how I can define variable name with " write " command in fortran

how I can define variable name with " write " command in fortran

I need to define a variable name for different files inside a fortran
code, while I 'm using this commands
open(unit=5,file="plot.sm")
write(unit=zbin_str,fmt='(f5.2)') zbin
plotname="LF_z"//zbin_str//".ps"
write(5,"dev postencap" plotname)
write(5,"toplabel LF for",//zbin_str//)
I'm receiving these errors :
Syntax error, found '//' when expecting one of: ( * ...
write(5,"toplabel LF for",//zbin_str//)
error #6355: This binary operation is invalid for this data type. [PLOTNAME]
write(5,"dev postencap" plotname)
An arithmetic or LOGICAL type is required in this context.
write(5,"dev postencap" plotname)
how I can define the available name inside the fortran code?? Thanks