Triplet states intersystem crossing from

A further possibility is that molecules undergo a nonradiative transition from the excited singlet states to the triplet states (intersystem crossing). This is the origin of some loss mechanisms for dye lasers. 

Deactivation processes competing with fluorescence are mainly nonradiative deactivation to the S0 state (IC) and nonradiative transition to a triplet state (intersystem crossing, ISC). Photochemical products are often formed from this triplet state. Important photochemical reactions are the E—yZ isomerization of ethylene, the oxidation of pyrazoline to pyrazole, and the dimerization of cou-marins. 

Skeletal motions can also induce transitions from the lowest excited singlet state to the lowest triplet state (intersystem crossing). Thus, changes in k may be due not only to internal conversion but also to Intersystem crossing. It should be noted that Injection can also take place from the triplet state, as discussed by Spltler et al. (29). 

The excited molecule can cross over to its triplet state (intersystem crossing), and from its triplet state, lose energy in the form of heat (internal conversion) and/or light phosphorescence, as it returns to its ground state. 

Intersystem crossing from Si to and higher triplet states has also been reported in some aromatic molecules Li, R. Lim, E.C. Chem. Phys., 1972, 57, 605 Sharf, B. Silbey, R. 

Subsequent to the formation of a potentially chemiluminescent molecule in its lowest excited state, a series of events carries the molecule down to its ground electronic state. Thermal deactivation of the excited molecule causes the molecule to lose vibrational energy by inelastic collisions with the solvent this is known as thermal or vibrational relaxation. Certain molecules may return radia-tionlessly all the way to the ground electronic state in a process called internal conversion. Some molecules cannot return to the ground electronic state by internal conversion or vibrational relaxation. These molecules return to the ground excited state either by the direct emission of ultraviolet or visible radiation (fluorescence), or by intersystem crossing from the lowest excited singlet to the lowest triplet state. 

Both l3A"-4b and 23A"-4b are predicted to be minima on the potential surface at the CASSCF level.61 This is in contrast with the situation for 4a, where one triplet state is calculated to be a transition state for pseudorotation of the other.55-57 A derivative of triplet 4b has been implicated in solution trapping experiments,49 but there have been no reports of the direct observation of 4b. Facile intersystem crossing from the triplet ground state (13A") to the open-shell singlet state ( A"), followed by vibrational relaxation to one of the enantiomers of 3b (Fig. 12), would be expected to rapidly depopulate the 13A" state of 4b. Thus, direct detection of triplet 4b may prove to be much more difficult than detection of 4a was.lla... 

Because the triplet state is produced by intersystem crossing from Si, not all absorbed photons result in the Ti state that is able to emit phosphorescence. 

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