Synthesis, spectroscopy, and electrochemistry of ionic hosts for organic electronics

Article abstract of DOI:10.1016/j.molstruc.2014.10.034

We report on charge- and ion-transport ionic hosts made from an imidazolium-cation-modified aryl-1,2,4-triazole, phosphineoxide-carbazole, and phosphineoxide. The hosts are white solids that have short-wavelength absorption cut-off at <355 nm (high-energy

Full text of DOI:10.1016/j.molstruc.2014.10.034

Journal of Molecular Structure 1081 (2015) 244–247
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, spectroscopy, and electrochemistry of ionic hosts for organic
electronics
⇑
⇑
Nail M. Shavaleev , Mohammad K. Nazeeruddin
Laboratory of Photonics and Interfaces, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
New charge- and ion-transport ionic
hosts.
The hosts are imidazolium-cation-
modified heterocycle chromophores.
The hosts exhibit high-energy optical
gap and a wide redox gap.
O
O
N
N
N
N
N
N
N N
O
O
P
P
a r t i c l e i n f o
a b s t r a c t
Article history:
We report on charge- and ion-transport ionic hosts made from an imidazolium-cation-modified
aryl-1,2,4-triazole, phosphineoxide-carbazole, and phosphineoxide. The hosts are white solids that have
short-wavelength absorption cut-off at <355 nm (high-energy optical gap of >28,200 cmÀ1) and that
exhibit oxidation at <1.44 V and reduction at >(À2.75) V against ferrocene with a wide redox gap of
>3.9 V.
Received 16 September 2014
Received in revised form 14 October 2014
Accepted 15 October 2014
Available online 22 October 2014
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
Heterocycle
Ionic liquid
Charge transport
Chromophore
Spectroscopy
Electrochemistry
Introduction
sufficiently high energy of excited states to avoid dissipative
energy- and charge-transfer processes between the host and
Host (matrix) materials are used in organic light-emitting
diodes (OLED) to facilitate the transport of charges (holes and elec-
trons) and to dilute the emitter in order to prevent its aggregation
and concentration quenching of its luminescence [1–8]. The host,
often a neutral polar organic heterocycle or a neutral/charged
metal complex [1–7], must have suitable redox potentials and
the emitter. Ionic liquids are used in organic electronics to facili-
tate the transport of ions [9]. Recently, we successfully applied
an ionic charge-transport carbazole host in blue light-emitting
electrochemical cells (LEC) to transport both the holes and the ions
in the device [8].
Here, we describe new charge- and ion-transport ionic hosts
made from an imidazolium-cation-modified aryl-1,2,4-triazole
(H1), phosphineoxide-carbazole (H2), and phosphineoxide (H3)
(Schemes 1–3).
⇑
Corresponding authors. Fax: +41 21 693 4111.
E-mail addresses: shava@mail.ru (N.M. Shavaleev), mdkhaja.nazeeruddin@epfl.
ch (M.K. Nazeeruddin).
http://dx.doi.org/10.1016/j.molstruc.2014.10.034
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

N.M. Shavaleev, M.K. Nazeeruddin / Journal of Molecular Structure 1081 (2015) 244–247
245
hydrogen peroxide gave 6 (Scheme 2). We note that neutral polar
heterocycles 3 and 6 themselves may be used as hosts in organic
electronics [2,5–7].
Results and discussion
The multi-step syntheses of the hosts H1–H3 are summarized
in Schemes 1–3.
The anisole group of 3 and 6 was converted to a phenol by de-
protection with pyridine hydrochloride [8] or with BBr3 [12]
(Schemes 1 and 2). The phenols 4 and 7 are soluble in dichloro-
methane when they are freshly prepared, but become insoluble
when they are dried and aged. The insolubility likely arises from
the formation of a network of hydrogen bonds between the
hydroxyl group of the phenol and the imine nitrogen of the
1,2,4-triazole in 4 or the oxygen of the phosphineoxide in 7.
The phenols 4 and 7 were alkylated with imidazolium salt 1 [8]
to give new ionic hosts H1 and H2 as hexafluorophosphate salts
after the anion exchange (Schemes 1 and 2). The side-products
The aryl-1,2,4-triazole 3 was prepared by cyclization of the
hydrazide 2 with 3-anisidine and PCl3 (Scheme 1) [2]. The
tert-butyl groups improve solubility of 3. The aryl-carbazole 5
was obtained by a copper-catalyzed C–N coupling of 3,6-dibromoc-
arbazole with 3-iodoanisole (Scheme 2) [10,11]. The side-reactions
in the synthesis of 5 are de-bromination and oligomerization (self
C–N coupling) of 3,6-dibromocarbazole; however, low tempera-
ture favors the desired C–N coupling. Lithiation of
n-butyllithium, quenching with chlorodiphenylphosphine,
and oxidation of the crude phosphine to phosphineoxide with
5 with
Br
Br
N
N
1
O
O
O O
a
+
Cl H2N NH
HN NH
2
OCH3
OH
c
b
N
N
N N
N N
3
4
O
N
N
d
N
PF6
N N
H1
Scheme 1. Synthesis of H1: (a) THF/pyridine, under Ar, room temperature; (b) 1,2-dichlorobenzene, PCl3, 3-anisidine, under Ar, 100–200 °C; (c) pyridine hydrochloride,
under Ar, 200 °C; (d) (i) 1, K2CO3, DMF, under Ar, 50–60 °C; (d) (ii) KPF6, room temperature.
OCH3
OCH3
H
N
N
N
a
b
O
O
Br
Br
Br
Br
P
P
5
6
O
OH
N
N
N
N
PF6
c
d
O
O
O
O
P
P
P
P
H2
7
Scheme 2. Synthesis of H2: (a) 3-iodoanisole, Cs2CO3, Cu2O, DMF, under Ar, 120 °C; (a) [preferred alternative] 3-iodoanisole, K3PO4, CuI, ( )-trans-1,2-diaminocyclohexane,
1,4-dioxane, under Ar, 110 °C; (b) (i) n-butyllithium, chlorodiphenylphosphine, THF, under Ar, À78 °C to room temperature; (b) (ii) hydrogen peroxide, CH2Cl2, under air, 0 °C
to room temperature; (c) BBr3, CH2Cl2, under Ar, 0 °C to room temperature; (d) (i) 1, K2CO3, DMF, under Ar, 60 °C; (d) (ii) KPF6, room temperature.

246
N.M. Shavaleev, M.K. Nazeeruddin / Journal of Molecular Structure 1081 (2015) 244–247
Br
Cl
Br
N
N
N
N
a
+ N
Br
N
Cl
Br
Cl
8
OCH3
OCH3
OH
O
P
O
P
c
b
9
10
d
PF6
P O
O
N
N
H3
Scheme 3. Synthesis of H3: (a) neat, room temperature; (b) (i) Mg, chlorodiphenylphosphine, THF, under Ar, room temperature to reflux; (b) (ii) hydrogen peroxide, CH2Cl2,
under air, 0 °C to room temperature; (c) BBr3, CH2Cl2, under Ar, 0 °C to room temperature; (d) (i) 8, K2CO3, DMF, under Ar, 60 °C; (d) (ii) KPF6, room temperature.
that are observed in the synthesis of H1 likely originate from
60
N-alkylation or the N4-to-N1 substitution rearrangement [13] of
H1
the 1,2,4-triazole; however, low temperature and the excess of
base favor the desired O-alkylation of the phenol.
50
40
30
20
10
0
H2
We modified the hosts with an imidazolium cation because it
has high-energy excited states and it is electrochemically-inert
[9] (it is often used to make ionic liquids). The m-substitution in
the ‘‘anisole’’ disrupts the conjugation, while a hexyloxy chain
makes the hosts hydrophobic, increases their solubility in organic
solvents, and minimizes interaction of the imidazolium cation with
the core of the host.
The hosts were purified by chromatography and by recrystalli-
zation. They were characterized by elemental analysis and by 1H
and 31P NMR spectroscopy. Their ESI+ mass-spectra display the
cation {MÀPF6}+. The purity of the ionic hosts is not likely to be a
limiting factor in solution-processed organic electronics devices
that use polymers or charged metal complexes, which themselves
cannot be vacuum-processed by sublimation [8].
ε
260
280
300
320
340
360
Wavelength [nm]
Fig. 1. Absorption spectra of H1 (1.36 Â 10À4 M) and H2 (9.98 Â 10À5 M) in
dichloromethane.
The hosts are white solids. They are soluble in polar organic sol-
vents and have absorption cut-off (optical gap) at short wavelength
(high energy) at 330 nm (30,300 cmÀ1) for H1 and at 355 nm
(28,200 cmÀ1) for H2 in dichloromethane (Fig. 1 and Table 1).
Replacement of the 1,2,4-triazole with the carbazole chromophore
[3,14] red-shifts the absorption spectrum from H1 to H2.
In cyclic voltammetry (CV), the ionic hosts undergo oxidation
and reduction processes that are irreversible at scan rates of 0.1–
1 V/s in acetonitrile on glassy-carbon electrode (Fig. 2 and Table 1).
The redox irreversibility in the CV experiment does not necessarily
mean that the host will not function in the device.
The oxidation process is localized on the ‘‘anisole’’ fragment of
H1 and on the ‘‘anisole-carbazole’’ of H2 [3,8]; the reduction—on
the 1,2,4-triazole of H1 [2] (it is accompanied by adsorption on
the electrode) and on the phosphineoxide of H2 [4–7]. Unlike the
previously reported hole-transport ionic host that exhibits reduc-
tion at 0.77 V but no oxidation to À2.7 V against ferrocene [8],
the new ionic hosts exhibit both the reduction and the oxidation
and, therefore, are probably ambipolar, that is, they may support
the transport of both the electrons (through the 1,2,4-triazole [2]
Table 1
Absorption and redox properties.
a
kabs/nm (
e
/103 MÀ1 cmÀ1
)
E
ox/Vb
E
red/Vb
H1
H2
272 (25)
1.44
1.28
<(À2.5)
À2.75
254 (58), 278 (57), 327 (3.8), 341 (2.6)
a
See Fig. 1. In dichloromethane at room temperature. At 250–500 nm. Errors:
2 nm; 5%.
See Fig. 2. Irreversible processes. At scan rates of 0.1–1 V/s in acetonitrile with
kabs
b
,
e,
0.1 M NBu4PF6 on glassy-carbon working electrode. Peak potentials are reported
relative to the ferrocene couple at 0.1 V/s.
and the phosphineoxide [4–7]) and the holes (through the ‘‘ani-
sole’’ and the carbazole [3,8]). In fact, the neutral analogs of H2
are known to be ambipolar hosts [5–7].
The redox gap (
D
ER = Eox À Ered; >3.94 V for H1; 4.03 V for H2)
and optical gap (>28,200 cmÀ1) of the new hosts are large enough
for blue-fluorescent emitters; however, these parameters may need

N.M. Shavaleev, M.K. Nazeeruddin / Journal of Molecular Structure 1081 (2015) 244–247
247
Acknowledgement
European Union (CELLO, STRP 248043).
Appendix A. Supplementary material
H2
Experimental techniques, syntheses and characterization, and
NMR spectra. Supplementary data associated with this article can
be found, in the online version, at http://dx.doi.org/10.1016/
j.molstruc.2014.10.034.
H1
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a white hygroscopic solid
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