CSHARP

unity look at 2d

Vector3 dir = target.position - transform.position;
 float angle = Mathf.Atan2(dir.y,dir.x) * Mathf.Rad2Deg;
 transform.rotation = Quaternion.AngleAxis(angle, Vector3.forward);

2d look at unity

//this code will point the object to the mouse
Vector3 diff = Camera.main.ScreenToWorldPoint(Input.mousePosition) 
	- transform.position;
diff.Normalize();
 
float rot_z = Mathf.Atan2(diff.y, diff.x) * Mathf.Rad2Deg;
transform.rotation = Quaternion.Euler(0f, 0f, rot_z - 90);