J. You et al.
Dyes and Pigments 184 (2021) 108865
solid was filtered, and then recrystallized from ethanol. Dark yellow
solid. Yield:65%. Mp: > 200 ◦C. 1H NMR (400 MHz, CDCl3) δ 8.24 (dd, J
= 8.0, 1.5 Hz, 1H), 8.15–8.12 (m, 2H), 7.68 (ddd, J = 8.6, 7.1, 1.6 Hz,
1H), 7.55 (d, J = 8.4 Hz, 1H), 7.42–7.38 (m, 1H), 7.33–7.30 (m, 4H),
7.19–7.16 (m, 5H), 7.13–7.10 (m, 3H).13C NMR (101 MHz, CDCl3) δ
172.9, 155.2, 149.6, 146.8, 145.5, 137.6, 133.3, 129.5, 128.8, 125.5,
125.4, 124.4, 124.1, 123.5, 121.2, 120.7, 118.1. IR (KBr, (cmꢀ 1)): 3231
–
–
–
(O–H), 1596 (C= O), 1561 (C C), 1064 (C–O); HRMS (ESI) calculated
–
Scheme 1. ESIPT property of 3HF.
for C27H19NO3 406.1438 [M+H]+; found 406.1433 [M+H]+.
excited by light. Both the directly excited state (N *) and the base
tautomeric excited state (T *) formed by the ESIPT process can emit
fluorescence (Scheme 1). Compared with the electron transfer process,
the rate of proton transfer is faster, and the excited state T * of the base
tautomer produces long-wavelength strong fluorescence emission.
Therefore, flavonol and its derivatives are an important class of fluo-
rescent probe molecules.
2.1.2. Synthesis of 2-(4-(diphenylamino)phenyl)-4-oxo-4H-chromen-3-yl
benzenesulfonate (3HF–S)
The 3HF–OH (5 mmol) was added to a 100 mL round bottom flask,
dissolved in dichloromethane (20.0 mL), and then triethylamine (10.0
mmol) was added. Next, the benzenesulfonyl chloride (10.0 mmol) was
dissolved in dichloromethane. The mixed solution was slowly added
dropwise to the round bottom flask under ice bath conditions. After the
mixed solution is added dropwise, the reaction at room temperature for
4 h. The organic phase was washed 3 times with H2O. After drying over
anhydrous Na2SO4, the solution was concentrated and purified by silica
gel column chromatography using PE/EA to obtain the pure product
(3HF–S) as yellow crystals. Yellow solid. Yield:82%. Mp: 112–114 ◦C. 1H
NMR (400 MHz, CDCl3) δ 8.20 (dd, J = 8.0, 1.5 Hz, 1H), 8.01 (dd, J =
8.4, 1.2 Hz, 2H), 7.85–7.81 (m, 2H), 7.70–7.63 (m, 2H), 7.53–7.50 (m,
3H), 7.42–7.38 (m, 1H), 7.37–7.33 (m, 4H), 7.16 (dd, J = 12.6, 7.4 Hz,
6H), 7.01–6.99 (m, 2H). 13C NMR (101 MHz, CDCl3) δ 172.3,
158.7155.2, 150.8, 146.4137.5, 133.9, 132.6, 130.1, 129.6, 128.7,
128.5, 126.9, 126.2, 126.0, 125.2, 124.7123.9, 121.0, 119.7, 117.9. IR
´
Lalevee et al. found that some flavonol derivatives linked to conju-
gated aromatic hydrocarbons have photosensitivity activity under long-
wavelength LED light sources [21]. Flavonol (3HF) can be used with
amino acids (e.g., N-phenylglycine (NPG)) as a low-toxicity visible light
initiator for thick samples or free radical polymerization of methacrylate
in composites [8]. Furthermore, the effect of different substituents on
the photopolymerization of flavonols was discussed, and the initiation
rules and mechanisms of the different substituents of flavonols were
given [22]. It was found that the flavonol ESIPT process is a competitive
process of the photoinitiated process. The strong fluorescence process
generated by the hydrogen atom transfer limits the charge transfer
sensitization ability and direct initiation ability of the flavonol dye
molecules in photopolymerization.
(KBr, (cmꢀ 1)): 1591 (C O), 1503 (C C), 1059 (C–O), 978 (S O);
–
–
–
–
–
–
HRMS (ESI) calculated for C33H23NO5S 546.1670 [M+H]+, found
546.1683 [M+H]+.
If the hydroxyl group of flavonol compounds is etherified or esteri-
fied, it can inhibit the proton transfer process in its excited state mole-
cules, thereby reducing the fluorescence intensity and increasing the
triplet lifetime, which results in an increase in its activity during pho-
topolymerization. To obtain a low-toxicity initiator with high-efficiency
initiating activity under safe long-wavelength LED soft light sources, we
2.1.3. Synthesis of 2-(4-(diphenylamino)phenyl)-4-oxo-4H-chromen-3-yl
benzoate (3HF–C)
Compound 3HF–C was synthesized from benzoyl chloride by using a
similar procedure described above for 3HF–S. Yellow solid. Yield:85%.
Mp: > 200 ◦C. 1H NMR (400 MHz, DMSO‑d6) δ 8.16 (d, J = 7.5 Hz, 1H),
8.10 (d, J = 7.9 Hz, 1H), 7.96 (d, J = 7.4 Hz, 1H), 7.89 (t, J = 7.4 Hz,
2H), 7.79 (dd, J = 14.5, 7.9 Hz, 2H), 7.62 (t, J = 7.6 Hz, 2H), 7.53 (dt, J
= 19.4, 7.5 Hz, 2H), 7.37 (t, J = 7.7 Hz, 4H), 7.16 (dd, J = 18.2, 7.6 Hz,
6H), 6.93 (d, J = 8.9 Hz, 2H). 13C NMR (101 MHz, DMSO‑d6) δ
170.7167.3, 163.2, 155.5, 154.9150.3, 145.7, 134.6, 132.8132.3, 129.9,
129.5, 129.2, 128.5128.0, 125.9, 125.6, 124.9122.7, 120.5, 119.1,
synthesized
2-(4-(diphenylamino)phenyl)-3-hydroxy-4H-chromen-4-
one (3HF–OH). Through the introduction of triphenylamine electron
donor, we were able to improve the charge transfer ability in the
molecule and extend the absorption wavelength. Consequently, the
absorption wavelength requirements of long-wavelength LED soft light
sources are satisfied. Furthermore, we synthesized flavonol sulfonate
(3HF–S) and flavonol carboxylate (3HF–C) by replacing the 3-position
hydroxyl group with benzenesulfonyl chloride and benzoyl chloride.
3HF–S and 3HF–C inhibit the ESIPT process, reduce the fluorescence
intensity, and improve the photosensitivity with the introduction of
ester groups. Moreover, we performed acid production experiment,
density functional theory (DFT) calculations, and electron spin reso-
nance (ESR) experiments.
118.5. IR (KBr, (cmꢀ 1)): 1741, 1587 (C O), 1498 (C C), 1060 (C–O);
–
–
–
–
HRMS (ESI) calculated for
C
34H23NO4 510.1700[M+H]+, found
510.1709 [M+H]+.
2.2. Photoacid generation experiments
All 3HFs dissolved in acetonitrile were previously N2 degassed. The
photoacid generation progress was monitored via UV–visible absorption
spectra. Rhodamine B (RhB) was used as a sensor of photoacid genera-
tion [23].
2. Experimental details
2.1. Synthesis of the 3HFs
2.1.1. Synthesis of 2-(4-(diphenylamino)phenyl)-3-hydroxy-4H-chromen-
4-one (3HF–OH)
2.3. ESR experiments
Sodium hydroxide (2.0 g, 0.05 mol) was added to a 100 mL round
bottom flask, dissolved in water (5.0 mL), the temperature was lowered
to room temperature, and then ethanol (30 mL) was added; Next, 2-
hydroxyacetophenone (6.0 mmol) and 4-(diphenylamino)benzalde-
hyde (6.0 mmol) were added, and diluted with ethanol (20.0 mL). 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
neutralization. At that time, a large amount of solid precipitated. The
ESR experiments were carried out using a Bruker A300 ESR spec-
trometer. The radicals were produced at room temperature under the
405 nm LED exposure and trapped by phenyl N-tert-butylnitrone (PBN)
[24,25].
2.4. Cytotoxicity in a non-contact model
Five pieces (1 mm thick disc with 7 mm diameter) of polymer was
prepared from the photopolymerization of the Bis-GMA/PEGDMA
(70%/30%, wt/wt) blend initiated by the 3HFs (0.125%, wt), 3HFs/
2