Inlersystem crossing

Inlersystem crossing is most common in molecules that contain heavy atoms, such as iodine or bromine (the heavy-iUom effect). Spin and orbital interactions increase in the presence of such atoms, and a change in spin is thus more favorable. The presence of paranmg-nelic species such as molecular oxygen in solution also enhances inlersystem crossing with a consequent decrease in fluorescence. 

Deactivation of electronic excited slates may also involve phosphorescence. After inlersystem crossing to the triplet state, further deactivation can occur cither by internal or external conversion or by phosphores cencc. A triplet — singlet transition is much less probable than a singlet-singlel conversion. Transition probability and excited-state lifetime are inversely related, Thus, the average lifetime of the excited triplet stale with respect to emission is large and ranges from 10 to 10 s or more. Emission from such a transition may persist for some time after irradiation has ceased. 

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