Reversible photoswitching of triplet-triplet annihilation upconversion using dithienylethene photochromic switches

Article abstract of DOI:10.1021/ja504211y

Reversible photoswitched triplet-triplet annihilation upconversion (TTA UC) was demonstrated with dithienylethene (DTE) derivatives as the photochromic units, 2,6-diiodoBodipy as the triplet photosensitizer, and perylene as the triplet acceptor/emitter. T

Full text of DOI:10.1021/ja504211y

Communication
pubs.acs.org/JACS
Reversible Photoswitching of Triplet−Triplet Annihilation
Upconversion Using Dithienylethene Photochromic Switches
Xiaoneng Cui, Jianzhang Zhao,* Yuhan Zhou, Jie Ma, and Yilong Zhao
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2
Ling-Gong Road, Dalian 116024, P. R. China
S
* Supporting Information
The rationale for the current photoswitchable TTA UC
ABSTRACT: Reversible photoswitched triplet−triplet
annihilation upconversion (TTA UC) was demonstrated
with dithienylethene (DTE) derivatives as the photo-
chromic units, 2,6-diiodoBodipy as the triplet photo-
sensitizer, and perylene as the triplet acceptor/emitter.
The TTA UC is undisturbed by the open-form DTE but
can be switched OFF upon photoirradiation of the mixture
of the three components at 254 nm, i.e., by the closed-
form DTE. Subsequent visible light irradiation restores the
TTA UC. By studying the competitive triplet-state energy-
transfer processes with nanosecond time-resolved transient
difference absorption and fluorescence spectroscopy, we
confirmed that the quenching of the perylene triplet
excited state by closed-form DTE is dominant among the
four possible quenching processes.
protocol is that the open-form DTE (DTE-(o)) gives a high
T1-state energy level (ca. 1.97 eV);33 thus, the triplet-state-
related TTA UC photophysical processes will not be inhibited,
because both B-1 and perylene show much lower T1-state
energy levels (ca. 1.69 and 1.53 eV, respectively) than DTE-
(o).1,34 Upon UV irradiation, DTE-(o) will be reversibly
transformed to the closed-ring form (DTE-(c)), which has a
very low T1-state energy level (1.23 eV; see the Supporting
Information (SI) for the time-dependent density functional
theory calculations of T1-state energy levels).26 As such, the
triplet states involved in the TTA UC will be quenched by the
DTE-(c), and TTA UC will be switched OFF. By visible light
irradiation, DTE-(c) will be switched back to DTE-(o), and the
TTA UC will be recovered. With nanosecond time-resolved
spectroscopy, we confirmed that the singlet excited states of
DTE played a negligible role in switching the TTA UC.
Two DTE units were investigated for photoswitching of
TTA UC, i.e., DTE-1 and DTE-2 (Scheme 1). DTE-1 has
ethynyl moieties; thus, further molecular derivatization is
feasible.
riplet−triplet annihilation upconversion (TTA UC) has
Tattracted much attention due to its advantages of strong
harvesting of the non-coherent and low-power-density
excitation light, high UC quantum yield, and tunable
excitation/emission wavelength.1−5 To date, great efforts have
been devoted to the preparation of new efficient triplet
photosensitizers2,6,7 or new triplet acceptors8,9 and the study of
the kinetics of the photophysical processes.10−12 Applications of
TTA UC in luminescence bioimaging and solar cells have also
been studied.13−18 However, an external stimuli-responsive or
switched TTA UC protocol has never been reported, although
this kind of controllable TTA UC would offer unprecedented
spatial and temporal resolution, for example, in luminescence
bioimaging.
First, the intrinsic photochromism of DTE-1 and DTE-2 was
studied (Figure 1). The open forms of both compounds give
absorption bands only in the UV region (259 and 257 nm,
respectively). Upon photoirradiation with monochromic light
Scheme 1. Dithienylethene Derivatives (DTE-1 and DTE-2)
a
Used in the Photoswitching of TTA UC
Herein for the first time we demonstrate that the TTA UC
can be photoswitched. Dithienylethene (DTE) was selected as
the photochromic unit due to its fatigue resistance, thermal
stability of the two metastable states, and feasibly derivatizable
molecular structures.19−23 DTE has been used to switch the
fluorescence or electron transfer of organic fluorophores24−28
and the triplet excited state or nonlinear optical properties of
Ru(II), Os(III), Pt(II), and Ir(III) complexes,29−31 and recently
to control the production of singlet oxygen (1O2) by a
porphyrin complex.26 However, it has never been used for
photoswitching of UC. Here a new protocol of photoswitched
TTA UC is devised with DTE as the photochromic unit, 2,6-
diiodoBodipy as the organic triplet photosensitizer, and
perylene as the triplet acceptor/emitter.32
a
B-1 is the triplet photosensitizer, and perylene (Py) is the triplet
energy acceptor and emitter of the TTA UC. DTE-1(c) and DTE-
2(c) are triplet quenchers to photoswitch the TTA UC.
Received: April 28, 2014
Published: June 17, 2014
© 2014 American Chemical Society
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DTE-2, respectively. These values are close to previously
reported values obtained by 1H NMR (Figures S9 and
S10).29b,c The quantum yields of the photochromism of
DTE-1 and DTE-2 were measured with a chemical actinometer
(SI). For DTE-1, ΦO→C = 0.299 and ΦC→O = 0.045; for DTE-
2, ΦO→C = 0.267 and ΦC→O = 0.223.
Next, the photoswitching of TTA UC was studied with the
mixture of DTE-1 (or DTE-2), 2,6-diiodoBodipy, and perylene
(Figure 2). In the presence of DTE-1(o), the TTA UC of 2,6-
diiodoBodipy/perylene was not inhibited (Figure 2a). Upon
UV irradiation of the mixed solution at 254 nm, however, the
TTA UC emission intensity decreased sharply, and the TTA
UC was completely quenched after 3 min of 254 nm
photoirradiation. The residual fluorescence of 2,6-diiodoBodipy
was almost not decreased, indicating that the quenching is via
the triplet-state manifold, not the singlet excited state of
Bodipy. Subsequent visible light (>400 nm) photoirradiation
will recover the TTA UC (Figure 2b). Similar photoswitching
of the TTA UC was observed with DTE-2 (Figure 2e,f). The
present photoswitching protocol may find applications with
microcapsules (liquid core/solid shell).14
The reversibility of photoswitching was studied (Figure
2c,g). With DTE-1, substantial loss of UC intensity was
observed for the first cycle (which may be attributed to the
establishment of PSS, thus the persistent presence of the
closed-form DTE); thereafter, the reversibility did not change
significantly (Figure 2c). For DTE-2, no such loss of
reversibility was found (Figure 2g). Considering that the
photoswitch efficacy, that is, the UC emission contrast ratio
between the ON and OFF states, is substantialas high as 20−
50the minor loss of reversibility will not compromise any
application of the current protocol.
Figure 1. (a) UV−vis absorption spectra of DTE-1, DTE-2, and the
photostationary state produced with 254 nm irradiation (1.0 × 10−5
M). Inset: Photographs of the solutions taken at 5.0 × 10−5 M. (b)
Kinetics of photoconversion of DTE-1 (absorbance at 570 nm) and
DTE-2 (absorbance at 525 nm) with 254 nm and visible light
irradiation: [DTE-1 or DTE-2] = 1.0 × 10−5 M in CH3CN, 20 °C.
at 254 nm (hand-held UV lamp), new absorption bands
appeared at 570 and 525 nm, respectively. Correspondingly the
solution turned from colorless to blue and pink. The process is
reversible; that is, upon subsequent visible light irradiation
(>400 nm), the open form was recovered.
Interestingly, we found that the photochromism kinetics of
the two compounds are complementary (Figure 1b). DTE-1
gives relatively faster photocyclization, DTE-1(o)→DTE-1(c),
as compared with that of DTE-2 (Figure 1b). However, DTE-
1(c) gives relatively slower photoreversion kinetics, DTE-
1(c)→DTE-1(o), than DTE-2(c). The fast photocyclization of
DTE-1 may be due to the antiparallel geometry of the
1
molecules, indicated by the H NMR spectrum (SI, Figures S1
and S3).19,33 These complementary photochromic kinetics may
offer an addition dimension for the photoswitch concerning
temporal and spatial resolution or contrast.35 The open/closed
ratios of the two isomers at the photostationary state (PSS)
were determined with HPLC as 7:93 and 44:56 for DTE-1 and
The reversible switching of TTA UC can be seen with un-
aided eyes (Figure 2d,h). Such highly visible reversible
photoswitching TTA UC will be useful in many areas, such
as luminescence bioimaging. Such a photoswitched TTA UC
Figure 2. Photoswitching of the TTA UC with B-1 as photosensitizer and perylene as acceptor by using (a−d) DTE-1 or (e−h) DTE-2 for the
switch. (a,e) Upconversion was observed in the presence of open-form DTE-1 or DTE-2 and was switched off by irradiation of the mixture at 254
nm. (b,f) Upconversion was recovered by subsequent visible light irradiation at >400 nm. (c,g) The reversibility of the photoswitching of TTA UC
with DTE-1 and DTE-2; (d,h) The photographs of the photoswitching of TTA UC. Upconversion was performed upon excitation with a 532 nm
continuous laser (power density is 24.5 mW cm−2; asterisks in a, b, e, and f are the scattered laser): [B-1] = 1.0 × 10−5 M, [Py] = 4.0 × 10−5 M, and
[DTE-1 or DTE-2] = 1.0 × 10−5 M in deaerated CH3CN, 20 °C.
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Communication
will work well in microcapsules, etc.13 UV irradiation may be a
disadvantage, which can be overcome by using sensitized
photochromism so that visible-light-induced photochromism of
DTE can be performed.33
We found that the complementary photochromic kinetics of
DTE-1 and DTE-2 was transduced to the photoswitching of
the TTA UC (Figure 3). The switching OFF process of TTA
longer than that of 2,6-diodoBodipy, and thus resonance energy
transfer (RET) or an inner filter effect is possible. However, no
fluorescence quenching was observed (Figure S15). The same
result was observed with DTE-2 (Figure S16). Therefore, we
conclude that photoswitching of TTA UC does not occur via
quenching of the singlet excited state of perylene; thus, k4 in
Scheme 2 is negligible. RET in the mechanically mixed solution
of perylene and DTE-1(c) is impossible (Figure S15).37
Quenching of the singlet excited state of 2,6-diiodoBodipy
was studied with fluorescence spectroscopy. The fluorescence
of 2,6-diiodoBodipy cannot be quenched by DTE-(c) (Figures
S13 and S14); this is also demonstrated in TTA UC (Figure
2a,e). This non-efficient FRET was also supported by studying
the quenching of the singlet excited state of the un-iodinated
Bodipy (Figures S11 and S12). The singlet-state lifetime of 2,6-
diiodoBodipy (0.38 ns) is much shorter than that of
unsubstituted Bodipy (3.86 ns), which means the singlet
excited state of 2,6-diiodoBodipy is more difficult to quench;
thus, k1 is negligible.32
The quenching constant k2 was measured with nanosecond
time-resolved transient absorption spectroscopy by monitoring
the decreasing triplet-state lifetime of 3[I-Bodipy]* in the
Figure 3. Kinetics of the photoswitching of upconverted fluorescence
intensity with DTE-1 and DTE-2 as the photochromic units. The UC
emission intensity was monitored at 438 nm against the irradiation
time (254 nm or visible light). Data taken from Figure 2.
presence of DTE-1(c), as k2 = (3.05
0.11) × 105 M−1.
Similarly, the kTTET value was measured as (1.04 0.037) ×
106 M−1. Since kTTET is ca. 3-fold of k2, we propose that the
UC with DTE-1 is faster as compared with that of DTE-2.
However, thereafter the switching ON process with DTE-1(c)
is much slower than that with DTE-2 (Figure 3). These results
may offer additional switching dimensions to control the TTA
UC with higher spatial or temporal resolution by using different
DTE derivatives.19
3
quenching of [perylene]* by DTE-1(c) is more significant
3
than the quenching of [I-Bodipy]* by DTE-1(c). In support
of this postulation, the k3 value was measured for the quenching
of the delayed fluorescence lifetime (TTA UC emission, Figure
4b), k3 = (1.17 0.024) × 106 M−1. Therefore, we conclude
Multiple photophysical processes are involved in TTA UC,
such as intersystem crossing (ISC) of the triplet photo-
sensitizer, triplet−triplet energy transfer (TTET), annihilation
of the triplet acceptor, and radiative decay of the triplet
acceptor (perylene, Scheme 2).1−8,36 The possible mechanisms
Scheme 2. Mechanism of Photoswitching TTA UC,
a
Exemplified with DTE-1(c)
Figure 4. (a) Stern−Volmer plots of the quenching processes:
quenching the triplet lifetime of B-1 by DTE-1(c) (k2) or Py (kTTET),
quenching the delayed fluorescence lifetime of perylene (k3), and
quenching the fluorescence lifetimes of B-1 and perylene by DTE-1(c)
(k1, k4). (b) Quenching the delayed fluorescence lifetime of perylene
by DTE-1(c) (k3): [B-1] = 1.0 × 10−5 M in deaerated CH3CN, 20 °C.
a
Quenching (with DTE-1 as quencher) is denoted with rate constants
of k1, k2, k3, and k4. The rate constants for the photophysical processes
involved in TTA UC are denoted with kISC (intersystem crossing),
kTTET (triplet−triplet energy transfer), and kTT (triplet−triplet
annihilation rate constant). The TTA UC pathway is highlighted
with yellow background.
3
that it is by quenching of the [perylene]* state that the TTA
UC is photoswitched by DTE-1(c). It should be clarified that
long-lived delayed fluorescence was normally observed (up to
ca. 142.0 μs);38,39 however, this long fluorescence lifetime was
due to the energy pooling effect of the TTET process, not the
inherent lifetime of the singlet state of perylene. All the
bimolecular quenching constants (kq) were calculated (see SI
for details), and the same conclusion was derived. For example,
the kq of k3 was calculated as (8.24 0.17) × 109 M−1 s−1,
which is higher than the typical kTT values (up to 10-fold).12
In summary, photoswitching of triplet−triplet annihilation
upconversion (TTA UC) was achieved for the first time. The
method uses dithienylethene (DTE) as the photochromic unit,
2,6-diiodoBodipy as the triplet photosensitizer, and perylene as
of photoswitching of the TTA UC with DTE-1(c) include
quenching the triplet state of the photosensitizer, quenching
the triplet state of the triplet energy acceptor, and quenching
the singlet state of the perylene (Scheme 2). Therefore,
mechanistic studies were carried out on the photoswitching of
TTA UC with steady-state and nanosecond/picosecond time-
resolved spectroscopy.
First, perylene and DTE-1 were mixed, and photoswitching
of the fluorescence of perylene by DTE-1(c) was studied,
because the absorption wavelength of DTE-1(c), 570 nm, is
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(15) Wohnhaas, C.; Mailander, V.; Droge, M.; Filatov, M. A.; Busko,
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the triplet energy acceptor/emitter. In the presence of open-
form DTE, TTA UC was not perturbed. Upon photoirradiation
of the mixture at 254 nm, the closed form of DTE was
produced, and the TTA UC was completely quenched. The
TTA UC can be photoswitched ON upon subsequent visible
light irradiation of the solution, and the photoswitch is
reversible. With steady-state and nanosecond time-resolved
spectroscopy, we confirmed that the closed form of DTE acts as
a triplet energy acceptor, i.e., mainly as the quencher of the
perylene triplet excited state among several possible quenching
pathways. Therefore, the photoswitching of TTA UC is not via
the singlet excited state of DTE. These results will offer
significant additional, unprecedented dimensions for TTA UC
with controllable spatial or temporal resolution.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures and spectroscopic data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
zhaojzh@dlut.edu.cn
(26) Hou, L.; Zhang, X.; Pijper, T. C.; Browne, W. R.; Feringa, B. L.
J. Am. Chem. Soc. 2014, 136, 910−913.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the NSFC (21273028), Royal Society (UK)-NSFC
(21011130154), and Ministry of Education (SRFDP-
20120041130005, IRT_13R06 and DUT14ZD226) for finan-
cial support.
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