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Direct measurement of the lifetime of 1Δ oxygen in solution
Volume 12, number 1
I December
CHEMICAL PHYSICS LETTERS
DIRECT I’+iEASWREMENT OF THE LIFETIME
OF
‘A
1971
OXYGENIN SOLUTION
P.B. MERKEL and D.R. KEARNS Depcrhnent of Chemistry, hisersity
of Califcmia, Rivem’de, Califotiia 92502, USA
Keceivcd 17 September
1971
by a laser pulse technique provides a value for the lifetime of *A mokm&u ox~&en in 7 * 1 ysec. Absolute reaction rote mnSmts of kp, = 7 f 1 X LO*Mm1sec’l and 4 t 1 X 107 M-1 see”1 are obA&d for the h&t o~ygm acceptors, 1,3_diphenylisobenzo~uran and tetramethylethylene respectively. Additional acceptor rare constants may be calculated by reference to published P values. Direct measurement
mthanO1 of T =
Much of the current interest in the properties of singlet oxygen is due to the discovery that electronically excited singlet oxygen molecules, produced by energy transfer from triplet state sensitizers to oxygen, are the reactive intermediates in dye sensitized photooxygenation of unsaturated hydrocarbons [l--4]. Although both 1Z and *A oxygen are generated by the quenching of triplet state sensitizers [5,6], only *A is important for reactions in solution, due to the rapid decay of lz1 (estimated lifetime 2 IO-10 see) [7] . Under kw!
photo-mygtnati&conditionsthe two
important competitive decay modes for ‘0 are
where A denotes a chemical acceptor and A& a peroxide product. Since the lifetime of IA in solution was unknown, reactitiries have usutiy been reported ti terms of ,0 = (:/Tk*). Recently, Foote and co. workers [S] observed that p-carotene was a highly efficient inhibitor of singlet oxygen reactions and concluded that this was due to quenching of lA. By assuming that the quenching was diffusion controlled they estimated that the li~atime of IA is about 10-s s+c or larger in a benzene-methanol solution. We wish to report here a direct measurement of the lifetime of singlet oxygen in solution using kinetic spectroscopy. I20
A 1J, 20 nsec pulse of 694 run photons from a ruby laser was used to excite the sensitizer methyIene blue in an oxygenated methanol solution. The methylene blue triplets thus formed are quenched by ground s&ateoxygen to produce approximately 5 X 10s5 M IA in less than 1O-7 set *. As a probe of the singlet oxygen concentration the coIored f&,_,* = 410 nm) acceptor i ,3diphenyJisobenzofuran @ 2 10-J) (91 was added to the solution, and the rate of decrease in absorption accompanying peroxide formation was monitored
syncl~onou.sIy
with
the laser pulse.
An ac-
UJP~CU with a WY IOW B VSIW is
required since the mz&m.rrn fractional bleaching can be shown to equal [lA]o/@, [iA],-, being the concentration of singlet oxygen produced by thf pulse. Under conditions where [.4] does not change substintiahy during the experiment, solution of the kinetic equations describing processes (1) and (2) yields
kA [Al @I (]/7+kA[A])
o
““p
i--(l~r+kA~Al)f)
’
where [AOz] p) is the concentration
of peroxide after all singlet oxygen has decayed. From this it follows
* 271: Hetime of triplet methykne blue in deoxygenated methanol is ? WC and the second order rate constant for quuncting of the triplet by oxygen is 2.2 X 10’ M” seF’.
CHEMICAL PHYSICS LETTERS
Volume 12, number 1
Fig. 1. Bleaching in an oxygenated solution of methylcne blue (10” M) in methanol of
monitored
at 410 nm in a 1 cm ceU. (b) 1.5 x
IO4 hi I$diphenylisobenzofu;an 2 mm ceil and (c) as (a)
monitored
but with S X
10m3 M
5
t (r.secl
10
i97 1
J 15
at 435 nm in a
tetramethyt
ethylene added. Arrows indicate % T before flash.
that 1ogAOD = -2.3(1/7+kA [A]) t + conk, where AOD is the optical density of the acceptor at time t minus the asymptotic value. Plots of AOD versus t at two acceptor concentrations provide values for both WldkA.
Photobleaching of 1,3diphenylisobenzofuran under three sets cf conditions is illustrated in fig. 1. In (a) a low acceptor concentration (2 X 10m5 M) results in a decay rate limited primarily by ‘i-..The initial short lived increase in absorption arises from the methylene blue triplet-triplet absorption. In a deoxygenated solution no bleaching is observed and themethylene blue triplet decays at rhe characteristic rate. This eliminates the possibility of a direct reaction between the acceptor and triplet methyIene b!ue. Increasing the acceptor concentration (1.5 X 10W4 M) predictably accelerates the decay rate by consumption of a significant portion of 1A as is demonstrated in (b). The effect of adding a second oxklizabie acceptor is depicted in (c). Addition of a sufficient amount of tetramethylethylene (5 X 10e4 M) results in a decrease in both the duration and yield of 1,3diphenylisobenzofuran photo-oxidation in accordance with expectations [compare amounts of bleaching in (a) and (c)l. First order plots qf the decay curves in figs.. 1(a) dnd (b) appear in fig .2. The slight curvature reflects the consumption of acceptor as photo-oxidation progresses. From the slopes we obtain values of T = 7 + 1 WC and k, = 7 + 1 X 108 M-l SK-~*. This yields * Foote, in ref. [8] , quotes unpublished results of F. Wilkinson which gave a Bfetimc of approximately
0
I December
10 psec.
Fig. 2. First order decay plots of (a) -and(b)-----of
&. I.
a fi value of 2 X IOB4 for 1,3diphenylisobenzofur~ reasonably close to that observed in l,l ,Ztrichlorotrifluorethane [9]. The absolute reaction rate constant for tetramethylethylene (TME) is calculated to be k, = 4 + 1 X 10’ M-I set-t,fl being in agreement with the previously determined value [I 1. The abso!ute values of the rate constants for 1,3-diphenylisobenzofuran and TME can now be used with tabuked p values to obtain absolute rate consUn& for other acceptors. In the present experiments where the transient singlet oxygen concentration is considerably greater than in steady state photo-oxidations, *A-IA annihilation might possibly affect the obsc=cd dec;iy. This is ruled out by the observation that a reduction of [IA] by a factor of two, produced by decreasing the methylene blue concentration, produces no change in the singlet oxygen lifetime, Since air saturated and ‘oxygen saturated solutions give similar lifetimes, quenching of :A by 3Z is also ruled out as a competitive decay mode. Jn retrospect the agreement between the IA lifetime measured directly and that estimated by Foote substantiates the proposal that the quenching of ‘A by hrotene is diffusion controlled [S] . Lifetime measurements in a variety of other solvents are currently being conducted and this information, along with additional quenching rate constants, will appear in a subsequent more detailed publication. ‘Ihe support of the U.S. Pubiic Health Service (Grant No. CA 11459) is most gratefuily acknowledged.
;
-:
:;
” [IJ C.&Fob& Acco&tiC!h& Res. i(t96&) 104.’ [2) KGolinick, Advan. Chem. Br, 77 (1968) 78. ” ,’ [3] C&Foote and,S.Wekler, !. Am. Chem. Sot, 86 (1964) 3879.
77(1968)133.
[S] C.S.Foote, R.W.Denny, L.Weaver, Y.chmg and J.Peters, AN+ Acad;Sci. N.Y. 17J (1970) 139. 19] LB.C.Matheson and J-Lee, Chem. Phys. Letters 7 (1970)
.- ,475.
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:
I December
CHEMICAL PHYSICS LETTERS
DIRECT I’+iEASWREMENT OF THE LIFETIME
OF
‘A
1971
OXYGENIN SOLUTION
P.B. MERKEL and D.R. KEARNS Depcrhnent of Chemistry, hisersity
of Califcmia, Rivem’de, Califotiia 92502, USA
Keceivcd 17 September
1971
by a laser pulse technique provides a value for the lifetime of *A mokm&u ox~&en in 7 * 1 ysec. Absolute reaction rote mnSmts of kp, = 7 f 1 X LO*Mm1sec’l and 4 t 1 X 107 M-1 see”1 are obA&d for the h&t o~ygm acceptors, 1,3_diphenylisobenzo~uran and tetramethylethylene respectively. Additional acceptor rare constants may be calculated by reference to published P values. Direct measurement
mthanO1 of T =
Much of the current interest in the properties of singlet oxygen is due to the discovery that electronically excited singlet oxygen molecules, produced by energy transfer from triplet state sensitizers to oxygen, are the reactive intermediates in dye sensitized photooxygenation of unsaturated hydrocarbons [l--4]. Although both 1Z and *A oxygen are generated by the quenching of triplet state sensitizers [5,6], only *A is important for reactions in solution, due to the rapid decay of lz1 (estimated lifetime 2 IO-10 see) [7] . Under kw!
photo-mygtnati&conditionsthe two
important competitive decay modes for ‘0 are
where A denotes a chemical acceptor and A& a peroxide product. Since the lifetime of IA in solution was unknown, reactitiries have usutiy been reported ti terms of ,0 = (:/Tk*). Recently, Foote and co. workers [S] observed that p-carotene was a highly efficient inhibitor of singlet oxygen reactions and concluded that this was due to quenching of lA. By assuming that the quenching was diffusion controlled they estimated that the li~atime of IA is about 10-s s+c or larger in a benzene-methanol solution. We wish to report here a direct measurement of the lifetime of singlet oxygen in solution using kinetic spectroscopy. I20
A 1J, 20 nsec pulse of 694 run photons from a ruby laser was used to excite the sensitizer methyIene blue in an oxygenated methanol solution. The methylene blue triplets thus formed are quenched by ground s&ateoxygen to produce approximately 5 X 10s5 M IA in less than 1O-7 set *. As a probe of the singlet oxygen concentration the coIored f&,_,* = 410 nm) acceptor i ,3diphenyJisobenzofuran @ 2 10-J) (91 was added to the solution, and the rate of decrease in absorption accompanying peroxide formation was monitored
syncl~onou.sIy
with
the laser pulse.
An ac-
UJP~CU with a WY IOW B VSIW is
required since the mz&m.rrn fractional bleaching can be shown to equal [lA]o/@, [iA],-, being the concentration of singlet oxygen produced by thf pulse. Under conditions where [.4] does not change substintiahy during the experiment, solution of the kinetic equations describing processes (1) and (2) yields
kA [Al @I (]/7+kA[A])
o
““p
i--(l~r+kA~Al)f)
’
where [AOz] p) is the concentration
of peroxide after all singlet oxygen has decayed. From this it follows
* 271: Hetime of triplet methykne blue in deoxygenated methanol is ? WC and the second order rate constant for quuncting of the triplet by oxygen is 2.2 X 10’ M” seF’.
CHEMICAL PHYSICS LETTERS
Volume 12, number 1
Fig. 1. Bleaching in an oxygenated solution of methylcne blue (10” M) in methanol of
monitored
at 410 nm in a 1 cm ceU. (b) 1.5 x
IO4 hi I$diphenylisobenzofu;an 2 mm ceil and (c) as (a)
monitored
but with S X
10m3 M
5
t (r.secl
10
i97 1
J 15
at 435 nm in a
tetramethyt
ethylene added. Arrows indicate % T before flash.
that 1ogAOD = -2.3(1/7+kA [A]) t + conk, where AOD is the optical density of the acceptor at time t minus the asymptotic value. Plots of AOD versus t at two acceptor concentrations provide values for both WldkA.
Photobleaching of 1,3diphenylisobenzofuran under three sets cf conditions is illustrated in fig. 1. In (a) a low acceptor concentration (2 X 10m5 M) results in a decay rate limited primarily by ‘i-..The initial short lived increase in absorption arises from the methylene blue triplet-triplet absorption. In a deoxygenated solution no bleaching is observed and themethylene blue triplet decays at rhe characteristic rate. This eliminates the possibility of a direct reaction between the acceptor and triplet methyIene b!ue. Increasing the acceptor concentration (1.5 X 10W4 M) predictably accelerates the decay rate by consumption of a significant portion of 1A as is demonstrated in (b). The effect of adding a second oxklizabie acceptor is depicted in (c). Addition of a sufficient amount of tetramethylethylene (5 X 10e4 M) results in a decrease in both the duration and yield of 1,3diphenylisobenzofuran photo-oxidation in accordance with expectations [compare amounts of bleaching in (a) and (c)l. First order plots qf the decay curves in figs.. 1(a) dnd (b) appear in fig .2. The slight curvature reflects the consumption of acceptor as photo-oxidation progresses. From the slopes we obtain values of T = 7 + 1 WC and k, = 7 + 1 X 108 M-l SK-~*. This yields * Foote, in ref. [8] , quotes unpublished results of F. Wilkinson which gave a Bfetimc of approximately
0
I December
10 psec.
Fig. 2. First order decay plots of (a) -and(b)-----of
&. I.
a fi value of 2 X IOB4 for 1,3diphenylisobenzofur~ reasonably close to that observed in l,l ,Ztrichlorotrifluorethane [9]. The absolute reaction rate constant for tetramethylethylene (TME) is calculated to be k, = 4 + 1 X 10’ M-I set-t,fl being in agreement with the previously determined value [I 1. The abso!ute values of the rate constants for 1,3-diphenylisobenzofuran and TME can now be used with tabuked p values to obtain absolute rate consUn& for other acceptors. In the present experiments where the transient singlet oxygen concentration is considerably greater than in steady state photo-oxidations, *A-IA annihilation might possibly affect the obsc=cd dec;iy. This is ruled out by the observation that a reduction of [IA] by a factor of two, produced by decreasing the methylene blue concentration, produces no change in the singlet oxygen lifetime, Since air saturated and ‘oxygen saturated solutions give similar lifetimes, quenching of :A by 3Z is also ruled out as a competitive decay mode. Jn retrospect the agreement between the IA lifetime measured directly and that estimated by Foote substantiates the proposal that the quenching of ‘A by hrotene is diffusion controlled [S] . Lifetime measurements in a variety of other solvents are currently being conducted and this information, along with additional quenching rate constants, will appear in a subsequent more detailed publication. ‘Ihe support of the U.S. Pubiic Health Service (Grant No. CA 11459) is most gratefuily acknowledged.
;
-:
:;
” [IJ C.&Fob& Acco&tiC!h& Res. i(t96&) 104.’ [2) KGolinick, Advan. Chem. Br, 77 (1968) 78. ” ,’ [3] C&Foote and,S.Wekler, !. Am. Chem. Sot, 86 (1964) 3879.
77(1968)133.
[S] C.S.Foote, R.W.Denny, L.Weaver, Y.chmg and J.Peters, AN+ Acad;Sci. N.Y. 17J (1970) 139. 19] LB.C.Matheson and J-Lee, Chem. Phys. Letters 7 (1970)
.- ,475.
: :
,‘, :
‘.
.
._
: ; .
:
.’
..’
,.
..
,.
‘.
.
,.
‘.
:
.. :
. :
‘,
,’ :
: