Photooxyge
natio
ns of PhSMe a
nd Bu
2S se
nsitized by
N-methylqui
noli
nium (NMQ
+) a
nd 9,10-dicya
noa
nthrace
ne (DCA) i
n O
2-saturated MeCN have bee
n i
nvestigated by laser a
nd steady-state photolysis. Laser photolysisexperime
nts showed that excited NMQ
+ promotes the efficie
nt formatio
n of sulfide radical catio
ns with both substrateseither i
n the prese
nce or i
n abse
nce of a cose
nsitizer (tolue
ne). I
n co
ntrast, excited DCA promotes the formatio
n ofradical io
ns with PhSMe, but
not with Bu
2S. To observe radical io
ns with the latter substrate, the prese
nce of acose
nsitizer (biphe
nyl) was
necessary. With Bu
2S, o
nly the dimeric form of the radical catio
n, (Bu
2S)
2+ntities/bull.gif">, was observed,while the absorptio
ns of both PhSMe
+ntities/bull.gif"> a
nd (PhSMe)
2+ntities/bull.gif"> were prese
nt i
n the PhSMe time-resolved spectra. The decayof the radical catio
ns followed seco
nd-order ki
netics, which i
n the prese
nce of O
2, was attributed to the reactio
n of theradical catio
n (presumably i
n the mo
nomeric form) with O
2-ntities/bull.gif"> ge
nerated i
n the reactio
n betwee
n NMQ
ntities/bull.gif"> or DCA
-ntities/bull.gif"> a
ndO
2. The fluoresce
nce que
nchi
ng of both NMQ
+ a
nd DCA was also i
nvestigated, a
nd it was fou
nd that the fluoresce
nceof the two se
nsitizers is efficie
ntly que
nched by both sulfides (rates co
ntrolled by diffusio
n) as well by O
2 (
kq = 5.9 ×10
9 M
-1 s
-1 with NMQ
+ a
nd 6.8 × 10
9 M
-1 s
-1 with DCA). It was also fou
nd that que
nchi
ng of
1NMQ* by O
2 led to theproductio
n of
1O
2 i
n sig
nifica
nt yield (
= 0.86 i
n O
2-saturated solutio
ns) as already observed for
1DCA*. The steady-state photolysis experime
nts showed that the NMQ
+- a
nd DCA-se
nsitized photooxyge
natio
n of PhSMe afford exclusivelythe correspo
ndi
ng sulfoxide. A differe
nt situatio
n holds for Bu
2S: with NMQ
+, the formatio
n of Bu
2SO was accompa
niedby that of small amou
nts of Bu
2S
2; with DCA, the formatio
n of Bu
2SO
2 was also observed. It was co
nclusively show
nthat with both se
nsitizers, the photooxyge
natio
ns of PhSMe occur by a
n electro
n tra
nsfer (ET) mecha
nism, as
nosulfoxidatio
n was observed i
n the prese
nce of be
nzoqui
no
ne (BQ), which is a trap for O
2-ntities/bull.gif">, NMQ
ntities/bull.gif">, a
nd DCA
-ntities/bull.gif">. BQalso suppressed the NMQ
+-se
nsitized photooxyge
natio
n of Bu
2S, but
not that se
nsitized by DCA, i
ndicati
ng that theformer is a
n ET process, whereas the seco
nd proceeds via si
nglet oxyge
n. I
n agreeme
nt with the latter co
nclusio
n,it was also fou
nd that the relative rate of the DCA-i
nduced photooxyge
natio
n of Bu
2S decreases by i
ncreasi
ng thei
nitial co
nce
ntratio
n of the substrate a
nd is slowed by DABCO (a
n efficie
nt si
nglet oxyge
n que
ncher). To shed light o
nthe actual role of a persulfoxide i
ntermediate also i
n ET photooxyge
natio
ns, experime
nts i
n the prese
nce of Ph
2SO(a trap for the persulfoxide) were carried out. Cooxidatio
n of Ph
2SO to form Ph
2SO
2 was, however, observed o
nly i
nthe DCA-i
nduced photooxyge
natio
n of Bu
2S, i
n li
ne with the si
nglet oxyge
n mecha
nism suggested for this reactio
n.No detectable amou
nts of Ph
2SO
2 were formed i
n the ET photooxyge
natio
ns of PhSMe with both DCA a
nd NMQ
+a
nd of Bu
2S with NMQ
+. This fi
ndi
ng, coupled with the observatio
n that
1O
2 a
nd ET photooxyge
natio
ns lead to differe
ntproduct distributio
ns, makes it u
nlikely that, as curre
ntly believed, the two processes i
nvolve the same i
ntermediate,i.e., a
nucleophilic persulfoxide. Furthermore, the cooxidatio
n of Ph
2SO observed i
n the DCA-i
nduced photooxyge
natio
nof Bu
2S was drastically reduced whe
n the reactio
n was performed i
n the prese
nce of 0.5 M biphe
nyl as a cose
nsitizer,that is, u
nder co
nditio
ns where a
n (i
ndirect) ET mecha
nism should operate. This observatio
n co
nfirms that a persulfoxideis formed i
n si
nglet oxyge
n but
not i
n ET photosulfoxidatio
ns. The latter co
nclusio
n was further supported by theobservatio
n that also the i
ntermediate formed i
n the reactio
n of thia
nthre
ne radical catio
n with KO
2, a reactio
n whichmimics step d (Scheme 2) i
n the ET mecha
nism of photooxyge
natio
n, is a
n electrophilic species, bei
ng able to oxidizePh
2S but
not Ph
2SO. It is thus proposed that the i
ntermediate i
nvolved i
n ET sulfoxidatio
ns is a thiadioxira
ne, whoseproperties (it is a
n electrophilic species) seem more i
n li
ne with the observed chemistry. Theoretical calculatio
nsco
ncer
ni
ng the reactio
n of a sulfide radical catio
n with O
2-ntities/bull.gif"> provide a ratio
nale for this proposal.