Jerry A. Bell and James D. MacGillivrayl
Simmons College Boston, Massachusetts 021 15
Photosensitized Oxidation
by Singlet Oxygen An adaptable photochemical system
-
While develoninz . a course concerned with entropy and its practical i m p o r t a n ~ ewe , ~ thought it would be useful to do an exneriment that would illustrate some of the important characteristics of photosynthesis (and many $her photochemical systems) Trapping of the sun's (photochemical)energy Creation of low-entropy products from high-entropy reactants (in particular, from an atmospheric constituent) The role of photosensitizers (to show that energy absorbed by one species may be transferred to another which actually reacts) That the rate of product formation is directly proportional to the amount of light absorbed (the Stark-Einstein principle) Our experimental system does these things nicely and also can be used in different contexts to illustrate other aspects of both photochemical and conventional kinetics: concentration dependencies of reaction rates, the effect of temperature in photochemical systems, and the effect of changes in actinic wavelength (i.e., the energy of the absorbed photons) on the identity of the products. With appropriate variations, the system would be useful for both high school and college laboratories, for science and nonscience concentrators, and for beginning and advanced students. We chose methylene-blue (MB)-photosensitized oxidation of 1,3-diphenylisobenzofuran(A) by singlet oxygen in methanol solutions as the system to develop for several reasons. Its kinetics are known in some detail (1-3) and it is extremely fast, an important criterion for a system that is to be studied in limited laboratory time. Furthermore, because the compound A is yellow and the product of the reaction with singlet oxygen, 0-dihenzoylbenzene (P), is colorless, the reaction is easily followed visually in demonstrations or with inexnensive snectrocolorimeters a t wavelengths around 420 nm. (since the reactive solutions also contain MB, they are initially preen-yellow plus blue-and become bluer-as the reaction proceeds.) Finally, it is one of the few rapid and visual singlet oxygen reactions that yields only one major, well-characterized product (4). There are some drawbacks to this svstem. which are indicated by the lower two reactions in ~ i g r e1. With or without sensitizer, A can absorb blue (and ultraviolet) radiation, which decolorizes both aerated (51 and deaerated solutions. Perhaps, in the latter case, the product is a dimer or an intramolecularly cyclized compound; these suggestions are based upon analogy with the photochemistry of anthracene and cis-stilbene (6). In any case, solu'Deceased. A senior at Boston Latin School when this work was done, he would have entered Massachusetts Institute of Technology in Fall 1973. Bell, J. A,, J. CHEM. EDUC., in press. The simplifications omit the various possible excited dimeric complexes (erciplexes) of MB present in such a system, the formation of the 'S of 0,. -. which auicklv . - decavs to or the eround state, 3V, and the mechanistic role of the solvent and other species in the MB* and 0.* quenchingand decay reactions.
\ DIMER ( ? ) + P Figure 1. Samepossiblereactionsof 1.3-diphenylisobenzofuran.
tions of A must be protected from light, especially sunlight and fluorescent light. Solutions of A are not stable for extended periods, even in the dark, and appear to oxidize thermally to P. This may even occur in the solid: the infrared spectrum of A, as received and used, shows a small carhonyl absorption for a compound that is supposed to have only a furan oxygen. Finally, there is the warning that A may be carcinogenic (by analogy with other phenyl-substituted, fused ring systems). A somewhat simplified3 reaction scheme for the reactions that occur is MB hdred) MB** (excited singlet state) R, = tGIdMBJ MB** MB* (triplet state)
+
MB*
+ O?
- +-
-+ MB 02* (singlet, ' A , state) k, MB* MB kg OX* A -+ AO, P k, O,* 0, kd where R, is the rate of excitation of MB, M s-', which is a function of: 6 , the molar extinction coefficient of MB; 1, the path length for absorption of light; lo, the rate a t which radiation enters the solution in einsteins I-* s-'; and [MB]. For our purposes we can assume that essentially all the initially formed MB** goes to MB*, i.e., that the second reaction is totally efficient. With these definitions and assumptions, application of the steady state hypothesis to this kinetic scheme yields
+
The first term on the right of eqn. (1)is the rate of formation of MB*, the second represents the fraction of MB* that transfers its energy to produce OZ*, and the third is the fraction of OZ* that reacts with A. (This result may easily be rationalized simply on the basis of these fractions, rather than through the steady state analysis, if one wishes to avoid such complexity with a particular class.) In methanolic solution, when [A] > M, k,[A] > kd, kd[A] (2, 3). Under these condiso we have kr[A] + k d tions
--
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Irradiation Time, minutes Figure 2. Results for MB-photosensitized ( A = 624 nm) oxidations of A at two initial concentrations near M , The concentrations of MB and the negative slopes of the lines are: m , 2.5 X M. 0 0 0 9 min-'; 0 . 5.0 X 10-8 M. 0 . 0 1 7 m i n - 3 : A . 10.0 X lo-", 0.035 min-'.
and eqn. (1) becomes independent of [A]; the rate of reaction, with all other variables held constant, should not he a function of [A], i.e., the reaction is zero-order in A, so M. Further simplification is also possilong as [A] > ble because kr[Oz] > k, for [OzI > M and, therefore, kt[02], which leads to ki[02] k,
+
-
the Coming CS2-60, that cuts out light of wavelengths shorter than about 600 nm should he satisfactory. (We have not tried using filtered sunlight directly as an excitation source, hut there should he no problems associated with this variation.) The distance between the light source (the light bulb, not the filter) and the reaction vessel (a Spectronic 20 tube) was varied to test whether the rate is directly proportional to lo, as eqns. (1) and (2) demand. The results, Table 1, confirm (or derive experimentally) eqns. (1) and (2). T o test the dependence of the reaction rate on [Oz] predicted bv. eon. . (1). one would need a vacuum svstem. which is generafiy'availahle only in physical chekistr; and other advanced courses. There is. however. a s i m ~ l e test that students in most laboratoriescan car& out. 1 f 0 z is eliminated from the solution, both eqn. (1) and common sense indicate that the rate of disappearance of A should he zero. Bubbling Nz slowly through the solution for an hour (or preferably longer) by means of a Pasteur pipet inserted through a small hole in a stopper will sweep the Oz out of the solution. After the Nz treatment, the reaction vessel should be well-stoppered and the reaction run as usual. Results from such an experiment are shown in Fieure 3 and confirm. dramaticallv. *. that 09 is essential for tce reaction. To Drove that the Droduct is indeed P. we irradiated 20 ml of-a M solution of A (plus MB) until the A had completely reacted, evaporated the solvent, and took up the residue in a few drops of diethyl ether to separate the product from ionic MB. The ether was evaporated and the residue, dissolved in a few drops of CHzC12, had an infrared spectrum identical to authentic P. All mechanistic deductions that may easily be derived from the concentration variations suggested above (with [A] 2 10W4 M ) can be summed up in a simple reaction
-
Tabla 1. DeDmdenca of the Reaction Rate on 10
Distance of cell from lamp ld (cm))
-
-Slope Rate imin-?
-100 -lo dl
Thus the reaction rate is independent of [Oz], so long as it is greater than M, and we may rewrite eqn. (11, under these conditions, as
The initial concentrations of A, for the experiments and 10-3 M and 1 and shown in Figure 2 are between lo are the same for all runs. Because methanol saturated with 0 2 from the air is about 10-3 M in Oz, the [Oz] never falls below M in these runs, even when they are carried to completion, so eqn. (2) is applicable. We find that the rates of the reactions, directly proportional to the slopes of the lines, are the same for both initial concentrations of A shown, when we look a t any particular value of [MB]. Equations (1) and (2) also predict that the rate should be directly proportional to [MB], the sensitizer concentration, and Figure 2 confirms this p r e d i ~ t i o n (No .~ reaction occurs upon irradiation with red light in the ahsence ofMB.) In our experimental set-up the light source is a 60-W light bulb filtered through an interference filter that passes a narrow band of light a t 624 nm, but any filter, e.g.,
0.9'
0
4Ta ensure that the reaction is first-order in [ M B ] , the eoncentration must he very low, so that only a small fraction of the incident light, la, is absorbed. It is easy to see why this restriction is necessary for, if [ M B ] were so high that all the light was absorbed, an increase in [ M B ] would not further affect the rate. 678
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10
15
Irradiation Time, minutes
.,
20
Figure 3. Effect of deaerating the reaction solutions: not deaerated: 0 , deaerated initially and then opened and shaken after the first 10 min of irradiation. [MB] = 14.0 X t O - * M in both SOIUtionS.
scheme that is entirely suitable for many pedagogical situations MB Mred) -+ MB* R, = dZdMB] MB* O2 MB 02* kg 0,* A AO, + P k,
+
+ +
--
Tabla 2.
+
+
- -+
A* P
"Proof' t h a t this is not the mechanism is difficult to ohtain simply, but we can argue that direct excitation of A in aerated solutions does not seem to yield P efficiently (an experimentally testable statement) as would be required by the mechanism. (It is also known that the lifetime of excited MB is not affected by [A] (3). hut this alternate mechanism would predict such a dependence.) Obviously there are many other experiments one can do and many questions that can be asked and experimentally answered in this system. For example, what sort of temperature dependence would such a reaction have? (Generally, photochemical reactions like this show little temperature sensitivity.) We tested this variable with the results shown in Table 2. Care must be taken in carrying out this experiment because the solubility of Oz in methanol is quite temperature sensitive. We passed a stream of Nz over, not through, the solutions for a minute and stoppered them well a t room temperature before placing them in water baths (beakers) a t other temperatures for the irradiation. Other experiments that would he easy to try are to vary the solvent, the absorbing path length, and the sensitizer. More ambitious experiments would be to develop techniques for studying the rate a t very low [A] or as a function of [02] to test eqn. (1) even further. The mechanism of the reaction that transforms A into P is probably similar to other reactions of singlet oxygen with dienes to yield, initially, endoperoxides; quantum yield measurements on the appearance of P or careful isolation techniques to obtain and characterize intermediates, e.g., AOz, would he useful for further elucidation of the reaction mechanism for advanced student^.^ Perhaps other
-Slop -Rate
0 10 20 25
0.050 0.050 0.049 0.051
30
Advanced students are sometimes concerned that we have not considered a plausible alternative mechanism A MB MB* A* + 0, + AO,
Dependence of the Reaction Rate on Temperature
Temperature (OC)
(mi"-?
0.049
reaction conditions (non-photolytic) could be found that cause this reaction. For example, A in the presence of Hz02 and the enzyme peroxidase is oxidized to P (7). The identity of the product(s) formed upon direct irradiation of A could be determined for both aerated and deaerated solutions. The list is limited only by one's curiosity, ingenuity, and previous chemical background. Experimental Hints The possible variations, even on the basic experiment, are so many that only some general suggestions will be given here; an experiment can then be designed that will fit the course being given by the reader. 1.3-Diphenylisabenzofuran (Aldrich) is dissolved in methanol (in subdued light) M stack solution of A - to give . about a (0.27 g/l). The actual concentration will vary, depending upon the purity of sample. For use with an instrument like the Speetmnic 20, the concentration is adjusted empirically to give an absorbance between 1.2 and 1.5 (protect from light at all times). Methylene blue is dissolved in methanol and successively diluted to give a stack solution that is about 5 X 10-7 M in MB (0.16 mg/l). A typical reaction run is carried out by mixing 1-5 ml of the A stock solution and 0.2-1 ml of the MB stock solution with enough methanol to give a total of 5 ml in a spectrocolorimeter cuvet, measuring the ahsarhance at 420 nm, and irradiating with periodic checks on the absorbance. The rate can easily be adjusted for any particular purpose by varying the [MB], the wattage of the lamp, or the distance from the lamp to the reaction vessel. The only precaution that must he observed rigidly is to protect the sample from any radiation except that passing through the red filter and the very low intensity monitoring beam in the spectrocalorimeter. Literature Cited (11 Merkel, P. B.. andKeams.D.R.,J, Amer Chem Soc.. 94. L029119121. (21 Morkel. P. B.. Nilrm. R.. and Kearns, D . R.. J. Amar. Chsm. Sor.. 9 4 1030 (19721. (31 Merkel. P.B., a n d K e e r n ~ . D . R .J, . A m w Chrm Sac.. S i . 7244i19721. (41 Foole, C. 6..Aerounfr Chrm R e s . 1. 104 (196.91: Kearna. D. R., Chrm R e v . 71. ?nc,,",.,
9z.,,La,L,.
5The pure endoperoxide intermediate, formally the monozonide of a bievclobutadiene derivative, thermally reacts to give P in solution, but is explosive and must be handled only with great care, if isolated; see Ref. (8).
(51 OlrnsUd, J.. sndAkarhsh,T., J. Amma. Chem Soe., 95.6211 (19731. 161 Bowen, E. J . , Adumn. in Photochem.. 1, 23 119631: Turro, N. d.. "Maleeular Photochemistry," W , A. Benjamin. NeuYork, 1967. pp. 205,233-4. (71 Chan. H . W.-S.il Amer Chrm. Sac.. 93,4632 il9lIl. (81 Oufreiue. C., andEeani. S.. Compf. rend.. 223.735 119461.
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