Towards the IR limit of the triplet-triplet annihilation-supported up-comersion: Tetraanthraporphyrin

Article abstract of DOI:10.1002/chem.200801305

A study was conducted to demonstrate the infrared (IR) limit of the triplet-triplet annihilation (TTA)-supported up-conversion (UC) of tetraanthraporphyrin. The fundamental advantage of the TTA-supported UC is its independence on the coherence of the excitation light. It was demonstrated that the TTA-supported UC approach also resolves another demanding limitation of the conventional methods for UC. Another significant advantage of the new approach, was the low intensity needed to achieve high quantum yields on the order of 2-4% in inorganic solutions. The combination of these characteristics and possibilities make energetically conjoined TTA-UC suitable for diverse applications, such as all-organic, flexible, transparent displays, and up-converter devices.

Full text of DOI:10.1002/chem.200801305

DOI: 10.1002/chem.200801305
Towards the IR Limit of the Triplet–Triplet Annihilation-Supported
Up-Conversion: Tetraanthraporphyrin
Vladimir Yakutkin,[a] Sergei Aleshchenkov,[b] Sergei Chernov,[b] Tzenka Miteva,*[c]
Gabriele Nelles,[c] Andrei Cheprakov,*[b] and Stanislav Baluschev*[a]
The processes by which locally (or in situ) up-converted
photons are generated by NIR or IR excitation sources have
been very intensively studied and have remarkable applica-
tion potential in fields like up-conversion displays,[1] biologi-
cal imaging and sensing,[2] and photodynamic therapy of
cancer.[3] The blue-shifted emission generated in the known
and long-time studied up-conversion processes results from
either two-photon absorption (TPA) in organic molecules,
quantum dots or in proximity of metallic clusters,[1–3] or se-
quential energy transfer (ETU) in rare-earth ion-doped
glasses.[4] All these processes have a common characteristic:
they need an excitation source with very high brightness—in
the case of TPA-based processes because of the virtual
energy level used, in the case of the ETU-based processes
because of the finite width of the ionic energy levels used.
Additionally, both these processes need moderate or strong
optical pumping, normally in order of many kWcmÀ2 up to
MWcmÀ2.
coherence of the excitation light.[6] The TTA-supported UC
resolves also another demanding limitation of the above de-
scribed “conventional” methods for UC (e.g., the ETU and
all types of TPA)—the necessity to excite the samples with
extremely bright optical sources (e.g., lasers). In contrast,
for excitation of an efficient TTA–UC, optical sources with
spectral power density of 125 mWnmÀ1 are sufficient[7] and,
in particular, the excitation source can be the Sun.
The next advantage revealing the enormous application
potential of the energetically conjoined TTA–UC is the very
low intensity needed (on the order of 100 mWcmÀ2) to ach-
ieve high quantum yields, on the order of 2–4% in organic
solutions.[7,8] In a further step, the efficiency of the TTA–UC
in bulk solid-state films, composed of the sensitizer and
emitter molecules blended in inactive polymer matrix, has
to be optimized as it is significantly lower than in solu-
tions.[6b,9]
The TTA-supported up-conversion devices, based on or-
ganic solutions are very efficient, but cannot be easily sealed
for the long term. The solid-state devices of this kind can be
sealed easily, but they are not efficient enough. This obstacle
can be avoided when highly viscous matrices are used. In
fact, the energetically conjoined TTA–UC in highly viscous
matrices[10] possesses all the required characteristics: high
quantum yield (comparable with those in liquid organic so-
lution of the active species), very low excitation intensity
(ꢀ25 mWcmÀ2), extremely low spectral power density opti-
cal sources (ꢀ200 mWnmÀ1), and versatility in excitation
and emission wavelengths. These devices can be also sealed
easily. The combination of all these unique characteristics
and possibilities make energetically conjoined TTA–UC
ready for diverse applications, such as all-organic, flexible,
and transparent displays,[10] up-converter devices for increas-
ing the efficiency of, for example, dye-sensitized solar cells
and local, in situ, generator of blue-shifted photons.
Recently, a different approach for up-conversion (UC),
based on energetically conjoined triplet–triplet annihilation
(TTA) was demonstrated.[5] The fundamental advantage of
the TTA-supported UC is its inherent independence on the
[a] Dr. V. Yakutkin, Dr. S. Baluschev
Max-Planck-Institute for Polymer Research
Ackermannweg 10, 55128 Mainz (Germany)
Fax : (+49)6131-379-100
E-mail: balouche@mpip-mainz.mpg.de
[b] S. Aleshchenkov, S. Chernov, Dr. A. Cheprakov
Department of Chemistry, Moscow State University
119899 Moscow (Russia)
Fax : (+7)495-939-1854
E-mail: avchep@elorg.chem.msu.ru
[c] Dr. T. Miteva, Dr. G. Nelles
Sony Deutschland GmbH
Materials Science Laboratory, Hedelfingerstr. 61
70327 Stuttgart (Germany)
Fax : (+49)711-5858-484
To explore the above describe applications in their full,
the IR limit, that is, the lowest energy photons able to serve
as pumping source for the studied energetically conjoined
TTA is of crucial importance. The highest excitation wave-
E-mail: miteva@sony.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801305.
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COMMUNICATION
length reported so far is l=700 nm (FWHMꢀ20 nm).[8] In
this communication we report for direct photon up-conver-
sion of low-intensity IR light (with maximum at l=790 nm,
FWHMꢀ23 nm) into yellow emission (l=570 nm, FWHM
ꢀ65 nm).
et al.[13] into 1,4-dihydroanthracene (3). Compound 3 was
then transformed by the procedure earlier used in the syn-
thesis of TNP derivatives (see, for example, reference [12a]
into allylic sulfone 4. As we have shown, allylic sulfones are
perfectly useful as substrates in place of vinylic sulfones in
the sulfone-modification of the Barton–Zard reaction, if a
small excess of strong base is present in the reaction mixture
furnishing the conditions of reversible allylic–vinylic sulfone
isomerization. The obtained naphthoisoindolecarboxylate
(6) was cleanly transformed into 4,11-dihydro-2H-naphtho-
To access this region of the light spectrum, we need the
respective sensitizer. The p-extended porphyrins used in ear-
lier studies (Pd or Pt derivatives of tetrabenzo- and tetra-
naphthoporphyrins) cannot reach as far into NIR with their
Q-bands, the most red-shifted of all these falling short by
almost 100 nm. Thus, a further extension of the p-system by
an extra layer of benzo rings is required, leading us to the
so far elusive tertraanthraporphyrin (TAP) system. Report-
ed earlier by Luk’yanets and Kobayashi et al.,[11] the 5,10,15-
triarylsubstituted TAP derivative was obtained as a Zn com-
plex by high-temperature templated condensation. Tetraar-
yl–TAPs have so far been unknown. To develop a reliable
synthetic approach to TAP, we have employed the dihydroi-
soindole strategy, earlier elaborated for the synthesis of vari-
ous derivatives of tetrabenzoporphyrins (TBP) and tetra-
AHCTRE[UNG 2,3-f]isoindole (7), which was introduced to Lindseyꢁs por-
phyrin synthesis. The intermediate porphyrin could not be
isolated, as the aromatization of annelated rings takes place
readily under the conditions of Lindseyꢁs method, and the
target TAP (8) was obtained in good yield. In comparison
with the respective TNP,[12a] TAP shows rather unusual prop-
erties; in spite of the huge conjugated system, which could
be expected to render even higher liability to aggregation
and poor solubility, the porphyrin (8) is readily soluble in
common organic solvents both as a free base and the dipro-
tonated form. Good solubility is likely to account for unpre-
cedented ease of Pd insertion, which takes place at room
temperature within a few hours, as compared to other por-
phyrins, which usually require prolonged reflux in high boil-
ing solvents like PhCN. On the
[12]
naphthaloporphyrin (TNP).
The synthesis (shown on Scheme 1) started from a readily
available benzynefurane adduct (1), which was transformed
via compound 2 by the procedure described by Dehaen
other hand, TAP possesses a
much more marked sensitivity
towards photooxidation, so all
manipulations with solutions
should be done under inert at-
mosphere or in the dark.
The absorption spectra of the
sensitizer investigated in this
work is shown in Figure 1
(line). The absorption of the
emitting compound (Rubrene;
Figure 1 filled circles) is negligi-
ble at wavelengths longer than
620 nm. Consequently, no sin-
glet emission can be observed
when the emitter molecules are
exposed to light of l=785 nm
with the intensities used in the
experiment described further
below (around 1 WcmÀ2, cw ex-
citation). The energy position
of the triplet state of PdTAP is
around 1.12 eV, with quantum
yield of the phosphorescence
less than 0.5% in toluene at
room temperature. The energy
position of the sensitizer triplet
Scheme 1. Synthesis
of
tetrakis-5,10,15,20-(p-methoxycarbonylphenyl)tetraanthra[2,3-b,g,l,q]porphyrin
state matches well the triplet
energy position of the emitter,
Rubrene.[7a]
The hypsochromic shift be-
tween the exciting and the
(PdTAP) (8). Reagents and conditions: a) sulfolene, NaHCO3, py, 1208C, 10 h (64%); b) HCl, EtOH, reflux,
24 h (79%); c) i. PhSCl, CH2Cl2, RT, 2 h; ii. Oxoneꢂ, MeOH-H2O, RT, 7 days (78%); d) DBU, CH2Cl2, RT,
1 h, (97%); e) CNCH2COOEt, tBuOK, THF, RT, 12 h (67%); f) NaOH, ethyleneglycol, reflux, 1 h (73%); g)
i. p-MeOOCC6H4CHO, BF3·Et2O, CH2Cl2, RT, 1 h; ii. DDQ, CH2Cl2, RT, 1 h (36%); h) Zn
CH2Cl2-py, 10 min (90%); i) Pd(OAc)2, Et3N, dioxane, RT, 5 h (75%).
ACHTRE(UNG OAc)2·2H2O,
AHCTREUNG
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S. Baluschev, A. Cheprakov, T. Miteva et al.
zo- and tetranaphthoporphyrins[7a] will allow for up-conver-
sion of the whole red band of the Solar irradiation with
wavelengths between 620–820 nm.
As shown in Figure 4, for excitation intensities between
20 mWcmÀ2 and 10 WcmÀ2 the intensity dependences can be
fitted with power law equation: PupÀconv =aPbexc in which b=
1.65Æ0.05 for the different relative molar concentrations of
Rubrene/PdTAP. Please, note that the demonstrated sublin-
ear intensity dependence of the UC signal is observed for
more than three orders of magnitude change of the excita-
tion intensity.
Figure 1. Normalized absorption spectra of PdTAP (line) and Rubrene
(filled circles) in toluene.
emitted photons is approximately DE=0.6 eV (Figure 2). In
Figure 3, the dependence of the UC quantum yield on the
relative concentration of the emitter/sensitizer is shown. The
UC quantum yield of the couple Rubrene/PdTAP at concen-
tration of 10À3 m/5”10À5 in toluene is 1.2%, estimated ac-
cordingly to the definition given in reference [7b] Therefore,
adding the anthraporphyrin-based sensitizer with Q-band
absorption centered at l=800 nm to the family of tetraben-
Figure 4. Dependence of the energetically-conjoined TTA–UC on the ex-
citation laser intensity for three different molar concentrations of the UC
emitter and fixed concentration of the sensitizer. Solid lines are power
law fits. Excitation spot size is 4”10À4 cm2.
The sublinear dependence of the UC signal on the excita-
tion intensity is at a first glance contradictory to the classical
representation of the TTA process.[14] Actually, in the classi-
cal experiments for TTA in which emitter triplets are creat-
ed through direct absorption of a UV photon in the emitter
singlet state, followed by the strongly prohibited process of
ISC in the emitter molecule, there is linear dependence be-
tween the created triplet states and the excitation intensity.
Taking into account, that the probability for TTA depends
on square of the concentration of the excited emitter trip-
lets, the known quadratic dependence between the excita-
tion intensity and the intensity of the TTA signal is ob-
served. It is important to note, that in the classical TTA ex-
periments, delayed fluorescence is observed at excitation in-
tensities comparable with those for the other nonlinear opti-
cal processes,[15] namely by intensities on the order of many
kWcmÀ2.[14]
Figure 2. Normalized fluorescence of Rubrene excited in up-conversion
regime at room temperature in toluene, excitation intensity 250 mWcmÀ2
(laser power 10 mW, collimated beam with diameter d=2 mm), l=
785 nm. Inset: A CCD-camera image of the up-converted fluorescence
inside the 1 mm cuvette, no optical filters were used.
In the energetically conjoined TTA reported herein, the
concentration of the excited emitter triplet states is propor-
tional to the concentration of the excited sensitizer triplet
states. On other hand, the phosphorescence intensity of the
porphyrin molecules (related to the degree of population of
the sensitizer triplet states) is known to depend as the
square-root of the excitation intensity.[14] This dependence,
together with the process of triplet–triplet transfer (TTT)
between molecules with large differences in the ISC coeffi-
cients, is most probably the rectification mechanism, leading
Figure 3. Dependence of the UC quantum yield on the relative concen-
trations of the emitter Rubrene, at fixed concentration of the sensitizer -
PdTAP, 5”10À5 m.
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Energy Transfer
COMMUNICATION
to the observed sublinear intensity dependence, shown in
Figure 4. Very importantly, the reported mechanism of ener-
getically conjoined TTA is efficient at intensities more than
six orders of magnitude lower than those of the “classical”
TTA.[14]
shifted photon up-conversion based on energetically con-
joined TTA. The reported molecular system shows stable
operation for more than 60 days.
As shown in Figure 5, the rise time of the up-conversion
emission decreases with the increase of the relative concen-
tration of the emitter molecules from about 12 ms at 0.1”
Experimental Section
The solutions were prepared and sealed in a nitrogen-filled glove-box
with de-aerated toluene as solvent. Note the crucial importance to keep
the oxygen amount less than 0.1 ppm. When the preparation is done at
0.1 ppm oxygen content in the glove-box, the degradation of the samples
is not observed even after 60 days. In contrast, when the samples are pre-
pared in 5–10 ppm oxygen-containing nitrogen atmosphere, the degrada-
tion of the samples starts immediately (ms) after their exposure to the
excitation light.
A single-mode cw diode laser (l=785 nm) was used as the pumping
source for the measurement of excitation intensity dependence. The
emission spectra were registered by Optical Multichannel Analyzer (Ha-
mamatsu Inc.). The excitation laser wavelength was effectively sup-
pressed (more than 106 times) by using a notch filter designed for l=
785 nm (Semrock Inc.).
The time-resolved spectra were measured on a Streak-Camera, Hama-
matsu C5680 in the single-pulse regime. Both, the rising and the decaying
pulse slope times of the excitation laser (l=785 nm) were less than
200 ns.
Keywords: energy transfer · IR excitation · palladium ·
porphyrinoids · triplet–triplet annihilation
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Received: June 30, 2008
Published online: October 1, 2008
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Chem. Eur. J. 2008, 14, 9846 – 9850