J. Phys. Chem. 1993,97, 5643-5648
5643
Laser Flash Photolysis Studies of the Contact Complex between Molecular Oxygen and 1-Methylnaphthalene Stephan L. Logunovt and Michael A. J. Rodgers' Center for Photochemical Sciences, Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403 Received: January 25, 1993
The contact complex (3%) between 1-methylnaphthalene (MN) and molecular oxygen in several solvents was excited at 355 nm, and the subsequent processes were observed by absorption spectrometry on subnanosecond and suprananosecond time scales. Experimental evidence for a significant amount of charge-transfer character in the upper 3E2is presented. The lifetime of this charge-transfer state (CT) is 20 f 10 ps, and there are two competitive pathways for decay of this radical pair: (i) to the localized triplet state and (ii) to the triplet ground state. In the nonpolar solvent cyclohexane, decay to the localized triplet state dominates and CPT = 1; in the polar solvent acetonitrile, the competition between two recombination pathways is approximately equal and reduces the quantum yield of the triplet state to one-half. The quantum yields of singlet oxygen (02(lAg)) in the two solvents reflect these triplet yields, which indicates that the precursor of singlet oxygen is the free triplet state of MN. This is supported by kinetic evidence. There is no evidence that intersystem crossing in the CT state results in direct entry to the singlet manifold.
Introduction It is well-known that molecular oxygen in its triplet ground state has a tendency to form contact complexes with a variety of organic compounds in their ground states.' This contact state, also of triplet multiplicity and labeled 3Eo, has no binding energy, but it shows a spectroscopic transition to higher lying exciplex states. In the case of aromatic hydrocarbons, these transitions are of lower energy than the SO SItransition and are found as weak bands at the red edge of the SO SIabsorption band. The upper state has been described as possessing characteristics of the aromatic radical cation and superoxideradical anion states; Le., it hascharge-transfer (CT) nature.2 Spectroscopictransitions into the upper exciplex states formed between 0 2 and aromatic hydrocarbonshave been the subject of much theoretical debate? but little is known about the dynamic consequences of pumping these transitions. Tokumaru and co-workers irradiated the CT transition in contact complexes of 0 2 with benzene, styrene, and naphthalene derivatives in polar solvents and inferred the formationof free radical^.^ More recently, Ogilby and colleagues have shown that singlet oxygen, O2(lAg), can be formed in significant yields4 when the CT transition was pumped. They concluded that the singlet oxygen yield after irradiation into the contact band of organic moleculeoxygen complexes is lower in polar solvents than in nonpolar ones? Particularly, for l-methylnaphthalene (MN), where the CT state is energeticallyclose to the triplet excited state, Kristiansen et al. reported4bthat a lower singlet oxygen yield in polar solvents (acetonitrile) arises from the direct coupling of the CT state with the ground state of the complex. They also suggested that this CT state could relax directly to form singlet oxygen without passing through the localized triplet state. Because of our continuing interest in the interaction of molecular oxygen with electronically excited states and the processes devolving therefrom, we have initiated a program to investigate the dynamic events that occur in such exciplex states. Previously we have shown that the exciplex formed between naphthalene (N) and oxygen, upon photoexcitation, undergoes subsequent rapid ( T 200 ps) deactivation into the localized
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* Author to whom correspondence should be addressed.
Permanent address: Biophysics Department, Faculty of Biology, M. V. Lomonosov State University, 1 19899 Moscow, Russia. +
triplet state.s In this paper, we focus on the excitation of the methylnaphthaleneoxygen [MN-*02] contact band and follow the subsequent deactivation processes in solvents of different polarity.
Experimental Section Instrumentation. Picosecond laser flash photolysis studies were carried out with the third harmonic (355 nm, 30 ps) of a modelocked Nd:YAG laser system (Quantel YG571). The basic features of the pumpprobe double-diode array spectrography and the single-wavelength kinetic setup have been described.5-6 The convolution of excitation and probe pulses in collinear geometry for single-exponential processes yielded a response time of ca. 10 ps. The incident excitation energy used was 2 mJ. The excitation beam diameter at the sample was no less than 3 mm. Nanosecond laser flash photolysis experiments were performed using the third harmonicof a Q-switchedNd:YAGlaser (Quantel YG661). Thedetailsofthisand theassociatedkineticabsorption spectrophotometry instrument have been described.' Singlet oxygen yields were measured with a Gephotodiodeplus-amplifier combination as published.' The 355-nm output of the Q-switched Nd:YAG laser was employed as the excitation source. Groundstate absorption spectra were measured using a Perkin-Elmer UV-vis diode-array spectrometer (Lambda 3840). Methods. Triplet Quantum Yields. An oxygen-saturated solution of MN at 2 M concentration was irradiated at 355 nm, and the value of the absorbance change at 420 nm (AA420) was measured. Another solution containing benzophenone (absorbance 0.5 at 355 nm) and MN (2 M) in the same solvent in argon-saturated solution was irradiated with the same laser intensity, and its hA420 value was read. The two U 4 2 0 values were compared after normalization to a constant value of Ass5. In the photosensitized experiment, = 1 for benzophenone; and at 2 M MN this is quantitatively converted into MN triplet. Thus, a direct comparison of the AA420 values leads to % for the direct excitation process. This same procedurewas employed for both nanosecond and picosecond experiments. In the picosecond experiments, the AA420 values were taken directly from the kinetic plots when a constant level of absorption had been reached-typically ca. 200 ps postpulse. In the nanosecond experiments,M 4 2 0 values were determined from the t = 0 points of the extrapolated exponential decays of the triplet population.
0022-3654/93/2097-5643S04.00/0 0 1993 American Chemical Society
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5644 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 0.68
7
i\
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Wavelength, nm Figure 1. Absorption spectra of 2 M MN in acetonitrile. Upper spectrum: oxygen-saturatedsolution. Lower spectrum: argon-saturated solution. The arrow indicates the wavelength of excitation.
Since both the test and reference samples were 2 M in MN in thesamesolvent,anypossibleeffectsof the high MNconcentration on extinction coefficients, etc., are canceled. Singlet Oxygen Quantum Yields. Plots of the initial 1.27-rm luminescence (Lo)vs laser intensity were generated according to procedures outlined earlier.' The slopes of the linear portions of such plots are proportional to @A. values were taken as the t = 0 point of the extrapolated exponential decay of the luminescence signals. Both phenazine and benzophenone were employed independently as references for determinations of @A. Phenazine is reported to have a constant @A value in several solvents,*and we have assumed this to be true for cyclohexane and acetonitrile that are both 2 M in MN. The triplet energy of phenazine is 1.95 eV, some 0.64 eV lower than that of MN, thus excluding the possibility of energy transfer from phenazine triplet to MN, thereby ensuring that the only source of singlet oxygen in the phenazine referencesamples is the phenazinetriplet state. The value of @T for benzophenone is unity, independent of solvent, but its triplet energy is higher than that of MN by 1.05 eV. Thus at [MN] = 2 M, benzophenone triplets will be converted quantitatively to 3MN* states, which will subsequently transfer to oxygen. We showed earlier9 that SAfor naphthalene was 1.0 in the solvents used here, and this is expected to hold for MN (vide infra). Therefore, in both solvent systems containing 2 M MN, we anticipate that the reference value of @A to use in the case of benzophenone as sensitizer is unity. 4 values depend also on k,,the rate coefficient for radiative decay of singlet oxygen. This rate parameter is solvent dependent, and normally it would be necessary to correct for this. However, in our experiments each solvent system used (2 M MN in Cyclohexane, and 2 M MN in acetonitrile) individually provides an absolute value of aA. Thus, no corrections are necessary. Materials. Methylnaphthalene (Aldrich) was purified by passage through a silica gel column; cyclohexane (Fluka), acetonitrile (Aldrich), and mineral oil (Aldrich) were used as received. RWult.9
Ground-State Absorption Spectra. The absorption spectra of 2 M solutions of MN in oxygen-saturatedand in argon-saturated acetonitrile are shown in Figure 1. At the laser excitation wavelength of 355 nm, the absorbance of the oxygen-saturated solution was much higher (0.12 per cm) than that of the argonsaturated solution (0.015 per cm). This contact CT absorption
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band was similarly located at the red side of the SO SItransition in cyclohexane and mineral oil. The M N singlet-state threshold energy is higher (3.92 eV) than the energy of the 355-nm (3.49 eV) excitationlight, which is itself higher in energy than the MN triplet energy (2.58 eV).Io The fact that 2 M MN shows absorbance at 355 nm in the absence of oxygen (Figure 1) may arise from hot bands. However, excitation of an oxygen-saturated solution of MN at 355 nm will predominantly lead to the population of the excited state of the (MN-02) complex. Picosecond Flash Photolysis. Excitation of oxygen-saturated solutions of MN (2 M) in cyclohexane with 30-ps pulses at 355 nm led to the formation of an absorption band with maximum around 420 nm shortly after the laser pulse (Figure 2A). Within the period during which the pump and probe pulses were overlapping, it was possible to observe a band with a maximum around 427 nm, but the conversion to the 420-nm absorption was fully developed soon after the end of the pulse. No further absorbance changes over the time scale of ca. 2 ns were apparent. The deconvolution of absorbance changes at 420 and 427 nm with the laser pulse profile showed that the rise of the 427-nm peak was indistinguishablefrom the pulse profile, but the growth at 420 nm was delayed with a rise time (l/e) of 20 10 ps (inset in Figure 2A). The 420-nm absorption is reminiscent of the T I T, absorption of MN. In order to ascertain whether the use of such a high concentration of MN may perturb the spectral features of MN (TI), we sensitized its formation via benzophenone (TI), from which triplet-triplet energy transfer to MN proceeds with unit efficiency. In an argon-saturated cyclohexane solution of benzophenone (absorbance 0.5 at 355 nm) in the presence 2 M MN, 355-nm excitation (30 ps) yielded the spectrum (Figure 3) of the triplet-triplet absorption of MN, fully formed within ca. 200 ps. This spectrum shows a strong absorption band at 420 nm similar to that formed by direct excitation of the (MN-02) complex at 50 ps postexcitation. We therefore assign the 420-nm peak as belonging to the TI T, transition in MN (seethe next subsection also). The quantum yield of formation of MN (TI) at 50 ps postexcitation was determined to be 1.1 f 0.15 by the procedure outlined in the Experimental Section. In addition to using cyclohexane as the solvent, a polar solvent (acetonitrile) and a high-viscosity nonpolar solvent (mineral oil) to limit diffusion were employed. The dynamics of the development of the absorbance changes were essentially the same in all solvents (Figure 2B,C). However, in the case of acetonitrile the value of the triplet yield was found to be 0.4 f 0.1 5 , only ca. 40% that in cyclohexane. As above, the reference was benzophenone in 2 M MN, argon-saturated. In an attempt to identify the absorption band at 427 nm that was generated and decayed very close to the laser pulse after excitation of 3&, we studiedthe spectral changesupon irradiation into the charge-transfer band formed between MN and dicyanobenzene (DCNB) in acetonitrile. Rapid formation of an absorption band with maximum at 430 nm was observed (Figure 4), corresponding to the cation radical of MN+.I1 It seems reasonable therefore to assign the ultrafast species at 427 nm in irradiated oxygen-saturated solutions of MN as being the absorption of the initially formed CT state, which has a large contribution from the cation radical spectrum of MN. Nanosecond Flash Photolysis. Oxygen-saturated solutions of MN in cyclohexane, acetonitrile, and mineral oil were subjected to 355-nm laser excitation (8-ns pulse), and transient absorbance changes after excitation of were measured. The absorption in the wavelength region 370-470 nm was seen to conform to the spectrum obtainedupon excitation of these solutions under argonsaturated conditions in the presence of benzophenone as the photosensitizer (Figure 5 ) . Thus, excitationinto thecontact band generates the TI T,, spectrum of MN, in confirmation of that described above for the picosecond time regime. In the oxygensaturated solutions, the lifetime of MN (TI) was 50 i 5 ns in
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Molecular Oxygen and 1-Methylnaphthalene
The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 5645
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Wavelength, nm Wavelength, nm
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Figure 3. Superposition of spectra generated by excitation of 2 M MN in oxygen-saturated acetonitrile (open circlcs) and by excitation of benzophenone in anargon-saturatedsolutionofacetonitrilein the presence of 2 M MN (continuous line), divided by a factor of 9. Spectra are taken at 200 ps after excitation by a 30-ps, 355-nm pulse.
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wavelength, nm 370
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Wavelength, nm Figure4. Spectrum generated by 30-ps, 355-nm excitationof thechargetransfer band between MN and DCNB in acetonitrile. Solution contained 2 M MN and 10 mM DCNB. The spectrum is taken 100 ps after excitation. The arrow indicates the position of the peak at 430 nm.
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reaction between the free MN (TI)and oxygen. As before, using benzophenone (TI)sensitization in 2 M MN solutionsas reference, we determined the quantum yields of this free MN (TI). The measured yields were 1.0 f 0.1 in cyclohexane, 1.1 0.1 in mineral oil, and 0.5 f 0.1 in acetonitrile, in confirmation of the picosecond results. Singlet Oxygen Yields. Using the procedure stated in the Experimental Section and using @A = 0.85 for phenazine in both solvent systems, we found that the singlet oxygen yields after irradiation into the contact band are equal to 0.87 f 0.1 in cyclohexane and 0.55 f 0.1 in acetonitrile. After sensitizing singlet oxygen by benzophenone under the same conditions, @A was determined as 1.0 0.1 in cyclohexane and 1.05 f 0.1 in acetonitrile (Figure .7A,B). The lifetime of singlet oxygen in the presence of such high concentrationsof MN is significantly shorter than its lifetime in the pure solvents, in accord with the findings of Kristiansen et al.4b For acetonitrile ( T A = 80 ps), the presence of 2 M MN reduces the lifetime to 20 1s. In the case of cyclohexane ( T A = 24 ps in the absence of the MN), T A was reduced to 19 at [MN] = 2 M.
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Wavelength, nm Figure 2. (A) Absorption difference spectra after irradiation with a 30ps, 355-nm pulse of 2 M MN in oxygen-saturated cyclohexane. Spectra were taken at -15-ps (lowest curve), 15-ps (middle curve), and 45-ps (upper curve) delaysbetween excitationand probe pulses. Arrows indicate themaximum wavelength at the different delays. The inset showskinetic profiles at wavelengths indicated by arrows in Figure 2A (427 nm, open circles; 420 nm, closed circles). Solid lines are the laser profile and its convolution with growth exponents having lifetimes of 0 and 20 ps. (B) Absorption difference spectra of 2 M MN in oxygen-saturated mineral oil under the same conditions as for cyclohexane. Spectra are taken at -20-ps and 90-psdelays between the excitation and probe pulses. (C) As for (B) but with acetonitrile as solvent. Spectra are taken at -5O-ps, -3O-ps, and 60-psdelays between the excitation and probe pulses.
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cyclohexane and in acetonitrile and 110 10 ns in mineral oil (Figure 6). This quenching corresponds to the bimolecular
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Discussion Direct excitation of the (MN-aO2) contact ground state by a 30-ps pulse of 355-nm radiation yields an immediate transient
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5646 The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 0.02
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Wavelength, nm Figure 5. Spectra generated by &ns, 355-nm laser pulse in oxygensaturated acetonitrile in the presence of 2 M MN (open circles) and in argon-saturated acetonitrile in the presence of benzophenone (A355= 0.5), divided by a factor of 8, and 2 M MN (triangles). Spectra are taken 20 ns after excitation.
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Laser energy, arb. units Figure 7. Dependence of singlet oxygen luminescence on laser power after excitation by an 8-ns, 355-nm laser pulse in 2 M MN in oxygensaturated cyclohexane (A) and oxygen-saturated acetonitrile (B). The open circles show these data. On the same plot are shown the references used: benzophenone (triangles) and phenazine (closed circles) in the same solvent in the presence 2 M MN.
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Time, ns Figure 6. Kinetic traces generated by 8-ns, 355-nm laser pulse excitation of 2 M M N in oxygen-saturatedacetonitrile (top),cyclohexane (middle), and mineral oil (bottom). All traces were monitored at 420 nm. The fitted curves arise from exponential fits of the experimental data.
state absorbing maximally at 427 nm. In a few picoseconds this converts to a second species that absorbs maximally at 420 nm.
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This 427 420 nm conversion is convoluted in our instrumental time response, but the 427-nm species has a lifetime that is not greater than 20 f 10 ps. The similarity of the spectral properties of the 427-nm species to those of the M'+ radical cation assists the reasonable assumption that this absorption corresponds to an optical transition originating from the 3(MN+--02-)state formed by 355-nm light absorption into the contact ground state. A few picoseconds thereafter, the spectral properties measured are reminiscent of those of the 3MN* localized state produced by energy transfer from benzophenone (TI)in oxygen-free solution. There are no subsequent spectral changes over an elapsed time of several nanoseconds. Presumably, between the collapse of the ion-pair state and the appearance of the M N (TI)localized state, free of oxygen involvement, there are contact and solvent-separated states of the (3MN*-.02) complex prior to the two molecules diffusing apart to become independent entities. No spectral changes are evident beyond ca. 50 ps; therefore, we must conclude that either the free triplet is formed in the initially observed event or the intermediate stages are not spectrally distinguishable. Fortunately, the quantum yield data provide extra information. The nanosecond time scale experiments show that excitation of the contact ground state generates 3MN* with a quantum yield of unity in cyclohexane and mineral oil and approximately one-half of that value in acetonitrile. These yields were measured at the end of the 8-ns pulse by extrapolation of the 3MN* decay
Molecular Oxygen and 1-Methylnaphthalene
SCHEME I
MN
E
back to the center of the pulse (Figure 6). These evaluated quantum yields depend on the reasonable assumption that on the multiple nanosecond time scale the species being measured is identical to that produced by energy transfer from benzophenone (TI) in oxygen-free solutions. If so, then in the nonpolar media the initial ion-pair state converts quantitatively to the free triplet state, 3MN*, which can subsequently involve itself in energytransfer reactions with oxygen (vide infra). In the polar acetonitrile, however, this conversion is only ca. 50% efficient. These observations are supported by the measured @T values at 50 ps. Our results allow us to propose a reaction scheme based on the energy diagram shown in Scheme I. Only intramolecular processes are depicted in Scheme I. As envisioned, the primary ion-pair state has three channelsthrough which it can lose its identity. It can radiationlessly decay to the ground-state surface as a consequence of charge recombination ( k l ) , it can reach the 3MN* state in an analogous charge recombination step (kz),or it can undergo intersystem crossing (k3) to its singlet-state counterpart, which could subsequently form singlet oxygen in a spin-allowed reaction. In the nonpolar solvents, the fact that aT= 1 indicates that kz >> kl + k3. Further, k2must beof theorder of 1011s-I since the picosecondinformation tells us that the ion-pair state moves onto the neutral triplet surface (perhaps with an attendant 0 2 molecule) with a lifetime of 20 f 10ps. Subsequentstaticquenching by attendant 0 2apparently does not occur. Recent work in this laboratory16 indicates that at room temperature in these solvents the MN-02 exciplex collapses to free 3MN* and O2species much more rapidly than it does to give Oz(lA,). It is worthwhile commenting on where the energy of the ionpair state lies in relationship to that of the localized 3MN* state. This latter has an energy of 2.58 eV, and the fact that our picosecond studies indicate that this is reached from the ion-pair state in a few picoseconds is consistent with the conclusion that the ion-pair state lies close to 3 M V . Then, the energy gap law helps us understand why k2>> kl (see Scheme I). In agreement, we note that the ion-pair state is populated by absorption of 355nm light (3.49 eV), and it has been stated to be of higher energy in naphthalene (and presumably MN) by Birks12 and by Kristiansen et al.4b In a recent report, McGarvey et al.13employed thermodynamic data to estimate that the charged-pair state in MN is 25 f 10kJ mol-', or 270 f 110 mV, below the 3MN*state. Our evaluated rate constant for the CT 3MN* conversion implies that the thermodynamic value may be inaccurate. The data recorded with acetonitrile as solvent indicate a somewhat modified picture from that shown by cyclohexane. As with the nonpolar solvent results, the rapidly formed 3MN*states are quenched by 0,(3ZJ, and the quantum yield for singlet oxygen is in quantitative agreement with the triplet quantum yield; however, in acetonitrile both depart significantly from unity. That the value of @T at 50 ps is 0.5 0.15 is indicative that the rate parameter kl (Scheme I) is no longer dominant. Now, quenching to the ground state (kl) or intersystem crossing (k3)
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The Journal of Physical Chemistry, Vol. 97, No. 21, 1993 5647 to ,E4becomes competitive. As far as we are able to tell given our instrumental time resolution (ca. 10 ps), the rate constant for the deactivation of 3(MN+*-Oz-) is the same in acetonitrile as it is in cyclohexane. It is instructive to compare our new nanosecond scale data with those from Ogilby's laboratoryb and with earlier work from this laboratory? There is complete agreementthat in cyclohexane solvent the quantum yields for 3MN* production and 02(IA8) formation are unity after excitation into the contact absorption band. In this respect, such a method of producing these species is quantitatively equivalent to producing them via an energytransfer process that employs a photosensitizer having @T = 1. In other words, the CT route is more effective in producing 3MN* than is the intersystem crossing from an unperturbed IMN* species. All singlet oxygen arises from bimolecular energytransfer processes, and thus every oxygen-quenching process involving 3MN* leads to singlet oxygen, Le., SA= 1 for MN, by analogy with an identical value for naphthalene in cycl~hexane.~ The acetonitrile solution case is different and contains some inconsistencies between the published work of Kristiansen et al. and the current and earlier data from this laboratory. Our current data show that the triplet and the singlet oxygen quantum yields are O S . As noted above, the lower triplet yield at 10 ns is unchanged from the 50 ps value, indicating that an alternative decay route exists for the ion-pair state and implying that, as with cyclohexane, all 02('A,) arises from collisional interactions between 0 , ( 3 Z ~ )and 3MN* and, further, that SA = 1 in acetonitrilein accord with our earlier conclusionsfor naphthalene? Kristiansen et al.4b did not report @T values (or relative triplet yields) in acetonitrile, but they did report that the singlet oxygen yield from the photosensitized process was more efficient than that from excitation into the ion-pair state. They conclude that in polar solvents such as acetonitrile there will be a CT character enhancement that can result in more efficient production of radical ions and a decrease in the CT-state energy, thereby facilitating coupling of excited compound states to the ground triplet state, resulting in Sa values less than unity. Our data, however, show that the lifetimeof theexcited 3(MN+.-02-) in acetonitrile is the same (within experimental error) as its lifetime in cyclohexane, so any differences between the MN triplet state and CT state in the different media have no noticeableeffect on the rate constant of 3M* formation and fail to explain the lower triplet and singlet oxygen yields in acetonitrile. The present observations clearly show that SAis equal to unity and that the lower @A value in acetonitrile arises from the lower value of @T. The fact that the sole source of singlet oxygen from 355-nm excitation of the ground-state contactcomplex is by energy transfer from the triplet state of MN indicates that the lower value of @T in acetonitrile arises because k2 and kl are of similar magnitude in that medium. In other words, the intersystemcrossing process (k3 in Scheme I) is not competitive with the other modes that deactivate the CT state; otherwise, @a would exceed @T since crossing into the singlet version of the CT complex comprises an entry into an intramolecular route to 02(lAg). All the evidence, then, points to the situation that in polar solvents such as acetonitrile k2 and kl are close to equal and in the region of (2.5 & 1.2) X 10'0 s-1, whereas in cyclohexane ka >> kl but remains near to (5.0 f 2.5) X 1010 s-1, The processes controlled by kl and k2 are
3(M+:O;) '(M+:O;)
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3M* (localized T I )
3E0 (contact ground state)
k,
k,
Both are charge recombination processes, but the driving force for kl is ca. 2.6 eV greater than that for kz. This opens the possibility that the rate constants are on different slopes of a parabolic In k vs AG ~ u r v e . ~Electron-transfer ~J~ processes with
5648 The Journal of Physical Chemistry, Vol. 97, No. 21, 195'3
rate constants of ca. 5 X 1010 s-1 and above probably have a high degree of adiabaticity, and the parabolas would be expected to beof the flat-topped typel4J5for a polar solvent suchasacetonitrile. In this case, in Marcus terminology, k2 will be located in the normal region and klin the inverted region of this parabola. In such a situation, it is possible that the rate constants k2 (AG2 = 0 eV) and kl (AGI = 2.6 eV) will have the same value, close to 2.5 X 10'0 s-I. In the case of nonpolar solvents such as cyclohexane, the application of the Rips-Jortner formulaI5 is not valid, but the driving force dependence of the rate constant will follow a parabolic shape with normal and inverted regions. In nonpolar solvents, the reorganization energy will be significantly smaller than it will be in acetonitrile, which will shift the In k vs AG curve to the left (lower values of AG). In this situation, k2 can assume a much greater value than kl,because the latter will be located more deeply in the inverted region compared to the acetonitrile case. Moreover, in the polar solvent the CT state will be lower than the CT state in nonpolar solvents. This will tend to decrease the value of k2 in acetonitrile compared to that in cyclohexane. Thus, in cyclohexane k2 >> kl,and in acetonitrile k2 kl. The uncertainty in the CT 3MN*energy gap prevents analysis of this kind from being rigorous. Nevertheless, the concept is soundly based on accepted treatments of the electron-transfer question, and the analysis provides an understanding of the differences in the @T values deriving from the CT-state deactivation in solvents of different polarity.
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Conclusions We provide experimental evidence to show that the contact state MN-02 has a significant amount of charge-transfer character. In MN, where the CT state is located close to the triplet state, the lifetime of the contact ion pair is about 20 ps. Weargue that thereare twocompetitive pathways for deactivation of this radical pair: (i) to the localized triplet state and (ii) to the triplet ground state (3E0). In a nonpolar solvent, because of a favorableenergetic situation, decay to the localized triplet state dominates. In a polar solvent, the competition between the two recombination pathways is approximately equal and reduces the quantum yield of the triplet state and thereby that of singlet oxygen. There is no evidence that intersystemcrossing in the CT state results in direct entry to the singlet manifold; therefore, the rate constant for intersystem crossing in the CT state must be at least 1 order of magnitude less than that for the decay of the CT state, viz., 5 X lo9s-I. In all cases, the precursor of singlet oxygen is the free triplet state of MN. Acknowledgment. This work was supported in part by the National Institutes of Health (Grant GM24235) and by the Center
Logunov and Rodgers for Photochemical Sciences at Bowling Green State University. We are grateful to Drs. A. A. Gorman and A. J. McLean for helpful discussions.
Note Added in Proof. After this paper was submitted, a report by McGarvey et al. appeared'' which closely parallels this work. One major difference concerns the origin of O2(lAg) after irradiation into the contact band in CH3CN solutions. Our conclusion is that all the singlet oxygen is produced from MN triplet state; McGarvey et al. state that there must be an additional source of 02(]A,) over that from the ethylnaphthalene (EN) triplet, perhaps a doubly-excited complex state. One discrepancy arises from Sa measurements in the benzophenone-sensitized experiments. Our value of unity is in agreement with published data for naphthalene from this laborat~ry.~ The McGarvey et al. value of 0.51 seems to be low. Furthermore, their quoted values of @A at 1.5 M EN appear to be overestimated since they do not record any correction of their 1.27-pmluminescence data for medium refractive index in accordance with the work of Kristiansen et al.4b References and Notes (1) (a) Tsuboruma, H.; Milliken, R. S. J . Am. Chem. SOC.1960, 82, 5966. (b) Evans, D. F. J . Chem. SOC.1953, 345. (2) (a) Birks, J. B.; Pantos, E.; Hamilton, T. D. S. Chem. Phys. Lett. 1973,20,544. (b) Kawaoka, K.; Khan, A. U.; Kearns, D. R. J . Chem. Phys. 1967, 47, 1883. (c) Murrell, J. N . Mol. Phys. 1960, 3, 319. (d) Khan, A. U.; Kearns, D. R. J . Chem. Phys. 1968,48, 3272. (3) Sakuragi, H.; Furusawa, G.; Ueno, K.; Tokumaru, K. Chem. SOC. Jpn. 1982, 1213. (4) (a) Scurlock, R. D.; Ogilby, P. R. J . Phys. Chem. 1989, 93, 54935500. (b) Kristiansen, M.; Scurlock, R. D.; Iu, K.-K.; Ogilby, P. R. J. Phys. Chem. 1991, 95, 5190. (5) Logunov, S. L.; Rodgers, M. A. J. J . Phys. Chem. 1992, 96,2915. (6) Shand, M. A.; Rodgers, M. A. J.; Webber, S. E. Chem. Phys. Lett. 1991, 177, 11. (7) Firey, P. A.; Ford, W. E.; Sounik, J. R.; Kinney, M. E.; Rodgers, M. A. J. J . Am. Chem. SOC.1988, 110, 7626. (8) (a) Redmond, R. W.; Braslavsky, S. E. In Photosensitisation.
Molecular, CellularandMedical Aspects; Moreno, G., Pottier, R. H., Truscott, T. G., Eds.; NATO AS1 Series; Springler-Verlag: Berlin, 1988; pp 93-97. (b) McLean, A. J. Personal communication. (9) Gorman, A . A.; Hamblett, I.; Lambert, C.; Prescott, A. L.; Rodgers, M. A. J.; Spence, H. M. J . Am. Chem. SOC.1987, 109, 3091. (IO) McClure, D. C. J. Chem. Phys. 1949, 17, 905. (11) (a) The DCNB anion radical has no absorption in the range 400-450 nm, and the cation radical of MN+ has characteristic peaks in the visible part of spectra.'Ib (b) Rodgers, M. A. J. Trans. Faraday SOC.1971, 67, 1029. (12) Birks, J. B. Photophysics of Aromatic Molecules; Wiley-Interscience: London, 1970; pp 403-412, 492-496. (13) McGarvey, D. J.; Szekeres, P. G.; Wilkinson, F. Chem. Phys. Lett. 1992, 199, 314. (14) (a) Gould, I. R.; Farid, S. J . Phys. Chem. 1992,96,7635. (b) Gould, I. R.; Young, R. H.; Moody, R. E.; Farid, S. J . Phys. Chem. 1991,95,2068. (15) Rips, I.; Jortner, J. J . Chem. Phys. 1987, 87, 2090. (16) McLean, A. J.; Rodgers, M. A . J. J . Am. Chem. Soc., in press. (17) McGarvey, D. J.; Wilkinson, F.; Worrall, D. R.; Hobley, J.; Shaikh, W. Chem. Phys. Lett. 1993, 202, 528.
