Evidence of a Photoinduced Electron-Transfer Mechanism in the Fluorescence Self-quenching of 2,5-Substituted Selenophenes Prepared through in Situ Reduction of Elemental Selenium in Superbasic Media

Article abstract of DOI:10.1021/acs.joc.1c00874

A series of new 2,5-disubstituted selenophene derivatives are described from elemental selenium and 1,3-diynes in superbasic media. The activation of elemental selenium in a KOH/DMSO system allows cyclization with conjugated diynes at room temperature. The cyclization reaction is extended to a broad range of functional groups, for which photophysics were experimentally and theoretically investigated. The selenophene derivatives present absorption maxima in the UV-A region and fluorescence emission in the violet-to-blue region. Fluorescence decay profiles were obtained showing a monoexponential decay with fast fluorescence lifetimes (～0.118 ns), as predicted by the Strickler-Berg relations. In general, in both investigations, no dependence on the solvent polarity on the absorption and emission maxima location was observed. On the other hand, solvents and substituents are shown to play a role in the fluorescence quantum yield values. In addition, a fluorescence self-quenching behavior could be observed, related to a photoinduced electron-transfer mechanism. Theoretical calculations performed at the MP2/ADC(2)/cc-pVDZ level of theory were performed in order to investigate the photophysical features of this series of selenophene derivatives.

Full text of DOI:10.1021/acs.joc.1c00874

pubs.acs.org/joc
Article
Evidence of a Photoinduced Electron-Transfer Mechanism in the
Fluorescence Self-quenching of 2,5-Substituted Selenophenes
Prepared through In Situ Reduction of Elemental Selenium in
Superbasic Media
̃
Helena Domingues de Salles, Felipe Lange Coelho, Douglas Bernardo Paixao,
Cristina Aparecida Barboza, Daniel da Silveira Rampon, Fabiano Severo Rodembusch,
and Paulo Henrique Schneider*
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ABSTRACT: A series of new 2,5-disubstituted selenophene derivatives are described from
elemental selenium and 1,3-diynes in superbasic media. The activation of elemental selenium
in a KOH/DMSO system allows cyclization with conjugated diynes at room temperature.
The cyclization reaction is extended to a broad range of functional groups, for which
photophysics were experimentally and theoretically investigated. The selenophene derivatives
present absorption maxima in the UV-A region and fluorescence emission in the violet-to-
blue region. Fluorescence decay profiles were obtained showing a monoexponential decay
with fast fluorescence lifetimes (∼0.118 ns), as predicted by the Strickler−Berg relations. In
general, in both investigations, no dependence on the solvent polarity on the absorption and
emission maxima location was observed. On the other hand, solvents and substituents are
shown to play a role in the fluorescence quantum yield values. In addition, a fluorescence self-
quenching behavior could be observed, related to a photoinduced electron-transfer
mechanism. Theoretical calculations performed at the MP2/ADC(2)/cc-pVDZ level of
theory were performed in order to investigate the photophysical features of this series of
selenophene derivatives.
In general, some synthetic methods involve precursors in
which the selenium atom is attached to its structure, as in the
case of electrophilic cyclization of selenoenynes (Scheme 1b)
or homopropargyl selenides (Scheme 1c), providing di- or
trisubstituted selenophenes19,20 or involve diynes (Scheme 1a)
or dienes (Scheme 1d) in the presence of dialkyl diselenides
and Lewis acid or selenocyanate and iodine, respectively,
resulting in mono- and tetrasubstituted selenophenes.2,21
Recently, the in situ activation of elemental selenium was
reported and the generated reactive Se2− species were used to
perform selenium functionalization of indoles22 and diorganoyl
selenide23 preparation. The same strategy can be investigated
to the selenophene synthesis as an alternative to the reported
methodologies that require iodine to promote cyclization or
metal catalysis combined with dialkyl diselenides or the
INTRODUCTION
■
The development in organochalcogen chemistry in the past
few decades has provided a series of new synthetic protocols
where chalcogen compounds emerge as synthetic intermedi-
ates to construct new chemical bonds or as new products with
promising application in different science fields.1 In this
context, chalcogen heterocycles stand out due to the diversity
of classes of compounds presenting the chalcogen as part of the
structure, such as, but not restricted to, chalcogenophenes,2
chalcogeno-indolizines,3 benzochalcogenadiazole,4 dichalcoge-
nazoles,5 and chalcogenapyrans,6 presenting high applicability
in materials science,7 medicinal chemistry, and biology.8
Among all these, selenophenes, in particular, are of great
relevance since their derivatives have shown biological
activities related to antidepressant,9 antibacterial,10 antitumor-
al,11,12 anti-inflammatory, and analgesic effects13 and interest-
ing electro-optical properties.14−17 Thus, it is expected that
novel and efficient methodologies have to emerge to support
the growing interest for new utilities to these heterocycles.
Since the pioneering studies reported by Paal in 1885,18 their
obtention is based on the addition of selenium species to
unsaturated acyclic reagents, as shown in Scheme 1.
Received: April 15, 2021
Published: July 20, 2021
https://doi.org/10.1021/acs.joc.1c00874
J. Org. Chem. 2021, 86, 10140−10153
© 2021 American Chemical Society
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Scheme 1. General Protocols (a−d) for Selenophene
Synthesis
Table 1. Reaction Optimization
Se/base
ratio
temp.
(°C)
time
(h)
yield
b
a
entry
base
solvent
(%)
1
4.5:9
4.5:9
4.5:9
4.5:9
4.5:9
4.5:9
1:1
2:2
1:2
2:4
2:4
2:4
2:4
2:4
2:4
2:4
2:4
2:4
2:4
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
NaOH
t-BuOK
K2CO3
Cs2CO3
K3PO4
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
EtOH
toluene
MeCN
THF
DMF
DMF
110
110
110
110
25
25
25
25
25
25
25
25
25
25
25
60
25
25
25
25
25
0.33
0.33
0.33
0.33
4
18
4
4
4
4
4
4
4
4
4
16
4
4
4
53
c
2
d
3
e
4
70
84
78
16
18
28
84
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
previous preparation of selenium-containing substrates. In
addition to the characteristics cited above, selenophenes also
present excellent electrical properties, processibility, and
environmental stability, allowing potential application as
optical materials,17 semiconductors,24 solar cells,25 film
transistors,26 and energy-storage applications.27 In addition,
the preparation of π-conjugated selenophenes, as 2,5-diary-
lselenophenes, can improve their photoactive properties due to
a rigid aromatic and planar structure with a very dense and
small architecture.28 On the other hand, it is reported that the
selenium atom can affect the photophysical properties of the
compounds by emission quenching or even shifting the
emission maxima.29
In this sense and aiming our continuous interest in the
synthesis of chalcogen compounds,30−34 the present study
describes a novel methodology involving in situ reduction of
elemental selenium to generate a nucleophilic Se2− species in
superbasic media [KOH and dimethylsulfoxide (DMSO)] to
produce 2,5-disubstituted photoactive selenophenes. The
scope of the methodology was investigated using 1,3-diyne
derivatives containing different substituents with electron-
donating and electron-withdrawing features in different
positions to verify electronic and steric effects. Additionally,
their photophysical properties were also studied using UV−vis
absorption and steady-state fluorescence emission spectros-
copies. Theoretical exploration of structural and photophysical
properties of this series of selenophenes was also performed.
33
75
62
DMSO
DMSO
DMSO
DMSO
DMSO
2:4
2:4
4
4
a
Reaction conditions: argon atmosphere, solvent (2.0 mL), Se0
(equiv), base (equiv), and 1a (0.3 mmol) for the respective reaction
b
c
times and temperatures indicated in Table 1. Isolated yields. MeOH
(1 equiv). H2O (1 equiv). TEMPO (4.5 equiv).
d
e
since they enhance the cyclization step efficiency,37 so instead
of the selenophene heterocycle, the corresponding furan
analogue was obtained in both cases. Further investigation of
the initial test (entry 1, Table 1) revealed the total conversion
of the starting material and the formation of some side
products of difficult characterization. To elucidate the presence
of radical intermediates that lead to side products or
decomposition of the product 2a, a similar reaction was
performed adding TEMPO and resulting to an increase in
reaction yield to 70% (entry 4, Table 1). Thus, to avoid the
formation of radical species, the reaction was carried out at
room temperature and cyclization successfully occurred at 25
°C reaching 84% yield after 4 h (entry 5, Table 1). Longer
times resulted in lower yields (entry 6, Table 1).
In order to evaluate the reaction stoichiometry (entries 7−
10, Table 1), both the amount and ratio of selenium and the
base were varied showing that similar results are achieved when
2 equiv of selenium and 4 equiv of the base are used. Other
solvents such as ethanol, toluene, acetonitrile, and tetrahy-
drofuran (entries 11−16, Table 1) and other bases such as
NaOH, t-BuOK, K2CO3, Cs2CO3, and K3PO4 (entries 17−21,
Table 1) were evaluated attesting the better efficiency of the
KOH/DMSO system. The change in solvent to N,N-
dimethylformamide (DMF) seems to slow down the reaction
where only 33% yield was obtained in 16 h while no product
was observed with other solvents. On the other side, the tests
carried out with NaOH and t-BuOK showed lower efficiency
(75 and 62% yield, respectively), whereas weaker bases did not
lead to the formation of the desired product. Thus, our best
RESULTS AND DISCUSSION
■
Synthesis. In this study, the synthetic methodology is
based on the in situ activation of elemental selenium in
superbasic media into Se2− reactive species to undergo
nucleophilic attack to conjugated diynes.35,36 In this sense,
the reactions were carried out in two steps in which, first, the
elemental selenium is submitted to the potassium hydroxide
solution in DMSO (superbasic media), followed by the
addition of the conjugated diyne. The 1,4-diphenylbuta-1,3-
diyne (1a) was chosen as a model substrate, the initial
cyclization test was carried out at elevated temperature with an
excess of selenium and base (entry 1, Table 1), and the
selenophene 2a was obtained in 53% yield. The influence of
proton source additives was verified (entries 2−3, Table 1),
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Scheme 2. Synthetic Route for Preparation of 2,5-Substituted Selenophenes, Where (a) I2, CuI, DMF, 80 °C, 5 h; (b) CuI,
PPh3, K2CO3, EtOH, 100 °C, 14 h; and (c) Se (2 equiv), KOH (4 equiv), DMSO, 4 h, 25 °C
a b
,
Table 2. Reaction Scope
a
Reaction conditions: argon atmosphere, DMSO (2.0 mL), Se0 (2.0 equiv), KOH (4.0 equiv), TEMPO (2 equiv), and 1 (0.3 mmol) for 4 h at 80
b
c
d
e
°C. Isolated yields. Reaction performed at room temperature. 20 h. 48 h.
reaction condition to synthesize the desired product 2,5-
diphenylselenophene (2a) was using 2 equiv of elemental
selenium, 4 equiv of potassium hydroxide, and DMSO as a
solvent under room temperature for 4 h (entry 10, Table 1).
The scope of the methodology was next investigated
employing other derivatives of 1,3-diynes containing different
substituents with electron-donating and electron-withdrawing
features as in different positions (ortho, meta, and para) to
verify electronic and steric effects. The symmetrical and
unsymmetrical 1,3-diynes used in scope investigation were
prepared according to Glaser−Hay coupling38 between
terminal alkynes promoted by copper(I) or from coupling
between a bromoalkyne and a terminal alkyne under similar
conditions (Scheme 2).39
The 2,5-disubstituted selenophenes were obtained in
moderate-to-good yields and are presented in Table 2. The
reaction for preparation of compounds 2a and 2c was carried
out using the optimized conditions. For the remaining
precursors, a low solubility of the diynes was observed in the
reaction media, resulting in limited yields. To circumvent this
limitation, the reaction temperature was increased and the
complete solubilization of the starting material was achieved at
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80 °C. In addition, TEMPO was added as an additive since the
optimization study (entry 4, Table 1) revealed that the
presence of TEMPO at elevated temperatures affords the
product in higher yields.
methanol, and acetonitrile). The relevant data from the
ground-state characterization are summarized in Table 3. In
Table 3. Photophysical Data of 2,5-Substituted Selenophene
Derivatives, Where λabs is the Absorption Maxima (nm), ε is
the Molar Extinction Coefficient (×104 M−1·cm−1), fe is the
Experimental Oscillator Strength, k0e is the Calculated
Radiative Rate Constant (108 s−1), and τ0 is the Calculated
Pure Radiative Lifetime (ns)
In general, the methodology is more extended to aromatic
1,3-diynes rather than aliphatic ones as can be observed by low
yield achieved for 1k. Using precursors with substituents on
the benzene ring, such as p-CH3, p-F, and m-OCH3, leads to
the formation of selenophenes in good yields. In other cases, as
for 2e, 2g, 2h, and 2i, the moderate solubility of the starting
material may affect the reaction yields where steric effects can
be additionally associated in the lower yields presented for 2e
and 2i. It is worth mentioning that while the symmetrical diyne
with a strong donating methoxy group in the para position
does not lead to the formation of 2f, the unsymmetrical
analogue with the methoxy group was obtained in 77% yield.
Furthermore, an example of a selenophene ring conjugated
with thiophene rings was successfully achieved (2j). Given the
reaction conditions, the product was not obtained from
precursors containing sensitive functional groups under basic
conditions such as −CN and −COOMe that probably undergo
hydrolysis, making the purification hard. Additionally, a large-
scale reaction of 1a (3 mmol) was performed at 80 °C with the
addition of selenium, potassium hydroxide, and TEMPO
affording the desired product 2a in 85% yield (0.72 g), which
demonstrates the practical utility of the method.
compound
solvent
hexane
λabs
ε
fe
ke0
τ0
2a
328
332
330
330
332
338
336
334
324
328
336
326
350
356
352
352
332
336
334
334
328
332
332
332
334
338
336
336
334
338
338
342
356
362
360
358
335
341
340
338
3.15
3.25
3.38
2.97
1.95
4.40
2.61
3.57
4.37
3.97
3.23
3.59
0.64
2.90
2.17
2.02
3.01
3.10
2.30
2.58
2.08
2.26
1.93
2.97
2.00
5.14
2.21
3.79
1.07
0.96
1.34
1.04
1.63
0.66
2.40
5.27
4.01
4.36
4.39
4.70
0.637
0.641
0.678
0.631
0.409
0.886
0.509
0.676
0.847
0.832
0.633
0.713
0.147
0.506
0.368
0.329
0.689
0.548
0.416
0.495
0.374
0.434
0.338
0.601
0.455
0.888
0.422
0.750
0.212
0.202
0.314
0.204
0.322
0.144
0.426
0.891
0.914
0.873
0.975
0.990
5.92
5.81
6.23
5.79
3.71
7.76
4.51
6.06
8.07
7.74
5.60
6.71
1.20
4.00
2.97
2.66
6.25
4.86
3.73
4.44
3.47
3.94
3.07
5.45
4.08
7.77
3.74
6.64
1.90
1.77
2.75
1.75
2.54
1.10
3.29
6.95
8.14
7.51
8.44
8.66
0.169
0.172
0.161
0.173
0.270
0.129
0.222
0.165
0.124
0.129
0.178
0.149
0.833
0.250
0.337
0.376
0.160
0.206
0.268
0.225
0.288
0.254
0.326
0.184
0.245
0.129
0.267
0.151
0.525
0.566
0.364
0.572
0.393
0.912
0.304
0.144
0.123
0.133
0.119
0.115
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
2b
2c
2d
2e
2g
2h
2i
Based on a previous report for the chalcogen activation in
superbasic media35,36 and the reaction with alkynes,40
a
plausible mechanism for the selenophene ring formation is
proposed, as depicted in Scheme 3. Under a superbasic
Scheme 3. Proposed Mechanism for Selenium Activation
and Selenophene Ring Formation
2j
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
2m
DMSO/KOH system, elemental selenium undergoes an
oxidation−reduction disproportionation giving the corre-
sponding selenide ion. Recently, the hydroselenation reaction
between the selenide ion and alkyne derivatives affording
divinyl selenides was described.38 In a similar way, the addition
of the selenide ion to the triple bond of 1,3-diyne generates the
intermediate 3a (Scheme 3). The intramolecular cycloaddition
of the Se moiety across carbon−carbon triple bonds finishes
the reaction pathway.
this sense, Figure 1a depicts the UV−vis absorption spectra of
the studied selenophene derivatives in dichloromethane. It is
worth mentioning that these compounds presented a similar
behavior in the additional organic solvents (data not shown,
see Figures S55−S63).
The selenophene derivatives present absorption maxima in
the UV-A region (328−362 nm) depending on the solvent
(Figure 1a), without any clear dependence on the solvent
polarity on the absorption maxima location (Figure 1b). In
general, the electronic character of the substituents on the 2,5-
positions seems not to significantly affect the electronic
PHOTOPHYSICAL CHARACTERIZATION
■
The photophysical study of the selenophene derivatives was
performed in solution (10−5 M) using organic solvents with
different dielectric constants (hexane, dichloromethane,
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spin- and symmetry-allowed electronic transitions, which could
be related to 1π → π* transitions. Moreover, an almost
constant radiative lifetime τ0 indicates that these compounds
after radiation absorption populate the same excited state. In
addition, based on the previous reports on the literature related
to the strong intermolecular interactions in the selenophene-
containing small molecules,43−45 an additional investigation at
different dichloromethane concentrations was performed using
compound 2d as a model (Figure 1c), to better study the
photophysics of these compounds in solution. It can be
observed that the studied compound presents structured
spectra with absorption maxima located around 350 nm, as
already observed. Any additional absorption band could be
observed increasing the solution concentration discarding the
possibility of aggregates in solution in the ground state.
The excited-state photophysics of the synthesized seleno-
phenes in solution (10−5 M) were also evaluated. Figure 2a
Figure 1. UV−vis absorption spectra of (a) 2,5-substituted
selenophene derivatives in dichloromethane (DCM) (10−5 M) and
compound 2d (b) in different organic solvents (10−5 M) and (c) at
different concentrations in dichloromethane (1.45 × 10−4 up to 2.27
× 10−6 M). The insert presents the magnification of the initial
concentrations.
Figure 2. (a) Steady-state fluorescence emission spectra of 2,5-
substituted selenophene derivatives in dichloromethane (10−5 M) and
(b) excitation and emission spectra of compound 2d in different
organic solvents (10−5 M).
transitions of these compounds. An exception can be observed
for compound 2d, which presents structured absorption
spectra, if compared to the studied phenyl-substituted
selenophenes. In this case, despite all selenophenes that are
predicted to be out of plane, it is believed that this particular
behavior can be related to the presence of two possible
structures for this compound. For more details, see the
theoretical calculation section. Regarding the compound 2j, its
absorption maxima red shift is believed to be due to the
coplanarity of thiophene and selenophene rings, which favors
the electronic delocalization through the heterocycle.
The nature of the observed electronic transitions is related
to the experimental extinction coefficient (ε) values, as well as
to the theoretical rate constant for emission (k0e) and the
respective experimental oscillator strengths (fe), given by the
Strickler−Berg relations, described in eqs 1 and 241
presents the fluorescence emission of the selenophene
derivatives in dichloromethane. These curves were obtained
by exciting the compounds at their respective absorption
maxima (Table 3). It is worth mentioning that these
compounds presented a similar behavior in the additional
organic solvents (data not shown, see Figures S64−S72).
Additionally, Figure 2b depicts the excitation and emission
spectra of compound 2d in all studied solvents. The relevant
data from this investigation are summarized in Table 4.
It can be observed that the selenophene derivatives
presented emission maxima located between 374 and 452
nm depending on the electronic nature of the substituents, as
expected. Red shift emission could be observed for compounds
2i and 2j, presenting aminophenyl and thiophene moieties,
respectively. The Stokes shift values (∼5000 cm−1) indicate
that these compounds do not present negligible energy loss in
the excited state, probably due to the rotational motion of the
substituents. This latter can also be related somehow with the
observed very low fluorescence quantum yields. Among this
series of compounds, only 2i presents significant solvatochrom-
ism, for which a significant red shift of emission bands is
observed with the increase in the polarity of the solvent. This
effect is more important for methanol, probably due to the
hydrogen bonds formed between the amino hydrogens and the
oxygen atom of the solvent.
fe ≈ 4.3 × 10−9 ε dv
∫
̅
(1)
0
−9
2
′
ke ≈ 2.88 × 10 v0
ε dv
̅
∫
(2)
In these equations, the area under the absorption curve from
a plot of the molar absorptivity coefficient ε (M−1·cm−1)
against wavenumber v (cm−1) provides the oscillator strength
̅
fe and the theoretical rate constant for emission (ke0),
respectively, where v0 is the wavenumber (energy in 1/λ
units) of the absorption band maximum. In addition, the pure
radiative lifetime τ0 can be obtained as 1/k0e.42 In this sense, the
molar absorptivity coefficient values (∼104 cm−1·M−1), as well
as the calculated radiative rate constants (∼108 s−1), indicate
Based on the steady-state fluorescence results, time-resolved
fluorescence was applied to better understand the excited-state
photophysics of these compounds. The relevant data are
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The charge distribution in the ground and excited states of
the compounds was also investigated presenting higher
dependence on the solvent polarity (2c, 2i, and 2m), which
were chosen as representative compounds in this study, the
difference in the dipole moment between the excited and
ground states was obtained applying the simplified Lippert−
Mataga relation presented in eq 347
Table 4. Photophysical Data of 2,5-Substituted Selenophene
Derivatives, Where λem is the Emission Maxima (nm), ΔλST
is the Stokes Shift (nm/cm−1), and ϕFL is the Fluorescence
Quantum Yield (%)
compound
solvent
hexane
λem
ΔλST (nm/cm−1
)
ϕFL
2a
396
404
394
397
405
408
402
405
394
400
393
394
410
418
411
412
390
394
396
391
374
378
379
380
406
412
406
408
425
440
452
448
436
442
436
436
407
416
411
416
68/5235
72/5368
64/4922
67/5114
73/5429
70/5076
66/4886
71/5249
70/5483
72/5488
57/4317
68/5294
60/4181
62/4166
59/4078
60/4137
58/4479
58/4381
62/4688
57/4365
46/3750
46/3665
47/3735
48/3805
72/5310
74/5314
70/5131
72/5252
91/6411
102/6859
114/7462
106/6918
80/5154
80/5000
76/4842
78/4997
72/5281
75/5287
71/5081
78/5547
0.6
0.7
0.5
0.5
0.6
0.8
0.5
0.4
0.5
0.6
0.4
0.2
0.8
1.5
0.9
2.6
1.2
2.1
0.8
1.2
1.6
2.5
1.1
4.7
1.6
0.9
0.4
1.3
0.2
6.2
1.8
4.7
0.2
0.6
0.3
0.3
0.4
1.2
0.4
1.3
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
hexane
2(μ − μ )2
e
g
Δv =
Δf + Δv0
̅
st
̅
hca3
(3)
2b
2c
2d
2e
2g
2h
2i
where h is the Planck’s constant, c is the speed of light, a is the
Onsager cavity radius in which the chromophore resides, and
μg and μe are the dipole moment of the solute in the ground
and excited states, respectively. The original equation
compares the solvatochromic shifts from the Stokes shift
(Δvst) versus the orientation polarization function (Δf),
̅
obtained from eq 442
(ε − 1)
(2ε + 1)
(n2 − 1)
Δf =
−
(2n2 + 1)
(4)
In this equation, the dielectric constant (ε) and the
refractive index (n) for a mixture of solvents are calculated
as presented in eqs 5 and 648
εmix = fA ·εA + fB ·εB
(5)
2
2
2
nmix = fA nA + fB nB
(6)
where fA and f B are the volumetric fractions of the two
solvents. By applying the Lippert−Mataga correlation, it can be
observed that all studied compounds present a small
solvatochromism regarding the absorption maxima against
Δf, where the wavenumber values increase with increasing
environment polarity (Figure 3a). On the other hand, it can be
observed that the selenophenes show no evidence of charge
transfer in the excited state, as well as seems to be low polar in
the excited state if compared to the ground state (Figure 3b).
In addition, Figure 3c that relates the Stokes shift versus Δf
indicates that compounds 2c and 2i seem to be the most
sensitive fluorophores due to the largest change in the dipole
moment upon excitation.
Fluorescence self-quenching dependent on the dye concen-
tration could be observed for these selenophenes, as presented
in Figure 4a, using compound 2d as a model. Increasing the
dye concentration, the fluorescence emission increases as well
(data not shown, see the full set of concentrations in Figure
S77), reaching a maximum around 3.63 × 10−5 M (pink
curve). At higher concentrations, a decrease in the fluorescence
intensity can be observed. Finally, an almost absence emission
could be observed when the concentration is increased up to
5.8 × 10−4 M. In even higher concentrations, a similar intensity
value could be observed. All studied samples presented a
similar quenching behavior (Figures S78−S86). Based on the
observed quenching, the fluorescence intensity on the dye 2d
concentration could be fitted as a Gaussian curve (Figure 4b).
It is worth mentioning that the fluorescence spectra only vary
in intensity, where no change is observed in their shape or
maxima location, discarding aggregation in solution. Moreover,
the absence of additional broad red-shifted emission bands also
excludes an excimer formation in the excited state.
Furthermore, the energy-transfer (ET) mechanism can also
be discarded as the origin of the observed self-quenching since
these compounds, due to their relatively large Stokes shift
2j
dichloromethane
methanol
acetonitrile
hexane
dichloromethane
methanol
acetonitrile
2m
summarized in Table S1 and the respective time decay curves
and residuals are presented in Supporting Information (Figures
S73−S76). The fluorescence decay profile of compound 2d
could be fitted with good χ2 values, showing a very fast
(∼0.118 ns) and monoexponential decay. Despite the lack of
time-resolved data concerning selenophenes, the obtained
values are consistent with data reported in the literature for
selenophene-based heteroacenes.28 In addition, it could also be
observed that the solvent does not affect significantly its
fluorescence lifetime (Δτ = 0.037 ns). The calculated radiative
and nonradiative decay constant values, around 108 and 109
s−1, respectively (Table S1), as well as the fast fluorescence
lifetime are an indicative that the singlet excited state decays
preferably nonradiatively and/or by triplet excited-state
population, as already observed in the literature for
selenium-based compounds.46
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Figure 3. Lippert−Mataga relation regarding (a) absorption, (b) fluorescence emission maxima, and (c) Stokes shift for the selenophenes 2c, 2i,
and 2m. The solvent polarity function Δf is given in eq 4.
(Figure S87), presented small overlap integral between the
absorption and fluorescence spectra, that is, a fundamental
requirement to ET.
To better investigate the fluorescence quenching process
present in these compounds, eq 7 was used
where at high concentrations, the formation of the non-
fluorescent selenophene complex takes place (Figure S89).
This latter can be envisaged as an association of selenophene
molecules probably promoted by a strong interaction by π−π
stacking between the aromatic rings of the selenophenes.
Additionally, to corroborate with this mechanism, fluorescence
lifetimes were obtained at different dye concentrations
(Figures S90−S93 and Table S2). Since the static quenching
mechanism removes a fraction of fluorophores from observa-
tion and the remaining fluorescence is from the uncomplexed
fluorophores that are unperturbed (Figure S89),48 it is
expected that there is no lifetime change when the final
product is nonfluorescent, as it could be observed
experimentally (τ0/τ ∼ 1). Considering that fluorescence
quenching via photoinduced electron transfer (PET) has
already been established in the literature for selenium-based
compounds,50−53 it is believed that a similar behavior takes
place in the excited state of the selenophene complex (S−Q)*
(Figure S89).
I0/I = 1 + KSV[Q]
(7)
where I0 is the fluorescence intensity observed at 3.6 × 10−5 M,
I is the observed fluorescence intensity at different quencher
concentrations [Q], and KSV is the Stern−Volmer constant.
The latter can also be envisaged as kqτ, where kq is the self-
quenching constant and τ is the radiative lifetime of the excited
dye. For this investigation, the fluorescence emission measure-
ments were performed in acetonitrile using the absorption
maxima (λexc = 352 nm) as the excitation wavelength. Since the
experiment aims to investigate a self-quenching mechanism in
the absence of an additional quencher molecule, the quencher
concentration (selenophene itself) was considered as the
additional amount of selenophene used to increase the
concentration of the starting sample (3.6 × 10−5 M).
THEORETICAL CALCULATIONS
The plot of I0/I versus [Q] presented in Figure 4c allowed
the obtention of a linear correlation (R2 = 0.977) with the
intercept value close to unity in accordance with the theoretical
equation and supporting the simple nature of self-quenching in
these compounds. The original emission curves can be found
in Supporting Information (Figure S88). From the slope of the
plot of I0/I versus [Q], a Stern−Volmer constant around 2 ×
104 M−1 could be obtained. The quenching rate constant (kq)
in the excited state was calculated by employing the
experimental fluorescence lifetime in acetonitrile (0.102 ns),
with a value (2.04 × 1014 M−1·s−1), which exceeds the
maximum values to the diffusional mechanism (5.0 × 109 to
1.0 × 1010 M−1·s−1).49 This result indicates a static mechanism,
■
According to calculations performed at the MP2/cc-pVDZ
level of theory, ground-state molecular structures of all
selenophenes are predicted to be out of plane, as can be
observed for the representative compounds 2a, 2d, and 2i
(Figure 5). The full set of compounds is presented as
Supporting Information (see Figure S94). It is worth
mentioning that in the excited state, all structures seem to
be planar (data not shown, see Figure S95), at least the
chromophoric units, which can lead to the observed intra-
molecular charge transfer in some derivatives.
In order to better investigate the structured absorption band
observed experimentally for compound 2d (Figure 1a),
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Figure 4. Fluorescence self-quenching study of the 2,5-substituted selenophene derivative 2d, (a) from 3.63 × 10−5 to 1.16 × 10−3 M (decrease in
the fluorescence intensity), (b) general profile of the fluorescence intensity vs dye concentration, and (c) (a) Stern−Volmer relation to obtain the
fluorescence self-quenching constant from eq 7, with a linear plot equation y = 0.96 + 20,817.36x.
expected not to be dependent on the microenvironment,
considered in the calculations within the COSMO approx-
imation (Table S4), in agreement to the observed
experimentally (Figure 1b). Additionally, the theoretical results
also corroborate with the experimental evidence that the
electronic character of the substituents on the 2,5-positions
seems not to affect significantly the electronic transitions of
these compounds. The computed excitation energies at the
ADC(2)/cc-pVDZ level of calculation varies less than 0.12 eV
along the studied series of selenophenes. However, there are
two exceptions: 2d and 2j, for which excitation energies are up
to 0.4 eV lower than the nonsubstituted selenophene 2a.
Inspection of the data available in Tables 5 and 6 shows that
the polarity of these compounds is expected to be low (0.5−3
debye). Their electronic transitions are of ππ* nature,
involving molecular orbitals delocalized over the whole
selenophene framework (Figure S96). Compounds 2d and 2j
present red-shifted absorption maxima (∼350 and 360 nm,
respectively) if compared to their parent compounds.54
Different than other selenophenes, absorption spectra of 2d
seem more structured, showing two peaks, separated by ∼10
nm, besides on a shoulder at higher wavelength (∼370 nm).
This feature may be assigned to the population of more than
one species on the ground state, since the rotation barrier
between ortho-anisole rings is predicted to be less than 0.3 eV,
what would be compatible to the thermal interconversion in
the ground state. The difference between excitation energies
Figure 5. Top and lateral view of the compounds 2a, 2d, and 2i in the
ground state computed at the MP2/cc-pVDZ level of theory.
minimum energy profiles for the twist between two benzene
rings of the derivatives substituted in the ortho position, 2d
(Figure 6a) and 2i (Figure 6b), were evaluated. The
replacement of a hydrogen atom to a bulky −OCH3 group
in the ortho position 2d results in two possible structures, as
can be seen in Figure 2. On the other hand, the same is not
observed for 2i, for which only one ground-state minimum
structure was found with the dihedral angle between two
aniline rings predicted to be around 120°.
Due to the low dipole moment of obtained molecules
optimized at the MP2/ADC(2)/cc-pVDZ level of theory
(Table 5), excitation energies of this series of compounds are
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Figure 6. Ground-state minimum energy profile for the twist between phenyl rings of (a) 2d and (b) 2i computed at the MP2/cc-pVDZ level of
theory.
Table 5. Vertical Transition Energy (ΔE), Theoretical Oscillator Strength (f), Dipole Moment (μ), and Leading Electronic
Configurations Computed with the ADC(2)/cc-pVDZ Method at the MP2/cc-pVDZ Equilibrium Geometry of the Ground
State for 2a−2m
molecule
state
ΔE/eV
f
μ/debye
el. conf.
(71a)2
2a
S0
0.49
1.20
0.53
0.56
0.57
0.57
0.29
0.89
1.17
1.63
2.33
2.29
2.98
3.14
0.62
0.73
1.85
2.97
1.91
4.56
0.87
0.79
1.72
2.54
gas phase
1ππ*
1ππ*
1ππ*
1ππ*
1ππ*
S0
1ππ*
S0
1ππ*
S0
1ππ*
S0
1ππ*
S0
1ππ*
S0
1ππ*
S0
4.26
4.19
4.19
4.20
4.20
1.01
1.09
1.10
1.09
1.09
0.98(71a−72a)
0.98(71a−72a)
0.98(71a−72a)
0.98(71a−72a)
0.98(71a−72a)
(79a)2
0.98(79a−80a)
(79a)2
0.98(79a−80a)
(87a)2
0.97(87a−88a)
(87a)2
0.94(87a−88a)
(87a)2
0.96(87a−88a)
(135a)2
0.98(135a−136a)
(79a)2
Hexane
Dichloromethane
Acetonitrile
Methanol
2b
2c
2d
4.18
4.28
4.12
3.97
4.23
4.14
4.21
3.87
4.21
1.17
1.02
0.91
0.87
1.02
1.03
0.39
0.95
1.09
“cis”
“trans”
2e
2g
2i
1ππ*
S0
1ππ*
S0
1ππ*
0.80(79a−80a)
(73a)2
0.98(73a−74a)
(79a)2
2j
2m
0.98(79a−80a)
computed at the ADC(2)/cc-pVDZ level of theory considering
both ground-state-optimized structures of 2d is 0.15 eV.
aromatic 1,3-diynes in which a broad range of substituents
are tolerated to selenophene synthesis. The photophysical
properties of the compounds present spin- and symmetry-
allowed electronic transitions in the UV-A region. The
compounds present emission maxima located in the UV−
visible region, depending on the electronic nature of the
substituents. Any additional absorption or emission band could
be observed increasing the solution concentration, discarding
the possibility of aggregates in solution in the ground and
excited states. Fluorescence self-quenching with a very large
CONCLUSIONS
■
In summary, the Se2− species generated from elemental
selenium in a KOH/DMSO system successfully undergo the
addition to 1,3-diyne and intramolecular cyclization affording a
series of 2,5-disubstituted selenophenes in moderate-to-good
yields. The methodology presents higher efficiency with
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Table 6. Relevant Molecular Orbitals Involved into Their Lowest Electronic Computed at the ADC(2)/cc-pVDZ Level of
Theory in the Gas Phase
temperature (25 °C) at a concentration of 2 × 10−5 M. The
fluorescence decay curves were analyzed using the software EasyLife
constant was observed in these compounds indicating an
efficient static mechanism, probably involving PET promoted
by a strong interaction by π−π stacking between the aromatic
rings of the selenophenes.
V from OBB. A nonlinear least square method was employed for the
fit of the decay to a sum of exponentials. The values of χ2, residuals,
and the autocorrelation function were used to determine the quality
of the fit.
EXPERIMENTAL SECTION
The fluorescence self-quenching study was performed by dilution
of stock solution of each compound. The stock solution was prepared
dissolving 2 mg of selenophene derivative in 5 mL of acetonitrile. The
final concentration of each stock solution is presented in Table S3
(see Supporting Information). The first measurement was obtained
with the stock solution and subsequently twofold dilutions were
performed until the absence of fluorescence emission was observed. In
general, the concentration range analyzed started from 10−3 to 10−6
M.
Synthesis. General Procedure for Preparation of 1,3-Diynes
1a−n. Method A. To a Schlenk tube were added dimethylformamide
(3 mL), phenylacetylene (3 mmol, 306 mg, 0.11 mL), CuI (1 equiv, 3
mmol, 570 mg), iodine (1 equiv, 3 mmol, 762 mg), and potassium
carbonate (2 equiv, 6 mmol, 828 mg). The reaction mixture was
stirred under heating in an oil bath (80 °C) for 4 h. The crude
mixture was, first, diluted with ethyl acetate and filtered in Celite and,
then, washed with saturated sodium thiosulfate aqueous solution and
aqueous sodium chloride (brine). The resulting organic phase was
dried over anhydrous sodium sulfate, filtered, and purified by column
chromatography on silica gel using hexanes or petroleum ether as the
eluent to afford the desired products.38
Method B. To a round-bottom flask were added acetonitrile (60
mL), trimethylamine (30 mL), triphenylphosphine (0.09 equiv, 0.36
mmol, 94 mg), CuI (0.03 equiv, 0.12 mmol, 23 mg), PdCl2(PPh3)2
(0.03 equiv, 0.12 mmol, 84 mg), and the respective terminal alkyne (1
equiv, 4 mmol). The reaction mixture was stirred under room
temperature for 48 h. The crude mixture was, first, diluted with ethyl
acetate and filtered in Celite and, then, washed with aqueous
hydrochloric acid (1 M). The resulting organic phase was dried over
anhydrous sodium sulfate, filtered, and purified by column
■
Materials and Methods. Hydrogen nuclear magnetic resonance
(1H NMR) and carbon-13 nuclear magnetic resonance (13C NMR)
spectra were recorded in CDCl3 solutions on Varian 400 MHz
spectrometers. Chemical shifts (d) are given in parts per million from
the peak of tetramethylsilane (δ = 0.00 ppm) as an internal standard
1
in H NMR or from the solvent peak of CDCl3 (δ = 77.23 ppm) in
13C NMR. Data are reported as follows: chemical shift (δ),
multiplicity, coupling constant (J) in Hertz, and integrated intensity.
High-resolution mass spectra (HMRS) were recorded on a Micro-
mass Q-Tof spectrometer, using electrospray ionization (ESI). All
commercially available reagents were used without further purifica-
tion. Toluene was distilled from sodium. DMSO was distilled from
CaH2. Triethylamine (NEt3) was distilled from potassium hydroxide.
Column chromatography was performed on silica gel (230−400
mesh).
Photophysical Characterization. Spectroscopic-grade solvents
were used in the photophysical characterization. UV−vis absorption
spectra were acquired on a Shimadzu UV-2450 spectrophotometer at
a concentration of 10−5 M, and the steady-state fluorescence spectra
were measured on a Shimadzu spectrofluorometer model RF-5301PC.
The maximum absorption wavelength (WL) was used as the
excitation WL for fluorescence emission measurements. The relative
quantum yield of fluorescence (ϕFL) was determined in the dilute
optical method. Quinine sulphate in H2SO4 (ϕFL = 0.55) was used as
the quantum yield standard.55 All measurements were performed at
room temperature (25 °C). Fluorescence time decay curves of
compound 2d, used as a model, in acetonitrile, methanol, hexane, and
dichloromethane were obtained with an EasyLife V spectropho-
tometer (OBB). All measurements were performed at room
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chromatography on silica gel using hexane or petroleum ether as the
eluent to afford the desired products.56
(s, 2H). 13C{1H} NMR (100 MHz, CDCl3) δ: 148.5, 137.8, 132.7 (q,
1
2JC−F = 33.5 Hz), 128.1, 126.0, 123.2 (q, JC−F = 272.9 Hz), 121.5.
77Se NMR (76 MHz, CDCl3, (PhSe)2 as internal standard δ = 463.00
ppm) δ: 597.81. HRMS (ESI) m/z [M + H+] calcd for C20H9F12Se,
556.9678; found, 556.9688.
Method C. To a Schlenk tube under an argon atmosphere were
added CuI (0.1 equiv, 0.2 mmol, 0.0381 g), triphenylphosphine (0.2
equiv, 0.4 mmol, 0.105 g), K2CO3 (2 equiv, 4 mmol, 0.553 g), 1-
ethynyl-4-methoxybenzene (1 equiv, 2 mmol, 0.253 mL), 1-(2-
bromoethynyl)benzene (1.3 equiv, 2.6 mmol, 0.468 g), and
anhydrous ethanol (3 mL). The reaction mixture was stirred for 14
h, at 100 °C and then cooled to room temperature. The crude mixture
was diluted with ethyl acetate and filtered in Celite and then
concentrated in vacuo. The compound was purified by column
chromatography on silica gel using hexane as the eluent to afford the
desired products.57
2,5-Bis(4-pentylphenyl)selenophene (2h).61 The compound was
purified by column chromatography on silica gel using hexanes as an
1
eluent. Yield: 55%, 69 mg as a beige crystal, mp 159−161 °C. H
NMR (400 MHz, CDCl3) δ: 7.50 (d, J = 8.2 Hz, 4H), 7.41 (s, 2H),
7.19 (s, J = 8.2 Hz, 4H), 2.63 (t, J = 7.9 Hz, 4H), 1.67−1.63 (m, 4H),
1.44−1.29 (m, 8H), 0.93 (t, J = 7.0 Hz, 6H). 13C{1H} NMR (100
MHz, CDCl3) δ: 149.5, 142.6, 134.0, 129.1, 126.0, 125.8, 35.8, 31.6,
31.2, 22.7, 14.2. HRMS (ESI) m/z [M + H+] calcd for C26H33Se,
425.1747; found, 425.1737.
General Procedure for Preparation of Selenophenes 2a−m. The
reaction is performed in three steps in a Schlenk tube, under an argon
atmosphere. First, dried dimethylsulfoxide (2 mL) was added and was
bubbled with argon for 10 min. Then, selenium (0.6 mmol, 2 equiv)
and potassium hydroxide (1.2 mmol, 4 equiv) were added and the
mixture was stirred under room temperature for 20 min. Finally, the
respective 1,3-diynes (0.3 mmol) 1a and 1c were added to the
reaction mixture under room temperature for 4 h. For 1,3-diynes 1b
and 1d−m (0.3 mmol), the reaction mixture was heated at 80 °C (oil
bath) and TEMPO (0.6 mmol, 2 equiv) was previously added. The
crude mixture was diluted with ethyl acetate and washed with water,
dried, and purified by column chromatography on silica gel to afford
the desired products.
2,5-Bis(2-aminophenyl)selenophene (2i). The compound was
purified by column chromatography on silica gel using 10:90 ethyl
acetate:hexanes as an eluent. Yield: 40%, 37 mg as a brown solid, mp
139−141 °C. 1H NMR (400 MHz, CDCl3) δ: 7.35 (s, 2H), 7.30 (dd,
J = 7.6 and 1.5 Hz, 2H), 7.15 (td, J = 7.7 and 1.5 Hz, 2H), 6.85−6.75
(m, 4H), 4.08 (s, 4H). 13C{1H} NMR (100 MHz, CDCl3) δ: 147.2,
143.5, 130.9, 129.0, 128.6, 122.1, 118.6, 116.0 77Se NMR (76 MHz,
CDCl3, (PhSe)2 as internal standard δ = 463.00 ppm) δ: 639.11.
HRMS (ESI) m/z [M + H+] calcd for C16H15N2Se, 315.0400; found,
315.0380.
2,5-Di(thiophen-2-yl)selenophene (2j).62 The compound was
purified by column chromatography on silica gel using hexanes as
an eluent. Yield: 75%, 66 mg as a yellow crystal, mp 198−200 °C. 1H
NMR (400 MHz, CDCl3) δ: 7.33 (s, 2H), 7.22 (dd, J = 5.1 and 1.2
Hz, 2H), 7.12 (dd, J = 3.6 Hz and 1.2 Hz, 2H), 7.01 (dd, J = 5.1 and
3.6 Hz, 2H). 13C{1H} NMR (100 MHz, CDCl3) δ: 141.2, 139.5,
128.0, 126.5, 124.8, 124.4. HRMS (ESI) m/z [M + H+] calcd for
C12H9S2Se, 296.9311, found 296.9299.
2,5-Diphenylselenophene (2a).31 The compound was purified by
column chromatography on silica gel using petroleum ether as an
1
eluent. Yield: 84%, 71 mg, as a white crystal, mp 175−177 °C. H
NMR (400 MHz, CDCl3) δ: 7.60 (d, J = 8.0 Hz, 4H), 7.48 (s, 1H),
7.40 (t, J = 8.0 Hz, 4H), 7.31 (t, J = 8.0 Hz, 2H). 13C{1H} NMR (100
MHz, CDCl3) δ: 149.8, 136.3, 128.9, 127.6, 126.2, 126.1. HRMS
(ESI) m/z [M + H+] calcd for C16H13Se, 285.0182; found, 285.0176.
2,5-Di-p-tolylselenophene (2b).58 The compound was purified by
column chromatography on silica gel using hexanes. Yield: 68%, 63
2,5-Diheptylselenophene (2k). The compound was purified by
column chromatography on silica gel using hexanes as an eluent.
1
Yield: 7%, 7 mg as pale-yellow oil. H NMR (400 MHz, CDCl3) δ:
1
mg as a light-yellow crystal, mp 203−205 °C. H NMR (400 MHz,
6.70 (s, 2H), 2.87−2.72 (m, 4 H), 1.27−1.23 (m, 20H), 0.88 (t, J =
6.8 Hz, 6H). 13C{1H} NMR (100 MHz, CDCl3) δ: 150.8, 152.4, 32.9,
32.5, 31.9, 29.3, 29.1, 22.7, 14.1. 77Se NMR (76 MHz, CDCl3,
(PhSe)2 as internal standard δ = 463.00 ppm) δ: 565.85. HRMS
(ESI) m/z [M + H+] calcd for C18H33Se, 329.1747; found, 329.1718.
2-(4-Methoxyphenyl)-5-phenylselenophene (2m). The com-
pound was purified by column chromatography on silica gel using
3:97 ethyl acetate/hexanes as an eluent. Yield: 77%, 72 mg as a beige
CDCl3) δ: 7.45 (d, J = 7.9 Hz, 4H), 7.38 (s, 2H), 7.16 (d, J = 7.9 Hz,
4H), 2.35 (s, 6H). 13C{1H} NMR (100 MHz, CDCl3) δ: 149.5,
137.6, 133.8, 129.7, 126.0, 125.8, 21.3. HRMS (ESI) m/z [M + H+]
calcd for C18H17Se, 313.0495; found, 313.0486.
2,5-Bis(4-fluorophenyl)selenophene (2c).59 The compound was
purified by column chromatography on silica gel using hexanes as an
1
eluent. Yield: 84%, 80 mg as a white crystal, mp 169−171 °C. H
1
NMR (400 MHz, CDCl3) δ: 7.51 (dd, J = 8.9 Hz and 5.2 Hz, 4H),
7.35 (s, 2H), 7.07 (t, J = 8.9 Hz, 4H). 13C{1H} NMR (100 MHz,
CDCl3) δ: 162.4 (d, 1JC−F = 247.8 Hz), 148.6, 132.50 (d, 4JC−F = 3.4
solid, mp 144−146 °C. H NMR (400 MHz, CDCl3) δ: 7.58−7.53
(m, 2H), 7.50 (d, J = 8.7 Hz, 2H), 7.45−7.41 (m, 1H), 7.39−7.31 (m,
4H), 7.29−7.27 (m, 1H), 3.84 (s, 6H). 13C{1H} NMR (100 MHz,
CDCl3) δ: 159.3, 149.7, 148.7, 136.4, 129.2, 128.9, 127.4, 127.3,
126.2, 125.9, 125.1, 114.3. 77Se NMR (76 MHz, CDCl3, (PhSe)2 as
the internal standard δ = 463.00 ppm) δ: 572.87. HRMS (ESI) m/z
[M + H+] calcd for C17H15OSe, 315.0288; found, 315.0284.
Large-Scale Synthesis of 2a. The reaction is performed in three
steps in a Schlenk tube, under an argon atmosphere. First, dried
dimethylsulfoxide (20 mL) was added and was bubbled with argon for
10 min. Then, selenium (0.47 g, 6 mmol, 2 equiv) and potassium
hydroxide (0.67 g, 12 mmol, 4 equiv) were added and the mixture was
stirred under room temperature for 20 min. Finally, TEMPO (0.94 g,
6 mmol, 2 equiv) and the respective 1,3-diyne 1a (3 mmol, 0.61 g)
were added and the reaction mixture was heated at 80 °C (oil bath)
for 4 h. The crude mixture was diluted with ethyl acetate and washed
with water, dried, and purified by column chromatography on silica
gel using petroleum ether as the eluent to afford the desired product
2a in 85% yield (0.72 g).
3
2
Hz), 127.6 (d, JC−F = 8.0 Hz), 126.2, 115.8 (d, JC−F = 21.8 Hz).
HRMS (ESI) m/z [M + H+] calcd for C16H11F2Se, 320.9994; found,
320.9972.
2,5-Bis(2-methoxyphenyl)selenophene (2d).60 The compound
was purified by column chromatography on silica gel using 10:90
ethyl acetate:hexanes as an eluent. Yield: 16%, 16 mg as a yellow
crystal, mp 94−96 °C. 1H NMR (400 MHz, CDCl3) δ: 7.74 (dd, J =
7.7, 1.5 Hz, 2H), 7.71 (s, 2H), 7.27−7.21 (m, 2H), 7.04−6.96 (m,
4H), 3.96 (s, 6H). 13C{1H} NMR (100 MHz, CDCl3) δ: 155.0,
143.9, 127.9, 127.0, 126.1, 125.5, 120.9, 111.6, 55.5. HRMS (ESI) m/
z [M + H+] calcd for C18H17O2Se, 345.0394; found, 345.0397.
2,5-Bis(3-methoxyphenyl)selenophene (2e).31 The compound
was purified by column chromatography on silica gel using 10:90
ethyl acetate:hexanes as an eluent. Yield: 87%, 89 mg as a yellow
crystal, mp 67−69 °C. 1H NMR (400 MHz, CDCl3) δ: 7.42 (s, 2H),
7.27 (t, J = 7.9 Hz, 2H), 7.18−7.13 (m, 2H), 7.11−7.06 (m, 2H),
6.83 (dd, J = 8.2 and 2.5 Hz, 2H), 3.83 (s, 6H). 13C{1H} NMR (100
MHz, CDCl3) δ: 160.1, 149.9, 137.7, 130.0, 126.5, 118.9, 113.2,
111.8, 55.5. HRMS (ESI) m/z [M + H+] calcd for C18H17O2Se,
345.0394; found, 345.0385.
Theoretical Calculations. The equilibrium geometries of all
studied selenophenes in their closed-shell ground state (S0) were
determined with the MP2 method without imposing any symmetry
constrains.63 Excitation energies, equilibrium geometries, and
response properties of the lowest excited singlet states were calculated
using the second-order algebraic diagrammatic construction ADC(2)
method.64−66 The correlation−consistent valence double zeta basis
set with polarization functions on all atoms (cc-pVDZ) was used.67
2,5-Bis(3,5-di(trifuoromethyl)phenyl)selenophene (2g). The com-
pound was purified by column chromatography on silica gel using
hexanes as an eluent. Yield: 51%, 84 mg as a white solid, mp 158−160
1
°C. H NMR (400 MHz, CDCl3) δ: 7.98 (s, 4H), 7.82 (s, 2H), 7.64
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Article
All calculations were performed with the TURBOMOLE 7.3 program
package,68 making use of the resolution of the identity (RI)
approximation for the evaluation of the electron-repulsion integrals.69
Additional calculations using the conductor-like screening model
(COSMO) to include solvent effects on the electronic excitation and
emission energies of the compound 2a were also performed.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.1c00874.
■
sı
Copies of NMR spectra for all compounds and
additional photophysical data (PDF)
AUTHOR INFORMATION
Corresponding Author
Paulo Henrique Schneider − Instituto de Química,
Departamento de Química Organica, Universidade Federal
■
̂
do Rio Grande do Sul (UFRGS), 91501-970 Porto Alegre,
Rio Grande do Sul, Brazil; orcid.org/0000-0003-4082-
7152; Email: paulos@iq.ufrgs.br
Authors
Helena Domingues de Salles − Instituto de Química,
(8) Carrol, L. D.; Davies, M. J. Organoselenium Compounds in Biology
and Medicine: Synthesis, Biological and Therapeutic Treatments; Jain, V.
K., Priyadarsini, K. I., Eds.; Royal Society of Chemistry: London,
2018.
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W.; Zeni, G. Synthesis and antidepressant-like activity of seleno-
phenes obtained via iron(III)-PhSeSePh-mediated cyclization of Z-
selenoenynes. Org. Biomol. Chem. 2012, 10, 798.
(10) Wiles, J. A.; Phadke, A. S.; Bradbury, B. J.; Pucci, M. J.;
Thanassi, J. A.; Deshpande, M. Selenophene-containing inhibitors of
type IIA bacterial topoisomerases. J. Med. Chem. 2011, 54, 3418.
(11) Wilhelm, E. A.; Jesse, C. R.; Prigol, M.; Alves, D.; Schumacher,
R. F.; Nogueira, C. W. 3-Alkynyl selenophene protects against carbon-
tetrachloride-induced and 2-nitropropane-induced hepatic damage in
rats. Cell Biol. Toxicol. 2010, 26, 569.
(12) Wilhelm, E. A.; Jesse, C. R.; Bortolatto, C. F.; Nogueira, C. W.;
Savegnago, L. Anticonvulsant and antioxidant effects of 3-alkynyl
selenophene in 21-day-old rats on pilocarpine model of seizures. Brain
Res. Bull. 2009, 79, 281.
̂
Departamento de Química Organica, Universidade Federal
do Rio Grande do Sul (UFRGS), 91501-970 Porto Alegre,
Rio Grande do Sul, Brazil
Felipe Lange Coelho − Instituto de Química, Universidade
Federal de Goiás (UFG), Campus Samambaia, 74690-900
Goaînia, Goiás, Brazil
Douglas Bernardo Paixao − Instituto de Química,
̃
̂
Departamento de Química Organica, Universidade Federal
do Rio Grande do Sul (UFRGS), 91501-970 Porto Alegre,
Rio Grande do Sul, Brazil
Cristina Aparecida Barboza − Institute of Physics, Polish
Academy of Sciences, 02-668 Warsaw, Poland
Daniel da Silveira Rampon − Laboratório de Polímeros e
Catálise (LAPOCA), Departamento de Química,
Universidade Federal do Paraná (UFPR), 81531-990
Curitiba, Paraná, Brazil; orcid.org/0000-0003-0712-
6645
(13) Wilhelm, E. A.; Jesse, C. R.; Bortolatto, C. F.; Nogueira, C. W.;
Savegnago, L. Antinociceptive and anti-allodynic effects of 3-alkynyl
selenophene on different models of nociception in mice. Pharmacol.,
Biochem. Behav. 2009, 93, 419.
(14) Lu, B.; Ming, S.; Lin, K.; Zhen, S.; Liu, H.; Gu, H.; Chen, S.; Li,
Y.; Zhu, Z.; Xu, J. [1,2,5]Chalcogenodiazolo[3,4-c]pyridine and
selenophene based donor−acceptor−donor electrochromic polymers
electrosynthesized from high fluorescent precursors. New J. Chem.
2016, 40, 8316.
(15) Cuesta, V.; Vartanian, M.; Malhotra, P.; Biswas, S.; de la Cruz,
P.; Sharma, G. D.; Langa, F. Increase in efficiency on using
selenophene instead of thiophene in π-bridges for D-π-DPP-π-D
organic solar cells. J. Mater. Chem. A 2019, 7, 11886.
(16) Mahdavifar, Z.; Tajdinan, S.; Shakerzadeh, E. Exploring the
electro-optical properties of conjugated polymers based on oligo-
selenophene and oligo(3,4-ethylenedioxyselenophene). Appl. Organo-
met. Chem. 2019, 33, 4962.
Fabiano Severo Rodembusch − Instituto de Química,
̂
Departamento de Química Organica, Universidade Federal
do Rio Grande do Sul (UFRGS), 91501-970 Porto Alegre,
Rio Grande do Sul, Brazil; orcid.org/0000-0001-6528-
2318
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.joc.1c00874
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to Coordenaca
■
̧
o
̃
de Aperfeico
̧
amento de
Pessoal de Nível SuperiorBrasil (CAPES)Finance Code
001, CNPq, INCT-CMN, and FAPERGS for financial and
technical support. This research was supported in part by the
PL-Grid infrastructure.
(17) Hollinger, J.; Jahnke, A. A.; Coombs, N.; Seferos, D. S.
Controlling phase separation and optical properties in conjugated
polymers through selenophene-thiophene copolymerization. J. Am.
Chem. Soc. 2010, 132, 8546.
(18) Paal, C. Ueber die einwirkung von phosphorpentaselenid auf
das acetonylaceton. Ber. Dtsch. Chem. Ges. 1885, 18, 2255.
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