Novel UV-sensitive bis-chalcone derivatives: Synthesis and photocrosslinking properties in solution and solid PMMA film

Article abstract of DOI:10.1007/s11164-010-0236-0

A series of chalcone derivatives in which two chalcone groups are attached by alkyldioxy chains were synthesized and characterized by ¹H NMR, UV-Vis, and Fourier transform infrared (FTIR) spectral analysis. Upon the irradiation of 365-nm UV lig

Full text of DOI:10.1007/s11164-010-0236-0

Res Chem Intermed (2011) 37:635–646
DOI 10.1007/s11164-010-0236-0
Novel UV-sensitive bis-chalcone derivatives: synthesis
and photocrosslinking properties in solution and solid
PMMA film
•
Zheng Kai Si Qing Zhang Min Zhao Xue
•
•
•
Qiao Rong Sheng Yan Gang Liu
Received: 13 December 2010 / Accepted: 30 December 2010 / Published online: 29 January 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A series of chalcone derivatives in which two chalcone groups are
1
attached by alkyldioxy chains were synthesized and characterized by H NMR,
UV–Vis, and Fourier transform infrared (FTIR) spectral analysis. Upon the irradi-
ation of 365-nm UV light, the chalcone groups in the molecules underwent
[2p ? 2p] photodimerization. The photocrosslinking properties were investigated
both in solution and in polymethyl methacrylate (PMMA) solid films on quartz
plate. It has been found that the photocrosslinking rates of the compounds depended
on the different flexibility of the functional groups, which was determined by the
length of the spacer chain between the two chalcone moieties. The longest soft chain
containing derivatives has a faster photocrosslinking rate both in solution and in
solid film. Irradiated by polarized ultraviolet light (PUV), all of the films doped with
bis-chalcone derivatives showed an anisotropic absorption property, which may
give a promising application as LC-alignment materials.
Keywords Chalcone ꢀ UV irradiation ꢀ Photocrosslinking ꢀ Photodimerization
Introduction
Photosensitive materials have attracted considerable attention in the past several
decades, since the photochemical reactions in organic materials may induce many
changes in the properties, such as solubility, optical behavior, photomechanical
effect, refractive index, and dielectric constant [1–6]. Among the photosensitive
materials, the UV-sensitive polymers have been highlighted because of their
excellent properties and wide applications in many fields. The technological
Z. K. Si ꢀ Q. Zhang ꢀ M. Z. Xue ꢀ Q. R. Sheng ꢀ Y. G. Liu (&)
School of Chemistry and Chemical Engineering, Shanghai Jiaotong University,
Shanghai 200240, People’s Republic of China
e-mail: ygliu@sjtu.edu.cn
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Z. K. Si et al.
applications of these photoactive materials are in the fields of lithography [7–10],
liquid crystals [11–14], nonlinear optical materials [15–18], optical anisotropy
[19–21], and UV curing process [22–24].
Photocrosslinking is one of the most attractive photochemical reactions. The
polymers containing a, b-unsaturated carbonyl groups, such as cinnamoyl and
chalcone groups, are very sensitive to UV light and undergo crosslinking reaction
upon UV light irradiation [25–28]. These polymers would have the properties of
high photosensitivity, good solubility, ability to form films, good resistance towards
solvents, and thermal stability after [2p ? 2p] photodimerization reaction, which
are very important for the practical applications. As the chalcone groups possess
very high absorption coefficients of around 300–365 nm, its photocrosslinking
reaction rate is relatively faster compared to that of cinnamoyl groups under the
irradiation of UV light [15]. This will enable chalcone derivatives to have more
interesting properties, including better thermal stability and resistance capacity than
cinnamoyl groups. However, most research work has focused on compounds with
chalcone groups embedded in the main chain or pended as side chains [29–36]. As
the chalcone groups of the dichalcone functionalized monomer can be dimerized
under UV light irradiation, it is theoretically possible to obtain linear polymers if the
reaction takes place intermolecularly [19, 37, 38]. Nevertheless, to the best of our
knowledge, few research works have been reported on the photocrosslinking
properties of molecules containing double chalcone moieties.
Herein, we, for the first time, report the synthesis and characterization of a series
of new photosensitive bis-chalcone derivatives with a structure of two chalcone
units connected by different alkyldioxy spacer chains. Upon irradiation of 365-nm
UV light, the chalcone units crosslinked each other and the photocrosslinking
properties were studied both in solution and in solid film, as well as the effect of the
spacer chain on the rate of crosslinking. The research would be seen as preceding
exploration to evaluate the applications of these materials.
Experimental
Materials
4-Hydroxybenzaldehyde, acetophenone, 1,2-dibromoethane, 1,3-dibromopropane,
1,4-dibromobutane, and 1,6-dibromohexane (CP) were purchased from Sinopharm
Chemical Reagent Co., Ltd and used as received. Polymethyl methacrylate (PMMA)
was kindly provided by the Shanghai Zhibi Chemical Factory. N,N-dimethylform-
amide was dried by 4A molecular sieves. Anhydrous potassium carbonate and
potassium iodide (AR) were obtained from Sinopharm Chemical Reagent Co., Ltd.
Ethanol, chloroform, and other solvents were used without further purification.
Synthesis of 4-hydroxyphenyl styryl ketone (4-hydroxychalcone)
In a three-necked flask equipped with a magnetic stirrer and dropping funnel, a
solution of 2.0 g (50 mmol) NaOH in 3 mL deionized water was added to 2.40 g
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Novel UV-sensitive bis-chalcone derivatives
637
(20 mmol) acetophenone in 60 mL of ethanol. The mixture was cooled to room
temperature. A solution of 2.45 g (20 mmol) 4-hydroxybenzaldehyde in 40 mL
ethanol was then added dropwise with constant stirring and the temperature was not
allowed to exceed 25 °C. After stirring for 24 h, the mixture was poured into
500 mL water. The pH of the solution was adjusted to about 6 by adding 1 mol/L
HCl and the solution was set aside for more than 12 h. The precipitated product was
filtered, washed several times with ice-cold water, and purified by recrystallization
from 95% ethanol into a yellow crystalline product. Yield: 57.8%, mp 186–187 °C.
1H NMR (400 MHz, CDCl3), d (ppm): 8.94 (s, 1H, –OH), 8.02–8.00 (d, 2H, Ar–H),
7.80–7.76 (d, 1H, =CH–CO–), 7.58–7.55 (m, 3H, Ar–H), 7.52–7.48 (t, 2H, Ar–H),
7.43–7.39 (d, 1H, =CH–Ar), 6.91–6.88 (d, 2H, Ar–H). Elemental analysis (%):
Calculated: C 80.34, H 5.39; Found: C 81.37, H 5.36. FTIR (KBr, cm-1): 3216
(–OH), 3020 (Ar–H), 1650 ([C=O), 1599 (s, –C=C– olefinic), 1557, 1512
(s, –C=C–Ar), 834, 725, 690 (–C–H).
General procedure for the synthesis of dichalcone derivatives
In a dry two-necked flask fitted with a dropping funnel and condenser closed with a
calcium chloride tube, 5 mmol of the corresponding dibrominated alkane was
added dropwise with stirring to a solution of 2.47 g (11 mmol) 4-hydroxychalcone
dissolved in 20 mL DMF in the presence of 3.04 g (22 mmol) anhydrous potassium
carbonate and a small amount of potassium iodide. The mixture was reacted at
80 °C with stirring for 24 h and was then poured into 300 mL deionized water. The
solution was set aside overnight. The product was collected by filtration and purified
by recrystallization from a mixture of ethanol and chloroform.
1,2-Di (20-benzoyl-4-styryloxy) ethane (BSOE)
1
Yield: 32.5%, mp 189–191 °C. H NMR (400 MHz, CDCl3), d (ppm): 8.02–8.00
(d, 4H, Ar–H), 7.81–7.77 (d, 2H, =CH–CO–), 7.63–7.61 (d, 4H, Ar–H), 7.58–7.56
(d, 2H, Ar–H), 7.52–7.48 (t, 4H, Ar–H), 7.45–7.41 (d, 2H, =CH–Ar), 7.01–6.98
(d, 4H, Ar–H), 4.40 (s, 4H, –O–CH2–). LC–MS: m/z 474.0 (M?). Elemental analysis
(%): Calculated: C 80.99, H 5.52; Found: C 81.01, H 5.48. FTIR (KBr, cm-1): 3066
(Ar–H), 2941, 2879 (–C–H), 1654 ([C=O), 1599 (s, –C=C– olefinic), 1589, 1570,
1511, 1426 (–C=C– aromatic), 1175, 1033 (s, –C–O–C–), 834, 724, 696 (–C–H).
1,3-Di (20-benzoyl-4-styryloxy) propane (BSOP)
1
Yield: 46.2%, mp 184–185 °C. H NMR (400 MHz, CDCl3), d (ppm): 8.02–7.99
(d, 4H, Ar–H), 7.80–7.76 (d, 2H, =CH–CO–), 7.61–7.59 (d, 4H, Ar–H), 7.57–7.56
(m, 2H, Ar–H), 7.51–7.48 (t, 4H, Ar–H), 7.44–7.40 (d, 2H, =CH–Ar), 6.96–6.94
(d, 4H, Ar–H), 4.24–4.21 (t, 4H, –O–CH2–), 2.34–2.28 (m, 2H, –C–CH2–C–).
LC–MS: m/z 488.3 (M?). Elemental analysis (%): Calculated: C 81.12, H 5.78;
Found: C 81.14, H 5.75. FTIR (KBr, cm-1): 3055 (Ar–H), 2941, 2880 (–C–H), 1661
([C=O), 1600 (–CH=CH– olefinic), 1572, 1511, 1422 (–C=C– aromatic), 1176,
1018 (–C–O–C–), 826, 720, 686 (–C–H).
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1,4-Di (20-benzoyl-4-styryloxy) butane (BSOB)
1
Yield: 84.6%, mp 156–158 °C. H NMR (400 MHz, CDCl3), d (ppm): 8.02–8.00
(d, 4H, Ar–H), 7.81–7.77 (d, 2H, =CH–CO–), 7.61–7.59 (d, 4H, Ar–H), 7.58–7.56
(d, 2H, Ar–H), 7.52–7.48 (t, 4H, Ar–H), 7.44–7.40 (d, 2H, =CH–Ar), 6.94–6.92
(d, 4H, Ar–H), 4.10–4.08 (t, 4H, –O–CH2–), 2.03–2.01 (m, 4H, –O–C–CH2–).
LC–MS: m/z 502.4 (M?). Elemental analysis (%): Calculated: C 81.25, H 6.02;
Found: C 81.28, H 5.99. FTIR (KBr, cm-1): 3059 (Ar–H), 2929, 2870 (–C–H), 1654
([C=O), 1598 (–CH=CH– olefinic), 1589, 1569, 1512, 1473 (–C=C– aromatic),
1174, 1018 (–C–O–C–), 831, 724, 696 (–C–H).
1,6-Di (20-benzoyl-4-styryloxy) hexane (BSOH)
1
Yield: 85.2%, mp 166–168 °C. H NMR (400 MHz, CDCl3), d (ppm): 8.02–8.00
(d, 4H, Ar–H), 7.81–7.77 (d, 2H, =CH–CO–), 7.61–7.58 (d, 4H, Ar–H), 7.57–7.55
(d, 2H, Ar–H), 7.51–7.48 (t, 4H, Ar–H), 7.43–7.39 (d, 2H, =CH–Ar), 6.93–6.91
(d, 4H, Ar–H), 4.02 (t, 4H, –O–CH2–), 1.85 (m, 4H, –O–C–CH2–), 1.58 (m, 4H,
–O–C–C–CH2–). LC–MS: m/z 530.1 (M?). Elemental analysis (%): Calculated:
C 81.48, H 6.46; Found: C 81.50, H 6.44. FTIR (KBr, cm-1): 3056 (Ar–H), 2943,
2899, 2868, 2851 (–C–H), 1653 ([C=O), 1599 (–CH=CH– olefinic), 1589, 1567,
1511, 1473(–C=C– aromatic), 1173, 1038 (–C–O–C–), 833, 725, 696 (–C–H).
Measurements
1H-NMR spectra of the dichalcone compounds were obtained on a Mercury Plus
400 (400 MHz for proton) spectrometer with TMS as the internal standard reference
in CDCl3 solution. Fourier transform infrared (FTIR) spectra were recorded using a
Perkin Elmer Paragon 1000 spectrophotometer by incorporating dichalcone
compounds in the potassium bromide tablets. Elemental analysis was performed
with a Vario EL II analyzer. Mass spectra were recorded on an Agilent 1100LC/
MSD system. The UV–Vis spectra of the spin-coated films on quartz plate were
determined on a Beijing Purkinje TU-1901 spectrophotometer. The melting points
were taken from an X-4 digital apparatus without correction. The intensity of the
UV light was detected with a broadband UV light radiometer (365 nm, Photoelec-
tric Instrument Factory of Beijing Normal University, China).
Film preparation and UV light irradiation
For fabricating the thin films, the synthesized chalcone derivatives and PMMA
(ratio: 1:10 wt) were dissolved in CHCl3 to give solutions with a concentration of
0.04 g/mL. The solutions were filtered through membranes with the pore size of
0.2 lm and were then spin-coated onto quartz plates (20 9 20 mm). The spin-
coating rate was controlled to be 600 rpm for 9 s and then 1,500 rpm for 30 s to
obtain films with similar thickness. The films were then dried at 60 °C under a
vacuum condition overnight to remove the residual solvent completely.
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Novel UV-sensitive bis-chalcone derivatives
639
A high-pressure mercury lamp equipped with a band pass filter to obtain an
emission of wavelengths mainly between 300 and 380 nm was used for the
irradiation of derivatives in solution and film on quartz plate. The intensity of the UV
light on the exposed surface was 3.44 mW/cm2 at 365 nm. The polarized ultraviolet
light (PUV) was achieved by the addition of a polarizer in the light route, the
intensity of which was 0.52 mW/cm2 at 365 nm. After each exposure interval,
the UV spectrum of the sample was recorded immediately. The rate of disappearance
of the UV-reactive chalcone group was monitored according to the following
equation:
Extent of conversion (%) ¼ ðA0 ꢁ AT Þ=A0 ꢂ 100
ð1Þ
where A0 and AT are the absorption intensities due to[C=C\after irradiation times
t = 0 and t = T, respectively.
Results and discussion
Synthesis
A series of UV-sensitive compounds with a structure of two chalcone moieties
connected by alkyldioxy soft spacer chains were synthesized as illustrated in
Scheme 1. The 4-hydroxychalcone was prepared by reacting 4-hydroxybenzalde-
hyde with acetophenone in ethanol/water solution in the presence of sodium
hydroxide as the base [27]. The reaction of 4-hydroxychalcone with the correspond-
ing alkyl dihalide, such as 1,2-dibromoethane, 1,3-dibromopropane, 1,4-dibromo-
butane, and 1,6-dibromohexane, through Williamson reaction in the presence of
anhydrous K2CO3 and KI led to the target bis-chalcone compounds. The solubility of
the prepared chalcone derivatives in various solvents was tested since it is one of the
most important factors for practical applications of these compound; all of the
derivatives are easily soluble in polar aprotic solvents and chlorinated solvents such
as chloroform, dichloromethane, N,N-dimethylformamide, N-methyl-2-pyrrolidone,
and dimethyl sulfoxide.
The chemical structures of the four chalcone derivatives were confirmed by
1
elemental analysis, H NMR, and FTIR spectra; the details have been given in the
1
experimental part. The typical H NMR spectra will be presented here for further
1
discussion as a sample. Figure 1a shows the H NMR spectrum of 4-hydroxychal-
cone. The chemical shifts and the integral areas are clearly consistent with the
proposed structure. After reaction with dibrominated alkanes, resonance at 8.94,
which belongs to the proton on the hydroxyl, disappeared completely and new
resonances at the high field of 4.40 in BSOE, 4.22 in BSOP, 4.10 in BSOB, and 4.02
in BSOH indicate the formation of aralkyl ether bond. The spectral variations of
2.31 (Fig. 1c), 2.02 (Fig. 1d), 1.85, and 1.58 (Fig. 1e) can be assigned to the other
methylene protons of spacer chains. By comparing the proton peaks and integral
area, which are all very consistent with the chemical structure of bis-chalcone
molecules, it can be concluded that the new chalcone derivatives were obtained.
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Z. K. Si et al.
O
O
O
EtOH, NaOH/H2O
25 °C
+
H
OH
OH
K2CO3/KI/DMF
80 °C
Br-(CH2)n-Br
O
O
O
(CH2)n O
n= 2, 3, 4, 6
Scheme 1 Synthetic procedure of dichalcone moieties containing molecules BSOE, BSOP, BSOB, and
BSOH. ChemDraw Ultra 9.0 was used to create the artwork
a
b
c
d
e
10
9
8
7
6
5
4
3
2
1
0
d (ppm)
Fig. 1 1H NMR spectra of (a) 4-hydroxychalcone, (b) BSOE, (c) BSOP, (d) BSOB, (e) BSOH.
MestReNova (version: 5.3.1) and OriginPro 7.5 were used to create the artwork
Photocrosslinking behavior in solution
The photosensitivity of the chalcone derivatives toward UV light was investigated
by measuring the UV absorption of the chalcone moieties in the molecules. The bis-
chalcone compounds undergo crosslinking reaction by dimerization between the
double bonds in chalcone groups (Scheme 2). The UV–Vis absorption spectra of
BSOP in CHCl3 solution were recorded for different time intervals during UV
irradiation (Fig. 2). The UV absorption spectroscopy of BSOP showed a maximum
absorption at 341 nm, corresponding to the p–p* transition of the chalcone moiety.
Under irradiation, the decrease of the absorption at 341 nm was evident, which can
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Novel UV-sensitive bis-chalcone derivatives
641
O
O
O
O
UV: 365nm
+
O
O
O
O
(CH2)n
O
O
O
O
n= 2, 3, 4, 6
Scheme 2 Structural change of bis-chalcone derivatives during UV light irradiation. ChemDraw Ultra
9.0 was used to create the artwork
1.0
0.8
0.6
0.4
0.2
0.0
250
300
350
400
450
500
Wavelength (nm)
Fig. 2 Absorbance spectral change of BSOP in CHCl3 solution irradiated with 365-nm UV light. Top to
bottom: t = 0, 5, 10, 20, 40, 60, 90, 120, 180, 240, 300, 390, 480, 600, and 900 s
be attributed to the decrease in the conjugated system and a concomitant increase in
the absorbance at a shorter wavelength of 259 nm due to the formation of
cyclobutane rings through [2p ? 2p] cycloaddition of the double bond in the
chalcone unit. The maximum absorbance at 341 nm of the double bond decreased
drastically upon UV irradiation. Therefore, the bis-chalcone compounds react
photochemically according to a mechanism that is well studied for cinnamic acid
and its derivatives in which cyclobutane structures are formed [39].
The photocrosslinking rate of the chalcone derivatives was studied in solutions in
terms of the disappearance of photoactive double bond in the chalcone unit. BSOH
has the fastest and highest rate, while BSOE has the lowest compared with the other
samples. This result may be attributed to the fact that the spacer units between the
two chalcone moieties could determine the free movements of the photosensitive
groups. The long and soft alkyoxy chain gives more flexibility of chalcone units to
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Z. K. Si et al.
BSOE
BSOP
BSOB
BSOH
1.0
0.8
0.6
0.4
0.2
0.0
0.7
0.6
0.5
0.4
0.3
0.2
0
40
80
120 160 200
0
500
1000
1500
2000
2500
Time (s)
Fig. 3 Photocrosslinking rate of the chalcone units of BSOE, BSOP, BSOB, and BSOH with UV
irradiation in CHCl3 solutions
increase the rate of crosslinking, which is similar to the results described in the
literature [27–29]. Thus, the photocrosslinking rate of the chalcone derivatives is
BSOH [ BSOB [ BSOP [ BSOE, as shown in Fig. 3.
Photocrosslinking behavior in PMMA film
The photocrosslinking reactions of the chalcone derivatives were also carried out
in PMMA films (Fig. 4). It is notable that the photoreactivity order of the four
chalcone derivatives in PMMA films is similar to that observed in solutions,
while the difference in photocrosslinking rates between each sample is not so
obvious. It may be explained by the fact that the PMMA chains restrict the
movement of chalcone groups, thus, leaving very little free volume for
crosslinking and the influence of the soft spacer chain is not as significant as
that in solution. The geometrical hindrance also leads to the photocrosslinking
rates being much slower in solid films than those in solutions and the relevant
photocrosslinking degree is smaller than that in solution also (Fig. 5). It is
reasonable to consider that the bis-chalcone derivatives could move freely and the
cis–trans configuration of chalcone moieties may transform easily in solution,
which leads to increased rate of crosslinking. As the exposure time was
prolonged, the unreacted chalcone molecules could move closely to crosslink,
which results in a much higher crosslinking extent. However, the chalcone groups
in solid PMMA film would be restricted by the polymer chains and the
photocrosslinking reaction could only occur between the adjacent chalcone units.
So, the procedure of photoreaction in film is much slower and the reaction
conversion is lower than that in solution.
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Novel UV-sensitive bis-chalcone derivatives
643
BSOE
BSOP
BSOB
BSOH
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.35
0.30
0.25
0.20
0.15
0.10
0
40
80
120
160
200
0
500
1000
1500
2000
2500
Time (s)
Fig. 4 Photocrosslinking rate of chalcone units of BSOE, BSOP, BSOB, and BSOH with UV irradiation
in PMMA films
BSOP Solution
1.0
0.8
0.6
0.4
0.2
0.0
BSOH Solution
BSOP Film
BSOH Film
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
40
80
120
160 200
0
500
1000
1500
2000
2500
Time (s)
Fig. 5 Photocrosslinking rate of BSOP and BSOH in CHCl3 solution and in PMMA films
Anisotropic behavior after PUV irradiation
In order to investigate the UV-induced optical properties of the chalcone
derivatives, the prepared PMMA films were also exposed to PUV for a certain
amount of time and then shown anisotropic spectra absorption by monitoring the
maximum absorbance. In Fig. 6, the differences of absorption spectra between the
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Z. K. Si et al.
BSOE
0
330
30
0.60
0.55
0.50
0.45
0.40
0.40
0.45
0.50
0.55
0.60
300
60
270
90
240
120
210
330
150
30
180
0
BSOH
0.60
0.55
0.50
0.45
300
60
0.40 270
0.45
90
0.50
240
120
0.55
0.60
210
150
180
Fig. 6 Circular diagrams of the absorbance of chalcone units in BSOE and BSOH after irradiation with
PUV in PMMA films. OriginPro 7.5 was used to create the artwork
perpendicular and parallel polarization direction of the PUV were illustrated by
samples of BSOE and BSOH. The parallel absorbance was observed to be smaller
than the perpendicular absorbance in the whole range of the irradiation, which may
attributed to the directional selectivity of photocrosslinking of the double bond in
chalcone groups. It can be assumed that the anisotropically UV-dimerized
cyclobutane rings of the C=C in chalcone moieties and the rearrangement of the
groups attaching to them will induce the directional orientation of liquid crystal,
although further studies are necessary in order to measure the parameters of the
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Novel UV-sensitive bis-chalcone derivatives
645
PUV-exposed films. Thus, these molecules could give a potential application as
LC-alignment materials.
Conclusion
Four chalcone derivatives with a structure of two chalcone moieties connected by
1
different alkyldioxy chains have been synthesized and characterized by H-NMR,
UV–Vis, and Fourier transform infrared (FTIR) spectra. Upon irradiation of the
365-nm UV light, the molecules, which have an absorption maximum at about
340 nm, undergo [2 ? 2] photodimerization reaction. The photocrosslinking
properties were studied both in solutions and polymethyl methacrylate (PMMA)
films on quartz plate. It is found that the photocrosslinking rates of the bis-chalcone
compounds in the PMMA films were much slower than those in the solutions, and
the derivatives containing the longest spacer chain had a faster photocrosslinking
rate. Irradiated with polarized ultraviolet light (PUV), films doped with the chalcone
derivatives showed an anisotropic absorption property. It is reasonable to assume
that the chalcone derivatives doped into commercially available polymer matrix
would give promising applications as photoresist and photoalignment materials.
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