Restricting assign user to access work item untill the state of work item get changed

Hi All,

I hope all are doing well, i want your assistance in following scenario, kindly look into this

Scenario:
In a customize Change Request workitem there are 2 different type of users i.e. Team Lead & Dept. Head. when the Change Request being created then Team Lead will be able work on the Change Requests and he will change the status of change request after changing status, system will generate Email notification to the Dept. Head and the same change request will be assign to Dept. head and he will be taking care of things against which he assigned.

What i have implemented so far:
1, i have created a customized RTC work item of Change Request.
2, In Change Request work item i have created two custom attributes called Team Lead & Dept. Head which are Contributor type attributes
3, Both custom attributes are in the same workitem
4, After selecting user in Team lead & Dept. Head dropdown then system assign same Change Request to the both users but i want to assign change request on the basis of state change and also i want to generate email notification to the Team Lead & Dept. Head when the state of work item get changed.

Regards,
SAJJAD