Resonance energy transfer-enhanced rhodamine-styryl Bodipy dyad triplet photosensitizers

Article abstract of DOI:10.1039/c3tc32456g

Organic triplet photosensitizers (R-1 and R-2) enhanced with the resonance energy transfer (RET) effect were prepared. Rhodamine was used as an intramolecular energy donor, and iodo-styryl-Bodipy was used as intramolecular energy acceptor/spin converter.

Full text of DOI:10.1039/c3tc32456g

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PAPER
Resonance energy transfer-enhanced rhodamine-styryl bodipy dyad
triplet photosensitizers
Jie Ma,a,b‡ Xiaolin Yuan,c‡ Betül Küçüköz,d Shengfu Li,c Caishun Zhang,a Poulomi Majumdar,a Ahmet
Karatay,d Xiaohuan Li,c H. Gul Yaglioglu,d Ayhan Elmali,d Jianzhang Zhaoa* and Mustafa Hayvali e*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Organic triplet photosensitizers (R-1 and R-2) enhanced with resonance energy transfer (RET) effect
were prepared. Rhodamine was used as intramolecular energy donor and iodo-styryl Bodipy was used as
intramolecular energy acceptor/spin converter. Both the energy donor and the energy acceptor in R-1 and
R-2 give strong absorption in visible region, but at different wavelength (e.g. for R-1, ε = 120000 M-1 cm-
1 at 557 nm for the energy donor, and ε = 73300 M-1 cm-1 at 639 nm for the energy acceptor). As a result
the photosensitizers show broadband absorption in visible spectral region. In comparison, the
conventional triplet photosensitizers contain only one visible light-harvesting chromophore, thus there is
usually only one major absorption band in visible spectral region. With steady state and time-resolved
spectroscopy, we demonstrated that photoexcitation into the energy donor was followed by intramolecular
singlet energy transfer, then via the intersystem crossing (ISC) of the energy acceptor (spin converter),
triplet excited states localized on the iodo-styryl-Bodipy were produced, confirmed by nanosecond time-
resolved transient difference absorption spectroscopy. The organic dyad triplet photosensitizers were used
for photoredox catalytic organic reactions to prepare pyrrolo[2,1-a]isoquinoline and we found that the
photocatalytic capability was improved with the RET effect. The dyads were used as fluorescent stains
for LLC cancer cells. Photodynamic effect was observed with the same cells, which were killed upon
photoirradiation with 635 nm red emitting LED after incubation with the triplet photosensitizers.
Therefore these photosensitizers can be potentially developed as dual functional theranostic reagents.
With the molecular structural protocol reported herein, organic triplet photosensitizers with strong
broadband absorption in visible spectral region and predictable ISC can be easily designed. These results
are useful for study of organic triplet photosensitizers in the area of organic photochemistry/photophysics,
photoredox catalytic organic reactions and photodynamic therapy (PDT).
properties, (1) strong absorption of visible light, or more
preferably broadband absorption in visible spectra; (2) long-lived
triplet excited states, so that the intermolecular triplet-triplet-
energy-transfer (TTET) and electron transfer can be enhanced; (3)
tunable molecular structures so that the photophysical and redox
properties can be readily optimized. Unfortunately, investigations
on triplet photosensitizers are rare and the above challenges are
left to overcome.5,24
Conventional triplet photosensitizers include the porphyrin
derivatives, aromatic ketones, iodo- or bromo-organic
chromophores, such as Bodipy, or the transition metal complexes
contain Pt(II), Ir(III) or Ru(II) atoms.7 While widely used, these
triplet photosensitizer still suffer from some drawbacks, such as
high cost, difficulties to modify the molecular structure or to
prepare and purify the compounds. Furthermore, all these
conventional triplet photosensitizers share a common molecular
structural protocol, that is, there is only one kind of visible light-
harvesting chromophore in these compounds. As a result, these
compounds show only one major absorption band in the visible
Introduction
Recently triplet photosensitizers have attracted much attention,
owing to the applications in photodynamic therapy (PDT),1–7
photosensitizing of singlet oxygen (1O2),6,8–10
photoredox
catalytic organic reactions,11–15 photocatalytic hydrogen (H2)
production,16–19 and triplet-triplet annihilation upconversions.20–23
Triplet photosensitizers are the compounds with triplet excited
state being produced upon photoexcitaion.1-7 Fluorescent dyes
alone are not applicable to the aforementioned applications,
because the production of the triplet excited state yield by these
compounds upon photoexcitation is inefficient.1,5,7,24 Thus, design
of triplet photosensitizers with high triplet excited state yield, or
efficient intersystem crossing (ISC), is crucial.7
On the other hand, different from the fluorescent dyes, for
which the main application is luminescence, triplet
photosensitizers are usually involved in triplet energy transfer or
electron transfer.7,25,26 Accordingly, an ideal triplet
photosensitizer should be with the following photophysical
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Journal of Materials Chemistry C
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spectral region.1–7 This is a clear disadvantage if a panchromatic
acceptor show similar absorption wavelength, thus the absorption
light source was used for photoexcitation, such as solar light.
Moreover, it is difficult to design new triplet photosensitizers
especially those containing no heavy atoms.7 Subtle alternation of
the molecular structures of an organic chromophore may
eliminate the ISC property completely. In order to prepare triplet
photosensitizers that show predictable ISC efficacy, one strategy
is to use an intramolecular spin converter.7 But this strategy was
rarely used for construction of organic triplet photosensitizers.
Previously C60 was used as such spin converter in dyad triplet
photosensitizers.7,25,27,28 C60 shows efficient ISC but very weak
absorption in visible spectral region (ε < 2000 M-1 cm-1 in the
region > 400 nm), hence a dyad of C60 with visible light-
harvesting chromophore cannot be used for construction of triplet
photosensitizers that show broadband absorption. Recently we
reported Bodipy-iodo-Bodipy dyad triplet photosensitizers,
however, the intramolecular energy donor and the energy
band is still ‘narrow’.29a Recently we prepared styryl Bodipy-iodo
aza-Bodipy triad triplet photosensitizers which showVbierwoAardtibclaenOdnline
DOI: 10.1039/C3TC32456G
absorption of visible light.29b But much room is left for exploring
other chromophores for construct broadband absorbing triplet
photosensitizers.
In order to overcome the above challenges in the designing of
triplet photosensitizers, herein we designed a resonance energy
transfer (RET)-based organic triplet photosensitizers, in which
Rhodamine chromophore is used as visible light-harvesting
energy donor, the iodo-styryl-Bodipy as energy acceptor, and at
the same time, the intramolecular spin converter.7,29 Both
Rhodamine and the Bodipy units show strong absorption of
visible light, but at different wavelength. As a result, the
Rhodamine/iodo styryl Bodipy dyad triplet photosensitizers show
broadband absorption in the visible spectral region, and
predictable ISC.
N
O
N
O
O
O
Br
b
c
Br
a
OH
O
O
Br
N3
O
N3
1
2
N
R-0
O
N
O
h
R-0
6
O
O
N
O
O
N
O
N
d
f
g
e
O
Br
O
3
I
I
N
F
N
F
I
I
OH
N
F
N
F
N
N
F
B
6
B
4
B
5
I
I
F
N
N
B
F
F
R-1
N
O
O
B
N
O
B
i
I
I
j
I
I
N
N
N
N
O
I
I
N
F
N
B
B
I
I
F
N
N
F
F
N
F
N
F
B-1
B-0
N
F
F
O
B-2
N
I
I
I
I
N
F
N
N
N
N
N
B
B
B
F
B-4
F
F
F
F
R-2
N
Scheme 1. Preparation of the RET triplet photosensitizer R-1 and R-2.a Key: (a) K2CO3 and DMF, reflux, 6 h. (b) DMF, 100 °C, 2 h. (c) 3-
(diethylamino)phenol, p-TsOH, 70 °C. (d) K2CO3 and DMF, reflux, 6 h. (e) nitrogen condition, CH2Cl2, TFA, DDQ, Et3N and BF3⋅Et2O. (f) NIS and
CH2Cl2, r.t., 5 h. (g) benzaldehyde, acetic acid and piperidine, microwave irradiation. (h) CuSO4·5H2O, Sodium ascorbate. 24 h. (i) p-N,N-dimethylamino-
benzaldehyde, acetic acid and piperidine, microwave irradiation. (j) CuSO4·5H2O, Sodium ascorbate. 24 h.
2
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With steady state and time-resolved spectroscopy, as well as
Bodipy B-1 and B-2 were selected as the energy acceptor, which
View Article Online
DFT calculations, we proved that upon photoexcitation,
intramolecular energy transfer occurred for the dyad triplet
photosensitizers and the triplet excited state localized on the iodo-
styryl-Bodipy part was produced following the ISC of the spin
converter. The dyad triplet photosensitizers were used as
photocatalysts in photoredox catalytic organic reactions and we
proved that the photocatalytic ability of the dyad triplet
photosensitizer was improved as compared with the reference
triplet photosensitizer which contains only one visible light-
harvesting chromophore. With fluorescence confocal microscopy
we found that the dyad triplet photosensitizers can enter cytosol
of LLC cancer cells. Photodynamic effect was observed with the
dyad triplet photosensitizers, that is, the LLC cells were killed
upon photoirradiation with 635 nm red-emitting LED after
incubation with the photosensitizers. Thus the photosensitizers
can be developed as theranostic reagents. These results are useful
for the designing of new triplet photosensitizers and the
application of these compounds in the area of photocatalysis,
solar cell and PDT.
gives absorption at 637 nm, so that there is substantial overlap
DOI: 10.1039/C3TC32456G
between the emission of Rhodamine and the absorption spectrum
of the iodo-styryl Bodipy (Scheme 1). Iodo-substitution was
introduced at the 2,6-position of the π-system of the energy
acceptor Bodipy to maximize the ISC effect, because the ISC of
the Bodipy chromophore without any heavy atom is non-
efficient.1,3,5,24,35–37 The energy donor and the energy acceptor
modules in R-1 can be easily changed to different structures,
which was demonstrated by the preparation of R-2 (Scheme 1).
All the preparation was based on established synthetic protocols
and the compounds were obtained in satisfactory yields (Scheme
1 and experimental section).
UV-Vis absorption spectra and fluorescence spectra
The UV-Vis absorption spectra of the compounds were studied
(Fig. 1). The energy donor R-0 shows strong absorption at 556
nm (ε = 85000 M-1 cm-1). The energy acceptor B-1 shows
absorption at longer wavelength of 637 nm (ε = 93600 M-1 cm-1).
The light-harvesting profiles of the typical mono-chromophore
compounds can be demonstrated by R-0 and B-1, i.e. they show
only one major absorption band in the visible region. On the
contrary, the dyad R-1 shows strong absorption at both 557 nm
and 639 nm. The UV-Vis absorption spectrum of R-1 is
superimposable to the sum of the absorption spectra R-0 and B-1
(Fig. 1a). Therefore the electronic interaction of the energy donor
and the energy acceptor in R-1 is weak at ground state.33,38-42
Notably R-1 shows strong broad absorption in the region of 500 –
700 nm. Similar absorption profiles were observed for R-2 (Fig.
1b). B-2 shows broad absorption at 639 nm, which is slightly
different from B-1.
Results and Discussion
Design and synthesis of the triplet photosensitizers
The principle molecular designing rationales are (1) using two
different visible light-harvesting chromophores which give
drastically different absorption wavelength to achieve broadband
absorption; (2) singlet energy transfer (RET) occur from one
chromophore to another one; (3) efficient ISC at the singlet
energy acceptor to ensure production of triplet excited state upon
photoexcitation. Previously we reported dyad triplet
photosensitizers with unsubstituted Bodipy used as energy donor
and iodo-Bodipy was used as energy acceptor.29a However, those
triplet photosensitizers show relatively narrow absorption band in
the visible region because the two chromophores show absorption
band close to each other. In order to broaden the absorption band
of the dyad triplet photosensitizer, herein Rhodamine was
1.5
1.0
0.5
0.0
1.0
0.8
0.6
0.4
0.2
0.0
B-1
R-0
R-1
B-2
R-0
R-2
a
b
selected as the one of the visible light harvesting moiety (λabs
=
556 nm) and the energy donor. Iodo-styryl-Bodipy as the energy
acceptor (λabs = 640 nm), and at the same time, the spin converter
(R-1 and R-2, Scheme 1).7,29 The two moieties were connected
together by the Cu(I)-catalyzed azide alkyne Huisgen
cycloaddition (Click reaction).
Rhodamine shows strong absorption in visible region and was
extensively used in fluorescence studies (related to singlet excited
states), such as FRET dyads,30–33 but its application in triplet
excited state manifold is very rare.34 To the best of our
knowledge, Rhodamine has not been used for construction of
RET triplet photosensitizers.29 Normally rhodamine is in the non-
fluorescent tautomer of lactam in aprotic solvents, which also
show very weak absorption in visible region. In order to avoid
this complexity, we prepared a rhodamine compounds which is
devoid of the −COOH group (R-0, Scheme 1). The simple
synthetic procedure may be useful for preparation of new
rhodamine derivatives.
300 400 500 600 700 800
Wavelength/nm
400
600
800
Wavelength/nm
Fig. 1 UV-vis absorption of (a) B-1, R-1 and R-0 (b) B-2, R-2 and R-0.
c = 1.0×10-5 M in CH2Cl2, 20 °C.
1000
800
R-0
600
400
200
R-1
R-2
0
550 600 650 700 750
Wavelength / nm
Rhodamine shows absorption at 556 nm and emission at 575
nm. In order to construct efficient FRET molecular array, styryl-
Fig. 2 Emission spectra of R-0, R-1 and R-2 (λex= 520 nm, at which the
three samples show the same absorbance A = 0.18. The concentration of
the solutions is slightly different). In CH2Cl2. 20 °C.
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Journal of Materials Chemistry C
Page 4 of 13
In order to evaluate the intramolecular energy transfer in R-1
intramolecular energy transfer from the rhodamine moiety to the
and R-2, the fluorescence spectra of the compounds were studied
(Fig. 2. The solutions are with same optical density at the
excitation wavelength). R-0 gives strong fluorescence emission at
575 nm (ΦF = 43 %). However, this intense emission was
completely quenched in R-1 and R-2 (Fig. 2). This significant
quenching effect demonstrated the intramolecular energy transfer
from the singlet excited state of rhodamine to the styryl Bodipy
unit.33,38-41,43,44 We studied the fluorescence emission of the
mixture of R-0 and B-1, no quenching effect on the emission of
R-0 was found (Figure S36), thus the RET effect in R-1 can be
confirmed.
iodo-styryl-Bodipy unit in B-1. Similar results were observed for
R-2, but the enhancement of the fluoresceDncOeI: 1e0m.1i0s3sVi9oie/nCw3AiTrstCict3leo2O4a5nl6inGe
much less extent (Fig. 4b).
The photophysical properties of the compounds were
summarized in Table 1. The photophysical properties of the
compounds in different solvents were studied. Normally the
compounds give hypochromatic shift in polar solvent as
compared with that in apolar solvent. For the fluorescence
emission, B-2 and R-2 show red-shifted emission band in polar
solvent, such as CH3CN. These results indicated more significant
intramolecular charge transfer (ICT) in B-2.
Quenching of the fluorescence of energy donor does not
gurantee intramolecular energy transfer because electron transfer
can also induce quenching.39,40,45,46 Thus, the fluorescence of B-1
was compared with that of R-1, upon selective excitation into the
energy accetpor. Similar emission intensity was observed (Fig.
3a). For R-2, however, the emission intensity is smaller as
compared with that of B-2 (Fig. 3b). These results indicated that
the electron transfer in the dyads are not significant.
Table 1. Photophysical Properties of R-1, R-2, R-0, B-1 and B-2 a
h
λabsb /nm ε c
λemd/nm
Φi %
τT f (μs)
air Ar
Solvents
τ e/ ns ΦΔ
j
PhCH3
DCM
551
641
52700
67400
659
1.66
8.6 0.15 1.64
R-1
−
557 120000 660
639 73300
554 106600 653
1.89 0.738 k 9.0 0.29 1.53
200
100
0
0.524 m
600
400
200
0
B-2
R-2
a
b
B-1
R-1
j
CH3CN
PhCH3
DCM
1.41
1.93
4.6 0.14 1.46
5.8 0.18 3.29
−
631
552
643
557
641
555
633
564
556
554
60800
29400
39100
89400
40600
78000
35200
14700
85000
81800
j
698
739
577
R-2
−
1.37 0.396 k 4.2 0.36 4.09
0.087 m
600
700
Wavelength / nm
800
600
700
800
g
j
g
Wavelength / nm
CH3CN
0.14 4.19
−
−
−
Fig. 3 Emission spectra of (a) B-1 and R-1 (λex=585 nm, at which ε B-1 =ε
=30000 M-1 cm-1); (b) B-2 and R-2 (λex=581 nm, ε B-2 = ε R-2 = 24000
j
j
j
j
g
g
g
g
g
g
R-1
PhCH3
DCM
584
575
579
1.49
4.19
1.60
1.93
22.8
43.0
34.0
R-0
B-1
B-2
−
−
−
−
−
−
−
−
−
−
M-1 cm-1). c = 1.0 × 10-5 M in CH2Cl2. 20 °C.
CH3CN
PhCH3
DCM
300
30
B-2
R-2
b
643 103400 663
11.1 0.15 1.73
a
B-1
R-1
637
629
647
639
630
93600
90400
77900
60600
59900
660
655
706
738
1.84 0.624 k 10.1 0.28 1.80
200
100
0
20
10
0
j
CH3CN
PhCH3
DCM
1.73
9.9 0.12 1.26
−
−
j
1.88
0.8 0.17 3.48
1.57 0.585 k 3.4 0.31 3.95
g
g
j
g
g
CH3CN
4.47
−
−
−
−
−
650 700 750 800
Wavelength / nm
650 700 750 800 850
Wavelength / nm
a
The excited wavelength for R-1, R-2, R-0, B-1 and B-2 were 520 nm,
520 nm, 520 nm, 600 nm and 600 nm respectively (1.0 × 10-5 M, 20 °C). b
Fig. 4 Emission of the compounds upon excitation at the energy donor. (a)
B-1 and R-1 (λex= 557 nm; at which ε
= 10200 M-1 cm-1, ε
Absorption wavelength. Molar extinction coefficient. In M-1cm-1.
c
d
=
R-1
= 11400
B-1
121000 M-1 cm-1); (b) B-2 and R-2 (λex= 557 nm; at which ε
Fluorescence emission wavelength. e Fluorescence lifetimes. f Triplet state
B-2
M-1 cm-1, ε R-2 = 89400 M-1 cm-1). c = 1.0 × 10-5 M in CH2Cl2, 20 °C.
g
h
i
lifetimes. No signal.
Quantum yield of singlet oxygen (1O2).
Fluorescence quantum yields with B-4 as the standard. j Not determined. k
With methylene blue (MB) as standard (ΦΔ= 0.57 in CH2Cl2, λex=580 nm).
m With B-0 as standard (ΦΔ= 0.83), λex=543 nm for R-1 and λex = 545 nm
for R-2.
The fluorescence emission of the dyads was also recorded by
selectively excitation at the absorption of the energy donor (Fig.
4). The dyad gives much stronger absorption at the energy donor
absorption band, than the energy acceptor alone (Fig. 1). On the
other hand, the emission of the energy acceptor in the dyads will
not be enhanced without the intramolecular energy transfer, even
the dyads give stronger absorption at the energy donor band
position. For R-1, the emission of the energy acceptor is much
stronger than that of the reference compound B-1 when excited at
the same wavelength (Fig. 4a). Thus, we propose there is
Recently, fluorescence excitation spectra were used for
evaluation of the intramolecular energy transfer in FRET
molecular arrays.40 The fluorescence excitation spectra of R-1
and R-2 were compared to the UV-Vis absorption spectra (Fig. 5).
The excitation at the energy donor is less efficient to produce the
fluorescence of energy acceptor. The values of the intramolecular
energy transfer efficiency was calculated as 56.3% and 53.2 %,
4
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Page 5 of 13
Journal of Materials Chemistry C
respectively (calculated by the intensity ratio of the excitation and
localized on the iodo-styryl-Bodipy unit, not the rhodamine
the absorption spectra, at the donor absorption peak
wavelength).40 These values are comparable to the intramolecular
unit.25,27 The selective localization of the triplet excited state (T1
View Article Online
state) of R-1 is due to the lower energy level of the T1 state of the
DOI: 10.1039/C3TC32456G
energy transfer efficiencies in FRET molecular arrays.40
iodo-styryl-Bodipy unit (1.13 eV) as compared with the T1 state
3.0
of the rhodamine unit (1.72 eV).
Absorption
Excitation
Absorption
Excitation
Similar results were observed for R-2 (See ESI † Fig. S41).
The triplet excited state of R-2 shows a lifetime of 4.09 μs, which
is close to that of the reference compound B-2 (3.95 μs).
Interestingly, these triplet excited state lifetimes of styryl Bodipy
are much shorter than that observed in styryl Bodipy-C60 dyads
(up to 123.2 μs).27 We propose the different triplet excited state
lifetime is due to the presence of iodo-substituent on the styryl-
Bodipy unit in R-1 and R-2. Direct attachment of the iodo atoms
to the π-system is essential for efficient ISC. However, this
efficient ISC may reduce the lifetime of the triplet excited state.47
In order to study intramolecular electron transfer, the redox
potentials of the triplet photosensitizers were studied (see ESI †
Fig. S35).42,45,48–50 Calculation of the free energy changes of the
electron transfer show that electron transfer is possible at the
singlet excited state of the energy donor, which is similar to the
results of Bodipy-C60 dyad (see ESI †for detail calculations).45
b
a
2.0
1.0
0.0
2.0
1.0
0.0
400
500
600
700
400 500 600 700
Wavelength / nm
Wavelength / nm
Fig. 5 Normalized UV-Vis absorption and excitation spectra: (a) R-1, λex
= 700 nm; (b) R-2, λex = 750 nm. c = 1.0 × 10-5 M in CH2Cl2, 20 °C.
Nanosecond time-resolved transient different absorption
spectra: population of the triplet excited states
In order to prove the generation of the triplet excited states of the
triplet photosensitizers upon photoexcitation, the nanosecond
time-resolved transient difference absorption spectroscopy were
studied (Fig. 6).7,25,27,28,36 Upon pulsed laser excitation at 532 nm,
bleaching band at 630 nm was observed for R-1 (Fig. 6a), where
the iodo-styryl-Bodipy gives the steady state absorption.
Furthermore, positive absorption at 390 nm, 514 nm and in the
region of 650 − 800 nm were observed. These features are
attributed to the triplet excited state absorption of styryl-
Bodipy.27,42 The lifetime of the transient was determined as 1.53
μs in deaerated solution. In aerated solution, however, the
lifetime was substantially reduced to 0.29 μs. The lifetime of R-1
is close to that of B-1 (1.80 μs), and the transient feature of R-1 is
similar to B-1 (Fig. 6c). These results indicated that the transients
observed for R-1 is due to the triplet excited state, which is
DFT Calculations: intramolecular energy transfer and
localization of the T1 states.
In order to study the localization of the triplet excited states of the
dyads from a theoretical perspective, the spin density surfaces of
the dyads triplet photosensitizers and the reference compounds
were calculated with the density functional theory (DFT) (Fig.
7).7,51-56 For the reference compounds B-1 and B-2, the spin
density surface was fully spread on the π-system of the
chromophore. The meso phenyl rings of B-1 and B-2 were not
involved in the spin density surface. For rhodamine R-0, similar
results were observed. However, the meso phenyl ring is more
involved in the spin density surface (see ESI† Fig. S53).
0.2
a
b
0.0
-0.1
-0.2
0.1
0.0
R-1
R-2
7.4μs
...
0 μs
-0.1
τT = 1.53 μs
400
600
800
5
10
Time / μs
15
Wavelength / nm
0.3
0.2
0.1
0.0
-0.1
0.05
c
d
0.00
-0.05
-0.10
-0.15
Fig. 7 Isosurfaces of spin density of compounds B-1, B-2, R-0, R-1 and
R-2. At the optimized triplet state geometries. DCM was used as solvents
in the calculations. Calculation was performed at B3LYP/6-
31G(d)/genecp level with Gaussian 09W. .
10.6μs
...
0 μs
τT = 1.80 μs
The spin density surfaces of the dyad triplet photosensitizers
R-1 and R-2 are localized on the styryl Bodipy units (Fig. 7). The
rhodamine unit does not contribute to the spin density surfaces.
Therefore, we propose that the triplet state of R-1 and R-2 are
highly localized on the strytyl Bodipy moiety.28,29,57-59 This
conclusion is in full agreement with the nanosecond time-
0
5
10
15
20
400
600
800
Time / μs
Wavelength / nm
Fig. 6 Nanosecond time-resolved transient difference absorption of (a) R-
1 and (c) B-1 after pulsed laser excitation (λex = 532 nm). Decay trace of
(b) R-1 and (d) B-1 at 630 nm. c = 1.0×10-5 M in deaerated CH2Cl2. 20
°C.
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Journal of Materials Chemistry C
Page 6 of 13
View Article Online
DOI: 10.1039/C3TC32456G
Fig. 8 Selected frontier molecular orbitals involved in the excitation, emission and triplet excited states of R-1. CT stands for conformation transformation.
The calculations are at the TDDFT/B3LYP/6-31G(d)/Genecp level using Gaussian 09W. DCM was employed as solvent in the calculation.
resolved transient absorption spectroscopy of the dyad triplet
photosensitizers (Fig. 6 and ESI †, Fig. S37 and S38).
energy from the S3 state to S1 state. This senario is in full
agreement with the conventional FRET concept.44
The ground state geometries of the compounds were studied.
The result shows that the Rhodamine and the iodo-styryl Bodipy
moieties keep far away from each other, for which the steric
hindrance exerted by the Rhodamine and the iodo-styryl Bodipy
parts is minimal. This result is in agreement with the steady state
and the time-resolved spectroscopic studies, that is, there is no
significant interaction for the two chromophores at ground state
and the excited states.
The triplet states of R-1 were also studied with TDDFT
calculations, based on the optimized ground state geometry.65 T1
state is identified as localized on the iodo-styryl-Bodipy part, T2
state was localized on the Rhodamine part. The energy difference
of the T1 and T2 state is 0.61 eV (Fig. 8 and Table S4). Therefore,
triplet excited state equilibirum was not observed.29a
Femto-second transient absorption spectroscopy
Ultrafast pump probe experiments were performed for both R-
1 and R-2 at 555 nm and 640 nm pump wavelengths to excite the
Rhodamine and iodo-styryl-Bodipy units, respectively.39 Pump
pulse at 555 nm excites the Rhodamine (S0→S1). It is expected
ultrafast spectroscopy results give the evidence of singlet state
intramolecular energy transfer and formation of triplet state of
iodo-styryl-Bodipy units in R-1 (Fig. 9). The bleaching signals
around 560 nm and 640 nm were observed upon femtosecond
pulsed laser excitation at 555 nm, where the Rhodamine give the
steady state absorption, respectively. Energy transfer may be
responsible for the decay of the bleaching band of R-1.39 On the
other hand, while the bleaching signal around 560 nm decreases,
the second bleaching signal around 640 nm slightly increases for
R-1. Based on the steady state UV-Vis absorption spectra, the
Bodipy part will also be excited with the 555 nm pump, therefore
the singlet state of the Bodipy part can be generated by the direct
photoexcitation. Energy transfer from the Rhodamine part
contributes partially to the production of the singlet excited state
of the Bodipy part.39 Similar results were observed for R-2 (see
ESI † Fig. S45).
Time-dependent DFT was used for study of the excited states
of the dyad triplet photosensitizers.44,53,60-64 For R-1, two low-
lying singlet excited states were indentified for R-1, i.e. S1 and S3
state, with absorption maximum at 604 nm and 475 nm,
respectively (see ESI †, Table S4). Inspection of the molecular
orbitals indicated that transitions are localized on the iodo-styryl
Bodipy part and the Rhodamine part, respectively. This result is
in agreement with the absorption bands of R-1 (Fig. 1).44
A charge transfer state is also indentified (S2 state), which is at
2.09 eV (594 nm). S2 state is a dark state (f = 0). Since this charge
transfer state is not the lowest-lying state, we propose the
photophysical properties of R-1 will be hardly affected by this
state. This postulation is in agreement with the fluorescence
lifetime of R-1.
The geometries of the singlet excited states were also
optimized with the TDDFT method, and the fluorescence of R-1
was calculated. The fluorescence emission of R-1 is predicted as
710 nm, which is close to the experimental result (660 nm). The
orbitals involved in S1 state is localized on the iodo-styryl Bodipy
part (Fig. 8). S3 state is identified as an excited state localized on
Rhodamine. According to Kasha's rule, however, this state will
not fluoresce. Therefore, the internal conversion will funnel the
The kinetics of the singlet energy transfer in R-1 upon
photoexcitation were followed by monitoring the transient
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Journal of Materials Chemistry C
View Article Online
iodo styryl Bodipy was produced due toDthOeI: 1h0e.a10v3y9/aCto3mTC3e2ff4e5c6tG.
This state is with lower energy level than the T1 state of
Rhodamine, thus the T1 state of the dyad is localized on the iodo-
styryl Bodipy part, which is confirmed by the nanosecond
transient absorption spectra and the DFT calculations on the spin
density surfaces (Fig. 7 and Fig. 8).
0.10
0.05
0.00
-0.05
-0.10
-0.15
-0.20
-0.25
616 ps
52.0
...
1.56
1.03 ps
Singlet oxygen (1O2) photosensitizing ability
To verify the efficiency of the triplet state yields of the dyad
triplet photosensitizers R-1 and R-2, the singlet oxygen (1O2)
quantum yields (ΦΔ) of the dyad triplet photosensitizers were
studied.8,25,35,66 With selective photoexcitation at the energy donor
absorbance (557 nm), ΦΔ values of 73.8% and 39.6 % were
observed for R-1 and R-2, respectively. These values are
400
500
600
700
800
Wavelength / nm
Fig. 9 Femtosecond time-resolved transient absorption spectra of R-1
upon excitation 555 nm wavelength in DCM.
comparable to the reference compounds B-1 and B-2 (ΦΔ
=
absorbance at 450 nm, 560 nm and 640 nm (Fig. 9 and Fig. S44).
The decrease of the transient absorption at 450 nm and the
bleaching band at 560 nm were companied by the development of
the transient at 640 nm. Rate constants of 6.70×1011 s-1 was
obtained for the decay of the bleaching band at 560 nm.
62.4% and 58.5 %, respectively). The lower ΦΔ value of R-2 (or
B-2) than R-1 (or B-1) may be due to the more significant
intramolecular charge transfer feature of the dimethylaminostyryl
moiety in R-2 (or B-2), which may quench the triplet excited
state.
In order to observe triplet formation of iodo-styryl-Bodipy
units, 640 nm pump pulses were used. Triplet state formation of
iodo-styryl-Bodipy units in R-1 and R-2 were observed around
1000 ps and 300 ps time delays, respectively (see ESI† Fig S48
S49). Observed triplet state formation times are another
indications of ISC rate differences of both compounds. In order to
obtain deep insight about ISC rates, decay components of bleach
signals around 640 nm for both compounds were compared upon
photoexcitation at 640 nm. R-2 has faster decay component when
compared to R-1 (ESI, Fig S46) indicating different ISC rates.
1.25
R-1
B-1
R-2
B-2
a
b
1.1
1.0
0.9
1.00
0.75
0
10 20 30 40 50 60
0
10 20 30 40 50 60
Time / s
Time / s
Fig. 10 Singlet oxygen (1O2) generation with photoexcitation at the
absorption of the energy donor of the dyad triplet photosensitizers.
Absorbance decrease of DPBF with increasing photoirradiation time in
the presence of photosensitizers. (a) R-1 and B-1. (b) R-2 and B-2. λex
=
557 nm, c = 1.0 × 10-5 M in CH2Cl2, 20 °C. Deeper slop indicates more
efficient 1O2 photosensitizing ability.
The effect of intramolecular energy transfer on the kinetics of
1O2 production with different triplet photosensitizers were
presented in Fig. 10 (the energy donor was selectively
1
photoexcited). R-1 is more efficient to produce O2 as compared
with that of B-1, with excitation at 557 nm, where the energy
donor rhodamine absorbs. However, the 1O2 production
efficiency of R-2 is lower than that of B-2, indicating that the
energy transfer in R-2 is less efficient.
Scheme 2. Qualitative Jablonski energy diagram of the photophysical
processes involved in dyad triplet photosensitizer R-1. RET stands for
resonance energy transfer and ISC stands for intersystem crossing, Rho
stands for Rhodamine moiety and BOD stands for iodo Styryl Bodipy part.
Photoredox catalytic organic reaction
The photophysical processes involved in R-1 and R-2 with
photoexcitation can be summarized in Scheme 2. The two
chromophores in R-1 give absorption at different wavelength,
thus broadband absorption was observed. Moreover, the energy
level of the S1 state of Rhodamine part is higher than the styryl
Bodipy part (by the fluorescence emission wavelength and the
absorption wavelength), thus RET from the Rhodamine part to
the iodo-styryl Bodipy is ensured. Due to the lack of heavy atoms
on Rhodamine part and the fast efficient RET, population of the
T1 state of the Rhodamine part is neglectable. The T1 state of the
The triplet photosensitizers were also used for photoredox
catalytic organic reactions.12–14,67,68 One of the major challenge
in the area of photoredox catalytic organic reactions is the limited
availability of the triplet photosensitizers, i.e. the photocatalysts.
The most popular photocatalyst are the Ru(II) complexes, such as
Ru(bpy)3Cl2, Ir(ppy)3, Eosin Y and Rose Bengal. Tailor designed
triplet photosensitizer with tunable photophysical properties are
highly desired, but were rarely reported. Recently we used iodo-
Bodipy,26 or C60-Bodipy derivatives for photoredox catalytic
organic reactions.25,27 However, these photocatalysts are still
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Journal of Materials Chemistry C
Page 8 of 13
suffered from the limitation of mono visible light-harvesting
indicated that the DCM is the optimal solvent, and the reaction
chromophore profile. Since R-1 and R-2 show broadband
absorption, these compounds are used as photocatalysts for
photoredox catalytic organic reaction. Herein we selected the
oxidation/[3+2] cycloaddition–oxidative aromatization sequence
to construct pyrrolo[2,1-a]isoquinolines (via electron transfer
mechanism), which are the important skeleton of bioactive
natural products.25,69
time is 90 min which is much shorter than the previously reported
View Article Online
value of 9 h with the conventional Ru(bpy)D3COlI2: 1p0h.1o0t3o9c/aCta3lTyCs3ts2.46956G
The enhancement may be due to the strong broadband absorption
of the dyad triplet photosensitizers in visible light region.
Satisfactory yields were observed for substrates with different
electron pushing or electron withdrawing substituents (Table 3).
The mechanism of the photoredox catalytic reactions was
studied with ESR (electron spin resonance spectroscopy), with
Table 3. Oxidation/[3+2] cycloaddition/aromatization tandem reaction
with tetrahydroisoquinolinederivatives catalyzed by organic triplet
photosensitizers R-1, B-1
5,5-dimethyl-1-pyrroline-N-oxide
(DMPO)
and
2,2,6,6-
tetramethylpiperidine (TEMP) as the scavengers for superoxide
anion radical (O2•−) and singlet oxygen (1O2), respectively (see
ESI , Fig. S43 and Scheme S1). The results show that the
reactions proceed via O2•−. The production of O2•− is significant
in the presence of tetrahydroisoquinoline, whereas the production
of 1O2 is completely inhibited in the presence of the
tetrahydroisoquinoline substrate.
COOEt
R-1 and R-2
Visible Light
O
O
N
N
O
O
N
+
N
NBS,Air
O
O
R
R
Intracellular photodynamic studies
Entry catalyst
Substrate
Product
t[h] [c]Yield[d]
1.5 83%
[b]
It is known that the normal Bodipy and Rhodamine show very
weak intersystem crossing (ISC), as a result, these compounds
cannot be used as triplet photosensitizers in photocatalysis or the
photodynamic therapy (PDT).1-7,21,24 Instead, the dyad triplet
photosensitizers R-1 and R-2 show red fluorescence and high
singlet oxygen quantum yield (ΦΔ), thus these compounds can be
used as dual-functional materials for luminescence bioimaging as
well as PDT studies.
As a preliminary study, the near IR luminescence bioimaging
and PDT effect of the dyad photosensitizers R-1 and R-2 on a
lung cancer cell line of LLC cells were studied (Fig. 11 and Fig.
12). With incubation, the fluorescence microscopy show that the
dyad triplet photosensitizers can enter the cytosol. Red
fluorescence was observed, which is beneficial for luminescence
bioimaging because the long excitation and emissive wavelength
can ensure deep penetration of the tissue (Fig. 11).
COOEt
O
N
1
R-1
N
O
N
2a
O
O
3a
O
COOEt
O
2
R-1
1.5 80%
N
N
Br
N
O
2b
O
3b
Br
COOEt
O
3
4
O
R-1
R-1
N
3c
N
1.5 72%
1.5 53%
N
OCH3
NO2
N
O
2c
O
OCH3
NO2
Br
COOEt
O
O
N
N
O
2d
O
3d
N
Fluorescence
Brightfield
Overlay
COOEt
O
5
O
R-2
R-1
a
1.5 trace
1.5 32%
N
N
O
3b
2a
O
6e
O
COOEt
O
N
N
N
O
2a
2a
3a
N
O
O
7 e
COOEt
O
B-1
1.5 36%
N
N
O
O
3a
b
a Reaction conditions: 1 (0.15 mmol), 2 (0.10 mmol), R-1 (2% mol) and
NBS (1.2 equiv) were mixed in CH2Cl2 (3.0 mL), the mixture was
irradiated with 35 W Xe lamp (λ > 385 nm, 300 W/m2), 20 °C. b Catalyst
in the reaction. c Reaction time with photosensitizers. d Yield of isolated
products catalyzed with different catalyst. e The mixture was irradiated
with 35 W Xe lamp (470 > λ > 570 nm,100 mW/cm2).
Fig. 11 Confocal fluorescence images in LLC cells (λex= 543 nm). The
cells were incubated with the photosensitizers (a) R-1 (b) R-2 (1.0 × 10-6
M) at 37 °C in the dark for 24 h..
The Rhodamine-Bodipy dyad triplet photosensitizers show
high ΦΔ values (Table 1), strong absorption broadband visible
Firstly, the reaction conditions were optimized. The results
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Journal of Materials Chemistry C
light and long-lived triplet excited state. Therefore, these
photocatalytic organic reactions, photodynamic therapy and for
complexes are used for PDT study (Fig. 12). The LLC cells were
incubated with the dyad photosensitizers. Then the cells were
irradiated with 635 nm red emitting LED for 4 h. Trypan blue
staining of the cells indicated that most of the cells were killed. In
a control experiment, however, the cells were not killed without
the photosensitizers. These results show that the triplet
photosensitizers can be developed as theranostic agents.
study of the organic photochemistry.
View Article Online
DOI: 10.1039/C3TC32456G
Experimental Section
General Methods
Fluorescence lifetimes were measured with time correlated single
photon counting technique (TCSPC) on OB920 spectrometer
(Edinburgh instruments, UK). Nanosecond time-resolved
transient absorption spectra were measured on LP920 laser flash
photolysis spectrometer (Edinburgh Instruments, UK). All the
samples in flash photolysis experiments were deaerated with N2
for ca. 15 min before measurement. The compounds R-1 and R-2
were prepared following the reported methods. For preparation of
compounds 1−3, see ESI † materials.
a
b
c
Compound 4
Fig. 12 Photocytotoxic activity of the sensitizers with Trypan blue
staining images of LLC cells. (a) cells were incubated with R-1 and were
kept in the dark for 24 h in the same incubator; (b) cells were illuminated
with 635 nm LED for 4 h after addition of R-1 and were incubated for a
further 24 h in the dark in the same incubator; (c) cells were illuminated
for 4 h without the sensitizers and incubated for another 24 h in the dark
in the incubator. The dead cells are preferentially stained with Trypan
blue because of increased cellular permeability. The concentration of R-1
was at 1.0 × 10-6 M. 37 °C.
Under N2 atmosphere, a mixture of 3 (1.60 g, 10 mmol ) and 2,4-
dimethylpyrrole (1.88 g, 20 mmol) in dry CH2Cl2 (250 mL) was
stirred at rt. On cooling with ice bath, TFA (0.1 mL) was added
via syringe. The mixture was stirred overnight at rt. A solution of
DDQ (1.13 g, 5 mmol) in THF (30 mL) was added via addition
funnel), then the mixture was stirred at rt for 7 h. Triethylamine
(15 mL) was added dropwise with cooling on ice bath. The
mixture was stirred for another 0.5 h. Then BF3⋅Et2O (15 mL)
was added dropwise with syringe. The reaction mixture was
stirred overnight. The solution was concentrated under reduced
pressure. Then water (200 mL) was added, and the mixture was
stirred for 24 h. The solution was extracted with CH2Cl2 and the
organic layer was dried over anhydrous Na2SO4. The solvent was
evaporated under reduced pressure. The crude product was
purified by column chromatography (silica gel, CH2Cl2 : hexane
= 1:1, v/v) to give 4 as red solid. 600 mg, 16.0 %. 1H NMR (400
MHz, CDCl3) δ 7.21 (d, 2 H, J = 8.0 Hz), 7.10 (d, 2H, J = 8.0 Hz),
5.98 (s, 2H), 4.76 (d, 2H), 2.55 (s, 7H), 1.42 (s, 6H).
Conclusions
In summary, we prepared resonance energy transfer (RET)-
enhanced organic triplet photosensitizers, which show broadband
absorption in visible spectral region. Rhodamine was used as
intramolecular energy donor and iodo-styryl-Bodipy was used as
energy acceptor and at the same time, the intramolecular spin
converter. The two visible light-harvesting chromophores of the
dyad gave different absorption wavelength, as a result, the dyad
triplet photosensitizers show strong broadband absorption in the
visible light region of 450 – 700 nm. The dyad molecular
structure protocol of the new triplet photosensitizers is in stark
contrast from the conventional triplet photosensitizers.
Traditionally the triplet photosensitizers are all based on mono-
chormophore profile, resulting in only one major absorption band
in the visible spectral region. The photophysical properties of the
Rhodamine/iodo-styryl-Bodipy dyad triplet photosensitizers were
studied with steady state and nanosecond time-resolved
spectroscopy, as well as DFT calculations. Upon excitation at the
absorption band of the energy donor (Rhodmaine part), we found
that the triplet excited state localized on the iodo-styryl-Bodipy
moiety was produced upon visible photoexcitation. The organic
triplet photosensitizers were used as photocatalysts and we
proved that the dyads triplet photosensitizers are more efficient
than the reference triplet photosensitizers that are based on the
conventional mono-chromophore profile. The photosensitizers
were used for fluorescent staining of cancer cells. Photodynamic
therapeutic (PDT) effect was observed. These results show that
the triplet photosensitizers can be developed as theranostic agents.
Moreover, based on our molecular design methods, it becomes
feasible to prepare new organic triplet photosensitizers that show
broadband absorption in visible spectral region and predictable
ISC. Our results are helpful for designing new organic triplet
photosensitizers and for the application of these compounds in
Compound 5
4 (250 mg, 0.66 mmol ) and N-iodosuccinimide (NIS. 468 mg,
2.64 mmol) was dissolved in dry CH2Cl2 (50 mL). The mixture
was stirred at room temperature. The proceeding of the reaction
was monitored by TLC. Then the mixture was concentrated under
reduced pressure and the crude product was purified by column
chromatography (silica gel. CH2Cl2 : hexane = 1:2, v/v). The
second band was collected to give the product as a red solid.
Yield: 300 mg, 62.0 %. Mp > 250 °C. 1H NMR (400 MHz,
CDCl3): δ 7.18−7.11 (m, 4 H), 4.78 (s, 2H), 2.46 (s, 6H), 2.57 (s,
1H), 1.44 (s, 6H). MALDI-HRMS: calcd ([C22H19BF2I2N2O]+)
m/z = 629.9648, found m/z = 629.9628.
Compound 6
Under Ar atmosphere, 5 (90.0 mg, 0.14 mmol), benzaldehyde
(60.0 mg, 0.56 mmol), acetic acid (3 drops) and piperidine (3
drops) were dissolved in dry DMF (5 mL). Then the resulting
solution was subjected to microwave irradiation (5 min, 150 °C, 1
min prestirring). After washing with water, the mixture was
purified by column chromatography (silica gel, CH2Cl2: PE = 1: 4,
v/v) to give a dark solid. Yield: 45.0 mg (40.0%). Mp > 250 °C.
1H NMR (400 MHz, CDCl3): δ 8.18( d, 2H, J = 20.0 Hz), 7.73 ( s,
1H), 7.68 ( d, 4H, J = 8.0 Hz), 7.43 ( t, 4H, J = 8.0 Hz),
This journal is © The Royal Society of Chemistry [year]
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Journal of Materials Chemistry C
Page 10 of 13
7.38−7.34 ( m, 2H),7.22 (d, 3H, J = 8.0 Hz), 7.15 (d, 2H, J = 8.0
[C65H61N7O3BF2I2]+ 1290.2957; found: 1290.2987.
Hz), 4.80 (s, 2H), 2.59 (s, 1H), 1.51 (s, 6H). MALDI-HRMS: m/z
Compound R-2
View Article Online
DOI: 10.1039/C3TC32456G
calcd for [C36H27N2OBF2I2]+ 806.0274; found: 806.0278.
Under Ar atmosphere, R-0 (29.0 mg, 0.06 mmol ) and 7 (45 mg,
Compound B-2
0.06 mmol) were dissolved in the mixed solvent (14 mL, CHCl3 :
MeOH: H2O = 12: 1: 1, v/v) then CuSO4·5H2O (1.5 mg) and
sodium ascorbate (2.4 mg) were added. The resulting mixture
was stirred at rt for 24 h. Then the reaction mixture was washed
with brine and extracted with CH2Cl2. The organic layer was
dried over Na2SO4. The solvent was evaporated under reduced
pressure. The crude product was purified by column
chromatography to give purple solid. Yield: 40.0 mg (53.5 %)
Under Ar atmosphere, the mixture of 5 (90.0 mg, 0.14 mmol), p-
N,N-dimethylamino-benzaldehyde (83.4 mg, 0.56 mmol), acetic
acid (3 drops) and piperidine (3 drops) was dissolved in dry DMF
(5 mL). The mixture was subjected to microwave irradiation (5
min, 150 °C, 1 min stirring before reaction). After washing with
water, the mixture was purified by column chromatography
(silica gel, CH2Cl2: PE = 1: 1, v/v) to give a dark solid. Yield:
1
1
35.0 mg (32.9 %). Mp > 250 °C. H NMR (400 MHz, CDCl3): δ
(silica gel, CHCl3 : CH3OH = 8 : 1, v/v). H NMR (400 MHz,
8.19 ( s, 1H, J = 16.0 Hz), 7.60−7.51 (m, 3H), 7.26 (m, 2H), 7.20
(d, 2H, J = 12. 0 Hz), 7.13 (d, 2H, J = 8.0 Hz), 4.79 (s, 2H), 3.07
(s, 6H), 2.68 (s, 3H), 2.58 (s, 1H), 1.49 (s, 3H), 1.44 (s, 3H).
MALDI-HRMS: m/z calcd for [C31H28N3OBF2I2]+ 761.0361;
found: 761.0383.
CDCl3): δ 8.35 ( s, 1H), 8.17 (1H, d, J = 16.0 Hz), 7.56−7.46 (m,
6H), 7.34 (d, 2H, J = 8.0 Hz), 7.23 (d, 2H, J = 8.0 Hz), 7.16−7.10
(m, 4H), 6.96 (d, 2H, J = 8.0 Hz), 6.83 (s, 2H), 6.76 (s, 2H), 5.30
(s, 2H), 4.99 (s, 2H), 4.69 (s, 2H), 3.65−3.60 (m, 8H), 3.06 (s,
6H), 2.65 (s, 3H), 1.45 (s, 3H), 1.41 (s, 3H), 1.33(m, 12H). 13C
NMR (100 MHz, CDCl3): 160.05, 159.37, 158.09, 157.65, 155.45,
151.47, 151.26, 146.68, 143.36, 140.33, 139.00, 132.41, 131.60,
129.51, 128.94, 127.55, 125.08, 124.36, 115.95, 115.49, 114.18,
113.39, 112.38, 96.43, 67.02, 62.07, 49.95, 46.21, 40.34, 29.81,
17.87, 17.10, 12.80. HRMS (ESI): m/z calcd for
[C60H62N8O3BF2I2]+ 1245.3062; found: 1245.3096.
Compound R-0
A mixture of 2 (570 mg, 3 mmol), 3-(diethylamino)phenol (990
mg, 6 mmol), p-TsOH (78 mg, 0.45 mmol) and acetic acid (15
mL) was heated at 70 °C and stirred for 7 h. The reaction mixture
was cooled to r.t., and the pH was adjusted to above 7.0 with 10%
NaOH solution. The precipitate was filtered and washed with
water (30 mL). The solid was dissolved in CH2Cl2 (30 mL), to
which chloranil (366 mg, 1.5 mmol) was added. The mixture was
stirred for 2 h. After removal of the solvent, the residue was
purified by column chromatography (silica gel; CH2Cl2/methanol,
DFT calucaltions
The density functional theory (DFT) calculations were used for
optimization of the ground state geometries, for both singlet
states and triplet states. The energy level of the T1 state (energy
gap between S0 state and T1 state) were calculated with the time-
dependent DFT (TDDFT), based on the ground state geometry.
These TDDFT calculations were used for the prediction of the
UV-Vis absorption of the T1 state of the organic triplet
photosensitizers, in our case it is the transient absorption of the
organic triplet photosensitizers after the laser flash. All the
calculations were performed with Gaussian 09.70
1
20:1, v/v) to give a purple solid. Yield: 500 mg (34.5 %). H
NMR (400 MHz, CDCl3): δ 7.45( d, 2H, J = 12.0 Hz), 7.36 ( d,
2H, J = 12.0 Hz), 7.21 (d, 2H, J = 12.0 Hz), 6.94−6.91( m, 2H),
6.78 (s, 2H), 4.33 (t, 2H, J = 8.0 Hz), 3.71(t, 2H, J = 8.0 Hz),
3.67−3.61 (m, 8H), 1.32 (t, 12H, J = 8.0 Hz). 13C NMR (100
MHz, CDCl3): 159.95, 157.83, 157.30, 155.23, 132.11, 131.24,
124.04, 115.08, 114.19, 113.96, 113.14, 96.24, 67.39, 50.05,
46.01, 12.55. H RMS (ESI): m/z ([C29H34N5O2]+) m/z = 484.2707,
found m/z = 484.2705.
Femtosecond transient difference absorption spectroscopy.
The ultrafast wavelength dependent pump probe spectroscopy
Compound R-1
measurements were performed using
a Ti:Sapphire laser
Under Ar atmosphere, R-0 (24.2 mg, 0.05 mmol ) and 6 (32.0 mg,
0.05 mmol) were dissolved in the mixed solvent (14 mL,
CHCl3:EtOH:H2O = 12:1:1, v/v). Then CuSO4·5H2O (1.5 mg)
and sodium ascorbate (2.4 mg) was added. The resulting mixture
was stirred at rt for 24 h. Then the reaction mixture was washed
with brine and extracted with CH2Cl2. Organic layer was dried
over anhydrous Na2SO4 and evaporated under reduced pressure.
The crude product was purified by column chromatography
(silica gel, CH2Cl2 : CH3OH = 8 : 1, v/v) to give purple solid.
Yield: 32.0 mg, 49.6%. 1H NMR (400 MHz, CDCl3): δ 8.41 (s,
1H), 8.14 (2H, d, J = 16.0 Hz), 7.70 (s, 1H), 7.66 (d, 4H, J = 8.0
Hz), 7.52−7.47 (m, 2H), 7.44−7.41 (m, 4H), 7.35 (d, 4H, J = 8.0
Hz), 7.23 (s, 3H), 7.19−7.13 (m, 4H), 6.96 (s, 2H), 6.76 (s, 2H),
5.33 (s, 2H), 5.01 (s, 2H), 4.72 (s, 2H), 3.64−3.58 (m, 8H), 1.49
(s, 6H), 1.33(t, 12H, J = 4.0 Hz). 13C NMR (100 MHz, CDCl3):
159.99, 159.56, 158.03, 157.61, 155.41, 150.34, 146.31, 139.46,
136.64, 133.42, 132.45, 131.56, 129.47, 128.89, 127.70, 124.33,
118.84, 116.09, 115.45, 114.14, 113.34, 96.39, 66.96, 62.03,
49.76, 46.14, 17.75, 12.74. H RMS (ESI): m/z calcd for
amplifier-optical parametric amplifier system (Spectra Physics,
Spitfire Pro XP, TOPAS) and commercial setup (Spectra Physics,
Helios). Pulse duration was measured as 100 fs. Wavelengths of
the pump beam were chosen according to the absorption spectra
of Rhodamine unit as 555 nm and iodo-styryl-Bodipy unit as 640
nm for R-1 and R-2 compounds. White light continuum was used
as a probe beam.
Singlet oxygen quantum yield (ΦΔ)
The 1O2 quantum yields (ΦΔ) of the photosensitizers were
measured with Methylene Blue trihydrate (MB) as standard (ΦΔ =
0.57 in DCM) and B-0 as standard (ΦΔ= 0.83).71 The absorbance
of 1,3-diphenylisobenzofuran (DPBF. 1O2 scavenger) was
adjusted around 1.0 in air saturated dichloromethane. Then, the
photosensitizer was added to cuvette and photosensitizer’s
absorbance was adjusted around 0.2−0.3. The cuvette was
irradiated with monochromatic light at the peak absorption
wavelength for 10 seconds. Absorbance was measured for several
times after each irradiation. The slope of absorbance maxima of
10
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Journal of Materials Chemistry C
DPBF at 414 nm versus time graph for each photosensitizer were
calculated. Singlet oxygen quantum yield (ΦΔ) were calculated
according to a modified equation:
Dalian 116001, P. R. China;
d
Department of Engineering Physics, Faculty of Engineering, Ankara
View Article Online
DOI: 10.1039/C3TC32456G
University, 06100 Beşevler, Ankara, Turkey;
e
Department of Chemistry, Faculty of Science, Ankara University, 06100
F
kbod
kref
ref
bod
ref
Eq. 1
Beşevler, Ankara, Turkey. E-mail: hayvali@science.ankara.edu.tr
ΦΔ = ΦΔ
×
×
F
‡ These authors contribute equally to the work.
where ‘bod’ and ‘ref’ designate the photobsoednsitizers and ‘MB’
respectively. k is the slope of difference in change in absorbance
of DPBF (414 nm) with the irradiation time, F is the absorption
correction factor, which is given by F = 1−10−O.D. (O.D. is the
optical density of the solution at the irradiation wavelength).
Contribution of the authors: J. Ma carried out the preparation of the
compounds and the spectroscopy studies; X. Yuan, S. Li and X. Li did the
studies on cellular imaging, Caishun Zhang contributes to the synthesis of
some intermediate compounds; P. Majumdar helped in the spectroscopy
and cellular studies; J. Zhao is responsible for the research, Ahmet
Karatay and Betül Küçüköz conducted pump-probe experiments together.
Ayhan Elmali, H. Gul Yaglioglu, and Mustafa Hayvali are responsible for
the femto-second spectroscopy studies.
Cell Culture
LLC cells (lung cancer cell) were grown in RPMI-1640 Medium
in an atmosphere of 5% CO2, 95% air at 37 °C supplemented
with 10% fetal bovine serum (FBS), and 1% antibiotics
(penicillin /streptomycin, 100 U/mL).
†
Electronic Supplementary Information (ESI) available:
molecular structure characterization data, UV/Vis absorption and
emission spectra, time-resolved transient absorption spectroscopy
and DFT calculations. See DOI: 10.1039/b000000x/
Laser scanning Confocal fluorescence microscope
Fluorescent images were acquired on Nikon ECLIPSE-Ti
confocal laser scanning microscopy. The excitation wavelength
was 543 nm. Cell imaging was carried out in confocal Petri
dishes.
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Acknowledgement
We thank the NSFC (20972024, 21073028 and 21273028), the
Royal Society (UK) and NSFC (China-UK Cost-Share Science
Networks, 21011130154), Science Foundation Ireland (SFI E.T.S.
Walton Program 11/W.1/E2061) and Ministry of Education
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Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C3TC32456G
Graphical contents entry:
Resonance energy transfer-enhanced rhodamine-styryl bodipy
dyad triplet photosensitizers
Jie Ma,a,b‡ Xiaolin Yuan,c‡ Betül Küçüköz,d Shengfu Li,c Caishun Zhang,a Poulomi Majumdar,a Ahmet
Karatay,d
Xiaohuan Li,c H. Gul Yaglioglu,d Ayhan Elmali,d Jianzhang Zhaoa* and Mustafa Hayvali e*
Broadband visible light-absorbing triplet photosensitizers with Rhodamine as energy donor
and styryl Bodipy as energy acceptor/spin converter were prepared.
Keyword: Bodipy, photochemistry, Rhodamine, Triplet photosensitizer, Resonance energy
transfer