Singlet oxygen generation properties of an inclusion complex of cyclic free-base porphyrin dimer and fullerene C60

Article abstract of DOI:10.1039/c7ra02699d

To gain insight into the singlet oxygen (¹O₂) generation properties of supramolecular complexes of cyclic free-base porphyrin dimer with fullerene C₆₀, we evaluated the ¹O₂ quantum yield (Φ_Δ) and rate constant (K_obs) of ¹O₂ generation for a cyclic free-base porphyrin dimer (CPD) linked by butadiyne bearing four 4-pyridyl groups and its inclusion complex (C₆₀?CPD) with C₆₀. We demonstrate that CPD and C₆₀?CPD possess the ability to generate ¹O₂ under visible light irradiation. Moreover, it was found that the Φ_Δ value of C₆₀?CPD is lower than that of CPD. Based on the kinetic and thermodynamic consideration concerning the electron transfer processes between the porphyrin dimer and C₆₀, this work revealed that the lower Φ_Δ value of the C₆₀ inclusion complex would be attributed to the formation of the charge-separated state c₆₀^·?-CPD^·+, leading to a low intersystem crossing (ISC) efficiency for the formation of the triplet excited state ³(CPD)*.

Full text of DOI:10.1039/c7ra02699d

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Singlet oxygen generation properties of an
inclusion complex of cyclic free-base porphyrin
Cite this: RSC Adv., 2017, 7, 18690
dimer and fullerene C †
6
0
a
a
a
b
Yousuke Ooyama, * Toshiaki Enoki, Joji Ohshita, Takuya Kamimura,
b
b
b
Shuwa Ozako, Taro Koide and Fumito Tani*
1
To gain insight into the singlet oxygen ( O
2
) generation properties of supramolecular complexes of cyclic
1
free-base porphyrin dimer with fullerene C60, we evaluated the O
2
quantum yield (F
D
) and rate constant
1
(K
obs) of O
2
generation for a cyclic free-base porphyrin dimer (CPD) linked by butadiyne bearing four 4-
pyridyl groups and its inclusion complex (C603CPD) with C60. We demonstrate that CPD and C603CPD
1
possess the ability to generate O
2
under visible light irradiation. Moreover, it was found that the F
D
value
of C603CPD is lower than that of CPD. Based on the kinetic and thermodynamic consideration
Received 6th March 2017
Accepted 22nd March 2017
concerning the electron transfer processes between the porphyrin dimer and C60, this work revealed
that the lower F
D
value of the C60 inclusion complex would be attributed to the formation of the
DOI: 10.1039/c7ra02699d
60$ꢀ
$+
charge-separated state
C
-CPD , leading to a low intersystem crossing (ISC) efficiency for the
3
rsc.li/rsc-advances
formation of the triplet excited state (CPD)*.
molecular design and development of porphyrin photosensi-
Introduction
7–11
tizers have been made to further improve the F value so far.
D
Photosensitizers possessing the ability to generate singlet Some researchers reported that porphyrin–fullerene C60 dyads
1
oxygen ( O
2
) have created considerable interest in recent years as well as boron dipyrromethene (BODIPY)-C60 dyads exhibits
value than each porphyrin or C60, which is attrib-
from the viewpoint of fundamental studies in photochemistry a higher F
and their potential applications in photodynamic therapy uted to the formation of long-lived triplet excited state ( S*) by
D
3
1
–6
1
(
PDT). These photosensitizers generally produce O through the effective ISC due to intramolecular energy transfer or
2
the following processes: initially the photosensitizer absorbs
light (hn) to generate the singlet excited state of the photosen-
1
1
sitizer ( S*), then the photoexcited sensitizer ( S*) undergoes
intersystem crossing (ISC) to generate the triplet excited state
3
(
S*). Subsequent energy transfer from the photoexcited sensi-
3
3
1
2 2
tizer ( S*) to triplet oxygen ( O ) produces O . Thus, to generate
3
S* efficiency is one of the most effective strategies to give high
O quantum yield (F ). In particular, porphyrin dyes have been
1
2
D
regarded as promising candidates for photosensitizers as
a result of their strong Soret (400–500 nm) and moderate Q
band (500–700 nm) absorption properties, as well as their
electrochemical, photochemical and thermal stabilities. The
typical photosensitizer, free-base tetraphenylporphyrin (H
2
TPP)
7
exhibits F
D
value of 0.6–0.7 (in benzene). Much effort in
a
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima
University, Higashi-Hiroshima 739-8527, Japan. E-mail: yooyama@hiroshima-u.ac.
jp; Fax: +81-82-424-5494
b
Institute for Materials Chemistry and Engineering, Kyushu University, 744 Motooka,
Nishi-ku, Fukuoka 819-0395, Japan. E-mail: tanif@ms.ifoc.kyushu-u.ac.jp; Fax: +81-
Fig. 1 Chemical structures of cyclic free-base porphyrin dimer (CPD)
linked by butadiyne bearing four 4-pyridyl groups, its inclusion
complex (C603CPD) with C60 and ABAB porphyrin monomer H2PyP
as a reference.
9
†
1
2-802-6224
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra02699d
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1
electron transfer (charge separation) between photosensitizer the photoabsorption spectral change of the known O
2
scavenger
1
1,12
and C .
1,3-diphenylisobenzofuran (DPBF) accompanied by the reaction
6
0
1
1
Recently, we have designed and developed cyclic free-base of DPBF with the generated O , that is, DPBF can trap O2
porphyrin dimer CPD linked by butadiyne bearing four 4-pyr- through its photooxidation. CH Cl /MeOH was bubbled with
2
14
2
2
idyl groups and its inclusion complex C603CPD with C60 air for 15 min. The air-saturated solution containing CPD,
1
3
(
Fig. 1). It was expected that C603CPD has favorable photo-
C603CPD or H2PyP and DPBF was irradiated with 509 nm (300
ꢀ
2
ꢀ1
ꢀ1
abs
chemical and electrochemical properties for PDT through the mW cm , 3 ¼ 27 300 M cm @l ¼ 509 nm for CPD and 3 ¼
ꢀ1
ꢀ1
abs
electrochemical measurements and the transient absorption 17 000 M cm @l ¼ 509 nm for H2PyP, respectively) ob-
spectroscopy, based on the fact that the singlet excited state tained by passage of xenon light through monochromator. For
1
C - (CPD)* undergoes intrasupramolecular electron transfer to both CPD and C 3CPD as well as H2PyP the absorption band of
give a completely charge-separated state C
this work, to gain insight into the O generation properties of photoirradiation time (Fig. 3), which indicate the reaction of
supramolecular complex of cyclic free-base porphyrin dimer DPBF with O
6
0
60
$ꢀ
$+
-CPD . Thus, in DPBF at around 410 nm decreased with the increase in the
6
0
1
2
1
2
generated upon the excitation of the porphyrin
1
with C60, we evaluated the F
D
and rate constant (Kobs) of O
2
dimer. To gain insight into the effect of the cyclic porphyrin
generation for CPD and C603CPD. Here we reveal that cyclic dimers on the efficiency of DPBF photooxidation, the changes in
free-base porphyrin dimer and its inclusion complex with optical density (DOD) of DPBF are plotted against the photo-
1
fullerene C60 possess the ability to generate O
2
under visible irradiation time (Fig. 4a), and the slope (msl) is used to estimate
light irradiation, based on the kinetic and thermodynamic the F value for CPD, C 3CPD and H2PyP. The m value (ꢀ1.5
D
60
sl
ꢀ2
ꢀ2
consideration concerning the electron transfer processes ꢁ 10 ) of H2PyP is larger than those of CPD (ꢀ1.2 ꢁ 10 ) and
ꢀ
3
between the porphyrin dimer and C .
C 3CPD (ꢀ9.8 ꢁ 10 ). Moreover, it was revealed that the m
6
0
60
sl
value of CPD is larger than that of C603CPD. Thus, the F
D
values
of CPD, C603CPD and H2PyP were estimated by the relative
ꢀ
2
Results and discussion
method using Rose Bengal (RB) (F ¼ 0.80, m ¼ ꢀ1.5 ꢁ 10
,
D
sl
15
see Fig. S1†) in methanol as the standard (Table 1). The F
value of CPD, C603CPD and H2PyP is 0.62, 0.52 and 0.91
respectively, which is in good agreement with the m value. This
result suggests that as for the ABAB porphyrin monomer H2PyP
the ISC efficiency from S* to the S* may be higher than in the
cyclic free-base porphyrin dimer CPD. Moreover, it is worth
noting that the F value of C603CPD is lower than that of CPD.
D
The cyclic free-base porphyrin dimer CPD in CH
exhibits strong Soret band at around 420 nm and relatively weak
2 2
Cl /MeOH
sl
abs
ꢀ1
ꢀ1
Q band in the range 500–650 nm (Fig. 2, lmax/nm (3/M cm ) ¼
16 (708 000), 514 (31 200), 548 (7400), 587 (9000), 642 (2900)).
The molar extinction coefficients (3) of Soret and Q bands for
4
1
3
abs
ꢀ1
ꢀ1
CPD are higher than those of H2PyP (lmax/nm (3/M cm ) ¼
D
13e
418 (419 000), 513 (19 200), 548 (6200), 588 (6000), 643 (3000))
Our previous work demonstrates that the decay of photoexcited
as an ABAB porphyrin monomer with two pyridyl groups and two
phenyl groups. The fact is attributed to the porphyrin dimer
structure of CPD with two porphyrin units. For the C60 inclusion
complex C603CPD, it is difficult to obtain its exact absorption
spectra because the 1 : 1 complex of CPD with C60 is in dissoci-
ꢀ
5
ꢀ6
ation equilibrium in solution of 10 to 10 M concentration
which is suitable for the measurement of photoabsorption
spectra of porphyrins. In our previous work, however, we have
demonstrated that upon addition of C60 to the solution of the
cyclic porphyrin dimer, its Soret band was redshied with
a decrease in intensity, whereas its Q band was slightly redshied
13
1
but increased in intensity.
2
O generation by CPD, C603CPD or
H2PyP in CH Cl
2
2
/MeOH (¼1/1, v/v) was evaluated by monitoring
Fig. 3 Photoabsorption spectral changes for the photooxidation of
ꢀ6
DPBF (Abs. ¼ ca. 1.0) using (a) CPD (1.3 ꢁ 10 M), (b) C603CPD (1.0 ꢁ
ꢀ
6
ꢀ6
1
0
M) and (c) H2PyP (1.4 ꢁ 10 M) as photosensitizer under pho-
ꢀ2
Fig. 2 Photoabsorption spectra of CPD and H2PyP in CH
Cl
2 2
/MeOH. toirradiation with 509 nm (300 mW cm ) in CH
2
Cl
2
/MeOH (¼1/1, v/v).
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3
formation of C60*-(CPD). Thus, on the basis of the photody-
namics of the cyclic free-base porphyrin dimer and its inclusion
complex with C , the lower F value of the C60 inclusion
6
0
D
complex would be attributed to the formation of charge-
separated state, leading to low ISC efficiency because the ISC is
in kinetically competition with the intrasupramolecular electron
3
transfer, that is, the formation of triplet excited state (CPD)* is
in kinetically unfavorable compared to that of the charge-
$ꢀ
$+
separated state C60 -CPD .
In order to evaluate the photosensitizing ability of the cyclic
free-base porphyrin dimer and its inclusion complex with C ,
Fig. 4 (a) Plots of DOD for DPBF against the photoirradiation time for
the photooxidation of DPBF using CPD, C603CPD, H2PyP or Rose
Bengal as photosensitizers under photoirradiation with 509 nm (300
6
0
the ln(C /C ) is plotted against the photoirradiation time, where
t
0
ꢀ2
C
t
is a concentration of 1,5-dihydroxynaphthalene (DHN) at the
mW cm ) in CH
Plots of ln(C /C
2
2
Cl /MeOH (¼1/1, v/v) and MeOH, respectively. (b)
) for DHN against the photoirradiation time for the reaction time (t) and C is the initial concentration of DHN
0
t
0
photooxidation of DHN using CPD, C603CPD, H2PyP or Rose Bengal before photoirradiation. CH
Cl
/MeOH (¼1/1, v/v) were
2
2
as photosensitizers under photoirradiation with visible light (>385 nm,
bubbled with air for 15 min. The air-saturated solution con-
ꢀ
2
3
0 mW cm ) in CH
Cl
2 2
/MeOH (¼1/1, v/v).
taining CPD or C603CPD and DHN was irradiated with visible
ꢀ2
light (>385 nm, 30 mW cm ) obtained by passage of xenon
light through a 385 nm long path lter. The photoabsorption
spectral changes for the photooxidation of DHN using CPD,
1
Table 1
O
2
quantum yield (F
D
) and first-order rate constant (Kobs) for
Cl
/MeOH (¼1/1, v/v),
respectively, using CPD, C603CPD or H2PyP as photosensitizer
the photooxidation of DPBF and DHN in CH
2
2
C
603CPD or H2PyP under photoirradiation with the visible
light in CH Cl
/MeOH (¼1/1, v/v) are shown in Fig. 6. Evidently,
the absorption band of DHN at around 300 nm decreased with
the increase in the photoirradiation time. The plots of ln(C /C
2
2
a
D
b
ꢀ1
Photosensitizer
F
K
obs /min
t
0
)
ꢀ3
CPD
C
H2PyP
0.62
0.52
0.91
6.6 ꢁ 10
5.8 ꢁ 10
9.5 ꢁ 10
ꢀ3
against the photoirradiation time indicate that for CPD,
C 3CPD and H2PyP the ln(C /C ) decreased almost linearly with
603CPD
ꢀ3
60
t
0
the increase in the photoirradiation time (Fig. 4b). Thus, this
a 1
O quantum yield (relative decomposition rate of DPBF), with Rose
2
15
result indicates the ln(C /C ) bears a linear relationship with the
t
0
Bengal (RB) as standard (F
diphenylisobenzofuran (DPBF) as
constant for the reaction of DHN with
photoexcitation of the photosensitizer. The Kobs for RB is 5.3 ꢁ 10
D
¼ 0.80 in methanol, see Fig. S1) and 3-
1 b
2
O
scavenger. First-order rate photoirradiation time to provide the rst-order rate constants
1
O
2
generated upon
(Kobs) for the photooxidation of DHN using the cyclic free-base
ꢀ3
ꢀ
1
porphyrin dimer or its inclusion complex with C60 as the
min (see Fig. S2).
state of C603CPD has two steps (Fig. 5): the rst step has a life-
time of 18 ps, which corresponds to the disappearance of the
1
singlet excited state of C60- (CPD)* (ca. ꢀ3.6 eV), that is,
1
C
60- (CPD)* undergoes intrasupramolecular electron transfer to
$ꢀ
$+
give a completely charge-separated state C60 -CPD (ca. ꢀ3.7
13
$ꢀ
0
$+
eV).
C
6
-CPD decays with a lifetime of 470 ps to the ground
1
state. The singlet excited state C - (CPD)* has a slower ISC to its
6
0
3
triplet excited state C - (CPD)* (ca. ꢀ4.0 eV), in addition, the
6
0
3
C
60- (CPD)* would undergo energy transfer to C60, leading to the
Fig. 6 Photoabsorption spectral changes for the photooxidation of
ꢀ
4
ꢀ6
DHN (1.0 ꢁ 10 M) using (a) CPD (2.5 ꢁ 10 M), (b) C603CPD (2.5 ꢁ
ꢀ6
ꢀ6
1
0
M) and (c) H2PyP (5.0 ꢁ 10 M) as photosensitizer under pho-
ꢀ2
2 2
toirradiation with visible light (>385 nm, 30 mW cm ) in CH Cl /
1
Fig. 5 The photodynamics of C603CPD and O
2
generation.
MeOH (¼1/1, v/v).
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photosensitizer (Table 1). Obviously, the higher Kobs values of the
cyclic free-base porphyrin dimer and its inclusion complex with
Conclusions
1
C60 relative to RB (see Fig. S2†) are due to the contribution of the To investigate singlet oxygen ( O ) generation properties of
2
strong Soret band of the cyclic free-base porphyrin skeleton, cyclic free-base porphyrin dimer and its inclusion complex with
1
although the Kobs values of CPD and C603CPD are lower than fullerene C , we evaluated the O quantum yield (F ) and rate
6
0
2
D
ꢀ
3
ꢀ1
1
that of H2PyP (9.5 ꢁ 10 min ). It is worth noting here that the constant (Kobs) of O generation for cyclic free-base porphyrin
2
ꢀ
3
ꢀ1
K
obs value (6.6 ꢁ 10 min ) of CPD is greater than that (5.8 ꢁ dimer CPD and its inclusion complex C 3CPD with C . It was
6
0
60
ꢀ3 ꢀ1
10
min ) of C603CPD. Therefore, this result demonstrates found that the F value of C 3CPD is lower than that of CPD.
60
D
that CPD exhibits more efficient photosensitizing ability due to The lower FD value of the C60 inclusion complex would be
$
ꢀ
C
60
the effective ISC compared to C 3CPD.
attributed to the formation of charge-separated state
-
6
0
$+
In addition, we performed an electron paramagnetic reso- CPD , leading to low intersystem crossing (ISC) efficiency for
3
nance (EPR) method with 2,2,6,6-tetramethyl-4-piperidone (4- the formation of triplet excited state (CPD)*, although it was
1
$ꢀ
$+
1
oxo-TEMP) as the spin-trapping agent, which can react with O
2
expected that the formation of C60 -CPD is favorable for O2
16
to produce 4-oxo-TEMPO as a stable nitroxide radical. When generation. Consequently, this work demonstrates that the
the air-saturated solution containing CPD, C603CPD or H2PyP cyclic free-base porphyrin dimer and its supramolecular
1
and 4-oxo-TEMP was irradiated with visible light (>385 nm, 30 complex with C60 possess the ability to generate O2 under
ꢀ2
mW cm ) obtained by passage of xenon light through a 385 nm visible light irradiation.
long path lter, for both the free-base porphyrin dimer and its
inclusion complex with C60 as well as H2PyP the ESR spectra of
4
-oxo-TEMPO were clearly observed as a characteristic 1 : 1 : 1
Experimental
triplet (Fig. 7). Consequently, this work demonstrated that the
cyclic free-base porphyrin dimer and its inclusion complex with Evaluation of O quantum yield
C
1
2
1
60 possess the ability to generate
O
2
under visible light
1
D
2
Quantum yield (F ) for singlet oxygen ( O ) generation by
irradiation.
cyclic free-base porphyrin dimer CPD, its inclusion complex
C 3CPD with fullerene C and H2PyP in CH Cl /MeOH (¼1/
6
0
60
2
2
1
, v/v) was evaluated by monitoring the photoabsorption
1
spectral change of the known O
2
scavenger 1,3-diphenyliso-
benzofuran (DPBF) accompanied by the reaction of DPBF with
1
1
the generated
photooxidation. CH
5 min. The absorbance of DPBF was adjusted to around 1.0 in
air-saturated solvent. Concentration of CPD, C 3CPD or
O
2
, that is, DPBF can trap
2
O through its
2
Cl /MeOH was bubbled with air for
2
1
6
0
H2PyP was adjusted with an absorbance of ca. 0.03 at the
irradiation wavelength (509 nm). The air-saturated solution
containing the photosensitizer (CPD, C603CPD or H2PyP) and
ꢀ
2
DPBF was irradiated with 509 nm (300 mW cm ) obtained by
passage of xenon light through monochromator. The photo-
absorption spectral change of DPBF with the photoirradiation
was monitored with an interval of 1 min up to 10 min. The
absorption band of DPBF at around 410 nm decreased with the
increase in the photoirradiation time. The changes in optical
density (DOD) of DPBF are plotted against the photo-
irradiation time, and the slope is used to estimate the F
values of CPD, C603CPD and H2PyP. The F values of CPD,
603CPD and H2PyP were estimated by the relative method
using Rose Bengal (RB) (F
D
D
C
D
¼ 0.80) in methanol as the stan-
1
dard. Therefore, the F
D
values were calculated according to
Fig. 7 The ESR spectra of 4-oxo-TEMPO which is formed by the
1
the following eqn (1):
reaction of 4-oxo-TEMP with O
2
which was generated by (a) CPD, (b)
C
603CPD and (c) H2PyP under irradiation with visible light (temper-
F
Dsam ¼ FDref ꢁ [(msam/mref) ꢁ (Lref/Lsam)]
(1)
ature 298 K, microwave power 1 mW, microwave frequency 9.439
GHz, and field modulation 0.2 mT at 100 kHz). The air-saturated
CH
ꢀ6
ꢀ6
1
Cl
2 2
solution containing CPD (2.5 ꢁ 10 M), C603CPD (2.5 ꢁ 10
where FDsam and FDref are the O
quantum yield of photo-
2
ꢀ
6
M) or H2PyP (5.0 ꢁ 10 M) as the photosensitizer and 4-oxo-TEMP
sensitizer (CPD, C603CPD or H2PyP) and RB, respectively,
msam and mref are the slope of the difference (DOD) in the
change in the absorption maximum wavelength of DPBF
around 410 nm) which are plotted against the photo-
irradiation time, Lsam and Lref are the light harvesting
(
(
50 mM) as the spin-trapping agent was irradiated with visible light
ꢀ
2
>385 nm, 30 mW cm for 1 h) obtained by passage of xenon light
Cl as low polar solvent
was used because it was difficult to obtain a clear ESR signal in polar
solvent such as CH Cl
/MeOH (¼1/1, v/v).
through a 385 nm long path filter, where CH
2
2
(
2
2
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ꢀ
A
efficiency, which is given by L ¼ 1 ꢀ 10 (“A” is the absorbance
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Phys. Chem. A, 2002, 106, 6445; (b) P. C. Lo, J. D. Huang,
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at the photoirradiation wavelength).
Photosensitizing ability
Photosensitizing ability of CPD, C603CPD and H2PyP in
CH
2
Cl
2
/MeOH was evaluated by plotting the ln(C
t 0
/C ) against
the photoirradiation time, where C
t
is a concentration of 1,5-
dihydroxynaphthalene (DHN) at the reaction time (t) and C0
is the initial concentration of DHN before photoirradiation.
CH Cl /MeOH was bubbled with air for 15 min. The air-
2
2
saturated solution containing the photosensitizer (2.5 ꢁ
ꢀ
6
ꢀ6
ꢀ4
1
2
0
M for CPD and C603CPD, 5.0 ꢁ 10 M for H2PyP and
ꢀ6
.5 ꢁ 10 M for RB) and DHN (1.0 ꢁ 10 M) was irradiated
9
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D. Ramaiah, T. K. Chandrashekar, A. Srinivasan and
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ꢀ2
with visible light (>385 nm, 30 mW cm ) obtained by
passage of xenon light through a 385 nm long path lter. The
photooxidation of DHN with the photoirradiation was
monitored by following the decrease in the photoabsorption
at around 300 nm with an interval of 1 min up to 10 min and
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then an interval of 5 min up to 30 min. The concentration (C
of DHN at the reaction time (t) was calculated based on
/C ) decreased
t
)
Lambert–Beer law (ADPBF ¼ 3cl). The ln(C
t
0
almost linearly with the increase in the photoirradiation time
due to the photooxidation of DHN, that is, the slope was used
to estimate the rate constants (Kobs).
1
0 (a) M. Prein and W. Adam, Angew. Chem., Int. Ed., 2014, 53,
1
2
O detection by EPR spin-trapping method with 4-oxo-TEMP
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ꢀ
6
CH Cl solution containing CPD (2.5 ꢁ 10 M), C 3CPD (2.5
2
2
6
60
ꢀ
ꢀ6
ꢁ
10 M) or H2PyP (5.0 ꢁ 10 M) as the photosensitizer and
F.
Royer,
S.
Achelle,
C.
Fiorini-Debuisschert,
4
-oxo-TEMP (50 mM) as the spin-trapping agent was irradiated
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ꢀ2
with visible light (>385 nm, 30 mW cm for 1 h) obtained by
passage of xenon light through a 385 nm long path lter. The
ESR spectrum of 4-oxo-TEMPO which is formed by the reaction
1
of 4-oxo-TEMP with O
2
, was clearly observed as a characteristic 11 M. E. Milanesio, M. G. Alvarez, V. Rivarola, J. J. Silber and
1
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