Singlet-triplet conversion

UV photolysis (Chapman et al., 1976 Chedekel et al., 1976) and vacuum pyrolysis (Mal tsev et al., 1980) of trimethylsilyldiazomethane [122]. The silene formation occurred as a result of fast isomerization of the primary reaction product, excited singlet trimethylsilylcarbene [123] (the ground state of this carbene is triplet). When the gas-phase reaction mixture was diluted with inert gas (helium) singlet-triplet conversion took place due to intermolecular collisions and loss of excitation. As a result the final products [124] of formal dimerization of the triplet carbene [123] were obtained. 

According to this sequence, formation of cis- and trani -stilbenes is preceded by formation of a magnetosensitive ion-radical by a singlet-triplet conversion. This means that spin polarization must be observed in cis- and trani -stilbene, and the isomerization rate must depend on the intensity of the magnetic held. These predictions were conhrmed experimentally (Lyoshina et al. 1980). Hence, the ion-radical route for trans —f cis conversion is the main one under photoirradiation conditions. Until now, the mechanisms assumed for such processes have involved energy transfer and did not take into account single-electron transfer. The electron transfer takes place in reality and makes the... 

Fig. 34 Photosensitized singlet oxygen production 1/ r is the general (radiative and non-radiative) rate constant of the transition Si So fcsi is the rate constant of singlet-triplet conversion tt is the lifetime of the triplet, T1, electronic state of PS kj is the second-order rate constant of singlet oxygen quenching of the Ti state of PS tl and nr are the radiative lifetime and rate constant of all intramolecular nonradiative energy relaxation processes of O2 ( Ag)...

The rates for the reverse intersystem crossing (from upper singlet to triplet) can also be calculated if we make the following assumptions. We first assume that the singlet-triplet conversion takes place only after the molecule in the excited singlet state has lost its excess of vibrational energy. The singlet-triplet conversion (rate = kt) is then in direct com-... 

V. S. Gladkikh, A. I. Burshtein, G. Angulo, and G. Grampp. Quantum yields of singlet and triplet recombination products of singlet radical ion pairs. Phys. Chem. Chem. Phys., 5(12) 2581-2588, 2003. A. I. Burshtein and A. Y. Sivachenko. Photochemical accumulation and recombination of ion pairs undergoing the singlet-triplet conversion. Chem. Phys., 235(1-3) 257-266, 1998. 

Berson and coworkers have reported that the addition of 9-phenylanthracene to MA is not affected by ferric-dipivaloylmethide. The latter is a known catalyst for singlet-triplet conversion. From this and other kinetic evidence on the sterochemistry of cyclopentadiene-acrylate adducts, Berson and coworkers argue that a diradical intermediate can be eliminated from consideration in the Diels-Alder reaction. Different products formed from photochemical versus thermal DA reaction may be considered as evidence of the absence of a triplet intermediate as well, although a singlet diradical cannot be ruled out. ... 

Intersystem Crossing(ikisc). Generation of Radical Pairs (koX Singlet-Triplet Conversion (Itst). Escape of Radicals (ks). 

In cocrystals 3 and 6, both phosphors are Phe. It is well known that fluorescence can be easily observed in a pure crystal of Phe, which emits blue fluorescence under 365-nm light (Fig. 13). The heavy atom effect can promote the singlet-triplet conversion by enhancing spin-orbit coupling between the excited-state electrons of a compound and the massive nucleus of the heavy atom [3, 35]. Thus, in cocrystals 3 and 6, 1,4-DITFB and 1,4-DBrTFB functional tectons are introduced into the Phe crystal, respectively. After co-crystallization, Phe in cocrystal 3 emits a strong orange phosphorescence under 365-nm light (Fig. 14a). The vibrational... 

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