Synthesis and photophysical properties of highly fluorescent 2-aryl-6-(aryleneethynylene)-1H-indoles

Article abstract of DOI:10.1016/j.dyepig.2016.05.017

Nine novel 2-aryl-6-(aryleneethynylene)-1H-indoles were prepared by one pot Sonogashira cross-coupling in DMSO and fluoride promoted cyclization followed by N-alkylation. The photophysical properties of these compounds are described. Absorption and excita

Full text of DOI:10.1016/j.dyepig.2016.05.017

Dyes and Pigments 133 (2016) 41e50
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis and photophysical properties of highly fluorescent 2-aryl-6-
(aryleneethynylene)-1H-indoles
,
*
b
Mariana Flores-Jarillo a, Alejandro Alvarez-Hernandez a , Rosa Angeles Vazquez-García ,
ꢀ
,
**
c
c
Eduardo Arias c , Ivana Moggio , Jose Roman Torres
ꢀ
ꢀ
a
ꢀ
ꢀ
Area Academica de Química, Universidad Autonoma del Estado de Hidalgo, Carr. Pachuca-Tulancingo km 4.5, Hidalgo, 42184, Mexico
Area Academica de Ciencias de la Tierra y Materiales, Universidad Autonoma del Estado de Hidalgo, Carr. Pachuca-Tulancingo km 4.5, Hidalgo, 42184,
b
ꢀ
ꢀ
ꢀ
Mexico
c
ꢀ
Centro de Investigacion en Química Aplicada, Blvd. Enrique Reyna 140, 25294, Saltillo, Coahuila, Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
Nine novel 2-aryl-6-(aryleneethynylene)-1H-indoles were prepared by one pot Sonogashira cross-
coupling in DMSO and fluoride promoted cyclization followed by N-alkylation. The photophysical
properties of these compounds are described. Absorption and excitation spectra of these compounds
were independent of the solvent polarity, while their emission spectra showed a pronounced depen-
dence. Fluorescence quantum yields in solution were very high and decreased with solvent polarity;
Received 14 December 2015
Received in revised form
1 March 2016
Accepted 13 May 2016
Available online 24 May 2016
possible processes that account for excessive values of
4 are discussed. Cyclic voltammetry studies
indicate irreversible redox processes and DFT calculations suggest they occur in the indole segment.
Keywords:
Arylindoles
© 2016 Elsevier Ltd. All rights reserved.
Intense fluorescence
Quantum yield
Solvatofluorochromism
Cyclic voltammetry
1. Introduction
arylindoles [12], we set out to prepare derivatives with an addi-
tional structural group that could expand the conjugated system
Indoles are privileged structures because of their ubiquity in
natural products and wide span of biological activity. In addition,
indoles are unsaturated, electron rich compounds capable to
display physical properties with great potential for many different
applications. Fluorescent indole derivatives are common structures
used for the design of fluorescent probes for a myriad of analytical
applications [1e4]. Some of these compounds display nonlinear
optics and have served to prepare two photon fluorescent probes
[5] for cellular imaging and even used for photothermal ablation of
cancer cells [6,7]. Indole-derived compounds may also display
photochromism [8,9] and electroluminescence [10] and thus have
potential for application in optoelectronic materials and solar cells
[11]. These remarkable properties of indole derivatives prompted
us to start our own studies aimed to obtain highly fluorescent small
conjugated molecules. Based on the known high fluorescence of 2-
and modify its properties, and the arylalkyne moiety was chosen
for its well documented participation in aryleneethynylene (AE)
chromophores of high quantum yield [13,14]. This led to the
preparation of 2-aryl-6-(aryleneethynylene)-1H-indoles of type 1
and herewith we report on the synthesis, photophysical and elec-
tronic properties of these novel, highly fluorescent compounds
(Scheme 1).
2. Experimental
2.1. Synthesis
Full experimental details for preparation of all of the compounds
involved in this study, spectroscopic characterization data and
selected NMR spectra can be found in the supplementary
information.
* Corresponding author.
** Corresponding author.
2.2. Theoretical calculations
E-mail addresses: alvarez@uaeh.edu.mx (A. Alvarez-Hernandez), eduardo.arias@
ciqa.edu.mx (E. Arias).
All calculations were performed with Spartan 14 v.1.1.2
http://dx.doi.org/10.1016/j.dyepig.2016.05.017
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

42
M. Flores-Jarillo et al. / Dyes and Pigments 133 (2016) 41e50
(Wavefunction. Inc). First the equilibrium conformer at ground
state was found at AM1 level and then this geometry was further
optimized through Density Functional Theory (DFT) calculations at
the restricted B3LYP level with 6-31G(d) basis set. The coordinates
generated during the DFT optimization processes were used as
input for the electronic excited state calculations and the first six
singlet electronic excited states were calculated by TDDFT with the
same basis set and in vacuum. The UV/Vis spectra were calculated
by running a single point TDDFT calculation using the Tamm-
Dancoff Approximation (TDA). By default only pairs of filled/un-
filled orbitals which have amplitudes larger than 0.15 are
considered.
deoxygenated by bubbling nitrogen at least for 15 min. The ex-
periments were carried out under nitrogen atmosphere at a scan-
ning rate of 50 mV/s. The molecular orbital energies HOMO and
LUMO were calculated from the first oxidation (Eox) and reduction
(Ered) potentials with the relationship [17,18] E HOMO (LUMO) ¼ [-
e(Emax (Ox[red] vs. Ag/AgCl)]-4.8.
3. Results and discussion
3.1. Synthesis
The synthesis of the target indole compounds 1a-i was envis-
aged as a straightforward sequence of Sonogashira couplings be-
tween the arylcarbamates 2 or 5 [19] with arylalkynes 3a-g,
followed by cyclization and N-alkylation (Scheme 2). This synthetic
strategy could be applied to obtain even more conjugated mole-
cules if an arylethynylene oligomer is used as the arylalkyne 3 in
Scheme 2.
2.3. Photophysical properties
The photophysical characterization was carried out in spectro-
scopic grade CH2Cl2, DMSO and toluene from Aldrich. UVeVis ab-
sorption spectra were measured on
a Shimadzu 2401PC
Carbamate 2 allowed introduction of identical aryl groups by
Sonogashira coupling, while carbamate 5 made possible the
sequential introduction of different aryl groups through a regio-
selective coupling at the CeI bond [19], followed by a second
coupling at the CeBr bond [20]. Substituents R1 and R3 on the aryl
groups were designed to modulate electronic donation/withdrawal
on these molecules and the N-alkylation step was deemed neces-
sary to make these compounds soluble in organic solvents (i.e.
these compounds are soluble in toluene, dichloromethane and
DMSO as representative common organic solvents of different po-
larity). The attempted N-alkylation of indoles 1g and 1h containing
carbonyl groups, showed extensive decomposition under experi-
mental conditions and consequently these less soluble, free NH
indoles were studied as such.
spectrophotometer. Optical band gap (Egopt) was determined from
the intercept with the X axis of the tangent of the absorption
spectrum drawn at absorbance of 0.1. Stokes shift (Dn) was deter-
mined as the difference between the lower energy absorption and
higher energy emission bands in wavenumbers. Emission and
excitation spectra have been recorded with a Perkin-Elmer LS 50B
spectrofluorometer. Excitation spectra were obtained by fixing the
maxima of fluorescence as the emission wavelength. Fluorescence
quantum yields in solution (
reported procedure [15] using quinine sulphate in H2SO4 0.1 M
¼ 0.54 at 310 nm) as the standard. The excitation wavelength
4) were determined according to the
(4
was 10 nm lower than the absorption maximum. Measurements
were carried out by controlling the temperature at 25.0 0.5 ꢀC
with a water circulating bath. Three solutions with absorbance at
the excitation wavelength lower than 0.1 were analyzed for each
sample and the quantum yield was averaged. Lifetimes were ob-
tained by TCSPC (Time-correlated single photon counting) with a
Tempro Horiba equipment with 370 nm nanoLEDs. A 0.01% sus-
pension of Ludox AS40 (Aldrich) in ultrapure water was used for the
prompt signal. Calibration of the equipment was realized with a
POPOP [1,4-Bis(4-methyl-5-phenyl-2-oxazolyl)benzene] methanol
solution (optical density <0.1 and lifetime of 0.93 ns [16]). Data
were fit in the software DAS6 available in the equipment.
3.2. Theoretical calculations
All of the compounds 1a-i synthesized and studied in this work
are depicted in Scheme 3, they share the same general structure,
where a central indole is conjugated to an aryleneethynylene at the
C-6 position and to a phenyl group at C-2, variations in their
structure include the substituents on the aryl groups and the indole
N-substituent. The structural modulation obtained by changing
these substituents affected properties such as geometry at the
ground state, dipole moment, molecular orbitals, and excitation
energies as shown below.
2.4. Electrochemical studies
The electrochemical properties of all of the compounds were
investigated by cyclic voltammetry in a C3 Stand cell from Basi,
coupled to an ACM Gill AC potentiostat/galvanostat. The system
consisted in a conventional three-electrode cell: platinum or glassy
carbon as a working electrode (polished with alumina and diamond
powder after each run), Pt wire as the counter electrode, Ag/AgCl as
reference electrode and ferrocene/ferrocenium (FOC) as internal
reference (Eox ¼ 0.58 V, Ered ¼ 0.701 V); a value of ꢁ4.8 eV below
the vacuum level was considered. Voltammetric measurements
were performed at room temperature in CH2Cl2 containing
Bu4NPF6 (0.1 M) as the supporting electrolyte. Prior to recording the
voltammograms, all of the solutions (~0.5 mmol) were
3.2.1. Geometry at the ground state
For sake of discussion, Fig. 1 shows the optimized geometry of
1a as example of the series, where the aryl units have been
labelled according to the planes they form. Three planes can be
visualized: plane I of the aryleneethynylene segment, plane II of
the indole and plane III belonging to the phenyl directly
attached to the indole. Table 1 displays all the calculated dihe-
dral angles, where
(considering atoms C3, C5, C10 and C16 in Fig. 1) and
dihedral angles between planes II and III (considering atoms
C13, N, C18 and C22 in Fig. 1). We observed that
is close to 0ꢀ,
a
is the dihedral angle between plane I and II
b
is the
a
indicating that the indole is almost coplanar with the aryle-
neethynylene segment; this supported the assumption that this
part of the molecule is highly conjugated. On the contrary, a
larger deviation from zero is observed for the dihedral angle
between planes II and III; for instance, molecules 1f and 1g for
which R3 is hydrogen have a dihedral angle of around 27ꢀ and
the rest, for which R3 is a hexyl group, exhibit a larger distortion
with
being around 50ꢀ due to steric repulsion between the N-
b
b
Scheme 1. 2-aryl-6-(aryleneethynylene)-1H-indoles.
b

M. Flores-Jarillo et al. / Dyes and Pigments 133 (2016) 41e50
43
Scheme 2. Synthesis of indoles 1a-i.
Scheme 3. Chemical structure of indoles 1a-i bearing different electron donor and withdrawing groups synthesized and studied in this work.
hexyl and C-2 aryl group. This means that at the ground state,
the equilibrium conformer for these molecules has this phenyl
ring clearly out of the plane with respect to the
aryleneethynylene-indole unit. It is worth noting the larger
steric effect of this N-hexyl group (R2) on the dihedral angle
compared to the electronic effect of the aryl substituent R1
group.
withdrawing (A), indole is an electron donor (D) and the phenyl
group can either be a weak electron donor or electron acceptor,
depending on the electronic nature of the substituent functional
groups (R3 and R1). Based on this assumption, compound 1h
(R3 ¼ OCH3, R1 ¼ CN) has an alternated electronic configuration D-
A-D-A with a high dipole moment, while its isomeric compound 1i
(R3 ¼ CN, R1 ¼ OCH3) displays an electronic configuration A-A-D-D
that aligns the electronic push-pull effects in only one direction,
consequently this compound has the highest dipole moment in this
series. On the other side, 1a and 1b have intermediate dipole
moment values as the substituents R1 and R3 do not have a sig-
nificant electronic effect and thus the push-pull effect is derived
only from the two main systems, i.e. aryleneethynylene and aryl-
indole. In general, compounds with high electron donor character
3.2.2. Dipole moment
The electronic nature of substituents R3 and R1 on the aryl rings
is important in what concerns to the dipole moment
m of the
molecule, (Table 1). Structural fragments in molecules 1a-i behave
electronically different: phenyleneethynylenes act as electron

44
M. Flores-Jarillo et al. / Dyes and Pigments 133 (2016) 41e50
delocalization is practically divided in two blocks: the electronic
density of the HOMO is distributed along the
methoxyphenyleneethynylene-indole segment, indicating the high
electron donor character of this segment. On the other hand, the
LUMO is more localized on the cyanophenyl, due to the electron
withdrawing nature of the CN group. Distribution of frontier or-
bitals in the isomeric compound 1i is reversed; the HOMO is
distributed along the indol-ethynylene segment, while the LUMO
covers the cyano phenyleneethynylene fragment. All of the HOMO
and LUMO calculated values are collected in Table 1, from which it
can be observed that their values increase (i.e. they are less nega-
tive) as the electron donor character of the molecule increases.
According to these values, the best electron donor compound of the
series is 1e, while 1d is the best electron acceptor compound.
Compounds 1h and 1i present intermediate value for both HOMO
and LUMO values, which is in agreement with their more struc-
tured push-pull character. The band gap (Eg) was calculated as the
Fig. 1. DFT optimized geometry at ground state for equilibrium conformer of 1a.
DEHOMO-LUMO and the lowest value corresponds to compound 1d,
Table 1
Dihedral angles between planes I and II (a) and between II and III (b), dipole moment m, high occupied molecular and lowest unoccupied molecular orbitals (HOMOth, LUMOth),
optical band gap (Eg,th), TDDFT calculated absorption maximum (labs,th) and oscillator strength f of compounds 1a-i.
Compound
a
[ꢀ]
b
[ꢀ]
m
[D]
HOMOth [eV]
LUMOth [eV]
Eg,th [eV]
labs,th [nm]
f
1a
1b
1c
1d
1e
1f
1g
1h
1i
0.48
1.20
0.46
ꢁ0.32
1.52
0.83
2.61
1.90
0.54
51.57
51.48
49.23
48.90
53.00
27.33
27.26
48.41
52.33
3.03
3.17
2.43
2.58
5.08
1.55
4.87
5.56
9.85
ꢁ5.02
ꢁ4.91
ꢁ5.62
ꢁ5.75
ꢁ4.76
ꢁ5.57
ꢁ5.43
ꢁ5.10
ꢁ5.22
ꢁ1.22
ꢁ1.13
ꢁ2.21
ꢁ2.78
ꢁ0.97
ꢁ2.36
ꢁ2.15
ꢁ1.82
ꢁ1.82
3.80
3.78
3.41
2.97
3.79
3.21
3.28
3.28
3.40
343
342
383
454
344
405
398
407
386
1.5201
1.7081
1.6264
0.9652
1.8319
1.8329
1.8487
0.9696
1.3838
such as 1e (R3 ¼ R1 ¼ OCH3), have a larger dipole moment than
those with substituents with a negligible electronic contribution
such as 1a and 1b; in contrast, compounds 1c, 1d, 1f with R3 ¼ R1
groups with electron attractor character have the lowest dipole
moment. An intriguing case is the large difference of dipole
moment between the structurally related methylketone 1f (1.55)
and aldehyde 1g (4.87). An important observation is that the di-
rection of the dipole moment vector changed within the series. For
positioning these molecules as potential semiconducting materials.
3.2.4. Excitation energies
The first six excitation energies were calculated at the TTDFT/
B3LYP/6-31G* level and the simulated UVeVis spectra were con-
structed with Spartan using Gaussian convolution with half height
band width (HHBW) of 30 nm (see Supporting material). All com-
pounds show a main electronic transition in the UV-blue region,
with absorption maximum (labs,th, Table 1) ranging in opposite
trend to the Eg, i.e. the compound with the lowest Eg 1d corre-
sponds to the absorption at the longest wavelength. According to
the TDDFT formalism, this band that corresponds to the first excited
state is due to the HOMO/LUMO electronic transition. Other
bands in the higher energy region, around 270e320 nm, were also
found and they are due to the contribution of different electronic
transitions. More detailed, specific data for these higher energy
bands can be found in the Supporting information.
compounds 1a, 1b, 1e and 1g, with intermediate values of
vector is directed towards the indole, as shown in Fig. 2. For com-
pounds 1c, 1d, and 1f with rather low values of , the vector is
directed towards the aryleneethynylene. Finally, for compounds 1h
and 1i with high values of , the vector is oriented towards the
m, the
m
m
electron acceptor cyano group, where the negative side of the
dipole of the molecule is located.
3.2.3. Molecular orbitals
The frontier (HOMO and LUMO) and subsequent molecular or-
bitals (HOMO-1 and LUMOþ1) were calculated for all compounds
studied in this work by DFT methods. In general, the HOMO-LUMO
electronic distribution is clearly modified by the push-pull effects
of the aryl substituent R3 and R1 in this series. The HOMO is mainly
centered on the indole-ethynylene segment. On the contrary, the
LUMOs are delocalized over the entire molecule, with a tendency to
center on the electron withdrawing groups. Three particular cases:
1d, 1h and 1i worth to be visualized are shown in Fig. 3. Thus, for
1d, the LUMO is more localized on the nitrophenyl rather than on
the nitrophenyleneethynylene. The isomeric compounds 1h and 1i,
with strong electron donor and electron withdrawing groups
swapped at the extremities, show a particularly pronounced
change in HOMO-LUMO distribution. Thus, for 1h the electronic
3.3. Photophysical properties
The indole and aryleneethynylene chromophore fragments of
compounds 1a-i were expected to play a role in their photophysics.
Indole has been extensively studied due to its chemical similarity to
tryptophan. The photophysics of indole derivatives is complex [21];
two overlapping
p
-
p
* electronic transitions 1La and 1Lb (also known
0
as S1 and S1 ) [22e24] are responsible for their observed behavior.
Experimental evidence suggests that the 1Lb (S1) transition has a
dipole moment similar to the ground state, requires lower energy
and hence, occurs first. The La (S1 ) transition requires more energy
to occur, has a dipole moment larger than the ground state and
consequently is sensitive to solvent polarity [25]. Although it is rare
1
0

M. Flores-Jarillo et al. / Dyes and Pigments 133 (2016) 41e50
45
Fig. 2. Calculated dipole moment vectors (represented by yellow arrows) obtained from the DFT optimized geometries of selected molecules. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. DFT simulated frontier molecular orbitals, spatial distribution and energy levels for representative compounds 1d, 1h and 1i.

46
M. Flores-Jarillo et al. / Dyes and Pigments 133 (2016) 41e50
to observe both transitions separated in the same UV spectrum
there are a few cases, for example with 5- or 6-hydroxyindole [21],
in which this has been possible. Among different indole derivatives,
2-phenyl indoles are fluorescent molecules with quantum yields
larger than 0.8 [12]. On the other hand, phenyeleneethynylenes
(PEs) are fluorophores which display medium-high quantum yields
in solution [13,14] and usually have only one emitting state due to a
be considered for some selected molecules in future work. Whereas
no general feature can describe the emission behavior of the whole
series, there are some interesting observations: i) 1a, 1b show
excitonic like spectra and no significant solvatochromism as the
differences in the emission maxima and quantum yields are within
the experimental error of the determination, ii) the spectra of 1c,
1e, 1f, 1g, 1h and 1i, which are excitonic in toluene, become broader
in dichloromethane and even more in DMSO. For example, the
HHBW of 1f increases from 49 nm in toluene to 79 nm in CH2Cl2
and to 89 nm in DMSO, accordingly, the Stokes shift also increases,
iv) for all these molecules, the quantum yield decreases with the
polarity of the solvent.
p-p* electronic transition. This localized excited state makes the
maximum emission wavelength independent of the solvent. As
found in the theoretical calculations, Fig. 1, the indole and the
phenyleneethynylene are practically coplanar in the ground state
with an extended electronic delocalization.
When absorption of compounds 1a-i was studied in toluene,
dichloromethane and DMSO the absorption coefficient was high
and no significant changes in the wavelength maxima were
observed. For these compounds the main peak in the UV spectra
The Stokes shift values (Dn) in toluene are in the range of mol-
ecules that undergo geometry change after excitation [26,27], but
in general lower to those usually reported for ICT states [31].
Intramolecular charge transfer can eventually occur in DMSO, and
in particular for 1h and 1i, which is in agreement with their
observed large HHBW and Stokes shift and can explain their lower
fluorescence quantum yields found in this solvent.
(labs) is located between 330 nm and 406 nm, depending on the
overall conjugation of the molecule that in turn is modulated by the
substituents on the aryl groups. (Tables 2e4). There is a good
agreement between the experimental and simulated spectra and
between the absorption maxima collected in Table 1 (theoretical)
and 2 (experimental), theoretical values are slightly larger than
experimental ones as usually found when TDDFT simulation is
carried out. [ 26,27] On the other hand, pronounced changes in the
emission spectra of these compounds due to change in solvent
polarity were observed (Tables 2e4). For example, while compound
1c showed nearly identical UV absorption spectra in all solvents
A particular case is compound 1d, for which the fluorescence
spectrum strongly red shifts and decreases in intensity with
increasing excitation wavelength (Fig. 4). Contrary to what was
observed for the rest of the compounds, the excitation spectrum of
1d does not show the electronic transition that gives rise to the
absorption band at 403 nm. This result could be explained on the
basis that a non-emissive state is reached after excitation, perhaps
related to the nitrophenyl chromophore which by coincidence ap-
pears at the same wavelength (400e405 nm). There is overlap
between the absorption and emission spectra and then we cannot
discard that the fluorescence bands are derived from internal
charge transfer. This interpretation could explain why the emission
band shifts and the large Stokes shift; the shift of the emission band
in a very polar solvent (DMSO) and the low quantum yield found. As
pointed out previously, indoles present two excited states S1 and
(labs 363 nm in toluene, 367 nm in dichloromethane and 360 nm in
DMSO, see entry 3 in Tables 2e4), its emission spectra showed a
bathochromic shift with the polarity of the solvent (lem 416,
431 nm in toluene, 426 nm in dichloromethane and 460 nm in
DMSO). This solvatofluorochromism is in line with reported ob-
servations in indole derivatives [21e24,28]. It is worth mentioning
that solvent dependent spectral shifts of dyes are ordinarily studied
by applying the Lippert-Mataga equation [29,30]. This equation
describes the Stokes shifts as a function of the change in the dipole
moment of the dye upon excitation. Plots of the Stokes shift vs. the
polarizability function of several solvents are drawn, and the slope
of the fitted lines can give the difference between the ground and
excited state dipole moment Dmeg. When the ground state dipole
moment is known or calculated (as in our case), the excited state
dipole moment can be derived. In this paper, we used DCM, DMSO
and toluene as they are good solvents for the entire family, allowing
a quantitative and overall picture of the photophysical properties of
the molecules. Several solvents of different polarity, normally 10 to
20, are needed to build those plots. However, some of them such as
alkanes and alcohols do not dissolve any of the molecules studied in
this work, others such as THF and acetonitrile dissolve, at least
partially, some molecules but not all of them. Based on these lim-
itations, the determination of the excited state dipole moment
could not be realized at this time by Lippert-Mataga plots but will
0
ꢀ
S1 , which according to the recent model by Catalan [24] the lower
0
energy excited state S1 is highly dipolar giving a structureless
emission that shifts depending of the polarity of the medium.
However, the theoretical study of the molecules in this work
already mentioned, suggested that conjugation involves both the
indole and phenyleethynylene moieties, i.e. the indole group is not
acting as an isolated chromophore and in consequence we do not
relate entirely the behavior of these molecules to the S10 properties
of the pure indole chromophore.
Besides 1d, all of the other molecules had very high fluorescence
quantum yields (
times between 1 and 2 ns? Notably, the measured quantum yield
) for 1a, 1 b and 1c (in toluene) 1a, 1e (in dichloromethane) and
4) with monoexpontential time decay and life-
(4
1b (in DMSO) gave a value larger than 1, which for definition must
be ꢂ 1. It is important to specify that the fluorescence quantum
yield was determined by the indirect dilution method of Williams
[15] using quinine sulfate as standard and full precautions were
Table 2
Optical properties of compounds 1a-i in toluene.
Compound
Egopt [eV]
labs [nm]
ε [104 Mꢁ1cmꢁ1
]
lem [nm]
HHBWem [nm]
Dn [cmꢁ1
]
F
[%]a
t
[ns]
krad [sꢁ1
]
1a
1b
1c
1d
1e
1f
1g
1h
1i
3.31
3.29
3.04
2.69
3.27
2.94
2.89
2.98
2.99
330
333
363
397
333
382
385
360
357
3.4
3.5
4.6
3.9
4.3
4.7
2.5
3.6
2.0
382, 399
385, 403
416, 431
495
384, 402
419, 440
427, 450
422
56
56
55
75
56
49
53
63
45
4125
4056
3510
4987
3989
2312
2555
4081
4030
108.0
124.5
123.0
15.7
101.9
91.2
76.3
60.0
43.7
1.21
1.14
1.34
1.51
1.14
1.10
1.12
1.51
1.18
8.92$108
1.09$109
0.92$109
0.10$109
0.89$109
0.83$109
0.68$109
0.39$109
0.37$109
417, 435
a
The quantum yield has an accuracy of 10% of the value shown.

M. Flores-Jarillo et al. / Dyes and Pigments 133 (2016) 41e50
47
Table 3
Optical properties of compounds 1a-i in CH2Cl2.
Compound
Egopt [eV]
labs [nm]
ε [104 Mꢁ1cmꢁ1
]
lem [nm]
HHBWem [nm]
Dn [cmꢁ1
]
F
[%]a
t
, [ns]
krad [sꢁ1
]
1a
1b
1c
1d
1e
1f
1g
1h
1i
3.33
3.28
2.98
2.58
3.27
2.92
2.86
2.99
3.04
332
333
367
403
335
381
390
364
353
3.8
3.9
5.0
4.0
4.5
5.1
2.8
3.7
2.1
387, 399
387, 400
426
456
386
449
460
463
444
59
59
62
64
59
79
76
79
67
4281
4190
3774
2884
3944
3975
3902
5874
5806
120.0
94.0
99.2
0.2
114.4
77.2
62.7
100.0
100.7
1.35
1.23
1.43
0.42
1.18
1.47
1.66
2.08
2.06
8.88$108
7.64$108
6.94$108
4.76$106
9.69$108
5.25$108
3.78$108
4.81$108
4.89$108
a
10% of error.
Table 4
Optical properties of compounds 1a-i in DMSO.
Compound
Egopt [eV]
labs [nm]
ε [104 Mꢁ1cmꢁ1
]
lem [nm]
HHBWem [nm]
Dn [cmꢁ1
]
F
[%]a
t
[ns]
krad [sꢁ1
]
1a
1b
1c
1d
1e
1f
1g
1h
1i
3.27
3.23
2.88
2.51
3.22
2.76
2.71
2.87
2.93
333
335
360
406
337
386
395
367
353
3.5
3.8
4.1
3.5
4.6
4.7
2.6
3.1
1.8
403
405
460
458
410
493
506
499
451
58
58
82
73
58
89
86
86
69
5216
5159
6039
2766
5283
5623
5554
7208
6156
89.5
1.36
1.23
1.92
0.17
1.23
1.40
0.58
2.32
2.22
0.66$109
0.91$109
0.23$109
1.64$106
0.46$109
0.91$108
0.76$108
0.28$109
0.18$109
111.6
43.4
2.8$10ꢁ2
56.9
12.7
4.4
64.8
40.0
a
10% of error.
proposed the existence of triplet states. These triplets by means of
intersystem crossing [32] can repopulate the first excited state
[33,34]. This process can occur by two mechanisms: thermally
activated delayed fluorescence (TADF, in which a small energy gap
between singlet and triplet states allows reversible intersystem
crossing and thermal repopulation of S state from T state) and
triplet-triplet annihilation (which consists of the combination of
two triplets to form a singlet state from which emission is
observed). Very high quantum yields and quite small Stokes shifts
attributed to thermally activated delayed fluorescence (TADF) have
been reported [34]. The critical point to achieve TADF is the com-
bination of a small energy gap between singlet and triplet excited
states and a radiative decay constant (krad) larger than 106 sꢁ1
;
according to our data, the observed krad~ 107 sꢁ1 in our compounds
satisfies this requirement. However, at present time there is no
experimental evidence to unambiguously support (or rule out) any
of these possible pathways. The observation that the measured
value of
4 for this series is generally lower in DMSO is likely due to
charge transfer, which is also indicated by the larger Stokes shifts
observed in this solvent, this makes possible non-radiative deac-
tivation pathways. However, not only the solvent polarity plays an
active role on the deactivation process, structural factors are
important as well [22e24]. For example, contrary to all other
congeners, compound 1b shows high quantum yield in the three
solvents tested despite their different polarity.
Fig. 4. Fluorescence spectrum of 1d in toluene obtained at 320 (red solid line), 360
(black dash line) and 400 nm (black dotted line) of excitation wavelength. The inserted
figure corresponding to excitation spectra, including 1a (green circles), was added as
reference. *Indicates the peak related to the emission scattering. (For interpretation of
the references to colour in this figure legend, the reader is referred to the web version
of this article.)
3.4. Electrochemical studies
taken to avoid experimental artifacts (see Supporting information).
Cyclic voltammetry. Table 5 summarizes the electrochemical
parameters of all of the molecules, which exhibit the following
behavior: a) when oxidation or reduction (if any) processes are
separately analyzed, both are irreversible in most of the cases; b)
products are electroactive only when forward (from þ3 to ꢁ3 V)
scans are analyzed, with the exception of 1c, 1d and 1f whose
voltammograms could be obtained in either direction (from þ3 to
ꢁ3 V or by reverse scan from -3 V to þ3 V); c) all of the other
compounds tend to deposit on the electrodes during the oxidation
However, it is possible that these excessive
4 values arise from
other sources; it may be that the absorption and emission bands
used to calculate the quantum yield do not entirely correspond to
the same process, since the excited state 1Lb gives rise to 1La and the
observed emission may have components from both states, this has
been called an anomalous emission [21]. In addition, other simul-
taneous emission processes can also occur. For example, by means
of ultrafast electron diffraction studies on indole, Zewail has

48
M. Flores-Jarillo et al. / Dyes and Pigments 133 (2016) 41e50
process. These deposits explain the high current peak intensity
observed in the voltammograms and suggests that transport by
adsorption is the dominant process, Fig. 7S.
In order to better understand the electrochemical behavior of
these compounds, Fig. 5 shows the voltammograms for 1h, repre-
sentative of the whole series, along with a phenyleneethynylene
trimer (3PEBz) and the 5-bromo-2-methylindole (BMI), used for
comparison as references to the electrochemical behavior of the
ethynylene and indole fragments contained in the indoles 1a-i re-
ported herein. Thus, it is observed that the phenyleneethynylene is
either difficult to be oxidized (þ1.78 V) or reduced (ꢁ2.09 V) [35].
On the contrary, the first oxidation (þ1.13 V) and reduction
(ꢁ1.04 V) peaks of the 5-bromo-2-methyl indole (BMI) indicate that
these processes are more likely to occur, but still they are irre-
versible. It has been reported that upon anodic electro oxidation
indoles form soluble or insoluble products depending on the
electron donor/acceptor ability of the substituting group on the
indole [36]. Furthermore, it is known that polymerization of indoles
can occur at the C-2 and C-3 positions [37,38]. Thus, 5-bromo-2-
methylindole (BMI), used as reference in this work, underwent
electro oxidation evidenced by appearance of a light green color in
solution and a green-black solid deposit on both the GC and plat-
inum wire electrodes; in addition, the anodic peaks were very
broad. In contrast, only one peak was observed in the first cathodic
cycle at ꢁ1.04 V. This peak was shifted to more negative values at
ꢁ1.67 V for subsequent cycles, likely due to electrochemical
modification of the structure. From these data and Fig. 5 it is clear
that the most energetically favorable oxidation and reduction oc-
curs in the indole fragment, while these processes require sub-
stantially more energy to occur in the acetylene group. Accordingly,
the first anodic and cathodic peaks in all of the compounds 1a-i on
this series were assigned to the oxidation-reduction of the indole
group and their values were used to calculate their HOMO and
LUMO energies. Variations in the magnitude of these energies are a
consequence of the electronic push/pull effects of the functional
groups at the extremities. It is worth mentioning that the expected
aryleneethynylene reduction band was only observable for com-
pounds 1a, 1d, 1f, 1g, and 1h, substituted with strong electron
withdrawing groups. The formation of insoluble films deposited on
the working electrode during electro oxidation indicates electro-
chemical modification of indoles 1a-i. According to literature [39]
the presence of two or more peaks in the voltamogramms of con-
jugated macromolecules can be due to the formation of polarons
(lower voltage oxidation peaks) and bipolarons (oxidation peaks at
higher voltages) or to the separate contribution of the different
functional groups on the molecule [37,40] For example, in isomeric
compounds 1h and 1i, it may be considered that the first group to
be electrooxidized is the indole (peak þ I, in curve (b) in Fig. 5),
followed by the electron donor OCH3 (peak þ II) and, at last, the
Fig. 5. Comparative electrochemical oxidative and reductive potential of a) 5-bromo-
2-methylindole (MBI), b) 1h and c) phenyleneethynylene (3PEBz).
phenyleneethynylene group (peak þ III).
4. Conclusion
In summary, novel 2-aryl, 6-(aryleneethynylene)-1H-indoles
were readily prepared by one pot Sonogashira coupling and
Table 5
Electrochemical properties of compounds 1a-i studied in this work and 5-bromo-2-methylindole (BMI) and a phenyleneethynylene trimer (3PEBz) used as reference; V vs. Ag/
AgCl.
II
II
III
Compound
E
(V)
EIred (V)
EIox (V)
E
(V)
E (V)
ox
HOMO (eV)
LUMO (eV)
Eg (eV)
red
ox
1a
1b
1c
1d
1e
1f
1g
1h
1i
e
e
ꢁ1.07
ꢁ0.99
ꢁ1.01
ꢁ1.11
ꢁ1.09
ꢁ1.02
ꢁ1.04
ꢁ0.99
ꢁ1.09
ꢁ1.04
ꢁ2.09
þ1.07
þ1.07
þ1.32
þ1.31
þ0.89
þ1.04
þ1.07
þ0.96
þ1.19
þ1.13
þ1.78
þ1.42
þ1.48
þ1.70
þ1.70
þ1.66
þ1.60
þ1.52
e
þ2.37
þ2.25
þ2.26
þ2.38
þ2.26
þ2.52
þ1.99
þ2.45
þ2.17
e
ꢁ5.87
ꢁ5.87
ꢁ6.12
ꢁ6.11
ꢁ5.69
ꢁ5.84
ꢁ5.87
ꢁ5.76
ꢁ5.99
ꢁ5.93
ꢁ6.58
ꢁ3.73
ꢁ3.81
ꢁ3.79
ꢁ3.69
ꢁ3.71
ꢁ3.78
ꢁ3.76
ꢁ3.81
ꢁ3.71
ꢁ3.76
ꢁ2.71
2.14
2.06
2.33
2.42
1.98
2.06
2.11
1.95
2.28
2.17
3.87
ꢁ2.23
ꢁ1.64
e
ꢁ1.93
ꢁ1.98
ꢁ2.14
e
þ1.52
þ1.61
e
BMI
3PEBz
e
e
e

M. Flores-Jarillo et al. / Dyes and Pigments 133 (2016) 41e50
49
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MFJ is thankful to CONACyT for a graduate fellowship, authors
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2013-221360).
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