J. Phys. Chem. 1993, 97, 177-180
177
Volume of Activation for the Radiationless Deactivation of Singlet Oxygen in Aromatic Hydrocarbons Masami Okamoto' Faculty of Engineering and Design, Kyoto Institute of Technology, Matsugasaki, Sakyo- ku, Kyoto 606, Japan
Fujio Tanaka College of Integrated Arts and Science, University of Osaka Prefecture, Gakuencho, Sakai 593, Japan Received: August 4, 1992; In Final Form: October 5, 1992
Phosphorescence lifetimes of singlet oxygen ('02; I A,) were measured in six aromatic hydrocarbons a t pressures of up to 400 MPa. The lifetimes decreased significantly with increasing pressure in the solvents examined. The observed activation volumes, AVD*,for the quenching by solvent were in the range from -8.3 cm3/mol (benzene) to-20.1 cm3/mol (mesitylene), and they decreased with decreasing ionization potential of solvents. The activation volumes for the quenching by solvent in alkane and acetonitrile were nearly equal to the volume changes for encounter+omplex formation estimated by using a hard-sphere model, A V I ( ~ while ~ ) , the discrepancy between AVD*and AV,(HS)in aromatic hydrocarbons increased with decreasing the ionization potential of the solvents. The enhanced decrease of AVD*in aromatic hydrocarbons was interpreted by assuming the formation of exciplexes between '02and the solvent. The effect of the ionization potential of the solvent on the magnitude of the values for AVD* is discussed.
Introduction The lifetime of singlet oxygen (102; 'A8) in solution varies over a time range extending from microseconds to milliseconds, depending on so1vent.l The deactivation processes have been recognized as the largely nonradiative collisional energy transfer from the lowest electronically excitedstateof I 0 2 to thevibrational levels of the solvent molecules.2 It has been shown that the bimolecular quenching rate constant by solvent can be built up additively from the quenching constants for the atom pair and/or the atom groups involved in the solvent m o l e c ~ l e .Such ~ ~ ~group ~ additivity relations of the quenchingconstants have been recently discussed theoretically! Singlet oxygen is also quenched effectively by many substrates with low ionization potential such as amine^,^ phenols: furan^,^^^ and hydrazines.9 For these systems, the quenching involves physical as well as chemical processes that lead to the reaction products. The charge-transfer (CT) character of these processes has been shown to be important, and it has been propo~eds-~ that the quenching mechanism involves an exciplex. Further, it has been recently demonstrated that excited-state complexesbetween molecular oxygen and an aromatic hydrocarbon have a substantial amount of CT character.I0 We have already shown that the lifetime of singlet oxygen in solution decreases with increasing pressure up to 400 MPa" and also reported that the cycloaddition reaction between I 0 2 and diphenylisobenzofuran competes with diffusion at elevated pressure.I2 These high-pressure studies have revealed the factors controlling the lifetime and the reactivity of singlet oxygen in solution. In the present work, we focus on the influence of pressure upon the physical quenching of IO2by aromatic hydrocarbons. The effect of pressure on the lifetime of I 0 2 in solution is characterized by the energy-transfer probability involving intersystem crossing (ISC) from IO2to 302 (3Zc)and the collision frequency between 1 0 2 and solvent molecules. The contribution of the former is very small when no specificinteraction is involved in the quenching." The formation of the exciplex by a CT interaction would be expected to cause considerable changes in the rate of ISC and the collision frequency, depending on the polarity of the exciplex.I0 In order to study the effects of a CT interaction on the lifetime, 0022-3654/58/2097-0177%04.00/0
we measured the phosphorescence lifetime of 1 0 2 in six aromatic hydrocarbons. The results are discussed in terms of the effect of changes in the ionization potential of the solvent on the activation volume for quenching.
Experimental Section Reagent grade aromatic hydrocarbons (Tokyo Kasei) were purified by the usual method. Tetraphenylporphine(TPP, Aldrich Chemicals) and acridine (zone refined grade, Tokyo Kasei) were used without further purification. A pulsed nitrogen laser (337 nm, fwhm = 8 ns) was used as the excitation source. Singlet oxygen was created by irradiating an aerated solution containing TPP or acridine as a sensitizer with the laser pulse. Near-infrared phosphorescence of singlet oxygen was isolated by using the combination of an IR glass filter (Toshiba Glass Co., IRD 80A) and an interference filter centered at 1270 nm (Vacuum Optics Co. Japan, BFF-4) and focused on a 3. l-mm2germanium photodiode (Hamamatsu, B2614-02) that was reversely biased at 4.5 V. The phosphorescence intensities were measured at right angles to the direction of the excitation pulse. The output signals were amplified by using a LH0032 operational amplifier (50 dB) and averaged 256 times on a digitizing oscilloscope (Hitachi VC 6024). Overall time response of the IR intensity measuring system was less than 2 ps. All data were analyzed by using a NEC PC9801 microcomputer that was interfaced to the digitizer. The high-pressure system and the associated experimental techniques have been described e1~ewhere.l~ Temperatures were controlled at 25.0 f 0.1 OC. Pressures were measured with a calibrated manganin wire. Results
The phosphorescence of singlet oxygen showed a singleexponential decay over the experimental conditions used in this work. The lifetime, kd-l, and its pressure dependence did not depend on the sensitizers used in aromatic hydrocarbons. For example, the values of kd-l at 0.1 MPa are 28.7 ps for acridine and 29.3 ps for TPP in toluene. TPP was used throughout as the sensitizer in this study since a very small laser power dependence of kd-l was observed for acridine in methylcyclohexaneand hexane. 0 1993 American Chemical Society
Okamoto and Tanaka
178 The Journal of Physical Chemistry, Vol. 97,No. 1. 1993
TABLE I: Activation Volumes and the Parameters Associated with the h a y Process of Singlet Oxygen in Solution at 0.1 MPa and 25 OC IP.' Elj2,b
Ilk&
(I/kd)ad,
solvent
cV
V
PS
FS
benzene toluene m-xylene +.xylene p-xylene mesitylene
9.24 8.82 8.59 8.56 8.44 8.39
2.30 1.98 1.91 1.89 1.77 1.81
30.9 f 0.3 29.3 f 0.2 25.0 f 0.3 22.9 f 0.2 19.9fO.l 15.6 f 0.4
35 40 44 44 45 48
AVD',
A V I ( " ~ ) , AVCT': cmA/mol cm'/mol
cmJ/mol -8.3 -10.0 -13.0 -14.2 -19.2 -20.7
-8.9 -7.7 -7.3 -7.5 -7.9 -7.6
f 0.4
0.1 f 0.6 f 0.2 f 0.2 f 1.2 f
13
i
4
'A,
-2.1 -8.6 -13.3 -13.2 -16.7 -15.9
Ionization potential o f ~ o l v e n t . ~Oxidation ~ potential in acetonitrile.2s AVcr' = AV2 AV~SC,'. (I
+
VI i
a
1
*I
2
' 0 -
m W
-
I
U
15
0.1 MPa
130
0
300
200
100
400
pressure1 MPa Figure2. Pressuredependenceon kd in benzene ( O ) ,toluene (W), m-xylene (A),o-xylene (0),p-xylene (O), and mesitylene (A)at 25 O C . The solid lines merely represent the trend in the data.
TABLE 11: Activation Volumes for the Decay Process of Singlet Oxygen with No CT Interaction between ' 0 2 and the Solvent' A VD'
0 0.0
.--
I
I
I
20.0
40.0
60.0
TIME
/
t
80.0
bleac hingb
hexane methylcyclohexane acetonitrile
-9 -6 -6
phosC
* *
-7.8 1.0 - 6 . 9 0.5 -7.5 f 0.4 (-7*)
AV~*(HS)
-9.7 -7.8 -7.5
Unit is cm3/mol. The values from the bleaching experiments." Present work. Reference 15a.
MICROSECOND
Figure 1. Phosphorescence decay curves of singlet oxygen in m-xylene a t six pressures and 25 OC. T P P was used as sensitizer. The solid lines are the best-fit exponential decays.
The lifetimes at 0.1 MPa are summarized in Table I, together with relevant parameters. Figure 1 shows a typical example of the pressure effect on the phosphorescence decay of IO2 in m-xylene. Figure 2 shows the pressure dependence on kd in six aromatic hydrocarbons. As seen in Figure 2, kd increases monotonically with increasing pressure, which is the same trend as that found for nonaromatic solvents.11J5a Since kd depends on the concentration of solvent, [SI,the bimolecular decay constant, kD ( = k d / [ S ] ) ,should be used in order to determine the intrinsic pressure dependence.2J I The volumes of activation for kD, AVD', were determined from the plots of In k Dagainst pressure via eq 1, where K is the isothermal compressibility of the solvent. The results are listed in Table I.
RT(d In k , / d P ) , = -AVD* - R T K
solvent
(1)
Discussion The lifetime of singlet oxygen in solution is determined by the collisional energy-transfer rate from 1 0 2 to the vibrational levels of the terminal oscillators in the surrounding solvent molecules.2 In the case of no specific interaction between I 0 2 and the solvent molecules, such as a CT interaction, the lifetime is related to the collision frequency and the energy-transfer probability per collision. By assuming a hard-sphere solute molecule of diameter do and a hard-sphere solvent molecule of diameter d,, we have shown that the pressure effect on the lifetime of I 0 2 can be
interpreted satisfactorily on the basis of the radial distribution function,g(d,), a t the closest approachdistance, d,.ll According to this model,l4 the activation volume, A V D * ( ~is~given ) , by
AVD*(HS) = -RT[d In g(d,,)/dP] - RTK
(2)
In eq 3, y is the packing fraction, given in terms of the solvent density, p,, by
where NAand M sare Avogadro's number and the molecular weight of the solvent, respectively. As a preliminary experiment, we measured the influence of pressure upon the phosphorescence lifetimes of 1 0 2 in hexane, methylcyclohexane, and acetonitrile in order to compare with the previous results by the indirect method (bleaching of diphenylisobenzofuran).'' The results are listed in Table 11, together with the values of A V D ' ( ~ ~As) .can be. seen in Table 11, the values for AVD*obtained experimentally are in good agreement with those for A V D * ( "estimated ~) by the hard-sphere model according to eq 2. This indicates that the energy-transfer probability per collision is independent of pressure. This result supports our conclusion reported previously.Il Similar discussion has been made by other workers.Is The decay process of I 0 2 can be described kinetically by assuming the formation of encounter complexes, l J ( 0 2 S ) , with
The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 179
Radiationless Deactivation of Singlet Oxygen singlet and triplet multiplicities.I6
+
-
ki
kisc
1 ~ 2 ( 1 s~ = ~ l(02-s) ) k-1
-
3 ( ~ 2 - ~ )
302(32,-)
+s
(5)
In this case, the observed bimolecular decay constant, kD, is expressed by eq 6. Since k-l >> klsc, kD = Klkrsc, where KI
(=kl/k-l) is the equilibrium constant for the encounter complex formation. Thus, AVD*= AVl AVlsc*. AVI can be calculated by using the radial distribution function g(d,,) as f01lows:~~
+
AVl(HS)= -RT(d In g(d,,)/dP),- R T K
(7) Here, AVD*is nearly equal to A V I ( ~ ~Hence, ). AVISC* 0; that is, klsc is independent of pressure. As a result, the activation volume for kD,AVD*,withnospecificinteractioncan beestimated from the pressure dependence on g(d,). However, from the data presented in Table I, one can see a large discrepancy between AVD*and AVl(HS)in aromatic hydrocarbons, which increases with decreasing ionization potential of the solvent. This may suggest the existence of a CT interaction between I 0 2 and the solvent molecules as reported for the systems involving solutes with low ionization potential.5-10 If no C T interactions are involved in the quenching mechanism, the lifetime of singlet oxygen a t 0.1 MPa, (l/k&d, can be estimated by using the quenching constant, k,(XY), intrinsic to the individual terminal oscillator XY and the number of XU, N(XY), in a given molecule according to the equation (kd)ad = [S]ZxviV(XY)k,(XY).2a,3 Since k,(CH) and k,(CH3) are 420 M-1 s-I 2aand 550 M-1 s-1,3 respectively, onecan obtain thevalues of (l/kd)ad that are listed in Table I. It can be seen from the values in Table I that (1 /kd)ad increases slightly with decreasing ionization potentials, whereas theexperimentally determined 1/kd decreases significantly with decreasing ionization potential of solvent. These facts may also suggest that the CT interaction is involved in the quenching processes by aromatic hydrocarbons. When C T interactions are involved in the reaction mechanism, the quenching may occur through the singlet and triplet exciplexes:
-
0
1
I
I
I
100
200
300
400
pressure1 MPa Figure 3. Plots of In ( k ~-, Klklsc) against pressure in benzene (a),
toluene (M), m-xylene (A),o-xylene (0),p-xylene (a), and mesitylene (A)at 25 O C . The solid lines in benzene andp-xylene are drawn by the equation In ( k -~Klklsc) = A + EP and those in other solvents by the equation In ( k -~Klklsc) = A ' + B'P + CY2. From the initial slopes of these plots, the values of at 0.1 MPa were evaluated. decreases almost linearly with decreasing ionization potential and oxidation potential of the solvents. In general, the volume change consists of two major contributions, one due to structure change for the reaction AV(st) and theother duetosolvationchangeAV(so1v). AV2for the formation of '(exciplex) from the encounter complex may involve AV,(st) as well as AVz(solv), but the major part of AV2 may be due to AVz(so1v) as expected for typical exciplex systems.17 When a solute molecule having a dipole moment ~c is transferred from vacuum to a spherical cavity of radius r in a continuous dielectric medium of dielectric constant D, AVz(s01v) is calculated by the Kirkwood theory (eq 10).l8
where
Since the formation of '(exciplex) is nearly diffusion-controlled for typical exciplex systems and the rate of intersystem crossing, klsc, would be expected to be much slower than the dissociation of the '(exciplex), the observed rate constant, kD, is given by
(9) = K l klSC + K l K2k1SC' where KI (=kl/k-l) is the equilibrium constant for theencounter complex formation and K2 (tkzlk-2) that for the singlet exciplex formation with some degree of polarity. The first term in eq 9 is due to the contribution to kD in the case of no C T interaction. This values (KIklsc) at pressure P can be evaluated by using the ratioofg(d,)at apressureofPMPa tothatat 0.1 MPa, together with the values of (1/kJad at 0.1 MPa, which were estimated from the additivity rule in aromatic hydrocarbons (Table I). Thus, one can determine the pressure dependence on K1K2klsct. Figure 3 shows the plots of In (kD - Klklsc) against pressure, whose slopesgive theactivationvolumes (AVl + AV2 + AVISC,'). Since AV, is known (AVl(HS)in Table I), one can obtain the values of AVCT*(=AV2 + AVIsc.*). The results are listed in Table I. It can be seen from Table I that AV2 + AVlsc,' is negative and kD
In eq 10, the subscripts e and c refer to the singlet exciplex and the encounter complex, respectively. The dipole moment of '(exciplex) in eq 10, which would be expected to increase with decreasing ionization potential of the solvent, can not be estimated from the present data. If we assume that pe in mesitylene is in the range from 10 to 15 D, as is the case for typical exciplex systems,I9 then the values of AVz(so1v) are, in turn, estimated to be between -7 and -15 m3/mol. Such an estimation, together with the data presented in Table I, implies that the activation volume with a C T interaction (AVCT*)cannot beexplained simply by the solvation of '(exciplex), AV2(solv), although the value calculated for the maximum dipole moment (15 D) is close to AVcT' (-16 cm3/mol). Another term in AVCT' is referred toas the pressuredependence on the intersystem crossing rate from l(excip1ex) to 3(exciplex), klsc,. In fact, such an increase due to spin-rbit coupling by heavy atoms has been observed.ld To the authors' knowledge, there is no available information concerning AVISC.'. The activation enthalpy for the intersystem crossing has been suggested to be nearly zero from studies on the temperature dependence on the physical quenching of singlet oxygen by phenols.6b If this is
180 The Journal of Physical Chemistry, Vol. 97, No. 1, 1993
-
the case for AVIsc* (AVISC,* O), then the dipole moments are estimated to be 4, IO, 12, 13, 15,and 15 D for benzene, toluene, m-xylene, o-xylene, p-xylene, and mesityIene,*O respectively, by using eq 10 and the values for AVO* (see Table I). In conclusion, it has been shown in the present work that the activation volume determined from the measurements of the phosphorescence lifetimes of singlet oxygen was correlated with the ionization potential of aromatic solvents. This was interpreted to arise mainly from the solvation of the exciplexes formed between singlet oxygen and the solvents.
Acknowledgment. This work was partly supported by a Grantin-Aid for Scientific Research from the Ministry of Education of Japan (No. 62540331). References and Notes (1) (a) Wilkinson, F.; Brumer, J. B. J . Phys. Chem. ReJ Data 1981,10, 809. (b) Scurlock, R. D.; Ogilby, P. R. J . Phys. Chem. 1987, 91, 4599. (c) Schmidt, R.; Seikel, K.; Brauer, H.-D. J . Phys. Chem. 1989,93,4507. (d) Schmidt, R. J . Am. Chem. SOC.1989,111,6983. (e) Schmidt, R.; Afshari, E. J. Phys. Chem. 1990, 94, 4377. (2) (a) Hurst, J. B.;Schuster, G. B.J. Am. Chem. SOC.1983,105,5756. (b) Schmidt, R.; Brauer, H.-D. J . Am. Chem. SOC.1987, 109, 6976. (3) Rodgers, M. A. J. J. Am. Chem. SOC.1983, 105, 6201. (4) Lin, S. H.; Lewis, J.; Moore, T. A. J . Phofochem. Phorobiol. A: Chem. 1991, 56, 25. (5) (a) Ogryzlo, E. A.; Tang, C. W. J. Am. Chem. SOC.1970,92, 5034. (b) Furukawa. K.:Oarvzlo. E. A. J . Phorochem. 1972/1973. 1. 163. (c) Young, R. H.; Martin;R. L.; Feriozi, D.; Brewer, D.; Kayser, R. Phorochem. Phorobiol. 1973,17.233. (d) Young, R. H.; Brewer, D.; Kayser, R.; Martin, R.; Feriozi, D. Can. J . Chem. 1974, 52, 2889. (e) Manring, L.E.; Foote, C. S.J . Phys. Chem: 1982, 86, 1257. (f) Saito, I.; Matsuura, T.; Inoue, K. J . Am. Chem. SOC.1983, 105, 3200. (9) Encinas, M. V.;Lemp, E.; Lissi, E.
Okamoto and Tanaka A. J . Chem. SOC.,Perkin Trans. I1 1987, 1125. (h) Clennan, E. L.;Noe, L. J.; Wen, T.; Szneler, E. J . Org. Chem. 1989, 54, 3581. (6) (a) Thomas, M. J.; Foote, C. S. Phorochem. Phofobiol. 1978, 27, 683. (b) Gorman, A. A.; Could, I. R.; Hamblett, I.; Standen, M. C. J . Am. Chem. SOC.1984, 106,6956. (7) Gorman, A. A.; Hamblett, I.; Lambert, C.; Spencer, B.; Standen, M. C. J. Am. Chem. SOC.1988, 110, 8053. (8) Clennan, E. L.; Mehrsheikn-Mohammadi, M. E. (a) J. Am. Chem. Sor. 1983, 105, 5932; (b) J . Org. Chem. 1984, 49, 1321; (c) J. Am. Chem. SOC.1984, 106, 7112. (9) Clennan, E. L.; Noe, L.J.; Sznler, E.; Wen, T. J . Am. Chem. SOC. 1990, 112, 5080. (10) (a) Scurlock, R. D.; Ogilby, P . R. J . Phys. Chem. 1989, 93, 5493. (b) Kristiansen, M.; Scurlock, R.D.; Iu, K.-K.; Ogilby, P. R. J. Phys. Chem. 1991, 95. 5190. (1 1) Okamoto, M.; Tanaka, F.; Teranishi, H. J . Phys. Chem. 1990, 94, 669. (12) Okamoto, M. J . Phys. Chem. 1992, 96, 245. (13) Okamoto, M.; Teranishi, H. J . Phys. Chem. 1984,88, 5644. (14) Yoshimura, Y.; Nakahara, M. J . Chem. Phys. 1984, 81, 4080. (151 . (a) . , Schmidt. R.: Seikel. K.: Brauer. H.-D. Ber. Bunsenpes. Phvs. Chem. 1990, 94, 1100. (b) Seikel, K.; Brauer, H.-D. Ber. Bunsen&. Phys. Chem. 1991, 95, 900. (16) Stevens, B. Acc. Chem. Res. 1973, 6, 90. (17) OConnor, D. V.;Ware, W. R. J . Am. Chem. Soc.1979, 101, 121. (18) Kirkwood, J. K. J . Chem. Phys. 1934, 2, 351. (19) Been, H.; Knibbe, H.; Weller, A. J . Chem. Phys. 1967, 47, 1183. (20) The values of q p in eq 10 were evaluated from the available data for benzene2' and toluene,22 and those for xylenes and mesitylene were estimated by using the density data23 according to the Clausius-Mossoti equation. (21) Kasiwagi, H.; Fukunaga, T.; Tanaka, Y.;Kubota, H.; Makita, T. Rev. Phys. Chem. Jpn. 1979, 49, 70. (22) Mopsik, F. I. J . Chem. Phys. 1969, 50, 2559. (23) Bridgman, P. W. Proc. Am. Acad. Arrs Sci. 1949, 77, 129. (24) Birks, J. B. Photophysics of Aromafic Molecules; Wiley-lnterscience: New York, 1970; p 457. (25) Pysh, E. S.; Yang, N. C. J . Am. Chem. SOC.1963, 85, 2124. I
