Inorg. Chem. 2009, 48, 9853–9860 9853
DOI: 10.1021/ic901401t
Syntheses and Photophysical Behavior of Porphyrin Isomer Sn(IV) Complexes
Daisuke Maeda, Hisashi Shimakoshi, Masaaki Abe, and Yoshio Hisaeda*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
Fukuoka 819-0395, Japan
Received July 17, 2009
2,3,6,7,12,13,16,17-Octaethylhemiporphycenato Sn(IV) chloride, [SnIV(OEHPc)Cl2], and 2,3,6,7,12,13,16,17-octaethylporphycenato Sn(IV) chloride, [SnIV(OEPc)Cl2], were synthesized in high yields and fully characterized by
various spectroscopic methods. The X-ray crystal structures of the Sn(IV) complexes with porphycene and
hemiporphycene were determined for the first time. The photophysical and photochemical properties of the singlet
state of the Sn(IV) porphycene and hemiporphycene complexes, structural isomers of porphyrin, have been
investigated by fluorescence and phosphorescence spectroscopies. The relatively strong emission and long
fluorescence lifetime of the Sn(IV) porphycene complexes indicated by the fluorescence quantum yield and lifetime
measurement were observed in the case of the Sn(IV) porphycene ([SnIV(OEPc)Cl2], ΦF = 0.125, τs = 2681 ps;
[SnIV(OEP)Cl2], ΦF = 0.010, τs = 438 ps; [SnIV(OEHPc)Cl2], ΦF = 0.027, τs = 733 ps). The triplet state of the Sn(IV)
complexes was investigated by transient absorption spectroscopy. It became clear that the triplet lifetime of the Sn(IV)
porphycene (τT = 49.9 μs) was longer when compared to those of the Sn(IV) porphyrin (τT = 32.6 μs) and
hemiporphycene complexes (τT = 28.3 μs). These porphyrin isomer Sn(IV) complexes showed the high singlet
oxygen generating ability, and the photo-oxidation of the 1,5-hydroxynaphthalene mediated by the Sn(IV) porphycene
was the most effective among the complexes. This result is due to its more effective light absorption in the visible region
and indicated that the porphycene is an excellent candidate as a photosensitizer.
Introduction
Porphyrins and phthalocyanines have strong visible absorptions, and they have been attracting much attention,
especially because of their potential application as efficient
photosensitizers for the photodynamic therapy of cancer.1,2
Thus, inspired by the significance of the porphyrins, a new
research direction has emerged that is devoted to the preparation and study of non-porphyrin tetrapyrrolic macrocycles.3-5 Recently, many porphyrin isomers with enhanced
and unique characteristic properties have been reported.6
Porphycene and hemiporphycene are some of the artificial
tetrapyrrole ligands, and these ligands were first synthesized
*To whom correspondence should be addressed. E-mail: yhisatcm@mail.
cstm.kyushu-u.ac.jp.
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by Vogel et al.7-9 It is known that these porphyrin isomers
have several interesting properties attributed to their ringframework modification.10 These differences have been elucidated from structural, theoretical, and electrochemical
studies.11-16 For example, these porphyrin isomers have a
small HOMO-LUMO gap and stronger absorption bands
::
(8) Vogel, E.; Broring, M.; Weghorn, S. J.; Scholz, P.; Deponte, R.; Lex,
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Published on Web 09/23/2009
pubs.acs.org/IC
9854 Inorganic Chemistry, Vol. 48, No. 20, 2009
than those of the porphyrins in the visible region. Therefore,
because they are very attractive compounds from a photochemical viewpoint and excellent candidates as photosensitizers,17 a significant effort has been put into fully characterizing their photochemical properties. Although specific
characteristics of porphyrin isomers have been revealed, the
photophysical and photochemical properties reflecting the
features derived from the ring skeleton change have not been
systematically discussed between porphyrin isomers in detail.
Therefore, a new insight and knowledge of the photophysical
and photochemical properties of the porphyrin isomers are
crucial to a better understanding of their functions in various
applications.
In the present study, we focused on the influence that a
different macrocycle has on the photophysical properties of
the porphyrin isomers, and we reported how variations in
their electronic structure and nitrogen-core size in the freebase forms of these three systems are reflected in the properties of their corresponding metal complexes, especially focused on the Sn(IV) hemiporphycene and porphycene
complexes. As numerous Sn(IV) porphyrins and corroles18
offer many advantages because of the particular properties
conferred by the highly charged main group metal center,
they can be possibly used in a variety of applications, that is,
in photoinduced electron transfer reactions,19 in supramolecular chemistry,20 and as a photocatalyst.21 Herein, we
synthesized the Sn(IV)-porphyrin, hemiporphycene, and
porphycene complexes having the same substituent and axial
ligands and investigated by several techniques the differences
due to the skeleton change. In particular, we compared the
photochemical properties of the octaethyl (OE) derivatives in
the form of their Sn(IV) complexes (Chart 1).
Experimental Section
Chemicals. For the UV-vis and fluorescent spectroscopy
studies, toluene and dichloromethane were purchased from
DOJINDO (Japan) and were of spectroscopic grade. For the
phosphorescent spectroscopy study, the bromobenzene was
purchased from WAKO (Japan) and distilled before use. For
the syntheses of 2 and 3, decahydronaphthalene (decalin) was
stirred for 1 day in the presence of sodium metal under a
nitrogen atmosphere, then distilled under reduced pressure,
and used immediately. The other organic solvents were dried
and distilled under a nitrogen atmosphere when necessary.
(17) (a) Ca~nete, M.; Ortiz, A.; Juarranz, A.; Villanueva, A.; Nonell, S.;
Borrell, J. I.; Teixido, J.; Stockert, J. C. Anti-Cancer Drug Des. 2000, 15, 143.
(b) Ca~nete, M.; Lape~na, M.; Juarranz, A.; Vendrell, V.; Borrell, J. I.; Teixido, J.;
Nonell, S.; Villanueva, A. Anti-Cancer Drug Des. 1997, 12, 543. (c) Villanueva,
A.; Ca~nete, M.; Nonell, S.; Borrell, J. I.; Teixido, J.; Juarranz, A. Anti-Cancer
Drug Des. 1996, 11, 89. (d) Rubio, N.; Martínez-Junza, V.; Estruga, J.; Borrell,
J. I.; Mora, M.; Sagrista, M. L.; Nonell, S. J. Porphyrins Phthalocyanines 2009,
13, 99.
(18) (a) Walker, D.; Chappel, S.; Mahammed, A.; Brunschwig, B. S.;
Winkler, J. R.; Gray, H. B.; Zaban, A.; Gross, Z. J. Porphyrins Phthalocyanines 2006, 10, 1259. (b) Wagnert, L.; Berg, A.; Stavitski, E.; Berthold, T.; Kothe,
G.; Goldberg, I.; Mahammed, A.; Simkhovich, L.; Gross, Z.; Levanon, H. Appl.
Magn. Reson. 2006, 30, 591.
(19) (a) Jang, J. H.; Kim, H. J.; Kim, H.-J.; Kim, C. H.; Joo, T.; Cho,
D. W.; Yoon, M. Bull. Korean Chem. Soc. 2007, 28, 1967. (b) Kim, H. J.; Park,
K.-M.; Ahn, T. K.; Kim, S. K.; Kim, S. K.; Kim, D.; Kim, H.-J. Chem. Commun.
2004, 2594.
(20) (a) Slagt, V. F.; van Leeumen, P. W. N. M.; Reek, J. N. H. Dalton
Trans. 2007, 2302. (b) Arnold, D. P.; Blok, J. Coord. Chem. Rev. 2004, 248, 299.
(21) (a) Kim, W.; Park, J.; Jo, H. J.; Kim, H.-J.; Choi, W. J. Phys. Chem.
C. 2008, 112, 491. (b) Monteiro, C. J.; Pereira, M. M.; Azenha, M. E.; Burrows,
H. D.; Serpa, C.; Arnaut, L. G.; Tapia, M. J.; Sarakha, M.; Wong-Wah-Chung, P.;
Navaratnam, S. Photochem. Photobiol. Sci. 2005, 4, 617.
Maeda et al.
Chart 1. Chemical Structures of Sn(IV) Complexes 1, 2, and 3
Chart 2. Chemical Structures of Porphyrin, Hemiporphycene, and
Porphycene
Measurements. The elemental analyses were obtained from
the Service Center of Elementary Analysis of Organic Compounds at Kyushu University. The 1H NMR spectra were
recorded by a Bruker Avance 500 spectrometer installed at the
Center of Advanced Instrumental Analysis at Kyushu University, and the chemical shifts (in ppm) were referenced relative to
the residual protic solvent peak. The UV-vis absorption spectra
were measured using a Hitachi U-3300 spectrophotometer at
room temperature. The fluorescence and phosphorescence spectra were measured by a HORIBA SPEX Fluorolog-NIR spectrophotometer in toluene and bromobenzene at room temperature. Bromobenzene was chosen to readily observe the
phosphorescence spectra by the external heavy atom effect.
The phosphorescence spectra as shown in this paper are differential spectra of the observed spectra under the anaerobic and
aerobic conditions (Supporting Information, Figure S1). The
MALDI-TOF mass spectra were obtained using a Bruker
Autoflex II without the matrix. The electrospray ionization
(ESI) mass spectra were obtained using a JMS-T100CS AccuTOF.
Porphyrin and Porphyrin Isomers. The chemical structures of
the porphyrin and porhyrin isomers used in this study are shown
in Chart 2. 2,3,6,7,12,13,16,17-Octaethylporphyrin [H2(OEP)]
was purchased from TCI (Japan) and used as received.
2,3,6,7,12,13,16,17-Octaethylhemiporphycene [H2(OEHPc)] and
2,3,6,7,12,13,16,17-octaethylhemiporphycene [H2(OEPc)] were
synthesized as reported in the literature.8,22
Preparation of Sn(IV) Complexes. 2,3,6,7,12,13,16,17-Octaethylporphyrinate Sn(IV) Chloride [SnIV(OEP)Cl2] (1). [SnIV(OEP)Cl2] (1) was prepared from H2(OEP) in a manner similar to that
previously reported.23,24 2,3,6,7,12,13,16,17-Octaethylporphyrin
(65 mg, 1.22 10-4 mol) and tin(II) chloride dihydrate (260 mg,
1.15 10-3 mol) were dissolved in pyridine (10 mL) and refluxed
at 115 °C for 2 h. The reaction was monitored by UV-vis
spectroscopy until the apparent four-Q bands were transformed
into the apparent two-Q bands typical of porphyrin metal complexes. After the reaction solution was evaporated to dryness at
(22) (a) Vogel, E.; Koch, P.; Hou, X.; Lex, J.; Lausmann, M.; Kisters, M.;
Aukauloo, M. A.; Richard, P.; Guilard, R. Angew. Chem., Int. Ed. Engl.
1993, 32, 1600. (b) Sessler, J. H.; Hoehner, M. C. Synlett 1994, 211.
(23) Gouterman, M.; Schwart, F. P.; Smith, P. D. J. Chem. Phys. 1973, 59,
676.
(24) Iwamoto, H.; Hori, K.; Fukazawa, Y. Tetrahedron 2006, 62, 2789.
Article
room temperature, the residue was extracted with chloroform and
washed 2 times with 5% HCl aq. The solution was dried with
Na2SO4 and evaporated to dryness. The residue was recrystallized
in chloroform/cyclohexane and formed violet prism-like crystals
(58.8 mg). Yield 67%. UV-vis (in dichloromethane): [λmax/nm] (ε)=
403 (400000); 538 (15300); 575 (14300). Anal. Calcd for C36H44N4Cl2Sn: C, 59.86; H, 6.14; N, 7.76. Found: C, 59.49; H, 6.00; N,
7.75. TOF-MS (MALDI): m/z [M-Cl]þ, 687.23 ([M-Cl]þ calcd
for 687.23). 1H NMR (CDCl3, 293 K): δ [ppm] = 2.06 (t, 24H,
-CH3), 4.24 (q, 16H, β-CH2-), 10.59 (s, 4H, -CH-).
2,3,6,7,12,13,16,17-Octaethylhemiporphycenato Sn(IV) Chloride [SnIV(OEHPc)Cl2] (2). [SnIV(OEHPc)Cl2] (2) was prepared
from H2(OEHPc) in a manner similar to that previously reported.25 2,3,6,7,12,13,16,17-Octaethylhemiporphycene (30 mg,
5.61 10-5 mol) and tin(II) chloride dihydrate (210 mg, 9.31
10-4 mol) were dissolved in dry decalin (36 mL) and refluxed at
200 °C for 0.5 h under a nitrogen atmosphere. The reaction was
monitored by UV-vis spectroscopy. The mixture was filtered
and then evaporated to dryness in vacuo at room temperature.
The residue was purified by column chromatography over
neutral silica gel (4 15 cm) using dichloromethane/methanol
(20:1) as the eluent. The product was then recrystallized from
chloroform/cyclohexane to obtain [SnIV(OEHPc)Cl2] (2) in the
form of violet prism-like crystals (32.4 mg). Yield 80%. UV-vis
(in dichloromethane): [λmax/nm] (ε) = 391 sh (56800); 416
(231900); 515 (4500); 545 sh (5000); 555 (6600); 587 (37900);
601 (35600). Anal. Calcd for C36H44N4Cl2Sn: C, 59.86; H, 6.14;
N, 7.76. Found: C, 59.34; H, 6.18; N, 7.72. TOF-MS (MALDI):
m/z [M-Cl]þ, 687.24 ([M-Cl]þ calcd for 687.23). 1H NMR
(CDCl3, 293 K): δ [ppm] = 1.99-2.09 (m, 24H, -CH3),
4.20-4.30 (m, 16H, β-CH2-), 10.47-10.68 (m, 4H, -CH-).
2,3,6,7,12,13,16,17-Octaethylporphycenato Sn(IV) Chloride
[SnIV(OEPc)Cl2] (3). 2,3,6,7,12,13,16,17-Octaethylporphycene
(14 mg, 2.61 10-5 mol) and tin(II) chloride dihydrate (93 mg,
4.12 10-4 mol) were dissolved in dry decalin (18 mL) and
refluxed at 200 °C for 0.5 h under a nitrogen atmosphere. The
reaction was monitored by UV-vis spectroscopy. The mixture
was cooled at room temperature under aerobic conditions and
then filtered. The solution was evaporated to dryness under
reduced pressure. The residue was extracted with chloroform
and washed 2 times with 5% HCl aq. The solution was dried
with Na2SO4 and evaporated to dryness at room temperature.
The residue was recrystallized from chloroform/benzene and
formed violet prism-like crystals (17.3 mg). Yield 92%. UVvis (in dichloromethane): [λmax/nm] (ε) = 377 sh (52000); 400
(141000); 608 (50000); 626 (46000). Anal. Calcd for C36H44N4Cl2Sn: C, 59.86; H, 6.14; N, 7.76. Found: C, 58.08; H, 5.84; N,
7.42. TOF-MS (MALDI): m/z [M-Cl]þ, 687.14 ([M-Cl]þ calcd
for 687.23). 1H NMR (CDCl3, 293 K): δ [ppm]=1.98-2.03 (m,
24H, -CH3), 4.21-4.30 (m, 16H, β-CH2-), 10.37 (s, 4H, methine).
X-ray Crystallography. All crystals suitable for X-ray analysis
were obtained by recrystallization at room temperature. The
crystals were mounted on a glass fiber, and used for the X-ray
diffraction study. The measurements were made using a Bruker
SMART APEX CCD detector with graphite-monochromated
Mo KR radiation (λ = 0.71073 Å) and a 2 kW rotating anode
generator. The data were collected at 223 K to a maximum 2θ
value of 28.28° in 0.30° oscillations.
The data frames were integrated using SAINT (Version 6.45)
and merged to give a unique data set for the structure determination. Empirical absorption corrections by SADABS26 were
carried out. The structure was solved by a direct method, and
refined by the full-matrix least-squares method on all F2 data
using the SHELX suite of programs.27 The non-hydrogen atoms
::
::
(25) Scholz, P. J. Ph.D. Thesis, University of Koln, Koln,
:: Germany,
:: 1998.
(26) Sheldrick, G. M. SADABS; University of Gottingen: Gottingen,
Germany, 1996.
(27) Sheldrick, G. M. SHELXL97 and SHELXS97; University of
::
::
Gottingen: Gottingen, Germany, 1997.
Inorganic Chemistry, Vol. 48, No. 20, 2009
9855
were anisotropically refined. The hydrogen atoms were included
in the structure factor calculations, but not refined. As for the
complex 2, it has two elements and the ring structure displays
disorder around the central metal, Sn. Thus, an alternative
structure is shown in Figure 1b. The crystal data and details of
the structure determinations are summarized in Table 1.
Photophysical Measurements. The fluorescence quantum
yield (Φf) value of 3 was measured using an absolute photoluminescence quantum efficiency measurement system
(Hamamatsu C9920-02) incorporating an integrating sphere.
To measure the Φf, degassed solutions of 3 in toluene were
prepared and the concentration was adjusted so that the absorbance of the solution at 337 nm would be less than 0.1. The
excitation was performed at 337 nm. In contrast, the determined
Φf values of 1 and 2 are less than 3% by using this method. In the
absolute photoluminescence quantum efficiency measurement,
these values are not correct values from the viewpoint of
sensitivity. Therefore, the Φf values of 1 and 2 were determined
by the comparative method of Williams et al. in which 2,7,12,17tetra-n-propyl porphycene was used as the standard.28 The
solutions of the standard and test samples (1 and 2) with
identical absorbances at the same excitation wavelength can
be assumed to be absorbing the same number of photons. Thus,
a simple ratio of the integrated fluorescence intensities of both
solutions will yield the ratio of the quantum yield values
(Supporting Information, Figure S2). To measure Φf using this
method, degassed solutions of the Sn(IV) complexes (1 and 2) in
toluene were prepared, and the concentration was also adjusted
so that the absorbance of the solution at 337 nm would be 0.1.
The excitation was performed at 337 nm.
The transient photoluminescence was measured using a
streak camera (Hamamatsu C4780) with a laser diode (LD)
(λ=375 nm, pulse width ∼200 ps, and repetition rate ∼100 MHz)
as the excitation source. The fluorescence lifetimes (τS) were
determined by curve fitting using a microcomputer. The measurement was carried out using a toluene solution under an
argon atmosphere.
The transient absorbance spectra were obtained using a laser
flash photolysis system (Unisoku TSP-1000M). To measure the
transient absorbance spectra, strictly degassed solutions
through several freeze-pump-thaw cycles of the Sn(IV) complexes in toluene were prepared, and the concentration was
adjusted so that the absorbance of the solution at 355 nm would
be 0.4-0.5. A Xe arc lamp was employed as the source of the
probe light to follow the spectral changes. For the laser flash
photolysis, a sample was excited with 5 ns pulses (355 nm) from a
Q-switched Nd:YAG laser (Surelite I, Continuum). The triplet
state lifetimes (τT) of Sn(IV) complexes were determined by
fitting the decay of each triplet-triplet absorption, 1: 440 nm, 2:
452 nm, and 3: 418 nm, respectively. The time course of the
absorbance decay was analyzed by single-phase kinetics to
determine the lifetimes of the triplet state (τT). The rate constant
for the oxygen quenching (kq) was determined from a
Stern-Volmer analysis of the triplet lifetime in degassed, air-,
and oxygen-saturated solutions as shown in Supporting Information, Figure S3. The measurements for both the air- and
oxygen-saturated solutions were performed under similar conditions. The concentration of oxygen for the samples in air
(0.0021 M-1) and dioxygen (0.00988 M-1) was used from
published oxygen solubility data.29
For the singlet oxygen phosphorescence measurements, an
air-saturated toluene solution containing the sample in a quartz
cell (optical path length 10 mm) was excited at 354 nm using a
HORIBA SPEX Fluorolog-NIR at room temperature. Each
singlet oxygen quantum yield (ΦΔ) value (1-3) was determined
(28) Williams, A. T. R.; Winfield, S. A.; Miller, J. N. Analyst 1983, 108,
1067–1071.
(29) Montalti, M.; Credi, A.; Prodi, L.; Gandolfi, M. T. Handbook of
Photochemistry, 3rd ed.; CRC Press: Boca Raton, FL, 2006.
9856 Inorganic Chemistry, Vol. 48, No. 20, 2009
Maeda et al.
Figure 1. ORTEP drawing of Sn(IV) complexes (a) 1, (b) 2, and (c) 3 with thermal ellipsoids at 50% probability. Hydrogen atoms and the crystallization
solvents have been omitted for clarity.
Results and Discussion
by treatment of the free base ligands with tin(II) chloride
dehydrate in a high-boiling solvent, such as pyridine or
decalin. The reactions were monitored by UV-vis spectroscopy until the apparent three or four-Q bands are
transformed into the apparent two-Q bands typical of
porphyrin and porphyrin isomer metal complexes.23 The
obtained complexes were characterized by UV-vis, 1H
NMR, and mass spectroscopies, and the crystal structures were determined by X-ray analysis. These complexes show a parent ion resulting from the loss of one
chloro ligand under the MALDI-TOF mass spectral
analysis conditions. All the Sn(IV) complexes suitable
for X-ray analysis were obtained by recrystallization at
room temperature. The crystal structure of the porphyrin
complex, 1, has already been reported,30 but the crystal
structure with chloroform as the solvent is a new one.
Syntheses and Structural Studies of Sn(IV) Complexes.
The Sn(IV)-dichloro complexes, 1, 2, and 3, were obtained
(30) (a) Cullen, D. L.; Meyer, E. F. Chem. Commun. 1971, 616. (b) Cullen,
D. L.; Meyer, E. F. Acta Crystallogr. 1973, B29, 2507.
from the slope of the plot with an intensity at 1270 nm versus the
concentrations of the Sn(IV) complexes based on 2,7,12,17tetra-n-propyl porphycene as the standard (ΦΔ =0.36) as shown
in Supporting Information, Figure S4.
Catalytic Reaction. The catalytic photo-oxygenation of 1,5dihyroxynaphthalene was investigated under aerobic conditions
at room temperature: ([sensitizer]=1.0 μM, [substrate]=3.33 mM,
solv. CH2Cl2-MeOH (9: 1 v/v)). These reaction solutions were
stirred during irradiation by a 500 W tungsten-lamp through a
cutoff filter (TOSHIBA Y-46, >460 nm). The progress of the
reaction was monitored by absorption at 427 nm, typical for the
product, 5-hydroxy-1,4-naphthoquinone (Juglone). The product
was isolated and identified according to previous method.3,5
1
H NMR (CDCl3, 293 K): δ [ppm] = 6.94 (s, 2H), 7.27 (dd,
1H), 7.60-7.65 (m, 2H), 11.90 (s, 1H, OH), GC-MS (EI):
m/z = 174 (Mþ).
Inorganic Chemistry, Vol. 48, No. 20, 2009
Article
9857
Table 1. Crystallographic Data for 1, 2, and 3
parameter
formula
1
2
C37H45N4SnCl5
841.71
monoclinic
P2(1)/n
8.208(2)
17.645(5)
15.195(5)
105.633(6)°
2119.4(11)
2
223(2)
1.391
0.947
10333
3058
0.0718
2510
0.0723,
0.1817
0.0877,
0.1926
C36H44N4SnCl2
722.34
monoclinic
P21
8.4442(15)
13.808(3)
14.573(3)
97.748(3)°
1683.7(5)
2
223(3)
1.425
0.949
9777
5851
0.0280
3308
0.0716,
0.1904
0.1223,
0.2430
3
C36H44N4SnCl2
formula weight
722.34
crystal system
monoclinic
space group
P2(1)/c
a/Å
8.2024(10)
b/Å
13.9702(16)
c/Å
14.3172(17)
β/deg
100.202(3)°
1614.7(3)
V/Å3
Z
2
T/K
223(3)
1.486
Fcalc/g cm-3
-1
μ/mm
0.989
reflections measured
7906
unique reflections
2304
0.0225
Rint
observed data [I > 2σ(I )]
2032
R1 awR2b [I > 2σ(I )]
0.0236,
0.0637
0.0269,
R1 awR2b (all data)
0.0670
P
P
P
2
2 2 P
a
b
R1 = ||Fo| - |Fc||/ |Fo. wR2 = [ w(Fo - Fc ) / w(Fo2)2]1/2.
Figure 2. UV-vis spectra of 1, 2, and 3 normalized at each Soret band in
toluene.
Table 2. Bond Lengths (Å) and Bond Angles (deg) of 1, 2, and 3
1
Sn-Cl
Sn-N1
Sn-N2
Sn-N3
Sn-N4
2.463
2.079
2.087
2.079
2.087
N1-N2
N2-N3
N3-N4
N1-N4
2.941
2.950
2.941
2.950
N1-Sn-N2
N2-Sn-N3
N3-Sn-N4
N1-Sn-N4
89.81
90.19
89.81
90.19
2.593
3.119
3.086
2.881
N1-Sn-N2
N2-Sn-N3
N3-Sn-N4
N1-Sn-N4
81.49
93.96
92.16
91.00
2.630
3.189
2.630
3.189
N1-Sn-N2
N2-Sn-N3
N3-Sn-N4
N1-Sn-N4
79.02
100.98
79.02
100.98
Figure 3. Fluorescence spectra for 1, 2, and 3 in toluene. The optical
densities of all samples are adjusted to be the same at the excitation
wavelength of 400 nm (Abs. = 0.5).
2
Sn-Cl
Sn-N1
Sn-N2
Sn-N3
Sn-N4
2.441
1.990
1.987
2.230
2.049
N1-N2
N2-N3
N3-N4
N1-N4
3
Sn-Cl
Sn-N1
Sn-N2
Sn-N3
Sn-N4
2.462
2.068
2.065
2.068
2.065
N1-N2
N2-N3
N3-N4
N1-N4
Moreover, the X-ray crystal structure of the Sn(IV)
complexes with porphycene and hemiporphycene have
not been revealed and are the first examples of an X-ray
analysis. The X-ray crystal structures of these complexes,
1, 2, and 3, are shown in Figure 1. The central metal Sn for
all the complexes coordinates with the N4 core and has
dichloride ions as the axial ligands.
Table 2 lists the parameters for the coordination environments of the Sn(IV) complexes, 1, 2, and 3. Comparison of all the Sn(IV) complexes show many similarities
but some striking differences. For example, the central Sn
ion is displaced in the mean plane of the four coordination
pyrrole N atoms, but these core shapes are different
(1: square, 2: quadrilateral, and 3: rectangular). Furthermore, the bond length of Sn-Cl (2.463 Å) in 1 is slightly
longer than those of 2 (2.441 Å) and 3 (2.462 Å). This is
presumably attributed to the average Sn-N distances
(1: 2.083 Å; 2: 2.064 Å; 3: 2.067 Å). Therefore, the interaction
Figure 4. Phosphorescence spectra for 1, 2, and 3 in bromobenzene.
between the central Sn and the chloride ion may be
weaker in 1 compared to those in 2 and 3.
Spectroscopic Properties of Sn(IV) Complexes. Figure 2
shows the absorption spectra of 1, 2, and 3 in toluene.
These complexes have characteristic absorption bands
around 400 nm derived from the Soret band and around
600 nm due to the Q-band with large extinction coefficients (ε). In particular, the porphycene Sn(IV) complex 3
exhibits extraordinary large absorption bands (ε ≈
100000) for the Q-band compared to the other Sn(IV)
complexes, 1 and 2. Also, the absorption bands in the
visible region are red-shifted in the order 1, 2, and 3. In
general, it is known that the absorption band of porphyrin and porphyrin isomers are mainly raised from π-π*
transitions of the macrocycle.13 Thus, the red-shifted Qband of 2 and 3 are caused by significant stabilization of
the lowest unoccupied molecular orbital (LUMO) energy
level with a decrease in the symmetry of the tetrapyrrolic
macromolecule.
9858 Inorganic Chemistry, Vol. 48, No. 20, 2009
Maeda et al.
Table 3. Spectroscopic Data of Sn(IV) Complexesa
compound
1
2
3
λabsb/nm (10-4 ε/M -1cm-1)
403(40.0), 538(1.53), 575(1.43)
416(23.2), 587(3.79), 601(3.56)
400(25.0), 608(9.10), 626(8.50)
λfluoc/nm
ΦFc,d
τSc/ps
580.6
604.5
631.5
0.010
0.027
0.125
438 (100%)
733 (100%)
2681 (100%)
λphose/nm
ESf/kJ mol-1
ET /kJ mol-1
713.5
927.0
945.5
207.4
198.5
190.6
167.6
129.0
126.5
a
λabs: absorption maximum; λfluo: fluorescence maximum; λphos: phosphorescence maximum; ES: lowest excited singlet energies; ET: lowest excited
triplet energies. b Solvent, dichloromethane. c Solvent, toluene. d The absolute fluorescence quantum yield value was based on photoluminescent
measurements using an integrating sphere with excitation wavelength at 337 nm. e Solvent, bromobenzene degassed in vacuo. f The singlet energy was
estimated from the intersection point of the normalized absorption and fluorescence spectra.
As shown in Figure 3, the relatively strong emissions for
1, 2, and 3 are exhibited by an excitation wavelength at
400 nm in toluene, and their fluorescence intensities are
significantly influenced by a difference of the ring structure. Also, the phosphorescence bands in the near-infrared
region are red-shifted in the order 1, 2, and 3 for similar
reasons (Figure 4). The fluorescence quantum yield (ΦF)
and singlet lifetime (τS) of the Sn(IV) complexes are
summarized in Table 3. Complex 3 shows a fluorescence
with a maximum at 631.5 nm and quantum yield of ΦF =
0.125 in an argon-saturated solution. This value is significantly higher than that of H2(OEPc) (ΦF = 0.017).8 It is
proposed that the internal conversion of the excited state is
accelerated by the steric repulsion of the ethyl group at the
pyrrole β-position of the H2(OEPc). Thus, it is attributed
to the fact that metalation flattened the ring skeleton of 3,
and the internal conversion was inhibited.
When the fluorescence lifetimes (τS) were measured in
the same solutions, the porphycene Sn(IV) complex 3
showed a very long lifetime (2681 ps) compared to those
of the other complexes (1: 438 ps, 2: 733 ps). The
fluorescence rate constant (kf) were readily calculated as
kf = ΦF/τS using the measured values. As a result, there is
a slight difference in kf for Sn(IV) complexes (1: 2.3 107,
2: 3.7 107, 3: 4.7 107 s-1, respectively). This is
consistent with the higher molar absorption coefficient
(Strickler-Berg equation).31 Moreover, it is expected
that the internal conversion rate constants (kic) are almost
the same values based on the energy-gap law because their
singlet energies are slightly different (207.4, 198.5, and
190.6 kJ mol-1, respectively).32 It is thus conceivable that
the intersystem crossing rate constant (kisc) of 3 is critically slower than those of 1 and 2. Therefore, the fluorescence lifetime (2681 ps) of 3 is quite long by comparison
with that of the other complexes. Similar trends in the
excited state are observed in the case of [Sn(TPrPc)Cl2],
in which TPrPc denotes the 2,7,12,17-tetra-n-propyl
porphycene ligand.33 Therefore, these spectroscopic measurements lead to the suggestion that the long fluorescence lifetime and the slow intersystem crossing rate constant are specific properties of the metalloporphycene.
Triplet Lifetime (τT) of Sn(IV) Complexes and Reactivity with Molecular Oxygen. The photophysical property
(31) Rubio, N.; Prat, F.; Bou, N.; Borrell, J. I.; Teixido, J.; Villanueva, A.;
Juarranz, A.; Ca~nete, M.; Stockert, J. C.; Nonell, S. New J. Chem. 2005, 29,
378.
(32) Englman, R.; Jortner, J. J. Lumin. 1970, 1-2, 134.
(33) [Sn(TPrPc)Cl2] was prepared in a manner similar to that of
[Sn(OEPc)Cl2]. UV-vis (in dichloromethane): [λmax/nm](ε) 391 (134000);
404 (125000); 606 (54000); 625 (89000). Anal. Calcd for C36H44N4Cl2Sn: C,
57.69; H, 5.45; N, 8.41. Found: C, 57.58; H, 5.41; N, 8.38. The fluorescence
quantum yield (ΦF) and lifetime (τs) were measured using the same methods:
ΦF = 0.092, τs = 3397 (98%), 9720 (2%) ps.
Table 4. Triplet Lifetimes (τT) in Aerated and Deaerated Solutions with Respective
Oxygen Quenching Rate Constants (kq), and Singlet Oxygen Quantum Yield (ΦΔ)a
compound τO2/ns τN2/μs
82.1
72.8
52.8
1
2
3
32.6
28.3
49.9
kq(O2)/
M-1 s-1
kp þ knr/s-1
1.23 109 4.81 104
1.39 109 4.75 104
1.92 109 0.14 104
ΦΔb
PTO2c
0.925 0.982
0.971 0.984
0.673 0.999
a
Solvent, toluene. b The quantum yield for singlet oxygen generation
was relatively determined using TPrPc as standard (λex = 354 nm, ΦΔ =
0.36). c The quenching efficiency by molecular oxygen for the excited
triplet state.
Figure 5. Photoirradiation time profile of the photo-oxygenation of 1,5dihydroxynaphthalene (3.33 mM) in the presence of photosensitizer
(1.0 μM) in CH2Cl2-MeOH (9: 1 v/v) in air at room temperature.
data, in particular, the triplet state of the porphyrin,
hemiporphycene, and porphycene complexes, are important to understand the influence derived from a different
macrocycle. The transient absorption spectra for Sn(IV)
complexes were obtained by laser flash photolysis, and
show common bands around 400-450 nm derived from
the triplet-triplet absorption bands (Supporting Information, Figure S5). The triplet lifetimes of the Sn(IV)
complexes were determined by fitting the decay of the
triplet-triplet absorption excited at 355 nm (Abs. = 0.5).
Table 4 lists the values obtained for the triplet lifetimes
and oxygen quenching rate constants (kq), which were
calculated from a Stern-Volmer analysis of the triplet
lifetime in aerated, deaerated, and oxygen-saturated solutions (Supporting Information, Figure S4). The triplet
state of 3 is 49.9 μs in a deaerated solution, much longer
than those of 1 (τT = 32.6 μs) or 2 (τT = 28.3 μs). This
result is the sum of the non-radiative decay and phosphorescence rate constant (1: 4.81 104, 2: 4.75 104,
and 3: 0.14 104 s-1, respectively). Thus, it presumably
suggested that the spin-inversion process from the triplet
state to the ground state is slower than those of 1 and 2
similar to the intersystem crossing process. In contrast,
the oxygen quenching rate constant (kq) of 3 is slightly
Inorganic Chemistry, Vol. 48, No. 20, 2009
Article
9859
Figure 6. Approximate energy-level diagram and schematic representation for the Sn(IV) complexes 1, 2, and 3 based on UV-vis absorption, emission,
and lifetime data of singlet and triplet state. The size of the arrow represents the probable rate of spin-inversion processes (S1-T1, T1-S0).
Scheme 1. Catalytic Photo-oxygenation of 1,5-Dihyroxynaphthalene
by the Singlet Oxygen in Air at Room Temperature
higher by comparison with 1 and 2. It is known that
sensitizers should have a triplet state of appropriate
energy (ET g 95 kJ mol-1) to allow for efficient energy
transfer to the ground state oxygen.1 It was reported that
the rate constant of the energy transfer to the ground state
oxygen depends on the energy of the triplet state. Thus,
the rate of energy transfer is fast enough so that the energy
gap between the triplet state of the sensitizer and the
singlet state of oxygen is small.34 Therefore, the respective
oxygen quenching rate constants are reflected in the ET
values for the Sn(IV) complexes.
The singlet oxygen generating ability of a photosensitizer is evaluated by its quantum yield (ΦΔ). The singlet
oxygen quantum yields (ΦΔ) following laser excitation of
the sensitizer solutions can be obtained from a comparison of the singlet oxygen phosphorescence intensity at
1270 nm with that of an optically matched reference
sensitizer. In the present study, the ΦΔ values of the
Sn(IV) complexes were determined from 2,7,12,17-tetran-propyl porphycene (TPrPc) as the standard (ΦΔ =
0.36).35 These singlet oxygen quantum yields (ΦΔ) are
summarized in Table 4.
The singlet oxygen production ability of the Sn(IV)
complexes are enhanced by the internal heavy-atom
effect, and the highest value of 0.97 for its quantum yield
(ΦΔ) was obtained for the Sn(IV) hemiporphycene complex, 2. On the other hand, the Sn(IV) porphycene complex, 3 showed a lower quantum yield value (ΦΔ = 0.67)
than those for the other Sn(IV) complexes. As described
(34) (a) Ozoemena, K.; Kuznetsova, N.; Nyokong, T. J. Mol. Catal. A
2001, 176, 29. (b) Engelmann, F. M.; Mayer, I.; Araki, K.; Toma, H. E.; Baptista,
M. S.; Maeda, H.; Osuka, A.; Furuta, H. J. Photochem. Photobiol. A 2004, 163,
403.
(35) (a) Redmond, R. W.; Valduga, G.; Nonell, S.; Braslavsky, S. E.;
::
Schaffner, K.; Vogel, E.; Pramod, K.; Kocher, M. J. Photochem. Photobiol.
B 1989, 3, 193. (b) Aramendia, P. F.; Redmond, R. W.; Nonell, S.; Schuster, W.;
Braslavsky, S. E.; Schaffner, K.; Vogel, E. Photochem. Photobiol. 1986, 44, 555.
above, this result is attributed to the fact that the intersystem-crossing rate of 3 is much slower than those of the
other complexes. However, the quenching efficiency
(PTO2) by molecular oxygen for the excited triplet state
of 3 is quite high at about 1. Therefore, the Sn(IV)
porphycene complex 3 is an attractive compound and
excellent candidate as a photosensitizer because of its
extraordinary large absorption bands (ε ≈ 100000) in the
visible region.
Photoreaction Catalyzed by a Sensitizer. The catalytic
photo-oxygenation of 1,5-dihydroxynaphthalene was
investigated as shown in Scheme 1. The reaction was
monitored by absorption at 427 nm, typical for the
product, 5-hydroxy-1,4-naphthoquinone (Juglone, ε =
3370).36 The reaction of singlet oxygen resulted in the
quantitative conversion of 1,5-dihyroxynaphthalene to
5-hydroxy-1,4-naphthoquinone. And also, the catalyst
bleaching of these Sn(IV) complexes were not observed
during the reaction. The respective chemical yields of the
product are shown in Figure 5. In the case of 3, the
photoreaction most effectively proceeded, and the substrate was completely converted to the oxygenated product in 4 h. As summarized in Table 4, the higher values of
0.97 and 0.93 for its ΦΔ are obtained for 1 and 2,
respectively. In contrast, 3 showed a lower quantum yield
value (ΦΔ = 0.67) than those for the other complexes.
Nevertheless, the Sn(IV) porphycene complex 3 showed
very high chemical yields. This result is caused by an
extraordinary large absorption band of 3 in the visible
region. Thus, the advantage of the porphycene over porphyrin and hemiporphycene in the photoreaction is due to
its more effective light absorption in the visible region.
Conclusion
A new insight into the photophysical and photochemical
properties of the porphyrin isomers is crucial to better
understand their function during various photoreactions.
In this study, the Sn(IV)-porphyrin, hemiporphycene, and
porphycene complexes with analogous skeletons were
synthesized, and the differences in their photochemical and
photophysical properties due to the skeleton change were
compared in detail. It became clear that there is a considerable difference between the singlet (S1) and the triplet (T1)
(36) Suchard, O.; Kane, R.; Roe, B. J.; Zimmermann, E.; Jung, C.;
::
Waske, P. A.; Mattay, J.; Oelgemoller, M. Tetrahedron 2006, 62, 1467.
9860 Inorganic Chemistry, Vol. 48, No. 20, 2009
states of 3 and the other complexes, although the spininversion processes (S1-T1, T1-S0) are enhanced by the internal heavy-atom effect of the central Sn metal. The long
fluorescence (τS), the triplet (τT) lifetime, and relatively small
singlet oxygen quantum yield (ΦΔ) of 3 compared to those of
the other complexes are attributed to the porphycene skeleton. Therefore, these results suggested that the heavy-atom
influence on the spin-inversion processes of porphycene is
mildly effective in comparison to those of the porphyrin and
hemiporphycene as shown in Figure 6. Thus, the spin-inversion processes in 3 could be slower than those of 1 and 2.
However, porphycene has an advantage in the photoreaction because of its more effective light absorption in the
visible region and thus is an excellent candidate as a photosensitizer.
Acknowledgment. We are grateful to Prof. E. Vogel of
::
Koln University for a gift of a sample of H2(OEHPc) and
Maeda et al.
useful information. We thank Prof. K. Sakai and Dr. S.
Masaoka of Kyushu University for helping to measure
the transient absorption spectra, Prof. C. Adachi and
Mr. A. Endo of Kyushu University for helping to measure the fluorescence quantum yield and lifetime. This
study was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (No. 460, “Chemistry of
Concerto Catalysis”), the Global COE Program “Science
for Future Molecular System” from the Ministry of
Education, Culture, Sports, Science and Technology
of Japan (MEXT), the a Grant-in-Aid for Scientific
Research (20.02314) and a Grant-in-Aid for Scientific
Research (A) (No. 21245016) from the Japan Society for
the Promotion of Science (JSPS).
Supporting Information Available: The crystallographic data
in CIF format and experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.