Singlet-triplet coupling

Marian CM, Schneidaer F, Kleinschmidt M, Tatchen J (2002) Electronic excitation and singlet-triplet coupling in uracil tatutomers and uracil-water complexes. Eur Phys J D 20 357... 

Clearly, much activity may be expected in the specific area of rotational coherence and the more general one of time-resolved and polarization-analyzed fluorescence. One area of interest, not touched upon here, concerns the influence of rotational coherence in electronic relaxation processes. In this we regard it is pertinent to note the polarization-dependent decays reported first by Matsumoto et al.72 in their studies on the nanosecond time scale of singlet-triplet coupling in pyrazine. 

The essential step in understanding collisionally induced intersystem crossing processes has been made by Gelbart and Freed. In small molecules, the optical excitation does not prepare pure spin states but the quasistationary states resulting from the intramolecular singlet-triplet coupling. In a simple two-level model... 

Under conditions of rapid relaxation among the three triplet levels, the singlet-triplet coupling is frequently characterized by the "total spin-orbit coupling strength" for the Sj state, defined by... 

Electronic Excitation and Singlet—Triplet Coupling in Uracil Tautomers and Uracil-Water Complexes, a Quantum Chemical Investigation. 

Summary Complicated nature of aromaticity in all transition metal cyclic systems can be understood more easily on simplified model cyclic triatomic and tetratomic systems as examples of cyclic systems composed out of odd or even number of atoms, respectively. Counting rules for a-, n-, 8-, and systems depend on the nature of atomic orbitals involved in the formation of corresponding bonding/antibonding molecular orbitals. 

Tn many molecular systems, other transitions (i.e. singlet-triplet) can also occur, including intersystem crossing and radiative singlet-triplet coupling (phosphorescence). 

Figure 7.8 Energy levels and relevant coupling scheme for singlet-triplet coupling with virtual transition between two triplet states caused by vibronic interactions.

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