Organic photosynthetic reactions

Transcription

1 rganic photosynthetic reactions Singlet oxygen 2 " excited state! excited state 3 2 ground state atomic orbitals molecular orbitals atomic orbitals 5 rganic photosynthetic reactions Singlet oxygen lifetime The radiative deactivation pathway of singlet oxygen electronic excited state takes place through a spin-forbidden transition. Spin-orbit coupling in oxygen is very inefficient, thus leading to a radiative rate constant for!$ 3 " phosphorescence of kr 0! 4!"0 4 s. 2 ( g) 2 ( 3 "g) energy transferred to the solvent The only other decay pathway is internal conversion (intersystem back-crossing), which implies that at least 94 kj mol (nearly! ev) must be released into the environment of the 2 molecule. This energy transfer requires collisions with surrounding molecules. In the gas phase, at low pressure, no collision can efficiently scavenge the excess vibrational energy and IC process is ineffective. The lifetime of singlet oxygen depends only upon radiative decay and # = # r 0! 45 minutes (!) for 2 ( g). " state is shorter lived with # 7-2 s due to an allowed transition "!$. The state is therefore the most chemically reactive form of singlet oxygen. 6

2 rganic photosynthetic reactions Singlet oxygen lifetime In a solvent, the rate of internal conversion depends upon the number of collisions that are required to scavenge 2 excess vibrational energy 94 kj mol 8,000 cm. Solvent molecules possessing vibrational modes of higher frequency (C-H, -H) will cause a rapid non-radiative decay of 2, while singlet oxygen will live much longer in solvents characterized by only low frequency vibrations (C-Cl, for instance). Solvent # [µs] Solvent # [µs] C-Cl C-H, -H 2 acetone acetonitrile hexafluorobenze 9400 ne methanol 0.4 acetonitrile-d methanol-d 37 benzene methanol-d4 227 benzene-d chloroform chloroform-d cyclohexane 23 CH2Cl ethanol ethyl-ester C2Cl4 200 CCl toluene 29 toluene-d8 320 C2Cl3F water water-d ! 670 cm! 3,000 cm! 8,000 cm 7 rganic photosynthetic reactions Singlet oxygen production Singlet oxygen can be prepared by physical or by chemical methods. The physical methods of preparative value include electrical corona discharge in 2 at low pressure (right), direct two-photons excitation, and photosensitization. Among the chemical methods, the most important are the decomposition of hydrogen peroxide by hypochlorite (Cl ) or hypobromite (Br ) and the decomposition of some ozonides. H22 + Cl! H Cl P 3 P P + 2 8

3 rganic photosynthetic reactions Singlet oxygen production E / kj mol!g phosphorescence Simultaneous transition Direct photoexcitation of 3 2 to 2 is a spin-forbidden transition. Absorption coefficients of oxygen are therefore very low. Direct excitation is possible only in particular cases. He-e laser radiation at % = 633 nm, for instance, corresponds almost exactly to the energy of a so-called simultaneous two-photon transition: hν 2 2 ( 3 ") 2 2 (!) The energy scheme on the left shows that singlet oxygen! can possibly be detected by its phosphorescence in the near IR at % = nm. Such measurement is usually feasible only at low temperature (77 K). 9 rganic photosynthetic reactions Photosensitized singlet oxygen production e e Singlet oxygen can be readily formed by energy transfer from a triplet sensitizer in a spin-allowed process. 2 ( 3 ") 2 (!) S0 hν S* ISC 3 S* Sens (T) Sens (S0) 3 S* S E / kj mol S S0 hν ISC 3 S Energy transfer 2 "! 3 " This sensitization to yield the! and 3 " states may be considered to occur by an electron-exchange (Dexter) type of mechanism for intermolecular energy transfer. Because of the small energy difference between the ground 3 " and! excited states, many compounds are capable of acting as sensitizers for singlet oxygen formation. 20

4 rganic photosynthetic reactions Photosensitized singlet oxygen production Rose Bengal Acridin yellow Hematoporphyrin Eosin-Y Methylene blue Chlorophyl a Efficient sensitizers for 2 formation are characterized by a large triplet quantum yield, a long triplet lifetime and a triplet energy ET > 94 kj mol. For technical applications, the compound should not be destroyed by 2, nor intervene in other reactions. Sensitizer ET [kj mol ] Rose Bengal 64. Eosin-Y 80.8 Acridine yellow 88.9 Methylene blue 40.2 Hematoporphyrin 55.7 Chlorophyl 58.8 Benzophenone rganic photosynthetic reactions Singlet oxygen reactions Electrons of singlet oxygen!g are paired (unlike in 32 and 3, for instance). Most reactions of 2 with unsaturated organic compounds, thus, ressemble those of ethene. Direct cycloaddition of 2 on a C=C double bond and formation of,2-dioxetanes 2 Addition on alcenes possessing one H atom in allylic position leading to the formation of hydroperoxides. This reaction is called the ene, or sometimes Schenck ene, reaction. Diels-Alder type [2+4] cycloaddition of 2 on a diene leading to the formation of endoperoxides. 22

5 rganic photosynthetic reactions Singlet oxygen reactions examples Mono-unsaturated fatty acids (oleic acid), along with &-tocopherol (vitamin E) and squalene (a precursor of vitamin D) are important constituants of olive oil. Traces of chlorophyl and pheophytin (chlorophyl without the central Mg + ion) are responsible for the green color of the oil. Both these dyes are also efficient singlet oxygen photosensitizers. In the presence of oxygen and under visible light irradiation, prompt photodegradation of oleic acid, &-tocopherol, pheophytin and squalene is observed, which is due to the ene reaction of 2 with ethenes and aromatic moieties. paque or dark-colored packaging is thus needed to block light and preserve the precious characteristics of the product. pheophytin oleic acid &-tocopherol squalene 23 rganic photosynthetic reactions Singlet oxygen reactions Singlet oxygen reacts with an alkene C=C CH by abstraction of the allylic proton in an ene reaction yielding the allyl hydroperoxide H--R (R = alkyl), which can then be reduced to the allyl alcohol. This reaction is used preparatively in the synthesis of rose oxide, a base of numerous fragrances and flavors. The industrial production of rose oxide is carried out by the photo-oxygenation of citronellol. Rose Bengal immobilized on a fixed inorganic or polymeric bed serves as 2 photosensitizer. hν, 2 Rose Bengal Citronellol Rose oxide 24

6 rganic photosynthetic reactions Catalyzed 2 production from H22 Hydrogen peroxide slowly decomposes at room temperature to yield singlet oxygen : H22 H2 + /2 2 ΔGr 0!=! 23,6 kj mol ; Dismutation of H22 is catalyzed by various transition metal ions, such as Fe 3+, Cu 2+, Co Catalase (hydrogen peroxide reductase) is an enzyme present in mamals' blood. It catalyzes the decomposition of H22 very efficiently, according to the sequence: H22 + E Fe(III) H22 + E Fe(IV)= H2 + E Fe(IV)= [ 2 ]* + E Fe(III) Rather then producing heat (ΔHr 0!=!!96,0 kj mol ), the exothermal reaction yields the electronic excited state of oxygen 2. Singlet oxygen can be detected by its phosphorescence or by chemical reactions producing chemiluminescence. 25 rganic photosynthetic reactions Singlet oxygen reaction with luminol: chemiluminescence luminol H H 2 H + 2 * + * + 2 * + 2 H + H H + lumière light 26

7 rganic photosynthetic reactions Singlet oxygen reaction with organic oxalates: chemiluminescence diphenyl-oxalate ( Cyalume ) + H 2 2 DEHP H 2 + * (non-emissive) * + BPEA fluorescent dye 2 C 2 + BPEA* 2 C 2 + BPEA + light 27