Masakazu Anpo and Yutaka Kubokawa
2448
that in the gaa it seems very difficult to attribute such a largo difference in B only to the difference in k 11. In other words, ( [ H ] k l o [ c O C H ~ ] k 1 2 for ) 2-pentanone should be smaller than that for methyl ethyl ketone. I t may therefore be concluded that in the adsorbed layer the lifetime of the n-propyl radical is longer than that of the ethyl radical. With respect to the nature of such a difference in the lifetime, it is at present impossible to decide between the two possiMities; recombination ([COCHsIk13) and hydrogen abstraction ( [ H l k l o )of the radicals.
+
Acknowledgment. The authors wish to thank Professor 13. Tsubomura and Dr. T. Sakata of Osaka University for making the spectrophotofluorimeter available for their present work.
References and Naites (1) J. Hide Boer Z.Phys. Chem., B 15, 281 (1932). (2) A. N. Terenin, J. Phys. Chem. USSR, 14, 1362 (1940). (3) H. P. Leflin end E. Hierman, Proc. int. Congr. Catai. 3rd, 1964, 1, 1064 ( 1965). (4) Y. Kubokawa, M. Kubo, and G. Nanjo, Bull. Chem. SOC.Jap., 43, 3968 (1970). ( 5 ) S. Brunauer, P. H. Emmett, and E. Teller, J. Amer. Chem. SOC.,60, 309 (1938).
(6) J. G. Calvert and J. N. Pitts, Jr., “Photochemistry,” Wiley, New York, N.Y., 1966, p 377. (7) N. J. Turro, J. C. Dalton, K. Daws, G. Farrington, R. Hautala, D. Morton, M. Niemczyk, and N. Schore, Accounts Chem. Res., 5,92 (1972). (8) P. Ausloos and E. Murad, J. Amer. Chem. SOC.,60, 5929 (1958). (9) R. B. Cundall and A. S. Davies, Trans. Faraday SOC.,62, 2444 (1966). (IO) R. P. Borkowski and P. Ausloos, J. Phys. Chem., 65, 2257 (1961). (11) M. O’Sullivan and A. C. Testa, J. Amer. Chem. Soc., 92, 258(1970). (12) R. F. Borkman and D. R. Kearns, J. Chem. Phys., 44, 945 (1965). (13) N. C. Yang, S . P. Elliot, and B. Kim, J. Amer. Chem. SOC.,91, 7551 (1969). (14) P. Ausloos and E. Murad, J. Phys. Chem., 65, 1519 (1961). (15) R. D. Rauh and P. A. Leermakers, J. Amer. Chem. SOC, 90, 2246 (1968). (16) M. Robin, J. Chem. Educ., 33, 536 (1956). (17) M. Robin and K. N. Trueblood, J. Amer. Chem. SOC.,79, 5138 (1957). (18) J. Kobayashi, Nippon Kagaku Zashi, 80,29 (1959). (19) R. B. Cundall and A. S. Davies, Roc. Roy SOC., Ser A, 290, 563 (1966). (20) R K Boyd, G. B. Carter, and K. 0 Kutschke. Can. J. Chem. 46, 175 (1968). (21) N. C. Yang, E. D. Feit, Man Him Hui, N. J. Turro, and J. C. Dalton, J. Amer. Chem. SOC.,92, 6974 (1970). (22) J. C. Dalton, K. Dawes, N. J. Turro, D. S. Weiss, J A. Barltrop. and J. D. Coyle, J. Amer. Chem. Soc., 93, 7213 (1971). (23) In the photolysis of 2-pentanone propane formation is quenched more efficiently than ethylene formation (Figure I), suggesting that B should be larger than A. Thus, the assignment of the two constants is possible. A similar relatlon Is expected to hold for the photolysis of methyl ethyl ketone. Some evidence supporting it will be given later (24) The temperature dependence of A appears to be determined overall by the temperature dependence of the quenching rate constant rather than that of the triplet lifetime, since the latter should decrease with increasing temperature. (25) M. I. Christie and J. S. Frost, Trans. Faraday Soc., 81, 468 (1965).
Photochernistry in the Adsorbed Layer. II. Energy Transfer in the Adsorbed Layer Masakazu Anpo and Yutaka Kubokawa” Vepetrtment of Applied Chemistry, University of Osaka Prefecture, Sakai, Osaka, Japan 59 1 (Received December 4, 1973; Revised Manuscript Received March 28, 1974)
PfJhliCatiOn costs assisted by the University of Osaka Prefecture
In the photolyses of acetone-methyl ethyl ketone and acetone-2-pentanone mixtures adsorbed on porous Vycor glass, the presence of acetone enhanced the rate of photolysis of methyl ethyl ketone as well as that of 2-pentanone. For a methyl ethyl ketone-2-pentanone mixture the rate of photolysis of 2-pentanone was increased by added methyl ethyl ketone, while the photolysis rate of methyl ethyl ketone was decreased by added 2-pentanone. Such behavior suggests that intermolecular energy transfer occurs in the adsorbed layer, its direction being acetone methyl ethyl ketone 2-pentanone. The occurrence of this energy transfer was correlated with the lowering of the ground state of ketones owing to hydrogen bond formation with surface OH groups.
-
Introduction The intermdecular energy transfer process in gas phase and solution has been studied extensively in the recent y e a r ~ . l -However, ~ there seems no report on such energy transfer im the adsorbed layer. It would be expected that the investigation of energy transfer in the adsorbed layer will give useful information on the energy levels of the excited and ground states of molecules on a solid surface. Such information appears to be important for understanding photochemistry in the adsorbed layer. During tihe study of the photolysis of alkyl ketones adThe Journal of Physical Chemistry, Val. 75, No. 24, 1974
-
sorbed on porous Vycor glass,*r5 a phenomenon has been found which suggests the occurrence of intermolecular energy transfer. The results are described in the present work.
Experimental Section Details of the materials, apparatus, and procedures used in the present work have been described in part 1 of this series. The absorption spectra of adsorbed ketones were determined with a Hitachi EPS 3T type spectrophotometer, measuring transmission through the sample. A quartz cell
Photochemisiry in the Adsorbed Layer
2447
TABLE I: Effect of Hydrogen Bond upon the n - ~ * Transition (nm)of t h e Alkyl Ketones at 25"
0.
_,methyl
Compounds
Acetone
Gas phase Heptane" Methanola Vycor glass
276.5 276.7 270 . O 262 . O
278 . O 277.5 272 . O 270 .Q
279 . O 279 .O 273.5 273 . O
Reference 9.
ethyl ketone
'"t
(302 X I O - * ~ O I O )
Acetone
Methyl ethyl 2ketone Pentanone
( x 10-5 mote )
Figure 1. Photolysis of 2-pentanone-acetone and methyl ethyl ketone-acetone mixtures. Q and Qo are the rate of photolysis at 25' in the presence and absence of acetone, respectively. These ketones were adsorbed at 25' for several hours. The amount adsorbed was kept constaint at 2.47 X loe5 mol for 2-pentanone and nios for methyl ethyl ketone. at 3.02 X
350 400 450 500 550 600
Methyl ethyl ketone ( x mole) ;i! 7 6 8 IO 12 14 16 r
I
I
l
nrn
I
I 5
increase in the phosphorescence intensity of 2-pentanone by added acetone: excitation wavelength, 313 nm. The amounts of 2-pentanone and acetone adsorbed are 3.29 X lop5 and 1.29 X
Figure 3.
mol/g, respectively: (- - - - -) pure %pentanone; (-) none-acetone mixture.
0
IO
20
30
2-pentanone
40
50
( X I O - ~mote)
Flgure 2. Photolysis of 2-pentanone-methyl ethyl ketone mixtures: Q, rate increase Qf 2-pentanone photolysis by added methyl ethyl ketone; 0,rate decrease of methyl ethyl ketone photolysis by added 2-pentanone. The amount adsorbed was kept constant at 3.02 X I O V 5 mol for methyl ethyl ketone and at 2.29 X mol for 2-pen-
tanone.
having two planar windows 10 mrn apart was used. The cell was placed just before the photomultiplier in order to minimize scattering error. Another porous Vycor glass sample was pretreated under the same conditions in a separate cell and used as a blank in the reference beam. Results ( 11 Photolysis of Aixtone-Methyl Ethyl Ketone and Acetone- 2-Pentaizone Mixtures. As described in part I of this series, the main gaseous products of the photolysis of alkyl ketones in the adsorbed layer are CHI and C2H6 for acetone, @&is for methyl ethyl ketone, and CsH8, CzH4, and CH3COCH3 for Z-pentanone. It is to be noted that 2-pentanone undergoes a Norrish type I as well as a type I1 proC ~ S S . The ~ . ~ relative quantum yields obtained from the gaseous products are 0.01 (CH4) for acetone, 1.0 (C2H6) for methyl ethyl ketone, and 10.0 (CzH4 C3Hs) for 2-pentanone. This indicates that in the photolysis of mixtures containing acetone the products from acetone are negligible, leading to the concluaion that it is impossible to determine
+
2-penta-
how the rate of photolysis of acetone changes in the mixture. The changes in the rate of photolysis o f methyl ethyl ketone as well as of 2-pentanone caused by added acetone are shown in Figure 1. The rates are increased with increasing amount of acetone added, the increase being larger in the case of the acetone-2-pentanone mixture, Essentially the same results were obtained for the case where the order of adsorption of the ketones was reversed. This suggests that adsorption equilibrium is established after adsorption for several hours. (2) Photolysis of 2-Pentanone-Methyl Ethyl Ketone Mixtures. In this case, the effect of added methyl ethyl ketone upon the rate of photolysis of 2-pentanone was investigated by a series of experiments using a constant amount of 2-pentanone and varying amounts of methyl ethyl ketone adsorbed, since the rate of photolysis of 2-pentanone was dependent upon its amount adsorbed. The results are shown in Figure 2. The effect of added 2-pentanone upon the rate of photolysis of methyl ethyl ketone was investigated in a similar manner. As shown in Figure 2, the rate of photolysis of methyl ethyl ketone is decreased with increasing amount of 2-pentanone,while that of 2-pentanone is increased with increasing amount of methyl ethyl ketone. The extent of the increase is much smaller than that of the decrease. (3) Phosphorescence Spectra of Acetone-&Pentanone Mixtures. Phosphorescence spectra of 2-pentanone as well as of a 2-pentanone-acetone mixture are shown in Figure 3. It is seen that the phosphorescence intensity for the mixture is higher than that for pure 2-pentanone. The phosphorescence of acetone was very weak and not detectable. This suggests that such an increase in the phosphorescence intensity may be attributed to an increase in the intensity of 2-pentanone phosphorescence. The Journal of Physical Chemistry, Val. 78, No. 24, 1974
Masakazu Anpo and Yutaka Kubokawa
448
( 4 ) Absorptic: n Spectra in the Adsorbed Layer. Acetone, methyl ethyl ketone, and 2-pentanone adsorbed on porous Vycor glass absorb light at shorter wavelengths than the corresponding compound in the gas phase. The wavelengths of maximum absorption are shown in Table I. A marked blue shift is observed with acetone, its magnitude being similar to that found for the acetone-Silica Gel SYStern investigated by Leermakers and Thomas.8 Blue shifts of the (n, n*) bands are observed with ketones in polar solvents, being attributed to hydrogen bond formation with s o l v e n t ~ . ~From J ~ a study of ir spectra, Low, et al.,ll have concluded that the adsorption force in the ketone-porous Vycor glass s j stern is mainly attributable to hydrogen bonding betweten the surface OH groups and the C=O groups of the ketones It appears that the strength of hydrogen bonding is determined by the acidity of the OH groups and the basicity of the solvent~.8~9 The blue shifts observed with porous Vycor glass are greater than those obtained with solvents (Table I), suggesting a strong proton-donating power of the surface OH groups. As shown in Table I, the blue shift observed with the adsorbed ketones decreases in the order acetone > methyl1 ethyl ketone > 2-pentanone. Such a trend can be explained in terms of the concept that an increase in wing power of the alkyl groups in the ability to accept the hydrogen bond as has been suggested by Balasubramanian and Rao.lo The spectra of the adsorbed ketones increased in intensit y proportional to the amounts adsorbed without changes in relative intensity for the bands from 240 to 320 nm. In Figure 4, the absorbance at the wavelength of maximum absorption for the ketones is plotted against the amount adsorbed, a linear relationship being obtained in the range of amount adsorbed from 1.60 X to 3.20 X rnol/g. It is seen that there is a marked difference in the relative extinction coefficients, the order being 2-pentanone > methyl ethyil kt5torae >> acetone. Although the true nature of such a difference is unclear a t present, a marked reduction in the extinction coefficient on adsorption has been already reported by ,a number of workers.12-14
iseussilo The results concerning the photolyses and phosphorescence spectra i n the adsorbed layer described above suggest the possibility that ain intermolecular energy transfer process takes place in the adsorbed layer, i.e., excitation energy absorbed b,y an acetone molecule is transferred to a 2pentanone moiecule to give an excited state, the direction of energy transfer being acetone methyl ethyl ketone %-pentanone. ‘The situation in the adsorbed layer is similar to that in s0luti0n.l~The donor-acceptor pair is held in a fixed position for a relatively long time (“cage effect”), more efficient energy transfer being expected to occur compared to that in the gas phafJe. It i!; well known that the efficiency of the energy tran,sfes process is closely associated with the exot h e r ~ i ~ c iof t ythe transfer process. From the similarities between the adsorbed layer and solution, the blue shifts observed for the ketones adsorbed on porous Vycor glass may be ascribed to the lowering of the ground states rather than of the excited states.10J6-1s From the results shown in Table J it is concluded that in the adsorbed layer the ground state if4 Powered by 5.5 kcal/mol for acetone, by 3.0 kcal/rnol for methyl ethyl ketone, and by 2.0 kcal/mol for ~ - ~ e ~ ~ cm~pared a n o n ~Lo~that in the gas phase, although
-
The Journal of Physical Chemistry, Val. 78, NO. 24, 1974
-
Amount
adsorbed
( xIQ6 mole)
Figure 4. Relationship between absorbance and amounts adsorbed of the alkyl ketones at wavelength of maximum absorption. Porous Vycor glass used in these experiments was about 0.6 g: 0 ,acetone; 0 , methyl ethyl ketone; 0, 2-pentanone.
the excited state levels are almost the same throughout the three ketones. Such considerations are applicable to the singlet excited states. In the present system the most probable energy transfer process appears to be triplet-triplet transfer. Although no information is available on the excited triplet state in the adsorbed layer, a similar lowering of the ground state is expected for the transition between the singlet and triplet ~ t a t e . ~ This ~ . ~ *suggests that the energy difference between the excited triplet and the ground state is larger than in the gas phase, the difference being the largest for acetone and the smallest for 2-pentanone. On such a basis it appears to be understood that the intermolecular energy transfer in the present system occurs in the direction acetone methyl ethyl ketone +. 2-pentan0ne.~~ In order to discuss the energy transfer process quantitatively, the following mechanism is proposed
-
D
-t-
D*
D*
hv
kl
k2
D -k heat
(1
products
(2
D*
--3
A
+ hv
A*
3
A*
-4.
A*
k4
kg
kg
D* 4- A
-
A*
heat
(4)
products
(5
A
A
-k
+ hv”
k7
D
+ A‘.
(6
(7)
The rate of photolysis of the ketone A (acceptor) is increased by addition of the ketone D (donor) as described above. Using steady-state treatment such an increase in the rate of photolysis is
Photochemistry in the Adsorbed
Layer
TABLE 11: Values #ofk7/ (k,
2449
+ kz + k3) ( X
104 g/mol) for Mixtures of t h e Alkyl Ketones at 25”
___
CHaCOCHs
+ CH-COC~HP k?/i:kl a
c kz -t- k,)
CH&OCH,
+ CH&OC~H,P
CH&OC*H,
+ CH~COC~HP
2.70
25.70
1.55
CH~COC~HT
+
CH~COC~HP 1.64
The amount adsorbed was kept constant in the mixtures of alkyl ketones.
where Q arid 80arc‘ the rate of photolysis of ketone A in the presence and absence of ketone D, respectively, ID and I A are the intensity of the light absorbed by ketone D and ketone A, respectively. [A] represents the amount of ketone A adsorbed. As seen in Figure 4, the absorbed light intensity varies linearly with the amount of adsorbed ketones, leading to the eq ID a q)[D] and IA Q[A] where t is the molar extinction coefficient, Hence, eq I becomes
From Figure 4, values of t ~ / are t ~ determined as follows: Z-pentanone)/t(acetone) = 7.21, e( methyl ethyl ketone)/ €(acetone) =: 1.70, t(2-pentanone)/C(methyl ethyl ketone) = 4.00. Thus, values of k 7 / ( k 1 k z k 3 ) are determined from the slopes of Figures 1 and 2, since the value of Q / E A and the amount adoorbed of ketone A are known. The results are shown in Table 11. For the case where the rate of photolysis of ketone D is decreased by the presence of ketone A, a sirnilm treatment gives the equation E(
+
+
+
Hence, from tlie slope of Figure 2, the value of k 7 / ( k 1 122 k 3 ) for a methyl ethyl ketone-Z-pentanone mixture is obtained as 1.64 X IO4 .g/mol, in agreement with the value determined from the rate increase by addition of ketone D (1.55 X lo4 g/i,nol, in Table 11). Such coincidence suggests the plausibility o f the energy transfer process represented by step 7. From the values of k 7 / ( k 1 k 2 k 3 ) (Table II), it is possible to compare the value of k7 for the methyl ethyl ketone-acetone and the 2-pentanone-acetone systems, since the denominator is the same in both cases. Thus, it is con-
+
-+ -+
cluded that the excitation energy of acetone is transferred about 10 times more efficiently to 2-pentanone than to methyl ethyl ketone. References and Notes (1) F. Wilkinson, Advan. Photmhem., 3, 241 (1964). (2) A. A. Lamola and N. J. Tuno. “Energy Transfer and Organic Photochemistry,” Why, New York. N.Y., 1969, p 17. (3) F. S. Wettack and W. A. Noyes, Jr., J. Amer Chem. Soc., 90, 3901 (1968). (4) Y. Kubokawa, M. Kubo, and G. Nanjo, Bull. Chem. SOC.Jap., 43, 3968 (1970). (5) Y. Kubokawa and M. Anpo. J. Phys. Chem., 78,2442 (1974). (6) J. G. Calvert and J. N. Pitts. Jr., “Photochemistry.” Wiley, New York, N.Y., 1966, p 377. (7) N. J. Turro, J. C. Dalton, K. Dawes, G. Farrington. R. Hautala, D. Morton, M. Niemczyk, and N. Schore, Accounts Chem. Res., 5,92 (1972). (8) P. A. Leermakers and H. T. Thomas, J. Amer. Chem, SOC., 87, 1620 (1965). (9) C. N. R. Rao. G. K. Goldman, and A. Balasubramanian, Can. J. Chem., 38, 2508 (1960). (10) A. Balasubramanian and C. N. R. Rao, Spectrochim. Acta, 18, 1337 (1962). (11) J. C. Mcmanus. Y. Harano, and M. J. D. Low, Can. J. Chem., 47, 2545 (1969). (12) M. Robin, J. Chem. Educ., 33, 526 (1956). (13) M. Robin and K. N. Trueblood, J. Amer. Chem. Soc., 79, 5138 (1957). (14) J. Kobayashl, NippOn KagakuZasshi, 80, 29 (1959). (15) N. J. Turro. “Molecular Photochemistry.” W. A. Benjamin, New York, N.Y.. 1967, p 92. (16) H. Baba. L. Goodman, and P. C.Valentl, J. Amor. Chem. Soc., 88, 5410 (1966). (17) G. C. Pimentel, J. Amer. Chem. Soc., 79, 3323(1957). (18) T. Abe, Bull. Chem. SOC.Jap., 39, 936(1966). (19) D. M. Hercules, “Fluorescence and Phosphorescence Analysis.” Wiley, New York, N.Y., 1966, p 81. (20) W. 0.Herkstroeter, A. A. Lamola, and G. S.Hammond, J. Amer. Chem. SOC., 86, 4537 (1964). (21) In the case where the triplet lifetime of 2-pentanone is shorter than that of acetone, an increase in the rate of photolysis of 2-pentanone by added acetone would be expected, even if both ketones had comparable triplet energies, /.e., the rate constant of the energy transfer from 2-pentanone to acetone was almost the same as that of the reverse transfer. However, such an explanation may be excluded, since, as described in part l of this series, the triplet lifetime increases in the order acetone < methyl ethyl ketone < 2-pentanone
The Journal of Physical Chemistry, Vol. 78, No. 24, 1974
