G Model
JPC 9925 No. of Pages 6
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S. Mathew et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
as expected by each pKa value, but is almost constant within each
pH region as seen in Fig. 2. The Pourbaix diagram indicates that the
oxidation of each species of Al(III)TMP in protolytic equilibria
among their axial ligands is not coupled with proton transfer
processes, which should be much slower than the electron transfer
on the BDD electrode. The spectroscopic, photophysical, and
electrochemical characteristics of each species of Al(III)TMP with
different axial ligands are summarized in Table 1.
3.3. Photochemical oxygenation of cyclohexene with water sensitized
by aluminium(III) porphyrins with visible light
On the basis of the spectroscopic, photophysical, and electro-
chemical information obtained above, photochemical oxygenation
of substrate sensitized by Al(III)TMP was examined. As mentioned
above, the two-electron oxidation of water by one-photon
excitation Eq. (1) would be the more plausible alternative
compared to the four-electron oxidation and another crucial
problem to be resolved would be a possible utilization of major
elements rather than rare earth ones. Aluminum is the most
abundant metal and the third most abundant element on Earth. It
should be thus most interesting to explore a photochemical
reaction of aluminum porphyrin with water. When the Al(III)TMP
as a sensitizer, having a similar but less positive redox potential
(+0.71 V vs. SHE under the neutral condition) than that of Ru(II)
TMP (+1.03 V vs. SHE), was irradiated with visible light, the
photochemical oxygenation of cyclohexene as the substrate with
water in the presence of hexachloroplatinate (IV) (0.5 mM) as an
electron acceptor was found to be induced in deaerated aqueous
acetonitrile (CH3CN/H2O (9/1 v/v)). (Scheme 2) The photochemical
reaction products were the corresponding epoxide (cyclohexene-
oxide (1), 2-chlorocyclohexanol (2), 2-cyclohexenol (3), cyclohex-
anone (4), 2-cyclohexenone (5), and 1, 2-dichlorocyclohexane (6),
depending on the reaction conditions. (Table 2) The photochemical
oxygenation proceeds under either Q-band (560 nm) or Soret-band
(420 or 430 nm) excitation. As in the case of Ru(II)TMP, a weakly
alkaline condition ([OHꢀ] <1.5 mM) favoured the epoxide (1)
formation [14–17], while the alcohol (3) was selectively formed
under a stronger alkaline condition ([OHꢀ] = 5 mM). (Table 2) The
selectivity of the “epoxide” formation increased with decreasing
[OHꢀ] (Table 2). Under the neutral and the weakly alkaline
conditions ([OHꢀ] = 0.5 mM), another product, 2-chlorocyclohex-
anol (2), appeared. With no addition of KOH, the pH value of the
reaction mixture after the photoreaction decreased to 3.8, owing to
the formation of HCl Eq. (1). The “epoxide” was actually confirmed
to be slowly converted into 2-chlorocyclohexanol (2) under the low
pH condition (3.8) in the presence of HCl; thus, the newly appeared
(2) was thought to be derived from the reaction of the “epoxide”
with HCl generated during the photoreaction Eq. (1). This suggests
that the epoxide selectivity, including the formation of 2 (18.5%
(1) + 20.9 % (2) = 39.4%), is the highest under an alkaline condition
([OHꢀ] = 0.5 mM) (Table 1).
Scheme 2. Photochemical oxygenation of cyclohexene with water under alkaline
condition sensitized by Al(III)TMP.
(87.9:12.1) at [KOH] = 1.5 mM for each reaction product were 1
(9.9%), 2 (12.4%), 3 (10.6%), 4 (11.6%), and 5 (10.1%). The lmax of Al
(III)TMP in each alkaline condition was different: 427 nm for
[OHꢀ] = 1–5 mM, 422 nm for [OHꢀ] = 0.5 mM, and 419 nm under the
neutral condition. During the photoreaction, the lmax values under
alkaline conditions gradually shifted to shorter wavelengths and
finally reached 419 nm, which corresponds to that for neutral or
acidic conditions. The lmax of Al(III)TMP under each condition and
its shifting during the photoreaction well coincide with the
protolytic equilibria among the two axial ligands (Scheme 1) and
the pH change during the reaction as anticipated by Eq. (1), while
there was no induction period for the formation of oxygenated
reaction products. The quantum yields were thus estimated at an
early stage of the reaction, within several minutes after the start of
irradiation before the shifting of the lmax of Al(III)TMP. The
apparent quantum yields under various conditions are summa-
rized in Table 2. They are relatively low compared to those obtained
with Ru(II)TMP [14,15]. When other substrates such as norbornene
(E1/2 = 2.17 Volt vs SHE), styrene (E1/2 = 1.92 Volt vs SHE), and
1-hexene (E1/2 = 2.71 Volt vs SHE) were examined for the
photochemical oxygenation sensitized by Al(III)TMP, very inter-
estingly, almost no reaction was observed, indicating the sensitizer
Al(III)TMP exhibited
a high selectivity for cyclohexene (E1/
2 = 2.08 Volt vs SHE). The apparent lower quantum yield with
high selectivity can be ascribed to the lower oxidation ability of the
sensitizer Al(III)TMP (Eox = +0.71 V vs. SHE under the neutral
condition) than that of Ru(II)TMP (Eox = +1.03 V vs. SHE). Since
cyclohexene was the most reactive substrate in the case of Ru(II)
TMP, the lower oxidation ability of Al(III)TMP might not sensitize
the oxygenation of other less reactive substrates. In the case of
styrene with similar oxidation potential with cyclohexene, steric
factor of substrate might be also operating as observed in the case
of RuTMP(CO) [14,15].
3.4. Excited state of Al(III)TMP responsible for the photochemical
oxygenation of cyclohexene and the reaction mechanism
The oxygen atom of water was confirmed to be incorporated in
the oxygenated reaction products by means of experiments using
H218O: The 18O uptakes (%) under the experiment in H216OꢀꢀH218O
The excited singlet state of Al(III)TMP under the neutral
condition was determined to situate at 2.07 eV by the estimation
from the crossing point between the normalized absorption and
fluorescence spectra, while the excited triplet states of metal-
loporphyrins are considered to have their energy around ꢁ1.8 eV
[20]. The oxidation potential of Al(III)TMP (Eox = 0.71 Volt vs SHE),
the reduction potential of K2PtCl6 (Ered = 0.76 Volt vs SHE), and the
excited state energy clearly indicate that an electron transfer to
K2PtCl6 is sufficiently exo-ergonic either from the excited singlet
Table 1
Characteristics of Al(III)TMP with different axial ligands in CH3CN /H2O (9/1 v/v).
Al(III)TMP
pKa
Eox
Ered
tF/ns tT/ms
vs SHE
vs SHE
Al(III)TMP(H2O)2
pKa1 = 9.6
0.71 1.10
ꢀ1.53 ꢀ1.83
9.5
1.1
–
Al(III)TMP(OH)(H2O) pKa2 = 10.1 0.70 1.08 ꢀ1.53 ꢀ1.83 8.7
(D
G = ꢀ2.12 eV) or the triplet states ( G = ꢁꢀ1.85 eV) of Al(III)TMP.
D
Al(III)TMP(OH)2
Al(III)TMP(OH)(Oꢀ)
Al(III)TMP(Oꢀ)2
pKa3 = 10.7 0.69 0.97 ꢀ1.55 ꢀ1.83
pKa4 = 11.0 0.69 0.99 ꢀ1.56 ꢀ1.83
0.67 1.05 ꢀ1.58 ꢀ1.83
8.7
7.3
6.6
–
To examine which excited state is actually responsible in the
reaction mixture, quenching experiments were carried out. Very
interestingly fluorescence (tF = 9.5 ns, 1/tF = 1.1 ꢂ108 sꢀ1) of Al(III)
1.1
1.0
Please cite this article in press as: S. Mathew, et al., Photochemical oxygenation of cyclohexene with water sensitized by aluminium(III)