Enhancing triplet sensitization ability of donor-acceptor dyads: Via intramolecular triplet energy transfer

Article abstract of DOI:10.1039/c9tc06337d

Two covalently linked porphyrin palladium (PdPor)-9,10-diphenylanthracene (DPA) donor-acceptor (D-A) systems, connected through the 2-position (PdPor-2-DPA) or 9-position (PdPor-9-DPA) of DPA, are prepared as sensitizers of triplet-triplet annihilation upconversion (TTA-UC). The steady-state absorption spectra demonstrate that PdPor and DPA in these compounds are electronically independent in the ground state. Phosphorescence spectra and femtosecond transient absorption spectra reveal that ultrafast intramolecular triplet energy transfer (iTET) from the PdPor unit to the DPA unit can be conducted in PdPor-9-DPA but not in PdPor-2-DPA. This may be caused by the stronger electronic coupling between PdTPP and DPA in PdPor-9-DPA that is essential for Dexter type triplet energy transfer. In addition, these two dyads are used as a triplet energy donor in TTA-UC for sensitizing DPA. The maximum upconversion efficiency for PdPor-9-DPA is about 23.59%, which is much larger than that of PdPor-2-DPA (10.48%). These results confirm that the iTET efficiency would affect the upconversion efficiency when the D-A system is adopted as a triplet donor. This provides new insights into designing D-A systems with highly efficient iTET as candidates for triplet energy donors in TTA-UC.

Full text of DOI:10.1039/c9tc06337d

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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
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DOI: 10.1039/C9TC06337D
Journal Name
ARTICLE
Enhancing triplet sensitization ability of donor-acceptor dyads via
intramolecular triplet energy transfer†
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
c,*
a
a
b,*
Shanshan Liu , Xiangyang Wang , Heyuan Liu , Li Shen , Dezhi Zhao , and Xiyou Li
DOI: 10.1039/x0xx00000x
Two covalently linked porphyrin palladium (PdPor)-9,10-diphenylanthracene (DPA) donor-acceptor (D-A) systems,
connected through 2-position (PdPor-2-DPA) or 9-position (PdPor-9-DPA) of DPA, are prepared as sensitizers of triplet-
triplet annihilation upconversion (TTA-UC). The steady-state absorption spectra demonstrate that PdPor and DPA in these
compounds are electronically independent in the ground state. Phosphorescence spectra and femtosecond transient
absorption spectra reveal that ultrafast intramolecular triplet energy transfer (iTET) from PdPor unit to DPA unit can be
conducted in PdPor-9-DPA instead of PdPor-2-DPA. This may be caused by the stronger electronic coupling between PdTPP
and DPA in PdPor-9-DPA that is essential for Dexter type triplet energy transfer. In addition, these two dyads are used as
triplet energy donor in TTA-UC for sensitizing DPA. The maximum upconversion efficiency for PdPor-9-DPA is about
www.rsc.org/
2
3.59%, which is much larger than that of PdPor-2-DPA (10.48%). These results confirm that the iTET efficiency would
affect the upconversion efficiency when the D-A system is adopted as triplet donor. This provides new insights into
designing D-A systems with highly efficient iTET as candidates for triplet energy donor in TTA-UC.
complexes with porphyrin, phthalocyanine, polyimines, and
cyclometalated ligands) and organic dyes (iodo- or bromo-
substituted dyes and C60 derivatives), have been successfully
Introduction
Triplet–triplet annihilation upconversion (TTA-UC) is promising
for application in photovoltaics, photocatalysis, and many
other light-driven photophysical and photochemical processes
due to its low excitation power density demand (solar light is
sufficient), high upconversion quantum yield and readily
tunable excitation/emission wavelength.1,2 TTA-UC is
bimolecular diffusion-controlled process, needs two molecules,
a triplet sensitizer and a triplet acceptor.3,4 The sensitizer
absorbs exciting light and is excited to triplet excited state.
Then it transfers excitation energy to the acceptor via triplet-
triplet energy transfer (TET) and creates triplet excited state of
acceptor. When two adjacent excited triplet acceptors
annihilate (TTA) via collision and form one acceptor in singlet
state and another one in the ground state.5,6
developed as triplet sensitizers.4 Recently, bichromophoric
triplet sensitizers, composed of covalently linked triplet donor-
acceptor (D-A) systems, have drawn lots of research
interests.9-14 In these systems, triplet donor harvests the
excitation energy to generate its singlet excited state, which
subsequently converts into its triplet excited state through ISC.
Followed by a rapid intramolecular TET (iTET) process, the
triplet excited of acceptor is generated.9,10,11 Triplet lifetimes of
these sensitizers could be prolonged by establishing a triplet-
state equilibrium between the triplet states of donor and
acceptor or replacing of the short-lived triplet state of donor
with long-lived triplet state of acceptor. For example, Zhao’s
group attached the triplet acceptor pyrene or anthracene to
ruthenium(II) polyimine complexes and got the extended
triplet lifetime.12,13 When these systems were used as triplet
sensitizers, the TTA-UC efficiency is promoted approximately
a
An ideal triplet sensitizer should efficiently absorb visible
and near-infrared light, have high triplet state quantum yields
(efficient intersystem crossing (ISC)) and long-lived triplet
excited states.7,8 At the beginning, some molecules, including
heavy-metal complexes (Pt(II), Pd(II), Ru(II), Re(I), and Ir(III)
10 times compared with that of the corresponding
ruthenium(II) polyimine complexe. Besides the extended
triplet lifetime, Ma’s group found out that iTET could also
effectively out-compete the radiative and nonradiative decays
of sensitizer and a dual-path intermolecular TET mechanism
was developed.10 Both the extended triplet lifetime and the
dual-path intermolecular mechanism will lead to the improved
a. College of Science, China University of Petroleum (East China), Qingdao,
Shandong, 266580, China.
b.
School of Materials Science and Engineering, China University of Petroleum (East
China), Qingdao, Shandong, 266580, China.
College of New Energy, China University of Petroleum (East China), Qingdao,
c.
TTA-UC efficiency. Many other D-A systems have also been
designed and showed enhanced sensitization ability.14
Shandong, 266580, China.
E-mail: xiyouli@upc.edu.cn; liuheyuan123@upc.edu.cn
†
Electronic
Supplementary
Information
(ESI)
available.
See Nevertheless, all the TTA-UC systems with an intramolecular
DOI: 10.1039/x0xx00000x
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triplet sensitization step reported so far involve also
intermolecular sensitization process, which normally involve
collision of two molecules in solution and affected by the
diffusion rate. Because diffusion rate of a molecule in solution
depends on its molecular weight and size, therefore, to assess
more exactly the advantage of intramolecular sensitization
over intermolecular sensitization, two systems with similar
molecular weight and dimensions, but different intramolecular
sensitization efficiency should be more applicable.
View Article Online
DOI: 10.1039/C9TC06337D
Fig.
1
Molecular
structures
of
these
compounds.
The solvents used for preparing solutions for photophysical
measurement were degassed by the Freeze–Pump–Thaw
method prior to use. All samples for the photophysical
measurements were prepared in a glovebox.
Herein, we designed two porphyrin-anthracene dyads with
tetraphenylporphyrin palladium (PdTPP) as the triplet energy
donor and 9,10-diphenylanthracene (DPA) as acceptor.
Femtosecond transient absorption spectroscopy
According to the literature,15 the connected position of triplet The pump beam was generated from a regenerative amplified
donor to acceptor has a significant influence on the iTET Ti:sapphire laser system from Coherent (800 nm, 100 fs, 6 Mj per
efficiency between donor and acceptor. So, the PdTPP was pulse, and 1 kHz repetition rate). The 800 nm output pulse from the
introduced onto the 2-position (PdPor-2-DPA) and 9-position regenerative amplifier was split into two parts with a beam splitter.
The reflected part was used to pump a TOPAS Optical Parametric
Amplifier (OPA) which generates a wavelength-tunable laser pulse
from 250 nm to 2.5 mm as the pump beam. The transmitted 800
nm beam was attenuated with a neutral density filter and focused
(
PdPor-9-DPA) of DPA, respectively (Fig. 1). By changing
connecting position, the torsion angle, orbital overlap,
electronic coupling, and triplet energy transfer between the
donor and acceptor can be changed. We found that ultrafast
iTET from PdTPP to DPA is conducted in PdPor-9-DPA instead
of PdPor-2-DPA. When these two dyads are used as triplet
energy donor in TTA-UC for sensitizing DPA, the maximum
upconversion efficiency for PdPor-9-DPA is about 23.59%,
which is much larger than that for PdPor-2-DPA (10.48%).
Because PdPor-9-DPA and PdPor-2-DPA are very similar in
molecular weight and molecular dimensions, this difference on
upconversion efficiency can be attributed exclusively to the
intramolecular sensitization in PdPor-9-DPA. This information
should be useful in the design of new TTA-UC systems.
2
into a rotating CaF disk to generate a white light continuum (WLC)
from 350 nm to 800 nm used for the probe beam. The probe beam
was focused with an Al parabolic reflector onto the sample. After
the sample, the probe beam was collimated and then focused into a
fiber-coupled spectrometer and detected at a frequency of 1 kHz.
The intensity of the pump pulse used in the experiment was
controlled by a variable neutral-density filter wheel. The delay
between the pump and probe pulses was controlled by a motorized
delay stage. The pump pulses were chopped by a synchronized
chopper at 500 Hz. The global analysis of the femtosecond resolved
transient absorption (fs-TA) data was conducted in glotaran
software.
TTA-UC experiment
Expeimental section
The TTA-UC emission spectra were also measured on an Edinburgh
Instrument FLS980 with a continuous diode laser (532 nm) as the
excitation resource. The diameter of the laser beam was about 3
mm. The solvent used for preparing solutions for the TTA-UC
experiment was degassed by the Freeze–Pump–Thaw method prior
to use. The sample was prepared in a glovebox. The upconversion
quantum yields were determined with rhodamine B as the
quantum-yield standard. The absolute fluorescence quantum yield
of rhodamine B was determined to be 67.94% by using the
General methods
1H NMR spectra were recorded on a Bruker 400 MHz NMR
spectrometer with chemical shifts reported in ppm (TMS as an
internal standard). MALDI-TOF mass spectra were recorded
with a Bruker/ultra flex instrument. Absorption spectra were
measured on a Hitachi U-3900 spectrophotometer. Steady-
state luminescence spectra were recorded on an Edinburgh
Instruments
FLS980
with
three-monochromator
spectrophotometers and three photomultiplier detectors. The
emission spectra were corrected for the wavelength
dependence of the sensitivity of the detection system. The
phosphorescence lifetimes of porphyrins were measured on
FLS980 with a microsecond lamp (50 Hz, 100W), and 532 nm
integrating sphere. The quantum yields were calculated with eqn
4
(
1), where ΦUC, Asam, Isam and ηsam represent the quantum yield,
absorbance, integrated photoluminescence intensity and the
refractive index of the samples and the solvents. The equation is
multiplied by a factor of two in order to make the maximum
(0.46 mJ) was chosen as the excitation wavelength. The
quantum yield be unity.
phosphorescence quantum yields were measured with an
integrating sphere (by using a system for the measurement of
absolute
퐀퐬퐭퐝 퐈퐬퐚퐦 훈퐬퐚퐦
UC = 2Φstd(퐀퐬퐚퐦)(퐈퐬퐭퐝 )(훈퐬퐭퐝 )2
Φ
(1)
The Stern–Volmer curves were obtained based on eqn (2),12
where I , I and [emitter] represent the integrated phosphorescence
fluorescence
quantum
yield).
0
intensity of pure sensitizer, the integrated phosphorescence
intensity of sensitizer in the mixture and the concentration of DPA.
I
0
/I = 1 + KSV [emitter]
(2)
2
| J. Name., 2012, 00, 1-3
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Materials
View Article Online
DOI: 10.1039/C9TC06337D
All commercially obtained reagents/solvents were used as
received without further purification unless otherwise noted.
Synthetic procedures of PdPor-2-DPA and PdPor-9-DPA are
shown in Scheme 1. Both of them were fully characterized by
MALDI-TOF mass spectra and 1H NMR (Fig. S1 and S2,
).16,17,18,19
ESI†
5
-(4-bromophenyl)-10,15,20-triphenylporphrinato
palladium(II):
-Bromobenzaldehyde (1.85 g, 10 mmol) and benzaldehyde
3.20 g, 30 mmol) were dissolved in propanoic acid (20 mL) in a
50 mL flask. After stirring at 138 °C for 10 min, 15 ml
Scheme 1. The synthesis of PdPor-2-DPA and PdPor-9-DPA.
4
(
2
product was extracted with dichloromethane, and a solid was
obtained upon drying under vacuum, and successfully purified
by column chromatography using (dichloromethane/hexane
propanoic acid including pyrrole (3.35 g, 50 mmol) were added
to the solution by dripping slowly within 20 min-30 min. The
reaction mixture was continually stirred at 138 °C for 30 min.
After cooling, added 200 ml methanol and refrigerate at least
=
4/5) as the eluent, and the isolate was recrystallized from
hexane to give PdPor-2-DPA as a orange powder (20 mg,
1
3
6
4
7
1
3
8.0%). H NMR (400 MHz, CDCl ): δ 8.89-8.87 (d, 2H), 8.81 (s,
H), 8.24 (s, 1H), 8.21 (s, 1H), 8.18-8.16 (d, 6H), 7.94-7.92 (d,
H), 7.77-7.71 (m, 11H), 7.70-7.66 (m, 5H), 7.64-7.59 (m, 5H),
1
2 hours for recrystallization. The resulting mixture was
washed with methanol after filtration. Then purified by column
chromatography on silica gel (dichloromethane/hexane =1/1.5)
to give the metal free porphyrin (140 mg, 2.2%). A mix of
metal free porphyrin and palladium(II) acetate (90 mg, 0.40
mmol) were dissolved in toluene (50 mL) in a 250 mL flask.
After reflux for 4 hours, The combined organic extracts were
evaporated by rotavapor and purified by column
chromatography on silica gel (dichloromethane/hexane =1/2)
44 4
.40-7.37 (m, 3H). C70H N Pd, MALDI-TOF: m/z:, calcd:
+
+
046.26 [M] ; found: 1045.29 [M] .
PdPor-9-DPA:
A mixture of 5-(4-boronic acid pinacol ester phenyl)-10,15,20-
triphenylporphrinato palladium(II) (42.2 mg, 0.05 mmol), 9-
bromo-10-phenyl-anthracene (19.92 mg, 0.06 mmol),
Pd(PPh ) (5.78 mg, 5 μmol), K CO (0.5 mL, 138 mg, 1.0 mmol),
3 4 2 3
and nitrogen-saturated toluene/ethanol (15 mL/7.5 mL) were
1
to give the target compound (145 mg, 82.5%). H NMR (400
o
mixed in a flask. The reaction mixture was heated at 80 C for 4
MHz, CDCl
3
): δ 8.83-8.77 (m, 8H), 8.17-8.15 (d, 6H), 8.05-8.03
h under nitrogen. After it was cooled to room temperature,
the reaction mixture was quenched with distilled water and
extracted with dichloromethane. The product was extracted
with dichloromethane, and a solid was obtained upon drying
under vacuum, and successfully purified by column
chromatography using (dichloromethane/hexane =4/5) as the
eluent, and the isolate was recrystallized from hexane to give
(d, 2H), 7.88-7.86 (d, 2H), 7.78-7.71 (m, 9H). MALDI-TOF: m/z:
+
+
calcd: 798.05 [M] ; found: 798.13 [M] .
-(4-boronic acid pinacol ester phenyl)-10,15,20-
triphenylporphrinato palladium(II):
mixture of 5-(4-bromophenyl)-10,15,20-
triphenylporphrinato palladium(II) (159.61 mg, 0.2 mmol),
bis(pinacolato)diboron (254 mg, 1.0 mmol), PdCl (dppf) (14.62
5
A
1
2
PdPor-2-DPA as a orange powder (9 mg, 18.5%). H NMR (400
MHz, CDCl ): δ 9.11-9.09 (d, 2H), 8.94-8.93 (d, 2H), 8.85-8.83
m, 4H), 8.44-8.42 (d, 2H), 8.23-8.18(m, 6H), 8.16-8.14(d, 2H),
mg, 0.02 mmol) and CH
dissolved in DMF (15 mL) heated at 80 C for 5 h under N
3
COOK (58.8 mg, 0.6 mmol) were
3
o
2
and
(
then cooled to room temperature, after which 50 mL of
distilled water was added to the flask. The product was
extracted with dichloromethane, and a solid was obtained
upon drying under vacuum. The product was purified by
7
7
.87-7.82(d, 2H), 7.80-7.75 (m, 9H), 7.68-7.65(m, 3H), 7.62-
.55(m, 5H), 7.44-8.48(m, 3H). C64 Pd, MALDI-TOF: m/z:
40 4
H N
+
+
calcd: 970.23 [M] ; found: 969.62 [M] .
column
chromatography
on
silica
gel
(
4
7
dichloromethane/hexane =2/1) to deliver a red solid (75 mg, Result and discussion
1
4.3%). H NMR (400 MHz, CDCl
3
): δ 8.72 (s, 8H), 8.11 (s, 8H),
Design and Synthesis of Compounds
.65 (m, 11H), 1.47 (s, 12H). MALDI-TOF: m/z: calcd: 845.12
M] ; found: 845.32 [M] .
PdPor-2-DPA:
A mixture of 5-(4-boronic acid pinacol ester phenyl)-10,15,20-
triphenylporphrinato palladium(II) (42.2 mg, 0.05 mmol), 2-
bromo-9,10-diphenyl-anthracene (24.56 mg, 0.06 mmol),
TET process includes a triplet state energy donor and a triplet
state energy acceptor.20 In this work, we choose 9,10-
diphenylanthracene (DPA) as the triplet state energy acceptor
+
+
[
4
because of its high fluorescence quantum yield. Furthermore,
5
,10,15,20-tetraphenylporphyrin (PdTPP) is used as the triplet
energy donor due to its easy synthesis and molecular
modification, near unity intersystem crossing efficiency (ISC)
and proper triplet state energy (1.78 eV) with respect to that
of DPA (1.77 eV).21,22 PdTPP was attached to the 2- or 9-
position of DPA (Fig. 1, PdPor-2-DPA and PdPor-9-DPA). By
3 4 2 3
Pd(PPh ) (5.78 mg, 5 μmol), K CO (0.5 mL, 138 mg, 1.0 mmol),
and nitrogen-saturated toluene/ethanol (15 mL/7.5 mL) were
o
mixed in a flask. The reaction mixture was heated at 80 C for 4
h under nitrogen. After it was cooled to room temperature,
the reaction mixture was quenched with distilled water. The
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changing connecting position and the torsion angle, orbital
overlap and electronic coupling between the donor and
acceptor are changed. As a result, the iTET rate and efficiency
will be altered.
View Article Online
DOI: 10.1039/C9TC06337D
The synthetic route is shown in Scheme 1. 5-(4-bromophenyl)-
1
0,15,20-triphenylporphrinato palladium(II) was synthesized
following the standard method of Alder and Longoby.16 Then
-(4-boronic acid pinacol ester phenyl)-10,15,20-
5
triphenylporphrinato palladium(II) was prepared according to
the previous report.17,18 At last, under the Suzuki coupling
protocol,23 5-(4-boronic acid pinacol ester phenyl)-10,15,20-
triphenylporphrinato palladium(II) reacted with 2-bromo-9,10-
diphenyl-anthracene or 9-bromo-10-phenyl-anthracene to get
PdPor-2-DPA or PdPor-9-DPA. These two compounds are
highly soluble in conventional organic solvents, which can
effectively avoid molecular aggregation in solution and allow
us to study the single molecule photophysical properties of
them.
Fig. 2 The minimized molecular structures of PdPor-2-DPA (A) and PdPor-9-
DPA (B). α and α are the dihedral angle between the porphyrin unit and
DPA unit.
1
2
To study iTET process from PdTPP to DPA unit in these two
dyads, we selectively excite the PdTPP part of PdPor-2-DPA or
PdPor-9-DPA with a 532 nm excitation light. As shown in Fig.
3
B, two main emission bands located at 697 and 772 nm are
Minimized molecular structure
observed for both PdPor-2-DPA and PdPor-9-DPA in deaerated
toluene, which can be ascribed to the phosphorescence of
PdTPP unit following the literature.21 This assignment is also
supported by the significant quenching of these two bands in
The minimized molecular structures of PdPor-2-DPA and
PdPor-9-DPA are calculated at the B3LYP/GENECP/LANL2DZ
level using Gaussian 09W and shown in Fig. 2.24,25 As shown in
aerated toluene (Fig. S4, ESI
†
). Moreover,
a
weak
Fig. 2, the center-to-center distances between DPA and PdPor
in PdPor-2-DPA and PdPor-9-DPA are 12.92 Å and 10.72 Å,
respectively. The different connection position did not
significantly change the center-to-center distance between
DPA and PdPor unit in these two compounds. However, the
dihedral angle between the DPA ring and porphyrin ring in
these two compounds is obviously different. The DPA ring is
nearly perpendicular to the porphyrin ring with a dihedral
angle of 74.47° in PdPor-2-DPA. Instead, the dihedral angle in
PdPor-9-DPA is only 29.66° (side view). It has been widely
recognized that a small interplanar dihedral angle between
luminescence band centered at 610 nm is also detected for
PdTPP, PdPor-2-DPA and PdPor-9-DPA in deaerated toluene.
This band remains unchanged in aerated toluene, which
indicates that this band is the fluorescence of PdTPP.21 Notably,
the phosphorescence quantum yields of PdTPP (0.41±0.05%),
PdPor-2-DPA (0.26±0.03%) and PdPor-9-DPA (0.23±0.03%) are
extremely low (Table S1, ESI†), which indicates that the decay
of the triplet excited state of these three compounds is mainly
caused by the nonradiative transition, but not the radiative
phosphorescence emission. Interestingly, the intensities of
phosphorescence bands centered at 697 and 772 nm of PdPor-
2-DPA and PdPor-9-DPA are obviously decreased compared to
the intensity of phosphorescence band of PdTPP in the same
adjacent units will bring a large molecular orbital overlap and
electronic coupling.26,27 In other words, the molecular orbital
overlap between DPA and PdPor unit in PdPor-9-DPA is larger
than that in PdPor-2-DPA.
concentration
phosphorescence quantum yields of PdPor-2-DPA and PdPor-
-DPA decrease a lot in comparison with that of PdTPP. This
(1.0×10-5
M).
Furthermore,
the
UV-vis absorption and luminescence emission spectra
9
The UV−vis absorption spectra of PdPor-2-DPA, PdPor-9-DPA
and their monomeric model compounds (PdTPP and DPA)
were measured and shown in Fig. 3A. As shown in Fig. 3A, two
main absorption bands centered at 417 and 524 nm are
observed in the absorption spectra of PdPor-2-DPA and PdPor-
may be attributed to the increased nonradiative transition
caused by the more flexible molecular structure after linking of
DPA.31,32,33 In addition, the phosphorescence intensity of
PdPor-9-DPA decreases slightly compared to that of PdPor-2-
DPA, as well as the decreased phosphorescence quantum yield
of PdPor-9-DPA, which may be caused by the extra iTET
process from PdTPP unit to DPA unit in PdPor-9-DPA.29,30 In
addition, the phosphorescence quantum yield of PdTPP
measured by us is identical to the previous report,21 which
suggests that our measurement is reliable.
9
-DPA, which match well with that of PdTPP and can be
assigned to the Soret band and B band of palladium porphyrin,
respectively.28 Besides those bands, three weak absorption
bands located at 338, 356 and 374 nm can be identified in the
enlarged spectra (the inset of Fig. 3A), which can be attributed
to the absorption of DPA.22 More importantly, the absorption
spectrum of PdPor-2-DPA or PdPor-9-DPA is nearly identical to
the sum of the absorption spectra of PdTPP and DPA (Fig. S3,
Furthermore, the phosphorescence dynamics of PdPor-2-
DPA and PdPor-9-DPA at 293 K (Fig. 3C) were also determined
and the fitting results were shown in Table S1 (ESI†). We can
ESI†), suggesting no electronic coupling between PdTPP and
see that the phosphorescence of PdPor-2-DPA and PdPor-9-
DPA at the ground state.
4
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DOI: 10.1039/C9TC06337D
Fig. 3 (A) Absorption spectra of PdPor-2-DPA, PdPor-9-DPA, PdTPP and DPA in toluene. (B, C) Phosphorescence spectra (B) and dynamics (C) of PdPor-2-DPA, PdPor-
-DPA and PdTPP in deaerated toluene (λex = 532 nm, c = 1.0×10-5 M, 293K).
9
DPA decay faster than that of PdTPP, which agrees with the 480 nm shows a fast decay (<100 ps). Nevertheless, no distinct
decrease on phosphorescence intensity as shown in Fig. 3B.
decay is detected in the dynamics of PdPor-2-DPA and PdTPP
probed at 417 and 480 nm. This suggests the presence of extra
process that quenches the triplet state of PdTPP unit in PdPor-
Femtosecond transient absorption spectra
In order to reveal the iTET process within PdPor-2-DPA and
PdPor-9-DPA, femtosecond transient absorption (fs-TA)
spectra of these two compounds were measured in deaerated
toluene with the excitation at 532 nm (150 nJ) (Fig. 4). Only
PdTPP unit in these two dyads can be excited with the 532 nm
excitation. The photo-induced bleach (PIB) bands were
observed at 417 and 525 nm in the fs-TA spectra of both
9
-DPA, but not in PdPor-2-DPA. Based on the previous
29,30,36
report,
TET process can be proceeded from PdTPP to
DPA. So the extra process in PdPor-9-DPA should be iTET from
PdTPP unit to DPA unit. Unfortunately, we cannot distinguish
the signal of the triplet state of DPA unit due to the spectral
overlap between the triplet state of PdTPP and DPA.
Furthermore, due to the molar extinction coefficient of GSB
and ESA bands of PdTPP are much larger than those of the
35,37
PdPor-2-DPA
and
PdPor-9-DPA
immediately
after
photoexcitation, which is close to the ground-state absorption
peaks of PdTPP unit (Fig. 3A). So these two PIB bands can be
assigned to the ground state bleach (GSB) of PdTPP unit.
Moreover, a broad photoinduced absorption (PIA) band
ranged from 430 nm to 520 nm was found and can be
attributed to the excited-state absorption (ESA) of singlet state
3
4
of PdTPP derivatives. With the delay of the time, the
intensity of the ESA band centered at 450 nm decreased
slightly, meanwhile the intensity of the ESA band located at
4
80 nm increased in the fs-TA spectra of these two dyads.
From the single-wavelength dynamics of 450 and 480 nm (Fig.
C-D), we can see that this process is completed in 10 ps.
4
Moreover, this conversion can also be seen in PdTPP (Fig. S5,
ESI†)) and the lifetime of the ESA band centered at 480 nm can
reach to microsecond range (Fig. S6, ESI†). This ESA band with
long lifetime should be assigned to the triplet state of PdTPP
3
4,35
according to the previous report.
should be intersystem crossing (ISC).
So this fast process
After ISC, a significant difference can be seen in the fs-TA
spectra of these two dyads. The intensity of the GSB band at
4
17 nm and the ESA band at 480 nm in PdPor-9-DPA have a
huge decrease after 10 ps (Fig. 4B). However, the TA spectrum
of PdPor-2-DPA keeps almost unchanged and no obvious
decay is observed after 10 ps (Fig. 4A), which is similar to that
of PdTPP (Fig. S5A, ESI†). This huge discrepancy can also be
clearly seen from the single-wavelength dynamics probed at
Fig. 4 (A, B) fs-TA spectra of PdPor-2-DPA (A) and PdPor-9-DPA (B). (C, D)
Single-wavelength dynamics probed at 450 and 480 nm of PdPor-2-DPA (C)
and PdPor-9-DPA (D). (E, F) Normalized single-wavelength dynamics of
4
4
17 and 480 nm of PdPor-9-DPA, PdPor-2-DPA and PdTPP (Fig.
E and 4F). The dynamics of PdPor-9-DPA probed at 417 and
PdPor-2-DPA and PdPor-9-DPA probed at 417 nm (E) and 480 nm (F). λex
32 nm, c = 2.0×10-5 M in deaerated toluene, 293K.
=
5
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Journal Name
View Article Online
DOI: 10.1039/C9TC06337D
Fig. 5 Species-associated spectra and population dynamics of PdPor-2-DPA (A, D), PdPor-9-DPA (B, E) and PdTPP (C, F) obtained from global analysis.
signals of DPA unit, the GSB and ESA peaks of DPA cannot be the fs-TA spectra of PdPor-9-DPA. As for PdPor-2-DPA and
identfied.3
5,37
This fast iTET process would quench the PdTPP,
phosphorescence of PdTPP unit in PdPor-9-DPA, which is DPA)→T
sequential
(PdPor *-DPA) and S
two-species
model
(S
(PdPor *-
1
1
3
1
3
1
1
(PdTPP *)→T
1
(PdTPP *)) was
consistent with the reduced lifetimes and quantum yield employed. The obtained spectra and dynamics of each
compared to those of PdTPP (Fig. 3B). However, due to the population were shown in Fig. 5. The fitting result agrees very
absence of iTET process in PdPor-2-DPA, the quenching of the well with the raw data (Fig. S7-S9, ESI†), which confirms our
phosphorescence of PdTPP unit in PdPor-2-DPA should be deconvolution method is reliable. As shown in Fig. 5, the
mainly caused by the flexible molecular structure after linking spectral signature of the first state in these three compounds
1
DPA, which results in the increasing on non-radiation resembles the typical singlet (S ) state absorption of PdPor,
transitions.3
1-33
Notably, the GSB band of PdTPP unit centered with a board absorption band at 430 nm to 520 nm.35,37 So we
at 417 nm show certain intensity after a very long time delay ascribed the first state to the S
1
state of PdPor unit. The
(5 ns), even after the end of iTET process in PdPor-9-DPA (Fig. second state in these three compounds shows an enhanced
4
1
B), suggesting that a triplet-state equilibrium between PdTPP absorption band at 480 nm compared with that of the S state.
1
unit and DPA unit may be established. This is reasonable This change is ascribed to the formation of the triplet state (T )
3
7
because of the similar triplet energy between PdTPP and DPA of PdPor. The spectrum of the third component in PdPor-9-
2
1,22
(
1.78 eV for PdTPP and 1.77 eV for DPA).
the minimized molecular structure, the orbital overlap in that of the second state (Fig. 5B), which may be caused by the
PdPor-9-DPA is larger than that in PdPor-2-DPA. In addition, state of DPA with a main absorption band at about 450
As revealed by DPA has an enhanced absorption at 450 nm with respect to
T
1
3
5
the 9-position of anthracene is more active to exchange charge nm. But the GSB of the PdPor unit is also resolved and so we
compared to 2-position anthracene. Both these two factors ascribe the third state to the triplet equilibrium state between
lead to a stronger electronic coupling in PdPor-9-DPA, which is PdPor unit and DPA unit.
1
0,11
3
8,39,40
necessary for efficient Dexter-based TET process.
As a
In addition, from the population dynamics of these three
1
result, PdPor-9-DPA shows an efficient iTET process instead of compounds, we can see that the S state decays rapidly with a
PdPor-2-DPA.4
1,42,40
This is similar with the observation by Tang time constant of about ~ 10 ps (Fig. 5D-5F), accompanied by a
1
group in which TET process between CdSe nanocrystal and 9- rise of the T state of PdPor unit. This is consistent with the ISC
3
7
position substituted anthracene is more efficient in rate of PdTPP in the previous report, suggesting the reliability
comparison with those in 1-position and 2-position of our deconvolution method. As for PdPor-9-DPA, the T
1
state
anthracene.15
of DPA unit is generated within 36.05±0.11 ps from the T
1
Next, we used global analysis to decompose the fs-TA data state of PdPor unit (Fig. 5E).
of PdPor-2-DPA, PdPor-9-DPA and PdTPP with the help of
Triplet-triplet annihilation upconversion (TTA-UC)
Glotaran software.43 Based on the above discussion, a
1
3
sequential three-species model (S
1
(PdPor *-DPA)→T
1
(PdPor *-
3
1
DPA)→T (PdPor-DPA *)) was adopted in the deconvolution of
6
| J. Name., 2012, 00, 1-3
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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
Journal Name
ARTICLE
View Article Online
DOI: 10.1039/C9TC06337D
Fig. 6 Dependence of luminescence spectra on the concentration of DPA at a constant PdPor-2-DPA (A), PdPor-9-DPA (B) or PdTPP (C) concentration (10-5 M) in
toluene with 532 nm laser excitation (25 mW).
We first measured the emission spectra of PdPor-2-DPA or more efficient compared to those from the PdPor unit in
PdPor-9-DPA without adding free DPA molecules (Fig. S10, PdPor-2-DPA or PdPor-9-DPA to DPA. As we known, the
ESI†), but no DPA fluorescence was observed no matter what Dexter-based intermolecular TET is diffusion controlled
concentration of PdPor-2-DPA or PdPor-9-DPA was used. This process46,47 and the full process can be formally described as a
phenomenon is also observed in other covalently linked direct collision between a triplet donor and an acceptor. In
photosensitizer-acceptor systems in which the ultrafast such a case, the TET rate (KTET) is proportional to diffusion
4
7
intramolecular re-absorption of photosensitizer to the coefficient (D) according to the previous report (KTET
=
acceptor emission may lead to the disappearance of the 4DR ). Furthermore, diffusion coefficient (D) is inversely
0 A
C
9
,11
47
m m
upconversion emission. As for PdPor-2-DPA or PdPor-9-DPA, proportional to the molecular radii (R ) (D=3kT/6πηR ). Thus,
we can see that the fluorescence spectrum of DPA overlaps PdTPP with smaller radii should have a faster and more
significantly with the absorption spectrum of PdTPP (Fig. S11, efficient TET with respect to those of PdPor-2-DPA and PdPor-
ESI†). So the reason for no DPA emission in PdPor-2-DPA and 9-DPA. However, the TET efficiency in PdPor-2-DPA and PdPor-
PdPor-9-DPA may be also the intramolecular re-absorption of 9-DPA should be the same if only above factors are considered.
PdTPP to the DPA emission.
Surprisingly, the Stern–Volmer quenching constant of PdPor-2-
According to the previous report,9,44,45 this re-absorption DPA is much larger than that of PdPor-9-DPA. This is because
effect can be suppressed effectively by changing that the diffusion controlled intermolecular TET from the
intramolecular process into intermolecular process. So we PdPor unit in PdPor-9-DPA to free DPA cannot compete with
added extra free DPA molecules into PdPor-2-DPA or PdPor-9- the fast iTET process (36.05±0.11 ps) in PdPor-9-DPA. So the
DPA solution and observed an emission band ranged from 400 phosphorescence quenching of PdPor unit in PdPor-9-DPA is
nm to 500 nm when a 532 nm laser was used as excitation mainly due to the iTET process. Instead, the intermolecular TET
light (Fig. 6). However, this emission was not detected when process is the main quenching pathway for the
only DPA was presented in the solution with the same phosphorescence of PdPor unit in PdPor-2-DPA due to the
excitation wavelength (Fig. S11, ESI†). Furthermore, the shape absence of iTET process. This leads to a smaller Stern–Volmer
of the emission spectrum is consistent with that of DPA after quenching constant for PdPor-9-DPA with respect to PdPor-2-
subtracting the strong absorption of Soret band (417 nm) of DPA.
2
2
PdTPP (Fig. S11, ESI†). These results indicate that the
emission from 400 nm to 500 nm is originated from TTA-UC.
As shown in Fig. 6, the intensity of the UC fluorescence is
enhanced as the concentration of DPA increases, meanwhile
the phosphorescence of PdTPP unit decreases. This suggests
the existence of an efficient intermolecular TET from PdPor-2-
DPA or PdPor-9-DPA or PdTPP to DPA. In order to
quantitatively study the TET process, the phosphorescence
intensity of PdPor-2-DPA or PdPor-9-DPA or PdTPP was
measured as a function of the concentration of DPA at a
constant PdPor-2-DPA or PdPor-9-DPA or PdTPP concentration
1
2
Fig. 7 (A) Stern-Volmer plots for phosphorescence quenching of the PdPor
unit in the mixture of PdPor-2-DPA, PdPor-9-DPA or PdTPP and DPA. (B)
and then the Stern-Volmer curves were obtained (Fig. 7A).
The Stern–Volmer quenching constant of PdTPP (KSV= 1.26×10
5
Dependence of the upconversion quantum yield (ΦUC
) on the
-1
4
-1
M ) is 1.5 times that of PdPor-2-DPA (KSV= 8.26×10 M ) and
concentration of DPA at a constant PdPor-2-DPA, PdPor-9-DPA or PdTPP
concentration (10-5 M) in toluene with 532 nm laser excitation (25 mW).
4
-1
nearly three times that of PdPor-9-DPA (KSV= 4.67×10 M ),
which indicates that the TET process from PdTPP to DPA is
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J Po lue ran sael od fo Mn aot te rai ad l jsu Cs ht emm ai rs gt ri yn sC
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ARTICLE
Next, the upconversion quantum yields (ΦUC
Journal Name
were to the stronger electronic coupling between DPVAiewanArdticlePOdnPlioner
)
calculated and shown in Fig. 7B. The highest ΦUC values of units within PdPor-9-DPA. When these tDwOoI: d10y.a10d3s9/aCr9eTCu0se63d37aDs
PdPor-2-DPA, PdPor-9-DPA and PdTPP are 10.48%, 23.59% and triplet energy donor in TTA-UC for sensitizing free DPA, the
2
3.56%, respectively. The highest ΦUC value of PdTPP is more maximum upconversion efficiency of PdPor-9-DPA (23.59%) is
than two times larger than that of PdPor-2-DPA, which is about two times larger than that of PdPor-2-DPA (10.48%).
consistent with the larger KSV value of PdTPP compared with This is most likely due to the extra triplet-triplet annihilation
that of PdPor-2-DPA. However, the highest ΦUC value of PdPor- pathway existing in PdPor-9-DPA by the collision between the
3
3
9
-DPA is much larger than that of PdPor-2-DPA even though PdPor-9-DPA * and the free DPA * except for the collision
3
PdPor-9-DPA has a smaller KSV. When PdPor-2-DPA was used between the two free DPA * as like in PdPor-2-DPA. These
as donor, the upconversion fluorescence can only be results confirm that the iTET process in covalently linked D-A
generated by the collision between two free DPA triplets systems can be controlled by changing the relative orientation
3
3
(
DPA *) intermolecularly sensitized by PdPor unit (PdPor *-2- and linked position between the donor and acceptor. This
DPA) of PdPor-2-DPA. However, in PdPor-9-DPA, the provides new insights into designing D-A systems with highly
upconversion fluorescence can be produced not only by the efficient iTET as candidates for triplet donor in TTA-UC. In
3
collision between two free DPA * intermolecularly sensitized addition, intramolecular TET in D-A dyads is more rapid and
3
by PdPor unit (PdPor *-9-DPA) of PdPor-9-DPA, but also by the more efficient in comparison with intermolecular TET so that it
3
collision between the DPA triplet (PdPor-9-DPA *) of PdPor-9- can effectively compete and inhibit the radiative and
3
DPA and the free DPA * intermolecularly sensitized by nonradiative decays of the triplet excited states of donor
3
10
PdPor *-9-DPA. Therefore, the upconversion efficiency of moiety. However, due to the increased molecular dimension
PdPor-9-DPA is higher than that of PdPor-2-DPA. If the collision when connecting the donor to the acceptor, the diffusion rate
3
3
between PdPor-9-DPA * and the free DPA * is not efficient, of the D-A dyad will be inevitably reduced in solution.
TTA-UC efficiency of PdPor-2-DPA should be close to that of Therefore, the advantage of intramolecular TET in TTA-UC
PdPor-9-DPA due to the same diffusion rate induced by their might be demonstrated more clearly in energy migration
same dimension and weight. In addition, it is notable that the mechanism in solid state instead of the molecular diffusion
TTA-UC efficiency of PdPor-9-DPA is lower than that of PdTPP mechanism in solution.
-
5
-4
in low concentration of DPA (10 M-10 M) while close to that
-4
of PdTPP at high DPA concentration (>2×10 M). Due to the
presence of the triplet state equilibrium between the PdPor
unit and the DPA unit, PdPor *-9-DPA and PdPor-9-DPA * can
both be formed in PdPor-9-DPA after photoexcitation. At low
Conflicts of interest
There are no conflicts to declare.
3
3
DPA concentration, the TTA-UC efficiency is determined by the
3
diffusion controlled TET from PdPor *-9-DPA to free DPA Acknowledgements
3
3
and/or the collision between PdPor-9-DPA * and free DPA *.
Financial support from the National Natural Science
Foundation of China (21703287), the Natural Science
3
3
However, no matter PdPor *-9-DPA or PdPor-9-DPA *, its
diffusion rate is slower significantly than that of PdTPP due to
the larger molecular dimension and weight of PdPor-9-DPA.
Therefore, PdPor-9-DPA shows a lower TTA-UC efficiency with
respect to that of PdTPP at low DPA concentration. When the
DPA concentration increases significantly, the surrounding of
Foundation
of
Shandong
Province
(ZR2017MB006,
ZR2017BB027), the Major Program of Shandong Province
Natural Science Foundation (ZR2017ZB0315), the Fundamental
Research Funds for the Central Universities (18CX02052A,
1
9CX05002A) and Post-graduate innovation project of China
3
3
the sensitizer (PdPor *-9-DPA or PdTPP *) is filled with many
DPA molecules, the diffusion controlled TET is no longer the
rate-limiting step for the TTA-UC process. So PdPor-9-DPA and
PdTPP exhibit a similar TTA-UC efficiency. In other words, the
diffusion controlled intermolecular TET plays an important role
in TTA-UC process at low acceptor concentration. However,
this diffusion limiting effect can be abated at the region of high
acceptor concentration. Similar phenomenon was also found
university of Petroluem (East China) (YCX2019094) are
acknowledged. X. Li thanks Taishan Scholar program of
Shandong Province (ts201712019) for financial support.
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Journal of Materials Chemistry C
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DOI: 10.1039/C9TC06337D
Table of Contents:
Enhancing triplet sensitization ability of donor-acceptor dyads via
intramolecular triplet energy transfer
a
a
c,*
a
a
b,*
Shanshan Liu , Xiangyang Wang , Heyuan Liu , Li Shen , Dezhi Zhao , and Xiyou Li
Efficient intramolecular triplet-triplet energy transfer is beneficial for improving upconversion efficiency when the donor-
acceptor system is adopted as triplet donor.
1