Acid-base-responsive intense charge-transfer emission in donor-acceptor-conjugated fluorophores

Article abstract of DOI:10.1002/asia.201402463

Herein we report on the synthesis and acid-responsive emission properties of donor-acceptor (D-A) molecules that contain a thienothiophene unit. 2-Arylthieno[3,2-b]thiophenes were conjugated with an N-methylbenzimidazole unit to form acid-responsive D-A-t

Full text of DOI:10.1002/asia.201402463

FULL PAPER
DOI: 10.1002/asia.201402463
Acid–Base-Responsive Intense Charge-Transfer Emission in Donor–
Acceptor-Conjugated Fluorophores
Toshifumi Inouchi, Takuya Nakashima,* and Tsuyoshi Kawai*[a]
Abstract: Herein we report on the syn-
thesis and acid-responsive emission
properties of donor–acceptor (D–A)
molecules that contain a thienothio-
acid. The effect of the substitution on
their photophysical properties as well
as their solvent-dependence indicated
non-twisting ICT emission in protonat-
ed D–A molecules. The quinoidal char-
acter of 2-arylthienothiophene as
a donor part is discussed, as it is as-
sumed that it contributes to suppres-
sion of the molecular twisting in the
excited state, therefore decreasing the
nonradiative rate constant, thereby re-
sulting in the intense ICT emission.
Acid–base-sensitive triple-color emis-
sion was also achieved by the introduc-
tion of a base-responsive phenol group
in the donor part.
phene unit. 2-ArylthienoACTHNUTRGNE[NUG 3,2-b]thio-
phenes were conjugated with an N-
methylbenzimidazole unit to form acid-
responsive D–A-type fluorophores.
The D–A-conjugated fluorophores
showed intense intramolecular charge-
transfer (ICT) emission in response to
Keywords: acid–base response
charge transfer donor–acceptor
systems · fluorophores · protonation
·
·
Introduction
Twisting in the excited state often sacrifices the emission ef-
ficiency[6,7] because of the nonradiative decay processes cou-
pled with the rotation. To avoid such a twisting in the excit-
ed state, Yang and co-workers[8] introduced a spiroconjuga-
tion-like ICT emission, although it gave suppressed emission
quantum yields in polar solvents. Meanwhile, non-twisting
planar ICT emission was demonstrated by covalently rigi-
dized molecules[9] and donor-substituted tridurylboranes.[10]
In our previous work, we reported a T-shaped conjugated
molecule with an N-methylbenzimidazole[11] junction that
showed TICT emission in response to acid, whereby the
charge transfer took place between two orthogonally ar-
ranged p systems.[12] The increase in quinoidal character in
a p component of the T-shaped conjugated molecule led to
non-twisting ICT emission with a higher emission quantum
yield.[13] Thus we proposed the introduction of quinoidal
character into the excited state for the design of fluoro-
phores that showed non-twisting ICT emission, whereas
the emission efficiency of the T-shaped molecule was 0.43 at
the most. We herein describe a simple D–A conjugated
system composed of an acid-responsive N-methylbenz-
Understanding and tuning the photophysical properties of
organic fluorophores is of importance for a wide range of
applications.[1] Recently, with an eye on the excited-state
properties that are a result of electronic and geometric
structures, much effort has been made to develop intensively
fluorescent molecules. For example, aggregation-induced
emission (AIE) molecules exhibit restricted intramolecular
rotations in the excited state, which leads to a highly emis-
sive aggregate with suppressed nonradiative decay process-
es.[2] Thermally activated delayed fluorescence (TADF) em-
ploys twisted molecular structures to separate HOMO and
LUMO spatially, thereby decreasing the energy gap between
S1 and T1 states to enhance the probability of thermal acti-
vation from the T1 to S1 states.[3] Excited-state intramolecu-
lar proton transfer (ESIPT) usually takes place faster than
fluorescence, thus leading to a redshifted emission with
a large Stokes shift and a strong solvatochromic effect.[4]
Emission from the intramolecular charge-transfer (ICT)
states has also been an important subject because of the en-
vironment-sensitive emission derived from a large dipole
moment in the excited CT state.[5] Excited-state relaxation
to an ICT state often accompanies the rotation between
a donor (D) and an acceptor (A) part, which is known as
twisted intramolecular charge-transfer (TICT) emission.[6]
imidazole moiety and a 2-arylthieno
(Scheme 1).
ACHTUNGERTN[NUNG 3,2-b]thiophene group
We envisage that the contribution of quinoid structure of
a p component suppresses the rotation about the D–A
bridge with the aid of NH/S hydrogen bonding in the excit-
ed state (Scheme 1a), thus leading to intense charge-transfer
emission in protonated fluorophores. To vary the quinoidal
and electron-donating properties, several thiophene deriva-
tives were introduced as a p component. The order of in-
crease in the quinoidal character may follow the order
shown in Scheme 1b, which roughly corresponds to the
orders of decrease in the aromatic resonance energy of the
donor part as well as the increase in the extent of charge
[a] Dr. T. Inouchi, Dr. T. Nakashima, Prof. T. Kawai
Graduate School of Materials Science
Nara Institute of Science and Technology, NAIST
8916-5 Takayama, Ikoma, Nara 630-0192 (Japan)
E-mail: ntaku@ms.naist.jp
tkawai@ms.naist.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402463.
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Tsuyoshi Kawai et al.
or borylated donor units (see
the Supporting Information for
details).
Photophysical Properties
Absorption and fluorescence
spectra of fluorophores were
measured in dichloromethane
(CH2Cl2). To study the acid-re-
sponsive photophysical proper-
ties, an excess amount of tri-
fluoroacetic
acid
(TFA,
0.01 mL) was added to the solu-
tions in CH2Cl2 (1.0ꢁ10À5 m,
3 mL) that was excessive
enough to protonate all the
molecules in CH2Cl2.[13] The op-
tical properties of compounds
in neutral and protonated forms
Scheme 1. a) Design concept for planarizing the molecular geometry in the excited state of a protonated fluo-
rophore by a combination of quinoid character and an NH/S interaction. b) Molecular structures of benzimida-
zole-based fluorophores.
separation in the whole structure.[14] An electron-donating
and an electron-withdrawing unit, methoxy and trifluoro-
methyl groups, respectively, were introduced as a substituents
on the 2-arylthienoACHTUNGTRENNUNG[3,2-b]thiophene moiety to study their
effect on the photophysical property. The acid-responsive
optical properties of these molecules were evaluated. Both
the acid- and base-responsive dual ICT emission with quan-
tum yields as high as 0.8 were also demonstrated by intro-
ducing a base-responsive phenol group in the donor part.
Results and Discussion
Synthesis
Arylthienothiophene–benzimidazole-conjugated D–A mole-
cules were synthesized as shown in Scheme 2. Newly synthe-
sized molecules were characterized by 1H and 13C NMR
spectroscopy, high-resolution mass spectrometry, and ele-
mental analysis. The arylthienothiophene parts with sub-
stituents were synthesized according to the modified method
reported in the literature,[15] followed by the stannylation at
the 5-position of 2-substituted thienothiophene. Since the
stannylated arylthienothiophenes (6) were easily decom-
posed to the protonated ones (5) in a silica gel column, they
were purified through a silica gel column that contained
10 wt% anhydrous potassium carbonate.[16] Stille cross-cou-
pling reactions between N-methyl-2-bromobenzimidazole[13]
and the stannylated arylthienothiophene unit led to the syn-
thesis of arylthienothiophene–benzimidazole conjugated
molecules. OHPhTTIm was readily derivatized by the de-
protection of the methoxy group in OMePhTTIm by using
BBr3. Other molecules that contained various thiophene de-
rivatives as a donor part were also synthesized as reference
compounds through Pd-catalyzed cross-coupling reactions
between N-methyl-2-bromobenzimidazole and stannylated
Scheme 2. Synthesis of arylthienothiophene–benzimidazole conjugated
molecules: a) K2CO3 (2m, aqueous), [Pd
ACHTUNGTRENNUNG
b) 1-iodo-4-(trifluoromethyl)benzene, KF, [PdCl CTHUNGTRENNUNG
c) nBuLi, DMF, dry THF; d) ethyl mercaptoacetate, K2CO3, DMF;
e) NaOH, ethanol, water; f) CuO, quinoline; g) nBuLi, nBu3SnCl, dry
THF; h) [PdACTHUNTRGNEUNG(PPh3)4], DMF; and i) BBr3, CH2Cl2.
are summarized in Table 1 together with the data on emis-
sion quantum yield and time-correlated single-photon count-
ing (TCSPC) measurements. All compounds exhibited mod-
erate emission intensity with quantum yields (Ff) in the
range of 0.2–0.6. The absorption and emission bands shifted
to longer wavelength depending on the extent of p-conjugat-
ed systems both for the neutral and protonated states (see
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Tsuyoshi Kawai et al.
Table 1. Photophysical properties of benzimidazole-based fluorophores.
Ff (FfH+/Ff)[a] t [ns][a] kr [nsÀ1
]
knr [nsÀ1
]
kr/knr
Dm [debye][c]
[a,b]
[a]
[a]
[a]
Compound
labs [nm][a] lem [nm][a] Dnst ꢁ10À3 [cmÀ1
]
OMePhTTIm
367
444
500
431
453
438
433
420
433
376
378
338
353
380
396
4.81
5.26
3.25
4.08
4.52
4.36
5.37
5.10
5.62
5.23
4.30
5.46
4.95
5.20
0.40
0.86 (2.15)
0.31
0.91 (2.93)
0.35
0.88 (2.51)
0.60
0.94 (1.57)
0.32
0.32 (1.00)
0.25
0.19 (0.76)
0.24
0.08 (0.33)
0.59
1.7
0.52
1.6
0.58
1.4
1.3
1.8
1.1
1.1
0.67
0.48
0.59
0.58
0.60
0.64
0.45
0.53
0.29
0.30
0.36
0.31
0.36
0.12
1.0
0.66
6.1
0.45
1.0ꢁ101
0.54
7.0
4.81
10.7
4.07
7.02
5.79
3.54
3.24
4.11
2.98
2.25
2.79
1.00
1.35
3.12
OMePhTTIm+H+[d] 398
7.8ꢁ10À2
1.3
PhTTIm
361.5
382
366
375.5
343.5
356
312
316.5
296
PhTTIm+H+[d]
CF3PhTTIm
CF3PhTTIm+H+[d]
PhThIm
5.6ꢁ10À2
1.1
9.2ꢁ10À2
0.30
1.5
15
PhThIm+H+[d]
2-ThIm
3.5ꢁ10À2
0.61
0.48
0.48
0.33
0.24
0.32
0.082
2-ThIm+H+[d]
3-ThIm
0.63
1.1
1.3
1.1
0.69
0.62
0.67
0.63
3-ThIm+H+[d]
BzThIm
296.5
320.5
328.5
BzThm+H+[d]
1.5
[a] In CH2Cl2 (1.0ꢁ10À5 m). [b] Obtained using l2-corrected fluorescence spectra. [c] Obtained from the solvent-dependent Stokes shifts. [d] In the pres-
ence of TFA (0.01 mL) in CH2Cl2 (3 mL).
the Supporting Information). The protonation on the N-
methylbenzimidazole moiety caused a redshift in absorption
spectra for all molecules, thus suggesting the stabilization of
the LUMO level by the introduction of a cationic charge.[17]
BzThIm showed the reduced Ff (0.08) and an increase in
the Stokes shift after protonation, which are indicative of
the TICT emission in BzThIm+H+. Owing to the aromatic
character of the annulated benzene ring, BzThIm appears to
have the least quinoid character among the present mole-
cules.[18] Compounds 2-ThIm and 3-ThIm seem to have an
enhanced quinoid character relative to BzThIm, whereas
they possess shorter p systems than BzThIm. Therefore,
those molecules gave higher emission quantum yields than
BzThIm+H+ in their protonated forms. The comparison
between 2-ThIm and 3-ThIm may well demonstrate the ef-
fects of quinoid nature and intramolecular NH/S hydrogen
bonds[19] in their protonated states. The former is expected
to have larger quinoid character and involve the NH/S inter-
action in the excited state, thus giving the unchanged Ff and
nonradiative rate constant (knr) after the protonation. Mean-
while, the protonation led to a decrease in the Ff and an in-
crease in the knr for the latter probably due to the absence
of NH/S interaction and the less quinoid character anticipat-
ed from the shorter p system in the excited state. The exten-
sion of a p system from 2-ThIm to PhThIm gave an increase
in the quinoid character, thereby resulting in the highest Ff
of 0.94 in the protonated form (PhThIm+H+). The sup-
pression of rotation in the excited state by the cooperative
effects of the NH/S hydrogen bonding and the quinoid char-
acter of p components are also suggested by the prominent
decrease in the knr after protonation for the molecules with
phenylthiophene and arylthienothiophene units.
property. The protonation increases the electron-accepting
capability of the benzimidazole unit and also the extent of
charge separation in the ground state. The substituent at the
donor part had an impact on their photophysical properties.
OMePhTTIm+H+ led to a redshift both in the absorption
and emission spectra with respect to PhTTIm+H+, whereas
CF3PhTTIm+H+ gave a blueshift (Figure 1). The stabiliza-
Figure 1. a) Absorption and b) emission spectra of 2-arylthienoACHTUNGTRENNUNG[3,2-
b]thiophene-containing conjugated molecules in the presence of TFA in
CH2Cl2 (1.0ꢁ10À5 m with 0.01 mL of TFA). Solid lines: PhTTIm+H+,
broken lines: CF3PhTTIm+H+, and dotted lines: OMePhTTIm+H+.
The compounds that contain an arylthienothiophene unit
showed a marked increase in the Ff values upon protona-
tion with the ratios of FfH+/Ff over 2, which is larger than
that of PhThIm (FfH+/Ff =1.57). The introduction of an ar-
ylthienothiophene unit gave an elevation in the HOMO
level as estimated by cyclic voltammetry (see the Supporting
Information), which suggests an increased electron-donating
tion and the destabilization of the excited-state energy with
the electron-donating (OMe) and electron-withdrawing
(CF3) groups, respectively, clearly indicated the contribution
of quinoidal CT property to the excited state (Scheme 3).
The degree of spectral shift from PhTTIm+H+ to substitut-
ed ones was larger in the emission spectra than in the ab-
sorption spectra. This result suggested that the quinoidal
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Acid–Base-Responsive Emission
The phenol group becomes a strong electron-donating unit
specifically in the deprotonated state under basic condi-
tions,[21] thus giving
a base-enhanced D–A character.
OHPhTTIm was thus expected to form D–A systems re-
sponsive to both acid and base. Scheme 4 shows the acid–
base response of OHPhTTIm and subsequent resonances
with quinoidal structures. Interestingly, both the
(OÀ)PhTTIm and OHPhTTIm+H+ states could give a very
similar p-conjugation backbone to their quinoidal resonance
structures, which would contribute to stabilizing the excited
states with CT character.
Scheme 3. Diagram of the emission process for the protonated forms of
2-arylthienothiophene–benzimidazole molecules. EDG: electron-donat-
ing group, EWG: electron-withdrawing group.
structure most likely contributes to the state responsible for
emission rather than the Franck–Condon state formed right
after the excitation. The fact that CF3PhTTIm gave a redshift
in the absorption spectrum but a blueshift in the emission
band upon protonation also supports the proposed destabili-
zation of the quinoidal ICT state by the electron-withdraw-
ing group. Furthermore, the significant elongation of the
emission lifetime for these molecules upon protonation
clearly implies the adiabatic excited state reaction from the
locally excited state to the ICT state.[13]
We also measured photophysical properties in various sol-
vents to gain further insight into the emission mechanism.
The solvent-polarity-dependent Stokes shifts afforded
a change in the dipole moment (Dm) between the ground
and the excited states through a Lippert–Mataga plot (see
the Supporting Information for details).[20] All compounds
without TFA showed solvent-independent structured emis-
sion, which could be clearly assigned to the LE emission.
Meanwhile, Stokes shift showed an increase with increasing
solvent polarity for the protonated compounds. The absorp-
tion peak shifted to shorter wavelength in polar solvents for
the protonated forms, thus indicating the stabilization of the
ground state of the ionic forms. However, polar solvents
gave the emission band at longer wavelength than less polar
solvents, which suggests the stabilization of the excited state
in polar media in a similar manner to the effect of electron-
donating group on the quinoidal ICT state (Scheme 3).
While polar solvents are expected to stabilize both the
ground and excited state of protonated molecules, they af-
forded the larger values of Stokes shift than less polar sol-
vents. This result is suggestive of the more polarized nature
in the excited state than in the ground state. PhTTIm
showed an increase in the Dm value from 4.07 to 7.02 upon
protonation. CF3PhTTIm exhibited a decrease in the Dm
after protonation, whereas OMePhTTIm+H+ showed the
most prominent solvent dependence with a Dm value of
10.7 debye. The electron-donating character of the OMe
group could facilitate the formation of polarized CT state, in
which the positive charge was accepted by the arylthieno-
thiophene unit (Scheme 3). These results also supported the
introduction of ICT character in the excited state for the ar-
ylthienothiophene-containing fluorophores.
Scheme 4. Acid–base response of OHPhTTIm and their resonance with
quinoid structures.
Protonation and deprotonation in the ground state on the
N-methylbenzimidazole and phenol unit, respectively, were
confirmed by 1H NMR spectroscopic measurements
(Figure 2). The assignment of peaks was based on the result
1
of correlation H NMR spectroscopic experiments (see the
Supporting Information). The addition of equimolar TFA
1
was enough to complete the spectral shift in the H NMR
spectroscopic profile, whereas the large excess amount of
Figure 2. Parts of 1H NMR spectra and signal assignment (CD3OD,
600 MHz, 298 K) of OHPhTTIm under different conditions. Top: With
TFA (1.0 equiv); middle: blank; bottom: with tBuOK (200 equiv).
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Tsuyoshi Kawai et al.
potassium tert-butoxide (tBuOK) was needed for the depro-
tonation of phenol group. The relative acidity between TFA
and N-methylbenzimidazole and between tBuOK and the
phenol part might determine the amount of acid and base to
complete protonation and deprotonation, respectively. The
N-methyl signal (d) and protons on the benzimidazole
moiety (a–c) were shifted downfield in the presence of TFA
owing to the deshielding effect of the protonated benzimida-
zolium cation.[22] The cationic charge also had an effect on
the protons (e,f) attached to the thienothiophene ring to
give downfield shifts. However, the deprotonation of the
phenol group with the excess amount of tBuOK resulted in
the upfield shifts of protons on the donor moiety including
the phenol (h,g) and thienothiophene (e,f) units. It should
be noted that the protons on the phenol and thienothio-
phene units still appeared in the aromatic region above d=
6.5 ppm under both the acidic and basic conditions, which
suggested the small contribution of nonaromatic quinoidal
structure in the ground state.
The spectral changes of OHPhTTIm in the presence of
TFA and tBuOK were measured in solutions in methanol
(1.0ꢁ10À5 m, 3 mL). Greater amounts of TFA (2 equiv) and
tBuOK (2000 equiv) were needed to quench the original
emission from the neutral state, probably on account of the
intermolecular proton transfer in the excited states (see the
Supporting Information). Both the acidic and basic states
gave a similar redshifted absorption profile (Figure 3a). The
blue-colored emission in the neutral state changed to green
by the addition of TFA, whereas it turned to yellow-green
emission with tBuOK (Figure 3c). The degree of redshift in
the fluorescence spectra was thus more pronounced in the
basic state (Figure 3b). The most plausible explanation
might be that the ICT state is more stabilized for
(OÀ)PhTTIm by the electron-donating phenolate group rel-
ative to OHPhTTIm+H+. While the Ff increased from
0.62 to 0.82 upon protonation in a similar manner to
OMePhTTIm, it was also enhanced under the basic condi-
tions to 0.77. The nonradiative rate constant decreased from
0.36 nsÀ1 in the neutral state to 0.10 nsÀ1 in the deprotonated
one (see the Supporting Information). Since the NH/S inter-
action did not operate without TFA, the intense green-
yellow emission under basic conditions could be explained
by the contribution of the quinoidal ICT state. The emission
lifetime increased from 1.0 to 2.0 and 2.3 ns under the acidic
and basic conditions, respectively, as in the case with other
molecules that contained the arylthienothiophene unit (see
the Supporting Information). Finally, we investigated the
pH-dependent emission spectra of OHPhTTIm under aque-
ous conditions (see the Supporting Information). OHPhT-
TIm successfully gave triple-color emission depending on
the pH conditions with a sharp spectral response in aqueous
solutions.
Conclusion
We synthesized acid-responsive D–A conjugated molecules
that contained an arylthienothiophene unit as a donor com-
ponent. The contribution of a quinoidal resonance structure
in the excited state promoted the non-twisting ICT emission
with strong intensity. The acid–base-responsive ICT emission
from the phenol-containing fluorophore under aqueous con-
ditions suggested the potential application of the present
system to biological sensing.
Experimental Section
Benzimidazole-based D–A-conjugated molecules with a variety of donor
components were synthesized through Pd-catalyzed cross-coupling reac-
tion between N-methyl-2-bromobenzimidazole and stannylated or bory-
lated donor moieties (see the Supporting Information for details).
1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were recorded with
a JEOL JNM-AL300 instrument. 1H NMR (600 MHz) spectra were re-
corded with a JEOL JNM-ECA600. Recycling preparative GPC and
normal-phase HPLC were performed with a LC-9110NEXT (Japan Ana-
lytical Industry) and LaChrom Elite (Hitachi), respectively. Elemental
analysis were performed for C, H, and
N with a Perkin–Elmer
2400IICHNS/O instrument. Mass spectroscopy and high-resolution mass
spectroscopy (HRMS) were measured with a JEOL JMS-Q1000TD and
JMS-700 MStation, respectively. UV/Vis absorption and fluorescence
spectra were obtained with a JASCO V-660 spectrophotometer and a Hi-
tachi F-7000 fluorescence spectrophotometer, respectively. Fluorescence
lifetimes and absolute fluorescence quantum yields of compounds were
measured with a FluoroCube 3000U (Horiba) and Hamamatsu C9920-02,
respectively. Cyclic voltammetry was performed with a m-AUTOLAB III
potentiostat/galvanostat equipped with an electrochemical analysis
system software in an argon-filled glovebox. A platinum wire was used as
a counter electrode, an Ag/Ag+ electrode was used as a reference elec-
Figure 3. a) Absorption and b) emission spectral change of OHPhTTIm
(black) in the presence of 2.0 equiv TFA (red) and 2000 equiv tBuOK
(blue) in methanol (1.0ꢁ10À5 m). c) Photographs of OHPhTTIm in meth-
anol taken under 365 nm UV light.
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Acknowledgements
This work was partly supported by a JSPS Research Fellowship for
Young Scientist to T.I. The authors thank F. Asanoma, Y. Nishiyama, and
Y. Nishikawa for their assistance in elemental analysis, MS, and NMR
spectroscopic measurements.
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