emacs 18 mapping function keys

[Glynn Smith]

In version 18 of emacs
    (global-set-key (quote [f1]) (quote undo))
produces the followng error:
Wrong type argument: string p, [f1]
This global-set-key command works just fine in version 19 or later.

Using describe-key-briefly for F1 just results in a 11~ written at the
first line first column of the current file being visited.

I believe the vt100 key code for F1 is ESC11~ or ^[11~ so I tried
changing the global-set-key statement to:
    (global-set-key (quote "\e11~") (quote undo))
This produced the following error:
Key sequence ^[11~ uses invalid prefix characters.

I hope someone can help.
Regards