I'm not sure if it would be fully hardy for you but the absolute best butterfly magnet in my own garden has been butterfly bush, Buddleia. B. 'Royal Red' is tall-growing but is supposed to be one of the best buddleias for attracting butterflies.

Another good butterfly plant is Verbena bonariensis. It won't be a perennial in your climate, but can be grown as an annual to bloom in late summer and combines very well with most tropicals. Plant densely for the best effect.

BTW you might want to post your question on the butterfly gardening forum for more info.

MAYPOP!
Passionvine looks tropical, can go to zone 5 (you might try it!), and if it's the cold-hardy American native (don't just buy any passionvine) then the butterflies and catterpillars love it!

The problem with "tropical-looking" plants is that many have large leaves and flowers, and butterflies often like smaller flat flowers that are easy for them to perch on and to get nectar.

Although, the butterflies here love my neighbors' HIBISCUS.
This won't be hardy for you, but investigate whether hibiscus will over-winter inside.

You can overwinter hibiscus inside - I've been doing it for years. They need a LOT of light and a fair amount of attention, they usually have pests, and they are rather messy, dropping leaves and flowers. Not a great living room plant, but OK if you have a hidden away area.