Macromolecules
Article
with interesting properties such as large Stokes shifts, high
quantum yields, and tunable structures.
2. EXPERIMENTAL SECTION
2
.1. General Methods and Materials. All commercial reagents
Dipolar chromophores containing electron donor (D) and
electron-acceptor (A) parts linked through a conjugated π-unit
have been broadly investigated and applied in optoelectronic
and electronic devices, in which the D−π−A system known as
push−pull molecules can tune the intramolecular charge
transfer, π-electron density, and HOMO−LUMO band gap
and solvents were purchased from J&K Scientific Ltd. (Beijing,
China). Tetrahydrofuran (THF) was distilled from sodium under dry
nitrogen immediately prior to use. Reactions were followed with TLC
(0.254 mm silica gel 60-F plates). Flash chromatography was carried
out on silica gel of 200−300 mesh. All NMR spectra were obtained at
an ambient temperature using a Bruker 400 MHz spectrometer.
Chemical shifts (δ) were reported as parts per million (ppm) on the δ
scale downfield from TMS. Multiplicities are reported as follows: s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet. UV−vis
spectra were recorded using a spectrophotometer UV-2550 model
35−48
leading to unique photophysical and chemical properties.
Attributed to the advantage of the D−π−A system, it would
provide a potential strategy to design and synthesize novel
organoboron fluorescent molecules with interesting properties,
such as large Stokes shifts, high quantum yields, and tunable
structures. The syntheses and structural modifications of many
fluorophores are relatively complex and sometimes involve
tedious multiple steps and also low yields. A combinatorial
strategy is useful for the exploration and optimization of
fluorophores to overcome these shortcomings. A fluorescent
scaffold with effective tunable domain can be strategized
through some efficient and accessible reaction, which can
further adjust the molecular structures, electron density, and
optical properties. On the other hand, compared to fluorescent
small molecules, fluorescent polymers show desirable advan-
tages such as tuned structure, topology, morphology, and
functionality. Tang et al. achieved in situ monitoring of RAFT
polymerization using tetraphenylethylene-containing agents
(
Shimadzu, Japan). Fluorescence spectra were acquired on
Fluorescence Spectroscopy-F4600. All of the quantum yields of
samples were determined in solution using Rhodamine 6G as a
reference λexc = 488 nm, Φ = 0.88 in ethanol. Dynamic light scattering
(
DLS) measurements were made with a Zetasizer Nano-ZS (Malvern
Instruments, U.K.). The samples were filtered using a microfilter
(
(
0.45 μm) prior to measurement. Transmission electron microscopy
TEM) was performed with an FEI Talos F200X microscope (FEI).
Gel permeation chromatography (GPC) measurements were
conducted using a system equipped with a Waters 515 pump, a
Waters UV−vis detector, and a column temperature controller with a
flow rate of 1.0 mL min in THF (HPLC grade) at 25 °C. All
calculations were carried out using Gaussian 03 program.
−
1
2.2. Synthesis of N,N-Dimethyl-Substituted Boron Ketoimi-
nates (NBKI 3a−3i). 2.2.1. Synthesis of 1-(4-Dimethylamino-
phenyl)-propynone (Compound 1). (1) A solution of ethynylmag-
nesium bromide (0.5 M in THF, 8 mL, 4 mmol, 2.0 equiv) was added
at 0 °C to a solution of 4-dimethylamino-benzaldehyde (298 mg, 2
mmol) in dry THF. After the mixture was stirred at room temperature
49
with AIE characteristics. Thus, the design and synthesis of
fluorescent polymers has drawn great attention as fluorescent
labels or sensors in many fields. Recently, coupling reaction,
click chemistry, and free-radical polymerization have been used
for 6 h under an Ar atmosphere, a saturated solution of NH Cl was
4
added and the THF was evaporated under vacuum. The aqueous
phase was extracted with EtOAc, and the organic phase was washed
with water and brine and then dried with Na SO . The solvent was
50−53
to synthesize fluorescent polymers.
However, ATRP/
RAFT initiators with fluorescent characteristics and their usage
in the preparation of multicolor fluorescent polymers were less
investigated.
2
4
removed in vacuo and the purification of the crude product by silica
gel column chromatography eluting with EtOAc/petroleum (1:5) was
performed to afford the expected product 1-(4-dimethylamino-
phenyl)-prop-2-yn-1-ol (yield: 86%). (2) IBX (1.4 g, 5 mmol, 2.5
equiv) was added to a solution of 1-(4-dimethylamino-phenyl)-prop-
Herein, a series of new multicolor fluorescent molecules
N,N-dimethyl-substituted boron ketoiminates (NBKI) involv-
ing a D−π−A conjugated system in the presence of electron-
2-yn-1-ol (350 mg, 2 mmol) in EtOAc (10 mL). The mixture was
donating amino substituent and unsymmetric OBF N part as
stirred overnight at 80 °C. Then, the mixture was allowed to reach
room temperature and filtered. After evaporation of the solvent, the
resulting crude product was purified by EtOAc/petroleum (1:10) to
give compound 1 (yield: 90%).
2
electron-acceptor unit were presented. The relationship
between their substituents and optical properties was system-
atically studied. First, owing to the introduction of N,N-
dimethyl group on the aryl group with strong electron-
donating ability, NBKI molecules formed a D−π−A system
and resulted in a high luminescence efficiency in solution.
Second, the incorporation of various tunable substituents on
the nitrogen atom of NBKI can be easily executed through the
click reaction between alkynes and primary amine compounds.
The nature of substituent at N-position affected the electron
density and optical properties of NBKI. The as-synthesized
NBKI showed versatile applications in atom transfer radical
polymerization (ATRP) and reversible addition−fragmenta-
tion chain transfer polymerization (RAFT). Meanwhile, a
series of fluorescent polymers with tunable chemical
composition and optical properties were prepared using
multicolor fluorescent initiators. The amphiphilic fluorescent
block copolymer i-NBKI-5-P(DBA-PEG) was demonstrated to
be self-assembled into micelle nanoparticles with about 144
nm diameter. Furthermore, due to their high water
dispersibility, good fluorescence, and excellent biocompati-
bility, the as-prepared fluorescent PEG-based homopolymers
2
.2.2. Synthesis of N,N-Dimethyl-Substituted Ketoenamines
Ligands 2a−2i). Example synthetic procedure: An amine (1.2
mmol) was added to a solution of 1-(4-dimethylamino-phenyl)-prop-
-yn-1-ol (compound 1) (173 mg, 1.0 mmol) in EtOH (5 mL), and
(
2
then the solution was stirred at room temperature until completion of
the reaction (TLC). The solution was diluted with EtOAc, washed
with H O and brine, and dried over anhydrous MgSO . After
2
4
evaporation of the solvent, the resulting crude product was
recrystallized with EtOH or purified by silica gel column
chromatography.
2
.2.3. Synthesis of N,N-Dimethyl-Substituted Boron Ketoimi-
nates (NBKI 3a−3i). BF ·Et O (2.5 mL, 20 mmol) was added to a
3
2
solution of ligand 2 (1 mmol) in the mixed solvent of CH Cl (2.5
2
2
mL) and NEt (2.5 mL). The reaction mixture was stirred under a
3
nitrogen atmosphere at 50 °C, and the course of the reaction was
monitored by TLC. Then, the mixture was allowed to reach room
temperature and extracted with CH
Cl , washed with a saturated
2
2
solution of NaHCO and brine, and dried over anhydrous Na SO .
3
2
4
The solvent was evaporated in vacuo, and the purification of the
reaction mixture by silica gel column chromatography (using EtOAc/
petroleum ether 1:1 and EtOAc as eluent) was performed to afford
the expected product.
(
i-NBKI-PEG) and block copolymer (i-NBKI-5-P(DBA-
2
.3. Synthesis of the Fluorescent Initiators (i-NBKI 1−4) for
PEG)) are attractive for application in cell imaging, biological,
Atom Transfer Radical Polymerization (ATRP). (1) 2-Bromoiso-
or environmental fields.
butyryl bromide (2.3 g, 10 mmol) was added to a solution of 4-[(2-
B
Macromolecules XXXX, XXX, XXX−XXX
Yang, Dong
