It all depends on the types of tropicals. If your barn is open, it will probably be the same temp as outside, just no frost. You could throw frost blankets over them in the barn. That might be enough but still a gamble. I had frost blankets on my peppers and papaya but lost them anyway. My lime had christmas light under the blanket and made it. I think one cold night at a time isn't too bad, but when you have repeated nights of freezing like the last few years, they just can't make it. I've lived in my house 15 years and the last couple years have finished off most of my hibiscus's.

Thats what I was afraid of....