Spin-orbit induced intersystem crossing

The collisional fluorescence quenching and phosphorescence induction processes have been later observed for a large number of small and medium-size molecules. The interpretation of these results was however rather confusing collision-induced intersystem crossing considered as a transition from the pure singlet to the pure triplet state is in apparent contradiction with the Wigner rule of spin conservation, at least in the case of light collision partners that cannot affect the intramolecular spin-orbit interaction. 

C. Daniel, M.-C. Heitz, J. Manz, and C. Ribbing, Spin-Orbit Induced Radiationless Transitions in Organometallies Quantum Simulation of the E- Ai Intersystem Crossing Process in HCo(CO)4 , J. Chem. Phys. 102, 905 (1995). 

Effect of the Intermolecular Spin-Orbit Interaction in the Collision-Induced Intersystem Crossing of Si State Glyoxal by Ar. 

The mechanistic rationalization of the highly exo-selective formation of oxetanes has been exempHfied by Griesbeck et al. The idea was originally used for rationalization of the endo-selectivity in the reaction with 2,three-dimensionalihydrofuran derivatives (Scheme 10). " Namely, the stereochemical outcome is proposed to come from the relative stabihty of the intermediary triplet 1,4 diradicals (T-1,4-DR), whose conformations are suitable for the spin-orbit coupling (SOC) induced intersystem crossing (ISC) process. The Conformational Memory Effects (Griesbeck Model) in the stereoselectivity can be appH-cable to the stereochemical outcome for the reaction with furan (Scheme 11). 

Fig. 2.5. Transmission difference spectra AT/T of quaterthiophene in solution. Photobleaching (PB) is seen as a rise towards the low wavelength edge and develops outside the experimentally accessible spectral range. Stimulated emission (SE) and photoinduced absorption (PA) from Si are seen at 500 nm and 675 nm, respectively. They decay in about 600 ps and a new PA band appears, at 560 nm, assigned to triplet-triplet transitions. A clear isosbestic point occurs at 600 nm. The data are an example of intersystem crossing in photoexcited molecules induced by spin-orbit wavefunction mixing...

The phenomenon of electron spin polarization in photoexcited triplet molecules in solids has been known for quite some time (39,51,52). The mechanism is associated with the unequal populations of the triplet sublevels induced by the spin-selective nature of the spin-orbit coupling interactions which couple the excited singlet and triplet states during the intersystem crossing (ISC) process. In the presence of an external magnetic field the spin polarization in the molecular frame can be transferred to the laboratory frame for esr observation. Kim and Weissman (83,84) have recently demonstrated beautifully that the initial polarization following photoexcitation of the triplet molecules such as pentacene in dilute solid solution can be readily observed up to a temperature of 275°K. 

Nakajima and Kato used the CASSCF(8,6)/DZP approximation to examine intersystem crossing from the state of glyoxal (Structure 3.4) induced by collisions with argon atoms. They evaluated the and interaction potentials and the spin-orbit coupling matrix elements between these two states at each geometry by using the full Breit—Pauli A semi-... 

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