I think you can be honest without badmouthing anyone necessarily: "startup went bankrupt due to reasons beyond my control".
As for letting your startup founders know why you're leaving - it really depends on your relationship with them but i would got for a professional - "i'd like to expole other opportunities and i wish you all the very best!"
"startup went bankrupt due to reasons beyond my control"
Unless they had a position in the C suite, that can probably be shortened to "Company went bankrupt" with zero further explanation volunteered. Though it helps to think things through ahead of time and figure out how to address any questions that may come up.
The most PC thing to say would be along the lines of "Starting a new business is always a gamble and lots of new businesses simply don't succeed."
As for letting your startup founders know why you're leaving - it really depends on your relationship with them but i would got for a professional - "i'd like to expole other opportunities and i wish you all the very best!"