Pd–Porphyrin Oligomers Sensitized for Green-to-Blue Photon Upconversion: The More the Better?

Article abstract of DOI:10.1002/chem.201504498

A series of directly meso-meso-linked Pd–porphyrin oligomers (PdDTP-M, PdDTP-D, and PdDTP-T) have been prepared. The absorption region and the light-harvesting ability of the Pd–porphyrin oligomers are broadened and enhanced by increasing the number of Pd–porphyrin units. Triplet–triplet annihilation upconversion (TTA-UC) systems were constructed by utilizing the Pd–porphyrin oligomers as the sensitizer and 9,10-diphenylanthracene (DPA) as the acceptor in deaerated toluene and green-to-blue photon upconversion was observed upon excitation with a 532 nm laser. The triplet–triplet annihilation upconversion quantum efficiencies were found to be 6.2 %, 10.5 %, and 1.6 % for the [PdDTP-M]/DPA, [PdDTP-D]/DPA, and [PdDTP-T]/DPA systems, respectively, under an excitation power density of 500 mW cm^?2. The photophysical processes of the TTA-UC systems have been investigated in detail. The higher triplet–triplet annihilation upconversion quantum efficiency observed in the [PdDTP-D]/DPA system can be rationalized by the enhanced light-harvesting ability of PdDTP-D at 532 nm. Under the same experimental conditions, the [PdDTP-D]/DPA system produces more³DPA* than the other two TTA-UC systems, benefiting the triplet–triplet annihilation process. This work provides a useful way to develop efficient TTA-UC systems with broad spectral response by using Pd–porphyrin oligomers as sensitizers.

Full text of DOI:10.1002/chem.201504498

DOI: 10.1002/chem.201504498
Full Paper
&
Photosensitizers
Pd–Porphyrin Oligomers Sensitized for Green-to-Blue Photon
Upconversion: The More the Better?
Zhiqing Xun,[a] Yi Zeng,*[a] Jinping Chen,[a] Tianjun Yu,[a] Xiaohui Zhang,[a] Guoqiang Yang,[b]
and Yi Li*[a]
Abstract: A series of directly meso-meso-linked Pd–porphyrin
oligomers (PdDTP-M, PdDTP-D, and PdDTP-T) have been pre-
pared. The absorption region and the light-harvesting ability
of the Pd–porphyrin oligomers are broadened and enhanced
by increasing the number of Pd–porphyrin units. Triplet–trip-
let annihilation upconversion (TTA-UC) systems were con-
structed by utilizing the Pd–porphyrin oligomers as the sen-
sitizer and 9,10-diphenylanthracene (DPA) as the acceptor in
deaerated toluene and green-to-blue photon upconversion
was observed upon excitation with a 532 nm laser. The trip-
let–triplet annihilation upconversion quantum efficiencies
were found to be 6.2%, 10.5%, and 1.6% for the [PdDTP-M]/
DPA, [PdDTP-D]/DPA, and [PdDTP-T]/DPA systems, respec-
tively, under an excitation power density of 500 mWcmÀ2.
The photophysical processes of the TTA-UC systems have
been investigated in detail. The higher triplet–triplet annihi-
lation upconversion quantum efficiency observed in the
[PdDTP-D]/DPA system can be rationalized by the enhanced
light-harvesting ability of PdDTP-D at 532 nm. Under the
same experimental conditions, the [PdDTP-D]/DPA system
3
produces more DPA* than the other two TTA-UC systems,
benefiting the triplet–triplet annihilation process. This work
provides a useful way to develop efficient TTA-UC systems
with broad spectral response by using Pd–porphyrin oligo-
mers as sensitizers.
Introduction
from the excited singlet acceptor gives delayed fluorescence,
achieving photon upconversion. However, given that the tran-
sition from ground state to triplet state is spin forbidden, ab-
sorption coefficients are too small to generate the triplet state
of the acceptor by direct excitation. The triplet state of accept-
or is obtained through photosensitization, an indirect method
of excitation, by electronic energy transfer from triplet sensitiz-
ers, which absorb low energy photons. Therefore, the TTA-UC
efficiency is affected not only by the TTA processes and fluo-
rescence quantum yield of acceptors, but also by the triplet
production and the triplet–triplet energy transfer, which are
deeply dependent on the properties of the triplet sensitizers,
such as the light-harvesting ability, the efficiency of intersystem
crossing, and the lifetime of the triplet states.
Photon upconversion, which refers to converting photons of
lower energy to those of higher energy, has attracted much at-
tention because of its potential applications in photovolta-
ics,[1,2] bioimaging,[3,4] displays,[5,6] and photodynamic therapy
(PDT).[7,8] Triplet–triplet annihilation upconversion (TTA-UC) has
been at the forefront of upconversion in recent years,[9,10] be-
cause TTA-UC can take place upon excitation with low-intensi-
ty, noncoherent light sources and the upconversion emission
wavelengths can be simply tuned by molecular tailoring. TTA-
UC involves a bimolecular system consisting of sensitizers and
acceptors. Acceptors, also known as emitters, give upconvert-
ed and delayed fluorescence through triplet–triplet annihila-
tion, in which two triplet acceptors diffuse and meet within
their lifetime, and this effective encounter produces a ground-
state singlet and an excited singlet. The radiative transition
Many contributions to TTA-UC systems are made through
the modification of the sensitizers,[11,12] including increasing the
light-harvesting ability, the intersystem crossing efficiency, and
the lifetime of the triplet states. PtII, IrIII, RuII, and ReII complexes
are commonly used triplet sensitizers because the intersystem
crossing efficiencies of these complexes are nearly 1. However,
their absorptions are in the relatively short wavelength region
with low molar absorption coefficients (e) and the lifetimes of
their triplet excited states are not very long (usually a few mi-
croseconds or shorter), which limit the application of these
sensitizers in TTA-UC systems. A strategy for improving the
photophysical characteristics of these metal complex sensitiz-
ers has been reported that introduces conjugated chromo-
phores such as BODIPY, perylenediimide, and coumarin to the
complexation ligands. The triplet states of these sensitizers
[a] Dr. Z. Xun, Dr. Y. Zeng, Dr. J. Chen, Dr. T. Yu, Dr. X. Zhang, Prof. Dr. Y. Li
Key Laboratory of Photochemical Conversion and Optoelectronic Materials
Technical Institute of Physics and Chemistry, Chinese Academy of Sciences
University of Chinese Academy of Sciences, Beijing 100190 (P.R. China)
E-mail: zengyi@mail.ipc.ac.cn
yili@mail.ipc.ac.cn
[b] Prof. Dr. G. Yang
Beijing National Laboratory for Molecular Sciences (BNLMS)
Key Laboratory of Photochemistry
Institute of Chemistry, Chinese Academy of Sciences
University of Chinese Academy of Sciences, Beijing 100190 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504498.
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thus change from the metal-to-ligand charge-transfer (3MLCT)
excited states to the ligand-localized/intraligand (3IL) excited
states, resulting in prolonged lifetimes of the triplet state. Fur-
thermore, the absorptions of these complexes were extended
into the visible region with high molar extinction coefficients.
Modification of these complexes used in TTA-UC systems as
sensitizers effectively enhances the upconversion efficien-
cies.[13–16]
Porphyrin complexes with heavy metal complexation or
heavy atom substitution are another popular type of triplet
sensitizer used in TTA-UC systems.[17–22] The Q band absorp-
tions of porphyrin complexes locate in the green region and
the lifetimes of their triplet states can last tens to hundreds of
microseconds, which benefit the TTA-UC process. Furthermore,
porphyrinoids with strong and broad absorption have also
been applied to improve the TTA-UC performance.[23–26] Devel-
oping and improving TTA-UC systems based on porphyrin
complexes with broad absorption bands of long wavelength
and intense absorption coefficients is an attractive focus. Re-
searchers have developed a series of meso-meso-linked por-
phyrin complexes, which exhibit broad and enhanced absorp-
tion bands arising from the excitonic interactions between
constituent porphyrin units.[27–33] It is anticipated that promis-
ing triplet–triplet annihilation upconversion systems can be de-
veloped by utilizing meso-meso-linked porphyrin complexes as
sensitizers.
Figure 1. Structures of the Pd–porphyrin oligomers.
Figure 2. (a) Absorption and (b) emission spectra of the Pd–porphyrin oligo-
mers and DPA in deaerated toluene. The emission spectra of the Pd–porphy-
rin oligomers are normalized to the specific absorbance of the sample at the
excitation wavelength (532 nm).
M exhibits a strong absorption band at 406 nm and two weak
ones at 515 and 546 nm, which are assigned to the Soret band
and the Q bands, respectively. With increasing the number of
porphyrin units, the Soret bands of PdDTP-D and PdDTP-T ex-
hibit splitting as a result of exciton coupling between the adja-
cent porphyrin units,[27] and the Q bands are enhanced and
broadened along with a slight bathochromic shift. Conse-
quently, the light-harvesting ability and the excitation region
of the TTA-UC systems using PdDTP-D or PdDTP-T as photosen-
sitizers will be enhanced and broadened in comparison to that
using PdDTP-M. The molar extinction coefficients at 532 nm
are determined to be 6.62”103, 5.26”104, and 5.91”
104 mÀ1 cmÀ1 for PdDTP-M, PdDTP-D, and PdDTP-T, respectively.
All the absorption bands of DPA lie below 420 nm, indicating
that the Pd–porphyrin oligomers can be selectively excited by
their Q band in the TTA-UC systems.
In the present work, a series of meso-meso directly linked
Pd–porphyrin oligomers with broadened and enhanced ab-
sorption bands, that is, the Pd–porphyrin monomer, dimer, and
trimer complexes (PdDTP-M, PdDTP-D, and PdDTP-T), were
synthesized. TTA-UC systems were successfully constructed by
using the Pd–porphyrin oligomers and 9,10-diphenylanthra-
cene (DPA) as the sensitizer and the acceptor, respectively, and
the best TTA-UC efficiency was observed in the [PdDTP-D]/DPA
system. Thorough photophysical studies indicate that the en-
hance absorption at the excitation wavelength can evidently
improve the TTA-UC efficiencies.
Results and Discussion
Synthesis and characterization of the Pd–porphyrin oligo-
mers
The emission spectra of the Pd–porphyrin oligomers and
DPA in deaerated toluene are shown in Figure 2b (non-normal-
ized emission spectra shown in Figure S10 in the Supporting
Information for comparison). Dual weak fluorescence bands
with maxima at approximately 570 and 620 nm and dual in-
tense phosphorescence bands with maximum at approximate-
ly 680 nm and a shoulder at approximately 740 nm were de-
tected for all three Pd–porphyrin oligomers upon excitation of
the Q band with 532 nm light. The phosphorescence quantum
yields of PdDTP-M, PdDTP-D, and PdDTP-T were measured to
be 1.1%, 1.4%, and 0.98%, respectively, by using PdII octae-
thylporphyrin (FP =0.022 in acetone) as the standard. The pa-
rameters of the absorption spectra and the emission spectra
are summarized in Table 1. The fluorescence of DPA is located
at a higher energy region in comparison to the emission of the
Pd–porphyrin oligomers and the fluorescence quantum yield
The meso-meso-linked Pd–porphyrin oligomers PdDTP-M,
PdDTP-D, and PdDTP-T were synthesized by replacing the
metal of zinc(II) 5,15-di(3,5-di-tert-butylphenyl)porphyrin oligo-
mers with palladium(II) acetate. The precursor zinc(II) 5,15-
di(3,5-di-tert-butylphenyl)porphyrin oligomers were synthe-
sized according to literature reports.[29,34] The details of the syn-
thesis and the characterization of the Pd–porphyrin oligomers
are described in the Supporting Information. The structures of
the Pd–porphyrin oligomers are depicted in Figure 1, which
1
were characterized by H NMR and IR spectroscopy as well as
MALDI-TOF mass spectrometry (Figures S1–S3 in the Support-
ing Information).
The absorption spectra of the Pd–porphyrin oligomers and
DPA were measured in deaerated toluene (Figure 2a). PdDTP-
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ments. The phosphorescence Stern–Volmer plots of the Pd–
porphyrin oligomers with DPA are shown in Figure 3b. When
the concentration of DPA is less than 50 mm, the plots are
nearly linear, which is in accordance with the Stern–Volmer
plots measured with the phosphorescence lifetimes of the Pd–
porphyrin oligomers in the presence of different concentra-
tions of DPA. The linear plots below 50 mm DPA indicate that
the dynamic quenching is responsible for the decrease in the
phosphorescence intensity. Upon further increasing the con-
centration of DPA, the phosphorescence intensity continues to
be quenched, but the quenching efficiency decreases. When
the concentration of DPA reaches 100 mm, over 90% of the
phosphorescence of the Pd–porphyrin oligomers have been
quenched by DPA, giving the quenching efficiencies of 0.96,
0.93, and 0.91 for PdDTP-M, PdDTP-D, and PdDTP-T, respective-
ly.
Table 1. Absorption and emission properties of the Pd–porphyrin oligo-
mers in deaerated toluene.
Compound
Absorption
(labs) [nm]
emax
[105 mÀ1 cmÀ1
Emission
(lmax) [nm]
FP
]
[%]
PdDTP-M
PdDTP-D
PdDTP-T
407, 515, 546
2.2, 0.24, 0.08
570, 598, 672,
734
565, 613, 682,
747
574, 625, 686,
759
1.1
417, 440, 527,
563
408, 459, 537,
570
1.8, 2.3, 0.63,
0.18
1.5, 1.8, 0.64,
0.22
1.4
0.98
of DPA in toluene was determined to be 0.85 by reference to
that in cyclohexane (FF =0.95).
Triplet–triplet annihilation upconversion
The TTA-UC systems were constructed in deaerated toluene
by using the Pd–porphyrin oligomers and DPA as the triplet
photosensitizer and the acceptor, respectively. Selective excita-
tion of the Pd–porphyrin oligomers in the TTA-UC systems
([Pd–porphyrin oligomer]=5 mm, [DPA]=50 mm) by using
a green laser (l=532 nm) results in blue emission (inset of Fig-
ure 4a), indicative of the occurrence of the TTA-UC process.
The emission spectra of the TTA-UC systems upon different ex-
The quenching experiments of the triplet excited state of the
Pd–porphyrin oligomers by DPA were carried out in deaerated
toluene; these were performed to ensure that the TTA-UC
system can be constructed from the Pd–porphyrin oligomers
and DPA. As shown in Figure 3a and Figure S4 (in the Support-
ing Information), the phosphorescence of the Pd–porphyrin
Figure 3. (a) The phosphorescence spectra of the PdDTP-D evaluated with
DPA in different concentrations in deaerated toluene; (b) Stern–Volmer plots
of Pd–porphyrin oligomers and DPA in deaerated toluene; [PdDTP-
D]=5 mm, lex =532 nm.
oligomers was clearly quenched by DPA. To classify the reason
for the phosphorescence quenching of the Pd–porphyrin olig-
omers by DPA, the feasibility of the electron transfer process
from the triplet state of the Pd–porphyrin oligomers to DPA as
well as the triplet energy transfer from the Pd–porphyrin oligo-
mers to DPA was evaluated. The free-energy change involved
in an electron transfer process from the triplet state of the Pd–
porphyrin oligomers to DPA were determined to be positive (~
1.36 eV, see the Supporting Information) by calculation with
the Rehm–Weller equation, suggesting that the electron trans-
fer is endothermic and thermodynamically unfavorable. The
triplet energy levels of the Pd–porphyrin oligomers are esti-
mated from their phosphorescence spectra to be 1.82, 1.81,
and 1.79 eV for PdDTP-M, PdDTP-D, and PdDTP-T, respectively,
which are higher than that of DPA (1.77 eV).[35] Thus, the ther-
modynamic triplet–triplet energy transfer from the triplet state
of the Pd–porphyrin oligomers to DPA accounts for the
quenching of phosphorescence of the Pd–porphyrin oligomers,
which is also confirmed by the transient absorption experi-
Figure 4. (a) Photoluminescence spectra at different excitation power. Inset
is the photo obtained from PdDTP-D (left, red emission) and PdDTP-D/DPA
(right, blue emission) upon excitation with a 532 nm laser. (b) Double loga-
rithmic plots of upconversion intensity at 430 nm measured as a function of
power density of a 532 nm incident laser for [PdDTP-D]/DPA in deaerated
toluene. [PdDTP-D]=5 mm, [DPA]=50 mm.
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citation power are presented in Figure 4a and Figure S5 (in the
Supporting Information). With increasing the excitation power
density from 60 to 1800 mWcmÀ2, a rapid enhancement of the
blue upconversion emission with maximum at 430 nm was ob-
served, accompanied by weak phosphorescence with maxi-
mum at 680 nm emitted from the Pd–porphyrin oligomers.
The maxima of the upconversion emission spectra obtained in
the [PdDTP-M]/DPA, [PdDTP-D]/DPA, and [PdDTP-T]/DPA sys-
tems are the same, but the detailed structures of the spectra
show slight difference, which can be ascribed to reabsorption
by the Pd–porphyrin oligomers.
The double logarithmic plots of the upconversion fluores-
cence emanating from DPA at 430 nm as a function of the inci-
dent light power density are shown in Figure 4b and Figure S6
(in the Supporting Information). A slope of near 2.0 is obtained
in the low power density region for all three TTA-UC systems,
which means that the triplet–triplet annihilation process is
much less than the spontaneous decay of the triplet state of
DPA. With increasing the power density on the samples, the
plots incline towards the x-axis and approach a slope of 1 in
the high power density region, indicating that the triplet–trip-
let annihilation reaches a saturated region and is the main
decay process of the triplet of DPA.[36,37] The slope decrease in
the TTA-UC system containing PdDTP-D or PdDTP-T caused by
the possibly intramolecular TTA of the sensitizers is excluded
by the phosphorescence measurements at different laser
power. The dependence of phosphorescence of the porphyrin
dimer or trimer on laser power demonstrates that the power
threshold of TTA within the porphyrin dimer or trimer is about
500 mWcmÀ2 in the absence of DPA. By taking account of the
effective quenching of the sensitizers by large amounts of
DPA, the power threshold will be much higher and the TTA
within the porphyrin dimer and trimer is not likely evident in
the TTA-UC systems.
Figure 5. (a) The concentration-dependent upconversion spectra of DPA
combined with PdDTP-D. (b) Upconversion efficiencies (FUC) as a function of
DPA concentration with the sensitizer at fixed concentration (5 mm) in deaer-
ated toluene (532 nm, 500 mWcmÀ2).
phyrin oligomer]/DPA systems with the concentration of DPA
are depicted in Figure 5b. Under the same experimental condi-
tions, the upconversion quantum yield of [PdDTP-D]/DPA is
higher than those of [PdDTP-M]/DPA and [PdDTP-T]/DPA. Upon
excitation with 532 nm light of 500 mWcmÀ2, the upconver-
sion quantum yields of the [Pd–porphyrin oligomer]/DPA sys-
tems ([Pd–porphyrin oligomer]=5 mm, [DPA]=100 mm) were
determined to be 6.2%, 10.5%, and 1.6% for [PdDTP-M]/DPA,
[PdDTP-D]/DPA, and [PdDTP-T]/DPA, respectively. Considering
solar irradiation intensity (ca. 100 mWcmÀ2) and to allow com-
parisons with other TTA-UC systems, the upconversion quan-
tum yields at a power density of 100 mWcmÀ2 have also been
examined and were found to be 3.4%, 8.3%, and 0.47% for
[PdDTP-M]/DPA, [PdDTP-D]/DPA, and [PdDTP-T]/DPA, respec-
tively. The combination of palladium octaethylporphyrin
(PdOEP) and DPA was used as a TTA performance reference.
The upconversion quantum yields of PdOEP/DPA under the
same conditions were found to be 6.8% and 9.2% upon the
excitation power densities of 100 and 500 mWcmÀ2, respec-
tively. The stability of the TTA-UC systems under irradiation
was also carried out and the results are shown in Figure S11 (in
the Supporting Information). The upconversion emission is
stable for more than 2 h under 100 mWcmÀ2 irradiation, and
a negligible change in the upconversion emission can be ob-
served, indicative of good stability of the TTA-UC systems.
The dependence of the upconversion efficiency on the DPA
concentration was investigated by measuring the emission
spectra of the TTA-UC systems at different concentrations of
DPA upon excitation of the Pd–porphyrin oligomers with
a 532 nm laser of 500 mWcmÀ2. As shown in Figure 5a and
Figure S7 (in the Supporting Information), when the concentra-
tion of DPA increases from 5 to 50 mm, the upconversion fluo-
rescence of DPA is significantly enhanced, and is accompanied
by a decrease in the phosphorescence of the Pd–porphyrin
oligomers. This can be ascribed to the increased chance of the
collision between the Pd–porphyrin oligomers and DPA, which
promotes the triplet–triplet energy transfer from the photosen-
sitizer to the acceptor and produces more triplet states of the
acceptor, favoring the triplet–triplet annihilation process and
then the upconversion fluorescence. When the concentration
of DPA increases from 50 to 100 mm, the increment of the up-
conversion intensity diminishes distinctly and the variation of
the phosphorescence intensity of the Pd–porphyrin oligomers
is small. This indicates that the redundant DPA makes little
contribution to the triplet–triplet energy transfer from the Pd–
porphyrin oligomers to DPA, which is consistent with the re-
sults of the phosphorescence quenching experiment. The
changes in the upconversion quantum yields of the [Pd–por-
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Photophysical processes in triplet–triplet annihilation up-
conversion
The kinetic traces of the triplet of the Pd–porphyrin oligo-
mers in the absence and presence of DPA were obtained by
taking the transient absorption of the Pd–porphyrin oligomers
at the maxima (455, 470, and 485 nm for PdDTP-M, PdDTP-D,
and PdDTP-T, respectively) as a function of time, as shown in
Figure 7 and Figure S9 (in the Supporting Information). In the
The upconversion quantum efficiency (FUC) is related to the in-
tersystem crossing efficiency of the sensitizer (FISC), the triplet–
triplet energy transfer efficiency from the sensitizer to acceptor
(FTTET), the triplet–triplet annihilation efficiency of the acceptor
(FTTA), as well as the fluorescence quantum yield (FF) of the
acceptor. To understand the effect of each parameter on FUC
in the [Pd–porphyrin oligomer]/DPA systems, the photophysi-
cal processes between the Pd–porphyrin oligomers and DPA
were studied by means of transient absorption spectra. The
transient absorption spectra of Pd–porphyrin oligomers were
carried out in deaerated toluene (5 mm) by using 532 nm exci-
tation light and are given in Figure 6a and Figure S8 (in the
Figure 7. The kinetic traces of PdDTP-D (5 mm) in the absence or presence of
DPA (100 mm); the intensity of the signal in the absence of DPA is normal-
ized to that in the presence of DPA, lex =532 nm.
absence of DPA, the kinetic traces can be fitted mono-expo-
nentially, giving the triplet lifetimes of 99.6, 107.3, and 62.8 ms
for PdDTP-M, PdDTP-D, and PdDTP-T, respectively. In the pres-
ence of DPA (100 mm), the kinetic traces can be visually divided
into two parts, a fast decay and a slower one, which can be ra-
tionalized by the overlap of the transient absorption of the
Pd–porphyrin oligomers and DPA. The fast and the slow decay
can be assigned to the triplet state of the Pd–porphyrin oligo-
Figure 6. Transient absorption spectra of PdDTP-D (5 mm) in the absence (a)
and presence (b) of DPA (100 mm) upon excitation with 532 nm light in dea-
erated toluene.
Supporting Information). Three absorption bands with maxima
at 336, 453, and 535 nm are observed in the transient absorp-
tion spectrum of PdDTP-M. The transient absorption spectra of
PdDTP-D and PdDTP-T are similar to that of PdDTP-M and the
maxima of the transient absorption bands of PdDTP-D and
PdDTP-T are 338, 466, 549 nm and 337, 483, 559 nm, respec-
tively. The transient absorption spectra of the Pd–porphyrin
oligomers are similar to those of Pd–porphyrin sensitizers re-
ported before[36] and can be quenched by oxygen and triplet
acceptors. Thus, these absorption bands are assigned to the
transient absorption of the Pd–porphyrin oligomer triplet
states. The negative band that appears in the transient absorp-
tion spectra is assigned to ground-state bleaching according
to the ground-state absorption spectra. The transient absorp-
tion spectra of the Pd–porphyrin oligomers (5 mm) in the pres-
ence of 100 mm DPA were further investigated under the same
conditions (Figure 6b and Figure S8). The transient absorption
of the triplet Pd–porphyrin oligomers in the presence of DPA
decays much faster than that in the absence of DPA and nearly
disappear within 20 ms. A new transient absorption band locat-
ed at 400–500 nm arises simultaneously, which is assigned to
the transient absorption from the lowest triplet state of DPA to
its higher triplets.[35,38] Only the Pd–porphyrin oligomers in the
[Pd–porphyrin oligomer]/DPA systems absorb the light under
the experimental conditions, therefore, the formation of triplet
DPA must be attributed to the triplet–triplet energy transfer
from the Pd–porphyrin oligomers to DPA.
3
mers and DPA*, respectively.[38] The triplet lifetimes of PdDTP-
M, PdDTP-D, and PdDTP-T in the presence of DPA are deter-
mined to be 5.1, 5.9, and 6.4 ms, respectively, by double-expo-
nential fitting of the decay traces. The triplet lifetimes in the
present of DPA are much shorter than those without DPA be-
cause of the triplet–triplet energy transfer from the Pd–por-
phyrin oligomers to DPA. The rate constants and the efficien-
cies of the triplet–triplet energy transfer from the sensitizer to
the acceptor are calculated to be 1.8”105, 1.5”105, 0.9”
105 sÀ1 and 0.95, 0.94, 0.90 for the [PdDTP-M]/DPA, [PdDTP-M]/
DPA, and [PdDTP-T]/DPA systems, respectively. The triplet–trip-
let energy transfer efficiencies obtained from the transient ab-
sorption data are consistent with those estimated from the
phosphorescence quenching. The triplet lifetimes of the Pd–
porphyrin oligomers in the absence and presence of DPA and
the rate constants and efficiencies of the triplet–triplet energy
transfer are summarized in Table 2.
The FUC of the [Pd–porphyrin oligomer]/DPA systems is also
affected by the triplet–triplet annihilation process of 3DPA*.
The triplet–triplet annihilation process of the [Pd–porphyrin oli-
gomer]/DPA system was also analyzed by using the transient
3
absorption spectra. In the TTA-UC systems, DPA* undergoes
decay through two pathways: one is a first-order decay pro-
cess, including a combination of the intrinsic phosphorescence
and the intersystem crossing to the singlet ground state; the
other is the triplet–triplet annihilation decay process. The time
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Table 2. The triplet lifetimes of the Pd–porphyrin oligomers in the ab-
sence and presence of DPA and the energy transfer efficiencies and rate
constants for the [Pd–porphyrin oligomer]/DPA systems.
Table 3. Photophysical data in the [Pd–porphyrin oligomer]/DPA upcon-
version systems.
[PdDTP-M]/DPA
[PdDTP-D]/DPA
[PdDTP-T]/DPA
[a]
[b]
Compound t [ms]
Without DPA With DPA
kTTET [105sÀ1
]
FTTET
FTTET
DA(3DPA*)0
0.007
0.45
4.3
0.9
1.4
0.08
1.0[a]
0.95
7.7
0.028
1.8
3.9
1.7
2.6
0.26
0.89
0.94
14.7
10.5
0.003
0.96
5.4
1.1
1.7
0.11
0.81
0.90
2.4
[3DPA*]0 [10À6 m]
PdDTP-M
PdDTP-D
PdDTP-T
99.6
107.3
62.8
5.1
5.9
6.4
1.8
1.5
0.9
0.96
0.93
0.91
0.95
0.94
0.90
kT [103 sÀ1
]
k’TT [105 sÀ1
]
kTT [109 mÀ1 sÀ1
fTT
FISC
]
[a] From the phosphorescence quenching spectra. [b] From the transient
absorption spectra.
FTTET
FTTA [%]
FUC [%]
6.2
1.6
[a] From ref. [42].
3
dependence of the transient absorption of DPA* (DA(3DPA*)t)
and the fraction (fTT) of 3DPA* decaying through the triplet–
triplet annihilation pathway can be described by equations (1)
and (2)[39–41]
:
FTTA can be calculated according to equation (4) (see below)
with the known values of FUC, FISC, FTTET, and FF of DPA. FUC
and FTTET have already been determined and FF of DPA is 0.85
in toluene. The intersystem crossing efficiencies (FISC) of the
Pd–porphyrin oligomers are required to calculate the triplet–
triplet annihilation efficiency (FTTA). Assuming the intersystem
crossing efficiency of PdDTP-M to be unity,[42] the FISC of
PdDTP-D and PdDTP-T are estimated to be 0.89 and 0.81, re-
spectively, by comparing [3DPA*]0 in the [PdDTP-D]/DPA and
[PdDTP-T]/DPA systems with that of the [PdDTP-M]/DPA
system under the same experimental conditions. Thus, FTTA is
estimated to be 7.7%, 14.7%, and 2.4% for the [PdDTP-M]/
DPA, [PdDTP-D]/DPA, and [PdDTP-T]/DPA systems, respectively.
1
3
*
DAð DPA Þ1 ¼
kT0 T
kT0 T
kT
ð1Þ
ð2Þ
1
ð
þ
Þ expðkTtÞ À
3
kT
DAð DPA Þ0
*
3
*
*
kT
DAð DPA Þ0 Â kTT
fTT ¼ 1 À
lnð1 þ
Þ
3
DAð DPA Þ Â k0
kT
*
0
TT
in which kT is the first-order triplet decay rate constant, k’TT =
kTT/eTl is the apparent rate constant of triplet–triplet annihila-
tion, kTT is the triplet–triplet annihilation rate constant, the eT is
the molar extinction coefficient of 3DPA*, and l is the path
length of the laser passed through the sample, DA(3DPA*)0 and
DA(3DPA*)t are the initial and time-dependent transient absorp-
tion intensities. Nonlinear fitting of the transient absorption ki-
Determinant of the difference of FUC in the [Pd–porphyrin
oligomer]/DPA systems
3
netic traces of DPA* with equation (1) yields the three param-
eters DA(3DPA*)0, kT, and k’TT, and then fTT can be calculated
with equation (2).
FUC is the product of the FISC of the sensitizer, FTTET from the
triplet sensitizer to the acceptor, and FTTA and FF of the ac-
ceptor. Because the same acceptor, DPA, is used in the three
[Pd–porphyrin oligomer]/DPA systems, their FUC values are
only affected by FISC, FTTET, and FTTA. FISC and FTTET in the [Pd–
porphyrin oligomer]/DPA systems show a downward trend
with the increasing number of linked porphyrin units, whereas
the order of FUC is [PdDTP-D]/DPA (10.5%)> [PdDTP-M]/DPA
(6.2%)> [PdDTP-T]/DPA (1.6%). Evidently, the intersystem
crossing and the triplet–triplet energy transfer processes
should not be the main causes for the difference of FUC. FTTA
was found to be 7.7%, 14.7%, and 2.4% for the [PdDTP-M]/
DPA, [PdDTP-D]/DPA, and [PdDTP-T]/DPA systems, respectively.
The same tendency for FTTA and FUC to vary with the number
of linked porphyrin units is observed, therefore, it can be infer-
red that FTTA is the main determinant for the difference in FUC
in the [Pd–porphyrin oligomer]/DPA systems. FTTA in the
[PdDTP-D]/DPA system is higher than those in the other two
systems, resulting in higher upconversion quantum efficiency.
We further looked into the factors affecting the triplet–trip-
let annihilation process. The triplet–triplet annihilation process
Although the transient absorption of the Pd–porphyrin oli-
gomer triplet state (3PS*) overlaps that of DPA* in the range
3
3
3
400–500 nm, PS* decays much faster than DPA*. The transi-
3
ent absorption of PS* faded away within 30 ms after the exci-
tation pulse. Therefore, the kinetic traces at the maximum of
the transient absorptions (455, 455, and 485 nm for the
[PdDTP-M]/DPA, [PdDTP-M]/DPA, and [PdDTP-T]/DPA systems,
3
respectively) after 30 ms belong to DPA*, which can be fitted
nonlinearly with equation (1) and DA(3DPA*)0, kT, and k’TT of the
TTA-UC systems can be obtained. The data for fTT of the TTA-
UC systems are calculated by using equation (2). The molar ex-
tinction coefficient of 3DPA* at 485 nm is estimated to be
3370mÀ1 cmÀ1 by using its transient absorption spectrum and
its molar extinction coefficient at 455 nm (1.55”104 mÀ1 cmÀ1)
reported in the literature.[35,38] With eT of DPA*, the initial con-
centration of DPA* ([3DPA*]0) and kTT in the [Pd–porphyrin oli-
3
3
gomer]/DPA systems can be calculated from DA(3DPA*)0 and
k’TT =kTT/eTl, respectively. All the fitted and calculated data,
DA(3DPA*)0, [3DPA*]0, kT, k’TT, kTT, and fTT are summarized in
Table 3. Under the same experimental conditions, the [PdDTP-
D]/DPA system exhibits the highest [3DPA*]0, fTT, and kTT in
these three TTA-UC systems.
3
involves two DPA* molecules and is concentration-dependent,
3
3
thus, it is advanced by higher concentrations of DPA*. DPA*
is generated from the sensitization by the Pd–porphyrin oligo-
Chem. Eur. J. 2016, 22, 8654 – 8662
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mers, which begins with the formation of the singlet excited
state of the Pd–porphyrin oligomers upon excitation and then
an intersystem crossing process to the triplet sensitizer, fol-
lowed by a triplet–triplet energy transfer from the Pd–porphy-
rin oligomer to DPA. Under the same experimental conditions,
design and selection of sensitizer and acceptor. Another issue
for application of TTA-UC systems is the aging caused by mo-
lecular oxygen and chromophore photobleaching, which may
be overcome by adopting oxygen scavenger compounds,
oxygen shielding components, as well as robust molecular
design.[6,43–45]
3
the concentration of the generated DPA* relates to the molar
extinction coefficient, the intersystem crossing efficiency of the
sensitizers, and the triplet–triplet energy transfer efficiency
from the sensitizer to DPA. Usually, the longer the triplet life-
time of the sensitizer is, the more chance it has of colliding
with DPA within its lifetime, resulting in higher sensitization ef-
ficiency. In the present work, the concentration of the acceptor
is high enough to quench the triplet sensitizer (FTTET >0.9), so
the effect of the triplet lifetime of the sensitizer can be ignor-
ed. Although FISC of PdDTP-M is near unity, its molar extinction
coefficient at 532 nm is low (~1/8 of those of PdDTP-D or
PdDTP-T), which limits the photons absorbed by the sensitizer
at certain excitation intensities, resulting in low concentrations
Conclusion
A series of directly meso-meso-linked Pd–porphyrin oligomers
have been prepared by utilizing zinc(II) 5,15-di(3,5-di-tert-butyl-
phenyl)porphyrin oligomers and palladium(II) acetate. The ab-
sorption region and the light-harvesting ability of the Pd–por-
phyrin oligomers are broadened and enhanced by increasing
the number of porphyrin units. The TTA-UC systems were con-
structed by using the Pd–porphyrin oligomers and 9,10-diphe-
nylanthracene as the sensitizer and the acceptor, respectively,
and their photophysical processes were investigated thorough-
ly. The triplet–triplet annihilation upconversion quantum effi-
ciencies of the TTA-UC systems under our experimental setup
were measured to be 6.2%, 10.5%, and 1.6% when using
PdDTP-M, PdDTP-D, and PdDTP-T as the sensitizer, respectively.
The higher TTA-UC quantum efficiency observed in the
[PdDTP-D]/DPA system can be attributed to the enhanced
light-harvesting ability of PdDTP-D, which produces more
3DPA*, consequently promoting the triplet–triplet annihilation
process. Furthermore, reabsorption of the upconversion fluo-
rescence by the sensitizer decreases the measured TTA-UC
quantum efficiency, which must be taken into consideration
when designing TTA-UC systems. These findings provide a new
perspective on the construction of effective triplet–triplet anni-
hilation upconversion systems.
3
of DPA*. The absorbance of PdDTP-D is similar to PdDTP-T and
is about seven times higher than PdDTP-M, which means the
excited dimer is seven times that of the monomer under the
same experimental conditions. The TTA-UC quantum yields
were found to be 8.3% and 10.5% for [PdDTP-D]/DPA, and
3.4% and 6.2% for [PdDTP-M]/DPA under 100 and
500 mWcmÀ2 irradiation, respectively. The TTA-UC quantum
yield of PdDTP-D/DPA increases by factors of 2.4 and 1.7 com-
pared with PdDTP-M/DPA under weak and strong irradiation,
respectively, which can be mainly ascribed to the enhanced
absorption of the sensitizer. The molar extinction coefficients
at 532 nm, the efficiency of intersystem crossing, and the spec-
tral overlap integral with DPA emission for PdDTP-D and
PdDTP-T are comparable, but the TTA-UC quantum yields of
PdDTP-D/DPA are 17.7 and 6.6 times higher than those of
PdDTP-T/DPA under 100 and 500 mWcmÀ2 irradiation, respec-
tively. The higher quantum yields of PdDTP-D/DPA can be ac-
3
counted for by the higher kTTET, which produces more DPA*
per unit time, thus giving higher [3DPA*]0 in the [PdDTP-D]/
DPA system. The higher [3DPA*]0 favors the two-molecule pro-
Experimental Section
Materials
3
cess, resulting in higher fraction of DPA* being involved in the
Reagents were purchased from Aldrich or Acros or J&K chemical
and were used without further purification, unless otherwise
noted. Palladium(II) acetate was purchased from Beijing Ouhe
Technology Co. Ltd. Toluene was distilled over Na/benzophenone
under a nitrogen atmosphere.
TTA decay pathway and consequently much better kTTA and
3
FTTA. The dynamic difference in DPA* generation and fTT ampli-
fies the discrepancy in the TTA-UC performance for PdDTP-D
and PdDTP-T, which is more pronounced under weak excita-
tion. [3DPA*]0 in the [PdDTP-T]/DPA system is higher than that
in the [PdDTP-M]/DPA system, but the magnitudes of FUC in
these two systems are opposite, which can be rationalized by
the more efficient energy transfer from the singlet state of
DPA to PdDTP-T than to PdDTP-M owing to the larger spectral
overlap in the PdDTP-T/DPA system. The overlap between the
absorption of the sensitizers and the fluorescence of the ac-
ceptor exists in all three [Pd–porphyrin oligomer]/DPA systems,
which leads to energy transfer from the singlet state of DPA to
the ground state of the Pd–porphyrin oligomers, decreasing
the measured FUC and the calculated FTTA. For potential appli-
cations, TTA-UC systems should be further improved by avoid-
ing the severe spectral overlap between the absorption of sen-
sitizers and the emission of acceptors through molecular
Instrumentation
1H NMR spectra were recorded with a Bruker Avance P-400
(400 MHz) spectrometer with tetramethylsilane as an internal stan-
dard. IR spectra were performed with an Excalibur 3100 IR spec-
trometer. MALDI-TOF-MS spectra were recorded with a Bruker
BIFLEX III spectrometer. Steady-state absorption and emission spec-
tra were measured by using a Shimadzu UV-2550PC spectrometer
and a Hitachi F-4500 spectrometer, respectively. Luminescence
decay processes were recorded with a single photon counting
technique with an Edinburgh FLS920 lifetime system. The transient
absorption spectra were performed with an Edinburgh LP 920
pump-probe spectroscopic setup.
Chem. Eur. J. 2016, 22, 8654 – 8662
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A diode-pumped solid-state laser (MGL-III-532 nm, Changchun New
Industries Optoelectronics Tech. Co. Ltd.) was used as the excita-
tion light source in the TTA-UC measurements. The laser power
was measured with an Ophir Nova II power meter with a PD300–
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The upconversion quantum yields (FUC) were determined with the
prompt phosphorescence of tris(4,4’-dimethyl-2,2’-bipyridine)ruthe-
nium bis(hexafluorophosphate) (FP =7.4% in acetonitrile) as the
standard. The upconversion quantum yields were calculated by
equation (3), where FUC stands for the upconversion quantum
yield, and Fstd is the phosphorescence quantum yield of the stan-
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minescence intensity, and the refractive index of the solvent, sam
and std refer to the sample and the standard, respectively. The
equation is multiplied by a factor of 2 to make the maximum
quantum yield be unity.
Astd Isam hsam
Asam Istd hstd
2
ð3Þ
ð4Þ
FUC ¼ 2Fstd
ð
Þð Þð
Þ
FUC ¼ FISCFTIETFTTAFF
The upconversion efficiency can also be obtained by the product
of the sensitizer intersystem crossing efficiency (FISC), the triplet–
triplet energy transfer efficiency (FTTET), and the triplet–triplet anni-
hilation efficiency (FTTA), as well as the fluorescence quantum yield
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Acknowledgments
Financial support from the 973 Program (Nos. 2013CB834703,
2013CB834505), the National Natural Science Foundation of
China (Nos. 21573266, 21172229, 21233011, and 21273258),
and the Chinese Academy of Sciences (KGZD-EW-T05) are
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Keywords: energy transfer · light harvesting · sensitizers ·
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Received: November 10, 2015
Published online on May 3, 2016
Chem. Eur. J. 2016, 22, 8654 – 8662
www.chemeurj.org
8662
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim