J. Phys. Chem. 1993,97, 5001-5006
5001
Temperature Dependence of the Oxygen Quenching of mr*-Singlet and mr*-Triplet States of Singlet Oxygen Sensitizers Christof Crewer and Hans-Dieter Brauer' Institut fur Physikalische und Theoretische Chemie der Universitat FrankfurtlM, Niederurseler Hang, 6000-FrankfurtlM, Germany Received: October 7, 1992; In Final Form: February 12, 1993
The rate constant for the oxygen quenching of the lowest m*-singlet state of chrysene (CHR) kS and the rate constant for the oxygen quenching of the lowest ar*-triplet states of CHR, 4-benzoylbiphenyl (BBP),and perinaphthenone (PN)k;f were determined in toluene. At room temperature k: was found to be (3.2 f 0.3) X 1O1O M-l s-l, indicating that oxygen quenching of the &-state of CHR is diffusion controlled. The k: values of the three sensitizers were found to be smaller than 1/9kt at room temperature. With time-resolved luminescence measurements the efficiencies SAof singlet oxygen (IO*) production from the triplet states of the three sensitizers were measured. For the ketones, SAvalues near unity were found at room temperature, whereas C H R exhibits a lower IO2 production efficiency. In addition the temperature dependence of the different quenching processes was studied in the temperature range -90 OC < T < 90 OC. For all compounds, deviation of triplet quenching from the diffusion-controlled reaction was observed at high temperatures. The data were interpreted with a collision complex model leading to the assumption of exciplex formation. We also determined temperature-dependent S p values. In the case of BBP and PN,evidence for intersystem crossing between singlet and triplet exciplexes was found. The measurements were quantified by applying a kinetic scheme, which allows the determination of probabilities of exciplex intersystem crossing.
1. Introduction
SCHEME I
Since the theoretical work of Kawaoka et al.,' the mechanism of singlet and triplet quenchingof organic molecules by molecular oxygen in solution was the subject of many s t u d i e ~ ~(see - ~ ref 5 for more references). It has been found that kg' is less than the diffusion-controlled rate constant of singlet quenching by a factor of 300 K indicates exciplex formation in solution as well.'3 However, in condensed media volume and enthalpy changes during contact complex formationhave to be considered,I4 complicating the interpretation of activation parameters. In this work we report the temperature dependence of kg"and k t for CHR and of kT of the ketones BBP and PN in the temperature range -90 B C < T < 90 OC to get further insight into the quenchingmechanism and to prove Scheme I of Gijzeman et al.3 For all three sensitizers the '02-formation results only by the oxygen quenching of the lowest m*-triplet state. In the case of the ketones the quantum yield Qlscfor SI-TIintersystemcrossing amounts to unity,lS whereas Qiscof CHR depends strongly on the oxygen quenching of the Sl-state.16 Furthermore Sa values were measured at different temperatures, allowing the determination of the quenching probabilities of the different quenching pathways (2) and (3). 2. Experimental Section
Chrysene (CHR, PAH-Forschung, Greifenberg), perinaphthenone (PN, Aldrich) and 5,10,15,20-tetraphenylporphine(TPP,
0022-3654/93/2097-5001%04.00/0 0 1993 American Chemical Society
Grewer and Brauer
5002 The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 9
3. Results and Discussion
8
3.1. Room Temperature Measurements. k; of CHR was determined by measuring the dependence of ks = 117s on oxygen concentration
7 T.
-0
Y
6
(4)
h
i 0 0
1
5 4
3
2
Figure 1. Dependence of the rate constants ks and k~ on oxygen concentrations at room temperature in toluene. Graph A: ks(CHR) vs [02]. Graphs B (CHR), C (BBP), and D (PN) show kT vs [04.
Aldrich) were used as received. 4-Benzoylbiphenyl (BBP, Aldrich) was purified by recrystallization from ethanol. Toluene (Merck) was purified by column chromatography. Oxygen concentration was adjusted by applying a definite partial pressure of oxygen gas (Messer Griesheim) to a degassed solution. The concentration in solution was calculated with Bunsen's solubilitycoefficienta = 0.202'' for toluene. The partial pressure of oxygen was calculated by subtracting the vapor pressure of toluene from the measured total pressure. Correction for thermal expansion of the solvent was performed in the temperature-dependent measurements. The temperature dependence of the solubility of oxygen in toluene is weak and can be ignored.') To diminish solubility effects, measurements were made with nonequilibrium conditions. This was accomplished by reducing the surface of the liquid phase to a minimum value, preventing gas exchange between vapor and liquid phase. Additionally, the gas volume was kept low compared to the liquid volume. The TT absorption apparatus was similar to the one described elsewhere.Is Excitation was performed with a XeCl excimer Laser (Lambda Physics) at 308 nm. The light of a 100-W Xe lamp was crossed in 90° angle and detected with a photomultiplier (Hamamatsu) wired for high-speed response. The analysis light path was 2 cm. The time resolution of the spectrometer was approximately 20 ns. Analysis of the decay traces was accomplished with iterative reconvolution by using the Marquardt a1g0rithm.I~ Fluorescence lifetimes were measured with a Si avalanche photodiode (Optical Electronics) perpendicular to excitation with 6 4 s pulses of a 337-nm nitrogen laser (Lambda Physics) in 1-cm square quartz cuvettes. The wavelength of fluorescence was selected with interference and cutoff filters. The response time of the Si diode is -800 ps. Lifetimes were calculated with the same program as for TT decay curves. Lifetimes > 2 ns could be measured with a accuracy of *0.3 ns. QA the singlet oxygen quantum yield was measured with a timeresolving IR spectrometer as described earliereZ0The intensity of 1 0 2 luminescence at 1276 nm was extrapolated to the time of the laser pulse and its dependence on laser energy was measured. The slope of the plot was compared to the slope of a compound with known QA. We used PN as standard with QA = 0.97.21*22 This value was proved by comparative measurements with a steady-state IR luminescence apparatus using TPP as standard (QA = 0.622i) in toluene. The samples were cooled and heated with a liquid nitrogen cryostat (Oxford Instruments). The temperature was measured directly in the cuvette with a chromel-alumel thermocouple. All measurements were performed in toluene because of the wide range of accessible temperatures in the liquid phase and good solubilities of the used compounds even at low temperatures.
where 7 s and 7; denote the lifetimes of the SIstate of CHR in the presence and the absence of oxygen, respectively. The data of the measurements at T = 295 K are presented in Figure 1 (curve A). The obtained value for kt of (3.2 0.3) X 1010 M-I s-I is in good agreement with the literature data of fluorescence quenching of a variety of aromatic hydrocarbon~,69*~ indicating a diffusion-controlled quenching reaction. The triplet quenching constants k: were estimated by eq 5,
*
where TT and 7; denote the lifetimes of the lowest triplet state in the presence and the absence of oxygen, respectively. Since the lowest triplet states of BBP and PN are of mr* character no ketyl radicals are observed in transient absorption. The k: values at 295 K evaluated for the three sensitizersfrom the straight lines presented in Figure 1 are summarized in Table I. All kT-values are significantly lower than Ilgkd. The difference of k9 between CHR, BBP, and PN cannot be due to diffusional ejfects only. kd should not vary significantly for all substances because of the major contributionof the high diffusivityof oxygen. Therefore the difference must be related to varying quenching probabilities of the compounds. The quenching constant k: can be expressed according to Scheme I as
(7) where PI and P3are the quenching probabilities of reactions 3 and 2, respectively. It is clearly seen that the evaluation of k: leads to the sum of PI and P3, giving no further information about the contribution of the two possible quenching reactions. However, if SA,the quenching efficiency of IO2 generation accompanying the oxygen quenchingof the triplet state, is known, PI and P1 can be evaluated. According to Scheme I SA is defined as
SA=PI
p,
+ 3P3
Together with eq 7 this yields the expressions for P I and P3:
9kT
P,= A S a kd
(9)
3kg' P, = -( 1 - SA) kd
For kd the value k: = 3.2 X 1Olo M-I s-l found for oxygen quenching of the SIstate of CHR will be used in the following evaluations. SAcan be experimentallydetermined if QA,the quantum yield of I 0 2 generation, and Q s c are known. For SAeq 11 holds:
SA Q d Q i s c (1 1) For the ketones BBP and PN with QiE = 1, SA= QAresults. Because of long m*-triplet lifetimes of BBP and PN in degassed
Oxygen Quenching of Singlet Oxygen Sensitizers
The Journal of Physical Chemistry, Vol. 97, No. 19, 1993 5003
TABLE I: Rate Constants and Quenching Probabilities of rr*-Triplet States in Toluene at Room Temperature According to Scheme I ET,cm-I k:," M-I s-I k:," M-1 s-I QA~.' SA PI P3 PN 1 84OOe 2.2 x 109 0.97 0.97f 0.10 0.60 f 0.10 (6 4 20) X l ( F 3 BBP 2 1 ooor 9.2 X lo8 0.96 0.96 f 0.10 0.25 f 0.04 (3 4 8) X 0.30 i 0.06 CHR 2000oB 3.2 X 1O'O 1.5 x 109 0.67 0.70 4 O.lld 0.04 4 0.02 410%. 410%. In air-saturated solution. Corrected for oxygen-induced isc. Reference 22. /Reference 28. 8 Reference 3. 300
191 25
Figure 2. Dependence of ' 0 2 phosphorescence intensity at time zero after the laser pulse on laser energy for CHR (A), BBP (B), and PN (C) at room temperature in air-saturated toluene. A308 = 0.4, QA(PN) = 0.97.22
solution (>30 ps), 100% of the triplet states are quenched by oxygen in air-saturated solution. The plots for determining QA with a comparativemethod are shown in Figure 2. The downward curvature at higher laser energy is due to the saturation behavior of sensitizer triplet state producti0n.2~ Only the linear regions of the plots are used for QAevaluation. For CHR Qsc has to be corrected for singlet-state quenching by oxygen. In this case Qisc is given as
Qisc
QL
QL+ &v[021 + K,,[O,]
= 1
where is the triplet quantum yield in the absence of O2and KSVis the Stern-Volmer constant of singlet-state quenching,KSV = kf7:. For Q;, a value of 0.85 was used.25 The large KSVvalue of CHR 1344 f 50 M-' results in significant enhancement of Q s c in air-saturated toluene solution (Qssc = 0.96). The measured Q A and SAvalues listed in Table I are in good agreement with those previously reported.7q8*22 The P I and P3 values evaluated from eqs 9 and 10 are also given in Table I. For BBP and P N the SAvalues of about 1 at room temperature can be explained with a negligible contribution of reaction 2. The low kf; values are effected by a low PI value, meaning that k,, < kd. The BBP triplet state is of high energy (21 000 cm-I). It is known that for high-energy triplets the quenching constant decreases with increasing triplet energy because of a reduced probability in the formation of 02(lLg),193 resulting in a clear deviation of P I from unity. The intermediate energy triplet of CHR is quenched with energy transfer probability of 0.3,lying between the values for BBP and PN. These values support the assumption of Gijzeman et aL3that P I = 1 for triplets with energy between 9000 and 14 000 cm-I and that PIshould decrease with triplet energy for higher energy triplets due to decreasing FC factors. SA< 1 for CHR indicates the quenching probability P3 being substantially greater than zero, whereas the ketones do not show internal conversion from the 3(3Ml-.302)to the 3(1M,+02) complex. To explain this behavior, a correlation to energies of charge-transfer states can be assumed as proposed in the l i t e r a t ~ r e . ~ . ~ $As 8 - 2pointed ~ * ~ ~ out by McGarvey et al. in the case of substitutedna~htalenes,~~ increasingexothermicityof formation
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3
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35
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103/T CK-'I
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45
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5
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1
55
Figure 3. Plots of In(&:) for CHR (A) and In(&:) for CHR (B), BBP (C), and PN (D) versus 1/T in toluene. The broken lines indicate In(1/9kd)(E) and h(4/9kd) (F) versus 1/T. The solid lines (9, C, and D) are calculated according to eq 6 for CHR and eq 19 for the ketones assuming P-,%
