A guanidine derivative of naphthalimide with excited-state deprotonation coupled intramolecular charge transfer properties and its application

Article abstract of DOI:10.1039/c3tc30853g

A new fluorophore based on guanidine substituted 1,8-naphthalimide was synthesized and characterized. In aqueous solution, the guanidine group undergoes deprotonation/protonation with a pK_a of ～8.5 in the ground-state and ～0.9 in the excited-state. The emission of its protonated and deprotonated forms exhibits a large Stokes shift (Ex/Em: 350/460 nm and 400/580 nm) due to the excited-state intramolecular charge transfer (ICT) process. The protonated form of this fluorophore exhibits dual fluorescence emission (Em: 460 and 580 nm; Ex: 350 nm) that is contributed to by an excited-state deprotonation coupled ICT process. The emission properties of this fluorophore are strongly dependent on the solvent environment, which make it possible to tune the luminescence of the materials made using this fluorophore. The absorption and emission spectra of this fluorophore respond to fluoride ions ratiometrically, showing the potential application as a fluoride ion sensor.

Full text of DOI:10.1039/c3tc30853g

Journalof
Materials Chemistry C
Materials for optical and electronic devices
www.rsc.org/MaterialsC
Volume 1 | Number 29 | 7 August 2013 | Pages 4399–4506
ISSN 2050-7526
PAPER
Dihua Shangguan et al.
A guanidine derivative of naphthalimide with excited-state deprotonation coupled
intramolecular charge transfer properties and its application
2050-7526(2013)1:29;1-A

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Materials Chemistry C
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PAPER
A guanidine derivative of naphthalimide with excited-
state deprotonation coupled intramolecular charge
transfer properties and its application†
Cite this: J. Mater. Chem. C, 2013, 1,
4427
Jin Zhou,ac Huiying Liu,b Bing Jin,ac Xiangjun Liu,a Hongbing Fub
a
*
and Dihua Shangguan
A new fluorophore based on guanidine substituted 1,8-naphthalimide was synthesized and characterized.
In aqueous solution, the guanidine group undergoes deprotonation/protonation with a pKa of ꢀ8.5 in the
ground-state and ꢀ0.9 in the excited-state. The emission of its protonated and deprotonated forms
exhibits
a large Stokes shift (Ex/Em: 350/460 nm and 400/580 nm) due to the excited-state
intramolecular charge transfer (ICT) process. The protonated form of this fluorophore exhibits dual
fluorescence emission (Em: 460 and 580 nm; Ex: 350 nm) that is contributed to by an excited-state
deprotonation coupled ICT process. The emission properties of this fluorophore are strongly dependent
on the solvent environment, which make it possible to tune the luminescence of the materials made
using this fluorophore. The absorption and emission spectra of this fluorophore respond to fluoride ions
ratiometrically, showing the potential application as a fluoride ion sensor.
Received 7th May 2013
Accepted 30th May 2013
DOI: 10.1039/c3tc30853g
www.rsc.org/MaterialsC
intermolecular ESPT process has also been found in biological
systems, such as green uorescence proteins and luciferin.8
Introduction
Electron transfer and proton transfer are the most fundamental
chemical reactions that play important roles in numerous
chemical and biological processes. They are also the basis of
different signaling mechanisms of various optical probes, such
as, photoinduced electron transfer, metal–ligand charge trans-
fer, twisted intramolecular charge transfer, electronic energy
transfer, intramolecular charge transfer (ICT) and excited-state
proton transfer (ESPT).1 Recently, organic molecules with ESPT
have received considerable attention because of their wide
range of applications in laser materials,2 photostabilizers,3
optical sensors,1c,4 energy storage systems5 and information
storage devices,6 etc. Most of the ESPT reactions take place in
molecules containing a vicinal proton donor and acceptor, i.e.
intramolecular ESPT.1c If a molecule only has a proton donor
(without an acceptor), or the proton donor is positioned too far
from the acceptor, the ESPT reactions cannot spontaneously
occur within the molecule and may occur between two such
molecules or from one molecule to adjacent molecules, such as
solvent molecules, i.e. intermolecular ESPT.1b,7 The
Compared with many molecules with intramolecular ESPT
properties,9 many fewer molecules showing intermolecular
ESPT properties have been reported.7,10 The reported intra-
molecular ESPT chromophores are limited to the structures of
naphthols,11 phenols,7a,8a–c,8e,8f,10a,12 hydroxyquinolinuims,13 etc.
Because of the great potential of intermolecular ESPT processes
in biological research and uorescent probe design,1c,14 the
discovery of new chromophores with intermolecular ESPT
properties is necessary.
Guanidines are the strongest organic bases (pKa ¼ 13.5) due
to the resonance stabilization of their conjugated acids.15 The
guanidinium cation is very stable in aqueous solution over a
wide pH range. Because the guanidinium group is capable of
forming both electrostatic and hydrogen bond interactions with
polar and anionic molecules,15 guanidine derivatives have been
used successfully as therapeutic agents,16 DNA intercalating
agents,17 ligands for ion recognition,18 and catalysts in organic
synthesis.19 4-Amino-1,8-naphthalimides have been widely used
as uorescent and colorimetric sensors because of their
advantageous optical properties, such as strong absorption and
emission in the visible region, high photostability, large Stokes
shi, and insensitivity to pH. These optical properties are due to
the ICT process that is caused by “push–pull” substituent pairs
(electron donor–acceptor pairs).20
aBeijing National Laboratory for Molecular Sciences, Key Laboratory of Analytical
Chemistry for Living Biosystems, Institute of Chemistry, Chinese Academy of
Sciences, Beijing, 100190, P.R. China. E-mail: sgdh@iccas.ac.cn; Fax: +86 10
62528509
bState Key Laboratory for Structural Chemistry of Unstable and Stable Species,
Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China
cUniversity of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc30853g
In the research on sensor design, we observed a dual uo-
rescence emission phenomenon from a guanidine substituted
naphthalimide, N-hydroxyethyl-4-guanidino-1,8-naphthalimide
(ENG). This phenomenon has been rarely reported in the
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studies of naphthalimides and guanidine derivatives.17 In this guanidine has a pKa of 13.5 in water, the naphthyl substitution
paper, we measured the optical behaviors of the guanidine on the guanidine can greatly lower its pKa,21 and the electron-
substituted naphthalimide, including steady-state spectra and withdrawing imide moiety of naphthalimide is expected to
time-resolved uorescence, and investigated the pH-dependent further enhance the acidity of the guanidinium group. The large
spectral response, and the solvent effect on the spectral red shi of the absorption spectra of ENG in basic buffer is due
behaviors. The guanidine substituted naphthalimide was to the electron-rich neutral guanidine group (deprotonated
identied to be a new uorophore with intermolecular ESPT guanidinium), which causes a signicant increase in the push–
properties. The application of ENG for Fꢁ recognition was also pull effect of the ICT transition of naphthalimide. This electron
investigated.
transfer effect can be proved by the considerable upeld shi of
the H NMR signals from the aromatic protons of the depro-
1
tonated form of ENG (Fig. S2†).
Results and discussion
The emission spectra of ENG (Fig. 2b) excited at the iso-
absorptive point (365 nm) showed two emission bands (with
maxima at 470 and 580 nm) at acidic pH; the emission band at
470 nm decreased gradually with an increase of the pH and
showed a linear pH response in the range 6.5–10.0. However the
emission band at 580 nm did not show signicant change upon
the pH increasing. At pH of above 10, the emission spectrum
only showed the band with a maximum at 580 nm, and the
absorption spectrum only showed the deprotonated NG band
(with a maximum at 400 nm), suggesting that the band at
580 nm is the emission from the exited-state NG (NG*).
Therefore the emission band at 470 nm (Fig. 1a, 2b and d) can
be assigned to the emission from the excited-state NGH+
(NGH+*). The large Stokes shis of the NGH+* emission
(120 nm) and NG* emission (180 nm) can be attributed to the
efficient excited-state ICT process. However, at pH lower than
6.0, the emission spectrum of ENG also showed both NG* and
NGH+* bands, but the absorption and excitation spectra only
showed the NGH+ band (350 nm), and the Stokes shi of the
NG* emission band was up to 230 nm. This phenomenon
suggests that an excited-state deprotonation process (i.e. inter-
molecular ESPT from ENG to solvent) coupled ICT process
occurred. As far as we know, this is the rst report of a guani-
dinium group associated with an ESPT process.
The uorescence spectra of ENG remained almost
unchanged within the pH range from 3.0–6.5, because almost
all the ENG was present in the NGH+ form. But in the more
acidic buffer, both emission bands changed with pH. As shown
in Fig. 2c and d, the NG* band (580 nm) greatly decreased and
the NGH+* band (470 nm) slightly increased as the pH dropped
from 3.38 to 0.3, while the absorption spectrum did not change.
This set of results suggests that the ESPT process was inhibited
in the more acidic solution. The excited state pKa value (pKa*) of
ENG was calculated to be 0.895 ꢂ 0.03 based on the uores-
cence change with pH at 580 nm (Fig. S1†).
The synthesis of guanidine substituted naphthalimides was
carried out by the substitution reaction of 4-bromine-1,8-
naphthalimides with guanidine (Scheme 1). Two compounds,
N-hydroxyethyl-4-guanidino-1,8-naphthalimide (ENG) and N-
butyl-4-guanidino-1,8-naphthalimide (TNG) were prepared for
investigation.
The emission spectrum of ENG in acidic buffer (Fig. 1a)
showed two emission bands with maxima at 470 nm and 580 nm,
but the excitation spectra recorded at 470 nm and 580 nm only
exhibited a single excitation band with a maximum at 350 nm,
which suggests that two excited-state species occurred aer the
excitation of the ground-state ENG. Interestingly, the emission
spectrum of ENG at pH 10.32 only shows a single emission band
with a maximum at 580 nm, and the excitation spectrum recor-
ded at 580 nm shows a red shied band with a maximum at
400 nm (Fig. 1b). In order to understand the pH response
mechanism of the ENG, the pH-dependent absorption spectra
and emission spectra were investigated.
As shown in Fig. 2a, with the pH increasing from 3.59 to
11.32, the absorption band with a maximum at 350 nm
decreased gradually, while a red shied band with a maximum
at 400 nm appeared and increased. The color of the solution was
observed to change from colorless to yellow. The changes of
both absorption bands showed a linear pH response in the
range 6.5–10.5. The isoabsorptive point at 365 nm indicates that
two distinct chromophores (acidic and basic forms of ENG)
were present at the equilibrium. Therefore, the absorptions at
350 nm and 400 nm could be considered as the contributions of
the acidic form (NGH+) and basic form (NG) of ENG, respec-
tively. The ground-state pKa value of ENG was calculated to be
8.53 ꢂ 0.03 based on the data of the absorbance change with pH
at 350 nm (See ESI, Fig. S1†). This pKa value can be assigned to
the deprotonation of the guanidinium group of ENG. Although
In order to exclude the possibility that the ESPT resulted
from the hydroxyl proton of ENG, we synthesized N-butyl-4-
guanidino-1,8-naphthalimide (TNG), an analogue of ENG
without the hydroxyl group (Scheme 1). TNG showed almost
identical optical properties to ENG in buffers with different pH
values and various solvents (Fig. S3 and S4†). The pKa and pKa*
of TNG were determined to be 8.49 ꢂ 0.1 and 0.904 ꢂ 0.08.
These results suggest that the optical response to pH and the
ESPT properties of ENG and TNG only relied on the moiety of
guanidine substituted naphthalimide, and the substituents on
the imide N-atom do not signicantly affect these properties.
Scheme 1 Synthetic route of guanidine substituted naphthalimides.
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Fig. 1 Steady-state spectra of ENG were measured in glycine–HCl–NaOH buffer at pH (a) 3.59 and (b) 10.32; fluorescence spectra with excitation at 365 nm
(up triangles), excitation spectrum detected at 460 nm (diamonds) and excitation spectra detected at 600 nm (circles).
The low pKa* values of ENG and TNG indicate that the guani-
dine substituted naphthalimides can be considered as a new tion process of ENG, time-resolved uorescence spectroscopy of
class of strong photoacid. ENG in acidic buffer (pH 5.35), dichloromethane (DCM),
In order to further demonstrate the excited-state deprotona-
The excited-state deprotonation process of ENG can be dimethyl sulphoxide (DMSO) and ethanol were measured (Fig. 3).
illustrated as shown in Scheme 2. In neutral and acidic solution, In these solvents, the NGH+* emission showed an instant rise
most ENG molecules were present in the protonated form and an exponential decay, and the NG* emission showed an
(NGH+), upon excitation at 350 nm, some excited molecules in exponential rise followed by an exponential decay, which
the protonated form (NGH+*) quickly converted into the excited suggests the occurrence of an excited-state deprotonation process
deprotonated form (NG*) through the intermolecular ESPT of ENG.22 In polar solvents (acidic buffer, ethanol and DMSO), the
process. The NG* molecules returned to the ground state (NG) rise time of NG* emission is comparable to the corresponding
by emission with a band maximum around 580 nm. Aer decay time of NGH+* (Table 1), and both times are increased in
returning to the ground state, the NG molecules reverted to the the order of acidic buffer < ethanol < DMSO, which may suggest a
protonated form (NGH+) via reverse proton transfer.
decrease of the deprotonation rate of NGH+*. The decay time of
Fig. 2 (a) Absorption spectra of 30 mM ENG in glycine–HCl–NaOH buffer with pH 3.59, 5.52, 6.11, 7.3, 7.65, 7.82, 7.93, 8.05, 8.17, 8.32, 8.5, 8.62, 8.74, 8.94, 9.08, 9.21,
9.45, 9.71, 10.05, 10.32, 11.12; (b) fluorescence spectra of 30 mM ENG, excited at 365 nm; (c) absorption spectra of 50 mM ENG in glycine–HCl buffer at pH 3.8, 2.57,
1.37, 0.86, 0.46; (d) fluorescence spectra of 50 mM ENG in glycine–HCl buffer at pH 0.3, 0.39, 0.77, 1.11, 1.39, 1.79, 2.33, 2.62, 3.38, excited at 350 nm.
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Since the efficiency of ESPT reaction, especially the inter-
molecular ESPT is strongly dependent on the environment,23
the solvent effect on the steady-state spectral properties of ENG
was also investigated. As shown in Fig. 4a and b, in toluene, a
low concentration of ENG (1 mM) only exhibited the NGH+*
emission peak (450 nm). When the concentration of ENG was
increased, the NG* peak appeared and became stronger. Both
emission bands were red-shied with the increase of ENG
concentration in toluene. This set of results suggests that the
ESPT process could not occur from ENG to toluene. The NG*
emission and the red-shied emissions at high concentration of
ENG may originate from the intermolecular interaction between
two ENG molecules. As shown in Fig. 4c and d, ENG (1 mM)
exhibited a high NGH+* emission peak (450 nm) and a low NG*
emission peak (530 nm) in dichloromethane. The NG* peak red
shied to 545 nm and became stronger than the NGH+* peak in
dioxane.
Scheme 2 Proton transfer reaction between the protonated form and depro-
tonated form of guanidine substituted 1,8-naphthalimides.
As summarized in Table 2, both emission peaks of ENG were
red shied as the solvent polarity was increased, but the
absorption peak of ENG did not show signicant shi in
different solvents. Since 4-amino-1,8-naphthalimide is a typical
uorophore with ICT nature,20 these results suggest that
increasing solvent polarity stabilizes the ICT excited-state of
both forms (NGH+* and NG*) of ENG relative to the ground
state and further lowers the energy of the excited-states. The
uorescence quantum yield of ENG decreased upon increasing
the solvent polarity, and markedly decreased in a protic solvent,
which is attributed to the increase in nonradiative decay
NG* emission increased in the same order, which is also
consistent with the increase of the quantum yield (Ff) of ENG in
these solvents (Table 2), suggesting the reduction of nonradiative
rates of ENG. However, in dichloromethane, the rise time of NG*
emission (1.57 ꢂ 0.26 ns) is similar to that in DMSO, but is much
faster than the decay of the NGH+* emission (4.80 ꢂ 0.06 ns),
which may be due to the fact that only some of the NGH+*
molecules can undergo the deprotonation process. The slow
decay of NGH+* emission in dichloromethane suggests the
signicant suppression of the deprotonation process and the
nonradiative decay of NGH* form.
Fig. 3 The fluorescence dynamics of ENG (excitation at 350 nm) in (a) glycine–HCl–NaOH buffer with pH 5.35, emission: 460 and 560 nm; (b) DCM (dichloromethane),
emission: 460 and 560 nm; (c) DMSO, emission: 460 and 560 nm; (d) ethanol, emission: 500 and 600 nm. Black curve, NGH+* emission at 460 or 500 nm; red curve, NG*
emission at 560 or 600 nm.
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Table 1 Summary of time resolved fluorescence data of ENG in different solvents
Solvent
NGH+* decay nsꢁ1
NG* increase nsꢁ1
NG* decay nsꢁ1
Buffer (pH 5.35)
DCM
DMSO
0.09 ꢂ 0.59
4.80 ꢂ 0.06
1.78 ꢂ 0.01
0.81 ꢂ 0.01
0.11 ꢂ 0.02
1.57 ꢂ 0.26
1.47 ꢂ 0.02
0.57 ꢂ 0.08
1.03 ꢂ 0.02
7.55 ꢂ 0.57
7.57 ꢂ 0.05
3.81 ꢂ 0.14
Ethanol
induced by the interactions between the ICT excited-state
The emission behaviour of ESPT molecules in micelles was
molecules and solvent molecules. (e.g. hydrogen bonding found to be markedly different to that in aqueous solution.27 We
between the carbonyl oxygen and protic solvent)24 The rapid further investigated the absorption and emission spectra in
nonradiative decay in a polar solvent is also demonstrated by solution of surfactants like cetyl trimethylammonium bromide
the short lifetime of the both emissions (Fig. 3).
(CTAB), Triton X-100 and sodium dodecyl sulfate (SDS). The
The ratio of the uorescence intensity of NG* and NGH+* absorption spectra of ENG only showed a 350 nm band in these
changed in different solvents (Table 2). Surprisingly, in a polar solutions, and did not show a notable change with the variation
solvent, acetonitrile, the NG* emission of ENG was very weak of surfactant concentration (Fig. S5†), suggesting that ENG was
compared with the NGH+* emission, suggesting that the ESPT mainly present in the protonated form. As shown in Fig. 5c and
from ENG to acetonitrile was unfavourable. In water, ENG d, both emission bands of ENG were not affected by CTAB and
exhibited the lowest quantum yield (0.018) and the fastest Triton X-100, but were greatly enhanced with an increase of the
uorescence decay (Fig. 3), but the ratio of the uorescence SDS concentration and reached a plateau near the critical
intensity of NG* and NGH+* was very high (1.70). In order to micelle concentration (SDS, CMC 8 mM). Compared with in
explain this phenomenon, the emission spectra of ENG were water, both emission bands of ENG in SDS solution were blue-
collected in DMSO or acetonitrile solution containing different shied. These results suggest that the protonated ENG did not
amounts of water. As shown in Fig. 5a and b, both the NGH+* bind to cationic (CTAB, CMC 0.89 mM) and neutral (Triton
and NG* emissions decreased signicantly upon increasing the X-100, CMC 0.2 mM) micelles, and strongly bound to anionic
water in both solvents, but the decrease of the NGH+* emission SDS even before its CMC. The binding to SDS provided a less
was much larger than that of the NG* emission, which caused polar microenvironment for ENG, and reduced the nonradiative
the low quantum yield and high ratio of NG* and NGH+* decay by lowering the accessibility to bulk water.
emissions in water. Obviously, there are many hydrogen-
The above results demonstrate that the emission properties
bonded sites in the ENG molecule, the multiple hydrogen- of ENG can be tuned by changing its environment, which shows
bonding interactions with water molecules cause the rapid the potential application in versatile luminescent materials,
nonradiative decay of the NGH+* and NG* forms. Compared to such as white-light emitting materials.4a,14 For this application,
the neutral NG* form, the hydration interaction of the cation different color luminescence from blue to red was observed
NGH+* further enhances the rapid nonradiative decay of from ENG in different solvents (see Fig. S6†).
NGH+*, as well as the rapid deprotonation. A large deuterium
Recently, anion recognition and sensing have attracted
isotope effect25 on the quantum yields and the ratio of NGH+* considerable interest, because anions play important roles in
and NG* emission was also observed in D2O (Table 2) relative the environment and in biological systems.28 Guanidinium
to H2O.
groups are extensively applied in anion recognition.21 Fluorine
Table 2 Spectral properties of ENG in different solvents at room temperature (lex ¼ 350 nm)
lem (nm) (NGH+a
)
lem (nm) (NGa)
INGa/INGH
Ffa
+a
Solvent
labs (nm)
Toluene (2 mM)
CH2Cl2 (2 mM)
CHCl3 (2 mM)
Acetic ether (2 mM)
Dioxane (2 mM)
Acetone (2 mM)
Acetonitrile (2 mM)
DMF (2 mM)
DMSO (2 mM)
Ethanol (5 mM)
Methanol (5 mM)
H2O (5 mM)
355
350
345
345
345
345
345
345
345
345
345
345
345
450
448
450
457
455
452
445
462
469
460
467
475
472
—
—
0.11
0.29
0.32
0.27
0.26
0.24
0.21
0.22
0.13
0.05
0.06
0.018
0.038
530
535
548
538
551
—
564
567
576
570
581
582
0.75
1.38
1.47
1.54
1.83
—
0.43
0.57
0.53
0.35
1.70
1.33
D2O (5 mM)
a
Quinine sulfate (Ff ¼ 0.56) in 1.0 N sulfuric acid as standard.26
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Fig. 4 Static excitation and fluorescence spectra of ENG in different solvents: (a) toluene (1 mM), (c) DCM (dichloromethane, 1 mM), and (d) dioxane (1 mM); fluo-
rescence spectrum excited at 350 nm (up triangles), excitation spectrum detected at 460 nm (diamonds) and excitation spectrum detected at 600 nm (circles). (b)
Fluorescence spectra of different concentrations of ENG (lex ¼ 350 nm) in toluene: 0.3 mM, 1.5 mM, 3 mM, 15 mM, 45 mM, 90 mM.
is the most electronegative atom, the anion of which usually excitation of the NG form (Fig. S7†). In acetic ether, ENG
forms the strongest H-bond interaction with an NH or OH showed NGH+* (457 nm) and NG* (548 nm) emission in the
group of an articial receptor. Several receptors of Fꢁ have been absence of Fꢁ, the NG* emissions in the presence of a low
reported under the proton transfer signaling mechanism.29 concentration of Fꢁ (<2 mM) originated from the excitation of
Therefore the interaction of ENG with Fꢁ in acetonitrile and the NG form and the ESPT from the NGH+* form. As shown in
acetic ether was investigated by absorption and emission Fig. 6b and d, the NG* emission in acetic ether was much
spectroscopy. As shown in Fig. 6a and c, upon the addition of Fꢁ higher than that in acetonitrile when all the ENG molecules in
to ENG solutions (acetic ether or acetonitrile), the absorption the NGH+ form were converted into the NG form by Fꢁ, sug-
band of ENG around 345 nm gradually decreased, while a band gesting that acetonitrile may suppress the NG* emission of
around 410 nm appeared and increased. An isosbestic point ENG. This result may also explain the low ESPT efficiency of
near 375 nm was observed. At the same time, the color of the ENG in acetonitrile.
ENG solution changed from colorless to yellow. Based on the
Other halide anions could not signicantly change the
above results, the two absorption bands can be assigned to absorption and emission spectra of ENG (see Fig. S8†),
the NGH+ and NG forms of ENG. The change in the absorption indicating that ENG has a high selectivity for Fꢁ sensing. The
spectra suggests that Fꢁ induced the deprotonation of ENG in linear or ratiometric responses of the absorption and emis-
the ground state.
sion spectra to Fꢁ suggest that ENG is a potential chemo-
Although the emission spectra of ENG (excitation at iso- sensor for the highly sensitive detection of Fꢁ (see Fig. S9 and
sbestic point) in acetic ether and acetonitrile were totally S10†). The apparent equilibrium dissociation constants
different in the absence of Fꢁ, they showed a similar change between ENG and Fꢁ in acetic ether and acetonitrile were
upon the addition of Fꢁ, i.e. the NGH+* emission gradually calculated to be 0.45 ꢂ 0.02 mM and 0.78 ꢂ 0.07 mM based on
decreased and the NG* emission gradually increased (Fig. 6b the curve of the uorescence change at 457 nm and 445 nm
and d). Because Fꢁ induced the deprotonation of ENG in the (Fig. S9 and S10†). Neither the absorbance nor emission
ground state, the NGH+* emission decreased with the addition spectra of ENG were changed upon the addition of Fꢁ in
of Fꢁ in both solvents. In acetonitrile, ENG mainly showed aqueous solution, which may due to the fact that water
NGH+* (450 nm) emission in the absence of Fꢁ, therefore the molecules compete for Fꢁ with ENG as water molecules have
NG* emission in the presence of Fꢁ originated from the a stronger acidity than ENG.
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Fig. 5 Fluorescence spectra of ENG in solution containing different amounts of water, (a) H2O/(DMSO + H2O)%: 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 5.0, 10, 15, 20, 24, 28, 33,
and 36; (b) H2O/(CH3CN + H2O)%: 0.1, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 10, and 20; (c) steady-state emission spectra of ENG in water, 5 mM Triton X-100,
4 mM CTAB, 10 mM SDS; (d) steady-state emission spectra of ENG at different concentrations of SDS: 0 mM, 1 mM, 2.3 mM, 3.5 mM, 5 mM, 10 mM. lex ¼ 350 nm.
solutions of ENG were obtained by dissolving it in DMSO. Tet-
rabutylammonium uoride (TBAF), tetrabutylammonium
Conclusions
A new uorophore based on guanidine substituted 1,8-naph-
thalimide was synthesized. The steady-state spectra and time-
resolved uorescence measurements demonstrated that the
guanidinium group on this uorophore possessed photoacidic
properties with pKa of ꢀ8.5 and pKa* of ꢀ0.9. In acidic/neutral
buffers or organic solvents, this uorophore exhibits dual emis-
sion with large Stokes shis in the wavelength range from 400–
700 nm because of the ESPT coupled ICT process. This ESPT
coupled ICT process is largely sensitive to the solvent microen-
vironment, implying the potential application in versatile lumi-
nescent materials. Fꢁ can specically induce the deprotonation
of the guanidinium group of this uorophore, resulting in a
signicant change in its absorption and emission spectra, sug-
gesting the potential application as a chemosensor for Fꢁ.
chloride, tetrabutylammonium bromide, and tetrabutylammo-
nium iodide were purchased from Acros and used without
further purication. All stock solutions of anions and other
molecules were dissolved in deionized water puried by a
UPHW-III-90T Milli-Q water purication system (Chengdu,
China). All of the titration experiments were performed at room
temperature.
Instruments
1H NMR spectra were recorded at 300 MHz, and 13C NMR spectra
were recorded at 75.5 MHz on a Brucker AM 300 spectrometer
with tetramethylsilane (TMS) as the internal standard. J values
were given in hertz. Low-resolution mass spectra (MS) were
recorded on an LC-MS 2010A (Shimadzu) instrument using
standard conditions. High-resolution MS were obtained on a
Bruker Daltonics Flex-Analysis. UV-visible absorption spectra
were recorded on a Hitachi U-2550 UV-vis spectrophotometer
(Kyoto, Japan). Fluorescence emission spectra were recorded on a
Experimental section
Chemicals
4-Bromo-1,8-naphthalic anhydride was purchased from Liao Hitachi F-4600 uorescence spectrouorometer (Kyoto, Japan).
Yang Lian Gang Dye Chemical Co. Ltd (Liaoyang). Guanidinium The excitation laser pulses (350 nm) for the picosecond time
hydrochloride was purchased from JK-chemical company (Bei- resolved PL experiment were supplied by an optical parametric
jing). Ethanolamine, ethanol, DMSO and other reagents were amplier (OPA-800CF, Spectra Physics), which was pumped by
purchased from Beijing Chemical Plant (Beijing). Distilled- the output from a regenerative amplier (Spitre, Spectra
deionized water was used throughout this work. All chemical Physics). The excitation pulse energy was ꢀ100 nJ per pulse.
reagents were used without further purication. The stock Fluorescence collected with a 90ꢃ geometry was dispersed by a
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Journal of Materials Chemistry C
Paper
Fig. 6 (a) Absorption spectrum of ENG (40 mM) in acetic ether with different concentrations of Fꢁ (TBAF in acetic ether solution): 0 mM, 1 mM, 2 mM, 3 mM, 4 mM, 5 mM,
6 mM, 7 mM, 8 mM, 9 mM, 10 mM, 11 mM; (b) fluorescence spectra of ENG (5 mM) in acetic ether with different concentrations of Fꢁ (TBAF): 0 mM, 0.25 mM, 0.5 mM,
0.75 mM, 1.0 mM, 1.25 mM, 1.75 mM, 2.0 mM, 2.25 mM, 2.5 mM (excitation at 375 nm); (c) absorption spectra of ENG (40 mM) in acetonitrile with different concentrations
of Fꢁ (TBAF in acetonitrile solution): 0 mM, 3 mM, 5 mM, 7 mM, 11 mM, 15 mM, 19 mM, 23 mM, 27 mM, 31 mM, 37 mM, 41 mM, 45 mM; (d) fluorescence spectra of ENG (5 mM)
in CH3CN with different concentrations of Fꢁ (TBAF): 0 mM, 1 mM, 2 mM, 3 mM, 4 mM, 5 mM, 6 mM, 7 mM, 8 mM, 9 mM, 10 mM (excitation at 365 nm).
polychromator (250is, Chromex) and detected with a photon- of ethanolamine (11.9 mmol) was added to the mixture slowly.
counting type streak camera (C5680, Hamamatsu Photonics). The resulting mixture was heated to reux with stirring for 1 h.
The spectral resolution was 0.2 nm, and the temporal resolution The reaction was over when the solution became clear. Aer the
was ꢀ50 ps. Analysis of the kinetic traces derived from time- solution was cooled to room temperature,
a precipitate
resolved spectra was performed individually using nonlinear emerged. A white solid was obtained by vacuum ltration, and
least-squares tting to a general sum-of-exponentials function then washed with water and ethanol three times each, and
aer deconvolution of the instrument response function (IRF). nally dried by vacuum (yield 93.2%);
All the spectroscopic measurements were carried out at room
temperature.
1H NMR (CDCl3, 300 MHz) d 8.65 (dd, J ¼ 7.5 Hz, 1.5 Hz, 1H),
8.56 (dd, J ¼ 7.5 Hz, 1.5 Hz, 1H), 8.40 (d, J ¼ 9 Hz, 1H), 7.84 (dd,
J ¼ 7.5 Hz, 3 Hz, 1H), 4.42 (t, J ¼ 6 Hz, 2H), 3.98 (t, J ¼ 6 Hz, 2H),
2.09 (s, 1H); 13C NMR (CDCl3, 75.5 MHz) d 162.9, 162.9, 132.5,
131.5, 131.3, 130.8, 129.7, 128.9, 128.7, 128.2, 122.8, 122.0, 57.7,
41.9; MS (ESI): m/z 320.1 (M + H)+.
Spectral measurements
The absorption and uorescence spectra were recorded in
20 mM glycine–HCl–NaOH buffer solution in a 1.0 cm quartz
cuvette. Buffered solutions of different pH values were prepared
by adding appropriate volumes of NaOH solution or HCl solu-
tion to the 20 mM glycine solution. A stock solution of ENG was
prepared (10 mM) in DMSO. UV-vis absorption spectra and
uorescence spectra used the same concentration of solution
made by adding 3 mL of stock solution to 1 mL of glycine–HCl–
NaOH buffer solution.
Synthesis of N-hydroxyethyl-4-guanidino-1,8-naphthalimide
(ENG)
430 mg (1.3 mmol) N-hydroxyethyl-4-bromine-1,8-naph-
thalimide, 200 mg (2.1 mmol) guanidinium hydrochloride and
88 mg (2.2 mmol) NaOH were dissolved in DMSO (6 mL). The
resulting mixture was heated to 80 ꢃC for 12 h. Aer cooling to
room temperature, the solution was evaporated under reduced
pressure. The residue was puried by column chromatography
using silica-gel (100–200 mesh) and 10% methanol in
Synthesis of N-hydroxyethyl-4-bromine-1,8-naphthalimide
(ENA)
3.0 g (10.8 mmol) 4-bromine-1,8-naphthalic anhydride was dichloromethane as eluent to give a yellow solid compound,
dissolved in ethanol (160 mL), and heated to reux. Then 730 mL then further puried by HPLC with chromatographic grade
4434 | J. Mater. Chem. C, 2013, 1, 4427–4436
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Journal of Materials Chemistry C
methanol and 0.1% TFA in deionized water. A white solid
compound was obtained (yield 29.7%).
Notes and references
1H NMR (DMSO-d6, 300 MHz) d (ppm): 10.57 (s, 1N–H), 8.58
(d, J ¼ 6 Hz, 1H), 8.54 (d, J ¼ 9 Hz, 1H), 8.42 (d, J ¼ 9 Hz, 1H), 7.97
(t, J ¼ 7.5 Hz, 1H), 7.82 (d, J ¼ 9 Hz, 1H), 7.79 (s, 4–NH–), 4.83 (t,
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J ¼ 12 Hz, 1–OH), 4.18 (t, J ¼ 12 Hz, 2H), 3.63 (t, J ¼ 6 Hz, 2H); 13
C
NMR (DMSO-d6, 75 MHz) d (ppm): 163.4, 163.0, 156.6, 137.6,
131.4, 131.2, 129.0, 128.6, 127.7, 127.4, 124.8, 122.6, 120.8, 57.7,
1
41.8. H NMR (DMSO-d6 + NaOH solid, 300 MHz) d (ppm): 8.55
(d, J ¼ 6 Hz, 1H), 8.45 (d, J ¼ 6 Hz, 1H), 8.33 (d, J ¼ 9 Hz, 1H), 7.72
(t, J ¼ 7.5 Hz, 2H), 7.31 (d, J ¼ 6 Hz, 1H), 6.56 (s, 4–NH–), 4.80 (s,1–
OH), 4.15 (t, J ¼ 6 Hz, 2H), 3.59 (t, J ¼ 6 Hz, 2H); 13C NMR (DMSO-
d6 + NaOH solid, 75 MHz) d (ppm): 163.8, 163.1, 156.1, 132.5,
130.7, 130.6, 129.3, 127.4, 125.2, 121.9, 118.6, 113.3, 57.9, 41.5;
MS (ESI): m/z 299.0 (M + H)+; HRMS (ESI): m/z calcd for
C15H15N4O3 (M + H)+ 299.11387, found, 299.11356.
Synthesis of N-butyl-4-guanidino-1,8-naphthalimide (TNG)
2.5 g (9.03 mmol) 4-bromine-1,8-naphthalic anhydride was
dissolved in 150 mL ethanol, and heated to reux. Then 610 mL
of butylamine (8.6 mmol) was added to the mixture slowly aer
the temperature was reduced to 50 ꢃC. The resulting mixture
was heated to reux with stirring for 1 h. The reaction was over
when the solution became clear. Aer the solution was cooled to
room temperature, water was added to the solution and then a
precipitate emerged. A white solid was obtained by vacuum
ltration, and then washed with water and ethanol three times
each, and nally dried by vacuum. N-Butyl-4-bromine-1,8-
naphthalimide was obtained (yield 85.3%). Then 100 mg
(0.302 mmol) N-butyl-4-bromine-1,8-naphthalimide, 78 mg
(0.80 mmol) guanidinium hydrochloride and 44 mg (1.1 mmol)
NaOH were dissolved in DMSO (4 mL). The resulting mixture
was heated to reux with stirring for 1 h. The resulting mixture
was heated to 80 ꢃC for 12 h. Aer cooling to room temperature,
the solution was added to water and precipitation appeared in
the solution. A brown solid was obtained by vacuum ltration.
The residue was puried by column chromatography using
silica-gel (100–200 mesh) and 5% methanol in dichloromethane
as eluent to give a yellow solid compound, then further puried
by HPLC with chromatographic grade methanol and 0.1% TFA
in deionized water. A white solid compound was obtained
(yield 33.5%).
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We gratefully acknowledge the nancial support from the grant
973 Program (2011CB935800 and 2013CB33700) and NSF of
China (21075124 and 21275149).
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