I think the cause is the functionalization of pyrene with functional groups having double bonds with 2 carbon atoms both by sp hybridization and triple bonds (1 sigma and 2 pi bonds). This condition can extend the π-conjugation system leading to a bathochromic shift from the emission band to the visible region. Furthermore, the shift results in the vibrational structure of the emission spectrum and at the same time prove that the ground state interactions occurring of the pyrene with the potential host molecule are provided by the emission spectrum.

Hopefully, this answer can provide some insight.