Photochemistry in the Adsorbed Layer
2225
Photochemistry in the Adsorbed Layer. IV. Effects of Oxygen upon the Photolysis of the Adsorbed Alkyl Ketones
Yutaka Kubokawa" and Masakazu Anpo Department of Applled Chemistry, University of Osaka Prefecture, Sakai, Osaka, Japan 59 1 (Received February 13, 1975)
The effects of oxygen upon the photolysis of acetone-de, methyl ethyl ketone, and 2-pentanone adsorbed on porous Vycor glass have been investigated. For all the ketones, with increasing oxygen pressure the rate of formation of each product passes through a maximum and then decreases. Such enhancements by oxygen of Norrish type I1 (2-pentanone) and type I reactions would be tentatively attributed to the interaction of oxygen with the triplet biradical intermediate and to suppression of the recombination of the acetylalkyl radical pairs, respectively. Applying the Stern-Volmer relationship to the rate decrease by oxygen, it has been concluded that in the adsorbed layer the lifetime of the alkyl radicals increases in the order methyl < ethyl < n-propyl radicals.
Introduction In the previous paper1 the present authors investigated the photochemistry of alkyl ketones adsorbed on porous Vycor glass and found that their photochemical reactivity was markedly different from that in the gas phase, leading to some general characteristics of the photochemistry in the adsorbed layer. Information on the nature and reactivity of the excited states as well as the free radicals in the adsorbed layer was obtained from the studies of the effect of nitric oxide upon the photolysis of adsorbed alkyl ketones. In the present work similar studies have been carried out using oxygen. During the course of those investigations, it has been found that a trace of oxygen enhances the photolysis of adsorbed alkyl ketones. Although the interpretation of such phenomena has not yet been settled, it seems worthwhile to report these results a t the present stage, since there seems to be very few reports concerning such an oxygen effect. Experimental Section Details of the apparatus, procedures, and materials were described in the previous paper.l A conventional vacuum system was used. The specimen of porous Vycor glass (Corning, No. 7930) which had been heated in oxygen to remove carbonaceous impurities was introduced to the cell and degassed a t 500"C for 7 hr. A small amount of ketone remaining in the gas phase after admission of the sample to the cell at room temperature was removed by a liquid nitrogen trap. Subsequently, oxygen was introduced into the cell, its pressure being adjusted in the range of 0.001-100 Torr. Then, photolysis was carried out using an ultra-high-pressure mercury lamp without filter. The products were analyzed by gas chromatography using a flame-ionization detector.
Results In Figure 1the rates of the ethylene and propane formation derived from the photolysis of adsorbed 2-pentanone are plotted against the oxygen pressure. As described previously,l the major gaseous products in this case are propane (Norrish type I reaction) and ethylene (Norrish type I1 reaction). I t is seen in Figure 1 that for both types of pri-
mary steps the rate increases a t first with increasing oxygen pressure, passes through a maximum, and then decreases at higher pressures of oxygen. It is to be noted that the pressure where the rate shows a maximum is higher for the ethylene than for the propane formation. We have shown1 that the rate of ethylene formation is decreased by added nitric oxide, which is attributed to quenching of the excited triplet state. Accordingly, the decrease in the rate of the ethylene formation observed a t an oxygen pressure above 1 X 10-I Torr is attributable to quenching of the triplet ketone molecules. In fact, it has been found that the phosphorescence of adsorbed 2-pentanone is quenched by oxygen. As regards to the propane formation, the decrease in the rate of formation observed above 1 X Torr oxygen is attributable t o quenching af the propyl radicals as well as their precursor, i.e., the triplet excited 2-pentanone molecules, as was done in the quenching studies by nitric oxide described in the previous paper.l Similar experiments were carried out with adsorbed acet o n e - d ~and methyl ethyl ketone. In Figures 2 and 3 the rates of the methane and ethane formation from acetone-& as well as the ethane formation from methyl ethyl ketone are plotted against oxygen pressure. It is seen that for all the plots the rate of formation passes through a maximum a t a particular pressure of oxygen. The decrease in the rates of products formation with increasing oxygen pressure may be explicable on a similar basis to that in the case of the propane formation described above. In the gas-phase photolysis of acetone,2 methane and ethane formation is comTorr in contrast to pletely quenched by oxygen at 6 X the behavior of the photolysis in the adsorbed layer.
Discussion (1) Enhancement of the Rates of Products Formation by Oxygen. As regards to the increase in the rates of products formation observed in the range of low pressures of oxygen, there seems some possibility that formation of a chargetransfer complex between molecules of oxygen and ketone would result in such a phenomenon. The absorption spectra of adsorbed 2-pentanone showed no change on addition of oxygen a t high pressures. Furthermore, according to the work of Tsubomura and M ~ l l i k e n ,acetone ~ shows no The Journal of Physical Chemistry, Vol. 79, No.21, 1975
2226
Yutaka Kubokawa and Masakazu Anpo charge-transfer absorption when it is saturated with oxygen. This suggests that such a possibility can be excluded. As described in the previous paper,l the results of the type I reactions of adsorbed alkyl ketones can be explained by taking into consideration only the reaction from the triplet excited state. The type 11 reaction of adsorbed 2pentanone occurs from both singlet and triplet excited states. Considering a marked difference in the reactivity of oxygen toward the singlet and triplet excited states, however, it may be allowed to assume that only the type I1 reactions from the triplet excited state are affected by oxygen. Thus, only reactions from the triplet state are included in the following scheme:
o
10-3
Initial
io2
10-1
Oxygen
io
I
102
Pressure, torr
Figure 1. Effect of oxygen upon the yield of products of 2-pentanone photolysis at 25OC. The amount of 2-pentanone adsorbed was 1.65 X mol/g. During the photolysis oxygen was somewhat consumed. It was confirmed, however, that the pressure decrease caused by it did not affect the values of 7kqand ~ ' k :in the SternVolmer equation: (0)ethylene; ( 0 )propane.
0
e
a
1.0
0
2 Initial
4
6
8
1
0
Oxygen Pressure, torr
Figure 2. Effect of oxygen upon the yield of products of acetone-& photolysis at 25OC. The amount of acetone-d6adsorbed was 1.83 X mol/g: (0)methane; ( 0 )ethane.
sc
Pressure, torr Figure 3. Effect of oxygen upon the yield of ethane in the photolysis of methyl ethyl ketone at 25OC. The amount of methyl ethyl ketone mol/g. adsorbed was 3.86 X Initial
Oxygen
The Journal of Physical Chemistry, Vol. 79, No. 21, 1975
K3*
--
K3*
(1)
+ KO+ h u
(2)
1,4biradical
(4)
KO heat
K3* K3*
+ C3H7
CH&'0
1,4biradical -+ CzH4 1,4biradical
-
+ CH3COCH3
+
KO heat
(3) (5)
(6)
Reactions 4-6 are included, since there seems little or no doubt that the type I1 reaction proceeds via the 1,4biradical geherated by the intramolecular hydrogen abstraction rea~tion.~ It seems very difficult to expect that an interaction of oxygen with the excited triplet ketone molecules would result in an increase in the rate of products formation, since this interaction would probably lead to a rate decrease. Consequently, it seems probable that some kind of interaction of oxygen with the triplet biradical would result in an enhancement of the type I1 reaction. For example, the following possibility emerges: the triplet biradical is incapable of undergoing a bond-breaking process without spin con~ e r s i o nI.t~is well known that the singlet-triplet transition is enhanced by paramagnetic species such as oxygen and nitric oxide.6 Accordingly, reactions 5 and 6 would be expected to be accelerated by oxygen. In the case where reaction 5 is accelerated by oxygen more efficiently than reaction 6,such an increase in the ethylene formation would result. It has been shown by O'Neal et al.' that in the gas phase the lifetime of the triplet 2-pentanone molecule is much shorter than the triplet 1,4 biradical lifetime. Furthermore, the more strongly hydrogen bonded to the surface a ketone molecules is, the longer the triplet 1,4biradical lifetime becomes.8 This suggests that a similar situation would be expected for the adsorbed layer. Thus, the rate maximum such as shown in Figure 1 could be explained. A similar enhancement of the type I1 reaction with oxygen has been found by Grotewold et a1.: who investigated -one and atthe photolysis of 4-methyl-1-phenylpentan-1 tributed it to an interaction of oxygen with the 1,4biradical intermediate. As was described previously,' in the rase of the nitric oxide quenching, no maximum in the rate of ethylene formation was observed. Such a different behavior may be ascribed to the fact that the excited triplet and/or biradical are deactivated to the ground state of 2-pentanone more efficiently by nitric oxide than by oxygen. Such a difference in the quenching efficiencies between nitric oxide and oxygen has already been found by Backstrom and StenerylO and by Rebbert and Ausloos.ll As for the increase in the rate of the type I reactions caused by a trace amount of oxygen, it seems unlikely that
Photochemistry in the Adsorbed Layer
2227
TABLE I: Values of ~ k and , 7 ‘ k q ffor NO and 0 2 Quenching ( M - l ) 7
Quenchers 2-Pentanone
-6
NO T k Q
-5
7’kq’ O2 7kQ ?kg’
-4;
4.9
X
10’
3.6 x 1 0 5 2.2 x l o 6 1.2 x lo5
Methyl ethyl ketone Acetone-d, 2.4 X 10’ 9.5 x 104
5.4
1.2 x lo5 3.2 x l o 4
8.8 x l o 3
X
lo4
-3 -2
0
02
04
06
08
IO
1
0
2
4 Initial
6
8
Pressure
1
0
, torr
Figure 4. Stern-Volmer plots for products quenching in the photolyses of alkyl ketones adsorbed on Vycor glass. The upper abscissa refers to quenching of propane and ethane formation by oxygen and methane formation by nitric oxide. The lower abscissa refers to quenching of methane formation by oxygen: ( 0 )propane; ((3)ethane: (0) methane (oxygen quenching); (0)methane (nitric oxide quenching).
the possibility of 01 cleavage of alkyl ketones is enhanced in the presence of oxygen. This suggests that oxygen will affect the secondary reactions of the radicals produced by the a cleavage. According to the work of Hoare and Whytock,12 in the gas phase the radical reactivity toward oxygen is much higher for acetyl than for alkyl radicals. Assuming a similar difference in the reactivities for the adsorbed layer, it is expected that only acetyl radicals are removed by oxygen a t low pressure. As a result, recombination of the geminate radical pairs is suppressed, which results in enhancement of the formation of alkane. On increasing oxygen pressure, both alkyl and acetyl radicals are scavenged. Thus, the maximum rates for alkane formation would be explicable. (2) Lifetime of Adsorbed Radicals. For quenching by oxygen of the methane formation from acetone-ds the following Stern-Volmer equation holds in the range below 10 Torr of oxygen (see Figure 4):
where QOis not the rate of formation in the absence of oxygen but the maximum rate of formation. r and h , are the lifetime of the methyl radicals and the quenching rate constant, respectively. A similar Stern-Volmer plot holds for the quenching by nitric oxide of methane formation described in the previous paper1 as shown in Figure 4. Such linear Stern-Volmer plots are in contrast with the photolysis of methyl ethyl ketone and 2-pentanone where the Stern-Volmer plots are concave upward, Le., quadratic Stern-Volmer plots are applicable (Figure 4).This suggests that in the case of acetone little or no quenching of the excited triplet acetone molecules takes place, only scavenging of the radicals being observed. Such a short lifetime of the triplet acetone molecules is in agreement with the fact that no phosphorescence is observed for acetone adsorbed on porous Vycor g1ass.l Thus, it is confirmed that in the order 2-pentanone < methyl ethyl ketone < acetone radiationless decay becomes more efficient, i.e., the excited triplet lifetime becomes shorter.
In the case of the quenching by nitric oxide of the ethane formation from methyl ethyl ketone and the propane formation from %pentanone, we have already obtained the r k , values for the ethyl and propyl radicals using the Stern-Volmer quadratic equati0n.l Accordingly, in the case of quenching by oxygen, the same equation is expected to hold:
Qo/Q = (1+ rkq[021)(1+ 7’kqf[021) where T and h , are the radical lifetime and the quenching rate constant for the radicals, respectively. T’ and k,’ are the corresponding values for the triplet ketone molecules. QO is again the maximum rate of the products formation. As described in the previous paper,l the triplet excited state is quenched three times more efficiently by nitric oxide than by oxygen. Accordingly, using the rfkq/ values for nitric oxide given previously,’ the # k q f values for oxygen is estimated a t 1.2 X lo5 M-l for 2-pentanone and 3.2 X lo4 M - l for methyl ethyl ketone, respectively. From these rfhqfvalues the r k , values are determined such that best fits to the experimental curves are obtained. The values of r k , and r f h q f including , the corresponding values determined from the nitric oxide quenching, are shown in Table I. Although it is expected that in the adsorbed layer the reactivity of the radicals is different from that in the gas phase, it seems very difficult to attribute such a large difference in the Tk, values only to the difference in h,. In other words, it can be concluded that the radical lifetime increases in the order methyl < ethyl < n-propyl radicals. By assuming that the relative reactivity of oxygen or nitric oxide toward these three radicals in the adsorbed layer is approximately equal to that in the gas phase,13 it is possible to estimate the relative lifetime of three radicals in the adsorbed layer as follows: Tmethyl:Tethyl:Tn.propyl = 1:30:1000. From the investigation of the effect of surface pretreatments upon the rate of products formation, the authors have concluded that the lifetime of radicals formed on the solid surface is mainly determined by the probability of the recombination of the geminate radical pairs.8 It is therefore concluded that in the adsorbed layer efficiency of the recombination of geminate radical pairs increases in the order 2-pentanone < methyl ketone < acetone.
References and Notes (1) (2) (3) (4)
Y. Kubokawa and M. Anpo, J. Phys. Chem., 78, 2442 (1974). G. S. Pearson, J. Phys. Chem.. 67, 1686 (1963). H. Tsubomura and R. S. Mulliken, J. Am. Chem. Soc.,82, 5960 (1960). P. J. Wagner, Tetrahedron Lett., 5385 (1968); 1753 (1967); J. A. Barltrop and J. 0. Coyle, ibid., 3235 (1968); N. J. Turro and P. A. Wriede. J. Am. Chem. Soc., 92, 321 (1970); N. C. Yang and S. P. Elliott, bid., 91,
7550 (1969). (5) P. J. Wagner, Acc. Chem. Res., 4, 169 (1971); L. M. Stephenson and J. I. Braurnan, J. Am. Chem. Soc.,93, 1988 (1971); P. D. Bartlett and N. A. Porter, ibid., 90, 5317 (1968).
The Journal of Physical Chemistry, Vol. 79, No. 2 1, 1975
S.H. Ehrlich
2228 (6)G. Porter and F. Wright, Discuss. Faraday SOC.,27, 18 (1959). (7)H. E. O’Neal, R. G. Miller, and E. Gunderson, J. Am. Chem. Soc., 96, 3351 (1974). (8)M. Anpo, T. Wada. and Y. Kubokawa, Eull. Chem. SOC.Jpn., in press. (9)J. Grotewold, C. M. Previtall, and D. Soria, J. Chem. SOC.,Cbem. Com-
mun., 207 (1973). (IO)H. L. J. Backstrorn and A. Stenery, Acta Cbem. Scand., 12,8 (1958). (11)R. E. Rebbertand P.Ausloos, J. Am. Chem. Soc., 87, 1847 (1965). (12)D. E. Hoare and D. A. Whytock, Can. J. Chem., 45,865 (1967). (13) M. I. Christie and J. S. Frost, Trans. Faraday SOC.,61,488 (1965).
Mechanisms Involving the Transient Absorptions of Cyanine Dyes in Gelatin. 1. Temperature Dependence S. H. Ehrlich Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 (ReceivedJune 2, 1975) Publication costs assisted by the Eastman Kodak Company
The temperature dependence of transient absorptions and steady-state spectral responses of four cyanine dyes dispersed in gelatin films was used to characterize two stereoisomers in 3,3’-diethyl-9-methylthiacarbocyanine bromide and anhydro-3-ethyl-9-methyl-3’-sulfobutylthiacarbocyanine hydroxide. The decay rates involving relaxation of the induced absorptions and rates of recovery to the ground state are interpreted in terms of kinetic reaction mechanisms, structural, and thermodynamic relationships.
Introduction Flash photolysis has enabled the detection of transient species formed during the exposure of photographic silver halide emulsion systems that might appear in the photographic process.1,2 The four cyanine dyes were previously studied at 2-4 X loF3M in - 3 0 - ~ m thick gelatin films with and without AgBr(1) (0.05-pm microcrystals, sulfur and @=CH-C=CH-C/B N’ H I C,H, 1 (ThC pts)
I
\; I
pts-
C,H,
+
gold [S Au] surface treated):lS2 (1) ThC pts (3,3’-diethylthiacarbocyanine p-toluenesulfonate); (2) 9-MeThCRr (3,3’-diethyl-9-methylthiacarbocyanine bromide); (3) sulfo9-MeThC (anhydro-3-ethyl-9-methyl-3’-sulfobutylthiacarbocyanine hydroxide); (4) 2,2’-cyanine (1,l’-diethyl-2,2’cyanine chloride). In general,1-4 pulsed irradiation within the visible absorption bands temporarily bleached the ground-state absorption bands and formed transient absorptions at longer wavelengths (Figures 1 and 2), which were assigned to triplets, free radicals, or metastable isomers. Several investigations monitored the short-wavelength transient absorption band formation and relaxat i ~ n . ~Optical -~ density changes after excitation of the dyes in gelatin (and adsorbed to silver halide crystals in gelatin) were described by
AOD
I C P
CH
CH2CH2CH-CHJ
I
I _
SO,
3 (sulfo-9-MeThC)
acm3 I
l
l
c1-
C2H6 C2H, 4 (2,Y-cyanineC1)
The Journal of Pbysical Chemistry, Vol. 79, No. 21, 1975
= A exp(-h,t)
+ B exp(-h,t)
(1)
where t is arbitrarily set equal to zero at the time the excitation pulse is interrupted. The complex decay curves were resolved into two exponential components: (1)a long-lived and (2) a short-lived process described by the specific rate constants k l and K z , respectively, or the reciprocal constants as lifetimes. The short-lived kinetics in regeneration and relaxation introduce the possibility of consecutive or concurrent reaction mechanisms from the excited state. The pulsed photolysis of all the cyanine dyes in gelatin and when adsorbed to silver halide microcrystals in gelatin showed apparent reversible transient processes on repeated flash excitation; however, approximately 1%of the dye in gelatin was irreversibly bleached after five flash excitations in air and at room temperature.lTZA corresponding increase of 0.2% in transient density was observed in the dyed crystals dispersed in gelatin owing to silver formation. The excitation frequency/sample was, therefore, limited to three exposures.lV2The rates of decay involving relaxation of the induced absorption and rates of recovery (regeneration) to the ground state were the same for the cyanine dyes in gel-
