Design and synthesis of efficient electrogenerated chemiluminescent emitters derived from pyrene

Article abstract of DOI:10.1149/2.0881814jes

The design and synthesis of novel dendrimeric structures derived from a pyrene core with substituted triphenylamine and carbazole periphery demonstrated how molecular engineering allowed the fine tuning of the electrogenerated chemiluminiscence (ECL) properties. These two compound (Py-CBZ and Py-TPA) in comparison with the parent structures (Py and Py-Core) showed better chemical stability and anti-aggregation capability. At the same time, the photophysical (^Absλ_max, E₀₀^FLλ_maxΦ_FL), photochemical (oxygen quenching), electrochemical (redox potentials) and optoelectronic (HOMO-LUMO gap and their energy position) properties were modulated, resulting in obtaining two highly ECL emitting molecular structures (Py-CBZ and Py-TPA). This study showed that functionalization of organic dyes with redox moieties (with the proper linker and position) improved the radical stability and also, modulated the optoelectronic properties with the concomitant enhancement of the ECL performance and their potential use in applications such as organic light-emitting devices.

Full text of DOI:10.1149/2.0881814jes

Journal of The Electrochemical Society, 165 (14) G163-G170 (2018)
G163
0013-4651/2018/165(14)/G163/8/$37.00 © The Electrochemical Society
Design and Synthesis of Efficient Electrogenerated
Chemiluminescent Emitters Derived from Pyrene
3
Maria V. Cappellari,1,2 Mar´ıa I. Mangione, 3,z Rolando A. Spanevello,
Gabriela Marzari,1,2 Gustavo M. Morales,1,2 and Fernando Fungo
1,2,z
1Departamento de Qu´ımica, Universidad Nacional de R´ıo Cuarto, R´ıo Cuarto, Argentina
2Instituto de Investigaciones en Tecnolog´ıas Energe´ticas y Materiales Avanzados, UNRC-CONICET, Argentina
3Instituto de Qu´ımica Rosario, Facultad de Ciencias Bioqu´ımicas y Farmace´uticas, Universidad Nacional de Rosario -
CONICET., S2002RLK Rosario, Argentina
The design and synthesis of novel dendrimeric structures derived from a pyrene core with substituted triphenylamine and carbazole
periphery demonstrated how molecular engineering allowed the fine tuning of the electrogenerated chemiluminiscence (ECL)
properties. These two compound (Py-CBZ and Py-TPA) in comparison with the parent structures (Py and Py-Core) showed better
FL
chemical stability and anti-aggregation capability. At the same time, the photophysical (Absλmax, E00
,
λ
max
, ꢀFL), photochemical
(oxygen quenching), electrochemical (redox potentials) and optoelectronic (HOMO-LUMO gap and their energy position) properties
were modulated, resulting in obtaining two highly ECL emitting molecular structures (Py-CBZ and Py-TPA). This study showed
that functionalization of organic dyes with redox moieties (with the proper linker and position) improved the radical stability and
also, modulated the optoelectronic properties with the concomitant enhancement of the ECL performance and their potential use in
applications such as organic light-emitting devices.
© 2018 The Electrochemical Society. [DOI: 10.1149/2.0881814jes]
Manuscript submitted August 6, 2018; revised manuscript received October 12, 2018. Published November 6, 2018.
ECL is a light emission process1–4 which has been used in the field
of sensors,5 development of analytical methods,6,7 bioanalysis and
bioimaging,8 among others.9,10 Recently, the use of the electrochem-
ical processes to operate light-emitting devices with unusual func-
tionality has gained significant interest; for example, light-emitting
electrochemical cells (LECs) containing a small amount of electrolyte
within the emissive active layer.11 The liquid nature of this kind of
ECL systems allows the ECL emitter encapsulation between flexible
substrates electrodes, which is a simple way to fabricate deformable
light-emitting devices. Also, through the right selection of the ECL
dyes mixtures, it is possible to effectively tune the emissive color of
these devices by controlling the potential applied to the electrodes.2
Despite of the remarkable ECL possibilities for real applications,
challenges still remain to be solve in the development of ECL mate-
rials. Two main issues to be approached are to increase the limited
ECL fluorophores library and to improve the dyes stability toward the
degradation originated by the ECL process. Regarding stability, it is
known that small organic molecules used in optoelectronic applica-
tions undergo chemical degradation induced by water and oxygen.12,13
This deleterious process can increase when the dyes excited states have
long lifetime, which allows reaction to take place in the diffusional
times of the species. Good candidates for ECL must be excellent
emitting centers with good cation-anion radical chemical stability.2
Among the organic ECL active materials, polycyclic aromatic hydro-
carbons (PAH) and their derivatives are classical examples, for in-
stance, 9,10-diphenylanthracene (DPA) and rubrene are used as ECL
standards compounds.2 Pyrene (Py) is an electrochemical active cen-
ter with unique photophysical properties, presenting high quantum
fluorescence yields with a large Stokes shift and its light emission is
sensitive to micro-environmental changes, among others.4,14–16 These
properties make Py an excellent candidate to be used in ECL applica-
tions. However, Py has problems associate with its intrinsic properties
that decrease its capability to produce efficiently ECL signal. Like
many other organic centers with ECL potential, Py is a flat molecule
that favors electrostatic interaction between molecular excited states,
which decreases its quantum fluorescence yields. In addition, Py is
a center that produces radical cation and anion with poor chemical
stability.17 The long lifetime of the Py excited states facilitates strong
interaction with oxygen, hence quenching the light emitting process
and producing singlet oxygen, a highly reactive species undesired in
operating ECL devices.12,18 Based on these factors, a Py center is not
a good candidate for ECL.16 This weaknesses can be overcome using
Py core as dendrimeric framework of new molecular structures with
improved optoelectric properties. Developments in Py core derivatives
have been mushrooming in recent years.19 Several synthetic modifica-
tions and functionalizations methodologies were reported in order to
construct fluorescent probes and organic semiconductor materials.19
This work describes the synthesis, electrochemical reduction and
oxidation and radical ion annihilation ECL of three Py derivatives,
which were modified through the introduction of different electron
donor-acceptor groups, conjugated bridges and bulky moieties (See
Figure 1). The Py was utilized as a platform to introduce acetylene
bonds to extend the intramolecular π-conjugation, block the reactive
Py positions and bridged the conjugated core with triphenylamine
(TPA) and carbazole (CBZ) electron donor groups. In addition, these
electron donor groups were substituted with bulky moieties to pro-
duce steric hindrance effect in order to control the intermolecular
interactions and to avoid the typical dimerization process that affects
both aromatic-amine centers.20–22 It has been reported that the use of a
weakly donating group (n-propyl) or alkylamines as a substituent at the
para-position of the phenyl group for a series of phenylethynylpyrene
derivatives greatly improved ECL of Py core.23–25 However, alky-
lamines substituents showed lack of chemical stability that induces
film formation on the electrode surface, which limits the use of these
moieties for ECL applications.24 Therefore, it is expected that the
functionalization with redox moieties (in appropriate position and
with the proper linker) would improve the radical stability and also,
modulate the optoelectronic properties of organic dyes with the con-
sequent enhancement of the dye ECL performance. The structures
of the Py-CBZ and Py-TPA molecules are shown in Figure 1 where
Py-Core is included as a model compound for comparative analysis.
Materials and Methods
Instrumentation and characterization techniques.—The optical
characterization was made using a diode array spectrometer Agilent
8453 and a spectrofluorometer Spex FluoroMax. Spectra were acquire
using quartz cells (path length: 1 cm) at room temperature in 1,2-
dichloroethane (DCE) and toluene (PhMe) solutions. The emission
spectra were measured between ∼280–800 nm, exciting the maxi-
mum of the lower light absorption energy bands. For relative quan-
tum fluorescence yields calculations 9,10-diphenylanthracene (DPA,
Sigma-Aldrich) was used as an actinometer (ꢀ = 0.90), which was
dissolved in cyclohexane (Cicarelli, refractive index, η = 1.427), pre-
viously purified by simple distillation.
zE-mail: mangione@iquir-conicet.gov.ar; ffungo@exa.unrc.edu.ar
Downloaded on 2018-11-09 to IP unknown address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

G164
Journal of The Electrochemical Society, 165 (14) G163-G170 (2018)
Figure 1. Pyrene derivatives molecular structures.
Electrochemical properties.—The electrochemical experiments
were made in a potentiostat CH Instrument, model 700 E. The con-
centrations of the dyes under study were in the range of 0.5–1.0 mM
in 1,2-dichloroethane (DCE, Sintorgan), which was purified by sim-
ple distillation and stored over molecular sieves (Biopack, 3 Å) and
CaCO3 (Riedel-de Hae¨n, 95%) containing 0.1 M of Tetrabutylam-
monium hexafluorophosphate (TBAHFP, purity >99%, Fluka) as the
support electrolyte. The voltammetric experiments were carry out us-
ing as working electrode an inlaid platinum disk (0.0314 cm2) polished
on a felt pad with 0.3 μm alumina and sequentially sonicated in water
and absolute ethanol. Before starting each electrochemical measure-
ment, the Pt electrodes were tested by cyclic voltammograms in 1 M
sulfuric acid (Cicarelli, 95–98%) in order to verify the correct clean-
ing of its surface. Silver wire was used as pseudo-reference electrode
and a platinum coil as the counter electrode. Before each experiment
electrolyte blank was made to discard possible electrochemically ac-
tive interferences. All potential values reported are expressed relative
to ferrocene/ferrocenium redox couple (Fc/Fc+ = 0.40 V vs SCE),
which was used as an internal standard.26
The Ullmann C-N and Sonogashira coupling reactions were the two
main chemical transformations applied to construct the core, dendron
and dendrimer structures. Starting with commercial Py (1), reaction
with bromine as electrophile in nitrobenzene produced the tetrabromo
derivative 2 in excellent yield.30 This light green solid is insoluble
in common organic solvents and its identity was verified by ESI, ob-
serving the pattern of masses corresponding to a compound with four
bromine atoms. The coupling of 2 with commercial trimethylsily-
lacetylene under Pd-catalyzed conditions generated compound 3 as a
bright orange solid. The trimethylsilyl protecting group was removed
with anhydrous K2CO3 in methanol, yielding Py-Core, with excellent
overall yield. The structures of 3 and Py-Core were confirmed by 1H
NMR spectroscopy and corroborated with the data published in the
literature.31 Py-Core was obtained as a yellowish solid and was the
nucleus used for the preparation of the dendrimers (Scheme 1).
On the other hand and concomitantly with the construction of the
pyrene nucleus, the synthesis of dendrons 7 and 11 were achieved.
The preparation of 3,6-di-tert-butyl-9-(4-iodophenyl) 7 started with
a Friedel-Crafst alkylation of commercial carbazole 4 with tert-butyl
chloride to afford compound 5 in good yield.32 Di-tert-butyl deriva-
tive 5 was treated under Ullmann modified conditions with 1,4-
diiodocarbazole 6 to obtain compound 7 in 67% yield. Dendron 7
ECL characterization.—All the ECL measurements were made in
a three electrode homemade electrochemical cell, which is adapted
to a spectrofluorometer Spex Fluoromax. The cell holds a working
electrode (Pt disc, diameter: 2mm) supported on glass, which has a
90◦ curvature that allows the Pt disc (electro-active region of the elec-
trode) to be focused on the fluorometer detector. The Pt electrode was
conditioned in the same way than the electrodes utilized for the elec-
trochemical studies. The dyes ECL emission efficiencies (ꢀECL) were
determined by the number of photons emitted per injected electron
relative to the known rubrene ECL efficiency under the same exper-
imental conditions.2,27,28 The electrochemical current and ECL were
generated by applying three potential step waveform to the working
electrode and the generated signals were registered in function on
time. The three-step pulsed amperometric waveform was defined by
the following parameters E1 = 0 V by 4 s, E2 = dye oxidation poten-
tial 0.2 s and E3 = dye reduction potential 0.2 s. This perturbation was
applied three consecutive time and the obtained ECL and currents sig-
nals were integrated and used to estimate the dyes ꢀECL. The ECL was
recorded at the maximum wavelength corresponding to the ECL emis-
sion of each molecule and the fluorometer slit was opened 7 mm in
order to collect as much light as possible. For the ECL measures, the
co-reactants tripropylamine (nPr3N, Sigma-Aldrich) and tetrabuty-
lammonium persulfate ((TBA)2S2O8) were used. (TBA)2S2O8 was
synthesized by a metathesis reaction between ammonium persulfate
(Cicarelli) and tetrabutylammonium hydroxide (Sigma-Aldrich).29
1
was characterized by H and 13C NMR and the data collected were
according to those published in the literature33 (Scheme 2).
The synthesis of the second dendron: N,N-di(4-tolyl)-4’-
iodophenylamine 11 was also achieved in a straightforward manner.
A ready available di-tolylamine 8 was coupled with iodobenzene 9
using a modified Ullmann protocol.34 The substituted triphenylamine
derivative 10 obtained in this way was subjected to an aromatic iodi-
nation procedure to afford 11 in good yield (Scheme 3).35
In a convergent synthetic strategy, Py-Core and the dendrons 7
and 11 were coupled applying Sonogashira reaction conditions to ob-
tain dendrimers Py-CBZ and Py-TPA respectively in moderated yields
(Scheme 4).36 It is important to point out that it was necessary to place
the terminal aryl alkyne on the core Py system and the aryl halide on
the dendrons in order to avoid Glasser coupling37 between dendrons
and improve the yields of dendrimers. Dendrimer Py-CBZ is a bright
orange solid while dendrimer Py-TPA is a bright red one. These com-
pounds were fully charactherized by uni- and bi-dimensional 1H and
13C NMR spectroscopy, IR and MALDI-TOF spectrometry, all the
data collected confirmed the expected structures.
Optical properties.—UV-visible light absorption and fluorescence
emission spectra were used to analyze the photophysical properties
of the molecules. The spectra showed in Figure 2 were obtained in
DCE diluted solutions where dyes aggregation effect was not detected
and therefore, the reported fluorescence data represent the monomer
emission. Also, to facilitate a comparative analysis in this figure the
typical Py absorption spectrum is included18 and the main optical
parameters obtained for the dyes are summarized in Table I. The
Results and Discussion
Synthesis and characterization.—The synthetic pathway and the
structures of the compound involved are depicted in Schemes 1 to 3.
Downloaded on 2018-11-09 to IP unknown address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 165 (14) G163-G170 (2018)
G165
Scheme 1. Synthesis of Py-Core.
Scheme 2. Synthesis of dendron 7.
Scheme 3. Synthesis of dendron 11.
Table I. Photophysical and electrochemical properties.
Py
Py-Core
Py-CBZ
Py-TPA
photophysic
Abs
λma´x (nm)a
262, 270, 276, 306, 321, 337
370, 381, 392
21,625
295, 308, 373, 392, 419
287, 297, 337, 388, 458, 485
300, 344, 414, 482, 521
FL
λma´x (nm)b
420, 445
14,085
0.986
511
68,316
0.835
2.53
573
44,987
0.456
2.28
εabs (M−1 cm−1
)
c
d
ꢀFL
E00 (eV)e
0.008–0.261∗
3.57
2.96
Electrochemistry
Epc (V)f
0.98
-
1.01
-
0.71–1
0.68–0.96
−1.75
0.52–0.92
0.47–0.88
−1.85
Ec1/2 (V)f
Epa (V)f
Ea1/2 (V)
−2.54
−1.79
−2.47
−1.77
−1.71
−1.82
ECL
-ꢁHan (eV)g
HOMO (eV)h
LUMO (eV)h
3.42
−6.08
−2.93
472
2.70
2.36
−5.78
−3.25
531
2.27
−5.57
−3.29
606
−6.11
−3.15
ECL
λma´x (nm)i
ꢀECL
-
-
j
0.0008
0.24
0.41
∗Fluorescence quantum yield in presence of oxygen and in degassed solution by bubbling Ar.
aAbsorption spectra obtained in DCE.
bEmission spectra obtained in DCE.
cCalculated by Lambert-Beer law.
dFluorescence quantum yield obtained in DCE. Standard used were 9,10-diphenylanthracene. ꢀFL (9,10-diphenylanthracene) = 1 in cyclohexane.47
eE00 calculated from UV/vis absorption and fluorescence spectra.
fAll electrochemical data referenced versus ferrocene/ferrocenium couple.26
gCalculated from the difference between the two redox peak potentials.2,26
hHOMO calculated from voltammetric data as of HOMO = -(Ec1/2 + 5.1) eV. and LUMO = -(E00-HOMO) eV.2
iECL spectra obtained in DCE.
jECL quantum yield obtained in DCE. Standard used were rubrene. ꢀECL = 1 in DCE.2
Downloaded on 2018-11-09 to IP unknown address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

G166
Journal of The Electrochemical Society, 165 (14) G163-G170 (2018)
Scheme 4. Synthesis of dendrimers Py-CBZ and Py-TPA.
Py-Core presented absorption maxima at 308 nm, 373 nm, 392 nm
and 419 nm, which are red shifted regarding Py (See Figure 2 and
Table I). Photophysical properties of Py exhibit strong positional sub-
stitution dependence since these parameters are affected by the π-
conjugated length of its molecular structure.18,36,38 Thus, the observed
bathochromic displacement in Py-Core was due to an increase in the
molecular π-conjugation by the addition of the alkyne groups in posi-
tions 1-, 3-, 6- and −8.30 On the other hand, Py-CBZ and Py-TPA have
the electron donating CBZ and TPA moieties, respectively, linked by
the acetylene bridge conjugated to the Py framework (See Figure 1).
Consequently, it was expected that the strong electron-donating na-
ture of these aromatic amines groups could affect the dye electron
density and HOMO-LUMO distribution with a concomitant effect on
the optical gap and the HOMO-LUMO energy position (see below the
electrochemical characterization).36,38,39 Py-CBZ showed absorption
maxima in the same UV-energy region of the spectrum like Py and
Py-Core (See Figure 2 and Table I). However, new absorption bands
at 458 nm and 485 nm appeared, which could not be assigned to the
introduction of the acetylene-CBZ residue (See spectrum of dye 1 in
Ref. 40). Therefore, the spectrum was not a linear combination of the
chromophores present in the dye molecular structure and this indi-
cated a strong interaction between the constituent functional groups
of the molecule. All the orbitals involved in the Py optical transitions
have nonzero contributions at the 1-, 3-, 6-, and 8- positions, hence,
significant influence in the Py-CBZ UV-spectra were expected.18 In
similar way, the introduction of a TPA substituent, which is a bet-
ter electron donating group than CBZ,40 produces light absorption
red shifted regarding the observed in the Py-CBZ (see Figure 2 and
Table I). All studied compounds showed no solvent dependence in
their light absorption maxima values, which indicates a small dipole
moment in their ground state.41
The fluorescence spectra of the dyes were studied in DCE and
PhMe diluted solutions where the formation of aggregates was mini-
mized and the results obtained are shown with a colored solid line in
Figure 2 and summarized in Table I. Py-Core in PhMe solution showed
maximum emission (FLλmax) at 420 nm and 445 nm and a shoulder at
477 nm. These maxima were red shifted regarding the Py as conse-
quence of the increase in the molecular conjugation length due to the
alkyne groups linked at 1-, 3-, 6- and 8- positions. Similar to Py, the
Py-Core light absorption and emission behaviors held fine structures
and they were quite insensitive to changes of solution polarity, indi-
cating that there were not important differences between ground and
excited states dipole moments. These photophysical characteristics
were in good agreement with those ones reported for related molecu-
lar structures.18,31,42 For Py-CBZ and Py-TPA their emission maxima
were found at 506 nm and 538 nm in nonpolar PhMe solutions, respec-
tively. As it was pointed out in the light absorption behavior analysis, it
was expected that Py-TPA emited at a low energy than Py-CBZ. How-
ever, Py-TPA showed fluorescence maximum more sensitive to the sol-
vent polarity changes than Py-CBZ. Figure 2 showed that the emission
maxima of Py-TPA presented a bathochromic shift of ∼35 nm, while
Py-CBZ only moved its maximum ∼5 nm, which would indicate that
there was a moderate change of the dipole moment between the ground
and excited state. The charge transfer strength variation between
Downloaded on 2018-11-09 to IP unknown address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 165 (14) G163-G170 (2018)
G167
-2,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
1.0
0.5
Py-TPA
DCE
PhMe
0,2
0,1
Py-TPA
Py-CBZ
Py-Core
Py
0,0
0.0
1.0
-0,1
Py-CBZ
0,16
0,08
0,00
-0,08
DCE
PhMe
0.5
0.0
1.0
Py-Core
0,42
0,21
0,00
DCE
PhMe
i
0.5
0.0
1.0
-0,21
2,22
Py
1,11
0,00
DCE
PhMe
0.5
0.0
-1,11
300
400
500
600
700
800
-2,5 -2,0 -1,5 -1,0 -0,5 0,0
0,5
1,0
λ (nm)
+
E (V)
Fc/Fc
Figure 2. Normalized light absorption spectra (black solid line) obtained
in DCE solution (5.10−7 M). Emission spectra obtained in DCE solution
(5.10−7 M and 1.10−4 M, red solid and red dashed line, respectively) and
PhMe solution (5.10−7 M, green solid line).
Figure 3. Cyclic voltammograms of the Py (∼1 mM) and Py-Core (∼0.5 mM)
in 3:1 toluene/acetonitrile mixture and for Py-CBZ and Py-TPA (∼0.5 mM)
in TBAHFP/3:1 toluene-acetonitrile mixture electrolyte. Pt electrode and scan
rate = 100 mV s−1
.
Py-CBZ and Py-TPA was due to the smallest conjugation length
present in Py-CBZ, which it was affected by the nonplanar orientations
of the carbazolyl moieties38,39,43 (See Figure 1). Also, TPA substituent
is a better electron donor than CBZ centers,40 which increases the
intramolecular charge transfer character. Thus, the solvatochromism
phenomenon effect in Py-TPA was more pronounced. It is interesting
to note that commonly, the modulation of this solvatochromic prop-
erty in Py derivatives is achieved by non-symmetrical substitution
pattern and in the dyes under study, this effect was accomplished with
symmetrical structures.24,44
The fluorescence quantum yields (ꢀFL) for each molecule were de-
termined at room temperature in degassed and aerated DCE solutions
and the observed values relative to the DPA are also listed in Table I.
All the synthesized compounds have higher quantum yields values
than the precursor Py. The rigid structure of Py allows efficient inter-
system crossing pathway, and also produces singlet state with lifetime
long enough to permit fluorescence quenching by oxygen diffusion,
which decreases its emission efficiency.13 Therefore, the high ꢀFL of
the fluorophores shown in Figure 1 suggests that these light emission
deleterious processes are inefficient. In agreement with these assump-
tions, when ꢀFL of the dyes were measured in presence of atmospheric
oxygen, the fluorescence intensities of compounds Py-Core, Py-TPA
and Py-CBZ were unaffected obtaining the same yields values be-
fore and after degassing; while Py ꢀFL was dramatically affected (See
Table I). Notably, this oxygen insensibility is a highly desired property
in optoelectronic application, because produces chemical stability in
the excited states of the materials. On the other hand, ꢀFL values de-
crease in the following order Py-Core, Py-CBZ and Py-TPA in good
concordance with the structural molecular characteristic. Py-Core was
the most rigid structure, while the introduction of the aromatic amines
in Py-CBZ and Py-TPA favors the free rotation of the phenyl sub-
stituents (specially for Py-TPA) in conjugation with the central pyrene,
which allows competing non-radiative pathways for the decay of the
excited singlet state.42
The dyes aggregated formation tendency was checked by taking
the fluorescence spectra at different molar concentrations. In Figure
2 the emission spectra in DCE solution at 5.10−7 M (red solid line)
and 10−4 M (red dashed line) are shown. As it was expected for the
concentrated solution of Py, the typical light emission band related
to the excimer formation appeared around at 450 nm.12 A similar
behavior was observed with Py-Core since the steric effect of the
acetylenic substituents were not able to prevent the face-to-face π
molecular interaction in solution. However, for Py-CBZ and Py-TPA
the fluorescent spectra were not affected by concentration changes and
the observed emission corresponded only to the monomers. Therefore,
in these dyes the CBZ and TPA groups could avoid that two molecules
get close enough to produce excimer emission, and similar results
were observed even at high concentrations above 10−4 M. Thus, the
fluorophores structural characteristics provided good explanations for
the photophysical behaviors observed.
Electrochemical properties.—To perform the electrochemical
measurements, TBAHFP/3:1 toluene-acetonitrile mixture electrolyte
was used due to the low solubility that have Py and Py-Core.
Figure 3 describes the cyclic voltammograms (CV) corresponding
to the molecules showed in Figure 1, where differentiated redox be-
haviors were observed for each system. In a comparative analysis with
related structures, the Py CV showed a reversible reduction process
at ∼−2.54 V and an oxidation wave with maximum at ∼+0.98 V,
which corresponds to the cation radicals formation with chemical in-
stability in the CV time scale.17 In the case of the remaining dyes,
it was clear that both anodic and cathodic redox processes were af-
fected, which indicates that functionalization effectively modified the
Py electrochemical properties. It was notorious the shift on the re-
duction potentials, more than 0.75 V (See Table I), observed between
the three modified dyes and Py itself. However, the differences of
reduction potential values between Py-Core, Py-CBZ and Py-TPA
Downloaded on 2018-11-09 to IP unknown address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

G168
Journal of The Electrochemical Society, 165 (14) G163-G170 (2018)
dyes were only ∼0.1 V (See Table I). Therefore, these results sug-
gested that the acetylene moieties were the main factor responsible
for the observed reduction potential shift, while the nitrogen atoms
of Py-CBZ and Py-TPA have a minor effect on the dyes reduction
processes. The increment of the conjugation length has a greater in-
fluence on the orbitals energy level that participates in the reduction
process than the electronegative effect that can exercise the nitrogen
atom. On the other hand, in the analysis of the dyes’ electrochemical
behavior at positive potential (Figure 3), Py-Core showed an oxi-
dation peak around +1.0 V with a wave shape characteristic of an
irreversible electrochemical process. Comparing it with Py, revealed
that the acetylene substituents do not affect the peak oxidation poten-
tial and also do not improve the stability of the formed radical cations.
This lack of chemical stability can due to the presence of the labile
hydrogen on terminal alkynes that facilitates the deprotonation of the
electrochemically formed Py-Core radical cation.
Py-CBZ
Py-TPA
Py
Alternatively, the introduction of the aromatic amines in Py-TPA
and Py-CBZ, changed radically the oxidation behavior of these dyes
regarding Py and Py-Core. In the case of Py-CBZ, the detection of an
oxidation wave with a shoulder at +0.71 V and +1.00 V, respectively,
was observed in the current trace corresponding to the positive poten-
tial scan showed in Figure 3. When the potential sweep direction was
inverted, both oxidation processes showed their complementary re-
duction waves at +0.64 V and +0.95 V, respectively, which indicated
a good chemical stability of the generated charged species. CBZ had
lower oxidation potential than Py,20–22 therefore, the first electrochem-
ically removed electron from Py-CBZ could be assigned to the CBZ
moiety oxidation; while the second one corresponds to the Py ring. The
good chemical stability observed for the Py-CBZ radical cation and
dication compared with Py and Py-Core can be explained by two fac-
tors: a.- the effective increment in the conjugation extension achieved
by bonding good electron donating groups (CBZ) to the Py ring via
a triple bond in the 1-,3-,6-,8- reactive positions (See Figure 1); b.-
the introduction of tert-butyl moieties in CBZ skeleton that stabilize
its radical cation avoiding the dimerization process.21,22 The analysis
of the Py-TPA electrochemical behavior was similar to the one made
with Py-CBZ. Likewise, Py-TPA showed two oxidation voltammetric
waves with potential peaks at +0.52 V and +0.92 V (Figure 3). But in
this case, the potential peaks maxima were shifted to lower potential
and the waves are less overlapping than those observed for Py-CBZ.
The TPA moiety is a better electron donor and it is easier to oxidize
than CBZ group.22 Therefore, the two oxidation waves were expected
to move at lower potentials with respect to Py-CBZ. Nevertheless, this
shift was more marked in the first wave than in the second one, which
produced a definite separation of the voltammetric waves. Similarly
to Py-CBZ, the TPA reactive positions were blocked with alkyl sub-
stituents, which stabilized its radical cation, avoiding the formation
of tetraphenylbenzidine through a coupling-dimerization reaction.21,22
Summarizing the electrochemistry of all the compounds, it could be
observed a stability improvement of radical cation and anion species
derived from Py-CBZ and the Py-TPA, which proved the success
of the molecular design strategy to enhance the performance of the
generated charged species. In this sense, Py-CBZ and the Py-TPA,
which hold tertiary aromatic amines in their structure (See Figure 1),
showed better radical cation behavior than tetrakis(ethynyl)pyrenes
functionalized with N,Nꢀ -dialkylanilines. The last one showed insta-
bilities in their charged species, which tends to form thin film on
the electrode surface during the voltammetric cycling.24 The HOMO
and LUMO energy levels were calculated and the results are listed in
Table I together with other electro-optical parameters. In agreement
with the energy-gap (E00) trends obtained from the spectroscopic stud-
ies, it was observed that the electrochemical gap (HOMO−LUMO)
gradually decreased as the π-conjugation length and electron donor
capability of the substituents increased. In another way, the position
of the LUMO-HOMO in the absolute energy scale was shifted conse-
quently with the electrochemical characteristic of the moieties bonded
to the Py ring. For example, the dyes HOMOs were in the energy range
expected for commonly phenylamine derived hole transporter layer
materials used in optoelectronic applications.22,45
400 500 600 700 800 400 500 600 700 800 400 500 600 700 800
λ
(nm)
Figure 4. ECL spectra of pyrene derivatives system (∼0.5 mM) in
TBAHFP/DCE electrolyte with pulsing (10 Hz) between dyes reduction and
oxidation peak potentials.
Finally, it was calculated the enthalpy energy (-ꢁHan) available
in the radical cation/radical anion annihilation reaction from the ob-
tained dyes redox parameters.1,3 This is an important ECL parameter
that allows to evaluate whether the studied systems release enough en-
ergy to produce the emitting excited state through electrochemically
induced reactions. Comparing the -ꢁHan values showed in Table I
with the corresponding E00, it could be noted that Py and Py-TPA
are able to exergonically produce ECL (-ꢁHan>E00).1,3 Considering
that only Py-TPA combines a high quantum fluorescence yield, good
charged species stability and sufficient ion annihilation enthalpy en-
ergy to populate directly their emitting excited states, it can be stated
that this molecule is a promising candidate to efficiently produce ECL
phenomenon.
Electrogenerated chemiluminescence properties.—In Figure 4
are shown the Py, Py-CBZ and Py-TPA ECL spectra registered un-
der the same conditions used for electrochemical experiments (dyes
concentration, electrolyte composition). The electrochemical cell was
perturbed by a 10 Hz potential square-wave, which it was modulated
by jumps between the oxidation and reduction potentials values, where
radical cation and anion forms are alternately produced. These applied
potentials were already determined and presented in the electrochem-
ical section (Table I ). The maximum of Py ECL signal (ECLλmax
)
was redshifted ∼80 nm regarding its fluorescence maxima spectrum
showed in Figure 1 and this behavior was ruled by the excimer forma-
tion, which emitted light at 470 nm.46 While for the Py-Core, it was
not possible to detect the ECL phenomenon in the used experimental
conditions, even after the addition of anodic or cathodic co-reactants.
This result was mainly due to the poor chemical stability of Py-Core
charged species, which avoided in the experimental time scale their
encounter, annihilation and formation of the excited light-emitting
state processes. On the other hand, Py-CBZ and Py-TPA showed ECL
activity with emission maxima at 531 nm and 606 nm, respectively.
Previously, it was estimated that Py-CBZ had insufficient energy to
populate their main emitting excited states (-ꢁHan<E00, See Table I).
However, the fact that this dye was able to generate ECL signal, with-
out use of co-reactants, it was an indicative that Py-CBZ could be
classified as an energy-deficient system, where -ꢁHan is smaller than
the first excited singlet state, but larger than its triplet state energy.2
Py-TPA was an active ECL substrate with enough energy to pro-
duce emitting states through electrochemical stimulation (-ꢁHan>E00,
See Table I). Consequently, in Figure 4 it is shown a Py-TPA
ECL spectrum with a broad band with maximum at 606 nm. In
Table I could be observed that the Py-CBZ and Py-TPA ECL
Downloaded on 2018-11-09 to IP unknown address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 165 (14) G163-G170 (2018)
G169
Scheme 5. ECL emission mechanism of Py derivatives,
where D is CBZ or TPA. (1) and (2) radical cation
and anion electrochemical formation. (3) and (4) radical
cation/radical anion annihilation reactions. (5) Triplet-
triplet annihilation reaction. (6) ECL emission.
maxima were shifted about 20 nm and 33 nm to lower energies regard-
ing its fluorescence emission, respectively. This shifting could be at-
tributed to self-absorption that allows the Stokes shift of these systems
(See Figure 2). This filter effect was magnified due to the high con-
centration (mM range) used in the ECL experiments. Therefore, there
is a good correlation between dyes ECL spectra and the correspond-
ing fluorescence spectra. Py-TPA also showed a broadness spectrum,
which can be associated to the previously observed intramolecular
charge transfer character of the excite state of this molecule. More-
over, the low aggregation effect observed for Py-CBZ and Py-TPA in
the photophysical study was retained in the ECL experiments due to
their ECL spectra are insensitive to the dyes concentration changes,
which it is a property wanted from an optoelectronic applications point
of view. The mechanism showed in the Scheme 5 is consistent with
the described ECL behavior for Py-CBZ and Py-TPA.
On the other hand, dyes ECL efficiencies were determined by
the number of photons emitted per injected electron relative to the
known rubrene ECL emission2 and the calculated values are listed in
Table I. A clear difference of several order of magnitude was observed
between the ECL relative yields of Py and the ones measured for Py-
CBZ and Py-TPA. Moreover, it is interesting to compare the relative
ECL quantum yield: the reached value by Py-TPA was approximately
twice as higher as the one found for Py-CBZ under the same ex-
perimental conditions, although Py-TPA photophysically emits half
of light than Py-CBZ (See ꢀECL and ꢀFL in Table I). This observed
behavior could be related with the differences between the electron
donating capability of TPA and CBZ moieties. As it was mentioned be-
fore, Py-CBZ was deficient in energy to directly produce ECL, while
Py-TPA was not (See Scheme 5). It is expected that the TPA moi-
eties bonded to the Py ring shift both, the oxidation potential and the
emission maximum toward lower energy values regarding the CBZ;
while the dyes reduction potential moves to more negative values. All
these changes can be corroborated in Table I, however, these displace-
ments were not affected in the same magnitude. Thus, the Py-TPA
formal potential difference (Ec1/2-Ea1/2) was larger enough to provide
the required driving force in the annihilation reaction for the efficient
formation of ECL emissive excited states. It was also observed for
Py-TPA (Figure 3) that its radical cation formation occur reversibly at
a well-defined potential, while the Py-CBZ first oxidation overlapped
with the dication formation processes. Thus, Py-TPA has remarkable
optoelectronic characteristics, which makes it the best ECL emitter
within the studied dyes.
ing molecular engineering all these drawbacks were systematically
overcome by bonding electron donor moieties (CBZ or TPA) through
unsaturated bridges (aryl alkyne) to Py reactive sites (1-, 3-, 6- and 8-
positions). In addition, the blockade of reactive positions of CBZ and
TPA with bulky alkyl groups avoids the typical coupling-dimerization
reaction, stabilizing the radical cations on the dendritic structures.
Simultaneously, the photophysical (Absλmax, E00, FL λmax, ꢀFLd), pho-
tochemical (low aggregation effect and oxygen quenching), electro-
chemical (redox potentials) and optoelectronic (HOMO-LUMO gap
and their energy position) properties were tuned in order to obtain two
molecules with a high ECL emitting yield (Py-CBZ and Py-TPA).
Hence, the used molecular design strategy succeed in produce new
dyes which efficiently emit ECL light. This study demonstrate that
the functionalization of organic dyes with redox moieties (in the ad-
equate positions and with the proper linker) allows to improve the
radical stability and modulate the optoelectronic properties with the
consequent enhancement of the ECL performance and their potential
use in applications such as organic light-emitting devices.
Acknowledgments
We thank Consejo Nacional de Investigaciones Cient´ıficas y
Te´cnicas (CONICET-Argentina), Agencia Nacional de Promocio´n
Cient´ıfica y Tecnolo´gica (ANPCYT Argentina), Universidad Nacional
de Rosario y Universidad Nacional de R´ıo Cuarto. M.I.M, R.A.S,
M.V.C, G.M.M, and F.F. are scientific members of CONICET.
ORCID
Mar´ıa I. Mangione https://orcid.org/0000-0003-4264-4392
Rolando A. Spanevello https://orcid.org/0000-0003-3701-5807
Fernando Fungo https://orcid.org/0000-0003-3291-0829
References
1. A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundamentals and Appli-
cations, 2nd Edition, John Wiley & Sons, Inc., New York (2001).
2. A. J. Bard, Electrogenerated Chemiluminescence, Marcel Dekker, New York (2004).
3. L. R. Faulkner and A. J. Bard, in Electroanal. Chem., A. J. Bard, ed., p. 1, Dekker,
New York, (1977).
4. M. Hesari and Z. Ding, J. Electrochem. Soc., 163, H3116 (2016).
5. J. Sun, H. Sun, and Z. Liang, ChemElectroChem, 4, 1651 (2017).
6. G. Marzari, M. V. Cappellari, G. M. Morales, and F. Fungo, Anal. Methods, 9, 2452
(2017).
7. J. Sun, W. Gao, L. Qi, Y. Song, P. Hui, Z. Liu, and G. Xu, Anal. and Bioanal. Chem,
410, 2315 (2018).
8. J. L. Liu, M. Zhao, Y. Zhuo, Y. Q. Chai, and R. Yuan, Chem. Eur. J., 23, 1853 (2017).
9. K. Hong, M. G. Kim, H. M. Yang, D. C. Lim, J. Y. Lee, S. J. Kim, I. Lee, K. H. Lee,
and J. L. Lee, Adv. Energy Mater., 6 (2016).
10. T. Kasahara, S. Matsunami, T. Edura, R. Ishimatsu, J. Oshima, M. Tsuwaki, T. Imato,
S. Shoji, C. Adachi, and J. Mizuno, Sens. Actuators, B, 207, 481 (2015).
11. S. H. Kong, J. I. Lee, S. Kim, and M. S. Kang, ACS Photonics, (2017).
12. N. J. Turro, V. Ramamurthy, and J. C. Scaiano, Principles of molecular photochem-
istry: an introduction, University science books (2009).
13. M. Hermenau, M. Riede, K. Leo, S. A. Gevorgyan, F. C. Krebs, and K. Norrman,
Sol. Energy Mater. Sol. Cells, 95, 1268 (2011).
Conclusions
Polycyclic aromatic hydrocarbons are promising candidates for
ECL applications due to the combination of an electrochemical ac-
tive center with a high quantum fluorescence yields. However, some
disadvantages such as the flat molecular structure, which favored
light quenching through intermolecular interactions or formation of
charged species with poor chemical stabilities, among others, decrease
the efficiency to generate the ECL phenomenon. Notwithstanding, us-
Downloaded on 2018-11-09 to IP unknown address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

G170
Journal of The Electrochemical Society, 165 (14) G163-G170 (2018)
14. T. M. Figueira-Duarte and K. Mu¨llen, Chem. Rev., 111, 7260 (2011).
15. K. Kalyanasundaram and J. Thomas, J. Am. Chem. Soc., 99 (1977).
16. J. Maloy and A. J. Bard, J. Am. Chem. Soc., 93, 5968 (1971).
17. M. M. Richter, Chem. Rev., 104, 3003 (2004).
18. A. G. Crawford, A. D. Dwyer, Z. Liu, A. Steffen, A. Beeby, L.-O. Paa˚lsson,
D. J. Tozer, and T. B. Marder, J. Am. Chem. Soc., 133, 13349 (2011).
19. X. Feng, J. Y. Hu, C. Redshaw, and T. Yamato, Chem. Eur. J., 22, 11898 (2016).
20. F. Fungo, K.-T. Wong, S.-Y. Ku, Y.-Y. Hung, and A. J. Bard, J. Phys. Chem. B, 109,
3984 (2005).
31. Z.-Q. Liang, Z.-Z. Chu, J.-X. Yang, C.-X. Yuan, X.-T. Tao, and D.-C. Zou, Synthetic
Metals, 161, 1691 (2011).
32. M. I. Mangione and R. A. Spanevello, Tetrahedron Lett., 56, 465 (2015).
33. Y. Xing, X. Xu, P. Zhang, W. Tian, G. Yu, P. Lu, Y. Liu, and D. Zhu, Chem. Phys.
Lett., 408, 169 (2005).
34. C. K. Tseng, C. R. Lee, M. C. Tseng, C. C. Han, and S. G. Shyu, Dalton Trans., 43,
7020 (2014).
35. J. Chen, S. Ko, L. Liu, Y. Sheng, H. Han, and X. Li, New J. Chem., 39, 3736 (2015).
36. J. y. Hu, M. Era, M. R. Elsegood, and T. Yamato, Eur. J. Org. Chem., 2010, 72
(2010).
21. J. Natera, L. Otero, L. Sereno, F. Fungo, N.-S. Wang, Y.-M. Tsai, T.-Y. Hwu, and
K.-T. Wong, Macromolecules, 40, 4456 (2007).
37. C. Glaser, Ber. Dtsch. Chem. Ges., 2, 422 (1869).
22. D. Heredia, J. Natera, M. Gervaldo, L. Otero, F. Fungo, C.-Y. Lin, and K.-T. Wong,
38. C.-Z. Wang, X. Feng, Z. Kowser, C. Wu, T. Akther, M. R. Elsegood, C. Redshaw,
Org. Lett., 12, 12 (2009).
23. Y. O. Lee, T. Pradhan, K. Choi, D. H. Choi, J. H. Lee, and J. S. Kim, Tetrahedron,
69, 5908 (2013).
and T. Yamato, Dyes Pigm., (2018).
39. S. N. Keller, A. D. Bromby, and T. C. Sutherland, Eur. J. Org. Chem., 2017, 3980
(2017).
24. Y. O. Lee, T. Pradhan, S. Yoo, T. H. Kim, J. Kim, and J. S. Kim, J. Org. Chem., 77,
11007 (2012).
40. M. I. Mangione, R. A. Spanevello, D. Minudri, D. Heredia, L. Fernandez, L. Otero,
and F. Fungo, Electrochim. Acta, 207, 143 (2016).
25. J. W. Oh, Y. O. Lee, T. H. Kim, K. C. Ko, J. Y. Lee, H. Kim, and J. S. Kim, Angew.
Chem. Inter. Ed., 48, 2522 (2009).
26. C. M. Cardona, W. Li, A. E. Kaifer, D. Stockdale, and G. C. Bazan, Adv. Mater., 23,
2367 (2011).
27. K. M. Omer, S. Y. Ku, K. T. Wong, and A. J. Bard, Angew. Chem. Inter. Ed., 48, 9300
(2009).
28. M. Hesari, K. N. Swanick, J.-S. Lu, R. Whyte, S. Wang, and Z. Ding, J. Am. Chem.
Soc, 137, 11266 (2015).
29. I.-S. Shin, S. Yoon, J. I. Kim, J.-K. Lee, T. H. Kim, and H. Kim, Electrochim. Acta,
56, 6219 (2011).
41. J. Sung, P. Kim, Y. O. Lee, J. S. Kim, and D. Kim, J. Phys. Chem. Lett., 2, 818 (2011).
42. H. Maeda, T. Maeda, K. Mizuno, K. Fujimoto, H. Shimizu, and M. Inouye, Chem.
Eur. J., 12, 824 (2006).
43. K. C. Moss, K. N. Bourdakos, V. Bhalla, K. T. Kamtekar, M. R. Bryce, M. A. Fox,
H. L. Vaughan, F. B. Dias, and A. P. Monkman, J. Org. Chem., 75, 6771 (2010).
44. Y. O. Lee, T. Pradhan, K. No, and J. S. Kim, Tetrahedron, 68, 1704 (2012).
45. Y. Shirota and H. Kageyama, Chem. Rev., 107, 953 (2007).
46. B. Fleet, G. Kirkbright, and C. Pickford, J. Electroanal. Chem. Interfacial Elec-
trochem., 30, 115 (1971).
47. I. Berlman, Handbook of Fluorescence Spectra of Aromatic Molecules, Academic
30. G. Venkataramana and S. Sankararaman, Eur. J. Org. Chem., 2005, 4162 (2005).
Press, New York and London (1971).
Downloaded on 2018-11-09 to IP unknown address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).