25 1
J. Phys. Chem. 1800, 84, 251-255
Photochemistry of the 1,4-Aiiphatic Diketones E. A. Llssl," M. V. Enclnas, F. Castafieda, and F. A. Olea Departamento de Qulmlca, (JnlversMad Tecrilca del Estado, Santiago, Chile, and Facultad de Clenclas Qdmlcas y Farmacol6glcas, Unlverskiad de Chile, Santiago, Chile (Recelved August 27, 7979)
The plhotochemistry of ketones with general formula RC(=O)CH2CH2C(=O)R', where R and R' represent the methyl, propyl, n-butyl, and isoamyl groups was investigated. The results obtained provide evidence of fast energy migration between the chromophores both in the singlet and triplet excited states. The rate of energy migration can be estimated as 2 X lo8 and 3 X lo8 s-l for the singlet and triplet states, respectively. The rate of energy migration between similar chromophores is a matter of current interest. Aromatic n-x* carbonyl excitation has been shown to migrate rapidly in exotherimic intramolecular processesi,lt2in deriviitives of the 1,4-diben~oylbutane,~ in macromolecules bearing pendant carbonyl group^,^ and intermolecularly between acetophenone derivativesas On the other hand, studies of aliphatic ketones in solution6~'and in gas phase' show that intermolecular energy hopping between similar aliphatic carbonyl groups is a rather slow process. Intramolecular energy migration between carbonyl groups located in small aliphatic ketones has not been investigated. i3ince these compounds afford the possibility of measuring the rate of energy migration both in the excited linglet and tripIet states, a systematic study of the photochiemistry of compounds of the general formula RCO(CIH,),COR' has been initiated. Preliminary results obtained in the photochemistry of 4,7-undecanedionehave been In this work we report a study of the photochemistry of a series of 1,4-aliphatic diketones whose main decomposition pathway is intramolecular y hydrogen abstraction. The results obtained show that energy migration is a relevant process both in the singlet and triplet excited states.
Chart I RCOCH,CH,COR' R R'
ketone I
n-butyl
n-butyl
-~
isoamyl
methyl
isoamyl isoamyl
Propyl propyl n-butyl
isoamyl
I1
I11 IV
V VI VI1
n-butyl isoamyl
methyl
methyl
TABLE I: Experimental Results
ketone
product
I 3-heptanone
-
(slopes
@sa
@ r a SVh
0.098 0.85 0.066 0.084 0.96 0.057 0.058 0.28
0.042
I1
6-methyl-3-heptanone
I11
IV
V
0.12 0.01 0.06 0.03 0.06
0.15
0.076
0.16
0.04
VI VI1
0.26
0.004 0.2 0.009 0.28
0.04
4.2
ResultII a For unsymmetrical diketones the quantum yields were The ketones1 investigated are shown in Chart I. Ketone obtained from @ = 2(rate of product %rmat.on)/l, which assumes equal fistrhtion of the initial excitation. VI1 was! a commercial product (Eastman) purified by distillation and gas chromatography. All other ketones several ketones bearing tertiary y hydrogens are given in were prepared by a method similar to that previously Figure 4. Details of the synthesis are given in the ExIn order to determine if the photobehavior of a given perimental Section. RCO group in a diketone is similar to that in a monoketone The results obtained in the photolysis at 20 "C in nwith similar R groups, a comparison was carried out behexane eolution are given in Table I. Products quantum tween the photobehavior of ketone I and that of monoyields were measured by gas-liquid clhromatography with ketones bearing secondary y hydrogens. The absorption 2-heptanone := 0.2)9 as the actinometer. In order and fluorescence spectra of ketone I and 2-heptanone were to obtain the singlet and triplet yields, we used 1,3-pensimilar in position and intensity. The relative valuef3 of tadiene as a selective triplet quencher. their oscilator strengths, as given by the relative values of Stern-Volmer type plots for triplet photoproducts were their integrated absorption bands, were obtained by employing 1,3-pentadiene as a triplet quencher. The results obtained are given in Figures 1-3. fI/fZ-heptanone = 1.1 Quenching experiments on the singlet state photoproSince the relative fluorescence quantum yields for both ducts were carried out by the double quenching technique compounds was found to be nearly one, it can be concluded by employing a fixed amount of 1,3-pentadiene or a-methat thylstyrene and different amounts of triethylamine. CYMethyleityrene is particularly suitable as selective triplet (7S)I = 0-%7S)2-heptanone (1) quencher since it quenches the triplet state at ,R nearly This relationship allows an estimation of the singlet lifediffusion-controlledrate, and even in pure a-methylstyrene only 20% of the 2-heptanone singlets are quenched by it.lo time for diketone I. From this value and the slope of the The slopes of the Stern-Volmer plota of the symmetrical Stern-Volmer plot given in Table I, it can be concluded diketones are included in Table I. The value given for that quenching reaction 2 takes place with a rate constant ketone VI1 was obtained by measuring the change in l(1) + Et3N quenching (2) fluorescence quantum yield as a function of the itriethylamine concentration. The values of ( t # ~ ~ ~ , ~ ~ /for $ ~ , ~ sof) ~1.1 X lo9 M-l s-l. By a similar method a value of 1.4
-
0022-36~4/80/2084-0251$01.OO/O
0 1980 Amerlcan Chemical Soclety
252
The Journal of Physical Chemistry, Vol. 84, No. 3, 1980
Lissi et al.
I A
I
30
15 I
0.0 2 5
[TRIETHYLAMINEI, M
005
1 3 PENTADlEYEi M
Figure 1. Values of
for the triplet state of ketone V against 1,Spentadlene concentration. (The straight line gives the results expected for 2-pentanone.*O)
Flgure 4. Values of (4 oc4HJ$ C,H ) for the singlet states of ketones 11, V, and VI as a function of triethylamine concentration. These results were obtained in the presence of 1.0 M styrene.
TABLE 11: Comparison between Ketone I and 3-Heptanone ketone I 3-heptanone
PS
@T
(slope SV)T, M-’ PT
0.2
(4 Oca
IV, and V I as a function
/$Jc3y) for the triplet states of ketones I,
9 1,Bpentadiene concentration.
A
Y
I1 3 - ’ E N T A D l E N E :
M
Figure 3. Values of (4 OW,/4C,HI) for the triplet states of ketones 11, 111, V, and V I as a function of 1,3-pentadiene concentration.
was obtained for the quenching of ketone VI1 singlets by triethylamine. These values are of the same order of magnitude as those obtained previously for singlet quenching for 2-pentanone.’l The triplet quantum yield of ketone I was obtained from the sensitized cis-trans isomerization of 1,3-pentadiene. Acetone (& = 1)was used as an actinometer. The value obtained is given in Table 11. From this value and the singlet lifetime, the values of kn and kIsc given in Table I1 can be derived. Assuming a triplet quantum yield of one for ketone VII, one obtains a value of kIsc for this ketone of 3.3 X lo8 s-’. The results obtained for ketone I are compared in Table I1 with those obtained for 3-heptanone, which can be X lo9 M-’ s-’
1.1 x 0.9 x 5.3 x 3.6 x 0.11 0.40 82 0.21
10-9 109 lo* 10’ (ref 1 3 )
considered as a reasonable “model” monoketone; this table includes the values of P, the fraction of l,4-biradicals which cleaves to propylene and the corresponding ketone.12
[ 1 3-”ENTADlENEI, M
Figure 2. Values of
0.72 x 10-9 1.1 x 109 8.6 X 10’ 5.3 x 10s 0.07 0.38 40 0.25
Discussion The similarity between the absorption and emission spectra of monoketones and 1,4-diketones indicates that in the diketones the two carbonyl groups can be considered as isolated in the ground and in the first excited states. The data shown in Table I1 corroborates this point since it shows that the photobehavior of the diketones is quite similar to that of a monoketone with the same type of y hydrogens. The aliphatic ketones do not show then the different behavior observed for monomeric and dimeric aromatic ketones by Faure et aLl4 The fact that both carbonyl groups can be considered as isolated allows treatment of the energy migration between the chromophores in terms of a weakly interacting m0de1.l~ The experimental results obtained for the unsymmetrical ketones can then be treated in terms of the following reaction scheme: K + hv ‘A (3) lB (4)
lA
--
‘A-
lbir(A) 3A lB lbir(B) 3B lA 3bir(A) 3B 3bir(B) 3A
---
‘A
‘B ‘B IB 3A 3A 3B 3B
-+
-+
(5) (6)
(7)
(8) (9)
(10) (11) (12)
(13) (14)
The Journal of Physical Chemistry, Vol. 84, No. 3, 1980 253
Photochemistry of the 1,tAliphatic Diketones
TABLE HI: Comparison between Calculated and Experimentally Determined Values for the Singlet State ketone product
exptl no migrn migrna equilibrm
IV
0.009 0.009 0.005 C2H, 0.01 0.043 0.05 0.06 C,H, 0.06 0.009 0.007 0.002 V C2H, 0.003 0.064 0.089 0.06 0.05 C,H, 0.04 0.022 0.04 0.043 VI C,H, 0.06 0.08 0.05 C4HU 0.076 0.10 0.05 0.07 IIIb C4H8 0.12 a Calculated with h,$ = 2 x l o 8 s-l (see following discussion), Equal absorption assumption may not be justified in this case.
*
i
/A'
C
30
$ 5 [ TRIIITHYLAMINC
seems sound for those ketones with similar R and R' groups (ketones IV-VI), but may not hold for ketone 111. In order to simplify the treatment we will assume the following: (i) kII,kISC,and /3 in unsymmetrical ketones are equal to the values obtained for symmetrical diketones, and (ii) k7 = k10 (kmig)s k12 = k14= (kmigh (16) when R and €1' are similar. If a steady state treatment is applied to the reaction scheme comprised of reactions 3-14 with the above assumptions, the following equations can be obtained for the singlets:
where 7.4 and 7 B are defined by 7-l = (kII + kIsc). The treatment leading to these equations is similar to that given by Wagner.18 These equations simplify considerably in two extreme situations: negligible migration, where kfig > k5 and k8. In this case
M
Flgure 5. Values of (4 OGb/$ c,b)/(4oc,f$ c,HB) for ketone V I plotted against triethylamine concentration. urves A-D were calculated according to eq 21, with k, equal to 0, 0.2, 0.5,and m , respectively.
where A and El represent the same ketone with the excited energy localized in different carbonyl groups. The different biradicals (bir) can react according to bir p(o1efine + ketone) + (1- 0)other products (15) The value of ,b will depend upon the biradical considered. For symmetrical ketones, where A is equal to B, migration steps 7, 10, l%, and 14 are irrelevant and the reaction slcheme becomes similar to that of a monoketone. The results given in Tables I and I1 show that the photobehavior of the symmetric 1,4-diketones is very similar to that of the corresponding monoketones. Furthermore, the data of Table I1 also show that the 1,4-biradical behaviors are the same as those expected from previous results, with the most noticeable difference between the biradicals in diketone I and those derived from 3-heptanone being the decrease in PS which can be related to the difference in the size of the group opposite to R.16 The results of Table I and Figures 1-4 show that the photobehavior of the unsymmetrical 1,4-diketones is quite different from that expected from the results obtained in "model" monoketones or in symmetrical diketones. The photobehavior of a given RCO group in a diketone depends then upon the characteristic of the other carbonyl group. This dependence can be attributed to the migration steps. Singlet State Reactions. The rate constants for intramolecular hydrogen abstraction for ketones I and I1 are 4.6 X 10' and 4 X lo9 s-l per hydrogen atom. These values compare well with the value expected from the reactivity in monoketones (4.6 X lo8 and 2.4 X lo9s-l, respe~tively).~' These results corroborate the independence of the intramolecular hydrogen abstraction rate constant of aliphatic ketones on factors other than y substitution. The relevance of energy migration can be quantitatively evaluated if it is assumed that both chromophores have equal probability of initial excitation. This assumption
-
The values of ($oA)s predicted by these two extreme models are given in Table 111, where the experimental results also have been included. Since the symmetrical diketone bearing two n-propyl groups was not investigated, a value of (3s equal to 0.07 was assumed for the n-propyl group. This value was selected from a consideration of the behavior of related monoketones.16 In order to evaluate the importance of energy migration from the quenching experiments it is useful to rearrange eq 17 to ~ O A / ~ A 1 + a[&] -$OB/$B
1 + b[Q1
(21)
where
(122)
In this equations kR = kmig/kq. From data obtained for symmetrical ketones and/or for model compounds, the values of a and b can be obtained for a given value of kR. A set of plots for different values of kR for ketones V and VI are given in Figures 5 and 6, where have been also included the experimental values. These figures show that energy migration has to be taken into consideration and that the best fit corresponds to kR = 0.2, which leads to kmlg= 2 X lo8 s-l. With the values of hmigin eq 18 the values given in Table I11 were callculated. It can be concluded then that the data obtained for the singlet reactions of the unsymmetrical ketones can be reasonably interpreted if a rate constant for energy mi-
254
The Journal of Physical Chemistry, Vol. 84, No. 3, 1580
[TIIIETHYLAMINEI
M
Figure 6. Values of (4 OCA/4 )/(4 OcSc/4cA) for the singlet state of ketone V plotted against trlet$larnine concentration. Curves A-D were calculated according to eq 21 with values of k, equal to 0, 0.2, 0.5, and m , respectively.
gration between the chromophores of approximately 2 x lo* s-l is considered. This is the first reported value for the rate of intramolecular singlet energy migration between carbonyl chromophores. Triplet State Reactions. The photobehavior of the triplet state in ketones I and I1 is similar to that shown by monoketoneswith the same type of y substitution. The kpTT values (40 M-' for I and 9 M-' for 11) are similar to those obtained for several ketones with the same type of y sub~titution.'~If the same k, value is assumed for monoand diketones, then k,/n (hydrogens) = 1.3 X lo8 M-l s-l and kbfi/n (hydrogens) = 1.2 X lo9 M-' s-l values that compare favorably with the average values obtained for aliphatic ketones.17 The data of Table I and Figures 1-3 show that energy migration is a relevant process in the triplet state.8 The quantitative treatment of the triplet data is rather difficult since the input of 3A and 3B depends on the singlet photoprocesses, including energy migration. It is, nevertheless, interesting to note that the isobutene quantum yield from the triplet state of ketone V is nearly 5 times larger than that of ketone 11. This result requires lack of equilibration in the singlet to obtain a rather large triplet quantum yield (which is in agreement with the previous discussion) and a migration rate in the triplet manifold faster than the hydrogen abstraction from the primary y hydrogens. Most of the isobutene arises then through the sequence of reactions 24-27. V + hv '(C3H,CO*CH2CH2COC5Hli) (24)
-
-
'(C3H&O*CH&H2COC5H11) 3(C3H7CO*CH2CH2COC5Hll) (25) 3(C3H7CO*CHzCH2COC6Hll) 3(C3H7COCH2CH2CO*CJ111)(26) 3(C3H&OCH2CH2CO*C5H1$ C&COCH2CH2C(OH)CH2CH2C(CH& (27) The relationship between (4O/4)T for a given compound and the quencher concentration will be given by an equation similar to eq 17 which incorporates the differences in initial input of 3A and 3B. This expression will be too complex to be treated quantitatively, but it takes a particularly simple form at low quencher concentrations when (7T)A
