DiiodoBodipy-perylenebisimide dyad/triad: Preparation and study of the intramolecular and intermolecular electron/energy transfer

Article abstract of DOI:10.1021/jo502899p

2,6-diiodoBodipy-perylenebisimide (PBI) dyad and triad were prepared, with the iodoBodipy moiety as the singlet/triplet energy donor and the PBI moiety as the singlet/triplet energy acceptor. IodoBodipy undergoes intersystem crossing (ISC), but PBI is dev

Full text of DOI:10.1021/jo502899p

Article
pubs.acs.org/joc
DiiodoBodipy-Perylenebisimide Dyad/Triad: Preparation and Study
of the Intramolecular and Intermolecular Electron/Energy Transfer
Zafar Mahmood,† Kejing Xu,† Betul Kucukoz,‡ Xiaoneng Cui,† Jianzhang Zhao,*,† Zhijia Wang,†
̧
̈ ̈ ̈
̈
Ahmet Karatay,‡ Halime Gul Yaglioglu,‡ Mustafa Hayvali,*,§ and Ayhan Elmali‡
†State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, E-208 West Campus, 2
Ling-Gong Road, Dalian 116024, People’s Republic of China
§
‡Department of Engineering Physics, Faculty of Engineering, and Department of Chemistry, Faculty of Science, Ankara University,
06100 Besȩ vler, Ankara, Turkey
S
* Supporting Information
ABSTRACT: 2,6-diiodoBodipy-perylenebisimide (PBI) dyad and triad
were prepared, with the iodoBodipy moiety as the singlet/triplet energy
donor and the PBI moiety as the singlet/triplet energy acceptor.
IodoBodipy undergoes intersystem crossing (ISC), but PBI is devoid of
ISC, and a competition of intramolecular resonance energy transfer
(RET) with ISC of the diiodoBodipy moiety is established. The
photophysical properties of the compounds were studied with steady-
state and femtosecond/nanosecond transient absorption and emission
spectroscopy. RET and photoinduced electron transfer (PET) were
confirmed. The production of the triplet state is high for the iodinated
dyad and the triad (singlet oxygen quantum yield ΦΔ = 80%). The
Gibbs free energy changes of the electron transfer (ΔGCS) and the energy level of the charge transfer state (CTS) were analyzed.
With nanosecond transient absorption spectroscopy, we confirmed that the triplet state is localized on the PBI moiety in the
iodinated dyad and the triad. An exceptionally long lived triplet excited state was observed (τT = 150 μs) for PBI. With the
uniodinated reference dyad and triad, we demonstrated that the triplet state localized on the PBI moiety in the iodinated dyad
and triad is not produced by charge recombination. These information are useful for the design and study of the fundamental
photochemistry of multichromophore organic triplet photosensitizers.
absorption of visible light and long-lived triplet excited states
have also been developed and have been studied for triplet−
triplet annihilation (TTA) upconversion.6,15,26−29
1. INTRODUCTION
Recently triplet photosensitizers have attracted much atten-
tion,1−7 due to the applications of these versatile materials in
photodynamic therapy,4 photocatalyses such as photocatalytic
hydrogen (H2) production,8 photoredox catalytic synthetic
organic reactions,9 photoinitiated polymerization,10 and more
recently triplet−triplet annihilation assisted upconversion.6,11−15
Conventional triplet photosensitizers are limited to porphyrin
derivatives (including Zn(II), Pt(II), or Pd(II) complexes)16−19
and halogenated xanthane dyes, such as Rose Bangle or
Methylene blue,6,20 as well as some ketone compounds, such
as benzophenone, diacetyl, ketocoumarin, etc.6,21,22 Transition-
metal complexes such as Pt(II), Ir(III), and Ru(II) complexes
have also been used as triplet photosensitizers.6,23 However,
these compounds suffer from drawbacks such as difficulties in
preparation/purification, loss of ISC upon derivatization
(although in some cases careful functionalization may leave
ISC intact), and weak visible light absorption.6
However, all these new triplet photosensitizers suffer a
common drawback: that is, the molecular structure is based on
a single chromophore protocol and, as a result, these compounds
show only one major absorption band in the visible spectral
region. Thus, these compounds are inefficient for harvesting the
photoexcitation energy from a broad-band light source, such as
solar light.6 Therefore, it is important to prepare a broad-band
visible-light-absorbing triplet photosensitizer.30 Inspired by the
popular method of fluorescence resonance energy transfer
(FRET),7a,3b,31−34 we proposed to prepare triplet photo-
sensitizers on the basis of the RET of chromophore dyads or
triads to attain broad-band visible light absorption. Following this
method, we have prepared broad-band visible-light-absorbing
triplet photosensitizers based on Bodipy dyads or triads.35,36
Previously Bodipy dyads showing broad-band visible light
absorption were studied from the perspective of molecular
logic gates.30,37 However, the fundamental photophysical
properties for these multichromophore triplet photosensitizers
New triplet photosensitizers have been developed: for
example, the bromo- or iodo-Bodipys and the heavy-atom-free
Bodipy dimers.1,4,5,24,25 These compounds show strong
absorption of visible light and long-lived triplet excited states
and have been used in photodynamic therapy (PDT)
studies.4,5,24 Moreover, transition-metal complexes with strong
Received: December 22, 2014
Published: February 24, 2015
© 2015 American Chemical Society
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a
Scheme 1. Synthesis of the Triplet Photosensitizers B-1 and B-2 and the Reference Compounds L-1, L-2, and L-3
a
Legend: (a) 2-ethylhexylamine, imidazole, refluxed at 160 °C for 8 h, under Ar; (b) Br2, CHCl3, stirred at room temperature for 48 h under Ar; (c)
Br2, CHCl3, stirred at 60 °C for 48 h under Ar; (d) 4 equiv of N-iodosuccinimide (NIS), DCM, stirred for 30 min at room temperature; (e) DDQ,
TFA, stirred overnight at room temperature under Ar, and then BF3·Et2O, Et3N, stirred for 2 h under ice-cold conditions; (f) phenylboronic acid,
K2CO3, Pd(PPh3)4, PhCH3/C2H5OH/H2O (4/2/1, v/v), refluxed at 90 °C for 8 h, under Ar; (g) K2CO3, Pd(PPh3)4, PhCH3/C2H5OH/H2O (4/2/
1, v/v), reflux 90 °C for 8 h, under Ar; (h) K2CO3, Pd(PPh3)4, PhCH3/C2H5OH/H2O (4/2/1, v/v), refluxed at 90 °C for 8 h under Ar; (i)
phenylboronic acid, K2CO3, Pd(PPh3)4, PhCH3/C2H5OH/H2O (4/2/1, v/v), refluxed at 90 °C for 8 h, under Ar; (j) excess NIS, stirred for 5 h at
room temperature; (k) excess NIS, stirred for a long time at 30 °C.
have still not been fully elucidated, such as the competition of
resonance energy transfer (RET) and intersystem crossing
(ISC); these processes have not yet been studied with
femtosecond or nanosecond transient absorption spectroscopy
for multichromophore organic triplet photosensitizers. Pre-
viously it was proposed that the ISC of intramolecular singlet
energy donors was inhibited by intramolecular RET.30,37
However the rich photophysical properties of multichromo-
phore dyads/triads have yet to be fully studied. Visible-light-
harvesting dyads or triads with intramolecular energy transfer
and production of triplet excited states were reported, but none
of these compounds were designed to be with competitive RET
and ISC processes.38−42 Establishment of such a competition
between ISC and FRET requires that the singlet energy donor is
also a spin converter. Such dyads/triads have never been
reported.43
In order to take an in-depth look into the uncharted
photophysical processes involved in the multichromophore
organic triplet photosensitizers, herein we have prepared an
iodoBodipy-perylenebisimide (PBI) dyad and triad (B-1 and B-
2, Scheme 1). For these compounds, both the PBI and the
Bodipy moiety show strong absorption of visible light;44−49 the
singlet state energy levels ensure RET. The singlet energy donor
iodoBodipy shows ISC capability, whereas the singlet energy
acceptor PBI is devoid of ISC. Moreover, the ISC of the
diiodoBodipy will be competed by the FRET, with which the
production of the triplet excited state is presumably inhibited.
The photophysical properties of the dyad and triad were studied
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with steady-state and femto-/nanosecond transient absorption
and emission spectroscopy, as well as electrochemical character-
ization. Photoinduced electron transfer (PET) was confirmed for
the dyad and triad. We found that the ISC of the iodoBodipy part
is unable to be completely inhibited by the RET to PBI, and
surprisingly the dyad and triad show high singlet oxygen
quantum yields (ΦΔ). The charge-separated state (CST) lies at
a higher energy level than the triplet excited state.
results may be due to the close proximity of the Bodipy and PBI
chromophores.52 The UV−vis absorptions of the compounds in
other solvents were also studied (see the Supporting
Information, Figures S31−S33).
In order to study the RET process, the fluorescence emissions
of the singlet energy donor L-3 and that of dyad B-1 were
compared (Figure 2a). With optically matched solutions (i.e., the
Nanosecond transient absorption spectroscopy indicated that
the triplet excited state of the dyad and the triad is exclusively
localized on the PBI moiety, and the triplet state lifetime of the
PBI moiety (150 μs) is much longer than that previously
observed. With uniodinated dyad and triad we confirmed that the
production of the triplet state in the dyad and triad is not due to
charge recombination. These studies are useful for the design of
new broad-band visible-light-absorbing organic triplet photo-
sensitizers and study of the photophysical properties of these
organic chromophores.
Figure 2. Quenching of the fluorescence of the energy donor
(iodoBodipy) in dyad B-1 and triad B-2: (a) Fluorescence spectra of
B-1 and L-3, λex 508 nm; (b) fluorescence spectra of B-2 and L-3, λex 480
nm. In (a) and (b) optically matched solutions were used (i.e., the
absorbance of the two samples at the excitation wavelength are same).
Conditions: c = ca. 1.0 × 10−5 M in toluene (slight variation of the
concentration is necessary to prepare optically matched solutions); 20
°C.
2. RESULTS AND DISCUSSION
2.1. Design and Synthesis of the Compounds. 2,6-
diiodoBodipy was selected as the singlet energy donor/spin
converter, due to its strong absorption of visible light and
efficient ISC.4,50 PBI was used as the singlet energy acceptor.38
On the basis of the fluorescence emission of the Bodipy and
iodoBodipy and the absorption spectra of PBI, we envisage that
RET will occur with Bodipy as an energy donor and PBI as an
energy acceptor.51,52 As a result, ISC of iodo-Bodipy will
compete with RET. Note that the PBI moiety is devoid of ISC
capability. Moreover, the T1 state energy level of the PBI moiety
(1.2 eV) is lower than that of Bodipy (ca. 1.7 eV).38,53,54 Thus,
the triplet state of the dyad or triad, if populated by any
mechanism, will be located on the PBI moiety, not the Bodipy
moiety.55,56 To the best of our knowledge, PBI-Bodipy molecular
dyads or triads have never been used for studies of the triplet state
manifold. The only example of a Bodipy-PBI assembly (a
dendrimer) was used for a singlet energy transfer study: i.e.,
fluorescence-related studies.52 All of the compounds were
synthesized by routine procedures (Scheme 1).
solutions show the same optical density, or absorbance, at an
excitation wavelength of 508 nm), the two compounds give
drastically different emission intensities: i.e., the emission of L-3
is substantially quenched in B-1. Similar results were observed for
the comparison of L-3 and B-2 (Figure 2b; the emission of B-2 is
attributed to the emission of the PBI moiety). The quenching of
the emission of L-3 in B-1 is most probably due to intramolecular
energy transfer,59−61 although electron transfer cannot be
excluded. Femtosecond transient absorption spectroscopy
indicated that the quenching is due to the FRET effect (see
below). Similar quenching effects were also observed in other
solvents such as dichloromethane and acetonitrile (see the
Supporting Information, Figure S36).
2.2. UV−Vis Absorption and Fluorescence Emission
Spectra. The UV−vis absorption spectra of the compounds
were studied (Figure 1). For L-1, moderate absorption bands at
The PET effects in B-1 and B-2 were studied by comparison of
the fluorescence emission of the energy acceptor in the dyad/
triad, i.e. the PBI moiety L-1/L-2, with that of B-1/B-2 (Figure
3a).32,47,59 Quenching of the energy acceptor emission in the
Figure 1. UV−vis absorption spectra of (a) B-1, L-1, and L-3 and (b) B-
2, L-2, and L-3. Conditions: c = 1.0 × 10−5 M in toluene; 20 °C.
500 nm (ε = 32000 M−1 cm−1) and 535 nm (ε = 47000 M−1
cm−1) were observed. For L-3, a strong absorption at 538 nm was
observed (ε = 106000 M−1 cm−1), which is the signature
absorption of the Bodipy chromophore.57,58 The absorption
spectrum of B-1 is not the sum of L-1 and L-3 with regard to the
molar absorption coefficients, although the absorption wave-
lengths are similar. Similar results were observed for B-2. These
Figure 3. Confirmation of the photoinduced electron transfer by
fluorescence quenching of the energy acceptor in B-1 and B-2: (a)
fluorescence spectra of B-1 and L-1 in toluene, λex = 515 nm; (b)
fluorescence spectra of B-2 and L-2 in toluene, λex = 460 nm. In (a) and
(b), optically matched solutions were used. Conditions: c = ca. 1.0 ×
10−5 M in toluene (slight variation of the concentration is necessary to
prepare optically matched solutions); 20 °C.
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Table 1. Photophysical Properties of the Compounds
a
b
c
d
e
f
λabs
537
541
ε
λem
566
ΦF
τT
τF
ΦΔ
B-1
B-2
5
1.03
1.32
0.18
0.05
1.04
0.16
150
148
g
0.96
0.80
0.78
g
597
0.84
504/534
504/552
534
1.09/0.37
1.73/0.25
0.47
514/549
520/595
574
2.8/3.2
3.3/4.5
5.96
6
g
g
L-1
L-2
L-3
Bodipy
42.4
g
0.08
g
552
0.31
597
50.5
2.7
g
7.30
h
h
h
538
1.06
554
90
0.29
0.83
504
1.09
514
72.0
g
3.86
g
a
b
c
d
In toluene (1.0 × 10−5 M), in nm. Molar absorption coefficient, in 105 M−1 cm−1. Fluorescence quantum yield, in percent. Triplet excited state
e
f
lifetimes, in μs; measured by nanosecond transient absorption in deaerated solutions. Luminescence lifetimes, in ns. Singlet oxygen quantum yield
(ΦΔ) with diiodoBodipy as a standard (ΦΔ = 0.83 in DCM) at λex 531 nm. Not applicable. Literature value.
g
h
dyad or triad indicates a PET effect. With an optically matched
solution at the excitation wavelength (λex = 515 nm), the
emission of B-1 is much weaker in comparison with that of L-1.
Similar results were observed for the comparison of L-2/B-2
(Figure 3b). The fluorescence quantum yields (ΦF) of L-1 and L-
2 were determined as being 42.4% and 50.5%, respectively. The
fluorescence quantum yields decreased to less than 1% in B-1 and
B-2 (Table 1). The quenching of the acceptor emission is
attributed to intramolecular electron transfer.59 A similar
quenching effect was also observed in other solvents such as
dichloromethane and acetonitrile (Figures S38 and S39,
Supporting Information). In a previously reported PBI-Bodipy
assembly, such electron-transfer-induced quenching of the
acceptor fluorescence was not studied.52
emission intensity of B-1 in a nonpolar solvent such as toluene is
stronger than that in polar solvents, such as dichloromethane and
acetonitrile (Figure 4a). For the components of dyad B-1, i.e. L-1
and L-3, however, fluorescence emission intensity is much less
dependent on the solvent polarity (Figure 4b,c). These results
confirm the photoinduced electron transfer in B-1, which is more
significant in polar solvents.32 Similar results were observed for
B-2 and the reference compounds (Figure S40, Supporting
Information). The photophysical properties of the compounds
are summarized in Table 1.
2.3. Redox Properties: Cyclic Voltammogram of the
Compounds. The electrochemical properties of the dyad B-1
and the triad B-2 were studied with cyclic voltammograms (CV;
Figures 5 and 6). These electrochemical data can be used for
The intramolecular electron transfer rate constants were
calculated with eq 1 based on the fluorescence lifetimes of L-1
and L-2, and the quenched fluorescence quantum yields of L-1
and L-2 in B-1 and B-2, respectively.59
⎡ΦPL(B‐1 or B‐2)⎤
⎢
⎣
⎥
⎦
ΦPL
intra
kCS
=
τ(L‐1 or L‐2)
(1)
The photoinduced intramolecular electron transfer rate
intra
constants of B-1 and B-2 in toluene were calculated as kCS
Figure 5. Cyclic voltammograms of L-3, B-1, and L-1. Ferrocene (Fc)
was used as internal reference (E1/2 = +0.64 V (Fc+/Fc) vs standard
hydrogen electrode). Conditions: in deaerated CH2Cl2 solutions
containing 0.5 mM photosensitizers with the ferrocene, 0.10 M
Bu4NPF6 as supporting electrolyte, Ag/AgNO3 reference electrode,
scan rate 50 mV/s, 20 °C.
= 3.8 × 1010 and 1.4 × 1011 s−1, respectively. These values are
close to the electron transfer rate constant observed in
intra
calix[4]arene-PBI dyad (kCS
= 4.0 × 1010 s−1 in toluene;
kCSintra is solvent-dependent).62
To further confirm the intramolecular electron transfer in B-1,
the fluorescence of B-1 in solvents with different polarities was
studied (Figure 4). The electron transfer induced fluorescence
quenching will be more significant in polar solvents.32 The
evaluation of the free energy changes of the photoinduced
intramolecular electron transfer, as well as the energy level of the
Figure 4. Fluorescence emission spectra: (a) B-1 (λex 508 nm); (b) L-3 (λex 508 nm); (c) L-1 (λex 515 nm). Optically matched solutions were used.
Conditions: 20 °C.
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The redox potentials of triad B-2 were studied and were
compared with those of the reference compounds L-2 and L-3
(Figure 6). Different from the case for B-1, the reduction waves
of B-2 are anodically shifted in comparison to that of L-2. A
reversible oxidation wave at +1.57 V was recorded for B-2, which
is assigned to the oxidation of the 2,6-diiodoBodipy. Three
reversible reductive waves were observed at −0.36, −0.60 V and
at −0.84 V, which are attributed to the 1,7-diphenylPBI moiety
and the diiodoBodipy moiety, respectively. The redox potentials
of the compounds are collected in Table 2. In comparison with
B-1 and B-2, similar redox potentials were observed for the
uniodinated dyad 5 and triad 6 (Figure S47, Supporting
Information).
It was reported that the triplet state of PBI derivatives can be
produced by charge recombination.63 Similar results were also
observed with the Bodipy-C60 dyad and Si-phthalocyanine
complex.64 Therefore, the PET of the dyad and triad was studied.
Electrochemical data were combined with the spectroscopic data
to study the Gibbs free energy changes (ΔG°CS) of the
photoinduced electron transfer and the energy level of the
charge transfer state (CTS) of B-1 and B-2.
Figure 6. Cyclic voltammograms of L-3, L-2, and B-2. Ferrocene (Fc)
was used as internal reference (E1/2 = +0.64 V (Fc+/Fc) vs standard
hydrogen electrode). Conditions: in deaerated CH2Cl2 solutions
containing 0.5 mM photosensitizers with the ferrocene, 0.10 M
Bu4NPF6 as supporting electrolyte, Ag/AgNO3 reference electrode,
scan rate 50 mV/s, 20 °C.
charge transfer state (CTS),64 which are useful for explanation of
the photophysical processes of B-1 and B-2. A reversible
oxidation wave at +1.52 V was observed for L-3 (Figure 5).
Conversely, a reversible reduction wave at −0.80 V was observed
(vs Ag/AgNO3 electrode).
The free energy changes of the intramolecular electron transfer
process can be calculated with the Weller equation (eqs 2 and
For reference compound L-1, two reversible reduction waves
at −0.45 and −0.64 V were observed (vs Ag/AgNO3 electrode).
No oxidation waves were observed within the potential range
used in the electrochemical measurement. These results indicate
that the PBI moiety in B-1 is more likely an electron acceptor,
rather than an electron donor. Moreover, the PBI moiety is more
easily reduced than the 2,6-diiodoBodipy unit. Therefore, in a
later discussion on the intramolecular electron transfer, the PBI
moiety was considered as an electron acceptor and the
diiodoBodipy moiety was treated as an electron donor.32
The redox potentials of B-1 were compared with those of the
components of the dyad, i.e. L-3 and L-1 (Figure 5). L-3 and B-1
show similar oxidation potentials. This result indicates that
oxidation of the diiodoBodipy part was not affected by linking to
the PBI moiety. For the reduction, however, the reduction wave
of B-1 is cathodically shifted in comparison to that of L-3. This
result is reasonable, since the PBI moiety is reduced in a less
negative range and, with the formation of the PBI anion, it is
more difficult to inject the third electron into this dyad: i.e., into
the diiodoBodipy moiety. The reduction waves of B-1 were
compared with those of L-1. The two compounds show identical
reduction potentials (Table 2).
ΔG°CS = e[EOX − ERED] − E00 + ΔGS
(2)
e2
e2
8πε0
1
1
RA
1
1
εS
⎛
⎞⎛
⎞
⎟
⎠
ΔGS = −
−
+
−
⎜
⎝
⎟⎜
⎠⎝
4πεSε0RCC
R
ε
D
REF
(3)
3),64a where ΔGS is the static Coulombic energy, which is
described by eq 3, e = electronic charge, EOX = half-wave potential
for one-electron oxidation of the electron-donor unit, ERED
=
half-wave potential for one-electron reduction of the electron-
acceptor uni, E00 = energy level approximated with the
fluorescence emission (for the singlet excited state), εS = static
dielectric constant of the solvent, RCC = center-to-center
separation distance between the electron donor (diiodoBodipy)
and electron acceptor (PBI), determined by DFT optimization
of the geometry, RCC(B-1) = 10.76 Å, RCC(B-2) = 11.54 Å, RD is
the radius of the electron donor, RA is the radius of the electron
acceptor, εREF is the static dielectric constant of the solvent used
for the electrochemical studies, and ε0 is the permittivity of free
space. The solvents used in the calculation of free energy of the
electron transfer are toluene (εS = 2.38), CH2Cl2 (εS = 8.93), and
acetonitrile (εS = 37.5).
Table 2. Electrochemical Redox Potentials of the
a
Compounds
compound
oxidation (V)
reduction (V)
Energies of the charge-separated states (ECS) and charge
recombination energy states (ΔGCR) can be calculated with eqs 4
and 5. The data are collected in Table 3.
5
+1.43
+1.43
+1.52
b
−0.42, −0.62, −1.09
−0.38, −0.60, −1.08
−0.80
−0.45, −0.64
−0.40, −0.61, −0.85
−0.45, −0.64
6
L-3
L-1
B-1
L-2
B-2
ECS = e[EOX − ERED] + ΔGS
(4)
+1.57
b
ΔGCR = −(ΔGCS + E00)
(5)
+1.57
−0.36, −0.60, −0.84
a
Cyclic voltammetry in Ar-saturated CH2Cl2 containing a 0.10 M
The free energy changes of the intramolecular electron transfer
and the charge recombination are compiled in Table 3. In
principle, electron transfer is thermodynamically allowed, even in
nonpolar solvents such as toluene. This prediction was
confirmed by the quenching of the fluorescence of energy
Bu4NPF6 supporting electrolyte. The counter electrode is Pt, and the
working electrode is glassy carbon, with the Ag/AgNO3 couple as the
reference electrode. Conditions: c [Ag+] = 0.1 M, 0.5 mM compounds
in CH2Cl2, 25 °C. Ferrocene (Fc) was used as internal reference (E1/2
b
= +0.64 V (Fc+/Fc) vs standard hydrogen electrode. Not observed.
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different transient absorption profile (Figure 7): for example, the
bleaching band and the transient absorption band wave-
length.51,67
Table 3. Driving Forces of Charge Recombination (ΔGCR)
and Charge Separation (ΔGCS) for the Dyad B-1, Triad B-2,
and the Reference Compounds L-1, L-2, and L-3 in Toluene,
Dichloromethane, and Acetonitrile
The decay trace at 470 nm gives a triplet excited state lifetime
of 150 μs (Figure 7b). This lifetime is much longer than that
observed with PBI-containing Pt(II) complexes (0.246 μs)68 or
with a different complex (0.37 μs).69 With the PBI-containing
Ir(III) complex, for which the T1 state is an intraligand triplet
state (not the conventional MLCT state), we observed a triplet
state lifetime of 22.3 μs.66 The production of the PBI triplet state
in these complexes is based on the heavy-atom effect of the
transition-metal atoms. Interestingly, we observed a lifetime of
the PBI triplet state up to 105.9 μs with the PBI-C60 dyad.65
Therefore, we propose that the inherent triplet state lifetime of
an organic chromophore, which is devoid of ISC, such as PBI, can
be probed by a triplet energy transfer approach (or sensitizing
method), as for B-1.38 The heavy-atom effect will produce the
triplet excited state of an organic chromophore, but the lifetime is
highly likely to be reduced due to the enhanced ISC process.
Similar results were observed for B-2 (Figure 7c,d), for which
the T1 state is localized on the PBI moiety, and the triplet state
lifetime was determined as 148 μs. The bleaching band of B-1 at
533 nm is more significant in comparison with that of B-2. This is
due to the larger molecular absorption coefficient of L-1 in
comparison to that of L-2.
a
a
b
b
c
c
ΔGCS
ΔGCR
ΔGCS
ΔGCR
ΔGCS
ΔGCR
(eV)
(eV)
(eV)
(eV)
(eV)
(eV)
d
e
d
e
d
e
B-1
B-2
5
−0.09
0.00
−2.15
−2.15
−2.03
−2.03
−0.42
−0.33
−0.45
−0.36
−1.82
−1.79
−1.70
−0.51
−0.42
−0.54
−0.45
−0.79
−0.54
−0.82
−1.73
−1.70
−1.61
−1.58
d
e
d
e
d
e
−0.09
+0.04
−0.37
−0.12
−0.37
d
e
d
d
e
−0.7
e
−0.45
−0.73
−0.48
d
e
d
e
d
e
6
−1.67
−0.12
−0.57
a
b
c
d
e
1
1
In toluene. In CH2Cl2. In acetonitrile. Via BDP*. Via PBI*.
acceptors in dyad B-1 and triad B-2, even in a nonpolar solvent
such as toluene (Figure 3). The energy level of the CTS is close
to or lower than the singlet state of 1[PBI]*; thus, the quenching
of the PBI emissions in B-1 and B-2 is rationalized.32 The
photophysics of the dyad (B-1) and the triad (B-2) can be fully
rationalized with the electrochemical data. Similarly, PET is
thermodynamically allowed for the uniodinated dyad 5 and triad
6 (Table 3).
The nanosecond transient absorption spectra of L-3 (2,6-
diiodoBodipy) was also studied (Figure 7e), for which the
transient bleaching and absorption motif is substantially different
from those of B-1 and B-2. For example, the intensity ratio of the
bleaching band to the transient absorption bands is much larger
than that observed with B-1 and B-2.51 These results give
unambiguous support for the assignment of the T1 state in B-1
and B-2 as the PBI-localized triplet excited state, not the Bodipy
moiety localized triplet state.
Since we have shown that the triplet formation by the
reference compound L-1 is negligible (no nanosecond transient
signal was detected for L-1, L-2, and the uniodinated analogue
compounds 5 and 6)62 and that the T1 state of the Bodipy moiety
2.4. Nanosecond Transient Absorption Spectroscopy:
Triplet Excited States of Dyad B-1 and Triad B-2. In order
to investigate the triplet excited state of the dyad (B-1) and the
triad (B-2), the nanosecond transient absorption spectroscopy of
the compounds was studied (Figure 7).
Upon 532 nm pulsed laser photoexcitation, a bleaching band at
533 nm was observed for B-1. This band overlaps with the
transient absorption band in the range 400−650 nm. This excited
state absorption (ESA) profile is a ypical T1 → Tn absorption of
the PBI triplet excited state.53,65,66 The transient absorption
spectra of B-1 are devoid of any significant absorption bands at
700 nm; thus, the transient species detected are not due to the
PBI radical anions.62 The iodoBodipy part gives a drastically
Figure 7. Nanosecond transient absorption of B-1, B-2, and L-3: (a) transient absorption spectra of B-1; (b) decay trace of B-1 at 470 nm; (c) transient
absorption difference spectra of B-2; (d) decay trace of B-2 at 470 nm; (e) transient absorption difference spectra of L-3; (f) decay trace of L-3 at 520
nm. Conditions: λex 532 nm (nanosecond pulsed laser), c = 1.0 × 10−5 M in deaerated toluene, 20 °C.
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Figure 8. Intermolecular triplet energy transfer: nanosecond transient absorption spectra of a mixture of L-2 and L-3, with L-3 as the triplet energy
donor and L-2 as the triplet energy acceptor. The concentration of L-3 is fixed at 1.0 × 10−5 M, and the concentration of L-2 was increased to (a) 1.0 ×
10−6 M, (b) 5.0 × 10−6 M, (c) 1.0 × 10−5 M, and (d) 2.0 × 10−5 M. (e) and (f) Selected transient absorption spectra at a few specific delay times, to show
the faster TTET with greater L-2/L-3 molar ratio. Conditions: excitation with a pulsed nanosecond laser at 532 nm, in deaerated toluene at 20 °C.
is 1.70 eV,54 whereas the T1 state of PBI is 1.2 eV,53 as a result,
Bodipy is the triplet energy donor and PBI is the triplet energy
acceptor; thus, the production of a triplet state by B-1 is most
probably via intramolecular triplet state energy transfer (TTET)
from iodo-Bodipy to the PBI unit. Since the intramolecular
triplet energy transfer rate constant can be up to 109 s−1,40,41 we
postulate that these rate constants are applicable for B-1 and B-2,
in which the triplet energy donor (iodoBodipy) and the triplet
energy acceptor (PBI) are in close vicinity, which is beneficial for
the TTET (usually via the Dexter mechanism, i.e. electron
exchange); thus, no slow development of the PBI transient
feature can be detected with our nanosecond transient
absorption spectrometer (the time resolution is 10 ns, which is
unable to detect photophysical processes with rate constants
larger than 108 s−1).
intermolecular triplet state energy transfer between the 2,6-
diiodoBodipy unit and the triplet acceptor PBI unit. The
intermolecular triplet energy transfer was monitored by
following the evolution of the transient of L-3 upon increasing
the concentration of L-2 in the L-2/L-3 mixture (Figure 8).72
When the concentration of L-2 was increased, the transient signal
of L-3 at 532 nm diminished more quickly, and the featured
transient absorption spectrum of the PBI triplet state developed
(Figure 8). For example, the bleaching band shifted from 530 to
556 nm with an increase in the concentration of L-2 (Figure 8b−
d). Concomitantly, the transient absorption band at 430 nm for
diiodo-Bodipy disappeared and a new ESA band at 490 nm
developed (Figure 8b−d), which is characteristic for the transient
absorption of the triplet state of PBI. Changes were observed in
the range 600−800 nm.
It should be noted that intramolecular electron transfer was
demonstrated by a steady-state fluorescence study, as well as
electrochemical data. However, the triplet production by B-1 and
B-2 was not affected significantly, concerning the aspects of both
the triplet state lifetime of PBI and the triplet state yield
(approximated by the singlet oxygen quantum yields; Table 1).
Electrochemical data show that the CTS has an energy level of
1.82 eV (in CH2Cl2), which is well above the triplet state energy
level of PBI (1.2 eV); thus, the triplet state of PBI is not
perturbed by any electron transfer process.71 Similar results were
obtained with the solvents toluene and acetonitrile (Table 3).
Previously it was reported that the triplet state of PBI
derivatives can be produced by charge recombination (CR).63
Similar formation of the triplet state of C60 was observed with
Bodipy-C60 dyad and Si-phthalocyanine complex.64 However, for
B-1 and B-2, the triplet excited states of the PBI moiety were not
populated via the CR process. Note that the electrochemical
characterization (Table 3) and fluorescence spectra (Figures S44
and S45, Supporting Information) indicated PET for compounds
5 and 6. However, no significant production of triplet excited
states was observed for the uniodinated dyad 5 and the triad 6.
2.5. Intermolecular Triplet Energy Transfer. Long-range
energy transfer is important in many aspects; thus, we studied the
On the basis of these observations, we propose that
intermolecular triplet−triplet energy transfer (TTET) occurred
for the L-2/L-3 mixture, with L-3 as the triplet state energy
donor and L-2 as the triplet energy acceptor. No triplet state
equilibrium was established due to the significant triplet state
energy gap of iodoBodipy and the PBI moiety (ΔE = 0.5 eV).
Similar intermolecular TTET was observed for a mixture of L-3
and L-1 with transient absorption spectra (Figure S48,
Supporting Information). These results support the ultrafast
intramolecular TTET in B-1 and B-2.
In order to study the kinetics of the transient absorption
evolvement for a mixture of L-2 and L-3, i.e. the TTET process,
the transient absorption (the ΔOD) at 500 nm was monitored
(Figure 9).72 L-3 gives the ground-state bleaching at 500 nm; for
L-2, however, ESA absorption was observed. Therefore, the most
significant information will be attained by monitoring the decay
trace at 500 nm, that is, the instant production of the triplet state
of L-3 and thereafter the formation of the triplet state of L-2.
Therefore, in order to study the intermolecular triplet energy
transfer between L-3 and L-2, the decay trace of the mixture at
500 nm was monitored. The information revealed by the
evolution of the transient at 500 nm includes the kinetic
information on the decay of the triplet state of L-3 (triplet energy
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presence of L-2, however, it is clear that the lifetime of the triplet
state of iodoBodipy is much shorter, and significant production
of the triplet state of PBI was observed, indicated by the
production of a positive transient absorption at 490 nm. The
latter decay trace at 490 nm is due to the decay of the T1 state of
L-2. Thus, these results indicated that the T1 state of L-2 was
produced by the intermolecular TTET.72
2.6. Femtosecond Transient Absorption Spectroscopy.
Ultrafast pump-probe experiments were performed for both B-1
and B-2 at 530 nm pump. It is expected that ultrafast
spectroscopy studies will give evidence of singlet state intra-
molecular resonance energy transfer (RET) and formation of the
triplet state of diiodo-BODIPY (via ISC) and PBI moieties in B-1
and B-2 (Figure 11).64a,38 Although there are minor differences,
compounds B-1 and B-2 both exhibit similar characteristics in
their pump-probe data.
Figure 9. Comparison of decay curves at 500 nm for a mixture of L-2
and L-3 with different L-3/L-2 molar ratios. The concentration of L-3
was fixed at 1.0 × 10−5 M, and the concentration of L-2 is (a) 1.0 × 10−6
M, (b) 5.0 × 10−6 M, (c) 1.0 × 10−5 M, and (d) 2.0 × 10−5 M.
Conditions: in deaerated toluene at 20 °C.
donor) and finally the decay of the triplet excited state of L-2 (the
triplet energy acceptor). More importantly, the intermolecular
TTET will be revealed by the ΔOD trace at 500 nm, i.e. the
appearance of the positive ΔOD values. Therefore, the decay
trace at 500 nm will reveal at least three photophysical processes.
At low L-2 concentration, the decay of the transient of the L-
2/L-3 mixture is identical with that of the L-3 alone (Figure 9a).
Under this circumstance the intermolecular energy transfer is
negligible (Figure 8b). Upon an increase in the concentration of
L-2, following the normal recovery of the bleaching, the OD
increased positively in the range 360−550 nm, indicating the
significant production of the triplet state of PBI. Note that the
triplet state of PBI shows a positive transient absorption at 500
nm.65,66 Then the third phase of the trace at 500 nm indicated the
decay of the triplet state of PBI. This three-phase feature of the
decay trace at 500 nm becomes more distinct with an increas in
the concentration of L-2: i.e., with more significant intermo-
lecular TTET (Figure 9c,d).
Figure 11. Ultrafast transient absorption spectra of (a) B-1 and (b) B-2.
Conditions: c = 1.0 × 10−5 M in toluene, excited with 530 nm
femtosecond pulsed laser, 20 °C.
The iodo-BODIPY and the PBI moieties were both excited
with a pump beam instantaneously due to overlapping of
absorption bands, indicated by the bleaching bands in the range
450−650 nm (centered at 530 nm, Figure 11). This bleaching
signal represents depletion of singlet states of both the diiodo-
BODIPY and the PBI moieties. The 530 nm bleaching signal
decreases until about 2 ps and increases from 2 to 250 ps, as
presented in Figure 11. This behavior could be due to
competition between FRET from iodo-BODIPY to PBI and
ISC of iodo-BODIPY mechanisms.
There are several positive signals (ESA) in the ultrafast pump
probe spectra representing transient absorptions in the
investigated samples (Figure 11). The absorption around the
420 nm region occurring simultaneously with the pump pulse is
attributed to the S1 → Sn transition of the diiodo-BODIPY
moiety. Another simultaneous transient absorption signal with
the pump pulse is observed around the 720 nm region. The
transient absorption band centered at 710 nm (B-1) or 725 nm
(B-2) is due to the S1 → Sn absorption of the PBI moieties in B-1
and B-2.73−75 It should be noted that the radical anion of PBI also
shows transient absorption in the same region.62 However, the
position of the transient absorption bands at 710/725 nm did not
change during 0−2626 ps after excitation; thus, it is unlikely that
other species, such as the radical anion PBI•−, were produced via
the singlet excited state of PBI.75 Moreover, an intermolecular
interaction, such as the π−π stacking, can be excluded since the
transient absorption band at 710 nm is not broad.75 The fact that
this signal increases after 2 ps until about 250 ps indicates that
there is FRET from iodo-BODIPY to the PBI moiety (Figure
11).
The traces of the transient at 490 nm for L-3 and the mixture of
L-3/L-2 (1/2 molar ratio) were compared (Figure 10). For L-3,
a decay trace with a long lifetime of 90.5 μs was observed. In the
Figure 10. Comparison of the decay trace of L-3 and the mixture of L-3
and L-2 at 490 nm upon nanosecond pulsed laser excitation.
Conditions: c[L-3] = 1.0 × 10−5 M and c[L-2] = 2.0 × 10−5 M in
deaerated toluene, at 20 °C.
In order to reveal the kinetics of the photophysical processes in
detail, the decay traces at a few critical wavelengths were
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monitored (Figure 12). For both B-1 and B-2, the decay trace at
715 and 725 nm had a triple-phase feature (Figure 12b). The
Figure 13. (a) Comparison of decay curves of B-1 at 476, 507, and 576
nm, respectively. (b) Comparison of decay curves of B-1 and B-2 at 531
and 542 nm, respectively. The data are extracted from Figure 11.
Figure 12. (a) Decay curves of B-1 and B-2 at 715 and 722 nm probe
wavelength, respectively. (b) Magnified early stage of the decay curves to
show the triple-phase feature. The data are derived from parts a and b of
Figure 11, respectively.
i.e., the ΔOD value will not change with time. Third, after the
ISC of diiodoBodipy, intermolecular TTET to PBI may occur;
thus, the bleaching band intensity will decrease because the
steady-state absorption of PBI in the region 475−600 nm is
much weaker than that of the diiodoBodipy part. All of these
processes may result in a complicated kinetics in the time range
1−100 ps (Figure 13a).
On the basis of the above analysis, the FRET process takes
about 50 ps to finish, which is in agreement with the result
derived from the transient absorption at 710 nm. Thus, the rate
constant of the FRET process can be determined as kRET = 5 ×
1010 s−1.62 After 100 ps, the bleaching band at 507 nm decreased
and recovered toward the baseline. We noted that the singlet
state of the PBI moiety does not absorb at this wave-
length,62,73−75 Instead, the triplet state of PBI absorbs at this
wavelength.53,65,70 On the other hand, the triplet state of
diiodoBodipy does not absorb at 507 nm (Figures 7 and 8).61
Therefore, we conclude that the recover of the bleaching at 507
nm is due to the consumption of the triplet state of diiodoBodipy
and the production of the triplet state of PBI: i.e. the
intramolecular triplet state energy transfer. This downhill
TTET was supported by the intermolecular TTET studied with
nanosecond transient absorption spectroscopy (vide supra).
Interestingly, this is a slow process which takes ca. 1 ns; thus, the
quickly increasing phase of the curves is due to the instant
excitation of the PBI moiety in B-1 and B-2 upon femtosecond
pulsed laser excitation. Then a relatively slow increasing phase
was observed, which is complete within ca. 50 ps. This process is
due to the production of a PBI singlet excited state via FRET,
with the diiodoBodipy unit as the singlet energy donor. The rate
constant for this process is ca. kRET = 5 × 1010 s−1 for B-1.
Previously, a larger value was reported for the Bodipy-azaBodipy
triad (ca. 1.0 × 1011 s−1).32 For Bodipy-styrylBodipy triads, the
calculated kRET value is up to 2.3 × 1011 s−1.60 In a bichromic
Bodipy-Pt(porphyrin) complex, the singlet energy transfer rate
constant is up to 7.8 × 1011 s−1.40 The relatively slow FRET may
be attributed to the poor spectral overlap between the emission
of the diiodoBodipy part and the absorption of the PBI
moiety.3,30,76−78
The third phase of the decay curve, i.e. the decay of the
transient absorption at 710/725 nm, is slow (Figure 12b). The
rate constant for this decay process was approximated as k = 1 ×
109 s−1. The lifetime of this transient is ca. 1 ns, which is very close
to the fluorescence lifetimes of B-1 and B-2 determined in
separate experiments (i.e., the lifetime of the emissive singlet
excited state, 0.96 or 0.84 ns, Table 1). These results, and the
electrochemical characterization data, all support that the
transient absorption at 710/725 nm is due to the singlet excited
state of PBI,73 for which three phases were observed: i.e., the
instant production of the PBI singlet excited state by excitation
and then by FRET and finally the decay of the S1 state to the
ground state (S0 state).
rate constant of the intramolecular TTET was estimated as kTT
=
1 × 109 s−1. Previously for a bichromic Bodipy-Pt(porphyrin)
hybrid, the intramolecular TTET process was reported with a
rate constant of kTT = 1.0 × 1010 s−1.40 For a Bodipy-N∧N Pt(II)
complex, the kTT value is up to 6.0 × 1010 s−1.41 A much slower
TTET was found for a hydrogen-bonding C60-ferrocene
assembly (kTT = 9.2 × 105 s−1).56
The decay traces of the transient in the characteristic region
(450−650 nm) of the triplet excited state of PBI were also
monitored (Figure 13).53,65,66,70 For B-1, the fast development of
the bleaching at 507 nm is due to the prompt excitation of the
diiodoBodipy unit upon femtosecond laser excitation. Thereafter
a slow increasing of the bleaching was observed; the process was
finished in ca. 100 ps. A few photophysical manifolds may be
involved in this slow process, such as the FRET, intersystem
crossing of diiodoBodipy and the following TTET from
diiodoBodipy to the PBI moiety. The rationalization for this
postulation is given in the following section. First, the steady-
state absorption wavelengths of PBI and the diiodoBodipy
moiety are similar to each other (Figure 11); thus, no clear trend
for the change of the transient absorption at 507 nm can be
observed with the FRET process. Second, the intersystem
crossing of diiodoBodipy (kISC = 7.7 × 109 s−1, 130 ps)61 will not
significantly alter the bleaching band of diiodoBodipy at 507 nm:
On the other hand, there are transient absorption signals
appearing after 100 ps. A similar observation is also valid for a
transient absorption around 830 nm. Therefore, this signal could
be related to the S1 → Sn transition of PBI moieties as well. The
transient absorption spectrum of B-1 has three positive signals at
480, 505, and 575 nm appearing at delay times of 250, 1000, and
100 ps, respectively (Figure 13). The lifetimes of these signals are
on the order of microseconds (Figure 13), and therefore these
signals can be attributed to T1 → Tn transitions.53,65,70
A
transient absorption signal appearing at the earliest time at 575
nm can be attributed to the T1 → Tn transition of iodo-BODIPY.
We expect the T1 → Tn transition related with this state to be
delayed in comparison to the T1 → Tn transition of iodo-
BODIPY. This is why the transient absorption signal appearing at
a later time at 505 nm is attributed to the T1 → Tn transition of
PBI. Note that the amplitude of the T1 → Tn transition of iodo-
BODIPY is smaller than that of other transient absorption signals
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Figure 14. Selected frontier molecular orbitals of compounds B-1 (upper row) and B-2 (lower row). The calculations are based on the optimized
ground state geometry (S0 state) at the B3LYP/6-31G(d)/level using Gaussian 09W.
after 100 ps; this may be more proof that ISC of the iodo-
BODIPY is quenched to some extent by FRET.
Similar DFT calculation results were observed for B-2 (Figure
14).
The spin density surfaces of the dyad B-1 and triad B-2 were
calculated with DFT methods (Figure 15). The spin density
These transient absorption signals of B-2 are slightly different
from those of B-1. The two bands around 480 and 505 nm
become a single absorption band for the B-2 sample due to the
presence of two iodo-BODIPY groups. The other differences
between B-1 and B-2 are the lifetimes of the bleaching signals
around 530 nm. The B-2 sample exhibits a faster and therefore
more efficient energy transition due to the two donor iodo-
BODIPY moieties (Figure 13b). This result may be the reason B-
2 shows more efficient fluorescence quenching than B-1, as seen
from fluorescence measurements (Figure 2).
In summary, the rate constant of the FRET and the ISC of B-1
indicated that 87% of the singlet excited state of diiodoBodipy in
B-1 will decay via FRET, whereas 13% of the singlet excited state
of diiodoBodipy will decay via ISC, as estimated by the respective
rate constants of the two processes. However, the decay curves at
710 nm and the singlet oxygen sensitizing experiments (ΦΔ =
80%, Table 1) all show that a much smaller ratio of FRET/ISC is
occurring for B-1. We confirmed that the triplet yield via CR is
negligible with the uniodinated reference compounds 5 and 6.
Thus, we postulate that the singlet excited state of diiodoBodipy
may be trapped by some unknown mechanisms, such as a
vibrational barrier (or small Franck−Condon factor), to prevent
a complete FRET from occurring.79 A detailed investigation of
this issue requires extensive theoretical calculations which are
beyond the scope of this paper.
2.7. DFT Calculations. The ground state geometries of B-1
and B-2 were optimized with the DFT method (Figure 14). The
phenyl ring, as a linker between the PBI and the BODIPY
moieties, takes a perpendicular geometry toward the two
moieties. Thus, the Bodipy moiety and the PBI moiety in the
dyad and triad are almost coplanar. However, the frontier
molecular orbitals of B-1 and B-2 show that there is no π
conjugation between the Bodipy and PBI moieties. Moreover, for
dyad B-1, HOMO is localized on the Bodipy moiety, whereas the
LUMO is confined on the PBI moiety. Thus, photoinduced
electron transfer is the feature of the electronic transition upon
photoexcitation. This conclusion is in full agreement with the
weak fluorescence of B-1 (quenching of the PBI moiety emission
in B-1), as compared with that of compound 5 (Figure 3a).
Figure 15. Isosurfaces of spin density of PBI-BODIPY sensitizers B-1
and B-2. Calculations were performed at the B3LYP/6-31G(d) level
with Gaussian 09W.
surface of the dyad/triad will indicate the localization of triplet
excited states. The DFT calculation results show that the triplet
excited states of B-1 and B-2 are exclusively confined on the PBI
moiety, not the Bodipy moiety. These theoretical results are in
full agreement with what we observed with nanosecond transient
absorption spectroscopy, which indicated that the triplet excited
states of B-1 and B-2 are exclusively localized on the PBI moiety
and the Bodipy moiety does not contribute to the triplet excited
states of B-1 and B-2 (Figure 7).
2.8. Energy Level Diagram and the Photophysical
Processes. On the basis of the spectral and electrochemical data,
the energy levels of the excited states, as well as the photophysical
processes, are summarized in Scheme 2 (exemplified with B-1).
Upon excitation into the diiodoBodipy unit, ultrafast FRET
competes with the ISC of the diiodoBodipy moiety. In CH2Cl2,
the CTS lies below the S1 state of the PBI moiety; as a result, the
fluorescence of the PBI unit should be significantly quenched.
This postulation is in agreement with the fluorescence of B-1
(Figure 3). Note that the PBI moiety shows very weak ISC,
supported by the high fluorescence of the reference compound
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affected by the CTS. Since the PBI moiety has a T1 state energy
level (1.2 eV) much lower than that of Bodipy moiety (1.7 eV),
the triplet−triplet energy transfer (TTET) process produces the
PBI triplet state, and an exceptionally long-lived triplet state
lifetime was observed (150 μs). Previously the triplet state
lifetime of PBI observed with a Pt(II) coordination approach was
much shorter (<1 μs). Thus, we propose to use the TTET
approach, instead of the conventional heavy-atom effect, to study
the inherent triplet state properties of organic chromophores
which are devoid of ISC capability. The intermolecular triplet
state energy transfer was studied with nanosecond transient
absorption spectroscopy. The intramolecular triplet energy
transfer is ultrafast (kTT > 108 s−1), whereas the intermolecular
TTET is relatively slow. The FRET effect of the dyad and the
triad was studied with femtosecond pump-probe transient
absorption spectroscopy, which indicates that the FRET has a
rate constant of 5 × 1010 s−1. Previously the ISC of the
diiodoBodipy moiety was determined with much slower kinetics
(kISC = 7.7 × 109 s−1). Thus, we propose the singlet excited state
of the energy donor of the RET is trapped somehow from
undertaking the fast FRET process. Moreover, with uniodinated
reference dyad and triad, we confirmed that the triplet excited
state localized on the PBI moiety is not produced by charge
recombination. This information will be useful for designing
multichromophore organic triplet photosensitizers and inspiring
in-depth insights into the fundamental photophysical processes
of multichromophore molecular assemblies.
Scheme 2. Simplified Jablonski Diagram Illustrating the
Photophysical Processes Involved in B-1 (in CH2Cl2)
a
a
The energy levels of the excited state are derived from the
spectroscopic data and the electrochemical data. [BDP-PBI] stands
for the diiodoBodipy and PBI units in B-1. The component at the
excited state is designated in red. The number of the superscript
designates the spin multiplicity.
L-1 (42.4%, Table 1) and the low singlet oxygen quantum yields
(ΦΔ = 0.08). The charge recombination does not likely produce
the PBI triplet state efficiently, indicated by the results of the
references dyad 5 and triad 6 (iodo-free). Furthermore, the dyad
B-1 and triad B-2 do not show any 1O2 photosensitizing ability in
dichloromethane and acetonitrile, indicating that the triplet
excited state is not produced by the charge recombination (CR).
Excitation into the PBI moiety will give a similar photophysical
process, except for the lack of FRET. Since the CTS lies well
above the T1 state of the PBI unit, the production of the triplet
state is not affected by CTS; for example, the lifetime of the
triplet state B-1 is very long and the triplet state yield is high (ΦΔ
= 0.80 for B-1, Table 1). Similar photophysical processes were
proposed for B-1 in toluene and acetonitrile (see the Supporting
Information, Schemes S49 and S50); note that the fluorescence
of the PBI moiety in B-1 was quenched under these
circumstances, but not the triplet state of the PBI moiety.
Similar photophysical processes were proposed for B-2 (see the
Supporting Information, Figures S54 and S55). Thus, the
photophysical properties of B-1, such as the quenching of the
fluorescence of the PBI unit, the ultrafast FRET yet high triplet
yield (approximated by the singlet oxygen yield of B-1), and the
independence of the triplet state property of solvent polarity, are
fully rationalized by the excited state and the CTS energy levels.
2.9. Conclusion. In summary, diiodoBodipy-perylenebisi-
mide dyads/triads were prepared, in order to study the
competing intersystem crossing (ISC) and resonance energy
transfer (RET) processes. The diiodoBodipy part is the singlet
energy donor and, at the same time, the spin converter to
produce a triplet state upon photoexcitation. The PBI moiety is
the singlet/triplet energy acceptor; as a result, the ISC of the
diiodoBodipy moiety competes with RET. With steady state and
femto/nanosecond transient absorption and steady state
fluorescence spectroscopy, we demonstrated the singlet energy
transfer and electron transfer for these iodinated dyad and the
triad. ISC is not completely outcompeted by the intramolecular
RET process. On the basis of electrochemical data, the charge
transfer state (CTS) is identified residing between the S1 state
and the T1 state of the PBI moiety; thus, the fluorescence of PBI
in the dyad and triad is drastically quenched in comparison with
the reference PBI compound, yet the triplet state of the PBI is not
3. EXPERIMENTAL SECTION
3.1. Analytical Measurements. All of the chemicals used in
synthesis are analytically pure and were used as received. Solvents were
dried and distilled before use for synthesis. Luminescence quantum
yields of the compounds were measured with L-3 as the standard (ΦF =
2.7% in CH3CN).
3.2. Synthesis of Compound 1. The mixture of 3,4,9,10-
pyrelenetetracarboxylic dianhydride (2.0 g, 5.1 mmol), imidazole
(10.0 g, 146.9 mmol), and 2-ethylhexylamine (3.8 mL, 21.0 mmol)
was stirred under an Ar atmosphere at 160 °C for 6 h. Then the mixture
was cooled and chloroform was added. The organic layer was dried, and
the solvent was evaporated under reduced pressure. The crude product
was not further purified and was directly used in the next reaction. Mp: >
250 °C. 1H NMR (CDCl3, 500 MHz): δ 8.56 (d, 4H, J = 5.0 Hz), 8.44
(d, 4H, J = 5.0 Hz), 4.17−4.09 (m, 4H), 1.98−1.93 (m, 2H), 1.43−1.33
(m, 16H), 0.97 (t, 6H, J = 7.2 Hz), 0.91 (t, 6H, J = 7.09 Hz). TOF
MALDI-HRMS: calcd ([C40H42N2O4]−), m/z 614.3145; found, m/z
614.3186.
3.3. Synthesis of Compound 2. Under an Ar atmosphere, a
mixture of N,N′-bis(2-ethylhexan-1-amine)perylene-3,4,9,10-tetracar-
boxylic dianhydride (2.0 g, 3.3 mmol), bromine (9.0 mL, 180.0 mmol),
and chloroform (50 mL) was stirred at room temperature for 48 h. The
mixture was washed with saturated Na2S2O3·5H2O aqueous solution.
The organic layer was dried over anhydrous Na2SO4, and the solvent was
removed under reduced pressure. The residue was purified with column
chromatography (silica gel, CH2Cl2) to give a red solid (500 mg, yield
22%). Mp: > 250 °C. 1H NMR (CDCl3, 500 MHz): δ 9.73 (d, 1H, J =
5.0 Hz), 8.85 (s, 1H), 8.64 (d, 3H, J = 5.0 Hz), 8.52 (d, 2H, J = 8.2 Hz),
4.18−4.07 (m, 4H), 1.98−1.92 (m, 2H), 1.42−1.32 (m, 16H), 0.97−
0.89 (m, 12H). TOF HRMS ESI: calcd ([C40H41N2O4Br]+), m/z
692.2250; found, m/z 692.2245.
3.4. Synthesis of Compound 3. Under an Ar atmosphere, a
mixture of N,N′-bis(2-ethylhexan-1-amine)perylene-3,4,9,10-tetracar-
boxylic dianhydride (2.0 g, 3.3 mmol), bromine (9.0 mL, 180.0 mmol),
and chloroform (50 mL) was stirred at 60 °C for 48 h. The mixture was
washed with saturated Na2S2O3·5H2O aqueous solution. The organic
layer was dried over anhydrous Na2SO4, and then the solvent was
evaporated under reduced pressure. The residue was purified with
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Article
column chromatography (silica gel, CH2Cl2) to give a red solid (760 mg,
yield 30%). Mp: > 250 °C. 1H NMR (CDCl3, 500 MHz): δ 9.46 (d, 2H, J
= 8.0 Hz), 8.90 (s, 1H), 8.67 (d, 2H, J = 8.0 Hz), 4.19−4.09 (m, 4H),
1.97−1.92 (m, 2H), 1.42−1.32 (m, 16H), 0.97−0.89 (m, 12H). TOF
MALDI-HRMS: calcd ([C40H40N2O4Br2]−), m/z 770.1355; found, m/z
770.1373.
3.5. Synthesis of Compound 4. Under an Ar atmosphere, 4-
formylbenzeneboronic acid (1.50 g, 10.0 mmol) and pinacol (1.18 g, 10
mmol) were placed with toluene (150 mL) in a round-bottom flask. The
mixture was refluxed for 12 h at 120 °C. Evaporation of the solvent
under reduced pressure lead to formation of a white-yellow solid (2.3 g,
yield 99%). This crude product was used for the next step of the
synthesis without further purification.
14.1, 10.7. TOF MALDI-HRMS: calcd ([C78H76B2N6O4F4]−), m/z
1258.6050; found, m/z 1258.6025.
3.8. Synthesis of Compound L-1. Under an Ar atmosphere,
compound 2 (110.0 mg, 0.15 mmol), phenylboronic acid (18.0 mg, 0.15
mmol), K2CO3 (65.0 mg, 0.45 mmol), and EtOH/toluene/water (50
mL, 2 4 1, v/v) were mixed together. Then Pd(PPh3)4 (9.0 mg, 0.007
mmol, 5 mol %) was added. The reaction mixture were refluxed and
stirred under Ar for 8 h. After completion of the reaction, the mixture
was cooled to room temperature. The reaction mixture was extracted
with CH2Cl2, and the organic layer was washed with water (2 × 100 mL)
and dried over Na2SO4. The solvent was evaporated to dryness under
reduced pressure to give a crude solid. The crude product was further
purified with column chromatography (silica gel, CH2Cl2) to give a red
solid (58 mg, yield: 56%). Mp: >250 °C. 1H NMR (CDCl3, 400 MHz):
δ 8.56−8.69 (m, 5H), 8.10 (d, 1H, J = 10.0 Hz), 7.83 (d, 1H, J = 10.0
Hz), 7.54−7.47 (m, 5H), 4.20−4.05 (m, 4H), 1.98−1.90 (m, 2H),
1.42−1.29 (m, 16H), 0.96−0.87 (m, 12H). 13C NMR (100 MHz,
CDCl3) δ 163.9, 163.7, 142.5, 141.7, 136.1, 134.8, 134.7, 134.4, 132.5,
131.0, 130.9, 130.4, 130.1, 130.0, 128.9, 128.7, 128.4, 128.3, 128.0, 127.4,
123.5, 123.2, 123.1, 122.6, 122.2, 44.3, 38.0, 30.8, 28.7, 24.1, 23.1, 14.1,
10.6. TOF MALDI-HRMS: calcd ([C46H46N2O4]−), m/z 690.3458;
found, m/z 690.3463.
3.9. Synthesis of L-2. Under an Ar atmosphere, compound 3 (116.0
mg, 0.15 mmol), phenylboronic acid (36.0 mg, 0.3 mmol), K2CO3
(125.0 mg, 0.9 mmol), and EtOH/toluene/water (50 mL, 2/4/1, v/v)
were mixed together. Then Pd(PPh3)4 (9.0 mg, 0.007 mmol, 5 mol %)
was added. The reaction mixture was refluxed and stirred under Ar for 8
h. After completion of the reaction, the mixture was cooled to room
temperature. The reaction mixture was extracted with CH2Cl2, the
organic layer was washed with water (2 × 100 mL), and the organic
layers were dried over Na2SO4. The solution was evaporated to dryness
under reduced pressure to give a crude solid. The crude product was
further purified with column chromatography (silica gel, CH2Cl2/
petroleum ether 3/1, v/v) to give a solid (60 mg, yield 52%). Mp: >250
°C. 1H NMR (CDCl3, 500 MHz): δ 8.59 (s, 2H), 8.12 (d, 2H, J = 5.0
Hz), 7.77 (d, 2H, J = 10.0 Hz), 7.55−7.46 (m, 10H), 4.17−4.08 (m,
4H), 1.95−1.90 (m, 2H), 1.40−1.29 (m, 16H), 0.95−0.87 (m, 12H).
13C NMR (100 MHz, CDCl3): δ 163.8, 142.0, 141.1, 135.4, 134.8, 132.5,
130.3, 130.2, 129.4, 129.1, 129.0, 128.7, 127.6, 122.2, 121.8, 44.2, 37.9,
30.7, 28.7, 24.0, 23.1, 14.2, 10.6. TOF MALDI-HRMS: calcd
([C52H50N2O4]−), m/z 766.3771; found, m/z 766.3750.
3.10. Synthesis of L-3. To a solution of Bodipy (200.0 mg, 0.62
mmol) in anhydrous CH2Cl2 (25 mL) was added excess N-
iodosuccinimide (NIS. 558.0 mg, 2.48 mmol). The mixture was stirred
at room temperature for about 30 min. After completion of the reaction,
the mixture was washed with saturated Na2S2O3·5H2O aqueous solution
and was extracted with DCM. The organic layer was dried over
anhydrous Na2SO4, and then the solvent was evaporated under reduced
pressure. The residue was purified with column chromatography (silica
gel, CH2Cl2/petroleum ether, 2/1, v/v) to give a red solid (325 mg, yield
91%). Mp: 194.3−194.9 °C. 1H NMR (CDCl3, 500 MHz): δ 7.53−7.50
(m, 3H), 7.26−7.24 (m, 2H), 2.65 (s, 6H), 1.38 (s, 6H). 13C NMR (100
MHz, CDCl3) δ 156.8, 145.4, 141.4, 134.8, 131.3, 129.6, 129.5, 127.8,
85.7, 17.0, 16.1. TOF MALDI-HRMS: calcd ([C19H17BN2F2I2]+), m/z
575.9542, found, m/z 575.9528.
3.11. Synthesis of B-1. Compound 5 (28 mg, 0.03 mmol) and
excess NIS (558 mg, 2.48 mmol) were dissolved in dry CH2Cl2 (25 mL).
The mixture was stirred at 30 °C for 5 h. After completion of the
reaction, the mixture was washed with saturated Na2S2O3·5H2O
aqueous solution and was extracted with DCM. The organic layer was
dried over anhydrous Na2SO4, and then the solvent was removed under
reduced pressure. The residue was purified with column chromatog-
raphy (silica gel, CH2Cl2) to give a red solid (34 mg, yield: 95%). Mp:
>250 °C. 1H NMR (CDCl3, 400 MHz): δ 8.76−8.64 (m, 4H), 8.57 (s,
1H), 8.10 (dd, 2H, J1 =8.0 Hz, J2 = 20 Hz), 7.75 (d, 2H, J = 8.0 Hz), 7.49
(d, 2H, J = 8.0 Hz), 4.20−4.07 (m, 4H), 2.68 (d, 6H, J = 12.0 Hz), 1.98−
1.95 (m, 2H), 1.43−1.31 (m, 16H), 1.26 (s, 6H), 0.98−0.88 (m, 12H).
13C NMR (100 MHz, CDCl3): δ 163.8, 163.5, 144.5, 140.1, 139.9, 136.1,
135.2, 134.9, 134.4, 134.4, 132.7, 131.4, 131.4, 130.1, 130.0, 129.5, 129.1,
128.5, 128.1, 127.6, 123.9, 123.3, 123.1, 122.8, 122.5, 86.2, 44.4, 38.0,
Under an Ar atmosphere, the above crude product (2.0 g, 8.7 mmol)
and 2,4-dimethylpyrrole (2 mL, 20 mmol) were dissolved in dry CH2Cl2
(150 mL). A few drops of trifluoroacetic acid were added to the solution,
and the mixture was stirred overnight at room temperature. After
completion of the reaction (monitored via TLC), a solution of DDQ
(2.0 g, 8.7 mmol) in freshly distilled THF was added to the reaction
mixture. The reaction mixture was stirred for 2 h. Absolute triethylamine
(10 mL) was added to the reaction mixture. After this mixture was
stirred for 15 min, BF3·Et2O (10 mL) was added dropwise under ice-
cold conditions. After the reaction mixture was stirred for 2 h more, the
mixture was washed with water several times and then extracted with
DCM. The organic phase was dried over anhydrous Na2SO4, and then
the solvent was evaporated under reduced pressure. The residue was
purified with column chromatography (silica gel, CH2Cl2) to give a red
solid (1.2 g, yield 31%). 1H NMR (CDCl3, 500 MHz): δ 7.91 (d, 2H, J =
5.0 Hz), 7.30 (d, 2H, J = 5.0 Hz), 5.97 (s, 2H), 2.55 (s, 6H), 1.39 (s,
12H), 1.37 (s, 6H). TOF MALDI-HRMS: calcd ([C25H30N2O2B2F2]+),
m/z 450.2461; found, m/z 450.2446.
3.6. Synthesis of Compound 5. Under an Ar atmosphere,
compound 2 (110.0 mg, 0.15 mmol), compound 4 (70.0 mg, 0.15
mmol), K2CO3 (65.0 mg, 0.45 mmol), and EtOH/toluene/water (50
mL, 2/4/1, v/v) were mixed together. Then Pd(PPh3)4 (9.0 mg, 0.007
mmol, 5 mol %) was added. The reaction mixture was refluxed and
stirred under Ar for 8 h. After completion of the reaction, the mixture
was cooled to room temperature. The reaction mixture was extracted
with CH2Cl2, washed with water (2 × 100 mL), and dried over Na2SO4.
The solution was evaporated to dryness under reduced pressure to give a
crude solid. The crude product was further purified with column
chromatography (silica gel, CH2Cl2) to give an orange solid (80 mg,
yield 57%). Mp: > 250 °C. 1H NMR (CDCl3, 400 MHz): δ 8.74−8.62
(m, 4H), 8.58 (s, 1H), 8.05 (dd, 2H, J1= 8.0 Hz, J2 = 24 Hz), 7.69 (d, 2H,
J = 8.0 Hz), 7.52 (d, 2H, J = 8.0 Hz), 6.14 (s, 1H), 6.04 (s, 1H), 4.21−
4.06 (m, 4H), 2.60 (d, 6H, J = 8.0 Hz), 1.99−1.93 (m, 2H), 1.45−1.30
(m, 16H), 1.25 (s, 6H), 0.98−0.88 (m, 12H). 13C NMR (100 MHz,
CDCl3): δ 163.7, 163.5, 156.2, 143.9, 143.1, 140.6, 140.4, 136.1, 135.6,
134.9, 134.5, 134.3, 132.7, 131.3, 130.4, 130.2, 129.6, 129.0, 128.5, 128.0,
127.5, 123.8, 123.3, 123.1, 123.0, 122.7, 122.4, 121.6, 44.4, 38.1, 30.8,
29.7, 28.7, 24.1, 23.1, 14.7, 14.1, 10.7. TOF MALDI-HRMS: calcd
([C59H59BN4O4F2]−), m/z 936.4597; found, m/z 936.4595.
3.7. Synthesis of Compound 6. Under an Ar atmosphere,
compound 3 (116.0 mg, 0.15 mmol), compound 4 (135.0 mg, 0.3
mmol), K2CO3 (125.0 mg, 0.9 mmol), and EtOH/toluene/water (50
mL, 2/4/1, v/v) were mixed together. Then Pd(PPh3)4 (9.0 mg, 0.007
mmol, 5 mol %) was added. The reaction mixture was refluxed and
stirred under Ar for 8 h. After completion of the reaction, the mixture
was cooled to room temperature. The reaction mixture was extracted
with CH2Cl2, washed with water (2 × 100 mL), and dried over Na2SO4.
The solution was evaporated to dryness under reduced pressure to give a
crude solid. The crude product was further purified with column
chromatography (silica gel, CH2Cl2) to give a solid (120 mg, yield 64%).
Mp: >250 °C. 1H NMR (CDCl3, 500 MHz): δ 8.66 (s, 2H), 8.05 (dd,
4H, J1 = 8.0 Hz, J2 = 16 Hz), 7.76 (d, 4H, J = 8.0 Hz), 7.51 (d, 4H, J = 8.0
Hz), 6.09 (d, 4H, J = 40 Hz), 4.18−4.09 (m, 4H), 2.60 (s, 12H), 1.97−
1.92 (m, 2H), 1.43−1.32 (m, 16H), 1.25 (s, 12H), 0.97−0.88 (m, 12H).
13C NMR (100 MHz, CDCl3): δ 163.8, 163.5, 156.2, 143.3, 140.3, 135.7,
135.6, 134.7, 133.9, 133.1, 132.6, 130.6, 130.3, 130.0, 129.8, 129.3, 127.9,
122.5, 122.4, 121.7, 44.5, 38.1, 31.9, 30.8, 29.7, 28.8, 24.1, 23.1, 14.7,
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30.8, 30.7, 28.7, 24.0, 23.1, 17.4, 16.2, 14.2, 10.7. TOF MALDI-HRMS:
calcd ([C59H57N4O4F2BI2]−), m/z 1188.2530; found, m/z 1188.2512.
3.12. Synthesis of B-2. The compound 6 (38 mg, 0.03 mmol) and
NIS (558 mg, 2.48 mmol) were dissolved in dry CH2Cl2 (25 mL). The
mixture was stirred at 30 °C for 12 h. After completion of the reaction,
the mixture was washed with saturated Na2S2O3·5H2O aqueous solution
and extracted with DCM. The organic layer was dried over anhydrous
Na2SO4, and then the solvent was removed under reduced pressure. The
residue was purified with column chromatography (silica gel, CH2Cl2)
to give a red solid (34 mg, yield: 65%). Mp: >250 °C. 1H NMR (CDCl3,
500 MHz): δ 8.66 (s, 2H), 8.10 (dd, 4H, J1 = 8.0 Hz, J2 = 24 Hz), 7.82 (d,
4H, J = 8.0 Hz), 7.49 (d, 4H, J = 8.0 Hz), 4.21−4.09 (m, 4H), 2.68 (s,
12H), 1.98−1.93 (m, 2H), 1.45−1.33 (m, 16H), 1.26 (s, 12H), 0.97−
0.88 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 163.9, 163.6, 157.6,
144.0, 140.1, 135.8, 135.6, 134.7, 132.7, 131.3, 130.7, 130.6, 130.2, 129.5,
129.5, 128.2, 122.8, 122.7, 86.4, 44.7, 38.2, 31.0, 28.9, 24.3, 23.3, 16.3,
14.3, 10.9. TOF MALDI-HRMS: calcd ([C78H72B2N6O4F4I4]−), m/z
1762.1916, found, m/z 1762.1892.
3.13. Nanosecond Transient Absorption Spectra. The nano-
second transient absorption spectra were measured on an LP920 laser
flash photolysis spectrometer (Edinburgh Instruments, U.K.) and
recorded with a Tektronix TDS 3012B oscilloscope and a nanosecond
pulsed laser (OpoletteTM 355II+UV nanosecond pulsed laser, typical
pulse length 7 ns, pulse repetition 20 Hz, peak OPO energy 4 mJ,
wavelength tunable in the range of 410−2200 nm; OPOTEK, USA).
The lifetime values (by monitoring the decay trace of the transients)
were obtained with LP900 software. All samples in flash photolysis
experiments were deaerated with N2 for ca. 15 min before measurement,
and the gas flow was maintained during the measurements.
3.14. Cyclic Voltammetry. Cyclic voltammetry was performed
using a CHI610D electrochemical workstation (Shanghai, People’s
Republic of China). Cyclic voltammograms were recorded at scan rates
of 0.05 V/s. The electrolytic cell used was a three-electrode cell.
Electrochemical measurements were performed at room temperature
using 0.1 M tetrabutylammonium hexafluorophosphate (Bu4N[PF6]) as
supporting electrolyte, after purging with N2. The working electrode was
a glassy-carbon electrode, and the counter electrode was a platinum
electrode. A nonaqueous Ag/AgNO3 (0.1 M in acetonitrile) reference
electrode was contained in a separate compartment connected to the
solution via a semipermeable membrane. Dichloromethane was used as
the solvent. Ferrocene was added as the an internal reference.
and Dalian University of Technology (DUT2013TB07) for
financial support.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, and tables giving general experimental methods,
molecular structure characterization, and additional spectra. This
material is available free of charge via the Internet at http://pubs.
acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for J.Z.: zhaojzh@dlut.edu.cn.
*E-mail for M.H.: hayvali@science.ankara.edu.tr.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSFC (21073028, 21273028, 51202207,
21473020 and 21421005), the Royal Society (U.K.) and NSFC
(China-UK Cost-Share Science Networks, 21011130154),
Science Foundation Ireland (SFI ETS Walton Program 11/
W.1/E2061), Program for Changjiang Scholars and Innovative
Research Team in University [IRT_13R06], State Key
Laboratory of Fine Chemicals (KF1203), the Fundamental
Research Funds for the Central Universities (DUT14ZD226),
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