Aggregation-induced emission, multiple chromisms and self-organization of N-substituted-1,8-naphthalimides

Article abstract of DOI:10.1016/j.dyepig.2017.08.013

Most aggregation-induced emission (AIE) luminogens possess propeller-like aromatic stator-rotor structures, such as silole, tetraphenylethylene (TPE) and triphenylamine (TPA), to restrict intra-molecular motions (RIM) and avoid aggregation-caused quenchin

Full text of DOI:10.1016/j.dyepig.2017.08.013

Accepted Manuscript
Aggregation-induced emission, multiple chromisms and self-organization of N-
substituted-1,8-naphthalimides
Shusheng Ge, Baoyan Li, Xinlei Meng, Han Yan, Mei Yang, Ben Dong, Yun Lu
PII:
S0143-7208(17)30886-0
10.1016/j.dyepig.2017.08.013
DYPI 6175
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 19 April 2017
Revised Date: 9 July 2017
Accepted Date: 9 August 2017
Please cite this article as: Ge S, Li B, Meng X, Yan H, Yang M, Dong B, Lu Y, Aggregation-induced
emission, multiple chromisms and self-organization of N-substituted-1,8-naphthalimides, Dyes and
Pigments (2017), doi: 10.1016/j.dyepig.2017.08.013.
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Aggregation-Induced Emission, Multiple Chromisms and
Self-Organization of N-substituted-1,8-Naphthalimides
3
Shusheng Ge, Baoyan Li, Xinlei Meng, Han Yan, Mei Yang, Ben Dong, Yun Lu*
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Department of Polymer Science and Engineering, State Key Laboratory of
Coordination Chemistry, Collaborative Innovation Center of Chemistry for Life
Sciences, Key Laboratory of High Performance Polymer Materials and Technology
(Nanjing University), Ministry of Education, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, P. R. China
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*Corresponding author.
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E-mail: yunlu@nju.edu.cn
11 Abstract
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Most aggregation-induced emission (AIE) luminogens possess propeller-like aromatic
stator-rotor structures, such as silole, tetraphenylethylene (TPE) and triphenylamine
(TPA), to restrict intra-molecular motions (RIM) and avoid aggregation-caused
quenching (ACQ) effects. In our work, two simple N-substituted naphthalimides were
designed and synthesized by the amide condensation to create novel AIE systems.
Except for the same aromatic stator (naphthalimide), N-phenyl-1,8-naphthalimide
(PhNI)
contained
an
aromatic
rotor
(phenyl
group)
while
N-cyclohexyl-1,8-naphthalimide (CyNI) contained an aliphatic rotor (cyclohexyl
group). The PhNI samples obtained from different preparation processes showed
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multiple chromisms (MC) effects and varied levels of luminescence. Despite of the
same intra-molecular structure, the crystal of PhNI had staggered parallel
inter-molecular conformation with weak emission but the precipitation of PhNI had
cross one with strong emission. Although without MC effects, CyNI gave an excellent
fluorescence quantum yield (Φf = 0.55). The experimental results and theoretical
analyses suggested that the AIE effects were decided by intra- and inter-molecular
structure simultaneously and the construction and destruction of inter-molecular π-π
stacking between aromatic rotors in PhNI resulted in the MC effects.
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Keywords: Naphthalimide; Aggregation-induced emission; Multiple chromisms;
Self-organization; Inter-molecular structure.
31 Introduction
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Most common planar luminogens, such as pyrene, naphthalimide and others, are
emissive in solution state but non-emissive in solid state due to aggregation-caused
quenching (ACQ). According to the reported quantum-chemical calculations, the
cofacial configurations (π-π stacking) in the molecules are responsible for the decay
of luminescence.[1] Theoretically, effective methods to solve this problem are
separating these molecules through the introduction of large steric hindrance groups
or achieving the rotation of the molecules around the stacking axis while keeping the
parallelism between the molecular planes.[2,3]
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Nowadays, an abnormal phenomenon called aggregation-induced emission (AIE)
is observed, in which some non-planar luminogens are emissive in solid state while
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none-emissive in solution state.[4,5] These AIE luminogens are usually composed of
a central stator, normally aromatic planar fluorescent molecule, and several steric
aromatic planar rotors linked to the stator.[6] The restriction of intra-molecular motion
(RIM) in the aggregation is accounted as the reason for the brightening. This new
progress offers a large platform for exploring practically useful luminescent materials
in solid state. So far diverse AIE luminogens with tuneable emission colors and high
quantum yields in solid state have been generated, which show the potential
applications in numerous fields including optics, electronics, energy and
bioscience.[7-9] Nevertheless, there are still some issues that need to be improved.
First of all, most non-planar AIE luminogens reported in recent years are prepared
based on coupling reactions with noble metal catalyst, which are neither convenient
for preparation nor friendly to environment.[10,11] Secondly, for most stator-rotor
systems, the steric aromatic planar rotors, which are used to break up the π-π stacking
between stators and increase the fluorescence quantum yield, could form extra π-π
stacking between themselves and reduce the fluorescence quantum yield. Thirdly, the
relation between inter-molecular stacking modes and photoluminescent properties for
AIE luminogens is still not so clear owing to the uncertainty, complexity and
variability of the stacking modes.
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On the other hand, the multiple chromisms (MC) effect resulting from external
stimuli often appears in the poly-aryl AIE systems with plenty of π-π stacking
structures.[12] These MC materials have showed the promising applications in
lighting, sensing and displaying.[13-17] But their complex inter- and intra-molecular
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structures based on large amounts of aromatic rings make it difficult to describe the
mechanism clearly.
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Herein, the naphthalimide moieties were first introduced into AIE systems as
twisted structures rather than connecting ones or donors/acceptors.[18-24] On the
basis of reported calculation, N-phenyl-1,8-naphthalimide (PhNI) and
N-cyclohexyl-1,8-naphthalimide (CyNI) shared the similar twisted cross
conformation.[25] Compared with phenyl group, cyclohexyl group could be not only
a huge group with the occupation functionality to break up the π-π stacking between
stators but also a non-conjugated one without forming extra π-π stacking between
rotors during aggregation. The samples of PhNI obtained from different preparation
processes showed changeable emission while the corresponding samples of CyNI
showed stable emission. The results demonstrated that the π-π stacking between
conjugated rotors (phenyl groups) affected the photoluminescent properties. Further
structural characterization illustrated that the crystals of PhNI and CyNI adopted
J-aggregation and X-aggregation respectively, suggesting that the inter-molecular
conformation had a profound impact on AIE and MC effects.
80 Experimental section
81 Measurements
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82
83
84
1H and C NMR measurements were recorded on a Bruker AVANCE 400 MHz
spectrometer, using tetramethylsilane (TMS) as an internal standard. Electron
ionization mass spectra (EI-MS) was measured using ThermoFisher spectrometers
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(TRACE 1300 GC-MS). Fourier transform infrared (FT-IR) spectra were obtained on
a BRUKER TENSOR 27 spectrometer (KBr pellet). UV-Vis spectra were recorded on
a SHIMADZU UV-3600 spectrophotometer. Photoluminescence emission spectra
were obtained with HORIBA Fluoromax-4 spectrofluorometer. The absolute
luminescence quantum yield (ΦF) was measured by a FLS920 fluorescence
spectrophotometer. Thermogravimetric analysis (TGA) was completed on
PerkinElmer Pyris 1 TGA with heating rate of 20 °C/min from 50 °C to 600 °C under
nitrogen atmosphere. Differential scanning calorimetry (DSC) was conducted by
PerkinElmer Pyris 1 DSC with heating rate of 10 °C/min from 50 °C to 300 °C under
nitrogen atmosphere. X-ray diffraction (XRD) patterns were obtained by using a
Bruker X-ray diffractometer (D8 ADVANCE, Germany) with Cu Kα radiation source
(40kV, 40mA). Morphologies of the samples were observed by scanning electron
microscopy (SEM, Hitachi S-4800). The single-crystal X-ray diffraction data for the
crystal of PhNI prepared with 50 mg sample and 50 mL THF/water (1:1 by volume)
as solvent were collected from a BRUKER D8 VENTURE system with Cu-Kα
radiation (λ=1.54178 Å) at 280(10) K. The structure was solved using direct methods
following the difference Fourier syntheses. All non-hydrogen atoms were
anisotropically refined through least-squares on F2 using the SHELXTL program
suite. The anisotropic thermal parameters were assigned to all non-hydrogen atoms.
The hydrogen atoms attached to carbon were placed in idealized positions and refined
using a riding model to the atom from which they were attached. The pictures of
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structure were produced using Diamond 3.1. CCDC 1537487 contains the
supplementary crystallographic data of this paper.
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108 Synthesis
109
110
N-phenyl-1,8-naphthalimide (PhNI)
1,8-Naphthalic anhydride (6.00 g, 30 mmol) and aniline (5.50 mL, 60 mmol)
111 were added to 50 mL acetic acid. After reflux for 6 h, the white suspension
112 turned into a black homogeneous solution. Then the reaction system was cooled
113 to room temperature with continuous stirring and a white precipitate formed.
114 After filtration, the residue was washed with 20 mL acetic acid twice and dried
115 with a vacuum drying oven at room temperature for 24h. The PhNI compound
1
116 was obtained as a white powder in 82% yield (6.76 g). H NMR (400 MHz,
117 CDCl3, 25 °C, TMS) δ (ppm):8.63 (dd,
J = 7.3, 1.0 Hz, 2H); 8.25 (dd, J = 8.3,
118 1.0 Hz, 2H); 7.77 (dd,
J
= 8.1, 7.4 Hz, 2H); 7.62 – 7.53 (m, 2H); 7.53 – 7.46 (m,
13
119 1H); 7.38 – 7.30 (m, 2H). C NMR (101 MHz, CDCl3, 25 °C, TMS) δ (ppm):
120 164.18, 135.33, 134.14, 131.56, 131.41, 129.26, 128.56, 128.53, 128.33, 126.89,
121 122.62. FT-IR (KBr) υ (cm-1): 3403, 3353, 3137, 3071, 1870, 1777, 1703, 1662,
122 1488, 1583, 1357, 1239, 1190, 888, 779, 701. EI-MS: m/z: calcd: 273.08.
123 Found: 272.07 [M-1]+. Anal. Calc. for C18H11NO2: C 79.11%, H 4.06%, N
124 5.13%; found: C 79.14%, H 4.25%, N 5.18%.
125
126
N-cyclohexyl-1,8-naphthalimide (CyNI)
1,8-Naphthalic anhydride (6.00 g, 30 mmol), cyclohexylamine (6.90mL, 60
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mmol), and piperidine (1.0 mL) were added to 50 mL 2-ethoxyethanol. After reflux
for 6 h, the white suspension turned into a black homogeneous solution. Then the
reaction system was cooled to room temperature with continuous stirring and a pale
yellow precipitate formed. After filtration, the residue was washed with 20 mL
2-ethoxyethanol twice and dried with a vacuum drying oven at room temperature for
24h. The CyNI compound was obtained as a pale yellow powder in 55% yield (4.63
g). 1H NMR (400 MHz, CDCl3, 25 °C, TMS) δ (ppm): 8.55 (dd, J = 7.3, 1.0 Hz,2H),
8.16 (dd, J = 8.3, 0.9 Hz,2H), 7.72 (dd, J = 8.1, 7.4 Hz,2H), 5.03 (tt, J = 12.2, 3.7
13
Hz,1H), 2.55 (qd, J = 12.3, 3.3 Hz,2H), 1.95 – 1.22 (m, 8H). C NMR (101 MHz,
CDCl3, 25 °C, TMS) δ (ppm): 164.57, 133.40, 131.36, 130.96, 128.10, 126.85, 123.23,
53.70, 29.06, 26.52, 25.42. FT-IR (KBr) υ (cm-1): 3400, 3133, 2928, 2853, 2866,
1694, 1655, 1589, 1399, 1237, 1185, 1101, 845, 774, 665, 542. EI-MS: m/z: calcd:
279.13. Found: 279.10 [M]+. Anal. Calc. for C18H17NO2: C 77.40%, H 6.13%, N
5.01%; found: C 77.43%, H 6.20%, N 5.07%.
141 Results and discussion
142 Synthesis of PhNI and CyNI
143
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147
Scheme 1 described the synthesis of PhNI and CyNI. Both of them were
synthesized via an one-step amide condensation[26,27]. Considering the different
nucleophilicity of aniline and cyclohexylamine, different acid-base catalysts were
used in the synthetic reactions. The structures of the two molecules were confirmed
1
by H NMR,13C NMR, gas chromatography-mass spectrometry (GC-MS) and
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148
elemental analysis.
149
150
Scheme 1. Synthesis of PhNI and CyNI.
151
152 AIE properties of PhNI and CyNI
153
The UV-vis absorption spectra and PL spectra of PhNI and CyNI
154 respectively in dilute THF solution and in THF solution with increasing water
155 content (fw) were provided in Fig. 1. For both PhNI and CyNI, a characteristic
156 absorption maximum (λmax) appeared at ~330nm, implying their similar rigid
157 conjugated mother compound of naphthalimide. With increasing water content
158
(fw) in THF solution, CyNI exhibited only one emission peak at ~470 nm (Fig.
159 1B), while PhNI showed two peaks at ~400 nm and ~470 nm respectively (Fig.
160 1A), suggesting its polymorphism or the possible solute-solvent interaction.
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161 The photographs taken under UV light (inset in Fig. 1A and 1B) revealed that
162 PhNI and CyNI aggregated and brightened with increasing water content,
163 showing their typical AIE effects.
164
165
166
167
168
Fig. 1. Normalized UV-PL spectra of (A) PhNI and (B) CyNI in THF with increasing
fw (the solution concentrations are 10-5 M for UV test and 10-3 M for PL test; λex = 330
nm), the insets in (A) and (B) are fluorescence images of PhNI and CyNI
respectively in THF with increasing fw under UV light (λex = 365 nm).
169
170
The PhNI samples prepared under different conditions and with various
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171 external stimuli showed different photophysical properties in solid state (Fig.
172 2A), suggesting the presence of MC effect. For instance, the crystal of PhNI
173
174 sandpaper, it partly changed to white powder (PhNI-G) with strong
175 bluish-green emission. The precipitation of PhNI PhNI-P) made by titrating
(PhNI-C) was white solid with weak violet emission. After grinding with
(
176 saturated THF solution of PhNI into large amount water quickly and filtrating
177 immediately gave pale-yellow powder with strong blue emission. After fuming
178 with THF vapor for 24h, it changed to white powder (PhNI-F) with weak violet
179 emission. Treating the crystal at 210 for 10 min, it melted into a transparency
180 liquid. If the cooling process was slow (annealing), the liquid turned into white
181 powder (PhNI-A) with weak violet emission. But with a fast cooling process
182 (quenching), the liquid turned into yellow powder (PhNI-Q) with strong blue
183 emission. Such mechanochromism and thermochromism effects of PhNI
184 samples were further verified by PL spectra (Fig. 2B). After grinding, the
185 emission peak of PhNI-C red-shifted from 430 nmto 505 nm. After fuming, the
186 emission peak of PhNI-P blue-shifted from 455nm to 420 nm. Through
187 different cooling process, the emission peak of PhNI-A was located at 430 nm
188 while PhNI-Q at 455 nm. Based on the fluorescence images and
189 photoluminescence spectra of PhNI samples, the two emission peaks in Fig. 1A
190 could be attributed to the polymorphism of PhNI instead of the solute-solvent
191 interaction. By contrast, the luminous intensities and wavelengths of CyNI
192 samples were stable, disregarding preparation processes and external stimuli
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193 (Fig. 2C and 2D), indicating the absence of MC effect. The UV-Vis absorption
194 spectra of PhNI samples (Fig. S1) showed that the stable samples (crystal, fume
195 and anneal) have the maximum absorption peaks at about 350nm while the
196 unstable samples (precipitation, grind and quench) at about 390nm, indicating
197 their different energy gaps between the ground state and the lowest excited state
198 as well as thus resulting different brightness. The photophysical data
199 (absorption, emission and fluorescence quantum yields) of PhNI and CyNI
200 samples in different solid states and solution states were listed in Table 1. Since
201 PhNI and CyNI shared the same fluorescence core of naphthalimide, the
202 difference in their photophysical properties should be relevant to the different
203 stacking modes caused by the different substituents and the different
204 preparation process.
205 Table 1 Photophysical data (absorption, emission and fluorescence quantum
206 yields) of PhNI and CyNI samples in different solid states and solution states.
state
Sample
PhNI
Abs. (nm)
330nm
330nm
355nm
385nm
393nm
Em. (nm)
-
Φf
0
solution
CyNI
-
0
PhNI-C
PhNI-P
PhNI-G
PhNI-F
430nm
455nm
505nm
420nm
0.06
0.51
0.45
0.10
solid
353nm
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PhNI-A
PhNI-Q
CyNI
345nm
396nm
398nm
430nm
455nm
470nm
0.15
0.45
0.56
207
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208
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Fig. 2. Fluorescence images (λex = 365 nm) and photoluminescence spectra (λex = 330
nm) of the different PhNI samples (A and B) and the different CyNI samples (C and
D).
209
210
211
212
213 MC properties of PhNI
214
To recognize the difference in inter-molecular structure ( π-π stacking)
215 between different PhNI samples and clear up the cause of the MC effect, the
216 phase identification of PhNI was undertaken with powder X-ray diffraction
217 (XRD). In Fig. 3A, the XRD pattern of PhNI-C exhibited a sharp profile,
218 indicating its well-ordered microcrystalline structures. According to Bragg
219 equation, the peak at 2
220 corresponded with the interlayer distance of 3.48Å. Similarly, the peak at
221 =30.3° (blue circle) corresponded with the interlayer distance of 2.95Å. Both
θ=25.4° (green circle) in the pattern of PhNI-C
2
θ
222 of them matched well with the calculated interlayer distance from face index
223 (3.49 Å for the interlayer distance of π-π stacking between naphthalimides and
224 2.96 Å for the interlayer distance of π-π stacking between phenyl groups in Fig.
225 4) and the simulated pattern from single crystal diffraction of PhNI-C (Fig.
226 3B). But in the pattern of PhNI-G, the decrease of Ph-Ph peak (the interlayer
227 distance of π-π stacking between phenyl groups) and the retained NI-NI peak
228 (the interlayer distance of π-π stacking between naphthalimides) implied the
229 destruction of π-π stacking between phenyl groups. There were no difference
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230 between the pattern of PhNI-P and PhNI-F, which suggested that only the
231 surface structure of PhNI-P was changed by fuming with THF vapor, thus
232 leading to the difference in fluorescence data but the similarity in XRD pattern.
233 Besides, PhNI-A had more peaks than PhNI-Q, indicating its more stability.
234 Moreover, the location of NI-NI peak and Ph-Ph peak in the pattern of
235 heat-treated PhNI samples changed a little, different from other PhNI samples.
236 The above-mentioned XRD results (Fig. 3A) and fluorescence data (Fig. 2A
237 and 2B) of PhNI samples illustrated that the samples with strong Ph-Ph peak
238 exhibited weak fluorescence and vice versa. Briefly, the fluorescence intensities
239 showed negative correlation with the presence of π-π stacking between phenyl
240 groups.
241
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Fig. 3. (A) XRD curves of the different PhNI samples, (B) simulated pattern from
single crystal diffraction of PhNI-C and (C) DSC curves of the different PhNI
samples.
242
243
244
245
246
Further characterization on the π-π stacking of the different PhNI samples
247 was done with differential scanning calorimetry (DSC). Comparing the first
248 heating curves of PhNI samples (Fig. 3C), besides the common melting peak at
249 202 , PhNI-Q exhibited an extra exothermic cold crystallization peak (1.5
250 kJ/mol) than PhNI-A at 168 , implying its more instability. PhNI-C showed
251 an extra endothermic phase transformation peak (2.0 kJ/mol) than PhNI-A at
252 181 , explaining their different locations of NI-NI and Ph-Ph peaks (Fig. 3a).
253 PhNI-P showed a set of strange overlap peaks at about 175 like a noise.
254 Since the exothermic cold crystallization peak of PhNI-Q and the endothermic
255 phase transformation peak of PhNI-C were located at the adjacent position in
256 DSC curves and had the same order of magnitudes in intensity, it could be
257 imagined that the overlap peaks of PhNI-P resulted from the existence and
258 superposition of the above two peaks. Ignoring the same endothermic phase
259 transformation peak, PhNI-P possibly contained an extra exothermic cold
260 crystallization peak than PhNI-C, implying that PhNI-P is relatively unstable
261 compared with PhNI-C. Because the energy of cold crystallization peak (1.5
262 kJ/mol) and the energy of forming π-π stacking between phenyl groups (8.0
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263 kJ/mol) were in the same order of magnitudes,[28] we could infer that PhNI-P
264 had less π-π stacking between phenyl groups than PhNI-C, agreeing with the
265 XRD results.
266
To accurately explore the structure (inter-molecular π-π stacking) of PhNI
267 samples and to figure out the MC mechanism, it was necessary to understand
268 the single-crystal structural information of PhNI-C (with π-π stacking between
269 phenyl groups) and PhNI-P (without π-π stacking between phenyl groups). As
270 shown in Fig. 4, PhNI-C adopted the twisted cross intra-molecular
271 conformation with dihedral angle near 90º. In addition, there existed π-π
272 stacking between naphthalimides (3.49Å) and π-π stacking between phenyl
273 groups (2.96Å) simultaneously, forming the J-aggregate with staggered parallel
274 inter-molecular conformation. Although PhNI-P was unavailable for
275 single-crystal structural analyses owing to its structure disorder, its
276 inter-molecular structure could be inferred by the conclusion of XRD that there
277 were π-π stacking between naphthalimides but no π-π stacking between phenyl
278 groups in PhNI-P. Therefore, PhNI-P could take cross inter-molecular
279 conformation. In such case, naphthalimides stacked with each other while
280 phenyl groups repulsed each other, which was called X-aggregate. The
281 emission of PhNI-C (J-aggregate) was weaker than that of PhNI-P
282 (X-aggregate), which has been confirmed from their fluorescence quantum
283 yields displayed in Table 1 and corresponded with the reported computation
284 results that the inter-molecular parallel cofacial configurations are detrimental
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285 to luminescence in the solid state.[1] On the other hand, the single crystal
286 structure of CyNI-C was supposed to be helpful for the structure analysis of
287 PhNI-P due to their similarity without the π-π stacking between phenyl groups.
288 However, the disorder in the crystal structure of CyNI hampered the
289 measurement of its accurate structure which, from another side, reflected its
290 unstable structure similar to that of PhNI-P
.
291
292
293
Fig. 4. Packing modes of PhNI-C.
294 Self-assembly of PhNI and CyNI
Both PhNI and CyNI were capable of self-assembling due to the
295
296 inter-molecular π-π stacking. In the case of dissolving in a good solvent (e.g.,
297 THF) mixed with a poor solvent (e.g., water), by evaporating the good solvent
298 slowly, microfibers with a diameter of several microns for CyNI-C (Fig. 5A)
299 and hundred microns for PhNI-C (Fig. 5C) were formed respectively.
300 According to the calculation results of reported articles,[29] the constraint in
301 one direction resulted in fiber-like structures and the constraint in two
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302 directions resulted in plate-like structures. In our case, such difference in
303 diameter suggested that plate-like structure (PhNI-C) was composed of
304 two-dimensional (2-D) π-π stacking while fiber-like structure (CyNI-C) was
305 composed of one-dimensional (1-D) π-π stacking. Precipitations obtained by
306 titrating saturated THF solution of PhNI/CyNI into water quickly and filtrating
307 immediately respectively were broken microfibers with several microns in
308 diameter (Fig. 5B and 5D), suggesting that PhNI-P and CyNI-P might be built
309 by 1-D π-π stacking. It could be imagined that PhNI-P, CyNI-P and CyNI-C
310 formed the similar X-aggregate for lacking the π-π stacking between rotors
311 (Ph/Cy). Moreover, like PhNI-C and PhNI-P, the diameter of PhNI-A (Fig.
312 5E) was much larger than that of PhNI-Q (Fig. 5F), which implied the
313 difference in π-π stacking (1-D/2-D) and corresponded well with the XRD data
314 (Fig. 3A).
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315
316
317
Fig. 5. SEM images of (A) CyNI-C, (B) CyNI-P, (C) PhNI-C, (D) PhNI-P, (E)
PhNI-A and (F) PhNI-Q.
318
319 Interaction analysis
320
Based on the above results, we inferred that the AIE effects of PhNI and
321 CyNI were influenced by intra-molecular and inter-molecular structures
322 simultaneously. Up to now, there have been several theories about AIE, for
323 example, restriction of intra-molecular motions (RIM), J/X-aggregate formation
324 (JXAF), excited state intra-molecular proton transfer (ESIPT) and twisted
325 intra-molecular charge transfer (TICT).[30] For the as-prepared two
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326 naphthalimides without any hydrogen bonds interaction or cofacial
327 intra-molecular conformation, the RIM and JXAF theories should be more
328 appropriate. In Fig. 6A, the excited state energy of PhNI and CyNI was
329 consumed by the intra-molecular motion in solution state, leading to weak
330 fluorescence. But in solid state, the intra-molecular motion was restricted
331 (RIM) by π-π stacking and the energy could be transformed into emission,
332 resulting in the fluorescence of PhNI and CyNI. Instead of changeless lacking
333 π-π stacking between cyclohexyl groups in CyNI samples, the changeable
334 presence and absence of π-π stacking between phenyl groups in different PhNI
335 samples brought about varied inter-molecular structures (J/X-aggregate) and
336 changed the energy gap between the ground state and the lowest excited state,
337 resulting in the varied fluoresce quantum yields (ΦF). To sum it up, AIE effects
338 were associated with both intra-molecular and inter-molecular structures. The
339 former provided the foundation of RIM and afforded the energy for generating
340 fluorescence, while the latter transformed the aggregate states, influenced the
341 energy gaps and adjusted the fluorescence intensity. As shown in Fig. 6B, the
342 fluorescence of AIE molecules with cross intra-molecular structures (RIM)
343 could be hidden due to the form of J-aggregate with parallel cofacial
344 inter-molecular configurations, which could be named implicit AIE (imAIE)
345 effect. By means of different preparations and various external stimuli, some
346 implicit AIE effects (J-aggregate) could be transformed into explicit AIE effects
347 (X-aggregate), like PhNI-C and PhNI-P. As illustrated in Fig. 6C, PhNI-C
,
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348 PhNI-A and PhNI-F could be obtained by thermodynamic controlling process
349 and exhibited stable J-aggregates with weak fluorescence due to the π-π
350 stacking between substituents. To the contrary, PhNI-P and PhNI-Q might be
351 acquired by kinetic controlling process and displayed instable X-aggregates
352 with strong fluorescence owing to the repulsion between substituents. Besides,
353 PhNI-G benefited from shear force and gained a similar instable structure. The
354 MC effect could be obtained by transformation between implicit AIE (imAIE)
355 effect and explicit AIE (exAIE) effect, which was controlled by
356 thermodynamic/kinetic process and external stimuli.
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357
358
Fig. 6. Proposed mechanisms of AIE/MC effects for PhNI and CyNI.
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359
360 Conclusions
361
362
363
364
365
366
367
368
369
370
371
372
373
In summary, two N-substituted naphthalimides (PhNI and CyNI) with AIE effects
were synthesized. Among them, PhNI showed an interesting MC property, which was
confirmed by PL spectra. Although without useful MC effect, CyNI showed stable
strong AIE effect. The studies by XRD, DSC and SEM indicated that the solid
fluorescent properties had a close relation with inter-molecular structures. The
single-crystal X-ray structural analyses verified that conjugated and non-conjugated
substituents resulted in different π-π stacking modes and different photoluminescent
properties. For the different PhNI samples, their MC property resulted from their
different inter-molecular structures and could be controlled by thermodynamic/kinetic
processes. These results suggested that AIE effect was decided by intra-molecular
(RIM) and inter-molecular (JXAF) structures simultaneously. We believed that our
studies would be helpful to obtain deeper insights into the AIE/MC phenomena and
mechanisms, and develop much more AIE/MC materials based on naphthalimides.
374 Acknowledgements
375
We are grateful for funding from the National Natural Science Foundation
376 of China (NO 21374046), Program for Changjiang Scholars and Innovative
377 Research Team in University, Open Project of State Key Laboratory of
378 Supramolecular Structure and Materials (SKLSSM201718) and the Testing
379 Foundation of Nanjing University. We thank Dr Y. Zhao for the single crystal
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380 measurements, Dr Z. H. Xi for the DSC measurements and Dr L. L. Zhang for
381 the fluorescence quantum yield measurements.
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Highlights
ò We explore a new AIE system using naphthalimide as twisted structure instead of
linker or donor/acceptor.
ò Two naphthalimides avoid noble metal catalyst in the synthesis.
ò
The relations between properties and structures of two naphthalimides reveal an
opinion that the AIE effects were influenced by intra- and inter-molecular structures
simultaneously.
ò
ò
The multiple chromisms effects of AIE materials may result from the transformation
of inter-molecular structures.
Two naphthalimides exhibit excellent AIE effects (Φf = 0.55) and interesting multiple
chromisms effects.