Visible light induced oxygenation of alkenes with water sensitized by silicon-porphyrins with the second most earth-abundant element

Article abstract of DOI:10.1016/j.jphotochem.2015.07.016

Silicon as the second most abundant element on Earth was effectively utilized as the central atom in the porphyrin to induce photochemical oxygenation of alkenes as the first example of photocatalytic reaction through activation of water molecule in the presence of K₂PtCl₆ as an electron acceptor. Oxygen atom of water was confirmed to be incorporated in the oxygenated product by the photoreaction with H₂¹⁸O. The excited triplet state of silicon porphyrin was revealed to be responsible for the photochemical oxygenation. The one-electron oxidized silicon porphyrin was predicted by DFT calculation to have its spin population mostly on the axially ligated hydroxyl oxygen atom. The oxyl radical character of the axial ligand could rationalize the oxygenation reaction.

Full text of DOI:10.1016/j.jphotochem.2015.07.016

Journal of Photochemistry and Photobiology A: Chemistry 313 (2015) 176–183
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Visible light induced oxygenation of alkenes with water sensitized by
silicon-porphyrins with the second most earth-abundant element
Sebastian Nybin Remello, Takehiro Hirano, Fazalurahman Kuttassery, Yu Nabetani,
Daisuke Yamamoto, Satomi Onuki, Hiroshi Tachibana, Haruo Inoue*
Department of Applied Chemistry, Graduate School of Urban Environmental Sciences, Center for Artificial Photosynthesis, Tokyo Metropolitan University, 1-1
Minami-ohsawa, Hachioji, Tokyo 192-0397, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 16 June 2015
Received in revised form 27 July 2015
Accepted 28 July 2015
Available online 4 August 2015
Silicon as the second most abundant element on Earth was effectively utilized as the central atom in the
porphyrin to induce photochemical oxygenation of alkenes as the first example of photocatalytic reaction
through activation of water molecule in the presence of K
water was confirmed to be incorporated in the oxygenated product by the photoreaction with H
2 6
PtCl as an electron acceptor. Oxygen atom of
18
2
O. The
excited triplet state of silicon porphyrin was revealed to be responsible for the photochemical
oxygenation. The one-electron oxidized silicon porphyrin was predicted by DFT calculation to have its
spin population mostly on the axially ligated hydroxyl oxygen atom. The oxyl radical character of the axial
ligand could rationalize the oxygenation reaction.
Keywords:
Artificial photosynthesis
Water oxidation
Oxygenation
Silicon
ã 2015 Elsevier B.V. All rights reserved.
Visible light
Porphyrin
2
ꢀ hv
MP
42ꢀ
1. Introduction
“S” þ H
2
O þ PtðIVÞCl
4
! “S” þ PtðIIÞCl
þ 2HCl
ð1Þ
where “S”, “SO”, MP denote substrate, oxygenated substrate, and
metalloporphyrins, respectively. As a half reaction in the oxidation
side of artificial photosynthesis, the two-electron oxidation of
water by one-photon excitation (Eq. (1)) would be the more
plausible alternative compared to the four-electron oxidation by
stepwise four-photon excitation under actual sunlight radiation
with rather low light intensity, which faces with the “photon-flux-
density problem [5].” Another crucial issue, furthermore, to be
resolved would be to devise ways to utilize major elements for the
artificial photosynthetic system rather than rare elements. We
have already found that aluminum(III) porphyrins can induce
photochemical oxygenation of substrate with water [9]. Here we
will report that another promising major element, silicon, can be
also utilized as Si(IV)- porphyrins to induce the photoreaction.
Silicon should be the most promising element for the purpose.
Porphyrins are well known to incorporate almost all kinds of
element within their ring as the central atom [1]. Silicon
porphyrins have attracted much attention, since silicon is the
second most abundant and easily available element on Earth [2,3].
Though many reports on their synthesis have appeared [2],
photochemical behavior of silicon porphyrins has been rarely
reported [3]. Here we report a visible light induced oxygenation of
alkenes such as cyclohexene, norbornene, and styrene with water
as an oxygen atom donor sensitized by tetra(2, 4, 6-trimethyl)
phenylporphyrinatosilicon (SiTMP) as the first example of a
photocatalytic reaction with water induced by silicon porphyrins.
One of the crucial view points to the visible light induced
oxygenation of substrate with water would be on artificial
photosynthesis. Artificial photosynthesis by visible light is one
of the most desirable chemical systems at present. Among the
problems to be resolved for the realization of artificial photosyn-
thesis, how could water molecules be incorporated at the oxidation
terminus of the system is one of the most crucial issues [4,5]. We
have recently focused our attention on the two-electron oxidation
of water by one-photon excitation to form oxygenated products of
various substrates sensitized by metalloporphyrins [5–8] (Eq. (1)).
2
2
2
. Experimental
.1. Materials
.1.1. Synthesis of trans-dihydroxy[5,10, 15, 20-tetra(2,4,6-trimethyl)
phenyl porphyrinato] silicon (IV): Si(IV)TMP(OH)
2
Tetra(2, 4, 6-trimethyl) phenylporphyrinatosilicon (SiTMP) as a
new compound was synthesized from the free base tetramesi-
tylporphyrin through four steps of lithiation, insertion of silicon,
*
Corresponding author. Fax: +81 42 653 3416.
E-mail address: inoue-haruo@tmu.ac.jp (H. Inoue).
http://dx.doi.org/10.1016/j.jphotochem.2015.07.016
010-6030/ã 2015 Elsevier B.V. All rights reserved.
1

S.N. Remello et al. / Journal of Photochemistry and Photobiology A: Chemistry 313 (2015) 176–183
177
2
Scheme 1. Synthesis of SiTMP(OH) .
dechlorination, and hydrolysis of axial ligands. The synthesis was
carried out as a multi-step single pot synthesis till step 3 as shown
chemicals) was distilled under nitrogen before use and was stored
under nitrogen. Acetonitrile (HPLC grade) was used as received
IV
2
in Scheme 1. The free base tetramesitylporhyrin (H TMP: 399 mg,
from Nakalai Tesque. K
2
Pt Cl
6
was used as received from Aldrich.
0
.51 mmoles) was taken in a reaction pot and kept under vacuum
Distilled water was passed through an ion-exchange column (G-10,
ORGANO Co.). The electrical conductivity of the water was below
0.1 mS/cm.
for 30 min. 80 ml of dimethoxyethane (DME) was vacuum
transferred to the reaction vessel and stirred at 80 C for 1 h
ꢁ
under nitrogen atmosphere. To the solution, lithium bis(trime-
thylsilyl) amide (LHMDS: 350 mg, 2.1 mmoles) was added and
stirred under 80 C for an hour. The completion of step 1 was
2.2. Measurements
ꢁ
confirmed by a red shift in UV–vis spectrum (416–436 nm) of
reaction mixture measured in dry DME. The reaction mixture is
UV–vis spectra were measured on a Shimadzu UV-2550 spec-
trophotometer. Fluorescence spectra were measured on a JASCO
FP-6500 spectrofluorometer. Oxidation potential of SiTMP was
measured by cyclic voltammetry with an electrochemical analyzer
(Model 611DST, BAS), with a boron doped diamond/glassy carbon
as a working electrode, Ag/AgCl as a reference one, and Pt wire as a
counter one in acetonitrile containing 0.1 M supporting electrolyte,
ꢁ
then cooled to ꢀ20 C and then HSiCl
3
(0.1 ml, 0.99 mmoles) was
added carefully and stirred under the same condition for 1 h. The
reaction mixture was then slowly warmed up to room temperature
and stirred under room temperature for 12 h. The completion of
step 2 was confirmed by a blue shift in UV–vis spectrum (436-
+
ꢀ
3+
432 nm) of the reaction mixture. Then a little excess amount of
4 9 4 6
(C H ) N PF . An Nd YAG laser-pumped OPG (EKSPLA, PL
silver trifluoromethanesulfonate (AgOTf: 1800 mg, 7 mmoles) was
2210JE + PG432JE; FWHM 26 ps, 1 kHz Hz) as the excitation source
for measuring the fluorescence lifetime of the Si(IV)TMP. The
fluorescence was monitored by a streak camera (Hamamatsu,
C4334) equipped with a polychromator (CHROMEX, 250IS).
Nanosecond laser flash photolysis was performed with a dye-
laser (LUMONICS H-300D, coumarin 540A dye, 590 nm, 8-
ns fwhm) pumped by an XeCl excimer laser (LUMONICS Hyper
EX-300, 308 nm, 12-ns fwhm) and a 500W Xe arc lamp (USHIO
500-DKO)/a light emitting diode (470 nm, Mightex systems) as a
monitoring light source equipped with a monochromator (RITSU
OYO KOGAKU MC-30, 1200 G/mm), and a photomultiplier tube
(HAMAMATSU PHOTONICS R-636). The amplified signal was
recorded on a digital storage oscilloscope (GOULD DSO4072,
100 MHz). Transient absorption spectra were obtained with a
spectrometric multichannel analyzer (SMA: Princeton Instruments
IRY-512) equipped with a polychromator (Jarrell-Ash Monospec-
27). The timing was controlled by a digital time delay (STANFORD
DG535). All spectral measurements were carried out at room
temperature (294 K). Gas chromatographic analyses were per-
formed on a Shimadzu GC-17A equipped with a TC-17 column (GL
Sciences Inc. 30 m, 60–250 C), and a mass spectrograph (Shimadzu
QP-5000) as a detector. The practical detection limit of the GC-MS
added to the reaction mixture and stirred for one overnight at
ꢁ
8
0 C. The completion of step 3 was confirmed by a blue shift in
UV–vis spectrum (432–415 nm) of the reaction mixture. The
reaction mixture was then filtered through PTFE membrane (pore
size: 0.1 mm) and celite successively to remove solid inorganic
impurities. The purple colored solution thus obtained was vacuum
dried to get purple powder which was then dissolved in 100 ml of
dichloromethane. Water (100 ml) was further added and the
mixture was kept stirring for one overnight. The completion of Step
4
was confirmed by a red shift in UV–vis spectrum (415–422 nm).
The organic layer was then separated and purified by passing
through SiO column using 1:3 ethyl acetate/ hexane as eluent to
get pure SiTMP as purple crystal (295 mg, 68% yield). The Si
porphyrin synthesized was identified as SiTMP(OH) which have
2
2
two hydroxy groups as axial ligands on the central Si atom.
EA: Obsd. C 77.97%, H 7.17%, N 5.55%, Calcd. for [Si (IV)TMP
1
1
1
(
OH)
NMR (CDCl
3
3.04 (s, 1.4H). C NMR (CDCl ): d= 21.36 (s), 21.60 (s), 115.17(s),
2
]( /
2
H
2
O
ꢂ
C
6
H
14
ꢂ
/
4
CH
2 2
Cl ), C 77.92%, H 7.30%, N 5.84%. H
3
): d= 8.63 (s, 8H), 7.18 (s, 8H), 2.54 (s,12H),1.85(s, 24H),
1
3
ꢀ
2
9
1
27.77(s), 131.16(s), 136.23(s), 137.92(s), 139.30(s), 142.48(s). Si
NMR (CDCl
max = 422.5 nm in CHCl
5
ꢀ1
ꢀ3
3
):
d
= ꢀ202.77(s). UV–viz:
e= 5.06 ꢃ10 M dm
ꢀ7
(l
3
). ESI-MS: m/z = 842.29
was ca. 10 M. Quantitative analysis was carried out in the
selected ion monitoring (SIM) detection mode.
2.1.2. Other materials
Dimethoxyethane (DME) purchased from TCI chemicals was
2.3. Photochemical oxygenation reaction
stored over molecular sieves (4A), and dried over Na and vacuum
transferred immediately prior to use.
Lithium bis(trimethylsilyl) amide (LHMDS) solid was purchased
3
from Aldrich and stored in dry condition. Trichloro silane (HSiCl )
was purchased from TCI chemicals. Silver trifluoromethanesulfo-
nate (AgOTf) was purchased from TCI chemicals. Cyclohexene (TCI
All of the samples for the photoreactions were degassed by
ꢀ5
seven repeated freeze-pump-thaw cycles under 10 torr. The
degassed samples in a 1 ꢃ1 ꢃ4.5 cm quartz cell (EIKO-SHA) were
irradiated with monochromatic light (420 nm/430 nm) through an
interference filter MX0420/a sharp cut-off filter L-39 for 420 nm

178
S.N. Remello et al. / Journal of Photochemistry and Photobiology A: Chemistry 313 (2015) 176–183
and MX0430/L-39 for 430 nm from a 500W Xe arc lamp (USHIO
00-DKO). The photoreaction was monitored with UV–vis
spectroscopy. The reaction mixture after the photoreaction was
vacuum-distilled and the products were analyzed by GC-MS
spectroscopy. All of the procedures were performed at ambient
temperature.
state of Si(IV) than Al(III). As a potential half reaction of artificial
photosynthetic system, SiTMP can add an option of operating the
system under the acidic conditions, while AlTMP does it under
basic condition.
Before examining the photoreaction of SiTMP, electrochemical
characterization was carried out. By means of cyclic voltammetry
using boron-doped diamond electrode with wide electrochemical
5
18
2.4. Labeling experiment with the use of
O
3 2
window [11] or glassy carbon in CH CN/H O (9/1 v/v)), each
oxidation peak potential (Eox ) of each SiTMP species with
different axial ligands in protolytic equilibria was estimated
(Table 1). Pourbaix diagram between the oxidation peak potential
p
The 18O content of the water, H
adjusted to 12.1% by mixing H
18
O/ (H
16O + H
18O), was
2
2
2
16
18
2 2
O and H O. The photoreactions
were carried out under exactly the same conditions as the
(Eox
When a one-electron oxidation process is coupled with proton
transfer, the oxidation peak potential (Eox ) should be linearly
dependent upon pH with a slope of 59 mV according to Nernst
Equation. The Pourbaix diagram indicates that only SiTMP(OH)
p
) for each species versus the pH condition is shown in Fig. 3.
16
experiment carried out with H
2
O. The reaction mixtures after the
18
photoreaction were vacuum- distilled, and the content of O in the
reaction product was determined by GC-MS (Shimadzu QP-5000).
p
2
3
. Results and discussion
species exhibit a proton-coupled one-electron oxidation, while
one-electron oxidation of all four other protolytic species are
independent on pH conditions.
3.1. Protolytic behaviour of axial ligands of tetra(2,4,6-trimethyl)
phenylporphyrinatosilicon(IV) (Si(IV)TMP(OH)
2
)
The excited singlet state of each SiTMP (E00) with different axial
ligands in protolytic equilibria under the neutral condition was
determined to situate at 2.07 eV by the estimation from the
crossing point between the normalized absorption and fluores-
cence spectra, while the phosphorescence spectrum of SiTMP
The two hydroxy groups as axial ligands exhibit protolytic
equilibria as observed in other metalloporpyrins [10]. Visible
absorption spectra of SiTMP under various pH conditions in
aqueous acetonitrile solution have clear changes in four-step with
four isosbestic points as show in Figs. 1 and 2. From the inflection
points of the absorbance at the fixed wavelength, four pKa values
were deduced (Fig. 2,Table 1). Fluorescence spectra also changed in
four-step and the corresponding four pKa values were also
2
(OH) in methylcyclohexane at 77 K (lmax = 755 nm) indicates
that the excited triplet state has its energy at 1.68 eV. The oxidation
potential of SiTMP at around +0.7 ꢄ 0.9 Volt vs SHE indicates that
2 6
an electron transfer to K PtCl = (Ered = +0.76 Volt vs SHE) from the
excited SiTMP state should be sufficiently exoergonic either from
estimated (Table 1, SI-1). 1H NMR spectra of SiTMP in CD
O (9/1 v/v) exhibited systematic peak shifts under the various
pH conditions as shown in the supplemental material SI-2.
CN/
the S
eV).
(D
G = ꢀ1.93 ꢄ ꢀ2.13 eV) or the T
states (
D
G = ꢀ1.54 ꢄ ꢀ1.74
3
1
1
D
2
All these results indicate that SiTMP has the protolytic
equilibria among the two axial ligands, hydroxy groups, as shown
in Table 1. It should be noted here that the four pKa’s of SiTMP are
all in acidic region. This is very contrasting to the cases of
aluminum-porphyrins that have their pKa’s in basic region [9,10].
The large difference between SiTMP and Al-porhyrins could be
rationalized by the substantial difference of their electronegativ-
3.2. Photochemical oxygenation of cyclohexene with water sensitized
by silicon (IV) porphyrins with visible light
When SiTMP (1 ꢃ10ꢀ M) in aqueous acetonitrile (CH
5
3
2
CN/H O
(9/1 v/v)) was irradiated with monochromatic visible light of
l
= 420 nm (through glass filters of L39/MX0420) or 560 nm (VY54/
MZ0560) from Xe lamp (500 W Ushio 500DKO) in the presence of
cyclohexene (1 ꢃ10ꢀ M) as a substrate, K
2
PtCl
(5 ꢃ10 M) as an
ꢀ4
ities (E
a
(Si) = 1.74, E
a
(Al) = 1.47) in addition to the higher valence
2
6
3 2
Fig. 1. Visible absorption spectra of SiTMP in CH CN/H O (9/1 v/v) under various pH conditions. Four isosbestic points (circled red) were observed during the reversible
change between pH 0.1 and 7. [SiTMP] = 1 ꢃ10ꢀ M.
6

S.N. Remello et al. / Journal of Photochemistry and Photobiology A: Chemistry 313 (2015) 176–183
179
Fig. 2. Estimation of pKa's of SiTMP in CH
3 2
CN/H O (9/1 v/v) under various pH conditions. The pKa values were estimated from the inflection point of the plot between
ꢀ
6
absorbance at = 423, 418, 415 nm,
l
lmax in the spectral change and pH values of the solution. [SiTMP] = 1 ꢃ10 M.
Table 1
3 2
Protolytic equilibria of SiTMP among their axial ligands and oxidation potential of each species in CH CN/H O
(
9/1 v/v).

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S.N. Remello et al. / Journal of Photochemistry and Photobiology A: Chemistry 313 (2015) 176–183
Time courses of the visible absorption spectra during the
photoreaction are shown in Fig. 4. The concentration of K PtCl
= 300 nm,
decreased efficiently to be consumed completely with the light
2
6
ꢀ
4
(
5 ꢃ10 M), which can be monitored at around
l
ꢀ5
irradiation, while the absorbance of SiTMP (1 ꢃ10 M) at around
l
= 420 nm gradually decreased down to ꢄ30% under the neutral
condition. (Fig. 4(a)) Under the basic condition, however, SiTMP
was more stable (Fig. 4(b)–(d)) and remained unchanged at
[
KOH] = 3 ꢄ 5 mM during the photoreaction (Fig. 4(e) and (f)). The
apparent turnover number of the catalyst, SiTMP, for the total
reaction products on the basis of the catalyst concentration
(
ꢀ5
1 ꢃ10 M) was larger than 30 under the neutral condition
(
Table 2). The di-chlorinated product (6) was predominant under
the neutral and weakly alkaline condition. Under the basic
condition, however, the formation of 6 was almost suppressed.
It should be noted here that the time of light irradiation required
2 6
for the complete consumption of K PtCl is the longest under the
neutral condition, while under basic conditions the reactivity was
much enhanced with the increase of the initial concentration of
KOH. (Fig. 4) The total amount of the reaction products derived
from cyclohexene, on the other hand, decreased with the increase
Fig. 3. Pourbaix diagram of SiTMP for the first (Eox
oxidation peaks against pH value of the solution. [SiTMP] = 5 ꢃ10
p
1: v) and second (Eox
p
2:
)
M,
ꢀ
5
4 3 2
6ꢀ
N PF ] = 0.1 M in CH CN/H O (9/1 v/v). Working electrode: Glassy carbon,
+
[
Bu
counter electrode: Pt wire, reference electrode: Ag/AgNO
3
.
ꢀ
of the initial [OH ]. (Table 2) These obviously indicate that
hydroxide ion itself is actually involved in the oxidation pathway to
compete with cyclohexene, resulting in the suppression of the
oxygenation of cyclohexene. Hydroxide ion should exert crucial
roles in the process. Though the point is very interesting from the
view point of oxidative activation of water and the further study is
now in progress, attention is focused here on the oxygenated
reaction products. As discussed later, the di-chlorinated product
(
6) might have a different reaction pathway than that for the other
Scheme 2. Photochemical oxygenation of alkene with water sensitized by SiTMP.
oxygenated reaction products. Among the reaction products, the
oxygenated ones except the di-chlorinated product (6) have similar
tendency of formation with that in the case of Ru(II)TMP [7,8]. The
formation of epoxide (1) was favoured under a weakly alkaline
condition ([OH ] < 1.5 mM). (Table 2) The formation of the
“
electron acceptor, and KOH (0 ꢄ 5 ꢃ10ꢀ M) under the degassed
condition, formation of oxygenated products of cyclohexene were
observed (Scheme 2, Table 2). The photochemical reaction
products were the corresponding epoxide (cyclohexeneoxide (1),
3
ꢀ
ꢀ
epoxide” (1) was the highest at [OH ] = 1.0 mM, while under
2
2
-chlorocyclohexanol (2), 2-cyclohexenol (3), cyclohexanone (4),
-cyclohexenone (5), and 1, 2-dichlorocyclohexane (6), depending
ꢀ
the neutral and the weakly alkaline conditions ([OH ] < 1.0 mM),
2
among the oxygenate products. When the photoreaction was
started under the neutral condition without adding KOH, the
-chlorocyclohexanol (2), was the major component in place of 1
on the reaction conditions. (Table 2) The photoreaction proceeds
under either Q-band (560 nm) or Soret-band (420 nm) excitation.
Table 2
3 2
Photochemical oxygenation of cyclohexene sensitized by SiTMP in CH CN/H O (9/1 v/v). Yield of each
reaction product,1–6, with or without KOH added to the reaction mixture with visible light irradiation
ꢀ5
ꢀ4
(
1
l
= 420 nm) under degassed condition. [SiTMP] = 1 ꢃ10 M, [K
2
PtCl
6
] = 5 ꢃ10 M, [Cyclohexene] =
ꢀ
2
ꢃ10 M.

S.N. Remello et al. / Journal of Photochemistry and Photobiology A: Chemistry 313 (2015) 176–183
181
ꢀ
2
ꢀ5
Fig. 4. Time course of visible absorption spectra during the photochemical oxygenation of cyclohexene (1 ꢃ10 M) sensitized by SiTMP (1 ꢃ10 M) in the presence of
ꢀ
4
K
2
PtCl
6
(5 ꢃ10 M) with KOH in CH
3 2
CN/H O (9/1 v/v) with optical length corresponding to 2.5 mm: (a) [KOH] = 0 mM, the photoreaction for 365 min, (b) [KOH] = 0.5 mM, the
photoreaction for 237 min, (c) [KOH] = 1 mM, the photoreaction for 224 min, (d) [KOH] = 1.5 mM, the photoreaction for 112 min, (e) [KOH] = 3 mM, the photoreaction for 91
min, (f) [KOH] = 5 mM, the photoreaction for 30 min.
reaction mixture became acidic after the photoreaction with the
pH value to be ꢄ3, due to the HCl formation as expected by Eq. (1).
Under the acidic condition (pH 3) in the presence of HCl, the
epoxide (1) was observed to be transformed into 2-chlorocyclo-
hexanol (2). The formation of 2 in the photoreaction was thus
considered to be derived from the epoxide (1) through the reaction
with HCl generated during the photoreaction (Eq. (1)). The
selectivity of epoxide formation among the oxygenated products
except the dichlorinated one (6) in the photoreaction was thus
suggested to be modified by adding the contribution of 2 to be
rather contrasting to the case of AlTMP which showed highly
selective reaction with cyclohexene, but has no reaction with other
alkenes [9]. The quantum yield of the photochemical oxygenation
of cyclohexene by SiTMP under the neutral condition as a typical
case was measured at the early stage of the photoreaction to be
1.2%, rather low compared to those obtained with Ru(II)TMP [7,8].
The lower quantum yield might be ascribed to the lower oxidation
ability of the sensitizer SiTMP (Eox = +0.74 V vs. SHE under the
neutral condition) than that of Ru(II)TMP (Eox = +1.03 V vs. SHE).
7
(
2.3% (4.2% (1) + 68.1% (2) = 72.3%) under the neutral condition
Table 2).
The experiments using H 1
oxygen atom in the oxygenated reaction products was derived
3.3. Excited state of Si(IV)TMP responsible for the photochemical
oxygenation of cyclohexene and the reaction mechanism
8O indicated that the source of
2
To get insight into the excited state of SiTMP responsible for the
photoreaction and the reaction mechanism, quenching experiment
for both the singlet and triplet excited state of SiTMP were carried
18
from water. The degree of O uptakes (%) in each reaction product
using H
16
18
2 2
O–H O (90:10) at [KOH] = 1.5 mM were 1 (14.9%), 2
(
8.2%), 3 (9.7%), 4 (8.4%), and 5 (10.5%). When other substrates such
out. As discussed above an electron transfer to K
sufficiently exo-ergonic either from the excited singlet (
1.93ꢄ -2.13 eV) or the triplet state ( G = -1.54ꢄ -1.74 eV) of SiTMP.
The excited states quenching experiments by K PtCl were carried
2
PtCl
6
should be
as norbornene, styrene, and 1-hexene were examined for the
photochemical oxygenation sensitized by SiTMP in the similar
D
G = -
D
condition ([KOH] = 1.5 mM in CH
ingly, highly selective epoxidation was observed for norbornene
97.6%) and moderate one for styrene (45.9%), while almost no
reaction was observed for 1-hexene (Table 3). The reactivity is
3
2
CN/H O (9/1 v/v)), very interest-
2
6
out to examine which excited state is actually responsible in the
ꢀ
ꢀ1
(
reaction mixture. The fluorescence of SiTMP(O )
2
under the
) was not
8
neutral conditions
(t
F
= 5.9 ns, 1/
t
F
= 1.7 ꢃ10 s

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S.N. Remello et al. / Journal of Photochemistry and Photobiology A: Chemistry 313 (2015) 176–183
Table 3
Photochemical oxygenation of alkenes sensitized by SiTMP in CH
Yield of each reaction product, selectivity in parenthesis with addition of KOH
3
2
CN/H O (9/1 v/v).
ꢀ
3
(
[
1 ꢃ10 M) with visible light irradiation (
l
= 420 nm) under degassed condition.
SiTMP] = 1 ꢃ10ꢀ M, [K
5
PtCl
6
] = 5 ꢃ10 M.
ꢀ3
2
Scheme 3. Mechanism of the visible light induced photooxygenation of
cyclohexene sensitized by SiTMP in aqueous acetonitrile at pH 4.
The similar spin densities on the axial –OH group and the further
ꢀ
deprotonated –O were also suggested in the case of Ru(II)TMP
[7,8,12] and AlTMP [9] to rationalize the selective epoxide
formation. The formation of the oxygenated reaction products in
Tables 2 and 3 could be thus explained by the similar mechanism as
revealed in the photochemical oxygenation of cyclohexene with
water sensitized by Ru(II)TMP [7,8]. The one electron oxidized form
2
ꢀ
of SiTMP generated through an electron transfer to PtCl
6
has
oxyl-radical nature of the axial ligand and thus suffers the attack of
alkene to form the corresponding oxygenated compound through
quenched by the addition of Pt(IV)Cl 2
excited state = 2.2 ms, 1/
quenched by Pt(IV)Cl
ꢀ
(0.5 mM), while the triplet
the subsequent second electron transfer from the intermediate to
6
2
ꢀ1
2ꢀ
(t
T
t
T
= 4.5 ꢃ10 s
with a quenching constant k = 9.6 ꢃ10
. The photochemical oxygenation is thus concluded to
proceed through an electron transfer to K PtCl from the excited
)
was effectively
PtCl
6
as shown in Scheme 3.
2
ꢀ
6
As regards the formation of di-chlorinated product (6) only
observed under the neutral and weakly alkaline condition, the
following mechanism could be operating. The electron spin in the
6
ꢀ
1 ꢀ1
M
s
2
6
triplet state of the SiTMP. Since the rate of electron transfer is
one-electron oxidized species of SiTMP with different axial –H
–OH, –O group, depending on the pH condition of the reaction
mixture, has different population as shown in Fig. 4. Since the non-
2
deprotonated species, SiTMP(OH) , which is predominantly exist
2
O,
6
ꢀ
estimated to be rather slow in the range of 5 ꢃ10
ꢀ
1
10
ꢀ4
s
(10 (diffusion controlled rate constant) ꢃ [K
2
PtCl
6
](5 ꢃ10
6
ꢀ1
3
ꢀ1
6
ꢀ1 ꢀ1
M) = 5 ꢃ10 s ) ꢄ 4.8 ꢃ 10 s
3
ꢀ1
(ket = k(9.6 ꢃ10 M
s
) ꢃ [
ꢀ
4
K
2
PtCl
6
](5 ꢃ10 M) = 4.8 ꢃ 10 s ), it cannot compete with the
in acidic conditions (1.2 < pH < 3.7) as the pKa measurements
indicate in Table 1, the electron spin is delocalized on the porphyrin
ring without activating the axial hydroxy group (Fig. 5(c)). The
photooxygenation is considered to proceed on the activated oxyl-
radical form of the axial ligand of the one-electron oxidized SiTMP
8
ꢀ1
decay of the singlet state (1.7 ꢃ10 s ). As regards the reaction
mechanism, four intermediates were actually detected in the case
of Ru(II)TMP [7,8]. The similar ones are presumed to have key roles
also in the photochemical oxygenation sensitized by SiTMP. A
deeper insight can be obtained by estimating the electronic
structure of the one-electron oxidized species of SiTMP generated
by the electron transfer from the triplet excited SiTMP. Spin
ꢀ
ꢀ
2
(OH)O ) or SiTMP(O ) , but is silent for the delocalized spin on the
porphyrin ring of SiTMP(OH)
transfer from Cl ion in the reaction mixture to form chloride
radical inducing the dichlorinated reaction product.
2
which rather suffers electron
ꢀ
densities of the one-electron-oxidized SiTMP(OH)
2
(at pH 3),
ꢀ
ꢀ
2
SiTMP(OH)(O ) (at pH 4) and SiTMP(O )
(at pH 7) were estimated
In the present system, the sacrificial electron acceptor, Pt(IV)
2
ꢀ
,
by density functional theory (DFT) calculations (Gaussian09 rev.
D01 B3LYP/6-31G*). The electron spin densities have been revealed
to have exclusive populations localized on the deprotonated axial
Cl
6
serves as a crucial role to drive the photoreaction. Since the
photoreaction with most abundant Si atom within the molecule
can be accepted as one of the promising candidates of the half
reactions for oxidative activation of water molecule, further
development to visible light induced artificial photosynthesis
ꢅ
ligand as –O in the latter two cases. These strongly suggest that the
axially ligating hydroxy group on the central Si atom suffers an
oxidative activation to become oxyl-radical through the deproto-
nation and one-electron oxidation as shown in Fig. 5(a) and (b).
2
system coupled with TiO , as reported in the case of Sn(IV)TCPP
(tetra(4-carboxy) phenylporphyrinatotin(IV))/TiO2 [13] to evolve
ꢀ
ꢀ
Fig. 5. Spin population of the one-electron oxidized SiTMP(O )
2
(a), SiTMP (O )(OH) (b), and SiTMP(OH)
2
(c) calculated by Gaussian09 rev.D01 B3LYP/6-31G*.

S.N. Remello et al. / Journal of Photochemistry and Photobiology A: Chemistry 313 (2015) 176–183
669;
183
hydrogen in the reduction terminal end with simultaneous
(
e) J.A.T.P. Cissell Vaid, A.L. Rheingold, J. Am. Chem. Soc. 127 (2005) 12212;
f) D.C. Lee, G.M. Morales, Y. Lee, L. Yu, Chem. Commun. 1 (2006) 100.
formation of useful oxygenated substrate in the oxidation terminal
end, is now in progress. Si-porphyrins with easily available,
abundant element would serve as most promising sensitizers
which enable to induce the two-electron oxidative activation of
water.
(
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Supplementary information
Supplementary information (SI) available: fluorescence spec-
tral changes of Si(IV)TMP (SI-1), 1H NMR spectra of Si(IV)TMP
under various pH condition (SI-2).
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This work was partially supported by a Grant-in-Aid for
Scientific Research on Innovative Areas from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
[
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(
Appendix A. Supplementary data
(
5
(
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
jphotochem.2015.07.016.
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