What you are trying to do doesn't work because dropdowns are made to be clicked, and have no action but to open a menu. You can clearly see that in the examples here in the documentation
If you made your button go to
about.php when you clicked
About you would never be able to open the dropdown menu to select other items.
If you want a button you can click AND a dropdown menu you should look at adding a Split Button Dropdown (documentation here) which will give the use a clickable button, and a dropdown arrow to select more options.