Design, synthesis, photophysical properties and pH-sensing application of pyrimidine-phthalimide derivatives

Article abstract of DOI:10.1039/c7tc03472e

On the basis of the molecular design of the donor-π-acceptor (D-π-A) pyrimidine-phthalimide derivatives, two new atypical AIE chromophores, 2-(4,6-dimethylpyrimidin-2-yl)isoindoline-1,3-dione (PB) and 2-(4,6-bis(4-(dimethylamino)styryl)pyrimidin-2-yl)isoindoline-1,3-dione (NPB), were synthesized and characterized by using IR, ¹H NMR, ¹³C NMR and HRMS. Both PB and NPB exhibited obvious solid-state fluorescence emission due to their twisted geometries and positive solvatochromism due to their different molecular conformations in various solvents. Owing to the different push-pull electronic effects of substituents on the pyrimidine moiety, PB and NPB, acting as D-π-A compounds, showed different HOMO-LUMO gaps and a variable red-shifted emission in their solid state, substantiating the possibility to tune effectively the photophysical properties of these AIE chromophores by rational molecular design. In addition, PB and NPB could be easily and reversibly protonated at the site of the nitrogen atoms, causing dramatic color changes. This phenomenon opens up the potential avenues of developing novel colorimetric pH sensors and logic gates for specific applications.

Full text of DOI:10.1039/c7tc03472e

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PAPER
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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
the Hofmeister effects
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DOI: 10.1039/C7TC03472E
1
2
3
Design, synthesis, photophysical properties and
pH-sensing application of the pyrimidine-phthalimide
derivatives
4
5
6
7
8
Han Yan, Xinlei Meng, Baoyan Li, Shusheng Ge, Yun Lu*
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
9
*Author to whom correspondence should be addressed: Nanjing University, Nanjing 210023, China, Eꢀmail:
yunlu@nju.edu.cn
10
11 ABSTRACT
12 On the basis of molecular design about the donorꢀπꢀacceptor (DꢀπꢀA)
13 pyrimidineꢀphthalimide
14 2ꢀ(4,6ꢀdimethylpyrimidinꢀ2ꢀyl)isoindolineꢀ1,3ꢀdione
15 2ꢀ(4,6ꢀbis(4ꢀ(dimethylamino)styryl)pyrimidinꢀ2ꢀyl)isoindolineꢀ1,3ꢀdione
derivatives,
two
new
atypical
AIE
chromophores
and
(PB)
(NPB),
were
1
13
16 synthesized and characterized by using IR, H NMR, C NMR and HRMS. Both PB and
17 NPB exhibited the obvious solidꢀstate fluorescence emission due to their twisted geometries
18 and positive solvatochromism due to their different molecular conformations in various
19 solvents. Owing to the different pushꢀpull electronic effects of substituents on the pyrimidine
20 moiety, PB and NPB, acting as the DꢀπꢀA compounds, showed different HOMOꢀLUMO
21 gaps and a variable redꢀshifted emission in their solid state, substantiating the possibility to
22 tune effectively photophysical properties of these AIE chromophores by rational molecular
23 design. In addition, PB and NPB could be easily and reversibly protonated at site of the
24 nitrogen atoms, causing dramatic color changes. This phenomenon opened up the potential
25 avenues of developing novel colorimetric pH sensors and logic gates for specific
26 applications.

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27 Introduction
28 The molecules with DꢀπꢀA structures, that is, connecting electronꢀdonating (D) and
29 electronꢀaccepting (A) groups via a πꢀconjugated linker, have attracted increasing attention
30 since they can serve as electroactive and photoactive materials in biochemical fluorescent
31 technology,1ꢀ4 efficient nonlinear optical (NLO),5ꢀ7 electrogenerated chemiluminescence,8ꢀ11
32 organic lightꢀemitting diodes (OLEDs),12ꢀ14 solar cells,15ꢀ18 etc. Such functional molecular
33 materials possess the intramolecular charge transfer (ICT) property, in which a number of
34 functional donors (such as dimethylamino, diphenylamino, carbazoles, pyrroles, and
35 dialkylamino) and acceptors (such as ketone, nitro, pyridine, and phthalimide) have been
36 employed.19
37
Pyrimidine, as a heterocyclic molecule, has high electron affinity and good coplanarity,
38 making it an appropriate building block in the construction of chromophores for optical
39 sensors and pHꢀresponsive materials.20 The methyl groups located at positions 4ꢀ and 6ꢀ of
40 the pyrimidine core have a wellꢀknown reactivity, and can easily undergo reaction with an
41 aromatic aldehyde. Meanwhile, the amino group located at positions 2ꢀ can also be
42 derivatized by many reactions, increasing not only the solubility of the molecule but also the
43 extent of electron delocalization.
44
On the other hand, since aggregationꢀinduced emission (AIE)21,22 phenomenon was
45 discovered, many organic molecules with AIE feature have been developed for the
46 application of fluorescent sensors, cell imaging and organic lightꢀemitting diodes.23ꢀ28 In
47 general, AIEꢀactive molecules are characterized by the strongly twisted conformations,
48 which enable them to emit more efficiently in the aggregated state by hindering
49 intermolecular close stacking and intense πꢀπ interaction.29,30 Additionally, these molecules
50 can also be explored further by the introduction of the appropriate functional units and used

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51 as stimuliꢀresponsive fluorescent solidꢀstate probes for pH determination or solvent
52 sensing.31ꢀ34 Thus, AIE compounds have opened up a new path for the development of novel
53 stimuliꢀresponsive fluorescent materials. A series of molecules with AIE characteristics, such
54 as 1ꢀmethylꢀ1,2,3,4,5ꢀpentaphenylsilole (MPPS), tetraphenylethene (TPE), triphenylamine
55 (TPA), etc. have been investigated in detail.27,30 However, there are only limited reports on
56 atypical AIE fluorescent materials because of the lack of clear guidelines on the
57 structureꢀproperty relations of the molecules.31ꢀ36
58
In this work, we designed and synthesized two new atypical fluorescent molecules,
59 2ꢀ(4,6ꢀdimethylpyrimidinꢀ2ꢀyl)isoindolineꢀ1,3ꢀdione
(PB)
and
by
60 2ꢀ(4,6ꢀbis(4ꢀ(dimethylamino)styryl)pyrimidinꢀ2ꢀyl)isoindolineꢀ1,3ꢀdione
(NPB),
61 attaching pyrimidineꢀbased derivatives to the phthalimide moity. These two molecules have
62 a similar framework of Yꢀshaped DꢀπꢀA chromophore structure, in which the
63 N,Nꢀdimethylaniline units acted as donors and the pyrimidin derivative moiety as an acceptor.
64 The effect of difference in electropositivity for two groups (methyl and N,Nꢀdimethylaniline)
65 in PB and NPB respectively on the AIE activity was investigated, and for these asꢀprepared
66 new functional molecules, their solvatochromic properties, nanoaggregates, the response to
67 pH and the potential application in logic gates, as well as the theoretical calculation about
68 structures were studied as well.
69 Results and discussion
70 Synthesis
71 In this work, two new chromophores, PB and NPB, consisting of the phthalimide and
72 pyrimidineꢀbased derivatives were synthesized (Scheme 1). The reaction was carried out
73 under refluxing conditions in toluene, and products were readily obtained by recrystallization
74 in good yield. Also, the procedure is simple and convenient. All the intermediates and final

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1
13
75 products were carefully purified and fully characterized by IR, H NMR, C NMR and
76 HRMS, which confirmed their expected molecular structures. These compounds were found
77 to be soluble in common organic solvents such as CH2Cl2, CHCl3, toluene, DMF, but
78 insoluble in methanol and water.
79
80
81
82
Scheme 1 Synthetic routes to PB and NPB.
83
84 Thermal stability
85 Thermal stability is one of the key requirements for practical applications of organic
86 fluorophores. It can be seen that, from the thermogravimetric analysis (TGA) curves (Fig. S1,
87 see Supporting Information), the degradation temperatures (Td ) of 5% weight loss of the PB
88 and NPB were 210 and 345 °C respectively. There was a total loss of about 40% for NPB
89 when being heated to close to 400 °C. NPB exhibited a higher stability due to its more planar
90 molecular configuration, which led to a longer conjugation length and stronger
91 intermolecular interactions. The results indicated that such pyrimidineꢀphthalimide
92 derivatives can provide desirable thermal properties through rational molecular structure
93 design.
94
95 AIE properties

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96 Both PB and NPB showed the solidꢀstate emission maxima under UV light of 365 nm and
97 the fluorescence emission peaks for their respective crystals and powders were located at 512
98 and 621 nm, respectively (Fig. 1a). Their solidꢀstate fluorescence observed could be ascribed
99 mainly to contribution of the whole twisted molecular structure. Compared to PB, NPB
100 showed a redꢀshifted emission peak due to the replacement of the strong electronꢀdonating
101 groups N,Nꢀdimethylaniline units on the 4ꢀ and 6ꢀ position of the pyrimidine core and the
102 enlarged conjugation segment by C=C bond linked with the N,Nꢀdimethylaniline units and
103 pyrimidine core. Different samples gave varied fluorescence quantum yields (Φ) such as
104 0.35 for PB crystal, 0.31 for PB powder, 0.44 for NPB crystal and 0.43 for NPB powder.
105 The CIE chromaticity coordinates of PB and NPB were depicted in Fig. 1b, and their values
106 were calculated to be (0.1662, 0.6322) and (0.5988, 0.3237) respectively, which were
107 consistent with the colors displayed by these two chromophores.
108
109
110
111
112
113
114
Fig. 1 (a) Solidꢀstate photoluminescence (PL) spectra (λex = 365 nm), inset: photo images of
the powder solids and crystals for PB and NPB respectively (λex = 365 nm). (b) Commission
Internationale de l’Eclairage (CIE) chromaticity coordinates of PB and NPB.
We used THF and water respectively as good and poor solvent for further investigating

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115 the aggregationꢀinduced emission of PB and NPB. Fig. 2 showed the PL spectra and the
116 change in fluorescence intensity of PB and NPB in various waterꢀTHF mixtures. In the case
117 of PB, its pure THF solution emitted weak blue light, which was ascribed to the free rotation
118 of pyrimidine ring and phthalimide ring around the CꢀC bond facilitating a nonꢀradiative
119 relaxation process. As water was gradually added to the THF solution, the PL intensity of PB
120 molecules displayed first the drop, then the rise, and then again the fall (Fig. 2a and 2b). The
121 enhanced PL intensity with increasing water fraction (fw) to 50% indicated that the dissolved
122 molecules began to aggregate with a restriction of intramolecular motion (RIM) process
123 resulting in obvious aggregationꢀinduced emission. The luminescence behavior of PB
124 molecules that showing a decrease rather than increase in intensity at fw < 10% is hard to
125 explained and the cause was unclear. For the phenomenon of lowering again the PL intensity
126 at fw > 50%, a possible reason was that at higher water content, molecules come out from the
127 solvent and tend to aggregate, which in turn creates a nonpolar microenvironment around the
128 molecules thus decresing the fluorescence.37ꢀ39 In the case of NPB, its AIEꢀbehavior was
129 found to be different to that of PB (Fig. 2c and 2d). The PL intensity remained unchanged
130 below the fw values of 30%. Due to the introduction of the polar groups N,Nꢀdimethylaniline
131 units, NPB exhibited a relative higher solubility in the solvents with low water fraction than
132 PB, that could be why the emission spectra of NPB were not affected dramatically by
133 solvent effect with the increase of water fraction. Then the PL intensity continues to increase
134 suddenly until the fw of 50%, and after that the rate of increase slowed down. Obviously, the
135 pronounced fluorescence enhancement phenomenon could be interpreted as the aggregation
136 of NPB at higher fw values. According to the RIM mechanism, when NPB was dissolved in
137 THF or THF/water mixtures with a relative high ratio, the active intramolecular rotations of
138 dimethylamino groups and phthalimide ring exhausted the energy of the molecules under

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139 excited state and resulted in fluorescence quenching. When more water was added into the
140 solution, NPB tended to form aggregates, in which the intramolecular rotations were
141 restricted and the excited states were decayed to the corresponding ground states through
142 irradiative channels.40 In such case, relative to PB, the enlarged molecular structure and
143 twisted conformation of NPB molecules increased steric hindrance and reduced
144 intermolecular contact, thus prevented redundant πꢀπ stacking and presented the AIE
145 behavior.
146
Except for the short wavelength (SW) peaks ranging from 400 to 470 nm, it is also
147 noticed that long wavelength (LW) peaks at 619 nm appeared in the PL spectra of NPB in
148 the aqueous mixtures (Fig. 2c). According to the reported literature,41 the phthalimide moiety
149 and the pyrimidine derivative groups may affect respectively the SW and the LW
150 excitedꢀstate energy levels. Thus it is believed that the LW peak was probably associated
151 with the TICT emission of the enlarged conjugation pyrimidine unit, and/or the equilibrium
152 between the AIE and the TICT in the excited state of NPB molecules.
153
154
155
Fig. 2 PL spectra of (a) PB and (c) NPB in waterꢀTHF mixtures with different ratios, and the

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156
157
corresponding plot of fluorescent intensity against fw of (b) PB and (d) NPB. (λex = 365 nm).
158
In addition, the SEM images showed that below fw = 10% (PB, Fig. 3a) and 0% (NPB,
159 Fig. 3c), regular large quadrate and blockꢀplate like nanoparticles formed, meaning that PB
160 and NPB had sufficient solubility in THF / THF with low water fraction. Attributing to the
161 slow process of molecular aggregation, ordered assemblies based on the πꢀπ interactions
162 between molecules could be generated, as a result, the detrimental species of excimers
163 formed and led to the fluorescence weakening.42 And at fw = 90%, few ordered
164 microstructures were observed for both PB (Fig. 3b) and NPB (Fig. 3d). At this point, the
165 molecules became insoluble and aggregated quickly so that no time to arrange in a regular
166 πꢀπ packing mode. In such case, the resulted restriction of intramolecular rotation effect was
167 most responsible for the AIE behavior and the enhenced fluorescence emission.
168
169
170 Fig. 3 SEM images for the solids formed by PB and NPB in THF/water mixtures with
171 different fw. (a) PB, fw = 0%; (b) PB, fw = 90%, (c) NPB, fw = 0%, and (d) NPB, fw = 90%.

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172
173 Crystalline Characteristic
174 Determination of Xꢀray singleꢀcrystal structures is the most effective and powerful tool for
175 acquiring the groundꢀstate molecular structures and revealing the AIE mechanism. As shown
P
176 in Fig. 4a, the Xꢀray crystal structure of PB was found to be triclinic with space group as 1.
177 The dihedral angle angle between the adjacent rings A and B was 69.96°. Upon aggregating,
178 these intramolecular rotations were highly restricted, thus enhancing the observed
179 fluorescence emission. The stacking structures of PB in the crystals were shown in Fig. 4a,
180 except partial antiparallel π···π stacking interaction between pyrimidine rings separated at a
181 distance of 4.03 Å, few intermolecular πꢀπ stacking was observed due to the twisted
182 arrangement.43 Such aggregation mode could be classified as Jꢀaggregate. Therefore, the
183 twisted geometry had important influence on the AIE property, in which the Jꢀaggregate
184 formation probably caused the further increase of the emission intensity.44
185
A similar type of randomness was absent in NPB also mainly due to its twisted
186 structure. The dihedral angle angle between the adjacent rings C and D was 75.88° (Fig. 4b).
187 NPB molecules aligned themselves in antiparallel geometry in adjacent layers with a small
188 partial πꢀoverlap between the aromatic units due to the Jꢀaggregate packing (Fig. 5b). The
189 layers were separated by a distance of 3.98 Å. Herein, the AIE of PB and NPB can be
190 attributed to their twisted conformations, which were conducive to prevent the formation of
191 excimers or exciplexes and boost the emission of samples in the aggregate state.
192

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193
194
Fig. 4 Molecular structure (ORTEP diagram) of (a) PB and (b) NPB.
195
196
197
198
Fig. 5 Packing structure of (a) PB and (b) NPB single crystal.
199
200 Solvent induced emission changes
201 The donorꢀacceptor property of PB and NPB in various solvents was exhibited in their
202 characteristic dualꢀband absorption spectra where the high energy band was attributed to the
203 π→π* transition and the low energy band ascribed to intramolecular charge transfer
204 transition. Compared to PB, the absorption of NPB ranging from 271 to 403 nm indicated
205 that the proposed DꢀπꢀA compound NPB is able to absorb over an extended absorption range,
206 leading to excellent lightꢀharvesting. Moreover, absorption spectra of PB and NPB were not
207 dramatically solventꢀdependent, implying that the effect of solvent polarity was negligible
208 for the molecules in the ground state.

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209
210
211
Fig. 6 UVꢀvis absorption spectra of (a) PB and (b) NPB in various solvents.
212
213
The emission behaviors of PB and NPB in solvents with different polarities were
214 checked as shown in Fig. 7. In nonꢀpolar hexane, PB showed an emission with a peak at 402
215 nm and two vibronic couplings at 387 and 428 nm, which indicated three closeꢀlying excited
216 vibration states with small energy gap. With the increasing the solvent polarity, redꢀshifted
217 emissions with varying degrees were observed in midꢀpolar solvents (EA, THF and DCM)
218 and polar solvents (DMF and acetonitrile), suggesting the presence of positive
219 solvatochromism (Fig. 7a). In addition, a single fluorescence emission peak in EA and DCM
220 as well as a dual peak in THF, DMF and acetonitrile with different red shift were observed.
221 Among the dual peak, the one at shorter wavelength (400 ∼ 420 nm) could be assigned to the
222 contribution of the locally excited (LE) state, while the one at longer wavelength (435 ∼ 474
223 nm) could be ascribed to that of twisted intermolecular charge transfer (TICT) state.45 The
224 variation of the PL spectra for PB with the change of solvent polarity probably related to the
225 potential energy surfaces of the molecule in diverse settings (Fig. 8). As a DꢀπꢀA system, PB
226 is in the LE state with the lowest energy in nonpolar solvents, and its emission came mainly
227 from the LE state. With the increase in polarity of the solvent, the TICT state with higher
228 energy is stabilized by solvation of the polar solvents, making the energy level of two states

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229 closer to each other. The red shift of emission peak in the PL spectra in solvents with
230 medium polarity such as THF and DCM just reflected the presence of TICT state. Especially,
231 in the high polar solvents such as DMF and acetonitrile, the level of TICT state became even
232 lower than that of the LE state and thus the emission was mainly from the TICT state.46 At
233 this point, due to the intramolecular rotation caused by dipolar soluteꢀsolvent interactions,
234 the energy level of PB completed its transformation from the LE to the TICT state after the
235 total charge separation between the donor and acceptor units was completed. The resulting
236 twisted conformation was stabilized by surrounding polar solvents, leading to the narrowed
237 band gap and consequently large redꢀshifts of 130 nm in the PL spectra.47 Compared with PB,
238 the PL spectra of NPB exhibited a larger redꢀshift in their maxima with increasing solvent
239 polarity (Fig. 7b). This indicated that due to introduction of the N,Nꢀdimethylaniline groups
240 acted as a strong donor, the stronger pushꢀpull effect made molecular dipole moment in the
241 excited state much larger than that in the ground state.
242
243
244
245
246
Fig. 7 Normalized PL spectra of (a) PB and (b) NPB in various solvents (5 × 10 ꢀ5 M). λex =
365 nm.

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247
248
Fig. 8 Schematic of the potential energy surfaces describing the LE→TICT reaction.
249
250
In order to further reveal the polarity effect on the molecular excited state relaxation and
251 the fluorescence property, NPB was chose to measure its PL spectra in the mixed solvents
252 due to its larger positive solvatochromism effect relative to PB. As shown in Fig. 9, with
253 titrating EA into dilute hexane solution of NPB (~10−5 M), the two peaks (448, 476 nm)
254 decreased gradually, and at the same time the peak at 454 nm with a relative lower intensity
255 emerged and also redꢀshifted as the ratio of EA to hexane increased. This fact indicated that,
256 for NPB, its emission band position is very sensitive to the solvent polarity (just with a little
257 amount of polar solvent) due to the ICT character of the excited state.
258
259
260
Fig. 9 PL spectra of NPB in mixed solvents mixing hexane (5 × 10 ꢀ5 M) with EA. λex = 365
nm.

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261
262
Further, the photophysical data of PB and NPB in different solvents were summarized
263 in Table 1. The results showed that PB possessed the highest quantum yield of 14% in DCM
264 and the lowest one of 0.5% in hexane. The results may be relative to the equilibrium between
265 the LE and the TICT state in the excited state.48 When in nonpolar or midpolar solvents
266 (hexane, EA, THF and DCM), PB is mainly LE state which has a fast radiative rate constant
267 causing the high quantum yields. As the solvent polarity increased, the equilibrium shifted to
268 the TICT state, emission intensity was normally weakened because of the susceptibility of
269 the TICT state to various nonradiative quenching processes,49 which explained the low
270 quantum yield of PB in DMF and acetonitrile. Similar quantum yield change was observed
271 for NPB, here we just discussed the photophysical data of PB as an example.
272
273
Table 1. Photophysical data of PB and NPB in various solvents.
PB
NPB
solvent
λab
λem
Stoke’s shift Ф*
λab
λem
Stoke’s shift Ф*
(nm) (nm) (cm ꢀ1)
(nm) (nm)
(cm ꢀ1)
hexane
EA
286
289
291
293
293
402
425
450
453
462
474
10089
11072
12142
12130
12485
13033
0.005 398
448
532
533
533
563
578
2805
6266
6301
6301
7238
7699
0.005
0.065 399
0.071 399
0.142 399
0.032 400
0.024 400
0.072
0.085
0.200
0.042
0.035
THF
DCM
DMF
acetonitrile 293
274 * The quantum yield (Ф) was measured by using the quinine sulfate solution (in 0.1 M

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275 H2SO4, literature quantum yield 54% at 360 nm) as the standard and calculated with the
2
276 equation of Ф = ФR (I/IR)(AR/A)(n2/nR ). Where, ФR is the quantum yield, I is the measured
277 integrated emission intensity, n is the refractive index, A is the optical density and the
278 subscript R refers to the quinine sulfate.
279
280
The effects of solvents on the emission features could be further evaluated by the direct
281 correlations between the solvent polarity parameter (ꢁf) and Stokes shift of the absorption
282 and
also
emission
maxima
via
LippertꢀMataga
equations
of
−1 n2 −1
2ꢁf
ꢁ
υ
= ꢁ
υ
abs − ꢁ
υ
em
=
(ꢀe − ꢀ
g )2 + constant (1)
ε
2ε
50
283
and ꢁf = f (
ε
) − f (n2 ) ≈
−
(2)
,
hca3
+1 2n2 +1
284 where ꢁυ was the Stokes shift; ꢁυabs and ꢁυem were wavenumbers of the absorption and
285 fluorescence peaks; h was Planck’s constant; c was the velocity of light; a was Onsager
286 cavity radius; ꢀe and ꢀg were the excited and ground state dipole moments; ꢁf was the
287 orientational polarizability of solvent which could be calculated using the dielectric constant
288 e and refractive indices n of the solvent. The plot (ꢁυꢀꢁf) for both PB and NPB exhibited a
289 linear relationship (Fig. 10), suggesting that the dipoleꢀdipole interaction between the solute
290 and solvent were mainly responsible for the solventꢀdependent fluorescence shift. In addition,
291 the slopes of the fitting line for PB and NPB were as high as 4389 and 9300, respectively.
292 NPB exhibited much larger slope than PB, suggesting its larger dipole moment changes
293 between the ground and excited states (ꢀeꢀꢀg) under photoexcitation. The result revealed that
294 NPB had an extremely polar structure in the excited state and stronger TICT effect, which
295 were beneficial to nonlinear optical properties.51
296

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297
298
Fig. 10 LippertꢀMataga solvent polarity graphs for PB and NPB.
299
300 Electronic Structure.
301 The density functional theory (DFT) computation technique52 has been used to optimize the
302 geometries of PB and NPB, and the highest occupied molecular orbital (HOMO) and lowest
303 unoccupied molecular orbital (LUMO) were shown in Fig. 11. The HOMO for PB was
304 distributed on the 4,6ꢀdimethylpyrimidine unit and parts of the phthalimide moiety, whereas
305 the HOMO for NPB was distributed on the N,Nꢀdimethylaniline units and C=C bond. In
306 view of the fact that the LUMO, for both PB and NPB, was mainly distributed over the
307 phthalimide moiety, the HOMO and LUMO energy levels were well separated which was in
308 favor of a facile charge migration.53 In addition, the theoretical HOMOꢀLUMO gap for PB
309 and NPB were 3.394 eV and 1.897 eV respectively, verifying the status for both PB and
310 NPB as DꢀπꢀA compounds. Besides, the bandgap value of NPB was a lower comparing with
311 that of PB, indicating its larger delocalization and pushꢀpull effect.
312

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313
314 Fig. 11 B3LYP/6ꢀ31G(d) calculated molecular orbital amplitude plots and energy levels of
315 the HOMOs and LUMOs of PB and NPB.
316
317 pH sensing
318 The nitrogen atoms of the asꢀprepared pyrimidine units are alkaline centers that can be
319 protonated. Thus, the effect of protonation on the optical properties of PB and NPB with
320 pyrimidine structure was also studied. As PB and NPB solutions in DCM experienced a
321 acidification by the addition of TFA (Fig. 12), a fully reversible change can be observed after
322 a subsequent neutralization with base (Et3N), and the cycle depicted can be repeated at least
323 10 times without loss of sensitivity to pH change.

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324
Instrestingly, the absorbance change of PB induced by TFA was much smaller than that
325 of NPB and only the NPB solution turned its colors from yellow to dark brown then to blue
326 with increasing concentration of TFA, indicating that NPB was quite a sensitive probe for
327 acid detection in nakedꢀeye observation. The changes observed in the PL spectra of PB upon
328 addition of acid are illustrated in Fig. 12a. The increase in the concentration of TFA led to
329 the appearance of a new, about 10 nm redꢀshifted band corresponding to the protonated
330 species, undergoing a slight color change. This bathochromic shift can be explained by an
331 increased charge transfer from the new DꢀπꢀA system in which dimethyl groups acted as
332 weak donor and remanent protontated moity acted as accepter (Fig. 14a). In the case of NPB,
333 the emission (Fig. 13) is a consequence of radiative deactivation from both excited TICT and
334 LE state at this particular concentration.54 On addition of TFA, the nitrogen atoms at position
335 1ꢀ and 3ꢀ of the pyrimidine unit and phthalimide moiety were first protonated (Fig. 14b), thus
336 the effect of acceptor part increased, the pushꢀpull effect got stronger and TICT became
337 more prominent, consequently, the fluorescence color of the solution changed from yellow to
338 orange. Compared to PB, not only the nitrogen atoms at position 1ꢀ and 3ꢀ of the pyrimidine
339 ring and phthalimide moiety but also the two dimethyl amino groups can be protonated, as a
340 result, the pushꢀpull effect was destroyed because the further protonated NPB was no longer
341 a DꢀπꢀA molecule, the emission came mainly from the LE state and the fluorescence color of
342 the solution changed in turn to dark blue. In this case, upon further addition of TFA, a new,
343 more intense blueꢀshifted band emerged at 457 nm corresponding to the protonated species.
344

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345
346
Fig. 12 Changes in the absorbance of (a) PB and (b) NPB (5 × 10 −5 M in DCM) with
347
348
increasing concentration of TFA.
349
350
Fig. 13 Changes in the PL intensity of (a) PB and (b) NPB (5 × 10 −5 M in DCM) with
351
352
increasing concentration of TFA. λex = 365 nm.

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353
354
Fig. 14 The pHꢀsensing mechanism of (a) PB and (b) NPB (5 × 10 −5 M in DCM) with
355
increasing concentration of TFA.
356
357
The optical performance of PB and NPB in the solid state is also sensitive to change in
358 surrounding TFA vapor. PB and NPB were wrote onto a nonꢀemissive filter paper when
359 exposed to TFA vapor for 30 min and 5s, respectively (Fig. 15). Consequently, the track with
360 blueꢀgreen fluorescence for PB was almost unchanged while the track with orange
361 fluorescence for NPB was changed to green under day light and quenched under 365 nm
362 ultraviolet. The result indicated that the solid state of NPB was quick responsive to TFA
363 atmosphere.
364
To understand the fluorochromic mechanism for PB and NPB in the presence of pH
365 stimuli, PXRD measurements were carried out (Fig. 16). When PB and NPB were exposed
366 to TFA vapor, only the liquid film for NPB was rapidly formed because the protonated NPB
367 was hard to be solidified just as many quaternary ammonium ionic liquids, which may be
368 resulted from the collapse of crystalline lattices induced by the protonation. The fluorescence
369 emission of NPB was switched off under acid stimuli, also due to the deaggregation of AIE

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370 gens or destruction of the ICT process. Furthermore, it was found that the color of PB neither
371 under day light nor 365 nm ultraviolet showed any change in presence of TFA vapor,
372 implying that the quenched fluorescence in the solid state should be ascribed to protonation
373 of dimethylamino rather than the pyrimidin unit. Indeed, the protonated dimethylamino loses
374 its ability to construct a DꢀπꢀA system and turns NPB to a nonꢀemissive phase. Within this
375 context, the NPB could recognize TFA vapor with a detection limit of 20 ppm, exhibiting a
376 clear colorimetric response to an acid environment both in solution and in the gas phase.
377 Such excellent pHꢀsensing property made NPB promising for developing some smart
378 materials in the further.
379
380
381
382
383
384
385
Fig. 15 Images of PB and NPB fabricated on a piece of filter paper (size: 1 cm × 1 cm)
before and after fumed with TFA vapor under day light (upper) and the illumination with
365 nm UV light (lower).

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386
387
Fig. 16 PXRD patterns of (a) PB and (b) NPB.
388
389 Logic gate application
390 Inspired by the pHꢀresponsive fluorescence, we employed the PB solution, whose positions
391 of the emission peaks have the oneꢀtoꢀone corresponding relations when the amine or acid
392 were added respectively, as a representative to construct a logic system. For most reported
393 molecular logic gates, with the drawback of the only single signal as an output, they are just
394 able to perform the single logic operation. In this work, on the basis of pHꢀmediated
395 multisignal response, multiple logic systems can be designed by rationally defining logic
396 states. In these logic operations, the PB aqueous solution served as a gate, while amine and
397 acid were used as inputs and fluorescent signals as outputs. The presence and absence of
398 inputs were defined as 1 and 0, respectively. Since the optical response to pH is highly
399 reversible, construction of more useful and multipleꢀconfigurable logic gates is possible.
400 When amine and acid were used as chemical inputs, the INH logic gate (447 nm as output)
401 and IMPLICATION (IMP) logic gate (437 nm as output) were constructed (Fig. 17) and
402 could display quick and sensitive optical behavior. A logic gate with two output signals may
403 improve its stability greatly, and so far, dualꢀoutput logic gates have been barely reported,55
404 thus, the logic system based on the pHꢀresponsive PB with high sensitivity and fast response

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405 could also hold potential applications in the field of material science and biomedicine.
406
407
In 1 (amine)
In 2 (acid)
Out 1(447 nm) Out 2 (437 nm)
0
1
0
1
0
0
1
1
0
0
1
0
1
1
0
1
408
409
Fig. 17 Colorimetric INH and IMP logic gates and their truth table of PB.
410 Conclusions
411 Two new atypical AIE chromophores PB and NPB were designed and synthesized, also
412 characterized by IR, 1H NMR, 13C NMR and HRMS. Both PB and NPB showed the positive
413 solvatochromism because of their different molecular conformations in various solvents. PB
414 with two methyl groups and NPB with C=C bond and N,Nꢀdimethylaniline substituting
415 groups showed a variable redꢀshifted emission in their solid state due to their distinctive
416 levels of intramolecular pushꢀpull electronic effect. Thanks to the twisted structures, PB and
417 NPB experienced little πꢀπ stacking interaction in their condensed phase and therefore
418 exhibited obvious fluorescence emission in the solid state. The density functional theory
419 calculations indicated that the HOMO and LUMO energy levels of PB and NPB were well
420 separated, verifying the status for both PB and NPB as DꢀπꢀA compounds. Besides, the
421 bandgap value of NPB was a little lower comparing with that of PB, indicating its larger

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422 delocalization and pushꢀpull effect. Moreover, as one would expect, PB and NPB underwent
423 a dramatic color change upon addition of acid as a result of the protonation of the nitrogen
424 atoms. Furthermore, based on the pHꢀsensitive properties, a dualꢀoutput INH logic gate was
425 developed successfully. The novel pyrimidineꢀphthalimide derivatives with sensitive
426 response to pH both in water solution and in the gas phase may hold great potential for
427 applications in the material science fields.
428 Experimental Section
429 Materials and Measurements. THF was distilled under normal pressure from sodium
430 benzophenone ketyl under nitrogen prior to use. All chemicals were purchased from Acros or
1
431 Aldrich and used as received. H and 13C NMR spectra were collected on a Bruker
432 Spectrospin Avance 400 MHz NMR spectrometer in CDCl3 with tetramethylsilane (TMS; δ
433 = 0 ppm) as internal standard. HRMS was acquired on MicromassꢀGTC spectrometer. UV
434 absorption spectra were obtained on a Varian CARY 100 Bio spectrophotometer. PL spectra
435 were recorded on a spectrofluorophotometer (RFꢀ5301PC, SHIMADZU, Japan). The relative
ꢀ5
436 fluorescence quantum yields were estimated relative to solutions of 5 × 10 M quinine
437 sulfate with ФR = 0.55 in 0.1 M sulfuric acid solution as a standard sample. The
438 singleꢀcrystal Xꢀray diffraction data for the crystals of PB and NPB prepared with 50 mg
439 samples and 50 mL THF/water (1:1 by volume) as solvent was collected from a BRUKER
440 D8 VENTURE system with CuꢀK a radiation ( l ¼ 1.54178 Å) at 280 (10) K. The structure
441 was solved using direct methods following the difference Fourier syntheses. All
442 nonꢀhydrogen atoms were anisotropically refined through leastꢀsquares on F2 using the
443 SHELXTL program suite. The anisotropic thermal parameters were assigned to all
444 nonꢀhydrogen atoms. The hydrogen atoms attached to carbon were placed in idealized
445 positions and refined using a riding model to the atom from which they were attached. The

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446 pictures of structure were produced using Diamond 3.1. CCDC 1566171 and 1566173
447 contained the supplementary crystallographic data of this work. Scanning electron
448 microscope (SEM) images were taken on a S4800 (Hitachi, Japan) scanning electron
449 microscope. Thermogravimetric analysis (TGA) was carried out under a N2 atmosphere at a
450 heating rate of 10 °C/min. The groundꢀstate geometries were optimized with the
451 B3LYP/6ꢀ31G(d) program, using the Gaussian 03 package.
452 Synthetic procedures
453
PB. Isobenzofuranꢀ1,3ꢀdione (1.2 equiv) and 4,6ꢀdimethylpyrimidinꢀ2ꢀamine (1.0 equiv)
454 were added in 3 mL of dry toluene and 1ꢀ2 mL of triethylamine (TEA). The mixture was
455 heated at 200°C for 2 h, and allowed to cool to room temperature. Further heating was
456 conducted to remove the solvent, and the residues were purified by silica gel column
457 chromatography, gaining a pale white solid powder PB (yield: 86%), mp 210.9ꢀ211.6 °C.
1
458 FTꢀIR (ν/cm ꢀ1): 3321, 3057, 3026, 2923, 2866, 1579, 1485, 1282, 501. H NMR (400 MHz,
13
459 CDCl3) δ 7.96 (dd, J = 5.5, 3.0 Hz, 1H), 7.82ꢀ7.76 (m, 1H), 7.12 (s, 1H), 2.59 (s, 1H). C
460 NMR (400 MHz, CDCl3) δ 134.58 (s), 124.02 (s), 119.92 (s), 23.94 (s). MS: Calculated =
461 254.0930, found = 254.0932 (M+H) +.
462
463
NP. Compounds NP were synthesized according to literature method.56
NPB. Replacing 4,6ꢀdimethylpyrimidinꢀ2ꢀamine by NP, compound NPB was obtained
464 using similar procedure described for compound PB as a red solid (yield: 78%), mp
ꢀ1
1
465 269.2ꢀ269.7 °C. FTꢀIR(ν/cm ): 3078, 2960, 2929, 2873, 1741, 1593, 1284, 1143. H NMR
466 (400 MHz, CDCl3) δ 8.03ꢀ7.96 (m, 2H), 7.85ꢀ7.79 (m, 4H), 7.52 (d, J = 8.7 Hz, 4H), 7.26 (s,
467 1H), 6.94 (d, J = 15.8 Hz, 2H), 6.85ꢀ6.64 (m, 4H), 3.04 (s, 12H). 13C NMR (400 MHz,
468 CDCl3) δ 166.44 (s), 165.64 (s), 153.34 (s), 150.86 (s), 138.21 (s), 134.36 (s), 132.18 (s),
469 129.34 (s), 123.90 (s), 120.72 (s), 113.63 (s), 112.24 (s), 40.37 (s). MS: Calculated =

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470 516.2400, found = 516.2396 (M+H) +.
471
472 Acknowledgements
473
This work was supported by the National Natural Science Foundation of China (NO
474 21374046), Program for Changjiang Scholars and Innovative Research Team in University,
475 Open Project of State Key Laboratory of Supramolecular Structure and Materials
476 (SKLSSM201718) and the Testing Foundation of Nanjing University.
477
478 Supporting Information
479 Available: Experimental details and characterization data and CIF.
480
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