Unexpected SiMe3 effect on color-tunable and fluorescent probes of dendritic polyphenyl naphthalimides with aggregation-induced emission enhancement

Article abstract of DOI:10.1039/c5tc03344f

In this paper, two new organic dyes derived from 1,8-naphthalimide and dendritic polyphenyl were designed and synthesized. Both the dyes exhibited unique aggregation-enhanced emission enhancement properties in methanol/water mixtures. The traditional fluorescent materials 1,8-naphthalimides were successfully transformed to AIE-active materials based on the dendritic polyphenyl structure. Furthermore, NPI-Ph and NPI-Si displayed excellent optical properties, such as solvent-induced emission changes from deep blue to light green, and the sensitive fluorescence response to nitroaromatic explosives. Interestingly, an unexpected SiMe₃ effect was found in the two dyes: the SiMe₃-containing compound NPI-Si exhibited remarkably enhanced optical properties compared with the non-SiMe₃ compound NPI-Ph such as a wider color-tunable range and higher sensitivity for the fluorescence detection of nitroaromatic explosives. The dendritic polyphenyl strategy and the SiMe₃ effect reported in this work will provide guidance to the design of AIE-active molecules and fluorescent materials for detecting nitroaromatic explosives.

Full text of DOI:10.1039/c5tc03344f

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Unexpected SiMe3 effect on color-tunable and
fluorescent probes of dendritic polyphenyl
naphthalimides with aggregation-induced
emission enhancement†
Cite this:DOI: 10.1039/c5tc03344f
Hua Wang, Yan Liang, Huanling Xie, Haifeng Lu, Shigui Zhao* and Shengyu Feng*
In this paper, two new organic dyes derived from 1,8-naphthalimide and dendritic polyphenyl were
designed and synthesized. Both the dyes exhibited unique aggregation-enhanced emission enhancement
properties in methanol/water mixtures. The traditional fluorescent materials 1,8-naphthalimides were
successfully transformed to AIE-active materials based on the dendritic polyphenyl structure. Furthermore,
NPI-Ph and NPI-Si displayed excellent optical properties, such as solvent-induced emission changes from
deep blue to light green, and the sensitive fluorescence response to nitroaromatic explosives. Interestingly,
an unexpected SiMe3 effect was found in the two dyes: the SiMe3-containing compound NPI-Si exhibited
remarkably enhanced optical properties compared with the non-SiMe3 compound NPI-Ph such as a wider
color-tunable range and higher sensitivity for the fluorescence detection of nitroaromatic explosives. The
dendritic polyphenyl strategy and the SiMe3 effect reported in this work will provide guidance to the design
of AIE-active molecules and fluorescent materials for detecting nitroaromatic explosives.
Received 15th October 2015,
Accepted 15th December 2015
DOI: 10.1039/c5tc03344f
www.rsc.org/MaterialsC
(HPB) was synthesized, and its optical properties were investigated
to prove our thought in this work. The synthesis process and
Introduction
Dendritic polyphenyl derivatives, which contain benzene with characterization were shown in the ESI.† It was exciting that HPB
multiple contiguous phenyl substituents, can be applied in exhibited excellent aggregation-enhanced emission (AIEE) (Fig. S1,
various areas in science and technology, including materials ESI†). According to the result, we believe that the HPB derivatives
for optoelectric devices,1–3 template materials of supramolecular with propeller molecular structures may probably exhibit AIEE
self-assembly,4,5 and grapheme precursors.6,7 In these compounds, properties.
aryl substituents are not constrained by the additional bonds in
1,8-Naphthalimides (NPIs) are recently considered as important
the aromatic core plane. Thus, aryl substituents exhibit large and organic fluorophores and can be extensively applied in fluorescent
twisted angles with the central benzene. These nonplanar topologies sensors, optoelectronic materials, and imaging agents, among
can hold conjugated molecules. Therefore, these compounds others.13–19 However, the applications of NPIs are greatly limited
typically show greater gaps between the highest-occupied (HOMO) because of the ‘‘aggregation-caused quenching’’ of fluorescence,
and lowest-unoccupied molecular orbitals (LUMO), lower degrees which can be attributed to the tendency of charge-separated
of self-association, less efficient packing, and higher solubility.8,9
planar p surfaces of NPIs to cause p–p stacking interactions in the
In 2007, Tang et al. found that compounds with biphenyl aggregated state.20 Therefore, the feasibility of transforming NPIs
derivatives exhibit aggregation-induced emission (AIE) proper- to AIE-active molecules will be considered as great development.
ties.10–12 The mechanism can be attributed to the phenyl However, only a few NPI-based AIE-active molecules have been
restriction of intramolecular rotations. In our opinion, dendritic reported up to now.21,22
polyphenyl derivatives which have similar structures to biphenyl
In this paper, two new NPI derivatives, namely, NPI-Ph and
probably have AIE properties. Therefore, hexaphenylbenzene NPI-Si, with molecular structures similar to that of HPB were
designed and synthesized (Scheme 1). It was exciting that both
School of Chemistry and Chemical Engineering, School of Material Science &
Engineer, Key Laboratory of Special Functional Aggregated Materials & Key
Laboratory of Colloid and Interface Chemistry (Shandong University), Ministry of
Education, Shandong University, Jinan, 250100, People’s Republic of China.
E-mail: fsy@sdu.edu.cn; Fax: +86-531-88564464; Tel: +86-531-88364866
† Electronic supplementary information (ESI) available: The synthesis, character-
ization of HPB. See DOI: 10.1039/c5tc03344f
the dyes exhibited unique AIEE properties. The result was a
good prove to our previous hypothesis: the dendritic polyphenyl
structure was a successful strategy which transforms the traditional
fluorescent materials to AIE-active materials. Furthermore, NPI-Ph
and NPI-Si displayed excellent optical properties, such as solvent-
induced emission changes from deep blue to light green,
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Synthesis of N-butyl-4-phenylacetylene-1,8-naphthalimide (3a)
The synthesis was conducted according to the published references.
2 (1.00 g, 3.00 mmol), phenylacetylene (0.306 g, 3.00 mmol),
Pd(PPh3)2Cl2 (112 mg, 0.16 mmol), PPh3 (40 mg, 0.16 mmol),
CuI (32 mg, 0.16 mmol), and 2 mL of triethylamine were added
to anhydrous ethanol (50 mL). The reaction was stirred under
reflux for 12 h under the argon atmosphere, cooled to room
temperature, and then filtered to obtain the product. Subsequently,
the crude product was purified by chromatography on silica gel
using a hexane/EA (10 : 1) mixture as an eluent. A light yellow
solid was then obtained. Yield: 0.66 g (55%).
Scheme 1 Molecular structures of HPB, NPI-Ph, and NPI-Si.
1H NMR (CDCl3, 400 MHz, ppm): d 8.66 (d, 1H), 8.59 (d, 1H),
8.57 (d, 1H), 7.98 (d, 1H), 7.86 (d, 1H), 7.70 (m, 1H), 7.46 (m, 1H),
4.21 (t, 2H), 1.73–1.79 (m, 2H), 1.44–1.53 (m, 2H), 0.99 (m, 3H).
HRMS (FAB) calcd C24H19NO2 for [M + H]+ 354.1494, found
354.1273.
and sensitive fluorescence response to nitroaromatic explosives.
Interestingly, an unexpected SiMe3 effect was found in the two
dyes: the SiMe3-containing compound NPI-Si exhibited remark-
ably enhanced optical properties compared with the non-SiMe3
compound NPI-Ph such as a wider color-tunable range and
higher sensitivity for the fluorescence detection of nitroaromatic
explosives.
Synthesis of N-butyl-4-trimethylsilylethynyl-1,8-naphthalimide (3b)
The synthesis was performed according to the published references.
2 (1.00 g, 3.00 mmol), ethynyltrimethylsilane (0.306 g, 3.00 mmol),
Pd(PPh3)2Cl2 (112 mg, 0.16 mmol), PPh3 (40 mg, 0.16 mmol), CuI
(32 mg, 0.16 mmol), and 2 mL of triethylamine were added to
anhydrous ethanol (50 mL). The reaction was stirred under reflux
for 12 h under the argon atmosphere, cooled to room temperature,
and then filtered to obtain the product. The crude product was
then purified by chromatography on silica gel using a hexane/EA
(10 : 1) mixture as an eluent. A light yellow solid was then
obtained. Yield: 0.66 g (55%).
Experimental section
Material
4-Bromo-1,8-naphthalic anhydride, n-butylamine, phenylacety-
lene, Pd(PPh3)2Cl2, PPh3, CuI, triethylamine, and 2,3,4,5-tetra-
phenylcyclopenta-2,4-dienone were purchased from Aladdin
Industrial Company. Ethynyltrimethylsilane was obtained from
Beijing HWRK Chem. Co. LTD and used after fractionation
(Scheme 2).
1H NMR (CDCl3, 400 MHz, ppm): d 8.66 (d, 1H), 8.59 (d, 1H),
8.57 (d, 1H), 7.98 (d, 1H), 7.86 (d, 1H), 7.70 (m, 1H), 7.46 (m, 1H),
4.21 (t, 2H), 1.73–1.79 (m, 2H), 1.44–1.53 (m, 2H), 1.00 (m, 3H).
HRMS (FAB) calcd C21H23NO2Si for [M + H]+ 350.1576, found
350.1540.
Synthesis of N-butyl-4-bromo-1,8-naphthalimide (2)
The synthesis was performed according to the published references.
4-Bromo-1,8-naphthalic anhydride (1) (1.38 g, 5 mmol) and
n-butylamine (0.73 g, 10 mmol) were dissolved in 100 mL of
ethanol. The reaction mixture was stirred and refluxed for 5 h.
The crude product was obtained by filtration after the mixture
was cooled to room temperature. The crude product was then
purified by silica gel column chromatography using CH2Cl2/
hexane (2 : 1, v/v) as an eluent to afford a white solid product.
Yield: 1.22 g (70%).
Synthesis of N-butyl-4-pentaphenylbenzene-1,8-naphthalimide
(NPI-Ph)
3a (0.5 g, 1.42 mmol) and 2,3,4,5-tetraphenylcyclopenta-2,4-
dienone (0.54, 1.42 mmol) were added to 10 mL of diphenyl
ether. The reaction was stirred under reflux for 30 h under the
argon atmosphere and cooled to room temperature. The mixture
was then poured into 200 mL of methanol to obtain the crude
product. The crude product was purified by chromatography on
silica gel using a hexane/EA (10 : 1) mixture as an eluent. A light
yellow solid was then obtained. Yield: 0.63 g (62%).
1H NMR (CDCl3, 400 MHz, ppm): d 8.67 (d, 1H), 8.60 (d, 1H),
8.58 (d, 1H), 8.07 (d, 1H), 7.85–7.89 (m, 1H), 4.19 (t, 2H),
1.70–1.78 (m, 2H), 1.44–1.55 (m, 2H), 1.00 (t, 3H). HRMS (FAB)
calcd for C16H14BrNO2 [M + H]+ 332.0286, found 332.0027.
1H NMR (DMSO, 400 MHz, ppm): d 8.33 (d, 1H), 8.25 (d, 1H),
8.08 (d, 1H), 7.77 (m, 1H), 7.66 (d, 1H), 6.98–7.00 (m, 5H), 6.76–6.93
(m, 14H), 6.62–6.69 (m, 4H), 6.52 (m, 2H), 3.91 (t, 2H), 1.50–1.54
(m, 2H), 1.26–1.32 (m, 2H), 0.88 (m, 3H). 13C NMR (CDCl3, 400 MHz,
ppm): d 164.31, 164.22, 146.07, 141.64, 140.80, 140.74, 140.20,
139.96, 139.66, 136.56, 133.41, 131.33, 131.29, 130.83, 130.70,
130.59, 130.14, 129.58, 127.45, 126.75, 126.70, 126.63, 126.61,
125.83, 125.75, 125.48, 122.33, 120.71, 40.22, 30.20, 20.42, 13.85.
HRMS (FAB) calcd C52H39NO2 for [M + H]+ 710.3059, found
710.2901. Anal.calcd for C52H39NO2: C 87.98%, H 5.54%, N
1.97%, O 4.51%; found: C 87.91%, H 5.63%, N 2.01%, O 4.42%.
Scheme 2 The synthesis process of NPI-Ph and NPI-Si.
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Synthesis of N-butyl-4-trimethylsilyl-tetraphenylbenzene-1,8-
naphthalimide (NPI-Si)
3b (0.5 g, 1.42 mmol) and 2,3,4,5-tetraphenylcyclopenta-2,4-
dienone (0.54, 1.42 mmol) were added to 10 mL of diphenyl
ether. The reaction was stirred under reflux for 30 h under the
argon atmosphere and cooled to room temperature. The mixture
was then poured into 200 mL of methanol to obtain the crude
product. The crude product was purified by chromatography on
silica gel using a hexane/EA (10 : 1) mixture as an eluent. A light
yellow solid was then obtained. Yield: 0.63 g (62%).
1H NMR (DMSO, 400 MHz, ppm): d 8.42 (d, 1H), 8.33 (d, 1H),
8.22 (d, 1H), 7.88 (d, 1H), 7.82 (t, 1H), 7.03–7.24 (m, 7H),
6.69–6.91 (m, 12H), 6.53 (t, 1H), 6.45 (t, 1H), 3.99 (t, 2H),
1.58–1.61 (m, 2H), 1.31–1.37 (m, 2H), 0.90–0.94 (m, 3H), 0.01
(s, 1H). 13C NMR (CDCl3, 400 MHz, ppm): d 143.35, 142.28,
141.67, 140.58, 140.21, 139.92, 139.44, 137.39, 134.02, 132.09,
132.01, 131.63, 131.31, 130.96, 130.89, 130.68, 130.55, 130.31,
129.62, 127.64, 126.93, 126.80, 126.65, 126.58, 126.55, 126.29,
126.20, 126.13, 125.97, 125.44, 125.27, 122.41, 121.48, 40.33,
30.24, 20.44, 13.86, 3.06. 29Si NMR (CDCl3, 400 MHz, ppm):
d À4.26. HRMS (FAB) calcd C49H43NO2Si for [M + H]+ 706.3141,
found 706.3000. Anal. calcd for C49H43NO2Si: C 83.37%, H 6.14%,
N 1.98%, O 4.53%; found: C 83.40%, H 6.17%, N 1.99%, O 4.60%.
Fig. 2 (a) Normalized emissions of NPI-Ph (a) and NPI-Si (b) in different
solvents and digital images of NPI-Ph in different solvents under UV
irradiation at 365 nm.
Table 1 The fluorescence emission data of NPI-Ph and NPI-Si in different
solvents
Hexane
em/nm
Toluene
em/nm
CH2Cl2
em/nm
DMF
em/nm
CH3OH
lem/nm
Compounds
l
l
l
l
NPI-Ph
NPI-Si
396
401
409
415
427
441
437
458
462
500
these results were different from NPI-Si. The dye NPI-Si displayed
blue fluorescence (lem = 401 nm) in hexane and green emission
(lem = 500 nm) in CH3OH, and the shifts of these emissions were
as large as 103 nm. These large shifts in different solvents
demonstrate that the two dyes show a solvatochromic effect.
However, the effects of the polarities of the solvents on the two
dyes were quite different. NPI-Si exhibited a wider color-tunable
range than NPI-Ph. NPIs were the electron acceptors in the two
dyes. Contrary to NPI-Ph, NPI-Si showed an asymmetrical molecular
structure. Moreover, ‘‘dp–pp’’ existed between the Si and benzene
ring which reduced the electron density of the center phenyl ring
and enhanced the molecular polarity. Therefore, NPI-Si showed
larger molecular polarity than NPI-Ph, which resulted in more
remarkable solvent-induced emission changes.
Results and discussion
Fig. 1 shows the UV-visible absorption and fluorescence emission
spectra of NPI-Ph and NPI-Si in CH2Cl2 solution. The absorption
bands for NPI-Ph were observed at 242, 346, and 363 nm, whereas
those for NPI-Si were observed at 240, 345, and 363 nm. The
fluorescence emission bands in CH2Cl2 solution for NPI-Ph and
NPI-Si were observed at 416 and 427 nm at 298 K. And the
fluorescence bands at 77 K for two dyes were 406 and 407 nm.
The two compounds exhibited similar UV-visible absorptions
and the fluorescence emission spectra in dilute CH2Cl2 solution.
The emission bands of NPI-Ph and NPI-Si showed a significant
bathochromic shift when solvent polarity was increased (Fig. 2).
Detailed data are provided in Table 1.
The large shifts in the different solvents and at different
temperatures demonstrate that the two dyes show twisted
intramolecular charge transfer (TICT) activities.22 The fluorescence
intensities of NPI-Ph and NPI-Si decreased when the solvent
polarity increased. Interestingly, the emission intensity of NPI-Ph
and NPI-Si dramatically increased when the H2O fraction of
50% (v/v%) was added into the CH3OH solution (Fig. 3). This
phenomenon is a characteristic of AIEE.
NPI-Ph displayed blue fluorescence (lem = 396 nm) in hexane
and green emission (lem = 463 nm) in CH3OH, and the shifts
of these emissions were as large as 67 nm. Interestingly,
The AIEE process was directly investigated by plotting the
graph of emission area versus different H2O fractions in CH3OH,
and a clear trend was observed. According to the Tang’s report,
the restriction of intramolecular rotation was inferred to be the
primary factor of the AIEE effect.10–12 The TICT feature dominated
the emission intensity of NPI-Ph and that of NPI-Si decreased at
small fractions of H2O. By contrast, NPI-Ph and NPI-Si aggregated
in CH3OH mixed solvents with the 50% H2O fraction, and
the emission intensity quickly increased. However, emission
intensity began to decrease when the H2O fraction exceeded 50%.
Fig. 1 UV-visible absorption spectra (left) and the fluorescence emission
spectra (right, lex = 360 nm) of NPI-Ph and NPI-Si in 10 mM CH2Cl2
solution.
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Fig. 4 The SEM pictures of NPI-Ph (a) and NPI-Si (b) aggregates in
CH3OH–H2O (H2O fraction: 50%); the DLS spectra of NPI-Ph (c) and
NPI-Si (d) aggregates in CH3OH–H2O (H2O fraction: 50%).
Fig. 3 Fluorescence spectra of NPI-Ph (a) and NPI-Si (b) in the CH3OH–H2O
mixture with different H2O fractions. (c) Plots of fluorescent areas of NPI-Ph
and NPI-Si versus the water fraction in the CH3OH–H2O mixture (d) digital
images of NPI-Ph and NPI-Si with different H2O fractions under UV
irradiation at 365 nm.
This phenomenon can also be attributed to TICT. The variation
of fluorescence intensity exhibited a unique ‘‘Z’’ shape when the
H2O fraction was increased from 0% to 90%. The absorptions of
NPI-Ph and NPI-Si in H2O fractions of 0%, 50%, and 90% are
illustrated in Fig. S2 (ESI†). The absorption spectra of the two
dyes at the 50% H2O fraction show bands that differ from those
at H2O fractions of 0% and 90%. Therefore, the NPI-Ph and NPI-Si
molecules have different packing aggregation states which lead to
AEE. In addition, the aggregate of NPI-Si in H2O was stable for
about two months, whereas that of NPI-Ph subsided after three Fig. 5 Frontier-molecular-orbitals of the density functional theory-
optimized ground state structures of NPI-Ph and NPI-Si.
days. Therefore, NPI-Si can be well used in water or other mixed
solvents with strong fluorescence.
The scanning electron microscope (SEM) and dynamic light
scattering (DLS) were proceeded to further confirm the formation This finding indicates that the two dyes have similar FMOs
of molecular (NPI-Ph and NPI-Si) aggregation. From the SEM and energy gaps, which resulted in the similar absorption and
pictures in Fig. 4(a) and (b), it could be clearly seen that the emission spectra in dilute solution.
molecular aggregates were formed when water was added. As
In recent days, the development of highly sensitive fluorescent
revealed by DLS analysis, the average particle diameter becomes sensor materials for the detection of explosives at low concen-
larger with the increase of the H2O fraction. The tendency was tration is still a challenge.23,24 The fluorescent AIE-active materials
same to Tang’s reports.11,12 The average particle diameters for have been proved to be the fantastic selection for fluorescence
NPI-Ph were 93 nm, 119 nm and 153 nm, whereas those for detection.25,26 Thus, NPI-Ph and NPI-Si with excellent fluorescence
NPI-Si were 78 nm, 102 nm and 139 nm in the H2O–CH3OH properties are expected to work as excellent fluorescent sensors for
(H2O fraction: 40%, 50% and 60%) mixture, respectively.
detecting electron-deficient quenchers, such as nitroaromatic
The structures of NPI-Ph and NPI-Si in the gas phase were explosives.27,28 In this work, we used picric acid (PA) as a model
optimized at the B3LYP/6-31G* levels. The frontier-molecular- explosive. The quenching processes of the PL of the dyes were
orbitals (FMOs) of NPI-Ph and NPI-Si were concentrated on the investigated by monitoring the changes in their PL in response
NPI moiety rather than on the phenyl substituent (Fig. 5). to PA addition.
Hence, their photophysical characteristics, i.e., spectral absorption
The PL intensities of the NPI-Ph and NPI-Si were both
and emission regions, were expected to be similar and dominated progressively weakened when an increasing amount of PA
by the NPI unit. The phenyl groups moderately contributed to was added into the solution with nanoaggregates in the mixture
the HOMO levels but did not contribute to the LUMOs. This solvents. The fluorescence spectra in Fig. 6 show that virtually
characteristic can be considered as a signature of a weak electron no light was emitted from the nanoaggregates in the 50% aqueous
donor. The energy levels of the HOMO and LUMO for NPI-Ph were mixture when the PA concentration was increased to 60 mM.
À6.09 and À2.25 eV, respectively, whereas the corresponding
The Stern–Volmer plot of relative PL intensity (I0/I) versus PA
values for NPI-Si were À6.08 and À2.28 eV. The energy gaps concentration in the aqueous mixtures with the 50 vol% H2O
for NPI-Ph and NPI-Si were 3.84 and 3.80 eV, respectively. content shows a curve bending upward, instead of a linear
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this work. The two dyes showed unique AIEE characteristics.
Therefore, the dendritic polyphenyl structure was a successful
strategy which transforms the traditional fluorescent materials
to AIE-active materials. Furthermore, NPI-Ph and NPI-Si displayed
excellent optical properties, such as solvent-induced emission
changes from deep blue to light green, and the sensitive
fluorescence response to nitroaromatic explosives. Interestingly,
an unexpected SiMe3 effect was found in the two dyes: the
SiMe3-containing compound NPI-Si exhibited remarkably
enhanced optical properties compared with the non-SiMe3
compound NPI-Ph such as a wider color-tunable range and higher
sensitivity for the fluorescence detection of nitroaromatic explosives.
The dendritic polyphenyl strategy and the SiMe3 effect reported in
this work will provide guidance to the molecular structural design
for detecting nitroaromatic explosives.
Fig. 6 Changes in the PL spectra of NPI-Ph (a) and NPI-Si (b) with the
addition of different amounts of PA in the THF–water mixture (5 : 5 v/v);
lex = 322 nm; c = 5 mM. (c) Stern–Volmer plots of I0/I versus [PA] in
THF–water mixtures with fw of 50 vol%.
Acknowledgements
This work was financially supported by the Shandong special
fund for independent innovation and achievements transfor-
mation (No. 2014ZZCX01101), the National Natural Science
Foundation of China (No. 21274080) and the China Postdoc-
toral Science Foundation (No. 2015M580590).
Notes and references
Fig. 7 (a) The fluorescence emission spectra of NPI-Ph and NPI-Si in the
solid state; (b) the XRD spectra of NPI-Ph and NPI-Si solid powder.
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dyes have a superamplification effect in the detection of nitro-
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