Macromolecules
Article
added dropwise (3−4 s per drop) into the flask through a dropping
funnel. After the addition, the reaction temperature was increased to
25 °C and the temperature maintained for 4 h, and the progress of the
reaction was monitored by thin-layer chromatography (TLC).
Subsequently, the reaction solution was poured into 50 mL of ice-
water and stirred for 5 min, and the organic layer was separated and
collected; then, the aqueous layer was extracted by dichloromethane
(3 × 10 mL). The combined organic layer was washed with sodium
chloride saturated solution and dried by anhydrous sodium sulfate.
The dichloromethane solvent was removed by vacuum distillation and
the crude product was purified by silica gel column chromatography
(petroleum ether/ethyl acetate = 12/1, volume ratio) to obtain pure
F-MBF (cream-colored solid, overall yield 61%). The synthesis
methods of S-MBF, C-MBF, Cl-MBF, O-MBF, N-MBF, and DC-
MBF are similar to that of F-MBF. Their specific synthesis methods
and characterization data are shown in the Supporting Information
Methyl p-trifluoromethylbenzoylformate (TF-MBF) was synthe-
sized according to the following procedure.36 First, the p-
trifluoromethyl acetophenone (0.564 g, 3 mmol) and selenium
dioxide (SeO2, 0.501 g, 4.5 mmol) were added to anhydrous pyridine
(10 mL) in the flask flushed with nitrogen. The reaction mixture was
heated to 110 °C and the temperature maintained for 1 h. Then, the
temperature was reduced to 90 °C and kept for an additional 4 h, and
the progress of the reaction was monitored by TLC. The solution was
filtered using a suction funnel to remove precipitated selenium, and
the residue was washed with ethyl acetate (50 mL). The combined
filtrate was treated with 20 mL of HCl with a concentration of 1 mol
L−1 until the pH of the aqueous layer was 3; then, the organic layer
was separated and the aqueous layer was extracted with ethyl acetate
(5 × 20 mL). Subsequently, organic layers were combined, dried by
anhydrous sodium sulfate, and used for the next step of the synthesis.
Next, methanol (0.192 g, 6 mmol), dicyclohexylcarbodiimide
(DCC, 0.928 g, 4.5 mmol), and 4-dimethylaminopyridine (DMAP,
0.037 g, 0.3 mmol) were dissolved in a flask with 20 mL of ethyl
acetate and stirred at 25 °C; then, the crude solution obtained in the
first step was dropped into the reaction flask (1 s per drop) using a
dropping funnel. The white precipitate was removed by a suction
funnel after the dropping and the organic layer was washed with
water. The organic layer was separated and the aqueous layer was
extracted with ethyl acetate (3 × 20 mL); then, the organic layers
were combined and dried by anhydrous sodium sulfate. The ethyl
acetate was removed by vacuum distillation and the crude product
was purified by silica gel column chromatography (petroleum ether/
ethyl acetate = 12/1, volume ratio) to obtain pure TF-MBF (pale
yellow liquid, overall yield 46%). The synthesis methods of DF-MBF
and DM-BD-F are similar to that of TF-MBF. Their specific synthesis
methods and characterization data are shown in the Supporting
Figure 1. UV−vis absorption spectra of MBFs and BAPO in
anhydrous acetonitrile (concentration = 1 × 10−4 mol L−1).
is greater, which is because the trifluoromethyl group in the
TF-MBF molecule is a very strong electron-withdrawing group
and cannot be conjugated with the benzene ring. Significantly,
N-MBF possesses the most excellent absorption property
under the common LED emission wavelengths, followed by
the commercial BAPO, and other investigated PIs exhibit
lower molar extinction coefficients such as 10 M−1 cm−1 at 405
nm for MBF and F-MBF, but that does not mean they do not
possess photoinitiating ability under LED emission wavelength
because the initiating performances of PIs do not depend only
on their light absorption properties.26
Two lowest-energy electronic transitions, transitions type,
energy gap of S0 → S1, and the frontier molecular orbits for
lowest-energy transitions of MBFs for S0 → S1 are n−π*
transition characteristics or π−π* transition characteristics. In
general, PIs with a small energy gap of S0 → S1 have more
excellent absorption performance in the visible-light region.29
3.2. Photochemistry of MBFs. The steady-state photol-
ysis curves of MBF, F-MBF, and S-MBF in anhydrous
acetonitrile are displayed in Figure 2. Their maximum
absorption peaks decline significantly without co-initiator
after 120 s of LED irradiation at 405 nm under nitrogen
atmosphere, which is key evidence for MBFs as cleavable PIs.29
Scheme 4 reveals the types of radicals generated potentially
by the cleavage of MBFs under LED irradiation according to
the literature.32 1H NMR (400 MHz) spectra of the steady-
state photolysis products for S-MBF are displayed in Figure 3.
Newly generated peaks (e, f, and g) belonging to 4,4′-
dimethylthiobenzyl could be apparently seen when S-MBF
solution was irradiated for 120 s; at the same time, the peak h
belonging to dimethyl oxalate was also discovered, demon-
strating distinct evidence of the cleavage reaction that occurred
in the C−C bond between dicarbonyls.
3. RESULTS AND DISCUSSION
3.1. Light Absorption Property. The UV−vis absorption
spectra of MBFs and BAPO in anhydrous acetonitrile are
exhibited in Figure 1, and the molar extinction coefficients (ε)
of MBFs and BAPO at the maximum absorption wavelength
and common LED emission wavelengths (385, 395, 405, and
455 nm) in anhydrous acetonitrile are presented in Table S1.
The maximum absorption wavelengths of MBFs all show a red-
shift compared with MBF whether the benzene ring of MBFs
link electron-withdrawing groups or electron-donating groups
expect for TF-MBF. For N-MBF and S-MBF, the introduction
of strong electron-donating groups dimethylamino and
methylthio causes a significant red-shift of the maximum
absorption wavelength and a high molar extinction coefficient
(43800 M−1 cm−1 at 360 nm for N-MBF and 29660 M−1 cm−1
at 326 nm for S-MBF). However, the absorption wavelength of
TF-MBF is blue shifted by more than 9 nm compared with
MBF even though the molar extinction coefficient of TF-MBF
The ESR spectra of MBFs in the presence of PBN as a
trapping reagent under 405 nm LED exposure and the
simulations of ESR spectra for MBF, F-MBF, and S-MBF are
exhibited in Figures 4 and S32. The hyperfine coupling
constant of benzoyl radical (αN = 14.20 G, αH = 5.80 G) was
obtained by DB upon 385 nm LED exposure for 300 s trapped
hyperfine coupling constants of the p-fluorobenzylbenzoyl
radical (αN = 13.30 G, αH = 1.60 G), the p-methylthiobenzoyl
radical (αN = 14.21 G, αH = 4.80 G), and the methyl radical
3857
Macromolecules 2021, 54, 3854−3864
He, Xianglong
