Achieving Purely-Organic Room-Temperature Aqueous Phosphorescence via a Two-Component Macromolecular Self-Assembly Strategy

Article abstract of DOI:10.1002/asia.202000965

Manipulation of supramolecular behaviors and aggregation states represents an important topic in devising intriguing photofunctional systems. Here we report a two-component macromolecular self-assembly strategy for achieving aqueous room-temperature phosphorescence (RTP) in purely organic systems. Amphiphilic triblock copolymers are used to modulate the self-assembly of planar RTP molecules in aqueous solution, leading to the formation of sheet-like RTP objects with well-defined morphology, uniform crystalline nanostructures and excellent aqueous dispersity. In contrast, the addition of the planar RTP molecules into aqueous medium only leads to precipitation and quenching of RTP properties. Powder X-ray diffraction and single-crystal X-ray diffraction studies reveal that the amphiphilic triblock copolymers can assist supramolecular columnar packing of the planar RTP molecules where multiple non-covalent interactions stabilize the triplet excited states. Interestingly, it is found that luminescent signals of the sheet-like RTP objects can be extracted from strong fluorescent environments by phosphorescence mode and emission lifetime measurement.

Full text of DOI:10.1002/asia.202000965

CHEMISTRY
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Accepted Article
Title: Achieving Purely-Organic Room-Temperature Aqueous
Phosphorescence via Two-Component Macromolecular Self-
Assembly Strategy
Authors: Wang Guo, Xuepu Wang, Bei Zhou, and Kaka Zhang
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Achieving Purely-Organic Room-Temperature Aqueous
Phosphorescence via Two-Component Macromolecular Self-
Assembly Strategy
Wang Guo, Xuepu Wang, Bei Zhou and Kaka Zhang*
[a]
Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules,
Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, People’s Republic of China
E-mail: zhangkaka@sioc.ac.cn
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: Manipulation of supramolecular behaviors and
RTP molecules have a large influence on their phosphorescence
properties. Most of the reported RTP phenomena in purely
organic systems can only be observed in molecular crystalline
states where the nonradiative deactivation of the triplet excited
states can be effectively suppressed.10-35 However, the
molecular crystals usually lack solution dispersity,10-39 which
limits the applications of these RTP materials.
aggregation states represent an important topic in devising intriguing
photofunctional systems. Here we report
a
two-component
macromolecular self-assembly strategy for achieving aqueous room-
temperature phosphorescence (RTP) in purely organic systems.
Amphiphilic triblock copolymers are used to modulate the self-
assembly of planar RTP molecules in aqueous solution, leading to
the formation of sheet-like RTP objects with well-defined morphology,
uniform crystalline nanostructures and excellent aqueous dispersity.
In contrast, the addition of the planar RTP molecules into aqueous
medium only leads to precipitation and quenching of RTP properties.
Powder X-ray diffraction and single crystal X-ray diffraction studies
reveal that the amphiphilic triblock copolymers can assist
supramolecular columnar packing of the planar RTP molecules
where multiple non-covalent interactions stabilize the triplet excited
states. Interestingly, it is found that luminescent signals of the sheet-
like RTP objects can be extracted from strong fluorescent
environments by phosphorescence mode and emission lifetime
measurement.
Nanoprecipitation has been reported to endow these RTP
materials with aqueous dispersity.14 RTP dispersion in aqueous
medium can be obtained by injection of the RTP solution into
water, whereas the RTP dispersion can undergo further
aggregation to form macroscopic precipitates upon long-term
storage due to the hydrophobic nature of the RTP molecules, as
well as the absence of steric or electrostatic repulsion effects for
particle dispersion. With the aid of surfactant and sonication,20,22
well-dispersed spherical RTP nanoparticles have been obtained
by bottom-up pathways, while anisotropic nanostructures with
well-defined crystalline structures that can maintain RTP
properties have been rarely reported. A recent study shows that
the bottom-up pathway may cause the loss of RTP performance
in the resultant products.22 Therefore, it remains a challenging
task to fabricate RTP nanostructures with excellent aqueous
dispersity and well-defined crystalline structures where the
nonradiative deactivation of the triplet excited states can be
largely inhibited.
In the present study, based on molecular design and
extensive screening, an efficient RTP system of 9-acetyl-3,6-
diiodocarbazole in the crystalline solid state has been found to
exhibit highly efficient intersystem crossing and pure
phosphorescence (fluorescence free) in its steady-state
emission spectra. The addition of the planar RTP molecules into
aqueous medium has been found to only lead to the
precipitation and quenching of RTP properties. In striking
contrast, sheet-like crystalline nanostructures that show well-
defined morphology, excellent aqueous dispersity and maintain
RTP properties have been fabricated through the
macromolecular self-assembly of 9-acetyl-3,6-diiodocarbazole
and an amphiphilic triblock copolymer. It has been also found
that, in a strong fluorescent environment, the luminescent
signals of the sheet-like crystalline nanostructures can be
extracted by phosphorescence mode and emission lifetime
measurements, exhibiting characteristics for promising
phosphorescence and lifetime imaging applications.
Introduction
The manipulation of supramolecular behaviors and
aggregation states plays an important role for the fabrication of
luminescent materials with intriguing photophysical properties.1,2
Block copolymers have been reported by us and others to
modulate the supramolecular behaviors of small molecules in
macromolecular self-assembly systems where the polymers can
noncovalently interact with the small molecules.3-9 This two-
component design strategy allows a flexible choice of building
blocks with less synthetic effort.
Recent
studies
show
that
room-temperature
phosphorescence (RTP) in purely organic systems has emerged
as an important topic in the field of luminescent materials.10-15
Characteristic of their long-lived excited states, organic RTP
materials show superior performance in oxygen sensing,16,17
advanced
anti-counterfeiting14,18,19
and
high-contrast
bioimaging,20-26 when compared to conventional organic
fluorescent materials. Due to the intrinsically spin-forbidden
nature of the phosphorescent emission, the RTP properties in
purely organic systems can be easily quenched by nonradiative
deactivation processes. The aggregation states of the organic
1
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Results and Discussion
nm UV lamp (Figure S7). When compared to the bright yellow
emission of RTP-8 crystalline solids with relatively high
crystallinity (Figure 1A), these observations suggest that the
luminescent properties of RTP-8 samples are strongly related to
their aggregation states, that is, sample crystallinity.
9-Acetyl-3,6-diiodocarbazole (RTP-8) was prepared by a
three-step reaction from diphenylamine according to the
procedures as shown in Scheme 1A. RTP-8 was purified by
column chromatography followed by recrystallization in
dichloromethane/methanol for three times to give white solids.
Colorless needle-like single crystals can be grown from
dichloromethane/hexane. 1H NMR, 13C{1H} NMR, LRMS, HRMS,
FT-IR, melting point measurement, elemental analysis and
single crystal X-ray diffraction of RTP-8 all give satisfactory
results (Figure S1-S6), which reveal its structure and high purity.
Figure 2. (A) Room-temperature steady-state emission spectra of RTP-8
crystalline solids (λexi = 400 nm). (B) Room-temperature emission decay profile
at 539 nm of RTP-8 crystalline solids excited at 400 nm.
Macromolecular self-assembly of RTP-8 and amphiphilic
triblock copolymers has also been performed in aqueous
solution to fabricate RTP-8 nanostructures. Upon mixing RTP-8
(0.3 mg/mL) with Pluronic F-127 (2.7 mg/mL, PEO100-b-PPO68-b-
PEO100, the subscript represents the degree of polymerization of
each block) in a mixed solvent of tetrahydrofuran-water (1:9, v/v),
the clear aqueous solution first turns bluish and then becomes
milky after incubation at room temperature for 10 minutes. RTP-
8 nanostructures were collected by centrifugation after room-
temperature incubation for 1 day and then redispersed in
deionized water.
Scheme 1. (A) Synthesis of 9-acetyl-3,6-diiodocarbazole from diphenylamine.
(B) Structure of RTP-8 and control samples. (C) Structure of block copolymers.
At room temperature, RTP-8 solid samples emit bright
yellow light under a 365 nm UV lamp (Figure 1A). With the aid of
UV curable resin and silicone molds (Experimental Section),
RTP-8 powders can be readily processed into objects with
arbitrary shapes and tailorable sizes (Figure 1B). Figure 1C
displays SIOC-shaped RTP-8/resin composites which emit
bright yellow light upon 365 nm irradiation. The steady-state
emission spectra of RTP-8 solids exhibit a structured emission
band with three peaks at 497, 539 and 578 nm (Figure 2A).
Emission lifetimes (Figure 2B) fit to double-exponential decay (τ1
= 0.1 ms, τ2 = 1.6 ms) probably due to heterogeneous
microenvironments in the RTP-8 powders where domains with
different crystallinities coexist.10
Figure 3. (A) TEM, (B) SEM and (C) AFM images of the sheet-like RTP-8
nanostructures. (D) PXRD pattern of RTP-8 nanostructures and RTP-8
precipitates. (E) Proposed packing of RTP-8 planar molecules within the
crystalline nanostructure (top view). (F) RTP-8 nanostructures dispersed in
deionized water, pH 4 water, pH 7 Tris-HCl buffer, pH 10 water and 150 mM
NaCl aqueous solution.
Figure 1. Digital photographs of (A) RTP-8 solids under 365 nm UV lamp, (B)
SIOC-shaped RTP-8/resin composites under ambient light and (C) SIOC-
shaped RTP-8/resin composites under 365 nm UV lamp.
Organic RTP materials dispersed in aqueous solution,
especially in physiological conditions, is essential for their
potential application in diverse areas. To endow the RTP
materials with aqueous dispersity, nanoprecipitation method was
first tested. The addition of 0.2 mL of RTP-8 solution in THF (3
mg/mL) to 1.8 mL deionized water only leads to the macroscopic
precipitation of RTP-8 solids (Figure S7) with low crystallinity
(PXRD pattern shown in Figure 3). The RTP-8 precipitates with
low crystallinity show the absence of yellow emission under 365
TEM and SEM studies exhibit sheet-like RTP-8
nanostructures with well-defined morphologies (Figure 3A and
3B). The nanosheets possess a width of 2.2±0.2 μm and a
length of 7.3±0.5 μm. AFM measurement shows that the
nanosheets have a thickness of 30 nm (Figure 3C and Figure
S8). The similar morphologies observed by different methods
and the overlap between the nanosheets as shown in Figure 3B
demonstrate that the nanosheets were formed in the solutions.
Selected area electron diffraction studies that can be used to
2
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investigate the molecular orientation within the RTP
nanostructures gave no signals because of their instability in the
electron beam. The powder X-ray diffraction (PXRD) pattern of a
dried film of RTP-8 nanostructures exhibit a series of Bragg
peaks with d-spacing following the ratios of 1 : 1/2 : 1/3 : 1/4 :
1/5 (Figure 3D). The appearance of higher order diffraction
peaks is evidence of the high quality of crystalline growth of the
sheet-like nanostructures. The diffraction peak at 2θ = 26.0° (d =
0.342 nm) shows the π–π stacking of RTP-8 planar molecules in
the nanostructures. It is proposed that, during the
macromolecular self-assembly, RTP-8 planar molecules stack
into supramolecular columns by non-covalent π–π interactions
and simultaneously the supramolecular columns undergo side-
to-side packing in the lateral direction, leading to the formation of
sheet-like crystalline nanostructures (Figure 3E). These
observations indicate the important role of the Pluronic F-127
amphiphilic triblock copolymers in assisting the regular packing
of planar RTP-8 molecules, when compared to the case of RTP-
8 precipitates. It is noteworthy that crystalline nanostructures
with such a well-defined morphology and regular molecular
packing in the present study (Figure 3A-3C) have been rarely
reported in purely organic RTP systems.10-35 Other block
copolymers (Scheme 1C) have also been used to assemble with
RTP-8 in aqueous solutions. In the case of poly(ethylene oxide)-
b-poly(acrylic acid) (PEO113-b-PAA65, the subscript represents
the degree of polymerization of each block), macroscopic
precipitates of RTP-8 formed (Figure S9). PEO113-b-PAA65 block
copolymers are double hydrophilic and cannot stabilize the
neutral hydrophobic RTP-8 molecules. Poly(ethylene oxide)-b-
poly(4-vinylpyridine) (PEO113-b-P4VP123) block copolymers are
found to stabilize the hydrophobic RTP-8 molecules, whereas no
yellow emission was observed in the resultant RTP-8/PEO113-b-
P4VP123 aqueous suspension under 365 nm UV lamp (Figure
S10).
acidic, neutral and basic conditions, as well as in 150 mM NaCl
solution (Figure 3F), which can be attributed to the steric
repulsion effect of PEO polymer chains of Pluronic F-127 for
particle dispersion. The RTP-8 nanostructures have also been
found to maintain excellent aqueous dispersity after storage in
the ambient condition for weeks. Upon excitation at 405 nm, the
steady-state emission spectra of RTP-8 nanostructures
prepared from the assembly of RTP-8 and Pluronic F-127 exhibit
emission band at 538 nm (Figure 4A). Emission lifetimes (Figure
4B) fit to double-exponential decay (τ1 = 0.05 ms, τ2 = 0.4 ms).
When compared to RTP-8 precipitates (Figure S7), the emission
band of RTP-8 nanostructures in the yellow region indicates that
the photophysical properties of the RTP systems are strongly
related to the aggregation states of the RTP molecules;
emission intensity increases with sample crystallinity.
The luminescent properties of the RTP-8 nanostructures
have also been tested in strong fluorescent environments. Upon
the mixing of RTP-8 nanostructures and fluorescent spheres
(carboxylate-modified polystyrene latex beads, 2.0 μm mean
particle size, λemi ~ 505 nm, fluorescent yellow-green), the
steady-state emission spectra of the mixture measured in the
fluorescence mode are found to be dominated by the fluorescent
spheres (Figure 4C). Interestingly, in the phosphorescence
mode (Figure 4C), the emission spectra of the mixture recorded
by a Hitachi FL-7000 spectrometer at chopping speed of 40 Hz
are found to be very similar to those of the RTP-8
nanostructures (Figure 4A). Moreover, room-temperature
emission decay profile of the mixture of RTP-8 nanostructures
and fluorescent spheres exhibit double-exponential decay
behaviors in the time region of several milliseconds, which is
similar to that of the RTP-8 nanostructures (Figure 4B and 4D).
These observations indicate that, because of their long-lived
excited states, the aqueous dispersable RTP-8 nanostructures
show the capability of removing the background fluorescent
interference by phosphorescence mode and emission lifetime
measurements.
To get further insight into the phosphorescence properties
of the RTP-8 nanostructures, more photophysical studies of the
RTP-8 crystalline solids have been performed. Figure 2A shows
the structured emission band with three peaks at 497, 539 and
578 nm in the steady-state emission spectra of RTP-8 solids. It
is found that the shape of the emission band and the peak
wavelength show insignificant change upon the variation of the
excitation wavelength (Figure S11), which suggests that the
emission band in the range of 460 to 700 nm are originated from
the same excited state. The excitation spectrum of RTP-8 solids
shows a peak at 340 nm (Figure S12), similar to the UV-vis
absorption spectrum of RTP-8 in thin film state (Figure S13).
The large Stokes shift from 340 nm to 539 nm (10859 cm-1) and
the long-lived excited states (Figure 2B) confirm that RTP-8
represents an organic room-temperature phosphorescent
material. Phosphorescence spectra of RTP-8 solids further
confirm that the emission band in the range of 460 to 700 nm
are originated from the triplet excited state of RTP-8 solids
(Figure S14). The emission maximum of RTP-8 nanostructures
is consistent with that of RTP-8 crystalline solids. The emission
band of RTP-8 nanostructures (Figure 4A) is less structured
than that of RTP-8 crystals (Figure 2A), which again suggest
that the photophysical properties of RTP-8 are dependent on
their aggregation states; the packing geometry of RTP-8
crystalline solids has been found to be different from that of
Figure 4. (A) Room-temperature steady-state emission spectra of RTP-8
nanostructures (λexi = 405 nm). (B) Room-temperature emission decay profile
of RTP-8 nanostructures. (C) Room-temperature steady-state emission
spectra (black spectrum, fluorescence mode; red spectrum, phosphorescence
mode, chopping speed 40 Hz) of the mixture of RTP-8 nanostructures and
fluorescent spheres (λexi = 405 nm). (D) Room-temperature emission decay
profile of the mixture of RTP-8 nanostructures and fluorescent spheres.
RTP-8 nanostructures formed by the assistance of
Pluronic F-127 are found to show excellent aqueous dispersity in
3
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RTP-8 nanostructures (vide infra). The photoluminescent
quantum efficiency of RTP-8 powders has been measured to be
4%. In control experiments, both 9-acetylcarbazole and 3,6-
diiodocarbazole (Scheme 1B) don’t show bright yellow emission
under 365 nm UV lamp (Figure S15). These suggest the
collaboration of heavy atom effect and n–π* transition in
improving the phosphorescence efficiency in RTP-8 system. It is
noteworthy that the reported carbazole RTP systems have been
found to show fluorescence/phosphorescence dual emission
behaviors.27-32 A distinct feature of the present RTP system is
the absence of fluorescence bands in its steady-state emission
spectra, which indicates the highly efficient singlet-to-triplet
intersystem crossing.
All the above photophysical studies and structural analysis
indicate the importance of the two-component design strategy
for achieving purely-organic room-temperature aqueous
phosphorescence. The amphiphilic triblock copolymers not only
provide excellent aqueous dispersity but also assist the regular
packing of planar RTP molecules into crystalline RTP
nanostructures. In contrast, the addition of the RTP-8 molecules
into aqueous medium only leads to the precipitation and
quenching of RTP properties. The efficient RTP properties of the
9-acetyl-3,6-diiodocarbazole systems are associated with the
introduction of heavy atom effect and n–π* transition, as well as
the formation of supramolecular columnar packing of carbazole
molecules where multiple non-covalent interactions in the
crystalline nanostructures stabilize the triplet excited states.
Taking advantage of their relatively long-lived excited state when
compared to fluorescent systems, the aqueous dispersable
RTP-8 nanostructures exhibit the capability of removing the
background fluorescent interference and show characteristics for
promising phosphorescence imaging and emission lifetime
imaging applications. Further study will also be placed on the
fabrication of purely organic aqueous RTP systems with higher
photoluminescence quantum yields and longer luminescent
lifetimes.
Experimental Section
Figure 5. Single-crystal structures of RTP-8 showing (A) the head-to-head
columnar stacking and (B) the multiple non-covalent interactions between
neighboring RTP-8 supramolecular columns.
Physical measurements and instrumentation
1H NMR (400 MHz) and 13C{1H} NMR (100 MHz) spectra were
recorded on a JEOL Fourier-transform NMR spectrometer (400 MHz)
with chemical shifts reported relative to tetramethylsilane. Mass spectra
were performed on Agilent Technologies 5973N and Thermo Fisher
Scientific LTQ FT Ultras mass spectrometer. FT-IR spectra were
recorded on a Nicolet AVATAR-360 FT-IR spectrophotomerter with a
Single crystal X-ray diffraction studies reveal that planar
RTP-8 molecules exhibit a head-to-head columnar stacking
through π–π interactions (Figure 5A). The RTP-8 columns
associate with neighbouring columns via multiple non-covalent
interactions such as C–I···I–C halogen bonding, C–H···O
interactions and others (Figure 5B). In the crystalline structures
of RTP-8 nanostructures, the RTP-8 molecules retain
supramolecular columnar stacking via π–π interactions (Figure
3D and 3E). Interestingly, the packing geometry of the
supramolecular columns of RTP-8 nanostructures (Figure 3D
and 3E) are different from that of RTP-8 single crystals (Figure
5B), which suggest that Pluronic F-127, as well as the solvent
mixture used for macromolecular self-assembly, can influence
and modulate the supramolecular and crystalline behaviors of
the planar RTP-8 molecules. In the crystalline state where the
microenvironment of RTP-8 is rigidified due to the presence of
multiple non-covalent interactions, the nonradiative deactivation
of triplet excited state can be suppressed in a large extent.
Besides, the crystalline structure with close packed RTP-8
columns can protect the triplet excited state from oxygen
quenching in the ambient condition. Further experiments show
that the luminescent properties of the RTP-8 crystalline solids at
ambient and in vacuum possess insignificant difference (Figure
S16). It is also found that the steady-state emission spectra
exhibit insignificant change when the RTP-8 crystalline solids
are exposed to different aqueous environments (Figure S17).
resolution of
4
cm-1. Differential scanning calorimetry (DSC) was
performed on a TA Q200 DSC instrument in nitrogen with a heating rate
of 20 °C/min. Elemental analysis was carried out on a Carlo-Erba1106
system. UV-Vis absorption spectra were recorded on a Hitachi U-3310
UV-vis spectrophotometer. Emission spectra were recorded using
Edinburgh FLS1000 fluorescence spectrometer, Hitachi FL-7000 and FL-
2700 fluorescence spectrometer. The phosphorescence spectra were
collected by a Hitachi FL-7000 fluorescence spectrometer. During the
measurements, chopping speed was fixed at 40 Hz and the PMT voltage
is fixed at 700 V. Photoluminescence quantum yield was measured by a
Hamamatsu absolute PL quantum yield measurement system.
Transmission electron microscopy (TEM) experiments were performed
on
a JEOL JEM-1230 transmission electron microscope with an
accelerating voltage of 80 kV. Scanning electron microscopy (SEM)
experiments were carried out on a JSM-6390LV scanning electron
microscope. Step-scanned powder X-ray diffraction (PXRD) data were
collected on a PANalytical X’Pert powder system using monochromated
Cu/Kα (λ = 0.154 nm).
Synthesis of 9-acetyl-3,6-diiodocarbazole
9-Acetyl-3,6-diiodocarbazole
(RTP-8)
was
synthesized
from
diphenylamine by a three-step reaction. (Step 1) Diphenylamine (0.50 g,
2.95 mmol), palladium(II) acetate (0.26 g, 1.18 mmol) and 2.0 mL acetic
acid were added to a round-bottle flask to react at 80 °C for 7 h.40
Carbazole (0.30g, 60% yield) was purified by column chromatography
(15:1, v/v, petroleum ether/ethyl acetate). (Step 2) Carbazole (0.25 g,
1.49 mmol), potassium iodide (0.32g, 1.93 mmol), potassium iodate (0.48
g, 2.24 mmol) and 2.0 mL acetic acid were added to a round-bottle flask
to react at 80 °C for 2 h.41 3,6-Diiodo-9H-carbazole (0.50 g, 80% yield)
Conclusion
4
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was purified by column chromatography (10:1, v/v, petroleum ether/ethyl
acetate). (Step 3) 3,6-Diiodo-9H-carbazole (0.4 g, 0.95 mmol), acetic
anhydride (2 mL) and boron trifluoride diethyl etherate (20 µL) were
added to a round-bottle flask and refluxed at 80 °C for 1h.41 9-Acetyl-3,6-
diiodocarbazole (0.36 g, 85% yield) was purified by column
The macroinitiator PEO113-Cl (the subscript represents the degrees of
polymerization) was first synthesized according to the literature
methods,44 and then the poly(ethylene oxide)-b-poly(4-vinylpyridine)
(PEO-b-P4VP) diblock copolymers were synthesized via ATRP.45 For the
polymerization of 4-vinylpyridine (4VP), the mixture of PEO113-Cl, CuCl
and isopropanol, and the mixture of tris[2-(dimethylamino)ethyl]amine
(Me6TREN) and 4VP, were degassed respectively via nitrogen bubbling
and then mixed together. The molar ratio of PEO113-Cl/CuCl/Me6TREN in
the reaction mixture was 1/0.5/0.5, and the degrees of polymerization of
P4VP were controlled by 4VP/PEO113-Cl feed ratio. The polymerization
was conducted at 50 °C in an oil bath and finally quenched in an ice bath.
The copper complex in the reaction mixture was removed by passing the
reaction mixture through neutral Al2O3 column using dichloromethane as
an eluent. PEO113-b-P4VP123 block copolymers (Mw/Mn = 1.25) were
purified by three cycles of dissolution in dichloromethane/precipitation in
diethyl ether. The degrees of polymerization of the P4VP blocks and
Mw/Mn of the PEO113-b-P4VP123 block copolymers were determined by 1H
NMR and GPC, respectively.
chromatography
(10:1,
v/v,
petroleum
ether/ethyl
acetate).
Recrystallization in dichloromethane/methanol for three times was
applied for the further purification of the 9-acetyl-3,6-diiodocarbazole
samples to give white solids. Melting point of 9-acetyl-3,6-
diiodocarbazole was measured by DSC to be 228 °C. 1H NMR (400 MHz,
CDCl3, 298 K, δ/ppm): δ = 2.83 (s, 3H), 7.75 (dd, J = 8.8, 1.8 Hz, 2H),
7.94 (d, J = 8.8 Hz, 2H), 8.23 (d, J = 1.8 Hz, 2H). 13C{1H} NMR (100 MHz,
CDCl3, 298 K, δ/ppm): δ = 27.7, 87.6, 118.0, 127.2, 129.0, 136.5, 138.0,
169.7. FT-IR (KBr, cm-1): ν 3060, 2998, 2924, 1737, 1706, 1670, 1607,
1574, 1468, 1424, 1393, 1379, 1345, 1304, 1278, 1209, 1067, 1058,
1044, 1025, 1015, 980, 868, 820, 811, 798. LRMS, m/z 461. HRMS m/z
found (calcd for C14H10ONI2): 461.8852 (461.8846). Elemental analysis
calcd (%) for C14H10ONI2: C 36.47, H 1.97, N 3.04; found: C 36.03, H
1.75, N 2.93.
Preparation of RTP-8 nanostructures
Preparation of SIOC-shaped RTP-8/resin composites
0.2 mL of RTP-8 solution in tetrahydrofuran (3.0 mg/mL) was mixed with
1.8 mL of Pluronic F-127 solution in deionized water (3.0 mg/mL). The
mixture was homogenized by gentle shaking. The clear aqueous solution
first turns bluish and then becomes milky after incubation at room
temperature for 10 minutes. RTP-8 nanostructures were collected by
centrifugation after room-temperature incubation for 1 day and then
redispersed in deionized water.
RTP-8 powders were added to the liquid precursor of UV curable
polyacrylate resin by stirring. The obtained suspension of RTP-8 powders
in the liquid precursor was transferred to silicon molds. After irradiation
under 365 nm UV light for 20 min, the SIOC-shaped RTP-8/resin
composites were obtained.
Preparation of Pluronic F-127 aqueous solution
Formation of strong fluorescent environments for RTP-8
nanostructures
Pluronic F-127 (poly(ethylene oxide)-b-poly(propylene oxide)-b-
poly(ethylene oxide), PEO100-b-PPO68-b-PEO100, the subscript represents
the degree of polymerization of each block, Sigma-Aldrich) was dissolved
in deionized water at 3.0 mg/mL by magnetic stirring before use.
Carboxylate-modified polystyrene latex beads (Sigma-Aldrich L4530,
2.5% aqueous suspension, 2.0 μm mean particle size, λemi ~505 nm,
fluorescent yellow-green) were mixed with RTP-8 nanostructures at a
sphere/RTP-8 mass ratio of approximately 1:1. The mixture shows bright
yellow-green emission color under 365 nm UV lamp. Room-temperature
steady-state emission measurements in both fluorescence and
phosphorescence modes, as well as the emission decay properties of the
mixture of fluorescent spheres and RTP-8 nanostructures were
performed to investigate the photophysical properties of RTP-8
nanostructures in the strong fluorescent environments formed by the
fluorescent spheres.
Synthesis of PEO-b-PAA block copolymers
The macroinitiator PEO113-Br (the subscript represents the degrees of
polymerization) was first synthesized according to the literature methods,
42
and then the poly(ethylene oxide)-b-poly(tert-butyl acrylate) (PEO-b-
PtBA) diblock copolymers were synthesized via atom transfer radical
polymerization (ATRP).43 For the polymerization of tert-butyl acrylate
(tBA), the mixture of PEO113-Br, CuBr and toluene, and the mixture of
N,N,N',N'',N''-pentamethyldiethylenetriamine (PMDETA) and tBA, were
degassed respectively via nitrogen bubbling and then mixed together.
The molar ratio of PEO113-Br/CuBr/PMDETA in the reaction mixture was
1/0.5/0.5, and the degrees of polymerization of PtBA were controlled by
tBA/PEO113-Br feed ratio. The polymerization was conducted at 100 °C in
an oil bath and finally quenched in an ice bath. The copper complex in
the reaction mixture was removed by passing the reaction mixture
through neutral Al2O3 column using dichloromethane as an eluent.
PEO113-b-PtBA65 block copolymers (Mw/Mn = 1.20) were obtained. The
degrees of polymerization of the PtBA blocks and Mw/Mn of the PEO113-b-
PtBA65 block copolymers were determined by 1H NMR and gel
permeation chromatography (GPC), respectively. To prepare
poly(ethylene oxide)-b-poly(acrylic acid) (PEO-b-PAA), PEG113-b-PtBA65
block copolymers were first dissolved in dichloromethane (DCM) at
approximately 50 mg/mL, and then trifluoroacetic acid (TFA) was added
at a TFA/DCM volume ratio of 1/10 to the solution to selectively
hydrolyze the tert-butyl ester groups. After hydrolysis for 2 days, the
reaction mixture was evaporated under reduced pressure to dryness.
The PEO113-b-PAA65 block copolymers obtained were purified by four
cycles of dissolution in methanol/precipitation in hexane. The
deprotection of the tert-butyl ester groups was confirmed by FTIR and 1H
NMR spectroscopy.
Acknowledgements ((optional))
We thank the financial supports from Shanghai Scientific and
Technological
Innovation
Project
(20QA1411600,
20ZR1469200), and Hundred Talents Program from Shanghai
Institute of Organic Chemistry (Y121078). The National Facility
for Protein Science (NFPS) of the Shanghai Synchrotron
Radiation Facility (SSRF) is acknowledged for providing
beamline time (BL17B beamline). We thank Prof. Vivian W.-W.
Yam and Mr. Liangliang Yan for their help in single crystal X-ray
diffraction study.
Keywords: aggregation • block copolymers • nanostructures •
phosphorescence • self-assembly
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Entry for the Table of Contents
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A room-temperature phosphorescence (RTP) system of 9-acetyl-3,6-diiodocarbazole in the crystalline state has been found to show
efficient intersystem crossing and pure phosphorescence (fluorescence free) in its steady-state emission spectra. Sheet-like RTP
crystalline nanostructures with excellent aqueous dispersity have been obtained by macromolecular self-assembly of 9-acetyl-3,6-
diiodocarbazole and amphiphilic triblock copolymers.
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