Blue-green up-conversion: Noncoherent excitation by NIR light

Article abstract of DOI:10.1002/anie.200700414

(Figure Presented) An emitter/sensitizer couple (see picture; C red/green, H white, N blue, Pd gray) was specially designed for the process of noncoherently excited photon up-conversion. The hypsochromic shift between the energy of the excitation photons

Full text of DOI:10.1002/anie.200700414

Angewandte
Chemie
DOI: 10.1002/anie.200700414
Fluorescence
Blue-Green Up-Conversion: Noncoherent Excitation by NIR Light**
Stanislav Baluschev,* Vladimir Yakutkin, Tzenka Miteva,* Yuri Avlasevich, Sergei Chernov,
Sergei Aleshchenkov, Gabriele Nelles, Andrei Cheprakov, Akio Yasuda, Klaus Müllen, and
Gerhard Wegner
The phenomenon of photon up-conversion—the generation
of photons, spectrally blue-shifted to the wavelength of the
excitation photons by simultaneous or sequential absorption
of two or more photons with lower energy—has always been
associated with the use of coherent light sources (lasers).
Already well-developed methods[1] for up-conversion, such as
two-photon absorption, second (or higher-order) harmonic
generation, parametric processes, and sequential multiphoton
absorption, all have a common serious limitation: the require-
ment for very high intensity light excitation (on the order of
MWcmÀ2). Some requirements that are specific for those
processes, like phase-matching conditions, for example,[1] are
fulfilled only for a limited spectral region. Therefore, even
when the necessary light intensity was lowered substantially
(on the order of kWcmÀ2), the requirement of extremely high
spectral power density of the excitation source could not be
overcome.[2–4]
considered as an inherently connected chain of three pro-
cesses already studied in the past. The first process in this
chain is intersystem crossing (ISC), which is strongly
enhanced by the spin–orbit coupling to the metal center of
metalated macrocyclic sensitizer molecules.[9] The second
process in the chain is the transfer of the excitation energy of
the sensitizer triplet to the emitter triplet.[10] The third process
is the subsequent triplet–triplet annihilation, which in our
system occurs mostly between emitter-molecule triplets.[11]
The key feature of the annihilation up-conversion process
is the large population of the triplet states of the emitter
molecules created in a chain of events following absorption of
single photons. Therefore, the sensitizer molecules have to
supply a long-lived triplet state highly populated by single-
photon absorption. Moreover, the strong overlap of the triplet
levels of the sensitizer and the emitter molecules is of crucial
importance for this type of up-conversion.
In comparison to the above-described methods, the
fundamental advantage of the triplet–triplet annihilation-
supported bimolecular photon up-conversion process is its
inherent independence[5] on the coherence of the excitation
light. Another principal advantage[5] of this up-conversion
process is the very low intensity (as low as 1 WcmÀ2) and
extremely low spectral power density (as low as 600 mWnmÀ1)
needed from the excitation source; thus, the source can be the
sun.
Herein we report the direct photon up-conversion of the
near-infrared part of the solar spectrum (around 700 nm) into
blue-green emission (480–580 nm). The external quantum
efficiency of the studied system is as high as 0.04.
Extension of the p-conjugated system in metalated
porphyrins by annelated aromatic rings leads to a noticeable
red shift[12] in the absorption and emission spectra. Our
interest in porphyrins such as tetrabenzoporphyrins[13] arises
from the ability to shift the excitation spectra for up-
conversion deep into the red region of the visible spectrum.
Accordingly, the triplet levels of the emitter molecules have
to be tuned into the infrared region too. The structures of the
materials investigated in this work are shown in Figure 1. The
The process of photon up-conversion, based on triplet–
triplet annihilation in multimolecular systems,[5–8] is to be
[*] Dr. S. Baluschev, Dr. V. Yakutkin, Dr. Y. Avlasevich, Prof. Dr. K. Müllen,
Prof. Dr. G. Wegner
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
Fax: (+49)6131-379-100
E-mail: balouche@mpip-mainz.mpg.de
Dr. T. Miteva, Dr. G. Nelles, Dr. A. Yasuda
Sony Deutschland GmbH
Materials Science Laboratory
Hedelfingerstrasse 61, 70327 Stuttgart (Germany)
Fax: (+49)711-5858-484
E-mail: miteva@sony.de
S. Chernov, S. Aleshchenkov, Dr. A. Cheprakov
Department of Chemistry
Moscow State University
Figure 1. Structures of 1 and 2; X=OMe.
Vorob’evy gory, 119992 Moscow (Russia)
[**] S.C., S.A., and A.C. thank the Russian Foundation of Basic Research
for the support through grants RFBR-04-03-32650-a and RFBR-07-
03-01121-a.
palladium–porphyrin complex 1 and the bis(tetracene) 2 were
synthesized in our group.
Compound 1 was synthesized by using a recently devel-
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
oped general method[14] by the condensation of 4,9-dihydro-
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2H-benzo[f]isoindole with 3,5-dimethoxybenzaldehyde and
subsequent in situ aromatization with 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone in 34% yield. The introduction
of 3,5-substituents in the meso-aryl rings, even groups as
simple as MeO, helps in increasing the solubility of the p-
extended porphyrins,[15] which are well-known to show
enhanced aggregation and low solubility in organic solvents.
The prepared porphyrin turned out to be readily soluble in
common organic solvents. Palladium was inserted by heating
the porphyrin with Pd(OAc)2 in benzonitrile for 0.5 h, and the
Pd complex was isolated by column chromatography in 60%
yield.
Compound 2 was synthesized in two steps: bromination of
tetracene to 5-bromotetracene with one equivalent of N-
bromosuccinimide, under the conditions described previ-
ously,[16] and Suzuki coupling of 5-bromotetracene with
commercially available 4,4’-biphenylene diboronic acid. It
was shown that Suzuki coupling of 5-bromotetracene with
commercial boronic acids using [Pd(PPh3)4] as a catalyst
formed the target compounds only in low yield (ca. 10%).[17]
Varying the reaction conditions did not improve the yield.
However, the Suzuki coupling of 5-bromotetracene with
diboronic acid was successfully applied by using the following
catalyst: a combination of [Pd2(dba)3] with the ligand bis(2-
(diphenylphosphino)phenyl)ether (DPEPhos), which has
been described as an efficient catalyst for the Suzuki coupling
of sterically hindered, ortho-disubstituted benzene deriva-
tives.[18] The application of this catalytic system afforded 4,4’-
bis(5-tetracenyl)-1,1’-biphenylene in 58% yield. Aqueous
potassium carbonate was used instead of potassium phos-
phate in the original procedure. Compound 2 was purified by
column chromatography on silica gel and had greater than
98% purity (TLC, MS, NMR, elemental analysis).
Figure 3. Comparison of the phosphorescence of a toluene solution
containing only the sensitizer 1 (10À4 m; red lines) and the fluores-
cence of a blended toluene solution of 2”10À3 m 2 and 10À4 m 1 (green
lines) at different excitation intensities. Inset: integral phosphores-
cence (red circles) and integral fluorescence (green circles) as a
function of the excitation intensity P.
573 nm), together with bands from phosphorescence (local
maxima at about 916 and 942 nm) of the sensitizer 1. The
fluorescence of the sensitizer (maxima at about 720 and
790 nm) is completely suppressed by the use of edge filters. It
is important to note that the phosphorescence of the sensitizer
is almost completely quenched.
Figure 3 shows also the dependence on excitation inten-
sity of the phosphorescence of the solution containing only
sensitizer, compared to the up-converted fluorescence of the
blended solution containing both sensitizer and emitter. The
sensitizer concentration in both solutions was the same. It is
important to note that the amount of the absorbed photons in
both the nonsensitized and the sensitized solution was kept
the same. The inset of Figure 3 shows the integral phosphor-
escence of the pure sensitizer solution (red circles, integration
window 850 nm–960 nm) and the integral fluorescence (green
circles, 480 nm–620 nm) as a function of the excitation
intensity, at constant emitter and sensitizer concentration.
It is evident from Figure 3 that for the given concentration
of the emitter, sensitizer, and constant excitation intensity, the
total emitter fluorescence in the blended solution exceeds the
total phosphorescence of the solution of pure sensitizer by a
factor of more than 2–5. Taking into account that at least two
triplet states of the sensitizer need to be used for the emission
of one up-converted photon, the amount of nonradiative
decaying triplet states is reduced much more strongly than
could be estimated directly from Figure 3. These facts lead to
the conclusion that adding 2 to a solution of 1 strongly reduces
the number of the porphyrin triplets that decay in a non-
radiative way. To the best of our knowledge, there are no
previous examples for enhanced optical triplet harvesting of
such deep-lying triplets by means of up-conversion fluores-
cence.
The absorption spectra of 1 and 2 are shown in Figure 2.
The absorption of the blue-green-emitting 2 is negligible at
wavelengths longer than 520 nm. Consequently, no singlet
*
*
Figure 2. Normalized absorption spectra of 1 ( ) and 2 ( ) in toluene.
emission can be observed from 2 when the emitter molecules
are exposed to light of 700 nm with intensities less than about
10 WcmÀ2, as described below.
The steady-state luminescence spectra, generated through
up-conversion for a solution of 2 in toluene blended with
5 mol% 1, is shown in Figure 3. The spectra clearly show
fluorescence of 2 (with local maxima at about 500, 533, and
The potential for possible applications of the triplet–
triplet annihilation-supported bimolecular process of photon
up-conversion is demonstrated in the experimental results for
a noncoherent excitation of a solution of 5 ” 10À4 m 2
sensitized with 2.5 ” 10À5 m 1 (Figure 4). The excitation
source was the NIR part of the terrestrial solar spectrum as
shown in Figure 4 (red). The observed up-conversion fluo-
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excitation source, the NIR part of the terrestrial solar
spectrum, with an intensity as low as 1 WcmÀ2 and a spectral
width of Dl ꢀ 20 nm, was used. To the best of our knowledge,
the demonstrated triplet harvesting of low-energy triplet
states (as low as 1.3 eV) by means of photon up-conversion
has not been achieved previously.
Experimental Section
The solutions were prepared and sealed in a glove box by using
degassed toluene. A single-mode continuous-wave diode laser (l =
695 nm) was used as the excitation source for the measurement of
excitation-intensity dependence. The emission spectra were recorded
on an Optical Multichannel Analyzer (Hamamatsu C7223, 16-bit
analog-to-digital acquisition). The excitation laser wavelength was
effectively suppressed (by a factor of more than 106) by using suitable
edge filters (Semrock Inc.). The emission spectra of the sensitizer and
the emitter molecules in individual solutions under single-photon
excitation as collected in the conditions used for the up-conversion
experiments are shown in the Supporting Information, together with
all known quantum yields.
Figure 4. Normalized excitation part of the solar spectrum (red line);
normalized fluorescence of 2 (green line) excited through up-conver-
sion at room temperature in toluene at an excitation intensity of
1 WcmÀ2. Inset: a charge-coupled device (CCD) camera image of the
up-converted fluorescence inside the 1-cm-wide cuvette; the fluores-
cence was excited with the near-infrared part of the solar spectrum,
and no filters were used.
A Dobsonian telescope (12’’ Lightbridge, Meade Ins. Corp.) was
used to collect the sunlight and couple it into a fiber (Multimode,
1000 mm, NA 0.48, Thorlabs Inc.). The infrared tail of the solar
spectrum (wavelengths longer than 750 nm) was removed with a large
interference filter (AHF Analysentechnik GmbH) before focusing
into the fiber. The part of the solar spectrum used as an excitation
source was selected using a broadband interference filter at the
output of the optical fiber.
rescence spectrum is also shown in Figure 4 (green). The
hypsochromic shift between excitation and emitted energy is
estimated to be as large as DE ꢀ 0.7 eV. The emission spectra
were registered by an optical-fiber spectrometer in the lateral
direction without using any blocking optical filter. It is
important to mention that the noncoherent excitation inten-
sity was 14 mW, collected from an excitation band with more
than Dl ꢀ 20 nm (FWHM) spectral width. The excitation spot
diameter was 1/e ꢀ 1.8 ” 10À3 m; thus, the excitation intensity
was lower than 1 WcmÀ2. The quantum yield for the up-
conversion fluorescence for this experiment was estimated to
be more than 0.04 (obtained by standard actinometry;
Figure 5).
Received: January 30, 2007
Revised: May 2, 2007
Published online: August 23, 2007
Keywords: energy transfer · fluorescence · macrocycles ·
.
NIR solar light · triplet–triplet annihilation
[1] Y. R. Shen, The Principles of Nonlinear Optics, Wiley-VCH,
Weinheim, 2002.
[2] M. M. Fejer, G. A. Magel, D. H. Jundt, R. L. Byer, IEEE J.
Quantum Electron. 1992, 28, 2631.
[3] J.-C. Boyer, L. A. Cuccia, J. A. Capobianco, Nano Lett. 2007, 7,
847–852.
[4] a) J. F. Suyver, A. Aebischer, D. Biner, P. Gerner, J. Grimm, S.
Heer, K. W. Krämer, C. Reinhard, H. U. Güdel, Opt. Mater.
2005, 27, 1111; b) F. Auzel, Chem. Rev. 2004, 104, 139.
[5] S. Baluschev, T. Miteva, V. Yakutkin, G. Nelles, A. Yasuda, G.
Wegner, Phys. Rev. Lett. 2006, 97, 143903.
[6] P. E. Keivanidis, S. Baluschev, T. Miteva, G. Nelles, U. Scherf, A.
Yasuda, G. Wegner, Adv. Mater. 2003, 15, 2095 – 2098.
[7] D. V. Kozlov, F. N. Castellano, Chem. Commun. 2004, 2860 –
2861.
[8] R. R. Islangulov, F. N. Castellano, Angew. Chem. 2006, 118,
6103 – 6105; Angew. Chem. Int. Ed. 2006, 45, 5957 – 5959.
[9] a) M. Kasha, Discuss. Faraday Soc. 1950, 9, 14 – 19; b) R. L.
Fulton, M. Gouterman, J. Chem. Phys. 1961, 35, 1059 – 1064.
[10] A. Terenin, V. Ermolaev, Trans. Faraday Soc. 1956, 52, 1042 –
1052.
[11] a) G. Porter, M. W. Windsor, Discuss. Faraday Soc. 1954, 17,
178 – 182; b) C. A. Parker, C. G. Hatchard, T. A. Joyce, Nature
1965, 205, 1282; c) P. Avakian, R. E. Merrifie, Mol. Cryst. 1968, 5,
37 – 41.
[12] The Porphyrin Handbook (Eds.: K. Kadish, K. M. Smith),
Elsevier, Amsterdam, 2003.
Figure 5. Dependence of the up-conversion quantum yield F on the
sensitizer concentration at constant sensitizer-to-emitter molar ratio
(ca. 1:10). As an actinometric standard, a solution of perylene in
toluene with a quantum yield of 0.95[19] was used. The highest external
up-conversion quantum yield (0.04) was observed for 6”10À5 m 1 and
6”10À4 m 2.
In summary, an emitter/sensitizer couple, specially
designed for the process of noncoherently excited photon
up-conversion, was investigated. The hypsochromic shift
between the energy of the excitation photons and the emitted
photons is about 0.7 eV. The up-conversion process has a very
high external quantum yield of 0.04. As the noncoherent
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[13] S. A. Vinogradov, D. F. Wilson, J. Chem. Soc. Perkin Trans. 1
[16] Yu. Avlasevich, K. Müllen, Chem. Commun. 2006, 4440 – 4442.
[17] A. M. Müller, Yu. Avlasevich, K. Müllen, C. J. Bardeen, Chem.
Phys. Lett. 2006, 421, 518 – 522.
[18] J. Yin, M. P. Rainka, X. X. Zhang, S. L. Buchwald, J. Am. Chem.
Soc. 2002, 124 1162.
1995, 2, 103 – 111.
[14] a) O. S. Finikova, S. E. Aleshchenkov, R. P. Brinas, A. V. Che-
prakov, P. J. Carroll, S. A. Vinogradov, J. Org. Chem. 2005, 70,
4617; b) O. S. Finikova, A. V. Cheprakov, P. J. Carroll, S. A.
Vinogradov, J. Org. Chem. 2003, 68, 7517; c) O. S. Finikova, A. V.
Cheprakov, S. A. Vinogradov, J. Org. Chem. 2005, 70, 9562.
[15] O. S. Finikova, S. Y. Chernov, A. V. Cheprakov, M. A. Filatov,
S. A. Vinogradov, I. P. Beletskaya, Dokl. Chem. 2003, 391, 222.
[19] F. Lewitzka, H.-G. Liihmannsrijben, M. Strauch, W. Litke, J.
Photochem. Photobiol. A 1991, 61, 191 – 200.
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