Water-soluble porphyrin easily derived from tetraphenylporphyrin: alkyloxo(methoxo)porphyrinatoantimony bromides

Article abstract of DOI:10.1246/cl.2008.886

In order to develop water-soluble porphyrins, alkyloxo-(methoxo) porphyrinatoantimony bromides (alkyl = decyl, dodecyl, and octadecyl) were prepared. These complexes have more than 200 mg/100 g of solubility in aqueous solution. From the analysis of absor

Full text of DOI:10.1246/cl.2008.886

886
Chemistry Letters Vol.37, No.8 (2008)
Water-soluble Porphyrin Easily Derived from Tetraphenylporphyrin:
Alkyloxo(methoxo)porphyrinatoantimony Bromides
Jin Matsumoto,ꢀ Tsutomu Shiragami, and Masahide Yasuda
Department of Applied Chemistry, Faculty of Engineering, University of Miyazaki,
Gakuen-Kibanadai, Miyazaki 889-2192
(Received May 19, 2008; CL-080502; E-mail: jmatsu@cc.miyazaki-u.ac.jp)
In order to develop water-soluble porphyrins, alkyloxo-
Table 1. The solubilities of 1a–1e and 2a–2e in water
(methoxo)porphyrinatoantimony bromides (alkyl = decyl,
dodecyl, and octadecyl) were prepared. These complexes have
more than 200 mg/100 g of solubility in aqueous solution. From
the analysis of absorption spectra and surface tension, it was elu-
cidated that the porphyrin complexes are present as aggregates in
aqueous solution.
na
MWb
Solubility (Cs)c
"=105d
1a
1b
1c
1d
1e
2a
2b
2c
2d
2e
0
1
6
10
18
1
848
862
932
988
1100
876
946
1002
1030
1114
6.9 (0.08)
8.6 (0.10)
7.1 (0.08)
98.6 (1.00)
1.2 (0.01)
11.3 (0.13)
103 (1.09)
210 (2.10)
228 (2.21)
213 (1.92)
5.62
5.01
5.37
5.13
4.90
5.01
3.98
4.61
3.74
5.12
6
There has been a great amount of interest in water-soluble
chromophores utilized as fluorescent probes which are selective-
ly incorporated into specific microorganisms sites in connection
with photodynamic therapy.1 Porphyrins and metalloporphyrins
are typical fluorescent chromophores with a variety of functions.
However, porphyrins are, in general, poorly soluble even in or-
ganic solvents, because of the formation of aggregates through
ꢀ–ꢀ stacking (J- and H-aggregations) between porphyrin rings.2
So far, the synthesis of water-soluble porphyrins has been
achieved through the introduction of ionic groups such as pyri-
dinium,3 sulfonate,4 and phosphonium5 in porphyrin rings. Tet-
raphenylporphyrin (TPP) is a typical and commercially available
porphyrin and can easily be converted into tetraphenylporphy-
rinatoantimony(V) complexes (SbTPP). SbTPP is relatively
soluble in relatively polar solvents such as CH2Cl2, MeCN,
and MeOH due to its cationic character. However, solubility is
still low in water. Here, we report on the synthesis of water-
soluble SbTPP through introduction of long alkyl chains as an
axial ligand (Scheme 1).
According to a previously reported method,6 the prepara-
tions of alkyloxo(hydroxo)tetraphenylporphyrinatoantimony
bromides 1 and alkyloxo(methoxo)tetraphenylporphyrinatoanti-
mony bromides 2 were started from the partial solvolysis of Br
ligand on dibromotetraphenylporphyrinatoantimony bromide
with H2O and MeOH, respectively. This was followed by ligand
exchange of Br with alcohols in the presence of pyridine.7 1 or 2
(5 mg) was suspended in pure water (1 cm3) and was left to stand
for 3 days. The supernatant solution was moved to another vessel
10
12
18
aNumber of methylene units on an axial ligand. Molecular
weight. Saturated concentration in mg/100 g of water. The
values in parenthesis are the saturated concentration in
b
c
mM. Molar absorptivity of Soret band in Mꢁ1ꢂcmꢁ1 (M =
d
molꢂdmꢁ3).
and was diluted with MeOH to measure the absorption spectra of
the solution. Solubility was defined as the saturated concentra-
tion (Cs) which was calculated using absorbance and molar ab-
sorptivity (") with a Soret band. The values of Cs are summariz-
ed in Table 1.
Di(hydroxo)tetraphenylporphyrinatoantimony bromide (1a)
dissolve well in organic solvents with the exception of nonpolar
solvents (e.g. hexane and toluene). However, the Cs of 1a in wa-
ter is low: Cs (1a) = 0.08 mM, even though 1a is cationic and
contained hydrophilic hydroxy groups. Moreover, the Cs’s of
1b–1e are lower than 0.1 mM with the exception of 1d. The pres-
ence of axial HO ligand lowers the Cs, probably because of an
interaction between the porphyrin rings through axial HO li-
gands. On the other hand, the Cs’s of 2b–2e are more than 10
times larger when compared with 1a, as shown in Figure 1.
Usually, the aggregation of porphyrins results in a shift of
the maximum absorption wavelength and/or broadening of the
2
1
0
O-(CH2)n-H
Ph
+
N
N
N
N
Ph
Ph
Sb
Br-
Ph
RO
1a; R = H, n = 0 2a; R = Me, n = 1
1b; R = H, n = 1 2b; R = Me, n = 6
1c; R = H, n = 6 2c; R = Me, n = 10
1d; R = H, n = 10 2d; R = Me, n = 12
1e; R = H, n = 18 2e; R = Me, n = 18
0
4
8
12
16
20
Number of CH2 units (n)
Figure 1. Dependence of Cs on the number (n) of methylene
units on axial ligands of 1a–1e ( ) and 2a–2e ( ).
Scheme 1.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.8 (2008)
887
(CH2)n-H
18
16
14
12
10
OMe
Ph
O
Ph
Ph
N
Ph
N
Sb N
N
N
N
N
N
OMe
Ph
Sb
Ph
N
Ph
Ph
Ph
O
N
N
Sb
N
Ph
O
Ph
H
H
O
Ph
O
Ph
N
N
N
N
Sb
Ph
Ph
A
10-6
10-5
10-4
Ph
O
[2c]/M
(CH2)n-H
B
Figure 2. Dependence of HW on [2c] in water ( ) and in
MeOH ( ).
Figure 4. (A) Edge-to-edge aggregation and (B) face-to-face
dimer.
dimer structure rather than a micelle-like structure, presumably
due to the formation of hydrogen bonds formed between the
HO ligands or OH–ꢀ interactions between the HO ligands and
the porphyrin rings (Figure 4B). In the case of 1d, hydrophobic
van der Waals interaction predominantly stabilizes the aggre-
gates, resulting in high Cs.
In conclusion, the water-soluble porphyrin complexes 2b–
2e12 have been conveniently synthesized from TPP. Moreover,
Cs was controlled by the number of methylene units (n) from
6 to 12 in the alkyl chain.
70
60
50
40
10-6
10-5
10-4
[2c]/M
References and Notes
1
R. P. Haugland, in Handbook of Fluorescent Probes and Research
Products, 9th ed., ed. by J. Gregory, Molecular Probes, Eugene,
OR, 2002.
Figure 3. Plots of surface tension (ꢁ) against [2c] in aqueous
solution.
band.3,8 In the case of 2b–2e, the broadening takes place but the
maximum absorption wavelength does not shift. Figure 2 shows
an example of the dependence of the peak-width at the half
height (HW) of the Soret band on the concentration of 2c
([2c]).9 The HW of 2c in water remains constant at 14 nm when
[2c] < 8 ꢃ 10ꢁ6 M. With an increase of [2c] up to 2:0 ꢃ 10ꢁ5 M,
HW increases from 14 to 17.4 nm. HW remains constant at
17.4 nm when [2c] > 2:0 ꢃ 10ꢁ5 M. On the other hand, the
HW of 2c in MeOH remains constant at 11.7 nm irrespective
of the increase of [2c], suggesting that 2c does not aggregate
in MeOH. Therefore, it is suggested that 2c behaves as aggre-
gates in aqueous solution.
The surface tensions (ꢁ) were measured for 2c in aqueous
solution (Figure 3).10 Upon the increase of [2c], ꢁ decreases
until [2c] reaches 1 ꢃ 10ꢁ5 M and remains constant when [2c]
> 1 ꢃ 10ꢁ5 M. It is well known that the breakdown point corre-
sponds to the critical micelle concentration of various surfac-
tants.11 Therefore, the results of HW and ꢁ shows that 2c affords
aggregates of a uniform size and shape in aqueous solution at
concentrations above 1 ꢃ 10ꢁ5 M.
2
a) P. Hambright, in The Porphyrin Handbook, ed. by K. M.
Kadish, K. M. Smith, R. Guilard, Academic Press, New York,
2000, Vol. 3, Chap. 18, pp. 129–210. b) S. Okada, H. Segawa,
J. Am. Chem. Soc. 2003, 125, 2792.
K. Kano, M. Takei, S. Hashimoto, J. Phys. Chem. 1990, 94, 2181.
X. Zhang, K. Sasaki, Y. Kuroda, Bull. Chem. Soc. Jpn. 2007, 80,
536.
R.-H. Jin, S. Aoki, K. Shima, J. Chem. Soc., Faraday Trans. 1997,
93, 3945.
T. Shiragami, J. Matsumoto, H. Inoue, M. Yasuda, J. Photochem.
Photobiol., C 2005, 6, 227.
3
4
5
6
7
A typical example was the preparation of 2c that was performed by
the reaction of bromo(methoxo)tetraphenylporphyrinatoantimony
bromide (40 mg) with decanol (50 cm3) in MeCN–pyridine
(40:1, 41 cm3) at 65 ꢄC. After evaporation, 2c was isolated by
column chromatography on SiO2. Yield 55%. 1H NMR (400
MHz, CDCl3): ꢂ ꢁ2:57 (t, J ¼ 6:1 Hz, 2H), ꢁ2:19 (s, 3H),
ꢁ2:00{ꢁ1:93 (m, 2H), ꢁ1:63 (quint, J ¼ 7:6 Hz, 2H), ꢁ0:34
(quint, J ¼ 7:6 Hz, 2H), 0.33 (quint, J ¼ 7:6 Hz, 2H), 0.68 (quint,
J ¼ 7:6 Hz, 2H), 0.81 (t, J ¼ 7:3 Hz, 3H), 0.89–0.96 (m, 2H),
1.00–1.07 (m, 2H), 1.16 (sextet, J ¼ 7:3 Hz, 2H), 7.92–8.02 (m,
12H), 8.29 (d, J ¼ 6:8 Hz, 4H), 8.36 (d, J ¼ 6:8 Hz, 4H), 9.56
(s, 8H); 13C NMR: ꢂ 14.03, 22.54, 23.17, 27.67, 28.25, 28.82,
29.02, 29.03, 31.68, 45.86, 58.02, 122.96, 127.95, 128.11,
130.03, 133.87, 134.73, 134.83, 138.12, 146.01; UV–vis (MeOH)
High Cs’s are achieved in 2b–2e with axial long alkyloxo
ligands, but Cs’s of 2a and 1 are extremely lower than that of
2b–2e. Therefore, the presence of axial long alkyloxo ligands
as well as the absence of an axial HO ligand are requisite for
higher Cs in water. 2b–2e prefer the micelle-like structure
through the hydrophobic interaction of long alkyl chains and
an edge-to-edge interaction of porphyrin rings (Figure 4A). This
is supported by 1H NMR spectra of 2c (1 mM) in D2O.9 The al-
kyloxo ligand is strongly affected by the neighboring porphyrins,
resulting in the higher field shifts of methylene protons com-
pared with those in CD3OD. Conversely, 1 forms a face-to-face
ꢃ
K. Kano, K. Fukuda, H. Wakami, R. Nishiyabu, R. F. Pasternack,
max/nm ("=104 Mꢁ1ꢂcmꢁ1) 419 (46.1), 551 (1.89), 590 (1.09).
8
9
J. Am. Chem. Soc. 2000, 122, 7494.
Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/.
10 The surface tensions were measured on a Kyowa Kaimen Kagaku
CBVP-Z at 25 ꢄC.
11 M. J. Rosen, Surfactants and Interfacial Phenomena, 2nd ed.,
Wiley, New York, 1989.
12 2b–2e are also soluble in organic solvents (CH2Cl2, MeOH, and
MeCN).
Published on the web (Advance View) July 19, 2008; doi:10.1246/cl.2008.886