The Quenching of Singlet Oxygen ( I & ) by Transition Metal Chelatesl

D. J . CARLSSON, T. SUPRUNCHUK, AND D. M. WILES Di1,ision of Chemistry, National Research Colrncil of Canada, Ortnwo, Canada K I A OR9

Received May 17, 1974

D. J. CARLSSON, T. SUPRUNCHUK, and D. M. WILES. Can. J. Chem. 52,3728(1974). Various transition metal chelates were found to quench '0, in the liquid phase. '0, was

generated either photochemically, from an ozonide, or with a microwave source and reacted with rubrene or 3-methyl-2-pentene. Rate constants for '02 quenching by the chelates were measured in various solvents using the three '0, sources? and compared with the constants of previously known '02 quenchers. The most efficient chelates quenched '0, at close to the diffusion controlled rate, as did B-carotene. Some quenchers of '0, in the liquid phase were qualitatively shown to quench in the solid state. The possible '0, quenching mechanisms of the chelates are discussed.

D. J. CARLSSON, T. SUPRUNCHUK et D. M. WILES. Can. J. Chem. 52.3728 (1974). Des chelates de divers metaux de transition permettent l'extinction de '0, en phase liquide.

On produit '0, soit par une reaction photochimique a partir d'un ozonide, soit par une source microonde, et il reagit avec le rubrene ou le methyl-3 pentene-2. On mesure les constantes de vitesse de l'extinction de '0, par les chelates dans des solvants varies en utilisant les trois sources de '0, et on les compare avec les constantes d'extincteurs connus. L'extinction de '0, par les chelates les plus efficaces se fait pres de la vitesse contralee de diffusion, comme le fait le (3-carotene. On a n~ontre qualitativement que quelques extincteurs de '0, en phase liquide provoquent aussi l'extinction en phase solide. On discute des mecanismes possibles de I'extinction de '0, par les chelates. [Traduit par le journal]

Since the initial discovery of singlet oxygen ('A,, '0,) reactions with organic substrates in 1939 (1)- and their rediscovery in 1963 (2), the study of '0, reactions has grown at an explosive rate (3-5). Commercial interest in singlet oxygen chemistry has now reached a high level because of known involvement, or postulated intervention of '0, in processes such as air pollution, cancer formation, rancidifica- tion of edible oils, and deterioration of many polymeric materials (rubbers, plastics, fibers, biopolymers, etc. (6)). Although '0, has been suggested to be an important component of photochemical smogs (7a), its atmospheric concentration is probably always low in com- parison with other airborne oxidants such as ozone (76). Nevertheless, significant quantities of '0, may be generated in solids or liquids either under the influence of ozone (S), or by the near u.v. (sunlight) excitation of impurities, which are then quenched by 302 to give '0, (6).

The commercial importance of '0, should lead to a growing interest in the prevention of '0, reactions, especially by quenching to ground state ( 3 ~ 2 ) oxygen by reaction 1.

[l] '0, + Quencher % Quencher + 302

'NRCC No. 14264.

Various compounds have been reported to quench '0, in the liquid or gas phases. These compounds include the carotenoids, aliphatic and aromatic amines, sulfides, phenols, etc. (9). However, all of these compounds are either inefficient quenchers (k, < 10' M - ' s- ' in the liquid phase) or are themselves unstable and subject to photo or thermal oxidative degradation (especially the carotenoids and the aromatic amines). In recent preliminary cornmunicatioiis we have reported that certain transition metal chelates are very efficient '0, quenchers both in liquid and solid states (10, 11). Since many of these chelates also possess extremely high u.v. stability and good thermal stability, they should find wide commercial application, and aiso be useful in the diagnosis of '0, reactions. In this paper we discuss in detail the quenching ability of these chelates in the presence of various '0, acceptors, using '0, derived from widely different sources.

Experimental Singlet Oxygen Genevation

Photochemical '02 was generated in the liquid phase either by the

irradiation of rubrene (9,10,11,12-tetraphenylnaph- thacene) at 520 i 20 nm or tetraphenylporphine at 390 f 20 nm. Air saturated solutions (-1 x M

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CARLSSON ET AL.: THE QUENCHING OF SINGLET OXYGEN

FIG. 1. Microwave system

in 0 2 ) were irradiated in 1 cm path fluorescence cells, in a Farrand Spectrofluorometer (MKI xenon arc source, IP28 Photomcltiplier). Sensitizer fluorescence emissions in the presence of '0, quenchers were period- ically monitored at 545 nni (rubrene) and 650nm (tetraphenylporphine).

Thermal Ozone was added to solutions of triphenylphosphite in

methanol at -70°C to give a solution of the ozonide adduct, (+O),P. . .O,. A rubrene or rubreneiquencher solution containing traces of methanol and pyridine (final concentration 5% in each case) was cooled to -70°C and then added to the ozonide solution. A rapid evolution of '0, resulted (reaction 2).

Microwace Oxygen at 6 Torr was led over a mercury droplet and

through a microwave discharge (2450 MHz, 70 W) in a quartz tube. Atomic oxygen, ozone, 'A, and 'Z,+ singlet oxygen species were generated (reaction 3).

The trace of mercury vapor formed mercuric oxide rings inside the quartz tubc, and prevented 0, and 0 . concentrating in the emergent gas stream (12a). Any 'X,+ (emerging from the discharge zone) was deactivated to 'A, by a low pressure of water vapor (from a trap containing ice at -40 ' C ) and \later vapor was removed by a trap at -78 "C. However, a low stationary con- centration of 'Z,+ is rapidly established in the flow system by an energy pooling process between 'A, mole- cules (12b). We are grateful to a referee for pointing O L I ~ that the H,O trap system probably does not in- fluence this low 'Z,+ concentration in the reaction cells at our pumping rate (-500cm s- ' flow). The '0, stream was split into two identical gas streams, each of which passed over a stirred rubrene solution (2.0 ml), one solution with a quencher, and one quencher free (Fig. 1). This procedure allowed comparative experiments to be performed, so as to compensate for fluctuations in the '0, concentration coming from the discharge. Owing to the low pressure in the microwave system, only involatile solvents can be used.

Reagents Rubrene was recrystallized from benzene. Hexadecane

was rendered hydroperoxide- and olefin-free by repeated

and twinned reaction cells.

tumbling with concentrated H,SO,, and fractionation ~ ~ n d e r reduced pressure. All hydrocarbons were passed through activated alumina immediately before use. 3-Methyl-2-pentene was Phillips Research Grade. Squalene (2,6,10,15,19,23-hexamethyltetracosane-2,6,10, 14,18,22-hexaene) was d~stilled under reduced pressure. All solvents were spectroscopic grade. Chelates were purified by recrystalliration or column chromatography.

Product Analysis Rubrene cons~nnption was measured spectrophoto-

metrically at 520nm, since singlet oxygen reaction produces a colorless endoperoxide. In the case of the tetraphenylporphinel3-methyl-2-pentene system, hydro- peroxide concentrations were estimated spectrophoto- metrically after the liberation of 1 , - from NaI solution in isopropanol, as described previously (13).

Treatment of Results Rubrene and many olefins react rapidly with

'0, to yield respectively an endoperoxide (14) and a hydroperoxide (4).

Photochemical ' 0 , Gerierafion Rubrene can act as both sensitizer and

substrate in photooxidations (14) whereas an olefin requires a sensitizer such as tetraphenyl- porphine. For the oxidation of the rubrene or olefin substrates, concentration [A], in the presence of a quencher, concentration [Q], reactions 1 and 4 to 6 are now well established (9, 149

I41 ,O2 + [Sensitizer]* + '0, + Sensitizer (Rate = R,)

151 ' 0 2 + A 5 AO,

and lead to the rate expression

A stationary concentration of '0, is assumed and R, is the rate of '0, generation in solution.

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3730 CAN. J. CHEM. VOL. 5 2 , 1974

Then in the absence of quencher

I t follows from 171

where subscript 0 refers to initial values, and superscripts Q and 0 refer to rates in the presence and absence of quencher respectively.

Therrzal '0 , Generafion The kinetics of the generation of '0, by

the decomposition of (+O),P. . .O, in the presence of pyridine have been discussed by Mendenhall (15), who has shown that '0, is generated quantitatively from the ozonide. If no '0, is lost from the solution, then from reactions 1 , 2, 5, and 6,

Integration of eq. 10 yields the following expression after complete ozonide decomposi- tion (at time a).

k, ; 1 [Ill k,, CQI

In the absence of quencher, kd/k,, can be derived from expression 1 1.

Microwave ' 0 , Generation Reactions 1, 3, 5, and 6 lead to the rate

expression 7. Integrating this expression up to a time t , both in the presence and absence of quencher leads to the relationship

Results Photochemical '0, Generatio~z

The loss of rubrene during irradiation at 520 nm in air-saturated solutions was followed spectrophotometrically. Some rubrene consump- tion curves are shown in Fig. 2. In the rubrene photo-oxidation, the rate of rubrene consump-

I 1 I I I I 0 0 20 30 40

IRRADIATION TIME (rnin)

FIG. 2. Rubrene photooxidation in the presence of '02 quenchers. Irradiation at 520 i 20 nm. Rubrene in iso-octane; quencher: 0, none; V, DABCO (4 x &I); 0, Ni(l1) bis[2,2'-thiobis-4-(1,1,3,3- tetramethylbutyl)phenolate] (4 x 10-"W); A, Ni(I1) [2,2'-thiobis-4-(1,1,3,3 -tetramethylbutyl)phenolate]-n- butylainine (4 x M). Rubrene in methylene chlo- ride; quencher: 8, none; a, 0-carotene (1 x lo-' M); A, Ni(1I) bis-acetylacetonate (1 x M ) .

tion is a complex function of the rubrene concentration, since rubrene is involved as both sensitizer (reaction 4) and substrate for '0, (reaction 5). Consequently the ratios of initial rates, (d[~]/dt) ,O/(d[Al/dt),~ were always used in the calculation of rate constant ratios. Two distinct kinetic effects were observed depending on the solvent employed. In iso- octane, the rate ratios measured at fixed quencher concentrations were virtually in- dependent of the rubrene concentrations, as shown by the examples in Table 1. I11 methylene chloride the rate ratio was dependent on rubrene concentration; the rnethylene chloride data are plotted in Fig. 3. From expression 9, the slopes and intercepts shown in Fig. 3 lead to kd/k,, and k,/k,, values of (1.76 f 0.4) x

M and 117 F 30 (p-carotene), (0.90 +_

0.07) x M and 0.19 f 0.02 [Ni(II)bis(O- butyl 3,5-di-t-butyl-4-hydroxybenzyl phospho- nate)], and (1.25 0.08) x M and 2.0 i 0.4 (Ni(I1)bis-acetylacetonate) respectively Kinetic data for rubrene photo-oxidation at various quencher concentrations are shown in Table 2.

Hydroperoxide is produced on the irradiation at 390 f 20 nm of a tetraphenylporphine solu- tion containing the olefin 3-methyl-2-pentene (Fig. 4). In the absence of a quencher, the reciprocal rate is linearly dependent on the

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CARLSSON ET AL.: THE QUEKCHING OF SINGLET OXYGEN

TABLE 1. Rubrene photooxidation in the presence of quenchers in i-octane

Rubrene Rate rubrene loss ( x lo7) ( M s-') -

(d tAlldt)oO [A] x lo4 kdkd

(MI - (d[Alldt)oO* - (d[Alldt)oQt (d[Alldt)oQ ( x lo-3) ( M - ' )

1,4-Diazabicyclo[2.2.2]octane (4 x M) 1 .O 3.8 1.15 3.30 0.57 0.75 2.25 0.75 3 .OO 0.50 0.59 1.74 0.55 3.16 0.54 0.43 0.96 0.34 2.82 0.45 0.22 0.30 0.093 3.23 0.56

Nickel(I1) bis[2,2'-thiobis-4-(l,l,3,3-tetramethylbutyl)phenolate] (4 x M ) 1 .O 3.80 0.38 10.0 2.2 0.89 3.49 0.32 11.2 2.5 0.78 2.25 0.25 9.0 2.0 0.61 1.74 0.18 9.7 2.2 0.43 0.96 0.11 8.7 1.9

Nickel(I1) [2,2'-thiobis-4-(l,l,3,3-tetramethylbutyl)phenolate]-n-butylan1ne (4 x M ) 1 .O 3.80 0.31 12.2 2.8 0.89 3.49 0.27 12.9 3.0 0.78 2.25 0.19 11.8 2.7 0.61 1.74 0.14 12.4 2.8 0.43 0.96 0.079 12.2 2.8

N-Isopropyl-N'-phenyl-p-phenylene diamine (4 x M ) 1 . O 2.29 0.80 2.86 4.6 0.78 1.74 0.60 2.90 4.7 0.65 1.19 0.42 2.83 4.6 0.43 0.64 0.23 2.78 4.4 0.33 0.43 0.155 2.77 4.4

*Without quencher. ?With quencher, at same light intensity as *.

I 1 I I 0 0.2 0.4 0.6 0.8 I .O

[ A ] , M ( x l o4 )

FIG. 3. Rubrene photooxidation in methylene chlo- ride. x , 0-carotene (1.5 x M); 0, Ni(l1) bis[O- butyl 3,5-di-t-butyl-4-hydroxybenzyl phosphonate] (4 x l o 3 M ) ; A, Ni(I1) bis-acetylacetonate (1.0 x 114).

reciprocal olefin concentration (Fig. 5) . From eq. 8 the intercept of this plot gives the rate of '0, generation in solution (4.3 x lo-' M s-') and the slope allows the calculation of k,/k,, = 1.64 x M in chlorobenzene solutions. Sensitized photo-oxidation of 3-methyl-2-pen- tene (1.0-5.2 x M ) in the presence of nickel(I1) [2,2'-thiobis-4-(l,l,3,3-tetramethylbu-

tyl)phenolate]-n-butylamine (7.4-32 x M ) led to an average k,/k,, of 80 k 5.

lrradiation of either a sensitizer-free solution of 3-methyl-2-pentene in chlorobenzene or an olefin-free solution of tetraphenylporphine in chlorobenzene yielded no detectable hydro- peroxide (4 x M ) . Many metal chelates are rapid decoinposers of hydroperoxides (6, 16). However, separate dark experiments with oxi- dized 3-methyl-2-pentene solutions showed that the hydroperoxide concentrations did not de- crease either in the presence of nickel(I1) [2,2'-thiobis-4-(I, l,3,3-tetramethylbuty1)pheno- late]-12-butylam~ne or of the sensitizer.

Tlierinal '0, Generation Rubrene is bleached by the '0, evolved

rapidly from (+O),PO, on the addition of a trace of pyridine. The validity of expression 1 1 for the (+O),PO, production of '0, in a rubrene solution has been confirmed by Menden- hall (15). Rubrene concentrations before and after ozonide decomposition are shown in

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3732 CAN. J . CHEM. VOL. 5 2 , 1974

TABLE 2. Effect of quencher concentration on rubrene photooxidation*

(d[Alidt)oO (d[Alldt)oQ [Quencher] k,/k, x lo-3

Quencher x lo4 (M) x 10' (MS-') x 10' (M S - I ) (M-l)

Ni(J1) bis(di-rz-buty1dithiocarbamate)f 40 5.3, 0.0089 150 2.1 2.54 0.0443 270 0.21 2.54 0.230 470

Ni(I1) bis(2-hydroxy-5-~i~ethoxyphenyl- 10 1.68 0.0875 18 hr-rz-butyl aldimine)? 2 . 4 1.68 0.215 28

0.60 1 .95 0.425 60 0.060 1 .21 0.90, 57

Ni(I1) [2,2'-thiobis-4-(1,1,3,3- 50 5.35 0.337 3 .O tetramethylbuty1)-phenolatel-n- 10 2.2, 0.35, 5 .3 butylaminei 1.7 1 . j 6 0 . go5 4.3

Fe(1II) tri(di-isopropyldithio- 10 5.2, 0.0236 0.0535 carbaniate)$ 1 . O 4 .2 , 0.203 0.052s

Wnquenched initial rate fluctuations due to variations in Xe arc intensity. tRubrene concentration 1.0 r lo-" IM in ;-octane. lRuhrene concentration 1.0 x M in CH,CI,. Skqlkox,

IRRADIATION TIME imin)

FIG. 4. Sensitized photooxidation of 3-methy1-2- pentene. Sensitizer tetraphenylporphine (7.5 x I M ) in chlorobenzene, irradiated at 390 & 20 nm. Quencher: Ni(I1) [2,2'- thiobis-4-(I,], 3 ,3 - tetramethylbuty1)pheno- late]-n-butylamine. Concentrations, olefin: 0, 0.52 x 10-'M; A, 1.02 x 10-'M; 0, 5.2 x 10-'M; olefin, quencher: 9, 1.02 x lo-', 3.25 x M ; A, 5.2 x

7.49 x 10-3 M.

Table 3 for various quenchers. The rate ratios k,/ko, for '0, deactivation in methylene chloride and iso-octane are calculated from [l 1 ] in the absence of a quencher. In the presence of a quencher, kq/ko, ratios were also calculated

from expression 11, using the appropriate experimental k,/k, value.

Microvt,ave ' 0 , Generation Rubrene is rapidly bleached by oxygen which

has passed through a microwave discharge. Although a microwave discharge is a convenient source of '0, it is difficult to control con- sistently the concentration of '0, dissolved in a liquid exposed to the '0,-containing stream. This problem was largely overcome by the simuitaneous exposure of solutions in the twinned reaction cells (Fig. l ) , one cell con- taining only rubrene solution, the other rubrene and a dissolved quencher. The discharge was usually continued until 60-70% of the rubrene in the quencher free solution had been oxidized to the colorless endoperoxide. Rubrene bleaching

FIG. 5. Sensitized photooxidation of 3-methyl-2- pentene. Tetraphenylporphine (7.5 x M) sensitizer in chlorobenzene. No quencher.

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CARLSSON ET AL.: THE QUENCHING OF SINGLET OXYGEN 3733

TABLE 3. '02 from (+O),P. . .O, in the presence of quenchers

Rubrene x lo4 (M) Initial

[(+O),P. . .O,] Initial Final k,/k, x lo4 Quencher*' x lo3 (M) [AID [A1 0 (MI kqlkox

-

DABCO DABCC

Ni(1I) bis(0-butyl 3,5-di-t-butyl-4- hydroxybenzyl phosphonate)

Ni(1I) bisj2-hydroxy-5-methoxyphenyl- N-n-butyl aldimine)

TABLE 4. 'Cz Fro111 the microwave discharge in the presence of quenchera ---

[Rubrene] x lo4 ( M )

Quencher

Final [Quencher] Initial -

x 103 (M) IAIo [AlrO LAICQ kqlkox

Ni(I1) bis(2-hydroxy-5-methoxyphenyl- 1.05 0.19 0.96 5.7* N-n-butyl aldimine) 1.45 0.19 1 .OO 4.8*

Ni(I1) bis-acetylacetonate

MnCI24H20 4.83 1.89 0.27 0.27 <O.O1t

*In hexadecane, using kdk , , = 9.0 Y lo4 s - ' , calculated from DABCO quenching data. +In 2-butoxyethanol, using kdlk,, = 38 x lo4 s-', calculated from DABCO quenching data.

data are shown in Table 4 for the microwave system.

Experimental difficulties and the complexity of the integrated rate expression 12 prevent the direct estimation of k,/ko, ratios for the solvents employed in the microwave work. Indirect k,/ko, ratios were calculated from competitive experiments involving rubrene and rubrene/

DABCO (1,4-diaza-bicyclo [2.2.2]octane) solu- tions. k,/ko,(DABCO) was assumed to be 0.49 and solvent independent as found from the ozonide deco~nposition data (Table 3), and this value was used together with eq. 12 to calculate the respective kd/ko, values. The k,/ko, values derived from the various '0, generation techniques are collected in Table 5.

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3734 CAN. J . CHEM. VOL. 5 2 , 1974

TABLE 5 . Experi~nental kd values in various solvents --

This work* Literature values -- -

kd x lo2 Source/substrate Solvent kd/k,, x lo3 (s-I) Solvent kd x (SKI)

Photo/rubrene CH2Clz 0.13 0 .95 CHCI3 1.73 (+0)3P03irubrene CH2CI2 0.10 0.73 (+O),PO3/rubrene i-Octane 0 .64 4 . 7 C Y C S H I ~ 5.93 Microwaveirubrene Hexadecane 1 .2 9 .0 Microwaveirubrene 2-Butoxyethanol 5.27 3 8 C H 3 0 H 143 Photo +,porphine/3-Me-2-pentene +C1 0.16f 1 . 1 C6Hs 4.0$§

*Assuming k,, = 7.3 x lo7 M-' s - ' (rubrene) and 1.0 x 106 M-' s-' (3-Me-2-pentene). tCalculated from DABCO qi~enched solutions, assuming k,(DABCO) = 3.5 x 107 M-1 s-I. $Reference 25. §Reference 18.

k,/k,, values were calculated from the micro- wave data using expression 12 and are shown in Table 4.

Since alkanes have been suggested to react with '0, (I?), hexadecane (3.4 M) was exposed for 5 h to microwave-generated '0,. Hydro- peroxide was not detectable after this treatment ( < 5 x M). However, when undiluted squalene was similarly exposed -2.0 M hydro-

\\

peroxide was formed, but no C=O products /

could be. detected by i.r. spectroscopy \

(< 1 x M from - 1715 cm-' C=O

absorption). /

Exposure of chelate solutions in hexadecane to 1 h of '0, from the n~icrowave source resulted in no detectable changes in the u.v. absorption of the solutions, for all of the chelates listed in Table 6. A similar resistance to '0, attack has been reported for nickel(I1) Schiff-base complexes (1 8).

Discussion All convenient methods for the generation

of '0, are plagued by experimentai problems. Of the three methods used in this work, problems can arise from the following sources.

In the photochemical routes, deactivation of the excited singlet or triplet states of sensitizers by the '0, quencher under investigation can lead to high apparent k,/k,, values for '0, quenching, since (RJOQ is no longer equal to (R,),', but instead smaller. Singlet quenching can be detected by observing the fluorescence of the sensitizer (at 545 nm for rubrene and

650 nm for tetraphenylporphine). All of the quenchers examined had negligible effect on the fluorescence of these sensitizers ( < l o % de- crease in emission). Unfortunately the phos- phorescence emissions from the sensitizer triplets were experimentally inaccessible. However, from the data of Adaniczyk and Wilkinson (19), who showed that polynuclear aromatic triplets are quenched by Ni(l1) Schiff base con~plexes with rate constants of -3 x 109 M -' s-' and of Porter and co-workers (201, who showed that 30, quenching of polynuclear aromatic tripletc occurred with a rate constant of - 2 x lo9 M - ' s- ', chelate quenching of rub- rene triplets can be expected to be important in comparison with 30, quenching (reaction 4) only when the chelate concentration exceeds - 1 x M (dissolved 30, concentration - 1 x M in air saturated solutions (20)). In fact, the three upper chelates listed in Table 2 show k,/k,, values which increase as the chelate concentration decreases (the converse of the effect expected to result from rubrene triplet quenching), but became approximately con- stant at below 1 x lo-" M. This unexpected effect might stem from chelate association (trimer, tetramer formation) at higher con- centrations as hac been reported for some Ni(I1) chelates in solution (21).

Problems occur in the decomposition of (+O),P. . . 0 3 to '0, because of the possibility of direct reaction between the ozonide and either the substrate or the quencher. Although the catalytic decomposition of the ozonide at -70 OC will minimize direct attack, nickel(I1) bis(di-n-butyldithiocarbamate), a well known antiozonant, did react directly with the ozonide to give a brown precipitate. Ozonide reaction

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CARLSSON ET AL.: THE QUENCHING OF SINGLET OXYGEN 3735

TABLE 6. Quenching of singlet oxygen ('A,) in rubrene solutions*

k,x (M-I s-I)* -

Rubrene* (+o),po, Microwave discharge - - - -

i-Octane Cl12C1, i-Octane CH,C12 Hexadecane 2-BuEtOH

DABCO N-Isopropyl-N'-phenyl-p-phenylene diamine fJ-Carotene Ni(I1) bis(di-n-butyldithiocarbamate) Ni(1I) bis(2-hydroxy-5-methoxyphenyl-

AT-n-butyl aldimine) Fe(II1) tri(di-isopropyldithiocarbamate) Co(1I) bis(di-isopropyldithiocarbamate) NiC126Hz0 Ni(II) [2,2'-thiobis-4-(l,l,3,3-

tetramethylbuty1)-phenolatel-n-butylamine,l Ni(I1) bis[2,2'-thiobis-4-(1,1,3,3-

tetran~ethylbutyl)phenolate] Ni(I1) bisacetylacetonate CoClz6H,O Ni(I1) bis(0-butyl 3,5-di-t-butyl-4-

kydroxybenzyl phosphonate) MnC124H20 Zn(I1) bis(di-n-butyldithiocarbamate)

*Averaged values at 25 ' C in the indicated solvent, calculated from k, values taken from our data in Table4 and k,,(rubrene) = 7.3 / lo7 M - 1 q - 1 " .

+Quencher concentration 4 x to 1 i M unless specified. :Quericlier concentration < 2 x M. §Interference by some chelate-ozonide reaction. k , = 0.8 x 10n M - I s-I from tetraphenqlporphine,'3-methyl-2-pentene system.

with rubrene has been shown to be negligible (22).

The effluent from an oxygen microwave discharge contains 0 , , O., 1X,+('02) etc. as well as 1Ag(102). The use of a slow mercury distillation into the discharge to give mercuric oxide rings and the presence of a low vapor pressure of water can minimize the concentra- tions of O,, O., and 'Xgf in the effluent stream (12). The absence of 0, and 0. in the ' ~ ~ ( ' 0 , ) stream from our apparatus was confirmed by the absence of carbonyl products (< 1 x M) on exposure of squalene so as to generate a high hydroperoxide level (2.0 M). Both 0, and 0. would have produced large amounts of carbonyl products.

Absolute k, and k, values may be calculated from the k,/k,, and k,/k,, ratios if literature values for k,, are assumed. For rubrene, k,, = 6.0 x lo7 M-' S- ' can be calculated from the kd/k,, value of 1.4 x M (23) in methanol, where k, = 9 x lo4 s 1 (23). A very similar value of k,, (rubrene) = 7.8 x lo7 M-I s- ' can be derlved from p-carotene quenching data (k,,(rubrene) = 3k,,(tetramethylethylene) (24) = 150k,,(2-methyl-2-pentene) (3), k,(P-

carotene)/k,,(2-methyl-2-pentene) = 2 x lo4 (9) and k,([3-carotene) = 1.3 x 1 0 ' ~ IM-' s- ' (18)). The k,, value for 3-methyl-2-pentene was aszumed to be identical to that of 2-methyl-2- pentene (1 .O x lo6 M ' s ') as derived above.

Experimental k,/k,, and calculated kd values for the various solvents employed are collected in Table 5. The calculated kd values (Table 5) are in fair agreement with values published for comparable solvent systems (18, 25). The indirectly estimated hexadecane and 2-butoxy- ethanol k, values seem somewhat high, and may indicate the presence of some '0, quenching impurities, or some solvent dependence of k,/k,, for DABCO.

In iso-octane. the rate ratio ( d ~ / d t ) ~ / ( d ~ / d t ) Q was found to be independent of the rubrene concentration, under fixed photolysis conditions (Table 1). This result is to be expected from the relative values of k, in iso-octane (4.7 x lo4 s- l ) and k,,[A] (always < 7 x lo3 s-'), when (from eq. 9)

k CQI [I31 ( d ~ / d t ) , ~ / ( d ~ / d t ) , ~ - 1 + LL- kd

(The limited solubility of rubrene in iso-octane

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3736 CAN. J . CHEM. VOL. 5 2 , 1974

precludes working at higher concentrations.) The k,/k, values calculated from expression 13 for the iso-octane solutions of various chelates are shown in Tables 1 and 2. k , values were calculated from these kq/kd ratios, using k, =

4.7 x lo4 s- ' (Table 5), and are listed in Table 6. In methylene chloride, k, = 8 x lo3 s-', and is con~parable to ko,[A] so that [9] is valid, as is confirmed by the linearity of the plots shown in Fig. 3. The slopes and intercepts of these lines in Fig. 3 allow the direct estimation of k,/ko, and k,/k,,. In addition, the linearity of these rate plots is indicative of the absence of any significant quenching of '0, by rubrene itself.

Despite the inherent problems with each method of '0, generation the fair agreement between the k, values collected in Table 6 for a given quencher is an indication of both the absence of significant side reactions and the general validity of the assumptions made in the calculation of k,/k,, for each technique. The collected k, values for the metal chelates are in fair agreement with the few published values available. The semiquantitative data of Guillory and Cook (21) and Felder and Schumacher (27) are in good agreement with Table 6, and agree- ment with the k, data of Flood et al. (28) who used Inethylene blue sensitization of 1,3- diphenylbenzofuran oxidation is excellent. Very recently Farmilo and Wilkinson (18) have also shown that certain nickel(1I) Schiff-base com- plexes can quench '0, at close to the diffusion controlled rate in benzene solution.

The k, values for DABCO, p-carotene, and N-isopropyl-N'-phenyl-p-phenylenediamine in Table 6 are included largely for comparison with literature values of 1.6 x lo7 (in 1 :1:1 benzene, methanol, diglyme) (9), 1.3 x 10'' in benzene (18), and 1.5 x lo9 M - ' s-' in ethanol respectively. The latter value is calculated from the data of Dalle et al. (26), using a literature value (25) of Ic, = 8.3 x lo4 s-' in ethanol. Aliphatic and aromatic amines have been shown (23) to quench '0, by the formation of a charge transfer complex (reaction 15).

Our amine k, values are lower than the published values perhaps because of a solvent polarity effect as suggested by Young et al. (23). Our low polarity solvents would not stabilize a '0,-amine charge transfer complex as effectively

as the high polarity solvents employed previously (9, 26). p-Carotene has been proven (18) to quench by an energy transfer process to a low lying p-carotene triplet (reaction 16), and quenching is accompanied by cis-trans isomeriz- ation.

It is interesting to speculate on the mechanism by which the chelates quench '0,. Chelate quenching seems to be a specific property of the chelate as a whole, since the free ligands (where stable) either do not quench 'O,, or quench a t a much lower efficiency (10, 28). The quenching effect ic apparently not related to coordination or magnetism, gince octahedral (paramagnetic) (e.g, NiC126H,0) and square planar (dia- magnetic) (e.g. nickel(1I) bis(di-n-butyldithio- carbamate)) complexes quench 'O,, as does an octahedral trimeric complex (e.g. nickel(I1) bis-acetylacetonate). For the hydrated chlorides (Table 6) '0, quenching efficiency increases in the sequence Mn(I1) << Co(I1) < Ni(I1). Both cobaltous and nickelous chlorides have vacant electronic energy levels at 8000-9000 cm-I, whereas the d5 nlanganous complex has none (29). This infers that energy transfer might be important for these chlorides, since the oxygen 'A, + 3C,+ transition occurs at --8000cm-'. A similar conclusion was reached by Fari~lilo and Wilkinson from a consideration of Ni(I1) and Pd(1II) chelates, and ferrocene quenching of '0, (18). However, both the charge transfer process [IS] and the energy transfer reaction 16 might well operate for different transition metal chelates. Another attractive source of '0, quenching by some of the metal chelates in- volves the formation of a loose complex, possibly analogous to those formed between 302 and many Co, Ni, and Fe complexes (30,31). Although the high stability of many cobalt- oxygen complexes would preclude efficient '0, quenching, a transient association could lead to efficient quenching.

Since '0, attack is now being considered as an important source of degradation of rubbers and plastics (6), it is important to know whether any of the compounds listed in Table 6 can quench '0, in the solid state. This was in- vestigated by either co-depositing a solid layer of a quencher with rubrene by evaporation of a solution in the cells of the microwave apparatus or by coating one of the '0, delivery tubes

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CARLSSON ET AL.: THE QUEI \JCHING OF SINGLET OXYGEN 3737

(Fig. 1) with a thin solid coating of the quencher, just above a stirred rubrene solution. With both methods, on exposure to lo,, appreciable reduction in rubrene bleaching was found with the quenchers N-isopropyl-N'-phenyl-p-phenyl- enediamine, p-carotene, nickel(I1) bis(di-uz-butyl- dithiocarban~ate), nickel(I1) bis(2-hydroxy-5- methoxyphenyl-N-n-butyl aldimine), and nickel- (11) [2,2'- thiobis -4- (1 , 1 , 3 , 3 - tetramethylbutyl) - phenolatel-12-butylamine. Meaningful quantita- tive solid state quenching data would require the deposition of uniform mono-molecular layers, rather than the polycrystalline layers produced by the above techniques.

The authors wish to express their appreciation for the help and guidance given by Dr. G. D. Mendenhall during the early stage of this work.

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