Thiol-activated triplet-triplet annihilation upconversion: Study of the different quenching effect of electron acceptor on the singlet and triplet excited states of bodipy

Article abstract of DOI:10.1021/acs.joc.5b00557

Thiol-activated triplet-triplet annihilation (TTA) upconversion was studied with two different approaches, i.e., with 2,4-dinitrobenzenenesulfonyl (DNBS)-caged diiodoBodipy triplet photosensitizers (perylene as the triplet acceptor/emitter of the upconver

Full text of DOI:10.1021/acs.joc.5b00557

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Article
Thiol-Activated Triplet-Triplet Annihilation Upconversion:
Study of the Different Quenching Effect of Electron
Acceptor on the Singlet and Triplet Excited States of Bodipy
Caishun Zhang, Jianzhang Zhao, Xiaoneng Cui, and Xueyan Wu
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.5b00557 • Publication Date (Web): 05 May 2015
Downloaded from http://pubs.acs.org on May 7, 2015
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ThiolꢀActivated TripletꢀTriplet Annihilation
Upconversion: Study of the Different Quenching
Effect of Electron Acceptor on the Singlet and
Triplet Excited States of Bodipy
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Caishun Zhang, Jianzhang Zhao,* Xiaoneng Cui and Xueyan Wu
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of
Technology, Eꢀ208 West Campus, 2 Ling Gong Rd., Dalian 116024, P. R. China.
Eꢀmail: zhaojzh@dlut.edu.cn
RECEIVED DATE (automatically inserted by publisher)
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Abstract: Thiolꢀactivated tripletꢀtriplet annihilation (TTA) upconversion was studied with two
different approaches, i.e. with 2,4ꢀdinitrobenzenenesulfonyl (DNBS) caged diiodoBodipy triplet
photosensitizers (perylene as the triplet acceptor/emitter of the upconversion), and DNBSꢀcaged
Bodipy fluorophore as the triplet accepter/emitter (PdTPTBP as the triplet photosensitizer,
TPTBP = tetraphenyltetrabenzoporphyrin). The photophysical processes were studied with
steadyꢀstate UV−vis absorption spectroscopy, fluorescence spectroscopy, electrochemical
characterization, nanosecond transient absorption spectroscopy and DFT/TDDFT computations.
DNBS caged triplet photosensitizer shows shorter triplet state lifetime (24.7 ꢀs) than the uncaged
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triplet photosensitizer (86.0 ꢀs), the quenching effect is due to photoinduced electron transfer
(PET). TTA upconversion was enhanced upon cleavage of the DNBS moiety by thiols. On the
other hand, the DNBS caged Bodipy shows no fluorescence, but the uncaged fluorophore shows
strong fluorescence, thus TTA upconversion is able to be enhanced with the uncaged fluorophore
as the triplet energy acceptor/emitter. The results indicate that the DNBS moiety exerts
significant quenching effect on the singlet excited state of Bodipy, but the quenching on triplet
excited state is much weaker. Calculation of the Gibbs free energy changes of the photoꢀinduced
electron transfer indicates that the singlet state gives large driving force for the PET process than
the triplet state does.
Keywords: Bodipy; Electron transfer; Quenching; Triplet state; Triplet−Triplet Annihilation
Upconversion
1. INTRODUCTION
Modulation of the excited states of chromophores is crucial for development of novel functional
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molecular materials, such as fluorescent molecular probes,
molecular switches,
and
external stimuliꢀresponsive molecular devices. 2−18 In this context, the methodologies of
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switching the singlet excited state have been well developed. However, switching of the triplet
excited states is rarely reported. 4,19−22 Switching of the triplet excited states will be very useful
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in the areas such as target activatable photodynamic therapy (PDT),20,23−27 molecular logic
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gates, as well as for study of fundamental photochemistry of organic chromophores.
Quenching of the triplet excited state may follow different principles as compared with that of
quenching the singlet excited states, even with the same quencher. But this fundamentally
important effect was rarely studied. For example, given that the quenching of the excited state is
due to photoꢀinduced electron transfer (PET), the Gibbs free energy changes (ꢁG° ) of the PET
CS
processes for quenching of the singlet and triplet excited state will be different, because the
driving forces, the E values (the energy level of the excited state by which the PET is driven),
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are substantially different, because the S state and the T state of typical fluorophores are
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substantially different, i.e. the S /T state energy gap is usually large for polycyclic aromatic
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hydrocarbons. In this case the singlet excited state will offer much larger driving force for the
PET process than the triplet excited state of the same chromophore. However, investigations of
such different quenching effect on the triplet and singlet excited state was not reported.
Concerning switching of triplet excited state of organic chromophores, previously aminoꢀ
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azaBodipy was prepared as acid activatable PDT reagents. The triplet state of the
iodoazaBodipy chromophore was presumably quenched by PET from the appended N atom to
the Bodipy chromophore. Protonation of the amino group will inhibit the PET, thus the triplet
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excited state yield is increased. Fluorescenceꢀenergyꢀresonanceꢀtransfer (FRET) was also used
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for switching of the triplet state of a Bodipy dyad. In the presence of acid/base, one of the
components in the dyad will be protonated so that the energy levels will be changed. As a result,
a FRET process will be activated to compete with the intersystem crossing (ISC). On the other
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hand, photochromic chromophores, such as dithienylethene (DTE) was incorporated into the
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molecular structure of Ru(II) or Os(III) complexes to modulate the triplet excited states.
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via an photoswitchable intermolecular triplet energy transfer.
Previously 2,4ꢀ
dinitrobenzenesulfonylamide (DNBS) moiety as electron trap to switch the triplet excited state of
transition metal complex to develop phosphorescent thiol probes or thiolꢀactivatable PDT
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reagents.
For most of these studies, the switching of the triplet excited states was not studied
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in detail, for example, with the nanosecond transient absorption spectroscopy.
Recently we used DTE for preparation of a iodoꢀBodipyꢀDTE triad, for which the triplet state
manifold is able to be photoswitched (in aspects of both triplet state lifetime and yield) by the
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photochromism of the DTE unit. The switching effect was used for tripletꢀtripletꢀannihilation
(TTA) upconversion. We also used an intermolecular tripletꢀtripletꢀenergyꢀtransfer (TTET)
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approach for switching the TTA upconversion with DTE as the photo responsive chromophore.
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Controlling the singlet oxygen ( O ) photosensitizing of iodoBodipy with acid was reported. In
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order to switch the visible lightꢀabsorption property, as well as the triplet state property, we
prepared rhodamineꢀC dyads for which the visible lightꢀabsorption and the triplet state can be
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switched on by addition of acid. However, much room is left to develop new methodologies for
the triplet state switching, especially with chemical stimulates.
Herein we report a new strategy for switching of the triplet excited state property of an organic
chromophore, and its application in controllable TTA upconversion, with a chemical input. The
approach is exemplified with thiolꢀcleavable DNBS caged triplet photosensitizer (2,6ꢀ
diiodoBodipy, Scheme 1). Thiol compounds are important for keeping the intracellular redox
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homeostasis, and thiol compounds have been the targets for many molecular probes. Herein our
method is based on the designing of a ‘caged’ triplet photosensitizer, in which the triplet state
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was quenched by the PET from the diiodoꢀBodipy chromophore to the intraꢀmolecular electron
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acceptor, DNBS (C−1, Scheme 1).
Previously DNBS was used for controlling of the triplet
excited state of metal complexes or bromoBodipy,30−32 although the switching effect was not
studied with nanosecond transient absorption spectroscopy. We envisage this modulation can be
conveyed to the triplet state manifold of organic chromophore for switching of the TTA
upconversion. The triplet state lifetime of C−1 will probably be shorter than that of the reference
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compound 5 (Scheme 1). The TTA upconversion will be less efficient with C−1 as triplet
photosensitizer because the critical step involved in TTA upconversion, the intermolecular
tripletꢀtripletꢀenergyꢀtransfer (TTET) will be less efficient with shorter triplet state lifetime of the
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photosensitizer.
In the presence of thiols, the DNBS moiety will be cleaved off from C−1
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(Scheme 2), as a result, the triplet state lifetime was extended as compared with that of C−1.
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Correspondingly the TTET will be enhanced.
On the other hand, previously DNBS was used as an electron trap for designing of fluorescent
molecular probes for detection of thiols. 0,39−42 The fluorescence of this kind of molecular probes
are usually quenched significantly with DNBS. In the presence of thiols analytes, such as cysteine,
the DNBS moiety will be cleaved off from the fluorophore, the fluorescence will be
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enhanced.
Inspired by these studies, herein we also studied a complementary approach for
switching of TTA upconversion, that is, to switch the emissive singlet excited state of Bodipy
chromophore (compound 2, Scheme 1). Compound 2 was used as triplet acceptor/emitter of TTA
upconversion, with Pd (II) tetraphenyltetrabenzoporphyrin (TPTBP) as triplet photosensitizer.
The results show that DNBS is more effective for quenching of the singlet excited state of Bodipy,
than quenching of the triplet excited state of the same Bodipy chromophore.
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The photophysical properties of the compounds were studied with steady state UV−vis
absorption and fluorescence emission spectra, nanosecond transient absorption spectroscopy,
electrochemical characterization (cyclic voltammetry) and DFT/TDDFT computations. We
demonstrated that the switching of the triplet state and singlet excited state with the thiolꢀ
cleavable cage moiety can be used for switching of the TTA upconversion. These information are
useful for designing new switchable triplet photosensitizers and for application of these
compounds in activatable PDT and chemical stimuliꢀcontrolled TTA upconversion.
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2. RESULTS AND DISCUSSIONS
2
.1. Molecular Designing Rationales. Previously it was shown that DNBS moiety is an strong
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electron acceptor for quenching of fluorescence.
This property was used in designing of thiolꢀ
selective fluorescent molecular probes. With DFT calculation we show that the S state of the
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chromophore was modulated with the attachment and the cleavage of the DNBS moiety. 0−42 The
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S state of the caged fluorophore is a dark state, whereas the uncaged fluorophore gives S state
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as emissive state.40−42 However, the quenching effect of DNBS on the singlet excited state and
triplet excited state of the same chromophore was not compared.
Herein we designed compound C−1 (Scheme 1), in which the Bodipy moiety was iodinated at
the 2,6ꢀposition for triplet formation. Bodipy was selected as the chromophore, due to its
satisfactory photophysical properties. 5−50 We envisaged that the triplet excited state of 2,6ꢀ
diiodoBodipy may be quenched by the DNBS moiety (in both aspects of lifetime and yield),
whereas cleavage of the DNBS moiety may lead to the recovery of the triplet excited state of the
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2,6ꢀdiiodoBodipy. In order to study the mechanism of the triplet state switching, we prepared
C−2 (Scheme 1), in which the styryl diiodoꢀBodipy shows much lower triplet state energy level
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than the 2,6ꢀdiiodoBodipy unit in C−1. The preparation of the compounds is based on routine
synthetic methods. The compounds were obtained in moderate to satisfactory yields.
Scheme 1. Preparation of the DNBS Caged Triplet Photosensitizer C-1 and C-2, as well as
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a
the DNBS Caged Fluorophore Compound 2
NO2
NO
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O
OH
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OH
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NO
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O NO
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BODIPY
a
Key: (I) dry CH Cl , TEA, 2,4ꢀdinitrobenzenesulfonyl chloride, 50 °C, 10 h, yield: 59.7%.
II) dry CH Cl , NIS, 5 h, 30 °C, yield 70%. (III) dry CH Cl , NIS, 5 h, 30 °C, yield: 72.6%.
IV) H O , yield: 43%. (V) dry toluene, piperidine, acetic acid and pꢀhydroxybenzaldehyde, 10
min, yield: 12%. (VI) dry CH Cl , TEA, 2,4ꢀdinitrobenzenesulfonyl, 50 °C, 10 h, yield: 51%.
VII) dry CHCl and TEA, yield: 65%. (VIII) dry CH Cl and TEA, 50 °C, 10 h, yield: 54%.
2
2
(
2
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2
2
(
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2
2
2
(
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2
2
As a complementary study, we investigated the quenching effect of DNBS moiety on the
singlet excited state of Bodipy, i.e. the fluorescence, with compounds 1 and 2 (Scheme 1). The
fluorescence emission of 1 and 2 were compared. The quenching effect of DNBS on the singlet
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excited state of Bodipy (fluorescence) is more significant than that on the triplet excited state.
Quenching of singlet excited state of Bodipy was also used for switching of TTA upconversion.
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Scheme 2. Cleavage Mechanism of the DNBS Moiety in C-1 by Thiols
NO2
O
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OH
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SO
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C-1
2.2 UV−vis Absorption and Fluorescence Emission Spectroscopies. Firstly the UV−vis
absorption of the compounds were studied (Figure 1). The dyad C−1 and the reference
compounds 5 and 6 show similar UV−vis absorption spectra. This result indicates that there is no
41
significant electronic interaction between the chromophores in C−1 at the ground state. For
C−2, however, different UV−vis absorption spectrum was observed as compared to the reference
compound 7. This result is reasonable since the DNBS moiety is directly attached on the πꢀ
conjugation framework in C−2, which is different from the molecular structural profile of C−1.
The fluorescence of the compounds was studied (Figure 1b and 1d). Reference compounds 5
and 6 show similar emission property (note optically matched solutions were used in the
comparison of the fluorescence of the compounds). For C−1, however, much weaker
fluorescence was observed as compared with that of compounds 5 and 6. This result indicates
that the fluorescence of the diiodoꢀBodipy part in C−1 was quenched, most likely by the PET
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process.
Similar results were found for C−2 and the reference compound 7 (Figure 1d). The
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fluorescence emission of C−2 is much weaker than the reference compound 7. It should be noted
that the switching effect of DNBS moiety on the fluorescence of the unꢀiodinated Bodipy
compounds is more significant.40−42 Generally the singlet excited state lifetimes of C−1 and C−2
are much shorter than the unꢀiodinated compounds, thus it is more difficult for the electron
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transfer to compete with the fast radiative decay of the singlet excited state of C−1 and C−2.
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550 600 650 700 750 800
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Figure 1. (a) UV−vis absorption spectra of C−1, 5 and 6. (b) Fluorescence emission spectra of
C−1, 5 and 6 (λex = 480 nm. Optically matched solution was used). (c) UV−vis absorption
spectra of 7 and C−2. (d) Fluorescence emission spectra of 7 and C−2 (λex = 530 nm. Optically
−5
matched solutions were used). For the absorption spectra, c = 1.0 × 10 M in CH CN. 20 °C.
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The UV−vis absorption and the fluorescence emission spectral changes of C−1 and C−2 in the
presence of thiol compound (2ꢀmercapitoethanol) were studied (Figure 2). For C−1, the UV−vis
absorption spectrum did not show significant change upon addition of thiol. 0−42 For C−2,
however, the absorbance decreased upon addition of 2ꢀmercapitoethanol. Fluorescence
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enhancement was observed for both C−1 and C−2 in the presence of 2ꢀmercapitoethanol (Figure
2c and 2d). This result is in agreement with the previously studied fluorescent thiol molecular
probes.40−42
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
1
0
0
0
.5
.2
.9
.6
.3
C-2
C-2 + Thiol
a
1.00
0.75
0.50
0.25
b
d
C-1
C-1 + Thiol
0.0
4
0
.00
4
00
500
600
700
800
00
500
600
700
800
Wavelength / nm
Wavelength / nm
40
20
15
10
5
C-1
C-1 + Thiol
C-2
C-2 + Thiol
c
3
2
1
0
0
0
0
500
0
600
700
800
550 600 650 700 750 800
Wavelength / nm
Wavelength / nm
Figure 2. UV−vis absorption spectra of (a) C−1 and (b) C−2 before and after thiol added.
Fluorescence emission of (c) C−1 ( λ = 470 nm) and (d) C−2 ( λ = 530 nm) before and after
ex
ex
thiol added. Optically matched solutions were used. For UV−vis absorption spectra, c = 1.0 ×
−
5
10 M. For fluorescence emission, c [Cꢀ1 or Cꢀ2] : c [thiol] = 1 : 200 and the thiol used in the
study is mercaptoethanol. In CH CN. 20 °C.
3
The UV−vis absorption and fluorescence properties of compounds 1 and 2 were also studied
(Figure 3). The two compounds show similar absorption, with maximum absorption at 503 nm
and 506 nm for 1 and 2, respectively (Figure 3a). The two compounds show drastically different
fluorescence property (Figure 3b), compared to compound 1, the fluorescence of compound 2
was completely quenched. The fluorescence of compound 1 was compared with the unsubstituted
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Bodipy (Figure 3c, with optically matched solution). Compound 1 gives lower fluorescence
emission (Φ = 50 %) than unsubstituted Bodipy (Φ = 90 %).
F
F
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
.00
300
150
20
a
2
2
50
00
c
b
1
0.75
0.50
0.25
9
6
0
0
150
1
2
1
2
1
1
00
50
0
Bodipy
30
0
0
.00
300
400
500
600
5
00
550
600
650
500
550
600
650
Wavelength / nm
Wavelength / nm
Wavelength / nm
−
5
Figure 3. (a) UV−vis absorption spectra of compounds 1 and 2. c = 1.0 × 10 M. (b)
Fluorescence emission of compound 1 and 2. λ = 470 nm. Optically matched solutions were
ex
used. (c) Fluorescence emission of 1 and Bodipy. λ = 470 nm. Optically matched solutions were
ex
used. In toluene. 20 °C.
a
Table 1. Photophysical Parameters of the Compounds
c
d
e
f
h
λabs
503
506
529
590
533
528
576
580
ε
λ
em
Φ
F
/ %
τ
F
/ns
τ
T
/ ꢁs
−
Φ
T
/ %
−
b
b
1
2
5
7
9.04
9.26
8.92
8.91
9.10
8.86
8.88
515
519
547
624
556
550
594
596
50.0
0.6
4.28
1.67
0.18
1.53
0.10
0.15
0.37
0.50
−
−
4.1
171.3
4.4
92.5
64.7
88.6
23.6
1.7
C−1
C−1
C−2
C−2
24.7
86.0
2.7
g
1.9
−
57.6
−
3.4
g
6.60
8.9
3.1
a
−5
b
c
4
−1
−1
d
In CH CN (1.0 × 10 M). In toluene. Molar absorption coefficient.
Fluorescence quantum yields. Bodipy (Φ = 90.0 % in toluene) was used as standard for 1 and 2.
ε
: 10 M cm .
3
F
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2
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
6
(Φ = 2.7 % in CH CN) was used as standard for 5, C−1 and C−1 after cleavage of the DNBS
F
3
moiety by thiols. 14 (Φ = 9.5 % in toluene) was used as standard for 7, C−2 and C−2 after
cleavage of the DNBS moiety by thiols. Fluorescence lifetimes. Triplet state lifetimes,
measured by transient absorptions. After cleavage of the DNBS moiety by thiols. Triplet state
quantum yield, with Rose Bengal as stand (Φ = 0.9 in methanol).
F
e
f
g
h
T
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The photophysical properties of the compounds were summarized in Table 1. The fluorescence
quantum yields of C−1, C−2 and the cleavaged products are low, which is due to the iodination
of the Bodipy chromophores.52−54 It is noted that the fluorescence lifetimes of C−1, C−2 and the
cleavage product are short, which are due to the influence of the DNBS electron acceptor (PET)
2
0,55
and the iodination of the Bodipy chromophore (ISC effect).
These results are drastically
different from the previously reported BodipyꢀDNBS based fluorescent thiol probes, those probes
show fluorescence switching effect. 2−54
5
a
Table 2. Rate Constants for Singlet Energy Transfer
c
−1 d
e
Φ (ref)/Φ (sen)
k (s )
τF (ns)
PL
PL
ET
9
f
C−1 2.4
7.83 × 10
0.18
9
g
h
C−2 6.9
3.88 × 10
2.13 × 10
c
1.53
3.87
b
10
2
83.3
a
b
In acetonitrile. In toluene. Ratio of the fluorescence quantum yields of the reference
d
compounds (5 and 7) and the corresponding DNBS caged photosensitizers (C−1 and C−2).
Photoinduced intramolecular electron transfer rate constants.
Fluorescence lifetime for compound 5. Fluorescence lifetime for compound 7. Fluorescence
lifetime for compound 1.
e
f
Fluorescence lifetime.
g
h

ΦRef

(Eq. 1)
k =
−1 /τ(
ET



Ref
)
ΦSen

The photoinduced intramolecular electron transfer rate constants in the caged fluorophore and
5
6
the triplet photosensitizers were calculated with Eq. 1, when k is the electron transfer rate
ET
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constant, ΦRef and τRef is the fluorescence quantum yield and the lifetime of the reference
compounds (5 or 7), and ΦSen is the fluorescence lifetime of the corresponding caged
photosensitizers (C−1 or C−2). It was found the PET process in compound 2 is faster than that in
C−1 and C−2. This result can be used to rationalize the fully quenched fluorescence in C−1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2.3. Nanosecond Transient Absorption Spectroscopy. In order to study the switching of the
triplet excited state of the compounds upon caging with DNBS and cleavage of the DNBS moiety
by thiols, the nanosecond transient absorption spectroscopy of the compounds C−1 and C−2 in
the absence and in the presence of thiols were studied. Firstly nonꢀpolar solvent toluene was used
for study of the triplet state of C−1 (Figure 4). The TA spectra of C−1 in toluene were
0
0
.04
.00
a
b
0.0
0.1
0.2
0.3
-0.04
-0.08
-0.12
533.2
99.9
...
199.9
0 ꢀs
ꢀ
s
s
-
-
-
4
ꢀ
ꢀs
τT = 187.7 ꢀs
-0.16
300 400 500 600 700 800
Wavelength / nm
0
200 400 600 800 1000
Time /
ꢀs
0.05
c
d
0.00
0.0
-0.05
0.10
-0.15
0.20
633.2
99.9
...
ꢀ
s
s
-
0.1
0.2
4
ꢀ
-
199.9
ꢀs
-
τT = 189.9 ꢀs
0
ꢀs
-
-0.3
3
00 400 500 600 700 800
0
200 400 600 800 1000
Time /
Wavelength / nm
ꢀ
s
Figure 4. (a) Nanosecond transient absorption spectra of C−1 and (b) the corresponding decay
trace at 533 nm. (c) Nanosecond transient absorption spectra of C−1 upon addition of
mercaptoethanol and (d) the corresponding decay trace at 533 nm (λ = 529 nm). c [C−1] : c
ex
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
[
mercaptoethanol] = 1 : 200. After the thiols added, the solution was standed for 30 min before
−
5
measurement of the spectra. c [C−1] = 1.0 × 10 M in toluene. 20 °C.
recorded (Figure 4a). Bleaching band at 538 nm was observed upon pulsed laser excitation,
which is due to the depletion of the ground state of the diiodoBodipy moiety upon photoꢀ
excitation. Excited state absorption (ESA) in the region of 380 nm − 480 nm and 584 nm − 774
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
nm were observed, which are attributed to the absorption of the triplet state of 2,6ꢀdiiodoBodipy
52,57
moiety (spinꢀallowed T →T transitions).
In toluene, the triplet state lifetime of C−1 was
1
n
determined as 187.7 ꢀs (Figure 4b). In the presence of thiol, the TA spectra hardly give any
changes (Figure 4c). The triplet state lifetime was determined as 189.9 ꢀs (Figure 4d), very close
1
to the caged triplet photosensitizer (C−1). The singlet oxygen ( O ) quantum yields (Φ ) of C−1
2
ꢁ
and the cleaved product were determined as 0.59 and 0.65 respectively (Table S1). Thus, we
conclude that there is no significant switching effect upon addition of thiol for C−1 in toluene. It
was well known that the PET of multiꢀchromophore compounds is unlikely to occur in non−polar
5
8−60
solvents such as toluene,
thus we postulate that the there is no significant intra molecular
PET for C−1 in toluene.
Similar results were observed in polar solvent such as dichloromethane (DCM), the triplet state
lifetimes of C−1 before and after cleavage with thiol is 166.6 ꢀs and 168.1 ꢀs, respectively
(Supporting Information, Figure S22). Note the triplet state lifetime of 5 in DCM and toluene is
151.4 ꢀs and 183.4 ꢀs, respectively (Supporting Information, Figure S23), which indicated that
the different triplet lifetime of C−1 in toluene and DCM is due to the diiodoꢀBodipy moiety, not
any PET process between the diiodoꢀBodipy and DNBS moiety.
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The TA spectra of C−1 in polar solvent, such as acetonitrile, were studied (Figure 5). Although
the bleaching and the transient positive absorption profiles of C−1 in the absence and in the
presence of thiols are similar (Figure 5a and 5c), the triplet state lifetime show substantial
difference (Figure 5b and 5d). The triplet state lifetime of C−1 in acetonitrile was determined as
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2
4.7 ꢀs, but the lifetime was significantly extended to 86.0 ꢀs in the presence of thiols, i.e.
cleavage of the DNBS moiety. Thus we propose that the PET in C−1 is significant in polar
solvent such as acetonitrile. This is a known fact for intramolecular PET in multiꢀchromophore
5
8−60
compounds.
The Φ value of C−1 changed from 0.74 to 0.88 upon cleavage with thiol in
ꢁ
CH CN (Table S1). Thus, switching of the triplet excited state of C−1 is implemented with thiol
3
as an external chemical input. Previously we studied a thiol−selective phosphorescent molecular
probe, in which the DNBS moiety is an electron acceptor and Ru(II) complex as the
0
.06
.00
0
0
.03
.00
a
b
0
-0.06
0.12
-0.18
0.24
2
85.0
250.0
..
ꢀ
ꢀ
s
s
-
-
0.03
-
.
2
0
05.0
ꢀ
s
τT = 24.7
ꢀ
s
0.06
3
ꢀ
s
-
00 400 500 600 700 800
Wavelength / nm
0
50
100
Time /
150
200
ꢀs
0.06
.00
0
0
.03
.00
c
d
0
-0.03
-0.06
-0.06
-0.12
-0.18
-0.24
3
3
...
233.3
0 ꢀs
99.9
33.3
ꢀ
ꢀ
s
s
-
-
-
0.09
0.12
ꢀ
s
τ = 86.0 ꢀs
T
0.15
3
00 400 500 600 700 800
Wavelength / nm
0
200 400 600 800 1000
Time /
ꢀs
Figure 5. Nanosecond transient absorption of (a) C−1 and (b) the decay trace at 533 nm.
Nanosecond transient absorption of (c) C−1 after cleavage of the DNBS moiety by
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
mercaptoethanol and (d) the decay trace at 533 nm, In all case, c [C−1] : c [thiol] = 1 : 200. λ =
ex
529 nm. In CH CN. After mercaptoethanol was added, the solution was standed for 30 min
3
−5
before measurement of the spectra. c [photosensitizers] = 1.0 × 10 M. 20 °C.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
0
phosphorescent chromophore. In order to reveal the different triplet state life time of 5 and
C−1 after cleavage of the DNBS moiety by thiols, SternꢀVolmer quenching plots of 5 in the
presence of compounds 10 and 13 (quenchers) was studied. Both of the compounds 10 and 13
have a strong quenching effect on the triplet state lifetime of 5 (see Supporting Information,
Figure S33−S35).
0.02
0.01
0.00
a
b
0
.01
.00
0
-0.01
-0.02
-0.03
-0.04
3
99.9
333.3
...
ꢀ
ꢀ
s
s
-0.01
0.02
-0.03
-
2
0
33.3
ꢀ
s
τT = 2.7
30
ꢀs
ꢀ
s
3
00 400 500 600 700 800
0
10
20
40
Wavelength / nm
Time / ꢀs
0.005
.000
0
0
0
.006
.003
.000
c
d
0
-0.005
-0.010
-0.015
1
5.0
14.7
..
10.7
ꢀ
s
s
-
-
-
0.003
0.006
ꢀ
.
τT = 3.1 ꢀs
ꢀs
0 ꢀs
0.009
3
-
0.020
00 400 500 600 700 800
Wavelength / nm
0
4
8
12 16 20
Time /
ꢀs
Figure 6. Nanosecond transient absorption of (a) C−2 and (c) after cleavage of the DNBS moiety
by mercaptoethanol. Decay traces of (b) C−2 and (d) after cleavage of the DNBS moiety by
thiols at 586 nm, (c[Cꢀ2] : c [thiol] = 1 : 200) excited with nanosecond pulsed laser (λ = 589
ex
nm). After the thiols added, the solution was standed for 30 min before measurement of the
−
5
spectra. c [photosensitizers] = 1.0 × 10 M. In CH CN. 20 °C.
3
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Similar studies were carried out for C−2 (Figure 6). Interestingly, no substantial triplet state
lifetime changes were observed for C−2 in the presence of thiol, even in polar solvent such as
acetonitrile. For example, the triplet state lifetime of C−2 is 2.7 ꢀs in acetonitrile in the absence
of thiol. In the presence of thiol, the triplet state lifetime was only slightly extended to 3.1 ꢀs.
Similar triplet state lifetimes were also observed for C−2 in toluene and dichloromethane
(Supporting Information, Figure S24 and S25). The cleaved chromophore, i.e. compound 7,
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
shows similar triplet excited state lifetime (Supporting Information, Figure S26). In CH CN, the
3
Φ was determined as 0.20 for C−2, as well as for the cleaved product (Table S1).
ꢁ
0
0
.1
.0
0
.0
a
9
66.5
933.2
..
ꢀ
ꢀ
s
s
τT = 170.8 ꢀs
-
-
0.2
0.4
-
0.1
0.2
.
3
0
3.3
ꢀ
s
ꢀ
s
-
b
400
500
600
700
0
200 400 600 800 1000
Time /
Wavelength / nm
ꢀs
0
.0
0.05
.00
0.05
-0.10
0.15
c
0
τT = 101.6 ꢀs
9
66.5
ꢀ
ꢀ
s
s
-
0.2
0.4
-
933.2
...
33.3
ꢀ
s
0
ꢀs
d
-
-
0
200 400 600 800 1000
Time /
400
500
600
700
ꢀ
s
Wavelength / nm
Figure 7. Nanosecond transient absorption of (a) PdTPTBP and (b) the corresponding decay
trace at 440 nm. (c) Nanosecond transient absorption of PdTPTBP in the presence of compound
−
6
−6
2
and (d) the decay trace at 440 nm. c [PdTPTBP] = 5.0 × 10 M. c [compound 2] = 2.0 × 10
M in toluene. λ = 445 nm. 20 °C.
ex
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Compounds 1 and 2 are devoid of any heavy atoms, thus no significant triplet state formation
was observed with the compounds. Instead, as triplet state accepter for the TTA upconversion,
quenching of the triplet state of PdTPTBP with compounds 1 and 2 was studied (Figure 7). With
compound 2, substantial quenching effect on the triplet state of PdTPTBP was observed (Figure
7c and 7d). For example, nanosecond transient absorption spectra of PdTPTBP upon pulsed laser
excitation show two bleaching bands at 442 nm and 627 nm. These bleaching bands are due to
the depletion of the ground state of PdTPTBP. The inherent triplet state lifetime of PdTPTBP
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
−
6
was determined as 170.8 ꢀs (Figure 7b). In the presence of compound 2 (2.0 × 10 M. 1:0.4
molar ratio for PdTPTBP/compound 2), the triplet state lifetime was reduced to 101.6 ꢀs (for
detail SternꢀVolmer quenching plots, see later section Figure 14, Supporting Information Figure
S38 and S39).
Interestingly, no bleaching band of the Bodipy moiety of compound 2 (at ca. 500 nm) was
observed, this result indicates that Bodipy moiety in compound 2 is not the ultimate triplet energy
trap. We postulate the quenching of the triplet excited state of PdTPTBP by the DNBS moiety is
6
1
due to intermolecular electron transfer.
.4. Electrochemical Studies: Free Energy Changes of the Photoinduced Electron
2
Transfer (PET). The electrochemical properties of the complexes were studied by cyclic
voltammetry (Figure 8). For the reference compound 5, a reversible oxidation wave was observed
at +0.94 V and a reversible reduction wave was observed at −1.26 V. For reference compounds
10 and 13, irreversible reduction peaks at −1.31 V and −0.95 V were observed. The reversible
oxidation wave at +0.97 V of C−1 is attributed to the 2,6ꢀdiiodoBodipy moiety (electron donor).
The irreversible oxidation wave of C−1 at −0.93 V can be attributed to the electron acceptor
moiety (Table 3). For the reference compound 7, a reversible oxidation wave was observed
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which halfꢀwave potential is +0.68 V and a reversible reduction wave can also be observed
which halfꢀwave potential is −1.15 V. Due to poor solubility of C−2, no signal can be observed.
The free energy changes of the intramolecular electron transfer process, can be calculated with
10
11
12
13
14
15
16
17
18
19
20
21
22
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24
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26
27
28
29
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49
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52
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5
9
the RehmꢀWeller equation (Eq. 2 and Eq. 3).
+
a
+
Fc /Fc
Fc /Fc
b
c
+
Fc /Fc
1
.2 0.6 0.0 -0.6 -1.2 -1.8
Potential / V
1
.2 0.6 0.0 -0.6 -1.2 -1.8 1.2 0.6 0.0 -0.6 -1.2 -1.8
Potential / V
Potential / V
e
f
d
+
+
Fc /Fc
Fc /Fc
+
Fc /Fc
C-2
1
.2 0.6 0.0 -0.6 -1.2 -1.8
Potential / V
1.2 0.6 0.0 -0.6 -1.2 -1.8
Potential / V
1.2 0.6 0.0 -0.6 -1.2 -1.8
Potential / V
Figure 8. Cyclic voltammogram of the dyad photosensitizer 5, 7, 10, 13, C−1 and C−2.
Ferrocene (Fc) was used as internal reference. (a) compound 5, (b) compound 7, (c) compound
1
0, (d) compound 13, (e) compound C−1, (f) compound C−2. Due to poor solubility, no
−
4
satisfactory signal was observed for C−2. In deaerated CH CN solutions containing 5.0 × 10 M
3
photosensitizers, 0.10 M Bu NPF as supporting electrode, Ag/AgNO reference electrode, Scan
4
6
3
rates: 0.1 V/s. 20 °C.
0
ꢁG CS = e[E − E ]− E + ꢁG
(Eq. 2)
OX
RED
00
S
2
2
e
e  1
πε0 RD RA εREF εS
1  1
1 
ꢁ
G = −
−


+


−


S
4πεSε0RCC
8



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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
Eq. 3)
S
Where ꢁG is the static Coulombic energy which is described by eq.3. e = electronic charge, EOX
=
halfꢀwave potential for oneꢀelectron oxidation of the electronꢀdonor unit, ERED = halfꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 3. Redox Potentials of Bodipy Photosensitizers for Study of the Potential
a
Intramolecular Electron transfer. Anodic and Cathodic Peak Potential Were Presented
E (ox) (V)
E1/2(red) (V)
1
/2
1
0.82
0.87
0.94
0.68
−1.53
−0.88
−1.26
−1.15
−1.31
−0.95
−0.93
2
5
7
b
10
−
b
13
−
C−1 0.97
b
b
C−2 −
−
a
−4
In deaerated CH CN solutions containing 5.0 × 10 M photosensitizers, 0.10 M Bu N[PF ] as
supporting electrode, Ag/AgNO reference electrode, Scan rates: 0.1 V/s. 20 °C. Not observed.
3
4
6
b
3
wave potential for oneꢀelectron reduction of the electronꢀacceptor unit, E0,0 = energy level
approximated with the intersection of fluorescence emission and UV−vis absorption after
Normalization (for the singlet excited state), εS = static dielectric constant of the solvent, R =
CC
centerꢀtoꢀcenter separation distance between the electron donor (diiodoBodipy) and electron
acceptor (PBI), determined by DFT optimization of the geometry, RCC (C−1) = 8.6 Å, RCC (C−2)
=
13.8 Å, R is the radius of the electron donor, R (C−1) = 4.3 Å, R (C−2) = 8.4 Å. R is the
D
D
D
A
radius of the electron acceptor, R (C−1) = 4.9 Å, R (C−2) = 4.9 Å. εREF is the static dielectric
A
A
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constant of the solvent used for the electrochemical studies, ε0 is permittivity of free space. The
solvents used in the calculation of Gibbs free energy of the electron transfer is toluene ( S
ε = 2.4),
CH Cl (εS = 9.1) and acetonitrile (εS = 37.5).
2
2
10
11
12
13
14
15
16
17
18
19
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53
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60
Energy levels of the chargeꢀseparated states (E ) and charge recombination energy state
CS
2
(
ꢂG(CR)) can be calculated with the Eq. 4 and Eq. 5. The data were collected in Table 3.
E = e[E − E ]+ ꢁGS
CS
OX
RED
Eq. 4
Eq. 5
ꢁG = −(ꢁG + E )
CR
CS
00
Table 4. Free Energy Changes of Charge Separation (ꢁG ), Free Energy Changes of Charge
CS
Recombination (ꢁG ) and Energy Levels of Charge Separation Energy States (E ) of
CR
CS
Compounds 2, C−1 and C−2.
a
a
a
b
b
b
c
c
c
ꢁ
GCS
eV)
ꢁ
GC
E
CS
ꢁ
GCS
ꢁ
GC
E
CS
ꢁ
GCS
ꢁ
GC
E
CS
(
(eV) (eV) (eV)
(eV) (eV) (eV)
(eV) (eV)
d
d
d
2
−0.27 −1.52 +1.52 −0.61 −1.17 +1.17 −0.71 −1.71 +1.71
e
e
e
+0.63
+0.29
+0.19
d
d
d
C−1 +0.14 −2.42 +2.42 −0.30 −1.98 +1.98 −0.43 −1.85 +1.85
e
e
e
+0.90
+0.46
+0.33
d
d
d
C−2 −0.02 −2.10 +2.10 −0.40 −1.71 +1.71 −0.51 −1.61 +1.61
e
e
e
+0.79
+0.40
+0.30
a
b
c
d
In toluene. In CH Cl . In acetonitrile. E0,0 = energy level approximated with the
2
2
intersection of fluorescence emission and UV−vis absorption after Normalization at the singlet
e
excited state. E0,0 = energy level approximated with the triplet state energy level by DFT
calculation. All the calculations (except C−2) are base on the first oxidation reduction potential.
For C−2, the calculation is base on the first oxidation reduction potential of compound 7 and 13.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
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5
5
5
5
5
5
5
5
5
6
The Gibbs free energy changes of the electron transfer in C−1 and C−2 indicate that electron
transfer is inefficient in nonꢀpolar solvent, such as toluene. This conclusion derived from the
electrochemical data is in agreement with the nanosecond transient absorption spectra of C−1
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(Figure 4 and 5). The driving force for PET in mediumꢀpolarity solvent is also small. The driving
force is large in polar solvent such as acetonitrile (Table 4). The Gibbs free energy changes of the
PET process of C−1 was calculated as −0.38 eV in acetonitrile. In less polar solvents, smaller
driving force for PET was observed (Table 4). Similar trend was observed for C−2. It should be
noted that the triplet state property of C−1, such as the lifetime, was not affected by PET, unless
the energy level of the chargeꢀtransferꢀstate (CST) is lower in energy level than that of the triplet
excited state (see later section).
+
Fc /Fc
a
b
+
Fc /Fc
1
.6 0.8 0.0 -0.8 -1.6 -2.4
Potential / V
1
.2 0.6 0.0 -0.6 -1.2 -1.8
Potential / V
Figure 9. Cyclic voltammogram of photosensitizers (a) 1 and (b) 2. Ferrocene (Fc) was used as
−
4
internal reference. In deaerated CH CN solutions containing 5.0 × 10 M photosensitizers with
3
the ferrocene, 0.10 M Bu NPF as supporting electrode, Ag/AgNO reference electrode, Scan
4
6
3
rates: 0.1 V/s. 20 °C.
In order to reveal the mechanism of the quenching of fluorescence with DNBS moiety in
compound 2, the electrochemical data of compounds 1 and 2 were also recorded (Figure 9).
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Following similar methods to that of compounds C−1 and C−2, the Gibbs free energy changes
for the PET process in compound 2 were calculated and the data were listed in Table 4. The
results show that for compound 2, the driving force for the PET process always exists, even for
nonꢀpolar solvent such as toluene, and in comparison the driving force for the PET in other
solvents are larger than that of compounds C−1 and C−2. Thus, we postulate that quenching of
the singlet excited state of Bodipy by DNBS moiety is more efficient than quenching of the
triplet excited state of Bodipy. The reason for this different quenching behavior is the different
energy level of the singlet and triplet excited state of Bodipy moiety. These results indicate that
designing triplet photosensitizers which show higher triplet state energy level is beneficial for
switching/activation with external stimulus.
10
11
12
13
14
15
16
17
18
19
20
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59
60
2
.5. DFT Calculations: Rationalization of the Photophysical Properties. DFT calculations
were carried out for rationalization of the photophysical properties of the compounds. 2−65 First
6
the spin density surfaces of C−1 and C−2 were calculated (see Supporting Information, Figure
6
6,67
S43).
The T triplet excited states of both C−1 and C−2 are confined on the diiodoꢀBodipy
1
moieties, which are in agreement with the nanosecond transient absorption spectra of the
compounds. For 2, the spin density surface is confined on the Bodipy moiety, not the DNBS
moiety. Thus the T state of compound 2 is localized on Bodipy part, not DNBS part.
1
In order to study the photophysical processes such as the electron transfer, the groundꢀstate
geometries of the compounds were optimized, and the UV−vis absorption and the virtual S →
0
Tn excitations of the triplet photoꢀsensitizers were calculated based on the optimized groundꢀstate
geometry with the TDDFT method (see Supporting Information, Figure S44 and Table S2). The
energyꢀminimized geometry of C−1 and C−2 at ground state indicated that the electron
withdrawing group (2,4ꢀdinitrobenzene part) keeps away from electronꢀdonating group (2,6ꢀ
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1
1
1
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1
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1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
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3
3
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3
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5
5
5
5
5
5
6
diiodoBodipy part). For C−1, the phenyl moiety connected to the Bodipy core takes a
perpendicular geometry against the π−core of the 2,6ꢀdiiodoBodipy moiety. For C−2, the styryl
moiety is almost coplanar with Bodipy π−core thus large π–conjugation was resulted.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The calculated UV−vis absorption bands for C−1 (S → S and S → S ) are located at 575nm
0
1
0
1
and 468 nm and HOMO→LUMO and HOMO→LUMO+1 are the respective major components
of the transitions. For both process, the electron density transfer from electronꢀdonating group
43
(
2,6ꢀdiiodoBodipy part) to electronꢀwithdrawing part (2,4ꢀdinitrobenzenesulfunyl part). The
oscillator strengths for both transitions are zero. Thus direct population of these states upon
6
8,69
photoꢀexcitation is prohibited.
The calculated UV−vis absorption band (S → S ) is located at
0
3
458 nm. HOMO → LUMO + 2 is the respective major components of the transition.
The electron density is still distributed on electronꢀdonating group (2,6ꢀdiiodoBodipy part), and
the transition is not a charge transfer transition. These results are in agreement with the UV−vis
absorption experimental results.
The triplet excited states of the compounds were calculated with the TDDFT calculations (see
Supporting Information, Figure S27, S44, Table S2). For T state, HOMO → LUMO+2 is the
1
main component of the transition. The MOs are localized on the Bodipy unit. T state is a charge
2
transfer state, for which HOMO → LUMO transition is involved, indicating that the electron is
transferred from the electronꢀdonating moiety (2,6ꢀdiiodoBodipy part) to electron withdrawing
moiety (2,4ꢀdinitrobenzenesulfunyl part). Thus the triplet state of C−1 will not be quenched by
any charge transfer process. This conclusion is in agreement with the nanosecond transient
absorption spectra of C−1, indicated that the T state is not completely quenched by PET
1
processes. Similar DFT/TDDFT calculation result was obtained for C−2 (see Supporting
Information, Figure S27 and Table S2)
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The fluorescence difference of compounds 1 and 2 were studied with DFT/TDDFT method
previously. The results indicated that the S state of compound 2 is a dark state, due to the
1
4
1
electron transfer feature. The S state of compound is an emissive state.
1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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49
50
51
52
53
54
55
56
57
58
59
60
2
.6. Jablonski Energy Diagram. The photophysical processes of the compound C−1 was
presented in Scheme 3. For C−1, there is a charge transfer state (CST) lying between the singlet
state (S state) and the triplet state (T state). TDDFT calculations indicate the CTS are an
1
1
electron transfer from the iodoꢀBodipy unit to the DNBS unit in C−1 (Figure 8 and Table 4). The
energy level of the CTS derived from the electrochemical data is in full agreement with the
TDDFT calculations, and it is fully supported by the nanosecond transient absorption spectra
(
Figure 5).
Scheme 3. Simplified Jablonski Diagram Illustrating the Photophysical Processes Involved
in (a) C−1 in the Absence of Meracptoethanol, (b) Compound 5 (the Cleaved Product of C−1
a
in the Presence of Meracptoethanol)
1
1
[
5]
[
C-1]
a
b
2
.34 eV
2
.33 eV
CTS
.85 eV
ISC
3
ISC
[5]
3
[C-1]
.52 eV
15 nm
1
1.52 eV
1
533 nm
529 nm
815 nm
8
S0
S0
S0
S
0
a
The energy levels of the excited states are designated based on spectral data, electrochemical
data and TDDFT calculations. The number of the superscript designated either the singlet or the
triplet excited state. In CH CN.
3
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Thus the fluorescence of C−1 may be quenched by the electron transfer. Moreover, the energy
gap between CTS and T state is small, thus thermal population of the CTS is possible from the
1
T1 state. As a result, both mechanisms can quench the triplet state of C−1. However, the energy
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
level of the CTS is higher than the T triplet excited state energy level, thus the T state of the
1
1
Bodipy moiety in C−1 is not expected to be completely quenched by the PET process. This
postulation is in full agreement with the nanosecond transient absorption studies (Figure 5). We
1
studied the singlet oxygen ( O ) photosensitizing ability of C−1 in different solvent (see
2
Supporting Information, Table S1, Figure S29). The result shows that the triplet excited state of
C−1 is unable to be completely quenched in polar solvent such as acetonitrile (Similar results
were observed for C−2, Supporting Information, Table S1, Figure S28 and S30).
It should be noted that in toluene, the energy level of the CTS state is much higher (2.42 eV,
Table 4), thus neither the fluorescence nor the triplet state of C−1 are quenched (Figure 4). This
theoretical prediction is in agreement with the fluorescence studies. In the presence of thiols, the
DNBS moiety was cleaved off the Bodipy moiety, as a result, the CTS is eliminated, thus the
triplet state lifetime of the diiodoBodipy moiety was recovered and the lifetime was
5
8,59
extended.
2
.7. Switching of the Triplet-Triplet Annihilation Upconversion. In recent years, new triplet
photosensitizers for TTA upconversion were developed. 8,70−74 TTA upconversion has been also
3
7
5
76,77
used for luminescence bioimaging, and to enhance the photovoltaics.
However, switching
of the TTA upconversion is rarely reported. Herein the chemicalꢀactivated TTA upconversion
with C−1 as triplet photosensitizer was studied (Figure 10 and Table 6). The upconversion
emission was compared with the samples before and after addition of mercaptoethanol. For C−1
in the absence of thiol, the upconversion fluorescence emission in the range 550 − 551 nm was
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observed (with perylene as the triplet acceptor). This result is reasonable because C−1 show
triplet excited state (Figure 5). In the presence of thiol, the DNBS moiety of C−1 was cleaved,
the upconversion was intensified (Figure 10), the upconversion quantum yield increased from
0.2 % to 0.5 %. The upconversion with compound 5 as triplet photosensitizer was also studied
and the upconversion quantum yield is 5.9 % (see Table 6 and Supporting Information, Figure
S31).
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
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50
51
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53
54
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56
57
58
59
60
1
50
20
C-1
C-1 + Thiol
1
90
60
30
400
500
600
700
800
Wavelength / nm
Figure 10. TTA upconversion with C−1 as triplet photosensitizer, before and after cleavage of
the DNBS moiety by mercaptoethanol. Excited with 532 nm CW laser (5 mW, power density: 28
−
2
−5
mW cm ). c[C−1] = 1.0 × 10 M. The optimized perylene concentrations were used c[perylene]
−
4
−5
=
1.1 × 10 M for C−1 and c[perylene] = 9.5 × 10 M after cleavage of the DNBS moiety by
o
mercaptoethanol. In CH CN. 20 C.
3
TTA upconversion with compounds 1 and 2 as triplet acceptor and PdTPTBP as triplet
photosensitizer were studied (Figure 11, Table 6 and Figure S32). With compound 1,
upconversion emission at 528 nm was observed. With compound 2, however, no upconversion
can be observed. Due to the significant quenching effect of the cleavage side product, the
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1
2
2
2
2
2
2
2
2
2
2
3
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4
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4
4
4
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5
5
5
5
5
5
5
5
5
6
switching of the TTA upconversion with compound 2 in the presence of thiol was failed (Figure
11d). In order to study the lack of TTA upconversion with compound 2 in the presence of thiol,
the quenching of the triplet state of PdTPTBP with compound 13 was studied (Figure 14b,
supporting information, Figure S40 and S41). The results show that the phosphorescence of
PdTPTBP was significantly quenched by compound 13. Therefore no upconversion was
observed with compound 2 in the presence of mercaptoethanol.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
600
450
300
150
0
a
compound 1
b
-4
8
x 10
x 10
-4
6
...
-5
1 x 10
0
I
II III
IV
500 600 700 800 900
Wavelength / nm
6
4
3
1
00
600
c
2 and thiols
d
compound 2
50
00
50
450
300
150
-
4
-
4
4
8
x 10
8
6
x 10
x 10
-4
-
6
x 10
.
..
...
-5
-5
1
0
x 10
1
0
x 10
0
5
0
5
00 600 700 800 900
Wavelength / nm
00 600 700 800 900
Wavelength / nm
Figure 11. TTA upconversion with PdTPTBP as triplet photosensitizer and compounds 1 or 2 as
the triplet acceptor/emitter with (a) increasing amount of compound 1. (b) Photograph of TTA
upconversion. (I) without any triplet accepter; (II) with compound 1 as triplet accepter; (III) with
compound 2 as triplet accepter; (IV) with compound 2 after cleavage of the DNBS moiety by
mercaptoethanol as triplet accepter. The upconversion spectra with (c) 2 as triplet accepter and
(d) 2 upon cleavage by mercaptoethanol . Excited with 635 nm CW laser (5 mW, power density:
ꢀ2
−6
o
28 mW cm ). c[photosensitizer] = 5.0 × 10 M in toluene. 20 C.
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Page 29 of 44
The Journal of Organic Chemistry
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8
9
2
.8. The Mechanism of the Thiol-Switched TTA Upconversion. Quenching of the triplet
state lifetime of the photosensitizer by triplet energy acceptor (perylene) was studied to reveal the
origin of different TTA upconversion quantum yields (Figure 12). With perylene as the triplet
10
11
12
13
14
15
16
17
18
19
20
21
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23
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54
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60
6
−1
acceptor (quencher), quenching constant (K ) of 1.6 × 10 M was observed for compound 5
SV
5
−1
(
triplet energy donor), which is much higher than that of C−1 (KSV = 2.0 × 10 M ). The
different quenching constant is due to the different triplet state lifetimes of compound 5 (τ ＝
T
51
1
71.3 ꢀs) and C−1 (τ ＝ 24.7 ꢀs). Longer triplet state lifetime is beneficial for the TTET
T
process, thus higher TTA upconversion quantum yield was observed with compound 5 (ΦUC
.9%) than that of C−1 (ΦUC = 0.2%) (for more detail, see Figure 14, Table 6 and supporting
information Figure S36 and Figure S37).
The quenching efficiency was studied (Eq. 6), where k is the diffusionꢀcontrolled bimolecular
=
5
0
5
1
quenching rating constants, can be calculated with the Smoluchowski equation (Eq. 7).
(Eq. 6)
f = k k
Q
q
0
6
0
16
a
b
1
2
8
4
0
45
5
C-1
1
2
1
1
3
0
0
3
15
0
-
5
-5
-5
-5
-5
0.0
0
.0
4.0x10
c[quencher]/M
8.0x10
1
.0x10 2.0x10 3.0x10
c[quencher]/M
Figure 12. SternꢀVolmer plots for quenching of the triplet lifetime of photosensitizers with
triplet energy acceptor. (a) Perylene as the triplet acceptor, 5 and C−1 as triplet photosensitizers.
29
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