Synthesis of furan derivative as LED light photoinitiator: One-pot, low usage, photobleaching for light color 3D printing

Article abstract of DOI:10.1016/j.dyepig.2019.03.011

A type II photoinitiator, containing furan structure, 2,6-bis(furan-2-ylmethylidene) cyclohexan-1-one (BFC) was synthesized via one step reaction between furan and cyclohexanone. This photoinitiator has large molar extinction coefficient and long wavelength absorption attributing to its large conjugation structure. Photopolymerization kinetics reveals that photoinitiator BFC at low concentration exhibits the high polymerization rate and conversion. UV-vis absorption and fluorescence spectrum demonstrate that BFC has a big red shift in the polyethylene glycoldiacrylate (PEGDA) solution and the light absorption region could be extended to 500 nm because of exciplex formed by BFC and PEGDA. Moreover, PEGDA is the excellent hydrogen donor to photoinitiator BFC compared with traditional coinitator amine. This BFC/PEGDA photoinitiator system has an outstanding photobleaching performance, which has potential to be used in the light-color materials with visible light photopolymerization, such as 3D photopolymerization printing material with long wavelength LED lamp.

Full text of DOI:10.1016/j.dyepig.2019.03.011

Accepted Manuscript
Synthesis of furan derivative as LED light photoinitiator: One-pot, low usage,
photobleaching for light color 3D printing
Jingfang Li, Yajuan Hao, Meijuan Zhong, Liqun Tang, Jun Nie, Xiaoqun Zhu
PII:
S0143-7208(18)32065-5
https://doi.org/10.1016/j.dyepig.2019.03.011
DYPI 7400
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 18 September 2018
Revised Date: 15 January 2019
Accepted Date: 5 March 2019
Please cite this article as: Li J, Hao Y, Zhong M, Tang L, Nie J, Zhu X, Synthesis of furan derivative
as LED light photoinitiator: One-pot, low usage, photobleaching for light color 3D printing, Dyes and
Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.03.011.
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Synthesis of furan derivative as LED light photoinitiator:
one-pot, low usage, photobleaching for light color 3D
printing
Jingfang Li, Yajuan Hao, Meijuan Zhong, Liqun Tang, Jun Nie*, Xiaoqun Zhu*
State Key Laboratory of Chemical Resource Engineering & Beijing Laboratory of Biomedical
Materials, Beijing University of Chemical Technology, Beijing, 100029
ABSTRACT
:
A
type
II
photoinitiator,
containing
furan
structure,
2,6-bis(furan-2-ylmethylidene) cyclohexan-1-one (BFC) was synthesized via one step reaction
between furan and cyclohexanone. This photoinitiator has large molar extinction coefficient and
long wavelength absorption attributing to its large conjugation structure. Photopolymerization
kinetics reveals that photoinitiator BFC at low concentration exhibits the high polymerization rate
and conversion. UV-vis absorption and fluorescence spectrum demonstrate that BFC has a big red
shift in the polyethylene glycoldiacrylate (PEGDA) solution and the light absorption region could
be extended to 500 nm beacuse of exciplex formed by BFC and PEGDA. Moreover, PEGDA is
the excellent hydrogen donor to photoinitiator BFC compared with traditional coinitator amine.
This BFC/PEGDA photoinitiator system has an outstanding photobleaching performance, which
has potential to be used in the light-color materials with visible light photopolymerization, such as
3D photopolymerization printing material with long wavelength LED lamp.
Keywords: Type II, LED, Low usuage, Exciplex, 3D printing, photobleaching

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1. Introduction
As new generation of green technology, photopolymerization has attracted great interests to
scientists and industries, for it has excellent advantages such as high efficiency, energy saving,
room temperature polymerization, and environmental friendly. It is widely used in coatings,
adhesives, paints and inks.1-9 In recent years, the government and people have been increasing
concerns to the environment, and the policy of environmental protection is getting stricter. Lots of
corporations involving in traditional solvent coatings, inks, adhesives or paints have switched to
the technique of photopolymerization, which has been accepted and generalized by more and more
countries and corporations.
In the context of global environmental protection, regards to the development of
photopolymerization, developing photoinitiators with the absorption wavelength above 365 nm or
visible light is a popular subject.10-14 That’s because the traditional light source is the mercury
lamp and it’s well-known the mercury is toxic to creatures. In the year of 2017, the “Minamata
convention” came into force and mercury lamp would be banned. Instead, LED lamp would be
used as the main light source. However, at present, limited by the bottleneck of LED technology,
the LED lamp with the wavelength over 365 nm could be used widely and steadily, which means
those photoinitiator with short absorption wavelength would not meet with the LED lamp.
Therefore, developing photoinitiator with LED or visible light absorption is urgent.
Several works about visible light photointiators with novel structure have been reported. Xiao
Pu and Jacques Lalevees’ groups have reported a series of visible light photoinitiators, such as
aminothiazonaphthalimide derivatives,15 naphthalic anhydride derivatives,16 curcumin,17 one of
the natural product. These photoinitators have absorption wavelength above 400 nm. Yusuf
Yagci’s group have reported a series of visible light photoinitator with novel structure, such as
graphitic carbon nitride (g-C3N4),18 benzoyltrimethylgermane (BTG).19 However, the efficiency of

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these photoinitiators is not very high. The synthesis process of the compounds is complex or it
would be expensive. And also, the used co-initiator or additives including diphenyliodonium
hexafluorophosphate (Iod), triphenylphosphine (TPP) was toxic and the application would be
confined in some areas.20
In this research, we report a new kind of LED photoinitiator (BFC) with furan ring structure,
which was synthesized by one-step through Claisen-Schmidt reaction of furan and cyclohexanone.
Attributing to the large conjugate structure of the BFC, it has absorption of long wavelength and
the maximum absorption region reaches to 460 nm. To our surprise, low usage of photoinitiator
BFC can achieve high polymerization rate and conversion of acrylate. Moreover, the result of
UV-vis absorption spectrum demonstrates that BFC can form exciplex with PEGDA which make
the maximum absorption region reach to 500 nm. Unlike the other type II photoinitiator which
usually prefers amine as co-initiator, BFC prefers PEGDA as co-initiator rather than amine.
PEGDA is no toxic, no odor, and could be polymerized together with monomer. On account of the
excellent photobleaching performance of BFC/PEGDA system, this initiating system would be
expected using in the light color materials with long wavelength irradiation.
2.Experimental
2.1.Materials
Furfural was purchased from Energy Chemical (Shanghai, China). Cyclohexanone, NaOH,
ethyl alcohol was purchased from Sinopharm Group Chemical Reagent Co. (Beijing, China). The
monomers polyethylene glycoldiacrylate (PEGDA, M=400), hydroxyethyl acrylate (HEA),
hexamethylene diacrylate (HDDA), trimethylolpropane triacrylate (TMPTA) were purchased from
Sartomer (Guangzhou, China). ITX, Irgacure 784 and EDAB were purchased from Hubei Gurun
technology company. All reagents were used as received. The chemical structures of the
compounds in this research are shown in Scheme 1.
Scheme 1 Chemical structures of monomers and additives

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Scheme 2 Synthesis route of BFC
2.2. Synthesis and characterization
The synthesis route of BFC is shown in scheme 2. The details of synthesis and
characterization are provided below. A mixture of furfural (2.5g 0.01mol), cyclohexanone (0.98g
0.01mol), and methyl alcohol (20mL) were added into a 100mL round-bottom flask equipped with
stirrer. NaOH/water solution (10% wt) was dropped into the solution and reacted at room
temperature for 5h. The process was monitored by TLC. The recrystallization was used for
purification (methyl alcohol, 3 times) to obtain yellow product (2.63 g, 75% yield). 1H-NMR and
elemental analysis were used to characterize chemical structures of the target molecules.1H-NMR
spectra of the sample were determined by a Bruker 500 MHz spectrometer at room temperature,
elemental analysis of the samples was performed by a CE44 in the measure center of Beijing
university of chemical technology.
2.3. Irradiation source and fluorescence experiments
The LED lamp with emission wavelength 365 nm (incident light intensity: 70mw/cm2) and 405
nm (incident light intensity: 70mw/cm2) were used as irradiation device. And the light intensity
was measured by light intensity meter (ST-86LB) purchased by Beijing normal university. The
fluorescent property of the BFC in different solvents was studied by using a JASCO FP-750
spectrofluorometer purchased by Beijing university of chemical technology.
2.4. Steady state photolysis experiments
The photoinitiator BFC in acetonitrile and PEGDA was irradiated with the 405 nm LED
lamp and the UV-vis spectra were recorded using Hitachi U-3010 UV spectrophotometer at
different irradiation times. The light intensity was 70mw/cm2.
2.5. Photopolymerization experiments
The real-time Fourier Transform IR (FT-IR) spectrometer (Nicolet 5700) equipped with an
MCT/A KBr detector-beam splitter combination was used to monitor the photopolymerization
kinetics. The spectrometer was operated in the absorbance mode between 650 and 4000 cm-1, and
the polymerization kinetics were determined by the FT-IR spectrometer working in the rapid
mode with average of 4 scans s-1 collection rate (4cm-1). The mixtures were dropped onto a piece
of KBr crystal with 2±0.1 mm thickness and was covered by another clear piece of KBr crystal.

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The mixture was irradiated by the LED lamp in the absence of air. The light intensity was
70mw/cm2 determined by a UV-visible light Radiometer (Beijing Normal University, China). The
double bond conversion of monomers was calculated by the decrease of the area of the peak of
acrylate absorption around 812 cm-1. The double-bond conversions, directly related to the decrease
in NIR absorbance, was calculated from the equation as following:
DC%=[1-(A812)t/(A812)0]×100%
where (A812)0 and (A812)t were the area of the absorption peak at 812 cm-1 of the sample before
and after photopolymerization respectively.
2.6. 3D printing.
The runwo-DLP 6500 3D printing machine with the lamp 425 nm was used. This machine
was fabricated by RUN WING M&E Co. LTD. The printed monomer was PEGDA with
photoinitiator 784 (0.7 %wt)or BFC (0.0625 %wt) respectively. The thick of the slice was 50 µm
and the resolution was 1080 dpi. The irradiation power was 6 W.
3.Results and discussion
3.1. Synthesis, characterization and absorption properties of BFC.
The target product BFC was prepared just through one step reaction of furfural and
cyclohexanone. The mixture of aldehyde and ketones reacted mildly at room temperature. Except
the target product, the reaction has no other by-product. Moreover, for both the reactants are cheap
and easily obtained, this target product has potential in industrial application. The structure was
1
characterized by H NMR (400 MHz, CDCl3) (Figure.S1).d (ppm): 7.59 (s,2H), 7.55(s,2H),
6.69(d,2H), 6.53 (s,2H), 3.05(m,4H), 1.92 (m,2H). Elem. anal. (%) calcd. for C16H14O3: C, 75.59,
H, 5.51, O, 18.89. Found: C, 75.61, H, 5.47, O, 18.95.
Fig.1 UV-vis spectra of BFC in acetonitrile
Table 1 Light absorption properties of BFC and its molar extinction coefficients

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λmax
(nm)
373
εmax
ε365
ε385
ε405
(M-1cm-1)
(M-1cm-1)
(M-1cm-1)
(M-1cm-1)
38200
35100
32600
11800
The
UV-vis absorption
spectrum of BFC (1×10-5mol/L) in acetonitrile is shown in Figure 1. The maximum absorption
peak of BFC is located at λmax=373 nm (εmax=38200 M-1cm-1) and the maximum absorption
wavelength could reach to 460 nm (Table 1). The absorption at 373 nm is mainly attributed to the
π-π* transition of the whole π-conjugate structure of BFC. The broad range of absorption and the
coverage of visible light is attributed this large π-conjugate structure (Table 1). This absorption
character of BFC indicates that BFC could be motivated by LED light.
3.2 The photo-initiating performance of BFC
To investigate the initiation efficiency of the BFC as photoinitiator, the photopolymerization
of different monomers in the presence of the different content of BFC are conducted by the
irradiation of 365 nm and 405 nm LED with the light intensity of 70mw/cm2 for the BFC showed
considerable extinction coefficients within this range. The results are shown in Figure.2. At first,
the different content of BFC to initiate the monomers of PEGDA and HDDA under the 365 nm
LED lamp irradiation was investigated, the conversion of double bond is shown in Figure. 2(a),
(b) respectively. As photoinitiator, BFC could initiate the photopolymerization of PEGDA and
HDDA. Under the same condition, the initiating efficiency of BFC to PEGDA is much higher
than to HDDA. The conversion of PEGDA can reach 70% while only 30% of HDDA. As the
usage of BFC decreases, the conversion of double bond increases until the usage declines to
0.0625%wt the conversion reaches the maximum. This usage of photonitiator is very much lower
than common photoinitiator, which is great advantage for photoinitiator, for the low usage of the
photoinitiator. it can be expected to solve the problems induced by photoinitiator, such as
migration, toxic and durability of the coating.
By reason of the broad absorption of BFC and the significance of the visible light
polymerization, using BFC to initiate the PEGDA and HDDA under 405 nm LED lamp
irradiation was also conducted. The result is shown in Figure.2(c). It is different from the results
of 365 nm irradiation, the conversion of double bond and polymerization rate of PEGDA under
405 nm irradiation is much higher than HDDA. Within several seconds, the conversion of double
bond of PEGDA reaches close to 80%, while, to the HDDA, it is only 10%. These results may
illustrate the PEGDA is good coinitiator for BFC or BFC had different initiating activity to
different monomer. In addition, the color of the solution of BFC in PEGDA was much deeper than
in HDDA, which declared there was interaction between BFC and PEGDA.
To verify it, the contrast experiments which through adding the common coinitiator EDAB and
PEGDA respectively to initiate the polymerization of HDDA were conducted, and the result are
shown in Figure.2(d), (e), (f). With adding the coinitiator EDAB, the conversion of double bond of
HDDA increases, while, it is not very obvious and the polymerization rate is still slow, which

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means the initiating efficiency of BFC with 5% EDAB is low. However, with adding the PEGDA,
both the polymerization rate and conversion increases. By adding 5% PEGDA into the HDDA
with the initiator BFC, both the polymerization rate and conversion of double bond of HDDA
increases obviously and the conversion of double bond can reach more than 60% within short
time. Aside from HDDA, BFC initiating the monomer HEA and TMPTA also had the faster
polymerization rate and higher conversion with the adding 5%wt of PEGDA than without adding
PEGDA. These results prove that the PEGDA is better coinitiator to the photoinitiator BFC than
EDAB under the 405 nm irradiation.
Fig.2 Conversion of double bond initiated by different content of BFC in different monomers with
different irradiation wavelength, (a)PEGDA and (b)HDDA with different content BFC without
coinitiator; (c) PEGDA and HDDA with content of 0.0625%wt BFC. (d) HDDA with BFC
content of 0.0625%wt with coinitiator EDAB (5%wt). (e) BFC/HDDA using different content of
PEGDA as coinitiator @405nm. (f) HEA and TMPTA with 0.0625%wt BFC with and without
5wt % PEGDA as coinitiator.
As a new developed photoinitiator, it’s necessary to compare the performance of initiation woth
the commercial photoinitiator. Thus, the thioxanthone(ITX) was chosen as contrast for it could
also initiate the photopolymerization under 405 nm LED lamp and the result was shown in
Fig.S2. From the Fig. S2, it could be seen that BFC or BFC/PEGDA initiating system has an
obviously better initiating effect than ITX or ITX/EDAB under the same condition, which
proved photoinitiator BFC has the potential application in the field of photopolymerization.
Usually, the usage of photoinitiator in photopolymerization is 1~5%wt. However, the
photoinitiator BFC with low content of 0.0625%wt is totally enough for photopolymerization.
Why the photoinitator BFC has high initiation efficiency with low usage? It may be explained
from two factors, fluorescence quantum yield and light shielding. On the one hand, BFC had high
fluorescence performance with high content in the monomers induced low double bond
conversion, which could be seen from the Figure.3. It is well known that high fluorescence
quantum yield declines the triplet yield while the radicals are generated from the triplet state,14

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which means that much BFC with high usage produced fluorescence and did not transform into
radicals. On the other hand, BFC has high molar extinction coefficient which could block the light
absorption of bottom layer of the coating with high content and the conversion of the whole
coating would decline. Thus, the high content of BFC in this system is not helpful for
photopolymerization.
Fig.3 In the monomers PEGDA and HDDA, the relationship of fluorescence quantum yield of
solution and conversion of monomers.
3.3 The mechanism of photoinitiator BFC and monomer PEGDA.
UV-vis absorption (Figure.4(a)) and fluorescence (Figure.4(b)) spectra of BFC in the
monomer PEGDA and HDDA are investigated firstly. It can be seen that in the monomer HDDA,
the shape of the UV-vis absorption spectrum of BFC is the same with in the solution of
acetonitrile. While, when the same concentration of BFC is dissolved in the PEGDA, the UV-vis
absorption spectrum had a big red shift and the molar extinction coefficients increased greatly.
The extinction coefficient of BFC at 405 nm (ε405=42800 M-1cm-1) is higher than the extinction
coefficient at 365 nm. (ε365=35900 M-1cm-1) in PEGDA. Moreover, the absorption region is
broadened to 500 nm. Accordingly, the fluorescence spectra of BFC in the PEGDA had big red
shift compared to the BFC in the HDDA. The color of fluorescence is green in the HDDA with or
without EDAB, while it turns into yellow in the PEGDA. As it can be seen from Figure.4 (b),
BFC/PEGDA exhibits
a
yellow emission (λem=575nm) while BFC/HDDA and
BFC/EDAB/HDDA exhibits a green emission (λem=502nm). What induces this unusual result? It
is well known the PEGDA itself has no absorption over 300 nm, there must be reaction between
PEGDA and BFC. Analysis from the chemical structure of the BFC and PEGDA, the oxygen
atoms on furan ring and carbonyl exhibit electronnegativity, while the active hydrogen on PEGDA
presents electropositivity, and this result can be explained by exciplex22 formed by BFC and
PEGDA according to electrostatic interaction. The exciplex of BFC and PEGDA has a wide
absorption region which could be initiated by long wavelength light.

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Fig.4 (a) UV-vis absorption spectra; (b) Fluorescence spectra of BFC in PEGDA, HDDA,
HDDA/EDAB. ([BFC]=20mM);
To better understand the mechanism of exciplex formed by BFC and PEGDA, a
solvatochromic of BFC was studied and shown in Figure.5. The protonic solvents, like ethyl
alcohol, glycerin, which have active hydrogen and non-protonic solvent, like hexane,
dichloromethane, which has no active hydrogen were selected to illustrate the solvatochromic of
BFC. From Figure.5(a), it could be seen that the UV-vis absorption spectra of BFC in protonic
and non-protonic solvents are similar, no red or blue shift was found. However, it’s different from
the UV-vis absorption spectra, the fluorescence spectra of BFC in different solvents are not
similar any longer. There is the red shift of BFC in the protonic solvents compared with in the
non-protonic solvents (Figure. 5(b)). It revealed that the influence of solvent effect on excited state
is greater than that of ground state. BFC may form exciplex with protonic solvent. It also could be
speculated that the possible interaction between BFC and PEGDA is existed.
Fig.5 (a)UV-vis absorption spectra; (b) Fluorescence spectra of BFC in Hexane, Dichloromethane,
Ethyl alcohol, Glycerin.
Therefore, the possible initiation mechanism can be described in Figure.6: In the BFC/PEGDA
system, BFC and PEGDA can form exciplex at the ground state. Once exposed to LED lamp, the
complex can make a transition to triplet state, resulting into the photopolymerization by the
mechanism of hydrogen abstraction.

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Fig.6 Proposed mechanisms of BFC/PEGDA
3.4 Photobleaching property of BFC/PEGDA and for 3D printing of light color materials.
The photoinitiator BFC/PEGDA has long and broad absorption wavelength, which covers
the wavelength from 325 to 500 nm. Hence, obviously, this photoinitiator system exhibits yellow
before irradiation. However, this photoinitiator BFC/PEGDA presents excellent photobleaching
property under visible light irradiation, shown in Figure.7. The PEGDA solution of BFC is yellow.
After irradiation for 2 min, the UV-vis absorption declined quickly, and the color of the solution
turns from yellow into colorless. Identically, accompany with the disappearance of the color of the
solution, the fluorescence also fades away during photopolymerization. Once the fluorescence
disappeared, the solution is cured too. And the cured material has no or light color. As we know,
the photobleaching performance is a big advantage to visible photoinitiator to be used in the
colorless or light color materials. It is also can be expected to be used in the 3D printing of
colorless or light color materials with the long wavelength LED lamp printing. To shape complex
three dimensions structure, it is no doubt that 3D printing technology is a high-efficiency method.
Photopolymerization 3D printing is one method to fast print high precision mold. Due to the
limitation of the LED encapsulation technology or LCD screen service life, photopolymerization
3D print machine is inclined to use long wavelength LED lamp. While, the matching
photoinitiator of long wavelength absorption is bound to bring color to the printing subject.
Therefore, it is a challenge to print colorless or light color material using the long wavelength
material and the photobleaching photoinitiator is the way to solve this problem. In order to
confirm the photobleaching performance in the 3D printing technology, BFC/PEGDA is used to
3D print a fly's-eye lens model with the 425 nm LED lamp. At the same time, a control
experiment that the long wavelength photoinitiator 784 is used to 3D print model is carried out.
From the Figure.8, the printed model with photoinitiator 784 is deep red, while the model with
BFC, the color is light, and the round of the printed model, where is thin, it is almost no color.
This result demonstrates that the BFC/PEGDA is good photobleaching photoinitiator.

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Fig.7 Steady state photolysis of BFC in PEGDA upon UV LED exposure ([BFC]=20 mM/0.5%
wt) @405nm indicated by UV-vis spectra recorded at the indicated irradiation times(left);
photobleaching property of BFC (0.0625% wt) in PEGDA (right).
Fig.8 3D printing models with phtoinitiator 784 (right) and BFC (left)
4.Conclusion
A novel LED photoinitiator, BFC, is synthesize by one-step reaction of furan and
cyclohexanone. Low usage of photoinitator BFC in the monomer is enough to initiate the
photopolymerization becasuse BFC at low content has a low fluorescense quantum efficiency
which provides itself a high capacity of intersystem crossing to move to triplet state to produce
free radicals. BFC has higher initiating activity with the coinitiator ether compound, such as
PEGDA, which is nontoxic, environmental friendly, instead of amine compound. Moreover, BFC
has the rapid photobleaching property, it can be used in preparing light color materials with LED
light irradiation, such as in 3D printing light color materials.
Acknowledgments
This study was financially supported by National Key R&D Plan 2017YFB0307800 and the
National Natural Science Foundation of China (No. 51603007, 51573011). The authors also
appreciate the support of the Beijing Laboratory of Biomedical Materials.
Supplementary data

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1H NMR spectra and the picture of 2,6-bis(furan-2-ylmethylidene) cyclohexan-1-one
(BFC).
Photopolymerization kinetics of BFC(0.0625%wt)/PEGDA, ITX(0.0625%wt)/PEGDA,
BFC(0.0625%wt)/PEGDA(5%wt)/HDDA, ITX(0.0625%wt)/PEGDA(5%wt)/HDDA upon
LED 405nm, 70mw/cm2.
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Supporting Information
Synthesis of furan derivative as LED light photoinitiator:
one-pot, low usage, photobleaching for light color 3D
printing
Jingfang Li, Yajuan Hao, Meijuan Zhong, Liqun Tang, Jun Nie*, Xiaoqun Zhu*
State Key Laboratory of Chemical Resource Engineering & Beijing Laboratory of Biomedical
Materials, Beijing University of Chemical Technology, Beijing, 100029, P.R. China
E-mail: zhuxq@mail.buct.edu.cn; Tel: (+86)10-6442-1310
Fig.S1 1HNMR spectra of BFC

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Fig.S2
Photopolymerization
kinetics
of
BFC(0.0625%wt)/PEGDA,
ITX(0.0625%wt)/PEGDA,
BFC(0.0625%wt)/PEGDA(5%wt)/HDDA, ITX(0.0625%wt)/EDAB(5%wt)/HDDA upon LED 405nm,
70mw/cm2.

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High lights
Simple synthesis: One pot reaction to prepare photoinitiator.
Low usage: Low content of photoinitiator can achieve high
efficiency.
Application: Photobleaching for light color 3D printing with
long wavelength LED light.