Flavonol dyes with different substituents in photopolymerization

Article abstract of DOI:10.1016/j.jphotochem.2019.112097

To further expand the applications of flavonol dyes (3HFs) in photopolymerization, six flavonol dyes with different substituents were prepared by using the Algar–Flynn–Oyamada method. The steady-state photolysis and fluorescence quenching of 3HFs under the 385 nm LED light source showed that the proton transfer reaction preceded the charge transfer reaction between 3HFs and triethanolamine (TEOA) or iodonium salts (ONI), and groups with different electron properties could affect the photochemistry of 3HFs. The influence of substituents on the free radical polymerization efficiencies of 3HFs/TEOA and 3HFs/ONI was evaluated. Results showed that charge transfer occurred in the oxidation or reduction processes between 3HFs and TEOA or ONI. The possible mechanism was speculated, and the thermal feasibility of charge transfer between 3HFs and TEOA or ONI was calculated on the basis of the free energy changes of photoinduced electron transfer.

Full text of DOI:10.1016/j.jphotochem.2019.112097

JournalofPhotochemistry&PhotobiologyA:Chemistry386(2020)112097
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Flavonol dyes with different substituents in photopolymerization
⁎
Jian Youa, Hongyuan Fua, Di Zhaoa, Tianyu Hua, Jun Niea, Tao Wanga,b,
a Department of Organic Chemistry, College of Chemistry, Beijing University of Chemical Technology, Beijing, 100029, PR China
b State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yatsen University, Guangzhou, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
To further expand the applications of flavonol dyes (3HFs) in photopolymerization, six flavonol dyes with
different substituents were prepared by using the Algar–Flynn–Oyamada method. The steady-state photolysis
and fluorescence quenching of 3HFs under the 385 nm LED light source showed that the proton transfer reaction
preceded the charge transfer reaction between 3HFs and triethanolamine (TEOA) or iodonium salts (ONI), and
groups with different electron properties could affect the photochemistry of 3HFs. The influence of substituents
on the free radical polymerization efficiencies of 3HFs/TEOA and 3HFs/ONI was evaluated. Results showed that
charge transfer occurred in the oxidation or reduction processes between 3HFs and TEOA or ONI. The possible
mechanism was speculated, and the thermal feasibility of charge transfer between 3HFs and TEOA or ONI was
calculated on the basis of the free energy changes of photoinduced electron transfer.
Photopolymerization
Photochemistry
Flavonol dyes
1. Introduction
cationic polymerization of epoxides upon exposure to near-UV light
LED. However, only a few 3HF with different structures were in-
Flavonol dyes (3HFs) have been extensively studied in food and
health sciences for their low toxicity and antioxidant and anticancer
activities [1–9]. 3HFs also exhibit excellent optical properties because
they are typical molecules of excited state intramolecular proton
transfer (ESIPT) properties [10–16] (Scheme 1). The ESIPT properties
of 3HFs enable them to have large Stokes shifts, dual emission, and
reasonable fluorescence quantum yield, which have been used in mi-
crobial fluorescence cell imaging studies [17–21]. In 2017, Ushakou
et al. demonstrated that 3-hydroxyflavone (3HF) was effective for the
on-line monitoring of the photopolymerization process in polymer E-
Shell 300 with good fluorescence properties [22].
vestigated in the photopolymerization.
Considering 3HFs will have important application in the photo-
polymerization technique, we prepared a series of 3HFs with different
substituents and discussed the effect of different substituents on the
charge transfer reaction between 3HFs and amine or iodonium salts.
The effect of substituents of 3HFs on their spectral properties and
photoreactivity were also investigated. The structure and abbreviation
of 3HFs with different substituents on rings A and B are shown in Fig. 1.
In the abbreviation of 3HFs, the letter A represents the substituents on
ring A, while B represents the substituents on ring B. This study will
further expand the applications of 3HFs in the photopolymerization
techique.
In recent years, using natural products as photosensitizers is im-
portant in the photopolymerization technique. For example, curcumin
and its derivatives exhibit excellent performance in both free radical
polymerization (FRP) and cationic polymerization [23,24]. 3HFs have
attracted considerable attention because they are natural products and
exhibit photoactivity. Jacques Lalevée et al. synthesized and evaluated
some 3HFs linked to a conjugated system, such as arene, pyrene, or
anthracene, as photo-initiator (PI) to achieve a high photosensitivity
under visible light [25]. They reported that 3HF could be used as a
versatile high-performance visible light PI in combination with an
amino acid (N-phenylglycine) for the FRP of methacrylates in thick
samples or composites [26]. Moreover, 3HF can be used in three-
component systems with an iodonium salt (ONI) and an amine for the
2. Experimental section
2.1. Materials and characterization
All reagents and solvents used were of reagent grade, which were
obtained from commercial sources and used without further purification.
N-Methylpyrrolidone (NMP), 2-hydroxyacetophenone, benzaldehyde, 4-
methylbenzaldehyde, 4-fluorobenzaldehyde, 4-chlorobenzaldehyde, 4-
bromobenzaldehyde, 2-hydroxy-5-fluoroacetophenoe, and 2-hydroxy-5-
bromoacetophenone were purchased from Tianjin Seans Technology
Company (Tianjin,China). Triethanolamine (TEOA), triethylamine (TEA)
⁎ Corresponding author at: Department of Organic Chemistry, College of Chemistry, Beijing University of Chemical Technology, Beijing, 100029, PR China.
E-mail address: wangtwj2000@163.com (T. Wang).
https://doi.org/10.1016/j.jphotochem.2019.112097
Received 8 June 2019; Received in revised form 12 September 2019; Accepted 18 September 2019
Availableonline20September2019
1010-6030/©2019PublishedbyElsevierB.V.

J. You, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry386(2020)112097
Scheme 1. ESIPT property of 3HF-H.
Fig. 1. Stucture and abbreviation of 3HFs with different substituents.
Fig. 2. Abbreviations and structures of the compounds used in this study.
and morpholine (MP) were purchased from Beijing Chemical Works
(Beijing, China). Iodonium bis (4-methylphenyl) hexafluorophosphate was
used as a reference iodonium salt (ONI). Tripropylene glycol diacrylate
(TPGDA, from Guangzhou Lihou Trading Co. Ltd, China) was chosen as
the monomer for free radical photopolymerization. The abbreviations and
structures of the compounds used in this study are listed in Fig. 2.
1H NMR (400 MHz) and 13C NMR (101 MHz) spectra were recorded
on a Bruker AV400 NMR spectrometer. IR spectra were recorded on a
Nicolet 5700 instrument (Thermo Electron Corporation, Waltham, MA).
Melting points were determined on an XT-4 microscopy melting point
apparatus. The 385 nm LED light sources were used for the irradiation
of the photocurable samples. I0 ≈ 64 mW cm−2 (3 cm). The UV–Vis
spectra were recorded using a UV-5200 (UNICO) UV–vis spectro-
photometer. Fluorescence spectra and fluorescence quantum yield were
investigated using a FS5 (EDINBURGH INSTRUMENTS) fluorescence
spectrometer. LC–MS spectra were recorded on a Agilent HPLC-6500
Series Q-TOF chromatograph.
2.2. Synthesis of 3HF dyes
Process of the preparation of 3HFs is shown in Scheme 2. The 3-HFs
were synthesized by the method of Algar-Flynn-Oyamada (AFO) [27].
Sodium hydroxide (2.0 g, 0.050 mol) was added to a 100 ml round
bottom flask, dissolved in 5.0 ml of water, the temperature was lowered
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J. You, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry386(2020)112097
Scheme 2. Process of the preparation of 3HFs.
to room temperature, and then 30 ml of ethanol was added; Next, weigh
6.0 mmol of 2-hydroxyacetophenone or substituted 2-hydro-
xyacetophenone and 6.0 mmol of benzaldehyde or substituted benzal-
dehyde were added, and diluted with 20.0 ml of ethanol. After 12 h of
reaction, 30% Hydrogen peroxide (H2O2) (2.0 ml) was added directly,
and the reaction was continued for 12 h. The reaction solution was
neutralized with concentrated hydrochloric acid to neutrality. Water
was then added to dissolve the inorganic salts produced by the neu-
tralization. At that time, a large amount of solid precipitated. The solid
was filtered, and then recrystallized from ethanol.
127.68, 122.29, 122.16, 121.90, 121.28, 109.04; IR (KBr, (cm−1)):
3266, 1126 (CeOeH), 1611 (C]O), 1570 (C]C); Yield: 55%. Mp:
159–160 °C (160–161 °C) [31];
2.2.7. 6-Bromo-3-hydroxy-2-phenyl-4H-chromen-4-one (3HF-A-Br)
1H NMR (400 MHz, DMSO-d6) δ: 8.29 (d, J =7.7 Hz, 2 H), 8.18 (d, J
=2.4 Hz, 1 H), 7.93 (dd, J = 9.0, 2.5 Hz, 1 H), 7.77 (d, J =9.0 Hz, 1 H),
7.52 (m, 3 H); 13C NMR (101 MHz, DMSO-d6) δ: 172.76, 153.30,
145.44, 140.93, 135.91, 131.43, 129.64, 128.42, 127.42, 126.65,
122.75, 121.12, 116.59; IR (KBr, (cm−1)): 3289, 1115 (CeOeH), 1600
(C]O), 1554 (C]C); Yield: 58%; Mp: 182–183 °C (180–181 °C) [31].
2.2.1. 3-Hydroxy-2-phenyl-4H-chromen-4-one (3HF)
1H NMR (400 MHz, DMSO-d6) δ: 8.47 – 7.95 (m, 3 H), 7.96 – 7.22
(m, 6 H); 13C NMR (101 MHz, DMSO-d6) δ 172.97, 154.56, 145.15,
139.05, 133.71, 131.27, 129.85, 128.49, 127.63, 124.77, 124.54,
121.28, 118.39; IR (KBr, (cm−1)): 3210, 1130 (CeOeH), 1608 (C]O),
1562 (C]C); Yield: 70%; Mp:178–179 °C (170 °C) [28];
2.3. Free radical polymerization (FRP) [24]
The two-component photoinitiating systems (PISs) are mainly based
on 3HFs/TEOA (1.0%/3.0% w/w) and 3HFs/ONI (0.20%/1.0% w/w)
or (1.0%/0.20% w/w) for the free radical polymerization of metha-
crylates. The weight percent of the photoinitiating system is relative to
the monomer content. The free radical photopolymerization experi-
ments were carried out under laminated conditions. The photosensitive
formulations were photocured in 0.6 mm thick plastic molds (Black
opaque and does not absorb 385 nm LED light) with a 5 mm diameter
center. The molds were clamped between two glass slides. The distance
between irradiation sources and formulations was 3 cm (I0 ≈ 64 mW
cm−2). The specimens were irradiated at different time intervals by
manually controlling the curing light.
2.2.2. 3-Hydroxy-2-(p-tolyl)-4H-chromen-4-one (3HF-B-CH3)
1H NMR (400 MHz, DMSO-d6) δ : 8.16 (d, J =8.1 Hz, 2 H), 8.12 (dd,
J = 8.0, 1.6 Hz, 1 H), 7.85 – 7.72 (m, 2 H), 7.47 (ddd, J = 8.0, 6.6,
1.5 Hz, 1 H), 7.38 (d, J =8.1 Hz, 2 H), 2.40 (s, 3 H); 13C NMR (101 MHz,
DMSO-d6) δ : 173.10, 154.45, 145.31, 139.61, 139.22, 133.51, 129.08,
128.61, 127.47, 124.72, 124.41, 121.24, 118.34, 21.01. IR (KBr,
(cm−1)): 3282, 1110 (CeOeH), 1608 (C]O), 1563 (C]C); Yield: 75%;
Mp: 197–199 °C (206 °C) [28];
The near-infrared spectra of uncured resin were collected by using a
Fourier transform near-infrared spectrometer (Nicolet 5700,
4000–7000 cm−1 wavelength range); the spectra were obtained im-
mediately after each exposure interval. For each sample, the RT-NIR
runs were repeated three times. The double bond conversion profiles
were calculated from the decay of the absorption intensities located at
6165 cm−1 as described by Stansbury and Dickens [32]. The double
bond conversion was calculated using the following equation (Eq. (1)):
2.2.3. 2-(4-Fluorophenyl)-3-hydroxy-4H-chromen-4-one (3HF-B-F)
1H NMR (400 MHz, DMSO-d6) δ: 8.30 (dd, J = 8.7, 5.6 Hz, 2 H),
8.13 (d, J =8.0 Hz, 1 H), 7.80 (dt, J = 16.3, 8.3 Hz, 2 H), 7.46 (dt,
J = 16.4, 8.1 Hz, 3 H); 13C NMR (101 MHz, DMSO-d6) δ: 172.94,
163.83, 161.36, 154.49, 144.37, 138.83, 133.72, 130.06, 127.83,
124.76, 124.56, 121.29, 118.38, 115.70, 115.48; IR (KBr, (cm−1)):
3299, 1126 (CeOeH), 1637 (C]O), 1620 (C]C); Yield: 61%; Mp:
147–149 °C (152–154 °C) [29];
St
S0
Conversion% = 1
× 100%
(1)
2.2.4. 2-(4-Chlorophenyl)-3-hydroxy-4H-chromen-4-one (3HF-B -Cl)
1H NMR (400 MHz, DMSO-d6) δ: 8.26 (d, J =8.3 Hz, 2 H), 8.13 (d, J
=7.9 Hz, 1 H), 7.80 (dt, J = 7.4, 8.3 Hz, 2 H), 7.65 (d, J =8.3 Hz, 2 H),
7.48 (t, J =7.4 Hz, 1 H); 13C NMR (101 MHz, DMSO-d6) δ : 172.97,
154.49, 143.93, 139.30, 134.39, 133.82, 130.16, 129.28, 128.62,
124.78, 124.59, 121.26, 118.39; IR (KBr, (cm−1)): 3271, 1127
(CeOeH), 1608 (C]O), 1570 (C]C); Yield: 64%; Mp:186–187 °C
(192 °C) [28];
where St is the area of the C = C characteristic absorbance peak and S0
is the initial area of the C = C characteristic absorbance peak.
2.4. Steady state photolysis experiments
In this study, the UV–vis absorption spectrum was used to char-
acterize the photolysis rate of 3HFs with the change of illumination
time. The experimental process is as follows：
The [3HFs] = 5.0 × 10−5 M solution is prepared.
2.2.5. 2-(4-Bromophenyl)-3-hydroxy-4H-chromen-4-one (3HF-B -Br)
1H NMR (400 MHz, DMSO-d6) δ : 8.23 – 8.15 (m, 2 H), 8.12 (dd,
J = 8.0, 1.6 Hz, 1 H), 7.87 – 7.72 (m, 4 H), 7.53 – 7.44 (m, 1 H); 13C
NMR (101 MHz, DMSO-d6) δ : 172.97, 154.49, 143.98, 139.35, 133.83,
131.55, 130.51, 129.46, 124.78, 124.60, 123.27, 121.26, 118.39; IR
(KBr, (cm−1)): 3265, 1125 (CeOeH), 1610 (C]O), 1586 (C]C); Yield:
62%; Mp: 186–188 °C (163–167 °C) [30];
The samples were irradiated for 0 s, 30 s, 60 s, 120 s, 180 s, 240 s,
300 s, the UV–vis absorption spectra were measured. The distance be-
tween the 385 nm LED light source and cuvette was 3 cm. The absor-
bance at 345 nm of the same sample taken at different times was re-
corded as At. The photolysis progress was followed by plotting At/A0 as
a function of irradiation time, where A0 was the initial absorbance of
3HFs at 345 nm.
2.2.6. 6-Fluoro-3-hydroxy-2-phenyl-4H-chromen-4-one (3HF-A-F)
1H NMR (400 MHz, DMSO-d6) δ 8.23 (d, J =7.6 Hz, 2 H), 7.89 (dd,
J = 9.3, 4.2 Hz, 1 H), 7.78 (dd, J = 8.5, 3.1 Hz, 1 H), 7.72 (td, J = 8.7,
3.1 Hz, 1 H), 7.56 (m, 3 H); 13C NMR (101 MHz, DMSO-d6) δ: 172.45,
159.60, 157.18, 151.07, 145.63, 138.91, 131.12, 129.97, 128.49,
2.5. Fluorescence quenching experiment
Fluorescence quenching experiments were tested using an FS5
fluorescence spectrometer. The experimental method was similar to the
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J. You, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry386(2020)112097
Fig. 3. UV–vis absorption spectra (a) and fluorescence spectra (b) of the 3HFs in methanol (concentration of 3HFs is 6 × 10−5 M).
impurities prior to use. All the solutions were purged with N2 gas for
Table 1
Absorption and fluorescence data of 3HFs in methanol: maximum absorption
wavelengths (λmax), molar extinction coefficients (ε) at λmax and at the emis-
sion wavelengths of the LEDs, maximum emission wavelengths (λem), Stokes
shift and fluorescence quantum yields (Φf).
10 min before measuring and an N2 gas blanket was maintained over
the solution during the experiments. The measured electrode potentials
are all converted to electrode potentials with reference to the saturated
calomel electrode. The free energy change ΔGS for the electron transfer
between the excited 3HFs and TEOA/ONI was calculated from the
classical Rehm–Weller equation (Eqs. (2) and (3)): [35]
3HF
Stokes
f
(mMa−x 1*cm−1
)
(3M85−318*5cm−1
)
shift(cm−1
)
max
(nm)
em
(nm)
(%)
ESCE = E
− E1 + E2
(2)
(3)
3HF - H
342
17789
14733
16225
19931
20515
16879
13815
1880
2607
1844
2120
2237
2390
3223
532
534
529
536
535
534
539
5263
5347
5348
5263
5291
5348
5263
5.91
11.20
5.57
8.39
8.09
9.33
11.44
(Ag/AgCl)
3HF - B-CH3 347
ΔGS= F(Eox − Ered) − ES − C
3HF - B-F
3HF - B-Cl
3HF - B-Br
3HF - A-F
3HF - A-Br
342
345
346
347
349
where ESCE, E(Ag/AgCl) are electrode potential of a reference electrode
with a saturated calomel electrode and Ag/AgCl electrode, E1, E2 are
ferrocene electrode potential of a reference electrode with a Ag/AgCl
electrode and saturated calomel electrode.
E1 = E(Fc+/Fc) = 0.29 V vs. Ag/AgCl, E2 = E(Fc+/Fc) = 0.38 V vs.
SCE
photolysis rate test. Fluorescence spectrum was measured at different
times for the same sample at the same slit width. The number of pho-
tons emitted was represented by the peak area from 360 nm to 670 nm.
The number of photons emitted of the same sample taken at different
times was recorded as Ct. The reaction progress was by plotting Ct/C0 as
a function of irradiation time, where C0 was the initial emission in-
tensity.
where Eox, Ered, ES, F and C are the oxidation potential of the 3HFs
or TEOA, the reduction potential of ONI or 3HFs, the excited singlet
state energy of the 3HFs, faraday constant and the electrostatic inter-
action energy for the initially formed ion pair. (this latter parameter is
considered as negligible in polar solvents), respectively.
2.6. 3HF-H/TEOA and 3HF-H/ONI interaction
3. Results and discussion
The solution of TEOA (1.00 M) and the solution of ONI (1.00 M)
were prepared. Then, took 30 μL of TEOA solution or ONI solution to
3.0 ml of the solution of 3HFs dye(1.0 × 10−4 M) and mixed. After the
samples were irradiated for 0 s, 30 s, 60 s, 120 s, 180 s, 240 s, 300 s, the
UV–vis absorption spectra were measured. Then, took 0 μL, 20 μL,
40 μL, 60 μL, 80 μL, 100 μL, 120 μL, 140 μL, 160 μL, 180 μL, 200 μL,
240 μL of TEOA solution or ONI solution to 3.0 ml of the solution of
3HF-H dye (1.0 × 10−4 M) and mixed. So the solutions of 3HF-H dye
with different concentrations of TEOA or ONI were obtained.
Fluorescence spectrum was measured for the same sample at the same
slit width and voltage.
3.1. Absorption and fluorescence emission of 3HFs
The UV–vis absorption spectra of the studied 3HFs in methanol are
presented in Fig. 3a, and the main data are listed in Table 1. The ab-
sorption peaks of 3HFs with different substituent groups occurred at
approximately the same wavelengths from the UV to visible regions.
The absorption peaks attributed to the conjugated system in 3HFs could
be observed at 280–450 nm, which showed different absorption
strength with different substituent groups. For example, the order of
absorption strength at 345 nm is 3HF-B-Br > 3HF-B-Cl > 3HF-A-
F > 3HF-H > 3HF-B-F > 3HF-A-Br > 3HF-B-CH3, whereas that at
385 nm is 3HF-A-Br ≈ 3HF-B-Br > 3HF-A-F > 3HF-B-Cl ≈ 3HF-B-
CH3 > 3HF-H ≈ 3HF-B-F.
2.7. Redox potentials [24]
The fluorescence spectra of 3HFs in methanol are presented in
Fig. 3b, and the main data are listed in Table 1. The 3HFs have a double
emission peak. The two emission bands that were observed corre-
sponded to the fluorescence from the Franck–Condon excited state
(emission at approximately 450 nm) and the tautomer produced by
ESIPT (emission at approximately 550 nm) [10,14]. The substituent
group slightly affected the emission of 3HFs at 550 nm. However, the
emission at approximately 400 nm showed obvious differences for dif-
ferent substituent groups. For example, the emissions of 3HF-A-F and
3HF-A-Br at approximately 400 nm disappeared, probably due to the
rapid ESIPT. The emission property of 3HF-B-CH3 was almost the same
Oxidation and reduction potentials in methanol (CH3OH) were
measured by cyclic voltammetry (CV) using a CHI760E electrochemical
workstation and tetrabutylammonium hexafluorophosphate (0.1 M) as
a supporting electrolyte. The concentrations of the 3HFs were 1 × 10−3
M. Glassy carbon, Ag/AgCl electrode and a platinum wire were used as
the working, reference, and auxiliary electrode, respectively. The fer-
rocenium/ferrocene (Fc+/Fc) couple was used as the internal standard
[33,34]. The glassy carbon electrode was polished with an alumina
slurry of 0.3 μm on a polish cloth before use. The platinum wire was
immersed in HNO3 solution for 30 min at 80 °C to remove metal
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J. You, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry386(2020)112097
Fig. 4. Absorption spectra of 3HF-H in methanol at different illumination time under the 385 nm LED (a); Photolysis rates of different substituted 3HFs in methanol
under the 385 nm LED (b). ([3HFs] = 5.0 × 10−5 M).
obtained. As an example, the changes of the absorption of 3HF-H in
methanol with illumination time are shown in Fig. 4a. The changes of
the absorption of other 3HFs in methanol with illumination time are
shown in the supplementary part (Figs. S34–S39). As the illumination
time increased, the absorbance decreased, but the peak shape remained
almost unchanged, which indicates that most of the molecules may
undergo photobleaching by hydrogen atom transfer reactions during
Scheme 3. Hydrogen atom transfer process of 3HF-H.
the illumination (Scheme 3).
According to the change of absorption at 345 nm, the photolysis
as that of 3HF-H.
rates were obtained. Fig. 4b shows that substituents have a large in-
fluence on the hydrogen atom transfer in 3HFs. The photolysis rate
sequence is 3HF-A-Br > 3HF-B-Br > 3HF-B-CH3 > 3HF-B-Cl > 3HF-
A-F > 3HF-H > 3HF-B-F.
3.2. Steady state photolysis of 3HFs, 3HFs /TEOA and 3HFs /ONI
3HF-H, 3HF-B-CH3 and 3HF-A-Br were selected as three re-
presentative molecules. The photolysis rates of 3HFs /TEOA and 3HFs
/ONI in methanol were also obtained at the same irradiating condition
to study the effect of TEOA and ONI on the photolysis of 3HFs (Fig. 5).
It can be seen that the addition of TEOA to the 3HFs solution has little
effect on the photolysis rates of 3HF-H, 3HF-B-CH3 and 3HF-A-Br, but
the addition of ONI to the 3HFs solution has large effect on the pho-
tolysis rates.
Many reports showed that 3HFs are unstable [36–40]. Under the
light conditions, intramolecular proton transfer [10–13,41–46], reverse
proton transfer [47], photorearrangement reaction [48,49], photo-
oxidation [50–52], and other reactions can occur. To study the photo-
chemical properties of 3HFs, photolysis experiments of 3HFs were
carried out under the 385 nm LED. The effects of triethanolamine
(TEOA) and ONI on the photolysis of 3HFs were investigated.
The absorption spectra of 3HFs with different irradiating times were
Fig. 5. Absorption spectra of 3HF-H in me-
thanol at different illumination time under the
385 nm LED (a); Absorption spectra of 3HF-H/
ONI in methanol at different illumination time
under the 385 nm LED (b). Absorption spectra
of 3HF-H /TEOA in methanol at different il-
lumination time under the 385 nm LED (c).
Photolysis rate of 3HF-H, 3HF-H/ONI, 3HF-H/
TEOA, 3HF-A-Br, 3HF-A-Br/ONI, 3HF-A-Br/
TEOA, 3HF-B-CH3, 3HF-B-CH3/ONI, 3HF-B-
CH3/TEOA in methanol under the 385 nm LED
(d).([3HFs]=1.0 × 10−4
M,
[TEOA]
=10 mM, [ONI] =10 mM).
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J. You, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry386(2020)112097
Fig. 6. Fluorescence spectra of 3HF-H in methanol at different illumination time under the 385 nm LED (a); fluorescence quenching rate of 3HF, 3HF-B-CH3, 3HF-A-
Br in methanol under the 385 nm LED (b). ([3HF-H] = 1.68 × 10−4 M, [3HF-B-CH3] = 1.59 × 10−4 M, [3HF-A-Br] = 1.26 × 10−4 M).
Fig. 7. Fluorescence quenching of 3HF-H by different concentrations of TEOA in methanol (a);Fluorescence quenching of 3HF-H by different concentrations of ONI
in methanol(b). ([3HF-H] = 1.0 × 10−4 M).
Fig. 8. Liquid chromatography of 3HF-H (a) in methanol before and after 5 min of illumination at 385 nm LED; 3HF-H /TEOA (b) and 3HF-H/ONI (c) in acetonitrile
before and after 5 min of illumination at 385 nm LED; The MS results of 3HF-H/ONI in acetonitrile after 5 min of illumination at 385 nm LED (d).
6

J. You, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry386(2020)112097
Fig. 9. Photopolymerization profiles of TPGDA in the presence of 3HFs/TEOA (1.0%/3.0% w/w) under the 385 nm LED.
The fluorescence spectra of 3HF-H, 3HF-B-CH3, and 3HF-A-Br in
To investigate the product of photolysis of 3HFs, 3HFs/TEOA and
3HFs/ONI in solution, the LC–MS of these solutions after being irra-
diated 5 min were obtained. The results of liquid chromatography of
3HF-H in methanol before and after 5 min of illumination at 385 nm
LED are shown in the Fig. 8a. The results show that there is only one
new product from photolysis of 3HF-H in methanol and the content of
this new product is < 2.0%. The results from the liquid chromato-
graphy of 3HF-H/TEOA in acetonitrile before and after 5 min of illu-
mination at 385 nm LED are shown in Fig. 8b. Only two new products
with less than 2.0% of content were found. So, it can be concluded that
the photolysis of 3HF-H and 3HF-H/TEOA is difficult to carry out. The
liquid chromatography of 3HF-H/ONI in acetonitrile before and after
5 min of illumination at 385 nm LED is shown in Fig. 8c. It can be seen
that three new products were produced. According to the MS results
(Fig. 8d), one of these products is the isomer of 3HF- toluene derivatives
from the reaction of 3HF-H with toluene free radical from the photo-
lysis of ONI. The real structure of 3HF- toluene derivatives can’t be
characterized.
methanol were studied with illumination time. In Fig. 6a, the emission
peak produced by the 3HFs tautomer is quenched with the illumination
time. Fig. 6b shows that substituents have a large influence on the
fluorescence quenching of 3HFs tautomer with the illumination time.
These results show that hydrogen atom transfer may occur during
photolysis and form a non-fluorescent substance by destroying the
conjugated structure of the ring, so the absorbance is reduced and the
fluorescence is quenched. Hydrogen atom transfer process of 3HF-H is
shown in Scheme 3.
3.3. 3HF-H/TEOA and 3HF-H/ONI interaction
Fluorescence quenching experiments of 3HF-H by high concentra-
tions of TEOA and ONI were carried out and the results were shown in
Fig. 7. Fluorescence quenching processes of 3HF-H by TEOAor by ONI
was noted. The emission of the tautomer produced by ESIPT (emission
at approximately 550 nm) decreased with the increase of the con-
centration of TEOA or ONI.
Fig. 10. Photopolymerization profiles of TPGDA in the presence of 3HFs/ONI under the 385 nm LED. (a) and (b) is 3HFs/ONI (0.20%/1.0% w/w); (c) and (d) is
3HFs/ONI (1.0%/0.20% w/w).
7

J. You, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry386(2020)112097
Fig. 11. Proposed mechanisms of polymerization initiation by 3HFs/TEOA and 3HFs/ONI.
3.4. Photopolymerization using 3HFs/TEOA and 3HFs/ONI as initiation
withdrawing group accelerated the polymerization. When R1 and R2
were substituted by the same atom, the polymerization rate was lower
at the R1 position. In the 3HFs/TEOA system, 3HFs was photoreduced
by TEOA. When the substituents make more electrons in 3HFs, they are
more difficult to be reduced by TEOA. Therefore, the electron donating
groups reduce the initiation efficiency of the 3HFs/TEOA initiation
system.
system
As amine co-initiators are important to 3HFs, the effect of different
amines including TEOA, MP, TEA and NPG on free radical poly-
merization was studied in our study. The photopolymerization profiles
of TPGDA in the presence of 3HF-H/TEOA, 3HF-H /MP, 3HF-H/TEA
and 3HF-H/NPG under the 385 nm LED light sources are shown in
supporting information S26-S28. It has been found that the photo-
polymerization has the highest polymerization rate using TEOA as the
co-initiator and it has the lowest polymerization rate using NPG as the
co-initiator. So TEOA was chosen as an electron donor to discuss the
effect of substituents on polymerization efficiency in this study.
Upon irradiation with the 385 nm LED, the double bond conversion
rates versus the time curves of the photocurable resin films containing
different 3HFs were obtained, and the results are shown in Figs. 9 and
10.
In the 3HFs/ONI initiation system, 3HFs can be used as both a co-
initiator and a primary initiator. As a co-initiator, the polymerization
curve of 3HFs/ONI (0.20%/1.0% w/w) is shown in Fig. 10a and b.
Their conversions and rates are similar. When the B ring had sub-
stituents, the order of FRP rate was 3HF-B-CH3 > 3HF-B-Cl > 3HF-B-
Br > 3HF-B-F > 3HF-H. When R1 and R2 were substituted by the same
atom, the order of FRP rate was 3HF-A-Br > 3HF-A-F > 3HF-B-Br >
3HF-B-F > 3HF-H. When 3HFs was used as the main initiator, the
polymerization curve of 3HFs/ONI (1.0%/0.20% w/w) was obtained,
as shown in Fig. 10c and d. The conversion exceeded 80%. When the B
ring had substituents, the order of FRP rate was 3HF-B-CH3 > 3HF-B-
F > 3HF-B-Cl > 3HF-B-Br > 3HF-H. When R1 and R2 were substituted
by the same atom, the order of FRP rate was 3HF-A-Br > 3HF-A-
F > 3HF-B-F > 3HF-B- Br > 3HF-H. When R1 and R2 were replaced,
the polymerization rate was accelerated. When R2 was substituted, the
electron-donating group had the greatest influence on the polymeriza-
tion rate. When R1 and R2 were substituted by the same atom, they had
a greater influence on the initiation efficiencies at the R1 position.
In the 3HFs/ONI system, 3HFs were photo-oxidized by ONI. When
As seen from Fig. 9a, the 3HFs/TEOA (1.0%/3.0% w/w) two-
component system could effectively initiate the FRP of TPGDA, and the
double bond conversions for 3HF-H, 3HF-B-F, 3HF-B-Cl, and 3HF-B-Br
were similar, approximately 83%. The order of polymerization rate was
3HF-B-Cl > 3HF-B-Br > 3HF-B-F > 3HF-H > 3HF-B-CH3. Fig. 9b
shows that the conversion and polymerization rate of 3HF-B-F and
3HF-B-Br are higher than those of the corresponding 3HF-A-F and 3HF-
A-Br. When the R2 group was substituted, the electron-donating group
caused
a decrease in the polymerization rate, but the electron-
8

J. You, et al.
JournalofPhotochemistry&PhotobiologyA:Chemistry386(2020)112097
produce the the corresponding aminoalkyl radical, capable for initia-
tion of polymerization, and a less reactive 3HF-derived radical, which
probably underwent other radical reactions (r2). In the case of 3HFs/
ONI initiating system, proton transfer occurred to generate tautomers
(r1). Then, the tautomer and the ONI underwent charge transfer to
generate a radical cation (3HF.+) and an unstable onium salt radical.
Unstable onium salt radical produced tolyl radical-initiated poly-
merization. The radical cation (3HF.+) probably underwent deproto-
nation and other chemical reactions.
ESR-ST spectra of tert-butylbenzene solution of 3HF/ONI/PBN
were obtained upon irradiation (385 nm LED). Following electron
transfer in the 3HFs and ONI under light irradiation, radical cations
(e.g. 3HF.+) and toluene radicals Ph. were detected in ESR experiments
(Fig. 12) [25].
3.6. Free energy changes of photoinduced electron transfer
Fig. 12. ESR-ST spectra of radical generated in 3HF-H/ONI and trapped by PBN
The Eox (V/SCE) and Ered (V/SCE) values of the 3HFs were mea-
sured by cyclic voltammetry by using a CHI760E electrochemical
workstation. As seen from Table 2, the ΔGS (TEOA) and ΔGS (ONI)
values are all negative, which prove that the charge transfer between
3HFs and TEOA/ONI is thermodynamically favorable.
in tert-butylbenzene ([ONI] = 0.01 M) [25].
Table 2
Parameters characterizing the photochemical reactivity of the 3HFs with ONI
or TEOA: oxidation and reduction potentials (Eox and Ered); the excited state
energy (ES); free energy changes for the 3HFs/ONI interaction (ΔGS(ONI)); free
4. Conclusions
energy
changes
for
the
3HFs/TEOA
interaction
(ΔGS(TEOA));
E
red(ONI) = 0.06 V, Eox (TEOA) = 0.87 V in the methanol.
3HFs with different substituents show a wide absorption from 280
to 450 nm and different absorption strengths caused by the substituents.
The substituents also affect the emission at approximately 400 nm
caused by the ESIPT process. The steady-state photolysis and fluores-
cence quenching of 3HFs under the 385 nm LED light source showed
that the proton transfer reaction preceded the charge transfer reaction
between 3HFs and triethanolamine (TEOA) or iodonium salts (ONI),
and groups with different electron properties could affect the photo-
chemistry of 3HFs. The 3HFs/TEOA and 3HFs/ONI two-component
systems can effectively initiate the FRP of TPGDA. Substituents affect
the initiation efficiency differently in the 3HFs/TEOA and 3HFs/ONI
system.
3-HF
Eox (V/SCE)
Ered (V/SCE)
ES
ΔGS(eV)
ΔGS(eV)
(TEOA)
(eV)
(ONI)
3HF-H
1.05
0.99
1.01
1.02
1.01
1.03
1.01
−0.92
−0.91
−0.95
−0.90
−0.89
−0.88
−0.81
3.27
2.85
3.26
3.25
3.24
3.13
2.77
−2.28
−1.92
−2.31
−2.29
−2.29
−2.16
−1.82
−1.48
−1.07
−1.44
−1.48
−1.48
−1.38
−1.09
3HF-B-CH3
3HF-B-F
3HF-B-Cl
3HF-B-Br
3HF-A-F
3HF-A-Br
+
+
Eox and Ered values vs. SCE (E(Fc/Fc) = 0.38 V vs. SCE [35], E(Fc/Fc) = 0.29 V vs.
Ag/AgCl). Singlet state energies ES extracted from the UV–vis absorption and
fluorescence emission spectra of the 3HFs in the methanol. Free energy changes
(ΔGS) were calculated from the classical Rehm–Weller equation.
Declaration of Competing Interest
The authors declare no competing financial interest.
Acknowledgements
the substituents increase electron density in 3HFs, 3HFs are more likely
to be oxidized by ONI. Therefore, the electron donating groups accel-
erate the polymerization of the 3HFs/ONI initiation system. The elec-
tron withdrawing group also accelerates the polymerization compared
to unsubstituted because the ONI has higher reactivity.
The authors thank the National Key Research and Development
Program of China (2017YFB0307800) and Science and Technology
Planning Project of Guangdong Province, China (project No.
2017B090911002) for financial support. We also thank the Beijing
University of Chemical Technology CHEMCLOUDCOMPUTING
Platform for support with calculations. The authors also thank the State
Key Laboratory of Optoelectronic Materials and Technologies (Sun Yat-
sen University) (project no. OEMT-2018-KF-12).
3.5. Free radical polymerization mechanism
On the basis of the above study, we found that the laws of photo-
polymerization were inconsistent with those of the photolysis rate.
Hence, the first photopolymerization process was excitation of 3HFs,
and then the charge transfer between 3HFs and TEOA or ONI generated
free radicals, which were the active species that initiated photo-
polymerization. The charge transfer between 3HFs excited state and
TEOA or ONI was the rate determining step of photopolymerization.
The proposed mechanism is shown in Fig. 11. In the 3HFs/TEOA
system, 3HFs was photoreduced by TEOA. In the 3HFs/ONI system,
3HFs was photo-oxidized by ONI. Oxidation and reduction are two
opposite processes. Hence, substituents should have different effects on
the photoinitiation efficiencies. Although halogen also accelerated the
rate of polymerization in the 3HFs/ONI system, it might be due to the
high oxidative activity of the iodonium salt.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.jphotochem.2019.
112097.
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