Naphthalimide derivatives containing benzyl-sulfur bond as cleavable photoinitiators for near-UV LED polymerization

Article abstract of DOI:10.1080/17415993.2020.1795175

Three naphthalimide aryl benzyl sulfide derivatives (NABSs) cleavable photoinitiators (PIs) containing benzyl-sulfur bonds were designed and prepared. The effect of the benzyl-sulfur moiety and substituents located on benzene ring of the benzyl group on p

Full text of DOI:10.1080/17415993.2020.1795175

Journal of Sulfur Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gsrp20
Naphthalimide derivatives containing benzyl-
sulfur bond as cleavable photoinitiators for near-
UV LED polymerization
Xiuyuan Hu , Jia Yu , Shengling Jiang , Yanjing Gao & Fang Sun
To cite this article: Xiuyuan Hu , Jia Yu , Shengling Jiang , Yanjing Gao & Fang Sun (2020):
Naphthalimide derivatives containing benzyl-sulfur bond as cleavable photoinitiators for near-UV
LED polymerization, Journal of Sulfur Chemistry, DOI: 10.1080/17415993.2020.1795175
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JOURNAL OF SULFUR CHEMISTRY
https://doi.org/10.1080/17415993.2020.1795175
Naphthalimide derivatives containing benzyl-sulfur bond as
cleavable photoinitiators for near-UV LED polymerization
Xiuyuan Hua, Jia Yub, Shengling Jiangc,d, Yanjing Gaoa and Fang Suna,e,f
aCollege of Chemistry, Beijing University of Chemical Technology, Beijing, People’s Republic of China;
bLiming Research Institute of Chemical Industry, Luoyang, People’s Republic of China; cCollege of Materials
Science and Engineering, Beijing University of Chemical Technology, Beijing, People’s Republic of China; dKey
Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education (Beijing University of Chemical
Technology), Beijing, People’s Republic of China; eState Key Laboratory of Chemical Resource Engineering,
Beijing University of Chemical Technology, Beijing, People’s Republic of China; fAnqing Research Institute,
Beijing University of Chemical Technology, Anqing, People’s Republic of China
ABSTRACT
ARTICLE HISTORY
Received 2 February 2020
Accepted 7 July 2020
Three naphthalimide aryl benzyl sulfide derivatives (NABSs) cleav-
able photoinitiators (PIs) containing benzyl-sulfur bonds were
designed and prepared. The effect of the benzyl-sulfur moiety
and substituents located on benzene ring of the benzyl group
on photochemical behavior, involving the light absorption prop-
erties, potential mechanisms of the photolysis of NABSs under
UV LED at 405 nm, and the electron transfer reaction between
NABSs and diphenyliodonium hexafluorophosphate (Iod) were
investigated in detail. The NABSs have different photolysis mech-
anisms as compared to PIs containing only an aryl-sulfur bond.
NABS3 is most efficient for initiating free radical photopolymer-
ization of acrylate monomers under UV LED exposure at 405 nm.
Furthermore, NABSs/Iod systems can initiate the cationic pho-
topolymerization of epoxides. Interestingly, the polymers pre-
pared by using NABSs as the PI exhibit a great photolumines-
cence band at about 460 nm under UV LED irradiation at 405 nm.
KEYWORDS
Naphthalimide; C–S bond;
cleavable photoinitiator;
photoluminescence; UV LED
polymerization
CONTACT Fang Sun
sunfang60@yeah.net
State Key Laboratory of Chemical Resource Engineering, Beijing
University of Chemical Technology, Beijing 100029, People’s Republic of China
This article makes reference to supplementary material available on the publisher’s website at
https://doi.org/10.1080/17415993.2020.1795175
© 2020 Informa UK Limited, trading as Taylor & Francis Group

2
X. HU ET AL.
1. Introduction
Nowadays, UV polymerization technology is widely used because of its advantages of envi-
ronmental protection and energy-saving, such as the curing of protective and decorative
coatings, printing inks or photoresists [1–4]. Recently, a more important application can
be found for batterie construction in the preparation of anion exchange membranes for
alkaline fuel cells and photopolymer electrolytes for sodium-ion batteries [5,6]. At present,
the near-UV or visible light – emitting diodes (LEDs), which have many advantages such
as low heat generation, long lamp life, low costs, no ozone release and environmental
preservation in comparison with traditional mercury UV lamps, has been widely used
[7,8]. The near-UV or visible LEDs emission wavelengths are above 365 nm. Hence, the
absorption wavelengths of photoinitiators as a crucial component of photopolymerza-
tion systems need to match the emission wavelengths of near-UV or visible LEDs, so
that the photoinitiators can produce active species [9]. To this end, considerable effort
has been devoted to the design of various new Norrish type II photoinitiators with good
absorption in LEDs emission range and they are mainly dyes [9,10]. However, although
currently available commercial photoinitiators used for near-UV or visible LEDs are all
cleavable (Norrish type I) photoinitiators, such as bisacylphosphine oxide (BAPO) and
2,4,6-trimethylbenzoyldiphenylphosphine oxide (TPO), the development of novel cleav-
able photoinitiators for these applications has received little attention. Only a small amount
of effort has been recently made to develop novel cleavable photoinitiators used for
near-UV or visible LEDs, such as acylgermane-based photoinitiators [11–14], cleavable
thioxanthones [15] and some co-initiators that can be used as cleavable photoinitia-
tors [16,17]. Therefore, it poses a great challenge to design and develop novel cleavable
photoinitiators with good absorption capacities in near-UV or visible LEDs wavelength
range.
As mentioned above, photoinitiators must have excellent absorption in the near-UV
or visible LEDs emission range. In addition, if a weak chemical bond is introduced, a
high photoinitiating efficiency can be expected. Our previous work [18] reported a series
of naphthalimide aryl sulfide derivatives containing two aryl-sulfur bonds and naphthal-
imide moieties as photoinitiators used for UV LEDs and demonstrated that the weak

JOURNAL OF SULFUR CHEMISTRY
3
Scheme 1. Chemical structures of NABSs and previously investigated NAS3 and NAS6 [18].
aryl-sulfur bonds of the photoinitiators can break under LED irradiation at 405 nm to pro-
duce aryl and thiyl radicals which can initiate photopolymerization of acrylate monomers.
Compared with the aryl-sulfur bond, the benzyl-sulfur bond is weaker [19], thus, the
introduction of a benzyl-sulfur bond into the photoinitiators instead of an aryl-sulfur
bond would improve the initiating ability of the photoinitiators. However, the electron-
donation capability of the -S-CH2-Ph group is higher than that of the -S-Ph group, which
results in a higher fluorescence quantum yield for photoinitiators containing a -S-CH2-
Ph group compared with the photoinitiators containing a -S-Ph group [20]. The higher
fluorescence quantum yield may bring a negative impact on the initiation efficiency of pho-
toinitiators. This contradiction motivated us to study how benzyl-sulfur bond influences
the properties of photoinitiators and whether the photoinitiators containing benzyl-sulfur
bond have some special functionality to provide some inspiration for the molecular design
of new photoinitiators. To the best of our knowledge, similar studies have not yet been
reported.
Herein, we designed and synthesized a series of naphthalimide aryl benzyl sulfide
derivatives photoinitiators (NABSs) (Scheme 1). Then, the influences of NABSs structures
on the light absorption properties of NABSs were investigated. The possible mechanism of
the photolysis of NABSs was researched through steady-state photolysis, nuclear magnetic
resonance (NMR), high-resolution mass spectra (HRMS) and electron spin resonance
(ESR), and by calculating the bond dissociation energies (BDEs) of the C–S bonds of
NABSs. We also studied the mechanism of the electron transfer reaction between NABSs
and diphenyliodonium hexafluorophosphate (Iod) by fluorescence spectroscopy, cyclic
voltammetry and laser flash photolysis. The photoinitiation capabilities of NABSs for
the free radical polymerization of 1,6-hexanediol diacrylate (HDDA) under UV LED
irradiation at 405 nm were investigated and compared with previously reported naph-
thalimide aryl sulfide derivative using real-time infrared spectroscopy (RTIR). The ini-
tiating efficiencies of NABSs in cationic polymerization of 3,4-epoxycyclohexylmethyl
3ꢀ,4ꢀ-epoxycyclohexane carboxylate (E-4221) were also examined. More importantly, we
explored the photoluminescence property of polymers prepared by using NABSs under
UV LED irradiation at 405 nm. Figure S1 shows the structures of monomers, additives
and other photoinitiators used for experiments.

4
X. HU ET AL.
2. Experimental
2.1. Materials and characterization
4-Bromo-1,8-naphthalic anhydride, bis(dibenzylideneacetone) palladium [Pd(dba)2],
1,1ꢀ-bis(diphenylphosphino) ferrocene (dppf), potassium phosphate (K3PO4) and acetic
anhydride were obtained from Beijing Ouhe Technology Co., Ltd. 2,6-Diisopropylaniline,
anhydrous acetonitrile, tert-butylbenzene, phenylmethanethiol, p-tolylmethanethiol and
(4-methoxyphenyl)methanethiol were purchased from J&K Scientific Ltd. BAPO, anhy-
drous nickel chloride (NiCl2) and Iod were supplied by Energy Chemical. 1,6-Hexanediol
diacrylate (HDDA) and 3,4-epoxycyclohexylmethyl 3ꢀ,4ꢀ-epoxycyclohexane carboxylate
(E-4221) were obtained from Ciba Geigy Co.. 5,5-Dimethyl-1-pyrroline N-oxide (DMPO)
was purchased from Dojindo Laboratories.
Shenzhen Lamplic household UV LED bulb (emission wavelength centered at 405 nm,
incident light intensity: 100 mW cm−2) was used as the irradiation device.
NMR spectra were recorded on a Bruker AV400 unity spectrometer (400 MHz). The
high-resolution mass spectra in dichloromethane were obtained with an Agilent 6540 Q-
ToF mass detector equipped with an electrospray ionization source. Light absorption prop-
erties of NABSs and the UV–vis absorption spectra of polymer films prepared by NABSs
and HDDA were explored by using a Shimadzu UV-3600 UV–vis-NIR spectrophotometer.
The light degradation of NABSs was studied by UV/vis spectroscopy. NABSs in anhy-
drous acetonitrile were irradiated by UV LED at 405 nm (∼100 mW cm−2) under nitro-
gen atmosphere, and their UV–vis spectra were recorded on the Shimadzu UV-3600
UV-vis-NIR spectrophotometer at different irradiation times.
ESR experiments were performed on a JEOL JES-FA200 spectrometer (X-band) at
9.06 GHz, 100 kHz field modulation, and a microwave power of 0.998 mW was used. The
radicals were produced at room temperature upon UV LED exposure at 405 nm under
an argon protection and trapped by DMPO. Solutions with concentrations of 1 × 10−4
mol L−1 were achieved by adding DMPO and NABSs into tert-butylbenzene. Then, the
solutions were irradiated. The molar ratio of NABSs to DMPO was 1:5.
Laser flash photolysis experiments of NABSs were carried out with a Spectra − Physics
Quanta – Ray Q-switched nanosecond Nd/YAG laser at λ = 355 nm (10 ns pulses;
exc
energy reduced down to 10 mJ) and an analyzing system consisted of a ceramic xenon lamp,
an Acton SpectraPro SP-2300 spectrometer, a monochromator, a fast photomultiplier and
a transient digitizer (Princeton Instruments PI-MAX 4). Liquid samples with 10−5 M of
NABSs in nitrogen-saturated acetonitrile were contained in a quartz cell with an optical
path length of 1 cm.
The fluorescence excitation spectra of polymer films prepared by NABSs and HDDA
and the fluorescence emission spectra of NABSs in acetonitrile and the polymer films
were obtained using Hitachi F-7000 fluorescence spectrophotometer. The interaction
rate constants kq between the studied compounds and Iod were obtained from classical
Stern–Volmer treatments [21] (I0/I = 1 + kqτ [Iod]; where I0 and I stand for the fluo-
0
rescent intensity of the studied compounds in the absence and the presence of the Iod
quencher, respectively; τ stands for the first excited singlet state lifetime in the absence of
0
Iod). The absolute fluorescence quantum yields and the fluorescence lifetimes of NABSs
and NAS6 in acetonitrile were investigated using Edinburch FLS980 spectrometer.

JOURNAL OF SULFUR CHEMISTRY
5
The oxidation potentials (Eox vs SCE) of NABSs were measured in acetonitrile by
cyclic voltammetry using tetrabutylammonium hexafluorophosphate (0.1 M) as a support-
ing electrolyte (CH Instruments Model 600E Series Electrochemical Workstation). The
procedure has been presented in detail in ref 14.
The free energy changes ꢀG for the electron transfer between NABSs and Iod can be cal-
culated from the classical Rehm−Weller equation [22]: ꢀG = Eox − Ered − ES (or ET) + C,
where Eox, Ered, ES (or ET), and C are the oxidation potential of the electron donor, the
reduction potential of the electron acceptor, the excited singlet (or triplet) state energies of
NABSs, and the electrostatic interaction energy for the initially formed ion pair, generally
considered as negligible in polar solvents.
Gaussian 09 package was used in the molecular orbital calculations. All molecular
structures of NABSs were optimized in the ground state by the density functional the-
ory (DFT) at B3LYP/6-31G**. Whether optimized geometries had energy minima (zero
imaginary frequency) were determined through frequency calculations. Frontier molec-
ular orbital properties were analyzed and visualized with Gaussview 5.0 software [23].
DFT calculations of the dissociation energy of C–S bonds of NABSs were performed with
Gaussian 09 software. All geometry optimizations were performed without imposing any
geometry constraints by using a B3LYP hybrid density functional with a 6-311G* basis
set [24]. The results were analyzed through GaussView 5.0. In all molecules, frequency
was calculated at the same level of theory to confirm that structures had energy minima.
Reported energies were zero-point corrected.
Photopolymerization kinetics of monomers (i.e. HDDA and E-4221) initiating NABSs
or NABSs/Iod as the photoinitiators were explored by using a Nicolet 5700 FT-IR spec-
troscopy. Photocurable liquids were coated on a KBr plate under air atmosphere or
clamped in laminate between two KBr plates ( ∼30 μm) for the exposure with UV LED
at 405 nm (∼100 mW cm−2). The conversions of acrylate, and epoxy functional groups
were evaluated through calculating the proportion of the peak areas of 831–792 cm−1 and
756–738 cm−1, respectively. Each sample was run three times, and errors on the reported
double bond and epoxy conversions as a function of polymerization time were lower than
2%. In most cases, the errors were lower than 1%.
2.2. Preparation of polymer films
A circular hole with a diameter of 3 mm or a square hole with a side length of 1 cm was
punched on a silicone film with a thickness of 1 mm. A silicone film was then pasted onto
a rectangular slide glass substrate. Next, the photocurable liquid was dripped into the hole
and covered with a coverslip. Finally, the photocurable liquid was irradiated by LED at
405 nm for 200 s to obtain the polymer film.
2.3. Synthesis
Scheme 2 displays the synthetic route of NABSs. 6-bromo-2-(2,6-diisopropylphenyl)-1H-
benzo[de]isoquinoline-1,3(2H)-dione (3) was synthesized as reported [25].
Firstly, S-benzyl ethanethioate (2a), S-(4-methylbenzyl) ethanethioate (2b) and S-(4-
methoxybenzyl) ethanethioate (2c) were prepared by the following procedure. A mixture

6
X. HU ET AL.
of phenylmethanethiol (4.96 g, 40 mmol) or p-tolylmethanethiol (5.53 g, 40 mmol) or (4-
methoxyphenyl)methanethiol (6.17 g, 40 mmol), acetic anhydride (8.16 g, 80 mmol) and
anhydrous NiCl2 (518 mg, 4 mmol) were stirred at room temperature for 4 h. Then the
reaction mixture was diluted with water (100 mL) and extracted with dichloromethane
(3 × 100 mL). Organic layers were combined and washed with 10% sodium bicarbonate
and water, dried over anhydrous sodium sulfate, and filtered. The solvent was removed
under vacuum, and the resulted crude product was purified by flash chromatography on
silica gel to obtain the desired product as colorless liquid.
2a: 4.96 g, 87%. 1H NMR (400 MHz, CDCl3, ppm) δ 7.30–7.21 (m, 5H), 4.11 (s, 2H),
2.33 (s, 2H). 13C NMR (100 MHz, CDCl3, ppm) δ 195.1, 137.6, 128.8, 128.7, 127.3, 33.5,
30.3. HRMS (ESI) [M + Na]+ calcd for C9H10NaOS+ 189.0345, found 189.0425 (see
Supplementary Information Figure S2 and Figure S3).
1
2b: 6.49 g, 90%. H NMR (400 MHz, CDCl3, ppm) δ 7.18–7.16 (d, J = 8.0 Hz, 2H),
7.10–7.08 (d, J = 7.9 Hz, 2H), 4.08 (s, 2H), 2.33 (s, 3H), 2.31 (s, 3H). 13C NMR (100 MHz,
CDCl3, ppm) δ 195.2, 137.0, 134.5, 129.3, 128.7, 33.2, 30.5, 21.1. HRMS (ESI) [M + H]+
calcd for C10H13OS+ 181.0682, found 181.0671 (see Supplementary Information Figure
S4 and Figure S5).
1
2c: 7.22 g, 92%. H NMR (400 MHz, CDCl3, ppm) δ 7.25–7.23 (d, J = 8.6 Hz, 2H),
6.87–6.85 (d, J = 8.7 Hz, 2H), 4.11 (s, 2H), 3.81 (s, 3H), 2.37 (s, 3H). 13C NMR (100 MHz,
CDCl3, ppm) δ 195.3, 158.8, 130.0, 114.1, 55.3, 32.97, 30.4, 28.4. HRMS (ESI) [M + Na]+
calcd for C10H12NaO2S+ 219.0450 found 219.0488 (see Supplementary Information
Figure S6 and Figure S7).
Then, NABS1, NABS2 and NABS3 were prepared by the following procedure. Com-
pound 3 (873 mg, 2 mmol), compound 2a (333 mg, 2 mmol) or compound 2b (362 mg,
2 mmol) or compound 2c (393 mg, 2 mmol), Pd(dba)2 (115 mg, 0.2 mmol), dppf (155 mg,
0.28 mmol), and K3PO4 (1.02 g, 4.8 mmol) were added into a round-bottomed flask.
Toluene (4.0 mL) and acetone (2.0 mL) were then added. The reaction mixture was stirred
for 12 h at 110 °C under nitrogen. Subsequently, the mixture was cooled to room tem-
perature, poured into 10 mL of saturated aqueous ammonium chloride, and extracted
with dichloromethane (3 × 10 mL). Organic layers were combined and washed with brine
Scheme 2. Synthesis route of NABSs.

JOURNAL OF SULFUR CHEMISTRY
7
(3 × 15 mL), dried over anhydrous magnesium sulfate, and filtered. Then, the solvent was
removed under vacuum, and the resulted crude product was purified by flash chromatog-
raphy on silica gel to obtain the desired product as yellow solid.
1
NABS1: 777 mg, 81%. H NMR (400 MHz, CDCl3, ppm) δ 8.70 (ddd, J = 10.7, 7.9,
1.1 Hz, 2H), 8.53 (d, J = 7.8 Hz, 1H), 7.82 (dd, J = 8.5, 7.3 Hz, 1H), 7.66 (d, J = 7.8 Hz,
1H), 7.52–7.46 (m, 2H), 7.42–7.31 (m, 5H), 4.44 (s, 2H), 2.75 (hept, J = 6.9 Hz, 2H), 1.17
(d, J = 6.8 Hz, 12H).13C NMR (100 MHz, CDCl3, ppm) δ 164.0, 164.0, 145.7, 144.9, 135.3,
132.0, 131.3, 130.9, 130.4, 129.8, 129.5, 129.0, 128.9, 128.4, 127.9, 126.8, 124.0, 123.8, 123.2,
119.6, 37.4, 29.1, 24.0. HRMS (ESI) [M + H]+ calcd for C31H30NO2S+ 480.1992 found
480.2045 (see Supplementary Information Figure S8 and Figure S9).
NABS2: 839 mg, 85%. 1H NMR (400 MHz, CDCl3, ppm) δ 8.69 (dd, J = 11.6, 7.9 Hz,
2H), 8.53 (d, J = 7.8 Hz, 1H), 7.82 (t, J = 7.9 Hz, 1H), 7.66 (d, J = 7.8 Hz, 1H), 7.49 (t,
J = 7.7 Hz, 1H), 7.36 (dd, J = 12.2, 7.8 Hz, 4H), 7.20 (d, J = 7.7 Hz, 2H), 4.41 (s, 2H),
2.75 (hept, J = 6.9 Hz, 2H), 2.38 (s, 3H), 1.17 (d, J = 6.8 Hz, 13H). 13C NMR (100 MHz,
CDCl3, ppm) δ 164.1, 164.0, 145.7, 145.2, 137.8, 132.2, 132.0, 131.3, 130.4, 129.8, 129.6,
129.5, 129.0, 128.8, 126.7, 124.0, 123.6, 123.2, 123.2, 119.5, 37.1, 29.1, 24.0, 21.2. HRMS
(ESI) [M + H]+ calcd for C32H32NO2S+ 494.2148 found 494.2188 (see Supplementary
Information Figure S10 and Figure S11).
NABS3: 897 mg, 88%. 1H NMR (400 MHz, CDCl3, ppm) δ 8.69 (dd, J = 12.5, 7.9 Hz,
2H), 8.53 (d, J = 7.8 Hz, 2H), 7.85–7.78 (m, 1H), 7.66 (d, J = 7.9 Hz, 2H), 7.49 (t,
J = 7.8 Hz, 2H), 7.40 (d, J = 8.3 Hz, 2H), 7.34 (d, J = 7.8 Hz, 2H), 6.92 (d, J = 8.6 Hz,
2H), 4.40 (s, 2H), 3.84 (s, 3H), 2.91–2.65 (hept, J = 6.9 Hz 2H), 1.17 (d, J = 6.8 Hz, 12H).
13C NMR (100 MHz, CDCl3, ppm) δ 164.1, 164.0, 159.3, 145.7, 145.2, 132.0, 131.3, 130.9,
130.4, 130.1, 129.8, 129.5, 129.0, 128.4, 127.1, 126.7, 124.0, 123.8, 123.2, 114.3, 55.3, 36.9,
29.1, 24.0. HRMS (ESI) [M + H]+ calcd for C32H32NO3S+ 510.2097 found 510.2093 (see
Supplementary Information Figure S12 and Figure S13).
Figure 1. UV–vis absorption spectra of NABSs in anhydrous acetonitrile (concentration = 1 × 10−5
mol L−1).

8
X. HU ET AL.
Table 1. Molar extinction coefficients (ꢁ) of NABSs in anhydrous acetonitrile at typical UV
LED emission wavelengths.
NABSs
ꢁ385nm × 104(M−1 cm−1
)
ꢁ395nm × 104(M−1 cm−1
)
ꢁ405nm × 104(M−1 cm−1
)
NABS1
NABS2
NABS3
1.30
1.49
1.39
1.27
1.48
1.39
1.00
1.20
1.15
3. Results and discussion
3.1. Synthesis
Three naphthalimide aryl benzyl sulfide derivatives, NABS1, NABS2 and NABS3, were
prepared from 6-bromo-2-(2,6-diisopropylphenyl)-1H-benzo[de]isoquinoline-1,3(2H)-
dione in two steps (Scheme 2). First, acetylation reactions [26] were performed
with benzyl mercaptan or para-substituted benzyl mercaptan to produce the S-benzyl
ethanethioate derivatives in high yield (over 85%). Then, C–S bonds were built
with S-benzyl ethanethioate derivatives and 6-bromo-2-(2,6-diisopropylphenyl)-1H-
benzo[de]isoquinoline-1,3(2H)-dione catalyzed by Pd(dba)2 in one step [27] thereby
obtaining the photoinitiators NABS1, NABS2 and NABS3. The structures of NABS1,
1
NABS2 and NABS3 were determined by H NMR, 13C NMR and HR-MS as shown in
Figure S8–S13.
3.2. Light absorption studies of NABSs
The UV – vis spectra of NABSs in anhydrous acetonitrile are shown in Figure 1. The
maximum absorptions of NABSs are centered at 389, 391 and 390 nm, respectively. The
maximum absorption peaks of NABS2 and NABS3 show a slight red-shift compared with
that of NABS1, due to the presence of the electron-donating group (methyl group and
methoxy group) at the para position of the benzene ring in their molecules. It can be seen
from Table 1 and Figure 1 that NABSs all have exellent absorption at 385, 395 and 405 nm
and their molar extinction coefficients at these wavelengths are all greater than or equal to
1 × 104 M−1 cm−1, indicating NABSs perfectly match the typical UV LEDs sources. The
reason for this phenomenon can be attributed to the charge transfer from the sulfur atom
or the p-methoxybenzyl group (for NABS3) linked to the sulfur atom (for HOMO) to the
naphthalimide moiety (for LUMO) as shown in Figure 2. This charge transfer transition
makes the light absorption properties of these derivatives significant in the region of the
LED emissions [28]. In addition, NABSs and previous reported NASs except NAS3 [18]
with nitro have similar maximum absorption and molar extinction coefficients at 385, 395
and 405 nm, indicating that naphthalimide-S moiety plays a major role in the light absorp-
tion properties of the molecules, unless there is an strong electron-withdrawing group
located in benzene ring linked with sulfur atoms.
3.3. Photochemistry of NABSs
Figure 3 displays the steady state photolysis of NABSs in anhydrous acetonitrile. When the
irradiation time was increasing, the intensity of the UV absorption at 390 nm for NABSs
was decreasing, while the intensity of the UV absorption at 340 nm was increasing with a

JOURNAL OF SULFUR CHEMISTRY
9
Figure 2. HOMO and LUMO of NABSs at B3LYP/6-31G** level (isovalue = 0.02).
blue-shift. The results are caused by the increase of the concentration of the photocleavage
products (Scheme 3) whose absorption peak centered at 340 nm. Two isosbestic points at
360 and 287 nm for NABS1 and NABS2, and the isosbestic point at 360 nm for NABS3 were
observed, demonstrating that photodegradations of NABSs proceeded selectively without
side reaction [29].
To confirm the potential mechanism of NABSs photolysis, the photolysis products of
1
NABS2 as a model were tested by NMR and HRMS. Figure S14 shows the H NMR
spectra of irradiated solutions of NABS2 after different irradiation time. The character-
istic peaks (10.00 and 2.47 ppm) belonging to 4-methylbenzaldehyde which was produced
by oxidation of 4-methylbenzyl radical by oxygen [30,31], and the characteristic peaks
(2.71–2.74 ppm) belonging to 1,2-di-p-tolylethane appeared and increased with the light
irradiation. This result is ascribed to the cleavage of S-CH2 bond of NABS2. Besides,
the characteristic peak (4.07 ppm) of di-p-tolylmethane also appeared and increased
with the light irradiation, which is attributed to the cleavage of C (aryl) – S bond of
NABS2. The HRMS spectra of the photolysis products of NABS2 upon UV LED expo-
sure at 405 nm for 30 min were displayed in Figure S15. The [M + H]+ ions of m/z
358.1790 and 390.1519 are assigned to photolysis products 2-(2,6-diisopropylphenyl)-6-
mercapto-1H-benzo[de]isoquinoline-1,3(2H)-dione and 2-(2,6-diisopropylphenyl)-1H-
benzo[de]isoquinoline-1,3(2H)-dione, respectively. The formation of these two photolysis
products proved that the two C–S bonds of NABS2 all can be broken to produce free
radicals under the light irradiation.

10
X. HU ET AL.
Figure 3. Steady state photolysis of NABSs in anhydrous acetonitrile upon UV LED at 405 nm (concen-
tration = 2.1 × 10−5 mol L−1).
Furthermore, ESR tests were conducted to detect the radicals produced from NABSs
under the UV LED irradiation at 405 nm. As shown in Figure 4, the hyperfine coupling
constants of the two free radicals are α = 1.45 mT, α = 2.13 mT and α = 1.26 mT,
N
H
N
•
α
= 1.30 mT. These signals are assigned to the adducts of DMPO with RPhCH2 and
H
ArS• radicals formed from direct photodecomposition of NABSs, respectively [32,33].
In addition, we calculated the BDEs of the C–S bonds of NABSs, and Table 2 presents
the results. The BDEs of the C–S bonds at position 1 for NABSs are all about 332 kJ mol−1
,
whereas such bonds at position 2 are 267.12 kJ mol−1 for NABS1, 265.63 for NABS2 and
282.21 for NABS3, indicating that C–S bonds at position 2 more likely break than those at
position 1 for NABSs. The results are consistent with the ESR tests results that the signals
of the adducts of DMPO with RPhCH2 and ArS•, respectively, were mainly observed.
•
Remarkably, the BDEs of the C–S bonds at position 2 for NABSs are lower than those of
the C–S bonds at position both position 1 and 2 for NAS6, which is consistent with our
expectation.
Based on the above information, the potential mechanism of photolysis of NABSs is pro-
posed (Scheme 3). NABSs can produce free radicals in two ways under UV LED irradiation
at 405 nm, but mainly in the second way. 2-(2,6-Diisopropylphenyl)-6-mercapto-1H-
benzo[de]isoquinoline-1,3(2H)-dione, 2-(2,6-diisopropylphenyl)-1H-benzo[de]isoquino
line-1,3(2H)-dione, 1,2-diphenylethane derivatives, diphenylmethane derivatives and ben-
zaldehyde derivatives (in the presence of oxygen) are produced by the photolysis reaction

JOURNAL OF SULFUR CHEMISTRY
11
Scheme 3. Potential mechanism of photolysis of NABSs.
of NABSs. Moreover, the free radicals produced by NABSs can be expected to initiate
free radical photopolymerization (FRP) of acrylates under the irradiation of UV LED at
405 nm.
Obviously, NABSs have different photolysis mechanism from NASs [19]. It can be
explained by the fact that the substituted benzyl radicals produced by NABSs are more
easily formed and stable than the substituted aryl radicals produced by NASs, which will
result in the fact that the substituted benzyl free radicals have more opportunities to attack
monomers to initiate photopolymerization, thereby enhancing the initiating rate of NABSs
[34–36]. Moreover, based on the electron-donating ability of -S-CH2-Ph, -S-CH2-Ph-CH3
and -S-CH2-Ph-OCH3 groups, absolute fluorescence quantum yields (ꢂfluo) of NABS1,
NABS2 and NABS3 theoretically should increase in turn. However, as can be seen from
Table 3, the ꢂ presents an opposite trend and ꢂ of NABS3 is lower about 10 times
fluo
fluo
than those of NABS1 and NABS2, which implies NABS3 is easier to decompose under the
irradiation. The result may relate to the higher stability of the p-methoxybenzyl radical
compared with p-methylbenzyl radical and p-methoxybenzyl radical.
Additional experiments have been carried out to explore the mechanism of the reaction
between NABSs and Iod. Table 3 gathered the free energy changes for the 1NABSs/Iod elec-
tron transfer reaction, the interaction rate constants kq between the NABSs and Iod, and the
characteristics of the fluorescence emission. The kq values are almost diffusion-controlled
[37]. The free energy changes ꢀG for 1NABSs/Iod electron transfer reactions are negative
(−1.02 to −1.13), which makes the processes favorable. The electron transfer quantum

12
X. HU ET AL.
Figure 4. ESR spectra of the radicals generated by (A) NABS1, (B) NABS2 and (C) NABS3 upon UV LED
exposure at 405 nm for five minutes and trapped by DMPO in tert-butylbenzene: (a) experimental and
(b) simulated spectra.
yields ꢂeT are high (0.34–0.94). In laser flash photolysis experiment, transient absorption
spectra of NABSs were obtained after excitation at 355 nm (Figure S16). There is a broad
absorption bond in 600–650 for NABS1 and NABS2, and 450–650 for NABS3. These results
prove that NABSs have an excited triplet state [38]. Hence, the possible mechanism for
NABSs/Iod electron transfer reaction is depicted in the following reactions.
NABS → 1NABS(hν) and 1NABS → 3NABS
(1)
1,3NABS + Ph2I+ → NABS•+ + Ph2I•
Ph2I• → Ph• + Ph - I
(2)
(3)
3.4. Photopolymerization kinetics
3.4.1. Free radical photopolymerization (FRP)
As shown in Figure 5(a), NABSs all can initiate FRP of HDDA under the UV LED irradia-
tion. The final conversions of double bonds of the systems with NABS1-3 are 76%, 64% and
80%, respectively. By contrast, the final conversions of the systems with NAS6 and BAPO
are 84% and 75%, respectively. The maximum rate of polymerization [see in Figure 5(b)] of

JOURNAL OF SULFUR CHEMISTRY
13
Table 2. BDEs of C–S bonds in NABSs calculated at the B3LYP/6-
311G* level of theory.
BDE (kJ mol−1
)
Bond
NABS1
NABS2
NABS3
NAS6 a
1
2
331.65
267.12
331.42
265.63
332.71
282.21
292.46
331.01
a From ref. [18].
Table 3. Parameters (rate constants, free energy changes) characterizing the reactivity of NABSs with
Iod; absolute fluorescence quantum yields (ꢂfluo), fluorescence lifetimes (τ), singlet (ES) state energies,
and oxidation potentials of the NABSs in acetonitrile.
Eoax(V vs SCE)
ESb(eV)
ꢀGcS(eV)
kqd(×1010
M
−1 s−1
)
ꢂeT
ꢂfluo
τ (ns)
e
NABS1
NABS2
NABS3
NAS6 [18]
1.71
1.68
1.63
2.93
2.92
2.96
−1.02
−1.04
−1.13
1.1
9.1
0.5
0.74
0.94
0.34
0.7342
0.5515
0.0679
0.0968
5.6
4.0
2.0
4.4
a Eox values measured by cyclic voltammetry for NABSs.
b Singlet state energies ES extracted from the UV-vis absorption and fluorescence emission spectra of NABSs (see in Figure
S17).
c ꢀGS = free energy change for the NABSs/Iod singlet interaction; reduction potential Ered = −0.2 V for Iod [39].
d kq
=
1NABSs/Iod interaction rate constant measured by fluorescence quenching experiments (see in Figure S18–S20).
eꢂeT = NABSs/Iod electron transfer quantum yields in the singlet state calculated according to
ꢂeT = kqτ0[Iod]/(1 + kqτ0[Iod]) [39], ([Iod] = 4.7 × 10−2 M in the formulations).
NABS3 system (6.50% s−1) is much higher than those of NABS1 (1.37% s−1) and NABS2
(1.55% s−1) systems, which further proves NABS3 more easily decompose to produce free
radicals under the irradiation. The result is consistent with the fluorescence tests results
that ꢂfluo (see in Table 3) of NABS3 (0.0679) is much lower than those of NABS1 (0.7342)
and NABS2 (0.5515). Low ꢂ
is good for bond breaking and free radical generation
fluo
[40]. Moreover, because the electron-donating ability of methoxy group is greater than
that of methyl group, the p-methoxybenzyl radical produced by the photolysis of NABS3
is easier to react with the electron-deficient double bond of acrylate than p-methylbenzyl
radical produced by the photolysis of NABS2 and benzyl radical produced by photolysis of
NABS1 [41]. Obviously, the initiation efficiency of NABS2 is lower than that of NAS6, while
that of NABS3 is higher than that of NAS6, although the BDE of C–S bonds at position 2
for NABS2 and NABS3 are lower than those of all C–S bonds in NAS6. The findings are
attributed to the fact that ꢂfluo of NABS2 is higher than that of NAS6, while that of NABS3

14
X. HU ET AL.
Figure 5. Photopolymerization profiles of HDDA in laminate in the presence of NABSs, BAPO and NAS6
upon UV LED exposure at 405 nm: (a) curves of double bond conversion; (b) curves of polymerization
rate. (concentration of photoinitiators is 0.5 wt%).
Figure 6. Photopolymerization profiles of E-4221 under air in the presence of NABSs/Iod (0.5 wt%
/2 wt%) upon UV LED exposure at 405 nm.
is lower than that of NAS6. In addition, the maximum polymerization rate of NABS3 sys-
tem is lower than that of BAPO system (6.50% s−1 vs 19.83% s−1), which is beneficial to the
enhancement of the double bond conversion, therefore, NABS3 system has higher double
bond conversion than BAPO system (80% vs 75%).
3.4.2. Cationic photopolymerization
We also investigated the cationic photopolymerization of epoxide initiated by NABSs/Iod
systems. It is found from Figure 6 that the epoxy conversion of NABS3/Iod system (16%) is
lower than those of NABS1/Iod (36%) and NABS2/Iod (35%) systems, indicating NABS3
with low ꢂ
more easily produce free radical instead of reacting with Iod. It is also
fluo
found that the NABS1/Iod and NABS2/Iod can initiate the cationic photopolymerization
of E-4221 under air upon UV LED exposure at 405 nm but with a low initiating efficiency
(conversion < 40%), which may be attributed to the low initiating efficiency of the radical
cations (NABS1•+ and NABS2•+).

JOURNAL OF SULFUR CHEMISTRY
15
Figure 7. Photoluminescence of PHDDA polymer prepared by NABSs/HDDA (0.5 /100 wt%); Reference:
polymer prepared by NAS6/HDDA (0.5 /100 wt%) and BAPO/HDDA (0.5 /100 wt%).
3.4.3. Photoluminescence of polymer prepared by NABSs/HDDA
Interestingly, we found that the polymers prepared by using NABSs as the photoinitiator
and HDDA as the monomer exhibit great photoluminescence under UV LED irradiation
at 405 nm as shown in Figure 7. Contrarily, the photoluminescence of polymers pre-
pared by using BAPO and NAS6 as the photoinitiator under the same conditions is not
observed. The UV-vis absorption test, the fluorescence emission and excitation tests of the
poly(1,6-hexanediol diacrylate) (PHDDA) polymer films prepared by NABSs/HDDA were
conducted to explore their photophysical properties. It is observed from Figure 8 that the
maximum absorption peaks of those polymers are located at about 320 nm and the shoul-
der absorption bands are located at about 426 nm, indicating that most of the NBASs in
the system are decomposed into free radicals, and then free radicals react with HDDA to
produce new structures. As shown in Figure 9, the emission spectra of polymer films pre-
pared by using NABS1 and NABS2 present maximum photoluminescence peaks at 461 and
467 nm, respectively. Interestingly, there are two peaks located at 454 and 463 nm, respec-
tively, in the emission spectrum of polymer film prepared by using NABS3. Moreover,
Figure 8. UV–vis absorption spectra of PHDDA polymer films prepared by NABSs and HDDA (concen-
tration of NABSs in HDDA is 0.5 wt%).

16
X. HU ET AL.
the excitation spectra of polymer films prepared by using NABSs all have broad excita-
tion bands in the 375–425 region. In conclusion, NABSs can be used as photoinitiators
to prepare polymers with a broad excitation band at 375–425 nm and a photolumines-
cence band at about 460 nm by photopolymerization. This may provide a convenient and
environmentally friendly approach to prepare the polymers with photoluminescence. The
polymers with strong photoluminescence may have great potential in outdoor advertising,
decoration, construction, packaging, traffic signs and other industries [42,43].
4. Conclusions
Three cleavable photoinitiators (NABSs) containing benzyl-sulfur were successfully
designed and synthesized. NABSs exhibit excellent absorption at 385, 395 and 405 nm.
NABS2 with methyl and NABS3 with methoxy group show longer absorption wavelength.
The NABSs can produce substituted benzyl and aryl thiyl radicals by the cleavage of
benzyl-sulfur bonds under UV LED irradiation at 405 nm, and these free radicals can effi-
ciently initiate the polymerization of acrylate monomers. By comparing the mechanism of
NABSs and NASs, and the initiation efficiency of NABSs and NAS6, it is demonstrated
that the mechanism of the NABSs photolysis is different from that of photoinitiators
containing only aryl-sulfur bond. The introduction of benzyl-sulfur bond makes NABSs
exhibit a great fluorescence (ꢂ
> 0.5), but when there is a strong electron-donating
fluo
group at para-position of the benzene ring of benzyl, NABSs show an enhanced ability
Figure 9. Fluorescence emission spectra and excitation spectra of PHDDA films prepared by (a)
NABS1/HDDA, (b) NABS2/HDDA and (c) NABS3/HDDA (concentration of NABSs in HDDA is 0.5 wt%).

JOURNAL OF SULFUR CHEMISTRY
17
to initiate photopolymerization of acrylates under UV LED irradiation at 405 nm and
a very weak fluorescence (ꢂfluo ∼0.06). As a result, as-prepared photoinitiator NABS3
with methoxy group is the most efficient for initiating free radical photopolymerization
of the acrylate monomer under UV LED exposure at 405 nm. Moreover, NABSs/Iod sys-
tems can initiate the cationic photopolymerization of E-4221 under UV LED irradiation
at 405 nm. Interestingly, polymer films prepared by using NABSs as the photoinitiator
exhibit a great photoluminescence band at about 460 nm under UV LED irradiation at
405 nm, whereas the polymers by using photoinitiators containing two aryl-sulfur bonds
do not possess photoluminescence property. NABSs may provide a convenient and envi-
ronmentally friendly approach to prepare the polymers with photoluminescence. The
photoluminescence mechanism and more properties of polymer films will be given in the
future work.
Acknowledgements
The financial support from the National Key Research and Development Program of China
(2017YFB0307800), the Fundamental Research Funds for the Central Universities (XK1802-6)
and the National Natural Science Foundation of China (grant number 51273014) is gratefully
acknowledged.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Funding
This work was supported by National Natural Science Foundation of China [grant num-
ber 51273014]; National Key Research and Development Program of China [grant number
2017YFB0307800]; Fundamental Research Funds for the Central Universities [grant number
XK1802-6].
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