Organic Letters
even red, and the design of compounds with satisfying activity
in the visible range remains an open challenge.
Boron dipyrromethene (BODIPY) chromophores have
promising optical properties, such as high fluorescence
quantum yields, large molar extinction coefficients, and narrow
1
8−20
emission bands, as well as flexible chemical structures.
the basis of previous knowledge,
On
21−23
we designed PAGs using
a BODIPY chromophore as the donor and triarylsulfonium as
the acceptor. Here we report the synthesis, characterization,
and photophysical and photochemical properties of BODIPY-
based PAG-1 (D−A) and PAG-2 (D−π−A). We studied the
acid-generating ability of the newly synthesized PAGs upon
irradiation with green and red LEDs, respectively.
In general, photoinduced polymerization is one of the most
exciting technologies for advanced manufacturing of polymers,
where one of the key components is a photosensitive moiety
that can generate active species upon light absorption. In the
past decade, to go beyond photoradical generators, intensive
Figure 2. (a) Absorption and (b) emission spectra of PAG-1 and
compound 2 and (c) absorption and (d) emission spectra of PAG-2
and compound 4.
10
research has been devoted to new structures of PAGs. PAGs
are capable of triggering a polymerization process by releasing
nm, respectively, and the emission was centered at 521 and
5
79 nm, respectively. Compared with PAG-1, PAG-2 has an
+
H upon light irradiation, are insensitive toward oxygen, and
extended conjugation system and displays red-shifted photo-
24,25
can lead to a considerable variety of polymer backbones.
In
physical properties as well as a larger absorption coefficient (ε).
this study, we used PAG-2 to demonstrate the capability to
initiate ring-opening cationic polymerization using red LED
light.
PAG-1 and PAG-2 were synthesized following the
sequences of chemical reactions shown in Scheme 1. First,
The fluorescence quantum yields (Φ ) were calculated using
f
fluorescein and crystal violet perchlorate as standards. The Φ
f
values for PAG-1 and PAG-2 in methanol were 0.11 and 0.02,
respectively (Table 1), whereas those of the precursor
compounds 2 and 4 were 0.30 and 0.45, respectively. This
clearly indicates that the emission decreases dramatically after
the formation of the triphenylsulfonium salt as a result of the
electron transfer from the BODIPY chromophore to the
triphenylsulfonium moiety in both the D−A and D−π−A
structures. Furthermore, the decrease in fluorescence intensity
correlates well with the decrease in fluorescence lifetime (τ)
Scheme 1. Synthetic Routes to PAG-1 and PAG-2
Initially the dark stability was checked, and the results
showed that after 24 h in the dark both PAG-1 and PAG-2 had
dark stabilities (Figure S3). Then we studied the photo-
chemistry of these BODIPY-based PAGs. On the basis of their
maximum absorption wavelengths and the availability of LED
light sources, we used a green LED (505 ± 30 nm) to irradiate
PAG-1, while a red LED (595 ± 30 nm) was employed to
−
5
excite PAG-2. PAG-1 (2 mL of a 1 × 10 M solution in 1:1
v/v MeOH/water) in a quartz cuvette (10 mm path) was
placed in front of the green LED light source (Thorlabs
M505L3). Upon irradiation, samples were analyzed at regular
time intervals by UV−vis spectrophotometry and LC−MS
analysis. The absorption peaks of PAG-1 decreased as the
irradiation time increased, indicating the photolysis under LED
light exposure. Similarly, under red LED light (Thorlabs
irradiation-time-dependent manner (Figure S4). Though the
In addition, the photolysis sample solution of PAG-1 was
subjected to study by reversed-phase HPLC. Originally, pure
PAG-1 exhibited a retention time of 0.84 min, but after 10 min
under green LED irradiation we observed four other
photobyproducts, all having longer retention times than
with increasing irradiation time (Figure S7). Furthermore, one
of the photobyproducts matched with the standard sample of
thiophenol was reacted with 4-bromobenzaldehyde to afford
compound 1. Then BODIPY diphenyl sulfide (2) was obtained
by reacting compound 1 with 2,4-dimethylpyrrole. Next, PAG-
1
was synthesized by microwave irradiation of compound 2
with diphenyliodonium hexafluorophosphate in a closed vessel.
Next, we synthesized PAG-2 with the D−π−A structure in a
similar way. The BODIPY core (3) was synthesized using a
literature method by treating 3,4,5-trimethoxybenzaldehyde
26
with 2,4-dimethylpyrrole. Then trimethoxy BODIPY diphen-
yl sulfide (4) was obtained by a condensation reaction between
compounds 1 and 3. Finally, PAG-2 was obtained by
microwave irradiation of compound 4 with diphenyliodonium
hexafluorophosphate.
Next, the UV absorption and emission spectra of PAG-1,
PAG-2, and their precursor compounds 2 and 4 in methanol
were recorded (Figure 2). The absorption spectra of PAG-1
and PAG-2 showed an intense peak centered at 509 and 566
B
Org. Lett. XXXX, XXX, XXX−XXX