It's beyond common understanding that the luminescent lifetime of

4

T

1

6

A

1

transition of Mn

2+

doped ZnS decrease drastically by 5 orders of maganitude from bulk to nanacrystal because the magnetic interactions which usuall relax the spin forbiddenness can not be so strong.In this paper

we present a possible mechanism involving in the exchange coulomb interaction between the d electrons of Mn

2+

and electrons of host.Assuming that the spin of the ground state of host is not zero

and that the exchange coulomb interaction cause mixing between the excited

4

T

1

state of Mn

2+

and certain excited state of host whose energy is slightly higher than

4

T

1

state of Mn

2+

we found that the spin-forbidden transition could be almost allowed.Morever

the mixing degree increases as the grain size of host decreases.A rough numerical estimation about this mechanism agrees with the results of experiment.