of the supramolcular complex from X-ray
analysis was used. In this case, one of the strongest non-bonding interactions was indicated.35
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Moreover, for proving the exciplex formation, the energies of the lowest singlet (S1) and triplet
(T1) excited states were obtained to be 2.22 and 2.19 eV, respectively, by TD-DFT with Tamm-
Dancoff approximation (TDA) using the B3LYP/6-31+G(d) method in vacuum. The following
data have a good correlation with the experimental PL spectrum of CbzNI in the solid state (Fig.
3b). In addition, the calculations showed that this structure contributes to the small ∆EST (i.e. 25
meV), enabling efficient exciton-harvesting through TADF.
Figure 4. Cyclic voltamperogram (a) and electron photoemission spectrum of the thin film (b) of
CbzNI.
For the determination of the energy levels, the electrochemical properties of the resulting
chromophores were examined by cyclic voltammetry (CV) under argon. The onset of the
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oxidation potential (Eox) and reduction potential (Ered) was estimated by the intersection point of
the tangent line. The ferrocene/ferrocenium redox couple (Fc/Fc+) was used as the reference.
Under the same experimental conditions as those for the resulting chromophores, the redox
potential (Efer) of Fc/Fc+ was determined to be 0.31 V. The highest occupied molecular orbital
energy level HOMO and the lowest unoccupied molecular orbital energy level LUMO were
calculated according to the equation HOMO = -1.40⨯(Eox + 4.60 - Efer)36 (eV) and LUMO = -
1.19⨯(Ered + 4.78 - Efer)37, respectively. The HOMO and LUMO energy values were estimated to
be -5.83 and -2.95 eV from the cyclic voltammetry data (Fig. 4a). Ionization potential (IP) in the
solid state was measured to be 5.81 eV by the electron photoemission spectrometry (Fig. 4b).
Moreover, the calculated HOMO and LUMO energies of the most energetically favorable
conformer using DFT/B3LYP/6-31G(d,p) and polarizable continuum model (PCM) in
tetrahydrofuran (THF) were found to be -5.34 eV and -2.44 eV, respectively. The geometry of
the CbzNI was optimized by B3LYP functional and 6-31G(d,p) basis set starting from X-ray
analysis data. In the HOMO, π-electrons are localized on the carbazole moiety, while in LUMO,
the electron orbitals have to be localized on the naphthalimide moiety, creating a donor-acceptor
characteristic (Fig. 5).
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Figure 5. Electron density distributions in the HOMO (left) and LUMO (right) states of the most
energetically favorable conformer calculated by the B3LYP/6-31G(d,p) level and PCM of THF.
3.4 Charge carrier drift mobilities
Because CbzNI includes the n- and p-type moieties, the electron and hole mobilities were
estimated at different electric fields so an experimental study of the CbzNI layer charge
transporting properties was performed by the time of fight (ToF) technique. As indicated by the
ToF transients in Fig. 6a, hole and electron transport in the layers of CbzNI is dispersive which
could be due to the presence of traps. The hole and electron mobilities for CbzNI taken from the
transit times and the film thickness obeyed the Poole–Frenkel electric field dependence (Fig. 6b).
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Figure 6. Charge carrier drift mobilities of CbzNI layer. (a) ToF transit times and (b) electric
field dependences of charge carrier mobilities.
The electron mobility of the layer of CbzNI was found in the range from 2.2×10-5 cm2V-
1s-1 at the electric field of 2.5×105 V cm-1 to 1.6×10-4 cm2V-1s-1 at the electric field of
7.0×105 V cm-1. The electric field dependence for electrons was found to be weaker than
that for holes showing that the electron transport is less dispersive then the hole transport.
At the low electric fields, the electron mobility of CbzNI is by ca. an order of magnitude
larger than the hole mobility while at the high electric fields the values of hole and
electron mobilities of CbzNI which was found to be comparable to ca. 10-4 cm2V-1s-1.
These values are comparable to hole and/or electron mobilities of the conventional
materials
such
as
tris(8-hydroxyquinoline)
aluminium
(Alq3),38
1,8-
naphtalenedicarboxylicimide derivatives,11
m
-terphenyl oxadiazole derivatives,39 1,2,3,5-
tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN)40 which were used for preparing
very efficient fluorescence, phosphorescence or TADF OLEDs.
The crystal structure of CbzNI was used to generate the wide variety of possible charge hopping
pathways (dimers) (Fig. S5). The standard Marcus-Hush model was used for the estimation of
the probability of charge transfer between the molecules.41 The reorganization energy (λ) values
were calculated using the normal-mode analysis and B3LYP/6-31+G(d) method. The
reorganization of CbzNI energy was found to be 441 meV for holes (λ+) and 341 meV for
electrons (λ−). These results show that the reorganization energies for holes and electrons are
somewhat similar and indicative of the ambipolar charge-transporting properties. The electronic
coupling (transfer) integrals Hi for the pathways (see Table S2) were estimated by the site-
energy overlap correction method42 at the long-range corrected hybrid density functional
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wB97X-D and 6-311G(d) basis set in vacuum 43
.
However, the lowest electronic coupling
integrals which stabilized via the hydrogen bond and electron donor-acceptor
intermolecular interactions (exciplex complex) were estimated to be 12.5 and 1.6 meV for
holes and electrons, respectively. Presumably, the zone of electron-hole recombination could be
localized on this dimer (exciplex complex). On the basis of the theoretical calculations the
strong dependency of the charge carrier mobility on electric field can be explained by a
comparatively large distance between the mass centre of the donor-donor (ca. 10 Å between the
carbazole moieties) and acceptor-acceptor moieties (ca. 10-15 Å between the naphthalene imide
moieties) in the carriers hopping pathways (see Table S2).
3.5 Electroluminescence properties
The undoped orange OLED was fabricated using MoO3 and tris(4-carbazoyl-9-ylphenyl)amine
(TCTA) for the preparation of the hole injection and transporting layers, CbzNI for emitting
layer and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) for the electron transporting
layer. Since Ca is highly reactive and corrodes quickly in the ambient atmosphere, Ca
layer was topped with 85 nm aluminum (Al) layer was used as the cathode. The energy
levels of the materials used for the layers of device are displayed in Fig 7a.
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Figure 7. Electroluminescence properties. (a) Energy-band diagram and (b)
electroluminescence spectrum of OLED containing the emitting layer of CbzNI, (c)
current density/luminance-versus-voltage characteristics and (d) current efficiency/power
efficiency-current density (inset: external quantum efficiency-current density) of the
device.
The electroluminescence maximum of the device was red-shifted by 30 nm in comparison
with photoluminescence intensity maximum of the film of CbzNI. The device emitted an
orange emission with the CIE 1931 coordinates (0.47, 0.49). (Fig. S3) The switch-on
voltage for the device was found to be 3.2 V with maximal brightness near 100 cd m-2,
current efficiency of 2.08 cd A-1, power efficiency of 1.87 lm W-1 and maximum external
quantum efficiency (EQE) of 1.02%. All OLED characteristics are show in Fig. 7.
The EQE value of OLEDs can be predicted by the equation (3):
ꢂꢗꢂ = ꢘ ⨯ ꢗꢙ ⨯ ꢚ ⨯ ꢛꢜꢝꢞ
(3)
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where ρ is the charge balance of the injected holes and electrons, QY is the quantum yield of the
emitting material, ϕ is the efficiency of electrically generated excitons conversion to light or
efficiency of radiative exciton production and ηout is the light-out-coupling efficiency.
Considering the ideal of ρ = 1, QY = 6.0 %, and estimating ηout as 0.20-0.25 at maximum EQE of
the device, the efficiency of electrically generated excitons conversion to light (ϕ) was calculated
to be in the range 68 to 85%. This result clearly indicates that the device overcame the theoretical
limit of 25% for the singlet exciton production efficiency (ϕ) that is assumed for fluorescence-
based OLEDs. In the case of the triplet-triplet annihilation (TTA)44 or of thermally activated
delayed fluorescence (TADF)45 mechanisms that can harvest both singlet and triplet excited
states, the theoretical limit are of 62.5% and 100%, respectively.
4. Conclusion
In summary, we have presented a new concept of construction of the hydrogen bonding
stabilized exciplex formed donor-acceptor compound. The luminescence lifetimes of 7 ns and
0.3 µs in the UV region and the visible region have been observed which confirms the
fluorescence and phosphophorescence characters of emission bands due to monomolecular and
exciplex emission, respectively. This compound showed the ionization potential in the solid state
of 5.81 eV and the ambipolar semiconductor properties. The hole and electron drift mobilities
have been estimated to be 10-4 cm2 V-1 s-1 at high electric fields. The photophysical and charge
mobility features have been supported by using the (TD)-DFT methods. Exciplex of CbzNI
based organic light-emitting diode emitted an orange emission with the CIE 1931 coordinates
(0.47, 0.49). This device exhibits the current efficiency of 2.08 cd A-1, power efficiency of 1.87
lm W-1 and maximum external quantum efficiency of 1.02%. The high efficiency of electrically
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generated excitons conversion to light has been calculated to be from 68 to 85 %. We envisage
that this concept can be applied to other structures.
Acknowledgment
This research was supported by FP7 REGPOT-2012-2013-1 ICT project CEOSeR under
grant agreement No 316010. K. Ivaniuk thanks for the support from “AmbiPOD” project
under grant agreement No 612670 (IRSES-GA-2013-612670). We are grateful to D.
Gudeika for MS, L. Peciulyte for the TGA, DSC measurements.
Supporting Information
NMR, MS, IR spectra, X-ray crystallographic data, conformation analysis and possible carrier
hopping pathways are presented. This material is available free of charge via the Internet.
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Highlights
•
•
•
•
Semiconductor consisting of the 1,8-naphthalimide and carbazole moieties was
synthesized and characterized.
For this compound, the ambipolar semiconductor properties and recombination of exciton
in exciplex complex are indicated.
The exciplex complex is stabilized by the intermolecular hydrogen bond and electron
donor-acceptor interactions.
OLED from the compound as an emitter exhibited an orange color emission.