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Langmuir 1998, 14, 7072-7075
Notes Oxygen-Quenching Process of the Triplet State of Diprotonated Tetraphenylporphine at a Liquid-Liquid Interface Satoshi Tsukahara* and Hitoshi Watarai Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama, Toyonaka, Osaka 560-0043, Japan Received June 16, 1997. In Final Form: September 25, 1998
Introduction Nowadays the liquid-liquid interface is extensively investigated, because it shows specific functions in electrochemical, biological, and photochemical systems and because it plays important roles in phase-transfer catalysis, colloidal chemistry, ion- and electron-transport processes, and pharmacology.1 Most of these studies treated either adsorption equilibria at interfaces by classical methods, for example, interfacial tension measurements, or charge-transfer kinetics by electric or electrochemical methods. On the other hand, the roles of interfacial reactions in the extraction of metal ions have been demonstrated by means of a high-speed stirring method,2-5 dynamic interfacial tension measurements,6 and an interfacial stopped-flow method.7 The high-speed stirring method and dynamic interfacial tension measurements are quite useful to study the kinetics at the liquid-liquid interface, but they are only available for somewhat slow reactions (time constant τ ) 1 s to 100 min and 0.1-100 s, respectively). The interfacial stoppedflow method improved the limitation, extending the applicability to relatively rapid reactions, that is, τ ) 10200 ms. However, there have been no methods to measure faster reactions (τ < 1 ms) of a solute adsorbed at the liquid-liquid interface, so far. Laser flash photolysis is one of the powerful relaxation methods for investigating rapid reactions (τ ) 0.1 ns to 1 µs) in solutions, but it has not been applied to the study of kinetics at the liquidliquid interface yet. This method has a high potential to detect more rapid reactions at the liquid-liquid interface than the interfacial stopped-flow method. The purpose of the present report is to measure kinetics at the liquidliquid interface by laser flash photolysis for the first time. It has been observed that triplet porphyrins generated by photolysis are quenched by dissolved oxygen following the reaction8-10 (1) Liquid-Liquid Interfaces. Theory and Methods; Volkov, A. G., Deamer, D. W., Eds.; CRC Press: Boca Ration, FL, 1996 and references therein. (2) Watarai, H.; Sasaki, K.; Takahashi, K.; Murakami, J. Talanta 1995, 42, 1691. (3) Watarai, H.; Satoh, K. Langmuir 1994, 10, 3913. (4) Watarai, H. Trends Anal. Chem. 1993, 12, 313 and references therein. (5) Cantwell, F. F.; Freiser, H. Anal. Chem. 1988, 60, 226. (6) Shioya, T.; Tsukahara, S.; Teramae, N. Chem. Lett. 1996, 469. (7) Nagatani, H.; Watarai, H. Anal. Chem. 1996, 68, 1250. (8) Feitelson, J.; Mauzerall, D. J. Phys. Chem. 1996, 100, 7698. (9) Tanielian, C.; Wolff, C. J. Phys. Chem. 1995, 99, 9825. (10) Bonnett, R. Chem. Soc. Rev. 1995, 19 and references therein.
T1 + O2(3Σg) f S0 + O2(1∆g)
(1)
where T1 and S0 indicate a porphyrin in its triplet and ground states, respectively. Since active O2(1∆g) produced through the reaction attacks tumors in vivo, water soluble porphyrins, for example, hematoporphyrin derivatives and tetraphenylporphine derivatives, are widely investigated as a reagent for photodynamic therapy.10-12 In this study, reaction 1 is examined at the liquid-liquid interface, because the liquid-liquid interface is considered to be one of the models of a cell membrane, and therefore the quenching mechanism of triplet porphyrins by dissolved oxygen at the interface is also an important subject in medicine. 5,10,15,20-Tetraphenylporphine (tpp) is known to be converted to its diprotonated species H2tpp2+ under acidic conditions. When a toluene solution of tpp was shaken with an aqueous HClO4 or HCl solution, an ion-pair, H2tpp2+‚2ClO4- or H2tpp2+‚2Cl-, was generated and dissolved in the toluene phase.13,14 However, when H2SO4 was used as an acid, an ion-pair, H2tpp2+‚2HSO4-, was not dissolved in the toluene phase but adsorbed to the toluene-aqueous H2SO4 interface.13,14 In these systems, H2tpp2+ was not dissolved in the aqueous phase, due to the high hydrophobicity of its porphyrin ring and four phenyl groups. We examine the quenching of triplet H2tpp2+‚2HSO4- adsorbed at the toluene-aqueous H2SO4 interface by dissolved oxygen and compare it with that of triplet H2tpp2+‚2ClO4- or H2tpp2+‚2Cl- in some organic solutions. Experimental Section Samples. Tpp was synthesized by the method of Adler et al.15 and purified by the method of Barnett et al.16 Water was distilled and purified with an ion-exchange system (Milli-Q Sp. Toc., Millipore). Other reagents were of analytical reagent grade. Tpp in toluene solution was quantitatively converted to the diprotonated species, H2tpp2+‚2ClO4- or H2tpp2+‚2Cl-, by shaking 3 mL of toluene solution of tpp (5.0 × 10-6 M) with an equal volume of aqueous 6 M HClO4 or 6 M HCl solution for 30 min, respectively. Each acetone, ethanol, and methanol solution of H2tpp2+‚2ClO4- was prepared by adding a small amount (5 µL) of aqueous 6 M HClO4 solution to 2 mL of each solution of tpp (2.0 × 10-6 M). The quantitative formation of H2tpp2+ in each solution was confirmed by its absorption spectrum. Transients at the liquid-liquid interface were measured with the following emulsion. A small amount (10 µL) of toluene solution of tpp (1.07 × 10-3 M) was added to 3 mL of aqueous 2-9 M H2SO4 solutions in a PFA bottle, which was employed for preventing loss of H2tpp2+ by adsorption to a glass wall. The mixture was sonicated for 5 s with an ultrasonicator (UTB-152, SHARP, 150 W, 28 kHz) to prepare an emulsion of toluene droplets and aqueous medium, and then the emulsion was (11) Kongshaug, M.; Moan, J.; Brown, S. B. Br. J. Cancer 1989 59, 184. (12) Kessel, D.; Thompson, P.; Saatio, K.; Nantwi, K. D. Photochem. Photobiol. 1987, 45, 787. (13) Watarai, H.; Chida, Y. Anal. Sci. 1994, 10, 105. (14) Chida, Y.; Watarai, H. Bull. Chem. Soc. Jpn. 1996, 69, 341. (15) Adler, A. D.; Longo, F. R.; Finavelli, J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. J. Org. Chem. 1967, 32, 478. (16) Barnett, G. H.; Hudson, M. F.; Smith, K. M. J. Chem. Soc., Perkin Trans. 1 1975, 1401.
10.1021/la970635k CCC: $15.00 © 1998 American Chemical Society Published on Web 11/06/1998
Notes
Langmuir, Vol. 14, No. 24, 1998 7073
Figure 1. Transient absorption spectra of H2tpp2+ under aerated conditions. (a) H2tpp2+‚2ClO4- in toluene (solid line) and in acetone (dotted line). Spectra at 0.3 µs after the laser pulse. Seventy (toluene) and 50 (acetone) laser shots were averaged. (b) H2tpp2+‚2HSO4- at the toluene-aqueous 9 M H2SO4 interface. Spectrum averaged from 1.3 to 3.0 µs after the laser pulse. One hundred laser shots were averaged. transferred to an optical cell. Transients of tpp in toluene droplets were also observed in a similar emulsion system; 10 µL of a toluene solution of tpp (1.07 × 10-4 M) and 3 mL of water were sonicated together in the PFA bottle. Microscopic observations revealed that these toluene droplets were about 2-3 µm in radius. The emulsions were stable for at least 30 min. Apparatus. Laser photolysis was carried out with an Nd: YAG laser (Surelite I-10, Continuum, 10 Hz) equipped with a second-harmonic generator (532 nm). The laser pulse width and power were about 6 ns and 18 mJ/pulse, respectively. Transients were monitored by a detection system (Unisoku, Japan). The intensity of a light beam from a 150 W xenon lamp (L2195, Hamamatsu Photonics) passed through a sample cell at a right angle to the laser light was measured by a photomultiplier (R2949, Hamamatsu Photonics) attached to a monochromator. The output of the photomultiplier was connected to a digital oscilloscope (TDS-320, SONY Tektronix), which was triggered by split laser light. Transients from 50 to 100 laser shots were averaged for each measurement. The time constant for decay of a transient species, τ, was obtained by applying the nonlinear least-squares method to the absorbance changes after the laser irradiation. The transients were measured within the wavelength range 350650 nm in 10 nm steps. To control the concentration of dissolved oxygen, the partial pressure of oxygen, P(O2), in the gas phase was changed. A mixture of a known volume of O2 and N2 gases was equilibrated with the solutions or the emulsions in a sealed cell. All experiments were carried out in a thermostated room at 25 ( 1 °C.
Results and Discussion Quenching of Triplet H2tpp2+ in Organic Solvents. Figure 1a shows a transient absorption spectrum observed in air-saturated toluene solution. The absorbance maximum (500 nm) is red-shifted by about 60 nm from that of H2tpp2+ in the ground state (440 nm). The time constant for decay (τ) of the transient species was obtained by analyzing each absorbance change at 420-510 nm as a single-exponential function. A linear relationship between τ-1 of transient H2tpp2+ in toluene and P(O2) was obtained, as shown in Figure 2. Therefore, τ-1 can be expressed by using Henry’s law as
τ-1 ) km + kq[O2]
(2)
where km and kq are the rate constant for intrinsic
Figure 2. Proportional relationships between the reciprocal time constant (τ) for transient H2tpp2+ and the partial pressure of oxygen (P(O2)): O, in toluene; b, at the toluene-aqueous 6 M H2SO4 interface.
monomolecular decay of the transient species and that for the bimolecular reaction between it and dissolved oxygen, respectively. The negligibly small intercept in Figure 2 means that the transient species is quenched only through the bimolecular reaction with dissolved oxygen, that is, km , kq[O2]. The concentration of dissolved oxygen, [O2], at each P(O2) was calculated with the [O2] value17 at P(O2) ) 1.01 × 105 Pa. The kq in toluene was obtained as (1.71 ( 0.52) × 109 M-1 s-1 from the slope. It was observed that triplet tpp was generated by photolysis and that the absorption maximum of it (440 nm in cyclohexane)18 was red-shifted by about 24 nm from that of tpp (416 nm) in the ground state. A number of organic compounds19 as well as porphyrins8-10 in their triplet states are quenched by oxygen molecules, and their quenching rate constants were in the range (1-3) × 109 M-1 s-1.19 The km value for triplet tpp in cyclohexane was reported as 1.6 × 103 s-1,18 which was much smaller than the kq[O2] values. An ESR study has shown that triplet H2tpp2+ is generated by photolysis.20 No reports on the absorption spectrum of triplet H2tpp2+ appeared, but the above facts imply that the transient species of H2tpp2+ observed here is triplet H2tpp2+. Figure 1a also shows a transient spectrum of H2tpp2+ in air-saturated acetone. Transient spectra in the other solvents as well as in acetone were quite similar to that in toluene, meaning that triplet H2tpp2+ was also generated in these solutions. kq values were generally obtained with τ values measured in air-saturated solutions and with an assumption that km , kq[O2].19 In this study, kq values in the solutions were also obtained in a similar way. The concentrations of dissolved oxygen in the solutions were calculated with reported values.17 The quenching process of a triplet species (T1) by an oxygen molecule (3O2) can be expressed as19 1/
9kdiff
ket
T1 + 3O2 y\ z 1(T1‚3O2)* 98 S0 + O2(1∆g) k -d
(3)
(17) Wilhelm, E.; Battino, R. Chem. Rev. 1973, 73, 1. (18) Kikuchi, K. JOEM Handbook 1 Triplet-triplet Absorption Spectra; Bunshin: Tokyo, 1989. (19) Gijzeman, O. L. J.; Kaufman, F.; Porter, G. J. Chem. Soc., Faraday Trans. 2 1973, 69, 708. Garner, A.; Wilkinson, F. Chem. Phys. Lett. 1977, 45, 432. (20) Hamacher, V.; Wrachtrap, J.; von Maltzan, B.; Plato, M.; Mo¨bius, K. Appl. Magn. Reson. 1993, 4, 297.
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Notes
Table 1. kq Values for the Quenching of Triplet H2tpp2+ by Dissolved Oxygen in Organic Solvents and at the Toluene-Aqueous H2SO4 Interface at 25 °C solvent toluene acetone ethanol methanol interface b
kqa/109 M-1 s-1
(1/9)kdiff/109 M-1 s-1
η/10-3 Pa‚s
1.69 ( 0.04 (7) 1.66 ( 0.09b (6) 1.36 ( 0.05 (5) 1.39 ( 0.06 (6) 1.31 ( 0.01 (6) 1.8 ( 0.5c (6)
1.33 1.33 2.39 0.673 1.32
0.552 0.552 0.307 1.09 0.554
a Average ( standard deviation (number of data points). H2tpp2+‚2Cl-; c H2tpp2+‚2HSO4-; others, H2tpp2+‚2ClO4-.
where 1/9 is a spin statistical factor of the singlet encounter complex, 1(T1‚3O2)*, and kdiff, k-d, and ket are the rate constants for the diffusion-controlled reaction, for the dissociative reaction to give back the initial reactants, and for the energy-transfer reaction, respectively. kdiff is expressed as
kdiff ) 8kT/(3η)
(4)
where k is the Boltzmann constant, T is the absolute temperature, and η is the solvent viscosity. Under the steady-state condition of 1(T1‚3O2)*, the following relation is drawn:
-
Figure 3. Absorbance changes of the transient H2tpp2+ at 430 nm at the toluene-aqueous H2SO4 interfaces and the residuals that were calculated as the differences between the observed and calculated values. One hundred measurements were averaged.
1 /9kdiff d[T1] ) ket[1(T1‚3O2)*] ) ket [T ][3O2] ) dt ket + k-d 1
kq[T1][3O2] (5) In general, ket . k-d, and thus kq ≈ (1/9)kdiff.19 Table 1 shows the kq values obtained in the organic solvents as well as the (1/9)kdiff values calculated with η.21 The kq values are comparable to the (1/9)kdiff values, meaning that the rate-determining step in solutions is the generation of the singlet encounter complex, 1(T1‚3O2)*, with the rate constant (1/9)kdiff. Quenching of Triplet H2tpp2+ at a Liquid-Liquid Interface. When water was used instead of H2SO4, tpp remained in toluene droplets. The τ value of triplet tpp in the toluene droplets was found to be 0.28 ( 0.03 µs (n ) 3, n is the data number) under aerated conditions, which well agreed with that in air-saturated toluene, 0.31 ( 0.02 µs (n ) 5). This implies that triplet tpp in toluene droplets is normally quenched by dissolved oxygen in the droplets even in the emulsion system. When diluted H2SO4 was used, the diprotonated tpp, H2tpp2+‚2HSO4-, was adsorbed at the liquid-liquid interface of the toluene droplets.13,14 From absorption spectra of the emulsions, it was confirmed that H2tpp2+ was generated quantitatively in the range 2-9 M H2SO4. For instance, a transient absorption spectrum at the toluene-aqueous 9 M H2SO4 interface is shown in Figure 1b. Transient spectra at the other toluene-aqueous 2-8 M H2SO4 interfaces were almost similar to that of H2tpp2+‚2ClO4- in toluene. A proportional relationship between τ-1 for the interfacial quenching and P(O2) was also obtained, as shown in Figure 2. This implies, according to eq 2, that the transient species, that is, triplet H2tpp2+, is quenched only by the bimolecular reaction with dissolved oxygen even at the liquid-liquid interface. The quenching rate at the interface is considerably slower than that in toluene. Figure 3 shows absorbance changes at the toluene-aqueous 6 and 9 M H2SO4 interfaces, in which (21) CRC Handbook of Chemistry and Physics, 62nd ed.; Weast, R. C., Astle, M. J., Eds.; CRC Press: Boca Raton, FL, 1981; pp F43-52.
Figure 4. Dependence of the reciprocal time constant (τ) for triplet H2tpp2+ at the toluene-aqueous H2SO4 interface (O) on the H2SO4 concentration under aerated conditions, that is, P(O2) ) 0.21. The concentrations of dissolved oxygen (b) in the aqueous H2SO4 solutions under aerated conditions were also shown.
the fitting curves were obtained by regarding them as single-exponential decay functions. Since absorbance changes at the other toluene-aqueous H2SO4 interfaces were also successfully fitted to the function, it is confirmed that the concentrations of dissolved oxygen were almost constant during the quenching process. This means that the concentration of oxygen at the liquid-liquid interfaces is enough higher than that of triplet H2tpp2+. The τ-1 values of triplet H2tpp2+ at the toluene-aqueous H2SO4 interfaces under aerated conditions (that is, under constant P(O2) conditions) were plotted against the H2SO4 concentration, as shown in Figure 4. If triplet H2tpp2+ is quenched by both dissolved oxygen in the organic (toluene) side and that in the aqueous side of the interface, τ-1 can be expressed as
τ-1 ) km + kq,o[O2]o + kq,a[O2]a
(6)
where kq,o and kq,a are the quenching constants of triplet
Notes
Figure 5. Proportional relationships between the reciprocal time constant (τ) for triplet H2tpp2+ at the toluene-aqueous H2SO4 interfaces and the concentration of dissolved oxygen in the aqueous H2SO4 phases under aerated conditions.
H2tpp2+ by dissolved oxygen in the organic and in the aqueous sides of the interface, respectively, and [O2]o and [O2]a are the concentrations of dissolved oxygen in toluene droplets and in the aqueous phase, respectively. Since oxygen is considered to show no specific adsorption to the liquid-liquid interfaces, it is reasonable to suppose that the concentrations of dissolved oxygen in the organic and aqueous sides of the interfaces are almost equal to those in the organic and aqueous phases, respectively. Henry’s law reveals that the [O2]o value is independent of aqueous phase composition unless the physical properties of toluene were influenced. To confirm the independency, tpp was dissolved in toluene that was shaken with 6 M H2SO4, and the τ value of triplet tpp was measured under aerated conditions. It was 0.30 ( 0.02 µs (n ) 7), which well agreed with the τ value in nontreated toluene, 0.31 ( 0.02 µs (n ) 5), implying that the concentration of dissolved oxygen in toluene was not influenced by the H2SO4 concentration in the aqueous phase. In other words, [O2]o is constant regardless of the H2SO4 concentration. The concentrations of dissolved oxygen22 in aqueous H2SO4 solutions, [O2]a, are also shown in Figure 4. The [O2]a value decreases with an increase in the H2SO4 concentration, and the variations in the quenching rate (τ-1) are considered to be caused by the variations in [O2]a. Figure 5 shows a linear relationship between τ-1 and [O2]a according to eq 6. All the quenching can be accounted for by the [O2]a, with a negligible intercept, (1 ( 6) × 104 s-1, which would represent the contribution from km and the constant [O2]o. The slope value in Figure 5, (1.8 ( 0.5) × 109 M-1 s-1, corresponding to kq,a, is almost equal to those in bulk organic solutions listed in Table 1, and this means that the interfacial quenching of triplet H2tpp2+ progresses through a mechanism similar to that in solution, that is, (22) The concentrations of dissolved oxygen were obtained by interpolating the solubility data of oxygen in 0-15 M H2SO4 at 21 °C. Solubility of Inorganic and Organic Compounds; Silcock, H. L., Ed.; Pergamon Press: Oxford, 1979; Vol. 3, pp 38-39. Original: Bohr, C. Z. Phys. Chem. 1910, 71, 47. (23) van Esch, J. H.; Feiters, M. C.; Peters, A. M.; Nolte, R. J. M. J. Phys. Chem. 1994, 98, 5541.
Langmuir, Vol. 14, No. 24, 1998 7075
Figure 6. Quenching scheme of triplet H2tpp2+ and triplet tpp by dissolved oxygen. By using 2-9 M H2SO4, tpp is adsorbed to the interface as H2tpp2+‚2HSO4- quantitatively, and triplet H2tpp2+ is quenched by dissolved oxygen in the aqueous side of the interface. By using water instead of sulfuric acid, triplet tpp generated in toluene droplets is quenched by dissolved oxygen in the droplets.
eq 3. The kq in toluene has no dependence on the kind of counterion, ClO4- and Cl-, as shown in Table 1, and thus the effect of HSO4- on the kq,a value would be negligible. The quenching rate at the interface was about 0.1 times that in toluene, as shown in Figure 2, and this can be explained by the fact that the concentration of dissolved oxygen in the aqueous H2SO4 solutions is lower than that in toluene droplets by a factor of about 10. Figure 6 shows the overall scheme for the quenching of triplet H2tpp2+ and triplet tpp by dissolved oxygen. We worried that the quenching of H2tpp2+ occurred not at the interface but just in the aqueous H2SO4 phases, because the τ-1 value is proportional to [O2]a. Aqueous 6 M H2SO4 was stirred with an excess amount of solid tpp for 1 week for saturation, at which the concentration of H2tpp2+ was found to be about 2 × 10-8 M. The effect of H2tpp2+ soluble in the H2SO4 phase on the transient absorbance of the toluene-aqueous H2SO4 system was only about 0.0005 at 430 nm and time ) 0, that is, negligibly small. Although the concentration of dissolved oxygen in toluene droplets is much higher than that in the aqueous solutions, oxygen in toluene is not effective for the quenching of triplet H2tpp2+ at the interface. As mentioned above, triplet tpp generated in the toluene droplets was normally quenched by dissolved oxygen in the droplets. These facts suggest that H2tpp2+ exists in the aqueous side of the interface. It was reported that a porphyrin derivative having no electric charge existed in the center of an LB film while another porphyrin derivative having a side chain of one positive charge was near the aqueous interface of the LB film.23 Therefore, H2tpp2+‚ 2HSO4- would be located in the aqueous side of the interface, since two positive charges of H2tpp2+ are delocalized in the whole porphyrin ring of the molecule. Acknowledgment. This work was supported by a Grant-in-Aid for General Scientific Research from the Ministry of Education, Science and Culture, Japan (No. 07404042). LA970635K
