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
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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)
Supporting information for this article is available on the WWW
Chem. Asian J. 2014, 9, 2542 – 2547
2542
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