J. Phys. Chem. 1994,98, 4230-4235
4230
Mechanism of the Triplet-State Quenching by Molecular Oxygen in Solution Christof Grewer and Hans-Dieter Brauer' Institut f i r Physikalische und Theoretische Chemie der Universitiit FrankfurtlM, Marie- Curie Strasse 1 1 , 60439-Frankfurt/M, Germany Received: November 3, 1993; In Final Form: January 12, 1994' Rate constants kT ranging between 0.3 X lo9 and 15 X lo9 M-l s-l for quenching of the lowest excited triplet state of several subtituted aromatic hydrocarbons and ketones by molecular oxygen were measured in toluene. The efficiencies of singlet oxygen (O2(lAg)) formation SAwere determined, ranging from 0.24 to 1.00. The results can be explained by C T interaction in that k: and SAcorrelate with the free enthalpy of electron transfer AGel, calculated from the redox potentials of the sensitizer and 0 2 . Quenching probabilities for the singlet and triplet channel were determined allowing the calculation of (ki/k+j)(where i = en, energy transfer, or ic, internal conversion), which exhibit an approximately linear dependence on AGe. In one case (9-methylcarbazole), direct evidence for encounter complex intersystem crossing is found and discussed in terms of reversible charge transfer (CT) complex formation and CT complex intersystem crossing to obtain a probability of the intersystem crossing process of 0.27.
Introduction The quenching of the first excited triplet state of aromatic molecules by molecular oxygen in the triplet ground state can be described by the kinetic scheme1 in Scheme 1
SCHEME 1
In this scheme, energy transfer generating singlet oxygen competes with internal conversion leading to both sensitizer and 0 2 in the ground state. In theoretical treatments ki, was shown to be about a factor 100-1000 lower than kenbecauseof lower FC factors.1.2 This indicates that every triplet molecule quenched by oxygen should lead to the formation of '02(SA= l ) , whereas SAvalues near unity are only found in few cases, e.g., for ketones with aa*-triplet states. In most cases, including aromatic hydrocarbons,SAisless than unity. Of special interest in this connection are ketones with na*-triplet states which exhibit SAvalues between 0.3 and 0.4.3 For example, in the case of benzophenone a triplet quenching constant k: = 2.36 X 109 M-ls-landSA = 0.34weremea~uredinbenzene.~ Although the k: value is lower than 1/9ka = 3.6 X 10'0 M-1 s-1, which would be the limitingvalue,if energy transfer is theonly quenching pathway, the '02-formation efficiency is lower than unity. k; values greater than 1/9kd, found for compounds with high triplet energy and low oxidation potential: are explained on the assumption of charge-transfer (CT) interaction, in which the authors postulated a kinetic scheme including reversible CTcomplex formation and an intersystem crossing equilibrium between the CT complexes of different multiplicity. Recently, McGarvey et a1.6measured kg' and SAfor a series of naphthalene derivatives with similar triplet energy and different oxidation potential. The k: values increase with decreasing oxidation potential, whereas SAshows the opposite behavior, indicating the ('O2('Z:,'AJ)
e Abstract published in
Advance ACS Abstracfs, March 15, 1994.
0022-3654/94/2098-4230504.50/0
participation of a CT-complex in the deactivation cascade. CT interaction seems to favor enhanced intersystem crossing compared to energy transfer which leads to the reduced SAof compounds with low oxidation potential.5 In the absence of CT interaction,encounter complex intersystem crossingis expected to beslow compared with complex dissociation or deactivation.' CT interaction is assumed to facilitate intersystem crossing of the encounter complexes of different multiplicity as found in the case of '02quenching by amines? In the literature, there are only two cases of direct evidence of complex intersystem crossing. Complex intersystem crossing was first postulated to explain the reduction of SAvalues accompanying the heavy atom substitution of na*-triplet-state ketones,3 and we recently proposed complex intersystem crossing to account for the high SAvalues of ketones with a a * - T ~state at low temperatures.* In this case, evaluation of the quenching measurements with Scheme 1 yields PI values greater 1 . According to Scheme 1, k t and SAcan be expressed as follows: '3
(5) If kT and SAare known, the quenching probabilities PI and P3 can& calculated as 9kT P , =Asa kd
(7) In this paper, we report the evaluation of the quenching data of a number of aromatic hydrocarbons and ketones with m*-or na*-triplet states according to Scheme 1. Furthermore, the dependence of SAand k: of the oxidation potential of the sensitizer is examined to determine the contribution of CT interaction. Encounter complexintersystem crossing is discussed using the kinetic scheme first postulated by Garner and Wilkinson.5
Experimental Section 6-Aminochrysene (6-AC, Aldrich) and fluoranthene (FLA, Merck) were sublimed. Acetophenone (AP, Aldrich), benzoyl@ 1994 American Chemical Society
Triplet-State Quenching by Molecular Oxygen
The Journal of Physical Chemistry, Vol. 98, No. 16, I994 4231
naphthalene (BN, Aldrich), perinaphthenone (PN, Aldrich), phenanthrene (PHE, Aldrich), 9-methylcarbazole (9-MC, Aldrich), 5,12-tetracenequinone (TQ, Aldrich), and triphenylene (TPH, Aldrich) were used as received. Benzophenone (BP, Aldrich), 9-cyanoanthracene (9-CA, Aldrich), and 9,lO-dichloroanthracene (DClA, Aldrich) were purified by recrystallization from ethanol. p-Aminobenzophenone (PAB, Merck) was recrystallized from methanol. 1,4-Dicyanonaphthalene(DCN) was donated by Prof. Matthay (University Miinster), and pentacenequinone (PQ) was received from Prof. Ried (University Frankfurt). The solvents (Merck) were purified by column chromatography. Solutions were degassed by the freezepumpthaw method to a residual pressure of l ( r Torr. After degassing, the solutions were exposed to a definite partial pressure of air or oxygen gas (Messer Griesheim). The oxygen concentration was calculated from Bunsen's solubilitycoefficienta (see Table 2) after correction of the total pressure for the partial pressure of the solvent. Absorption spectra were recorded with a Perkin Elmer 555 spectrophotometer. Fluorescence and phosphorescence spectra were measured with a Perkin Elmer 650-40 emission spectrometer. For the measurement of the phosphorescence spectra, the methylcyclohexane solutions were cooled to the temperature of liquid nitrogen. The decay kinetics of the triplet states were measured with a nanosecond-absorption spectrometer described elsewhere.* The soltuions were excited by a XeC1-Excimer laser (Lambda Physics) at 308 nm. Detection of theabsorbancechange was performed perpendicularto excitationwith a photomultiplier (Hamamatsu) and the light of a 100-W high pressure Xe lamp. The analysis light path was 2 cm. The time resolution of the spectrometer was about 20 ns. The decay curves were analyzed with iterative reconvolution using the Marquardt a l g ~ r i t h m . ~ Time-resolved fluorescence measurements were performed with a homebuilt apparatus described earlier.* The yield of '02-formation QA was measured with a steadystate IR spectrometeras described previously10by comparing the integrated 1 0 2 phosphorescence spectrum of a solution of the sensitizer with the spectrum of a standard (ST) under identical experimental conditions. If the same solvent and optically matched solutions are used for standard and probe, QAcan be calculated according to
PN was used as standard with QA= 0.97 in toluene.*J1.]2 Over concentration ranges of sensitizersused no reduction in 1 0 2 lifetime was observed.
Results The T I energiesof 9-MC and 6-AC, determined from the 0-0 band of the phosphorescence spectrum in methylcyclohexane solution at 77 K,are ET = 298 f 2 kJ/mol and 226 f 5 kJ/mol for 9-MC and 6-AC, respectively. The quantum yield of triplet formationin the absence of oxygen QL was determined by fitting the values a and b to the experimental saturation behavior of the absorbance change A (OD)! extrapolated to time zero after the laser pulse versus the number of exciting photons according to the Lachish equationI3 as described p r e v i ~ u s l y . ~ ~ J ~
A(OD)$ = a(1- exp(-bE))
(9)
TABLE 1: Photophysical Parameters for the First Excited Singlet and Triplet State and Quenching Parameters by 02 in Toluene kf/ 109 compd (I& s-l) AP BP BN PAB TQ PQ TPH FLA 6-AC 9-MC DCN DClA 9-CA
4.41b 2.11' 1.3W 11.Ea 1.28' 1.300 1.6W 1.93' 7.68b 14.6b 0.26b 0.706 0.71b
T$
(ws)
0.20 0.25 29Oe 6 1OC 5c
>55 4.105 2 10 30 10 40
Ti character n** nr* *u*
see text see text see text *A
*
*** *** UT* TU*
*** TU*
GV Pk (M-I) 1.00" 1.00" 1.W 0.98, 1.01f g
-
-
0.8gd 1795 0.25h 391 317 0.78f 0.36' 449 0.36' 0.2gk 218 0.04' 181
QA(air)
SA
0.16 0.23 0.80 0.05 0.89 0.92 0.51 0.49 0.22 0.33 0.77 0.42 0.41
0.25 0.34 0.80 0.24 0.90 0.92 0.54 0.73 0.27 0.45 1.00 0.70 1.10
&lo%. Because of small TT absorption +15%. Estimated from the intercept of the linear regression according to eq 12. Reference 21. CReference 20. f&lO%.KEstimated to be one in analogy to TQ. References 8 and 22. Value for 9-ethyl~arbazole.~~ Reference 24. Reference 25. Reference 26. Error for relative QA*lo%. No error can be given for SAdue to the unknown uncertainty of reported .e ,"
F i p e 1. Dependenceoftriplet decay constantkTof oxygen concentration for PAB, BP, TPH, and 9-CA in toluene. The inset shows the decay of TT absorbance of BP in ethanol at oxygen concentrations 0.2,0.45, and 0.92 mM (analyzing wavelength 520 nm).
and are the extinction coefficients of the ground state and the triplet state of compound S at the analysis wavelength and d is the optical light path length. e ' is ~ the ground-stateextinction coefficient at the excitation wavelength and d'the corresponding light path length. Vdenotes the volume of the excitation region. The measured Qo values for PAB, TQ, and 6-AC are listed in Table 1. The Qk0" of TQ of unity is in good agreement with the negligible fluorescence quantum yieldl6 and since PQ exhibits similar spectroscopical behavior with no observable fluorescence, QLis assumed to be 1. In the case of FLA, the reported = 0.2518 is not compatiblewith the QAmeasurementsand evaluation of the intercept according to eq 20 (vide infra) provides QL= 0.41 f 0.15. Triplet quenching constants k;f were obtained by measuring the triplet lifetime dependence on oxygen concentration and evaluation according to eq 12 where
The parameters a and b determined from the nonlinear fitting procedure are defined as follows: a = (eT - b ) d [ S ]
b = 1n(lO)Q~dad'/V
(10)
Tf and T! are the triplet lifetimes in the presence and absence of oxygen, respectively (Figure 1). The plots are linear for all compounds except PAB in toluene and isooctane at oxygen
4232 The Journal of Physical Chemistry. Vol. 98, No. 16, 1994
Grewer and Brauer
TABLE 2 Rate Constants for Triplet Quenching by 0 2 in Solvents of Different Polarity' $/io9 (~-1s-1) solvent a c q(cP) BP BBP PAB CHR 2.12 0.345 3.84 1.21 14.4 MeCN 0.181 38.5 1.078 3.50 1.04 2.56 1.72 ethanol 0.223 25.8 1.51 toluene 0.202 2.38 0.587 2.11 0.92 11.8 1.39 i-octane 0.340 1.94 0.504 1.76 0.79 10.7 0 a = Bunsen's solubility coefficient at T = 295 K c = dielectric constant; q = viscosity. concentrations >2 mM. In the case of PAB only the linear region at low 02 concentrations was used for the calculation of kg'. For the compounds with m*-triplet states, the triplet decay curves in the presence of oxygen were monoexponential. The n?r*-ketones undergo H-atom abstraction in ethanol, toluene, and isooctane, in which case the time dependence of the absorbance change can be described by the following eq 13,
A(OD)(t) = A, exp(-k,t)
+ A,(exp(-k,t)
-
exp(-kRIRHl r)) (l 3,
Figure 2. Plots for determination of SAof TPH,9-CA, PAB, BP, and TQ in toluene according to cqs 20 and 15, respectively.
quenching by oxygen takes place (aromatic hydrocarbons), the following bimolecular deactivation processes are possible in addition to Scheme 1 (Scheme 2). k3 and k4 are assumed to be
SCHEME2
where the rate constant of ketyl radical formation ~ R [ R Hcan ] be determined independently at [O,] = 0. kK, the rate constant of the decay of the ketyl radical, depends on oxygen concentration because the ketyl radical reacts with 3 0 2 , presumably with formation of ground-state ketone and HO2 radical.19 With the iterative reconvolution procedure, a fit according to eq 13 yields the rate constants kT and kK, from which the quenching constants k: and kg" can be obtained. k, = k i
kd
+ 3(1~,-.30,) kd
+ kt[O,]
kd F=
For BP in toluene and ethanol values of kg" = (5.0 f 0.4) X lo9 M-1 s-1 and (3.5 f 0.2) 109 M-1 s-1 are obtained. In isooctane, ketyl radical formation is too slow to compete with triplet quenching by oxygen. The estimated k: values are summarized in Table 1. PAB is assumed to have n**-triplet configuration in nonpolar solvents and CT character in polar solvents.20 The character of the lowest triplet state of 9-MC is m*. In aromatic amines, the nr*-triplet state is in general energetically higher than the m*triplet state. 6-AC is therefore assumed to have a mr*-lowest triplet state. TheTl stateofTQandPQisassignednr*-character because its phosphorescence lifetime is less than 0.5 s.16 The splitting of the lowest mr* absorption and the 0 4 transition of the phosphorescence is smaller than expected for the normal m * - S and AT*-T splitting in aromatic hydrocarbons, therefore the T1 state would be expected to be m * 1 6 in agreement with the SA values, which are near unity, and the lack of ketyl radical formation. For four compounds, kg' was measured as a function of solvent polarity; the results are shown in Table 2. The calculation of S A from the experimentally determined Q A depends on the nature of the sensitizer. For the TU* ketones with QL= 1 and long triplet lifetime SA= QAresults in air-saturated solution. In the case of the n?r*-ketones, the short triplet lifetime requires extrapolation of QA to infinite oxygen concentration for the determination of SA.
According to eq 15, SAcan be calculated from the intercept of the linear regression if 1/QA is plotted versus 1/ [O,] (see Figure 2). The plot is nonlinear for PAB in toluene at low oxygen concentrations and for the calculation of S A only the h e a r region at higher oxygen concentrations was used. When fluorescence
k3
k4
3(1M1.-302)
4
IM,, + 10,
(18)
'M0 + 30,
(19)
slow compared to complexdissociation because the corresponding processes are spin-forbidden (eq 18) and energetically unfavorable (eq 19). The general expression for determination of SAmay then be written as1' QA(1
+ &[O,I)
SAQk + (SA+ &&[021
(20)
gV
where the Stern-Volmer constant = is defined as the product of rate constant of oxygen quenching of the first excited singlet state and the &-lifetime in the absence of oxygen and 4; denotes the efficiency of ' 0 2 formation accompanying the fluorescence quenching.
Figure 2 shows a plot of Q A ( + ~ g v [ 0 2 ] ) versus [02] for TPH and 9-CA, respectively, according to eq 20, where linear leastsquares fitting yields SA@, as intercept leading to SAif @ , is known. SAcan also be obtained from the slope, if the &-TI energy gap is insufficient for 1 0 2 generation. The SA-values are listed in Table 1. Discussion It is well-known that k; decreases with increasing triplet energy of the sensitizer in the case of aromatic hydrocarbons with ET > 200 kJ/mol as expected from the energy gap law, whereas na*-ketones and compounds with high triplet energy and low oxidation potential exhibit the opposite behavior.ll4.5 We observe a reduction in k; with increasing ET also for the ketones with rr*-T1 states. A clear deviation from the expected ET dependence on k t is found in the case of the cyano- and chloro-
Triplet-State Quenching by Molecular Oxygen
The Journal of Physical Chemistry, Vol. 98, NO.16, 1994 4233
Probabilities and Energetic Parameters of TI Quenchins by 0 2 in Toluene Calculated on the Basis of Scheme 1 and Mwroremeats for Cbrysene (CHR),QPbenylbenzopbenOae (BBP), and Perinaphtbenone (PN) from a Previous Paper (Ref 8)
TABLE 3: compd ketones AP BP PAB PN BN
BBP TQ
PQ
ET (kJ/mol)
AGm (kJ/mol)
E(D/D+) (V)
AGa (kJ/mol)
P1
308" 287' 26gb 1 84' 251' 254" 233d 231d
-214 -193 -175 -90 -157 -160 -139 -137
2.34h 2.37h 0.95'
-7 19 -102 51m
0.27 0.18 0.73 0.55 0.27 0.23 0.30 0.3 1
kealk-d
P3 0.27 0.12 0.77 0.006 0.02 0.003 0.01 0.008
kk1k-d
0.36 0.22 2.69 1.22 0.37 0.29 0.42 0.45
0.36 0.13 3.32 0.006 0.02 0.003 0.02 0.008
aromatic TPH 278' -184 1.64' -45 0.22 0.06 0.29 0.07 CHR 237e -143 1.4P -27 0.27 0.04 0.37 0.04 FLA 227" -133 1.741 16 0.36 0.05 0.57 0.07 9-MC 296 -202 l.lV -115 1.69 0.69 -2.5 2.21 6-AC 226 -132 0.68' -85 0.53 0.48 1.14 0.93 DCN 2321 -138 0.07 -0 0.07 -0 DClA i69r -75 1.54k 54 0.13 0.02 0.14 0.02 9-CA 17W -76 1.78' 77 0.18 0.001 0.22 0.001 a Reference 21. * Reference 31. C Reference 1 1 and 22. Reference 16. 8 Reference 32. /Reference 24. Reference 33. h Reference 34.1 Reference 35. Reference 36 calculated from the reported value versus NH electrode +0.56 V. Ir Reference 37. Reference 38. Calculated using Ip = 8.36 eV with Eo = l p - 6.739 All redox potentials versus SCE in acetonitrile. For calculation of Pi kd = 3.5 X loLoM-l s-l was uscd. substituted anthracene and naphthalene derivatives, which show k: values much lower than those for unsubstituted aromatic hydrocarbons with the same triplet energy and similar effects are found for fluoranil and chloranils and p-cyanoacetophenone?O where the electron-withdrawingcyano and chloro groups increase the oxidation potential in comparison with the unsubstituted compounds. The CT states (1J(2M+-.z02-)) are therefore expected to lie energetically far above the initially formed l13.5(3M1-*302)states, which precludes mixing of CT character to the latter. Thus, DClA, 9-CA, and DCN represent compoundswhich show the dependence of k: on triplet energy without CT contribution, while the opposite behavior is observed for the compoundswith low oxidation potential (6-AC, PAB, and 9-MC), which exhibit k: values much greater than expected from the triplet energy. As mentioned above, the knowledge of kT and SAallows the calculation of the quenching probabilities %I and P3 according to Scheme 1. From the definition of PI and P3, the following equations can be derived:
0.8
d
-
b"'
CUE
I oE!
'
.IW
'
.lo
;
io AGe,ILJlmoll '
'
'
IL
'
i o
?A0
'
.loo
'
-50
'
0
'
io
AG,, [tllmoll
,A
'
1 0
Figwe.3. Plots of log(ki) and SAversus AGO!in toluene. The open symbolsrepresent literaturevalues for substitutednaphthalenederivatives in benzene?
and the reduction potential of SCE32)
0 2
(E(Oz/O;) = -0.78 V versus
AGel = 96.5(E(M/M+) - E(O,/O;)) - E T
The calculated values of PI,P3, k.,/kA, and ki,/k-d are listed in Table 3. As one can see from the PI and P3 values for most of the compounds, the results can be described by Scheme 1. Only for the 9-MC complex intersystem crossing has to be assumed, since for PI a value greater I is obtained (vide infra). It is interesting to note that the P1 values of ketones with nr*triplet state (AP and BP) do not differ remarkably from the P1 values found for ketones and quinones with u?r*-T1 character. Obviously the change in SA(see Table 1) is mainly caused by P3, which differs by more than an order of magnitude. Since k,/kd and ki,/kA do not correlate with triplet energy it can be assumed that CT interaction is masking the triplet energy dependenceof the quenching process. To support the assumption of CT interaction we can apply the common concept of photoinduced electron transfer to our quenching data. The energetics of completeelectrontransfer can be estimatedaccording to Rehm and WellelJOfrom the oxidation potential of the sensitizer
'
(24)
where the Coulombic term and correction for the heat of solution of the ion pair according to the Born equation41 are neglected; calculated AG,1 values are also listed in Table 3. Figure 3 shows the dependence of log (kt) and SAof AG,l. k;f increases with increasing exergonicity of the CT-state formation. The trend for SAis reversed. According to scheme 1 the maximum expected value for k: amounts to 4/9kd which can be given from our fluorescence quenching measurements as 1.55 X 1OloM-l s-l. This value is reached from 9-MC in toluene, indicating a complete diffusion-controlledquenching process. In the endergonic region, a minimum value of k: = 7 X 1@M-1 s-1 is found for DClA. SAapproaches a value of 1 at highly positive AGel (except DClA) and seems to reach a limiting value of 0.2 in the exergonic region (except 9-MC). Figure 4 shows the dependence of Pi and log(ki/kA) (i = en, ic) of AGel. P3 is nearly zero at positive AGa. With decreasing AG,l P3 increases and exceeds P1 at AGe1 -80 kJ/mol. This is a result of the stronger AG,l dependence of kh compared to k, as reflected by the SAvalues, which increase with increasing AG,l. If log(ki/kd) is plotted versus AG,l, a linear dependence can be observed at first approximation although, in principle, a linear dependence can only be expected in the endergonic region of the plot. At negative AGa a limiting value of log(Ai/kA)
-
4234 The Journal of Physical Chemistry, Vol. 98, No. 16, 1994 1
I,
I
*I
Grewer and Brauer
SCHEME 3 'C
-5 0
%O
1
,
, -100
,
, .50
,
, 0
,
l',,,
50
,
,
,
100
'E
(25)
I 150
AG,,ItJhnoll
Figure 4. Plots of dependence of PIand log(&/b) (filled symbols), and&andlog(&k/k-d)(opensymbols)onA&. Thebrokenliierepnsents the dependence expected for complete electron transfer according to ref 40. The rectangularsymbolscorrespond to literaturevaluesof substituted naphthalene derivatives.6
(28)
measured value SA= 0.45 exceeds the expected by a factor of 1.8 resulting in the physically meaningless PI value of 1.69. To explain this behavior, intersystem crossing between the CT complexes 3 ( M r * - 0 3 and '(MF-OF) has to be assumed, as postulated by Garner and Wilkinsons in terms of Scheme 3. 'C denotes the collision complex of multiplicity i and 'E denotes the encounter complex or exciplex with CT character. Under the assumption of quasistationary concentration of the intermediate complexes for k i and SA, respectively, the following equations can be deduced:
should be observed (Ai is the preexponentialfactor of CT-induced internal conversion and energy transfer, respectively,which should be a measure of the collision frequency of the molecules in the encounter complex). Linear regression yields slopes of -1.6 X 10-2 mol/kJ for the triplet channel and 4 . 5 X mol/kJ for the singlet channel compared with a slope of 4 , 1 7 8 mol/kJ expected in the case of complete electron transfer.42 Two explanations are possible for this behavior. 1. The enhancement of the singlet channel over the triplet channel in the slight exergonic and endergonic region of electron transfer is due to competing energy transfer without CT interaction. In thecase of the triplet channel, CT freedeactivation is much slower because of diminishing FC factors, leading to the increased slope of the log(kic/kA) versus AG,I plot. 2. Reduced slopes in the endergonic region compared to completeelectron transfer give evidencefor exciplex for~nation.~Z~~ This is in agreement with the results of the temperature-dependent measurements which show that exciplexes are involved in the The different efficiencies are defined as quenching of triplet aromatic hydrocarbons and The plots of log(ki) and S A versus AGel show much more scatter than the data reported by McGarvey et ala6for substituted naphthalene derivatives in benzene. This maybe due to the different triplet energies of the compounds studied here, whereas the naphthalene derivatives have approximately the same triplet energy. As stated above, the intrinsic complex deactivation without CT contribution is much slower than expected (&, is about 1/10 of k4). Thus log(ki) and SAshould not depend on triplet energy in the exergonic region of electron transfer. The scatter in the data can therefore also be explained with competing complex intersystem crossing discussed in the case of 9-MC (vide infra). The solvent polarity dependence of ki, measured for BP, BBP, PAB, and CHR, betrays a small increase of kT with solvent polarity with the strongest effect in the case of $AB, although this interpretation is complicated by the different viscosity of the solvents used. Thus in acetonitrile k d increases by a factor of 1.5 compared to toluene, and in ethanol a factor of 0.9 can be expected, suggesting that kT is solvent polarity independent in the case of BBP and CHR. of BBP decreases when changing the solvent from toluene to acetonitrile from 0.96 to 0.94 in agreement with theassumption that the CT-state formation for BBPis endergonic. As one can see, the equations are far too complex to be evaluated. For CHR, Qi,the quantum yield of ' 0 2 formation at [O,] a, In the case of 9-MC, the expressions can be simplified because is found to be 0.30 in acetonitrile,45 consistent with a reduction the quenching is at first approximation diffusion controlled (if in CT-state energy in the polar solvent acetonitrile in comparison thequintet complexdoesnotcontributeto thedeactivation). Under with toluene according to the Born equation. &;f of PAB in the further assumption that k k > k-3 and (ken k k ) >> k-1, to explain SAvalues less than 0.25 in the case of heavy atom substituted ketones with nr*-T1 state. The use of this scheme has the advantage that the expression for k: adopts the simple form similar to Scheme 1. However, the expression for SAisas complex as for Scheme3 (seesupplementary material for details). The simplification made by the authors gives therefore no further information regarding the competition of the different channels. To our knowledge, direct evidence for complex intersystem crossing was only given in one other case, e.g., the oxygen quenching of r?r*-triplet ketones at low temperature.*
+
+
Conclusions The rate constant of triplet-state quenching by oxygen and the efficiencies of 102 generationSAof a series of different sensitizers with nr*- or m*-triplet states can be described with the wellknown Scheme 1. A strong dependence of kT and SAon the free enthalpy change of electron transfer is founl, which implies the participation of CT interaction. The quenching channel with triplet multiplicity of the encounter complex is influenced stronger by CT interaction that the singlet channel, leading to reduced SA at negative AC.1. This can be explained by competing CT free energy transfer or exciplex formation. The quenching data of one compound, 9-MC, cannot be adequatelydescribed by Scheme 1 and give direct evidence for intersystem crossing between the CT complexes of triplet and singlet multiplicity. In the case of 9-MC. CT-complex intersystem crossing leads to an increase of SA.If the CT-complex intersystemcrossing is taken into account in the kinetic equations, a value of P-i, of 0.27can be given. The solvent polarity dependence of k: is significant only in the case of PAB and BP.
Acknowledgment. Financial support from the Fonds der Chemischen Industrie is gratefully acknowledged. Supplementary Material Available: Description of the kinetic scheme used by Darmanyan and Footes with derivation of the
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