Single Benzene Green Fluorophore: Solid-State Emissive, Water-Soluble, and Solvent- and pH-Independent Fluorescence with Large Stokes Shifts

Article abstract of DOI:10.1002/anie.201502365

Benzene is the simplest aromatic hydrocarbon with a six-membered ring. It is one of the most basic structural units for the construction of πconjugated systems, which are widely used as fluorescent dyes and other luminescent materials for imaging applicat

Full text of DOI:10.1002/anie.201502365

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201502365
German Edition: DOI: 10.1002/ange.201502365
Fluorescence
Hot Paper
Single Benzene Green Fluorophore: Solid-State Emissive, Water-
Soluble, and Solvent- and pH-Independent Fluorescence with Large
Stokes Shifts**
Teruo Beppu, Kosuke Tomiguchi, Akito Masuhara, Yong-Jin Pu, and Hiroshi Katagiri*
Abstract: Benzene is the simplest aromatic hydrocarbon with
a six-membered ring. It is one of the most basic structural units
for the construction of p conjugated systems, which are widely
used as fluorescent dyes and other luminescent materials for
imaging applications and displays because of their enhanced
spectroscopic signal. Presented herein is 2,5-bis(methylsul-
fonyl)-1,4-diaminobenzene as a novel architecture for green
fluorophores, established based on an effective push–pull
system supported by intramolecular hydrogen bonding. This
compound demonstrates high fluorescence emission and
photostability and is solid-state emissive, water-soluble, and
solvent- and pH-independent with quantum yields of F = 0.67
and Stokes shift of 140 nm (in water). This architecture is
a significant departure from conventional extended p-conju-
gated systems based on a flat and rigid molecular design and
provides a minimum requirement for green fluorophores
comprising a single benzene ring.
optoelectronic materials. As desirable features for fluorescent
molecules used in bioimaging, solid-state emission properties
help to prevent self-quenching,[5] and in general, a large
Stokes shift helps to avoid the reabsorption of emitted
photons, which allows higher contrast in fluorescence imag-
ing. There are sometimes disadvantages for basic scaffolds of
organic fluorophores. For example, cyanine dyes show
aggregation and photobleaching,[6] fluorescein is pH sensi-
tive,[7] and BODIPY has a small Stokes shift.[8] Therefore,
many researchers are intent on solving these problems
through synthetic modification.[9]
Herein, we report a new class of sulfonylaniline-based
fluorophores,
2,5-bis(methylsulfonyl)-1,4-diaminobenzene
(BMeS-p-A), as robust fluorescent scaffolds through
a push–pull system; the fluorophores are solid-state emissive,
water-soluble, environmentally insensitive, and display large
Stokes shift performances.
Our approach to install fluorescence at a longer wave-
length relied on our previous concept of a push–pull system
between amino and sulfonyl moieties supported by intra-
molecular hydrogen bonding, which showed good photo-
stability and efficient fluorescence in the blue range (about
400 nm) in both solution and solid state.[10] This observation
prompted us to explore more effective combinations of amino
and sulfonyl moieties demonstrating longer wavelengths and
smaller sizes. The 1,4-diaminobenzene unit is one of the
strongest donating groups with a high HOMO and is used as
a component in conductive polymers;[11] 1,4-bis(alkenyl)-2,5-
dipiperidinobenzenes have been reported as visible fluoro-
phores;[12] 2,5-bis(phenylsulfonyl)-1,4-diaminobenzene has
been reported as a yellow crystalline compound.[13]
BMeS-p-A was prepared according to Scheme 1. Com-
mercially available compound 1 was methylated at sulfur
atoms to give sulfide 2. The amino groups were then
protected with acetyl groups, the sulfur atoms were oxidized
with m-CPBA, and the acetyl groups were removed by
deprotection in basic conditions to afford the desired BMeS-
p-A in 62% overall yield over four steps. In addition to the
ease of synthesis, the oxidation and deprotection processes
were conducted at ambient conditions in a one-pot procedure,
and all products could be isolated without column chroma-
tography; synthetic intermediate compounds 2 and 3 were
isolated by filtration, and the target molecule BMeS-p-A was
purified by recrystallization (see the synthesis in the Support-
ing Information). Owing to the high yields and simple work-
up procedures, BMeS-p-A was conveniently prepared on
a gram scale.
N
ovel designs for fundamental fluorescent scaffolds have
been attracting the attention of chemists because of their
potential applications in fluorescent bioimaging[1] and as
light-emitting layers in organic light-emitting diodes.[2] There
are two general strategies for the construction of organic
fluorescent scaffolds; one is an extended p-conjugated system
based on a flat and rigid framework[3] and the other is a push–
pull system based on a donor-p-conjugated-acceptor struc-
ture.[4] The extended p-conjugated system leads to low
solubility in many solvents, and the ease of intermolecular
stacking at high concentrations leads to subsequent quench-
ing and lowered photosensitivity through intermolecular
energy-transfer processes. Conversely, the push–pull system
is constructed by relatively small p-conjugated systems and is
widely used as a fundamental scaffold in organic dyes and
[*] T. Beppu, K. Tomiguchi, Prof. Dr. A. Masuhara, Prof. Dr. Y.-J. Pu,
Prof. Dr. H. Katagiri
Graduate School of Science and Engineering
Yamagata University
4-3-16 Jonan, Yonezawa, Yamagata 992-8510 (Japan)
E-mail: kgri7078@yz.yamagata-u.ac.jp
[**] We thank Mr. R. Satake (Yamagata University) for technical support
in quantum yield measurements. This study was partially supported
by Izumi Science and Technology Foundation (grant number H26-J-
031 to H.K.).
Supporting information for this article, including 1H and 13C NMR
spectroscopic data, high-resolution mass spectrometry data for all
new compounds, X-ray crystallographic data and crystallographic
files, powder XRD data, UV/Vis and fluorescence spectra, MO
calculation details, and photostability measurements, is available
on the WWW under http://dx.doi.org/10.1002/anie.201502365.
BMeS-p-A is soluble in water and various organic solvents
and exhibits green fluorescence with high quantum yields
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Scheme 1. Synthetic route toward BMeS-p-A. Conditions and reagents:
a) sodium (4.1 equiv), iodemethane (2.1 equiv), methanol, RT, 5 h.
b) Triethylamine (10 equiv), acetic anhydride (10 equiv), dichlorome-
thane, RT, one day. c) 1. m-Chloroperbenzoic acid (10 equiv), dichloro-
methane, RT, two days. Sodium hydroxide aqueous solution (2. 6m),
tetrahydrofuran, reflux, one day.
Table 1: Absorption and emission properties of BMeS-p-A in selected
solvents.[a]
Solvent
lmaxabs [nm]
lmax [nm][b]
emax [mÀ1 cmÀ1
]
F[c]
em
THF
384
385
394
377
380[d]
490
505
509
517
477
4170
3990
4360
3820
–
0.51
0.47
0.70
0.67
0.69[e]
MeOH
DMSO
water
Figure 1. a) Absorption and fluorescence spectra of BMeS-p-A in
tetrahydrofuran (blue), methanol (red), dimethylsulfoxide (green), and
water (black). b) Absorption and fluorescence spectra of BMeS-p-A in
water (black) and solid state (crystalline powder, red). Solid-state
absorption spectrum was measured using the reflection method in an
integrating sphere with a lamp switching wavelength of 330 nm.
c) Plot of molar extinction coefficient (black square) and fluorescence
quantum yields (red circle) of BMeS-p-A in phosphate buffers (pH 2.8–
9.6).
powder
[a] UV/Vis spectra recorded at 1.5ꢀ10À4 m; fluorescence spectra
recorded at 5.0ꢀ10À6 m. [b] Excitation at l=350 nm. [c] Determined
relative to 9,10-diphenylanthracene in cyclohexane (F=0.95).
[d] Reflection spectra in an integrating sphere. [e] Determined in an
integrating sphere.
(Table 1). Notably, large Stokes shifts up to 140 nm were
observed. Of particular interest, a 140 nm shift was observed
in water, which is six times larger than that in fluorescein. In
addition, fluorescent properties such as lmaxabs, lmaxem, e, and F
showed comparatively small dependence on the solvent.
Moreover, the pH dependence of the fluorescence intensity
was quite small (Figure 1), which is quite different from that
of fluorescein (see absorption and fluorescence spectra in the
Supporting Information). The pH sensitivity is generally
a disadvantage because the fluorescence intensity drastically
decreases below pH 6; thus, the experimental conditions need
to be maintained between pH 6 and 10.[7] To avoid this, Tokyo
Green was developed by replacing the ionic carboxylic acid of
fluorescein with a non-ionic methyl group.[9j] BMeS-p-A is
non-ionic and composed of a single benzene platform, which
most likely contributes to the pH independence of the
fluorescence intensity.
The X-ray single-crystal structure of BMeS-p-A was
dominated by strong intramolecular and intermolecular
hydrogen bonding between amino and sulfonyl groups,
which remarkably adopt Etter’s empirical hydrogen bonding
rules for organic compounds.[14] Six-membered ring intra-
molecular hydrogen bonding prompts both methylsulfonyl
groups to adopt a syn configuration (Figure 2a); intermolec-
ular hydrogen bonding supports ladder structures along the a–
c plane (Figure 2b). These structural features prevented
intermolecular p-stacking between benzene units, allowing
the generation of solid-state emission (Figure 2c). The
powder X-ray pattern of the sample used for fluorescent
measurements was in agreement with the simulation from the
single-crystal X-ray diffraction (see the powder X-ray dif-
fraction pattern in the Supporting Information).
Density functional theory (DFT) calculations in water at
the PCM/B3LYP/6-31G(d) level were performed to inves-
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Figure 2. Crystal structure of BMeS-p-A. a) Thermal ellipsoid drawing
of side and top views at 50% probability level. b) Partial view with the
indication of intramolecular and intermolecular hydrogen bonding.
c) Packing structure along the c axis.
tigate the optical properties of BMeS-p-A. These results were
in good agreement with the absorption and emission spectra
(Figure 3).[9g] The anti conformer was the most stable, as
determined from the conformation search. The optimized
structures in both the ground state (DFT) and excited state
(time-dependent DFT) exhibited HOMOs that were located
at the central 1,4-diaminobenzene moieties and LUMOs that
were distributed over the entire molecule via the sulfonyl
groups. The resulting HOMO–LUMO asymmetry indicated
that an effective push–pull system was established for the
Figure 3. MO calculation of BMeS-p-A. a) Geometry of BMeS-p-A in
the ground and excited states. b) Rationalization for the UV/Vis
absorption, emission, and large Stokes shift of BMeS-p-A; the geome-
try relaxation and the frontier molecular orbitals involved in the vertical
excitation (i.e., UV/Vis absorption, left column) and emission (right
column) of BMeS-p-A. The vertical-excitation-related calculations are
based on the optimized geometry of the ground state (S0). The
emission-related calculations were based on the optimized geometry
of the excited state (S1). Water was used as the solvent for calculations
(polarizable continuum model) at the B3LYP/6-31G(d) level.
1
BMeS-p-A framework. Moreover, the H NMR spectrum of
BMeS-p-A showed that the proton signal assigned to NH2
resonated at 5.45 ppm, shifting downfield from that of sulfide
precursor 2, which resonated at 4.38 ppm. These results
suggest that the sulfonyl groups withdraw electron density
from nitrogen and its adjacent protons. The six-membered
ring intramolecular hydrogen bonds are also predominantly
controlled by the push–pull system, promoting the downfield
planar structure. Generally, the asymmetric push–pull system
exhibits significantly different dipole moments between the
ground and excited states, which gives strong solvatochromic
shifts of the absorption and emission bands.[15] Conversely,
BMeS-p-A has a symmetrical push–pull system, and the
difference in dipole moments between the ground and excited
state was almost zero (see MO calculations in the Supporting
Information), which is most likely a reason for the solvent
independence of the optical properties of BMeS-p-A. Fur-
thermore, the seemingly small structural transformation from
the planar geometry to non-planar geometry of the sulfonyl
groups from the ground state to excited state prompted
a large difference in BMeS-p-A because of its small molecular
size, which is reasonable and clearly explains the large Stokes
shift of BMeS-p-A.
1
shift of NH2 protons in the H NMR spectra. As a notable
feature of the optimized structures, structural differences
between the ground and excited states were observed. In the
excited state, the sulfonyl groups were 0.327 ꢀ above the
benzene plane and not in line with the aromatic plane
(Figure 3). In addition, the amino groups in the excited state
clearly had sp2 geometry, whereas the ground state had sp3
geometry. Moreover, the C-N distance in the exited state was
shorter than that in the ground state. These structural
properties indicate that the ground state has a neutral,
planar structure, and the excited state has a zwitterionic, non-
Finally, we demonstrated the photostability of BMeS-p-A
in water (Figure 4). BMeS-p-A showed higher stability than
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time for BMeS-p-A (black circle), fluorescein (red square), and BODIPY
(blue rhombus) obtained using a 150 W xenon lamp in water for
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(v/v) DMSO in water for BODIPY. BODIPY: [[(3,5-dimethyl-1H-pyrrol-2-
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solvent- and pH-independent, and large Stokes shift perform-
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departure from conventional extended p-conjugated systems,
may provide a means to construct a wide variety of
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Keywords: fluorescence · hydrogen bonds ·
solid-state emission · stokes shifts · water solubility
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Received: March 13, 2015
Published online: && &&, &&&&
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Fluorescence
Evergreen: An effective push–pull system
supported by intramolecular hydrogen
bonding between amino and sulfonyl
groups generated high fluorescence
properties. The achievement of a single
benzene core fluorescent system (see
picture) allowed steady green fluores-
cence, independent of the surrounding
environment such as solvent polarity and
pH, as well as extraordinarily large Stokes
shifts of up to 140 nm.
T. Beppu, K. Tomiguchi, A. Masuhara,
Y.-J. Pu, H. Katagiri*
&&&& — &&&&
Single Benzene Green Fluorophore:
Solid-State Emissive, Water-Soluble, and
Solvent- and pH-Independent
Fluorescence with Large Stokes Shifts
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!