J. Php. Chem. 1982, 86, 4759-4766
4759
Study of the Oscillatory Behavior in Irradiated 9,IO-Dimethylanthracene/Chloroform Solutions J. P. Lapiante’ and R. H. Pottier Deperfment of Chemlstty and Chemical Englneedng, Royal Mllltary college of Canada, Kingston, Ontario#Canada K7L 2W3 (Recelved: April 12, 1982; I n Final Form: June 25, 1982)
Upon local irradiation of dilute solutions of anthracene and 9,lO-dimethylanthracene (DMA) in chlorocarbon solvents, the fluorescence intensity can be observed to oscillate peripdidy with time (R. J. Bose and co-workers, J.Am. Chem. SOC.,99,6119 (1977)). The results of an investigation of the fluorescence oscillatory behavior in irradiated DMA/CHCls solutions are reported. The experimental conditions for which sustained oscillations can be observed are determined. Results are presented which illustrate a large variety of unusual kinetic behavior, such as single and multipeak oscillations, transitions from one type of oscillation to another, and transition between states of high and low average fluorescence intensity. It is also found that oscillations can sometimes be induced by physically perturbing the fluorescence cell. The fluorescence oscillations are shown to have a critical dependencyupon two types of variables: (1)the chemical or photochemical variables such as the excitation wavelength, the DMA concentration, and the nature of the solvent and (2) the physical variables such as the rate of stirring,the volume of solution, and also the fact that the cell has to be opened in order to observe sustained periodic oscillations. It is suggested that this critical dependency upon the cell not being stoppered in order to observe sustained oscillations can be related to the convection motion induced by solvent evaporation. The role played by the hydrodynamic motion in open cell systems is believed to be twofold. First, it determines the rate at which fresh DMA and 0 2 reactants are supplied to the irradiated portion of the solution and, second, it defines the thickness of the reaction zone stationary layer within which the instability giving rise to the oscillations is believed to be localized.
I. Introduction Oscillatory behavior in chemical reactions has received considerable attention in the past decade. Many examples of chemical and biochemical oscillators have been studied, both theoretically and experimentally.l In selected cases, remarkable agreement has been obtained between theoretical descriptions and experimental r e s u h 2 In the past 5 years, a few photochemical reactions have been reported to display unusual temporal oscillatory behavior. In 1976, Yamazaki and co-workers reported periodic oscillatory behavior in the photoreaction of 1,5napthyridine in cyclohexane solution^.^ Of particular interest was their observation that “...In the course of the photoreaction, the intensity of the fluorescence originating from the photoproduct(s) oscillates with a period of several minutes...”. Similar light-induced oscillatory phenomena were also reported in the emission from irradiated acetone solutions,4 in the absorbance of rhodamine B in solvents of low dielectric constants,5 and in the emission from irradiated chlorocarbon solutions of anthracene and 9,lOdimethylanthracene.6 Finally, oscillations have also been (1)Comprehensive reviews on oscillatory reactions can be found in (a) R. M. Noyes and R. J. Field, Acc. Chem. Res., 10,273,214 (1977); (b) D. 0.Cooke, Progr. React. Kinet., 8, 185 (1977); (c) B. F. Gray, ’Specialist Periodical Reports”, The Chemical Society, 1975,Chapter 8; (d) ‘Physical Chemistry of Oscillatory Phenomena”, Faraday Symp. Chem. Soc., 9 (1974); (e) P. H a n w e , J. Ross, and P. Ortoleva, Adu. Chem. Phys., 28,317 (1978);(f) G.Nicolie, T. Erneux, and M. Herschkowitz-Kaufman, Adu. Chem. Phys., 38,263 (1978). (2)This is especially true for the Belousov-Zhabotinskii reaction which is certainly the most well-known and best understood of all chemical oscillators;see (a) R. M. Noyes, J.Am. Chem. SOC.,102,4644(1980); (b) J. H. Tyson, J. Chem. Phys., 66,905(1977);(c) K.R. Graziani, J. L. Hudson, and R. A. Schmitz, Chem. Eng. J., 12,9(1976);(d) K. Wegmann and 0. E. Rossler, 2.Naturforsh. A , 33,1179(1978);Nature (London), 271,89 (1978). (3)I. Yamazaki,M.Fujita, and H. Baba, Photochem. Photobiol., 23, 69 (1976). (4)T.L. Nemzek and J. E. Gullet, J. Am. Chem. SOC.,98,1032(1976). (5)R. W. Bigelow, J . Phys. Chem., 81,88 (1977). 0022-3654/82/2086-4759$01.25/0
reported in the photostationary state sometimes attained when the peroxide of 9,lO-diphenylanthracene is simultaneously being photochemically produced and thermally destroyed.’ In all of these cases, a continuous light source is used to locally induce a photochemical reaction within a solution. The irreversible consumption of reactants is then accompanied by temporal oscillations in the radiation emitted from or absorbed by the system. Light is thus both the stimulus for, and the probe of, the oscillatory phenomena.8 The expression “photochemically driven oscillator$” (used in ref la) or simply “photochemical oscillators” seems appropriate to describe this class of oscillatory reactions. Compared to other types of oscillators, photochemical oscillators have received surprisingly little attention. That light can be used to induce or to influence chemical instabilities is, however, relatively well-documented.”12 In a pioneering series of papers, Ross and co-workers have examined selected mechanisms through which instabilities can be photoinduced in a system in which light is absorbed by one of the specie^.^ Light is also known to have an influence upon certain chemical oscillators. It has, for example, been reported that UV light can modify or suppess oscillations in the Belousov-Zhabotinskii (BZ) reaction.1° The effect of light on the Briggs-&usher oscillating (6)(a) R. J. Bose, J. Roes, and M. S. Wrighton, J.Am. Chem. Soc., 99, 6119 (1977);(b) see also P. H. Richter, I. Procaccia, and J. Ross, Adu. Chem. Phys., 43,217 (1980). (7)N. J. Turro (unpublished), reported in ref la. (8)This was first pointed out by Bose and co-workers in ref 6a. (9)(a) A. Nitzan and J. Ross, J. Chem. Phys., 59,241 (1973);(b) A. Nitzan, P. Ortoleva, and J. Roes, ibid., 60, 3134 (1974);(c) J. M.Deutch, S. Hudson, R. J. Ortoleva, and J. Ross, ibid., 57,4327(1972);(d) C.L. Creel and J. Ross, ibid., 65, 3779 (1976). (10)V. A.Vavilin, A. M. Zhabotiskii, and A. N. Zaikin, R u s . J. Chem., 42,1649 (1968). (11)P. De Kepper and W. Horsthemke, C.R. Acad. Sci. Paris, Ser. C,287,251 (1978)): (12)H.Tributach, ref Id, p 217.
0 1982 American Chemical Society
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provisions were made to ensure that the lamp housing exhaust blower and connecting hose did not touch the monochromators and cell holder. All chemicals were obtained from Aldrich Chemicals and used as received. Solvents used in this study were either Spectrograde or Analytical grade and were used without further purification (CHC1, contains up to 0.75% ethanol as a preservative). All experiments were carried out at room temperature. Full fluorescence spectra were also run on the Aminco-Bowmanspectrofluorometer. Absorption spectra were recorded on a Beckman Model DK-2A7double-beam, ratio-recording spectrophotometer.
Figure 1. Experimental configuration used in our fluorescence experiments. Slits a and b (w x h) = 3.0 mm X 5.0 mm.
reaction has also been noted." In addition, light has been used successfully in an oscillating electrochemical reaction to modulate the frequency of the electrode-current oscillations.12 Due to the paucity of experimental results and the lack of theoretical consideration given to photochemical oscillators, our understanding of the basic operative mechanisms is, however, still rather minimal.13 In order to better understand these mechanisms, we have decided to examine some of the features and characteristics of a given system. After having compared the various photochemical systems which have been reported to display oscillatory behavior?-' it was decided that the one reported by Bose and co-workers seemed the most promising. It was on this system that most of our efforts were concentrated. This system consists of a dilute solution of 9,lO-dimethylanthracene in chloroform (DMA/CHC13).
11. Experimental Section Fluorescence intensity measurements were carried out on an Aminco-Bowman emission spectrofluorometer. The design of this instrument is such that only a small portion of the solution is illuminated, the emission intensity being monitored at right angle to the incident beam (see Figure 1). The irradiated sample was always contained in a 1 X 1 x 4.5 cm (10 mm path length) Suprasil fluorescence cell. Excitation and emission slit widths (a and b in Figure 1) were fixed at 3 mm (16.8-nm band pass). The phototube entrance slit was fixed at 0.1 mm (0.55-nm band pass). The light source was a 150-W xenon arc lamp. our instrument was equipped with an off-axis ellipsoidal condensing system along with a varibale-excitation monochromator entrance slit. With this combination of source and lamp housing, the radiant power at the exit of the excitation monochromator was found to be typically of the order of 0.3 pW.cm-? at 260 nm and 60 pW.cm-2 at 377 nm (entrance slit = 3 mm). This corresponds to a photon flux of 4.0 X loll and 1.1X 1014photon.cm-2.s-1,respectively. Lamp fluctuations were minimized by the use of a magnetic arc stabilizer. Lamp stability was periodically checked with an NBS calibrated radiometer-photometer and also by looking at the stability of the fluorescence from a standard quinine sulfate solution. Great care was taken to suppress any mechanical vibrations in order to assure that the solution was undisturbed and free of external perturbation. To this end, (13) I t was indeed pointed out in ref l a that the oscillatory behavior reported by Turro7 could be accommodated to an "Oregonator" type of mechanism involving singlet oxygen as an intermediate. Unfortuantely, the argument was not further substantiated.
111. Results A . The Oscillations. The fluorescence oscillations depicted in Figure 2 were all obtained upon continuous irradiation of undisturbed DMA/ CHCl:, solutions under the following set of experimental conditions (hereafter referred to as set A): (a) excitation wavelength, A, = 260 nm; (b) fluorescence monitoring wavelength, A, = 410 nm; (c) DMA concentration, [DMA], = 4 X M; (d) slit widths as given in the Experimental Section; (e) room temperature (21-24 "C), in Figure 2h, room temperature was 18 "C; (f) nonoutgassed solutions; (g) nonstoppered solutions; (h) volume of sample, 3.00 =F 0.02 cm3. Although fluorescence oscillations can be observed for other experimental conditions, we have found set A to be the most "productive" of various types of periodic oscillations. For most of the experiments conducted under such conditions, the time dependency of the measured fluorescence intensity was found to go through two distinct phases: a preperiodic phase and a periodic phase. As a result of the photoreaction taking place (see Figure 3), this evolution is accompanied by a decline in the average value of the fluorescence intensity. 1. The Preperiodic Phase. The fluorescence oscillations were found to begin immediately after the irradiation is initiated (time = 0). These oscillations, however, were usually chaotic for the first few minutes after the start of the irradiation. This chaotic phase of the reaction, which we call preperiodic (or aperiodic6),was of variable duration and totally irreproducible from one experiment to the next. Sometimes, periodicity would emerge after only a short period of time (a few minutes) whereas, for other "identical" experiments, the fluorescence oscillations would remain chaotic for the whole photoreaction time (-60 min). At the other extreme, periodic oscillations were occasionally observed immediately after the irradiation was inititated (see Figure 2b). We believe that this last observation is important since if the emergence of periodic oscillations depends on the buildup of some intermediate(s), then they must be available very soon in the photoreaction. Typically, however, nonperiodicity was found to be the rule for the first few minutes. Periodic oscillations were very rarely observed to emerge in the final stage of the photoreaction. In other words, if periodicity was not achieved within the first 10-15 min, it was in general not observed later in that same experiment. In addition to oscillatory behavior, a sudden decrease in the average fluorescence intensity was sometimes noted during the irradiation. A typical example of such a behavior is provided in Figure 2c. As seen in this result, the average fluorescence intensity is definitely higher in the preperiodic phase than in the periodic phase of the reaction. This suggests that the transition from the aperiodic to the periodic oscillations can sometimes occur via a transition between two states in which the average fluorescence intensity assumes different values. In this particular example, however, it should be noted that the double peak
The Journal of Physical Chemistry, Vol. 86,No. 24, 7982 4781
Oscillatory Behavior of DMAICHCI, Solution
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Flgue 3. Absorption spectra of 4 X lo4 M DMA/CHCi3solutions: (a) before Irradiation (solid Ilne); (b) after 60 mln of Irradiation at 260 nm (- --): (c) after 90 mln of lrradiatlon at 377 nm (-): (d) after 5 h of Irradiation at 260 nm of an outgassed solution (. e). (All the lrradiations were carried out In the Amlnco-Bowman spectrofluorometer.)
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oscillation is already well-defined in the time preceeding the decay to the state of lower fluorescence. This observation would therefore seem to suggest as well that the two phenomena (Le., emergence of periodic oscillations and transition between these two states) can effectively occur independently. Many periodic oscillations were in fact observed to emerge smoothly, without experiencing this transition. On the other hand, the following observation is to be underlined: In most of the cases where this transition has been observed, the emergence of periodic oscillations has occurred immediately after the transition to the state of lower fluorescence. Concerning the emergence of periodic oscillations, a rather peculiar phenomenon waa observed in many experiments, i.e., if after a few minutes of irradiation, the oscillations were still more or less chaotic, the periodicity could sometimes be made to emerge by (physically) perturbing the solution (e.g., by gently tapping the cell a few times). This unusual behavior is illustrated in Figure 2d.
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The Journal of Physical Chemistry, Vol. 86,No. 24, 1982
An arrow indicates the time at which the solution was perturbed. This perturbation fmt causes the fluorescence intensity to increase; it then later decreases with periodic oscillations of larger and larger amplitude. The periodic oscillations then become stable within a few minutes. Interestingly enough, this behavior is similar to the one depicted in Figure 2c, where the solution had by no means been physically perturbed. Figure 2e also illustrates another result where the “perturbation method” (!) was used 6 min). Finally, the result successfully (see arrow at depicted in Figure 2j is, to say the least, intriguing. In that particular experiment, the oscillations were still very irregular after -10 mih (not shown). Then within 1min, the oscillations gradually stopped and a monotonic slow decline of the fluorescence intensity was observed. At time 15 min, the cell was perturbed (see arrow). Violent oscillations appeared immediately and, within a few minutes, the periodicity was emerging as large amplitude, periodic oscillations. The fluorescence behavior following the perturbation is here again very similar to the one already described for Figure 2d, i.e., an abrupt increase of Iffollowed by a decline to a stable periodic oscillation (this is easily seen if one examines the average value of If as a function of time). The preperiodic phase can certainly be thought of as being similar to the induction time observed for the BZ reaction and other oscillating reactions. For the BZ reaction, it is also known that log (l/induction time) is inversely proportional to the temperature.I4 Our results, however, do not show any relationship between the temperature (or any of the experimental variables considered in this study) and the duration of the preperiodic phase. 2. The Periodic Phase. The periodic phase is characterized by periodic temporal oscillations of the fluorescence intensity and, thus, by a high degree of order. As previously mentioned, this periodic phase is usually preceeded by a preperiodic phase of variable duration. Once the periodicity is fully developed, the evolution remains periodic until the photoreaction is over. The DMA/CHClB system was never observed to come back to a chaotic behavior unless seriously perturbed. Several types of periodic oscillations have been observed. The simplest types observed in this study, quasi-sinusoidal in shape, are illustrated in Figure 2, b and d. Among other types of single-peak oscillations, “pulses” such as those depicted in Figure 2a were also frequently observed. The oscillation amplitude was typically found to be of the order of 710% of the total fluorescence intensity but has reached ~ 4 0 %in some instances (see Figure 2j). In most cases, this amplitude decreases monotonously as the photoreaction proceeds (see Figure 2a). Intriguing oscillations in which the amplitude is itself oscillating smoothly were observed in many of our experiments. Such a behavior is illustrated in the oscillations depicted in Figure 2f. A low frequency wave, superimposed on the higher frequency oscillations, is clearly modulating the amplitude of the higher frequency oscillations. Whenever observed, the period of this amplitude modulating oscillation was typically found to be of the order of 90-120 s. Multipeak oscillations were also obtained quite frequently. In their most elementary form, the first harmonic accompanies the fundamental frequency. Typical examples are provided in Figure 2, c, e, and g. As illustrated in Figure 2e, one of the peaks was sometimes observed to gradually decrease in amplitude and almost vanish after a certain time (note that this does not affect the fundamental oscillation frequency). Interestingly, this evolution was sometimes ob-
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(14) V. J. Farage and D. Janjic, Helo. Chen. Acta, 63,433 (1980).
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served to develop in the opposite direction, i.e., from a single- to a double-peak oscillation. It should be pointed out that not all double-peak oscillations were “short-lived transients”. The periodic oscillation which is seen to emerge in Figure 2c, for instance, remained virtually unaltered for more than 30 min (see also Figure 2g). Intriguing and still more complex composed oscillations are shown in Figure 2, i and h. For most of the oscillations recorded, the period of oscillation was usually in the range 11-16 s. Oscillations with larger periods were, however, occasionally observed (e.g., Figure 2h). 3. Reproducibility of the Periodic Oscillations. Although nonperiodic oscillatory behavior could always be induced quite readily, we have found that periodic oscillations were not that easily induced and/or reproduced. In fact, most of our earlier attemps were rather frustrating. Later, it was realized that the reproducibility of the periodic regime critically depends upon the care being taken in reproducing all of the experimental conditions. These conditions include not only the “classical” variables such as the excitation wavelength, the DMA concentration, etc... (which are easy to control) but also some usually overlooked experimental variables such as the volume of sample, the time for which the sample is in the cell holder prior to irradiation, etc. A very careful control of all these parameters has allowed us to reproduce in two (and in some cases three) consecutive experiments some of the oscillations depicted in Figure 2. Unfortunately, a 100% reproducibility was not achieved. The frequent observation of patterns of the same family (Figure 2, c and g, for instance) and a (partial) reproduction of very complex oscillations such as the one depicted in Figure 2i is most encouraging at this stage of the work. B. Dependency on Selected Experimental Variables. 1. Excitation Wavelength. Experiments were conducted at several excitation wavelengths in the range 245-400 nm. In agreement with earlier observations,6 it was found that truly periodic oscillations can only be induced by excitation at wavelengths shorter than -290 nm, the maximum amplitude being obtained at A,, 260 nm. Upon moving away from 260 nm (within the range 245-290 nm), the oscillation amplitude is reduced but the oscillatory pattern remains the same. The same effect is observed upon decreasing the incident light flux. Although irradiation at longer wavelengths was found to induce a photoreaction as well (see Figure 3), no periodic oscillatory behavior was observed under such conditions. Instead, we observed small-amplitude, irregular fluctuations superimposed onto an otherwise monotonous decay of the fluorescence intensity. It might be of interest to point out that, although the photon flux is -200 times larger at 377 nm than at 260 nm (see Experimental Section), the total photoreaction time is very similar for the two excitation wavelengths, i.e., -60 min. Upon completion of the photoreaction, the solutions are in both cases slightly yellowish and no longer fluorescent. The absorption spectrum of the mixture of the resulting photoproducts then reveals a loss of the anthracenic structure as well as a new absorption band in the 275-500-nm range (see Figure 3). Although qualitatively similar, the spectrum obtained upon the photolysis of a DMA/CHCI, solution at 377 nm is still significantly different from the one obtained at 260 nm. In addition, none of the spectra recorded at various times during the photoreactions at 260 and 377 nm were found to show a close quantitative correspondance. A preliminary study of the nature of the photoproducts has also revealed a dependency upon the DMA concentration, as well as on the nature of the solvent and on the concentration of oxygen
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Oscillatory Behavior of DMA/CHCI, Solution I
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wavelength(nm) Flgure 4. Fluorescence spectra recorded at various times during the irradiation of a 4 X lo4 M DMA/CHCi, solution at 260 nm (set A). The numbers on each curve refer to the following Irradiation tlmes: (0) before irradiation; (1) 2 min; (2) 5 min; (3) 7 min; (4) 11 min; (5) 15 min; (6) 20 min; (7) 27 min.
in solution. In outgassed solutions (six freeze-pumpthaw cycles), the photoreaction is extremely slow and no oscillations are observed. Even after prolonged irradiation, most of the anthracenic structure is still present in the photoproducts absorption spectrum (see Figure 3). As well, some fluorescence persists even after prolonged irradiation of outgassed solutions. 2. Monitoring Wavelength. In most of our experiments, the fluorescence monitoring wavelength was fixed at 410 nm. This wavelength corresponds to a (near) maximum in the DMA fluorescence spectrum (see Figure 4). The observation that the oscillatory phenomenon is, to some extent, independent of the wavelength at which the fluorescence is monitored has previously been noted by Bose and co-workers! Similar oscillations can therefore be observed at different monitoring wavelengths. We have also found that the amplitude of the oscillations at a given monitoring wavelength closely parallels the corresponding fluorescence intensity at that wavelength. In order to better characterize this fluorescent emission, a series of experiments were carried out in which the whole emission spectrum was repeatedly scanned under the conditions for which short period oscillations are usually observed (Le., set A). Due to the relatively long time necessary for scanning the whole emission spectrum (approximately 20 s), it was not possible to follow the variation in fluorescence intensity of the whole spectrum during the course of a single oscillation (approximately 15 s). It might be pointed out however that, in none of these scans, have we obtained spectra significantly different from the DMA fluorescence spectrum. Although the relative intensities of the main fluorescence bands were often not constant with time (which is to be expected since the whole spectrum oscillates), the initial DMA fluorescence spectrum was always observed to retain its overall shape (see Figure 4). In particular, no new fluorescence band(s) were observed to emerge as the photoreaction proceeds. 3. DMA Concentration. The fluorescence oscillations were found to be critically dependent upon the DMA concentration. Although oscillations were recorded for all concentrations within the range M 5 [DMA] S M, the emergence of truly periodic and sustained oscillations was found to be generally restricted to the concenM. At 8 X M 5 [DMA] I6 x tration range 2 X M, oscillations were of much smaller amplitude and often irregular and chaotic. Beyond lo4 M, no oscillations were observed and the fluorescence intensity was found
to decay monotonously with time. Below M, the photoreaction was rapid, and regular oscillations were rarely obtained. 4 . Solvent. In an attempt to study the fluorescence oscillations of DMA in other solvents, a number of experiments were carried out in CCll and CH2Clp Although oscillatory behavior was observed in both solvents, these oscillations were found to be significantly less regular than the ones usually observed in CHC13under similar conditions. In a few experiments, ethanol (preservative) was removed from CHC13by a standard method.15 The irradiation of solutions prepared with this purified CHC1, produced results similar to those in which unpurified CHC1, had been used. A few trial experiments were also carried out in nonchlorinated solvents such as benzene, toluene, and cyclohexane. In all of these solvents, the photoreaction was much slower than in chloroform and no oscillations or fluctuations were observed. 5. Temperature. All experiments were carried out at room temperature and no deliberate attempt was made to specifically study the effect of temperature on the oscillatory behavior. The actual room temperature was, however, noted for most experiments. Since it varied significantly over the period in which our experiments were conducted, the following comment on the effect of temperature can be made: At relatively low room temperatures (16-18 "C), short-period regular oscillations such as those depicted in most of the traces of Figure 2 were very rarely observed. On the other hand, longer period ("smoother") oscillations such as those illustrated in Figure 2h, were much more frequent. Within the range 20-24 "C, most of the periodic oscillations were pulselike and much more violent than the lower temperature ones. In none of the experiments done within this temperature range were oscillations having periods larger than 25 s or less than e11 s observed. 6 . Effect of Stirring. In order to investigate the effect of stirring on the oscillatory behavior, the following simple stirring device was used: a 2-mm glass rod connected to a variable-speed motor was positioned over the solution such that the rod immersed into the liquid at a level just above the irradiated portion of the solution (e1cm from the bottom of the cell). Even at very low stirring rates, this simple device was efficient enough to destroy any periodicity in the oscillatory behavior. It must, however, be pointed out that as long as the stirring rate remained low, small amplitude irregular fluctuations were still observed. As the stirring rate was increased, their amplitude decreased rapidly and a nonfluctuating montonous behavior was obtained at moderate stirring rates. 7. Other Anthracene Derivatives. Some experiments were carried out with nonsubstituted anthracene (for which oscillations were also reporteds) and some of its alkyl and aryl derivatives (9-methyl, 9-phenyl-, and 9,lO-diphenylanthracene). Although oscillatory behavior was observed for all of these species, these oscillations wre found to be much less regular than the ones usually observed with DMA. The solvent used in these experiments was CHC1,. 8. Fluorescence Behavior: Stoppered us. Nonstoppered Solutions. Quite unexpectedly, the fluorescence behavior from irradiated stoppered solutions was found to be very different from the ones usually observed upon irradiation of nonstoppered (otherwise identical) solutions under the same conditions. While relatively short-period oscillatory behavior was found to be the rule for open cells (see Figure 2), monotonous, although nonregular, evolution was gen(15) A. I. Vogel, "Practical Organic Chemistry", Longmans, Green and Co., London, 1967, p 176.
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Flgure 5. (a, b) Fluorescence intensty vs. time pbts obtained upon irradiationof 4 X lo3 M stqoperedDMA/CHCI, solutions at 260 nm. Experimental conditions otherwise as given in section I I I . A , set A. (c) Fluorescence spectra recorded at various times during an experiment in which oscillations similar to 7b were observed: (0)initial DMA fluorescence spectrum; (1) 4 min; (2) 8 min; (3) 12 min; (4) 16 min; (5) 20 min; (6) 24 min.
erally observed for stoppered ones. Although Bose et have reported the use of stoppered solutions in their experiments, we have been unable to induce any truly sustained periodic behavior under these conditions. Instead, results such as depicted in Figure 5a were typically obtained: the fluorescence intensity increases and then decreases abruptly, remains constant for some time, and then declines quite monotonously with time. In a few experiments in which stoppered solutions were used, irregular low-frequency oscillations such as those depicted in Figure 5b were, however, sometimes observed. Figure 5c shows a set of fluorescence spectra recorded at various times during the course of an experiment that produced such an oscillatory pattern. The entire fluorescence spectrum is seen to oscillate at a unique low frequency. The spectrum is also seen to remain virtually identical with the initial DMA fluorescence spectrum. It was suspected that this critical dependency of the cell being open in order to produce short-period oscillations could be related to the internal fluid motion of the solvent.16 This fluid motion known as "evaporative convection" is induced by solvent evaporation." The surface cooling due to evaporation causes the density of the liquid near the surface to become greater than that of the bulk liquid beneath. An adverse temperature gradient is thereby created in the fluid. The fluid column thus becomes hydrodynamically unstable and buoyancy forces then induce convection. The theory of evaporative convection has been the subject of some interest in the past (16) I t may be of interest to point out an observation reported by Bigelow? "...In one notable exception, an open cell was necessary to induce fluctuations in a solution which otherwise exhibited an approximate logarithmic absorbance/time decay curve at 5600 A,..(17) J. C. Berg, A. Acrivos, and M. Boudart, Adu. Chem. Eng., 6 , 61 (1966), and references therein.
Flgure 6. Typical cross section of a convection current observed upon irradiation of a DMAICHCI, solution with a He/Ne laser,
years.l8 Unfortunately, the problem of convection in deep layers of fluids has received little attention. One would not, however, expect this problem to differ significantly from the classical Benard problem, i.e., convection in a fluid heated from below. In this case, the convection current usually takes the form of two- or three-dimensional convection Although we did not carry out any theoretical study of the evaporative convection current present in our solutions, we were able to obtain experimental evidence with regards to its existence and morphology. To this end, an attenuated beam from a 50-mW He/Ne laser was passed through a DMA/CHCl, solution contained in a fluorescence cell. By an examination of the light scattered from suspended microdust particles, at various locations in the fluid column, a three-dimensional picture of the convection flow could be obtained. The convection current morphology (18) G. P. Quinn and D. A. Saville, Lett. Heat Mass Transfer,3, 309 (1976). (19) C. Normand, Y. Pomeau, and M. G. Velarde, Rev. Mod. Phys., 49, 581 (1977), and references therein.
Oscillatory Behavior of DMA/CHCI, Solution
was usually found to assume a cylindrical symmetry and a schematic representation of a typical current cross section is given in Figure 6. Properties such as convection velocity and number of rolls were found to have a critical dependency upon the evaporation rate and the depth of the fluid layer (as in the Benard problem). The convection velocity could be momentarily increased by gently blowing over an open solution, or stopped by merely closing the cell. Smaller external perturbations, such as mechanical vibrations, made it very difficult for the hydrodyamic motion to reach or to sustain a steady convection regime. Also, the steady-state convection velocity was observed to be much larger at the center of the cell than near the cell walls. While relatively immobile in the neighborhood of the walls, the fluid at the center of the cell was typically moving, in the convection regime, with a velocity of 3-5 mm/s. No convection motion was observed if only a small volume of solution was being used, e.g., 1 cm3 in a 1 X 1 x 4.5 cm cell. It is of significance to note that each time the experimental conditions were such that a convection motion was most likely present in the solution, short period fluorescence oscillations could be observed. An examination of Figure 7a shows unequivocally the effect of closing and reopening the cell on the fluorescence oscillations. Figure 7b also shows the effect on the fluorescence oscillations when the volume of the DMA/CHC13 solution is too small to sustain a convection motion. This is to be compared with typical oscillations observed under identical conditions for a larger volume of solution (e.g., set A, Figure 2).
IV. Discussion The DMA/CHCl, system has been shown to display sustained fluorescence oscillations upon excitation by UV light (A 5 290 nm).6 Our results not only confirm these findings but also reveal a much greater diversity than previously suspected. The results depicted in Figure 2 illustrate a wide variety of unusual kinetic behavior such as single- and multipeak oscillations, transitions from one type of oscillation to another, “perturbation-induced” oscillations, transitions between states of high and low average fluorescence intensity, etc. The fluorescence oscillations were also shown to have a critical dependency upon two types of variables: (i) the chemical or photochemical variables such as the excitation wavelength, the DMA concentration, and the nature of the solvent, and (ii) the physical variables such as the rate of stirring and the hydrodynamic motion induced by solvent evaporation. The study of the DMA/CHC13 oscillator is still in its infancy stage and it must be emphasized that any discussion must, at this stage, necessarily be qualitative in nature. Factors that would be of potential relevance to a future understanding of this system may, however, be pointed out. Upon irradiation of DMA/CHC13 solutions at 260 nm, the incident photons can be absorbed by any of the following species: DMA, CHC13, and, as the reaction proceeds, by some of the photoproducts. Although the photophysical properties of the photoproducts are still unknown, it seem reasonable to assume that, because of its very high molar absorptivity at 260 nm = 1.85 X lo5 L.m-1-cm-1),20DMA is likely to absorb a very significant portion of those incoming photons. It is readily shown that upon passing through a 1-cm path length in a (nonreacting) 4X M DMA/CHC13 solution, 99% of the 260-nm photons would be absorbed by DMA. Since the DMA fluorescence quantum yield is also very large (in outgassed (20) J. B. Birks, ‘Photophysics of Aromatic Molecules”, Wiley, New York, 1970.
The Journal of Physical Chemistry, Vol. 86, No. 24, 7982 4765
ethanol solution, cjf = 0.8920),it is therefore likely that the observed fluorescence intensity originates predominantly from the first excited singlet state of DMA. The fluorescence spectra of the irradiated solutions tend to confirm this. In no experiments have we obtained fluorescence spectra which were significantly different from the initial DMA fluorescence spectrum. Upon irradiation of a DMA/CHC13 solution with UV photons, a complex sequence of photochemical reactions is likely taking place and the fluorescence oscillations are undoubtedly related to chemical reactions that are photoinduced within the irradiated portion of the solution. The primary reactants are suspected to be DMA, CHCl,, and O2 (at normal room temperature, dissolved oxygen is present in organic solvents at a concentration of approximately M). That oxygen is part of the photochemistry involved in the oscillations is supported by our result showing that, in outgassed solutions, the photoreaction is orders of magnitude slower than in air-saturated ones. Irradiation of oxygen-containing DMA solutions at wavelengths shorter than -400 nm is known to give rise to DMA photoperoxidation.21 The DMA endoperoxide is, however, light sensitive and can undergo further photochemistry under appropriate conditions.22 Upon irradiation at 260 nm, CHC13 photolysis can also occur and produce reactive chloromethyl radicals.23 This type of radical is known to react quickly with DMA at room temperature to give DMA chlorinated derivative^.^^ Due to its triplet ground state, molecular oxygen also readily adds to free radicals, to give relatively stable peroxy radicals.25 Although all of the above photoreactions are well documented in systems similar to ours, the relative importance of the various reactions in the DMA/CHC13 system is still to be determined. It is to be noted, however, that CHC13 photolysis needs only be considered for excitation wavelengths shorter than 290 nm.= At first pointed out by Bose et al.6 and confirmed in the present study, this is precisely the upper limit of excitation wavelength beyond which no sustained oscillations are observed. This would seem to indicate that solvent photolysis is indeed of key importance in the mechanism which is responsible for the fluorescence oscillations. The experimental observation that sustained oscillations can only be obtained when the experimental conditions are such that a convection motion is present within the solution deserves special consideration. The role played by the hydrodynamic motion in open cell systems is believed to be twofold. First, it controls the rate at which fresh DMA and O2reactants are supplied to the irradiated portion of the solution. Second, it defines the thickness of the reaction zone stationary layer. Because of the convection motion, only a thin portion of the solution can be considered as stagnant in open cell systems. A crude estimate of the thickness of this layer can be obtained if one assumes a parabolic radial velocity profile within the convecting solution.26 Since the convection velocity (21) B. Stevens, K. L. Marsh, and J. A. Baltrop, J. Phys. Chem., 85, 3049 (1981), and references therein. (22) That overirradiation of the DMA endoperoxide in quartz vessels can lead to ita decomposition was first noted by Southern and Waters (J. Chem. SOC., 4340 (1960)). More recently, it was suggested that this decomposition essentially takes place via 0-0 bond homolysis (J. Rigaudy, M. Moreau, and N. K. Cuong, C. R. Acad. Sei. Paris, ser. C, 274, 1589 (1972)). (23) J. G. Calvert and J. N. Pitta, “Photochemistry”, Wiley, New York, 1967. (24) J. D. Unruh and G. J. Gleicher, J. Am. Chem. Soc., 93, 2008 (1971). (25) J. A. Howard, Adu. Free Radical Chem., 4, 49 (1972). (26) R. B. Bird, W. E. Stewart, and E. N. Lightfoot, “Transport Phenomena”, Wiley, New York, 1960.
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Laplante and Pottier
b. [lee I
\A, -
trmecmin)
measured at the center of the cell was typically of the order of 5 mm/s, it is then readily shown that a layer 0.1 mm from the cell surface would move with a velocity of 0.2 mm/s. With a slit height of 5 mm, a point 0.1 mm away from the cell wall would therefore spend 25 s in the incident light beam. consequently, it seems reasonable to assume that a 0.1" thick layer of solution in the vicinity of the cell walls is essentially stationary with respect to both the incident light beam and the convecting bulk of solution. T h e instability giving rise to the oscillations in open cell systems is therefore believed to be localized within a thin stationary layer of solution which clings to the inner surfaces of the cell. In open cell systems, this thin layer would be the reaction zone. It should be noted that the absorption by the DMA present within such a thin layer can account for a significant portion of the total light absorption by DMA. When Beer's law is used, a simple calculation readily shows that, upon irradiation of a 4 X M (nonreacting) DMA/ CHC1, solution at 260 nm, 16% of the incoming photons are absorbed within the first 0.1 mm of solution.27 The fluorescence which originates from this thin layer is, however, unlikely to contribute very significantly to the measured fluorescence intensity. This is because this layer is far removed from the point at which the fluorescence intensity is monitored, i.e., at slit b (see Figure 1). On the other hand, it can be shown that a total depletion of DMA within a 0.1-mm reaction zone, where its concentration is initially 4 X lod M, would result in a 19% increase of the amount of photons available for excitation purposes at slit b. As a consequence, the measured fluorescence intensity would be found to increase by 19% upon such a depletion (27) A question raised by one of the referees was whether or not thermal convection could result from local warming by absorbed radiation. It was pointed out that this effect should be largest in what we believe to be the reaction zone in open cell systems. Although such thermal effects cannot be entirely ruled out at this stage, the following arguments would seem to indicate that such effects are expected to be very small. Local heating would come from two major sources: first, heating via absorption of IR radiation and, second, heating due to radiationless deactivation following DMA and/or solvent absorption. The first possibility can be ruled out since no change in the oscillatory behavior was observed upon placing a 2-cm path length water jacket between the incoming light beam and the DMA solution. The influence of the DMA radiationlea deactivation on the temperature of a 0.1-mm thick layer of solution can be calculated to be of the order of "C,for set A of experimental conditions. This would be too small a temperature difference to induce any thermal convection. One cannot entirely rule out, however, the possibility of a lens effect at the front surface of the cell, which would modify the above estimate.
5
I
process. If one tentatively assumes that the amplitude of the fluorescence oscillations is closely related to the amount of DMA consumed in each cycle of the reactions giving rise to the oscillations, the 19% figure obtained above is then seen to be in qualitative agreement with the typical oscillation amplitude observed in open cell systems, i.e., ~ 1 0 %It. should be noted that the hydrodynamic motion determines simultaneously both the reactant feed rate and the thickness of the reaction zone stationary layer. These variables are therefore not mutually independent. An increase in the convection velocity (or the stirring speed) will induce both an increase in the reactant feed rate and a decrease in the thickness of the reaction zone stationary layer. The interdependency of these two variables would, unfortunately, preclude their independent study. In nonconvecting solutions (closed cells of small volume of solution), the whole irradiated volume is relatively stagnant and the slow molecular diffusion is the only transport mechanism available for mass transfer. No such layer as defined for convecting solutions would exist. The boundary definition of the reaction zone in this case is likely to be much larger and probably includes m.ost of the bulk of the irradiated volume. The observation that the emergence of periodic oscillations could sometimes be initiated by perturbing the cell containing the irradiated solution poses certain unanswered questions. The mechanism through which such a perturbation would initiate periodicity is certainly not clear. A possibly significant observation is that the onset of the convection motion is not always spontaneous in open cells. This would seem to indicate that some sort of activation barrier must be overcome in order to have convection. If such is the case, it can be thought that the sudden increase in the evaporation rate which results from such a perturbation can activate the convection motion. An alternative hypothesis which might also be retained is that such a perturbation actually triggers some (yet unidentified) key phenomenon in the reaction zone. Further work will hopefully clarify the origin of this effect, a clear understanding of which, could possibly provide significant cltles toward the elucidation of the mechanism responsible fcr the fluorescence oscillations in open-cell systems. Acknowledgment. We acknowledge stimulating discussion with members of the Department of Chemistry and Chemical Engineering of RMC, in particular Drs. M. J. B. Evans, J. C. Amphlett, and S. S. Barton. This work was supported by CRAD Grant No. 3610-644.
