High quantum yield both in solution and solid state based on cyclohexyl modified triphenylamine derivatives for picric acid detection

Article abstract of DOI:10.1016/j.dyepig.2015.08.013

Five triphenylamine derivatives containing cyclohexyl have been designed and synthesized with a solvent-free green procedure. The compounds possess high quantum yields both in solution and solid state. Our investigation shows that the steric effects of th

Full text of DOI:10.1016/j.dyepig.2015.08.013

Dyes and Pigments 123 (2015) 257e266
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
High quantum yield both in solution and solid state based on
cyclohexyl modified triphenylamine derivatives for picric acid
detection
Yuyang Zhang a, Jianting Pan a, Chenyang Zhang a, Haowei Wang a, Gaobin Zhang a,
, b, *
Lin Kong a, Yupeng Tian a, Jiaxiang Yang a
a Department of Chemistry, Anhui University and Key Laboratory of Functional Inorganic Materials of Chemistry of Anhui Province, Hefei 230039, PR China
b State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Five triphenylamine derivatives containing cyclohexyl have been designed and synthesized with a
solvent-free green procedure. The compounds possess high quantum yields both in solution and solid
state. Our investigation shows that the steric effects of the chair conformation cyclohexyl and strong
intramolecular hydrogen bond play a key role for the high emission. Moreover, a conjugated triphe-
nylamine core and strong polar groups (pyridinyl and hydroxy) and their location in the molecule formed
a typical non-planar pushepull electronic structure. This special chemical structure endows the mole-
cules with solvatochromic effect and exhibit interesting fluorescent behavior in binary solvent systems of
THF-H2O due to the strong intramolecular charge transfer (ICT), which has been confirmed by DFT
calculation, absorption and fluorescence spectra of the compounds in different polar solvents. In addi-
tion, their high emission characters, both in solution and solid state are utilized for picric acid (PA)
detection.
Received 25 June 2015
Received in revised form
6 August 2015
Accepted 9 August 2015
Available online 18 August 2015
Keywords:
Triphenylanime
Cyclohexyl
Solvent-free reaction
Solvatochromic effect
High quantum yields
Picric acid detection
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
state, their fluorescence quantum yields are usually relatively lower
in diluted solution than that in the solid state [6]. There is still few
Highly efficient
p
-conjugated organic luminescent materials
blue or deep-blue emissions of organic molecules in both solution
and solid states were reported. Thus, it is an attractive topic that
design and synthesis molecules emitting efficiently both in solution
and in the solid-state. Recently, some luminophors emit efficiently
in dilute solution and aggregates have been reported [7]. However,
materials with high quantum yield both in solution and solid state
have rarely been reported in the literature to date.
Triphenylamine (TPA) is excellent blue luminescent material
and usually employed to construct organic photoelectric functional
materials with electron donoreacceptor structure [8]. Meanwhile,
the three-dimensional propeller structure of TPA can improve the
have received considerable attention due to their potential appli-
cations in organic light-emitting diodes (OLEDs), organic fluores-
cent chemosensors, photoconductor, switching devices, organic
field-effect transistors (OFETs) and solid-state dye laser [1]. How-
ever, in most cases, organic luminescent materials become weakly
emissive or no fluorescence in the aggregate or solid state [2]. This
kind of materials is known as the notorious aggregation-caused
quenching (ACQ) effect due to strong
pep stacking interactions,
which restricted the scope of the application fields [3]. To alleviate
the ACQ effect, various chemical and physical methods have been
developed [4]. Recently, the aggregation-induced emission (AIE) or
aggregation-induced emission enhancement (AIEE) materials have
been applied to overcome ACQ effect [5]. They are faintly or weekly
emissive in solutions, but were strongly luminescent upon aggre-
gations. Although AIEE materials emit both in solution and in solid-
fluorescence quantum yields by avoiding intermolecular
pep
stacking. In addition, there is another strategy that introducing
bulky groups to prevent molecular aggregation [9]. Many re-
searchers attempt to modify the triphenylamine via introducing a
conjugate moiety to achieve different fluorescence emission and
high quantum yield [10]. Nevertheless, the intricate synthesis
process, poor solubility and film-forming ability of extensive con-
jugate molecule hind their applications [11]. Thus, developing high
emission triphenylamine fluorogen with a simple method is a
fascinating work.
* Corresponding author. Department of Chemistry, Anhui University and Key
Laboratory of Functional Inorganic Materials of Chemistry of Anhui Province, Hefei
230039, PR China. Tel.: þ86 551 63861279.
E-mail address: jxyang@ahu.edu.cn (J. Yang).
http://dx.doi.org/10.1016/j.dyepig.2015.08.013
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

258
Y. Zhang et al. / Dyes and Pigments 123 (2015) 257e266
Recently, there is a critical need for detection of picric acid (PA),
AlCl3 0.55 g (4.12 mmol), yielding a black suspension. The mixture
was cooled to 0 ꢁC under ice bath. Acetic anhydride 1.20 mL (1.20 g,
20.00 mmol) dissolved methylene dichloride (10 mL) was slowly
added dropwise via syringe under vigorous stirring. On completion
of the addition, the ice bath was removed, and the dark green slurry
solution was warm to room temperature with stirring about 5 h.
The mixture was poured into a mixture of ice and concentrated HCl,
extracted three times with CH2Cl2, washed with 1.0 M Na2CO3 so-
lutions and saturated brine consequently, and the extracts was
separated, dried over MgSO4. After removing solvents under
reduced pressure, the residue was purified by flash chromatog-
raphy on silica gel using petroleum/ethyl acetate (5:1) as elution,
and got yellow solid 4-acetyltriphenylamine. Yield: 0.47 g, 80%.
FTeIR (KBr, cmꢀ1): 3036, 3005, 1944, 1673, 1584, 1488, 1417, 1357,
1332, 1269, 1173, 1150, 1074, 1029, 953, 843, 821, 755, 699, 675, 621,
which is a powerful explosive and easily contaminates soil and
groundwater [12]. Various techniques are employed for the detec-
tion of PA [13]. Among these, the fluorescence probe is regarded as
the ideal method for PA detection because of its cost-effectiveness
and inherent sensitivity [14]. Even though organic
p-conjugated
polymers, nanoparticles and metal-organic architectures have been
used to detect PA [15]. However, most of the fluorogen for PA
detection is only working in single system due to they could not
emit efficiently both in solution and solid state. Developing flores-
cence sensors with high quantum yield both in solution and solid
state for detection of PA in dual system is still a challenge.
Herein, our work is to introduce a cyclohexyl structure with a
chair conformation into triphenylamine group to gain functional
triphenylamine derivatives with high emission. The flexible cyclo-
hexyl could increase the intermolecular steric hindrance and film-
forming ability of the title compounds. In addition, in order to
obtain bond site for PA, we modify the cyclohexyl with two pyridine
groups. The strong polar hydroxyls and carbonyl appending in
cyclohexyl could form a strong intramolecular hydrogen bond,
which could stabilize the chair structure decrease the molecular
vibration. The other subsidiary groups, such as p-methylphenyl,
phenyl, thienyl, p-fluorophenyl, p-methoxylphenyl, were intro-
duced to adjust the photophysics properties and intermolecular
interaction. For the purpose of gaining these TPA derivatives with
high emission both in solution and solid state, we adopted a facile
and highly efficient method by aldol condensation and solvent-free
Michael addition reactions. The X-ray crystal structure, optical
properties, electrochemical properties, thermal properties and
picric acid (PA) detection were systematically investigated.
588 and 525. 1H NMR (CD3COCD3, 400 MHz)
d: 2.49 (s, 3H), 6.95 (d,
2H, J ¼ 8.8 Hz), 7.19 (m, 6H), 7.39 (t, 4H, J ¼ 7.9 Hz), 7.85 (d, 2H,
J ¼ 8.8 Hz).
2.2.2. Synthesis of compounds 3ae3e
To a stirred solution of substituted aldehyde (1.74 mmol) in
ethanol (50 mL) was added NaOH 0.10 g (2.50 mmol) and
14 mL H2O and the reaction mixture was room temperature for
overnight. The progress of the reaction was monitored by TLC. After
completion, the reaction mixture was cooled to room temperature
and the precipitate was filtered, washed with water and recrys-
tallized with ethanol, and then dried to give products 3. The com-
pounds 3ae3e were obtained in 64e85% yields, respectively.
Compound 3a: yellow solid, yield 78%; mp: 153.5 ꢁC. FTeIR (KBr,
cmꢀ1): 3037, 2920, 1655, 1586, 1508, 1490, 1419, 1384, 1337,
1284, 1225, 1175, 1024, 811, 755 and 697. 1H NMR (DMSO-d6,
2. Experimental
2.1. Materials and instruments
400 MHz)
d
: 2.35 (s, 3H), 6.91 (d, 2H, J ¼ 8.9 Hz), 7.20 (m, 6H),
7.26 (d, 2H, J ¼ 8.0 Hz), 7.41 (t, 4H, J ¼ 7.9 Hz), 7.66 (d, 1H,
All chemicals were obtained from commercial suppliers. The
solvents were purified by conventional methods before used. FTeIR
spectra were obtained on a Nicolet NEXUS 870 spectrometer
(400e4000 cmꢀ1, KBr pellets). 1H NMR and 13C NMR spectra were
recorded on a Bruker Avance 400 (400 MHz) spectrometer, using
CDCl3, DMSO-d6 or CD3COCD3 as solvent and tetramethylsilane
(TMS) as internal standard. Melting points were determined on a
MELTTLER Toledo type FP62. Identification and quantification of
compounds were carried out using a Agilent 6410 LC-MS/MS sys-
tem (USA) consisted of a Agilent 1200 liquid chromatographic
J ¼ 15.5 Hz), 7.74 (d, 2H, J ¼ 8.0 Hz), 7.83 (d, 1H, J ¼ 15.5 Hz), 8.04
(d, 2H, J ¼ 8.9 Hz). 13C NMR (DMSO-d6, 100 MHz)
d: 21.53, 119.16,
121.46, 125.51, 126.47, 129.17, 129.99, 130.39, 130.82, 132.63,
140.82, 143.28, 146.34, 152.06, 187.23.
Compound 3b: yellow solid, yield 77%. FTeIR (KBr, cmꢀ1): 1656,
1585, 1490, 1337, 1285, 1271, 1223, 1714, 763 and 697. 1H NMR
(DMSO-d6, 400 MHz)
d
: 6.91 (d, 2H, J ¼ 8.8 Hz), 7.20 (m, 6H), 7.42
(m, 7H), 7.71 (d, 1H, J ¼ 15.6 Hz), 7.85 (m, 2H), 7.89 (d, 1H,
J ¼ 15.6 Hz), 8.05 (d, 2H, J ¼ 8.8 Hz). 13C NMR (DMSO-d6,
100 MHz)
d: 119.12, 122.54, 123.93, 125.55, 126.50, 129.14,
system with the volume injection set to 5
m
L. And the triple
129.36, 129.90, 130.40, 130.89, 143.22, 146.31, 147.79, 152.14,
187.24.
quadrupole mass spectrometer equipped with an electrospray ion
source (ESI). The data were obtained by Mass Hunter software.
Column chromatography was carried out on silica gel. UVevis
spectra were recorded on a Shimadzu UV-3600 spectrometer.
Fluorescence spectra were obtained on Hitachi FL-7000 (Hitachi
high technologies corporation, Tokyo, Japan). The absolute photo-
luminescence quantum yield of solid state was measured on
HORIBA FluoroMax-4 spectrofluorometer using an integrating
sphere (HORIBA Scientific, F-3092 integrating sphere). Cyclic vol-
tammetry were carried on a CHI660D instrument. Electrochemical
analyzer with three-electrode cell glassy carbon was used as
working electrode and Platinum wire as counter electrode, and Ag/
AgCl as reference electrode at room temperature.
Compound 3c: yellow solid, yield 82%; mp: 183.4 ꢁC. FTeIR (KBr,
cmꢀ1): 1650, 1588, 1489, 1368, 1334, 1280, 1241, 1170, 1026, 820,
753, 722 and 699. 1H NMR (DMSO-d6, 400 MHz)
d: 6.90 (d, 2H,
J ¼ 8.8 Hz), 7.20 (m, 7H), 7.41 (t, 4H, J ¼ 7.8 Hz), 7.51 (d, 1H,
J ¼ 15.3 Hz), 7.64 (d, 1H, J ¼ 3.4 Hz), 7.75 (d, 1H, J ¼ 5.0 Hz), 7.86
(d, 1H, J ¼ 15.3 Hz), 7.98 (d, 2H, J ¼ 8.8 Hz). 13C NMR (DMSO-d6,
100 MHz)
d: 119.09, 120.79, 125.58, 126.53, 129.14, 130.07,
130.43, 130.45, 130.76, 132.94, 136.05, 140.41, 146.28, 152.09,
186.65.
Compound 3d: yellow solid, yield 85%. FTeIR (KBr, cmꢀ1): 3068,
2919, 1942, 1656, 1586, 1487, 1419, 1343, 1314, 1279, 1219, 1179,
1156, 1025, 976, 821, 748 and 696. 1H NMR (DMSO-d6, 400 MHz)
d
: 6.91 (d, 2H, J ¼ 8.9 Hz), 7.20 (m, 6H), 7.29 (t, 2H, J ¼ 8.9 Hz), 7.41
2.2. Synthesis
(t, 4H, J ¼ 7.9 Hz), 7.70 (d,1H, J ¼ 15.6 Hz), 7.85 (d,1H, J ¼ 15.6 Hz),
7.93 (m, 2H), 8.05 (d, 2H, J ¼ 8.9 Hz). 13C NMR (DMSO-d6,
2.2.1. Synthesis of 4-acetyltriphenylamine (1)
To a stirring of triphenylamine 0.50 g (2.04 mmol) in anhydrous
methylene dichloride (15 mL) was added immediately anhydrous
100 MHz)
d: 116.35 (d, J ¼ 21 Hz), 119.10, 122.44 (d, J ¼ 2 Hz),
125.55, 126.50, 130.23, 130.40, 130.89, 131.45 (d, J ¼ 9 Hz), 132.03
(d, J ¼ 3 Hz), 141.97, 146.30, 152.14, 163.72 (d, J ¼ 247 Hz), 187.16.

Y. Zhang et al. / Dyes and Pigments 123 (2015) 257e266
259
Compound 3e: yellow solid, yield 64%; mp: 150.5 ꢁC. FTeIR (KBr,
cmꢀ1): 1649, 1591, 1507, 1490, 1418, 1339, 1285, 1256, 1221, 1170,
1032, 980, 819, 750 and 695. 1H NMR (DMSO-d6, 400 MHz)
165.56, 198.79. MS (ESIþ, 4.5 kV, 200 ꢁC): m/z ¼ 624.3 ([MþH]þ),
calcd for C39H33N3O3Sþ ¼ 623.2 ([M]þ). MS (MALDI-TOF):
624.223 [MþH]þ, calcd for: 624.232.
d
: 3.82 (s, 3H), 6.91 (d, 2H, J ¼ 8.8 Hz), 7.01 (d, 2H, J ¼ 8.8 Hz), 7.19
Compound 4d: yellow solid, yield 63%. FTeIR (KBr, cmꢀ1): 3742,
3449, 3303, 3050, 2922, 2858, 1729, 1638, 1582, 1500, 1224, 1174,
(m, 6H), 7.41 (t, 4H, J ¼ 7.9 Hz), 7.67 (d, 1H, J ¼ 15.5 Hz), 7.75 (d,
1H, J ¼ 15.5 Hz), 7.81 (d, 2H, J ¼ 8.8 Hz), 8.03 (d, 2H, J ¼ 8.8 Hz).
1077, 835, 755 and 693. 1H NMR (DMSO-d6, 400 MHz)
d: 1.75 (d,
13C NMR (DMSO-d6, 100 MHz)
d: 55.82, 114.85, 119.26, 120.00,
1H, J ¼ 14.3 Hz), 1.91 (d, 1H, J ¼ 13.2 Hz), 2.66 (t, 1H, J ¼ 13.2 Hz),
3.17 (d, 1H, J ¼ 14.3 Hz), 3.99 (t, 1H, J ¼ 12.0 Hz), 4.64 (d, 1H,
J ¼ 12.0 Hz), 6.09 (s, 1H), 6.30 (s, 1H), 6.51 (d, 2H, J ¼ 8.8 Hz), 6.95
(m, 6H), 7.12 (m, 3H), 7.27 (m, 3H), 7.37 (m, 7H), 7.54 (t, 1H,
J ¼ 7.7 Hz), 7.72 (d, 1H, J ¼ 8.0 Hz), 7.81 (t, 1H, J ¼ 7.7 Hz), 8.40 (d,
1H, J ¼ 4.2 Hz), 8.54 (d, 1H, J ¼ 4.2 Hz). 13C NMR (DMSO-d6,
125.44, 126.42, 127.98, 130.38, 130.60, 130.73, 130.99, 143.21,
146.38, 151.94, 161.62, 187.18.
2.2.3. Synthesis of compounds 4ae4e
100 MHz) d: 38.59, 45.51, 46.66, 54.46, 76.38, 78.89, 115.25 (d,
Compound 3 (1.29 mmol) and 2-acetylpyridine 0.51 g (0.47 mL,
4.21 mmol) and power NaOH 0.28 g were crashed together with a
pestle and mortar at room temperature for 0.5 h, the mixture was
put overnight. The resulting mixture was purified by flash chro-
matography with petroleum/ethyl acetate (V:V ¼ 2:1) as elution,
and gave the compounds 4. The compounds 4ae4e were obtained
in 42e63% yields, respectively.
J ¼ 20 Hz), 119.48, 119.69, 120.20, 122.27, 122.48, 124.89, 125.60,
130.07, 130.22, 130.29, 131.59, 136.78, 137.35, 140.35 (d, J ¼ 2 Hz),
146.50, 148.46, 148.60, 150.96, 161.04 (d, J ¼ 240 Hz), 164.05,
166.26, 200.33. MS (MALDI-TOF): 636.258 ([MþH]þ), calcd for:
636.266.
Compound 4e: yellow solid, yield 42%; mp: 95.7 ꢁC. FTeIR (KBr,
cmꢀ1): 3743, 3319,1652,1588,1498,1335,1280,1222,1172,1029,
819, 751 and 695. 1H NMR (DMSO-d6, 400 MHz)
d: 1.73 (d, 1H,
Compound 4a: yellow solid, yield 55%; mp: 124.7 ꢁC. FTeIR (KBr,
cmꢀ1): 3741, 3443, 2918, 1640, 1583, 1495, 1277, 1224, 1178, 756
J ¼ 14.3 Hz), 1.90 (d, 1H, J ¼ 13.2 Hz), 2.63 (t, 1H, J ¼ 13.2 Hz), 3.17
(d, 1H, J ¼ 14.3 Hz), 3.63 (s, 3H), 3.92 (t, 1H, J ¼ 12.0 Hz), 4.60
(d, 1H, J ¼ 12.0 Hz), 6.08 (s, 1H), 6.31 (s, 1H), 6.50 (d, 2H,
J ¼ 8.7 Hz), 6.70 (d, 2H, J ¼ 8.6 Hz), 6.93 (d, 4H, J ¼ 7.8 Hz), 7.11
(m, 3H), 7.26 (m, 5H), 7.37 (m, 5H), 7.53 (t, 1H, J ¼ 7.3 Hz), 7.73
(d, 1H, J ¼ 7.9 Hz), 7.81 (t, 1H, J ¼ 7.2 Hz), 8.40 (d, 1H, J ¼ 4.2 Hz),
and 696. 1H NMR (DMSO-d6, 400 MHz)
d: 1.74 (d, 1H,
J ¼ 14.3 Hz), 1.90 (d, 1H, J ¼ 13.0 Hz), 2.16 (s, 3H), 2.61 (t, 1H,
J ¼ 13.0 Hz), 3.18 (d, 1H, J ¼ 14.3 Hz), 3.94 (t,1H, J ¼ 12.1 Hz), 4.61
(d, 1H, J ¼ 12.1 Hz), 6.04 (s, 1H), 6.31 (s, 1H), 6.50 (d, 2H,
J ¼ 8.8 Hz), 6.94 (t, 6H, J ¼ 8.9 Hz), 7.12 (m, 3H), 7.25 (m, 5H), 7.35
(t, 5H, J ¼ 7.9 Hz), 7.53 (t, 1H, J ¼ 7.3 Hz), 7.72 (d, 1H, J ¼ 8.0 Hz),
7.81 (t, 1H, J ¼ 7.7 Hz), 8.40 (d, 1H, J ¼ 4.2 Hz), 8.54 (d, 1H,
8.55 (d, 1H, J ¼ 4.2 Hz). 13C NMR (DMSO-d6, 100 MHz)
d: 37.89,
45.27, 46.15, 54.80, 55.07, 75.91, 78.41, 113.45, 118.97, 119.16,
119.66, 121.72, 121.93, 124.38, 125.10, 128.86, 129.55, 129.64,
129.71, 131.17, 135.56, 136.85, 146.03, 147.94, 148.07, 150.41,
157.45, 163.62, 165.85, 200.05. MS (ESIþ, 4.5 kV, 200 ꢁC):
m/z ¼ 648.3 ([MþH]þ), calcd for C42H37N3O4þ ¼ 647.3 ([M]þ). MS
(MALDI-TOF): 648.286 ([MþH]þ), calcd for: 648.286.
J ¼ 4.2 Hz). 13C NMR (DMSO-d6, 100 MHz)
d: 20.48, 38.18, 45.24,
46.16, 54.96, 75.91, 78.32, 118.96, 119.26, 119.68, 121.70, 121.95,
124.35, 125.04, 127.75, 128.69, 129.51, 129.70, 131.23, 134.91,
136.24, 136.85, 140.77, 146.05, 147.94, 148.08, 150.35, 163.55,
165.83, 199.60. MS (ESIþ, 4.5 kV, 200 ꢁC): m/z ¼ 632.1 ([MþH]þ),
calcd for C42H37N3Oþ3 ¼ 631.3 ([M]þ). MS (MALDI-TOF): 632.290
([MþH]þ), calcd for: 631.283.
3. Results and discussion
Compound 4b: yellow solid, yield 63%; mp: 200.1 ꢁC. FTeIR (KBr,
cmꢀ1): 3742, 3453, 3279, 2923, 2857, 1638, 1581, 1488, 1350,
3.1. Synthesis
1275, 1221, 1173, 753 and 693. 1H NMR (DMSO-d6, 400 MHz)
d:
The synthetic routes of the proposed compounds 3 and 4 are
outlined in Scheme 1. The chalcone 3ae3e can be easily prepared
by ClaiseneSchmidt condensation by 4-acetyltriphenylamine (1)
and aromatic aldehyde 2ae2e with adjusting water and the pro-
portion of ethanol. These compounds can also be prepared by the
reaction in ethanol, but obtained in lower yields. After optimizing
the reaction conditions, we further explored the next step reaction.
According to the literature, the highly useful CeC bond formation
1.76 (d, 1H, J ¼ 14.3 Hz), 1.94 (d, 1H, J ¼ 13.2 Hz), 2.66 (t, 1H,
J ¼ 13.2 Hz), 3.21 (d,1H, J ¼ 14.3 Hz), 4.01 (t,1H, J ¼ 12.0 Hz), 4.66
(d, 1H, J ¼ 12.0 Hz), 6.07 (s, 1H), 6.36 (s, 1H), 6.49 (d, 2H,
J ¼ 8.8 Hz), 6.93 (d, 4H, J ¼ 7.8 Hz), 7.10 (m, 6H), 7.26 (t, 3H,
J ¼ 7.8 Hz), 7.35 (m, 7H), 7.52 (t, 1H, J ¼ 8.4 Hz), 7.72 (d, 1H,
J ¼ 7.9 Hz), 7.81 (t, 1H, J ¼ 8.7 Hz), 8.41 (d, 1H, J ¼ 4.4 Hz), 8.55 (d,
1H, J ¼ 4.4 Hz). 13C NMR (DMSO-d6, 100 MHz)
d: 38.68, 45.23,
46.19, 54.82, 75.89, 78.35, 118.97, 119.21, 119.69, 121.72, 121.95,
124.35, 125.08, 126.07, 127.89, 128.12, 129.52, 129.70, 131.18,
136.23, 136.83, 143.82, 146.03, 147.96, 148.10, 150.40, 163.57,
165.82, 199.63. MS (ESIþ, 4.5 kV, 200 ꢁC): m/z ¼ 618.3 ([MþH]þ),
calcd for C41H35N3Oþ3 ¼ 617.3 ([M]þ). MS (MALDI-TOF): 618.264
([MþH]þ), calcd for: 618.276.
through Michael addition of 2-acetylpyridine to a, b-unsaturated
ketones always forms the diketone using 2.4 equivalents of the
2-acetylpyridine. Instead of the diketone, the compounds 4ae4e
were obtained in 42e63% yields by a mild, efficient, and solvent-
free green procedure by Michael addition, when 2-acetylpyridine
was used in excess. The structures of the products 3ae3e and
4ae4e were characterized by the analyses of their spectral data,
including 1H NMR, 13C NMR, FTeIR and MS comparing with X-ray
diffraction analysis.
Compound 4c: yellow solid, yield 49%; mp: 191.7 ꢁC. FTeIR (KBr,
cmꢀ1): 3742, 3472, 3344, 3052, 2916, 1633, 1580, 1491, 1278,
1223, 1174, 837,755 and 693. 1H NMR (DMSO-d6, 400 MHz)
d
: 1.74 (d, 1H, J ¼ 14.3 Hz), 2.12 (d, 1H, J ¼ 13.1 Hz), 2.65 (t, 1H,
J ¼ 13.1 Hz), 3.16 (d, 1H, J ¼ 14.3 Hz), 4.32 (t, 1H, J ¼ 12.0 Hz), 4.52
(d, 1H, J ¼ 12.0 Hz), 6.00 (s, 1H), 6.38 (s, 1H), 6.52 (d, 2H,
J ¼ 8.7 Hz), 6.79 (t, 1H, J ¼ 4.0 Hz), 6.94 (m, 5H), 7.14 (m, 3H), 7.19
(d,1H, J ¼ 4.9 Hz), 7.32 (m, 8H), 7.52 (t,1H, J ¼ 7.7 Hz), 7.72 (d,1H,
J ¼ 7.9 Hz), 7.82 (t, 1H, J ¼ 7.3 Hz), 8.41 (d, 1H, J ¼ 4.2 Hz), 8.55 (d,
3.2. X-ray crystallography analysis
Suitable single crystals of 4c for X-ray structural analysis were
obtained by slow evaporation of a dichloromethane solution of 4c
at room temperature for several days. The spatial structures of
compound 4c were determined by using X-ray diffraction analysis.
A summary of crystallographic data collection parameters and
refinement parameters for 4c are compiled in Tables S1eS3. The
1H, J ¼ 4.2 Hz). 13C NMR (DMSO-d6, 100 MHz)
d: 33.78, 46.08,
46.18, 56.48, 75.78, 78.16, 118.95, 119.12, 119.65, 121.74, 122.04,
123.20, 124.34, 124.41, 125.15, 126.61, 129.52, 129.72, 130.99,
136.25, 136.91, 146.00, 147.52, 148.02, 148.16, 150.44, 163.27,

260
Y. Zhang et al. / Dyes and Pigments 123 (2015) 257e266
Scheme 1. The synthetic routes of compounds 4ae4e. Reagents and conditions: (I) NaOH, EtOH/H2O, 25 ꢁC; (II) 2-acetylpyridine, NaOH, grind.
single crystal structure and atomic number chosen for 4c were
shown in Fig. 1. The structure of compound 4c was crystallized in
monoclinic space group P2(1)/c.
The triphenylamine group and cyclohexyl in 4c was propeller
structure and chair conformation, being consistent with our esti-
mates. The crystal packing was stabilized by intermolecular in-
teractions such as C(phenyl)eH$$$O(hydroxyl) and three kind
3.3. Photophysical properties
The UVevis spectra of the compounds 4ae4e in different sol-
vents are shown in Fig. 2 and Fig. S1. The normalized absorption
spectra of compounds 4ae4e in thin film was shown in Fig. S2. The
optical characteristics were summarized in Table 1. Compounds
4ae4e exhibited similar absorption spectra with two peaks. The
different C(11, 23, 37)eH$$$
(Å) is 2.432 and the C(11, 23, 37)eH$$$
3.597, 3.069, respectively. In addition, the cyclohexyl structure was
almost perpendicular the thienyl group and two pyridine moiety,
which are 72.25ꢁ, 88.83ꢁ and 83.60ꢁ, respectively.
p
bonds. The hydrogen bond distance
bond distances are 3.238,
290 nm absorption bands were due to pep* transition and the
p
360e375 nm absorption regions were attributed to intramolecular
charge transfer (ICT) [16]. The absorbance of compounds 4ae4e
were different depending on various substitutions. For example,
the molar absorption coefficients (εmax) of 4c are the highest and 4d
Fig. 1. (A) Crystal structure of compound 4c, (B) intermolecular interactions (C) show CeH$$$O and CeH$$$
p bonds along the a-axis (Hydrogen atoms except H2A and H3B are
omitted for the sake of clarity).

Y. Zhang et al. / Dyes and Pigments 123 (2015) 257e266
261
Fig. 2. The UVevis spectra and fluorescence spectra of 4a in solvents with different polar (3.0 mM).
Table 1
Photophysical properties of compounds 4ae4e in different polar solvents.
ε (ꢂ104)
F
Dn
Fs
a
b
c
d
e
f
Compound
Solvents
lmax
lmax
4a
Benzene
Dichloromethane
THF
370
372
366
366
366
364
360
372
372
368
368
366
364
358
372
374
370
368
366
364
358
376
374
370
368
366
364
364
370
370
366
366
366
366
358
466
516
495
511
496
525
523
468
516
496
511
499
523
524
471
520
501
515
497
526
523
485
518
502
515
502
523
524
468
513
495
504
495
522
521
2.9695
3.1752
2.8933
2.8648
2.9143
2.9771
2.7724
3.0387
2.8857
2.8143
2.6553
2.7867
2.7190
2.4943
3.1724
3.4724
3.1362
3.0857
3.1943
3.2695
2.8810
2.3748
2.3724
2.3467
2.3390
2.2524
2.4162
2.1695
2.5286
2.6419
2.4524
2.4695
2.6029
2.5914
2.3333
0.97
0.27
0.54
0.01
0.44
0.02
0.03
0.93
0.25
0.48
0.01
0.40
0.02
0.03
0.90
0.22
0.24
0.01
0.35
0.01
0.02
0.74
0.21
0.40
0.01
0.34
0.02
0.02
0.93
0.26
0.50
0.01
0.42
0.02
0.03
5568
7502
7120
7753
7161
8425
8657
5514
7502
7013
7604
7282
8352
8849
5650
7507
7107
7756
7202
8461
8813
5977
7433
7107
7756
7402
8352
8389
5660
7534
7120
7481
7120
8165
8739
0.43
0.32
0.19
0.49
0.23
Ethanol
Ethyl acetate
Acetonitrile
DMF
benzene
Dichloromethane
THF
4b
4c
4d
4e
Ethanol
Ethyl acetate
Acetonitrile
DMF
Benzene
Dichloromethane
THF
Ethanol
Ethyl acetate
Acetonitrile
DMF
Benzene
Dichloromethane
THF
Ethanol
Ethyl acetate
Acetonitrile
DMF
Benzene
Dichloromethane
THF
Ethanol
Ethyl acetate
Acetonitrile
DMF
a
Peak position of the longest absorption band.
b
c
d
e
f
Peak position of SPEF, excited at the absorption maximum.
Molar absorptivity (L/cm/mol).
Quantum yields determined by using quinine sulfate as standard.
Stokes' shift in cmꢀ1
.
Quantum yields in solid.
are the lowest than other compounds in the same solvents. It
indicated that the subsidiary groups had an influence on their
photophysics properties.
The emission spectra of compounds 4ae4e in different solvents
were shown in Fig. 2 and Fig. S1. The detailed data were summa-
rized in Table 1. The normalized fluorescence spectra of compounds
4ae4e in thin film were shown in Fig. S2. It can be found that the
fluorescence intensity differed from each other both in solution and
solid state, which illustrated that the different substituent groups
had an influence on the emission. On the other hand, the com-
pounds exhibited an evident solvatochromic effect. The emission
wavelength of the compounds were red-shifted companied with
the decreased of quantum yields with the increasing of solvent
polarity (Fig. 2 and Table 1). This phenomenon was commonly

262
Y. Zhang et al. / Dyes and Pigments 123 (2015) 257e266
observed in the molecules with ICT. The molecular polarity was
significant increased when it transferred from ground state to
excited state. This would induce the strong dipoleedipole interac-
tion between solute and solvent. The interaction forced the solvent
molecules around the excited solute molecule occurred realign-
ment resulting the emission from a relaxation state. Thus, the
fluorescence wavelength was bathochromic [17].
To better understand the photophysical processes, molecular
orbitals were investigated by hybrid density functional theory
(B3LYP) with 6-31G* basis set employing the Gaussian 09 program
package [18]. The frontier molecular orbital electron cloud distri-
butions of 4ae4e were similar. The electron cloud of highest
occupied molecular orbital (HOMO) was localized on the triphe-
nylamine unit and carbonyl. While, the lowest unoccupied molec-
ular orbital (LUMO) was localized on the cyclohexyl and carbonyl.
The compounds exhibited the obviously charge separation of the
HOMO and the LUMO (Table S6). It indicated a typical ICT effect.
Moreover, the calculated band gaps of 4ae4e had little difference
which was consistent with our experimental results that the
absorbance spectra of them were similar.
Fig. 3. The UVevis spectra of compound 4c in different water fractions.
The quantum yields in solid state of compounds 4ae4e had been
determined and analyzed (Table 1). The steric hindered effect of the
emission in pure THF. However, with an increasing content of water
from 0 to 5%, the emission intensity of compounds was decrease
swiftly and remains faint emission intensity until the water content
was increased to 80%. When fw was further increased to 80%, the
molecule aggregates formed, accompanied by the emission of
4ae4e drastically increased, which demonstrated that 4ae4e
emitted both in solution and solid state. Interestingly, the profiles of
the fw dependent fluorescence spectra of 4ae4e were different
from the traditional AIEEgens. When fw was less than 80%, the
aggregates were not formed. The strong polar groups possessed the
ability to form hydrogen bond interactions with themselves, as well
as with the polar solvents [22]. The polarity of the mixed solution
was increased with the addition of water in the binary solvent
systems of THF-H2O. Thus, the emission of the compounds was
gradually decreased due to the solvatochromic effects. Meanwhile,
the emission wavelengths of compound 4c were blue-shift about
28 nm between 60% ꢃ fw ꢃ 99%. We noted that the film emission of
compound 4c was hypochromatic about 10 nm compare to its pure
THF solution. The reason of blue-shift might be attributed to the
formation of crystalline aggregates in high water fraction as the
previous literature [23].
chair conformation cyclohexyl impeded the intermolecular
pep
stacking in aggregation state. In addition, the formation of intra-
molecular hydrogen bond between the hydroxyls and carbonyl
improved the electronic conjugation and coupling by both through-
bond and through-space interactions [19]. Thus, the solid 4ae4e
could emit stronger fluorescence and had a high quantum yield in
solid state than the precursor of TPA. For the different substituent
groups, the quantum yields of 4ae4e in solid state were in the
range of 0.19e0.49. However, the quantum yield of TPA in solid
state was just 0.08. Obviously, the solid quantum yields of TPA
derivatives were increased about 2.5e6.1 fold after functioned with
the cyclohexyl. The reasons of different Fs of 4ae4e were caused by
the influence of different subsidiary groups of cyclohexyl structure.
According to the single crystal structure analysis of compound 4c,
we could find that the thienyl group influence the stacking of TPA
by C (thienyl, 37)-H$$$p bonds of intermolecular. The substitutions
of six-membered affected the arrangement of TPA derivatives
4ae4e in solid state; therefore, the quantum yields were different
with each other.
These compounds were good soluble in organic solvent such as
THF, whereas insoluble in water. To further study the optical prop-
erties in aggregation state, we added deionized water to THF in
order to cause molecules formation of aggregates. The concentra-
3.4. Thermal properties
The compounds were heated at 10 ꢁC/min under a nitrogen
atmosphere to detect the decomposition temperatures (Td) by
thermogravimetric analysis (TGA). The detailed data were listed in
Table S4. The compounds 4ae4e exhibited high thermal stabilities
and the decomposition temperature at 236.6, 231.4, 238.4, 234.5
and 230.5 ꢁC, respectively. The high decomposition temperatures
revealed that the compounds were stable.
tion of 4ae4e was kept at 10 mM in THF-water mixtures. The UVevis
absorption and PL emission spectra of compounds 4ae4e in THF-
water with different water volume fraction (fw) were displayed in
Fig. 3, Figs. S3 and S4. The compound 4c was analyzed carefully as a
representative of the material. When the water fractions increased
from 0 to 70%, the absorption band almost remained at the same
position of 370 nm and the molar absorbance (εmax) not showing
obvious change. This indicates that most of the molecules were still
free. When reach up to 80%, it worth that the lifted level-off tail
when the water volume fraction was 80%e99% in the absorption
spectra, which were attributed to Mie scattering caused by nano-
sized particles [20]. It showed that the molecular start aggregating.
Moreover, the absorption peak of compounds 4ae4e red shifted
about 7 nm, 22 nm, 11 nm, 13 nm, and 12 nm, respectively. This
phenomenon suggests that the molecules were J-type aggregation
that arranged in a head-to-tail direction indicating a better conju-
gation when the compounds become nanosized particles [21].
The emission spectra of 4ae4e in different water contents were
investigated (show in Fig. 4). 4ae4e had an analogous change
process of fluorescence. They exhibited strong fluorescence
3.5. Electrochemical properties
The electrochemical properties of compounds 4ae4e were
analyzed by cyclic voltammetry (CV) in CH2Cl2 in the presence of
Bu4NClO4 (0.10 mol Lꢀ1) as supporting electrolyte. All CV mea-
surements were recorded at room temperature with a conventional
three-electrode configuration consisting of a glass carbon working
electrode, a platinum wire counter electrode, and an Ag/AgCl
reference electrode under argon. The subsidiary groups had a slight
effect on their electrochemical behavior. As shown in Fig. S6, all of
4ae4e showed better redox reversibility that there was an oxida-
tion peaks about 0.9 V and a reduction peaks about 1.1 V. The

Y. Zhang et al. / Dyes and Pigments 123 (2015) 257e266
263
Fig. 4. Fluorescence spectra of compound 4c in different water fractions (up left); effect of water fraction on the maximum fluorescence intensity and emission wavelength (up
right); optical photographs recorded under 365 nm UV irradiation with various fractions of water (down).
HOMO energy values of them were calculated by using the ex-
a new band at 425 nm of UVevis spectra was formed with the
titration of PA, suggesting an interaction between the compounds
4ae4e and PA occurred [25]. Almost 63% quenching of fluorescence
intensity happened when ten equivalents of PA were titrated. For
other NACs, however, no more than 18% when keeping the same
equivalents, indicated that compound 4c can be as a sensor for PA.
As shown in Fig. S9, SterneVolmer plots of compounds 4ae4e are
similar to each other. The detection limits of these compounds
OX
OX
pressions: HOMO ¼ [ꢀ(E
þ 4.38)] eV (E
is the first oxidation
onset
onset
onset potential) [9a,24], displayed in Table S5. The onset oxidation
potentials of the compounds 4ae4e were, 0.98, 0.95, 0.94, 0.97, and
1.08, respectively, nearly equal to one another. The calculated
HOMO energy values were ꢀ5.36, ꢀ5.33, ꢀ5.32, ꢀ5.35 and ꢀ5.46,
respectively,
which
were
consisted
with
the
values, ꢀ5.04, ꢀ5.14, ꢀ5.28, ꢀ5.19 and ꢀ5.07, respectively, obtained
from DFT calculation.
4ae4e were calculated to be 6.0 ꢂ 10ꢀ7, 1.8 ꢂ 10ꢀ6, 1.5 ꢂ 10ꢀ6
,
1.0 ꢂ 10ꢀ6, 7.5 ꢂ 10ꢀ7 mol Lꢀ1 for PA, respectively. The fluorescence
quenching mechanism of compound 4c for PA could be explained
by fluorescence resonance energy transfer (FRET) [15c,26].
As shown in Fig. S11, there was a clearly spectral overlap of the
absorption spectrum of PA and the emission spectrum of 4c in
the range of 400e480 nm in THF solution. For other compounds,
the similar phenomena were observed in THF solution, it indicated
that the fluorescence quenching mechanism was the same to 4c.
Based on the high emission of 4ae4e in solid state, to employ
the detection in a feasible method, test strips were prepared that
3.6. Explosive detection
Firstly, the recognition behaviors of the compounds 4ae4e
toward the selected explosive compounds (NACs) were investi-
gated by UVevis and fluorescence spectra in dilute solution, as
shown in Fig. 5 and Fig. S7. For compounds 4ae4e, the concentra-
tion was kept at 10 mM in THF and the NACs were kept at 5 mM in
acetonitrile. Among the various nitro derivatives, all of the com-
pounds 4ae4e exhibited selectivity toward PA (Fig. S8). In Fig. 5,
Fig. 5. Changes in UVevis and fluorescence spectra of compound 4c (10 mM) upon titration with picric acid (0e15 equiv.) in THF.

264
Y. Zhang et al. / Dyes and Pigments 123 (2015) 257e266
Appendix A. Supplementary material
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.08.013.
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