J . Phys. Chem. 1988, 92, 16-18
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Bistabllfty in an Isothermal Photochemical System. The Triphenylimidazolyl Radical Dimer in a CSTR D. Lavabre,t G. Levy,*J. P. Laplante,*+and J. C. Micheau*l Department of Chemistry and Chemical Engineering, Royal Military College of Canada, Kingston, Ontario K7K 5L0,and Laboratoire IMRCP, UA 470 CNRS. UniuersitZ Paul Sabatier, 31 062 Toulouse, Cedex, France (Received: October 12, 1987)
Hysteresis and bistability were observed when a solution of triphenylimidazolyl radical dimer (TPID) is irradiated in a CSTR. The reactor is open to a flow of TPID and irradiated at 360 nm. The transition between the two stable states can be induced by changing either the flow rate or the incominglight flux. This bistability is believed to be the first experimental observation of a genuine chemical instability in an isothermal photochemical system.
Introduction Bistability can be defined as the existence of two possible stable states for the same set of external constraints. It is one of the simplest “exotic” behaviors observable in far from equilibrium nonlinear systems. Despite this simplicity, its observation has so far been limited almost exclusively to systems involving inorganic reactions in a CSTR, Le., those open to a mass flow.’ Exceptions to this rule are the light-induced thermochemical instabilities studied by Ross et al. in various illuminated In the latter, the necessary feedback is thermal and originates from the coupling between light absorption, the temperature dependency of the equilibrium constant, and the heat loss to the surroundings. The bistability observed in the illuminated liquid phase o-cresolphthalein system4 is particularly significant since it provided the first example of nonlinear behavior in a chemical system strictly closed to mass flow but open to an energy flow. So far, however, photochemical systems have remained relatively unexploited in the search for new, interesting nonlinear p h e n ~ m e n a . ~This is, in fact, quite surprising since (i) light absorption in multicomponent systems is governed by the highly nonlinear Beer’s law and (ii) direct or indirect autocatalysis or inhibition is certainly present to some extent in many photochemical reactions. This paper reports on what is believed to be the first experimental observation of a genuine chemical instability in an isothermal photochemical systems6 Hysteresis and bistability were indeed observed when a soluticm of triphenylimidazolyl radical dimer (TPID) was irradiated in a CSTR. A summary of the known photochemistry of the TPID/chloroform system is presented in the next section, followed by the experimental procedure and typical results illustrating the bistability. A tentative feedback mechanism is suggested.
Photochemistry of the TPID/Chloroform System Photodimer TPID (Figure 1) is a member of a group of compounds known as photochromic.’ When irradiated, the compounds undergo a color change. The photochromic properties of TPID were first recognized by White and Sonnenberg* and more recently have been studied by Maeda and H a y a ~ h i .A ~ solution of TPID prepared in the dark is light yellow. Upon irradiation, however, the solution quickly turns reddish-purple. The purple color is due tc the triphenylimidazolyl radical (R’, Figure 1 ) immediately formed as a result of the TPID photodissociation. The radical absorbs strongly around 350 and 550 nm. Photodissociation is reversible, however, and on standing in the dark at room temperature the solution’s color gradually reverts to the original. The duration time of this reversible cycle varies with the solvent and with the coexistence of oxygen. In deaerated benzene solutions, the duration time was found to be as long as ‘Royal Military College of Canada. ‘Universite Paul Sabatier.
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1 year,9 whereas in ethanol or toluene it is much shorter. In chloroform solutions, the duration time is of the order of minutes. This loss of activity is thought to be due to hydrogen abstraction from the solvent by the triphenylimidazolyl radical, which then reverts to lophine (L, Figure 1) and/or to its oxidation by dissolved oxygen. Our HPLC studies of the photoreacting mixture have confirmed the presence of lophine as one of the intermediate photoproducts. Upon further irradiation, however, a variety of photoproducts are formed, one of which we have identified as 2-phenyl-9,lOphenanthroimidazole, a strongly fluorescent compound.1° We have also carried out a HPLC study of the photoproducts arising from the photooxidation of lophine in chloroform. The same phenanthroimidazole was found as one of the products of this photoreaction.
Experimental Section The experiments were carried out in a CSTR, the design of which is described in Figure 2. The reactor is constructed of quartz and has a volume of approximately 1.10 mL. Stirring is magnetic. The reactor is provided with ports for both in- and outflow of a TPID solution. As the solution leaves the reactor, it immediately flows through a 50-pL absorbance cell, the cell being positioned inside a diode array spectrophotometer. This system allows the monitoring of the whole absorption spectrum as a function of time. The monitoring wavelength of 554 nni was chosen. Since the TPI monomer is the only species which absorbs strongly in that region, the value of ASS4is essentially proportional to the radical concentration. Irradiation was carried out using a 150-W mercury lamp with the wavelength fixed at 360 nm (interferential filter, band-pass 40 nm). Pumping of the TPID solution was accomplished with a 70-mL linear syringe pump. In all experiments the concentration of TPID being pumped through the reactor was kept constant at 0.001 M in chloroform. The (1) (a) De Kepper, P.; Boissonade, J. Oscillations and Trauelling Waues in Chemical Systems; Wiley: New York, 1985; Chapter ?. (b) Vidal, C.; Hanusse, P. Int. Reu. Phys. Chem. 1986, 5 , 1-S5. (c) Nicolis, G.; Prigogine, I . Self-Organization in Nan-equilibrium Systems; Wiley: New York, 1977. (2) Nitzan, A,; Ross, J. J . Chem. Phys. 1973, 59, 241. Nitzan, A,; Ortoleva, P.; Ross, J. J . Chem. Phys. 1974, 60, 3134. (3) Zimmerman, E. C.; Schell, M.; Ross, J. J . Chem. Phys. 1984,81, 1327. (4) Kramer, J.; Ross, J. J. Phys. Chem. 1986, 90, 923. (5) Grtgoire, F.; Lavabre, D.; Micheau, J . C.: Gimenez, M.; Laplante, .I. P. J . Photochem. 1985, 28, 261. (6) A few years ago, some photochemical systems were reported to display oscillatory fluorescence upon continuous irradiation. The phenomena were studied by some of us and shown to be of hydrodynamic origin. See for example: Gimenez, M.; Micheau, J. C.; Laplante, J. P. J . Phys. Chem. 1985, 89, 1 and references therein. See also: Epstein, I. R.; Morgan, M.; Steel, C.: Valdes-Aquilera, 0. J . Phys. Chem. 1983, 87, 3955. ( 7 ) Exelby, R.; Grinter, R. Chem. Reu. 1965, 65, 247. (8) White, D. M.; Sonnenberg, J. J . Am. Chem. Sac. 1966, 88, 3825. (9) Maeda, K.; Hayashi, T. Bull. Chem. Sac. Jpn. 1970, 43, 429. (10) Hennessy, J.; Testa, T.C. J . Phys. Chem. 1972, 76. 3362.
0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 1, 1988 17
Letters
L
TPID (P2’
-3’
,,R)
LOPYINE ( R i :
OTHER PWWCTW)
The triphenylimidazolyl radical dimer system: TPID (R2), triphenylimidazolyl radical dimer; TPI (K), triphenylimidazolyl radical; lophine (RH), 2,4,5-triphenylimidazole;P, a mixture of photoproducts.
I F
i
:-
Figure 1.
:: t.
I
I
Figure 3. Figure 2. Schematicsof the CSTR used: (A) syringe pump, (B) quartz reactor, ( C ) magnetic stirrer, (D) flow-through absorbance cell, (E) diode array spectrophotometer.
photon flux was kept constant as well at 1.5 X M/s. Solid TPID was prepared from commercial lophine according to the procedure of White and Sonnenberg.8
Results Figure 3 shows the steady-state absorbances at 554 nm as a function of the residence time, T, of a TPID/chloroform solution flowing through the reactor. At very short residence times (T < 300 s), the TPID solution is flowing quite rapidly through the reactor. The “effective” irradiation time is then not long enough to allow the system to reach its maximum photostationary state. In this region, the TPI radical concentration therefore increases 300 s the radical concentration with the residence time. At T reaches a maximum at which point the solution in the reactor is then reddish-purple. In the range 300 s < T < 1100 s the photostationary concentration of TPI radicals decreases slowly. This decrease is due to the side photoreactions described previously (Figure I ) . As the residence time increases, the degradation becomes more pronounced, and at T N 1100 s a sudden drop in absorbance is observed. The solution then becomes strongly fluorescent and the purple color disappears. Further increases in the residence time have little effect on the absorbance or the fluorescence intensity. Hysteresis is observed as the residence time is decreased, stepwise, say, from T = 1600 s. Until the residence time reaches T E 500 s, the solution remains fluorescent, while the absorbance at 554 nm stays fairly constant and almost negligible. At T 500 s, a sudden jump in absorbance is observed; the solution becomes reddish-purple again and no longer fluoresces. This jump takes place at a much shorter residence time than the reverse transition, hence hysteresis. Bistability was confirmed by the following experiment. The reactor was first stabilized in the fluorescent state, at an intermediate residence time, e.g., T = 800 s. A shutter was then introduced in front of the lamp and the solution allowed to flow for at least 7 residence times, Le., long enough to replenish the reactor with fresh TPID solution. The shutter was then removed and the system allowed to reach its photostationary state. The absorbance was then recorded and found to correspond to the high-absorbance purple branch. Two stable states therefore exist for the same set of external constraints, hence bistability.
Discussion In order for any far from equilibrium nonlinear system to display bistability, some type of feedback must be present in the reaction mechanism. Although it is too early at this stage to propose a detailed mechanism, the following experimental facts should be considered: (1) Injecting approximately 100 WLof the photoproduct solution
Photostationary state absorbances at 554 nm as a function of
the residence time. (TPID)o = 0.001 M, Io = 1.5 X lo4 M/s. The absorbance at 554 nm is proportional to the TPI radical concentration. Data points shown as (+) were obtained in experiments where the reaction was first stabilized in the absence of any irradiation. On turning the light on, the absorbances shown as (+) were obtained. Data points shown as (*) were obtained in those runs where the reactor was first stabilized in its fluorescent state (long residence time). As the residence time was reduced, the steady states (*) were recorded. Note here that the absorbance scale shown in the figure above corresponds to the 1-mm light path spectrophotometer cuvette positioned in the outflow line, close to the reactor. The light path of the reactor itself is 1 cm.
into the reactor while the system is in its purple state instantaneously quenches the radical color. The fluorescent photoproduct solution therefore has the ability to quench the TPI radicals. (2) Pure lophine, by itself, was found to be unable to quench the purple color of TPI radicals. The quenching effect is therefore likely due to the photoproducts arising from the irradiation of the intermediate lophine, through a “two-photon” process. The exact nature of the quencher is, however, still unclear. (3) At 360 nm, the molar extinction coefficients of both TPID dimer and TPI radicals are much larger than that of lophine I ) L/(mol.cm); [e,,(TPID) = 1500 L/(mol-cm); C ~ ~ ~ ( T=P50000 c360(lophine)= 200 L/(mol-cm)]. As a result, the fraction of the incoming light absorbed by the lophine intermediate remains small as long as there is a significant concentration of dimer and radical present in the reactor. (4) Oxygen and/or hydrogen donor solvents are essential to the photoreaction. As mentioned previously, triphenylimidazolyl radical solutions are stable in carefully deaerated solutions. However, the purple color disappears quickly as air is admitted into a previously deaerated solution. ( 5 ) No significant temperature difference was observed as a function of the residence time of the TPID solution in the reactor. The system is therefore isothermal. On the basis of the experimental results presented above, the following tentative feedback mechanism may be suggested. At short residence times, both the TPID and radical concentrations are high. Most of the light is then absorbed by the radical and its dimer. Even though lophine is already present (as an impurity) at this stage, its concentration is low and remains low as long as the residence time is not long enough to allow a “sufficient” photoreaction time. As the residence time increases, the radical concentration decreases and more light becomes available to lophine. Photoproducts (P, Figure 1) then begin to be produced in significant amounts. Because of its radical quenching action, the photoproduct mixture further reduces the concentration of the TPI radicals. This overall decrease in the radical optical density then increases the fraction of the incoming light available to lophine, thereby accelerating the rate of production of quenchers. A positive feedback loop is thus present, and bistability becomes possible. A detailed study of the influence of various control parameters is now under progress in our laboratories (J.P.L., J.M.C.). Hopefully, these results will provide us with key elements of a more quantitative mechanism. It is nevertheless interesting to
J . Phys. Chem. 1988, 92, 18-27
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note that the TPID/chloroform photochemical system is a system involving two consecutive photochemical reactions. Such a scheme is similar, in its structure, to biphotonic photophysical processes. A simple photoionization kinetic scheme involving the successive absorution of two photons was studied some time ago by one of us.” It was then ihown that this type of system c&ldbecome bistable, under appropriate conditions.
Acknowledgment. J.P.L. acknowledges the financial support from CRAD Grant 3610-644:F4120. J.C.M. acknowledges a contribution from Centre National de la Recherche Scientifique, C N R S AIP 0693 1. (1 1) Micheau, J. C.; Boue, S.;VanderDonckt, E. J . Chem. SOC.,Faraday Trans. 2 1982, 78, 39.
FEATURE ARTICLE On the Dynamics of Abstraction, Insertion, and Addition-Elimination Reactions in the Gas Phaset J. J. Sloant National Research Council of Canada, 100 Sussex Drive, Ottawa, Canada K I A OR6, and Department of Chemistry, Carleton University, Ottawa, Canada iYlS 5B6 (Received: April 13, 1987; In Final Form: September I , 1987)
The dynamics of elementary abstraction, insertion, and addition-elimination reactions are discussed. For the case where the newly formed bond involves a light atom, special dynamical behavior occurs, and this is explored in depth. Following a brief outline of some early results on triatomic light-atom abstraction reactions, three examples of recent work, the F/HCO, F/NH3, and O(’D,)/H, reactions, are discussed. Although the mass combinations for the first two reactions are similar, their dynamics appear to be different. Furthermore, the dynamics of the third are dramatically different from those of the first two.
I. Introduction A fundamental advance in the study of physical chemistry occurred about 30 years ago with the introduction of two experimental techniques: molecular beam reactive scattering, which dates from 1954,’ and infrared chemiluminescence, introduced in 1958., The former permits the measurement of the kinematic and geometrical aspects of reactive collisions, and the latter gives the excitation in the products’ internal degrees of freedom. Together these techniques shifted the focus of physical chemistry from macroscopic kinetics and thermodynamics to the physics of chemical reactions at a molecular level, thereby creating the field of molecular reaction dynamics. This achievement was recognized by the award of the 1986 Nobel Prize in Chemistry to the founders and principal developers of the field, Professors J. C. Polanyi, D. R. Herschbach, and Y . T. Lee. In an exhaustive review of molecular beam reactive scattering published in 19663 it was reported that 20 reactions had been studied using that technique in the first decade of its existence. Less than half that number had been studied by infrared chemiluminescence at the time. A single 1979 review of molecular reaction dynamics: however, listed more than lo00 publications which had been generated during the field’s second decade. This explosive growth has placed the subject beyond comprehensive reportage in any but the most heroic format; therefore, the following discussion will deal with only that part of the field which is concerned with information drawn from measurements of internal energy distributions made by infrared emission spectroscopy. Since the results of current measurements are usually interpreted in terms of the simpler, triatomic reactions studied during Issued as NRCC No. 28300. *Present address: Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
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the development of the field, I shall first refer to a few of these historically significant measurements to provide a basis for the subsequent discussion. Following this, I shall present detailed descriptions of three problems that exemplify work presently under way. When J. C. Polanyi and co-workers introduced the technique of infrared chemiluminescence, the first result that they published2 was the unresolved HCl emission produced by the H + C1, reaction. Although not vibrationally resolved, this first emission spectrum demonstrated that chemical reactions are capable of releasing very large amounts of energy as product internal excitation. Later studies of this same reaction5 showed that it releases 37% of the total available energy as HCl vibration and causes the excitation of a broad distribution of vibrational levels, which extends up to the maximum energetically accessible level. Broadly similar results were found for the other H X2 (X = halogen atom) reactions as ell.^,^ All H + X2 reactions create total vibrational inversion. This refers to a vibrational distribution in which the population of at least one vibrational level, P(u), is greater than that of a lower level, P(u - n ) . (This may be distinguished from partial inversion, for which the vibrational excitation is greater than that appropriate to a Boltzmann distribution at the ambient temperature of the other degrees of freedom.’) Exploitation of vibrational inversion led to the chemically
+
(1) Bull, T. H.; moon, P. B. Discuss. Faraday SOC.1954, 17, 54. (2) Cashion, J. K.; Polanyi, J. C. J . Chem. Phys. 1958, 29, 455. (3) Herschbach, D. R. Adu. Chem. Phys. 1966, 10, 319. (4) Levy, M. R. Prog. React. Kinet. 1979, 10, 1 . (5) Anlauf, K. G.; Horne, D. S.; MacDonald, R. G.; Polanyi, J . C.; Woodall, K. B. J . Chem. Phys. 1962, 57, 1561. (6) (a) Polanyi, J. C.; Sloan, J. J. J . Chem. Phys. 1972, 57, 4988. (b) Jonathan, N . ; Okuda, S.;Timlin, D. Mol. Phys. 1972, 24. 1143. (c) Sung, J. P.;Malins, R. M.; Setser, D. W. J . Phys. Chem. 1979, 83, 1007. ( 7 ) Polanyi, J. C. J . Chem. Phys. 1961, 34, 347.
Published 1988 by the American Chemical Society
