The conjugation
length of PDAs causes the presence of absorption bands
in the visible spectra, but luminescence is generally not
found in PDAs, because the lowest excited energy level is
2Ag, which has a non-radiative transition to the ground
level 1Ag. However, some PDAs have shown luminescence
[1–5], in this case the lowest excited level is 1Bu, which has
a radiative channel to the ground level 1Ag