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Oxygen in Organic Photochemistry

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Oxygen in Organic Photochemistry a common statement: The reaction is quenched by oxygen, thus, it must be a triplet reaction true or false? 10.1 Electronic structure The basic electronic structure of the oxygen molecule in the ground state can be written as: 2 2 2 2 2 4 2 O2 (1s g) (1s µ) (2s g) (2s µ) (3s g) (1pµ) (1pg) or 2 2 2 2 2 4 2 O2 (1s ) (1s* ) (2s ) (2s* ) (3s ) (1p) (1p*) 10.2 Energy levels for molecular oxygen 1 1 -1 Dg, Dg 44.8 kcal/mol, 15764 cm 1 -1 Sg 37.5 kcal/mol, 13121 cm 1D 22.4 kcal/mol, 7882 cm-1 Energy g 3 Sg (ground state) Energy levels for molecular oxygen. Excited triplet states have not been included because they 1 are much higher in energy. The Dg state is the one normally refereed to as singlet oxygen. 10.3 Dimol emissions Oxygen shows several "dimeric" emissions, the best known of which is the dimol emission at ~635 nm. No dimer is formed under normal laboratory conditions. Kasha 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 1 1 3 3 O2( Dg) + O2( Dg ) ® O2( Sg ) + O2( Sg) + hn 10.4 Jablonski diagram for the excited states of molecular oxygen 1 1 6 -1 Sg ; t = 130 ns (sum of knr from Sg = 7.6 x 10 s ) 3 -1 krad = 3.4 x 10 s knr = -4 F em = 4.5 x 10 -1 krad = 0.40 s 1 Dg ; t = 87 ms knr = -8 F em = 5.2 x 10 -1 krad = 1.1 s -1 knr = 10.4 s F em = 0.087 CCl4 3 Sg 10.5 Singlet oxygen spectroscopy Solvent F em t oD (µs) krad (s-1) C6H6 4.7 x 10-5 31 CH3CN 7.1 x 10-5 75 CHCl3 3.6 x 10-4 207 CS2 0.040 34000 CCl4 0.087 87000 Freon-113 0.15 99000 H2O ~5 CH3OH 10.4 Sources: (Schmidt, 1989), Singlet oxygen (1Dg) lifetimes, emission quantum yields and radiative decay rate constants (krad) in various solvents at room temperature 10.6 Effect of deuteration on singlet oxygen lifetimes 1 O2 ( Dg) lifetimes in CH3OH 10.4 µs CH3OD 37 µs CD3OD 227 µs k [M] P = M r - 1 Probability of reaction t + kM[M] 10.7 Effect of deuterium on reaction yields k [M] P = M r - 1 Probability of reaction t + kM[M] There are 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 10.8 1 Simple rules let us anticipate changes in O2 ( Dg) lifetime as a function of the solvent • The longest lifetimes are observed in perhalogenated solvents. • t oD decreases on increasing the number of H atoms in the solvent molecule. • The shortest t oD values are observed with solvents having O–H groups, notably water. • The presence of heavy atoms reduces t oD . • Solvent deuteration invariably increases t oD . 10.9 Bond dissociation energies for selected oxygen containing species Molecule 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 ROH H – O 105 ROOH H – O R2O2 O – O 37 ketone C = O methanol H – O 104.4 10.10 Redox properties • Oxygen is a good electron acceptor, but a very poor donor. • • – • Reduction of oxygen can lead to O2 , H O2 , H O2 , H2O2 and HO• • It is usually the first electron transfer to O2 that is the rate limiting step. • • The O2/ O2 couple has an immense importance in nature. •It has an E˚ of -0.15 V in water and -0.60 in dimethylformamide. • • Under many conditions, O2 is itself a good reductant. • 2– • Superoxide is a poor oxidant, since E˚ (O2 / O2 ) < -1.7 V 10.11 Redox properties of singlet oxygen Singlet oxygen is a better oxidant than ground state oxygen. When the excitation energy of singlet oxygen is taken into 1 • consideration the values of E˚ ( O2/ O2 ) are 0.34 V in dimethylformamide and 0.79 V in water. Singlet oxygen oxidizes molecules such as N,N,N',N'-tetramethyl-p- phenylenediamine to its radical cation. Me2N NMe2 10.12 Energy transfer Singlet oxygen as a donor There are not many examples of energy transfer from oxygen, largely because few molecules have such a low excitation energy 1 1 3 3 3 O2 Dg + b-C O2 Sg + b-C* CH3 CH3 CH3 CH3 H3C CH3 CH3 CH3 CH3 CH3 b-carotene (b-C) 10.13 Singlet oxygen: chemical quenching H3C CH2 H3C CH3 1 O2 H C CH (A) H C 3 3 3 CH3 OOH 1 O2 O (B) O CH O CH3O 3 1 O O2 O (C) CH3O CH3O 10.14 The Schenck or ene reaction H3C CH2 H3C CH3 1O 2 (A) H3C CH3 H C CH 3 3 OOH O H O H O O 10.15 Singlet oxygen: reversible addition Addition to a conjugated system can be reversed thermally with regeneration of singlet oxygen O 1 O O2 1 D + O2 endoperoxide Provides a chemical mechanism for ‘storing’ singlet oxygen 10.16 Dioxetane formation is not reversible CH3 H3C CH3 H3C O 1 + O2 minor H3C O H3C CH3 CH3 dioxetane CH3 H C H C H3C O 3 3 D O + O* H3C O Ea ~ 27 kcal/mol H3C H3C CH3 F S ~ 0.25 F T ~ 0.35 CH3 H3C O• H3C O• CH3 ? 10.17 Interaction of oxygen with excited singlet states S1 T2 Possible but unlikely T1 1 * 3 3 * 1 X + O2 ® X + O 2 ( Dg) requires large S-T gap So Common Assisted intersystem crossing 1 * 3 3 * 3 X + O 2 ® X + O2 10.18 Energy transfer processes Quenching by oxygen of excited triplet states 1 1 singlet [X····O2]* X + O2 path 1/9 1/3 3 3 3 3 triplet X* + O2 [X····O2]* X + O2 path 5/9 5 quintet [X····O2]* path Spin statistics plays a key role indetermining the probabilities of the vatious reaction paths between an excited triplet state and molecular oxygen 10.19 Detecting singlet oxygen The emission at 1270 nm provides a 1S ; t = 130 ns 1 6 -1 convenient tool to study the chemistry of g (sum of knr from Sg = 7.6 x 10 s ) singlet oxygen in the 1Dg state. The next k = 3.4 x 103 s-1 higher electronic state is the 1Sg, 37.5 kcal/mol rad knr = -1 or 13121 cm above the ground state. It emits -4 F em = 4.5 x 10 1 weakly by decay to both the Dg state and the -1 krad = 0.40 s ground state. Its lifetime of 135 ns in carbon 1 Dg ; t = 87 ms knr = tetrachloride is surprisingly long for an upper -8 F em = 5.2 x 10 electronic state (Kasha's Rule) laser sample -1 krad = 1.1 s power -1 monitoring knr = 10.4 s F = 0.087 silicon filter em germanium diode 3 Sg signal monitoring 10.20 Paradigm for electronic energy transfer from a triplet sensitizer to molecular oxygen • It cannot occur if the sensitizer energy is significantly below 22 kcal/mol. • It can only populate the 1Dg level of molecular oxygen if the sensitizer energy is between 22 and 37 kcal/mol, since population of the 1Sg level would be energetically unfavorable. • If the sensitizer energy exceeds 38 kcal/mol, excitation of oxygen to either the 1Dg or 1Sg levels is possible. • If the energy of the sensitizer is between ~21 and ~25 kcal/mol, it is possible for the process 1 1 to be reversible, with O2 ( Dg) also transferring energy back to repopulate the triplet state of the sensitizer. • If the energy of the sensitizer is in the neighborhood of 37-40 kcal/mol – i.e. matching 1 1 reasonably well the energy of O2 ( Sg) – the process is not expected to show reversibility. This is due to the fact that the 1Sg state is too short-lived for reversible transfer to occur with any significant probability at the concentrations of organic solutes frequently used in photochemistry. The process is possible but not probable. 10.21 Reaction paths available in the interaction of sigma singlet oxygen with the substrate RX krxn Products RX kbypass 1 1 3 3 O2 Sg O2 Sg (The upper state of singlet oxygen) kSD 1 1 O2 Dg 10.22 Quenching by oxygen of excited triplet states. Chemical trapping In the majority of cases, interaction of oxygen with triplet states involves energy transfer. In a few examples, notably diketones, a chemical reaction occurs between the triplet state and O2: 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 Schenck mechanism 3* O O• O O 3 3 O2 Sg O R O• R R R O O 3 3 O + O2 Sg 2 R • R O O• R O O• 10.23 1 Efficiency of singlet oxygen, O2( Dg), generation: selecting a good singlet oxygen sensitizer kq [O2] F D = F ISC • SD • –1 t + kq [O2] SD is a true indicator of the 'quality' of a triplet sensitizer in the generation of singlet oxygen 10.24 Values of SD for some singlet oxygen sensitizers • The p,p* triplet states of polynuclear aromatics are generally highly efficient, frequently with SD ³ 0.8.
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