Porphyrin/MgCl2/silica gel composite as a cobalt-free humidity indicator

Article abstract of DOI:10.1246/cl.2007.1246

In order to develop cobalt-free humidity indicator, the composite consisted of porphyrin, MgCb, and silica gel was prepared. The composite exhibited a reversible color change from green to purple under dry and humid conditions, respectively. Copyright

Full text of DOI:10.1246/cl.2007.1246

1246
Chemistry Letters Vol.36, No.10 (2007)
Porphyrin/MgCl2/Silica Gel Composite as a Cobalt-free Humidity Indicator
Yoshiyuki Fueda,ꢀ1 Jin Matsumoto,2 Tsutomu Shiragami,2 Kazunori Nobuhara,1 and Masahide Yasudaꢀ2
1Fuji Silysia Chemical Ltd., 16303-3, Kihara, Hichiya, Hyuga, Miyazaki 883-0062
2Department of Applied Chemistry, Faculty of Engineering, University of Miyazaki, Gakuen-Kibanadai, Miyazaki 889-2192
(Received July 24, 2007; CL-070786; E-mail: yasuda@cc.miyazaki-u.ac.jp)
In order to develop cobalt-free humidity indicator, the
MCl2
PCl2TPPCl
SiO2
PCl2TPP+/SiO2
1a
H4TPP2+/SiO2/MCl2
composite consisted of porphyrin, MgCl2, and silica gel was pre-
pared. The composite exhibited a reversible color change from
green to purple under dry and humid conditions, respectively.
1) MeOH–H2O
2) dryness
at 130 °C
1) MeOH
2) dryness
at 130 °C
1b; M= Mg
1c; M= Ca
1d; M= Sr
Ph
Cl
Ph
+
2+
Ph
NH HN
N
N
N
P
Ph
Ph
Silica gel (SiO2) is most commonly used as a desiccant. The
ability of desiccant is usually checked by use of a CoCl2/SiO2
humidity indicator which is transformed to Co(H2O)6/SiO2 un-
der humid conditions, revealing a color change from blue to pur-
ple. Recently, considerable caution has been paid in using
CoCl2/SiO2 humidity indicators because IARC has determined
that the CoCl2 is carcinogenic to humans.1 In Europe, for exam-
ple, the use of CoCl2/SiO2 humidity indicators in desiccants
placed in the packages are controlled by by-law.2 Therefore,
we need to develop a safe humidity indicator without carcino-
genic chemicals.
Recently, commercially available cobalt-free humidity indi-
cators have been developed. However, these materials have
showed only small color change between dry and humid condi-
tions compared with CoCl2/SiO2. NeoBLUE3 and Chameleon
C,4 for example, showed the increase of absorbance at 533 nm
and the shift of the absorption maximum from 460 to 510 with
a decrease of absorbance under humid conditions, respectively.
Therefore, our attentions are paid to develop the humidity
indicator having large color change.
Porphyrins can absorb visible light to exhibit a characteristic
color depending on the central metal, the solvent, pH, and aggre-
gation with other chromophores.5 In particular, free-base tetra-
phenylporphyrin (H2TPP) are sensitive to pH. Indeed, by addi-
tion of aqueous HCl solution to CHCl3–MeOH solution of
H2TPP,6 the maximum absorbance of H2TPP largely shifted
from 514 to 658 nm, as shown in Figure 1A(a, b). The absorption
at 658 nm can be assigned to be the protonated porphyrin
(H4TPP2þ). If pH can be easily controlled with humidity in
SiO2, the porphyrin chromophores can operate as a humidity
N
Ph
NH HN
-
-
Ph
Cl
2Cl
Cl
PCl2TPP+Cl
Ph
-
H4TPP2+(Cl )2
-
Scheme 1. Synthesis of the composites (1a–1d).
indicator of SiO2.
As has been reported earlier,7,8 H2TPP has very low solubil-
ity in MeOH but dichloro(tetraphenylporphyrinato)phosphorous
chloride (PCl2TPPCl) is more soluble due to the existence of a
cationic complex. Moreover, the PCl2TPPCl was found to be
non-toxic: LD50 > 2 g/kg.9 At first, therefore, we began by
preparing the composite between SiO2 and PCl2TPPCl. Into a
MeOH solution (15 mL) of the PCl2TPP (1.25 mg), SiO2 (25 g,
particle size 1.7–4.0 mm, surface area 636 m2/g) were added.
Almost PCl2TPPCl was adsorbed on SiO2. The particles were
filtrated, and heated at 130 ꢁC overnight to yield the purple
PCl2TPPþ/SiO2 composite (1a; content of PCl2TPPCl =
50 ppm) (Scheme 1). Microscopic analysis10 showed that an ab-
sorption band of 1a appeared at 557 and 596 nm (Figure 1B(d)).
These absorptions corresponded to a characteristic absorption
band of PCl2TPP, which appeared at 566 and 610 nm in MeOH
(Figure 1A(c)).
Under dry conditions
O-
Si
Si
Si
OH
OH
M2+
+ 2 HCl
(1)
O
+
MCl2
O
O-
Si
M = Mg, Ca, Sr
∆, H+
H2TPP + H3PO4 + 3 HCl
H4TPP2+(Cl-)2
PCl2TPPCl + 4 H2O
H2TPP + 2 HCl
(2)
(3)
0.4
0.3
0.2
0.1
0
0.4
0.3
0.2
0.1
0
(A)
(B)
e
Under humid conditions
b
c
d
O-
Si
OH
Si
a
M2+
O
+ 2 H2O
O
+ M(OH)2
(4)
(5)
O-
OH
Si
Si
H4TPP2+(Cl-)2 + M(OH)2
H2TPP + MCl2 + 2 H2O
500
600
700
500
600
700
Wavelength/nm
Wavelength/nm
When 1a stood under the humid conditions for 1 h, however, 1a
maintained entirely its original absorption, showing no color
change (Figure 1B(e)).
Recently, it has been reported that acidic conditions were
made by the reaction of SiO2 with CaCl2 (eq 1).11 Therefore,
Figure 1. (A) Absorption spectra of H2TPP in MeOH (a),
H4TPP2þ in CHCl3–MeOH solution (b), and PCl2TPPCl in
MeOH (c). (B) Microscopic analysis of 1a under dry (d) and
humid conditions (e).
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.10 (2007)
1247
1
0.8
0.6
0.4
0.2
0
a
(A)
0.15
b
c
0.1
0.05
0
Dry RH20% 60%
90%
Green Blue Purple Purple
500
600
700
Wavelength/nm
0
20
40
RH/%
60
80
100
(B)
0.15
0.1
0.05
0
a
b
c
Figure 3. Dependence of absorption fraction (F) in absorption
spectra of 1b on relative humidity (RH): F ¼ A514=ðA514
þ
A649Þ.
reversibility of the coloration ability.
500
600
700
In conclusion, the porphyrin/MgCl2/SiO2 composite 1b
was developed as a new type of cobalt-free humidity indicator.14
It showed the spectral change of 135 nm between dry and humid
conditions. It was sensitive to the moisture on even low RH and
will be safely used since H2TPP is strongly adsorbed on SiO2
and 1b does not include the carcinogenic chemicals.
Wavelength/nm
Figure 2. Microscopic analysis of H4TPP2þ/MCl2/SiO2 1b–
1d under dry (A) and humid (B) conditions; 1b (a), 1c (b), and
1d (c): [MCl2] = 7.2 mM.
we intended to prepare the porphyrin/MCl2/SiO2 composites
1b–1d using alkaline earth metal chlorides (MCl2 = MgCl2,
CaCl2, and SrCl2). The composites were prepared by mixing
of SiO2 (25 g) with MeOH solution (15 mL) of PCl2TPPCl
(1.25 mg) and aqueous solution (1 mL) of MCl2 (7.2 mM), and
followed by filtration and dryness at 130 ꢁC overnight to yield
1b–1d (Scheme 1). Under dry atmosphere, 1b–1d had a strong
absorption band at 649 nm due to the formation of H4TPP2þ
(Figure 2A). Therefore, the elimination of the central metal
from PCl2TPPþ complexes (eq 2) and the protonation took place
nearly quantitatively during the preparation (eq 3). It is well
known that PCl2TPPþ undergoes demetallation under acidic
conditions.5 Among the three H4TPP2þ/MCl2/SiO2, the great-
est absorbance at 649 nm was observed in H4TPP2þ/MgCl2/
SiO2 (1b) (Figure 2A).12 However, SrCl2 was ineffective for
the formation of H4TPP2þ/MCl2/SiO2 composite at all. As
the results of the preparation of 1b using various concentrations
of MgCl2 (0–7.2 mM), the optimum concentration of MgCl2 was
determined to be 5.3 mM.
After 1b–1d stood under humid conditions for 1 h, the
absorbance at 649 nm (A649) decreased while new absorption
appeared at 514 nm with large absorbance along with weak
absorptions at 552, 582, and 640 nm, resulting in a color change
from green to purple (Figure 2B). The resulting absorptions can
be unambiguously assigned to be the absorptions of H2TPP (514,
550, 589, and 645 nm. See Figure 1A(a)). It is suggested that
MCl2/SiO2 reacted with H2O to give M(OH)2, reverting pH
from acidic to neutral, as has been reported (eq 4).8 Thus the
deprotonation from green H4TPP2þ took place to give purple
H2TPP under humid conditions (eq 5).
References and Notes
1
International Agency for Research on Cancer, 1991, Vol. 52,
WHO.
2
3
Consolidated Version of Directive EU/67/548/EEC.
a) Moisture indicator (NeoBLUE) using neutral red
(C15H17N4Cl; pH indicator) is commercially availabe. b)
T. Oe, M. Hamada, S. Yoshida, Jpn. Kokai Tokkyo Koho
2002350419, 2002; Chem. Abstr. 2002, 137, 384155.
S. Moreton, Mater. Res. Innovations 2002, 5, 226.
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.
4
5
6
Aqueous HCl solution (0.5 mL; pH 2.0) was added to
CHCl3–MeOH solution (1:1; 5 mL) of H2TPP (10ꢂ5 M) to
give H4TPP2þ
.
7
8
9
H. Segawa, K. Kunimoto, A. Nakamoto, T. Shimidzu, J.
Chem. Soc., Perkin Trans. 1 1992, 939.
R. P. S. Pandian, T. K. Chandrasekhar, V. Chandrasekhar,
Indian J. Chem. 1991, 30A, 579.
Y. Fueda, H. Suzuki, Y. Komiya, Y. Asakura, T. Shiragami,
J. Matsumoto, H. Yokoi, M. Yasuda, Bull. Chem. Soc. Jpn.
2006, 79, 1420.
10 Microscopic analysis was performed on an Olympus FV-300
equipped with a spectrophotometer (Seki Technotron, STFL-
250).
11 L. G. Gordeeva, I. S. Glaznev, E. V. Savchenko, V. V.
Malakhov, Y. I. Aristov, J. Colloid Interface Sci. 2006,
301, 685.
We investigated whether the color changes were dependent
on humidity. According to JIS method,13 1b was exposed for
48 h to the air whose relative humidity (RH) was adjusted to 0,
20, 50, and 90% by an aqueous solution of sulfuric acid. The
color change was evaluated by the fraction of A514 (F) in absorp-
12 The pH on the composite was estimated to be lower than
3.1 by the comparison of absorption spectra of H2TPP in
aqueous solution adjusted to various pH (see Supporting
Information).
13 Japanese Industrial Standard (JIS Z0701), Silica gel desic-
cants for packaging, Japanese Standards Association (1977).
14 a) Y. Fueda, Jpn. Kokai Tokkyo Koho 2007010642, 2007. b)
Y. Fueda, U.S. Patent 20060270047, 2006; Chem. Abstr.
2006, 145, 510809.
tion spectra of 1b after standing under given RH: F ¼ A514
=
ðA514 þ A649Þ. As shown in Figure 3, the F value was remarkably
changed up to 20% of RH. Moreover, the humid purple 1b was
dried again at 130 ꢁC and returned to a green color, showing the
Published on the web (Advance View) September 15, 2007; doi:10.1246/cl.2007.1246