193
J . Phys. Chem. 1993, 97, 193-195
Quenching of blZg+ Oxygen in Solution Bojie Wang and Peter R. Ogilby' Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 871 31 Received: August 31, 1992; I n Final Form: October 5, 1992
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b12,+02 has been directly monitored by its fluorescence a t 1926 nm (blBg+Oz alAg02). Stern-Volmer quenching studies have been performed for seven organic molecules in a host solvent of CC4. The quenching of b12,+O2 follows some of the same trends observed for the quenching of a1A,02, as expected for a process defined, in part, by electronic-to-vibrational energy transfer. Deuterium isotope effects for quenching, however, differ substantially for b12,+02 and a1Ag02. The data are interpreted within the context of an exchange mechanism for energy transfer.
Introduction The ground state of molecular oxygen is a triplet (X3Z,-02). The lowest excited electronic state, a1AK02,often simply called singlet oxygen, lies 22.5 kcal/mole above X3Z,02. The recent discovery that a1A,02 phosphorescence (alA,02- X3Z,-02; 1270 nm, 7874cm-I) can be directly monitored in both liquid solutions1q2 and solid polymers3 has been one of the more useful and informativeadvances in the field. A second singlet state, b1Zg+02, lies 15 kcal/mol above a1Ag02.It has been suggested, on the basis of theoretical arguments,4*5and documented by gas-phase experiments+* that, when a triplet-state organic molecule of sufficient energy (greater than -38 kcal/mol) is quenched by XjZ,-O2, the well-known energy-transfer process that forms alA,O2 can proceed via the intermediacy of b1ZK+02(eqs l-2).9
-
%ens
+ X32,-02
b'Z,+02
+M
-
Sens
+ b128+0,
1.00
-
0 7 1
-
050
-
025
-
>
lH Ln
A
z
W I-
5 W
>
H F
5 w
a: 0 00
(1)
12
TIME
alAgO,
+M
(2)
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Chou and Freilo have recently observed fluorescence from b1Z,+02 (b1ZK+O2 a1A,02; 1926 nm, 5192 cm-I) upon X3Z,-02 quenching of triplet benzophenone (ET = 69 kcal/mol) in a solution of CC14, thus indicating that, like a1A,02, b1Zg+02can be directly monitored in liquid solutions. They reported an upper limit for the bl2,+02 lifetime in C C 4 of 24 ps (determined by the time resolution of their detection system). As part of our research program to investigate excited-state interactions between oxygen and organic molecules, we would like a better understanding of the solution-phase bimolecular processes that result in b1Zg+02deactivation (e.&, eq 2). Thus, we set out to quantify b1ZK+02quenching by a variety of organic molecules and compare our results with those obtained previously for the quenching of a1A,02.132
Results and Discussion We have detected the 1926-nm fluorescence from b1Zg+02in benzophenone and acridine (ET = 45.2 kcal/mol1I) photosensitized experiments (Figure 1).12 Like those of Chou and Frei,lo our data are also limited by the time resolution of our detector/ amplifier combination (2 ps) rather than the b1Z,+02 lifetime (when monitored with the same detector equipped with a 1270nm interference filter, 7(alA802) > 1 ms, in agreement with previous reportsj.2). In addition to the fact that the emission was obtained at 1926 nm, which is coincident with the blZK+02alA,02 energy gap,lo several points were used to assign the data to bIZ,+02 fluorescence. The 1926-nm emission was not observed
* Author to whom correspondence should be addressed. 0022-3654/58/2097-0193$04.00/0
16
D
(microseconds)
Figure 1. Time-resolved emission signal (1926 nm) observed subsequent to the 355-nm photolysis of benzophenone in aerated CC14. The data (average of those obtained from 5 12 laser pulses) are limited by the time resolution of our detection system ( 2 p s ) .
when (1) a benzophenone-free, oxygenated CC14 solution was irradiated at 355 nm, (2) the benzophenone/CC14 solution was bubbled with nitrogen and irradiated, (3) meso-tetraphenylporphine was used as the sensitizer (ET = 33.5 kcal/mol14 < E(b1Z,+02)) under conditions in which the 1270-nm phosphorescence of a1A,02 was observed, and (4) benzene was used as the solvent instead of CCL. The latter result is consistent with gas-phase b12,+02 quenching data;I5J6CCl, is a comparatively poor b1ZK+02 quencher (kqKas = 7.8 X lo5 s-I M-I) and perdeuterated alkanes are better, whereas hydrocarbons are efficientquenchers (kqBas(C6H14)= 5.5 X lo8s-l M-1).15J6Thus, it is likely that the quantum efficiency of b1Z,+02 fluorescence is sufficiently small in benzene as to preclude detection with our system, a conclusion consistent with data presented below. Addition of small quantities of a hydrocarbon to CCl, results in a quantifiable decrease in the blZ,+O2 fluorescenceintensity.17 Stern-Volmer treatments for the "quenchers" methanol, n-pentane, benzene, toluene, and acetone yield linear plots (Figure 2)with a slope of k,ro (eq 3).l* Resultant data are presented in IJI = 1 + kq~o[Ql
(3)
Table 1. Flash absorption measurements indicated that the yield and lifetime of the triplet sensitizer did not change over the concentration range of added quencher, thus indicating the absence of deactivation pathways that could compete with quenching by X3Z,-02 and the subsequent formation of b'Zs+02. Data obtained for the quencher methanol yielded the largest 0 1993 American Chemical Society
Wang and Ogilby
194 The Journal of Physical Chemistry, Vol. 97, No. I, 1993
1
3 001
20,000 2 00-
15,000 H
1 00
\ 0
I
1
10,000 0 001
,
0
I
I
1
I
1
1.2
2 4
3 6
4 0
6
[Toluene]
(IO-*)
5,000
Figure2. Stern-Volmer plot (Io/]= 1 + kqro[Q])obtaineduponaddition of toluene to CCls.
TABLE I: b'Z,+02 Quenching Constants in CClr quencher methanol n-pentane toluene benzene
kaTO,
M-I
quencher acetone benzene-& acetone-&
7 9 f 1" 44 f 4 28 f 3 28 f 3
0
24 f 2" 10f 1 5.4 0.5"
*
These numbers may also reflect the presence of a small amount of water, which, at least in the gas phase,15J6is an excellent quencher of bl Zg+02.
TABLE 11: Deuterium Isotope Effects for the Deactivation of Oxvnen Excited States (k!/k!) ~~~
1.oo
k,ro, M-l
~
quencher
b'Zg+02
a1AgOf
benzene acetone
2.8 4.4
24 15
Calculated from a'Ag02 lifetime data in the neat solvent.l-2
Stern-Volmer slope. This is consistent with gas-phase results in which alcohols are known to be some of the best quenchers of blZ,+02; &,gas (CHSOH) = 2.4 X 109 s-l M-1.15J6This is also consistent with solution-phase a1A,02 data. For example, the a1A,02 lifetime in liquid methanol is one of the shortest known (- 10 ps), which is surpassed only by a few other solvents (e& water, ~(alA,Oz) -4 If we assume that k, for methanol is limited by diffusion, then dividing our methanol Stern-Volmer slope by kdirf = 2.5 X 1010s-1 M-1 19 yields a blZ,+02 lifetime (TO) in CC14 of 3.2 ns. This estimate is consistent with predictions made by Ogryzlo, among others,I5J6 in which the lifetime of blZ,+O2 in solution is expected to be very short, certainly much shorter than the a1A,02 lifetime in the same solvent. Stern-Volmer slopes were also determined for benzene-& and acetone-d6 (Table I). Like quenching data recorded for a1Ag02,132*20 the Stern-Volmer slopes are smaller for the perdeuterated analogues. These results are consistent with a deactivation process that is dependent, in part, on electronicto-vibrational energy transfer.2O.21 Because TOshould be constant for all the Stern-Volmer slopes obtained, ratios of the perdeuterated and corresponding perprotonated data in Table I should yield an accurate isotope effect for blZK+02quenching. Deuterium isotope effects are shown in Table I1 for both bIZK+02and alA,02 deactivation. It is clear that b12,+02 is less sensitive to this perturbation than a'Ag02. It is well documented that the overall lifetime of a1A,02 in solution is principally determined by nonradiative deactivation channels in which the a1A,02 energy of excitation is transferred to specific vibrational modes of the solvent.l.2 We have recently
1.eo
1.40
Internuclear Distance (A)
Figure 3. Potential energy curves for blZg+,alAg,and X3Zg- oxygen. The energies corresponding to typical values of the fundamentalstretching frequencies of three quencher bonds are also shown with an origin at the lowest vibrational levels of the blZg+and alAg states (0-H= 3600 cm-I, C-H = 3050 cm-I, C-D = 2240 cm-I).
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shown, however, that large solvent effects on the a1A,02 X3Z,-02 radiative rate constant can influence intensity measurements of a1A,02 in solution.13c Furthermore, we have determined that solvent parameters/characteristics that correlate with the a1A,02radiative lifetime do not correlate with the overall alA,02 lifetime and vice versa (i.e., the solvent-induced radiative and nonradiative processes are indeed independent).l3e By extension, it is possible that the blZ,+Oz --c a1A,02 radiative transition may likewise be perturbed by the quenchers used in this study and in turn will contribute to changes in the ratio &,/I shown in eq 3. Thus, even though it appears that, like a1A,02, our present b1Zg+02quenching data may principally reflect a process of nonradiative electronic-to-vibrational energy transfer, a rigorous interpretation of our results based solely on the latter phenomenon may not be appropriate. Furthermore, values of kqr, obtained by the present method may also reflect quencherinduced deactivation of blZK+02to X3Z,-02. (In the gas phase, the radiative transition blZK+02 X32,-02 is more probable than the blZK+02 aIA,O2 radiative process.22 In preliminary solution-phase experiments, however, we have not yet been able to detect the 762-nm phosphorescence from the transition blZ,+O2 X3Z,-02 under conditions in which the 1926-nm fluorescence of b1ZK+O2was observed. These data are consistent with conclusions presented below.) Although our values of k, may indeed reflect several different radiative and nonradiative quencher-induced b1Z,+02 deactivation channels, we will nevertheless proceed to examine our results within the context of an exchange mechanism for electronic-tovibrational energy transfer, as applied by Schuster and Hurst2' to the deactivation of a1A,02. In taking this step, we assume that, in solution, bIZ,+Oz deactivation is principally controlled by the nonradiative b12,+02- alA,02 channel. The key features of this model are shown in Figure 3. In this approach, the magnitude of the quenching rate constant is proportional to the product of three terms summed over all the available vibronic transitions (eq 4). F, is the Franck-Condon factor for the 0 s vibrational transition of the quencher (solvent) and F,,, is the corresponding
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The Journal of Physical Chemistry, Vof. 97, NO. 1 , 1993 195
Quenching of blZg+Oxygen (4)
0 --* m factor for the transitions between a1Ag02and X3Z> RE;D;vide supra), we propose that this near resonance between the oxygen 0 2 transition and the C-D stretching frequency may contribute to a less pronounced solvent isotope effect for bIZ,+Oz deactivation. Specifically, the inequality RE;D>> RE;Hfor blZ,+O2 deactivation may sufficiently mitigate the large Franck-Condon ratio FI/F2 such that the overall isotope effect for b1Zg+02deactivation will be smaller than that for a1A,02.
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In conclusion, we have demonstrated that, in solution, (1) the lifetime of b1Z,+02 is indeed very short, (2) even small amounts of a dissolved hydrocarbon can preclude direct detection of the b1Zg+02fluorescence a t 1926 nm, (3) the quenching of b1Zg+02 follows trends observed in the gas phase,l5J6 (4) the quenching of blZg+02 follows some of the same trends observed for the nonradiative quenching of a1A,02 as expected for a process defined, in part, by electronic-to-vibrational energy tran~fer,l-~,I3e ( 5 ) our quenching data are consistent, to a first-order approx-
imation, with a model based on exchange energy transfer and bl Z,+02 deactivation appears to be principally controlled by the bl Z,+02 a1A,02 nonradiative transition, and (6) deuterium isotope effects for b1Zg+02quenching are smaller than those for a1Ag02,a result that can likewise be interpreted within the context of the exchange energy transfer model.
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Acknowledgment. This work was supported by the U.S. Army Research Office and the National Science Foundation (NSF Grant CHE-882 1324). References and Notes (1) Singlet Oxygen: Frimer, A. A., Ed.; CRC Press: Boca Raton, FL, 1985; Vols. I-IV and references cited therein. (2) Gorman, A. A.; Rodgers, M. A. J. In CRC Handbook of Organic Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; Vol. 11, pp 229-247 and references cited therein. (3) (a) Ogilby, P. R.; Dillon, M. P.; Kristiansen, M.; Clough, R. L. Macromolecules 1992, 25, 3399-3405. (b) Ogilby, P. R.; Kristiansen, M.; Clough, R. L. Macromolecules 1990, 23, 2698-2704. (c) Clough, R. L.; Dillon, M. P.; lu, K.-K.; Ogilby, P. R. Macromolecules 1989,22,3620-3628. (d) Gao, Y.; Ogilby, P. R. Macromolecules 1992, 25, 4962-4966. (4) Kawaoka, K.; Khan, A. U.; Kearns, D. R. J . Chem. Phys. 1967,46, 1842-1853. (Erratum: J. Chem. Phvs. 1967. 47. 1883-1884). ( 5 ) Gijze'man, 0. L. J.; Kaufman, F. J . Chem. Soc., Faraday Trans. 2 1973,69, 721-726. (6) Duncan, C. K.; Kearns, D. R. J . Chem. Phys. 1971,55,5822-5823. (7) Andrews, L. J.; Abrahamson, E. W. Chem. Phys. Lett. 1971, I O , 113-116. (8) Davidson, J. A.; Kear, K. E.; Abrahamson, E. W. J . Photochem. 1972/73, I, 307-316. (9) Sens = organic molecule/photosensitizer. M = organic molecule/ buffer gas/quencher. (10) Chou, P.-T.; Frei, H. Chem. Phys. Lett. 1985, 122, 87-92. (1 1) Kellmann, A. J . Phys. Chem. 1977,81, 1195-1 198. (12) Theinstrumentationandapproach weusetomonitor (a) weakinfrared luminescence in photosensitized experiments and (b) organic molecule triplet states have beenpublished.I3 For the present experiments,a 77 K InSbdetector, equipped with a cold filter, was used to detect the 1926-nm fluorescence. The latter wasisolated withan interference filter (FWHM = 25 nm). All solutions were prepared to yielda sensitizerabsorbanceof 1.Oat thephotolysis wavelength (355 nm, 1.5 mJ/pulse). (13) (a) Iu, K.-K.;Scurlock, R. D.;Ogilby, P. R. J . Photochem. 1987,37, 19-32. (b) Iu, K.-K.; Ogilby, P. R. J . Phys. Chem. 1987, 91, 1611-1617. (Erratum: J . Phys. Chem. 1988, 92,5854.) (c) Scurlock, R. D.; Ogilby, P. R. J . Phys. Chem. 1989, 93, 5493-5500. (d) Kristiansen, M.; Scurlock, R. D.;Iu,K.-K.;Ogilby,P. R. J.Phys.Chem.1991,95,5190-5197. (e)Scurlock, R. D.; Ogilby, P. R. J . Phys. Chem. 1987, 91, 4599-4602. (14) Gouterman, M.; Khalil, G.-E. J . Mol. Spectrosc. 1974, 53, 88-100. ( 15) Ogryzlo, E. A. In Singlet Oxygen: Reactions with Organic Compounds and Polymers; Ranby, B., Rabek, J. F., Eds.; John Wiley: New York, 1978; pp 17-26 and references cited therein. (16) Ogryzlo, E. A. In Singlet Oxygen; Wasserman, H. H., Murray, R. W., Eds.; Academic Press: New York, 1979; pp 35-58 and references cited therein. (17) Because quencher-induced deactivation of b'Zp+Ol is so facile, it is essential to avoid contaminating the sample with impurities. For example, plasticizers from polymeric tubing can be entrained in oxygen/air streams used to bubblethesolutionandcan thus precludedetectionof b1Z,+02(tubing made of Teflon is satisfactory). Similarly, a high grade of oxygen, free of hydrocarbons and water, must be used. (1 8) Signal intensities were obtained by integrating data such as those in Figure 1. Rise and fall times for the observed signal are detector limited. (19) Ware, W. R. J. Phys. Chem. 1962, 66, 455-458. (20) Ogilby, P. R.; Foote, C. S. J . Am. Chem. Soc. 1983, IO5,3423-3430. (21) Hurst, J. R.; Schuster, G. B. J . Am. Chem. SOC.1983, 105,5 7 5 6 5760. (22) Krupenie, P. H. J. Phys. Chem. Re/. Data 1972, 1 , 423-534. (23) Davidson, J. A.; Ogryzlo, E. A. Can. J. Chem. 1974, 52, 240-245. (24) Nicholls, R. W. J. Res. Nat. Bur. Stand. 1965, 69A, 369-373.
