Hi Sourav,
As mentioned in the documentation for "simplify" there is no universal idea to the simplest form of an expression. You may want to try out different Name-Value Pair arguments mentioned in the documentation to get the desired form suitable for your use case
As an example,
executing
T=simplify(T,'Steps',20);
instead gives
T12=
-(exp(-a*k2*2i)*(k1^2 - k2^2)*(exp(a*k2*4i) - 1))/(4*k1*k2)
while
T=simplify(T,'Steps',30);
gives the result
T12=
-(sin(2*a*k2)*(k1^2 - k2^2)*1i)/(2*k1*k2)
Hope this helps