Contextualizing yellow light-emitting electrochemical cells based on a blue-emitting imidazo-pyridine emitter

Article abstract of DOI:10.1016/j.poly.2017.11.048

This work provides the synthesis, structural characterization, electrochemical and photophysical features, as well as the application in light-emitting electrochemical cells (LECs) of a novel small molecule belonging to the pyridilimidazo[1,5-a]pyridine family, namely 3-(2-methoxyphenyl)-5-methyl-1-(6-methylpyridin-2-yl)H-imidazo[1,5-a]pyridine (Me-impy). This compound shows a low-cost and facile synthesis, excellent redox properties, and a high photoluminescence quantum yield (Φ) of 0.4 associated to a blue emission (～436 nm) in solution. Despite these appealing features, the emission in solid state is red-shifted to the yellow region, owing to its aggregation character. Additionally, the electroluminescence response shows a broader and red-shifted emission even upon dilution of Me-impy thin films with polymethyl methacrylate (PMMA). Herein, we report an in-depth study on the aggregation features and their impact on the electroluminescence response of Me-impy, providing relevant information to design small-molecule based LECs.

Full text of DOI:10.1016/j.poly.2017.11.048

Accepted Manuscript
Contextualizing yellow light-emitting electrochemical cells based on a blue-
emitting imidazo-pyridine emitter
Elisa Fresta, Giorgio Volpi, Claudio Garino, Claudia Barolo, Rubén D. Costa
PII:
S0277-5387(17)30776-3
https://doi.org/10.1016/j.poly.2017.11.048
POLY 12950
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
11 November 2017
28 November 2017
Please cite this article as: E. Fresta, G. Volpi, C. Garino, C. Barolo, R.D. Costa, Contextualizing yellow light-emitting
electrochemical cells based on a blue-emitting imidazo-pyridine emitter, Polyhedron (2017), doi: https://doi.org/
10.1016/j.poly.2017.11.048
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Contextualizing yellow light-emitting electrochemical cells based on a
blue-emitting imidazo-pyridine emitter
Elisa Frestaa, Giorgio Volpib, Claudio Garinob, Claudia Barolob,c*, Rubén D. Costaa*
aIMDEA Materials Institute, Calle Eric Kandel 2, 28906 Getafe, Madrid, Spain
bDepartment of Chemistry, NIS Interdepartmental Centre and INSTM Reference Centre, Università degli Studi di Torino, Via Pietro Giuria 7, 10125 -
Torino, Italy
cICxT Interdepartmental Centre, Università degli Studi di Torino, Lungo Dora Siena 100, 10153 - Torino, Italy
A R T I C L E I N F O
A B S T R A C T
This work provides the synthesis, structural characterization, electrochemical and photophysical
features, as well as the application in light-emitting electrochemical cells (LECs) of a novel small
molecule belonging to the pyridilimidazo[1,5-a]pyridine family, namely 3-(2-methoxyphenyl)-5-
methyl-1-(6-methylpyridin-2-yl)H-imidazo[1,5-a]pyridine (Me-impy). This compound shows a
low-cost and facile synthesis, excellent redox properties, and a high photoluminescence quantum
yield (Φ) of 0.4 associated to a blue emission (~436 nm) in solution. Despite these appealing
features, the emission in solid state is red-shifted to the yellow region, owing to its prone
aggregation character. Additionally, the electroluminescence response shows a broader and red-
shifted emission even upon dilution of Me-impy thin films with polymethyl methacrylate (PMMA).
Herein, we report an in-depth study on the aggregation features and its impact on the
electroluminescence response of Me-impy, providing relevant information to design small-
molecule based LECs.
Article history:
Received 00 December 00
Received in revised form 00 January 00
Accepted 00 February 00
Keywords: light-emitting
electrochemical cells, blue-emission,
small molecules, pyridilimidazo[1,5-
a]pyridine,aggregation
shaped substrates based on metallic and/or semiconducting materials and
ii) new low-cost, easily synthetisable, and sustainable luminescent
materials spanning the whole visible range.[1–3,23] As far as the first
aspect is concerned, LECs fabricated with industry relevant based
1. Introduction
Light-emitting electrochemical cells (LECs) are regarded as one of
the most promising thin-film lighting sources, as they combine high
efficiency, a simple architecture with air stable electrodes, and a low-cost,
solution based up-scalable fabrication process.[1–4] In short, LECs are a
single-layer electroluminescent device consisting of a mixture of a
luminescent material and an ionic electrolyte. As such, the main
differences from the well-established organic light-emitting diode
(OLED) technology is the control of the charge injection using mobile
ions.[5] Upon biasing the device, the ion redistribution towards the
electrode interface assists the electrochemical doping forming a p-i-n
junction structure.[3,6–13] This particular operational mode leads to a
high tolerance towards i) working function of the electrodes paving the
way of using air-stable cathodes, such as Ag or Al, ii) different
thicknesses and minor defects of the active layer, iii) using low-cost and
up-scalable solution based techniques, and iv) applying a huge palette of
emitters, such as polymers, ionic transition metal complexes, small
molecules, quantum dots, and perovskites.[2,14–22]
techniques, such as inkjet printing,[24] roll-to-roll-coating,[25] spray-
coating,[26] and slot-die coating,[25] have been reported, as well as
wearable fiber-shaped devices.[27,28] Regarding the new generation of
emitters, small molecules have recently attracted much attention, due to i)
their wide variety using easily modifiable scaffolds, ii) their emission
covering the whole visible range with high Φ values that are not subjected
to ambient quenching, iii) their stable electrochemical and thermal
features, iv) their easy processability and high stability in solution, v) their
good carrier mobilities, and vi) the presence of a thermally activated
delayed fluorescence (TADF) behavior.[3,29–34]
Numerous works have been reported in the last few years regarding
various families of small molecules for LECs (hereafter abbreviated as
SM-LECs), such as ionic fluorine derivatives, phenanthroimidazoles,
borazines, carbazoles, benzodithiazoles, pyrene derivatives, porphyrins,
pentacenes, and cyanines;[16,29,35–47] covering all the visible spectrum
including white emission.[14,15] Concerning blue SM-LECs, only three
main families, namely ionic fluorine derivatives, imidazoles and
borazines, have been tested.[29,31,35–37,39,40,48,49] Moderate
The future advances in the field head into two directions, namely i)
the development of new device architectures, such as flexible and/or 3D
* Corresponding author.
E-mail address: claudia.barolo@unito.it, ruben.costa@imdea.org,

efficiencies of 0.18 cd/A were reported by Choe et al., along with a
maximum luminance of 711 cd/m2 under L-I-V assays.[39] However,
stability still remains a bottleneck, highlighting that many efforts still have
to be done in the field.
operating frequency 400 MHz), with chemical shifts referenced to residual
protons of the solvent. The following abbreviations are used: s, singlet; d,
doublet; t, triplet; m, multiplet. Mass spectra were recorded using an XCT
PLUS electrospray ionisation - ion trap (ESI - IT) mass spectrometer
(Agilent Italy, Milan).
In order to widen the horizon of candidates for blue SM-LECs, we
focused our attention on a pyridilimidazo[1,5-a]pyridine (impy), namely
3-(2-methoxyphenyl)-5-methyl-1-(6-methylpyridin-2-yl)H-imidazo[1,5-
a]pyridine), referred to as Me-impy in the work at hand – Figure 1.
Pyridilimidazo[1,5-a]pyridines have found applications mainly as diamine
(N^N) ligands in a large variety of fields, including organic thin-layer
field effect transistors (OFETs),[50] LEDs,[51] and OLEDs.[52] The wide
interest is fueled by i) low-cost, facile, and high-yield synthesis, ii) blue
emission centered around 430 nm with high  values, iii) excellent
solution stability, and iv) reversible electrochemical features.[53–57]
Although this family of blue emitters can be, at a first glance, regarded as
appealing candidates for SM-LECs, their electroluminescent features in
LECs have been counterintuitive up to date.[58] For instance, LECs based
on a blue emitting heteroleptic [Cu(impy)(POP)]PF6 complex, in which
POP is bis(2-(diphenylphosphanyl)phenyl) ether – Figure 1, showed a
yellow electroluminescent response due to the absence of an effective
TADF process, leaving the triplet excited state as the only radiative
deactivation pathway under electrical stimuli.[58] This is, indeed, rather
surprising in this type of emitters.[34,59–62]
2,2’-Dimethyldipyridilketone.
In
a 50 mL round-bottomed flask containing 15 mL dry
tetrahydrofuran solution of 2-bromopyridine (0.79 g, 5 mmol), 3 mmol
(0.19 g) of n-butyl lithium (2.5 M in hexane) was added over 20 min at –
78°C under N2 and the resulting mixture was stirred at –78°C for 5
minutes. The addition of 10 mmol (1.18 g) of diethyl carbonate followed.
The mixture was warmed to room temperature and quenched with HCl
10% until acidic. The mixture was then basified with a saturated solution
of Na2CO3 in water and extracted 3 times with dichloromethane (DCM),
anhydrificated and dried, giving yield to a yellow/orange oil. The oil was
purified via flash column chromatography using a mixture of DCM and
methanol (MeOH) in a ratio of 96:4, giving yield to the desired 2,2’-
Dimethyldipyridilketone. 1H-NMR (200 MHz, CDCl3): δ 2.62 (s, 6H),
7.33 (d, J=7.6, 2H), 7.74 (m, 2H), 7.79 (d, J=7.6, 2H) ppm.13C-NMR (50
MHz, CDCl3): δ 24.6, 123.0, 126.1, 136.6, 153.6, 158.3, 193.3 ppm. m/z
(ESI+) for C13H13N2O calcd 213.10 [M + H]+, found 212.98 [M + H]+.
3-(2-methoxyphenyl)-5-methyl-1-(6-methylpyridin-2-yl)H-imidazo[1,5-
a]pyridine
All of the aforementioned has encouraged us to further investigate the
inherent electroluminescent properties of Me-impy. Despite the excellent
blue emitting features, yellowish LECs were obtained due to its prone
aggregation behavior in thin films. Strikingly enough, the best performing
devices with Me-impy showed, however, similar figures-of-merit that
devices prepared with either copper(I) or iridium(III) complexes bearing
Me-impy as ancirally N^N ligand – i.e., [Ir(ppy)2(Me-impy)]PF6 where is
ppy is phenylpyridine and [Cu(impy)(POP)]PF6 as shown in Figure 1. As
such, this work contextualizes the unforeseen electroluminescent behavior
of pyridilimidazo[1,5-a]pyridine emitters employed in LECs, providing
relevant information to develop new candidates for SM-LECs.
3 mmol of 2,2’-dimethyldipyridilkethone (0.65 g) was mixed with 2-
methoxybenzaldehyde (0.61 g, 4.5 mmol) and ammonium acetate
NH4OAc (1.15 g, 15 mmol) in glacial acetic acid HOAc (15 mL). The
solution was stirred at 110 °C under inert atmosphere. After 5 h, the
reaction mixture was cooled to room temperature and the acetic acid was
removed by evaporation under reduced pressure. The obtained solid was
dissolved in a saturated aqueous solution of Na2CO3 and the mixture
extracted with DCM. The organic layer was separated, dried and the
solvent evaporated under vacuum. The obtained crude product (yellow-
orange solid, 0.91 g, yield 93%) was purified via column chromatography
1
on silica gel (DCM:MeOH 98:2). H NMR (200 MHz, Acetone-d6) 2.15
(s, 3H), 2.58 (s, 3H), 3.75 (s, 3H), 6.46 (d, J=6.9 Hz, 1H), 6.88 (dd, J=6.5,
2.7, 1H), 6.97 (d, J =9.1 Hz, 1H) 7.03-7.13(m, 2H), 7.49-7.60 (m, 3H)
7.97 (d, J = 9.6 Hz, 1H), δ 8.79 (d, J=9.8 Hz, 1H) ppm. 13C-NMR (50
MHz, Acetone-d6): δ 19.76, 20.28, 55.72, 111.40, 111.74, 117.20, 120.05,
124.72, 129.95, 131.85, 131.92, 133.52, 134.68, 136.45, 137.17, 155.96,
157.95, 159.77 ppm. m/z (ESI+) for C21H20N3O calcd 330.16 [M + H]+,
found 330.21 [M + H]+.
Figure 1. Chemical structures of Me-impy (left), [Ir(ppy)2(Me-impy)]+
(middle), and [Cu(impy)(POP)]+ (right).
[Ir(impy)(ppy)2]PF6
The iridium(III) complex [Ir(impy)(ppy)2]PF6 was synthesized following
standard procedures reported in literature.[63–65] 1H-NMR (400 MHz,
Acetone-d6): δ 2.42 (s, 3H), 2.55 (s, 3H), 3.58 (s, 3H), 5.64 (d, J=7.25,
1H), 6.17 (d, J=7.52, 1H), 6.32 (d, J=7.38, 1H), 6.38 (t, J=7.38, 1H), 6.45
(d, J=7.46, 1H), 6.52 (m, 2H), 6.68 (m, 2H), 6.82 (m, 2H), 7.15 (m, 2H),
7.21 (m, 2H), 7.30 (d, J=8.86, 1H), 7.34 (3, 1H), 7.67 (d, J=8.06, 1H),
7.86 (d, J=6.71, 1H), 7.96 (t, J=8.19, 1H), 7.05 (m, 3H), 8.34 (d, J=8.19,
1H), 8.40 (d, J=9.40, 1H), 8.47 (d, J=5.50, 1H) ppm. m/z (ESI+) for
C43H35N5OIr calcd 830.25 [M]+, found 830.32 [M]+. Elemental analysis
2. Experimental details
2.1 Synthesis
All chemicals were purchased from chemical suppliers and used without
further purification. All analytical reagent grade solvents were purified by
distillation. All reactions were carried out under inert nitrogen atmosphere
using standard vacuum lines techniques.1H- and 13C-NMR spectra were
recorded on a Bruker Avance 200 spectrometer (1H-NMR operating
frequency 200 MHz) and on a JEOL ECP 400 spectrometer (1H NMR

Electroluminescence spectra were recorded using the above mentioned
spectrophotometer.
calcd (%) for C43H35F6IrN5OP: C 52.97, H 3.62, N 7.18; found: C
52.49, H 3.89, N 6.93.
3. Results and discussion
2.2 Spectroscopic, electrochemical and morphology characterization
Steady-state absorption spectra were recorded with a Perkin Elmer
3.1 Synthesis
The synthesis of Me-impy was performed in a two-step synthetic
protocol reported elsewhere (Figure 2).[53,65] Its purity was confirmed
by mass spectrometry, 1H-NMR, and 13C-NMR.
Lambda 35. Steady-state emission spectra were recorded with
a
Fluoromax spectrometer from HORIBA Jobin Yvon IBH by using
fluorescence and phosphorescence mode (measurement after 10 ns).
Atomic Force Microscope (AFM) assays were performed with VEECO
DIMENSION 5000 with a NanoScope V probe head and the Gwyddion
evaluation software. Cyclic voltammetry was performed in acetonitrile
(MeCN) solution, using the redox couple Cp2Fe/Cp2Fe+ as internal
reference (E1/2=+0.40 V),
a
solution of tetrabuthylammonium
hexafluorophosphate 0.1 M as electrolyte and a glassy/carbon electrode as
working electrode. The scan speed was of 100 mV/s.
Figure 2. Two step synthesis of Me-impy. (3-(2-methoxyphenyl)-5-
methyl-1-(6-methylpyridin-2-yl)H-imidazo[1,5-a]pyridine).
2.3 Device Fabrication and analysis
3.2 Electrochemical characterization
ITO substrates were purchased from Naranjo Substrates with an ITO
thickness of 130 nm. They were firstly cleaned with detergent, water,
ethanol, and propan-2-ol as solvents in an ultrasonic bath (frequency 37-
70 Hz, 30°-40°C) for 15 min each. Afterwards, the slides were dried with
N2 gas and put in an UV-ozone cleaner for 8 min. A solution of poly(3,4-
As shown in Figure 3 and summarized in Table 1, Me-impy shows two
reversible oxidation waves located at 0.44 V and 0.85 V, while only one
quasi-reversible reduction wave centred at –1.48 V was noted. Both
oxidation and reduction are ascribed to the benzoimidazolic unit as
supported by previously theoretical studies.[53,54] Thus, the charge and
transport processes might be operative under device operative conditions.
ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)
was
filtered, sonicated and mixed with propan-2-ol in a ratio of 3:1. 50 μL of
the aforementioned solution was dropped onto the ITO while the spin
coater was rotating at a speed of 1500 rpm for 60 s giving rise to a
thickness of 70 nm after drying the covered substrates for 10 minutes at
120 °C. PEDOT:PSS (Clevios PAl4083) was purchased from Heraeus.
The slides were then stored in a N2-glovebox (O2 level <0.1 ppm, H2O
level <0.5 ppm). Me-impy was dissolved in acetonitrile with
a
concentration of 15 mg/mL. It was stirred in a closed vial for 30 minutes
and, if necessary, it was filtered. In order to obtain a thickness of around
100 nm of active layer, 75 μL of the solution was spread onto the
substrate that was later on spin coated with a speed of 1000 rpm for 50
seconds. The coated slides were then transferred into N2-glovebox and
dried on a hotplate for 30 minutes at 90°C. The active layer was deposited
by spin-coating from a tetrahydrofuran (THF) solution of the Me-impy. In
order to furnish the necessary ion mobility, a TMPE: LiOTf matrix was
added, in the following ratio: Me-impy:TMPE:LiOTf 1:0.15:0.06. The
thickness of the active layer was 100 nm. Two types of active layer were
characterized, in which the ratio wt% Me-impy:PMMA was 1:0 (named
100 in this work) or 0.2:0.8 (20). The active layer of the LEC was
prepared from an acetonitrile solution of the Ir(III) complex with the ionic
liquid ethyl methyl imidazolium hexafluorophospate ([EMIM][PF6]) in
ratio 1:1, reaching a thickness of 120 nm. Once the active layer was
deposited, the samples were transferred into an inert atmosphere glovebox
(<0.1 ppm O2 and H2O, Innovative Technology). Aluminum cathode
electrode (90 nm) was thermally evaporated using a shadow mask under
high vacuum (<1 x 10–6 mbar) using an Angstrom Covap evaporator
integrated into the inert atmosphere glovebox. Time dependence of
luminance, voltage, and current was measured by applying constant
and/or pulsed voltage and current by monitoring the desired parameters
simultaneously by using Avantes spectrophotometer (Avaspec-
ULS2048L-USB2) in conjunction with a calibrated integrated sphere
Avasphere 30-Irrad and Botest OLT OLED Lifetime-Test System.
Figure 3. Cyclic voltammogram of Me-impy in MeCN.
3.3 Spectroscopic characterization
Me-impy presents three main absorption peaks in MeCN located at
286, 330, and 367 nm associated to a molar extinction coefficient of
30,000 mol–1 cm–1  Figure 4 and Table 1. These values are in line with
the previously works on the family of emitters bearing the imidazo[1,5-
a]pyridine moiety.[53,54,63,66–68] Concerning the photoluminescence
features, Me-impy shows an emission maxima at 440 nm flanked by two
shoulders at 418 and 463 nm  Figure 4. The  is around 0.4 in MeCN,
underlying its potential as candidate for lighting applications. The
photoluminescence parameters above are similar to those in powder –
Figures 4 and 5 as well as Table 1. The emission spectrum in powder
consists of a broader and well-structured emission band featuring two
maxima at 435 and 456 nm followed by two less intense bands at 489 and

556 nm. This indicates the presence of aggregates in the powder sample.
maxima at 417, 432 and 453 nm (shoulder) was noted. Figure 5.
Upon cooling up to 77 K, a more defined vibrational spectrum with three
Table 1. Spectroscopic and electrochemical features of Me-impy.
Absorption
Solutiona
Emission
Powder
λmax(nm) λmax(nm)
Electrochemistry
Eox (V)b Ered (V)b
Thin film
Solution
Thin film
Φa
λmax(nm)
λmax(nm)
298 K
λmax(nm)
298K
λmax(nm)
[ε (105 Lmol-1cm-1)]
298 K
77 K
329, 370,
404
(shoulder)
418, 430,
454
450 (lower),
559
+0.44(r)
-1.46(r)
+0.85(r)
286 [3], 335[2.25], 370[2.55]
436, 418, 464
435, 456
0.39
a Measured in MeCN. Measured with integrating sphere. λexc=370 nm. b. qr=quasi reversible, r=reversible.
Next, we turned our attention to investigate the spectroscopic features
of thin-films (100 nm) used for device fabrication, that is, a blend of Me-
impy with ionic polyelectrolyte mixture
– i.e. trimethylolpropane
ethoxylate (TMPE): lithium triflate (LiOTf) ionic matrix in ratio
1:0.15:0.06 with Me-impy as 1. In contrast to both powder and solution
emissions, these films show a yellowish emission with two maxima
centered at 450 and 559 nm and x/y CIE of 0.34/0.40 – Figure 6. While
the high-energy emission peak is ascribed to the monomeric species of
Me-impy, the latter should be ascribed to the presence of aggregates. This
was confirmed by AFM characterization of the films. As shown in Figure
6, the presence of aggregates with a size of ~200 nm and a height of 6-8
nm is clearly observed. The nature of these aggregates can be ascribed to
π-π stacking interactions– Figure 6. The stacking angles and the
tridimensional arrangement are so far unknown and need further
corroboration. As a second experiment, we prepared diluted films with
different concentrations of polymethyl methacrylate or PMMA – i.e.,
100:0, 80:20, 50:50, 20:80 wt.% Me-impy:PMMA hereafter abbreviated
as 100, 80, 50 and 20 according to the wt% of Me-impy in the films – to
reduce the formation of aggregates in the thin-film above
– see
experimental section for more details. Please notice that both PMMA and
the ionic polyelectrolyte matrix show featureless absorption or emission
spectra. As shown in Figure 7, the absorption features of 100 films consist
of two well-defined peaks at 330 nm and 366 nm in concert with two low-
energy shoulders at around 400-425 nm. The latter are progressively
disappearing upon diluting the emitter into the PMMA matrix. Indeed, the
absorption spectrum of 20 and 50 films involve a well-defined band
peaking at 310 nm, while spanning the range from 300 to 400 nm. Much
more interesting is the photoluminescence features of the films above. In
particular, while 20 and 50 films show performances of the evaporated
devices, measured at the pulsed current of 15 mA approximately the same
blue-emission features as in solution – Figures 4 and 7, 80 films show a
broad emission with two maxima 450 and 535 nm related to the monomer
and aggregated species. Overall, both experiments indicate that the
emission of thin films is ruled by both monomer and aggregate species.
Figure 4. Absorption (top) and photoluminescence (bottom) of Me-impy
in MeCN (black line) and powder (blue line).
Figure 5. Photoluminescence spectrum of Me-impy as powder at room
temperature (black line) and at 77 K (blue line).

Indeed, we also prepared device with 20 films to investigate the
electroluminescence response that consisted of a featureless band centered
at 480 nm with x/y CIE color coordinates of 0.31/0.41 – Figure 8 and
Table 2. Thus the 20 LEC still does not show any blue response,
highlighting the relevance of the presence of aggregates under electrical
stimuli of 100 LECs.
Figure 6. Photoluminescence spectrum (top), a schematic draw of the
aggregates (middle) and AFM image (bottom) of 100 thin films.
3.4 Electroluminescent characterization of Me-impy LECs
Next, we decided to fabricate a series of LECs with 100 films. Prior to the
deposition of the active layer,
a
first layer of poly (3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (70 nm)
was deposited on top of a patterned ITO substrate. The devices were
finalized by the physical vapor deposition of 90 nm of aluminum as
cathode. The devices were driven at the pulsed current of 15 mA, using a
1000 Hz block-wave and a 50% duty cycle – see experimental section for
more details and Table 2 summarizing the device figures-of-merit. These
devices show the typical LEC behavior with an initial high voltage of 4.5
V that decays to reach a constant value at 4.3 V. As well, the luminance
behavior shows sub-second turn-on times and exponential decay with
time. The maximum luminance reached low values of 2.5 cd/m2
associated to efficiencies of 0.01 cd/A. The electroluminescent response
consists of a featureless band centered at 560 nm with a lack of
contribution at the high-energy region of the spectrum – Figure 8. The x/y
CIE color coordinates of these devices are 0.42/0.50 indicating that the
electroluminescence emission corresponds to orange-emitting devices.
This indicates that aggregates emission governs the electroluminescence
mechanism, even though both aggregates and monomer species are active
under photoexcitate stimuli-vide supra. Quite likely, the aggregates act as
electron-hole recombination centers where the emission is produced.

Figure 7. a) Absorption and b) photoluminescence of the thin films. c)
CIE diagram in which the x/y photoluminescence coordinates of 20 (pink
dot ), 50 (blue dot ), 80 ( red dot ) and 100 ( black dot) are displayed.
d)The deposited thin films on ITOs (from left to right, 100, 80, 50 and 20)
under a UV lamp (310 nm, 8 W).
Table 2. LEC performances of the evaporated devices, measured at the
pulsed current of 15 mA.
Turn
on
time
Luminance
Lifetime Efficacy
x/y
CIE
max
Figure 8. a) Normalized electroluminescence spectra of 20 and 100
devices under the driving current of 15 mA. b) Average Voltage and
Luminance versus Time curves under the driving current of 15 mA for the
devices 20 (empty squares) and 100 (filled squares). c) CIE diagram in
which the x/y electroluminescence coordinates of 20 (pink dot), and 100
(black dot) are displayed.
cd/m2
coord.
s
s
cd/A
[λmax (nm)]
20
0.3 [540]
2.5 [556]
0
0
10
0.001
0.01
0.31/0.41
0.42/0.50
100
300
Ir(Me-
impy)
2.5 [590]
0
900
0.01
0.48/0.52
3.5 Direct comparison of using Me-impy LECs with those with impy based
coordination complexes
In order to contextualize the use of the Me-impy for LECs, we have
synthesized the Ir(III) complex [Ir(ppy)2(N^N)2]PF6 (Ir-impy) and have
prepared LECs with this emitter – see experimental section for details.
Noteworthy, although Ir(impy) has been reported in the literature, its
application for lighting devices has not been explored.[63–65] In brief, the
absorption spectrum of Ir(-impy) shows an intense UV band located at
260 nm that is ascribed to mixed ligand-centered and metal-to-ligand
1
1
charge transfer ( LC and MLCT, respectively) transitions involving all
ligands and a shoulder at 356 nm is assigned to Ir-ppy → ppy singlet
transitions – Figure 9.[63] The emission features involves a highly
structured emission band that is ascribed to the LC phosphorescence
nature - Figure 9.[63] This is confirmed by the low  (0.05 in degassed
acetonitrile solution).

Figure 9. Absorption (top) and photoluminescence(bottom) spectra of
Ir(Me-impy) in dichloromethane degassed solution.
Figure 10. Electroluminescence spectrum of Ir(Me-impy)(top), recorded
under the driving current of 15 mA; Average voltage and Luminance
versus Time of an Ir(Me-impy) device (bottom), at the driving current of
15 mA.
Next, the same device architecture and driving conditions were
followed to investigate the electroluminescent response of the Ir(impy) in
LECs – see experimental section for details. The device performances are
shown in Figure 10 and the device figures-of-merit are summarized in
Table 2. The EL devices show the typical LEC features with a decrease of
the voltage after the switch-on until it reaches a steady state at 5.25 V -
Figure 9; the voltage remains approximately constant for more than 4
hours, thereby indicating that the emitter is not degrading.[3,7] However,
both the maximum luminance (2.4 cd/m2) and lifetime are quite low
compared to other Ir(III) complexes – Figure 10.[69,70] More strikingly,
the electroluminescence spectrum also consists of a broad shapeless
emission band centered at 590 nm that corresponds to a yellow-emitting
device (x/y color coordinates of 0.48/0.52) – Figure 10. This performance
is comparable with those of the Me-impy, underlining that in this case the
use of a metal core is of no use to improve the device performances. This
also holds if we directly compare the Me-impy performance with those
recently published for LECs with copper(I) complexes bearing this ligand
– i.e., luminance of 13.9 cd/m2, efficacy of 0.028 cd/A, and stability of
0.28 h for a yellow emitting device.[58]
Conclusions
A small molecule belonging to the pyrydilimidazo[1,5-a]pyridine
family has been for the first time applied in an LEC. This compound is a
blue emitter in solution, but it is prone to aggregate in thin films. The
aggregation is likely related to a π-π stacking. Here, a deeper study on the
tridimensional arrangement needs to be performed to further shed light
onto the nature of the aggregates. Although whitish emission is noted per
photo-excitation stimuli, the electroluminescence response is described as
purely yellow since the aggregates species act as hole/electron
recombination species. Even though the EL response is not yet efficient,
we contextualize its value with a direct comparison of LECs with Me-
impy and Ir(Me-impy) emitters. Both devices feature similar
performances, underlining that the use of the expensive metal is in this
case of no aid to the enhancement of the figures-of-merit. To conclude,
this work, while deeply analyzing the photophysical feature of
a
pyridilimidazo [1,5-a]pyridine, provides relevant information to further
study this family of compounds, towards stable and efficient blue LECs.
Acknowledgments
E.F. and R.D.C. acknowledge the program “Ayudas para la atracción de
talento investigador – Modalidad 1 of the Consejería de Educación,
Juventud y Deporte – Comunidad de Madrid with the reference number
2016-T1/IND-1463”. The authors acknowledge Marco Giordano for his
help with the synthesis.
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LECs
solution
1.0
0.8
0.6
0.4
0.2
0.0
400
500
600
700
Wavelength [nm]