Oxygen in Organic Photochemistry

Oxygen chapter (v13D) Page 1 April 13, 2004 at 13:22 PM Chapter 14 Oxygen in Organic Photochemistry In this chapter we present information on oxy...
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Oxygen chapter (v13D)

Page 1

April 13, 2004 at 13:22 PM

Chapter 14

Oxygen in Organic Photochemistry

In this chapter we present information on oxygen, its molecular structure, photophysics, photochemistry, and its interaction with various reactive intermediates, including excited states and some ground state species, such as free radicals, carbenes and biradicals. This knowledge will help us understand not just the spectroscopy of oxygen, but how it interacts within a photochemical system. Oxygen is a very special molecule from an environmental, as well as biological point of view. Just like water, it is an essential ingredient to support life on earth. Many of the roles of oxygen in the environment involve –directly or indirectly– the interaction with light, making the understanding of these processes essential.

1. The oxygen molecule The basic electronic structure of the oxygen molecule in the ground state can be written as: O2 (1g)2 (1)2 (2g)2 (2)2 (3g)2 (1)4 (1g)2 or O2 (1)2 (1*)2 (2)2 (2*)2 (3)2 (1)4 (1*)2 where the 3 orbitals result from the two 2pz orbitals (along the O-O bond) and constitute the -bond in molecular oxygen. The 1 orbitals result from the 2px and 2py orbitals in each oxygen atom and lead to two bonding (1) and two antibonding (1g) orbitals. All but the last pair of antibonding orbitals are fully occupied and constitute closed shell contributions; thus, they do not determine overall symmetry, angular momentum, or spin. The last pair of electrons [i.e. (1g)2 or (1*)2] is responsible for six different electronic substates. There

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are three triplets with orbital angular momentum of zero that correspond to the 3g ground state. There are two singlet states with orbital angular momentum of 2 and -2 corresponding to the 1g state,1 and finally, a singlet state of  geometry, i.e. the 1g state, or the second excited state of oxygen. Partly as a reflection that these three states result from different arrangements of the same two electrons in the same two degenerate orbitals, they all have nearly coincident potential minima, i.e., the O-O bond distance (~1.09 Å)) is almost the same for the three states. Excited triplet states are significantly higher in energy. On the basis of simple orbital occupancy arguments, Hückel has shown that the 1g energy should be approximately half-way between the two g states, i.e., E(1 g ) 

1 E(1  g ) + E(3  g ) 2

[

]

(1)

In reality, the 1g state is 22.4 kcal/mol above the ground state, while the 1g state is at 37.5 kcal/mol. On the basis of these energy levels (see Scheme oo.1), one would expect oxygen phosphorescence at 1269 nm (1g  3g) and at 762 nm (1g  3g); both are actually observed;2 however, singlet oxygen leads to emission at several other wavelengths, such as 1910 nm and 635 nm. These emissions reflect other transitions; for example, the weak emission at 1910 nm is due to fluorescence between the two singlet states of oxygen, i.e., 1g  1g.

1 The reader should note that this well known state, in fact, corresponds to two degenerate substates. 2 We have normally described emission from singlet states as fluorescence, not phosphorescence. The case of oxygen is special in that this emission involves a change in multiplicity. For this reason, it is correctly described as phosphorescence, in spite of originating from a singlet state.

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1

g, g 44.8 kcal/mol, 188.6 kJ/mol, 15764 cm

-1

1

S g 37.5 kcal/mol, 157 kJ/mol, 13121 cm-1

Energy

R = 6.7 min (to 1g)

1

g 22.4 kcal/mol, 94.3 kJ/mol 7882 cm

-1

R = 64.6 min

3

-

g (ground state)

Scheme 1: Gas phase energy levels for molecular oxygen.1 Excited triplet states have not been included because they are much higher in energy. The 1g state is the one normally refereed to as singlet oxygen.

The emission wavelengths given above correspond to the transition between the  = 0 vibrational levels of the initial and final states. Emission is also observed frequently to the  = 1 state of O2 (3g). This leads to a red shift of the emission by 1585 cm-1, which, in the case of the emission for 1g, corresponds to a weaker band at 1588 nm, which accompanies the band at 1269 nm mentioned before. Oxygen shows several "dimeric" emissions, the best known of which is the dimol emission at ~635 nm. The name conveys, incorrectly, the idea of emission from some form of dimer. In fact, no dimer is formed under normal laboratory conditions. Kasha

2

has

pointed out that the simultaneous transition for a pair of emitting species does not require an actual complex to exist, although the two molecules must be within contact distance; i.e., close enough for electron exchange to be possible. The process has also been described as energy pooling and is reminiscent of triplet-triplet annihilation processes discussed in Chapter z z

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O2 (1 g ) + O2 (1 g )  O2 (3  g ) + O2 ( 3  g ) + h

April 13, 2004 at 13:22 PM

(2)

where the emitted photon corresponds to twice the energy of the 1g state. An upper limit of 7 x 105 M-1s-1 has been estimated for the rate constant for reaction 2 in carbon disulfide.3 Interestingly, the 'dimol' emission may be responsible for red glows in the aurora borealis (northern lights). Other emissions, corresponding to [1g, 1g] (~476 nm) and [1g, 1g] (~381 nm) pairs are also known. Clearly, since these unusual transitions are rare in other systems, we may thus ask if there is any special reason why in the case of molecular oxygen they are so well established. This is in part due to the fact that oxygen has been the subject of close scrutiny because of its key role in atmospheric chemistry; further, singlet oxygen can be readily produced in moderately high concentration in thermal (chemiluminescent) reactions between ground state molecules, such as the hypochlorite-peroxide reaction.

Just as

importantly, the estimated emission (i.e., radiative) lifetimes in the gas phase at zero pressure are a remarkable 64.6 min. for O2 1g and 6.7 sec. for the 1g state (see Scheme 1). At higher pressure, or in the liquid phase, the lifetimes are much shorter, but frequently long enough to allow singlet oxygen bimolecular reactions to take place, particularly for the 1g state. The dynamics of these processes will be discussed in more detail as we analyze the reactions of singlet oxygen in Section zz. Ogilby 1 has summarized the effect of condensed media on oxygen spectroscopy as follows: "Solution-phase perturbations give rise to three principal changes in these transi-tions, the extent of which depends significantly on the solvent: 1. The transitions become more probable, as reflected by increases in kr, 2. the transition energies decrease, as reflected in red-shifted emission spectra, and,

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3. nonradiative decay channels become accessible and dominate the overall rate constants for O2 (b 1 g+) and O2 (a 1 g) deactivation which, in turn, results in emission quantum yields that are quite small (10 -3 to 10-7)."

When we discussed earlier the excited states of organic molecules, we frequently encountered situations where the excited state energy was comparable with typical bond dissociation energies (i.e., in the 40 – 100 kcal/mol). Thus, we were not surprised to find bond rupture as a frequent consequence of excited state processes. In contrast, the energy of the O2 1g state (~22.4 kcal/mol) is lower than any chemical bond that will survive at room temperature. Thus, we anticipate that bond cleavages will not be plausible reactions in singlet oxygen chemistry unless they are accompanied by other changes that compensate the energy requirements of bond rupture (e.g., the formation of new bonds). We note at this point that the singlet oxygen energy is comparable with the energy associated with high frequency vibrations (e.g., C-H or O-H). For example, 3500 cm-1 corresponds to ~10 kcal/mol. We will see later that this energy matching plays a role in the deactivation and characterization of singlet oxygen.

2. Thermodynamic and electrochemical properties of oxygen and oxygenrelated species A key parameter in interpreting the influence of oxygen on reactions in solution, is its concentration in solution. In general, we can expect a term of the type "k[O2]" to appear in our equations. As a general rule, the solubility of oxygen in solution follows the trend: halogenated solvents > hydrocarbons > aqueous systems Table 1 gives the concentration of oxygen in solution in various solvents at room temperature (22-25˚C) saturated with oxygen at a total pressure of 1 atm. The concentration

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under air is approximately one fifth of the value in Table 1. If one wants to calculate the concentration from the solubility parameters, it is important to note that the vapor pressure of the solvent needs to be subtracted from the atmospheric pressure before the partial pressure of oxygen is calculated. Table 1:

Concentration of oxygen in various solvents at room temperature under a total pressure of 1 atm.a,b

Solvent

[O2], mM

Water

1.0

Dimethylsulfoxide

2.1

Pyridine

4.9

Acetonitrile Benzene

a The partial pressure of oxygen taken as 1 atm minus the vapor pressure of the solvent. b Sources: ref. 4

In dealing with oxygen and oxygen related species, we may frequently need bond dissociation energies (BDE) related to these species. Table 2 summarizes some useful values.

Table 2:

Bond dissociation energies for selected oxygen containing species.

Species

Bond type

BDE (kcal/mol)

O2

O=O

119.0

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H2O2

O–O

51

H–O

H–O

102.2

H2O

H–O

119.3

H2O2

H–O

88.1

HO2

H–O

47.1

ROH

H–O

105

ROOH

H–O

R2O2

O–O

ketone

C=O

methanol

H–O

37 104.4

Sources: ref. 5,6

Oxygen is a good electron acceptor, but a very poor donor. Reduction of oxygen can lead to O2• , HO2• , HO2–, H2O2 and HO• . However, it is usually the first electron transfer to O2 that is the rate limiting step. In this sense, the O2/O2• couple has an immense importance in nature. It has an E˚ of -0.15 V in water and -0.60 V in dimethylformamide. The values in other polar organic solvents are generally within ± 0.1 V of that for dimethylformamide. Under many conditions, O2• is itself a good reductant. On the other hand, superoxide is a poor oxidant, since E˚ (O2• /O22–) < -1.7 V. Singlet oxygen is naturally a better oxidant than ground state oxygen. When the excitation energy of singlet oxygen is taken into consideration the values of E˚ (1O2/O2• ) are 0.34 V in dimethylformamide and 0.79 V in water. Most singlet oxygen interactions involve partial charge transfer (vide infra), although a few examples of full electron transfer are known; for example, singlet oxygen oxidizes aqueous N,N,N',N'-tetramethyl-pphenylenediamine to its radical cation. The pKa for HO2• (the conjugate acid of superoxide) is 4.8. Superoxide absorbs in the deep ultraviolet region, with a maximum at 245 nm 7 .

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3. The interaction of ground state oxygen with excited states 3a. Interaction of oxygen with excited singlet states The form of interaction of oxygen with excited singlet states will depend on the magnitude of the energy gap between the sensitizer singlet and triplet states. A possible energy transfer process is shown below. In this reaction oxygen is promoted from its ground to its 1g state, while the sensitizer singlet yields its triplet state. 1

X*

+

3

3

X*

O2

+

1

O 2 (1 g )

(3)

The H for this reaction is given by: H = ET - ES + 22.4 kcal/mol where ET - ES are the triplet and singlet energies of the sentitizer, respectively. When the S-T gap in the sensitizer is smaller than the triplet-singlet energy separation in molecular oxygen the reaction is not possible on energetic grounds. The excitation energy of O2 (1g) is 22.4 kcal/mol, a value that is too large compared with the singlet-triplet energy gap for n* states and uncommon even for ,* states (for a discussion of the magnitude and significance of these values, see Section ##). For systems where reaction (3) is not energetically feasible, the only spin allowed processes are quenching of the singlet to produce excited triplet sensitizer and ground state triplet oxygen –effectively an example of assisted intersystem crossing.3–, or to form both ground states, 1

X +3 O 2 

3

X  +3 O2

(4)

1

X +3 O 2 

1

X + 3 O2

(5)

ground state

3

A related example has already been illustrated in the case of biradicals, see section xxxx.

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There are many polynuclear aromatic hydrocarbons that behave as in equations 4 and 5 when the energy transfer process of reaction 3 is feasible. Table 3 gives a few representative rate constants for singlet quenching by oxygen; many approach diffusion control, particularly when charge transfer to oxygen is favored by the oxidation potential of the sensitizer, the solvent or both. Table 3 also gives the efficiencies of O2 (1g) generation due to sensitizer singlet quenching (SS). Note that in this table the only cases where the energy available is not sufficient to produce O2 (1g) are phenanthere and triphenylene. The methodology to determine values of S will be explained in more datail in relation to the use of triplet sensitizers (see section xxx).

Table 3:

Representative rate constants and singlet oxygen efficiencies for the quenching of excited singlets by oxygen in acetonitrile at room temperature.a, b

Substrate

Solvent

SS

k qS, 109 (M-1s-1)

Naphthalene

Acetonitrile

 0.09

31

Phenanthrene

Acetonitrile

0

33

triphenylene

Acetonitrile

 0.02

37

Pyrene

Acetonitrile

0.30

29

Fluoranthene

Acetonitrile

0.30

6.6

Perylene

Acetonitrile

0.27

38

Tetracene

Acetonitrile

0.25

42

Anthracene

Acetonitrile

 0.02

30

Anthracene

Cyclohexane

0.0

25

9-Cyanoanthracene

Cyclohexane

0.5

6.7

9,10-Dicyanoanthracene

Cyclohexane

1.0

4.7

9-Methoxyanthracene

Cyclohexane

0.3

27

9-Methylanthracene

Cyclohexane

0.1

30

a From: 8,9 b SS is the fraction of singlet quenching events that yield O2 (1g)

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A couple of interesting features in Table 3 are worth noting. While 9,10-dicyano anthracene is 100% efficient forming singlet oxygen by singlet sensitization (according to equation 3), anthracene is totally inefficient; this has been explained on the basis of a T 2 level in anthracene just below the S1 level, thus, the small ST gap favors the process of equation 4, which yields the T2 state of anthracene, which rapidly converts to T 1 in a radiationless process. The process is reminescent of intersystem crossing in benzophenone (see Chapter xxx), where the T2 state plays a key role. The other observation relates to the rate constants for singlet quenching; clearly electron deefficient singlets are quenched more slowly by oxygen. In order to determine the values of SS in Table 3 it is necessary to be able to monitor quantitatively singlet oxygen formation. During the last decade, methods involving the direct time-resolved detection of the near infrared luminescence from O2 (1g) have become readily available and a relatively inexpensive method of detection. In this technique, the emission from the sample following pulsed laser excitation is filtered through a silicon disk (to eliminate wavelengths O2(1g) > O2(3g-)].

Figure 3: Dependence of the multiplicity-normalized rate constants kTP/m of formation of O2(1g+ ) (circles), O2(1g) (triangles) and O2(3g-) (squares) in CCl4 during O2 quenching of ,* triplet states on the excess energy E for formation of the respective O2 product state and the free energy GCET for formation of an ion pair. From reference3 5 (Reproduced with permission from the copyright owner).

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The information presented above has very direct implications for an experimentalist trying to select a good singlet oxygen sensitizer. These considerations will be presented in the next section.

3d. Efficiency of singlet oxygen, O2 (1g), generation; selecting a good singlet oxygen sensitizer In order to carry out reactivity studies on singlet oxygen, or to perform oxygenations mediated by singlet oxygen, chemists have developed an arsenal of photosensitizers useful for a wide range of applications and experimental conditions The following are some of the parameters we may want to take into consideration in selecting the sensitizer and conditions best suited for singlet oxygen work. • High value of S (poor electron donors and modest excess energies above O (1 ) 2 g are best, see above). •

Long triplet lifetime in order to maximize triplet quenching.



High rate constant for triplet quenching by oxygen (true in almost all cases), and low rate constant for triplet quenching by substrate. This may require a careful analysis of the relative triplet energies of sensitizer and substrate (see Chapter ##). Note also thet trying to maximize this rate constant can lead to a lower value of , particularly if redox properties are used as a tool to maximize the rate, vide supra.



High sensitizer stability toward singlet oxygen.

Some good (i.e., high S)

sensitizers may also trap singlet oxygen efficiently, thus reducing their own usefulness. •

Good spectral properties making possible the selective excitation of the sensitizer (as opposed to the substrate) with a readily available light source.

Oxygen chapter (v13D)



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A sensitizer with efficient intersystem crossing under the experimental conditions (that could include oxygen-assisted intersystem crossing of the excited singlet).



A solvent with good solubility for oxygen (e.g., halogenated) and where singlet

oxygen has a long lifetime. Facile sensitizer removal. For synthetic applications it may be desirable to eliminate the sensitizer at the end of the reaction. Some heterogeneous sensitizers have been developed (e.g., tethered to polymer particles) that can be readily filtered at the end of the oxidation. No sensitizer aggregation. In general, sensitizers that aggregate in solution tend to yield lower values of S and  .3 6

Before we learn how singlet oxygen interacts with other molecules, we need to learn a bit more about its spectroscopy.

4. Spectroscopy and dynamics of singlet molecular oxygen We have learned earlier about the relative energies of various electronic states of O2 (see Scheme 1). We will now learn about the dynamics of the interconversion between these states.

4a. Dynamics of radiative and radiationless processes in singlet oxygen Singlet oxygen interacts strongly with C-H and O-H bonds; we therefore choose for our initial discussion carbon tetrachloride as a solvent, since it simplifies our analysis. Some rate parameters for this system have been known for many years, but the complete set of values was finally reported in 1995

2 0.

The Jablonski diagram for oxygen in carbon

tetrachloride at room temperature is shown in Figure 3.6

6

As usual, we show triplet states on the right side, which in this case, places the excited states on the left side of the diagram.

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(sum of knr from 1g = 7.6 x 10 6 s-1)

g ; t = 130 ns 3

krad = 3.4 x 10 s em = 4.5 x 10-4

-1

-1

1

krad = 0.40 s -8  em = 5.2 x 10

g ; t = 87 ms

-1

krad = 1.1 s knr = 10.4 s-1 e m = 0.087 3

g

Figure 4: Jablonski diagram for the excited states of molecular oxygen in carbon tetrachloride2 0.

Note the very low quantum yields associated with the various emissions from the two excited singlet states of oxygen. The lifetime of the 1g state shows a strong solvent dependence, as illustrated in Table 5.

Table 5: Singlet oxygen (1g) lifetimes in various solvents at room temperature

Solvent

 (s)

Solvent

 (s)

toluene

29

benzene

31

acetone

51

acetonitrile

75

diethyl ether

34

chloroform

207

pyridine

16

carbon disulfide

34000

Dioxane

27

Carbon tetrachloride

87000

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Hexafluorobenzene

3900

Freon-113

99000

chlorobenzene

45

water

~5

hexane

30

methanol

10.4

Sources: From ref. 16,17,37 There are many cases in which some of the lifetimes given in Table 5 are very difficult to achieve experimentally, particularly for values exceeding 1 ms. This can be due to impurity quenching of O2 (1g), or trapping of singlet oxygen by the sensitizer used for its generation. As a general rule, reported values show more dispersion for the longer values (i.e., in the more inert solvents); this is simply a reflection of the experimental difficulties, and as a general rule, the longer reported values are more likely to be accurate.7 The radiative lifetimes of O2 (1g) change significantly with the solvent

3 8.

For

example, a detailed study of these variations reveals a 16-fold change between 1methylnaphthalene (long) and water (short)

3 9.

The reader will recognize this as an unusual

situation, since radiative lifetimes are directly related to absorption spectra and oscillator strengths (see Chapter zz) and variations of this magnitude are not common. It has been proposed that these variations are due to oxygen-solvent interactions within the encounter complex, such that some of the intensity from the solvent transitions is introduced in the oxygen 1g  3g transition. It is also believed that the strong spin orbit coupling in the 1g-1g

transition plays a role on the transition intensity for 1g  3g. The effect,

generally not observed in most molecules, is expected to be detectable only in molecules with very low oscillator strength.8 The radiative rate constants for O2 (1g) show an empirical correlation with the solvent polarizability, (n2 - 1)/(n 2 - 2), where n is the refractive index of the solvent.94 0 7

There are many experimental factors that can cause a reduction of the lifetime, but it is difficult to find one that could lengthen the lifetime. 8 For example, for oxygen in benzene f = 2.5 x 10-8 3 9. 9 The term n2 appears in in the Einstein's expression for spontaneous emission between two states.

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Figure 4 illustrates this dependence in a plot of kr/n2 against n, the refractive index of the solvent 1,41.

Figure 4. Solvent dependence of the rate constant for radiative decay of singlet oxygen as a function of the solvent's refrative index. From1,41. Several expressions relating to the polarizability of the solvent (and thus its refractive index) have been proposed 1,38,40,42. The quantum yield of emission in any given solvent is given by the ratio of the actual lifetime (i.e., the reciprocal of kr) to the radiative lifetime, i.e.,  =  /o = kr •  In the case of hexane this leads to  = 1.8 x 10-6.

(10)

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When an experimentalist is trying to establish if a reaction is mediated by singlet oxygen, it is a very common practice to replace a protic solvent for the deuterated material; an increase in reaction efficiency/yield is taken as an indication that the process is probably mediated by singlet oxygen. This simple test reflects another important characteristic of singlet oxygen; its lifetime is greatly enhanced when C–H and O–H bonds in the solvent are replaced by C–D and O–D. For example, the O2 (1g) lifetimes in CH3OH, CH3OD and CD3OD are 10.4, 37 and 227 s, or an overall 22-fold increase in lifetime. The probability of reaction (Pr) with a substrate M will change as a result of these variations in singlet lifetime, i.e., Pr =

k M [M ]

 1 + k M [ M]

(11)

where kM is the rate constant for the interaction between M and singlet oxygen, and  the lifetime of singlet oxygen in the absence of M. Interestingly, these two extreme situations in which we would not expect an effect: (i) if the reaction does not involve singlet oxygen; and (ii) if the reaction with singlet oxygen is extremely fast, such that under all conditions:

kM [M] >> 1

(12)

In the second case, we expect the product yields to start showing dependence on the isotope composition of the solvent if the concentration of M is reduced such that kM [M] becomes comparable with -1. The following simple rules let us anticipate changes in O2 (1g) lifetime as a function of the solvent 4 3: •

The longest lifetimes are observed in perhalogenated solvents.

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o decreases on increasing the number of H atoms in the solvent molecule.



The shortest o values are observed with solvents having O–H groups, notably water.



The presence of heavy atoms reduces o .



Solvent deuteration invariably increases o .

The effect of H atoms in the solvent (or more generally in the quencher) in reducing the lifetime of singlet oxygen results from an electronic-to-vibrational energy transfer occurring by coupling to vibrational modes of solvent with a (0,m) transition associated with the deactivation of singlet oxygen, i.e. O2 (1g)  O2 3g .

5. Physical and chemical quenching of singlet oxygen In order to study the reactions and mechanisms of singlet oxygen, it is necessary to have reliable sources of this intermediate.

Fortunately, numerous well characterized

sensitizers are readily available (see Section 3 in this Chapter). For example phenalenone and phenazine are frequently employed as standards, while dyes such as Rose Bengal, porphyrins and phthalocyanines are convenient long wavelength sensitizers. All these molecules transfer energy to oxygen readily and efficiently and some have found applications in medicine in the field of photodynamic therapy (PDT) . Just like with any other excited species, singlet oxygen can decay by a variety of pathways. We have already described in some detail the radiative processes which are common for oxygen, and that are frequently employed to detect its presence. In addition, a number of interactions can lead to chemical change, radiationless decay, or energy transfer.

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5a. Intermolecular interactions leading to the deactivation of singlet oxygen (physical quenching) Three types of interactions frequently contribute to physical deactivation of singlet oxygen. If energetically feasible the rate of these processes follow the order (from high to low): 1. Energy transfer 2. Charge transfer interactions 3. Elentronic-to-vibrational energy conversion In spite of this order, the first one is rather uncommon, simply because few molecules have accessible electronic energy levels below that of singlet oxygen. We have already outlined how electronic-to-vibrational energy conversion can lead to O2 (1g) deactivation through interactions with the vibrational states of C–H or O–H bonds, frequently those of the solvent.4 4 The process is not common in other molecules because their (usually) much higher excitation energy makes the energy matching required for this process highly improbable. Another process by which the O2 (1g)  O2 3g can take place is by energy transfer to a suitable donor, so that this transformation can be achieved while obeying the energy and momentum conservation rules presented in earlier chapters. For most organic molecules the excitation energy of their lowest triplet state is significantly above 22 kcal/mol, and therefore energy transfer from singlet oxygen to them is thermodynamically unfavorable. A few exceptions are known, among them the best known example is the case of -carotene, for which the triplet state lies ~20 kcal/mol above the ground state. Here the following energy transfer can occur: 1

O2

1

g +

-C

3

O2

3

g +

3

-C*

(13)

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CH3 CH3

CH 3

CH3

CH 3 CH3

H3 C CH 3

CH3

CH3

-carotene (-C)

Another mechanism for physical quenching of singlet oxygen involves reversible charge transfer interactions, as illustrated below. Oxygen functions as a temporary electron acceptor. This is reminescent of the charge transfer interactions already discussed in relation to oxygen quenching of excited triplet states (see Section 3). 1

O2 1  g + Q

[O2



+

Q +•]

3

O2 3 g + Q

(14)

Clearly the overall process of equation 14 is not spin allowed; thus, we expect the radical ion pair or exciplex/encounter that mediates the reaction (in square brackets, [ ]) to live long enough for spin evolution to be possible. We have already discussed in Chapter z z the interactions that can make spin flip in a spin correlated radical pair (SCRP) a plausible process. For thermodynamically favorable processes, we can expect the order of rate contants to follow the order: Energy transfer > charge-transfer quenching > electronic-to vibrational conversion Thus, we may start with these three possible processes. Then, we may label as plausible those processes that are energetically feasible. From among this list, the criterion above may help us decide which is the most probable outcome.

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5b. Intermolecular interactions leading to chemical transformations (chemical quenching) One of the most important reactions of singlet oxygen involves its reactions with double bonds to lead the peroxides through the three reactions illustrated in Scheme 5.

H3C H3 C

CH3

1

H3C

CH 2

H3C

CH 3 OOH

O2

CH3

1

O2 O

(A)

(B)

O

CH3 O

1

CH3O

O

O2

CH3 O

O

CH3O

(C)

Scheme 5. Chemical trapping of singlet oxygen

The first reaction in Scheme oo.2 is known as the ene reaction, also called the Schenck reaction. This process, leads to stereospecific oxygenation and shift of the double bond, is mediated by a suprafacial hydrogen atom transfer, as illustrated below. Reaction (A) may be mediated by an exciplex or a perepoxide. We discuss this uniquely important reaction in some detail later in this section. O H

O

H O

O

(15) The second reaction in Scheme 5 is also common in polynuclear aromatic hydrocarbons. In some cases the addition to a conjugated system can be reversed thermally

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with regeneration of singlet oxygen, for example in the case of anthracene and its endoperoxide: O 1

O

O2



1

+ O2

(16) Thus, anthracene endoperoxide can b

a chemical reservoir for singlet

oxygen. In contrast with the case of endoperoxides, dioxetanes (reaction C in Scheme 5) – that form most easily by addition of singlet oxygen to electron rich alkenes– decompose to yield excited carbonyl compounds, as illustrated in reaction 17. The reaction has an activation energy of ca. 27 kcal/mol and is believed to involve a 1,4 biradical (•O–C–C–O•).

H3C

CH3 O

H3C

O

H3C



H3C O

O*

+

H 3C

CH3

H 3C S ~ 0.25 T ~ 0.35 (check values)

(17)

Singlet oxygen (an electrophillic reagent) also reacts readily with a variety of electronrich molecules, such as amines, a process that frequently competes with physical quenching (see section ##): RCH 2NH2

1

O2

R CHNH 2 OOH

(18)

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Thousands of rate constants have been determined for the interaction of singlet oxygen with organic and inorganic molecules10

1 7.Table

6 provides a small sample of these

values.

Table 6: Rate constants for the quenching of singlet oxygen in solution at room temperature. Molecule

Solvent

kq (M-1s-1)

2,3-dimethyl-2-butene -carotene

CH2Cl2 CH2Cl2

1,3-cyclohexadiene cyclohexene cyclohexane -tocopherol

CHCl3 CHCl3 CCl4

5.2 x 107 4.6 x 109 ~7 x 106 9 x 103 6.4 x 103 2.2 x 108

Acetic acid Triethyl phosphite Indole

toluene CCl4 acetone Toluene

2.3 x 103 2.5 x 107 7.7 x 105

Sources: from refs. 16,17.

5.c Using anthracene endoperoxides as a switch to detect singlet oxygen by fluorescence As already pointed out, the formation of enndoperoxides can be viewed as a way of storing singlet oxygen, since the addition can be reversed thermally. A novel application of this reaction has been developed by Nagano and coworkers. 4 5 The molecule of fluorescein is viewed as made up of a fluorophore (the xanthene ring) and a switch, as shown in Figure 6. In fluorescein itself (R = H) the S1 state of the benzoic acid moiety is at higher energy than S1 for the xanthene ring, that fluoresces without much perturbation from this 'inactive' switch. Adding aromatic rings to the switch raises its HOMO level, and in the case of an 10

A wide range of rate constants are freely available on the internet: see http://allen.rad.nd.edu/browse_compil.html

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thracene moiety places between the HOMO and the LUMO of the fluorophore. Under these conditions (as in the case of DMAX) the molecule is essentially non-fluorescent as a result of intramolecular quenching. Trapping of singlet oxygen by the switch to form the endoperoxide lowers the switch HOMO level below that of the fluorophore, and as a result quenching becomes unfavorable, causing a dramatic increase in fluorescence quantum yield.

Figure 6. A singlet oxygen switch based on HOMO level changes caused by endoperoxide formation. From ref. 4 5.

5.d The ene reaction. An important tool in organic synthesis Organic photochemists have been fascinated by the 'ene' or Schenck reaction for over three decades. This interest reflects the usefulness of the reaction in organic synthesis,11 its

11

Usually the initially formed hydroperoxide is converted to the alcohol.

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possible involvement in biochemical processes, and its role in the photodegradation of materials. The quenching of singlet oxygen luminescence by alkenes shows that while the rate constant is a function of the alkene structure, the activation energy is essentially negligible; in other works, it is the activation entropy that truly controls the reaction dynamics. These results have been rationalized on the basis of an initial formation of an exciplex that later converts to a perepoxide 4 6. Note that the cis-alkene is more reactive than the trans isomer. Similarly in the case of cis- and trans-2-butene, the cis isomer is 18 times more reactive than the trans 4 7, see Table 7.

Scheme dd: Illustrating the activation entropy dependence of the kinetics for singlet oxygen quenching by alkenes in carbon disulfide. From ref.4 6



H (kcal/mol) ‡ S (e.u.)

kq (M-1 s-1 )

2.0

0.4

-0.1

- 31

-39

-42

39,000

7,700

5,200

Adam and coworkers 4 8 have summarized the mechanistic aspects of this reaction. They propose that the ene reaction of electron rich and electron poor alkenes are controlled by a different transition state, as shown schematically in Figure 7.

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Figure 7: Different transition states control the reactivity of electron rich and poor alkenes. From ref. 4 8.

In the case of electron rich alkenes the first transition state, or exciplex-like encounter, controls the reaction. Scheme 6 shows some representative values of threoerythro selectivity and illustrates the diastereoselectivities of the ene reaction.

Oxygen chapter (v13D)

Page 39

Number

X=

R=

Threo/etrythro

1

OH

H

93:7

2

OH

CH3

93:7

10

CO2C2H5

H

22:78

18

C(CH3)3

CH3

29:71

April 13, 2004 at 13:22 PM

Scheme 6: Diastereoselectivities in the ene reaction of singlet oxygen with electron rich alkenes in carbon tetrachloride. The compound numbers are the same as in the original publication.From ref. 4 8. In general, these substituent effects reflect their directing influence on the incoming oxygen. Thus, for example in the case of X=OH hydrogen bonding to the polarized oxygen favors the threo exciplex, as shown below: -

O

H

+ O

O

R H3 C

H H threo-TS

CH3

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In protic solvents, competitive hydrogen bonding leads to a decrease in diastereoselectivities. Electron accepting substituents (e.g., CO2Et) carry a partial negative charge and would undergo an unfavorable interaction with the external (note ) oxygen in the threo transition state, leading to erythro products via the following transition state: CH3 H2 C O R

C

O H

H3 C

H

CH3

+ O O erythro-TS

Similarly, steric interactions (note the effect of tert-butyl in compound 18) favor erythro selectivity by directing the incoming oxygen molecule towards the opposite face. In the case of electron-poor alkenes exciplex formation is reversible (note the ralative energies of the two transition states in Figure 6) and is thus controlled by the second transition state. Here the selectivity is controlled by transfer of the less polarized hydrogen atom, and hydrogen bonding is a lot less important (solvent changes have little effect), and converting hydroxyl to methoxy substitution has little effect. In the example of ,unsaturated esters, threo selectivity is observed and is believed controlled by the transition state of Figure 8.

Oxygen chapter (v13D)

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H

April 13, 2004 at 13:22 PM

CH3

H

EtO2 C

H H

H

OX

O

O

Figure 8: Transition state for the ene reaction of singlet oxygen with electron poor alkenes

6. Chemical quenching of excited triplet states by oxygen In the vast majority of cases, the interaction of oxygen with triplet states involves energy transfer, as discussed above. There are a few examples, notably diketones, where a chemical reaction occurs between the triplet state and molecular oxygen, namely addition to a carbonyl carbon (Schenck mechanism) and subsequent C–C bond cleavage to an acylperoxy and an acyl radical, which itself is scavenged by molecular oxygen to yield a second acylperoxy radical (Scheme ##). The quantum yield for this reaction in benzene as solvent is sizable for biacetyl (23%) but insignificant for camphorquinone and benzil ( 2%). Schenck mechanism 3* O R

3

O

O2

3

O

g

R

R

O R

O •

3

+ R

O• O R O•

O O•

O2

3

g

O 2 R

O O•

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Scheme 7. Reaction of ground state oxygen with triplet diketones.

7. Reaction of oxygen with reaction intermediates: Mechanisms and kinetics Numerous oxidation processes that occur (or could occur) in nature are highly exothermic, yet, they do not occur spontaneously or instantaneously, in spite of constant exposure to an oxygen-rich atmosphere.

The reason for this is that it is not

thermodynamics, but rather kinetics that controls these processes. We normally need oxygen to be activated. A simple way to activate a molecule involves the use light to achieve this activation. Yet, as discussed earlier in this Chapter, oxygen is largely transparent to the sunlight reaching to earth's surface and to that emitted by the most common sources employed in the laboratory. In most cases activation is achieved by absorption of light by other chromophores that can latter transfer either energy or an electron to molecular oxygen. In other cases, oxygen reacts with ground state reaction intermediates (e.g. free radicals), which may in turn have been produced in photochemical reactions. To fully understand how oxygen influences photochemical reactions, we need to learn about some of these modes of interaction too. Molecular oxygen is a highly reactive species, which frequently interacts rapidly with reactive intermediates which either have unpaired electrons, or where spin-evolution processes can lead to thermodynamically favorable changes. The following sections provide a brief outline of the more common types of interaction observed. These processes can be rationalized on the basis of the spectroscopic and thermodynamic properties discussed before. When studying a photoreaction involving oxygen, an experimentalist may want to carry out some experiments to test for some of the processes described below. Our ability to choose the "right" experiments (i.e., those that can provide a definitive answer) will be in

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direct relationship to the quality and completeness of our paradigm, and to our understanding of the physical and chemical principles supporting it.

7a. Free radical scavenging by oxygen Carbon-centered free radicals frequently react with oxygen with rate constants exceeding 109 M-1s-1 in fluid solution, to yield a peroxyl radical, reaction 19 4 9. R• + O2

R-OO•

(19)

If the precursor of R• is a molecule R–H with moderately weak R–H bond dissociation energy, reaction 19 couples with reaction 20 to lead to the well known chain autooxidation of hydrocarbons and other organic molecules, i.e.,12 R-OO• +

RH

R• + ROOH

(20)

The chain autooxidation can be inhibited by introducing in the system a hydrogen donor molecule, such that the radical derived by hydrogen abstraction from it will not react with oxygen and will not abstract hydrogen readily from other molecules. Many phenols meet this criterion, and 'BHT' (for butyl-hydroxylated-toluene) is frequently used commercially, while -tocopherol (Vitamin E, also a quencher of singlet oxygen5 0) is the most important natural antioxidant. The reactive hydrogen in the structures below is shown in circled boldface.

12

This important reaction may be familiar to us in different contexts: e.g. the frequent cautionary note indicating that organic compounds, especially ethers, should not be distilled dry, reflects the danger posed by accumulated hydroperoxides. The industrial synthesis of acetone and phenol from cumene is mediated by cumene hydroperoxide, manufactured as indicated above. The autooxidation of fatty acids is responsible for their (flavor) deterioration in vitro and an important contributor to the aging process in vivo.

Oxygen chapter (v13D)

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CH3

O- H t

t

Bu

April 13, 2004 at 13:22 PM

Bu

H-O

CH 3

CH3

O CH3

CH3 BHT

CH3

CH3 CH3

CH3

Vitamin E

These two molecules serve to exemplify another important free radical property: Oxygen-centered radicals are frequently unreactive toward molecular oxygen. Table 8 summarizes a few representative rate constants for the reaction of carboncentered free radicals with oxygen.

Table 8:

Rate constants for radical reactions with oxygen at room temperature.a

Radical

Solvent

k (M-1s-1)

PhCH2•

cyclohexane

2.4 x 109

PhCH2•

acetonitrile

3.4 x 109

Ph2CH•

cyclohexane

6.3 x 108

(CH3)3C•

cyclohexane

4.9 x 109

cyclohexadienyl

benzene

1.6 x 109

CH3C•OHCH3

2-propanol

3.9 x 109

a From ref. 4 9

7b. Biradical scavenging by oxygen Our discussion in this section deals exclusively with triplet biradicals, largely reflecting that our current experimental knowledge is largely limited to them.13

13

For an example of a well characterized singlet biradical see ref.5 1.

Triplet

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biradicals share some of the properties of true free radicals along with some of those that we will later see as characteristic of excited triplet states. The three more common reaction paths when oxygen interacts with biradicals are illustrated in Scheme 8 A) Assisted intersystem crossing 3

1 3

O2 Products

H

H Ph

E.g.

Ph 3

OH

OH

O2

+

+ PhCOCH3

B) Hydroperoxide formation 3 OO•

3

O2

OOH

H

H Ph

Ph

E.g.

3

OH

O

O2

OOH

C) Peroxide formation 3 O

3

O2

O H

H

E.g.

Scheme 8. Biradical mechanisms of interaction with oxygen.

Note that the possible reaction paths in Scheme 8 are all determined by the rules of conservation of spin angular momentum discussed in Chapter xx. While the same is true to all reactions, their importance becomes more evident when we deal with a species with a ground state triplet.

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Reaction paths B and C combine the characteristic trapping of radicals that we have seen above, with disproportionations (B) or combination reactions (C) that are common in radical-radical reactions. The encounter of two triplet states has a 1/9 probability of leading to a singlet configuration, a 1/3 probability of leading to a triplet and 5/9 probability of resulting in an overall quintet configuration. Quintet encounters are expected to be dissociative (see Chapter ##), i.e. they are unreactive.

Only singlet encounters have the correct spin

configuration to lead to the ground state products of reaction paths B and C. The case of path A in Scheme 8 is particularly interesting. Singlet encounters cannot lead to successful events, since oxygen would have to result in its singlet state for the products to have a singlet spin configuration. Normally the singlet-triplet energy gap in biradicals is too small for this to be an accessible process.14 Thus, assisted intersystem crossing is normally mediated by the triplet encounters that allow products to be formed in their singlet ground states, while oxygen can remain in its triplet ground state. Interestingly, in the example of -methylvalerophenone (see example in case A, Scheme 8), oxygen results in an increase in the yields of products because, instead of excited triplet quenching, its main role is to assist the intersystem crossing of biradicals required for product formation. The overall process amounts to catalytic spin relaxation; other paramagnetic substrates (e.g. nitroxides and some transition metal ions such as Cu2+) can function in a similar way. They transform a process that would require a (highly unlikely!) violation of spin angular 14

An interesting example that at first sight may appear to be an exception is the case of ketones that lead to

photoenolization, such as ortho-methylacetophenone, whose "biradical" interacts with oxygen leading to singlet oxygen with an efficiency of xxx. However, this is a rather special case in which the "biradical" is the same species as the triplet state of the enol product. The energy gap with the ground state enol is sufficient to make singlet oxygen formation energetically feasible 5 2.

• OH •

C H2

3

OH

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momentum conservation, into a spin allowed process by changing their own spin angular momentum to accommodate the conservation of angular momentum (see Chapter ##). 7c. Reactions of carbenes with oxygen Carbenes are divalent carbon species; the simplest member of this group is methylene, :CH2. While many sources of carbenes are available, the more common ones involve elimination of molecular nitrogen (thermal or photochemical) from diazo compounds or diazirines (see Scheme 9). Methylene, CH2, has a triplet ground state and its first excited state is a singlet located xxx kcal/mol above the ground state. Substituted carbenes can have singlet (e.g. , ClCC 6H 5 ) or triplet (e.g., (C6H5)2C:) ground states, and occasionally in some carbenes the two states are very close (e.g. fluorenylidene), such that both spin states can play an important role in room temperature reactions 53,54. Diazo precursor of carbene with triplet ground state N N

1

3

h

fast

–N2

Diazirine precursor of carbene with singlet ground state N N Cl

1

h

Cl

–N 2

Scheme 9: Preparation of carbenes Singlet carbenes do not react readily with ground state molecular oxygen because no accessible reaction path is available that will satisfy spin and energy conservation requirements.15 In contrast, in the case of triplet carbenes, the same spin selection rules as in 15

We should remember, however, that in many cases "forbidden" reactions do take place, although their

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the case of biradicals apply (see Scheme 8). In this case, formation of a "carbonyl oxide" is an allowed process; for example, in the case of diphenylmethylene5 5.

3

[(C6 H5)2 C:]

+

3

O2

(C 6H5)2 COO

(21)

Carbonyl oxides are readily detectable spectroscopically and are very polar species. For example, in the case of (C6H5)2COO, its dipole moment is xx D 5 6. They have singlet ground states. The same intermediates can be formed by ozonolysis of olefins, a process studied in detail by Criegee. Carbonyl oxides are frequently called "Criegee intermediates". While no examples have been reported, it is clear that oxygen could catalyze intersystem crossing in carbenes in much the same way as it does in the case of biradicals. The lack of experimental examples probably reflects that, in general, systems where this would be spin allowed (i.e., spin angular momentum is conserved) are energetically not accessible. 7d. Other reaction intermediates While no attempt to cover all reaction intermediates is made, a few are worth mentioning. For example, radical cations, frequently produced by oxidation of their stable precursors tend to either not react with oxygen, or react very slowly. This reflects that oxygen is not a good reducing agent. Similarly, carbocations are usually unreactive towards oxygen. Radical anions are easily produced by photochemical or electrochemical reduction of their precursors. For example, benzophenone radical anions can be easily formed by electron transfer from a good donor to triplet benzophenone (see Chapter ##) or by reaction of kinetics are much slower than allowed processes. In reality, "forbidden" and "allowed" are best interpreted as "less probable" and "more probable", respectively. Our paradigm does not exclude them, but predicts that they will either not occur, or occur very slowly.

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benzophenone with alkali metals in inert solvents. Reaction of radical anions with oxygen is frequently rapid (>109 M-1s-1) and leads to the formation of O2• . For example in the case of benzophenone: • (C6 H5 )2 CO

+

O2

(C6 H5 )2 CO

+

• O2

(22)

It is important to examine the species not so much as to evaluate whether their overall charge is negative or positive, but rather, whether they are reduced or oxidized species, and on the basis of this, whether their reaction with oxygen is a probable process. Let's remember that oxygen is a good electron acceptor but not a donor (see Section 2 above). A case in point is that of the radical cation from methyl viologen which reacts with oxygen at close to the diffusion controlled limit. This radical cation is in fact the reduced form of the methyl viologen dication, i.e.,

N• Me

N+ CH3 MV+ •

+ O2

N+ Me

N+ Me MV2 +

+ O2



(23)

Depending on the solvent and pH, O2• may protonate to yield HO2• .

The

corresponding acid-base equilibrium (pKa 4.88) will ultimately determine the role played by these species. Other reaction intermediates, such as ortho-xylylenes tend to show little or no reactivity towards ground state molecular oxygen, although they react readily with singlet oxygen. H 3C

OH C

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8. Oxidative processes in biology The coverage of oxidative processes in biology is well beyond the scope of this book. However, it is important for the reader to be able to identify some of the nomenclature used in photobiology with the same type of processes described in this Chapter. Thus, this section is simply a glossary establishing the relationship between some common photochemical and photobiological nomenclature. Reactive Oxygen Species, or ROS, is a generic term which groups a relatively large number of reactive intermediates having in common that oxygen has been 'activated'. It includes singlet oxygen, hydroxyl radicals, peroxyl radicals, alkoxyl radicals, superoxide and HO2• Photosensitized oxidations are usually divided into Type I and Type II processes. Type II processes involve the participation of singlet oxygen, while Type I processes are mediated by free radical or electron transfer processes. In the latter case the actual oxidative process is a ground state reaction of the type discussed in Section 7. Wavelength ranges are labeled in photobiology as UVA, UVB and UVC. UVA refers to 315 to 400 nm. UVB to 280 to 315 nm and UVC is below 280 nm. In some cases the boundary between UVA and UVB is placed at 320 nm. Note that the low limit of the UVA region coincides with the transmission cut-off of window glass. Thus, when sunlight passes through a normal window, only the UVA and visible (400 to 700 nm) regions are transmitted. Outdoors exposure to the sun also includes UVB light, while UVC is widely available outside the atmosphere.

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9. Is evidence for oxygen quenching of a reaction good evidence for triplet involvement? The following is a common statement in photochemistry and photobiology: 'The reaction is quenched by oxygen, thus, it must be a triplet reaction'. We ask a simple question: True or false. The paradigms of organic photochemistry as it applies to oxygen, that were developed earlier in this Chapter clearly indicate that the statement above does not provide enough information to resolve this question.

Our

paradigms do suggest a number of questions that we may want to ask in an attempt to supplement our information so as to decide whether the statement is true or false. It is clear that the statement above describes a possible situation. As usual within the paradigm of organic photochemistry, moving from possible to plausible and probable interpretations requires information on the usual behavior of reactive species in photoreactions, and enough knowledge to develop a list of experiments that will allow the experimental verification of a given proposed reaction mechanism. Our leading statement is also somewhat vague. What is the specific meaning of "The reaction is quenched by oxygen"? Let's assume that the experimental observation is that a given photoproduct of the reaction is either not formed, or formed in lower yields, when oxygen is present. Clearly, one of the possibilities is that a triplet state is indeed involved, and that its lifetime is sufficiently long to be quenched by oxygen. Further, triplet quenching by oxygen yields no products, or at least different products.

Naturally, there may be

different possibilities and different experiments that could be performed to test different hypothesis. Below, we describe just three scenarios, ultimately providing an analysis along

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the terms of the paradigm presented at the beginning of this book (see Chapter xx). We start with the possibility that triplets do mediate the reaction: •

The reaction is mediated by a triplet state. Possible test: Measure luminescence from singlet oxygen. In general triplet quenching leads to its formation. Note that this confirms that a triplet with adequate lifetime is present. It does not ensure that it is responsible for product formation.



The reaction is mediated by free radicals (which may or may not have a triplet precursor). Possible tests: Determine if peroxides or hydroperoxides are formed. Use ESR techniques to monitor the radicals directly or after spin trapping. Use specific radical scavengers (this will require some knowledge of the anticipated radical structure).



The reaction is mediated by an excited singlet state. Possible tests: Since this will require a relatively long lived singlet state (particularly is air saturation is sufficient to quench the reaction), the molecule is likely to be fluorescent: in this case test for fluorescence quenching.

The tests above can and should be combined with kinetic analysis based on direct detection of the intermediates, or on concentration dependencies, such as those discussed earlier in the context of Stern-Volmer plots. Oxygen is a highly reactive species and interacts with numerous reaction intermediates including triplets. It should not be treated as a specific or diagnostic quencher.

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References

1. Ogilby, P. R.,"Solvent effects on the radiative transitions of singlet oxygen" Acc. Chem. Res. 1999, 32, 512. 2. Khan, A. U.; Kasha, M.,"Chemiluminescence Arising from Simultaneous Transitions in Pairs of Singlet Oxygen Molecules" J. Am. Chem. Soc. 1970, 92, 3293. 3. Scurlock, R. D.; Ogilby, P. R.,"Quenching of O2(a1g) by O2(a1g) in Solution" J. Phys. Chem. 1996, 100, 17226. 4. Battino, R. Solubility Data Series: Oxygen and Ozone; Pergamon Pre

Oxford, 1981;

Vol. 7. 5. Griller, D.; Kanabus-Kaminska, J. M. Homolytic bond dissociation energies for simple organic molecules; Scaiano, J. C., Ed.; CRC Press: Boca Raton, Florida, 1989; Vol. II, p p 359. 6. Benson, S. W. Thermochemical Kinetics; 2nd Ed. ed.; Wiley: New York, 1976. 7. Bielski, B. H. J.; Cabelli, D. E.; Arudi, R. L.; Ross, A. B.,"Reactivity of HO2/O2- radicals in aqueous solution" J. Phys. Chem. Ref. Data 1985, 14, 1041. 8. Abdel-Shafi, A. A.; Wilkinson, F.,"Charge transfer effects on the efficiency of singlet oxygen production following oxygen quenching of excited singlet and triplet states of aromatic hydrocarbons in acetonitrile" J. Phys. Chem., A 2000, 104, 5747. 9. Wilkinson, F.; McGarvey, D. J.; Olea, A. F.,"Factors Governing the Efficiency of Singlet Oxygen Production during Oxygen Quenching of Singlet and Triplet States of Anthracene Derivatives in Cyclohexane Solution" J. Am. Chem. Soc. 1993, 115, 12144. 10. Niedre, M.; Patterson, M. S.; Wilson, B. C.,"Direct Near-Infrared Luninescence Detection of Singlet Oxygen Generated by Phootodynamic Therapy in Cells In Vitro and Tissues In Vivo" Photochem. Photobiol. 2002, 75, 382. 11. Redmond, R. W.; Braslavsky, S. E.,"Time-Resolved Thermal Lensing and Phosphorescence Studies of Photosensitized Singlet Oxygen Formation. Influence of the

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Electronic Configuration of the Sensitizer on Sensitization Efficiency" Chem. Phys. Lett. 1988, 148, 523. 12. Brauer, H.-D.; Wagener, H.,"Untersuchung der selbst-sensibilisierten Photooxidation des Rubrens in verschiedenen Lösungsmitteln" Ber. Bunsenges. Phys. Chem. 1975, 79, 597. 13. McGarry, P.; Doubleday, C.; Wu, C.-H.; Staab, H.; Turro, N.,"UV—vis Absorption Studies of Singlet to Triplet Intersystem Crossing Rates of Aromatic Ketones: Effects of Molecular Geometry" J. Photochem. Photobiol., A: Chem. 1994, 77, 109. 14. McGarvey, D. J.; Szekeres, P. G.; Wilkinson, F.,"The Efficiency of Singlet Oxygen Generation by Substituted Naphthalenes in Benzene. Evidence for the Participation of Charge-transfer Interactions" Chem. Phys. Lett. 1992, 199, 314. 15. Boch, R.; Mehta, B.; Connolly, T.; Durst, T.; Arnason, J. T.; Redmond, R. W.; Scaiano, J. C.,"Singlet Oxygen Photosensitizing Properties of Bithiophene and Terthiophene Derivatives" J. Photochem. Photobiol. A: Chem. 1996, 93, 39. 16. Gorman, A. A.; Rodgers, M. A. J. Singlet Oxygen; Scaiano, J. C., Ed.; CRC Press: Boca Raton, Florida, 1989; Vol. II, pp 229. 17. Wilkinson, F.; Helman, W. P.; Ross, A. B.,"Rate constants for the decay and reactions of the lowest electronically excited singlet state of molecular oxygen in solution. An expanded and revised compilation" J. Phys. Chem. Ref. Data 1995, 24, 663. 18. Schmidt, R.; Tanielian, C.; Dunsbach, R.; Wolff, C.,"Phenalenone, a Universal Reference Compound for the Determination of Quantum Yields of Singlet Oxygen O2(1g) Sensitization" J. Photochem. Photobiol. A: Chem. 1994, 79, 11. 19. McLean, A. J.; Rodgers, M. A. J.,"Exciplexes Involving Aromatic Ketone Triplet States and Molecular Oxygen. The Lack of a Spin-Statistical Effect on Exciplex Formation Rate Constants" J. Am. Chem. Soc. 1993, 115, 9874. 20. Schmidt, R.; Bodesheim, M.,"Collision-Induced Radiative Transitions b1g+  a1g, b1g+  X3g-, and a1g  X3g- of O2" J. Phys. Chem. 1995, 99, 15919.

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21. Weldon, D.; Poulsen, T. D.; Mikkelsen, K. V.; Ogilby, P. R.,"Singlet Sigma: The "Other" Singlet Oxygen in Solution" Photochem. Photobiol. 1999, 70, 369. 22. Scurlock, R. D.; Wang, B.; Ogilby, P. R.,"Chemical Reactivity of Singlet Sigma Oxygen (b1g+) in Solution" J. Am. Chem. Soc. 1996, 118, 388. 23. Bodesheim, M.; Schmidt, R.,"Chemical Reactivity of Sigma Singlet Oxygen O2(1g+)" J. Phys. Chem. A 1997, 101, 5672. 24. Garner, A.; Wilkinson, F.,"Quenching of triplet states by molecular oxygen and the role of charge-tranfer interactions" Chem. Phys. Lett. 1977, 45, 432. 25. Grewer, C.; Brauer, H.-D.,"Mechanism of the triplet state quenching by molecular oxygen in solution" J. Phys. Chem. 1994, 98, 4230. 26. Smith, G. J.,"Enhanced Intersystem Crossing in the Oxygen Quenching of Aromatic Hydrocarbon Triplet States with High Energies" J. Chem. Soc., Faraday Trans. 2 1982, 78, 769. 27. Darmanyan, A. P.; Lee; W.; Jenks, W. S.,"Charge Transfer Interactions in the generation of singlet oxygen (1g) by strong electron donors" J. Phys. Chem., A 1999, 103, 2705. 28. Wilkinson, F.; Abdel-Shafi, A. A.,"Mechanism of Quenching of Triplet States by Oxygen: Biphenyl Derivatives in Acetonitrile" J. Phys. Chem. 1997, 101, 5509. 29. Wilkinson, F.; Abdel-Shafi, A. A.,"Mechanism of Quenching of Triplet States by Molecular Oxygen: Biphenyl Derivatives in Different Solvents" J. Phys. Chem., A 1999, 103, 5425. 30. Schweitzer, C.; Mehrdad, Z.; Shafii, F.; Schmidt, R.,"Common Marcus type dependence of the charge transfer induced processes in the sensitization and quenching of singlet oxygen by naphthalene derivatives" J. Phys. Chem., A 2001, 105, 5309. 31. Mehrdad, Z.; Noll, A.; Grabner, E.-W.; Schmidt, R.,"Sensitization of Singlet Oxygen via Encounter Complexes and via Exciplexes of ,* Triplet Excited Sensitizers and Oxygen" Photochem. Photobiol. Sci. 2002, 1, 263.

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32. Mehrdad, Z.; Schweitzer, C.; Schmidt, R.,"Formation of O2(1g+), O2(1g) and O2(3g-) during Oxygen Quenching of n,* Triplet Phenyl Ketones: The Role of Charge Transfer and Sensitizer-Oxygen Complex Structure" J. Phys. Chem., A 2002, 106, 228. 33. Abdel-Shafi, A. A.; Wilkinson, F.,"Electronic to vibrational Energy Conversion and Charge Transfer Contributions during quenching by Molecular Oxygen of Electronically Excited Triplet States" Phys. Chem. Chem. Phys. 2002, 4, 248. 34. Abdel-Shafi, A. A.; Beer, P. D.; Mortimer, R. J.; Wilkinson, F.,"Photosensitized Generation of Singlet Oxygen from Vinyl Linked Benzo-Crown-Ether-Bipyridyl Ruthenium (II) Complexes" J. Phys. Chem., A 2000, 104, 192. 35. Schweitzer, C.; Mehrdad, Z.; Noll, A.; Grabner, E.-W.; Schmidt, R.,"The mechanism of photosensitized generation of singlet oxygen during oxygen quenching of triplet states and the general dependence of the rate constants and efficiencies of O2(1g+), O2(1g) and O2(3g-) formation on sensitizer triplet state energy and oxidation potential" J. Phys. Chem., A 2002, in press. 36. Tanielian, C.; Heinrich, G.,"Effect of Aggregation on the Hematoporphyrin-Sensitized Production of Singlet Molecular Oxygen" Photochem. Photobiol. 1995, 61, 131. 37. Schmidt, R.,"Influence of Heavy Atoms on the Deactivation of Singlet Oxygen (1g) in Solution" J. Am. Chem. Soc. 1989, 111, 6983. 38. Scurlock, R. D.; Nonell, S.; Braslavsky, S. E.; Ogilby, P. R.,"Effect of Solvent on the Radiative Decay of Singlet Molecular Oxygen (a1g)" J. Phys. Chem. 1995, 99, 3521. 39. Schmidt, R.; Afshari, E.,"Effect of Solvent on the Phosphorescence Rate Constant of Singlet Molecular Oxygen (1g)" J. Phys. Chem. 1990, 94, 4377. 40. Hild, M.; Schmidt, R.,"The Mechanism of the Collision-Induced Enhancement of the a1g - X3g- and b1g+ - a1g Radiative Transtions of O2" J. Phys. Chem. A 1999, 103, 6091.

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