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J. Phys. Chem. 1996, 100, 17226-17231
Quenching of O2(a1∆g) by O2(a1∆g) in Solution Rodger D. Scurlock and Peter R. Ogilby*,† Department of Chemistry, UniVersity of New Mexico, Albuquerque, New Mexico 87131 ReceiVed: June 20, 1996; In Final Form: August 29, 1996X
The reaction in which O2(a1∆g) is quenched by another O2(a1∆g) molecule is examined in CS2. Spectroscopic data indicate that energy-pooling to form O2(b1Σg+) occurs in this process. This observation is important to the kinetics of O2(a1∆g)-O2(a1∆g) annihilation because O2(b1Σg+) is itself a precursor of O2(a1∆g). An upper limit of (7.0 ( 0.8) × 105 s-1 M-1 is determined for the O2(a1∆g)-O2(a1∆g) annihilation rate constant. Data are also presented to indicate that the quenching of O2(b1Σg+) by the fullerene C60 is limited by solute diffusion. This result is not consistent with a mechanism of electronic-to-vibrational energy transfer, which thus far has been the only documented deactivation process for O2(b1Σg+) in solution. O2(b1Σg+) quenching by C60 likely proceeds via electronic energy transfer facilitated by the near degeneracy of the O2(b1Σg+) and triplet C60 energy levels.
Introduction Under conditions in which the O2(a1∆g) lifetime is inherently long or when the concentration of O2(a1∆g) is high, quenching of O2(a1∆g) by O2(a1∆g) can be a dominant channel for O2(a1∆g) removal. This quenching process proceeds via the intermediacy of a O2(a1∆g) complex, the so-called O2(a1∆g) dimol. Evidence for the existence of oxygen dimols comes from unique emission bands, as shown in eqs 1 and 2 for the O2(a1∆g) dimol.1-7
O2(a1∆g)‚‚‚O2(a1∆g) f O2(X3Σg-)V)0‚‚‚O2(X3Σg-)V)0 + hν (635 nm) (1) O2(a1∆g)‚‚‚O2(a1∆g) f O2(X3Σg-)V)0‚‚‚O2(X3Σg-)V)1 + hν (703 nm) (2) These are one-photon transitions between discrete states of the complex. Although the complexes may not have a significant binding energy,8,9 orbital overlap between the interacting molecules is sufficient to define new states that derive from the states of the respective monomers. Thus, the complex formed as a result of the interaction between two O2(a1∆g) molecules has a state whose energy corresponds roughly to twice the energy of the O2(a1∆g) monomer. This state can then couple to the radiation field, with emission being observed at a wavelength (∼635 nm) that is one-half that for the monomer emission (∼1270 nm). Gas-phase experiments indicate that the quenching of O2(a1∆g) by O2(a1∆g) can also proceed via another energy-pooling process.2,10 In this case, the energy of one reacting O2(a1∆g) molecule is used to elevate the other molecule to the O2(b1Σg+) state in a nonradiative process (eq 3).
O2(a1∆g) + O2(a1∆g) f O2(b1Σg+) + O2(X3Σg-)
(3)
Derwent and Thrush10 report that, in the gas phase, the rate constant for this latter reaction is significantly larger (k ) 1.2 × 104 s-1 M-1) than that for the 635 nm radiative quenching channel (k ) 0.016 s-1 M-1). This is important with respect * To whom correspondence should be addressed. † Current address: Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark. X Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)01841-2 CCC: $12.00
to the kinetics of O2(a1∆g) because O2(b1Σg+) is itself a precursor to O2(a1∆g).11-16 Attempts to determine a rate constant kq for the quenching of O2(a1∆g) by O2(a1∆g) in solution-phase photosensitized reactions can be complicated by competing O2(a1∆g) deactivation channels that give rise to unrepresentatively large values of kq. Of particular concern are O2(a1∆g) quenchers created by the high irradiation fluences needed to produce O2(a1∆g) concentrations where O2(a1∆g)-O2(a1∆g) quenching can occur. Such quenchers may include ionic species produced as a result of multiphoton absorption,17-19 species derived from the reaction of photoproduced ions with the solvent,20,21 and neutral sensitizeroxygen adducts.22 To our knowledge, there are only two reports of a solutionphase O2(a1∆g)-O2(a1∆g) annihilation rate constant kq. Salokhiddinov et al.23 reported a rate constant of 1 × 109 s-1 M-1 for experiments performed in CCl4. Chou and Frei24 reported a smaller rate constant of (8.7 ( 3.8) × 106 s-1 M-1 for experiments performed in C6F6.25 Although competing processes that might influence kq were considered by Chou and Frei, the value reported was still larger than the gas-phase rate constant reported by Derwent and Thrush.10 Assuming that O2(a1∆g)-O2(a1∆g) annihilation produced O2(b1Σg+), Chou and Frei argued that the discrepancy between the gas- and solutionphase data was reasonable if the rate of this energy-pooling process depended on resonances between reactant and product energy levels. In solution, where collisions with the solvent can both supply and consume excess energy, the effect of a reactant-product energy mismatch on the reaction rate is less important. As a consequence, the annihilation reaction could indeed be expected to proceed faster in the condensed phase than in the gas phase. Events that result in both the formation and deactivation of the O2(a1∆g) dimol are important with respect to several problems of long-standing interest: (1) In 1966, Khan and Kasha26 predicted that the O2(a1∆g) dimol could transfer its energy of excitation to a second molecule and thus result in the fluorescence of this molecule. Although a number of investigators have addressed this issue, irrefutable evidence to support the Khan and Kasha prediction have yet to be provided.5,27-31 (2) In 1976, Deneke and Krinsky32,33 reported that certain molecules, particularly cyclic amines, enhanced the intensity of the O2(a1∆g) dimol emission. In these experiments, O2(a1∆g) was generated in the reaction of NaOCl and H2O2, and the amine © 1996 American Chemical Society
Quenching of O2(a1∆g) by O2(a1∆g) in Solution
J. Phys. Chem., Vol. 100, No. 43, 1996 17227
was added as a solute to the reaction mixture. Di Mascio and Sies34 later reported that cyclic amines also enhanced the O2(a1∆g) dimol emission using O2(a1∆g) generated via the thermolysis of an aromatic endoperoxide. A commonly accepted explanation for this phenomenon has yet to be provided.34,35 In this study, we set out to gain a better understanding of the quenching of O2(a1∆g) by O2(a1∆g). We were particularly interested in ascertaining whether or not O2(b1Σg+) was indeed formed as a result of this reaction in the solution phase. Experimental Section Details of the techniques and instrumentation we use to monitor O2(b1Σg+), O2(a1∆g), and organic molecule excited states have been published.36-38 Briefly, a pulsed Nd:YAG laser was used as the excitation source. The second harmonic (532 nm) of the fundamental lasing wavelength was used to excite the sensitizer, the fullerene C60. Neutral density filters (Schott) were used to vary the photon flux incident on the sample. Laser energies were quantified using a calibrated energy meter (Scientech). In an attempt to provide a homogeneous distribution of O2(a1∆g) molecules in the irradiated volume, the following measures were taken: (1) The excitation beam (TEM00) was passed through a 0.6 cm restricting aperture to remove the low flux portions of the beam. (2) Experiments were performed in 1 mm path length cells to avoid the larger concentration gradients of O2(a1∆g) found in a 1 cm cell. The 1270 nm phosphorescence of O2(a1∆g) was monitored with a liquid nitrogen-cooled, dc-coupled germanium detector (North Coast) whose response function was single exponential with a time constant of 400 ns. The 1926 nm fluorescence of O2(b1Σg+) was monitored with a liquid nitrogen-cooled InSb detector (Judson) whose response function was also single exponential, but with a time constant of 2 µs. C60 was obtained from Aldrich and used as received. CS2 was likewise obtained from Aldrich and distilled from P2O5 prior to use. Acridine (Aldrich) was recrystallized from ethanol/water and then sublimed under vacuum. Zn(II) octaethylporphine (Porphyrin Products, Inc.) was used as received. Results and Discussion 1. Quenching of O2(a1∆g) by O2(a1∆g). O2(a1∆g) was produced in CS2 using the fullerene C60 as a photosensitizer. C60 has several desirable features for this experiment: (1) The quantum yield of C60-sensitized O2(a1∆g) production in CS2 is 1.0.36 (2) C60 is not a good O2(a1∆g) quencher (vide infra). (3) C60 is stable upon irradiation in aerated solutions.39 Thus, O2(a1∆g) will not be quenched by adventitious species derived from sensitizer photochemistry. (4) The ionization energy of C60 is sufficiently high (∼7.7 eV)40,41 that a biphotonic process will not create a species that potentially could quench O2(a1∆g). Although 7.7 eV is a gas-phase value, it is unlikely that solvent stabilization could reduce the C60 ionization threshold to a value accessible under our experimental conditions. Specifically, upon 532 nm irradiation, a biphotonic process could impart a maximum of only 4.66 eV into C60. Upon irradiation of C60 at 532 nm, time-resolved O2(a1∆g) phosphorescence signals are readily observed. At low irradiation energies, the signals exhibit first-order decay kinetics. As the irradiation energy is increased, the rate of signal decay increases as does the extent of deviation from first-order kinetics (Figure 1). These data are consistent with the creation, upon sensitizer irradiation, of a transient species that can quench O2(a1∆g) over a time period that is coincident with the lifetime of O2(a1∆g).18,19,22,24,42 Given that C60 is a benign sensitizer, we believe that our energy-dependent data only reflect the quenching of O2(a1∆g) by O2(a1∆g). This assignment is further
Figure 1. (a, top) Time-resolved O2(a1∆g) phosphorescence signals in CS2. O2(a1∆g) was produced by energy transfer from 3C60. Data are shown for experiments in which the irradiation energy at 532 nm was 0.09, 0.83, 1.67, 2.27, 3.3, 4.75, and 7.0 mJ/pulse. The absorbance of ground-state C60 at 532 nm was 0.102 in a 1 mm path length cell, which corresponds to a C60 concentration of 9.4 × 10-4 M ( for C60 at 532 nm in CS2 ) 1.08 × 103 cm-1 M-1). The data have been scaled to yield the same intensity at time ) 0. (b, bottom) Single-exponential fitting functions (dashed lines) have been applied to two of the traces shown in (a). Data recorded at the comparatively high irradiation energy of 7 mJ/pulse clearly show a deviation from first-order decay kinetics.
substantiated upon comparison of the O2(a1∆g) and O2(b1Σg+) kinetics, as discussed below. 2. O2(a1∆g)-O2(a1∆g) Annihilation Does Produce O21 (b Σg+). The transition O2(b1Σg+) f O2(a1∆g) at 1926 nm provides a convenient method by which to monitor O2(b1Σg+).16,38,43-45 Under conditions, described above, in which O2(a1∆g) was presumably being quenched by O2(a1∆g), we were indeed able to detect O2(b1Σg+) fluorescence at 1926 nm in a time-resolved experiment (Figure 2). 3. O2(b1Σg+) Kinetics. The decay rate of the 1926 nm emission signal shown in Figure 2 indicates that O2(b1Σg+) is formed as a result of O2(a1∆g)-O2(a1∆g) annihilation. Specifically, as we now show, the kinetics of O2(b1Σg+) formed as a result of O2(a1∆g)-O2(a1∆g) annihilation are characteristically different from the kinetics of O2(b1Σg+) formed directly by energy transfer from a photosensitizer. 3.a. O2(b1Σg+) Decay. The lifetime of O2(b1Σg+) in solution is very short (
