Electronically excited quinones

The primary process of photosynthesis (in both photosystems) is an electron transfer reaction from the electronically excited chlorophyll molecule to an electron acceptor, which is in most cases a quinone. This primary electron acceptor can then hand over its extra electron to other, lower energy, acceptors in electron transport chains which can be used to build up other molecules needed by the organism (in particular adenosine triphosphate ATP). The complete process of photosynthesis is therefore much... 

Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Paterno-Buchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies involving quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate which appear on the ns/ps time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse ( c=108/s) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

The reaction starts with excitation of the quinone, followed by intersystem crossing and electron transfer from the thiophene to the triplet excited quinone. The ion radical pair collapses to a biradical which loses a chlorine and a hydrogen atom. Yields are high (65-78%) when R1 = halogen and R2 = H, fair (57%) when R1 = R2 = H and poor (2-17%) when R1 = H and R2 = halogen. The regioselectivity has been explained on the basis of calculated electron densities in the cation radicals of thiophenes. 

The electron excited away from Pggo in Photosystem II eventually reaches a quinone in that photosystem that accepts two electrons and also picks up two protons (H+) from the stroma (Fig. 5-19). This quinone transfers its two electrons and two protons to a mobile plastoquinone in the plastoquinone pool occurring in the lamellar membranes, and the mobile... 

Absorption spectra of electronically excited states may be observed in flash photolysis studies. Porter has established the existence of the triplet state in a wide range of organic compounds in the liquid and gaseous phases. For example, the first triplet state of anthracene is populated by radiationless conversion from a photochemically excited singlet molecule, and may be observed by the absorption to the second triplet level. Absolute measurements of the triplet concentration may be made by determinations, from the absorption spectra, of the depletion of the singlet state. Similar results have been obtained with a variety of hydrocarbons, ketones, quinones and dyestuffs. 

The hydroxy group at position 2 in 1,4-naphthoquinone (318) is involved in [3 + 2] addition, producing the 2,3-dihydronaphtho[2,3-6]furan-4,9-dione 319 in 50% chemical yield (Scheme 6.140).1002 The reaction proceeds by a two-step process from a triplet excited quinone via exciplex (Section 2.2.3), intramolecular electron transfer (Section 5.2) and possibly ionic intermediate 320 formation. 

The organic radical formed may give more semiquinone by reaction (9). Excited quinones may also abstract electrons from suitable donors. 

Free radical mechanisms also serve to explain the photo-cross-linking of various polymers, such as that of polyethylene accomplished with the aid of lightabsorbing additives such as benzophenone, quinone, benzoin, acetophenone, or their derivatives. When electronically excited by light absorption, these additives either directly abstract hydrogen from the polymer or decompose into free radicals capable of abstracting hydrogen, as shown in Schemes 7.10 and 7.11. 

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

To assess performance of the selected DFT techniques in predicting electronic absorption spectra of quinones, the authors computed excitation energies of... 

Anthraquinones are nearly perfect sensitizers for the one-electron oxidation of DNA. They absorb light in the near-UV spectral region (350 nm) where DNA is essentially transparent. This permits excitation of the quinone without the simultaneous absorption of light by DNA, which would confuse chemical and mechanistic analyses. Absorption of a photon by an anthraquinone molecule initially generates a singlet excited state however, intersystem crossing is rapid and a triplet state of the anthraquinone is normally formed within a few picoseconds of excitation, see Fig. 1 [11]. Application of the Weller equation indicates that both the singlet and the triplet excited states of anthraquinones are capable of the exothermic one-electron oxidation of any of the four DNA bases to form the anthraquinone radical anion (AQ ) and a base radical cation (B+ ). 

The flow of electrons occurs in a similar manner from the excited pigment to cytochromes, quinones, pheophytins, ferridoxins, etc. The ATP synthase in the mitochondria of a bacterial system resembles that of the chloroplast—chloroplast proton translocating ATP synthase [37]. 

J.R. Bolton In solution most photochemical electron transfer reactions occur from the triplet state because in the collision complex there is a spin inhibition for back electron transfer to the ground state of the dye. Electron transfer from the singlet excited state probably occurs in such systems but the back electron transfer is too effective to allow separation of the electron transfer products from the solvent cage. In our linked compound, the quinone cannot get as close to the porphyrin as in a collision complex, yet it is still close enough for electron transfer to occur from the excited singlet state of the porphyrin Now the back electron transfer is inhibited by the distance and molecular structure between the two ends. Our future work will focus on how to design the linking structure to obtain the most favourable operation as a molecular "photodiode . 

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