Why does selenophene not undergo aromatic substitution?
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 &quot;not aromatic&quot;, not anti-aromatic or non-aromatic as they both require cyclic conjugation. Words involving 'aromatic' are tricky.