Calling `/bin/pwd` like that will not work because the program is run by the same process (it is **not** your shell). If you are using `bash`, you can modify your prompt to send an escape sequence to `tmux` to change the window name using the escape sequence shown as an example:
printf '\033kWINDOW_NAME\033\\'
Here is a separate shell script to demonstrate the escape sequence:
#!/bin/sh
printf '\033k%s\033\\' $(pwd)
It is not the same escape sequence used for xterm titles, but you can apply the advice in the Bash Prompt HOWTO in the same way.
Further reading:
* Bash Prompt HOWTO