J. Phys. Chem. 1992, 96, 245-248
245
The Contribution by Diffusion to the Cycloaddition Reactions of Singlet Oxygen with Furans in Solution under High Pressure Masami Okamoto Faculty of Engineering and Design, Kyoto Institute of Technology, Matsugasaki, Sakyo- ku, Kyoto 606, Japan (Received: June 6, 1991)
The rate constants for the cycloaddition of singlet oxygen (IO2; IA,) to furan and 1,3-diphenylisobenzofuran(DPBF) have been determined in five solvents at pressures of up to 400 MPa. Plots of the logarithms of the observed rate constants against pressure are linear for furan, whereas those for DPBF are curved significantly downwards or show maximum values depending on the solvent. These results are attributed to the enhancement by pressure of the participation of diffusion in the cycloaddition reactions. The contribution of diffusion to the cycloaddition, for which two mechanisms have been proposed by other workers, is discussed, and it is concluded that exciplex formation between IO2and the furans is involved in the cycloaddition reaction. The rate processes associated with the exciplex formation and dissociation are discussed in terms of the solvent polarity and the effects of pressure-induced changes in the solvent viscosity.
Introduction Because of the photochemical and photobiological importance, the nature of the reactions of singlet oxygen with organic substrates 1,3-Diphenylisobenzofuran has been studied extensively. (DPBF), which reacts efficiently with singlet oxygen, yielding 1,Cendoperoxide, has been used as a monitor for determinations of the lifetime of singlet oxygen as well as the rate constants for the reactions of singlet oxygen with organic substrates.2 The mechanism for the cycloaddition includes a concerted Diels-Alder reaction3 and rapid reversible formation of an exciple^.^,^ In general, the bimolecular reaction competes more effectively with diffusion as the solvent viscosity increases. The retardation due to the contribution by diffusion to the reaction would be expected to be observed easily for solutes with high reactivity toward singlet oxygen. In fact, Gorman et a1.4d have shown in variable-temperature experiments that the slope of Arrhenius plots for the quenching rate constants of the DPBFsinglet oxygen system changes sign from positive to negative with decreasing temperature. The present work focuses on the effect of solvent viscosity on the cycloaddition of singlet oxygen to DPBF, where the viscosity of the solvent is varied by the application of pressure. Recently, we have measured the lifetime of singlet oxygen as a function of pressure by observing the rate of bleaching in the optical absorption of DPBF in four solvents and found that the bimolecular rate constant for the DPBFsinglet oxygen system increases significantly with increasing pressuree6 This suggests that the reaction may compete wjth diffusion at higher pressures because the solvent viscosity increases strongly with increasing pressure, and this pressure-induced increase in the solvent viscosity decreases remarkably the rate processes associated with diffusion.' The aim of the present work was to investigate the contribution by diffusion to the mechanism of cycloaddition by comparing results for two
furan derivatives that have different reactivities toward singlet oxygen. The results are discussed in terms of the volume of activation for solvent viscosity and that for the cycloaddition. Experimental Section Zone refined grade anthracene (A; Tokyo Kasei) was used as supplied. All solvents were of spectroscopic grade (Dojin) and used without further purification. 2,2,4,4,6,8,8-Heptamethylnonane (HMN; Tokyo Kasei) was purified by passing through a silica gel column twice. Furan (F; Tokyo Kasei) was purified by distillation. 1,3-Diphenylisobenzofuran(DPBF; Aldrich) was purified as described before.6 The laser photolysis system modified for measurements at high pressure and the associated experimental techniques have been described Singlet oxygen (lo2;lAg) was created by irradiating an aerated anthracene solution (ca. lo4 M) with an 8-ns pulse of a nitrogen laser (337 nm). The computer-assisted data collection and analysis have been described beforen6 The change in the concentrations of furan and DPBF with increasing pressure was corrected by taking into account the known compressibilities of the solvents?
ReSultS The sensitized singlet oxygen creation and its decay in the presence of DPBF and F have been established.* When the initial hu
A + 1A* + 3A*
-
3A* + 302(3zlg-)A
+ '02(lA,)
+ DPBF -% loss of DPBF IO2(IAg)+ F loss of '02('A,)
IOZ(lAg)
(1)
-
(4)
k,
(1) (a) Kearns, D. R. Chem. Rev. 1971,71,395. (b) Frimer, A. A. Chem. Rev. 1919, 79, 359. (2) (a) Merkel, P. B.; Kearns, D. R. J. Am. Chem. SOC.1972, 94, 7244. (bl Adams. D. R.: Wilkinson. F. J. Chem. SOC..Faraday Trans. 2 1972.68, 586. (c) Young, R. H.; Brewer, D.; Keller, R. A. J . Am. Chem. SOC.1973, 95, 375. (3) Monroe, B. M. J. Am. Chem. SOC.1981, 103, 7253. (4) (a) Gorman, A. A.; Loverring, G.; Rodgers, M. A. J. Am. Chem. SOC. 1979. 101, 3050. (b) Gorman, A. A.; Gould, I. R.; Hamblett, I. Ibid. 1982, 104, 7098. (c) Gorman, A. A.; Gould, I. R.; Hamblett, 1.; Standen, M. C. Zbid. 1984, 106, 6956. (b) Gorman, A. A.; Hamblett, I.; Lambert, C.; Spencer, B.; Standen, M. C. Ibid. 1988, 110, 8053. ( 5 ) Clennan, E. L.; Mehrsheikn-Mohammadi, M. E. (a) J. Am. Chem. SOC.1983,105,5932. (b) J . Org. Chem. 1984,49, 1321. (c) J . Am. Chem. SOC.1984, 106, 7112. (6) Okamoto, M.; Tanaka, F.; Teranishi, H. J . Phys. Chem. 1990,94, 669. (7) (a) Okamoto, M.; Teranishi, H. J . Phys. Chem. 1984,88, 5644. (b) Turley, W. D.; Offen, H . W. Ibid. 1984.88. 3605. IC) Okamoto. M.: Tanaka, F.; Teranishi, H. Ibid. 1986, 91, 1055.
0022-3654/92/2096-245$03.00/0
concentration of IO2 is much smaller than those of DPBF ([DPBF]) and F ([F]), the absorbance change of DPBF with time, A ( t ) , is given by eq 66since reactions 1 and 2 are much faster than those of 3,4, and 5 in the pressure range examined.6J0 In A ( t ) = A(m)
+ [A(O)- A ( - ) ] exp(-kobst)
kob = k,j + k,[F] + k,[DPBF]
(6)
(7)
(8) Okamoto, M.; Teranishi, H. J . Am. Chem. SOC.1986, 108, 6378. (9) (a) Bridgman, P. W. The Physics of High Pressure; Bell: New York, 1958; p 128. (b) Jonas, J.; Hasha, D.; Huang, S . G. J . Chem. Phys. 1979, 71, 3996. (c) Schroeder, J.; Schiemann, V. H.; Sharko, P. T.; Jonas, J. Ibid. 1977,66,3215. (d) Brazier, D. W.; Freeman, G. R. Can. J . Chem. 1%9,47, 893.
0 1992 American Chemical Society
246 The Journal of Physical Chemistry, Vol. 96, No. 1 , 1992 TABLE I: Activation Volumes (cm3/mol) at 25 O solvent m AVmrb
acetonitrile methanol MCH hexane HMN
31.5 32.1 2.02 1.88
-
8f 11 f 24 f 18 f 43 f
C
Okamoto
and 0.1 MPa and the Relevant Parameters for the S i l e t Oxygen-DPBF System AV,'*Obs AVO' AVain" AV-1' - AV?* (k-alk,)n -15 -20 -19 -14 -6
Id
le 18
If lh
f 1
-17 -18 -19 -12 -18
f2 f2 f2 f4
f1 f2 f2 f2 f3
5 f 1 ( 5 f 1) 7 f 1 ( 5 f 1) 14 f 1 (11 f 1) 11 f 1 26 f 2 (25 f 1)
22 25 33 23 44
f 1 f2 f2 f2 f4
2.6 4.4 8.1 20.0 3.1
f 0.5 f 0.8
f 1.0 f 3.0 f 0.6
+ +
"Dielectric constant at 0.1 MPa?O bAV,,' was estimated by the equation (a In ?/a P)T = AV,,*/RT, assuming that In TJ = A BP CP2. cAVdifl* was estimated by the equation AVdifl*= oAV,' (see eq 9). The values in parentheses were evaluated from the linear plots of In kdifragainst pressure. dReference 21a. 'Reference 21b. /Reference 7d. ZReference 9b. hReference 21c.
-
'
-
&
f,/
, /
1 U
.Y
-C 1
0
10
20
30
40
0
50
I
[furan] I m M
Figure 1. Plots of kobs against the concentration of furan, [furan], in
T 0
methylcyclohexane (MCH) at five pressures and 25 OC ([DPBF] = 51 rM).
I
I
I
I
100
200
300
400
pressure/ MPa
eq 6 , A ( 0 ) and A ( = ) are the absorbances at t = 0 and t = m, respectively. The values of kobswere determined from the measurements of A ( t ) according to eq 6.6 k, was determined from the plots of kobs against [DPBF] in the absence of F. k , was evaluated from the plots of kobsagainst [F] at a fixed [DPBF], which is shown in Figure 1. The pressure effects of k, and k, are shown in Figures 2 and 3, respectively. As seen from these figures, for the furan system with lower rate constants, the plot is linear (Figure 2), whereas for the DPBF system with higher rate constants they are curved in the pressure range examined (Figure 3). The pressure dependence of the rate constant ki is generally expressed in terms of the activation volume, AY', via eq 8, where K is the isothermal compressibility of the solvent. The observed
RT(d In k i / a P ) , = -AV,*'obs- RTK
Figure 2. Pressure dependence of kq for the furansinglet oxygen system in methylcyclohexane at 25 OC.
22
I
2
c Y
(8)
activation volume for the furan system, AVq*90bs,was determined to be -17 f 1 cm3/mol in methylcyclohexane (MCH), and those at 0.1 MPa for the DPBF system, AV,*,Obsevaluated from the initial slopes in Figure 3 are listed in Table I together with the relevant parameters.
Discussion In general, the observed rate constant changes monotonically with increasing pressure." However, the present results for the DPBF system (Figure 3) display downwards curvature at higher pressure which results in maximum values for k, in acetonitrile, MCH and HMN. Similar observations have been reported for the cycloaddition reactions of carbenes to alkenes12 and for the photoreduction of benzophenone triplet in alcohol solution^.'^
-c 2 1
0
100
200
300
400
pressure I MPa
Figure 3. Pressure dependence of k , for the DPBFsinglet oxygen system in hexane ( O ) , methylcyclohexane (0),HMN (A),methanol (m), and acetonitrile ( 0 )at 25 OC. The solid lines in solvents except hexane were drawn according to eq 14 by using the parameters listed in Table I. The solid line in hexane was drawn by the equation In k , = A EP CP2.
+
(IO) Yasuda, H.; Scully, A. D.; Hirayama, S.; Okamoto, M.; Tanaka, F. J . Am. Chem. SOC.1990, 112, 6847. ( I I ) (a) Asano, T.; Okada, T. J. Phys. Chem. 198488,238. (b) Isaacs, N. S. Liquid Phase High Pressure Chemistry; Wiley-Interscience: New York, 1981:. r1) 183. (12) Turro, N. J.; Okamoto, M.; Gould, I. R.; Moss,R. A,; Lawrynowicz, W.; Hadel, L. M. J . Am. Chem. SOC.1987, 109, 4913. (13) Okamoto, M. J. Phys. Chem. 1990, 94, 8182.
+
As seen from Figure 3, the value for k, in MCH is almost equal to that in H M N at 0.1 MPa, but the maximum value in H M N appears at lower pressure. Judging from the fact that the activation volume for viscosity, At'?*, in H M N is much larger than that in MCH (see Table I), the existence of the maxima for these plots may be related to the increase in the viscosity of the solvents
The Journal of Physical Chemistry, Vol. 96,No. 1, 1992 241
Reactions of Singlet Oxygen with Furans
2
22
c \
4t
I
31
.
I
I c
.-
I
-
N Y
A.
I
I
r
Y v
TI
1
-1
I
I
I
I
1
I
0
1
2
3
4
5
C
lnrl Figure 4. Plots of In k, against In q in hexane (O), methylcyclohexane (0),HMN (A), methanol (W), and acetonitrile (0) at 25 'c.
-31 T
with increasing pressure. The maxima may also depend on the magnitude of k, as noted from the value of AV,* (Table I) and the results of hexane in Figure 3, which is supported by the pressure dependence of the slower furan system (Figure 2) where the plot is linear. These results provide that the maxima or downward curvature displayed by the plots shown in Figure 3 may be the result of enhanced participation by diffusion in the cycloaddition reactions with increasing pressure. This implies that the activation volume at 0.1 MPa, AV,*.Obs(see Table I), involves the contribution of diffusion. The value for the rate constant for diffusion, kdiff,in a solvent of viscosity 7 is often estimated by using the equation, kdiff = 8RT/2000qlalthough evaluation of kdiffby this equation is not accurate. Another means of estimating the value for k&ffis by using eq 9 where the parameters A and a are constant for a given t e m p e r a t ~ r e . ~ When ~ . ~ ~ In . ~ k, ~ is plotted against In q, which is kdiff = AV-a
(9)
shown in Figure 4, the value of in k, at high viscosity falls into a linear line, irrespective of solvent polarity. The values for A and a estimated from the results in H M N at higher viscosity are 2.7 X lo9 M-I S-I and 0.60 f 0.02,respectively. The a value is in good agreement with those calculated recently for diffusioncontrolled fluorescence quenching of anthracene (0.57) and 9methylanthracene (0.64) by oxygen in MCH.l0 The volume of activation for kdiff, AVdiff', was determined from the equation, AVdif,' = aAV,*. These results are listed in Table I. As seen in Table I, the values for AVdirf'are similar to those obtained for fluorescence quenching by oxygen of anthracene (12 cm3/mol) and 9-methylanthracene (14 cm3/mol) in MCH,l0 and the quenching by 0-carotene of IO2 in M C H (10 ~ m ~ / m o l ) . ~ ~ Two types of the reaction mechanism for the cycloaddition have been proposed: one is a singlestep reaction2 and the other involves exciplex formation between IO2 and the furans as an intermediate.4.5 The participation by diffusion in each mechanism is discussed separately. (a) Singlestep Reaction. When the rate of reaction is comparable with that for diffusion, the observed rate constant, k,, is given by eq 10l6 where k, is the rate constant that would be observed if diffusion effects were absent. This equation indicates that k, = k, in the low viscosity limit (kdic>> k,), while k, = kdifl in high viscosity limit (kdiff > k2),k, (12)
+
= k l k 2 / k d l .The observed activation volume, AV,', (=AVl' AV2* - AV-,*) is negative as indicated by the plots shown in Figures 2 and 3. On the other hand, in the high viscosity limit (kI
