J. Phys. Chem. 1991, 95, 5190-5197
5190
fate is hydrogen abstraction from the solvent leading to regeneration of the parent molecule, phenol. The latter path is confirmed by the detection of phenol in the photolysis of anisole. The rate constant for the loss of the cation in methanol was determined to be 2.3 X lo5 s-I, and in hexane the same rate constant was measured as 0.8 X IO5 s-I. No significant change in these values was observed in the presence of oxygen, even with the use of oxygen as the purge gas during the course of the photolysis. Similar measurements for the decay of the phenoxy radical (spectral feature at 247 nm) were 0.6 X lo5 s-I and 1.0 X lo5 s-' in hexane and methanol, respectively. All values were for deoxygenated solutions. Measurements of rate constants via the transient absorption spectra are necessarily prone to error and these values should be taken as estimates of the relative rates of radical versus cation reactivity, rather than as absolute values. However, the approximate factor of 3 difference in the rate constants was reproducible and easily observed in the typical
transient absorption spectra presented in Figure 2. To summarize, it has been shown that resonantly enhanced multiphoton pumping of aromatic molecules in solution cleanly produces cationic species, which may then be used as reagents in ion-molecule chemistry. The MPIC technique may be thought of as a more selective type of ionizing radiation and may prove to be a useful technique in probing the nature and rate constants of ion-molecule reactions in solution. Further experimental work will involve the comparison of these solution-phase reactions with gas-phase photochemistry in the presence of the same solvent molecules in order to better characterize the relationship of these gas-phase "solvated" reactant studies to actual studies in solution. Acknowledgment. One of the authors (J.J.B.) acknowledges the financial support of the National Science Foundation through Grant Nos. CHE-8521664 and CHE-8852168. The technical assistance of D. Parsons and S. Brown was greatly appreciated.
Charge-Transfer State and Singlet Oxygen ('Ag 0,) Production in Photoexctted Organic Molecule-Molecular Oxygen Complexes Marianne Kristiansen, Rodger D. Scurlock, Kai-Kong Iu, and Peter R. Ogilby* Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131 (Received: December 20, 1990)
We examined charge-transfer (CT) state participation in an excited-state organic molecule (M)-molecular oxygen complex using a variety of timeresolved spectroscopicprobes. We showed earlier that singlet oxygen ('40,) is formed upon photolysis into the M-0, CT absorption band of molecules whose CT state energy is lower than that of the triplet state (3Ml). Our data were consistent with the production of 'A 0,upon dissociation of the photoexcited CT complex. We now examine a molecule whose CT state energy is substantialfy higher than that of 3MI. Relative 'Ag 0,and 'MI yields were determined upon (1) photolysis into the M-02 CT band of 1-methylnaphthaleneand (2) 3Z; 0,quenching of triplet 1-methylnaphthalene, 0,and 'MI yields were independent of the *(Ma,) production which was independently produced. In nonpolar solvents, method, indicating that relaxation of the 'JCT states to the *3(3M1...3Z, 0,) states is very efficient. In a polar solvent where the CT state is more stable, the data indicate that direct coupling between the CT and ground-state surface [3('Mo,3ZC; O,] may increase. CT-mediated indirect coupling of the ' V ~ ( ~ M ~ .O,), . . ~ Zand ; other M-0, excited states, to the ground-state surface should also increase in a polar solvent. 'A, 0,quantum yields obtained upon direct photolysis ('Mo 'MI) of six aromatic hydrocarbons support this interpretation. Our data are consistent with a model for oxygen-induced intersystem crossing in organic molecules in which coupling between singlet and triplet states is facilitated by mixing with a CT state.
'9
-
Introduction We are interested in understanding excited-state interactions between molecular oxygen and organic molecules. In an attempt to achieve this goal, we examined (a) ground-state oxygen (3Z; 0,) induced intersystem crossing in organic molecules (i.e., 'MI 3MI and 'MI 4 'Mo), focusing on the energy-transfer process that results in singlet molecular oxygen ('A, 02)rl*2 and (b) ground-state organic molecule ('Mo) induced deactivation of '$ O2to '2; Continued efforts in this field, however, require a better understanding of the coupling between electronic states of an oxygen-organic molecule complex. Upon exposure to oxygen, a new feature appears in the absorption spectra of organic molecules. It has been assigned to a transition from a ground-state oxygen (32;O2)-organic molecule complex to an oxygen-organic molecule charge-transfer (CT) state.5 The CT state is often represented as a complex with organic molecule radical cation and oxygen radical anion character (M'+02'-). Theoretical studies indicate that there is significant interaction between the CT state and other states of the M-O2 This is shown in Scheme I in which 1,3(M*+02*-) states mix with the 1*3*5(3Ml...3Z:g 0,) states that are formed when 3Z; O2 quenches a triplet-state organic molecule (3Ml).
-
0 2 3 9 4
*Author to whom correspondence should be addressed.
This system can also be represented in a model presented by Mullikenq in which the wave function for a particular electronic state of the M-0, complex can be expressed as a linear combination of zero-order wave functions for all available M a 2 states (eq 1). The coefficients of expansion (a, 6, c, ...) are such that \k(M.02) U ~ ~ ( ' M0,) ~ ,+~b41(M'+02'-) Z ~ c ~ ~ ( ~ M0,) I , +~ d&('Mo,'A, Z ~ 0,)+ (1)
+
e..
the wave functions (\k) for each state (e.g., ground, CT, or locally excited state) are orthogonal to each other. In the ground state, 9(M.02) is dominated by the 'no-bond" term (a >> b, c, d). In ~~
~
~~~
~
~
(1) (a) Iu, K.-K.; Ogilby, P. R. J. Phys. Chem. 1987,91, 1611-1617. (b) Iu, K.-K.; Ogilby, P. R. J. Phys. Chem. 1988, 92, 5854. (2) Iu, K.-K.; Ogilby, P. R. J. Phys. Chem. 1988, 92, 4662-4666. (3) Scurlock, R. D.; Ogilby, P. R. J. Phys. Chem. 1987,91,4599-4602. (4) Ogilby, P. R.; Footc, C. S. J. Am. Chem. Soc. 1983,105,3423-3430. ( 5 ) (a) Tsubomura, H.;Mulliken, R. S. J. Am. Chem. Soc. 1960, 82, 5966-5974. (b) Birks, J. B.; Pantos, E.;Hamilton, T. D. S.Chem. Phys. Le??. 1973, 20, 544-546. (6) (a) Kawaoka. K.; Khan, A. U.; Kearns, D. R. J. Chem. Phys. 1967, 46, 1842-1853. (b) Kawaoka, K.;Khan, A. U.;Keams.D. R. 1. Chem. Phys. 1967, 47, 1883-1884. (7) Khan, A. U.;Kearns, D. R. J. Chem. Phys. 1968. 48, 3272-3275. (8) Murrell. J. N. Mol. Phys. 1960, 3, 319-329. (9) Mulliken, R. S.; Person, W. B. Molecular Complexes; Wiley: New York, 1969.
0022-3654191 12095-5 190302.5010 0 1991 American Chemical Societv
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5191
Organic Molecule-Molecular Oxygen Complexes SCHEME I
SCHEME I1
1
Separated Molecules
Complex States
Separated Molecules
"REACTION COORDINATE' component of IA, 0,formation. (2) Rates of 'A, O2formation the C T state, the second, "dative", term dominates ( b > a, c, 4, upon CT band photolysis were independent of oxygen concenand so forth. tration, which is not consistent with a )MI photosensitized proWe recently showed that IA, 0,is formed upon photolysis into cess.ll (3) If )MI were formed, triplet energy migration could the M-O2 C T absorption band of liquid alkanes, arenes, ethers, yield a rate of 'A, O2formation much greater than that expected and alcohols.I0 In these molecules where the triplet-state energy is high, it is possible to photolyze into the broad M-O2 CT band from a case where mass diffusion is limiting. However, even where the concentration of M is high enough to yield an M-M distance and preclude )MI formation. The IA, O2yield was linearly dependent on the C T band absorbance a t a variety of photolysis of -3 A (e.g., polystyrene), the amount of time required for an energy hop (10-100 ps)', is still slower than the observed rate wavelengths, and discontinuities were not observed as the wavelength was shifted to a point energetically sufficient for 3MI of 'A, 0,formation (11.7 ps).I1 These points are significant to the discussion presented later in section I of Results and Discussion, production. It thus appears, at least on the basis of these data, that even if CT state dissociation to form 3MI and 3 X ; O2 is where data from corresponding experiments in a different system exothermic, I A. 0,is produced directly from the photoexcited support the production of 3MIupon CT band photolysis. CT complex. In an attempt to quantify matrix elements for M a 2 state Data we obtained from experiments with solid polystyrene and mixing, we set out to determine if yields of 'A8 02,)MI, and M" a low-temperature 1-phenylbutane glass are consistent with this were independent of the *(M-0,) entry point. We were parinterpretation." Upon photolysis of these neat, oxygenated solids, ticularly interested in a system where the CT state energy was the !A, 0,yield was again linearly dependent on the CT absorhigher than that of )MI so that we could address the following bance and discontinuities were not observed as the photolysis questions (Scheme 11): (1) Does dissociation of the l(M'+O;-) wavelength was shifted to a point that exceeded the )MI energy. state to yield 'A! O2(k,) compete with relaxation to 1*3(3Ml...3ZIn addition, (1) the rate of IA, O2 formation upon C T band 02), resulting In IA, 0,yields that depend on the method of photolysis was much faster than that for a known solid-state excited-state preparation? (2) Is CT state relaxation so efficient photosensitized process." In the latter, the bimolecular encounter that there is only one I$ 0,precursor, the l ( 3 M l . . . 3 q0,)state? frequency between )MI and )Z-0,is reduced due to a decrease In the latter, the )MI yield should also be independent of the in diffusion coefficients. A fast f4 O2formation rate is consistent method by which *(M-O,) was prepared. with facile coupling between the C T and l(lMO...lAg0,) states Relative IA, 02,)MI, and radical ion (Me+) yields were defollowed by rapid dissociation to yield free 'A, 0,.Admittedly, termined for l-methylnaphthalene ( M N ) subsequent to (1) irany )MI formed by CT relaxation would be in the immediate radiation into the MN-0, C T absorption band and (2) specific vicinity of O2and give rise to a fast static quenching comproduction of 3MN by energy transfer from a triplet ketone." The ponent. Nevertheless, separation of the M a 2 complex followed results of our experiments in polar and nonpolar solvents and in by a new 'MI-)Z, O2encounter should also give rise to a slow a low-temperature glass are presented in this paper. To provide additional and independent support for our M N study, we determined absolute l$ O2quantum yields for a variety of aromatic (10) (a) Scurlock, R. D.; Ogilby, P. R. J . Am. Chem. Soc. 1988, 110, (b) Scurlock, R. D.; Ogilby, P. R. J . Phys. Chem. 1989, 93, 5493-5500. (1 1) (a) Ogilby, P. R.;Kristiansen, M.; Clough, R. L. Macromolecules 1990, 23, 2698-2704. (b) Clough, R.L.; Dillon. M. P.; Iu, K.-K.; Ogilby, P. R. Macromolecules 1989, 22, 3620-3628.
640-641.
(12) Burkhart, R. D. Chem. Phys. Left. 1987, 133, 568-573. (13) Gorman, A. A,; Hamblett, 1.; Lambert, C.; Prcscott, A. L.; Rodgcrs, M. A. J.; Spence, H. M . J . Am. Chem. SOC.1987, 109, 3091-3097.
Kristiansen et al.
5192 The Journal of Physical Chemistry, Vol. 95, No. 13. 1991 TABLE I: #AgOs Quantum Yields (OA"), Singlet State Lifetimes (&) for Seven1 Organic Mokcuks' molecule MN chrysene pyrene phenanthrene triphenylene biphenyl DCA
(T,),
and Bimolecular Rate Constants for Oxygen Quenching of tbe Singlet State
solvent acetonitrile cyclohexane acetonitrile cyclohexane acetonitrile cyclohexane
ns 72.6 f 1.0 84.1 f 0.8 43.7 f 0.3 44.76 374 f 5 45ob
acetonitrile cyclohexane acetonitrile cyclohexane acetonitrile cyclohexane acetonitrile cyclohexane
55.1 f 0.2 57.9 36.0 f 0.3 38.0 f 1.0 15.9 f 0.2 17.2 f 0.5 14.7 f 0.2 11.0 f 0.5
kl0x1 s-I M-I
T,,
(3.6 f 0.2) X (2.2 i 0.2) x (2.8 f 0.2) X 2.9 X I O t o b (2.7 f 0.3) X 2.5 X l o l o b (2.8 f 0.2) X 2.5 X (3.3 f 0.4) X (2.3 f 0.1) X (4.3 f 0.6) X (2.6 f 0.2) X (6.4 f 0.3) X (3.6 f 0.5) x
@A-
0.30 f 0.08 0.77 f 0.15 0.30 f 0.13 0.65 f 0.08 0.42 f 0.17 0.81 f 0.04E 0.24 f 0.10 0.44 f 0.04 0.27 f 0.10 0.49 f 0.05 0.27 i 0.1 1 0.73 f 0.08 2.0 f 0.lt 1.9 f 0.5
1Olo
1010 1Olo
IOto IO'O 1Olo
1OIo 1Olo
IOto IO9 109
In several instances, available data from the literature are presented. The appropriate references are indicated. Data were determined at laser energies 1 1 . 5 mJ pulse-l c&. "eference 37. 'Average of two numbers reported in the literature, 0.79 f 0.04 and 0.83 f 0.05 (refs 31 and 32). This number was used as the standard in cyclohexane. dReference 38. CReference 30. This number was used as the standard in acetonitrile.
hydrocarbons in both acetonitrile and cyclohexane. These data are also presented here.
Experimental Section Our time-resolved I$ O2detection system and flash absorption spectrometer have been Under all conditions, '4 O2phosphorescence followed first-order decay kinetics. For 'Ag O2 intensity measurements, the time-resolved phosphorescence signal was integrated and normalized by the observed '$ O2 lifetime to yield a value of IAe O2phosphorescence intensity at "zero time". [At the MN concentrations used in the CT experiments (3.2 M), quenching of IA, O2did not occur in benzene and cyclohexane (T* = 31 and 23 ps, respectively), but the ]Ae O2 lifetime in acetonitrile was shortened from 80 to 21 ps.] We recently showed that the rate constant for '4O2radiative decay (k,) is solvent dependent and that a good correlation exists between k, and solvent polarizability.' The latter parameter is conveniently expressed as a function of the solvent refractive index? All absolute 'A, O2quantum yields (Table I) were determined against a standard in the same solvent; therefore, a correction for k, was not necessary. In the MN-sensitized and CT studies, however, although data from a common solvent were compared, the MN concentration in the CT experiment (3.2 M) was sufficiently high that k, was perturbed. In an independent series of sensitized experiments, we determined that k, increases with MN concentration Over the range 0.4-3.2 M (Figure 1A). These data are consistent with our solvent polarizability arguments.' For example, addition of MN (nZ0D = 1.6159) to either acetonitrile (PD = 1.344) or cyclohexane (#D = 1.426) results in an increase of the solution refractive index that is linear with MN concentration (Figure 1B). On the basis of this change in refractive index, it is possible to calculate an expected change in k, from data in ref 3. These calculated Akr values agree with Akr values directly measured in MN/acetonitrile and MN/cyclohexane solutions (Figure 1A). All the MN 'Ag O2 data were normalized for changes in k,. Finally, since IAe O2phosphorescence originates in a solvent of refractive index nl and is detected in air of refractive index no, the ratio of the phosphorescence intensity actually detected to the desired quantity will be a function of n,.' Under our conditions, the correction factor is nf. Our flash absorption data were not influenced by changes in the sample refractive index. Refractive indices were measured on a Bausch and Lomb Abbe refractometer. A reverse-biased silicon detector was used to monitor fluorescence. This system (IOO-ohm load resistor/lOO MHz unity gain buffer amplifier) had a response time of 8 ns. Relative intensities were determined by integrating the time-resolved fluorescence signal. Fluorescence lifetimes were determined by deconvoluting the laser pulse profile from the observed fluorescence
-
n
0.00
4 ~
/
8.00
.*
J
4.401
A 3.20 0
I
I
1
I
.E
1.6
2.4
3.2
concentration
(M)
1.42-
X
a, U C TI
a,
1.90-
> .*
1.331 0
.
I .0
.
I
1.6
,
I 2.4
,
I
,
3.2
c o n c e n t r a t i o n (MI O2 phosphorescence intensity (Ia) as a Figure 1. (A) Plot of the function of 1-methylnaphthalene (MN) concentration in acetonitrile. p-Methoxyacetophenone was used to sensitize the formation of '4 02. Under these circumstances, I A is directly proportional to k,, the rate constant for I adiative decay. (B) Plot of the refractive index of a solution of M %O in acetonitrile as a function of MN concentration. The solid lines are linear least-squares fits to the data.
decay. Fluorescence was spectrally resolved with an Oriel Model 7240 monochromator. Analog signals were digitized as described
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5193
Organic Molecule-Molecular Oxygen Complexes p r e v i ~ u s l y . ' - ' ~ ~ ~Absorption ~~'~ spectra were recorded on a Beckman Model DU-40spectrophotometer. With the exception of the low-temperature organic glass study (Figure 4), all experiments were performed at 25 OC in 1 cm path length cuvettes. Low-temperature measurements were made by using a refrigeration apparatus described e l ~ e w h e r e . ~ J ~ The method used to control sample oxygen concentrations has been described.'J I$ O2quantum yields reported in Table I were determined with sensitizer concentrations that did not exceed 3.0 X lo-' M. Acetonitrile, benzene, and cyclohexane (either HPLC or spectrophotometric grade) were used as received. Pyrene, biphenyl, benzophenone, and p-methoxyacetophenone (Aldrich) were recrystallized twice from ethanol. Chrysene (Aldrich) was recrystallized twice from cyclohexane. Phenanthrene (Aldrich, gold label) and triphenylene (Aldrich) were used as received. 9,lO-Dicyanoanthracene (Eastman Kodak) was recrystallized twice from benzene/methanol. Acridine (Aldrich) was recrystallized twice from ethanol/water and then sublimed under vacuum. Naphthalene (Aldrich) was sublimed. Mesoporphyrin IX dimethyl ester (Porphyrin Products) was used as received. 1-Methylnaphthalene (Aldrich) was purified by chromatography on silica gel immediately prior to use. 1-Phenylbutane (Aldrich) was distilled from sodium.
Results and Discussion I. IA, O2and 'MI Yields: CT Excitation vs Triplet SensifizatiOn 1-Methylnaphthalene (MN) is an appropriate substrate for this study: ( 1 ) It is a liquid at room temperature; thus solutions sufficiently concentrated to give an appreciable CT absorbance at the photolysis wavelength can be prepared ( A s n m= 0.07 at 3.2 M MN in oxygenatedt5cyclohexane or benzene). (2) The MN-02 CT absorption is red-shifted relative to the solvent-02 CT absorption: thus it is possible to selectively photolyze the former at the exclusion of the latter.16 (3) The MN singlet-state energy (90.6 kcal/mol)" is high enough that photolysis into the MN-02 CT band at 355 nm precludes 'MI formation. (4) The MN triplet-state energy (59.6 kcal/mol, 480 nm)17J*is significantly below that of the CT state. In an independent experiment, 'MN was prepared with the technique of Gorman et al." In this case, solutions of a ketone and MN were prepared. Pulsed laser photolysis (355 nm) of the ketone results in 3ketone formation within several picoseconds with unit efficiency. The concentration of MN in solution (-0.3 M) was adjusted such that (a) none of the incident photons would be absorbed by the MN-02 complex, and (b) all 'ketone molecules produced could be efficiently quenched to form 'MN (eq 2).19 ketone
- - hv
'ketone
3ketone
MN
'MN
+ ketone (2)
(14) Iu, 19-32.
K.-K.; Scurlock, R. D.; Ogilby, P. R. J. Photochem. 1987, 37,
(15) Our labortaories in Albuquerque are at an elevation of -1609 m; thus, the atmospheric pressure is typically -630 Torr. (16) Although the ratio of MN-02 CT absorbance to b e n z e n t 0 2 ,CT absorbance is 101 at 355 nm under our conditions. and it is indeed possible to produce 'A, O2upon 355-nm photolysis into the b e n z e n t 0 2 CT band,I0 the data recorded from the MN experiment in benzene were equivalent to those recorded in cyclohexane where the solvent-02 CT absorbance is negligible. The acetonitril4, complex does not absorb at 355 nm. (17) (a) Bcrlman, I. Handbook of Fluorescence Specrra of Aromatic Molecules; Academic Press: London, 1965. (b) McClure, D. S.J . Chem. P h p . 1949, 17,905-913. (18) Engel, P. S.;Monroe, B. M.In Aduances in Photochemistry; Rtts, J. N.. Hammond. 0. S.,Noyes, W.A., Eds.; Wiley: New York, 1971; Vol. 8, pp 245-313. (19) (a) In benzene, the rate constant for quenching of 3benzophenoneby M N IS 9.4 X I@ M-l s-I (reference 19b). Thus, at [MN] = 0.30 M, M N quenches 3bcnzophenone50 times faster than does oxygen in an air-saturated solution. Both of these processes are significantly faster than the hydrogen abstraction reaction between 3benzophenoneand benzene to make the ketyl radical (ref 19c). In cyclohexane at [MN] 0.30 M,the 'MN yield upon quenching of 3@-methoxyacetophenone)was independent of oxygen concentration. Thus, in this case as well, oxygen quenching of 3ketone does not interfere with the energy transfer process to form )MN. (b) Urruti, E.H.; Kilp, T. Macromolecules 1984, 17, 50-54. (c) Turro, N. J. Modern Molecular Photochemistry; Bcnjamin/Cummings: Menlo Park, CA. 1978; p 374.
-
p h o t o n s absorbed
(XlOi5)
photons absorbed
(x1014)
Figure 2. Plots of the relative '4 O2 phosphorescence intensity as a function of the number of laser photons absorbed. Data recorded subsequent to photolysis of the MN-02 C T absorption band (*) have essentially the same slope as data recorded subsequent to photolysis of a ketone, followed by energy transfer to form 'MN (0). The solid lines are linear least-squares fits to the data. (A) The solvent is benzene. The ketone is benzophenone. (B) The solvent is cyclohexane. The ketone is p-methoxyacetophenone.
By monitoring 'MN absorbance at 420 nm, we determined that, under our conditions in an air-saturated solution, 'MN is quenched entirely by oxygen and not by the ground-state ketone. 'Ap O2 was produced by the methods outlined above, and relative yields were quantified by monitoring the 'A, O2phosphorescence intensity as a function of the number of photons absorbed in each system. Where the MN-0, complex and ketone absorbances were not identical, phosphorescence intensities were normalized to account for? t ese lo differences. The number of photons absorbed by each system was varied by changing the photolysis laser energy over the range 0.26-1.5 mJ pulse-' cm-2. Data for the solvent benzene are shown in Figure 2A. In this experiment, benzophenone was used to sensitize )MN formation. The independent sets of data are linear with the expected y intercept of zero. Our results indicate that, within experimental error, the IAsO2yield is independent of whether * ( M a 2 ) was prepared by a triplet-sensitized process or by CT excitation. When the experiments described above were performed in cyclohexane, our results indicated that more I$ O2was produced upon M N a 2 CT band photolysis than in the benzophenone/MN triplet-sensitized process. It is known, however, that the rate constant for hydrogen abstraction by 'benzophenone is greater for reaction with cyclohexane than with benzene.'9c We therefore suggest that, in cyclohexane, 'benzophenone does not produce 'MN with unit efficiency due to a reaction with the solvent to form the
Kristiansen et al.
5194 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
0.75
++0.50
W
>
H
I-
W [I
photons absorbed
TIME
(x10i4)
Figure 3. Plots of the triplet state MN yield as a function of the number of laser photons absorbed. The solvent is cyclohexane. Data recorded subsequent to photolysis of the MN-02 CT absorption band (*) have essentially the same slope as data rccorded subsequent to photolysis of pmethoxyacttophenone, followed by energy transfer to form 'MN (0).2i The solid lines are linear least-squares fits to the data.
(microseconds)
Figure 4. Time-resolved '4 O2phosphorescence data in a solid 130 K glass of I-phenylbutane containing 3.2 M 1-methylnaphthalene (MN). The 'AgO2signals were recorded subsequent to 355-nm photolysis into the MN-02 CT absorption band in oxygen (---) and air (-) saturated samples. The sample pathlength was 2.0 cm.
ketyl radical. This interpretation was confirmed by repeating the experiment with the less reactive ketone p-methoxyacetophenone ['(7,7*) state]" in place of benzophenone ['(n,7*) state] Under these conditions, as in benzene, the 'A6 0,yield was independent of * ( M a , ) entry point (Figure 2B). Therefore, from the slopes of data shown in Figure 2, we determined the following ratio for M N in a nonpolar solvent:
1' A6 0 2 yield1triplet sensitizer [IA,
O2 yieldlCTphotolpis
= 1.00 i 0.04
(3)
These M N data are consistent with several interpretations. Mixing
between the 193(M*+02') and I*3(3Ml...3Z,4) states is significant, and as a result, both states appear to be equally coupled to the '('M ,-,...'A 0,) potential surface. In this case, the relative 'MN yield shoufd be less upon CT band photolysis. On the other hand, the data may indicate that mixing between the CT and 1.3(3M1,3Z- 0,) states facilitates rapid deactivation of the initially !( state to the 1*3(3M1...3Z-0,) states. From this prepared T perspective, where k* >> kcr, 1(3M1..!Z; 02)would be the only IA O2precursor, and both the 'A6 O2and 'MN yields would be idependent of the excitation method. To further characterize this system, we monitored 'MN in a flash absorption experiment subsequent to both energy transfer from p-methoxyacetophenone and MN-02 CT band photolysis. Although the triplet lifetime was substantially shortened by oxygen quenching, we were able to quantify relative triplet yields and determine the following ratio from the data in Figure 3.21 ['MN yieldltipiet sensitizer
= 1.10 f 0.13
(20) Lutz, H.; Breheret, E.; Lindquist, 1. 1. Phys. Chem. 1973, 77,
1758-1762.
(21) The extinction coefficient ( 6 ) of 'MN in a dilute cyclohexanesolution of M N is 14200 M-'cm-I at 420 nm?' Using this number as a standard, c is 6350 M-'cm-' in neat M N (7.0 M).**Assuming a linear relationship between e and M N , we calculate a 420-nm 'MN extinction coefficient of 1061 1 M-'cm- in a 3.2 M solution of MN. The data in Figure 3 have been
(22) Carmichael, I.; Hug, G. L.J . Phys. Chem. Re/. &tu 1986, IS, 1-250.
(x10i4)
Figure 5. Plots of the relative '4 O2 phosphorescence intensity as a function of the number of laser photons absorbed. The solvent is acetonitrile. Data rccorded subsequent to photolysis of the MN-02 CT absorption band (*) have a slope approximately 1.5 times smaller than that for data recorded subsequent to photolysis of p-methoxyacetophenone, followed by energy transfer to form 'MN (0).The solid lines are linear least-squares fits to the data.
obtained when M N was deposited in a low-temperature (130 K) glass of 1-phenylbutane. Because the observed (or manifestttb) time-resolved 'A 0,phosphorescence signal is a convolution of the intrinsic '4 decay and the decay of the IA, O2precursor, changes in the precursor lifetime can appear as changes in the rising and falling portion of the manifest '4O2phosphorescence signal.'' We have established that, in solid samples, the manifest I$ O2phosphorescence signal in a known triplet-photosensitized reaction is quite sensitive to changes in the sample oxygen concentration." With an increase in oxygen concentration, the encounter frequency between 'Mi and '2; O2increases, and the rates of signal appearance and disappearance likewise increase. Conversely, the IA6 O2phosphorescence signals obtained upon photolysis into the M a 2 CT bands of solid polystyrene and 1phenylbutane do not change with oxygen concentration (vide supra, Introduction). These latter data are consistent with our proposal that, in these samples, the CT state itself is the immediate 'A, O2precursor. We recorded '4 O2phosphorescence subsequent to pulsed laser photolysis into the MN-02 CT band in air- and oxygen-saturated samples of M N in a low-temperature glass (Figure 4). A change in the manifest 'A8 O2phosphorescence signal is clearly evident with a change in oxygen concentration,
8,
(4)
[3MN yieldlCT photolpb Since the ratio given above does not deviate substantially from unity, these data indicate that direct dissociation of 193(M*+Oz'-) does not compete with relaxation to the ' S ~ ( ~ M ~ 02) . . . ~states Z~ and that there is indeed only one IA6 0,precursor in this M N system, the 1(3Ml..,3Z; 02)state. Evidence consistent with relaxation of initially prepared C T states to the 193(3M,..?Z;0 2 ) states prior to I$ O2formation was
normalized accordingly.
i
photons absorbed
The Journal of Physical Chemistry, Vol. 95, No. 13, 1991 5195
Organic Molecule-Molecular Oxygen Complexes and we propose that these data reflect the following sequence of events: ( I ) rapid relaxation of the initially prepared C T state to the I.3(3MI,,.3Z; 0,) states; (2) cage escape of 3Z; O2 to yield "free" )MI (this process will compete with static quenching of 3Ml by 'Z- 02);(3) quenching of "free" 3MI by 'Zi O2 that now depends on the bulk oxygen concentration which, In turn, will be reflected in the manifest I As O2 phosphorescence signal. In acetonitrile, photolysis into the MN-0, C T band did not produce 0,as efficiently as when *(M-0,) was prepared by triplet sensitization (Figure 5).
'4
I'A,
~ieldlt,ip~ctscnsitizcr = 1.52 f 0.05 ['A* 0 2 YieldICT photolysis 0 2
(5)
These data may indicate that the 1.3(M*+02*-)states are better coupled to the potential surface for the solvated radical ions than are the 1-3(3Ml.,.3Z-0,) states (Scheme I).23 The following assumptions are imdicit in this hypothesis: (1) Although radical cations can interact with 0;-to yield I$ 02:526 the lA, 0,yield subsequent to ion recombination will be less than that subsequent to energy transfer from 3Ml,for example. In ion recombination, back electron transfer may occur in the solvent-separated ion pair, thus precluding efficient coupling with the '('M ,...'A, 0,) state. (2) In a time-resolved experiment where the yield of IAS 0,is correlated to the A, O2phosphorescence intensity at zero time, ion recombination may "slowly" provide a source of IA, 0,that is not detected. We were indeed able to detect spectroscopically MN'+ (and, in an independent experiment, naphthalene'+) subsequent to M-0, C T band photolysis and energy transfer to form 3Ml. The M" flash absorption spectra recorded in acetonitrile were identical with published data.2' [As expected, similar transients were not observed upon *(M-0,) preparation in a nonpolar solvent.] Unfortunately, (1) the M'+ absorbance of the samples increased rapidly with each consecutive photolysis pulse, (2) the signal to noise ratio for fresh, unirradiated samples was poor, and (3) the transient decay did not follow first-order kinetics. Thus, we are hesitant to draw conclusions from these data. Although we are presently unable to quantify the absolute yield of MN", the best estimates of the radical ion extinction coefficient:' coupled with our flash absorption data, indicate that the smaller ' A O2yield upon C T band photolysis (eq 5) cannot be accounted for entirely by radical ion formation. Tnus, we propose that, in a polar solvent where the MN-0, C T state is expected to be more stable, direct coupling of the C T state to the ground-state surface [3(1M,...3Z, O,)] may increase, providing a deactivation channel that will compete with '$ O2formation. Within the context of the Mulliken model embodied by eq 1, in acetonitrile, there will be an increase of C T character in the \k[3('Mo...3Z- O,)] state. Of course, as the C T state energy is lowered andt E(CT)-E(3Ml) becomes smaller, the indirect, CT-mediated, coupling between the '*3(3Ml...3Z; 0,)and 3(1Mo...3Z, 0,) states will also increase. We suggest, however, that these latter changes will not influence the IA 0,yield as much, thus contributing in part to the difference in t A, 0,yields shown in Figure 5. 11. Absolute IA, O2Quantum Yields: The Effect of Solvent. In section I, we indicated that 'v3CT states will be more stable in a polar solvent. Consequently, direct coupling of the CT state to the ground-state M-0, surface [3(1Mo...3Z,O,)]may be more
-
(23) In liquids, the transition '2' 0 2 '4 02 is extremely rapid (
