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Chemistry Letters Vol.35, No.1 (2006)
Photophysicochemical Properties of Chlorophyll-a Adsorbed on Mg-containing Mesoporous Silica
Takahiko Nakamura,1 Masato Takeuchi,1 Hiromi Yamashita,2 and Masakazu Anpoꢀ1
1Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University,
1-1 Gakuen-cho, Sakai, Osaka 599-8531
2Department of Materials Science and Processing, Graduate School of Engineering, Osaka University,
2-1 Yamadaoka, Suita, Osaka 565-0871
(Received October 14, 2005; CL-051310; E-mail: anpo@chem.osakafu-u.ac.jp)
Chlorophyll-a (chl-a) was found to easily change to pheo-
phytin-a during its adsorption on hexagonal mesoporous silica
(HMS) since the centered Mg ions of chl-a easily react with
silanol groups as Brꢀnsted acid sites (pheophytinization). This
pheophytinization process could be suppressed by adding small
amounts of Mg species into the frameworks of HMS. The highly
dispersed Mg species work as adsorption sites for chl-a and/or
the porphyrin ring of pheophytin-a, resulting in the formation
of an original chl-a like complex species.
washed with distilled water, dried at 373 K for 24 h, crushed with
an agate mortar and then calcined in air at 873 K for 6 h in order
to remove the template. Mg-HMS was prepared using the same
procedure by adding MgCl2 into a precursor solution. Mg-
HMS100, thus, refers to HMS including Mg in its framework
structure with a Si/Mg ratio of 100.
The adsorption of chl-a on the mesoporous silica was carried
out by adding HMS powders into 0.5 mL of 1:0 ꢁ 10ꢂ4 M chl-a/
toluene solution. After stirring at 295 K under dark conditions
for 2 h, the samples were dried at 323 K for 24 h to evaporate
the toluene solvent. Characterization studies were carried out
by XRD (Shimadzu, XRD–6100) and UV–vis absorption
(Shimadzu, UV–2200A) measurements in air at 295 K.
Figure 1 shows the XRD patterns of HMS and Mg-HMS100.
A diffraction peak at around 2 degrees attributed to the hexago-
nal mesoporous structure can clearly be observed, however, the
peak width was quite broad, showing that the wall of HMS is
much wider as compared to MCM-41 and FSM-16. Using the
Sherrer’s equation, the pore sizes of HMS and Mg-HMS100
were estimated to be 5.09 and 4.80 nm, respectively. Since the
size of chl-a is estimated to be ca. 2 nm, both HMS and Mg-
HMS can be considered to be good host materials to adsorb
and incorporate chl-a in their pores. The incorporation of Mg
ions within the HMS matrices was found to decrease the inten-
sity of the diffraction peak. Mg-HMS10 (Si/Mg ¼ 10) did not
show any diffraction peaks. The BET surface areas are summa-
rized in Table 1. The surface area of Mg-HMS100 became
smaller than that of the original HMS of ca. 580 m2/g by the
incorporation of Mg into its framework. These results clearly
indicate that some part of the mesoporous structure of HMS
has collapsed by including Mg ions within the SiO2 matrices,
although the mesostructure of the Mg-HMS100 has been
maintained.
Chlorophyll-a (chl-a) dye plays an important role in photo-
synthesis, one of the most efficient photochemical processes in
converting sunlight energy into chemical energy. In recent years,
researchers in organic chemistry have tried to develop artificial
photosynthetic procesess by using photofunctional dyes, fuller-
ene derivatives and carbon nano-tubes1,2 while the immobiliza-
tion of these photofunctional compounds onto the solid surfaces
has been actively investigated,3–8 since pioneering works9 by
Turro et al. and Anpo et al. The chl-a itself is unstable and easily
changes to pheophytin-a owing to the lack of centered Mg ions
in acidic conditions, as shown in Scheme 1.3 Kuroda et al. have
used mesoporous silica (e.g., C18FSM) as a host material to
adsorb chl-a and reported that the C18FSM modified with diol
can stabilize the chl-a adsorbed on such solid surfaces.10 In the
present work, we have investigated the stabilization of chl-a
on Mg-containing mesoporous silica.
The hexagonal mesoporous silica (HMS) was prepared
by procedures in previous literature.11 Tetraethoxyorthosilicate
(TEOS) as a SiO2 source, dodecylamine (DDA) as a template,
2-propanol, ethanol, H2O, and HCl were mixed at a molar
ratio of TEOS:DDA:EtOH:i-PrOH:HCl:H2O = 1:0.27:6.54:
1:0.02:36.3. The mixture solution was stirred at 295 K for 24 h
and then filtered. The obtained white powders were thoroughly
12000
10000
a
8000
b
6000
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θ / degree
Scheme 1. Molecular structure of: (a) chlorophyll-a; and (b)
pheophytin-a.
Figure 1. XRD patterns of: (a) HMS and (b) Mg-HMS100.
Copyright Ó 2006 The Chemical Society of Japan
Nakamura, Takahiko
