A Zinc(II) Benzamidinate N-Oxide Complex as an Aggregation-Induced Emission Material: toward Solution-Processable White Organic Light-Emitting Devices

Article abstract of DOI:10.1002/ejic.201800579

The synthesis, characterization, photophysical and redox properties of a dinuclear complex Zn₂(AMOX)₄ (AMOX = 4-bromo-N,N′-diphenylbenzamidinate N-oxide) are highlighted. This compound is a first example of a novel class of aggregation-induced emission (AIE) materials. Noticeably, solution-processed white-green organic light-emitting diodes were fabricated using this complex as dopant in a co-host matrix in the emissive layer. A luminance efficiency and power efficiency as high as 1.12 cd/A and 0.30 lm/W were obtained, respectively. The characteristics of this complex: high solubility, thermal stability, AIE behaviour, ease of synthesis, tunability options, and low-cost, reveal altogether great potential for the development of this new family of compounds as materials for applications in optoelectronic devices.

Full text of DOI:10.1002/ejic.201800579

DOI: 10.1002/ejic.201800579
Full Paper
Optoelectronic Materials
A Zinc(II) Benzamidinate N-Oxide Complex as an Aggregation-
Induced Emission Material: toward Solution-Processable White
Organic Light-Emitting Devices
Mihela Cibian,[a] Afshin Shahalizad,[b] Fathi Souissi,[b,c] Jessica Castro,[a] Janaina G. Ferreira,[a]
Daniel Chartrand,[a,d] Jean-Michel Nunzi,*[b] and Garry S. Hanan*[a]
Abstract: The synthesis, characterization, photophysical and emissive layer. A luminance efficiency and power efficiency as
redox properties of a dinuclear complex Zn2(AMOX)4 (AMOX = high as 1.12 cd/A and 0.30 lm/W were obtained, respectively.
4-bromo-N,N′-diphenylbenzamidinate N-oxide) are highlighted. The characteristics of this complex: high solubility, thermal sta-
This compound is a first example of a novel class of aggrega- bility, AIE behaviour, ease of synthesis, tunability options, and
tion-induced emission (AIE) materials. Noticeably, solution-proc- low-cost, reveal altogether great potential for the development
essed white-green organic light-emitting diodes were fabri- of this new family of compounds as materials for applications
cated using this complex as dopant in a co-host matrix in the in optoelectronic devices.
Introduction
phenolate,[7] quinolin-8-olate[8] (q), 2-pyridylcarboxylate[9]] with
metal ions such as IrIII,[9,10] AlIII,[7,8,11] and ZnII[6a–6c,7,8b,8d,11a,12]
have been successfully used in optoelectronic devices.[13] Of
these metal ions, the latter has attracted increased interest as
it is biocompatible, earth-abundant, and inexpensive, especially
when compared with iridium.[6,7,8b,8d,11a,12a,14] ZnII complexes
which exhibit AIE behavior have also been reported.[4,15] In par-
ticular, zinc(II) complexes have been targeted for fabrication of
low-cost single-compound white organic light-emitting diodes
(WOLEDs) as illumination sources[16] due to their broad emis-
sion bands and good charge transport properties.[11a,17] They
have been employed as host materials for phosphorescent
emitters and also as charge transport layers in a variety of OLED
structures.[6a,6d,18] It is important to note that those devices
were fabricated by thermal evaporation which is a complex
process requiring expensive infrastructure. Solution-processing
is technically less complicated, inexpensive, and therefore
highly attractive for large-area manufacturing purposes.[19]
However, to date, reports on solution-processed zinc(II) com-
plex-based OLEDs are scarce.[20] Moreover, to the best of our
knowledge, there has been no report on solution-processed
WOLEDs based on zinc(II) complexes with AIE behavior.
Over the last decade, researchers have been motivated to im-
prove the photophysical and morphological properties of com-
pounds used in optoelectronic applications in order to increase
the performance efficiency of the devices and to reduce their
production cost.[1] In this regard, aggregation-induced emission
(AIE) materials[2] present great potential for the development of
solid-state optoelectronic technologies such as organic light-
emitting diodes (OLEDs).[2c] While the first AIE compounds were
entirely organic aromatic compounds,[3] AIE materials based on
coordination complexes have been developed more re-
cently.[2c,4] The latter category presents additional advantages
brought by the metal ions, such as enhanced thermal and envi-
ronmental stability, the possibility of a synergic double contri-
bution for electron/hole transport and increased light emis-
sion.[2c,4,5]
Coordination complexes of anionic, bidentate N,O-ligands
[e.g., 2-(2-benzothiazolyl)phenolate (BTZ),[6] 2-(2-benzoxazolyl)-
[a] Département de Chimie, Université de Montréal,
2900 Edouard-Montpetit, Montréal, Québec, H3T-1J4, Canada,
E-mail: garry.hanan@umontreal.ca
www.greenenergygroup.ca
Part of our ongoing research on coordination compounds
focuses on amidine N-oxides (AMOXs, also called α-aminonitro-
nes and N-hydroxyamidines) and their transition metal com-
plexes. AMOXs are anionic, bidentate N,O-ligands and are good
chelators for metal ions. They exhibit extended conjugation in
the amidine backbone, structural rigidity, and present the possi-
bility to introduce C-aryl and N,N′-diaryl substituents,[21] key
features as platforms for AIE materials.[2] Here, we report the
synthesis and characterization of the dinuclear O-bridged
complex Zn2(AMOX)4 (1) (AMOX = 4-bromo-N,N′-diphenylbenz-
amidinate N-oxide) (Figure 1), which opens the route to a new
class of materials exhibiting AIE behaviour. The performance of
[b] Department of Physics, Engineering Physics and Astronomy,
Queen's University,
Kingston, Ontario, K7L 3N6, Canada,
E-mail: nunzijm@queensu.ca
http://faculty.chem.queensu.ca/people/faculty/Nunzi/
[c] Laboratory of Nanomaterials and Systems for Renewable Energies
(LaNSER), Center for Research and Technologies of the Energy Technopole,
Borj Cedria, Bp 95,
Hammam Lif, 2050, Tunisia
[d] Present address: LAMP - Laboratoire d'Analyse pour les Molécules et
Matériaux Photoactifs - Laboratory for the Analysis of Molecules' and
Materials' Photoactivity, Université de Montréal, Canada
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201800579.
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solution-processed OLEDs using 1 as the dopant in the EML
was also investigated and is discussed herein.
Complex 1 crystalized in the P21/c space group, as a DCM
solvated dimer [1·3(CH2Cl2)]. The center of the dimer and one
of the solvent molecules sit on inversion centers. Similar dimeric
structures exist for the complexes of zinc(II) with the BTZ ligand
[Zn(BTZ)2]2,[6b] as well as with substituted
q
ligands
Results and Discussion
[Zn(Rq)2]2.[25] Trimeric and tetrameric structures are also re-
ported for the latter.[8b,26] The solid-state structure of
1·3(CH2Cl2) highlights pentacoordinate zinc(II) centres with a
distorted square-pyramidal geometry (τ5 = 0.20).[27] The Zn···Zn
distance of 3.092(1) Å is comparable but at the lower limit of
those found in [Zn(Rq)2]2 and [Zn(BTZ)2]2 (3.06–3.29 Å) (Table 1).
The AMOX ligands show two coordination modes to the
zinc(II) ions: μ-AMOX-κN,κO:κO and AMOX-κN,κO. The Zn–O
bond lengths for the bridging AMOX ligands [2.152(2) and
2.076(2) Å] are longer than those in the non-bridging ones
[1.966(2) Å], indicating, as expected, weaker interactions be-
tween the zinc(II) centers and the bridging O atoms. However,
the Zn–N distances [2.073(2) Å and 2.075(2) Å] are statistically
identical in both types of AMOX ligands. For comparison, se-
lected bond lengths and angles in 1 and those in the
[Zn(BTZ)2]2 and [Zn(Rq)2]2 dimers are shown in Table 1. In terms
of packing forces, in the solid-state structure of 1·3(CH2Cl2),
intramolecular [C(C-aryl)–H···O] and intermolecular [C(DCM)–
H···Br; C(C-aryl)–H···Cl] hydrogen bonds exist (Figure S6, ESI, SI),
as well as networks of intermolecular C(sp2)–H···πC(sp2) and
Br···πC(sp2) interactions (Figure S7, SI). The latter patterns were
also identified in other AIE compounds.[34] The existence of the
dimeric form of the compound in the solid bulk powder was
confirmed by X-ray powder diffraction analyses (Figure S8, SI).
A distinction should be made between the molecular complex
1 in powder form which does not contain solvent molecules
(1), the single crystals of 1 containing co-crystalized solvent
[1·3(CH2Cl2)], and the sample of crushed and dried crystals
used for the comparison with the powder sample in the X-ray
powder diffraction experiments.
Synthesis and Characterization
The synthesis of 1 is outlined in Scheme 1. The parent benz-
amidine AM (4-bromo-N,N′-diphenylbenzamidine)[22] was pre-
pared in excellent yield (97 %) by condensation of the 4-bromo-
benzoic acid with two equivalents of aniline, in polyphosphoric
acid trimethylsilyl ester (PPSE), at 180 °C, followed by basic
treatment.[23] N-Oxidation of AM with meta-chloro-peroxy-
benzoic acid (m-CPBA) in dichloromethane (DCM) at room tem-
perature (r. t.) afforded the AMOX ligand in reasonable yield
(51 %).[24] The reaction of zinc(II) acetate with two equivalents
of AMOX in aqueous ethanol at r. t. readily gave a precipitate,
the dimeric complex 1, which was isolated in good yield (70 %)
as an air-stable yellow powder. Both 1 and AMOX were charac-
terized by 1H NMR and 13C NMR spectroscopy, mass spectrome-
try (MS) (Figure S1–S3, SI), and elemental analyses (EA). Com-
plex 1 has a good thermal stability, the decomposition temper-
ature (Td) at 5 % weight loss being 243 °C. (Figure S4, SI).
The solid-state structure of 1 is presented in Scheme 1 (inset)
and Figure S5–S7, SI. XRD quality single crystals were grown
from slow evaporation of a DCM/hexane solution of the com-
pound at –10 °C. Crystal data and refinement details are given
in Table S1, SI. Selected bond lengths and angles are tabulated
in Table 1.
Redox and Photophysical Properties
The electrochemical and photophysical data for 1 are tabulated
in Table 2, together with experimental data for
[6a,6b,6d]
[Zn(BTZ)2]2
and [Znq2]4,[8d,31–33] molecular luminescent
materials which have been successfully used in OLEDs. Cyclic
voltammetry measurements for 1 in DCM show two irreversible
ligand-based oxidation events at 0.97 V and 1.16 V vs. SCE. No
reduction event is observed in the solvent window, in line with
the anionic character of the AMOX ligand, as also found for
zinc(II) complexes with bulky formamidinate N-oxides.[35] The
electronic spectrum of 1 (Figure S10, SI and Table 2) features
π–π* ligand-centred transitions. The lowest-energy transitions
present a λmax at 336 nm with ε = 38800
M
–1 cm–1. DFT and
TD-DFT studies [vide infra; theory level: B3LYP/6,31-g(d,p), PCM:
DCM] were undertaken in order to better understand the elec-
tronic properties of the complex.
The molecular orbital (MO) manifold for 1 is influenced by
the symmetry of the molecule (inversion center at the center
of the dimer), resulting in symmetry-related pairing (Figure 1
and Figure S9, SI). The degenerate HOMO/H-1 are mainly local-
Figure 1. Frontier molecular orbitals (MOs) for 1 [isovalue at 0.03; occupied
orbitals in green and virtual orbitals in pink; theory level: B3LYP/6,31-g(d,p),
PCM: DCM].
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Scheme 1. Synthesis of the zinc(II) dinuclear complex 1 (AM = the 4-bromo-N,N′-diphenylbenzamidine precursor, AMOX = 4-bromo-N,N'-diphenylbenz-
amidinate N-oxide ligand; PPSE = polyphosphoric acid trimethylsilyl ester); the inset is showing the ORTEP view of 1·3(CH2Cl2) with ellipsoids drawn at 50 %
probability level. Hydrogen atoms and co-crystallized DCM molecules have been omitted for clarity.
Table 1. Selected bond lengths [Å] and angles [°] for 1·3(CH2Cl2) (from its solid-state structure and its DFT optimized structure), and for selected relevant
zinc(II) complexes from the literature.[28]
[g]
Cpd.
τ5
Bond lengths [Å]
Angles
Zn1–O1
Zn1–O10
Zn1–O10[i] Zn1–N2
Zn1–N4
Zn···Zn
1·3(CH2Cl2)
[Zn(BTZ)2]2
0.20
0.87
1.996(2)
1.962(2)
1.978(3)
1.959(2)
2.152(2)
2.058(2)
2.105(2)
2.090(2)
2.076(2)
2.016(2)
2.085(3)
2.023(2)
2.075(2)
2.095(3)
2.095(4)
2.131(3)
2.073(2)
2.170(3)
2.047(3)
2.222(3)
3.092(1) Zn1–O10–Zn1[i]
3.209(1) 94.0(1)
3.060(1) 103.9(1)
3.243(1) 93.8(1)
3.291(1) 104.1(1)
3.262(1) 104.7(1)–106.0(1)[j] 74.4(1)[i]
O10–Zn1–O10[i] O1–Zn1–N2
[a]
86.0(1)
76.1(1)
86.2(1)
74.9(1)
79.6(1)
87.4(1)
81.8(1)
82.7(1)
82.3(1)–84.4(1)[j]
82.7(2)
[Zn(Rq)2]2-1[b] 0.19
[Zn(Rq)2]2-2[c] 0.58
[Zn(Rq)2]2-3[d] 0.20 and 0.47[h] 1.978(3)[i]
[Zn(Rq)2]2-4[e] 0.54
1-dft[f]
0.30
2.118(3)–2.144(3)[j] 2.006(4)[i]
2.051(2)–2.113(2)[j] 2.191(4)[i]
1.954(4)
1.98
2.022(4)
2.20
2.078(4)
2.06
2.134(4)
2.13
2.140(5)
2.06
3.22
105.4(2)
74.6(2)
[a] [Zn(BTZ)2]2 [BTZ = 2-(2-benzothiazolyl)phenolate][6b]. [b] [Zn(Rq)2]2-1: Rq = 5-chloro-7-iodoquinolin-8-olato (DOBNEP).[25b] [c] [Zn(Rq)2]2-2: Rq = 2-[2-(2,6-dichloro-
phenyl)vinyl]quinolin-8-olato (CEBLOM).[25c] [d] [Zn(Rq)2]2-3: Rq = 2-[2-(4-methoxyphenyl)vinyl]quinolin-8-olato (MIMLIF).[29] [e] [Zn(Rq)2]2-4: Rq = 2-nonylquinolin-8-olato
(QUBNIK).[25a] [f] Optimized structure, theory level: B3LYP/6-31g(d, p), PCM: CH2Cl2. [g] As defined by Addison.[27] [h] Different geometries at the two zinc(II) centers. [i]
The average value is given when values are statistically (3σ) identical. [j] The range of values is given when values are statistically (3σ) different.
ized on the ONCN moieties of the two non-bridging AMOXs, gands. This distinction indicates that the energy of the LUMO
whereas the LUMO/L + 1 are found delocalized on the NCN can be fine-tuned by modifying the C-aryl substituents with
moieties and the C-aryl substituents of the two bridging li- EW/ED groups. The TD-DFT calculations for 1 (Figure S10, Table
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Table 2. Photophysical, electrochemical and theoretical data for 1, and selected experimental data for well-established Zn-based compounds for OLED
fabrication.
[a] Photophysical data were obtained in CH2Cl2, at room temperature unless otherwise stated. [b] Electrochemical data were obtained using the following
conditions: dry CH2Cl2, [nBu4N]PF6 0.1 M, compound concentration about 1 mM, glassy carbon electrode, scan rate 100 mV/s, room temperature, Ar atmos-
phere, ferrocene used as internal reference; all potentials are reported in volt, vs. SCE (Fc/Fc+ vs. SCE was considered 0.46V in DCM[30]). [c] In solid state
(powder). [d] Measured by absolute method, using an integration sphere (error is 20 %). [e] HOMO level was determined using the equation: EHOMO (eV) =
–(4.4 + Eox_onset). [f] No reduction process is observed. [g] LUMO level was determined using the equation: ELUMO (eV) = EHOMO + Eg. [h] Eg was obtained using
the absorption edge technique: Eg = 1240/λabs_onset. [i] BTZ = 2-(2-benzothiazolyl)phenolate. [j] In thin film. [k] From ref.[6b] [l] From ref.[6a] [m] Determined by
Ultraviolet Photoelectron Spectroscopy (UPS). [n] q = Quinolin-8-olate. [o] From ref.[8d] [p] In CH2Cl2. [q] From ref.[31] [r] From ref.[32] [s] From ref.[33] [t] Theory
level: B3LYP/6-31g(d, p), PCM: CH2Cl2. [u] HOMO level was obtained from DFT optimization, theory level: B3LYP/6-31g(d, p), PCM: CH2Cl2. [v] LUMO level was
obtained using ELUMO (eV) = EHOMO (DFT) + Eg(TD-DFT). [w] Eg(TD-DFT) is the TD-DFT calculated first singlet, which corresponds to the HOMO–LUMO transition;
theory level: B3LYP/6-31g(d, p), PCM: CH2Cl2.
S2, SI and Table 2) confirm the lowest energy absorption bands
Complex 1 is AIE active: it is non-emissive in DCM solution,
as principally interligand (HOMO/H-1 to LUMO/L + 1 at 384– but it is strongly emissive in the solid state (Figure 3 and Figure
388 nm), principally intraligand (HOMO/H-1 to L + 2/L + 3 at S11, SI). Its powder photoluminescence (PL) spectrum features
371 nm) or mixed inter- and intra-ligand (H-3/H-2 to LUMO/ a broad emission band (400–800 nm) with λmax em = 556 nm at
L + 1 at 362–368 nm) transitions. At higher energy, intraligand r. t. (λexc = 350 nm) (Figure S11, SI). At 77 K, a similar PL spec-
(HOMO/H-1 to L + 8/L + 9) and mixed inter- and intra-ligand trum is obtained, with the exception of a narrower bandwidth
transitions are also calculated (see Table S2, SI).
and a slight blue shift (λmax em = 548 nm) (Figure S11, SI). The
Furthermore, the theoretical value of the HOMO–LUMO gap
of 1 determined by DFT/TD-DFT (3.19 eV) is close to the experi-
mental value (3.06 eV), and it is in the range of those of
[Zn(BTZ)2]2[6b] and [Znq2]4,[8d,31–33] well-established compounds
in OLED fabrication (Table 2 and Figure 2).
Figure 3. PL spectra of 1 in hexane/DCM mixtures (concentration: 10–4
max em = 556 nm, λexc = 350 nm). Insets show the variation of the PL intensity
M;
λ
with the increasing fraction of hexane in the mixture and images of the
corresponding solutions under 365 nm UV light; the shoulder peak at 400–
450 nm for the emission spectra in this Figure is due to scattering from the
experimental setup and it is dissimilar to the shoulder peak observed in the
same region in the EL spectra (Figure 4).
Figure 2. HOMO–LUMO gap of 1 vs. those of well-established compounds for
OLED fabrication: [Zn(BTZ)2]2 [BTZ = 2-(2-benzothiazolyl)phenolate][6b] and
[Znq2]4, (q = quinolin-8-olate).[8d,36]
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AIE behaviour of 1 is shown in Figure 3. PL spectra of 1 in EL spectra of the devices A, B, and C were also calculated to be
various volumetric fractions of hexane/DCM mixtures are pre- (0.28, 0.51), (0.26, 0.53), and (0.27, 0.54), respectively. Obviously,
sented (Figure 3). Up to a volumetric ratio of 20 % DCM/ 80 % even though the EL spectra of the devices cover the entire visi-
hexane, complex 1 is non-emissive, but increasing the hexane ble spectral region, these color coordinates (particularly the y
proportion over 80 % results in formation of emissive aggre- coordinates) are still far from the ideal values for the ideal white
gates (as suspensions). In general, the AIE behaviour is based color (0.33, 0.33), due to the narrow bandwidths of the EL spec-
on the existence of excited states for which the non-radiative tra. For this reason, the thicknesses of the devices should be
decay pathways are suppressed due to the rigid environment optimized in order to avoid undesired microcavity effects and
provided by the aggregates restricting their intramolecular rota- to achieve ideal broad-band white color, which is the subject
tions.[2,3,37] Nevertheless, the AIE phenomenon is complex and of our future work.
the understanding and probing of the exact nature of these
excited states is yet to be fully established.[37a] Complex 1 has
an excited state lifetime τ < 1.0 ns and a photoluminescence
quantum yield (ΦPL) of ca. 2 %, in the solid state (powder). Its
short excited state lifetime, the similarity of its PL spectra (pow-
der) at room temp. and 77 K, and those obtained for the AIE
experiments, as well as the DFT and TD-DFT calculations, sug-
gest a fluorescence based interligand-type emission. However,
the contribution of other luminescence pathways (e.g., intra-
and intermolecular aromatic-dimer excited states) cannot be
excluded, especially considering the broadness of the PL spec-
trum.[37a]
It should also be noted that the EL spectra exhibit a shoulder
peak in the range of 400–450 nm. As can be seen in Figure S11,
SI, such a shoulder peak is missing in the solid-state PL spec-
trum of complex 1, indicating that it is not a characteristic emis-
sion of this material. Indeed, this shoulder peak is a characteris-
tic emission of the host matrix and can be ascribed to the
PVK:PBD exciplex excited state (formed at the interface be-
tween the LUMO of PBD and the HOMO of PVK-l, see Figure 6),
according to the literature.[40] [To avoid confusion, it should also
be mentioned that the shoulder peak at 400–450 nm in the
emission spectra in Figure 3 is due to scattering from the exper-
imental setup and it is dissimilar to the shoulder peak observed
The short excited state lifetime of 1 in the solid state (pow- in the same region in the EL spectra (Figure 4)]. Furthermore,
der), together with its broad PL spectrum, and its solubility in unlike electroplex emission, it is well-known that the exciplex
common solvents bode well with testing it in solution-proc- emission is not red-shifted upon increasing the driving voltage
essed OLEDs, despite its low photoluminescence quantum (or current density).[41] For instance, as displayed in Figure 4b,
yield.
Device Fabrication
At the first attempt to realize solution-processed WOLEDs based
on 1, three devices with different doping concentrations of
complex 1 (i.e. 3, 6, and 12 wt. %) were fabricated, hereafter
called device A, B, and C, respectively. The structure of the devi-
ces was ITO/PEDOT:PSS (30 nm)/PVK-h (15 nm)/PVK-l:PBD:com-
plex 1 (3, 6, and 12 wt. %) (30 nm)/BCP(10 nm)/Alq3(40 nm)/Al
(150 nm)/Ag (50 nm). Details concerning the fabrication process
are given in the Supporting Information (SI). The chemical struc-
tures of the materials used in the OLEDs are also shown in
Figure S12, SI. To fabricate the EML, complex 1 was doped into
a co-host system consisting of poly (N-vinylcarbazole) (PVK-l)
hole transporting and 2-(4-biphenyl)-5-phenyl-1,3,4-oxadiazole
(PBD) electron transporting materials with a weight ratio of
30:70 (PVK:PBD), kept fixed in all the devices. PVK-h and PVK-l
are respectively high and low-molecular weight PVK.
As shown in Figure 4a, the devices emitted a greenish-white
electroluminescence (EL) light with a characteristic emission
peak at 518 nm. This value is slightly blue-shifted compared
with the PL spectra shown in Figure 3 and Figure S11, SI, most
likely due to microcavity effects in the device structure.[6d,38]
In addition, the bandwidths of the EL spectra are narrower in
comparison to the PL spectra which can also be attributed to
the microcavity effect in the devices.[39] It is noteworthy to
mention that complex 1 emits a yellow light and the CIE (Com-
mission Internationale de l'Éclairage) chromaticity coordinates
(x,y) for the PL spectrum shown in Figure S11, SI, were calcu-
lated to be (0.40, 0.48). The CIE chromaticity coordinates for the
Figure 4. (a) The electroluminescence (EL) spectra of devices A, B, and C; (b)
the EL spectra of device A at different current densities. No shift in the EL
spectra in device A was observed at different bias current densities applied
to the device. The Full Widths at Half Maximum (FWHM) of the EL spectra in
devices A, B, and C are 79.9 nm, 72.4 nm, and 71.6 nm, respectively.
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Figure 5. The energy level alignment of the components used in the device structure. The electron and hole injection pathways from the corresponding
charge transport materials into the emissive layer and formation of excitons on complex 1 molecules are also shown. Formation of exciplexes at the interface
between the LUMO of PBD and the HOMO of PVK-l is also shown by the dashed arrow.
the EL spectra of device A for various current densities do not crease by increasing the doping concentration. This is obviously
show any significant variation with applied bias current, indicat- not in contradiction with the AIE behavior of complex 1, as
ing that the above-mentioned shoulder peak is not due to for- described earlier in Figure 3, where the PL emission enhances
mation of PVK:PBD electroplexes. Interestingly, the PVK:PBD ex-
ciplex emission covers the missing blue region in the solid-state
PL spectrum of complex 1, to achieve white emission.
Furthermore, emission from complex 1 overweighs the con-
tribution of PVK:PBD exciplex emission, indicating that direct
charge trapping on complex 1 molecules is highly favored in
these devices. It can be clearly understood from the energy
level alignment between the components (Figure 5) that injec-
tion of electrons and holes from the charge transport layers
into complex 1, and subsequently formation of excitons on
complex 1 molecules, can take place very efficiently. Moreover,
direct charge trapping on an emitter can be evidenced when a
shift in the current density-voltage (J–V) curves is present by
increasing the doping concentration.[42] Obviously, such a shift
is observed when looking at the J–V curves of the OLEDs,
shown in Figure S13, SI.
The luminance efficiency vs. current density-voltage (LE-J)
and power efficiency vs. voltage (PE-V) plots of the OLEDs are
illustrated in Figure 6a and Figure 6b, respectively. The maxi-
mum LEs obtained for devices A, B, and C are 1.12, 1.03, and
0.49 cd/A, which correspond to the current densities at 9.7, 10,
and 2.4 mA/cm2, respectively. The maximum power efficiencies
(PEs) of the WOLEDs were also measured to be 0.30, 0.25, and
0.13 lm/W for devices A, B, and C, respectively. Even though
these LE and PE values are low in comparison to those reported
for phosphoresce and TADF-based WOLEDs,[20,43] they are com-
parable with the best values reported in the literature for ther-
mally-processed fluorescent zinc(II)-based WOLEDs (0.12 to 2.1
cd/A for LE and 0.038 to 1.17 lm/W for PE).[44] It is important to
note that the gradual decrease in the device efficiency of devi-
ces B and C compared with device A is due to the fact that, as
mentioned above, the turn-on voltages of these devices in-
Figure 6. (a) The luminance efficiency vs. current density (LE-J); (b) the power
efficiency vs. voltage (PE-V) curves of devices A, B, and C.
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concentration of the compounds was about 1 mM. Tetrabutylam-
monium hexafluorophosphate (TBAP) was used as supporting elec-
with increasing the concentration. The luminance vs. voltage
plots of the devices are also shown in Figure S14. The maximum
luminance values for devices A, B, and C were obtained to be
91.1, 30.4, and 14.7 cd/m2, respectively.
trolyte and its concentration was 0.10
M. Cyclic voltammograms
were obtained at scan rate of 100 mV/s. For irreversible oxidation
processes, the anodic peak was used as E. Thermogravimetric analy-
ses (TGA) were carried out on a TGA2950 Thermogravimetric Analy-
zer (TA Instruments) under N2 atmosphere, in the 25–600 °C tem-
perature range, at a heating rate of 10 °C/min. High-Resolution Mass
Spectrometry (HRMS) analyses were performed on a Bruker micro-
TOF II instrument by electrospray ionization (ESI) method for the
ligand AMOX and using a cryosource (Bruker) for the complex 1.
The microanalyses were done by the Elemental Analysis Service at
Université de Montréal. Solvents, purchased from VWR and Fisher,
were removed under reduced pressure using a rotary evaporator,
unless otherwise stated. The m-CPBA from Acros Organics, P2O5,
hexamethyldisiloxane (HMDS), 4-bromobenzoic acid, and metal salt,
from Aldrich, were used without further purification. The aniline
from Aldrich was distilled before use.
Conclusions
In summary, a new zinc(II) coordination complex of a benz-
amidine N-oxide (AMOX) ligand was synthesized and character-
ized. This compound displays AIE behaviour and it is the proto-
type of a new family of AIE materials based on the AMOX ligand
platform, presently under development. Solution-processed
white-green OLEDs using this complex as the dopant in the
EMLs were successfully fabricated and characterized. Remarka-
bly for a fluorescent zinc(II)-based OLED, the device containing
3 wt. % of the complex shows the best LE and PE values (1.12
cd/A and 0.30 lm/W) in comparison to the other devices con-
taining higher concentrations of the complex. While these val-
ues are promising for the development of solution-processed
single-compound WOLEDs based on ZnII complexes exhibiting
AIE behaviour, the structure of the devices should be optimized
in order to enter into practical applications for the lighting tech-
nologies.[43a–43c] Therefore, further studies on device structures
are ongoing to improve the efficiencies of WOLEDs based on
zinc(II) AMOX complexes. Additionally, the CIE chromaticity co-
ordinates for all the devices are quite similar and exhibit the
coordinates of a greenish-white light. By optimizing the device
structure (for example, through device thickness variation), we
believe that achieving color coordinates close to those of ideal
white light would be possible. Overall, the characteristics of
complex 1: high solubility, thermal stability, AIE behaviour, ease
of synthesis, tunability options, and low-cost, already demon-
strate great potential for development of this family of com-
pounds for application in optoelectronic devices.
Details on the X-ray structure determination, X-ray powder diffrac-
tion measurements, computational studies and device fabrication
are given in Supporting Information (SI).
Synthetic Details
4-Bromo-N,N′-diphenylbenzamidine (AM): The previously re-
ported,[22] amidine precursor was synthetized based on modified
reported procedures.[23,45]
4-Bromo-N,N′-diphenylbenzamidine N-Oxide (AMOX): To a solu-
tion of the corresponding amidine (1.50 g, 4.3 mmol, 1 equiv.) in
DCM (100 mL), NaHCO3 (0.39 g, 4.7 mmol, 1.1 equiv.) was added as
a solid, followed by the slow addition at r. t. of a solution of m-
CPBA (0.81 g, 4.7 mmol, 1.1 equiv.) in DCM (50 mL). The reaction
mixture was stirred for 24 h, then was filtered and washed with an
aqueous solution of NaOH 1M (2 × 25 mL) and with water (2 ×
25 mL). The combined organic layers were dried with anhydrous
MgSO4 and filtered. After filtration and solvent evaporation, a
brown solid was obtained, further purified by flash chromatography
on silica [gradient of eluents: hexane/AcOEt (5:5), AcOEt 100 %,
AcOEt/EtOH (9:1), EtOH 100 %]. The purification yielded a yellow
1
solid, the desired product. Yield: 0.80 g, 51 %. m.p. 155–157 °C. H
Experimental Section
NMR (400 MHz, CDCl3): δ = 7.33 (d, J = 8 Hz, 2 H), 7.27–7.21 (m, 5
H), 7.15 (t, J = 7 Hz, 2 H), 7.01 (t, J = 7 Hz, 1 H), 6.96 (d, J = 8 Hz, 2
H), 6.71 (d, J = 8 Hz, 1 H) ppm. H NMR ([D6]DMSO, 400 MHz): δ =
7.43 (d, J = 8 Hz, 2 H), 7.36 (d, J = 8 Hz, 2 H), 7.29 (t, J = 8 Hz, 2 H),
7.16–7.13 (m, 3 H), 7.08 (t, J = 8 Hz, 2 H), 6.84 (t, J = 7 Hz, 1 H), 6.64
(d, J = 8 Hz, 2 H) ppm. 13C NMR ([D6]DMSO, 126 MHz): δ = 153.0,
145.5, 144.1, 131.7 (2 C), 131.0 (2 C), 130.4, 128.4 (2 C), 128.3 (2 C),
125.7, 123.7 (2 C), 122.4, 122.1, 121.7 (2 C) ppm. MS (ESI, DCM)
(m/z): 367.1 [M + H]+ (100 %). C19H15BrN2O (367.24): calcd. C 62.14,
H 4.12, N 7.63; found C 61.95, H 3.73, N 7.54.
Materials and Methods: Nuclear magnetic resonance (NMR) spec-
tra were recorded in CDCl3 and/or [D6]DMSO at room temperature
(room temp.) (unless otherwise stated) on the following spectrome-
ters: Bruker AV-400, AV-300, DRX-400, and ARX-300 MHz. Chemical
shifts (δ) are reported in part per million (ppm) relative to TMS,
using the residual solvent protons (δ =7.26 ppm and 2.50 ppm) as
reference. Absorption spectra were measured in dichloromethane
1
(DCM) (concentration range 10–4–10–6
M) at room temp. on a Cary
500i and a Cary 6000i UV/Vis-NIR Spectrophotometer. Lumines-
cence spectra were obtained using a Perkin–Elmer LS55 Lumines-
cence Spectrometer equipped with an accessory for measuring
solid state samples. Excited state lifetime measurements were per-
formed with an Edinburgh Instruments Miniature Fluorescence Life-
time Spectrometer (mini-τ). Quantum yield was determined for
solid state samples (powder) by absolute method, using a FLSP920
Spectrometer (Edinburgh Instruments) equipped with an integrat-
Bis[μ-(4-bromo-N,N′-diphenylbenzamidinate N-Oxide-κN,κO:κO)]-
bis[4-bromo-N,N′-diphenylbenzamidinate N-Oxide-κN,κO]dizinc-
(II, II) (Complex 1): The synthesis of 1 was performed based on
modified previously reported procedures.[35,46] To a solution of the
AMOX ligand (0.58 g, 1.6 mmol, 2 equiv.) in ethanol, aq. KOH was
added to form a clear orange solution, which was then added to a
solution of the metal salt Zn(AcO)2·2H2O (0.17 g, 0.79 mmol,
ing sphere. Electrochemical measurements were carried out in ar- 1 equiv.) in water, previously brought to pH 8 with aq. KOH. The
gon-purged dry DCM at room temperature with a BAS CV50W po-
tentiostat. The working electrode was a glassy carbon electrode.
The counter electrode was a Pt wire, and the pseudo-reference elec-
trode was a silver wire. The reference was set using an internal
1 mM ferrocene/ferrocinium sample at 0.46 V vs. SCE in DCM. The
total quantity of solvent used was 100 mL of EtOH 70 %. The forma-
tion of precipitate is observed almost instantly. The reaction mixture
was stirred at room temperature for 24 h before water was added,
and was kept at 4 °C for 2 h before being filtered. The resulted solid
was washed with hot water and aqueous ethanol 50 %, and was
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dissolved in DCM and dried with MgSO4. A second filtration and
evaporation of the solvent yielded the desired products as a pale
beige-yellow-green solid, which was further recrystallized from
DCM/hexane at –10 °C to yield the pure product as a pale yellow
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Acknowledgments
The authors acknowledge the Natural Sciences and Engineering
Research Council (NSERC), le Fonds du Québec pour la recher-
ché sur la nature et les technologies (FRQNT), the Centre for
Self-Assembled Chemical Structures (CSACS), the Centre in
Green Chemistry and Catalysis (CGCC), and the Université de
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grateful to Compute Canada, as well as to UdeM XRD, NMR, MS,
and EA services and personnel. Professor W. Skene, his research
group, and Baptiste Laramée-Milette are thanked for their help
with the quantum yield measurements. Research at Queen′s
University was also supported by Discovery Grants program
(RGPIN-2015-05485) and CREATE program (Novel Chiral Materi-
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Received: May 10, 2018
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Optoelectronic Materials
Zn2(AMOX)4 is a prototype for new AIE
materials based on the amidine N-ox-
ide ligand platform. Solution-proc-
essed white-green OLEDs were fabri-
cated using it as the dopant in the
emissive layer. The high solubility and
thermal stability, together with AIE be-
haviour, ease of synthesis, tunability
options, and low-cost demonstrate
great potential for developing this
family of compounds for optoelec-
tronic applications.
M. Cibian, A. Shahalizad,
F. Souissi, J. Castro, J. G. Ferreira,
D. Chartrand, J.-M. Nunzi,*
G. S. Hanan* ..................................... 1–10
A Zinc(II) Benzamidinate N-Oxide
Complex as an Aggregation-Induced
Emission Material: toward Solution-
Processable White Organic Light-
Emitting Devices
Hanan and co-workers present a zinc complex suitable for an OLED which produces emission upon aggregation
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DOI: 10.1002/ejic.201800579
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