Unique text index on a non-text column

I'd like to enforce text uniqueness on a non-text column (it's of type "molecule" from a third-party plugin). The third-party plugin doesn't support unique indexes. My guess was something like this, but it doesn't work:

The "molecule" column does contain an ordinary string, and when selected returns ordinary text data.

Is there a way to do this? (And if so, what did I miss in the documentation?)

Thanks,

Craig