Oxygen in Organic Photochemistry - Columbia Universityturroserver.chem.columbia.edu/courses/MMP_Chapter... · Oxygen chapter (v13D) Page 1 April 13, 2004 at 13:22 PM Chapter 14 Oxygen

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 (1 g)2 (1 )2 (2 g)2 (2 )2 (3 g)2 (1 )4 (1 g)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 (1 g) 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. (1 g)2 or (1 *)2] is responsible for six different electronic substates. There


are three triplets with orbital angular momentum of zero that correspond to the 3 g ground

state. There are two singlet states with orbital angular momentum of 2 and -2 corresponding

to the 1 g state,1 and finally, a singlet state of geometry, i.e. the 1 g 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 1 g

energy should be approximately half-way between the two g states, i.e.,

E(1 g )1

2E(1 g ) + E(

3g )[ ] (1)

In reality, the 1 g state is 22.4 kcal/mol above the ground state, while the 1 g 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 (1 g 3 g) and at 762 nm (1 g 3 g); 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., 1 g 1 g.

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 ofoxygen is special in that this emission involves a change in multiplicity. For this reason, it is correctlydescribed as phosphorescence, in spite of originating from a singlet state.


3g- (ground state)

1g 22.4 kcal/mol, 94.3 kJ/mol 7882 cm-1

R = 64.6 min

1Sg 37.5 kcal/mol, 157 kJ/mol, 13121 cm-1

R = 6.7 min (to 1 g)

1g,1 g 44.8 kcal/mol, 188.6 kJ/mol, 15764 cm-1

Energy

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 1 g 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 (3 g). This leads to a red shift of the emission by 1585 cm-1, which, in the

case of the emission for 1 g, 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 zz


O2 (1

g ) +O2 (1

g) O2 (3

g) +O2 (3

g ) + h (2)

where the emitted photon corresponds to twice the energy of the 1 g 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 [1 g, 1 g] (~476 nm) and [1 g, 1 g] (~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 1 g and 6.7 sec. for the 1 g 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 1 g

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,


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 1 g 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 oxygen-

related 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


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


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 37

ketone C = O

methanol H – O 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-p-

phenylenediamine 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.


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 1 g state, while the sensitizer singlet yields its triplet state.

1X* + 3O23X* + 1O2 (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 (1 g) 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,

1X +3O 2 3X +3 O2 (4)

1X +3O 2 1X + 3O2 (5)

ground state

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


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 (1 g) generation

due to sensitizer singlet quenching (S S). Note that in this table the only cases where the

energy available is not sufficient to produce O2 (1 g) 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 S Skq

S, 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 S S is the fraction of singlet quenching events that yield O2 (1 g)


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 T2 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 T1 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 S S 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 (1 g) 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 <900 nm) or an interference filter, and then detected by a fast

germanium photodiode. The weak signals are then amplified and monitored by an

oscilloscope or transient digitizer. With this technique, it is easy to achieve response times

around 1 s and sufficient sensitivity in most solvents, see Figure 1. Cooled, well shielded

detectors have represented a significant improvement, and new infrared photomultipliers

gratly improve time resolution and sensitivity, allowing detection even in water where the

lifetime is short and the yield low. This opens the opportunity for singlet oxygen work in

many biological systems.10


laser sample

silicon filter

germanium diode

signalmonitoring

powermonitoring

Figure 1. Simplified outline of a time-resolved system for monitoring the near-infrared

emission (1270 nm) from singlet oxygen.

Thermal lensing is another technique that can be employed to monitor singlet oxygen

processes.11 The technique relies on the heat released as a result of non-radiative processes

taking place.

Assisted intersystem crossing (see equations 3 and. 4) can ultimately result in

enhanced singlet oxygen yields, since the excited triplets formed in reactions 3 and 4 can in

turn sensitize singlet oxygen generation. Cases with an overall quantum efficiency of 2.0 for

singlet generation are known; in these cases a highly efficient reaction (3) is followed by

equally efficient singlet oxygen generation by the triplet.12

The most common cause of inefficient excited singlet quenching is short singlet

lifetimes. For example, benzophenone undergoes intersystem crossing with unit efficiency

and with a singlet lifetime around 15 ps.13 Thus, considering that typical oxygen

concentrations in solution under 1 atm oxygen pressure are in the 10-2 – 10-3 M range, it is

very unlikely that much singlet quenching will occur.4 We anticipate that molecules with

longer singlet lifetimes will be more sensitive to singlet quenching effects. Many polynuclear

aromatic hydrocarbons, such as those in Table 3, involving , * excited states and large

singlet-triplet gaps (see Chapter xx ) meet this criterion.

4 For example, if the singlet lifetime is 50 ps (i.e. kisc = 2 x 1010 s-1), the quenching rate constant 4 x 1010

M-1s-1 (see table 3), and the oxygen concentration 0.01 M, we only expect 0.2 % of the singlets to decay byinteraction with oxygen.


Experimentally, one can frequently determine if excited singlet states are quenched by

oxygen by monitoring the influence of oxygen on the fluorescence lifetime or intensity.

While not all molecules show fluorescence emission, those more prone to singlet quenching

(i.e., with long singlet lifetime) frequently fluoresce with significant quantum yield.

3b. Quenching by oxygen of excited triplet states. Energy transfer processes

We have delayed until now a detailed analysis of the determination of quantum yields

and efficiencies for 1O2 (1 g) generation. In order to understand best the following sections,

we need to introduce an important parameter of a sensitizer, its efficiency for singlet oxygen

generation, S 5. The quantum yield of singlet oxygen generation from a given sensitizer, ,

gives us the number moles of singlet oxygen formed per mole of photons (i.e., per einstein)

absorbed:

=moles of singlet oxygen formed

moles of photons(Einsteins)absorbed(6)

The value of is influenced by many experimental parameters, such as oxygen

concentration and intersystem crossing yield among others. In contrast, S is the fraction of

triplet quenching events that result in the formation of singlet oxygen, and is characteristic

for a given sensitizer/solvent system, i.e.,

S =moles of singlet oxygen formed

molesof excited triplets quenched by oxygen(7)

The parameters S and are related by the quantum yield of intersystem crossing

( ISC) and the efficiency of oxygen quenching of triplet states, i.e.,

5 By analogy with the case of singlet state sensitization, this term should be S T. However, in order tomaintain consistency with most of the literature we exclude the superscript 'T' and use S .


= ISC • S • kq [O2]

–1 + kq [O2](8)

The last term in equation 8 corresponds to the calculation for the fraction of triplets

quenched by oxygen. The triplet lifetime, , is the value under the experimental conditions

employed, but in the absence of oxygen; i.e., if one of the substrates (other than oxygen) in

the system reduces the triplet lifetime this needs to be taken into consideration. Similarly,

ISC should take into account any singlet quenching caused by either the substrate or

oxygen. Equation 8 does not include oxygen assisted intersystem crossing (excited singlet to

triplet, see Section ## in this Chapter); the equation can be readily modified to include this

term.

Thus, S is a true indicator of the 'quality' of a triplet sensitizer in the generation of

singlet oxygen. Numerous attempts to correlate S values with molecular and spectroscopic

properties have been carried out; some of these will be presented later in this chapter as we

discuss the role of charge transfer interactions in triplet quenching by oxygen. A few

guidelines are useful, even if exceptions to them are well known:

• The , * triplet states of polynuclear aromatics are generally highly efficient,

frequently with S 0.8. Many other , * triplet states are also very efficient.

• The n, * triplet states of ketones have low values of S , for example for

benzophenone in the 0.3-0.4 range.

4. There is usually a modest increase in the value of S with decreasing triplet

energy.

5. The value of S normally decreases with increasing solvent polarity.

6. S decreases with decreasing oxidation potential.14

Many values of S are well established. Table 4 gives a few selected values.


Table 4: Selected values of S for some common singlet oxygen sensitizers in various

solvents at room temperature.

Sensitizer Solvent S

naphthalene cyclohexane 1.0

anthracene benzene 0.8

benzophenone benzene 0.3

fluorenone benzene 0.8

tetraphenylporphyrin benzene 0.58

Ru(bipy)3Cl2 methanol 0.92

-Terthienyl benzene 0.8

phenazine benzene 0.83

acridine acetonitrile 0.82

phenalenone

Sources: 15-18

Oxygen is a well known and highly efficient quencher of excited triplet states; and

quenching by oxygen of a given reaction is frequently employed as a test for triplet

mediation. However, it is far from a specific quencher, and other reaction intermediates can

be responsible for the observed overall quenching of a reaction by oxygen. We will learn in

this section that there are specific characteristics that accompany triplet quenching by

oxygen; later in this chapter we will learn how those characteristics can be used to advantage

in determining if the intervention of oxygen in a photochemical reaction involves excited

triplet quenching.

In general, excited triplet quenching by ground state molecular oxygen is not a

reactive process,8 although it can trigger a series of events that can in turn lead to new

products. There are two dominant primary outcomes in the interaction of oxygen with

triplet states: radiationless deactivation and singlet oxygen formation. As usual, spin and


energetics play a key role in determining which path is favored. Scheme 2 shows an

oversimplified diagram illustrating the options available.

3X* + 3O2

1/9

1/3

5/9

1[X····O2]*

3[X····O2]*

5[X····O2]*

X + 1O2

X + 3O2

singletpath

tripletpath

quintetpath

Scheme ##

Scheme 2: Spin statistics plays a key role in determining the probabilities of the

various reaction paths between an excited triplet state and molecular oxygen. Note

that intersystem crossing processes between encounter pairs with different spin

configurations have been excluded. Recent studies suggest that this may be an

oversimplification (vide infra).

Back reactions (i.e., separation) have been included for the singlet and triplet path

with a dashed line, since whether or not they take place will be determined by a kinetic

competition among the various exit channels from the encounter complex. In the case of

quintet encounters, the most common output is dissociation,13 just as in the biradical

example of Scheme ## (vide infra).

It is interesting to note that from the perspective of spin, the triplet path would also

be consistent with 1X* + 3O2 as the outcome; however, this is not allowed on energetic

grounds, since for closed shell organic molecules, the lowest triplet state lies below the

lowest excited singlet state. An alternate outcome, giving the triplet precursor (i.e., 3X*) and

singlet oxygen would also satisfy spin requirements, but is energetically unfavorable by an

amount identical to the excitation energy of singlet oxygen.


The occurrence of dissociative (quintet) encounters, as well as any back reaction from

the other reaction paths, leads to a kinetic inefficiency; thus, instead of having the diffusional

rate constants (see Chapter ##) as an upper limit to the quenching rate constant, this limit is

anticipated to become the diffusional rate multiplied by the fraction of successful quenching

events. For example, if only singlet encounters lead to quenching, the limiting rate constant

would be 1/9 of diffusion control, or if singlet and triplet encounters participate, then the

limit would be 4/9 of diffusion control. In fact, there is clear evidence of intersystem crossing

between excited triplet and ground state oxygen encounters of different multiplicity; these

appears more common at low temperatures.19 Scheme 2, which is the basis for oxygen's rich

excited chemistry in nature –in spite of the fact that it cannot be excited by sunlight– is in

fact oversimplified as it excludes intersystem crossing between encounters of different

multiplicity.

The emission of 1g oxygen at 1270 nm provides a convenient tool to study the

chemistry of singlet oxygen in the 1 g state. The next higher electronic state is the 1 g, 37.5

kcal/mol or 13121 cm-1 above the ground state. It emits weakly by decay to both the 1g

state and the ground state. Its lifetime of 135 ns in carbon tetrachloride 20 is surprisingly

long for an upper electronic state (see Kasha's Rule in Chapter ##). A few lifetimes in other

solventa are 8.2 ps in water, 42 ps in D2O, 83 ps in cyclohexane, 134 ps in acetonitrile and

18.8 ns in CS2 21.

The paradigm for electronic energy transfer from a triplet sensitizer to molecular

oxygen can be summarized in the following rules (see Schemes 1 and 2):

(a) It cannot occur if the sensitizer triplet energy is significantly below 22

kcal/mol.

(b) It can only populate the 1 g level of molecular oxygen if the sensitizer energy

is between 22 and 37 kcal/mol, since population of the 1 g level would be

energetically unfavorable.


(c) If the energy of the sensitizer exceeds 38 kcal/mol, excitation of oxygen to

either the 1 g or 1 g states can occur.

(d) If the energy of the sensitizer is between ~21 and ~25 kcal/mol, it is possible

for the process to be reversible, with 1O2 (1 g) also transferring energy back to

repopulate the triplet state of the sensitizer. A few examples of this behavior are

well documented.

(e) If the energy of the sensitizer is in the neighborhood of 37-40 kcal/mol – i.e.,

matching reasonably well the energy of 1O2 (1 g) – the process is not expected to

show reversibility [compare with (d) above]. This is due to the fact that the 1 g

state is too short-lived for reversible transfer to occur with any significant

probability at the concentrations of organic solutes frequently used in

photochemistry. In other words, the process is possible but not probable.

The relative yields of 1O2 (1 g) and 1O2 (1 g) formation depend on the sensitizer

and solvent employed, but in some favorable cases the yield of the former can be quite high.

For example, decacyclene yields ~90% of the excited oxygen in the 1O2 (1 g) state22. One of

the principal modes of decay of 1O2 (1 g) is to form 1O2 (1 g). The latter can therefore be

formed by either direct transfer into this state, or by following rapid relaxation of the 1 g

state.

decacyclene, a very

efficient sensitizer

for 1O2 1

g

When 1O2 (1 g) interacts with organic molecules it can in principle do it by the three


mechanisms shown in Scheme 3. It has been shown that chemical reaction krxn is a possible

highly improbable path 22,23, simply because it does not compete kinetically with the other

two physical modes of quenching leading to either the ground state (kbypass), or to the O2

(1 g) state (k ). Naturally, the fact that k . corresponds to a spin allowed process, and

that the energy gap is smaller than for kbypass contributes to its dominance.

1O2 1

g

RX

krxn

kbypass

k

1O2 1

g

Products

3O2 3

g

Scheme 3: Various reaction paths are available in the interaction of sigma singlet oxygen

with the substrate RX.

Detection of emission at ~1270 nm from singlet oxygen provides strong evidence that

excited triplets are present, and a strong, but not conclusive, indication that the

photochemistry of the sensitizer involves the triplet state. Why this reservation? Review

of the last few sections will show that conceivably one may have promoted the formation of

triplets by assisted intersystem crossing of the singlet state (see reaction 5); thus, one may

have formed triplets that were not present before oxygen was introduced as a quencher.

3c. Charge transfer interactions in the triplet quenching process

If energy transfer from triplet sensitizers to oxygen to produce singlet oxygen was


the only effective quenching process, the following will be true:

The maximum quenching rate constant would be 1/9 of the diffusion controlled rate constant,

which can be estimated according to equation xxx.

An efficiency of singlet oxygen formation of 100 % could be achieved (i.e., S would

frequently reach 1.0)

On the other hand if all triplet encounters led to quenching the following conditions

would prevail:

1. The maximum quenching rate constant would be 4/9 of the diffusion controlled

rate constant, which can be estimated according to equation xxx.

2. An efficiency of singlet oxygen formation of 25 % would be common if both

singlet and triplet encounters were irreversible.

Neither one of the scenarios mentioned above fully reflects the experimental results,

suggesting a more complex situation. In particular, the quenching component mentioned

above changes greatly from system to system and it has been proposed that charge transfer

interactions play a role in this process 14,24-26. As already pointed out, oxygen is a good

electron acceptor, but not a donor. Consistently with this, quenching is quite efficient in the

case of good electron donors, such as amines. The contributions from energy and electron

transfer to overall triplet quenching can be readily determined if the value of S T is known or

can be estimated.

kqCT = (1 - S T) kq (9)

Figure 2 illustrates how the physical quenching rate constant (displayed as kqCT)

depends on the G for electron transfer; note that the fastest quenchers (left section of the

plot) are all amines, while for naphthalene log(kqCT) equals 8.9 and for 1-nitronaphthane the

value is 8.34, the lowest one in the graph 27.


Figure 2: Dependence of the quenching rate constant via charge transfer interactions on the

free energy for full electron transfer. From Darmanyan, Lee and Jenks27.

The data in Figure 2 fit well the Rehm-Weller equation (solid line, see Section xxx)

and is indicative of partial charge transfer. Several studies suggest that typical the typical

fraction of charge transfer in non-polar solvents is frequently around 10-20% of that required

for full electron transfer 8,27-29, and occasionally exceeds 25% 30. These are typical

parameters for exciplexes, a terminology that describes well the characteristics of these

sensitizer-oxygen encounter complexes.

Thus in the next level of elaboration, one has to allow for the efficiency of charge

transfer quenching being variable. Such a model would allow readily for values of S to fall

between 0.25 and 1.0, since charge transfer would mainly determine to what extent physical

quenching contributes to the overall inefficiency. However, these concepts are not sufficient

to explain all the data on triplet quenching rate constants and reported values of S . For

example, overall quenching rate constants exceeding 4/9 of kdiff are inconsistent with the


model. To reconcile these observations several groups have proposed that the singlet, triplet

and quintet channels (see Scheme 2) are not independent, but rather that some

communication exists. This communication takes the form of intersystem crossing between

encounter complexes of different multiplicity. A very general mechanism is shown in

Scheme 4. Note that the communication takes place at the encounter (E) level; once charge

separation takes place (1C and 3C) the interaction is stronger and intersystem crossing is no

longer viable.

3M* + 3O2

3M* + 3O2

3M* + 3O2

1/9 kd

3/9 kd

5/9 kd

1E

3E

5E

1kr

3kr

1C

3C

1kp

3kp

1P

3P

k-d

kd

1M + 1O2*

1M + 3O2

1k

3kGS

kTS kST

kQT kTQ

Scheme 4: General mechanism for triplet quenching by oxygen, allowing for communication

between encounters of different multiplicity. Adapted from Abdel-Shafi and Wilkinson 8.

While Scheme 4 clearly offers more flexibility, it also restricts out ability to interpret

data. In fact, few results in the literature truly require this level of complexity. Schmidt and

coworkers have proposed that oxygen quenching of excited triplet states occurs by

simultaneous dual mechanisms; one path involves encounter complexes with no charge

transfer (nCT) and the other exciplexes with partial charge transfer (pCT) 30-32. The nCT

channel follows a conventional energy gap law, while the pCT channel is controlled by the

free energy change for complete electron transfer, thus showing typical exciplex behavior

31,32. The dependence of quenching rate constants and singlet oxygen yields, while complex,


can be understood on the basis of their dependence on triplet energy, redox properties, the

nature of the excited state (n * vs. , *) and the solvent properties.

A simple inverse relationship between quenching rate constants and singlet oxygen formation

efficiencies and the rate constant for triplet quenching has been reported by Abdel-Shafi and

Wilkinson33 In cyclohexane, where the triplet channel (quenching) does not contribute

significantly the maximum value of S reaches one for poor electron donors. In acetonitrile,

S never reaches one, even for poor electron donors, but in contrast, the low value of S for

very good donors (such as 2,6-dimethoxynaphthalene) is around 0.25, a clear reflection of

spin statistical factors, showing that while singlet encounters lead to singlet oxygen

formation by energy transfer, the triplet encounters (statistically three times as probable)

lead to efficient quenching via a mechanism involving partial charge transfer. The quenching

rate constants show an inverse dependence with S , being larger in polar solvents, where

triplet encounters contribute to triplet decay; i.e., in the limit of non-polar solvents and poor

electron donors the limit for kqT is 1/9 of the diffusion controlled limit. In contrast, for good

donors in polar solvents the limit is 4/9 of the diffusion controlled limit. Similar correlations

have been reported for other systems 34.

A general mechanism for , * sensitizers in carbon tetrachloride has been

proposed.35. According to this interpretation, every quenching rate constant is composed of

three contributions:

7. Sensitization to yield O2(1g)

8. Sensitization to yield O2(1g+)

9. Quenching to yield O2(3g-)

According to this interpretation,35 the photosensitized generation of O2(1

g+),

O2(1

g) and O2(3

g-) have one common dependence on Eox and ET. Their work is illustrated

in Figure 3 (for , * sensitizers in carbon tetrachloride ). This graph shows that the rate


constants for energy transfer to generate both singlet excited states of oxygen, or quenching

to its triplet ground state (after multiplicity correction) all fit on the same kinetic surface.

Thus, for a given set of redox properties and a given E the rate constant is independent of

the electronic state of oxygen produced. Note however, that the value of E is in itself a

function of the electronic state produced; that is the excess energy, E, has an inverse

dependence depends on the fraction of the energy will be 'stored' by the form of oxygen

produced [in terms of storage O2(1

g+) > O2(

1g) > O2(

3g-)].

Figure 3: Dependence of the multiplicity-normalized rate constants kTP/m of formation of

O2(1

g+) (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 reference35 (Reproduced with

permission from the copyright owner).


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 O2(1g)

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.


• 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 .36

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 20. 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 leftside of the diagram.


1g ; t = 130 ns

1g ; t = 87 ms

3g

krad = 3.4 x 103 s-1

em = 4.5 x 10-4

krad = 0.40 s-1

em = 5.2 x 10-8

krad = 1.1 s-1

knr = 10.4 s-1

em = 0.087

(sum of knr from 1 g = 7.6 x 106 s-1)

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

tetrachloride20.

Note the very low quantum yields associated with the various emissions from the

two excited singlet states of oxygen. The lifetime of the 1 g state shows a strong solvent

dependence, as illustrated in Table 5.

Table 5: Singlet oxygen (1 g) 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


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 (1 g), 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 (1 g) change significantly with the solvent 38. For

example, a detailed study of these variations reveals a 16-fold change between 1-

methylnaphthalene (long) and water (short) 39. 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 1 g 3 g transition. It is also believed that the strong spin orbit coupling in the

1 g-1 g transition plays a role on the transition intensity for 1 g 3 g. 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 (1 g) show an empirical correlation with the

solvent polarizability, (n2 - 1)/(n2 - 2), where n is the refractive index of the solvent.940

7 There are many experimental factors that can cause a reduction of the lifetime, but it is difficult to find onethat could lengthen the lifetime.8 For example, for oxygen in benzene f = 2.5 x 10-8 39.9 The term n2 appears in in the Einstein's expression for spontaneous emission between two states.


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 • (10)

In the case of hexane this leads to = 1.8 x 10-6.


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 (1 g) 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 =kM 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 (1 g) lifetime as a function

of the solvent 43:

• The longest lifetimes are observed in perhalogenated solvents.


• 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 (1 g) O2 3 g .

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.


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 (1 g) deactivation through interactions with the vibrational states of C–H or O–H bonds,

frequently those of the solvent.44 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 (1 g) O2 3 g 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:

1O2 1 g + -C 3O2 3 g + 3 -C* (13)


CH3

CH3

CH3

CH3 CH3

CH3

CH3

H3C

CH3CH3

-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).

1O2 1 g + Q 3O2 3 g + Q [O2• + 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 zz

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.


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.

1O2

1O2

1O2

H3C CH3

H3C CH3

CH3O

CH3O

H3C CH2

H3C CH3OOH

OO

O

OCH3O

CH3O(C)

(A)

(B)

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.

H OO

OOH

(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


with regeneration of singlet oxygen, for example in the case of anthracene and its

endoperoxide:

1O2 + 1O2

OO

(16)

Thus, anthracene endoperoxide can be viewed as 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•).

O

OCH3

H3C

CH3

H3C

H3CO

H3C

H3CO*

H3C+

S ~ 0.25

T ~ 0.35

(check values) (17)

Singlet oxygen (an electrophillic reagent) also reacts readily with a variety of electron-

rich molecules, such as amines, a process that frequently competes with physical quenching

(see section ##):

1O2RCH2NH2 CHNH2R

OOH (18)


Thousands of rate constants have been determined for the interaction of singlet

oxygen with organic and inorganic molecules10 17.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 CH2Cl2 5.2 x 107

-carotene CH2Cl2 4.6 x 109

1,3-cyclohexadiene CHCl3 ~7 x 106

cyclohexene CHCl3 9 x 103

cyclohexane CCl4 6.4 x 103

-tocopherol toluene 2.2 x 108

Acetic acid CCl4 2.3 x 103

Triethyl phosphite acetone 2.5 x 107

Indole Toluene 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. 45 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: seehttp://allen.rad.nd.edu/browse_compil.html


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. 45.

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.


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 46. 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 47, see Table 7.

Scheme dd: Illustrating the activation entropy dependence of the kinetics for singlet oxygen

quenching by alkenes in carbon disulfide. From ref.46

H‡ (kcal/mol)

S‡ (e.u.)

kq (M-1 s-1)

2.0

- 31

39,000

0.4

-39

7,700

-0.1

-42

5,200

Adam and coworkers 48 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.


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

From ref. 48.

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 threo-

erythro selectivity and illustrates the diastereoselectivities of the ene reaction.


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

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. 48.

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

CH3H

H

H

H3C

R

O

O

threo-TS

-

+


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:

C

CH3H

H

H3C

R

O

O

erythro-TS

+

O O

H2C

CH3

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.


EtO2C

H

HH

OO

H

CH3

OXH

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%).

RR

O O3*

3O2 3 g

RR

O O•O

O•

O

R •+

O

R O O•

3O2 3 g 2O

R O O•

Schenck mechanism


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


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 49.

R-OO•R• + O2(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• + ROOHR-OO• + RH (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 oxygen50) 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 noteindicating that organic compounds, especially ethers, should not be distilled dry, reflects the danger posed byaccumulated hydroperoxides. The industrial synthesis of acetone and phenol from cumene is mediated bycumene hydroperoxide, manufactured as indicated above. The autooxidation of fatty acids is responsible fortheir (flavor) deterioration in vitro and an important contributor to the aging process in vivo.


BHT

O-HtBu But

CH3

O

CH3

H-O

CH3

CH3

CH3CH3

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 carbon-

centered 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. 49

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 Triplet

13 For an example of a well characterized singlet biradical see ref.51.


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

H

3O2

H

3O2

H

3O2

H

H

3O2

3O2

H

3 1

3

3

Ph

OH

Ph

OH

Ph

O

OOH

PhOH

O

O

OO•OOH

A) Assisted intersystem crossing

Products

E.g. + + PhCOCH3

B) Hydroperoxide formation

E.g.

C) Peroxide formation

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.


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 52.

CH2

OH OH

•

•3


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 6H5 ) 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.

NN

–N2

–N2Cl

NN

Cl

h1 3

Diazo precursor of carbene with triplet ground state

Diazirine precursor of carbene with singlet ground state

h1

fast

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


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 diphenylmethylene55.

3[(C6H5)2C:] + 3O2 (C6H5)2COO (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 56. 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.


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:

(C6H5)2CO• + O2 (C6H5)2CO + 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+•

N+Me N+Me

MV2 +

+ O2 + 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.

COHH3C


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.


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


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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