Organoboron copolymers containing thienothiophene and selenophenothiophene analogues: Optical, electrochemical and fluoride sensing properties

Article abstract of DOI:10.1039/c7ra01793f

Conjugated donor-acceptor (D-A) copolymers possessing alternating fused bicyclic aromatic rings thieno[3,2-b]thiophene, selenopheno[3,2-b]thiophene, thieno[2,3-b]thiophene and selenopheno[2,3-b]thiophene as donors, thiophene as a π-conjugated bridge and mesitylboron as an acceptor were synthesized and characterized by spectroscopic methods. Large Stokes shifts of 96-166 nm were recorded and the solution quantum yields of the polymers were in the range of 5-18%. Their ionization potentials and electron affinities were investigated by cyclic voltammetry. Optical band gaps varied between 2.26 and 2.78 eV. (TD)-DFT studies were performed to unveil their electronic structures, Kohn-Sham orbitals and electronic transitions. Absorption and emission measurements revealed that these polymers have high sensitivity to fluoride anions, among which the polymer possessing cross-conjugated thienothiophene units had the best sensing properties supported by orbital composition analysis with Mulliken partition. Thus, copolymers P1-P4 could be used as colorimetric and fluorescent sensors for small fluoride anions.

Full text of DOI:10.1039/c7ra01793f

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PAPER
Organoboron copolymers containing
thienothiophene and selenophenothiophene
analogues: optical, electrochemical and fluoride
sensing properties†
Cite this: RSC Adv., 2017, 7, 23197
ab
ac
Gulsen Turkoglu,a M. Emin Cinar
and Turan Ozturk
*
Conjugated donor–acceptor (D–A) copolymers possessing alternating fused bicyclic aromatic rings thieno
[3,2-b]thiophene, selenopheno[3,2-b]thiophene, thieno[2,3-b]thiophene and selenopheno[2,3-b]
thiophene as donors, thiophene as a p-conjugated bridge and mesitylboron as an acceptor were
synthesized and characterized by spectroscopic methods. Large Stokes shifts of 96–166 nm were
recorded and the solution quantum yields of the polymers were in the range of 5–18%. Their ionization
potentials and electron affinities were investigated by cyclic voltammetry. Optical band gaps varied
between 2.26 and 2.78 eV. (TD)-DFT studies were performed to unveil their electronic structures, Kohn–
Sham orbitals and electronic transitions. Absorption and emission measurements revealed that these
polymers have high sensitivity to fluoride anions, among which the polymer possessing cross-
conjugated thienothiophene units had the best sensing properties supported by orbital composition
analysis with Mulliken partition. Thus, copolymers P1–P4 could be used as colorimetric and fluorescent
sensors for small fluoride anions.
Received 13th February 2017
Accepted 10th April 2017
DOI: 10.1039/c7ra01793f
rsc.li/rsc-advances
studies.7 In addition, organic materials having “Se” atom in
their structures are supposed to have higher frontier molecular
Introduction
orbitals (FMO), i.e. lower oxidation and reduction potentials,
and easier polarizability. Existence of intermolecular Se/Se
interactions in their structures leads to plausible inter-chain
charge transfers,8 accepted as gorgeous properties for
optoelectronics.
Like thiophenes, selenophenes are categorized in the class of
chalcogenophenes.1 Fused selenophenes have been the focus of
scientists owing to their promising use in optoelectronic
materials.2 In spite of the extensive laboratory work devoted to
the synthesis and investigation of organic thin lm transistors
(OTFTs), to the best of our knowledge, the properties of oligo-
meric fused selenium heterocycles and fused selenophene
containing polymers have not been thoroughly explored,
possibly due to lack of synthetic methodologies and their low
solubilities.3 Replacement of sulphur with selenium enhances
the optoelectronic properties, such as an increase of conduc-
tivity as observed with tetraselenafulvalene (TSF) with respect to
tetrathiafulvalene (TTF) derivatives at room temperature.4
Moreover, polyselenophenes have higher conductivity and
mobility which might be based on better p-orbital overlap in Se
compounds.1,5 Furthermore, polyselenophenes have smaller
band gaps according to both computational6 and experimental
It is well known that fused thiophene containing polymers,
as an active layer, provide high charge mobilities in eld effect
transistors (FET).9 Besides, fused rings afford a high degree of
planarity and rigidity resulting in higher crystallinity and
extended pi overlap. Additionally, they do not only hinder
unfavourable chain folding but also diminish reorganization
energy of the molecules, and make feasible intermolecular
charge hopping, resulting in the increase of the mobility.
Heteroaromatic compounds with sulphur have been the one
of the best candidates as electron-donating scaffolds for low
band gap polymers. Their Se analogues afford high charge
mobilities arisen from strong intermolecular interactions
through Se atoms.10 Exchange of sulphur with selenium, known
as a less electronegative and more polarizable atom with respect
to sulphur, affects the electronic and optical properties.11
The low band gap polymers can be successfully achieved
through D–p–A approach, which requires copolymerization of
electron-rich donor and strong acceptor monomers.12 LUMO of
the obtained polymers is predominantly located on the electron
accepting unit, whereas HOMO is mainly placed on the donor
aDepartment of Chemistry, Istanbul Technical University, Maslak, Istanbul 34469,
Turkey. E-mail: ozturktur@itu.edu.tr
b
¨
Department Chemie-Biologie, OC1, Universitat Siegen, Adolf-Reichwein-Str., 57068
Siegen, Germany
cChemistry Group Laboratories, TUBITAK UME, PO Box 54, 41470, Gebze-Kocaeli,
Turkey
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra01793f
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material. In addition to the fused heteroaromatic systems with powdery solids in the yields of 39–50%. The polymers were
sulphur and/or selenium atoms, conjugated organic molecules found to be stable to air and moisture that no special precaution
and polymers containing three-coordinate boron have also was taken during the purication processes. The number-
drawn considerable attention owing to their convincing opto- average molecular weights (Mn) of P1–P4 were determined
electronic and sensor properties.13 As boron is an electron using light scattering technique and found to be 97.9 kDa, 11.2
decient atom, it has a strong electron acceptor character due to kDa, 40.5 kDa, and 40.7 kDa, respectively. As thermal gravi-
its empty p orbital. Its use in polymers having conjugated metric analysis (TGA) provides valuable information on thermal
electron donor units has generated a new type of donor– stability of materials, TGA analyses of the polymers P1–P4 were
acceptor conjugated polymeric systems for material science.14 conducted, which indicated the relative thermal stability of P1
The bulky mesityl group attached to the boron protects it from and P3 with respect to P2 and P4 (Fig. S7†). While main thermal
any nucleophilic attacks. Thus, boron containing compounds decomposition steps of P1 and P3 was observed around 178 and
with vacant pp orbital can also be used as Lewis acids to coor- 170 ꢁC, respectively, decompositions of P2 and P4 were recorded
dinate Lewis bases, such as uoride ions (Fꢀ), leading to to be at lower temperatures (78 and 154 ꢁC, respectively). Lower
a remarkable colour and/or luminescence changes, which have molecular weight could be a reason for the lower decomposition
been extensively exploited in chemical sensors.15
temperature of the polymer P2.
Herein, we report the synthesis, optical and electrochemical
properties of four novels D–p–A copolymers P1–P4, possessing
acceptor mesityl boron units and donor fused conjugated and
cross conjugated selenophenothiophene heteroaromatic
linkers. A detailed structure–property correlation suggests
insights into the inuence of linkers on the electronic struc-
tures and photophysical properties of organoboron polymers.
Finally, all the copolymers have been successfully applied to
detect uoride anion (Fꢀ) in order to probe their sensing
properties.
Photophysical properties
The UV-Vis absorption spectra of the polymers in THF and spin-
coated thin-lms were recorded (Fig. 1 and S8 in ESI†) and
therefrom obtained corresponding data are summarized in
Table 1. The lowest energy absorptions were assigned to be
localized p–p* transitions with a strong intramolecular charge
transfer (ICT) to boron acceptor. UV-Vis absorption maxima of
both P1 and P3 polymers were signicantly red-shied implying
that they had effective conjugation lengths with respect to P2
and P4. While the maxima were recorded for P1 and P3 to be 406
and 400 nm, respectively, those were 362 nm for P2 and 348 nm
for P4, which have cross-conjugated selenopheno[2,3-b]thio-
phene and thieno[2,3-b]-thiophene, respectively (Fig. 1). This
large hypsochromic shis of P2 (44 nm) and P4 (52 nm) are
consistent with a diminished effective conjugation length
compared to P1 and P3. Furthermore, the comparatively low
absorptions of P2 and P4 could be connected to the poor elec-
tronic couplings of cross-conjugated donors and mesitylboron
acceptor in the polymer chain. In addition, the absorption
maxima obtained from thin-lms of P1–P4 were about 6 to
12 nm red-shied compared to solution state, supporting well
organized intermolecular packings and sulfur–sulfur interac-
tions in solid states in solid states.19 Optical HOMO–LUMO gaps
of the polymers were determined from the onsets of UV-Vis
absorption spectra in solution state to be 2.26 eV for P1 as the
smallest one, and 2.60, 2.52 and 2.78 eV for P2–P4, respectively.
The smallest band gap of P1 mainly emerges from the strong
D–A interaction between the electron rich selenopheno[3,2-b]
thiophene, more efficiently increasing the HOMO level than the
Results and discussion
Synthesis of monomers and polymers
The syntheses of the polymers P1–P4 are shown in Scheme 1.
Dibrominated monomers 1–4 were synthesized according to the
established methods.3b,16–18 The precursors for the polymeriza-
tions, 2,5-di(thiophen-2-yl)selenopheno[3,2-b]thiophene (5),
2,5-di(thiophen-2-yl)selenopheno[2,3-b]thiophene
(6),
2,5-
di(thiophen-2-yl)thieno[3,2-b]thiophene (7) and 2,5-di(thio-
phen-2-yl)thieno[2,3-b]thiophene (8) were synthesized via
a Suzuki coupling in moderate to good yields (40–72%).
Syntheses of the polymers were performed through the reac-
tions of the monomers 5–8 with mesityldimethoxyborane
ꢁ
(Scheme 1). Lithiation of the monomers with n-BuLi at ꢀ78 C
was followed by addition of mestiyldimethoxyborane in THF at
room temperature providing the polymers P1–P4. They were
puried by precipitation from n-hexane leading to reddish-
orange (P1), yellow (P2), reddish-brown (P3) and orange (P4)
Scheme 1 Preparation of the polymers P1–P4.
23198 | RSC Adv., 2017, 7, 23197–23207
Fig. 1 UV-Vis (left) and emission (right) spectra of P1–P4 in THF.
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Table 1 Photophysical and electrochemical properties of polymers P1–P4
abs
a
em
b
abs
c
em
c
CV h
CV h
l
l
l
l
FF
Sg
[nm]
Eogpt
[eV]
E
E
red
HOMOi
[eV]
LUMOi
[eV]
ECgV
[eV]
HOMOj
[eV]
EDg FT k
[eV]
max
max
max
max
ox
[nm]
406
362
400
348
[nm]
[nm]
[nm]
[%]
[V]
[V]
P1
P2
P3
P4
572(s)
542
464
412
653(s)
547
504
430
511
10d
5e
166
102
100
96
2.26
2.60
2.52
2.78
1.28
1.62
ꢀ1.59
ꢀ1.79
ꢀ1.36
ꢀ1.41
ꢀ5.22
ꢀ5.35
ꢀ5.30
ꢀ5.37
ꢀ3.45
ꢀ3.28
ꢀ3.44
ꢀ3.26
1.77
2.09
1.86
2.11
ꢀ5.13
ꢀ5.30
ꢀ5.15
ꢀ5.33
2.66
2.97
2.70
3.00
437(s)
370
410
345
360
480(s)
500
444
18f
12f
1.17
1.89
1.32
502
476
a
b
c
Only longest wavelength absorption in THF. Emission in THF. Absorption and emission maxima of the polymer lms spin coated on glass.
d
e
f
g
Using rodhamine 6G as a standard in ethanol. Anthracene as a standard in cyclohexane. Coumarin1 as a standard in ethanol. Stokes
shi. All data reported vs. Fc/Fc+. For polymers P1–P4 recorded on thin lms at scan rates of 100 mV sꢀ1 with CH3CN/TBAPF6 (0.1 M) as
h
i
ox
onset
red
onset
supporting electrolyte; Eox and Ered were estimated from the oxidation and reduction peaks. HOMO ¼ ꢀE
+ 4.40 (eV), LUMO ¼ ꢀE
+
j
k
4.40 (eV). At (PCM:THF)B3LYP/6-31G(d) level. From (CPCM:THF)-CAM-TD-B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level computations.
corresponding conjugated sulphur containing thieno[3,2-b] in uorescence, which is obviously due to a better contribution
thiophene, and the electron poor mesitylboron units, dimin- of big selenium atom to the available proper conjugation
ishing the LUMO level and prompting strong ICT.13d
compare with sulphur atom.
The uorescence spectra of the solutions, having the same
The solid state uorescence of P1–P4 showed an intense
concentration used for the UV-Vis measurements and thin green uorescence with the maxima of 547, 504, 511, and
lms, were recorded (Fig. 1, S8,† and Table 1). While a strong 502 nm, respectively (Fig. S8,† Table 1). Such a clear red-shi
yellow-green emission at 542 nm with a shoulder peak at from 5 to 66 nm in comparison to the solution state could be
around 572 nm was recorded for P1, polymer P3 emitted a green attributed to boron acceptor and rigid fused aromatic donor
colour with lem maximum of 500 nm, which is about 42 nm blue core units having better interactions in solid state.
shied compared to that of P1 pointing out the more effective
conjugation and charge transfer in P1. Presence of cross-
conjugated selenopheno[2,3-b]thiophene and thieno[2,3-b]
Electrochemical properties
The redox properties of all the polymers were determined by
cyclic voltammetry on thin lms. The CV diagrams of the
polymers P1–P4 are depicted in Fig. 2 and electrochemical
results are listed in Table 1. While the oxidation processes were
irreversible for P1, P2 and P4, two quasi-reversible characteris-
tics (Eox ¼ +1.17 and +1.89 V, vs. Fc/Fc+) were recorded for P3,
the rst oxidation process of which was found to occur at the
lowest potential among the four polymers. Slightly lower
oxidation potential of P1 (Eox ¼ +1.28 V), having a cross-
conjugated selenophenothiophene unit, than P2 (+1.62 V)
indicated a low laying HOMO of P2 compared to P1. The same
trend was observed with P3 and P4. The oxidation of P4 (+1.32
V), possessing a cross-conjugated thienothiophene, was found
to be more difficult than P3. The reason for the difference could
thiophene in the main backbones of P2 and P4 caused
a signicant blue-shi with respect to P1 and P3. While P2
exhibited a deep-blue emission at 464 nm, P4 gave a blue-violet
emission at around 444 nm. Thus, the photophysical properties
of these emitters are strongly affected primarily by the p-
conjugation length and then with the presence of “Se” atom in
the ring. Absorption spectra of P2 and P4 had two maxima
above 300 nm. While low energy absorption bands arose from
p–p* transitions, high energy absorption bands were trigged by
charge transfer to mesitylboron unit. The uorescence
quantum yields (FF) of both copolymers were measured in THF
and found to be 0.18 for P3 and 0.12 for P4, whereas the other
polymers P1 and P2 exhibited slightly lower quantum yields of
0.10 and 0.05, respectively, due to higher rate of nonradiative
decays emerging from high degree of stabilization of charge
transfer state, by means of that making vicinity of this state to
ground charge transfer states. Moreover, P1 and P3 had the
same longest absorption wavelengths, the emission of P1 was
found to be much longer than P3, which had the largest Stokes
shi of 166 nm. While the polymers P1 and P2, having seleno-
phenothiophene units, exhibited the Stokes shi of 166 and
102 nm, respectively, those of the other polymers P3 and P4,
possessing thienothiophene units, had the Stokes shits of 100
and 96 nm, respectively. These results clearly indicated that
cross-conjugated ring of selenophenothiophene, i.e. seleno-
pheno[3,2-b]thiophene, is more effective in generating red shi
Fig. 2 Cyclic voltammograms of the polymers P1–P4 on glassy
carbon disk electrode in acetonitrile, 0.1 M Bu4NPF6, 100 mV sꢀ1 scan
rate, vs. Fc/Fc+.
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be explained not only with the presence of the different donor a uoride source was gradually added to a THF solution of each
units in the polymer back-bones but also with the structures of polymer and the response of P1–P4 to uoride anions was
the fused rings.
investigated by UV-Vis absorption and uorescence spectros-
All the polymers displayed irreversible reduction peaks in copy. Titration of organoborane polymers P1–P4 with uoride
the CV diagrams, which is possibly due to the known relaxation ion displayed an interesting binding process of uoride ion to
effects generating hysteresis phenomena (Fig. 2). The reduction boron atom in the absorption spectra. Upon addition of less
potentials of P1 and P2 were recorded to be ꢀ1.59 and ꢀ1.79 V, than 1.0 equiv. of TBAF to the polymer P1, while the absorption
whereas those of P3 and P4 were measured as ꢀ1.36 and at 406 nm started to decrease, new bands gradually appeared in
ꢀ1.41 V, respectively, indicating a more difficult reduction in the high-energy region at 267, 322 and 354 nm, intensities of
the case of P1 and P2 possessing electron rich and more which enhanced upon continuing the addition of TBAF
polarizable Se atoms with respect to P3 and P4 having more (Fig. 3a). An isosbestic point at 373 nm was observed. On the
electronegative and less polarizable S atoms. In addition, other hand, when more than 1.0 equiv. of TBAF was added,
compared to P1 and P2, the boron-copolymers P3 and P4 intensities of the new absorption bands started to decrease and
showed more positive shis in their rst reduction potentials, the absorption band at 406 nm, which already decreased upon
supporting improved electron-accepting ability.
addition of TBAF, almost disappeared along with the disap-
From the electrochemical data, the HOMO and LUMO pearance of the isosbestic point. This process was reected in
energy levels and there from calculated energy gap of P1–P4 are a colour change of the solution from yellow to complete col-
listed in Table 1. The HOMOs of P1–P4 were found to be in the ourless, which allowed a naked-eye detection of uoride anion.
range of ꢀ5.22 to ꢀ5.37 eV, while LUMO energy levels were Decrease of the band at 406 nm and appearance of the new
obtained at ꢀ3.45 to ꢀ3.26 eV. The HOMO energy levels of P1 bands at 267, 322 and 354 nm may indicate the interaction of
and P2 are lower than the corresponding S containing cross- boron atoms with uoride ions and interruption of electron
conjugated P3 and P4 pointing out a better stability of P2 and ow to boron, which causes shortening of the conjugation and
P4 against oxidative doping. The calculated electrochemical lowering of the absorption at 406 nm. Further addition of TBAF,
HOMO–LUMO gaps of selenophenothiophene derivatives i.e. above 1.0 equiv., may result in the quenching of the polymer
(1.77 eV for P1 and 2.09 eV for P2) are considerably smaller than and decrease of absorptions even at 267, 322 and 354 nm.
those of corresponding thienothiophene analogues (1.86 eV for
P3 and 2.11 eV for P4).
Fig. 3b shows the change in uorescence spectrum of P1
upon addition of various concentrations of TBAF. Aer the
addition of rst 0.05 equiv. of TBAF, a sharp decrease of emis-
sion intensity at 542 nm with a shoulder at 572 nm was observed
with an appearance of a new band with lower intensity at
560 nm. When reached the addition of 3.0 equiv. of TBAF,
almost a complete quenching took place. Thus, P1 works as
a “turn-off” sensor for uoride ion. This result is attributable to
the apparent amplied quenching effect, which has also been
described for other organoborane polymers,15b,22a–f involving the
efficient energy transfer to lower energy charge transfer states
which are generated by binding of uoride anion to boron.
Next, uoride sensing properties of P3 were examined
(Fig. S9a†). Similarly, upon addition of TBAF, the characteristic
intense charge-transfer absorption band at 400 nm steadily
decreased, while the absorption maximum at 242, 329 and
293 nm appeared and further addition up to 0.7 equiv. made
them intense bands. An isosbestic point at 350 nm was obtained
at this stage. Continuing the addition from 0.7 to 2.4 equiv.
caused all the absorption bands gradually decrease, except the
band at 242 nm, which slightly increased, and a new weak band
at 370 nm appeared. Colour of the solution turned to colourless
from yellow. Furthermore, the emission intensity of P3 was also
quenched in this process (Fig. S9b†).
Fluoride sensing properties
Presence of electron-decient boron centres could be a good
advantage for anion bindings such as uorides and cyani-
des.15c,15e,15g,20 Triarylboranes have been widely studied as
chemo-sensors for uoride anions with high selectivity and
sensitivity.21 Complexation of boron with anions typically leads
to a distinct change in the photophysical properties, and poly-
meric species are particularly interesting because of their
potential for reciprocal interactions with different binding
sites.15b,15g,22 Thus, tetrabutylammonium uoride (TBAF) as
The stepwise addition of TBAF resulted in a steady decrease
of the broad emission band between 375–600 nm, which was
almost completely quenched with 3.0 equiv. of TBAF; hence it is
also a “turn-off” sensor for uoride ion. Quenching of uores-
cence intensity was observed to be more effective with the
polymer P3 (78%) compare with the polymer P1 (69%) at the
addition of the same amount of TBAF (1.5 equiv.). Absorption
and emission changes of the polymers (P1 and P3) clearly
Fig. 3 (a) Absorption spectra and (b) emission spectra (lexc ¼ 470 nm)
of P1 (1 ꢂ 10ꢀ4 M based on boron sites) in THF in the presence of
various amounts of TBAF. (c) Pictorial view of interruption of charge
transfer (CT) by fluoride binding.
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sensitive accepting uoride ions. Concerning the best sensi-
tivity of P4, it has the least electron donation among the four
polymers due to both having “S” atom instead of “Se” and its
cross-conjugated structure.
Computation
The properties of the polymers were investigated further by
performing density functional theory (DFT) calculations on
monomers (M) and segments of polymers (S), possessing
mesitylboron acceptor between thienothiophene (TT) and
selenophenothiophene (SeT) donor units (Fig. 5). Geometry
optimizations of their S0 and S1 states were realized without any
symmetry constraints by means of Gaussian 09 package
program.23 B3LYP24 method with 6-31G(d) basis set was chosen
because of its good performance in the estimation of structures
and properties of optoelectronic compounds.25 Solvent effect
(THF) was included by using polarizable continuum model
(PCM).26 Analysis of the harmonic vibrational frequencies using
analytical second derivatives was performed to conrm the
minima. Orbital composition analysis with Mulliken partition
was carried out using Multiwfn program (a multifunctional
wavefunction analyser).27 TD-DFT calculations with solvent
effect (CPCM)28 were conducted on polymer segments at
coulomb-attenuating density functional theory (CAM-TD-
B3LYP)29 level with 6-311++G(d,p) basis set to obtain the lowest
singlet–singlet vertical excitations owing to the good perfor-
mance of CAM-B3LYP in the calculations of excitation ener-
gies.30 Absorption bands were collected for singlets with N states
of 50. While visualization of MOs with an isosurface value of
0.04 au was accomplished with GaussView 5.0, absorption and
emission spectra were produced with GaussSum 3.0.31
Computations performed on the monomers depicted that TT
and SeT units attached to the mesityl boron are almost planar to
the boron plane with a dihedral angle of less than 10ꢁ, whereas
the mesityl group attached to boron atom stayed vertical with
respect to TT and SeT groups (>60ꢁ) (Fig. 5). The localized
HOMOs are observed on TT and SeT groups of all the molecules,
spreading marginally over the thiophene substituents. On the
other hand, the LUMOs of the molecules are entirely located on
boron atoms with slight contributions of TT and SeT units.
Therefore, TT and SeT play as electron rich groups and mesityl
boron as an electron poor unit. In the case of monomers M2 and
M4 with cross-conjugated systems, LUMO orbitals are spread
over only half of the TT and SeT units. MOs undoubtedly
demonstrate an ICT between donor and acceptor units. The
presence of nodes on electron poor boron atoms in HOMOs
results in the charge density separation and consequently
hampers the delocalization of HOMOs over all the whole system
witnessed with the constructed HOMO surfaces for S1–S4.
The predicted HOMO energy levels of ꢀ5.13, ꢀ5.30, ꢀ5.15,
and ꢀ5.33 for S1–S4 are matching well with those of P1–P4
(Table 1), respectively, emphasizing the increase of HOMO
levels with incorporation of Se atoms in P1 and P3 compared to
P2 and P4. This phenomenon was reected to HOMO–LUMO
gaps. That is, Egap values of 2.66, 2.97, 2.70, and 3.00 eV were
slightly overestimated at TD-TDFT level of theory (Table 1), but
Fig. 4 (a) Absorption spectra and (b) emission spectra (lexc ¼ 350 nm)
of P4 (1 ꢂ 10ꢀ5 M based on boron sites) in THF in the presence of
various amounts of TBAF.
indicated the disruption of the p-conjugation through the
boron empty orbital by the formation of the corresponding
uoroborate in the polymer chain.
Considering the specic interactions between Fꢀ and the
boron centres of P1 and P3, the response of P2 and P4 to Fꢀ was
also investigated through absorption and uorescence spectra
(Fig. S10† and 4). With the initial addition of uoride ions up to
0.6 equiv. the main absorption peaks of P2 at 252, 312 and
364 nm increased dramatically, while the absorption peaks at
312 and 364 nm were slightly blue shied by 26 and 12 nm,
respectively. When the addition of the ion was continued from
0.6 to 2.4 equiv., all the bands were gradually decreased as
a result of coordination of the uoride ion to the empty p orbital
of the boron atom. A similar tendency was observed with the
polymer P4 in absorption spectra upon addition of uoride
anion (Fig. 4a). Moreover, in both P2 and P4, a colour change of
the solution from yellow to complete colourless was observed.
In the uorescence spectra, although initially P2 exhibited
a deep blue ICT emission at 464 nm, intensity of the emission
was steadily decreased resulting in 60% of uorescence
quenching efficiency with the increase of uoride ion concen-
tration and a violet-blue emission peak at 431 nm emerged
(Fig. S10†). This indicated the donor–acceptor interaction
between cross conjugated selenophenothiophene and boron
has stopped. In contrast to the behaviour of P2, the emission of
P4 at 444 nm was almost completely quenched and a red-shied
band at 480 nm, having weak intensity at 475 nm was developed
with an addition of small amount of uoride ion (0.05 equiv.)
(Fig. 4b). It was disappeared with further addition of TBAF. That
is, addition of only 0.05 equiv. of TBAF led to about 82% uo-
rescence quenching of P4 and the emission intensity was
almost completely quenched ca. 94% with 1.5 equiv. of TBAF.
Stern–Volmer plots shown in Fig. S11† for both P1 and P2 are
nearly identical, indicating that both molecules have a similar
affinity toward uoride ion in uorescence quenching mecha-
nism. On the other hand, the polymers P3 and P4 had higher
sensitivities toward uoride ions compare with P1 and P2. Thus,
polarizability difference of Se and S atoms and their orientation
in the ring could be affecting the sensing power of the whole
material. Comparison of the both systems, i.e. “Se” (P1, P2) and
“S” (P3, P4), indicated that as Se is a better electron donor
compare with S atom, it provides more electrons to the empty
orbital of boron making the material less sensitive toward
uoride ions. Thus, the systems containing Se atoms are less
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Fig. 5 FMOs (isosurface value ¼ 0.04 au) of monomers M1–M4 and segments S1–S4 at (PCM:THF)B3LYP/6-31G(d) level.
the trend well aligns with those obtained from the onset of UV- Table 2, respectively. The computed spectral properties are
Vis absorption spectra, demonstrating the smaller band gaps matching well with the experimentally observed values. The
for S1 and S3 with Se than their corresponding S2 and S4 having lmax values of compounds arise from the HOMO to LUMO
S atoms.
transitions pointing out the fact that the low energy transitions
Orbital composition analysis provided the highest LUMO emerge from HOMO / LUMO. Emission spectra of the polymer
orbital composition of 22.0% for boron atom of S4 and the segments were predicted by performing the calculations on the
lowest one for boron atom of S1 with 17.0% orbital composition optimized excited state geometries with solvent effect
supporting the experimentally obtained uoride quenching (CPCM:THF). The obtained emission spectra are illustrated in
efficiencies. The larger the LUMO orbital composition on boron Fig. S14.† In all segments (S1–S4), the emission predominantly
atom, the higher the sensitivity of boron atom toward Lewis originates from LUMO / HOMO transition, well in alignment
base.
with the experimental outcome (Table S1†). Fluoride attach-
The estimated UV-Vis absorption spectra of S1–S4 and ment on the electron decient boron atom (S1-F–S4-F) switches
vertical excitation energies are provided in Fig. S13† and the planar geometry of boron to tetrahedral structure. While the
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Table 2 Excited state electronic transitions at (CPCM:THF)-CAM-TD-B3LYP/6-311++G(d,p)//B3LYP/6-31G(d) level
States
labs [nm]
E [eV]
Fa
Major contributionb [%]
Exp. [nm]
S1
S1
S2
466
408
2.66
3.04
2.311
0.628
H / L (77%)
470
406
Hꢀ1 / L (55%), H / L+1 (36%),
Hꢀ1 / L+1 (30%)
S3
S24
336
244
3.68
5.08
0.199
0.124
H / L+2 (37%)
303
256
Hꢀ10 / L (14%), Hꢀ10 / L+1 (13%),
Hꢀ10 / L+2 (12%)
S2
S3
S4
S1
S2
S5
S16
S1
S2
S3
S15
S1
S2
418
355
322
263
459
400
331
252
413
351
318
268
234
2.97
3.49
3.85
4.71
2.70
3.10
3.75
4.92
3.00
3.54
3.90
4.63
5.29
1.734
0.362
0.447
0.148
2.343
0.582
0.276
0.084
1.755
0.438
0.586
0.308
0.127
H / L (76%)
—
H / L+1 (16%), Hꢀ1 / L (55%)
H / L+3 (23%), Hꢀ1 / L+3 (24%)
Hꢀ11 / L (32%), Hꢀ3 / L+3 (10%)
H / L (77%)
Hꢀ1 / L (56%), H / L+1 (34%)
Hꢀ1 / L+1 (32%), H / L+2 (38%)
Hꢀ1 / L+1 (13%), H / L+2 (19%)
H / L (76%)
362
312
252
400
320
292
243
—
348
308
248
—
H / L+1 (21%), Hꢀ1 / L (54%)
H / L+2 (30%), Hꢀ1 / L+3 (29%)
Hꢀ3 / L+3 (19%), Hꢀ2 / L+2 (22%),
Hꢀ9 / L+2 (26%), Hꢀ8 / L+3 (37%)
Hꢀ11 / L (20%)
S4
S12
S21
S1-F
S2-F
S1
S2
S5
S6
S1
S2
S3
415
400
291
290
350
343
337
2.99
3.09
4.26
4.27
3.53
3.61
3.68
2.144
0.663
0.101
0.094
1.048
0.152
1.097
H / L (49%)
—
H / L+1 (45%), Hꢀ1 / L (44%)
Hꢀ6 / L (62%), Hꢀ6 / L+1 (25%)
Hꢀ5 / L+1 (23%), Hꢀ7 / L+1 (19%)
H / L+2 (38%)
H / L (27%), Hꢀ1 / L (30%)
H / L+3 (18%), Hꢀ1 / L+3 (17%),
H / L+1 (12%)
354
322
267
364
—
—
S4
331
3.74
0.714
Hꢀ1 / L+2 (18%), H / L+1 (15%),
—
H / L+3 (14%)
S5
S6
S1
S2
S7
S8
S1
S3
294
292
406
392
277
276
343
330
4.21
4.25
3.06
3.16
4.48
4.49
3.62
3.76
0.188
0.174
2.238
0.640
0.083
0.101
1.231
0.460
Hꢀ3 / L (51%), Hꢀ4 / L (22%)
Hꢀ2 / L+1 (30%), Hꢀ4 / L+1 (28%)
H / L (49%)
H / L+1 (46%), Hꢀ1 / L (46%)
Hꢀ6 / L+1 (34%), Hꢀ6 / L (29%)
Hꢀ7 / L (38%), Hꢀ7 / L+1 (26%)
H / L+2 (50%)
312
252
—
329
293
243
321
300
S3-F
S4-F
Hꢀ1 / L (28%), H / L (20%),
H / L+1 (14%)
S4
S5
329
284
3.77
4.36
0.973
0.196
H / L+1 (20%), Hꢀ1 / L+1 (12%),
Hꢀ1 / L (11%)
—
Hꢀ3 / L+1 (27%), Hꢀ4 / L+1 (25%)
248
Oscillator strength. b H: HOMO, L: LUMO.
a
HOMOs and LUMOs were observed on the whole molecule, selenopheno[3,2-b]thiophene, thieno[2,3-b]thiophene and sele-
excluding mesityl boron units in S1-F and S3-F, HOMOs were nopheno[2,3-b]thiophene as donors and mesitylboron as an
speaded over the system up to cross-conjugated rings and acceptor connected through a thiophene p-spacer. Their prop-
LUMOs were localized on one of the terminal thiophene sepa- erties were investigated optically, electrochemically and as
rated by cross-conjugated systems in S2-F and S4-F (Fig. S12†). uoride ion sensors, comparing their structures and interac-
This results in the interruption of charge transfer. As a conse- tions of S and Se atoms with boron. As selenium atoms improve
quent, it gives rise to a blue shi in the predicted absorption the conjugation and facilitate charge-transfer processes due to
spectra by 51–70 nm, which is matching well with the results their more polarizable and less electronegative properties,
recorded in the uoride sensing investigations (Table 2).
polymer P1, having better optical properties compared to P3,
exhibited a broader absorption band with a larger red shi
emission band.
Moreover, P1 exhibited the longest emission wavelength,
and thus the largest Stokes shi among the four organoboron-
based polymers. Quantum efficiencies of the polymers were
Conclusions
Synthesis of new four D–p–A polymers was realized, which
possess fused bicyclic aromatic rings thieno[3,2-b]thiophene,
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obtained to be in the range of 5–18%. DFT studies shed light on (TGA) of the polymers were performed by EXSTAR-SII TG/
their electronic structures and frontier molecular orbitals, DTA7300 thermal gravimetry-differential thermal analysis (TG-
which supported the experimental outcome. CAM-TD-B3LYP DTA) under dynamic argon atmosphere. Temperature calibra-
calculations provided vertical excitations from HOMO to tion of the instrument was conducted according to indium
LUMO with energies varying from 2.66 to 3.00 eV. Fluoride melting point and enthalpy.
sensing properties of these polymers P1–P4 were investigated
through absorption and emission measurements, owing to the
high Lewis acidity of the boron centres to coordinate uoride
anions. The enhancement of strong sensitivity with P4 was
detected in comparison to the other polymers P1–P3, which was
2,5-Di(thiophen-2-yl)selenopheno[3,2-b]thiophene (5)
In a Schlenk tube, a mixture of 2-(thiophen-2-yl)-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (687 mg, 3.27 mmol), 2,5-
dibromoselenopheno[3,2-b]thiophene (1) (451 mg, 1.30 mmol)
and 2 M aqueous K2CO3 (2 mL) in a stirred THF (30 mL) was
interpreted to be due to the presence of less polarizable sulfur
atom compare with selenium and its cross-conjugated struc-
ushed with N2 for 15 min. Then, to this mixture was added
ture. Upon addition of TBAF as a uoride ion source, P1–P4
Pd(PPh3)2Cl2 (69.0 mg, 5% mmol) as catalyst and it was reuxed
exhibited a “turn off” sensor properties, observed by a colour
with vigorous stirring for 60 h. The excess solvent was distilled
change of the solution from yellow to completely colourless.
and the compound was extracted with dichloromethane (DCM)
(3 ꢂ 20 mL). The organic phase was washed with water, dried
Thus, the present polymers can be used as uorescent or
colorimetric uoride anion sensors. On the basis of the optical
over anhydrous MgSO4 and solvent was evaporated. The residue
and electrochemical studies, these polymers can be promising
was puried by column chromatography using n-hexane/DCM
candidates for the large area device fabrication as functional
materials.
(5 : 1, Rf ¼ 0.78) to obtain an orange solid in 55% (253 mg)
1
yield. M.p. 220–221 ꢁC; H NMR (500 MHz, CDCl3): d 7.45 (s,
1H), 7.33 (s, 1H), 7.24 (d, J ¼ 5.5 Hz, 2H), 7.21 (d, J ¼ 3.5 Hz, 1H),
7.15 (d, J ¼ 3.5 Hz, 1H), 7.04–7.01 (m, 2H); 13C NMR (125 MHz,
CDCl3): d 141.9, 139.7, 139.5, 138.5, 137.6, 137.4, 127.9, 126.8,
124.9, 124.7, 124.3, 123.8, 118.9, 117.9. MS (m/z): [M + 2]+ calcd
for C14H8S3Se, 351.89; found, 353.10.
Experimental
All reagents were used as received from commercial sources
without further purication. Tetrahydrofuran (THF) and diethyl
ether (Et2O) were dried over sodium in the presence of benzo-
phenone. All chemical reactions were performed under N2
Following the same procedure of 5, compounds 6–8 were
prepared.
atmosphere.
2,5-Dibromoselenopheno[3,2-b]thiophene
(1),3b,16,32,33 2,5-dibromoselenopheno[2,3-b]thiophene (2),17,18,32
2,5-dibromothieno[3,2-b]thiophene (3),16,34 2,5-dibromothieno 2,5-Di(thiophen-2-yl)selenopheno[2,3-b]thiophene (6)
[2,3-b]thiophene (4),16,34b 4,4,5,5-tetramethyl-2-(thiophen-2-yl)-
Compound 2 (736 mg, 2.10 mmol) was reacted with 2-
[1,3,2]dioxaborolane35 and mesityldimethoxyborane36 were
(thiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (942 mg
synthesized according to published procedures. 1H and 13C
4.48 mmol) in the presence of 2 M aqueous K2CO3 (10.7 mL) and
NMR spectra were recorded on a Varian model NMR (500 MHz).
PdCl2(PPh3)2 (212 mg, 5% mol) in THF (35 mL). The residue was
Proton and carbon chemical shis are reported in ppm down-
puried by column chromatography using n-hexane/DCM (5 : 1,
eld from tetramethylsilane (TMS). UV-Vis measurements were
Rf ¼ 0.70) affording a pale yellow solid in 48% (357 mg)
yield. M.p. 169–170 ꢁC; 1H NMR (500 MHz, CDCl3): d 7.40 (dd, J
studied on HITACHI U-0080D. Mass spectra were recorded on
Bruker MICROTOFQ and Thermo LCQ-Deca ion trap mass
instruments. CH-Instruments Model 400A was used as
a potentiostat for the CV studies. Cyclic voltammetry was per-
formed on a glassy carbon disk electrode coated with a polymer
thin lm was used as the working electrode, a platinum wire as
the counter electrode and an Ag wire as the reference electrode.
Tetrabutylammonium hexauorophosphate (Bu4NPF6) (0.1 M)
¼ 5.0 and 1.0 Hz, 1H), 7.38 (dd, J ¼ 3.8 and 2.5 Hz, 1H), 7.32 (s,
1H), 7.27 (d, J ¼ 1.0 Hz, 1H), 7.26 (d, J ¼ 1.5 Hz, 1H), 7.21 (dd, J ¼
3.8 and 1.2 Hz, 1H), 7.10 (dd, J ¼ 5.2 and 3.8 Hz, 1H), 7.04 (dd, J
¼ 5.2 and 3.8 Hz, 1H); 13C NMR (125 MHz, CDCl3): d 148.8,
140.9, 136.8, 136.3, 135.7, 130.6, 127.8, 127.4, 127.2, 126.7,
125.0, 124.3, 118.5, 101.8. MS (m/z): [M + 1]+ calcd for C14H8S3Se,
351.89; found, 352.00.
in acetonitrile, saturated with nitrogen, was used as the sup-
porting electrolyte. A ferrocene internal standard was used to
calibrate the results. The molar mass and molar mass distri-
2,5-Di(thiophen-2-yl)thieno[3,2-b]thiophene (7)37
bution of the polymers were determined using gel permeation Compound 3 (585 mg 1.90 mmol) was treated with 2-(thiophen-
chromatography, equipped with Perkin-Elmer 200 GPC high 2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (866 mg, 4.12
pressure pump, injector, THF columns (guard column + Styr- mmol) in the presence of 2 M aqueous K2CO3 (10 mL) and
agel HR 2 + Styragel HR 3 + Styragel HR 4E + Styragel HR 5E), PdCl2(PPh3)2 (140 mg, 5% mol) in THF (40 mL). The residue was
Wyatt Optilab differential refractive index detector (654 nm) puried by column chromatography using n-hexane/DCM (5 : 1,
and Dawn Heleos multi angle light-scattering detector. The Rf ¼ 0.75) furnishing a yellow solid in 72% (433 mg) yield. M.p.
mobile phase was THF (ow rate: 0.7 mL minꢀ1). Measurements 255–256 ꢁC; 1H NMR (500 MHz, CDCl3): d 7.46 (dd, J ¼ 3.5 and
were conducted at 25 ꢁC. Polymer concentrations were in the 1.2 Hz, 2H), 7.39 (dd, J ¼ 5.2 and 1.2 Hz, 2H), 7.28 (s, 2H), 7.11
range of 0.5–2.0 mg mLꢀ1 and all the samples were ltered (dd, J ¼ 5.2 and 3.5 Hz, 2H); 13C NMR (125 MHz, CDCl3): d 147.3,
through 0.2 mm lter prior to use. Thermal gravimetric analyses 140.7, 137.5, 135.5, 133.6, 128.0, 126.6, 125.2, 124.4, 116.2.
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respectively. We thank Prof. M. S. Eroglu for Light Scattering
measurements. We are indebted to National Center for High
Performance Computing (UYBHM) (tvddvb) and the High-
Performance-Computing (HPC) Linux Cluster HorUS of
University of Siegen for the computer time provided. Unsped
Global Logistic is gratefully acknowledged for nancial support.
2,5-Di(thiophen-2-yl)thieno[2,3-b]thiophene (8)
Compound 4 (414 mg 1.40 mmol) was reacted with 2-(thiophen-
2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (650 mg, 3.10
mmol) in the presence of 2 M aqueous K2CO3 (7 mL) and
PdCl2(PPh3)2 (100 mg, 5% mol) in THF (40 mL) following the
above procedure. The residue was puried by column chroma-
tography using n-hexane/DCM (5 : 1, Rf ¼ 0.68) providing
a yellowish orange solid in 40% (166 mg) yield. M.p. 195–196 ꢁC;
1H NMR (500 MHz, CDCl3): d 7.27 (s, 2H), 7.25 (dd, J ¼ 5.2 and
1.0 Hz, 2H), 7.21 (dd, J ¼ 3.5 and 1.0 Hz, 2H), 7.04 (dd, J ¼ 5.2
and 3.5 Hz, 2H); 13C NMR (125 MHz, CDCl3): d 146.9, 139.8,
137.3, 134.9, 127.8, 124.6, 124.3, 123.7, 116.3.
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Acknowledgements
G. T. and M. E. C. thank the Scientic and Technological
Research Council of Turkey (TUBITAK) for Postdoctoral
Research Fellowships BIDEB 2218 and 2216 programs,
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