Singlet oxygen generation by two-photon excitation of porphyrin derivatives having two-photon-absorbing benzothiadiazole chromophores

Article abstract of DOI:10.1039/b704499b

Porphyrin derivatives with four two-photon absorbing 2,1,3-benzothiadiazole chromophores at the meso positions provide large two-photon absorption cross-sections up to 735 GM as well as large quantum yields of one-photon singlet oxygen sensitization of ca

Full text of DOI:10.1039/b704499b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Singlet oxygen generation by two-photon excitation of porphyrin
derivatives having two-photon-absorbing benzothiadiazole chromophores{
Tsutomu Ishi-i,*ab Yoshiki Taguri,c Shin-ichiro Kato,c Motoyuki Shigeiwa,*d Hideki Gorohmaru,d
Shuuichi Maedad and Shuntaro Matakab
Received 26th March 2007, Accepted 8th May 2007
First published as an Advance Article on the web 18th May 2007
DOI: 10.1039/b704499b
Porphyrin derivatives with four two-photon absorbing 2,1,3-benzothiadiazole chromophores at
the meso positions provide large two-photon absorption cross-sections up to 735 GM as well as
large quantum yields of one-photon singlet oxygen sensitization of ca. 0.7, leading to generation
of singlet oxygen by two-photon excitation of 800 nm. The two-photon singlet oxygen
sensitization is discussed on the basis of a parameter of the normalized oxygen luminescence
intensity obtained from the two-photon experiment.
Recently, Rebane’s group reported that p-conjugated por-
Introduction
phyrin dimers show extremely high TPA cross-sections and
Two-photon absorbing (TPA) organic molecules have been of
high quantum yields of one-photon singlet oxygen sensitiza-
tion.11,12 Correlation between the two parameters ‘TPA cross-
much interest in recent years in view of their potential
applications for optical power limitation,1 microfabrication,2
section and quantum yield of one-photon singlet oxygen
three-dimensional optical data storage,3 two-photon laser
sensitization’ was discussed to characterize molecular ability in
scanning fluorescence imaging,4 and photodynamic therapy.5
In particular, in photodynamic therapy, the two-photon
excitation technique using red and near-IR region light sources
has three advantages: i) increase of penetration depth in
tissues, ii) less photodamage, and iii) pin-point excitation by a
focused laser beam, which can not be achieved by linear one-
photon excitation. Thus, in two-photon-induced photody-
namic therapy, TPA organic molecules are required to have
both large TPA cross-sections in the red and near-IR region
and high quantum yields of singlet oxygen generation. Till
now, a limited number of research groups have succeeded in
two-photon singlet oxygen sensitization by using difurano-
naphthalene,6 vinylbenzene,6–9 ethynylbenzene,9 and por-
phyrin derivatives.10–15
two-photon-induced photodynamic therapy.12 Another
approach was performed by Fre´chet’s group using fluores-
cence resonance energy-transfer (FRET) as a key step. In a
porphyrin derivative containing eight two-photon absorbing
AF-343 chromophores, singlet oxygen can be generated by
two-photon excitation of the AF-343 chromophore followed
by the subsequent FRET to the porphyrin core.13 Recently,
this system was developed for use in an aqueous medium.14
On the other hand, 4,7-diaryl-2,1,3-benzothiadiazoles are
known as strongly fluorescent dyes.16 Further, an electron-
withdrawing 2,1,3-benzothiadiazole (BTD) unit was used as an
important spacer in semiconducting oligomers.17 Recently, we
have developed BTD-based dyes to produce functional
materials such as dichroic fluorescent materials in liquid
crystal displays,18 and light-harvesting antennae as well as
energy-transfer reagents in fullerene–dyad systems.19 More
recently, we have created BTD-based two-photon absorbing
molecules with red-fluorescent ability on the basis of a
combination of the electron-withdrawing BTD unit and
electron-donating amino groups to enhance the intramolecular
charge transfer character.20 This finding led us to newly
develop BTD-based dyes to two-photon singlet oxygen
sensitizers. The strategy is based on the combination of a
two-photon absorbing BTD-based unit and a singlet oxygen
sensitizing porphyrin (Fig. 1). In this article, we report that
porphyrin derivatives having four diphenylamino group-
containing BTD chromophores at the meso positions show
high TPA activity as well as one- and two-photon oxygen
sensitization. The two-photon singlet oxygen sensitization is
discussed on the basis of the quantum yield of one-photon
excited singlet oxygen generation and the two-photon absorp-
tion cross-section as well as the parameter of the normalized
oxygen luminescence intensity obtained from the two-photon
experiment.
The first persuasive report on two-photon sensitized singlet
oxygen generation was developed by Ogilby’s groups.
Difuranonaphthalene and distyrylbenzene derivatives, which
provide moderate efficiencies on one-photon singlet oxygen
sensitization and two-photon absorbing, display singlet oxygen
sensitization by two-photon excitation as monitored by singlet
oxygen luminescence.6 Later on, they systematically studied a
series of water soluble porphyrins and vinylbenzenes.7
aDepartment of Biochemistry and Applied Chemistry, Kurume National
College of Technology, 1-1-1 Komorino, Kurume, 830-8555, Japan.
E-mail: ishi-i@kurume-nct.ac.jp; Fax: +81 942 35 9400;
Tel: +81 942 35 9404
bInstitute for Materials Chemistry and Engineering (IMCE), Kyushu
University, 6-1 Kasuga-koh-en, Kasuga 816-8580, Japan
cInterdisciplinary Graduate School of Engineering Sciences, Kyushu
University, 6-1 Kasuga-koh-en, Kasuga 816-8580, Japan
dMitsubishi Chemical Group Science and Technology Research Center,
Inc., 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502, Japan
{ Electronic supplementary information (ESI) available: synthesis of
materials, UV/Vis linear absorption, one-photon excited fluorescence,
fluorescence excitation, and two-photon excited fluorescence spectra.
See DOI: 10.1039/b704499b
This journal is ß The Royal Society of Chemistry 2007
J. Mater. Chem., 2007, 17, 3341–3346 | 3341

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Fig. 1 Structures of TPP, ZnTPP, 1a, 1b, 1c, 2a, and 2b.
Results and discussion
Preparation
Porphyrin derivatives 1a having four (diphenylamino)phenyl
group-containing BTD chromophores (DPAP-BTD) and 1b
having four (diphenylamino)phenylethenyl group-containing
BTD chromophores (DPAPE-BTD) were obtained from a
porphyrin derivative having four phenylboronate moieties, 6,21
by Suzuki coupling reactions with the corresponding BTD
bromides
4 and 5, respectively, in the presence of a
palladium(0) catalyst (Scheme 1). Subsequently, 1a was derived
to the corresponding zinc complex 1c by treatment with zinc
acetate. The synthetic intermediates 4 and 5 were derived from
4,7-dibromo-2,1,3-BTD 322 by Suzuki and Heck coupling
reactions, respectively. Diphenylamino group-containing BTD
dyes 2a and 2b, which correspond to the side chain moieties in
1a and 1b, respectively, were prepared as reference compounds
(Scheme 1). Identification of 1a–c was performed by means of
spectroscopic methods, MALDI-TOF mass spectrometry, and
elemental analysis.
One-photon absorption and emission properties
Scheme 1 Preparation of 1a, 1b, 1c, 2a, and 2b.
The UV/Vis linear absorption spectra of 1a in both toluene
and chloroform show a Soret band around 430 nm and Q
bands between 550–650 nm (Fig. 2 and Table 1). In addition, a
broad charge-transfer (CT) transition band arising from the
donor–acceptor DPAP-BTD chromophore in 1a around
440 nm, which coincides with that of reference 2a (ESI{), is
overlapped with the intense Soret band.19 In 1b with an
additional olefinic spacer, the CT band from the DPAPE-BTD
chromophore shifts bathochromically around to 465 nm,
whereas the Soret and Q bands remain at the same positions
(Table 1 and ESI{). The Soret band in 1a and 1b was
broadened along with a bathochromic shift of ca. 10 nm
compared to that of TPP, indicating an electronic perturbation
from the BTD chromophore to the porphyrin core.23 Similar
UV/Vis spectral behavior was obtained from a comparison of
1c with ZnTPP (Fig. 3, Table 1, and ESI{).24 The strong
absorption around 400 nm including both the Soret and
DPAP-BTD/DPAPE-BTD bands in 1a–c is very suitable for
3342 | J. Mater. Chem., 2007, 17, 3341–3346
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Fig. 3 UV/Vis linear absorption spectra of 1c and ZnTPP in
chloroform (1.0 mM).
Fig. 2 UV/Vis linear absorption spectra of 1a, 1b, and TPP in toluene
(1.0 mM).
Table 2 One-photon and two-photon excited fluorescence spectral
data, and two-photon absorption cross-sections (d) of 1a, 1b, 1c, 2a,
2b, TPP, and ZnTPP in toluene and chloroform
two-photon excitation at 800 nm by using a Ti : sapphire laser
source.
References 2a and 2b are strongly fluorescent, emitting an
orange-red color with good quantum yields of 0.36–0.72
(Table 2).19,20 Upon excitation of the DPAP-BTD and
DPAPE-BTD chromophores in 1a and 1b, respectively, such
red-orange emissions were quenched completely. Only
weak emissions from the porphyrin core were observable
around 660 and 720 nm as found in TPP (Fig. 4, Table 2,
and ESI{). The fluorescent quantum yields (0.06–0.08) in 1a
and 1b are comparable to those (0.04–0.05) of the parent
TPP (Table 2). Fluorescence excitation spectra of 1a and
1b at he porphyrin emission bands almost coincide with
the UV/Vis linear absorption spectra (ESI{). The foregoing
results indicate that in 1a and 1b the singlet excited state
of the DPAP-BTD/DPAPE-BTD moiety is quenched by
the porphyrin core via intramolecular energy transfer,23
then the generated singlet excited state of the porphyrin
Toluene
Chloroform
Comp. lSPEF
WFL dc/GM l2PEF lSPEF
WFL dc/GM l2PEF
a
b
g
a
b
g
d
1a
1b
1c
2a
2b
TPP
658, 721 0.07
659, 721 0.08
607, 653 0.04
573
588
—
—
—
569
598
—
658, 719 0.06 441f
658, 720 0.06 735f
614, 656 0.04 469f
665, 722
666, 724
621, 660
d
d
0.72 12e
0.70 23e
615
635
0.51
7.4e 617
0.36 38e
647
2.2e 675 (sh),
718
653, 717 0.04
—
653, 715 0.05
ZnTPP 598, 647 0.01
a
—
—
603, 653 0.02
—
—
Fluorescent maxima obtained from one-photon excitation (around
Q1 band: 550–558 nm) at 1.0 mM. Rhodamine B (WFL = 0.65) was
b
c
used as a reference. Two-photon absorption cross-section (d) at
800 nm was estimated by using AF-50 (d = 45 GM) as a reference.
Due to the low solubility of 1a, 1b, and 1c (,0.01 mM) in toluene,
d
e
values could not be measured. Measured at 5.0 mM.
the
s
f
g
Measured at 0.1 mM. Fluorescent maxima obtained from two-
photon excitation at 800 nm.
chromophore converts to
a
triplet excited state via
intersystem crossing as found in many porphyrin derivatives
such as TPP. Similar energy transfer and subsequent inter-
system crossing are observable in zinc complex 1c (Table 2
and ESI{).
Two-photon absorption and emission properties
The two-photon excited absorption and fluorescence spectral
properties of 1a–c are summarized in Table 2. By two-photon
Table 1 One-photon absorption spectral data of 1a, 1b, 1c, TPP, and ZnTPP (1.0 mM), and 2a and 2b (10.0 mM) in toluene and chloroform
max/nm (1024 e/M21 cm21
l
)
Solvent
Toluene
Comp.
DPAP-BTD/DPAPE-BTD
Soret
Q1
Q2
Q3
a
a
1a
1b
TPP
2a
2b
1a
1b
1c
429 (24.3)
428 (18.6)
419 (45.0)
—
557 (1.74)
557 (2.42)
549 (0.69)
—
592 (0.48)
594 (0.58)
590 (0.44)
—
652 (0.55)
652 (0.63)
648 (0.36)
—
—
650 (0.55)
650 (0.59)
—
439 (1.28)
464 (2.32)
—
—
—
a
Chloroform
427 (26.7)
427 (17.8)
430 (26.4)
418 (39.7)
424 (46.0)
—
558 (1.74)
558 (2.41)
557 (2.29)
550 (0.49)
554 (1.90)
—
594 (0.50)
592 (0.61)
599 (1.22)
590 (0.31)
595 (0.44)
—
a
a
TPP
ZnTPP
2a
—
—
648 (0.32)
440 (1.25)
465 (2.37)
—
—
2b
—
—
—
a
The DPAP-BTD/DPAPE-BTD absorption band overlapped with the Soret band.
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J. Mater. Chem., 2007, 17, 3341–3346 | 3343

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absorption spectra. The TPA maxima were observed at shorter
wavelength positions compared to the one-photon absorption
maxima, because the TPA-allowed excited state exists above
the one-photon absorption excited state for this kind of
centrosymmetrical molecule. The significant increase of d
values observed below 760 nm is interpreted as one-photon
resonance enhancement on two-photon transition at a short
wavelength near the single-photon transition, as reported
previously (Fig. 5).27
At 800 nm, 1a displays a maximum d value of 441 GM,
which is estimated on the basis of AF-50 (45 GM) used as a
TPA benchmark.28 By introducing an olefinic spacer in 1b, the
d value at 800 nm increases to 735 GM based on an expansion
of the p-system (Table 2).26 Zinc complex 1c displays a
maximum d value of 492 GM at the longer wavelength of
825 nm, which is larger than that (441 GM) of free base
porphyrin 1a (Fig. 5). A comparison with TPP having a low d
value of 2.2 GM29 indicates that the large TPA cross-sections
in 1a–c are attributed to the two-photon absorbing nature of
the peripheral DPAP-BTD/DPAPE-BTD moiety.20 Compared
to references 2a and 2b (7–12 and 23–38 GM, respectively), the
d values in 1a–c are larger by one order of magnitude (Table 2),
indicating an electron-donating effect of the porphyrin
core and a cooperative enhancement effect between the four
DPAP-BTD/DPAPE-BTD chromophores in the star-shaped
structure.30 The porphyrin core is conjugated with the
electron-withdrawing BTD moieties by through-bond electro-
nic communication.15 Such electronic communication can not
be created in non-conjugated system.13,31
Fig. 4 One-photon excited fluorescence spectra of 1a, 1b, and TPP in
toluene (1.0 mM). The fluorescence spectra were obtained by excitation
around Soret band (at 429 nm for 1a, at 428 nm for 1b, and at 419 nm
for TPP).
excitation with 800 nm laser pulses, 1a and 1b emitted, as
expected, frequency upconverted fluorescence from the por-
phyrin core around 665 and 720 nm (ESI{), which shifted
slightly to 3–8 nm longer wavelength compared with those
obtained by one-photon excitation (Table 2).25 In zinc complex
1c, the frequency upconverted fluorescence was observable
also upon excitation at 800 nm (Table 2).
Two-photon absorption cross-sections (d) at 700–900 nm
were measured by using an open aperture Z-scan method with
wavelength variable femtosecond laser pulses. In toluene, the d
values in 1a–c could not be measured because of their low
solubility in toluene (,0.01 mM). In contrast, 1a–c show good
solubility in chloroform (y1.0 mM). In chloroform, plots of
two-photon absorption cross-sections against the excitation
wavelength exhibited maxima around 800–825 nm (Fig. 5),26
which correspond to the CT transition of DPAP-BTD/
DPAPE-BTD chromophores observed in the UV/Vis linear
Singlet oxygen sensitization
Singlet oxygen generation by one- and two-photon excitation
of 1a and 1b was measured in toluene. The quantum yield of
one-photon singlet oxygen sensitization (WD) was estimated by
monitoring oxygen luminescence at 1275 nm generated by the
excitation of 400 nm laser pulses (Fig. 6a). The WD values of
0.68 in 1a and 0.65 in 1b are comparable to that (0.70) of the
parent TPP (Table 3),32 indicating the good performance of 1a
and 1b as singlet oxygen sensitizers. In 1c, reliable data could
not be obtained because of the very low solubility of 1c in
toluene. In chloroform, 1c displays a WD value of 0.62, which
seems to include a solvent effect of the chloroform.33 The
DPAP-BTD/DPAPE-BTD moiety in 1a–c scarcely affects the
oxygen sensitizing nature of the central porphyrin core. A
figure of d 6 WD expresses the merit of singlet oxygen
generation by two-photon excitation.12 The d 6 WD values of
300 GM in 1a and 478 GM in 1b are larger by two orders of
magnitude than that (1.5) of TPP (Table 3), indicating the
excellent performance of 1a and 1b as two-photon singlet
oxygen sensitizers.
In 1a and 1b, the oxygen luminescence is detected actually
by two-photon excitation of 800 nm laser pulses even at a low
concentration of y10 mM. In contrast, such oxygen lumines-
cence in TPP could not be detected at the same concentration,
and reliable detection required a high concentration of .5 mM
(Fig. 6b). The high performance in the two-photon-induced
singlet oxygen generation could be ascribed to the through-
bond electronic communication between the porphyrin core
Fig. 5 Two-photon absorption spectra of 1a, 1b, and 1c in chloro-
form. The s value (2130 GM) of 1b at 760 nm is out of the range of the
plot area to retain clarity of the other data.
3344 | J. Mater. Chem., 2007, 17, 3341–3346
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Fig. 7 Energy diagram for singlet oxygen generation by two-photon
excitation of the benzothiadiazole moiety (DPAP-BTD/DPAPE-BTD)
in 1a–c.
and the BTD moieties in 1a and 1b.15 The oxygen luminescence
intensity normalized by a concentration term indicates the
order of two-photon oxygen sensitization to be TPP (1.5) ,,
1a (298) , 1b (547) (Table 3). We compared the d 6 WD values
(TPP (1.5 GM) ,, 1a (300 GM) , 1b (478 GM) in Table 3)
with these normalized luminescence intensity values. Although
the coincidence is not perfect, the trend can be seen clearly.
This result demonstrates that Kasha’s rule is valid in this
oxygen sensitization process irrespective of the excitation
method and excited state.8,34
The energy diagram for singlet oxygen generation by two-
photon excitation in 1a–c is shown in Fig. 7. The singlet
excited state of the benzothiadiazole chromophore generated
by two-photon excitation is converted efficiently to the singlet
excited state of the porphyrin chromophore, from which the
corresponding triplet state is given via intersystem crossing to
generate singlet oxygen.
Fig. 6 Oxygen luminescence spectra obtained from (a) 400 nm and
(b) 800 nm laser irradiation of air-saturated toluene solutions of 1a
(16.7 mM), 1b (11.3 mM), and TPP (12.5 mM for 400 nm and 5.0 mM
for 800 nm).
Conclusions
In conclusion, we have demonstrated that porphyrin
derivatives with four benzothiadiazole chromophores
generate singlet oxygen by two-photon excitation at 800 nm.
The present two-photon-induced singlet oxygen generation
is ascribed to a combination of the two-photon absorbing
nature of the peripheral benzothiadiazole dye and the
singlet oxygen sensitizing ability of the porphyrin core.
This is a new application of benzothiadiazole dye as a
functional material. The two-photon oxygen sensitization can
be analyzed by a simple parameter of oxygen luminescence
intensity obtained only from the two-photon excitation
experiment. We believe that the present system will be
developed for photodynamic therapy initiated by two-photon
excitation.
Table 3 Singlet oxygen generation efficiency by one-photon (400 nm)
and two-photon (800 nm) excitation in toluene
One-photon Two-photon
db/GM
Luminescence
d 6 WDc/GM intensityd
a
Comp. WD
1a
1b
1c
TPP
a
0.68
0.65
0.62e
0.70f
441
735
469
2.2
300
478
291e
298
547
264e
1.5
1.5
Quantum yield of singlet oxygen generation (WD) was estimated on
the basis of the intensity of singlet oxygen luminescence at 1275 nm
(at 16.7 mM for 1a, at 11.3 mM for 1b, at 37.5 mM for 1c, and at
b
12.5 mM for TPP). Two-photon absorption cross-section (d) at
800 nm was estimated in chloroform by using AF-50 (d = 45 GM)
as a reference. In toluene, the d values of 1a–c could not be
measured because of the low solubility. Figure of merit for two-
photon singlet oxygen sensitization. The intensity of singlet oxygen
luminescence at 1275 nm normalized by the concentration term (at
Acknowledgements
c
d
We thank Professor Dr Shigeori Takenaka (Kyushu
Institute of Technology) and Dr Keiichi Otsuka (Kyushu
University) for the measurement of MALDI-TOF-MS. This
work was partially supported by Research for Promoting
Technological Seeds, Japan Science and Technology
Cooperation (JST).
16.7 mM for 1a, at 11.3 mM for 1b, at 37.5 mM for 1c, and at 5.0 mM
for TPP). The normalized value in TPP was given to be 1.5. Due
e
to the very low solubility of 1c in toluene, the measurement was
A reported value in
f
performed in chloroform at 37.5 mM.
reference 32.
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3346 | J. Mater. Chem., 2007, 17, 3341–3346
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