2874
J. Phys. Chem. 1994, 98, 2874-2876
Time-Resolved Measurement of O,( '2;) in Solution, Phosphorescence from an Upper Excited State R. Schmidt' and M. Bodesheim Institut fur Physikalische und Theoretische Chemie. Universitat FrankfurtlM, Marie-Curie-Strasse 11, D 60439 FrankfurtlM, Germany Received: August 10, 1993; In Final Form: December 22, 1993"
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The second excited state 'E: of 0 2 has been detected for the first time in time-resolved measurements at room temperature in solution. The lifetime 7 2 of O2('Z:) was determined via the 'El 'E; phosphorescence to be 130 f 10 ns in CC4. In addition rate constants of O2('Zl) quenching by several solvent molecules M have been measured in diluted solution in CC4,allowing the estimation of values of 7 2 in pure solvents of about 20 ps in methanol, 80 ps in cyclohexane, 120 ps in acetone, 130 ps in acetonitrile and in benzene, and 1.2 ns in chloroform.
Introduction The ground state of the dioxygen molecule is a triplet state (32-)whereas both lowest excited electronic states are singlets w i d excitation energies of 7882 cm-1 ('AB) and 13121 cm-l ('2;).' The 'A, 3Z- transition is strictly forbidden resulting B in extraordinary long lifetimes of Oz(lA,) ranging in solution from about 4 X 10-6 s (H20)to 3 X 10-1 s (perfluorodecalin).24 The large variation of with solvent was explained consistently by a collisional electronic to vibrational energy transfer from 02(1AB)to single terminal bonds (=oscillators) X-Yof the solvent molecule M occurring with rate constant k&.3*7-9kxAy correlates exponentially with the X-Y stretching energy EX^. The quantitativeanalysisof the 1602/'*02isotope effect on therateconstants revealed that overtone excitations of X-Y play an important role in radiationless 02(1AB)deactivation? Gas-phase measurements demonstrated that compared with Oz('A,), collisional deactivation of O,('Z;) occurs about 6 orders of magnitude faster.10 This enormous increase indicates that the spin-allowed quenching 0,('2; 'A,) is the dominant deactivation path. Again quenching rate constants are composed additively from rate constants k:y of 0,('2;) deactivation by terminal bonds X-Y of the quencher molecule M. Very short lifetimes r2 of O,(Z ' :) in solution have been estimated, ranging from about 6 X 10-12 s (H20) to 3.6 X s (CC14), by extrapolation of rate constants of O,('Z;) deactivation by M determined in the gas phase." O,('Z;) can be monitored by luminescence spectroscopy via two emissions. The O,('Z;-'A,) fluorescence is observed at 1908 nm in the gas phase.', The radiative lifetime 7; of this electric quadrupole transition is very sensitive to perturbations. 7 ; decreases from 400 s in the diluted gas phase', to 2.5 X s in an Ar matrix at 4 K where the transition acquires partial electric-dipole character.13 A further enhancement of 7; can be expected in organic solvents. Actually the first direct detection of O,('Z:) in solution at room temperature was performed by Chou and Frei14 monitoring the 02('Z8++'Ag) fluorescence at 1926 nm in CC14. Later Wang and Ogilby carried out 0, ('2;) quenching experiments in CC14 using the same technique.15 However, no time-resolved experiments were possible because of the slow rise time of the near-infrared detectors used ( 1 2 ps). The second radiative transition of O,('Z:) is its '2: 3Zi phosphorescence occurring at 762 nm in the gas phase16 (765 nm
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in liquid O2I7)in a spectral region where detection by fast photomultipliers is possible. However, the probability of emission of that magnetic dipole transition is unfortunately very small and rather insensitive to collisional perturbation as is demonstrated by comparison of the radiative lifetimes 7; of O,('Z:) of about 12 s in thevacuum and of 6 sin a perhalogenated solvent.'* Thus, time-resolved detection of 02('Z;) in solution is hampered by the low phosphorescence quantum yield which we estimate to reach a maximum value of about at room temperature in CC14. As we will show for the first time, it still is possible to follow the photosensitized formation and the decay of 0, ('2;) in CC14 via its phosphorescence and to determine directly rate constants of O,('Z:) deactivation. Results and Discussion
02('2;)was produced by N2 laser pulse excitation (pulse width 7 ns) of the aromatic ketone phenalenone (PH) dissolved in 02-containing CC14.19 PH undergoes efficient intersystem crossing with quantum yield QiSc= lZoand sensitizes 02(lAg) in air-saturated CC1, with high efficiency of Q A = 0.97.22 Its triplet energy ET = 15 500 cm-1 allows exothermic sensitization of 02('2;)which could be the precursor of Oz(lA,). Emissions observed at 765 nm were always dominated by a very strong but rapidly declining luminescence, resulting probably from emissions of sensitizer and of solvent impurities, followed by a slowly decaying tail, which we identify on the basis of our results (see below) as phosphorescence of 0,('2:).23At 695 nm, where O,('Z;) does not emit, we observed also a very strong, rapidly declining background luminescence. To eliminate the perturbing background emissions, a difference technique was used. For each solution a second sample was prepared which differed from the first only by the addition of a quencher of O,('Z+). For instance, 7 vol % cyclohexane or 1.5 vol % methanol are sufficient to quench O,('Z:) completely, whereas the Oz(lAs) sensitization efficiency by the triplet stateof PH is not affected by these Thus, the difference of the emission signals of PH solutions recorded at 765 nm without and with these amounts of quenchers should correspond to the phosphorescence of 0,('2:). Actually, we obtained net difference emission signals at 765 nm but not at 695 nm. Figure 1 presents as examples three difference curves AZ7a5(t) measured a t different concentrations [02] .z4 For evaluation the curves were fitted by convolutions of the laser pulse with eq 1.
Abstract published in Aduance ACS Abstracts, February IS, 1994.
0022-3654/94/2098-2874$04.50/0
0 1994 American Chemical Society
Time-Resolved Phosphorescence of
02(
Zg+)
The Journal of Physical Chemistry, Vol. 98, No. 11, 1994 2875
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LO, 3/mM Figure 2. [02] dependence of TT. Data of Table 1 .
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Figure 1. 02(1Zl+3Z-) phosphorescence curve A1765(t)following laser = 1.02 X pulse excitation of [Pd]= 1.7 X 1V M in CCld at [02] 6.8 X l e 3 ,and 2.3 X M (from top tobottom). Fits areconvolusions calculated with parameters given in Table 1 . Inset: phosphorescence curve AI,55o(t) of [PHI= 1.7 X 1V M. All phosphorescences recorded at 25 O C in air-saturated CCl4.
TABLE 1: Fit Parameters of the [02]-Dependent Transient Curves of 02(l2:) Given in Figure 1 ([PHI = 1.7 X 10-4 M) [021, M A, au 72,ns 10% TT, ns & 10% 2.3 x 10-3" 7.4 128 218 3.5 x 1 0 - 3 6.7 132 143 6.8 X 10-3 9.6 x 1 0 - 3 10.2 x 10-3
6.3 5.6 6.4
129 143 129
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Air-saturated solution. The relation has been derived for the time dependence of the 0,('2:) phosphorescence I&) sensitized by the triplet state T1 of PH with the lifetime rT, and the decay time of O,('Zl), 7 2 . The constant A is proportional to the efficiency of 0 2 ( l Z + ) formation. The biexponential fit to u 7 6 5 ( t ) results in rT = 2f8 f 20 ns and 72 = 128 f 13 ns for air-saturated samples.
To further substantiate the interpretation of N 7 6 5 ( f ) as timeresolved O,('Z;) phosphorescence, we determined the TIstate lifetime of the sensitizer by independent emission experiments. Because ET = 15 500 cm-l, the phosphorescence of PH can be monitored through a 650-nm interference filter. The emission signal consists of background luminescence L ( t ) and PH phosphorescence: & 5 0 ( t ) = L(t) + const [T1],. For elimination of L(t) a difference technique was used again. Assuming that 02saturation of the solution reduces only the triplet-state lifetime from the value rl for air-saturated solutions to the valus 7 2 , but not the fast decaying L(t),the difference &O(t) of emissions of air- and 02-saturated samples of otherwise identical solutions of PH in C C 4 corresponds to the difference of monoexponential phosphorescence decays of PH. The inset of Figure 1 shows AZ650(t)obtained at room temperature. From the fit of a convolution of the laser pulse with eq 2,71 = 220 f 10 ns results arsso(t)= B(e-'IT1 - e-+2)
(2)
for the PH triplet lifetime in air-saturated C C 4 , being equal to therise timeofthe . & , 5 ( t ) signal. Table 1 summarizes theresults of the evaluation of the transient curves &S(t) recorded with [PHI = 1.7 X 10-4 M a t different [O,]. The reciprocal rise time TT correlates linearly with [ 0 2 ] as can be seen from the plot of Figure 2. The slope of the straight line of kg = (2.2 f 0.1) X
lo9 M-1 s-l is the rate constant of T1 state quenching of PH by and fits into the range of values 1.5 X 109 5 k: 5 2.4 X IO9 M-l s-I determined for quenching by 0 2 of efficient sensitizers of 0z(lAg) in benzene.,5 Actually, PH efficiently sensitizes 0,(1Ag).20,22Therefore the excited species emitting N 7 6 5 ( t ) is produced by energy transfer from TI. The lifetime of the TI state of PH amounts to 38 ps in degassed benzene,2O indicating that the TI state is almost completely (-99.5%) quenched by 0 2 in air-saturated C C 4 . Since fit parameter A, being proportional to the efficiency of O,('Zl) sensitization, remains constant with increasing [O,] (Table l), the species emitting u 7 6 5 ( t ) must be an excited state of the quencher 0 2 itself. Since it emits at 765 nm, it is identified as O2('Z;). Thus, the difference signals of Figure 1 represent the first time-resolved measurements of the phosphorescence of 02( 'E:), which is a phosphorescence from an upper excited state. As can be seen from the data of Table 1, the lifetime of O,('Z:) does not depend on [O,]. To test whether 72 is influenced by [PHI, we performed additional experiments with air-saturated solutions containing [PHI = 8.5 X 10-5 M. No change in 72 was found in the limits of experimental uncertainty. Therefore, the mean value of 72 = 130 f 10 ns, which is 2.7 times smaller than thevalueof 360 nsextrapolated from therateconstant of O2(l2;) quenching by CC14 in the gase phase,11J6 is the lifetime of 02( '2;) limited by collisional deactivation by the solvent CC14. 72 is very sensitive to the presence of quenchers. Figure 3 illustrates the quenching of O,('Z:) by benzene. The lifetime 7T was taken constant 220 ns in the evaluation of 72 for air-saturated solutions. 72-1 varies linearly with [C~HS].The slope (6.6 f 0.5) X 108 M-1 s-l is the rate constant kg(CC14) of O,('Z;) quenching by benzene in CC14. We also have determined rate constants kg(CC1,) of collisional O,(' Z : ) deactivation by methanol, chloroform, acetonitrile, acetone, and cyclohexane by time-resolved measurements in CC4. Table 2 listsour results whichcorrelatequite well with recently determined Stern-Volmer constants K&(CC14) of O,(' Z l ) quenching in CC1415 and with gas-phase deactivation r a t e constants k:(g).'O The graduation of the values of K&(CCl,) and kg(CC1,) is the same, confirming mutually our and Wang and Ogilby's results of the quenching experiments with methanol, acetone, and benzene. However, from our data of 7 2 and kE(CC1,) we calculate values of K&(CCl,), which are about 3.3 times larger than the literature values of Table 2.15 This indicates that 7 2 was only around 40 ns in the CC14samples used by Wang and Ogilby. With exception of the rate constant of deactivation by CH30H values of kg(CC1,) are in the mean larger by a factor of about 2 than values of kE(g). A similar effect was already 0 2
Schmidt and Bodesheim
2876 The Journal of Physical Chemistry, Vol. 98, No. 11, I994
vary with quencher M, we estimate from the kg(CC1,) data of Table 2 values of ~g in pure solvents of about 20 ps in methanol, 80 ps in cyclohexane, 120 ps in acetone, 130 ps in acetonitrile and in benzene, and 1.2 ns in chloroform. We intend further to continue the investigation of 02('Z+) quenching in solution in order to (I) quantify and understand t i e effects of the phase (gas/liquid) and the solvent on kg and to (11) collect more accurate data of kg required for an improvement of the model of collisional O,('Z+) deactivation.ll First values of absolute efficiencies of OZ('$) sensitization have been published by us very recently.28
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Acknowledgment. Financial support by the Fonds der Chemischen Ind. is gratefully acknowledged. We thank Ch. Grewer and M. Hild for their support in the development of the convolution programs.
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References and Notes
tC6H6-7 / m M
Figure 3. [C&j] dependence of
72.
TABLE 2 Rate Constants of Oz('2;) Deactivation by Quencbers M Determined in Liquid CCb, g(CCl,), and in the Gas-Phase g ( g ) . Stem-Volmer Constants GV(CCl4)of O,('Zf) Quenching in Liquid CCb M ki(CCl,),' M-l s-l ki(g)? M-l s-l K&,(CC14),C M-I CCI4 CH30H CHClp CH3CN CH3COCHo CsH6 C6H I2
*
(7.3 0.6) X (2.2f 0.1) X (6.8 h 1.0) X (3.9 0.2)x (6.0& 0.6) X (6.6 0.5) X (1.3 f 0.1) X
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This work. Reference 10; erroneously instead of the original value of 2.7 X lo5 M-I s-Iez6 2.7 X lo6 M-l s-I is given in ref 10 for CC14. 0 Reference 15. a
observed for the quenching of 02(1A,) by O2('Z;) which occurs about 3 times faster in perhalogenated solvents than in the gas phase.6 Presumably this acceleration is caused by an increase of the normalized collision frequency owing to the increase of the value of the radial distribution function at collision distance on changing from the gas to the liquid phase.6.27 Only for CH3OH this seems not to be the case. However, because of the already mentioned agreement of the graduations of the kg(CC1,) and K&(CCl,) data of Table 2, this could be due to a too large value of kg(g) for CH30H. As has been derived from gas-phase data, kg is additively composed from rate constants kzy.ll This conclusion can also be drawn from the liquid-phase data as is demonstrated by the roughly linear correlation of the kg(CC1,) data with the number of C-H bonds of the quencher in the series CHCl3, CH3CN, CH3COCH3, C& and CsH12, in which C-H is the dominant deactivating oscillator X-Y. CH,OH, however, does not fit into that correlation, since 0-H quenches O,('Z+) much stronger b deactivation by than C-H. A detailed analysis of the O,(l Zg) small polyatomic molecules basing exclusively on gas-phase literature data was already given in ref 11. Rate constants of collisional deactivation of 02(IAg) by solvent molecules M differ slightly if they are determined in the pure liquid M or in a diluted solution of M in a weakly deactivating solvent. For example, the rate constant of quenching of Oz(lAg) by C6H6 has a Value Of 2880 M-' S-l in pure C6H6,2650 M-' S-l in C6D6,3 and 2430 M-l s-1 in Freon 113. Since the same mechanism of collisional deactivation operates for 02( lAg) as well as for O2(lZ~),l1 likewise only a weak solvent dependence is expected for rateconstants kE(1) of O,('Xi) deactivation in the liquid phase. Neglecting this solvent dependence, which may
(1) Herzberg, G. Spectra of Diatomic Molecules; Van Nostrand Reinhold: New York, 1950; p 560. (2) Ogilby, P. R.; Foote, C. S.J . Am. Chem. Sot. 1983,105, 3423. (3) Hurst, J. R.; Schuster, G. B. J . Am. Chem. Soc. 1983,105,5756. (4) Rodgers, M. A. J. J. Am. Chem. Soc. 1983,105,6201. ( 5 ) Egorov, S.Y.;Kamalov, V. F.; Koroteev, N. I.; Krasnovsky, A. A,, Jr.; Toleutaev, B. N.; Zinukov, S.V. Chem. Phys. Lett. 1989, 163, 421. (6) Afshari, E.;Schmidt, R. Chem. Phys. Lett. 1991,184,128. (7) Schmidt, R.; Brauer, H.-D. J . Am. Chem. Soc. 1987,109,6976. (8) Schmidt, R. J. Am. Chem. Soc. 1989,I l l , 6983. (9) Schmidt, R.; Afshari, E. Eer. Bunsen-Ges. Phys. Chem. 1992, 96, 788. (10) Wayne,R. P.SingletOz;Frimer,A.A.,Ed.;CRCPress: BocaRaton, FL, 1985;Vol. 1, Chapter 4. (11) Schmidt, R. Eer. Bunsen-Ges. Phys. Chem. 1992,96, 794. (12) Noxon, J. F. Can. J . Phys. 1961,39, 1110. (13) Becker, A. C.; Schurath;U.; Dubost, H.; Galaup, J. P.Chem. Phys. 1988,125, 321. (14) Chou, P. T.; Frei, H. Chem. Phys. Lett. 1985,122, 87. (15) Wang, B.; Ogilby, P. R. J . Phys. Chem. 1993,97,193. (16) Clyne, M. A. A.; Thrush, B. A.; Wayne, R. P. Photochem. Photobiol. 1965,4, 957. (17) PlBtz, J. Ph.D. Thesis, University of Regensburg, Germany, 1988. (18) Long, C.; Kearns, D. R. J. Chem. Phys. 1973,59, 5729. (19) PH (Aldrich) was purified as described in ref 20. CC4 (Janssen, 99+%, spectrophotometric grade) was dried by column chromatography with A1203 A from Woelm. Solutions were prepared and filled into sample cells in a glovebox under a dry atmosphere. Different concentrations of 0 2 were obtained by degassing of the sample solutions and subsequent addition of = 1.24 gaseous 02.The partial pressure of 02 of 1 bar corresponds to X 10-2 M.21 Solvents M used as quenchers of 0 ('2') have been purified by column chromatography with A1203 N from (20) Oliveros, E.;Suardi-Murasecco, P.;Aminian-Saghafi, T.; Braun, A. M.; Hansen, H.-J. Helu. Chim. Acta 1991, 74, 79. (21) Battino, R.; Rettich, T. R.; Tominaga, T. J . Phys. Chem. Ref.Dura 1983,12, 163. (22) Schmidt, R.; Tanielian, C.; Dunsbach, R.; Wolff, C. J. Phorochem. Phorobiol. A: Chem., in press. (23) The cross section of the laser beam in the rectangular fluorescence cells was about 2 X 3 mm. Pulse energy varied between 0.5 and 2 mJ after having passed a 337-nm interference filter. For most experiments the absorbanotofthesamplesolutionswasA3,= 1on I s m pathlength. Emissions were observed in 90° arrangement at 22 OC and focused on a red-sensitive, dry ice cooled R 1464 photomultiplier (Hamamatsu) through suited interference filters. 765 nm: maximum transmission Tm = 0.62,half-band width HBW = 19 nm; 695 nm: Tm = 0.52,HBW = 15 nm; 650 nm: T, = 0.47, HBW 14 nm. Signals were amplified, when necessary (Hamamatsu C 4543), stored in a transient digitizer (Gould 4072), and accumulated over 128 laser shots. (24) Emission intensities were choosen such that the 02('Zl) signal was at maximum about 30% of the full eight-bit scale. Consequently background emissions were out of scale resulting for the first 50-60 ns after the laser pulse in difference signals of zero. (25) Redmond, R. W.; Braslavsky, S.E. Chem. Phys. Lett. 1988, 148, 523. (26) Davidson, J. A.; Kear, K. E.; Abrahamson, E. W. J . Photochem. 1973,I, 307. (27) Chatelet, M.; Tardieu, A.; Spreitzer, W.; Maier, M. Chem. Phys. 1986, 102, 387. (28) Schmidt, R.; Bodesheim, M. Chem. Phys. Lett. 1993, 213, 111.
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