A dendritic DPA annihilator—syntheses, photophysical properties and application for co-assembling enhanced triplet-triplet annihilation upconversion

Article abstract of DOI:10.1016/j.dyepig.2020.108643

A dendritic annihilator D-2 containing 18 peripherical methyl ester units was synthesized by attaching the branches on the phenyl unit of 9,10-diphenylanthracene (DPA). The absorption and emission spectra of D-2 are very similar to the unbranched DPA diacid 5 and the lower branched homolog D-1, while the fluorescence quantum yield increased with the degrees of the branching and was determined to be 86.9percent for D-2 in MeOH. D-2 was used as the annihilator for triplet-triplet annihilation upconversion with a Schiff-base Pt(II) complex serving as the sensitizer. It was found that D-2 showed very weak UC emission in MeOH with a UC quantum yield of only 1.4percent. Interestingly, the UC emission intensity increased significantly by adding different amounts of water in MeOH, the UC intensity was enhanced by more than 9 fold by adding 40percent water into MeOH, and the UC quantum yield was determined to be 10.2percent in MeOH with 40percent water. A mechanism for the enhancing of UC emission was proposed: When in MeOH, the large molecule size of the dendrimer limited the movability of the acceptor, and the shielding effect of the dendritic branches on the DPA core limited the intimate contacts among DPA cores or between DPA core and the sensitizer. When the water was added, D-2 aggregated to nanoparticles with internal hydrophobic cavities induced by the hydrophobic effect and π-π stacking of DPA core, the sensitizer Pt-1 was captured in the internal hydrophobic cavities, and thus, both the TTET and TTA processes were greatly accelerated which attributed to the enhanced TTA-UC.

Full text of DOI:10.1016/j.dyepig.2020.108643

Journal Pre-proof
A Dendritic DPA Annihilator—Syntheses, Photophysical Properties and Application
for Co-assembling Enhanced Triplet-triplet Annihilation Upconversion
Qian Guo, Guojuan Li, Ming Rao, Lingling Wei, Fanrui Gao, Wanhua Wu, Qingshan
Yang, Guo Cheng, Cheng Yang
PII:
S0143-7208(20)31340-1
DOI:
https://doi.org/10.1016/j.dyepig.2020.108643
DYPI 108643
Reference:
To appear in: Dyes and Pigments
Received Date: 18 May 2020
Revised Date: 5 June 2020
Accepted Date: 10 June 2020
Please cite this article as: Guo Q, Li G, Rao M, Wei L, Gao F, Wu W, Yang Q, Cheng G, Yang C, A
Dendritic DPA Annihilator—Syntheses, Photophysical Properties and Application for Co-assembling
Enhanced Triplet-triplet Annihilation Upconversion, Dyes and Pigments, https://doi.org/10.1016/
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CRediT author statement
Qian Guo: Investigation, Methodology; Guojuan Li: Investigation, Methodology;
Ming Rao: Formal analysis; Lingling Wei: Formal analysis; Fanrui Gao: Formal
analysis; Wanhua Wu: Conceptualization, Writing –Original Draft, Supervision;
Qingshan Yang, Conceptualization, Writing - Review & Editing; Guo Cheng: Writing
- Review & Editing; Cheng Yang: Conceptualization, Writing - Review & Editing,
Supervision;

TOC:

A Dendritic DPA Annihilator—Syntheses, Photophysical Properties and
Application
for Co-assembling Enhanced Triplet-triplet Annihilation
Upconversion
Qian Guo,a Guojuan Li,a Ming Rao,a Lingling Wei,a Fanrui Gao,a Wanhua Wu,*a Qingshan Yang,*b Guo Cheng,*a and
Cheng Yang*a
a Key Laboratory of Green Chemistry & Technology, College of Chemistry and Healthy Food Evaluation Research
Center, Sichuan University, Chengdu 610064, China
b Chengdu University of Information Technology, Chengdu 610103, China
___________________________________________________________________________
and TTA processes were greatly accelerated
which attributed to the enhanced TTA-UC.
Abstract
A dendritic annihilator D-2 containing 18
peripherical methyl ester units was synthesized by
attaching the branches on the phenyl unit of
9,10-diphenylanthracene (DPA). The absorption
and emission spectra of D-2 are very similar to
Keywords:
Dendrimer; Annihilator; triplet-triplet annihilation
upconversion; DPA
1. Introduction
the unbranched DPA diacid
5 and the lower
branched homolog D-1, while the fluorescence
quantum yield increased with the degrees of the
branching and was determined to be 86.9% for
D-2 in MeOH. D-2 was used as the annihilator for
triplet-triplet annihilation upconversion with a
Schiff-base Pt(II) complex serving as the
sensitizer. It was found that D-2 showed very
weak UC emission in MeOH with a UC quantum
yield of only 1.4%. Interestingly, the UC emission
intensity increased significantly by adding
different amounts of water in MeOH, the UC
intensity was enhanced by more than 9 fold by
adding 40% water into MeOH, and the UC
quantum yield was determined to be 10.2% in
MeOH with 40% water. A mechanism for the
enhancing of UC emission was proposed: When
in MeOH, the large molecule size of the
dendrimer limited the movability of the acceptor,
and the shielding effect of the dendritic branches
on the DPA core limited the intimate contacts
among DPA cores or between DPA core and the
sensitizer. When the water was added, D-2
aggregated to nanoparticles with internal
hydrophobic cavities induced by the hydrophobic
The applications in solar energy, photocatalytic
and biological fields[1-6] have drawn the photo
upconversion (UC) technique based on triplet-triplet
annihilation (TTA) into the spotlight in recent years,
as the low-power, incoherent excitation light, such as
sunlight could be applicable for TTA-UC[7]. The
sensitizer-annihilator pair underwent a multistep
energy transfer/fusion process to achieve the UC
emission[8, 9]. In principle, the sensitizer harvested
the low energy incident light and got the triplet-state
population via intersystem crossing (ISC), the
subsequent triplet-triplet energy transfer (TTET) from
the sensitizer to the annihilator/acceptor populated the
triplet state of the annihilator, and two annihilators at
the triplet excited state collided and underwent the
electron spin conversion (i.e. TTA) to generate a
singlet state in a certain ratio. The radiative transition
from the singlet state of the annihilator thus formed
led to the delayed fluorescence with higher energy
than the incident light[10]. Both TTET and TTA
processes obey an electron-exchange Dexter
mechanism, which requires the proximity of the
components to each other to allow the overlap of
molecular orbitals[11].
effect and
π-π stacking of DPA core, the
To achieve an efficient TTA-UC, several factors,
including the photophysical properties of both the
sensitizer and the annihilators, should be well
matched. Moreover, a good dispersion at the
sensitizer Pt-1 was captured in the internal
hydrophobic cavities, and thus, both the TTET
1

molecular level of UC components and the use of a
medium of low viscosity is generally essential to
allow fast diffusion and collision of excited molecules
[12, 13]. Efficient UC emission is usually achieved in
nonviscous organic solvents as the annihilators are
often polycyclic aromatic hydrocarbons (PAHs)[7,
14-16], while in aqueous solution or a solid matrix,
such as polymers, the upconversion efficiency was
A dendritic annihilator with DPA as the core and
the Lin’s amine [46] as the building block (D-2,
3
scheme 1) was therefore devised. We expected that by
the addition of a poor solvent, D-2 should aggregate
to form particles with flexible internal cavities which
could accommodate the sensitizers without the
specific restriction on the molecule size and chemical
structures. A Schiff-base complex Pt-1 was selected
as the sensitizer due to the high ISC efficiency and
chemically tunable singlet and triplet state energies,
which makes the large anti-Stokes shift of TTA-UC
possible. Recently, we have achieved a large energy
gain of 1.08 eV with a high quantum yield of 21% by
using the Schiff-base complexes as the sensitizers in
toluene[7]. However, it is quite difficult to achieve
efficient UC emission in the highly polar solvents,
including alcohol or water[27], due to the poor
solubility/dispersibility of these planar complexes.
Herein, we report the UC emission of a dendric DPA
annihilator with Pt-1 as the sensitizer in MeOH or
aqueous solution by co-assembling Pt-1 into the
annihilator assemblies.
commonly low as
a
result of the poor
solubility/dispersibility of UC components in water or
in polymer matrix [17-20], which often leads to the
aggregation of the sensitizer and acceptor separately,
and the limited mobility of the triplet excitons
therefrom result in the inefficient energy transfer
[21-23].
The co-assembling of the sensitizer and the
annihilator has been proven to be an effective way to
solve this problem[24-27]. We have reported that the
TTET and TTA-UC efficiency was greatly enhanced
by the supramolecular host-guest complexation which
drew the sensitizer and acceptor in close proximity [4,
5, 24]. We have also modified the sensitizer or the
annihilator by attaching macrocyclic host molecule,
such as cyclodextrins, to achieve the efficient
TTA-UC in water, and the self-aggregation of the
water-soluble annihilators combined with the
host-guest interaction between the modified
sensitizers and annihilators led to a drastically
enhanced UC emission in water [26, 27]. However,
supramolecular macrocyclic hosts are usually featured
by confined cavity size, and the size matching is
prerequisite for achieving suitable complexation
between organic guest molecules and the host
molecules in aqueous solution [28-35]. Therefore, the
structures of the guests have to be sophisticatedly
devised and adapted to the cavities of hosts[36-39].
Thus, carefully tailoring the structures of both the
sensitizer and acceptor was required by employing the
host-guest complexation strategy to co-assembling
UC components. Developing a more general strategy
to co-assembling the sensitizer and annihilator is in
high demand.
Dendrimers[40,41], termed as well-defined,
highly branched, and nanoscopic molecules, have
been attracting sufficient attention. Put aside the
unique photochemical and photophysical properties of
the dendrimers due to the well-defined number of
functional end groups[42-45], we are particularly
interested in the architectural features of internal
cavities. It is reasonable to deduce that the
aggregation of the dendrimers will form multiple
three-dimensional spaces with different sizes, among
which the spaces close to the core will be the widest.
2. Experimental Section
2.1 Materials.
All the chemicals were obtained from commercial
suppliers and were used as received. 1H NMR and
13C NMR spectra were obtained with a Brucker
DRX-400 spectrometer in CDCl3 or CD3OD.
High-resolution mass spectra (HRMS) were
obtained on a Shimadzu LCMS-IT-TOF (ESI)
mass spectrometer. UV-vis spectra were recorded
using a JASCO V650 spectrometer. Fluorescence
spectra and fluorescence lifetime decays were
taken on a fluoromax–4 spectrofluoremeter. A
diode-pumped solid state (DPSS) laser (λ = 532
nm) was used as the exciting source for the
upconversion measurements.
2.2 Measurements.
Fluorescence quantum yields
measured using 9,10-diphenyl anthracene (DPA)
in cyclohexane (Φr 90%) as standard.
Time-resolved fluorescence decays were
(
Φf)
are
=
measured under the excitation of a pulsed nano
LED, the fluorescence lifetime (τf) was obtained
with acceptable statistics parameters, e.g. the
chi-square (χ2) is below 1.2.
The solutions used for upconversion
measurements were degassed for ca. 20 min with
Ar before measurement, and the airflow was kept
on during the measurement. The upconversion
quantum yields (ΦUC) were determined with the
2

ꢐ
prompt fluorescence of I-Bodipy-I (structure
shown in scheme S1, Φ = 2.7% in MeCN)[47] as
the standard, and was calculated by Eq. (1),
Φꢀꢁ = 2
Φ
ꢂꢃꢄ ꢅ ꢆꢇꢈꢉ ꢌ ꢅꢍꢍꢇꢊꢋꢌ ꢎηηꢇꢊꢋꢏ (1)
ꢆ
ꢇꢊꢋ
ꢇꢈꢉ
ꢇꢈꢉ
where Astd and Asam are the absorbance of the
reference (I-Bodipy-I) and the sample (sensitize
Scheme 1. The synthesis of the dendrimer annihilator D-2 and the structure of the sensitizer Pt-1. Conditions: (i) KOH,
1,4-dioxane (ii) MeOH, H2SO4; (iii) (4-(methoxycarbonyl)phenyl)boronic acid, Pd(PPh3)4, CsF; (iv) THF, KOH; (v)
DCC, HOBT
The synthesis of the compounds 2 and 3 was shown in
Pt-1), respectively. Isam stands for the integrated UC
the supporting information.
emission of the annihilator, while Istd stands for the
4:
Under
a
nitrogen
(1.7 g,
atmosphere,
5.0 mmol),
integrated fluorescence of I-Bodipy-I with absorbance
lower than 0.05. Φstd is the fluorescence quantum
yield of I-Bodipy-I. ηsam and ηstd are the refractive
index of the sample solution and the reference
solution. The constant 2 was to set the maximum
quantum yield to be 100%, as TTA-UC was a
two-photon process[8, 10].
9,10-dibromoanthracene
4-(methoxycarbonyl) phenylboronic acid (1.2 g, 6.3
mmol), K2CO3 (1.4 g, 10.0 mmol) and CsF (1.0 g, 6.0
mmol) were dissolved in
a mixed solvent of
toluene/THF/H2O (4:6:1, 50 mL). The solution was
degassed and re-filled with Ar for several times, and
then Pd(PPh3)4 (75.0 mg, 0.13 mmol) was added. The
reaction mixture was stirred and heated to reflux for
over 12 h. after the completion of reaction, the solvent
2.3 Preparation of the dendrimer D-2
3

was removed under reduced pressure, and then was
subjected to a silica gel column chromatography with
DCM:PE = 1:1 as the eluent, the target product
compound 4 was afforded in light yellow solid (yield,
1:1:0.1) to afford the target product (yield, 5.6%).1H
NMR (400 MHz, CDCl3) δ (ppm): 8.13 (d, J = 8.1 Hz,
4H), 7.66 – 7.63 (m, 4H), 7.56 (d, J = 8.1 Hz, 4H), 7.35
– 7.28 (m, 4H), 7.04 (s, 2H), 6.18 (s, 6H), 3.69-3.66 (m,
150H), 2.53-2.50 (m, 48H).
1
15.0 %). H NMR (400 MHz, CDCl3) δ (ppm): 8.31 (d,
J = 1.8 Hz, 2ꢀH), 8.29 (d, J = 1.9 Hz, 2ꢀH), 7.64–7.61 (m,
4ꢀH), 7.59 (d, J = 1.8 Hz, 2ꢀH), 7.57 (d, J = 1.8 Hz, 2ꢀH),
7.37–7.34 (m, 4ꢀH), 4.03 (s, 6ꢀH).
3. Results and discussion
3.1 Synthesis and characterization
5: NaOH (1.6 g, 40 mmol) was added to a solution of
compound 4 (580 mg, 1.3 mmol) in 1,4-dioxane and
water (5:1, 60 mL). The mixture was stirred vigorously
and heated to reflux at 90ꢀ°C for 6 h. The starting
material 4 was monitored with TLC, and the reaction
was terminated after the consumption of compound 4.
Then the 1,4-dioxane was removed under reduced
pressure, and the suspension was diluted by water and
adjusted the pH to ca. 7 with HCl. The light yellow
precipitation was filtered and dried to give compound 5
D-2 was synthesized by using compound 5 as starting
material. It was grown via an iterative repeat of DCC (N,
N'-dicyclohexylcarbodiimide)
and
HOBt
(1-hydroxybenzotriazole) mediated coupling and
subsequent saponification, which is a well-known
method in peptide synthesis. The monomer 3 with amino
group was reacted with diacid 5 to give a hexaester D-1
(Scheme 1). D-1 can be readily hydrolyzed in alkaline
conditions, herein a KOH solution was used to
hydrolyze D-1 to the hexaacid 6, which was again
treated with the monomer 3 to give D-2 as light yellow
oils. All the compounds were well characterized by
NMR spectroscopic and mass-spectrometric analysis.
1
(yield: 90%). H NMR (400 MHz, D2O) δ (ppm): 7.99
(d, J = 8.1 Hz, 4H), 7.56 (dd, J = 6.8, 3.3 Hz, 4H), 7.29
(dd, J = 6.9, 3.2 Hz, 4H), 7.26 (d, J = 8.1 Hz, 4H).
D-1: Compound 5 (330 mg, 0.8 mmol) and the
compound 3 (610 mg, 0.16 mmol) were dissolved in
50.0 mL dry THF, then HOBT (220 mg, 1.6 mmol),
DCC (420 mg, 1.7 mmol) were added. The mixture was
stirred at room temperature for 24 h. The solvent was
removed in vacuum, and residue was subjected to a
column chromatography on silica gel with DCM: EA:
MeOH = 1:1: 0.1 as the elution, white solid was
collected as the target product (220 mg, yield, 24.0
%).1H NMR (400MHz, CDCl3) δ (ppm): 8.02 (d, J = 8.2
Hz, 4H), 7.65 – 7.62 (m, 4H), 7.56 (d, J = 8.2 Hz, 4H),
7.35 – 7.27 (m, 4H), 6.77 (s, 2H), 3.92 (s, 12H), 3.80 (t,
J = 6.2 Hz, 12H), 3.68 (s, 18H), 2.62 (t, J = 6.2 Hz,
12H). HRMS (ESI+): m/z calcd for C60H72N2O20Na
[M+Na]+, 1163.4571, found [M+Na]+, 1163.4547.
3.2 Photophysical properties.
The UV–vis absorption as well as the
solvent-dependent absorption and emission spectra
of the dendrimer D-2 at room temperature (RT) was
displayed in Fig. 1, the absorption and emission
spectra of the selected sensitizer Pt-1 also showed.
the spectroscopic and photophysical parameters of
the dendrimers D-2 and Pt-1 was summarized in
Table 1. The photophysical data of the lower
branched homolog D-1 and precursors
5 were also
listed for comparison. D-2 showed typical sharp
peaks of DPA, and the absorptions of the two
dendrimers with different generations are almost the
same, demonstrating that attaching the branches on
the phenyl group did not significantly disturb the
electronic state of DPA as the dendrimer branches
did not possess any chromophoric groups. The
absorption and emission wavelengths were a little
bit red-shifted relative to DPA (Fig. S10b), due to
6: In a 100 ml flask, KOH (3.0 g, 53.6 mmol) was added
to a solution of compound D-1 (120 mg, 0.11 mmol) in
THF/water (5:1, 60 mL). The mixture was stirred
vigorously at 25ꢀ°C for 48 h. After the starting material
D-1 was completely consumed, the reaction was
stopped. Then THF was removed under reduced
pressure, and the suspension was diluted by water and
the conjugation effect of the amido group in D-2
Interestingly, D-2 showed a very high fluorescence
quantum yield of 86.9%, which is even higher
.
adjusted pH to
7 with HCl. The light-yellow
than its precursors
5 and D-1, and comparable to
precipitation was filtered and dried to give compound 6
in 95% yield. The compound was directly used for the
next reaction without further purification.
DPA in cyclohexane (90%). High fluorescence
quantum yield is one of the essential
photophysical properties for the annihilators to
apply to efficient TTA-UC systems. We
contributed the higher quantum yield of D-2 than
D-2: In a 50 ml flask, compound 6 (60.0 mg, 0.06
mmol) and 3 (260.0 mg, 0.7 mmol) were dissolved in 10
mL THF, then HOBt (100.0 mg, 0.73 mmol), DCC (140
mg, 0.57 mmol) were added. The mixture was stirred at
room temperature overnight. The solvent was removed
in vacuum, and the crude product was purified by silica
gel column chromatography (DCM: EA: MeOH =
compound
5 and D-1 to the protection of DPA
core by the dendric units which avoid collision of
DPA core with each other, and this proposal was
further verified by the longer fluorescence
4

lifetime of D-2 than D-1 and compound
5
(Table
polarity, and the emission maxima as well as the
fluorescence lifetime was almost unchanged in
the commonly used solvent such as CDCl3,
MeOH and EA. Especially, D-2 showed very
similar fluorescent intensity in the investigated
solvents, demonstrating that the quantum yield of
D-2 would not be affected by the solvents.
1).
Solvent-dependent photophysical properties
of D-2 were further investigated (Fig. 1b, 1c, and
table
1).
In
general,
D-2
showed
solvent-independent photophysical properties,
e.g., the absorption spectra showed little
hypochromic-shift upon increasing the solvent
0.6
CHL
EA
MeOH
D-1
D-2
0.9
0.4
0.2
0.0
0.6
0.3
0.0
300
400
500
300
400
Wavelength / nm
Wavelength / nm
Abs D-2
Em D-2
Abs Pt-1
Em Pt-1
D-2 in CHL
D-2 in EA
D-2 in MeOH
9.0x105
0.9
0.6
0.3
0.0
6.0x105
3.0x105
0.0
400
500
600
300 400 500 600 700
Wavelength / nm
Wavelength / nm
Fig. 1. (a) Normalized UV-vis absorption spectra of D-1 and D-2 in MeOH; (b) UV-vis absorption spectra of D-2 in
different solvent. [D-2] = 40 µM; (c) Fluorescence emission spectra of D-2 (10 µM) in different solution (λex = 380
nm) (d) Normalized absoption and emission of the sensitizer Pt-1 and acceptor D-2 in MeOH.
Table 1. Photophysical parameters of the acceptors and the sensitizer Pt-1.a
λabs / nm
354/372/392
355/373/394
355/372/393
355/373/394
358/376/397
516
λem / nmc
Φ / %d
ε
b /104 M-1 cm-1
τ / nse
0.88/1.42/1.38
428
81.7
4.70
5
0.84/1.36/1.27
0.76/1.23/1.17
0.80/1.29/1.20
0.80/1.31/1.25
0.66
426
82.8
86.9
--
4.80
4.99
4.87
4.87
3600
D-1
D-2
D-2f
D-2g
Pt-1h
427
427
427
--
616
13
[a] Results in methanol. [b] Molar extinction coefficient at the absorption maxima. [c] emission maxima. [d] Luminescence
quantum yield determined by a relative method with DPA (Φ = 90% in cyclohexane) as the standard. [e]Luminescence lifetimes.
[f] results in ethyl acetate. [g] results in CHCl3. [h] Literature data[27].
5

intensity of D-2 with the excitation power density.
By increasing the power density, the
upconversion intensity increased (Fig. S13), the
double logarithmic plots of the integral area of
UC emission and the power density fitted well to
linear regression, and showed two slopes of 2.13
and 1.22, respectively, demonstrating that the
TTA-UC intensity firstly followed a quadratic
process vs the light power, and switched to a
linear process. The cross point of the two fitting
curve was assigned as the threshold value, which
was determined as 43.3 mW cm-2 for Pt-1/D-2
system. After this threshold value, the TTA
process becomes a dominant decay pathway for
the triplet states of D-2, and the UC quantum
yield could not be increased simply by inceasing
the light power density. The quadratic relationship
of the intensity versus excitation power density
further verified the TTA based upconversion
mechanism.
3.3 Applications in Triplet-triplet annihilation
upconversion
Considering the good photophysical properties of D-2,
we tried its application as triplet annihilator in
TTA-UC. The normalized absorption and emission
spectra of the sensitizer Pt-1 and acceptor D-1 was
shown in Fig. 1d. When exciting with a 532 nm diode
pump solid state (DPSS) laser, strong orange
phosphorescence was observed for Pt-1 in MeOH, the
phosphorescence was quenched significantly when
D-2 was added and the blue emission ranging from
400- 500 nm appeared (Fig. 2a), this emission is very
similar with the prompt
a
6.0x105
Pt-1
Pt-1 and D-2
The upconverted emission with different
concentrations of D-2 by fixing the concentration
4.0x105
of the sensitizer Pt-1 was investigated (Fig. 3).
The results demonstrated that the intensity of UC
emission increased by adding the concentration of
2.0x105
0.0
D-2 at the range of 1.5-40
µ
M and reached a
M (Fig. 3b).
maximum at the concentration of 40
µ
Generally, the UC intensity is very weak, and the
maximal quantum yield was determined to be
only 1.4% in MeOH. This is a little bit
unexpected in terms of the high quantum yield of
the annihilator D-2 and the relatively good
solvability of the sensitizer Pt-1 in MeOH. We
attribute the relatively low UC quantum yield to
the following two aspects. Firstly, the large size
of D-2 caused the slow movability of molecules
in solutions and thus caused the low TTET
efficiency; Secondly, the highly branched units
attaching on the DPA unit prevented the collision
of DPA core which caused the low TTA
efficiency.
400
500
600
700
Wavelength / nm
b
I
1.22
2
107
th
= 43.3 mW/cm
2.13
106
To increase the TTA-UC efficiency, we tried
to assemble the sensitizer Pt-1 and acceptor D-2
together by adding water into MeOH. It is
interesting to find that the upconverted emission
of D-2 enhanced continuously by adding water
ranging from 0% to 40% into the MeOH solution
(Fig. 4a), the emission maxima intensity of D-2 in
MeOH with 40% water was more than 9-fold of
the intensity in pure MeOH. It should be pointed
out that, each sample was freshly prepared to
maintain the concentration of Pt-1 and D-2
precisely. The upconverison quantum yield was
enhanced from 1.4% in MeOH to 10.2% in 40%
MeOH aqueous solution (Fig. 4b), this is
significant, as efficient TTA-UC in aqueous
20
40
60 80
Power density / mW cm-2
Fig. 2. (a) Emission spectra of Pt-1 or D-2 alone and
Pt-1 with D-2 in MeOH. C[Pt-1] = 30 μM, C[D-2] = 40 μM
141 mW/cm2, λex = 532 nm, 25 oC. (b) The UC intensity
of Pt-1/D-2 plotted as a function of incident light power.
λex = 532 nm.
fluoresecene of D-2, and exciting D-2 or Pt-1
alone with 532 nm laser did not result in such
emissions, thus verified sensitized upconvertion
feature of D-2 [48, 49]. The threshold value of the
upconversion was further determined by
investigating the changes of the upconvertion
6

solution was rarely achieved due to the poor
solubility of organic sensitizers and emitters in
water which led to the separate aggregation of
these organic components[22, 27]. This is harmful
to the TTET and TTA processes because Dexter
energy transfer mechanism of the two processes
demanded the intimate contact of the two
components. For example, we have investigated
the upconverted emission of a DPA dimer
sensitized by Pt-1 in Na2SO3 aqueous solution,
and very weak UC emission was found due to low
triplet-triplet energy transfer efficacy caused by
the aggregation of the sensitizers and the
annihilator separately[27].
5x104
a
b
4x104
3x104
2x104
1x104
0
40 µM
3x106
2x106
1x106
0
1.5 µM
400
450
500
0
10
20
30
40
Wavelength / nm
Concentration of D-2 / µM
Fig. 3. (a) Change of the upconversion spectra upon the addition of different amounts of D-2 in MeOH; (b) The
upconverted emission intensity plotted as a function of concentration of D-2. C[Pt-1] = 30 μM, 141 mW/cm2, λex = 532
nm, 25 oC.
ΦUC = 10.2%
b
10
9.4%
a
40% H2O
3x105
30% H2O
20% H2O
2x105
1x105
0
5.2%
5% H2O
0%
5
0
2.7%
ΦUC = 1.4%
0
20
40
400
450
500
Content of water / %
wavelength / nm
2.5x106
2.0x106
1.5x106
1.0x106
5.0x105
0.0
d
c
5% H2O
1.0
20% H2O
30% H2O
40% H2O
0.8
0.6
0.4
0
20
40
600
700
800
Content of water / %
wavelength / nm
7

Fig. 4. (a) The upconverted fluorescence changes of D-2 sensitized by Pt-1 in MeOH by adding different amount of
water; (b) The upconverted efficiency of D-2 plotted as a function of the ratio of water in MeOH (data from Fig. 4a);
(c) The phosphorescence changes of the Pt-1 and D-2 solution in MeOH by adding different amount of water; and (d)
The phosphorescent integral area of Pt-1 plotted as a function of the ratio of water in MeOH (data from Fig. 4c). C[Pt-1]
= 30 μM, C[D-2] = 50 μM, 141 mW/cm2, λex = 532 nm, 25 oC.
Fig. 5. (a) The normalized UV−vis spectra of D-2 at
different concentrations in MeOH solution containing
In order to further understand the origin of
the enhanced UC emission by adding water into
MeOH, the remnant phosphorescent intensity of
the UC system Pt-1/D-2 was investigated. It is
interesting to find that the phosphorescent
intensity of Pt-1 was continuously quenched by
increasing the ratio of water in MeOH even the
apparent concentration of D-2 was not changed at
all (Fig. 4c and 4d). We propose that the
quenched phosphorescence should be attributed to
the spatial close of the sensitizer and the acceptor
caused by the aggregation, in other words, the
aggregation has brought more and more Pt-1
molecules close to the DPA core which caused
the quenching of the phosphorescence.
35% water at room temperature. (b) The DLS spectra of
compound D-2 at 50 µM and the dye pair of Pt-1 and
D-2 in MeOH or in MeOH containing 35% water. 25 .
The aggregation of D-2 in MeOH with water
was confirmed by the UV-vis absorption spectra
and the dynamic light scattering (DLS) studies
(Fig. 5 and Fig. S12). The UV-vis absorption
spectra of D-2 showed a slightly red-shifted when
added different ratios of water (0%-35%) in
MeOH at a concentration of 16
µM. Moreover,
the baseline of the absorption spectra of the
acceptor D-2 raised up obviously when increasing
the concentration from 16 µM to 50 µM in MeOH
containing 35% water (Fig. 5a). These results
implied an aggregation of D-2 in the aqueous
solution. This is reasonable considering the
a
hydrophobic aromatic core of D-2 and the
π-π
16 uM D-2 in 35% water
stacking interactions of D-2, despite the good
solubility of D-2 in most organic solvents and
acceptable solubility in water. Dynamic light
scattering (DLS) studies revealed that D-2 at the
50 uM D-2 in 35% water
1.0
concentration of 50
of a mean size of 220 nm, when 30
µ
M afforded nanostructures
M of Pt-1
µ
0.5
0.0
was added, the size was increased slightly to a
mean size of 255 nm, which is much larger than
the size of the dye pair in MeOH(Fig. 5b).
Based on the experimental results observed
above, a mechanism was proposed for the
enhancement of the UC emission by adding water
into MeOH. When in MeOH, the shielding effect
of the highly branched units attaching on both
rims of the DPA core prevent the collision of Pt-1
with DPA core or between DPA cores, which
caused the low TTA-UC efficiency (Fig. 6a).
When water was added into the solution, D-2
aggregated, and the DPA core stacked together
350
400
450
500
Wavelength / nm
Pt-1+D-2 in MeOH
D-2 in MeOH with 35% H2O
b
80
60
40
20
0
Pt-1+D-2 in MeOH with 35% H2O
due to the hydrophobic effect and the
π-π
stacking interactions. The unique dendritic
structure of D-2 formed hydrophobic cavities in
the interior of the assemblies (Fig. 6b), which
captured the highly hydrophobic sensitizer Pt-1
.
The hydrogen-bond interaction between the
carboxyl group in Pt-1 and D-2 should also
attribute to the formation of co-assemblings of
Pt-1 and D-2. Therefore, the TTET process
between the sensitizer and the acceptor facilitated
due to intimate contact in space. On the other
200
Size / nm
400
600
8

hand, the triplet exciton migrated efficiently
among the aggregations of D-2 facilitate TTA
process between the acceptors[26], so as to
improve the upconversion intensity.
Fig. 6. The proposed mechanisms for the enhancing of UC emission when adding water into MeOH solution.
Science Foundation of China (Nos. 21971169,
21871194, 21572142), National Key Research and
4. Conclusion
Development
217YFA0505903), and Science
Program
of
China
(No.
In summary, we have synthesized a dendritic
annihilator D-2 with DPA as the core by an
iterative DCC/ HOBt coupling reaction. The
&
Technology
Department of Sichuan Province (2019YJ0090,
2019YJ0160). Comprehensive Training Platform of
Specialized Laboratory, College of Chemistry and
Prof. Peng Wu of Analytical &Testing Center,
Sichuan University for characterization and lifetime
measurement, are highly appreciated.
photophysical
properties
of
D-2
were
demonstrated to be very similar to the core, with
an even higher fluorescence quantum yield of
86.9% in MeOH than the model compound
5,
which was beneficial for applications as
annihilator in TTA-UC. However, D-2 showed a
very weak UC emission in MeOH, with
Schiff-base Pt(II) complex Pt-1 as the sensitizer,
while adding water into the MeOH solution led to
the significant enhancement of the UC emission.
The UC quantum yield was increased to 10.2% by
adding 40% water into the MeOH solution, vs.
only 1.4% in pure MeOH. The co-assembling of
D-2 and Pt-1 with the addition of water was
responsible. e.g., D-2 aggregated to a nanoparticle
of 250 nm with internal cavities where the
sensitizer molecules were accommodated in a
MeOH/water mixed solution. Therefore both the
TTET and TTA process was accelerated, which
led to the enhanced TTA-UC emission. This work
provides new perspectives for the derivatization
of the annihilators to improve the TTA-UC
efficiency.
References
[1] Cheng Y-Y, Nattestad A, Schulze T-F, MacQueen R-W,
Fuckel B, Lips K, et al. Increased upconversion
performance for thin film solar cells: a trimolecular
composition. Chem Sci. 2016;7:559-68.
[2] Schulze T-F, Schmidt T-W. Photochemical
upconversion: present status and prospects for its
application to solar energy conversion. Energy Environ Sci.
2015;8:103-25.
[3] Ravetz B-D, Pun A-B, Churchill E-M, Congreve D-N,
Rovis T, Campos L-M. Photoredox catalysis using infrared
light via triplet fusion upconversion. Nature.
2019;565:343-6.
[4] Rao M, Kanagaraj K, Fan C, Ji J, Xiao C, Wei X, et al.
Photocatalytic
supramolecular
enantiodifferentiating
dimerization of 2-anthracenecarboxylic acid through
triplet-triplet annihilation. Org Lett. 2018;20:1680-3.
[5] Rao M, Wu W, Yang C. Effects of temperature and host
concentration on the supramolecular enantiodifferentiating
Acknowledgements
The work was supported by the National Natural
9

[4 + 4] photodimerization of 2-anthracenecarboxylate
through triplet-triplet annihilation catalyzed by Pt-modified
cyclodextrins. Molecules. 2019;24:1502.
assemblies designed for excitation energy transfer. Chem
Rev. 2015;115:7502-42.
[21] Simon Y-C, Weder C. Low-power photon
upconversion through triplet-triplet annihilation in
polymers. J Mater Chem. 2012;22:20817-30.
[22] El Roz K-A, Castellano F-N. Photochemical
upconversion in water. Chem Commun. 2017;53:11705-8.
[23] Singh-Rachford T-N, Lott J, Weder C, Castellano F-N.
Influence of temperature on low-power upconversion in
[6] Liu Q, Xu M, Yang T, Tian B, Zhang X, Li F. Highly
photostable near-IR-excitation upconversion nanocapsules
based on triplet-triplet annihilation for in vivo bioimaging
application. ACS Appl Mater Interfaces. 2018;10:9883-8.
[7] Fan C, Wei L, Niu T, Rao M, Cheng G, Chruma JJ, et al.
Efficient triplet-triplet annihilation upconversion with an
anti-stokes shift of 1.08 eV achieved by chemically tuning
sensitizers. J Am Chem Soc. 2019;141:15070-7.
rubbery polymer blends.
2009;131:12007-14.
J
Am Chem Soc.
[24] Fan C, Wu W, Chruma JJ, Zhao J, Yang C. Enhanced
triplet-triplet energy transfer and upconversion fluorescence
through host-guest complexation. J Am Chem Soc.
2016;138:15405-12.
[25] Yang D, Duan P, Liu M. Dual upconverted and
downconverted circularly polarized luminescence in
donor-acceptor assemblies. Angew Chem, Int Ed.
2018;57:9357-61.
[8] Singh-Rachford T-N, Castellano F-N. Photon
upconversion based on sensitized triplet-triplet annihilation.
Coord Chem Rev. 2010;254:2560-73.
[9] Zhao J, Wu W, Sun J, Guo S. Triplet photosensitizers:
from molecular design to applications. Chem Soc Rev.
2013;42:5323-51.
[10] Zhao J, Ji S, Guo H. Triplet-triplet annihilation based
upconversion: from triplet sensitizers and triplet acceptors
to upconversion quantum yields. RSC Adv. 2011;1:937-50.
[26] Xu W, Liang W, Wu W, Fan C, Rao M, Su D, et al.
Supramolecular
assembly-improved
triplet-triplet
[11]
Monguzzi
A, Tubino R,
Meinardi
F.
in
annihilation upconversion in aqueous solution. Chem-Eur J.
2018;24:16677-85.
[27] Lai H, Zhao T, Deng Y, Fan C, Wu W, Yang C.
Upconversion-induced
delayed
fluorescence
multicomponent organic systems: Role of Dexter energy
transfer. Phys Rev B. 2008;77:155122/1-/4.
Assembly-enhanced
upconversion in the aggregation formed by Schiff-base
Pt(II) complex grafting-permethyl-β-CD and
triplet-triplet
annihilation
[12] Ogawa T, Hosoyamada M, Yurash B, Thuc-Quyen N,
Yanai N, Kimizuka N. Donor-acceptor-collector ternary
crystalline films for efficient solid-state photon
upconversion. J Am Chem Soc. 2018;140:8788-96.
[13] Bharmoria P, Hisamitsu S, Nagatomi H, Ogawa T,
Morikawa M-a, Yanai N, et al. Simple and versatile
platform for air-tolerant photon upconverting hydrogels by
biopolymer-surfactant-chromophore co-assembly. J Am
Chem Soc. 2018;140:10848-55.
9,10-diphenylanthracence dimer. Chin Chem Lett.
2019;30:1979-83.
[28] Rekharsky M-V, Inoue Y. Complexation
thermodynamics
of
cyclodextrins.
Chem
Rev.
1998;98:1875-918.
[29] Yu X, Liang W, Huang Q, Wu W, Chruma J-J, Yang C.
[14] Wang Z, Zhao J, Barbon A, Toffoletti A, Liu Y, An Y,
et al. Radical-enhanced intersystem crossing in new Bodipy
derivatives and application for efficient triplet-triplet
Room-temperature phosphorescent γ-cyclodextrin-cucurbit
[6] uril-cowheeled [4] rotaxanes for specific sensing of
tryptophan. Chem Commun. 2019;55:3156-9.
[30] Ji J, Wu W, Liang W, Cheng G, Matsushita R, Yan Z, et
al. An ultimate stereocontrol in supramolecular
annihilation
upconversion.
J
Am
Chem
Soc.
2017;139:7831-42.
[15] Zhong F, Zhao J. Phenyleneanthracene derivatives as
triplet energy acceptor/emitter in red light excitable
triplet-triplet-annihilation upconversion. Dyes Pigm.
2017;136:909-18.
photochirogenesis:
photocyclo-dimerization
of
2-anthracene-carboxylate mediated by sulfur-linked
beta-cyclodextrin
2019;141:9225-38.
dimers.
J
Am
Chem
Soc.
[16] Lu Y, Wang J, McGoldrick N, Cui X, Zhao J, Caverly
[31] Yi J, Liang W, Wei X, Yao J, Yan Z, Su D, et al.
Switched enantioselectivity by solvent components and
C,
et
al.
Iridium(III)
complexes
bearing
Pyrene-functionalized 1,10-phenanthroline ligands as
highly efficient sensitizers for triplet–triplet annihilation
upconversion. Angew Chem Int Ed 2016;55:14688-92.
[17] Xu W, Liang W, Wu W, Fan C, Rao M, Su D, et al.
temperature
2-anthracene-carboxylate
in
photocyclodimerization
with
of
6A,
6X-diguanidio−γ-cyclodextrins.
Chin
Chem
Lett.
2018;29:87-90.
[32] Wei X, Wu W, Matsushita R, Yan Z, Zhou D, Chruma
JJ, et al. Supramolecular photochirogenesis driven by
Supramolecular
assembly-improved
triplet-triplet
annihilation upconversion in aqueous solution. Chem-Eur J.
2018;24:16677-85.
higher-order
complexation:
enantiodifferentiating
[18] Kerzig C, Wenger OS. Sensitized triplet–triplet
annihilation upconversion in water and its application to
photochemical transformations. Chem sci. 2018;9:6670-8.
[19] Kouno H, Sasaki Y, Yanai N, Kimizuka N.
Supramolecular crowding can avoid oxygen quenching of
photocyclodimerization of 2-anthracenecarboxylate to
slipped cyclodimers via a 2:2 complex with β-cyclodextrin.
J Am Chem Soc. 2018;140:3959-74.
[33] Yan Z, Huang Q, Liang W, Yu X, Zhou D, Wu W, et al.
Enantiodifferentiation in the photoisomerization of (Z, Z)-1,
3-cyclooctadiene in the cavity of γ-cyclodextrin–curcubit
[6] uril-wheeled [4] rotaxanes with an encapsulated
photosensitizer. Org Lett. 2017;19:898-901.
photon
upconversion
in
water.
Chem-Eur
J.
2019;25:6124-30.
[20] Peng H-Q, Niu L-Y, Chen Y-Z, Wu L-Z, Tung C-H,
Yang Q-Z. Biological applications of supramolecular
10

[34] Huang Q, Jiang L, Liang W, Gui J, Xu D, Wu W, et al.
Inherently chiral azonia [6] helicene-modified
electron-transfer reactions.
1999;121:6290-8.
J
Am
Chem
Soc.
β-cyclodextrin: Synthesis, characterization, and chirality
sensing of underivatized amino acids in water. J Org Chem.
2016;81:3430-4.
[35] Yao J, Yan Z, Ji J, Wu W, Yang C, Nishijima M, et al.
Ammonia-driven chirality inversion and enhancement in
[42] Zeng Y, Li P, Liu X, Yu T, Chen J, Yang G, et al. A
''breathing'' dendritic molecule-conformational fluctuation
induced by external stimuli. Polym Chem. 2014;5:5978-84.
[43] Li Y-Y, Han L, Chen J, Zheng S, Yi Z, Li Y, et al.
Study on the Extent of Folding Back Conformation in
Poly(aryl ether) Dendrimers by intramolecular electron
transfer and exciplex formation. Macromolecules.
2007;40:9384-90.
[44] Liu X, Zeng Y, Liu J, Li P, Zhang D, Zhang X, et al.
Highly emissive nanoparticles based on AIE-active
molecule and PAMAM dendritic "molecular glue".
Langmuir. 2015;31:4386-93.
[45] Zhang X, Zeng Y, Yu T, Chen J, Yang G, Li Y.
Advances in photofunctional dendrimers for solar energy
conversion. J Phys Chem Lett. 2014;5:2340-50.
[46] Newkome; GR, Lin X. Symmetrical, four-directional,
poly(ether-amide) cascade polymers. Macromolecules.
1991;24:1443-4
[47] Wu W, Guo H, Wu W, Ji S, Zhao J. Organic triplet
sensitizer library derived from a single chromophore
(BODIPY) with long-lived triplet excited state for
triplet-triplet annihilation based upconversion. J Org Chem.
2011;76:7056-64.
enantiodifferentiating
2-anthracenecarboxylate
diguanidino-γ-cyclodextrin.
2014;136:6916-9.
photocyclodimerization
of
by
Soc.
mediated
J
Am
Chem
[36] Yao J, Wu W, Liang W, Feng Y, Zhou D, Chruma JJ, et
al. Temperature driven planar chirality switching of a pillar
[5] arene based molecular universal joint. Angew Chem In
Ed. 2017;56:6869-73.
[37] Kanagaraj K, Xiao C, Rao M, Fan C, Borovkov V,
Cheng G, et al. A quinoline-appended cyclodextrin
derivative as a highly selective receptor and colorimetric
probe for nucleotides. iScience. 2020;23:100927.
[38] Xiao C, Wu W, Liang W, Zhou D, Kanagaraj K, Cheng
G, et al. Redox-triggered chirality switching and
guest-capture/release with a pillar[6]arene-based molecular
universal joint. Angew Chem Int Ed. 2020.
[39] Liu R, Zhang Y, Wu W, Liang W, Huang Q, Yu X, et
al.
Temperature-driven
braking
of
γ-cyclodextrin-curcubit[6]uril-cowheeled
Chin Chem Lett. 2019;30(3):577-81.
[4]rotaxanes.
[48] Liang H, Sun S, Zafar M, Yuan Z, Dong Y, Ji S, et al.
Tuning the SOCT-ISC of bodipy based photosentizers by
introducing different electron donating groups and its
application in triplet-triplet-annihilation upconversion.
Dyes Pigm 2020;173:108003
[49] Wan S, Lin J, Su H, Dai J, Lu W.
Photochemically deoxygenating solvents for triplet-triplet
annihilation photon upconversion operating in air. Chem
Commun 2018;54:3907-10.
[40] Issberner J, Vogtle F, DeCola L, Balzani V. Dendritic
bipyridine ligands and their tris(bipyridine)ruthenium(II)
chelates - Syntheses, absorption spectra, and photophysical
properties. Chem-Eur J. 1997;3:706-12.
[41] Vogtle F, Plevoets M, Nieger M, Azzellini GC, Credi
A, De Cola L, et al. Dendrimers with a photoactive and
redox-active [Ru(bpy)(3)](2+)-type core: Photophysical
properties, electrochemical behaviour, and excited-state
11

Highlights:
ò
ò
ò
A dendritic triplet annihilator containing 18 peripherical methyl ester units was
designed and synthesized.
Efficient UC emission with a ΦUC as high as 10.2% was obtained in MeOH aqueous
solution.
The co-assembling of the sensitizers into the internal cavities of the dendritic
annihilator aggregates and efficient energy transfer therefrom is responsible for the
high quantum yields.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests: