trigger for IsFocused - not working?

I just want to add a trigger for the main window, when it goes out of
focus, so I did a little test app, and it seems to be broken. It raises
an exception, in which it says that 'IsFocused' string is not a valid
value for 'Property' property of type 'DependencyProperty'. Here's the
code:

<Window x:Class="WindowsApplication1.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="WindowsApplication1" Height="300" Width="300">
<Window.Triggers>
<Trigger Property="IsFocused" Value="False">
<Setter Property="Title" Value="Not Focused" />
</Trigger>
</Window.Triggers>
<Grid>

</Grid>
</Window>

I hope someone knows why this is not working, maybe I'm doing something
wrong.

Thanks,
Cosmin.