Triplet-triplet annihilation upconversion from rationally designed polymeric emitters with tunable inter-chromophore distances

Article abstract of DOI:10.1039/c4cc07589g

We report an investigation of triplet-triplet annihilation upconversion (TTA-UC) based on polymeric emitters with tunable inter-chromophore distances. Poly[(9-anthrylmethyl methacrylate)-co-(methyl methacrylate)] (poly(AnMMA-co-MMA)) with different percentages of AnMMA was synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization, and used as an emitter in association with platinum octaethylporphyrin as a sensitizer to form TTA-UC systems. It is observed that the TTA-UC intensity first increases with increasing AnMMA percentage in the polymers then decreases, and ultimately disappears, upon further increasing the AnMMA percentage. The results shed light on the key factors affecting TTA-UC in polymers, and have implications for the design of polymer-based TTA-UC systems. This journal is

Full text of DOI:10.1039/c4cc07589g

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: X. Yu, X. Cao, X.
Chen, N. Ayres and P. Zhang, Chem. Commun., 2014, DOI: 10.1039/C4CC07589G.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

ChemComm
Page 1 of 4Journal Name
Dynamic Article Links ►
View Article Online
DOI: 10.1039/C4CC07589G
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
Triplet-Triplet Annihilation Upconversion from Rationally Designed
Polymeric Emitters with Tunable Inter-Chromophore Distances
Xinjun Yu,# Xian Cao,# Xiaoping Chen, Neil Ayres,* Peng Zhang *
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
mechanical support for TTA-UC.[13-15] In addition to the limited
We report an investigation of triplet-triplet annihilation
mobility of chromophores (sensitizers and/or emitters) in
50 polymers, the bottleneck of this strategy is the solubility and
aggregation of chromophores in the polymer. These initial studies
led to the proposition that the chromophores be covalently
attached to the polymer backbones, which could potentially
increase the amount of loaded chromophores in the polymer
55 while avoiding the aggregation or phase separation problem and
leading to efficient TTA-UC.[16] For instance, ruthenium
bipyridine and 9,10-diphenylanthracene as polymer core and
pendant ‘arms’, respectively, have been synthesized through
reversible addition-fragmentation chain transfer (RAFT)
60 polymerization method, producing both 2-arm and 6-arm
polymers containing both sensitizer and emitters in the same
polymer. It was observed experimentally that the 2-arm polymer
displayed more efficient TTA-UC than the 6-arm
counterpart.[17,18] Still, more experimental results are desired to
65 better understand the key factors involved in TTA-UC in
polymers.
Herein, we report an investigation of TTA-UC systems based
on polymeric emitters, which possess individual emitters
covalently attached to a polymer backbone. Specifically, we used
70 the monomer, 9-anthrylmethyl methacrylate (AnMMA), as
individual emitters in copolymers of poly[(9-anthrylmethyl
methacrylate)-co-(methyl methacrylate)] (Poly(AnMMA-co-
MMA)), while platinum (II) octaethylporphyrin (PtOEP) as the
sensitizer. Our results demonstrate that interactions between
75 chromophores on the same polymer chain play an important role
in affecting the overall TTA-UC intensity of these systems.
A series of Poly(AnMMA-co-MMA)s with different AnMMA
to MMA ratios were synthesized through RAFT polymerization.
The monomer, AnMMA, was synthesized by reacting 9-
80 anthracenemethanol with methacryloyl chloride, as shown in
Scheme 1a. AnMMA was polymerized with methyl methacrylate
upconversion (TTA-UC) based on polymeric emitters with
tunable inter-chromophore distances. Poly[(9-anthrylmethyl
methacrylate)-co-(methyl methacrylate)] (Poly(AnMMA-co-
10 MMA)) with different percentages of AnMMA were
synthesized using reversible addition-fragmentation chain
transfer (RAFT) polymerization, and used as emitters in
association with platinum octaethylporphyrin as sensitizer to
form TTA-UC systems. It is observed that TTA-UC intensity
15 first increases with increasing AnMMA percentage in the
polymers then decreases, and ultimately disappears, upon
further increasing the AnMMA percentage. The results shed
light on the key factors affecting TTA-UC in polymers, and
have implication to the design of polymer-based TTA-UC
20 systems.
The potential benefits for societal adoption of solar energy are
high in comparison to the adverse effects of climate change from
using fossil fuels. However, the Shockley-Queisser limit sets a
solar power conversion efficiency limit on all single bandgap
25 photovoltaic devices, such as silicon and dye-sensitized solar
cells.[1,2] Triplet-triplet annihilation upconversion (TTA-UC),
which can convert the sub-bandgap photons to above-bandgap
photons, has been proposed as a way to increase the solar cell
efficiency. TTA-UC can be achieved with low power-density
30 (<100 mW/cm2), and non-coherent light as the excitation
sources.[3] Enhancement of solar fuel production of ~130% using
a TTA-UC system has been reported in a recent study. [4]
The TTA-UC process is a sequence of events: a sensitizer
becomes excited and reaches a triplet state via intersystem
35 crossing (ISC); triplet-triplet energy transfer (TTET) between
sensitizer and emitter; triplet-triplet annihilation (TTA) between
two emitters leading to a singlet excited state of one of them; and
the final upconverted emission from the singlet excited acceptor
(Figure S6 in the Supporting Information).[5-7] The TTET and
40 TTA steps in the sequence require collisions between two triplets.
Thus mobility of the species in the medium is critically important
for both the occurrence and efficiency of TTA-UC.
(MMA)
in
a
statistical
copolymerization
with
azobisisobutyronitrile (AIBN) as initiator and cumyl
dithiobenzoate (CDB) as RAFT chain transfer agent (CTA),
85 shown in Scheme 1b. The use of a copolymer as the polymeric
emitter allows us to adjust the distance between the individual
AnMMA emitters. MMA is chosen as the co-monomer because it
can generate a tertiary carbon propagating radical, matching the
reactivity of the AnMMA propagating radical. By varying the
90 molar ratio of AnMMA/MMA in the monomer feed, a series of
copolymers were synthesized (Table S1 in the Supporting
Most studies on TTA-UC are carried out with sensitizers and
emitters in solution. Studies on TTA-UC in polymers have also
45 been reported in recent years, with varying degrees of success.[8-
13]
The most common strategy is to blend the sensitizer and
emitter into a polymer matrix, which merely serves as a
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

ChemComm
Page 2 of 4
View Article Online
DOI: 10.1039/C4CC07589G
Information). Hereafter, we denote the molar ratio of the two
monomers, AnMMA/MMA, as AnMMA ratio. Detailed
more emitting chromophores (AnMMA) are present in the
mixture, and the average distance between the chromophores in
information of synthesis and characterizations are included in the 50 the same polymer chain decreases. The probability of collision
Supporting Information.
between chromophore triplets increases, which would be
favorable to higher TTA-UC intensity. However, the TTA-UC
intensity in this series of mixtures does not increase
monotonically with the increase of the AnMMA ratio. It first
55 increases when the AnMMA ratio increases up to ~8.8%, then
decreases when the AnMMA ratio increases further, down to
almost zero at the AnMMA ratio of 40%. Note that it is unlikely
that the chromophores (AnMMA) in these polymers aggregate or
have phase separation when the AnMMA ratio increases, as they
60 are covalently attached to the polymer chains and all
Poly(AnMMA-co-MMA) polymers disperse well in the solvent.
This is supported by the fact that fluorescence spectra of
AnMMA in Poly(AnMMA-co-MMA) of different AnMMA
ratios (Figure 5) do not show much red-shift when the AnMMA
5
O
a)
OH
O
O
Et3N, THF
0 oC to r.t.
+
Cl
AnMMA
10
S
S
b)
O
n
m
O
MeO
O
O
O
OMe
+
AIBN, Anisol, 80 oC
O
Poly(AnMMA-co-MMA)
15 Scheme 1. a) Synthesis of AnMMA; and b) synthesis of Poly(AnMMA- 65 ratio is high. Therefore, simply increasing the number of
co-MMA).
chromophores in the polymer chain does not always improve the
TTA-UC efficiency.
b
The normalized absorption and fluorescence spectra of
Poly(AnMMA-co-MMA) and PtOEP are shown in Figure 1. Note
20 that PtOEP absorbs strongly at 380 nm (Soret band) and 536 nm
a
60
40
20
0
1.5
1.0
0.5
0.0
AnMMA Ratio (%)
2
4
8.8
12
17.5
28.4
40
(Q-band)
while
emits
predominantly
at
645
nm
[19]
(phosphorescence, quantum yield ~0.4).
Poly(AnMMA-co-
MMA) does not absorb in the region where PtOEP emits, while
emitting at 420 nm, shorter than the Q-band wavelength of
25 PtOEP.
0
10
20
30
40
350
400
450
500
AnMMA Ratio (%)
Wavelength (nm)
1.2
Figure 2. (a) TTA-UC spectra of mixtures containing Poly(AnMMA-co-
MMA) (0.25 mM polymer) with different AnMMA ratios and PtOEP (10
70 M) in deoxygenated DMF solutions under 532 nm excitation (32
mW/cm2). (b) Relationship of integrated TTA-UC intensity of the
mixtures vs. AnMMA ratio.
b
a
PAnMMA-co-MMA
PtOEP
1.0
0.8
0.6
0.4
0.2
0.0
PAnMMA-co-MMA
PtOEP
1.0
0.8
0.6
0.4
0.2
0.0
In the second series, the mixtures contained the same
75 concentrations of PtOEP, while the concentrations of
poly(AnMMA-co-MMA) with different AnMMA ratios were
adjusted so that all mixtures had the same concentration of the
AnMMA unit. Results are shown in Figure 3. Again, the TTA-
UC intensity in this series of mixtures first increases when the
80 AnMMA ratio increases up to 4%, then decreases when the
AnMMA ratio increases further.
300
400
500
600
400
500
600
700
Wavelength (nm)
Wavelength (nm)
Figure 1. Normalized absorption (a) and emission (b) spectra of
Poly(AnMMA-co-MMA) and PtOEP in deoxygenated DMF,
30 respectively, at room temperature.
Poly(AnMMA-co-MMA) and PtOEP were dissolved
separately in N,N-dimethylformamide (DMF) before being mixed
together. The final concentrations of PtOEP and Poly(AnMMA-
35 co-MMA) were as noted later for each mixture. All mixtures
were thoroughly deoxygenated by the freeze-pump-thaw method
before spectroscopic measurements, which were excited by a 532
nm laser. The double logarithmic plot of the UC intensity
measured as a function of the incident laser power (Figure S3)
40 illustrates the transition in the slope from ~2 at lower incident
power to ~1 at higher power, similar to what has been reported in
the literature.[20]
a
b
2.5
2.0
1.5
1.0
0.5
0.0
80
60
40
20
0
AnMMA Ratio (%)
2
4
8
12
17.5
28.4
40
350
400
450
500
0
10
20
30
40
Wavelength (nm)
AnMMA Ratio (%)
Two series of upconversion measurements were carried out. In
the first series, the mixtures contained the same concentrations of
45 PtOEP and Poly(AnMMA-co-MMA) with different AnMMA
ratios. Results are shown in Figures 2 and S2. With the increase
of the AnMMA ratio in the Poly(AnMMA-co-MMA) polymers,
Figure 3. (a) TTA-UC spectra of mixtures containing Poly(AnMMA-co-
MMA) (1.5 mM AnMMA unit) with different AnMMA ratios and PtOEP
85 (10 M) in deoxygenated DMF solutions under 532 nm excitation (32
mW/cm2). (b) Relationship of integrated TTA-UC intensity of the
mixtures vs. AnMMA ratio.
2
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 3 of 4
ChemComm
View Article Online
DOI: 10.1039/C4CC07589G
Both results indicate that there appears to be an optimal range
with various concentrations of Poly(AnMMA-co-MMA) of 66%
of the AnMMA ratio in Poly(AnMMA-co-MMA), where the 60 AnMMA ratio (Figure S5 in the Supporting Information). Little
TTA-UC is most efficient. Considering all triplet chromophores
(AnMMA) in the mixtures, there are three types of possible
collisions among them that could lead to annihilation, as
illustrated in the diagram shown in Figure 4: (1) two
chromophores attached to two different polymer chains, noted as
TTA I, (2) two adjacent chromophores attached to the same
polymer chain, noted as TTA II, and (3) two non-adjacent
10 chromophores attached to the same polymer chain, noted as TTA
III. TTA I is of inter-chain type, while TTA II and TTA III are of
intra-chain type. Under the conditions (excitation power and
concentrations of PtOEP and polymer) used in these experiments,
it is unlikely that a second triplet sensitization process takes place
15 within the lifetime (Figure S4 in the Supporting information) of
the triplet state of AnMMA in the same polymer chain, and the
inter-chain TTA can be the only significant UC process occurring
(Supporting Information). Yet simply increasing the number of
the polymer chains in the mixture does not always increase the
TTA-UC is observed despite the significant increase in the
polymer concentration in the mixture, indicating the dominance
of quenching over TTA-UC.
5
b
a
5
4
3
2
1
0
5
4
3
2
1
0
200
150
100
50
200
150
100
50
0
0
0
10
20
30
40
0
10
20
30
40
AnMMA Ratio (%)
AnMMA Ratio (%)
400
450
500
550
400
450
500
550
Wavelength (nm)
Wavelength (nm)
Figure 5. Fluorescence spectra of Poly(AnMMA-co-MMA) with different
AnMMA ratios in deoxygenated DMF solutions under 360 nm excitation.
(a) Polymer concentrations set at 0.25 mM. (b) AnMMA concentrations
20 observed TTA-UC intensity, as shown in Figure 3. This prompts 70 set at 1.5 mM. Inserts illustrate the relationship between the integrated
us to look into the possible self-quenching of the excited singlet
of the chromophores.
intensity and the AnMMA ratio.
These results combine to suggest that the observed UC
intensity is directly related to the net abundance of the excited
75 singlet of the chromophores. The generation of the excited singlet
of the chromophores is ascribed to the inter-chain TTA, while its
quenching is due to the intra-chain interaction. In other words,
TTA I processes promote UC while interaction among excited
singlets in the same polymer suppress UC, and the amount of
80 AnMMA in the polymer (and hence overall in the mixture)
controls the observed UC intensity. At low AnMMA ratios, the
intra-chain quenching is negligible, and increasing AnMMA ratio
is conducive to increasing the observed UC intensity. At high
AnMMA ratios, the intra-chain quenching of the excited singlet
85 of the chromophores becomes predominant, resulting in very low
UC intensity regardless of the total polymer concentration.
Therefore, a window exists of optimal AnMMA ratios where
TTA-UC is maximal. Such ratio varies slightly in the two series
of experiments shown here, which can be understood as follows.
90 At fixed polymer concentration, generation of the excited singlets
of the chromophores increases with increasing AnMMA ratio.
The decrease in UC intensity after AnMMA ratio of 8.8% reflects
the predominence of intra-chain quenching of the excited
singlets. In contrast, when the concentration of the AnMMA unit
95 in the polymers is fixed, the polymer concentration decreases,
leading to the reduced generation of the excited singlets, as the
AnMMA ratio increases. The effect on both generation and
quenching of the excited singlets results in a smaller optimal
AnMMA ratio of ~4%.
25
30
35
40
45
Polymer chain I
n
M
M
M
TTA II
TTA II
TTA III
TTA I
TTA I
N
N
N
N
Pt
M
M
Quenching
Polymer chain II
Figure 4. Schematic illustration of collisions among sensitizer and
emitters leading to TTA-UC in Poly(AnMMA-co-MMA)/PtOEP
mixtures. “M” represents methyl methacrylate.
Fluorescence spectra of the Poly(AnMMA-co-MMA) with
different AnMMA ratios directly excited at 360 nm, at either
fixed polymer concentration or fixed chromophore concentration,
were taken, with results shown in Figure 5. The fluorescence
50 intensity of the polymers of AnMMA ratio >4% decreases as the 100 We therefore suggest that, when designing and synthesizing
AnMMA ratio increases, suggesting that self-quenching of
excited singlet become significant when the AnMMA ratio is
above 4%.
polymeric emitters to be used in the TTA-UC systems to achieve
high UC efficiency, it is important to consider the distance
between the adjacent chromophores to avoid self-quenching of
the excited singlet of the emitter, more so than simply increasing
Based on the relationship of the UC intensity vs. AnMMA
55 ratio under the fixed polymer concentration conditions (Figure 2), 105 the number of emitting chromophores per polymer chain. One
we attribute the quenching to be mainly due to intra-chain
chromophores, and not inter-chain chromophores. This is further
supported by results of TTA-UC measurements on PtOEP mixed
simple way to test whether self-quenching occurs is to measure
the prompt fluorescence of the polymers.
In summary, we report the design and synthesis of a series of
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

ChemComm
Page 4 of 4
View Article Online
DOI: 10.1039/C4CC07589G
65 20 Haefele, A.; Blumhoff, J.; Khnayzer, R. S.; Castellano, F. N. J. Phys.
Chem. Lett. 2012, 3, 299-303.
Poly[(9-anthrylmethyl methacrylate)-co-(methyl methacrylate)]
through RAFT with different AnMMA ratios, resulting in tunable
inter-chromophore distances. These polymers can serve as
emitters, with PtOEP as sensitizer, in TTA-UC systems. TTA-UC
intensity of the Poly(AnMMA-co-MMA)/PtOEP mixtures
displays interesting dependence on the AnMMA ratio in the
polymer. It increases initially with increasing AnMMA ratio, and
decreases after the AnMMA ratio is above an optimal value,
ultimately disappearing when AnMMA ratio reaches 40%.
5
10 Interactions between chromophores on the same polymer chain
play the key role in affecting the TTA-UC intensity in these
systems. It is critical to minimize intra-chain chromophore
quenching in order to achieve high UC intensity.
15
N.A. acknowledges support from American Chemical Society
Petroleum Research Fund (51850-DN17). P.Z. acknowledges
supports from the National Science Foundation (CBET-1065633)
and DoD Award DM102420 (W81XWH-11-2-0103).
Notes and references
20 [#] X.Y and X.C contribute equally to this work.
[*] Correspondence should be addressed to P.Z. (peng.zhang@uc.edu) or
N.A. (neil.ayres@uc.edu). Department of Chemistry, University of
Cincinnati, Cincinnati, OH 45221.
† Electronic Supplementary Information (ESI) available: Experimental
25 details. See DOI: 10.1039/b000000x/
1
2
Smith, M. B.; Michl, J. Annu. Rev. Phys. Chem. 2013, 64, 361-386.
Nattestad, A.; Cheng, Y.Y.; MacQueen, R.W.; Schulze, T.F.;
Thompson, F.W.; Mozer, A.J.; Fückel, B.; Khoury, T.; Crossley,
M.J.; Lips, K.; Wallace, G.G.; Schmidt, T.W. J. Phys. Chem. Lett.
2013, 4 (12), 2073-2078.
30
35
3
4
5
Singh-Rachford. T. N.; Castellano, F. N. Coord. Chem. Rev. 2010,
254, 2560-2573.
Borjesson, K.; Dzebo, D.; Albinsson, B.; Moth-Poulsen, K. J. Mater.
Chem. A, 2013, 1, 8521-8524.
Baluschev, S.; Yakutin, V.; Miteva, T.; Wegner, G.; Roberts, T.;
Nelles, G.; Yasuda, A.; Chernov, S.; Aleshchenkov, S.; Cheprakov,
A. New J. Phy. 2008, 10, 013007.
6
Zhao, J.; Ji, S.; Guo, H. RSC Adv. 2011, 1, 937.
40 7 Cao, X.; Hu, B.; Zhang, P. J. Phys. Chem. Lett. 2013, 4, 2334-2338.
8
9
Singh-Rachford, T. N.; Lott, J.; Weder, C.; Castellano, F. N. J. Am.
Chem. Soc. 2009, 131, 12007.
Lee, S. H.; Lott, J. R.; Simon, Y. C.; Weder, C. J. Mater. Chem. C.,
2013, 1, 5142-5148.
45 10 Baluschev, S.; Jacob, J.; Avlasevich, Y. S.; Keivanidis, P. E.; Miteva,
T. ; Yasuda, A.; Nelles, G.; Grimsdale, A. C.; Müllen, K.; Wegner,
G. ChemPhysChem, 2005, 6, 1250-1253.
11 Wohnhaas, C.; Friedemann, K.; Busko, D.; Landfester, K.;
Baluschev, S.; Crespy, D.; Turshatov, A. ACS Macro Letters, 2013,
50
2, 446-450.
12 Miteca, T.; Yakutkin, V.; Nelles, G.; Baluschev, S. New J. Phys.
2008, 10, 103002.
13 Islangulov, R.R.; Lott, J.; Weder, C.; Castellano, F.N. J. Am. Chem.
Soc. 2007, 129, 12652-12653.
55 14 Singh-Rachford, T. N.; Lott, J.; Weder, C.; Castellano, F. N. J. Am.
Chem. Soc. 2009, 131, 12007-12014.
15 Merkel, P. B.; Dinnocenzo, J. P. J. Lumin. 2009, 129, 303-306.
16 Simon, Y. C.; Weder, C. J. Mater. Chem., 2012, 22, 20817-20830.
17 Tilley, A. J.; Kim, M. J.; Chen, M.; Ghiggino, K. P. Polym. J. 2013,
60
54, 2865-2872.
18 Boutin, P. C.; Ghiggino, K. P.; Kelly, T. L.; Steer, R. P. J. Phys.
Chem. Lett. 2013, 4, 4113-4118.
19 Bansal, A.K.; Holzer, W.; Penzkofer, A.; Taiju, T. Chemical Physics.
2006, 330, 118-129.
4
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]