Dear Mokhalad Ali :
Singlet to singlet transition is a spin-allowed transition, while triplet to singlet is a spin forbidden transition. The difference in the spin state makes the triplet to singlet transition less probable when compared to the singlet to singlet transition. As a result, the lifetime of the triplet state is relatively longer.
Thanks,
Nil Pandey
yes, the transition triplet-singlet means that the spin of the electron has to flip. That needs to occur. Otherwise the transition is not triplet-singlet. The lifetime therefore depends on the average time for the electron spin to flip which usually is the determining factor for such transitions. However there are possibilities to reduce such lifetimes. If there is closed interaction between triplet states of neighbouring molecules triplet-triplet annihilation can occur. Also heavy metal ions can take up electron spin and reduce triplet lifetime.
As mentioned by other two colleagues above, there are some rules governing the transition of electrons or electronic states called selection rules. According to these rules, the electronic transitions in which the spin of the ground and excited states are not the same will not be allowed. That's why these two transitions you mentioned have different life times.
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