A 4-aminonaphthalimide based environmentally sensitive fluorescence probe

Article abstract of DOI:10.1016/j.cclet.2013.03.027

A simply synthesized 4-aminonaphthalimide derivative 1 expresses both polarity and viscosity sensitive fluorescence spectra, indicating its potential usage as an environmentally sensitive fluorescence probe. By comparing the fluorescence behavior of 1 with that of a known 4-aminonaphthalimide derivative 2, it was found that the substitution of the 4-amino group has profound influence on the environmentally sensitive fluorescence properties of 4-aminonaphthalimide.

Full text of DOI:10.1016/j.cclet.2013.03.027

Chinese Chemical Letters 24 (2013) 359–361
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Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Original article
A 4-aminonaphthalimide based environmentally sensitive fluorescence probe
b,
a
Zhi-Yong Wu a, Jing-Nan Cui a, , Xu-Hong Qian , Tian-Yu Liu
*
*
a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China
b Shanghai Key Laboratory of Chemical Biology, East China University of Science and Technology, Shanghai 200237, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A simply synthesized 4-aminonaphthalimide derivative 1 expresses both polarity and viscosity sensitive
fluorescence spectra, indicating its potential usage as an environmentally sensitive fluorescence probe.
By comparing the fluorescence behavior of 1 with that of a known 4-aminonaphthalimide derivative 2, it
was found that the substitution of the 4-amino group has profound influence on the environmentally
sensitive fluorescence properties of 4-aminonaphthalimide.
Received 4 February 2013
Received in revised form 8 March 2013
Accepted 11 March 2013
Available online 12 April 2013
ß 2013 Jing-Nan Cui, Xu-Hong Qian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All
rights reserved.
Keywords:
4-Aminonaphthalimide
Environmentally sensitive
Fluorescence probe
Substitution effects
1. Introduction
investigated by comparing the fluorescence behavior of 1 with that
of a known 4-aminonaphthalimide derivative 2.
Environmentally sensitive fluorescence probes are functional
molecules whose fluorescence properties are dependent upon the
polarity or viscosity of the local environments and they were
widely used in biochemical research [1–3]. 4-Aminonaphthalimide
is an environmentally sensitive fluorophore and expressed
excellent photophysical properties such as long emission wave-
lengths, high fluorescence efficiency and good photostability [4–7].
Environmentally sensitive fluorescence probes based on 4-
aminonaphthalimide have been developed for different applica-
tions [8–10]. However, most of the studies focused on the effects of
solvent polarity on the fluorescence properties of 4-amino-
naphthalimide. The effects of solvent viscosity on the fluorescence
properties of 4-aminonaphthalimide were rarely concerned and
viscosity sensitive fluorescence probes based on 4-aminonaphtha-
limide have not been reported. The relationship between molecu-
lar structure and the fluorescence properties of 4-
aminonaphthalimide was also not well understood, even though
this subject is essential for the design of 4-aminonaphthalimide
based environmentally sensitive fluorescence probes. To develop
environmentally sensitive fluorescence probes, we synthesized a
4-aminonaphthalimide derivative 1 (Scheme 1) and examined its
fluorescence spectra in solvents with different polarity and
viscosity. The effects of substitution of the 4-amino group on
the fluorescence properties of 4-aminonaphthalimide were also
2. Experimental
All the reagents were analytical grade and used as received. 1H
NMR was measured on a Bruker AV-400 spectrometer. Mass
spectra were measured on a Micromass CA 156 GC-TOF MS
spectrometer. Melting points were determined by an X-6 micro-
melting point apparatus and are uncorrected. Fluorescence spectra
were determined on a Hitachi F-4500 spectrometer. Absorption
spectra were determined on
spectrophotometer. The fluorescence quantum yields (
estimated with quinine sulfate in 0.5 mol/L sulfuric acid as a
standard ( F = 0.546) [11]. Viscosity measurements were made at
a
PGENERAL TU-1901 UV–vis
FF) were
F
(22 ꢀ 2) 8C in ethylene glycol/glycerol mixtures using Ubbelohde
capillary viscometer. The viscosities of all other solvent were obtained
from literature values [12].
N-Butyl-4-bromo-1,8-naphthalimide (3) and N-butyl-4-n-
butylamino-1,8-naphthalimide (2) were synthesized according
to literature procedures [4]. N-Butyl-4-N0-methyl-n-butylamino-
1,8-naphthalimide (1) was synthesized according to similar
procedure of 2. A solution of 3 (332 mg, 1 mmol), N-methyl-n-
butylamine (120
mL, 1 mmol) and triethylamine (140 mL, 1 mmol)
in 10 mL of 2-methoxyethanol was refluxed for 4 h. After removal
of 2-methoxyethanol, the residue was purified by flash chroma-
tography on silica gel using ethyl acetate/hexane (1/10, v/v) as
eluent to afford 1 (284 mg, 84%) as a yellow crystal. Mp 97.5–
98.5 8C. 1H NMR (400 MHz, CDCl3):
(d, 1H, J = 8.0 Hz), 8.40 (d, 1H, J = 8.0 Hz), 7.64 (t, 1H, J = 8.0 Hz),
* Corresponding authors.
d 8.55 (d, 1H, J = 8.0 Hz), 8.47
E-mail addresses: Jncui@dlut.edu.cn (J.-N. Cui),
xhqian@ecust.edu.cn (X.-H. Qian).
1001-8417/$ – see front matter ß 2013 Jing-Nan Cui, Xu-Hong Qian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2013.03.027

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Z.-Y. Wu et al. / Chinese Chemical Letters 24 (2013) 359–361
I
Scheme 1. Synthesis of 1 and 2.
7.15 (d, 1H, J = 8.0 Hz), 4.17 (t, 2H, J = 8.0 Hz), 3.31 (t, 2H, J = 8.0 Hz),
3.04 (s, 3H), 1.71 (m, 4H), 1.45 (m, 2H), 1.35 (m, 2H), 0.97 (t, 3H,
J = 8.0 Hz), 0.93 (t, 3H, J = 8.0 Hz); 13C-NMR (100 MHz, CDCl3):
d
164.61, 164.09, 156.85, 132.41, 130.96, 130.83, 130.19, 125.95,
124.95, 123.18, 115.26, 114.58, 56.83, 41.60, 40.02, 30.28, 29.58,
20.42, 20.23, 13.89, 13.87; GC-MS (EI+), Calcd. for C21H26N2O2 [M+]
338.1994; found 338.2003.
Fig. 2. The change of the fluorescence efficiency of 1 and 2 as a function of ET(30).
The longest emission wavelength of 1 appears between 467 nm
and 546 nm. Increasing polarity of solvent from hexane to ethanol
leads to a bathochromic shift of about 70 nm of the emission
maximum of 1 (Fig. 1). This value is greater than that of the
absorption maximum which confirmed the formation of an
internal charge transfer (ICT) excited state. The emission maxi-
mum of 2 shows the same trend as that of 1, except that it is about
10 nm shorter in a given solvent (except in hexane). Their similar
emission maximums indicate that the emitting state of 1 and 2 is
similar in structure. Regarding to their long emission wavelengths,
the 4-amino group may be parallel to the naphthalimide ring on
the emitting state. The relatively longer emission wavelengths of 1
than that of 2 can be attributed to the inductive effect of the methyl
substituent which increases the electron donating ability of the 4-
amino group. As shown in Fig. 1, the emission maximum of both 1
and 2 exhibits a good linear relationship with ET(30).
3. Results and discussion
The photo-physical properties of 1 and 2 in solvents with
different polarity were investigated and the spectra data are
collected in Table S1 (Supplementary data). The longest wave-
length absorption of 1 appears at 391–443 nm with an extinction
coefficient in the range of 10,000–15,000 L molꢁ1 cmꢁ1. Increasing
polarity of solvent described by an empirical parameter ET(30) [13]
leads to a bathochromic shift of the absorption maximum (Fig. 1).
Changing the solvent from hexane to ethanol results in a red-shift
of the absorption maximum of about 30 nm. These observations
suggest that the maximum absorption of 1 arises from a charge
transfer (CT) transition [4] and that the ground state of 1 is
significantly polar. The absorption spectra of 2 show similar
dependence on solvent polarity. But, the maximum absorption of 2
is 10–20 nm longer than that of 1 in a given solvent. The molar
extinction coefficient of 2 (16,000–24,000 L molꢁ1 cmꢁ1) is also
greater than that of 1. This difference can be attributed to the steric
interaction between the methyl group of the 4-amino substituent
and the hydrogen atom at the 5-position (the peri effect), which
makes the 4-amino group of 1 less planarity with the naphtha-
limide ring [4]. This effect inhibits the CT from the 4-amino group
to the naphthalimide ring and destabilizes the Franck–Condon
excited state of 1.
Although the emission wavelengths of 1 and 2 are similar, their
fluorescence efficiencies are quite different. Both the fluorescence
efficiencies of 1 and 2 are higher in nonpolar solvents and decrease
with increasing the solvent polarity. But the fluorescence efficiency
of 1 decreases more steeply than that of 2 (Fig. 2). In polar solvents
such as ethanol, the fluorescence of 1 is almost fully quenched
(
(
F
F = 0.008), while the fluorescence efficiency of 2 remains high
FF = 0.62). It is obvious that there is an additional nonradiative
pathway of the excited state of 1. We attribute it to the formation
of a twisted internal charge transfer (TICT) state where the 4-
amino group is orthogonal to the naphthalimide ring and the
positive and negative charges are fully separated (Scheme 2) [14].
The formation of this TICT state may be facilitated by the steric
interaction between the peri H atom and the methyl group of the 4-
amino substituent. Therefore, the charge separation of the TICT
state is more complete than that of the ICT state. Increasing solvent
polarity increases the probability of the formation of the TICT state,
which leads to the sharp decrease of the fluorescence efficiency of
[(Schem2)_TD$FI]G
[(Fig._1)TD$FIG]
Fig. 1. The change of the absorption maximum of 1 (lab(1)) and 2 (lab(2)) and the
emission maximum of 1 ( em(1)) and 2 ( em(2)) as a function of ET(30). The straight
l
l
lines are the linear fitness of the emission maximum of 1 (solid) and 2 (dash) to
ET(30). The correlation coefficient is 0.947 (solid) and 0.981 (dash), respectively.
Scheme 2. The proposed mechanism of the formation of the TICT state of 1.

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Z.-Y. Wu et al. / Chinese Chemical Letters 24 (2013) 359–361
361
can be used as an environmentally sensitive fluorescence probe.
The alkyl substituent of the 4-amino group was found to have
profound influence on the environmentally sensitive fluores-
cence properties of 4-aminonaphthalimides. This study may be
helpful for the design of environmentally sensitive fluorescence
probes.
Acknowledgment
We are grateful to the National Science & Technology Pillar
Program of China (No. 2011BAE31B01).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.cclet.2013.03.027.
Fig. 3. Fluorescence spectra of 1 in ethylene glycol/glycerol (v/v) mixtures of varying
viscosity at 25 8C: (a) ethylene glycol only; (b) 8:2; (c) 6:4; (d) 5:5; (e) 4:6; (f) 3:7;
(g) 2:8; (h) 1:9; (i) glycerol only. The excitation wavelength is 441 nm. The inset is
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In summary, a simply synthesized small molecule 1 exhibits
both polarity and viscosity sensitive fluorescence spectra and