Unique solvent-dependent fluorescence of nitro-group-containing naphthalene derivatives with weak donor-strong acceptor system

Article abstract of DOI:10.1016/j.tetlet.2013.01.096

We synthesized nitro-group-containing π-conjugated naphthalene derivatives with a weak donor-strong acceptor system and investigated their photophysical properties. The nitro group was introduced into naphthalene through the phenyl or phenylethynyl moiety at the C2 and C7 positions as the strong acceptor moiety, and a methoxy group was introduced into naphthalene directly at the position opposite to the nitro group, as a weak donor moiety. While 2-(4-nitrophenyl)naphthalene did not show fluorescence in various solvents, 2-methoxy-6-(4-nitrophenyl)naphthalene showed fluorescence in weakly polar solvents (Φ_F = 0.11 in CH₂Cl₂), with a large Stokes shift (Δν = 12,000 cm^-1). Additionally, 2-methoxy-6-(4-nitrophenyl)naphthalene did not show fluorescence in polar solvents (acetonitrile) and non-polar solvents (toluene). This unique solvent-dependent fluorescence is remarkable for environmental fluorescence sensor applications.

Full text of DOI:10.1016/j.tetlet.2013.01.096

Tetrahedron Letters 54 (2013) 1839–1841
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Tetrahedron Letters
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Unique solvent-dependent fluorescence of nitro-group-containing
naphthalene derivatives with weak donor–strong acceptor system
Sojiro Hachiya a, Kengo Asai a, Gen-ichi Konishi a,b,
⇑
a Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
b PRESTO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
We synthesized nitro-group-containing p-conjugated naphthalene derivatives with a weak donor–strong
Received 17 December 2012
Revised 22 January 2013
Accepted 23 January 2013
Available online 30 January 2013
acceptor system and investigated their photophysical properties. The nitro group was introduced into
naphthalene through the phenyl or phenylethynyl moiety at the C2 and C7 positions as the strong accep-
tor moiety, and a methoxy group was introduced into naphthalene directly at the position opposite to the
nitro group, as a weak donor moiety. While 2-(4-nitrophenyl)naphthalene did not show fluorescence in
various solvents, 2-methoxy-6-(4-nitrophenyl)naphthalene showed fluorescence in weakly polar
Keywords:
Solvatochromic fluorescence
Nitro group
Large Stokes shift
Naphthalene
solvents (UF = 0.11 in CH2Cl2), with a large Stokes shift (Dm
= 12,000 cmÀ1). Additionally, 2-methoxy-6-
(4-nitrophenyl)naphthalene did not show fluorescence in polar solvents (acetonitrile) and non-polar
solvents (toluene). This unique solvent-dependent fluorescence is remarkable for environmental fluores-
cence sensor applications.
Ó 2013 Elsevier Ltd. All rights reserved.
Recently, fluorescent probes showing fluorescence solvatochro-
mism or on/off switching as environment-sensitive sensors or
reporters are investigated.1 In many cases, these probes have a
fluorophore moiety, which has a donor–acceptor system in the
environmental fluorescence by introducing a nitro group through
-conjugated linker moiety for constructing a donor–acceptor
system and by using a weak donor group.
a
p
In this study, we synthesized naphthalene derivatives in which
a nitro group was introduced through a phenyl or phenylethynyl
moiety as the acceptor moiety and a methoxy group as the weak
donor group and found that the resulting compounds show unique
solvent-dependent fluorescence. Naphthalene was used as the core
structure to construct simple nitro-group-containing donor–accep-
tor systems. Nitro group was introduced through phenyl moiety or
phenylethynyl moiety to naphthalene at the C2 and C7 positions
(1a–4a). In the previous study, phenyl group was an effective lin-
ker to induce unique fluorescence and ethynyl moiety was used
as an additional linker because that is a famous moiety to expand
p
-conjugated system.2,3 While many fluorescent dyes with various
functional groups as the donor and acceptor moieties have been
reported, very few Letters focus on nitro-group-containing fluores-
cent dyes.4 Generally,
p-extended compounds in which the nitro
group is introduced directly into the fluorophore show very weak
fluorescence as compared to unsubstituted compounds, since the
nitro group elongates the path for internal conversion immediately
after intersystem crossing.5 Nevertheless, in our previous study, we
found that a fluorene in which the nitro group is introduced
through a phenyl moiety shows specific solvent-dependent fluo-
rescence.6 Interestingly, the fluorescence properties of nitro-
group-containing compounds depend on the manner in which
the nitro group is introduced into the fluorophore. Introduction
p-conjugation. Additionally, a methoxy group was introduced to
naphthalene at the C3 and C6 positions (1b–4b) to construct a
donor–acceptor system as shown in Scheme 1. Their photophysical
properties were evaluated in various solvents.
of a nitro group into a fluorophore through a
p-conjugated linker
moiety such as a phenyl group is an effective method to induce
fluorescence in a nitro-group-containing dye. On the other hand,
the reported dyes having donor–acceptor systems have been syn-
thesized with a strong donor such as an amino group and a nitro
group. These dyes show fluorescence properties similar to those
of dyes with other strong acceptors such as a cyano group. Hence,
we believe that it is possible to synthesize a novel dye showing
The naphthalene derivatives were synthesized from
corresponding bromonaphthalenes using transmetal catalyzed
Suzuki–Miyaura7 and Sonogashira8 cross coupling reactions,
respectively.
UV–vis absorption and fluorescence spectra of 1–4 were mea-
sured in toluene, dichloromethane, tetrahydrofuran, chloroform,
acetonitrile, and methanol. Figure 1 shows the representative
UV–vis absorption and fluorescence spectra of all the compounds
in dichloromethane. Table 1 summarizes the photophysical prop-
erties of the compounds which showed fluorescence in various
solvents. The lowest-energy absorption band of 1a was observed
⇑
Corresponding author. Address: Department of Organic and Polymeric Mate-
rials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552 Japan.
E-mail address: konishi.g.aa@m.titech.ac.jp (G. Konishi).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.01.096

1840
S. Hachiya et al. / Tetrahedron Letters 54 (2013) 1839–1841
methoxy or ethynyl group. While the lowest absorption maxima
of 3 and 4 were almost the same as those of 1 and 2, the extinction
coefficients of the lowest-energy absorption bands of 3 and 4 were
greater than those of 1 and 2. These results indicated that the do-
nor–acceptor systems at the C2–C6 and C3–C7 positions are sepa-
rated by the p-electronic conjugation system.
In the case of 2a and 4a, fluorescence could be observed only in
dichloromethane or chloroform, and the fluorescence quantum
yield, too, was very low (UF <0.01). The weak fluorescence of 1a–
4a could be attributed to the nitrophenyl-group-induced rapid
intersystem crossing from a singlet excited state to a triplet excited
state.5 Therefore, the linker strategy was an effective method to
induce fluorescence of nitronaphthalene derivatives because
1-nitronaphthalene showed no fluorescence under normal condi-
tion. The Stokes shift of almost nitronaphthalene derivatives over
10,000 cmÀ1. This is because the rapid distortion of nitro group by
photoexcitation caused dramatic change in their structure. The
solvent-dependent fluorescence properties were unique to the
nitrophenyl-substituted naphthalene derivatives. Upon the intro-
duction of donor groups at the C2 and C6 positions of the nitrophenyl
naphthalene derivatives, the fluorescence spectra showed a redshift
and UF increased. The maximum fluorescence quantum yield of 1b
Scheme 1. Synthesis of nitrophenyl naphthalene 1 and 3, and nitrophenylethynyl 2
and 4.
(
UF = 0.11) was obtained in dichloromethane. The fluorescence
intensity decay followed single-exponential kinetics. The fluores-
cence lifetime ( ) increased with an increase in UF. The fluorescence
rate constant kf and nonradiative relaxation constant knr were calcu-
lated from UF and
.9 The main factor that contributed to the fluores-
s
s
cence enhancement was the decrease in knr. In particular, in the case
of 1b in dichloromethane and chloroform, knr was one order of mag-
nitude smaller than that in the other cases. These results indicated
that the donor–acceptor system served to decrease knr, thereby
increasing UF. However, in our previous study,6a 2,7-bis(4-nitro-
phenyl)fluorene derivative exhibited the highest UF in DMF, not in
chloroform. As a result, there is no specific interaction for suppress-
ing knr between nitro group and halogen solvent in the excited state.
It is difficult to classify the fluorescence properties of nitrophenyl-
group-containing chromophores in the present stage.
Figure 2 shows the UV–vis absorption and fluorescence spectra
of 1b in various solvents. While the absorption spectra did not
show any obvious dependence on the solvent, solvatochromic fluo-
rescence spectra, which showed strong dependence on the solvent
polarity, were observed. The Stokes shift of 1b in various solvents
was very large, with the maximum value approaching 306 nm
(13,870 cmÀ1) in CH3CN. This value of Stokes shift was larger than
Figure 1. UV–vis absorption (solid line) and fluorescence (dashed line) spectra of
1a–4b in CH2Cl2. The compounds 1a and 3a did not show fluorescence. Excitation
wavelength for fluorescence measurements was absorption maxima.
at around 320 nm in various solvents. The absorption maxima of
1b and 2a were redshifted about 20 nm as compared to that of
1a, indicating extension of the
p-electronic conjugation by the
Table 1
Photophysical properties of 1b–4b in various solvents at 25 °C
(cmÀ1
)
UF
sa (ns)
kf (108 sÀ1
)
knr (108 sÀ1
)
a
a,b
Compound
Solvent
kabs (nm)
kem (nm)
D
m
1b
THF
344
346
345
341
338
339
356
360
356
336
337
347
361
362
360
534
573
591
647
536
546
533
592
635
544
578
527
514
577
593
10,340
11,450
12,070
13,870
10,500
11,180
9330
10,890
12,340
9270
10,050
9840
8250
10,290
10,910
0.006
0.055
0.11
0.33
1.87
2.04
0.22
0.25
0.35
0.57
0.77
0.32
0.62
0.53
0.27
0.25
0.90
0.38
0.18
0.29
0.54
0.27
0.20
0.17
0.44
0.39
0.47
0.16
0.17
0.26
0.36
0.59
0.50
30.1
5.05
4.36
45.2
39.8
20.8
17.1
12.6
30.8
16.0
18.7
37.8
39.6
10.5
25.8
CHCl3
CH2Cl2
CH3CN
CHCl3
CH2Cl2
THF
CHCl3
CH2Cl2
CHCl3
CH2Cl2
CH2Cl2
THF
0.006
0.005
0.006
0.025
0.030
0.015
0.010
0.009
0.007
0.009
0.053
0.019
2a
2b
3b
4a
4b
CHCl3
CH2Cl2
Data corresponding to no-fluorescence cases have been omitted.
a
The solutions were prepared with 0.1 absorption at kabs. kex = kabs
Measured as absolute quantum yield.
.
b

S. Hachiya et al. / Tetrahedron Letters 54 (2013) 1839–1841
1841
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fluorescence emission maximum, EF, and the Dimroth–Reichardt
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between EF and ET(30) was observed for 1b. This result suggested
that the solvatochromic fluorescence resulting from the donor
groups at the C2 position of 1b induced ICT. In general, the UF va-
lue of a fluorescent dye showing ICT character increases with
decreasing solvent polarity. The maximum UF of 1b (UF = 0.11)
was observed in chloroform, which is a weakly polar solvent, and
1b showed very weak fluorescence quantum yields (UF <0.01) in
other polar (acetonitrile) or non-polar (toluene) solvents. Thus,
the unique fluorescence properties of nitrophenyl-substituted
naphthalene derivatives could be explained by the solvent depen-
dence of the fluorescence quantum yield induced by the nitro-
phenyl groups and ICT character.
In conclusion, we successfully synthesized nitro-group-contain-
ing naphthalene derivatives that showed unique solvent-depen-
dent fluorescence. In particular, compound 1b showed
fluorescence with a large Stokes shift in weakly polar solvents
but no fluorescence in polar and non-polar solvents. The com-
pound showing fluorescence in weakly polar solvent only with a
large Stokes shift is remarkable, and it is expected that abovemen-
tioned compounds applied fluorophore of high resolution fluores-
cence sensor with a small signal noise ratio because of their large
Stokes shift. Our results suggested that the combination of nitro
group with linker method and donor moiety has a potential to de-
velop novel high resolution and desirable environmentally-respon-
sive fluorescence sensor6a which was a useful tool for in vivo
imaging. Further investigations of the fluorescence properties of
nitrophenyl-substituted naphthalene derivatives, in addition to
quantum chemical calculations and transient absorption studies,
are in progress.
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Supplementary data (experimental details, 1H NMR and 13C
NMR spectra, UV–vis absorption and fluorescence spectra) associ-
ated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2013.01.096.