A single 2-(2′-hydroxyphenyl)benzothiazole derivative can achieve pure white-light emission

Article abstract of DOI:10.1002/asia.201402779

The synthesis and photophysics of two novel 2-(2′-hydroxyphenyl)- benzothiazole (HBT) derivatives are presented. The electron-withdrawing trifluoromethyl (CF₃) group in compound 1 facilitates the deprotonation of the phenolic hydroxy group. Well-resolved triple fluorescence from the enol, keto, and phenolic anion, which ranges from 350 to 600 nm, was detected for 1 in ethanol, which marks the first time triple fluorescence from an excited-state intramolecular proton transfer (ESIPT) molecule has been reported. Both triphenylamine and CF3 were introduced into derivative 2. Intramolecular charge transfer and the "red-edge effect" resulted in the bathochromic shift of dual fluorescence of 2. Triple fluorescence was also observed for 2 in ethanol. In mixed acetonitrile and ethanol, pure white-light emission with CIE coordinates of (0.33, 0.33) and a quantum yield of 0.25 was achieved for 2. This work provides a new avenue for the rational design of an ESIPT molecule to achieve white-light generation under mild conditions.

Full text of DOI:10.1002/asia.201402779

FULL PAPER
DOI: 10.1002/asia.201402779
A Single 2-(2’-Hydroxyphenyl)benzothiazole Derivative Can Achieve Pure
White-Light Emission
Jinling Cheng,[a, b] Di Liu,*[a] Lijun Bao,[a] Kai Xu,[a] Yang Yang,[b] and Keli Han*[b]
Abstract: The synthesis and photophy-
sics of two novel 2-(2’-hydroxyphenyl)-
benzothiazole (HBT) derivatives are
presented. The electron-withdrawing
trifluoromethyl (CF3) group in com-
pound 1 facilitates the deprotonation
of the phenolic hydroxy group. Well-re-
solved triple fluorescence from the
enol, keto, and phenolic anion, which
ranges from 350 to 600 nm, was detect-
ed for 1 in ethanol, which marks the
first time triple fluorescence from an
excited-state intramolecular proton
transfer (ESIPT) molecule has been re-
ported. Both triphenylamine and CF3
were introduced into derivative 2. In-
tramolecular charge transfer and the
“red-edge effect” resulted in the batho-
chromic shift of dual fluorescence of 2.
Triple fluorescence was also observed
for 2 in ethanol. In mixed acetonitrile
and ethanol, pure white-light emission
with CIE coordinates of (0.33, 0.33)
and a quantum yield of 0.25 was ach-
ieved for 2. This work provides a new
avenue for the rational design of an
ESIPT molecule to achieve white-light
generation under mild conditions.
Keywords: charge transfer · enols ·
fluorescence · hydrogen bonds ·
photophysics
Introduction
White-light generation is drawing wide attention in both
academic and industrial fields owing to its applications in
constructing lighting sources, flat-panel displays, and so
on.[1,2] White-light emission should ideally be composed of
three primary (blue, green, and red) or two complementary
(blue and yellow) colors and cover the whole visible range
from 400 to 700 nm. Traditional methods to generate white
light typically rely on mixing various primary-color lights
emitted from different materials.[3–5] An alternative ap-
proach is to use a single-component emitter, which can have
such advantages as simpler fabrication processes, higher sta-
bility, and better reproducibility.[6,7] However, for single-
component emitters, the self-absorption and energy transfer
between the different luminophores can inevitably quench
the fluorescence intensity.[8] Thus, the development of emit-
ting centers without an energy-transfer process would be the
most ideal strategy for white-light generation.
Figure 1. Schematic representation of the ESIPT mechanism of a 2-(2-hy-
droxyphenyl)benzothiazole derivative.
Molecules that feature an excited-state intramolecular
proton transfer (ESIPT) are usually characterized by fluo-
rescence that covers a wide spectral range, which has consid-
erable potential to form white light. 2-(2’-Hydroxyphenyl)-
benzothiazole (HBT) derivatives are a series of well-known
ESIPT compounds. As shown in Figure 1, the intrinsic four-
level photocycle (N!N*!T*!T!N) is involved in the
ESIPT process.[9,10] In non- or weakly polar solvents, the
enol form (I) is exclusively in the ground state owing to sta-
bilization of the intramolecular hydrogen bond. Upon pho-
toexcitation, the redistribution of charge evokes an increas-
ing acidity of the proton donor or the basicity of the proton
acceptor. Consequently, the fast proton-transfer reaction
takes place through the pre-existing intramolecular hydro-
gen bond to generate the keto tautomer (II), which emits
fluorescence in the long-wavelength region with a large
Stokes shift. Although in some strong polar solvents like
[a] J. Cheng, Dr. D. Liu, L. Bao, K. Xu
State Key Laboratory of Fine Chemicals
School of Chemistry, Dalian University of Technology
2 Linggong Road, Dalian 116024 (P.R. China)
E-mail: liudi@dlut.edu.cn
[b] J. Cheng, Dr. Y. Yang, Prof. K. Han
State Key Laboratory of Molecular Reaction Dynamics
Dalian Institute of Chemical Physics, CAS
Dalian 116023 (P.R. China)
E-mail: klhan@dicp.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402779.
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Scheme 1. Chemical structures and synthetic routes of HBT and compounds 1 and 2. Reagents: a) PCl3; b) benzyl chloride, K2CO3; c) Lawesson reagent,
toluene; d) K3[Fe(CN)6], NaOH; e) N-bromosuccinimide (NBS), CCl4; f) nBuLi/THF, BACTHUNGRTNENUG(OCH3)3; and g) [PdACHTUTGNERN(NUGN PPh3)4], K2CO3; h) HBr, acetic acid
(AcOH).
acetonitrile the solvent molecules can form hydrogen bonds
with the hydrogen in the hydroxyl groups and generates the
enol–solvent form (III), in this case the ESIPT process is
hindered. Owing to the intermolecular hydrogen-bonding
effect, the radiance from the keto tautomer (II) is restricted,
which consequently results in another band with a normal
Stokes shift that can be observed in the short-wavelength re-
gions. According to the previous studies, the intermolecular
hydrogen bond has a great influence on the absorption and
fluorescence spectra.[11] Moreover, the dual fluorescence will
be disturbed in protic solvents such as alcohol because of
the competition between intermolecular and intramolecular
hydrogen bonding. The competition of the two kinds of hy-
drogen bonds will redistribute the contents of conformers I
and III and might bring about the deprotonation of the phe-
nolic hydroxyl group to generate the luminescent phenolic
anions (IV). To the best of our knowledge, there has been
no report of such ESIPT molecules in which the phenolic
anion IV can coexist with the conformers I and III to form
the triple fluorescence.
In this paper, we report the design and synthesis of novel
HBT derivatives that are capable of emitting white light in
a simple solvent system. First the strong electron-withdraw-
ing trifluoromethyl group (CF3) was introduced into the
para position of the hydroxyphenyl ring of the parent HBT
to form derivative 1 (Scheme 1). The well-resolved triple
fluorescence that covers a wide spectral range from 350 to
600 nm was easily achieved in protic solvents. By attaching
an electron-donating triphenylamine (TPA) group into the
benzothiazole ring of molecule 1, we synthesized derivative
2, the emission spectrum of which can cover the whole visi-
ble-light region. In addition, derivative 2 can emit pure
white light in mixed acetonitrile and ethanol solvents.
Abstract in Chinese:
Results and Discussion
Preparation of Compounds
Scheme 1 illustrates the synthetic procedure of compounds
1 and 2. Compound 1 was prepared by the one-step cycliza-
tion reaction between 2-aminothiophenol and 4-trifluorome-
thylsalicylic acid at high yield of 80% according to the liter-
ature method.[12] Compound 2 was synthesized through mul-
tistep reactions according to the literature method.[13] Con-
densation of 4-bromoaniline with 4-trifluoromethylsalicylic
acid in the presence of PCl3 generated the amide (I). After
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treatment with benzyl chloride to protect the phenolic hy-
droxy, the amide was converted by Lawessonꢁs reagent into
the thiobenzamide (III). Jacobsen reaction of this thiobenza-
mide with potassium ferricyanide resulted in the important
intermediate 2-[2’-(benzyloxy)-4’-(trifluoromethyl)phenyl]-6-
bromobenzothiazole (IV). Finally, the Suzuki coupling reac-
tion of IV with [4-(diphenylamino)phenyl]boronic acid (VI)
followed by deprotection of the hydroxy group generated
the target compound 2.
bands (Figure 2d; FF =0.183 for all three bands). Triple
fluorescence with a wide spectrum is generated.
For the relationship between the additional absorption
band at 390 nm and the fluorescence emission at 455 nm in
ethanol to be explored, 1 was excited at 390 nm, and the
band at 450 nm was detected as the exclusive emission band
(see the Supporting Information). This indicates that the
390 nm absorption band and the 455 nm emission band
indeed originate from the same chemical species. Further-
more, when 1 was titrated with Bu4NOH in ethanol, the
original green and purple emission bands collapsed into the
blue emission at 450 nm (see the Supporting Information).
Hence, it is reasonable to ascribe the concurrent appearance
of the blue emission band at 455 nm and the 390 nm absorp-
tion band to the corresponding phenolic anions IV that are
formed in the ground state and the excited state. The excita-
tion spectra of 1 (see the Supporting Information) further
confirm these assignments. No such blue emission was de-
tected for the parent HBT in ethanol (Figure 2a). As shown
by the calculation results in the Supporting Information, in-
corporation of the electron-withdrawing CF3 group extends
Triple Fluorescence
Figure 2 illustrates the absorption and emission spectra of
the parent molecule HBT and derivative 1 in different sol-
ꢀ
the O H bond length and increases the charge density on
the O atom for 1, which results in the increased acidity of
the hydroxyl group and facilitates its deprotonation. The in-
corporation of the CF3 group into HBT causes the appear-
ance of the 455 nm emission; hereafter, the triple fluores-
cence of 1 in ethanol has a wide spectral range from 350 to
600 nm, as shown in Figure 2d. Further structural modifica-
tion of 1 might redshift this broad spectrum to cover the
whole visible-light region.
Figure 2. UV/Vis absorption (dashed lines) and emission spectra (solid
lines) for parent compound HBT and derivative 1. a) The spectra of HBT
in ethanol; b–d) the emission evolution of 1 from single via dual to triple
fluorescence in different solvents (3ꢂ10ꢀ5 molLꢀ1, lexc =330 nm).
White-Light Emission of a Single HBT Molecule
By introducing an electronic-donating triphenylamine
(TPA) group into 1, we designed and synthesized 2 with the
expectation that its broad spectrum would cover the entire
visible-light region. As can be seen from the Supporting In-
formation, 2 exhibits a broad absorption band centered at
380 nm in various solvents, which is attributed to the overlap
between the phenolic anion absorption band and the
charge-transfer absorption band. The complete separation
of the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) shown in
Figure 3 confirms the intramolecular charge-transfer feature
for 2.[15] Figure 4 illustrates the fluorescence spectra of 2 in
different solvents. Compound 2 exhibits the single fluores-
cence with a peak at 475 nm (FF =0.889) in nonpolar tolu-
ene, which should be due to the emission from the corre-
sponding keto tautomer. This deduction was confirmed by
time-dependent (TD) DFT calculations, in which the bandg-
ap and emission wavelength of the keto form of 2 in toluene
are calculated to be 2.80 eV and 443 nm, respectively
(Figure 3). Dual emission at 435 and 605 nm from the corre-
sponding conformer III and the keto tautomer was detected
in strong polar acetonitrile (FF =0.0594), and triple emission
peaks (435, 480, and 525 nm) in protic ethanol (FF =0.24).
The fluorescence quantum yields are much higher than
vents. Similar to the parent HBT, compound 1 in toluene
and tetrahydrofuran (THF) displays two major absorption
bands. In protic ethanol, one additional absorption band
centered at 390 nm is observed, which implies that a novel
chemical species is formed in the ground state in addition to
conformers I and III. As can be seen in Figure 2, upon pho-
toexcitation the emission evolution of 1 from single, via
dual, to triple fluorescence was easily achieved only by vary-
ing the solvent. In nonpolar toluene, 1 emits single fluores-
cence with an emission peak at 520 nm (Figure 2b; fluores-
cence quantum yield FF =0.444). In THF with increased po-
larity, a short-wavelength band at 380 nm was detected
along with the original band around 520 nm, thus leading to
the formation of the dual fluorescence (Figure 2c; FF =
0.028 for dual emission). According to the typical spectral
features of the general HBT derivatives, it is reasonable to
ascribe the above 380 and 520 nm emission bands of com-
pound 1 to the corresponding conformer III and tautomer
II, respectively.[9,13,14] If 1 is dropped into protic ethanol,
a third emission band at 455 nm can be simultaneously de-
tected, accompanied with the former purple and green
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Figure 3. The frontier molecular orbitals of enol forms and keto forms for compounds 1 and 2 at the optimized geometries and corresponding energies.
Toluene was used as solvent in these calculations. The fluorescence wavelength of the keto form of 2 was calculated to be 1240/2.80=443 nm.
Figure 4. Fluorescence spectra of compound 2 in different solvents with excitation at 345 nm. The inset describes the spectra by simply integrating the
emission spectra in acetonitrile and ethanol.
those of its parent molecule HBT and compound 1 (see the
Supporting Information). The fluorescence lifetimes of com-
pounds 1 and 2 are summarized and compared with the
parent HBT in the Supporting Information. It should be
noted that the fluorescence of conformer III of 2 in acetoni-
trile (at 435 nm) was successfully redshifted relative to that
of 1 (at 380 nm) owing to the intramolecular charge-transfer
(CT) effect, which is discussed in the Supporting Informa-
tion. In addition, an extra shift of the keto emission toward
longer wavelength, which corresponds to the increase in po-
larity of the solvent, indicates the existence of a red-edge
effect.[16] The origin of the red-edge effect lies in the change
in fluorophore–solvent interactions in the ground and excit-
ed states, which is brought about by a change in the dipole
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moment of the fluorophore upon excitation. The D–p–A
structure allows the creation of a large dipole moment in
the excited state that allows for the stabilization of the excit-
ed-state intramolecular charge transfer (ESICT) state in the
polar solvents. As a result, we can observe that the emission
shows a large bathochromic shift with increasing solvent po-
larity. The bathochromic shift of the keto emission of com-
pound 2 from 475 nm in toluene (with orientation polariza-
bility Df of 0.03) to 525 nm in ethanol (with Df of 0.29) and
further to 605 nm in acetonitrile (with Df of 0.31) remains in
line with the above rule. Most likely, it can be speculated
that compound 2 undergoes ESIPT prior to the ESICT reac-
tion. Compound 2 displays the largest Stokes shift with the
long-wavelength keto emission centered at 605 nm in aceto-
nitrile, and its redshifted triple fluorescence in ethanol en-
couraged us to simply combine the emission spectra in ace-
tonitrile and ethanol so that a broad spectrum that covers
the whole visible-light range from 400 to 750 nm can be ob-
tained (shown in the inset of Figure 4).
To realize pure white-light emission, we adopted the strat-
egy of mixing acetonitrile and ethanol in various proportions
as the solvents for compound 2. Owing to the perturbation
of intermolecular hydrogen bonds between ethanol and 2,
the efficiency of the ESIPT process would be restrained, as
shown by the transient fluorescence decay spectra and the
potential-energy curves in the Supporting Information. As
a result, the incorporation of ethanol in acetonitrile will
strengthen the intensity of the phenolic anion (IV) at the ex-
pense of the keto (II) emission. Hence, there is one optimal
proportion of the mixed solvents to balance the multiple
emission bands for white-light generation. In the mixed sol-
vents of acetonitrile and ethanol with a volume ratio of 5:1,
the fluorescence is located in the white region as the excita-
tion wavelength changes from 340 to 370 nm (Figure 5c–e).
In addition, a pure white emission is realized when the exci-
tation wavelength is 350 nm. And its CIE coordinates of
(0.33, 0.33) fall well within the white region of the 1931 CIE
diagram (Figure 5 and the Supporting Information; for pure
white x=0.33, y=0.33).[17] In our case, the quantum yield of
the white emission that spans the entire visible spectrum is
0.245. As far as we know, this novel compound is the first
example of a single ESIPT dye that can emit pure white
light under mild conditions without the help of low tempera-
ture or high pressure. Based on the easy realization of ideal
white light, we propose that compound 2 can find potential
applications as a white-light lighting or decorative source for
long-term usage, especially in deep-sea exploration or in
swimming pools, when a violet light is available as an excita-
tion source.
Conclusion
In summary, a single type of HBT derivative has been de-
signed and synthesized to realize pure white-light emission
composed of enol, keto, as well as phenolic anion emission.
The mechanism for white-light generation is based on inter-
actions of the electron-donating/-withdrawing framework
with the solvents, which involve the intermolecular hydro-
gen bond, the ESIPT process, as well as deprotonation. In
particular, deprotonation was demonstrated to be capable of
tuning the emission spectrum. By properly modulating the
content ratio of the binary solvents, we utilized compound 2
to achieve white-light emission with CIE coordinates of
(0.33, 0.33). This compound is the first example of a single
ESIPT dye that can emit pure white light under mild condi-
tions.
Figure 5. Fluorescence spectra of compound 2 (black solid line) in mixed acetonitrile and ethanol (5:1) (lex =350 nm) and the deconvolution curves
(left), and the CIE 1931 chromaticity coordinates of the fluorescence of 2 (right): A) in ethanol, lex =360 nm; B) in acetonitrile, lex =360 nm, W) in
mixed acetonitrile and ethanol (5:1), lex =350 nm; and C–E) in mixed acetonitrile and ethanol (5:1), lex =340, 360, 370 nm, respectively.
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Experimental Section
Instruments and Methods
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Theoretical Calculations
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ꢀ
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Acknowledgements
We thank the National Natural Science Foundation of China (21274016
and 21374013), the Program for Changjiang Scholars and Innovative Re-
search Team in University (IRT-13R06), the Program for DUT Innova-
tive Research Team (DUT2013TB07), and the Fundamental Research
Funds for the Central Universities (DUT13LK06 and DUT12ZD211) for
financial support.
Received: July 4, 2014
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Published online: && &&, 0000
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FULL PAPER
Exciting emission: On the basis of the
interactions of an electron-donating/-
withdrawing framework with solvents
that involve the intermolecular hydro-
gen bond, the excited-state intramolec-
ular proton transfer (ESIPT) process,
as well as deprotonation, a single HBT
derivative achieves white-light emis-
sion with CIE coordinates of (0.33,
0.33) under mild conditions in binary
solvents (see figure).
Fluorescence
Jinling Cheng, Di Liu,* Lijun Bao,
Kai Xu, Yang Yang,
Keli Han*
&&&& — &&&&
A Single 2-(2’-Hydroxyphenyl)benzo-
thiazole Derivative Can Achieve Pure
White-Light Emission
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