Selenium — like sulfur — is a chalkogen/member of group 16. It is heavier than sulfur, and thus more metalloid. In this regard similar to the trend in the adjacent group 17 (the halogens) where iodine has more metal-like character than the other members, or group 15 (the tetrels) with carbon; yet silicon, germanium, tin, and lead. Thus, formally, selenophene is isoelectronic to thiophene, a with six π-electrons over five ring atoms an electron enriched heteroaromatic compund (compared to benzene).
But, comparing pyrrole, furan, thiophene and selenophene, Yadav arguments
«The electron count [of π electrons for the Hueckel rule], however, is not enough. Along with the number of requisite electrons, all the bond lengths in the ring must be the same or very similar, as in benzene. In five-membered heterocylces such as pyrrole, furan, and thiophene, the size of the heteroatom lone pair orbital must be similar to that of the $p$ orbital on ring π bonds. Both the requirements are not fulfilled in thiophene because σC–S bond is significantly longer than σC–C bond and, in comparison to the p orbital on carbon, the size of lone pair orbital on sulfur is too large to allow effective overlap.»
(abstract)
Given the larger radius of selenium ($\pu{115 pm}$) vs. sulfur ($\pu{100 pm}$), this might be even more the valid for selenophene, than for thiophene.
The discussion however isn't a new one. By older recordings, the aromaticity of tellurophen and selenophen were considered between the one of furan and thiophene:
(Fringuelli et al., 1974)
References:
Fringuelli, F.; Marino, G.; Taticchi, A.; Grandolini, G. A comparative study of the aromatic character of furan, thiophen, selenophen, and tellurophen. J. Chem. Soc., Perkin Trans. 2 1974, 332-337; doi 10.1039/P29740000332.
Yadav V.K. (2021) Relative Aromaticity of Pyrrole, Furan, Thiophene and Selenophene, and Their Diels–Alder Stereoselectivity. In: Steric and Stereoelectronic Effects in Organic Chemistry, 191-216. Springer, Cham. doi 10.1007/978-3-030-75622-2_9.
Note, Yadav's publication is behind a paywall for which our library has no subscription. I only was able to access the abstract above.
