Switching of the triplet-triplet-annihilation upconversion with photoresponsive triplet energy acceptor: Photocontrollable singlet/triplet energy transfer and electron transfer

Article abstract of DOI:10.1021/jp5111828

A photoswitchable fluorescent triad based on two 9,10-diphenylanthracene (DPA) and one dithienylethene (DTE) moiety is prepared for photoswitching of triplet-triplet annihilation upconversion. The DPA and DTE moieties in the triad were connected via Click reaction. The DPA unit in the triad was used as the triplet energy acceptor and upconverted fluorescence emitter. The fluorescence of the triad is switched ON with the DTE moiety in open form [DTE-(o)] (upconversion quantum yield _UC = 1.2%). Upon UV irradiation, photocyclization of the DTE-(o) moiety produces the closed form [DTE-(c)], as a result the fluorescence of DPA moiety was switched off (_UC is negligible). Three different mechanisms are responsible for the upconverted fluorescence photoswitching effect (i.e., the photoactivated fluorescence resonance energy transfer, the intramolecular electron transfer, as well as the photoactivated intermolecular triplet energy transfer between the photosensitizer and DTE-(c) moiety). Previously, the photoswitching of TTA upconversion was accomplished with only one mechanism (i.e., the triplet state quenching of the photosensitizer by DTE-(c) via either the intermolecular or intramolecular energy transfer). The photophysical processes involved in the photochromism and photoswitching of TTA upconversion were studied with steady-state UV-vis absorption and fluorescence emission spectroscopies, nanosecond transient absorption spectroscopy, electrochemical characterization, and DFT/TDDFT calculations.

Full text of DOI:10.1021/jp5111828

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Article
Switching of the Triplet-Triplet-Annihilation Upconversion with
Photoresponsive Triplet Energy Acceptor: Photocontrollable
Singlet/Triplet Energy Transfer and Electron Transfer
Kejing Xu, Jianzhang Zhao, Xiaoneng Cui, and Jie Ma
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp5111828 • Publication Date (Web): 16 Dec 2014
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Switching of the TripletꢀTripletꢀAnnihilation
Upconversion with Photoresponsive Triplet Energy
Acceptor: Photocontrollable Singlet/Triplet Energy
Transfer and Electron Transfer
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Kejing Xu, Jianzhang Zhao* Xiaoneng Cui and Jie Ma
State Key Laboratory of Fine Chemicals,School of Chemical Engineering, Dalian University of
Technology, Dalian 116024 (China)
Eꢀmail: zhaojzh@dlut.edu.cn;
Web: http://finechem2.dlut.edu.cn/photochem
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Abstract: A photoswitchable fluorescent triad based on two 9,10ꢀdiphenylanthracene (DPA) and
one dithienylethene (DTE) moiety is prepared for photoswitching of tripletꢀtriplet annihilation
upconversion. The DPA and DTE moieties in the triad were connected via Click reaction. The
DPA unit in the triad was used as the triplet energy acceptor and upconverted fluorescence
emitter. The fluorescence of the triad is switched ON with the DTE moiety in open form (DTE-
(o)) (upconversion quantum yield ΦUC = 1.2%). Upon UV irradiation, photocyclization of the
DTE-(o) moiety produces the closed form (DTE-(c)), as a result the fluorescence of DPA moiety
was switched off (ΦUC is negligible). Three different mechanisms are responsible for the
fluorescence photoswitching effect, i.e. the photoactivated fluorescenceꢀresonanceꢀenergyꢀ
transfer (FRET), the intramolecular electron transfer, as well as the photoactivated
intermolecular triplet energy transfer between the photosensitizer and DTE-(c) moiety.
Previously the photoswitching of TTA upconversion was accomplished with only one
mechanism, i.e. the triplet state quenching of the photosensitizer by DTE-(c) via either the
intermolecular or intramolecular energy transfer. The photophysical processes involved in the
photochromism and photoswitching of TTA upconversion were studied with steady state UV−vis
absorption and fluorescence emission spectroscopies, nanosecond transient absorption
spectroscopy, electrochemical characterization, and DFT/TDDFT calculations.
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Keywords: Energy Transfer; Photosensitizer; Switch; Tripletꢀtriplet annihilation; Upconversion
INTRODUCTION
Photon upconversion is important for luminescent bioimaging,1,2 photovoltaics,3−5 and
photocatalytic hydrogen (H2) production,6 etc. Among the upconversion methods, tripletꢀtriplet
annihilation upconversion (TTA UC) has attracted much attention owing to the advantage of low
power and nonꢀcoherent excitation requirement (less than 100 mW cmꢀ2, solar light is sufficient),
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strong absorption of the excitation energy, and high upconversion quantum yields.7−10 However,
the known TTA upconversion is uncontrollable via external stimuli, such as pH, light irradiation,
or with chemical species.
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On the other hand, dithienylethene (DTE) is a versatile photochromic chromophore.11ꢀ29
Previously the fluorescence of Bodipy and other fluorophores were photoswitched by the
appended DTE unit, via the mechanisms of fluorescenceꢀresonanceꢀenergyꢀtransfer (FRET),30−34
or aggregation,35 photoswitched electron transfer,36−42 or disruption of πꢀconjugation
framework.11,43 For example, DTEꢀo gives absorption only in UV spectral region, whereas DTEꢀ
c normally gives absorption at ca. 600 nm.11 Thus the fluorescence emission of the covalently
attached fluorophore (which gives emission length shorter than ca. 600 nm) can be quenched by
FRET.44,45 Photoswitching of triplet state of transition metal complexes was also reported.46−49
Since fluorescent compounds are used in TTA upconversion as triplet acceptor/upconverted
fluorescence emitter,8 we envisaged that a triplet acceptor/emitter in TTA upconversion, such as
DPA, covalently linked with DTE chromophore, can be used as photoresponsive triplet
acceptor/emitter for photoswitching of TTA upconversion. Such investigation will present a new
strategy for photoswitching of TTA UC.
Recently we performed photoswitched TTA upconversion with dithienylethene (DTE)
photochromic unit as intermolecular triplet energy quencher.50 Such switchable TTA
upconversion will offer unprecedented modulability for the application of TTA UC, for example,
in photoꢀactivatable localization microscopy (PALM), a popular superꢀresolution fluorescence
microscopy.51,52 It will be also significant for construction of molecular devices, and for study of
photophysics of organic chromophores.53
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Our previous approach is based on the mechanism of intermolecular triplet state quenching
within the threeꢀcomponent mixture. The DTE compound shows switchable T1 state energy level,
thus in the closed form, the DTE compound acted as triplet energy acceptor to quench the triplet
excited state of the photosensitizers, thus the TTA UC is switched off. Note the
photocycloreversion DTEꢀ(c)→DTEꢀ(o) is usually nonꢀefficient, otherwise the quenching effect
of DTEꢀo will not last long. In the open form, the DTE moiety is with much higher T1 state
energy level, and the triplet state of the photosensitizer is unable to be quenched by DTEꢀo.50
Thus photoswitchable TTA upconversion was achieved. However, in order to make this
photoswitchable TTA upconversion more efficient, new photoswitching motif is desired.
Following this line, herein we prepared a 9,10ꢀdiphenylanthracene (DPA) derived triplet
acceptor/emitter with DTE unit attached (DD-o. Scheme 1). Triad DD-o contains two DPA
moieties and one DTE moiety. The three chromophores are linked together by the feasible Click
reaction. In the open form, DD-o is effective for TTA upconversion, with DPA unit as the triplet
energy acceptor/emitter. By photoswitching the DTE moiety in DD-o to the closed form (DDꢀc),
the fluorescence of the DPA moiety in DD-(c) is quenched by FRET, thus the TTA upconversion
is photoswitched off. Thereafter the TTA upconversion can be photoswitched ON upon
irradiation of the triplet acceptor at 580 nm. Moreover, we confirmed that the fluorescence of the
DPA moiety is not only quenched by FRET, the photoinduced intramolecular electron transfer
also contributes to quenching effect. Furthermore, intermolecular triplet state energy transfer
from the triplet photosensitizer to the DTEꢀ(c) unit is also responsible for the switching of TTA
UC. Thus the present photoswitchable TTA UC motif is with tripleꢀmechanism, i.e. the FRET,
intramolecular electron transfer, and the intermolecular triplet energy transfer. The photophysical
processes of the photoswitching system were studied with steady state UV−vis absorption and
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fluorescence emission spectroscopies, nanosecond
transient absorption spectroscopy,
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electrochemical characterization (cyclic voltammetry) and DFT/TDDFT calculations.
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RESULTS AND DISCUSSIONS
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Design and Synthesis of the compounds. In order to photoswitch the fluorescence of DPA,
the photochromic unit DTE was covalently linked with DPA. The feasible Click reaction was
used to link the chromophores together. The fluorescence of DPA is at 400 nm.8,54 DTEꢀ(o)
shows absorption only in UV region, but DTEꢀ(c) shows absorptions at ca. 600 nm,11 thus
photoswitchable FRET from DPA moiety to DTEꢀ(c) moiety is envisaged.32,55,56 With the DTE
moiety in the open form, the DPA unit is able to fluoresce, since the S1 state of DTEꢀo is much
Scheme 1. Dithienylethene Derivative (DTE) and DPA Derivatives (DD-o) Used in the
Photoswitching of TTA UC a
F2
C
F2C CF2
N
N
N
N
N
N
S
S
O
O
DD-o
F2
F2
C
C
254 nm
Relatively Fast
F2C CF2
F2C CF2
580 nm
Relatively slow
S
S
S
S
closed form
c
o
open form
N
N
N
PF6
O
O
H
N
F2
C
N
N
N
O
Ir
F2C CF2
N
N
N
S
S
DPA
DTE
Ir-1
6
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a
Ir-1 is the triplet photosensitizer and 9,10ꢀDiphenylanthracene (DPA) is the triplet energy
acceptor and emitter of the TTA upconversion.
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Scheme 2. Synthesis of Compounds 1 – 5 and DD-o a
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OH
Br
c
b
a
Br
1
2
3
d
O
N3
Br
N
N
O
O
N
f
6
NaN3
e
g
DD-o
DTE-o
5
4
F2
F2
C
C
F2C CF2
F2C CF2
N
N
N
N
N
N
S
S
O
O
S
S
DTE-o
DD-o
a Key: (a) Phenylboronic acid, K2CO3, Pd(PPh3)4, toluene/ ethanol/ water, reflux, 6 h. Yield: 93%;
(b) Br2, CHCl3, r.t., 75 min. Yield: 82%; (c) 4ꢀhydroxyphenylboronic acid, Na2CO3, Pd(PPh3)4,
toluene/ ethanol/ water, reflux, 6 h. Yield: 69%; (d) 1,2ꢀdibromoethane, anhydrous K2CO3,
TBAB, dry acetone, reflux, 3d. Yield: 50%; (e) NaN3, DMF, 100 °C, 1 h. Yield: 67%; (f)
Phenylacetylene, Et3N, CuSO4·5H2O, and sodium ascorbate, 24 h, at r.t., in Ar, Yield: 71%; (g)
similar with step f, Yield: 56%.
higher than the S1 state of DPA moiety, thus no FRET exists to quench the fluorescence of DPA.
Upon UV photoirradiation, DTEꢀo transformed into the closed form, DTEꢀc, for which the S1
state energy level (approximated with the UV−vis absorption maximum) is lower than the S1
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state of DPA moiety, thus FRET will quench the fluorescence of DPA moiety. It should be
pointed out that the photocycloreversion quantum yield of DTE-c is usually very low,11 this
property is crucial for DTE-(c) to be used as triplet state trap in the photoswitchable TTA
upconversion, otherwise DTEꢀo will be produced and accumulated and the TTA upconversion
OFF state will not last long. Thus the TTA upconversion can be photoswitched ON and OFF.32
An Ir(III) complex Ir-1 was used as the triplet photosensitizer due to its strong absorption of
visible light and the longꢀlived triplet excited state.57
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The synthesis of DD-o is with 9ꢀbromoanthracene as starting material (Scheme 2). Suzuki
cross coupling reaction with phenylboronic acid gives the product 9ꢀbromoꢀ10ꢀphenylanthracene.
A further Suzuki cross coupling reaction with 4ꢀhydroxylphenylboronic acid gives the
intermediate compound 3. After two steps of routine synthesis, the azide compound 5 was
obtained. The Cu(I)ꢀcatalyzed Click reaction between compound 5 and the ethynylated DTE
compound DTE-o gives the target compound DD-o. All the compounds were obtained with
moderate to satisfactory yields.
UV−vis absorption and fluorescence spectra. UV−vis absorption spectra of the compounds
were studied (Figure 1). With the DTE unit in the open form, the major absorption of DD-o is
attributed to the DPA moiety. Upon UV light irradiation (254 nm), a new broad absorption band
centered at 580 nm was observed, which is due to the closed form of DTE moiety (DD-c). The
absorption of DDꢀo/DD-c in the 350 nm − 450 nm is identical to that of DPA, thus we propose
that there is no significant interaction between the chromophores in DD at the ground state.32,58,59
The fluorescence of DD-o and DD-c were compared with that of DPA (Figure 1b). The
fluorescence of DD-o is close to that of DPA, whereas the fluorescence of DD-c is much weaker
than DPA (quenched to 19.4% of the fluorescence intensity of DD-(o)). Based on the
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composition of the photostationary state (PSS. The DD-(o) is of 26.4% of the total DD), the
fluorescence in DD-c is due to the residual DD-o in the PSS state.32 The fluorescence of DPA
moiety in the triad DD was significantly quenched in DD-c.32,46,55
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0.4
300
a
b
0.3
DD-c
200
DD-o
DPA
6
DD-o
DD-c
DPA
0.2
0.1
0.0
hv
100
0
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450
500
550
300
400
500
600
700
Wavelength / nm
Wavelength / nm
Figure 1. (a) UV−vis absorption spectra of DD, DPA and compound 6. (b) The fluorescence
emission spectrum of DD-o, DD-c and DPA, measured with optically matched solutions (A372 nm
= 0.12). The concentration is slightly different. λ
ex = 372 nm. c = 1.0 × 10−5 M in toluene; 20°C.
The UV−vis absorption changes of DD-o during the photochromism were studied (Figure 2).
Upon 254 nm irradiation of the DD-o solution, a new absorption band at 580 nm appeared
(Figure 2a). This is due to the photocyclization of DD-o thus generation of DD-c. The process
takes ca. 15 min to finish. The quantum yield of the photocyclization (ΦO→C) was determined as
0.78% with chemical actinometer potassium ferrioxalate actinometer (see Supporting
Information for details). The composition at the photostationary state (PSS) is with 26.4% of
DD-o and 73.6% of DD-c. After establishment of the PSS with UV light, the solution was
irradiated with 580 nm light, the absorption band at 580 nm decreased (Figure 2c). However, the
photocycloreversion (DD-c→DD-o) process is much slower than the photocyclization (DD-
o→DD-c). For the photocycloreversion, a very low quantum yield (ΦC→O) of 0.02% was
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determined.46,60 This nonꢀefficient photocycloreversion is crucial for DD-c as the triplet/singlet
energy trap for photoswitching of TTA UC (see later section for detail discussions). It should be
noted that the quantum yields of the photocyclization and the photoreversion of DTE may vary
substantially, depend on the molecular structures.11,61 For example, previously the ΦO→C and
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Φ
C→O of DTEꢀcontaining porphyrin derivatives were determined as 0.26% and 0.075%.55 The
color change of the solution accompanying the transformation is distinct. DD-o is almost
colorless, whereas DD-c is in purple color (Figure 2a and 2b).
0.6
0.15
UV
UV
254 nm
DD-o
a
b
DD-c
DD-o
DD-c
254 nm
0.4
0.2
0.0
0.10
0.05
0.00
0.15
960 s
......
0 s
UV
0
300
600
900
300 400 500 600 700
Wavelength / nm
Time / s
Vis
c
DD-c
DD-o
d
Vis
580 nm
580 nm
DD-c
DD-o
0.4
0.10
0.05
0.00
0 min
......
90 min
0.2
580 nm
0.0
0
20 40 60 80 100
Time / min
300
400
500
600
700
Wavelength / nm
Figure 2. (a) Changes of UV−vis absorption spectra of DD-o upon irradiation with UV light
(254 nm, 2.0 W/ m2) and (b) the corresponding kinetics trace by following the absorption at 580
nm. (c) Changes of UV−vis absorption spectrum of DD-c upon visible light irradiation at 580 nm
(10.0 W/ m2) and (d) the corresponding kinetics trace by following the absorption at 580 nm. c =
1 × 10−5 M in toluene. Inset: the photographs of the solution taken at 1.0 × 10−4 M; 20 °C.
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Photoswitching of the fluorescence of DD-o upon irradiation at 254 nm and 580 nm were also
studied (Figure 3). For DD-o, strong blue fluorescence was observed (ΦF = 82.9%). Upon 254
nm irradiation, the fluorescence intensity decreased (Figure 3a). At the PSS, the fluorescence
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1.00
0.75
0.50
0.25
0.00
UV 254 nm
UV 254 nm
DD-o
DD-o
DD-c
0 s
DD-c
300
200
100
0
......
UV
600 s
a
c
b
0
200
400
600
400
450
500
550
Time / s
Wavelength / nm
UV 580 nm
DD-o
UV 580 nm
DD-c
d
300
200
100
0
DD-c
DD-o
1.0
0.5
0.0
80 min
......
0 min
Vis
0
20
40
60
80
400
450
500
550
Wavelength / nm
Time / min
Figure 3. (a) Changes of fluorescence emission spectrum of DD-o upon irradiation with UV light
(254 nm, 2.0 W/ m2) and (b) the corresponding kinetic trace by monitoring the emission intensity
at 416 nm. Inset: the photographs of the photoswitching of the emission; (c) Changes of emission
spectrum of DD-o upon irradiation with visible light (580 nm, 10.0 W/ m2) and (d) the
corresponding kinetic trace by monitoring the emission intensity at 416 nm. Inset: the
photographs of the photoswitched emission. λ
ex = 372 nm, c = 1 × 10−5 M, in aerated toluene; 20
°C.
was decreased to 12.8% of the original value (ΦF = 17.1%). At PSS the fluorescence of DD-c
was not completely switched off, which may be due to the equilibrium established at the PSS
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(DDꢀo is of 26.4% of the PSS mixture).39,46,47,55,62 The emission color of the DD-o faded from
blue to dark (Figure 3b). The fluorescence recovery during the photocycloreversion upon 580 nm
irradiation was also observed (Figure 3c). The fluorescence is nearly completely recovered.
Correspondingly the emission of the solution was enhanced (Figure 3d). It is noticeable that the
photoreversion is much slower than the photocyclization process (Figure 3b and 3d).
The reversibility, or the fatigue resistance of the system was studied with the UV−vis
absorption and fluorescence responses (Figure S17, Supporting Information). The results show
that the switching of UV−vis absorption and the fluorescence is reversible, no significant loss of
the fluorescence intensity was observed after three cycles. It is known that DTE is resistant to
photodecomposition. Furthermore, we found that the photocyclization quantum yield in toluene
is much lower than that in dichloromethane and acetonitrile.46
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Table 1. Photophysical Parameters of the Compounds a
ε b
λ
abs/nm
λ
em/nm
τF /ns c
ΦF (%) d
Φ Φ
O→C (%) e C→O (%) f
g
g
436
5.3
489
416
416
3.35
4.40
4.52
0.7
Ir-1
−
−
−
g
375
2.9
82.9
17.1h
0.78
DD-o
DD-c
DTE-o
DTE-c
DPA
6
g
375/ 580
282
3.7/ 1.1
2.2
0.015
−
g
g
g
g
0.68
−
−
−
−
g
g
g
g
570
0.75
1.4
0.084
−
−
−
−
g
g
373
408
416
5.30
4.83
92.5
86.3
−
−
g
g
375
1.3
−
−
b
c
a In toluene, c =1.0 × 10−5 M. Molar absorption coefficient.
ε
: 104 M−1 cm−1. Fluorescence
d
lifetimes. Fluorescence quantum yields, determined with 9,10ꢀdiphenylanthracene (DPA) as
reference (ΦF = 95.0 % in cyclohexane). e The quantum yields of the photochromism of DTEO→C
f
g
and DDO→C
.
The quantum yields of the photochromism of DTEC→O and DDC→O
.
Not
applicable. h Determined at the photostationary state (PSS).
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The photophysical properties of the compounds were summarized in Table 1. The fluorescence
5
6
7
quantum yield of DD-o (ΦF = 82.9%) is slightly decreased as compared with that of DPA (ΦF =
8
9
92.5%). Upon photocyclization, the DD-c shows much smaller fluorescence quantum yield (ΦF
=17.1%). Thus the photoswitching effect is significant.
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Fluorescence Quenching Mechanism. We have shown that the fluorescence of DD-o is
switched off by the photocyclization of DTE moiety (Figure 3a). In order to study the quenching
mechanism, the fluorescence lifetimes of the compounds were determined with the time
correlated single photon counting technique (TCSPC) (Figure 4). It is known that kinetic
quenching will give reduced fluorescence lifetimes, whereas the static quenching of fluorescence
does not show any fluorescence lifetime changes.63 DPA gives fluorescence lifetime of 5.30 ns.
DD-o give slightly shorter fluorescence lifetime of 4.40 ns.
105
DPA,
τ
= 5.30 ns
DD-c, = 4.52 ns
6, = 4.83 ns
104
103
102
τ
τ
DD-o
= 4.40 ns
τ
0
10 20
30
40
50
Time / ns
Figure 4. The fluorescence lifetime curves of DD-o, DD-c and compound 6 by monitoring the
emission at 416 nm. Excited at λex = 405 nm (picosecond pulsed laser), c = 1.0 × 10−5 M in
toluene; 20 °C.
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Note the fluorescence quantum yield DD-(o) (ΦF = 82.9% ) is close to DPA (ΦF = 92.5%) and
compound 6 (ΦF = 86.3%). Interestingly, DD-c gives fluorescence lifetime of 4.52 ns, but the
7
8
9
fluorescence quantum yield was greatly reduced (ΦF =17.1%, Table 1, at PSS). Considering the
determination error, we can consider that the fluorescence lifetime of DD-o and DD-c are very
close. Note that the fluorescence of DD-c is much weaker than that of DD-o. Thus we propose
the quenching of the fluorescence in DD-c (at PSS) is similar to a static quenching,63 that is, for
the molecules with DTE unit in closed form, the fluorescence of the DPA unit is completely
quenched and this portion does not contribute to the lifetime value; the fluorescence of DD-c (at
PSS) is from the residual DD-o species in the PSS mixtures of the closed and opened isomers.
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800
600
400
200
0
1000
800
600
400
200
0
Toluene
CH2Cl2
c
b
100
50
0
a
Toluene
CH2Cl2
Toluene
CH2Cl2
CH3CN
CH3OH
CH3OH
CH3CN
CH3OH
CH3CN
Compound 6
DD-o
DD-c
400
450
500
550
600
400
450
500
550
600
400 450 500 550 600
Wavelength / nm
Wavelength / nm
Wavelength / nm
Figure 5. Fluorescence emission spectra of (a) DD-o, (b) DD-c and (c) compound 6 in solvents
ex = 372 nm, c = 1.0 × 10−5 M; 20 °C.
with different polarity.
λ
Besides the FRET, other photophysical processes may also contribute to the quenching of the
fluorescence of a multichromophore molecular assembly, such as photoinduced electron transfer
(PET). In order to study the PET effect, the fluorescence emission spectra of DD-o in solvents
with different polarity were compared with that of compound 6, which contains only the DPA
moiety (Figure 5). The fluorescence emission intensity of compound 6 is almost independent on
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the solvent polarity, for that similar fluorescence emission intensity was observed in solvents of
toluene, dichloromethane, acetonitrile and methanol (Figure 5c). For DD-o and DD-c, however,
the fluorescence emission intensity was quenched by half in acetonitrile than that in toluene
(Figure 5a and 5b). Note that the emission wavelength in toluene and acetonitrile are similar, thus
the quenching is due to intramolecular charge transfer.30,63 The UV−vis absorption spectra did
not show such significant variation in different solvents (Figure S21, Supporting Information).
Electrochemical Study: Cyclic Voltammetry. In order to study the photoinduced electron
transfer in DD-o and DD-c, the redox property of the compounds were studied with cyclic
voltammetry (Figure 6). The reference compounds were also studied. DPA shows reversible
oxidation wave at +0.87 V, and reversible reduction wave at −2.31 V was observed (Figure 6a).
For compounds 6, similar redox potentials were observed (Figure 6b). For DD-o, besides the
redox waves assigned to the DPA moieties, an irreversible reduction wave at −1.89 V was found
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Fc+ / Fc
a
b
Fc+ / Fc
DPA
Compound 6
1
0
-1
-2
1
0
-1
-2
Potential / V
Potential / V
d
c
Fc+ / Fc
Fc+ / Fc
DD-c
-2
DD-o
-2
1
0
-1
1
0
-1
Potential / V
Potential / V
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Figure 6. Cyclic voltammogram of (a) DPA, (b) compound 6, (c) DD-o and (d) DD-c. In
deaerated CH3CN containing 0.10 M Bu4NPF6 as supporting electrode, Ag/AgNO3 reference
electrode. Scan rates: 50 mV/s. Ferrocene (Fc) was used as internal reference. c = 1.0 × 10−3 M,
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20 °C.
(Figure 6c). For the DD-c, an pseudoꢀreversible oxidation wave at +0.59 V was observed. A few
irreversible reduction waves at −1.39 V, −1.72 V and −2.27 V were observed (Figure 6d).
In order to study the photoꢀinduced electron transfer in DD-o and DD-c, the Gibbs free energy
changes (ꢀGCS) of the electron transfer processes were calculated with the RohmꢀWeller
equation (eq. 1 and eq. 2. See the Supporting Information for the detail procedure of the
calculations),59
ꢀG0 = e[EOX − ERED ]− E00 + ꢀGS
CS
(Eq. 1)
(Eq. 2)
e2
e2
1
1
1
1









ꢀGS = −
−
+
−
4πεSε0RCC 8πε0 RD RA εREF εS



Where ꢀGS is the static Coulombic energy, which is described by eq. 2. e = electronic charge,
EOX = halfꢀwave potential for monoꢀelectron oxidation of the electronꢀdonor unit, ERED = halfꢀ
wave potential for oneꢀelectron reduction of the electronꢀacceptor unit; note herein the anodic
and cathodic peak potentials were used because in some cases the oxidation is irreversible
therefore the formal potential E1/2 cannot be derived; E00 = energy level approximated with the
fluorescence emission wavelength (for the singlet excited state). εS = static dielectric constant of
the solvent, RCC = (15.5 Å) centerꢀtoꢀcenter separation distance determined by DFT optimization
of the geometry, RD is the radius of the electron donor, RA is the radius of the electron acceptor,
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εREF is the static dielectric constant of the solvent used for the electrochemical studies, ε0
5
permittivity of free space. The solvents used in the calculation of free energy of the electron
6
7
8
9
transfer are CH3CN (ε = 37.5), CH2Cl2 (ε = 8.93), toluene (ε = 2.4, 20 °C). The redox potentials
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and the free energy changes of the electron transfer were compiled in Table 2.
Table 2. Redox Potentials of DPA, Compound 6, DD-o and DD-c a
E(ox) / V
+ 0.87
E(red) / V
DPA
− 2.31
+ 0.85
Compound 6
DD-o
− 2.27
+ 0.85
− 1.89, − 2.26
− 1.39, − 1.72, − 2.27
+ 0.85, + 0.59
DD-c
a
Cyclic voltammetry in N2 saturated CH3CN containing a 0.10 M BuNPF6 supporting
electrolyte; Counter electrode is Pt electrode; working electrode is glassy carbon electrode;
Ag/AgNO3 couple as the reference electrode.
Table 3. The Data of Charge Separation (ΔGCS) and Charge Separation Energy States (ECTS
)
for DD-o and DD-c in Different Solution a
E
CTS (eV)
ꢀGCS (eV)
Toluene CH2Cl2
CH3CN
− 0.26
− 0.77
− 0.14
Toluene
+3.36
CH2Cl2
+2.86
+2.36
+3.00
CH3CN
+2.72
+2.21
+2.84
1DPA∗ → DTEꢀo
−0.12
− 0.62
0.002
+0.38
1DPA∗ → DTEꢀc − 0.12
+2.86
DTEꢀc → 1DPA∗
+0.50
+3.48
a The arrow indicated the direction of electron transfer. “1 and *” means this part of the triad is at
its singlet excited state.
The results show that for DD-o in toluene, the photoinduced intramolecular electron transfer is
thermodynamically prohibited because the Gibbs free energy changes (ꢀGCS) is +0.38 eV. In
polar solvents such as CH2Cl2 and CH3CN, however, the values are −0.12 eV and −0.26 eV, thus
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PET is a thermodynamically allowed process for DD-o in polar solvents. This conclusion is in
agreement with the fluorescence emission of DD-o in solvents with different polarity (Figure 5),
which show that the fluorescence of DD-o was quenched in polar solvents. Previously it was
reported that the electron transfer in DTEꢀcontaining perylenebisimide (PBI) compounds and
Ru(II) complex was modulated by the photocyclization and photoreversion of DTE.36,41
The Gibbs free energy changes of the possible PET in DD-c were also calculated. It was found
that PET is possible even in toluene (ꢀGCS = −0.12 eV). The driving forces for PET in polar
solvents such as CH2Cl2 and CH3CN are larger. These data suggest that the fluorescence of DD-c
may be drastically quenched in polar solvents, which is in agreement with the experimental
results.
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Photoswitching of the TTA Upconversion. The photoswitching of the TTA upconversion
with DD-o as triplet acceptor/emitter was studied (Figure 7). An Ir(III) complex showing strong
absorption of visible lightꢀabsorption and longꢀlived triplet excited state was used as triplet
80
*
120
b
a
60
40
20
0
40 min
30 min
20 min
15 min
10 min
5 min
0 s
5 s
10 s
15 s
20 s
25 s
*
90
60
30
0
ON
Vis
ON
UV
254 nm
580 nm
0 min
OFF
OFF
400
500
600
700
400
500
600
700
Wavelength / nm
Wavelength / nm
Figure 7. Photoswitching of the TTA upconversion with Ir-1 as photosensitizer and DD-o/c as
triplet energy acceptor. (a) The upconverted fluorescence with DD-o as triplet energy
acceptor/emitter upon 254 nm photoirradiation (switching off of the upconversion); (b) The
upconverted fluorescence with DD-c as triplet energy acceptor/emitter upon 580 nm
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photoirradiation (switching off of the upconversion). Upconversion was performed upon
excitation with a 445 nm continuous laser (power density is 5.0 mW). c = 1.0 × 10−5 M in toluene,
20 °C.
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photosensitizer.57 TTA upconversion was observed with DD-o as the triplet acceptor/emitter.
Upon 254 nm irradiation, the upconversion emission intensity decreased, and was completely
switched off after 25 s of UV irradiation (254 nm irradiation).
We found that the open form→closed form photocyclization is unable to be sensitized via
intermolecular triplet energy transfer between Ir-1 and DD-o (see the Supporting Information,
Figure S19). Previously it was reported that the photocyclization of DTE was sensitized by
intramolecular triplet energy transfer in Ru(II) or Os(III) complexes.46−48,64,65 Furthermore, triplet
photosensitizer Ir-1 is stable under the continues photoirradiation with 445 nm laser (see
Supporting Information, Figure S20).
b
254 nm ON
a
1.0
0.5
254 nm ON
UV
Vis
0.0
580 nm ON
0
1
2
3
4
Cycle Time
Figure 8. (a) The reversibility of the photoswitching of TTA upconversion with DD-o/c as
photoactivatable triplet energy acceptor/emitter. (b) The photographs of the solutions in
photoswitching of TTA upconversion. Upconversion was performed upon excitation with a 445
nm continuous laser (power density: 16.8 mW/ cm2 ). c = 1.0 × 10−5 M in toluene; 20 °C.
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Upon 580 nm photoirradiation, the TTA upconversion intensity recovered (Figure 7b). The
switching process is reversible, although there is significant loss of the upconversion emission
intensity after the first circle (Figure 8). This is due to the persistent presence of the DD-c in the
mixture after the first time of UV irradiation (photocyclization).66 Moreover, the switching OFF
process of the TTA upconversion upon UV photoirradiation is much faster than recovery of the
TTA upconversion upon visible light irradiation. This result indicates that the TTA upconversion
is highly sensitive to the presence of the quencher (the DTEꢀc moiety). The photoswitching of the
TTA upconversion is visible to unaided eye (Figure 8b). Thus these photocontrolable TTA
upconversion may be used for photocontrollable luminescent bioimaging, or superꢀresolved
fluorescence microscope (such as PALM), etc.51
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In order to investigate the mechanism of the photoswitching of the TTA upconversion, the
quenching of the triplet state of photosensitizer Ir-1 were studied with the nanosecond transient
absorption spectroscopy. SternꢀVolmer quenching plots were constructed (Figure 9). The parent
compound DPA was used as a benchmark.
DTE-c
20
DTE-o
DPA
DD-c
15
DD-o
10
6
5
0
0.0
0.5
1.0
1.5
2.0
c / 10-5 M
Figure 9. Stern−Volmer plots of the quenching of triplet state lifetime of Ir-1 by DTE-c, DTE-o,
DD-c, DD-o, DPA and compound 6. The plots of DTE-c and DD-c were calculated according to
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the open form/closed form ratio at the PSS. The triplet state lifetimes were determined with
nanosecond transient absorption spectroscopy (λex = 445 nm). c (Ir-1) = 1.0 × 10−5 M in
deaerated toluene. 20 °C.
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Compound 6 shows smaller quenching constant than that of DPA, this decreased quenching
efficiency (i.e. the TTET efficiency) may be attributed to the large molecular size of compound 6
as compared with DPA. For DD-o, however, the quenching constant (KSV = 4.85 × 105 Mꢀ1) is
larger than DPA (KSV = 3.12 × 105 Mꢀ1). This enhanced quenching effect with DD-o may be
attributed to the dual DPA moieties in DD-o, which makes the quenching more efficient.
Table 4. Upconversion-related Parameters. Ir-1 as the Photosensitizer a
DD-o
1.27
DD-c
0.00
8.04
1.10
1.12
1.00
DTE-c
DTE-o
6
DPA
19.7
3.12
0.45
1.50
0.30
g
g
g
Φ
UC (%) b
KSV/M–1 [105] c
−
−
−
4.85
10.48
1.50
1.69
0.89
0.29
0.041
1.57
0.03
1.79
0.26
1.20
0.21
kq/M–1 s–1 [1010] d 0.69
k0/M–1 s–1 [1010] e 1.11
f
fQ
0.62
b
a In deaerated toluene. Upconversion quantum yields measured with Ir-1 as the standard (ΦP
= 21.3 % in CH3CN).57 c Stern–Volmer quenching constants. d Bimolecular quenching constants.
e
f
KSV = kqτ0, τ0 = 70.0 ꢁs. Diffusionꢀcontrolled bimolecular quenching rating constants. The
quenching efficiency. g Not applicable.
For the DD-c, the quenching constant (KSV = 8.04 × 105 Mꢀ1) is much larger than DD-o (KSV =
4.85 × 105 Mꢀ1). This result indicate that the DTEꢀc moiety in DDꢀc is an extra triplet energy
acceptor besides DPA moiety, i.e. the triplet state energy draining funnel, i.e. TTET, between the
Ir-1 and DTEꢀc moiety is activated. This enhanced quenching is supported with the parent DTE
compounds, for which DTE-c gives much larger quenching constant (KSV = 1.05 × 106 M−1) than
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the DTE-o (KSV = 2.9 × 104 M−1). Thus, our quenching study shows that the TTET efficiency can
be photoswitched by the DTE moiety. Larger triplet acceptor is disadvantageous as compared
with that of smaller triplet acceptor concerning the diffusionꢀcontrolled bimolecular quenching
constants, which are dependent on the molecular size. Previously DTE was used for
photoactivated quenching of the triplet state of Zn(II)ꢀporphyrin complex, but the quenching
constant was not reported.67
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In order to investigate the quenching efficiencies of the compounds, the diffusionꢀcontrolled
bimolecular quenching constants were calculated with equation 3 (Table 4).63
4πN
(Eq. 3)
k0 = 4πRND /1000 =
(Rf + Rq )(Df + Dq )
1000
D is the sum of the diffusion coefficients of the energy donor (Df) and quencher (Dq), N is
Avogadro’s number. R is the collision radius, the sum of the molecule radii of the energy donor
(Rf) and the quencher (Rq). Diffusion coefficients can be obtained from StokesꢀEinstein equation:
D = kT / 6πηR
(Eq. 4)
k is Boltzmann’s constant, η is the solvent viscosity, R is the molecule radius. The quenching
efficiency fQ can be obtained from eq.5:
fQ = kq / k0
(Eq. 5)
a
Table 5. The Parameters Related to Calculation of the Quenching Efficiency fQ
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DD-o
23.14
1.86
DD-c
15.37
1.86
DTE-c DTE-o
6
DPA
6.28
1.86
5.80
Rq/m [10−10]
5.03
1.86
7.24
5.73
1.86
6.35
10.9
1.86
3.34
Df/cm2s−1[10−6]
Dq/cm2s−1[10−6]
1.57
2.37
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a
In toluene,
η
= 5.90 × 10−4 Pa⋅s; T = 293.15 K; Rf = 1.96 × 10−9 m; Df is the diffusion
coefficients of the energy donor, Dq is the diffusion coefficients of the quencher.
The diffusion coefficients of DPA are larger than DD-o and DD-c (Table 5), due to the smaller
molecular size of DPA than DD-o and DD-c. The diffusionꢀcontrolled bimolecular quenching
constants of DD-o and DD-c are smaller than that of DPA (Table 4). However, the quenching
efficiency of DD-o and DD-c are 2ꢀ and 3ꢀfold of that of DPA. This result can be attributed to the
two DPA units in DD-o and DD-c, thus the quenching chance per collision is higher for DD-o
and DD-c than that of DPA.
DFT Calculations: the Excited State Energy Levels of the Triad. In order to study the
photophysical properties of DD-o and DD-c, and the photoswitching of the TTA upconversion,
DFT calculations were carried out. Firstly, the spin density surfaces of DD-o and DD-c were
calculated (Figure 10). For DD-o, the spin density is confined on the DPA unit. For DD-c,
however, the spin density surface is localized on the DTE-c moiety. This result is in agreement
with the experimental observation (the triplet state quenching studies).
DD-o
DD-c
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Figure 10. Isosurfaces of spin density of DD-o and DD-c at the optimized triplet state
geometries. Calculation was performed at B3LYP/6ꢀ31G(d)/LANL2DZ level with Gaussian
09W.
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The ground state geometry of DD-o was optimized, the excitation energy of the singlet excited
states were calculated with the TDDFT methods (Figure 11 and Table 6). Two lowꢀlying
isoenergetic singlet excited states (S1 and S2 states) were obtained, which are confined on the two
DPA units. The calculated excitation energy (3.09 eV, 400 nm) is in good agreement with the
UV−vis absorption experimental results. The S7 state is confined on the DTE-o unit, which
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Figure 11. Selected frontier molecular orbitals involved in the singlet and triplet excited states of
DD-o. The calculations were performed at the B3LYP/6ꢀ31G(d)/LANL2DZ level using Gaussian
09W. Toluene was used as solvent in the calculations.
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is with much higher excitation energy (3.53 eV). Thus the order of the singlet excited state is in
agreement with the experimental results.
The triplet state energy levels of DD-o were calculated with TDDFT method, based on the
optimized ground state geometry. Two degenerated triplet states were obtained (T1 and T2 states),
which are localized on the two DPA units in DD-o, respectively. The calculated energy levels
(1.73 eV) are in agreement with the experimental results.68 The T3 state is localized on the DTE-
o moiety, which is with much higher energy of 2.60 eV.
Table 6. Selected Parameters for the Vertical Excitation Energies (eV) (UV−vis absorption)
and Oscillator Strengths (f), Configurations of the Low-Lying Excited States of DD-o.
Calculated by TDDFT//B3LYP/6-31G(d)/LANL2DZ, Based on the Optimized Ground State
Geometries (Toluene Was Used as Solvent in All the Calculations).
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Electronic
transition a
Absorption
TDDFT/B3LYP/6ꢀ31G(d)
Excitation energy
3.09 eV (400 nm )
3.11 eV (398 nm )
3.53 eV (351 nm )
f b
Composition c
CI d
S0→ S1
S0→ S2
S0→ S7
0.2494
0.2207
0.0561
0.5317
0.0000
0.0000
0.0000
0.0000
0.0000
H→L
0.7018
0.7019
0.6938
0.6773
0.6984
0.6997
0.5536
0.4827
0.4331
H−1→ L + 1
H−2 → L+ 2
H−2→ L+ 4
H→ L
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S0→ S29 4.35 eV (285 nm )
T0→ T1 1.73 eV (717 nm )
T0→ T2 1.74 eV (715 nm )
T0→ T3 2.60 eV (478 nm )
T0→ T4 2.78 eV (447 nm )
T0→ T5 3.23 eV (384 nm )
Triplet
states
H−1→ L + 1
H−2→ L + 2
H−2→ L + 3
H−2→ L+ 4
a
Only selected excited states were considered. The numbers in parentheses are the excitation
energy in wavelength. b Oscillator strength. c H stands for HOMO and L stands for LUMO. Only
the main configurations are presented. d Coefficient of the wavefunction for each excitations. The
CI coefficients are in absolute values. e Based on the optimized S1 state geometry.
The excited states of DD-c were also studied with the TDDFT methods (Figure 12, Table 7).
The calculated S1 state is localized on DTE moiety, with excitation energy of 2.05 eV (604 nm).
This calculated excitation energy is very close to the experimental results (absorption band of
DD-c at 580 nm). The S4 and S5 states are localized on the two DPA moiety, respectively, which
are with much higher excitation energy 3.10 eV. Thus FRET in DD-c is possible with DPA as
the intramolecular singlet energy donor and the DTE-c moiety as the singlet energy acceptor.
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Figure 12. Selected frontier molecular orbitals involved in the excitation and emission of DD-c
in toluene. The calculations are at the B3LYP/6ꢀ31G(d)/LANL2DZ level using Gaussian 09W.
The triplet state of DD-c was calculated. T1 state is localized on the DTE moiety, with energy
level of 0.96 eV. The triplet states localized on the two DPA moieties are with energy level of
1.73 eV, which is much higher than that of T1 state. Thus the triplet state localized on the DPA
moiety may be quenched by the intramolecular triplet energy transfer to the DTE-c moiety. This
theoretical calculation results can be used for rationalization of the quenched TTA upconversion
with the DTE moiety in the closed form.
Table 7. Selected parameters for the vertical excitation (UV-vis absorption), electronic
excitation energies (eV) and oscillator strengths (f), configurations of the low-lying excited
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states of DD-c. Calculated by TDDFT//B3LYP/6-31G(d)/LANL2DZ, based on the optimized
ground state geometries (toluene was used as solvent in all the calculations)
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Electronic
transition a
Absorption
TDDFT/B3LYP/6ꢀ31G(d)
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Excitation energy
S0→ S1 2.05 eV (604 nm )
S0→ S4 3.10 eV (400 nm )
S0→ S5 3.11 eV (399 nm )
S0→ S13 3.59 eV (345 nm )
T0→ T1 0.96 eV (1298 nm )
T0→ T2 1.73 eV (716 nm )
T0→ T3 1.73 eV (715 nm )
T0→ T4 2.35 eV (527 nm )
T0→ T5 2.35 eV (526 nm )
f b
Composition c
CI d
0.5288
0.2463
0.2229
0.1934
0.0000
0.0000
0.0000
0.0000
0.0000
H→L
0.7070
0.7011
0.7009
0.5354
0.7169
0.6988
0.6995
0.6739
0.6455
H−1→ L + 2
H−2 → L+ 1
H→ L+ 3
H→ L
Triplet
states
H−1→ L + 2
H−2→ L + 1
H→ L + 3
H−1→ L
a
Only selected excited states were considered. The numbers in parentheses are the excitation
energy in wavelength. b Oscillator strength. c H stands for HOMO and L stands for LUMO. Only
the main configurations are presented. d Coefficient of the wavefunction for each excitations. The
CI coefficients are in absolute values. e Based on the optimized S1 state geometry.
The photophysical processes involved in the DD-o, DD-c and the photoswitch of the TTA UC
were summarized in Scheme 3. For DD-o, the S1 state of the DTEꢀo moiety is higher in energy
than the DPA moiety. Thus, no FRET exists to quench the fluorescence of DPA moiety.
Moreover, the charge transfer state (CST) is with energy level of 3.35 eV, thus the fluorescence
of DPA moiety is not perturbed by any electron transfer (in toluene). Therefore, DD-o shows
similar fluorescence quantum yield as compared with that of DPA (Table 1). However, the
energy level of CTS will decrease to 2.71 eV in polar solvents such as acetonitrile, as a result, the
energy level is lower than the S1 state energy level of the DPA moiety (2.98 eV), thus,
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fluorescence quenching of the DPA emission was observed in acetonitrile as compared to that in
nonpolar solvent, such as toluene.
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Scheme 3. Simplified Jablonski Diagram Illustrating the Photophysical Processes Involved
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in the Photoswitched TTA Upconversion, (a) DD-o as triplet acceptor/emitter, (b) DD-c as
triplet acceptor/emitter a
a
The component at the excited state was designated with red color. The number of the
superscript designated either the singlet or the triplet excited state. For clarity, the production of
the triplet state of the Ir(III) complex by photoexcitation into the singlet excited state of the
complex is omitted. The excitation of the Ir-1 photosensitizer was omitted. Toluene as the
solvent.
The DTEꢀo moiety is with T1 state energy level of 2.60 eV, which is much higher than the
DPA triplet state energy level (1.77 eV), therefore, the triplet state of DPA moiety is not
quenched by DTEꢀo. Based on the above analysis, TTA upconversion should be observed with
DD-o. It should be noted that the T1 state of the photosensitizer (2.06 eV),57 complex Ir-1,
should not be quenched by DTEꢀo moiety either.
Upon UV irradiation, photocyclization of the DTEꢀo moiety will give DD-c, with the DTE
moiety in closed form. Along with the photocyclization, the singlet excited state energy level of
DTE moiety significantly decreased, as well as the triplet excited state energy level. As a result,
the cascade photophysical processes involved in TTA upconversion were modulated. First, the S1
state energy level of the DTEꢀc moiety decreased to 2.14 eV, which is lower than DPA moiety
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(2.98 eV), thus FRET with DPA moiety as singlet energy donor and DTEꢀc moiety as energy
acceptor was activated, the fluorescence of the DPA moiety was quenched (Figure 8a). Second,
intermolecular triplet state energy from Ir-1 to DTE-c is also activated, because the DTE-c
moiety is with much lower triplet state energy level (0.96 eV). Moreover, the intramolecular
triplet state energy transfer in DDꢀo, with the DPA moiety as the triplet energy donor and the
DTEꢀc moiety as the triplet state energy acceptor, is also activated. Third, the CTS energy level
of DD-c is 2.79 eV (toluene), which is lower than the S1 state of DPA, thus the fluorescence of
DPA was quenched (Scheme 3b). Based on these analysis, the TTA upconversion will be
reduced with DD-c as the triplet state acceptor. The photoswitching is reversible, thus the
photophysical processes, as well as the TTA upconversion, can be photoswitched.
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CONCLUSIONS
In summary, a photoswitchable triplet energy acceptor/fluorescent emitter, herein exemplified
with a triad based on 9,10ꢀdiphenylanthracene (DPA) and dithienylethene (DTE), was developed
for the first time for photoswitching of tripletꢀtripletꢀannihilation (TTA) upconversion. The triad
was prepared by covalently linking DPA and DTE moieties by Click reaction. The fluorescence
of the DPA moiety in the triad, as well as the energy level and the confinement of the triplet state
of the triad, is able to be photoswitched via the reversible photocyclization and photoreversion of
the DTE moiety, as a result the TTA upconversion can be photoswitched ON and OFF. The
photophysical processes involved in the photochromism of the triad and the photoswitched TTA
upconversion were studied with steady state UV−vis absorption and fluorescence emission
spectroscopies, nanosecond transient absorption spectroscopy, electrochemical characterization,
and DFT/TDDFT calculations. We concluded that three different mechanisms are responsible for
the photoswitching of the TTA upconversion with the triad as the triplet acceptor, i.e. the
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