Photon upconverting liquids: Matrix-free molecular upconversion systems functioning in air

Article abstract of DOI:10.1021/ja411316s

A nonvolatile, in-air functioning liquid photon upconverting system is developed. A rationally designed triplet sensitizer (branched alkyl chain-modified Pt(II) porphyrin) is homogeneously doped in energy-harvesting liquid acceptors with a 9,10-diphenylanthracene unit. A significantly high upconversion quantum yield of ～28% is achieved in the solvent-free liquid state, even under aerated conditions. The liquid upconversion system shows a sequence of efficient triplet energy transfer and migration of two itinerant excited states which eventually collide with each other to produce a singlet excited state of the acceptor. The observed insusceptibility of upconversion luminescence to oxygen indicates the sealing ability of molten alkyl chains introduced to liquefy chromophores. The involvement of the energy migration process in triplet-triplet annihilation (TTA) provides a new perspective in designing advanced photon upconversion systems.

Full text of DOI:10.1021/ja411316s

Communication
pubs.acs.org/JACS
Photon Upconverting Liquids: Matrix-Free Molecular Upconversion
Systems Functioning in Air
Pengfei Duan, Nobuhiro Yanai,* and Nobuo Kimizuka*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Center for Molecular Systems (CMS), JST CREST,
Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan
S
* Supporting Information
solvents and deactivation of triplet states by molecular oxygen
ABSTRACT: A nonvolatile, in-air functioning liquid
photon upconverting system is developed. A rationally
designed triplet sensitizer (branched alkyl chain-modified
Pt(II) porphyrin) is homogeneously doped in energy-
harvesting liquid acceptors with a 9,10-diphenylanthracene
unit. A significantly high upconversion quantum yield of
∼28% is achieved in the solvent-free liquid state, even
under aerated conditions. The liquid upconversion system
shows a sequence of efficient triplet energy transfer and
migration of two itinerant excited states which eventually
collide with each other to produce a singlet excited state of
the acceptor. The observed insusceptibility of upconver-
sion luminescence to oxygen indicates the sealing ability of
molten alkyl chains introduced to liquefy chromophores.
The involvement of the energy migration process in
triplet−triplet annihilation (TTA) provides a new
perspective in designing advanced photon upconversion
systems.
significantly limit their practical applications. Although recently
the TTA-based UC has been investigated in solid polymer
films,4 such macromolecular matrices inevitably restrict the
diffusion of triplet molecules that limit the efficiency of UC. To
solve problems in the conventional dispersion systems, it is
essential to develop an oxygen-impermeable TTA system which
exerts migration of triplet excitons among densely clustered
chromophores. Here we present a first example of solvent-free
UC in liquid molecular networks; a set of organic donor and
liquid acceptor chromophores displays a sequence of efficient
triplet energy transfer, migration, and TTA in the matrix-free
liquid state (Figure 1). It is known that the modification of
aromatic chromophores with branched alkyl chains gives
nonvolatile organic liquids.5 Although luminescence character-
istics of some liquid chromophores have been reported,5 triplet
energy transfer, migration, and consequent photon UC in such
pconversion (UC), the population of luminescent higher-
Uenergy excited state with excitation at lower-energy light,
has attracted much attention because of its potential to
overcome the thermodynamic efficiency limits in solar energy
conversion devices.1 The most actively investigated UC systems
are based on nonlinear phenomena such as multiple-photon
absorption which, however, suffer from a fateful flaw in
requiring high excitation intensities (∼MW/cm2) that detracts
from its appeal.2 Consequently, an alternative UC mechanism
based on the triplet−triplet annihilation (TTA, Figure S1 in
Supporting Information [SI]) has come under the spotlight
since it offers numerous advantages over the aforementioned
techniques.3 For example, noncoherent light with low excitation
power density can be used to achieve UC, with excitation/
emission wavelengths tunable, depending on the independently
selected donor and acceptor molecules. A triplet excited state of
sensitizer (donor) is formed by intersystem crossing from the
excited singlet state, which exerts triplet−triplet energy transfer
(TTET) in the presence of suitable acceptors (Figure S1, SI).
When two acceptor molecules in the triplet state diffuse and
collide during their lifetimes, a higher singlet energy level is
populated by TTA which consequently produces delayed
fluorescence.
Figure 1. A schematic representation of the matrix-free liquid UC
system. Donor molecules (red) in acceptor liquid (yellow) are excited
by long-wavelength light. This is followed by a sequence of TTET
from the donor to the acceptor, triplet energy migration, TTA, and
delayed fluorescence from the upconverted singlet state of the
acceptor.
To date, efficient UC has been achieved in solution because
diffusion of triplet molecules is essential for both of the TTET
and TTA processes. However, the use of volatile organic
Received: September 12, 2013
Published: December 12, 2013
© 2013 American Chemical Society
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Communication
photofunctional liquids have not been achieved. With this in
mind, we synthesized an organic liquid acceptor with a 9,10-
diphenylanthracene moiety6 and a branched alkyl chain-
containing Pt(II) porphyrin derivative as a triplet sensitizer
(donor) (Figure 2). This pair of chromophores was selected
Figure 3. (a) Normalized absorption and emission spectra of CHCl3
Figure 2. (a) Chemical structures of liquid acceptor 1 and sensitizer 2.
Photographs of the doped liquid (2/1 = 0.01 mol %) upon being
exposed to (b) white light and (c) 532 nm green laser (incident laser
direction is indicated by a green arrow). Bright blue luminescence is
due to UC emission, and green spots are scattered incident laser.
solution of 1 (λex = 375 nm, 0.1 mM) and 2 in deaerated CHCl3 (λex
=
510 nm, 0.1 mM). (b) Normalized absorption and emission spectra of
liquid 1 (λex = 375 nm) and cast film of 2 (λex = 520 nm).
at 433 nm. On the other hand, 2 exhibited Soret- and Q-bands
at 403 and 510 nm, respectively. When 2 dissolved in deaerated
CHCl3 was photoexcited at 520 nm, phosphorescence was
observed at 660 nm. Absorption and fluorescence spectra of 1
without solvent are shown in Figure 3b. The liquid 1 showed a
less-structured absorption band, reflecting molecular crowding
in the liquid. Meanwhile, the fluorescence spectrum of 1 in its
pure liquid form is almost identical to that observed in CHCl3
solution.
Consistently, the fluorescence lifetimes and quantum yields
for 1 in pure liquid were comparable to those observed for
dilute CHCl3 solution (Figure S7, Table S1 in SI). These
observations indicate the absence of strong electronic
interactions among chromophores in the ground or excited
states in liquid 1 (Figure 3b). On the other hand, the sensitizer
2 doped in liquid acceptor 1 (2/1 = 0.01 mol %) showed a Q-
band at 511 nm (Figure S8, SI), which is almost identical to
that of 2 dissolved in CHCl3. Note that aggregates of 2 in cast
films showed spectral shift in λmax to 518 nm (Figure 3b).
Together with the polarized optical microscope observation, we
conclude that 2 is molecularly dispersed in liquid 1.
Figure 4a shows steady-state luminescence spectra of 2-
doped liquid 1 obtained at varied incident laser power (2/1 =
0.01 mol %). Very interestingly, blue UC emission peaks were
clearly observed upon excitation of 2 by a 532 nm green laser.
The spectral shape of UC emission (λex = 532 nm) is similar to
that of the normal fluorescence (λex = 375 nm; Figure 3b). It is
noteworthy that the phosphorescence of sensitizer 2 was
completely quenched regardless of the excitation power,
indicating the efficient triplet−triplet energy transfer from 2
to the acceptor liquid 1. To prove the TTA-based UC
mechanism, dependence on the power density of incident light
was investigated. In general, TTA-assisted UC shows a
quadratic incident light power dependence of emission
intensity in the weak-annihilation limit, which consequently
since 9,10-diphenylanthracene (acceptor and emitter) and
Pt(II) octaethylporphyrin (PtOEP, sensitizer) have been
popularly used as a benchmark in solution UC systems.3c,f
Branched alkyl chains were introduced in the periphery of 2, in
order to ensure good miscibility with the liquid acceptor 1.
The nonvolatile liquid 1 showed strong blue emission under
UV light (Figure S2, SI). Differential scanning calorimetry
(DSC), rheology, X-ray diffraction (XRD), and small-angle X-
ray scattering (SAXS) experiments confirmed the fluid nature
of 1 at ambient temperature (Figures S3−S5, SI). The DSC
and rheology results indicated a glass transition temperature at
−59 °C and a low-viscosity of 0.99 Pa·s, respectively. The
diphenylanthracene chromophores in liquid 1 showed an
averaged core-to-core distance of 2.1 nm by SAXS measure-
ment. When PtOEP was used as donor, it was not molecularly
dissolved in 1, and irregular crystalline structures were observed
under polarized optical microscopy (Figure S6, SI). Meanwhile,
the sensitizer 2 modified with branched alkyl chains was
homogeneously miscible with liquid 1 in the examined
concentration range up to 2/1 = 1 mol %. To optimize the
donor-to-acceptor ratio, we measured absolute emission
quantum yields of 1 at various mixing ratios (2/1 = 0.001−1
mol %, λex = 365 nm; Table S1 in SI). 1 in the pure liquid form
showed a high quantum yield of 0.68, while its fluorescence
underwent quenching upon increasing the molar ratio of 2
notably above 0.1 mol %. It indicates the occurrence of singlet
Forster resonance energy transfer from 1 to 2 at the higher
̈
doping ratio. Accordingly, 2 was added to 1 at the low molar
ratio of 0.01 mol % in all of the following experiments.
Figure 3a presents normalized absorption and emission
spectra of 1 and 2 in CHCl3 (concentration, 0.1 mM). 1
showed a typical vibrational structure of the La absorption band
(320−390 nm), with its fluorescence observed with a maximum
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characteristics of the TTA-based UC processes which occurred
via long-lived triplet states.
We then quantified the UC efficiency of the 2-doped liquid
(2/1 = 0.01 mol %) by using Nile red-dissolved liquid 1
(concentration, 0.01 mol %) as a reference (see SI for
details).3a,9 Figure 4c shows a dependence of quantum yield on
the excitation power density, in which each represents an
average of at least three independent measurements using
different batches. With increase in the power density, the
quantum efficiency increased and reached saturation at 28%.
Another set of experiments conducted by using rhodamine B in
liquid 1 as a reference also reproducibly gave similar quantum
yields. The observed remarkably high quantum yield is close to
the highest records reported for solvent-free UC systems,4f,8
showing a great potential of the present liquid upconverting
system. Although higher concentration of the sensitizer is
desirable to generate excited triplet states in higher density, the
donor/acceptor molecular ratio (2/1 = 0.01 mol %) employed
was optimal in terms of UC quantum yield. At the higher
donor/acceptor ratio of 2/1 = 0.1 mol %, the obtained
quantum yield showed a decrease to 13% because of the lower
fluorescence efficiency of 1 caused by the quenching via singlet
energy transfer to 2 (Table S1, SI).
As described before, strict deoxygenation treatment is
generally inevitable in solution to achieve UC phenomena
since oxygen molecules quench triplet excited states7 and it
severely hampers their real-life applications. Upconverted
emission has been exceptionally observed under aerated
conditions by using rubber polymers,4f,8 soybean oil,3e and a
hexadecane/polyisobutylene mixture3g as matrices, in which the
diffusion or concentration of oxygen was suppressed by these
matrices. We therefore investigated sensitivity of the present
liquid TTA-UC system to molecular oxygen. Remarkably, the
current liquid UC system is unperturbed by the presence of
oxygen molecules. The UC data obtained in vacuoUC
emission spectra, incident light power dependence of UC
emission, and UC luminescence decay for 2-doped liquid 1 (2/
1 = 0.01 mol %)were almost identical to those obtained in
air (Figures S7 and S9, SI). There was no obvious loss in the
quantum efficiency within 20 days of air exposure, and a good
quantum yield as high as 15% was still observed even after the
longer period of 80 days (Figure S10, SI). These results are
explicable by suppressed diffusion of oxygen molecules into
liquid chromophores, indicating an air-sealing effect of molten
alkyl chains introduced around the chromophores.
Figure 4. (a) Photoluminescence spectra of the doped liquid (2/1 =
0.01 mol %) with different incident power density of 532 nm laser in
air. (b) Dependence of UC emission intensity at 433 nm on the
incident power density (2/1 = 0.01 mol %). The dashed lines are
fitting results with slopes of 1.83 (purple) and 0.85 (green) in the low-
and high-power regimes, respectively. (c) Quantum yield of the doped
liquid (2/1 = 0.01 mol %) measured as a function of 532 nm incident
power density.
To prove the presence of triplet energy migration in the
liquid acceptor UC system, UC experiments were conducted in
the low-temperature glassy state. As shown in Figure S11 (SI),
the relative UC emission intensity at 433 nm decreased with
the decrease in temperature (2/1 = 0.01 mol %; λex = 532
nm).4b It is noteworthy that the UC emission is observed even
below the glass transition temperature (−59 °C), with ∼20% of
the intensity preserved compared to those observed at ambient
temperatures. This result clearly demonstrates that the triplet
energy migration occurs among frozen acceptor molecules,
without the help of molecular diffusion. This is a remarkable
feature contrasting strongly with the previous solution/polymer
dispersion systems in which UC emissions were completely
suppressed below the frozen temperature of matrices.4b The
involvement of triplet energy migration is a unique character-
istic of the photon UC in condensed chromophores. It is
understood that triplet energy transfer and energy migration
occur via the electron exchange mechanism (Dexter excitation
turns into first-order dependence in the saturated annihilation
regime.7 Figure 4b presents a double logarithm plot for the UC
emission intensity of the 2-doped liquid 1 (2/1 = 0.01 mol %)
as a function of incident light power density (λex = 532 nm). At
the lower-power regime of <50 mW cm−2, a slope close to 2
was observed, whereas it changed to ∼1 at higher-power
density (>50 mW cm−2). It provides decisive experimental
evidence for TTA-based photon UC in this solvent-free liquid
system. It is noteworthy that the crossover threshold was
observed at a relatively low power density around 50 mW cm−2,
which is comparable to those reported for the solvent-free
polymer systems.4f,8 The TTA-based UC in the present liquid
system was further characterized by the luminescence decay
measurements. The UC emission of the doped liquid (2/1 =
0.01 mol %) showed a decay in the millisecond time scale
under excitation at 531 nm, which is significantly longer than
the fluorescence lifetime of pure liquid 1 (τ = 7.6 ns; λex = 365
nm; Figure S7 in SI). Such prolonged luminescence decay is
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transfer).10 It requires overlap of wave functions between
donors and acceptors, and consequently, it has a much steeper
exponential dependence on distance compared to that
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triplet energy migration among the adjoining liquid acceptor
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neighboring molecules is required to maximize the overlap of
wave functions. The temperature dependence observed in
Figure S11 (SI) therefore demonstrates the involvement of
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ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and characterization of liquid 1 (DSC,
rheology, XRD, SAXS), luminescence decays, absolute
quantum yield, temperature-dependent UC emission. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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Soc. 2005, 127, 1358.
AUTHOR INFORMATION
■
Corresponding Authors
yanai@mail.cstm.kyushu-u.ac.jp
n-kimi@mail.cstm.kyushu-u.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid for Scientific
Research (S) (25220805) from the Ministry of Education,
Culture Sports, Science and Technology of Japan and by JST,
CREST. P.D. acknowledges JSPS postdoctoral fellowships for
foreign researchers.
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