A highly efficient solution and solid state ESIPT fluorophore and its OLED application

Article abstract of DOI:10.1039/d0nj05600f

We present herein the synthesis and photophysics of 2,2′-bipyridine-3,3′-diol-5,5′-dicarboxylic acid ethyl ester (BP(OH)₂DCEt₂), an excited state intramolecular proton transfer (ESIPT)-based fluorophore featuring two identical intramolecular hydrogen bonds. BP(OH)₂DCEt₂emits efficiently not only in solution, including protic solvents (λ_em= 521 nm,Φ_f= 40 to 75%), but also in crystalline state (λ_em= 530 nm,Φ_f= 51%). In addition, its saponified form (Na₂BP(OH)₂DC) is highly fluorescent in water (λ_em= 490 nm,Φ_f= 51%). Finally, the good electroluminescence performance of BP(OH)₂DCEt₂is also demonstrated in an OLED device.

Full text of DOI:10.1039/d0nj05600f

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A highly efficient solution and solid state ESIPT
fluorophore and its OLED application†
Cite this: New J. Chem., 2021,
45, 3014
a
a
Virgile Trannoy,‡a Anne Leaustic,a Sophie Gadan,
Regis Guillot,
´
´
b
b
cd
Clemence Allain,
Gilles Clavier,
Sandra Mazerat,a Bernard Geffroy
*
and
´
a
Pei Yu
*
We present herein the synthesis and photophysics of 2,20-bipyridine-3,30-diol-5,50-dicarboxylic acid
ethyl ester (BP(OH)2DCEt2), an excited state intramolecular proton transfer (ESIPT)-based fluorophore
featuring two identical intramolecular hydrogen bonds. BP(OH)2DCEt2 emits efficiently not only in solution,
including protic solvents (lem = 521 nm, Ff = 40 to 75%), but also in crystalline state (lem = 530 nm, Ff = 51%).
In addition, its saponified form (Na2BP(OH)2DC) is highly fluorescent in water (lem = 490 nm, Ff = 51%).
Finally, the good electroluminescence performance of BP(OH)2DCEt2 is also demonstrated in an OLED
device.
Received 16th November 2020,
Accepted 6th January 2021
DOI: 10.1039/d0nj05600f
rsc.li/njc
lasing,1,13–16 and OLEDs.1,17–20 However, ESIPT dyes generally
suffer from low emission efficiency, impeding their applications.
Introduction
Organic chromophores with intramolecular hydrogen bonds Synthetic efforts, intended to increase their fluorescence quan-
are prone to undergo, upon light absorption, an excited-State tum yield, have mainly focused on 2-(2-hydroxyphenyl)-benzazole
Intramolecular Proton-Transfer (ESIPT), an ultrafast photochemical scaffold (HBX), such as HBI (X = imidazole), HBO (X = oxazole)
process that leads to a new excited state with an electronic structure and HBT (X = thiazole) and produced some bright ESIPT emitters
quite different from that of the original one and gives rise thereby to in solution or/and in the solid state.5,21,22 Nevertheless, their
a unique four-level photo-cycle, as illustrated in Scheme 1 in the number is quite limited and their quantum yields are often low
case of an archetypal molecule of this family, 2-(2-hydroxyphenyl)- in protic solvents due to competing hydrogen bond formation
benzothiazole (HBT).
with the solvents. In this context, design of new ESIPT dyes with
As a result, ESIPT chromophores display remarkable fluores- high fluorescence efficiency both in solution and in the solid
cence properties, with either a low energy K* emission with a state is of high interest for expanding their structural diversity
large Stokes shift or a dual and tunable E* and K* emission.1–4 and applications.
Moreover, unlike most organic fluorophores, they also tend to be
Recently, we have prepared and characterized stable, porous
a more efficient solid state emitter as self-quenching is mini- and highly fluorescent MIL-140c type MOFs by using a newly
mized due to the absence of spectral overlap between absorption designed symmetric ESIPT linker, 2,20-bipyridine-3,30-diol-5,50-
and emission.5 All these attractive features make ESIPT chromo- dicarboxylic acid (H2BP(OH)2DC, Scheme 2).23
phores prime candidates for various applications such as sensing/
imaging,1,6–11 drug-delivery with real-time fluorescence monitoring,12
a
´
´
´
Universite Paris-Saclay, CNRS, Institut de Chimie Moleculaire et des Materiaux
d’Orsay, 91405, Orsay, France
b
c
´
Universite Paris-Saclay, ENS Paris-Saclay, CNRS, PPSM, 91190 Gif-sur-Yvette,
France
´
Universite Paris-Saclay, CEA, CNRS, NIMBE, LICSEN, 91191, Gif-sur-Yvette,
France
d LPICM, CNRS, Ecole Polytechnique, Institut Polytechnique de Paris,
route de Saclay, 91128 Palaiseau, France
† Electronic supplementary information (ESI) available: Detailed X-ray crystal-
lographic data, 1H NMR, 13C NMR and fluorescence spectra and AFM images.
CCDC 2004784. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d0nj05600f
‡ Present address: Instituto de Ciencia Molecular (ICMol), Universidad de
´
Scheme 1 Four-level photocycle of HBT.
Valencia, c/Catedratico Jose Beltran 2, 46980 Paterna, Spain.
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Scheme 3 Synthetic route of BP(OH)2DCEt2: (a) EtOH, SOCl2/reflux; (b)
aqueous Na2CO3, I2 at RT; and (c) Zn, LiCl, NiCl2ꢁ6H20 (cat)/DMF at RT and
then at 50 1C.
Scheme 2 The ground and emitting state of BP(OH)2 (top); BP(OH)2DCEt2,
its diacid and disodium forms (bottom).
The target linker derives from 2,20-bipyridine-3,30-diol
(BP(OH)2, Scheme 2), a molecule featuring two intramolecular
hydrogen bonds and well-documented for its intense double-
proton-transfer based fluorescence and applications.24–32 More
importantly, even in protic solvents, such as MeOH and EtOH,
in which most ESIPT dyes are either not or only weakly
fluorescent, BP(OH)2 keeps a more than decent quantum yield
of 10 and 18%, respectively.26 However, to our surprise, very few
fluorescent derivatives of BP(OH)2 have been reported in the
literature.33–38 In this work, we report the synthesis, crystal
structure and photophysics of a highly fluorescent and functio-
nalized BP(OH)2 derivative, namely 2,20-bipyridine-3,30-diol-
5,50-dicarboxylic acid ethyl ester (BP(OH)2DCEt2, (Scheme 2),
precursor of H2BP(OH)2DC. The new compound substantially
outperforms the parent BP(OH)2 in solution, including in protic
solvents, and emits efficiently in the crystalline state. Finally,
good electroluminescence performance of BP(OH)2DCEt2 is
also demonstrated in an OLED device.
Results and discussion
Synthesis and characterizations
Starting from the commercially available 5-hydroxy-3-
pyridinecarboxylate, BP(OH)2DCEt2 was synthesized according to
Scheme 3 in three straightforward steps with good overall yield.
By adapting
a phosphine-free nickel catalytic system
Fig. 1 Molecular structure of BP(OH)2DCEt2 (top); p stacking (middle);
and angle between the long axis of two adjacent stacked molecules
(bottom).
reported for the synthesis of symmetric and dissymmetric
bipyridines,39 5-hydroxy-6-iodo-niconic acid ethyl ester was
reductively homo-coupled to give BP(OH)2DCEt2 in good yield.
BP(OH)2DCEt2 was fully characterized using standard techniques
as well as by single crystal analysis (more details can be found in pyridinic rings of the central BP(OH)2 moiety (Fig. 1, top). On the
the ESI†). The molecule crystallizes in the C2/c space group, and other hand, there is no intermolecular hydrogen bonding nor
its molecular structure is shown in Fig. 1.
other close contacts between adjacent molecules except a signifi-
Similar to the parent BP(OH)2 and its 6,60-dimethyl substituted cant p stacking that runs along the [101] crystallographic direc-
derivative,40,41 BP(OH)2DCEt2 adopts a planar geometry due to tion, with a mean interlayer distance of ca. 3.25 Å (Fig. 1, middle).
the two strong and identical hydrogen bonds between the two Note that the two adjacent molecules are not co-facial but more in
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Table
1
Photophysical data of BP(OH)2DCEt2, Na2BP(OH)2DC and
a slip-stacking mode with, moreover, an angle of 481 between the
long axis of the molecules (Fig. 1, bottom).
BP(OH)2 in different solvents
lem
e (Mꢀ1 cmꢀ1) (nm) FF
Fluorescence
Solv.
lab (nm)
In organic solvents. The emission properties of BP(OH)2DCEt2
were investigated using steady-state fluorescence at room tem-
perature in four different and aerated solvents. Its absorption
and emission spectra are shown in Fig. 2 and the main spectro-
scopic data listed in Table 1 along with available data of BP(OH)2
for comparison.
BP(OH)2DCEt2 mainly absorbs in the UV region, with a
strong and rather structured low energy band located between
370 and 385 nm, while its emission is broad and centred at
521 nm. Both the absorption and emission spectra only slightly
vary with solvents of different polarity and proticity.
BP(OH)2DCEt2 C6H12
CH2Cl2
372, 389
372, 388
370, 385
370, 385
521 0.70
15 900
521 0.75
521 0.57
521 0.40
540 0.29
MeCN
MeOH
H2O + DMSO 370, 385
(3%)
C6H12
CH2Cl2
MeCN
MeOH
H2O
BP(OH)2
340–350
340–350
340–350
340–350
347, 403, 430
360, 426, 460
500 0.31a
500 0.21b
500 0.09c
480 0.10c
460 0.04c
490 0.51
Na2BP(OH)2DC H2O
a
b
c
Ref. 24. Ref. 27. Ref. 26.
In comparison with BP(OH)2, the major difference in
absorption is a redshift of ca. 30 nm for the low energy band
likely due to a more stabilized LUMO by the electron-
withdrawing ester groups, while its emission is red-shifted by
ca. 20 nm in aprotic solvents and ca. 40 nm in protic solvents
like MeOH (Table 1). However, the most remarkable difference
between the two is the quantum yield of emission. The new
emitter has a much higher efficiency in all solvents investigated
(Table 1). Particularly noteworthy is the still high quantum
yield of 40% in methanol, which is four times that reported for
BP(OH)2.26 The emission was also studied using time-resolved
fluorescence emission in dichloromethane. A single-exponential
decay was observed with a fluorescence lifetime of 4.64 ns, which
is superior to that of 3.2 ns reported for BP(OH)2 in the same
solvent.27 This emission is not sensitive to oxygen as the same
decay was obtained in degassed dichloromethane.
The reasons for such an enhancement in the fluorescence
efficiency are not yet clear. Due to the presence of an electron-
accepting ester group one could expect significant changes in
the strength of the intramolecular hydrogen bonds, which
directly affect the ESIPT process and its emission. Actually,
the measured chemical shift of the hydrogen atom engaged in
the hydrogen bond (d = 14.40 ppm) is very close to that reported
for BP(OH)2 (d = 14.54 ppm),35 indicating therefore a very
similar strength of the intramolecular hydrogen bonds within
the two fluorophores. This unusually small substitution effect
is, however, not unexpected since the enhancement of the
hydrogen donating ability of the hydroxyl group is offset at
least in part by a concomitant decrease in the hydrogen
accepting capability of the nitrogen atom located on the same
ring, leading thus to an overall limited impact on the hydrogen
bonding in symmetrically substituted BP(OH)2 derivatives.
One possible explanation to the observed fluorescence
enhancement could be a better ‘‘insulation’’ of the emitting
state from the solvent in BP(OH)2DCEt2 than that in BP(OH)2.
And this is best reflected in methanol where a blue shift in
emission, observed for BP(OH)2 (Table 1), is thought to be
related to some specific interactions of its excited state (DK*,
Scheme 2) through the formation of intermolecular hydrogen
bonds with protic solvents,27,29 while this is not the case for
BP(OH)2DCEt2 (Table 1) probably due to a different pattern of
intermolecular interactions between BP(OH)2DCEt2 and solvent
molecules.
In water. ESIPT dyes are generally either not or only weakly
fluorescent in water as the intramolecular hydrogen bonds of
the dye, mandatory for the ESIPT process, are disrupted or at
least weakened to a large extent by the formation of intermo-
lecular hydrogen bonds. The high quantum yield of
BP(OH)2DCEt2 in methanol suggests a better ESIPT emitter in
water than that of the weakly fluorescent BP(OH)2.26
As BP(OH)2DCEt2 is insoluble in water, dimethyl sulfoxide
(DMSO) was used as the co-solvent (3% in volume) to prepare
an aqueous solution of the dye for fluorescence characteriza-
tion studies. To our delight, the solution remains highly
fluorescent, with a slightly redshifted maximum at 540 nm
and a measured quantum yield still up to 29% (Table 1). In
addition, even if it is expected for ESIPT dyes, the high
quantum yield makes the new emitter potential candidate as
a sensitive pH probe. As a matter of fact, the fluorescence
underwent drastic changes when the pH was gradually raised from
6 to 12 (Fig. S1, ESI†), while the emission colour changed from
yellow-green to blue-cyan. Such changes are clearly associated
with a progressive deprotonation of the two OH functions of
BP(OH)2DCEt2. Using global analysis (ReactLabTM-EQUILIBRIA),
Fig. 2 Normalized absorption and emission spectra of BP(OH)2DCEt2 in
different solvents.
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Fig. 3 Absorption and emission spectra of Na2BP(OH)2DC in water
(6 ꢂ 10ꢀ6 M) at 25 1C.
Fig. 4 Fluorescence changes of Na2BP(OH)2DC versus pH in water at
25 1C.
the fluorescence responses could be satisfactorily fitted with two
proton dissociation constants of pKa = 8.1 and 11.3, respectively. form, H2BP(OH)2DC. Finally, analysis of the fluorescence
These pKa values are smaller than those (9.2, 12.4) reported for response vs. pH in the range from 5 to 10 allowed to determine
BP(OH)2,27 but consistent with the expected electron- the first proton dissociation constant (pKa) of the two OH
withdrawing effect of the ethyl ester group.
functions to be 8.0, similar to what is obtained for the ester
The dicarboxylic acid form of the new emitter, derivative BP(OH)2DCEt2.
H2BP(OH)2DC (Scheme 2), is not sufficiently soluble in water
In crystalline state. The emission of BP(OH)2DCEt2 in the
for fluorescence measurements. This very low solubility can solid state was investigated at room temperature by both
probably be attributed to the presence of multiple strong steady-state fluorescence spectroscopy with an integrating
intermolecular hydrogen bonds between neighbouring mole- sphere and time-resolved fluorescence measurements.
cules in the solid state. However, its disodium dicarboxylate
The emission spectrum of the pristine microcrystals is
form (Na2BP(OH)2DC, Scheme 2) is pretty soluble in water qualitatively very close to that in solution, with a maximum
owing to its twice negatively charged nature at neutral pH. Its centred around 530 nm and a measured quantum yield of 51%
absorption and emission spectra are shown in Fig. 3.
(Fig. S2, ESI†). Then, the decay was monitored using time-
The absorption spectrum of Na2BP(OH)2DC in water is resolved fluorescence measurements at three different emis-
characterized by an intense band at 360 nm and two others sion wavelengths: 510 nm (blue-edge), 530 nm (maximum) and
of much lower intensity located between 400 and 480 nm 580 nm (red-edge). The three decays could be well fitted using a
(Table 1). This spectrum resembles qualitatively that of two-exponential model with decay times of 2.06 ns and 4.74 ns.
BP(OH)2 in water27,29 and by analogy, the two bands at lower The long decay time, making up to ca. 95% of the total fluores-
energy can be reasonably assigned to the zwitterionic diketo cence, is very close to the fluorescence lifetime measured in
form stabilized in the ground state by the formation of inter- dichloromethane (4.64 ns), while the short decay (ca. 5%) could
molecular hydrogen bonds with water molecules.
likely be ascribed to the surface and near-surface molecules in the
The emission of Na2BP(OH)2DC peaks at 490 nm and microcrystals which experience a different environment. The
represents a redshift of ca. 30 nm in comparison with that of comparison of the decays at different emission wavelengths shows
BP(OH)2 (Table 1). But the most remarkable feature of this that the relative contributions of the two decay times to the overall
ESIPT emission is its quantum yield of 51%, which is more fluorescence only slightly vary with the emission wavelength,
than ten times that of BP(OH)2 in water.26
Like BP(OH)2DCEt2, the fluorescence of Na2BP(OH)2DC is excimers or resonance energy transfer occurring in the solid.
which indicates that there are no excited-state processes such as
particularly sensitive to the pH of the solution, as shown in
Fig. 4.
The high solid-state quantum yield, comparable to those in
solution, is clearly due to the absence of the so called ‘‘Aggrega-
In the pH range between 7 and 10, the observed changes in tion Caused Quenching’’ (ACQ), generally encountered in the
fluorescence are linked to the progressive deprotonation of the solid state for organic dyes. Two reasons can be put forward.
two OH functions and suggest the generation of a new blue Firstly, resonance energy transfer, one of the main contributors
emitting species, corresponding probably to the triple to ACQ, is suppressed due to the absence of spectral overlap
negatively charged species after first proton removal. The rapid between the absorption and emission of the dye. Secondly, the
decrease in intensity of the initial fluorescence when lowering crystal structure has favourable intermolecular arrangements
pH below 5 is likely due to the combination of two effects: (1) (Fig. 1). Indeed, significant intermolecular interactions occur
partial breakdown of the intramolecular hydrogen-bonds; and only between molecules involved in a kind of p slip-stacking,
(2) the formation of the scarcely water-soluble dicarboxylic which is believed to enhance solid state emission.1
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Table 2 Thin film PL efficiency
Electroluminescence
The electroluminescence properties of BP(OH)2DCEt2 were
investigated to access its potential as an emitting dye in OLED
device the structure of which is shown in Scheme 4. The
emitting layer (EML) is composed of BP(OH)2DCEt2 molecules
either as neat film or host–guest system of 20 nm thick (see
experimental for details of OLED fabrication and characterization).
The solid-state photoluminescence yields (PL) of the emitting
layer are given in Table 2 and the PL spectrum is shown in Fig. 5
for a 70 nm thick neat film. The PL emission peak is centred at
536 nm with a FWHM around 60 nm. The PL yields of the neat
films are very high (B80%) showing the interest in
BP(OH)2DCEt2 molecules for OLED fabrication. For the guest–
host films, the PL yield increases with the concentration of the
BP(OH)2DCEt2 molecule in the host and particularly, with mCP
as the host material. At a 10% doping ratio, the PL efficiency is
higher with mCP as the host than with DPVBi as the host. This
could be explained by self-absorption or the particular morphology
of the film as discussed below.
Host
No
Doping ratio (%)
Film thickness (nm)
PL yielda (%)
100
100
20
70
77
85
mCP
1.5
4
10
20
20
20
22
20
54
DPVBi
0.7
4
8
20
20
20
20
25
31
35
32
10
a
PL measurement accuracy is 10%.
The electroluminescence (EL) performance of the OLED
devices is reported in Table 3 for the neat film and for the
guest–host systems as a function of the host material and the
doping ratio, and the EL spectra are shown in Fig. 6.
The EL spectra are similar to the PL spectrum of the neat
film (Fig. 5) showing that the EL emission is due to the
BP(OH)2DCEt2 molecule. For the neat film (100% doping ratio),
the devices show a short-circuit and therefore no light is
emitted. This result is explained by the crystallization of
BP(OH)2DCEt2 molecules after evaporation, as revealed by
AFM (Fig. S3, ESI†), and the diffusion of aluminum electrode
through the layer. When mCP is used as the host, the EL device
characteristics are almost constant from a doping ratio of 1.5%
to 10%. The external quantum efficiency (EQE), luminous effi-
ciency (cd Aꢀ1) and power efficiency (lm Wꢀ1) are 1.5 ꢃ 0.2%,
5.3 ꢃ 0.5 cd Aꢀ1 and 1.7 ꢃ 0.2 lm Wꢀ1 respectively. As shown in
Fig. 6a, the EL emission is from the BP(OH)2DCEt2 molecule with
a complete transfer from the host to the guest and the emission
peak position is stable at 540 nm. The CIE chromaticity coordi-
nates are slightly shifted by increasing the doping ratio.
Fig. 5 PL spectrum of a neat film of 70 nm thick (excitation at 405 nm).
(o1.8%), the device efficiency is improved by increasing the
doping ratio and the EL emission peak is shifted from 532 to
536 nm. As shown in Fig. 6b, the exciton transfer from the
DPVBi host to the guest molecule is not complete for such low
doping ratios and some light is coming from the DPVBi layer in
the 430–480 nm range. At a doping ratio 41.8%, the device
performance decreased until a doping ratio of 8% where no EL
emission was obtained. Note that at a doping ratio of 5.6%, the
EL peak is at the same position as that of the mCP host and the
CIE coordinates have almost reached the ones with mCP host.
Surprisingly, the EL emission stopped at a doping ratio of 8%
and 10% and the devices were short-circuited similarly to the
results reported previously for the neat emitting layer. By increas-
ing the doping ratio up to 8%, the guest–host system tends to
crystallize like for the neat film (Fig. S4, ESI†). For the best device
(1.4–1.8% doping ratio range), EQE, cd Aꢀ1 and lm Wꢀ1 are
2.3 ꢃ 0.2%, 7.5 ꢃ 0.5 cd Aꢀ1 and 2.6 ꢃ 0.2 lm Wꢀ1 respectively.
As shown in Fig. 7, the luminous efficiency is almost constant as
a function of the current density and the luminance is well
proportional to the current density. A luminous efficiency of 7.5
cd Aꢀ1 is a very good value for a pure fluorescent emitter.
When DPVBi is used as the host, the results are different as
reported in Table 3 and Fig. 6b. The device performance is
dependent on the doping rate with a maximum efficiency
reached at a doping ratio of 1.4–1.8%. At a low doping ratio
Conclusions
In summary, we have designed and fully characterized a new
ESIPT emitter derived from the well-known BP(OH)2. It is found
Scheme 4 OLED device structure and molecular materials used.
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Table 3 OLED device performance
Host
No
Doping ratio (%)
100
EQEa (%)
—
cd Aꢀ1 a
—
lm Wꢀ1 a
—
EL peak (nm)
—
Luminanceb (cd mꢀ2
—
)
CIE chromaticityb (x; y)
—
mCP
1.5
4
10
1.4
1.5
1.5
5.3
5.1
5.3
1.7
1.7
1.7
540
540
540
1422
1515
1510
0.336; 0.592
0.346; 0.600
0.347; 0.600
DPVBi
0.7
1.4
1.8
4
5.6
8
1.7
2.3
2.2
1.8
1.3
—
5.4
7.5
7.2
6.0
4.6
—
1.6
2.6
2.6
2.0
1.2
—
532
536
536
536
540
—
1571
2206
2104
1819
1260
—
0.304; 0.554
0.325; 0.569
0.318; 0.577
0.335; 0.593
0.342; 0.588
—
10
—
—
—
—
—
—
a
b
Recorded at 10 mA cmꢀ2
.
Recorded at 30 mA cmꢀ2, error bar ꢃ 10%, – not measurable.
properties.18,19 Finally, with two ester functions that can be easily
further derivatized the new emitter constitutes an appealing and
versatile entrance for the design of novel luminescence systems.
Experimental
General
Fig. 6 EL spectra recorded at 30 mA cmꢀ2. (a) mCP as the host material,
Solvents and reagents were used as received unless otherwise
stated. Elemental analyses were performed using a Service de
Microanalyse, ICSN, 91198, Gif sur Yvette, France. Fluorescence
spectra were recorded either using a Fluromax-4 or a Fluorolog
spectrometer (Horiba Jobin Yvon). Fluorescence quantum yield
in solution was determined using 9,10-diphenylanthracene in
cyclohexane as the reference (F = 0.9). Absolute fluorescence
quantum yields of crystalline BP(OH)2DCEt2 were measured
using a Quanta-Phi integrating sphere (Horiba Jobin Yvon).
Fluorescence decay curves were obtained by using the time-
correlated single-photon counting (TCSPC) method with a
femto-second laser excitation composed of a Titanium Sapphire
laser (Tsunami, Spectra-Physics) pumped by a doubled Nd:YVO4
laser (Millennia Xs, Spectra-Physics). Light pulses at 800 nm
from the oscillator were selected by an acousto-optic crystal at a
repetition rate of 4 MHz, and then doubled at 400 nm by a
nonlinear crystal. Fluorescence photons were detected at 901
through a monochromator and a polarizer at magic angle using
a Hamamatsu MCPR3809U photomultiplier, connected to a SPC-
630 TCSPC module from Becker & Hickl. The instrumental
response function was recorded before each decay measurement
with a FWHM (full width at half-maximum) of B25 ps. The
fluorescence data were analyzed using the Globals software
package developed at the Laboratory for Fluorescence Dynamics
at the University of California, Irvine, which includes reconvolu-
tion analysis and global nonlinear least-squares minimization
method. UV-vis absorption spectra were recorded using a Varian
Cary 5000 spectrometer equipped with a temperature control unit.
and (b) DPVBi as the host material.
to be a robust ESIPT emitter in all solvents investigated,
including protic ones, with the fluorescence quantum yields
exceeding by far those of BP(OH)2. More interestingly, its
dicarboxylate form fluoresces efficiently in pure water with a
quantum yield up to 51%, and this fluorescence is highly pH
dependent. In the crystalline state, it is also found to be an
excellent ESIPT emitter and performs quite decently as a pure
fluorescent emitter in an OLED device. Of course, its EQE is far
below the state of the art, but recent studies show that much
higher EQE could be achieved with ESIPT chromophores dis-
playing TADF (Thermally Activated Delayed Fluorescence)
Synthesis
The commercially available but quite expensive ethyl 5-hydroxy-
3-pyridinecarboxylate and ethyl 5-hydroxy-6-iodo-3-pyridine-
caroxylate can be readily prepared from the much cheaper
Fig. 7 EL performance of DPVBi: 1.4% BP(OH)2DCEt2 OLED device. Inset
shows a photograph of the OLED device recorded at a current density of
30 mA cmꢀ2
.
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corresponding acid and used without chromatographic purifi- thick, sheet resistance of 15 O &ꢀ1). Prior to organic layer
cation for the synthesis of BP(OH)2DCEt2 as follows.
Ethyl 5-hydroxy-3-pyridinecarboxylate. To
deposition, the ITO substrates were cleaned by sonication in a
mixture of detergent solution, rinsed twice in de-ionized water and then in
a
5-hydroxy-3-pyridinecarboxylic acid (5.00 g, 36 mmol) in absolute isopropanol solution and finally treated with UV-ozone for
ethanol (100 ml) cooled at 0 1C thionyl chloride (10.5 ml, 72 mmol) 15 minutes. The OLED stack used in this study is as follows:
was added dropwise. The mixture was left to warm up to room Glass/ITO/CuPc (10 nm)/a-NPB (40 nm)/EML 20 nm/BCP
temperature and refluxed overnight. After the removal of the (10 nm)/Alq3 (40 nm)/LiF (1.2 nm)/Al (100 nm). Copper II
solvent under reduced pressure the residue was dissolved in water phthalocyanine (CuPc) is used as the hole injection layer
(100 ml) and the pH of the solution was adjusted to 7 by the (HIL), N,N0-bis-(1-naphthalenyl)-N,N0-bis-phenyl-(1,10-biphenyl)-
careful addition of solid sodium hydrocarbonate in small por- 4,40-diamine (aNPB) as the hole transport layer (HTL), bath-
tions. After filtration, the title compound was obtained by washing ocuproine (BCP) as the hole blocking layer (HBL), tris-(8-
with water as an off-white microcrystalline solid and used for the hydroxyquinoline)aluminum (Alq3) as the electron transport
1
next step without further purification (5.0 g, 83% yield). H NMR layer (ETL), lithium fluoride as the electron injection layer
(CDCl3, 7.26 ppm): d = 8.77 (s, 1H), 8.46 (s, 1H), 7.90 (s, 1H), 4.41 (EIL) and 100 nm of aluminum as the cathode, respectively.
(q, 3J = 7.3 Hz, 2H) and 1.41 (t, 3J = 7.3 Hz, 3H).
The architecture of the device as well as the molecular structure
Ethyl 5-hydroxy-6-iodo-3-pyridinecarboxylate. Ethyl 5-hydroxy- of the materials are shown in Scheme 4. The emitting layer
3-pyridinecarboxylate (5.00 g, 30 mmol), sodium carbonate (6.36 g, (EML) is a BP(OH)2DCEt2 molecule either as a neat film or a
60 mmol) and iodine (7.61 g, 30 mmol) in water (200 ml) were host–guest system 20 nm thick. The materials used as hosts
stirred vigorously at room temperature overnight. The pH of the are 1,3-bis(N-carbazolyl)benzene (mCP) and 4,40-bis(2,2-
orange solution was then adjusted to ca. 7 with hydrochloric acid diphenylvinyl)-1,10-biphenyl, (DPVBi). BP(OH)2DCEt2 is used as
(1 M) and the resulting beige microcrystalline precipitate was the guest with a doping ratio of 0.7 to 10% weight in the guest–
filtered, washed with water and dried under vacuum, affording the host EML. All the organic materials were purchased from com-
title compound (7.30 g, 83% yield). 1H NMR (CDCl3): d = 8.56 mercial companies except for the BP(OH)2DCEt2 molecule.
4
4
(d, J = 1.9 Hz, 1H), 7.75 (d, J = 1.9 Hz, 1H), 5.87 (bs, 1H), 4.40 Organic layers were sequentially deposited onto the ITO sub-
(q, 3J = 7.0 Hz, 2H) and 1.40 (t, 3J = 7.0 Hz, 3H). strate at a rate of 0.2 nm sꢀ1 under high vacuum (10ꢀ7 mbar).
BP(OH)2DCEt2. Ethyl 5-hydroxy-6-iodo-3-pyridinecarboxylate The doping rate was controlled by simultaneous co-evaporation
(2.93 g, 10 mmol), LiCl (0.43 g, 10 mmol) and NiCl2ꢁ6H2O (0.24 g, of the host and the dopant. An in situ quartz crystal was used to
1.0 mmol) were dissolved in DMF (20 ml). To the resulting monitor the thickness of the layers deposited with an accuracy of
blue solution was added zinc dust (0.78 g, 12 mmol) and the 5%. The active area of the devices defined by the Al cathode was
mixture was stirred at room temperature under argon. After a 0.3 cm2. The organic layers and the LiF/Al cathode were deposited
few minutes the mixture turned deep red with a concomitant in a one-step process without breaking the vacuum.
rise of temperature to 40–50 1C. Once subsided, the stirring
After deposition, all the measurements were performed at
continued for 1 h at RT and 1h at 50–55 1C to ensure the room temperature and under ambient atmosphere with no
completion of the reaction. The cooled mixture was made further encapsulation of devices. The current–voltage–lumi-
acidic (pH = 1 to 2) with aqueous HCl (B1 M) and was nance (I–V–L) characteristics of the devices were measured with
vigorously stirred for 30 min before the pH was brought back a regulated power supply (ACT100 Fontaine) combined with a
to ca. 4 with solid NaHCO3. CH2Cl2 (100 ml) was added and the multimeter (Keithley) and a 1 cm2 area silicon calibrated
organic phase was collected. The aqueous phase was extracted photodiode (Hamamatsu). Electroluminescence (EL) spectra
with CH2Cl2 (3 ꢂ 30 ml) and the combined organic phase was and chromaticity coordinates of the devices were recorded
dried over Na2SO4 and concentrated. The residue was subjected using a PR650 SpectraScan spectrophotometer, with a spectral
to column chromatography (silica gel, CH2Cl2) to afford the title resolution of 4 nm. Photoluminescence (PL) properties of the
compound as brilliant yellow microcrystalline solid (1.215 g, EML layer are obtained at room temperature with an integrat-
1
4
73%). H NMR (CDCl3): d = 14.40 (s, 2H), 8.70 (d, J = 1.8 Hz, ing sphere (LabSphere) and illumination with a 405 nm laser
4
3
3
2H), 8.03, J = 1.8 Hz, 2H), 4.44 (q, J = 7.0 Hz, 4H), 1.42 (t, J = diode. For PL measurements, the films were deposited on pre-
7.0 Hz, 6H). 13C NMR (CDCl3, 77 ppm): 164.3, 156.2, 141.7, 137.6, cleaned glass substrates in the same deposition sequence as
128.1, 127.0, 61.8, 14.3. HRMS (ESI): C16H16N2O6 found: that of the OLED devices.
331.0931[M-H]ꢀ, calculated: 331.0936. Elemental analysis calcu-
lated (%) for C16H16N2O6: C 57.83, H 4.85, N 8.43; found: C 57.87,
H 4.93, N 8.35.
Single crystals suitable for structure determination were grown
by slow evaporation of BP(OH)2DCEt2 in a mixture of CH2Cl2 and
Conflicts of interest
There are no conflicts to declare.
MeCN at room temperature.
Acknowledgements
OLED device fabrication and characterization
The OLED devices were fabricated onto indium tin oxide (ITO) We acknowledge the LabEx CHARMMMAT (ANR-11-LABX-0039)
glass substrates purchased from Xin Yang Technology (90 nm for the postdoctoral fellowship of Virgile Trannoy. Arnaud
3020 | New J. Chem., 2021, 45, 3014ꢀ3021
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Paper
NJC
18 M. Mamada, K. Inada, T. Komino, W. J. Potscavage, Jr.,
H. Nakanotani and C. Adachi, ACS Cent. Sci., 2017, 3, 769.
19 K. Wu, T. Zhang, Z. Wang, L. Wang, L. Zhan, S. Gong,
C. Zhong, Z.-H. Lu, S. Zhan and C. Yang, J. Am. Chem. Soc.,
2018, 140, 8877.
Brosseau (PPSM) is gratefully acknowledged for his help with
the time-resolved fluorescence measurements.
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