Turkish Airlines is currently running a promotion offering a 15% Discount for bookings through Mobile App however it seems that the application has some technical issues at the moment.
Turkish Airlines has some very good business class fares to begin with (depending on the city pair) but the extra 15% discount made me think that I might want to try them again, especially since they now use a A330 wide body aircraft between Istanbul and Frankfurt.
You can access their promotional page here.
Unfortunately the furthest I get on their app is the date selection:
I wish that all these travel providers had a bit more quality control related to their IT services but especially Turkish Airlines isn’t really famous for having a reliable website or application for that matter.
If you can get it to work then the 15% might be a nice discount in case you’re able to find a decent fare on TK for your desired city pair.