Reactive Oxygen Species (ROS)-vs Peroxyl-Mediated Photosensitized Oxidation of Triphenylphosphine: A Comparative Study

Article abstract of DOI:10.1021/acs.joc.6b02088

The oxidation of triphenylphosphine in the presence of various photocatalytic systems (dicyanoanthracene/biphenyl, N-methylquinolinium, triphenylpyrylium, and thiatriphenylpyrylium tetrafluoroborate) was investigated by means of both steady state and lase

Full text of DOI:10.1021/acs.joc.6b02088

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Article
Reactive Oxygen Species (ROS)- vs peroxyl-mediated photosensitized
oxidation of triphenylphosphine. A comparative study.
Sergio Mauricio Bonesi, Stefano Protti, and Angelo Albini
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02088 • Publication Date (Web): 28 Oct 2016
Downloaded from http://pubs.acs.org on October 28, 2016
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Reactive Oxygen Species (ROS)-vs peroxyl-mediated photosensitized oxidation
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of triphenylphosphine. A comparative study.
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Sergio Mauricio Bonesi,*a,b Stefano Protti,b Angelo Albinib
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aCIHIDECAR – CONICET, Departamento de Química Orgánica, 3er Piso, Pabellón 2, Ciudad
Universitaria, FCEyN, University of Buenos Aires, Buenos Aires, 1428, Argentina.
Phone/FAX:+541145763346.
bPhotoGreen Lab, Department of Chemistry, V.Le Taramelli 12, 27100 Pavia, Italy.
E-mail: smbonesi@qo.fcen.uba.ar
Table of Contents/Abstract Graphic.
Abstract.The oxidation of triphenylphosphine in the presence of various photocatalytic systems
(dicyanoanthracene/biphenyl,
Nꢀmethylquinolinium,
triphenylpyryliumium
and
thiatriphenylpyryliumtetrafluoborate) was investigated by means of both steady state and laser flash
photolysis experiments. The effect of different additives (including 1,4ꢀbenzoquinone,
diphenylsulfoxide, tetramethylethilene and sodium azides) on the photosensitized oxidation was
investigated in order to fully characterize the involved intermediates. Photoinduced electron transfer
and final regeneration of the catalyst occur when dicyanoanthracene and Nꢀmethylquinolinium are
used, while in cage oxygen transfer to the photoexcited (thio)pyrylium derivatives have been
characterized in the last two cases.
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INTRODUCTION
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Molecular oxygen (3O2) is the ideal reagent for sustainable oxygenation processes because of its
abundant availability and its nontoxicity.1 Since direct concerted addition of (singlet) organic
molecules and triplet oxygen (3O2) is a spinꢀforbidden process, activation of at least one of the
reagents is required. Activation of molecular oxygen can occur either via transition metal
photocatalysis2 or by using a suitable photocatalyst to produce singlet oxygen (1O2) via sensitization
in a spinꢀallowed process3 or odd electron species (either Reactive Oxygen Species, ROS, or
radicals or radical ions from the organic molecules) via photoinduced electron transfer.4 It is not
always easy to recognize the mechanism involved in any specific case and actually oxidizable
molecules may react through a variety of paths according to conditions. Although metal complexes
are increasingly used in photocatalyzed reactions, organic photocatalyst/sensitizers still offers a
promising alternative and the mechanism is often more easily characterized. Apart from organic
dyes, largely used organic compounds used in this capacity include heteroaromatic cations
(pyrylium and thiapyrylium,5 quinolinium6 and acridinium salts7) as well as aromatic nitriles,8 due
to their high oneꢀelectron oxidation potential from the lowꢀlying excited state (either singlet or
triplet). Among oxidizable substrates, aliphatic and aromatic sulfides have been largely studied,9
and a variety of different mechanisms have been demonstrated to be involved. Another class of
electron donors that have been deeply investigated is that of triarylphosphines [for the parent
molecule, Eox(Ar3P/Ar3P+.)=1.00V]10 and are of interest as excellent electron donors. The radical
cation then evolved to a stable photoproduct, the phosphine oxide (Ar3P=O). This intermediate
exhibits a dual reactivity, as an electrophile (in the presence of different nucleophiles such as
alcohols, water, halides, hydroxyl anions and bases11) as well as a free radical (with thiols).12
It thus seemed appropriate to choose the easily oxidized triphenylphosphine as a probe for
comparing the photooxidation by various photocatalysts and look for experimental support for the
mechanism. We report below a combined steady state and time resolved investigation by using four
different sensitizers, viz 9,10ꢀdicyanoanthracene (DCA, E* vs SCE = 1.97 V),13 Nꢀ
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methylquinolinium (NMQ, E* vs. SCE = 2.70 V),13 2,4,6ꢀ triphenylpyrylium (TPP, E* vs. SCE =
3.10 V for the singlet excited state)14 and 2,4,6ꢀtriphenylthiapyrylium (SꢀTPP, E* vs. SCE = 3.09 V
for the singlet excited state)15 in different solvents (CH2Cl2, MeCN, MeOH and MeCN/Water 9/1
mixture, Scheme 1). With the photocatalysts used oxidation of triphenylphosphine is in every case
an exergonic process (ꢀG = – 22.4 Kcal molꢀ1 for DCA; ꢀ62.3 Kcal molꢀ1 for NMQ; ꢀ48.4 Kcal molꢀ
1for TPP and ꢀ48.2 Kcal molꢀ1 for SꢀTPP). In the case of DCA, the use of biphenyl as coꢀsensitizer
was used to improve the process, as previously observed by Majima and coꢀworkers.16
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Scheme 1.Photosensitized oxidation reaction of triphenylphosphine and chemical structures of the
photosensitizers.
RESULTS
Fluorescence quenching. The fluorescence parameters of the photocatalysts considered have been
recently collected in a review by Miranda and coꢀworkers.17 DCA, NMQ and TPP exhibit a medium
to high fluorescence quantum yield (from 0.47 to 0.90) in acetonitrile, whereas SꢀTPP shows a
significantly lower emission (ΦF = 0.06). In every case, the fluorescence quenching by
triphenylphosphine occurred efficiently (kq ranging from 1.9 to 7.9 × 1010 Mꢀ1sꢀ1 see Table 1).
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Table 1. Fluorescence quantum yield, fluorescence lifetime and rate constants for the
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quenching (kq) of the photocatalysts used by Ph3P in MeCN.
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Photosensitizer
DCA
ΦF
τ/ ns
15.3b
20c
kq / Mꢀ1sꢀ1
1.9×1010
2.2×1010
7.9×1010
5.1×1010
a
0.90
0.85
0.47
0.06
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NMQ
TPP
2.9d
3.1e
SꢀTPP
a From ref 17.b From ref. 18.c From ref. 19.d From ref. 5d.e From ref. 20.
Photochemical reactions. The irradiation of O2ꢀsaturated solutions of Ph3P in the presence of the
photocatalysts gave phosphine oxide as the only stable product when the reaction mixture was
analyzed by GC. In every case, the sum of the amounts of Ph3P and Ph3PO accounted for more than
90% of the material balance. No other product was detected by the GC. Irradiations were performed
at 410 nm (multilamp reactor, 4 lamps), except for NMQ, where the same reactor with lamps
emitting at λ = 365 nm has been used, due to the negligible absorbance of this photocatalyst at 410
nm. The quantum yield of photo oxidation of a PPh3 solution (0.011 M) was measured in different
solvents in the presence of a chosen photocatalyst (10ꢀ3M, with the exception of DCA, for which a
concentration of 10ꢀ5 M was used). The effect of some additives (1,4ꢀdimethoxybenzene (DMB),
1,4ꢀbenzoquinone (BQ) and diphenylsulfone (Ph2SO)) on the irradiation was likewise explored. In
every case, the oxidation rate was strongly reduced in oxygenꢀfree solutions. The results thus
obtained have been summarized in Tables 2 and 3.
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Table 2. Quantum yield of consumption of Ph3P with different photosensitizers.
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φa
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9
DCAb
0.033
NMQc
Solvent
MeCN
TPP
S-TPP
0.143
0.019
0.184
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< 1×10ꢀ3
0.015
0.013
0.056
0.169
0.128
0.007
0.090
0.048
0.009
0.091
MeCN (N2)
MeOH
DCE
0.023
0.103
0.265
0.077
MeCN – H2O (9:1)
0.007
a[Ph3P]: 0,011 M; [Photosensitizer]: 1.0x10ꢀ3 M; λexc: 410 nm. [DCA]: 1x10ꢀ5 M (in the presence
b
of biphenyl 10ꢀ2M as coꢀsensitizer). cλexc: 365 nm. All data were obtained with a ± 0.001 error
Table 3.Rate of Ph3PO [ꢁmol.min-1] formation in the presence of different photocatalysts and
additives.a
DCA
NMQ
BQ
None
1.40
1.81
2.88
DMBb BQc
Ph2SOd
1.34
None
9.6
DMB
6.6
Ph2SO
9.6
MeCN
MeOH
DCE
1.49
1.53
2.73
0.89
1.05
1.96
4.8
3.6
1.66
6.0
4.8
7.2
1.10
11.4
11.4
2.4
12.6
TPP
SꢀTPP
7.2
MeCN
MeOH
DCE
9.0
6.0
5.4
7.2
5.4
4.8
6.6
3.0
4.8
10.8
6.0
11.4
3.6
11.4
10.2
4.8
3.6
4.2
3.0
5.4
4.2
4.8
4.2
aAmounts of triphenylphosphineoxide formed in ꢁmol.minꢀ1.b5×10ꢀ2 M 1,4ꢀdimethoxybenzene
(DMB). c1.25×10ꢀ3 M 1,4ꢀBenzoquinone (BQ). d2.5×10ꢀ3 M Ph2SO.
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Photocatalyzed oxidation via DCA. Consistent with what was observed in previous reports, 5e, 17 the
DCA/Biphenyl photosensitized oxidation of Ph3P was found to be quite inefficient (Φ < 0.05). The
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use of a protic environment further lowered the reaction efficiency (from Φ = 0.033 to 0.015 when
moving from neat acetonitrile to methanol). The oxidation rate was not unaffected by the presence
of 1,4ꢀdimethoxybenzene (DMB), whereas the quenching effect of diphenylsulfoxide (Ph2SO) was
found to be significant only in dichloromethane solution, where the oxidation rate decreased from
2.88 to 1.10 ꢁmol minꢀ1. The addition of 1,4ꢀbenzoquinone reduced the rate value of ca. one third in
all of the systems tested.
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Photocatalyzed reactions via NMQ. With NMQ as the photosensitizer, the oxidation quantum
yields improved, with maximal values in polar aprotic solvents (up to 0.169 in dichloromethane). In
the absence of oxygen, the photo oxidation rate was decreased to one tenth of the initial value.
Again, the use of a protic medium resulted in a lowering of the quantum yield value. As for
quenchers, DMB and Ph2SO had little or no effect on the rate of phosphine oxide formation, while
the use of BQ generally led to a significant decrease of the rate value, as previously observed with
DCA.
Photocatalyzed reactions via TPP and S-TPP. With the heterocyclic salts TPP and SꢀTPP as the
photocatalysts the oxidation quantum yield varied from 2 to 9% with the former (maximal value in
methanol and in dichloromethane) and from 9 to 26.5% with the latter (maximal value in
acetonitrileꢀwater 9:1). None of the above additives caused a large effect, with some slowing down
of the reaction only with BQ in the oxidation in polar solvents.
However, these photocatalysts were partially consumed during the irradiation in oxygen saturated
nucleophilic media, such as MeOH, MeCN and aqueous MeCN solutions of the phosphine (see an
example in Figure(1a)). The formation of Ph3PO remained quantitative, though. In fact,
photosensitization of Ph3P with TPP in O2ꢀsaturated MeCN – H2O (9:1) at 405 nm on an optical
bench provides 95 % yield of Ph3PO at a fixed reaction time while TPP is only consumed in 23 %
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yield. Likewise, with SꢀTPP Ph3PO is formed in 92 % yield while SꢀTPP in consumed in only 3 %
yield. Finally, no significant consumption of both TPP and SꢀTPP was observed in apolar solvents.
The use of biphenyl as a coꢀoxidant of all of the examined catalysts has been also considered.
Interestingly, an enhancement of the photo oxidation rate has been observed with all of the
photosensitizers used, with a maximum increase in the case of NMQ, for which the oxidation rate
increased by a factor of five (Figure 1b).
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(a)
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S-TPP (λ
: 369 nm)
abs
60
%A
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0
TPP (λ
: 404 nm)
abs
0
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t / s
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ꢁmol.s-1
(Ph3PO)
(b)
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NMQ+
1.2
1.0
0.8
0.6
0.4
0.2
TPP
S-TPP
DCA
0.0
0.00
0.02
0.04
0.06
0.08
0.10
0.12
[Biphenyl] / M
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Figure 1.(a) Relative consumption of photosensitizers TPP and SꢀTPP during irradiation with λexc:
405 nm of O2ꢀsaturated solutions of PPh3 (0.010 M) in MeCN – H2O (9:1).(b) Effect of addition of
biphenyl on the photosensitized oxidation of O2ꢀsaturated acetonitrile solution of PPh3 (0.010 M).
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Time-resolved spectroscopic measurements. The steady state irradiations have been supplemented
by laser flash photolysis experiments. The spectra obtained in the DCA/biphenyl system in the
presence of PPh3 clearly show the formation of three different absorption bands located at 530 nm,
600 nm and 730 nm (Figure 2a). In accord with the literature18 we assigned the last two bands to the
biphenyl radical cation (BP.+) and DCA.ꢀ, respectively. Similar results were obtained with the other
photocatalysts in all of the chosen media. In all of the cases studied, formation of the two absorption
bands located at 380 and 530 nm was observed but, due to the interference of other intermediates
involved, only the former band was clearly visible in the presence of NMQ (Figure 2b).
Experiments in the presence of sodium azide showed that both absorption bands were quenched by
azide anion with similar kq values (kq = 2.4ꢀ2.8 × 108 Mꢀ1sꢀ1). Therefore, these bands can be
univocally assigned to the absorption spectrum of a single transient, the triphenylphosphine radical
.+
cation (PPh3 ). Likewise, the reduced forms of the photoꢀoxidants (the corresponding Nꢀ
methylquinolyl, pyryl and thiapyryl radicals) were also detected along with the transient radical
cation. Representative examples are shown in Figure 2 for MeCN.
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0.03
0.02
(a)
+.
Ph
P
-.
3
DCA
+.
Bf
6
7
OD
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9
0.01
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0.00
400
500
600
700
λ / nm
0.06
(b)
0.04
OD
+.
Ph3P
.
NMQ
0.02
0.00
400
450
500
λ / nm
550
600
650
.
TPP
0.006
(c)
+.
Ph3P
0.004
OD
0.002
0.000
450
500
550
λ / nm
600
650
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(d)
+.
Ph P
3
.
S-TPP
0.02
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OD
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0.01
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0.00
450
500
550
λ / nm
600
650
Figure 2. Transients absorption spectra recorded after a laser pulse (10 ꢁs, λexc: 355 nm) of N2ꢀ
saturated acetonitrile solution of PPh3 (0.0110 M) in the presence of: (a) DCA and BP; (b)
NMQ;(c) TPP and (d) SꢀTPP.
The transient Ph3P+. was found to decay with a secondꢀorder rate constant (k2) of 109 – 1010 Mꢀ1sꢀ1
(see Table 4) in N2ꢀsaturated solution, whereas in O2ꢀsaturated solution a pseudoꢀfirstꢀorder decay
was observed. In the latter case, a secondꢀorder rate constant of 109 – 1010 Mꢀ1sꢀ1 was calculated by
considering the concentration of O2 solubilized in each of the solvents used. Representative decay
+.
curves of transient PPh3 in N2ꢀ and O2ꢀsaturated solutions are shown in Figure 3 for SꢀTPP and
NMQ photosensitization. It should be noticed that triphenylphosphine oxide is the only
photoproduct formed in the Ph3P/TPP system after flashing the oxygenated solution with a laser
pulse (λexc: 355 nm; 10 consecutive shots). Analogous results have also been obtained with the
other sensitizers.
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0,006
0,004
(a)
DCM
6
MeCN
7
OD
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0,002
MeCN-H O (9:1)
2
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0,000
-0,002
0
50
100
150
t / ꢁs
0,02
OD
(b)
N
2
0,01
O
2
0,00
0
50
100
150
t / ꢁs
Figure 3.(a) Decay traces of PPh3+. after a laser pulse (λexc: 355 nm) of N2ꢀsaturated solutions of
PPh3 (0.010 mol.dmꢀ3) and NMQ (1.0 × 10ꢀ4mol dmꢀ3) in dichloromethane, MeCN and MeCNꢀH2O
+.
9:1 mixture recorded at 510 nm. (b) Decay traces of PPh3 after a laser pulse (λexc: 355 nm) of N2ꢀ
and O2ꢀsaturated acetonitrile solutions of PPh3 (0.010 mol dmꢀ3) and SꢀTPP (1.0×10ꢀ4mol dmꢀ3)
recorded at 510 nm.
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Table 4.Second-order decay parameters of Ph3P+.sensitized by DCA, TPP, S-TPP and NMQ
in N2- and O2-saturated organic solvents.
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k2 / Mꢀ1.sꢀ1
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Solvent
MeCN
Atmosphere
DCA
NMQ
TPP
S-TPP
1.6×1010
8.9×109
1.6×1010
N2
1.6×1010 3.4×1010
2.8x1010 1.6×1010
7.3×109
2.4×1010
a
O2
N2
1.7×1010 Not deter. 3.4×109
MeOH
DCM
a
O2
6.0x1010 Not deter. 1.1×1010 1.5×1010
N2
2.5×1010
5.6×109
1.7×1010 3.7×1010
4.9×1010 1.5×1010
2.0×1010 2.8×1010
a
O2
1.4×1010 2.0×1010
3.9×1010 3.9×1010
4.2×1010 1.2×1010
N2
MeCN – H2O (9:1)
a
O2
6.7×109
6.5×1010
aCalculated from the observed pseudoꢀfirst order plot, by taking into account the
concentration of oxygen dissolved.
As done in steady state experiments, the interaction of the intermediates generated by flash
photolysis with different additives was investigated. Thus, addition of tetramethylethylene (TME)
in the DCA/BP system quenched the transient lifetime (see Figure 4(a)). On the contrary, addition
+.
of TME to a solution of NMQ/Ph3P caused a noticeable decrease of the amount of the PPh3
transient observed, while its lifetime remained constant (Figure 4(b)).
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(a)
9e+6
8e+6
7e+6
6e+6
5e+6
4e+6
λ
+. = 510 nm
Ph P
3
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8 -1 -1
: 5,37x10 M .s
k
Q
r ² 0,9989
0.000
0.002
0.004
0.006
0.008
[TME] / M
0.020
0.016
0.012
0.008
(b)
0.000
0.002
0.004
0.006
0.008
0.010
0.012
[TME] / M
Figure 4.(a) Quenching of Ph3P+. transient generated at 510 nm in the DCA/BP system by TME.
(b) Decrease of the endꢀofꢀpulse absorption of Ph3P+. transient at 380 nm by TME observed in the
case of NMQ; no change of the transient lifetime.
DISCUSSION
Photoxidation of PPh3 via DCA/BP system and via NMQ. As hinted above, the photoxidation of
triphenylphosphine by DCA is quite inefficient, but improves when using biphenyl as coꢀsensitizer
.+
(Scheme 2, path a). Formation of the radical cation PPh3 is clearly identified under these
conditions by two absorption bands located at 380 nm and 530 nm.16,18 The same transient was
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detected here when using NMQ (see Figures2(a) and 2(b)). In both cases, the intermediate decayed
through a second order kinetics, reasonably attributed to back electron transfer (BET). In oxygenꢀ
saturated solutions, however, trapping of photogenerated Ph3P.+ with molecular oxygen occurred
and led to a pseudoꢀfirst order kinetics with both sensitizers. This behavior has been previously
observed under pulse radiolysis21 and it has been proposed that it involves formation of the peroxyl
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radical cation Ph3PO2 (Scheme 2, path b).10,18 According to Majima, reaction of this intermediate
.+
with a molecule of PPh3 and ensuing mesolytic cleavage yields triphenylphosphine oxide (Ph3PO)
and radical cation Ph3PO.+ (II) (Scheme 2, path c). Finally, intermediate (II) reacted with
superoxide ion yielding another molecule of Ph3PO and molecular oxygen (Scheme 2, path d).
Although molecular oxygen was found to be involved in the first oxidative step, other related
Reactive Oxygen Species (ROS) such as singlet oxygen and superoxide ion, can be involved in the
overall processes. Energy transfer leading to singlet oxygen is known to proceed efficiently
(limiting Φꢀ = 2 and 0.86 for DCA and NMQ, respectively)5e,17 but an active role of singlet oxygen
has been previously excluded by Majima, (for DCA/BP photocatalyzed oxidation of phosphines)16
and by Baciocchi for NMQꢀmediated oxidation of sulfides,13 respectively, because the reaction is
faster than expected. In our case, a comparison between the bimolecular rate constant (kr) between
Ph3P and singlet oxygen reported (1.6×107 Mꢀ1.sꢀ1) 22 and the values shown in Table 4, highlights
that this process does not compete efficiently with the reaction of Ph3P.+ and molecular oxygen.
Furthermore, singlet oxygen photosensitization provides intermediate Ph3PO2, which is a
phosphodioxirane intermediate23 that is quenched by electrophilic additive Ph2SO giving Ph2SO2.
However, in our experimental conditions no Ph2SO2 was formed. Therefore, we excluded reaction
of singlet oxygen with Ph3P from the reaction mechanism as a competitive oxidative pathway. On
3
the other hand, the reduced forms of both sensitizers transfer efficiently an electron to O2
generating superoxide ion.13 The fate of this anion has been the subject of some controversy in the
case of sulfides, and both back electron transfer between O2.ꢀ and R2S.+ as well as addition reaction
.ꢀ
of O2 with R2S.+have been proposed.9,13 The present results with phosphines, however, support
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none of the above paths, since addition of the ions would give the corresponding
phosphodioxiranes, the same intermediates involved in the singlet oxygen addition23 that is
efficiently trapped by electrophiles viz. diphenylsulphoxide (Ph2SO). In the experiment, addition of
Ph2SO had no effect, (see Table 3), while electron acceptor 1,4ꢀbenzoquinone, a selective quencher
of superoxide ion, had (see Table 2). The role of superoxide ion in the photosensitization with
DCA/Bp or NMQ, is further evidenced by the decay traces of Ph3P.+ measured at 510 nm under
oxygen atmosphere which shows a pseudoꢀfirst order kinetics in all the solvents studied. Indeed, in
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.ꢀ
the case of a direct reaction of O2 with Ph3P.+ as the primary oxidation pathway, a secondꢀorder
kinetic would be observed. Therefore, coupling of superoxide ion with Ph3P.+ has been excluded as
a possible oxidation pathway from the proposed reaction mechanism. Then, O2.ꢀ has a role, but does
.ꢀ
not add to the phosphine radical cation. A reasonable alternative is that O2 operates as electron
carrier in the reduction of Ph3PO.+(II) (Scheme 2, path d) so that DCA.ꢀ and NMQ. are efficiently
oxidized to DCA and NMQ, respectively, and therefore, the two sensitizers are not consumed
during the reaction. The different effect of a donor such as tetramethylethylene (TME) on NMQ and
DCA/BP photosensitized reaction is explained by the redox parameters of the species involved. In
the case of NMQ+, the strongly positive oxidation potential (2.70 V vs. SCE) in the excited state
causes competition between the phosphine (1.0 V) and the olefin (1.74 V),24 resulting in a decrease
of the initial concentration of Ph3P.+ (see Figure 4b). On the other hand, with BP.+ (E vs. SCE = 1.8
.+
V) oxidation of PPh3 is the only thermodynamically allowed reaction. However, the PPh3
produced was reduced by TME, resulting in an efficient quenching of the intermediate.
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Scheme 2.Proposed mechanism for the photosensitization of triphenylphosphine with: (a) DCA/Bp
system and (b) NMQ.
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Photoxidation of PPh3 via TPP and S-TPP. As in the case of the previously examined
photosensitizers, electron transfer from triphenylphosphine to yield the corresponding Ph3P+. was
found to be the first step. The participation of both spin states of TPP and SꢀTPP in photooxidation
reactions has been reported in the literature. The singlet excited state of TPP is involved in the
photo oxidation electron transfer of different benzoic acid derivatives25 whereas the participation of
26
both TPP and SꢀTPP triplet excited states in electron transfer5e,
is wellꢀknown and was
demonstrated in the presence of different substrates, including cinnamic acids.26 In our case, the
efficient quenching of the fluorescence emission by triphenyl phosphine highlighted the active role
of singlet excited state of TPP. On the other hand, the SꢀTPP fluorescence was likewise quenched,
with a quenching rate constant of 5 × 1010 Mꢀ1sꢀ1. However, intersystem crossing quantum yield
(0.94) largely overshadow fluorescence quantum yield (0.06) and hence, it is expected that the
triplet excited state has a predominating role in the process. As observed for NMQ and the DCA/Bp
system, whereas bimolecular back electron transfer is the elective pathway in nitrogen saturated
solution, in the presence of oxygen the photogenerated Ph3P.+ reacted following a pseudoꢀfirst order
decay, which highlighted again the role of molecular oxygen as the primary oxidant. Two
competitive pathways have been taken into account. As proposed by Clennan in the photo oxidation
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of sulfides,27 photoinduced electron transfer resulted in the formation of a radical/radical ion
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3
intimate pair. We suggested that bimolecular reaction with O2 occurs either at this stage or after
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solvation, depending on ion solvating power of the solvent. In the case of polar solvents (MeCN,
8
9
+.
3
MeOH and MeCN/H2O mixture), reaction of solvated PPh3 with O2 afforded the oxygenated
intermediate Ph3POO+.(I) (Scheme 3, path a), which was previously proposed for DCA/Bp and
NMQ sensitization. As hinted above, intermediate I reacts with Ph3P to produce a Ph3PO molecule
and the radical cation PPh3O.+. (II) and then, the last intermediate oxidizes TPP. radical regenerating
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the sensitizer TPP+ (Scheme 3, paths b and c).On the other hand, addition of O2 to the reduced
3
(thia)pyrylium radical (TPP. or SꢀTPP.) affords the peroxyl radical TPPꢀOO. (III), and this in turn
reacts with PPh3 to produce the triphenylphosphine oxide (Scheme 3, path d). This has analogy with
the observed oxygen transfer to aromatic and aliphatic sulfides by radicals of this type, for example,
the cumoylperoxyl radical or halogenated peroxyl radicals.28 Thus, reaction via a TPPꢀOO. radical is
a possibility, but in such a case oxygen transfer involves consumption of the sensitizer, since the
alkoxyl radicals (V) resulting from oxygen transfer (TPPꢀOO.+ PPh3ꢀTPPꢀO. + Ph3PO; Scheme 3,
path e) would further react with irreversible degradation of the pyrylium ring. In fact, electron
transfer from water that has an oxidation potential (E = 1.28 V) comparable to that of PPh3 and thus
a further oxidizing species, hydroxyl radical, is formed and increases the reaction efficiency.29
Intermediate (III) or any other oxidizing species may contribute to the oxidation of Ph3P as a side
pathway (Ph3PO is formed quantitatively while the consumption of the sensitizers take place in a
few percent). Previous work by Clennan using sulfides as probes showed that these sensitizers did
not form neither singlet oxygen nor superoxide (ꢀGET< 10 kcal molꢀ1).27 Thus, the cyclic inner
sphere mechanism shown in Scheme 3, viz. paths f, g and h, is the exclusive path in aprotic
solvents, while formation of radicals occurs in polar and protic media. The exclusive presence of III
in polar solvents is also evidenced by the addition of quencher BQ. In fact, the reaction rate is only
diminished in polar and protic solvent in the presence of BQ, whereas no effect was measured in
dichloromethane.
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Scheme 3.Reaction mechanism for the photosensitization of triphenylphosphine with TPP and Sꢀ
3
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TPP.
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Finally, addition of biphenyl is required when DCA is the sensitizer, but an increase of rate in the
presence of biphenyl is observed for all of the examined photocatalysts (see Figure1(b)). Thus, BP
is an efficient coꢀoxidant, and the improvement depends on the oxidation potential of the sensitizer
used. The effect is largest when NMQ is used (up to six times the initial rate).
CONCLUSIONS.
The photosensitized oxidation reactions examined in the present paper take place, at least, through
two different reaction mechanisms, depending both on the nature of the photosensitizer employed
and on the solvent used. The first step is in every case an electronꢀtransfer process from the electron
donor (triphenyl phosphine) to the photoexcited sensitizer providing a triphenylphosphine radical
cation, as confirmed by both preparative photochemical experiments and laser flash photolysis
analyses. In the case of photosensitization with DCA/Bp system and NMQ, respectively, the Ph3P
radicalꢀcation then reacts with molecular oxygen (3O2) following a pseudoꢀfirst order kinetics and
affording the oxygenated intermediate Ph3POO.+ (I). Then, such a transient reacts with Ph3P to
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afford Ph3P=O and intermediate Ph3PO.+ (II). Superoxide ion (O2 ) is also generated during the
irradiation and operates as electron carrier in the reduction of Ph3PO.+(II) to Ph3P=O, thus no
sensitizers consumption is observed during the reaction. Formation of Ph3PO.+(II) can be observed
also in the case of cationic photosensitizers (TPP and SꢀTPP) in polar solvents, where molecular
oxygen plays again the role of the primary oxidant. On the other hand, in apolar media, a cyclic
inner sphere mechanism is the exclusive path that provides triphenylphosphine oxide via the
formation of a peroxidic intermediate (IV) and the subsequent consumption of the sensitizer.
Finally, the finding that triphenylphosphine radical cation reacts efficiently with molecular oxygen
sheds some light on the reactivity of triarylphosphines under photosensitization conditions.
.ꢀ
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EXPERIMENTAL DETAILS
Materials .Compounds triphenylphosphine, triphenylphosphine oxide, diphenyl sulfoxide, 1,4ꢀ
dimethoxybenzene, benzoquinone, tetramethylethylene, biphenyl and the photocatalyst 2,4,6ꢀ
triphenylpyrylium tetrafluoroborate (TPP) were commercially available and used as received. TPP
was washed with water and dried before use to eliminate traces of acids. DCA,30 Nꢀ
methylquinolinium tetrafluoroborate31 and 2,4,6ꢀtriphenylthiapyrylium tetrafluoroborate (SꢀTPP)32
were prepared by published procedures. All the solvents used were of spectroscopic grade and used
as received.
Photochemical reactions. The photooxidations were carried out by using 0.01– 0.1 M solutions (2
mL) of the triphenylphosphine in the presence of DCA, NMQ, SꢀTPP, or TPP (5×10ꢀ4 to 1×10ꢀ3 M)
in acetonitrile, methanol, 1,2ꢀdichloroethane and acetonitrile water (9:1). The solutions were
contained in rubberꢀstoppered Pyrex tubes (1 cm diameter) and exposed to four phosphorꢀcoated 15
W lamps with emission centered at 400 nm (for DCA, SꢀTPP and TPP) or 366 nm (for NMQ) while
a stream of dry oxygen saturated with the appropriate solvent was passed into the solution through a
needle. The products were determined by means of GC analyses on the basis of calibration curves
in the presence of cyclododecane as internal standard and by HPLC analyses. Experiments for
quantum yield determination were limited to ≥10% conversion and the DCAꢀsensitized oxidation of
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1,1 diphenylethene (Φꢀ1 = 0.8) was used as actinometer to measure the quantum yields.33 These
experiments were performed with an optical bench provided with a high pressure Hg lamp and the
appropriate interference filters. Irradiation of such solutions with photosensitizers DCA, TPP and Sꢀ
TPP were carried out with a filter centered at 405 nm, while for the case of NMQ the filter was
centered at 366 nm. The samples were placed in quartz cells (10 ×10 mm) while a stream of dry
oxygen saturated with the appropriate solvent was passed into the solution through a needle.
Experiments at a higher conversion (>50%) were also carried out, and no significant difference in
the product distribution was noted. A solution of TPP in MeOH was appreciably bleached and the
addition of further amounts of the sensitizer was required in order to continue the oxidation. In
order to exclude any contribution by spurious light to the photooxidation of triphenyl phosphine, all
the irradiations were performed in a dark room and inside of a laboratory hoodboard where the
temperature was between 20 – 22 °C.
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Steady-State Fluorescence Quenching. Solutions of the photosensitizers in MeCN were placed in a
quartz cell (10 × 10 mm section) at a concentration adjusted to obtain an A value of 0.5 at the
maximum emission wavelength characteristic of each photosensitizer. The fluorescent spectra were
recorded with a fluorimeter. The quenching rate constants (kQ) were obtained using the Sternꢀ
Volmer method applying leastꢀsquare linear regression fitting procedure and the r2 values were used
to judge the goodness of fit.
Time-resolved laser flash spectroscopy. Nanoꢀtoꢀmicrosecond transient absorption experiments
were performed using a nanosecond laser flash photolysis apparatus equipped with a 20 Hz
Nd:YAG laser (20 ns, 1 mJ at 355 nm) and a 150 W Xe flash lamp as the probe light. Samples were
placed in a quartz cell (10 × 10 mm section) at a concentration adjusted to obtain an OD value of
1.0 at 355 nm. Furthermore, the samples were flushed with a stream of either argon or oxygen
during 20 minutes in order to create an inert and an oxidative atmosphere, respectively. Leastꢀ
squares linear regression fittings were carried out in order to judge the goodness of the fit with the
correlation coefficients (square root) better than 0.99. Monoꢀexponential fittings procedures were
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carried out and the quality of the data obtained was judged from the χ2 parameter and distribution of
the residuals. All of the laser flash photolysis experiments were carried out at 20 °C. Quenching of
the Ph3P+.transient by TME were performed with the same apparatus using the SternꢀVolmer
method.
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ACKNOWLEDGMENTS.
S.M.B. is a research member of CONICET (Argentinean National Research Council).
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