Visible light induced photo-oxidation of water. Formation of intermediary hydroxyl radicals through the photoexcited triplet state of perfluorophenazine

Article abstract of DOI:10.1039/a604920f

1,2,3,4,5,6,7,8-Octafluorophenazine (F-Phen) has an absorption spectrum in the longer wavelength extending to the visible-light region and has a more positive oxidation potential than for unfluorinated phenazine. F-Phen has been photolysed with water in a

Full text of DOI:10.1039/a604920f

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Visible light induced photo-oxidation of water. Formation of
intermediary hydroxyl radicals through the photoexcited triplet state
of perÑuorophenazine
Takayuki Kitamura,a Hiroyuki Fudemoto,a Yuji Wada,a Kei Murakoshi,a Mitsuhiro Kusaba,b¤
Nobuaki Nakashima,b* Tetsuro Majimac and Shozo Yanagidaa*
a Material and L ife Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565,
Japan
b Institute of L aser Engineering, Osaka University, Suita, Osaka 565, Japan
c T he Institute of ScientiÐc and Industrial Research, Osaka University, Ibaraki , Osaka 567,
Japan
1,2,3,4,5,6,7,8-OctaÑuorophenazine (F-Phen) has an absorption spectrum in the longer wavelength extending to the visible-light
region and has a more positive oxidation potential than for unÑuorinated phenazine. F-Phen has been photolysed with water in
acetonitrile under visible-light irradiation to produce stoichiometrically 1,3,4,5,6,7,8-heptaÑuoro-2-hydroxyphenazine (F-Phen-2-
OH). Photolysis of F-Phen with water in the presence of benzene leads to the formation of phenol followed by the disappearance
of F-Phen. EPR analysis and laser Ñash photolysis reveal that the photoexcited triplet state of F-Phen participates in water
oxidation to hydroxyl radicals with concurrent conversion to F-Phen-2-OH. The mechanism is discussed with the results of
semi-empirical molecular orbital calculations.
The photoassisted splitting of water molecules to hydrogen
and oxygen has attracted much attention as a potential solar
energy conversion and storage system.1 In such systems, the
development of efficient photosensitizers or photocatalysts to
split water under visible-light irradiation is one of the impor-
tant topics. We reported recently that perÑuorinated poly-
and oligo-(p-phenylene)s (F-PPP-n, F-OPP-n; n denotes the
number of phenyl rings in the chain) photosensitized catalytic
oxidation of water to hydroxyl radicals under UV-light irra-
diation in the presence and absence of oxygen and that the
resulting hydroxyl radicals were scavenged by benzene to give
phenol.2h5 Spectroscopic analysis of transient spectra of per-
Ñuorinated p-terphenyl (F-OPP-3) as a model molecule of
F-PPP-n and F-OPP-n revealed that the radical cation of
F-OPP-3 formed from the photoexcited singlet state.5 Electro-
chemical analysis conÐrmed that F-OPP-3 had a more posi-
tive redox potential (high enough to oxidize water to hydroxyl
radicals) than p-terphenyl (OPP-3).2 In addition, semi-
empirical molecular orbital calculations with MOPAC also
suggested that perÑuorination of aromatic molecules like
OPP-3 should have resistance to oxidative degradation.2
In linear aromatic compounds e.g. a series of p-phenylenes,
however, perÑuorination induces a hypsochromic shift, and
the bathochromic shift which is anticipated from an increase
of chain length is found to be negligible.2 This is interpreted as
owing to an increase of the dihedral angles between the neigh-
bouring phenyl rings because of the steric repulsion between
large Ñuorine atoms at ortho-positions of the neighbouring
phenyl rings. Such repulsion forbids their conjugation of the n
electron system, resulting in the negligible bathochromic shift
in absorption spectra even when the chain length is increased.
On the other hand, we attempted to calculate the absorption
spectra of perÑuorinated condensed aromatic compounds by
semi-empirical molecular orbital calculations with ZINDO.
The results indicated that they would have absorption spectra
shifted to longer wavelength than the original aromatics. With
these facts in mind, we aim to examine some perÑuorinated
condensed aromatics as photosensitizers for water oxidation
under visible light irradiation.
In this paper, we deal with the photochemistry of 1,2,3,4,5,6,
7,8-octaÑuorophenazine (perÑurophenazine, F-Phen) as an
example of perÑuorinated condensed aromatics that can
absorb visible light, because its synthesis has been reported.6
The photophysical and electrochemical properties of F-Phen
are characterized for the purpose of water oxidation under
visible-light irradiation. We have now found that F-Phen
induces photooxidation of water in its photoreaction.
Experimental
Materials
For the synthesis of perÑuorophenazine, commercially avail-
able 2,3,4,5,6-pentaÑuoroaniline (Aldrich, 99%) and lead tetra-
acetate (WAKO, Chemical reagent grade) were used without
further puriÐcation, and benzene was used after distillation in
the presence of calcium hydride (CaH : EP grade). For spec-
2
troscopic measurements, acetonitrile (AN: GR grade), cyclo-
hexane (CH: Spectro grade) and 1-chlorobutane (BuCl:
Spectro grade) were distilled in the presence of CaH , and 2-
2
methyltetrahydrofuran (MTHF: GR grade) was distilled in
the presence of sodium metal. Finally all the solvents were
puriÐed further by repeated bulb-to-bulb distillation in the
presence of CaH or Na. Tetrabutylammonium hexa-
2
Ñuorophosphate (TBAPF : GR grade) was recrystallized three
6
times from an EtOHÈH O \ 1 : 1 solution and dried at 45 ¡C
in vacuo for 3 h before use for electrochemical measurements.
2
AN used for electrochemical measurements was obtained by
distillation three times in the presence of CaH . Iron(II)
¤
Present address: Laser Engineering Laboratory, Department of
2
ammonium sulfate [Fe(NH ) (SO ) É 6H O: EP Grade] and
Electrical Engineering and Electronics, Faculty of Engineering, Osaka
Sangyo University, Diato, Osaka 574, Japan.
”
4 2
4 2
2
hydrogen peroxide (H O : EP grade) were used for Fenton
2
2
Present address: Department of Chemistry, Faculty of Science,
reaction as received. 5,5-Dimethyl-1-pyroline N-oxide
Osaka City University, Sumiyoshi-ku, Osaka 558, Japan.
(DMPO: Dojin, specially prepared reagent) as a spin trapping
J. Chem. Soc., Faraday T rans., 1997, 93(2), 221È229
221

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evaporated to dryness under reduced pressure, giving a black
residue (4.71 g). The residue was chromatographed over silica
gel. After orange perÑuoroazobenzene was eluted by
cyclohexaneÈbenzene \ 9 : 1, light orange F-Phen was eluted
by benzene. Recrystallization from benzene gave light-orange
coloured F-Phen (0.92 g, 20%), mp 234 ¡C (lit.,6 239, and lit.,7
2
34); d (CDCl ) [ 150.36 (d, 2-F) and [146.13 (d, 1-F); m/z
F
3
(E ) 324 (M`, 100%).
I
Electrochemical measurements
The redox potentials of F-Phen and Phen in AN solution
were measured by cyclic voltammetry using an argon-purged
AN solution containing F-Phen or Phen (2 ] 10~3 mol
dm~3) and TBAPF (0.2 mol dm~3) as a supporting electro-
6
lyte. Redox potentials were standardized by Ag/0.01 mol
dm~3 AgNO as a reference electrode and platinum wires
3
(
/ \ 0.3 mm) were used as working and counter electrodes.
Photoreactions of F-Phen with water
Photoreaction monitored by UV–VIS spectroscopy. In a
quartz cuvette cell (optical pathlength: d \ 10 mm), an AN
solution (3 ] 10~3 dm3) containing F-Phen (1 ] 10~4 mol
dm~3) and H O (0.06 ] 10~3 dm3: 1.1 mol dm~3) was
2
degassed by freezeÈpumpÈthaw cycles and sealed, and the
solution was then irradiated by a 300 W halogenÈtungsten
lamp through a saturated sodium nitrite Ðlter (j [ 400 nm).
The reaction was followed by UVÈVIS spectrometer.
Scheme 1
Photoreactions followed by products analysis. In a Pyrex
tube (id \ 8 mm), an AN (2 ] 10~3 dm3) solution containing
F-Phen (1 ] 10~3 mol dm~3) and water (0.04 ] 10~3 dm3:
reagent for EPR detection was used without further puriÐ-
cation.
1.1 mol dm~3) was irradiated after the system was purged
Instruments
with argon to eliminate dissolved air and oxygen. Photolysis,
in the presence of benzene, was carried out in a Pyrex tube
Because perÑuorinated compounds sublime so easily, melting
points were obtained as extrapolated onsets by a di†erential
scanning calorimetry method recorded on a Seiko Instru-
ments, DSC 22C calorimeter (sample weight: 5 mg; tem-
perature range: 30È300 ¡C; increase rate: 10 ¡C min~1). 19F
NMR spectra were recorded in CDCl , CD CN and
(id \ 8 mm), on an AN (2 ] 10~3 dm3) solution containing
F-Phen (1 ] 10~3 mol dm~3), water (0.04 ] 10~3 dm3: 1.1
mol dm~3) and benzene (0.4 ] 10~3 dm3: 4.5 mol dm~3)
under an atmosphere of argon or oxygen gas.2 The amount of
F-Phen was determined by HPLC [column: Nacalai Tesque,
COSMOSIL 5C18Èwater (4.6 mm ] 250 mm); eluant:
AN : water (w/w) \ 50: 50; Ñow rate: 600 ] 10~3 dm3 min~1;
UV detector: j \ 254 nm]. Phenol was analysed by GLC
3
3
CD COCD at 254.05 MHz on a JEOL, JNM-EX 270 instru-
3
3
ment. Chemical shift values were calibrated to triÑuoroacetic
acid at d [ 77.000 or 1,1,1-trichloro-2,2,2-triÑuoroethane
[
column: PEG-20M (5%) (3 mm ] 1 m); carrier gas: N ; Ñow
(
CCl CF ) at d [ 81.890 used as standards. UVÈVIS absorp-
2
3
3
rate: 30 ] 10~3 dm~3 min~1: column temperture: 160 ¡C;
injection temperature: 190 ¡C; detector: FID], and hydrogen
peroxide by colorimetry using a methanolic Ti(SO )
tion.2
tion spectra, steady-state Ñuorescence spectra and lifetime of
Ñuorescence were determined as reported previously.5 The
Ñuorescence quantum yield was determined by using 9,10-
diphenylanthracene in CH (0.90) as a standard. FTIR spectra
were recorded on a Perkin-Elmer, System 200 FTIR spectro-
meter. GCMS analyses were conducted on a Shimadzu,
GCMS-QP2000A instrument combined with a GC-14A [GC/
column: Hi-Cap CBP-1 (equivalent to OV-1, 0.2 mm ] 25 m);
carrier gas: He; Ñow pressure: 1.0 kg cm~2; column tem-
perature: 100È180 ¡C; injection temperature: 250 ¡C, MS/
2 solu-
4
Photoreaction monitored by 19F NMR spectroscopy. A
CD CN solution (1 ] 10~3 dm3) containing F-Phen
3
(5.4 ] 10~3 mol dm~3) and H O (0.05 ] 10~3 dm3: 2.8 mol
2
dm~3) was placed in an NMR tube with triÑuoroacetic acid
sealed in a glass capillary as an external standard and then
purged with argon gas. The solution was irradiated for 9 h
under the same conditions described in the above section. One
couple of doublet signals of F-Phen faded out and then eight
signals appeared by visible-light irradiation, assigned as/due
to the formation of 1,3,4,5,6,7,8-heptaÑuoro-2-hydroxy-
phenazine (F-Phen-2-OH) as described in the identiÐcation of
the photoproduct as 1,3,4,5,6,7,8-heptaÑuoro-2-hydroxy-
phenazine (F-Phen-2-OH) and in the molecular orbital calcu-
lations sections. The signal observed at d [ 177.42 was
ascribed to hydrogen Ñuoride (HF). 19F NMR before irradia-
tion: d (aqueous CD CN) [ 151.86 (d, 2-F) and [148.69 (d,
temperature: 250 ¡C; ionization: E (70 eV)]. EPR spectra
I
were recorded on a JEOL, JES-RE2X spectrometer.
Preparation of 1,2,3,4,5,6,7,8-octaÑuorophenazine (F-Phen)
1,2,3,4,5,6,7,8-OctaÑuorophenazine (perÑuorophenazine, F-
Phen) was synthesized by HaszeldineÏs method6 as follows.
In a three-necked Ñask with a condenser, lead tetraacetate
(
25.0 g, 55 mmol) was added to a benzene solution (0.15 dm3)
of 2,3,4,5,6-pentaÑuoroaniline (5.0 g, 27 mmol), and the
mixture was heated under reÑux for 1 h. The resulting dark
brown suspension was washed successively with 50% aqueous
acetic acid, water, saturated aqueous sodium hydrogen car-
bonate, and saturated aqueous sodium chloride, and the
resulting organic phase was dried with magnesium sulfate and
F
3
1-F); 19F NMR after 9 h irradiation: d (aqueous CD CN)
F
3
[ 177.42 (s, br, HF), [161.26 (d, 1-F), [152.93 (d, 3-F),
[152.48 (dd, 6-F), [151.91 (dd, 7-F), [150.93 (dd, 8-F),
[150.44 (dd, 5-F), [148.37 (dd, 4-F).
222
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

View Article Online
Fenton reaction.8 Into a mixture of the same quantity
10 ] 10~3 dm3) of an aqueous solution of Fe(NH ) (SO )
with hydroxide ion (OH~) by assuming that the transient p-
quinonoid structure of the OH~ adduct might be favourable
for production of F-Phen-2-OH.9 However, the HudsonÏs
product we prepared was assigned to 2,3,4,5,6,7,8-heptaÑuoro-
1-hydroxyphenazine (F-Phen-1-OH) on the basis of the fol-
lowing analysis; mp 267 ¡C (decomp.) (lit.,9 272È273); m/z (EI)
(
(
4
2
4 2
0.36 mol dm~3) and an AN solution of F-Phen (1 ] 10~3
mol dm~3), an aqueous solution of 30% H O (10 ] 10~3
2
2
dm3) was added dropwise under stirring at room temperature
The resulting red solution was condensed under reduced pres-
sure and then the aqueous residue was extracted with diethyl
ether. The extract was dried with sodium sulÐde, condensed
and analysed by GCÈMS.
322 (M`, 100); l (KBr)/cm~1 3100 (br, OwH stretching),
max
1680, 1580, 1520, 1500 and 1360 (aromatic ring stretching),
1300 (OwH in-plane bending), 1090 and 1050 (CwF
stretching); d (CD COCD ) [155.59 (d, 2-F), [155.37 (dd,
F
3
3
IdentiÐcation of the photoproduct as 1,3,4,5,6,7,8-
heptaÑuoro-2-hydroxyphenazine (F-Phen-2-OH). The reaction
mixture was evaporated to dryness under reduced pressure
and the residue was puriÐed by using recycling gel permeation
chromatography as described previously.5 The collected frac-
tion was evaporated in vacuo and the orange-coloured photo-
product was analysed by IR, MS and 19F NMR spectroscopy:
mp 259 ¡C (decomp.); MS: m/z (EI) 322 (M`, 100); IR:
3-F), [153.88 (dd, 6-F), [153.82 (dd, 7-F), [153.55 (dd, 8-F),
[152.53 (dd, 5-F), [145.83 (d, 4-F) (see Scheme 1). 19F NMR
analysis gave the seven signals with the same integral value,
which were di†erent from those of our product (F-Phen-2-
OH).
The assignment was also supported by MO calculations
using MOPAC (see the molecular orbital calculations section)
as follows; (i) C(1) of F-Phen was more favourable for nucleo-
philic attack than C(2), because the partial charge of the
former was more positive than the latter (ii) the heats of for-
mation of the intermediary OH~ adduct at C(1) were smaller
by 1.9È5.3 kcal mol~1 than that of the OH~ adduct at C(2).
The di†erence in the wavenumber of the OwH bond
l
(KBr)/cm~1 3360 (br, OwH stretching), 1680, 1580, 1520,
max
1
490 and 1360 (aromatic ring stretching), 1330 (OwH in-
plane bending), 1080 and 1060 (CwF stretching). 19F NMR
analysis gave the following seven signals with the same inte-
gral value. From the 19FÈ19F COSY spectrum the signals
were assigned on the basis of the speciÐc coupling between
ortho- and para-Ñuorine atoms with J \ 15 Hz at the benze-
noid rings and the charge density at each Ñuorine atom deter-
mined by semi-empirical molecular orbital (MO) calculations
using MOPAC (see the molecular orbital calculations section
and Scheme 1) as follows; d (CD CN) [ 162.54 (d, 1-F),
bending between F-Phen-1-OH (l \ 1300 cm~1) and
max
F-Phen-2-OH (l \ 1330 cm~1) was worth noting for valid-
max
ity of the assignment. This observation could be explained by
the fact that the OwH bending vibration should be restricted
by the intramolecular interaction of hydroxy group with the
lone pair electron of the nitrogen atom in F-Phen-1-OH
rather than in F-Phen-2-OH. In fact, the OwH bond faced
toward the nitrogen atom in the energetically stable structure
of F-Phen-1-OH optimized by the MO calculations [Fig.
1(a)].
F
3
[
[
154.46 (d, 3-F), [153.92 (dd, 6-F), [153.41 (dd, 7-F),
152.25 (dd, 8-F), [151.94 (dd, 5-F), [149.92 (dd, 4-F).
Hudson et al. reported that F-Phen-2-OH should be pro-
duced by the nucleophilic substitution reaction of F-Phen
Fig. 1 Energetically optimized geometries of monohydroxylated F-Phen (F-Phen-1-OH and F-Phen-2-OH) by MOPAC with various Hamilto-
nians: upper MNDO; middle AM1; lower PM3, bond lengths and atom distances in A
Ž
and bond angles in degrees.
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
223

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Table 1 Heats of formation of F-Phen, products and intermediary species from F-Phen calculated by MOPACa
species
F-Phen
MNDO
AM1
AM1ÈMNDO
PM3
PM3ÈMNDO
[277.40242
[353.14860
[174.86385
[175.86014
[405.22582
[403.36323
[283.29969
[281.36712
[281.94545
[281.34804
[239.54184
[308.21576
[135.45257
[136.49589
[362.74984
[360.33016
[241.54721
[240.30860
[241.57796
[241.04784
[269.74069
[349.04179
[168.15920
[169.31097
[395.63411
[393.34903
[275.25095
[272.57516
[273.18574
[272.54397
[251.55998
Èb
[271.49621
Èb
F-Phen~~
F-Phen-1~
[147.10089
[146.66409
[380.12684
[375.79105
[255.32942
[253.81621
[253.85426
[253.28083
[168.54403
[169.79025
[399.00656
[393.75391
[276.94425
[275.08952
[275.62634
[275.02574
F-Phen-2~
F-Phen-1-OH~
F-Phen-2-OH~
F-Phen-1-OH(a)
F-Phen-1-OH(b)
F-Phen-2-OH(c)
F-Phen-2-OH(d)
a Structural geometries were optimized energetically by using MNDO, AM1 and PM3 Hamiltonians, respectively. The 1SCF energy calculations
were repeated by using MNDO Hamiltonian about the optimized structures obtained with AM1 and PM3 Hamiltonians. b Optimization of
structure of F-Phen~~ failed in the SCF calculations.
Table 2 Partial charges of atoms of F-Phen calculated by MOPACa
position
MNDO
AM1
AM1ÈMNDO
PM3
PM3ÈMNDO
C(1),(4),(5),(8)
C(2),(3),(6),(7)
N(9),(10)
C(11),(12),(13),(14)
F(1),(4),(5),(8)
F(2),(3),(6),(7)
0.1717
0.1298
0.0982
0.0575
[0.0468
[0.0239
[0.0506
[0.0578
0.1773
0.1382
0.0840
0.0475
0.0490
[0.0566
[0.0485
[0.0509
0.1763
0.1352
[0.1071
[0.1015
[0.1092
0.0169
0.0189
0.0194
[0.1288
[0.1360
[0.1390
[0.1446
[0.1355
[0.1409
a Structural geometries were optimized energetically by using MNDO, AM1 and PM3 Hamiltonians, respectively. The 1SCF energy calculations
were repeated by using MNDO Hamiltonian about the optimized structures obtained with AM1 and PM3 Hamiltonians.
EPR analysis
pulse duration: 30 ns). Other instruments and conditions were
the same as in the previous paper.5
An AN solution containing F-Phen (1 ] 10~3 mol dm~3),
water (1.1 mol dm~3), and DMPO (0.1 mol dm~3) was placed
in an EPR tube (id \ 2.6 mm) and degassed by freezeÈpumpÈ
thaw cycles. The spectrum was recorded at room temperature
during irradiation with a 500 W high-pressure mercury arc
lamp through a 390 nm cut-o† Ðlter under the following con-
ditions: magnetic Ðeld, 338 ^ 7.5 mT; Ðeld modulation width,
c-Radiolysis
Each solution in a Supersil cell (d \ 1.5 mm) containing
F-Phen (5 ] 10~3 mol dm~3) in dry BuCl or MTHF was
degassed by freeze-pump-thaw cycles and sealed in vacuo. c-
Radiolysis was conducted in the same way as in the previous
paper.5
0
.01 mT; microwave power, 4 mW. Isotropic simulations of
the EPR signals were performed on a JEOL, System ESPRIT-
30 ver. 1.205 instrument.
Molecular orbital calculations
3
Procedure of molecular orbital calculations. The geometric
and electronic structures of F-Phen, products and inter-
mediary species from F-Phen, i.e., F-Phen, F-Phen~~, two
isomers of the F~-eliminated neutral radicals (F-Phen-1~ and
F-Phen-2~), two isomers of the intermediary OH~ adducts of
F-Phen (F-Phen-1-OH~ and F-Phen-2-OH~) and two con-
formations of two isomers of monohydroxylated F-Phen (F-
Phen-1-OH and F-Phen-2-OH) (Scheme 1), were obtained by
semi-empirical molecular orbital (MO) calculations using
MOPAC (ver. 6.10) in the CAChe Worksystem (ver. 3; CAChe
ScientiÐc, Inc.). Geometries were optimized energetically by
using MNDO, AM1 and PM3 Hamiltonians, respectively.
Then the 1SCF energy calculations were made again by using
MNDO Hamiltonian about the optimized structures obtained
with AM1 and PM3 Hamiltonians. The results of the calcu-
lations are summarized in Table 1È4 and Fig. 1. Optimization
of the structure of F-Phen~~ by the PM3 Hamiltonian failed
in the SCF calculations.
Laser Ñash photolysis
An AN solution containing F-Phen was placed into a rec-
tangular quartz call (d \ 40 ] 10 ] 10 mm) and then
degassed by repeated freezeÈpumpÈthaw cycles. The sample
was irradiated by the fundamental of an XeF-excimer laser
(
Lambda Physik, EMG 201 MSC; peak power at j \ 351 nm,
Table 3 Partial charges of atoms of F-Phen~~ calculated by
MOPACa
position
MNDO
AM1
AM1ÈMNDO
C(1),(5)
C(2),(6)
C(3),(7)
C(4),(8)
N(9),(10)
C(11),(13)
C(12),(14)
F(1),(5)
F(2),(6)
F(3),(7)
F(4),(8)
0.0598
0.1190
0.0213
[0.0020
0.0438
0.0662
0.1289
0.0366
[0.0308
0.1403
0.0597
0.1407
Heats of formation. Table 1 lists the heats of formation of
all the species. Among the two F~-eliminated neutral radicals
from F-Phen~~, F-Phen-2~ was more stable than F-Phen-1~ by
[0.2275
[0.1730
0.0283
[0.2245
0.0738
0.0738
[0.0148
[0.1610
[0.1732
[0.1745
[0.1631
[0.0397
[0.0901
[0.1021
[0.1026
[0.0915
[0.0066
[0.1729
[0.1834
[0.1836
[0.1750
1.0È1.2 kcal mol~1. The heats of formation of F-Phen-1-OH~
were smaller by 1.9È5.3 kcal mol~1 than that of F-Phen-2-
OH~. Both F-Phen-1-OH and F-Phen-2-OH had two optim-
ized conformations with local minima in energy, where all of
the atoms were in the same plane and the directions of the
OwH bonds were di†erent to each other as shown in Fig. 1.
The di†erences in the heats of formation between the respec-
tive conformations were only 1.3È2.7 kcal mol~1. In F-Phen-
a Structural geometries were optimized energetically by using MNDO
and AM1 Hamiltonians, respectively. The 1SCF energy calculations
were repeated by using the MNDO Hamiltonian about the optimized
structures obtained with AM1 Hamiltonians.
224
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Table 4 Partial charges of atoms of F-Phen-1-OH and F-Phen-2-OH calculated by MOPACa
F-Phen-1-OH
MNDO
AM1
AM1ÈMNDO
(a) (b)
PM3
PM3ÈMNDO
position
(a)
(b)
(a)
(b)
(a)
(b)
(a)
(b)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
N(9)
N(10)
C(11)
C(12)
C(13)
C(14)
F(2)
F(3)
F(4)
F(5)
F(6)
F(7)
F(8)
O
0.1341
0.1048
0.1545
0.1366
0.1759
0.1238
0.1335
0.1622
[0.1509
[0.1086
[0.0019
0.0315
0.0170
0.0202
[0.1403
[0.1383
[0.1347
[0.1284
[0.1370
[0.1369
[0.1354
[0.2125
0.2306
0.1575
0.0303
0.1641
0.1349
0.1699
0.1289
0.1221
0.1761
[0.0955
[0.1168
0.0238
0.0312
0.0218
0.0838
0.0524
0.0759
0.0716
0.1016
0.0533
0.0614
0.0901
[0.0876
[0.0459
[0.0426
[0.0131
[0.0236
[0.0189
[0.0580
[0.0595
[0.0558
[0.0499
[0.0583
[0.0582
[0.0568
[0.2173
0.2553
0.1046
[0.0264
0.0863
0.0695
0.0956
0.0585
0.0512
0.1028
[0.0299
[0.0549
[0.0114
[0.0124
[0.0186
[0.0365
[0.0815
[0.0603
[0.0559
[0.0519
[0.0592
[0.0599
[0.0517
[0.1983
0.2402
0.1180
0.1187
0.1611
0.1437
0.1814
0.1324
0.1418
0.1682
[0.1448
[0.1023
0.0012
0.0336
0.0185
0.1430
0.0422
0.1724
0.1420
0.1752
0.1378
0.1308
0.1818
[0.0890
[0.1115
0.0257
0.0327
0.0241
0.1325
[0.0596
0.0809
0.0474
0.0819
0.0473
0.0400
0.0884
0.0642
0.1111
0.0168
0.0721
0.0483
0.0881
0.0421
0.0517
0.0740
0.0077
0.1453
0.0397
0.1683
0.1414
0.1745
0.1345
0.1278
0.1806
[0.0998
[0.1189
0.0273
0.0326
0.0240
0.1195
0.1138
0.1585
0.1428
0.1804
0.1294
0.1390
0.1669
[0.1526
[0.1097
0.0002
0.0346
0.0195
0.0358
0.0509
[0.0591
[0.0408
[0.0495
[0.0719
[0.0660
[0.0546
[0.0525
[0.0495
[0.0524
[0.0529
[0.0503
[0.1727
0.2136
[0.0844
[0.0420
[0.0574
[0.0520
[0.0548
[0.0537
[0.0523
[0.0482
[0.0514
[0.0515
[0.0530
[0.1893
0.2273
0.0020
0.0225
0.0043
0.0062
0.0229
[0.1603
[0.1394
[0.1349
[0.1301
[0.1379
[0.1386
[0.1310
[0.1989
0.2209
[0.1470
[0.1475
[0.1447
[0.1387
[0.1455
[0.1455
[0.1454
[0.2010
0.2212
[0.1686
[0.1480
[0.1450
[0.1403
[0.1464
[0.1471
[0.1411
[0.1859
0.2110
[0.1650
[0.1445
[0.1413
[0.1369
[0.1428
[0.1434
[0.1377
[0.1810
0.2092
[0.1452
[0.1437
[0.1411
[0.1351
[0.1418
[0.1418
[0.1417
[0.1934
0.2186
H
F-Phen-2-OH
MNDO
AM1
AM1ÈMNDO
(c) (d)
PM3
PM3ÈMNDO
position
(c)
(d)
(c)
(d)
(c)
(d)
(c)
(d)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
N(9)
N(10)
C(11)
C(12)
C(13)
C(14)
F(1)
F(3)
F(4)
F(5)
F(6)
F(7)
F(8)
O
0.0773
0.1149
0.1397
0.1731
0.1770
0.1212
0.1294
0.1679
[0.1209
[0.1036
0.0466
0.0022
0.0028
0.1482
0.0930
0.0847
0.1922
0.1751
0.1224
0.1290
0.1687
0.0193
0.0609
0.0762
0.1015
0.1024
0.0512
0.0583
0.0948
0.0934
0.0414
0.0160
0.1184
0.1008
0.0524
0.0577
0.0958
[0.0540
[0.0444
[0.0102
[0.0400
[0.0331
[0.0179
[0.0508
[0.0786
[0.0520
[0.0524
[0.0598
[0.0592
[0.0516
[0.2138
0.2418
0.0869
0.1005
0.1486
0.1800
0.1829
0.1297
0.1385
0.1732
[0.1155
È0.0985
0.0487
0.0050
0.0050
0.1588
0.0773
0.0945
0.1991
0.1810
0.1310
0.1378
0.1743
[0.1116
È0.0988
0.0366
0.0000
0.0064
[0.0177
0.0964
0.0498
0.0875
0.0888
0.0396
0.0475
0.0801
0.0309
0.0548
0.0757
0.0865
0.1023
0.1447
0.1782
0.1815
0.1270
0.1351
0.1726
[0.1226
È0.1057
0.0478
0.0057
0.0063
0.1562
0.0794
0.0923
0.1966
0.1796
0.1283
0.1344
0.1736
[0.1184
[0.1059
0.0362
0.0004
0.0078
[0.0066
0.1036
0.0866
0.0410
0.0465
0.0815
0.0375
0.0539
[0.0372
[0.0773
[0.0682
[0.0491
[0.0523
[0.0640
[0.0508
[0.0503
[0.0528
[0.0525
[0.0496
[0.1863
0.2158
[0.1167
[0.1035
0.0350
[0.0576
[0.0439
0.0002
0.0550
[0.0255
[0.0730
[0.0698
[0.0487
[0.0635
[0.0529
[0.0509
[0.0499
[0.0528
[0.0527
[0.0505
[0.1868
0.2190
[0.0036
[0.0359
[0.0342
[0.0187
[0.0748
[0.0549
[0.0523
[0.0520
[0.0599
[0.0595
[0.0529
[0.2129
0.2449
0.0042
0.0229
0.0244
0.0249
0.0258
0.0249
0.0261
[0.1532
[0.1369
[0.1316
[0.1305
[0.1387
[0.1382
[0.1312
[0.2147
0.2245
[0.1331
[0.1571
[0.1312
[0.1308
[0.1385
[0.1379
[0.1301
[0.2156
0.2213
[0.1634
[0.1442
[0.1420
[0.1407
[0.1470
[0.1467
[0.1415
[0.1985
0.2143
[0.1415
[0.1659
[0.1413
[0.1411
[0.1469
[0.1464
[0.1403
[0.1996
0.2108
[0.1597
[0.1424
[0.1385
[0.1372
[0.1434
[0.1430
[0.1380
[0.1948
0.2127
[0.1397
[0.1621
[0.1381
[0.1375
[0.1433
[0.1428
[0.1368
[0.1955
0.2092
H
a Structural geometries were optimized energetically by using MNDO, AM1 and PM3 Hamiltonians, respectively. The 1SCF energy calculations
were repeated by using MNDO Hamiltonian about the optimized structures obtained with AM1 and PM3 Hamiltonians.
1
-OH, the conformation (a) whose OwH bond faced the
1.321 A
F-Phen~~, the CwF bonds at the a-positions were shortened
drastically [from 1.474 to 1.348 (1- and 5-position) or 1.353 A
(4- and 8-position)] and those of the b-positions were
lengthened (from 1.349 to 1.353 A).
The charge distribution of F-Phen also had D symmetry,
Ž (b-positions)], but in the AM1-optimized F-Phen and
nitrogen atom was found to be more stable than another con-
formation (b). In F-Phen-2-OH, the conformation (c) was
more stable than (d).
Ž
Ž
2
h
Comparison between F-Phen and F-Phen~—. The optimized
geometries of F-Phen had D symmetry, but F-Phen~~ had
and the partial charge on Ñuorine atoms was slightly more
negative at F(2) than at F(1). The partial charge on carbon
atoms was more positive at C(1) than at C(2) (Table 2) leading
to favourable OH~ attack at C(1). In F-Phen~~, the charge
2
h
only C symmetry. The CwF bonds in the MNDO-optimized
2
F-Phen~ were slightly lengthened when compared with those
of F-Phen [from 1.316 to 1.322 (a-positions) and from 1.318 to
distribution had C symmetry and the partial charges on all
2
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
225

View Article Online
Ñuorine atoms increased and the di†erence in charge distribu-
tion at a- and b-positions became more remarkable (Table 3).
On the other hand, the average partial charge of C(2) and C(3)
was less positive than that of C(1) and C(4). These calculations
suggest that the CwF bond should be more weaker at the
b-position than at the a-position in F-Phen~~.
in AN at room temperature (Fig. 3). The lifetime and quantum
yield of the emission were determined to be 23 ns and 0.1,
respectively, regardless of the excitation bands. The shape of
the emission in AN was unchanged within the range of con-
centration of F-Phen from 1 ] 10~6 to 5 ] 10~3 mol dm~3.
The emission spectrum in cyclohexane (Fig. 3) shifted to
shorter wavelength than in AN with vibrational structures due
to the aromatic ring stretching (ca. 1500 cm~1). These results
suggest that the emission of F-Phen is ascribed to the nÈn*
transition. F-Phen did not show phosphorescence in an
ethanol glass matrix at 77 K, while Phen has a phosphor-
escence at around j \ 640 nm under the same conditions.10
Explanation of the signal of F-Phen-1-OH and F-Phen-2-
OH in 19F NMR. In order to assign the signals of the present
photoproduct and the HudsonÏs product in 19F NMR, the
charge densities of Ñuorine atoms on F-Phen-1-OH [(a) and
(
b)] and F-Phen-2-OH [(c) and (d)] were obtained as listed in
Table 4. F(8) in both conformers of the two isomers were
more negatively charged than F(5), so that its 19F NMR
signal should appear at higher magnetic Ðeld. In F-Phen-1-
OH, F(4) was more negatively charged than F(2) in the two
conformers. The assignment in F-Phen-2-OH was made by
using the conformation (c), where F(1) was more electronically
shielded than F(3).
Electrochemical properties
Redox potentials for reduction of F-Phen and Phen in AN
were obtained by cyclic voltammetry. The reduction potential
of Phen was [1.45 V vs. Ag/AgNO , as observed by Sawyer
3
et al. ([1.13 V vs. SCE11). F-Phen was reduced reversibly to
its radical anion by a cathodic sweep, and the redox potential
These MO calculations support that fact that the F~ elimi-
nation should occur on the 2-position of F-Phen~~, and F-
Phen~~ and HO~ should be converted selectively into F-Phen-
was obtained as [0.89 V vs. Ag/AgNO . This value is in con-
3
trast with the value of [1.45 V vs. Ag/AgNO for Phen, indi-
3
cating that perÑuorination induces a positive shift in the redox
2
-OH. In contrast, the nucleophilic addition of OH~ to
potential by 0.6 V.
F-Phen should occur at C(1) rather than C(2), giving F-Phen-
-OH selectively.
From the reduction potential and the di†erence between the
energy of HOMO and LUMO, i.e., the 0È0 band energy, 2.8
eV, determined from the excitation and emission spectra (Fig.
2), the redox potential of F-Phen/F-Phen~` couple was calcu-
lated to be ]2.2 V vs. SCE. It is difficult to measure redox
potentials of the F-Phen~~/F-Phen* and F-Phen*/F-Phen~`
couples. However, the redox potential of the F-Phen/F-
Phen~` couple could approximate to the energy level of not
only the HOMO of F-Phen, but also either the lower SOMO
of F-Phen* or the SOMO of F-Phen~`. Accordingly, the
potential of the oxidizing species conceivable in the photo
process of F-Phen would be much more positive than the
redox potential of HO~/HO~ in AN (]0.8 V vs. NHE in
AN12).
1
Results and Discussion
Photophysical characteristics
PerÑuorophenazine (F-Phen) was prepared in 20% yield by
the method of oxidation coupling of 2,3,4,5,6-pentaÑuoroanil-
ine using lead tetraacetate.6 F-Phen showed absorption bands
at j \ 256 and 368 nm, which were assigned to nÈn* and
max
nÈn* transitions, respectively. When compared with those of
phenazine (Phen), they shifted to longer wavelength and the
onset of the nÈn* band reached the visible-light region
(j [ 450 nm) (Fig. 2) as anticipated from MO calculations
(ZINDO). These results are rationalized as owing to conjuga-
Photolysis of F-Phen with water
tion of aromatic n electrons in F-Phen with unshared p-
electron pairs of the Ñuorine atoms. Molar absorption
coefficients of each band of F-Phen were found to be lower
Taking into account that F-Phen absorbed visible light
(j [ 400 nm) and that it had the redox potential for F-Phen/
[
j
(AN) \ 256 and 368 nm (e \ 110 000 and 9 000 dm3
F-Phen~` positive enough to oxidize water, we examined a
visible-light photolysis of F-Phen in an aqueous AN solution
under degassed conditions. Fig. 4 shows the change in the
absorption spectrum of the photolysate. The absorption spec-
trum of F-Phen was changed with two isosbestic points at
j \ 330 and 420 nm. Under anhydrous conditions, such a
max
mol~1 cm~1)] than those of Phen [j (AN) \ 248 and 363
nm (e \ 140 000 and 15 000 dm3 mol~1 cm~1)].
max
It is well known that Phen has very weak Ñuorescence and
strong phosphorescence, because the singlet excited state of
Phen (1Phen*) undergoes fast intersystem crossing, giving its
triplet excited state (3Phen*). F-Phen showed an emission at
j
\ 475 nm by excitation of both bands of nÈn* and nÈn*
max
Fig. 3 Excitation and emission spectra of F-Phen: excitation spec-
trum monitored at j \ 473 nm (ÈÈ); emission spectra irradiated at
j \ 256 nm (ÈÈ) and j \ 368 nm ( - - - - - ) in AN; emission spectrum
irradiated at j \ 258 nm in CH (È - È); emission intensities were
normalized.
Fig. 2 Absorption spectra of F-Phen (ÈÈ) and Phen ( - - - - ) in an
AN solution (1 ] 10~5 mol dm~3). Insert: MagniÐcation of the
absorption spectra.
226
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

View Article Online
Fig. 4 Spectral change of F-Phen (1 ] 10~4 mol dm~3) in AN in the
presence of water (2 vol.%: 1.1 mol dm~3) under degassed conditions
during the photo-irradiation with j [ 400 nm, the spectra were
recorded after 0, 5, 20, 40, 60 and 80 min irradiation. Insert: Time
conversion plots of photolysis of an AN solution (2 ] 10~3 dm3) of
F-Phen (1 ] 10~3 mol dm~3) in the presence (…) and absence (>) (2
vol.%: 1.1 mol dm~3) of water with j [ 400 nm under degassed con-
ditions.
Fig. 6 Observed (a) and simulated [(b), (c), (d)] EPR spectra in an
AN solution of F-Phen (1 ] 10~3 mol dm~3) containing water (2
vol.%) in the presence of DMPO (0.1 mol dm~3) during irradiation
(
j [ 390 nm): (b) consisted of two signals (c) and (d) with an intensity
ratio of 10 : 1; (c) g \ 2.006, a \ 14.00, ab \ 12.45; (d) g \ 2.006,
change was found to be negligible (Fig. 4, insert). No change
was observed when Phen was used instead of F-Phen.
N
H
a \ 14.10, ab \ 21.50, ac \ 1.30.
N
H
H
These observations suggest that F-Phen should undergo
stoichiometric transformation into another compound only in
the presence of water by visible-light irradiation. During the
photolysis monitored by 19F NMR, the signals due to F-Phen
disappeared with the appearance of eight new signals with
same integral values. On examination of the photoproduct by
was formed only in the early stages of photolysis. F-Phen dis-
appeared gradually during the photolysis, and the quantities
of the consumed F-Phen and of the formed PhOH were com-
parable. In order to suppress the possibility of reductive deg-
radation of F-Phen, the photolysis was conducted in the
1
9F NMR, IR spectroscopy and MS, we speculated the forma-
tion of hydroxylated F-Phen as a sole product. Further, the
photoproduct was identiÐed as 1,3,4,5,6,7,8-heptaÑuoro-2-
hydroxyphenazine (F-Phen-2-OH) on the basis of the com-
parison with the previously reported hydroxylated F-Phen9
and the mechanistic analysis with the semi-empirical MO cal-
culations (see Experimental). The characterization of the sole
hydroxylated photoproduct suggests that hydroxyl radicals
presence of O as an electron acceptor. The formation of
2
PhOH and H O was enhanced but the degradation of
2
2
F-Phen was unavoidable (Fig. 5).
EPR detection of hydroxyl radicals (HO~)
In order to conÐrm the formation of HO~, F-Phen was irradi-
ated in an aqueous AN solution containing DMPO as a spin-
trapping reagent with visible light (j [ 390 nm) under
degassed conditions, and EPR signals were measured as
reported in the previous work.4 A main signal with six lines
was obtained as shown in Fig. 6(a), and was assigned as the
HO~ adduct of DMPO by simulating its hyperÐne splitting
constants [hfsc, Fig. 6(c)].¤ 13 In the absence of either F-Phen
or water, no EPR signal was observed. These EPR analyses
support the fact that F-Phen should play a crucial role in the
formation of HO~ due to the water oxidation in the present
photoreaction system.
(
HO~) should be formed through oxidation of H O in the pho-
2
tolysis with F-Phen under visible-light irradiation.
We also examined the visible-light photolysis of F-Phen
with water by using benzene as a scavenger of HO~ in an
atmosphere of argon gas, as reported in the F-OPP-3-sensi-
tized photohydroxylation of benzene to phenol (PhOH).2
PhOH was formed and increased with reaction time (Fig. 5),
and hydrogen peroxide (H O ) derived from unremovable O
2
2
2
Determination of absorption spectra of intermediary radical
ions from F-Phen by c-radiolysis
An absorption spectrum of the radical anion of F-Phen (F-
Phen~~) was obtained by c-radiolysis in an MTHF glassy
matrix cooled at 77 K and upon annealing to room tem-
perature. In the MTHF matrix system at 77 K, the absorption
spectrum gave some peaks in the region between j \ 550 and
700 nm, and these peaks decreased when the cold matrix was
gradually warmed to room temperature (Fig. 7, insert). The
di†erence spectrum obtained from each spectrum at 77 K and
at room temperature showed four peaks at j \ 460, 570, 620
and 660 nm as F-Phen~~ (Fig. 7). The spectrum pattern was
Fig. 5 Time conversion plots of photolysis of an AN solution of
F-Phen (1 ] 10~3 mol dm~3) containing water (2 vol.%: 1.1 mol
dm~3) and benzene (20 vol.% : 4.5 mol dm~3) under degassed (…, >,
¤ The small signal observed on EPR simulations as in Fig. 6(d) was
not assigned, but judging from hfsc values, it was probably due to a
DMPO adduct with an alkyl radical derived from DMPO itself.
=
) and O -saturated (L, |, K) conditions: (…, L) F-Phen; (>, |)
2
PhOH; (=, K) H O .
2
2
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
227

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Fig. 7 Di†erence absorption spectra between 77 K and room tem-
perature obtained after 1 h c-radiolysis of F-Phen (2 ] 10~3 mol
dm~3) in MTHF. Insert: Spectra obtained at 77 K and change on
annealing to room temperature.
Fig. 9 Transient absorption spectra obtained by laser Ñash photoly-
sis of an AN solution of F-Phen (1 ] 10~3 mol dm~3) in the presence
of water (5 vol.%: 2.8 mol dm~3) at various time delays after the 30 ns
pulse irradiation at j \ 351 nm: (È -…È - È) 0.5 ls, ( - - - -| - - - - ) 1
ls, (È=È) 2 ls. Insert: Change of transient absorption monitored at
j \ 490 and 630 nm, and their Ðtting curves.
very similar to that of the radical anion of Phen,14 although it
shifted to longer wavelength.
On the other hand, we failed to obtain the spectrum of the
radical cation of F-Phen by c-radiolysis in BuCl. The absorp-
tion spectrum observed at 77 K was identical with that of the
radical cation of BuCl and warming of the system gave the
spectrum of F-Phen only in the wavelength range 200È800
nm.
We monitored the decay of the absorbance of 3F-Phen* at
j \ 490 nm and the build-up of the absorbance of F-Phen~~
at j \ 630 nm in an aqueous AN solution. Both of them fol-
lowed almost Ðrst-order kinetics and gave the rate constants
1
.5 and 0.7 ] 106 s~1, respectively (Fig. 9, insert). Both rate
constants were in the same order, 106 s~1, supporting the fact
that 3F-Phen* should contribute to the formation of F-
Phen~~ in the presence of water. In other words, 3F-Phen*
Observation of intermediary species from F-Phen by laser Ñash
photolysis
should abstract one electron from H O to give HO~ followed
2
We examined laser Ñash photolysis of F-Phen to ascertain
short-lived intermediates photoformed from F-Phen. Fig. 8
shows a transient absorption spectrum of F-Phen in an AN
solution obtained after pulse irradiation at j \ 351 nm under
degassed conditions. This absorption with maxima at j \ 430
and 560 nm was assigned to the transition between excited
triplet states of F-Phen (3F-Phen*), because it was similar to
that of unÑuorinated phenazine (3Phen*15 ) in shape and was
by the formation of F-Phen~~.
Mechanism
A mechanism of photolysis of water with F-Phen is depicted
in Schemes 2 and 3. The photoexcited triplet state of F-Phen
(
3F-Phen*) is formed by intersystem crossing from the singlet
excited state of F-Phen (1). 3F-Phen* leads to oxidation of
water to hydroxyl radicals in the reductive quenching process
quenched readily by addition of O (Fig. 8, insert). The change
2
(2). The formation of HO~ was ascertained by both EPR
of the absorbance of the transient spectrum monitored at
analysis and the phenol production in the photolysis with
benzene. On the other hand, the reductively quenched 3F-
Phen* results in the production of F-Phen~~ through electron
transfer from water.
We propose here that both F-Phen~~ and HO~ should par-
ticipate in the formation of F-Phen-2-OH on the basis of the
following facts; (i) the formation of F-Phen-2-OH was also
detected in c-radiolysis of F-Phen in aqueous MTHF where
F-Phen~~ and HO~ were formed, (ii) F-Phen-2-OH was not
detected in FentonÏs reaction where only HO~ formed effi-
ciently.
j \ 430 and 560 nm followed a single exponential decay. The
lifetime of 3F-Phen* was determined as 6.3 ls (Fig. 8, insert).
Introduction of water into the system induced a change of
the decay of 3F-Phen*. The transient absorption of 3F-Phen*
was converted into a new one that showed absorption around
j \ 550È700 nm (Fig. 9). The resulting spectrum was similar
to that of F-Phen~~ obtained in the c-radiolysis in MTHF
matrix (see Fig. 7).
With regard to the bimolecular reaction between F-Phen~~
and HO~, two mechanisms are conceivable (Scheme 3), i.e., the
direct attack of HO~ to F-Phen~~ followed by Ñuoride elimi-
nation [S 2-like mechanism, (3) and (4)]16 and the coupling
RN
of HO~ with F-Phen-2~ formed by loss of Ñuoride from
F-Phen-2~~ [S 1-like mechanism, (5) and (6)].17,18
RN
On the MO calculations, the charge densities of carbon
atoms of F-Phen~~ are less positive at b-positions than at a-
positions (Table 3), causing hydroxyl radicals to attack prefer-
ably the carbon of b-positions. On the other hand, radical
anions of polyÑuoroaromatics fragment generally to form
polyÑuoroaromatic radicals and Ñuoride ion.17 Fluoride
Fig. 8 Transient absorption spectrum obtained by laser Ñash pho-
tolysis of an AN solution of F-Phen (1 ] 10~4 mol dm~3) at 2 ls
after the 30 ns pulse irradiation at j \ 351 nm. Insert: Decay of tran-
sient absorption monitored at j \ 430 nm under degassed (ÈÈ) and
aerated conditions ( - - - - - ).
Scheme 2
228
J. Chem. Soc., Faraday T rans., 1997, V ol. 93

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Scheme 3
elimination from F-Phen~~ may occur at b-positions rather
than a-positions, since the MO calculations indicate that the
CwF bond at the b-position is more dissociative than the
CwF bond at the a-position, and that the formation of
F-Phen-2~ from F-Phen~~ is energetically more favourable
than that of F-Phen-1~. Thus, both of the S 2-like and S 1-
Laser Technology under the commission of the Kansai Elec-
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References
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RN
RN
like mechanisms are supported by the MO calculations.
In the present system, however, the stoichiometric forma-
tion of F-Phen-2-OH was conÐrmed spectroscopically, and we
did not detect any hydrogenated or dimeric products which
might form by reactions of F-Phen-2~ with hydrogen donors
2
K. Maruo, Y. Wada and S. Yanagida, Bull. Chem. Soc. Jpn., 1992,
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4
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like H O and other F-Phen-2~ radicals. These facts imply that
2
RN
5
the S 2-like mechanism mainly contributes to the present
system. In any event, the rapid coupling of HO~ with F-
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cage, giving F-Phen-2-OH.
6
7
8
9
A. G. Hudson, A. C. Pedler and J. C. Tatlow, T etrahedron L ett.,
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yield HO~ under visible-light (j [ 400 nm) irradiation. Long
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the oxidation of water. The photoexcited F-Phen is stoichio-
metrically converted into F-Phen-2-OH through the rapid
coupling of HO~ with F-Phen~~ and F-Phen-2~ in a solvent
cage. Success in the construction of electron mediating
systems which suppress the concurrent reaction of F-Phen~~
could hopefully bring about water-splitting systems driven by
visible light irradiation.
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1
5
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
n,

This work was supported by a Grant-in-Aid for ScientiÐc
Research (A) from the Ministry of Education, Science, Sports,
and Culture of Japan (No. 06403023), partly defrayed by the
Grant-in-Aid on Priority-Area-Research on “Photoreaction
DynamicsÏ from the Ministry of Education, Science, Sports,
and Culture of Japan (No. 07228246), and the Institute of
9
1
8
L. C. T. Shoute and J. P. Mittal, J. Phys. Chem., 1993, 97, 379.
Paper 6/04920F; Received 12th July, 1966
J. Chem. Soc., Faraday T rans., 1997, V ol. 93
229