THELASERPHOTOLYSIS OF METHYLENE BLUE
energy of the competing hydration equilibrium. The observed increase of the association constants with decreasing water content indicates that bromide ions displace nitrate ions from the coordination sphere of cadmium ions more readily than they displace water
2633
molecules and is a measure of the tendency of the cadmium ions to become hydrated. In a subsequent paper, we shall present a statistical interpretation of the competing hydration and association equilibria of cadmium ions in aqueous molten salts.
The Laser Photolysis of Methylene Blue
by Robert M. Danziger, Kedma H. Bar-Eli, and Karl Weiss Photochemistry and Spectroacopy Laboratory, Department of Chemistry, Northeastern University, Boston, Massachusetts 021 16 (Received Janiulry SO, 1967)
~
~~
The giant pulse ruby laser flash photolysis of plain aqueous methylene blue solutions reveals three transient species (A, B, and C) with half-lives of -2, 30, and 140-psec, respectively. With a 5.5 X M dye solution a 0.5-joule, 30-nsec pulse causes almost complete conversion into transients. The photochemical change is completely reversible. M indicate that Experiments a t various concentrations in the range 5.5-294 X transients A and C are derived from the dimeric form of the dye, which exists in equilibrium with the monomer. It is proposed that transients A and B are the triplet states of the dimer and monomer, respectively, and that transient A decays primarily into the longer lived transient C, which is viewed as a charge-transfer state of the dimer. The creation of C by reaction of the monomer triplet (B) with the ground-state monomer is estimated to be slower than diffusion controlled. The establishment of the groundstate monomer-dimer equilibrium appears to be slower than all the transient decay processes. The results obtained by laser photolysis and conventional flash photolysis are compared.
Introduction The pulsed ruby laser constitutes an ideal flash photolysis source. The output is strictly monochromatic at 6493 A and with &-spoiling techniques flash durations sec are readily achieved.’ These properof 30 X ties have obvious advantages for the study of very shortlived transients.2 I n this paper we report the giant pulse laser flash photolysis of aqueous methylene blue solutiohs. The dye shows strong absorption in the 5000-7000-A region, which encompasses the ruby laser emission line (eaara -1 X lo4 in water). Extensive previous photochemical studies indicate that methylene blue is readily p h o t o r e d u ~ e d ~and - ~ conventional flash photolysis has revealed two transient
It was not anticipated that the dye would show markedly different behavior with laser excitation, Rather, this study was designed to develop the tech(1) B. A. Lengyel, “Introduction t o Laser Physics,” John Wiley and Sons, Inc., New York, N. Y., 1966. (2) Chemical applications of lasers, actual and potential, have recently been reviewed by D. L. Rousseau, J. Chem. Educ., 43, 566 (1966). (3) M. Koizumi, H. Obata, and 5. Hayashi, Bull. Chem. SOC. Japan. 37, 108 (1964),and previous papers cited. (4) G. Oster and N. Wotherspoon, J . Am. Chem. Soc.. 79, 4836 (1957). (5) C. A. Parker, J . Phys. Chem., 63, 26 (1959). (6) S.Kato, M. Morita, and M. Koizumi, B d . Chem. SOC..lapan, 37, 117 (1964). (7) S. Matsumoto, ibid., 37, 491 (1964).
Volzime 7 1 , Surnlrer 8 July 1967
2634
nique of laser flash photolysis and to make measurements in the time range not accessible by conventional means.
Experimental Section Methylene Blue. The dye (Fisher Certified reagent, 88-90% dye content) was purified by the method of Bergmann and O'Konski.* The ratio of the optical densities a t 6650 and 62200 A was 2.0, indicative of the absence of demethylated forms. Concentrations were computed to *2% from the reported8 extinction coefficients. The transient characteristics of the unpurified dye were quite similar to those of the purified dye, although the decay rates were faster. Apparatus and Procedure. The photolysis apparatus is shown diagramatically in Figure 1. Central to its operation is a Maser Optics, Inc., Model 869 watercooled laser head which accommodates a 3/&n. diameter, 6.5-in. long ruby rod. Energy for the two flash tubes is supplied by a circuit consisting of an 8.5-kv power supply, a 1560-pf capacitor bank, and a 400-phenry inductance. The triggering mode is internal by means of an Atlas Engineering Co. transformer No. 7831, which amplifies an 800-v trigger pulse to 2.4 kv. Q spoiling is accomplished with a methanolic solution of cryptocyanine.9 Single 30-nsec pulses are obtained by adjusting the concentration of the cryptocyanine solution and the input voltage in the range 2.22.9 kv. The energy is 0.5 joule (*5%)/pulse, as measured calorimetrically, corresponding to ca. lo7 w. The operation of the unit is extremely reproducible. Even the scattered laser light is extremely intense and its elimination a t the detection end is critically dependent on the placement of the components. It was possible to achieve freedom from stray light below 6500 A. The laser beam makes an angle of ca. 15" to a monitoring beam which originates from a General Electric Type CPR 108-W battery-operated tungsten projection lamp (LS) and terminates in a Bausch and Lomb 250-mm grating monochromator. Masks, which have to be carefully placed so as not to be hit by the ca. 12-mrn diameter laser beam, serve to confine the monitoring light to the irradiated portion of the reaction cell. Firing of the photolysis unit is accomplished as follows. A wave form and pulse generator (Tektronix Types 162 and 161) actuate the 800-v trigger pulse generator, which causes the flash lamps to fire. The flash duration is ca. 2 msec. Scattered light from the Q-spoiled laser flash, which is produced during this interval, is registered on photomultiplier PM 1 (RCA 1P28, emitter follower). The rising current pulse in this tube triggers a Fairchild 777 dual beam oscilloThe Journal of Physical Chemistry
R. 14. DANZIGER, K. H. BAR-ELI,AND K. WEISS
IMONO-
4 REACTION CELL
I
I
)o)
TRIGGER FROM PMI
J
U OSClLLOSCOPE
Figure 1. Laser flash photolysis apparatus.
scope which displays the laser spike and the transient absorption changes. The latter are registered on photomultiplier Phl 2 (RCA 1P28, EM1 6256B, or DuMont 6911, and cathode follower). After a 1-sec delay, the absorption display is triggered again to provide a base line. Tests established that the position of this trace is the same as before lasing. The time resolution is ca. 2 psec for PM 2 and ca. 0.5 psec for PM 1. The laser pulse is thus partially integrated, but it was shown that the maximum pulse height remains proportional to the energy. The quartz reaction cells are cylindrical with a 2.5cm diameter and a 1-cm path length. Dye solutions were prepared in distilled water. Degassing was carried out in bulbs attached to the reaction cell by first expelling dissolved carbon dioxide by bubbling purified nitrogen through the solution followed by freeze( -20O0)-pump-thaw cycles. The photolyses were run a t ambient temperature (23 f 1") without specific temperature control. A single &-spoiled pulse produced no measurable temperature change. With cells of longer path length (3-14 cm) the transient decay curves showed a superimposed, high-frequency (100-kc) sinusoidal pattern whose origin remains obscure. The oscilloscope traces were recorded on Polaroid film and digital information was obtained directly from the photographic record with a Gerber data reduction system. These data were evaluated in terms of AD, the change in optical density relative to that of the unexcited solution. Conventional spectrophotometric measurements were made with a Beckman DK-1 spectrophotometer. For comparison with the laser results and literature r e p ~ r t s , ~conventional J flash photolysis experiments were conducted under one set of conditions. The apparatus for this purpose, which will be described in detail elsewhere, incorporates xenon-filled flash tubes which provide ca. 20-psec pulses with input energies (8) K. Bergmann and C. T. O'Konski, J . Phys. Chem., 67, 2169 (1963). (9) P.Kafalas, J. I. Masters, and E. 31. E. Murray, J . A p p l . Phys., 3 5 , 2349 (1964).
THELASERPHOTOLYSIS OF METHYLENE BLUE
in the range 100-5600 joules. A 7.0 X lo-” M solution of methylene blue was used, which was freed of oxygen by bubbling water-saturated, purified nitrogen through it for 45 min. Since the unfiltered flash causes irreversible bleaching of the dye, the cylindrical “Pyrex” reaction cell (14 cm, 2-cm diameter) was covered with a Roscoe No. 809 filter, which confines light to the visible absorption band. The input energy was 250 joules/flash.
2635
I
0.06
Results I . General Features. Plain aqueous solutions of methylene blue in the concentrated range 294 X to 5.5 X M were studied. With the most dilute solution employed, the 0.5-joule laser pulse suffices to convert the ground-state dye almost completely into transients. The changes which occur are completely reversible; thus there is no detectable change in optical density after many laser flashes and after prolonged exposure to the glass-filtered monitoring light. From Bergmann and 9’Konski’s spectral data,* a radiative lifetime of 9.8 nsec can be calculated for the methylene blue singlet excited state. The quantum yield of fluorescence is low10 so that the actual lifetime of this state is at least an order of magnitude smaller. This lifetime, in conjunction with the 30-nsec pulse duration and the high intensity, constitutes the conditions for absorption saturation with respect to the singletsinglet transition and for the inapplicability of Beer’s law.” Although no measurements using the splitbeam technique” were made with methylene blue, there is reason to believe that initial conversion to the singlet excited state was essentially complete even in the most concentrated solution. The number of light quanta is always in excess of the number of molecules in the irradiated volume. 2. Transient Spectra. The general features of the transient spectra are similar at all the concentrations examined. Maxima appear at 420, 520, and in the 700-900-mp region, which parallels the results obtained by conventional flash phot~lysis.~J The transient behavior of the 5.5 X 10+ M solution is shown in Figures 2-5. Decay a t 520 mp is slower than that at 420 mp (Figure 2), which is indicative of a t least two transients with absorption in this region. The bleaching region (570-695 mp) is shown in Figure 3. In this case the transient optical density, obtained by adding the ground-state optical density to m, is plotted to portray the extent of bleaching. The 8-psec curve stops at 660 mp, since beyond this wavelength a photomultiplier circuit with slower response was used. The ground-state absorption curve of the dye (dotted line) was obtained by spectrophotometer. The curves
I
t -
5 0 0 -
0
350
400
450
c
2
550
500
Wavelength, mp.
Figure 2. Transient absorption in the 350-550-rnp region, 5.5 X 10- M methylene blue. The numbers indicate the time in microseconds after the laser flash.
0.4
.-d 0.3 2
-t 4
’3 0.2
0”
0.1
* 575
600
I
625
*
I
650
675
1 700
Wavelength, mp.
Figure 3. Transient bleaching in the 570-700-mp region, 5.5 X 10-8 M methylene blue.
corresponding to bleaching resemble the ground-state curve quite closely. The apparent -5-mp shift in the maximum is probably due to a calibration discrepancy between the flash apparatus monochromator and the spectrophotometer and it is probable that there is no absorption due to transient species in the 600-68O-mp range. Extrapolation of log AD vs. time curves to zero time for wavelengths in the vicinity of the maximum indicates at least 90% bleaching. Figure 4 represents (10) N. Wotherspoon and G. Oster, J . Am. Chem. SOC.. 79, 3992 (1957). (11) Absorption saturation by laser radiation has been observed with phthalocyanine and cryptocyanine solutions. Cf. J. A. Armstrong. J . AppE. Phys., 36, 471 (1965); F. Gires and F. Combaud, J . Phy8. Radium,26, 325 (1965); F. T.Arecchi, V. Degiorgio, and A. Sona, Nuovo Cimento, 38, 1096 (1965); V. Degiorgio and G. Potenza, dbid., 41, 254 (1966); C.R. Giuliano and C. D. Hess, Appl. Phu8. h t t e r 6 , 9, 196 (1966).
Volume 71, Number 8 July 1967
R.M. DANZIGER, K. H. BAR-ELI,AND K. WEISS
2636
0.12
0.09
0.06 547.5 mp.
550 mp.
552.5 mp.
555 mfi.
557.5 mp.
560 mp.
0.03
0
I
J
-0.03
700
800
750
850
900
Wavelength, mfi.
Figure 4. Transient absorption in the 7 0 0 - 9 0 0 . ~region, 5.5 x 10-6 M methylene blue. The numbers indicate the time in microseconds after the laser flash.
Figure 6. Oscilloscope traces in the isosbestic region. The ordinates are 0.05 v/major division and the abscissas 5 psec/major division, except at 547.5 mp where it is 10 pseclmajor division. The voltage deflection due to the monitoring light is 8.0 v in all cases. The horizontal traces represent the base lines. For the laser spikes a t 547.5-555 mp, the axes are 2.0 v/cm and 20 psec/cm.
0.01
0
9 -0.01
-0.02
which is not detectable under ordinary flash conditions. After -10 psec an isosbestic point due to the longer lived transients appears at 552.5 mp. This is shown clearly in the oscilloscope traces of Figure 6, which indicate that the decay of the transient is complete in ca. 10 psec. The apparent half-life is 2 psec or less, which is just about the time resolution of the measurements. The fast transient is not distinctly manifested in other spectral regions; below 420 mp, maximum absorption is reached after less than 2 psec, whereas near 520 mp it is reached after ca. 6 psec. In the region above 660 mp, the fast transient is outside the time resolution of detection and consequently an uncomplicated isosbestic point appears at 705 mp. The lower wavelength isosbestic point with respect to the longer lived species changes regularly with concentration, being 552.5, 550, 546, 544, and 540 mp at 5.5, 23.8, 55.0, 130, and 294 X lousM , respectively. The relative proportion of the transients changes with the concentration of the dye. At 546 mp, the isosbestic point for a 55 X lom6M solution of methylene
. u 635
545
555
565
Wavelength, mp.
Figure 5. Transient changes in the 535-570-mp isobestic region, 5.5 X lo* M methylene blue.
transient absorption in the near-infrared region. Maxima appear at 740, 780, and 840 mp. The decay is rapid, as it is near 420 mp, although a slow component is again apparent above 800 mp. By placing a cryptocyanine filter solution in front of the monochromator entrance slit it was possible to observe methylene blue fluorescence12above 720mp. From the shape of the absorption curves, it is clear that isosbestic points appear near the short-wavelength and long-wavelength edges of the ground-state band. I n the 540-56o-m~ isosbestic region an extremely short-lived transient is apparent (Figure 5 ) The Journal of Physical Chemistry
(12) G . N. Lewis,0. Goldschmid, T. T. Magel, and J. Biegeleisen, J . Am. Chem. SOC.,65, 1150 (1943).
THELASERPHOTOLYSIS OF METHYLENE BLUE
2637
blue, the maximum AD due to the fast transient, is 0.018, For the 5.5 X M solution, the fast transient contribution to AD is 0.0025 at the same wavelength. Measurements in the ground-state absorption region indicate a maximum of 30% bleaching and conversion into the longer lived transient states for the more concentrated solution. Since conversion is essentially complete in the dilute solution, AD,,, 0.08 is expected for 55 X M . The larger than anticipated AD,,, value thus reflects a change in the composition of the solution with increasing concentration. Methylene blue exists in monomer-dimer equilibrium in aqueous s ~ l u t i o n ~ *and ~ ~ the J * most reliable value of mole/l. the dissociation constant is (1.7 i 0.2) X at 25°.8 With this information, one can calculate the fractions of dimer to be 0.30 and 0.055 for the 55 X and 5.5 X M solutions, respectively. These considerations strongly suggest that the fast transient is derived from the dimeric dye species. The decay kinetics (vide infra) indicate that the 400 and 800-9 regions represent absorption by two components and that a single species absorbs near 500 mp. The increases with increasing fraction ratio AD,,x520/AD,~x820 of dimer as shown in Figure 7. This implies that the slow 520-mp transient is also dimeric and that absorption at 820 mp is predominantly due to a monomerderived species. Owing to the slower detection circuit response, the maximum absorption change is observed at a longer time after the flash at 820 mp than at 520 mp and this probably accounts for the failure of the plot of Figure 7 to extrapolate t o a zero value for the AD ratio. 3. Decay Kinetics. A detailed kinetic analysis was made only for the 5.5 X M solution, although some
-
0
+
+
hD = ~ A C A ~ B C B ciccc ~M(CM CM')
+ +~
C M- ,CY,')
(1)
where M and Mz represent the monomeric and dimeric dye, respectively, e the molar extinction coeficients, and C the concentrations. The superscript degree refers to initial concentrations. The expression becomes simpler in different spectral regions for the dilute solution since EM and e ~ are , negligible between 350 and 540 mp and above 720 mp and it is assumed that there is no transient absorption between 600 and 680 mp. Further, Ca = 0 after 10 psec. I n the region 485-540 mp, the decay of AD is strictly first order for at least three half-lives. Rate constants for the decay of the slowest transient (C), to which absorption in this range is ascribed, are quoted in Table I. The tenfold change in concentration is seen to be without effect on the rate. ~~
Table I: Rate Data for Transient C A, mp
485 500 520 535 540 543
3
2
1
results for the 55 X M solution will be quoted. Analysis for the more concentrated solutions is complicated by substantial ground-state dye absorption throughout the 350-720-mp region. Every indication is that, apart from the fast transient, there aye two longer lived transients generated. Labeling these A, B, and C in order of increasing lifetime, we have
a
--kc 5.5
x
x 10-s
M
10-1 sec-1 a 55
x
lo-' M
5 . 2 (3) 4 . 9 (3) 4 . 4 (3) 4 . 9 (2, 4 . 4 (1) 5 . 7 (1) 5 . 2 (2) 5 . 5 (1) 5 . 3 (2) Av kc = (5.0 i 0 . 5 ) X los 8ec-I
Average value for the number of runs quoted in brackets.
At wavelengths where two transients absorb (below 480 mp and above 720 mp), the data were found t o fit two concurrent first-order processes. The rate constant for the faster transient (B) was obtained by extrapolation and subtraction of the contribution of the slower transient, as illustrated for decay at 400 mp in Figure 8. Similar behavior is shown in the bleaching
I 0
I
0.2
t
0.4
Fraction of dimer.
Figure 7. Variation of the relative amounts of transients as a function of the concentration and composition of the solution.
i 0.6
(13) G. Holst, 2. Physik. Chem. (Leipzig), A182, 321 (1938). (14) (a) E. Rabmowitch and L. F. Epstein. J . Am. Chem. doc., 6 3 , 69 (1941); (b) B. Broyde and G. Oster, ib&i., 81,5099 (1959), report
that the irradiation of glasses containing a high concentration of thiazine dyes and a mild reducing agent leaves a species with strong absorption in the 600-600-mr region which they identify as the entrapped dimer of the dye.
Volume 71, Number 8 July 1967
R. M. DANZIGER, K. H. BAR-ELI, AND K. WEISS
2638
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I
sients by separate excitation. The ground-state absorption spectra of the two species indicate an isosbestic point close to the excitation (laser) wave= Consequently, the fraclength a t which tions of the total absorbed light which is absorbed by the monomer and dimer are simply a and 1 - a, respectively, where a = CM/(CM 2cM2). Since the fast transient (A) appears to be derived from the dimer, the excitation processes may be formulated as
+
M2
+M2* +A
M+M*+B 200
100
400
300
Time, psec.
Figure 8. Decay kinetics at 400 mM, 5.5 X 10-6 M methylene blue.
region, where the recovery of absorption mirrors the disappearance of transients B and C. These results are summarized in Table 11. The value of k, given here is in satisfactory agreement with the one listed in Table I. The data in Table I1 refer to the dilute solution. For the 55 X M solution, evaluation of the decay a t 420 mp provides the value k , = 4.2 X lo3 sec-I for the slow transient, which is comparable to the tabulated values. However, the extracted contribution of transient B does not show first-order behavior, the apparent first-order constant decreasing from 50 X lo3 sec-I at 20 psec to -20 X lo3 sec-I at 100 psec. Evidently, another decay mode becomes important in concentrated solutions.
-
Discussion The results leave little doubt that the monomeric and dimeric forms of methylene blue give rise to tranTable 11: Rate Data for Transients B and C in the A
< 480-mp, Bleaching, and Near-Infrared Regions"
375 400 420 470 600 610 620
630 820 860 Av O
23 20 26 26 25 23 25 25 27 25 (25f2)
5.5 X 10-6 M methylene blue solution.
The J o u r a l of Physical Chemistry
5.1 4.9 4.3 5.1 3.9 4.2 4.1
4.4
(2)
(3)
The starred species represent the first excited singlet states and, for reasons set forth below, we assign A and B to the corresponding triplet states.16 Competing with the rapid conversion to A and B are the processes Mz* + M2 or 2M and M* + Ill. Little can be said about the former reaction other than, reasoning by analogy with the behavior of thionine,14" that it is probably entirely radiationless. On the other hand, the competing decay of M* proceeds with and without emission.'" On the basis of photochemical reduction studies with methylene blue in dilute aqueous solution (where CM >> CHJ, the quantum yield of the singlettriplet intersystem crossing has been estimated as 0.Z4J6 This value renders the observed extensive long-lived depletion of the ground state entirely reasonable. The identification of A and B as triplets is based on their lifetimes (tl,* 2 and 30 psec, respectively) and their sensitivity to oxygen. I n air, the decay of A is so rapid that its absorption is below the limit of detection. At the same time, the amount of the slowest transient (C) is decreased, but its decay rate is about the same as in the degassed solution. Air causes a decrease in the amount of transient B formed and substantially increases its decay rate. This behavior has already been noted for the 420 and 520-mp peaks.6 The lifetime of transient B agrees within a factor of 2 with that obtained by quenching the photoreduction of methylene blue.* The simplified decay scheme is proposed
