Functionalization of silica surface by tetrahydroxyporphyrin via Si-olinkages

Article abstract of DOI:10.1246/bcsj.20100317

To construct an interface between a π-conjugated organic molecule and an inorganic/metal surface, 5,10,15,20-tetrakis[4-(5-hydroxypentyloxy)phenyl] porphyrin was chemisorbed on silica gel by refluxing in pyridine. Thermogravimetric analysis confirmed that

Full text of DOI:10.1246/bcsj.20100317

794
Bull. Chem. Soc. Jpn. Vol. 84, No. 7, 794-801 (2011)
© 2011 The Chemical Society of Japan
Functionalization of Silica Surface by Tetrahydroxyporphyrin
via Si-O Linkages
Nagisa Minamijima, Nao Furuta, Shintaro Wakunami, and Tadashi Mizutani*
Department of Molecular Chemistry and Biochemistry, Faculty of Science and Engineering,
Doshisha University, Tatara-miyakodani, Kyotanabe, Kyoto 610-0321
Received November 15, 2010; E-mail: tmizutan@mail.doshisha.ac.jp
To construct an interface between a ³-conjugated organic molecule and an inorganic/metal surface, 5,10,15,20-
tetrakis[4-(5-hydroxypentyloxy)phenyl]porphyrin was chemisorbed on silica gel by refluxing in pyridine. Thermogravi-
metric analysis confirmed that 1.5-6.3 mg tetrahydroxyporphyrin was adsorbed on 50 mg of silica gel. To functionalize
the surface of a silicate glass plate with the porphyrin, a spin-coated film of the porphyrin on the glass plate was heated at
80-296 °C, followed by sonication in pyridine to remove the unreacted porphyrin. Heating above 130 °C gave a
monolayer film. The reaction proceeds between two solid phases since the melting point of the porphyrin was 293 °C. The
threshold temperature for the reaction to proceed was 90, 160, and 250 °C for 5,10,15,20-tetrakis[4-(5-hydroxypentyl-
oxy)phenyl]porphyrin, 5,10,15,20-tetrakis[4-(5-acetoxypentyloxy)phenyl]porphyrin, and 5,10,15,20-tetrakis(4-hexyloxy-
phenyl)porphyrin, respectively. The different reactivities of the alcohol, ester, and ether functional groups could be used
to construct highly sophisticated organic-inorganic surfaces.
For the development of electronic devices by use of
Hoffman, and co-workers have demonstrated that multivalency
of the organic molecule can regulate the orientation of discotic
molecules to the metal surface.29 The object of this study is to
establish a procedure to covalently bind a porphyrin molecule
onto the surface of silicon oxide.30,31 We have studied the
reaction between tetrahydroxyporphyrin and silica to function-
alize the silica surface with porphyrin via silicate ester
linkages, and have characterized the resulting porphyrin layer
on the SiO2 surface.
Reaction of alcohols with silica gel has been the subject
of active investigations.32,33 The alcohols employed have
been limited to relatively simple ones such as ethanol, butanol,
and octanol.34,35 The reaction shown in Scheme 1 is a revers-
ible reaction, and proceeds in both directions depending on the
reaction conditions. The esterification proceeds by a nucleo-
philic attack of alcohol oxygen on silicon, while hydrolysis
occurs by a nucleophilic attack of water on silicon, i.e., via
silyl-oxygen fission.36 Ballard and co-workers have reported34
that the esterification reaction is endothermic, and proceeds
at elevated temperatures under anhydrous conditions.
The reaction can also be performed by continuous removal
of water by azeotropic distillation.37 Recently, the structure
of alcohols and the stability of the resulting silicic acid ester
was studied, and branched alcohols and polyols such as
poly(vinyl alcohol) formed a more stable ester than did a
simple alcohol.38
semiconducting organic molecules, control of the interfacial
structure between the organic active layer and metallic/
inorganic electrodes is important, particularly for nanoelec-
tronics fabrication.1-4 For instance, electric conducting poly-
mers have been used to cover an electrode to have good
communication between the electrode and the active layer of
electronic devices.5-7 The structures of the interfaces have been
characterized by spectroscopic8-11 and microscopic tech-
niques.12,13 Ishii and co-workers have studied the UV photo-
emission spectra of several porphyrin derivatives deposited on
Mg, Ag, and Au, and have revealed the electronic structures of
porphyrins and the interface between the porphyrins and the
metals.14,15
There have been several approaches to construct an interface
between organic molecules and metal surfaces. Reaction of
thiols with Au was used to construct metal-molecule-metal
junctions.16 Zaera and co-workers have reported the preparation
of a porphyrin monolayer on Si(100) via Si-C bonds, and
revealed that the electron-transfer rate between the porphyrin
and silicon depended on the linker structures.17 It is well-
known that the oxide layer forms on the surface of metallic
Si under ambient conditions.11 Our strategy18-20 is to use
the thin oxide layer on the surface of silicon21-25 to attach
³-conjugated molecules covalently where the oxide layer can
be formed under controlled conditions. The reaction is a
reverse of the sol-gel process, in which the organic groups
are eliminated from the inorganic particle as an alcohol.26,27 We
envisioned that covalent attachment of the organic molecules
could result in a stable and more ordered interface than simple
deposition of organic layer on the metal surface.28 We
employed porphyrin with four reactive groups to attach to
the SiO2 surface through multiple covalent bonds. Mirkin,
Other approaches to the preparation of porphyrin-silica
hybrid materials are to use trialkoxysilane as a coupling
reagent. Battioni and co-workers have reported39 that poly-
condensation of triethoxypropylamino-substituted iron porphy-
rin or cocondensation of it with tetraethoxysilane to prepare a
new catalyst for alkene epoxidation or alkane hydroxylation.
Compared to using silane coupling reagents, advantages of
Published on the web June 25, 2011; doi:10.1246/bcsj.20100317

N. Minamijima et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 7 (2011)
795
Si
Si
O
Si
O
O
Si
Si
O
OH
ROH
+
O
Si
O
OR
+ H2O
Si
Si
Scheme 1.
R
O
OH
R
NH
N
NH
N
O
O
HO
OH
N
HN
N
HN
R
O
OH
1: R = OH
4
2: R = OAc
3: R = CH3
R
Chart 1. Porphyrin derivatives 1, 2, 3, and 4.
using alcohol to modify silica gel and silicate glass are twofold:
(1) alcohols are more stable than silicate esters, which are
susceptible to hydrolysis, and the preparation and purification
of alcohols is more facile than silicate esters, and (2) reaction
of trialkoxysilane with silica gel gives the self-condensation
products of trialkoxysilane itself in a side reaction in the
presence of water, and once they are formed, their removal is
difficult. Alcohols do not give such by-products under the
condensation reaction conditions.
We report here that the tetrahydroxyporphyrin was adsorbed
on silica gel and silicate glass either in pyridine or by use of a
solid-solid reaction. The resulting porphyrin on silicate glass
was stable in various solvents, and potentially useful as a
substrate for further chemical modifications. In the solid-solid
reaction between the tetrahydroxyporphyrin and silicate glass,
the porphyrin forms a J-aggregate, as the coverage with the
porphyrin was higher. The alcohol, ester, and ether functional
groups exhibited different reactivity in the reaction with silicate
glass, which could be used to build an organic interface on
silicate glass in a more sophisticated fashion.
a Shimadzu Multispec-1500 spectrophotometer. Silica gel,
CARiACT Q-6, was purchased from Fuji Silysia Chemical Ltd.
It had a specific surface area of 451 m2 g¹1. 18 mm © 18 mm
Borosilicate cover glass plates, Matsunami Trophy, were used
for reactions with porphyrins.
Reaction of 1 with Silica Gel. Porphyrin 1 (9 mg, Chart 1)
was dissolved in pyridine (0.5 mL) and 50 mg CARiACT Q-6
was added. The suspension was heated at 110 °C for 1 h and the
pyridine was evaporated over a period of 2 h to dryness. The
silica gel was collected by suction filtration, and was washed in
pyridine in a bath sonicator. The red silica gel was collected by
suction filtration and dried in vacuo. The silica gel was
subjected to TG-DTA.
Reaction of 1 with Silicate Glass. A glass plate was
washed with a solution of H2SO4:H2O2 (3/1) for 1 h at 80 °C,
with water for 30 s, and then with water for 10 min in a
bath sonicator. The glass plate was then dried with N2 gas.
A pyridine solution of 1 (60 ¯L, 4.2 mM) was spin-coated on
the silicate glass plate at 3000 rpm for 60 s. The porphyrin film
on the glass was heated at 80-296 °C for 10 min using a hot
stage. The silicate glass was then washed in pyridine in a bath
sonicator to remove unreacted porphyrin. The resultant glass
adsorbing porphyrin was analyzed by UV-visible spectroscopy.
Experimental
Instruments.
Thermogravimetric (TG) analysis and
differential scanning calorimetry (DSC) were performed using
a Shimadzu DTG-60A and DSC-60, respectively. A hot stage
Mettler-Toledo FP82HT was used for heat treatment of
the spin-coated films. UV-visible spectra were obtained
with a Perkin-Elmer Lambda 950 spectrophotometer or with
Results and Discussion
Preparation of Tetrahydroxyporphyrin. We prepared a
porphyrin bearing four hydroxy groups in the periphery
according to Scheme 2. Four hydroxy groups were attached

796
Bull. Chem. Soc. Jpn. Vol. 84, No. 7 (2011)
Functionalization of Silica by Porphyrin
OR
CHO
NH
N
N
a
+
RO
OR
N
H
HN
OCH3
OR
5: R = CH3
b
4: R = H
Figure 1. Silica gel, CARiACT Q-6, functionalized with 1.
OR
100
w1'
O
80
c
RO
NH
N
N
60
O
O
1 on SiO2
1
HN
SiO2
OR
w0'
40
O
2: R = Ac
20
d
1: R = H
OR
Scheme 2. Synthesis of tetrahydroxyporphyrin 1. Reagents
and conditions: a propionic acid, reflux; b pyridinium
hydrochloride, 22 h, reflux; c Br(CH2)5OCOCH3, K2CO3,
18-crown-6, 60 °C, 2 h; d NaOCH3, THF, reflux 3 h.
200
400
600
800
Temperature/°C
Figure 2. TG traces of 1 adsorbed on the silica gel, 1, and
the silica gel. The sample was heated at a rate of 15
°C min in air. Porphyrin 1 (9 mg) was allowed to react
with CARiACT (50 mg) at 110 °C in pyridine (0.5 mL).
Evaporation of the pyridine, addition of another portion of
pyridine, and heating at 110 °C was repeated three times.
to the porphyrin core through a flexible spacer of the
pentamethylene groups. Porphyrin 1 was characterized by
1H NMR, mass spectroscopy, and combustion analysis. Porphy-
rin 1 was soluble in pyridine, while it was sparingly soluble in
chloroform and dichloromethane. For control experiments,
porphyrins having ester, ether, and phenolic OH functional
groups 2, 3, and 4 were employed (Chart 1).
¹1
Reaction of Porphyrin 1 with Silica Gel. A reaction of 1
with silica gel in dry pyridine gave the 1-silica hybrid. It was
deep red as shown in Figure 1. The amount of adsorbed
porphyrin was determined by TG analysis. The typical TG
curves of the 1-silica hybrid, the silica gel, and 1 are shown in
Figure 2. A sample of the 1-silica hybrid showed a weight loss
in the temperature range 40-220 °C and another weight loss in
the temperature range 380-620 °C. A sample of silica alone
showed a weight loss in the temperature range 40-130 °C and
another weight loss in the temperature range 360-650 °C. We
ascribe the first weight loss of both 1-silica and silica alone to
the loss of adsorbed water. The second weight loss of silica
alone should then be due to the condensation reaction of Si-OH
to Si-O-Si. We observed an exothermic reaction of 1-silica in
the temperature range 340-650 °C by the use of DTA measure-
ments (data not shown). Therefore the second weight loss of
1-silica was ascribed to the combustion of 1. The sample of
1-silica after heat treatment at 650 °C was colorless, showing
that all organic components were eliminated. Thus, we
determined the amounts of 1 adsorbed onto silica based on
the second weight loss.
The calculation of the absorbed amount of porphyrin was
performed as described in Electronic Supporting Information
(ESI). The amounts of the porphyrin adsorbed onto the silica,
wads, calculated according to eq 2 (ESI) are listed in Table 1.
The values of wads ranged from 0.4 to 6.3 mg/50 mg of SiO2
depending on the reaction conditions. As shown in Figure 3,
the values of wads increased as the initial concentration of 1 was

N. Minamijima et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 7 (2011)
797
Table 1. Amounts of Adsorbed 1 on Silica Gel, CARiACT
Q-6 (50 mg) in the Reaction in Pyridine
Table 2. Amounts of Adsorbed 1 on Silica Gel (50 mg) in
the Solvent-Free Reaction
1 in the initial Pyridine Temperature Time 1 adsorbed,
Temperature
Time
/min
1 adsorbed,
Run
Run
1/mg
solution, [1]
/mL
/°C
/h
wads/mg
/°C
w
ads/mg
1
2
3
4
5
6
7
8
9
4.9 mg,
0.4
110
4
2.8
1
2
3
4
9.0
9.0
9.0
9.0
50
110
130
296
10
10
10
2
0.4
1.2
1.4
6.0
2.5 © 10¹2 M
3.0 mg,
3
110
110
110
110
110
110
110
110
rt
2.5
2.5
1.5
2.2
0.4
2.2
2.5
3.7
6.3
4.7
®
9.4 © 10¹4 M
9.0 mg,
3
2.8 © 10¹3 M
3.0 mg,
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
O
O
5.7 © 10¹3 M
3.0 mg,
O
N
N
O
O
NH
Si
HN
O
O
2.0
O
Si O
O O
O
5.7 © 10¹3 M
3.0 mg,
O
O
2.0 © 3a)
2.0 © 3a)
2.0 © 3a)
2.0 © 3a)
2.0
O
O Si
O O
Si
O
O
5.7 © 10¹3 M
6.0 mg,
1.1 © 10¹2 M
9.0 mg,
Figure 4. Schematic representation of 1 adsorbed onto
silica surface.
1.7 © 10¹2 M
12.0 mg,
2.3 © 10¹2 M
Since the BET surface area of CARiACT Q-6 is 451 m2 g
,
¹1
50 mg of CARiACT has a surface area of 22.5 m2. The
geometry of 1 with all trans-configuration of the methylene
chains optimized using ab initio calculations at the B3LYP/
6-31G(d) level showed that the distance between two cis-
oxygens is 2.31 « 0.014 nm. If we assume that 1 is a 2.3-nm-
square, 3 mg of 1 occupies the area of 9.3 m2, assuming that the
porphyrin plane is parallel to the silica surface (Figure 4). The
calculation shows that ca. 41% of the BET surface area is
occupied by the adsorbed porphyrin. The BET surface area is
obtained by use of adsorption of dinitrogen, which is a
much smaller molecule than porphyrin. Therefore the surface
available for dinitrogen to be bound is not necessarily available
for 1 to be bound, depending on the surface roughness of the
silica. Therefore the discrepancy between the BET surface area
and the surface area covered by 1 may be caused by the
difference in molecular size between 1 and N2. The surface
coverage of simple alcohols are reported in literature.40 The
10 3.0 mg,
5.7 © 10¹3 M
a)
A cycle of heating at the designated temperature,
evaporation of the solvent, and addition of pyridine was
repeated three times.
7
6
5
4
3
2
1
0
¹2
25x10-3
0
5
10
15
20
value of 3.22 ¯mol m reported for 1-butanol is larger than
0.16 ¯mol m observed for 1. The difference may be ascribed
¹2
[1]/M
to the larger size of 1 and also to the polyvalency of 1, where
the molecular orientation should be more restricted.
Figure 3. Plot of adsorbed 1 onto 50 mg of CARiACT
against the concentration of 1 in pyridine. Data taken from
Runs 6-9 in Table 1. The dotted line is drawn on the basis
of the Langmuir isotherm: wads = wmaxa[1]/(1 + a[1]).
To evaluate the stability of the 1-SiO2 hybrid and to confirm
that the porphyrin structure was intact other than silicate ester
formation during the reaction with silica, the hybrid (Run 8,
Table 1) was added to boiling water and heated at reflux for 2 h.
The hybrid was collected by filtration and washed with
pyridine. The pyridine turned red, thus showing that some of
1 were liberated due to hydrolysis of the silicic ester linkages.
The 1-silica hybrid after washed with pyridine was still red,
and the TG analysis revealed that about 30% of 1 remained on
the silica surface. The UV-visible spectrum of the recovered
porphyrin showed that the Soret band and the four Q-band
appeared at the same wavelengths as original 1, showing that
the porphyrin was not metallated and the structure was
unaltered when adsorbed onto the silica surface. Interestingly,
when the hybrid obtained in the solvent-free reaction (Run 4,
Table 2) was similarly boiled in water for 2 h, 89% of 1
increased. The amounts of adsorbed 1 were higher when
solvent evaporation and addition were repeated, indicating that
water removal during evaporation drove the equilibrium shown
in Scheme 1 to the silicate ester formation (Run 5 and Run 6 in
Table 1).
Adsorption of 1 was also carried out in a solvent-free
system: 1 (9 mg) was dissolved in a minimum amount of
pyridine, and silica gel (50 mg) was added. The pyridine was
evaporated in vacuo, and the resulting powder was heated at
50-296 °C for 2-10 min. The amounts of adsorbed 1 was
determined by TG and are listed in Table 2. Compared to the
reaction in pyridine, similar amounts of 1 was adsorbed in a
much shorter reaction period.

798
Bull. Chem. Soc. Jpn. Vol. 84, No. 7 (2011)
Functionalization of Silica by Porphyrin
50x10-3
40
210 °C
230 °C
250 °C
190 °C
1
0
-1
30
20
10
0
170 °C
150 °C
1 on SiO2
-2
-3
-4
-5
1
130 °C
110 °C
293 °C
300
300
400
500
600
700
Wavelength/nm
50
100
150
200
250
Temperature/°C
Figure 6. UV-visible spectra of porphyrin alcohol 1 spin-
coated on silicate glass followed by heat treatment at 80,
90, 100, 110, 130, 150, 170, 190, 210, 230, and 250 °C for
10 min. The absorption maximum at 130 °C was 421 nm
while that at 250 °C was 436 nm.
Figure 5. The DSC traces of 1 and the 1-SiO2 hybrid.
remained on the silica, showing that the hybrid obtained in the
solvent-free system at a higher temperature is much more stable
against hydrolysis.
Table 3. UV-visible Spectra of 1 on Silicate Glassa)
The DSC traces of 1 and the 1-SiO2 hybrid are shown in
Figure 5. 1 showed a melting point at 293 °C, while 1-SiO2 did
not show any phase transition up to 400 °C. No phase transition
of the 1-silica hybrid demonstrated that 1 was attached to the
surface of silica and was not able to move freely. Crystal to
liquid transition of 1 at 293 °C was also confirmed by polarized
optical microscopic observation.
Temperature
-
/nm
Absorbance
at -max
Number of porphyrin
max
/°C
molecules per 100 nm2
80
90
®
®
®
2
3
418
421
421
426
430
433
435
437
436
436
0.002
0.003
0.008
0.018
0.027
0.041
0.045
0.045
0.049
0.103
110
130
150
170
190
210
230
250
296
9
20
30
46
50
50
55
115
¹1
Reaction of 1 with Silicate Glass in Pyridine. A similar
procedure was applied to the functionalization of silicate glass
with 1. To a pyridine solution of 1 was placed a silicate glass,
and the pyridine was heated at 110 °C to evaporate the solvent
over a period of 1 h. When the solvent was evaporated, a
portion of pyridine was added and the solution was heated
again at 110 °C. Evaporation and pyridine addition was
repeated three times. The resulting glass was rinsed with
pyridine thoroughly. Adsorption of 1 could not be detected by
using UV-visible spectroscopy but was confirmed by using the
fluorescence emission spectrum as shown in Figure S1 (Sup-
porting Information). The chloroform solution of 1 showed
fluorescence emission maxima at 658 and 718 nm, while the
silicate glass treated with 1 showed those at 653 and 709 nm.
Reaction of 1 with Silicate Glass Using a Solid-Solid
Reaction. We attempted to perform the reaction in a solvent-
free system: a spin-coated film of 1 on silicate glass was heated
at various temperatures. The heat-treated glass was sonicated in
pyridine to remove unreacted 1. The resulting silicate glass was
then analyzed with UV-visible spectroscopy.
a) -max = 426 nm, ¾max = 5.38 © 105 cm¹1 M in pyridine.
Number of porphyrin molecules per 100 nm2 = ANA/¾
©
max
1015 where NA is the Avogadro number, and A is the
41
absorbance at -max
.
glass plate formed under the conditions of heating temperatures
of 90-130 °C possibly exist as an isolated molecule, while
those of heating temperatures above 190 °C form aggregates on
the glass surface. The red shift also suggests that the porphyrin
molecules form a head to tail aggregate, i.e., a J-type
aggregate.42 The temperatures at which the reaction occurs
are well below the melting point of 1 (293 °C), indicating that
the reaction occurs between the two solid phases.
In Figure 6, the UV-visible spectra of the glass after heat
treatment at 80-250 °C for 10 min followed by sonication in
The kinetics of the solid-solid reaction of 1 with silicate
glass at 100-170 °C is shown in Figure 7. The reaction was
slow at 100 °C, while saturation was observed above 130 °C,
and the absorbance at saturation was 0.045 regardless of the
reaction temperatures. The plot of the absorbance against time
can be fit using Langmuir rate equation: ¹ln(1 ¹ A(t)/A(¨)) =
kt, where A(t), A(¨), k, and t are absorbance at time t,
absorbance after the reaction was complete, the rate constant,
and the heating period, respectively. The rate constants k were
pyridine to remove unreacted 1 are shown. The values of -max
,
absorbance at -max, and the surface coverage calculated by the
equation shown in the table footnote are listed in Table 3. The
absorbance of the Soret band increases with heating temper-
ature. In the glass heated at 80 °C, the Soret band could hardly
be detected. The absorption maximum of 1 in pyridine was
426 nm and the absorption maximum of the glass heated at
130 °C was 421 nm. On the other hand, the absorption
maximum of the glass heated above 190 °C was red-shifted
to 433-437 nm, with increasing the absorbance in the Soret
band. These observations imply that the molecules 1 on the
¹1
¹1
¹1
0.0008 min at 100 °C, 0.0106 min at 130 °C, 0.0274 min
¹1
at 150 °C, and 0.104 min at 170 °C. The Arrhenius plot of
¹1
ln k vs. 1/T gave the activation energy of 93 kJ mol for the
reaction.

N. Minamijima et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 7 (2011)
799
4x10-2
3
100 °C
130 °C
140 °C
150 °C
170 °C
2
1
0
50
100
150
200
250
300
Time/min
Figure 7. Plot of absorbance of the absorption maxima of the Soret band at 423-436 nm of a silicate glass spin-coated with 1 and
heated at 100, 130, 140, 150, and 170 °C against heating periods. The curve obtained by fit the data to Langmuir adsorption rate
equation: Abs = 0.046(1 ¹ exp(¹kt)) is shown, where k = 0.0008 (100 °C), 0.0106 (130 °C), 0.0097 (140 °C), 0.0274 (150 °C), and
¹1
0.104 min (170 °C).
0.9 nm
Aggregate Structure Deduced from Kasha’s Exciton
Coupling Theory. According to the Kasha’s exciton coupling
theory, the spectral shift observed for a dimer with a distinct
geometry can be calculated using the equation:
®2ð1 ꢀ 3 cos2 ªÞ
ꢀE ¼
ð1Þ
4³¾0r3
where ® is the transition dipole moment, ª is the angle between
the two transition dipoles, ¾ is the permittivity in vacuo, and
0
r is the distance between the two chromophores. Putting all the
physical constants in the above equation, we obtain the
following equation:
3.3 nm
Figure 8. Schematic representation of a dimer structure
deduced from the UV-visible spectral shift for an
aggregate of 1. The molecule of 1 is shown as a square.
The distance between the centers of 1 was calculated to be
0.9 nm on the basis of Kasha’s theory.
2
3
ꢀE=J molꢀ1 ¼ 5:17 ꢁ 1026ð®=DÞ ð1 ꢀ 3 cos2 ªÞ=ðr=mÞ ð2Þ
The shift of 15 nm corresponds to the exciton coupling
energy ¦E of 10 kJ mol¹1. According to the above equation,
r can be calculated to be 0.9 nm assuming that the transition
moment ® = 8 D and the angle ª = 0. Figure 8 shows
schematically the aggregate structure 1 on the silicate glass,
where the porphyrin molecule is represented by a square.
Stability of 1 Bound to Silicate Glass in Various Organic
Solvents. The porphyrin layer on silicate glass was immersed
in various organic solvents at 25 °C for 5 h, and the decrease in
absorbance of the Soret band was determined after washing
with pyridine. The amounts of 1 immobilized on silicate glass
after 5 h were 87%, 88%, 88%, 98%, 81%, 96%, 92%, and
86% in toluene, chloroform, THF, pyridine, DMF, DMSO,
acetone, and methanol, respectively (Figure 9). Therefore the
porphyrin layer is rather stable in broad ranges of organic
solvents. The porphyrin-silicate glass hybrid can be subjected
to further chemical reactions, and such reactions will be
reported elsewhere.
(%)
100
80
60
40
20
0
Reaction of Ester Porphyrin and Ether Porphyrin with
Silicate Glass. In order to determine the role of the hydroxy
groups of 1 in the reaction with silicate glass, we performed
similar experiments using reference porphyrins, ester porphy-
rin, 5,10,15,20-tetrakis[4-(5-acetoxypentyloxy)phenyl]porphy-
rin (2), and ether porphyrin, 5,10,15,20-tetrakis(4-hexyloxy-
phenyl)porphyrin (3). The UV-visible spectra of a spin-coated
PhCH
THF
DMF
acetone
MeOH
3
pyridine DMSO
CHCl
3
Figure 9. Absorbance of the Soret band of 1 on silicate
glass after immersion in various organic solvents for 5 h at
25 °C and washing with pyridine.

800
Bull. Chem. Soc. Jpn. Vol. 84, No. 7 (2011)
Functionalization of Silica by Porphyrin
Table 4. UV-visible Spectra of 3 on Silicate Glassa)
40x10-3
Temperature
-
/nm
Absorbance
at -max
Number of porphyrin
molecules per 100 nm2
max
240 °C
250 °C
260 °C
270 °C
280 °C
296 °C
/°C
240
250
260
270
280
296
®
®
®
5
10
17
32
30
20
10
0
426
426
426
430
430
0.003
0.006
0.011
0.020
0.029
46
¹1
a) -max = 425 nm, ¾max = 3.79 © 105 cm¹1 M in pyridine.
Table 5. UV-visible Spectra of 2 on Silicate Glassa)
350
400
450
500
550
Temperature
-
/nm
Absorbance
at -max
Number of porphyrin
molecules per 100 nm2
max
Wavelength/nm
/°C
Figure 10. UV-visible spectra of ether porphyrin 3 spin-
coated on silicate glass followed by heat treatment at the
designated temperatures between 240 and 296 °C for
10 min. The absorption maximum heated at 296 °C was
430 nm while that heated at 250 °C was 426 nm.
140
160
180
190
200
®
®
®
7
26
32
44
426
427
429
431
0.006
0.021
0.026
0.035
¹1
a) -max = 425 nm, ¾max = 4.84 © 105 cm¹1 M in pyridine.
40x10-3
70x10-3
140 °C
160 °C
180 °C
190 °C
200 °C
60
1 (OH)
2 (OAc)
3 (CH3)
4 (phenolic OH)
30
20
10
0
50
40
30
20
10
0
300
350
400
450
500
550
Wavelength/nm
100
150
200
250
300
Figure 11. UV-visible spectra of ester porphyrin 2 spin-
coated on silicate glass followed by heat treatment at the
designated temperatures between 140 and 200 °C for
10 min. The absorption maximum heated at 200 °C was
431 nm while that heated at 160 °C was 421 nm.
Temperature / °C
Figure 12. Plot of absorbance of the Soret band against the
reaction temperatures for 1, 2, 3, and 4. The reaction period
was 10 min. The curves fit to the Arrhenius equation,
Abs = A exp(¹¦E/(temp + 273.15)), are shown. The ab-
sorbance of 1 heat-treated above 210 °C was much lower
than expected from the Arrhenius equation.
film on silicate glass after heat treatment and sonication in
pyridine are shown in Figures 10 and 11.
When the spin-coated film of ether porphyrin 3 was heated
below 240 °C, no absorption of the Soret band was detected.
The porphyrin was detected when the heating temperature was
raised to 250 °C, and the absorbance of the Soret band increased
as the temperature was raised further. A red shift from 426 to
430 nm was also observed as the temperature was higher. The
values of -max, absorbance, and the surface coverage are listed
in Table 4. The sluggish reaction of the ether porphyrin 3 with
silicate glass demonstrated that the hydroxy groups in 1 played
an important role in the reaction with SiO2.
The reaction of 2 with silicate glass proceeded when the
temperature was above 160 °C. The melting point of 2 was
198 °C. Thus, the reaction of 2 with silicate glass proceeded
across the solid-solid interface. Porphyrin 1 having OH groups
reacted with silicate glass at a lower temperature than did 2
having OAc groups. Figure 12 shows the plot of absorbance of
the Soret band of the porphyrins after reaction with silicate
glass against the reaction temperatures. Four porphyrins reacted
with silicate glass at different temperatures: the reactivity
decreases in the order 1 (OH) > 2 (OAc) > 4 (phenolic OH) º
3 (CH3). The reaction of 1 with silicate glass could be initiated
by a nucleophilic attack of the oxygen atom of alcohol of 1 on
the Si atom, while the reaction of 2 may proceed via the acetyl
transfer from 2 to SiOH followed by the nucleophilic attack of
Figure 11 shows the UV-visible spectra of ester porphyrin 2,
which has acetate groups at the termini, spin-coated on the
glass, heated at 140-200 °C, and sonicated in pyridine. Table 5
summarizes the spectral data of 2 adsorbed on silicate glass.

N. Minamijima et al.
Bull. Chem. Soc. Jpn. Vol. 84, No. 7 (2011)
801
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Conclusion
Tetrahydroxyporphyrin was allowed to react with silica gel
and silicate glass either by use of pyridine as a solvent or by
use of a solid-solid reaction. In the reaction in pyridine,
removal of water from the reaction mixture by evaporating
solvents assisted the adsorption reaction. Tetrahydroxyporphy-
rin was allowed to react with silicate glass below the melting
point, i.e., in a solid-solid reaction. A spin-coated film of the
porphyrin was heated at 90-250 °C to yield a glass modified
with the porphyrins. The amounts of porphyrins adsorbed
increased with increasing temperature and the reaction periods,
and showed saturation, displaying that monolayer film of
porphyrin formed. As the coverages of porphyrin increased, the
Soret band was shifted from 421 to 436 nm, suggesting that the
porphyrin was adsorbed as a J-type aggregate. The reaction of
tetraalkoxyporphyrin with silicate glass occurred at much
higher temperature around 250 °C, and the reaction of
tetraacetoxyporphyrin at 160 °C. The different reactivities of
the ether, ester, and alcohol functional groups can be useful for
multiple functionalization of silica with organic molecules.
We thank Tokyo Ohka Foundation for the Promotion of
Science and Technology for support of this research. This work
was also supported by “Creating Research Center for Advanced
Molecular Biochemistry,” Strategic Development of Research
Infrastructure for Private Universities, the Ministry of Educa-
tion, Culture, Sports, Science and Technology (MEXT), Japan.
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Supporting Information
Synthesis of porphyrin derivatives, 1, 2, and 3; calculation of
amounts of adsorbed porphyrin on silica gel based on TG
traces, and fluorescence spectra of 1 adsorbed on silicate glass
(PDF). This material is available free of charge via the web at
http://www.csj.jp/journals/bcsj/.
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