Other derivatives like thiophene, pyrrole give electrophilic substitution but why not selenophene? Is it because selenophene does not have enough electron donating effect and not having its electrons in conjugation, making the ring anti-aromatic?
The Se atom has such diffuse valence orbitals compared with carbon that there is not good pi overlap, so we don't really have a conjugated cycle. By definition when there isn't a conjugated cycle, it's "not aromatic", not anti-aromatic or non-aromatic as they both require cyclic conjugation. Words involving 'aromatic' are tricky.