Photophysical and photosensitizing properties of brominated porphycenes

Article abstract of DOI:10.1039/b802730g

A heavy atom, bromine, was directly substituted into the porphycene macrocycle to promote intersystem crossing by way of spin-orbit coupling. The singlet oxygen production ability of the porphycene is dramatically enhanced, and the highest value of 0.95 for the quantum yield of singlet oxygen generation (Φ_Δ) was obtained for the dibrominated porphycene by visible light excitation. The Royal Society of Chemistry.

Full text of DOI:10.1039/b802730g

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Photophysical and photosensitizing properties of brominated
porphycenesw
Hisashi Shimakoshi,a Tatsushi Baba,a Yusuke Iseki,a Isao Aritome,a Ayataka Endo,b
Chihaya Adachib and Yoshio Hisaeda*a
Received (in Cambridge, UK) 18th February 2008, Accepted 25th March 2008
First published as an Advance Article on the web 18th April 2008
DOI: 10.1039/b802730g
A heavy atom, bromine, was directly substituted into the porphycene
macrocycle to promote intersystem crossing by way of spin–orbit
coupling. The singlet oxygen production ability of the porphycene is
dramatically enhanced, and the highest value of 0.95 for the
quantum yield of singlet oxygen generation (UD) was obtained for
the dibrominated porphycene by visible light excitation.
Brominated porphycenes (2, 3, 4, 5) were synthesized by the
reactions between bromine and 1.9 The number of bromine
substituents was controlled by the amount of added bromine
(ESIw Table S1). All of the brominated porphycenes were
characterized by NMR, HRMS, and elemental analyses as well
as X-ray analysis. Their crystal structures are shown in Fig. 2.10z
The positions of bromine on each pyrrole ring of the porphy-
cenes were clearly confirmed. The absorption, fluorescence and
phosphorescence spectral data are summarized in Table 1. The
Q-type absorption bands are red-shifted with increasing number
of bromine groups of the porphycene (Fig. 3) while the fluores-
cence intensities decreased with increasing number of bromine
groups (ESIw Fig. S1). Therefore, the rate of intersystem cross-
ing would be expected to become greater.
Inspired by the significance of the porphyrins, a new research
direction has emerged devoted to the preparation and study of
non-porphyrin tetrapyrrolic macrocycles.1 Among them, por-
phycene, a structural isomer of porphyrin, was first synthe-
sized by Vogel and co-workers in 1986.2 This isomer exhibits
geometries of the N4 core which deviate from the ideal square
shape of the porphyrin core, resulting in an interesting elec-
tronic structure of the porphycene.3 For example, porphycenes
show a higher absorption than porphyrins in the red spectral
region due to the lower molecular symmetry.3 Therefore,
porphycenes are excellent candidates as photosensitizers.4 In
particular, the photosensitized production of singlet oxygen
has significance in the areas of photo-oxidation, DNA
damage, and photodynamic therapy of cancer.5 The singlet
oxygen generating ability of a photosensitizer is measured by
its quantum yield (FD). Vogel reported a FD value of 0.36 for
2,7,12,17-tetra-n-propylporphycene (1).6 This value is not very
high compared to those for other sensitizers including por-
phyrins.7 Therefore, our work was motivated by the quest for
a better porphycene sensitizer that generates singlet oxygen in
high yield. In this study, we incorporated a heavy atom,
bromine, directly substituted into the porphycene macrocycle
as shown in Fig. 1, and that can promote intersystem crossing
by way of spin–orbit coupling.8 As a consequence, the singlet
oxygen production ability via energy transfer from the triplet
state of the porphycene will be enhanced. Furthermore, bro-
mine substituents afford many advantages for photochemical
reactions, not only to enhance the rate of intersystem crossing
but also robust enough to prevent oxidation of the sensitizer
itself, and causing a bathochromic shift of its absorption due
to its electron withdrawing nature.
Thus, we next measured the lifetimes of the singlet state (tS)
for the porphycenes, and the results are also summarized in
Table 1. The lifetimes decreased with increasing number of
bromine groups. This result indicates the enhanced yield for
the intersystem crossing to the triplet state. As a consequence,
the yields of the singlet oxygen generation are expected to
become greater. The quantum yields of the singlet oxygen
generation (FD) by a series of porphycenes were determined by
the phosphorescence at 1270 nm in air-saturated solutions.
The values increased with increasing number of bromine
substituents, and the highest value of 0.95 was obtained for
the dibrominated porphycene (3) as shown in Fig. 4. The
values then decreased to 0.71 and 0.49 for the tri- (4) and tetra-
(5) brominated porphycenes, respectively. To elucidate this
bromine substitution effect, we obtained all the photophysical
parameters and the results are summarized in Table 2.11
The fluorescence rate constants (kf) are readily calculated
using the measured values (Ff and tS) and eqn (1).
kf = Ff/tS
(1)
The internal conversion rate constants (kic) and intersystem
crossing rate constants (kisc) are expressed by eqn (2) and (3).
kic = Fic/tS
(2)
a Department of Chemistry and Biochemistry, Graduate School of
Engineering, Kyushu University, Motooka, Fukuoka, 819-0395,
Japan. E-mail: yhisatcm@mbox.nc.kyushu-u.ac.jp;
Fax: +81-92-802-2827
b Center for Future Chemistry, Kyushu University, Motooka,
Fukuoka, 819-0395, Japan
w Electronic supplementary information (ESI) available: Details of
experimental procedures. CCDC 677018 (2), 677019 (3), 677020 (4),
677022 (5). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b802730g
Fig. 1 Molecular structures of a series of porphycenes.
ꢀc
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Fig. 3 UV-Vis spectra of a series of brominated porphycenes; (a) 1,
(b) 2, (c) 3, (d) 4, (e) 5 from front to back.
1/tT = kp + knr + kq[O2]
(7)
where kp is the phosphorescence rate constant, and knr is the
rate constant of nonradiative decay. By using these para-
meters, PTO2 was then calculated by eqn (6). The fTD value of
1 was determined to be 1.13 In the case of 2–5, the fTD were also
estimated as 1 since the free energy change of the charge-
M
transfer process DGCT {= F(Eox ꢁ EredO2) ꢁ ET} where F is
M
the Faraday constant, Eox is the half-wave oxidation poten-
tial of the sensitizer in the ground state, and EredO2 is the half-
wave reduction potential of oxygen} of 2–5 are higher than
that of 1, so that the charge transfer process is suppressed by
bromination to induce the enhancement of the sensitization
efficiency fTD. Thus FT can be calculated from eqn (4). Finally,
the singlet state (S1) may decay via a radiative process, which
produces a fluorescence emission, or through non-radiative
processes, including internal conversion (ic) to the ground
state (S0) or intersystem crossing (isc) to the excited triplet
state (T1), Fic can be expressed by eqn (8).
Fig. 2 Crystal structures of a series of brominated porphycenes (top
and side views); (a) 2 (b) 3 (c) 4 (d) 5, showing 50% displacement
ellipsoids (H atoms are omitted for clarity). Atoms labelled with the
suffix A are at symmetry related positions.
kisc = FT/tS
(3)
Fic = 1 ꢁ (FT + Ff)
The calculated parameters, (kp + knr), kq, PTO2 and DGCT for
(8)
The triplet quantum yield (FT) and the internal conversion
quantum yield (Fic) are obtained as follows. As FD can be
expressed using eqn (4),12
1–5 are summarized in Table S2 of ESI.w
FD = FTPTO2fTD
FT can be expressed by eqn (5),
FT = FD/PTO2fTD
(4)
As expected, the rate of intersystem crossing (kisc) increased
with the increasing number of bromines even for the tri- (4)
and tetra- (5) brominated porphycene. Remarkably, the rate
of internal conversion (kic) is significantly increased between
the di- (3) and tri- (4) brominated porphycenes. The effects of
the bromine groups are shown in Fig. 5.
(5)
3
where PTO2 is the proportion of the triplet quenched by O2,
and fTD is the fraction of the triplet quenched by 3O2 leading to
For 4 and 5, the increase in the kic value reduced their ratio
of intersystem crossing, so that the quantum yield of the
singlet oxygen generation decreased. Steric hindrance between
two neighboring bromine groups in 4 and 5 distort the
porphycene macrocycle.14 The flexibility caused by this dis-
tortion should then enhance the internal conversion rate.15
1
the formation of O2. As PTO2 can be expressed by eqn (6),
PTO2 = kq[O2]/(kp + knr + kq[O2])
(6)
(kp + knr) and kq were determined from the Stern–Volmer
analysis (ESIw Fig. S3) using eqn (7).
Table 1 Photophysical data of a series of porphycenesa
Compound
labsb/nm (10ꢁ3e/Mꢁ1 cmꢁ1
)
lfluoc/nm
lphosde/nm
tScf/ps
tTceg/ms
1
2
3
4
5
371 (142), 562 (35.3), 601 (33.3), 633 (46.4)
644, 702
648, 712
658, 719
659, 683
649, 716
978
990
8920
1400
754
103
53
85
39
23
8
371 (125), 383 (102), 567 (34.7), 607 (28.4), 641 (43.8)
372 (132), 385 (133), 571 (40.7), 613 (29.0), 649 (47.3)
375 (113), 387 (108), 579 (34.0), 625 (23.8), 665 (34.6)
379 (107), 590 (36.0), 640 (20.6), 684 (29.0)
996
1053
h
—
2
a
l
abs: absorption maximum; lfluo: fluorescence maximum; lphos: phosphorescence maximum; tS and tT: lifetimes of the singlet and triplet
b c d e f g
states. Solvent, CH2C12. Solvent, toluene. Solvent, bromobenzene. Degassed under vacuum. Degassed under nitrogen. Excitation at
h
532 nm. Low intensity peak around 1100–1200 nm.
ꢀc
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In conclusion, the photophysical properties of a series of
brominated porphycenes were clarified such that their photo-
physical parameters are dependent on the number of bro-
mines. Especially, the dibrominated porphycene shows the
highest sensitization efficiency for singlet oxygen production
in response to visible light.
We thank Prof. K. Sakai and Dr S. Masaoka, Kyushu
University, for helping to measure the transient absorption
spectra. This work was partially supported by Global COE
Program ‘‘Science for Future Molecular Systems’’ from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT) of Japan and Research for Promoting Technology Seeds
(15-007) from Japan Science and Technology Agency (JST).
Fig. 4 (a) Phosphorescence spectra of singlet oxygen sensitized by a
series of porphycenes in air-saturated toluene at r.t. with excitation at
600 nm. (b) Comparison of FD values for a series of porphycenes.
Table 2 Photophysical parameters of a series of porphycenesab
Notes and references
Compound Ffc
Fic
FT
FD
10ꢁ7
10ꢁ7
10ꢁ7
z Crystallographic data are given in the ESI.w Compounds 3 and 4
kf/sꢁ1 kic/sꢁ1 kisc/sꢁ1
have crystallographically imposed inversion symmetry.
1
2
3
4
5
0.37
0.08
0.03
0.27 0.36 0.36 4.1
0.02 0.90 0.90 5.7
0.02 0.95 0.95 4.0
3.0
1.4
2.7
260
870
4.0
64
130
700
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Fig. 5 Schematic representations of effects of bromine groups. The
rate constants (kf, kicand kisc/107 sꢁ1) are from Table 2.
ꢀc
This journal is The Royal Society of Chemistry 2008
2884 | Chem. Commun., 2008, 2882–2884