Triplet ordering scheme

Almost all triplet-state photocycloadditions studied by Wagner s group [93-103] yield ortho adducts that undergo thermal ring opening to a cyclooctatriene followed by photochemical 4-rr ring closure. Many examples of these reactions are shown in the section devoted to additions via the triplet state. Scheme 51 schematically illustrates the basic reactions for intermolecular as well as intramolecular cycloadducts. A substituted benzene has been chosen for the intermolecular reaction in order to demonstrate the two modes of ring closure. 

Fig. 16 Ordering scheme for triplet emitters [12,49, 63,108]. The diagram illustrates the relation of MLCT character of the emitting state to the magnitude of zero-field splitting AL(ZFS) of the Tj state. The positions of compounds with AL(ZFS) < 1 cm-1 are for most emitters only roughly estimated. The molecular structures are summarized in [49]. The diagram is adapted from [49]. Properties of the compounds in frames are discussed in detail in Sects. 2-5...

Fig. 7 Ordering scheme for organometallic triplet emitters according to the amount of zero-field splitting (ZFS) of the emissive triplet state. This splitting reflects the size of metal participation (MLCT and/or d-orbital character) and spin-orbit coupling in the corresponding wavefunctions. The diagrams at the bottom show energy levels of the relevant frontier molecular orbitals for the different compounds. For details see text. (Compare [79] and for the Ir(III) compounds [25, 87, 88])...

Fig. 8 Photophysical properties of a representative organic molecule compared to two or-gano-transition-metal emitters. The emissive triplets of Pd(thpy)2 and Pt(thpy)2 exhibit small and significant MLCT admixtures to the LC(7ztz ) states, respectively. The positions of the compounds in the ordering scheme and the molecular structures are found in Fig. 7. Photophysical properties of Pd(thpy)2 and Pt(thpy)2 are discussed in detail in [79]. lig. vibr.=ligand vibrations, M-L vibr.=metal-ligand vibration...

An alternative programme of instraction aimed at reinforcing the use of the triplet relationship when describing and explaining the seven types of chemical reactions was developed by the second anthor and incorporated into the prescribed scheme of work. In order to delineate the content of the alternative instructional prograrmne, the concept map in Fig. 7.1 (Chandrasegaran, 2004) was developed. The concept map encapsulates the characteristics of the seven chemical reactions and indicates... 

Reaction step 5 in Scheme 3.1 can be rnled ont becanse the flnoranil ketyl radical (FAH ) reaches a maximum concentration within 100 ns as the triplet state ( FA) decays by reaction step 2 while the fluoranil radical anion (FA ) takes more than 500 ns to reach a maximum concentration. This difference snggests that the flnoranil radical anion (FA ) is being produced from the fluoranil ketyl radical (FAH ). Reaction steps 1 and 2 are the most likely pathway for prodncing the flnoranil ketyl radical (FAH ) from the triplet state ( FA) and is consistent with the TR resnlts above and other experiments in the literatnre. The kinetic analysis of the TR experiments indicates the fluoranil radical anion (FA ) is being prodnced with a hrst order rate constant and not a second order rate constant. This can be nsed to rnle ont reaction step 4 and indicates that the flnoranil radical anion (FA ) is being prodnced by reaction step 3. Therefore, the reaction mechanism for the intermolecular hydrogen abstraction reaction of fluoranil with 2-propanol is likely to predominantly occur through reaction steps 1 to 3. 

Bis(2,4,6-tribromophenyl)carbene (106f) was easily generated by photolysis of the precursor diazomethane (105f) and was characterized by EPR spectroscopy (Scheme 9.14). The triplet carbene generated in a degassed solution at room temperature decayed very slowly, persisting for at least 30 s. The decay was found to be second order Ik/el = 8.9 s ). The parameter fj/2, is estimated to be Is (Table 9.14). 

Irradiation in the presence of MDEA completely inhibits the formation of products. The amine quenches the fluorescence of Eosin with a rate constant of 8 x 108 M-1s-1 and quenches the Eosin triplet with a rate two orders of magnitude lower. A summary of rate constants for the decay of the triplet is presented in Table 8. In addition to the reactions shown in Scheme 3, with Am = (V-methyl diethanolamine, the rate constants for reaction of PDO with Eosin triplet and semioxidized Eosin radical in aqueous solution (Eqs. 19 and 20) are included in the table. 

The energy gap between the two levels (3 (3-carotene + C6o and (3-carotene + + (7,o) is also dependent on the solvent polarity, becoming smaller in polar solvents (Scheme 3) [141], The decay of the 3-carotene +-absorption follows second-order kinetics suggesting that back-electron transfer occurs from C 60 [141], Similar results were observed for mixtures of fullerenes with phthalocya-nines as donors [148]. In nonpolar solvents triplet energy transfer from the triplet-excited fullerenes to the phthalocyanine becomes prominent [148],... 

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